© 2016 Junlin Jiangufdcimages.uflib.ufl.edu/UF/E0/05/06/44/00001/JIANG_J.pdfI also want to give special thanks to Weijia Niu, Guagua and Niannian for the happy memories we had together

CHARGE TRANSFER STUDIES IN MOLECULAR DONOR-ACCEPTOR SYSTEM AND APPLICATIONS IN POLYMER BASED SOLAR ENERGY CONVERSION

By

JUNLIN JIANG

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2016

© 2016 Junlin Jiang

To my families

4

ACKNOWLEDGMENTS

I want to give my sincere appreciations to all those who helped me in the past

five years at University of Florida. First of all, I want to express my deepest thanks to my

research advisor, Dr. Kirk S. Schanze, for his parental guidance, inspirational

encouragement and endless supports through my PhD research career. It has been a

privilege and honor for me to work with him, we shared lots of failures and successes on

the way towards splitting the water.

I would like to give my sincere thanks to my committee members, Prof. Brent S

Sumerlin, Prof. Jiangeng Xue, Prof. Kenneth B. Wagener and Prof. Valeria D. Kleiman,

for their help, support, valuable time providing suggestions and revising documents, and

strong appreciations to Prof. Brent S Sumerlin to serve as my co-advisor. I want to also

give my thanks to Prof. Thomas Meyer, Prof. John Reynolds, Prof. John Papanikolas,

Prof. Gerald. Meyer, Dr. Fang Zhen, Dr. Ben Sherman and Zachary A. Morseth for their

advice and collaborations in the EFRC project. Equally, I would like to thank Prof. Omar

F. Mohammed Abdelsaboor and Ms. Amani Alsam from KAUST for their contributions in

studying OPE-NDI project.

I am so proud and grateful to work in a group with so many great colleagues. Dr.

Zhuo Chen trained me in controlled polymer synthesis and characterization, Dr.

Zhenxing Pan helped me to start the OPE-NDI project, Dr. Shanshan Wang helped me

in photophysical measurements. Dr. Anand Parthasarathy, Dr. Randi Price, Dr.

Subahdip Goswami. Dr. Ali Gundogan. Yajing Yang, Xiaofeng Chen are my lab mates,

thank them for the wonderful working experiences together. I want to express my

special thanks to Dr. Gyu Leem for his collaborations in EFRC project (Dr.Gyu Leem

accomplished the work in DSSC device fabrication and characterizations in Chapter 4

5

and photoanode implementation by “Layer-by-Layer method in Chapter 5). I am so glad

to work with him on EFRC projects for 5 years. Also, I want to thank Austin Jones for

the collaboration in diblock project (Austin accomplished all the ultrafast measurements

for dilcock oligomers in Chapter 3).

There are a lot of current students and former graduates in the Schanze groups I

want to thank. Dr. Dongping Xie, Dr. Danlu Wu, Dr. Xuzhi Zhu, Dr. Jie Yang helped me

adapt to the new working environment and are always willing to give me assistance. Dr.

Seda Cekli, Dr. Shanshan Wang and I joined the Schanze group at the same year, we

had lots of fun working together. I also want to thank Dr. Raj, Dr. Russ Winkel, Dr. Jan-

Moritz Koenen, Dr. Galyna Dubinina, Yun Huang, Jiliang Wang, Zhiliang Li, Ethan D.

Holt, Bullock James D, Bethy Kim, Ru He and Yan Zhao for their valuable advice and

friendship. I also want to give special thanks to Weijia Niu, Guagua and Niannian for the

happy memories we had together in Gainesville and their supports on my research.

Last, but not least, I want to thank my parents, Mr. Daohui Jiang and Mrs Peixiu

Jiang and two elder sisters, Miss Chunhua Jiang and Miss Lihua Jiang for their endless

love, support and encouragement through my life. This dissertation is dedicated to

them.

6

TABLE OF CONTENTS page

ACKNOWLEDGMENTS .................................................................................................. 4

LIST OF TABLES ............................................................................................................ 9

LIST OF FIGURES ........................................................................................................ 10

LIST OF ABBREVIATIONS ........................................................................................... 14

ABSTRACT ................................................................................................................... 16

CHAPTER

1 INTRODUCTION .................................................................................................... 18

Conjugated Donor-Acceptor Oligomers .................................................................. 18

Conjugated Oligomers ...................................................................................... 18 Energy Transfer and Electron Transfer ............................................................ 19

Energy Transfer ......................................................................................... 19

Electron Transfer ....................................................................................... 22 Donor-Acceptor Conjugated Systems .............................................................. 25

π-Conjugated Bridge .................................................................................. 25 Donor and Acceptor ................................................................................... 27 Detection of Electron Transfer Process ..................................................... 27

Light Harvesting Polymers ...................................................................................... 30 Metal Chromophore Based Polymer ................................................................ 30

Synthesis of Metal Chromophore Based Polymers .......................................... 30 Metal Complexes ....................................................................................... 31

Polymer Backbone ..................................................................................... 34 Ru(II) Chromophore Based Polymers .............................................................. 34

Site-Site Energy Transfer ........................................................................... 35

Competitive Charge and Energy Transfer .................................................. 36 Artificial Photosynthesis .......................................................................................... 37

Natural Photosynthesis..................................................................................... 38 Artificial Photosynthesis Design ....................................................................... 41 Dye-Sensitized Photoelectrosynthesis Cell ...................................................... 44

Design and Mechanism of DSPEC ............................................................ 44

Chromophore-Catalyst Assembly .............................................................. 45 Chromophore ............................................................................................. 47 Catalyst ...................................................................................................... 49

Semiconductor ........................................................................................... 51 Immobilization ............................................................................................ 53 Device Implementation of DSPEC ............................................................. 55

7

2 EFFECT OF CONJUGATION LENGTH ON PHOTOINDUCED CHARGE-

TRANSFER IN A -CONJUGATED OLIGOMER-ACCEPTOR DYADS ................. 57

Background ............................................................................................................. 57 Results and Discussion........................................................................................... 59

Oligomer Structures, Synthesis, and Characterization. .................................... 59

Photophysical Studies ...................................................................................... 60 Energetics of Photoinduced Charge Transfer. .................................................. 64 Intermolecular Photoinduced Charge Transfer ................................................. 66 Charge Separation and Charge Recombination (fs-TA) ................................... 68 Charge Separation and Charge Recombination (TRIR) ................................... 72

Distance Effect ................................................................................................. 73 Solvent Effect ................................................................................................... 75

Electron Transfer .............................................................................................. 76 Summary ................................................................................................................ 78 Experiments and Materials ..................................................................................... 79

Instrumentation and Methods ........................................................................... 79

Materials ........................................................................................................... 82 Synthesis and Characterization ........................................................................ 82

3 WAVELENGTH CONTROL OF PHOTOINDUCED CHARGE TRANSFER IN A

-CONJUGATED DIBLOCK MOLECULAR PHOTODIODE ................................... 93

Background ............................................................................................................. 93 Results and Discussion........................................................................................... 93

Photophysical Study ......................................................................................... 95

Bimolecular Charge Transfer Study ................................................................. 99 Electrochemistry Study ................................................................................... 101

Femtosecond Transient Absorption Spectroscopy for CT/CR ........................ 102 Energetics for Selective Charge Transfer States ............................................ 106

Summary .............................................................................................................. 109 Experiments and Materials ................................................................................... 109

Instrumentation and Methods ......................................................................... 109

Materials ......................................................................................................... 111 Synthesis and Characterization ...................................................................... 112

4 THE SYNTHESIS OF RUTHENIUM(II) CHROMOPHORES GRAFTED POLYSTYRENES AND APPLICATIONS IN DYE-SENSITIZED SOLAR CELLS . 121

Background ........................................................................................................... 121 Results and Discussion......................................................................................... 122

Structure Design and Synthesis ..................................................................... 122 Photophysical Properties ................................................................................ 127 DSSC Solar Cell Applications ......................................................................... 132

Summary .............................................................................................................. 137 Experiments and Materials ................................................................................... 137

Instrumentation and Methods ......................................................................... 137

8

Materials ......................................................................................................... 139

Synthesis and Characterization ...................................................................... 140

5 POLYMERIC CHROMOPHORE WATER OXIDATION CATALYST ASSEMBLY FOR SOLAR FUEL SYSTEM ............................................................................... 147

Background ........................................................................................................... 147 Results and Discussion......................................................................................... 150

Target Design and Synthesis ......................................................................... 150 Photophysical Study ....................................................................................... 155

Electrochemistry Study ................................................................................... 156 Oxidation Reactions of Organic Compounds .................................................. 159 Water Oxidation Study ................................................................................... 163

Summary .............................................................................................................. 166

Experiments and Materials ................................................................................... 166 Instrumentation and Methods ......................................................................... 166

Materials ......................................................................................................... 169 Synthesis and Characterization ...................................................................... 170

6 A NEW GENERATION OF POLYMERIC CHROMOPHORE WATER OXIDATION CATALYST ASSEMBLY FOR SOLAR FUEL SYSTEM ................... 173

Background ........................................................................................................... 173

Results and Discussion......................................................................................... 174 Target Design and Synthesis ......................................................................... 174

Photophysical Study ....................................................................................... 181 Electrochemistry Study ................................................................................... 183

Water Oxidation Reaction Study .................................................................... 185 Summary .............................................................................................................. 188 Experiments and Materials ................................................................................... 188

Instrumentation and Methods ......................................................................... 188 Materials ......................................................................................................... 190

Synthesis and Characterization ...................................................................... 190

7 CONCLUSION ...................................................................................................... 193

LIST OF REFERENCES ............................................................................................. 199

BIOGRAPHICAL SKETCH .......................................................................................... 218

9

LIST OF TABLES

Table page 2-1 Summary of the photophysical properties .......................................................... 62

2-2 Energetics of PEn-NDI ....................................................................................... 65

2-3 Charge separation/recombination kinetics from fs-TA and TRIR studies ........... 71

2-4 Charge separation/recombination kinetics for PE6-NDI from TRIR .................... 76

3-1 Summary of the photophysical properties .......................................................... 96

3-2 Summary of the charge transfer efficiency ......................................................... 98

3-3 Electrochemistry study. .................................................................................... 102

3-4 Summary of electrochemistry properties .......................................................... 106

4-1 Photophysical properties of pomplexes and polymers ...................................... 130

10

LIST OF FIGURES

Figure page 1-1 The common π-conjugated oligomer. ................................................................. 18

1-2 Presentation of energy transfer mechanisms. .................................................... 20

1-3 Schematic representation of electron transfer. ................................................... 22

1-4 Graphical comparison of the McConnell super-exchange and Gamow tunneling mechanism .......................................................................................... 24

1-5 Example of a donor-bridge-acceptor (D-B-A) system ......................................... 25

1-6 Summary with common π-conjugated bridges applied in D-B-A systems .......... 26

1-7 Schematic diagram for femtosecond-TA apparatus ............................................ 28

1-8 Charge transfer dynamics studied by fs-TA ........................................................ 29

1-9 Summary of metal containing polymers .............................................................. 31

1-10 Presentation of common synthesis techniques................................................... 32

1-11 Simplified molecular orbital diagram. .................................................................. 32

1-12 Representation of possible deactivation channels of excited states ................... 34

1-13 Polystyrene based Ru(II) chromophore grafted polymer. ................................... 35

1-14 Site-site energy migration along PS-Ru(II) polymer ............................................ 36

1-15 Competitive energy transfer and charge transfer in PF-Ru assembly ................ 37

1-16 The illustration of photosynthesis processes ...................................................... 39

1-17 The structure of Mn4CaO5 cluster ....................................................................... 40

1-18 The S-state cycle for water oxidation reaction .................................................... 40

1-19 Cobalt thin-film assembly ................................................................................... 41

1-20 Preparation of IrO2-modified LaTiO2N electrode and water oxidation properties ........................................................................................................... 42

1-21 The illustration of the PV electrolysis device ...................................................... 43

1-22 The demonstration of DSPEC ............................................................................ 45

11

1-23 The summary of chromophore-assembly architecture ........................................ 46

1-24 Ru(II) polypyridyl chromophores. ........................................................................ 48

1-25 Structure of blue dimer and the representative pathways of O–O bond formation. ........................................................................................................... 49

1-26 Summary of some pioneering single-site Ru(II) catalyst ..................................... 50

1-27 Depiction of catalyst onto semiconductor ........................................................... 52

1-28 Demonstration of SnO2/TiO2 core-shell substrate............................................... 53

1-29 Summary of bound modes of chromophore-catalyst assemblies onto semiconductor. ................................................................................................... 54

1-30 Demonstration of DSPEC device. ....................................................................... 56

2-1 Oligomer structures ............................................................................................ 60

2-2 Photophysics study ............................................................................................. 61

2-3 Intersystem crossing study ................................................................................. 64

2-4 Schemaic illustation of the energy levels for OPE-NDI oligomers. ..................... 66

2-5 Intermolecular charge transfer study .................................................................. 67

2-6 Charge separation and charge recombination study for OPE-NDI. .................... 69

2-7 Dynamics study of PE4-TIPS and PE4-NDI. PE4-TIPS ....................................... 70

2-8 Kinetics of charge separate and charge recombination processes ..................... 71

2-9 Transient Infrared spectra of OPE-NDI ............................................................... 73

2-10 Distance factor study in OPE-NDI ...................................................................... 74

2-11 Solvent factor study in PE6-NDI ......................................................................... 76

3-1 Structures of diblock oligomers. .......................................................................... 94

3-2 UV-visible absorption and fluorescence spectra. ................................................ 95

3-3 Fluorescence emission spectra for T4PE4TIPS and PE4TIPS under different excitation wavelengths ....................................................................................... 97

3-4 Time correlated single photon counting (TCSPC) experiments of T4PEnTIPS and T4PEnNDI ..................................................................................................... 97

12

3-5 Fluorescence quantum yields under different excitation wavelengths of T4PE4NDI. ........................................................................................................... 98

3-6 Nanosecond transient absorption spectra of T4PEnTIPS oligomers with MV2+. 100

3-7 Electrochemistry study ..................................................................................... 101

3-8 Picosecond transient absorption spectra of T4PE4 and T4NDI .......................... 103

3-9 Picosecond transient absorption of T4PE4NDI under different exitation wavelengths...................................................................................................... 104

3-10 Transient absorption kinetics of T4PE4NDI ....................................................... 105

3-11 Molecular modeling .......................................................................................... 107

3-12 Energy level diagram for T4PE4NDI. ................................................................. 108

4-1 Structure of the model complex and polymer. .................................................. 123

4-2 Synthesis scheme for polypyridylruthenium derivatized polystyrene. ............... 124

4-3 GPC characterization ....................................................................................... 125

4-4 1H NMR spectra of polymers ............................................................................ 126

4-5 ATR-IR spectra of PVBA-170 and Poly-170-Cl. ................................................ 127

4-6 Absorption and emission spectra. ..................................................................... 128

4-7 Plot of the lifetime and quantum yields versus degree of polymerization of Poly-n-Cl ........................................................................................................... 129

4-8 Stern-Volmer plots ............................................................................................ 131

4-9 Structure for Ru-A model compound and PS-Ru-A .......................................... 132

4-10 The synthesis of PS-Ru-A ................................................................................ 133

4-11 Adsorption profiles of TiO2//PS-Ru-A films ....................................................... 135

4-12 IPCE and J-V curve for DSSC. ......................................................................... 136

5-1 Structure of polystyrene based catalyst-chromophore assembly. .................... 150

5-2 Synthesis procedure of functional catalyst ....................................................... 152

5-3 Synthesis procedure of polymeric catalyst-chromophore assembly (Poly-10 and Poly-11) ..................................................................................................... 153

13

5-4 1H-NMR and FT-IR for Poly-11 ......................................................................... 154

5-5 Photophysical studies ....................................................................................... 156

5-6 Electrochemistry studies ................................................................................... 158

5-7 UV-visible absorption spectra for FTO//TiO2//(PAA/Poly-10)10 and FTO//TiO2//(PAA/Poly-8)10. ............................................................................... 160

5-8 Photocatalytic oxidation of PhOH ..................................................................... 161

5-9 Photocatalytic oxidation of BnOH ..................................................................... 162

5-10 Photocatalytic oxidation of Water ..................................................................... 164

6-1 The illustration of the polymer assembly deposited photoanode. ..................... 175

6-2 Synthesis for the ethyne functionalized catalyst ............................................... 176

6-3 Synthesis of Poly-1. .......................................................................................... 177

6-4 1H-NMR of Poly-1.. ........................................................................................... 178

6-5 Synthesis of Poly-2 ........................................................................................... 180

6-6 UV-visible absoprtion study .............................................................................. 181

6-7 Emission study and life time measurements. ................................................... 182

6-8 Electrochemistry studies. .................................................................................. 184

6-9 UV-visible absorption study for photoanodes. .................................................. 185

6-10 Photocatalytic oxidation of Water ..................................................................... 186

14

LIST OF ABBREVIATIONS

A Acceptor

AQS

CB

D

bpy

DCM

DMF

DMSO

DP

DSSC

DSPEC

ESA

FTO

FF

FRET

FTIR

GPC

IPCE

ITO

HOMO

LUMO

MLCT

Mn

MV

9,10-Anthraquinone-2,6-disulfonate

Conduction band

Donor

Bypyridine

Dichloromethane

Dimethylformamide

Dimethylsulfoxide

Degree of polymerization

Dye sensitized solar cell

Dye sensitized photoelectrosynthesis cell

Excited-state absorption

Fluorine doped tin oxide

Fill factor

Förster resonance energy transfer

Fourier transform infrared spectroscopy

Gel permeation chromatography

Internal photon to current efficiency

Indium tin oxide

Highest occupied molecular orbital

Lowest unoccupied molecular orbital

Metal to ligand charge transfer

Number average molecular weight

Methyl viologen

15

MW

NDI

NHS

NIR

NMP

NMR

OPE

PDI

PET

PEC

PF

PMDETA

PS

PVBA

PVBC

SnO2

TA

TBAF

TCSPC

terpy

THF

TiO2

TMS

TIPS

VB

Molecular weight

Naphthalene diimide

N-hydroxysuccinimide

Near-infrared

Nitroxide mediated polymerization

Nuclear magnetic resonance spectroscopy

Oligo(phenylene-ethynylene)

Polydispersity index

Photo-induced electron transfer

Photoelectrosynthesis cell

Polyfluorene

N,N,N’,N’’,N’’-Pentamethyldiethylenetriamine

Polystyrene

Poly (4-vinylbenzyl azide)

Poly (4-vinylbenzyl chloride)

Tin dioxide

Transient absorption

Tetrabutyl ammonium fluoride

Time-correlated single photon counting

Terpyridine

Tetrahydrofuran

Titanium dioxide

Trimethylsilyl

Triisopropylsilyl

Valence band

16

Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

CHARGE TRANSFER STUDIES IN MOLECULAR DONOR-ACCEPTOR SYSTEM

AND APPLICATIONS IN POLYMER BASED SOLAR ENERGY CONVERSION

By

Junlin Jiang

December 2016

Chair: Kirk S. Schanze

Cochair: Brent S. Sumerlin Major: Chemistry

Photoinduced electron transfer (PET) has been studied intensively in physics,

chemistry and biology for its key roles in artificial photosynthesis and photovoltaics. One

of the key goals in PET is to produce a long-lived charge separated state.

Herein, we present how to produce long-lived charge separated states in a -

conjugated donor-acceptor oligomer system and study their kinetics via two

approaches. First, three naphthalene diimide (NDI) end-capped oligo-phenylene

ethynylene (OPE) oligomers were synthesized., with variable conjugation length

controlled, from which, we studied the charge separate and chare recombination

processes under different conjugation lengths. Second, we prepared a diblock oligomer

end capped by one acceptor in configuration of T4PE4NDI, which features

tetrathiophene (T4) and tetra-phenyleneethynylene (PE4) as electron rich -conjugated

segments, possessing different electron donor strengths (ΔEHOMO (T4-PE4)=0.76eV,

capped with a naphthalene diimide unit as an electron acceptor. When selectively

exciting the PE4 segment (λex = 370 nm), the electron transfers to NDI in 44% yield,

competing against a concurrent energy transfer to T4 section. However, when

17

selectively exciting the T4 segment (λex = 420 nm), no obvious charge transfer

observed.

Furthermore, we prepared polystyrene based Ru(II) complexes based polymer

(PS-Ru) and studied its photophysical properties, including site-site energy transfer and

amplified quenching effects. A new Ru(II) complexes based polymer featuring

carboxylic acid anchoring groups were also synthesized to implement a dye-sensitized

solar cell. The antenna effects in PS-Ru polymer inspired us to incorporate water

oxidation catalyst (WOC) and Ru(II) chromophore into polymeric assembly to achieve

the artificial photosynthesis (harvest sunlight and generate solar fuels) in form of dye

sensitized photoelectrosynthesis cell (DSPEC). The polymer based catalyst-

chromophore assemblies were characterized by 1HNMR, IR, UV-visible spectroscopies

and electrochemistry studies. The polymer assemblies were deposited onto FTO//TiO2

and FTO//(SnO2/TiO2) substrate to prepare photoanode and fabricate into DSPEC.

Based on our work on photo-catalytic activities and stabilities, the polymer assemblies

deposited photoanodes are capable to harvest and convert sunlight into solar fuels

actively through oxidizing water and organic substrates (phenol and benzyl alcohol).

18

CHAPTER 1 INTRODUCTION

Conjugated Donor-Acceptor Oligomers

Conjugated Oligomers

Conjugated oligomers have been used in numerous applications in materials

science for their interesting optical, electrical, and optoelectronic properties. Moreover,

conjugated oligomers are usually considered as model compounds for the

corresponding conjugated polymers to study their possible properties.1-6 Different from

polymers, which are made up of a large number of repeating building blocks,

conjugated oligomers constitute their lower homologs, containing only one or a few π-

conjugated units (Figure 1-1).

Figure 1-1. The common π-conjugated oligomer. Reprinted with permission from ref 3. Copyright 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

It appears valueless to place an absolute border between oligomers and

polymers in terms of their repeating units, while it is certainly a critical feature that

increasing the chain length of an oligomer changes its properties until a convergence

limit reached where further increase of the chain length brings no obvious property

differences. For example, the effective conjugation length of oligothiophenes is about

10-20 repeating units, and it is similar for other conjugated heterocyclic oligomers as

well.1, 7 Actually, one of the significances of oligomers is to elucidate the polymer

19

structures and even its properties, since conjugated oligomer is monodispersed, while

polymer is not well structured. Based on the shape, oligomers could be categorized into

linear oligomers, star-shaped oligomers and dendritic oligomers.

Energy Transfer and Electron Transfer

Electron transfer and energy transfer are two fundamental steps to achieve many

important chemical and biological processes, ranging from light harvesting, energy

conversion and energy storage. In those processes, light harvesting molecules collect

and transfer energy to reaction centers, where cascades of electron transfer reactions

take place. These processes are also important in studying molecular electronics.8, 9

The molecule that gives energy or electron is called donor (D) while the molecule that

accepts energy or electron is called acceptor (A). The system incorporating donor and

acceptor units are normally called D-A system or D-B-A system if a bridge exits

between donor and acceptor.

Energy Transfer

Electronic excitation energy is transferred between molecules by two possible

ways, emission of a photon or a non-radiative pathway. The nonradiative energy

transfer process, where a donor and an acceptor molecule exchange excitation energy

(a virtual photon) in a short distance. The energy transfer occurs in either between

freely diffusing donor and acceptor molecules or between D and A linked together, in

form of D-A or D-B-A, as shown in Equation 1-1. Förster energy transfer occurs via a

dipole-dipole interaction, while Dexter energy transfer involves an electron exchange

interaction.

D* + A → D + A*

20

D*-B-A → D-B-A* (1-1)

The requirement for energy transfer is that the relevant transitions, D* → D and A → A*

of the donor and acceptor are in resonance and that the states are coupled by suitable

donor–acceptor interactions. It could be fulfilled if the spectral overlap integral is

sufficiently large. The electronic coupling for energy transfer can be divided into a

Coulomb and an exchange part, and results in two mechanisms with focuses on either

Coulomb part or exchange part called Förster energy transfer or Dexter energy transfer

(Figure 1-2).

Figure 1-2. Presentation of energy transfer mechanisms. (A) Dexter energy transfer, (B) Förster energy transfer. Figures are adapted with modification 10. Copyright 2014 Royal Society of Chemistry.

In Förster energy transfer mechanism, the driving force comes from the

electronic coupling interaction between the dipole moments of the excited donor and the

acceptor (dipole-dipole interaction). During the Förster energy transfer process, the

energy from the donor (D*) is transferred to the acceptor (A) via a Columbic dipole-

21

dipole interaction. As a result, the electron in donor (D*) relaxes from its lowest

unoccupied molecular orbital (LUMO) relaxes to its highest occupied molecular orbital

(HOMO), and the electron in acceptor (A) gets excited from HOMO to LUMO. This

mechanism does not involve the exchange of electrons and therefore occur over a

relatively long distance (30 - 100 Å). The electrostatic interaction energy (E) is

proportional to the strength of both transition dipoles (μD and μA) and inversely

proportional to the cube of the distance between the donor and acceptor, as shown in

Equation 1-2.

(1-2)

And, the energy transfer rate (kET) is proportional to the square of the electrostatic

interaction energy (E), as shown in Equation 1-3.

(1-3)

Based on Equations 1-2 & 1-3, energy transfer rate is proportional to the inverse

sixth power of the distance between donor and acceptor. So energy transfer efficiency

increases with the magnitude of dipole moments and decreases significantly with

increasing donor-acceptor separation.

In the Dexter energy transfer process, there is direct electron exchange occurring

between the donor and acceptor. The excited electron from the LUMO of D* transfers to

the LUMO of A, and an electron from the HOMO of A transfers to the HOMO of D*. The

transfer rate is exponential of the distance of D and A (RDA), so Dexter energy transfer is

22

efficient only when D and A are in relative short distances (5-10 Å), with the energy

transfer rate equation is given in Equation 1-4:

(1-4)

Where K is related to specific orbital interactions, J is the spectral overlap integral, R0 is

the separation of D* and A when they are in van der Waals contact and RDA is the

distance between D and A.

Electron Transfer

Unlike energy transfer, electron transfer involves actual electron transfers from a

donor to an acceptor, producing a charge-separated state (Figure 1-3). During the

photo-induced electron transfer process, an electron in the donor is excited to its LUMO

and then transfers to the LUMO of the acceptor, forming the oxidized donor (D+) and the

reduced acceptor (A-). The charge-separated state (D+ - A-) is deactivated to the ground

state (D-A) by either charge recombination or other subsequent electron transfer

processes.

Figure 1-3. Schematic representation of electron transfer.

Marcus developed a theory to explain electron transfer, in which an electron

transfer process was considered as a transition state, the excited donor and acceptor

pair (D-A)* and the charge separation state (D+-A-) are treated as reactant and product

23

respectively.11 Accordingly, the rate for electron transfer can be described by the

Marcus equation (Equation 1-5)

(1-5)

where V is the electronic coupling matrix element, λ is the reorganization energy, and

∆G0 is the Gibbs free energy change during the reaction. As shown from the Equation

(1-5), the electron transfer rate is controlled by two parameters: 1) the electronic

coupling and 2) the reorganization energy. Maximizing the electronic coupling or/and

minimizing the reorganization energy enhances the electron transfer rate.

At the avoided crossing geometry, where the energies of the reactant equals to

that of product states, the electron tunnels with a rate determined by the electronic

coupling (V). The electronic coupling depends on several factors, including the involved

states, the donor and acceptor, and their spatial separation (RDA). There are two models

to calculate the electronic coupling , which are Gamow model12, 13 and McConnell

model.14-16

From the Gamow model, a quantum mechanical particle can reside in regions

where the potential energy is larger than the total energy of the particle, with electronic

coupling calculated with Equation 1-6:

(1-6)

From the McConnell model, the electronic coupling is mediated by the low lying

virtual states possessed in bridging structure composed of several identical units (either

24

monomer units of an oligomeric molecular bridge or solvent molecules), with electronic

coupling calculated with Equation 1-7, which is a simplified function applied when bridge

length between the connecting the donor and acceptor is a linear function of subunits.

(1-7)

It is clearly that both the Gamow tunneling model and the McConnell super-

exchange model conclude that the electronic coupling depends on the donor– acceptor

energy gap, so that the coupling decreases approximately exponentially with distance.17

For molecular bridges with different lengths, the Gamow tunnelling barrier is different for

the π -conjugated bridge with different lengths, while McConnell superexchange energy

gap is constant (Figure 1-4).

Figure 1-4. Graphical comparison of the McConnell super-exchange (A) and Gamow tunneling mechanism (B). Reprinted from reference 14. Copyright 2007 Royal Society of Chemistry

B)

A)

25

Donor-Acceptor Conjugated Systems

Molecular donor-bridge-acceptor (D-B-A) systems(Figure 1-5) have been studied

intensively to study the electron transfer/energy transfer and applications in creating

long lived radical cation-anion pair states, aiming at making molecular electronics18 or

mimicking artificial photosynthesis.19

Figure 1-5. Example of a donor-bridge-acceptor (D-B-A) system. The dimers ZnP-nB-AuP+ and ZnP-RB-AuP+. Reprinted with permission from ref 20. Copyright 2006 American Chemical Society.

π-Conjugated Bridge

In those D–B–A systems, donor (D) and acceptor (A) are covalently linked

through a π-conjugated bridge (B).21 The π-conjugated linkers hold the donor and

acceptor in specific positions with controlled linker lengths and types, allowing well-

defined geometries obtained. Moreover, they are effectively involving promoting

different types of transfer processes. So far, several types of conjugated bridges have

been developed towards constructing molecular D-B-A systems (Figure 1-6), including

oligo-phenylenevinylenes (OPV),22, 23 oligo-phenyleneethynylene (OPE),17, 20, 24

oligofluorene25-27 and oligothiophene.28 These bridges are not just applied as spacers to

separate donor and acceptor, instead, they are actively involved in mediate electronic

coupling for electron transfer process, which is well known now, as summarized in a

single exponential decay constant, attenuation factor β.21 For example, βCS = 0.31 Å-1

and βCR = 0.39 Å-1 were reported for an OPE bridged ZnP-nB-AuP+ system.20

26

Figure 1-6. Summary with common π-conjugated bridges applied in D-B-A systems. Reprinting from reference 14. Copyright 2007 Royal Society of Chemistry.

Wasielewski and co-workers investigated the balance between coherent

superexchange and incoherent hopping mechanisms through spacers of oligo-p-

phenylene (OP) in phenothiazine (PTZ)-OP-perylene-3,4:9,10-bis(dicarboximide) (PDI)

system.29, 30 They observed a switch in two electron transfer mechanisms: a coherent

super-exchange mechanism dominates during charge separation and recombination in

the shorter bridged D-B-A system with stronger length dependence, while a hopping

mechanism dominates the charge recombination in a longer π-conjugated bridge (n > 3)

linked D-B-A system with weaker length dependence. The longer bridges therefore

operated as molecular wires to combine the donor and acceptor. This clearly indicates

that in addition to the length dependence, the electronic properties of the bridge and the

donor–bridge energy gap must also be considered when designing a D–B–A system.

From all the π-conjugated bridge related studies, the attenuation factors span a

large range from fully conductive (β ~zero) to quite insulating (β > 0.5 Å). However, the

27

non-conjugated spacers, uniformly have similar β values, close to unity.31 Even for the

same type of π-conjugated bridges, the attenuation factor also differs in different D-B-A

systems.

Donor and Acceptor

Series of donors and acceptors were also intensively studied. Commonly used

donor-acceptor pairs include phenothiazine (PTZ)- perylene-3,4:9,10-bis(dicarboximide)

(PDI),32 C60-extended tetrathiafulvalene (exTTF),33 and tetracene-pyromellitimide.22

Some porphyrins, with energies tuned via coordinating with different transition metal are

often used as electron donors or/and acceptors as well.20, 24

Detection of Electron Transfer Process

There are several ways to monitor the electron transfer and energy transfer

processes, such as transient absorption (TA) spectroscopy and transient Infrared

spectroscopy (TRIR).

Transient absorption (TA) spectroscopy is also known as “pump-probe”

spectroscopy and flash photolysis, is a powerful technique for the study of short-lived

transient species. These transient species include photoreaction intermediates, higher

excited states of a molecule. Spectral and dynamic information are acquired from the

evolution of excited states, populated upon excitation with light. The absorption spectra

acquired by TA depend on the energy gap of the excited states involved in the

transition. Since it was First used by Norrish and Porter to study reactive intermediates

in 1949, TA spectroscopy has been used widely to study photophysical profiles for the

most important electronic transitions of a molecule or processes, together with the other

spectroscopic techniques. The time resolution for transient absorption spectroscopy

was significantly improved with the introduction of laser technology and other

28

techniques, owning the power to monitor the charge transfer dynamics34 in the

femtosecond (10-15 s) and picosecond (10-12s) timescales. Based on the time detection

limit, we have nanosecond-TA and femtosecond-TA (Figure 1-7), which cover transient

processes occurring from femtosecond (10-15 s) to microsecond (10-6 s).

Figure 1-7. Schematic diagram for femtosecond-TA apparatus. Copyright Dr.Wai-Ming Kwok, Hongkong Polytechnic University.

Transient absorption instrumentation works by using a pump source (laser) to

create a detectable population of the transient species, a probe source (lamp) with a

detector such as a photomultiplier tube (PMT) to detect differences in the transmitted

light, and software for data analysis. The signal is amplified and collected on an

oscilloscope. To achieve the signal in high intensity, an intense laser pulse and high

power lamp are usually used as the pump and probe beams respectively.

29

Figure 1-8. Charge transfer dynamics studied by fs-TA. Reprinting based on reference 34. Copyright 2015 American Chemical Society.

Depending on the instrumentation that is used, different transient species in

different life times can be detected. For example, the development of femtosecond-

picosecond lasers has pushed the detection of transient absorption to shorter-lived

species, allowing the investigation of the dynamics of charge separation with lifetimes in

picoseconds. It supplies powerful capability to study the charge transfer and energy

transfer processes (Figure 1-8).

Transient Infrared spectroscopy is another widely used transient technique, with

strong sensitivity to the changes in molecule’ geometry and bond order. Probing the

molecular system with a broadband mid-infrared pulse after the optical excitation allows

the detecting and monitoring the valence changes in lots of systems.35, 36 In our

research work, we also used transient Infrared spectroscopy to probe the charge

transfer process (Chapter 5).

30

Light Harvesting Polymers

Metal Chromophore Based Polymer

Polymers containing transition metal complexes have attracted much attention,

since the utility of these metal complexes, in particular of Ru(II), Ir(III), Os(II), Re(I) et al

(Figure 1-9), leads to important applications.37 Most of those complexes have exhibited

exceptional spectroscopic properties featuring photostability and a broad absorption in

the visible region, together with good performances in chemical stability, redox

properties, luminescence, excited-state lifetime and excited-state reactivity. The

relatively long-lived excited state, which is a unique character from metal-to-ligand

charge transfer (MLCT), has established comprehensive photophysical and

photochemical applications in solar energy harvesting and conversion.38-42

In the perspective of structures, metal containing polymers incorporate structure

from both components, so the properties from metal complexes (strong UV-visible

absorption, strong fluorescent or/and phosphorescent emission, strong stability) and

properties from polymers (flexibility, processablility, solubility) are well maintained in a

single assembly system.43 Moreover, by embedding proper metal complexes to different

polymers, the photophysical and electrochemical properties of the resulting material can

be tuned.

Synthesis of Metal Chromophore Based Polymers

In the perspective of synthesis of metal containing polymers, there are two

primary procedures applied (Figure 1-10): First, the transition metal complex is attached

to a polymer backbone after the polymerization process (grafting); Second, direct

polymerization of the metal complex, which served as a monomer itself. Regarding the

grafting approach, there are generally two possible pathways to graft a transition metal

31

complex onto a polymer. Pathway 1, the polymer backbone reacts with metal

complexes directly if polymer and metal complexes own proper chemical functional

groups. Pathway 2, the polymer bears appropriate ligands, which can further react with

the precursor complex.

Figure1-9. Summary of metal containing polymers. (A) Ruthenium based polymer, (B) Osmium based polymer. (C) Rhenium based polymer. (D) Iridium based polymer. Reprinted with permission from ref 37. Copyright 2012 Royal Society of Chemistry.

Metal Complexes

Many transition metal complexes have been studied in photophysics, while the

second and third row d6 transition metals, in particular Ru(II), Os(II), Re(I) and Ir(III) are

primarily discussed below as examples to illustrate the photophysical processes. The

metal complexes are unusually prepared with coordinated N-heterocycles ligands, and

their properties are easily tuned by ligand structure design. One of the most widely used

ligand is polypyridine ligands, which are usually colorless possessing s-donor orbitals

B)

A)

C) D)

32

localized on the nitrogen atoms and p-donor and p*-acceptor orbitals delocalized on the

aromatic rings to some extent.37

Figure 1-10. Presentation of common synthesis techniques. (A) Grafting approach. (B) Co-polymerization approach. Reprinted with permission from ref 37. Copyright 2012 Royal Society of Chemistry.

Figure 1-11. Simplified molecular orbital diagram. Diagram for d6 metal complexes in octahedral symmetry showing the three types of electronic transitions without metal–ligand p-bonding interaction. Reprinted with permission from ref 37. Copyright 2012 Royal Society of Chemistry.

B)

A)

33

For metal complexes, Jablonski diagrams are constructed based on their

electronic configurations. The various MOs can be conveniently classified based on

their predominant atomic orbital contributions. Six orbitals could be classified: (1)

strongly bonding, ligand centered σL orbitals; (2) bonding, ligand-centered πL orbitals;

(3) non-bonding, metal-centered πM orbitals; (4) anti-bonding, metal-centered σM∗

orbitals; (5) anti-bonding, ligand-centered πL∗ orbitals; and (6) strongly antibonding,

predominantly metal-centered σM∗ orbitals (Figure 1-11). Accordingly, the promotion of

an electron from a πM metal orbital to the πL∗ ligand orbitals gives rise to metal to ligand

charge transfer (MLCT) excited states, whereas promotion of an electron from πM to σM∗

orbitals gives rise to metal-centered (MC) excited states, and promoting an electron

from πL orbital to πL∗ orbital obtain Ligand-centered (LC) excited states. The lowest MC,

MLCT and LC states energies determine the excited states properties. For example,

MLCT state of Ru(II) complexes is commonly well above the ground state, results in

strong luminescence, with inefficient radiationless deactivation. In contrast, Os (II)

complexes has lower MLCT energy than Ru (II) complexes, making Os(II) complexes

excited states quenched to the ground state is more efficient manner. As a result, Os(II)

complexes reveal much shorter emission lifetimes than the Ru(II) analogs.44 The

phototphysical processes involving a metal complex in an excited state could be

explained by a regular Jablonski diagram in several channels (Figure 1-12): (1)

depletion of the original molecule (photochemical reaction), (2) radiationless

deactivatiob, (3) emission of light (fluorescent or phosphorescent luminescence), and

(4) interaction with another species present in the environment (quenching process).45

34

Figure 1-12. Representation of possible deactivation channels of excited states (M: molecule, Q: quencher). Reprinted with permission from ref 37. Copyright 2012 Royal Society of Chemistry.

Polymer Backbone

The design of macromolecular structure is determined by a number of factors,

including the size and spacing of the pendant groups, the torsional flexibility of the

backbone, the length of the side chains, and the solvent. Polystyrene (PS) is a widely

used polymer backbone with strong flexibility. The conjugated π-conjugated

polyfluorene (PF), polythiophene (PT) have reduced torsional flexibility, resulting in

more extended structures and larger average separations between adjacent complexes

compared with PS based metal polymer.46 The conjugated poly(phenylene ethynylene)

backbone was also reported as a scaffold to graft with pendant Ru(II) and Os(II)

polypyridine chromophores.47

Ru(II) Chromophore Based Polymers

Ruthenium (II) polypyridine complex based polymer is one of most widely studied

metal containing polymers. It can function as a light-harvesting antenna system, with a

35

combination of Ru(II) complexes beneficial properties such as excellent photostability,

luminescence, high optical cross section, long excited state lifetime, and efficient charge

and/or exciton transport over nano-scale distances.48

Site-Site Energy Transfer

Figure 1-13. Polystyrene based Ru(II) chromophore grafted polymer. (A) Ether bonding

Ru(II) based polymer. (B) Amide bonding Ru(II) based polymer. Reprinted with permission from ref 49. Copyright 1998 American Chemical Society.

Previously, Meyer and co-workers reported synthetic methods to attach transition

metal complexes to polystyrene backbones by ether or amide linkages (Figure 1-13).49,

50 The photophysical and electrochemical properties of these polymers were

investigated to understand the mechanism and dynamics of charge and exciton

transport within polychromophore assemblies. Based on the studies of the co-polymers

having Ru(II) and Os(II) pendants linked to the polymer by combined approach utilizing

experimental spectroscopic techniques and Monte Carlo simulations, it concludes that

MLCT excitons in the polymer are transported along the polymer chains by a site-to-site

hopping mechanism (energy from an excited Ru(II) site transfers to Ru(II) sites in

ground states), with a life time of 1−4 ns.51, 52

B) A)

36

The amplified quenching effects observed when mixing the polymer and

quencher (even in small amount) confirms the energy transfer among the sites grafted

on the polymer backbone.53 48 The excitons generated in the polychromophore are

diffusing along the polymer chain in a quick manner, with energy transfer from site to

site (Figure 1-14).48 As long as one chromophore site gets quenched, any exciton

transferred to such site is get quenched as well. The longer the exciton diffuses, the

more obvious the quenching effect observed.

Figure 1-14. Site-site energy migration along PS-Ru(II) polymer. Reprinted with permission from ref 48. Copyright 2012 American Chemical Society.

Competitive Charge and Energy Transfer

In assemblies utilizing π-conjugated polymers, photoexcitation of polymer in the

visible gives rise to competitive charge transfer and energy transfer. Schanze and co-

workers reported the synthesis of a polyfluorene (PF)-based Ru(II) polypyridyl assembly

bound onto TiO2 and its multifunctional characteristics (Figure 1-15). After excitation of

any one of the Ru(II) complexes grafted on the polymer, energy transports through site-

to-site hopping to the TiO2 interface, where electrons inject from excited Ru(II) site into

the TiO2. After the injection, there is a hole transfer process from the oxidized complex

37

(Ru(III)) to the PF backbone, when the chromophore get regenerated at the interface in

~ps scale. The holes reside on the PF could sustain for >100 μs. The electron transfer

(hole transfer) also possibly occur among the Ru(II) sites. Meyer group reported a Ru(II)

chromophore polymer, end capped by a water oxidation catalyst 54. Upon the excitation

of a single chromophore in the polymer chain, energy transport between adjacent units,

Ru(II)*–Ru(II) → Ru(II)–Ru(II)*, ultimately with electron injection into the semiconductor,

(TiO2–Ru(II)* → TiO2(e−)–Ru(III). After the injection, electron migrates along the

polymer, ends with oxidation of the catalyst, Ru(III)–Ru(II)cat → Ru(II)–Ru(III)cat.

Figure 1-15. Competitive energy transfer and charge transfer in PF-Ru assembly. Reprinted with permission from ref 46. Copyright 2015 American Chemical Society

Artificial Photosynthesis

Generation of clean energy in a more economical way has become one of

several critical challenges in the twenty-first century.55 The fact that the sun is supplying

more energy in one hour than what was consumed one the planet for the whole year

(2001) inspired lots of researchers dedicate to the work towards sustainable conversion

38

of solar energy into fuels or electricity.56 One of the most studied approaches to utilize

solar energy (solar capture, conversion and storage) is artificial photosynthesis process,

borrowed from the nature, in which chemical bonds are broken and formed to produce

solar fuels.56-61

Natural Photosynthesis

In natural photosynthesis, the sunlight is captured and converted into fuels,

reducing CO2 into carbohydrates while releasing O2, summarized in Equation 1-8.56, 62,

63

6CO2 +12H2O -> C6H12O6 + 6O2 + 6H2O (1-8)

Intensive studies have been accomplished to understand the mechanism of

photosynthesis processes. Light harvesting and water oxidation involved processes

occur within photosystem II (PSII). In year 2004, Dr. Ferreira and coworkers reported

the first complete and refined crystal structure of PSII, which revealed valuable

information about the organization of the Mn4Ca-cluster and the details of its protein

environment.64 In 2011, Dr. Yasufumi Umena reported the crystal structure of oxygen-

evolving photosystem II at a resolution of 1.9 Å and located all the metal atoms of the

Mn4CaO5 cluster, together with all of their ligands 65. In photosystem (PS) II (Figure 1-

16), when P680 (a central pair of chlorophylls) is excited, it transfers an electron to the

acceptor system, which reduces CO2 subsequently in photosystem I (PSI).63 The P680 is

oxidized into P680•+, which is an oxidant with oxidation potential of +1.2 V versus the

normal hydrogen electrode (vs NHE). The P680•+ will be reduced back to P680 through

electron transfer from a Mn4Ca-cluster in the oxygen-evolving complex (OEC). After four

consecutive electron transfers from the OEC, one molecule of O2 and four protons will

be generated from two molecules of H2O.64, 66, 67

39

Figure 1-16. The illustration of photosynthesis processes. Located in the chloroplasts’ membranes. Copyright Copyright 2012 InTech.

The structure of OEC has been characterized by crystallography and other

spectroscopic techniques, such as X-ray absorption spectroscopy. It is comprised of

one calcium atom and four manganese and, and the surrounding μ-oxo and μ-hydroxo

ligands hold the meta atoms in a cubane-like arrangement, where three of the

manganese centers are capped by the calcium atom, while the fourth dangling

manganese atom is connected to the cubane via two of its oxo-groups (Figure 1-17).

Such OEC structural motif is preserved in essentially all kinds of photosynthetic

organisms to achieve water oxidation reactions.63, 65, 68, 69

The mechanism of how the OEC catalyzes H2O oxidation has also been

extensively studied.69, 70 It has been known that the OEC cycles through five oxidation

states denoted S0−S4, as depicted in the S-state cycle (Figure 1-18). In this cycle, the S0

state represents the most reduced state, and S4 represents the most oxidized state.

After four subsequent oxidations from P680•+, the OEC reaches its S4 state, which will be

reduced back to S0 by four electrons derived from two substrate water molecules with

the concurrent formation of one oxygen molecule.67

40

Figure 1-17. The structure of Mn4CaO5 cluster. Reprinted with permission from 65. Copyright 2011 Macmillan Publishers Ltd

Figure 1-18. The S-state cycle for water oxidation reaction. The illustration shows how the absorption of four photons by P680 drives the formation of one molecule O2 via splitting two water molecules through a consecutive series of five intermediates (S0, S1, S2, S3 and S4). Reprinted with permission from Ref 67. Copyright 2009 Royal Society of Chemistry.

41

Artificial Photosynthesis Design

Inspired by the natural photosynthesis process, lots of efforts have been spent on

artificial photosynthesis device design in mimicking photosynthesis in artificial systems.

The artificial photosynthesis processes generally include splitting water to hydrogen and

oxygen gas or reducing carbon dioxide to carbon monoxide, formate, or more energy

rich products, supported by multiple electron transfer and proper hierarchical

arrangement of light harvesting sensitizers and catalytic sites.71

Figure 1-19. Cobalt thin-film assembly. (a) Cobalt thin-film. (b) Thin-film cobalt electrode immersed in electrolyte solution. (c) Current density traces for bulk electrolysis at 1.1 V (vs. Ag/AgCl, and SEM images at 6500 times magnification (top) and AFM images (bottom) .Reprinted with permission from Ref 72. Copyright 2010 Royal Society of Chemistry.

42

A critical design of photosynthesis is the separation of the light harvesting and

energy conversion, which dictated by the thermodynamics of the water-splitting

reaction.73 To overcome such challenges, several artificial photosynthesis processes

have been achieved using molecular or heterogeneous approaches.34, 71, 74-77 A

heterogeneous self-assembly of a cubane cluster has been reported owning analog

structure as OEC in PS(II) (Figure 1-19).72 The artificial OEC could be made from

cobalt, oxygen, and phosphate self-assembles upon the oxidation of Co2+ to Co3+ ion in

aqueous solution78 or, the Co-OEC may be grown from cobalt metal films under

anodization conditions.72

Figure 1-20. Preparation of IrO2-modified LaTiO2N electrode and water oxidation properties 79. (A) IrO2-modified LaTiO2N electrode. (B) Current density under illumination and variable bias potentials. Reprinted with permission from Ref 79 .Copyright 2013. Royal Society of Chemistry.

Fabrication of photo electrodes with non-oxide semiconductor and co-catalyst, in

the absence of light sensitizers was also reported as another example of heterogeneous

artificial photosynthesis approach.76, 79 In this study, IrO2 was used as a catalyst for the

B) A)

43

oxygen evolution reaction, the LaTiO2N worked as semiconductor, and the IrO2-

modified LaTiO2N electrode was prepared with particle transfer method. The electrode

showed large currents (3.5 mA /cm2) under irradiation with potential at 1.23 VRHE (Figure

1-20).

Integration of solar cells and catalysts for water electrolysis is a proven

technology, which demonstrated long-term device efficiencies. In this design, the PV

device generates an electrochemical bias in an electrochemical cell with catalytic

anodes and cathodes, to help to achieve the water splitting.80 One of the most recent

examples was reported by Nocera and co-workers,81 the Co-borate WOC was

deposited onto ITO as anode to from O2, NiMoZn was deposited on a Ni mesh

substrate as cathode to generate H2, and connected with commercial triple-junction

amorphous silicon (3jn-a-Si) solar cell to implement the wire type device. The wireless

type device was made when depositing the NiMoZn catalyst on the opposing stainless

steel surface of the 3jn-a-Si solar cell (Figure 1-21).81

Figure 1-21. The illustration of the PV electrolysis device 34. (A) Wire type device. (B) Wireless type device. Reprinted with permission from Ref 34. Copyright 2015 American Chemical Society.

44

Meanwhile, lots of progresses have been achieved towards solar fuel conversion

in the molecular artificial photosynthesis approach.34, 77, 82 One of those pioneering

design is dye-sensitized photoelectrosynthesis cell (DSPEC).

Dye-Sensitized Photoelectrosynthesis Cell

The dye sensitized photoelectrosynthesis cell (DSPEC) approach has been

widely used to study the artificial photosynthesis process since 1999 for its great

simplifications in cell implementation with variable semiconductors, sensitizers and

catalysts available.83-85 This photoelectrosynthesis cell (PEC) basically mimics dye-

sensitized solar cells (DSSC) but with the goal of producing oxygen and solar fuel at two

physically separated electrodes rather than photopotential and photocurrent. 86 In a

DSPEC, absorption of sunlight by a sensitizer (dye or chromophore) deposited on the

surface of a metal oxide semiconductor triggers a series of molecular and interfacial

electron transfer events that drive water oxidation and solar fuel generation half

reactions in the two separate electrodes of the PEC. Among this two half reactions, the

photo-catalytic oxidation towards O2 is much more challenging compared to reduction of

protons, so most of the water splitting research efforts have been focused on half

reaction of water oxidation, and achieved more progress compared to cathode side

reactions. Based on that, the introduction discussed here is mostly focusing on anode

reactions.

Design and Mechanism of DSPEC

DSPEC relies on the combination of chromophore−catalyst assemblies and

stable high-band-gap semiconductor oxides (Figure 1-22). As in dye-sensitized solar

cells (DSSCs), water splitting in a DSPEC is initiated by sun light absorption by a

surface-bound chromophore. After chromophore absorbs sun lights and get excited, it

45

injects electron into the conduction band of the semiconductor with the formation of

oxidized chromophore. In a DSSC, injected electrons are transferred to a separate

cathode to generate a photocurrent, in the presence of a redox carrier couple, typically

I3−/I−. However, in a DSPEC, injected electrons are transferred to a separate cathode to

reduce H+ to H2 or CO2 to CO or organic solar fuels (like methanol, methane et al). The

oxidative equivalents on the chromophore sides are accumulated by catalyst in a

chromophore−catalyst assembly based on multiple-step electron transfer to oxidize

water into O2. 87-89

Figure 1-22. The demonstration of DSPEC. Copyright 2015 American Chemical Society.

Chromophore-Catalyst Assembly

At the heart of a DSPEC is a chromophore−catalyst assembly, which bounds to

the semiconductor surface, incorporating light harvesting sensitizer and catalytic sites.

46

There are several essential principles when designing a chromophore-catalyst

assembly; 1) select a sensitizer with strong UV-visible absorption; 2) a catalyst is

essential efficiently to oxidize the water to molecular oxygen; 3) the chromophore and

catalyst are “connected” somehow to make the intra-electron transfer/hole transfer

possible, including covalent bonding, metal oxide bridging or even cross-surface by co-

deposition; 4) the assembly could bound to semiconductor, so photo-excited sensitizers

could inject electrons into semiconductors (Figure 1-23). So far, several approaches

have been developed to assemble catalysts and light sensitizers, including molecular

co-deposition,90 “layer-by-layer” with Zr(IV)-phosphonate bridges,91, 92 molecular

covalent bond,34, 85 and cross-linked electropolymerization via reductive vinyl coupling.93

Figure 1-23. The summary of chromophore-assembly architecture. (A) Covalent type bridging. (B) Co-deposition type. (C) Electro-polymerization type. (D) Metal oxide assisted layer-by-layer type. Copyright 2015 American Chemical Society

B) A)

C) D)

47

Chromophore

When choosing a chromophore for DSPEC application, several characteristics

need to be considered. First, the excited states after light absorption must be

thermodynamically capable of injecting an electron into the conduction band of an n-

type oxide. Second, the oxidized chromophore must be thermodynamically capable of

driving water oxidation at the catalyst site. Third, chromophore should own strong

stability and strong molar absorption in UV-visible absorption. This could be explained

by the free energy change for the water splitting reaction, the redox equivalent limit for

water splitting into H2 and O2 is ΔG° = 1.23 eV, which corresponds the wavelength

threshold 1008 nm. In order to drive the water oxidation reaction, the wavelength must

be <1008 nm. Considering sunlight gave dominant photons in 400-800 nm, a good

chromophore should have strong absorption in 400-800 nm. Several good

chromophores have been developed, including organic dyes, 94-96 transition metal

complexes83, 97, 98 or quantum dots.

Among all the chromophores, Ru(II) polypyridyl complex has been clearly

understood in light absorption, excited state, reactivity, stability and applications in

DSSC.45, 89, 99-101 Moreover, the Ru(III/II) oxidation potentials for Ru(bpy)3 is 1.26 V vs

NHE, and it could increase up to ~1.50 V vs NHE when the ligand is modified with

withdrawing electron groups, like carboxylate. So it features strong potentials to initiate

water oxidation reactions. Based on that, so far, Ru(II) polypyridyl complexes have been

the dominated chromophore used in DSPEC studies (Figure 1-24).

Briefly, the UV absorption is dominated by ligand based transitions, while the

visible absorption is caused by MLCT transition, with electronic excitation from dπ

orbitals on the metal to π* acceptor levels on the polypyridyl ligand. The disadvantages

48

in Ru(II)−polypyridyl complexes as chromophores is the limited absorptivity in the visible

region (400 - 500 nm). Take the most classic Ru(bpy)3 2+ for an example (CH3CN ) in

Figure 1-10. The molar absorptivity at the visible maximum wavelength (λmax = 449 nm)

is ~1.4 × 104 M−1 cm−1, which decreases to 5.1 × 102 M−1 cm−1 by 550 nm.34 The light

absorption disadvantage can be manipulated systematically by varying the ligands. A

“black MLCT absorbers” was reported with such approach, which absorbs light

throughout the near-UV−Vis and near-IR regions.102

Figure 1-24. Ru(II) polypyridyl chromophores. Ru(II) polypyridyl chromophores derivatives structure and corresponding oxidation potentials (A). UV-visible absorption spectra for Ru(II)(bpy)3

2+ 103 in acetonitrile (B). Reprinted with permission from Ref 103. Copyright 2011 Royal Society of Chemistry.

B)

A)

49

Catalyst

In order to incorporate the catalyst into chromophore-catalyst assembly, the catalyst

should be in molecular form instead of heterogonous form. In 1985, Dr. Meyer and co-

workers reported the first molecular water oxidation catalyst, cis,cis-[(bpy)2(H2O)Ru(III)-

ORu(III)(OH2)(bpy)2]4+ also called “blue dimmer ”.104 The detailed mechanistic insight

gained from the blue dimer concludes that O---O bond formation occurs between a

single Ru(V)=O site and an external water molecule in the solvent environment, which

inspires the design of single-site Ru(II)−polypyridyl catalysts. Since 2005, after

Thummel and co-workers reported the first single-site Ru(II)−polypyridyl catalyst,105 the

family of single-site Ru(II)−polypyridyl catalysts has been significantly expanded.105-110

Figure 1-25. Structure of blue dimer 1 and the representative pathways of O–O bond formation 103. Reprinted with permission from Ref 103 .Copyright 2011 Royal Society of Chemistry.

The mechanism how the ruthenium metal based complex oxidize water is not

quite clearly so far, while two widely accepted pathways were proposed (Figure 1-

25).111 Pathway A is water nucleophilic attack: the oxygen from water molecule

nucleophilically attacks the metal-oxo group in the catalyst site, leading to 2-electron

reduction of the metal center and O–O bond formation. Pathway B is interaction of two

50

M–O units, the interaction of two mono-radical M–O units affords a peroxo intermediate

based in formation of O–O bond. Some catalysts are believed involved in pathway A

while some undergo mechanism in pathway B.

Notably, Sun and co-workers have reported Ru(bda)(L2) type catalyst with

impressive catalytic rates at relatively low over potentials (Figure 1-26, bda = 2,2′-

bipyridine-6,6′-dicarboxylate, L = picoline, isoquinoline, or other substituted pyridine).108,

112, 113 Based on experimental and calculated results, it concludes that O---O bond

formation occurs between two adjacent complexes, as shown in pathway B.114

Figure 1-26. Summary of some pioneering single-site Ru(II) catalyst. (A) from Thummel group, reprinted with permission from Ref 105 .Copyright 2005 American Chemical Society. (B) from Meyer group, reprinted with permission from Ref 115. Copyright 2013 American Chemical Society. (C) from Sun group, reprinted with permission from Ref 108. Copyright 2012 Nature Publishing Group

B)

A)

C)

51

Semiconductor

Intrinsic properties of semiconductor are their electric structures, expressed in

form of electronic energy bands. Each semiconductor owns their unique highest

occupied band (valence band) and lowest empty band (conduction band). For n-type

semiconductors, the Fermi level lies just below the conduction band, while the Fermi

level lies just above the valence band for a p-type semiconductor. When two different

semiconductors, or a semiconductor and electrolyte solution are put in contact, their

different chemical potentials supply the driving force for the movement of charges,

electrons and electrolyte, across the interface until reaching equilibrium.86 When

electrons are injected into the semiconductor, they are basically pushed to transport

from anode to cathode via wires by such driving forces.

The semiconductor applied in DSPEC should be transparent and own proper

valence band to accept electrons from excited chromophore at the same time, the

semiconductor is prepared in form of mesoporous structure to get sufficient surfaces

binding chromophores, and annealed onto transparent conductive substrates (ITO or

FTO, Figure 1-27). So far, lots of metal oxide based semiconductors have been applied,

including WO3,116 ZnO,117 Nb2O5,

118 Zn2SnO4,119 SrTiO3,

120 TiO2 and SnO2. 87, 121 Among

them, TiO2 and SnO2 are the two most commonly used metal oxides for photoanodes

due to their low conduction-band potentials, ease of synthesis, and stability. In

perspective of energy levels, ECB = −0.1 V (vs NHE) for TiO2 and ∼0.28 V for SnO2 at

pH = 0,121 while E°′(Ru(bpy)3 3+/Ru(bpy)3 2+*) = −0.8 V (vs NHE).100 In other words, the

driving force of electron injection into TiO2 from Ru(bpy)3 2+* at pH 0 is ∼0.6 V while

injection into SnO2 is ∼1.0 V. From dynamics study based on [Ru (4,4′-

52

(PO3H2)2bpy)(bpy)2] 2+* deposited TiO2 or SnO2 electrode, the injection efficiencies was

measured of ∼1.122

Figure 1-27. Depiction of catalyst onto semiconductor. Reprinted with permission from Ref 34. Copyright 2015 American Chemical Society.

SnO2/TiO2 core/shell structure was designed to slow down the back electron

transfer,123 from semiconductor to oxidized chromophores, based on the difference in

CB energy levels between SnO2/TiO2. SnO2 has a conduction band minimum (CBM)

~380 millivolts below that of TiO2, leading to better electron injection yield.124 Meyer and

co-workers prepared a SnO2/TiO2 core/shell structure by atomic layer deposition (ALD)

of TiO2 on SnO2 nanoparticles, and demonstrated an electron transfer cascade from the

dye’s excited state into TiO2 and subsequently into the SnO2 CB (Figure 1-28). This

architecture results in enhanced electron injection, slower recombination kinetics, and

an overall device performance enhancement.93, 125

53

Figure 1-28. Demonstration of SnO2/TiO2 core-shell substrate (A) and semiconductor energy levels (B) Reprinted with permission from Ref 126. Copyright 2016 American Chemical Society.

Immobilization

When a proper chromphore-catalyst assembly and semiconductor are selected

to make DSPEC, the next key question will be how to bound the chromphore-catalyst

assembly onto semiconductor of the electrode. Several factors need to be considered

regarding this question.103 First, strong stability under strong oxidative conditions

(anode), and reductive conditions (cathode). Second, high tolerance against basic

condition (anode) and acidic condition. Third, successful transfer electron from

chromophore-catalyst to semiconductor. Under those principles, several approaches

have been developed, aiming to immobilizing chromophore-catalyst assembly onto a

semiconductor (Figure 1-29).

The covalent bond is able to provide a stronger interlinkage between molecules

and the semiconductor surface rather than other binding modes. So far, several

B) A)

54

anchoring approaches, including carboxylic acids, phosphonates, ethers, amides,

siloxanes, acetyl acetonates, and cyanides have been tested, while carboxylic acids

and phosphonic acid still remain the most two successful approaches with the formation

of carboxylate or phosphonate linkages,127-129 as shown in Figure 1-29.

Figure 1-29. Summary of bound modes of chromophore-catalyst assemblies onto semiconductor. (A) Covalent bonding. Reprinted with permission from Ref 130. Copyright 2009 Wiley-VCH Verlag Gmbh & Co. (B) Electrostatic interaction. Reprinted with permission from Ref 125. Copyright 2016 American Chemistry society. (C) Electro-polymerization. Reprinted with permission from Ref 131.Copyright 2008 Wiley-VCH Verlag Gmbh & Co .(D) Physical adsorption. Reprinted with permission from Ref 107. Copyright 2000 Wiley-VCH Verlag Gmbh & Co.

B) A)

C) D)

55

Electrostatic interaction based deposition is also a good way for deposition,

especially for those assemblies without anchoring groups. Considering most of the

Ru(II) based complex chromophore and catalysts are ionic, they are good targets to

deposit onto semiconductor by electrostatic interaction, especially for the polymer

electrolytes. Layer-by-layer (LbL) polyelectrolyte self-assembly is such an approach

involving the sequential deposition of oppositely charged polymers to build up multilayer

polymer structures.125

Another straightforward strategy for catalyst immobilization is electro-

polymerization of molecular WOCs directly on the electrode surface. One of the

examples is anchoring the Hbbp-based dinuclear Ru complex on a conductive surface

reported by Llobet and co-workers, with vitreous carbon sponges (VCS) and FTO

electrodes were chosen for this electro-polymerization. It was concluded that the

polymer hybrid material catalyzes O2 evolution in a less efficient while more durable

way.131

Physical adsorption of molecules on solid surfaces is a simple method for

immobilization. Tanaka’s group reported the direct deposit of dinuclear Ru complex

[Ru(II)2(OH)2(3,6- tBu2sq)2(btpyan)]2+ (btpyan = bis(terpyridyl)anthracene; tBu2sq =

di(tert-butyl)-1,2-semiquinone) onto the ITO electrode, which is active to oxidize water to

dioxygen under an applied potential of 1.91 V (vs. NHE) electrochemically.107

Device Implementation of DSPEC

The photoanode was prepared with the depositing chromophore-catalyst

assembly onto semiconductor surface. The binding mode could be further protected by

atomic layer deposition (ALD) technique to enhance the device performances.132

56

Photocathodes consist of p-type semiconductors and H2 generation catalyst.

Since a reduction reaction takes places on the photocathode side, the possibility of

oxidative degradation is lower than that for photoanodes. So far, still limited number of

p-type semiconductors are reported to be active as cathode materials, like NiO.

Besides, some Cu-based chalcogenides exhibit good p-type conductivity with potential

to make photoanodes as well.76 In the current DSPEC study, Pt wire is normally applied

as the cathode (also the H2 generation catalyst) for its high catalytic activities. The

complete DSPEC apparatus is constructed when connecting the photoanode and a

photo cathode (for example, Pt wire) by wire, and immersed in electrolyte solution

(Figure 1-30).86, 90

Figure 1-30. Demonstration of DSPEC device. Reprinted with permission from Ref 90. Copyright 2013 American Chemical Society.

57

CHAPTER 2 EFFECT OF CONJUGATION LENGTH ON PHOTOINDUCED CHARGE-TRANSFER

IN A -CONJUGATED OLIGOMER-ACCEPTOR DYADS

Background

Electron transfer and energy transfer are two fundamental steps to achieve many

important chemical and biological processes, ranging from light harvesting to energy

conversion and energy storage. For generations, researchers have spent extensive

efforts designing and developing conjugated based materials to study electron transfer

and energy transfer processes, and apply them to artificial electronic device

developments such as solar cells, light emitting diodes, field effect transistors and

artificial photosynthesis cells.133-136 Among them, molecular donor-bridge-acceptor (D-B-

A)“wires” were developed to create long lived radical cation-anion pair states, aiming at

making molecular electronics18 or mimicking artificial photosynthesis.19 In those D–B–A

systems, the donor (D) and the acceptor (A) are covalently linked through a molecular

bridge (B).21 So far, many donor-bridge-acceptor (D-B-A) systems were reported:

Wasielewski and co-workers reported PTZ-Phn-PDI type D-B-A system (PTZ:

phenothiazine, PDI: perylene-3,4:9,10-bis(dicarboximide), Ph: p-phenylene bridge) and

its molecular wire-like charge recombination properties.29 The Albinsson group applied

porphyrins as an electron donor and acceptor to conduct systematic studies of photo-

induced electron transfer rates under variable donor-acceptor distance and controlled

donor-bridge energy gaps.20, 24 Other donor-bridge-acceptor (D-B-A) systems include

donor-acceptor pairs like tetracene-pyromellitimide,22 porphyrin-C60,137 Ru(II)

chromophpre-phenotiazine.138 Several π-conjugated oligomers were studied as bridges

to link donor /acceptors in D-B-A systems, including oligo-phenylenevinylenes (OPV),22,

23 oligo-phenyleneethynylene (OPE),17, 20, 24 oligofluorene25-27 and oligothiophene.28

58

These bridges are not just applied as spacers to separate donor and acceptor, instead,

they are actively involved in mediating electronic coupling for the electron transfer

process, which is well known now, as summarized in a single exponential decay

constant, attenuation factor β.21 For example,βCS=0.31 Å-1/βCR=0.39 Å-1 were reported

for a OPE bridged ZnP-nB-AuP+ system.20 In these D-B-A systems, varying bridge

length is a straightforward approach to study electron transfer rate constants, KET, under

different donor–acceptor distances, RDA, as supported by the McConnell model, as

shown in Equation 2-1.

KET = K0 exp (-β RDA) (2-1)

To our knowledge, all the molecular wire systems studied so far were focused on D-B-A

structures. It is still unclear about how conjugated “wire” donor communicates with

acceptor electronically, and how the wired conjugation length affects such

communication. The only related report came from Nicola Armaroli group, where they

studied the photo-induced energy and electron transfer in fullerene–

oligophenyleneethynylene systems.139 Therefore, it would be very interesting to

investigate how the conjugation length affects charge separation and charge

recombination processes in donor-acceptor (D-A) systems. When the conjugation

length increases, the radical cations formed from charge transfer would be delocalized

in an extended area, which was expected to elongate the radical ions’ lifetime. More

importantly, it will also supply intrinsic information to study the charge separation/charge

recombination processes in complex conjugated systems for applications ine solar cells

or artificial photosynthesis.

59

In this work, we report the synthesis of a series of oligophenyleneethynylene-

naphthalene diimide derivative oligomers (PEn-NDI, n = 4, 6, 8, or overall called OPE-

NDI sometimes in the paper) featuring an oligophenyleneethynylene (OPE) backbone

(with variable repeating units) as donor and naphthalene diimide derivative (NDI) as an

acceptor. This type of D-A structure was chosen due to the fact that there is no obvious

overlap of the emission spectra of OPE and absorption of NDI. Furthermore, the

conjugation length of OPE is easily controllable in synthetic prospective, and the NDI is

a well-studied acceptor without obvious fluorescence. Therefore, OPE-NDI oligomers

with controlled wired conjugation lengths supply an ideal model system to study the

CS/CR properties, especially under the influences of variable conjugation lengths.

Several transient techniques, including femtosecond transient absorption (fs-TA)

spectroscopy and femtosecond transient infrared spectroscopy (TRIR) were conducted

to study the CS/CR processes.

Results and Discussion

Oligomer Structures, Synthesis, and Characterization.

In this study, a series of oligomers, PEn-NDI (n = 4, 6, 8) were synthesized which

feature a conjugated OPE backbone as an electron donor capped with NDI as an

electron acceptor. The NDI was chosen for its good ability as an electron acceptor and

because its radical anion has distinct absorption features.140-143 Meanwhile, three

corresponding model oligomers end-capped with a triisopropylsilylacetylene unit (TIPS)

were prepared for control studies (PEn-TIPS, n = 4, 6, 8, or overall called OPE-TIPS

sometimes in the paper). The structures of all the oligomers are outlined in Figure 2-1.

The oligomers were synthesized under Sonogashira reaction conditions, and

characterized by 1H-NMR/13C-NMR spectrometry with the key target compounds also

60

characterized by Mass spectrometry. Detailed synthesis procedures and

characterizations are described in the synthesis and characterization section in Chapter

2.

Figure 2-1. Oligomer structures

Photophysical Studies

UV-visible absorption and fluorescent emission of the oligomers were studied in

chloroform. All the PEn-NDI oligomers show combined absorption properties from both

the OPEand NDI segments; however, the absorption is dominated by OPE backbone

due to its relatively large absorption coefficient (Figure 2-2). Taking PE4-NDI as an

example, as shown by Figure 2-1-A, the NDI moiety has two major absorption peaks at

358 nm and 378 nm while PE4-TIPS has a broad absorption from 300 nm to 400 nm,

with the absorption maximum at 350 nm. Meanwhile, the absorption spectrum of PE4-

NDI has a broad absorption from 300 nm to 400 nm, but with two peaks at 358 nm and

378 nm. When n increases from 4 to 8, the absorption of PEn-TIPS and PEn-NDI

oligomers exhibit slight red shifts, with increased molar absorptivity. The NDI does not

61

have any absorption beyond 395 nm while the emission of the OPE backbone onsets

near 390 nm. The mismatch of the emission from OPE backbone and absorption from

NDI block ensures that the energy transfer in the OPE-NDI system is minimized. The-

Figure 2-2. Photophysics study. A) UV-visible absorption spectra of the PE4-TIPS (solid black line), PE4-NDI (solid red line), and fluorescence emission spectra of PE4-TIPS (dashed black dash) and PE4-NDI (dashed red dash). B) UV-visible absorption spectra of the PE6-TIPS (solid black line), PE6-NDI (solid red line), and fluorescence emission spectra of PE6-TIPS (dashed black dash) and PE6-NDI (dashed red dash). C) UV-visible absorption spectra of the PE8-TIPS (solid black line), PE8-NDI (solid red line), and fluorescence emission spectra of PE8-TIPS (dashed black dash) and PE8-NDI (dashed red dash). D) Fluorescence quantum yields of PE4-TIPS/PE6-TIPS/PE8-TIPS and PE4-NDI/PE6-NDI/PE8-NDI. All the UV-visible absorption and fluorescence emission spectra were measured in HPLC chloroform. The fluorescence emission spectra were normalized based on their fluorescence

-relative fluorescence quantum yields were measured relative to quinine sulfate in 0.1 M

H2SO4 aqueous solution as a standard (fl =54%).144 All of the PEn-TIPS oligomers

have high fluorescence quantum yields, and show a slight decrease from 84% to 76%

62

when conjugation length increases from n = 4 to n = 8. On the contrary, all the PEn-NDI

oligomers exhibit much weaker fluorescence quantum yields, but with slight increase

from 0.2% to 2.5% when n = 4 increases to n = 8 (Figure 2-2-D). The PEn-TIPS

oligomers feature decreased fluorescence lifetime with increasing oligomer length, from

= 0.82 ns (n= 4) to 0.45 ns (n = 8) (Table 2-1), which is consistent with their radiative

decay rate trend. Not surprisingly, the fluorescence lifetimes for all the NDI capped

oligomers were below the instrument response for dominated charge transfer process.

Table 2-1. Summary of the photophysical properties

a All the photophysics data were obtained in chloroform. b The excitation wavelength for all fluorescent studies is 355 nm. c Fluorescent emission lifetime measured by TCSPC. d With anthrathene as quantum yield standard, ϕ =0.27 in ethanol at room temperature. e Measured by nanosecond transient absorption spectroscopy in mixture solvents (67% THF + 33% Acetonitrile). f Measured in deuterium chloroform, with

tetra-thiophene as standard 159. g Electron transfer efficiencies (η) were calculated as η = 1−fl(PEn-

NDI)/fl(PEn-TIPS) (n=4,6,8).

The low fluorescence quantum yields also indicate efficient electron in OPE-NDI

oligomers. The electron transfer efficiency can be estimated from Equation 2-2, where

fl(PEn-NDI) is the fluorescence quantum yield for the PEn-NDI oligomers, and

fl(PEn-TIPS) is the quantum yield for the corresponding PEn-TIPS oligomers. Based

on calculations, all the three PEn-NDI oligomers exhibit high electron transfer efficiency

(>96.7%) (Table 2-1). The highly efficient charge separation efficiency makes OPE-NDI

Oligomera λabs

nm

ε 105

cm-1 M-

1

λfb nm τf

c ns f d τT

e s λT-T nm e Δ f g

PE4-TIPS 354 1.08 398 0.82 0.84 3.45 623 0.19 --

PE4-NDI 362 1.21 412 -- 0.0021 -- -- -- 0.998

PE6-TIPS 372 1.32 410 0.51 0.77 4.17 696 0.10 --

PE6-NDI 381 1.82 413 -- 0.012 -- -- -- 0.984

PE8-TIPS 377 2.53 415 0.45 0.76 3.13 712 0.08 --

PE8-NDI 382 2.97 413 -- 0.025 -- -- -- 0.967

NDI 381 0.51 403 -- 0.0018 -- -- -- --

63

type oligomers excellent targets to investigate their charge separation and

recombination kinetics and how they are influenced by the donor conjugation length,

which are discussed in charge separation and charge recombination section in Chapter

2.

= 1-f (OPE-TIPS)/ f (OPE-NDI) (2-2)

Singlet-triplet intersystem crossing is another possible pathway for the

deactivation of the excited states in OPE,145-147 which is normally characterized by

nanosecond transient absorption spectroscopy (ns-TA). We also carried out the similar

experiments to study the singlet-triplet intersystem crossing in OPE-TIPS and OPE-

NDO oligomers, in which, the oligomers (degassed THF solution) were prepared with

controlled optical density as 0.7 at excitation wavelength (355 nm). As shown in Figure

2-3, all the OPE-TIPS oligomers exhibit moderately intense transient absorptions in

visible regions (500 nm- 700 nm) that persists into the µs time domain. The transient

spectra are similar to those previously assigned to the triplet state of oligo(phenylene

ethynylene)s, and by analogy they are assigned to the triplet excited states of the OPE-

TIPS oligomers.145, 147 On the contrary, photolysis of the OPE-NDI oligomers under the

same conditions does not give transient absorption that can be observed on timescales

longer than 20 ns. The lack of a triplet absorption in OPE-NDI further confirms the rapid

photoinduced charge transfer taking place from the singlet state, suppressing triplet

formation. Interestingly, with conjugation length increased, the triplet-triplet absorption

peak is red-shifted from 623 nm to 712 nm, with the absorption intensity (t= 20 ns)

reduced as well. The singlet oxygen quantum yield studies of OPE-TIPS oligomers

confirm the reduced triplet yields for OPE oligomer with increasing conjugation length\.

64

In particular, PE4-TIPS exhibits singlet oxygen quantum yield of 19%, followed by 10%

for PE6-TIPS, and 8% for PE8-TIPS (Table 2-1).This finding is consistent with earlier

work.145

Figure 2-3. Intersystem crossing study. Nanosecond triplet-triplet transient absorption

spectra of PE4-TIPS (solid red line), PE6-TIPS (blue dashed line), (C) PE8-TIPS (green dotted line). The samples were prepared with controlled optical density as 0.7 at 355 nm. Transient absorption spectra were obtained at 20 ns after photolysis at 355 nm

Energetics of Photoinduced Charge Transfer.

In order to estimate the energetics of the charge-separated states for OPE-NDI

oligomers, cyclic voltammetry (CV) and differential pulse voltammetry (DPV)

measurements were performed in dichloromethane, with all potentials calibrated against

Fc+/Fc, and summarized in Table 2-2. In details, the cyclic voltammetry of PE4-NDI

features two quasi-reversible reduction peaks at -1.05 V and -1.48 V, along with a single

irreversible oxidation peak at Eox 1.04 V. The other NDI capped oligomers give similar

results (Table 2-2). The reduction waves are clearly observable due to the NDI centred

one- and two-electron processes,141, 148 while the oxidation peak is irreversible due to

65

the large conjugated OPE segment. The driving forces for CS/CR processes in OPE-

NDI series can be approximated by using the Rehm-Weller equation (Equation 2-3 and

2-4).149, 150

G0CS = (E0

ox - E0red - E0,0 - e2/(4πɛ0ɛRDA) (2-3)

G0CR = (E0

ox - E0red - e2/(4πɛ0ɛRDA) (2-4)

The E0ox is the oxidation potential of the donor, E0

red is the reduction potential of

the acceptor, and E0,0 is the singlet excited state energy. The last term is the Coulomb

stabilization energy in the charge separated state, where ε is the dielectric constant in

the solvent and RDA is the distance between the donor and acceptor units. In our study,

RDA were estimated as center-to-center distances, calculated from DFT B3LYP 6-31G*

(2.36 nm for PE4-NDI, 3.04 nm for PE6-NDI and 3.73 nm for PE8-NDI).

Table 2-2. Energetics of PEn-NDI (n=4, 6, 8)

Eox/Va (Eox/V)b Ered/Va (Ered/V)b ∆E0,0

c G0CS

d G0CR

e

PE4-NDI 1.04 (0.98, 1.24) -1.05, -1.48 (-1.05, -1.48) 3.20 -1.19 -2.01

PE6-NDI 1.03 (0.99) -1.05, -1.49 (-1.07, -1.50) 3.12 -1.10 -2.02

PE8-NDI 0.96 (0.91) -1.05, -1.51 (-1.05, -1.51) 3.12 -1.16 -1.96

PE6-TIPS 1.06 (0.97) -- -- 3,12 -- --

aObtained from cyclic voltammetry measurements. bObatained from differential pulse voltammetry measurements .cZero-zero transition energy, estimated on the basis of ∆E0,0 = ((Eabs(max) + Eem(max))/2. dCalculated from equation 2-3. eCalculated from Equation 2-4. (Eox and Ered were obtained from cyclic voltammetry measurements, Coulombic stabilization energies were calculated from Weller method, 0.05 eV for PE8-NDI, 0.06 eV for PE6-NDI and 0.08 eV for PE4-NDI).

The G0CS, as listed in Table 2-2, range from -1.19 eV to -1.10 eV across the

OPE-NDI series. Note that G0CS does not vary much across the series, and this is a

reflection of the fact that the oxidation and reduction potentials do not vary much with

oligomer length. The G0CR, also listed in Table 2-2, range from -2.02 eV to -1.96 eV.

Taken together, the thermodynamic data suggests that both CS and CR are highly

exothermic, and not obviously affected by OPE conjugation length.

66

In order to complete a full picture of the states involved in photophysics of OPE-

NDI oligomers, it is necessary to pinpoint the triplet levels. Previous studies indicate that

the singlet-triplet splitting (EST) in phenylene ethynylene conjugated systems is ~ 0.75

eV.151 Using this value, combined with the singlet energies, we estimate that the triplet

states in the OPE-NDI oligomer lie within the range 2.37 - 2.45 eV. It is important to

note that the triplet states are higher in energy than the charge separated states in all of

the OPE-NDI series. Figure 2-4 below summarizes the states involve in the

photophysics of the OPE-NDI oligomers, along with their approximate energies. The

important aspect is that the charge transfer state is at lower energy compared to the

singlet and triplet states. Therefore, excited state decay of both states is anticipated to

occur via the OPE NDI charge transfer excited state.

Figure 2-4. Schematic illustration of the energy levels for OPE-NDI oligomers.

Intermolecular Photoinduced Charge Transfer

Intermolecular charge transfer between the OPE-TIPS oligomers and methyl

viologen (MV2+) was studied by nanosecond transient absorption (ns-TA) to gain insight

concerning the spectra of the oligomer radical cations (OPE+•.). The excited OPE-TIPS

67

oligomers are expected to transfer one electron to MV2+, forming OPE+• and MV+•

radical cations in equal yields (Equation 2-5). The photolysis experiments were carried

out for deoxygenated PEn-TIPS solutions in the presence of paraquat (MV2+, 1.0 mM)

under 355 nm with transient spectra collected in timescale of 20 ns- 2000 ns. During the

photolysis, a new absorption peak (λmax= 590 nm ~ 600 nm) is becoming more

pronounced in all three MV2+ mixed oligomer samples as the triplet transient absorption

peaks decay. The new absorption peak is assigned to OPE+•. The Eigen spectra were

obtained from principal component analysis of time resolved transient absorption

spectra using SpecFit software, from which, we are able to see the triplet transient

absorption and OPE+• transient absorption separately in a clearer manner.

Figure 2-5. Intermolecular charge transfer study. (A) Eigen spectra for triplet transient absorption. (B) Eigen spectra for cation radical absorption. Photolysis experiments of PEn-TIPS and paraquat (MV2+, 1.0 mM) were carried out in a mixture solvent of THF (66%) and acetonitrile (34%), with the optical density for oligomers were controlled at 0.7 at 355 nm. Transient absorption spectra used for principal component analysis were obtained at 20 ns after laser illumination at 355nm. The Eigen spectra were obtained from principal component analysis of time resolved transient absorption spectra using SpecFit software.

In the photolysis experiment for bimolecular system of PE4-TIPS and MV2+, the

intrinsic peak for MV+• is also detected at 400 nm, which furthers confirms the

68

intermolecular electron transfer in the bimolecular system. The decay kinetics monitored

at the maximum triplet transient absorption peak exhibits accelerated (3-5 times faster)

triplet state decay rates in the presence of MV2+ (1.5*106 M-1 s-1 for PE4-TIPS, 1.4*106

M-1 s-1 for PE6-TIPS and 1.1*106 M-1 s-1 for PE8-TIPS) than the OPE-TIPS without MV2++

(2.9 – 3.2*105 M-1 s-1).

OPE-TIPS + hv(355 nm) → 1OPE-TIPS* → 3OPE-TIPS*

3OPE-TIPS + MV2+ → OPE+• + MV+•

(2-5)

One more thing need to note is that the fluorescent lifetimes were checked before and

after adding MV2+, from which, fluorescence lifetime stay unchanged. Based on that, we

think the intermolecular electron transfer process occur between MV2+ and the triplet

states of PEn-TIPS oligomers, as shown in Equation 2-5.

Charge Separation and Charge Recombination (fs-TA)

Femtosecond transient absorption (fs-TA) experiments were carried out to collect

the transient absorption spectra in range of 350 – 650 nm for both OPE-TIPS and OPE-

NDI oligomers to study their intra-charge separation and recombination processes and

how the conjugation length affects their kinetics (Figure 2-6). The transient spectra of all

PEn-TIPS samples behave similarly in UV-visible region of 300 – 600 nm: negative

transient spectra around 400 nm ~ 500 nm come from stimulated emission,152 the broad

absorption (λ > 500 nm) in high intensity are assigned to excited state absorption

(ESA).145, 152 Interestingly, even without deoxygenation, singlet-triplet intersystem

crossing is clearly observed for PEn-TIPS oligomers, especially for PE4-TIPS where a

broad absorption band was observed with λmax= 625 nm. No surprisingly, no charge

transfer signals were observed for all the TIPS protected oligomers.

69

Figure 2-6. Charge separation and charge recombination study for OPE-NDI. Femtosecond transient absorption spectra of charge separation and charge recombination processes for PEn-NDI oligomers (n=4, 6, 8). Samples were excited at 380 nm in dichloromethane. The dashed vertical line indicates the formation of NDI-• at 480 nm.

For the PEn-NDI oligomers, the spectra behave completely different from the

TIPS protected samples. Under photolysis (Figure 2-6), the rising transient absorption

spectra were detected in range of 450 - 650 nm in the first 10 ps, followed by steady

decaying to baseline in 1 ~5 ns. All three NDI end capped oligomers exhibit a major

absorption peak ~ 600 nm, together with a shoulder peak ~ 480 nm. Based on earlier

studies, the shoulder peak ~ 480 nm is attributed to the NDI-• .141, 153 And the absorption

70

peak ~ 600 nm is contributed by OPE+• supported by bimolecular study (Figure 2-5,

Eigen spectra of OPE+• from three bimolecular systems have absorption peak ~ 600

nm). The process with rising population of OPE+• in timescale of ~10 ps is assigned as

CS process while the process with decaying population of OPE+• in timescale of ~ ns is

assigned as CR process.

Figure 2-7. Dynamics study of PE4-TIPS and PE4-NDI. PE4-TIPS (A) and PE4-NDI (B) were studied by fs-TA in dichloromethane for the short time (1 ps ~5 ns) and by nanosecond transient absorption spectroscopy in mixture solvent (67% THF + 33% acetonitrile) in long time (20 ns ~10 µs).

From nanosecond transient absorption and femtosecond transient absorption

spectroscopy techniques, we are able to study the dynamics of OPE-TIPS and OPE-

NDI oligomers in time scales of fs - µs. In our work, we used PE4-TIPS and PE4-NDI as

examples to illustrate the behavior differences under photolysis (Figure 2-7).Under

illumination, the exited PE4-TIPS exhibits an ultrafast component of ~1.6 ps (structure

71

reorganization or other ultrafast process), followed by fluorescent decay, in competition

with the intersystem crossing with τ= ~675 ps. Afterwards, triplet states (Δ=0.19)

decays to the ground state with τ= 3470 ns. However, when PE4-NDI gets exited,

charge separation process occurs with τCS= 1 ps, forming the OPE+• and NDI-•, followed

by charge recombination with τCR= 316 ps.

kET = 1/τET (2-6)

Figure 2-8. Kinetics of charge separate and charge recombination processes. Data were obtained from fs-TA (A) and TRIR (B) for PEn-NDI (n=4, 6, 8). For fs-TA, all data were extracted at 480 nm in dichloromethane. For TRIR, data were extracted at 2080 cm-1 in dichloromethane The detail charge transfer rates were summarized in Table 2-3.

Table 2-3. Charge separation/recombination kinetics from fs-TA and TRIR studies

a Measured from femtosecond transient absorption spectroscopy(fs-TA) in dichloromethane. b Measured from time resolved infrared (TRIR) spectroscopy in dichloromethane.

The kinetics of CS and CR processes can be studied via monitoring the

population dynamics for either OPE+• (480 nm) or NDI-• (600 nm). In this work, we used

τCSa

ps

kCSa

s-1

τCRa

ps

kCRa

s-1

τCSb

ps

kCSb

s-1

τCRb

ps

kCRb

s-1

PE4-NDI 1.0 1.0*1012 316.0 3.2*109 1.0 1.0*1012 446.5 2.2*109

PE6-NDI 2.3 4.3*1011 1004.0 1.0*109 4.4 2.3*1011 1123.0 8.9*108

PE8-NDI 8.4 1.2*1011 1176.0 8.5*108 9.1 1.1*1011 1005.0 1.0*109

A)

B)

72

the population dynamics of NDI-•, with detailed kinetics summarized in Figure 2-8 &

Table 2-3. In details, the τCS increases from 1 ps to 2.3 ps and τCR increases from 316 ps

to 1004 ps from PE4-NDI to PE6-NDI.From PE6-NDI to PE8-NDI, lifetime for CS

continue to increase from 2.3 ps to 8.4 ps, while the lifetime for CR maintains at ~1176

ps. The rate constants for CS/CR are calculated from the inverse of the measured

CS/CR lifetimes,24 as shown in Equation 2-6, where kET is charge transfer rate, and τET

is lifetimes for CS/CR processes measured from fs-TA. The calculated results are

summarized in Table 2-3.

Charge Separation and Charge Recombination (TRIR)

Ultrafast time-resolved infrared (TRIR) spectroscopy is another technique to

probe transient processes.154-157 In our work, TRIR studies were also conducted to

provide more information in understanding the CS/CR processes in PEn-NDI (n=4, 6, 8)

oligomers. As shown in Figure 2-9, upon laser illumination, new raising infrared spectra

(~ 2080 cm-1) observed for all the OPE-NDI oligomers within a timescale of 3 ~50 ps,

followed by bleaching in relatively slower process within the time scale of 1 ~ 3.5 ns.

This peak was ~ 70 cm-1 lower than the ground state IR absorption of an internal

acetylene vibration mode in both PE4-TIPS and PE4-NDI, and attributed to the internal

acetylene vibration in radical species (OPE+•) formed from CS process, based on similar

results.155-157 The dynamics of CS/CR processes were obtained through monitoring the

kinetics at 2080 cm-1 (Figure 2-8-B). To be specific, the PE4-NDI shows the fastest CS

rate (τCS =1.04 ps) along with a much slower CR rate τCR =446.5 ps, PE6-NDI shows a

decreased CS rate (τCS =4.35 ps) and CR rate τCR =1123 ps, and PE8-NDI shows the

further decreased CS time constant with τCS = 9.05 ps while the CR lifetime is

73

comparable with PE6-NDI with τCR=1005 ps. In short, the kinetics from TRIR study

confirms conclusions obtained from fs-TA experiments.

Figure 2-9. Transient Infrared spectra of OPE-NDI. The data were measured in dichloromethane with excitation wavelength at 400 nm. The optical density of sample solutions was controlled with 1 mOD at 350 nm with a 350 nm spacer.

Distance Effect

The distance-dependent effects in CS/CR processes could be evaluated by an

exponential expression,20 Equation 2-7, where k0 is the hypothetical rate at contact

distance, RDA is the distance between donor and acceptor and kET is the CS/CR rates

summarized in Table 2-3.

kET = k0 exp (-β RDA) (2-7)

In our work, RDA were estimated as center-to-center distances, calculated from

DFT B3LYP 6-31G* (2.36 nm for PE4-NDI, 3.04 nm for PE6-NDI and 3.73 nm for PE8-

NDI). The attenuation factor β could be calculated from plot of lnkET vs RDA. As we can

74

see from Figure 2-10, in the CS process, lnkET is linearly related to the RDA for all three

OPE-NDI oligomers, with the β is calculated as 0.16 Å-1 (both fs-TA and TRIR give the

same result). This implies that kET decreases exponentially when the OPE conjugation

length increases (RDA is directly determined by OPE conjugation length) in the CS

process.

Figure 2-10. Distance factor study in OPE-NDI. The rates for charge separation (A) and charge recombination (B) processes were obtained from lifetimes measured by fs-TA and TRIR techniques. The center-center distances were calculated by DFT.

Notably, in the CR process, the kET does not related to R in an exponential manner

throughout all the three OPE-NDI oligomers. Instead, kET stays almost the same for

PE6-NDI and PE8-NDI, even though their RDA increases from 30.4 Å to 37.3 Å. As a

result, the β of CR was calculated as 0.14 Å-1 ~ 0.17 Å-1 when RDA increases from 23.6

Å to 30.4 Å, and -0.016 Å-1 ~ 0.023 Å-1 when R increases from 30.4 Å to 37.3 Å. Based

on Coulomb’s law, the CS/CR is driven by the Coulomb interaction between the cation

and anion radicals, and its driving force is proportional to the inverse square of the

distance between the interacting charge centers (Scheme 2-1). The distance between

the cation and anion radicals is expected to increase when extending the OPE

75

conjugation length where hole could be delocalized further away from the NDI end-caps.

The Coulomb’s law explains the CS process for all three samples and CR for PE4-NDI

to PE6-NDI. However, the similarity in CR rates between PE6-NDI and PE8-NDI

indicates there might be some distance boundaries for Coulomb’s law, within the

boundary, longer conjugated OPE segment reduces the CR rates exponentially.

However, conjugation length would no more play a critical role in determining the CR

rates beyond that boundary. Based on our studies, we assume the transition point for

OPE-NDI type oligomer is between 3.04 nm and 3.83 nm. Furthermore, we assume the

distance boundaries also exist in CR process, though we may need to make further

longer OPE-NDI oligomer to prove it.

Scheme 2-1. Demonstration of CS/CR in OPE-NDI Solvent Effect

To investigate the solvent effects, photolysis experiments for PE6-NDI conducted

in three polarity-variable solvent systems by TRIR technique (Table 2-4), includes

dichloromethane, mixture of dichloromethane and acetonitrile (v:v 60:40) and mixture of

76

dichloromethane and methanol (v:v 60:40). In dichloromethane, the lifetime for CR is

measured as ~1000 ps. When using mixture of dichloromethane and acetonitrile, the

CR rate almost doubled, with lifetime decreases to ~540 ps. CR lifetime was further

lowered in a mixture of dichloromethane and methanol, detected as 213 ps ~ 260 ps

(Figure 2-11). Interestingly, the CS kinetics were not significantly influenced in those

three solvent systems, with CS lifetimes maintain 4 ps - 6 ps.

Figure 2-11. Solvent factor study in PE6-NDI. A) Transient IR absorption spectra of PE6-NDI in 21 ps, B) Kinetics monitoring at 2080 cm-1 of PE6-NDI under variable solvent systems (Dichloromethane, mixture of dichloromethane and acetonitrile and mixture of dichloromethane and methanol).

Table 2-4. Charge separation/recombination kinetics for PE6-NDI from TRIR

Solvents τCS (ps) kCS (s-1) τCR(ps) kCR (s-1)

DCM 4.4 2.3*1011 1123.0 8.9*108

DCM/ACN 4.9 2.0*1011 540.3 1.9*109

DCM/MeOH 5.6 1.8*1011 213.6 4.7*109

The data were extracted from Figure 2-11.

Based on the lifetime variances in different solvent systems, it is fair to conclude

more polar solvent accelerates CR rates in the OPE-NDI oligomers, while CS rates are

not much affected. Since the CR rates are determined by strength of Coulomb

interaction between the cation and anion radicals, and their center-center distances

A) B)

77

(Discussed in distance effect section in Chapter 2). Solvents with higher polarity would

cause stronger Coulomb interactions between cation-anion radicals, resulting in

stronger forces to pull the electron from NDI-• back to OPE+•, in form of accelerate CR

rate. The accelerated CR rate in dichloromethane/methanol, compared to

dichloromethane/acetonitrile is probably caused by the hydrogen bonding in

methanol.158

Electron Transfer

A Jablonski energy diagram (Scheme 2) was outlined to show the photophysics

processes after exciting the PEn-NDI oligomer systems. Upon excitation, small portion

of excited states will decay to ground states through fluorescent emission. From

fluorescence quantum yield study, less than 2.5% excited states were estimated to

decay through this process. Most of the excited OPE block will transfer electron to NDI

group, forming OPE+• and NDI-• in <10 ps during the CS process. The Equation 2-8

illustrates the CS/CR processes in OPE-NDI systems. Considering the ultrafast intra-

charge transfer, the intersystem crossing from singlet to triplet states will be suppressed.

The free electron from OPE+• would migrate and delocalize along the conjugated OPE

block until NDI-• transfer its electron back to OPE+• with lifetime τCR = 450~1100 ps, and

the CR lifetime is dependent on conjugation conditionally: shorter center-center distance

would accelerate the CR rates, while there might be a boundary limit for OPE-NDI type

oligomer between 3.04 nm and 3.83 nm, beyond which, the CR rates were not

influenced by center-center distance. Besides, the CR lifetime is also affected by its

environment: the rate is accelerated in a more polar solvent system, for example, when

changing solvents from pure dichloromethane to mixtures of dichloromethane/methanol,

CR rate enhanced 4 times.

78

OPE-NDI + hv → 1OPE-NDI* → 3OPE+•-NDI-•→ OPE-NDI (2-8)

Scheme 2-2. A Jablonski energy diagram of OPE-NDI system

Summary

A series of oligomers (PEn-NDI) with varying conjugation lengths were prepared

which featured a conjugation length controlled oligo(phenylene ethynylene) (OPE)

conjugated backbone and end capped naphthalene diimide derivative (NDI) group.

Under illumination, intra-electron transfer process from OPE backbone to NDI was

proved as the dominated process (96.7% < < 99.8%). The energetics of oligomers

was investigated by electrochemistry, steady state absorption and fluorescent emission

spectroscopy studies. Charge separation energies range from -1.19 eV to -1.10 eV and

charge recombination energy ranges from -2.02 eV to -1.96 eV. Taken together, the

thermodynamic data suggests that both CS and CR are highly exothermic, and not

obviously affected by OPE conjugation length. Bimolecular studies of TIPS protected

OPE oligomer and MV2+ indicates the charge transfer based quenching effects between

79

triplet states of oligomers and quenchers. Femtosecond transient absorption (fs-TA) and

transient IR (TRIR) studies clearly demonstrate the ultrafast charge separation process

occurring in the oligomer in a time scale of <10 ps to form OPE+• and NDI-• , followed by

relatively slower back electron transfer process in 400 ps ~ 1100 ps. Notably, we

demonstrated that Coulomb’s law is not always applicable in molecular donor-acceptor

systems, and the increasing of OPE conjugation length does not always exponentially

slow down the charge recombination rates. Furthermore, the charge recombination

rates are also influenced by other factors, such as solvent or hydrogen bonding.

Experiments and Materials

Instrumentation and Methods

1H and 13C NMR spectra were measured on a Mercury 300, a Gemini 300, or an

Inova 500. Chemical shifts were referenced to the residual solvent peaks. 1H NMR data

recorded with residual internal CD2Cl2 (δ 5.32) and CDCl3 (δ 7.26). 13C NMR data

recorded with references (CD2Cl2 (δ 53.84) and CDCl3 (δ 77.16). High-resolution mass

spectrometry was collected with either an Agilent 6200 ESI-TOF or an AB Sciex 5800

MALDI TOF/TOF in the Chemistry Department at the University of Florida.

Steady-state absorption spectra were recorded on a Shimadzu UV-1800 dual

beam spectrophotometer. Corrected steady-state emission measurements were

performed on a Photon Technology International (PTI) spectrophotometer.

Fluorescence lifetimes were collected in anhydrous chloroform (HPLC grade) on a

Picoquant FluoTime 100 time-correlated single photon counting (TCSPC) instrument

and analyzed with FluoFit software. Fluorescence quantum yields were reported relative

to known standards and estimated to have 10% error. Singlet oxygen quantum yield

measurements were conducted in deuterated chloroformunder 10 mins of purging with

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oxygen, and reported relative to terthiophene (∆=84%),159 and estimated to have15%

error.

Nanosecond transient absorption spectroscopy measurements were performed

on an in-house apparatus that is described in detail elsewhere.160 The third harmonic of

a Continuum Surelite series Nd:YAG laser (λ= 355 nm, 10 ns FWHM, 10 mJ per pulse)

was used as the excitation source. Probe light was produced by a xenon flash lamp and

the transient absorption signal was detected with a gated-intensified CCD mounted on a

0.18 M spectrograph (Princeton PiMax/Acton Pro 180). The optical density of the

solutions was adjusted to ~0.7 at the excitation wavelength. Samples were measured in

a cell that holds a total volume of 10 ml and the content was continuously recirculated

through the pump–probe region of the cell. Samples were prepared in solvents (THF or

mixture solvents of THF and acetonitrile (v:v 33%:67%) and degassed by bubbling

argon for 45 min before the acquisition. The transient absorption (TA) spectrum was

collected from 350 nm to 850 nm with a 20 ns initial camera delay and with different

subsequent delay time increments depending on the triplet lifetime of the molecule. Fifty

averages were obtained at each delay time.

Ultrafast pump–probe experiments were performed with femtosecond (fs)

transient absorption spectroscopy with broadband capabilities. Detailed information of

the experimental setup can be found elsewhere.161 An Ultrafast Systems Helios

femtosecond transient absorption spectrometer equipped with UV-visible and near-

infrared detectors was used to measure the samples in this study. The white light

continuum probe pulse was generated in a thick sapphire plate (800–1300 nm) and in a

CaF2 crystal (350–700 nm spectral range) using a few mJ pulse energy of the

81

fundamental output of a Ti:sapphire fs regenerative amplifier operating at 800 nm with

35 fs pulses and a repetition rate of 1 kHz. The pump pulses at 355 nm were created

from fs pulses generated in an optical parametric amplifier (Newport Spectra-Physics).

The sample solution was constantly stirred to avoid photo degradation in scanned

volume. The pump and probe beams were overlapped both spatially and temporally on

the sample solution, and the transmitted probe light from the samples was collected on

the broad-band UV-visible-near-IR detectors to record the time resolved excitation-

induced difference spectra.

Time-resolved IR experiments were carried out using a Helios-IR spectrometer

with broadband capability (Ultrafast Systems, U.S.A.). The UV pump pulses at 400 nm

were straightforwardly obtained by the second harmonic of a 120 fs Ti:sapphire

regenerative amplifier operating at 1 kHz (Spectra-Physics). The tunable mid-IR probe

pulses were generated by difference frequency mixing of the signal and idler pulses

from a near-infrared optical parametric amplifier. The experimental setup is detailed

elsewhere156.In the transient IR measurements, the photo-induced reaction was

recorded on a solution of PEn-NDI in DCM pumped through a rotational cell with

nominal thickness of 350 µm, ensuring that for every laser shot a fresh sample was

excited. The optical density of the solution was 1 mOD at 350 nm with 350nm spacer.

The cyclic voltammetry (CV) and differential pulse voltammetry (DPV)

experiments were performed on a Bio Analytical Systems CHI750 electrochemical

analyzer at a sweep rate of100 mV/s respectively, by using a platinum button as a

working electrode, a platinum wire as a counter electrode, and a silver wire as a

pseudo-reference electrode. Solutions of samples were prepared in dichloromethane

82

with 0.1 M tetrabutylammonium hexafuorophosphate (TBAPF6) as a supporting

electrolyte. The electrochemical potentials were internally calibrated against the

standard ferrocenium/ferrocene redox couple (Fc+/Fc). The highest occupied molecular

orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels for each

complex were reported with respect to the potential of a Fc+/Fc redox couple (5.1 eV vs.

vacuum).162

Materials

Unless specified, all compounds and solvents were purchased from commercial

sources (Aldrich, Acros, Strem Chemicals, et al) and used without further purification.

Tetrakis(triphenyl phosphine) palladium (Pd(PPh3)4) was purchased from Strem

Chemical, triisopropylsilyl acetylene and trimethylsilylacetylene was purchased from

TCI. Copper (I) iodide (CuI), 1-Ethynyl-4-methylbenzene, diisopropylamine ((i-Pr)2NH),

Tetra-n-butylammonium fluoride in THF (1M), Na2CO3, Methanol, tetrahydrofuran (THF)

and all other chemicals were purchased from either Sigma-Aldrich or Fisher Chemicals.

Synthesis and Characterization

Scheme 2-3. Synthesis scheme of OPE-NDI oligomers

83

Scheme 2-4. Synthesis scheme of OPE-NDI oligomers

Compounds 1163, compound 2164, and compound 8165 were synthesized

according to references.

84

Compound 3

Compound 2 (2.33 g, 4.67 mmol) and Compound 1 (1.1 g, 3.89 mmol) were

dissolved in a mixed solvent of THF (7 5ml) and diisopropylamine (45 ml) and degassed

with argon for 30 min. Pd(PPh3)4 (219 mg, 0.19 mmol) and CuI (74 mg, 0.39 mmol) were

added under argon protection. The mixture was degassed for another 30 min and

stirred at R.T. for overnight, under argon protection. The mixture was filtered and the

solution was evaporated under vacuum. The oil was purified by column with hexane as

eluent to yield compound 3 (1.2 g, 46%). 1H NMR (300 MHz, CDCl3): δ 7.67 (s, 1H),

7.44 (dd, 4H), 7.30 (s, 1H), 2.73 (t, 2H), 2.65 (t, 2H), 1.56-1.67 (m, 4H), 1.26-1.39 (m,

12H), 1.14 (s, 21H), 0.89 (m, 6H). 13C NMR (75 MHz, CDCl3): 11.35, 14.13, 18.70,

22.64, 29.08, 29.24, 30.23, 30.67, 31.70, 33.89, 40.25, 89.64, 92.86, 93.18, 101.20,

106.67, 122.51, 123.26, 123.40, 131.18, 132.02, 132.22, 139.54, 142.82, 144.12 ppm.

Compound 4

Compound 2 (5 g, 10 mmol) and 1-Ethynyl-4-methylbenzene(1.16 g, 10 mmol)

were dissolved in mixed solvent of THF (100 ml) and diisopropylamine (50 ml) and

degassed with argon for 30 min. Pd(PPh3)4(347 mg, 0.3 mmol) and CuI (114 mg, 0.6

mmol) were added under argon protection. The mixture was degassed for another 30

min and stirred at R.T. for overnight, under argon protection. The mixture was filtered

and the solution was evaporated under vacuum. The oil was purified by column with

hexane as eluent to yield compound 4 (2.2g, 45%). 1H NMR (300 MHz, CDCl3): δ 7.66

(s, 1H), 7.41 (d, 2H), 7.30 (s, 1H), 7.16 (d, 2H), 2.74 (t, 2H), 2.65 (t, 2H), 1.57-1.67 (m,

4H), 1.31-1.39 (m, 12H), 0.89 (m, 6H).13C NMR (75 MHz, CDCl3):δ 14.50, 21.93, 23.01,

85

29.43, 29.60, 30.61, 30.97, 32.11, 34.26, 40.62, 87.59, 94.05, 101.01, 120.70, 123.32,

129.55, 131.72, 132.56, 138.85, 139.81, 143.08, 144.41 ppm.

Compound 5

Compound 4 (1.9 g, 3.9 mmol) was dissolved in mixed solvents of THF (45 ml) and

diisopropylamine (25 ml) and degassed with argon for 30 min. Pd(PPh3)4(218 mg, 0.19

mmol), CuI (74 mg, 0.39 mmol) and trimethylsilylacetylene (0.58 g, 5.85 mmol) were

added under argon protection. The mixtures were stirred at R.T. for overnight, under

argon protection. The mixture was filtered and the solution was evaporated under

vacuum to obtain crude products. The crude products were dissolved in mixed solvents

of 4 ml methanol and 8 ml THF, 2 ml KOH aqueous solution (10%) was added to the

reaction solution. The mixtures were stirred at R.T. for overnight. After the reaction, the

organic solvents were evaporated and dissolving in 30ml dichloromethane, followed by

washing with water (30 ml) for 3 times. The organic solution was collected and dried

with Na2SO4. The mixture was filtered and the solution was evaporated under vacuum.

The oil was purified by column with hexane as eluent to yield compound 5 (1.1 g, 73%).

1H NMR (300 MHz, CDCl3): δ 7.41 (d, 2H), 7.32 (d, 2H), 7.17 (d, 2H), 3.29 (s, 1H),

2.75(dt, 4H), 2.36 (s, 3H), 1.60-1.70 (m, 4H), 1.30-1.40 (m, 12H), 0.88-0.91 (m, 6H). 13C

NMR (75 MHz, CDCl3): δ 14.12, 21.54, 22.65, 29.16, 29.24, 30,52, 30.58, 31.70, 31.77,

33.88, 34.10, 81.30, 82.56, 87.57, 94.20, 120.35, 121.15, 123.31, 129.17, 131.38,

132.14, 133.00, 138.46, 142.07, 142.75 ppm.

Compound 6 (PE4-TIPS)

Compound 3 (0.75 g, 1.15 mmol) and compound 5 (0.49 g, 1.26mmol) was

dissolved in mixed solvent of THF (30 ml) and diisopropylamine (20 ml) and degassed

86

with argon for 30 min. Pd(PPh3)4 (65 mg, 0.058 mmol), CuI (22 mg, 0.115 mmol) were



vacuum. The solid was purified by column with hexane as eluent to yield compound 6

(0.87g, 84%). 1H NMR (300 MHz, CDCl3): δ 7.41-7.46 (m, 6H), 7.34-7.37 (m, 4H), 7.16

(d, 2H), 2.75-2.84 (m, 8H), 2.38 (s, 3H), 1.70 (dd, 8H), 1.33-1.40 (m, 21H), 1.14 (m,

16H), 0.88 (m, 12H). 13C NMR (75 MHz, CDCl3): δ 11. 34, 14.13, 14.16, 18.69, 21.54,

22.63, 22.67, 29.07, 29.22, 29.28, 29.29, 30.50, 30.61, 30.67, 30.70, 31.68, 31.76,

31.78, 31.85, 33.97, 34.09, 34.19, 78.24, 81.71, 87.64, 87.82, 90.34, 92.8093.22, 93.61,

94.19, 94.86, 106.68, 120.29, 120.44, 120.96, 122.22, 122.54, 122.80, 123.08, 123.31,

123.36, 123.70, 129.17, 129.18, 131.21, 131.37, 131.41, 132.02, 132.21, 132.30,

132.45, 132.49, 133.28, 138.42, 138.57, 141.88, 141.93, 142.18, 142.32, 143.64 ppm.

MALDI-MS: m/z=908.6653 [M-H]+ (calcd: 908.6650).

Compound 7

Compound 6 (0.75 g, 0.825 mmol) was dissolved in chloroform (30 ml) and

degassed with argon for 30 min. Tetrabutylammonium fluoride solution (2ml, 1M in THF)

was added under argon protection. The mixtures were stirred at R.T. for 24 hs, under

argon protection. After the reaction, the organic solvents were evaporated and

dissolving in 20ml dichloromethane, followed by washing with water (20 ml) for 3 times.

The organic solution was collected and dried with Na2SO4. The mixture was filtered and

the solution was evaporated under vacuum. The crude products were purified by

column with hexane as eluent to yield compound 7 (350 mg, 56%). 1H NMR (300 MHz,

CDCl3): δ 7.46-7.49 (m, 4H), 7.42 (d, 2H), 7.35-7.36 (m, 4H), 7.17 (d, 2H), 3.18 (s, 1H),

87

2.79-2.83(m, 8H), 2.38 (s, 3H), 1.67-1.73 (m, 8H), 1.25-1.41 (m, 24H), 0.86-0.89

(m,12H). 13C NMR (75 MHz, CDCl3): δ14.12, 21.59, 22.66, 29.31, 30.66, 30.70, 31.77,

31.78, 31.84, 34.18, 78.95, 83.31, 87.81, 90.56, 92.80, 93.27, 93.32, 94.22, 120.43,

121.84, 122.10, 122.52, 122.82, 123.18, 124.00, 129.17, 131.32, 131.38, 132.12,

132.21, 132.33, 132.45, 132.50, 138.43, 141.90, 141.94, 142.18, 142.35 ppm.

Compound 9 (PE4-NDI)

Compound 7 (154 mg, 0.20 mmol) and compound 8 (100 mg, 0.17 mmol) were

dissolved in mixed solvent of THF (20 ml) and diisopropylamine (12 ml) and degased

with argon for 30 min. Pd(PPh3)4 (9.7 mg, 0.0085 mmol), CuI (3.3 mg, 0.017 mmol)

were added under argon protection. The mixtures were stirred at R.T. for overnight,

under argon protection. The mixture was evaporated under vacuum. The crude product

was purified by column with dichloromethane as eluent to yield compound 9 (174 mg,

85%). 1H NMR (300 MHz, CDCl3): δ 8.81 (m, 4H), 7.72 (d, 2H), 7.54 (m, 4H), 7.42 (d,

2H), 7.31-7.34 (m, 6H), 7.17 (d, 2H), 4.2 (t, 2H), 2.78-2.85(m, 8H), 2.38 (s, 3H), 1.68-

1.75 (m, 10H), 1.29-1.44 (m, 34H), 0.87-0.91 (m, 15H).13C NMR (75 MHz, CDCl3): δ

14.11, 14.14,14.15, 27.12, 28.10, 29.22, 29.30, 29.31, 30.66, 30.68, 30.71, 31.79,

31.82, 31.86, 34.19, 34.20, 41.09, 90.36, 90.42, 90.61, 92.86, 93.27, 93.63, 94.23,

120.41, 122.18, 122.51, 122.72, 122.79, 123.12, 123.63, 124.18, 126.51, 126.82,

127.03, 127.05, 128.74, 129.16, 130.98, 131.37, 131.40, 131.42, 131.70, 132.17,

132.31, 132.43, 132.49, 132.67, 134.44, 138.42, 141.88, 142.16, 142.34, 162.68,

162.84 ppm. MALDI-MS: m/z=1205.7106 [M]+ (calcd: 1205.7085).

88




with argon for 30 min. Pd(PPh3)4(16.5 mg, 0.014 mmol), CuI (5.4 mg, 0.028 mmol) were



vacuum. The crude product was purified by column with hexane as eluent to yield

compound 10 (230mg, 63%). 1H NMR (300 MHz, CDCl3): δ 7.53 (s, 4H), 7.47 (m, 4H),

7.44 (d, 2H), 7.39 (t, 6H), 7.19 (d, 2H), 2.79-2.85(m, 12H), 2.39 (s, 3H), 1.70-1.75 (m,

12H), 1.28-1.44 (m, 36H), 1.16 (m, 21H), 0.88-0.91 (m, 18H).13C NMR (75 MHz,

CDCl3): δ 11.32, 14.11, 18.67, 21.52, 22.64, 22.65, 29.28, 30.66, 31.77, 31.84, 34.19,

87.81, 90.28, 90.41, 90.53, 92.84, 93.25, 93.67, 93.76, 93.79, 94.20, 106.67, 120.42,

122.20, 122.52, 122.80, 123.11123.24, 123.33, 129.14, 131.20, 131.36, 131.42, 132.00,

132.19, 132.30, 132.35, 132.43, 132.48, 138.39, 141.87, 141.92, 142.16, 142.34 ppm.

MALDI-MS: m/z=1277.9205[M]+ (calcd: 1277.9186).

Compound 11

Compound 10 (250 mg, 0.20 mmol) was dissolved in THF (10 ml) and degassed

with argon for 30 min. Tetrabutylammonium fluoride solution (1ml, 1M in THF) was

added under argon protection. The mixtures were stirred at 40 degrees overnight, under

argon protection. After the reaction, the organic solvents were evaporated and dissolved

in 20 ml dichloromethane, followed by washing with water (20 ml) for 3 times. The

organic solution was collected and dried with Na2SO4. The mixture was filtered and the

solution was evaporated under vacuum. The crude products were purified by column

89

with hexane: dichloromethane (20:1) as eluent to yield compound 11 (187 mg, 83%).1H

NMR (500 MHz, CDCl3): δ 7.57 (s, 4H), 7.49 (m, 4H), 7.44 (d, 2H), 7.39 (dd, 6H), 7.18

(d, 2H), 3.19 (s, 1H), 2.80-2.87(m, 12H), 2.39 (s, 3H), 1.70-1.75 (m, 12H), 1.28-1.45 (m,

36H), 0.89-0.92 (m, 18H).13C NMR (75 MHz, CDCl3): δ 14.14, 22.69, 29.21, 29.32,

30.66, 30.70, 31.77, 31.87, 34.15, 34.20, 78.94, 83.29, 87.82, 90.37, 90.48, 90.56,

92.88, 93.24, 93.48, 93.66, 93.85, 94.22, 120.44, 121.89, 122.21, 122.42, 122.54,

122.62, 122.82, 123.13, 123.23, 123.37, 123.95, 129.17, 131.33, 131.38, 131.44,

132.12, 132.21, 132.32, 132.40, 132.45, 132.50, 138.42, 141.88, 141.95, 142.18,

142.32, 142.38, 142.39 ppm.


Compound 8 (51.7 mg, 0.09 mmol) and compound 11 (50 mg, 0.045 mmol) were

dissolved in mixed solvent of THF (8 ml) and diisopropylamine (4 ml) and degassed with

argon for 30 min. Pd(PPh3)4(4.6 mg, 0.0045 mmol), CuI (1 mg, 0.005 mmol) were added

under argon protection. The mixtures were stirred at R.T. for overnight, under argon

protection. The mixture was evaporated under vacuum. The crude product was purified

by column with dichloromethane as eluent to yield compound 12 (52mg, 73%). 1H NMR

(300 MHz, CDCl3): δ 8.80 (m, 4H), 7.72 (d, 2H), 7.53 (m, 8H), 7.42 (d, 2H), 7.36 (m,

6H), 7.32 (d, 2H), 7.17 (d, 2H), 4.2 (t, 2H), 2.80-2.84(m, 12H), 2.38 (s, 3H), 1.69-1.76

(m, 14H), 1.29-1.44 (m, 46H), 0.88-0.92 (m, 21H). 13C NMR (75 MHz, CDCl3): δ 14.12,

21.55, 22.67, 22.70, 27.12, 28.09, 29.23, 29.29, 29.31, 30.66, 30.69, 30.71, 31.79,

31.83, 31.86, 34.18, 41.09, 87.82, 90.35, 90.43, 90.51, 90.55, 92.86, 93.27, 93.71,

93.77, 93.87, 94.22, 120.42, 122.21, 122.50, 122.53, 122.56, 122.78, 122.81, 123.12,

123.24, 123.34, 123.57, 124.16, 126.50, 126.81, 127.04, 128.75, 129.17, 130.96,

90

131.38, 131.44, 131.71, 132.19, 132.30, 132.38, 132.44, 132.50, 132.67, 134.45,

138.43, 141.88, 141.93, 142.16, 142.32, 142.55, 162.65, 162.82 ppm. MALDI-MS:

m/z=1573.9569 [M-H]+ (calcd: 1573.9589).


Compound 3 (217 mg, 0.33 mmol) and compound 11(187 mg, 0.167 mmol) were


with argon for 30 min. Pd(PPh3)4(9.6 mg, 0.0084 mmol), CuI (3.2 mg, 0.017 mmol) were



vacuum. The crude product was purified by column with hexane: dichloromethane

(10:1) as eluent to yield compound 10 (170mg, 62%).1H NMR (300 MHz, CDCl3): δ 7.53

(s, 8H), 7.44-7.47 (m, 6H), 7.39 (m, 8H), 7.18 (d, 2H), 2.85(m, 16H), 2.39 (s, 3H), 1.75

(m, 16H), 1.28-1.44 (m, 48H), 1.16 (m, 21H), 0.88-0.91 (m, 24H). 13C NMR (75 MHz,

CDCl3): δ 11.35, 14.15, 18.70, 21.57, 22.65, 22.68, 29.27,29.32, 30.67,30.74, 31.77,

31.81, 31.88, 34.18, 87.82, 90.29, 90.43, 90.55, 92.86, 93.27, 93.69, 93.79, 93.83,

94.22, 106.67, 120.45, 122.22, 122.52, 122.55, 122.83, 123.14, 123.26, 123.32, 123.36,

129.17, 131.23, 131.39, 131.43, 132.03, 132.22, 132.33, 132.37, 132.46, 132.51,

138.43, 141.90, 141.96, 142.19, 142.38 ppm. MALDI-MS: m/z=1646.1683 [M]+ (calcd:

1646.1689).

Compound 14

Compound 13 (120 mg, 0.073 mmol) was dissolved in THF (12 ml) and

degassed with argon for 30 min. Tetrabutylammonium fluoride solution (2 ml, 1 M in

THF) was added under argon protection. The mixtures were stirred at 40 degrees

91

overnight, under argon protection. After the reaction, the organic solvents were

evaporated and dissolved in 20ml dichloromethane, followed by washing with water

(20ml) for 3 times. The organic solution was collected and dried with Na2SO4. The

mixture was filtered and the solution was evaporated under vacuum. The crude

products were purified by column with hexane: dichloromethane (10:1) as eluent to yield

compound 14 (100 mg, 92%).1H NMR (300 MHz, CD2Cl2): δ 7.58 (s, 8H), 7.53 (m, 4H),

7.42-7.48 (m, 10H), 7.24 (d, 2H), 3.29 (s, 1H), 2.87 (m, 16H), 2.42 (s, 3H), 1.75 (m,

16H), 1.30-1.47 (m, 48H), 0.88-0.91 (m, 24H). 13C NMR (75 MHz, CD2Cl2): δ

13.86,21.22, 22.68, 29.20, 29.65, 30.67, 30.67, 31.77, 31.78, 31.89, 34.09, 78.84,

83.03, 87.63, 90.28, 90.34, 92.80, 93.17, 93.38, 93.60, 93.72, 94.17, 120.27, 121.89,

122.21, 122.42, 122.52, 122.53, 122.58, 122.79, 123.09, 123.23, 123.25, 123.28,

123.32, 123.90, 129.18, 131.27, 131.32, 131.41, 132.09, 132.14, 132.28, 132.35,

132.37, 132.40, 132.45, 138.73, 142.03, 142.07, 142.26, 142.45, 142.49, 142.52 ppm.


Compound 8 (50.6 mg, 0.088 mmol) and compound 14 (65 mg, 0.044 mmol)

were dissolved in mixed solvent of THF (10 ml) and diisopropylamine (4 ml) and

degassed with argon for 30 min. Pd(PPh3)4 (4.6mg, 0.0045 mmol), CuI (1 mg, 0.005

mmol) were added under argon protection. The mixtures were stirred at R.T. for

overnight, under argon protection. The mixture was evaporated under vacuum. The

crude product was purified by column with dichloromethane as eluent to yield

compound 12 (65 mg, 76%). 1H NMR (300 MHz, CDCl3): δ 8.81 (m, 4H), 7.73 (d, 2H),

7.54 (m,12H), 7.43 (d, 2H), 7.36 (m, 8H), 7.33 (d, 2H), 7.17 (d, 2H), 4.18 (t, 2H), 2.78-

2.85(m, 18H), 2.38 (s, 3H), 1.68-1.75 (m, 18H), 1.29-1.44 (m, 58H), 0.87-0.91 (m, 27H).

92

13C NMR (75 MHz, CDCl3): δ 14.13, 21.56, 22.69，27.12，28.11, 29.22, 29.28, 29.30,

30.66, 30.69, 30.72, 31.79, 31.80, 31.82, 31.86, 34.17, 41.10, 87.74, 90.35, 90.42,

90.51, 90.53, 92.85, 93.27, 93.68, 93.76, 93.84, 94.21, 120.43, 122.21, 122.53, 122.78,

122.81, 123.12, 123.24, 123.29, 123.35, 123.58, 124.19, 126.52, 126.84, 127.07,

128.74, 129.17, 130.99, 131.37, 131.45, 131.71, 132.18, 132.29, 132.36, 132.39,

132.45, 132.50, 132.68, 134.44, 138.43, 141.88, 141.94, 142.17, 142.32, 142.36,

142.39, 162.70, 162.84 ppm. MALDI-MS: m/z=1942.2120 [M]+ (calcd: 1942.2091).

93

CHAPTER 3

WAVELENGTH CONTROL OF PHOTOINDUCED CHARGE TRANSFER IN A -CONJUGATED DIBLOCK MOLECULAR PHOTODIODE

Background

Photoinduced electron transfer (PET) has been studied intensively in physics,

chemistry and biology for its key roles in artificial photosynthesis and photovoltaics.60,

166, 167 One of the key goals in PET is to produce a long-lived charge separated state.168

So far, different varieties of donor-acceptor (D-A) linked systems have been developed

for PET studies to learn how various parameters including driving force, electronic

coupling, reorganization energy and temperature et al effect the charge transfer

kinetics.2, 22, 169, 170 However, to our knowledge, most of the related studies are more

focused on changing the bridged spacer or screening D-A pairs with appropriate energy

levels. Herein, we present an example of controlling charge transfer through selective

excitation of a -conjugated diblock oligomer system.


In this work, a diblock molecular photodiode, T4PE4NDI, was studied, which

features tetrathiophene (T4) and tetra-phenyleneethynylene (PE4) as electron rich -

conjugated segments, which possess different electron donor strengths (ΔEHOMO (T4-

PE4)=0.76 eV), capped with a naphthalene diimide unit as an electron acceptor. The

diblock oligomers were synthesized with the vision of creating molecules that can be

selectively excited to form a charge transfer complex which is then stabilized through

delocalization of the cation over the T4 segment. T4NDI, T4PE2NDI and three

corresponding TIPS-protected oligomers, T4PEnTIPS (n=0, 2, 4), were also prepared

and used as model compounds (Figure 3-1). The oligomers were synthesized based on

segment subsequent growing approach through Suzuki & Sonogashira coupling

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reactions The detailed synthesis procedures and characterization information were

described in synthesis and characterization section in Chapter 3.

Figure 3-1. Structures of diblock oligomers.

The multiple hexyl side groups attached to the oligomers make them highly

soluble in organic solvent systems. All the oligomers were characterized by 1H-

NMR/13C-NMR and the key target compounds were also characterized by Mass

spectroscopy. As predicted, we achieved the controlling the photoinduced charge

transfer in T4PE4NDI by switching wavelengths between 370 nm and 420 nm, as

outlined in Equation 3-1&3-2. Charge transfer was observed under excitation at 370 nm

via detection of a radical T4+• cation in the near IR region, while excitation at 420 nm

resulted in no obvious charge transfer.

95

T4PE4NDI + hv(370 nm) → T4(1PE4*)NDI → T4(PE4

+)NDI- → (T4+) PE4NDI- → T4PE4NDI

(3-1)

T4PE4NDI + hv(420 nm) → (1T4*) PE4NDI →

(3T4*)PE4NDI → T4PE4NDI (3-2)

Photophysical Study

Figure 3-2. UV-visible absorption (a,c) and fluorescence spectra (b,d) of T4PE4TIPs,

T4TIPS, PE4TIPS, T4PE4NDI and T4NDI in chloroform. The fluorescence spectra were obtained under an excitation of 370 nm and normalized based on relative fluorescent quantum yields.

From photophysics study in chloroform, ground state absorption of the diblock

oligomers exhibit the sum of their molecular conjugated components, which gives 50 nm

between 400-450 nm where the T4 segment can be selectively excited without exciting

PE4 or NDI (Figure 3-2-a).171 T4PE4TIPS exhibits adequate fluorescent emission

quantum yields (ϕfl=0.32, λex=370 nm),with emission spectrum ranges from 450 nm to

650 nm, but significantly less when compared to PE4TIPS (ϕfl=0.84) with emission

spectrum ranges from 375 nm to 475 nm. (Figure 3-1-b). No fluorescent emission was

96

detected from T4PE4TIPS in region of 400nm-450nm (the intrinsic emission spectra

wavelength range from PE4-TIPS segment), such fluorescence quenching of the PE4

segment indicates energy transfer from PE4 to the T4 segment (Figure 3-1-b and Figure

3-3). As shown from Figure 3-1-c, T4PE4NDI has much higher molar absorptivity than

T4NDI, with the extra molar absorptivity contributed from PE4 segment. Under excitation,

T4NDI is almost non-emissive for its dominated photo-induced charge transfer process.

Notably, T4PE4NDI exhibits strong fluorescent emission in similar spectral region as

T4PE4TIPS with ϕfl=0.16. From time correlated single photon counting (TCSPC)

measurements (Figure 3-4), the lifetime of the TIPS protected oligomers were recorded

around 0.5 ns, while NDI end capped T4NDI and T4PE2NDI exhibit much shorter

lifetimes ( < 100 ps). Notably, T4PE4NDI exhibits a fluorescent lifetime around 0.4 ns,

which implies that the charge transfer process is significantly suppressed.

Table 3-1. Summary of the photophysical properties

a Fluorescence decay life time measured by TCSPC. bWith anthrathene as fluorescence quantum yield standard, ϕ =0.27.172 in ethanol at room temperature. cElectron transfer efficiencies (η) were calculated as η = 1 − ϕ(T4PEnNDI)/ϕ(T4PEnTIPS), in which ϕ(T4PEnNDI) was the fluorescence quantum yield of NDI end cupped oligomers, while ϕ(T4PEnTIPS) was the fluorescence quantum yield of TIPS group protected oligomers.

Oligomers λabs (nm) λf (nm) ε (105 cm-1M-1) τfa (ns) f

b c

PE4TIPS 354 398 1.08 0.82 0.84 --

T4TIPS 394 479 0.39 0.48 0.20 --

T4NDI 362,382 N.A 0.84 (382 nm) -- -- 99.8

T4PE2TIPS 407 502 0.60 0.53 0.26 --

T4PE2NDI 362,382 N.A 1.10 (382 nm) -- -- 98.3

T4PE4TIPS 377 503 1.22 0.51 0.29 --

T4PE4NDI 364,382 502 1.53 (382 nm) 0.42 0.16 44.8

97

Figure 3-3. Fluorescence emission spectra for T4PE4TIPS and PE4TIPS under different

excitation wavelengths. T4TIPS gave fluorescence emission spectra, maximized around 400 nm when excited at 350 nm (black dash line). T4PE4TIPS gave similar fluorescence emission spectra (maximum peak around 500 nm) when excited at 350 nm (black solid line), 375 nm (red solid line) and 450 nm (blue solid line).

Figure 3-4. Time correlated single photon counting (TCSPC) experiments of T4PEnTIPS

and T4PEnNDI (n=0, 2, 4). Experiments were conducted in chloroform (controlled absorption=0.2 at 375 nm).

The excitation wavelength dependent fluorescence quantum yields were also

studied under wavelength at 370 nm and 420 nm for T4PE4TIPS and T4PE4NDI in

98

chloroform. It turns out that TIPS protected oligomers have comparable quantum yields

when exited differently (0.32 at 370 nm and 0.31 at 420 nm). However, the NDI capped

oligomers show different behaviors that 370 nm excitation gives lower quantum yield-

Figure 3-5. Fluorescence quantum yields under different excitation wavelengths of

T4PE4NDI. Fluorescence quantum yields were measured in chloroform. Electron transfer efficiencies (η) were calculated as η =

1−fl(PEnNDI)/fl(PEnTIPS).

Table 3-2. Summary of the charge transfer efficiency Oligomers

(Excitation wavelength) Charge transfer efficiency ()

T4PE4TIPS (370 nm) --

T4PE4NDI (370 nm) 34.4%

T4PE4TIPS (420 nm) --

T4PE4NDI (420 nm) 9.7%

Fluorescence quantum yields were measured in chloroform. Electron transfer efficiencies (η) were

calculated as η = 1−fl(PEnNDI)/fl(PEnTIPS).

-around 0.21 while 420 nm excitation gives 0.28 (Figure 3-5). This clearly show that

fluorescent emission process was suppressed in some extent when excited at higher

energy wavelength. In other words, there might be other excited states decay process

occurring in higher yields when exciting T4PE4NDI with 370 nm, compared to 420 nm.

We assume that process is the charge transfer process. When considering T4PE4TIPS

as a control, we can calculate the electron transfer efficiency as 34.4% at 370 nm and

99

drops to 9.7% at 420 nm (Table 3-2). The excitation wavelength dependent

fluorescence quantum yields preliminarily indicate the selective charge transfer in

controlling excitation wavelength for diblock oligomer

Bimolecular Charge Transfer Study

To gain a better understanding of the diblock oligomers’ cation radicals formed

from PET processes and how they “look like” and “behave” in perspectives of dynamics

and kinetics, a bimolecular electron transfer experiment was performed with T4PE4TIPS

(7.0 uM) in the presence of paraquat (MV2+, 0.7 mM), based on the nanosecond

transient absorption spectroscopy technique. As shown in Figure 3-6, in the absence of

paraquat, triplet states decay processes of TIPS protected oligomers were observed by

nanosecond transient absorption spectroscopy. However, in the presence of paraquat

(MV2+, 0.7 mM), a new rising peak was observed along with the quenching of the

oligomers’ triplet states. Take T4PE4TIPS as an example, a rising peak with λmaxT4PE4+•

= ~725 nm was observed, assigned as T4PE4+•, during the quenching of the oligomer’s

triplet states at λmax 3T4PE4* = ~ 675 nm. Bimolecular studies of T4PE2TIPS and T4TIPS

deliver similar results as well (Figure 3-6). The MV+• absorption peak was also detected

around 390 nm in all the three bimolecular systems. We are confident to conclude there

are inter-charge transfer occurring between TIPS protected oligomers and MV2+. There

are two possible pathways for the electron-transfer based quenching phenomenon.

First, oligomers in singlet states are involved in inter-charge transfer. Second, oligomers

in triplet states are involved in such inter-charge transfer. Based on the time-correlated

single-photon counting (TCSPC) experiments of the bimolecular system, there are no

quenching effects observed between singlet states and MV2+, which leaves only one

100

reasonable pathway that quenching interactions only occur between MV2+ and

oligomers in triplet states.

Figure 3-6. Nanosecond transient absorption spectra of T4PEnTIPS oligomers with

MV2+ (Left column, a) T4TIPS (35.0 uM)), b) T4PE2TIPS (15 uM), c) T4PE4TIPS (7.0 uM)). Nanosecond transient absorption spectra of T4PEnTIPS oligomers with MV2+ (Right column, d) T4TIPS (35.0 uM) + MV2+ (3.5 mM)), e) T4PE2TIPS (15 uM) + MV2+ (1.5 mM), f) T4PE4TIPS (7.0 uM) + MV2+ (0.7 mM)). The experiments were conducted in mixture solution of THF (66%) and Acetontrile (34%) with controlled absorption around 0.7 at 355 nm.

The kinetics of T4TIPS - MV2+ bimolecular systems were monitored at 680nm,

where T4+• radical canion dominated the transient absorption and the overlap from triplet

states could be ignored. The intermolecular charge separation was measured with KCS

=3.5*106 s-1, followed by intermolecular charge recombination with KCR =1.5*104 s-1

Consider that the triplet states quenching rates by MV2+ are determined by molecule

diffuse rates in solvent environments. The diblock oligomers (T4PE2TIPS, T4PE4TIPS)

were expected to have similar bimolecular charge transfer behavior with slightly

101

reduced CS/CT rates for their larger molecule sizes. We got similar results from

conjugated oligo-phenylene ethynylene (OPE) systems.

Electrochemistry Study

Figure 3-7. Electrochemistry study. CV (black solid line) and DPV (red dashed line) of

a) T4PE4NDI, b) T4TIPS, c) PE4TIPS, measured in dry dichloromethane, with 0.1M TBA(PF)6 as electrolyte, and referenced to Fc/Fc+ as an internal standard.

Electrochemistry studies, including cyclic voltammetry (CV) & differential pulse

voltammetry (DPV) were conducted to study the energy levels for oligomers. The

energetics of these two NDI end capped oligomers will supply intrinsic information to

study the dynamics in both charge separation (CS) and charge recombination (CR)

processes. The values from DPV measurements, with vs Ferrocenium /Ferrocene

(FC+/FC) as external references are applied for energy level calculations in this work.

The T4-PE4-NDI has shown two typical reduction peaks -1.05 V vs FC+/FC, and -1.48 V

102

vs FC+/FC from NDI group, meanwhile two oxidation peaks for both T4 segment (0.45 V

vs FC+/FC) and PE4 segments (1.05 V vs FC+/FC). T4 has multiple oxidation peaks, and

the oxidative value from the first peak was applied in this work (0.44 V vs FC+/FC). The

HOMO and LUMO energy levels were also calculated from Equation 3-1 and

summarized in Table 3-3. The donor ability difference from T4 and PE4 segments were

calculated to as. ΔEHOMO (T4-PE4) = -5.58 eV-(-6.34) eV = 0.76 eV

Table 3-3. Electrochemistry study.

Eox/Va (Eox/V)b Ered/Va (Ered/V)b EHOMO/eVc ELUMO/eVc

PE4TIPS 1.06 0.97 -- N.A -6.07 N.A

T4TIPS 0.59, 0.87 (0.50, 0.79)

-- N.A -5.60 N.A

T4PE4NDI 0.48,0.75,

1.24 (0.45,

0.67,1.05) -1.06, 1.54

(-1.05, -1.52)

-5.58 (T4),-6.34 (PE4)

-4.04

aObtained from cyclic voltammetry (CV) measurements. bObtained from differential pulse voltammetry (DPV) measurements. cCalculated from equations above (Equations 3-3 and 3-4), the Eox and Ered values are data measured from DPV.

EHOMO = - (E(ox VS FC+

/FC) +5.1) (eV)162 (3-3)

ELUMO = - (E(red VS FC+

/FC) +5.1) (eV)162 (3-4)

Femtosecond Transient Absorption Spectroscopy for CT/CR

Systematic ultrafast transient absorption spectroscopy experiments were

conducted on all oligomers to gain a better understanding of the excited state dynamics

and absorptions. Two key oligomers (T4NDI and T4PE4TIPS) were used as models to

help predict the excited state pathway for the NDI capped diblock oligomers. Figure 3-8

shows both of the aforementioned standard oligomers’ ultrafast transient absorption.

The visible spectrum of T4PE4 shows a decaying absorption band between 600-800 nm

that is representative of the (T4)*PE4 singlet state. The absorption band that grows in

with a maximum of 680 nm is the triplet state that persists through the entirety of the TA

experiment (8 ns). There are two bands that appear in the negative regime of the TA

spectrum, the high energy band with a maximum around 400 nm is the ground state

103

bleach, and the band with a maximum around 550 nm is the stimulated emission. The

IR region for T4PE4 only reveals one broad band with a maximum around 900 nm, which

is simply a continuation of the singlet state band mentioned earlier when describing the

visible region.

Figure 3-8. Picosecond transient absorption spectra of T4PE4 (a,b) and T4NDI (c,d). Spectra obtained with excitation at 370 nm, 100 nJ/pulse.

Ultrafast transient absorption spectrums of T4NDI reveal new charge-transfer

absorption bands that can be used to probe charge-transfer kinetics for all oligomers

capped with NDI (Figure 3-8c). The visible region shows a strong absorption band

around 480 nm characteristic of the NDI radical anion absorption.141 In the IR region of

T4NDI, a broad intense absorption decays rapidly around 1275 nm, this band is

assigned to the T4 cation absorption (Figure 3-8d). The T4 cation absorption peak in IR

region is so unique without interruption from other species that we used it as a spectral

feature to analyze charge separation processes in our work.

104

Figure 3-9. Picosecond transient absorption of T4PE4NDI excited at 370, 420 and 440 nm in DCM (100 nJ/pulse). Each row represents a different time slice including 5, 50, 500 ps and 1 ns with the left column displaying a visible probe (440-800 nm) and the right column displaying an IR probe (800-1400 nm).

Wavelength dependent excitation to manipulate excited state charge transfer

was achieved in T4PE4NDI (Figure 3-9). More specifically, charge transfer was observed

when the excitation was tuned to 370 nm (PE4 absorption), but was not observed when

it was tuned to 420 or 440 nm (T4 absorption). This selective excitation was not

observed in either T4NDI or T4PE2NDI due to the insufficient bridge length of the PE

segment. Figure 3 shows the ultrafast TA spectra of T4PE4NDI when excited at 370, 420

and 440 nm. The spectra in red (370 nm) clearly show the formation of the NDI anion

indicated by the band at 480 nm, whereas, the NDI anion does not form when excitation

105

was performed at 420 or 440 nm. The IR region also clearly reveals the formation of a

T4 cation, as evidenced by the broad band at 1350 nm, when T4PE4NDI is excited at

370 nm, although the absorption looks red shifted compared to T4NDI. This red shift is

due to the cation delocalizing over the PE4 segment. The absorption band that resides

with a maximum around 680 nm is the triplet state that formed by the competing PE4

T4 energy transfer process.

Charge recombination was significantly slowed down as the PE unit increased in

length. When probing for kinetics at 1200 nm, it was found that charge recombination

occurred in 8 ps for T4NDI, 350 ps for T4PE2NDI and 1038 ps for T4PE4NDI

Figure 3-10. Transient absorption kinetics of T4PE4NDI at 900 nm (red) and 1200 nm (blue) when excited at 420 nm (top) and 370 nm (bottom)

Selective excitation was also confirmed in T4PE4NDI because the kinetics for

T4NDI and T4PE2NDI were very similar at both 370 and 420 nm whereas the kinetic

traces of T4PE4NDI were significantly different. As shown in Figure 3-10, the kinetics

monitored at 900 nm represents the decay of the singlet state for T4PE4NDI when

106

excited at 370 and 420 nm, with a lifetime between 450-500 ps. The lifetime is close to

the time recorded when the molecule is excited at 420 nm when probing at 1200 nm

(Figure 3-10-top). It implies there is not obvious charge transfer, while the life time with

~511 ps comes from the decay of the singlet stats. However, when T4PE4NDI is excited

at 370 nm a new kinetic component with a lifetime of 1038 ps occurs (Figure 3-10-

bottom), which is assigned as charge recombination. Notably, under excitation of 420

nm, an ultrafast process with lifetime ~14 ps is observed, which is much longer than the

ultrafast process when exciting at 370 nm, featuring the lifetime of ~ 3 ps. We think the

~3 ps corresponds the charge separation process (excite at 370 nm) while the ~ 14 ps

is assigned as the possible exciton migration process along the conjugated oligomer.

Table 3-4. Summary of electrochemistry properties

E1/2ox/Va (E1/2ox/V)b E1/2red/Va (E1/2red/V)b ∆Eexc ∆G0

CSd ∆G0

CRe

PE4TIPS 1.06 0.97 -- N.A 3.12 -- --

T4TIPS 0.59, 0.87 (0.50, 0.79) -- N.A 2.59 -- --

T4PE4NDI 0.48,0.75,1.24 (0.45, 0.67,1.05) -1.06, -1.54 (-1.05, -1.52)

2.47 .99 1.48

T4PE2TIPS 0.53,0.79 (0.46, 0.72) -- N.A 2.47 -- --

T4PE4TIPS 0.51, 0.75, 1.23 (0.46,0.71,1.21) -- N.A 2.47 -- --

T4NDI 0.53 (0.44, 0.60, 0.74,

0.87) -1.04, -1.49

(-1.05, -1.48)

2.59 1.18 1.41

T4PE2NDI 0.96 (0.45, 0.70) -1.05, -1.51 (-1.05, -1.51)

2.47 1.02 1.45

aObtained from cyclic voltammetry measurements (CV). bObtained from differential pulse voltammetry measurements (DPV). eExcited states energy, estimated from the first emission peak, T4NDI and T4PE2NDI were estimated from corresponding TIPS protected oligomers. dCalculated from Equation 3-5. eCalculated from Equation 3-6.173 (E1/2ox and E1/2red were obtained from CV measurements).

Energetics for Selective Charge Transfer States

In order to under the selective charge transfer properties in T4PE4NDI. The

energetics for CS and CR processes of all the oligomers were calculated, based on data

from electrochemistry studies and Whener method (Equations 3-5 & 3-6), with the

energy levels were summarized in Table 3-4

107

ΔGC S= Eox - Ered - Eex(D) - ΔECoul (3-5)

ΔGCR = Eex(D) - ΔGCS (3-6)

Where Eox is the first one-electron oxidation potential of the donor, Ered is the first one-

electron reduction potential of the acceptor, Eex(D) is the energy of the lowest singlet

excited state of the donor, and ΔECoul is the correction term for the Coulombic interaction

energy within the ion-pair state at a specific distance. The Coulombic term is quantified

using the equation e2/4πɛ0ɛr, where r is the center-to-center distance of the ions, e is

the charge of an electron, ɛ0 is the vacuum permittivity constant and ɛ is the dielectric

constant of the solvent (CH2Cl2 = 8.93). Using the data gathered from CV in DCM,

steady-state emission and molecular modeling, the charge transfer state energies were

calculated. The center-to-center distances of the ions were calculated based on

molecular modeling as below (Figure 3-11):

Figure 3-11. Molecular modeling. (A) T4NDI, (B)T4PE2NDI, (C)T4PE4NDI.

A)

C)

B)

108

Figure 3-12. Energy level diagram for T4PE4NDI.

The energetics of T4PE4NDI was shown in Figure 3-12, along with the

mechanistic route of the excited state dynamics. The energy of the (T4)*PE4 singlet state

is 2.47 eV acquired from the first emission maximum. There are multiple paths the

excited state of T4PE4NDI can travel once excited. First, it can return to the ground state

via fluorescence. Second, it will intersystem cross to the triplet state and then

nonradiatively decay to the ground state. The triplet state energy is estimated to be

smaller than T4’s triplet energy (~1.8 eV) calculated by de Melo and coworkers.174

Finally, depending on the excitation wavelength, the compound can form a charge

transfer complex that nonradiatively decays to the ground state via charge

recombination. The higher energy excitation (370 nm) promotes the PE4 segment of the

diblock to its singlet state, which relaxes to a charge transfer state. Then the

T4(PE4)+•NDI-• state relaxes to (T4)+•PE4NDI-• through delocalization of the cation over

the highly electron rich T4 segment (Equation 3-1). When the T4 segment is selectively

excited using 420 nm, the energy goes directly to the triplet state and returns to the

ground state with minimal to no charge transfer presence (Equation 3-2).

109

Summary

In conclusion, a series of diblock oligomers were synthesized and probed for

wavelength selective excitation for the purpose of controlling photoinduced charge

transfer. It was found that a bridge length of four PE units was necessary to create a

molecule with such desired properties. Although selective excitation was possible, it

was found that the efficiency of charge transfer significantly diminished with increasing

PE length due to competition with fluorescence and intersystem crossing. Future plans

include creating diblock oligomers that have wavelength selective excitation with highly

efficient charge transfer properties, and applications in making molecular-scale

electronics.



1H and 13C NMR spectra were measured on a Mercury 300, a Gemini 300, or an

Inova 500. Chemical shifts were referenced to the residual solvent peaks. 1H NMR data

recorded with residual internal CD2Cl2 (δ 5.32) and CDCl3 (δ 7.26). 13C NMR data

recorded with references (CD2Cl2 (δ 53.84) and CDCl3 (δ 77.16). High-resolution mass

spectrometry was collected with either an Agilent 6200 ESI-TOF or an AB Sciex 5800

MALDI TOF/TOF in the Chemistry Department at the University of Florida.

Steady-state absorption spectra were recorded on a Shimadzu UV-1800 dual

beam spectrophotometer. Corrected steady-state emission measurements were

performed on a Photon Technology International (PTI) spectrophotometer.

Fluorescence lifetimes were collected in anhydrous chloroform (HPLC grade) on a

Picoquant FluoTime 100 time-correlated single photon counting (TCSPC) instrument

and analyzed with FluoFit software. Fluorescence quantum yields were reported relative

110

to known standards and estimated to have 10% error. Singlet oxygen quantum yield

measurements were conducted in deuterated chloroformunder 10 mins of purging with

oxygen, and reported relative to terthiophene (∆=84%),159 and estimated to have15%

error.

Nanosecond transient absorption spectroscopy measurements were performed

on an in-house apparatus that is described in detail elsewhere.160 The third harmonic of

a Continuum Surelite series Nd:YAG laser (λ= 355 nm, 10 ns FWHM, 10 mJ per pulse)

was used as the excitation source. Probe light was produced by a xenon flash lamp and

the transient absorption signal was detected with a gated-intensified CCD mounted on a

0.18 M spectrograph (Princeton PiMax/Acton Pro 180). The optical density of the

solutions was adjusted to ~0.7 at the excitation wavelength. Samples were measured in

a cell that holds a total volume of 10 ml and the content was continuously recirculated

through the pump–probe region of the cell. Samples were prepared in solvents

(tetrahydrofuran or mixture solvents of 67% tetrahydrofuran and 33% acetonitrile) and

degassed by bubbling argon for 45 min before the acquisition. The transient absorption

(TA) spectrum was collected from 350 nm to 850 nm with a 20 ns initial camera delay

and with different subsequent delay time increments depending on the triplet lifetime of

the molecule. Fifty averages were obtained at each delay time.

Ultrafast pump–probe experiments were performed with femtosecond (fs)

transient absorption spectroscopy with broadband capabilities. Detailed information of

the experimental setup can be found elsewhere.161 An Ultrafast Systems Helios

femtosecond transient absorption spectrometer equipped with UV-visible and near-

infrared detectors was used to measure the samples in this study. The white light

111

continuum probe pulse was generated in a thick sapphire plate (800–1300 nm) and in a

CaF2 crystal (350–700 nm spectral range) using a few mJ pulse energy of the

fundamental output of a Ti:sapphire fs regenerative amplifier operating at 800 nm with

35 fs pulses and a repetition rate of 1 kHz. The pump pulses at 355 nm were created

from fs pulses generated in an optical parametric amplifier (Newport Spectra-Physics).

The sample solution was constantly stirred to avoid photo degradation in scanned

volume. The pump and probe beams were overlapped both spatially and temporally on

the sample solution, and the transmitted probe light from the samples was collected on

the broad-band UV-visible-near-IR detectors to record the time resolved excitation-

induced difference spectra.

The cyclic voltammetry (CV) and differential pulse voltammetry (DPV)

experiments were performed on a Bio Analytical Systems CHI750 electrochemical

analyzer at a sweep rate of100 mV/s respectively, by using a platinum button as a

working electrode, a platinum wire as a counter electrode, and a silver wire as a

pseudo-reference electrode. Solutions of samples were prepared in dry

dichloromethane with 0.1 M tetrabutylammonium hexafuorophosphate (TBAPF6) as a

supporting electrolyte. The electrochemical potentials were internally calibrated against

the standard ferrocene/ferrocenium redox couple (Fc+/Fc). The highest occupied

molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels

for each complex were reported with respect to the potential of a Fc+/Fc redox couple

(5.1 eV vs. vacuum).162

Materials

Unless specified, all compounds and solvents were purchased from commercial

sources (Aldrich, Acros, Strem Chemicals, et al) and used without further purification.

112

For all palladium-catalyzed reactions, the solvents were carefully degassed with argon

for 30 min.Tetrakis(triphenyl phosphine) palladium (Pd(PPh3)4) was purchased from

Strem Chemical, triisopropylsilyl acetylene and trimethylsilylacetylene was purchased

from TCI. Copper (I) iodide (CuI), diisopropylamine ((i-Pr)2NH), 1-Ethynyl-4-

methylbenzene, Tetra-n-butylammonium fluoride in THF (1M), N-Bromosuccinimide

(NBS), Na2CO3, Na2SO4, methanol, tetrahydrofuran (THF), dichloromethane (DCM),

dimethylformamide (DMF) and all other chemicals were purchased from either Sigma-

Aldrich or Fisher Chemicals. All reagents were used without further purification unless

specified.


Compounds tetrathiophene, 4165, 6165, 12164 were synthesized according to

previous references. Compound PE4-TIPS was prepared as described in Chapter 2.

Scheme 3-1. Synthesis procedure

Scheme 3-1. Synthesis of T4NDI

Compound 1 (T4-Br)

Tetrathiophene (3 g, 6.0 mmol) was dissolved in 200 mL of DMF and cooled

down to 0 °C. Then NBS was slowly added in portions (1.07 g, 6.0 mmol) and left to

react at room temperature for 12 hs. The organic solvent was removed under reduced

113

pressure. Water (100 ml) was added to the crude product and was extracted with

CH2Cl2 (100 ml) twice. The organic phase was dried using brine and Na2SO4. Solvents

were removed under reduced pressure. The resulting crude product was purified by

silica gel column chromatography using hexane as eluent to afford a yellow oil (1.39 g,

40%). 1H NMR (500 MHz, CDCl3): δ 7.19 (d, 1H), 7.13 (d, 1H), 7.11 (d, 1H), 7.03 (d,

1H), 6.97 (d, 1H), 6.95 (d, 1H), 6.91 (s, 1H), 2.79 (t, 2H), 2.72 (t, 2H), 1.64 (m, 4H), 1.36

(m, 12H), .90 (t, 6H); 13C NMR (125 MHz, CDCl3): δ 140.56, 140.05, 137.49, 136.58,

135.73, 133.92, 132.82, 131.94, 130.36, 130.22, 127.07, 126.64, 124.15, 124.04,

123.90, 110.70, 31.81, 31.76, 30.77, 30.65, 29.44, 29.36, 29.25, 22.77, 22.74, 14.25,

14.24 ppm. MALDI-TOF MS (m/z) [M]+ Calcd for

Compound 2 (T4-TIPS)

Compound 1 (0.58 g, 1.0 mmol) and triisopropylsilyl acetylene (0.91 g,5.0 mmol)

was dissolved in mixed solvent of THF (30 ml) and diisopropylamine (20 ml) and

degassed with argon for 30 min. Pd(PPh3)4(57.8 mg, 0.05 mmol), CuI (19.5 mg, 0.10

mmol) were added under argon protection. The mixtures were stirred at 70 °C for

overnight, under argon protection. After the reaction, the mixture was filtered and the

solution was evaporated under vacuum. The mixture was purified by column with

hexane as eluent to yield compound 2 (0.53 g, 92%).1H NMR (500 MHz, CD2Cl2): δ

7.20 (d, 1H), 7.15 (dd, 2H), 7.10 (s, 1H), 7.06 (dd, 2H), 6.97 (d, 1H), 2.81(t, 2H), 2.76 (t,

2H), 1.64-1.69 (m, 4H), 1.33-1.43 (m, 12H), 1.11-1.18 (m, 21H), 0.90-0.94(m, 6H). 13C

NMR (125MHz, CD2Cl2): δ13C NMR (75 MHz, CD2Cl2) δ 140.51, 140.09, 137.65,

136.82, 136.09, 135.94, 134.74, 132.39, 130.64, 130.57, 127.34, 126.92, 124.50,

124.35, 124.32, 121.63, 99.76, 97.07, 32.15, 32.12, 31.10, 30.87, 29.75, 29.73, 29.69,

114

29.64, 23.12, 23.10, 18.92, 14.37, 11.82 ppm. MALDI-MS: m/z=678.2845 [M*]+ (calcd:

678.2872).

Compound 3

Compound 2 (0.45 g, 0.66 mmol) was dissolved in THF (20 ml) and degassed

with argon for 30 min. Tetrabutylammonium fluoride solution (3.3 ml, 1 M in THF) was

added under argon protection. The mixtures were stirred at 40 °C overnight, under


in 20 ml dichloromethane, followed by washing with water (20 ml) for 3 times. The



with hexaneas eluent to yield compound 3 (300mg, 87%).1H NMR (500 MHz, CDCl3): δ

7.19 (d, 1H), 7.12-7.15 (m, 3H), 7.04 (dd, 2H), 6.96 (d, 1H), 3.39 (s, 1H), 2.79(t, 2H),

2.74 (t, 2H), 1.62-1.69 (m, 4H), 1.33-1.42 (m, 12H), 0.90-0.92(m, 6H). 13C NMR (125

MHz, CDCl3): δ14.49,22.99, 29.52, 29.61, 29.69, 30.78, 31.02, 32.02, 32.06, 82.43,

120.10, 124.22, 124.31, 124.46, 126.90, 127.44, 130.48, 130.61, 132.90, 134.42,

136.04, 136.41, 136.81, 137.86, 139.78, 140.32 ppm.

Compound 5 (T4-NDI)



with argon for 30 min. Pd(PPh3)4 (11.6 mg, 0.01 mmol), CuI (3.8 mg, 0.02 mmol) were

added under argon protection. The mixtures were stirred at r.t. for overnight, under

argon protection. After the reaction, the mixture was filtered and the solution was

evaporated under vacuum. The mixture was purified by column with mixture solvents of

115

hexane (50%) and dichloromethane (50%) as eluent to yield compound 5 (120 mg,

65%).1H NMR (500 MHz, CD2Cl2): δ 8.71 (s, 4H), 7.69 (d, 2H), 7.34 (d, 2H), 7.17-7.20

(m, 2H), 7.11(dd, 2H), 7.04(dd, 2H), 6.96(d, 1H), 4.15(t, 2H), 2.78(m, 4H), 1.66-1.73 (m,

6H) 1.35-1.41 (m, 22H), 0.91(m, 9H). 13C NMR (125 MHz, CD2Cl2): δ 163.20, 162.96,

140.44, 140.26, 137.60, 136.59, 136.06, 135.96, 135.28, 134.57, 133.10, 132.52,

131.48, 131.08, 130.63, 130.47, 129.36, 127.41, 127.32, 127.27, 127.12, 126.84,

126.77, 124.47, 124.37, 124.30, 124.26, 124.08, 120.77, 93.62, 84.24, 41.27, 32.25,

32.08, 30.99, 30.73, 29.75, 29.71, 29.66, 29.63, 29.60, 28.41, 27.54, 23.09, 23.06,

14.32, 14.31 ppm. MALDI-MS: m/z=974.3241 [M*]+ (calcd: 974.3274).

Scheme 3-2. Synthesis of T4PE2NDI and T4PE4NDI.

116

Compound 7 (T4-PE2-TIPS)

Compound 3 (300 mg, 0.57 mmol) and Compound 6(430 mg, 0.66 mmol) were


with argon for 30 min. Pd(PPh3)4 (33.0 mg, 0.029 mmol), CuI (10.9 mg, 0.057 mmol)

were added under argon protection. The mixtures were stirred at r.t. for overnight, under


evaporated under vacuum. The mixture was purified by column with hexane as eluent to

yield compound 7 (570 mg, 95%).1H NMR (500 MHz, CDCl3): δ 7.47 (m, 4H), 7.35(d,

2H), 7.18 (d, 1H), 7.15 (t, 2H), 7.11 (s, 1H), 7.06 (d, 1H), 7.04 (d, 1H), 6.96 (d, 1H),

2.77-2.82(m, 8H), 1.67-1.71 (m, 8H), 1.33-1.43 (m, 24H), 1.16(s, 21H), 0.89-0.93 (m,

12H). 13C NMR (125 MHz, CDCl3): δ142.47, 142.30, 140.06, 139.87, 137.46, 136.65,

135.76, 134.84, 134.51, 132.57, 132.48, 132.14, 132.12, 131.34, 130.38, 130.23,

126.97, 126.67, 124.17, 124.05, 124.03, 123.46, 122.57, 121.32, 106.83, 93.95, 93.47,

92.98, 90.48, 87.51, 34.31, 31.92, 31.90, 31.82, 31.81, 30.81, 30.78, 30.56, 29.49,

29.45, 29.41, 29.40, 29.37, 29.34, 22.84, 22.78, 22.76, 18.82, 18.77, 14.32, 14.27,

14.25, 11.48 ppm. MALDI-MS: m/z=1046.5350 [M*]+ (calcd: 1046.5375).

Compound 8

Compound 7 (400 mg, 0.38 mmol) was dissolved in THF (20 ml) and degassed

with argon for 30 min. Tetrabutylammonium fluoride solution (1.9 ml, 1M in THF) was

added under argon protection. The mixtures were stirred at 40 °C overnight, under


in 20ml dichloromethane, followed by washing with water (20 ml) for 3 times. The


117


with mixture solvents of hexane (90%) and dichloromethane (10%)as eluent to yield

compound 8 (302 mg, 89%). 1H NMR (500 MHz, CD2Cl2): δ 7.49 (s, 4H), 7.35(d, 2H),

7.21 (d, 1H), 7.17 (t, 2H), 7.14 (s, 1H), 7.10 (d, 1H), 7.05 (d, 1H), 6.97 (d, 1H), 3.25 (s,

1H), 2.77-2.82(m, 8H), 1.64-1.70 (m, 8H), 1.33-1.43 (m, 24H), 0.87-0.92(m, 12H). 13C

NMR (125 MHz, CD2Cl2): δ 14.29, 14.36, 23.05, 23.06, 23.12, 29.58, 29.62, 29.63,

29.66, 29.68, 29.73, 30.80, 31.04, 31.08, 31.11, 32.07, 32.18, 34.48, 34.51, 79.30,

83.47, 87.75, 90.79, 93.59, 93.85, 121.37, 122.29, 122.74, 122.90, 124.33, 124.35,

124.51, 126.94, 127.34, 134.49, 130.64, 131.74, 132.35, 132.51, 132.77, 132.85,

134.72, 135.36, 136.07, 136.77, 137.66, 140.42, 140.55, 142.68, 142.94 ppm.

Compound 9 (T4-PE2-NDI)



with argon for 30 min. Pd(PPh3)4 (6.9 mg, 0.006 mmol), CuI (2.1 mg, 0.01 mmol) were





88%).1H NMR (500 MHz, CD2Cl2): δ 8.77 (s, 4H), 7.73 (d, 2H), 7.55(dd, 4H), 7.34-7.38

(m, 4H), 7.21 (d, 2H), 7.17(t, 2H), 7.13(d, 1H), 7.04(d, 1H), 6.96(d, 1H), 4.18(t, 2H),

2.78-2.84(m, 8H), 1.63-1.76 (m, 10H) 1.32-1.41 (m, 34H), 0.89-0.91(m, 15H). 13C NMR

(125 MHz, CD2Cl2): δ 163.33, 163.09, 142.93, 142.68, 140.54, 140.42, 137.63, 136.74,

136.06, 135.41, 135.36, 134.71, 132.88, 132.83, 132.76, 132.34, 132.10, 131.85,

118

131.58, 131.17, 130.64, 130.48, 129.35, 127.57, 127.46, 127.31, 127.26, 126.94,

126.92, 124.50, 124.34, 124.29, 123.98, 123.17, 122.84, 122.81, 121.36, 94.13, 93.62,

90.84, 90.72, 90.57, 87.73, 54.27, 54.06, 53.84, 53.62, 53.41, 41.31, 34.51, 34.50,

32.24, 32.18, 32.07, 31.11, 31.08, 31.03, 30.78, 29.73, 29.72, 29.67, 29.64, 29.61,

29.57, 28.42, 27.53, 23.12, 23.07, 23.04, 14.36, 14.31, 14.28.ppm. MALDI-MS:

m/z=1342.5720[M*]+ (calcd: 1342.5780).

Compound 10 (T4-PE4-TIPS)

Compound 6 (220 mg, 0.34 mmol) and Compound 8 (200 mg, 0.22 mmol) were


with argon for 30 min. Pd(PPh3)4 (12.7mg, 0.011 mmol), CuI (4.2 mg, 0.022 mmol) were





59%).1H NMR (500 MHz, CD2Cl2): δ 7.54 (s, 4H), 7.48(s, 4H), 7.37-7.39(m, 4H), 7.20

(d, 1H), 7.17 (t, 2H), 7.14 (s, 1H), 7.10 (d, 1H), 7.06 (d, 1H), 6.98 (d, 1H), 2.79-2.82(m,

12H), 1.67-1.72 (m, 12H), 1.33-1.43 (m, 36H), 1.16(s, 21H), 0.90-0.93(m, 18H). 13C

NMR (125 MHz, CD2Cl2): δ142.93, 142.91, 142.69, 140.53, 140.41, 137.67, 136.80,

136.08, 135.35, 134.75, 132.88, 132.77, 132.36, 131.86, 131.68, 130.65, 130.53,

127.32, 126.94, 124.51, 124.35, 123.77, 123.73, 123.70, 122.96, 122.87, 121.42,

107.02, 94.24, 94.19, 94.11, 93.68, 93.39, 90.83, 90.79, 90.68, 87.80, 34.56, 34.54,

32.23, 32.21, 32.11, 31.15, 31.13, 31.07, 30.82, 29.77, 29.72, 29.70, 29.68, 29.65,

119

29.62, 23.16, 23.11, 23.10, 23.08, 18.88, 14.40, 14.35, 14.33, 11.78 ppm. MALDI-MS:

m/z=1414.7881 [M*]+ (calcd: 1414.7880).

Compound 11 (T4-PE4-NDI)

Compound 10 (120 mg, 0.085 mmol) was dissolved in THF (20 ml) and

degassed with argon for 30 min. Tetrabutylammonium fluoride solution (0.5 ml, 1 M in

THF) was added under argon protection. The mixtures were stirred at 40 °C overnight,

under argon protection. After the reaction, the organic solvents were evaporated and

dissolved in 20 ml dichloromethane, followed by washing with water (20 ml) for 3 times.


the solution was evaporated under vacuum. The crude products (95 mg) were obtained

and used without further purification. Compound 4 (66 mg, 0.11 mmol) and crude

product (95 mg) were dissolved in mixed solvent of THF (15 ml) and diisopropylamine

(5 ml) and degassed with argon for 30 min. Pd(PPh3)4 (4.6 mg, 0.004 mmol), CuI (1.5

mg, 0.008 mmol) were added under argon protection. The mixtures were stirred at r.t.

for overnight, under argon protection. After the reaction, the mixture was filtered and the

solution was evaporated under vacuum. The mixture was purified by column with

mixture solvents of hexane (30%) and dichloromethane (70%) as eluent to yield

compound 12 (100 mg, 77%).1H NMR (500 MHz, CD2Cl2): δ 8.76 (s, 4H), 7.72 (d, 2H),

7.55-7.58(m, 8H), 7.34-7.38 (m, 6H), 7.21 (d, 2H), 7.16(t, 2H), 7.13(s, 1H), 7.08(d, 1H),

7.04(d, 1H), 6.97(d, 1H), 4.17(t, 2H), 2.78-2.84(m, 12H), 1.63-1.76 (m, 14H), 1.32-1.41

(m, 46H), 0.89-0.91(m, 21H). 13C NMR (125 MHz, CD2Cl2): δ163.30, 163.06, 142.92,

142.91, 142.67, 140.52, 140.39, 137.62, 136.73, 136.06, 135.38, 135.35, 134.72,

132.86, 132.84, 132.78, 132.77, 132.75, 132.73, 132.33, 132.10, 131.93, 131.84,

120

131.56, 131.15, 131.03, 130.64, 130.49, 129.35, 127.55, 127.43, 127.29, 127.24,

126.91, 126.90, 124.49, 124.34, 124.27, 123.98, 123.68, 123.18, 122.96, 122.91,

122.84, 122.82, 121.36, 94.23, 94.19, 94.11, 93.65, 90.82, 90.79, 90.76, 90.73, 90.57,

87.76, 54.27, 54.06, 53.84, 53.62, 53.41, 41.31, 34.52, 32.25, 32.20, 32.08, 31.11,

31.08, 31.03, 30.79, 29.75, 29.72, 29.69, 29.67, 29.66, 29.62, 29.59, 28.42, 27.54,

23.13, 23.09, 23.05, 14.37, 14.33, 14.32, 14.29 ppm. MALDI-MS: m/z=1710.8285[M*]+

(calcd: 1710.8282)

121

CHAPTER 4 THE SYNTHESIS OF RUTHENIUM(II) CHROMOPHORES GRAFTED

POLYSTYRENES AND APPLICATIONS IN DYE-SENSITIZED SOLAR CELLS

Background

The study of polymer-based chromophores has been driven by their potential use

in solar energy conversion and photovoltaic systems.52, 167, 175, 176 For these uses,

extensive fundamental studies of photoinduced electron and energy transfer within

polymer-based chromophores assemblies have been done for the use of light-

harvesting antennae.177-179 Interpretation of these studies is significant for designing

novel assemblies for further uses in dye-sensitized solar cells, artificial photosynthesis,

and solar fuel cells.52, 180 Of special interest are polystyrene-based polymers that

contain ruthenium(II) polypyridyl (RuIIL3) chromophores as light harvesting materials,

due to their efficient electron and energy transfer processes, excellent photostability and

thermal stability, as well as long excited state lifetimes, that give rise to efficient long-

range energy and electron transfer processes.181

Previously, Meyer and co-workers reported synthetic methods that have been

used to attach polystyrene backbones to transition metal complexes by ether or amide

linkages using coupling chemistry50, 182, 183. Very recently, we reported the synthesis and

characterization of novel poly-chromophore assemblies by using “Click chemistry” to

attach Ru(II)-polypyridyl (RuIIL3) units to polystyrene backbones, prepared by reversible

addition−fragmentation chain transfer (RAFT) polymerization.48 The resulting polymers

exhibited comparatively low MLCT emission yields and lifetimes, especially in the

shortest polymers (i.e. Mn ∼ 3100 and 8600). It was concluded that the excited states of

the polymer-bound chromophores were quenched by the thiol (–SH) end-group in the

polymer that was introduced from the RAFT chain transfer agent. Thus, to avoid the

122

quenching effect of the –SH end groups arising from the RAFT initiator, there is a need

to develop efficient synthetic methods to assemble Ru(II)-polypyridyl polystyrenes that

are free of the thiol end group functionality.

Towards this goal, we developed a nitroxide-mediated controlled radical

polymerization (NMP) method to assemble polystyrene backbone polymers that are

functionalized with Ru(II) polypyridyl (RuIIL3) pendant chromophores.184, 185 This

approach was used to prepare a series of polymers with defined molecular repeating

units, with degree of polymerization (DP) ~ 35 -170. At the same time, the

corresponding mono Ru(II) polypyridyl chromophore was also prepared for control study

(Figure 4-1). Systematic photophysics studies were conducted based on different

polymer chain lengths.

Based on the nitroxide-mediated controlled radical polymerization (NMP)

method, we also prepared polystyrene-based assemblies with carboxylic acid groups

functionalized Ru(II) polypyridyl (RuIIL3) pendant chromophores (PS-Ru-A) (Figure 4-9).

PS-Ru-A was successfully anchored to mesoporous TiO2 films to implement DSSC

solar cells. Illumination of the films in a conventional dye-sensitized solar cell

configuration gave rise to moderate photocurrent.


Structure Design and Synthesis

The synthesis scheme towards the Ru(II) polypyridyl chromophore grafted

polystyrene is shown in Figure 4-2. The synthesis scheme is based on a modified

approach used in our earlier investigation to construct the functionalized polystyrene via

RAFT method.10 In detail, the polystyrene backbone was prepared by the NMP method

by reacting 4-vinylbenzyl chloride (VBC) as the monomer in the presence of N-tert-

123

butyl-O-[1-[4-(chloromethyl)phenyl]ethyl]- N-(2-methyl-1-phenylpropyl)hydroxylamine as

the initiator under heating of 120 °C (Figure 4-2).186 Nitroxide radicals employed as

effective mediators are highly stable and act as persistent radicals resulting in well-

controlled radical polymerization. The polymerization afforded chloro-methyl-substituted

polystyrenes with nitroxide end groups (PVBC-n-NMP). By controlling the

polymerization reaction times and ratios between monomers and initiators, three PVBC-

n-NMP were obtained with number average molecular weights, Mn ∼ 5500, 12,000, and

24,000 g mol-1, corresponding to DP of 35, 80, and 170 with polydispersity index (PDI)

of 1.30, 1.50 and 1.45, respectively (Figure 4-3).

Figure 4-1. Structure of the model complex and polymer. (A) Model-PF6 and Model-Cl) and (B) Ruthenium derivatized polystyrene (Poly-n-PF6 and Poly-n-Cl, n=35,

80, 170).

Subsequent to the polymerization, the nitroxide end group on PVBC-n-NMP was

eliminated by treatment with m-chloroperbenzoic acid (m-CBPA) in toluene solution,

resulting in the end-group free chloromethyl-substituted polystyrene (PVBC-n) (Figure

4-2). The conversion of PVBC-n-NMP to PVBC-n was monitored by 1H-NMR. After

treatment with mCPBA, the weak NMR peaks between δ ∼ 0.5 and 0.8 ppm (assigned

A) B)

124

from the nitroxide end group in PVBC-n-NMP) disappeared (Figure 4-4), while the

polymer was unaffected by mCPBA treatment. The azide substitution reaction from

PVBC-n to PVBA-n was conducted by treatment of the chloromethyl polystyrene with

sodium azide.

Figure 4-2. Synthesis scheme for polypyridylruthenium derivatized polystyrene.

Analysis of PVBA-n by 1H-NMR and IR spectroscopy confirmed that the reaction

gave rise to essentially quantitative conversion of the chloromethyl to azidomethyl

substituents (Figure 4-4 and Figure 4-5). In particular, the observed resonance at 4.24

ppm for the benzylic CH2N3 group in the 1H-NMR, and a strong band at ∼2090 cm−1 in

125

the infrared spectrum for the azido stretching, appeared in PVBA-n and the peaks

correspond well with the characteristic peaks reported in our previous study.48

A) B) C) Figure 4-3. GPC characterization. GPC elution curve of (A) PVBC-n-NMP, (B) PVBC-n

and (C) PVBA-n in THF

Gel permeation chromatography (GPC) on samples of PVBC-n-NMP, PVBC-n,

and PVBA-n showed that there was no significant change in Mn or polydispersity index

during the oxidation and azide substitution reactions (Figure 4-3). Attachment of the

Ru(II) chromophores onto the polymer backbone was accomplished by reacting the

ethynyl substituted complex (Ru(II), Figure 4-2) with the azido-methyl substituted

polystyrene backbone using the azide–alkyne “click” reaction, by analogy with our

previous work on RAFT polystyrenes.

Similar to the metal complex monomers, the solubility properties of the Ru(II)

polypyridyl chromophore functionalized polystyrenes (which are polycations) are

controlled by the nature of the counter anions. In particular, with chloride as the counter

anion, polymer PSn-Ru(Cl) was soluble in water and methanol (MeOH). By reaction of

polymer PSn-Ru(Cl) with ammonium hexafluorophosphate in water, the polymer was

converted to the corresponding hexafluorophosphate salt PSn-Ru(PF6), which is soluble

in acetone and acetonitrile. After the reaction, the IR spectra indicated the attachment of

10 11 12 13 14 15 16 17

PVBC-35

PVBC-80

PVBC-170

Retention time (min.)

10 11 12 13 14 15 16 17

PVBC-35

PVBC-80

PVBC-170


10 11 12 13 14 15 16 17

PVBA-35

PVBA-170

PVBA-80


126

the Ru(II) polypyridyl chromophores onto the polymer backbones is quantitatively

complete, because of the disappearance of peak for azido groups at 2050 cm-1 (Figure

4-5). Also, from the 1H-NMR spectra of PSn-Ru(PF6), the aromatic region between δ ∼7

and 8 ppm were assigned to the presence of the bipyridyl ligands of RuIIL3 in the

polymer, and the broad and weak resonances at ∼4.5 ppm and ∼5.3 ppm are attributed

to the methylene groups next to the triazole rings. We were unable to conduct GPC

analysis on PSn-Ru, due to poor solubility in common eluent solvents for GPC

measurement. However, our previous work on related “click” modified PVBA

polychromophores shows that the resulting functional polymers have the expected Mn

value based on the PVBA structure, and nearly the same PDI as the PVBA precursor187

So we anticipate that the DP and PDI values from the PSn-Ru samples are nearly

comparable to the PVBA precursors.

Figure 4-4. 1H NMR spectra of (1) PVBC-170-NMP, (2) PVBC-170 and (3) PVBA-170 in

CDCl3, (4) Poly-170-Cl in D2O, and (5) Poly-170-PF6 in CD3CN

127

Figure 4-5. ATR-IR spectra of PVBA-170 and Poly-170-Cl.

Photophysical Properties

The UV-visible absorption spectra were measured for Ru(PF6) and PSn-Ru(PF6) in

acetonitrile and Ru(Cl) and PSn-Ru(Cl) (n = 35, 80 and 170) in methanol solutions at

room temperature, as presented in Figure 4-6. All solutions displayed similar spectra

with characteristics of the Ru(II) polypyridyl chromophores. The absorption spectra in

acetonitrile and methanol feature strong, ligand- based π→π* absorption bands with

maxima at λ ∼ 288 nm in the UV and the strong π(Ru) → π*(bpy) metal-to-ligand

charge transfer (MLCT) bands at λ ∼ 455 nm in the visible region.48 The extinction

coefficients of the model complexes and polymers are listed in Table 1. Steady-state

emission spectra were characterized in acetonitrile for Ru(PF6) and PSn-Ru(PF6) and

methanol solutions for Ru(Cl)and PSn-Ru(Cl) at room temperature under ~455 nm

excitation, as shown in Figure 4-6. The emission maxima of the model complex and

polymers exhibit a similarly broad and moderately intense luminescence at λmax ∼650 –

660 nm, arising from the 3MLCT excited state.

128

Figure 4-6. Absorption and emission spectra. (A) UV-visible absorption, (B) Emission spectra of Model-Cl, Poly-35-Cl, Poly-80-Cl, and Poly-170-Cl in MeOH and (C) UV-visible absorption of Model-PF6, Poly-35-PF6, Poly-80-PF6, Poly-170-PF6 and (D) Emission spectra in acetonitrile. Emission spectra acquired with excitation at λex = 455 nm.

The fact that the band shape and emission maxima from the polymers are

identical to that for the monomer complexes indicates that the polymer backbone does

not strongly interact with the pendant ruthenium complexes in the emissive excited

state, which is in agreement with the observation in the absorption spectra described

above. Emission quantum yields and excited-state lifetimes for the model complexes

and polymer samples are also listed in Table 4-1. These results reveal that the emission

A) B)

250 300 350 400 450 500 550 600 650

0

1x104

2x104

3x104

4x104

5x104

6x104

7x104

8x104

9x104

1x105

Wavelength (nm)

Mo

lar

Ab

so

rpti

viy

(M

-1cm

-1)

Model-Cl

Poly-35-Cl

Poly-80-Cl

Poly-170-Cl

500 550 600 650 700 750 800 850

0.0

0.2

0.4

0.6

0.8

1.0

Wavelength (nm)

No

rmali

zed

Em

issio

n

Model-Cl

Poly-35-Cl

Poly-80-Cl

Poly-170-Cl

250 300 350 400 450 500 550 600 650

0

1x104

2x104

3x104

4x104

5x104

6x104

7x104

8x104

9x104

1x105

Wavelength (nm)

Mo

lar

Ab

so

rpti

viy

(M

-1cm

-1)

Model-PF6

Poly-35-PF6

Poly-80-PF6

Poly-170-PF6

500 550 600 650 700 750 800 850

0.0

0.2

0.4

0.6

0.8

1.0

No

rmali

zed

Em

issio

n

Wavelength (nm)

Model-PF6

Poly-35-PF6

Poly-80-PF6

Poly-170-PF6

C) D)

129

quantum yields and lifetimes for Ru(PF6) and PSn-Ru(PF6) in acetonitrile are generally

larger than those for Ru(Cl) and PSn- Ru(Cl) in methanol; the difference is due to a

solvent effect on the non-radiative decay rate constant.188, 189 More interestingly, as

shown in Figure 4-7, the emission quantum yields and lifetimes for the polymers in both

acetonitrile and methanol systematically decrease with increasing molecular weight.

Dupray and co-workers reported an analogous correlation between emission efficiency

and the density of attached metal chromophores on a polymer backbone.50 In their

study, the emission quantum yields and lifetimes decreased as the loading of metal

chromophores on the polymer increased. By analogy, the behavior observed in our

system can be explained in emission quantum yields and lifetimes as a function of the

number of Ru(II) polypyridyl chromophores per polymer chain. All the photophysical

data were summarized in Table 4-1.

Figure 4-7. Plot of the lifetime and quantum yields versus degree of polymerization of

Poly-n-Cl in MeOH. Lifetime is represented by black square while quantum yield represented by blue circle.

0

200

400

600

800

1000

1200

1400

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Lif

e T

ime (

ns)

Poly-30a

Poly-35 Poly-80 Poly-170Model

Qu

an

tum

Yie

ld

130

Table 4-1. Photophysical properties of pomplexes and polymers

Absmax /nm (ε/104 M-1•cm-1) λem/nm τ /nsc ϕem

d

π-π* dπ-π*

Model-PF6a 287(8.80) 455(1.82) 657 τ =1.41 µs 0.116

Poly-35-PF6a 288 (7.66) 456 (1.64) 657

<τ> = 0.91 µs τ1=1.13 µs (0.72) τ2=0.35 µs (0.28)

0.065

Poly-80-PF6a 288 (8.34) 456 (1.78) 657

<τ> = 0.86 µs τ1=1.05 µs (0.72) τ2=0.36 µs (0.28)

0.048

Poly-170-PF6

a 288 (7.99) 457 (1.73) 655

<τ> = 0.78 µs τ1=1.02 µs (0.67) τ2=0.30 µs (0.33)

0.049

Model-Clb 287(5.73) 455(1.23) 657 τ =1.05 µs 0.084

Poly-35-Clb 289 (5.61) 457 (1.24) 658 <τ> = 0.61 µs

τ1=0.86 µs (0.37) τ2=0.33 µs (0.63)

0.044

Poly-80-Clb 289 (5.78) 457 (1.27) 658 <τ> = 0.55 µs

τ1=0.72 µs (0.50) τ2=0.22 µs (0.50)

0.030

Poly-170-Clb 289 (5.84) 457 (1.29) 656 <τ> = 0.52 µs

τ1=0.73 µs (0.57) τ2=0.23 µs (0.43)

0.035

Measured in aacetonitrile (ACN) solution and bMeOH solution. c<τ> is median lifetime calculated as <τ> =∑αiτi. dEmission

quantum yield (±10%) determined by [Ru(bpy)3]Cl2 as quantum yield standard in air saturated water, Φ= 0.0379.190

It is interesting to note that our previous work on PS-Ru poly-chromophores

prepared by RAFT revealed that the emission yields and lifetimes increased with

molecular weight.48 In the RAFT PS-Ru system, the quenching was attributed to

electron migration and quenched by the thiol (–SH) polymer end-groups. The longer the

polymer chain is, the less quenching caused by end groups. However, in our polymer

assemblies, site-to-site energy transfer occurs among the pendant Ru(II) polypyridyl

131

chromophores. This allows the exciton to migrate to trap sites within the

polychromophores (i.e., sites with close-packed pendants) where the rate of non-

radiative decay is enhanced. We hypothesize that the occurrence of these traps within a

given chain is low, but as the chain length increases the probability of a trap existing in

a given chain is increased.

Figure 4-8. Stern-Volmer plot (I0/I vs [AQS]) for emission quenching of Model-Cl, Poly-

35-Cl, Poly-80-Cl, and Poly-170-Cl with AQS by monitoring the emission

intensity at 673 nm. (ex = 455 nm)

To confirm the existence of exciton hopping along the polymer chains, we

conducted emission quenching studies of the Ru(Cl) and PSn-Ru(Cl) in degassed

aqueous solution. In previous work, we and other groups have demonstrated the

“amplified quenching” effect that occurs when the luminescence of an ionic

polychromohore (a polyelectrolyte) is quenched by an oppositely charged quencher,48,

191 This amplified quenching is attributed to arise in part due to diffusion of the exciton

along the polychromophore chain. In the study, Sodium 9,10-anthraquinone-2,6-

disulfonate (AQS) was used as the anionic quencher. Figure 4-8 illustrates the Stern–

132

Volmer plots and lists the quenching constant values (Ksv). Stern–Volmer constants

showed significant amplified quenching with Ksv> 106 M-1 in comparison to the

quenching of the Ru model complex (Ksv ∼ 2.1 × 104 M-1). This represents a ∼100-fold

amplification in quenching efficiency for the polymers compared to the model complex

(Figure 4-8). Besides, all the three polymers with different chain lengths have exhibited

similar quenching effects with comparable Ksv values. Based on that, we conclude that

the MLCT exciton diffuses among pendants that are tethered along the non-conjugated

polymer backbone.

DSSC Solar Cell Applications

Figure 4-9. Structure for Ru-A model compound and PS-Ru-A

Based on similar approach, we prepared carboxyl acids containing Ru(II)

polypyridyl chromophores grafted polystyrene polymers(PS-Ru-A) and corresponding

Ru(II) model complex (Ru-A), with structures shown in Figure 4-9, and the synthetic

scheme summarized in Figure 4-10. The precursory carboxyl ester containing

Ruthenium functionalized polymers (PS-Ru-E) feature a polystyrene backbone (Mn

N N

N

N Ru2+

N

N

ONH

HOOC

HOOC COOH

COOH

2PF6-

N N

N

N N

NRu2+

HNO

14

NN

N

OH

O

OHHO

HO

OO

O

2PF6-

133

~2100 g/mol) with approximately 14 pendant ruthenium units per polymer chain. The

carboxylic acid derivatized polymer, PS-Ru-A, was obtained by treatment with base

hydrolysis of PS-Ru-E. The carboxylic acid derivatized Ru(II) polypyridyl chromophores

positioned along the polymer chain afford multiple surface binding sites for adsorption

on mesoporous TiO2 films.

Figure 4-10. The synthesis of PS-Ru-A

The FTO/TiO2 substrate was prepared according to literatures.176 The average

size of TiO2 nanoparticles and thickness of the TiO2 layer were approximately 32 nm

and 12 µm as measured by SEM and TEM. The temporal dependence of adsorption of

134

PS-Ru-A to mesoporous TiO2 films from ACN and MeOH solutions was measured by

using UV-visible spectroscopy according to a reported procedure.192, 193 The absorption

changes of PS-Ru-A in ACN and MeOH were monitored at 465 nm and the absorptance

is plotted in Figure 4-11. The polymer coverage on the films increased with time, and

saturated to the films after 4h for ACN and 1h for MeOH. The surface coverage of PS-

Ru-A on the TiO2 films (mol·cm-2) was determined by the reported equation.194 At

saturation surface coverage, the maximum uptake of PS-Ru-A was calculated

approximately as 0.74 X10-8mol·cm-2 in ACN and 0.53 X10-8 mol·cm-2 in MeOH (based

on polymer repeating units) respectively (Figure 4-11). It is noted that the surface

coverage of PS-Ru-A is lower compared to Ru-A in ACN solution (~ 5.0 * 10-8 mol·cm-2)

about 10-fold.

Interestingly, it is seen that the adsorption rate and coverage at saturation is

different for these two solvents. In particular, the surface coverage of PS-Ru-A on the

TiO2 films from methanol solution increases more rapidly during the initial phase (~1 h)

compared to acetonitrile. However, the maximum surface coverage by the polymer in

MeOH is reached after approximately one hour (~0.5 x 10-8 mol·cm-2), while deposition

of the polymer from ACN occurs more slowly, reaching saturation after ~4 hours but

with approximately 40% higher surface coverage (~0.74 x 10-8 mol·cm-2). It is possibly

that the better solubility of polymer in ACN led to less aggregation of polymer in ACN.

Therefore, for a TiO2 binding density and mesoscopic porosity, the amount of polymer

adsorption to the films could be related to polymer aggregation via different solubility of

polymer in ACN and MeOH. In addition, a previous investigation reported a similar

observation, where an organic dye gave rise to higher surface coverage when

135

deposited from ACN compared to MeOH.195 In our previous studies, we reported

thatwhen polystyrene-based Ru(II) polypyridyl (RuIIL3) polymers are dissolved in MeOH,

they may exist in an aggregated state due to the lower dielectric constant of MeOH

compared to acetonitrile.184 This effect may prevent the diffusion of the polymer into

mesoporous TiO2 films leading to a reduced surface coverage on the TiO2.

Figure 4-11. Adsorption profiles of TiO2//PS-Ru-A films in acetonitrile and methanol measured for a period of 12 h (solid line represents the numerical regression fit).

Dye sensitized solar cells (DSSC) were implemented with the configuration

FTO//TiO2//PS-Ru-A as photoanode, where the polymer (PS-Ru-A) was deposited on

electrode (FTO//TiO2//PS-Ru-A) in mixtured solvent of ACN (33%) and MeOH (67%) for

24 hs. The cells used the 0.05 M I2 and 0.1 M LiI electrolyte in a hydrous nitrite solvent

and a platinized platinum counter electrode. Figure 4-12-A illustrates the incident photon

to current efficiency (ICPE) plot of a typical cell, compared to the absorption spectrum of

the FTO//TiO2//PS-Ru-A photoanode. The IPCE spectrum for the PS-Ru-A sensitized

DSSC correlates well with that of the Ru(II) polypyridyl chromophores. The IPCE

136

spectrum exhibited a maximum value of 24% at a wavelength that corresponds closely

to the MLCT band maximum for the Ru(II) polypyridyl chromophores. By using the

absorption of the TiO2//PS-Ru-A film, it is possible to compute the absorbed photon-to-

current efficiency (APCE) via the equation APCE = IPCE/(1–T), where T =

transmittance); and the calculation reveals APCE ∼27% at 480 nm.

Figure 4-12. IPCE and J-V curve for DSSC. (A) Incident photon to current efficiency (IPCE) spectra of a photoelectrochemical cell based on a FTO//TiO2//PS-Ru-A photoanode (black solid line with squares) and UV-visible absorption spectrum of the same photoanode (blue solid curve). (B) Current-voltage (J−V) cell characteristic for the cell based on the FTO//TiO2//PS-Ru-A photoanode under AM 1.5, 100 mW/cm2 illuminations.

As shown in Figure 4-12-B, the J-V measurement of the cell based on the

FTO//TiO2//PS-Ru-A photoanode measured under AM 1.5, 100 mW cm−2 illumination

gives a short-circuit photocurrent density (Jsc) of 1.03 mA cm−2, an open circuit voltage

(Voc) of 0.51 V, fill factor (FF) of 0.63, and an overall power conversion efficiency (η) of

0.33%. Taken together, the results reveal that the light harvesting and electron injection

occurs with moderate efficiency in the PS-Ru sensitized cell. Under the same conditions,

the model complex Ru-A gives a peak IPCE value of ∼47%, which is higher than the

polymer DSSC. The reasons are obvious that, there are more deposition of model

137

complex of Ru-A on the film since it has much smaller sizes compared to polymers, and

the DSSCs cell performance is related to the total amount of chromophore absorbed on

TiO2 and the absorption of the harvesting chromophore.192, 194

Summary

A series of Polystyrene based Ru(II) polypyridyl (RuIIL3) chromophores have

been prepared by using an NMP polymerization method and copper CuBr -catalyzed

“click” reactions. The photophysical properties were studied as a function of the polymer

chain lengths. The polymers showed similar absorption and emission spectra in

methanol and acetonitrile that were characteristic of Ru(II) polypyridyl (RuIIL3)

complexes. However, the excited state quenching of the MLCT was observed

depending on the polymer chain lengths. Longer polymer chain lengths result in shorter

emission lifetime and quantum yields for MLCT states. These effects could be attributed

to energy migration and exciton trapping within the polymers. With the similar synthesis

approach, polystyrene-based Ru(II) polypyridyl chromophores with carboxylic acid (–

COOH) anchoring groups was also prepared and immobilized onto metal oxide films to

make DSSC solar cells. The IPCE of the DSSC was measured as 24% at 480 nm,

which corresponds closely to the MLCT band maximum for the Ru(II) polypyridyl

chromophores, and the overall power conversion efficiency (η) was measured as

0.33%.



1H-NMR and 13C-NMR spectra were obtained on a Varian VXR300 instrument

utilizing CDCl3, CD3CN, and D2O as solvents. The IR spectra were recorded on a

Perkin-Elmer Spectrum One FTIR spectrometer equipped with an attenuated total

138

reflection (ATR) accessory using typically 128 scans at a resolution of 4 cm-1 in the

range of 4000-450 cm-1.

Gel permeation chromatography (GPC) analyses were carried out on a system

comprised of a Shimadzu LC-6D pump, Agilent mixed D column and a Shimadzu SPD-

20A photodiode array (PDA) detector, with THF as eluent at 1 mL/min flow rate. The

system was calibrated against linear polystyrene standards in THF.

All sample solutions in HPLC grade organic solvents or deionized water were

degassed with argon gas bubbling for 30 min (organic solution) or 90 min (aqueous

solution) and conducted using 1 cm2 quartz cells for photophysical experiments. UV-

visible spectra were collected using a Shimadzu UV-1800 dual beam absorption

spectrophotometer. Steady-state emission spectra were recorded on a

spectrofluorometer from Photon Technology International (PTI). Photoluminescence

lifetimes were obtained by using a single photon counting Fluo Time 100 (Picoquant)

Fluorescence Lifetime Spectrometer and excitation was provided using a PDL 800-B

Picosecond Pulsed Diode Laser. The emission quantum yields (ϕem) were measured

relative to [Ru(bpy)3]Cl2 in air saturated H2O (ϕem = 0.0379 in air saturated water and

0.055 in degassed water).190.

Electrochemistry was performed in acetonitrile solutions containing 0.1 M

tetrabutylammonium hexafluorophosphate ((TBA)PF6) as the supporting electrolyte

using a single-compartment three-electrode cell with a platinum button, platinum flag,

and non-aqueous Ag/AgCl electrode as the working, counter, and reference electrodes,

respectively. The scan rate was 50 mV/s, and the concentration of the analyte was 1

mM. Potentials were reported vs. a ferrocenium/ferrocene (Fc+/Fc) external standard.

139

DSSCs were fabricated using a method previously reported in the literature.176

Briefly, the TiO2 films were prepared on conductive fluorine doped glass (FTO) by a

doctor blade method. The films were then sintered by slowly heating the films at a rate

of 1 °C/min to a final temperature of 500 °C and maintaining this temperature for 30

minutes. The films are then cooled at a rate of 10 °C/min until 80 °C. Then, the TiO2

substrate (the active area of cells = ~0.18 cm2) was immersed in a solution of 0.1 mg/ml

PS-Ru-A dissolved in a 1:2 (v/v) mxture of anhydrous ACN:MeOH for 24 hs. Then the

PS-Ru-A coated TiO2 (PS-Ru-A//TiO2) electrodes were rinsed with anhydrous methanol

and acetonitrile to remove unbound PS-Ru-A. The PS-Ru-A//TiO2 electrode was

mounted in a sandwich configuration with a Pt-coated FTO glass electrode by heating

the assembly at ~ 80 °C with a 25 µm Surlyn (Solaronix Meltonix 1170-25) as a spacer.

Finally, an electrolyte solution containing 0.05 M I2, 0.1 M LiI, 0.6 M 1-methyl-3-(n-

propyl) imidazolium iodide, and 0.5 M 4-tert-butylpyridine in a dry nitrite solution was

injected into the sealed device via holes in the Pt counter electrode side with two holes

using a drill. For the IPCE response of the PS-Ru-A-based DSSC cells, an Oriel

Cornerstone monochromator as a light source was used to illuminate the cells and the

device current response was recorded under short-circuit condition using a Keithley

2400 source meter at 10 nm wavelength intervals. The light intensity at each

wavelength was calibrated with an energy meter (S350, UDT Instruments). The

current−voltage (J-V) characteristics of the cells were measured with a Keithley 2400

source meter under illumination with AM1.5 (100 mW/cm2) solar simulator.

Materials

All solvents and chemicals were purchased from the indicated suppliers and

used without purification unless noted otherwise: 4,4’-dimethyl-2, 2’-bipyridyl, 2, 2’-

140

bipyridine-4,4’-dicarboxylic acid, selenium dioxide, silver nitrate, potassium hydroxide,

4-dimethylaminopyridine (DMAP), copper(I) bromide (CuBr, 99.999%), N,N'-

dicyclohexylcarbodiimide (DCC), N-hydroxysuccinimide (NHS), propargyl amine,

N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA), 4-vinylbenzyl chloride, N-tert-

butyl-O-[1-[4-(chloromethyl)phenyl]ethyl]-N-(2-methyl-1-phenylpropyl)hydroxylamine,

meta-chloroperoxybenzoic acid (m-CPBA), tetrabutylammonium chloride, ammonium

hexafluorophosphate (NH4PF6), and sodium azide were purchased from Sigma-Aldrich.

cis-bis-(2,2’-bipyridine)-dichlororuthenium(II) dihydrate (Ru(bpy)2Cl2·2H2O) and

Ruthenium(III) chloride hydrate (RuCl3.xH2O) were purchased from Alfa Aesar. Copper

(I) bromide (CuBr) was treated with acetic acid and washed with acetone before use.

Deionized water was purified by passage through a Millipore purification system. All

organic solvents used for photophysical studies were HPLC grade.


Poly (4-vinylbenzyl chloride) (PVBC-170-NMP, DP = 170)

A solution of 4-vinylbenzyl chloride (13.10 mmol, 2 g), N-tert- butyl-O-[1-[4-

(chloromethyl) phenyl]ethyl]-N-(2-methyl-1-phenylpropyl)hydroxylamine (NMP) (0.067

mmol, 25 mg), and xylene (2 mL) were mixed together in a 10 mL of round bottom flask.

The solution was deoxygenated by three freeze–pump–thaw cycles, after which the

reaction mixture was stirred at 120 °C for 4 hs. Upon completion, the reaction flask was

placed into liquid nitrogen to quench the polymerization reaction. The crude product was

dissolved into THF and the solution was slowly added to methanol to afford a white solid

precipitate. Yield: 0.98 g (49%). 1H-NMR (300 MHz, CDCl3): 6.85–7.25 (sb, 2H), 6.20–

6.85 (sb, 2H), 4.51 (sb, 2H), 1.10-2.10 (mb, 3H). GPC: Mn ∼ 24000 g mol−1, PDI = 1.45.

141


The title compound was modified from the standard experimental procedure in order to

control the degree of polymerization. Following the procedure above, the reaction time

was reduced to 1 h. Yield: 0.76 g (38%). GPC: Mn ∼ 12000 g mol−1, PDI= 1.50.


The title compound was modified from the standard experimental procedure in

order to control the degree of polymerization. Following the procedure above, the

reaction time was reduced to 0.5 h and 4-vinylbenzyl chloride (13.10 mmol, 2 g), NMP

(0.134 mmol, 50 mg), and xylene (4 mL) was used. Yield: 0.42 g (21%). GPC: Mn ∼

5500 g mol−1, PDI = 1.30.

Poly (4-vinylbenzyl chloride) (PVBC-170)

PVBC-170-NMP (1) (0.027 mmol, 0.64 g) and meta-chloroperoxybenzoic acid

(0.27 mmol, 46.22 mg) were mixed together in 8 mL of toluene and stirred then for a

period of 24 h at room temperature. The crude product was dissolved into THF and the

solution was slowly added to methanol to afford a white solid precipitate. Yield: 0.50 g

(78%). 1H-NMR (300 MHz, CDCl3): 6.85–7.25 (sb, 2H), 6.20–6.85 (sb, 2H), 4.51 (sb,

2H), 1.10–2.10 (mb, 3H). GPC: Mn ∼ 22000 g mol−1, PDI = 1.58.


The title compound was prepared following the synthetic procedure for PVBC-

170. Yield: 0.32 g (75%). GPC: Mn ∼ 12 000 g mol−1, PDI = 1.48.



170. Yield: 0.16 g (55%). GPC: Mn ∼ 5500 g mol−1, PDI = 1.39.

142

Poly (4-vinylbenzyl azide) (PVBA-170)

PVBC-170 (2) (0.66 mmol of polymer repeating units, 0.10 g) and NaN3 (1.94

mmol, 0.13 g) were mixed together in 10 mL of DMF and stirred then for a period of 24

h at room temperature. (Caution! Sodium azide is highly toxic and presents a severe

explosion risk when shocked, heated or treated with acid.) Afterwards, the polymer

solution was precipitated with water. Yield: 90 mg (86%). 1H-NMR (300 MHz, CDCl3):

6.85–7.25 (sb, 2H), 6.20–6.85 (sb, 2H), 4.24 (sb, 2H), 1.10–2.10 (mb, 3H). GPC: Mn =

22000 g mol−1, PDI = 1.60. IR (cm−1): 2090 cm−1 (azide group).



170. Yield: 93 mg (89%). GPC: Mn ∼ 12 000 g mol−1, PDI= 1.47.


The title compound was prepared following the synthetic procedure for PVBA-

170 except PVBC (n = 40) (0.33 mmol of polymer repeating units, 0.05 g) and NaN3

(0.97 mmol, 0.06 g) were used. Yield: 24 mg (46%). GPC: Mn ∼ 5600 g mol−1,PDI =

1.51.

PSn-Ru(Cl) (n = 170, 80, 35)

Under nitrogen, PVBA-n (3) (0.08 mmol of polymer repeating units, 13 mg),

compound 6 (0.12 mmol, 0.11 g) and PMDETA (0.12 mmol, 19.6 mg) were mixed

together in 2 mL of DMF and stirred then for 1 h at room temperature. Afterwards,

copper (I) bromide (0.12 mmol, 16.4 mg) was added into the polymer solution. The

resulting solution was stirred at room temperature for 48 h. Ethanol (20 mL) was added

to the reaction flask. A brown-reddish precipitate was isolated by repeated

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centrifugation and washed with 20 mL of ethanol. The precipitate product was dissolved

in acetone and the impurities were isolated by centrifugation to the bottom of the tube.

The clear acetone supernatant solution was transferred into a round-bottom flask and

the solvent was removed using a rotary evaporator. Purification of the resulting polymer

product was achieved by repeated centrifugation, re-dispersion, and evaporation. The

polymer product underwent anion metathesis using a saturated solution of

tetrabutylammonium chloride in acetone. The chloride salt of the polymer was dissolved

in Milli-Q water and purified in dialysis bags (cutoff = 12–14 kDa), which were bathed for

72 h in periodically replenished Milli-Q water. Yield: 54 mg (75%). 1H-NMR (300 MHz,

D2O): 6.5–8.5 (sb, 2H).

PSn-Ru(PF6) (n = 170, 80, 35)

The PSn-Ru(Cl) (24.3 mg) was dissolved in Milli-Q water. The saturated NH4PF6

aqueous solution was dropped into the polymer solution to produce a solid precipitate.

Yield: 23.8 mg (95%). 1H-NMR (300 MHz, CD3CN): 7.5–9.0 (sb, 2H), 6.83(sb), 6.31(sb),

5.25(sb), 4.47(sb).

[Ru(2,2′bipyridine)2(4′-methyl-2,2′-bipyridine-4-carbonyl propargyl amine)] (PF6)2

(Ru(II))

The title compound was synthesized as described in the corresponding literature

sources.177 1H NMR (300 MHz, acetonitrile-d3), δ (ppm): 2.53 (t, 1H, J = 2.6 Hz); 2.57

(s, 3H); 4.20 (dd, 2H, J1 = 5.4 Hz, J2 = 2.6 Hz); 7.30 (dd, 1H, J1 = 5.7 Hz, J2 = 1.0 Hz);

7.38–7.46 (m, 4H); 7.57 (d, 1H, J=5.7Hz); 7.63 (dd, 1H, J1=6.0Hz, J2=2.0Hz); 7.68–

7.76 (bm, 5H); 7.87 (d, 1H, J = 6.0 Hz); 8.04–8.09 (m, 4H); 8.53(d, 5H, J=7.4Hz);

8.80(d, 1H, J=1.5Hz). 13CNMR (75 MHz, acetonitrile-d3), δ (ppm): 21.31, 30.12, 72.54,

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80.55, 122.61, 125.32, 125.45, 126.60, 128.62, 128.68, 129.82, 138.93, 138.97, 142.74,

151.78, 151.89, 152.61, 152.69, 152.73, 152.79, 153.57, 156.96, 157.84, 157.96,

158.01, 158.04, 159.14, 164.19. ESITOF MS m/z calculated for C35H29N7ORu2+PF6-

1[M1+]: 810.1123, found: 810.1134.

[Ru(deeb)2(4'-methyl-2,2'-bipyridine-4-carbonyl propargyl amine)] (PF6)2 (Ru(II)-E)

was prepared as described in the corresponding literature sources.176

Ru(4-(COOH),4’-(COOH)-bpy)2(4’-methyl-2,2’-bipyridine-4-carbonyl propargyl

amine) Cl2 (RuII-A) was synthesized with modified procedure described in the

corresponding literature sources.196, 197

The cis-Dichlorobis (2,2’-bipyridyl- 4,4’-dicarboxylic acid) ruthenium(II) (0.140

mmol, 100 mg) was dis-solved in 2ml water and neutralized to pH = 7 by addition of

sodium hydroxide. A solid was precipitated by addition of 10 mL acetone, filtered off and

dried under vacuum. The solid collected and 4’-methyl-2,2’-bipyridine-4-carbonyl

propargyl amine (0.150 mmol, 37.7 mg) were dissolved in 40 mL of MeOH/H2O (1:1).

The reaction mixture was degassed for 30 min and refluxed for 5 hs under argon. The

solution was concentrated to 2 mL, loaded onto a silica column, and eluted with NaCl-

saturated methanol. The first fraction was collected and evaporated to dryness to obtain

Na4Ru(4-(COO-),4’-(COO-)-bpy)2(4’-methyl-2,2’-bipyridine-4-carbonyl propargyl amine)]

Cl2. The crude product was precipitated by the addition of dilute HCl to the aqueous

solution to obtain the final product. Yield (45 mg, 32 %). 1H NMR (500 MHz, D2O): d

(ppm) = 2.70 (s, 3H); 2.88 (t, 1H, J=2.6 Hz); 4.36 (d, 2H, J=5.5 Hz); 7.49 (d, 1H, J=5.7

Hz); 7.78 (d, 1H, J=5.8 Hz); 7.84 (d, 1H, J=6.0 Hz); 7.86–7.91 (m, 4H), 8.04–8.08 (m,

4H); 8.12 (d, 1H, J=5.8Hz); 8.66 (s, 1H); 9.01 (s, 1H), 9.05 ppm (s, 4H). 13C NMR (125

145

MHz, D2O): d= 20.55, 29.76, 35.00, 49.31, 49.37, 72.64, 79.63, 121.46, 123.33, 123.53,

123.64, 125.79, 126.40, 129.05, 141.42, 150.79, 151.26, 151.96, 152.17, 152.48,

152.53, 155.51, 157.29, 157.35, 157.44, 157.50, 158.22, 166.41, 171.11 ppm. ESI-TOF

MS m/z calculated for C39H25N7O9Ru2 [M2-]: 418.5390, found: 418.5400.

PS-Ru-E.

PVBA (0.05 mmol of polymer repeating units, 8.0 mg), Ru(II)-E (0.06 mmol, 74.6

mg) and PMDETA (0.01 mmol, 1.7 mg) with 3 mL of DMF were mixed together and

stirred for 1 h at room temperature under nitrogen. After adding copper(I) bromide (0.01

mmol, 1.4 mg) into the polymer solution, the mixture was then stirred at room

temperature for 48 h. After 48h, ethanol was added to the stirred reaction mixture. A

brown-red precipitate was isolated by centrifugation. Several post-precipitative washing

steps with 20 mL of ethanol were applied. The precipitated product was dissolved in

acetone and the insoluble impurities tube were removed by centrifugation. The solvent

in the clear acetone supernatant solution was removed using a rotary evaporator to

yield a brown reddish solid (60 mg, 86%). IR (cm-1): 1722 (C=O, ester)

PS-Ru-A.

PS-Ru-E (5) (32 mg, 0.026 mmol) was dissolved in the mixture of 8 mL of

acetonitrile and 2 mL of methanol. Tetrabutylammonium hydroxide (1 mL) was added to

the reaction mixture and the resulting solution was stirred at 40 °C for 24 hours under Ar

purge. After stirring the solution for 24 hs, acetic acid (1 mL) was added via a syringe

and the mixture was stirred for an additional 2 hrs. Then, the resulting solution was

concentrated by a nitrogen stream and precipitated from anhydrous diethyl ether to

obtain a brown-red solid (20 mg, 59%). In the FT-IR spectrum of PS-Ru-A, the band of

146

the carbonyl stretching at 1722 cm-1 of PS-Ru-E disappears, indicating quantitative

ester hydrolysis. The product was insoluble in common NMR solvents to obtain high-

resolution spectra.

147

CHAPTER 5 POLYMERIC CHROMOPHORE WATER OXIDATION CATALYST ASSEMBLY FOR

SOLAR FUEL SYSTEM

Background

Generation of clean energy in a more economical way has become one of

several critical challenges in the twenty-first century.55 The fact that the sun is supplying

us more energy in one hour than what was consumed one the planet for the whole year

(2001) inspired lots of researchers dedicate their work towards sustainable conversion

of solar energy into fuels or electricity.56 One of the most studied approaches to utilize

solar energy (solar capture, conversion and storage) is artificial photosynthesis, inspired

by nature, in which chemical bonds are broken and formed using sunlight as the input

energy to produce solar fuels.56-61 In natural photosynthesis, sunlight is captured and

converted into fuels, reducing CO2 into carbohydrates while releasing O2, summarized

in equation 5-1.56, 62, 63 However, in artificial photosynthesis process, a more

straightforward approach is to accomplish the water splitting to generate molecular O2

and H2.62 The water splitting process requires the coupling of the two half-reactions: (1)

oxidation of H2O to generate the molecular oxygen (Equation 5-2), (2) reduction of

protons to generate molecular hydrogen (Equation 5-3).

6CO2 + 12H2O→ C6H12O6 + 6O2 + 6H2O (5-1)

2H2O → O2 + 4H+ (5-2)

4H+ + 4e- → H2 (5-3)


widely used to study the artificial photosynthesis process since 1999 for its great

simplifications in cell implementation with variable semiconductors, sensitizers and

catalysts available.83-85 This photoelectrosynthesis cell (PEC) basically mimics dye-

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sensitized solar cells (DSSC) but with the goal of producing oxygen and solar fuel at two

physically separated electrodes rather than photopotential and photocurrent.86 In a

DSPEC, absorption of sunlight by a sensitizer (dye or chromophore) deposited on the

surface of a metal oxide semiconductor triggers a series of molecular and interfacial

electron transfer events that drive water oxidation and solar fuel generation half

reactions in the two separate electrodes of the PEC.

Among these two half reactions, the photo-catalytic oxidation of water to

generate O2 is much more challenging compared to reduction of protons, and therefore

most of the ongoing research efforts have been focused on half reaction of water

oxidation. In order to conduct photo-catalytic oxidation of water at a photoanode,

several components are essential: 1) a sensitizer is required to maximize the absorption

of photons from sunlight; 2) a catalyst is essential efficiently to oxidize the water to

molecular oxygen; 3) the electrons from photoexcited sensitizers need to be injected

into a semiconductor interface. The photoanode will be fabricated by these three

components, on the basis of electron transfer events.

The complete DSPEC apparatus is constructed when connecting the photoanode

and a photocathode (for example, Pt wire) by wire.59, 86 According to the previous report

from Dr. Meyer’s research group for “blue dimer ” in 1982,63 ruthenium based molecular

water oxidation catalysts have been attracted towards water oxidation reactions105-110.

At the same time, even more light sensitizers have been developed to harvest sunlight

with strong molar absorption in the visible region, including organic dyes,94, 96 transition

metal complexes 83, 97, 98 and quantum dots. Among them, the Ru(II) polypyridyl complex

and derivatives are the most widely used photo-sensitizers for their strong metal-to-

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ligand charge transfer (MLCT) absorption in UV-visible region and good photostability.

The potential difference between contacted semiconductor and electrolyte solution

drives the transport of charges and electrolytes across the interfaces. Mesoporous

nanostructured TiO2 and core/shell structured SnO2/TiO2 onto a conductive, transparent

substrate, typically indium tin oxide (ITO) or fluorine doped tin oxide (FTO) are two most

popular semiconductors used in implementing DSPEC.86

The orientation between water oxidation catalyst, light sensitizer and

semiconductor substrate is critical in solar energy conversion for the DSPEC device.

Researchers have designed several approaches to assemble catalysts and light

sensitizers, including molecular co-deposition, 90 “Layer-by-Layer” with Zr(IV)-

phosphonate bridges91, 92, molecular covalent bond,34, 85 and cross-linked

electropolymerization via reductive vinyl coupling.93 These approaches could require

large efforts of synthetic work or lack control of the catalyst-chromophore configuration.

To the best of our knowledge, for the first time, here we describe an approach to

prepare a polymeric chromophore-catalyst assembly, followed by deposition onto a

semiconductor for water oxidation at a model DSPEC photoanode. This design is rooted

at light harvesting and intra-electron transfer process in the polymer assembly. In order

to let the polymer do the work, several factors need to be considered when selecting

catalyst, chromophore and polymer backbone; 1) catalyst, chromophore and polymer

are easy to synthesize; 2) catalyst and chromophore are assembled to polymer

efficiently, 3) polymer assembly owns good solubility in variable solvents, especially in

the aqueous solution, 4) strong stability during synthesis and irradiation, 5) catalyst has

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high catalytic efficiency and stability, 6) the light sensitizer features strong UV-visible

light absorption coefficients.


Target Design and Synthesis

In our study, we used Ru(phenq)(tpy)2+ (tpy = 2,2′;6,2″- terpyridine, phenq = 2-

(quinol-8′-yl)-1,10-phenanthroline) as water oxidation catalyst,198 Ru(II)(bpy)32+

(bpy=2,2'-bipyridine) type complex as chromophore-sensitizer, and polystyrene as the

scaffold to construct the polymeric chromophore-catalyst assembly. The chemical

structure of the polymer assembly is shown in Figure 5-1.

Figure 5-1. Structure of polystyrene based catalyst-chromophore assembly.

There are several obvious advantages for those targets: Ru(phenq)(tpy)2+

catalyst has no aquo ligand (e.g., M-OH2), and a seven-coordinate intermediate is

formed when water molecule attacks equatorial plane of the complex. Unlike aquo-

ligand type catalysts, we are able to tune its solubility through exchanging the counter

ions easily. Moreover, the catalyst is cationic, the exchange of counter ions (between Cl-

and PF6- ) makes it possible to control its solubility in organic vs. aqueous solution,

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especially when attached to polymer backbone systems, together with Ru(II) polypyridyl

complex.184, 189 At the same time, we have chosen to use the Ru(II)(bpy)32+ complex as

solar energy harvesting sensitizer due to its good thermal and photo-stability and strong

absorption in UV-visible region. Previously we have studied the energy/electron transfer

and interfacial electron injection mechanisms in Ru(II)(bpy)32+ complex based molecular

and polymeric systems.176, 184, 185, 199, 200 We used polystyrene as the polymer backbone

for its easy controlled polymerization process. Considering the conclusions from our

previous studies that polymer with longer chains would have higher possibility of

aggregation caused excited quenching effects, we proposed to make polymer with 10-

20 repeating units.184

In order to synthesize the polymeric chromophore-catalyst assembly, the ethyne

phenyl-containing Ru(phenq)(tpy)2+ derivatives (compound 5) was prepared via a

modified approach reported initially (Figure 5-2).198 Note that the synthesis of ligand

phenq: we made the phenq from 2-qinoline-boronic acid and 10-bromophenanthroline in

one step with yield of 76% based on a Suzuki reaction, instead of the previously

reported multi-step synthesis based on a Friedlander condensation reaction.201 The

Ru(phenq)(tpy)2+ was also prepared with the similar procedure as model catalyst

(compound 7). The azide substituted polystyrene (Poly-9, DP=15, Mn=2300, PDI= 1.2)

was prepared via nitroxide-mediated living radical polymerization (NMP), and ethyl

ester-containing Ru(II)(bpy)32+ derivatives (compound 6) was prepared from initial

studies185. Click assembly via an azide-alkyne Huisgen cycloaddition reaction allowed

attachment of ethyne ester-containing Ru(phenq)(tpy)2+ derivative and ethyne phenyl-

containing Ru(II)(bpy)32+ derivative onto the polystyrene scaffold, with the formation of

152

polymeric catalyst-chromophore assembly (Poly-10, Figure 5-3). Under counter ion

exchange, polymer assembly in Cl- form was converted into PF6- form (Poly-11, Figure

5-3). Consider the fact that generation of 1 mole O2 require 4 mole electron transfer and

1 mole excited Ru(II) chromophores are only able to inject 1 mole electrons into

semiconductor, the click reaction was carried out with 20% catalysts and 80%

chromophore.

Figure 5-2. Synthesis procedure of functional catalyst

The Poly-11 was characterized by 1H-NMR and FT-IR spectroscopies (Figure 5-

4). The NMR peak around 2.25 ppm is attributed to 3 protons of –CH3 group of

Ru(II)(bpy)32+ derivatives, and the broad and weak resonances at ∼4.5 ppm and ∼5.3

ppm are attributed to the methylene groups next to the triazole rings. The aromatic

153

resonances from 7.0 - 9.0 ppm are highly similar to the peaks from Ru(II)(bpy)32+ based

polymer, therefore assigned to bipyridine ligands of Ru(II)(bpy)32+ derivatives, 2,2′;6,2″-

terpyridine and 2-(quinol-8′-yl)-1,10-phenanthroline ligands of Ru(phenq)(tpy)2+

derivatives grafted on the polymer backbone in Poly-11

Figure 5-3. Synthesis procedure of polymeric catalyst-chromophore assembly (Poly-10 and Poly-11)

At the same time, a new broad peak was also observed ~9.35 ppm, which was

assigned to 5 protons from the Ru(phenq)(tpy)2+ units, whose resonances were

measured over 9.0 ppm (synthesis and characterization section). The integrations of

protons at ~2.25 ppm (3 protons from Ru(II)(bpy)32+ sites) and ~9.35 ppm (5 protons

from Ru(phenq)(tpy)2+ sites) were used to calculate the ratio of catalyst and

chromophore units attached to the polymer. The percentage of catalyst sites was

calculated as (1.58/5)/(1.58/5+3.00/3)=24%, and the percentage of chromophore sites

was calculated as 1-24%=76%. Those calculated results are comparable to the reaction

ratio, with 20% catalyst and 80% chromophore complexes. The FT-IR spectrum for

154

Poly-11 clearly verified a substantial diminution of the peak from the azide groups at

2050 cm-1 while maintaining the IR features from both catalyst and chromophores units

after the azide-alkyne Huisgen cycloaddition reaction.184 In short, we successfully

synthesized a polymer based catalyst-chromophore assembly with controlled

chromophore/catalyst ratio on the polystyrene scaffold.

Figure 5-4. 1H-NMR and FT-IR for Poly-11. (A) 1H-NMR in CD3CN. (B) FTIR in KBr crystal.

A)

B)

155

Photophysical Study

The UV-visible absorption of the Poly-10 and corresponding Ru(II) chromophore

and catalyst units were measured in a mixture solvent of acetonitrile (50%) and

methanol (50%) at room temperature. As shown in Figure 5-5A, the chromophore

features a strong ligand-based π→π* absorption band with maxima at λ ∼ 288 nm and

a strong dπ(Ru) → π*(bpy) metal-to-ligand charge transfer (MLCT) band at λ ∼ 455 nm

in the visible region.184 The catalyst exhibits unique absorption features at 300-350 nm,

and a red-shifted MLCT absorption compared with the chromophore at λmax ∼ 480 nm.

As a result, the Poly-10 displayed combined features from both catalyst and

chromophores. Even though Ru(II) chromophores contributed the major absorption

features for the Poly-10, the features from catalysts were still observable at 300-350 nm

and 500-550 nm.

Photoluminescence emission studies were also carried out in the mixture of 50%

acetonitrile and 50% methanol for all the three samples. the samples were prepared

with identical absorption intensity at 450 nm, equals 0.2. As shown in Figure 5-5B, an

emission peak was observed in chromophore, with at λmax ∼ 650 nm (Intensity

normalized to 1), while the catalyst is almost non-emissive. Interestingly, the Poly-10

shows a much lower emission, with 90% intensity (λ ∼ 650 nm) quenched compared to

the chromophore. The significant quenching effect is caused by electron transfer and/or

energy transfer between excited chromophore sites and catalyst sites along the polymer

backbone. From the electrochemistry study described afterwards, we are convinced

there are intra-electron interactions between catalyst and chromophore sites, though we

do not eliminate the possibility of intra-polymer energy transfer.

156

Figure 5-5. Photophysical studies. (A) UV-visible absorption spectra for the Poly-10 (Solid blue line), Ru(II) chromophore (Dashed red line) and catalyst (Dotted black line). (B) Emission spectra of the Poly-10 (Solid blue line), Ru(II) chromophore (Dashed red line) and catalyst (Dotted black line). Excitation wavelength for three samples = 450 nm. (C) Simulated UV-visible absorption spectra under different Ru(II) chromophore/ Ru(II) catalyst composites (The percentage of Ru(II) catalyst increases from 10% to 40%). (D) Experimental UV-visible absorption spectra for Poly-10 (Black solid line) and simulated UV-visible absorption spectra (Red dotted line) for mixtures of 70% Ru(II) chromophore and 30% Ru(II) catalysts. All the photophysical experiments were conducted in mixed solvent of methanol (50%) and acetonitrile (50%).

With the molar absorption coefficients available, we are able to obtain simulated

absorption spectra for the polymer assembly under different estimated composite

(Figure 5-5C and Figure 5-5D). When the composition was set as 30% catalyst and 70%

chromophore, we obtained a simulated UV-visible absorption spectrum, which was

A) B)

C) D)

157

nearly identical to the experimentally measured spectrum of Poly-10 (normalized at the

maximum absorption peak). In other words, from the perspective of UV-visible

spectroscopy, the catalyst sites on the polymer were estimated as 30%, and the

chromophore sites were estimated as 70%, which is comparable with the ratio

calculated from 1H-NMR spectroscopy.


Complete electrochemistry studies for the Poly-10, Ru(phenq)(tpy)2+ and

Ru(bpy)32+ were conducted in phosphate buffer solution (pH=7, Figure 5-6A) and

potentials are listed vs. Ag/AgCl reference. Reversible waves were observed for

Ru(phenq)(tpy)2+ units at 1.05 V and the Ru(bpy)32+ units at 1.25 V. The ~0.20 V

potential difference makes the electron transfer from RuII(phenq)(tpy)2+ to RuIII(bpy)33+

thermodynamically favorable. At potential of 1.6 V, the catalyst shows a significant

anodic current (~60 µA, at 50 mV/s), compared to chromophore (~5 µA at 50 mV/s),

such enhanced current is expected to arise from water oxidation. Interestingly, for the

Poly-10, a strong anodic current was observed during the oxidation process, when both

Ru(phenq)(tpy)2+ units and Ru(bpy)32+ units in the polymer were oxidized to

(Ru(phenq)(tpy)3+ and Ru(bpy)33+ or higher oxidation states. However, for the reduction

process, no obvious cathodic current was observed at 1.25 V, until a small shoulder

appears at ~1.10 V. This indicates that during the cathodic process, no large amount of

oxidized chromophores, Ru(bpy)33+, exist around the working electrode surface. We

think such disappearance of Ru(bpy)33+ species was caused by the hole transfer from

Ru(bpy)33+ to Ru(phenq)(tpy)2+ (Ru(phenq)(tpy)2+ was regenerated from catalytic water

oxidation). After the hole transfer, the Ru(phenq)(tpy)2+ was oxidized to

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Ru(phenq)(tpy)3+, while the Ru(bpy)33+ is reduced to Ru(bpy)3

2+ (Figure 5-6C).

.

Figure 5-6. Electrochemistry studies. (A) Cyclic voltammetry measurement for Poly-10, Ru(phenq)(tpy)Cl2 and Ru(bpy)3Cl2 in 0.1 M phosphate buffer aqueous solution (pH=7), scan rate = 50 mV/s, carbon electrode as working electrode. (B) Cyclic voltammetry measurement for ITO//(PAA/Poly-10)5 under different scan rates (20 mV/s, 50 mV/s and 100 mV/s) in 0.1 M HClO4 aqueous solution (pH=1). (C). The scheme showing the electron interaction between catalyst and chromophore sites. For both solution and film cyclic voltammetry studies, Pt wire works as counter electrode and Ag/AgCl as reference electrode.

In short, the electrochemistry study of Poly-10 clearly implies the electronic

interaction between catalyst and chromophore units in the polymer. At the same time,

the relatively higher anodic current at 1.60 V in Poly-10 (~28 µA at 50 mV/s) compared

C)

A) B)

159

to Ru(bpy)32+ (~5 µA) also proves some chemical reactions occurring around the

working electrode interfaces, and those reactions were attributed to water oxidation

reactions

In order to understand the electrochemistry properties for the film, the Poly-10

deposited ITO substrate was prepared with “Layer-by-Layer” (LBL) method125 (detailed

description of LBL deposition method was introduced in instrumentation and methods

section in Chapter 5). After 5 bilayer deposition of polyacrylic acid (PAA, pH= ~4.2) and

Poly-10 (pH=6.8) alternatively on ITO substrate, the film was prepared in form of

ITO//(PAA/Poly-10)5, and measured by cyclic voltammetry under three different

scanning rates, 20 mV/s, 50 mV/s and 100 mV/s (Figure 5-6B). We observed oxidation

peaks under all three scanning rates, with current intensities decreased under reduced

scan rates. The strong anodic currents were detected ~1.15 V and mainly contributed

by the oxidation of Ru(bpy)32+ and Ru(phenq)(tpy)2+ species. The Ru(III/II) potentials

from the ITO//(PAA/Poly-10)5.is comparable with Ru(III/II) potentials measured in

solution. Noticeably, because of the low loadings of Poly-10 on the ITO substrate, the

anodic currents are ~200 fold smaller than the anodic current measured from solution

electrochemistry of Poly-10. Compared to the electrochemistry study of Poly-10 in

solution, the ITO//(PAA/Poly-10)5 exhibits some observable cathodic current, even

though it is still much lower than its corresponding anodic current. This indicates that

electron transfer (hole transfer) also occurs the polymer deposited film, though the

electron transfer rates might be relatively lower than rates in solution (Figure 5-6C).

Photocatalytic Oxidation of Organic Compounds

Photochemical organic oxidation reactions have previously been studied for

photo sensitizer-catalyst co-systems, especially for the polypyridyl Ru−aquo catalysts.84,

160

202, 203 As we studied in electrochemistry of Poly-10, oxidation of Ru(phenq)(tpy)2+ by

Ru(bpy)33+ is a thermodynamically favorable process, the formation of oxidized catalytic

species in higher state opens up pathways towards oxidations of organic compounds,

especially for the phenol and benzyl alcohol.

Figure 5-7. UV-visible absorption spectra for FTO//TiO2//(PAA/Poly-10)10 and FTO//TiO2//(PAA/Poly-8)10.

Phenol and benzyl alcohol are also two examples used in our study with the

purpose to test the photocatalytic activity of Poly-10. We used the LBL method to

deposit the Poly-10 onto FTO//TiO2 substrate with 10 bilayers, fabricating the

photoanode in form of FTO//TiO2//(PAA/Poly-10)10. The substrate (FTO//TiO2) was

prepared according to a previous report185 and dipped into

polydiallyldimethylammonium chloride solution (PDADMAC, pH= ~6.6) for 15 min before

further use. At the same time, Ru(II) chromophore loaded polymer (Poly-8) was also

deposited onto TiO2 substrate with 10 bilayers to fabricate the control photoanode for

control studies, in form of FTO//TiO2//(PAA/Poly-8)10. The FTO//TiO2//(PAA/Poly-10)10

and FTO//TiO2//(PAA/Poly-8)10 were characterized by UV-visible absorption (Figure 5-7).

161

The maximum absorption for both films are nearly ~0.12 at ~475 nm. Compared to

FTO//TiO2//(PAA/Poly-8)10, the FTO//TiO2//(PAA/Poly-10)10 exhibits enhanced

absorption in region of 500 nm - 550 nm, contributed by the catalyst sites of Poly-10.

Based on the literature,194 the surface coverage of polymer on FTO//TiO2//(PAA/Poly-

8)10 and FTO//TiO2//(PAA/Poly-10)10 were estimated as ~6.9 x 10-9 mol/cm2.

Phenol (PhOH) was chosen as the first target to study the organic oxidation

performance for FTO//TiO2//(PAA/Poly-10)10, as it has been previously shown to be

highly reactive with polypyridyl Ru−aquo catalysts.96 The organic oxidation activities

were measured in form of photocurrent density, upon the irradiation of the photoanode

with visible light (λ > 400 nm, 100 mW-cm−2) under a positive bias potential (Eappl = 0.2

V), when the anode was connected with cathode (Pt wire) and Ag/AgCl reference

electrode, and immersed in the 0.02 M acetate buffer solution (pH=4.6).

Figure 5-8. Photocatalytic oxidation of PhOH. Photocurrent–time traces with 20 second light off/on cycles for FTO//TiO2//(PAA/Poly-10)10 (A) and FTO//TiO2//(PAA/Poly-8)10 (B). The data were collected in 20 mM acetate buffer (PH=4.6) under illumination (1 sun, 100 mW cm−2, 400 nm cutoff filter) with an applied bias of 0.2 V versus. Ag/AgCl). The phenol was added into solution with 4mM, 8mM and 12mM.

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As shown in Figure 5-8A, without adding any PhOH, the photocurrent density

(data extracted at t = 30 s for all the photocurrent densities in organic oxidation reaction

studies) was measured ~4.5 µA/cm2 when light was on, and down to 0 when light was

off. After adding 4 mM PhOH into the solution, the photocurrent density increased to 9

µA/cm2, which is ~100% increase. Based on literatures,96 the enhancement of

photocurrent was assigned to oxidation of PhOH around the anode interface. Continual

increase of PhOH concentration up to 12 mM resulted in further photocurrent

enhancement and stabilized at ~11.5 µA/cm2. To the contrary, the

FTO//TiO2//(PAA/Poly-8)10 does not give much enhanced photocurrent density. Briefly,

the photocurrent density was detected as ~4.0 µA/cm2 before adding PhOH, and

increased to ~5.0 µA/cm2 after injecting 4 mM PhOH, with 25 % enhancement. The

photocurrent finally stabilized ~6.0 µA/cm2 after adding 12 mM PhOH. The generation of

extra photocurrents on photoanode were caused by the oxidation reactions of PhOH.

Figure 5-9. Photocatalytic oxidation of BnOH. Photocurrent–time traces with 20 second light off/on cycles for FTO//TiO2//(PAA/Poly-10)10 (A) and FTO//TiO2//(PAA/Poly-8)10 (B). The data were collected in 20 mM acetate buffer (PH=4.6) under illumination (1 sun, 100 mW cm−2, 400 nm cutoff filter) with an applied bias of 0.2 V versus. Ag/AgCl). The BnOH was added into solution with 0.1M.

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Oxidation of benzyl alcohol (BnOH) at the FTO//TiO2//(PAA/Poly-10)10 electrode

was subsequently investigated (Figure 5-9A). When adding 0.1 M BnOH into the

electrode solution, the photocurrent density was measured with an enhancement from

~4.5 µA/cm2 to ~9.0 µA/cm2, with 100% enhancement. However, for the control study of

FTO//TiO2//(PAA/Poly-8)10 (Figure 5-9-B), only 25% photocurrent increase was

observed when adding 0.1 M BnOH, from ~4.0 µA/cm2 to ~5.0 µA/cm2. The extra

photocurrent generated was also attributed to the oxidation of BnOH around the anode

interface. Consider that the concentration of BnOH (0.1 M) is 25 times more than the

concentration of PhOH (4 mM) in the solution when FTO//TiO2//(PAA/Poly-10)10

achieves the similar photocurrent density, it is confident to conclude the PhOH is much

easier to undergo photo-catalytically oxidation compared to BnOH. In photocatalytic

oxidation studies of both PhOH and BnOH, FTO//TiO2//(PAA/Poly-8)10 also generated

observable photocurrent (25% increase), we think this may be caused by the

dissociation of Ru(bpy)32+ complexes grafted on the Poly-8. The similar reactions may

also occur for Poly-10, though its current was overlapped with photocurrent from

catalytic oxidation reactions. However, we cannot eliminate the possibility of oxidation

reaction of PhOH or BnOH by Poly-8.

Water Oxidation Study

In previous work, Leem and co-workers reported the use of a core-shell substrate

containing a mesoporous SnO2 film onto a top fluorine doped tin oxide (FTO), which is

modified with a TiO2 overlayer deposited by atomic layer deposition (ALD), in form of

FTO//(SnO2/TiO2). The FTO//(SnO2/TiO2) was reported to block the back electron

transfer efficiently, therefore enhancing the photo-catalytic efficiency.125 So we used

FTO//(SnO2/TiO2) core-shell substrate to fabricate the photoanode towards studying the

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water oxidation reactions. The core-shell substrate was prepared according to the

literature,84 followed by deposition of Poly-10 through LbL method with 5 bilayers to

fabricate photoanode, in form of FTO//(SnO2/TiO2)//(PAA/Poly-10)5. The photoanode

was connected with cathode (Pt wire) and Ag/AgCl reference electrode, and immersed

into electrolyte solution (0.5 M KNO3 aqueous solution with 0.1 M phosphate buffer at

pH= 7). The core-shell electrode from Poly-8 was prepared with the similar procedure,

in form of FTO//(SnO2/TiO2)//(PAA/Poly-8)5 for control studies.

Figure 5-10. Photocatalytic oxidation of water. (A) Photocurrent–time traces with 20 second light off/on cycles for three anodes: FTO//(SnO2/TiO2)//(PAA/Poly-10)5 (Solid red line), FTO//(SnO2/TiO2)//(PAA/Poly-8)5 (Dashed blue line), and bare FTO//(SnO2/TiO2) (Dotted black line). (B) Photocurrent–time traces with 250 second light on for two anodes: FTO//(SnO2/TiO2)//(PAA/Poly-10)5 (Solid red line) and bare FTO//(SnO2/TiO2) (Dotted black line). The data were collected in 0.5 M KNO3 aqueous solution with 0.1 M phosphate buffer (PH= 7) under illumination (1 sun, 100 mW cm−2, 400 nm cutoff filter) with an applied bias of 0.2 V versus. Ag/AgCl.

The photocatalytic water oxidation activity was studied in form of measuring the

generation of the photocurrent on the photoanode under illumination with 100 mW-cm−2

visible light (λ > 400 nm). From the light on-off experiment (Figure 5-10A), under applied

bias potential (Eappl = 0.2 V), FTO//(SnO2/TiO2)//(PAA/Poly-10)5 generated peak

B) A)

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photocurrent density (t= 20 s) with ~18.5 µA/cm2 when light was on, followed by gradual

decrease to ~13 µA/cm2 after irradiation for 20 s (t= 40 s). The photocurrent decreased

to 0 simultaneously when the light was off and stayed in the same state for another 20

seconds (t= 60 s). After turning on the light for the second cycle (t= 60 s), the peak

photocurrent (t= 60 s) was observed with ~12.5 µA/cm2. After three cycles of light on-off

switch, the photocurrent density was measured as ~11 µA/cm2 (t= 120 s). Under the

same conditions, Poly-8 deposited core shell substrate (FTO//(SnO2/TiO2)//(PAA/Poly-

8)5 was detected to generate peak current with ~7.5 µA/cm2 when the light was on (t=

20 s), which is around ~41% of the peak current from FTO//(SnO2/TiO2)//(PAA/Poly-10)5.

After three-cycle light on-off switches, the photocurrent stayed at ~7.0 µA/cm2.As

similarly discussed in previous section, we think photocurrent from

(FTO//(SnO2/TiO2)//(PAA/Poly-8)5 was the result of dissociation of Ru(bpy)32+

complexes from the Poly-8 (even though there might be some water oxidation

reactions), while the extra photocurrent from (FTO//(SnO2/TiO2)//(PAA/Poly-10)5 was

contributed by the water oxidation reactions near the photoanode. Bare

FTO//(SnO2/TiO2) anode was also studied, which gave slight photocurrent under

irradiation at the same condition, such weak photocurrent might be produced by the

water oxidation reaction under excitation of TiO2 semiconductor after absorbing small

amount of photons with λ < 400 nm (residual photons pass through the 400 nm cutoff

filter)

The photostability of the FTO//(SnO2/TiO2)//(PAA/Poly-10)5 was also tested under

the same condition (Figure 5-10B, Eappl = 0.2 V, 100 mW·cm−2 visible light, λ > 400 nm)

for 250 seconds. After a sharp decrease from peak current of ~18 µA/cm2 to 12 µA/cm2

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for the first 45 seconds (from t= 30 s to t= 75 s), the FTO//(SnO2/TiO2)//(PAA/Poly-10)5

maintained a stable photocurrent generation with slight decrease. At end of the test, a

photocurrent with ~10µA/cm2 was still observed (t= 280 s). From both water oxidation

and stability studies, it is confident to conclude that the Poly-10 could work as an active

photo catalyst to oxidize the water, with strong photo stability.

Summary

A polymer based catalyst-chromophore assembly (Poly-10) was synthesized for

the first time. The chemical structure and orientation of catalysts-chromophores were

studied from 1HNMR, FT-IR and UV-visible absorption spectroscopies. The intra-

electron interactions between chromophores and catalysts within the polymer were

confirmed by electrochemistry study. The polymer assembly was deposited onto

FTO//TiO2 and FTO//(SnO2/TiO2) semiconductor substrates by “Layer-by-Layer” method

to fabricate photoanode, followed by connecting with cathode (Pt wire) and Ag/AgCl

reference electrode to implement DSPEC. Several studies were carried out for the

photoanode regarding electrochemistry properties, photo-catalytic activities and

stabilities. Based on our work, the polymer assembly deposited photoanode was

capable to harvest and convert sun lights actively into solar fuels through oxidizing

organic substrates (PhOH and BnOH) and water. The photocurrent stability test also

confirms the strong stability of the polymer assembly in the photo-catalytic reactions.




utilizing CDCl3, CD3CN, CD3OD and D2O as solvents. The IR spectra were recorded on

a Perkin-Elmer Spectrum One FTIR spectrometer equipped with an attenuated total

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reflection (ATR) accessory using typically 128 scans at a resolution of 4 cm-1 in the

range of 4000-450 cm-1.





spectrophotometer. Steady-state emission spectra were recorded on a spectro-

fluorometer from Photon Technology International (PTI). Photoluminescence lifetimes

were obtained by using a single photon counting Fluo Time 100 (Picoquant)


Picosecond Pulsed Diode Laser.

Cyclic voltammetry experiments were performed using a CH Instruments 760E

bipotentiostat, with platinum wire as the counter electrode and Ag/AgCl as reference

electrode respectively. For the solution samples, experiments were performed in

aqueous solutions containing 0.1 M phosphate (pH= 7), with carbon electrode as

working electrode, the scan rate was 50 mV/s, and the concentration of the sample was

1 mM. Electrochemistry for ITO//(PAA/Poly-10)5 film was carried out in 0.1 M HClO4

aqueous solution (pH=1) at 20, 50, 100 mV/s scan rates.

The core-shell substrate FTO//(SnO2/TiO2) was prepared according to the

previous report.84 The deposition of the Poly-10 on the FTO//(SnO2/TiO2) substrate via

Layer-by-Layer method was performed according to similar procedures previously

reported.125 Briefly, the FTO//(SnO2/TiO2) electrode was dipped into PDADMAC solution

(pH ~6.6) for 15 min., followed by deposition of PAA (pH ~4.2) and Poly-10 (pH ~6.8) for

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15 min. each. The concentration of these polyelectrolyte solutions were controlled as1

mM with respect to the polymer repeat unit, and prepared by dissolution in Millipore

water and adjusted pH with HCl. After dipping in a polyelectrolyte solution, the

electrodes were rinsed three times with Millipore water, followed by dipping into

alternative polyelectrolyte solution. Multi-bilayers were implemented by repeating the

deposition of alternate layers of PAA polyanion and Poly-10 polycation on the electrode,

to get photo electrode in form of FTO//(SnO2/TiO2)//((PAA/Poly-10)5 (where 5 = the

number of bilayers). After the implementation, the film was dried under vacuum

overnight. The Poly-8 deposited core-shell electrode was prepared by the similar

procedure described above, in form of FTO//(SnO2/TiO2)//((PAA/Poly-8)5 (where 5 = the

number of bilayers).

The substrate (FTO//TiO2) was prepared according to previous report,185 and the

deposition of the Poly-10 and Poly-8 were performed by LBL method described above,

to make FTO//TiO2//((PAA/Poly-10)10 and FTO//TiO2//((PAA/Poly-8)10.

To investigate the cyclic voltammetry of polymer deposited film, the Poly-10 was

deposited onto ITO substrate using the Layer-by-Layer procedure described above,

denoted as ITO//(PAA/Poly-10)5.

Water oxidation study. Current–time (i–t) profiles were recorded at the

photoanode of a photoelectrochemical cell. The electrodes

(FTO//(SnO2/TiO2)//((PAA/Poly-10)5 or FTO//(SnO2/TiO2)//((PAA/Poly-8)5, Pt wire and

Ag/AgCl) were immersed in 25 ml 0.5 M KNO3 with 0.1 M phosphate buffer at pH = 7

under illumination (1 sun, 100 mW cm−2 from the Sun simulator light source, 400 nm

cutoff filter) with an applied bias of 0.2 V (versus. Ag/AgCl). The light was on and off

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every 20 seconds. For the photostabilty test, the experimental conditions are the same

as described in the water oxidation experiments except the photoanode was illuminated

for 280 seconds.

Oxidation reactions of PhOH or BnOH. Current–time (i–t) profiles were recorded

at the photoanode of a photoelectrochemical cell. The electrodes

(FTO//TiO2//((PAA/Poly-10)10 or FTO//TiO2//((PAA/Poly-8)10, Pt wire and Ag/AgCl) were

immersed in 25 ml 20 mM acetate/acetic acid buffer (pH= 4.6) under illumination (1 sun,

100 mW cm−2, from the Sun simulator light source, 400 nm cutoff filter) with an applied

bias of 0.2 V (versus. Ag/AgCl). The light was on and off every 20 seconds. The PhOH

or BnOH was added into the electrolyte solution and the solution was stirred for 5

minutes before light illumination. The concentration of PhOH in the solution was 4mM,

8mM and 12 mM after each injection, and the concentration of BnOH in the solution was

0.1M.

Materials


used without purification unless noted otherwise: Tetrakis(triphenyl phosphine)

palladium (Pd(PPh3)4), RuCl3 were purchased from Strem Chemical, triisopropylsilyl

acetylene and trimethylsilylacetylene was purchased from TCI. Copper (I) iodide (CuI),

Copper (I) bromide (CuBr), diisopropylamine ((i-Pr)2NH), triethylamine, tetra-n-

butylammonium fluoride in THF (1M), Na2CO3, Methanol, tetrahydrofuran(THF),

ammonium hexafluorophosphate, tetra-n-butylammonium chloride (TBACl), 2-

Acetylpyridine, 4-bromobenzaldehyde, KOH, ammonium solution, K2CO3, N,N,N′,N′′,N′′-

pentamethyldiethylenetriamine (PMDETA), polydiallyldimethylammonium chloride

(PDADMAC, Mw = 200,000- 350,000), polyacrylic acid (PAA, Mv ~450,000), and all

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other chemicals were purchased from either Sigma-Aldrich or Fisher Chemicals. Copper

(I) bromide (CuBr) was treated with acetic acid and washed with acetone before use.

Deionized water was purified by passage through a Millipore purification system. All

organic solvents used for photophysical studies were HPLC grade.


Compounds 1204, 4205, 6177, 7198 were synthesized according to corresponding

references. Ru(bpy)32+derivatized polystyrene (Poly-8, DP=20)185 was synthesized from

earlier study in the group. Azide substituted polystyrene (Poly-9, DP=15, Mn=2300,

PDI=1.20) was available from earlier study in the group.185

Compound 2

2-Bromo-1,10-phenanthroline (100mg, 0.39mmol) and 8-Quinolinylboronic acid

(80mg, 0.46mmol) were dissolved in mixture solvents of THF (15ml) and water (2ml)

and degassed with argon for 30mins. Pd(PPh3)4 (23mg, 0.02mmol) and K2CO3 (269mg,

1.95mmol) were added under argon protection. The mixtures were refluxed for 48hs,

under argon protection. After the reaction, the solvents were evaporated and re-

dissolved in water 20ml, followed by washing with dichloromethane (20ml) for 3 times.


the solution was evaporated under vacuum. The crude products were purified by silica

gel column with mixture solvents of hexane (20%) and ethyl acetate (80%) as eluent to

yield compound 2 (95mg, 79%).1H NMR (500 MHz, CD3Cl): δ 9.20 (dd, 1H), 8.95 (dd,

1H), 8.48 (dd, 1H), 8.41 (d, 1H), 8.34(d, 1H), 8.25(m, 2H), 7.93(dd, 1H), 7.87(d, 1H),

7.80(d, 1H), 7.74(t, 1H), 7.61(dd, 1H), 7.43(dd, 1H). 13C NMR (75MHz, CD3Cl): δ

121.10, 122.83, 126.44, 126.68, 126.93, 127.02, 127.85, 128.62, 128.91, 129.09,

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132.61, 135.06, 136.14, 136.55, 139.62, 146.32, 146.43, 146.67, 150.33, 150.47,

158.20ppm. ESI-MS: m/z=308.1197 [M+H]+ (calcd: 308.1182).

Compound 3

The compound 3 was synthesized with similar approach reported in literature.198

RuCl3·3H2O (84mg, 0.29mmol) and 2- (quinol-8’-yl)-1,10-phenanthroline (phenq, 90 mg,

0.29 mmol) were suspended in ethanol (25 ml) and refluxed for 6hs. After the reaction,

the mixture was cooled to room temperature and centrifuged. The precipitate was

collected and washed with ethanol, and dried. The [Ru(phenq)Cl3] was isolated as a

dark black solid (123 mg, 82%) and used without further purification.

Compound 5


dissolved in mixture solvents of EtOH (25 ml) and water (8 ml), followed by adding

triethylamine (1.5 ml). The mixtures were refluxed overnight. After the reaction, the

solution volume was reduced, followed by adding NH4PF6 (350 mg) to get precipitates.

The precipitate was collected, washed with water, and dried. The crude products were

purified by alumina column with mixture solvents of toluene (50%) and acetonitrile

(50%) as eluent to afford compound 5 as a red solid (22 mg, 10%). 1H NMR (500 MHz,

CD3CN): δ 9.12-9.13 (m, 2H), 9.07 (m, 1H), 9.02 (t, 2H), 8.61 (d, 2H), 8.47(d, 2H), 8.38

(d, 1H), 8.30 (d, 1H), 8.29 (d, 2H), 8.25 (d, 1H), 8.12-8.13 (m, 2H), 7.94 (d, 2H), 7.88

(m, 2H), 7.61 (d, 2H), 7.53 (dd, 1H), 7.41 (d, 2H), 7.23 (dd, 1H), 7.03 (m, 2H), 3.71 (s,

1H). 13C NMR (75MHz, CD3CN): δ 80.5, 82.6, 122.1, 122.3, 124.2, 124.9, 125.1, 126.6,

127.6, 127.8, 128.1, 129.9, 130.4 130.7, 133.1, 135.1, 136.0, 136.5, 138.2, 138.3,

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140.1, 143.2, 147.2, 147.3, 147.9, 152.2, 153.0, 154.8, 155.8, 157.2, 158.0ppm. ESI-

MS: m/z=368.0732 [M]2+ (calcd: 368.0720) (Overlapped with [M+2]2+).

Poly-10

Compound 5 (13.4 mg, 0.013 mmol), compound 6 (47.7 mg, 0.050 mmol) and

Poly-9 (8.5 mg, 0.053 mmol) were dissolved in 2.5 ml DMF and degassed for 1 h at

room temperature. Afterwards, copper (I) bromide (0.01 mmol, 1.4 mg) and PMDETA

(0.01 mmol, 1.4 mg) were added into the mixture solution. The resulting solution was

stirred at room temperature for 36 hs. After the reaction, ethanol (15 ml) was added to

the reaction flask. A brown-reddish precipitate was isolated by repeated centrifugation

and washed with 15 ml of ethanol. The precipitate product was dissolved in 3ml

acetone, followed by adding tetrabutylammonium chloride (50 mg) to get precipitates.

The precipitate was collected, washed by acetone and dried. The dry solid was

dissolved in Milli-Q water (10 ml) and purified in dialysis bag (cutoff = 5-6 kDa). The

dialysis bag was bathed in periodically replenished Milli-Q water for 72 hs. Water was

removed through freeze pump technique to afford orange-red solid (Poly-10, 40 mg,

75%).

Poly-11

Polymer 10 (15 mg) was dissolved in Milli-Q water (2 ml). The saturated NH4PF6

aqueous solution (1 ml) was dropped into the Poly-10 aqueous solution to obtain a solid

precipitate. The precipitate was collected, washed by methanol and dried, to afford

Poly-11 (16 mg, 86%).The Poly-11 was characterized by 1H-NMR and FT-IR (Figure 5-

4).

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CHAPTER 6 A NEW GENERATION OF POLYMERIC CHROMOPHORE WATER OXIDATION

CATALYST ASSEMBLY FOR SOLAR FUEL SYSTEM

Background


studied intensively to harvest and convert solar energy into chemical energy through

splitting water photo-chemically.34, 83-85 To achieve these goals, many approaches have

been developed to implement the photoanode that incorporates both light harvesting

sensitizers and water oxidation catalysts onto the surface of semiconductors. That

includes molecular co-deposition,90 “layer-by-layer” with Zr(IV)-phosphonate bridges,91,

92 molecular covalent bond,34, 85 cross-linked electropolymerization via reductive vinyl

coupling.93

In chapter 5, we discussed a rarely reported technique to prepare a polymeric

catalyst-chromophore assembly (Poly-10) and made a photoanode through “layer-by-

layer” method afterwards. From photocurrent studies, the photoanode was proven to be

active in harvesting and converting visible light actively into solar fuels through oxidizing

water and organic substrates. However, there are couples of weaknesses in Poly-10,

preventing its wider applications in artificial photosynthesis. First, the oxidation catalyst

applied is Ru(phenq)(tpy)2+, which is a great candidate to be incorporated to polymeric

system regarding stability and solubility, while relative low catalytic turnover rate limits

its application in making a robust photoanode. Second, the photoanode was prepared

via layer-by-layer method involving the sequential deposition of oppositely charged

polymers based on electrostatic interactions. The weaker bonds may render adsorption

quasi-reversible and unstable compared to regular covalent bonding linkage.129

Moreover, the polyelectrolyte multilayer structure, especially under large number of

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layers, may block the charge transfer and reduce charge injection yields. We report

here the preparation of a new generation polymeric assembly with strong charge

injection yields and robust catalytic efficiency through modifying structures for both

water oxidation catalyst and light harvesting sensitizer.

Ru(bda)(pic)2 (H2bda = 2,2′-bipyridine-6,6′ -dicarboxylic acid, pic = 4-picoline) has

been reported as one of the most robust molecular water oxidation catalysts since 2009

(turnover frequency = 19 – 32 s-1).90, 108, 114 It has been reported that a seven-coordinate

Ru(IV) dimer complex is involved as an intermediate in catalytic water oxidation in

based on experiments and calculations.108, 114 From initial studies, the electron injection

and back electron transfer not only depend on the molecular structures of sensitizer or

metal oxide, but also on the mode of binding to the metal oxide surface.128 Generally,

anchoring the light sensitizer to the semiconductor surface (e.g. TiO2, NiO) has been

proven to be a practical approach to achieve interfacial electron injection in high yield.206

Several anchoring groups, including carboxylic acids, phosphonates, ethers, amides,

siloxanes, acetyl acetonates, and cyanides have been tested, while carboxylic

acids/ionic carboxylate still remains ideal anchoring functional groups in terms of

absolute conversion efficiency from dye-sensitized solar cell (DSSC), with the formation

of ester-type linkages.127-129 However, the TiO2//HOOC-R linkage is reported not to be

stable in strongly acid or basic media.


Target Design and Synthesis

In our study, we employed the Ru(bda)(pic)2 (complex 1) as water oxidation

catalyst and carboxylate-derivatized Ru(II) polypyridyl complex (complex 2) as light

sensitizer, which were assembled onto azide substituted polystyrene scaffold, followed

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by depositing onto a metal oxide type semiconductor to fabricate photoanode, with the

target structure shown in Figure 6-1. The Ru(bda)(pic)2 complex can efficiently oxidize

water.90 And the carboxylate-derivatized Ru(II) polypyridyl complex is able to absorb

sun lights in UV-visible region, with anchoring groups bound onto TiO2 surface.

Figure 6-1. Illustration of the polymer assembly deposited photoanode.

Notably, we use carboxylate ester groups as anchors instead of carboxylic acid

groups for three reasons. First, the carboxylate ester-derivatized Ru(bpy)32+ type

complexes have higher Ru(III/II) oxidation potentials with ~1.40 - 1.54 V (versus NHE)

than 1.26 V (versus NHE) for Ru(bpy)32+, which can generate stronger thermodynamic

driving forces to oxidize catalysts, with further boost towards photocatalytic water

oxidation.103 Second, the carboxylate ester has been reported as an anchoring group to

immobilize onto semiconductor for the carboxylate linkage formed from its reaction with

surface hydroxyl groups on the semiconductor surface.207 Third, carboxyl acid group is

sensitive to its environment. For example, it stays in neutral form (-COOH) at low pH,

while it is deprotonated (-COO-) in neutral or basic pH. The neutral form (-COOH) in

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polymer may cause insolubility issues while salt form (-COO-) might introduce

challenges in operating counter ion-exchange for the polymer based on our previous

studies. Compared to the possible issues from carboxyl acid group, the carboxylate

ester groups would be more stable to the reaction environment.

Figure 6-2. Synthesis for the ethyne functionalized catalyst

The synthesis of the polymer assembly was first attempted by “object-oriented”

approach (Figure 6-2), the same approach we used to make Poly-10 in Chapter 5.

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Briefly, the ethyne ester-containing Ru(bda)(pic)2 derivatives (complex 4) and ethyne

ester-containing carboxylate-functionalized Ru(II) chromophores (complex 2) need to be

synthesized individually, followed by assembled onto polyvinyl azide backbone (PVBA)

in one pot. However, this approach did not work properly because of the failed

preparation of (Ru(bda)(pic)(DMSO) (complex 3) as shown in Figure 6-2. Complex 3 is

an important precursor to make complex 4, while it is hard to separate from complex 1

with high yield.

Figure 6-3. Synthesis of Poly-1.

Instead, we used a “graft to” approach (Figure 6-3) to prepare the

polychromophores. In this approach the ethyne functionalized pyridine (compound 5,

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20%) and complex 2 (80%) were reacted with PVBA polymer first to obtain the

polymeric chromophore-ligand assembly (Poly-1), followed by further metalation

reaction with complex 3 to prepare the polymeric chromophore-catalyst assembly (Poly-

2). In order to better identify the catalyst-chromophore ratio of the polymer, we would

like to make a catalyst-chromophore assembly with 30% catalyst sites and 70%

chromophore sites, so the reaction ratio for compound 5 is 30% and the ratio for

complex 2 is 70%. Consider that only complex 3 would react with the free pyridine

ligand along the polymer and unreacted complex 4 easily removed from polymer

assembly afterwards, we used the mixture of complex 3 and complex 4 to react with

Poly-1 to prepare Poly-2. In this way, we are able to avoid the separation work of

complex 3 from complex 4.

Figure 6-4. 1H-NMR of Poly-1. The Poly-1 was characterized by 1H-NMR spectroscopy in mixture solvents of CD3OD (90% in volume) and CD3CN (10% in volume).

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The compound 5 was prepared from isonicotinic acid in two steps with 50% yield

(synthesis and characterization section in Chapter 6). Complex 2 and PVBA (Mn= 2300,

DP= 15, PDI= 1.2) were available from earlier studies (Chapter 4 & 5). Click assembly

via an azide-alkyne Huisgen cycloaddition reaction allows attachment of ethyne -

containing compound 5 and ethyne ester-containing complex 2 onto polystyrene

scaffold, with the formation of Poly-1 (Figure 6-3). The Poly-1 was characterized by 1H-

NMR spectroscopy in a solvent mixture of CD3OD and CD3CN (90:10 v:v). The peak at

~2.25 ppm is attributed to 3 protons of –CH3 group (green color in Figure 6-4) of Ru(II)

polypyridyl complexes, with the assigned integration of 2.06, the broad peak at ~5.5

ppm is attributed to the protons from methylene group between triazole ring and

benzene ring (red color in Figure 6-4), and a minor peak at ~4.2 ppm is attributed to 2

protons of methylene group between pyridine ligand and triazole ring (blue color in

Figure 6-4). The proton from methylene group between triazole ring and Ru(II)

polypyridyl complex is overlapped by water peak at ~4.6 ppm. We were able to

calculate the ratio between pyridine ligand and chromophore sites from the resonance

peaks by two ways. First, we can accomplish the calculation based on pyridine sites:

Percentage of pyridine sites = (0.67/2)/(2.00/2)=0.34, therefore the percentage of

chromophore sites =1 – 0.34 = 0.66. Second, we can also calculate the ratio based on

chromophore sites: Percentage of chromophore = (2.06/3)/(2.00/2)=0.69, and the

percentage of pyridine sites = 1-0.69 = 0.31. From both calculating approaches, we get

similar results, with both comparable to reaction ratios (30% ligands and 70%

chromophore). The aromatic resonances from 7.0 - 9.0 ppm are highly similar with

Ru(II) polypyridyl complexes based polymer, and assigned to protons in bipyridine

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ligands of Ru(II) polypyridyl complex, overlapping some protons from free pyridine

ligand.

Figure 6-5. Synthesis of Poly-2

At the same time, complex 3 (mixed with complex 1) was prepared from

metalation reaction between picoline and Ru(bda)(DMSO)2 (Figure 6-5), and further

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react with Poly-1 for 5 hours to obtain Poly-2. Poly-2 was also characterized by 1H-NMR

spectroscopy, with all the nonaromatic resonance features remained while the aromatic

features displayed significant differences. We think the difference in aromatic resonance

was caused by successful metalation reaction, though we are unable to assign those

peaks due to of the overlapping of the aromatic features from both catalyst and ester-

containing Ru(II) polypyridyl complexes.

Photophysical Study

Figure 6-6. UV-visible absoprtion study.(A) UV-visible absorption spectra of model

catatyst and complex 2. (B) UV-visible aboprtion spectra for Poly-1 and Poly-2 normalzied at maximum MLCT absorption peak.

The UV-visible absorption of Poly-1, Poly-2 and corresponding Ru(II)-COOEt

chromophore (complex 2) and Ru(bda)(pic)2 catalyst (complex 1) units were measured

in a solvent mixture of acetonitrile and methanol (50:50 v:v). As shown in Figure 6-6-A,

the chromophore featured a strong ligand-based π→π* absorption band with maxima at

λ ∼300 nm and a strong dπ(Ru) → π*(bpy) metal-to-ligand charge transfer (MLCT)

band at λ ∼475 nm in the visible region. At the same time, the catalyst exhibited a

unique absorption features at 300 nm, and a weak absorption at λ ∼480 nm. Obviously,

complex 2 has higher molar absorptivity than complex 1 in MLCT region. As shown in

A) B)

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Figure 6-6B, where the UV-visible aboprtion spectra for Poly-1 and Poly-2 were

normalzied at maximum MLCT absorption peak, the Poly-1 basically reflected the

absorption features from Ru(II)-COOEt chromophore, while Poly-2 exhibited enhanced

absorption in the visible region of 500 nm - 600 nm, which was assigned to the

absorption from catalyst units. Interestingly, a broad peak with weak intensity was also

observed at ~700 nm, caused by the oxidized catalyst species. In perspective of UV-

visible absorption spectroscopy, both the chromophore and catalyst sites were

incorporated into polymer assembly successfully.

Figure 6-7. Emission study and life time measurements. Emission specra (excite at 455 nm) (A) and life time measurement (B) for Poly-1 and Poly-2.

The emission studies were also carried out for Poly-1 and Poly-2 in a solvent

mixture of acetonitrile and methanol (50:50 v:v) to study the possible energy or electron

transfer between the catalyst and chromophore sites in the polymer. As shown in Figure

6-7A, the Poly-1 was measured with an emission spectrum (λmax ∼ 665 nm), while the

emission of Poly-2 was 89% quenched compared to Poly-1. As discussed in Chapter 5,

we assumed that this quenching effect was caused by electron or energy transfer

between chromophore units and catalyst units along the polymer backbone. From the

A) B)

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time correlated single photon counting (TCSPC) experiment (Figure 6-7B), the excited

Poly-1 displayed two decay processes: The first one was 𝜏 = ~1100 ns with 83%

amplitude, which was assigned for normal emission decay. The lifetime is comparable

to small Ru(II) polypyridyl complex,184 the second component was 𝜏= ~320 ns with 17%

amplitude, which was possibly caused by the aggregation between metal-chromophore

sites along the polymer.184 However, in the case of Poly-2, there was an extra fast

decay process observed. We think such fast decay process was caused by site-site

energy or electron transfer between chromophore and catalyst units attached on the

polymer.


To study the oxidation potentials and further confirm catalyst-chromophore ratio

in polymer assembly, electrochemistry studies were carried out for Poly-1, Poly-2 and

Ru(bda)(pic)2. Particularly, Poly-1 and Poly-2 were studied on ITO film (preparation of

polymer deposited ITO film was described in instrumentation and methods section in

Chapter 6), while Ru(bda)(pic)2 was measured in solution with ITO glass as working

electrode. As shown in Figure 6-8, a reversible oxidation peak was recorded for

Ru(bda)(pic)2 with Ru(III/II)cata at 0.45 V, and a reversible wave was recorded for

ITO//Poly-1 with Ru(III/II) chrom at 1.35V as well. Notably, the ITO//Poly-2 exhibited

combined electrochemistry features from both Ru(bda)(pic)2 and Poly-1, with a

reversible oxidation peak at ~0.55 V (Contributed from catalyst sites) and the other

oxidation peak at ~1.35 V (Contributed from chromophore sites). These measured

potentials were in agreement with Ru(bda)(pic)2 and carboxylate-derivatized Ru(II)

polypyridyl complex from previous reports.90, 103

184

Figure 6-8. Electrochemistry studies. (A) ITO//Poly-2. (B) ITO//Poly-1. (C) Ru(bda)(pic)2

in solution. The polymer deposited ITO film (ITO//Poly-1 or ITO//Poly-2) was measured in 0.1 M HClO4 aqueous solution. The solution electrochemistry of Ru(bda)(pic)2 was measured in 0.1 M HClO4 aqueous solution, with ITO glass as working electrode, concentration of sample is ~1mM. All the electrochemistry studies conducted with scan rates = 50 mV/s, Pt as counter electrode and Ag/AgCl as reference electrode.

Interestingly, different from ITO//Poly-1, ITO//Poly-2 generated more intensive

anodic currents under potential of 1.2 V - 1.6 V. Particularly, at the potential of 1.6 V

when both the catalyst and chromophore sites were oxidized to higher states electro-

chemically, ITO//Poly-2 generated electric current density with ~12 µA/cm2, while Poly-1

only obtained current density with ~2 µA/cm2. It clearly implies that some chemical

reactions were occurring in oxidized ITO//Poly-2, which we assume are water oxidation

reactions.

185

Water Oxidation Reaction Study

Figure 6-9. UV-visible absorption study for photoanodes. (A). UV-visible absorption spectra for FTO//(SnO2/TiO2)//Poly-1 and FTO//(SnO2/TiO2)//Poly-2. (B) Image of FTO//(SnO2/TiO2)//Poly-2 and of FTO//(SnO2/TiO2)

The FTO//(SnO2/TiO2) was reported to block the back electron transfer efficiently,

therefore enhancing the photo-catalytic efficiency. Briefly, a mesoporous SnO2 film was

deposited onto a top fluorine doped tin oxide (FTO), with further modifying with a TiO2

overlayer deposited by atomic layer deposition (ALD). So we used FTO//(SnO2/TiO2)

core-shell substrate to fabricate the photoanode towards studying the water oxidation

reactions. The core-shell substrate was prepared according to literatures,84 followed by

immersing in Poly-2 aqueous solution(1 mM in 50% meOH and 50% acetonitrile) for

24hs to fabricate the photoanode, in form of FTO//(SnO2/TiO2)//Poly-2. The carboxylate

ester would undergo hydrolysis and react with the hydroxyl groups on the TiO2 surface

to form covalent carboxylate linkages. Poly-1 was also deposited onto core-shell

substrate using the similar method to fabricate FTO//(SnO2/TiO2)//Poly-1 as control

study. The photoanodes were characterized with UV-visible absorption spectroscopy

(Figure 6-9). Both photoanodes exhibited strong MLCT absorption in visible region at

~400 -500 nm with comparable absorption intensity. The λmax in MLCT for films is ~3 - 5

B) A)

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nm red shift compared to data measured in solution. In details, the MLCT intensity of

Poly-2 was ~0.35 at ~480 nm, while the intensity for Poly-1 was ~0.32 at ~478 nm. In

perspective of UV-visible absorption intensity, Poly-1 and Poly-2 are successfully

immobilized onto core-shell surface. If we ignore the molar absorptivity difference

between catalyst and chromophore, the surface coverage of Poly-1 in

FTO//(SnO2/TiO2)//Poly-1 was calculated as ~2.1*10-8 mol/cm2 and Poly-2 in

FTO//(SnO2/TiO2)//Poly-2 was calculated as ~2.3*10-8 mol/cm2.

Figure 6-10. Photocatalytic oxidation of Water. (A) Photocurrent–time traces with 20 second light off/on cycles for three anodes: FTO//(SnO2/TiO2)//Poly-2 (Solid red line), FTO//(SnO2/TiO2)//Poly-2 (Dashed blue line), and bare FTO//(SnO2/TiO2) (Dotted black line). (B) Photocurrent–time traces with 250 second light on for two anodes: FTO//(SnO2/TiO2)//Poly-2 (Solid red line) and bare FTO//(SnO2/TiO2) (Dotted black line). The data were collected in 0.5 M KNO3 aqueous solution with 0.1 M phosphate buffer (PH= 7) under illumination (1 sun, 100 mW cm−2, 400 nm cutoff filter) with an applied bias of 0.2 V versus. Ag/AgCl.

DSPEC device was implemented afterwards by connecting photoanode with

counter electrode (Pt wire) and Ag/AgCl as reference electrode, and immersed into

electrolyte solution (0.1 M phosphate buffer at pH 7 and 0.5 M KNO3 aqueous solution).

The photo-catalytic water oxidation activities were studied by measuring the

photocurrent densities generated from the photoanodes under illumination. From light

A) B)

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on-off experiments (Figure 6-10-A), the FTO//(SnO2/TiO2)//Poly-2 generated high

photocurrent density upon irradiation (t= 20 s) with visible light (1 sun, 100 mW cm−2,

400 nm cutoff filter) with an applied bias of 0.2 V versus. Ag/AgCl, while no obvious

photocurrent was detected when the irradiation light was off (t= 40 s). In details, the

peak current density was detected as ~30 µA/cm2 at the point when light was on (t=20

s), followed by gradual decrease to ~24 µA/cm2 after irradiation for 20 s (t= 40 s). The

photocurrent decreased to 0 simultaneously when the light was off and stayed

unchanged for another 20 seconds (t= 60 s). When the light was turned on again, the

peak photocurrent was back with ~24 µA/cm2 and reduced to ~22 µA/cm2 after 20

seconds (t= 80 s). After three-cycle of light on-off switches, the photocurrent density

was measured ~22 µA/cm2 (t= 120 s). Differently, FTO//(SnO2/TiO2)//Poly-1 only gave

weak photocurrent with ~4 - 6 µA/cm2 under irradiation, with ~20% of the current density

generated from FTO//(SnO2/TiO2)//Poly-2. The bare FTO//(SnO2/TiO2) substrate was

also studied at the same condition without any obvious photocurrent observed. The

extra photocurrent from FTO//(SnO2/TiO2)//Poly-2, compared to FTO//(SnO2/TiO2)//Poly-

1 clearly implied some water oxidation involved processes occurring around the

FTO//(SnO2/TiO2)//Poly-2 anode under the light illumination. The photo stability of

FTO//(SnO2/TiO2)//Poly-2 was also tested under the same irradiation conditions (Figure

6-10-B). After irradiation for 250 seconds, photocurrent density was observed with ~10

µA/cm2 (t= 280 s). Based on these two experiments, it is confident to conclude that the

FTO//(SnO2/TiO2)//Poly-2 could work as an active photoanode to oxidize the water, with

medium stabilities.

188

Summary

A polymer based catalyst-chromophore assembly assembly (Poly-2) was

synthesized with incorporation of (Ru(bda)(pic)2 catalyst and carboxylate-derivatized

Ru(II) polypyridyl complex (complex 2). The orientation of the Poly-2 was studied by

UV-visible absorption spectroscopy, and further confirmed by electrochemistry study.

From the electrochemistry studies, Poly-2 displayed intrinsic electric features from both

catalyst and chromophore units. Poly-2 was deposited onto FTO//(SnO2/TiO2) to

fabricate photoanode (FTO//(SnO2/TiO2)//Poly-2) with high surface coverage of

~.2.3*10-8 mol/cm2. Photo-catalytic activities and stabilities were studied for

FTO//(SnO2/TiO2)//Poly-2, from which, we conclude FTO//(SnO2/TiO2)//Poly-2 is

capable to harvest and convert sun lights actively into solar fuels through oxidizing

water.




utilizing CDCl3, CD3CN, CD3OD as solvents. The IR spectra were recorded on a Perkin-

Elmer Spectrum One FTIR spectrometer equipped with an attenuated total reflection

(ATR) accessory using typically 128 scans at a resolution of 4 cm-1 in the range of

4000-450 cm-1.





spectrophotometer. Steady-state emission spectra were recorded on a

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spectrofluorometer from Photon Technology International (PTI). Photoluminescence

lifetimes were obtained by using a single photon counting Fluo Time 100 (Picoquant)


Picosecond Pulsed Diode Laser.

Cyclic voltammetry experiments in this work were performed using a CH

Instruments 760E bipotentiostat, with platinum wire as counter electrode, and aqueous

Ag/AgCl electrode as reference electrode, measured in 0.1 M HClO4 aqueous solution

at 50 mV/s scan rate. For the solution samples, the ITO is the working electrode, the

concentration of the sample is 1 mM. For the electrochemistry of films, the ITO//Poly-1

and ITO//Poly-2 films work as working electrodes. The ITO//Poly-1 or ITO//Poly-2 film

was prepared with immersing the ITO glass (1 cm*2.5 cm) into a 5 ml vial containing 1

mM polymer solution (mixture solvent of methanol and acetonitrile (50:50 v:v)) for 24

hours, followed by rinsing with acetonitrile and methanol alternatively for 3 times and dry

under vacuum for 5 hours.

The core-shell substrate (FTO//(SnO2/TiO2)) was prepared according to the

references.84 The FTO//(SnO2/TiO2)//Poly-1 and FTO//(SnO2/TiO2)//Poly-2 were

prepared by dipping method.185 Briefly, the FTO//(SnO2/TiO2) core-shell substrate was

immersed in Poly-1 or Poly-2 solution (1 mM in mixture solvent of methanol and

acetonitrile (50:50 v:v)) for 24 hours, followed by resining with clean methanol and

acetonitrile sequentially for 3 times, and dried under vacuum for 5 hours. The

carboxylate ester would undergo hydrolysis and react with the hydroxyl groups on the

TiO2 surface to form covalent carboxylate linkages.

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Water oxidation study. Current–time (i–t) profiles were recorded at the

photoanode of a photoelectrochemical cell. The electrodes (FTO//(SnO2/TiO2)//Poly-1 or

FTO//(SnO2/TiO2)//Poly-2), Pt wire and Ag/AgCl) were immersed in 25 ml 0.5 M KNO3

with 0.1 M phosphate buffer at pH = 7 under illumination (1 sun, 100 mW-cm−2 from the

Sun simulator light source, 400 nm cutoff filter) with an applied bias of 0.2 V (versus.

Ag/AgCl). The light was on and off every 20 seconds. For the photostabilty test, the

experimental conditions are the same as described in the water oxidation experiments

except the photoanode was illuminated for 280 seconds.

Materials


used without purification unless noted otherwise: Copper (I) iodide (CuI), Copper (I)

bromide (CuBr), diisopropylamine ((i-Pr)2NH), triethylamine, chloroform, Na2CO3,

methanol, tetrahydrofuran(THF), tetra-n-butylammonium chloride (TBACl), isonicotinic

acid, ammonium hexafluorophosphate, propargylamine, N,N,N′,N′′,N′′-

pentamethyldiethylenetriamine (PMDETA), Thionyl chloride, picoline and all other

chemicals were purchased from either Sigma-Aldrich or Fisher Chemicals.

Ru(bda)(DMSO)2 was prepared according to literature.90 Copper (I) bromide (CuBr) was

treated with acetic acid and washed with acetone before use. Deionized water was

purified by passage through a Millipore purification system. All organic solvents used for

photophysical studies were HPLC grade.


Azide substituted polystyrene (PVBA, Mn=2300, DP=15, PDI=15).185 Ru-COOEt

chromophore(complex 2) were available from initial research projects.176 Ru(bda)(pic)2

(complex 1) was prepared from reported references.114

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Ethyne-pyridine(compound 5) was synthesized from modified procedure.208

Isonicotinic acid (343 mg,2.8 mmol) and thionyl chloride (15 ml) with 25 ml CHCl3

were mixed in a 50 ml flask and refluxed for 3 hours. Redundant thionyl chloride was

distilled off under reduced pressure to afford quantitative yield of the corresponding

chloride which was used directly in the next step. The residues collected, triethylamine

(5 ml) and propargylamine (5 ml) were mixed in 30 ml CHCl3 and stirred for 24 hours at

room temperature. After the reaction, the organic solvents were evaporated and

dissolved in 20 ml dichloromethane, followed by washing with water (20 ml) for 3 times.


the solution was evaporated under vacuum. The crude products were purified by

column with hexane: dichloromethane (20:1) as eluent to yield target compound (223

mg, 50%).

Poly-1

PVBA (0.093 mmol based on polymer repeating units, 15.0 mg), complex 2 (67

mg, 0.071 mmol), ethyne-pyridine (5.0 mg, 0.031 mmol) and PMDETA (0.02 mmol, 3.4

mg) with 3 ml DMF, 3 ml acetonitrile and 20 ml THF were mixed together and stirred for

1 h at room temperature under nitrogen. After adding copper (I) bromide (0.02 mmol,

2.8 mg) into the polymer solution, the mixture was then stirred at room temperature for

36 hours. After the reaction, evaporate most solvents and added into ether. A brown-red

precipitate was isolated by centrifugation and under drying overnight. The precipitated

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product was dissolved in mixture solvents of 2 ml acetonitrile and several drops of

methanol, the insoluble impurities were removed by centrifugation. Saturated TBACl

acetonitrile solution was added into the solution, a brown-red precipitate was isolated by

centrifugation, followed by washing with acetonirtile and ether, and dried under vacuum

overnight. The precipitated product was dissolved in 10ml Milli-Q water, filtered and

purified in dialysis bags (cutoff=6000-8000) for 3 days, and dried with free pump. 40 mg

Poly-1 was obtained in orange red color.

Poly-2

20 mg Poly-1 in Cl- form was converted back to PF6- form (26 mg collected). A

mixture of 4-picoline (18.6 mg, 0.2 mmol) and Ru(dba)(DMSO)2 (100 mg, 0.2 mmol) in

mixture solvents of methanol (10 mL) and acetonitrile (10 ml) was refluxed for 1 hour

under N2 protection. 20 mg Poly-1 (PF6- form) was added into the reaction solution. The

solution was refluxed for another 5 hours. After the reaction, the mixture solution was

filtered to obtain a dark red solution. Most organic solvents were evaporated, and the

rest solution was added into ether. Orange red precipitates were isolated by

centrifugation and washed with acetonitrile and ether, the precipitate was collected by

centrifugation and dried overnight. The precipitated product was dissolved in 10 ml Milli-

Q water, filtered and purified in dialysis bags (cutoff = 6000-8000) for 3 days, and dried

with free pump. 13 mg brown solid was collected (Poly-2).

193

CHAPTER 7 CONCLUSION

π-Conjugated molecular donor-acceptor systems have attracted a lot of interests.

Their interesting optical and electronic properties allow the possibility for tremendous

applications in molecular electronics and organic solar cells. Also, their well-defined

conjugated structures are good models for theory studies in understanding electron

transfer and energy transfer processes and implementing complex device architectures,

such as dye sensitized photoelectrosynthesis cell. In this dissertation, two research

projects are First discussed about the electron transfer and electron recombination in

donor-acceptor type oligomers, and how their rates are affected by conjugation length

and conjugation block with different donating ability. Based on the understanding in

energy transfer/electron transfer processes, three research projects are further

discussed about the device implementation for dye sensitized solar cell and dye

sensitized photoelectrosynthesis cell with the goal of solar energy harvesting and

conversion.

In Chapter 2, we studied conjugation length dependent electron

transfer/recombination processes in oligo-phenylene ethynylene (OPE)-naphthalene

diimide derivative (NDI) oligomer system. A series of oligomers (PEn-NDI) with different

conjugation length were prepared which featured the conjugation length controlled

oligo-(phenylene ethynylene) (OPE) conjugated backbone and end capped naphthalene

diimide derivative (NDI) group. Under illumination, intra-electron transfer process from

OPE backbone to NDI was proved as the dominated process (96.7% < < 99.8%). The

energetics of oligomers was investigated by electrochemistry, steady state absorption

and fluorescent emission spectroscopy studies. With the increasing of conjugation

194

length, all PEn-NDI oligomers show similar charge separation free energy change (-2.02

eV ~ -1.96 eV) and charge recombination free energy change (-1.19 eV ~ -1.10 eV).

Bimolecular studies of TIPS protected OPE oligomer and MV2+ indicates the

charge transfer based quenching effects between triplet states of oligomers and

quenchers. Femtosecond transient absorption (fs-TA) spectroscopy and transient

infrared (TRIR) spectroscopy studies clearly demonstrate the ultrafast charge

separation process occurring in the oligomer in time scale <10 ps to form OPE+• and

NDI-•, followed by the relatively slower back electron transfer process in 400 ps ~

1100ps.

Notably, we demonstrated that increasing of conjugation length exponentially

slow down the charge recombination rates in OPE-NDI system owning low conjugation,

which could be explained well by Marcus super-exchange mechanism. In the OPE-NDI

system where donor has long conjugation length, no apparent changes of

recombination rates were not observed even though conjugation gets further expanded.

This indicates that some other electron transfer mechanisms may be involved for in long

conjugated bridged donor-acceptor system. Furthermore, based on solvent dependent

kinetics study, the charge recombination kinetics were proven to be sensitive with

solvent polarity and hydrogen bonding.

In Chapter 3, we studied the electron transfer and recombination processes for

naphthalene diimide derivative end capped tetrathiophene-tetraphenyleneethynylene

diblock oligomer system (T4PE4NDI) and its derivatives with shorter

phenyleneethynylene bridge (T4PE2NDI and T4NDI. The diblock oligomers exhibited

combined photophysical and electrochemistry properties. Under illumination, intra-

195

electron transfer process from T4 conjugation block to NDI was proved to be a

dominated process (98.3% < < 99.8%) for T4PE2NDI and T4NDI, while a competitive

energy transfer process observed when exciting the T4PE4NDI, resulting in much lower

electron transfer efficiency in 44.8%.

Time correlated single photon counting (TCSPC) study confirmed the results

from fluorescent quantum yield study that T4PE4NDI has a lifetime = ~0.4 ns, which is

very close to the TIPS protected oligomers, with = ~0.5 ns. Whereas, both T4PE2NDI

and T4NDI have exhibited extreme short life times (within the instrument detection limit).

The significant differences in photophysical properties are caused by their different

charge transfer kinetics patterns. Bimolecular electron transfer experiments between

T4PE4TIPS and paraquat (MV2+) demonstrated the absorption spectra for T4 based

radical anions (T4+•, T4 PE2

+•, T4 PE4+• ), with λmax=700 nm – 725 nm. Moreover, the

bimolecular charge transfer was proven to be a triplet states involved process.

Distinctive UV-visible absorption for T4 and PE4 absorption allow the selective

excitation for either conjugation block, from which, different charge separation and

recombination kinetics were observed. This is First confirmed by excitation wavelength

dependent fluorescent quantum yield studies (f = ~ 0.21, exciting at 370 nm and f = ~

0.28 when exciting at 420 nm), which implies the wavelength dependent charge transfer

behaviors with 34.4% when exiting PE4 block and 9.7% when exciting T4 conjugation

block. The fs-TA spectroscopy supplies clear evidences for excitation wavelength

dependent charge transfer: no absorption spectra observed for generation of T4+• when

exciting 370 nm, while a rising spectra observed forT4+• production at ~1200-1300 nm,

along with the unique absorption peak at ~ 480 nm for NDI-• formation. Moreover, the

196

kinetics monitored at 1200 nm gives charge separation life time with = ~ 14 ps,

followed by charge recombination process with = ~1000 ns (exiting at 370 nm). The

“long” 14 ps charge separation ( = ~3 ps excitation at 420nm) implies some concurrent

energy transition states when charge separates.

In Chapter 4, we synthesized polystyrene based Ru(II) metal chromophores and

studied their photophysical properties and applications in energy conversion. Because

of site-site hopping transfer mechanism, polymer based light harvesting become more

practical for application of energy harvesting and conversion. The polymer backbones

were prepared by nitroxide-mediated radical polymerization with controlling average

molecular weight. Pendant Ru(II) polypyridyl complexes were grafted to the polymer

backbone by azide–alkyne click chemistry to afford full chromophore loaded polymers.

The photophysical and electrochemical properties of the series of PS-Ru polymers were

characterized in solution and investigated as a function of polymer chain length and

solvent, with the characteristics of each individual Ru(II) polypyridyl unit maintained in

the polymer. Emission quantum yield and lifetime studies reveal that the metal-to-ligand

charge transfer (MLCT) excited states are quenched to a variable extent depending on

the molecular weight of the polymers, consistent with facts that polymers withlonger

chain lengths bring self-quenching in higher possibility.

With the similar synthesis approach, polystyrene-based Ru(II) polypyridyl

chromophores with carboxylic acid (–COOH) anchoring groups was also prepared and

immobilized onto metal oxide films to make DSSC solar cells. The IPCE of the DSSC

was measured as 24 % at 480nm, which corresponds closely to the MLCT band

197

maximum for the Ru(II) polypyridyl chromophores, and the overall power conversion

efficiency (η) was measured as 0.33%.

In Chapter 5, we designed and synthesized a polymeric chromophore-catalyst

assembly (Poly-10) to achieve the water oxidation reactions. The 20% water oxidation

catalysts, Ru(phenq)(tpy)2+, and 80% Ru(II) polypyridyl complexes were assembled to

azide substituted polystyrene scaffold by azide–alkyne click chemistry reaction. The

catalysts and polymer structures were characterized by NMR and FT-IR spectroscopies.

IR spectroscopy confirmed the click reaction is quantitatively complete.

The UV-visible absorption studies of polymeric chromophore-catalyst assembly

show combined absorption features from both chromophore and catalysts. The

significantly quenched fluorescence emission indicates the interactions between

catalyst and chromophores, caused by energy transfer or/and electron transfer, while

the electron interactions between catalysts and chromophore units were confirmed by

electrochemistry experiments. With “Layer-by Layer” method, polymers were deposited

on TiO2 and TiO2/SnO2 substrates, to prepared photoanodes, and implement DSPEC

device. The polymer assembly deposited photoanode was capable to harvest and

convert sun lights actively into solar fuels through oxidizing organic substrates (PhOH

and BnOH) and water

In Chapter 6, we developed a novel polymer based catalyst-chromophore

assembly assembly (Poly-2), with the incorporation of (Ru(bda)(pic)2 catalyst and

carboxylate-derivatized Ru(II) polypyridyl complex, with the aim to boost the catalytic

activities and charge injection efficiency. The orientation of the polymer assembly was

studied by NMR and UV-visible absorption spectroscopies, and further confirmed by

198

electrochemistry study. From the electrochemistry studies, the polymer assembly

displayed intrinsic electric features from both catalyst and chromophore units. The

polymer assembly was deposited onto FTO//(SnO2/TiO2) by dipping method to prepare

photoanode, followed by implementing into DSPEC. Photo-catalytic activities and

stabilities were studied for photoanode. Based on our work, the polymer assembly

deposited photoanode is capable to harvest sun lights and convert actively into solar

fuels through oxidizing water.

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BIOGRAPHICAL SKETCH

Junlin Jiang was born in 1987 in Bazhong, Sichuan, China, where he was raised

and went to high school. At the age of 18, he moved to Shanghai for college and got a

B.S degree in chemistry in the year of 2010. After that, he worked in the solar energy

industry for 1 year as a project manager and moved to Gainesville, FL for his graduate

study at University of Florida in 2011, pursuing a Doctor of Philosophy degree in

chemistry. Under the supervision of Dr. Kirk S. Schanze, he focused his research on

charge transfer studies and applications in light harvesting and solar fuels generation.

After his graduation, he will go back to China and pursue a career in chemical industry.

Documents

© 2016 Junlin Jiangufdcimages.uflib.ufl.edu/UF/E0/05/06/44/00001/JIANG_J.pdfI also want to give special thanks to Weijia Niu, Guagua and Niannian for the happy memories we had together