CHARGE TRANSPORT MECHANISMSIN ORGANIC MATERIALS

Michael R. WasielewskiDepartment of Chemistry and Institute for Nanotechnology

Northwestern University, Evanston, IL 60208-3113

+ -+ -

hν e-

NN

O

O

O

O

O

O

N

N N

NZn

hν e-hν e-

NN

O

O

O

O

O

O

N

N N

NZn

+ -

Energy Transfer, 21 ps

Electron Transfer, 7 ps

Energy Transfer, 21 ps

Electron Transfer, 7 ps

SUPEREXCHANGE: COHERENT ET

D B1 B2 B3 A

kET = k0 e-βr

HOPPING: INCOHERENT ET

D B1 B2 B3 A

kET = k0 (1/r)

Molecular Wire Behavior inp-Phenylenevinylene Oligomers

Charge Separation

WIRE N

O

O

N

O

O

C8H17

Donor Acceptor

RO

OR

RO

OR

RO

OR

R = 2-ethylhexylWire = 1.

2.

3.

4.

5.

Charge Recombination

Davis, W. B., W. A. Svec, Ratner, M.A., Wasielewski, M.R., Nature, 1998, 396, 60-63.

Rylenes Absorbing the Entire Solar Spectrum• Chemically robust, easily oriented due to their rectangular shape• Excellent chromophores as well as electron donors and acceptors• Strong tendency to π stack in a variety of solvents and in the solid

R = 3,5-di-t-butylphenoxy

N

O

O

R

R

N(CH2)nNN

O

O

O

O

R

R

PDI 5PMI (n=0), 6PMI (n=1)

5PDI (n=0), 6PDI (n=1)

N

O

OR

R

PMI

NN

O

O

O

O

N

N

(CH2)n

(CH2)n

R

R

R

R

NN

O

O

O

O

QDI

Acceptor-Bridge-Donor Series

N N

RO

ORO

O

O

O

N N

RO

ORO

O

O

O

N N

RO

ORO

O

O

O

N N

RO

ORO

O

O

O

N N

RO

ORO

O

O

O

N S

N S

N S

N S

N S

Michael Ahrens

p-Phenylene Oligomer Properties

Eox. (vs. SCE) UV-vis250

350

2.4 V

1.85 V

1.60 V

1.47 V

~1.4 V

1.34 V

Meerholz, K., Heinze, J., Electrochimica Acta, 1996, 41(11/12), 1839-1854.

UV-Vis Spectra andSpectroelectrochemistry

300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Abs

orba

nce

(nor

mal

ized

)

Wavelength (nm)

1 2 3 4 5

400 500 600 700 800

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Wavelength (nm)

∆A

NN

O

O

O

O O

O

C8H17C8H17

potential = 0 eVpotential = -0.6 eV

E0X= 0.83 eV vs. SCE

PDI Properties:RobustEred = -0.53 V Eox. = 1.7 VEs = 2.25 eVFluorescence Quantum yield ~ 1Intense Radical Anion Absorption

Daub, J., et. al., J. Phys. Chem. A, 2001, 105, 5655-5665.

500 600 700 800-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

∆ A

Wavelength, nm

3 ps 400 ps

Stimulated emissionfrom 1*PDI

Transient Spectroscopy of PTZ+-Ph2-PDI-

Observation of PDI radical anionat 720 nm

Stimulated emission Decay from 1*PDI at 620 nm

NN

OR

RO

O

O O

O

C8H17NS

0 100 200 300 400 500 600 700 8000.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

∆ A

Time, ps

τ = 202 ps

Louise Sinks

Distance Dependence of Charge Separation forPTZ-(Ph1-5)-PDI

β = 0.46 Å-1

12 14 16 18 20 22 24 26 28 30 32107

108

109

1010

1011

n=4

n=5

n=3

n=2

n=1k C

S (

s-1)

rDA (Å)

Energy Levels Relevant to Charge Separation

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

n = 5n = 4n = 3

n = 2

PTZ-Phn-3*PDI

PTZ-Phn+.-PDI-.

PTZ+.-Phn-PDI-.

PTZ-Phn-PDI

PTZ-Phn-1*PDI

Ene

rgy

(eV

)n = 1

n = 5n = 4n = 3n = 2n = 1

PDI Radical Anion Decay and 3*PDI Formation

NN

OR

RO

O

O O

O

C8H17NS

50 100 150 200 250 300

0.0

0.2

0.4

0.6

0.8

1.0

∆A (N

orm

aliz

ed)

Time (ns)

723 nmτ = 21 ns

50 100 150 200 250 300

-0.015

-0.010

-0.005

0.000

0.005

0.010

∆A

Time (ns)

450 nmτ = 20 ns

Singlet and Triplet Radical Ion Pair States

B(2J)

(S)

(T+1)

(T0)

Splitting of Triplet Levelswith Applied B-Field

(T-1)

2J

Ener

gy (a

rbitr

ary

units

)

Magnetic Field Strength

∑−∆

ΨΨ=∆=

nnRP

nelRPST E

VEJ

2

2

VRPS-S0

VRPT-T1

hfc

3(D-A)

VRPS-S1

3(D+-A-)1(D+-A-)

D-A

1*D-A

Ene

rgy

(arb

itrar

y un

its)

[ ] ( )∑∞

= ⎥⎥⎦

⎤

⎢⎢⎣

⎡ ++∆−−=

0

22

4exp

!exp

412

n B

TCR

n

BDAET Tk

nGnSS

TkVk

λωλ

πλπ ηη

Spin-Spin Interactions in Wire-Like Systems

0 50 100 150 2000.7

0.8

0.9

1.0

1.1

1.2

1.3

Rel

ativ

e Tr

iple

t Yie

ld (T

/T0)

Magnetic Field (mT)0 2 4 6 8 10 12

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

Rel

ativ

e R

P Y

ield

(RP

/RP 0)

Magnetic Field (mT)

PTZ+. Ph3 PDI-. PTZ+. Ph4 PDI-.

2J = 31 mT 2J = 6.2 mT

Emily Weiss

Distance Dependencies in Wire-Like Phn Systems

Spin-Spin Exchange Interactionin PTZ+.-(Ph)n-PDI-.

Charge Recombination Rate toTriplet PDI

β = 0.67 Å-1

16 18 20 22 24 26 28 30 321

10

100

1000

n= 5

n= 4

n= 3

2J (

mT

)

rDA (Å)

n= 2

12 14 16 18 20 22 24 26 28 30 32106

107

108

109

n= 5n= 4

n= 3

n= 2

k CR (s

-1)

rDA (Å)

n= 1

Energy Levels Relevant to Charge Recombination

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

n = 5n = 4n = 3

n = 2

PTZ-Phn-3*PDI

PTZ-Phn+.-PDI-.

PTZ+.-Phn-PDI-.

PTZ-Phn-PDI

PTZ-Phn-1*PDI

Ene

rgy

(eV

)n = 1

n = 5n = 4n = 3n = 2n = 1

PTZ – (FL)n – PDI

NN

O

O

O

O

O

ONS

n

C6H13C6H13

Eox (FL)n n = 1-4: ~1.3 V vs. SCE (in DCM)

Eox (PTZ) = 0.8 V vs. SCEEox (PDI) = -0.5 V vs. SCE

300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Abs

orba

nce

(nor

mal

ized

)

Wavelength (nm)

n = 1 n = 2 n = 3 n = 4

PTZ

FL1

FL2

FL3

FL4 PDI

Randall Goldsmith

Charge Recombination through Oligofluorenes

100 200 300 400 500

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

∆ (N

orm

aliz

ed)

nanoseconds250 500 750 1000 1250 1500

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

∆A

nanoseconds100 200 300 400 500

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

∆A

nanoseconds100

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

200 300 400 500

nanoseconds

∆A

n = 1 n = 2 n = 3

τCR = 32 ns τCR = 266 ns τCR = 51 ns τCR

Kinetic Traces monitoring the formation of 3*PDI at 455 nm

n = 4

= 22 ns

-200 0 200 400 600 800 1000 1200 1400 1600

0.6

0.8

1.0

1.2

1.4

1.6

Rel

ativ

e Tr

iple

t Yie

ld (T

/T0)

Magnetic Field (mT)0 100 200 300 400 500

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

Rel

ativ

e Tr

iple

t Yie

ld (T

/T0)

Magnetic Field (mT)0 20 40 60 80

0.80

0.85

0.90

0.95

1.00

1.05

1.10

Rel

ativ

e Tr

iple

t Yie

ld (T

/T0)

Magnetic Field (mT)

n = 2 n = 4n = 3

2J = 240 2J = 40 2J = 6 mT

Magnitude of the Spin-Spin Exchange Interaction in PTZ+.-(FL)n-PDI-.

Distance Dependencies in Wire-Like FLn Systems

Spin-Spin Exchange Interactionin PTZ+.-(FL)n-PDI-.

Charge Recombination Rate toTriplet PDI

20 22 24 26 28 30 32 34 36 38 401

10

100

1000

n = 4

n = 3

2J (

mT)

rDA (Å)

n = 2

10 15 20 25 30 35 40106

107

108

n = 4

n = 3

n = 2k C

R (s

-1)

rDA (Å)

n = 1β = 0.23 Å-1

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

n = 1-4

PTZ-FLn-3*PDI

PTZ-FLn+.-PDI-. PTZ+.-FLn-PDI-.

PTZ-FLn-PDI

PTZ-FLn-1*PDI

Ene

rgy

(eV

)

n = 5n = 4n = 3n = 2n = 1

Energy Levels Relevant to Charge Recombination

Charge Transport in Self-AssembledArtificial Photosynthetic Systems

hν e-

NN

O

O

O

O

O

O

N

N N

NZn

hν e-hν e-

NN

O

O

O

O

O

O

N

N N

NZn

ZnTTPEox= 0.82 eVEs = 2.06 eVΦf = 0.035

PDIEred= -0.53 VEs = 2.25 eVΦf = 1

T. van der Boom et al., J. Am. Chem. Soc.124, 9582-9590 (2002).

• The first two equations predict that• B1/2 = 1.1 mT.• The observed B1/2 = 68 mT is 60x larger.• This could be due to:

1) uncertainty broadening2) a distribution of 2J values

2/12 )1( ⎥

⎦

⎤⎢⎣

⎡+= ∑

kkkiki IIaB

21

22

21

2/1)(2

BBBBB

++

=

Magnetic Field Effect on Charge Recombination in(ZnTTP-PDI4)n Nanoparticles in Solution

B(2J)

(S)

(T+1)

(T0)

Zeeman Splitting of theTriplet Energy Levels

(T-1)

2J

Ene

rgy

Magnetic Field 0.0 0.1 0.2 0.3 0.4 0.50.92

0.94

0.96

0.98

1.00

1.02

Rel

ativ

e Tr

iple

t Yie

ld

Magnetic Field (T)

17 mT

68 mT

η≅⋅τ2/1B

Radical Ion Pair Distances within (ZnTPP-PDI4)n

1.48 nm

1.60 nm

1.78 nm

2.01 nm

2.27 nm

2.55 nm

2.84 nm

3.15 nm

3.46 nm

1.44 nm

3.78 nm

ZnTPP PDI

The fact that 2J = 17 mTindicates that the averageradical ion pair distance is21 Å. This corresponds toa PDI molecule 5 layersremoved from thelayer in which the initialradical ion pair wasgenerated.

hν e-

NN

O

O

O

O

O

O

N

N N

NZn

hν e-hν e-

NN

O

O

O

O

O

O

N

N N

NZn

The observed B1/2 = 68 mTImplies that khop = 4 x 1010 s-1

Symmetry-Breaking in the Excited State Leads to Quantitative Charge Separation in Dimers of 5PDI, a Green Chlorophyll a Mimic

• Strong absorber of 600-800 nm light.• EOX = 0.68 V and ERED = -0.76 V vs. SCE• The π-stacked cofacial chromophores

undergo symmetry breaking in theexcited state leading to quantitativecharge separation.

N N

N

N

O

O

O

O

5PDI

O O

N

O O

O

N

N

N

O

N

O O

O

N

N

N

300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

(AU

)

W avelength (nm )

cof-5PDI2 (to luene) 5PDI (to luene)

cof-(5PDI)2

J. M. Giaimo et al., J. Am. Chem. Soc. 124, 8530-8531 (2002).

cof-(5PDI)2

O

O N

O

O

O

N

N

N

O N

O

O

O

N

N

N

hν

τCS <150 fs τCR =880 ps

500 550 600 650 700 750 800-0.03

-0.02

-0.01

0.00

0.01

∆A

Wavelength (nm)

500 600 700 800

5PDI

cof-(5PDI)2

Combining Light-harvesting and Charge Separation in a Self-assembledArtificial Photosynthetic system Based on Perylenediimide Chromophores

NN

N

N

O

O O

O

N

N

O

O

O

O

O

O

N

N

O

O

O

O

O

ON

N

O

O

O

O

O

O

N

N

O

O

O

O

O

O

5PDI

PDI

PDI

400 500 600 700 8000.0

0.5

1.0

1.5

Abs

orba

nce

Wavelength (nm)

Chloroform Toluene

B. Rybtchinski et al., J. Am. Chem. Soc. 126, 12268-12269 (2004).

Small-Angle X-ray Scattering Structural Studies of 2 x 10-4M 5PDI-PDI4 in Toluene Solution

Guinier Plot PDF PlotScattering Intensity

0.1

100

Inte

nsity

q, Å-1

0.002 0.004 0.006 0.008

100

150

200

Rg = 18.1± 0.03

Inte

nsity

q20 20 40 60

0.0

0.4

0.8

P(r)

r, Å

Compound 1 Calculated dimer

Small-Angle X-ray Scattering Structural Studies of 2 x 10-4M 5PDI-PDI4 in Toluene Solution

Simulated AnnealingReconstruction of the Aggregate Shape

500 600 700 800

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

∆A

Wavelength (nm)

0.6 ps 2.1 ps 9.1 ps 29 ps 306 ps 506 ps

A) 680 nm excitation

500 600 700 800-0.06

-0.04

-0.02

0.00

0.02

∆A

Wavelength (nm)

0.6 ps 0.9 ps 4.0 ps 20 ps 157 ps

B) 550 nm excitation

Transient Absorption Spectra of (5PDI-PDI4)2 in Toluene following Laser Excitation at 680 nm and at 550 nm

Energy Transfer, 21 ps

Electron Transfer, 7 ps

Energy Transfer, 21 ps

Electron Transfer, 7 ps

τCS = 7 psτCR = 420 ps

Summary:

•Wire-like behavior in electron donor-bridge-acceptor systems requires near-resonant energy level matching between a weakly-coupled electron donor and bridge.

•Using direct measurements of magnetic exchange interactions as a function of distance, the superexchange and hopping contributions to the overall electron transfer rate can be dissected.

•Photoexcitation of self-assembled (ZnTPP-PDI4)n arrays initiates an ultrafast charge separation that occurs with near unit efficiency to generate a coherent radical ion pair state. One of the spins hops between several closely-coupled electron acceptors.

•Self-assembly of two types of robust perylenediimide chromophores5PDI (red-absorber) and PDI (green absorber) are used to produce an artificial light-harvesting antenna structure that in turn induces self-assembly of a functional special pair that undergoes ultrafast, quantitative charge separation, (5PDI-PDI4)2 .

Wasielewski Group – Winter 2004

Funding: DOE, NSF, ONR, DARPA, PRF

Collaborations: David Tiede and Andrew GosheArgonne National Laboratory