Introduction

A Thesis submitted in partial fulfillment of the requirements for the degree of

Doctor of Philosophy

Qureshi Mohammad

!"

#$

Introduction

STATEMENT

I hereby declare that the work embodied in this thesis entitled, “Design & Fabrication of

benzthiazole based white light emitting diodes: Synthesis, spectroscopic and semi

empirical studies” has been carried out by me under the supervision of Dr. S. Sundar

Manoharan.

In keeping with the scientific tradition, whatever work done by others has been utilized,

due acknowledgement has been made.

IIT Kanpur Qureshi Mohammad

December 2004 Candidate

Introduction

Department of Chemistry

Indian Institute of Technology Kanpur, INDIA

CERTIFICATE 1

Certified that the work presented in this thesis entitled “Design & Fabrication of benzthiazole based white light emitting diodes: synthesis, spectroscopic and semi empirical studies” by Mr. Qureshi Mohammad, Department of Chemistry, Indian Institute of Technology, Kanpur has been carried out under my supervision and has not been submitted elsewhere for a degree.

IIT Kanpur Dr. S. Sundar Manoharan

December 2004 Thesis Supervisor

Department of Chemistry

Indian Institute of Technology Kanpur

Kanpur – 208 016

Introduction

DEPARTMENT OF CHEMISTRY

INDIAN INSTITUTE OF TECHNOLOGY KANPUR, INDIA

CERTIFICATE II

This is to certify that Mr. Qureshi Mohammad has satisfactorily completed all the courses required for the Ph. D. degree programme. The courses include:

CHM 637 Molecular Spectroscopy

CHM 664 Modern Physical Methods in Chemistry

CHM 693 Chemical Approaches to the Synthesis of Advanced Materials

MS 601 Structural & Magnetic Properties of Materials

CHE 633 Principles of Heterogeneous Catalysis

CHM 670 Scientific Instrumentation

Mr. Qureshi Mohammad was admitted to the candidacy for Ph.D. degree in 2000 after he successfully completed the written and oral qualifying examinations.

Head, Convener,

Dept. of Chemistry Departmental Post – Graduate Committee

Department of Chemistry

IIT Kanpur.

Introduction

Acknowledgements

My sincere thanks are due first to my thesis supervisor and Sir, Professor S. Sundar Manoharan for guiding me in scientific way for the last five years and who taught me that the worst sin is the lack of aggression, ethics and enthusiasm. I am always thankful to him for keeping confidence in me while introducing to this challenging field. His valuable suggestions and positive criticism always kept me on top gear throughout my research stay at IITK.

I would like to take this opportunity to express my gratitude to Drs. Matthias Koeberg, Jingsong Haung, Donal Bradley, Imperial College, London for helping me in some of the time decay, time resolved and electroluminescent measurements. I am also grateful to Dr. C. M. Schneider and Dr. Stefan Cramm, IFF, Juelich, Germany for helping me in XPS and UPS measurements and their useful discussions. I also thank Dr. Periasamy, TIFR, Mumbai for letting me use the time decay facilities in his laboratory. I wish to thank Dr. Y. N. Mohapatra and Samarender Pratap Singh for their help in electroluminescent measurements. Thanks due to Dr. Asima Pradhan and Sharad Gupta for their help in PL measurements. Thanks due to Mr. Kuntal Pal for helping me in electro chemical measurements.

Thanks are due to my senior colleagues Dr. Ranjan K Sahu and John Prassana who taught me basics in the initial stages of my research, to my colleagues, Dr. Manju Lata Rao for her support and encouragement throughout my stay. Thanks are due to Brajendra Singh, Vimlesh Chandra, Sonia Arora, Yona Paul, Sreya Dutta, Rishi, Avani, Tanima, Rohit, Karishma, Tapas, Bijay lakshmi, Chandrani, Kanpriya, Vijayant for their assistance and keeping the lab in good humor. A special note of thanks to Dr. Roli Saxena and Sonia Arora for helping me in the initial stages of organic synthesis. Thanks are due to Dr. Chin, Jacob for their help in proof reading of the thesis.

I wish to thank Sir, Madam Sheeba Manoharan and Himani for keeping a homely atmosphere and care they took during my stay at IITK.

I am lucky to have friends Srinu, Jyothi, Varam, Sulochana, Sandhya, Rehana without whom I would not have come to this stage of my career. They all mean a lot to me.

I wish to thank Mr. Uma Shankar and Mr. Nayab Ahmed for their help in collecting powder X – ray data and NMR data. Thanks due to Glass blowing staff for their cooperation in making some of the specially designed glass ware.

I wish to thank all my table tennis partners and coaches for letting my academic pressure off during my practice sessions.

I take pleasure to thank Gopal, Madhaviah, Shikha, Srirammurthy, Bhanu, Chandrasekhar, Yamuna, Aditi, Anita, Rajesh, Prasanjit, Natarajan, Hari, Shayan, Anil, Bama Prasad, Neelam, Jhunpa, Peru and all my southern lab, core lab colleagues and friends for their friendly suggestions and some of the technical discussions.

Finally, I don’t have words to express my profound gratitude to my parents, brothers and sisters for their support, patience and love.

The financial assistance and facilities provided by the Indian Institute of Technology, Kanpur, is duly acknowledged. Qureshi

Introduction

SYNOPSIS

Thesis Title: Design & fabrication of benzthiazole based white light

emitting diodes: Synthesis, Spectroscopic and Semi -

empirical studies

Name of the Student: Qureshi Mohammad Roll Number: 9910772

Department: Chemistry

Thesis Supervisor: Professor S. Sundar Manoharan

Degree for which submitted: Doctor of Philosophy.

!"#$% & !'( % )* ++ , !!-$. /0 0 $,1 1 2 / 0 0 $% 1 1 0 $ 0 1 1 $

Introduction

CHAPTER I presents the literature survey on the basic concepts of the

electroluminescent devices and the issues related to the fabrication of the light emitting

diodes. A brief overview on the selective history of the organic luminescence is

presented. Molecular materials, which are used in the literature for the use in light

emitting diodes as electron transport, hole transport and emissive materials are

highlighted. A brief discussion on the device strategies to produce broad white light

emission is discussed. A summary of the literature reported so far on benzthiazole based

compounds are discussed. The motivation and the objectives of the presented work are

highlighted.

CHAPTER II deals with the aspects of synthesis and characterization techniques. A host

of benzthiazoles, oxazoles and imidazoles prepared either using microwave or by wet

method is described and tabulated. The advantages of microwave synthesis over wet

route are highlighted. The structural characterization of these molecules synthesized has

been carried out using NMR, IR, Elemental Analysis and thermo gravimetric techniques.

Reduction – Oxidation behavior of these molecules were evaluated employing cyclic

voltamograms. Absorption and emission spectra were studied in both solid and solution

form to understand the features of electronic transitions. Fluorescence decay

measurements using Time Correlated Single Photon Counting (TCSPC) spectroscopy

were carried out employing N2 pico second pulsed laser. Time resolved spectral studies

were done using ultra fast response CCD camera. Photo generated time – of – flight

(TOF) electron mobility measurements were carried out using frequency tripled Nd:

YAG laser with incident power ~ 5mJ/cm2. We highlighted the use of Ultraviolet

photoemission spectroscopy (UPS) and X – ray photoemission spectroscopy (XPS)

studies to understand the interfacial energy level alignments in organic hetero junctions

and valence band spectra for the electronic structures of the molecules.

CHAPTER III deals with the stereo selective white light emission among the 1,2 –

dibenzthiazolyl ethylene isomers. The cis - form of the isomer distinguishes itself from

the trans - form in showing white light emission, where as the trans form shows a single

component emission in the green wavelength region. Time decay measurements of both

the isomers were studied. In the solid-state, the cis form of the isomer shows a faster

Introduction

decay in comparison to the trans isomer indicating an extended energy transfer between

the benzthiazolate chromophoric groups. Nano second time resolved spectral studies

shows a strong time dependency during the first few nano seconds (1 – 6 ns) in the case

of cis isomer leading to a red - shifted emission where as no time dependency was

observed in the case of the trans isomer. This suggests a distinct energy transfer

processes operating in the cis form of the isomer. Energy minimized structure suggests an

angle of 400 around the –C=C-C=N where as trans isomer is coplanar with an angle of

~ 1740. AM1 – CI calculations suggest electron density distribution showing a strong

energy transfer from the adjacent benzthiazolato groups in the cis isomer through the

olefinic linkage with a predominant ‘space charge transfer’ in the second excited state,

LUMO +1. On the other hand, for the trans isomer there is no change in the distribution

in the electron density. In the device structure such as ITO/PEDOT: PSS/PFO: CDBE

(10%) /Al, the cis - form of the isomer, which shows white light emission was employed

as an emissive layer. It is noteworthy that the electroluminescent spectrum of this device

shows a broad white light emission similar to the PL spectra of the cis – DBE that

confirms that the generation of excitonic zone is in the cis – DBE that is responsible for

the white light emission. The threshold voltage of the device structure is at 8 V with an

initial brightness of 30 cd/m2.

CHAPTER IV describes the electroluminescent properties of the dimeric bis (2-(2-

hydroxyphenyl) benzthiazolato) Zinc (ZBZT) complex. The ZBZT complex in the

device structure ITO/TPD/ZBZT (80 nm)/Al shows an unusual broadening leading to a

white light emission (~ 240 nm) which is based on the exciplex formation at the interface

of the hole transport layer, TPD, and the ZBZT complex. The exciplex formed is a

consequence of charge transfer interaction between the excited state of ZBZT and the

ground state of the TPD layer. Under electrical bias conditions the lower IP value of TPD

(-5.4 eV) than ZBZT (-5.62eV) injects holes into the ZBZT/TPD interface. The higher

electron affinity of ZBZT (-2.85 eV) provides the preferable electron transport pathway

through the LUMO of ZBZT. When injected, holes and electrons form TPD+ - ZBZT-

exciplex and recombine radiatively to give broad emission feature at higher wavelengths.

On the contrary, photo luminescent properties of the ZBZT complex are distinctly

different from that of the electroluminescent spectra of the device, where in PL is

Introduction

showing a much narrowed emission (FWHM ~ 88nm) in comparison to that of EL.

Single crystal data suggest two different kinds of ligand present in the dimeric complex,

one directly attached to the metal ion, and the other through a oxo - bridge. Semi

empirical calculations suggest dominant electron density distribution particularly on

bridged benzthiazole ligand, which is controlling the photo luminescent properties. Nano

second time decay analysis fits to a two-component exponential decay, one of which

closely resembles to that of the free ligand, which confirms the dominance of ligand in

controlling the PL properties.

CHAPTER V describes the observations made on unusual electroluminescent properties

of Gallium complex; tris (2-(2-hydroxyphenyl) benzthiazolato) Gallium (III).

Photoluminescent spectra shows a peak maxima around 480 nm where as the

electroluminescent spectra of the Gallium complex based ITO/TPD/GBZT/Al device

structure shows a peak maxima at 610 nm with almost no overlap between the PL and EL

spectra. Ultraviolet photoemission spectroscopy (UPS) and X - ray photoemission

spectroscopy (XPS) studies give a clue about the strange shift in the electroluminescent

peak maxima in comparison with the Photoluminescent spectra. The HOMO (5.0 e V)

and LUMO (2.1 e V) levels of the GBZT complex relative to TPD (HOMO (5.2 eV) and

LUMO (2.2 eV)) are located such that there is no barrier between the GBZT and TPD. In

the absence of any barrier between GBZT and TPD interface, a donor – acceptor

mechanism (electroplex formation) appears to be responsible for large shift in the EL

peak. We have improved on the emission features of the above device by employing 2,5

– dibenzthiazolyl thiophene (TBZT) as an electron transport material. The time of flight

mobility (TOF) measurement of TBZT shows a high mobility of 2.4 X 10-4 cm2/Vs

compared to Alq3 (1.21 x 10–6 cm2/Vs). The high electron mobility of TBZT enhances the

device features by lowering the threshold voltage significantly in Zn and Ga complexes.

CHAPTER VI summarizes the results of our studies on benzthiazole based molecular

materials. The mechanistic aspects leading to the white light emission in these classes of

compounds under diode environment is highlighted. A simple way to design organic

molecules to get white light emission has been discussed.

Introduction

CONTENTS

Acknowledgements ---- V

SYNOPSIS ---- VII

CONTENTS ---- XI

CHAPTER I INTRODUCTION ---- 1

I.1 Introduction to organic luminescence ---- 2

I.2 The structure and operation of OLEDs ---- 3

I.3 Amorphous and polycrystalline Organic

Devices ---- 6

I.4 Polymeric light – emitting devices ---- 8

I.5 Metal free light emitting devices ---- 9

I.6 Full color active matrix displays ---- 10

I.7 White light emission, a simple way to Achieve

Full color emission ---- 12

I.8 Can white light emission be possible with the

Steric control of the organic molecules ---- 13

I.9 Brief literature review on molecular organic

Electroluminescent materials ---- 15

I.10 Hole – injection/transport materials ---- 15

I. 10. (a) Biphenyl diamine derivatives ---- 17

I.11 Electron – transport and host emitting Materials ---- 18

I.12 Anodes ---- 20

I.12 (a) Surface treatments of ITO ---- 21

I.13 Non – ITO anodes ---- 21

I.14 Cathode ---- 21

I.15 A comment of reliability ---- 22

I.16 “LIGHT” on benzthiazole based Molecules

used for electro luminescence ---- 23

Introduction

SCOPE OF THE PRESENT WORK ---- 24

Motivation and objectives of the present work ---- 24

References ---- 26

Summary of the literature survey ---- 26

CHAPTER II EXPERIMENTAL ---- 23

II. 1 Introduction ---- 34

II. 2 General Procedure ---- 34

II. 2(a) General scheme of reaction for Benzheterazoles ---- 34

II. 2(b) General Procedure for the synthesis of Poly

phosphoric acid (PPA) ---- 35

II. 3 Mechanism of Microwave interaction ---- 36

II. 4 Synthesis of 2-(2-hydroxy phenyl)Benzthiazole ---- 38

II. 5 Synthesis of cis-1,2-dibenzthiazolyl ethylene ---- 40

II.6 Synthesis of 2,5-dibenzthiazolyl Thiophene ---- 42

II.7 Synthesis of 2,5-dihydroxy-1,4-Dibenzthiazolyl

Benzene ---- 43

Table II Various benzheterazoles synthesized using

Conventional and microwave conditions ---- 45

II.8 Characterization ---- 46

II.9 Physico – Chemical techniques ---- 48

II.9.A. Thermo gravimetric analysis ---- 48

II.9.B. Differential Scanning Calorimetry (DSC) ---- 49

II.9.C. Cyclic Voltammetry ---- 50

II.9.D. Time correlated single photon counting method ---- 50

II.9.E. Time resolved emission spectroscopy ---- 52

II.9.F. Time – of – Flight technique ---- 54

II.9.G. Ultraviolet photoelectron and X – ray

photoelectron Spectroscopy ---- 55

II. 10 Electroluminescence Characterization ---- 59

II.11 References ---- 61

Introduction

CHAPTER III STEREOSELECTIVE WHITE LIGHT EMISSION

IN 1,2 –DIBENZTHIAZOLYL ETHYLENE ---- 62

III.1 Introduction ---- 63

III.2 Experimental ---- 64

III.3 Results and Discussion ---- 67

III.3.A. Theoretical Studies ---- 74

III.4. Electroluminescent Characterization ---- 76

III.5. Summary ---- 78

III.6. Notes and references ---- 79

CHAPTER IV WHITE LIGHT EMISSION BASED ON THE

BENZTHIAZOLE BASED ZINC COMPLEX ---- 82

IV. 1. Introduction ---- 83

IV. 2. Experimental ---- 85

IV.2.A. Synthesis 2 –(2-hydroxy phenyl) Benzthiazole ---- 85

IV.2.B. Bis (2-(2-hydroxy phenyl) benzthiazolate) Zinc ---- 85

IV.3 Results and Discussions ---- 88

IV.4. Semi empirical calculations ---- 92

IV.5 Quantum yield calculations ---- 94

IV.6 Electroluminescent Characterization ---- 94

IV.7. References ---- 103

CHAPTER V LUMINESCENT PROPERTIES AND SPECTROSCOPIC

INVESTIGATIONS OF BENZTHIAZOLE BASED HALF

CROSS COMPLEXES; TRIS (2-(2’-HYDROXY PHENYL)

BENZTHIAZOLATE) M(III); (M = Al, Ga, In) ---- 105

V.I Introduction ---- 106

V.II. A. Synthesis ---- 107

V.II.B. Powder X – ray patterns of the metal complexes ---- 109

V.II.C. Photo luminescent studies ---- 111

Introduction

V.II.D. Photo luminescent decay measurements ---- 112

V.III. Electroluminescent properties of tris (2-(2-hydroxy

Phenyl) benzthiazolate) Gallium (III) complex ---- 115

V. IV. X –ray photo electron spectroscopy – Ultraviolet photo

Electron spectroscopic studies on GBZT – TPD

Interfaces --- 116

V.V. Current – Voltage characteristics of

ITO/TPD/GBZT/Al ---- 124

V.VI. Influence of 2,5 – dibenzthiazolyl thiophene as an

Electron Transport layer on the GBZT device

Structures ---- 125

V.VII. XPS and UPS characterization of

ITO/TPD/GBZT/TBZT ---- 126

V. VIII. References ---- 121

SUMMARY AND CONCLUSIONS ---- 133

Appendix .V – 1 ---- 135

V.A.I.1 UPS and XPS investigations on Tris (2-(2-hydroxy

phenyl) benzthiazolate) Gallium (III) complex ---- 135

Appendix. V – 2 ---- 141

V.A.II.1 Introduction ---- 141

V.A.II.2 Experimental ---- 141

V.A.II.3 Electrochemical Characterization ---- 142

V.A.II.4 Photo generated Time – of – flight mobility

Measurements ---- 142

V.A.II.5 References ---- 148

1

2

I. 1: Introduction to organic luminescence:

Organic materials are characterized by an immense variation in structure and

properties, and such a flexibility in tuning its properties is one of the principal reasons for

studying their application to electrical engineering problems. For example, many

molecules and polymers have pronounced optical qualities that can be adjusted by

modifications of chemical structure. Electrical properties such as conductivity are also

dependent on molecular design. Although the intramolecular conductivity of molecules

and polymer chains may be enhanced by delocalized electrons from sp2 hybridized

carbon atoms, the intermolecular overlap of van der Waals bonded solids generally limits

charge transport mobilities, 10 cm2 /Vs at room temperature; increasing to ~ 105 cm2/Vs

at low temperature. Thus, the transport characteristics of organic films are strongly

dependent on the molecular order. To avoid the need for very large voltages, organic

devices should employ thin films of molecules designed for optimum molecular overlap.

Most of the initial studies on the solid-state properties of these materials concentrated on

the molecular crystals because ordered crystals possess better electronic transport

properties. The first reported observation of electro luminescence in organic crystals

under steady-state bias was made by Helfrich and Schneider using anthracene. The

applied potential was over 100 V for the onset of EL since used crystals of several

microns thick, thereby limiting the possible practical applications of this technology.

Subsequently, light emitting devices were fabricated using thermal evaporation of

polycrystalline anthracene thin films. The ability to make much thinner devices using this

technique significantly reduced the voltage required to generate light. However, in these

3

early devices, electron-hole combination occurred very close to an injecting contact and

the quantum efficiencies were limited to less than 0.1%.

I. 2: The structure and operation of OLEDs:

The process of light emission in organic light emitting diodes is based on the injection of

positive and negative carriers from electrodes that sandwich an organic layer. These

recombine to form excitons that can radiatively decay to produce electro luminescence

from the device. The emission color can be tuned over a wide range by appropriate

choice of polymer and small molecule based organic materials. The basic structure of

the organic light emitting diodes is shown in figure I.1. Electrons injection from the

cathode is from a low work function material and hole injection is facilitated by the Hole

transport layers. The device is placed on an ITO substrate that is transparent and

provides electrical contact to the hole injection layer.

Figure I.1 : Typical device architecture of an Organic Light Emitting Diode consisting of hole transport and electron transport layers for the ideal recombination of electrons and holes.

Electron transport layer

ITO-Glass substrate

Hole transport layer

Emissive layer

Cathode

V

4

In order to understand and therefore, maximize the efficiency of an OLED, we

need to consider the various loss mechanisms. Figure I.2 shows the schematic of the

processes involved in EL devices. The charge carriers recombine in the organic layer to

produce excitons. The first loss process occur due to carriers that do not combine, the

probability of which is related to the balance between the number of positive and

negative charges injected into the organic layer. The excitons that are formed can be of

two types, singlets that have a radiative decay and triplets that decay through non -

radiative processes and thus lead to a loss in efficiency. All the singlet excitons do not all

radiatively decay, due to the presence of non - radiative pathways for the excitonic states

to relax. The loss in efficiency is related to the intrinsic photoluminescence efficiency of

the organic materials as well as the additional losses due to the exciton quenching

mechanism. Finally, a large portion of the light generated in the device is unable to

escape, thereby introducing a loss that is referred to as the output coupling.

These four major losses described above can be combined to determine the external

quantum efficiency (ext) of OLED device, as shown in the following equation I. 1;

---------- ------------- (I.1)

ext = x rst x q x coupling

5

Figure I. 2: Schematic diagram for the electro luminescent processes describing various loss mechanisms involved in controlling the efficiency of the emission process

h e

h – e

Singlet exciton

Triplet Exciton

Thermal deactivation

Emission

Internal reflection loss

External emission

Radiationless deactivation

6

I. 3: Amorphous and polycrystalline organic devices:

A significant step forward in organic technology came with the publication by Tang and

vanslyke of solar cells and light-emitting devices fabricated from thin amorphous and

polycrystalline layers deposited by thermal evaporation in vacuum. [1] Although

disordered films possess inferior electron transport characteristics, they can satisfy the

requirement for extremely thin, low voltage, organic devices:

Mg:Ag Alq3 Diamine ITO Glass N

O

N

O

N

O

Al

Alq3

N N

S

Diamine

NN

αααα- NPD

NN

TPD

Fig. I.3: Tang and van Slyke’s (1987) organic light emitting device is built on a glass substrate coated with a transparent indium tin oxide layer acting as the anode. A diammine was used as the hole transporting material since this class of materials was found to have stable conductivity properties in earlier work on photoconductors. Many variations on the basic diamine structure were later investigated; for example, both TPD and -NPD were found to make successful hole transport layers. The electron transporting and luminescent material employed was Alq3. In this hetero - structure devices electrons and holes combine at the diamine/Alq3 interface, but whereas there is minimal electron injection into the diamine layer, some holes penetrate into the first 100Å of Alq3. Thus, excitons are formed in the Alq3 and emission is observed from Alq3 fluorescence. Electrons are injected into the organic layers from an Mg: Ag alloy cathode with an additional layer of Ag to protect the Mg from oxidation.

7

Indeed, thermal evaporation is capable of growing continuous, molecularly -

smooth, films as thin as 100Å, allowing vertical device feature sizes approaching

molecular scales.

As with the anthracene thin-film devices, the reduction in organic thickness to

~1000Å in the solar cells and light-emitting devices enabled a dramatic reduction in

operating voltage. A significant step was to use amorphous films to fabricate the first

organic hetero - structure light emitting device, increasing the quantum efficiency of

luminescence by approximately two orders of magnitude to 1%, at an operating voltage

of less than 10V; the hetero - structure design was also used in a 1% power efficient solar

cell. With this work, organic materials first showed their potential as the basis for an

efficient emissive technology applicable to all aspects of the display industry. An intense

examination by chemists and electrical engineers followed.

Figure I.4: The transport and luminescence characteristics of the first organic electro luminescent device. From Helfrich and Schneider (1965).

8

I. 4: Polymeric light-emitting devices:

Following the successful demonstration of organic electro luminescent devices

using small molecular weight materials, attention turned to the development of polymeric

electro luminescence. Polymers are too large to be thermally evaporated, so unlike small

molecules they are often cast from solution by spin coating technique, complicating the

creation of hetero structure and other devices employing multiple layers.

Figure I.5: Proposed energy structures for the single layer and hetero – structure PPV devices. In (a) the electrons and holes combine close to the calcium electrode reducing the quantum efficiency. However, in the hetero – structure device shown in (b), the holes are trapped at the PPV/PBD interface and the overall quantum efficiency is nearly two orders of magnitude higher. After Brown et al (1992).

ITO PPV Ca

Holes

Electrons

EF

EF

(a)

Hole

ITO PPV

PBD Ca

Holes

Electrons EF

EF

(b)

9

However, a widespread interest in polymer chemistry and the belief that polymer

structures were more structurally robust spurred the development of the first polymer

electro luminescent device. The material used was poly (p - phenylene vinylene) (PPV),

which is fluorescent in the yellow-green. Partly due to lack of a hetero structure, the

initial polymer device possessed a quantum efficiency of only 0.05%, similar to the

thermally deposited anthracene electro luminescent devices that predated the hetero

structures of Tang and van Slyke.

Since PPV preferentially conduct holes, the original devices constructed from

films of PPV were limited by electron injection. This deficiency was addressed the by a

polymeric hetero structure using an electron transport layer of the molecular material 2-

(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) dispersed in an insulating

polymer, poly (methyl methacrylate) (PMMA). This improved the quantum efficiency of

PPV electro luminescence to 0.8%, roughly the same as the undoped Alq3 devices.

I. 5: Metal free light emitting devices:

The development of organic light emitting devices based on small-molecules and

polymers proceeded incrementally after the initial demonstrations of 1% quantum

efficiency using hetero structure. For example, the range of substrates was expanded to

include flexible plastic and new luminescent dyes were introduced to improve the device

performance across the visible spectrum. One aspect of the organic technology that

remained unrealized, however, was the potential for highly transparent devices.

10

Semitransparent devices had been developed earlier using very thin metal contacts

[2] but the metal-free device [3] of Parthasarathy et al. shown in Fig. I.6 employed a

protective layer of CuPc and a sputtered film of indium tin oxide. Not only did the device

have important technological implications, it also demonstrated that the electron injection

contact could be made of virtually any material.

I.6: Full color active matrix displays:

Although Tang et al. established the essential principles of organic electro

luminescence in the initial papers; the commercial development of this technology was

retarded by reliability problems. The organic materials are sensitive to both oxygen and

water, and maintaining the devices in an inert atmosphere is critical to their long-term

stability. But with improvements in material purity and packaging technology,

commercially acceptable lifetimes of ~ 10,000 hours were ultimately realized and several

Figure I. 6: The device structure of the metal-free transparent organic light-emitting device together with its transmission spectra. A thin layer of CuPc underneath the top ITO cathode protects the remaining organic layers from damage during the ITO sputtering process. Subsequently, it was discovered that materials with lower optical absorption than CuPc had similar protective qualities, while improving the optical transmission of the device. From Parthasarthy et al (1998).

11

prototype full-color organic displays have now been demonstrated. One such display is

shown below in Fig.I.7, fabricated on a poly silicon thin film transistor back plane. Note

that the organic emissive devices are Lambertian emitters and therefore the color is

relatively independent of viewing angle. This further facilitates note the very thin and flat

display.

Semi conducting organic materials have found an important application in organic

light emitting devices (OLEDs). Brightly emissive across the visible spectrum, organic

materials are readily incorporated in electrical devices that are inexpensive to fabricate on

a large scale. Organic devices may be grown on glass or flexible transparent plastic and

they can operate below 10V. Due to these advantages, it is envisaged that OLEDs may

provide the platform for the next generation of video displays. But organic materials must

not only match the color purity and long-term stability of competing technologies, they

must also possess a significant advantage in efficiency, especially in low power, portable

applications.

Figure I.7: 15” - Organic active matrix displays, Eastman Kodak Company and Sanyo Electric Co. This material used molecular organic materials for producing full color emission

12

I. 7: White light emission, a simple way to achieve full color emission:

Owing to the difficulty in controlling the electronics for three different pixels [4] in

achieving full color display, research is now directed towards the simple device

fabrication methods, which can reduce the complication in the device making. One of the

major set backs in the commercialization is the lifetime of the devices in particular

concerning the blue light emitting materials. Larger bandgap materials often pose the

problems of getting degraded near the cathodic surfaces, which is of concern to the

industry [5]. A variety of strategies have been developed to make white LED, in most

cases by combining several emitting species of different colors [6]. However, multi layer

devices fabricated by vapor deposition are not only difficult to assemble, but also require

suitable blockers at the interfaces to control the flow of electrons. Blending several

dyes/polymers together faced other kinds of complexities, such as color instability due to

voltage changes and / or undesired Fröster – type energy transfers among chromophores

[7]. Most of these problems can be avoided if single component material can be used as

emitting species. Several approaches to achieve full color displays are shown pictorially

in figure I.8. Achieving full color emission based on the white light emission seems to be

more practical because of the availability of established cheaper filter technology; also

device structure becomes very simple so as to reduce operating voltages as well as

electronics part considerably. One of the primary requisite to obtain white light emission

is to achieve (x, y) CIE – color coordinates of 0.33, 0.33.

13

I. 8: Can white light emission be possible with the steric control of the organic

molecules:

When a molecule consists of two aromatic groups, separated by a saturated a

molecular structure, then they do not interact in the ground state and practically the –

electronic absorption spectrum is equivalent to the sum of the absorption spectra of the

separated entities [8]. For example, the absorption spectra of the 1- naphtyl – 9 anthryl

alkanes, are very similar to that of an equi - molar mixture of 1-methyl naphthalene and 9

– methyl anthracene. In such compounds different groups can be excited separately by

the choice of appropriate excitation wavelengths.

0.33, 0.33

Figure I.8: shows different approaches to Full color display. Conventionally RGB pixels were used to produce the full color emission, alternatively using low energy filter can be used to deduce all the three colors from the blue emitting materials. One of the viable methods is to extract all the three colors using color filter from the white light emitting material.

14

If the steric conformation of the molecular chain is such that the excited segment

cannot approach a second unexcited segment, then the excited species emits characteristic

molecular fluorescence spectrum, which is approximately mirror image of the absorption

spectrum. On the other hand, if during the lifetime of the excited species it can move into

the vicinity of an unexcited segment, it may form an intramolecular excimer, which

would give a structureless fluorescence spectrum at lower energies than the molecular

spectrum. This typical behavior has been observed in the diphenyl alkanes, the triphenyl

alkanes, and the ditolyl alkanes [9].

CH2 n

CH2 n

15

I. 9: Brief literature review on the molecular organic electro luminescent materials:

One of the key enablers in the history of OLED advancement can be attributed to

the continuing discovery of new and improved electro luminescent materials, which were

made possible by the dedication, and ingenuity of many organic chemists who provide

the design and skilled synthesis. Indeed, from small molecules, oligomers to conjugated

polymers; intense research in both academia and industry has yielded OLEDs with

remarkable color fidelity, device efficiencies and operational stability.

I.10: Hole – injection/transport materials:

Oxidation of the ITO surface by O2 plasma, CF4/O2 plasma [10] or UV ozone

treatment can reduce the carrier injection energy barrier, remove residual organic

contaminants and get its work function up to near 5eV which is still about 0.5 eV lower

than most of the HOMO of the hole – transport materials. A layer of hole – injection

material, which reduces the energy barrier in between ITO/HTL, is therefore beneficial to

enhancing charge injection at the interfaces and ultimately improving power efficiency of

the device. Thus, hole – injection can be promoted by introduction of new hole –

transport layers with optimized HOMO levels and by inserting a thin layer of copper

phthalocyanine (CuPc) [11], starburst polyamines [12], polyaniline [13] and SiO2 [14]

between the ITO/HTL interface. In addition, HTLs doped with oxidizing agents such as

FeCl3 [15], iodine [16], tetra (flouro) – tetra (cyano) quinodimethane (TF – TCNQ) [17]

and tris (4 – bromophenyl) aluminum hexachloroantimonate (TBAHA) [18] have been

reported as an effective material for hole – injection. One of the widely used polymers for

promoting hole injection is poly (3,4 – ethylenedioxythiophene) – poly (styrene) known

16

as PEDOT/PSS which has been found to be useful in a hybrid OLED architecture

combining both the advantages of polymer LED (PLED) and multi – layered small

molecule OLED [19]. One of the potential drawbacks of using PEDOT/PSS is its acidity

which could get as high pH as ~ 3. Nuesch et al. [20] found that the surface treatment

using grafting of molecule and adsorption of acids or bases [21] on ITO could also

modify its work function. In case of 2 – chloroethyl phosphoric acid, a self – assembled

mono layer (SAM) can be readily formed on the ITO substrate, which significantly

reduced the threshold voltage of a standard OLED device of [ITO/TPD/Alq3/Al][22].

Recently, p – doped aromatic diamines have been found to be excellent injection

materials such as SbCl5 – doped N, N′-bis (m- tolyl) – 1,1′ – biphenyl –4, 4 – diamine

(TPD) thin film [23] as well as the amorphous starburst amine, 4,4′, 4′′ – tris (N, N′-

diphenyl amino) triphenylamine (TDPTA) doped with a very strong acceptor TF – TCNQ

by controlled co – evaporation [24].

NN

CH3

H3C

SbCl-6

+

TPD

S

OO

n

SO3H

m

PEDOT/PSS

s

s

s

ss

s

R' R'

R'

R'

– 6T

Figure I.9: Hole – injecting materials based on molecular species

17

The discovery of using tri – aryl amines with a “bi-phenyl” center core as the hole

– transport layer which greatly improved both EL efficiency and operational stability of

OLED [25], led to most of the new hole transport materials (HTM). The creativity of

synthetic chemists as well as material scientists throughout the world continue to provide

the OLED device community with their ever improved products having superb properties

and elegant design. By far, one of the most widely used HTM in OLED is still N, N′ - bis

(1 – naphthyl) – N, N′ - diphenyl – 1, 1′ - biphenyl – 4,4′ - diamine (NPB) and TPD.

Therefore, studies on the design and synthesis of new HTMs have been continually

focused on finding materials with high thermal and thin film morphological stabilities

and on finding ways to control and optimize carrier injection and transport. These

approaches to molecular design can be roughly categorized into biphenyl diamine

derivatives; starburst amorphous molecular glass; spiro – linked biphenyl diamines.

I.10a: Biphenyl diamine derivatives:

Heat treatment of organic multi – layers has been found to cause an inter - diffusion

between organic layers in OLED [26] which ultimately affects the stability of the device.

Therefore, to increase the Tg of HTM is critical in obtaining a more thermally durable

display. Using thermo dynamical consideration, Sato has proposed a molecular design

rule according to which high Tg materials can be obtained by increasing the number of

– electrons and by decreasing rotational moment by placing a heavy moiety at the center

of the molecule [28].

18

I. 11: Electron – transport and host emitting materials:

To date, the most widely used electron – transport and host material in OLEDs is still tris

(8 – hydroxy quinolinato) aluminum (Alq3). This is because Alq3 is thermally and

morphologically stable to be evaporated into thin films, easily synthesized and purified.

Arguably, it is still one of the most robust electron – transport backing layers in OLED,

particularly with the help of the hole blocker to trap the hole carriers from injecting into

Alq3 [28] or doping with lithium or other alkali metals [29] to assist electron injection to

lower the drive voltage [30]. But it has also many shortcomings such as quantum

efficiency, mobility, band gap and the ashing problem during sublimation. It has been

found by the time – of – flight technique that the drift mobility of electrons in Alq3 is

increased by about two orders of magnitude (10-4cm2/Vs) as the deposition rate decreased

from 0.7 to 0.2 nm/s. The electron drift mobility in Alq3 is found to increase linearly

with the square root of the applied electric field. Other metal chelates which showed

decent device performances and interesting fluorescent properties are shown below.

Sano et al from Sanyo [31] have prepared several kinds of 2:1 complexes with 8 –

SNN

BFA – I T

NN

CH3

H3C

TTBND

Figure I. 10: Hole transport Biphenyl diamine derivatives

19

hydroxyquinoline derivatives and a variety of metal ions, such as Be, Mg, Ca, Sr, Sc, Y,

Cu or Zn. Amongst them, the beryllium complex was found to be the most fluorescent in

the green and Zinc complex was found to have a strong yellow fluorescence. Some of the

metal complexes are shown below. One of the most widely used electron – transport and

hole blocking materials is 2 – biphenyl – 4 – yl – 5 – (4 – t – butyl phenyl) – 1,3,4, -

oxadiazole (PBD) which has been branched, spiro – linked [32] and starburst to prevent

from crystallization in these films. In oligothiophene, it was shown that by substituting

electron – withdrawing and bulky dimesityl boryl groups [33,34] one could produce a

superior electron transport emitter.

Current glassy and amorphous organic molecular materials have low – electron

mobility in the range of 10–6cm2/Vs. Moreover, effective electron – transport molecules

are usually more chemically sensitive to their environments, hence, very few useful

materials with superior electron mobility have been reported.

N

O

B

R

Ph2Bq

N

NO

OAl

N

N

OO

NN

O

O

Al (ODZ) 3

N N

O NN

O

Zn

Zn(BIZ)2

N

O N

O

Be

Bepp2

N N

O

O

NN

O

OZn

Zn(ODZ)3

N

O

N

O

Be

Bebq2

Figure I.11: Highly fluorescent metal chelates for electroluminescence devices

20

Oxidiazole containing organic materials generally possess good electron – transport

properties, and a number of oxadiazole derivatives have been utilized as electron –

transport materials in OLEDS [35-38]. Two soluble tris (phenyl) quinoxalines have been

identified in the literature as the effective electron – transport materials, which are tris

(phenyl) quinoxalines 1,3,5 – tris (3 – phenyl – 6 – trifluoro – methyl) quinoxaline – 2 –

yl] benzene (TRQ1) and 1,3,5 – tris [3- (4 – tert – butyl phenyl) – 6 – trifluro methyl –

quinoxaline – 2 yl] benzene (TRQ2) [39]. The electron mobilities for both compounds

approach 10–4cm2/Vs at electric fields of 106 V/cm at room temperature. The electron

mobility for 4,7 – diphenyl – 1, 10 – phenanthroline (bathophenanthroline, or BPhen) is

in the range of 3.9 x 10-4cm2/Vs at 2 X 105 V/cm to 5.2 x 10-4 cm2/Vs at 2x 105 V/cm

with a weak dependence on the electric field [40]. Although the electron mobilities in

those materials are one or two orders magnitude greater than that of Alq3, only a few

experiments have been carried out to replace Alq3 with the new materials in OLEDs.

I. 12: Anode:

In an OLED, the barrier to hole – injection from an electrode is normally taken as

the energy difference between the electrode work function and the ionization potential of

the organic material. This is generally a very poor approximation and has been shown to

be quantitatively incorrect for many electrode/organic interfaces. Moreover, recent

experiments on some new hole – transport materials showed that with OLEDs on ITO

glass the device turn – on voltage and quantum efficiency were not correlated well with

the ionization potential of the hole – transport materials, indicating that injection of

carriers cannot be fully controlled by simply varying the energy offset barrier at the

electrode.

21

I.12a: Surface treatments of ITO:

The work function of a metal is strongly influenced by the electrostatic conditions

at its surface. The work function shift induced by a uniform dipolar surface layer is

determined by the change in electrostatic potential (V) created at the surface and can be

derived from classical electrostatics as = - V = - e N (µmol r o), where N is the

surface number density, r the dielectric constant of the polar adsorbate molecules, and

µmol is the molecular dipole normal to the surface [41]. There are few important ways one

can efficiently do the surface treatment of ITO, namely (i) acid – base treatments, [42],

plasma treatments [43].

I.13: Non – ITO anodes:

Over the past few years, increasing activity has focused on improving charge

injection efficiency at electrode / organic interfaces in OLEDs. In contrast, relatively few

materials have been explored as alternatives to ITO as OLED anodes. ITO has the virtue

of optical transparency, but it is not a well – controlled material. Several alternative

materials have been recently examined as anode, however all suffer from some

unfavorable characteristics. For example, Fluorine – doped tin oxide, germanium based

tin oxide [44], Al – doped zinc oxide [45].

I.14: Cathode:

Elemental metals with low work functions are ideally suited for the use in OLED

as the cathodes, since, having a low they can inject electrons easily. Stossel et al [46]

have investigated the Alq3 – base OLEDs prepared at 5 x 10 –9 mbar with the cathode

work function ranging from 2.63 to 4.70 eV. They have showed that lower the work

function of the metal higher is the current density obtained in the device structure. The

22

attempt to use Ca, K, and Li for effective cathode materials revealed that they exhibit

poor corrosion resistance and high chemical reactivity with the organic medium. Thus, a

variety of low work function metal alloys such as Mg – Ag and Al – Li are used for

cathodes. [47]

I. 15: A comment on the reliability of OLEDs:

Commercialization of the OLEDs depends on many factors such as efficiency,

color, and most importantly stability. There has been extensive research efforts aimed at

understanding the degradation mechanisms of small molecule based OLEDs. The

majority of these reliability studies concerned OLEDs employing the common electron –

transport material, Alq3. The operational instability is a long – term intrinsic decay in EL

intensity leading to a uniform loss of efficiency over the device emitting area. The

storage instability occurs through the formation and growth of non-emissive regions or

“dark spots”

One of the main issues concerning small molecules is the crystallization owing to

their low molecular weights. Since organic thin films prepared by vapor evaporation are

glassy and amorphous, crystallization is considered as one of the dominant degradation

mechanisms. In particular, most organic materials commonly used in an OLED,

especially those used for HTL, have relatively low glass transition temperatures (Tg). For

example, while using TPD as a hole transport layer, light output gradually decreased with

increase in temperature and dramatically dropped at 700C, which corresponds well to the

glass transition temperature, Tg ~ 600C [48]. As a result, many methods have been

employed to prevent or minimize the crystallization of the organic layers. The

introduction of bulky molecules resistant to crystallization offers one solution to this

23

problem. Molecular dopants were also proven to suppress crystallization, along with

other benefits, such as improved emission efficiency and color tuning.

Apart from the above mentioned problems, operational stability due to unstable

anodic contacts [49], excited state reactions [50], self – heating during operation [51],

presence of non – emissive sites [52] and reaction of cathodes with moisture [53] will be

the main problems related to the reliability of devices.

I. 16: “LIGHT” on benzthiazole based molecules used for Electro luminescence:

Inspite of good efficiencies and light output, there are very few reports available

in the literature on benzthiazole-based compounds. The main reason being the lack of

understanding about the photo – physics and physics involved at the interface of the

organic devices. Some of the literature reports wherein use of benzthiazole materials as

EL devices are highlighted below.

1. First report on the white light emission based on benzthiazole moieties was by

Hamada et al where in they have shown white light emission based on single

chromophoric emission. But due to lack of structural understanding, mechanism

of photoluminescence and electro luminescence was not explained properly. [54]

2. Lim et al [55] reported white light emission and bis [2-(2-hydroxy phenyl)

benzothiazolato) Zinc doped in 4 – dicyano methylene – 2 – methyl – 6 – [2 –

(2,3,6,7 – tetra hydro – 1H, 5H – benzo – [i, j] quinolizin – 8 – yl) vinyl] – 4H –

pyran (DCM2) dye based on the partial energy transfer from dye to the Zinc

complex giving white light emission.

24

3. Zhang et al demonstrated a benzthiazole based blue light emitting diodes in the

device structure using 1,4 – bis (m – methyl – benzthiazole – vinyl) benzene as

the active layer. [56]

4. Other studies based on photo physical aspects of benzthiazole, in particular based

on the proton transfer kinetics were studied by P.F. Barbara et al [57] and Ding et

al. [58].

5. Takashi et al studied the photo - physics of 2,6 – disubstituted benzobisthiazoles

derivatives where in they demonstrated the energy transfer from the benzthiazole

group to the anthracene contributing to the enhanced low temperature emission in

these systems in solution states. [59]

Scope of the present work:

Molecular materials having benzthiazoles moieties as the active

lumophoric groups have been subject of recent interest because of the high

internal quantum efficiencies observed in the related bio luciferin systems.

Molecular structures of these bio systems mainly consist of benzthiazole as

common active groups. Further more there are few reports by Hamada et al on

the complexes based on benzthiazole moieties which shows promise to use as

electron transport and emissive materials in white light emitting diodes.

Motivation and objectives of the present work:

White light emitting materials holds promise as candidates for full color

display technology by virtue of its flexibility and stability. Understanding the photo

physics and device physics of such phenomenon become important to further

improve the approaches to device fabrication. Exploring new materials, which will

25

give white light emission in diode environments, forms the basis for the present

work. In this context understanding the photo and device physics involved in

benzthiazole based molecular materials is the main focus of the presented work.

The following points outline the objectives of the present work:

To synthesize new benzthiazole derivatives which emits light over entire

visible region.

To attempt new complexes based on benzthiazole derivatives

To understand the photo physics of the newly synthesized molecules.

To evaluate the electroluminescent performance of simple benzthiazoles

molecules.

To understand the electroluminescent behavior of benzthiazole based metal

complexes

26

I.17: References:

1. C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51, 913, 1987; C. W. Tang, US

Patent No. 4356429, 1982.

2. Y. Shirota, J. Mater. Chem. 10, 1, 2000.

3. U. Mitschke, P. Bauerle, J. Mater. Chem. 10, 1471, 2000.

4. G.Gu, G. Parthasarathy, P. Tian, P.E. Burrows, and S.R. Forrest, J. Appl. Phys.

86, 4076, 1999.

5. C. H. Chen, J. Shi, J. Coord. Chem. Rev. 171, 161, 1998.

6. M. Granstroem and O. Inganas, Appl. Phys. Lett. 68, 147, 1996; M.Berggren,

G.Gustafsson, O. Inganaes, M.R.Anderson, T. Hjertberg, and O. Wennerstroem,

J.Appl. Phys. 76, 7530, 1994; M.Berggren, O.Inganaes, G.Gustafson, J.C.

Carlberg, J.Rasmusson, M. R. Anderson, T. Hjertberg, and O.Wennerstroen,

Nature (London) 372, 444, 1994; F.Hide, P. Kozodo, S. P. DenBaars, and A.J.

Heeger, Appl. Phys. Lett. 70, 2664, 1997.

7. J. Kido, M.Kimura, and K.Nagai, Science 267, 1332, 1995; J.Kido, M. Kimura

and K.Nagai, Appl. Phys. Lett. 67, 2281, 1995; S.Tasch, E.J. W. List, O.

Ekstroem, W. Graupner, G. Leising, P. Schlichting, U. Rohr, Y. Geerts, U.

Scherf, and K.Muellen, Appl. Phys. Lett. 71, 2883, 1997; C. –I. Chao and S.-A.

Chen, Appl. Phys. Lett 73, 426, 1998; A. –A. Chen and C. –I.Chao, Synth. Met.

79, 93, 1996; T.-W. Lee, O.O. Park, H.N. Cho, J.-M.Hong, C.Y.Kim and

Y.C.Kim, Synth. Met.122, 437, 2001; J. P. Yang, Y.D. Jin, P.L. Heremans, R.

Hoefnagels, P.Dieltiens, F. Blockhuys, H.J. Geise, M. Vander Auweraer, and

G.Borghs, Chem. Phys. Lett. 325, 251, 2000.

27

8. O. Schnepp and M.Levy, J. Am. Chem.Soc, 84, 172, 1962.

9. F. Hirayama, J. Chem. Phys. 42, 3163, 1965.

10. I. –M. Chan, W. -C. Cheng, F. C. Hong, Asia Display/IDW’01, 1483, 2001.

11. S.A. Vanslyke, C.H. Chen, C.W. Tang, Appl. Phys. Lett. 69, 2160, 1996.

12. Y. Shirota, Y.Kuwabara, H. Inada, Appl. Phys. Lett 65, 807, 1994.

13. Y.Yang, A.J. Heeger, Appl. Phys. Lett 64, 1245, 1994, Y.Cao, G.Yu, C. Zhang,

R. Menon, A.J.Heeger, Synth. Met. 87, 171, 1997.

14. Z.B.Deng, X.M.Ding, S.T.Lee, W. A. Gambling, Appl. Phys. Lett. 74, 2227,

1999.

15. D.B. Romero, M. Schaer, L.Zuppiroli, B. Cesar, B.Francois, Appl. Phys. Lett, 67,

1659, 1995.

16. F.Haung, A.G.MacDiamind, B.R.Hsieh, Appl. Phys. Lett 71, 2415,1997.

17. J. Blochwitz, M.Pfeiffer, T. Fritz, K. Leo, Appl. Phys. Lett. 73, 729, 1998.

18. A.Yamamori, C. Adachi, T. Koyama, Y. Taniguchi, Appl. Phys. Lett. 72, 2147,

1998.

19. A. Elschner, F. Bruder, H. – W. Heuer, F. Jonas, A. Karbach, S. Kirchmeyer, S.

Thurm, R. Wehrmann, Synth. Met. 111, 139, 2000.

20. F. Nuesch, F. Rotzinger, L. Si-Ahmed, L. Zupioli, Chem. Phys. Lett. 288, 861,

1998.

21. F. Nuesch, E.W.Forsythe, Q.T.Le, Y.Gao, L.J.Rothberg, J. Appl. Phys. 87, 7973,

2000.

22. S.F.J. Appleyard, S.R.Day, R.D. Pickford, M.R. Willis, Opt. Mater. 9, 120, 1998.

23. C. Ganzorig, M. Fujihira, Appl. Phys. Lett, 77, 4211, 2000.

28

24. X. Zhou, M. Pfeiffer, J. Blochwitz, A. Werner, A. Nollau, T. Fritz, K.Leo, Appl.

Phys. Lett. 78, 410, 1998.

25. S. A. Vanslyke, C.W. Tang, US Patent No. 5,061,569; 1991.

26. Y. Sato, S. Ichinosawa, H. Kanai, IEEE, J. Selected Top. Quantum Electron. 4,

40, 1998.

27. Y. Shirota, Y.Kuwabara, H. Inada, Appl. Phys. Lett. 65, 807, 1994.

28. Y. Hamada, N. Matsusue, H. Kanno, H. Fujii, Jpn. J. Appl. Phys. 40, L753, 2001.

29. J. Kido, T. Mizukami, US Patent No.6,013,384; 2000.

30. Z. Liu, J. Pinto, J. Soares, E. Pereiara, Synth. Met. 122, 177, 2001.

31. T. Sano, Y. Nishio, Y. Hamada, H.Takahashi, T. Usuki, K. Shibata, J. Mater.

Chem. 10, 157, 2000.

32. N. Johansson, J.Salbeck, J. Bauer, F.Weissortel, P.Broms, A. Anderson, W.R.

Salaneck, Adv. Mater. 10, 1136, 1998.

33. T. Noda, Y. Shirota, J. Am. Chem.Soc.,120, 9714, 1998.

34. T. Noda, H. Ogawa, Y. Shirota, Adv. Mater. 11,283, 1999.

35. H. Tokuhisa, M.Era, T. Tsutsui, S. Saito, Appl. Phys. Lett. 66, 3433, 1995.

36. J. Bettenhausen, P. Strohriegl, W. Brutting, H. Tokuhisa, T. Tsutsui, J. Appl.

Phys. 82, 4957, 1997.

37. H. Tokuhisa, M.Era, T. Tsutsui, Adv. Mater. 10, 404, 1998.

38. H. Tokuhisa, M.Era, T. Tsutsui, Appl. Phys. Lett.72, 2639, 1998.

39. M. Redecker, D.D.C.Bradley, M.Jandke, P.Strohriegl, Appl. Phys.Lett. 75, 109,

1999.

40. S. Naka, H. Okada, H. Onnagawa, T. Tsutsui, Appl.Phys.Lett.76, 197, 2000.

29

41. P.A. Cox, The Electronic Structure & Chemistry of Solid, Oxford university

Press, Oxford, 1987, p. 231

42. F. Nuesch, L.J. Rothberg, E. W. Forsythe, Q.T. Le. Y. Gao, Appl. Phys. Lett. 74,

880, 1999; Q. T. Le, F. Nuesch, L.J. Rothberg, E. W. Forsythe, Y. Gao, Appl.

Phys. Lett. 75, 1357, 1999.

43. C. C. Wu, C. I. Wu, J.C. Strurm, A. Kahn, Appl. Phys. Lett. 70, 1348, 1997.

44. M. Mizno, T. Miyamoto, Jpn. J. Appl. Phys. 39, 1849, 2000; A. Anderson, N.

Johansson, P. Broms, N. Yu, D. Lupo, W. R. Salaneck, Adv. Mater. 10 , 859,

1998.

45. H. Kim, C. M. Gilmore, J. S. Horwitz, A. Pique, H. Murata, G. P. Kushto, R.

Schlaf, Z.H. Kafafi, D.B. Chrisey, Appl. Phys. Lett. 76, 259, 2000; H. Kim, A.

Pique, J. S. Horwitz, H. Murata, Z. H. Kafafi, C.M. Gilmore, D.B. Crisey. Thin

Solid Films, 377, 798, 2000; J. Zhao, S. Xie, S. Han, Z. Yang, L. Ye, T. Yang,

Synth. Met. 114, 251, 2000.

46. M. Stossel, J. Staudigel. F. Steuber, J. Simmerer, A. Winnacker, Appl. Phys. A

68, 387,1999; M. Stossel, J. Staudigel, F. Steuber, J. Blassing, J. Simmerer, A.

Winnacker, H. Neuner, D. Metzdorf, H. –H. Johannes, W. Kowalsky, Synth. Met.

111 ,19,2000.

47. C. W. Tang, S.A. Vanslyke, C.H. Chen, J. Appl. Phys. 65, 3610, 1989.

48. S. Tokito, H. Tananka, K. Noda, A. Okada, Y. Tga, Appl. Phys. Lett. 70, 1929,

1997.

49. C. Adachi, K. Nagai, N. Tamoto, Appl. Phys. Lett. 66 , 2679, 1995.

30

50. Z. D. Popovis, S. Xie, N. –X. Hu, A. M. Hor, D. Fork, G. Anderson, C. Tripp,

Thin Solid Films 363, 6, 2000; G. Sakamoto, C. Adachi, T. Koyama, Y.

Taniguchi, C. D. Merritt, H. Murata, Z.H. Kafafi, Appl. Phys. Lett. 75, 766, 1999.

51. X. Zhou, J. He, L.S.Liao, M. Lu, X. M. Ding, X. Y. Hou, X. M. Zhang, X. Q. He,

S. T. Lee, Adv. Mater. 12, 265, 2000.

52. P. E. Burrows, V. Bulovic, S.R. Forrest, L.S. Sapochak, D.M. McCarty, M.E.

Thompson, Appl. Phys. Lett , 65 , 2922, 1994.

53. J. McElavain, H. Antoniadis, M.R. Hueschen, J. N. Miller, D.M.Toitman, J. R.

Sheats, R.L. Moon, J. Appl. Phys. 80, 6002, 1996; H. Aziz, Z. D. Popovict, S.

Xie, A. –M. Hor, N. –X. Hu, C. Tripp, G.Xu, Appl. Phys. Lett. 72, 756, 1998; H.

Aziz, Z. D. Popovict, C. Tripp, N. –X. Hu, S. Xie, A. –M. Hor, G.Xu, Appl. Phys.

Lett. 72, 2642, 1998; Y. F. Liew, H. Aziz, N. -X. Hu, S. –O, Chan, G. Xu, Z. D.

Popovic, Appl. Phys. Lett. 77, 2650, 2000.

54. Yuji Hamada, Takeshi Sano, Hiroyuki Fujii, Yoshitaka Nishio, Hisakazu

Takahashi and Kenichi Shibata, Jpn. J. Appl. Phys. 35, L1339, 1996.

55. J. T. Lim, N.H. Lee, Y.J. Ahn, G.W. Kang and C.H.Lee, Current Appl. Phys. 2,

295, 2002.

56. X. H. Zhang, O.Y. Wong, Z.Q. Gao, C.S.Lee, H.L. Kwong, S.T. Lee and S.K.

Wu, Mater. Sci. and Engg. B, 85, 182, 2001.

57. P.F.Barbara, L.E.Brus and P.M. Rentzepis, J. Amer. Chem.Soc.,102, 5631, 1980.

58. K.Ding, S.J. Courtney, A.J. Stranjord, S.Flom, D.Friedrich and P.F. Barbara,

J.Phys.Chem. 87, 1184, 1983.

31

59. Takashi Sasaki, Takashi Inoue, Yukari Komori, Satoshi Irie, Kensuke Sakurai and

Kyoji Tsubakiyama, Phys.Chem.Chem.Phys., 5, 1381, 2003.

60. C. Ganzorig, M.Fujihira, Appl.Phys.Lett. 77, 4211, 2000.

61. I. –Y.Wu, J.T.Lin, Y.-T.Tao, E. Balasubramaniam, Adv. Mater.12, 668, 2000.

62. I. –Y.Wu, J.T.Lin, Y.-T.Tao, E. Balasubramaniam, Y.Z.Su,C.-W.Ko.

Chem.Metr.13, 2626, 2001.

63. J.Salbeck, N.Yu, J.Bauer, F.Wiessotel, H.Bestgen, Synth.Met.91, 209, 1997.

64. U.Back,K.D.Cloedt, H.Spreitzer, M.Gratzel, Adv.Mat.12,1060,2000.

65. U.Mitschke, P.Bauerle, J.Mater.Chem.10, 1471, 2000.

66. Beniot Domerq, Caro Gresso, Jose – Luis Maldonado, Marcus Halik, Stephan

Barlow, Seth R. Mardes and Bernard Kippelen, J. Phys. Chem B, 108, 5147,

2004.

67. Q. Lin, C.Y. Shi, Y.J. Liang, Y.X.Zheng, S.B. Wang and H.J. Zhang, Synthetic

Metals 114, 373, 2000.

68. Kexhi Wang, Ling Huang, Lihua Gao, Chunhui Haung and Linpei Jin, Solid State

Commun, 122, 233, 2002.

69. J.-K.Lee, D.S.Yoo, E.S.Handy and M.F.Rubner, Appl. Phys. Lett.69, 1686, 1996.

70. J.L.Fox, C.H.Chen, US Patent No. 4,736,032, 1998.

71. T.Inoe, K.Nakatani, Japanese Patent No. 6,009,952, 1994.

72. J.Ito, Japanese Patent No. 7,166,160, 1995.

73. K.Yamashita, J.Futenma, T. Mori, T.Mizutani, Synth.Met. 111, 87, 2000.

74. C.H.Chen, J.Shi, K.P.Klubek, US Patent No. 5908581, 1999.

32

75. X.T.Tao, S.Miyata, H.Sasabe, G.J.Zhang, T.Wada, M.H.Jiang, Appl. Phys.Lett.

78, 279,2001.

76. M.Mitsuya, T. Suzuki, T.Koyama, H.Shirai, Y.Taniguchi, Appl. Phys.

Lett.77,3272,2000.

77. L.C.picciolo, H.Murata, Z.H.Kafafi, Appl. Phys. Lett.78, 2378, 2001.

78. S.Toguchi, Y.Morioka, H.Ishikawa, A.Oda, E.Hasegawa, Synth. Met.111, 57,

2000.

79. M. Ichimura, T.Ishibashi, N.Ueda, S.Tamura: Proceedings of the 3rd International

Conference on EL Mol. Mater. Relat. Phenom. (ICEL – 3), O – 29, Los Angelo’s,

CA, USA, 2001.

33

34

!

ππππ"ππππ#"ππππ#

$

% !

&" '

(

))*

* +

,-./,0..

&1/2&

35

""# $""# %

3 ) '""# (

4 5

% 67 ,.

+

"&' &()*&+,-.,,/"& &,-, &0

NH2

SH+ R - COOH

I II

S

NR

36

10 0

/ '

(

' /(

'(

/

+

/$2,,/%&2&' 0 &, &

H3PO4 +P205 H2P2O7

I

II

37

'&1-. ! 8 (

+

))*

&1-+ 8 9..: *

3 1!

7 ,-/&.

&1 / 2& 7

; 8 1< 8

))* *

38

( )$4-5%

3

! & /

))*

% (6 ,.

=.#

Scheme II. 3: Schematic of the mechanism involved in the microwave assisted reactions of benzheterazoles

R - COOH +

NH2

XH

O P O P O

O

O

O

O

+ +

R C O O P O P O

O

O O

OO

- -

- -

+ +NH3

+PPA-

XH

+

X

NR

39

%# )

6

; 8 1< 8

))*

5 )

>! >,&..

(&'&5/(

/&' &.,,/&.6 7,8 0 & &, 6 7(8

NH2

SH

+

OH

COOH

I II

S

NHO

1H NMR (CDCl3) δ 7.2351, 7.3632 (q, J = 6.84Hz), 6.9312 (td, J = 7.8 Hz, 8.56 Hz), 7.0838 (d, J

= 8.2 Hz), 7.6567 (d, J = 7.84 Hz), 7.8569 (d, J = 8.04 Hz), 7.9539 (d, J = 8.04Hz). IR, (KBr,

ν/cm-1) 3056.9, 1908.2, 1784.75, 1583.94, 1311.5, 969.23, 734.7

40

,...

,2.. ,9

2 -& -

$ &/

?

))* &.@ 9..:

,. 3 2! 1.@

; 8 1< 8

))*

A - & -

/

&/

))* ,- -1@

41

;

5

cis – 1,2 – dibenzthiazolyl ethylene - 1H NMR (DMSO – d6) δ 7.2473 (1H, d, J = 12.4

Hz), 7.9868 (1H, d, J = 7.6Hz), 7.8093 (1H, d, J = 8.04Hz), 7.3504 (1H, t, J = 3.9Hz),

7.456 (1H, t, J = 3.94Hz). IR (KBr; ν/cm-1) 1513, 1435, 937, 758, 647; Elemental

Analysis: %C: 65.23, %H: 3.4; %N: 9.48; %S: 21.20.

trans – 1,2 – dibenzthiazolyl ethylene - 1H NMR (DMSO – d6) δ 6.9031 (1H, d, J =

16.98 Hz), 7.8312 (1H, d, J=8.28 Hz), 7.7501 (1H, d, J = 8.28 Hz), 7.5017 (1H, t, J = 8

.84 Hz), 7.4377 (1H, t, J = 9.0 Hz), IR (KBr, ν/cm-1) 1510, 1429, 920, 745, 667;

Elemental Analysis: %C: 65.12, %H: 3.1; %N: 9.32; %S: 21.80.

Scheme II. 5: synthesis of stereoisomers of 1, 2 – dibenzthiazolyl ethylene I: Microwave radiation, 10 minutes, Yield: 40% II: Microwave radiation, 15 minutes, Yield: 54%

NH 2

SH

COOH

HOOC

HOOC COOH

S

N

S

N

S

N

N

S

I II

42

+ &2 -

$ & - /

&/

3 +!B ))*

7

&.@ 9..:

,& *

; 8 1< 8

C> 91@ ,1- "

,-.

7 ,.. .

,9..,-C9.@

43

* &2 -5-&( )>

$ & - "

& /

1H NMR (CDCl3) – δ 7.5243 (t, J=4.06 Hz), 7.6035 (t, J=3.96 Hz), 8.0869 (d, J

=4.99Hz), 8.187 (d, J=4.95 Hz), 7.974 (s), 13C – NMR: δ 160.33, 153.855, 140.297,

135.056, 128.787, 126.722, 125.71, 123.415, 121.58; IR (KBr, ν/cm-1) – 3059.32,

1593.78, 1426.16, 1307.65, 914.94, 796.24, 693.09. Elemental Analysis: C18H10N2S3:

%C – 61.62, %H – 2.84, %N – 7.94, % S – 27.44%.

+$ &-/

/ & +,,/& . ' & 6.,8 0 & &, 6 .(8

NH2

SH+

S COOHHOOC

I II

S S

NN

S

44

,...

- &...D2

C =-@

5

II. 7: Synthesis of 2, 5 – dihydroxy – 1, 4 – dibenzthiazolyl benzene I: Conventional heating, 200oC, nitrogen atmosphere, 72 hrs

NH2

SH+

S

N

N

S

OH

HO

COOH

COOH

HO

OH

45

)

9 ' 0 : 'E ! $ < "D(δδδδ9,229'F G9.18 (9.9-,'F G9.18 (

9&0-D'(20DD,'(2-9DD'F G9.98 (2-&=2'F G2=&8 (

: 'HI ννννJ",( / =.-0=& ,-0=29 ,1&D,D ,=.2D- 0,101

20D&1D0=.0

*

*

,

4

:

: " 6$; < &= &' 9 %/

0

NH2

XH

OH

HOOC

X

NHO

7,$(.%

+2$(.%

*,$(.%

7 $,%

12 $,%

..$,%

46

COOH

HOOC

X

N

N

X

) 7,$2%

(,$2%

.,$2%

COOHHOOC

NX

XN

)))) 7$,%

12$,%

..$,%

SHOOC COOH

N

XS

N

X

*,$(.%

*2$(.%

*($(.%

7,$,%

(2$,%

7$,%

COOH

HO

COOH

OH

X

N

N

X

OH

HO

+,$*%

+.$*%

*,$*%

)))))

./

.

! F $ + : ' (

,8 ; ! 7 ,= ; ! 7 F < K

F ; ! K! $ 7

I 4 && 37

4

K,,.9

47

,../&..

K

4

'&-1=1-(

./

&/&-/

&- /

$ E ! 3

4 5

7 "-/ >

>

',( ; ! 77

48

'&( + * '+ * ( E $

'E $ (

'=( '4 (

'1( B 4 /4 $

'-( '$ )(

'D(

'2( 3 '< 3(

'9( B ")$ 'B )$ (

'0( L /)$ 'L )$ (

',.(

7

'+ * (

+ *

"

"

.,µµµµ+ *

+ *

49

+ *

7? / $? /%

*

*

:

$

7/@

50

*

)7; < ; / ,...

3 * J*

.,! '/I (1; < 1J; ; 5/

'E ! 3(

7

$ ) '$ )(

"

$ )

A

51

) :

M,5

*

3

M5MI5

E

A.

/

/ /

'8 &9.0(

=,- )K

52

1.../-...J'N.-@ (

-/,.,.=J

7

B

E

" J

> K$ * 3 *

.,OF 1=. - 8

53

+ <

+ < "

0. "

CW,ML, KTP

SPCD

DYE SAC

SHG

PMT DELAY

CFD

TA BA MCA

PDP 11/03

Cyber 170

RM CFD PM

M

BS M

M

OSC R

M

M

M

SC

S ST

A

ST

1.06 532 nm

UV VISIBLE

Figure II. 8a: Schematic experimental set up

A – Aperture, BA – Biased amplifier, BS – Beam Splitter, CFD – Constant Fraction Discriminator, FB – Fiber Bundle, KTP – KTP Crystal, L – Lens, M – Mirror, MC – monochromator, MCA – Multi Channel Analyzer, NDF – Neutral Density Filters, OSC – Oscilloscope, P – Polarizer, PMT – Photo multiplier, R – Polarization rotator, RM – Rate meter, S – Sample, SAC – Scanning Auto Correlator, SC – Scatter Plate, SHG – Second Harmonic Generator ( KDP Crystal), ST – Beam Stop, TA – Time Analyzer

54

E

E .-,D

,D &

7 - -A>

/"

/; >C* + 'λλλλG=-- ττττGD&8 (

)N-F "&B

' ,.. ( <

3A 7! *

/

1 n sec light pulse N2 laser (337 nm) Dye laser (357 ~ 710 nm)

Bias Voltage

ITO transparent electrode

Sample

Metal electrode

R

Measurement of transient current

Figure II. 9: Block diagram of photo generated Time – of – Flight technique used.

55

K

µµµµ

µ<BτC """""""""""""""""""""""""',(

: ττττ ',N

&µµµµ(

7 D $D " % ; - $; " %

L )$

' (

I B )$

'

(

56

>"

A + hν →ν →ν →ν → A+ + e- ------------------------- (2)

Conservation of energy then requires that:

E (A) + hνννν = E (A+) + E (e-) ------------------------- (3)

Since the electron's energy is present solely as kinetic energy (KE) this can be rearranged

to give the following expression for the KE of the photoelectron:

KE = hνννν - [E (A+) - E (A)] ------------------------- (4)

The final term in brackets, representing the difference in energy between the ionized and

neutral atoms is generally called the binding energy (BE) of the electron - this then leads

to the following commonly quoted equation:

KE = hνννν - BE ------------------------- (5)

The basic requirements for a photoelectron experiments (XPS or UPS) are:

1. A source of fixed-energy radiation (an x-ray source for XPS or, typically, a He discharge lamp for UPS).

2. An electron energy analyzer (which can disperse the emitted electrons according

to their kinetic energy, and thereby measure the flux of emitted electrons of a

particular energy).

57

3. A high vacuum environment (to enable the emitted photoelectrons to be analyzed

without interference from gas phase collisions).

Such a system is illustrated schematically below:

Figure II. 11: Schematic diagram of the experimental set up used for the X – ray and Ultraviolet photoemission spectroscopy.

Figure II. 10: Schematic block diagram of processes involved in the photo emission spectroscopy

58

3

"

" >

! HP>QG,&-=D4 * HP>QG,19DD4

* * HP

. " ,&-. 4 . " ,19. 4

B )$

A 8 "8

&,&4

59

$

"

B 4 L "

,C /

,?

Vacuum deposition by resistive heating is most appropriate for depositing small

molecule based materials. Organic vapor phase deposition has also been demonstrated to

deposit organic materials on large substrates. It is also common to deposit cathode

materials using the same vacuum evaporation from filaments. For the deposition of high

– temperature metals one may employ e – beam evaporation or sputtering. The later is

particularly useful for large substrates and high throughput production. However,

OLEDs are extremely sensitive to radiation, and special care needs to be taken. In e –

beam deposition, a magnetic field is applied across the substrate to repel electrons and

ions. In sputter deposition, a buffer layer is required to minimize the radiation damage

inflicted on the OLED organic layer stack.

Typical device fabrication occurs by the following sequence: Devices are

grown on glass slides pre – coated with transparent ITO with a sheet resistance of 15 -

100/. Substrates are ultrasonically cleaned in detergent solution, followed by thorough

rinsing in deionized water. They are then cleaned in organic solvents and dried in pure

nitrogen gas. After cleaning, the ITO glass is subject to an oxygen treatment either using

60

UV ozone or oxygen plasma to enhance hole – injection. Single hetero structure devices

are formed by sequential high vacuum vapor deposition of a hole – transport layer such

as TPD, followed by an electron – transport layer. Deposition is carried out by thermal

evaporation from a baffled Tantalum or Molybdenum crucible at a nominal deposition

rate of 0.2 – 0.4 nm/s. An electron-injecting electrode is subsequently deposited by from

the Ta boats. The device preparation is completed with encapsulation in a dry argon box.

OLEDs are constructed using glassy and amorphous organic films and thus provide

significant advantages in device fabrication and cost reduction. They are pronouncedly

different in structures from inorganic LEDs consisting of epitaxial semiconductor thin

films.

All the electroluminescent characterizations were done with the help of

indigenously built measurements systems. Electroluminescent spectra and CIE

Power supply

Vacuum chamber

Source boats

Shutter Substrate

Thickness monitor

GND

Figure II. 12: Schematic of the experimental chamber used in our experiments

61

coordinates were recorded using MINOLTA – C 1000 A which has the wavelength range

of 380 – 780 nm, with a wavelength resolution of 0.9.nm/pixel, spectral band width of 5

nm (half band width) and has an accuracy in luminance with ± 2. Current – Voltage

characteristics were recorded on Keithley 2400 source meter coupled with the Keithley

voltmeter interfaced to a computer for data recording.

:

, : E * 7 ; ! 7$ F

: R $ * < KK,09D,",=.1

& I7$ 1,.11,0D,

= 4 * ) < -

KKI$ ,090

3

$ K KS

1. S. Caddick, Tetrahedron, 51, 10403, 1995.

2. Laurence Perreux and Andre’ Loupy, Tetrahedron, 57, 9199, 2001.

3. Pelle Lidström, Jason Tierney, Bernard Wathey, and Jacob Westman,

Tetrahedron, 57, 9225, 2001.

62

$ ,&/

63

III. 1. Introduction:

Device architecture employing organic molecules to achieve full color display holds

potential to construct cost effective, high brightness light emitting devices (OLED) [1- 4].

Organic / small molecules can play a pivotal role in such architectures, provided,

synthesis and photo physics of such molecules can be tailored. Among many candidates

that are currently being explored, for example: organic – polymer blends [5 - 12], organic

metal chelates in combination with organic dyes have shown interesting display

properties [13 - 17]. Organic dyes on the other hand have strong preference for red and

green light emission [18,19], while stable blue light dyes are scarce and are still being

explored. Alternately organic dyes, which can emit white light, can become crucial for

achieving full color emission [20 - 24]. To design a white light-emitting molecule it is

important to understand the type of charge transfer that is involved. It is well known that

the emission characteristics of the organic molecules are known to change with the

structural packing of the molecules [25 - 29]. Therefore molecular design should be such

that, during the lifetime of the excited species, the electron density could move to the

vicinity of the unexcited segment, in order to fluoresce entirely different, from that of the

molecular species. Such a charge delocalisation can lead to an emission at lower energies

to give a broadened spectral feature. We have attempted a rational synthesis of

stereoisomers of benzthiazole based organic molecules in order to demonstrate white

light emission, largely based on its structural conformation. The simplest of such

structures could be obtained by condensing fumaric/maliec acids with unsubstituted

benzthiazoles. We document here the emission features of 2,5 – dibenzthiazolyl ethylene

stereoisomers, where in selective white light emission is observed in one of the stereo

64

isomeric form, namely the cis form of 2,5 – dibenzthiazolyl ethylene. Dynamic photo

luminescent spectral studies give conclusive evidence for the existence of extensive

energy transfers more specifically in cis form of the isomer than the trans isomer. Semi –

empirical calculations show a strong intramolecular energy transfer in the solid form

arising due to an out of plane geometry for the cis isomer. We further demonstrate the

use of cis isomer as an emissive layer in a PFO blended electro luminescent device where

in the photo luminescent features of the cis isomer is best reflected.

III. 2. Experimental:

Synthesis of stereo specific isomers of DBE in high yields was obtained for the

first time at ambient conditions by a quick synthetic route involving microwave radiation.

In a typical experiment, as shown in scheme III.2, starting materials, 2 – amino

thiophenol and maleic/ fumaric acid were taken in a borosil vessel along with

polyphosphoric acid [30] which acts as an acid catalyst as well as a solvent for

microwave thermal heating. After 20 minutes of exposure, the compounds were obtained

by the work – up of the reaction mixture and subsequent washing with dilute NH4OH to

remove excess of polyphosphoric acid. Crude products were recrystallized from methanol

*

Scheme III. 1: Excited state interactions in molecular packed organic molecules

65

and purified using silica column chromatography. Formation and chemical purity of

these isomers were verified using NMR and FT – IR [31].

NH 2HS

COOH

HOOC COOHHOOC

(i) Polyphosphoric acidMicrowave radiation,(20%)15 min.(ii) HydrolysisYield: 64%

(i) Polyphosphoric acidMicrowave radiation (20%),20min.(ii) HydrolysisYield: 40%

S

N

N

S

S

N

S

N

Scheme III.2.: Synthesis of cis – DBE and trans – DBE using microwave

reaction conditions.

10 20 30 40 50 60 70 80 90

11.6

5

16.2

5

23.3

27.1

32.6

5

41.3

45.0

5

53.4

65 76.7

85.8

5

2θθθθ(deg.)

**

*

10 20 30 40 50 60 70 80 90

10.1

9

15.4

4

23.1

4 30.5

4

46.3

9

61.8

4

2θθθθ (deg.)

Figure III. 2.1: shows the powder X – ray diffraction patterns for the cis and trans conformations. Note the extra reflection, which arises due to the out of plane geometry of the molecule in comparison to the trans isomer.

66

Figure III. 2.1. shows the powder X – ray diffraction patterns for the

stereoisomers using Cu – K, radiation. Owing to its expected bent crystal structure cis –

isomer is expected to scatter more in the X – ray analysis which is evident from the

diffraction patterns shown. Cis – DBE gives extra reflections compared tot he trans –

DBE, indicative of more distorted structure. Absorption measurements were carried out

using Perkin – Elmer spectrophotometer. Photo luminescent emission measurements

were carried out on Hamamatsu C4334. The fluorescence decay was recorded by

stroboscopic sampling using a gating pulse applied to the photo cathode of the GOI,

which could gate the detector for on-times of as little as 90 ps. The fluorescence decay

could then be built-up by varying the delay between the excitation and detection and

recording a series of images on the CCD camera.

Theoretical measurements were done using molecular mechanical MM+ and

AM1/CI empirical models using Hyper Chem. 7.5 molecular modeling software. Figure

50 100 150 200 250 300

0

-20

-40

-60

-80

-100

-120

-140

-160

-180

50 100 150 200 250 300

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2 200 0C

Hea

t Flo

w E

ndo

Up

(m W

)

Delta H = 437.5 J/gm.

140oC

Temperature (oC)

Wei

ght (

mg)

Figure III.2. 2: TG – DSC curves of cis – form of the isomer showing phase transition from cis - to trans – form of the isomer at 140oC.

67

III. 2. 2 shows the TG, DSC curves of CDBE which shows a phase transition from cis –

form of the isomer to the trans form of the isomer indicated by an endothermic peak at

140oC with a H value of 437 Jg-1.

III. 3. Results and discussion:

Absorption spectra of the cis – DBE and the trans – DBE in solid state and in solution

form are shown in Figure III. 3.1. The cis – DBE absorbs below 380 nm and the trans –

DBE absorbs below 270 nm in the solid state. Absorption spectra of the cis –DBE and

trans - DBE in the N, N’ – dimethyl formamide (DMF) at a concentration of 2 x 10-6 M

were recorded. Absorption spectra of the cis – isomer gives absorption edge unusually at

lower energies than that of the trans – isomer wherein conventionally it is expected that

trans form shows absorption at lower energies than cis form of the isomer. This red shift

in the absorption gives clue about the extended delocalisation of electron density in the

cis isomer compared to that of the trans isomer. Absorption spectra in the solid and

solution at low concentrations show similar trend, which is indicative of absence of any

dimeric structure in solid form that can lead to the lower energy absorption in the cis

form.

250 300 350 400 450300 400

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.00

0.01

0.01

0.01

0.02

0.03

0.03

0.04

0.04

trans - DBE in solid state

cis - DBE in solid state

Abs

orba

nce

(sol

id st

ate)

cis - DBE in DMF

Absorbance (2x10

-6M)

W a v e l e n g t h ( n m ) Figure III.3.1: Absorption spectra of the cis – DBE and trans – DBE in solid and in solution state. Solution spectra was recorded in N, N’ – dimethyl formamide at a concentration of 2 x 10-6M.

68

The possibility of aggregate formation is ruled out based on the observance of emission

spectra with different excitation wavelength. It is well known that when there is

aggregate formation in the excited state the photoemission characteristics will alter with

different excitations. Figure III.3.2 shows photo emissive features at different excitations.

Solid-state photo luminescent (PL) emission characteristics of the cis and the

trans isomers are shown in Figure III. 3. 3.a. When excited at the absorption maximum

(370nm), cis –DBE shows a structured, broad (FWHM ~ 170nm) emission, while the

trans – DBE (absorption maximum 280 nm) shows an unstructured emission (FWHM~

130nm) at higher energy. Figure III.3.3.b, depicts the emissive spectra of CDBE in DMF

solution at a concentration of 2 x 10 –6 M, which shows narrowing of the emission. The

trans form of the DBE doesn’t shows any considerable emission which gives clue about

the existence of intra molecular charge transfer in case of cis – DBE compared to the

trans form of the isomer, which could arise from their differences in the solid state

packing of the isomers [32]. The mechanistic aspects of this interesting behaviour gets

clearer by looking at the nano – second time decay and time resolved photo luminescence

dynamics. The life - time decay traces of the integrated emission between 450 and 600

nm measured in solid state, are shown in the Figure III. 3.4. The PL decay of both the

trans and the cis isomer can be fitted with a double exponential decay model with life

400 450 500 550 600 650 7000.0

0.1

0.3

0.4

0.6

0.7

0.9

1.0 cis - DBE at 300nm (closed)cis - DBE at 400nm (open)

I n t

e n

s i t

y

W a v e l e n g t h ( n m )

Figure III.3.2: Excitation dependant photo luminescent spectra of cis - DBE showing nochange in the emission features with different excitation wavelengths indicating absenceof aggregate formation in the excited state

69

times of 0.7 and 3.1ns, and 0.7 and 2.4 ns respectively. However, for the cis – DBE

amplitude of the fast component in the decay is larger compared to that of the trans –

DBE. The differences in decay rates in the cis and the trans isomers give a clue to the

extended energy transfer in case of the former as compared to the trans form of the

isomer as reported by Peng et al. [33]. It is known that with the increase in the

concentration of the species there will be more intra molecular energy transfers which

leads to the faster decay processes.

Figure III. 3.3.b The emission spectra of cis – DBE in N, N’- dimethyl formamide at a concentration of 2 x 10 –6 M.

400 450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

I n t

e n

s i t

y

W avelength(nm)

450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

Figure III. 3.3.a. Photo luminescent emission spectra of cis-DBE (solid line)and trans - DBE (dashed line) in the solid state with FWHM ~ 170nm and130nm, respectively.

trans - DBEcis - DBE

I n t

e n

s i t

y

W a v e l e n g t h ( n m )

70

This phenomenon is observed specifically in the cis form of the isomer owing to

its more closed structure, which resulted in the extended energy transfers in the cis -

isomer, evident from its faster decay compared to that of trans isomer. Nano second time

decay in dilute conditions in DMF solvent in comparison with the decay spectra in the

solid form is shown in Figure III.3.5. The trans form of the isomer doesn’t show any

emission in dilute DMF, but the cis – isomer shows considerable emission, which is

much slower as compared to the solid-state cis – DBE. This can be understood by

considering the fact that intramolecular interactions which become considerably small as

the dilution increases, because of the structural relaxation in the solution [33], contributes

to the slow emission as compared to the decay in the solid state. This also gives clue

about the possibility of non – existence of any intra molecular interactions in case of the

trans – isomer which doesn’t show any emission in the dilute conditions.

0.1

1

10

0 5 10 15 20

I n t

e n

s i t

y trans - DBE

cis - DBE

T i m e ( n s )

Figure III. 3.4. Fluorescence decay traces of cis – DBE and trans – DBE in the solid state.

71

To further support our conclusions, we performed nano second time resolved

spectroscopy. The spectral features of both decay components can be seen in figure

III.3.6. Time resolved spectra are built-up by varying the delay between the excitation

and detection and recording a series of images on the CCD camera (see figure III. 3.6.a).

We observed drastic differences between the time resolved properties of the cis and the

trans form of the isomers. The trans – DBE has no time dependence of its emission

spectrum, while the cis – DBE shows a strong red – shift during the first few

0 5 10 15 20 25 30 35 40 45 50

0.01

0.1

1

0 5 10 15 20 25

0.1

1

10

I n t

e n

s i t

y

Time (ns)

cis - DBE in dmf

I n t

e n

s i t

y

T i m e ( n s )

Dmalsolcis - DBE in solid

Figure III.3.5: Fluorescence decay patterns of cis – DBE in solid state and in N, N’ –dimethyl formamide

72

nanoseconds. The marked difference between the two isomers can be explained by the

differences in either inter – or intra molecular interactions which could arise out of

structural packing of the two materials. The planar character of the trans – DBE (-C=C-

C-N dihedral angle ~1770) and the out – of - plane twisted structure of the cis – DBE (-

C=C-C-N dihedral angle ~ 400) are likely to introduce differences in the solid – state

packing of these two materials. A planar system like trans – DBE is more likely to

introduce strong interactions coming from co – facial orientations in the solid state. This

could possibly lead to the formation of exciplex or aggregate in the excited state. These

exciplex or aggregate type interactions usually lead to a decrease in the optical band gap;

hence we expected a low band gap in case of trans over the cis isomer. However, the

fact that the out – of - plane cis – DBE shows considerable shift in the spectra towards

red region giving low band gap than the trans form of the isomer suggest that the

differences are probably of an intra – molecular nature.

0

5

10

15

20

25

30

450 500 550 600 650

T i m

e

( n s

)

Wavelength (nm)

CDBE

450 500 550 600 6500

5

10

15

20

25

30

Wavelength (nm)

T i m

e

( n s

)

TDBE

73

450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

cis - DBE

6 ns

5 ns

4 ns

3 ns

2 ns

I n

t e

n s

i t

y

W a v e l e n g t h ( n m )

0.0

0.2

0.4

0.6

0.8

1.0 trans - DBE

2-6 nsI n t

e n

s i t

y

Figure III. 3. 6: Time resolved photo luminescence spectra of cis – DBE and trans – DBE in the solid state.

74

III. 3.A. Theoretical studies:

The geometries of the molecules in the ground state were optimised with the

semi empirical Austin Model 1 (AM1) method. To calculate the geometries associated

with the lattice relaxation followed by excited state formation, the AM1 Hamiltonian was

coupled to a configuration interaction (CI) technique. Figure III.3.7 shows the frontier

orbitals of both the cis – and the trans – DBE, based on the semi – empirical AM1/CI

calculations [34]. From the frontier orbitals it is evident that the cis – DBE HOMO level

has the charge localized on the benzthiazolate rings while the excited states have an

increased pi – electron density distributed along –N=C-C=C- linkage leading to a reduced

torsion degree of freedom. This results in a decrease in the twist angle between the

vinylene unit and the benzthiazole rings. These orbitals show a redistribution of the

electron density predominantly in the core region of the cis – DBE. However, the

frontier orbitals for the trans isomer gives an entirely different feature wherein even in the

ground state, the HOMO level has a symmetrical electron distribution. The excited state

does not show any significant change in the π – electron distribution, leading to a

predominantly single component emission. This augments our PL spectra as shown in

Figure III.3.3.a. The energy level diagram calculated from AM1/CI model is shown

Figure III.3.8. The optical band gap of the trans form is higher compared to the cis form.

However, the higher LUMO levels of cis isomer namely LUMO +1 and LUMO +2 levels

have discrete energies leading to a featured emission as seen in Figure III.3.3.a. This

argument goes well with our dynamic PL studies (cf. Figure III.3.6.b), which shows a

time dependent red shift in the case of cis - DBE. However in the case of trans isomer

the emission appears to be predominantly from the first excited state to the ground state.

Therefore, we attribute the drastic changes in the emission characteristics of the cis –

DBE to its structural features, compared to the trans – isomer.

75

HOMO

LUMO

HOMO - 1

LUMO + 1

LUMO

HOMO

HOMO - 1

LUMO + 1

Figure III.3. 7: RHF/AM1-CI electron density distribution of the cis – DBE and the trans – DBE. The cis – DBE shows strong electron density reorientation in the excited state which results in strong time dependant red shifted spectra, where as trans – DBE doesn’t show any redistribution in the electron density.

LUMO+2 LUMO+1 LUMO HOMO

trans-

LUMO+2 LUMO+1 LUMO HOMO

Figure III.3.8: The predicted energy level of the cis – DBE and the trans –

DBE. The emission decay in the cis form appears to originate from many of

the excited singlet states, leading to a featured broad emission.

76

III. 4. Electro luminescent Characterization:

Electro luminescent characteristics of the stereo - isomers were studied in detail. Trans –

form of the isomer doesn’t give appreciable light emission when fabricated using

different ways. Being sensitive to the heat treatment we chose solution-processing

methods to deposit the cis – DBE. We fabricated the organic light emitting diode

structure using CDBE as an active layer with polyflourene blend using spin coating

method (see Figure III. 4. 1). Owing to the large HOMO level of PFO ( ~ 5.0 eV), we

employed PEDOT: PSS, as the hole injection layer as well as for the smoothening of the

ITO surface. A layer thickness of 100 nm was spin coated on to the substrate using 10%

CDBE as an active layer doped in polyflourene matrix. The device structure is

ITO/PEDOT: PSS/PFO: CDBE/Al. We expected a energy transfer from CDBE to PFO

because of the partial overlap between the absorption band of CDBE and the emission

spectra of PFO which facilitates efficient energy transfer and hence the brightness

efficiency. As expected, this device configuration gave a white light emission with full

width half maxima of about 170 nm at an applied voltage of 8V as shown in Figure III.

4. 2. To rule out the possibility of observed electro luminescence from the oxidized PFO

matrix, we performed all the measurements in strict nitrogen atmosphere under repeated

conditions [35]. Figure III. 4.2.a shows the current – voltage characteristics of the device

structure, which has a threshold voltage of 8V with an initial brightness of 30 cd/m2

reaching 50 cd/m2 at an applied voltage of 14V.

C8H17 C8H17n

PFO

S

OO

n

PEDOT

S O3-

PSS

SN

S

N

CDBE

Figure III. 4. 1: Organic materials used in electroluminescent devices

77

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

Ele

ctro

lum

ines

cenc

e

Wavelength(nm)

Figure III .4.2: Electro luminescent (EL) spectra of ITO/PEDOT: PSS/PFO: CDBE/Al device is shown. cis – DBE is the emissive layer in a PFO host matrix as shown in the device. Note the close resemblance of the emission curves between the EL device and the PL spectra of the cis – DBE (cf. Figure 1).

Figure III.4.3: Current – Voltage characteristics of the device, ITO/PEDOT:PSS/PFO:CDBE/Al, operating at a threshold voltage of 8V.

0 2 4 6 8 10 12 14

0.00

0.01

0.02

0.03

0.04

0.05

0.06

Cur

rent

den

sity

(mA

/cm

2 )

Voltage (V)

Al CDBE:PFO PEDOT:PSS ITO

78

III.5: Summary: In summary, for the first time we have shown the influence of stereo

specificity, which controls the emission characteristics of two DBE stereoisomers. These

are mainly governed by intra – molecular steric effects, leading to differences in the

solid-state packing order and, more importantly, differences in the intra – molecular

electron density distribution causing ‘charge resonance’ in the excited state.

Understanding organic molecule in the light of intra molecular charge transfers helps in

designing organic molecule, which can give desirable emission. We have demonstrated

white light electro luminescence based on the cis form of the isomer that shows a full

width half maxima of ~ 170 nm at an applied voltage of 8V.

79

III. 6: Notes and references:

1. M. Gross, D. C. Mueller, H. – G. Mothofer, U. Scherf, D. Neher, C. Braeuchle, K.

Meerholz, Nature, 405, 661,2000.

2. M. A. Baldo, D.F.O’. Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, S. R.

Forrest, Nature (London), 395, 151, 1998.

3. R. C. Aaron, I. L. Medintz, M. Mawro, B. R. Fisher, M. G. Bawendi, H. Mattougi, J.

Amer. Chem. Soc, 126, 310, 2004.

4. L. D. Carlor, R. A. Sa Ferreira, J. P. Rainho, V. de zea Bermudez, Adv. Funct. Mater,

12, 1, 2002.

5. J. L. Fox, C. H. Chen, US Patent No, 4,736,032, 1998.

6. T. Inoe , K. Nakatani, Japanese Patent No. 6,009,952, 1994.

7. J. Ito, Japanese Patent No. 7,166,160, 1995.

8. D. Thetford , A.P. Chorlton, Dyes and Pigments, 61,49, 2004.

9. M. A. Metwally, E. Abdel-latif, A. M. Khalil, F. A. Amer, G. Kaupp, Dyes and

Pigments, 62 , 181,2004.

10. T. G. Pavlopoulos, P. R. Hammond, J. Am. Chem. Soc., 96, 6568, 1974.

11. H. Tokailin, M. Matsuura, H. Higashi, C. Hosokawa, T. Kusumoto, SPIE, 38, 1910,

1995.

12. C. Hosokawa, S. Sakamoto, T. Kusumoto, US Patent No. 5,389,444, 1995.

13. M. A. Baldo, D.F.O’.Brien, Phys. Rev.B., 60, 14422, 1999.

14. D.F. O’.Brien, M. A. Baldo, M.E.Thompson, S. R. Forrest, Appl. Phys. Lett., 75, 4.

1999.

80

15. V. Adamovich, J. Brooks, A. Tamayo, A. M. Alexander, P. I. Djurovich, B. W.

D’Andrade, C. Adachi, S. R. Forrest and M. E. Thompson, New J. Chem.,26, 1171,

2002.

16. J. T. Lim, N.H. Lee, Y.J. Ahn, G.W. Kang, C. H. Lee, Current Appl. Phys., 2,295.

2002.

17. X. Y. Jiang, Z. L. Zhang, B. X. Zhang, W. Q. Zhu, S. H. Xu, Synt. Met. 129, 9. 2002.

18. J. Qiao, Y. Qui, L. Wang, L. Duan, Y. Li, D. Zhang, Appl. Phys. Lett., 81, 4913,

2002.

19. K. O. Cheon and J. Shinar, Appl. Phys. Lett, 81, 1738. 2002.

20. H. Yuji, S. Takeshi, F. Hiroyuki, N. Yashitaka, T. Hisakazu, S. Kenichi. Jpn. J. Appl.

Phys, 35, L 1339,1996.

21. J. T. Lim, N. H. Lee, Y.J. Ahm, G. W. Kang, C. H. Lee, Current Applied Physics, 2,

295, 2002.

22. G. Yu, S. Yin, Y. Liu, Z. Shuai, D. Zhu. J. Am. Chem. Soc., 125, 14816, 2003.

23. P.F. Barbara, L.E. Brus, P.M. Rentzepis, J. Am. Chem. Soc., 102, 5631, 1980.

24. Q. Mohammad and S.S. Manoharan, Phys. Stat. Solidi (a).(accepted)

25. F. J. Hirayama, Chem. Phys. 42, 3163, 1965.

26. A. Hartschuh, I.B. Ramsteiner, H. Port, J. M. Endtner, F. Efenberger, J.

Luminescence, 108, 1, 2004.

27. T. Hirsch, H. Port, H.C. Wolf, B. Miehlich, F. Effenberger, J. Phys. Chem. B, 101,

4525, 1997.

28. N. Lokan, N. Micheal, P.–Row, A. T. A, Smith, M. La Rosa, P. K. Ghiggino, S.

Speriser, J. Am. Chem. Soc., 121, 2917, 1999.

81

29. J. A. Hudson, R. M. Hedges, Molecular Luminescence, W.A. Benjamin Inc. New

York, p. 667. 1969.

30. To synthesize PPA, stiochiometric amounts of phosphorous pentoxide and

orthophosphoric acid (S.D. Fine Chemicals, India) were taken in a borosil beaker

(200ml), kept in the microwave oven. Exposure time is 7 minutes in one-minute pulses at

a power level of 20% of total 800W.

31. cis – 1,2 – dibenzthiazolyl – ethylene; 1H NMR (DMSO – d6) δ 7.2473 (1H, d, J =

12.4 Hz), 7.9868 (1H, d, J = 7.6Hz), 7.8093 (1H, d, J = 8.04Hz), 7.3504 (1H, t, J =

3.9Hz), 7.456 (1H, t, J = 3.94Hz). IR (KBr; ν/cm-1) 1513, 1435, 937, 758, 647; Elemental

Analysis: %C: 65.23, %H: 3.4; %N: 9.48; %S: 21.20; trans – 1,2 – dibenzthiazolyl –

ethylene; 1H NMR (DMSO – d6) δ 6.9031 (1H, d, J = 16.98 Hz), 7.8312 (1H, d, J=8.28

Hz), 7.7501 (1H, d, J = 8.28 Hz), 7.5017 (1H, t, J = 8 .84 Hz), 7.4377 (1H, t, J = 9.0 Hz),

IR (KBr, ν/cm-1) 1510, 1429, 920, 745, 667.; Elemental Analysis: %C: 65.12, %H: 3.1;

%N: 9.32; %S: 21.80.

32. We resorted to the powder X –ray diffraction data due to unsuccessful attempts to get

single crystals of both the isomers. Powder X – ray diffraction patterns are shown in

Figure III.2.1. Note the extra diffraction peaks for the cis form which indicates a more

distorted structure compared to the trans isomer.

33. K. –Y. Peng, S. -A.Chen, W. –S.Fann, J. Am. Chem. Soc., 123, 11388, 2001.

34. For initial geometrical minimization we employed molecular mechanical MM+

model, the geometry is further optimised using Restricted Hartree Fock AM1/CI.

35. M. Sims, A. Marilu, A. Aristidis, M. Koeberg, S. Mathias, M. Fox, D.D.C. Bradley,

Proc. SPIE Int. Soc. Opt. Eng, 216, 5214, 2004.

82

/@

:

T

- $)$4-5

%%E $% 5&

F 5

50

5

83

IV. 1. Introduction:

Bis (2 – (2’ – hydroxyl phenyl) benzthiazolate) Zinc (II) complex, ZBZT, has

been studied as an effective white light emissive and electron transport material in

electroluminescent (EL) devices [1,2]. An alternate strategy of fabricating white light

emitting diodes is to use electro phosphorescent materials involving a singlet – triplet

annihilation, for example Pt [3] and Ir [4] complexes. Alq3 is one of the promising

materials for the use in as an electron – transport and emissive material, in combination

with different dye conjugates such as DMQA, and with metal complexes such as PtOEp,

for the full color displays [5]. Single chromophoric white light emission has been a key

issue in ZBZT complex with respect to the electro luminescent emission, which offers

flexibility in the device-making phenomenon [6]. However, the physical properties of this

complex were not well understood; since the photo and electro luminescent properties of

this material were explained based on the presumed monomeric structure [7]. More

recently, Yu et al have reported the crystal structure of this compound, demonstrating the

unique dimeric nature of this complex, as shown in Figure IV.1, wherein it exits as

[ZBZT] 2 in both powder and in the amorphous thin film form, [8]. Using the

photoemission, emission decay and the electrochemical methods, we show that within the

dimeric form of the complex, the electron density distribution was localized on the non-

bridged benzthiazolate moiety in the excited state and hence the emission and the

transport properties were controlled by the benzthiazolate groups. Mechanistic aspects

related to the broadband emission were understood based on the exciplex formation at the

organic – organic interface. We further substantiate the experimental results with semi-

empirical calculations (ZINDO/S-SCF-CI), which indicate the electron density

84

distribution in LUMO levels of the complex to be more localized on the non - bridged

benzthiazolate groups, controlling the emission characteristics.

Figure IV. 1: Crystal structure of bis (2-(2’ – hydroxyl phenyl) benzthiazolate) Zinc (II) [taken from Reference: 8]

O

Zn

O

NO

N

Zn

NO

N

85

IV. 2. Experimental:

IV. 2.A: 2 - (2’ - hydroxyl phenyl) benzthiazole, (BZT): BZT was synthesized for the

first time employing microwave radiation as shown in scheme IV.1. Figure IV.2 shows

the thermal analysis of the BZT.

IV. 2.B: bis (2 - (2 - hydroxyl phenyl) benzthiazolate) Zinc(II): Zinc complex of 2,2 –

hydroxy phenyl benzthiazole was prepared by taking stiochiometric amount of Zinc

Acetate and BZT in ethanol and stirred for half an hour at 60oC. A pH of 8 – 9 was

maintained for complexation. Zinc complex was vacuum sublimed prior to use. m.p.

450oC. 1H – NMR, 7.2483, 7.432(q, J = 6.84Hz), 6.9312 (td, J = 7.8 Hz, 8.56 Hz),

7.4112(d, J = 3.78 Hz), 7.6567 (d, J = 7.84 Hz), 7.8569 (d, J = 8.04 Hz), 8.2993 (d, J =

6.67Hz). IR, cm-1 (KBr): 3058.06, 1709.6, 1541.8, 1343.43, 975.23, and 750.7. Figure

IV.3 shows the DSC curve of ZBZT. As can be seen a large endothermic transition is

observed at 310oC indicative of dimeric to monomeric transition.

Zn (CH3COO)2. 2H20 + BZT ZBZT60OC, 2hrs. stirring

NH2

SH

OH

COOH

S

NHO

Poly phosphoric acidMicrowave radiation, Ambient conditions15 minutes.

Poly phosphoric acid160-1700C, N2 atmosphere24 hours,Mechanical stirring

+

Scheme IV. 1: Synthesis of 2,2- hydroxy phenyl benzthiazole using conventional and microwave reaction conditions as described in chapter II.

86

Figure IV.2: TG – DTA curves of 2 - (2’ – hydroxyl phenyl) benzthiazole

87

Optical absorption and Emission measurements were carried out on PERKIN –

ELMER and SPEX-FLOUROLOG spectro - flourimeter respectively. Differential

Scanning Calorimetric measurements were performed using Perkin – Elmer Differential

Scanning Calorimeter. The cyclic voltamograms (CV) of the ligand and complex were

measured at room temperature under argon atmosphere, using Ag/AgCl electrode as

reference electrode and platinum wire as an auxiliary electrode supported in 0.1M (n-Bu)

4 NClO4/ N, N’ – dimethyl Formamide (DMF) solvent. The time-decay fluorescence

measurements were made using a high- repetition rate pico second laser coupled to a time

- correlated single photon counting spectrometer, currently using a micro-channel plate

photo multiplier (Hamamatsu 2809). The samples were excited at 375 nm laser pulses

and the emission at their PL peak maxima were collected. The full-width at half-

100 200 300 400

0

5

10

15

20

25

30

35

40

45

Hea

t Flo

w E

ndo

Up

(mW

)

Temperature (oC)

DSC- ZBZT

Figure IV.3: DSC curves of ZBZT under Nitrogen atmosphere

88

maximum of the instrument response function was approximately 200 ps. Typical count

rate for fluorescence decay measurements was about 4000-5000/s (~ 0.5% of the

excitation rate) and the typical peak count was 5-10 x 103[9]. All the semi – empirical

calculations were carried out using Hyperchem molecular - modeling lab software.

IV.3: Results and Discussions: The crystal structures of the reported [8] dimeric ZBZT involve Zinc ions in five

coordination with the benzthiazolate ligands and have distorted trigonal bipyramidal

geometry. Energy minimized structure of the dimeric Zinc complex is shown in Figure

IV.1. Two out of four benzthiazolate ligands are involved in a bridged conformation

between two Zinc atoms while the other two-benzthiazole ligands are solely attached to

either of the Zn atoms. From the single crystal X – ray data, calculated Zn – O bond

lengths for the non – bridging ligands are 1.962 Å, where as those in the two bridging

ligands lengthen to 2.058 Å. Similar observations are predicted for Zn – N bond lengths.

DSC curve (Figure IV.3) shows a single endothermic transition at 3100C, which confirms

the existence of a dimeric species in accordance with the literature value [8]. Photo

luminescent behavior of the ligand and complex studied in solid state are shown in Figure

IV.4a. BZT and [ZBZT] 2 were excited at 325 nm owing to their broadband absorption

spectra. Figure IV. 4b shows the optical image of the vacuum-evaporated film of [ZBZT]

2 on a glass substrate. Film evaporation was done at a base pressure of ~10–6 mbar at a

rate of 0.2 – 0.3 nm / sec. Optical micrograph shows smooth and uniform [ZBZT] 2 films.

Figure IV .4 shows the photo luminescent spectra of the ligand, BZT, and of the

complex in solid state, when excited at 325 nm. In the case of BZT molecule, an

unsymmetrical structured emission with peak maxima at 511 nm having a shoulder at 550

89

nm was observed. BZT has been studied extensively in the solution state, owing to its

interesting proton transfer kinetics. Barbara et al demonstrated that the two intense

emission peaks at 480 nm and 520 nm could be attributed to the H – transferred keto –

enol tautomeric structures respectively in the solution state [10,11]. However, in the

solid state, energetically favored “enol” form is preferred. As a result, in the solid state,

the peak at 480 nm completely disappears due to the fact that the torsional motion is

virtually frozen so that the H – bonded enol conformer is preferred energetically. The

shoulder at 550 nm is attributed to the n - π* transition due to presence of hetero - atoms

in the molecule. However, when complexed with the Zinc metal ion, we observe a blue

shift (λmax = 485 nm), in contrary to the usual red shift observed for the complexes. This

could be due to the localization of electron density in the excited state.

.

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

400 500 600 700

BZT [ZBZT]

2

I n t

e n

s i t

y

W a v e l e n g t h ( n m )

Figure IV. 4: (a) Photo luminescent spectra of ligand and the complex in solid state. (b) shows the 80 nm film forming ability of vacuum evaporated [ZBZT] 2 at a rate 0.2 – 0.3 nm/s at 10-6 mbar pressure.

90

The fluorescence decay curves of the ligand and complex in solid state on

excitation at 325nm are shown in figure IV. 5. The analysis of the decay curves was

performed by non - linear least squares routines minimizing the χ2 parameters (~ 1) using

the supplied software. BZT fits to a single exponential decay pattern with a lifetime of

4.8 ns, where as ZBZT fits to a double exponential decay model with life times of 2.3 ns

0.0 0.5 1.0 1.5 2.0 2.5 3.0

10

100

1000

0.0 0.5 1.0 1.5 2.0 2.5 3.0

10

100

1000I n

t e

n s

i t y

fluor. lifetime(ττττ1) = 2.354ns;

fluor. lifetime(ττττ2) = 5.236 ns;

Average Lifetimes: 3.027ns

fluor. lifetime = 4.822 ns.

BZT

[ZBZT]2

T i m e ( n s )

Figure IV. 5: Photo luminescent time decay spectra of BZT and [ZBZT]2 excited

at 315 nm. BZT follows a single exponential decay pattern while [ZBZT]2 follows

a double exponential decay profile with average lifetimes of 4.82 ns and 3.02 ns

respectively. Also shown the residual fits of ligand and complex, which gives a

good minimum χ2 value.

91

(τ1) and 5.2 ns (τ2) respectively. The second lifetime component of the complex has a

close match with that of ligand that reveals the dominance of the benzthiazole moiety in

controlling the emission process of the Zinc complex.

The cyclic voltammogram of the Zinc complex is shown in Figure IV. 6. On

sweeping ZBZT in cathodic direction one oxidation peak at –1.82 V which matches

closely with that of oxidation potential of 2, 2’ – hydroxy benzthiazole ligand at -1.80 V,

which gives clue about the ligand controlled transport in the complex which was in good

agreement with the observations of the time decay PL measurements that the

participation of ligand moiety was more prominent.

-1.4 -1.6 -1.8 -2.0-0.006

-0.004

-0.002

0.000

0.002

0.004

0.006

C u

r r

e n

t (

a r

b .

u n

i t s

)

V o l t a g e ( V )

Scan Speed =100mV/s Std. Electrode= Ag/AgCl Aux. Electrode= Pt wire Cathodic sweep.

Figure IV. 6: Cyclic voltammograms of [ZBZT]2 in (n – Bu)4 NClO4 / N,

N’ – Dimethyl Formamide solvent. Concentration: 1 x 10–3M.

92

HOMO

LUMOMO

IV. 4: Semi empirical calculations:

PM3 – CI calculations [12] shows (see Figure IV. 7) that the electron density

distribution in the filled π -orbitals (HOMO’s) and the unfilled orbitals (LUMO’s) were

mainly dominated by orbitals originating from those of the 2- (2- hydroxyphenyl)

benzthiazolate ligand in the complex.

Figure IV. 7: Electron density distribution in HOMO and LUMO levels of

[ZBZT]2 calculated from PM3-CI based on UV/VIS absorption energy spectrum

93

Structural Parameter Calculated* Reported** Zn (1) – O (1) 1.9708 1.962

Zn (1) – O (2A) 1.9977 2.017 Zn (1) – O (2) 2.201 2.058 Zn (1) – N (1) 1.9955 2.095 Zn (1) – N (2) 2.0487 2.170

O (1) – Zn (1) – O (2A) 99.1 97.4 O (1) – Zn (1) – O (2) 164.21 172.18 O (2A) – Zn (1) – O (2) 62.49 76.13

O (1) – Zn (1) – N (1) 90.72 87.37 O (2A) – Zn (1) – N (1) 106.76 120.09 O (2) – Zn (1) – N (1) 87.01 99.63 O (1) – Zn (1) – N (2) 96.24 97.26

O (2A) – Zn (1) – N (2) 120.05 117.12 O (2) – Zn (1) – N (2) 79.21 82.13 N (1) – Zn (1) – N (2) 118.61 121.38 Zn (1A) – O (2) – Zn (1) 100.61 103.87

The contribution from the Zn2+ ions was distinctly small. We observe the HOMO’s of

[ZBZT] 2, which were localized on the phenoxide ring of the two non-bridging ligands,

whereas the LUMO’s electron density distributes on the phenoxide and thiazolyl rings of

the two bridging ligands. Therefore, the electronic π-π* transitions in [ZBZT] 2 molecule

were localized on the benzthiazolate rings, from the non-bridging ligands which

contributes to the blue shift in the emission spectrum. Theoretical UV/VIS spectral

calculations show energy of 26331.3cm-1, 379.8 nm between the Highest Occupied

Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) that

exactly matches with that of the experimental value (absorption edge below 380nm), with

Tabel IV. I: showing PM3 calculated (this work) in comparison to the reported literature from Yu et al.

94

an oscillator strength length of 0.1782. The above conclusions were in agreement with

the pico second time decay spectroscopic studies.

IV. 5. Quantum yield calculations:

The quantum yield of [ZBZT] 2 in N, N’ – Dimethyl formamide (DMF) was

measured relative to a standard solution of Quinine sulfite in 1N H2SO4 whose absolute

quantum efficiency φs ~ 0.54 [13]. The quantum yield was calculated according to the

reported literature [14]. All solutions were taken at 2x10-6 M dilutions. The quantum

yield obtained for [ZBZT] 2 is 0.242, which shows that [ZBZT] 2 has very high photo

luminescence efficiency.

IV. 6. Electro luminescence characterization: The OLED structure used here has a configuration of ITO/TPD (50 nm)/ [ZBZT] 2 (80

nm) /Al (1000 nm). TPD was used as a hole transport material in the EL device, [ZBZT] 2

was used as the electron – transport and emissive layer. ITO – coated glass with a sheet

resistance of 50 Ω/ was used as the transparent anode. All the organic layers were

deposited successively onto the ITO glass at 5 x 10 -6 Torr. The deposition rates for the

organic layers and the metallic layers were 0.2- 0.3 nm/Sec. The emissive area was 4 mm

x 4 mm. Figure IV.8a shows the EL spectrum of [ZBZT]2 at various current densities.

Figure IV. 8b shows the current – voltage – luminance spectra of the device which shows

typical diode characteristics. The EL emission spectrum shows a broad emission with a

full width half maximum of more than 220 nm with CIE coordinates of 0.33 and 0.29,

which is a close match to a clear white emission. The color coordinates of the

electroluminescent spectra remained constant with the applied voltage, which is good for

the use as electroluminescent devices as shown in figure IV. 9. The broadening of the

spectra compared to the photoluminescence spectra gives clue about the emission from

95

the interface between TPD and [ZBZT]2. The probable mechanism could be the

formation of exciplex of TPD – [ZBZT]2 at the interface of the TPD and [ZBZT]2 which

leads to the broadening of the emission spectra. The exciplex is a result of charge transfer

interaction between the excited/ground state of the donor and ground/excited state of the

acceptor [15]. Under the electrical excitation, the exciplex should be formed by the

confinement of the carriers injected from the opposite electrodes between donor and

acceptor molecules. Figure IV.10, shows the exciplex emission process of ITO/ TPD:

[ZBZT]2 /Al device under electrical bias. The lower IP value of TPD (-5.4 e V) than

[ZBZT]2 makes holes injected readily into and transported via the HOMO of TPD. The

higher electron affinity of [ZBZT]2 provides the preferable electron transport pathway

through the LUMO of [ ZBZT]2. When injected, holes and electrons encounter each

other, they can form a hole – electron pair, i.e., TPD+ - [ZBZT]-2 exciplex, and they

recombine radiatively. To further prove our point we fabricated EL devices with three

different thickness of active layer, [ZBZT]2, since the formation of exciplex is sensitive

to the thickness of the active layer [16]. Figure IV. 11 shows the EL spectra of different

thickness of [ZBZT]2 active layer. As can be seen recombination zone and hence the

spectra width of the

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

Ele

ctro

lum

ines

cenc

e

Wavelength (nm)

0.2 mA 0.5 mA 1.0 mA 1.6 mA

96

30 35 40 45 50 55-1

0

1

2

Voltage (Volts)

Cur

rent

(mA

)

0

1

2

3

Luminance (C

d/m2)

Figure IV. 8: (a) Electro luminescent emission spectra of ITO/TPD/ [ZBZT]2/Al at

various current densities. (b) shows the dependence of current density and luminance to

the voltage.

303540455055

0.0

0.2

0.4

0.6

0.8

1.0

X - Co-ordinate Y - Co-ordinate

Co

- ord

inat

es

V o l t a g e

Figure IV.9: CIE co – ordinates as a function of applied voltage

97

electro luminescence peak maximal shifted with either decreasing or increasing thickness

of the active layer gives the clue about the formation of exciplexes at the interface of

ZBZT – TPD. The device with ZBZT layer thickness of 60nm, 70 nm and 95 nm shows

much narrowed EL emission than that of the one with 80 nm thickness.

Figure IV. 10: Band structure and exciplex emission process of ITO/TPD/

[ZBZT]2/Al.

98

Multiple Gaussian fits on the observed electroluminescent spectra yields a fair conclusion

that there might be five different species involved in the electroluminescent spectra with

a broad peak emission at lower energies, which is a result of exciplex formation. Figure

IV. 12 show the Gaussian fits to the electroluminescent spectra along with the analysis

table.

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0E

lect

rolu

min

esce

nce

W a v e l e n g t h ( n m )

60 nm 70 nm 95 nm 80 nm

Figure IV. 11: Electro luminescence spectra of ITO/ TPD/[ZBZT] 2 /Al, with 60

nm, 70 nm, 80nm, 95 nm active layer thickness

99

Further we show that the introduction of a insulating layer in the form of thin LiF which

has a large band gap of 12eV, which is know to facilitate the electron injection to a

greater extent should appreciably shift the peak maxima and also the broadening of the

electroluminescent spectra if it was because of the exciplex formation at the interface of

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

Ele

ctro

lum

ines

cenc

e

W a v e l e n g t h ( n m )

ITO/TPD/ZBZT/Al

Peak Area Center Width Height --------------------------------------------------------------------------- 1 10 452 24 0.35 2 31 490 54 0.47 3 56 595 93 0.47 4 26 547 59 0.35 5 122 666 185 0.52 ---------------------------------------------------------------------------

Figure IV. 12: Peak Area fitting of ZBZT – Electroluminescence, which fits to multiple Gaussian fit showing a broad emission feature at lower wavelength as a result of exciplex formation between the interface of ZBZT and TPD.

100

the ZBZT-TPD. Figure IV. 13 below shows the effect of LiF (5nm) insertion on the

properties of electroluminescent spectra, as can be seen spectra maxima got shifted to 475

nm in comparison with the electroluminescent spectra with out LiF insertion which peaks

at 583 nm.

300 400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0475

ITO/TPD/Zn(BZT)2/LiF/Al

E L

I n

t e

n s

i t y

W a v e l e n g t h ( n m )

I0.05mA I0.11mA I0.15mA

0 10 20 30 40 501E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

Cur

rent

(Am

p)

Voltage (Volts)

Figure IV. 13: (a) shows the effect of LiF insertion in the device structure (b) shows I- V characteristics of the LiF inserted device.

101

The full width half maxima of the spectra get reduced from ~ 200 nm to 90 nm indicative

of exciplex formation at the interface. However this device structure is not very reliable

as our experimental limitations in depositing thin film of LiF yield poor film quality.

As for our best devices with simple device structures we obtained a maximum

luminance of 200 cd/m2 (corrected) an applied voltage of 24 Volts as shown in the Figure

IV. 14. Inset to Figure IV.14 shows the picture of pixel appearance of 5 x 5 mm at an

applied voltage of 24V. Figure IV. 15 shows the current – Voltage – Luminance

characteristics of the device with 95 nm active layer thickness.

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

EL

Inte

nsity

Wavelength (nm)

16 Volts 20 Volts 24 Volts 25 Volts

Figure IV. 14: Electro luminescent spectra of ITO/TPD/ZBZT (95 nm)/Al. inset shows the 5x 5 mm pixel at an applied voltage of 24 V.

102

In summary, we have explained the luminescent behavior of dimeric bis (2 - (2’

- hydroxyl phenyl) benzthiazolato) Zinc using pico-second time decay photo

luminescent spectroscopy. Photoluminescent emission and transport properties

of the Zinc complex is mainly dominated by the non – bridged benzthiazole

connected to Zn2+. We have shown the electron density distribution in HOMO

and LUMO levels which is dominant on the benzthiazole rings resulting in an

intra-ligand energy transfer contributing to the enhanced electron transport

property of the complex, [ZBZT]2 which otherwise is not clear when considering

the monomeric structure of the complex. Broadband electroluminescent behavior

of [ZBZT]2 is explained based on the possible exciplex formation considering

devices at different thickness of [ZBZT]2 at the interface of TPD- [ZBZT]2.

14 16 18 20 22 24 26

0.0

2.0x10-4

4.0x10-4

6.0x10-4

8.0x10-4

1.0x10-3

1.2x10-3

1.4x10-3

Voltage (Volts)

Cur

rent

(Am

p)

0

5

10

15

20

25

30

35

40

Luminance (C

d/m2)

Figure IV. 15: shows the I – V – L characteristics ITO/TPD/ZBZT/Al of the device structure.

103

References:

1. Y. Hamada, T. Sano, H. Fujii, Y. Nishio, H. Takahashi and K. Shibata, Jpn. J.

Appl. Phys. 35, L 1339,1996.

2. T. Sano, Y. Nishio, Y. J. Hamada, H. Takahashi, T. Usiki, K.J. Shibata, Mater.

Chem., 10, 157.2000.

3. M. A. Baldo, D.F.O’ Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.

R. Forrest, Nature, 395, 151,1998.

4. S. Lamansky, P. Djurovich, D. Murphy, F. Abdel – Razzaq, H. – E. Lee, C.

Adachi, P. E. Burrows, S. R. Forrest, M.E. Thompson, J. Am. Chem. Soc. 123

4304,2001.

5. G. Gu, G. Parthasarathy, P. Tian, P.E. Burrows, S.R. Forrest, J. Appl. Phys. 86

4076,1999.

6. M. – L. Tsai, C. – Y. Liu, M. – A. Hsu and T. J. Chow, Appl. Phys. Lett. 82

550,2003.

7. J. T. Lim, N. H. Lee, Y. J. Ahm, G. W. Kang, C .H. Lee, Current Applied Physics

2 ,295,2002.

8. G. Yu, S. Yin, Y. Liu, Z. Shuai and D. Zhu, J. Am. Chem. Soc., 125, 14816,

2003.

9. N. Periasamy, S. Doraiswamy, G.B. Maiya, and B. Venkataraman, J.Chem. Phys.

88, 1638, 1988.

10. K. Ding, S. J. Courtney, A. J. Strandjord, S. Flom, D. Friedrich, and P. F. Barbara,

J. Phys. Chem. 87, 1184, 1983.

104

11. P. F. Barbara, L. E. Brus, and P. M. Rentzepis, J. Am. Chem. Soc, 102, 563,1980.

12. Geometry optimizations were done using a combination of molecular dynamics

and quantum mechanical calculations (PM3, RHF, closed shell, default settings).

Excited energy state analysis and analysis of electronic absorption spectra were

carried out using ZINDO/S-SCF-CI calculations using Argus Lab 3.01 molecular

Modelling software.

13. M. Lesiecki and J. Drake, Appl. Opt. 12, 557, 1982.

14. J. N. Demas and G. Crosby, J. Phys. Chem. 75, 991,1971.

15. R. Ballardini, G. Varani, M.T.Indelli, F. Scandola, Inorg. Chem. 25, 3858,1986.

16. S .A. Jenekhe, J. A. Osaheni, Science 265, 765,1994.

17. Z. R.Hong, O. Lengyel, C.S.Lee and S.T.Lee, Organic Electronics, 4, 149, 2003.

105

8 * )74

K

A'&"

'&5"((! '(A'! G*

+ (

This chapter deals with a study on the synthesis and optical properties of benzthiazole

complexes of group 13 elements namely Aluminum, Gallium and Indium metal ions

together with Zinc benzthiazole complex as they form a family of complexes showing

systematic and interesting luminescent properties. These are grouped as HALF CROSS

COMPLEXES. To highlight on the feasibility of achieving electroluminescent device, we

have studied in detail the Gallium based complexes. The Gallium based device has been

extensively characterized using UPS and XPS studies.

106

V. I. Introduction:

Group 13 elements in the periodic table has always been fascinating to both

chemists and physicists due to their interesting properties related to the inorganic

semiconductors like InAsP [1] , GaN [2], InP [3], InGaAs [4] etc., and a variety of

organo metallic complexes [5-10], more specifically Alq3, Gaq3, Inq3 [11,12] complexes.

A rich chemistry and physics involved in these metal ion complexes enthusiasts’

researchers to explore new molecules. Recent research on possible complexes as

candidates for the use in light emitting diodes show promise because of their distinct

transport and emission properties. Our interest lies in exploring benzthiazole-based

complexes of group 13 metal ions. We have studied Aluminum, Gallium, Indium and Zinc

benzthiazole complexes, which we would hereafter refer to as “Half Cross Complexes”

(HCC). All these metals occupy distinct positions in the group 12 and 13 of the periodic

table.

One of our objectives in studying these HCC is to investigate the PL behavior in

the solid state. The other objective is to spectroscopically understand the interfacial

effects of the metal – organic (M –O) and organic – organic (O – O) interfaces in the

electro luminescence devices, with specific reference to Gallium (III) complex. Another

Scheme V. 1: Half cross metal complexes discussed in this chapter include benzthiazole complexes of Al, Ga, In and Zn.

107

objective is to improve on the device performance employing benzthiazole molecules

based on their charge mobility values. To this end we show 2,5 – dibenzthiazolyl

benzthiazole as an effective electron injection layer.

V. II .A. Synthesis:

V. II.1. : Tris (2-(2’ – hydroxy phenyl) benzthiazolato) Aluminum (III) complex

(ABZT): was synthesized by refluxing stiochiometric amounts of 2-(2’ – hydroxy phenyl)

benzthiazole (BZT) (synthesis of BZT was described in chapter II) and Aluminum iso –

propoxide in ethanol under nitrogen atmosphere. Total reaction time was 5 hours.

Melting point: > 2000C; Appearance: Yellowish white solid. Solubility: Hot N, N’ –

dimethyl formamide; Molecular formula: C39H24Al N3O3S3; Elemental Analysis: C-

66.56, H – 3.45, Al – 3.80, N – 6.01, O – 6.78, S – 13.43.

V. II. 2: Tris (2 – (2’ – hydroxy phenyl) benzthiazolato) Indium (III) complex (IBZT):

was synthesized by refluxing stiochiometric amounts of 2 –(2’ – hydroxy phenyl)

benzthiazole (BZT) (synthesis of BZT was described in chapter II) and Indium Chloride

in ethanol under nitrogen atmosphere. Total reaction time was 3 hours. Melting point: >

2000C; Appearance: Bright green solid. Solubility: Hot N, N’ – dimethyl formamide.

Molecular formula: C39H24In N3O3S3; Elemental Analysis: C-59.12, H – 3.10, In –

14.50, N – 5.31, O – 6.21, S – 12.63.

---------------(1) Al (i - OC3H7)3 + C13H9NSO Al -( C13H8NSO)3Ethanol, reflux5 hours, N2 atms.

In Cl3 + C13H9NSO In -( C13H8NSO)3Ethanol, reflux3 hours, N2 atms. --------------- (2)

108

V. II.3: Tris (2-(2’ – hydroxy phenyl) benzthiazolate) Gallium (III). dihydrate complex

(GBZT): was synthesized by refluxing stiochiometric amounts of 2-(2’ – hydroxy phenyl)

benzthiazole (BZT) (synthesis of BZT was described in chapter II) and Gallium Chloride

in ethanol under nitrogen atmosphere. (As handling GaCl3 is difficult, all transfers and

weighing were done in glove bag under nitrogen atmosphere). Total reaction time was 5

hours. Melting point: 2400C; Appearance: Bright green solid. Solubility: Hot N, N’ –

dimethyl formamide. Molecular formula: C39H24Ga N3O3S3; Elemental Analysis: C-

62.52, H – 3.2, Ga – 9.28, N – 5.66, O – 6.42, S – 12.8. Thermogravimetric analysis of

GBZT was done in the inert atmosphere. Figure V.II. shows the TG analysis of GBZT

complex. The first weight loss of the TG corresponds to two water molecules, which were

probably, involved as the water of crystallization in the complex. It is interesting to note

that the weight loss in the TG goes to zero unlike other metal complexes. This could be

due to the low melting point (!) of Gallium metal 29oC, which sublimes at even ambient

temperatures.

GaCl3 + C13H9NSO Ga -( C13H8NSO)3.2H2OEthanol, reflux5 hours, N2 atms.

------------- (3)

50 100 150 200 250 300 350 400 450 500 550 60010

5

0

-5

-10

-15

-20

-25

-30

50 100 150 200 250 300 350 400 450 500 550 6000.0

0.5

1.0

1.5

2.0

2.5

Hea

t Flo

w E

ndo

Up

(mW

)

Temperature (oC)

Wei

ght (

mg)

Figure V.II.1: Thermo gravimetric and differential thermal analysis curves of GBZT under N2 atmosphere.

109

Table V. 1: showing the thermo gravimetric analysis of GBZT complex

V.II.4: bis (2-(2’ – hydroxy phenyl) benzthiazolate) Zinc (II) complex (ZBZT):

Although complete description of synthesis and properties of this complex has been

extensively discussed in Chapter IV, we show the synthetic conditions of Zinc complex for

the sake of comparison.

V.II.B: Powder X – ray patterns of the metal complexes:

Powder X – ray diffraction patterns of the Aluminum, Gallium, Indium and Zinc

benzthiazole complexes are shown in Figure V. II.2. All the complexes indicate a weakly

crystalline pattern with more reflections in the lower angles indicating on an average a

larger unit cell, >5 Å. The patterns are indicative of a more distorted structure. Although

there are some similarities in the X – ray patterns (for example, see the patterns of

Indium and Zinc complexes), none of the complexes appear to be clearly isomorphous

one with the other, restricting our efforts to prepare solid solutions of these complexes.

Zn(CH3COO)2.2H20 + C13H9NSO Zn -( C13H8NSO)2Ethanol, 600C2 hours, ambient conditions.

------------(4)

110

20 30 40 50 60 70 80

24.8

27.8

5

31.5

35.1

39.6

46.3

5

50.8

5

56.6

63.2

68.1

5

22.7 25

.25

34.4

41.5

5

45.6

48.3

53.5

5

57.7

5

63.8

68.6

70.7

5

74.8

20.8

5 22.5

5

26.9

31.3

5

41.1

44.8

5

48.5

5

57.2

5

63.0

565

.35

71.8

73.1

77.1

20.5

22.5

5

26

32.7

5

39.3 44

.746

.8

51.6

56.8

61.3

5

67.1

5

77.2

5

GBZT

ABZT

IBZT

2θθθθ(degree)

Inte

nsity

(a.u

.)

ZBZT

Figure V.II.2: shows the cumulative powder X – ray diffraction patterns of the Half Cross Complexes showing similar diffraction patterns indicative of isomorphism

111

V. II.C: Photo luminescent studies:

Figure V.II.3. shows the photo luminescent spectra of the half cross complexes in

the solid state. It is noteworthy to observe a systematic shift in the emission maxima with

the size of the metal ion. The observations are tabulated in Table V. 2

Figure V.II.3: shows the cumulative photoluminescent spectra of the HALF CROSS complexes showing systematic shift in the emission maxima with the ionic size. For clarity expanded spectra is shown below.

400 450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity

Wavelength (nm)

IBZT GBZT ZBZT ABZT

450 465 480 495 510 5250.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity

Wavelength (nm)

IBZT GBZT ZBZT ABZT

112

Table V. 2: Photoluminescent emission parameters of the half cross complexes

V. II. D: Photo luminescent decay measurements:

Photoluminescent decay measurements were carried out using Time correlated

single photon counting method-using Nd: YAG laser under ambient conditions. Figure V.

II.4 shows the photo luminescent decay patterns of ABZT, GBZT, and ZBZT at room

temperature and recorded at 77K. Photoluminescent decay analysis of the ABZT at room

temperature fits to a double exponential decay model (refer to equation V. II.1) with life

times of τ1 = 0.13 ns and τ2 = 0.49 ns with an average lifetime of 0.301 ns. The first

lifetime component of the complex was faster in decay, which was due to the possible

ligand to metal charge transfer. Low temperature decay (77K) fits to a double

exponential decay with lifetimes of 4.07 ns and 9.02ns respectively. The decay

measurement at 77K shows much slower decay with an average lifetime of 5.136 ns.

F (λ, t) = A1.exp (-k1τ1 ) + A2.exp (-k2 τ2) ------------------------V.II.1.

113

GBZT photoluminescence decay measured at room temperature fits to a double

exponential decay with lifetimes τ1 = 1.41 ns and τ2 = 5.38 ns with an average lifetime of

3.35 ns. Decay profile at 77K shows decay which fits to double exponential with life times

τ1 = 3.74ns and τ2 = 7.549 ns with an average lifetime of 3.78 ns. It is interesting to note

that in the case of GBZT, the average life times are independent of temperatures showing

Figure V.II.4: shows the PL decay patterns of the half cross complexes at room temperature and at liquid nitrogen temperature.

0 5 10 15 20 25 30 3510

100

1000

GBZT

ABZTZBZT

at 300K

Inte

nsity

Time (ns)

0 5 10 15 20 25 30

10

100

1000

GBZT

ABZT

ZBZT

at 77K

Inte

nsity

Time (ns)

114

that the spin – orbit coupling contribution from the metal ion is less, which differs from

the photo physical behavior of the Aluminum complex, although they belong to the same

group in the periodic table. ZBZT PL decay measured at room temperature fits to a

double exponential decay with lifetimes of τ1 = 2.35 ns and τ2 = 5.23 ns with an average

lifetime of 3.02 ns. Decay profile at 77K shows decay which fits to double exponential

with life times τ1 = 2.90 ns and τ2 = 4.66 ns with an average lifetime of 3.34 ns. It is

interesting to note that in case of ZBZT complex the average life times are independent of

temperatures similar to Gallium complex. Table V.3 tabulates all the observations on the

decay analysis for comparison.

COMPLEX

300K 77K

τ1 τ2 τave τ1 τ2 τave

ABZT 0.13

1.41

2.35

0.49

5.38

5.23

0.30

3.35

3.02

4.07

3.74

2.90

9.02

7.54

4.66

5.13

3.78

3.34

GBZT

ZBZT

IBZT

- - - - - -

Table V.3: shows the PL decay analysis of the half cross complexes. Note the similarities in the lifetime trend variation of ZBZT and GBZT

115

V. III: Electroluminescent properties of tris (2-(2 – hydroxy phenyl) benzthiazolate) Gallium (III) complex:

Similar to ZBZT complex (as discussed in the preceding chapter), which was

employed as an emissive layer in the EL device, we have made a device employing GBZT

complex in the device structure such as ITO/TPD (50nm) /GBZT (80 nm)/Al (100 nm). As

in the case of ZBZT, ITO acts as the anode, TPD as a hole transport layer and GBZT as

an electron transport and emissive layer. Aluminum was used as a cathode. From the TG

data we have optimized the evaporation conditions for the GBZT complex. However

during the device fabrication, we observed that GBZT needed slower evaporation for the

deposition at the rate of 1-2 Å/sec, probably due to its poor thermal conductivity. The

pixel size was 3 x 3 mm2. All usual procedures were adopted, like patterning and

ozonization of ITO surface before depositions. ITO used here has a sheet resistance of 50

Ω/ .

The electro luminescent spectrum of the device (Figure V.III.1) shows an

emission around 620 nm with a FWHM of around 170 nm, a feature completely different

from the photo luminescent emission (PL emission maxima was around 483 nm with a

FWHM ~ 80nm). We observe a distinct change in the electro luminescent emission curve

compared to the photo luminescent emission is shown in Figure V.III.1 This difference

between the PL and EL spectrum suggest that the recombination zone is completely

dominated by the interfacial properties of GBZT and TPD in the device structure. Such a

contrast between photo luminescent and electro luminescent spectra is noted for the Ir

based complexes as reported by Jason et al [13].

116

The differences in PL and EL features in the actual device structure can be

interpreted based on the interfacial energy levels alignment. In the subsequent sections,

we show the Organic - Organic (O – O) interfaces deposited on the Gold substrate and

the ITO substrate using X – ray photoelectron and Ultraviolet Photo electron

spectroscopy to investigate the mechanism for the observed red shift in the EL spectra

compared to the PL.

V. IV: X – ray Photoelectron Spectroscopy – Ultraviolet Photoelectron Spectroscopic studies on GBZT-TPD interfaces:

Before proceeding to the actual interfacial alignments used in the device structures,

we would study the O –O interfacial properties on poly crystalline gold (Aupoly) substrate.

These studies were intended to address the issues, namely,

Figure V.III.1: Electro luminescent spectra of ITO/TPD/GBZT/Al, where in TPD was used as a hole transport and GBZT was used as an electron transport and emissive layer.

Al GBZT TPD ITO

400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0 EL PL

Inte

nsity

Wavelength (nm)

117

1. To determine the HOMO level of GBZT and TPD on the Au substrate.

2. To investigate, the chemical interactions between organic – organic interfaces

which could lead to the red shifted EL emission.

Based on the UPS valence band spectra and the onset spectra we have constructed the

energy level diagram for the interfaces of Au/GBZT/TPD. (The detailed experimental

investigations of XPS and UPS on the Au/GBZT/TPD are presented in Appendix:

V.1) The HOMO level of GBZT deposited on Au was at 0.81 eV with respect to Au

Fermi level, showing a value of 5.31 eV. The LUMO energy level was calculated

from the difference in optical band gap and the HOMO level was 2.11 eV, (EHOMO –

EOPT = 5.31-3.2=2.11). Ionization Potential was calculated by taking the difference

between the vacuum level and the HOMO onset of GBZT. The value of ionization

potential of GBZT is 4.81 eV. Figure V.IV.1 shows the energy level diagram of the

Au/GBZT/TPD.

Au

HOMO 0.81eV

2.11eV

0.5eV

GBZT

1.8 eV

4.5 eV 2.9 eV

TPD

0.6 eV

EV

EF

LUMO

Au/GBZT/TPD

I.P = 4.81 eV I.P = 5.7 eV

Figure V. IV. 1: shows the energy level diagram of Au/GBZT/TPD/Au. The ionization potential of GBZT was 4.81 eV. Calculated HOMO and LUMO levels were 5.31 eV and 2.11 eV respectively.

118

The low energy cut off shows a vacuum level shift of 0.5 eV at Au/GBZT interface,

where as the vacuum level shift of 0.1 eV is observed between GBZT /TPD interface.

Such a large shift in the vacuum level observed for the metal – Organic (M – O)

interfaces due to the formation of large interface dipoles in the literature. [14]

From the above energy level diagram (Figure V. IV.1) it is evident that the HOMO

of TPD agrees well with the reported literature [15]. Further this give a clue that the

O – O interface between TPD and GBZT doesn’t seem to alter the HOMO levels,

implying that there is no distinct interfacial effects. However we observe distinct

shifts in the vacuum levels of both GBZT and TPD with reference to Au substrate

indicative of formation of interface dipole between M – O and O – O interfaces. In

order to see if any chemical effects which could arise in the O – O interface we look

at the core level spectra of C – 1s. Figure V.IV.2 shows C – 1s spectra of XPS with

the sequential deposition of GBZT, TPD and Au. The C – 1s peak undergoes a 0.5

eV shift to higher BE with the first few angstrom (100Å) of GBZT deposition on Au

surface and its position and shape remain basically unchanged thereafter. The shift in

the C –1s spectrum is indicative of the formation of interface dipole between Au and

GBZT. All the spectra were similar in shape with no substantial change in the peak

broadening indicative of the absence of chemical interaction between the interfaces.

Similar shift of 0.23 eV was also observed between the TPD and Au interface

towards the lower energy region. Thus based on the above observations, we rule out

the possibility of any distinct chemical interactions occurring between the layers,

which could result in a red shift in the EL when compared to the PL.

119

The other possible reason could be the interfacial energy alignments in the actual

device structure rather than the interfacial interactions. We have further investigated the

interfacial effects by studying the actual device structure, which we have used for the

electroluminescent studies. In the following discussion we highlight some of the XPS –

UPS investigations made on ITO/TPD (500Å) /GBZT (240 Å).

Figure V.IV.3 shows the valence band spectra of ITO/TPD/GBZT. From the

valence band spectra it can be seen that evolution of respective HOMO levels at the

interfaces for TPD and GBZT from which we calculated the HOMO and from the optical

band gap the LUMO levels. TPD valence band spectra show a HOMO of 1.81 eV relative

282 284 286 288 290

B inding Energy (eV)

Au- partial G BZT layer

0.5eV

Au- w ith fu ll GBZT coverage

0.45eV

GBZT-Partial TPD

0.49eV

G BZT-TPD

0.23eV

C- 1sTPD - Au

Figure V.IV.2: shows the XPS spectrum of C –1s. Note the absence of peak shift and peak shape between organic – organic interfaces, which is indicative of absence of interface dipole and chemical interaction between the GBZT-TPD interfaces respectively.

120

to the ITO. With the deposition of GBZT, a peak at 0.81 eV, corresponding to the HOMO

level of GBZT is observed. It is note worthy that the HOMO levels remains practically

unchanged with both Au and ITO substrates.

The onset spectra of the layer deposited ITO/TPD/GBZT is shown in Figure

V.IV.4. The vacuum level shift in the ITO/TPD/GBZT doesn’t follow the usual “ladder”

(a systematic increase/decrease in the ionization potential values) sequence typically

needed for the smooth electron/hole transport. For example the vacuum level shift in the

0 2 4 6 8Binding Energy (eV)

ITO/TPD

ITO/TPD/GBZT

Figure V.IV.3: shows the UPS - Valence band spectra of ITO/TPD/GBZT demonstrating the evaluation of HOMO on TPD and GBZT.

121

ITO/TPD interface is positive (0.82 eV) while the vacuum level shift in the

ITO/TPD/GBZT interface is negative (- 0.91 eV).

Based on our results on the valence band and onset spectra we show the energy level

diagram in the device structure ITO/TPD/GBZT (as seen in Figure V.IV.5.) It is known

in the literature that dipoles occur at hetero layer interfaces between materials of different

ionisation energies and electron affinities, which were referred to as donor/acceptor

interfaces [15]. In such cases, a partial charge is expected to get transferred from the low

ionisation energy constituent to the high electron affinity molecule. Such donor/ acceptor

interfaces usually leads to the formation of electroplex at the interfaces leading to

9 8 7 6 5 4 3 2 1Binding Energy (eV)

ITO

0.82 eV

TPD

0.19 eV

GBZT

Figure V. IV.4: shows the onset spectra of ITO/TPD/GBZT showing a vacuum level shift of 0.82 eV between ITO/TPD interface where as it shows a shift of –0.19 eV between TPD/GBZT.

122

emission at lower energies. This argument goes well with our experimental data on

electro luminescence. Recall that the photo luminescent and electro luminescent spectra

of the GBZT differ drastically, “where a broad totally red shifted EL” is observed

compared to the photo luminescent spectra. From the energy level diagram it is evident

that there could be a partial energy transfer from low ionisation potential of TPD to the

high electron affinity of GBZT resulting in the formation of donor/acceptor interfaces

leading to the emission at lower energies causing a red shift in the electroluminescent

spectra. This cumulative effect could be attributed to the ITO/TPD and TPD/GBZT

interfacial energy alignments leading to electroplex formation in the TPD/GBZT

interface.

ITO

4.2 eV

1.81 eV 0.81 eV

TPD GBZT

0.82 eV 0.19 eV

2.61 eV 1.81 eV

I.P. = 5.19 eV 5.2 eV

Figure V.IV.5: shows the energy level diagram of the device ITO/TPD/GBZT

123

Figure V.IV.6 shows the C – 1s spectra of the TPD/GBZT interfaces in ITO/TPD/GBZT

device structure. The C – 1s spectra shows a marginal shift of 0.09 eV towards low

binding energy that gives clue about the formation of interfacial dipole between O – O

interface ie., TPD/GBZT. Note that the shape of the C – 1s spectra for all depositions

remain unchanged indicative of absence of any chemical modifications between the

interfaces. Figure V.IV.6 also shows the N – 1s spectra of TPD/GBZT interface. N – 1s

shows no shift in the peak, but the shape of the spectra changed considerably forming a

low binding energy shoulder. The change in the spectral shape gives clue about the

formation of different chemical environment around the nitrogen atom probably due to

more localization of electron density around nitrogen atom.

282 284 286 288 290 390 395 400 405

Binding Energy (eV)

TPD

0.09 eV

N-1sGBZT C-1s

Binding Energy (eV)

TPD

GBZT

Figure V.IV.6: shows the C -1s and N-1s spectra of XPS studies on ITO/TPD/GBZT deposited layer sequence. Formation of small interface dipole between the O – O interface is evident from the spectral shift in the C – 1s. The chemical environment around the nitrogen was different showing more localization of charge around nitrogen atom in GBZT layer.

124

V.V: Current – Voltage characteristics of the ITO/TPD/GBZT/Al:

Figure V.V.1 shows the current – voltage characteristics of the device structure

showing a threshold voltage of 35 V, which follows the behavior of the typical diode. (A

diode current density will linearly increase with the light out put as seen in Figure

V.V.1). This device shows an initial brightness of 20 cd/m2 and reaches to 200 cd/m2 at

an applied voltage of 40 V. Inset to Figure V.V.1 shows the 3 x 3 mm2 pixel at an

applied voltage of 40 V.

The operating voltage of the device ITO/TPD/GBZT/Al is higher compared to any

desired values. In our efforts to further improve on the performance of this device, we

show the use a high electron mobility benzthiazole molecule namely 2,5 – dibenzthiazolyl

thiophene as a electron injection layer in order to reduce the threshold voltage.

4

6

8

10

12

20 30 40 500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Lum

inan

ce (a

rb.u

nit)

Cur

rent

(mA

)

Voltage (V)Figure V.V.1: Current – Voltage – Luminance characteristics of the EL device structure with the configuration ITO/TPD/GBZT/Al, following typical diode characteristics.

125

V.VI: Influence of 2,5 – dibenzthiazolyl thiophene as an electron transport layer on the GBZT device structures:

Figure V. VI.1 shows the Current – Voltage characterstics of TBZT (100 Å)

deposited between GBZT/Al interface in the ITO/TPD/GBZT/Al device structure. As can

be seen in the Figure V.VI.1 the threshold voltage of the device gets considerably

reduced with the TBZT deposition from 35 V to 15 V. This also highlights the electron

transport ability of TBZT in the device structure. The detials of electron mobility

measurements are documented in Appendix V.2. Figure V.VI.2. shows the cumulative

electroluminescence spectra of ITO/TPD/GBZT/Al and ITO/TPD/GBZT/TBZT/Al. One

of the remarkable feature observed apart from the lowering of the threshold voltage is the

movement of the recombination zone towards the anodic direction resulting in a blue

shift of the order of 25 nm, which demonstrates the uniqueness of TBZT as an electron

injection layer.

Figure V.VI.1: shows the I – V characteristics of TBZT deposited OLED with device structure: ITO/TPD/GBZT/TBZT/Al. Note the drastic change in the threshold voltage from 35 volts to 15 volts with the insertion of 100Å thick TBZT.

13 14 15 16

0.000

0.002

0.004

0.006

Cur

rent

(mA

)

Voltage (v)

+

- Al TBZT GBZT TPD ITO

126

V.VII: XPS and UPS characterization of ITO/TPD/GBZT/TBZT:

Having observed a distinct change in the threshold voltage with the incorporation

of TBZT layer, it is worthwhile to probe the spectral features of the GBZT/TBZT

interface. Although a complete description of XPS – UPS studies are described in

Appendix-2; we will show here the important observations from the XPS and the UPS

studies on ITO/TPD/GBZT/TBZT.

Figure V.VII.1 shows the valence band spectra of ITO/TPD/GBZT/TBZT/Au.

From the valence band spectra it can be clearly seen that evolution of respective HOMO

levels at the interfaces for TPD, GBZT and TBZT from which we calculated the HOMO

levels. The LUMO levels of the respective molecules were calculated by taking

differences between the optical band gap and the HOMO level. TPD shows the HOMO

400 450 500 550 600 650 700 7500.0

0.2

0.4

0.6

0.8

1.0

GBZT/TBZT

GBZT

Inte

nsity

Wavelength (nm)

Figure V.VI.2: shows the effect of TBZT on the GBZT electroluminescent properties with device structure ITO/TPD/GBZT/TBZT/Al. An improvement of threshold voltage of the order of 20 V was obtained by introducing a thin layer of TBZT (10nm) into the device structure ITO/TPD/GBZT/Al.

127

level at 1.81 eV relative to the ITO. With the deposition of GBZT, a peak at 0.81 eV,

corresponding to the HOMO level of GBZT was observed. Similarly with the deposition

of TBZT a peak at 1.0 eV was observed indicative of HOMO of TBZT.

Figure V.VII.1: shows the UPS – valence band spectra of the ITO/TPD/GBZT/TBZT device, with the spectra recorded after each layer deposition

0 2 4 6 8

UPS

Binding Energy (eV)

TPD

GBZT

Partial TBZT

TBZT

128

Energy level diagram is constructed based on the UPS valence band spectra of

ITO/TPD/GBZT/TBZT/Au and the ONSET spectrum for the same device. Charge

exchange, chemical bonds and chemistry – induced effects are usually expected at

metal – organic interfaces and are not expected to occur at O - O interfaces. However,

as can be seen from the energy level diagram as shown in Figure V.VII.2, we

observed large dipoles at the O - O interfaces in our device structure. The energy

barrier for charge injection between the TBZT and GBZT is only 0.2 eV facilitating

the enhanced electron injection, which is contributing to the lowering of the threshold

voltages in the device structure.

Figure V.VII.2 shows the N – 1s spectrum of ITO/TPD/GBZT/TBZT/Au using XPS.

The N - 1s spectra at the TPD/ GBZT interface is virtually unchanged. However with the

deposition of TBZT, the N – 1s develops a low binding energy component (chemical

ITO

4.2 eV

1.81 eV 0.81 eV

TPD GBZT TBZT

1.0 eV

0.82 eV 0.19 eV

2.1 eV

0.25 eV

2.61 eV 1.81 eV

I.P. = 5.19 eV 5.2 eV 4.95 eV

700 Figure V.VII.2: showing the energy level band diagram of the layer deposited ITO/TPD/GBZT/TBZT/Au

129

shift ~ 1.42 eV), which is indicative of excess of electronic charge in and around the

nitrogen atom. A similar shift towards a lower binding energy is observed in S – 2p core

level spectra due to a dominant electron density around the ‘S’ atom. This is evident with

full TBZT coverage on the GBZT layer. (see Figure V.VII.4).

Figure V.VII.3: shows the N – 1s spectrum of ITO/TPD/GBZT/TBZT/Au using XPS. A low binding energy component developed at the low binding energy is indicative of excess electron density around nitrogen atom in the TBZT

394 396 398 400 402 404

Binding Energy (eV)

TPD

GBZT

0.59 eV

Partial TBZT

1.42 eV

N-1sTBZT

130

Figure V.VII.4 shows the S – 2p spectra ITO/TPD/GBZT/TBZT using XPS. As can be

seen with the partial coverage of TBZT on GBZT a shift of 1.11 eV is observed in the

XPS spectra of S – 2p. Such a large shift in the spectra in the S – 2p spectra is unusual. It

is interesting to note that with the complete coverage of TBZT on GBZT the shift in S-

2p get reduced to 0.62 eV ( a shift of 0.49 eV from the partially covered TBZT). Such a

trend could arise out of high mobility of TBZT, which could localize the electron density

more on the sulphur atom.

0.8

Figure V.VII.4: shows the S – 2p spectrum of ITO/TPD/GBZT/TBZT/Au using XPS. A low binding energy component developed at the low binding energy is indicative of excess electron density around nitrogen atom in the TBZT

160 164 168 172

1.11 eV

Partial TBZT

0.49 eV

S - 2pTBZT

Binding Energy (eV)

GBZT

131

In summary, we have shown a series of benzthiazole based Half Cross

Complexes and a detailed study on the EL and PL properties of the Gallium

complex. The chemistry behind the electro luminescence, which differs

significantly from that of the photoluminescence, has been explained, based on

the XPS and UPS studies. We have shown the effect of 2,5 – dibenzthiazolyl

thiophene as a electron transport material in tuning the electro luminescent

properties of GBZT.

V. VIII: References:

1. E. Ribeiro, R.L.Maltez, W. Carvalho, D. Ugarte and G. Medeiros – Ribeiro, Appl.

Phys. Lett. 81, 2953, 2002.

2. Rafal Dylemicz, Seqiusz Z. Patela and Regina Paszkiewicz, Proc. SPIE Int. Soc.

Opt.Eng. 5484, 328, 2004.

3. S.H.Pyun, J.Appl. Phys. 96, 5766, 2004.

4. C. Baker, I.S.Gregory. W.R.Tribe, I.V. Bradley, M.J. Evans, E.H. Linfield, and

M. Missous, Appl. Phys. Lett. 85, 4965, 2004.

5. J. Kido, K. Hongawa, K.Okuyama and K. Nagai, Appl. Phys. Lett, 64, 815, 1994.

6. G Gu, G. Parthasarathy, P.Tian, P.E. Burrows, and S. R. Forrest, J. Appl. Phys,

86, 4076,1999.

7. Y. He, S. Gong, R. Hattori, and J. Kanicki, Appl. Phys. Lett, 74, 2265, 1999.

132

8. T. P. Nguyen, P. Jonnard, F. Vernard, P. F. Staub, J. Thiion, M. Lapkowski, V. H.

Tran, Synth. Met. 75, 175, 1995.

9. Y. Hamada, T. Sano, H. Fujii, Y. Nishio, K. Takahashi, K. Shibata, Jpn. J. Appl.

Phys, 35, L1339 , 1996.

10. X. H. Zhang, O.Y. Wong, Z.Q.Gao, C.S. Lee, H.L. Kwong, S.T. Lee and S.K.

Wu, Materials Science, and Engineering B, 85, 182, 2001.

11. C. W. Tang and S. A. Vanslyke, Appl. Phys. Lett. 1, 913,1997.

12. P.E. Burrows, L. S. Sapochak, D.M. McCarty, S.R. Forrest, and M. E. Thompson,

Appl. Phys. Lett. 64, 2718, 1994.

13. Jason D. Slinker, Alon A. Gorodetsky, Michael S. Lowry, Jingjing Wang, Sara

Parker, Richard Rohl, Stefan Bernhard, and George G. Malliaras, J Am. Chem.

Soc. 126, 2763, 2004.

14. H. Peisert, M. Knupfer, T. Schwiegen, G.G.Fuentcs, O. Olligs and J. Fink, J.

Appl. Phys. 93, 9683, 2003.

15. H. Ishii, K. Sugyama, D. Yoshimura, E. Ito, Y. Ouchi and K. Seki, IEEE J. Sel.

Top. Quantum Electron. 4, 24, 1998.

0.6 0.4 0.2 0.0 1.0 500 400 800 600

133

Summary and conclusions

With the emphasis of current display activities surrounding challenging issues such as synthesis and understanding of white light emitting organic molecules for the display industries. The present work highlights the synthesis of novel benzthiazole based organic molecules and exploring them for its use in white light emitting diodes. Understanding the photo physics involved in the benzthiazole based molecules give clue about the broadened emission and a way to rationally synthesize organic molecule for the white light emission

Some important observations documented in the preceding chapters

include:

Synthesis of benzthiazole based organic molecules, which show white light emission and superior transport properties for the use in light emitting diodes.

For the first time we have demonstrated the white light emission of molecular

materials exploiting the structural features of the stereo isomers, namely, 1,2

– dibenzthiazolyl ethylene.

Dynamic photo luminescent and semi – empirical studies prove the criticality of understanding intra molecular interactions for designing white light emitting molecules, specifically choosing the structural features in 1,2 – dibenzthiazolyl ethylene.

The photo luminescent and electro luminescent properties of bis (2-(2’-

hydroxy phenyl) benzthiazolate) Zinc (II) are explained based on the dimeric

structure. An unusually broad white light emission with a full width half

maximum of ~ 220 nm is observed. The active species involved in the broad

white light are explained based on the exciplex formation at the interface of

ZBZT and TPD layers.

Explore the possibility of benzthiazole based complexes of Al, Ga, In and Zn

for the use in light emitting diodes, referred as Half Cross Complexes (HCC).

Tris (2 – (2’- hydroxy phenyl) benzthiazolate) Gallium (III) complex has been

studied extensively. For the first time we have shown that the shift in the EL

compared to PL is based on the possible electroplex formation at the

interfaces using XPS and UPS as a tool of investigation.

134

First report on thiophene based benzthiazole molecule for the use in electron

transport layers, which has the mobility, 3 – 6 X 10-4 cm2/Vs, two orders of

magnitude higher than the well-known Alq3, characterized by photo generated

time of flight mobility measurements.

This work is largely to understand the physical phenomenon involved in novel

benzthiazole molecule, which shows promise for the use in white light emitting

and as good transport layers in light emitting diodes. Some of the issues like the

interfacial phenomenon that can lead to unusual electro luminescence for

example ZBZT and GBZT which shows broad light emissions irrespective of their

PL are critically evaluated using spectroscopic techniques. In our further quest to

explore new molecules for the use in light emitting diodes we have synthesized

TBZT as one of the candidate, which has superior properties than AlQ3 in terms

of electron mobility.

135

Appendix V - 1

V. A.I.1. Ultraviolet Photo Electron and X – ray photoelectron spectroscopic investigations on Tris (2-(2’ – hydroxyl phenyl) benzthiazolate) Gallium (III) complex and related device interfaces:

The experimental investigations underlying XPS and UPS were to predict the HOMO

level and ionization potential of the GBZT complex and to investigate the interfacial

energy alignments in the device structures. First we show the investigations on the

Au/GBZT, GBZT/TPD and on TPD/Au interfaces. These sequential deposition and

analysis was done to predict the HOMO level of the GBZT by depositing on the Au

substrate. For the investigation of interfacial properties between GBZT and TPD in

absence of any active substrate like ITO etc. For this purpose we carried out sequential

deposition in the following order.

Au/GBZT (240Å)/TPD (500 Å)/Au

After each deposited layer we carried out insitu XPS and UPS analysis. Thicknesses of

the layers were insitu monitored by quartz thickness monitor. The energy scale for XPS

was calibrated to reproduce the binding energy (BE) of Au 4f7/2 (84.00 eV). Note that the

work function of the instrument was grounded to the work function of Au in all the

measurements.

Figure V. 5.5.1 shows the overview of the XPS spectra of all deposited layers in the

sequence Au/GBZT/TPD/Au. Insitu XPS studies were carried out with different thickness

deposition. As can be seen the peak around 86 eV, 335 eV which is assigned to Au – 4f,

Au – 4d5/2 and Au – 4d3/2 respectively are completely disappeared indicative of total

coverage of GBZT on the Au surface. Peak at 285 eV became stronger with the

136

deposition of GBZT which is assigned as C – 1s peak position, which is highlighted in the

discussion of Figure V.5.5.5. With the subsequent deposition of TPD over GBZT, the

peak at 426 eV disappears which is due to the Auger – Ga, indicates the complete

coverage of TPD.

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0

B i n d i n g E n e r g y ( e V )

Au-

4d 3/

2

Au

- 4d 5/

2Au-

4f7/

2

( d )

( a )

C-1

s

( b )

Aug

er-G

a

N-1

S

( c )

Ga-

2p3/

2

Ga

- 2p 1/

2

( e )

Figure V.A - 1.1: shows the XPS – overview spectra of (a) Au deposition (b) GBZT (100Å) on Au layer (c) 240 Å GBZT on Au (d) TPD over layer on GBZT/Au (e) Au deposited on TPD/GBZT/Au layers.

137

The peaks at 229 eV (S – 2s), 110 eV (Ga – 2p), 21.80 eV (Ga – 3d), 1116 eV (Ga – 2p1/2)

and 1144 eV (Ga – 2p3/2) also disappear as a consequence of TPD deposition. With the

partial coverage of Au on TPD the peaks at 85.9, 335, 354 eV, which are characteristic

of Au peaks, reappears.

Figure V. 5.5.2 show the valance band spectra of Au – GBZT and Au/GBZT/TPD.

The first HOMO level of GBZT was located at 0.81 eV below the Gold Fermi level, and

TPD first HOMO level is located at 2.01 eV below the Gold Fermi level. To determine

the ionization potentials of the GBZT and TPD in the device structure, it is essential to

have an idea about the vacuum level alignments in the device structure.

-2 0 2 4 6 8 100.0

0.2

0.4

0.6

0.8

1.0UPS

Au/GBZT/TPD

Au/GBZT

I n t

e n

s i t

y

Binding Energy (eV)

Figure V.A – 1.2: Ultra violet photoemission spectra of Au/GBZT and Au/GBZT/TPD layer sequences. Predicted HOMO levels for GBZT complex was 0.81 eV below the Au Fermi level and 1.8 eV for TPD in the layer structure.

138

Figure V.5.5.3 shows the low energy onset of the sequential layers using UPS

technique. ONSET spectra essentially give the relative position of vacuum level at the

interfaces. According to Schotky model the vacuum level will get aligned themselves

between the interfaces for an ideal interfacial barrier, i.e., with no interfacial dipolar

formation.

Figure V.A – 1.3: shows the ONSET spectra of the Au/GBZT and Au/GBZT/TPD interfaces. Note the formation of interfacial dipolar formation between the Au and GBZT interface leading to an ONSET shift of 0.5 eV in the vacuum level.

-1 -2 -3 -4 -5 -6 -7 -8

0.5eV

Binding Energy (eV)

Au

Au-GBZT

ONSETGBZT-TPD

139

But in real cases they will be formation of interfacial dipolar between the

interface which leads to the change in the relative positions of the vacuum levels and

hence the ionization potentials in the device structure will be difference from the of the

individual ionization potential.

As can be seen there is a shift in the onset spectra of the order of 0.5 eV between

the Au – GBZT interface which is due to the interfacial dipole formation between the

metal and organic interface. This difference is very marginal between the Organic –

Organic interface that is of the order of 0.09 eV in the present study. It is known in the

literature that the vacuum level shifts in case of M – O will be considerably high which is

accounted to the formation of large dipole formation when compared to the O – O

interfaces. Such effect is also observed in our device structure.

Figure V.5.5.5 shows the XPS spectra of C – 1s with the sequential deposition of

GBZT, TPD and Au. The C – 1s peak undergoes a 0.5 eV shift to higher BE with the

first few angstrom (100Å) of GBZT deposition on Au surface and its position and

shape remain basically unchanged thereafter. The shift in the C –1s spectrum was

indicative of formation of interface dipole at the interface of Au and GBZT. All the

spectra are similar in shape with no substantial change in the peak broadness

indicative of absence of chemical interaction between the interfaces. Similar shift of

0.23 eV also observed between the TPD and Au interface towards the lower energy

region.

140

280 282 284 286 288 290

Binding Energy (eV)

Au- partial GBZT

0.5eV

Au-Full GBZT

0.45eV

GBZT-Partial TPD

0.49eV

GBZT-TPD

0.23eV

C- 1sTPD - Au

Figure V.A – 1.4: shows the XPS spectrum of C –1s on the device Au/GBZT/TPD/Au. Note the shift in the C –1s peak position with the partial deposition of GBZT indicative of a dipole formation at the interface of Au – GBZT interface.

141

Appendix - 2

V. A .II.1: Introduction:

The motive behind the fabrication of TBZT lies on the fact that polythiophene

units which are known for their superior hole transport abilities are sensitive to the

substitution of hetero atoms like nitrogen and sulphur in the main chain and end capping

in controlling the transport properties. Based on this analogy, we have end - capped the

thiophene with benzthiazole group at both the ends [1,2]. First we show the synthesis and

the complete characterization of TBZT.

V. A .II. 2: Experimental:

TBZT synthesis was carried out under microwave conditions. Stiochiometric

amounts of starting materials 2, 5 – thiophene dicarboxylic acid and 2 – amino thiophenol

were taken in a borosil beaker along with the calculated amount of poly phosphoric acid,

which acts as a solvent and acid catalyst. Use of polyphosphoric acid is recommended

since it is a good microwave absorber due to its high polarity, results in a superheating of

the system, giving good yields. Reaction was carried out in a microwave oven at a power

S

Z

YXS

Z

YX

S

Z

YXS

Z

YX

S

S

Scheme V.A –2.1: Some of the “ strategical” molecules discussed in the literature

142

level of 20% of total 800 W at ambient conditions. The total reaction time was 12

minutes. After the reaction was over, the crude product was obtained by work up of the

reaction mixture with cold water. Subsequently the product was washed with dil. NH4OH

and recrystallized twice in hot toluene. Yield: 84%. m.p. 145 - 150oC. 1H-NMR (400 M

Hz) – (t, 7.5243, J 4.06 Hz), (t, 7.6035, J 3.96 Hz), (d, 8.0869, J 4.99Hz), (d, 8.187, J 4.95

Hz), (s, 7.974) IR – (cm-1) – 3059.32, 1593.78, 1426.16, 1307.65, 914.94, 796.24,

693.09. Elemental Analysis: C18H10N2S3: %C – 61.62, %H – 2.84, %N – 7.94, %

S – 27.44%. Note that the synthesis of TBZT can be carried out using conventional oil

bath method, which was described in detail in Chapter II.

V. A.II. 3: Electrochemical Characterization:

Cyclic voltametric measurements were performed in N, N’ – dimethyl formamide

using SCE as standard electrode and Pt as the working electrode in 0.001M tetra butyl

ammonium per chlorate electrolytic conditions. Cyclic voltammogram shows two fully

reversible reduction waves at – 1.28 V and – 1.7 V, corresponding to two one-electron

transfer steps, indicating the ability of the molecule to undergo reduction easily, making

it a good electron transport material. This can be understood by the fact that the

introduction of heteroatom in the system, which suppresses the oxidation of the system as

reported for the oligothiophene systems, promotes the reduction pathway of the

molecules [1,2].

V. A.II.4: Photo generated Time – Of – Flight mobility measurements:

Time of flight measurements were carried out using frequency tripled Nd – YAG

laser ( = 355 nm, = 6 ns, 2 Hz) with incident power of 5 mJcm-2. Figure V.A – 2.2.

143

shows the current transient for ITO/TBZT (1.2~1.7µm)/Al (100nm) device as a function

of time under forward bias conditions for a specific carrier mobility. Under these

conditions, photo generation is confined to a layer (a few 100 nm thickness) close to the

ITO electrode. An optical pulse incident on the material through the transparent anodic

contact creates a thin sheet of electron – hole pairs next to the contact. The absorption

depth of the optical excitation is small compared to the film thickness and the optical

pulse duration is short compared to the transit time of the charge carriers across the

sample.

Low intensity optical pulses were used so that the photo generated charge carrier

density does not significantly perturb the spatially uniform electric field in the structure,

i.e. TOF measurements were performed in the quasi space – charge – free conditions. The

carrier mobility is given by

µe = d/ E -------------------------- (1)

where is the transit time of the carriers, d is the film thickness (1.7 m), and E is the

applied voltage (2.5 x 105V/cm). It can be seen that there is an initial current peak at

short times, commonly attributed to an initial relaxation of the charge carriers towards

their intrinsic density – of – states distribution [3,4]. For dispersive transients, where

inflection point is not visible, ttr is taken at the point of intersection of the linear

asymptotes [5,6]. Figure V.A – 2. 3 shows the electron mobility as a function of electric

field determined from the TOF measurements and a least square fit (solid line) to the

mobility. The field dependence of the mobility provides information about the carrier

transport mechanism in dispersive and non – dispersive systems.

144

0.01 0.1 1

2E-6

2E-5

2E-4 ITO/TBZT(1.2 ~ 1.7µµµµm)/Al (100nm)

ttr

C u

r r

e n

t (

a . u

)

T i m e ( µµµµ s)

Figure V.A – 2. 2: Time - of - flight photo current transient for ITO/ 2,5 – dibenzthiazolyl thiophene / Al shows dispersive transients. Note that transit time, ttr was taken at the point of intersection of the linear asymptotes.

450 500 550 600 650 700 7500.01

1E-3

1E-4

1E-5

Mob

ility

(cm

2 /Vs)

E1/2(V/cm)1/2

Figure V.A – 2. 3: Experimental field dependence of electron mobility, solid line shows best Poole – Frenkel fit.

145

Values of µe computed from Eq. (1) are plotted against E1/2. All the data can be fitted to a

phenomenological Poole - Frenkel equation [7]:

µ e = µo exp (β √E) ------------------------ (2)

where β is the PF slope. The error bars on the mobility shown in the inset to Figure V.A

– 2. 3 are estimates from TOF measurements on several different devices. As can be seen,

µe has relatively weak field dependence and has a value between 3 – 6 x 10 –4 cm2 V-1s-1

in the field range 450 < E0.5 < 700 (V cm-1) which is equivalent to the commonly used

hole transport layers TPD and NPD.

Theoretical studies have shown that such weak field dependence is characteristic

of a system with little geometrical flexibility [8,9]. To find out the structural rigidity we

carried out AM1-CI calculations on the neutral and anionic species of TBZT, which show

that there is a strong electron delocalization in the ground and excited states making the

molecule rigid under the photo - excited conditions in the anionic form. Figure V.A – 2.

4 shows the electron density mapping of HOMO, LUMO levels of the molecule, which

shows that the total energy of the system in the anionic form is strongly dependant on the

torsional angle. For neutral TBZT, the total energy is almost independent of the torsion

angle.

The mobility value of TBZT is high compared to that of the conventionally used

Alq3, which has a value of 1.4 x 10-6 cm2/Vs, hence TBZT can be used as an effective

146

HOMO

LUMO

Figure V.A – 2. 4: Electron density distribution in HOMO and LUMO levels of anionic 2,5 – dibenzthiazolyl thiophene using semi – empirical AM1 calculations

0 1 2 3 40.000

0.002

0.004

0.006

0.008

0.010

C u

r r

e n

t (

A m

p )

V o l t a g e ( V o l t s )

3.5 V

TPD

TBZT

ITO

Figure V.A - 2.5: Current – Voltage characteristics of bi - layer device, ITO/TPD/TBZT/Al showing a threshold voltage of 3.5 V.

Al

147

electron-injecting layer compared to the Alq3 to carry high current at low voltage.

Figure V.A - 2.5, shows the representative I- V characteristics of a bi - layer device

showing low threshold voltage of 3.5 V with high current output values. The schematic of

the device is shown as an inset to Figure V.A - 2.5 where in TPD is used as a hole

transport layer and TBZT as an electron transport and emissive layer. The device

structure does give high current values but doesn’ t emit appreciable light out put at

various thicknesses. One of the possible reasons is that the recombination zone of the

light emission is moved to the anodic region owing to the high mobility value of TBZT.

We evaluated the TBZT as an electron transport/injection layer by using ZBZT

device structure. Figure V.A - 2.6 shows the electroluminescent spectra of

ITO/TPD/ZBZT/Al and ITO/TPD/ZBZT/TBZT/Al, where in ZBZT, (bis (2 –(2’ –

hydroxyl phenyl) benzthiazolate) Zinc (II) complex ) acts as an electron transport and

emissive layer. The electroluminescence spectra of ITO/TPD/ZBZT/Al shows an

unusually broad band emission with a full width half maxima of more than 220 nm (filled

symbols) at an threshold voltage of 35 V. The broadened spectral contribution at the low

energies were attributed to the formation of exciplex at the interface of TPD and ZBZT

[10,11]. The electro luminescence spectra of ITO/TPD/ZBZT/TBZT (10nm)/Al shows a

much narrowed emission (open symbols) with a FWHM of 190 nm and a large drop in

the threshold voltage from 35 V to 16 V. A spectral shift of 25 nm is observed towards

higher energy region. The shift in the electroluminescence

characteristics can be attributed to the presence of TBZT layer in the device structure.

Owing to its high electron mobility, TBZT brings the recombination zone away from the

148

interface reducing the effect of exciplex to cause the narrowing of the emission spectra.

This confirms the electron transport capability of TBZT in the device structures.

V. A.II. 5: References:

1. P. Baeuerle, U. Mitschke, G. Gruener, and G.Rimmel, Pure. Appl. Chem. 71,

2153, 1999.

2. J. Caldas, E. Pettenati, G. Goldoni, and E. Molinari, Appl. Phys. Lett. 79, 2505,

2001.

3. H. Baessler, Phys. Status Solidi (b) 175, 15 ,1993.

4. E. Muller – Horsche, D. Haarer, and H. Scher, Phys. Rev. B 35, 1273, 1987.

400 450 500 550 600 650 700 7500.0

0.2

0.4

0.6

0.8

1.0ITO/TPD(50nm)/ZBZT(80nm)/TBZT(10nm)Al

ITO/TPD (50nm)/ZBZT( 80 nm) /Al

Ele

ctro

lum

ines

cenc

e

W a v e l e n g t h ( n m )Figure V.A - 2.6: shows the effect of TBZT on the ZBZT electroluminescent properties using the device structure ITO/TPD/ZBZT/Al.

149

5. I. H. Campbell, D.L. Smith, C.J. Neef, and J.P. Ferraris, Appl. Phys. Lett. 74,

2809, 1999.

6. B. Demrcq, C. Grasso, J. –L. Maldonado, M. Halik, S. Barlow, S.R.Marder, and

B. Kippelen, J. Phys. Chem. B, 108, 8647,2004.

7. D. Poplavskyy, T. Kreouzis, A. Campbell, J. Nelson, D. D. C. Bradley Mat. Res.

Soc. Symp. Proc. Vol. 725, P1.4.1.(2002).

8. Z.G. Yu, D.L. Smith, A. Saxena, R. L. Martin, and A.R. Bishop, Phys. Rev. Lett.

84, 721, 2000.

9. J. P. J. Markham, T. D. Anthopoulos, I. D. W. Samuel, G. J. Richards, H.

Baessler, Appl. Phys. Lett. 81,3266, 2002.

10. Y. N. Mahapatra, S.P. Singh, S.S. Manoharan, M. Qureshi, Indian Patent,

1774/DEL/2004.

11. Qureshi Mohammad, S. Sundar Manoharan, S. P. Singh and Y. N. Mahapatra,

Solid State Commun. (in print).