New Carbazole-, Indole-, and Diphenylamine-Based Emissive

NEW CARBAZOLE-, INDOLE-, AND DIPHENYLAMINE-BASED EMISSIVE

COMPOUNDS: SYNTHESIS, PHOTOPHYSICAL PROPERTIES, AND FORMATION OF

NANOPARTICLES

Krishna K. Panthi

A Dissertation

Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY

May 2011

Committee:

Thomas H. Kinstle (Advisor)

Carmen F. Fioravanti Graduate Faculty Representative

Marshall Wilson

John R. Cable

Krishna K. Panthi

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ABSTRACT

Thomas H. Kinstle, Advisor

The electronic and optical behavior of conjugated small molecules constitutes one of the most extensively studied properties of this class of organic compounds. In particular, electron donor-acceptor compounds separated by π-conjugation have attracted much research interest because of their applications as electroactive and photoactive materials in molecular electronic areas such as fluorescent technology, chemoluminescence, and photovoltaics. This work aims to demonstrate the synthesis and some of the interesting properties of new carbazole, indole, and diphenylamine donor-based donor-acceptor compounds.

Basically, we have divided our compounds into two groups: (a) aromatic fumaronitrile core-based compounds and (b) 2,7-carbazole linker-based compounds. We designed and synthesized these compounds for possible applications in electroluminescent devices (such as

OLEDs), organic sensors, organic nanoparticles, sensitizers in organic dye-sensitized solar cells

(DSSCs), and other optoelectronic devices.

Compounds containing an aromatic fumaronitrile core have attracted significant attention as candidates in electroluminescent devices because of their strong emissions in the solid state.

Five different compounds with a highly fluorescent and stable carbazole, indole, 2-phenylindole, diphenylamine, or 3,6-disubstituted carbazole donor with an aromatic fumaronitrile core were synthesized and characterized. They showed absorption ranging from Amax 306 nm to 450 nm and emission ranging from λmax 360 nm to 637 nm in medium polar solvent DCM. These compounds emitted blue, green, and red light. In most of the compounds, red-shifted emission in the solid state relative to that in solution was observed. The red shift in the solid state was as high as 114 nm. These compounds showed molar extinction coefficients ranging from 20881 dm3cm-1

-1 3 -1 -1 mol to 73266 dm cm mol in DCM, quantum yields of fluorescence (ФF) from 0.01 up to 0.80 in solution and 0.38-0.80 in the solid state, and lifetimes of fluorescence (τF) ranging from less than 0.1 ns up to 7 ns. Some of these compounds have significant potential for use in organic light-emitting diode devices since their emission covers nearly the entire visible region of the spectrum with high quantum yields both in solution and in solid state. The correlation between functional groups and optical properties of some of the compounds has been established. The ability of some of the compounds to function as colorimetric and luminescence pH sensors is demonstrated with color change and luminescence switching upon the addition of trifluoroacetic acid.

Fumaronitrile core-based compounds have a high propensity to form Fluorescent Organic

Nanoparticles (FONs) in appropriate superior/inferior solvent mixtures. All of these five aromatic fumaronitrile core-based compounds formed FONs in THF/water mixtures. These

FONs emit light from the visible region to the near IR region. These FONs show remarkable change in terms of intensity in both absorption and emission. Some of the nanoparticles of these compounds absorb up to 580 nm and emit from 350 nm up to the near infrared (NIR) region.

This is the first example that NIR emission was achievable upon the formation of nanoparticles from pure organic compounds. In some compounds, the emission intensity of nanoparticles is increased by 19 fold, and in some of the nanoparticles the emission is red shifted by 256 nm.

A new class of 15 different organic donor-acceptor compounds with a 3,6-disubstituted carbazole or diphenylamine donor, 2,7-functionalized N-substituted carbazole linker and either

(i) an aldehyde, (ii) cyanoacetic acid, (iii) malononitrile acceptors or (iv) diphenylamino or 3,6- disubstituted carbazolyl donors–all with or without phenylethynyl extenders-has been synthesized. The effect on the photophysical properties of these compounds caused by changing

v acceptors while keeping the donor and linker constant has been studied. These compounds, also, absorb from the UV into the visible region and emit intensely from blue to green. A study of the solvent effects of some of these compounds on their photophysical properties has shown that an increase in polarity of the solvent causes a reduction of fluorescence quantum yields. Solid state fluorescence quantum yields are generally greater than those in DCM. The compounds having a cyano group in the acceptor formed fluorescent nanoparticles in THF/water mixtures.

The properties of these 2,7-functionalized N-substituted carbazole linker-based compounds were also investigated using a combination of conventional steady-state absorption spectroscopy and tools of computational photochemistry. Time-dependent density functional theory calculations provide a deep insight into the photochemistry of these compounds in terms of molecular orbitals and the changes in electron density accompanying low-lying electronic transitions. One noticeable difference is that in the absence of phenylethynyl linker extenders, delocalization along the π-framework is generally more effective. These findings point out that less efficient electronic communication between the donor and acceptor is achieved when the phenylethynyl extender is inserted by design. Compounds having cyanoacrylic acid as an acceptor have potential as sensitizers in organic DSSCs; thus, we have fabricated them into devices and obtained the power conversion efficiency of 2.7% in the blue region of the spectrum.

All these data indicate that these compounds and their nanoparticles can be utilized in different applications such as photonics, electronics, sensors, and organic DSSCs.

To my father and mother

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ACKNOWLEDGMENTS

I would like to express my sincere and heartfelt gratitude to my advisor, Dr. Thomas H.

Kinstle, for his guidance, motivation, and support throughout my doctoral study at Bowling

Green State University. I greatly appreciate the freedom he gave me and the opportunity to pursue my research in my own way. His passion for science always encouraged me to think freely and logically, and motivated me to get more knowledge. Words are insufficient to express my appreciation, but still I would like to say thank you Dr. Kinstle for everything you did for me to achieve this goal. I feel honored to acknowledge your seminal role for this dissertation.

I would like to express my gratitude to Dr. Marshall Wilson, one of my committee members as well as my spectroscopy teacher, for his invaluable support. I also extend my gratefulness to Dr. John R. Cable for supporting me in my class work, and accepting my request to be on my committee after Dr. Neckers’ retirement, and I am grateful to Dr. Carmen F.

Fioravanti for serving on my committee.

It has my pleasure to acknowledge Dr. Neckers for being a committee member and encouraging me in my research before his retirement. I would also like to acknowledge Dr.

Anzenbacher and Dr. Castellano for their support and suggestions and I thank Dr. Ravi Adhikari for encouraging and critiquing the research.

I would like to thank all the present and former group members of Dr. Kinstle, who have been very helpful, collaborative, and friendly in lab during my work. Besides them, I would like to thank Dr. Neckers’ research students and students from other groups such as Patrick, Anthony, and others for their kind help during my research work.

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I would like to thank Nora, Alita, Stacey, and Lisa for their administrative help, and

Craig, Doug, Larry, Chen, and Romanowicz for assistance in all the technical issues regarding the research. I would like to highly acknowledge the Department of Chemistry, for providing me an assistantship and a scholarship during my graduate work. I am also indebted to the graduate student development fund for supporting me to attend conferences. The help of Center for

International Programs is highly appreciated and finally I would like to thank the Center for

Photochemical Sciences and the Bowling Green State University for giving me an opportunity to pursue a PhD degree in Photochemical Sciences.

I would not have been at this stage without encouragement, guidance, and support of my late father Bhima Kanta Panthi. I would like to express my gratitude to my mother Neel Maya, brothers, sisters, Father-in-law, Mother-in-law, brother-in-law, sister-in-law, and all other family members and teachers for their love, support, and encouragement.

My special thanks to my wife, Sapana, and sons, Kaustuv and Kovid, for their enormous love, care, support, and encouragement throughout my study. Without their emotional support, I could not have accomplished what I have today.

TABLE OF CONTENTS

Page

CHAPTER 1. CARBAZOLE-, INDOLE-, AND DIPHENYLAMINE-BASED COMPOUNDS:

BACKGROUND, INTRODUCTION, AND PHOTOPHYSICAL PROCESSES ...... 1

1.1. Background ...... 1

1.2. Introduction...... 3

1.3. Principles of Photophysical Processes ...... 8

1.3.1. Energy Transfer Processes ...... 10

1.3.2. Organic Semiconductor ...... 11

1.4. Organic Electronic Devices ...... 13

1.4.1. Organic Light-Emitting Devices (OLEDs) ...... 13

1.4.2. Photovoltaic Devices ...... 15

1.4.2.1. Organic Dye-Sensitized Solar Cells ...... 16

1.4.3. Organic Field Effect Transistors (OFETs) ...... 18

1.5. Goal of this Dissertation and Scope of the Projects ...... 19

1.6. References ...... 21

CHAPTER 2. AROMATIC FUMARONITRILE CORE-BASED DONOR-LINKER-

ACCEPTOR-LINKER-DONOR (D-π-A- π-D) COMPOUNDS: SYNTHESIS AND

PHOTOPHYSICAL PROPERTIES ...... 25

2.1. Introduction ...... 25

2.2. Experimental Section ...... 27

2.2.1. Synthesis of Compounds 2.10, 2.11, and 2.1-2.5...... 27

2.2.2. Fluorescence Quantum Yields (ФF)...... 31

2.2.3. Fluorescence Lifetime (τF) Measurements ...... 32

2.3. Results and Discussion ...... 32

2.3.1. Synthesis ...... 32

2.3.2. Absorption and Emission Spectra in Solution ...... 34

2.3.3. Solvatochromism: Optical Switching With Solvents ...... 37

2.3.4. Correlation between Quantum Yield and Magnitude (Aπ) of the π-Conjugation Length

...... 42

2.3.5. Fluorescence Switching With Concentration- Concentration Quenching ...... 45

2.3.6. Excitation Energy Dependence Fluorescence; Edge Excitation Red Shift (EERS) .... 46

2.3.7. Fluorescence Switching With a Change in pH ...... 47

2.4. Conclusions ...... 49

2.5. References ...... 50

Appendix 1. Absorption, emission, excitation, concentration quenching, emission during the

1 13 addition of TFA and Et3N, H NMR and C NMR spectra of compounds 2.1-2.5...... 52

CHAPTER 3. CARBAZOLE DONOR-CARBAZOLE LINKER-BASED COMPOUNDS:

PREPARATION AND PHOTOPHYSICAL PROPERTIES ...... 64

3.1. Introduction ...... 64

3.2. Experimental Section ...... 66

3.2.1. Synthesis of Compounds 3.1-3.4, 3.6, and 3.7 ...... 66

3.2.2. Fluorescence Quantum Yields (ΦF) Measurement ...... 70

3.2.3. Fluorescence Lifetime (τF) Measurement ...... 70

3.3. Results and Discussion ...... 71

3.3.1. Synthesis ...... 71

3.3.2. Absorption and Emission Spectra in Solution ...... 72

3.3.3. Solid State Photoluminescence ...... 77

3.3.4. Optical Switching with Concentration ...... 78

3.3.5. Excitation Energy Dependent Fluorescence; Edge Excitation Red Shift ...... 80

3.3.6. Temperature-Dependent Emission ...... 82

3.3.7. Lippert-Mataga Plot and its Significance on Specific and General Solvent Effects ... 84

3.4. Conclusions ...... 85

3.5. References ...... 86

Appendix 2. Absorption, emission, excitation, concentration quenching, emission with change in temperature, 1H NMR, and 13C NMR spectra of compounds 3.1-3.4...... 89

CHAPTER 4. CARBAZOLE DONOR AND CARBAZOLE OR BITHIOPHENE-BRIDGED

SENSITIZERS FOR DYE-SENSITIZED SOLAR CELLS ...... 101

4.1. Introduction ...... 101

4.2. Experimental Section ...... 104

4.2.1. General ...... 104

4.2.2. Characterization, Physical Measurements, and Instrumentation ...... 105

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4.2.3. Synthesis and Characterization ...... 106

4.2.4. Preparation of Nanocrystalline TiO2 Electrode and Transparent Platinum Cathode 109

4.2.5. Sandwiched Solar Cell Assembly ...... 109

4.3. Results and Discussion ...... 110

4.3.1. Synthesis and Characterization ...... 110

4.3.2. Photophysical Properties ...... 111

4.3.3. Electrochemistry ...... 113

4.3.4. Photovoltaic Measurements ...... 114

4.3.5. Quantum Chemical Calculations ...... 117

4.4. Conclusions ...... 119

4.5. References ...... 120

Appendix 3. 1H NMR, and 13C NMR spectra of compounds 4.3, 4.5, 4.6, 4.1, and 4.2...... 124

CHAPTER 5. SYNTHESIS AND COMPUTATIONAL STUDIES OF DIPHENYLAMINE

DONOR-CARBAZOLE LINKER BASED DONOR-ACCEPTOR COMPOUNDS ...... 129

5.1. Introduction ...... 129

5.2. Results and Discussion ...... 131

5.3. Conclusions ...... 142

5.4. Experimental and Computational Methods ...... 143

5.4.1. Synthesis ...... 143

5.4.2. Computational Methodology ...... 147

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5.5. References ...... 148

Appendix 4. 1H NMR and 13C NMR spectra of compounds 5.1-5.6 and tables showing

calculated TD B3LYP/6-31G* vertical transition energies ...... 151

CHAPTER 6. FLUORESCENT ORGANIC NANOPARTICLES; FUTURE MICRO-

OPTOELECTRONIC DEVICE MATERIALS ...... 159

6.1. Introduction ...... 159

6.2. Preparation of Nanoparticles ...... 160

6.2.1. Reprecipitation Method ...... 161

6.3. Fluorescent Organic Nanoparticles ...... 162

6.3.1. Molecular Exciton Model ...... 165

6.4. FONs: Importance and Applications ...... 166

6.5. References ...... 168

CHAPTER 7. VISIBLE AND NEAR IR EMITTING ORGANIC NANOPARTICLES OF

AROMATIC FUMARONITRILE CORE-BASED DONOR-ACCEPTOR COMPOUNDS .... 171

7.1. Introduction ...... 171

7.2. Experimental ...... 173

7.2.1. Fluorescence Lifetime (τF) Measurement ...... 173

7.2.2. Preparation of Nanoparticles ...... 173

7.2.3. SEM Images of Nanoparticles ...... 173

7.2.4. Fluorescent Microscopy (FM) Images of Nanoparticles ...... 174

7.2.5. Confocal Microscopy Images of the Nanoparticles ...... 174

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7.3. Results and Discussion ...... 174

7.4. Conclusions ...... 185

7.5. References ...... 185

Appendix 5. Absorption and emission spectra, scanning electron, fluorescent, and confocal

microscopy images, and solutions in THF and THF/water mixtures of compounds 2.1-2.5. . 188

CHAPTER 8. NANOPARTICLE FORMATION OF CARBAZOLE LINKER AND

CARBAZOLE OR DIPHENYL/TRIPHENYLAMINE-DONOR-BASED COMPOUNDS ..... 192

8.1. Introduction ...... 192

8.2. Experimental Section ...... 194

8.2.1. Synthesis of Compounds 8.1 and 8.2 ...... 194

8.2.2. Preparation of Nanoparticles ...... 195

8.2.3. SEM Images of Nanoparticles ...... 196

8.2.4. Fluorescent Microscope (FM) Images of Nanoparticles ...... 196

8.2.5. Confocal Microscope Images of Nanoparticles ...... 196

8.3. Results and Discussion ...... 196

8.3.1. Synthesis ...... 196

8.3.2. Formation of Nanoparticles ...... 197

8.4. Conclusions ...... 200

8.5. References ...... 201

Appendix 6. Absorption and emission spectra, scanning electron, fluorescent, and confocal microscopy images of compounds 3.2-3.3, and solutions in THF and THF water mixtures of compounds 3.2 -3.3 and others...... 203

Appendix 7. List of abbreviations, acronyms, and symbols.…………………………….….208

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LIST OF FIGURES

Page

Figure 1.1. Structures of carbazole, indole, and diphenylamine...... 3

Figure 1.2. Structures of compounds used to study the tunable optical and electrical properties

(due to substitution at various positions) of carbazole...... 5

Figure 1.3. Jablonski diagram ...... 9

Figure 1.4. Energy band diagram of metal, semiconductor, and insulator...... 12

Figure 1.5. The working principle of an OLED device...... 14

Figure 1.6. Typical aromatic amines used for hole-transport in OLED devices. Bis-(4-carbazol-9- yl)-biphenyl (CBP) and N,N'-di(naphthalen-1-yl)-N,N'-diphenyl-biphenyl-4,4'-diamine (α-NPD).

...... 15

Figure 1.7. The working principle of organic photovoltaics...... 16

Figure 1.8. Principle of operation of dye-sensitized nanocrystalline solar cell. The Figure is adopted from reference 42...... 17

Figure 1.9. Dyes 1-5 adopted from references 43-47 have efficiencies of 8.2%, 9.5%, 9.1%,

5.2%, and 6.0% respectively...... 18

Figure 2.1. Structures of compounds 2.1-2.5. ……………………………………………………27

Figure 2.2. Normalized absorption spectra of 2.1-2.5 in DCM...... 35

Figure 2.3. Normalized emission spectra of 2.1-2.5 in DCM (Compounds were excited at the corresponding Amax. Compounds 2.1-2.5 were excited at the 418 nm, 306 nm, 450 nm, 440 nm, and 347 nm respectively)...... 35

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Figure 2.4. Normalized Emission spectra of 2.3 in hexanes, toluene, DCM, and ACN. (Emission spectra are recorded at the respective Amax of the samples. They are excited in hexanes at 456 nm, toluene at 479 nm, DCM at 450 nm, and ACN at 480 nm)...... 40

Figure 2.5. Emission spectra of 2.4 in hexanes, toluene, DCM, and ACN excited at 340 nm. .... 40

Figure 2.6. Emission spectra of 2.4 in hexanes, toluene, DCM, and ACN excited at 440 nm. .... 41

Figure 2.7. Solutions of compound 2.4 in different solvents (A) hexanes, (B) toluene,(C) DCM, and (D) ACN. Left side Figure is under day light and right side is under UV-365 nm...... 42

Figure 2.8. A plot of quantum yield versus magnitude (Aπ) of π-conjugation length in hexanes and toluene for compounds 2.1-2.5...... 43

Figure 2.9. Emission spectra of 2.4 recorded in THF at different concentrations: (C1) 7.3x10-4 M,

(C2) 7.3x10-5 M, (C3) 7.3x10-6 M, and (C4) 7.3x10-7 M. (Inset: the enlarged spectrum of 2.4 recorded at 7.3x10-4 M)...... 45

Figure 2.10. Emission spectra of compound 2.4 in DCM at different concentrations. A = excitation wavelength 340 nm, B = excitation wavelength = 400 nm, and C = excitation wavelength = 440 nm. C1 = 5.55x10-4 M, C2 = 2.22x10-4 M, C3 = 6.93x10 -5 M, C4 2.22x10-5

M, and C5 = 2.22 x10-6 M concentration...... 47

Figure 2.11. Emission spectra of DCM solutions of compound 2.4 in presence of TFA and Et3N at various pH. A=DCM, B = TFA (pH = 4), C = TFA (pH = 3), D =TFA (pH = 2), E = TFA (pH

= 1), F = Et3N (pH = 9), and G = Et3N during neutralization of D (pH = 6)...... 48

Figure 2.12. Change in color of DCM solutions of compound 2.4 in presence of TFA and Et3N at various pH. A=DCM, B = TFA (pH = 4), C = TFA (pH = 3), D =TFA (pH = 2), E = TFA (pH =

1), F = Et3N (pH = 9), G = Et3N during neutralization of D (pH = 6), and H = Et3N during the neutralization of E (pH = 5)...... 48

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Figure 3. 1. Structure of compounds 3.1-3.4...... 66

Figure 3. 2. Normalized absorption spectra of compounds 3.1-3.4 in DCM...... 73

Figure 3.3. Normalized Emission spectra of compound 3.1-3.4 in DCM. (note: λex = Amax) ...... 74

Figure 3.4. Emission spectra of 3.1 recorded in THF at different concentrations: (C1) 3.1 x10-4

M, (C2) 3.1 x 10-5 M, (C3) 3.1 x 10-6 M, and (C4) 3.1 x10-7 M. (Inset: the enlarged spectrum of

-4 3.1 recorded at 3.1 x 10 M). (note: λex = Amax) ...... 76

Figure 3.5. Normalized solid-state emission of the thin solid films of 3.1-3.4. (note: λex = Amax in

DCM)...... 77

Figure 3.6. Emission of compound 3.2 on excitation at 350 nm with dilution in toluene.

Concentrations of the solutions are C0 = 2.7 x10-4 M, C1 = 2.22 x10-5 M, C2 = 4.5 x10-7 M, C3

=18.0x10-11 M, C4 = 36.0x10-13 M, C5 = 72.0x10-15 M, and C6 = 144.0x10-17 M...... 79

Figure 3.7. Normalized Excitation spectra of compound 3.2 in toluene by monitoring emission at

480 nm. Concentrations of the solutions are C0 = 2.7x10-4 M, C1 = 2.22x10-5 M, C2 = 4.5x10-7

M, C3 = 18.0x10-11 M, C4 = 36.0x10-13 M, C5 = 72.0x10-15 M, and C6 = 144.0x10-17 M...... 80

Figure 3.8. Jablonski diagram showing the possible excitation and de-excitation pathways for compound 3.2...... 80

Figure 3.9. Emission of compound 3.2 in hexanes (1x10-5 M) on excitation at different wavelengths...... 81

Figure 3.10. Emission of compound 3.2 in isopropanol (1x10-5 M) on excitation at different wavelengths...... 82

Figure 3.11. Emission spectra of compound 3.2 in DCM on excitation at 350 nm...... 83

Figure 3.12. Lippert-Mataga plot of compounds 3.1-3.4...... 85

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Figure 4.1. Chemical structures of the sensitizers 4.1, 3.3, and 4.2………………………….…104

Figure 4.2. Absorption-Emission spectra of sensitizers 4.1, 3.3, and 4.2 in DCM...... 112

Figure 4.3. Photoaction spectra of sensitizers 4.1, 3.3, and 4.2 (a) and N3 (b)...... 115

Figure 4.4. Current-voltage curves of sensitizers 4.1, 3.3, and 4.2 (a) and N3 (b)...... 115

Figure 4.5. HOMO-LUMO structures of the sensitizers 4.1, 3.3, and 4.2...... 119

Figure 5.1. Molecular structures of compounds 5.1-5.6…………………………………….….131

Figure 5.2. Experimental and computed UV-vis absorption spectra of compounds 5.1-5.6. The spectra are shown in red and blue for hexanes and DCM, respectively. The calculated vertical transition energies in hexanes and DCM are represented by circles and squares, respectively. The stick spectra were calculated using both the B3LYP (dashed bars, open symbols) and CAM-

B3LYP (dotted bars, full symbols) functionals...... 134

Figure 5.3. The highest 4 occupied molecular orbitals of compounds 5.1-5.3 calculated at the

CAM-B3LYP/6-31G* level of theory...... 137

Figure 5.4. The lowest 4 unoccupied molecular orbitals of compounds 5.1-5.3 calculated at the

CAM-B3LYP/6-31G* level of theory...... 138

Figure 5.5. The highest 4 occupied molecular orbitals of compounds 5.4-5.6 calculated at the

CAM-B3LYP/6-31G* level of theory...... 139

Figure 5.6. The lowest 4 unoccupied molecular orbitals of compounds 5.4-5.6 calculated at the

CAM-B3LYP/6-31G* level of theory...... 140

Figure 6.1. Methods of preparation of nanoparticles. …………………………………………..161

Figure 6.2. SEM images of PPB nanocrystals showing the size as average diameters (a) 50 nm,

(b) 150 nm, and (c) 300 nm obtained from suspensions of nanoparticles obtained for 65%, 70%, and 75% volume fractions of water added to THF, respectively...... 162

Figure 6.3. Structure of some compounds used for FONs...... 165

Figure 6.4. Band splitting of parallel vs antiparallel aggregation...... 166

Figure 7.1. Structures of compounds 2.1-2.5. ……………………………………………….….172

Figure 7.2. Absorption spectra of 2.1 nanoparticles solution (1x10-5 M) recorded at different THF and THF/H2O mixtures...... 177

Figure 7.3. Emission spectra of 2.1 nanoparticles solution (1x10-5 M) recorded at different THF and THF/H2O mixtures. Sample solutions were excited at 440 nm. (Inset: the enlarged spectrum of 2.1 recorded in THF)...... 177

Figure 7.4. Absorption spectra of 2.4 nanoparticles solution (1x10-5 M) recorded at different THF and THF/H2O mixtures...... 178

Figure 7.5. Emission spectra of 2.4 nanoparticles solution (1x10-5 M) recorded at different THF and THF/H2O mixtures. Sample solutions were excited at 440 nm...... 179

Figure 7.6. SEM images of nanoparticles of compound 2.2 (a) and 2.4 (b) in THF/H2O_1:8. .. 181

Figure 7.7. Fluorescent Microscopy (FM) image of nanoparticles of compound 2.4 in

THF/H2O_1:8...... 182

Figure 7.8. Confocal Microscopy image of nanoparticles of compound 2.4 in THF/H2O_1:8. . 182

Figure 7.9. Solutions of compound 2.1 from left to right in THF and THF/H2O mixtures in the ratio of 1:0, 1:4, 1:8, 1:12, and 1:16. The concentrations of all solutions were 1x10-5 M...... 184

Figure 7.10. Solutions of compound 2.4 from left to right in THF and THF/H2O mixtures in the ratio of 1:0, 1:4, 1:8, 1:12, and 1:16. The concentrations of all solutions were 1x10-5 M...... 184

Figure 8.1. Structures of compounds 3.2 and 3.3………………………………………………194

Figure 8.2. Solutions of nanoparticles of compound 3.2 (a) and 3.3 (b) under UV 365 nm...... 197

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Figure 8.3. Emission of compound 3.3 in THF and THF/H2O in the ratio of 1:5 and 1:8. (note:

λex = Amax)...... 198

Figure 8.4. SEM image of nanoparticles of compound 3.3 in THF/H2O_1:5...... 199

Figure 8.5. Confocal microscope image of nanoparticles of compound 3.3 in THF/H2O_1:5. .. 199

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LIST OF TABLES

Page

Table 2.1. Absorption maximum (Amax), emission maximum (λmax), molar absorptivity (ε),

Stokes shifts, and solid state quantum yields of 2.1-2.5 recorded in four different solvents.

Excitation is at Amax for each compound in corresponding solutions. Solid state ΦF values were measured by using an integrating sphere (errors within 15% range)...... 38

Table 2.2. Photophysical data of π-conjugated compounds 2.1-2.5 in different solvents ...... 44

Table 3.1. Absorption maximum (Amax), molar absorptivity (ε), emission maximum (λmax),

Stokes shift, ФF, and τF of compounds 3.1-3.4 in DCM. Excitation is at Amax for each compound in corresponding solutions……………………………………………………………………….75

Table 3.2. Emission maxima (λmax) and quantum yields fluorescence ΦF of 3.1-3.4 recorded in the solid state...... 78

Table 3.3. Emission maxima of compound 3.2 in isopropanol at different excitation wavelength.

...... 82

Table 3.4. Change in emission maxima of compounds 3.2 and 3.4 at different temperature...... 83

Table 4.1. Summary of photophysical properties of sensitizers 4.1, 3.3, and 4.2……………...113

Table 4.2. Summary of photovoltaic properties of the Sensitizers 4.1, 3.3, 4.2, and N3...... 117

Table 5.1. A description of the lowest three vertical transitions of compounds 5.1-5.6 in terms of major contributing molecular orbitals calculated at the CAM-B3LYP/6-31G* level of theory.142

Table 7.1. Absorption maximum (Amax), emission maximum (λmax), molar extinction coefficient

(ε), red shift of emission, and lifetime (τF) of compounds 2.1-2.5 in THF and THF/H2O mixtures…………………………………………………………………………………………183

Table 8.1. Absorption maximum (Amax), emission maximum (λmax) and red shift of emission during the formation of nanoparticles of compounds 3.2 and 3.3……………………………...199

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LIST OF SCHEMES

Page

Scheme 2.1. Synthesis of compounds 2.11, 2.16, and 2.18……………………………………..33

Scheme 2.2. Synthesis of compounds 2.1-2.5…………………………………………………...33

Scheme 3.1. Synthesis of compounds 3.7 and 3.8 ………………………………………………71

Scheme 3.2. Synthesis of compounds 3.1-3.4…………………………………………………..72

Scheme 4.1. Synthesis of sensitizers 4.1 and 4.2……………………………………………….111

Scheme 5.1. Synthesis of compounds 5.1-5.6………………………………………………….132

Scehme 8.1. Synthesis of compounds 8.1 and 8.2……………………………………………...197

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PUBLICATIONS

• Panthi, K.; Adhikari, R. M.; Kinstle, T. H. Aromatic Fumaronitrile Core-based Donor- Linker-Acceptor-Linker-Donor (D-π-A-π-D) compounds: Synthesis and Photophysical Properties. J. Phys. Chem. A. 2010, 114, 4542-5549.

• Panthi, K.; Adhikari, R. M.; Kinstle, T. H. Carbazole Donor-Carbazole Linker Based Compounds: Preparation, Photophysical Properties and Formation of Fluorescent Nanoparticles. J. Phys. Chem. A. 2010, 114, 4550-4557.

• Panthi, K.; Adhikari, R. M.; Kinstle, T. H. Visible and Near IR emitting Organic Nanoparticles of Aromatic Fumaronitrile Core-Based Donor-Acceptor Compounds. J. Photochem. Photobiol. A: Chem. 2010, 115, 179-184.

• Panthi, K.; El-Khoury, P. Z.; Tarnovsky, A. N.; Kinstle, T. H. Synthesis and a Comparative Studies of Diphenylamine Donor-Carbazole Linker-Based Donor-Acceptor Compounds. Tetrahedron. 2010, 66, 9641-9649.

• Panthi, K.; Onicha, A. C.; Castellano, F. N.; Kinstle, T. H. Carbazole Donor and Carbazole or Bithiophene-Bridged Sensitizers for Dye-Sensitized Solar Cell. Manuscript in preparation.

• Panthi, K.; El-Khoury, P. Z.; Kinstle, T. H. Synthesis and a Comparative TD DFT Study of Carbazole Donor Carbazole-Bridged Compounds. Manuscript in preparation.

Part I

CHAPTER 1. CARBAZOLE-, INDOLE-, AND DIPHENYLAMINE-BASED

COMPOUNDS: BACKGROUND, INTRODUCTION, AND PHOTOPHYSICAL

PROCESSES

1.1. Background

Organic compounds have a long history for human uses. A large number of small organic molecules are utilized as drugs, dyes, food additives, etc. More recently, large assemblies of organic molecules called materials, such as biological and synthetic polymers, have attained a position of prominent importance in science. After the discovery of conductive polymers by Alan

J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa, the possibility of using organic semiconducting materials for applications in optoelectronics and the semiconductor industry has been of great scientific and technological interest.1-3 Easy processability (i.e. from solution),

large area covering, and the possibility to use flexible substrates make organic semiconductors

ideal candidates for low-cost electronic applications. During the last 19 years, rapid progress has

taken place in the fields of materials development, device fabrication, deposition, and molecular

modeling.4 In the area of organic thin-film devices, very active research is underway covering

many subjects such as organic light-emitting diodes (OLEDs),5 organic field-effect transistors

(OFETs),6 organic sensors,7 and organic photovoltaics.8 Electronic devices, such as field-effect transistors using organic materials, are examples of applications of these materials. Photonic

organic materials for light-emitting devices and photovoltaic cells are currently under intensive

development. These and related materials are important in basic research as well as in industrial

applications because it is believed that they provide more versatility for device application,

energy conservation, and energy generation than alternative materials. Recent development of

solar cells using organic sensitizers is a fast-growing and crucial area due to expected energy

cost and possible shortage in the near future.

The development of such electroactive and photoactive organic materials has progressed

rapidly not only because of their potential applications cited above, but also because of the

advantages these materials have over inorganic ones, i.e., their ease of processing and the

tunability of their properties through simple chemical modifications. Such functional group

tuning has endowed the molecular materials with unique and interesting optoelectronic

properties.9-11

An intramolecular charge-transfer compound is one of the most important types of

molecular materials. They typically consist of an electron-donating (D) and an electron-accepting

(A) group connected through a π-conjugated linker. In their initially excited states, charge transfer and charge separation in these compounds endow them with unique optical and electrical properties. Molecules with donor-linker-acceptor (D-π-A) structures have attracted much technological research interest during the past few years because of their application as electroactive and photoactive materials in such molecular electronics, application in biochemical fluorescent technology,12 nonlinear optics,13,14 chemiluminescence,15 OLEDs,16,17 and

photovoltaic cells.9,18 The special advantage of these donor-acceptor materials are that their

physical properties can be easily tuned over wide ranges by appropriate chemical modification to

the structures of donors or acceptors, as stated earlier. Among such organic compounds,

carbazole-, indole-, and diphenyl/triphenylamine-based compounds are being increasingly

employed in various optoelectronic device applications. They have exhibited many exciting

results and should be more fully explored. Thus, we were interested to design, synthesize, and

study the photophysical properties and applications of different carbazole, indole, and

diphenyl/triphenylamine-based donor-acceptor compounds. The results of our photophysical

studies will help research chemists and applied chemists in their attempts to design and produce

better optoelectronic devices.

In this dissertation, I describe the synthesis and the investigation of novel compounds

based on symmetrical donor-linker-acceptor-linker-donor (D-π-A-π-D) and unsymmetrical

donor-linker-acceptor (D-π-A) structures. We have chosen carbazole, indole, or

diphenyl/triphenylamine groups as donors and most often carbazole as a linker. These molecules

have shown interesting optoelectronic properties. The strong electron withdrawing cyano group

is used as an acceptor in most of the compounds. We also fabricated three carbazole donor-based

compounds into a device and studied the performance of the resulting organic dye-sensitized

solar cell (DSSC).

1.2. Introduction

Carbazole is an organic heterocyclic compound consisting of a dibenzopyrrole structure,

and indole is an organic heterocyclic compound consisting of a simpler monobenzopyrrole

structure. Diphenylamine, or diphenyl substituted ammonia, is also called N-phenylbenzamine.

Carbazole, indole, and diphenylamine (Figure 1.1) derivatives are widely used as in

electroluminescent (EL) materials and in hole-transporting layers5,19,20 of organic light-emitting diodes (OLEDs), due to their high charge mobility.

N N N H H H

Carbazole Indole Diphenylamine

Figure 1.1. Structures of carbazole, indole, and diphenylamine.

Carbazole is a conjugated unit that has interesting optical and electronic properties such

as photoconductivity and photorefractivity. It has strong absorption in the near-UV region and

has a low redox potential. The electrochemical and spectroscopic properties of carbazole and its

derivatives have been extensively investigated. Due to its unique optical, electrical, and chemical

properties, carbazole has been used widely as a functional building block or substituent in the

construction of organic molecules for use as light-emitting and hole-transporting layers in OLED devices, as host materials for electrophosphorescent applications, and as active component in solar cells. The carbazole derivatives are often used as the materials for hole-transporting layers, utilizing the high charge mobility. A number of carbazole derivatives have been designed and synthesized. The properties of carbazoles can be tuned by substitution on the 2-, 3-, 6-, 7-, and

9H- positions.21,22 Chemically, carbazole can be easily functionalized at its 3-, 6-, or 9- positions

and covalently linked to other molecular moieties. Many carbazole derivatives have a

sufficiently high triplet energy making them efficient host materials where they can serve as

blue, green, and red triplet emitters.

Brunner et al.23 investigated the effect of substitution at the aromatic benzene rings or the

nitrogen of carbazole on the optical and electrochemical properties and correlated these

properties with various applications such as hole-transporting materials in OLEDs. Carbazoles

substituted on the 4,4’-positions of biphenyl through their nitrogen atoms, for example 4,4’-

bis(9-carbazoyl)-biphenyl (CBP), are good host materials for small-molecule OLEDs.23 Coupling

via the 9H- position of carbazole shifts the HOMO level to lower energy in comparison to the

coupling via the 3-position. Replacement of alkyl groups at the 9H- positions (1, 3, and 4)

(Figure 1.2) by aryl groups (5, 6, and 7) on carbazole oligomers exhibit a similar effect. The 9H-

substituted alkyl group activates the carbazole ring system by increasing its electron density,

5 thereby making the compound easier to oxidize than the corresponding 9H-substituted aryl group does. Since the aryl group is twisted with respect to the carbazole ring system, it is not involved in delocalization of π-electrons in the carbazole.

Substitution of an electron withdrawing group on conjugated molecules decreases the electron density in the π-system. As a result, the molecule is stabilized, and the oxidation potential increases, thereby shifting the HOMO level to lower energy. Such a stabilizing effect is stronger for the 3- and 6- substituted carbazole than for the 2- and 7- substituted carbazole which is because the electronic density at the 2- and 7- positions is less than at the 3- and 6- positions.

H H C8H17 C8 17 C8 17 C8H17 C H N N N 8 17 N N

Br Br N N N C H 3 C H N C H 2 8 17 8 17 1 8 17 4 C8H17

OCH3 OCH3 OCH3 OCH3

N N N N Br Br

N N N 7 5 6 OCH3 OCH3 OCH3

Figure 1.2. Structures of compounds used to study the tunable optical and electrical properties (due to substitution at various positions) of carbazole.

Brunner et al.23 found that the effects of substitution at the 9H- position and at the 3- and

6- positions are cumulative. The substitution effects can act cumulatively on the HOMO level. It is also found that the effects of substitution can oppose each other as evident by comparing

compounds 1, 3, and 7. In this series, upon attachment of a third carbazole, the HOMO level first

shifts to higher energy. Again, on replacing the alkyl groups at the 9H- position by aryl groups,

the HOMO shifts back. This has been found useful in tuning the HOMO level of carbazole

compounds by substitution at the 3-, 6-, and/or 9H- positions. Connecting carbazole molecules

through their 3- position shifts the HOMO level to higher energy, while replacing alkyl groups at

the 9H- position by aryl groups shifts the HOMO level to lower energy.23

Among the great number of papers relating to the well-known carbazole chemistry, only

a few papers have reported the synthesis and properties of 2,7-carbazole-based organic

compounds. Nevertheless, this type of material could be very interesting for electroactive and photoactive devices, because carbazole units linked at the 2- and 7- positions should lead to materials having a longer conjugation length than all other known carbazole derivatives.

However, the synthesis of 2,7-carbazole-based materials is not as straightforward as that of 3,6- carbazole-based materials. These 2,7-positions cannot be directly functionalized by standard electrophilic aromatic substitution.

Indole and indole-based electron donor groups have been utilized to build new second order nonlinear optical chromophores. These indole-based chromophores exhibit superior nonlinear optical properties in comparison with their more generally used aniline-donor analogues. Indole moieties have been reported as good building blocks for red electroluminescent materials.24 Indole-containing compounds are prominent in the literature because indole is a very important building block not only for naturally occurring compounds but also for many synthetic chemicals for their potential applications.25-28 Indole has been used for

the preparation of luminescent chelate compounds used as sensitizers in solar cells.29 Indole

compounds have also been used as light-emitting polymers,24 chemosensors,30 organic

sensitizers, and other devices.

The triarylamine (e.g. triphenylamine) and diphenylamine units are well known for their

ease in oxidation of the nitrogen center and the ability to transport charge carriers via radical

cation species with high stability. These have been widely investigated and applied in various

electro-optical materials such as OLEDs and organic field effect transistors. Diphenylamine is a

parent compound of many derivatives used for the production of dyes, pharmaceuticals, red

electroluminescent materials, photography applications, and other small-scale applications.

In recent years, it was found that the excellent photoelectric function of triarylamine was

desirable for organic sensitizers, and a substantial number of dyes with triarylamine as electron

donor have been developed for organic solar cells.

While, carbazole, indole, and triphenyl/diphenylamine derivatives are already proven as potential candidates in various types of optoelectronic applications, still more exploration of their derivatives is necessary for the development of different optoelectronic devices including organic solar cells, OLEDs, organic sensors, and other devices. Furthermore, our focus is on fumaronitrile core-based and 2,7-carbazole linker-based compounds. This is because diphenylfumaronitrile core in the trans-form consists of antiparallel dipoles so it helps to reduce fluorescence quenching in the solid state. Moreover, there are no reported D- π-A compounds in

which carbazole linker is directly linked to donor carbazole, indole, or diphenyl/triphenylamine

groups. In this dissertation, we have reported 20 new compounds of which 16 compounds have

carbazole either as donor or linker or both and 8 have either triphenylamine or diphenylamine

donors.

1.3. Principles of Photophysical Processes

A physical process resulting from the electronic excitation of a molecule by light or photons is called a photophysical process.31-33 The electronic states and the transitions involved

between them can be illustrated from the Jablonski diagram as shown in the Figure 1.3. The

singlet ground and first and second excited states are abbreviated as S0, S1, and S2. Within each

of these states, several vibrational levels can be assigned as 0, 1, 2, etc. Usually, light absorption

populates a higher vibrational level of S1 or S2. As given in the Figure 1.3, the electronic states

are arranged vertically by energy and horizontally by spin multiplicity. There are two types of

electronic transitions involved, i.e., radiative and nonradiative. The radiative transitions consist

of absorption, fluorescence, and phosphorescence, and are shown by straight arrows. The

nonradiative transitions are internal conversion (IC), intersystem crossing (ISC), and vibrational

relaxation.

The radiative excitation transition, in which a photon is absorbed, is called absorption.

When a photon is absorbed the molecule is excited from a lower to a higher electronic state. The

S1←S0 or Sn←S0 transitions are spin allowed and mostly appear in the absorption spectra. The

T1←S0 or Tn←S0 transitions are spin forbidden, but can be observed by using long light paths or

intense light. The Sn←S1 or Tn←S1 absorptions are mostly observed using time-resolved flash

photolysis. First, S1 is populated by an intense light source, such as a pulsed laser, and the

transient absorption of S1 can be observed during its lifetime. Similarly, when T1 is populated via

intersystem crossing from S1, the transient absorption of T1 can be observed by time-resolved

spectrometry. Such experiments yield the optical absorption spectra of the transient states, their

lifetimes, and rates of any reactions.

Sn IC

S1 ISC Vibrational relaxation

Energy T1 Phosphorescence Absorption Fluorescence

S0 Electronic Ground State

Figure 1.3. Jablonski diagram

The opposite of absorption is luminescence (emission), which can be defined as the radiative de-excitation transition in which the molecule is de-excited from a higher electronic state to a lower electronic state by the emission of light. Luminescent processes are of two types:

When a transition occurs between states with the same multiplicity, the process is called fluorescence, whereas the process involving a transition between states of different multiplicity is called phosphorescence. Generally, S0←S1 transition is fluorescence and S0←T1 transition is phosphorescence. Phosphorescence is a spin-forbidden process and is of relatively longer duration than fluorescence. Emission from higher excited states (S2 or T2) is also known, but occurs rarely.

There are different kinds of nonradiative transitions and they come through different mechanisms. Relaxation of the excited state to its lowest vibrational level is called vibrational relaxation. This process involves the dissipation of energy from the molecule to its surroundings, usually a solvent. Another type of nonradiative transition is internal conversion (IC), which occurs when an electronically excited vibrational state can couple to a lower electronic

vibrational state. A third type is the intersystem crossing (ISC), which is a transition to a state

with a different spin multiplicity such as T1←S1. ISC is facilitated in molecules with large spin-

orbit coupling and is responsible for efficient phosphorescence.

Emission is usually observed at longer wavelengths with respect to excitation. This is a

universal phenomenon for emission in solutions called Stokes shift. The explanation for this

effect is the rapid decay of the molecule to the lowest excited state. Very often, photophysical

and photochemical processes are carried out in solutions. Solvent molecules can interact with the

excited molecules of interest. For example, if the molecule is polar, the solvent polarity

environment has a large effect on the emission spectrum. Spectral shifts in such a cases can be

accounted for by the general effect of solvent polarity, where the excited state energy decreases

with increasing solvent polarity. Spectral shifts may be due to specific chromophore-solvent

interactions and to charge separation in the excited state. Overall, these effects are very complex,

and there is no single theory to predict all of them simultaneously.

1.3.1. Energy Transfer Processes

Energy transfers between photons and electrons are considered in optical materials, i.e.,

photoelectron generation occurs by light absorption, and emission after hole-electron

recombination. Energy transport after photo-excitation is also important, especially in

photovoltaic devices.

For photoluminescence, a radiative relaxation process occurs, i.e., electronic energy is converted to photonic energy. Fluorescence is luminescence from the recombination of single hole-electron pairs. This process is quantum-mechanically allowed to be fast, within 1 nanosecond. Phosphorescence appears when the recombination of holes and excited electrons is

11 quantum-mechanically forbidden because hole-electron pairs are triplet. Extra interactions are required to recombine these triplet hole-electron pairs, which causes the longer lifetime.

In light-emitting devices, recombination of excited electrons in holes is the most important process. About one-fourth of hole-electron pairs are ready to recombine to emit fluorescence, and the other pairs cannot recombine to emit light. Thus, converting triplet pairs to singlet pairs is desirable in light emitting devices.

For photovoltaic cells, recombination of hole-electron pairs is not desirable; therefore, triplet pairs are more useful to preserve photo-generated charge carriers. However, just after photo-absorption in optical materials, singlet hole-electron pairs are generated without transformation of these pairs, and they will recombine quickly to lose their electronic energies.

Thus, other processes for these photo-generated pairs are required to separate electrons from them. In addition, transport of separated charges should be considered. The energy transfer from an excited donor to an acceptor can be radiative or nonradiative. Nonradiative energy transfer between the excited donor and the acceptor can occur by two pathways. The first is a Forster type, a long-range resonance interaction, and the second is a Dexter type, a contact exchange energy transfer process. The Forster process is energy transfer via radiation or dipole-dipole interaction between a photo-excited system and an energy-receiving system,34 whereas in the

Dexter process the two systems are linked by a linker, and an energy transfer through this linker occurs by exchanging electrons.35 In both mechanisms, the energy level of the donor should be higher than the energy level of the acceptor.

1.3.2. Organic Semiconductor

Almost all organic compounds are insulators, but when their constituent molecules have

π-conjugated systems, electrons can move. Organic materials having such compounds can act as

12 semiconductors. An organic semiconductor is an organic material that has semiconducting properties. There are two major classes of organic semiconductors: organic charge-transfer complexes and various linear backbone conductive polymers, derived from polypyrrole, and polyaniline. Charge transfer complexes often exhibit similar conduction mechanisms to inorganic semiconductors. The simplified energy band diagrams of insulators, semiconductors, and metals are shown in Figure 1.4.

Figure 1.4. Energy band diagram of metal, semiconductor, and insulator.

The lowermost, almost fully occupied band in an insulator or semiconductor is called the valence band by analogy with the valence electrons of individual atoms, which is similar to the highest occupied molecular orbital (HOMO). The unoccupied energy level just above this is called a conduction band, which is similar to the lowest unoccupied molecular orbital (LUMO).

The gap between conduction band and valence band or LUMO and HOMO is called the forbidden gap or band gap energy. The transport of electrons is possible in the conduction band, while the holes, which are the counterparts of electrons, move along the valence band.

The band gap is one of the most useful aspects of the band structure, as it strongly influences the electrical and optical properties of a material. Electrons can transfer from one

band to the other by means of carrier generation and recombination processes. The amount of energy required for an electron to transfer from the valence band to the conduction band depends on temperature, the kind of material, and its purity. The band gap and defect states created in the band gap by doping can be used to create semiconductor devices, such as solar cells, diodes, transistors, and laser diodes. In our projects, we basically focused on photoconductivity, which is a phenomenon in which a material conducts energy due to the free charge carriers generated by light absorption.

1.4. Organic Electronic Devices

Organic electronics is a field related to the study of the properties of organic or polymeric materials utilized in active parts of electronic and optoelectronic devices. Among the devices that can be fabricated using science and technology, organic electronics are the OLEDs, transistors, solar cells, and sensors. Organic materials which have good fluorescence quantum yields are

useful for OLEDs. Compounds having suitable molecular packing and enhanced charge mobility

from one to another are useful for organic field-effect transistors (OFETS). They also have

applications in other electronic devices such as photovoltaic solar energy collectors and p-or n-

type semiconductors in p-n hetero-junctions. Organic semiconductors have advantages over their

inorganic counterparts since they have structural flexibility, low temperature solution processing,

and low cost.

1.4.1. Organic Light-Emitting Devices (OLEDs)

OLEDs have attracted considerable attention given their great potential application in flat

panel displays.36,37 The basic principle of an OLED is simple. When a layer of emitting material

is sandwiched between a cathode and an anode (Figure 1.5), light is emitted due to the

14 recombination of holes and electrons. However, many modifications are needed in order to enhance the performance of OLED devices.36,37 The structure of the organic layers and the choice of anode and cathode are selected to maximize the recombination process in the emissive layer, thus maximizing the light output from the OLED device.

Light

Glass ITO Input Organic material Al, Ca, Mg

Figure 1.5. The working principle of an OLED device.

The efficiency of OLEDs is determined by the number of charge carriers that are injected and the number of holes and electrons that actually recombine during emission of light. The materials used in single layer devices are usually better hole conductors than electron conductors.

As the holes are moving faster through the emitting layer, the recombination zone is shifted towards the cathode, which usually leads to a non-radiative loss of energy. Consequently, the efficiency of the device decreases. In order to improve device efficiency, the multilayered OLED architecture was introduced. In such multilayers, by varying transport properties and thickness, the recombination zone can be shifted towards the emission layer.

Highly emissive compounds with good chemical and electrochemical stability as well as a high thermal stability are important prerequisites for OLED materials. Compounds with bulky substituents are more suitable for a hole-transporting layer, and they have to exhibit HOMO levels in the order of -5.3 eV, therefore have low ionization potential. Aromatic amines, such as

NPD and CBP, are typical hole conducting materials for OLED applications (Figure 1.6).

N N N N

α− CBP NPD

Figure 1.6. Typical aromatic amines used for hole-transport in OLED devices. Bis-(4-carbazol- 9-yl)-biphenyl (CBP) and N,N'-di(naphthalen-1-yl)-N,N'-diphenyl-biphenyl-4,4'-diamine (α- NPD).

1.4.2. Photovoltaic Devices

Conversion of sunlight energy directly into electric energy via photovoltaic (PV) devices

is being recognized as an essential part of future global energy production. One possible solution

is adaption of light-harvesting biological methods. This biomimetric process has been recently

transformed into technological advances like solar cells.38,39 Although silicon-based

semiconductors40 dominated solar cell applications for decades, recent demonstration of DSSCs

based on nanocrystalline TiO2 by Gratzel et al. opened up the opportunity for use of organic dyes

in this area.38,41 Basically, the underlying principle of a light-harvesting organic PV cell is the

reverse of the principle in light emitting diodes (LEDs) (Figure 1.7).

In photovoltaics, when light is absorbed by a molecule, an electron is promoted from the

HOMO to the LUMO forming an exciton; this process must be followed by exciton dissociation.

The electron must then reach one electrode, whereas the hole must reach the other electrode.

Light

Glass ITO Output Organic material Al, Ca, Mg

Figure 1.7. The working principle of organic photovoltaics.

1.4.2.1. Organic Dye-Sensitized Solar Cells

A schematic presentation of the operating principle of an organic DSSC is given in

Figure 1.8.42 A photon enters the cell and traverses until it is absorbed by a dye molecule. The

dye will be promoted into its excited state (S*), from where it is energetically able to inject an

electron into the TiO2 conduction band. The electron then flows into an external circuit through a

load so that the energy can be utilized. After this, the electron, which now carries less energy,

enters the cell via the counter electrode. From the counter electrode/electrolyte interface, it

reduces the oxidized mediator (commonly an iodide/triiodide couple). The remaining oxidized

+ dye S on the TiO2 surface is then reduced back to its original state by the redox mediator to complete the cycle.

Figure 1.8. Principle of operation of dye-sensitized nanocrystalline solar cell. The Figure is adopted from reference 42.

Organic DSSCs have been the target of intense research in recent years due to their capability to convert solar light directly into electricity with inexpensive materials and low production costs. Compared with the traditional ruthenium dyes, metal-free organic dyes have

many advantages, such as lower cost, easier processing, and higher molar extinction coefficients.

Recently, novel organic dyes based on coumarin, merocyanine, cyanine, indoline,

triphenylamine, tetrahydroquinoline, and carbazole have been used in DSSCs with exciting

results. Expansion of the π-conjugated backbone to extend the absorption spectrum to the red is

one way to improve the solar cell’s performance. It does, however, complicate the synthetic

procedure, and decrease the stability of the dye molecules. The introduction of π-conjugated ring

moieties, such as benzene, thiophene, or carbazole into the methane backbone is, therefore used

as an economical way to simultaneously expand the π-conjugation system and sustain the

stability of the dye molecule. Here are the structures of some organic dyes43-47 which show

exciting solar cell performances (Figure 1.9).

C8H17 S N S COOH O O N N O NC S O S N Dye 1 O COOH Dye 2 N

COOH N NC Dye 3

N S COOH N CN COOH N NC

Dye 4 Dye 5

Figure 1.9. Dyes 1-5 adopted from references 43-47 have efficiencies of 8.2%, 9.5%, 9.1%, 5.2%, and 6.0% respectively.

1.4.3. Organic Field Effect Transistors (OFETs)

The first transistor based on inorganic semiconductor was invented by Bardeen, Brattain, and Shockley in 1947 using germanium and for which all three were awarded the 1956 Nobel

Prize in physics. Organic carbon based materials were not used for field effect transistors until in the middle 1980’s. Discoveries of semiconducting and conducting organic materials and the development of inorganic transistors gave motivations to develop organic semiconductors.

Though the organic semiconductors have too low charge carrier mobility, their processing advantages, and structural flexibility help make them competitive in various thin film transistor applications, such as in active matrix flat panel displays based on Liquid Crystal Displays

(LCDs) or OLEDs. Several novel applications have been proposed for sensors, smart cards, and radio frequency identification tags.

1.5. Goal of this Dissertation and Scope of the Projects

In the last 12 years, organic compounds/materials have emerged as potential candidates in the optoelectronic field, and they are now competing with their inorganic counterparts.

Organic materials can be employed in a variety of optoelectronic applications. This dissertation addresses some research issues in the field of organic optoelectronics. The objective of this research is multifold: the synthesis and study of the photophysical properties of compounds whose specific properties might be applicable to use as (i) fluorescent nanoparticles (ii) sensitizers in solar cells and (iii) sensors. We will undertake a study of time dependent density functional theory (TD DFT) calculations of these compounds in order to understand the effect of structure, particularly on the insertion of phenylethynyl linker/extender, in electron transfer.

The past decade has seen great progress in the synthesis of luminescent materials for

OLEDs. Fluorophores emitting light in the long wavelength region are usually polar and are highly emissive in solution, but weakly emissive in the solid state. This is because of aggregation by either intermolecular π-stacking or dipole-dipole interactions. Relevant to this problem, we have designed, synthesized, and reported two classes of luminescent compounds containing either a diphenylfumaronitrile core or a carbazole core. Diphenylfumaronitrile is used as the core in some of the compounds because, in the trans form, it consists of antiparallel dipoles. Thus, concentration-related fluorescence quenching in the solid state should be reduced due to dipole- dipole interaction. Also, the presence of the polar and sterically disruptive cyano group induces the J-aggregation that enhances the emission in the solid state. Additionally, a series of 2,7- carbazole linker, carbazole or diphenylamine/triphenylamine donor, and different acceptors- based DA compounds were synthesized and characterized. These compounds were tested for

20 various photophysical properties, nanoparticle formation, and dye-sensitized solar cell performance.

The next important application was make fluorescent organic nanoparticles (FONs) using diphenylfumaronitrile core-based or carbazole core-based compounds. Similarly, the D-π-A compounds having a cyano group as an electron acceptor also formed nanoparticles. These nanoparticles show emission that is red shifted and an intensity that is decreased. We reported near IR emitting organic FONs for the first time in the history of organic FONs. Also, we have studied the organic solar cell performance of some of these carbazole linker-based compounds.

These subjects will be elaborated in the subsequent chapters organized in the following ways.

• Chapter 1 describes the general background and principles related to the content of the

dissertation.

• Chapter 2 provides the synthesis and photophysical properties of aromatic fumaronitrile

core-based donor –acceptor compounds. This chapter describes the relationship between

quantum yield and magnitude of π-conjugation length, fluorescence switching with

change in pH, edge excitation red shift, concentration quenching emission, and other

topics.

• Chapter 3 describes the synthesis and photophysical properties of carbazole donor-

carbazole linker DA compounds containing phenylethynyl extenders. This chapter

illustrates solid state emission, concentration quenching emission, Lippert Mataga plot,

edge excitation red shift, and change in emission with temperature, among other

properties.

• Chapter 4 presents a synthesis and solar cell applications of carbazole donor and

cyanoacrylic acid acceptor based compounds. An energy conversion efficiency of 2.7%

in the blue region of the spectrum has been obtained for those compounds.

• Chapter 5 describes the synthesis and quantum calculations of diphenylamine donor

based DA compounds. We have elucidated the role of a phenylethynyl extender in the

DA compounds and also explained this role using calculations at the CAM-B3LYP level

of theory.

The chapters 1-5 mentioned above are included under Part I and the chapters 6-8 are included under Part II in this dissertion.

• Chapter 6 contains the general background and principle of organic nanoparticles related

to the context of the dissertation.

• Chapter 7 describes the formation of nanoparticles of aromatic fumaronitrile core-based

donor-acceptor compounds. We have observed for the first time organic nanoparticles

which emit in the near IR region, an intensity increase of 19 fold, and a red shift of 256

nm during particle formation.

• Chapter 8 describes the formation of nanoparticles for carbazole linker-based donor-

acceptor compounds. We have found that only compounds having a strong electron

withdrawing cyano group form nanoparticles.

1.6. References

(1) Miller, L. S.; Mullin, J. B., Editors, Electronic Materials: From silicon to organics, 1991.

(2) Farchioni, R.; Grosso, G., Editors, Organic Electronic Materials: Conjugated polymers and low molecular weight organic solids, 2001.

(3) Forrest, S. R. Nature 2004, 428911-428918.

(4) Kelley, T. W.; Baude, P. F.; Gerlach, C.; Ender, D. E.; Muyres, D.; Haase, M. A.; Vogel, D.

E.; Theiss, S. D. Chem. Mater. 2004, 16, 4413-4422.

(5) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913-915.

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CHAPTER 2. AROMATIC FUMARONITRILE CORE-BASED DONOR-LINKER-

ACCEPTOR-LINKER-DONOR (D-π-A- π-D) COMPOUNDS: SYNTHESIS AND

PHOTOPHYSICAL PROPERTIES

2.1. Introduction

Organic compounds with good photoluminescent responses are promising candidates for a variety of optoelectronic applications. Conjugated organic compounds exhibit a variety of interesting optical, electrical, and photoelectric properties in the solid state.1 The advantages of

organic compounds over inorganic ones are their cost and the ease of processing. Moreover, the

use of functional groups has endowed the molecular materials with unique and interesting

optoelectronic properties.2-4 Many conjugated organic light-emitting compounds are highly

emissive in their dilute solutions but become weakly luminescent when fabricated into devices.5

In the solid state the molecules aggregate to form less emissive species, such as excimers,

leading to a reduction in their luminescence efficiency.6 The conjugated materials are practically utilized in the solid state commonly as thin films. Many groups have attempted to diminish aggregate formation through elaborate chemical, physical, and engineering approaches. Swager and co-workers found 3.5 times higher quantum yield of fluorescence (ΦF) for poly(p-

phenylenethylene) film than that for its solution.7,8

Donor-acceptor compounds having an aromatic fumaronitrile core have attracted significant attention as candidates in electroluminescent (EL) devices because they emit efficiently in the solid state.9-11 It is assumed that the enhanced emission in the solid state is due

to the intramolecular planarization or a specific aggregation (H- or J-aggregation) in such

compounds. Such aggregation-enhanced emissive materials are promising as emitters in highly

efficient OLEDs.8,12 Motivated by their likely application for use in electroluminescent devices,

we chose to synthesize and study the optical properties of electron donor acceptor compounds

containing an aromatic fumaronitrile core showing higher ΦF in solid state than in solution.

Fumaronitrile core-based compounds have already attracted significant attention as candidates in

EL devices.9,10

We describe here the synthesis and characterization of a novel series of aromatic

fumaronitrile core-based electron donor-acceptor compounds 2,3-bis(4-(1H-indol-1-

yl)phenyl)fumaronitrile (2.1), 2,3-bis(4-(2-phenyl-1H-indol-1-yl)phenyl)fumaronitrile (2.2), 2,3- bis(4-(diphenylamino)phenyl)fumaronitrile (2.3), 2,3-bis(4-(2-(4-

(diphenylamino)phenyl)ethynyl)phenyl)fumaronitrile (2.4), and 2,3-bis(4-(2-(4-(3,6-di-tert- butyl-9H-carbazol-9-yl)phenyl)ethynyl) phenyl)fumaronitrile (2.5) (Figure 2.1). We chose the carbazolyl, indolyl, and diphenylaminyl donors with and without a π system extending phenylethynyl moiety to compare the optical effect of phenylethynyl on the same donor and acceptor. Not only do these compounds exhibit intense luminescence in solution and in solid state, but also show excellent stability. In the pursuit of stable and efficient organic compounds for optoelectronic application, it is important to establish some correlation between the fluorescent quantum yield and the π structure of fluorophores containing donor and acceptor.13

So, we also tried to find such a direct correlation between the functional group, π-conjugation

length, and donor/acceptor strength versus ΦF.

Since diphenylfumaronitrile was used as the core, concentration related fluorescence

quenching in the solid state should be greatly reduced due to the interaction of antiparallel

dipoles.14 Carbazole derivatives show high emission quantum yields in the solid state and the

compounds can easily be modified at the 3-, 6-, and 9H- positions to tune the optical

properties.15,16 Likewise, the diphenylamino group shows high emission efficiency as a dopant

in electroluminescence studies when attached to a cyano acceptor.17 Finally, indole containing

compounds have been used for the preparation of luminescent chelate compounds and can be

used as effective red electroluminescent materials.18

N N N NC N N CN NC CN NC 2.4 t-Bu CN NC CN t-Bu NC N N N N N CN 2.5 t-Bu t-Bu 2.1 . 2 2 . 2 3

Figure 2.1. Structures of compounds 2.1-2.5.

2.2. Experimental Section

2.2.1. Synthesis of Compounds 2.10, 2.11, and 2.1-2.5.

General: All solvents and reagents, unless otherwise stated, were of pure quality and used as

received. All reactions and manipulations were carried out under argon gas with the use of

standard inert atmosphere. The starting materials 2.6, 2.7, and 2.8 were purchased. The starting

material 2.16 was prepared by adapting literature procedure.15 Synthetic procedures for

compounds 2.10, 2.11 and compounds 2.1-2.5 are described here. 1H and 13C NMR spectra were

recorded at 300 MHz or 500 MHz using tetramethylsilane as the internal standard.

Compound 2.18 was synthesized according to the literature reported method.9

Synthesis of compound 2.10 (4-(4-(diphenylamino)phenyl)-2-methylbut-3-yn-2-ol). To a

solution of 4-bromotriphenylamine (6.48 g, 20 mmol) in dry piperidine (60 ml) was added 2-

methyl-3-butyn-2-ol (2.52 g, 30 mmol). After the solution was degassed with nitrogen for 30 min

while stirring, trans-dichlorobis(triphenylphosphine)palladium(II) (Pd(PPh3)2Cl2) (140 mg, 0.2

mmol), copper iodide (CuI) (95 mg, 0.5 mmol), and triphenylphosphine (PPh3) (130 mg, 95

mmol) were added. The reaction mixture was then refluxed under nitrogen for 10 h. After the

reaction was completed, the crude mixture was filtered at room temperature and the precipitate

was rinsed with diethyl ether, and the combined filtrates were evaporated to dryness. The residue

was purified by flash column chromatography (silica gel, 20% ethylacetate in petroleum ether) to

+ 1 give the product (6.4 g, 97%) as a dark syrup. MS: m/z 327 (M ). H NMR (300 MHz, CDCl3): δ

1.6 (s, 6H), 6.08 (d, 2H), 7.05 (d, 2H), 7.10 (d, 4H), 6.27 (t, 4H), and 7.29 (t, 2H). 13C NMR (300

MHz, CDCl3): 31.6, 65.7, 82.2, 93.0, 115.7, 122.4, 123.5, 124.9, 129.4, 132.6, 147.2, and 147.9.

+ HRMS (TOF MS ES ) calculated for C23H22NO 328.1701, measured 328.1697.

Synthesis of compound 2.11 (N-(4-ethynylphenyl)-N-phenylbenzenamine). The compound

2.10 (5.56 g, 12.75 mmol) was then dissolved in 75 ml of 2-propanol, to which pulverized KOH

(2.25 g, 37.5 mmol) was added. The mixture was refluxed under nitrogen with stirring for about

2 h. After hydrolysis, the product was extracted with diethyl ether, dried over MgSO4, and the

solvent was evaporated. The residue was purified by column chromatography (silica gel, 15%

ethyl acetate in petroleum ether) to give the product (3 g, 87%) as a golden yellow solid. MS:

+ 1 m/z 269 (M ). H NMR (300 MHz, CDCl3) δ 3.05 (s, 1H), 6.98 (d, 2H), 7.07 (d, 2H), 7.12 (d,

13 4H), 7.28 (t, 4H), and 7.34 (t, 2H). C NMR (300 MHz, CDCl3): 76.1, 83.9, 114.8, 122.0, 123.6,

+ 125.0, 129.4, 133.0, 147.1, and 148.4. HRMS (TOF MS ES ) calculated for C20H16N 270.1283,

measured 270.1279.

Synthesis of compound 2.1 (2,3-bis(4-(1H-indol-1-yl)phenyl)fumaronitrile). Bis(4-

bromophenyl)fumaronitrile (300 mg, 0.77 mmol) and indole (198 mg, 1.69 mmol) were mixed

with dry toluene (80 ml) in a two necked round bottomed flask containing a stir bar. The

Pd(OAc)2 (3%), tri-tert-butylphosphine (P(t-Bu)3) (7%), and cesium carbonate (Cs2CO3) (990

mg, 3.04 mmol) were also added and stirred under argon at 110 oC for about 30 h. The reaction

mixture was then cooled to room temperature and toluene was removed completely under

vacuum. The solid mixture was dissolved in tetrahydrofuran (THF) and unreacted Cs2CO3 was

removed under gravity filtration. The organic residue was then purified by column

chromatography (silica gel, 20% ethyl acetate in petroleum ether) to give the product (500 mg,

84%). 1H NMR (DMSO) δ 6.81 (d, 2H), 7.2 (t, 2H), 7.3 (t, 2H), 7.71 (d, 2H), 7.75 (d, 2H), 7.85

(d, 2H), 7.94 (d, 4H), and 8.14 (d, 4H). 13C NMR (500 MHz, DMSO): 105.4, 111.2, 121.4,

121.7, 123.3, 124.1, 124.5, 128.8, 129.7, 130.0, 131.0, 135.2, and 142.0. HRMS (EI) calculated

for C32H20N4 460.16879, measured 460.16865.

Synthesis of compound 2.2 (2,3-bis(4-(2-phenyl-1H-indol-1-yl)phenyl)fumaronitrile). Bis(4- bromophenyl)fumaronitrile (300 mg, 0.77 mmol) and 2-phenyl indole (326 mg, 1.69 mmol) were mixed with dry toluene (80 ml) in a two necked round bottomed flask containing a stir bar. The

Pd(OAc)2 (3%), P(t-Bu)3 (7%), and Cs2CO3 (990 mg, 3.04 mmol) were also added and stirred

under argon at 110 oC for about 50 h. The reaction mixture was then cooled to room temperature

and toluene was removed completely under vacuum. The solid mixture was dissolved in THF

and unreacted Cs2CO3 was removed under gravity filtration. The residue was then purified by

column chromatography (silica gel, 10% ethyl acetate in petroleum ether) to give the compound

1 (270 mg, 57%). H NMR (500 MHz, CDCl3) δ 7.23-7.30 (m, 4H), 7.3-7.35 (m, 10H), 7.17 (t,

13 2H), 7.46 (m, 6H), 7.73 (m, 2H), and 7.94 (d, 4H). C NMR (500 MHz, CDCl3): 100.0, 110.9,

120.3, 120.7, 122.4, 125.2, 127.7, 128.3, 129.0, 129.1, 129.3, 129.8, 132.4, 136.8, and 137.9.

HRMS (EI) calculated for C44H28N4 612.2314, measured 612.2316.

Synthesis of compound 2.3 (2,3-bis(4-(diphenylamino)phenyl)fumaronitrile). Bis(4-

bromophenyl)fumaronitrile (300 mg, 0.77 mmol) and diphenylamine (286 mg, 1.69 mmol) were

mixed in dry toluene (80 ml) in a two necked round bottomed flask containing a stir bar. The

Pd(OAc)2 (3%), P(t-Bu)3 (7%), and Cs2CO3 (990 mg, 3.04 mmol) were added and refluxed under argon for about 30 h. The reaction mixture was cooled to room temperature and toluene was removed completely under vacuum. The solid mixture was dissolved in THF and any unreacted

Cs2CO3 was removed under gravity filtration. The residue was purified by column

chromatography (silica gel, 10% ethyl acetate in petroleum ether) to give the compound (332

1 mg, 76%). H NMR (500 MHz, CDCl3) δ 7.07 (d, 4H), 7.16 (t, 4H), 7.20 (d, 8H), 7.35 (t, 8H),

13 and 7.70 (d, 4H). C NMR (500 MHz, CDCl3): 117.8, 120.1, 124.8, 125.9, 126.0, 129.6, 129.7,

130.4, 146.2, and 150.1. HRMS (EI) calculated for C40H28N4 564.23139, measured 564.23192.

Synthesis of compound 2.4 (2,3-bis(4-(2-(4(diphenylamino)phenyl)ethynyl)phenyl)

fumaronitrile). Bis(4-bromophenyl)fumaronitrile (75 mg, 0.19 mmol) and compound 2.11 (114

mg, 0.42 mmol) were mixed with dry toluene (20 ml) in a two necked round bottomed flask

containing a stir bar. The Pd(OAc)2 (3%), P(t-Bu)3 (7%), and Cs2CO3 (248 mg, 0.76 mmol) were also added and stirred under argon at 110 oC for 30 h. The reaction mixture was then cooled to room temperature and toluene was removed completely under vacuum. The solid mixture was dissolved in THF and any unreacted Cs2CO3 was removed under gravity filtration. The residue

was purified by column chromatography (silica gel, 15% ethyl acetate in petroleum ether) to give

1 the compound (106 mg, 72%). H NMR (500 MHz, CDCl3) δ 7.04 (d, 4H), 7.11 (d, 4H), 7.15

13 (d, 8H), 7.31 (t, 8H), 7.42 (d, 4H), 7.66 (d, 4H), and 7.86 (d, 4H). C NMR (500 MHz, CDCl3):

117.9, 121.9, 123.8, 124.3, 125.2, 125.3, 128.7, 129.4, 129.5, 132.0, 132.8, 147.01, and 148.5.

HRMS (EI) calculated for C56H36N4 764.29399, measured 764.29341.

Synthesis of compound 2.5 (2,3-bis(4-(2-(4-(3,6-di-tert-butyl-9H-carbazol-9- yl)phenyl)ethynyl) phenyl)fumaronitrile). Bis(4-bromophenyl)fumaronitrile (50 mg, 0.13 mmol) and compound 2.16 (107 mg, 0.28 mmol) were mixed with dry toluene (20 ml) in a two necked round bottomed flask containing a stir bar. The Pd(OAc)2 (3%), P(t-Bu)3 (7%), and

Cs2CO3 (165 mg) were also added and stirred under argon and refluxed for about 30 h. The

reaction mixture was cooled to room temperature and toluene was removed completely under

vacuum. The solid mixture was dissolved in THF and unreacted Cs2CO3 was removed under

gravity filtration. The residue was purified by column chromatography (silica gel, 15% ethyl

acetate in petroleum ether) to give compound 2.5 (82 mg, 60%). Mass spectrum (MALDI-TOF)

+ 1 m/z M = 984. H NMR (500 MHz, CDCl3) δ 1.5 (s, 36H), 7.43 (d, 4H), 7.51 (d, 4H), 7.63 (d,

13 4H), 7.72 (d, 4H), 7.79 (t, 8H), and 8.17 (s, 4H). C NMR (500 MHz, CDCl3): 32.0, 34.8, 109.2,

113.3, 116.4, 123.6, 123.8, 126.4, 126.4, 126.5, 132.4, 133.2, 138.8, and 143.3.

2.2.2. Fluorescence Quantum Yields (ФF). Fluorescence quantum yields in solutions were

measured following a general method using riboflavin (0.3 in ethanol) and anthracene (0.27 in

ethanol) as the standards.19,20 Dilute solutions of these compounds in appropriate solvents were

used for recording the fluorescence. Sample solutions of these compounds in quartz cuvettes

were degassed for ~15 min. The degassed solutions had an absorbance of 0.06-0.09 at

absorbance maxima. The fluorescence spectra of each of the sample solutions were recorded 3-4

times and an average value of integrated areas of fluorescence used for the calculation of ФF.

The refractive indices of solvents at the sodium D-line were used.

21 Values of ФF in the solid state were measured following a literature method. A DCM solution of sample was cast as a thin film on a spherical quartz plate and then allowed to dry. The plate was then inserted into an integrating sphere, and the required spectra were recorded. The

samples were excited at their absorption maxima in DCM. It is well-known that for compounds

showing an overlap of the absorption and the emission spectra (a small Stokes shift), the use of

an integrating sphere results in a substantial loss of emission because of absorption of the emitted

light.

2.2.3. Fluorescence Lifetime (τF) Measurements. To measure the fluorescence lifetimes (τF), the samples in different solvents were put in quartz cuvettes. Fluorescence decay profiles of the argon-degassed samples were measured using a single photon counting spectrofluorimeter.

Decays were monitored at the corresponding emission maximum of the samples. In-built software allowed the fitting of the decay spectra (χ = 1-1.5) and produced the fluorescence lifetimes.

2.3. Results and Discussion

2.3.1. Synthesis

Compounds 2.6 (indole), 2.7 (2-phenylindole), and 2.8 (diphenylamine) were purchased from Sigma-Aldrich and used without purification. Compound 2.11 (N-(4-ethynylphenyl)-N- phenylbenzenamine) was prepared from commercially available 2.9 (4-bromotriphenylamine) by

Sonogashira coupling with 3-methyl butyn-3-ol followed by reverse addition (Scheme 2.1).

Similarly, compound 2.16 (3,6-di-tert-butyl-9-(4-ethynyl-phenyl)-9H-carbazole) was synthesized from commercially available 2.12 (carbazole) in four steps involving alkylation, aromatic substitution, Sonogashira coupling, and deprotection (Scheme 2.1).15 Compound 2.18 (bis(4-

bromophenyl)fumaronitrile) was synthesized from compound 2.17 (4-bromophenylacetonitrile)

in one step (Scheme 2.1) following a literature procedure.9 Compounds 2.6, 2.7, 2.8, 2.11, and

2.16 were then coupled with compound 2.18 under similar conditions to obtain compounds 2.1-

2.5 (Scheme 2.2 and the Appendix 1).

Scheme 2.1. Synthesis of compounds 2.11, 2.16, and 2.18.

a b N Br N OH N H N N N H H H

2.6 2.7 2.8 2.11 2.9 2.10

- u t B t-Bu t-Bu t-Bu

NH NH c d N r N OH B e f N H - u t B t-Bu t-Bu t-Bu 2.12 2.15 . 2.13 2.14 2 16 Br

Br I2 (1 equiv) in diethyl ether, NaOCH3 (2.1 equiv) NC 30 min at -78 0C, 4 h at 0 0C, 3 % aq HCl CN

r CN B 2.18 2.17

Reagents and conditions: (a) 2-Methyl-3-butyn-2-ol, CuI, PPh3, Pd(PPh3)2Cl2; (b) KOH; (c) t- o BuCl, ZnCl2; nitromethane, heat at 50 C 5h; (d) 1,4-Dibromobenzene, K2CO3, Cu, 18-Crown-6; (e) 2-Methyl-3-butyn-2-ol, CuI, Pd(PPh3)2Cl2; (f) KOH.

Scheme 2.2. Synthesis of compounds 2.1-2.5.

Products Yield

2.6 2.1 84% 2.7 2.2 57% Pd OAc P t-Bu 2. 76% + ( )2, ( )3, 2.8 3 2.18 + 72 Cs2CO3 (4 equi.), reflux 2.11 2.4 % 2.16 2.5 60%

All compounds 2.1-2.5 are solid and soluble in common organic solvents such as toluene, dichloromethane (DCM), acetonitrile (ACN), and THF. The visible color of the compounds 2.1-

2.5 are yellow, deep orange, red, dark red, and orange respectively. These materials are perfectly stable in the solid state and could be stored without the need for any special precautions.

2.3.2. Absorption and Emission Spectra in Solution

The optical properties of the synthesized D-π-A-π-D compounds were investigated by

UV/vis absorption and photoluminescence spectroscopy using DCM solutions at room temperature. Representative examples of the absorption and emission spectra recorded in DCM are shown in Figures 2.2 and 2.3, respectively, and the data obtained are summarized in Table

2.1. The absorption maxima of these compounds range from 300 nm to 484 nm. The peak around

290-350 nm is due to absorption by carbazole, indole, or biphenyl moiety. Similarly, peaks at

390-425 nm are absorption by the intramolecular charge transfer complex (ICT) formed by some compounds even in the ground state.22 The presence of a strong electron donating (ED) group and an electron withdrawing (EW) group in 2.3 and 2.4 induces the formation of strong ICT.

Compound 2.5 forms the twisted intramolecular charge transfer state (TICT) in the presence of polar solvents.23,24 So, a weak band in the region of 425 nm is due to the absorption by TICT for

2.5. Interestingly, the absorption of compound 2.4 is blue shifted from that of compound 2.3 even though the former has two π-extending phenylethynyl moieties. This indicates that the ethynyl groups allow free rotation in the molecule to conformations in which electrons cannot be delocalized easily from the nitrogen atom of the donors to the acceptors (vide infra).

1.0 2.1 2.2 2.3 0.8 2.4 2.5

0.6

0.4

0.2 Normalized Absorbance 0.0 300 400 500 600 Wavelength (nm)

Figure 2.2. Normalized absorption spectra of 2.1-2.5 in DCM.

2.1 1.0 2.2 2.3 2.4 0.8 2.5

0.6

0.4

0.2 Normalized Emission 0.0 400 500 600 700 800 Wavelength (nm)

When excited at their Amax, dilute solutions of 2.1-2.5 in DCM showed violet to red

emission (Figure 2.3). In spite of the similar linker length and acceptor the emission maxima

vary from 360 nm to 614 nm. This indicates that the emission is mostly dependent on the linkers

and donors of these compounds (vide infra). Compound 2.1 has an indole moiety as ED and has

the most red shifted emission of all. It forms strong ICT upon excitation. So, in this compound an

excited ICT state is responsible for the emission in the red region with large Stokes shift.

Compound 2.2 has a weaker phenyl substituted indole as an ED group because the nitrogen lone

pair of electrons is delocalized by the phenyl group. Additionally, the presence of the bulky

phenyl group causes planarity distortion of the phenylindole with the rest of the molecule thereby

lowering the conjugation. Formation of ICT excited state is thus impeded. On keeping EA and

linker constant ΦF in solution increases with the stronger ED (Table 2.2). But in solid state ΦF

follows a reverse order. A similar trend is observed in 2.3, which has a stronger ED than 2.1 or

2.2. In compounds 2.4 and 2.5, which have same EA and linker, ΦF is higher for the compound

with weaker ED both in solution and in solid state. Red shifted emission was expected for

compounds 2.4 and 2.5 as compared to others because of the presence of the extra phenylethynyl

moiety in the linker. However, we observed a blue shifted emission for them. The increased

length of linkers distorts the planar structure and hence conjugation is impeded. Similar behavior

of linkers has been reported in similar compounds.25 Their strange emissive behavior could also

be due to the different kind of hybridization of the bond. The triple bond is sp hybridized which

is of linear configuration and substituents can rotate freely in solution because of the small

energy barrier, thus leading to a more twisted structure for 2.4 and 2.5 than similar double

bonded compounds.26

The magnitudes of the Stokes shifts indicate large (vibrational, electronic, and

geometric) differences between the excited state reached immediately after absorption and the

excited state from which the emission starts. Charge-transfer processes should be fairly effective due to the presence of a π-donor and a π-acceptor groups on these compounds.27 This effect also leads to a very low degree of self-absorption of emitted light. Hence, these compounds are predicted to perform well in organic light-emitting diodes and as laser dyes.

2.3.3. Solvatochromism: Optical Switching With Solvents

In an effort to gain further insight into the photophysical processes in these compounds, we investigated their absorption and emission behaviors in different solvents. The results of these investigations are summarized in Table 2.1. The absorption spectra are nearly independent of solvent polarity, except for a slight, insignificant shift that indicates a negligible intramolecular interaction between donor and acceptor groups in the ground state. In contrast, the emission spectra exhibit distinct solvent dependence. Broad structureless emission and larger Stokes shifts were observed for compounds 2.1-2.3 on increasing the solvent polarity, along with a successive decrease in the fluorescence intensity. Compounds 2.4 and 2.5 showed the reverse behavior. This kind of behavior has been explained fully for various chromophores containing donor-acceptor units.14 In general, the absorption and emission maxima are gradually red shifted when the compounds are dissolved in non polar hexanes compared to polar ACN. Surprisingly, both absorption and emission are either the same or blue shifted in ACN with respect to medium polar solvents like toluene or DCM for compounds 2.4 and 2.5 (vide infra).

Table 2.1. Absorption maximum (Amax), emission maximum (λmax), molar absorptivity (ε), Stokes shifts, and solid state quantum yields of 2.1-2.5 recorded in four different solvents. Excitation is at Amax for each compound in corresponding solutions. Solid state ΦF values were measured by using an integrating sphere (errors within 15% range).

Compound solvent Absorption Molar ext. Emission Stokes shifts Solid (Amax) (nm) coefficient (λmax)(nm) (nm) state 3 -1 -1 ε(dm cm mol ) ΦF

Hexanes 391 - 516 125

Toluene 414 - 550 136

2.1 DCM 418 19147 631 213 0.80

ACN 400 - 552 152

Hexanes 304 38933 351 47

2.2 Toluene 306 46058 360 54

DCM 306 29953 360 54 0.49

ACN 305 62183 360 54

Hexanes 456 - 560 104

2.3 Toluene 479 25177 603 122

DCM 450 18693 637 177 0.38

ACN 480 20881 700 220

Hexanes 440 47178 566 126

2.4 Toluene 450 43060 630 180

DCM 440 73266 614 174 0.55

ACN 420 69523 528 108

Hexanes 346 16496 477 131

2.5 Toluene 346 25237 513 177 0.64

DCM 347 29812 446 99

ACN 345 41966 446 101

The compound 2.2 shows a much smaller solvatochromic effect for absorption and emission. Also, the Stokes shift in all solvents for this compound is much smaller (50 nm) in comparison to other compounds for the reason described above. The compounds 2.1 and 2.3 show positive solvatochromism. The cases of compounds 2.4 and 2.5 are different since a systematic solvatochromic effect is not observed among solvents like in hexanes, toluene, and

DCM (Figure 2.5, 2.6, and the Appendix 1). Compound 2.4 shows positive solvatochromic effect from hexanes to toluene and negative solvatochromism from toluene to ACN (Figure 2.5 and

2.6). In toluene a red shifted emission is observed which is probably due to the stabilization of excited state by aromatic electrons of toluene. The excitation spectra obtained by monitoring the emission at the emission maximum of 2.4 (Appendix 1) shows that the emission is due to radiative deactivation of both local excited (LE) and ICT excited state in non-polar solvents

(hexanes and toluene) but in polar solvents such as ACN and DCM the emission occurs predominantly from LE state. In its ground state, 2.4 forms ICT state in polar solvents. On excitation, a red shifted emission is observed in non polar solvents but in polar solvents a blue shift emission, a property called negative solvatochromism, is observed.28 This suggests that the dipole moment of 2.4 decreases on excitation. Thus, less polar ICT excited state becomes more stable in non-polar solvents and less stable in polar solvents. The same holds true for 2.5 where

TICT forms instead of ICT (Appendix 1). The non-planar alignment of the carbazolyl moiety with 9H-substituted phenyl group is responsible for formation of TICT.29 Thus, this abnormal solvatochromic behavior of compounds 2.4 and 2.5 is probably due to their different donors and to formation of TICT or ICT in their singlet excited states.23,24

Hex 1.0 Tol DCM ACN 0.8

0.6

0.4

0.2

Normalized Emission 0.0 500 550 600 650 700 750 Wavelength (nm)

Hex 1.0 Tol DCM 0.8 ACN

0.6

0.4

0.2 Normalized Emission 0.0 400 500 600 700 800

Wavelength (nm)

Figure 2.5. Emission spectra of 2.4 in hexanes, toluene, DCM, and ACN excited at 340 nm.

Hex 1.0 Tol DCM 0.8 ACN

0.6

0.4

Normalized Emission 0.2

0.0 450 500 550 600 650 700 750 800 Wavelength (nm)

Figure 2.6. Emission spectra of 2.4 in hexanes, toluene, DCM, and ACN excited at 440 nm.

Compound 2.4 shows interesting emission properties. When a DCM solution is excited at

340 nm emission in the blue region was observed, and on exciting the same solution at 440 nm we observed emission in the red region (Figure 2.5 and 2.6). DCM is less polar than ACN, so upon excitation very little formation of ICT excited state occurs. When exciting DCM solution of

2.4 at 340 nm we observed emission mainly in the blue region with a shoulder around 575 nm.

The emission in the blue region is due to the LE state and that around 575 nm is from the ICT excited state. When exciting the same solution at 440 nm we observed emission primarily in the red region which is emission from the ICT state only. The same results were not observed in other solvents since only LE state was formed in ACN and predominantly ICT state was formed in toluene and hexanes. Changes in emission color of 2.4 in different solvents on excitation at

365 nm is illustrated in Figure 2.7.

Significantly high Stokes shifts of 213 nm for 2.1, 220 nm for 2.3, and 180 nm for compound 2.4 were observed. The absorption and emission spectra of compound 2.3 shows 104

nm Stokes shifts in hexanes, 122 nm in toluene, 177 nm in DCM, and 220 nm in ACN (Figure

2.4).

Figure 2.7. Solutions of compound 2.4 in different solvents (A) hexanes, (B) toluene,(C) DCM, and (D) ACN. Left side Figure is under day light and right side is under UV-365 nm.

2.3.4 Correlation between Quantum Yield and Magnitude (Aπ) of the π-Conjugation

Length

We tried to establish correlations among ΦF, donor strength, and length of linker in these

compounds. As no simple correlations could be observed, possible correlation between the ΦF and the magnitude (Aπ) of the π-conjugation length in the excited singlet state has been

discussed. We derived Aπ from the radiative (rate constant: kr) and radiationless process (rate

13 constant: knr). The obtained kr and knr values are shown in Table 2.2. On absorption of light,

unshared electrons on the nitrogen atom of the donor group promote to the acceptor. Since the

acceptor and donor are strong, the excited state is a charge transfer state with partial positive

charge on the N atom of donor and partial negative charge on nitrile moiety. The kr is the rate

constant for the return process of an n-electron from EA group promoted to the excited singlet

(S1) state to the ED in the ground (S0) state of a D-π-A-π-D system molecule accompanying

fluorescence emission.

1.5 1.0 0.5 0.0

-0.5 Aπ -1.0 -1.5 -2.0 Hexanes -2.5 Toluene 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Quantum Yield of Fluorescence

Figure 2.8. A plot of quantum yield versus magnitude (Aπ) of π-conjugation length in hexanes and toluene for compounds 2.1-2.5.

Similarly, knr is the rate constant for the radiationless process due to internal conversion and intersystem crossing. The magnitude of conjugation length may be defined as the distance between dipoles arising from So to S1 transition (light absorption). Thus knr is considered as the rate constant of radiationless decay due to the electron transfer (ET) from the negative charge to the positive charge. As shown in Table 2.2, the kr values increase and the knr values decrease with π-conjugation length of the D-π-A-π-D system, suggesting that the kr/knr value might be related to π extension in the S1 state of the D-π-A-π-D molecule. The magnitude of conjugation length (Aπ) is derived from kr and knr. On plotting ΦF against Aπ (Figure 2.8), a line with a positive slope is obtained. From this graph, we can see on increasing the strength of ED or length of linker or both, increases the ΦF.

Table 2.2 Photophysical data of π-conjugated compounds 2.1-2.5 in different solvents.a

Compound solvent Ф K (s-1) K (s-1) A (Å) F τF (ns) r nr π

Hexanes 0.15 0.88 1.70x108 9.65x108 -1.73 Toluene 0.43 6.25 6.88x107 9.12x107 -0.28

2.1 DCM 0.1 1.34 7.46x107 6.71x108 -2.20

ACN 0.02 3.51 5.69x106 4.99x108 -2.43

Hexanes 0.12 1.63 7.36x107 5.39x108 -1.99 2.2 Toluene 0.10 0.01 1.0x1010 9.10x1010 -2.20

DCM 0.01 1.01 9.90x106 9.80x108 -4.59

ACN 0.01 1.42 7.042x106 6.97x108 -4.74

Hexanes 0.16 1.00 1.6x108 8.4x108 -1.65 2.3 Toluene 0.10 1.42 7.04x1010 7.0x1011 -2.29

DCM 0.01 0.35 2.85x107 2.82x109 -4.59

ACN 0.01 0.07 1.42x108 8.00x1010 -6.33

Hexanes 0.49 2.24 2.18x108 2.28x108 -0.20 2.4 Toluene 0.80 3.20 2.50x108 6.12x107 1.40

DCM 0.74 7.65 9.64x107 3.36x107 1.05

ACN 0.01 2.09 4.78x108 4.74x108 -4.59

Hexanes 0.43 2.33 1.84x108 2.45x108 -0.28 2.5 Toluene 0.77 3.42 2.25x108 6.7x107 1.21

DCM 0.12 3.40 3.5x107 2.5x108 -1.96

ACN 0.08 3.42 2.33x107 2.68x108 -2.44

a Compounds were excited at the corresponding Amax representing the π-π* transition for τF and ΦF. The τF and ΦF were measured from argon-saturated solutions and decay way monitored at the corresponding λmax. The ΦF for 2.1, 2.3, and 2.4 are relative to that of riboflavin (0.30 in ethanol). The ΦF values for 2.2 and 2.5 are relative to that of anthracene (0.27 in ethanol).

2.3.5. Fluorescence Switching With Concentration- Concentration Quenching

A significant fluorescence quenching of compounds 2.1-2.5 has been observed in THF at higher concentrations (Appendix 1). For example, the fluorescence intensity almost triples when the concentration of compound 2.5 is increased from 4.5x10-7 M to 4.5x10-6 M but it decreases by almost the same amount when the concentration is increased to 4.5x10-5 M and is almost completely quenched at a concentration of 4.5x10-3 M. Although the fluorescence intensity is significantly decreased at a higher concentration, the λmax remains unchanged. Similar type of behavior is shown by all other compounds (Appendix 1). Although, no considerable quenching behavior of fluorescence was observed in compounds 2.2, 2.3, and 2.4, in compound 2.1

-5 quenching was observed even at 3.1x10 M concentrations. However, in these cases the λmax values were found to be slightly blue-shifted. This might be due to aggregation at higher concentration. In compound 2.5, there is no shift on emission with concentration change, which is probably because the tertiary butyl groups prevent aggregation.

6 6x10 18000 C1 16000 14000 6 C2 12000 C3 10000 5x10 8000 C4 6000 6 4000 2000 4x10 0 500 600 700 6

3x10

Intensity 2x106 1x106 0 450 500 550 600 650 700 750 800 Wavelength (nm)

Figure 2.9. Emission spectra of 2.4 recorded in THF at different concentrations: (C1) 7.3x10-4 M, (C2) 7.3x10-5 M, (C3) 7.3x10-6 M, and (C4) 7.3x10-7 M. (Inset: the enlarged spectrum of 2.4 recorded at 7.3x10-4 M).

In Figure 2.9, the emission spectra of 2.4 in THF at different concentrations are plotted.

At higher concentration, the emission appears mainly in the 500-600 nm region, and on dilution

the emission appears mainly in the region of 400-500 nm with an increase on intensity. We did

not observe such significant shift on emission with the other compounds though there is

concentration quenching in all of them. To further investigate this interesting property of 2.4, we

measured emission spectra in DCM and found a similar but more significant effect (Appendix 1).

2.3.6. Excitation Energy Dependence Fluorescence; Edge Excitation Red Shift (EERS)

Contrary to Kasha’s rule, fluorescence of 2.4 depends on the excitation energy.

Compound 2.4 in DCM was excited with different energies–340 nm, 400 nm, and 440 nm–at the

same concentration. The emission spectra obtained are plotted in Figure 2.10 (A, B, C). The

switching of emission with excitation energy was observed. On excitation at 340 nm, emission

was found mainly on the blue region with lower intensity in red region. On excitation of the

same solution at 400 nm, a perfect white emission is observed, and on excitation of the same

solution at 440 nm, we observed the emission mainly in the red region. This effect has been

referred to as EERS or red-edge excitation shift.30 This phenomenon can be described by

considering the emission contribution from additional excited species. In a case when the

reorientation relaxation time is larger than the fluorescence time, the total fluorescence emission

spectrum is a composite of fluorescence emission from differently solvated species governed by

Franck-Condon excited-state distribution (F-CESD) that is a function of excitation energy.30-32

For a polar compound in polar solvent, the energy required to excite the solvated species is a

function of solvent orientation. If the excitation energy is smaller, only limited configurations of

the ground state may be excited. The excitation with particular energy will excite only the

47 particular fraction of total fluorophore population which is surrounded by solvent dipoles.

6 6 3.5x10 1.4x10 C1 C1 A C2 6 C2 1.2x106 C3 3.0x10 C3 C4 6 6 2.5x10 C4 1.0x10 C5 C5 6 8.0x105 2.0x10

6 6.0x105 1.5x10 B 6 Intensity Intensity 4.0x105 1.0x10 2.0x105 5.0x105 0.0 0.0 350 400 450 500 550 600 650 450 500 550 600 650 700 750 Wavelength (nm) Wavelength (nm)

6 1.6x10 C1 1.4x106 C2 6 C3 1.2x10 C4 1.0x106 C5 5

8.0x10 6.0x105

Intensity C 4.0x105 2.0x105 0.0 450 500 550 600 650 700 750 Wavelength (nm)

2.3.7. Fluorescence Switching With a Change in pH

The nitrogen atoms of donors are basic centers that can be protonated. Thus the effect of protonation on the optical properties of DCM solutions of 2.1-2.5 was also studied. DCM

48 solutions of some compounds underwent a significant color change in the presence of trifluoroacetic acid (TFA). This color change was found to be reversible. Representative spectra for the change in optical properties with a change in pH for compound 2.4 are illustrated in

Figure 2.11 and the color change under UV light at 365 nm are given in Figure 2.12.

1x107 6 9x10 A 6 B 8x10 C 6 7x10 D 6 E 6x10 F 5x106 G

4x106 6 Intensity 3x10 2x106 1x106 0 350 400 450 500 550 600 650 Wavelength (nm)

Figure 2.11. Emission spectra of DCM solutions of compound 2.4 in presence of TFA and Et3N at various pH. A=DCM, B = TFA (pH = 4), C = TFA (pH = 3), D =TFA (pH = 2), E = TFA (pH = 1), F = Et3N (pH = 9), and G = Et3N during neutralization of D (pH = 6).

Figure 2.12. Change in color of DCM solutions of compound 2.4 in presence of TFA and Et3N at various pH. A=DCM, B = TFA (pH = 4), C = TFA (pH = 3), D =TFA (pH = 2), E = TFA (pH = 1), F = Et3N (pH = 9), G = Et3N during neutralization of D (pH = 6), and H = Et3N during the neutralization of E (pH = 5).

There is not a remarkable change in absorption spectra on addition of TFA to a DCM

solution of 2.4. However, the emission completely switches from the white region to the blue

region upon addition of TFA. In DCM, the emission is a consequence of radiative deactivation

from both excited ICT and LE state at this particular concentration. The ICT state is responsible

for emission in the red region and LE state is responsible for emission in the blue region. When

TFA is added the ICT state becomes less prominent as H+ from TFA consumes the lone pair of

electrons from N of the donor to form N+, so no charge separated ICT is possible. The

protonated species emits in the blue region. Again, on addition of triethylamine (Et3N) the proton

is removed and ICT becomes more prominent and emission by both LE and ICT state occurs in

blue and orange-red region. This property of 2.4 could find application as a pH sensor. Similar

but less significant effects have been observed for compounds 2.1 and 2.5. Compounds 2.2 and

2.3 did not show the similar change in optical properties on addition of the same amounts of TFA

(Appendix 1).

2.4. Conclusions

A class of aromatic fumaronitrile core based D-π-A-π-D compounds was synthesized,

fully characterized, and their optical properties were studied. A direct correlation between the functional group, π-conjugation length, and donor/acceptor strength versus ФF has been studied.

Fluorescence switching with concentration, excitation energy, and solvent polarity is illustrated.

Potential application of these compounds as a pH sensor is optically assessed with a change in

emission color upon change in pH. Their optical properties in solution and in solid state indicate

that these compounds could find application in optoelectronic device fabrication.

2.5. References

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(19) Scaiano, J. C. Hand book of Organic Photochemistry, CRC Press: Boca Raton, FL, 1989;

Vol. I.

(20) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229-235.

(21) de Mello, J. C.; Wittmann, H. F.; Friend, R. H. Adv. Mater. 1997, 9, 230-232.

(22) Fu, H.; Loo, B. H.; Xiao, D.; Xie, R.; Ji, X.; Yao, J.; Zhang, B.; Zhang, L. Angew. Chem. Int. Ed. 2002, 41, 962-965. (23) Yu, M.-X.; Chang, L.-C.; Lin, C.-H.; Duan, J.-P.; Wu, F,-I.; Chen, I.-C.; Cheng, C.-H. Adv. Funct. Mater. 2007, 17, 369-378. (24) Nikolaev, A. E.; Myszkiewicz, G.; Berden, G.; Meerts, W. L.; Pfanstiel, J. F.; Pratt, D. W. J. Chem. Phys. 2005, 122, 084309-1-084309-10. (25) Adhikari, R. M.; Shah, B. K.; Neckers, D. C. J. Org. Chem. 2009, 74, 3341-3349.

(26) Shao, H.; Chen, X.; Wang, Z.; Lu, Ping. J. Lumin. 2007, 127, 349-354. (27) Achelle, S.; Nouira, I.; Pfaffinger, B.; Ramondenc, Y.; Ple, N.; Lopez, J. R-. J. Org. Chem. 2009, 74, 3711-3717. (28) Allen, D. W.; Li, X. J. Chem. Soc., Perkin Trans.2 1997, 1099-1104. (29) Lakowicz, J. R. Principles of Fluorescence Spectroscopy: Springer: New York, NY, 2006. (30) Itoh, K.-i.; Azumi, T. J. Chem. Phys. 1975, 62, 3431-3438. (31) Fletcher, N. J. Phys. Chem. 1968, 72, 2742-2749. (32) Khalil, O. S.; Seliskar, C. J.; McGlynn, S. P. J. Chem. Phys. 1973, 58, 1607-1612.

Appendix 1. Absorption, emission, excitation, concentration quenching, emission during

1 13 the addition of TFA and Et3N, H NMR and C NMR spectra of compounds 2.1-2.5.

Absorption and emission spectra of 2.1-2.5

Hex Hex 1.0 Tol 1.0 Tol DCM DCM ACN 0.8 ACN 0.8

0.6 0.6

0.4 0.4

0.2 Normalized Emission 0.2 Normalized Absorbance 0.0 0.0 250 300 350 400 450 500 550 450 500 550 600 650 700 750 800 Wavelength (nm) Wavelength (nm)

Figure A1.1. Absorption spectra Figure A1.2. Emission spectra of of 2.1 in hexanes, toluene, DCM, 2.1 in hexanes, toluene, DCM, and ACN. and ACN.

Hex 1.0 1.0 Hex Tol Tol DCM DCM 0.8 ACN 0.8 ACN

0.6 0.6

0.4 0.4

0.2 0.2 Normalized emission Normalized Absorbance 0.0 0.0 250 300 350 400 450 500 550 350 400 450 500 Wavelength (nm) Wavelength (nm) Figure A1.3. Absorption spectra of Figure A1.4. Emission spectra of 2.2 2.2 in hexanes, toluene, DCM, and in hexanes, toluene, DCM, and ACN. ACN.

1.0 Hex 1.0 Hex Tol Tol DCM DCM 0.8 ACN 0.8 ACN

0.6 0.6

0.4 0.4

0.2 0.2 Normalized Absorbance Normalized Absorbance 0.0 0.0 300 350 400 450 500 550 600 300 400 500 Wavelength (nm) Wavelength (nm)

Figure A1.5. Absorption spectra of Figure A1.6. Absorption spectra of 2.4 in hexanes, toluene, DCM, and 2.5 in hexanes, toluene, DCM, and ACN. ACN.

1.0 Hex Tol DCM 0.8 ACN

0.6

0.4

0.2 Normalized emission 0.0 350 400 450 500 550 600 650 Wavelength (nm)

Figure A1.7. Emission spectra of 2.5 in hexanes, toluene, DCM, and ACN.

Excitaion spectra of 2.4 and 2.5

1.0 Hex 1.0 Hex Tol Tol DCM DCM 0.8 ACN 0.8 ACN

0.6 0.6

Intensity 0.4 0.4 Intensity

0.2 0.2

0.0 0.0 300 350 400 450 500 550 300 350 400 450 Wavelength (nm) Wavelength (nm)

Figure A1.8. Excitation spectra of 2.4 Figure A1.9. Excitation spectra of 2.5 in hexanes, toluene, DCM, and ACN. in hexanes, toluene, DCM, and ACN.

Concentration quenching spectra

5 8 5.0x10 C1 1.6x10 500000 5 4.5x10 C2 1.4x108 400000 5 C3 4.0x10 8 300000 C4 1.2x10 5 200000 3.5x10 8 5 1.0x10 3.0x10 100000 5 7 0

2.5x10 8000 8.0x10 350 400 450 500 5 7000 7 2.0x10 6000 6.0x10 C1 5000

5 Intensity Intensity C2 1.5x10 4000 7 5 3000 4.0x10 C3 2000 1.0x10 7 C4 4 1000 0 2.0x10 5.0x10 450 500 550 600 650 700 750 0.0 0.0 450 500 550 600 650 700 750 800 350 400 450 500 Wavelegth (nm) Wavelength (nm) Figure A1.11. Emission spectra of 2.2 Figure A1.10. Emission spectra of 2.1 recorded in THF at different recorded in THF at different -3 -5 concentrations: (C1) 1.3x10 M, (C2) concentrations: (C1) 3.1x10 M, (C2) -4 -5 -7 -6 -6 1.3x10 M, (C3) 1.3x10 M, and (C4) 7.1x10 M, and (C3) 1.5x10 (C4) 4.6x10 -6 1.3x10 M. (Inset: the enlarged spectrum M. (Inset: the enlarged spectrum of 2.1 -3 of 2.2 recorded at 1.3x10 M.) recorded at 3.1x10-5 M.)

18000 1.8x10 120000 6 16000 C1 14000 6 100000 2.5x10 12000 C2 1.6x10 10000 80000 8000 C3 6 6 6000 1.4x10 60000 4000 C4 2.0x10 2000 6 40000 0 500 550 600 650 700 750 1.2x10 20000

6 6 0 1.5x10 1.0x10 350 400 450 500 550 600 650

5 6 8.0x10 C1 1.0x10 6.0x105 C2

Intensity C3

Intensity 5 5.0x105 4.0x10 C4 2.0x105 0.0 0.0 500 550 600 650 700 750 800 350 400 450 500 550 600 650 Wavelength (nm) Wavelength (nm) Figure A1.12. Emission spectra of 2.3 Figure A1.13. Emission spectra of 2.5 recorded in THF at different concentrations: -4 -5 recorded in THF at different concentrations: (C1) 6.2x10 M, (C2) 6.2x10 M, (C3) (C1) 4.5 x 10-4 M, (C2) 4.5 x 10-5 M, (C3) 6.2x10-6 M, and (C4) 6.2x10-7 M. (Inset: the -6 -7 -4 4.5 x 10 M , and (C4) 4.5 x10 M. (Inset: enlarged spectrum of 2.3 recorded at 6.2x10 the enlarged spectrum of 2.5 recorded at 4.5 M.) -4 x 10 M.)

Normalized Emission of compounds 2.1 and 2.5 during the addition and TFA and Et3N in DCM maintaining different pH

A A 1.0 B 1.0 B C C D 0.8 D 0.8 E E F G F 0.6 H 0.6 G H

0.4 0.4

0.2 0.2 Normalized Emission Normalized Emission 0.0 0.0 450 500 550 600 650 700 750 800 350 400 450 500 550 600 650 Wavelength (nm) Wavelength (nm)

Figure A1.14. Emission spectra of DCM Figure A1.15. Emission spectra of DCM solutions of compound 2.1 in presence of solutions of compound 2.5 in presence of TFA and Et N at various pH. A = DCM, B TFA and Et3N at various pH. A = DCM, B 3 = TFA (pH = 5), C = TFA (pH = 4), D = TFA (pH = 5), C = TFA (pH = 4), D =TFA (pH = 1), E = TFA (pH = 2), F = =TFA (pH = 2), E = TFA (pH = 1), F = Et N (pH = 9), G = Et N during Et3N (pH = 9), G = Et3N during 3 3 neutralization of D (pH = 6), and H = Et N neutralization of D (pH = 1), and H = Et3N 3 during the neutralization of E (pH = 5). during the neutralization of E (pH = 3).

Change in color of DCM solutions of compounds 2.1-2.5 in the presence of TFA

Figure A1.16. Change in color of DCM Figure A1.17. Change in color of DCM solutions of compound 2.1 in presence of solutions of compound 2.2 in presence of TFA and Et3N at various pH. A=DCM, B TFA and Et3N at various pH. A=DCM, B = TFA (pH = 5), C = TFA (pH= 4 ), D = TFA (pH = 5), C = TFA (pH = 4), D = =TFA (pH = 1 ), E = TFA (pH = 2), F = TFA (pH = 3), E = TFA (pH = 1), F = Et3N (pH = 9), G = Et3N during Et3N (pH = 8), G = Et3N during neutralization of D (pH = 1), and H = neutralization of D (pH = 7), and H = Et3N Et3N during neutralization of E (pH = 7). during the neutralization of E (pH = 2).

Figure A1.18. Change in color of DCM Figure A1.19. Change in color of DCM solutions of compound 2.3 in presence of TFA solutions of compound 2.5 in presence of and Et3N at various pH. A = DCM, B = TFA (pH TFA and Et3N at various pH. A = DCM, B = = 4), C = TFA (pH = 3), D = TFA (pH = 2), E = TFA (pH = 5), C = TFA (pH = 4), D =TFA TFA (pH = 1), F = Et3N (pH = 8), G = Et3N (pH = 2), E = TFA (pH = 1), F = Et3N (pH = during neutralization of D (pH = 5), and H = 9), G = Et3N during neutralization of D (pH Et3N during the neutralization of E (pH = 2). = 6), and H = Et3N during the neutralization of E (pH = 5).

1H and 13C NMR spectra of compounds 2.1-2.5, 2.10, and 2.11

N OH

Figure A1.20. 1H NMR of compound 2.10.

Figure A1.21. 13C NMR of compound 2.10.

N H

Figure A1.22. 1H NMR of compound 2.11.

Figure A1.23. 13C NMR of compound 2.11.

H2O

DMSO

N CN

N NC

Figure A1.24. 1H NMR of compound 2.1.

Figure A1.25. 13C NMR of compound 2.1.

acetone

CN N N H2O NC

Figure A1.26. 1H NMR of compound 2.2.

Figure A1.27. 13C NMR of compound 2.2.

CN N N NC

Figure A1.28. 1H NMR of compound 2.3.

Figure A1.29. 13C NMR of compound 2.3.

CN N N NC

H2O

Figure A1.30.1H NMR of compound 2.4.

Figure A1.31. 13C NMR of compound 2.4.

t-Bu t-Bu NC N N CN t-Bu t-Bu H2O

Figure A1.32. 1H NMR of compound 2.5.

Figure A1.33. 13C NMR of compound 2.5.

CHAPTER 3. CARBAZOLE DONOR-CARBAZOLE LINKER-BASED COMPOUNDS:

PREPARATION AND PHOTOPHYSICAL PROPERTIES

3.1. Introduction

Electron donor-acceptor compounds separated by π-conjugation (D-π-A) have attracted

much research interest during the past few years because of their applications as electroactive

and photoactive materials in molecular electronics areas such as fluorescent technology,1 chemoluminescence,2 and photovoltaics.3,4 The strong emission and absorption properties of

carbazole and their derivatives5,6 have been exploited as electroluminescent and hole transporting

layers of OLEDs,7,8 as wide band gap energy transfer materials to lower band gap emitters,9,10

and in photovoltaics.11-13 The molecular and optical properties of carbazole can be tuned by

modifying its 2-, 3-, 6-, 7-, and 9- positions.14-16

There have been many studies of carbazole-based compounds in devices7-16 but fewer

studies of their photophysical properties in fluids17 and solid state. So, we focused our study on

the fundamental photophysical properties of some carbazole based D-π-A molecules in fluid and

solid state. We also focused on a study of formation of FONs 18-21 and their optical properties.

Edge excitation red shifts (EERS) have been observed in various excited-state reactions leading

to photoisomerization,22 electron transfer,23 and proton transfer.24 The application of EERS in

viscous and glass-forming fluids,25 binary solvent mixtures of different polarity proteins,26 and polymers27 has been demonstrated. Studies of luminescence as a function of temperature allow

estimation of the activation energy for the deactivation of the excited states.28 FONs have

received less attention, even though they allow wider variability and flexibility as materials. The

electronic and optical properties of nanoparticles differ from those of bulk materials because they

are structurally distinct and show confinement effects caused by their finite size.29,30

In studies using carbazole compounds, the molecular design has been mostly focused on

networking through the 3- and 6- positions,31-34 and only few examples have the conjugation

pattern through the 2-, 7-, and 9- positions.35-38 There are in fact no reported D-π-A compounds

in which carbazole donor is linked through its 9- position to the 2- and 7- linked carbazole. In

view of this we studied three D-π-A molecules (3.1-3.3) which have N-substituted carbazole

bonded through its 2- and 7- positions to the 9- position of another carbazole and an acceptor

moiety, respectively. Also, we have synthesized one more compound 3.4 by bonding N-

substituted carbazole through its 2- and 7- positions to the 9- position of two other carbazoles.

Compounds 3.1, 3.2, 3.3, and 3.4 differ only in one group, i.e. they have aldehyde in 3.1,

dicyanoethylene in 3.2, cyano acetic acid in 3.3, and 3,6-di-tert-butyl-9-(4-ethynyl-phenyl)-9H-

carbazolyl group in 3.4. In this dissertation, we report the synthesis of 4-(2-(7-(2-(4-(3,6-di-tert- butyl-9H-carbazol-9-yl)phenyl)ethynyl)-9-(4-(trifluoromethyl)phenyl)-9H-carbazol-2- yl)ethynyl)benzaldehyde (3.1), 2-((4-(2-(7-(2-(4-(3,6-di-tert-butyl-9H-carbazol-9- yl)phenyl)ethynyl)-9-(4-(trifluoromethyl)phenyl)-9H-carbazol-2- yl)ethynyl)phenyl)methylene)malononitrile (3.2), 2-cyano-3-(4-(2-(7-(2-(4-(3,6-di-tert-butyl-9H- carbazol-9-yl)phenyl)ethynyl)-9-(4-(trifluoromethyl)phenyl)-9H-carbazol-2- yl)ethynyl)phenyl)acrylic acid (3.3), and 3,6-di-tert-butyl-9-(4-(2-(7-(2-(4-(3,6-di-tert-butyl-9H- carbazol-9-yl)phenyl)ethynyl)-9-(4-(trifluoromethyl)phenyl)-9H-carbazol-2-yl)ethynyl)phenyl)-

9H-carbazole (3.4) compounds (Figure 3.1).

t-Bu t-Bu

NC N OHC N N COOH N t-Bu t-Bu t-Bu 3.3 t-Bu CF 3.1 t-Bu 3 CF3 N N N NC N N CN t-Bu t-Bu . t-Bu 3 4 CF 3.2 3 CF3

Figure 3.1. Structure of compounds 3.1-3.4.

We also report the solvatochromic properties, fluorescence switching with concentration,

temperature-dependent fluorescence behavior, and solid state photoluminescence of these

compounds. The specific and general solvent effects on optical properties of these compounds

are discussed with the help of Lippert-Mataga plots. By varying the acceptors, the absorption is

tuned from 353 nm to 386 nm, and emission is tuned from 422 nm to 454 nm in DCM.

Compound 3.2 with its electron-withdrawing (EW) group showed red-shifted emission in DCM

with decrease in temperature while 3.4 which has no EW group showed only a small change.

3.2. Experimental Section

3.2.1. Synthesis of Compounds 3.1-3.4, 3.6, and 3.7.

General: The synthesis of compounds 3.1-3.4, 3.6, and 3.7 is described here. Synthesis of the

remaining precursors are described elsewhere.39,40 Hexanes, toluene, DCM, ACN, THF, and

isopropanol are HPLC grade. Compounds 2.16 (described in chapter 2) and 3.5 were synthesized

by adapting literature procedures.39,40

Synthesis of 2,7-dibromo-9-[4-(trifluoromethyl)phenyl]-9H-carbazole (3.6). 2,7-Dibromo-

9H-carbazole (2.9 gm, 9 mmol), dimethylformamide (100 ml), potassium carbonate (54 mmol),

and 1-fluoro-4-(trifluoromethyl)benzene (2.9 mg, 18 mmol) were mixed in a round bottom flask.

The mixture was refluxed for 5 hours then cooled to room temperature. Following the addition of

water the mixture was filtered and the residue washed with water several times. The resulting

1 white solid was dried under vacuum (yield 95%). H NMR (500 MHz, CDCl3): δ 7.47 (dd, 2H),

13 7.54 (d, 2H), 7.70 (d, 2H), 7.95 (d, 2H), and 7.98 (d, 2H). C NMR (CDCl3): δ 112.9, 120.3,

121.7, 122.0, 124.3, 127.2, 127.5, 127.6, 127.61, and 141.3. HRMS (EI) calculated for

C19H10NF3Br2 468.911, measured 468.911.

Synthesis of 4-(2-(7-bromo-9-(4-(trifluoromethyl)phenyl)-9H-carbazol-2-

yl)ethynyl)benzaldehyde (3.7). To a solution of compound 3.6 (1 gm, 2.13 mmol) in dry

diisopropylethylamine (12 ml) was added 4-ethynylbenzaldehyde (0.24 gm). After the solution

was degassed with argon for 30 min while stirring, Pd(PPh3)2Cl2 (15 mg), triphenylphosphine

(10 mg), and CuI (11 mg) were added. The reaction mixture was then refluxed under argon for

10 h. After the reaction was complete, the crude mixture was filtered at room temperature, the precipitate was rinsed with diethyl ether and the combined filtrates were evaporated. The residue was purified by flash column chromatography (silica gel, 20% ethyl acetate in petroleum ether)

1 to give compound 3.7 (51%) as a yellowish solid. H NMR (300 MHz, CDCl3) δ 7.47 (dd, 1H),

7.54 (dd, 1H), 7.58 (d, 2H), 7.70 (d, 2H), 7.74 (d, 2H), 7.87 (s, 1H), 7.90 (s, 1H), 7.96 (d, 2H),

13 8.01 (d, 1H), 8.12 (d, 1H), and 10.05 (s, 1H). C NMR (CDCl3): δ 89.0, 94.0, 112.9, 113.2,

120.4, 120.6, 120.7, 121.9, 122.2, 123.6, 124.3, 124.8, 127.3, 127.5, 127.6, 129.6, 132.1, 135.5,

140.3, 141.9, and 191.3. HRMS (EI) calculated for C28H15ONBrF3 517.0289, measured

517.0287.

Synthesis of 4-(2-(7-(2-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)ethynyl)-9-(4-

(trifluoromethyl)phenyl)-9H-carbazol-2yl)ethynyl)benzaldehyde (3.1). To a solution of

compound 3.7 (470 mg, 0.9 mmol) in dry diisopropylethylamine (8 ml) was added compound

2.16 (344 mg, 0.9 mmol). After the solution was degassed with argon for 30 min while stirring,

Pd(PPh3)2Cl2 (10 mg), triphenylphosphine (10 mg), and CuI (10 mg) were added. The reaction mixture was then refluxed under argon for 10 h. After the reaction was complete, the crude mixture was filtered at room temperature, the precipitate was rinsed with diethyl ether, and the

combined filtrates were evaporated. The residue was purified by flash column chromatography

(silica gel, 10% ethyl acetate in petroleum ether) to give compound 3.1 (55%) as a shining

1 yellow solid H NMR (500 MHz, CDCl3) δ 1.49 (s, 18H), 7.42 (d, 2H), 7.5 (dd, 2H), 7.57 (dd,

2H), 7.59 (dd, 2H), 7.65 (s, 1H), 7.66 (s, 1H), 7.72 (d, 2H), 7.78 (d, 2H), 7.81 (d, 2H), 7.9 (d,

13 2H), 7.99 (d, 2H), 8.16 (s, 2H), 8.17 (d, 2H), and 10.05 (s, 1H). C NMR (CDCl3): δ 32.0, 34.7,

109.2, 112.9, 113.1, 116.3, 120.5, 120.8, 121.5, 123.3, 123.6, 123.7, 124.7, 126.4, 127.4, 127.5,

129.6, 132.1, 133.0, 135.5, 138.9, 140.8, 140.9, 143.3, and 191.3. HRMS (EI) calculated for

C56H43ON2F3 816.332, measured 816.331.

Synthesis of 2-((4-(2-(7-(2-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)ethynyl)-9-(4-

(trifluoromethyl)phenyl)-9H-carbazol-2yl)ethynyl)phenyl)methylene) malononitrile (3.2).

In a dry two necked round bottomed flask was added compound 3.1 (100 mg, 0.12 mmol),

malononitrile (0.13 mmol), basic aluminium oxide (10 mmol), and dry toluene (30 ml). The

mixture was refluxed under argon for 5 h. The mixture was filtered hot and the residue was

washed several times with hot ethyl acetate. The filtrate was then dried and the solid obtained

was purified by chromatography (silica gel, 10% ethyl acetate in petroleum ether) to obtain pure

3.2 (78%) as a yellow solid . 1H NMR (500 MHz, DMSO) δ 1.47 (s, 18H), 7.39 (d, 2H), 7.51

(dd, 2H), 7.61 (d, 2H), 7.68 (s, 2H), 7.67 (d, 2H), 7.83 (d, 2H), 7.85 (d, 2H), 8.0 (d, 2H), 8.03 (d,

2H), 8.12 (d, 2H), 8.31 (s, 2H), 8.43 (d, 2H), and 8.56 (s, 1H). 13C NMR (500 MHz, DMSO): δ

32.3, 35.1, 89.7, 91.6, 96.0 109.6, 113.3, 113.6, 113.7, 117.3, 121.2, 122.1, 123.3, 123.6, 124.0,

125.6, 125.8, 126.8, 128.1, 128.3, 128.5, 128.7, 129.4, 130.3, 131.3, 131.6, 132.1, 132.7, 133.6,

138.1, 138.6, 140.7, 140.8, 140.9, 143.4, and 160.6. HRMS (MALDI-TOF) MS ES+ = 864.4

measured, and 864.34 for theoretical spectrum having molecular formula C59H43N4F3.

Synthesis of 2-cyano-3-(4-(2-(7-(2-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)ethynyl)-9-

(4-(trifluoromethyl)phenyl)-9H-carbazol-2-yl)ethynyl)phenyl) acrylic acid (3.3). To a round

bottomed flask containing a mixture of compound 3.1 (80 mg, 0.1 mmol), cyanoacetic acid (9

mg, 0.1 mmol), and ammonium acetate (1 mg) was added acetic acid (4 ml). The mixture was

heated at 120 oC for 6 h. and allowed to cool to room temperature. The resulting solid was

filtered and washed with distilled water, diethyl ether, and methanol to give a bright orange solid

(81%). Mass spectrum (MALDI-TOF) m/z M+ = 883. 1H NMR (500 MHz, DMSO) δ 1.48 (s,

18H), 7.38 (d, 2H), 7.50 (dd, 2H), 7.61 (d, 2H), 7.68 (s, 2H), 7.69 (d, 2H), 7.78 (d, 2H), 7.85 (d,

2H), 8.02 (d, 2H), 8.09 (d, 2H), 8.12 (d, 2H), 8.30 (s, 2H), 8.37 (s, 1H), and 8.42 (d, 2H). 13C

NMR (DMSO): δ32.3, 35.0, 90.0, 91.5, 94.6, 101.6, 109.6, 113.2, 113.5, 116.5, 117.2, 120.3,

121.0, 122.0, 123.4, 123.4, 123.6, 123.8, 124.9, 125.0, 125.6, 126.7, 127.2, 128.1, 128.2, 129.4,

131.4, 131.9, 132.5, 133.6, 138.0, 138.6, 140.7, 140.8, 143.4, 153.7, and 163.6.

Synthesis of 3,6-di-tert-butyl-9-(4-(2-(7-(2-(4-(3,6-di-tert-butyl-9H-carbazol-9- yl)phenyl)ethynyl)-9-(4-(trifluoromethyl)phenyl)-9H-carbazol-2-yl)ethynyl)phenyl)-9H- carbazole (3.4). To a solution of compound 3.6 (61 mg, 0.13 mmol) in dry diisopropylethylamine (6 ml), was added compound 2.16 (100 mg, 0.26 mmol). After the solution was degassed with argon for 30 min while stirring, Pd(PPh3)2Cl2 (1 mg),

triphenylphosphine (1 mg) and CuI (1 mg) were added. The reaction mixture was then refluxed

under argon for 6 h. After the reaction was complete, the crude mixture was filtered at room

temperature, the precipitate was rinsed with diethyl ether, and the combined filtrates were

evaporated. The residue was purified by flash column chromatography (silica gel, 10% ethyl

1 acetate in petroleum ether) to give 3.4 (78%) as a yellow solid. H NMR (500 MHz,CDCl3) δ

1.48 (s, 36H), 7.42 (d, 4H), 7.50 (dd, 4H), 7.59 (d, 4H), 7.60 (d, 2H), 7.67 (s, 2H), 7.78 (d, 4H),

13 7.83 (d, 2H), 8.00 (d, 2H), 8.16 (s, 4H), and 8.18 (d, 2H). C NMR (500 MHz, CDCl3): δ 32.0,

34.8, 90.0, 91.0, 109.2, 112.9, 116.3, 120.3, 120.5, 121.6, 121.9, 122.0, 123.7, 124.2, 124.3,

124.7, 126.4, 127.3, 127.4, 127.5, 127.6, 133.0, 138.2, 138.9, 141.4, and 143.2. HRMS

(MALDI-TOF) MS ES+ = 1065.3 measured, and 1065.5 for theoretical spectrum having molecular formula C75H66N3F3.

3.2.2. Fluorescence Quantum Yields (ΦF) Measurement. The ΦF values in solution were

measured following a general method using 9,10-diphenylanthracene (ΦF = 0.9 in cyclohexane) as the standard.41,42 Dilute solutions of 3.1-3.4 in DCM were used for recording the

fluorescence spectra. Sample solutions were placed in quartz cuvettes and degassed for about 15

minutes. The fluorescence spectra of each were recorded three times and an average value of

integrated areas of fluorescence was used for the calculation. The ФF values in the solid state

were measured following a literature method.41 A concentrated DCM solution of samples were

cast as thin films on quartz plates and then allowed to dry. The plates were inserted into an

integrating sphere and the spectra were recorded. The samples were excited at their Amax.

3.2.3. Fluorescence Lifetime (τF) Measurement.. The method of lifetime measurements is

described in chapter two. The samples in DCM were put in quartz cuvettes. Fluorescence decay

profiles of the argon-degassed samples were measured using a single photon counting

spectrofluorimeter. Decays were monitored at the corresponding emission maximum of the

samples. In-built software allowed the fitting of the decay spectra (χ = 1-1.5) and produced the

fluorescence lifetimes.

3.3. Results and Discussion

3.3.1. Synthesis

Following literature methods,39 carbazole was converted into 3,6-di-tert-butyl-9-(4-

ethynyl-phenyl)-9H-carbazole) (2.16). The preparation of 2-,7-functionalized carbazoles is not straightforward, since both the 2- and 7-positions are in the meta position of the amino group of the carbazole unit and cannot be directly functionalized by standard electrophilic aromatic substitution. Successful synthetic strategies usually require functionalized 4,4’-biphenyl precursors with an additional reactive group at the 2-position used for a subsequent ring closure reaction. In this way 4,4’-biphenyl was converted into 2,7-dibromocarbazole (3.5) following a literature procedure.40 N-Arylation of 3.5 was followed by the Sonogashira coupling with 4-

ethynylbenzaldehyde to obtain 3.7 (Scheme 3.1). Sonogashira coupling of 3.7 with 2.16

produced 3.1 which further react with malononitrile and basic aluminum oxide in toluene to

produce 3.2. Treatment of 3.1 with cyanoacetic acid, ammonium acetate, and acetic acid

produced 3.3. Sonogashira coupling of 2.16 with 3.7 produced 3.4 (Scheme 3.2).

Scheme 3.1. Synthesis of compounds 3.7 and 3.8.

t-Bu Br Br Br CHO N N N H r Br B OHC H N a H t-Bu . CF b CF 3.7 2.16 3 5 3.6 3 3

Reagents and conditions: (a) 1-bromo-4-trifluoromethylbenzene, K2CO3, DMF, reflux 12 h; (b) Pd(PPh3)2Cl2, CuI, PPh3, N,N-diisopropylethylamine, reflux 10 h.

Scheme 3.2. Synthesis of compounds 3.1-3.4.

a 2.16 + 3.7 3.1 b 3.1 3.2 c 3.1 3.3 d 2.16 + 3.6 3.4

Reagents and conditions: (a) Pd(PPh3)2Cl2, CuI, PPh3, N,N-diisopropylethylamine, reflux 10 h; (b) malononitrile, basic Al2O3, 2-methylpropanol, reflux 5 h; (c) cyanoacetic acid, ammonium acetate, CH3COOH, reflux 5 h; (d) Pd(PPh3)2Cl2, CuI, PPh3, N,N-diisopropylethylamine, reflux 6 h.

3.3.2. Absorption and Emission Spectra in Solution

Absorption spectra of 3.1-3.4 recorded in DCM are shown in Figure 3.2. The absorption

around 300 nm is due to the carbazole moiety. The absorption around 325 nm to 475 nm shows

that the EA group influences the absorption transition property significantly. Compound 3.4,

which has no EW group, shows absorption maximum at 350 nm. Addition of an EW group

increases the red shift in absorption transition. Order of red shift on absorption follows the order

of strength of EW group (vide infra). Compound 3.1 with the weakest EW moiety absorbs at 425 nm, 3.3 with one nitrile and one carboxylate group absorbs up to 450 nm, and 3.2 with the dinitrile group absorbs up to 475 nm. Even though compound 3.4 has longer conjugation length its absorption transition is blue-shifted relative to the other compounds because there is no strong

EW group. The red shifted absorptions in these compounds as compared to 3.4 are due to the formation of different kinds of chromophores in DCM solution. Compounds 3.1, 3.2, and 3.3 form intramolecular charge transfer complexes (intramolecular CT), even in the ground state, so the absorptions at lower energies are due to the transition from ICT state to excited state.

When excited at their absorption maximum, dilute solutions of 3.1-3.4 in DCM showed

violet to green emission (Figure 3.3). It is interesting that the compounds 3.2 and 3.4 have more

red shifted fluorescence emission than that of compounds 3.1 and 3.3. It is because compound

3.2 has a stronger EW group and the larger extension of conjugation in 3.4 might be reflected in

its emission. The fluorescence spectrum of compound 3.2 is most red-shifted among all the four

compounds. The formation of ICT in 3.2 is more prominent than in 3.1 and 3.3. The chance of

formation of ICT in 3.4 is nearly zero since it does not have an EW group. Even though

compounds 3.1, 3.2, and 3.3 show ICT absorption transition, emission resulted mostly from a local excited (LE) state 28,43 as shown by excitation spectra (Appendix 2-Figures A9-A11),

which suggests that the ICT excited state of some compounds is destabilized with increasing

solvent polarity (vide infra).

3.1 1.0 3.2 3.3 0.8 3.4

0.6

0.4

0.2 Normalized Absorbance 0.0 300 350 400 450 500 Wavelength (nm)

Figure 3.2. Normalized absorption spectra of compounds 3.1-3.4 in DCM.

1.0 3.1 3.2 3.3 0.8 3.4

0.6

0.4

0.2

Normalized Emission 0.0 350 400 450 500 550 600 650 Wavelength (nm)

Figure 3.3. Normalized Emission spectra of compound 3.1-3.4 in DCM. (note: λex = Amax)

The ΦF and the τF of 3.1-3.4 recorded in DCM are summarized in Table 3.1 along with their absorption maxima (Amax), emission maxima (λmax), and molar absorptivity (ε). The quantum yield values of 3.1-3.4 are found in the range of 0.12 to 0.72 relative to that of 9,10- diphenylanthracene (0.90 in cyclohexane).42,44 Fluorescence decays of 3.1-3.4 in DCM were fitted with monoexponential functions indicating emission from the singlet excited state in all cases.

Each compound shows significant solvatochromism (Appendix 2, Figures-A2.1-A2.8) both in absorption and emission spectra. The emission spectra exhibit a more significant red shift in polar solvents for compounds 3.1 and 3.4, and follow the exact order of solvent polarity in their red shift. The excitation spectra of 3.1 and 3.4 show that emission in ACN results from a wide range of absorption– from 300 nm to 450 nm. The emission in these compounds occurs from variously solvated chromophores including LE state, ICT excited state, and relaxed ICT excited state.45 The emission region in solvents with higher polarity is also broader and so are the excitation spectra in compounds 3.1 and 3.4. The same is not true for compounds 3.2 and 3.3.

The solvatochromism of compound 3.2 and 3.3 does not follow a smooth order with a

change on solvent polarity. With a medium polar solvent toluene, they show the most red shifted

emission among all solvents used. The excitation spectra in toluene are broad- ranging from

300–450 nm. The emission spectrum also extends from 400 nm to 600 nm- a perfect white

emission. The emission of compound 3.2 and 3.3 in toluene is red shifted because π-electron rich toluene interacts with π-electrons and lone pair electrons of these compounds. Compounds 3.2 and 3.3 have more π-electrons and a lone pair of electrons to interact with the π-electrons of toluene. Compounds 3.1 and 3.4 have fewer π-electrons and lone pair of electrons so their interaction with π-electrons of toluene is lessened. A similar observation was found in benzanthrone derivatives.46 Similarly, the red shift of emission of 3.1 and 3.4 follows the exact

order of solvent polarity. However, in compounds 3.2 and 3.3 the presence of lone-pair of

electrons in THF interacts with the lone pair of electrons of these compounds. The same is less

significant with compound 3.1 and 3.4 as they have less lone pair electron density (Figures 3.4

and A2.13-A2.15).

Table 3.1. Absorption maximum (Amax), molar absorptivity (ε), emission maximum (λmax), Stokes shift, ФF, and τF of compounds 3.1-3.4 in DCM. Excitation is at Amax for each compound in corresponding solutions.

Compound Amax ε λmax Stokes Shift Quantum Lifetime 3 -1 -1 (nm) (dm cm mol ) (nm) (nm) Yield (ΦF) (τF) (ns)

3.1 377 64813 422 45 0.21 0.84

3.2 355 73548 454 99 0.12 0.29

3.3 386 54741 426 40 0.12 1.27

3.4 353 72938 431 78 0.72 0.1

Emission spectra of 3.1-3.4 were recorded at different concentrations in THF to assess the extent of possible aggregate formation. For example, the fluorescence intensity becomes maximum for compound 3.1 when the concentration is 3.1x10-6 M (Figure 3.4) but it decreases by almost triple when the concentration is decreased to 3.1x10-7 M and also decreased about half when the concentration becomes 3.1x10-5 M and still less at a concentration of 3.1x10-4 M. We could see sharp concentration quenching of emission at very high concentration for 3.1, 3.2, and

3.3 but with no concomitant red shift in their emission spectra, which indicates that there was no aggregation even at high concentration. Compound 3.4 shows aggregation at a concentration of

1.4x10-3 M as indicated by red shifted emission (Appendix 2).

1.4x107 300000 250000 7 1.2x10 200000

150000 7 1.0x10 100000 8.0x106 50000

0 6 400 450 500 550 600 650 6.0x10 C1 Intensity 4.0x106 C2 C3 2.0x106 C4 0.0 400 450 500 550 600 650 Wavelength (nm)

Figure 3.4. Emission spectra of 3.1 recorded in THF at different concentrations: (C1) 3.1x10-4 M, (C2) 3.1x10-5 M, (C3) 3.1x10-6 M, and (C4) 3.1x10-7 M. (Inset: the enlarged spectrum of 3.1 -4 recorded at 3.1x10 M). (note: λex = Amax)

3.3.3. Solid State Photoluminescence

To assess the potential application of these compounds in solid state devices their

electroluminescent emission properties in the thin film were studied. Emissions of these

compounds in thin film are substantially red shifted in comparison to emission in DCM. The

solid state emission spectra of these compounds are broad (Figure 3.5) and quantum yields of

fluorescence were found to be good (Table 3.2). For example, λmax of compound 3.2 in the solid state was 559 nm but only 454 nm in DCM. This remarkable red shift in the solid state could be caused either by twisting of the compound in solution (due to the presence of phenylethynyl group) thereby decreasing the conjugation from ED through the EW group, or by intermolecular charge transfer in the solid state, or both. The ΦF in solid state for compounds 3.1-3.3 is found to

be higher than that in DCM. These compounds have both ED and EW groups so stabilization of

47 polar excitated chromophores by polar solvents reduces ΦF. The ΦF for 3.4 in solid thin film is

less than in DCM.

1.0 3.1 3.2 3.3 0.8 3.4

0.6

0.4

0.2 Normalized Emission 0.0 400 450 500 550 600 650 700 Wavelength (nm)

Figure 3.5. Normalized solid-state emission of the thin solid films of 3.1-3.4. (note: λex = Amax in DCM).

Table 3.2. Emission maxima (λmax) and quantum yields fluorescence ΦF of 3.1-3.4 recorded in the solid state.

a Compound λ max(nm) λmax(nm) Red shift ФF (nm) (DCM) (solid) (solid)

3.1 422 500 78 0.27

3.2 454 559 105 0.68

3.3 430 451 21 0.30

3.4 431 545 114 0.51

aRed shift of emission is between DCM solution and solid state of compounds.

3.3.4. Optical Switching with Concentration

To further understand the number and nature of emissive species of compound 3.2, we recorded emission spectra at different concentrations in toluene (Figure 3.6) and monitored excitation spectra at different emission maxima (Figure 3.7 and Appendix 2). When 3.2 was excited with 350 nm light and spectra collected, the highest concentration sample (C0, 2.7x10-4

M) showed only one peak at 500 nm with a small hump around 400-450 nm. On reduction of concentration, emission intensity increased in the 400-450 nm regions and decreased in the 500 nm region. At higher concentrations, molecules of 3.2 are close enough together to allow intermolecular charge transfer excited state (intermolecular CT*) formation. So, excitation spectra in 450 nm region correspond to the intermolecular CT*. Similarly, at higher concentration carbazole moieties of adjacent molecules assemble together and some electronic interaction between them causes the red shift of absorption. This is reflected by the excitation spectra of higher concentration samples. Consequently, the Franck-Condon excited-state distribution (F-CESD) of the intermolecular CT* state will increase with an increase in

79 concentration leading to a higher distribution of the intermolecular CT* state in the excited-state equilibrium. Similarly, on reduction of concentration the distance between the molecules of 3.2 in solution increases and consequently the formation of intermolecular CT* in excited state decreases. At lower concentration intramolecular CT* are formed. So, excitation spectra in 350-

400 nm regions correspond to the intrarmolecular CT*. Similarly, the peak around 300 nm corresponds to carbazole moiety. Thus, at lower concentration Franck-Condon excited-state distribution (F-CESD) of the intramolecular CT* state will increase with decrease in concentration leading to a higher distribution of the intramolecular CT* state in the excited-state equilibrium. Excitation spectra taken at different concentrations with monitoring at different emission maxima supports this claim. As seen in Figure 3.7, the peak of excitation obtained by monitoring emission at 480 nm increases around 300 nm and 450 nm with an increase on concentration. The peak at 350-425 nm increases on intensity with a decrease in concentration.

The broad spectrum at lower concentration is possibly due to the combined LE, intermolecular

CT*, and intramolecular CT*. A Jablonski diagram for the optical behavior of the LE, intermolecular CT*, and intramolecular CT* states is depicted in Figure 3.8.

C0 1.0 C1 C2 0.8 C3 C4 C5 0.6 C6

0.4

0.2 Normalized Emission 0.0 400 450 500 550 600 650 Wavelength (nm)

Figure 3.6. Emission of compound 3.2 on excitation at 350 nm with dilution in toluene. Concentrations of the solutions are C0 = 2.7x10-4 M, C1 = 2.22x10-5 M, C2 = 4.5x10-7 M, C3 =18.0x10-11 M, C4 = 36.0x10-13 M, C5 = 72.0x10-15 M, and C6 = 144.0x10-17 M.

C0 1.0 C1 C2 C3 0.8 C4 C5 0.6 C6

0.4

0.2 Normalized Emission 0.0 300 350 400 450 Wavelength (nm)

Figure 3.7. Normalized Excitation spectra of compound 3.2 in toluene by monitoring emission at 480 nm. Concentrations of the solutions are C0 = 2.7x10-4 M, C1 = 2.22 x10-5 M, C2 = 4.5x10-7 M, C3 = 18.0x10-11 M, C4 = 36.0x10-13 M, C5 = 72.0x10-15 M, and C6 = 144.0x10-17 M.

LE ≈ 350 nm

Lower Higher concentration concentration ≈ 400 nm Intramolecular CT* ≈ 450 nm

Intermolecular CT*

Figure 3.8. Jablonski diagram showing the possible excitation and de-excitation pathways for compound 3.2.

3.3.5. Excitation Energy Dependent Fluorescence; Edge Excitation Red Shift

In contrast to Kasha’s rule we observed that the fluorescence of 3.2 depends on the excitation energy. This effect has been referred to as the Edge Excitation Red Shift (EERS).48 We

81 observed this remarkable red shift only when excitation is at the longer wavelength edge of the lowest energy absorption band. Fletcher49 described this phenomenon by considering the emission contribution from additional excited species. In a case when the reorientation relaxation time is larger than the fluorescence time, the total fluorescence emission spectrum is a composite of fluorescence emission from differently solvated species governed by F-CESD that is a function of excitation energy.48

1.0 Ex 300 Ex 330 0.8 Ex 350 Ex 370 Ex 390 0.6 Ex 410 Ex 440 0.4

0.2 Normalized Emission 0.0 350 400 450 500 550 600 650 Wavelength (nm)

Figure 3.9. Emission of compound 3.2 in hexanes (1x10-5 M) on excitation at different wavelengths.

For a polar compound in a polar solvent, the energy required to excite solvated species is a function of solvent orientation. If the excitation energy is smaller, only limited configurations of the ground state may be excited. The excitation with lower energy will excite only the fraction of total fluorophore population which is surrounded by solvent dipoles to decrease the energy of fluorophores. Thus, the resultant total emission lacks some high-energy components and this causes the red shifted emission.

Compound 3.2 shifted as much as 128 nm in isopropanol (Figure 3.10 and Table 3.3).

Compound 3.2 shows no EERS effect in hexanes (Figure 3.9). This is due to the fact that polar

solvents solvate the fluorophore more strongly than nonpolar solvents do. Compound 3.4 did not

show any EERS in either solvent- isopropanol or hexanes (Supporting Information).

Table 3.3. Emission maxima of compound 3.2 in isopropanol at different excitation wavelength.

λex (nm) 300 330 350 370 390 410 440

λmax(nm) 412 414 415 416 416 490 540

Ex 300 1.0 Ex 330 Ex 350 Ex 370 0.8 Ex 390 Ex 410 Ex 440 0.6

0.4

0.2 Normalized Emission 0.0 400 450 500 550 600 650 700 Wavelength (nm)

Figure 3.10. Emission of compound 3.2 in isopropanol (1x10-5 M) on excitation at different wavelengths.

3.3.6. Temperature-Dependent Emission

With a decrease in temperature the emission spectra of 3.1 and 3.4 recorded in DCM

shows a red-shifted fluorescence emission accompanied by an increase of emission intensity. All

these nonplanar compounds have significant degrees of free rotation in solution. With decreased

temperature, the system attains some solid-state character and molecular packing restricts

44 intramolecular rotation. This explains why ΦF is also higher at lower temperature. The red-

shifted emission spectra of 3.2 (Figure 3.11 and Table 3.4) with decreasing temperature are in

83 contrast to some literature reports.45 Compound 3.2 shows a red shift of 23 nm in its emission spectra at reduced temperature whereas 3.4 shows a red shift of only 15 nm.

Table 3.4. Change in emission maxima of compounds 3.2 and 3.4 at different temperature.

Compound Temperature (oC)

-60 oC -40 oC -20 oC 0 oC 8 oC 25 oC

3.2 λex 350 nm 457 452 446 443 440 434

3.4 λex 350 nm 441 437 433 430 428 426

0 1.0 -60 C -40 0C -20 0C 0.8 0 0C 08 0C 0.6 25 0C

0.4

0.2 Normalized Emission 0.0 400 450 500 550 600 650 Wavelength (nm)

Figure 3.11. Emission spectra of compound 3.2 in DCM on excitation at 350 nm.

Compound 3.2 possesses strong EW and ED groups, thus an ICT state even in the ground state will be more stable at reduced temperature. Absorption spectra of these compounds show two bands (vide supra). Since the viscosity of the solvent increases at lower temperature, the reorientational relaxation of the solvent molecules is inhibited and enhanced stabilization of the

ICT* emitting state is expected because reduction of thermal motion allows better alignment of the solute and solvent dipoles. Thus, the ICT excited state dominates over the LE state at lower

temperature. Consequently, the F-CESD of the ICT excited state will increase with a decrease in

temperature. Compound 3.4 with no strong EW groups, shows smaller red shift of emission with

a decrease in temperature from 25 oC to -60 °C.

3.3.7. Lippert-Mataga Plot and its Significance on Specific and General Solvent Effects

The nonlinearity of Lippert-Mataga plots for 3.1-3.4 (Figure 3.12) indicates the presence of multiple excited states that produce multiple emitting species.33 Nonlinearity is also evidence

for a specific solvent effect on the spectral shift. The possible excited states might be the LE, the intra or intermolecular charge transfer excited state (ICT*) or the relaxed intra or intermolecular charge transfer excited (ICTr*) states.33 LE is the emitting species in nonpolar solvents and ICT*

and ICTr* are the emitting species in polar solvents. Each of the compounds show multiple

emitting species: one locally excited (LE or 1S) and the other ICT* and/or ICTr*. Compounds

3.1 and 3.4 have weaker or no EW groups, respectively. The number of excited state could be

less than that in 3.2 and 3.3. Compounds 3.1 and 3.4 form LE states and solvent relaxed excited

states whereas 3.2 and 3.3 forms LE, relaxed LE, ICT, and relaxed ICT states. So, there are more

possible emissive entities in 3.2 and 3.3, as shown by Lippert-Mataga plots.47

7000

) 6000 -1 5000

4000

3000 3.1 3.2 Stokes shiftStokes (cm 2000 3.3 3.4 1000 0.05 0.10 0.15 0.20 0.25 0.30 Orientation polarizability (∆f)

Figure 3.12. Lippert-Mataga plot of compounds 3.1-3.4.

3.4. Conclusions

A new class of carbazole donor-carbazole linker-based compounds (3.1-3.4) was synthesized. Compounds with EW and ED showed longer lifetime of fluorescence than those without EW. Solvatochromic effects of these compounds have also been studied. Formations of different emitting species in dilute and concentrated solution for compound 3.2 has been suggested on the basis of its emission and excitation spectra. In polar solvents, the effect of temperature on emission is higher than in non-polar solvents. Similarly, compounds with EW and ED groups showed more change in emission properties with change in temperature than that of a compound without an EW. EERS effects for D-π-A compounds are more significant than for the compound without EW. The specific and general solvent effects on optical properties of these compounds are discussed with the help of Lippert-Mataga plots. Their higher solid state ФF suggest their higher potential in fabrication of solid state electroluminescent devices.

3.5. References

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Appendix 2. Absorption, emission, excitation, concentration quenching, emission with change in temperature, 1H NMR, and 13C NMR spectra of compounds 3.1-3.4.

Absorption and emission spectra of 3.1-3.4

Hex Hex 1.0 Tol 1.0 Tol DCM DCM 0.8 ACN 0.8 ACN

0.6 0.6

0.4 0.4

0.2 Normalized emission 0.2 Normalized Absorbance 0.0 0.0 300 350 400 450 500 350 400 450 500 550 600 650 Wavelength (nm) Wavelength (nm) Figure A2.1. Normalized absorption Figure A2.2. Normalized emission spectra of 3.1 in hexanes, toluene, DCM, spectra of 3.1 in hexanes, toluene, DCM, and ACN. and ACN. (λex = Amax for all solvents).

Hex 1.0 Tol 1.0 Hex DCM Tol ACN DCM 0.8 0.8 ACN

0.6 0.6

0.4 0.4

0.2 Normalized emission 0.2 Normalized Absorbance 0.0 0.0 300 350 400 450 500 400 450 500 550 600 650 700 Wavelength (nm) Wavelength (nm)

Figure A2.3. Normalized absorption Figure A2.4. Normalized emission spectra of 3.2 in hexanes, toluene, spectra of 3.2 in hexanes, toluene, DCM, and ACN. DCM, and ACN. (λex = Amax for all solvents).

Hex Hex 1.0 1.0 Tol Tol DCM DCM ACN 0.8 ACN 0.8

0.6 0.6

0.4 0.4

0.2 0.2 Normalized Emission Normalized Absorbance 0.0 0.0 300 350 400 450 500 550 400 450 500 550 600 650 700 Wavelength (nm) Wavelength (nm) Figure A2.5. Normalized absorption Figure A2.6. Normalized emission spectra of 3.3 in hexanes, toluene, DCM, spectra of 3.3 in hexanes, toluene, and ACN. DCM, and ACN. (λex = Amax for all solvents).

1.0 Hex 1.0 Hex Tol Tol DCM DCM 0.8 ACN 0.8 ACN

0.6 0.6

0.4 0.4

0.2 0.2 Normalized Emission Normalized Absorbance 0.0 0.0 300 350 400 450 500 400 450 500 550 600 650 Wavelength (nm) Wavelength (nm) Figure A2.7. Normalized absorption Figure A2.8. Normalized emission spectra of 3.4 in hexanes, toluene, DCM, spectra of 3.4 in hexanes, toluene, DCM, and ACN. and ACN. (λex = Amax for all solvents).

Excitation spectra of compounds 3.1-3.4 in hexanes, toluene, DCM, and ACN

Hex 1.0 1.0 Tol DCM 0.8 0.8 ACN

0.6 0.6

0.4 Intensity 0.4 Intensity Hex 0.2 Tol 0.2 DCM ACN 0.0 0.0 300 325 350 375 400 425 300 350 400 450 Wavelength (nm) Wavelength (nm) Figure A2.9. Normalized excitation Figure A2.10. Normalized excitation spectra of 3.1 in hexanes, toluene, spectra of 3.2 in hexanes, toluene, DCM, DCM, and ACN by monitoring at their and ACN by monitoring at their emission emission maxima. maxima.

Hex Hex 1.0 Tol 1.0 Tol DCM DCM 0.8 ACN 0.8 ACN

0.6 0.6

0.4 0.4 Intensity Intensity

0.2 0.2 0.0 0.0 300 350 400 450 340 360 380 400 420 440 Wavelength (nm) Wavelength (nm) Figure A2.11. Normalized Figure A2.12. Normalized excitation excitation spectra of 3.3 in hexanes, spectra of 3.4 in hexanes, toluene, DCM, toluene, DCM, and ACN by and ACN by monitoring at their monitoring at their emission emission maxima. maxima.

Concentration quenching spectra of compounds 3.2-3.4 in THF

40000 6 6 3.0x10 140000 1.0x10 30000 120000 6 100000 20000 2.5x10 80000 5 60000 8.0x10 10000 6 40000 0 20000 450 500 550 600 650 700 750 800 2.0x10 5 0 6.0x10 450 500 550 600 650 700 750 800 C1 6

C2 1.5x10 C1 4.0x105 C3 C2 C4 6 C3 Intensity Intensity 1.0x10 C4 5 2.0x10 5 5.0x10 0.0 0.0 450 500 550 600 650 700 750 800 450 500 550 600 650 700 750 800 Wavelength (nm) Wavelength (nm) Figure A2.13. Emission spectra of 3.2 Figure A2.14. Emission spectra of 3.3 recorded in THF at different concentrations: -4 -5 recorded in THF at different concentrations: (C1) 4.2x10 M, (C2) 4.2x10 M, and (C3) -4 -5 -6 -7 (C1) 4.1x10 M, (C2) 4.1x10 M, and (C3) 4.2x10 M, (C4) 4.2x10 M. (Inset: the 4.1x10-6 M, (C4) 4.1x107 M. (Inset: the enlarged spectrum of 3.2 recorded at -4 enlarged spectrum of 3.3 recorded at 4.1x 4.2x10 M). (note: λex = Amax) -4 10 M). (note: λex = Amax)

7 6x10 600000 500000 7 5x10 400000

300000 7 200000 4x10 100000 0 7 400 450 500 550 600 650

3x10

Intensity C1 2x10 C2 7 C3 1x10 C4 0 400 450 500 550 600 650 Wavelength (nm) Figure A2.15. Emission spectra of 3.4 recorded in THF at different concentrations: (C1) 1.4x10-3 M, (C2) 1.4x10-4 M, (C3) 1.4x10-5 M, and (C4) 1.4x10-6 M. (Inset: the enlarged spectrum of 3.4 recorded at 1.4x -3 10 M). (note: λex = Amax)

Emission and excitation spectra at different excitation, concentration, and temperature

Ex 300 1.0 Ex 270 1.0 Ex 330 Ex 290 Ex 350 Ex 330 0.8 Ex 370 0.8 Ex 350 Ex 390 EX 360 EX 370 0.6 Ex 410 0.6

Ex 440

0.4 0.4

0.2 0.2 NormalizedEmission Normalized Emission 0.0 0.0 350 400 450 500 550 600 650 300 400 500 600 Wavelength (nm) Wavelength (nm) Figure A2.16. Emission of compound 3.2 Figure A2.17. Emission of compound 3.4 in toluene on excitation at different in hexanes on excitation at different wavelengths. wavelengths.

EX 270 1.0 EX 290 1.0 C0 EX 300 C1 0.8 EX 330 0.8 C2 EX 350 C3 EX 355 C4 0.6 EX 370 0.6 C5 C6 0.4 0.4 0.2

Normalized Emission 0.2 0.0 Normalized Emission 350 400 450 500 550 600 650 0.0 Wavelength (nm) 300 350 400 450 Wavelength (nm) Figure A2.18. Emission of compound Figure A2.19. Normalized excitation spectra of 3.4 in isopropanol on excitation at compound 3.2 in toluene at 400 nm. different wavelengths. Concentrations of the solutions are C0 = 2.7x10-4 M, C1 = 2.22x10-5 M, C2 = 4.5x10-7 M, C3 = 18.0x10-11 M, C4 = 36.0 x10-13 M, C5 = 72.0 x10-15 M, and C6 = 144.0x10-17 M.

o 1.0 -60 C -40 oC o 0.8 -20 C 0 oC 8 oC 0.6 25 oC

0.4

0.2 Normalized Emission 0.0 400 450 500 550 600 650 Wavelength (nm) Figure A2.20. Emission spectra of compound 3.4 in DCM at different temperature on excitation at 350 nm.

1H NMR and 13C NMR spectra of compounds 3.1-3.4, 3.6, and 3.7

Br Br N

CF3

Figure A2.21. 1H NMR of compound 3.6.

Figure A2.22. 13C NMR of compound 3.6.

OHC Br N

CF3

Figure A2.23. 1H NMR of compound 3.7.

Figure A2.24. 13C NMR of compound 3.7.

t-Bu

OHC N N t-Bu

CF3

Figure A2.25. 1H NMR of compound 3.1.

Figure A2.26. 13C NMR of compound 3.1.

t-Bu

NC N N CN t-Bu

CF3

Figure A2.27. 1H NMR of compound 3.2.

Figure A2.28. 13C NMR of compound 3.2.

t-Bu

NC N N COOH t-Bu

CF3

Figure A2.29. 1H NMR of compound 3.3.

Figure A2.30. 13C NMR of compound 3.3.

t-Bu t-Bu

N N N t-Bu t-Bu

CF3

Figure A2.31. 1H NMR of compound 3.4.

100

Figure A2.32. 13C NMR of compound 3.4.

101

CHAPTER 4. CARBAZOLE DONOR AND CARBAZOLE OR BITHIOPHENE-

BRIDGED SENSITIZERS FOR DYE-SENSITIZED SOLAR CELLS

4.1. Introduction

Dye-Sensitized Solar Cells (DSSCs) have been attracting a great deal of interest due to the potential of high energy conversion efficiency at low cost.1,2 Since the original breakthrough

in DSSCs,1,3 ruthenium polypyridyl complexes have been widely used as sensitizers in DSSCs, and the highest power conversion efficiency of 11.18% has been obtained for the N719

4 sensitizer, a derivative of cis-Ru(dcbpyH2)(NCS)2 (the N3 sensitizer). In addition to ruthenium

polypyridine sensitizers, other organometallic sensitizers such as osmium(II) polypyridine

sensitizers,5-7 platinum(II) sensitizers,8 and zinc(II) sensitizers,9,10 amongst others, have also been

investigated by researchers. Although the metal-complexed sensitizers exhibit high efficiency

and stability, they are expensive and difficult to purify compared to the metal-free organic

sensitizers. Metal-free organic sensitizers also show advantages such as low cost, high molar

extinction coefficient, and easily variable molecular design. Recently, an unprecedented power

conversion efficiency of 10.3% was reported for an organic sensitizer,11 thus indicating that with

further strategic optimization, metal-free organic sensitizers may overtake their organometallic

counterparts in photovoltaic performance.

Novel organic sensitizers such as coumarin,12-14 tetrahydroquinoline15,16 merocyanine17,18 cyanine,19,20 phenothiazine,21 indoline,22,23 and hemicyanine24,25 have been used in DSSCs with

good results. Still, more strategic molecular design of organic sensitizers is required to achieve

higher efficiency (η) values. The requirements17,24,26 of efficient sensitizers for use in DSSCs

include (a) wide absorption range and high absorption coefficient to give high light harvesting

efficiency, (b) excited-state redox potential should match the energy of the conduction band, (c)

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light excitation associated with vectorial electron flow from the light-harvesting moiety of the sensitizer towards the surface of the semiconductor, (d) conjugation across the donor and the anchoring group, (e) good anchoring group, and (f) electronic coupling between the lowest unoccupied molecular orbital (LUMO) of the sensitizer and the TiO2 conduction band. The

major factors that result in low conversion efficiency of many organic sensitizer in the DSSCs

are (a) the aggregation of sensitizer on the semiconductor surface and (b) recombination of

conduction-band electrons with the iodide/triiodide redox mediator.27 Many metal-free organic sensitizers usually exhibit excellent incident photon-to-current efficiency in the blue region of the spectrum with little or no photoaction in the lower energy regions.28,29 A tendency of π-stack

exists in π-conjugated planar molecules due to strong intermolecular interactions between π-

electrons and may result in a dissipative intermolecular energy transfer which could have adverse

effects on the photovoltaic performance of DSSCs which incorpore this type of molecules.30,31

Minimization of the extent of aggregation of organic sensitizers and charge recombination

through appropriate structural modification becomes necessary in order to achieve optimal

performance. On the basis of this strategy, we have designed and synthesized organic dyes for

use as sensitizers of DSSCs. The very few organic compounds incorporating carbazole32-34 which

have been used as sensitizers in DSSCs exhibit exciting results. Also, a few reports on senzitizers

incorporating a thiophene-based spacers and exhibiting good device efficiencies have been

appeared.35 So far there are no reports to date related to sensitizers bearing the carbazole moiety

functioning as both the donor and the linker. Therefore we are developing and evaluating the

potential of compounds bearing carbazole donor and carbazole or thiophene linker as sensitizers

in organic solar cells. Carbazole-based sensitizers have been shown to exhibit excellent

photovoltaic characteristics in the blue region of the spectrum.33,34 These sensitizers may

103 eventually be mixed with other sensitizers that exhibit excellent photovoltaic responses in the red in a tandem cell. This sort of tandem arrangement becomes necessary since extending the spectral response of individual sensitizers to low energy regions of the spectrum will ultimately result in ground and excited states that are not thermodynamically well aligned for sensitizer regeneration and electron injection. Also, we are interested in an excellent hole-transporting material such as carbazole moiety, and solid-state DSSCs29,36 so studied carbazole based compounds for the possible applications in DSSCs.

We describe here the design and synthesis of (2-cyano-3-[4-({3',6'-di-tert-butyl-9-[4-

(trifluoromethyl)phenyl]-9H-2,9'-bicarbazol-7yl}ethynyl)phenyl]acrylic acid) (4.1) and (2- cyano-3-(4-(2-(5-(2-(4-(3,6-di-tert-butyl-9H-carbazolyl)phenyl)ethynyl)-2,2’- bithiophene)ethynyl)phenyl)acrylic acid) (4.2). The design, synthesis, and photophysical properties of sensitizer 3.3 is discussed in chapter 3 and reported in our published paper.37

DSSCs using these carbazole donors based sensitizers (4.1, 3.3, and 4.2) which exhibit maximum photovoltaic performance in the blue region of the spectrum are discussed herein. The fabrication for solar cell performance was done by Anthony C. Onicha, a graduate student in Dr. Felix N.

Castellano’s research group. The chemical structures of these sensitizers are shown in Figure 4.1.

All sensitizers have the same acceptor cyanoacrylic acid which incorporates the carboxylic acid anchoring group, and identical carbazole donor but differ in the linker. Sensitizers 4.1 and 3.3 have 2-, 7-, 9-substituted carbazole linker and the sensitizer 4.2 has a bithiophene group as a linker. Cyanoacrylic acid has evolved as the acceptor group of choice in metal-free organic sensitizers since it functions both as an electron acceptor and as an anchoring group and has been shown to outperform the carboxylic acid group in these roles.38

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t-Bu

N NC N COOH 4.1 t-Bu t-Bu CF3

N NC N COOH t-Bu 3.3 t-Bu CF3

S N NC S COOH 4.2 - u t B

Figure 4.1. Chemical structures of the sensitizers 4.1, 3.3, and 4.2.

4.2. Experimental Section

4.2.1. General: Triphenylphosphine, 5,5’-dibromo-2,2’-bithiophene, CuI, N,N- diisopropylethylamine, Na2SO4, NaHCO3, CS2CO3, PdCl2(PPh3)2, cyanoacetic acid, and ammonium acetate were purchased from Sigma Aldrich. Lithium iodide, 4-tert-butylpyridine (4- tBupy), iodide, and 1-n-propyl-3-methylimidazolium iodide (PMII) were available from our previous study.7 3,6-Di-tert-butylcarbazole and 3,6-di-tert-butyl-9-(4-ethynylphenyl)-9H- carbazole were synthesized following the reported literature procedure.39 The synthesis of sensitizers 4.1 and 4.2, and their precursors 4.3, 4.5, and 4.6 are described here. Synthesis of sensitizer 3.3 and other precursors 3.7 and 2.16 are described elsewhere.37,40 1H and 13C NMR spectra for 4.1, 4.2, 4.3, 4.5, and 4.6 are given in Appendix 3.

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4.2.2. Characterization, Physical Measurements, and Instrumentation

1H and 13C NMR spectra were measured on a Bruker 500 MHz spectrometer internally referenced to TMS. UV – Vis absorption spectra were measured on a Shimadzu UV-2401 spectrophotometer and emission were recorded on Jobin-Yvon-Fluorolog-3 spectrometer.

Electrochemical data were obtained using a BAS Epsilon electrochemistry workstation with a conventional three-electrode arrangement. Cyclic voltammetry measurements were carried out in either anhydrous ACN or DCM solution containing 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte, a gold microdisk (1.6 mm dia.) working electrode (BAS model MF-2014), a platinum wire auxiliary electrode (BAS model MW-4130) and a Ag/AgCl (3 M NaCl) reference electrode (BAS model MF-2079), respectively.

Measurements were conducted in ca. 1 mM electroactive substrate in an inert gas atmosphere with a scan rate of 300 mV/s. Ferrocene was used as internal standard. For all measurements, potentials were recorded vs. the ferrocenium/ferrocene (Fc+/Fc0) internal standard, and finally

+ 0 were converted to E1/2 vs. the normal hydrogen electrode (NHE) using E1/2(Fc /Fc ) = +0.69 V

vs. NHE.41 Incident photon-to-current conversion efficiency (IPCE) measurements were carried out using a system from PV Measurements, Inc., equipped with a Xenon arc lamp and calibrated with a silicon reference photodiode. Current-voltage characteristics were measured on an I-V data acquisition system (PV Measurements, Inc.) equipped with a small area solar simulator

(AM 1.5 Global) and an NREL-certified silicon reference solar cell (PVM 274, NREL) for calibrating the intensity of the simulated sunlight to 100 mW/cm2, with the measured photocurrent being within 2% of its calibration value. Photocurrent density (JSC) values directly

measured using I-V curves were typically 10-15% larger than those estimated from the

integrated EQE (IPCE) spectra. Estimation of JSC was performed by the I-V software (PV

106

Measurements Inc.) according to the ASTM standard E1021. The sandwiched solar cells were

illuminated directly through the transparent conductive glass support containing the TiO2

photoanode. The photovoltaic characteristics are reported herein as overall yields that were not

corrected for losses due to light absorption and reflection by the conductive glass substrate.

4.2.3. Synthesis and Characterization

4-({3',6'-di-tert-butyl-9-[4-(trifluoromethyl)phenyl]-9H-2,9'-bicarbazol-7-

yl}ethynyl)benzaldehyde (4.3). To a solution of compound 3.7 (200 mg, 0.38 mmol) in dry

toluene (30 ml) was added 3,6-di-tert-butylcarbazole (160 mg, 0.42 mmol). The Pd(OAc)2 (3%),

P(t-Bu)3 (7%), and Cs2CO3 (495 mg, 1.5 mmol) were also added and stirred under argon at 110

oC for about 24 h. The reaction mixture was then cooled to room temperature and toluene was

removed completely under vacuum. The solid mixture was dissolved in THF and unreacted

Cs2CO3 was removed under gravity filtration. The organic residue was then purified by column

chromatography (silica gel, 8% ethyl acetate in petroleum ether) to give the product (180 mg,

1 66%). H NMR (500 MHz, CDCl3) δ 1.50 (s, 18H), 7.36 (d, 2H), 7.48 (dd, 2H), 7.55 (dd, 1H),

7.61 (dd, 2H), 7.68 (s, 1H), 7.30 (d, 2H), 7.78 (d, 2H), 7.90 (d, 2H), 7.91 (d, 2H), 8.18 (d, 2H),

8.22 (d, 1H), 8.34 (d, 1H), and 10.05 (s, 1H). 13C NMR (500 MHz, DMSO): δ32.0, 34.8, 88.9,

94.4, 108.3, 109.0, 113.2, 116.4, 120.1, 120.3, 120.6, 121.9, 122.2, 123.4, 123.7, 124.8, 127.1,

127.2, 127.5, 127.6, 129.5, 129.6, 129.7, 132.0, 135.4, 139.5,140.8, 142.0, 143.0, and 191.4.

HRMS EI+ calculated for C48H39ON2F3 716.30146, measured 716.30024.

4-(2-(5-bromo- 2,2’-bithiophene)ethynyl)benzaldehyde (4.5). 5,5’-Dibromo-2,2’-bithhiophene

(4.4) (0.45 g, 1.38 mmol), PdCl2(PPh3)2 (16 mg), CuI (12 mg) N,N-diisopropylethylamine (1ml),

and DMF (9 ml) were mixed together in 100 ml round bottom flask. Then Argon was bubbled

for 20 minutes. Ethylene benzaldehyde (0.1 g, 0.77 mmol) was added and stirred for 12-14 h at

107

80 oC under argon atmosphere. Then the reaction mixture was poured into distilled water and

extracted with DCM. The extract was washed with 0.5 M HCl, then it was washed using

saturated NaHCO3 and distilled water. The solution was dried using anhydrous Na2SO4, followed

by the addition of pentane (1/3 volume of DCM). The resultant solution was filtered using a

Buckner funnel with a layer of silica pad. The filtrate was dried under vacuum. The crude

product was purified by flash chromatography (silica gel, 30% DCM in hexanes) to obtain pure

1 compound (0.17 g, 65%). H NMR (500 MHz, CDCl3) δ 6.96 (d, 1H), 6.99 (d, 1H), 7.03 (d, 1H),

13 7.22 (d, 1H), 7.65 (d, 2H), 7.87 (d, 2H), and 10.03 (s, 1H). C (500 MHz, CDCl3) δ 86.5, 93.6,

120.1, 121.4, 123.0, 123.6, 129.0, 129.6, 130.0, 131.7, 133.7, 135.5, 137.9, 138.8, and 191.3.

HRMS EI+ calculated for C17H9OS2Br 371.92783, measured 371.92912.

4-(2-(5-(2-(4-(3,6-di-tert-butyl-9H-carbazolyl)phenyl)ethynyl)-2,2’-

bithiophene)ethynyl)benz-aldehyde (4.6). To a solution of compound 4.5 (100 mg, 0.27 mmol)

in diisopropylethylamine (0.5 ml) and DMF (7 ml), was added 3,6-di-tert-butyl-9-(4-

ethynylphenyl)-9H-carbazole (344 mg, 0.9 mmol). After the solution was degassed with argon

for 30 min while stirring, PdCl2(PPh3)2 (7 mg), CuI (5 mg), and triphenylphosphine (8 mg) were

added. The reaction was then stirred for 14 h at 80 oC under argon. The solvent was evaporated using vacuum and crude product was obtained. The product obtained was recrystallized from

1 DCM and hexane to obtain the product. (150 mg, 82%). H NMR (500 MHz, CDCl3) δ 1.5 (s,

18H), 7.17 (d, 1H), 7.18 (d, 1H), 7.27 (d, 1H), 7.28 (d, 1H), 7.42 (d, 2H), 7.50 (ds, 2H), 7.61 (d,

2H), 7.69 (d, 2H), 7.75 (d, 2H), 7.91 (d, 2H), 8.16 (d, 2H), and 10.08 (s, 1H). 13C (500 MHz,

CDCl3) δ 31.0, 32.0, 76.6, 76.8, 77.0, 77.3, 83.1, 86.7, 93.9, 94.3, 109.2, 116.3, 120.9, 121.6,

122.6, 123.6, 123.8, 124.2, 124.4, 126.4, 129.0, 129.7, 131.8, 132.6, 133.1, 133.9, 135.5, 138.0,

108

+ 138.5, 138.8, 139.2, 143.3, and 191.4. HRMS EI calculated for C45H37ONS2 671.23167,

measured 671.22981.

2-cyano-3-[4-({3',6'-di-tert-butyl-9-[4-(trifluoromethyl)phenyl]-9H-2,9'-bicarbazol-7-

yl}ethynyl)-phenyl]acrylic acid (4.1). To a round bottomed flask containing a mixture of

compound 4.3 (100 mg, 0.14 mmol), cyanoacetic acid (11 mg, 0.14 mmol), and ammonium

acetate (1 mg) was added acetic acid (5 ml). The mixture was heated at 120 oC for 6 h and allowed to cool to room temperature. The resulting solid was filtered and washed with distilled water, diethyl ether, and methanol to give a bright orange solid (82%). Mass spectrum (MALDI-

TOF) m/z M+ = 783. 1H NMR (500 MHz, DMSO) δ 1.40 (s, 18H), 7.38 (d, 2H), 7.47 (dd, 2H),

7.59 (dd, 1H), 7.61 (s, 1H), 7.64 (dd, 1H), 7.10 (s, 1H), 7.90 (d, 2H), 8.40 (s, 4H), 8.10 (d, 2H),

8.30 (d, 2H), 8.38 (s, 1H), 8.47 (d, 1H), and 8.59 (d, 1H). 13C NMR (DMSO): δ32.3, 35.0, 89.7,

94.5, 108.3, 109.6, 113.5, 117.2, 119.9, 121.9, 122.1, 123.1, 123.3, 124.0, 124.2, 125.6, 127.2,

128.0, 128.1, 131.3, 132.5, 136.9, 139.3, 140.7, 141.8,143.0, 153.5, and 163.6.

2-cyano-3-(4-(2-(5-(2-(4-(3,6-di-tert-butyl-9H-carbazolyl)phenyl)ethynyl)-2,2’-bithiophene)-

ethynyl)phenyl)acrylic acid (4.2). To a round bottomed flask containing a mixture of compound

4.6 (150 mg, 0.22 mmol), cyanoacetic acid (9 mg), and ammonium acetate (1 mg) was added

acetic acid (10 ml). The mixture was heated at 120 oC for 8 h and allowed to cool to room

temperature. The resulting solid was filtered and washed with distilled water, diethyl ether, and

methanol to give a bright red solid (142 mg, 86%). Mass spectrum (MALDI-TOF) m/z M+ =

738. 1H NMR (500 MHz, DMSO) δ 1.43 (s, 18H), 7.40 (d, 2H), 7.47-7.54 (m, 6H), 7.71 (d, 2H),

7.75 (d, 2H), 7.84 (d, 2H), 8.07 (d, 2H), 8.29 (s, 1H), and 8.1 (s, 2H). 13C NMR (500, DMSO):

δ31.2, 32.3, 83.4, 86.2, 94.8, 109.7, 117.3, 120.3, 121.2, 122.0, 123.6, 124.3, 126.1, 126.2, 126.6,

131.2, 132.2, 133.5, 134.8, 135.5, 137.7, 138.4, 138.5, 138.6, 143.5, and 163.6.

109

4.2.4. Preparation of Nanocrystalline TiO2 Electrode and Transparent Platinum Cathode

The sol-gel synthesis of the colloidal TiO2 paste has been described in detail

7,42 elsewhere. The prepared TiO2 paste was doctor-bladed onto the conductive glass substrate

(Hartford Glass, TEC-15) to give the transparent layer of TiO2 film with a typical thickness of 13

µm. The obtained nanoparticle film was then dried at 125 °C for 6 minutes and a 5 µm thick

scattering layer of mesoscopic TiO2 (Solaronix, Ti-Nanoxide 300) was doctor-bladed on top of

it. The resulting TiO2 films were subsequently annealed for 30 minutes at 500 °C under oxygen flow in a tube furnace with ramped heating control of 5 °C per minute. Upon cooling to 100 °C,

TiO2 electrodes were immersed in 0.5 mM sensitizer solution in ACN/tert-butanol (50: 50 v/v%)

for 48 hours at room temperature. Due to solubility limitations, sensitization of sensitizer 4.2 was

carried out in THF solution. Transparent platinum-coated FTO cathodes were prepared as

described elsewhere.7

4.2.5. Sandwiched Solar Cell Assembly

2 The active device area of the sensitized TiO2 photoanode was adjusted to 0.25 cm .

Stretched Parafilm-M was used as a spacer between the photoanode and the platinum counter

electrode. The typical thickness of the spacer was 20 - 30 µm. A few drops of the redox

electrolyte were placed on top of the active electrode area and a platinized FTO-glass counter

electrode was placed on top. The electrodes were then sealed together using binder clips. For

these studies, the redox electrolyte solution consisted of 0.2 M LiI, 0.05 M I2, 0.7 M PMII and

0.5 M 4-tBupy in anhydrous ACN.43

110

4.3. Results and Discussion:

4.3.1. Synthesis and Characterization.

The preparation of 2,7-functionalized carbazoles is not straightforward, since both the 2- and 7-positions are in the meta position of the amino group of the carbazole unit and cannot be directly functionalized by standard electrophilic aromatic substitution. Successful synthetic strategies usually require functionalized 4,4’-biphenyl precursors with an additional reactive group at the 2-position used for a subsequent ring closure reaction. In this way 4,4’-biphenyl compound was compound 3.7 by following literature procedure.40 and the detailed procedure is discussed in chapter 3. Compound 3.7 reacts with 3,6-di-tert-butylcarbazole in the presence of

Pd(OAc)2, P(t-Bu)3, and Cs2CO3 to obtain 4.3 which further reacts with cyanoacetic acid, ammonium acetate and acetic acid to give the sensitizer 4.1. Similarly 5,5’-dibromo-2,2’- bithiophene (4.4) was converted into compound 4.5 by Sonogashira coupling with 4- ethynylbenzaldehyde which further undergoes Sonogashira coupling with 3,6-di-tert-butyl-9-(4- ethynylphenyl)-9H-carbazole to obtain compound 4.6. Compound 4.6 reacts with cyanoacetic, ammonium acetate, and acetic acid to give the sensitizer 4.2.

111

Scheme 4.1. Synthesis of sensitizers 4.1 and 4.2.

t-Bu

t-Bu t-Bu OHC N Br CHO N N b N H t-Bu 4.1 a 4.3 3.7 CF3

CF3 t-Bu

CHO N S S Br Br Br CHO S c S (2.16) t-Bu 4.4 4.5 - u t B d

S OHC N e 4.2 S

4.6 t-Bu

Reagents and conditions: (a) Pd(OAc)2, P(t-Bu)3, Cs2CO3, toluene, reflux 22 h. (b) cyanoacetic acid, ammonium acetate, CH3COOH, reflux 5 h. (c) Pd(PPh3)2Cl2, CuI, PPh3, N,N- diisopropylethylamine/DMF, reflux 14 h. (d) Pd(PPh3)2Cl2, CuI, PPh3, N,N- diisopropylethylamine, heat at 80 oC for 14 h. (e) cyanoacetic acid, ammonium acetate, CH3COOH, reflux 5 h.

4.3.2. Photophysical Properties

These sensitizers (4.1, 3.3, and 4.2) are soluble in common organic solvents, such as

DCM, THF, dimethylformamide (DMF), and dimethylsulfoxide (DMSO). The absorption

spectra of the sensitizers in DCM are shown in Figure 4.2. All sensitizers have a strong

absorption in the ultraviolet and blue regions with maximum at 290 nm and approximately 350

nm-430 nm, respectively. The absorption bands near 290 nm (not shown) are due to the

carbazole moiety and are assigned to a π-π* transition, while the bands around 350-430 nm are due to intramolecular charge transfer (ICT) between the donor carbazole and the acceptor

112 moiety. The summary of photophysical properties of sensitizers 4.1, 3.3, and 4.2 are given in

Table 4.1. The excited state potential (ELUMO) and the minimum energy between the ground and

44 excited states (E0-0) were determined as described elsewhere.

1.0 4.1 1.0 3.3 4.2 0.8 0.8

0.6 0.6

0.4 0.4

0.2 0.2 Normalized Emission (a.u.) Emission Normalized

Normalized Absorption (a.u.) 0.0 0.0 300 400 500 600 700 800 Wavelength (nm)

Figure 4.2. Absorption-Emission spectra of sensitizers 4.1, 3.3, and 4.2 in DCM. (Emission spectra are obtained by exciting these compounds 4.1, 3.3, and 4.2 at 460 nm, 460 nm, and 450 nm respectively.)

These sensitizers are composed of three main parts: a carbazole as donor, a carbazole or bithiophene as a linker, and cyanoacrylic acid as an acceptor and anchoring group. The donor

- groups may efficiently inhibit I3 from approaching the surface of the nanocrystalline TiO2. The phenylethynyl spacer is a semi-rigid conjugated linker45 which should favor electron injection and the carbazole linker in sensitizers 4.1 and 3.3 can suppress the aggregation of dye molecules by steric hindrance caused trifluoromethyl benzyl group attached at the 9H position of it.

113

Table 4.1. Summary of photophysical properties of sensitizers 4.1, 3.3, and 4.2.

Sensitizer λabs, nm EOX Estimated ELUMO λem

-3 -1 -1 (Є x10 M cm ) (V vs NHE) E0-0 (eV) (V vs NHE) (nm)

4.1 350 (35.76), 395 (34.12) 1.43 2.71 −1.28 545 3.3 350 (48.43), 385 (54.74) 1.45 2.63 −1.18 524

4.2 350 (11.09), 420 (16.42) – 2.58 – 540

4.3.3. Electrochemistry

The cyclic voltammogram of 4.1 in ACN solution shows a quasi-reversible oxidation

process centered at 1178 mV vs Ag/AgCl with the Fc+/Fc0 wave centered at 443 mV vs

Ag/AgCl. The cyclic voltammogram of 3.3 in DCM solution shows a quasi-reversible oxidation

process centered at 1191 mV vs Ag/AgCl with the Fc+/Fc0 wave centered at 428 mV vs

Ag/AgCl. Due to solubility limitations, the electrochemical properties of 4.2 could not be

measured. Although sensitizer 4.2 has high solubility in THF, no oxidation waves were observed

in THF solution within the solvent’s potential window.46 The oxidation potentials were further

+ 0 41 converted to E1/2 vs NHE using E1/2(Fc /Fc ) = +0.69 vs. NHE yielding E1/2(4.1) = 1.43 V vs

NHE and E1/2(4.2) = 1.45 V vs NHE. The sensitizers have oxidation potentials that are more

- - 47 positive than that of the I /I3 redox mediator (0.4 V vs NHE) which provide a thermodynamic

driving force of 1.03 – 1.05 V for regeneration of the oxidized sensitizers by the iodide/triiodide

redox mediator. The excited state oxidation potentials of the sensitizers (Table 4.1) are

48 sufficiently more negative than the conduction band edge of TiO2, which is –0.5 V vs NHE, providing a great amount of thermodynamic driving force for electron injection.

114

4.3.4. Photovoltaic Measurements

The photoaction spectra and I-V curves were measured under identical conditions for all

the sensitizers investigated herein. Four independent DSSCs were assembled and measured in

parallel and the results reported herein are the average of the four cells which are reported as

overall yields which were not corrected for any kind of losses. All devices have an active area of

25 cm2. The photoaction spectra of dye-sensitized solar cells incorporating sensitizers 4.1, 3.3,

and 4.2, along with that of the N3 sensitizer, are shown in Figure 4.3. The carbazole-based

sensitizers convert visible light to photocurrent efficiently in the wavelength range of 350 nm to

550 nm. A maximum photon-to-current conversion efficiency of about 75% at 450 nm was

realized for sensitizers 4.1 and 3.3, while sensitizer 4.2 has a maximum IPCE of 66% at 440 nm

and N3 sensitizer shows a maximum IPCE of 71.5% (Figure 4.3, Table 4.2). The current-voltage

properties of the DSSCs based on sensitizers 4.1, 3.3, and 4.2, as well as N3 sensitizer, measured

under standard conditions (AM 1.5G, 100 mW/cm2) are shown in Figure 4.4. Devices based on

the carbazole-linked sensitizers 4.1 and 3.3 displayed the highest power conversion efficiency of

2.7%, those based on the thiophene-linked sensitizer 4.2 displayed a power conversion efficiency

of 2.23%. Importantly, DSSCs based on these metal-free sensitizers displayed excellent fill

factors which suggests that there is reduced contribution to the internal resistance within these

49,50 devices, especially from the charge transport at the TiO2/dye/eltctrolyte interface. Compared with sensitizers 4.1 and 3.3, sensitizer 4.2 has a higher short-circuit current despite having a lower value for the maximum IPCE. This is probably due to the fact that the sensitizer has a much broader absorption and photoaction spectra whose contributions are expected to enhance the value of the photogenerated current. Sensitizers 4.1 and 3.3 are structurally similar, except for the inclusion of an extra phenyl acetylene group in the motif of sensitizer 4.2, and as such

115

should display similar properties which are reflected in the photophysical, electrochemical, and photovoltaic properties of the sensitizers. The photogenerated short-circuit current and the values of the IPCE maxima are identical, within experimental error, for both sensitizers and the observed differences are in the open-circuit voltage. There is a ~ 25 mV increase in the value of the VOC of sensitizer 4.1 relative to sensitizer 3.3 which can be attributed to the effects of an

extended π-backbone which enhances charge separation between the carbazole donor and

cyanoacrylic acceptor moieties, enhances electron injection and occupancy of the conduction

band which raises the Fermi level and ultimately results in a greater open-circuit voltage.

80 (a) 80 70 70 (b) 60 60 50 50 N3 40 40 4.1 IPCE (%) 30 3.3 IPCE (%) 30 4.2 20 20 10 10 0 0 350 400 450 500 550 600 650 400 500 600 700 800 900 Wavelength (nm) Wavelength (nm)

Figure 4.3. Photoaction spectra of sensitizers 4.1, 3.3, and 4.2 (a) and N3 (b).

) 2 ) 8 2 (a) 18 (b) 16 6 14 12 N3 10 4

4.1 8 3.3 4.2 6 2 4 2 Photocurrent Density (mA/cm Photocurrent Density (mA/cm 0 0 0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700 Photovoltage (mV) Photovoltage (mV)

Figure 4.4. Current-voltage curves of sensitizers 4.1, 3.3, and 4.2 (a) and N3 (b).

116

All photovoltaic performances were obtained with TiO2 photoanodes incorporating 16 µm

of transparent layer and 5 µm of scattering gave the best photovoltaic performance (Table 4.2).

Optimization of the TiO2 film thickness became necessary in order to ascertain the optimum

thickness that would yield the best photovoltaic performance and in this case involved the use of

transparent layer of TiO2 film (13 and 16 µm thickness) with or without a second layer of a 5 µm scattering layer on top of it (Table 4.2). Thicker films are expected to give better light absorption from an optical point of view, but the thickness of a film affects its mechanical strength, adherence onto substrates, and the interconnectivity between particles and these pose a challenge when making thicker films. Interconnectivity between particles is essential for efficient electron transport and influences resistance within the bulk semiconductor. Inefficient charge transport

through the substrates results in losses through various recombination pathways. The optimal

thickness of any given photoelectrode, on the other hand, depends on the extinction coefficient of

the adsorbed sensitizer as well as on the semiconductor particle properties51 and these conditions

need to be optimized to get the best out of any combination of semiconductor and sensitizer.

The overall photovoltaic performance of the sensitizers may have been affected by π-

stacking, despite the presence of the tertiary-butyl groups, as we did not use any coadsorbents

while sensitizing the semiconductor nanocrystals. The effect of π-stacking in planar conjugated

sensitizers may be reflected in a dissipative intermolecular energy transfer which could have

adverse effects on the cell photovoltaic performance.30,31 Various approaches, such as

coadsorption of organic acids like 3R,7R-dihyroxy-5β-cholic acid11 and deoxycholic acid12 have been reported to improve device performance by breaking up of π-stacked aggregates. In view of

the excessive driving force for regeneration of the sensitizers (ca. 1.0 V vs NHE) by the

iodide/triodide redox mediator, the use of an alternate redox mediator with a more positive

117

oxidation potential might yield a greater photovoltaic performance by decreasing the

thermodynamic driving force and loss of the excess energy in the form of heat.

Table 4.2. Summary of photovoltaic properties of the Sensitizers 4.1, 3.3, 4.2, and N3.

TiO2 Film IPCEmax a 2 Sensitizer Thickness (µm) Voc (mV) Jsc(mA/cm ) FF(%) η(%) (%)

4.1 13+5 649±4.83 5.60±0.10 67.4±2.89 2.45±0.04 74.0 16 615±11.7 5.77±0.07 68.7±0.50 2.44±0.00 74.4

16+5 617±1.80 6.24±0.30 70.2±1.77 2.70±1.77 75.3

3.3 13+5 664±9.03 4.72±0.39 71.5±1.00 2.24±0.25 75.1 16 636±4.41 5.73±0.39 67.4±6.11 2.44±0.07 74.0 16+5 642±6.59 5.96±0.20 70.5±1.23 2.70±0.05 75.9

4.2 16+5 523±10.1 7.03±0.28 60.5±0.30 2.23±0.04 66.5

N3 16+5 683±4.09 15.9±1.01 55.9±1.58 6.04±0.31 71.5

a Thickness of the transparent layer was either 13 or 16 µm while the scattering layer is 5 µm thick. Four identical solar cells were prepared and evaluated in each case. The Redox electrolyte consisted of 0.2 M LiI, 0.05 M I2, 0.7 M PMII, and 0.5 M tbupy in ACN solution.

These sensitizers reported herein exhibit excellent photon-to-conversion efficiencies in

the blue region of the spectrum. Further work is focusing on developing metal-free sensitizers

with excellent photoconversion efficiencies in the lower energy region and assembling the blue-

and red-response sensitizers in a tandem arrangement to boost the energy conversion efficiency.

The sensitizers reported herein exhibit unique photovoltaic behaviors and show promises for

further optimization and application in the growing field of organic dye-sensitized solar cells.

4.3.5. Quantum Chemical Calculations

Density functional theory (DFT) calculations were performed on the sensitizers 4.1, 3.3,

and 4.2 to get a deep insight into the structure-photovoltaic cell performance relationships. The

118 calculations were done on a B3LYP level for the geometry optimization. Before the irradiation of light the electron density is located mainly on the donor units, whereas after light irradiation they move completely to the acceptor units near to the anchoring group, which favor the electron injection from the sensitizer to the conduction band of TiO2. From the electron distribution structures in Figure 4.5, it is obvious that the acceptor can withdraw almost all the electrons to the phenyl unit next to the anchoring group in all sensitizers 4.1, 3.3, and 4.2. The HOMO electrons are delocalized almost from donor carbazole to linker carbazole in sensitizer 4.1, but in sensitizer 3.3 it is not up to linker carbazole, whereas in sensitizer 4.2, the electrons are localized only in donor carbazole moiety. The absence of electronic communication between the thiophene linker and the donor carbazole in the HOMO on the one hand, and the linker and the acceptor cyanoacrylic acid on the other hand, in sensitizer 4.2 is closely related to the relatively higher energy of the linker, compared to the energies of the donor and acceptor groups. The electronic communication that exists in sensitizers 4.1 and 3.3 between the donor and the linker in the

HOMO, and the acceptor and the linker in the LUMO, is made possible by the relative stabilization of the energy levels of the linker by the electron-withdrawing trifluoromethyl group.

Therefore the results show that the sensitizers can have efficient electron injection from the

LUMO to the conduction band of TiO2 and improving the cell efficiency.

119

LUMO LUMO LUMO

HOMO HOMO HOMO

Sensitizer 4.1 Sensitizer 3.3 Sensitizer 4.2

Figure 4.5. HOMO-LUMO structures of the sensitizers 4.1, 3.3, and 4.2.

4.4. Conclusions

Organic sensitizers with a carbazole as a donor, carbazole or bithiophene as a π- conjugation linker, and cyanoacrylic acid group as an acceptor have been synthesized as for

DSSCs. Compared with the sensitizer having bithiophene in the linker, the sensitizers with carbazole in the linker showed more efficiency. Sensitizers 4.1 and 3.3 produce a power conversion efficiency of 2.70% with a maximum IPCE of 75% at 450 nm, while sensitizer 4.2 has a power conversion efficiency of 2.23% with a maximum IPCE of 66% at 440 nm. The sensitizers thus exhibit photon-to-current conversion efficiencies in the blue region of the spectrum and serve as candidates for further optimization in tandem cells. These sensitizers could be mixed with other sensitizers with optimized red response in a tandem arrangement to achieve excellent photoconversion efficiencies in the spectral regions of interest. All the results indicate that these organic compounds are promising candidates in the development and further optimization of metal-free organic sensitizers for application in DSSCs.

120

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124

Appendix 3. 1H NMR, and 13C NMR spectra of compounds 4.3, 4.5, 4.6, 4.1, and 4.2.

t-Bu

OHC N N

t-Bu

CF3

Figure A3.1. 1H NMR of compound 4.3.

Figure A3.2. 13C NMR of compound 4.3.

125

S Br CHO S

Figure A3.3. 1H NMR of compound 4.5.

Figure A3.4. 13C NMR of compound 4.5.

126

t-Bu

S OHC N S

t-Bu

Figure A3.5. 1H NMR of compound 4.6.

Figure A3.6. 13C NMR of compound 4.6.

127

t-Bu

N NC N COOH t-Bu

CF3

Figure A3.7. 1H NMR of sensitizer 4.1.

Figure A3.8. 13C NMR of sensitizer 4.1.

128

t-Bu

S N NC S COOH t-Bu

Figure A3.9. 1H NMR sensitizer 4.2.

Figure A3.10. 13C NMR of sensitizer 4.2.

129

CHAPTER 5. SYNTHESIS AND COMPUTATIONAL STUDIES OF DIPHENYLAMINE

DONOR-CARBAZOLE LINKER BASED DONOR-ACCEPTOR COMPOUNDS

5.1. Introduction

Linear π-conjugated systems constitute one of the most extensively studied classes of organic compounds. Research efforts in this field cover a broad spectrum of topics ranging from purely fundamental investigations to exploring the potential applications of individual compounds and their ensembles in nano-electronics and solar energy conversion.1

In this regard, the electronic and optical properties of conjugated small molecules have attracted much attention in the past few decades.2,3 Specifically, the molecular architecture and

strong absorption and emission properties of donor-acceptor (DA) complexes separated by

different lengths of linkers have rendered these molecules useful as emitters in organic light

emitting diodes (OLEDs)4,5 and sensitizers in DSSCs.6,7 Carbazole and diphenylamine based DA

compounds have recently found use as wide band gap energy transfer materials as well as

promising applications in the area of hole-transporting.8,9 To date, a number of derivatized

carbazole and diphenylamine-based DA compounds have been designed, synthesized, and characterized because of their tunable intramolecular charge transfer properties. It has been found that the molecular and optical properties of carbazole-based compounds can be controlled

by modifying the 2-, 3-, 6-, 7-, and 9- positions of the carbazole.10,11 There are only a few

examples in the literature of DA type compounds in which triphenylamine was linked to the 3-

position of a carbazole12 and/or diphenylamine was linked to a fluorene.13 However, there are no reported compounds in which diphenylamine or triphenylamine donors are linked to the 2- position of the linker carbazole to form D-π-A molecules. Previous molecular designs have been

130

mostly focused on networking through the 3-, 6- positions,14-16 and only a handful of examples have the conjugation through the 2-, 7-,17-20 and 9- positions.21,22

This work presents the synthesis and characterization of a novel series of diphenylamine

donor-based compounds (5.1-5.6) having 2-,7- carbazole linkers all with/without phenylethynyl

extenders attached to a terminal acceptor or donor group, Figure 5.1. Compounds 5.1 and 5.2

have terminal electron acceptor moieties, an aldehyde in 5.1 and malononitrile in 5.2, and

compound 5.3 has another diphenylamino donor group. Compounds 5.4, 5.5, and 5.6 are similar

to compounds 5.1, 5.2, and 5.3 respectively, conjugated without the incorporation of

phenylethynyl groups in the linkers. As the recorded absorption spectra of the structurally related

compounds 5.1-5.6 are fundamentally different, a qualitative description of the photochemistry

of these compounds in terms of molecular orbitals and the changes in electron density along the

π-conjugated framework was obtained using time dependent density functional theory, TD DFT,

calculations. The TD DFT calculation was done by Patrick Z. El-Khoury, a graduate student of

Dr. Alexander N. Tarnovsky’s research group and the results are already reported.23

Although N-vinylcarbazoles and 3,6-functionalized carbazoles can be readily synthesized

from 9H-carbazole, the preparation of 2,7-functionalized carbazoles is not straightforward. We

report the synthesis of 4-({7-{[4-(diphenylamino)phenyl]ethynyl}-9-[4-(trifluoromethyl)phenyl]-

9H-carbazol-2-yl}ethynyl)benzaldehyde(5.1), [4-({7-{[4-(diphenylamino)phenyl]ethynyl}-9-[4-

(trifluoromethyl)phenyl]-9H-carbazol-2yl}ethynyl)benzylidene]malononitrile (5.2), 4,4'-[{9-[4-

(trifluoromethyl)phenyl]-9H-carbazole-2,7-diyl}bis(ethyne-2,1-diyl)]bis(N,N-diphenylaniline)

(5.3), 4-({7-(diphenylamino)-9-[4-(trifluoromethyl)phenyl]-9H-carbazol-2-

yl}ethynyl)benzaldehyde (5.4), [4-({7-(diphenylamino)-9-[4-(trifluoromethyl)phenyl]-9H-

131 carbazol-2-yl}ethynyl)benzylidene]malononitrile (5.5), and N,N,N',N'-tetraphenyl-9-[4-

(trifluoromethyl)phenyl]-9H-carbazole-2,7-diamine (5.6).

OHC N OHC N N N . 5.1 5 4

CF3 CF3 N NC N N NC CN N CN 5.5

5.2 CF3 CF3 N N N N N N 5.3 5.6 CF3 CF3

Figure 5.1. Molecular structures of compounds 5.1-5.6.

5.2. Results and Discussion

Compound 2.11 (N-(4-ethynylphenyl)-N-phenylbenzenamine) was prepared from commercially available 2.9 (4-bromotriphenylamine) by Sonogashira coupling with 3- methylbutyn-3-ol followed by reverse addition24 and the details is already described in chapter 2.

The synthesis of compounds 3.6 and 3.7 is carried out from commercially available compound

(4,4’-biphenyl) by following the reported literatures25,26 and details are described in chapter 3.

Sonogashira coupling of 3.7 with 2.11 yielded 5.1 which were further reacted with malononitrile and basic aluminum oxide in toluene to produce 5.2. Sonogashira coupling of 2.11 with 3.6 produced the compound 5.3 (Scheme 5.1). Similarly, 3.7 was coupled with diphenylamine to produce 5.4 which was further reacted with malononitrile and basic aluminum oxide in toluene

132 to produce 5.5. Diphenylamine was coupled with compound 3.6 under standard conditions to produce compound 5.6.

Scheme 5.1. Synthesis of compounds 5.1-5.6.

N CHO Br CHO N H N N (2.11)

(3.7) Pd(PPh3)2Cl2, CuI, PPh3, . N,N-diisopropylethylamine, reflux 10 h 5 1 CF3 CF3 N malononitrile, basic Al2O3 CN 5.1 N 2-methylpropanol, reflux 5 h NC 5.2

Br Br N H CF3 N N N (2.11) N Pd(PPh3)2Cl2, CuI, PPh3, 5.3 (3.6) N,N-diisopropylethylamine, reflux 10 h

CF3

CF3 N CHO N diphenylamine 3.7 5.4 Pd(OAc)2, P(t-Bu)3 Cs2CO3 (4 equi.), reflux 22 h CF3 N CN malononitrile, basic Al2O3 N 5.4 5.5 NC 2-methylpropanol, reflux 5 h

CF3 N N diphenylamine N 3.6 Pd(OAc)2, P(t-Bu)3 Cs2CO3 (4 equi.), reflux 22 h 5.6 CF3

The experimental and calculated absorption spectra of compounds 5.1-5.6 in hexanes and

DCM are shown in Figure 5.2. There are three major factors that are expected to affect the nature

133 of the excited states of the reported compounds: (i) the choice of donor - held constant in this study, (ii) the presence/absence of phenylethynyl extenders which is expected to affect – to some extent – the degree of conjugation and hence the color of these molecules, and (iii) the choice of acceptor – varied as previously outlined. The effect of incorporating phenylethynyl extenders on the UV-vis spectra of these compounds can be discerned by comparing compounds 5.3 and 5.6, as all other variables are held constant in this case. Here, incorporation of the extenders causes an increase in the ratio of intensities of the red to blue absorption bands. Although the spectra are relatively more complex, the same trend is manifested when compounds 5.1 and 5.4 on one hand, and 5.2 and 5.5 on the other hand, are compared. The effect of changing the nature of the acceptor group on the absorption spectra can be monitored by comparing compounds 5.1 and 5.4 to compounds 5.2 and 5.5, respectively. It is not surprising that a stronger electron acceptor moiety results in an overall relatively red-shifted absorption spectrum. However, the overall changes in the absorption spectra as a function of changing the molecular and electronic structures of these compounds are more pronounced in going from compound 5.4 to 5.5, as opposed to compound 5.1 to 5.2. The latter is thought to be another consequence of the presence of the phenylethynyl extenders in compounds 5.4 and 5.5, and a clear indication that the low- lying electronically excited states in these molecules not only depend on the natures of the donor and acceptor moieties, but also on the distance separating the two groups.

134

1.0 1.0 15.1 45.4 0.8 0.8

0.6 0.6

0.4 0.4

0.2 0.2

Normalized Absorbance 0.0 0.0 300 350 400 450 500 300 350 400 450 500

1.0 1.0 hex 2 5.2 55.5 hexB3LYP 0.8 0.8 hexCAMB3LYP dcm dcmB3LYP 0.6 0.6 dcmCAMB3LYP

0.4 0.4

0.2 0.2 Normalized Absorbance 0.0 0.0 300 400 500 600 700 300 400 500 600 700

1.0 1.0 35.3 5.6 0.8 0.8 6

0.6 0.6

0.4 0.4

0.2 0.2 Normalized Absorbance 0.0 0.0 300 350 400 450 300 350 400 450 Wavelength (nm) Wavelength (nm) Figure 5.2. Experimental and computed UV-vis absorption spectra of compounds 5.1-5.6. The spectra are shown in red and blue for hexanes and DCM, respectively. The calculated vertical transition energies in hexanes and DCM are represented by circles and squares, respectively. The stick spectra were calculated using both the B3LYP (dashed bars, open symbols) and CAM- B3LYP (dotted bars, full symbols) functionals.

135

Because of the charge transfer character of the low-lying electronic states of compounds

5.1, 5.2, 5.4, and 5.5, the calculated stick spectra depicted in Figure 5.2 reveal that the B3LYP

functional and its time dependent analogue fail to correctly describe these states. Although the

major goal of this report is not to accurately reproduce the measured UV-vis absorption spectra using tools of computational photochemistry, a qualitative understanding of the nature of the low-lying electronically excited states in these molecules is sought after. We thus tested the performance of another density functional, namely CAM-B3LYP, in combination with one of the most commonly used basis set for the calculation of organic compounds, Figure 5.2. The calculated TD CAM-B3LYP vertical transition energies are in very close agreement with the experimental spectra, revealing the importance of the long range correction (the Coulomb-

attenuating method) to the B3LYP functional in describing the excited states of our DA type

compounds.

Figure 5.3 shows the highest four occupied molecular orbitals of compounds 5.1-5.3

calculated at the CAM-B3LYP/6-31G* level of theory. It can be directly observed that the nature

of these orbitals is qualitatively similar for compounds 5.1-5.2 (the DA complexes), both

different from compound 5.3 (the DD complex). Particular attention is drawn to two orbitals: (i)

the HOMO where electron density is localized at the donor moiety in compounds 5.1-5.2 but

delocalized throughout the entire π-framework in compound 5.3, and (ii) the HOMO-2 orbital in

compounds 5.1-5.2 in which electron density is localized at the carbazole moiety, this orbital

relatively stabilized to become HOMO-3 in compound 5.3. Moreover, the previously mentioned

HOMO in compound 5.3 is qualitatively similar to the HOMO-1 orbitals of compounds 5.1 and

5.2. The latter has direct implications on the photochemistry of the fundamentally different

constructs 5.3 and 5.6, and this will be discussed in the context of electronic transitions (vide

136

infra). Figure 5.4 shows the lowest four unoccupied molecular orbitals of compounds 5.1-5.3.

Here, electron density in the LUMO of compounds 5.1-5.2 is localized at the structurally

different acceptor moieties, whereas it is delocalized over the conjugated π-framework of 5.3 where no acceptor was attached by design. Qualitatively similar results were obtained for

(compounds 5.4-5.6), Figures 5.5 and 5.6. One noticeable difference is that in compounds 5.1-

5.2, delocalization extends along the π-framework only to the linker carbazole but in compounds

5.4-5.5 the electron density is delocalized over the linker carbazole as well as the donor. For instance, the electron density in the HOMO of the latter mentioned compounds is delocalized over the donor as well as the carbazole moieties. These findings point out that less efficient electronic communication between the donor and acceptor is achieved when the spacers are inserted by design. In other words, electron density is “trapped” at the donor moiety when a phenylethynyl extender is inserted between the donor and the carbazole mediator. As delocalization is expected to affect the overall color of π-conjugated systems, the effects of the

presence/absence of a phenylethynyl extender on delocalization is well-documented in this work.

137

Energy

5.1 5.2 5.3

Figure 5.3. The highest 4 occupied molecular orbitals of compounds 5.1-5.3 calculated at the CAM-B3LYP/6-31G* level of theory.

138

Energy

5.1 5.2 5.3

Figure 5.4. The lowest 4 unoccupied molecular orbitals of compounds 5.1-5.3 calculated at the CAM-B3LYP/6-31G* level of theory.

139

Energy

5.4 5.5 5.6

Figure 5.5. The highest 4 occupied molecular orbitals of compounds 5.4-5.6 calculated at the CAM-B3LYP/6-31G* level of theory.

140

Energy

5.4 5.5 5.6

Figure 5.6. The lowest 4 unoccupied molecular orbitals of compounds 5.4-5.6 calculated at the CAM-B3LYP/6-31G* level of theory.

141

The assignments of the first three electronic transitions in terms of major contributing

molecular orbitals of compounds 5.1-5.6 are summarized in Table 5.1. First, the lowest energy

transition in compounds 5.1-5.2 and 5.4-5.5 are HOMO/LUMO Donor-Acceptor transitions.

Second and when the results from this table are combined with Figures 5.3-5.6, it can be

immediately noticed that the nature of the first two electronic transitions is similar in compounds

5.1, 5.2, 5.4, and 5.5 all fundamentally different from the nature of the first two transitions in the

DD compounds 5.3 and 5.6. Third, the results of the DD compounds shed light on the fact that

the trifluoromethylphenyl- group does not participate in the DA-type transitions in the series of

compounds unless electron density is “confined in space” by design (for instance by inspecting

the HOMO to LUMO transition in compound 5.6). A careful inspection of Table 5.1 and Figures

5.3-5.6 reveals that none of the molecular orbitals associated with the lowest two lying electronic

transitions in compounds 5.1-5.2 and 5.4-5.5 have electron density localized at the

triflouromethylphenyl moiety. This is not the case for compounds 5.3 and 5.6 where low-energy

electronic transitions involve electron density shift toward the trifluoremethylphenyl group.

Overall, the computational findings in this work suggest that both the nature of the acceptor moiety and the presence/absence of a spacer are expected to influence the characters of the low-

lying electronic states and hence the “colors” of these compounds. Further experimental work

and computational works are needed to separate these two factors and to understand the overall

photophysics/photochemistry of these systems.

142

Table 5.1. A description of the lowest three vertical transitions of compounds 5.1-5.6 in terms of major contributing molecular orbitals calculated at the CAM-B3LYP/6-31G* level of theory.

S0/S1 S0/S2 S0/S3 5.1 Description HOMO/LUMO HOMO/LUMO+1 HOMO-2/LUMO

Coefficient 0.69 0.56 0.59 5.2 Description HOMO/LUMO HOMO-1/LUMO HOMO-2/LUMO

Coefficient 0.7 0.65 0.7 5.3

Description HOMO/LUMO HOMO-1/LUMO HOMO/LUMO+1

Coefficient 0.67 0.68 0.7

5.4 Description HOMO/LUMO HOMO-1/LUMO HOMO/LUMO+1

Coefficient 0.68 0.68 0.7 5.5 Description HOMO/LUMO HOMO-1/LUMO HOMO-2/LUMO

Coefficient 0.69 0.7 0.64

5.6 Description HOMO/LUMO HOMO/LUMO+1 HOMO/LUMO+2

Coefficient 0.7 0.67 0.7

5.3. Conclusions

The synthesis of a novel series of organic Donor-Acceptor compounds with a

diphenylamine donor, 2,7-functionalized carbazole-based linkers, and either (i) an aldehyde, (ii)

malononitrile, or (iii) diphenylamino acceptors - all with and without phenylethynyl extenders -

is reported. An insight into the nature of the first three electronic transitions in these compounds

is obtained from DFT and TD DFT calculations. Results suggest that in this series, less efficient

143

electronic communication between the donor and acceptor is achieved when the phenylethynyl groups are inserted. The latter has direct implications on the overall color of the compound in the reported series. Moreover, which major contributing molecular orbitals are involved in low-lying electronic transitions is found to depend on both (i) the nature and choice of the acceptor group, and (ii) the presence/absence of a phenylethynyl group in the linker.

5.4. Experimental and Computational Methods

5.4.1. Synthesis

General: All solvents and reagents were of reagent grade quality and used without further purification unless stated otherwise. All reactions and manipulations were carried out under argon gas with the use of standard inert atmosphere techniques. The precursors 2.11, 3.6, and 3.7 compounds were prepared by following the literature procedures24-26 and described in chapters 2

and 3. New synthetic procedures for synthesis of 5.1-5.6 are described here. 1H and 13C NMR

spectra were recorded at 500 MHz using tetramethylsilane as the internal standard. The general

synthetic methodologies are outlined in Scheme 5.1.

Synthesis of 4-({7-{[4-(diphenylamino)phenyl]ethynyl}-9-[4-(trifluoromethyl)phenyl]-9H-

carbazol-2-yl}ethynyl)benzaldehyde (5.1). To a solution of compound 3.7 (100 mg, 0.19

mmol) in dry diisopropylethylamine (6 ml) was added compound 2.11 (52 mg, 0.19 mmol).

After the solution was degassed with argon for 30 min while stirring, Pd(PPh3)2Cl2 (5 mg),

triphenylphosphine (5 mg), and CuI (5 mg) were added. The reaction mixture was then refluxed

under argon for 10 h. After the reaction was completed, the crude mixture was filtered at room

temperature, the precipitate was rinsed with diethyl ether, and the combined filtrates were

evaporated. The residue was purified by flash column chromatography (silica gel, 20% ethyl

acetate in petroleum ether) to give compound 5.1 (92 mg, 67% yield) as a shining yellow solid.

144

o 1 Mp: 197 C. H NMR (500 MHz, CDCl3): δ 7.03 (d, 2H), 7.09 (t, 2H), 7.14 (dd, 4H), 7.3 (dd,

4H), 7.4 (d, 2H), 7.52 (dd, 1H), 7.55 (dd, 1H), 7.58 (s, 1H), 7.62 (s, 1H), 7.71 (d, 2H), 7.78 (d,

2H), 7.89 (d, 2H), 7.96 (d, 2H), 8.11 (d, 1H), 8.14 (d, 1H), and 10.05 (s, 1H). 13C NMR (500

MHz, CDCl3): 88.92, 89.4, 94.2, 105.3, 113.2, 119.9, 120.6, 120.7, 122.0, 122.9, 123.6, 124.1,

124.3, 124.8, 126.9, 127.3, 129.3, 129.6, 132.0, 132.1, 140.2, 147.9, and 191.4. HRMS (MALDI-

TOF) MS ES+ = 707.231 measured, and 707.231 for theoretical spectrum having molecular

formula C48H30N2OF3.

Synthesis of [4-({7-{[4-(diphenylamino)phenyl]ethynyl}-9-[4-(trifluoromethyl)phenyl]-9H-

carbazol-2-yl}ethynyl)benzylidene]malononitrile (5.2). In a dry two necked round bottomed

flask was added compound 5.1 (100 mg, 0.14 mmol), malononitrile (9.4 mg, 1.1 mmol), basic

aluminium oxide (10 mmol), and dry toluene (10 ml). The mixture was refluxed under argon for

5 h. The mixture was filtered hot, and the residue was washed several times with hot ethyl

acetate. The filtrate was then dried, and the solid obtained was purified by chromatography

(silica gel, 20% ethyl acetate in petroleum ether) to obtain pure compound 5.2 (90 mg, 86%

yield) as a reddish brown solid. Mass spectrum (MALDI-TOF) m/z M+ = 754. Mp: 203 oC. 1H

NMR (500MHz, CDCl3): δ 7.03 (d, 2H), 7.09 (t, 2H), 7.14 (d, 4H), 7.3 (t, 4H), 7.4 (d, 2H), 7.52

(dd, 1H), 7.55 (dd, 1H), 7.58 (s, 1H), 7.62 (s, 1H), 7.68 (d, 2H), 7.76 (s, 1H), 7.78 (d, 2H), 7.92

13 (d, 2H), 7.97 (d, 2H), 8.12(d, 1H), and 8.14 (d, 1H). C NMR (500 MHz, CDCl3): 82.7, 89.3,

90.6, 96.2, 112.6, 112.7, 113.2, 119.8, 120.8, 122.2, 122.8, 123.7, 124.2, 124.7, 125.1, 127.3,

127.4, 127.5, 129.4, 130.0, 130.2, 130.7, 132.4, 132.5, 140.6, 147.1, and 158.5.

Synthesis of 4,4'-[{9-[4-(trifluoromethyl)phenyl]-9H-carbazole-2,7-diyl}bis(ethyne-2,1-

diyl)]bis(N,N-diphenylaniline) (5.3). To a solution of compound 3.6 (61 mg, 0.13 mmol) in dry

N,N-diisopropylethylamine (6 ml) was added 2.11 (100 mg, 0.26 mmol). After the solution was

145

degassed with argon for 30 min while stirring, Pd(PPh3)2Cl2 (1 mg), triphenylphosphine (1 mg),

and CuI (1 mg) were added. The reaction mixture was then refluxed under argon for 6 h. After

the reaction was completed, the crude mixture was filtered at room temperature, the precipitate

was rinsed with diethyl ether, and the combined filtrates were evaporated. The residue was

purified by flash column chromatography (silica gel, 10% ethyl acetate in petroleum ether) to

give pure 5.3 (78 % yield) as a yellow solid. Mp: 88 oC. Mass spectrum (MALDI-TOF) m/z M+

1 = 845. H NMR (500MHz, CDCl3): δ 7.03 (d, 4H), 7.09 (t, 4H), 7.14 (d, 8H), 7.3 (t, 8H), 7.4 (d,

4H), 7.5 (dd, 2H), 7.57 (dd, 2H), 7.77 (d, 2H), 7.96 (d, 2H), and 8.1 (d, 2H). 13C NMR (500

MHz, CDCl3): 106.3, 117.8, 119.4, 119.8, 120.6, 121.0, 121.1, 122.5, 122.6, 123.6, 123.8, 124.0,

126.4, 127.0, 127.1, 127.2, 127.3, 127.4, 141.7, 143.1, 146.0, and 148.1.

Synthesis of 4-({7-(diphenylamino)-9-[4-(trifluoromethyl)phenyl]-9H-carbazol-2-

yl}ethynyl)benzaldehyde (5.4). Compound 3.7 (200 mg, 0.38 mmol) and diphenylamine (60

mg, 0.38 mmol) were mixed with dry toluene (80 ml) in a two necked round bottomed flash

containing a stir bar. The Pd(OAc)2 (3 mole %), P(t-Bu)3 (7 mole %), and Cs2CO3 (495 mg, 1.52

mmol) were also added and the mixture was stirred under argon at 110 oC for about 10 h. The

reaction mixture was then cooled to room temperature and toluene was removed completely

under vacuum. The solid mixture was dissolved in THF, and unreacted Cs2CO3 was removed

under gravity filtration. The organic residue was then purified by column chromatography (silica

gel, 10% ethyl acetate in petroleum ether) to give the yellow compound 5.4. (180 mg, 77%

o 1 yield). Mp: 142 C. H NMR (500 MHz, CDCl3): δ 7.05 (t, 2H), 7.1 (dd, 1H), 7.13(d, 4H), 7.14

(d, 1H), 7.27 (d, 4H), 7.51 (dd, 1H), 7.57 (s, 1H), 7.64 (d, 2H), 7.7 (d, 2H), 7.7.83 (d, 2H), 7.89

13 (d, 2H), 8.0 (d, 1H), 8.4 (d, 1H), and 10.03 (s, 1H). C NMR (500 MHz, CDCl3): 82.5, 88.7,

90.6, 96.9, 105.2, 112.9, 118.4, 118.7, 118.9, 119.9, 121.5, 123.0, 124.2, 124.8, 126.9, 127.3,

146

129.1, 129.3, 130.0, 130.2, 130.7, 132.3, 140.3, 142.3, 147.8, and 158.5. HRMS (EI) calculated

for C40H25ON2F3 606.1919, measured 606.1910.

Synthesis of [4-({7-(diphenylamino)-9-[4-(trifluoromethyl)phenyl]-9H-carbazol-2-

yl}ethynyl)benzylidene]malononitrile (5.5). The synthetic procedure is exactly same as for the

synthesis of 5.2, but using 5.4 as the starting material. The product was obtained after

purification by column chromatography (silica gel, 20% ethyl acetate in petroleum ether) as an

o 1 orange solid 5.5 (82% yield). Mp: 212 C H NMR (500MHz, CDCl3): δ 7.05 (t, 2H), 7.1 (dd,

1H), 7.14 (d, 4H), 7.15 (d, 1H), 7.27 (d, 4H), 7.51 (dd, 1H), 7.57 (s, 1H), 7.66 (d, 2H), 7.68 (d,

2H), 7.75 (s, 1H), 7.84 (d, 2H), 7.92 (d, 2H), 8.0 (d, 1H), and 8.05 (d, 1H). 13C NMR (500 MHz,

CDCl3): 82.5, 88.7, 90.6, 96.9, 105.2, 112.9, 118.4, 118.7, 118.9, 119.9, 121.5, 123.0, 124.2,

124.8, 126.9, 127.3, 129.1, 129.3, 130.0, 130.2, 130.7, 132.3, 140.3, 142.3, 147.8, and 158.5.

+ HRMS (TOF MS ES ) calculated for C43H26N4F3 655.21, measured 655.20.

Synthesis of N,N,N',N'-tetraphenyl-9-[4-(trifluoromethyl)phenyl]-9H-carbazole-2,7- diamine (5.6). Compound 3.6 (200 mg, 0.38 mmol) and diphenylamine (120 mg, 0.77 mmol) were mixed with dry toluene (80 ml) in a two necked round bottomed flash containing a stir bar.

The Pd(OAc)2 (3 mole %), P(t-Bu)3 (7 mole %), and Cs2CO3 (495 mg, 1.52 mmol) were also

added and stirred under argon at 110 oC for about 18 h. The reaction mixture was then cooled to

room temperature and toluene was removed completely under vacuum. The solid mixture was

dissolved in THF and unreacted Cs2CO3 was removed under gravity filtration. The organic

residue was then purified by column chromatography (silica gel, 10% ethyl acetate in petroleum

1 ether) to give the yellowish white product (180 mg, 77% yield). HMR (500 MHz, CDCl3): δ7.01

(t, 4H), 7.07 (dd, 2H), 7.10 (dd, 8H), 7.15 (d, 2H), 7.25 (t, 8H), 7.7.52 (d, 2H), 7.69 (d, 2H), and

13 7.93 (d, 2H). C NMR (500 MHz, CDCl3): 106.3, 117.8, 119.4, 119.8, 120.6, 121.0, 121.1,

147

122.5, 122.6, 123.6, 123.8, 124.0, 126.4, 127.0, 127.1, 127.2, 127.3, 127.4, 141.7, 143.1, 146.0,

13 and 148.1. C NMR (500 MHz, CDCl3): 106.3, 117.9, 119.4, 119.8, 120.6, 121.1, 122.5, 123.6,

+ 123.8, 126.4, 127.0, 127.3, 129.2, 129.4, 141.7, 143.1, 146.1, and 148.1.MS (EI ) for C43H30N3F3

calculated 645.2391, measured 645.2391.

5.4.2. Computational Methodology

Unconstrained geometry optimization was used to locate the global minima of

compounds (5.1-5.6) on the ground state potential energy surface. Geometry optimization was

first performed using the B3LYP density functional in combination with the commonly used 6-

31G* basis set.27 These calculations were performed in the vacuum, hexanes, and DCM - the

solvent environments treated using the polarizable continuum solvation (PCM) model. Using the

fully optimized B3LYP/6-31G* minima, vertical transition energies were computed at the TD

B3LYP/6-31G* level of theory, Table A4.1 of the Appendix 4. The B3LYP functional is the standard methodology for simulating various processes in organic chemistry because it offers a

good compromise between computational cost and accuracy in the prediction of a variety of

molecular properties.28 However, as the low-lying electronic transitions in these compounds involve a large change in electron density or intramolecular charge transfer (vide infra), the approximation in the level of theory used in this work has little to do with the choice of basis set and mainly comes from the density functional.29 This is especially true for the first electronic

(HOMO/LUMO) transition in the reported D-A complexes where electron density shifts across these molecules from the donor moiety to the acceptor moiety through different lengths of π- conjugated mediators. However, these calculations provide a framework for a qualitative comparative description of the electronic transitions in compounds (5.1-5.6) in terms of

148 molecular orbitals and the changes in electron density accompanying electronic transitions.

These results are discussed in the “Results and Discussion” section of this work.

There is a growing need to obtain more quantitative agreement between the calculated and experimental spectra of such DA type compounds. This is why we also tested a more recently developed long range corrected version of the B3LYP hybrid functional, namely the

CAM-B3LYP functional.30 The CAM-B3LYP/6-31G* calculations yield a more quantitative agreement with the spectra, Table A4.1 and A4.2 of the of the Appendix 4. All calculations reported in this work were performed using the methodologies developed in the Gaussian 2009 package.31

5.5. References

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(12) Li, Z. H.; Wong, M. S. Org. Lett. 2006, 8, 1499-1502. (13) Huang, S.; Hsu, Y.; Yen, Y.; Chou, H.; Lin, J. T.; Chang, C.; Hsu, C.; Tsai C.; Yin, D. J. Phys. Chem. C 2008, 112, 19739-19747. (14) Ko, C.-W.; Tao, Y.-T.; Lin, J. T.; Thomas, K. R. J. Chem. Mater. 2002, 14, 357-361. (15) Siove, A.; Ades, D. Polymer 2004, 45, 4045-4049. (16) Kim, J. K.; Hong, S. I.; Cho, H. N.; Kim, D. Y.; Kim, C. Y. Polym. Bull. 1997, 38, 169- 176. (17) Blouin, N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19, 2295-2300. (18) Morin. J.-F.; Leclerc, M. Macromolecules 2001, 34, 4680-4682. (19) Morin, J.-F.; Drolet, N.; Tao, Y.; Leclerc, M. Chem. Mater. 2004, 16, 4919-4626. (20) Sonntag, M.; Strohriegl. P. Chem. Mater. 2004, 16, 4736-4742. (21) Promarak, V.; Ruchirawat, S. Tetrahedron 2007, 63, 1602-1609. (22) Shao, H.; Chen, X.; Wang, Z.; Lu, P. J. Lumin. 2007, 127, 349-354. (23) Panthi, K.; El-Khoury, P. Z.; Tarnovsky, A. N.; Kinstle, T. H. Tetrahedron. 2010, 66, 9641-

9649.

(24) Panthi, K.; Adhikari, R. M.; Kinstle, T. H. J. Phys. Chem. A 2010, 114, 4542-4549.

(25) Dierschke, F.; Grimsdale, A. C.; Mullen, K. Synthesis 2003, 2470-2472.

(26) Panthi, K.; Adhikari, R. M.; Kinstle, T. H. J. Phys. Chem. A 2010, 114, 4550-4557.

(27) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257-2261. (28) Riley, K. E.; Op’t Holt, B. T.; Merz, K. M. Jr. J. Chem. Theory Comput. 2007, 3, 407-433.

(29) Jensen, J. Introduction to Computational Chemistry: Wiley; Second edition, 2006. (30) Yanai, T.; Tew, D. P.; Handy, N. C. Chem. Phys. Lett. 2004, 393, 51-57. (31) Gaussian 03, Revision C.02, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;

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Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al- Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople, J. A.; Gaussian, Inc., Wallingford CT, 2004.

151

Appendix 4. 1H NMR and 13C NMR spectra of compounds 5.1-5.6 and tables showing calculated TD B3LYP/6-31G* vertical transition energies.

OHC N N

CF3

Figure A4.1. 1H NMR spectra of compound 5.1.

Figure A4.2. 13C NMR spectra of compound 5.1.

152

NC N NC N

CF3

Figure A4.3. 1H NMR spectra of compound 5.2.

Figure A4.4. 13C NMR spectra of compound 5.2.

153

N N N

CF3

Figure A4.5. 1H NMR spectra of compound 5.3.

Figure A4.6. 13C NMR spectra of compound 5.3.

154

OHC N N

CF3

Figure A4.7. 1H NMR spectra of compound 5.4.

Figure A4.8. 13C NMR spectra of compound 5.4.

155

N NC N CN

CF C 3

Figure A4.9. 1H NMR spectra of compound 5.5.

Figure A4.10. 13C NMR spectra of compound 5.5.

156

N N N

CF3

Figure A4.11. 1H NMR spectra of compound 5.6.

Figure A4.12. 13C NMR spectra of compound 5.6.

157

Table A4.1. Calculated TD B3LYP/6-31G* vertical transition energies (nm). The solvent environments are simulated using the PCM model.

Tv 5.1 5.2 5.3 5.4 5.5 5.6 S1/ GF 487.50 (0.9399) 651.12 (0.3723) 445.36 (2.5772) 466.29 (0.7598) 610.15 (0.5288) 387.02 (0.0121) HEX 498.31 (1.0977) 665.15 (0.4544) 454.74 (2.7738) 483.60 (0.8820) 640.31 (0.6379) 385.36 (0.8268) DCM 501.22 (1.0724) 657.82 (0.4767) 457.21 (2.7667) 494.50 (0.8433) 653.93 (0.6455) 385.06 (1.0461)

S2/ GF 395.85 (1.7429) 474.25 (0.4108) 402.98 (0.0136) 378.84 (0.0283) 463.03 (0.0270) 379.55 (0.8748) HEX 403.67 (1.8516) 489.01 (1.4927) 410.16 (0.0240) 386.87 (0.0419) 477.23 (0.0395) 383.98 (0.2041) DCM 405.89 (1.9115) 489.88 (1.4688) 413.29 (0.0259) 395.43 (0.0431) 488.71 (0.0393) 379.41 (0.0001)

S3/ GF 384.58 (0.0447) 461.53 (0.0138) 368.92 (0.0058) 369.74 (0.0002) 423.60 (1.3042) 346.23 (0.0001) HEX 391.94 (0.0305) 472.47 (0.0164) 363.99 (0.0039) 366.23 (0.0020) 439.87 (1.2981) 342.25 (0.0002) DCM 396.98 (0.0114) 479.34 (0.0556) 361.75 (0.0130) 367.40 (1.2455) 444.43 (1.2747) 336.34 (0.0010)

S4/ GF 378.99 (0.0976) 422.48 (0.9050) 357.11 (0.0110) 355.99 (1.0611) 379.38 (0.2712) 336.24 (0.0036) HEX 387.45 (0.0805) 428.28 (0.9943) 359.81 (0.0091) 363.73 (1.2034) 385.77 (0.3516) 335.42 (0.0047) DCM 390.62 (0.0824) 425.93 (1.0420) 355.59 (0.0019) 359.93 (0.0035) 386.14 (0.3867) 333.82 (0.0042)

S5/ GF 363.30 (0.0030) 379.48 (0.3599) 348.08 (0.0576) 351.94 (0.0000) 366.24 (0.0036) 331.50 (0.0106) HEX 357.53 (0.0019) 387.50 (0.3168) 352.19 (0.0871) 352.60 (0.0132) 363.81 (0.0019) 333.12 (0.0093) DCM 347.75 (0.0011) 386.25 (0.2893) 353.55 (0.0873) 355.39 (0.0073) 358.64 (0.0014) 332.50 (0.0070)

S6/ GF 353.08 (0.0000) 366.09 (0.0011) 340.58 (0.0001) 344.55 (0.0904) 330.37 (0.1714) 326.12 (0.0024) HEX 349.62 (0.000) 359.30 (0.0019) 341.08 (0.7601) 348.51 (0.0000) 337.51 (0.1525) 325.86 (0.0084) DCM 345.36 (0.000) 348.67 (0.0140) 341.93 (0.8413) 344.29 (0.0000) 340.43 (0.1393) 324.26 (0.0137)

S7/ GF 328.55 (0.0042) 341.20 (0.3038) 338.86 (0.7972) 332.76 (0.0002) 329.55 (0.0020) 316.14 (0.0254) HEX 327.53 (0.2138) 343.62 (0.2086) 337.51 (0.0002) 328.03 (0.0003) 327.69 (0.0295) 317.30 (0.0728) DCM 328.24 (0.2473) 345.22 (0.0671) 334.46 (0.0576) 320.67 (0.0005) 329.14 (0.0304) 317.06 (0.0478)

S8/ GF 325.31 (0.2137) 337.80 (0.2714) 332.68 (0.0011) 316.03 (0.0455) 325.69 (0.0179) 314.78 (0.0846) HEX 323.96 (0.0407) 340.28 (0.3984) 333.38 (0.1006) 316.57 (0.0514) 326.00 (0.0593) 316.45 (0.2288) DCM 326.66 (0.0109) 340.00 (0.4875) 331.54 (0.0008) 316.82 (0.0472) 327.87 (0.0715) 316.54 (0.3174)

S9/ GF 323.80 (0.0148) 333.19 90.0197) 329.46 (0.0579) 312.54 (0.0937) 320.40 (0.0959) 313.86 (0.0389) HEX 323.87 (0.0261) 335.37 (0.0206) 326.81 (0.0006) 312.46 (0.0959) 325.77 (0.0210) 315.67 (0.0635) DCM 323.27 (0.0227) 334.29 (0.0223) 324.10 (0.0159) 310.28 (0.0754) 326.89 (0.0422) 315.51 (0.0587)

S10/ GF 320.54 (0.0451) 329.08 (0.0077) 325.29 (0.0226) 307.12 (0.2028) 316.98 (0.0397) 312.83 (0.3210) HEX 321.84 (0.0094) 330.06 (0.0019) 325.05 (0.0053) 309.62 (0.2741) 323.17 (0.0358) 313.80 (0.2490) DCM 321.59 (0.0496) 329.08 (0.0073) 324.07 (0.0212) 309.04 (0.2891) 319.41 (0.0061) 311.80 (0.1729)

158

Table A4.2. Calculated TD CAM-B3LYP/6-31G* vertical transition energies (nm). The solvent environments are simulated using the PCM model.

Tv 5.1 5.2 5.3 5.4 5.5 5.6

S1/ GF 352.86 (2.9349) 381.42 (2.9438) 354.44 (3.3830) 344.24 (1.7922) 384.29 (1.9686) 362.26 (1.0515) HEX 359.08 (3.1613) 389.81 (3.0940) 359.92 (3.5666) 352.01 (2.0001) 395.41 (2.1424) 366.25 (1.2128) DCM 359.21 (3.1988) 389.10 (3.1418) 360.53 (3.5943) 353.40 (2.0068) 396.43 (2.1628) 366.28 (1.2292)

S2/ GF 322.05 (0.0001) 329.67 (0.0765) 316.42 (0.0339) 321.63 (0.0001) 315.66 (0.0539) 289.22 (0.0043) HEX 320.02 (0.0001) 337.67 (0.1931) 323.36 (0.0550) 319.61 (0.0001) 322.11 (0.0423) 289.50 (0.0055) DCM 317.27 (0.0001) 337.78 (0.2575) 324.64 (0.0583) 316.90 (0.0001) 322.63 (0.0349) 289.47 (0.0054)

S3/ GF 310.13 (0.0194) 318.47 (0.0551) 302.58 (0.0712) 300.69 (0.0331) 313.33 (0.0656) 281.77 (0.0312) HEX 315.55 (0.0290) 321.35 (0.0702) 304.72 (0.0932) 302.87 (0.0445) 317.22 (0.0428) 280.94 (0.0350) DCM 316.80 (0.0219) 322.46 (0.0777) 306.41 (0.0970) 304.57 (0.0457) 318.16 (0.0431) 279.28 (0.0416)

S4/ GF 305.02 (0.1060) 312.68 (0.5743) 278.80 (0.0328) 288.31 (0.1069) 298.05 (0.4187) 276.13 (0.0342) HEX 309.77 (0.1316) 313.42 (0.5374) 282.01 (0.0194) 293.61 (0.0857) 301.07 (0.4861) 277.94 (0.0352) DCM 311.47 (0.1433) 309.53 (0.4689) 282.49 (0.0180) 294.22 (0.0834) 300.92 (0.4995) 277.69 (0.0277)

S5/ GF 278.68 (0.0227) 280.64 (0.0461) 278.80 (0.0083) 274.73 (0.0129) 276.98 (0.0484) 271.36 (0.0385) HEX 279.13 (0.0337) 285.65 (0.0379) 279.18 (0.0141) 274.65 (0.0373) 277.26 (0.0596) 274.07 (0.0386) DCM 278.99 (0.0336) 285.88 (0.0287) 278.99 (0.0231) 273.78 (0.0644) 276.89 (0.0269) 274.15 (0.0359)

S6/ GF 268.07 (0.2311) 278.56 (0.0226) 277.91 (0.0174) 268.80 (0.1998) 271.27 (0.0131) 270.63 (0.4219) HEX 271.36 (0.1325) 279.04 (0.0379) 279.17 (0.0468) 271.30 (0.2590) 274.13 (0.0217) 273.46 (0.5878) DCM 271.60 (0.1208) 278.95 (0.0336) 278.97 (0.0388) 271.32 (0.2510) 276.80 (0.0726) 273.62 (0.6037)

S7/ GF 267.85 (0.1948) 271.26 (0.0226) 268.65 (0.2081) 263.31 (0.1868) 268.65 (0.2047) 266.34 (0.0887) HEX 269.81 (0.2791) 274.15 (0.0195) 270.44 (0.2219) 263.46 (0.2325) 271.28 (0.2642) 266.91 (0.0446) DCM 268.85 (0.2834) 276.98 (0.0255) 269.34 (0.2399) 261.88 (0.2052) 271.53 (0.2584) 266.59 (0.0394)

S8/ GF 266.09 (0.0548) 268.13 (0.0096) 268.64 (0.2583) 260.01 (0.0731) 263.18 (0.1054) 264.11 (0.0250) HEX 267.30 (0.1250) 270.41 (0.0195) 270.43 (0.3625) 260.26 (0.1866) 265.60 (0.0617) 264.90 (0.2604) DCM 267.26 (0.1338) 271.30 (0.0242) 269.33 (0.2399) 261.27 (0.2402) 265.79 (0.0403) 263.65 (0.2708) S9/ GF 261.43 (0.1765) 267.57 (0.2387) 263.26 (0.1902) 257.34 (0.1555) 262.76 (0.0373) 263.95 (0.1625) HEX 260.96 (0.2242) 269.52 (0.2916) 262.69 (0.2290) 258.59 (0.0769) 263.66 (0.0680) 263.95 (0.0132) DCM 258.15 (0.2207) 268.76 (0.2877) 259.55 (0.2308) 256.81 (0.0033) 262.79 (0.0290) 262.92 (0.0092)

S10/ GF 255.24 (0.0203) 263.77 (0.0881) 259.21 (0.0026) 254.29 (0.0163) 260.03 (0.0110) 247.09 (0.0119) HEX 255.48 (0.0236) 265.28 (0.0866) 256.79 (0.0057) 255.59 (0.0235) 260.80 (0.0979) 246.85 (0.0558) DCM 256.90 (0.0343) 264.22 (0.0805) 253.66 (0.0088) 256.65 (0.0912) 260.59 (0.0993) 246.60 (0.0664)

PART II

159

CHAPTER 6. FLUORESCENT ORGANIC NANOPARTICLES; FUTURE MICRO-

OPTOELECTRONIC DEVICE MATERIALS

6.1. Introduction

A nanoparticle is defined as a small object, ranging in size from about 10 nm to a few

hundred nanometers that behaves as a single whole unit in terms of its transport and properties.1

There is scientific interest in nanoparticles owing to their special properties, which lie between the properties of molecules and those of bulk material. Although nanoparticles are generally considered as an invention of modern science, actually, they have a long history. Material, now recognized as nanoparticles, were used by artisans as far back as the 9th century in Mesopotamia

for generating a glittering effect on the surface of pots.2 Nanoparticle research is now an area of

intense scientific investigation, and they appear to have a variety of potential applications in

different fields, including biomedical, optical, electronic, and other fields. Light emissive

nanoparticles have preferred applications over non-emissive ones in various fields in which they

can be traced by the emitted light.

The importance of nanoparticles is obvious: in the form of protein complexes and other

cell components, and as colloidal particles in drinking water, they affect human health. The

intense scientific work with nanoparticles has produced a variety of potential applications not only in biomedical, optical, electronic, and other fields mentioned above, but also in industrial applications where they play important roles in the formulation of pigments and production of catalysts. Numerous attempts are being made to deliver nanoparticulate forms of pharmaceutically active compounds, and nanoparticles find use as quantum dots with their special properties useful in electronic components.

160

The study of organic nanoparticles potentially bridge our understanding of the evolution from single molecule to bulk materials.1 The electronic properties of organic nanoparticles differ fundamentally from those of inorganic ones because of weak intermolecular forces accompanying interactions of van der Waals type or hydrogen bonding.3 Organic nanocrystals allow much increased variability and flexibility in material synthesis, nanoparticle preparation, and investigation of their physicochemical properties such as luminescence.4 They are thus expected to serve as novel functional materials in electronics and photonics.5 From a fundamental point of view, organic nanoparticles are fascinating because their optical properties in absorption and emission depend on size. With the advent of modern instruments such as single molecule spectroscopy, scanning electron microscopy, fluorescence spectroscopy, confocal microscopy, tunneling electron microscopy, fluorescence excitation, and others, the study and characterization of nanoparticles has become possible. Inorganic nanoparticles have been investigated for various potential applications including fluorescent biological labels,6 photovoltaic cells,7 light-emitting diodes,8 and optical sensors.9 In recent years, FONs have received increasing attention and are expected to play various roles in a wide variety of applications such as optoelectronic devices due to the flexibility of synthetic approaches to such organic small compounds.10-14 The switching of emission properties of FONs is often size- dependent and related to the effects of intramolecular planarization or specific intermolecular aggregation conformation. Presently, investigations on the FONs are only at their initial stages.

6.2. Preparation of Nanoparticles

Several methods have been reported for the preparation of nanoparticles (Figure 6.1).

Laser ablation, milling, and reprecipitation are the main processes for nanoparticle formation.

161

The reprecipitation method is most used for the formation of organic nanoparticles.13,15-17 since it is the most suitable and economical.

Molecular Dissolved solution educts

Precipitation Chemical reaction

Nanoparticles

Milling Surfactants Polymers

Coarse suspension

Figure 6.1. Methods of preparation of nanoparticles.

6.2.1. Reprecipitation Method

Reprecipitation is done by changing temperature or by changing the composition of solvents. In this method, the targeted compound is dissolved in a superior organic solvent like

THF or acetone, and then an inferior solvent like water is added into the solution with constant and vigorous stirring. The free molecules in the solution start to aggregate following the addition of a certain quantity of the inferior solvent.18,19 Initially, spherical particles are formed as the superior solvent is replaced by inferior solvent and dispersed in bulk inferior solvent. The reason for this spherical shape is the minimization of interfacial energies between the chromophoric compound and the inferior solvent. The reprecipitation method is also called the ouzo effect. The ouzo effect enables one to create a dispersion of small droplets in a surrounding liquid phase without the use of surfactant or dispersing agents. Generally, as the inferior solvent fraction increases, the molecules aggregate into larger structures. The change in particle size with the

162

change in percentage of THF in water for 1,4-di[(E)-2-phenyl-1-propenyl]-benzene (PPB)18 is illustrated in Figure 6.2. These structures may be spherical or non spherical because of the competition between anisotropic growth and spherical aggregation with time.

Figure 6.2. SEM images of PPB nanocrystals showing the size as average diameters (a) 50 nm, (b) 150 nm, and (c) 300 nm obtained from suspensions of nanoparticles obtained for 65%, 70%, and 75% volume fractions of water added to THF, respectively.

6.3. Fluorescent Organic Nanoparticles

The fluorescence of nanoparticles has engendered thriving interest because it is of

fundamental and technological importance in contemporary nano-science and nano-technology.

Though several FONs have been investigated over the years, they are only in the early developmental stage. FONs prepared from conventional fluorophores show rather diverse absorption and fluorescence properties, and comprehensive investigations have substantiated their reliability for various applications.

Systematic research in the field of organic fluorescent nanoparticles was initiated by a

Japanese scientist Hachiro Nakanishi two decades ago.20-22 He used perylene and phthalocyanine

to demonstrate the formation of FONs, and that they exhibited size-dependent fluorescent

properties.21 Subsequently, different types of organic compounds have been introduced to make

FONs. Inspired by the research on the preparation and optical properties of FONs initiated by

Nakanishi and co-workers,20-23 there has been a growing interest in the fundamental optical

163

properties of FONs. It has been demonstrated that FONs prepared from conventional organic

fluorophores show very different size-dependent absorption and fluorescent properties from

those of bulk.1,15,24 Organic fluorophores often quench their own emission by aggregate formation in the solid state due to the intermolecular interactions, which is the main drawback of

FONs. This quenching is because intermolecular vibronic interactions reduce the radiative

deactivation processes leading to the quenching of emission. In other words, the reason for the

low quantum yield in solution of some of these molecules is that the conjugated backbone of the

molecules is significantly twisted by steric hindrance and the radiative decay pathway of the

resulting twisted chromophores is generally suppressed. Therefore, planarization of the twisted

conjugated backbone upon aggregation in the solid state enhances the emissive property of the

molecules. This unusual fluorescence found in nanoparticles is correlated with aggregation

morphology and explained by the molecular exciton model.25-28 It is because aggregation

strongly depends on the molecular geometry.

Recently, a few papers have been published reporting that conjugated organic small

molecules showed higher emission quantum yield in the solid state than in solution. In other

words, self-quenching does not exist in these unique molecules. It has opened a new possibility

to develop organic nanoparticles with enhanced emission for various applications. For example,

three new classes of FONs prepared from silole,10 cyano-stilbene,29,30 and salicylidene-aniline31

compounds that overcome the aggregation quenching problem in the nanoparticles have been

reported. Tang et al.10 synthesized a silole derivative, 1-methyl 1,2,3,4,5-pentaphenylsilole, and prepared a fluorescent nanoparticle suspension using the simple reprecipitation method, using water as a non-solvent in ethanol. The silole compound showed negligible photoluminescence in

dilute ethanol solution, but its nanoparticles showed an extremely high photoluminescence by

164 increasing the intensity by 333 times, which is in contradiction to the common belief of

“aggregation quenches emission”.

This unusual behavior showed by the siloles FONs is attributed to the effect of intramolecular planarization induced by the molecular aggregation process. It is assumed that the molecule has a twisted conformation in the dilute solution of a good solvent. When the molecules get closer and form aggregates, however, coplanarization of chromophores can be induced by strong intermolecular forces, which lead to a better conjugation between its core and sides, thus intensifying and red shifting its absorption and emission. This striking feature of these

FONs is aggregation induced enhanced emission (AIEE) and later proved as a general feature for the siloles and other compounds. Park et al.30 reported strongly fluorescent organic nanoparticles

(FONs) derived from 1-cyano trans-1,2-bis-(4’-methylbiphenyl)ethylene (CN-MBE). They observed the intensity has been increased by 700 times during the formation of nanoparticles for

CN-MBE compound. Usually, compounds have higher fluorescence in solution and become lower in solid because of planar and conjugated conformation is favored in solid state, and the molecule is possibly twisted in solution. It is also speculated that the bulky and polar cyano groups in CN-MBE play an important role in restricting the parallel intermolecular interaction i.e. J-aggregation.19 Figure 6.3 shows some typical organic molecules capable of forming FONs which can have potential applications.

165

CN H3C H3C N

- - Py CN MBE CH3 PPB CN t-Bu H3C O CH3 S CN-MBE NH O N HO HN

DHBIA

HO S Si N CH3 O H N N H O 1-methyl-1,2,3,4,5-pentaphenylsilole p-BSP

Figure 6.3. Structure of some compounds used for FONs.

6.3.1. Molecular Exciton Model

When two molecules come close to each other, their transition dipole moments will start to interact when induced by light. When two dipoles are brought to close proximity side by side, their relative orientation causes two phenomena: (a) The energy of individual molecule E will split into two new states with the energies E1 and E2 (Figure 6.4). Parallel dipoles repel each other and acquire the higher energy state, while the antiparallel dipoles attract, which lowers the energy of that state and, (b) parallel dipoles make up an overall higher dipole moment, i.e., stronger absorption, while antiparallel dipoles cancel each other to make a weak absorption. The net result is a blue shift of absorption. Similarly for head-to-tail arrangement, the lower energy

state has larger transition dipole moment and the net effect is red shift of absorption. (The

intermediate, oblique orientation results in band splitting-two bands appearing at lower and

higher energy). This parallel aggregation is named as H-aggregation and head-to-tail aggregation is named as J-aggregation.19,29

166

E2 E2

E E

E1 E1

E0 E0 monomer dimer monomer dimer

Parallel aggregation Head-to-tail aggregation

Figure 6.4. Band splitting of parallel vs antiparallel aggregation.

The nature of the molecules such as substituents, their twisted nature, polarity, and also

other factors all play important roles in aggregation morphology. Changes in fluorescence also

have a direct correlation to the aggregation morphology. H-aggregation induces a nonradiative

deactivation process that quenches the fluorescence while J-aggregation enhances the

fluorescence due to a lack of strong intermolecular interactions.

6.4. FONs: Importance and Applications

Though inorganic and hybrid nanoparticles have been widely used, organic nanoparticles have also emerged as competitive and is now an area of intense research among organic researchers. It is considered that studies on organic nanoparticles are most important to bridge the structural gap of the better understood bulk materials and single molecules. Macroscopic properties of organic matter can be interpreted in terms of a microscopic molecular picture.

However understanding of molecular association, aggregation, and assembling requires more knowledge of the self-assembling and formation of stratified structures, which can explain new properties. This is scientifically new and interesting, and is the primary reason why organic

167 nanoparticles are receiving much attention at present. Emissive nanoparticles have preferred applications over non-emissive nanoparticles in various fields because they can be traced by the emitted light.

With their unique optical behavior, FONs are potentially utilized in optoelectronic devices, clinical devices, and sensors. For example, upon the formation of nanoparticles, the

FONs of arylethynyl derivatives and pentaphenylsilole10 give enhanced fluorescence, rather than the generally observed fluorescence quenching in the solid state of planar π-conjugated compounds.32 Lin et al.33 developed optimized conditions for the aggregation-induced enhanced emission properties of phenothiazine derivatives. Their related nanoparticles were observed in vitro. Most importantly, the bright fluorescent spots in lysosomes of cancer cells, which were formed from the FONs can be used in recognizing cancer cells due to their multiple fluorescent characteristics. Nakanishi et al. prepared microcrystals of perylene and polydiacetylene by the simple reprecipitation method and found that polydiacetylene microcrystals were a new type of material for third order non-linear optics. Because of their strong fluorescence properties, pyrazolines have been widely used as optical brightening agents for textiles and papers, and as a hole-conveying medium in photoconductive materials and electroluminescent (EL) devices. The

1-phenyl-3-((p-dimethylamino)-styryl)-5-((p-dimethylamino)phenyl)-2-pyrazoline (PDDP) nanoparticles have been proved useful in optoelectronic device applications, such as optical modualtors controlling the wavelength in organic photoconductors or electroluminescent materials.

Also, it has recently been reported that FONs are of potential use to detect macromolecules,34 heavy metal ions,35 anions in aqueous solutions,36 and so on. Recently Qu et al.37 prepared a thiourea type of fluorescent organic nanoparticles for use as fluorescent and

168 colorimetric probes for Ag+ ions and as a novel ratiometric fluorescent for Hg++ probe in aqueous media.

If organic small molecules can be made into nanoparticles, these nanoparticles will be capable of being directly used as fluorescent nanoparticle probes without modification.

Compared with small organic molecules, the organic nanoparticles simultaneously provide efficient fluorescence, a great reduction in photobleaching and colloidal stability in a variety of bioenvironments.38 The nanoparticles also usually have a longer fluorescence lifetime than organic small molecules from which they are derived.

6.5. References

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(3) Silinsh, E. A. Organic Molecular Crystals: Their Electronic States; Springer-Verlag: Berlin,

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(4) Yoshikawa, H.; Masuhara, H. J. Photochem. Photobiol. C 2000, 1, 57-58.

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Kasai, H.; Okada, S.; Oikawa, H.; Nakanishi, H. J. Phys. Chem. B 1999, 103, 11050-11056.

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Y.; Zhu, D.; Tang, B. Z. Chem. Commun. 2001, 1740-1741.

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(12) Sun, Y.-Y.; Liao, J.-H.; Fang, J.-M.; Chou, P.-T.; Shen, C.-H.; Hsu, C.-W.; Chen, L.-C.

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(13) Palayangoda, S. S.; Cai, X.; Adhikari, R. M.; Neckers, D. C. Org, Lett, 2008, 10, 281-284.

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(15) Fu, H.-B.; Yao, J.-N. J. Am. Chem. Soc. 2001, 123, 1434-1439.

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(17) Qian, Y.; Li, S.; Zhang, G.; Wang, Q. Wang, S.; Xu, H.; Li, C.; Li, Y.; Yang, G. J. Phys.

Chem. B 2007, 111, 5861-5868.

(18) Bhongale, C. J.; Chang, C.-W.; Lee, C.-S.; Diau, E. W.-G.; Hsu, C.-S. J. Phys. Chem. B

2005, 109, 13472-13482.

(19) Li, S.; He, L.; Xiong, F.; Li, Y.; Yang, G. J. Phys. Chem. B 2004, 108, 10887-10892.

(20) Kasai, H.; kamatani, H.; Okada, S.; Oikawa, H.; Matsuda, H.; Nakanishi, H. Jpn. J. Appl.

Phys. 1996, 35, L221-L223.

(21) Kasai, H.; Kamatani, H.; Yoshikawa, Y.; Okada, S.; Oikawa, H.; Watanabe, A.; Ito, O.;

Nakanishi, H. Chem. Lett. 1997, 1181-1182.

(22) Kasai, H.; Yoshikawa, Y.; Seko, T.; Okada, S.; Oikawa, H.; Mistuda, H.; Watanabe, A.;

Ito, O.; Toyotama, H.; Nakanishi, H. Mol. Cryst. Liq. Cryst. 1997, 294, 173-176.

(23) Komai, Y.; Kasai, H.; Hirakoso, H.; Hakuta, Y.; Okada, S.; Oikawa, H.; Adschiri, T.;

Inomata, H.; Arai, K.; Nakanishi, H. Mol. Cryst. Liq. Cryst. 1998, 322, 167-172.

(24) Gong, X. C.; Milic, T.; Xu, C.; Batteas, J. D.; Drain, C. M. J. Am. Chem. Soc. 2002, 124,

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(25) Levitus, M.; Schmieder, K.; Ricks, H.; Shimizu, K. D.; Bunz, U. H. F.; Garicia-Garibay, M.

A. J. Am. Chem. Soc. 2001, 123, 4259-4265.

(26) Oelkrug, D.; Tompert, A.; Gierschner, J.; Egelhaaf, H.-J.; Hanack, M.; Steinhuber, E. J.

Phys. Chem. B 1998, 102, 1902-1907.

(27) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371-392.

(28) Dähne, L.; Biller, E. Adv. Mater. 1998, 10, 241-245.

(29) An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410-

14415.

(30) Oelkrug, D.; Tompert, A.; Egelhaaf, H.; Hanack, M.; Steinhuber, E.; Hohloch, M.; Meier,

H.; Stalmach, U. Synth. Met. 1996, 83, 331-237.

(31) An, B.-K.; Lee, D.-S.; Lee, J.-S.; Park, Y.-S.; Song, H.-S.; Park, S. Y. J. Am. Chem. Soc.

2004, 126, 10232-10233.

(32) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: London,1970.

(33) Lin, H.-H.; Su, S.-Y.; Chang, C.-C. Org. Biomol. Chem. 2009, 7, 2036-2040.

(34) Wang, L.Y.; Dong, L.; Wang, L. Anal. Lett. 2004, 37, 1811-1822.

(35) Wang, L.; Bian, G. R.; Dong, L.; Xia, T. T.; Chen. H. Q. Spectrochim. Acta A: Mol.

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171

CHAPTER 7. VISIBLE AND NEAR IR EMITTING ORGANIC NANOPARTICLES OF

AROMATIC FUMARONITRILE CORE-BASED DONOR-ACCEPTOR COMPOUNDS

7.1. Introduction

In the recent past, several fluorescent inorganic and composite nanoparticles have been prepared and investigated for various applications such as in light emitting devices,1,2 in

photovoltaics,3 as biological nanosensors4 etc. For these applications, major efforts to generate

inorganic semiconductors and metal nanoparticles have met with considerable success, but

relatively few approaches have been reported using organic nanostructures.5 Recently, several

articles report that fluorescent organic nanoparticles (FONs) show size dependent absorption and

emission characteristics.6,7 Systematic research on FONs has been initiated by Nakanishi and

coworkers.8-10 Upon formation of nanoparticles, arylethynyl compounds,11 pentaphenylsilole,12

and 1-cyano-trans-1,2-bis-(4’-methylbiphenyl)ethylene6 give enhanced fluorescence, compared to the generally observed fluorescence quenching in the solid state.13 The emission enhancement

in nanoparticles is probably because of the effects of intramolecular planarization and a specific

intermolecular aggregation in the solid state.

There is considerable interest in the development of materials and devices that exhibit

wavelength tunability and efficient light emission in the near IR (NIR) wavelength region.14 The

first report of NIR emission from an organic light-emitting device (OLED) based on a lanthanide

complex15 has lead to the pursuit of applications in the defense, telecommunication, and

biomedical imaging fields.16 Light emitting nano-materials, if properly emissive, could provide a

source of white light at a greatly reduced cost for application in full color electronic displays.

Organic NIR emitting particles have the advantage of low cost, flexible synthesis, and lowered

toxicity, and their availability should stimulate new applications in various fields. In this article,

172

we have reported the first preparation of NIR emitting FONs, and studied their optical properties

to assess their potential applications. We herein report two different sub-classes of compounds

that form FONs- one class of compounds that retains optical purity upon the formation of FONs

accompanied by increased fluorescence intensity and the other class of compounds that show a

large red shift of emission into the near IR but diminished fluorescence intensity upon the

formation of FONs. We have chosen to study electron donor acceptor compounds containing an

aromatic fumaronitrile core because fumaronitrile core-based compounds have shown useful

emission properties in electroluminescent (EL) devices.17,18 Palayangoda et al.19 have reported

that aromatic fumaronitrile core-based carbazole compounds form fluorescent nanoparticles. We

have reported20 the synthesis and photophysics of the aromatic fumaronitrile core-based electron donor-acceptor compounds 2,3-bis(4-(1H-indol-1-yl)phenyl)fumaronitrile (2.1), 2,3-bis(4-(2-

phenyl-1H-indol-1-yl)phenyl)fumaronitrile (2.2), 2,3-bis(4-(diphenylamino)phenyl)fumaronitrile

(2.3), 2,3-bis(4-(2-(4-(diphenylamino)phenyl)ethynyl)phenyl)fumaronitrile (2.4), and 2,3-bis(4-

(2-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)ethynyl) phenyl)fumaronitrile (2.5) and

discussed details of their photophysical properties in chapter 2. Herein, we describe the

formation and properties of fluorescent nanoparticles formed from compounds 2.1-2.5 (Figure

7.1).

R 2.1 R = N N

. = 2 2 R 2. 4 R = CN N - u NC t B

N 2. 3 R = R N 2. 5 R = t-Bu

Figure 7.1. Structures of compounds 2.1-2.5.

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Nanoparticles of compounds 2.1-2.5 were prepared by a simple reprecipitation

technique.19 Identical concentrations of solutions (1x10-5 M) were prepared for all THF/water

mixtures (1:4, 1:8, 1:12, and 1:16). The suspension of nanoparticles formed was visibly

transparent and stable at ambient temperature. Spectral properties of all samples were unchanged

after three months.

7.2. Experimental

7.2.1. Fluorescence Lifetime (τF) Measurement. The compounds in THF and THF/water in the

ratio of 1:4, 1:8, 1:12, and 1:16 were put in quartz cuvettes. Fluorescence decay profiles of the

argon-degassed samples were measured using a single photon counting spectrofluorimeter.

Decays were monitored at the corresponding emission maximum of the samples. In-built

software allowed the fitting of the decay spectra (χ = 1-1.5) and produced the fluorescence lifetimes.

7.2.2. Preparation of Nanoparticles. Samples of predetermined concentrations of 2.1-2.5 in

THF were made. The appropriate volumes of these solutions were placed in different vials and the required amount of water was rapidly injected into those vials while maintaining the final concentrations of the solutions the same. Four different THF/water ratios, i.e., 1:4, 1:8, 1:12, and

1:16 for each compound were made. In all samples the formation of nanoparticles could be observed by irradiation using 365 nm UV lamp. In some samples formation of nanoparticles could be seen by the naked eyes because of change of color of the transparent solution.

7.2.3. SEM Images of Nanoparticles. Scanning Electron Microscopy (SEM) images were recorded on a FEI-FP2031/11 microscope at 15 eV using INCA Penta FETX3 detector. Samples for SEM were prepared by placing a few drops of nanoparticle suspension onto a glass with

174

cover slip placed on an aluminum stud. The samples were allowed to dry at room temperature

before observing them under the scanning electron microscope. To enhance the contrast and

quality of the SEM images, the samples were sputter-coated with gold/palladium.

7.2.4. Fluorescent Microscopy (FM) Images of Nanoparticles. Fluorescent Microscopy

images were recorded on a Q-imaging Epi Fluorescence Microscope using QICAM camera. The

data were collected using Metaphor software in computer. The samples were prepared by putting

a droplet of nanoparticles suspension on a glass slide and covering with a cover slip.

7.2.5. Confocal Microscopy Images of the Nanoparticles. Confocal microscopy images were

obtained using an Olympus FluoView 1000 microscope. The same method employed to prepare

the samples for FM images was used to prepare the samples for confocal microscopy images.

The excitation source was a diode blue laser (405 nm), and fluorescence was detected using

standard three confocal channels (three photomultiplier detectors). The emission filter (BA535-

565) at 405 nm laser light for green emission and the emission filter (BA 650 IF) at 488 nm laser

light were used for the image recording.

7.3. Results and Discussion

UV-visible spectra of 2.1 in THF and as nanoparticles suspended in solvents differing in

the THF/water ratio are displayed in Figure 7.2. Compound 2.1 shows two distinct peaks at 300

nm and 410 nm in THF. With the addition of water the absorption transition associated with the

indole moiety (300 nm) slightly red-shifts. This may be due to electronic coupling between

neighboring molecules as they approach each other in THF/water mixture because of the

hydrophobic nature of 2.1. 1,3-Diphenyl-5-(2-anthryl)-2-pyrazoline-based nanoparticles exhibit a

21 similar behavior. The peak around 410 nm, which can be assigned to the transition from So to

intramolecular charge transfer (ICT) state, shifted to red. As the ratio of water is further

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increased, a new absorption band (470 nm) appears. This is assigned to a transition from So to an

ICT state22 that forms as the molecules of 2.1 are assembled even more closely and begin nucleating into nanoparticles. The comparative intensity at 470 nm increases with the fraction of water. It is likely that intramolecular interaction originates from overlapping of the indole moiety and the nitrile group of the neighboring molecules (vide infra), which further increases as the

nucleation progresses.23-26 The molecular overlap is also strengthened by an increase in the

molecular dipole and the ICT state becomes more prominent. These result in the red-shifted

absorption (vide infra).21,22,27 Additionally, Mie scattering may also be responsible for the red

shift in the absorption transition.6

There are significant differences between the fluorescence spectrum of 2.1 in THF and

the spectra of 2.1 nanoparticles formed in the THF/water medium (Figure 7.3). The emission of

2.1 in THF around 550 nm is due to the radiative deactivation of ICT excited state to ground state. On increasing the fraction of water the emission intensity is enhanced and is accompanied by a slight red shift. The ICT state of the 2.1 nanoparticles is responsible for the broad emission

(500 nm to 700 nm) that appears at the higher water ratio. It is likely that an n-electron from nitrogen of the indole moiety transfers to the nitrile moiety of the same molecule, resulting in the

ICT state. This leads to an increase in the dipole moment of 2.1 in the nanoparticle state, which in turn enhances the intermolecular interaction. A similar charge transfer phenomenon was observed in 1-phenyl-3-((dimethylamino)styryl)-5-((dimethylamino)phenyl)-2-pyrazoline-based nanoparticles.5 The size of nanoparticles increases with an increase in the fraction of water.7 This

allows the intermolecular electronic interactions to extend over a large number of molecules and

to increase in magnitude. This is likely the reason for the significantly red-shifted emission of the

2.1 nanoparticles compared to that from the individual molecules of compound 2.1. A red-shifted

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emission is expected for an aggregate state20,21,28,29 arising from the extended orbital overlap of

closely stacked molecules in nanoparticles.17 According to the molecular exciton model, head-to-

tail alignment of transition dipole (J-aggregation) shifts emission to the red region and enhances

fluorescence. The main reason for an enhanced emission in J- aggregation is the planarization of

the molecule in nanoparticles.22 Similar results were observed for compound 2.3 in THF and as

nanoparticle suspensions. Compounds 2.1 and 2.3 have the same linker and are planar.

The red-shifted absorption of 2.1 and 2.3-2.5 in all THF/water ratios indicates J-type of aggregation. Compound 2.2 does not show any effect on continued addition of water to the THF solution other than slight intensity change in absorption and emission. It does not have a sufficiently strong electron donor (ED) to form ICT. In compounds 2.1 and 2.3, the emission intensity of nanoparticles in suspension is stronger than that in THF (Figure 7.3 and Appendix

5). For compounds 2.4 and 2.5, significant red shift of absorption and emission has been observed for nanoparticles compared to their THF solutions (Figures 7.4 and 7.5, Table 7.1, and

Appendix 5). In THF solution, 2.4 forms ICT and 2.5 forms TICT both in ground state and excited state as THF stabilizes the CT.20 But when they form nanoparticles on addition of water

they are pushed out of solvent cage. Compound 2.4 forms ICT in nanoparticle form as it is more

planar but compound 2.5, a non-planar structure,20 forms intermolecular charge transfer state.

Both 2.4 and 2.5 show diminished emission intensity as nanoparticles in THF/water suspension compared to that of THF solution. The main reason for an enhanced emission in J-aggregation is the planarization of the molecule in nanoparticles. It is likely that the 2.4 and 2.5 molecules undergo J-aggregation facilitating inter- or intra-molecular charge transfer, but retains the non- planar or twisted structure.

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0.9 THF 0.8 THF/H2O_1:4 0.7 THF/H2O_1:8 THF/H O_1:12 0.6 2 THF/H O_1:16 0.5 2 0.4 0.3 Absorbance 0.2 0.1 0.0 300 350 400 450 500 550 600 650

Wavelength (nm)

Figure 7.2. Absorption spectra of 2.1 nanoparticles solution (1x10-5 M) recorded at different THF and THF/H2O mixtures.

700000 7 THF THF THF/H O_1:4 600000 1.2x10 2 THF/H O_1:8 500000 2 400000 THF/H O_1:12 7 2 300000 THF/H O_1:16 1.0x10 2 200000 100000 6 0 8.0x10 500 600 700

6.0x106 Intensity 4.0x106 2.0x106 0.0 450 500 550 600 650 700 750

Wavelength (nm)

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Nanoparticle suspensions of 2.4 emit in the near IR region with a huge Stokes shift. This

is the first time to our knowledge where NIR emission from organic nanoparticles has been

reported. Since compound 2.4 is planar, intramolecular electron transfer from electron donor to

electron acceptor is significant even in the nanoparticle forms. Again, in the nanoparticle form

intermolecular electronic interactions extends over a large number of molecules and also

increases in magnitude arising from the extended orbital overlap of closely stacked molecules in

nanoparticles.

1.0 THF THF/H O_1:4 0.8 2 THF/H2O_1:8 THF/H2O_1:12 0.6 THF/H2O_1:16

0.4 Absorbance 0.2

0.0 300 400 500 600 Wavelength (nm)

Figure 7.4. Absorption spectra of 2.4 nanoparticles solution (1x10-5 M) recorded at different THF and THF/H2O mixtures.

It has been observed that for compound 2.4 the emission for nanoparticles has been red

shifted by 256 nm (Figure 7.5) and, for compound 2.1 the intensity of emission for nanoparticles

has increased by 19 times. In compounds 2.2 and 2.3 the intensity of emission has been increased

but with little red shift. In compounds 2.3 and 2.4, the formation of nanoparticles can be seen by

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the naked eye as a noticeable change of color due to the large red shift of absorption and

emission. Thus, we were able to maintain the color purity with an increase in fluorescence

intensity in compounds 2.1, 2.2, and 2.3 upon formation of nanoparticles, and in compounds 2.4

and 2.5, with the formation of nanoparticles, the tuning of emission toward the near IR region

from visible region was achieved.

6 4.0x10 THF 6 THF/H O_1:4 3.5x10 2 THF/H2O_1:8 6 THF/H O_1:12 3.0x10 2 THF/H2O_1:16 2.5x106 6

2.0x10 6

Intensity 1.5x10 1.0x106 5.0x105 0.0 450 500 550 600 650 700 750 800 Wavelength (nm)

Figure 7.5. Emission spectra of 2.4 nanoparticles solution (1x10-5 M) recorded at different THF and THF/H2O mixtures. Sample solutions were excited at 440 nm.

Interestingly, the absorption spectra of compound 2.4 are more blue shifted in all solvents compared to the absorptions of compound 2.3, even though the former compound has one more phenylethynyl moiety to increase conjugation length. Likewise, the emission maxima of compound 2.4 are also at higher energies than that of compound 2.3. The two additional phenylethynyl moieties in 2.4 increase its rotational flexibility so that the electrons from the

nitrogen atom of the diphenylamine group cannot be easily transported towards the cyano group.

In nanoparticles of 2.3 and 2.4, the emission of 2.4 is red shifted (up to 256 nm) but in compound

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2.3 there is no red shift in emission of nanoparticles but the intensity has increased 9 fold.

Similar results were observed for compound 2.5 having two more phenylethynyl moieties than

the 1,2-dicyano-trans-1,2-bis-4(3,6-di-tert-butylcarbazolyl)phenylethylene compound that

Palayangoda et al.19 reported.

The size of the nanoparticles was determined using scanning electron microscopy (SEM)

(Figure 7.6 and Appendix 5) and the particle sizes are about 233, 210, 630, 650, and 203 nm for

compounds 2.1-2.5 respectively. There is no remarkable change in absorption, emission, or

particle size with the higher fractions of water in THF. For all compounds, we obtained the best

nanoparticles in terms of emission intensity and in terms of red shift in a 1:4 THF/water solution,

but we obtained SEM images for nanoparticles in 1:8 THF/water solution since the size of

nanoparticles in the more THF rich solutions might have been too small to take images.7 We also scanned the fluorescent microscopy (FM) images of nanoparticles for all compounds for

THF/water 1:8 mixture to judge how fluorescent these nanoparticles were (Figure 7.7, also

Appendix 5). The size distribution of nanoparticles from all compounds was rather large. As reported in some literature,7 we did not find a direct correlation between the fraction of water

with the red shift of emission, though with higher fractions of water we observed red shift on

emission. But there are some reports which did not show the same correlation.19

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a b

Figure 7.6. SEM images of nanoparticles of compound 2.2 (a) and 2.4 (b) in THF/H2O_1:8.

The emission properties of all compounds were further investigated using a confocal microscope (Figure 7.8 and Appendix 5). Green light emission was observed from the nanoparticles of compounds 2.1 and 2.3, and red light emission was observed from the nanoparticles of compound 2.4. This assured us that the red emission is indeed from the nanoparticles since there is no solvent to interfere with the emission. These FONs were stable to a blue laser for hours.

The formation of nanoparticles is clearly associated with the addition of water. Water and

THF are miscible, so the solubility of hydrophobic compounds decreases with the increasing fraction of water, eventually reaching a critical nucleation condition at which nuclei form throughout the solution and begin to grow as particles.30 Once the concentration of solute in

solution falls due to the growing nuclei, particle growth stops so that the equilibrium mixture

contains both particles and dissolved molecules.

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Figure 7.7. Fluorescent Microscopy (FM) image of nanoparticles of compound 2.4 in THF/H2O_1:8.

Figure 7.8. Confocal Microscopy image of nanoparticles of compound 2.4 in THF/H2O_1:8.

We further measured the fluorescence lifetimes (τF) of compounds in dilute THF solutions and as nanoparticles dispersed in different fractions of THF/water (Table 7.1). The excitation source used was nano-LED and was excited using 350 nm nano-LED for compounds

3.2 and 3.3, and 560 nm for compounds 3.1, 3.4, and 3.5. Decays were monitored at the corresponding emission maxima (λmax). All compounds followed monoexponential decay.

Usually the lifetimes of fluorescence are higher for samples with higher fraction of water with some exceptions. The same result has been observed and reported elsewhere.7

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a Table 7.1. Absorption maximum (Amax), emission maximum (λmax), molar extinction coefficient (ε), red shift of emission, and lifetime (τ) of compounds 2.1-2.5 in THF and THF/H2O mixtures.

-1 -1 Compound Ratio of solvents Amax (nm) ε (M cm ) λmax (nm) Redshift(nm) Lifetime (τ) (ns) (emission)

THF/H2O_1:0 410 23174 542 - 2.97

THF/ H2O _1:4 387-472 16222 572 30 3.26

2.1 THF/ H2O _1:8 400-470 14167 572 30 5.34

THF/ H2O _1:12 400-470 10741 572 30 4.83

THF/ H2O _1:16 400-470 9127 572 30 4.69

THF/ H2O _1:0 306 45734 359 - 1.33

2.2 THF/ H2O _1:4 306 871187 363 4 0.18

THF/ H2O _1:8 306 66363 363 4 0.02

THF/ H2O _1:12 306 51371 363 4 0.16

THF/Water_1:16 306 33720 363 4 0.05

THF/ H2O _1:0 471 20749 644 - 0.34

2.3 THF/ H2O _1:4 490 30188 644 00 1.36

THF/ H2O _1:8 490 25254 644 00 1.81

THF/ H2O _1:12 490 26008 644 00 1.73

THF/ H2O _1:16 490 26145 644 00 1.76

THF/ H2O _1:0 436 49357 490 - 5.19

2.4 THF/ H2O _1:4 442 103371 746 256 1.04

THF/ H2O _1:8 450 53651 716 236 9.70

THF/ H2O _1:12 457 29034 693 203 12.69

THF/ H2O _1:16 470 39764 692 202 14.31

THF/ H2O _1:0 345 43749 418 - 1.99

2.5 THF/ H2O _1:4 345 53560 524 106 3.22

THF/ H2O _1:8 345 47687 620 202 3.75

THF/ H2O _1:12 345 40313 609 191 2.85

THF/ H2O _1:16 345 37956 605 186 1.81

aEmissions are measured by exciting at the absorption maximum of each compound.

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The emission of nanoparticles of compound 2.4 could be reversibly shifted yellow or

NIR, through an adjustment in the THF/water ratio. The fluorescence maximum of 2.4 nanoparticles (Figure 7.5), for example, shifted from 410 nm to 590 nm with an increase in water. In contrast, increasing the THF ratio with such samples caused the fluorescence emission to revert to its initial value (410 nm) (see the Appendix 5). Changes in emission color of compounds 2.1 and 2.4 in THF and THF water mixtures on excitation at 365 nm is illustrated in

Figures 7.9 and 7.10 respectively.

Figure 7.9. Solutions of compound 2.1 from left to right in THF and THF/H2O mixtures in the ratio of 1:0, 1:4, 1:8, 1:12, and 1:16. The concentrations of all solutions were 1x10-5 M.

Figure 7.10. Solutions of compound 2.4 from left to right in THF and THF/H2O mixtures in the ratio of 1:0, 1:4, 1:8, 1:12, and 1:16. The concentrations of all solutions were 1x10-5 M.

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7.4. Conclusions

The FONs were prepared from five different compounds and their optical properties were measured using absorption spectroscopy, fluorescence spectroscopy, scanning electron microscopy, fluorescence microscopy, and confocal microscopy. Their optical properties were further assessed by the measurement of lifetimes of fluorescence at various ratios of THF/water maintaining the same concentration. In some compounds we were able to retain the optical purity upon the formation of FONs. Compounds 2.4 and 2.5 showed tunable emission properties upon formation of nanoparticles. This is the first example that NIR emission was achievable upon the formation of nanoparticles from pure organic compounds. The potential application of this property is under active investigation. Stokes shift of as much as 256 nm was achieved using compound 2.4. Emission was enhanced by as much as 19 times in compound 2.1 upon the formation of nanoparticles. Reversible switching of emission with a change in THF/water ratio was illustrated. A possible alignment of molecules that is responsible for the high Stokes shift in nanoparticles was discussed.

7.5. References

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Appendix 5. Absorption and emission spectra, scanning electron, fluorescent, and confocal microscopy images, and solutions in THF and THF/water mixtures of compounds 2.1-2.5.

Absorption and emission spectra of nanoparticles of compounds 2.2, 2.3, and 2.5

1.0 THF 8 THF 1.0x10 THF/H O_1:4 THF/H2O_1:4 2 THF/H O_1:8 THF/H2O_1:8 0.8 2 7 THF/H2O_1:12 THF/H2O_1:12 8.0x10 THF/H2O_1:16 THF/H2O_1:16 0.6 6.0x107

0.4 4.0x107 Absorbance 7 0.2 2.0x10 Emission Intensity/a.u. 0.0 0.0 350 400 450 500 550 300 400 500 600 Wavelength (nm) Wavelength (nm) Figure. A5.1. Absorption spectra of Figure. A5.2. Emission spectra of compound 2.2 in THF and THF/H2O in compound 2.2 in THF and THF/H2O the ratio of 1:4, 1:8, 1:12, and 1:16. in the ratio of 1:4, 1:8, 1:12, and 1:16.

0.7 7 THF 1.8x10 THF THF/H2O_1:4 7 THF/H2O_1:4 0.6 1.6x10 THF/H O_1:8 THF/H2O_1:8 2 7 THF/H O_1:12 THF/H O_1:12 1.4x10 2 0.5 2 THF/H O_1:16 THF/H O_1:16 7 2 2 1.2x10 0.4 7 1.0x10

0.3 8.0x106 6.0x106

Absorbance 0.2 4.0x106 0.1 6 EmissionIntensity/a.u. 2.0x10 0.0 0.0 300 350 400 450 500 550 600 650 700 500 550 600 650 700 750 800 Wavelength (nm) Wavelength (nm) Figure. A5.3. Absorption spectra of Figure. A5.4. Emission spectra of compound 2.3 in THF and THF/H2O in compound 2.3 in THF and THF/H2O in the ratio of 1:4, 1:8, 1:12,and 1:16. the ratio of 1:4, 1:8, 1:12, and 1:16.

189

7 THF 2.0x10 THF 0.5 THF/H O_1:4 7 2 1.8x10 THF/H2O_1:4 7 THF/H O_1:8 THF/H2O_1:8 2 0.4 1.6x10 THF/H O_1:12 THF/H2O_1:12 7 2 THF/H2O_1:16 THF/H2O_1:16 1.4x10 0.3 1.2x107

1.0x10 0.2 8.0x106 Absorbance 6.0x106 0.1 4.0x106 6 0.0 Emission Intensity/a.u. 2.0x10 300 350 400 450 500 550 0.0 Wavelength (nm) 350 400 450 500 550 600 650 Wavelength (nm) Figure. A5.5. Absorption spectra of Figure. A5.6. Emission spectra of compound 2.5 in THF and THF/H2O in the ratio of 1:4, 1:8, 1:12, and 1:16. compound 2.5 in THF and THF/H2O in the ratio of 1:4, 1:8, 1:12, and 1:16.

Fluorescence spectra of nanoparticle solutions of compounds 2.1 and 2.5 recorded during the reverse process of adding THF to a THF/water solutions

7 THF 2.5x107 THF 1.2x10 THF/H O_1:16 THF/H2O_1:16 2 THF/H O_1:12 7 THF/H2O_1:12 7 2 1.0x10 THF/H O_1:8 THF/H2O_1:8 2.0x10 2 THF/H O_1:4 6 THF/H2O_1:4 2 8.0x10 1.5x107

6.0x106 1.0x107 4.0x106 6 6 5.0x10

2.0x10 Emission Intensity/a.u. Emission Intensity/a.u 0.0 0.0 450 500 550 600 650 700 750 400 450 500 550 600 Wavelength (nm) Wavelength (nm) Figure. A5.7. Emission spectra of Figure. A5.8. Emission spectra of compound 2.1 in THF and THF/H2O in compound 2.5 in THF and THF/H2O in the ratio of 1:4, 1:8, 1:12, 1:16 during the ratio of 1:4, 1:8, 1:12, 1:16 during reverse process. reverse process.

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Scanning Electron Microscope (SEM) images of nanoparticles of compounds 2.1, 2.3, and

2.5

Figure. A5.9. SEM image of Figure. A5.10. SEM image Figure. A5.11. SEM compound 2.1 in of compound 2.3 in image of compound 2.5 in THF/H2O_1:8. THF/H2O_1:8. THF/H2O_1:8.

Fluorescence Microscope (FM) images of nanoparticles of compounds 2.1, 2.3, and 2.5

Figure. A5.12. FM image Figure. A5.13. FM image Figure. A5.14. FM image of compound 2.1 in of compound 2.3 in of compound 2.5 in THF/H2O 1:8. THF/H2O 1:8. THF/H2O 1:8.

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Confocal Microscope images of nanoparticles of compounds 2.1 and 2.3

Figure. A5.15. Confocal Microscopy Figure. A5.16. Confocal Microscopy image of compound 2.1 in image of compound 2.3 in THF/H2O_1:8. THF/H2O_1:8.

Solutions of compounds 2.3 and 2.5 in THF and THF/H2O mixtures under UV 365 nm

Figure. A5.17. Solutions of compound 2.3 Figure. A5.18. Solutions of compound 2.5 from left to right in THF and THF/H2O from left to right in THF and THF/H2O mixtures in the ratio of 1:0, 1:4, 1:8, 1:12, mixtures in the ratio of 1:0, 1:4, 1:8, 1:12, and 1:16. and 1:16.

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CHAPTER 8. NANOPARTICLE FORMATION OF CARBAZOLE LINKER AND

CARBAZOLE OR DIPHENYL/TRIPHENYLAMINE-DONOR-BASED COMPOUNDS

8.1. Introduction

For the last 15 years, many kinds of luminescent nanoparticles have been prepared and

investigated for various potential applications including light-emitting devices,1,2 photovoltaic

cells,3,4 biological nanosensors,5,6 and others. In such applications, numerous attempts to generate

inorganic semiconductors and metal nanoparticles have been discovered but there are only a few

approaches to organic nanostructures.7-9 In comparison to inorganic nanoparticles, organic

nanoparticles have their own advantages and potential applications because they allow much

more variety and flexibility in material synthesis. It is well known that properties of many

nanoscale aggregates have unique size dependence and are substantially different from those of

bulk material composed of the same atoms or molecules.10 Regarding luminescent organic

nanoparticles, several articles reported that they show different and size dependent absorption

and emission.7-9,11,12

Attracted by the probable applications of the luminescent organic nanoparticles on

optoelectronic devices, we applied the precipitation method of making nanoparticles to all of our

twenty newly synthesized compounds. First, we applied this method to fumaronitrile core-based

compounds (2.1-2.5). We obtained very exciting results for these compounds and the details of

which are already reported13 and described in chapter 7 of this dissertation. We then became

more interested in making nanoparticles and applied the same method to the carbazole donor- based compounds (3.1-3.4), but we obtained and reported14 formation of nanoparticles only for

3.2 and 3.3. In other words, we obtained nanoparticles only for compounds having strong

electron acceptors. Then, we synthesized and reported15 the compounds 4.3, 8.1, 4.1, and 8.2

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(some of them are first reported here) which are similar to the compounds 3.1-3.4 , respectively,

conjugated without the incorporation of phenylethynyl groups in the linkers. For them we

obtained different emission in THF/water mixtures only for compounds having a cyano acceptor.

This might be due to the formation of nanoparticles. Furthermore, we applied the same method

to diphenylamine donor analogues of 2,7-carbazole linker-based compounds (5.1-5.6)16 and

obtained similar results, i.e., we obtained different emission for the compounds having at least one cyano group as an electron acceptor. In other words, we obtained different emission in

THF/H2O mixture than in THF solutions (which is probably because of nanoparticle formation) only for the compounds having a strong electron withdrawing group, i.e., compounds having at

least one cyano group. Then we randomly selected and measured emission of two compounds

(5.2 and 5.6) and found no change of emission for compound 5.6 but a significant change in

emission for 5.2 in THF/water mixtures. We obtained SEM images for the nanoparticles formed

from 5.2 and the data are presented in the Appendix 6. Adhikari et al. also reported nanoparticle

formation in THF/water mixtures for carbazole donor and dicyano acceptor-based compounds.17

In our case, all compounds having carbazole or diphenyl/triphenylamine donor and

dicyanoethylene or cyanoacrylic acid acceptor presumably formed nanoparticles.

We here present the synthesis of compounds 8.1 and 8.2, (Scheme 8.1) and the

nanoparticle formation for the compounds having carbazole donor and dicyano or cyanoacetic

acid acceptors (3.2 and 3.3). The structures of these compounds are given in Figure 8.1. We

already have reported these compounds, and discussed the synthesis and detailed photophysical

studies in chapter 3.

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t-Bu

NC N N CN t-Bu 3.2 t-Bu CF3

NC N N COOH t-Bu 3.3 CF3

Figure 8.1. Structures of compounds 3.2 and 3. 3.

8.2. Experimental Section

8.2.1. Synthesis of Compounds 8.1 and 8.2

Synthesis of [4-({3',6'-di-tert-butyl-9-[4-(trifluoromethyl)phenyl]-9H-2,9'-bicarbazol-7- yl}ethynyl)benzylidene]malononitrile (8.1). In a dry two necked round bottomed flask was added compound 4.3 (100 mg, 0.13 mmol), malononitrile (0.14 mmol), basic aluminum oxide

(10 mmol), and dry toluene (10 ml). The mixture was refluxed under argon for 6 h. The mixture was filtered hot, and the residue was washed several times with hot ethyl acetate. The filtrate was then dried, and the solid obtained was purified by chromatography (silica gel, 20% ethyl acetate in petroleum ether) to obtain pure compound 8.1 (90 mg, 90% yield) as a yellow solid. Mass

+ 1 spectrum (MALDI-TOF) m/z M = 764. H NMR (500MHz, CDCl3): δ 1.5 (s, 18H), 7.45 (d,

2H), 7.56 (dd, 1H), 7.60 (m, 2H), 7.67 (s, 1H), 7.7 (d, 2H), 7.77 (s, 1H), 7.79 (d, 2H), 7.91 (d,

13 2H), 7.94 (d, 2H), 8.17 (d, 2H), 8.23 (d, 1H), and 8.35 (d, 1H). C NMR (500 MHz, CDCl3):

32.0, 34.8, 82.7, 88.8, 96.1, 108.3, 109.0, 112.6, 113.3, 113.8, 116.4, 119.7, 120.3, 120.7, 122.0,

122.1, 123.4, 123.7, 124.2, 124.8, 124.9, 237.2, 127.5, 127.6, 130.0, 130.2, 130.7, 132.4, 137.3,

139.5, 140.1, 140.8, 142.0, 143.0, and 158.5.

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Synthesis of 3,3'',6,6''-tetra-tert-butyl-9'-[4-(trifluoromethyl)phenyl]-9'H-9,2':7',9''-

tercarbazole (8.2). Compound 3.7 (61 mg, 0.13 mmol) and diphenylamine (73 mg, 0.38 mmol)

were mixed with dry toluene (40 ml) in a two necked round bottomed flash containing a stir bar.

The Pd(OAc)2 (3 mole %), P(t-Bu)3 (7 mole %), and Cs2CO3 (0.52 mmol) were also added and

the mixture was stirred under argon at 110 oC for about 10 h. The reaction mixture was then

cooled to room temperature and toluene was removed completely under vacuum. The solid

mixture was dissolved in THF and unreacted Cs2CO3 was removed under gravity filtration. The

organic residue was then purified by column chromatography (silica gel, 20% ethyl acetate in

petroleum ether) to give the yellow compound 8.2. (101 mg, 80% yield). 1H NMR (500 MHz,

CDCl3): δ 1.5 (s, 36H), 7.36 (d, 4H), 7.48 (dd, 4H), 7.57 (dd, 2H), 7.63 (s, 2H), 7.74 (d, 2H),

13 7.82 (d, 2H), 8.17 (d, 4H), and 8.39 (d, 2H). C NMR (500 MHz, CDCl3): 32.0, 34.8, 108.4,

109.0, 109.1, 116.4, 120.4, 121.6, 122.4, 123.3, 123.7, 126.9, 127.0, 127.5, 127.6, 136.6, 141.9,

and 142.9. HRMS (EI) calculated for C43H30N3F3 645.23918, measured 645.23915.

8.2.2. Preparation of Nanoparticles. Samples of predetermined concentrations of 3.2 and 3.3 in

THF were made. Appropriate volumes of these solutions were taken in different vials and the

required amount of water was rapidly injected into those vials to maintain the final

concentrations of the solutions the same. Four different THF/water ratios, i.e. 1:0, 1:5, 1:8, and

1:12, we made for compounds 3.2 and 3.3 but 3.3 did not form any nanoparticles in

THF/water_1:12. In all samples the formation of nanoparticles could be observed under a 365

nm UV lamp. Furthermore we followed the same method of nanoparticle formation for rest of

the compounds in THF/H2O in the ratio of 1:4, 1:8, and 1:16 and the resulting solutions upon UV

365 nm irradiation are pictured in Appendix 6. In some samples formation of nanoparticles could

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be seen by the naked eye because of change of color of the solution though the solution is

transparent.

8.2.3. SEM Images of Nanoparticles. Scanning Electron Microscope (SEM) images were

recorded on a FEI-FP2031/11 microscope at 15 eV using INCA Penta FETX3 detector. Samples for SEM were prepared by placing a few drops of nanoparticle suspension onto a glass cover slip placed on an aluminum stud. The samples were allowed to dry at room temperature before observing under the scanning electron microscope. To enhance the contrast and quality of the

SEM images, the samples were sputter-coated with gold/palladium.

8.2.4. Fluorescent Microscopy (FM) Images of Nanoparticles. Fluorescent Microscopy images were recorded on a Qimaging Epi Fluorescence Microscope using a QICAM camera. The data were collected using Metaphor software. The samples were prepared by putting a droplet of nanoparticles suspension on a glass slide and covering with a cover slip.

8.2.5. Confocal Microscopy Images of Nanoparticles. Confocal microscopy images were performed using an Olympus FluoView 1000 microscope. The same method employed to prepare the samples for FM images was used to prepare the samples for confocal microscopy images. The excitation source was a diode blue laser (405 nm), and fluorescence was detected using standard three confocal channels (three photomultiplier detectors). The emission filter

(BA535-565) at 405 nm laser light was used for the image recording.

8.3. Results and Discussion

8.3.1. Synthesis

The synthetic details of compounds 3.6 and 4.3 are described in chapters 3 and 4 respectively. The compound 8.1 was synthesized from compound 4.3 by refluxing with malononitrile and basic aluminum oxide in toluene for 5 hours. The compound 3.6 was coupled

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with 3,6 di-tert-butylcarbazole in the presence of Pd(OAc)2, P(t-Bu)3,and Cs2CO3 upon refluxing in toluene for 22 hours to obtain 8.2 (Scheme 8.1).

Scheme 8.1. Synthesis of compounds 8.1 and 8.2.

t-Bu t-Bu

OHC N N NC N N CN t-Bu a t-Bu 4.3 8.1 t-Bu CF CF3 3 t-Bu

t-Bu Br Br t-Bu N N N N N t-Bu H t-Bu 8.2 CF3 3.6 b CF3

Reagents and conditions: (a) malononitrile, basic Al2O3, 2-methylpropanol, reflux 5 h. (b) Pd(OAc)2, P(t-Bu)3, Cs2CO3, (4 equiv.), reflux 22 h.

8.3.2. Formation of Nanoparticles

In this study nanoparticles of compounds 3.2 and 3.3 were prepared by a reprecipitation technique.18 Identical concentrations of solutions (1x10-5 M) were prepared for all THF/water mixtures. The nanoparticle suspensions formed were visibly transparent and stable at room temperature for several months. Suspensions of compounds 3.2 and 3.3 in THF and in

THF/water have the appearances shown in Figure 8.2 when placed under a UV light.

Figure 8.2. Solutions of nanoparticles of compound 3.2 (a) and 3.3 (b) under UV 365 nm.

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The sizes of nanoparticles formed were determined by scanning electron microscopy and

fluorescence microscopy (Appendix 6). Also, we obtained images from confocal microscopy and

obtained only green light showing that the nanoparticles emit in the green region (Figure 8.5).

Emission spectra of 3.3 in THF solution and as nanoparticles suspended in solvents differing in

the THF/water ratio are given in Figure 8.3 and for 3.2 the spectra are given in Appendix 6. In

both compounds there is no red shift of absorption on adding water in THF. Instead we obtained

blue shifted absorption for compound 3.3 which might be due to the interaction of water on

carboxylic acid group of the compound. Interestingly, the emission is red shifted by 126 nm for

compound 3.2 and by 120 nm for compound 3.3. For both compounds we observed a more and

more red shifted emission with increasing percentage of water in THF (Figure 8.3, Table 8.1 and

Appendix 6).

1.8x107 THF THF/H O_1:5 1.5x107 2 THF/H2O_1:8 1.2x107

9.0x10 6.0x106 Intensity 3.0x106 0.0 400 450 500 550 600 650 700 Wavelength (nm)

Figure 8.3. Emission of compound 3.3 in THF and THF/H2O in the ratio of 1:5 and 1:8. (note: λex = Amax).

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Figure 8.4. SEM image of nanoparticles of compound 3.3 in THF/H2O_1:5.

Figure 8.5. Confocal microscope image of nanoparticles of compound 3.3 in THF/H2O_1:5.

Table 8.1. Absorption maximum (Amax), emission maximum (λmax) and red shift of emission during the formation of nanoparticles of compounds 3.2 and 3.3.

Compound Ratio of solvents Amax λmax Red shift Lifetime (nm) (nm) (emission) (ns) (nm)

THF/Water_1:0 370 448 -- 0.30

THF/Water_1:5 370 506 58 2.60 3.2 THF/Water_1:8 370 533 85 2.43

THF/Water_1:12 370 574 126 --

THF/Water_1:0 380 431 -- 0.78

3.3 THF/Water_1:5 375 531 100 1.61

THF/Water_1:8 370 551 120 0.47

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We applied the same method of nanoparticle formation to the rest of our compounds and the solution of these compounds in THF and THF/water mixtures and the data are shown in

Appendix 6. These solutions under UV 365 nm irradiation shows that compounds having at least one cyano acceptor have different emissive color in THF/water mixture than the solution in THF

(Appendix 6, Figure A6.11). This result when compared with the results obtained with compounds 2.1-2.5, and 3.2-3.3, supports the conclusion of nanoparticle formation for compounds 8.1, 4.1, 5.2 and 5.5. In other words compounds having cyano group as an electron acceptor have change in emission color in THF/H2O mixtures. Furthermore, we randomly measured the emission of compounds 5.2 and 5.6 and obtained emission change for 5.2 and no emission change for 5.6 showing the indication of formation of nanoparticles for compound 5.2.

To further prove it we measured the SEM image of compound 5.2 (Appendix 6) for the solutions of THF/H2O_1:8 and obtained the particles size about 100 nm as a proof of nanoparticle formation.

8.4. Conclusions

The precipitation method for nanoparticle formation was applied to all carbazole donor - carbazole linker with phenylethynyl extended compounds (3.1-3.4) but we obtained the nanoparticles for only those compounds having a cyano group as an electron acceptor. Their optical properties were measured using absorption spectroscopy, fluorescence spectroscopy, scanning electron microscopy, fluorescence microscopy, and confocal microscopy. Optical properties were further assessed by measuring the lifetime of fluorescence at various ratios of

THF/water maintaining the same concentration. Photophysical studies indicate that compounds

3.2 and 3.3 with their strong electron acceptor form fluorescent nanoparticles and these nanoparticles switch fluorescence from blue to green so retained the optical purity upon the

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formation of FONs. Emission has been red shifted by 126 nm for 3.2 and by 120 nm for 3.3 upon

the formation of nanoparticles. We further followed the same procedure of nanoparticle

formation to all of our other carbazole linker based compounds. The change of color under UV

365 nm light supports our claim, i.e., only the compounds having strong electron withdrawing

cyano group can form organic nanoparticles.

8.5. References

(1) Schlamp, M.C.; Peng, X.; Alivisatos, A. P. J. Appl. Phys. 1997, 82, 5837-5842. (2) Bozano, L.; Tuttle, S. E.; Carter, S. A.; Brock, P. J. Appl. Phys. Lett. 1998, 73, 3911-3913. (3) Godovsky, D. Y. Adv. Polym. Sci. 2000, 153, 163-205. (4) Gratzel, M. Curr. Opin. Colloid Interface Sci. 1999, 4, 314-321. (5) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18-52. (6) Bergey, E. J.; Levy, L.; Wang, X. P.; Krebs, L. J.; Lal, M.; Kim, K. S.; Pakatchi, S.; Liebow, C.; Prasad, P. N. Biomed. MicroDevices 2002, 4, 293-299. (7) Kasai, H.; Kamatani, H.; Yoshikawa, Y.; Okada, S.; Oikawa, H.; Watanabe, A.; Ito, O.; Nakanishi, H. Chem. Lett. 1997, 1181-1182. (8) Taylor, J. R.; Fang, M. M.; Nie, S. Anal. Chem. 2000, 72, 1979-1986. (9) Fu, H. B.; Yao, J. N. J. Am. Chem. Soc. 2001, 123, 1434-1439. (10) Horn, D.; Rieger, J. Angew. Chem., Int. Ed. 2001, 40, 4331-4361. (11) Gong, X. C.; Milic, T.; Xu, C.; Batteas, J. D.; Drain, C. M. J. Am. Chem. Soc. 2002, 124, 14290-14291. (12) An, B. K.; Kwon, S. K.; Jung, S. D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410- 14415. (13) Panthi, K.; Adhikari, R. M.; Kinstle, T. H. J. Photochem. Photobiol. A: Chem. 2010, 215, 179-184. (14) Panthi, K.; Adhikari, R. M.; Kinstle, T. H. J. Phys. Chem. A 2010, 114, 4550-4557.

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(15) Panthi, K.; Onicha. A. C.; Castellano, F. N.; Kinstle, T. H. Manuscript in Preparation. (16) Panthi, K.; El-Khoury, P. Z.; Tarnovsky, A. N.; Kinstle, T. H. Tetrahedron. 2010, 66, 9641- 9649.

(17) Adhikari, R. M.; Shah, B. K.; Palayangoda, S. S.; Neckers, D. C. Langmuir, 2009, 25, 2402-2406. (18) Palayangoda, S. S.; Cai, X.; Adhikari, R. M.; Neckers, D. C. Org. Lett. 2008, 10, 281-284.

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Appendix 6. Absorption and emission spectra, Scanning Electron, Fluorescent, and

Confocal Microscopy Images of compounds 3.2-3.3, and solutions in THF and THF water mixtures of compounds 3.2 -3.3 and others.

Absorption and emission spectra of nanoparticles

3.5x107 1.2 THF THF THF/H2O_1:5 7 THF/H O_1:5 THF/H2O_1:8 3.0x10 2 1.0 THF/H O_1:8 THF/H2O_1:12 7 2 2.5x10 THF/H2O_1:12 0.8 2.0x107

0.6 1.5x107 0.4 1.0x107 Emission Absorbance 6 0.2 5.0x10

0.0 0.0 300 350 400 450 500 400 450 500 550 600 650 Wavelength (nm) Wavelength (nm) Figure A6.1. Absorption spectra Figure A6.2. Emission spectra of of compound 3.2 in THF and compound 3.2 in THF and THF/H2O THF/H2O in the ratio of 1:5, 1:8, in the ratio of 1:5, 1:8, and 1:12. and 1:12. (note: λex = Amax).

1.8 2.5x106 THF 1.6 THF/H2O_1:5 6 THF/H O_1:8 2.0x10 1.4 2

6 1.2 1.5x10 1.0

1.0x106 0.8 0.6 5 Emission 5.0x10 Absorbance THF/H2O_1:5 0.4 THF/H O_1:8 2 0.2 0.0 THF/H O_1:12 2 0.0 400 450 500 550 600 650 300 400 500 600 Wavelength (nm) Wavelength (nm) Figure A6.3. Emission of compound Figure A6.4. Absorption of 3.2 in THF/H2O in the ratio of 1:5, compound 3.3 in THF and THF/H2O 1:8, and 1:12. (note: λ = A ). in the ratio of 1:5, 1:8, and 1:12. ex max

204

THF 4 THF 1000 THF/H O_1:4 5x10 2 THF/H2O_1:4 THF/H O_1:8 2 4 THF/H2O_1:8 800 THF/H2O_1:16 4x10 THF/H2O_1:16 600 3x104

400 2x104 Emission Emission 200 1x104 0 0 400 450 500 550 600 650 700 400 420 440 460 480 500 Wavelength (nm) Wavelength (nm)

Figure A6.5. Emission of Figure A6.6. Emission of compound 5.2 in THF/H2O in the compound 5.3 in THF/H2O in the ratio of 1:4, 1:8, and 1:16. (note: ratio of 1:4, 1:8, and 1:16. (note: λex = Amax). λex = Amax).

SEM, FM, and Confocal images, and solution under UV

Figure A6.7. SEM image of Figure A6.8. FM image of nanoparticles nanoparticles of compound 3.2 in of compound 3.2 in THF/H2O_1:5. THF/H2O_1:5.

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Figure A6.11. Equal concentration of compounds in THF and THF/H2O_1:4, 1:8, and 1:16 to show the change during nanoparticle formation.

206

t-Bu

N NC N CN t-Bu

CF3

Figure A6.12. 1H NMR of compound 8.1.

Figure A6.13. 13C NMR of compound 8.1.

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t-Bu t-Bu N N N t-Bu t-Bu

CF3

Figure A6.14. 1H NMR of compound 8.2.

Figure A6.15. 13C NMR of compound 8.2.

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Appendix 7. List of abbreviations, acronyms, and symbols.

ACN acetonitrile

AM air mass

Amax absorption maximum

Al2O3 alumina

Aπ magnitude of the π-conjugation length

ASTM American Society for Testing and Materials

CBP bis-(4-carbazol-9-yl)-biphenyl

CDCl3 deuterated chloroform

oC degree Celsius

CH3COOH acetic acid

CN-MBE 1-cyano-trans-1,2-bis-(4-methylbiphenyl)ethylene

CS2CO3 cesium carbonate

CuI copper iodide d doublet

DCM dichloromethane

DD donor donor dd doublets of doublet

Δf orientation polarizability

DFT density functional theory

DMF dimethyl formamide

DMSO dimethyl sulfoxide

DSSC dye-sensitized solar cell

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D-π-A donor-linker-acceptor

D-π-A-π-D donor-linker-acceptor-linker-donor

ε molar absorptivity

EA electron accepting group

ED electron donating group

EERS edge excitation red shift

EI electron impact

EL electroluminescent

EQE external quantum efficiency

ET electron transfer

Et3N triethylamine

EtOAc ethyl acetate

EtOH ethanol

eV electron volt

EWG electron-withdrawing group

Fc+/Fc0 ferrocenium/ferrocene

F-CESD Franck-Condon excited-state distribution

FF fill factor

FM fluorescent microscopy

FONs fluorescent organic nanoparticles

G global

GCMS gas chromatography coupled with mass-spectrometry

gm gram

210 h hour

HCl hydrochloric acid

HOMO highest occupied molecular orbital

HRMS high resolution mass spectroscopy

IC internal conversion

ICT intramolecular charge transfer

ICTr relaxed intramolecular charge transfer

IPCE incident photon-to-current conversion efficiency

ISC intersystem crossing

JSC photocurrent density

K2CO3 potassium carbonate

KOH potassium hydroxide knr nonradiative rate constant of emission kr radiative rate constant of emission

λ wavelength

λmax emission maximum

LCD liquid crystal display

LE locally excited

LEDS light-emitting diodes

LUMO lowest unoccupied molecular orbital

M molar (mol/L) m multiplet

MALDI matrix-assisted laser desorption

211

mg milligram

MgSO4 magnesium sulfate

MHz megahertz

ml milliliter

mm millimeter

mv millivolt

µm micrometer

η solar cell efficiency

NaCl sodium chloride

NaHCO3 sodium bicarbonate

Na2SO4 sodium sulphate

NHE normal hydrogen electrode

NIR near infrared

nm nanometer

NPD N,N'-di(naphthalen-1-yl)-N,N'-diphenyl-biphenyl-4,4'-diamine

NMR nuclear magnetic resonance

Ns nanosecond

OFETs organic field effect transistors

OLEDs organic light emitting diodes

% percent

Pd(OAc)2 palladium acetate

Pd(PPh3)2Cl2 trans-Dichlorobis(triphenylphosphine)palladium(II)

PPh3 triphenyl phosphine

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ppm parts per million

P(t-Bu)3 tri-tert-butylphosphine

PV photovoltaic

ΦF quantum yield of fluorescence

S singlet

SEM scanning electron microscopy t triplet t-Bu tertiary butyl

TD DFT time dependent density functional theory

TFA trifluoroacetic acid

τF fluorescence lifetime

THF tetrahydrofuran

TICT twisted intramolecular charge transfer state

TiO2 titanium dioxide

UV ultraviolet

UV-vis ultraviolet-visible

V volts

Voc open circuit voltage