Carbazole-Based Emitting Compounds: Synthesis

CARBAZOLE-BASED EMITTING COMPOUNDS: SYNTHESIS,

PHOTOPHYSICAL PROPERTIES AND FORMATION OF

NANOPARTICLES

Ravi M Adhikari

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

December 2008

Committee:

Dr Douglas C. Neckers, Advisor

Dr Jeffrey G. Miner Graduate College Representative

Dr Thomas H. Kinstle

Dr John R. Cable

Ravi M Adhikari

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ॐ भूभुवःर् ःवः ।

तत ्सिवतुवरर् ेण्यं ।

भगोर् देवःय धीमिह ।

िधयो यो नः ूचोदयात ्॥

oṃ bhūr bhuvaḥ swaḥ

tat savitur vareṇyaṃ

bhargo devasya dhīmahi

dhiyo yo naḥ prachodayāt

Vakratunda Mahakaya Surya Koti Samaprabha

Nirvighnam Kurumedeva Sarvakaryeshu Sarvada

ABSTRACT

Dr Douglas C. Neckers, Advisor

Carbazole is a heterocyclic tricyclic aromatic organic compound consisting of two six- membered benzene rings fused on either side of a five-membered nitrogen-containing ring. A large number of carbazole derivatives have been designed and synthesized and organic electronic devices based on these derivatives such as organic light emitting diodes (OLEDs), have been investigated. Ever since Tang and VanSlyke constructed electroluminescent (EL) devices using organic materials as emitters, development of efficient and stable EL materials has taken good momentum. Carbazole derivatives are widely used as materials for EL and hole-transporting layers of OLEDs which use their high charge mobility.

The optical and electrical properties of carbazoles are affected by substitution on the 2-,

3-, 6-, 7- and 9H-positions. Many carbazole derivatives have sufficiently high triplet energy to make them an efficient host where they can serve as red, green, or blue triplet emitters.

Highly fluorescent and stable carbazole-based compounds were synthesized and characterized. Substitution of carbazoles at 3- and 6- position by tert-butyl group enhanced the solubility. They showed high extinction coefficients of absorption (Amax 328-353 nm) and quantum yields of fluorescence ( max 386-437 nm; ΦF 0.72-0.89; F 2.09-3.91 ns) in dichloromethane. The quantum yields of fluorescence of these compounds in the solid state were also high ( max 385-422 nm; ΦF 0.40-0.85).

Simple synthetic procedures were developed to prepare other stable carbazoles. These compounds emit blue, green, and orange-red light. The red-shifted emission in the solid state which can be as much as 120nm relative to that in solution is highly dependent on the nature and positions of the substituents.. The presence of a carbaldehyde or malononitrile on the

v carbazole moiety quenches fluorescence severely in solution and in the solid state. However, the effect is not the same for the fluorescence lifetime. Lowering the temperature from 25 0C to

0 -10 C causes a small but distinct red-shift in the emissions and a systematic increase in the ΦF values of blue and green emitters. A considerable edge excitation red shift was observed for some of these compounds. The emission of some of these compounds in solution showed both specific and general solvent effects.

Nanoparticle research is currently an area of intense scientific work, due to the wide variety of potential applications in biomedical, optical, and electronic fields. Today nanotechnology has been used in various fields ranging from optoelectronic devices to sensors, in biological imaging as well as in third-order non-linear optics. A nanoparticle is a small object, sized between 1 and 100 nanometers, that behaves as a single whole in terms of its transport and properties.

Suitably susbtituted carbazoles form highly stable fluorescent organic nanoparticles.

The emission of these nanoparticles was reversibly switched on/off in the blue-green and orange-red regions from a change in the ratio of the tetrahydrofuran/water system used in their preparation. The size of the nanoparticles was depends on the solvent ratio and the emissions were significantly red shifted compared to those of dilute solutions in tetrahydrofuran.

Similarly, highly stable composite fluorescent organic nanoparticles (CFONs) were prepared by co-reprecipitation of blue and red emitting carbazole-based organic compounds from water/tetrahydrofuran mixtures. SEM images showed diversity in particle size. Emission spectra of CFONs prepared from different ratios of red and blue emitters covered the entire visible region from 400 to 700 nm. Confocal microscopy measurements revealed composite

vi organic nanoparticles emitting a white light. CIE coordinates of these CFONs demonstrated high color purity (CIE X, Y: 0.34, 0.35).

These data indicate that these compounds and their nanoparticles have potentials as emitting materials in organic light-emitting diodes (OLEDs).

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DEDICATION

I would like to dedicate this dissertation to my parents, Yadu Nath Adhikari and Neem

Kumari Adhikari, my brother Prem Raj Adhikari, sister in-law Uma Devi Sapkota, my father- in-law Shreekanta Regmi and my cousin Tanka Prasad Subedi for their inspiration, devotion, faith and support throughout this process and for helping me for my dreams to come true.

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ACKNOWLEDGMENT

I would like to express my deepest gratefulness to my advisor Dr. Douglas C. Neckers for his leadership, professionalism, and continued optimism, which make him an exceptional advisor. I am grateful for his time, effort, and editing skills. His constant support, the trust he placed in my abilities, encouragement and timely interventions made this thesis possible and a reality. Thank you for your support to present my research in local, national and international conferences.

I highly appreciate my committee member Dr. Thomas H. Kinstle, who helped me in various ways: as an organic teacher, as a synthetic problem solver, as a landlord and so on. He paved my way to Dr. Neckers group by helping me maintain the good grades. I also extend my gratefulness to Dr John R. Cable for supporting me in my class work and Dr Jeffery G. Minor for serving as my committee member.

I voice my gratitude to Dr. Michael A. J. Rodgers for enlightening me with knowledge of photophysics and photochemistry. I highly appreciate my mentor Dr. Rajib Mondal who encouraged and motivated me in the field of synthetic chemistry. Had his constant support not been for me I would have never accomplished my dissertation. I equally appreciate Dr Bipin K.

Shah for his brotherly advice to me. He introduced me with the modern photophysics and photochemistry. Thanks to Ohio Laboratory for Kinetic Spectroscopy where I did some femtosecond and nanosecond experiments.

Thank goes to Dr Xichen Cai, Sujeewa S. Palayangoda, Puran De and Kelechi C.

Anyaogu for their part in synthesis, photophysics and instruments. Collaboration with them was highly fruitful. Thanks to Dr Abdul Hamza for his help with some calculations. I wish to thank

Nora Cassidy, Alita Frater, Karen Voland and Jadrez Romanowicz for all the administrative

ix and instrumental help in the department. Thanks to Craig, Doug, Larry for taking care of all the technical issues regarding the research.

I am indebted to SPIE (Spectra Physics Award), SPIE BGSU chapter, the department of chemistry (Graduate Student Professional Development Fund and Dissertation Research

Support Fund), center for photochemical sciences, graduate student senate (Charles E. Shanklin

Award), Book store fund, and professional development award. I wish to express my gratefulness to Dr. Deanne L. Snavely for her support to find the fund to attend a workshop.

I would like to give my deep appreciation to my siblings: Nabaraj Adhikai, Dasharath

Adhikari, Samjhana and Kalpana, and my first cousin Janak R. Subedi for standing by me throughout the life. I also would like to appreciate my maternal uncle Chakrapani Paudel and

Ashok Paudel, paternal uncle Bamdev Deep Adhikari for encouraging and inspiring me for higher studies. At this moment I cannot forget my primary (Ananda Jyoti Secondary School) school teacher: Kamala Thapa and Tikaram Pandey for their support in my childhood education. My neighborhood in my village Begnas (Nepal) deserves a token of appreciation for its natural beauty which enabled me to understand the world. I thank each and every person associated directly or indirectly with the success of this endeavor.

At last but not the least, I wish to extend my deepest gratefulness to my wife Indu D.

Adhikari for years of love and support and my son Amit Adhikari without whom this work would never be accomplished.

TABLE OF CONTENTS PART 1 ...... 1

1. Introduction ...... 1

2. History of electroluminescence devices ...... 2

3. Metal-carbazole based phosphorescent OLEDs ...... 4

4. Tuning the HOMO-LUMO by substitution on the carbazole ...... 7

5. Structure and working principle of OLED devices ...... 9

6. References ...... 11

CHAPTER 1 ...... 15

1.1. Introduction ...... 15

1.2. Results and discussion ...... 17

1.3. Photoluminescence in solution ...... 19

1.4. Solid-state photoluminiscence ...... 24

1.5. Effect of aging and annealing on the solid-state emission ...... 26

1.6. Conclusions ...... 27

1.7. Experimental section ...... 27

1.7.1. Instrumentation: ...... 27

1.7.2. Synthesis: ...... 28

1.7.3. Fluorescence quantum yields (ΦF): ...... 30

1.7.4. Fluorescence lifetime ( F) measurement: ...... 31

1.8. References ...... 31

APPENDIX 1 ...... 35

CHAPTER 2 ...... 46

2.1. Introduction ...... 46

2.2. Results and discussion ...... 48

2.2.2. Absorption and emission spectra in solution: ...... 50

2.2.3. Emission spectra in the solid state: ...... 55

2.2.4. Solvatochromism: ...... 57

2.2.5. Concentration dependent fluorescence switching: ...... 59

2.3. Effect of aging and annealing ...... 60

2.4. Temperature dependent emission spectra ...... 61

2.5. Experimental section ...... 63

2.5.1. Fluorescence Quantum Yields (ΦF): ...... 63

2.5.2. Synthesis: ...... 64

2.7. References ...... 70

APPENDIX 2 ...... 72

CHAPTER 3 ...... 91

3.1. Introduction ...... 91

Chart 3.1. Structures of compounds B1-R3 ...... 92

3.2. Experimental section ...... 92

3.2.1. Materials ...... 93

3.2.2. Measurements: ...... 93

3.3. Results and discussion ...... 93

3.3.1. Temperature dependent emission maxima and fluorescence quantum yields (ΦF): 93

3.3.4. Fluorescence switching with concentration in PMMA matrix; aggregation quenching: ...... 100

3.4. Intermolecular energy transfer ...... 101

APPENDIX 3 ...... 106

PART 2 ...... 111

7. Introduction ...... 111

8. FONs for biological applications ...... 112

9. FONs: Application in optoelectronic devices ...... 114

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10. FONs for studying structure-properties relationship; bridging the gap between bulk and the molecular level ...... 120

11. Preparation of nanoparticles ...... 122

12. References ...... 123

CHAPTER 4 ...... 127

4.1. Introduction ...... 127

4.2. Results and discussion ...... 128

4.3. Conclusions ...... 138

4.4. Experimental section ...... 138

4.4.1. Synthesis ...... 138

4.4.2. Fluorescence lifetime ( F) measurement: ...... 141

4.4.3. Preparation of nanoparticles ...... 141

4.4.4. SEM images of nanoparticles ...... 141

4.4.5. Confocal microscope images of the R2 nanoparticles ...... 142

4.5. References ...... 142

CHAPTER 5 ...... 150

5.1. Introduction ...... 150

5.2. Results and discussion ...... 151

5.3. Experimental section ...... 160

5.3.1. Fluorescence lifetime ( F) measurement ...... 160

5.4. Conclusions ...... 160

5.6. References ...... 161

LIST OF ABBREVIATIONS ...... 163

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LIST OF FIGURES

Figure 1 : Tang and VanSlyke’s configuration of an EL cell and molecular structures of emissive and electron transport material, adapted from reference 12 ...... 2

Figure 2: (A) A recent OLEDs display included in a cell phone set, (B) The world’s largest 32- inch television, (C) OLEDs prototype for lighting, adapted from reference 16 ...... 3

Figure 3: Structure of OLEDs device, adapted from reference 30 ...... 9

Figure 4: Light creation by OLEDs, adapted from reference 32 ...... 10

Figure 1.5 : Normalized absorption and emission spectra of 1-5 recorded in dichloromethane, λex = Amax ...... 19

Figure 1.6: Fluorescence decay of 1 monitored at max = 390 nm in dichloromethane ( F = 2.53 ns, = 1.08); λex = 340 nm...... 22

Figure 1.7: Normalized emission spectra of 2 recorded in different solvents, λex = Amax ...... 24

Figure 1.8 : Mataga-Lippert plot of 1 (red) and 2 (blue) (solvents: toluene, chloroform, methanol, and acetonitrile in the order of increasing Stokes shifts)...... 24

Figure 1.9 : Normalized solid-state fluorescence spectra of 1-5 recorded in thin films of poly (methyl methacrylate), λex = Amax ...... 25

Figure 1.10: Normalized solid-state fluorescence spectra of the thin films of 1-5, λex = Amax25

Figure 1.11: Fluorescence spectra recorded from thin films of PMMA containing 1: (a) pristine, (b) after exposing the film for more than 15 days at ambient conditions, and (c) after heating at 150 0C for 24 h and cooling to room temperature, λex = Amax ...... 27

Figure A1.12: 1H-NMR spectrum of 1 ...... 35

Figure A1.13: 13C-NMR spectrum of 1 ...... 35

Figure A1.14: 1H-NMR spectrum of 2 ...... 36

Figure A1.15: 13C-NMR spectrum of 2 ...... 36

Figure A1.16: 1H-NMR spectrum of 3 ...... 36

Figure A1.17:1H-NMR spectrum of 3 ...... 37

Figure A1.18:1H-NMR spectrum of 4 ...... 37

Figure A1.19:13C-NMR spectrum of 4 ...... 37

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Figure A1.20:1H-NMR spectrum of 5 ...... 38

Figure A1.21:13C-NMR spectrum of 5 ...... 38

Figure A1.22 :Normailized UV-visible spectra of 1 in different solvents ...... 38

Figure A1.23: Normalized fluorescence spectra of 1 in different solvents, λex = Amax ...... 39

Figure A1.24: Normalized UV-visible spectra of 2 in different solvents ...... 39

Figure A1.25 :Normalized UV-visible spectra of 3 in different solvents ...... 40

Figure A1.26: Normalized fluorescence spectra of 3 in different solvents, λex = Amax ...... 40

Figure A1 27: Normalized UV-visible spectra of 4 in different solvents ...... 41

Figure A1.28::Normalized fluorescence spectra of 4 in different solvents, λex = Amax ...... 41

Figure A1.29:: Normalized UV-visible spectra of 5 in different solvents ...... 42

Figure A1.30:: Normalized fluorescence spectra of 5 in different solvents; λex = Amax ...... 42

Figure A1.31: Fluorescence spectra of 1 recorded in dichloromethane at different concentrations; λex = Amax ...... 43

Figure A1.32:: Fluorescence decay of 2 monitored at λmax = 413 nm in dichloromethane; λex = 340 nm ...... 43

Figure A1.33: Fluorescence decay of 3 monitored at λmax = 389 nm in dichloromethane; λex = 340 nm ...... 44

Figure A1.34: Fluorescence decay of 4 monitored at λmax = 417 nm in dichloromethane; λex = 340 nm ...... 44

Figure A1.35: Fluorescence decay of 5 monitored at λmax = 437 nm in dichloromethane; λex = 340 nm ...... 44

Figure 2.36: Absorption spectra of B1, B4, G1, G3, R1, and R2 recorded in dichloromethane 51

Figure 2.37: Normalized emission spectra of B1, B4, G1, G3, R1, and R2 recorded in dichloromethane, λex = Amax ...... 51

Figure 2. 38: Normalized emission spectra of thin films of B1, B4, G1, G3, R1, and R2, λex = Amax ...... 56

Figure 2. 39: Normalized emission spectra of B1 recorded in different solvents, λex = Amax 57

Figure 2.40: Normalized emission spectra of G1 recorded in different solvents, λex = Amax . 58

Figure 2.41: Emission spectra of R3 recorded in dichloromethane at different concentrations (λex = 380 nm); Inset: the enlarged spectrum recorded at a higher concentration (2.6λ10-4 M) that appears a straight line in the main Figure, ...... 60

Figure2.42: Emission spectra recorded from thin films of R2: (a) pristine, (b) after exposing the film for more than 4 weeks at ambient condition, and (c) after heating at 150 0C for 24 hrs and cooling to room temperature, λex = Amax ...... 61

Figure 2.43: Emission spectra of B1 recorded at different temperatures, λex = 330 nm ...... 62

Figure A2.44: Absorption Spectra for B1-R3 in dichloromethane solution ...... 73

Figure A2.45:Emission spectra of B1-R3 in dichloromethane solution, λex = Amax ...... 73

Figure A2.46:Emission spectra of B1-R3 in solid thin films, λex = Amax ...... 73

Figure A2 47:Absorption spectra of B1 in different solvents ...... 74

Figure.A2 48:Absorption spectra of B2 in different solvents ...... 75

FigureA2 49:Absorption spectra of B3 in different solvents ...... 75

Figure.A2 50:Absorption spectra of B4 in different solvents ...... 75

FigureA2 51:Absorption spectra of G1 in different solvents ...... 76

Figure.A2 52:Absorption spectra of G2 in different solvents ...... 76

FigureA2 53:Absorption spectra of G3 in different solvents ...... 76

FigureA2 54:Absorption spectra of R1 in different solvents ...... 77

FigureA2 55:Absorption spectra of R2 in different solvents ...... 77

FigureA2 56:Absorption spectra of R3 in different solvents ...... 77

FigureA2 57:Emission spectra of B1 in different solvents, λex = Amax ...... 78

FigureA2 58:Emission spectra of B2 in different solvents, λex = Amax ...... 78

FigureA2 59:Emission spectra of B3 in different solvents, λex = Amax ...... 78

FigureA2 60:Emission spectra of B4 in different solvents, λex = Amax ...... 79

FigureA2 61:Emission spectra of G1 in different solvents, λex = Amax ...... 79

FigureA2 62:Emission spectra of G2 in different solvents, λex = Amax ...... 80

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FigureA2 63:Emission spectra of G3 in different solvents, λex = Amax ...... 80

FigureA2 64:Emission spectra of R1 in different solvents, λex = Amax ...... 80

FigureA2 65:Emission spectra of R2 in different solvents, λex = Amax ...... 81

FigureA2 66:Emission spectra of R3 in different solvents, λex = Amax ...... 81

FigureA2 67:Fluorescence life time decay profile of B1 in dichloromethane ...... 81

FigureA2 68:Fluorescence life time decay profile of B2 in dichloromethane ...... 82

FigureA2.69:Fluorescence life time decay profile of B3 in dichloromethane ...... 82

FigureA2.70:Fluorescence life time decay profile of B4 in dichloromethane ...... 82

FigureA2. 72:1HNMR of B4 ...... 83

FigureA2. 73:13CNMR of B4 ...... 83

FigureA2.71:Geometry optimized ground state structures ...... 83

FigureA2. 74:1HNMR of B1 ...... 84

FigureA2. 75:13CNMR of B1 ...... 84

FigureA2. 76:1HNMR of G2 ...... 84

FigureA2. 77:13CNMR of G2 ...... 85

FigureA2. 78:1HNMR of G3 ...... 85

FigureA2. 79:13CNMR of G3 ...... 85

FigureA2. 80:1HNMR of G1 ...... 86

FigureA2. 81:13CNMR of G1 ...... 86

FigureA2. 82:1HNMR of B2 ...... 86

FigureA2 83:13CNMR of B2 ...... 87

FigureA2. 84:1HNMR of B3 ...... 87

FigureA2. 85:13CNMR of R1 ...... 88

FigureA2.86:1HNMR of R2 ...... 89

FigureA2. 87:13CNMR of R2 ...... 89

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FigureA2. 88:1HNMR of R3 ...... 89

Figure3 89:Temperature dependent emission spectra of B1 recorded in dichloromethane, ex = 330 nm ...... 94

Figure3. 90:Jablonski diagram showing the possible excitation and de-excitation pathways for B1, B3 and G1-G3 ...... 94

Figure3 91:Temperature dependent emission spectra of G3 recorded in dichloromethane, λex = 370 nm ...... 96

Figure3. 92:Emission spectra of B5 recorded in DCM, excitation at different wavelengths. .... 97

Figure3 93:Absorption spectra of G3 recorded at different hexane methanol ratios ...... 99

Figure3 94:Emission spectra of G3 recorded in different ratios of hexane to methanol, ex = Amax ...... 99

Figure3 95:Emission spectra of G1 recorded in different fractions of G1 in PMMA matrices, ex = 370 nm ...... 100

Figure3 96:Emission spectra of G3 recorded in DCM, Absorbance of sample = 3 ...... 101

Figure3. 97:Emission spectra of G3 recorded in DCM, Absorbance of sample = 0.01 ...... 102

Figure3. 98:. Lippert-Mataga plots for B1 and B3 in solvents ACN, methanol, DCM, toluene and hexanes ...... 103

Figure3. 99:Lippert-Mataga plots for G1-R1 in acetonitrile, methanol, dichloromethane hexane , toluene and ...... 103

Figure3.100:Lippert-Mataga plots for R2 and R3 in solvents ACN, methanol, DCM, toluene and hexanes ...... 104

Figure A3.101: 1HNMR of B5 ...... 107

Figure A3.102: 13CNMR of B5 ...... 108

Figure A3.103: Temperature dependent emission spectra of G1 recorded in DCM, λex = 330 nm ...... 108

Figure A3.104:Temperature dependent emission spectra of G1 recorded in DCM, λex = 370 nm ...... 108

Figure 105:Temperature dependent emission spectra of G3 recorded in DCM, λex = 330 nm ...... 109

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Figure A3.106: Temperature dependent emission spectra of G2 recorded in dichloromethane, λex = 310 nm ...... 109

Figure A3.107: Temperature dependent emission spectra of G2 recorded in dichloromethane, λex = 370 nm ...... 109

Figure A3.108: Emission spectra of G2 recorded in dichloromethane, excitation at different wavelengths ...... 110

Figure A3.109: Emission spectra of G2 recorded in acetonitrile, excitation at different wavelengths ...... 110

Figure A3.110: Emission spectra of G3 recorded in dichloromethane at absorbance 0.5 ...... 110

Figure 111:Chemical structure of BDSA (top) and optimized geometry of a model structure with dimethylamino end groups by semiempirical calculation where the torsion angles = φ, adapted from reference 16 ...... 114

Figure 112:Photopatterned array of Py-CN-MBE nanoparticles. (A–C) Schematic diagram of the procedure for photopatterning Py-CNMBE nanoparticles. (a–g) Fluorescence emission and SEM images at each step. The inset photo in (f) shows a microscope image of the patt .. 115

Figure 113:Photographs of patterned glossy paper utilizing a luminescent microemulsion, which shows the logo of the Fudan University 18 ...... 116

Figure 114:The fluorescence excitation and emission spectra of A) DPP in acetonitrile with a concentration of 1.0x10-5 molL-1; and nanoparticles of B) NP1, C) NP2, D) NP3 and E) NP4; F) DPP bulk crystals (NP1, 65 nm; NP2, 120 nm; NP3, 180 nm, and NP4, 310 nm)...... 117

Figure 115:Schematic presentation of microemulsion conversion into nanoparticles by ink-jet printing, adapted from reference 20 ...... 118

Figure 116:Photograph of fluorescent water-insoluble molecule as a printed pattern, taken through a red viewing filter (max wavelength of 593 nm), while a polylight lamp, equipped with a 503-587 nm filter was used for excitation of the fluorescent printed text, ad ...... 119

Figure 4.117:UV-visible spectra of R2 nanoparticle solutions (3.7 × 10-6 M) recorded at different THF/water ratios by volume ...... 130

Figure 118: Fluorescence spectra of R2 nanoparticle solutions (3.7 x 10-6 M) recorded at different THF/water ratios by volume; λex = 350 nm ...... 131

Figure 4.119:(A) The intermolecular charge transfer (ICT) state of the R2 nanoparticles and (B) the absorption and emission transitions in R2 in THF and in THF/water solutionsigu ...... 132

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Figure A.120:A widefield confocal microscope image of the R2 nanoparticles ...... 133

Figure 4.121:(A) SEM of R2 nanoparticles. (B) R2 nanoparticles formed at the THF/water ratio of (i) 1/5, (ii) 1/11, and (iii) 1/90, showing the size dependence on the THF/water ratio; the bar of SEMs = 0.5 µm ...... 134

Figure 4.122:Fluorescence spectra of R3 nanoparticle solutions (5.2 x 10-6 M), (A) recorded at different THF/water ratios by volume; excitation at 380 nm and (B) recorded during the reverse process of adding THF to a THF/water solution of R3 nanoparticles, λex = 38 ...... 137

Figure A4.123:Fluorescence decay profile of a PBM solution in THF monitored at λmax 450 nm ...... 144

Figure A4 124: Fluorescence decay profile of a R2 nanoparticle solution (THF/water ratio = 1/3) monitored at λmax 590 nm; excitation wavelength = 340 nm ...... 144

Figure A4 125:Fluorescence decay profile of a R2 nanoparticle solution (THF/water ratio = 1/5) monitored at λmax 590 nm; λex = 340 nm ...... 144

Figure A4.126: Fluorescence decay profile of a R2 nanoparticle solution (THF/water ratio = 1/11) monitored at λmax 590 nm; λex = 360 nm ...... 145

Figure A4.127:Fluorescence decay profile of a R3 solution in THF monitored at λmax 460 nm; λex = 360 nm ...... 145

Figure A.128:Fluorescence decay profile of a R2 nanoparticles solution (THF/water ratio = 1/45) monitored at λmax 600 nm; excitation wavelength = 360 nm ...... 145

Figure A4.129:Fluorescence decay profile of a R2 nanoparticle solution (THF/water ratio = 1/40) monitored at λmax 600 nm; excitation wavelength = 360 nm...... 146

Figure A4.130:Fluorescence spectra of R2 nanoparticles recorded at different THF/water ratios by volume and different concentrations; λex = 345 nm ...... 146

Figure A4.131:Shift in emission maximum of R3 due to change in the THF/water ratio; arrows show the direction of emission shift ...... 146

Figure A4.132;Fluorescence spectra of R3 nanoparticle solutions (5.2 x 10-6 M) at different THF/water ratios by volume; λex = 380 nm ...... 147

Figure A4.133:Fluorescence spectra of R3 nanoparticles recorded during the reverse process of adding THF solutions to a THF/water solution, λex = 380 nm ...... 147

Figure A4.134:Emission spectra of R2 in different solvents; λex = 350 nm ...... 147

Figure A4.135:1HNMR of R2 ...... 148

Figure A4.136: 13CNMR of R2 ...... 148

Figure A4.137: 1HNMR of R3 ...... 148

Figure A4.138: 13CNMR of R3 ...... 149

Figure 5.139: Emission spectra of CFONs in the dispersion medium at different excitation energies ...... 152

Figure 5.140: Excitation spectra of composite nanoparticles monitored at different emission maxima ...... 154

Figure 5.141: Emission spectra of composite nanoparticles at different fractions of water and THF, λex = 340 nm ...... 155

Figure 5.142: Emission spectra of composite nanoparticles dispersed in the medium - reverse process at different fractions of water, λex = 340 nm ...... 156

Figure 5.143: Emission spectra of CFONs dispersed in the dispersion medium made from different ratios of B and R2, λex = 340 nm ...... 157

Figure 5.144: Emission spectra of CFONs, B, R2 in the dispersion medium and in solid state, λex = 340 nm ...... 157

Figure 5.145: Scanning electron micrograph of FONs ...... 158

Figure 5.146: Widefield fluorescence image of composite organic nanoparticles (excitation source: mercury lamp; emission filter: low - pass < 405 nm in frequency; excitation filter: broad band UV 300 – 400 nm, max = 365 nm; Objective lens - 100 X) ...... 159

Figure 5.147: Confocal images of the CFONs showing emission in the (a) blue: 430-470nm, (b) green: 500-570nm , and (c) red: 600-670nm channels; and (d) darkfield image...... 159

Figure 5.148: Chromacity diagram and CIE coordinates for the CFONs demonstrating their high color purity (CIE X, Y: 0.34, 0.35) ...... 160

PART 1

CARBAZOLE: PROPERTIES AND APPLICATIONS 1. Introduction

Carbazole is a heterocyclic aromatic organic compound. It has a tricyclic structure, consisting of two six-membered benzene rings fused on either side of a five-membered nitrogen-containing ring,1 (Chart 1). The structure and numbering of positions in carbazole is shown in Chart 1. Carbazole is a conjugated unit that has interesting optical and electronic properties such as its photoconductivity and photorefractivity.2,3

H 8 N 1 9 7 2

6 3 5 4

Chart 1: Structure of Carbazole

A number of carbazole derivatives have been designed and synthesized and organic electronic devices based on these derivatives such as organic light emitting diodes (OLEDs) investigated. Carbazole derivatives are widely used as the materials for hole-transporting layers of OLEDs, utilizing their high charge mobility. Carbazole derivatives are also used as light- emitting layers because they are thermally stable and show blue photo- and electroluminescence due to the large bang gap of the biphenyl unit.2 Carbazole-based compounds are known for intense luminescence and are widely used as blue, green, red, and white emitters.3 The optical and electrical properties of carbazoles are easily tuned by substitution on the 2-, 3-, 6-, 7- and 9H-positions.

OLEDs based on thin layers of small molecules4-6 and polymers6-8 continue to attract wide attention owing to the numerous possible applications including in flat-panel displays.

The design and synthesis of hole-transporting materials,9-11 an important constituent of an

OLEDs, have significantly advanced since multilayered OLEDs were demonstrated by Tang and VanSlyke.12,13 Organic electronics such as OLEDs, organic field effect transistors

(OFETs), and photovoltaic cells have progressed enormously in recent years. Displays based on

OLEDs were introduced to the scientific community more than 12 years ago, and commercialization has taken serious momentum in recent years.14

2. History of electroluminescence devices

The first electroluminescent (EL) property was observed in 1965 by Helrich et al. The luminescence was quite inefficient requiring a drive voltage of more than 100 V to achieve significant light output.12 Later, Vincet et al. attempted to reduce the drive EL operation below

30 V by using a thin organic film, but the quantum efficiency of the EL diodes was only about

0.05%. Significant progress on the performance of EL in devices was made by Tang and

12 VanSlyke in 1987. They used aluminum (III)tris(8-quinolinolate) (Alq3) (Figure 1) as an emissive and electron transport material to obtain an external efficiency of 1%. Three years later Holmes et al. reported that the conjugated polymer, poly(p-phenylenevinylene) could be used as a polymer emitting layer in an EL device. Though the Holmes group obtained a low efficiency, they opened the door for the use of polymers in EL devices.15

Figure 1 : Tang and VanSlyke’s configuration of an EL cell and molecular structures of emissive and electron transport material, adapted from reference 12

Recently, EL based devices have begun to obtain a little market share. OLED based displays include those in cell phones, i-pods, computers, televisions and other niche lighting

devices. Until now liquid crystal display (LCDs) technology has overwhelmingly controlled the display market. LCDs are non-organic, non-emissive lighting devices that need a back light. (In some instances OLEDs backlighting has been used). Back lighting accounts for about half of the power requirements of LCDs being the reason for their high power consumption. In contrast, when used to produce displays, OLEDs produce self-luminous displays requiring no backlighting. This means that one can construct thin, flexible and compact displays that have a wide viewing angle, up to 160 degrees and require very little power, only 2-10 volts. In addition to the display market, OLEDs are making inroads in the present lighting market for which six quadrillion BTUs of energy (1 BTU = 1055.05585 joules) is currently consumed per year in the world. 17% of this is used in building lighting. Incandescent bulbs convert 90% to heat and, hence go dead frequently because the filament, even in nearly degassed bulbs, is oxidized and consumed. Fluorescent lamps convert 70% to light. OLEDs do better both in their lifetime and in energy consumption.16a

C B A

Figure 2: (A) A recent OLEDs display included in a cell phone set, (B) The world’s largest 32-inch television, (C) OLEDs prototype for lighting, adapted from reference 16

As shown in Figure 2 OLED devices are thriving in various fields in the display and lighting industries. Red, green, and blue emitters are required for the fabrication of a full-color display.3

3. Metal-carbazole based phosphorescent OLEDs

Ever since Forrest el al.17 used fac-[Ir(ppy)3] (Hppy=2-phenylpyridine) (Chart 2) for organic green-OLEDs, the development of efficient phosphorescent materials for potential uses in full color flat-panel-display technology has taken on good momentum.18 The emission efficiency and the color of iridium-carbazole-based OLEDs can be tuned by structural modifications of the ligand chromophores.19 The performance of the device is greatly influenced by the charge balance between the electrons and holes from opposite electrodes.

Many carbazole derivatives are known to transport predominantly positive charge carriers in organic systems. Poly(9-vinylcarbazole) (PVK) is described as being a unipolar hole transporter (HT) whereas 4,4’-N,N’-dicarbazolebiphenyl (CBP) is known to have a more bipolar transport character.20 Typically, purely organic carbazole based compounds show high- mobility HT property. They have a tunable high triplet energy level. This is why they are widely used as the host materials for electrophosphors emitting different colors.

Chart 2: Structure of fac-[Ir(ppy)3] (Hppy = 2-phenylpyridine), first ever used iridium complex in OLEDs, adapted from reference 20

Lin et al. have reported some triplet-harvesting carbazolyl-substituted iridium complexes, (fac-[Ir(X-Cz-py)3] (X = H, 1; X = F, 2; Cz = carbazolyl, py = pyridyl) (Chart 3).

These complexes have advantages in terms of lower first ionization potential. This facilitates the hole transport, and enhances EL efficiencies relative to the prototypical fac-[Ir(ppy)3].22

Chart 3: Molecular structure of fac-[Ir(ppy)3] adapted from reference 22.

Holmes et al. synthesized phosphorescent iridium complexes (Chart 4) to get low yielding OLED devices.23

n = 5; 18 [Ir(btp-(FO)5)2(acac)]

n = 10; 19 [Ir(btp-(FO)10)2(acac)]

n = 20, 20 [Ir(btp-(FO)20)2(acac)]

n = 40; 21 [Ir(btp-(FO)40)2(acac)]

n= 10; 14 [Ir(ppy-(FO)10)2(acac)]

n =30; 15 [Ir(ppy-(FO)30)2(acac)] btpH = 2-(2’-benzo[b]thienyl)pyridine, acac = acetylacetonate, ppy = 2-phenylpyridinato, btp =

2-(2’-benzo[b]thienyl)- pyridinato, FO = 9,9-dioctylfluorene

Chart 4: Iridium complexes used by Holmes et al. as phosphorescent materials, adapted from reference 23

A carbazole derivative, 1,3,5-tris(2-(9-ethylcarbazyl-3)-ethylene)benzene (TECEB)

(Chart 5) was used as a hole transporting material in an OLED. Introduction of a carbazole moiety into the core molecule of TECEB added thermal stability. Carbazole is chemically stable and easily modified at the 2-, 3-, 6-, 7- or 9H-positions. These carbazole containing compounds have moderately high oxidative potential that makes them promising HTMs. The chemical structure of TECEB was designed in such a way that the carbazole moiety was introduced by C-C double bond connection at its 3- position.24

Chart 5: Structure of TECEB adapted from reference 24

Many carbazole derivatives have a sufficiently high triplet energy making them an efficient host where they can serve as red,25,26 green,27 or blue28 triplet emitters. Carbazole derivatives can be used as host materials for both small-molecule and polymer OLEDs. A polymer initially applied as a photoconductor: poly(9-vinylcarbazole) (PVK) is now widely used for polymer OLEDs.29 PVK consists of a non-conjugated main chain with carbazole units attached as side groups. PVK can be treated as a collection of carbazole molecules. For small- molecule OLEDs, a carbazole derivative that is often used as host for triplet emitters is CBP.

This host enhanced maximum internal quantum efficiencies to 60 to 80% in OLEDs device.29

Brunner et al.29 investigated the influence of substitution at the aromatic benzene rings of carbazoles as well as on the nitrogen on the optical and electrochemical properties and correlated these properties with various applications (HTM and emissive layer) in OLEDs. The authors showed that the position of the coupling site that connects the monomers to form oligomers determines the triplet energy. Furthermore, the HOMO level can be engineered by substitution at the 3-, 6-, and 9H- positions of the carbazoles.

4. Tuning the HOMO-LUMO by substitution on the carbazole

Carbazoles substituted on the 4,4’-positions of biphenyl via their nitrogen atoms, CBP, is a good host material for small-molecule OLEDs.29 In contrast to dimerization via the 3- position, dimerization via the 9H- position leads to a shift of the HOMO level to lower energy.

Replacement of alkyl groups at the 9H- positions (4, 6, and 7) (Chart 6) by aryl groups (8, 9, and 10) on carbazole oligomers shows a similar effect. The 9H- substituted alkyl group activates the carbazole ring system by increasing its electron density 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.

This is due to the fact that the electronic density at the 2- and 7- positions is less than at the 3- and 6- positions.

Chart 6: Chemical structures of the compounds to study the substitution/functional group-tunable optical and electrical properties adapted from reference 29

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

6- positions are additive. The substitution effects can act cumulatively on the HOMO level.

These authors also found that the effects of substitution can act against each other as evident from comparing compounds 4, 6, and 10. In this series, upon attaching a third carbazole the

HOMO level first shifted to higher energy. Again, on replacing the alkyl groups at the 9H- position by aryl groups the HOMO level shifted back. This method 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 via their 3- positions 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.29

5. Structure and working principle of OLED devices

The basic OLED consists of a stack of thin organic layers sandwiched between a transparent anode and a metallic cathode (Figure 3). The organic layers comprise a hole- injection layer, a hole-transport layer, an emissive layer, and an electron-transport layer. When an appropriate voltage (typically between 2 and 10 volts) is applied to the cell, the injected positive and negative charges recombine in the emissive layer to produce light

(electroluminescence). The structure of the organic layers and the choice of anode and cathode are designed to maximize the recombination process in the emissive layer, thus maximizing the light output from the OLED device. Holes are injected from the transparent anode, typically transparent indium/tin oxide. Electrons are injected from a low work function cathode, typically aluminum or calcium.30,31

Figure 3: Structure of OLEDs device, adapted from reference 30

Organic materials have been considered for fabrication of practical EL devices because they have extremely high fluorescence quantum efficiency, including in blue region and hence they are well suited for multi-color display applications.12

Figure 4: Light creation by OLEDs, adapted from reference 32

The process of light creation by OLEDs is as follows (Figure 4):

On applying a voltage across the OLEDs electrical current flows from the cathode to the anode through the organic layers. The cathode gives electrons to the emissive layer of organic molecules. The anode removes electrons from the conductive layer of organic molecules which is equivalent to giving electron holes to the conductive layer. At the boundary between the emissive and the conductive layers, electrons find electron holes. When an electron finds an electron hole, the electron fills the hole and the electron gives up energy in the form of a photon of light. The color of the light depends on the organic compound used in the emissive layer.32

A major challenge in OLEDs development is tuning the devices such that holes and electrons meet in equal quantities in the emissive layer. The mobility of holes (i.e. carbocationic charges) is much lower than that of electrons (carboanionic charges) in organic compounds. Light emission can only occur from singlet excitons. Only one in four excitons is formed as a singlet.33,34

Improvements in the efficiency and the stability (lifetime) of EL devices and the tuning of color using different emitting materials still remains a challenge. Considerable advances are also to be made for full-color displays which require three primary colors, i.e., blue, green, and red emitting materials. Due to the high band gap energy, blue light emitting materials have a low affinity for the electron from cathode in OLEDs. The design and synthesis of blue emitting materials suitable for the fabrication of stable OLEDs remain major obstacles. The life time of blue emitters is very low (7,000 hours) as compared to green (40,000 hours) and red (80,000 hours).35

6. References

1) http://en.wikipedia.org/wiki/Carbazole.

2) Yang, J.-X.; Tao, X.-T.; Yuan, C. X.; Yan, Y. X.; Wang, L.; Liu, Z.; Ren, Y.; Jiang, M. H.

J. Am. Chem. Soc. 2005, 127, 3278.

3) Adhikari, R. M.; Mondal, R.; Shah, B. K.; Neckers, D. C. J. Org. Chem. 2007, 72, 4727.

4) Segura, J. L. Acta Polym. 1998, 49, 319.

5) (a) Noda, T.; Shirota, Y. J. Am. Chem. Soc. 1998, 120, 9714. (b) Lu, P.; Hong, H.; Cai, G.;

Djurovich, P.; Weber, W. P.; Thompson, M. E. J. Am. Chem. Soc. 2000, 122, 7480.

6) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 371, 402.

7) Pistolis, G.; Andreopoulou, A. K.; Malliaris, A.; Kallitsis, J. K. Macromolecules 2004, 37,

1524.

8) Pei, J.; Yu, W.-L.; Huang, W.; Heeger, A. J. Macromolecules 2000, 13, 2462.

9) Forrest, S. R.; Burrows, P. E.; Thompson, M. E. Chem. Ind. 1998, 1022.

10) Katsumo, K.; Shirota, Y. Adv. Mater. 1998, 10, 223. (b) O’Brien, D. F.; Burrows, P. E.;

Forrest, S. R.; Koene, B. E.; Loy, D. E.; Thompson, M. E. Adv. Mater. 1998, 10, 1108.

11) Okumoto, H.; Yatabe, T.; Shimomura, M.; Kaito, A.; Minami, N.; Tanabe, Y. Adv. Mater.

2001, 13, 72.

12) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.

13) Thomas, K. R. J.; Lin, J. T.; Tao, Y.-T.; Ko, C.-W. Chem. Mater. 2002, 14, 1354.

14) Liu, Y.; Di, C.; Xin, Y., Yu, G.; Liu, Y.; Heb, Q.; Bai, F.; Xu, S.; Cao, S. Synt. Met. 2006,

156, 824.

15) Burroughs, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R.

H.; Burn, P. L.; Holmes, A. R. Nature 1990, 347, 539.

16) (a) Service, R. F. Science, 2005, 310, 1762. (b) http://electronics.howstuffworks.com/oled.htm.

17) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nature 2000, 403, 750.

18) (a) Gong, X.; Robinson, M. R.; Ostrowki, J. C.; Moses, D.; Bazan, G. C.; Heeger, A. J.

Adv. Mater. 2002, 14, 581. (b) KMhler, A.; Wilson, J. S.; Friend, R. H. Adv. Mater. 2002, 14,

701.

19) (a) Holder, E.; Langeveld, B. M. W.; Schubert, U. S. Adv. Mater. 2005, 17, 1109.

(b) Grushin, V. V.; Herron, N.; LeCloux, D. D.; Marshall, W. J.; Petrov, V. A.; Wang, Y.

Chem. Commun. 2001, 1494.

20) (a) Pai, D. M.; Yanus, J. F.; Stolka, M. J. Phys. Chem. 1984, 88, 4714. (b) Kanai, H.;

Ichinosawa, S.; Sato, Y. Synth. Met. 1997, 91, 195.

21) Yeh, S.-J.; Wu, M.-F .; Chen, C.-T.; Song, Y.-H.; Chi, Y.; Ho, M.-H.; Hsu, S.-F.; Chen,

C. H. Adv. Mater. 2005, 17, 285.

22) Wong, W.-Y.; Ho, C. L.; Gao, Z.-Q.; Mi, B.-X.; Chen, C.-H.; Cheah, K.-W.; Lin, Z.

Angew. Chem., Int. Ed. 2006, 45, 7800.

23) Sandee, A. J.; Williams, C. K.; Evans, N. R.; Davies, J. E.; Boothby, C. E.; Kohler, A.;

Friend, R. H.; Holmes A. B. J. Am. Chem. Soc. 2004, 126, 7041.

24) Li, J.; Liu, D.; Li, Y.; Lee, C.-S.; Kwong, H.-L.; Lee, S. Chem. Mater. 2005, 17, 1208.

25) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H. E.; Adachi, C.;

Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304.

26) Tsutsui, T.; Yang, M. J.; Yahiro, M.; Nakamura, K.; Watanabe, T.; Tsuji, T.; Fukuda, M.;

Wakimoto, T.; Miyaguchi, S. Jpn. J. Appl. Phys. 1999, 38, L1502.

27) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Appl. Phys.

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29) Brunner, K.; Dijken, A. V.; Borner, H.; Jolanda, J. A. M.; Bastiaansen, J. J. A. M.;

Kiggen, M. M. N.; Langeveld, B. M. W. J. Am. Chem. Soc. 2004, 126, 6035.

30) http://komar.cs.stthomas.edu/qm425/01s/Tollefsrud2.htm.

31) Shinar, J. Organic Light Emitting Devices-A Survey, Ed. Springer-Verlag: Berlin, 2003.

32) http://electronics.howstuffworks.com/oled2.htm.

33) Kraft, A., Grimsdale, A.; Holmes, A. B. Angew. Chem., Int. Ed.1998, 37, 428.

34) http://en.wikipedia.org/wiki/OLED.

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34, 7677.

CHAPTER 1

SYNTHESIS AND PHOTOPHYSICAL PROPERTIES OF CARBAZOLE-BASED BLUE LIGHT-EMITTING DENDRIMERS

A new class of highly fluorescent and stable carbazole-based dendrimers (1-5) that contain the ethynylbenzene and diethynylbenzene cores has been synthesized and characterized. They show very high extinction coefficients of absorption (Amax 328-353 nm) and high quantum yields of fluorescence (λmax 386-437 nm; ΦF 0.72-0.89; F 2.09-3.91 ns) in dichloromethane. The quantum yields of fluorescence of 1-5 in the solid state are equally high ( max 385-422 nm; ΦF 0.40-0.85). These data indicate their potential use as blue- emitting materials in organic light-emitting diodes (OLEDs).

1.1. Introduction

Ever since Tang and VanSlyke constructed electroluminescent (EL) devices using organic materials as emitters, development of new EL materials has become an active area of research because of their potential use in displays.1,2 A large effort to make organic light- emitting diode (OLED)-based technology a possible alternative to liquid crystal-based displays and to develop OLED displays for other niche lighting applications has also been underway.

Red, green, and blue emitters are required for the fabrication of a full-color display. The stability and efficiency of green and red emitters have approached commercially viable levels.3

But the design and synthesis of blue emitters suitable for fabrication of stable OLED devices remains a major challenge.4 The performance of blue emitters is not as efficient as that of red and green emitters.5

Carbazole-based compounds are known for their intense luminescence6,7 and widely used in OLEDs as blue emitters,8-12 white emitters,13 green emitters14 and red emitters.15 They also undergo reversible oxidation processes, making them suitable hole carriers.16a We have synthesized a new class of carbazole-based stable dendrimers that contain the ethynylphenyl core (1-5, Chart 1.1) in order to develop stable blue emitters that also have hole transporting properties. While 9-(4-(2-phenylethynyl)phenyl)-9H-carbazole (1) and 3,6-di-tert-butyl-9-(4-

(2-phenylethynyl)phenyl)-9H-carbazole (2) are monodenderons, 9-(4-(2-(3-(2-(4-(9H-carbazol-

9-yl)phenyl)ethynyl)phenyl)ethyny) phenyl)-9H-carbazole (3), 3,6-di-tert-butyl-9-(4-(2-(3-(2-

(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)ethynyl)phenyl)ethynyl)phenyl)-9H-carbazole

(4), and 3,6-di-tert-butyl-9-(4-(2-(4-(2-(4-(3,6-di-tert-butyl-9H-carbazol-9- yl)phenyl)ethynyl)phenyl)ethynyl)phenyl)-9H-carbazole (5) are meta- and para-substituted didendrimers. A dendrimer is a highly branched molecule that contains structurally three regions: the first region is the inner most encapsulated core, the second region is the tailored sanctuary, and the third region can hold a huge number of functionalities that interact with the bulk, hence defining the dendrimer’s macroscopic properties.16b

R R R

N N N R R 1. R = H R 2. R = t-Bu R R 3. R = H 4. R = t-Bu N N

R 5. R = t-Bu R

Chart 1.1: Structure of compounds 1-5

In designing these dendrimers, the ethynylphenyl group was preferred as the core because of its linearity, generally high fluorescence quantum yield, emission in the violet-blue

region, and synthetic advantage.17 A number of compounds containing the ethynylphenyl group as a conventional core have been synthesized and studied for their applications in electrooptical devices.18 The carbazole group has another advantage: it is easier to modify the 3- and 6- positions and tune the molecular properties.19 In this chapter an easy synthesis and discussion of the photophysical properties of 1-5 are presented. Not only do they exhibit intense luminescence both in solution and in the solid state, but they also show excellent stability. The solvatochromism exhibited by these dendrimers is also discussed.

1.2. Results and discussion

1.2.1. Synthesis: Following literature13 methods, carbazole (1.6a) and 3,6-di-tert-butyl-9H- carbazole (1.6b) were converted into 9-(4-bromophenyl)carbazole (1.7a) and 9-(4- bromophenyl)-3,6-di-tert-butylcarbazole (1.7b), respectively (Scheme 1.1). These compounds are the main intermediates for the synthesis of 1-5. Compounds 1 and 2 were synthesized by

Sonogashira coupling of 1-ethynylbenzene with 1.7a and 1.7b, respectively (Scheme 1.2).

Similar Sonogashira coupling of 1.7a and 1.7b with 1,3-diethynylbenzene yielded 3 and 4, respectively. Finally, 5 was prepared by coupling 1,4-diethynylbenzene with 1.7b (Scheme 2).

The coupling of 1.7a with 1,4-diethynylbenzene was not successful probably due to lower solubility of the final product (dendrimer similar to 5 without tert-butyl groups) in common solvents.

t-Bu t-Bu (a) N N H 1.6a H 1.6b t-Bu t-Bu

1.6a or 1.6b N (b)

Br 1.7a. R = H 1.7b. R = t-Bu

Scheme 1.1:a Reagents and conditions: (a) tert-butyl chloride, zinc chloride, nitromethane, 40-50 0C, 5 h; (b) Cu, potassium carbonate, 18-crown-6, o-dichlorobenzene, reflux, 12 h.

1.7a or 1.7b 1 or 2 (a)

1.7a or 1.7b 3 or 4 (a)

1.7b 5 (a)

Scheme 1.2a : Reagents and conditions: (a) CuI, Pd(PPh3)2Cl2, PPh3, triethylamine, tetrahydrofuran, reflux, 12 h (for 1 and 2), 72 h (for 3 and 4), and 7 days (for 5).

1.3. Photoluminescence in solution

Absorption spectra of 1-5 recorded in dichloromethane show a - * band at ~325-353 nm (Figure 1.5). The absorption maxima of 3 (329 nm) and 4 (337 nm) are not significantly red-shifted from that of 1 (326 nm) and 2 (334 nm), respectively. This indicates that there is only a slight conjugation between the carbazole arms when they are linked at the meta- positions of the central phenyl ring.20 Compound 5 having arms at the para-positions of the central phenyl ring expectedly showed a larger red-shifted absorption relative to that of 1-4, indicating a significant conjugation in 5.

Figure 1.5 : Normalized absorption and emission spectra of 1-5 recorded in dichloromethane, λex = Amax

When excited at 340 nm, dilute solutions of 1-5 in dichloromethane show violet to blue emission (Figure 1.1). It is interesting that dendrimers having tert-butyl groups (2 and 4) show significantly red-shifted fluorescence relative to those that have no tert-butyl groups (1 and 3).

On the other hand, the emission maxima of 3 (394 nm) and 4 (411 nm) are similar to those of 1

(390 nm) and 2 (410 nm), respectively. This indicates that the density of chromophores in a molecule can be enhanced through a meta linkage without significantly perturbing the purity of the emission. That there is a larger extension of conjugation in 5 than in 3 and 4 is reflected in its emission; the fluorescence spectrum of 5 ( max = 437 nm) was the most red-shifted among all the compounds studied.

The fluorescence quantum yields (ΦF) and lifetimes ( F) of 1-5 recorded in dichloromethane are presented in Table 1.1 along with their absorption (Amax) and emission ( max) maxima. The ΦF values of 1-5 were found in the range of 0.72-0.89 relative to that of 9,10- diphenylanthracene (0.90 in cyclohexane).21 Fluorescence decays of 1-5 were fitted with monoexponential functions indicating emission from the singlet excited state in each case. The nonradiative decay of the singlet state was calculated to be about 2 to 4 times lower than the radiative decay for 1-5.

1 -1 -1 compd Amax ε(dm3 cm- mol- λmax ΦF F koR (s ) koNR (s ) (nm) 1) (nm) (ns)

1 326 30100 390 0.72 2.53 2.80 × 1.15 × 108 108

2 334 32800 410 0.80 3.91 2.04 × 5.10 × 108 107

3 329 54700 394 0.80 2.44 3.27 × 8.28 × 108 107

4 337 48500 411 0.83 3.62 2.29 × 4.70 × 108 107

5 353 66700 432 0.89 2.09 4.26 × 5.24 × 108 107

Table 1 1. Photophysical data of 1-5a recorded in dichloromethane a λex = 340 nm for ΦF and F. The ΦF values are relative to that of diphenyanthracene (0.90 in cyclohexane). The F values were measured from argon-saturated solutions and decay was monitored at the corresponding λmax. ΦF values of 4 and 5 measured in various solvents are presented in Table 1.2. The ΦF values of 4 were found in the range of 0.76-0.88 irrespective of the solvent polarity, while those of 5 were comparatively lower in polar solvents than in nonpolar solvents. For example, the ΦF value of 5 in methanol (0.41) was almost half that observed in pentane (0.89) or toluene (0.86).

In each solvent, the fluorescence decay of 4 and 5 could be fit monoexponentially, indicating the singlet states are formed exclusively. Interestingly, the F values of both 4 and 5 showed strong solvent dependence and gradually increased with an increase in the solvent polarity. The

F value of 4, for example, was lowest in pentane (0.72 ns) and highest in acetonitrile (4.22 ns).

Similarly, the F value of 5 gradually increases from 1.38 ns in pentane to as high as 5.51 ns in acetonitrile.

Emission spectra of 1 were recorded at different concentrations in dichloromethane

(Appendix 1) to assess the extent of possible dimer formation. We could see sharp concentration quenching at very high concentrations but no red shift in emission spectra indicating no aggregation occurs up to concentrations as high as 10 mmol. Each dendrimer shows significant solvatochromism. The emission spectra exhibit a more significant red shift in polar solvents than in nonpolar solvents (Appendix 1). For example, the emission maximum of

2 in acetonitrile (434 nm) is about 75 nm red-shifted from that in pentane (359 nm) (Figure

1.7). Similarly the emission maxima of 5 gradually increases going from pentane (385 nm) to acetonitrile (470 nm). This solvatochromic effect can be attributed to a decrease in the energy of the singlet excited states as a function of an increase in the polarity of the solvents.

Figure 1.6: Fluorescence decay of 1 monitored at max = 390 nm in dichloromethane

( F = 2.53 ns, = 1.08); λex = 340 nm

4 5

solvents Amax λmax ΦF F Ama λmax ΦF F (ns) (nm) (nm) (ns) x (nm) (nm)

pentane 340 359 0.7 1.3 356 385 0.89 0.72 6 8

toluene 345 379 0.8 1.6 359 400 0.86 0.85 2 5

chloroform 344 400 0.8 2.5 356 419 0.69 1.34 4 1

dichloromethan 345 416 0.8 3.6 355 437 0.89 2.09 e 8 2

methanol 338 426 0.8 4.8 354 453 0.41 3.21

1 8

acetonitrile 340 435 0.8 5.5 353 470 0.67 4.22 7 1

Table 1.2: Photophysical data of 4 and 5 measured in different solvents a a λex = 340 nm for measuring ΦF and F. The ΦF values are relative to that of diphenylanthracene (0.90 in cyclohexane). The F values were measured from argon-saturated solutions and decay was monitored at the corresponding λmax.

Typically, a fluorophore has a larger dipole moment in the excited state than in the ground state. Following excitation, solvent dipoles can reorient or relax lowering the energy of the excited state.21 The solvent dependency of the emission of 1-5 can be described by plotting the Stokes shift against orientation polarizability.22 Mataga-Lippert plots of 1 and 2 are shown in Figure 1.8. The slope gives the variation of dipole moment upon excitation. The linear relationship suggests the presence of just one excited state. This was also revealed from the monoexponential decay of the fluorescence of the dendrimers.16 The solvatochromic effect is evidence that dendrimers demonstrate intramolecular charge transfer from the donor

(carbazole) to the acceptor (ethynyl) units.23 Compounds 3 and 4 have higher solvatochromic displacements than 2 because the former compounds have an additional donor acceptor unit.

On the other hand, although 5 also has an additional donor acceptor unit, it shows less solvatochromic shift. This may be due to 5 being more symmetrical than 3 and 4, which results in a lower excited state dipole of 5 which gets less stabilization by the polar solvent.24

Figure 1.7: Normalized emission spectra of 2 recorded in different solvents, λex = Amax

Figure 1.8 : Mataga-Lippert plot of 1 (red) and 2 (blue) (solvents: toluene, chloroform, methanol, and acetonitrile in the order of increasing Stokes shifts).

1.4. Solid-state photoluminiscence

Solid-state emissions of 1-5 as recorded in poly(methyl methacrylate) (PMMA) matrix

(Figure 1.10) are structured and similar to those obtained in solution. However, the emission spectra recorded from thin films of the dendrimers (Figure 1.9) are less structured and broader.

For example, thin films of 1 and 2 show emission from ~350 nm to more than 550 nm.

Interestingly, these broad solid-state emissions were observed mainly from 1 and 2. Thin films

of 3-5 showed more structured and narrower emissions like that observed in the PMMA matrix.

A head-to-tail compact packing may be responsible for excimer formation in the cases of 1 and

2. Thus, in these cases, the emission can be considered to be a combination of monomeric and excimeric emission. Prohibition of compact packing in the case of 3-5 due to two bulkier carbazole groups may be the reason why these compounds did not show broader emission.

Figure 1.9 : Normalized solid-state fluorescence spectra of 1-5 recorded in thin films of poly (methyl methacrylate), λex = Amax

Figure 1.10: Normalized solid-state fluorescence spectra of the thin films of 1-5, λex = Amax

Quantum yields of fluorescence (ΦF) in the solid state were measured by using an

25 integrating sphere and are presented in Table 1.3 along with the emission maxima ( max). The solid-state emission maxima of 1 and 2 were substantially red-shifted compared to those measured in the matrix PMMA, while such a shift was not observed for 3-5. The solid-state ΦF values of 1-5 ranged from 0.40 to 0.85 with 1 showing the lowest and 3 showing the highest.

1.5. Effect of aging and annealing on the solid-state emission

To evaluate the effect of aging and annealing on the solid-state emission of 1-5, thin films of PMMA containing the dendrimers were exposed to ambient light for several weeks as well as heated to 150 0C for 24 h. The emission spectra of 1 recorded immediately after forming the PMMA thin film, exposing the film to ambient conditions for two weeks, and annealing the film at 150 0C for more than 24 h are shown in Figure 1.11. There was no change in the emission spectra before and after such treatment. This indicates that 1 maintains its color purity under the conditions we used for aging and annealing. Similar results were obtained for

2-5. This suggests that they are reasonably stable compounds and suitable for use in OLED devices.

a b b compd Amax (nm) λmax (nm) ΦF

1 392 406, 431 0.40

2 393 434 0.65

3 392 402 0.85

4 395 392 0.62

5 422 426 0.68

Table 1.3: Emission maxima (λmax) and quantum yields of fluorescence ΦF of 1-5 recorded in the solid state

a Thin films of PMMA were used as matrices. bData obtained from thin films of 1-5; thin films were prepared by the spin casting method, λex = 340 nm; the ΦF values were measured by using an integrating sphere (errors within 15 % range).

1.6. Conclusions

Carbazole-based monomeric and dimeric dedrimers (1-5) were synthesized.

Photophysical studies of 1-5 indicate that chromophoric density may be increased in these dendrimers through a meta-linkage of the dendrons to the central ethnylphenyl core without perturbing the purity of the emission. However, a para-linkage of dendrons results in a significant red shift in the emission. Compounds 1-5 showed intense luminescence both in solution (ΦF 0.72-0.89) and in the solid state (ΦF 0.40-0.85). The solvatochromic effects observed in 1-5 can be explained on the basis of charge-transfer complex formation in the excited singlet state. These dendrimers were found to retain their color purity even when they were exposed to 150 0C for more than 24 h. The assessment of OLED device performance of some dendrimers is underway.

1.7. Experimental section

1.7.1. Instrumentation: Mass spectra were recorded on a Shimadzu GCMS-QP5050A instrument equipped with a direct probe (ionization 70 eV). Matrix assisted laser desorption

ionization (MALDI) spectra were obtained on a Bruker Daltonic Omniflex instrument (N2 laser, 337 nm). Melting points were uncorrected. A Bruker spectrometer (working frequency

1 300.0 MHz for H) was used to record the NMR spectra. CDCl3 was the solvent for NMR, and chemical shifts relative to tetramethylsilane at 0.00 ppm are reported in parts per million (ppm) on the δ-scale. Absorption and fluorescence spectra were recorded on a Shimadzu UV-2401 spectrophotometer and a Fluorolog-3 spectrometer, respectively. All measurements were carried out at room temperature unless otherwise specified.

1.7.2. Synthesis: 9-(4-Bromophenyl)carbazole (1.7a) and 9-(4-bromophenyl)-3,6-di-tert- butylcarbazole (1.7b) were synthesized following a literature method13 starting from carbazole

(1.6a).

9-(4-(2-Phenylethynyl)phenyl)-9H-carbazole (1). Compound 1.7a (2.405 g, 7.5 mmol), trans-dichlorobis(triphenylphosphine)palladium(II) (0.265 g, 0.375 mmol), triphenylphosphine

(0.0975 g, 0.375 mmol), CuI (0.07 g, 0.375 mmol), and anhydrous triethylamine were mixed in a degassed round-bottom flask. The mixture was heated to reflux. A solution of phenylacetylene (0.765 g, 7.6 mmol) in 10 ml of tetrahydrofuran (THF) was added to the above mixture. The mixture was refluxed overnight with stirring. After the reaction mixture was allowed to cool to room temperature, the solvent was evaporated under vacuum. The residue was mixed with dichloromethane and washed with water and then with brine. The organic layer was collected and dried under MgSO4. After filtration, the solvent was removed under vacuum.

The crude product was purified by chromatography (silica gel, 80% hexane in dichloromethane) to obtain pure 1 (0.75 g, 31%) as a yellowish white shining solid. 1H NMR

(300 MHz, CDCl3) δ 7.30 (m, 2 H), 7.35-7.38 (m, 3 H), 7.41-7.45 (m, 4H), 7.54-7.6 (m, 4H),

13 7.75 (m, 2H), 8.14 (d, J = 10 Hz, 2H); C NMR (300 MHz, CDCl3) δ 89, 91, 110, 120, 122,

122.5, 123, 124, 124, 126, 128, 132, 137, 140; mass spectrum (DIP-MS) m/z M+ 343 (100%);

HRMS (EI+) m/z 343.1362, calcd m/z 343.1361; mp 134-136 0C.

3,6-Di-tert-butyl-9-(4-(2-phenylethynyl)phenyl)-9H-carbazole (2). This compound was obtained in 25% yield as a white shiny solid, following the same procedure described for the

1 synthesis of 1 with 1.7b instead of 1.7a. H NMR (300 MHz, CDCl3) δ 1.45 (s, 18H), 7.3-7.39

(m, 4H), 7.44-7.5 (m, 1H), 7.52-7.6 (m, 6H), 7.73-7.78 (m, 2H), 8.15 (s, 2H); 13C NMR (300

MHz, CDCl3) δ 32, 35, 88.5, 90,109, 116.5, 122, 123.5, 124, 126.5, 128, 129, 132, 132.5,

133.5, 138, 139, 143.5; mass spectrum (DIP) m/e M+ 455 (60%), 440 (80%), 185 (100%);

HRMS (EI+) m/z 455.2621, calcd m/z 455.2613; mp 217-220 0C.

9-(4-(2-(3-(2-(4-(9H-Carbazol-9-yl)phenyl)ethynyl)phenyl)ethyny)phenyl)-9H- carbazole (3). Compound 3 was obtained following the same procedure described for the synthesis of 1 with 1,3-diethynylbenzene instead of phenylacetylene in a half molar ratio. 1,3-

Diethynylbenzene was added to 1.7a, and the mixture was refluxed for 3 days. This gave pure 3

1 in 20% yield as a white shiny solid. H NMR (300 MHz, CDCl3) δ 7.3 (m, 4H), 7.4-7.5 (m,

13 9H), 7.6 (m, 6H), 7.8 (m, 5H), 8.15 (d, J = 10 Hz, 4H); C NMR (300 MHz, CDCl3) δ 89,

119.5, 120, 122, 123.5, 126, 127, 129, 132, 133.5, 135, 138, 141; mass spectrum (MALDI-

TOF) m/z M+ 608; HRMS (FAB+) m/z 608.225, calcd m/z 608.2252; mp 217-219 0C.

3,6-Di-tert-butyl-9-(4-(2-(3-(2-(4-(3,6-di-tert-butyl-9H-carbazol-9- yl)phenyl)ethynyl)phenyl)ethynyl)phenyl)-9H-carbazole (4). Compound 4 was obtained following the same procedure described for the synthesis of 3. 1,3-Diethynylbenzene was treated with 1.7b, and the reaction mixture was refluxed for 3 days to obtain pure 4 in 20%

1 yield as a white shiny solid. H NMR (300 MHz, CDCl3) δ 1.45 (s, 36H), 7.38-7.42 (m, 5H),

7.47-7.51 (m, 4H), 7.53-7.62 (m, 7H), 7.75-7.8 (m, 4H), 8.15 (s, 4H); 13C NMR (300 MHz,

CDCl3) δ 32, 35, 89.5, 90, 109, 110, 116.5, 120.5, 121, 124, 126, 126.5, 127, 128, 132, 133.5,

135, 138, 138.5, 143.5; mass spectrum (MALDI-TOF) m/z M+ 833; HRMS (FAB+) m/z

832.4756, calcd m/z 832.4754; mp 202-205 0C.

3,6-Di-tert-butyl-9-(4-(2-(4-(2-(4-(3,6-di-tert-butyl-9H-carbazol-9- yl)phenyl)ethynyl)phenyl)ethynyl)phenyl)-9H-carbazole (5). Compound 5 was obtained by reaction of 1,4-diethynylbenzene (in half molar ratio) with 1.7b. The mixture was refluxed for

1 7 days to obtain pure 5 in 18% yield as a white shiny solid. H NMR (300 MHz, CDCl3) δ 1.45

(s, 36H), 7.38-7.52 (m, 8H), 7.58-7.6 (m, 8H), 7.72-7.78 (m, 4H), 8.15 (s, 4H); 13C NMR (300

MHz, CDCl3) δ 32, 35, 89.5, 90, 109, 117, 123, 127, 132, 133, 139, 143; mass spectrum

(MALDI-TOF) m/z M+ 833, HRMS (FAB+) m/z 832.4756, calcd m/z 832.4754; mp >360 0C.

1.7.3. Fluorescence quantum yields (ΦF): The ΦF values in solution were measured following a general method with 9,10-diphenylanthracene (ΦF = 0.9 in cyclohexane) as the standard.

Diluted solutions of 1-5 in appropriate solvents were used for recording the fluorescence spectra. Sample solutions were taken in quartz cuvettes and degassed for ~ 15 min. The degassed solutions had absorbances of 0.05-0.09 at 340 nm. The fluorescence spectra of each were recorded 3-4 times and an average value of integrated areas of fluorescence used for the calculation of ΦF. The refractive indices of solvents at the sodium D line were used. The ΦF values in the solid state were measured following a literature method.25 A concentrated dichloromethane solution of sample was cast as thin films on quartz plates and then was allowed to dry. The plate was inserted into an integrating sphere and the required spectra were recorded. The samples were excited at 340 nm. 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 due to reabsorption of the emitted

light. A method employed earlier was used to minimize the impact of this on the calculation of

3 the ΦF.

1.7.4. Fluorescence lifetime ( F) measurement: Solutions of 1-5 that showed absorbances of 0.1-

0.25 at 340 nm were placed in quartz cuvettes. Fluorescence decay profiles of argon-degassed

(~15 min) solutions were recorded with use of a single photon counting spectrofluorimeter.

Decays were monitored at the corresponding emission maximum of the compounds. In-built software allowed the fitting of the decay spectra ( 2 = 1-1.5) and yielded the fluorescence lifetimes.

1.8. References

1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.

2 (a) Shinar, J. Organic Light Emitting Devices-A Survey, Ed. Springer-Verlag: Berlin, 2003.

(b) Müllen, K.; Scherf, U. Organic Light-Emitting Devices. Synthesis, Properties and

Applications; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2006.

3) Mondal, R.; Shah, B. K.; Neckers, D. C. J. Org. Chem. 2006, 71, 4085.

4) Shah, B. K.; Neckers, D. C.; Shi, J.; Forsythe, E. W.; Morton, D. J. Phys. Chem. A 2005,

109, 7677.

5) Chan, L.; Lee, R.; Hsieh, C.; Yeh, H.; Chen, C. J. Am. Chem. Soc. 2002, 124, 6469.

6) Yu, H.; Zain, S. M.; Eigenbrot, I. V.; Phillips, D. Chem. Phys. Lett. 1993, 202, 141.

7) Howell, R.; Taylor, A. G.; Phillips, D. Chem. Phys. Lett. 1992, 188, 119.

8) Almeida, K. D.; Bernede, J. C.; Marsillac, S.; Godoy, A.; Diaz, F. R. Synth. Met. 2001,

122, 127.

9) Lee, J.; Woo, H.; Kim, T.; Park, W. Opt. Mater. 2002, 21, 225.

10) Ding, J.; Gao, J.; Cheng, Y.; Xie, Z.; Wang, L.; Ma, D.; Jing, X.; Wang, F. Adv. Funct.

Mater. 2006, 16, 575.

11) Morin, J.; Boudreault, P.; Leclerc, M. Macromol. Rapid Commun. 2002, 23, 1032.

12) Grigalevicius, S.; Ma, L.; Xie, Z.; Scheri, U. J. Polym. Sci., Part A: Polym. Chem. 2006,

44, 5987.

13) Liu, Y.; Nishiura, M.; Wang, Y.; Hou, Z. J. Am. Chem. Soc. 2006, 128, 5592.

14) Thomas, K. R.; Lin, J. T.; Tao, Y. T.; Ko, C. J. Am. Chem. Soc. 2001, 123, 9404.

15) Guan, M.; Chen, Z.; Bian, Z.; Liu, Z.; Gong, G.; Baik, W.; Lee, H.; Huang, C. Org.

Electron. 2006, 7, 330.

16) (a) Loiseau, F.; Campagna, S.; Hameurlaine, A.; Dehaen, W. J. Am. Chem. Soc. 2005,

127, 11352 and references therein. (b) Dykes, D. M. J. Chem. Technol. Biotechnol. 2001, 76,

903.

17) (a) Rodriguez, J. G.; Esquivias, J.; Lafuente, A.; Diaz, C. J. Org. Chem. 2003, 68, 8120.

(b) Yamaguchi, Y.; Kobayashi, S.; Wakamiya, T.; Matsubara, Y.; Yoshida, Z. Angew. Chem.,

Int. Ed. 2005, 44, 7040. (c) Yamaguchi, Y.; Ochi, T.; Miyamura, S.; Tanaka, T.; Kobayashi,

S.; Wakamiya, T.; Matsubara, Y.; Yoshida, Z. J. Am. Chem. Soc. 2006, 128, 4504.

18) (a) Schumm, J. S.; Pearson, D. L.; Tour, J. M. Angew. Chem., Int. Ed. Engl. 1994, 33,

1360. (b) Meier, H.; Ickenroth, D.; Stalmach, U.; Koynov, K.; Bahtiar, A.; Bubeck, C. Eur. J.

Org. Chem. 2001, 4431. (c) Anderson, S. Chem. Eur. J. 2001, 7, 4706.

19) Joule, J. A. Adv. Heterocycl. Chem. 1984, 35, 83.

20) Yamaguchi, Y.; Ochi, T.; Miyamura, S.; Tanaka, T.; Kobayashi, S.; Wakamiya, T.;

Matsubara, Y.; Yoshida, Z. J. Am. Chem. Soc. 2006, 128, 4504.

21) Sciano, J. C. Handbook of Organic Photochemistry; CRC Press: Boca Raton, FL, 1989;

Vol. 1, p 231.

22) Lakowich, J. R. Principles of Fluorescence Spectroscopy; Kluwer Academic/Plenum:

New York, 1999; p 187.

23) Fu, H.; Wu, H.; Hou, X.; Xiao, F.; Shao, B. Synth. Met. 2006, 156, 809.

24) Thomas, K. R.; Lin, J. T.; Velusamy, M.; Tao, Y.; Chuen, C. Adv. Funct. Mater. 2004,

14, 83.

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APPENDIX 1

Figure A1.12: 1H-NMR spectrum of 1

Figure A1.13: 13C-NMR spectrum of 1

Figure A1.14: 1H-NMR spectrum of 2

Figure A1.15: 13C-NMR spectrum of 2

Figure A1.16: 1H-NMR spectrum of 3

Figure A1.17:1H-NMR spectrum of 3

Figure A1.18:1H-NMR spectrum of 4

Figure A1.19:13C-NMR spectrum of 4

Figure A1.20:1H-NMR spectrum of 5

Figure A1.21:13C-NMR spectrum of 5

Figure A1.22 :Normailized UV-visible spectra of 1 in different solvents

Figure A1.23: Normalized fluorescence spectra of 1 in different solvents, λex = Amax

Figure A1.24: Normalized UV-visible spectra of 2 in different solvents

Figure A1.25 :Normalized UV-visible spectra of 3 in different solvents

Figure A1.26: Normalized fluorescence spectra of 3 in different solvents, λex = Amax

Figure A1 27: Normalized UV-visible spectra of 4 in different solvents

Figure A1.28::Normalized fluorescence spectra of 4 in different solvents, λex = Amax

Figure A1.29:: Normalized UV-visible spectra of 5 in different solvents

Figure A1.30:: Normalized fluorescence spectra of 5 in different solvents; λex = Amax

Figure A1.31: Fluorescence spectra of 1 recorded in dichloromethane at different concentrations; ex = Amax

Figure A1.32:: Fluorescence decay of 2 monitored at max = 413 nm in dichloromethane; ex = 340 nm

Figure A1.33: Fluorescence decay of 3 monitored at λmax = 389 nm in dichloromethane; λex = 340 nm

Figure A1.34: Fluorescence decay of 4 monitored at λmax = 417 nm in dichloromethane; λex = 340 nm

Figure A1.35: Fluorescence decay of 5 monitored at max = 437 nm in dichloromethane; ex = 340 nm

Table A1.1: Absorption and emission maxima of 1-3 observed in different solvents

TableA1.2: Fluorescence lifetimes ( F) of 1- 3 recorded in different solvents

CHAPTER 2

A PHOTOPHYSICAL STUDY OF BLUE, GREEN AND ORANGE-RED LIGHT- EMITTING CARBAZOLES

Simple synthetic procedures have been developed to prepare the stable carbazoles (B1-

B4, G1-G3, and R1-R3). These compounds emit blue, green, and orange-red light, respectively. Red-shifted emission in the solid state relative to that in solution is highly dependent on the nature and the positions of the substituents. For example, red-shifts in the solid state of B1-B4 and G1-G3 are in the range of ~ 15 nm and ~ 30 nm, respectively, but as high as 120 nm for R3. The presence of a carbaldehyde or malononitrile on the carbazole moiety is found to quench fluorescence severely in solution and in the solid state, as indicated by low fluorescence quantum yields of B1 (ΦF ~ 0.03), B3 (ΦF ~ 0.04), and G1-G3 (ΦF ~ 0.04-

0.15). However, the effect is not the same for the fluorescence lifetime (τF ~ 1 – 5.69 ns). The rate constants of radiative and nonradiative deactivation of B1-R3 have been found to be in the range of 6.40×106 - 9.50×108 and 1.38×108 – 9.84×108, respectively. Lowering the temperature from 25 0C to -10 0C causes a small but distinct red-shift in the emissions and a systematic increase in the ΦF values of blue and green emitters. Solvatochromism and concentration dependent emissions of the compounds are also discussed.

2.1. Introduction

The application of organic electroluminescent materials in flat-panel displays has led to an intensive search for stable organic materials that can emit light with high efficiency and brightness.1-7 A set of pure red, green, and blue emitters is required for full-color displays.

Carbazoles are known for intense luminescence and widely used in organic light-emitting diodes (OLEDs).8-19 They also undergo reversible oxidation processes, making them suitable

hole carriers.20 The molecular and optical properties of carbazole can be tuned by structurally modifying its 2, 3, 6, 7 and 9H-positions.21 In fact, the light-emitting property combined with the hole-transporting capability of carbazoles make them one of the important classes of compounds. They have been actively pursued for applications in OLEDs and other optoelectronic devices.

Although studies on carbazoles have appeared in the literature over the last few years, they are mostly focused on their use as materials and applications in devices.8-19 We have developed a new series (Chart 2.1, B1-B4, G1-G3, and R1-R3) that exhibit stable blue, green, and orange-red emission in solution and in the solid state and carried out a systematic study of

B1-B4, G1-G3, and R1-R3 in an attempt to understand photophysical properties and structure- property relationships in these compounds. While B1, B3, and R3 each possess mild electron- withdrawing (EW) carbaldehyde groups, B2 and B4 lack such groups. Compounds G1-G3 and

R2-R3 are substituted with strong EW malononitrile groups at various positions. We report on the solvatochromic properties, concentration dependent fluorescence switching, and the temperature dependent fluorescent behavior of the new systematically substituted carbazoles.

(A) Blue emitters

CHO N N N N N N N

OHC CHO CF3 B1 B2 B3 B4

(B) Green emitters (C) Red emitters

CN CN t-Bu t-Bu t-Bu t-Bu t-Bu t-Bu CN CN N N N N N

G1 G2

NC CN CHO NC CN NC CN N R1 R2

NC CN G3 R3

Chart 2.1: Structures of compounds B1-R3

Compounds B1-B4 emit intense blue light both in solution and in the solid state, while compounds containing the malononitrile group attached to the carbazole moiety (G1-G3) shows green emission. It is interesting that R1, which is the immediate synthetic precursor of

R2, shows an orange emission in solution, but a bluish-green emission in the solid state, while

R2 emits orange in solution and almost red in the solid state. On the other hand, R3 shows a green emission in dichloromethane and orange-red in the thin film. Moreover, R2 and R3 are rather different from B1-B4, G1-G3, and R1 in that the former compounds readily form nanoparticles under our now well defined tetrahydrofuran(THF)/water system and their emission can be switched on and off by manipulating the THF/water ratio.22 However, the latter compounds show no such behavior.

2.2. Results and discussion

2.2.1. Synthesis: Compounds B1-B4 were synthesized starting from carbazole (2.1) (Scheme

2.1). The first step involved N-substitution of 2.1 by the naphthyl and phenanthrynyl groups

resulting in 2.2 and 2.3, respectively. Bromination and iodination of 2.2 and 2.3 at the 3 and 6 positions provided 2.4-2.6. Compound 2.4 was treated with n-BuLi and dimethylformamide resulting in B1. Sonogashira coupling of 2.5 with phenylacetylene proved to be a difficult reaction. Thus, iodo-substituted 2.6 was synthesized, and this easily yielded B2. The reaction of

2.1 with 4-4’-dibromobibenzene (2.7) in the presence of potassium carbonate yielded 2.8.

Formylation of the latter introduces the aldehydic functionalities at the 3 and 3’-positions of the carbazoles, forming B3. Compound 2.7 was converted into 2,7-dibromocarbazole (2.10) following a literature procedure (Scheme 2.2).23 N-arylation of 2.10 was followed by substitution of bromine with the diphenylamine groups to obtain B4.

Br Br (a) (b) N N N H R R 2.1 2.2. R = naphthyl 2.4. R = naphthyl 2.3. R = phenanthrynyl 2.5. R = phenanthrynyl

I I (d) (e) 2.3 B2 N R 2.6

(f) (g) 2.1+ 2.7 N N B3

2.8

Scheme 2. 1: Synthesis of B1-B3§

§ Reactions and conditions: (a) 1-bromonaphthalene or 9-bromophenanthrene, K2CO3, nitrobenzene, reflux 48 hrs (b) N-bromosuccinimide, dichloromethane (c) n-BuLi, ether, DMF, 0 -70 C (d) N-iodosuccinimide, dichloromethane (e) phenylacetylene, Cl2PdP(PPh3)2, CuI, PPh3, triethylamine, reflux 12 hrs (f) 4-4’-dibromobibenzene (7), K2CO3, nitrobenzene, reflux 3 days (g) POCl3, DMF, 1,2-dichloroethane, reflux 12 hrs.

(a) Br Br Br Br NO2 2.7 2.9 (b)

Br Br (c) N Br Br N 2.11 2.10 H CF3 (d) B4

Scheme 2.2: Synthesis of B4

§ Reactions and conditions: (a) CH3COOH, HNO3, H2O, reflux 30 min (b) P(OEt)3 (c) 1-bromo- 4-trifluoromethylbenzene, K2CO3, DMF, reflux 12 hrs (d) Cs2CO3, Pd(PPh3)4, (t-Bu)3P, toluene, reflux 6 hrs.

Reaction of either of 2.4 and 2.5 with BuLi and DMF in ether (formylation) resulted in a mixture of mono and diformylated products (Scheme 2.3) that were separated using column chromatography. Treatment of mono formylated products (2.12 and B1) with malononitrile and basic aluminum oxide in toluene produced G1 and G2, respectively. A similar reaction of 3,6- di-formylated N-naphthyl product (2.13) yielded G3. The syntheses of R1-R3 are described elsewhere.22

Br Br CHO OHC CHO (a) + N N N R R R 2.4. R = naphthyl 2.12. R = naphthyl 2.13. R = naphthyl 2.5. R = phenanthrynyl B1. R = phenanthrynyl

(b) (b)

G1 and G2 G3

Scheme 2.3: Synthesis of G1-G3

§Reactions and conditions: (a) n-BuLi, DMF, ether, -70 0C (b) malononitrile, toluene, basic Al2O3, reflux 6 hrs.

2.2.2. Absorption and emission spectra in solution:

Representative examples of the absorption and emission spectra recorded in dichloromethane (DCM) are shown in Figure 2.36 and 2.37, respectively. The absorption in the 280 nm to 360 nm region can be assigned to the π-π* transition and the low energy absorption in the 360 nm to 500 nm region to an intramolecular charge-transfer (ICT) transition.22 Compounds with no EW groups (B2 and B4) showed no similar absorption behavior. The assignment of the low energy band to the ICT transition is further supported because this band underwent a small but gradual red-shift when solvents of increasing polarity were used. (Appendix 2). An ICT state is expected to be more stable in polar solvents. On the other hand, the high energy absorption bands showed no dependency on solvent polarity. The extinction coefficients (ε) of absorption at the peak were in the range of 12500-48000 L mol-1 cm-1, except for G2 and G3. The ε values of the latter compounds (293000 and 110000 L mol-1 cm-1, respectively) were much higher.

Figure 2.36: Absorption spectra of B1, B4, G1, G3, R1, and R2 recorded in dichloromethane

Figure 2.37: Normalized emission spectra of B1, B4, G1, G3, R1, and R2 recorded in dichloromethane, λex = Amax

The emission spectra of B1-B4, G1-G3, and R1-R3 recorded in DCM extend from the violet-blue to orange-red regions. The emission maxima of B1-B4 (λmax = 392-405 nm) were

similar, indicating that substitution of carbazole with either a smaller carbaldehyde group or conjugation extending ethynylphenyl groups at the 3 and 6 positions causes no significant red- shift. The geometry optimized structures of B1 and B2 show the phenynthrynyl group almost orthogonal to the carbazole moiety (Appendix 2). Interestingly, even though the ethynylphenyl group and the carbazole moiety are coplanar in B2, its emission (λmax = 392 nm) is not red- shifted significantly relative to that of B1 (λmax = 402 nm). This reinforces that extension of conjugation due to the ethynylphenyl groups at the 3 and 6 positions imparts a negligible effect on the emission. Nevertheless, N-substitution by the ethynylphenyl group that contains a carbaldehyde or malononitrile group causes a large red-shift in the emission (vide infra).

The emission spectrum of B4 was narrower than are those of B1-B3. This is because B4 is substituted at the 2 and 7 positions, while B1-B3 are substituted at 3 and 6 positions. It may also be due to the diphenylamine groups in B4. Nonetheless, significant differences were observed in the fluorescence quantum yields (ΦF) of B1-B4 (Table 2.1). The ΦF values of B1

(0.03) and B3 (0.04) were smaller by an order of magnitude than are those of B2 (0.32) and B4

(0.52). Carbaldehyde substitution significantly decreases the fluorescence quantum yield. This may be related with free bond rotation around C-CHO causing excitation diffusion and deactivation. In other words, rotational deactivation increases the chance of non-radiative relaxation of the excited singlet state, resulting in the decrease in the ΦF value. The effect of the carbaldehyde group on the fluorescence lifetime (τF) was not the same. Although the τF value of B3 (1 ns) was abnormally short, the τF value of B1 (4.68 ns) was found to be similar to those of B2 (4.27 ns) and B4 (3.48 ns).

Table 2.1 Photophysical properties of B1-B4, G1-G3, and R1-R3 recorded in dichloromethane and in the thin films§

Compound Solution Solid

0 -7 -8 Amax λmax τF ΦF kR 10 kNR010 λmax ΦF (ns) (nm) (nm) (s-1) (s-1) (nm)

B1 320 402 4.68 0.03±0.01 0.64 2.06 418 0.04±0.01

B2 329 392 4.27 0.32±0.03 10.5 2.23 400 0.45±0.06

B3 329 405 ~1 0.04±0.01 4.10 9.84 418 0.06±0.01

B4 377 403 3.48 0.52±0.05 14.9 1.38 414 0.37±0.04

G1 406 480 4.68 0.04±0.01 0.85 2.04 515 0.11±0.02

G2 405 473 <1 0.05±0.01 5.00 9.50 502 0.12±0.02

G3 418 453 <1 0.07±0.01 7.00 9.30 525 0.15±0.02

R1 351 528 5.69 0.10±0.01 1.78 1.60 445 0.21±0.03

R2 350 534 4.20 0.40±0.03 9.50 1.42 600 0.34±0.05

R3 385 474 3.70 0.30±0.02 8.10 1.80 594 0.25±0.04

§ Compounds were excited at or near the corresponding Amax for τF and ΦF. The ΦF and τF values were measured from argon-saturated solutions and decay was monitored at the corresponding λmax. The OD of the samples for τF and ΦF measurements were less than 0.13 and 0.09, respectively. The ΦF values for B1-B4 and G1-G3 are relative to that of diphenylanthracene (0.90 in cyclohexane). The ΦF values for R1-R3 are relative to that of riboflavin (0.3 in ethanol).25

The difference in the structure of B1 and G1 is that the later contains a malononitrile group instead of a carbaldehyde group (Chart 2.1). However, the emission of G1 (λmax = 480 nm) was red-shifted by about 80 nm relative to that of B1 (λmax = 402 nm). As pointed out above, the additional conjugation provided by the malononitrile groups in G1 does not account for such a significant red-shift in the emission. If this were the case, the emission of G3 would

have been red-shifted in comparison to that of G2 because the former contains one malononitrile group, while the latter has two such groups. In contrast, the emission of G3 (λmax

= 454 nm) is actually blue-shifted relative to that of G2 (λmax = 473 nm). The effect of the malononitrile group in causing a blue-shift in the emission is associated with its ability to influence the polarity of the excited singlet state. This is also supported by the strong solvatochromic effect observed in these compounds (vide infra). The singlet states of G1 and

G2 are less polar than that of G3 due to the presence of two EW malononitrile groups in the latter. A highly polar singlet state would be less stabilized in relatively nonpolar DCM, causing a blue-shifted emission from G3. The malonitrile group also causes a significant decrease in the fluorescence quantum efficiencies similar to the carbaldehyde group. The ΦF values of G1-G3

(0.04-0.07) were found to be much lower than that of carbazole (0.42).24 The effect of the malononitrile group on the fluorescence lifetime (τF) was, however, not similar. Compounds

G2 and G3 exhibited abnormally short lifetime τF (<1 ns), while that of G1 was ~ 4.68 ns.

N-phenylethynylphenyl substituted carbazoles exhibit an intense blue emission in DCM.

However, the presence of a carbaldehyde or malononitrile substitutent on the ethynylphenyl group completely changes the emission behavior, as exhibited by the large red-shifted emission of R1-R3. The emission characteristics of R1-R3 are, in fact, interesting. Compounds R1 (λmax

= 528 nm) and R2 (λmax = 534 nm) emit orange light in DCM. However, R3 emits bluish- green light (λmax ~ 474 nm) in DCM, although it contains an additional ethynylphenyl group relative to R1 and R2. The emission maximum of R3 is blue-shifted about 55-65 nm relative to those of R1 and R2. The geometry optimized structure of R3 expectedly shows that the long ethynylphenyl side chain is flat in this molecule. Thus, the additional ethynylphenyl group renders R3 more rigid, which may be the cause of the blue-shifted emission of the latter.

The ΦF values of R1 (~0.10), R2 (~0.40), and R3 (~0.30) were found to be larger than those of G1-G3 (~0.04-0.07). Compound R1 showed a longer τF (5.69 ns) compared to those of

R2 (τF ~4.20 ns) and R3 (τF ~3.70 ns). In fact, the fluorescence decays of R1-R3 were biexponential, while those of B1-B4 or G1-G3 were monoexponential. The lifetimes of the shorter components of R1 and R2 are less than 1 ns, while that of R3 is 1.80 ns. The two distinct lifetimes (1.80 ns and 3.70 ns) observed in the case of R3 are probably due to two separate singlet states associated with the carbazole and 1,4-diethynylphenylbenzene moieties.

It should be noted that there is no systematic difference in the rate constants of radiative

0 0 deactivation (kR ) of the three classes of compounds. For example, the kR values of B1-B4

(~6.40×106 - 1.49×108 s-1), G1-G3 (~8.54×106 – 7.00×107 s-1), and R1-R3 (~1.78×106 –

9.50×108) are in the same range, although the difference between individual compounds is as high as two orders of magnitude.

2.2.3. Emission spectra in the solid state: The emissions of all compounds except R1 in the solid state were found to be red-shifted compared to the corresponding emission in solution

(Figure 3.38). This is most probably due to molecular stacking in the solid state. Solid state packing has little effect on the shape of the emission however because the thin film emission of

B1 was broad while that of B4 was narrow, similar to the corresponding emissions in solution

(DCM). The red-shift in solid state emission results from interaction of neighboring polar substituents since the compounds are more polar in the singlet excited states than in the ground states (vide infra). Intermolecular interaction of neighboring polar substituents lowers the energy level of the excited states. This also explains why the extent of the red-shift was smaller for B1-B4 (~15 nm) which is either without or has weak EW substituents.

Figure 2. 38: Normalized emission spectra of thin films of B1, B4, G1, G3, R1, and R2, λex = Amax

G1-G3 contain more polar malononitrile groups and exhibit a moderate red-shift (~30 nm). The red-shift was large in the cases of R2 (66 nm) and R3 (120 nm). On the other hand,

R1 is strikingly different in that its emission showed a blue-shift of ~ 85 nm and even though the structures of R1 and R2 are similar. The replacement of the carbaldehyde group in R1 (λmax

= 445 nm) with a malononitrile group R2 (λmax = 600 nm) does not account for the emission difference in the solid state. Compound R1 probably forms H-aggregates in the solid state resulting in a blue-shift, while R2 and R3 undergo J-aggregation that accounts for the red- shift.25,26 All these compounds, except for B4 and G3, exhibit an excimer emission (spectra provided in Appendix 2). Very weak excimer emissions were observed in the case of B4 (500 nm) and G3 (630 nm).

ΦF values of these compounds in the solid state (integrating sphere) were found similar to the corresponding values observed in DCM (Table 2.1). The ΦF values of B2 (0.45) and B4

(0.37) were reasonable. But B2, B3, and G1-G3 showed very low ΦF values (<1%), indicating that the presence of the carbaldehyde or the malonitirle group on the carbazole moiety severely quenches the fluorescence. However, the ΦF values of R1-R3 (0.21-0.34) suggest presence of

these groups on the side phenylethynylphenyl function fails to quench fluorescence in the solid state.

2.2.4. Solvatochromism: A moderate solvatochromic effect was observed for B1, B3 and G1-

G3 each containing EW groups on the carbazole. Representative examples of normalized fluorescence of B1 and G1 recorded in hexanes, toluene, DCM, and acetonitrile are shown in

Figures 2.39 and 2.40. The emission maxima were gradually red-shifted when the compounds were dissolved in the non-polar pentane to the polar acetonitrile. The ground states of these compounds are relatively polar as indicated by their ICT absorptions. The red-shifted emissions in more polar solvents indicate that the singlet excited states are more polar than the ground states. The increased polarity of the singlet states stem from further charge separation following excitation, with the negative charge mainly on the carbonyl group and the positive charge on the nitrogen atom of the carbazolyl moiety. This solvatochromic effect is attributed to the decrease of the singlet excited state energy with the increases of the solvent polarity.

Figure 2. 39: Normalized emission spectra of B1 recorded in different solvents, λex = Amax

Figure 2.40: Normalized emission spectra of G1 recorded in different solvents, λex = Amax

The emission maximum of B1 is red-shifted from pentane (λmax = 376 nm) to acetonitrile (λmax = 416 nm) because of the EW group (Table 2.2). On the other hand, B2 and

B4 with no EW groups, show no solvatochromism (Appendix 2). The emission maximum of

B2 in pentane (λmax = 386 nm), for example, is similar to that observed in acetonitrile (λmax =

390 nm). The red-shift from pentane to acetonitrile was more prominent (~60 nm) for G1-G3, because of the more polar malonitrile group. However, such a systematic solvatochromic effect was not observed in R1-R3. Interestingly, R2 shows a negative solvatochromic effect from toluene to DCM (λmax = 535 nm) and acetonitrile (λmax = 490 nm), although it shows a large red-shift from pentane (λmax = 448 nm) to toluene (λmax = 554 nm). Such a behavior of R1-R3 is probably due to formation a twisted ICT in their excited singlet states.18,19

Table 2.2: Absorption maxima (Amax), emission maxima ( max), and fluorescence quantum yields ( F) of B1, G1, and R2 measured in different solventsa Compound B1b G1c R2d

Solvent Amax λmax ΦF Amax λmax ΦF Amax λmax ΦF (nm) (nm) (nm) (nm) (nm) (nm)

Pentane 313 376 0.02 393 447 0.16 345 448 0.50 5

Toluene 316 380 0.02 402 445 0.16 348 554 0.48 2 dichloromethan 320 402 0.02 405 481 0.11 350 535 0.40 e 0

acetonitrile 314 416 0.01 395 503 0.09 346 490 0.33 5

a b,c Excitation wavelength for B1 (λex = 320 nm), G1 (λex = 405 nm), and R2 (λex = 350 nm), 9,10-diphenylanthracene was used as standard, driboflavin was used as standard25

The ΦF values of B1, G1, and R2 – three representative examples - are included in

Table 2.2. The ΦF values decrease slightly going from a nonpolar to a polar solvent. The ΦF

value of B1 in pentane (0.025), for example, was slightly higher than that in acetonitrile

(0.015). Similarly, the ΦF value of R2 decreased from 0.50 in pentane to 0.33 in acetonitrile.

2.2.5. Concentration dependent fluorescence switching: Concentration dependent fluorescence

switching was observed in R3 (Figure 2.41) but in none of the other compounds. A dilute

-9 solution of R3 (4.10×10 M in DCM) emits blue (λmax ~ 430 nm). However, the emission

shows a red-shift and becomes structureless at higher concentrations. The emission becomes

green and the intensity keeps increasing in the concentration range of 10-6 to 10-5 M. In fact, the

-4 emission was green (λmax ~ 475 nm) at a concentration as high as 2.60×10 M. But severe

quenching of fluorescence was observed if the concentration was further increased. For example, the emission intensity was negligible at a concentration of 5.30×10-3 M.

Interestingly, the fluorescence of this compound was not quenched in the solid state, even though the molecules are closer than in solution. R3 contains an elongated, rigid, flat side chain comprised of multiple ethynylphenyl groups. As its concentration is increased, molecules of R3 come in close proximity forming stacked structures even in solution. This explains why the compound exhibits significantly red-shifted emission (120 nm) in concentrated solution and in the solid state. Although fluorescence was severely quenched at higher concentrations in solution, the ΦF measured in the solid state (0.25) was similar to that recorded in DCM (ΦF =

0.30) at a lower concentration.

2.3. Effect of aging and annealing Thin films of B1-R3 were exposed to ambient light for several weeks as well as heated to 150 0C for 24 hours. There was no change in the emission spectra before and after either treatment. For example, the fluorescence spectra of thin films of R2 recorded under different

conditions (pristine, after exposing the film for several weeks, and after heating the film at 150

0C for 24 h) are essentially the same (Figure 2.7). B1-R3 maintain their color purity and are reasonably stable under the conditions used for aging and annealing.

2.4. Temperature dependent emission spectra We also studied the effect of temperature on the absorption and emission spectra of B1,

B3, G2, and G3. There is no change in the absorption spectra, but these compounds show a small red-shift in the emission with decreasing temperature, Figure 2.43. For example, the

0 emission of B1 recorded at -10 C (λmax = 420 nm) is about 14 nm red-shifted from that at 25

0 C (λmax = 406 nm) (Table 2.3) likely because alignment of the solvent dipole around the molecule in the excited state is prevented at low temperature. A change in the dielectric constant of the solvent upon cooling may be important. ΦF values increase nominally but systematically as the temperature is decreased (Table 2.3). ΦF values of B1 (0.05) and G3

(0.07) at 25 0C, for example, are lower than those measured at -10 0C (0.20 and 0.25,

respectively). The rotational deactivations of the singlet excited states are minimized at lower temperatures, resulting in an increase in the fluorescence quantum yield.

Figure 2.43: Emission spectra of B1 recorded at different temperatures, λex = 330 nm

Table 2.3: Emission maxima (λmax) and fluorescence quantum yields (λF) of B1 and G3 recorded at different temperatures

Compound B1 G3

Temperature λmax ΦF (nm) λmax ΦF (nm) (0C) (nm) (nm) 25 406 0.05 453 0.07

10 412 0.10 454 0.15

5 414 0.12 456 0.17

0 414 0.15 456 0.19

-5 416 0.18 456 0.21

-10 420 0.20 458 0.25

a Excitation wavelength for B1 (λex = 330 nm) and G3 (λex = 410 nm); error in the ΦF values = ±0.01.

2.5. Experimental section

2.5.1. Fluorescence Quantum Yields (ΦF): The ΦF values in solution were measured following a general method with 9,10-diphenylanthracene (ΦF = 0.9 in cyclohexane) (for B1-G3) as the standard.17 For compounds R1-R3 riboflavin was used as standard.25 Dilute solutions of these compounds in appropriate solvents were used. Sample solutions were in quartz cuvettes were degassed for ~15 min. The degassed solutions had absorbances of 0.05-0.09 at absorbance maxima. The fluorescence spectra of each were recorded 3-4 times and an average value of integrated areas of fluorescence used for the calculation of ΦF in solution. The refractive indices of solvents at the sodium D line were used. The ΦF values in the solid state were measured following a literature method.17 A concentrated dichloromethane solution of sample was cast as thin film on a quartz plate and then was allowed to dry. The plate was inserted into an integrating sphere and the required spectra recorded. The samples were excited at absorption maxima in DCM solution. 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 due to re-absorption of the emitted light. A method

17 employed earlier was used to minimize the impact of this on the calculation of the ΦF.

2.5.2. Synthesis: Compounds B1-B4 and G1-G3 were synthesized starting from carbazole. The synthesis of R1-R3 is provided elsewhere.22

9-(Phenanthren-9-yl)-9H-carbazole (2.3). Carbazole (30 mmol), 9-bromophenanthrene

(60 mmol), potassium carbonate (200 mmol) and nitromethane (200 ml) were mixed in a dry round bottom flask and refluxed for 2 days. Solvent was distilled off and the product was purified by column chromatography (silica gel, dichloromethane/hexanes (1:5), affording pure

1 2.3 as a white solid (yield-70 %). HMR (300MHz, CDCl3): δ 7.16 ( d, 2H), 7.39 (m, 6H), 7.71

(m, 1H), 7.8 (m, 1H), 7.9 (d, 1H), 8 (s, 2H), 8.3 (d, 2H), 8.9 (t, 2H). 13CNMR (300 MHz,

CDCL3) δ 110.2, 119.7, 120.3, 122.8, 123.2, 124.1, 126, 127.2, 127.3, 127.5, 127.6, 127.8, 129,

129.5, 130.6, 131.8, 132.6 and 142.5. MS (EI) calculated 343.1334, measured 343.1364.

3,6-Dibromo-9-(naphthalen-1-yl)-9H-carbazole (2.4). 9-(Naphthalen-1-yl)-9H- carbazole (12 mmol) in 100 ml glacial acetic acid was subjected to sonication for 20 minutes and then stirred for 30 minutes in a round bottom flask. Bromine (6 mmol) in 20 ml glacial acetic acid was slowly added into the mixture over 20 minutes while stirring at room temperature. The mixture was stirred for an additional 20 minutes at the same temperature. Ice cold water (100 ml) was then added. The mixture was then subjected to suction filtration. The residue was washed several times with water and was then dried over vacuum to get pure 2.4

1 (yield-95 %). HMR (300MHz, CDCl3): δ 6.88 (d, 2H), 7.15 (d, 1H), 7.35 (d, 1H), 7.44 (d, 2H),

13 7.6 (m, 2H), 7.68 (d, 1H), 8.08 (m, 2H), 8.3 (s, 2H). CNMR (300 MHz, CDCL3): δ 112.5,

113.3, 123, 123.5, 124, 126, 126.5, 127, 127.5, 128.5, 129.5, 130.5, 133, 135 and 141.5. MS

(EI) calculated 448.9449, measured 448.9415.

3,6-Dibromo-9-(phenanthren-9-yl)-9H-carbazole (2.5). Compound 2.3 (20 mmol) was mixed with acetic acid in round bottom flask and subjected to sonication followed by stirring

for 30 minutes. Into this mixture bromine (10 mmol) in 30 ml glacial acetic acid was slowly dropped (30 minutes) while stirring. After 2 hours of additional stirring, ice-cold water was poured in. The mixture was subjected to suction filtration. The residue was washed with water several times to obtain 2.5 in the form of a yellowish white compound, which was dried under

1 vacuum (yield-95 %). HMR (300MHz, CDCl3): δ 6.9 (d, 2H), 7.11 (d, 1H), 7.41 (m, 3H), 7.7

13 (d, 2H), 7.79 (d, 1H), 8.29 (s, 2H), 8.8 (s, 2H). CNMR (300 MHz, CDCL3): δ 112, 113, 122.8,

123.3, 123.7, 123.9, 127.4, 127.8, 127.9, 128, 129, 129.1, 129.5, 130.8, 131.4, 131.6, 131.8 and

141. MS (EI) calculated 498.9606, measured 498.9575.

3,6-Diiodo-9-(phenanthren-9-yl)-9H-carbazole (2.6). Compound 2.3 (5 mmol) was dissolved in dichloromethane (100 ml). A solution of N-iodosuccinimide (11 mmol) in dichloromethane (10 ml) was slowly added to it. The whole mixture was stirred in dark for 6 hours at room temperature. The solvent was evaporated to get 2.6 (yield-90 %). 1HMR

(300MHz, CDCl3): δ 6.8 (d, 2H), 7.1 (d, 1H), 7.4 (m, 1H), 7.58 (d, 2H), 7.69 (m, 2H), 7.78 (m,

13 1H), 7.84 (s, 2H), 8.45 (s, 2H), 8.8 (s, 2H). CNMR (300 MHz, CDCL3): δ 82, 112, 122.8,

123.2, 123.5, 124.2, 127.3, 127.38, 127.6, 127.66, 128, 129, 129.1, 129.6, 130.6, 131.3, 131.5,

131.8, 135, 141. MS (EI) calculated 594.9341, measured 594.9302.

9-(Phenanthren-9-yl)-9H-carbazole-3-carbaldehyde (B1). Compound 2.5 (4 mmol) in dry tetrahydrofuran (60 ml) was stirred under argon and cooled to -78oC. n-Butyl lithium (8 mmol in pentane) was slowly added with vigorous stirring. Stirring was continued for two hours at the same temperature. Dry dimethylformamide (25 mmol) was added and the mixture allowed stirring for two more hours at the same temperature. It was mixed with 2N hydrochloric acid at room temperature, extracted with ether, and purified by chromatography

(silica gel, dichloromethane/hexanes (1:5)) to get pure B1 (yield-30 %). 1HMR (300MHz,

CDCl3): δ 7.1 (m, 2H), 7.2 (s, 1H), 7.4 (m, 3H), 7.7 (m, 2H), 7.8-8 (m, 4H), 8.3 (m, 1H), 8.75

13 (s, 1H), 8.84 (m, 2H), 10.15 (s, 1H). CNMR (300 MHz, CDCL3): δ 110.5, 111, 120.8, 121.3,

122.8, 123, 123.3, 123.6, 123.8, 127, 127.2, 127.5, 127.8, 128, 129, 129.5, 130.5, 131.5, 131.8,

131.9, 142, 145, 191. MS (EI) calculated 371.1283, measured 371.1309.

9-(Phenanthren-9-yl)-3,6-bis(2-phenylethynyl)-9H-carbazole (B2). Compound 2.6 (3 mmol), trans-dichlorobis(triphenylphosphine)palladium(II), diisopropylamine (50 ml), phenylacetylene (6.2 mmol), benzene (10 ml), copper iodide (0.1 mmol), and triphenyl phosphine (0.3 mmol) were added in a dry round bottom flask. The mixture was refluxed for 12 hours. The solvent was evaporated and the solid obtained was subjected to column chromatography (silica gel, dichloromethane/hexanes (1:9)). The fraction containing B2 was

1 evaporated to obtain pure B2 as white powder (yield-56 %). HMR (300MHz, CDCl3): δ 7.05

(d, 2H), 7.28 (s, 1H), 7.35-7.45 (m, 6H), 7.47 (d, 1H), 7.54-7.65 (m, 6H), 7.75 (m, 2H), 7.85 (t,

13 1H), 8 (s, 2H), 8.45 (s, 2H), 8.9 (t, 2H). CNMR (300 MHz, CDCL3): 88, 90, 111, 115, 122.8,

123.2, 123.5, 123.8, 124.2, 127.3, 127.5, 127.7, 128, 128.4, 129, 129.1, 130, 130.5, 131.5, and

131.7. MS (EI) calculated 543.196, measured 543.198.

4,4'-Bis((9H-carbazol-9-yl)-3,3’-dicarbaldehyde)biphenyl (B3). Phosphoryl chloride

(0.1 mol) was added drop wise into cooled DMF (0.1 mol) in ice bath. The mixture was maintained at room temperature for 1 h, and a solution of 2.8 (0.004) in 5 ml of DMF was added. The reaction mixture was heated at 130 0C with stirring for 24 h and then poured into cracked ice. After neutralizing with a base, the mixture was extracted with chloroform. The extract phase was dried with anhydrous magnesium sulfate, and the solvent was removed by distillation in a vacuum. The solid residue was purified by using silica-gel column chromatography (eluent: ethyl acetate/hexanes, (1:4) to obtain a whitish-yellow solid (yield-50

1 %). HMR (300MHz, CDCl3): δ 7.4 (m, 2H), 7.5-7.6 (m, 6H), 7.75 (d, 4H), 8.0 (m, 6H), 8.25

13 (d, 2H), 8.7 (s, 2H), 10.1 (s, 2H). CNMR (300 MHz, CDCL3): δ 110, 110.3, 121, 121.5,

123.3, 124.7, 125, 127, 127.5, 128, 129, 129.7, 136.5, 140, 142, 144.5 and 192. MS (EI) calculated 540.183, measured 540.183.

2,7-Dibromo-9-(4-(trifluoromethyl)phenyl)-9H-carbazole (11). 2,7-Dibromo-9H- carbazole (2.10, 9 mmol), dimethylformamide (100 ml), potassium carbonate (54 mmol), and

1-fluoro-4-(trifluoromethyl)benzene (18 mmol) were added in a round bottom flask. The mixture was refluxed for 5 hours and then cooled to room temperature. Following the addition of water the mixture was filtered and the residue washed with water several times. The

1 resulting white solid (2.11) was dried under vacuum (yield-95 %). HMR (300MHz, CDCl3): δ

13 7.42 (d, 2H), 7.5 (s, 2H), 7.65 (d, 2H), 7.94 (t, 4H). CNMR (300 MHz, CDCL3): δ 113, 120,

122, 123, 125, 126, 128, 128.5, 140 and 141.5. MS (EI) calculated 468.911, measured 468.910.

N2,N2,N7,N7-Tetraphenyl-9-(4-(trifluoromethyl)phenyl)-9H-carbazole-2,7-diamine

(B4). A two-necked dry round bottom was charged with 2.11 (5 mmol), tri-tert-butylphenyl phosphine (0.5mmol), cesium carbonate (35 mmol), palladium diacetate (0.5 mmol), and diphenylamine (11 mmol). Argon was passed through the flask. Dry toluene (50 ml) was injected by syringe. The mixture was refluxed for 6 hours. After cooling the mixture to room temperature, saturated ammonium chloride solution was added and the mixture extracted with ethyl acetate and purified by column chromatography (alumina, ethyl acetate/hexanes (1:5)) to

1 get pure B4 (yield-50 %). HMR (300MHz, CDCl3): δ 6.9-7.0 (m, 6H), 7-7.1 (m, 12H), 7.19-

13 7.27 (m, 6H), 7.48 (d, 2H), 7.65 (d, 2H), 7.9 (d, 2H). ). CNMR (300 MHz, CDCL3): δ 106,

117.5, 119, 120, 120.5, 121, 122.5, 123.5, 125.5, 126, 129.4, 129.6, 141.7, 143.3, 146.2 and

148.3. MS (ES+) calculated 646.2446, measured 646.2441.

9-(Naphthalen-1-yl)-9H-carbazole-3-carbaldehyde (2.12). This compound was obtained as a side product while synthesizing 2.13 (see below). It was obtained in 30 % yield as

1 white powder and dried under vacuum. HMR (300MHz, CDCl3): δ 7.04 (d, 2H), 7.2 (d, 1H),

7.35- 7.47 (m, 3H), 7.55 (d, 1H), 7.65 (m, 2H), 7.9 (d, 1H), 8.1 (dd, 2H), 8.28, (d, 1H), 8.8 (s,

13 1H), 10.15 (s, 1H). CNMR (300 MHz, CDCL3): δ 110.5, 111, 120.5, 121, 123, 123.3, 123.5,

123.8, 125.8, 126.8, 127.3, 127.6, 127.8, 128.5, 129.5, 129.8,, 130.5, 133, 135, 143, 146, 192.

9-(Naphthalen-1-yl)-9H-carbazole-3,6-dicarbaldehyde (2.13). This compound was synthesized in an alternative way to the reported method.27 Compound 2.4 (10 mmol) was dissolved in dry tetrahydrofuran in a dry two-necked round bottom flask that was charged with argon and cooled in an acetone-dry ice bath. tert-Butyllithium in pentane (20 mmol) was added slowly with vigorous stirring. The mixture was stirred for an additional two hours at the same temperature. Dry DMF (25 mmol) was added and the mixture allowed to stirring for two more hours at the same temperature. It was subsequently mixed with 2N hydrochloric acid at room temperature, extracted with ether, and purified by chromatography (silica, dichloromethane/hexanes (1:5)). Compound 2.13 was obtained as white powder (yield-40 %).

1 It was dried under vacuum before characterization. HMR (300MHz, CDCl3): δ 7.1 (m, 3H),

7.38 (d, 1H), 7.55- 7.7 (m, 3H), 7.95 (d, 2H), 8.05 (d, 1H), 8.15 (d, 1H), 8.8 (s, 2H), 10.2, (s,

13 2H). CNMR (300 MHz, CDCL3): δ 111, 122.5, 123.5, 124, 126, 126.5, 127.3, 127.8, 128.5,

129, 130.5, 130.7, 132, 135, 147 and 192. MS (EI) calculated 349.1076, measured 349.1106.

2-((9-(Phenanthren-9-yl)-9H-carbazol-3-yl)methylene)malononitrile (G1). Compound

B1 (4 mmol), malononitrile (4.2 mmol), basic aluminum oxide (20 mmol), and toluene (100 ml) were added to a round bottom flask. The mixture was refluxed for 6 hours. After cooling to room temperature, it was filtered and the residue washed with dichloromethane. The filtrate

was evaporated and the solid obtained purified by chromatography (silica, dichloromethane/hexanes (1:4)). The procedure afforded pure G1 as yellow powder (yield-80

1 %). HMR (300MHz, CDCl3): δ 7.1-7.2 (m, 3H), 7.4-7.48 (m, 3H), 7.55 (m, 2H), 7.8-7.98 (m,

13 5H), 8,3 (dd, 1H), 8.85 (m, 3H). CNMR (300 MHz, CDCL3): δ 111.1, 113.3, 114, 115, 120.8,

121.7, 122.5, 122.7, 123.5, 123.6, 124.3, 124.8, 127.5, 127.55, 127.6, 127.7, 127.75, 127.8,

128.3, 128.8, 129.1, 129.3, 130.8, 131.2, 131.4, 131.9, 143, 145 and 160. MS (EI) calculated

419.1436, measured 419.1419.

2-((9-(Naphthalen-1-yl)-9H-carbazol-3-yl)methylene)malononitrile (G2). The same synthetic procedure employed for G1 was used to prepare G2. Compound 2.12 (4 mmol) was used instead of B1. The procedure afforded pure G2 as yellow powder (yield-80 %). 1HMR

(300MHz, CDCl3): δ 7.05 (d,1H), 7.2, (d, 1H), 7.4 (m,3H), 7.55-7.7 (m, 4H), 7.92, (m, 2H),

13 8.05 (d, 1H), 8.13 (d,1H), 8.28 (d,1H), 8.82 (s,1H). CNMR (300 MHz, CDCL3): δ 111, 111.2,

121, 121.8, 122.6, 123.4, 124.1, 124.8, 126, 126.5, 127.1, 127.8, 127.9, 128.8, 129, 130, 130.3,

132.4, 135, 143, 145, and 160. MS (EI) calculated 369.1266, measured 369.1266.

2,2’((9-(Naphthalen-1-yl)-9H-carbazol-3,3’-yl)methylene)malononitrile (G3). This compound was prepared using the synthetic procedure also used for the preparation of G1.

Compound 2.13 (4 mmol) was used instead of B1. Double the amount of malononitrile (8.4 mmol) was used. Compound G3 was obtained as yellow powder (yield-80 %). 1HMR

(300MHz, CDCl3): δ 7.1 (d, 1H), 7.15 (d, 2H), 7.4 (t, 1H), 7.62 (m, 2H), 7.72 (t, 1H), 7.95 (s,

13 2H), 8.08 (d, 3H), 8.19 (s, 1H), 8.8 (s, 2H). CNMR (300 MHz, CDCL3): δ 80, 112.3, 113.4,

114.3, 122, 123.5, 124.7, 125.3, 125.9, 126.5, 127.4, 128.1, 129, 129.7, 130.8, 131.2, 135, 146 and 160. MS (EI) calculated 445.131, measured 445.132.

2.6. Conclusions

A photophysical study of the stable blue, green, and orange-red light-emitting carbazoles (B1-B4, G1-G3, and R1-R3) was carried out. The solid state emission was red- shifted from the solution (DCM) emission for each compound, except for R1 which showed an unusual blue-shift. The substitution pattern on the carbazole significantly affected the relative red-shift of the corresponding solution spectra and that observed in the solid state. ΦF values of

B2, B4 and R1-R3 in solution were (0.30-0.52 in DCM) and in the solid state (0.21-0.45).

However, ΦF values of B1, B3, G1-G3, and R1-R3 were extremely low (0.03-0.15), indicating that the EW groups such as the carbaldehyde and malononitrile groups effectively quench the fluorescence. The τF values, on the other hand, were not found to be affected by the substitution pattern. The singlet states of those compounds having EW groups are more polar than their corresponding ground states. This results in B1, B3, and G1-G3 exhibiting positive solvatochromism while B2 and B4 showing negligible solvatochromism. Such a systematic solvatochromism did not occur with R1-R3 because of the twisted ICT nature of their singlet states. Lowering the temperature from 25 0C to -10 0C caused a small but distinct red-shift in the emissions and a slight increase in the ΦF values of B1, B3, G2, and G3. When thin films containing B1-R3 were exposed to ambient conditions for at least four weeks or heated at 150

0C for 24 hours, the emission purity was retained indicating these are robust compounds.

2.7. References

(1) Mondal, R.; Shah, B. K.; Neckers, D. C. J. Org. Chem. 2006, 71, 4085.

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

(3) Shinar, J. Organic Light Emitting Devices-A Survey, Ed. Springer-Verlag: Berlin, 2003.

(4) Müllen, K.; Scherf, U. Organic Light-Emitting Devices. Synthesis, Properties and

Applications; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2006.

(5) 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.

(6) Shah, B. K.; Neckers, D. C.; Shi, J.; Forsythe, E. W.; Morton, D. J. Phys. Chem. A 2005,

109, 7677.

(7) Thomas, K. R.; Lin, J. T.; Tao, Y. T.; Ko, C. J. Am. Chem. Soc. 2001, 123, 9404.

(8) Yu, H.; Zain, S. M.; Eigenbrot, I. V.; Phillips, D. Chem. Phys. Lett. 1993, 202, 141.

(9) Howell, R.; Taylor, A. G.; Phillips, D. Chem. Phys. Lett. 1992, 188, 119.

(10) Almeida, K. D.; Bernede, J. C.; Marsillac, S.; Godoy, A.; Diaz, F. R. Synth. Met. 2001,

122, 127.

(11) Lee, J.; Woo, H.; Kim, T.; Park, W. Opt. Mater. 2002, 21, 225.

(12) Ding, J.; Gao, J.; Cheng, Y.; Xie, Z.; Wang, L.; Ma, D.; Jing, X.; Wang, F. Adv. Funct.

Mater. 2006, 16, 575.

(13) Morin, J.; Boudreault, P.; Leclerc, M. Macromol. Rapid Commun. 2002, 23, 1032.

(14) Grigalevicius, S.; Ma, L.; Xie, Z.; Scheri, U. J. Polym. Sci., Part A: Polym. Chem. 2006,

44, 598713.

(15) Liu, Y.; Nishiura, M.; Wang, Y.; Hou, Z. J. Am. Chem. Soc. 2006, 128, 5592.

(16) Guan, M.; Chen, Z.; Bian, Z.; Liu, Z.; Gong, G.; Baik, W.; Lee, H.; Huang, C. Org. Elect.

2006, 7, 330.

(17) Adhikari, R. M.; Mondal, R.; Shah, B. K.; Neckers, D. C. J. Org. Chem. 2007, 72, 4727.

(18) 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.

(19) Nikolaev, A. E.; Myszkiewicz, G.; Berden, G.; Meerts, W. L.; Pfanstiel, J. F.; Pratt, D. W.

J. Chem. Phys. 2005, 122, 84309.

(20) Loiseau, F.; Campagna, S.; Hameurlaine, A.; Dehaen, W. J. Am. Chem. Soc. 2005, 127,

11352 and references therein.

(21) Joule, J. A. Adv. Heterocycl. Chem. 1984, 35, 83.

(22) Adhikari, R. M.; Shah, B. K.; Palayangoda, S. S.; Neckers, D. C. Langmuir manuscript submitted

(23) Dierschke, F.; Grimsdale, A. C.; Müllen, K. Synthesis 2003, 2470.

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Vol. 1, p 22.

(25) Palayas, S. S.; Cai, C.; Adhikari, R. M.; Neckers, D. C. Org. Lett. 2008, 10, 281.

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Steinhuber, E. J. Phys. Chem. B 1998, 102, 1902.

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923.

APPENDIX 2

B1 2.0 B2 B3 B4 1.5 G1 G2 1.0 G3 R1 R2

Absorbance 0.5 R3

0.0 300 360 420 480 Wavelength (nm)

Figure A2.44: Absorption Spectra for B1-R3 in dichloromethane solution

B1 1.00 B2 B3 0.75 B4 G1

ty G2 i 0.50 G3 R1 ntens I 0.25 R2 R3

0.00 350 420 490 560 630 700 Wavelength (nm)

Figure A2.45:Emission spectra of B1-R3 in dichloromethane solution, λex = Amax

B1 1.0 B2 B3 0.8 B4 G1 0.6 G2 G3 0.4 R1 Intensity R2 0.2 R3

0.0 300 400 500 600 700 Wavelength (nm)

Figure A2.46:Emission spectra of B1-R3 in solid thin films, λex = Amax

Table A2.1 Photophysical data of B1-R3, λex = Amax

-1 - Amax λmax λmax ε(Lmol cm τ1 τ2 ΦF ΦF kR010- kNR01 1 7 -1 -8 -1 (nm) (nm) (nm) )(DCM) (ns) (ns) (DCM (solid) (s ) 0 (s ) DC , ) M solid B1 320 401 418 12500 4.6 _ 0.03 0.04 0.64 2.06

B2 329 392 400 34500 4.27 _ 0.32 0.45 10.5 2.23

B3 329 405 418 21000 1 _ 0.04 0.06 4.10 9.84

B4 377 403 414 5000 3.48 _ 0.52 0.37 14.9 1.38

G1 405 481 515 30000 4.68 <1 0.04 0.11 0.85 2.04

G2 406 475 502 293000 < 1 _ 0.05 0.12 5.00 9.50

G3 418 453 525 110000 < 1 _ 0.07 0.15 7.00 9.30

R1 351 528 445 23000 5.69 < 1 0.10 0.21 1.78 1.60

R2 350 535 600 48000 4.20 < 1 0.40 0.34 9.50 1.42

R3 385 474 594 35000 3.70 1.80 0.30 0.25 8.10 1.80

2.0 Acetonitrile Dichloromethan 1.5 Toluene Pentane 1.0

Absorbance 0.5

0.0 300 320 340 360 X Axis Title

Figure A2 47:Absorption spectra of B1 in different solvents

Acetonitrile Dichloromethane Toluene 0.8 Pentane

0.4 Absorbance

0.0 300 400 Wavelength (nm)

Figure.A2 48:Absorption spectra of B2 in different solvents

1.5 Acetonitrile Dichloromethane Toluene Pentane 1.0

0.5 Absorbance

0.0 280 320 360 400 Wavelength (nm)

FigureA2 49:Absorption spectra of B3 in different solvents

Acetonitrile 1.6 Dichloromet Toluenehane Pentane 1.2

0.8 Absorbance 0.4

0.0 300 350 400 Wavelength (nm)

Figure.A2 50:Absorption spectra of B4 in different solvents

1.0 Acetonitrile Dichloromethane Toluene Pentane 0.5

Absorbance 0.0

300 360 420 480 Wavelength (nm)

FigureA2 51:Absorption spectra of G1 in different solvents

Acetonitrile Dichloromethane 1.0 Toluene pentane 0.8

0.6

0.4 Absorbance 0.2

0.0 300 350 400 450 Wavelength (nm)

Figure.A2 52:Absorption spectra of G2 in different solvents

1.00 Acetonitrile Dichloromethane Toluene 0.75 Pentane

0.50

Absorbance 0.25

0.00 300 350 400 450 500 Wavelength (nm)

FigureA2 53:Absorption spectra of G3 in different solvents

Acetonitrile 1.8 Chloroform Dichloromethane 1.5 Toluene 1.2 Hexanes

0.9

0.6 Absorbance 0.3

0.0 300 330 360 390 420 Wavelength (nm)

FigureA2 54:Absorption spectra of R1 in different solvents

Acetonitrile Dichloromethan 0.75 Toluene Pentane 0.50

Absorbance 0.25

0.00 300 350 400 450 500 Wavelength (nm)

FigureA2 55:Absorption spectra of R2 in different solvents

1.0 Acetonitrile Dichlorometha 0.8 Toluene Pentane 0.6

0.4 Intensity 0.2

0.0 300 400 500 Wavelength (nm)

FigureA2 56:Absorption spectra of R3 in different solvents

1.00 Acetonitrile Dichloromethane 0.75 Toluene Pentane 0.50 Intensity 0.25

0.00 360 420 480 540 600 Wavelength (nm)

FigureA2 57:Emission spectra of B1 in different solvents, λex = Amax

1.00

0.75 Acetonitrile Dichloromethane Toluene 0.50 Pentane Intensity 0.25

0.00 350 400 450 500 Wavelength (nm)

FigureA2 58:Emission spectra of B2 in different solvents, λex = Amax

Figure A2.15.

1.2 Dichloromethane Toluene Pentane 0.8 Acetonitrile

Intensity 0.4

0.0 360 420 480 540 600 Wavelength (nm)

FigureA2 59:Emission spectra of B3 in different solvents, λex = Amax

1.0 Acetonitrile 0.8 Dichloromet Toluenehane 0.6 Pentane

0.4 Intensity 0.2

0.0 400 440 480 520 Wavelength (nm)

FigureA2 60:Emission spectra of B4 in different solvents, λex = Amax

Acetonitrile 1.00 Dichloromet Toluenehane Pentane 0.75

0.50 Intensity 0.25

0.00 420 490 560 630 700 Wavelength (nm)

FigureA2 61:Emission spectra of G1 in different solvents, λex = Amax

1.0 Acetonitrile 0.8 Pentane Toluene 0.6 Dichloromethane

0.4

Intensity 0.2

0.0 420 480 540 600 660 Wavelength (nm)

FigureA2 62:Emission spectra of G2 in different solvents, λex = Amax

1.00 Acetonitrile Dichloromethane Toluene 0.75 pentane

0.50 Intensity 0.25

0.00 420 480 540 600 660 Wavelength (nm)

FigureA2 63:Emission spectra of G3 in different solvents, λex = Amax

Acetonitrile Chloroform 1.0 Dichloromethane Toluene 0.8 Hexanes

0.6

0.4 Intensity

0.2

0.0 400 480 560 640 Wavelength (nm)

FigureA2 64:Emission spectra of R1 in different solvents, λex = Amax

Acetonitrile 1.0 Hexanes Toluene 0.8 Dichloromethane 0.6

0.4 Intensity 0.2

0.0 400 500 600 700 800 Wavelength (nm)

FigureA2 65:Emission spectra of R2 in different solvents, λex = Amax

1.0 Dimethylformamide 0.8 Acetonitrile Toluene 0.6 Hexanes

0.4 Intensity

0.2

0.0 400 500 600 700 Wavelength (nm)

FigureA2 66:Emission spectra of R3 in different solvents, λex = Amax

3 10 Decay3 IR3 Decay3F2 Decay3F2 102 Fit Results τ1 0.20ns τ2 4.65ns

Counts χ2 1.000 101

100 0 5 10 15 20 25 30 Time/ns 4.7 0.0 -4.7 Residuals

FigureA2 67:Fluorescence life time decay profile of B1 in dichloromethane

3 10 Decay1 IR1 Decay1F1 Decay1F1 102 Fit Results τ1 4.27ns χ2 0.916 Counts 101

100 0 5 10 15 20 25 30 35 Time/ns 3.8 0.0 -3.8 Residuals

FigureA2 68:Fluorescence life time decay profile of B2 in dichloromethane

3 10 Decay2 IR2 Decay2F1 Decay2F1 102 Fit Results τ1 0.41ns χ2 1.504 Counts 101

100 0 2 4 6 8 10 12 14 Time/ns 4.2 0.0 -4.2 Residuals

FigureA2.69:Fluorescence life time decay profile of B3 in dichloromethane

3 10 Decay4 IR4 Decay4F1 Decay4F1 102 Fit Results τ1 3.46ns χ2 0.959 Counts 101

100 0 5 10 15 20 25 Time/ns 3.0 0.0 -3.0 Residuals

FigureA2.70:Fluorescence life time decay profile of B4 in dichloromethane

B1 B3 B4 G2-Cis

R1 R2 G2-Trans

FigureA2.71:Geometry optimized ground state structures

FigureA2. 72:1HNMR of B4

FigureA2. 73:13CNMR of B4

FigureA2. 74:1HNMR of B1

FigureA2. 75:13CNMR of B1

FigureA2. 76:1HNMR of G2

FigureA2. 77:13CNMR of G2

FigureA2. 78:1HNMR of G3

FigureA2. 79:13CNMR of G3

FigureA2. 80:1HNMR of G1

FigureA2. 81:13CNMR of G1

FigureA2. 82:1HNMR of B2

FigureA2 83:13CNMR of B2

FigureA2. 84:1HNMR of B3

Figure A2.41. 13CNMR of B3

Figure A2.42. 1HNMR of R1 191.36 143.39 138.87 138.82 135.58 133.26 132.17 129.64 129.46 128.59 126.43 123.78 123.71 120.76 116.36 109.20 92.89 89.22 77.44 77.22 77.02 76.59 34.77 32.00 0.00 483- aldehyde Cnmr

200 180 160 140 120 100 80 60 40 20 0 ppm FigureA2. 85:13CNMR of R1 Figure A2.43.

531, mononitrile tertbitylcarbazol HNMR 7.646 7.618 7.515 7.491 7.486 7.438 7.409 7.278 7.177 7.143 7.090 7.037 7.013 6.927 6.828 6.486 5.340 5.320 5.128 4.759 4.696 4.242 4.132 4.095 4.045 3.811 3.730 3.600 3.577 3.554 3.265 2.068 1.879 1.696 1.577 1.489 1.279 1.158 1.096 1.078 0.996 0.975 0.902 0.879 0.756 0.643 0.631 0.582 0.558

8.2 8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 ppm

8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 ppm 2.00 1.93 5.20 1.96 4.20 18.00 FigureA2.86:1HNMR of R2 Figure A2.44. 158.50 143.44 139.12 138.70 133.38 132.50 130.72 130.31 129.82 126.39 123.79 123.71 120.32 116.38 113.71 112.59 109.17 94.55 77.44 77.22 77.01 76.59 34.77 31.98

531, mononitrile tertbutlycarbazoly C13

150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 ppm FigureA2. 87:13CNMR of R2

FigureA2. 88:1HNMR of R3

CHAPTER 3

UNUSUAL PHOTOPHYSICAL PROPERTIES OF CARBAZOLE-BASED EMITTING COMPOUNDS

Photophysical properties of a new class of fluorescent and stable carbazole-based compounds (B1-B3, G1-G3 and R1-R2) in matrices and solution, and as a function of excitation wavelength are reported. The emission maxima of B1 and B3 and G1-G3 show red shifts (6 nm-16 nm) and substantially increased fluorescence quantum yields with a decrease in temperature from 25oC to -10oC. A considerable edge excitation red shift has been observed in

B2. The emission of G1 shows both specific and general solvent effects while intermolecular excitation energy transfer occurs from the naphthyl moiety to the carbazolyl moiety in G3.

Lippert-Mataga plots confirm the existence of multiple emitting states in each of these compounds.

3.1. Introduction

There have been many studies of carbazole-based compounds in device fabrication1 but fewer of their photophysical properties in fluids and polymer matrices. Fluorescent organic compounds having large Stoke’s shifts have advantages because there can be remarkable light loss due to re-absorption. A major reason for high Stoke’s shift is formation of intramolecular charge transfer (ICT) states.2 Edge excitation red shift (EERS) phenomena have been observed in various excited state reactions leading to photoisomerization,3 electron transfer,4 and proton transfer.5 The application of EERS in viscous and glass forming fluids,6 binary solvent mixtures of different polarity proteins,7 polymers8 and micelles has been demonstrated.5

Studies of luminescence as a function of temperature allow estimation of the activation energy for the deactivation of the excited states. We have been pursuing various photophysical

properties of carbazoles in different media including as solids, in solution and in poly(methyl methacrylate) [PMMA] matrices.1,9a,b

(A) Blue emitters

CHO

N N N

OHC CHO B3 B5 B1

(B) Green emitters

CN CN NC CN CN CN NC CN

N N N N CHO

G1 G2 G3 G4

CN CN N CN N CN

R2 R3

Chart 3.1. Structures of compounds B1-R3

In this chapter, we report photophysical properties of the compounds in Chart 3.1 as a function of temperature, concentration and solvent polarity. EERS is observed only when the excitation is at the longer wavelength edge of the lowest energy absorption band. By performing solid state dilution experiments we also observe aggregation quenching in these compounds. Finally, a study of the energy transfer shows that the intermolecular energy transfer occurs from the naphthyl moiety to the carbazolyl moiety.

3.2. Experimental section

3.2.1. Materials: The synthesis of B5 is described in the Appendix 3. Syntheses of the remainder of the carbazoles are reported elsewhere.9b,c Acetonitrile (ACN), methanol, dichloromethane (DCM), pentane, toluene and hexanes are HPLC grade.

3.2.2. Measurements: Steady state absorption and fluorescence spectra were recorded on a

Shimadzu UV-2401 spectrophotometer and a Fluorolog-3 spectrometer, respectively. All measurements were carried out at room temperature unless otherwise specified. The quantum yields of fluorescence (ΦF) in solution were measured following a general method using 9,10-

1 diphenylanthracene (ΦF = 0.9 in cyclohexane) as the standard. Sample solutions were taken up in quartz cuvettes and degassed for ~15 minutes. The degassed solutions had an absorbance of

0.05-0.09 at the excitation wavelength. 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 ΦF. The refractive indices of solvents at the sodium D line were used. The

1 ΦF values in the PMMA were measured following a literature method. Compounds in dichloromethane were mixed thoroughly with a solution of PMMA in acetonitrile. We cast thin films of the mixture on a quartz plate and allow the sample to dry. The plate is subsequently inserted into an integrating sphere and the required spectra recorded.

3.3. Results and discussion

3.3.1. Temperature dependent emission maxima and fluorescence quantum yields (ΦF): With a decrease in temperature the emission spectra of B1, B3 and G1-G3 recorded in DCM show a red shifted fluorescence emission maxima accompanied by an increase of ΦF (vide infra). All of these non-planar compounds have significant degrees of free rotation in solution. With decreased temperature, the system attains some solid state character and molecular packing

10 restricts intramolecular rotation. This explains why ΦF is also higher at lower temperature.

The red shifted emission spectra (Figure 3.89, 3.91, 3.92 and Appendix 3) on decreasing the temperature from 25oC to -10oC are in contrast to some literature reports.2 B1 shows a red shift of 14 nm in its emission spectra at reduced temperature whereas B3 shows red shift only by 6 nm. B1 is smaller in size and has more freedom of rotation in solution than does B3.

1.0 -10oC 0.8 -5oC 0oC 0.6 10oC o 0.4 25 C Intensity 0.2

0.0 350 400 450 500 550 Wavelength (nm)

Figure3 89:Temperature dependent emission spectra of B1 recorded in dichloromethane, ex = 330 nm

These compounds possess strong electron withdrawing (EW) and electron donating

(ED) groups, thus predicting an intramolecular charge transfer (ICT) state even in the ground state- a state that will be even more stable at reduced temperature. Absorption spectra of these

1 compounds show two bands. The band of higher energy is associated with the So-S (π- π*)

9b transition and that at lower energy is associated with the So-ICT* transition. On lowering the temperature, the viscosity of the solvent increases and reorientational relaxation of the solvent molecules is inhibited. Enhanced stabilization of the ICT* emitting state is expected because the reduction of thermal motion of the solvents allows a better alignment of the solute and solvent dipoles. Thus, the ICT excited state dominates over the local excited (LE) state at lower

temperature.

Consequently, the

Franck-Condon

Figure3. 90:Jablonski diagram showing the possible excitation and de- excitation pathways for B1, B3 and G1-G3 95

excited state distribution (F-CESD) of the ICT excited state will increase with a decrease on temperature.

Temp(0C) -10 -5 0 10 25

B1 λmax (nm) 420 416 414 412 406

λex. 330nm ΦF % 10 8 6.2 4.5 3

B3 λmax (nm) 420 418 416 416 414

λex. 330nm ΦF % 10 8.5 7 5.5 3.2

G1 λmax (nm) 422 - 422 412 408

λex. 330nm ΦF % 12 - 10 8 6

G1 λmax (nm) 432 - 434 434 36

λex. 370 nm ΦF % 9 - 7 5 4

G2 λmax (nm) 390 386 386 382 378

λex. 330 nm ΦF % 21 18 15 12.5 10

G2 λmax (nm) 484 482 482 478 476

λex. 370 nm ΦF % 10 8 7 5 5

G3 λmax (nm) 458 456 456 456 452

λex. 370 nm ΦF % 20 15.5 13 9 7

G3 λmax (nm) 454 454 452 446 440

λex. 330 nm ΦF % 22 18.5 15 13 10

When this transition occurs following excitation, two parallel processes occur – excitation to LE and also to ICT*. The F-CESD for ICT* is higher than that for LE (Figure 3.90). This leads to a higher distribution of the ICT* state in the excited state equilibrium. The dielectric relaxation time of

Table 3.1. Photophysical data for compounds B1, B3, G1, G2, and G3 recorded at different temperatures and different excitation wavelengths the solvent at lower temperature might be longer than the fluorescence of both LE and ICT*.

Thus solvent relaxation rates from LE-ICT* (ks) and ICT*-ICTr* (relaxed intramolecular charge transfer excited state) (ks’) will also be lower. The fluorescence relaxation rate from

ICT* to So (kF) and LE to So (kF’) will be higher than ks and ks’, respectively. Fluorescence lifetimes of the compounds show biexponential decay; shorter lifetimes are assigned for radiative de-excitation of the ICT* state and longer lifetimes to the de-excitation of the LE state.9a Thus the fluorescence emission spectra include emission from all three excited states with ICT* the largest contributor. Lippert-Mataga plots suggest the existence of multiple excited states (vide infra) an effect that increases with the increasingly EW group (G3 when compared to G1 and G2). All the compounds show more red shifted emission at lower temperature upon excitation at higher energy than following excitation at lower energy. 11

o 1.00 -10 C -5oC o 0.75 0 C 5oC o 0.50 15 C 25oC Intensity 0.25

0.00 450 540 630 Wavelength (nm)

Figure3 91:Temperature dependent emission spectra of G3 recorded in dichloromethane, λex = 370 nm

It has been found that on lowering the temperature the lifetime increases as a result of the decrease in the non-radiative rate (Knr).12

3.3.2. Excitation energy dependent fluorescence; edge excitation red shift: In contrast to

Kasha’s rule we observed that the fluorescence of G2 and B5 depends on the excitation energy

(Figure 3.92, Appendix 3 and Table 3.2). This effect has been referred to as the B shift,13 the bathochromic luminescence effect,14 the red-edge effect15 and most pertinently edge excitation red shift (EERS)16 or red-edge excitation shift.17 We observe this remarkable red shift only when excitation is at the longer wavelength edge of the lowest energy absorption band without and serious change in spectral shape. Fletcher13 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.16

Excitation 300 nm Excitation 320 nm 0.9 Excitation 350 nm Excitation 370 nm Excitation 390 nm 0.6 Excitation 400 nm

Intensity 0.3

0.0 400 450 500 550 600 Wavelength (nm)

Figure3. 92:Emission spectra of B5 recorded in DCM, excitation at different wavelengths.

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.

Thus, the resultant total emission lacks some high energy components and this causes the red shifted emission. Mandal et at. explained two requirements for compounds to show EERS behavior; solute-solvent interaction energies leading to inhomogeneity and the excited state relaxation of the fluorescent species must be slower or comparable to the fluorescent lifetime of the species.5 G1, G2 and G3 appear to meet both requirements.

Table 3.2. Photophysical data for G2 and B5 recorded in different solvents and at different excitation wavelengths

G2 λex (nm) 335 355 375 395 405 415 430

CH2Cl2 λmax (nm) 382 476 480 481 481 481 488

G2 λex (nm) 335 355 375 395 405 415 425

CH3CN λmax (nm) 406 496 501 501 504 504 510

B5 λex (nm) 300 320 350 370 390 400 -

CH2Cl2 λmax (nm) 438 438 440 446 473 474 -

3.3.3. Specific and general solvent effects on emission: The emission maximum of G3 shifts from 417 nm in hexane to 468 nm in methanol (Figure 3.94). We observed no change in the absorption spectra (Figure 3.93). On addition of 2.5 % methanol to hexane, which does not significantly change the orientation polarizability, results in a shift of the emission maximum as a result of H-bonding to the minor solvent by 9 nm (Figure 3.94). The data in the Table 3.3 show a gradual red shift in the emission maxima with the fraction of methanol in hexane. So, specific solvent-fluorophore interactions occur only in the excited state while general solvent- fluorophore interactions occur both in the ground state and the excited state of this compound.

1.00 Methanol 2.5 % Methanol 75 % Hexanes 100 % 0.75 Methanol 100 %

0.50

Absorbance 0.25

0.00 315 360 405 450 Wavelength (nm)

Figure3 93:Absorption spectra of G3 recorded at different hexane methanol ratios

Methanol 2.5 % Methanol 75 % 0.9 Hexanes 100 % Methanol 100 %

0.6

Intensity 0.3

0.0 400 500 600 Wavelength (nm)

Figure3 94:Emission spectra of G3 recorded in different ratios of hexane to methanol, ex = Amax

Table 3.3. Photophysical data of G3 recorded in different ratios of methanol and hexanesa

% of methanol methanol 2.5% methanol 75% Methanol 100% Hexanes 100%

λmax (nm) 426 454nm 468 417

a λex= Amax for each sample.

It is note worthy that the emission spectra are broadened on increasing the fraction of methanol due to emission from both the Franck-Condon state and the relaxed state. A specific

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spectral shift occurs even at a low concentration of methanol in hexane due to the hydrogen- bonding of methanol to the nitrogen of nitrile group on G3.

3.3.4.Fluorescence switching with concentration in PMMA matrix; aggregation quenching: We cast thin films of G1 from a DCM solution to which PMMA in acetonitrile had been added. We observe a progressive red shift in the fluorescence spectrum with an increase in the concentration of G1 (Figure 3.95) and a fluorescence quantum yield enhanced 2.5 times on reducing the concentration of G1 from 5 % to 0.01 % in the PMMA matrix (Table 3.4). The linear decrease of fluorescence quantum yield with increase in the concentration of G1 indicates that, under these circumstances, the fluorescence is being quenched by aggregation.

A linear increase of red shift indicates that the fluorescence can be switched by a change in concentration of G1 in a PMMA matrix. A total shift of 42 nm is observed from changes in concentration.

5% by wt. in PMMA 1% by wt. in PMMA 1.0 0.1 by wt. in PMMA 0.8 0.01 by wt. in PMMA

0.6

0.4 Intensity

0.2

0.0 450 500 550 600 650 Wavelength (nm)

Figure3 95:Emission spectra of G1 recorded in different fractions of G1 in PMMA matrices, ex = 370 nm

Table 3.4. Photophysical data recorded for different concentrations of G1 in PMMA matrices at different wavelengths

% by wt 5% 1% 0.1% 0.01%

λex 370 nm 1.2 2 5 7

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λex 330nm 5 6.2 9 12

506 487 477 464 λmax at λex 370 nm

3.4. Intermolecular energy transfer

The absorption spectrum of G3 in DCM shows two distinct bands in the visible (400-

430 nm) and UV regions (300-330 nm).9a Absorption in the visible region is due to the carbazolyl moiety, while that in the UV region is due to naphthyl group. The sample with A = 3 was excited at 315 nm where almost all of the radiation is absorbed by the naphthyl moiety, though the only emission observed is that from the carbazolyl moiety (Figure 3.96). On excitation of the same sample at 415 nm, we observe similar emission spectra reduced in intensity (Figure 3.96). If we excite a sample A = 0.5 in DCM at 315 nm we see two emissions; that at 380 nm is residual fluorescence emission from the naphthyl moiety and the other at 460 nm from the carbazolyl group (Appendix 3). This suggests that excitation energy transfer has occurred from the higher energy naphthyl group to the lower energy carbazolyl moiety.

Excitation of a sample with a low concentration (absorbance 0.01) at 315 nm shows only one emission band with a maximum intensity at 380 nm (Figure 3.97).

4 2.8x10 Ex 315 nm Ex 415 nm 2.1x104

1.4x104

7.0x103

Unnormalized Intensity Unnormalized 0.0 420 480 540 600 Wavelength (nm)

Figure3 96:Emission spectra of G3 recorded in DCM, Absorbance of sample = 3

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Again, excitation of the same solution at 415 nm shows an emission with a maximum intensity at 460 nm. This shows there is no excitation energy transfer from the naphthyl moiety to carbazolyl moiety proving that the energy transfer must be intermolecular. Additionally, a

Gaussian view of the ground state structure of G3 shows that the naphthyl moiety is almost orthogonal to carbazolyl moiety9a and hence, the electronic communication between them is blocked.

8.0x104 Ex 315 nm Ex 415 nm

4.0x104

Unnormalized emission Unnormalized 0.0 350 400 450 500 550 Wavelength (nm)

Figure3. 97:Emission spectra of G3 recorded in DCM, Absorbance of sample = 0.01

3.5. Lippert-mataga plot and its significance on specific and general solvent effects

The nonlinearity of Lippert-Mataga plots for B1, B5, G1-G4, and R2-R3 plots (Figures

3.98-3.100) indicate the presence of multiple excited states that produce multiple emitting species.17 Nonlinearity is also evidence for a specific solvent effect on the spectral shift. The possible excited states might be the locally excited state (LE) and/or the ICT* and/or ICTr* states (described above). 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

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species; one locally excited (LE or 1S) and the other ICT, ICTr. G4, R2 and R3 probably form twisted intramolecular energy transfer states (TICT) in addition to LE, ICT and ICTr. TICT formation is possible in the excited state given that the carbazolyl and 9-phenyl moieties are positioned almost orthogonally.9

1.0x105

) B1 1 B3 8.0x104

6.0x104 Stoke's shift (cm-

4.0x104 0.0 0.1 0.2 0.3 Orientation Polarizability

Figure3. 98:. Lippert-Mataga plots for B1 and B3 in solvents ACN, methanol, DCM, toluene and hexanes

G1 G2 1.6x1010 G3

) R1 1 1.2x1010

8.0x109

4.0x109 Stoke's shift (cm- 0.0 0.0 0.1 0.2 0.3 Orientation Polarizability

Figure3. 99:Lippert-Mataga plots for G1-R1 in acetonitrile, methanol, dichloromethane hexane , toluene and

104

1x105 1x105

) 4 -1 9x10 R2 R3 8x104 7x104 6x104 Stoke's Shift (cm Stoke's 5x104 0.0 0.1 0.2 0.3 Orientation Polarizability

Figure3.100:Lippert-Mataga plots for R2 and R3 in solvents ACN, methanol, DCM, toluene and hexanes

3.6. Conclusion

Investigation of red shifted emissions of B1, B3 and G1-G3 with a decrease in temperature is explained with a Jablonski diagram. F-CESD for ICT* dominates over F-CESD for LE resulting in the higher contribution from ICT* to the total emission. The extent of EERS is remarkable. Dilution experiments in PMMA matrices show that aggregation quenching occurs at higher concentrations of the compound. Fluorescence switching to some extent occurs with the change in concentration in PMMA matrices. Specific solvent effects occur only in the excited state whereas general solvent effects occur both in the ground state and the excited state in these compounds. Lippert-Mataga plots suggest the existence of multiple emitting species.

Excitation energy transfer from the naphthyl moiety to the carbazolyl moiety occurs intermolecularly.

3.7. References

1) (a) Adhikari, R. M.; Mondal, R.; Shah, B. K.; Neckers, D. C. J. Org. Chem. 2007, 72,

4727. (b) Yang, J.-X.; Tao, X.-T.; Yuan, C. X.; Yan, Y. X.; Wang, L.; Liu, Z.; Ren, Y.; Jiang,

M. H. J. Am. Chem. Soc. 2005, 127, 3278.

2) Doroshenko, A. O.; Kyrychenko, A. V.; Waluk, J. J. Fluoresc. 2000, 10, 41.

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3) Braun, D.; Rettig, W. Chem. Phys. Lett. 1997, 268, 110.

4) Demchenko, A. P.; Sytnik, A. S. J. Phys. Chem. 1991, 95, 10518.

5) Mandal, P. K.; Paul, A.; Samanta, A. J. Photochem. Photobiol., A 2006, 182, 113.

6) Vincent, M.; Galley, J.; Demchenko, A. P. J. Phys. Chem. 1995, 99, 34931.

7) Lakowicz, J. R.; Nakamato, S. K. Biochemistry 1984, 23, 3013.

8) Al-Hassan, K. A.; El-bayoumi, M. A. J. Polym. Sci., B 1987, 25, 495.

9) (a) Adhikari, R. M.; Shah, B. K.; Neckers, D. C. Chem. Eur. J. submitted. (b) Adhikari,

R. M.; Shah, B. K.; Neckers, D. C. Langmuir. submitted.

10) Wang. Z.; Shao, H.; Ye, J.; Tang, L.; Lu, P. J. Phys. Chem. B 2005, 109, 19627.

11) Viard, M.; Gallay, J.; Vincent, M.; Meyer, O.; Robert, B.; Peternostre, M. Biophys. Chem.

1997, 73, 2221.

12) Barigelletti, F. J. Chem. Soc., Faraday Trans. 2, 1987, 83, 1567.

13) Fletcher, N. J. Phys. Chem. 1968, 72, 2742.

14) Rubinov, A. N., Tomin, V. I. Opt. Spektrosk.1970, 29, 1972.

15) Khalil, O. S.; Selinskar, C. J., McGlynn, S. P. J. Chem. Phys. 1973, 58, 1607.

16) Itoh, K-I.; Azumi, T. J. Chem. Phys. 1975, 62, 3431.

17) Lakowich, J. R. Principles of Fluorescence Spectroscopy, Academic/Plenum Publishing,

1999, pp. 194 and 448.

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APPENDIX 3

Scheme A3.1. Synthesis of B5

1,4-diiodobenzene, Cu N N 18-crown-6, K2CO3 sonogashira

1.14

3,6-Di-tert-butyl-9-(4-iodophenyl)-9H-carbazole (1.14). 3,6-di-tert-butyl-9H-carbazole

(1.5 mmol), 18-crown-6 (0.15 mmol), copper powder (0.2 gm), potassium carbonate (15 mmol), 1,4-diiodobenzene (3 mmol) and 1,2-dichlorobenzene were added in a dry round bottom flask. The mixture was refluxed for 12 hours and filtered. Solvent was evaporated.

Solid was purified by chromatography (silica gel, 80 % hexane in dichloromethane) to obtain pure 1.14 (80 %) as a yellowish solid

3,6-Di-tert-butyl-9-(4-(2-(4-ethynylphenyl)ethynyl)phenyl)-9H-carbazole (B5). In a dry round bottom flask flushed with argon were mixed of 3,6-di-tert-butyl-9-(4-iodophenyl)-9H- carbazole (1.14) (3 mmol), 1,4-diethynylbenzene (4 mmom), triphenylphosphine (0.03 mmol),

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dry and distilled triethylamine (40 ml), CuI (0.03 mmol), trans- dichlorobis(triphenylphosphine)palladium(II) (0.03 mmol) and stirred at O 0C for 3 hours. This mixture was then warmed to room temperature and stirred for four more hours. The solvent was evaporated. The crude solid was purified by chromatography (silica gel, 80% hexane in dichloromethane) to obtain pure B5 (81%) as a white solid. 1H NMR (300 MHz, CDCl3) δ 1.50

(s, 18 H), 3.2 (s, 1H), 7.3-7.6 (m, 10H), 7.7 (d, 2H), 8.18 (s, 2H); 13C NMR (300 MHz, CDCl3)

δ 31.5, 34.5, 78, 83, 89, 91, 109, 116, 120, 121, 123, 125, 130.5, 131, 132, 128, 139, 143; mass spectrum(DIP-MS) m/z M+ 479 (100%); HRMS (ES+) m/z 480.2691, calcd m/z 480.2691.

Figure A3.101: 1HNMR of B5 Figure A3.1 .

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Figure A3.102: 13CNMR of B5

0.9 25oC 10oC 0oC 0.6 -10oC

Intensity 0.3

0.0 350 400 450 500 550 600 Wavelength (nm)

Figure A3.103: Temperature dependent emission spectra of G1 recorded in DCM, λex = 330 nm

1.0 25oC o 0.8 10 C 0oC 0.6 -10oC

0.4 Intensity 0.2

0.0 400 450 500 550 Wavelength (nm)

Figure A3.104:Temperature dependent emission spectra of G1 recorded in DCM, λex = 370 nm

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1.0 25oC 15oC 0.8 o 5 C 0o 0.6 -5oC -10oC 0.4 Intensity

0.2

0.0 350 400 450 500 550 600 Wavelength (nm)

Figure 105:Temperature dependent emission spectra of G3 recorded in DCM, λex = 330 nm

1.00 -10oC -5oC 0.75 -0oC 10oC 0.50 25oC Intensity 0.25

0.00 360 420 480 540 600 Wavelength (nm)

Figure A3.106: Temperature dependent emission spectra of G2 recorded in dichloromethane, λex = 310 nm

1.00 -10oC -5oC 0.75 0oC 10oC 0.50 o 25 C Intensity 0.25

0.00 420 490 560 630 wavelength (nm)

Figure A3.107: Temperature dependent emission spectra of G2 recorded in dichloromethane, λex = 370 nm

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315 nm 335 nm 5 1.0x10 355 nm

4 375 nm 8.0x10 395 nm 405 nm 6.0x104 315 nm 4 430 nm 4.0x10 Intensity 440 nm 2.0x104

0.0 350 400 450 500 550 600 Wavelength (nm)

Figure A3.108: Emission spectra of G2 recorded in dichloromethane, excitation at different wavelengths

Ex 335 nm 375000 Ex 355 nm Ex 375 nm 300000 Ex 395 nm Ex 405 nm 225000 Ex 415 nm Ex 425 nm 150000 Intensity 75000

0 360 420 480 540 600 Wavelength (nm)

Figure A3.109: Emission spectra of G2 recorded in acetonitrile, excitation at different wavelengths

Ex 315 nm Ex 415 nm 1.5x105

1.0x105

5.0x104

Unnormalized intensity 0.0 360 450 540 Wavelength (nm)

Figure A3.110: Emission spectra of G3 recorded in dichloromethane at absorbance 0.5

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PART 2

FLUORESCENT ORGANIC NANOPARTICLES 7. Introduction

A nanoparticle is defined as a small object, sized between 1 and 100 nanometers, that behaves as a single whole unit in terms of its transport and properties.1 Historians have reported that as early as the ninth century nanoparticles were used by “craftsman” in Mesopotamia for generating a glittering effect on the surface of pottery. Nanoparticle research is currently an area of intense scientific work, and appears to have a variety of potential applications in biomedical, optical, and electronic fields. Though inorganic and hybrid nanoparticles have been widely used in these fields, organic nanoparticles have also emerged as competitive and this has enabled an area of intense research. Emissive nanoparticles have preferred applications over non-emissive nanoparticles in various fields in that they can be traced by the emitted light.

With the advent of modern instruments including single particle spectroscopy, scanning electron microscopy, fluorescence spectroscopy, laser, confocal microscopy, dynamic light scattering, tunneling electron microscopy, fluorescence excitation, scanning electron microscopy and various others, the study and characterization of nanoparticles has become possible.

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Parallel to the much explored molecular luminescence, the luminescence of nanoparticles has engendered thriving interest because it is of fundamental and technological importance in contemporary nano-science and nano-technology. Though several fluorescent organic nanoparticles (FONs) have been investigated over the years,2 these 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.2 The study of organic nanoparticle potentially bridges our understanding of the evolution from single molecules to bulk materials. Nanoparticles are studied as an ensemble. Consequently, only averaged information on nanoparticle size, shape, structure and property can be obtained.3 Light emitting nano-materials have been a sought-after target because if properly emissive, they could greatly reduce the cost of white light emission in full color electronic displays.

8. FONs for biological applications

Highly emissive biotin-functionalized 1,4-di(3-(benzoxazol-2-yl)-4-hydroxyphenyl)-

2,5-dihexyloxybenzene-10,12-pentacosadiynoic acid/2,2’-(ethylenedioxy)-bis(ethylamide)- biotin FONs have been investigated for possible applications in non-toxic immunofluorescence labeling.4 Kumar et al developed organically modified silica (ORMOSIL) nanoparticles by covalently incorporating the fluorophore rhodamine-B. These workers functionalized the silica surface with a variety of active groups such as hydroxyl, thiol, amine, and carboxyl.5 The nanoparticles thus synthesized were stable in aqueous suspension, and retained the optical properties of the incorporated fluorophore. These workers used the newly prepared particles for a variety of therapeutic applications, including in photodynamic therapy (PDT). The inert, optically transparent ORMOSIL can be doped with any desired fluorophore to get visible or

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NIR emission, and this leads to the generation of robust, fluorescent nanoparticles.6-9 The particles also can be incorporated with bioactive molecules such as enzymes, genetic materials, and chemotherapeutic drugs, etc., for a variety of biological uses.10,12

Photodynamic therapy (PDT) has emerged as one of the important areas in biophotonics. Two photons absorption (TPA)-induced excitation of photosensitizers is one of the promising approaches to increase the light penetration in to the living skin. TPA dyes, 9,10- bis[4′‐(4″‐aminostyryl)styryl]anthracene (BDSA) has severely distorted geometry (Figure 111).

The BDSA based FONs showed intense fluorescence in the aggregated state under two‐photon excitation.13 Surprisingly, aggregation did not lead to emission quenching. Because of the planarization of p-conjugation upon nano-aggregation, fluorescence was intensified due to an enhanced TPA cross-section. Additionally, aggregation causes the torsional motion to be locked, and this results in less quenching because the interaction of the loose packing of the partially distorted molecules.14 2-Divinyl-2-(1-hexyloxyethyl)pyropheophorbide (HPPH), which is a known, effective photosensitizer, was co-reprecipitated with polymeric ORMOSIL and BDSA to get composite nanoparticles. BDSA acts as the two photon energy donor and

HPPH acts as the acceptor. The use of HPPH/BDSA co-encapsulted nanoparticles as drug- carriers was evaluated by fluorescence imaging, in vivo, of live tumor cells under two-photon laser excitation at 850 nm (BDSA absorbs at around 425 nm). BDSA transfers energy to

HPPH. An intense fluorescence signal is observed from the cells. This confirms that the indirect two-photon excitation of HPPH through intraparticle fluorescence resonance energy transfer remains operative in the cellular environment and these nanoparticles are stable in vivo.

ORMOSIL nanoparticles encapsulating a varying amount of phase-separated

BDSA/HPPH aggregates were proven to be biocompatible15 and stable without releasing

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encapsulated hydrophobic molecules.16 The utility of the BDSA/HPPH/ORMOSIL composite nanoparticles for optical bioimaging, was also tested by incubating human cervical epitheloid carcinoma cells with the nanoparticles and then imaging using two-photon fluorescence microscopy. The authors have also confirmed the minimal cytotoxicity of the nanoparticles.

9. FONs: Application in optoelectronic devices

FONs are of potential use in optoelectronics.17 To realize the practical device applications, reliable methods of transferring and aligning FONs with large surface areas on solid substrates is required. Fine patterning of an FONs ensemble on a surface (Figure 6) by vapor-driven self-assembly (VDSA) on the substrate was successfully achieved using 1-cyano- trans-1-(4’-methylbiphenyl)-2-[4’-(2’-pyridyl) phenyl]ethylene (Py-CN-MBE), using the pyridine unit as a modulator.17 Py-CN-MBE, which is basically non-emissive in solution, shows strong fluorescence emission in nanoparticle form. Park et al. spin coated Py-CN-MBE with poly(methyl methacrylate) (PMMA) to get a thin film. They exposed the film to dichloromethane vapor to get nanoparticles in the matrix which were uniformly distributed.

These particles showed intense fluorescence emission in the blue-green region. In another experiment, these authors made thin films of Py-CN-MBE in the presence of the photoacid generator (PAG) triphenylsulfonium trifluoromethanesulfonate. The latter releases protons

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- (H+) in the presence of non-nucleophilic counter ions (CF3SO3 ) upon exposure to UV light.

They irradiated the film with 254 nm light for a minute. After irradiation Py-CN-MBE was converted to Py+HX-CN-MBE and this resulted in green fluorescence emission following the quaternization Figure 112. Exposing this film to dichloromethane did not change its emission characteristics. As a result these authors claimed that they had devised a simple and reliable method for the fabrication of photo-pattern assemblies of FONs on a solid surface. This new approach was an approximation to the realization of a practical optoelectronic nano-device.17

Huang et al.18 dissolved fluorescent 2-(anthracene-9-yl)-9,9’-dioctyl-fluorene(ANF)

(Scheme 1) in methyl methacrylate and water. They added the polymerizable surfactant, 4-ω- arcyloyloxy-β-hydroalkyl sulfonate (Cops-1) and butyl acrylate and the mixture was successfully polymerized. The authors found that the PMMA/polybutyl acrylate/ANF particles

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were uniformly dispersed in the microemulsion. These particles emit in the blue region. This emulsion was used as an ink formed micro-droplet for ink-jet printing onto a flexible substrate

Figure 113.

Scheme 1. Chemical structure of Cops-1 and ANF18

Figure 113:Photographs of patterned glossy paper utilizing a luminescent microemulsion, which shows the logo of the Fudan University 18

1-Phenyl-3-((p-dimethylamino)styryl)-5-((p-dimethylamino)phenyl)-2-pyrazoline

(DPP) nanoparticles were found to exhibit a special type of multiple emission ranging from the near-UV to the green. The emission can be tuned by altering either the excitation wavelength or the nanoparticle size. This phenomenon is of interest for tailoring properties of optical materials.19

Zhang et al. found that with increasing nanoparticle size, the absorption features of a pyrazoline chromophore become gradually more prominent in the region from 370 nm to 420

117

nm, while the absorption bands of pyrene shift to longer wavelengths. Moreover, a new feature gradually appears in the spectrum at ≈ 450 nm. There are significant differences between the fluorescence features of nanoparticles and those of monomers and bulk crystals. Figure 114 describes the fluorescence features of nanoparticles reported in the Zhang et al’s publication.

The nanoparticle emission (black line) divides into three parts at 385, 465, and 570 nm. The red, yellow, and blue lines are the excitation spectra obtained from monitoring the emissions at

385, 465, and 570 nm, respectively. The authors found that the intensity of the different emissions at various excitation wavelengths, λex, is proportional to the intensity at λex in their corresponding excitation spectra. When λex is fixed at 290 nm, the first section of the emission spectrum vanishes, while the second and third parts remain with the second part becoming strongest. The multiple emissions have different individual optical channels, and can be tuned easily by the selection of λex.

118

The colored lines are the excitation spectra obtained by monitoring the emission at 385 (red), 465 (yellow), and 570 nm (blue), except for the red line in E, which was obtained by monitoring the emission at 445 nm. Black lines are emission spectra obtained by excitation at 345 nm, I = relative intensity, adapted from reference 19

Magdassi and Moshe20 demonstrated direct patterning of water-insoluble organic molecules in the form of nanoparticles. They dissolved the functional organic molecules as the internal droplets of an oil-in-water microemulsion. The droplets are present in a volatile water- insoluble solvent mixture. By use of the proper surfactant, solvent, and co-solvent, a microemulsion is spontaneously formed. After printing, the droplets of the microemulsion that are printed on the surface of the substrate (glass, ceramic, etc.) are rapidly evaporated, leaving patterns of the organic functional molecules, in the form of nanoparticles. This direct patterning can be easily adapted to a large variety of compounds such as conducting polymers and those used in organic light-emitting diodes. Microemulsion droplets are converted into organic nanoparticles upon impact with a substrate surface through the evaporation of a volatile solvent.20 Figure 115 explains the conversion of microemulsion into nanoparticles schematically using the techniques of ink-jet printing.

Figure 115:Schematic presentation of microemulsion conversion into nanoparticles by ink-jet printing, adapted from reference 20

Figure 116 shows the photograph of a fluorescent water–insoluble molecule as a printed pattern.

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The stable fluorophores poly(fluoresceinyl terephthalyl benzoate) (Polymer 1) and poly(fluoresceinyl terephthalyl benzoate-co-bisphenol A terephthalate) (Polymer 2) (Scheme 2) as FONs were prepared from tetrahydrofuran (THF) solutions by slow evaporation of the solvent under mild vacuum conditions. A significant enhancement in fluorescence emission was attributed to the high level of molecular stacking in the fluorescent nanoparticles.21

Organic polymer-based materials are also finding application in light-emitting diodes (LEDs),22 light emitting electrochemical cells (LECs),23 plastic lasers,24 solar cells,25 field-effect transistors,26 and sensors.27

Scheme 2. Structures of fluorescent polymers

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. The fabrication of devices using FONs is not currently managed well. Because organic nanoparticles can show size-dependent optical properties, and because their preparations lead to diverse size populations, it is useful to investigate the individual nanoparticles one at a time.

Single-molecule/nanoparticle spectroscopy is an ideal tool to use to address this. Compared to bulk studies, single molecule spectroscopy (SMS) allows complex and heterogeneous systems to be studied in detail.28-31 Nakanishi et al. prepared microcrystals of perylene and polydiacetylene by a simple reprecipitation method and found polydiacetylene microcrystals to be a new type of material for third-order nonlinear optics.32,33

10. FONs for studying structure-properties relationship; bridging the gap between bulk and the molecular level

FONs based on N,N-bis(2,5- di-tert-butylphenyl)-3,4,9,10-perylenedicarboximide were studied in polyvinyl alcohol (PVA)/water solution by spectroscopy and in a PVA matrix by single-particle spectroscopy. These multichromophoric systems have coupled chromophores, leading to such phenomena as fluorescence blinking and energy funneling. Since such phenomena could be a limiting factor in the application of FONs in various fields, by virtue of single-particle spectroscopy, the authors were able to correlate this observation with the presence of two distinct types of nanoparticles in the samples: nanoparticles having blue emission and nanoparticles having red emission.34

The emission of compounds found to be strong in solution usually becomes weak in the solid state due to both intermolecular energy and electron transfer.35 Therefore, compounds that emit efficiently in the solid state is a continuing objective. Thus aggregation-induced emissive materials are promising as emitters for the fabrication of highly efficient OLEDs.36

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The strong effects of confinement on electron-hole pairs in all three dimensions results in the size-tunable optoelectronic properties of semiconductor quantum dots.37,38 Similar results are not expected from organic nanoparticles because of the small radius of the Frenkel exciton.

When a material's dielectric constant is very small, the Coulomb interaction between electron and hole become very strong and the excitons tend to be much smaller, so the electron and hole sit on the same cell.38b This condition of electron and holes is called Frenkel exciton. In the case of organic nanoparticles, because of weak van der Waals intermolecular forces the electronic and optical properties are fundamentally different from those of inorganic semiconductors,.39,40 The optical properties are size dependent with nanoparticles consisting of

1-phenyl-3-((p-dimethylamino)-styryl)-5-((p-dimethylamino)phenyl)-2-pyrazoline (PDDP).

This was shown to originate from charge-transfer (CT) exciton formation that increased with nanoparticle size.41

The emission spectra of FONs of 1,4-bis((E)-2-p-tolylprop-1-enyl)benzene showed that the emission intensity soars with increasing water fraction in the nanoparticle suspension. The compound is basically non-emissive in THF.42 This change is unusual in that organic fluorophores are generally highly fluorescent in solution and less fluorescent in the solid state due to concentration quenching. An observation of enhanced solid state emission can be attributed to a phenomenon called “aggregation induced enhanced emission” (AIEE).36 This is supported by restriction to the parallel face-to-face intermolecular interactions (H- aggregation by the methyl groups on this compound). 42-44

Switching of the emission region with a change in the ratio of solvent/non solvent has been achieved with 2-((4-(2-(4-(2-(4-(3,6-di-tert-butyl-9H-carbazol-9- yl)phenyl)ethynyl)phenyl)ethynyl)phenyl)methylene)malononitrile based FONs. The use of

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FONs as chemical vapor sensors has also been investigated. Aggregation diminished emission has been shown in 2-((4-(2-(4-(2-(4-(3,6-di-tert-butyl-9H-carbazol-9 yl)phenyl)ethynyl)phenyl)ethynyl)phenyl)methylene)malononitrile and 2-((4-(2-(4-(2-(4-(3,6- di-tert-butyl-9H-carbazol-9-yl)phenyl)ethynyl)phenyl)ethynyl)phenyl)methylene)malononitrile based FONs.2 Highly stable composite fluorescent organic nanoparticles (CFONs) were prepared by co-reprecipitation of blue and red emitting organic compounds from water/tetrahydrofuran mixtures. The formation of nanoparticles altered the emission intensity.

The emission region was reversibly tuned by using different solvent/nonsolvent ratios. SEM images showed diversity in particle size. Emission spectra of CFONs prepared from different ratios of red and blue emitters cover the entire visible region from 400 to 700 nm. Confocal microscopy measurements of single CFONs reveal that a composite organic nanocrystal can emit white light. The optical properties of FONs based on 2-(4-(2-(4-(3,6-di-tert-butyl-9H- carbazol-9-yl) phenyl)ethynyl)benzylidene)malononitrile have been tuned by altering the ratio of solvents/nonsolvents.45

11. Preparation of nanoparticles

Laser ablation, milling and reprecipitation are main processes used for formation of nanoparticles. The recprecipatation method is mostly applied when one is preparing organic nanoparticles. That method is described below. We shall use it in most of the work reported that follows.

Reprecitation method;- Mixing the compound in a solution of solvents such as acetone or tetrahydrofuran with a poorer solvent like water causes the reprecipitation and nanocrystallization of the organic compound. The nanocrystals are dispersed in the medium.3

The latter is called ‘the dispersion medium.”

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12. References

1) http://en.wikipedia.org/wiki/Nanoparticle.

2) Adhikari, R. M., Shah, B. K., Palayangoda, S. S. Neckers, D. C. Langmuir Manuscript submitted.

3) Masuhara, H.; Nakanishi, H.; Sasaki, K. Single Organic Nanoparticles Springer-Verlag

Berlin Heidelberg New York, 2003.

4) Kim, H.-J.; Lee, J.; Kim, T.-H.; Lee, T. S.; Kim, J. Adv. Mater. 2008, 20, 1117.

5) (a) Kumar, R.; Roy, I.; Ohulchanskyy, T. Y.; Goswami, L. N.; Bonoiu, A. C.; Bergey E. J.;

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127

CHAPTER 4

SOLVENT DEPENDANT OPTICAL SWITCHING IN CARBAZOLE-BASED FLUORESCENT NANOPARTICLES

Suitably susbtituted ethynylphenyl carbazoles (R2 and R3) form highly stable fluorescent organic

nanoparticles. The emission of the nanoparticles can be reversibly switched on/off in the blue-

green and orange-red regions by a change in the ratio of the tetrahydrofuran/water system used in

their preparation. The size of the nanoparticles was found to be dependant on the solvent ratio and

the emissions were significantly red shifted compared to those of dilute solutions of R2 and R2 in

tetrahydrofuran. This is attributed to the formation of intermolecular charge transfer complexes in

the nanoparticles state. The application of the nanoparticles as a chemical vapor sensor has been

investigated.

4.1. Introduction Fluorescent inorganic nanoparticles have found applications as biological labels,1,2 in photovoltaic cells,3 as light emitting diodes,4 and as optical sensors.5 Fluorescent organic

nanoparticles (FONs), on the other hand, have received less attention, even though they allow wider variability and flexibility as materials and in synthesis. Since Nakanishi et al. reported use

of FONs in third order nonlinear optics, composite and hybrid organic/inorganic nanoparticles

have been developed for applications as sensors and biological detectors.6-9 The electronic and

optical properties of nanoparticles differ from those of bulk materials because they are structurally distinct and exhibit confinement effects caused by their finite size.10,11 Thus, given the diversity of organic compounds, the development of FONs should stimulate new applications in many fields.

128

In this article, the formation and photophysical characteristics of highly fluorescent,

stable FONs based on 2-(4-(2-(4-(3,6-di-tert-butyl-9H-carbazol-9-

yl)phenyl)ethynyl)benzylidene) malononitrile (R2) and 2-((4-(2-(4-(2-(4-(3,6-di-tert-butyl-9H-

carbazol-9-yl)phenyl)ethynyl)phenyl)ethynyl)-phenyl)methylene)malono-nitrile (R3) (Scheme

4.1) is reported. We demonstrate for the first time, a direct correlation of the particle size with

the solvent/non-solvent (THF/water) ratio used to form the nanoparticles. We also report that the

emission of R2 and R3 in the blue, blue-green, and orange-red regions can be reversibly

switched on and off by changing the THF/water ratio and that these FONs may be used as

chemical vapor sensors.

4.2. Results and discussion R2 and R3 were synthesized following our previously developed method.12 Carbazole

(1.6a) was converted to 3,6-di-tert-butyl-9H-carbazole (1.6b), which was further converted into

3,6-di-tert-butyl-9-(4-iodophenyl)-9H-carbazole (1.14) (Scheme 4.1). Compound 1.14 was

subjected to Sonogashira couplings with suitable substrates to obtain R1 and 1.15. R2 and R3

resulted from refluxing R1 and 1.15, respectively, with malononitrile and basic aluminum oxide

in toluene.

Scheme 4.1.a

129

t-Bu t-Bu t-Bu t-Bu (a) (b) N N N H 1.6a H 1.6b 1.14 t-Bu t-Bu t-Bu t-Bu (c) I N N (e) (d) t-Bu t-Bu

N t-Bu R1 t-Bu

N R2 CN CHO CN t-Bu t-Bu (c)

N B5

(d)

R3 1.15 CHO

CN CN

aReagents and conditions: (a) tert-butyl chloride, zinc chloride, nitromethane, 40-50 0C, 5 h. (b) 1,4-diiodobenzene, Cu, potassium carbonate, 18-crown-6, o-dichlorobenzene, reflux, 12 h. (c) 4- ethynylbenzaldehyde, CuI, Pd(PPh3)2Cl2, triethylamine, room temperature, 12 h. (d) malononitrile, basic Al2O3, toluene, reflux 6 h. (e) 1,4-diethynylbenzene, CuI, Pd(PPh3)2Cl2, triethylamine, room temperature, 12 h

R2 and R3 nanoparticles were prepared by a reprecipitation technique.13 The nanoparticle

suspensions formed were visibly transparent and stable for weeks under ambient conditions. UV-

visible spectra of R2 in THF and as nanoparticles suspended in solvents differing in the

THF/water ratio are displayed in Figure 4.117. R2 shows three distinct peaks at 295 nm, 345 nm,

and 400 nm in THF. With the addition of water (THF/water ratio: 1/1), the absorption transition

associated with the carbazole moiety (295 nm) gradually blue-shifts (275 nm) with a broadening

of the peak. This may be due to the electronic coupling between the neighboring molecules as

they approach each other in THF because of the hydrophobic nature of the R2 molecules. 1,3-

Diphenyl-5-(2-anthryl)-2-pyrazoline-based nanoparticles exhibit a similar behavior.14 The peak at 345 nm, associated with the absorption transition of the N-substituted ethynylphenyl moiety, remains unchanged. The 400 nm peak can be assigned to the transition from So to twisted

intramolecular charge transfer (TICT) state.15,16 This band disappears on the addition of water

130 due to the destabilization of the TICT caused by an increase in the solvent polarity. This is further supported by the fact that R1 shows negative solvatochromism (Appendix 4).

Figure 4.117:UV-visible spectra of R2 nanoparticle solutions (3.7 × 10-6 M) recorded at different THF/water ratios by volume

As the ratio of water is further increased, a new absorption band (420 nm) appears. This

17 is assigned to a transition of So to an intermolecular charge transfer (ICT) state that forms as the

R2 molecules are cramped even more closely and begin nucleating into nanoparticles. It is likely that intermolecular interaction originates from overlapping of the carbazole moiety and the nitrile group of the neighboring molecules (vide infra), and further increases as the nucleation progresses and the size of the particles becomes larger.18-21 The molecular overlap is also strengthened by an increase in the molecular dipole and the ICT state becomes more prominent.

This results in the red-shifted absorption and emission (vide infra).2,14,17 Additionally, Mie scattering may also be responsible for the red shift in the absorption transition.4

There are significant differences between the fluorescence spectra of R2 in THF and the

R2 nanoparticles formed in the THF/water medium (Figure 4.118). The change in the emission

characteristics of the R2 samples at different water/THF ratios, in fact, follows the same pattern

observed for absorption. The emission changes from blue-green in THF (410 nm and 485 nm)

to blue (415 nm) and ultimately to orange-red as the water fraction is gradually increased, if

131 one maintains the same concentration (3.7×10-6 M). The initial two emission maxima (410 nm

and 485 nm) likely result from the locally excited state and the TICT, respectively. The latter

emission disappears upon addition of water (THF/water ratio: 1/1), and the locally excited state

becomes the only emitting state (~ 415 nm).

Figure 118: Fluorescence spectra of R2 nanoparticle solutions (3.7 x 10-6 M) recorded at different THF/water ratios by volume; λex = 350 nm

The ICT state of the R2 nanoparticles is responsible for the broad emission (525 nm to

700 nm) that appears at the higher water ratio. It is likely that an n-electron from nitrogen of the

carbazole moiety of one molecule transfers to the nitrile moiety of the adjacent molecule,

resulting in the ICT state. This leads to an increase in the dipole moment of R2 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.8 The size of nanoparticles increases

with an increase in the fraction of water (vide infra). 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 R2 nanoparticles compared to

that from the R2 individual molecules. A red-shifted emission is expected for an aggregate

132 state14-17 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 fluorescences.21 We

observed the red shifted emission of the R2 nanoparticles, but a diminished fluorescence

intensity. The main reason for an enhanced emission in J aggregation is the planarization of the

molecule in nanoparticles.21 It is likely that the R2 molecules undergo J aggregation facilitating

intermolecular charge transfer, but retains the non-planar or twisted structure in the ICT state. A

schematic of the ICT state responsible for the red-shifted broad emission and energy associated

with various excitations and emissions is depicted in Figure 4.119.

(A) t-Bu N N t-Bu N t-Bu N N t-Bu N t-Bu N N N t-Bu

(B) Intramolecular charge transfer Intermolecular excited state charge transfer in solution excited state in nanoparticles S1

410 nm 485 nm 350 nm 400 nm 590 nm 420 nm So Monomer Monomer Nanoparticle transition transition transition

Figure 4.119:(A) The intermolecular charge transfer (ICT) state of the R2 nanoparticles and (B) the absorption and The emission emission transitions properties in R2 inR2 THF were and infurther THF/water investigated solutionsigu us ing a confocal microscope. A

clear orange-red light emission was observed from the precipitated nanoparticles (Figure 4.120).

This assured us that the orange-red emission is indeed associated with the R2 nanoparticles, and

133 is not a solution emission of R2 induced by the solvent gradient. These FONs were stable to a blue laser for hours.

Figure A.120:A widefield confocal microscope image of the R2 nanoparticles

The formation of R2 nanoparticle is clearly associated with the addition of water. Water and THF are miscible, so the solubility of hydrophobic R2 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.22 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 solution. This is the point that determines particle size. In a system containing a greater

amount of water, the concentration at which particle growth stops is reached when most of the

substance has fallen out of the solution.

As mentioned earlier the color of the emission of R2 changes instantaneously from blue- green to blue and ultimately to orange-red as the water fraction increases. In fact, fluorescence switching to the orange-red region starts with a water ratio of ~75 % by volume. On increasing the water ratio to 92 % by volume, a complete transition to orange-red fluorescence occurs.

Similar results were obtained with samples of different concentrations in different THF/water

134 ratios (Appendix 4). Thus, the change in emission is due to a change in nanoparticle size and is irrespective of the concentration of R2. Figure 4.121 shows scanning electron microscope (SEM) images of R2 nanoparticles formed at different THF/water ratios. Nanoparticles of ~100 nm were observed from a THF/water ratio of 1/5 (83 % water by volume) with a polydispersity factor of 9 %. Nanoparticles of larger sizes were formed at higher water ratios. For example, the size of the particles increased to ~200 nm and ~400 nm, respectively, when the water ratio was increased to 92 % and 99.5 % by volume. Though control of nanoparticle size by aging has been reported,14,17 our observations clearly suggest that the size of nanoparticles can be controlled by altering the solvent/non-solvent ratio.

B (i)

(ii)

(iii)

R3, which contains one more phenylethynyl group than R2, forms similar nanoparticles,

the sizes of which depend on the THF/water ratio from which they are particulated. The water

content needed to induce nanoparticle formation is higher for R3 (THF/water ratio of 1/5) than

for R2 (THF/water ratio of 1/3). In the case of R2 a structured higher wavelength absorption

corresponding to the nanoparticles distinctly appears at a THF/water ratio of 1/11. However, in

the case of R3, the nanoparticle absorption appears distinctly only at a THF/water ratio of 1/50.

135

To obtain further information on the nature of the excited state, we measured the fluorescence life times (τF) of R2 and R3 in dilute THF solutions and as nanoparticles dispersed

in different fractions of THF/water (Table 4.1). Decays were monitored at the corresponding

emission maxima (λmax). Decays monitored in THF solution could be fit mono-exponentially,

indicating the singlet excited state is exclusively formed. R2 and R3 showed similar

lifetimes (τF) in THF (3.0 ns and 3.5 ns, respectively). Samples that contained different

amounts of water and THF showed biexponential decay suggesting involvement of multiple excited states.

Table 4.1. Fluorescence lifetimes (τF) of R2 and R3 recorded at different THF/water ratios (decay monitored at the corresponding λmax, excitation at 360 nm)

Compound THF/water λmax (nm) τF1 τF2 ratio (ns) (ns) R2 1/0 450 3.0 - 1/3 450 0.5 3.0 1/3 590 1.2 12.7 1/5 590 2.1 14.5 1/11 590 4.4 32.3 R3 1/0 460 3.5 - 1/40 590 10.5 45.0 1/45 590 13.0 49.0

The fluorescence decay of a R2 sample containing a 1/3 of THF/water was monitored at

two different emission maxima. The τF values (1.2 and 14.7 ns) observed at higher wavelength

(590 nm) were correspondingly longer than those (0.5 and 1.2 ns) monitored at shorter

wavelength (450 nm). The τF2 values were 12.7 ns, 14.5 ns, and 32.3 ns when the THF/water

ratios were 1/3, 1/5, and 1/11, respectively. This suggests that nanoparticles are responsible for

the longer lifetime components. In fact, the bi-exponential decay of the R2 nanoparticles

monitored at 450 nm and 590 nm is due to overlap between the S1 and the ICT state, the longer

136

τF originating from the ICT state. The charge transfer state of organic molecules generally

23 exhibits a longer decay time compared to the locally excited state. The τF values of the R2

nanoparticles increase with increasing fraction of water. This indicates that the larger the

nanoparticle, the longer is the fluorescence lifetime.

The R3 solutions exhibited similar fluorescence decay characteristics. Interestingly, the

R3 nanoparticles showed longer lifetimes than the R2 nanoparticles. For example, the shorter

(τF1 -10.5 ns) and longer (τF2 -45.0 ns) lifetimes of the R3 nanoparticles formed at a 1/40

THF/water ratio, respectively, were much longer than those of the R2 nanoparticles formed at a

1/11 THF/water ratio (τF1 -4.4 ns and τF2 -32.3 ns). This may be related to the differences in the ratios of THF/water used in these two cases, which could result in R2 and R3 nanoparticles of different sizes. It is noted that the shorter component of the lifetimes (τF1) of R2 and R3 also

gradually increase as the water content is increased.

Surprisingly, 2 (3,6-di-tert-butyl-9-(4-ethynylphenyl)phenyl)-9H-carbazole), R1 and

B5 (Chart 4.1), which are similar in structure to R2, form no nanoparticles under similar

experimental conditions. Similarly, 1.15 (similar in chemical structure to R3) forms no

nanoparticles. The presence of the malononitrile group on the side chain phenyl seems

crucial for the formation of nanoparticles. We obtained no nanoparticles from either G2 or

G3 (Chart 4.1) under similar conditions, although these compounds contain mono- and di-

malononitrile groups attached to the carbozole. Thus, it is clear that when the malononitrile

group part of the side chain phenyl, it is in a suitable position to induce the nucleation

process.

137

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

t-Bu 2 G2 G3

Chart 4.1

The emission of nanoparticles of R2 and R3 could be reversibly shifted red or blue, through an adjustment in the THF/water ratio. The fluorescence maximum of R3 nanoparticles

(Figure 4.122), 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).

To further investigate the on/off fluorescence switching of R2 and R3 nanoparticles, we spotted a suspension on a silica gel thin layer chromatography (TLC) plate. The emission from the plate was bright orange-red under illumination at 350 nm. Upon insertion of the plate into

THF vapor, the color gradually fades eventually to be replaced by blue indicating that the THF vapor was sensed by the R2 and R3 nanoparticles. The new FONs showed no solvent specificity

138 and the same fluorescence switching behavior was observed from exposure of the particles to dichloromethane, ethyl acetate, and acetonitrile vapors. Nonetheless, this observation implies that fluorescence switching of R2 and R3 nanoparticles may find application as inexpensive sensors for organic vapors.

4.3. Conclusions

We have prepared novel R2 and R3-based FONs that show significantly red-shifted emissions compared to those of the free R2 and R3 molecules. An ICT state originating from the interaction of the carbazole moiety and the nitrile group of adjacent molecules is responsible for this behavior. This is further supported by fluorescence lifetime data. While R2 and R3 in THF show a single lifetime associated with the locally excited state, R2 and R3 nanoparticles exhibit biexponential fluorescence decay, the longer lifetime arising from the ICT state. The sizes of

FONs formed were dependent on the THF/water ratio used for precipitation. A change in the

THF/water ratio can reversibly switch the emission of these FONs from orange-red to blue- green. Fluorescence switching behavior was used to sense certain organic vapors. Detailed studies of the application of these and similar FONs in selectively sensing organic vapors are underway.

4.4. Experimental section

4.4.1. Synthesis: Compound 3 was synthesized according to our previous method12 and

converted into R2 and R3.

4-(2-(4-(3,6-Di-tert-butyl-9H-carbazol-9-yl)phenyl)ethynyl)benzaldehyde (R1). A dry

round bottom flask was charged with 1.14 (1 mmol), 4-ethynylbenzaldehyde (1 mmol), triphenylphosphine (0.01 mmol), dry and distilled triethylamine (20 ml), CuI (0.01 mmol), and trans-dichlorobis(triphenylphosphine)palladium(II) (0.01 mmol). The reaction mixture was

139 stirred under argon for 5 hours at room temperature and the solvent was evaporated. The solid obtained was purified by chromatography (silica gel, 80 % hexane in dichloromethane) to obtain

1 pure R1 (60 %) as a yellowish solid. H NMR (300 MHz, CDCl3): δ 1.5 (s, 18H), 7.42 (d, 2H),

7.5 (d, 2H), 7.62 (d, 2H), 7.72-7.78 (m, 4H), 7.92 (d, 2H), 8.15 (s, 2H), 10.1(s,1H). 13C NMR

(300 MHz, CDCL3): 31, 35, 84, 93, 108, 116, 121, 123, 125, 130, 132, 134, 135, 139, 142, 192.

MS (ES) calculated 484.2640, measured 484.2642.

2-(4-(2-(4-(3,6-Di-tert-butyl-9H-carbazol-9-yl)phenyl)ethynyl)benzylidene)malononitrile

(R2). R1 (1 mmol), malononitrile (1.1 mmol), basic aluminum oxide (10 mmol), and toluene (30 ml) were added to a dry round bottom flask. The mixture was refluxed under argon for 5 hours then filtered hot as which the residue was washed several times successively with hot ethyl acetate and dichloromethane. The filtrate was then dried and the solid thus obtained purified by chromatography (silica gel, 80 % hexane in ethyl acetate) to obtain pure R2 (60 %) as a bright

1 yellow solid. H NMR (300 MHz, CDCl3): δ 1.5 (s, 18H), 7.44 (d, 2H), 7.5 (d, 2H), 7.64 (d,

13 2H), 7.72 (d, 2H), 7.79 (m, 3H), 7.95 (d, 2H), 8.15 (s, 2H). C NMR (300 MHz, CDCl3): 31,

35, 89, 95, 109, 112, 113, 116, 124, 127, 129, 129.5, 130, 132, 133, 139, 140, 143, 159. MS (EI) calculated 531.2674, measured 531.26742.

3,6-Di-tert-butyl-9-(4-(2-(4-ethynylphenyl)ethynyl)phenyl)-9H-carbazole (B5). 1.14 (3 mmol), 1,4-diethynylbenzene (4 mmol), triphenylphosphine (0.03 mmol), dry and distilled triethylamine (40 ml), CuI (0.03 mmol), and trans-dichlorobis(triphenylphosphine)palladium(II)

(0.03 mmol) were mixed in a dry round bottom flask and stirred under argon at O0C for 3 hours.

This mixture was allowed to warm to room temperature and stirred for 4 more hours. The solvent

was evaporated and the solid obtained was purified by chromatography (silica gel, 80% hexane

1 in dichloromethane) to obtain pure B5 (81 %) as a white solid. H NMR (300 MHz, CDCl3) δ

140

1.50 (s, 18 H), 3.2 (s, 1H), 7.3-7.6 (m, 10H), 7.7 (d, 2H), 8.18 (s, 2H); 13C NMR (300 MHz,

CDCl3) δ 31.5, 34.5, 78, 83, 89, 91, 109, 116, 120, 121, 123, 125, 130.5, 131, 132, 128, 139,

143; mass spectrum(DIP-MS) m/z M+ 479 (100%); HRMS (ES+) m/z 480.2691, calcd m/z

480.2691.

4-(2-(4-(2-(4-(3,6-Di-tert-butyl-9H-carbazol-9-

yl)phenyl)ethynyl)phenyl)ethynyl)benzaldehyde (1.15). A dry round bottom flask was charged

with 5 (1 mmol), 4-iodobenzaldehyde (1 mmol), triphenylphosphine (0.01 mmol), dry

triethylamine (20 ml), CuI (0.01 mmol), and trans-dichlorobis(triphenylphosphine)palladium(II)

(0.01 mmol). The mixture was stirred for 5 hours at room temperature under argon and the

solvent evaporated. The solid obtained was purified by chromatography (silica gel, 80 % hexane

1 in dichloromethane) to obtain pure 1.15 (60 %) as a yellowish solid. H NMR (300 MHz, CDCl3)

δ 1.50 (s, 18 H), 7.4 (d, 2H), 7.46 (d, 2H), 7.58 (m, 6H), 7.7 (m, 4H), 7.88 (d, 2H), 8.15 (s, 2H),

13 10 (s,1H); C NMR (300 MHz, CDCl3) δ 31.5, 34.5, 88.5, 89, 90, 92, 109, 116.5, 121, 122,

123.6, 123.8, 126.4, 129.3, 129.7, 131.5, 131.6, 132.3, 133, 135.4, 138.4, 139, 144, 191; mass

spectrum(DIP-MS) m/z M+ 583 (100%); HRMS (EI+) m/z 583.2878, calcd m/z 583.2875.

2-((4-(2-(4-(2-(4-(3,6-Di-tert-butyl-9H-carbazol-9-

yl)phenyl)ethynyl)phenyl)ethynyl)phenyl)-methylene)malononitrile (R3). To a dry round bottom

flask were added 1.15 (1 mmol), malononitrile (1.1 mmol), basic aluminum oxide (10 mmol),

and dry toluene (30 ml). The mixture was refluxed under argon for 5 hours, filtered hot and the

residue washed several times successively with hot ethyl acetate and dichloromethane. The filtrate was then dried and the solid obtained purified by chromatography (silica gel, 80 % hexane in ethyl acetate) to obtain pure R3 (60 %) as a bright yellow solid. 1H NMR (300 MHz,

CDCl3) δ 1.50 (s, 18 H), 7.4 (d, 2H), 7.46 (d, 2H), 7.7 (m, 6H), 7.64 (d, 2H), 7.72 (m, 3H), 7.89

141

(d, 2H), 8.15 (s, 2H); 13C NMR (300 MHz, CDCl3) δ 31.5, 34.5, 83, 89, 90, 91, 95, 109, 113,

114, 117, 120, 121, 123, 123.2, 125, 129, 129.3, 130, 131, 132, 133, 138.4, 139, 143, 158; mass spectrum(DIP-MS) m/z M+ 631 (100%); HRMS (EI+) m/z 631.29858, calcd m/z 631.29875.

4.4.2. Fluorescence lifetime ( F) measurement: The samples containing R2 and R3 in THF and nanoparticles dispersed in THF/water were placed in quartz cuvettes. Fluorescence decay profiles of the argon-degassed (~15 min) samples were recorded using a single photon counting spectrofluorimeter. Decays were monitored at the corresponding emission maximum of the compounds. In-built software allowed the fitting of the decay spectra ( 2 = 1-1.5) and yielded the

fluorescence lifetimes.

4.4.3. Preparation of nanoparticles: Samples of predetermined concentrations of R2 and R3 in

THF were prepared. Appropriate volumes of these solutions were taken in different vials and

different amounts of distilled water were rapidly injected into those vials so as to maintain the

same concentrations in the final solutions. The formation of nanoparticles at the appropriate

THF/water ratios could be clearly observed under a 366 nm UV lamp. In some experiments,

different volumes of water were injected into the same quantity of THF solution to prepare

nanoparticle solutions of different concentrations.

4.4.4. SEM images of nanoparticles: SEM images were recorded on a Hitachi S-2700 electron

microscope at 15 kV. Samples for SEM were prepared by placing few drops of nanoparticle solutions onto a glass cover slip placed on an aluminum stub. The samples were allowed to dry in an oven (45°C) before viewing under the electron microscope. To enhance the contrast and quality of the SEM images, the samples were sputter-coated with gold/palladium. Quartz PCI 7 imaging software was utilized to process the images and determine the size of the particles.

142

4.4.5. Confocal microscope images of the R2 nanoparticles: The same method employed to

prepare the samples for SEM images was used to prepare the sample for confocal images

(objective lens 100 X). The excitation source was a mercury lamp. The emission filter (low -

pass; < 405 nm in frequency) and the excitation filter (broad band UV 300 – 400 nm; max = 365

nm) were used for image recording.

4.5. References

1) Kim, H.-J.; Lee, J.; Kim, T.-H.; Lee, T. S.; Kim, J. Adv. Mater. 2008, 20, 1117.

2) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 2013.

3) Hagfeldt, A.; Gratzel, M. Chem. Rev. 1995, 95, 49.

4) An, B.-K.; Kwon, S-K.; Jung, S-D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410.

5) Wohltjen, H.; Snow, A. W. Anal. Chem. 1998, 70, 2856.

6) Fernandez-Arguelles, M. T.; Yakovlev, A.; Sperling, R. A.; Luccardini, C.; Gaillard, S.;

Medel, A. S.; Mallet, J.-M.; Brochon, J.-C.; Feltz, A.; Oheim, M.; Parak, W. J. Nano Lett. 2007,

7, 2613.

7) Su, X.; Zhang, J.; Sun, L.; Koo, T.-W.; Chan, S.; Sundararajan, N.; Yamakawa, M.; Berlin, A.

A. Nano Lett. 2005, 5, 49.

8) Fu, H.-B.; Yao, J.-N. J. Am. Chem. Soc. 2001, 123, 1434.

9) Kasai, H.; Kamatani, H.; Yoshikawa, Y.; Okada, S.; Oikawa, H.; Watanabe, A..; Itoh, O.;

Nakanishi, H. Chem. Lett. 1997, 9, 1181

10) Benzamin, G.; Huang, F.; Zhang, H.; Glenn, A. W.; Banfield, J. F. Science 2004, 305, 651.

11) Gesquiere, A. J.; Uwada, T.; Asahi, T.; Masuhara, H.; Barbara, P. F. Nano Lett. 2005, 5,

1321.

143

12) Adhikari, R. M.; Mondal, R.; Shah, B. K.; Neckers, D. C. J. Org. Chem. 2007, 72, 4727.

13) Palayangoda, S. S.; Xichen, C.; Adhikari, R. M.; Neckers, D. C. Org. Lett. 2008, 10, 281.

14) Xiao, D.; Xi, L.; Yang, W.; Fu, H.; Shuai, Z.; Yan, F.; Yao. J. J. Am. Chem. Soc. 2003, 125,

6740.

15) 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.

16) Nikolaev, A. E.; Myszkiewicz, G.; Berden, G.; Meerts, W. L.; Pfanstiel, J. F.; Pratt, D. W. J.

Chem. Phys. 2005, 122, 84309.

17) Fu, H. B.; Loo, B. H.; Xiao, D. B.; Xie, R. M.; Ji, X. H.; Yao, J. N.; Zhang, B. W.; Zhang, L.

Q. Angew. Chem., Int. Ed. 2002, 41, 962.

18) Alivisatos, A. P. Science 1996, 271, 933.

19) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997,

119, 7019

20) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos,

A. P. Nature 2000, 404, 59.

21) Oelkrug, D.; Tompert A.; Gierschner, J.; Egelhaaf, H.-J.; Hanack, M.; Hohloch, M.;

Steinhuber, E. J. Phys. Chem. B 1998, 102, 1902-1907.

22) Horn, D.; Rieger, J. Angew. Chem. Int. Ed. 2001, 40, 4330.

23) Schulman, S. G. Fluorescence and Phosphorescence Spectroscopy: Physicochemical

Principles and Practice; Oxford: New York, 1977.

144

APPENDIX 4

Figure A4.123:Fluorescence decay profile of a PBM solution in THF monitored at λmax 450 nm

Decay7 IR1 Decay7F2 Decay7F2R 102 Fit Results τ1 14.72ns τ2 1.00ns χ2 1.234 Counts 101

100 0 50 100 150 200 250 300 350 400 450 Time/ns 4.6

0.0

-4.6 Residuals

Figure A4 124: Fluorescence decay profile of a R2 nanoparticle solution (THF/water ratio = 1/3) monitored at λmax 590 nm; excitation wavelength = 340 nm

Decay4 IR1 Decay4F2 Decay4F2R 102 Fit Results τ1 14.47ns τ2 2.15ns χ2 1.005 Counts 101

100 0 50 100 150 200 250 300 350 400 450 Time/ns 2.9

0.0

-2.9 Residuals

Figure A4 125:Fluorescence decay profile of a R2 nanoparticle solution (THF/water ratio = 1/5) monitored at λmax 590 nm; λex = 340 nm

145

Decay2 IR1 Decay2F2 Decay2F2R 102 Fit Results τ1 32.34ns τ2 4.44ns χ2 0.928 Counts 101

100 0 50 100 150 200 250 300 350 400 450 Time/ns 3.7

0.0

-3.7 Residuals Figure A4.126: Fluorescence decay profile of a R2 nanoparticle solution (THF/water ratio = 1/11) monitored at λmax 590 nm; λex = 360 nm

103

102 Counts 101

100 0 5 10 15 20 25 Time/ns 3.2 0.0 -3.2 Residuals

Figure A4.127:Fluorescence decay profile of a R3 solution in THF monitored at λmax 460 nm; λex = 360 nm

3 10 6-Ex360Em600.t 6-Ex360Em600.t 6-Ex360Em600.t

Fit Results τ1 49.00ns 2 τ2 13.87ns 10 χ2 1.048 Counts 101

100 0 100 200 300 400 500 600 700 800 900 Time/ns 3.5

0.0

-3.5 Residuals

Figure A.128:Fluorescence decay profile of a R2 nanoparticles solution (THF/water ratio = 1/45) monitored at λmax 600 nm; excitation wavelength = 360 nm

146

1-Ex360Em592.t 1-Ex360Em592.t 1-Ex360Em592.t

Fit Results τ1 45.85ns 2 τ2 10.41ns 10 χ2 1.011 Counts 101

100 0 50 100 150 200 250 Time/ns 3.1

0.0

-3.1 Residuals

Figure A4.129:Fluorescence decay profile of a R2 nanoparticle solution (THF/water ratio = 1/40) monitored at λmax 600 nm; excitation wavelength = 360 nm.

THF only THF/water; 1/23 (1.9x10-6 M) THF/water; 1/45 (9.2x10-7 M) THF/water; 1/90 (4.6x10-7 M) Emission

400 450 500 550 600 650 700 750 Wavelength (nm)

Figure A4.130:Fluorescence spectra of R2 nanoparticles recorded at different THF/water ratios by volume and different concentrations; λex = 345 nm

100

% of in % of water mixture 0

400 450 500 550 600 Emission maximum (nm)

Figure A4.131:Shift in emission maximum of R3 due to change in the THF/water ratio; arrows show the direction of emission shift

147

Figure A4.132;Fluorescence spectra of R3 nanoparticle solutions (5.2 x 10-6 M) at different THF/water ratios by volume; λex = 380 nm

Figure A4.133:Fluorescence spectra of R3 nanoparticles recorded during the reverse process of adding THF solutions to a THF/water solution, λex = 380 nm

Acetonitrile Dimethylformamide 1.0 Hexanes Methanol 0.8 Toluene Dichloromethane

0.6

0.4 Intensity

0.2

0.0 400 500 600 700 Wavelength (nm)

Figure A4.134:Emission spectra of R2 in different solvents; λex = 350 nm

148

8.2 8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 ppm

8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 ppm 2.00 1.93 5.20 1.96 4.20 18.00 Figure A4.135:1HNMR of R2 Figure A4.13.

158.50 143.44 139.12 138.70 133.38 132.50 130.72 130.31 129.82 126.39 123.79 123.71 120.32 116.38 113.71 112.59 109.17 94.55 77.44 77.22 77.01 76.59 34.77 31.98

531, mononitrile tertbutlycarbazoly C13

150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 ppm Figure A4.136: 13CNMR of R2 Figure A4.14.

Figure A4.137: 1HNMR of R3 Figure A4.15.

149

Figure A4.138: 13CNMR of R3

150

CHAPTER 5

A SINGLE WHITE LIGHT EMITTING COMPOSITE ORGANIC NANOPARTICLE

Highly stable composite fluorescent organic nanoparticles (CFONs) were prepared by co- reprecipitation of blue and red emitting organic compounds from water/tetrahydrofuran(THF) mixtures. The emission intensity is altered by the formation of nanoparticles. The emission region can be reversibly tuned by using different solvent/nonsolvent ratios. SEM images show diversity in particle size. Emission spectra of CFONs prepared from different ratios of red and blue emitters cover the entire visible region from 400 to 700 nm. The fluorescence lifetime of the particles is 2 ns. Confocal microscopy measurements reveal composite organic nanocrystals emitting a white light. CIE coordinates of these CFONs demonstrate high color purity (CIE X,

Y: 0.34, 0.35).

5.1. Introduction

Inorganic nanoparticles and quantum dots1 have been explored as biological labels,2

components of photovoltaic cells,3 light emitting diodes,4 and optical sensors.5 Organic nanoparticles are anticipated to be more practical considering their greater flexibility in synthesis and in the preparation of nanoparticles. Several fluorescent organic nanoparticles (FONs) have been investigated over the years.6-15 However, investigations on the FONs are only in the

beginning stage. FONs prepared from conventional fluorophores show diverse absorption and

fluorescence properties, and comprehensive investigations have substantiated their reliability in

various applications. Parallel to the much explored molecular luminescence, the study of

luminescent nanoparticles has brought recent interest in quantum confinement effects.

Luminescent organic nanoparticles are anticipated to be useful for application in various fields including as fluorescent biological labels, in photovoltaic cells, for micro- and nano-electronics,

151 in catalysis, and as optical sensors.12,15,16 Nakanishi et al. showed their application in third order

nonlinear optics..

Incorporation of a single nano-material emitting white light has been a sought-after target

because it would greatly reduce the cost of white light emission in full color electronic displays.

17 Studying organic nanoparticles is an excellent tool that hopefully bridges the understanding of bulk materials and molecules in solution.16 Nanoparticles form an ensemble and only average

information of size, shape, structure and property can be obtained.18 Direct correlation between

the characteristics of individual particles and macroscopic properties is to be further

established.16

We report herein, for the first time, a novel method for the preparation of white light

emitting composite organic nanocrystals (CFONs). Two compounds previously synthesized, one

emitting blue19 (B, Chart 5.1) and the other emitting red light6 (R2, Scheme 5.1), were co-

reprecipiated to form white light emitting CFONs. A tetrahydrofuran (THF) solution of

compounds B and R2, at specific molar ratios, was mixed. The CFONs were formed upon

injection of water into the mixture.

CN CN N

R2 B

Chart 5.1

5.2. Results and discussion

Figure 5.139 shows the emission spectra of the CFONs in the dispersion medium. The

wavelength of emission is independent of excitation wavelength. The absorption maximum of B as a nanoparticle, previously reported, is at 290 nm19 and that of R2, also previously reported,6 is

152 at 350 nm. Excitation of composite nanoparticles at either of these wavelengths produces white light. On excitation of R at the absorption maxima of B (280 nm), we found almost no emission.

Similarly, on excitation of B at the absorption maxima of R (360 nm), little emission also resulted. But the composite nanoparticles show strong emissions when excited at each of these wavelengths suggesting that the co-reprecipitated particles behave as a single chromophore.

Excit 280 nm Excit 300 nm 2.75x105 Excit 340 nm Excit 350 nm 2.20x105 Excit 360 nm -5 1.65x105 Conc of B 3.52x10 M Conc of R2 10x10-6 M 1.10x105 % of water 90 % Intensity 5.50x104

0.00 400 500 600 700 . Wavelength (nm)

Figure 5.139: Emission spectra of CFONs in the dispersion medium at different excitation energies

Furthermore, excitation spectra monitored at the two different emission maxima gives

similar spectra (Figure 5.140) though there is a difference in intensity. This behavior is explained

as follows: Addition of water to the THF solution containing a mixture of B and R2 induces

nucleation of both. Since water and THF are miscible, the solubility of both B and R2, which are

hydrophobic, decreases with the addition of water. When the solvent composition reaches the

critical nucleation condition both begin to grow as particles.18 While growing, molecules of both

B and R2 come close and form composite particles in which the proportion of B and R2 is controlled by the concentration of the individual molecules in solution. Since the mixture is homogenous, each particle in the suspension will contain an equal proportion of B and R2.

153

According to molecular exciton model,6 parallel alignment of the transition dipole moments (H-aggregation) shifts absorption to the blue region and reduces the fluorescence rate as well as diminishes the emission intensity. In the process of nucleation both B and R2 pack themselves in CFONs possibly as shown in Figure 5.140(a).

(A) (B) (C) (D)

+ N N t-Bu N N N t-Bu t-Bu N N t-Bu _ N t-Bu N N N N N _N N t-Bu

(E)

S 1 S1

TICT S1

290 400 nm 425 400 340 440 nm 500 590

So B R Monomer Nanoparticle transition transition in THF

Figure 5.140(a). (A) The monomer of B responsible for emission in THF solution, (B) The monomer of R2 responsible for emission in THF solution, (C) The twisted intramolecular charge transfer (TICT) state responsible for emission at lower energy (D) The intermolecular charge transfer (ICT) state of the R2 aligned with the J coupled B in composite nanoparticles and (E)

154 the absorption and emission transitions in THF solution and composite nanoparticle in

THF/water suspension

We have previously reported J aggregated R2 formed an intermolecular charge transfer state in nanoparticles.6 In composite particles the molecules of R2 form J aggregates to each

other, while molecules of B also form J aggregates. If one considers the two adjacent chains of B

and R2, the molecules of B, which have aromatic rings, will arrange their transition dipoles

perpendicular to the transition dipole of R2. Since molecules of R2 form a strong ICT, the electron density in the nitrile group is high and the electron deficiency in the N of the carbazole moiety is strong. The portion of B near the carbazole moiety acts as an electron donor while that away acts as an electron donor. So, inside the nanoparticle there will be two different kinds of alignments of transition dipoles: parallel between adjacent B and R2, and head to tail between each of B and R2. The resultant effect on the photophysical properties of CFONs is now the combined effect from these two alignments of transition dipoles. Thus, the whole system behaves as one entity photophysically. This is further corroborated by the following experiments.

Emission at 430 nm 5.20x106 Emission at 550 nm

3.90x106

2.60x106 Intensity Intensity 1.30x106

0.00 250 300 350 400 Wavelength (nm)

Figure 5.140: Excitation spectra of composite nanoparticles monitored at different emission maxima

155

Figure 5.141 shows the emission spectra of CFONs at different water/THF compositions.

The emission intensity of the red emitting nanoparticles decreases with an increase in the water fraction. Whereas, in contrast, the emission intensity of the blue emitting nanoparticles increases with the water fraction. On increasing the water fraction, the emission intensity in the blue, green and red regions increase suggesting the CFONs are spectroscopically different from either B or

R2.

5 THF:water = 1:7 5.00x10 THF:water = 1:9 THF:water = 1:11 THF:water = 1:5 THF:water = 1:4

2.50x105 Intensity

0.00 400 480 560 640 Wavelength (nm)

Figure 5.141: Emission spectra of composite nanoparticles at different fractions of water and THF, λex = 340 nm

The emission of the CFONs could be reversibly shifted, white or blue, merely by

adjusting the THF/water ratio from which they were re-coprecipitated. It appears that the

THF/water ratio at which the system reaches critical nucleation concentration that causes

formation of the particles in the forward process is also the ratio at which it loses critical

nucleation conditions in the reverse process, Figure 5.142. If the ratio of water is increased, the

fluorescence maximum shifts from 400 nm to 560 nm (Figure 5.141). Increasing the THF ratio

with such samples causes the fluorescence emission to revert to the initial value (400 nm)

(Figure 5.142). The emission intensity increased in the violet-blue region with a higher fraction

of water and in the orange-red region with a lower water fraction.

156

THF:Water = 1:11 3.0x106 THF:Water = 1:6 THF:Water = 1:5 THF:Water = 1:4 6 2.4x10 THF:Water = 1:3 THF:Water = 1:2.5 1.8x106 THF:Water = 1:2 THF:Water = 1:1.8 THF:Water = 1:1.6 6 1.2x10 THF:Water = 1:1.2

Intensity THF:Water = 1:1 6.0x105 THF Only

0.0 360 420 480 540 600 660 Wavelength (nm)

Figure 5.142: Emission spectra of composite nanoparticles dispersed in the medium - reverse process at different fractions of water, λex = 340 nm

Fluorescence decays of the CFONs prepared by using various THF/water ratios were

monitored at two emission maxima. Lifetimes of 1.5 and 1 ns that were observed at shorter wavelengths (440 nm) with samples prepared from solutions at a higher THF ratio became correspondingly longer (12 and 3 ns) when monitored at longer wavelengths (580 nm). For

systems prepared with higher water ratios, when decays were monitored at the same wavelengths

(580 nm and 440 nm), similar lifetimes were observed (Table 5.1). At higher THF ratios, the

presence of monomers of B and R2 causes variability in lifetime of the fluorescence, but when there is complete CFONs formation at higher water ratio, the lifetimes of fluorescence were same when monitored at either of the emission maxima.

Table 5.1. Lifetimes of fluorescence of composite nanoparticles dispersed in the dispersion medium: λex = 340 nm, emission monitored at emission maxima

Water/THF 1:11 1:9 1:7 1:5 1:4 ratio Em at 440

τF1(ns) 0.1 0.1 0.6 0. 2 0.1

τF2(ns) 2 2 2 1 1.5

157

Em at 580

τF1 (ns) 0.6 0.1 1 1 3

τF2 (ns) 2 2 3 3 12

Emission spectra taken with different ratios B and R2 are shown in Figure 5.143. The

best emission spectra covering the region from 400 nm to 670 nm were found from particles with

12 parts B and 1 part R2. At higher ratios of B, emission spectra were overwhelmed by the blue region.

3.0x105

2.5x105

2.0x105

1.5x105 B:R = 3.5:1 5 B:R = 5:1 Intensity 1.0x10 B:R = 7.5:1 B:R = 12.:1 4 5.0x10 B:R = 20:1 B:R = 45:1 0.0 % of Water = 90 400 450 500 550 600 650 Wavelength (nm)

Figure 5.143: Emission spectra of CFONs dispersed in the dispersion medium made from different ratios of B and R2, λex = 340 nm

2.0x106 B+R2, excit. 340 nm R2 excit. 340 nm 6 1.5x10 B excit. 300 nm

y B+R2,solid, excit.340 nm 1.0x106 Intensit

5.0x105

0.0 360 420 480 540 600 660 Wavelength (nm)

Figure 5.144: Emission spectra of CFONs, B, R2 in the dispersion medium and in solid state, λex = 340 nm

158

The emission spectra of B alone and R2 alone in the same dispersion medium are shown in Figure 5.144. CFONs have emission maxima at 431 nm and 587 nm, and the solid state containing the B and R2 in same proportion as CFONs, have emission maxima at 455 nm and

565 nm. The shape of the emission spectrum of CFONs in the blue region does not resemble the shape of the emission spectrum of B in the same dispersion medium. The solid state emission spectra of a mixture of B and R2 in the same proportions used to prepare CFONs is also different from the emission spectra of CFONs. These CFONs are highly stable in laser light for hours.

Scanning electron microscopy (SEM) images of the composite nanoparticles show a wide particle size distribution, Figure 5.145.

Figure 5.145: Scanning electron micrograph of FONs

Widefield fluorescence images from confocal microscopy (Olumpus IX81) show bluish white

COFNs at when excited at 365 nm, Figure 5.146.

159

Figure 5.146: Widefield fluorescence image of composite organic nanoparticles (excitation source: mercury lamp; 6 emission filter: low - pass < 405 nm in frequency; excitation filter: broad band UV 300 – 400 nm, max = 365 nm; Objective lens - 100 X)

Figure 5.147 shows the confocal images of the CFONs. There is clear emission in the blue, green, and red channels with similar intensities confirming the white light emission of the

CFONs.

a b

2 µm c 2 µm d

Figure 5.147: Confocal images of the CFONs showing emission in the (a) blue: 430-470nm, (b) green: 500- 570nm , and (c) red: 600-670nm channels; and (d) darkfield image. 2 µm 2 µm

160

As shown in Figure 5.148, CIE coordinates of these CFONs are X, Y: 0.34, 0.35 suggesting perfect white light emission from these particles in the dispersion medium.

Figure 5.148: Chromacity diagram and CIE coordinates for the CFONs demonstrating their high color purity (CIE X, Y: 0.34, 0.35)

5.3. Experimental section

5.3.1. Fluorescence lifetime ( F) measurement: CFONs dispersed in dispersion medium were

placed in quartz cuvettes. Fluorescence decay profiles of argon-degassed (~15 min) samples were recorded with use of a single photon counting spectrofluorimeter. Decays were monitored at the corresponding emission maximum of the compounds. Built-in software allowed the fitting of the decay spectra ( 2 = 1-1.5) and yielded the fluorescence lifetimes

5.4. Conclusions

We have prepared CFONs by a co-reprecipitation method using a proper proportion of

R2 and B from a THF water mixture. Reversible switching of emission from violet-blue to white

resulted. The emission spectra, excitation spectra and confocal images show that the particles are

white emitters at the appropriate ratio of B and R2 and behave differently from either of B or R2.

The CIE coordinates suggest the pure white color emission from these FONs. The emission of

these CFONs is attributed to the formation of composite alignment: J aggregation and H

161 aggregation of B and R2. The fabrication of devices using these CFONs as emissive layer is under way.

5.6. References

1) http://en.wikipedia.org/wiki/Quantum_dot.

2) Banin, U.; Bruchez, M.; Alivisatos, A. P.; Ha, T.; Weiss, S.; Chemla, D. S. J. Chem. Phys.

1999, 110, 1195-1201

3) Hagfeldt, A.; Gratzel, M. Chem. Rev. 1995, 95, 49.

4) An, B.-K.; Kwon, S-K.; Jung, S-D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410.

5) Wohltjen, H.; Snow, A. W. Anal. Chem. 1998, 70, 2856.

6) Adhikari, R. M., Shah, B. K., Palayangoda, S. S.; Neckers, D. C. Langmuir manuscript submitted.

7) Su, X.; Zhang, J.; Sun, L.; Koo, T.-W.; Chan, S.; Sundararajan, N.; Yamakawa, M.; Berlin, A.

A. Nano Lett. 2005, 5, 49.

8) Fu, H.-B.; Yao, J.-N. J. Am. Chem. Soc. 2001, 123, 1434.

9) Palayangoda, S. S.; Xichen, C.; Adhikari, R. M.; Neckers, D. C. Org. Lett. 2008 10, 281.

10) Bruchez, M. Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013.

11) Mullen, T. J.; Srinivasan, C.; Shuster, M. J.; Horn, M. W.; Andrews, A. M.; Weiss, P. S. J.

Nanopart. Res. 2008, 10, 1231.

12) Kasai, H.; Nalwa, H. S.; Okada, S.; Oikawa, H.; Nakanish, H. Handbook of

Nanostructured Materials and Nanotechnology; Academic Press: New York, 2000; Vol. 5,

Chapter 8, 433-473.

13) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.; Kwok, H. S.; Zhang, X.; Liu,

Y.; Zhu D.; Tang, B. Z.; Chem. Commun. 2001, 1740.

162

14) Tong, H.; Dong, Y.; HauBler, M.; Lam, J. W. Y.; Sung, H. H.-Y.; William, I. D.; Sun, J.;

Tang, B. H. Chem. Commun. 2006, 1133.

15) Geckeler, K. E., Rosenberg, E. Functional Nanomaterials; American Scientific Publishers,

2006 Edition, Chapter 13.

16) Masuhara, H.; Nakanishi, H.; Sasaki, K. Single Organic Nanoparticles Springer-Verlag

Berlin Heidelberg New York, 2003.

17) Adhikari, R. M.; Mondal, R.; Shah, B. K.; Neckers, D. C. J. Org. Chem. 2007, 72, 4727.

18) Horn, D.; Rieger, J. Angew. Chem., Int. Ed. 2001, 40, 4330.

19) Bhongale, C. J.; Chang, C.-W.; Lee, C.-S.; Diau, E. W.-G.; Hsu, C.-S. J. Phys. Chem. B

2005, 109, 13472.

163

LIST OF ABBREVIATIONS

tert Tertiary

ISC Intersystem Crossing ps Picosecond ns Nanosecond

μs Microsecond s or sec Second min Minute h or hr. Hour nm Nanometer

M Milli mL Milliliter

L Liter mm Millimeter cm Centimeter g Gram

164

μ Micro

MHz MegaHertz

W Watt

Ȧ Angstrom v or V Volume kcal Kilo Calorie

Conc. Concentration mol Mole mmol Millimol

M Moles per liter

Mp Melting Point

0C Degree Celsius

Wt Weight

TLC Thin Layer Chromatography

GC Gas Chromatography

MS Mass Spectrometry

NMR Nuclear Magnetic Resonance

165 s (for NMR) Singlet d (for NMR) Doublet m (for NMR) Multiplet

UV or Uv Ultra Violet

IR Infra Red hν Photon

THF Tetrahydrofuran

DCM Dichloromethane

DMSO Dimethyl sulfoxide

DMF Dimethyl formamide

Ph Phenyl

Ar Aryl

Me Methyl

T Triplet

S Singlet