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 ii © 2008 Ravi M Adhikari All Rights Reserved iii 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 iv 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). vii 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. viii 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. x 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 .....................................................................................................................