The Directed Heavy Atom Effect: A Design Principle for Metal-Free Organic Phosphors by Onas James Bolton A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Materials Science and Engineering) in The University of Michigan 2010 Doctoral Committee: Associate Professor Jinsang Kim, Chair Associate Professor Lingjie Guo Assistant Professor Max Shtein Assistant Professor Anton Van der Ven © Onas James Bolton 2010 For Carrie, Charlotte, and Abby. ii Acknowledgements I first must thank Professor Jinsang Kim. When I first met with Professor Kim I was an under-utilized engineer at a struggling company, possessor of a modest academic record, four years removed from college, and clinging to uncertain hope that I be accepted into graduate school. I can only imagine now what he saw in me then, but nothing in my time at Michigan is more visible than his commitment to support and mentor me. The opportunity he gave me is perhaps the greatest professional, and in some ways personal, gift I have ever received and I will always be in his debt. Thank you, Professor Kim. Secondly I must thank my colleague, Dr. Kangwon Lee, whose unbridled success in his main research is the only reason I was lucky enough to inherit this most interesting and fruitful project. Without his keen observations none of these findings nor our success with them would have happened. Thank you, Kangwon. I would also like to thank the members of our research group and my other collaborators. In particular Dr. Hyong-Jun Kim, Dr. Myung-Su Kim, Dr. Eun Jeong Jeong, and Bong-Gi Kim contributed much time as they helped to make devices and new compounds. Dr. Jae Choel Cho taught me the finer points of chemical synthesis. Laura, Rob, Donghwan, and Jisama, I will always treasure your camaraderie. The members of the Shtein Group, Shaurjo Biswas and Steven Morris, were selfless with their time to fabricate devices and move our materials further. Thanks, all. The small army of undergraduate students that helped me can certainly not be forgotten. In the project’s early days Kevin Lin and Matt Gray suffered with me as we struggled to identify the critical compound. Later Jacqueline Harper, Young-Do Kim, Sarah Wang, Sunghyun Jeon, and Kevin Yien helped to take the work to new places and run lots and lots of tests. Lots. Their contributions were all critical. Thanks, guys. Of course, I thank my family for their support throughout the years. I am the product of their love and guidance and I hope I make them proud. Thanks to my Grandparents and siblings. Thanks to Fred and Jean for helping me juggle science and fatherhood in these last critical months. To Mom and Dad love and thanks for your enduring support. Thanks for band concerts, cars, food/shelter, tuition, and lessons learned as I watched from the kindergarten window. My daughter, Charlotte, and dog, Abby, didn’t contribute a great deal of direct help. In fact, their disregard for my privacy and pointed distain for my focus while writing this seemed, at the time, quite counterproductive. Still, they are both ridiculously cute and having two adorable cherubs at constant play on the center stage of my life gives the entire performance its truest purpose. Hugs and kisses. Lastly and most importantly, I must thank my wife for whom I dedicate all things I do. My success has come only through her support and patience. Thanks, Carrie. I love you. iii Table of Contents Dedication……………………………………………………………………………… ii Acknowledgements…………………………………………………………………….. iii List of Figures………………………………………………………………………….. viii List of Tables………………………………………………………………………….. xi List of Schemes………………………………………………………………………… xii Abstract………………………………………………………………………………… xiii Chapter 1: Introduction and Background……………………………………………… 1 1.1 Fluorescence and Phosphorescence..………………………………….. 2 1.1.1 Singlets and Triplets……………………………………………….. 2 1.1.2 Excited State Transitions………………………………………….. 4 1.2 Characterizations and Measurements………………………………….. 8 1.2.1 Absorption, Emission, and Excitation Spectra…………………….. 8 1.2.2 Quantum Yields……………………………………………………. 9 1.2.3 Quantum Lifetimes………………………………………………… 10 1.2.4 Rate Constants…………………………………………………….. 11 1.3 Phosphorescent Applications………………………………………….. 12 1.3.1 Organic Light Emitting Diodes……………………………………. 12 1.3.2 Photovoltaic Devices………………………………………………. 16 1.3.3 Sensors…………………………………………………………….. 16 1.3.4 Biological Imaging………………………………………………… 17 1.4 Known Organic Phosphors…………………………………………….. 18 1.4.1 Organometallic Chelates…………………………………………… 18 1.4.2 Aromatic Ketones………………………………………………….. 20 1.4.3 Confined Aromatics……………………………………………….. 20 1.4.4 Thiones……………………………………………………………. 21 1.5 Conclusions…………………………………………………………….. 22 1.6 References……………………………………………………………… 23 Chapter 2: Molecular Design of Metal-Free Organic Phosphors……………………… 26 2.1 Challenges to Organic Phosphor Design………………………………. 27 2.1.1 Intersystem Crossing……………………………………………….. 27 2.1.2 Vibrational Dissipation…………………………………………….. 27 2.2 Directed Heavy Atoms (DHAE)……………………………………….. 28 2.3 Aromatic Ketones……………………………………………………… 28 2.3.1 Intersystem Crossing……………………………………………….. 28 2.3.2 Spin-Orbit Coupling……………………………………………….. 30 2.3.3 Vibrational Suppression……………………………………………. 31 2.3.4 Applied in DHAE………………………………………………….. 32 2.4 The Heavy Atom Effect……………………………………………….. 32 2.4.1 Background………………………………………………………… 33 2.4.2 Internal and External Effects………………………………………. 33 2.4.3 Applied in DHAE………………………………………………….. 34 iv 2.5 Halogen Bonding………………………………………………………. 35 2.5.1 Background………………………………………………………… 35 2.5.2 Electron-deficient Halogens……………………………………….. 36 2.5.3 Distances and Angles………………………………………………. 37 2.5.4 Applied in DHAE………………………………………………….. 38 2.6 Vibrational Limitation…………………………………………………. 38 2.6.1 Low temperatures………………………………………………….. 38 2.6.2 Polymer/Crystal Embedding……………………………………….. 39 2.6.3 Applied in DHAE………………………………………………….. 39 2.7 Conclusions…………………………………………………………….. 40 2.8 References……………………………………………………………… 41 Chapter 3: The Directed Heavy Atom Effect at Work in Substituted Benzaldehydes……………………………………………………………. 42 3.1 Introduction…………………………………………………………….. 43 3.2 Results and Discussion………………………………………………… 43 3.2.1 Crystal Structure…………………………………………………… 43 3.2.2 Photophysical Properties: Solution………………………………… 44 3.2.3 Photophysical Properties: Crystal………………………………….. 47 3.3 Experimental…………………………………………………………… 50 3.3.1 General Methods…………………………………………………… 50 3.3.2 Synthesis and Characterization…………………………………….. 51 3.4 Conclusions…………………………………………………………….. 53 3.5 References……………………………………………………………… 54 Chapter 4: Cocrystallization for Enhanced Brightness from DHAE………………….. 55 4.1 Introduction……………………………………………………………. 56 4.2 Optical Results and Discussion………………………………………… 57 4.2.1 Photophysical Properties: Br6………………………………………57 4.2.2 Photophuscial Properties: Cocrystals………………………………. 59 4.3 Cocrystallization……………………………………………………….. 61 4.3.1 Crystal Comparisons……………………………………………….. 61 4.3.2 Cocrystallization Efficiency……………………………………….. 63 4.3.3 Size Constraints……………………………………………………. 67 4.4 Experimental…………………………………………………………… 68 4.4.1 General Methods…………………………………………………… 68 4.4.2 Synthesis and Characterization…………………………………….. 69 4.5 Conclusions…………………………………………………………….. 71 4.6 References……………………………………………………………… 73 Chapter 5: Principles for Color Tuning……………………………………………….. 74 5.1 Introduction………………………………………………………………. 75 5.2 Fine Tuning…………………………………………………………….. 75 5.2.1 Halogen Variations………………………………………………… 75 5.2.2 Halogen-free Chromophore……………………………………….. 77 5.3 Broad Tuning………………………………………………………….. 79 v 5.3.1 Blue………………………………………………………………… 80 5.3.2 Yellow……………………………………………………………… 82 5.3.3 Orange……………………………………………………………… 83 5.3.4 Red…………………………………………………………………. 86 5.4 Experimental…………………………………………………………… 88 5.4.1 General Methods…………………………………………………… 88 5.4.2 Synthesis and Characterization…………………………………….. 88 5.5 Conclusions…………………………………………………………….. 96 5.6 References……………………………………………………………… 98 Chapter 6: Delayed Phosphorescence………………………………………………….. 99 6.1 Introduction and Background………………………………………….. 100 6.1.1 Delayed Fluorescence……………………………………………… 100 6.1.2 Excimers…………………………………………………………… 101 6.2 Results and Discussion………………………………………………… 102 6.3 Experimental…………………………………………………………… 106 6.3.1 General Methods…………………………………………………… 106 6.3.2 Synthesis and Characterization…………………………………….. 107 6.4 Conclusions…………………………………………………………….. 107 6.5 References……………………………………………………………… 108 Chapter 7: Dimethoxybromobenzaldehyde……………………………………………. 109 7.1 Introduction……………………………………………………………. 110 7.2 Results and Discussion ……………………………………………….. 110 7.3 Experimental…………………………………………………………… 115 7.3.1 General Methods…………………………………………………… 115 7.3.2 Synthesis and Characterization…………………………………….. 116 7.4 Conclusions……………………………………………………………….. 117 7.5 References………………………………………………………………… 119 Chapter 8: Ester Functionalized Phosphors……………………………………………. 120 8.1 Introduction…………………………………………………………….. 121 8.2 Results and Discussion………………………………………………… 122 8.3 Experimental…………………………………………………………… 125 8.3.1 General Methods…………………………………………………… 125 8.3.2 Synthesis and Characterization…………………………………….. 126 8.4 Conclusion…………………………………………………………….. 128 8.5 Reference………………………………………………………………. 129 Chapter 9: Polymer Entanglement…………………………………………………….. 130 9.1 Introduction………………………………………………………………. 131 9.2 Results and Discussion…………………………………………………… 131 9.3 Experimental……………………………………………………………… 139 9.3.1 General Methods…………………………………………………… 139 9.3.2 Synthesis and Characterization……………………………………. 139