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High energy hosts and blue emitters for phosphorescent organic light emitting diodes

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High energy hosts and blue emitters for phosphorescent organic light emitting diodes

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Content HIGH ENERGY HOSTS AND BLUE EMITTERS FOR PHOSPHORESCENT ORGANIC LIGHT EMITTING DIODES by Wei Wei A Dissertation Presented to the FACULTY OF THE USC GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (CHEMISTRY) December 2010 Copyright 2010 Wei Wei ii DEDICATION Dedicated to My Parents: Mr. Jianhui Wei and Ms. Meizhen Liang iii ACKNOWLEDGMENTS First of all, I would like to thank my advisor, Prof. Mark Thompson for providing me this opportunity to work on my Ph. D. projects, and also for his guidance to me in my journey of pursuing science. I would also like to thank Prof. Surya Prakash, Prof. Edward Goo, Prof. Robert Bau and Prof. Kyung Jung for being my dissertation or my qualifying exam committee members, and also for helping me grow towards a real scientist. I would also like to thank Dr. Peter Djurovich for his knowledgeable support and scientific discussions, to Dr. Alberto Bossi for discussions and teaching me lab techniques, to Dr. Seogshin Kang and Dr. Slava Diev for sharing their organic chemistry experience, to Dr. Zhiwei Liu and Dr. Sergio Lima for discussions and reviewing my manuscripts, to Dr. Matthew Whited for X-ray crystallography, to Dr. Jason Brooks for collaborations on OLED lifetime measurements. I would also like to extend my gratitude to Dr. Tissa Sajoto, Dr. Chao Wu, Dr. Kenneth Hanson, Dr. Euegene Polikarpov, Dr. Paulin Wahjudi, Dr. Marco Curreli for their help in the lab and their friendships. There are also other members in the group that have helped me whenever I need it. So, in this case, I would like to thank Siyi, Rui, Yifei, Qiwen, Francisco, Vincent, Changhong, Carston, Cody, Dolores, Laurent, Valentina, Patrick and Cong for their constant support. In addition, I am thankful to Judy Hom, who has been very helpful for organizations and administrations. My thanks are also due to other staff members of the Chemistry Department especially Michele Dea, Heather Connor and Marie de la Torre for their tactical support. Allan Kershaw, Ross Lewis and Phil Sliwoski are also acknowledged for their technical iv help with NMR, DSC, PotentioStat and glass blowing. Finally, I would also take this opportunity to give my special thanks to Dr. Clement Do for his love and help in both chemistry and life, and also special thanks to my parents Mr. Jianhui Wei and Ms. Meizhen Liang who have granted me their endless support and help in my whole life. v TABLE OF CONTENTS DEDICATION ii ACKNOWLEDGMENTS iii LIST OF TABLES viii LIST OF FIGURES x LIST OF SCHEMES xv ABSTRACT xvi Chapter 1: Introduction……………………………………………………………….... 1 1.1 Organic Light Emitting Diode and its History…………………………………… 1 1.2 Advantages of OLEDs over LCDs……………………………………………….. 3 1.3 Functional mechanism of OLED…………………………………………………. 4 1.3.1 Charge Injection and Charge Transportation ................................................. 5 1.3.2 Recombination and emission ......................................................................... 6 1.3.3 Energy transfers in OLEDs ............................................................................ 8 1.3.4 Phosphorescent devices ................................................................................. 9 1.4 Materials for OLEDs ............................................................................................. 11 1.4.1 Hole transporter and electron transporter .................................................... 11 1.4.2 Hosts ............................................................................................................ 12 1.4.3 Iridium Cyclometalated Complexes as Dopants for PHOLEDs .................. 16 1.5 Summary ............................................................................................................... 21 1.6 Chapter 1: References ........................................................................................... 23 Chapter 2: Properties of Fluorenyl Silanes in Organic Light Emitting Diodes........ 27 2.1 Introduction……………………………………………………………………... 27 2.2 Experimental…………………………………………………………………..... 29 2.2.1 General. ........................................................................................................ 29 2.2.2 Procedures. ................................................................................................... 30 2.2.3 Differential Scanning Calorimetry. .............................................................. 31 2.2.4 Electrochemistry and Photophysics. ............................................................ 31 2.2.5 Device Fabrication. ...................................................................................... 32 2.3 Result and Discussion ............................................................................................ 33 2.3.1 Design, Synthesis and Characterization of the Molecules ........................... 33 2.3.2 OLED Studies .............................................................................................. 40 vi 2.4 Summary ................................................................................................................ 51 2.5 Chapter 2: References ............................................................................................ 52 Chapter 3: A Comparison between Methyl and Silyl Spirobifluorenyls: Synthesis, Photophysics and Performance in Phosphorescent Organic Light Emitting Diodes…………………………………………………………………. 55 3.1 Introduction……………………………………………………………………... 55 3.2 Experimental……………………………………………………………………. 58 3.2.1 Synthesis. ..................................................................................................... 58 3.2.2 X-ray Crystallographic Procedures. ............................................................. 60 3.2.3 Thermal Analysis. ........................................................................................ 61 3.2.4 Electrochemistry and photophysics. ............................................................ 61 3.2.5 Device Fabrication. ..................................................................................... 62 3.3 Result and Discussion. .......................................................................................... 63 3.3.1 Design, Synthesis and Characterization of the Materials. ........................... 63 3.3.2 OLED Studies .............................................................................................. 70 3.4 Summary ............................................................................................................... 78 3.5 Chapter 3: References ........................................................................................... 80 Chapter 4: Synthesis and Characterization of Isomeric, Blue Iridium Complexes with Cyclometalating Rigid Ligands…………………………………….. 83 4.1 Introduction……………………………………………………………………... 83 4.2 Experimental…………………………………………………………………..... 87 4.2.1 General. ........................................................................................................ 87 4.2.2 Electrochemistry and Photophysics. ............................................................ 92 4.2.3 X-ray crystallography. ................................................................................. 93 4.2.4 Sublimation. ................................................................................................. 93 4.3 Result and Discussion ......................................................................................... 94 4.3.1 Design and Synthesis of the Ligand and the complexes. ............................. 94 4.3.2 NMR Characterization. ................................................................................ 98 4.3.3 X-ray crystallography. ................................................................................ 101 4.3.4 DFT calculation. ........................................................................................ 101 4.3.5 Electrochemistry. ....................................................................................... 104 4.3.6 Electronic Spectra and Emission Spectra. ................................................. 107 4.3.7 Isomerization of Ir(tpzp) 2 pic. ..................................................................... 109 4.4 Conclusion ........................................................................................................... 111 4.5 Chapter 4: References ......................................................................................... 114 Chapter 5: Heteroleptic Fluorene based Iridium Complexes as Blue Emitters for Phosphorescent Organic Light Emitting Diodes……………………………...... 118 5.1 Introduction………………………………………………………………….....118 5.2 Experimental…………………………………………………………………… 121 vii 5.2.1 Synthesis and Characterization .................................................................. 121 5.2.2 Electrochemistry. ....................................................................................... 124 5.2.3 Photophysics. ............................................................................................. 124 5.2.4 Device Fabrication and Testing. ................................................................. 125 5.3 Result and Discussion ......................................................................................... 126 5.3.1 Synthesis .................................................................................................... 126 5.3.2 Electrochemistry ........................................................................................ 128 5.3.3 Photophysics .............................................................................................. 132 5.3.4 OLED studies ............................................................................................. 135 5.4 Summary ............................................................................................................. 140 5.5 Chapter 5: References ......................................................................................... 142 Chapter 6: Temperature Dependence Studies of Cyclometalated Ir Complexes with Phenylpyrazole Based Ligands………………………………………………… 144 6.1 Introduction……………………………………………………………………. 144 6.2 Experimental…………………………………………………………………… 147 6.2.1 General. ...................................................................................................... 147 6.2.2 X-ray crystallography. ................................................................................ 150 6.2.3 Photophysics. ............................................................................................. 150 6.3 Result and Discussion ......................................................................................... 151 6.3.1. Design of ligands ...................................................................................... 151 6.3.2 Synthesis and characterization of Ir(dpq) 3 ................................................ 152 6.3.2. Absorption, Excitaiton and Emission Spectra ........................................... 154 6.3.3 Temperature Dependence Studies .............................................................. 157 6.4 Chapter 6: References ......................................................................................... 163 BIBLIOGRAPHY……………………………………………………………………... 165 APPENDIX 1: Chapter 2 Supplemental Information……………………….………… 174 APPENDIX 2: Chapter 3 Supplemental Information…………….…………………… 177 APPENDIX 3: Chapter 4 Supplemental Information…………….…………………… 182 APPENDIX 4: Chapter 6 Supplemental Information…….…………………………… 199 viii LIST OF TABLES Table 2.1 Thermal properties of SiFln ...................................................................... 34 Table 2.2 Electrochemical and photophysical characteristics of SiFln. ................... 47 Table 2.3 Performance of undoped devices I and II. ................................................ 47 Table 2.4 Performance of phosphorescent devices III, IV and V . ............................. 50 Table 3.1 Percent type of intermolecular interactions (C-H, C-C or H-H) for Ph 3 CSBFL and Ph 3 SiSBFL crystals calculated using the Crystal Explorer software package. ....................................................................... 64 Table 3.2 Summary of the electrochemical and photophysical properties of Ph 3 CSBFL and Ph 3 SiSBFL. ...................................................................... 67 Table 3.3 OLED performance of the FIrpic device: ITO /NPD /mCP/ FIrpic: SBFL Host (10%) /BCP and the Ir(ppy) 3 device: ITO/ CHATP/ NPD/ Ir(ppy) 3 : SBFL Host (10%)/ BAlq/ Alq 3 . ........................................ 72 Table 4.1 Selected bond distance (Å) for Ir(tpzp) 2 pic (I1 and I3). The numbering of the atoms is shown in Fig. 4.3. ......................................... 102 Table 4.2 DFT calculated energies and HOMO/LUMO levels of Ir(tpzp) 3 and the isomers of Ir(tpzp) 2 pic ................................................................ 103 Table 4.3 Electrochemical and photophysical properties of tpzp, Ir(tpzp) 3 and Ir(tpzp) 2 pic (I1, I2 and I3) ................................................................ 106 Table 5.1 Summary of quantum yields and lifetimes of Ir(Pspiro) 3 , Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz. ................................................................ 134 Table 5.2 Performance of undoped devices I and II. .............................................. 137 Table 6.1 Bond lengths [Å] for Ir(dpq) 3 compared to Ir(ppz) 3 .............................. 155 Table 6.2 Photophysical parameters of Ir(ppz) 3, Ir(ppzMe 2 ) 3, Ir(dpq) 3 and Ir(tpzp) 3 ................................................................................................... 158 Table 6.3 Kinetic parameters for the excited-state decay of Ir(ppz) 3, Ir(ppzMe 2 ) 3, Ir(dpq) 3 and Ir(tpzp) 3 .......................................................... 160 Table A3.1 Crystal data and structure refinement for Ir(tpzp) 2 pic(I1). ..................... 197 ix Table A3.2 Crystal data and structure refinement for Ir(tpzp) 2 pic(I3). ..................... 198 Table A4.1 Crystal data and structure refinement for Ir(dpq) 3 . ................................. 200 x LIST OF FIGURES Figure 1.1 Typical OLED device structure .................................................................. 1 Figure 1.2 Generation of electroluminescence ............................................................. 5 Figure 1.3 Two different mechanisms of charge recombination mechanism ............... 7 Figure 1.4 Mechanism of Foster (left) and Dexter (right) energy transfer ................... 8 Figure 1.5 Singlet and triplet states formed after charge recombination ..................... 9 Figure 1.6 Jablonski diagram ..................................................................................... 10 Figure 1.7 Hole transporter NPD and electron transporter Alq 3 ................................ 12 Figure 1.8 Common hosts utilized in OLED devices ................................................. 14 Figure 1.9 Structures of fluorene based materials for OLEDs ................................... 15 Figure 1.10 Iridium phosphors utilized in PHOLEDs .................................................. 17 Figure 1.11 Strucutures of Ir(ppz) 3 and Ir(ppy) 3 .......................................................... 18 Figure 1.12 Energy diagram of radiative and non-radiative states for iridium complexes ................................................................................................. 20 Figure 1.13 Isomeric structures of homolepitc and heteroleptic 6-coordinated cyclometalated Iridium complexes ........................................................... 22 Figure 2.1 Dependence of the glass transition temperature, T g (triangle), and the sublimation temperature, T sub (circle), on the number of fluorenes in SiFln. ..................................................................................... 35 Figure 2.2 Cyclic voltammetric redox curves for SiFl4. ............................................ 36 Figure 2.3 (a) Absorption and (b) emission spectra of SiFln in CH 2 Cl 2 solution. (c) Emission spectra from neat films of SiFl4 recorded at room temperature (RT) and 77K with the phosphorescence spectrum of SiFl4 measured in 2-MeTHF solution at 77K. ..................................... 41 Figure 2.4 Energy diagrams of the fabricated OLED devices.................................... 45 xi Figure 2.5 Current density vs. voltage plots of devices I and II. ................................ 46 Figure 2.6 V oltage dependence of the EL spectra of the device: ITO / NPD (30 nm) /mCP (10 nm) /SiFl4 (20 nm) / Alq 3 (20 nm) /LiF (1 nm) /Al (100 nm).. ............................................................................... 46 Figure 2.7 EL spectra of (a) devices III and (b) device IV and device V .. ................. 49 Figure 3.1 X-ray crystal structures of Ph 3 CSBFL (left) and Ph 3 SiSBFL (right). ....... 63 Figure 3.2 Absorption (abs), excitation (ex) and photoluminescence (PL) spectra of Ph 3 CSBFL and Ph 3 SiSBFL from (a) dilute 2-methyl THF solution, (b) sublimed crystals and (c) 40 nm thin films. ................. 68 Figure 3.3 Energy diagrams of undoped devices: ITO / NPD / mCP / SBFL Host / Alq3 and FIrpic devices: ITO / NPD / mCP / FIrpic: SBFL Host (10%) / BCP. ..................................................................................... 70 Figure 3.4 Electroluminescence spectra of Ph 3 CSBFL and Ph 3 SiSBFL undoped devices: ITO / NPD / mCP / SBFL Host / Alq 3 at 9 V . .............. 71 Figure 3.5 Current density - voltage (J-V) characteristics of Ph 3 CSBFL and Ph 3 SiSBFL undoped devices: ITO / NPD / mCP / SBFL Host / Alq 3 ................................................................................................. 74 Figure 3.6 Comparison of (a) current density – voltage characteristic and (b) luminance – voltage characteristic of the FIrpic devices: ITO / NPD / mCP / FIrpic : SBFL Host (10%) / BCP and the Ir(ppy) 3 devices: ITO / CHATP / NPD / Ir(ppy) 3 : SBFL host (10%) / BAlq / Alq 3 . .......................................................................... 75 Figure 3.7 Electroluminescence spectra of the FIrpic devices: ITO / NPD / mCP / FIrpic : SBFL Host (10%) / BCP and the Ir(ppy) 3 devices: ITO / CHATP / NPD / Ir(ppy) 3 : SBFL Host (10%) / BAlq / Alq 3 . ........... 76 Figure 3.8 Quantum efficiency vs. current density of the FIrpic devices: ITO / NPD / mCP / FIrpic : SBFL Host (10%) / BCP and the Ir(ppy) 3 devices: ITO / CHATP / NPD / Ir(ppy) 3 : SBFL Host (10%) / BAlq / Alq 3 . ............................................................................................... 77 Figure 4.1 1 HNMR spectra of Ir(tpzp) 2 pic I1, I2 and I3 ............................................ 97 Figure 4.2 Thermal ellipsoid (ORTEP) plots of Ir(tpzp) 3 (top), Ir(tpzp) 2 pic I1 (middle) and I3 (bottom). ...................................................................................... 100 xii Figure 4.3 DFT calculated LUMO orbitals of Ir(tpzp) 3 ( top-left) and Ir(tpzp) 2 pic (I1) (top-left) and Differential pulse voltammetry (DPV) curves of Ir(tpzp) 3 and Ir(tpzp) 2 pic (I1, I2 and I3) (bottom) ....... 105 Figure 4.4 Absorptions and emissions of Ir(tpzp) 3 and Ir(tpzp) 2 pic (I1, I2 and I3) at room temperature (top) and emissions at 77 K (bottom). ..................................................................................... 109 Figure 4.5 1 H NMR chemical shifts of the tert-butyl groups of the Ir(tpzp) 2 pic isomers before and after sublimation of I1 ............................................. 110 Figure 5.1 HOMO and LUMO comparisons between FlzIr , Ir(ppy) 3 , PQIr and FIrpic ................................................................................................ 120 Figure 5.2 Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) curves of Ir(Pspiro) 3 .................................................................... 129 Figure 5.3 Differential pulse voltammetry (DPV) curves of Pspiro, Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz ................................................................. 130 Figure 5.4 HOMO and LUMO levels of Ir(Pspiro) 3 , Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz ................................................................................................ 131 Figure 5.5 Absorption spectrum and emission spectrum of Ir(Pspiro) 3 ................... 132 Figure 5.6 Absorption and emission spectra of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz ........ 133 Figure 5.7 Devices structure of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz devices: NPD(40 nm) /dopant: mCP(8%, 25 nm)/BCP (40 nm) .......................... 136 Figure 5.8 J-V characteristics of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz devices: ITO/NPD(40 nm)/dopant: mCP (25 nm)/ BCP (40 nm)/LiF(1 nm)/ Al (100 nm) ............................................................................................. 137 Figure 5.9 L-V characteristics of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz devices: ITO/NPD(40 nm)/dopant: mCP (25 nm)/ BCP (40 nm)/LiF(1 nm)/ Al (100 nm) ............................................................................................. 138 Figure 5.10 External quantum efficiency versus current density of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz devices: ITO/NPD(40 nm)/dopant: mCP (25 nm)/ BCP (40 nm)/LiF(1 nm)/ Al (100 nm) .................................................... 138 Figure 5.11 V oltage dependence of the electroluminescence spectra of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz devices: ITO/NPD(40 nm)/dopant: mCP (25 nm)/ BCP (40 nm)/LiF(1 nm)/ Al (100 nm) ............................ 139 Figure 6.1 DFT calculation of deactivation pathway of Ir(ppz) 3 excited state ......... 146 xiii Figure 6.2 X-ray crystal structure of Ir(dpq) 3 ........................................................... 155 Figure 6.3 Absorption spectra of Ir(ppzMe 2 ) 3 , Ir(dpq) 3 compared to Ir(tpzp) 3 ........ 156 Figure 6.4 Excitation and emission spectra of Ir(ppzMe 2 ) 3 , Ir(dpq) 3 and Ir(tpzp) 3 at 77 K ...................................................................................... 159 Figure 6.5 Temperature dependence of luminescence decay of Ir(ppzMe 2 ) 3 and Ir(tpzp) 3 ................................................................................................... 161 Figure 6.6 Temperature dependence of luminescence decay of Ir(dpq) 3 (77 K to 150 K) ................................................................................................. 161 Figure A1.1 Differential scanning calorimetric (DSC) thermograms of SiFln ........... 174 Figure A1.2 Differential pulse voltammetry curves of the reductions of SiFln .......... 174 Figure A1.3 DFT Calculated HOMO (left), LUMO (middle) and the triplet (right) surfaces of SiFln .......................................................................... 175 Figure A1.4 (a) Current density vs. voltage and (b) quantum efficiency vs. current density of devices III, IV and V .................................................. 176 Figure A2.1 Packing of Ph 3 CSBFL (top) and Ph 3 SiSBFL (bottom) molecules in a unit cell. ............................................................................................ 177 Figure A2.2 Calculated Hirshfeld surfaces of Ph 3 CSBFL and Ph 3 SiSBFL, which are shown in different viewing angles (front, left and back) ....... 178 Figure A2.3 DFT calculated HOMO (solid) and LUMO (mesh) surface and levels of Ph 3 CSBFL and Ph3SiSBFL. Calculated values are compared to the measured values. ......................................................... 178 Figure A2.4 Cyclic voltammetry curves of Ph 3 SiSBFL and Ph 3 CSBFL .................... 179 Figure A2.5 Differential scanning calorimetric (DSC) thermograms of Ph 3 SiSBFL and Ph 3 CSBFL ..................................................................... 179 Figure A2.6 V oltage dependence of EL spectra of the undoped devices: (a) NPD/mCP/Ph 3 CSBFL/Alq 3 and (b) NPD/mCP/Ph 3 SiSBFL/Alq 3 . As the voltage increases from 8 V to 15 V , the Alq 3 emission from the Ph 3 CSBFL device decreases, and the NPD emission from the Ph 3 SiSBFL device increases. ................................................... 180 Figure A2.7 Device performance of Ir(ppy) 3 device with 15% doped percentage. .... 181 Figure A3.1 1 HNMR of dichloro bridged dimer [Ir(tpzp) 2 Cl] 2 ................................... 182 xiv Figure A3.2 13 C NMR of Ir(tpzp) 3 and Ir(tpzp) 2 pic (I1, I2 and I3) ............................. 183 Figure A3.3 COSY of Ir(tpzp) 3 ................................................................................... 184 Figure A3.4 NOESY of Ir(tpzp) 3 ................................................................................ 185 Figure A3.5 COSY of Ir(tpzp) 2 pic I1 .......................................................................... 186 Figure A3.6 NOESY of Ir(tpzp) 2 pic I1 ....................................................................... 187 Figure A3.7 COSY of Ir(tpzp) 2 pic I2 .......................................................................... 188 Figure A3.9 COSY of Ir(tpzp) 2 pic I3 .......................................................................... 190 Figure A3.10 NOESY of Ir(tpzp) 2 pic I3 ....................................................................... 191 Figure A3.11 DFT calculated LUMO (top) and HOMO (bottom) orbitals of Ir(tpzp) 3 and the isomers of Ir(tpzp) 2 pic ................................................ 192 Figure A3.12 Cyclic voltammetry and differential pulse voltammetry (inset) curves of tpzp versus ferrocene/ferrocenium .......................................... 193 Figure A3.13 Cyclic voltammetry curves of Ir(tpzp)3 versus ferrocene/ferrocenium ............................................................................. 193 Figure A3.14 Cyclic voltammetry curves of Ir(tpzp)2pic I1 versus ferrocene/ferrocenium ............................................................................. 194 Figure A3.15 Cyclic voltammetry curves of Ir(tpzp)2pic I2 versus ferrocene/ferrocenium ............................................................................. 194 Figure A3.16 Cyclic voltammetry curves of Ir(tpzp)2pic I3 versus ferrocene/ferrocenium ............................................................................. 195 Figure A3.17 Absorption and emission of tpzp ligand ................................................. 196 Figure A3.18 HPLC chromatogram of the Ir(tpzp) 2 pic I1 sample before (top) and after (bottom) sublimation. The bottom spectrum shows the chromatogram of the unsublimed samples after heated for 24 h. ........... 196 Figure A4.1 1D and 2D COSY NMR spectra of Ir(dpq) 3 ........................................... 199 xv LIST OF SCHEMES Scheme 2.1 Structures of SiFln, Fln, NPD and Alq 3 .................................................... 28 Scheme 3.1 Synthesis of Ph 3 CSBFL ............................................................................ 57 Scheme 3.2 Synthesis of Ph 3 SiSBFL ........................................................................... 57 Scheme 4.1 Illustration of the isomeric structures of 6-coordinated Homoleptic and heteroleptic cyclometalated Ir Complexes ......................................... 86 Scheme 4.2 Illustration of the isomeric structure of 6-coordinated homoleptic and heteroleptic cyclometalated Ir complexes .......................................... 94 Scheme 4.3 Synthesis of tpzp ....................................................................................... 96 Scheme 5.1 Structures of flz and Ir(flz) 3 .................................................................... 119 Scheme 5.2 Structures of Pspiro, Ir(pspiro) 3 , Ir(ppz) 2 flz and Ir(ppz) 2 Pspiro ............. 121 Scheme 5.3 Synthesis of Ir(Pspiro) 3 ........................................................................... 127 Scheme 5.4 Synthesis of Ir(ppz) 2 flz and Ir(ppz) 2 Pspiro ............................................. 128 Scheme 6.1 Jablonski diagram ................................................................................... 144 Scheme 6.2 Structures of Ir(ppz) 3 , Ir(ppzMe 2 ) 3 , Ir(dpq) 3 , Ir(tpzp) 3 ........................... 147 Scheme 6.3 Structural comparison of ppz, ppzMe2, dpq and tpzp ............................ 152 Scheme 6.4 Synthesis of Ir(dpq) 3 ............................................................................... 153 xvi ABSTRACT Organic light emitting diodes (OLEDs) have achieved high levels of performance including high efficiencies, color purities and device lifetimes. It has been widely utilized in commercial applications except that the blue device lifetime is still poor compared to red and green devices. The stability of high energy OLED materials is a key issue to be addressed for achieving a long life and efficient blue device. In this dissertation, a great effort has been put into developing more stable and more efficient materials that can also fulfill the band gap and photophysical requirements. The materials studied include large bad gap hosts and blue phosphorescent emitters. In chapter 2 and 3, multi-fluorenyl silanes, silyl spirobifluorenes and methyl spirobifluorenes have been studied as potential hosts for phosphorescent OLEDs. These materials have high triplet energies, large HOMO-LUMO gaps and high glass transition temperatures and they also show ambipolar transport characteristics in OLED devices. The device current could be enhanced by increasing the fluorene ratios in the hosts. These materials were also tested in phosphorescent OLEDs and have been proved to be good hosts for red, green and blue devices. In the second part of this dissertation, studies on large band gap iridium complexes are demonstrated. These cyclometalated iridium complexes are all derivatives of iridium (III) tris(1-phenyl-pyrazole) (Ir(ppz) 3 ). Chapter 5 is dedicated to solve an electron trapping problem of fluorenyl pyrazolyl Ir complexes. A better electron trapping group, fluorene, was incorporated into the ligand through a spiro structure. Photophysical studies and OLED fabrications were utilitzed to test these spirobifluorene based Ir xvii cyclometalated complexes. Characterization and isomerization studies of rigid ligand heteroleptic Ir complexes will be discussed in Chapter 4. It was found that these heteroleptic complexes could isomerize during sublimation. In chapter 6, some results from temperature dependence studies will be discussed to discover the triplet deactivation of phenylpyrazole related Ir complexes. 1 Chapter 1: Introduction 1.1 Organic Light Emitting Diode and its History An organic light emitting diode (OLED) is a solid state semiconductor with several thin organic or organometallic layers (~100 nm) sandwiched between two electrodes as shown in Fig. 1.1. A typical OLED structure consists of an anode (generally a transparent indium tin oxide, ITO), a hole transporting layer (HTL), an emissive layer (EML), an electron transporting layer (ETL), and a cathode (Al or Ag:Mg). When a certain voltage is applied to an OLED, charge carriers (holes and electrons) are injected respectively from the anode and the cathode into the organic thin films. The charge carriers migrate through charge transport layers under the influence of an electrical field and then recombine in the emissive layer, forming excitons (excited state molecules) which will radiate from its excited energy level to its ground state by emitting light. This process is known as electroluminescence. 1,2 Figure 1.1 Typical OLED device structure 2 Electroluminescence in organic materials was discovered in 1950s by Bernanose and co-workers. 3,4 They applied a high-voltage alternating current (AC) field to thin layers of organics which produced electroluminescence. Between 1950s and 1980s, there have been researches focusing on improving organic materials for electroluminescent devices. Cheaper and more flexible materials were used to replace expensive and rigid semiconductors. 5-8 The term “OLED” was firstly introduced in 1987 at Eastman Kodak Inc. by Tang and Van Slyke when they fabricated light emitting diodes using small organic molecules. 2,9 This device has a two layer (single heterojunction) 10 structure in which hole transporting and electron transporting are separate, such that recombination and light emission could occur at the interface of the organic layers. This technology has greatly reduced operating voltage and improved device efficiency, and started a new era of OLED research and device production. In 1990 electroluminescent polymeric materials were developed by Cambridge Display Technology. These polymers were able to be processed using spin coating or ink jet printing techniques. 11-13 Also, devices with polymer materials could be permanently fold, 14 so they are more suitable for flexible displays. Although the picture quality of polymeric OLEDs (PLEDs) still lags behind, but ink jet printing is considered to be less expensive, so it could reduce manufacturing costs. In 2000, Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa won Nobel Prize for chemistry due to their pioneering work on conductive polymers. 15,16 In 1998, S. R. Forrest and M. E. Thompson et al. utilized phosphorescent materials in OLEDs. 17,18 These phosphorescent OLEDs (PHOLED) could harvest both singlet and triplet excitons of the emissive molecules, so that these devices could reach up to 100% internal efficiency, 19 while fluorescent OLEDs only utilize 25% of the totally excitons. During 3 the last 20 years, a completely new industry based on the semi conducting properties of organic materials has been created. A variety of OLED technologies, such as phosphorescent OLEDs (PHOLEDs), 18 white OLEDs (WOLED), 20 polymeric OLEDs (POLEDs) 21-23 , Active-matrix OLEDs (AMOLEDs) 24 and so on, have been utilized for manufacturing displays in industry. Recent OLED researches have been focused on reducing manufacturing costs and increasing device stabilities. There have been great efforts on maintaining good display qualities when using Ink jet printing for manufacturing. 11-13 Organic materials used for OLED devices especially blue emitters are still not stable enough to reach a long blue device lifetime. 1.2 Advantages of OLEDs over LCDs OLED technology has been developing rapidly in the last twenty years and enters its current stage. Nowadays, OLED applications for displays have ranged from small devices like cell phones and MP3 players to large displays like televisions and computers. Comparisons are usually established between OLED and LCD technologies, since OLEDs have been considered as next generation of displays and will eventually replace LCDs from the display market. In a LCD device, a white fluorescent backlight is needed behind the LCD panel and filtered by a polarizer to produce pixel colors. But OLED displays achieve RGB pixels through thin layers (~100 nm) of organic materials and they do not need a backlight. So OLEDs could have a much thinner structure and a much wider viewing range (~ 170 degrees). They also save the energies that LCD filters out and require very little external power. This is especially important for battery-operated 4 devices such as cell phones and MP3 players. Efficient ink jet printing technologies could be utilized for OLED manufacturing, 11 since the organic materials for OLEDs could be used directly as “inks”, so manufacturing cost could be reduced. The thin organic layers of OLEDs can be either deposited or spin coated onto plastic or glass substrates. So the OLED displays could be much thinner and lighter than the crystalline layers in LCD displays. The thickness of recently reported OLED display by Sony Inc. is only 0.2 mm (0.0079 in). Also, when plastic substrates are utilized, OLED displays could be designed as flexible or rollable. 25 Another significant feature of OLEDs is that organic molecules are more modifiable compared to inorganic materials, so the device properties i.e. colors, conductivity, efficiencies and stability could be improved by tuning the properties, such as emitting wavelength, energy levels, quantum efficiencies, of the materials. 1.3 Functional mechanism of OLED Several organic layers (HTL, ETL and EML etc.) are applied in an OLED device in order to achieve efficient charge injection, transportation, and recombination. 26 Heterojunction structures of the devices (Fig. 1.1) were utilized to increase electron to photon quantum efficiencies. Both single heterojunction and double heterojunction devices have been found to have good photon quantum efficiencies. The generation of electroluminescence in an OLED device includes four basic steps: charge injection, charge transportation, charge recombination and emission. 5 1.3.1 Charge Injection and Charge Transportation The device structure in Fig. 1.2 is defined as a single heterojunction structure, which consists of a HTL and an ETL. There is always an energy barrier between the electrodes and the charge transport layers. Holes and electrons have to overcome the energy barrier to reach the HTL and the ETL from the anode and cathode, respectively. The thermodynamic process that the charges overcome the energy barrier and move from one layer to another layer is called injection in an OLED device (Fig. 1.2). Figure 1.2 Generation of electroluminescence 6 Different from injection, transportation (Fig. 1.2) is a process in which charges hop from one molecule to another molecule in an organic layer. While hole transport is usually dominating in a donor type material, electron transportation is dominating in an acceptor type material. The rate of charge transportation (hopping) has been described in Marcus theory 27,28 as shown in equation 1: (1) Where, k et is the rate of charge transfer, λ is the reorganization energy and | H AB | is the electronic coupling between the initial and final states. 1.3.2 Recombination and emission When holes and electrons reach an interface, they recombine to form an exciton. The recombination mechanism involving in charge transfer processes are shown in Fig. 1.3. In the top diagram, an electron transferred from the excited state of the electron transport material (ETM) to the unoccupied orbital of the hole transport material (HTM), while in the bottom diagram. An electron transfers from the ground state of the ETM to the ground state of the HTM. Both processes form a ground state molecule and an excited state molecule. So an exciton forms in either HTL or ETL after the recombination. Excitons will be trapped in the material with the lowest excited state and can diffuse randomly in that material until it decays. 7 Electron Hole ∗ Ground State Exciton Recombination Electron Hole Recombination ∗ Ground State Exciton Figure 1.3 Two different mechanisms of charge recombination mechanism Double heterojunction structure is applied in order to trap excitons at a certain organic layer (the emissive layer), since in some cases, the excitons could diffuse to the electrodes and get quenched before they decay. In this structure an EML which has a lowest excited state is inserted between the HTL and ETL, so that the excitons could be efficiently trapped in this layer. Also this material usually has a lower lowest unoccupied molecular orbital (LUMO) and a higher highest occupied molecular orbital (HOMO), so the holes and electrons could be also trapped in this material, leading to good charge recombination. When charges recombine at a dopant molecule, a dopant exciton could form directly and emission happens when the dopant exciton radatively decay to its ground state. However, when excitons form at the host molecule, energy transfer could happen from hosts to the dopant and lead photoluminescent decay of the dopant. Decay from singlet excited states is defined as fluorescence while decay from the triplet excited state is defined as phosphorescence. 26 8 1.3.3 Energy transfers in OLEDs Foster transfer and Dexter transfer are the two different mechanisms of energy transfer in OLED devices. Föster transfer 29 involves a coulombic (dipole and dipole) interaction between the excited donor (host centered exciton) and the acceptor (ground state dopant molecule). 26 A coupling occurs between the transition state dipole of the exciton and the induced ground state dipole of the dye (Fig. 1.4). As the excited state donor relaxes, this coulombic interaction causes excitation of the acceptor dye. Through- space, coulombic interactions are advantageous since they can be influential over great distances (typically 30 Å or more). In a Dexter transfer, 30,31 the physical exchange of electrons occurs between the excited donor and the ground state acceptor either simultaneously or consecutively. In this system, energy transfer is realized through the formation of a transient exciplex between the exciton and acceptor through which electrons are transferred before the molecules dissociates. * Donor Acceptor * Donor Acceptor Foster Transfer * Donor Acceptor * Donor Acceptor Dexter Transfer Figure 1.4 Mechanism of Foster (left) and Dexter (right) energy transfer 9 In both Foster and Dexter transfers, partial overlap between the absorption spectrum of the acceptor and the emission spectrum of the donor are required 26 (as represented by the integral). Foster transfer could happen within 30 Å in space, it does not depend on the distance between two molecules. But the rate of Dexter charge transfer also depends on the distance between the two molecules in relation to the sum of the van der waals radii of the donor and acceptor. There is an inverse exponential dependence on the distance, so the Dexter rate drops quickly as the distance increases. Usually Dexter transfer is ineffective with a distance greater than 20 Ǻ. 1.3.4 Phosphorescent devices The excitons formed during charge recombination have been described in Fig. 1.3 without considering the spins of these charges. When arrows are added onto these electrons, one singlet excited states and three triplet excited states are formed as shown in Fig. 1.5. Singlet excitons cover only 25% of the excited states whereas triplet excitons cover 75% of the total. 32 () ↓↑> + ↓↑> ↓↓> ↑↑> | | 2 1 | | ( ) ↓↑> − ↓↑> | | 2 1 Triplet Singlet Hole electron Figure 1.5 Singlet and triplet states formed after charge recombination 10 Jablonski diagram is shown in Fig. 1.6, which represents the energy states of an OLED emitter. For most of organic and organometallic compounds, the radiative decay of the singlet excited state (S 1 ) to the ground state (S 0 ) is very efficient. However, the decay from triplet excited states (T 1 ) to the ground state (S 0 ) is forbidden, which is very inefficient and usually cannot be observed at room temperature. So 75% of the excitons will be lost if only fluorescence is utilized. S 1 S 0 ISC T 1 fluorescence phosphorescence h ν h ν ABS Figure 1.6 Jablonski diagram A solution to this problem is to look for efficient phosphorescent emitters. It has been found that for some heavy metal complexes, i.e. Os, Ru, and Ir, their strong spin- orbital coupling (SOC) could lead to a strong intersystem crossing (ISC) between the excited singlet and triplet states (Fig. 1.6). The strong ISC results in high radiative rates of the phosphorescence, leading to strong phosphorescence at room temperatures. Due to their promising quantum efficiencies, these complexes have been widely used as PHOLED emitters and have achieved an internal efficiency of 100% in PHOLED devices. 19 These Ir cyclometalated complexes will be discussed further in chapter 4, 5 and 6. 11 1.4 Materials for OLEDs 1.4.1 Hole transporter and electron transporter Except for the emissive molecules, there are other materials for transporting and injecting charges in OLEDs, which are charge transporters. Charge transporters include hole transporter, electron transporter. There are a number of requirements for charge transporters in a successful OLED. First of all, these materials in an OLED have to be thermally and chemically stable. High glass temperature is also preferred, since molecules with high glass transition temperature could prevent crystallization of the materials under high temperature and maintain the formation of an amorphous thin film. 33,34 Low device efficiencies usually occur when there is an energy level mismatch between the charge transporters and EML, where energy or charge leakages might take place leading to a poor device efficiency. So it is important to carefully consider the energy levels of each material i.e. HOMO/LUMO levels. Since the rate of charge migrating through the charge transporting materials is directly related to the device conductivity, it is also essential to discover high charge mobility transporter. Triarylamines such as NPD 35,36 (Fig. 1.7) are common hole transporters used in OLEDs due to their high hole mobilities. On the other hand, the most common electron transporting material used for OLED is tris(8-hydroxyquinoline) aluminum(III) (Alq 3 ), 37 which has a electron mobility of 10 -5 cm 2 /Vs and a very low hole mobility only about 1/100 as much (Fig. 1.7). 12 N N NPD N O Al 3 Alq 3 Figure 1.7 Hole transporter NPD and electron transporter Alq 3 1.4.2 Hosts Hosts are usually charge transporters with ambipolar charge transport abilities. They usually have both good hole mobilities and good electron mobilities. In phosphorescent OLEDs, usually only a small percentage of triplet emitters are utilized as to reduce the quenching associated with long excited-state lifetimes of triplet emitters and triplet-triplet annihilation. 26 This small portion of emitters is usually called dopant in an OLED device. Dopants with specific emissive wavelengths and high luminance quantum efficiencies are usually co-deposited with host to achieve good performances of OLEDs. The generated excited-state molecules are either emissive molecules or host molecules that could transfer their energy to the luminescent dopant. Therefore, the host materials to be used in the emitting layer should have higher triplet energy than the phosphors in the layer. So they can prevent the energy leakages from the forming host excitons and traps the excitons in the dopant layer. On the other hand, they also have to match the LUMO/HOMO energy level for charge carrier injection and acceptance of both holes and electrons, i.e., both electron-donating and electron-accepting, to permit the formation of both stable cation and anion radicals. 38 Large HOMO-LUMO gap of a host material could help trap charges to the dopant molecule and lead a dopant center charge 13 recombination. If the dopant is doped into a well-match material, the electroluminescence (EL) output from this device will come exclusively from the dopant itself and thus achieve high quantum efficiency. Therefore, efficient host materials are of equal importance as emitters for efficient phosphorescent OLEDs. There have been extensive studies on host molecules in the literature. 1.4.2.1 Carbazoles and Arylsilanes Carbazole derivatives such as mCP 39-41 and CBP 42-44 (Fig. 1.8) have been utilized as host materials in OLEDs since the 1990s. The ambipolar abilities of these compounds can achieve a balance of holes and electrons at the emissive layer. The high triplet energies of carbazoles allow them to be used as host materials for red, green and even blue PHOLEDs. For example, mCP has a triplet energy of 2.9 eV corresponding to a wavelength of 430 nm which locate at the deep blue region. This triplet energy is higher than typical red emitter PQIr (E T = 2.1, λ = 595 nm), green emitter Ir(ppy) 3 (E T = 2.4, λ = 512 nm) and blue emitter FIrpic (E T = 2.6, λ = 468 nm). These materials also have proper HOMO and LUMO levels, where “proper” means good HOMO/LUMO levels that match most HTLs, ETLs and emitters in OLEDs. A good host chosen allows holes and electrons getting injected from either HTL or ETL into the EML, and also helps trap charges into the emissive layer. But carbazoles have low glass transition temperatures. The low glass transition temperature could increase the probability of crystallization of the amorphous film in organic electronic devices, leading to a poor device performance. These materials have also been reported unstable in a PHOLED device producing radicals or charge species leading to a short device lifetime. 14 N N mCP N N CBP Si Si UGH2 Figure 1.8 Common hosts utilized in OLED devices Arylsilanes were first developed in 2004 by Ren et al as large band gap host materials for blue PHOLEDs. 45 Arylsilanes are considered to be very stable materials due to there Si-C backbone. One of the examples is UGH2 as shown in Fig.. 1.8. The aromatics of this molecule have been highly isolated, so this material has a ultra high triplet energy of 3.2 eV. This high energy could prevent the energy transfer from the dopant to the host materials. However, when large band gap was introduced to these materials, their HOMO levels become very deep (7.2 eV). This lead to an inefficient hole injection. A hole injection layer, for example, an mCP layer, may solve this problem and charge recombination could be achieved at the EML. But in other cases, even an injection layer could not balance the charges in a UGH2 device. When holes get blocked at the HTL/EML interface, energy leakage happens at the HTL, leading to an inefficient device. 15 1.4.2.3 Fluorenes High charge mobility and large HOMO-LUMO gap of fluorene derivatives have been developed as charge transport materials for OLEDs (Fig. 1.9). Fluorene triarylamines species with high charge mobilities and thermal stabilities have been proven to have good charge transporting characteristics. 46-49 Some fluorene materials have been utilized as hosts in OLEDs. For examples, phosphorescence Ir(ppy) 3 device has been fabricated with a fluorenyl carbazole host (DFC in Fig. 1.9) with an external quantum efficiency of 9%. 27BPSF 27BPSF n Spirofluorene oligomers (n=2,3,4) n Spirofluorene oligomers (n=2,3,4) n poly(DBF) n poly(DBF) N MeO OMe DFC N MeO OMe DFC Figure 1.9 Structures of fluorene based materials for OLEDs Spirobifluorene oligomers are recently reported to have excellent ambipolar charge mobilities (in the order between 10 -2 -10 -3 cm 2 N -s ) by Chung-Chi’s group (Fig. 1.9), 46,50 while NPD has only a hole mobility in the order of 10 -3 and Alq 3 has an electron 16 mobility in the order of 10 -5 . It was found that the charge mobility drops when the oligomer length increases. Also, as the conjugation increases, the oligomers will have decreased triplet energies. To maintain the high triplet energy of fluorenes, there have been studies on developing fluorene monomer derivatives and utilized them as host materials. Some materials has been reported to be good OLED hosts with good device performance. 51 1.4.3 Iridium Cyclometalated Complexes as Dopants for PHOLEDs A large number of materials have been reported to efficiently produce EL luminescence. The color of the OLEDs could be controlled by alternating the color of the emissive materials. The efficiency of all of the fluorescent material is limited by the singlet/triplet exciton ratio be in the limit of 25%. Due to the self quenching of these materials, the quantum efficiencies in neat film are even lower than this value. The solution to this problem is to dope a small portion of luminescent dye (dopant) into another material. This material could be a hole transport material, a electron transport material or a specific host material. Charge recombination at the dopant molecule is desired in an OLED device, so the excited state of the dopant needs to be lowest at the emissive layer. Also a lower LUMO level and a higher HOMO level of the dopant are also required to confine charges in the dopant layer, which could lead to an efficient charge recombination. In a PHOLED, the dopant materials usually have a lower triplet state than its host. An efficient decay from the triplet excited state to the ground state is required for efficient phosphorescent dopants. For most of organic materials the decay from T 1 to S 0 is 17 forbidden, so the phosphorescence is very inefficient. However, some of the d 6 heavy metal complexes such as ruthenium, iridium and osmium complexes could produce very efficient phosphorescence due to their efficient intersystem crossing between their singlet excited state and triplet excited states, leading to high phosphorescence efficiencies. For these complexes, they can trap both singlet and triplet excitons and achieve internal quantum efficiencies (up to 100%) in a PHOLED device. 19,44 In 1999, a heterojunction device with 6% Ir(ppy) 3 doped in to CBP was fabricated. This green PHOLED achieved 8% of the external efficiency. 18 Since then, a lot of iridium complexes were developed as emitters for PHOLEDs. The colors of these complexes are throughout whole visible regions. Fig. 1.10 shows the molecular structure of some common Ir phosphors. Figure 1.10 Iridium phosphors utilized in PHOLEDs dfppy ppy bt pq btp N Ir 2 N Ir 2 S N Ir 2 N Ir S 2 Ir 2 N F F 18 1.4.3.1 Blue Iridium Cyclometalated Complexes Investigation of blue phosphorescent materials is one of the focuses of OLED researches nowadays, since the blue PHOLED lifetime is still much shorter than its red and green counterparts. The short PHOLED lifetime could be resulted from the instability of the fluorinated blue phosphorescent emitters. Iridium bis[(4,6-difluorophenyl)- pyridinato-N, C 2 ] picolinate (FIrpic) is a derivative of fac-iridium (III) tris(2-phenyl- pyridine) (Ir(ppy) 3 ). 18 (Fig. 1.10) Incorporating electron withdrawing groups fluoro onto the phenylpyridine (ppy) has successfully blue shifted the green emission of from 516 nm to 468 nm, leading to blue color of FIrpic. But it has been previously found that the fluoro group could be cleaved from the difluorophenylpyridine (dfppy) ligand in an operating OLED device leading to device degradation. N Ir 3 N Ir 3 N Ir(ppy) 3 Ir(ppz) 3 Figure 1.11 Strucutures of Ir(ppz) 3 and Ir(ppy) 3 Researches on improving properties of blue phosphorescent emitters have been focused on phenylpyrazole (ppz) based irdium complexes. fac-iridium (III) tris(1-phenyl- pyrazole) (Ir(ppz) 3 ), which is shown in Fig. 1.11, phosphorescence at 415 nm and shows high quantum efficiency at low temperature. However, Ir(ppz) 3 shows very low quantum 19 efficiency at room temperature. The great difference between the quantum efficiency of Ir(ppy) 3 and Ir(ppz) 3 at room temperature leads to investigations on the radiative state and non radiative states of these complexes. Temperature dependence studies on emissions and lifetimes have provided detailed information of the photophysics of metal complexes. Since 1970s, both G. D. hager et al and S. R. Allsop et al have utilized temperature dependence studies to investigate the photophysics of Ru(II) complexes and obtained their radiative and non-radiative decay rates. 52-54 In 2009, by measuring temperature dependent lifetime of iridium complexes, T. Sajoto et al found that deactivation of Ir(ppz) 3 may occur by rupture of Ir-N bond on ppz ligand. 55 A energy diagram as shown in Fig.1.12 was utilized to explain the behaviors of these Ir complexes. The triplet states of Ir cyclometalated complexes could be divided into three substates. This usually happens at low temperature. The energy difference between the lowest energy substate and the highest energy substate in the absence of magnetic field is defined as zero-field splitting (zfs). Increase of zfs is correlated to increase of metal to ligand charge transfer (MLCT) characters of the complex. In the diagram, an activation energy E a appears in between the triplet state (T 1 ) and the non-radiative state (NR) as shown in Fig. 1.12. There is a equilibrium between the triplet state and the NR state, dependent on E a , k r (T), and k nr (T). When k r (T) >> k nr (T), no non-radiative decay is observed, and the complex could radiate efficiently at room temperature like Ir(ppy) 3 . In case of k r (T) << k nr (T) or E a is very small, the energy could overcome the barrier to the NR state, resulting in non-radiatively decay, like Ir(ppz) 3 which does not emit at room temperature. 20 S 1 S 0 ISC NR E a k nr k r (T) k nr (T) T 1 Figure 1.12 Energy diagram of radiative and non-radiative states for iridium complexes The report from T. Sajoto et al is consistent with the DFT calculation by G. Treboux et. al. on the excited states of Ir(ppy) 3 and Ir(ppz) 3 . Their calculation shows that a five coordinate non-radiative state could form from its triplet excited state. This non- radiative state has a trigonal bipyramidal geometry, in which one of the Ir-N has broken. Given that the formation of NR state could greatly reduce the Ir quantum efficiency, it is essential to look for an approach to shut down this deactivation pathway and improve the quantum efficiency of Ir(ppz) 3 based complexes at room temperature. 1.4.3.2 Isomers of 6-coordinate Iridium Cyclometalated Complexes With symmetric ligands, such as phenathroline and acetylacetonate, the homoleptic 6-coordinate irdium complexes will have only stereo isomers. But the iridium complexes we studied for OLED purposes are usually with asymmetric cycolmetalated ligand such as phenylpyrazole (N^C), picolinate (O^N), carbene (C^C). The iridium complex with three identical ligands is called homoleptic complex. Homoleptic complex could have two different structural isomers, facial and meridional as shown in Fig. 1.13. In facial structure three nitrogens are in a same plane that is not across the Ir center, while 21 in meridional isomer, the plane with three nitrogens is across the Ir center. Facial isomer usually has a higher quantum efficiency than the meridional isomer, and it has been proven that the facial isomer is lower in energy than the meridional isomer and it is stable in the PHOLED. Some other iridium complexes contain two different types of ligands. For example, FIrpic contains two difluoro-phenylpyridine (dfppy) ligands and one picolinate ligand. These complexes are called heteroleptic complexes. There are more possible structural isomers for heteroleptic complexes as shown in Fig. 1.14, due to the complexity from the arrangements of two different types of ligands. Both homoleptic and heteroleptic iridium complexes have been utilized as PHOLED emitters, while the homoleptic complexes have been better studied. Even though the heteroleptic complexes (i.e. FIrpic) are widely used to fabricate PHOLEDs, the possible isomerization of these heteroleptic complexes have never been considered until very recently, Baranoff et al. reported the isomerization of a ppy based heteroleptic complex during OLED fabrication. Isomerization of heteroleptic iridium complexes will be investigated and demonstrated in Chapter 5. 1.5 Summary The OLED technology has advanced rapidly in the last thirty years to its current stage. OLED devices nowadays have achieved high levels of performance including high efficiencies, color purities and device lifetimes. However, stability of high energy OLED materials is still one of the problems that need to be addressed to achieve a long device lifetime of blue devices. This includes an effort to search for more stable moieties that can also fulfill the bad gap and photophysical requirements. As for blue emitters, a deeper 22 Figure 1.13 Isomeric structures of homolepitc and heteroleptic 6-coordinated cyclometalated Iridium complexes understanding of their photophysical behavior and their behavior in devices are necessary. The first part of my thesis, chapter 2 and chapter 3, is dedicated to investigate and develop better blue phosphorescent host materials and utilized them in blue PHOLEDs. In the second part (chapter 4, chapter 5 and chapter 6), studies on large band gap iridium complexes will be demonstrated. My dissertation studies include synthesis of organic and organometallic compounds, electrochemical and photophysical characterizations of organic materials, and also PHOLED fabrications using these newly designed materials. Characterization and isomerization studies of heteroleptic Ir complexes will be discussed in chapter 4, while attempt to increase charge trapping of iridium complexes will be demonstrated in chapter 5. 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Journal of the American Chemical Society 2009, 131, 9813- 9822. 27 Chapter 2: Properties of Fluorenyl Silanes in Organic Light Emitting Diodes 2.1 Introduction Ever since the doping strategy has been developed to prevent self-quenching of the emissive molecules, the technique has been widely utilized to optimize the efficiencies of organic light emitting diode (OLED) devices. 1 Typical doped OLED devices consist of several discrete molecular or polymeric layers: an electron transport layer (ETL), an emissive layer (EML) and a hole transport layer (HTL). While the ETL and HTL are used to inject and transport electrons and holes into the EML, the EML is where the charges recombine and form the dopant (emissive molecule) excitions. In a doped device, a host material is also employed in the EML to inhibit self-quenching by the dopant and to confine the energy on the dopant molecules. The host plays an important role in an OLED device because its energy levels and charge mobilities determine 1) whether the holes and electrons recombine at the emissive layer; and 2) whether the emission can be confined into the dopant exclusively. The host material used in an OLED thus has to be energetically matched with a specific dopant. The most common hosts for OLEDs are carbazole derivatives, i.e. N,N’-dicarbazolyl-3,5-benzene (mCP) 2-4 and 4, 4′-N,N ′-dicarbazole-biphenyl (CBP). 2,5,6 However, carbazoles have relatively low glass transition temperatures (T g ), and have been shown to be chemically unstable in the devices, which can ultimately lead to a shortened device lifetimes. 7,8 Compounds that have large energy band gaps and contain no easily oxidized nitrogen donor groups, bis(triphenylsilyl)benzenes, have been developed as host materials, 28 particularly for blue phosphorescent emitters. 9-13 Unfortunately, these materials have low glass transition temperatures and the deep energy level of their HOMOs hinder hole injection into heterojunction devices. Therefore, further investigations are still needed to develop host materials that can improve the performance of OLEDs. Fln (n = 2, 3, 4) n-2 SiFln (n = 1, 2, 3, 4) Si n 4-n N N N O Al 3 NPD Alq 3 Scheme 2.1 Structures of SiFln, Fln, NPD and Alq 3 Fluorene derivatives have been studied in OLEDs due to their robust thermal stability, high charge transport mobilities, large HOMO-LUMO gaps and relatively high T g s. 14-17 Spirobifluorene oligomers (Fln, Scheme 2.1) have excellent thermal and chemical stabilities as well as high charge transport mobilities (µ = 10 -5 –10 -3 cm 2 /V·s) for both holes and electrons. 14-17 Mobilities for the conjugated Fln oligmers are comparable to one of the most common OLED hole transporters, NPD (Scheme 2.1), and two orders of magnitude better than the common OLED electron transporter Alq 3 (Scheme 2.1). 18,19 Wu et al. found that the number of the fluorene units in the oligomer is inversely 29 correlated with the charge transport mobility of the molecule; as the number decreased from four to two, the charge carrier mobility increased. 19 While the data reported by Wu et al. suggested that the best properties would be found for simple monofluorenes, this idea could not be investigated due to the high volatility of such low molecular weight species. Monofluorene containing materials with low volatility, such as triphenyl-(4-(9- phenyl-9H-fluoren-9-yl)phenyl)silane (TPSi-F), 20 have been reported, however, variants of this molecule with increased fluorene content have not been examined. In order to explore the properties of monofluorenyl materials in OLEDs, we have chosen to study compounds with multiple-linked, non-conjugated fluorenes. The materials examined here are fluorenylsilanes, SiFln, n = 1, 2, 3, 4 (Scheme 2.1). While the use of arylsilanes in OLEDs has been previously reported, 9-13,20,21 these compounds have not been used to assemble multiple fluorenes into a single molecule as described here. The fluorenes in the molecules presented here are electronically isolated, leading to a high triplet energy and large HOMO/LUMO gap. These materials have high glass transition temperatures and show ambipolar charge transport characteristics, making them promising host materials for OLED devices. 2.2 Experimental 2.2.1 General. All chemicals, reagents, and solvents were received from commercial sources without further purification except for tetrahydrofuran (THF) that had been distilled over sodium/benzophenone. All glassware used was oven dried and the reactions were performed under N 2 . 30 2.2.2 Procedures. The 2-Bromo-9,9’-dimethylfluorene (1, Scheme 2.2) was synthesized according to literature procedures. 22,23 The fluorenylsilanes were synthesized as shown in Scheme 2.2, according to the literature reported conditions. 24,25 To a solution of 1 (C 1 = 0.25 M) in anhydrous THF, 1.2 eq. nBuLi (2.5 M in hexanes) was added dropwise at -78 °C. The reaction mixture was allowed to warm up to room temperature and the corresponding phenylchlorosilane (Ph 4-n SiCl n , n = 1, 2, 3 and 4, 1/n eq., Scheme 2.2) was added dropwise after 1 hour. The reaction mixture was stirred at room temperature overnight. The organic layer was extracted by ethyl acetate, washed with brine and dried over magnesium sulfate. The resulted solution was evaporated to dryness under reduced pressure to give the crude products. Silica gel column chromatography with pentane and ethyl acetate (10:1) was performed to obtain white solids of the pure products. 2-Bromo-9, 9-dimethylfluorene (1) (Yield: 94 %): 1 H NMR (CDCl 3 , 250MHz) δ (ppm) 7.73-7.68(m, 2H). 7.60-7.56 (m, 2H), 7.48-7.32 (m, 3H), 1.48 (s, 6H). SiFl1. Triphenyl-(9,9’-dimethylfluoren-2-yl)-silane (Yield 92 %): 1 H NMR (CDCl 3 , 250MHz): δ (ppm) 7.56-7.81 (m, 10H), 7.41 -7.52 (m, 10H), 7.32-7.41 (m, 2H), 1.48 (s, 6H). MS (m/z): 452, calcd. for C 33 H 28 Si: 452.2. Ana. Calcd. for C 33 H 28 Si: C, 87.56; H, 6.23. Found: C, 87.45; H, 6.23. SiFl2. Diphenyl-di(9,9’-dimethylfluoren-2-yl)-silane (Yield 67%): 1 H NMR (CDCl 3 , 250MHz): δ (ppm) 7.54-7.80 (m, 12H), 7.39 -7.50 (m, 8H), 7.30-7.39 (m, 4H), 1.44 (s, 12H). MS (m/z): 568, calcd. for C 42 H 36 Si: 568.2. Ana. Calcd. for C 42 H 36 Si: C, 88.68; H, 6.38. Found: C, 88.65; H, 6.37. 31 SiFl3. Phenyl-tri(9,9’-dimethylfluoren-2-yl)-silane (Yield 58 %): 1 H NMR (CDCl 3 , 250MHz): δ (ppm) 7.58-7.80 (m, 14H), 7.40 -7.49 (m, 6H), 7.30-7.39 (m, 6H), 1.45 (s, 18H). MS (m/z): 684, calcd. for C 51 H 44 Si: 684.3. Ana. Calcd. for C 51 H 44 Si: C, 89.43; H, 6.47. Found: C, 89.45; H, 6.45. SiFl4. Tetra(9,9’-dimethylfluoren-2-yl)-silane (Yield 52 %): 1 H NMR (CDCl 3 , 250MHz): δ (ppm) 7.61-7.81 (m, 16H) 7.40 -7.47 (m, 4H), 7.30-7.39 (m, 8H), 1.46 (s, 24H). MS (m/z): 607, calcd. for C 45 H 39 Si (M-C 15 H 13 ): 607.3. Ana. Calcd. for C 60 H 52 Si: C, 89.95; H, 6.54. Found: C, 89.91; H, 6.47. 2.2.3 Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) was performed using a TA Instruments DSC Q10 instrument with a scanning range from room temperature to 300 °C. The sample was first scanned at a heating rate of 10 °C min -1 and was cooled down to room temperature rapidly using liquid N 2 . The second and third scans were performed at a heating rate of 5 °C min -1 . The glass transition temperatures were determined from either the second or the third scan for each compound. 2.2.4 Electrochemistry and Photophysics. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed using an EG&G potentiostat/galvanostat model 283 under N 2 atmosphere. Anhydrous DMF was used as solvent for the scan from - 3.7 to 1.0 V to detect the reduction signals, while anhydrous acetonitrile was used as solvent for the scan from - 0.5 to 2.5 V to detect the oxidation signals, and 0.1 M tetrabutylammonium hexafluorophosphate (TBAH) was used as the supporting electrolyte. A glassy carbon rod 32 was used as the working electrode, a platinum wire was used as the counter electrode, and a silver wire was used as a pseudo reference electrode. The reversibility and the redox potentials were determined by the cyclic voltammetry and differential pulse voltammetry, respectively. The redox potentials were calculated relative to an internal reference ferrocenium/ferrocene (Cp 2 Fe + /Cp 2 Fe). The UV-visible spectra were measured in dichoromethane using a Hewlett-Packard 4853 diode array spectrometer. Steady state emission measurements of both solutions and thin films were performed at room temperature and 77 K using a Photon Technology International (PTI) QuantaMaster model C-60 fluorimeter. Room temperature emission lifetimes were measured using an IBH Fluorocube instrument equipped with a 281 nm diode excitation source. Low temperature phosphorescent lifetimes of thin films was measured using a PTI QuantaMaster fluorimeter on samples immersed in liquid nitrogen. 2.2.5 Device Fabrication. All materials used for vapor deposition were purified by temperature gradient vacuum sublimation. Indium tin oxide (ITO) on glass was provided by Thin Film Devices Inc.. ITO substrates were cleaned by detergent, rinsed with deionized water, sonicated in organic solvents (tetrachloroethylene, acetone and ethanol), and finally treated with UV ozone for 10 min. The organics were vapor-deposited onto the substrates in a high- vacuum chamber, followed by the LiF (10 Å) and the aluminum cathode (1000 Å) deposition using a shadow mask with 2-mm wide stripes. The electrical and optical characteristics of the devices were measured with a Keithly 2400 source/meter/2000 multimeter coupled to a Newport 1835-C optical meter, equipped with a UV 818 Si photo 33 detector. The electroluminescence (EL) spectra were measured on a Photon Technology International QuantaMaster model C-60 fluorimeter. 2.3 Result and Discussion 2.3.1 Design, Synthesis and Characterization of the Molecules The molecular structures of the four fluorenylsilanes SiFln (n = 1, 2, 3, 4) were specifically designed to achieve a high level of fluorene incorporation in the molecules, while keeping them electronically isolated to prevent the red shift associated with conjugated oligo- and polyfluorenes. Tetraarylsilanes were chosen for their thermal stability, relatively high T g s and the flexibility for structural modification. 26-28 The synthetic approach and structures of the SiFln compounds are shown in Scheme 2.2. The compounds were synthesized using a lithium-halogen exchange reaction of 2-bromo-9,9’- dimethylfluorene with n-butyllithium, followed by the addition of phenylchlorosilane to the lithiated fluorene, in good-to-excellent yields (52–92%). 24,25 The yields decreased as Br Br Li Si n 4-n CH 3 I, NaOH DMSO nBuLi THF Ph 4-n SiCl n 1 SiFln (n = 1, 2, 3, 4) THF Scheme 2.1 Synthesis of SiFln 34 the number of fluorenyl groups in the molecule is increased, presumably due to steric hindrance that slows the rate of addition of each successive fluorenyl group to the silyl center. 2.3.1.1 Thermal Properties. Amorphous materials with high glass transition temperatures are beneficial for OLEDs since crystallization can introduce deleterious grain boundaries in homogenous films. The glass transition temperature of the molecules is related to their molecular structures. 29-32 The SiFln compounds have been designed to possess high glass transition temperatures because 1) they have symmetrical tetrahedral structures; 2) they have relatively high molecular weights and 3) they contain rigid and bulky aromatic groups. The glass transition temperatures of the fluorenyl silanes were measured using differential scanning calorimetry (DSC). Samples were rapidly cooled from a melt after the first DSC scan to obtain glassy materials. Glass transition temperatures for all four materials were clearly observed on subsequent second and third scans (Table 2.1). The values for SiFl2, SiFl3 and SiFl4 range from 76–126 °C (see Table 2.1, Fig. 2.1 and Fig. A1.1) and exceed those of bis(triphenylsilyl)benzenes 44 (26 - 53 °C), mCP (65 °C), and CBP (60 °C). Table 2.1 Thermal properties of SiFln a measured by differential scanning calorimetry; b measured by gradient vacuum sublimation at 5×10 -6 Torr n 1 2 3 4 T g (ºC) a 45 76 103 126 T sub (ºC) b 179 222 245 302 35 12 34 40 80 120 160 200 240 280 320 T sub T g Temperature (°C) n Figure 2.1 Dependence of the glass transition temperature, T g (triangle), and the sublimation temperature, T sub (circle), on the number of fluorenes in SiFln. For example, SiFl2 has a molecular weight similar to that of 1,3- bis(triphenylsilyl)benzene (UGH3), yet the former has a glass transition temperature that is 30 °C higher. A linear relationship found between the number of fluorenyl groups in the molecule and the glass transition temperature (Fig. 2.1) supports our prediction that fluorenyl groups act to stabilize the glassy states of these tetraarylsilanes. Another important characteristic of these fluorene based materials, with respect to OLED fabrication, is their high sublimation temperatures, which range from 179 °C to 302 °C at 5×10 -6 torr (Table 2.1 and Fig. 2.1). The sublimation temperature also increases with the increasing fluorene content in the SiFln molecules, similar to the trend observed for the T g values. 36 2.3.1.2 Electrochemistry The electrochemical properties of these compounds were studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The reduction and oxidation potentials of these molecules versus Cp 2 Fe + /Cp 2 Fe are listed in Table 2.2. Reversible reduction waves were observed for all four compounds, with first reduction potentials observed at -2.8 to -3.1 V (Fig. 2.2 and Fig. A1.2). Multiple waves were observed for SiFl2, SiFl3 and SiFl4 corresponding to the reduction of each fluorenyl moiety in the molecule (Fig. A1.2). The fourth reduction of SiFl4 was not observed, as it is presumably obscured by solvent reduction. The shift to higher potential for the first reduction process as the number of fluorene groups increases suggests that weak electronic coupling occurs between the individual fluorene units. -3 -2 -1 0 1 2 3 -0.6 -0.4 -0.2 0.0 0.2 Current (a. u.) Voltage vs. Cp 2 Fe + /Cp 2 Fe Oxidation,SiFl4 Reduction,SiFl4 Figure 2.2 Cyclic voltammetric redox curves for SiFl4. 37 Oxidation waves for the SiFln compounds occur at ~1.4 eV and are irreversible, as shown in Fig. 2.2 for SiFl4. The absence of a reversible couple leads to some ambiguity as to whether the measured potential represents the thermodynamic value for oxidation process. Rathore et al. investigated a series of π-stacked, non-conjugated oligofluorenes that undergo reversible electrochemical oxidation and showed a linear correlation exists between the oxidation potential and the number of fluorene units in the chain, giving progressively higher oxidation potentials for shorter chains. 33 Even though oxidation of the fluorenyl group is irreversible in the SiFln compounds, the observed potential fits the length-potential correlation of olgioflourenes found by Rathore. Therefore, we consider that, for the fluorenylsilanes reported here, the irreversible oxidation wave provides a valid estimate of the thermodynamic oxidation potential. In order to design efficient OLEDs, it is important to determine the HOMO and LUMO energies of a new compound so that appropriate transport materials can be selected to match the energetic requirements for carrier injection and transport throughout the device. The HOMO and LUMO levels of all the molecules were determined using both density functional theory (DFT) and electrochemical experimental data. While DFT calculations do not give a direct measure of the HOMO or LUMO energy of the neat material, they are useful for predicting the sites of oxidation and reduction for each molecule. DFT calculations for the fluorenylsilanes show HOMO/LUMO orbitals and triplet surfaces primarily localized on the fluorenyl groups (see Fig. A1.3). The calculated energies of the frontier orbitals are -5.64 eV to -5.75 eV for HOMOs and -0.96 eV to -1.04 eV for LUMOs. Electrochemical methods have been commonly used to determine molecular 38 frontier orbital energy levels. 34 The HOMO energies can be estimated from the correlation between the UPS measured HOMO energy and the oxidation potential of the molecule (Eq. 1), 34 while the LUMOs can be estimated from the correlation between the IPES measured LUMO energy and the reduction potential (Eq. 2). 35 A correlation between the LUMO levels calculated using DFT methods and the IPES measured LUMO energies has also been reported (Eq. 3). 35 E HOMO = - (1.4 ± 0.1) E ox – (4.6 ± 0.08) (2.1) E LUMO = - (1.19 ±0.08) E red – (4.78± 0.17) (2.2) E LUMO = (0.92 ± 0.04) E DFT LUMO - (0.44 ± 0.11) (2.3) Here, E HOMO and E LUMO are the HOMO and LUMO energies in eV, E ox and E red are the oxidation and reduction potentials in V, and E DFT LUMO is the LUMO energy calculated using DFT at the B3LYP level using either 6-31G* (hydrocarbons) or LACVP** (organometallics) basis sets (Titan v1.0.7, Wavefunction, Inc). Using the UPS/HOMO correlation, the HOMO energies are determined to be -6.6 eV for all the fluorenylsilanes. Using the IPES/LUMO correlation, the LUMO energies are estimated to be -1.4 eV for SiFl1, SiFl2 and SiFl3, and -1.1eV for SiFl4. A similar LUMO energy (-1.3 eV) is obtained from the IPES/DFT LUMO correlation. Transport gaps of ca. 5.2 eV were calculated from the difference between the LUMO and HOMO energies. An optical gap energy of 4.0 eV was estimated from the long wavelength edge of the absorption spectra of the fluorenylsilanes. These two values fit well into the correlation found for the optical gap and the transport gap of related organic semiconductors. 35 The HOMO and LUMO levels of other molecules used for the OLED fabrications in this paper were determined using the same methodology. 39 2.3.1.3 Photophysics The SiFln compounds were analyzed by UV-visible absorption and emission spectroscopy, data and spectra are shown in Table 2.2 and Fig. 2.3. The lowest-energy absorption transitions of all the fluorenylsilanes are close to 310 nm and show little shift in energy as a function of the number of fluorene groups (Fig. 2.3a). The absorptivity in the region from 250 nm to 320 nm increases monotonically with the number of fluorene groups in the molecule, as expected for non-interacting chromophores. Fluorescence was observed for all of the compounds in dilute solution, with very small Stokes shifts from the lowest absorption bands (Table 2.2, Fig. 2.3b). All of the SiFln compounds show phosphorescence at low temperatures ( λ max of 435 nm, Table 2.2, Fig. 2.3b inset) corresponding to a triplet energy of 2.9 eV. While the electrochemical data suggests a weak intramolecular interaction between the fluorenyl moieties, the emission spectra recorded in solution show no significant differences between the SiFln molecules. Photoluminescence spectra from thin films of SiFln were measured at both room temperature and 77K (Figs. 2.3c and A1.4). The emission properties of the neat films are markedly different from those of the isolated molecules. Three different types of emission can be resolved in the spectra from the films. High energy bands are observed between 300–400 nm (strongest for SiFl2 and SiFl3) and assigned to singlet emission from the monomer. A strong, featureless band is also observed between 400–500 nm. This transition is significantly red shifted from spectra recorded in dilute solution and is thus assigned to singlet emission from excimer-like states in aggregates. At low 40 temperature, a new band appears between 500–600 nm (Fig. 2.3c). This low energy band has a long lifetime (~700-800 msec), which leads us to assign the feature to triplet emission from the aggregates in the SiFln films. 2.3.2 OLED Studies The charge transport properties of the SiFln compounds were investigated using three- and four-layer OLEDs. The structures used were ITO / NPD (40 nm) / SiFln (20 nm, n = 2, 3, 4) /Alq 3 (20 nm) /LiF (1nm) /Al (100 nm) (device I), and ITO / NPD (30 nm) / mCP (10 nm) / SiFln (20 nm, n = 2, 3, 4) /Alq 3 (20 nm) /LiF (1nm) /Al (100 nm) (device II). The low sublimation temperature of SiFl1 precluded OLED studies using this material. Wu et al. showed that oligofluorenes have ambipolar carrier conduction properties, 14-17 suggesting that the location of charge build-up and recombination in these devices should be controlled by injection barriers at the HTL and ETL interfaces. Fig. 2.4 shows the relative HOMO/LUMO energies of the materials used in the devices. The LUMO of Alq 3 (2.0 eV) lies below that of the fluorenylsilanes (1.1–1.4 eV) presenting a 0.6–0.9 eV barrier for injecting electrons from Alq 3 into SiFln. The HOMO of NPD (5.3 eV) is 1.3 eV higher than that of the fluorenylsilanes (6.6 eV), giving a much higher barrier to the hole injection into SiFln than for electron injection in device I (Fig. 2.4a). Thus, for device I we do not expect to see emission from Alq 3 . OLEDs with device I structures using SiFl2, SiFl3 and SiFl4 give only blue emission ( λ max = 432 nm) at voltages ranging from 7 V to 15 V , as expected based on the high barrier to the hole 41 240 260 280 300 320 340 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 SiFl1 SiFl2 SiFl3 SiFl4 Wavelength (nm) ε (10 5 M -1 cm -1 ) 300 320 340 360 380 400 0 1 Normallized Intensity (a.u.) Wavelength (nm) SiFl1 SiFl2 SiFl3 SiFl4 300 350 400 450 500 550 600 0 1 Normalized Intensity (a.u.) Wavelength (nm) RT-film 77K-film 77K-solution Figure 2.3 (a) Absorption and (b) emission spectra of SiFln in CH 2 Cl 2 solution. (c) Emission spectra from neat films of SiFl4 recorded at room temperature (RT) and 77K with the phosphorescence spectrum of SiFl4 measured in 2-MeTHF solution at 77K. 42 43 injection into the fluorenylsilane layer. The turn-on voltages (at 0.1 cd/m 2 ) also decrease as the number of fluorenyl groups in the molecule increase. While the SiFl2 device turns on at 7.2 V, the SiFl3 and SiFl4 devices turn on at 4.7 V and 4.5 V, respectively, as seen in Table 2.3. Presumably, the increased fluorene content in the SiFl3 and SiFl4 layers increases the density of charge conducting states at NPD / SiFln interface and thus, lowers the potential for charge injection into the film. The large barrier for hole injection into the SiFln film can mitigated by inserting a thin layer of mCP between the NPD and SiFln materials (device structure II). The HOMO energy of the mCP layer (5.9 eV) is intermediate between that of NPD and SiFln, thereby creating an energetic “step” to facilitate hole injection into SiFln (Fig. 2.4b). A closely related device architecture has been previously employed to enhance hole injection in OLEDs. 36 OLEDs with device structure II have turn-on voltages of 3.9–4.5 V (Table 2.3). For devices made using the same fluorenylsilane material, holes are injected more efficiently with an mCP layer present than without. For example, the current density of device I using SiFl2 was 171 mA/cm 2 at 13 V, while for device structure II the measured value was nearly three times greater at the same bias. The current densities of the devices with structure II also increase with increasing fluorene content in the SiFln layer (Fig. 2.5), which implies that the fluorenyl group is the dominant charge carrying moiety in the film. The SiFln layer exhibits ambipolar charge transport characteristics based on the voltage dependence of the EL spectra. The emission spectra measured for device ITO / NPD / mCP / SiFl4 / Alq 3 / LiF / Al is shown in Fig. 2.6. Pure Alq 3 emission was observed at a low voltage (8 V) indicating that recombination takes place at or near the 44 Alq 3 layer. As the voltage increases, the contribution of blue emission increases, thus more charge recombination takes place away from the Alq 3 interface. At 15 V, the electroluminescence (EL) spectrum is dominated by blue emission (from either NPD, mCP or SiFln). We also compared the charge transport properties of the SiFln to another arylsilane, 1,4-bis(triphenylsilyl)benzene (UGH2), 11 by fabricating an analogous device: ITO / NPD / mCP / UGH2 /Alq 3 /LiF /Al. As shown in Fig. 2.6, all of the devices using SiFln give markedly higher current densities at a given voltage than the related UGH2 device. This is likely due to greater charge carrier mobilities and the shallower HOMO of SiFln (6.6 eV) relative to UGH2 (7.2 eV). A comparison between EL spectra at 12 V from OLEDs using either SiFl2 or UGH2 is shown in Fig. 2.6, inset. The SiFl2 molecule is shown for 45 Device I. ITO/ NPD (40 nm) / SiFln (20 nm) (n = 2, 3, 4) /Alq 3 (20 nm) /LiF (1 nm) /Al (100 nm) Device III. ITO / NPD (40 nm) / dopant: SiFl4 (20 nm), (dopant = PQIr, Ir(ppy) 3 or FIrpic) /BCP (40 nm) /LiF (1 nm) /Al (100 nm) Device II. ITO / NPD (30 nm) / mCP (10 nm) / SiFln (20 nm) (n = 2, 3, 4) / Alq 3 (20 nm) /LiF (1 nm) /Al (100 nm) Device IV. ITO / NPD (30 nm) / mCP (10 nm) / FIrpic: SiFl4 (20 nm) /BCP (40 nm) /LiF (1 nm) /Al (100 nm) Device V . ITO / NPD (30 nm) / Ir(ppz) 3 (10 nm) / FIrpic: SiFl4 (20 nm) /BCP (40 nm) /LiF (1 nm) /Al (100 nm) Figure 2.4 Energy diagrams of the fabricated OLED devices. The scheme for Device III shows the energies for three different dopants. Devices utilize only a single dopant, not mixtures. (b) NPD mCP SiFl4 1.5 5.3 5.9 6.6 6.5 E (eV) LUMO HOMO 1.5 1.6 FIrpic 1.9 5.7 1.1 Alq 3 6.0 2.0 BCP 0.7 5.0 Ir(ppz) 3 SiFl2 or SiFl3 (a) Alq 3 6.0 2.0 NPD SiFl4 1.5 5.3 6.6 E (eV) LUMO HOMO Ir(ppy) 3 PQIr 1.9 2.3 5.1 5.0 1.1 FIrpic 1.9 5.7 6.5 1.6 BCP SiFl2 or SiFl3 46 0 1 2 3 4 5 6 7 8 9 10 11 12 13 0 200 400 600 800 350 400 450 500 550 600 650 0 1 Alq 3 Wavelength (nm) Normalized Intensity (a.u.) UGH2, device II SiFl2, device II NPD SiFl2, device I SiFl2, device II SiFl3, device II SiFl4, device II UGH2,device II Voltage (V) Current Density (mA/cm 2 ) Figure 2.5 Current density vs. voltage plots of devices I and II. The inset spectrum shows the EL spectra measured at 12 V for the UGH2 and SiFl2 devices. 400 450 500 550 600 650 0.0 0.5 1.0 8V 11-14 V 15V Wavelength (nm) Normalized Intensity (a.u.) Figure 2.6 Voltage dependence of the EL spectra of the device: ITO / NPD (30 nm) /mCP (10 nm) /SiFl4 (20 nm) / Alq 3 (20 nm) /LiF (1 nm) /Al (100 nm). As the voltage increases from 8 V to 15 V, the peak at 432 nm increases, while the peak at 516 nm decreases. 47 Table 2.3 Performance of undoped devices I and II. Device n EL maxima (nm) turn-on voltage a (V) Maximum luminance (cd/m 2 ) η ext, max (%) 2 432 7.2 464 (at 15 V) 0.3 I 3 432 4.7 648 (at 13 V) 0.3 4 432 4.5 392 (at 15 V) 0.3 2 432, 516 4.5 3945 (at 15 V) 0.4 II 3 432, 516 4.5 3984 (at 13 V) 0.5 4 432, 516 3.9 5911 (at 14 V) 0.5 UGH2 432 8.7 385 (at 15 V) 0.2 a Determined at a luminance of 0.1 cd/m 2 this comparison since it has both a size and molecular density close to that of UGH2. Very little Alq 3 emission is observed from the device made with UGH2 . Even with the presence of an mCP layer, a large energy barrier prevents hole injection from the mCP into the UGH2 layer. On the other hand, the EL spectra from the device using SiFl2 displays a substantial contribution from Alq 3 , which indicates improved hole injection from the HTL due to the relatively high HOMO level of the SiFl2 layer. Green and red phosphorescent devices were fabricated using SiFl4 as the host for emissive dopants in heterojunction devices. The structure used for these devices (device III) was ITO / NPD (40 nm) / dopant: SiFl4 (20 nm) /BCP (40 nm) /LiF (1nm) /Al (100 nm), where BCP = 2,9–dimethyl-4,7– diphenyl-1,10-phenanthroline. The fac- tris(2-phenylpyridyl)iridium (Ir(ppy) 3 , 6 %) and bis(2-phenylquinolyl)iridium acetylacetonate (PQIr, 6 %) complexes were used as dopants for the green and red OLEDs, respectively. Table 2.4 summarizes the performance of all these doped devices. 48 The HOMO energy levels of Ir(ppy) 3 and PQIr are well above those of the fluorenylsilanes and close to that of NPD, which facilitates hole injection into the emissive layer and eliminates the need for a mCP hole injection layer (Fig. 2.4). Pure dopant emission was observed for both the green and red emissive devices (Fig. 2.7a). These two devices turn on at roughly 3.0 V and the highest quantum efficiencies (8 %) were measured for both devices at a current density of 0.01 mA/cm 2 (Fig. A1.5b). The triplet energies of the SiFln molecules (2.9 eV) are greater than the triplet energies of the blue phosphorescent dopants, such as bis[(4,6-difluorophenyl) pyridyl]iridium picolinate (FIrpic, E T = 2.65 eV), making the fluorenylsilanes potential hosts for blue phosphorescent emitters. However, the low triplet energy of the aggregate states in film (Fig. 2.4c) could compromise the performance of the OLEDs. To examine this possibility, blue phosphorescent devices using device structure III were prepared using FIrpic (10 wt%) as the emissive dopant. No emission from Firpic was observed in these devices, suggesting that a sufficiently large barrier to hole injection is present that limits charge recombination to the HTL layer. FIrpic has a markedly deeper HOMO energy than either Ir(ppy) 3 or PQIr (Fig. 2.4). In order to promote hole injection, an OLED with Firpic was also prepared using device structure IV (ITO / NPD (30 nm) / mCP (10 nm) / FIrpic: SiFl4 (10%, 20 nm) /BCP (40 nm) /LiF (1 nm) /Al (100 nm)), in which a mCP layer is inserted between the NPD layer and the EML (Fig. 2.4b). Emission from FIrpic can be observed for this device (Fig. 2.7b), suggesting that recombination takes place in both the HTL and the intended emissive layer (FIrpic : SiFl4). The EL spectrum was observed to be strongly 49 Figure 2.7 EL spectra of (a) devices III and (b) device IV and device V. In device IV, the peak at 432 nm increases, while the peaks at 468 nm and 492 nm decrease from 8 V to 14 V. The EL spectra of device V do not change from 8 V to 14 V. voltage dependent (Fig. 2.7b), with predominant FIrpic emission at low voltage (8V) and predominant emission from the HTL layer at high voltage (14 V). The significant emissive contribution from HTL layer leads to a relatively low efficiency in this device, peaking at roughly 2% (Table 2.4). (b) 400 500 600 700 0.0 0.5 1.0 8 V - 14 V 8 V 10 V, 12 V 14 V FIrpic, device IV Wavelength (nm) Normalized Intensity (a.u.) FIrpic, device V (a) 400 500 600 700 0.0 0.5 1.0 Normalized Intensity (a.u.) W avelength (nm) PQIr, device III Ir(ppy)3, device III FIrpic, device III 50 Table 2.4 Performance of phosphorescent devices III, IV and V . Device Dopant EL maxima (nm) turn-on voltage a (V) maximum luminance (cd/m 2 ) 　 ext, max (%) III Ir(ppy) 3 510 3.0 9514 (at 13 V) 8 PQIr 595 3.2 4814 (at 14 V) 8 FIrpic 432 4.2 2605 (at 15 V) 0.4 VI FIrpic 432(strong), 3.9 3053 (at 14 V) 2 V FIrpic 468 4.4 458 (at 15 V) 3 a Determined at a luminance of 0.1 cd/m 2 The fac-tris(1-phenylpyrazolyl)iridium complex, Ir(ppz) 3 , has been shown to be both a good hole injecting and electron blocking material. 9,37 Substituting Ir(ppz) 3 in place of mCP is expected to prevent electron leakage into the HTL layer and give pure dopant emission. Device V, i.e. ITO / NPD (30 nm) / Ir(ppz) 3 (10 nm) / FIrpic: SiFl4 (10%, 20 nm) /BCP (40 nm) /LiF (1nm) /Al (100 nm), was prepared to explore the effect of Ir(ppz) 3 incorporation on device performance (Fig. 2.4b). As expected, device V gives only FIrpic emission ( λ max = 468 nm) at all voltages examined (8–14 V) (Fig. 2.7b). While this device gives emission exclusively from the phosphorescent dopant, the peak efficiency (3% at 3 V) is still well below values achieved for the OLEDs using green and red dopants (Table 2.4). The poor performance of the blue devices could be due to the presence of host aggregates. As mentioned previously, the aggregates have triplet state energies (510 nm, 2.43 eV) that are much lower than that of FIrpic. While the energy of triplet state of the aggregates is too high to quench either the green or red dopants, the value is low enough to adversely affect the efficiency of the devices doped with FIrpic. 51 2.4 Summary We have designed and synthesized four fluorenylsilanes with varying number of fluorenyl moieties, ranging from one to four. These materials give high glass transition temperatures (for n = 2, 3 and 4) and reversible reductions, making them good host candidates for OLED applications. Systematic studies including electrochemistry, photophysics and OLED device studies have been performed to investigate the charge transport abilities of these molecules. Device data suggest the fluorenylsilanes have ambipolar characteristics. Their large HOMO-LUMO gap and the high triplet energy confine excitons to green and red phosphorescent emitters in doped OLEDs. High external quantum efficiencies were observed for the green and red phosphorescent devices fabricated with Ir(ppy) 3 and PQIr doped into SiFl4. We also found that these molecules could be utilized as host materials for blue OLEDs. However, the presence of aggregated states with low triplet energies in neat films of SiFl4 limits the performance of the latter devices. 52 Chapter 2 References 1. Tang, C. W.; Van Slyke, S. A., Appl. Phys. Lett. 1987, 51, 9131. 2. Holmes, R. J.; Forrest, S. R.; Tung, Y. J.; Kwong, R. C.; Brown, J. J.; Garon, S.; Thompson, M. E., Applied Physics Letters 2003, 82, 2422. 3. D'Andrade, B. 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S.; Cahill, P. A., Applied Physics Letters 1995, 66, 3618. 19. Wu, C. C.; Liu, T. L.; Lin, Y . T.; Hung, W. Y .; Ke, T. H.; Wong, K. T.; Chao, T. C., Applied Physics Letters 2004, 85, 1172. 20. Shih, P. I.; Chien, C. H.; Chuang, C. Y.; Shu, C. F.; Yang, C. H.; Chen, J. H.; Chi, Y ., Journal of Materials Chemistry 2007, 17, 1692. 21. Tsai, M. H.; Lin, H. W.; Su, H. C.; Ke, T. H.; Wu, C. C.; Fang, F. C.; Liao, Y. L.; Wong, K. T.; Wu, C. I., Advanced Materials 2006, 18, 1216. 22. Zhan, X. W.; Risko, C.; Amy, F.; Chan, C.; Zhao, W.; Barlow, S.; Kahn, A.; Bredas, J. L.; Marder, S. R., Journal of the American Chemical Society 2005, 127, 9021. 23. Xie, N.; Zeng, D. X.; Chen, Y ., Synlett. 2006, 5, 737. 24. Liao, Y.; Baskett, M.; Lahti, P. M.; Palacio, F., Chemical Communications 2002, 252. 25. You, Y .; An, C. G.; Kim, J. J.; Park, S. Y ., Journal of Organic Chemistry 2007, 72, 6241. 26. Uchida, M.; Izumizawa, T.; Nakano, T.; Yamaguchi, S.; Tamao, K.; Furukawa, K., Chemistry of Materials 2001, 13, 2680. 27. Chan, L. H.; Lee, R. H.; Hsieh, C. F.; Yeh, H. C.; Chen, C. T., Journal of the American Chemical Society 2002, 124, 6469. 28. Yu, G.; Xu, X.; Liu, Y.; Jiang, Z.; Yin, S.; Shuai, Z.; Zhu, D., Appl. Phys. Lett. 2005, 87, 222115. 29. Shirota, Y ., Journal of Materials Chemistry 2000, 10, 1. 30. Shirota, Y ., Journal of Materials Chemistry 2005, 15, 75. 54 31. O'Brien, D. F.; Burrows, P. E.; Forrest, S. R.; Koene, B. E.; Loy, D. E.; Thompson, M. E., Advanced Materials 1998, 10, 1108. 32. Koene, B. E.; Loy, D. E.; Thompson, M. E., Chemistry of Materials 1998, 10, 2235. 33. Rathore, R.; Abdelwahed, S. H.; Guzei, I. A., Journal of the American Chemical Society 2003, 125, 8712. 34. D'Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.; Polikarpov, E.; Thompson, M. E., Organic Electronics 2005, 6, 11. 35. Djurovich, P. I.; Mayo, E. I.; Forrest, S. R.; Thompson, M. E., Organic Electronics 2009, 10, 515. 36. Loy, D. E.; Koene, B. E.; Thompson, M. E., Advanced Functional Materials 2002, 12, 245. 37. Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E., Journal of the American Chemical Society 2003, 125, 7377. 55 Chapter 3: A Comparison between Methyl and Silyl Spirobifluorenyls: Synthesis, Photophysics and Performance in Phosphorescent Organic Light Emitting Diodes 3.1 Introduction Phosphorescent organic light emitting diodes (PHOLEDs) that harvest both singlet and triplet excitons have been recognized as an efficient technology to achieve 100% internal efficiency of organic light emitting diodes. 1-3 Efficient and stable devices have been developed for green and red PHOLEDs. However, the performance of blue PHOLEDs is still not comparable with their red and green counterparts, especially with respect to long-term device stability. 4-8 Premature aging of blue devices remains one of the most serious obstacles to wide acceptance of PHOLEDs in commercial applications. The poor performance of blue PHOLEDs is partly a consequence of the properties of current host materials for blue dopants. These materials, which include carbazoles, 9-12 phenylsilanes, 13,14 and fluorenes, 15,16 have been widely used as hosts for PHOLEDs and each has its own disadvantage. For example, carbazole hosts like (4,4’-N,N’- dicarbazole)biphenyl (CBP) and 4,4’,4”-tris(carbazol-9-yl)triphenylamine (TCTA) have been reported to be unstable in blue OLEDs. 4-6 Both Scholz et al. 4,5 and Kondakov et al. 6 found that the C–N bonds between the phenyls and the carbazoles in these hosts dissociated after an extended operation time and produced charged or radical species. Phenylsilanes are potentially more stable host materials due to the strong Si-phenyl bonds and they have been utilized as large band gap host materials for blue PHOLEDs. 13,14 However, phenylsilanes are poor charge carriers and have deep HOMO levels (7.2 eV) that make it difficult to inject holes into the doped emissive layer. 56 Fluorenylsilanes, which have higher HOMO levels (6.6 eV) and improved charge mobilities, have been developed for blue PHOLEDs, but poor hole injection in these materials still leads to a low device efficiency. 15,16 Arylmethane hosts materials have been recently reported by Ye et al. who described two fluorenylbenzene-based compounds, 17 pDPFB and mDPFB (DPFB = bis(9-phenyl-9H-fluoren-9-yl)benzene). While these two molecules have high triplet energies (2.8 eV) their OLED performance has not yet been optimized. Compared to the fluorenes, spirobifluorenes have been reported to exhibit higher glass transition temperatures 13 and hole mobilities, 14 whereas fluorene species have slightly higher electron mobilities. As mentioned above, the deep HOMO of the fluorenes (6.2–6.6 eV) can make it difficult to inject holes into the emissive layer of blue OLEDs. Considering the comparable HOMO levels of the spirobifluorenes to the fluorenes, the increased hole transport ability of the former species may lead to improved charge balance in the OLED devices. However, since most of the prior spirobifluorene derivatives have been conjugated species with low triplet energies, few simple spirobifluorene hosts have been reported for blue PHOLEDs. 18,19 Non-conjugated carbon- based spirobifluorene hosts have not yet been discovered. In this study, a carbon-based spirobifluorene host Ph 3 CSBFL (Scheme 3.1) and its silicon analog Ph 3 SiSBFL (Scheme 3.2) were synthesized and utilized as hosts for blue OLED devices. The compounds were designed to maintain high singlet and triplet energies of the spirofluorenyl moiety by using the central C or Si to isolate the aromatic groups of these two molecules. A series of studies have been carried out to compare the properties of these two materials, including thermal analysis, electrochemical studies and photophysical studies. Data are also 57 included to evaluate the performance of these two materials in blue and green PHOLED devices. Br MgBr NH 2 OH O Mg THF THF H 2 N HCl/HOAc 1. tBuONO 2. H 3 PO 2 1 2 3 Ph 3 CSBFL Scheme 3.1 Synthesis of Ph 3 CSBFL nBuLi THF Ph 3 SiCl THF Br Li Si 1 Ph 3 SiSBFL Scheme 3.2 Synthesis of Ph 3 SiSBFL 58 3.2 Experimental 3.2.1 Synthesis. All chemicals and solvents were purchased from commercial sources without further purification except for tetrahydrofuran (THF), which was distilled from sodium/benzophenone. Triphenyl-9,9-spirobifluoren-2-yl methane (Ph 3 CSBFL). A Grignard solution was prepared by adding 2-bromo-9,9’-spirobifluorene (1, 1.74 g, 4.4 mmol) in 8 ml THF dropwise to magnesium turnings (0.11 g, 4.4 mmol) in 7 ml THF. A solution of benzophenone in 4 mL THF was then slowly added to the Grignard reagent at room temperature and the mixture was refluxed for 3 h. After the mixture was cooled to the room temperature, saturated ammonium chloride (aq.) was added to quench the reaction. The organic layer was extracted into CH 2 Cl 2 and dried over MgSO 4 . After the solvent was removed using rotary evaporation, the crude product was passed through a silica gel column with CH 2 Cl 2 /hexanes (1:3) to give diphenyl-spirobifluorenyl methanol (3) as a white solid (1.63 g, 81%). Compound 3 (3.17 mmol, 1.58 g) was next refluxed with aniline (4.8 mmol, 0.44 ml), 37% aq. HCl (0.5 ml) and acetic acid (10 ml) at 140 °C for 3 days. The reaction mixture was allowed to cool to the room temperature and 100 mL water was added, followed by solid NaOH until the solution became neutral. The product, diphenylspirobifluorenylaniline methane (4), was isolated by filtration as a white solid (1.65 g, 91%). To a solution of 4 (0.574 g, 1 mmol) in 30 mL THF, H 3 PO 2 (6 M in H 2 O, 1 mL) was added. After stirring for 10 min, a solution of tBuONO (0.185 mL, 1.5 mmol) in 5 ml THF was added and the mixture was stirred for 16 h at 40 °C. After the THF was 59 removed, the reaction mixture was extracted with CH 2 Cl 2 and water. The organic layer was washed by brine and dried over MgSO 4 . The solvent was removed under reduced pressure and the product was recrystallized from DCM/ hexanes to give Ph 3 CSBFL as a white solid (0.25 g, 45%). Triphenyl-9,9-spirobifluoren-2-yl silane (Ph 3 SiSBFL). 20,21 To a solution of 2- bromo-9,9-spirobifluorene (1, 0.78 g, 2 mmol) in dry THF (20 mL) was added n-BuLi (1.6 M in hexanes, 1.38 mL, 2.2 mmol) under N 2 atmosphere to get an orange solution. After one hour, triphenylchlorosilane (0.56 g, 1.9 mmol) dissolved in THF (20 mL) under N 2 atmosphere was slowly added by syringe and the solution was stirred overnight. Ethyl acetate and water were used to extract the organic layer, which was washed with brine and dried over magnesium sulfate. The organic solvents were removed by rotary evaporation to form an orange liquid and recrystallization from CH 2 Cl 2 afforded Ph 3 SiSBFL as a white solid (0.89 g, 78%). 1 H NMR, mass spectrometry and elemental analysis data of these compounds are shown as follows: Diphenyl-9,9-spirobifluoren-2-yl methanol (3), Yield: 81%. 1 H NMR (CDCl 3 , 400 Hz) δ: 7.79 -7.81 (1H, d, Ar-H), 7.77-7.79 (2H, d, Ar-H), 7.69-7.71 (1H, d, Ar-H), 7.33-7.38 (1H, d, Ar-H), 7.31-7.36 (2H, dd, Ar-H),7.18-7.22 (4H, d, Ar-H), 7.18-7.21 (2H, dd, Ar-H), 7.12-7.17 (4H, dd, Ar-H), 7.04-7.10 (1H, dd, Ar-H), 6.92 (1H, s, Ar-H), 6.72- 6.75 (2H, d, Ar-H), 6.67-6.70 (1H, d, Ar-H). 13 C NMR (400 Hz): MS (m/z): 498, 482, 421, 315. p-(Diphenyl-9,9-spirobifluoren-2-yl)methyl aniline (4), Yield: 91%. 1 H NMR (CDCl 3 , 250Hz): δ 7.75-7.79 (1H, d, Ar-H) 7.69 -7.73 (2H, d, Ar-H), 7.25 -7.32 (3H, m, 60 Ar-H), 7.21-7.24 (1H, d, Ar-H), 7.01-7.10 (H, m, Ar-H), 6.80-6.83 (2H, d, Ar-H), 6.68- 6.71 (2H, d, Ar-H), 6.65-6.68 (1H, d, Ar-H), 6.56-6.58 (1H, s, Ar-H), 6.40-6.43 (2H, d, Ar-H). MS (m/z): 572, 481, 258. Triphenyl-9,9-spirobifluoren-2-yl methane (Ph 3 CSBFL), Yield: 45%. 1 H NMR (CDCl 3 , 400Hz) δ: 7.76 -7.79 (1H, d, Ar-H), 7.71-7.73 (2H, dd, Ar-H), 7.68-7.70 (1H, d, Ar-H), 7.30-7.34 (1H, dd, Ar-H), 7.27-7.31 (2H, dd, Ar-H), 7.23-7.26 (1H, d, Ar-H), 7.03-7.12 (18H, m, Ar-H), 6.68-6.70 (2H, d, Ar-H), 6.62-6.68 (1H, d, Ar-H), 6.55 (1H, s, Ar-H). MS (m/z): 558, 481, 403, 316. Anal. calcd. for C 44 H 30 : C 94.59 H 5.41; Found: C 94.45; H 5.31. Triphenyl-9,9-spirobifluoren-2-yl silane (Ph 3 SiSBFL) Yield: 78%. 1 H NMR (CDCl 3 , 250Hz): δ 7.80 -7.87 (2H, dd, Ar-H), 7.74-7.78 (2H, d, Ar-H), 7.45-7.49 (1H, d, Ar-H), 7.29-7.41 (12H, m, Ar-H), 7.20-7.27 (6H, m, Ar-H),7.08-7.15 (4H, dd, Ar-H), 7.05(1H, s, Ar-H), 6.75-6.79 (2H, d, Ar-H), 6.71-6.75 (1H, d, Ar-H). MS (m/z): 574, 497, 316, 259. Anal. calcd. for C 43 H 30 Si: C 89.85 H 5.26; Found: C 90.07 H 5.12 . 3.2.2 X-ray Crystallographic Procedures. X-ray quality crystals grown from CH 2 Cl 2 /hexanes (Ph 3 CSBFL) or CH 2 Cl 2 (Ph 3 SiSBFL) were mounted on a glass fiber with Paratone-N oil. X-ray diffraction data was collected on a Bruker SMART APEX diffractometer using graphite-monochromated Mo K α radiation, and structures were determined using direct methods with standard Fourier techniques using the Bruker AXS software package. 61 3.2.3 Thermal Analysis. Differential scanning calorimetry (DSC) was performed on a TA Instruments DSC Q10 instrument with a scanning range from room temperature to 300 °C. The sample was first scanned at a heating rate of 10 °C min -1 and was cooled down to room temperature rapidly using liquid N 2 . The second and third scans were performed at a heating rate of 5 °C min -1 . The glass transition temperatures were determined from either the second or the third scan for each compound. 3.2.4 Electrochemistry and photophysics. Cyclic voltammetry was performed using an EG&G potentiostat/galvanostat model 283 under N 2 atmosphere. A glass carbon rod was used as the working electrode. Tetrabutylammonium hexafluorophosphate (TBAH, 0.1 M) was used as the supporting electrolyte. Anhydrous acetonitrile was used as the solvent for the oxidation measurements and anhydrous DMF was used as the solvent for the reduction measurements. The redox potentials are calculated relative to an internal reference ferrocenium/ferrocene (Cp 2 Fe + /Cp 2 Fe). The UV-visible spectra were measured by Hewlett-Packard 4853 diode array spectrophotometer. Steady state emission measurements were performed on a Photon Technology International QuantaMaster model C-60SE spectrofluorimeter at room temperature and 77 K. 62 3.2.5 Device Fabrication. Temperature gradient vacuum sublimation was used to purify all the materials used for vapor deposition. Undoped and Firpic doped devices were fabricated using pre- patterned indium tin oxide (ITO) provided by Thin Film Devices Inc. The ITO substrates were coated with a photo resist, baked in the oven for 20 min, cleaned by acetone, dried with nitrogen, and treated with UV ozone for 10 min. The organic materials were vapor- deposited onto the substrates in a high-vacuum chamber (<10 -6 Torr), followed by the deposition of 10 Å LiF and 1000 Å aluminum cathodes using a shadow mask with 2-mm wide stripes. The electrical and optical characteristics of the devices were measured with a Ketithly 2400 source/meter/2000 multimeter coupled to a Newport 1835-C optical meter, equipped with a UV 818 Si photo detector. The electroluminescence (EL) spectra were measured using a Photon Technology International QuantaMaster model C-60SE spectrofluorimeter. A 1200 Å layer of sputtered indium tin oxide (ITO) on glass was used as the substrate for the Ir(ppy) 3 doped devices. All organic layers were deposited by high vacuum (<10 -7 Torr) thermal evaporation. The organic stack consisted of sequentially, from the ITO surface, 100 Å thick layer of 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12- hexaazatriphenylene as the hole injection layer (HIL), 300 Å of NPD as the hole transporting layer (HTL), 300 Å of Ph 3 CSBFL or Ph 3 SiSBFL doped with 9 and 15 wt% of the dopant emitter, Ir(ppy) 3 , as the emissive layer (EML), 100 Å of BAlq and 400 Å Alq 3 as the ETL layers. The cathode consisted of 5 Å of LiF followed by 1,000 Å of Al. All devices were encapsulated with a glass lid sealed with an epoxy resin in a nitrogen glove box (<1 ppm of H 2 O and O 2 ) immediately after fabrication, and a moisture getter 63 was incorporated inside the package. Device life-testing was performed under constant current conditions of 40 mA/cm 2 . 3.3 Result and Discussion. 3.3.1 Design, Synthesis and Characterization of the Materials. The compounds Ph 3 CSBFL and Ph 3 SiSBFL were prepared using two different synthetic methodologies. The synthesis of Ph 3 CSBFL follows a four step route shown in Scheme 3.1. Benzophenone was added to a Grignard reagent prepared from 2-bromo-9, 9’-spirobifluorene (1) to produce diphenyl-9,9-spirobifluoren-2-yl methanol (2). A subsequent Friedel-Crafts reaction between 2 and aniline under strongly acidic conditions afforded p-(diphenyl-9,9-spirobifluoren-2-yl)methyl aniline (3). 22 The amino group of 3 was then removed using H 3 PO 2 and tBuONO to give the final product, Ph 3 CSBFL. The overall yield of this multi-step synthesis was 33%. Ph 3 SiSBFL was synthesized as shown in Scheme 3.2. Addition of Ph 3 SiCl to a solution of lithiated 9,9’-spirobifluorene in THF gave Ph 3 SiSBFL as a pure product in 78% isolated yield. 20,21 Ph 3 CSBFL Ph 3 SiSBFL Figure 3.1 X-ray crystal structures of Ph3CSBFL (left) and Ph3SiSBFL (right). 64 Table 3.1 Percent type of intermolecular interactions (C-H, C-C or H-H) for Ph 3 CSBFL and Ph 3 SiSBFL crystals calculated using the Crystal Explorer software package. C-H C-C H-H Ph 3 CSBFL 36 0.5 63.5 Ph 3 SiSBFL 37.2 1.1 61.7 3.3.1.1 X-ray Structures. The molecular structures of Ph 3 CSBFL and Ph 3 SiSBFL were determined using X- ray crystallography, the structures are shown in Fig. 3.1. Crystals of both compounds are isomorphous (trigonal space group P-3c1) with Ph 3 SiSBFL having slightly larger unit cell parameters. The aryl-C (Si) distances, whether Ph or SBFL, are similar to each other, however, the SBFL-C distance (1.546(3) Å) is much shorter than the corresponding SPFL–Si distance (1.8729(19) Å). Fig. A2.1 shows the crystal packing structure in a unit cell for each compound. Even though there are short contacts found between C and H in the crystals, there are no close contacts between aromatic rings, which indicate the absence of intermolecular π- π interactions in these two materials. In order to quantify the solid-state interactions, Hirshfeld surface calculations 23 based on the X-ray data were applied to determine the intermolecular contacts in these two materials. Fig. A2.2 shows the Hirshfeld surfaces of these two molecules in their crystal lattices, in which red regions indicate a stronger intermolecular interaction while blue regions indicate a weaker interaction. A “fingerprint plot” generated from the Hirshfeld surface provides more detailed information of the types and the percentages of these interactions. The type and percentage of intermolecular contacts are listed in Table 3.1. Ph 3 SiSBFL crystals 65 show higher ratios of both C-C type and C-H type interactions between molecules than the Ph 3 CSBFL crystals, which instead have a higher ratio of H-H type intermolecular interactions. 3.3.1.2 HOMO and LUMO energies. Ph 3 CSBFL and Ph 3 SiSBFL display similar electrochemical behavior. Both species show irreversible waves at 1.1 V and 1.7 V (Table 3.2 and Fig. A2.4). The first oxidation at 1.1 V is assigned to the unsubstituted fluorene ring, the second to oxidation of the fluorene adjacent to the C or the Si center. Irreversible oxidation has been reported for single spirobifluorenes and non-conjugated derivatives at similar potentials, where the difference between the first and the second wave is dependent on the groups attached to the spirobifluorene. 17,24 The reduction potentials of the Ph 3 CSBFL and the Ph 3 SiSBFL are almost identical (-2.9 ± 0.1 V for both compounds, Table 3.2 and Fig. A2.4). DFT calculations of HOMO and LUMO surfaces and energies of these two molecules are shown in Fig. A2.3. The HOMO (-5.6 eV) and LUMO (-0.9 eV for Ph 3 CSBFL and -1.0 eV for Ph 3 SiSBFL) surfaces are localized on the spirobifluorenyl groups. Values for the HOMO (-6.2 eV) and LUMO (-1.3 eV) energies for both molecules were estimated from the electrochemical data using the correlations between redox potentials and the HOMO/LUMO levels 25,26 and lead to transport gaps of 4.9 ± 0.1 eV. The transport gaps reported for the fluorenylsilane hosts are larger (5.5 eV), mainly due to the deeper HOMO energy (6.6 eV). A typical hole transport material, N,N’-bis(naphthalene-1-yl)- N,N’-bis(phenyl)-benzidine (NPD), has a much higher HOMO energy (5.2 eV), which results in a large injection barrier from the hole transport layer (HTL) to the emissive layer (EML). Hence, when compared with their fluorene counterparts, the 0.4 eV higher 66 HOMO levels of Ph 3 CSBFL and Ph 3 SiSBFL should improve the hole injection process into the EML. 3.3.1.3 Thermal Properties. An important parameter for a material applied as a amorphous thin film in OLED devices is a high glass transition temperature (T g ). 27-30 A melting transition at 284 °C was found for Ph 3 CSBFL, 31 °C higher than that of Ph 3 SiSBFL (253 °C). We were not able to detect a T g for Ph 3 CSBFL, however, did observe one for Ph 3 SiSBFL at 87 °C (Fig. A2.3). Both compounds can be sublimed at 245 °C in high yields (80–90%) using a gradient vacuum sublimator (Fig. A2.3). Due to the steric bulk and symmetry of the spirobifluorene substituent, the T g for Ph 3 SiSBFL is higher than the fluorenylsilane analogs and also greater than typical OLED hosts such as N,N’-dicarbazolyl-3,5-benzene (mCP), (4, 4′-N,N ′-dicarbazolyl)biphenyl (CBP) and phenylsilanes. 13 3.3.1.4 Photophysical characterization. The absorption and photoluminescence spectra of Ph 3 CSBFL and Ph 3 SiSBFL in solution are shown in Fig. 3.2a, data is listed in Table 3.2. Absorption bands between 290 nm and 320 nm are attributed to transitions on the spirobifluorene moiety. 31 The optical gap for both compounds, determined from the long wavelength edge of the absorption spectra, is 317 nm (3.9 eV). This value is ca. 1 eV smaller than the transport gap. The luminescence spectra display features typical for fluorene singlet emission ( λ max = 320 nm for Ph 3 CSBFL and 318 nm for Ph 3 SiSBFL). Phosphorescence from the fluorenyl group is observed at low temperature (77K) (E 0-0 = 443 nm (Ph 3 CSBFL) and 438 nm (Ph 3 SiSBFL)) (Fig. 3.2a, inset). The triplet energies for both molecules are lower than 67 that of fluorenylsilanes (E 0-0 = 435 nm), yet still higher than such typical blue phosphorescent emitters as iridium(III) bis[2-(4, 6-difluorophenyl)-pyridinato-N,C 2 ]- picolinate (FIrpic, E 0-0 = 465 nm). Table 3.2 Summary of the electrochemical and photophysical properties of Ph 3 CSBFL and Ph 3 SiSBFL. E ox (V) a E red (V) a λ abs (nm) b λ PL (nm) c λ PL,crystal (nm) d λ PL, film (nm) e , τ (ns) f Ph 3 CSBFL 1.1 -3.0 315 320, 443 356 342 (1.7, 5.1) Ph 3 SiSBFL 1.1 -2.8 315 318, 438 386 360 (1.1, 2.6), 428 (1.7 , 3.7) a Oxidation and reduction potentials were measured in anhydrous DMF and acetonitrile, respectively. b Measured in CH 2 Cl 2 . c Measured in 2-methyl-THF. Fluorescence was recorded at room temperature. Phosphorescence was recorded at 77K. d Crystals obtained from gradient sublimation. e 40 nm films by organic vapor phase deposition . f Double exponential fit. Emission and excitation spectra from microcrystalline samples of both materials were recorded (Fig. 3.2b) so that a direct correlation between the molecular arrangements and optical properties could be made using the X-ray and photophysical data. The emission spectrum of crystalline Ph 3 CSBFL displays a peak maximum at 356 nm, which is 36 nm red-shifted from its solution absorption. On the other hand, the peak maximum in the excitation spectrum is blue-shifted 3 nm from its value in solution. The emission spectrum of crystalline Ph 3 SiSBFL is strongly red-shifted ( λ max = 386 nm) from its solution absorption and displayed an excitation peak maximum at 362 nm. The red-shifts 68 0 1 2 0 1 2 250 275 300 325 350 375 400 0 1 2 0 1 2 440 480 520 560 600 λ (nm) PL - triplet 443 Ph 3 SiSBFL 315 ABS PL - singlet Intensity (a.u.) Absorbance (a.u.) 320 Ph 3 CSBFL (a) 440 480 520 560 600 λ (nm) PL - triplet 436 315 ABS PL - singlet Wavelength (nm) 317 0 1 0 1 200 250 300 350 400 450 500 550 600 0 1 0 1 362 314 386 Ph 3 CSBFL crystals EX PL 356 Intensity (a.u.) Wavelength (nm) Ph 3 SiSBFL crystals EX PL Intensity (a.u.) (c) 0 1 0 1 250 300 350 400 450 500 550 0 1 0 1 318 342 428 360 Ph 3 CSBFL film PL ABS EX 317 Intensity (a.u.) Wavelength (nm) Ph 3 CSBFL film PL ABS EX Absorbance (a.u.) (b) Figure 3.2 Absorption (abs), excitation (ex) and photoluminescence (PL) spectra of Ph3CSBFL and Ph3SiSBFL from (a) dilute 2-methyl THF solution, (b) sublimed crystals and (c) 40 nm thin films. 69 in the emission spectra of Ph 3 CSBFL and Ph 3 SiSBFL come from the intermolecular interactions between molecules in the crystal lattice. The larger red-shifts in both excitation and emission spectra of Ph 3 SiSBFL suggest that more extensive intermolecular interactions are present in its crystals. The Hirshfeld surface calculation discussed in the previous section also provides evidence of greater number C-C type and C-H type intermolecular interactions in the Ph 3 SiSBFL crystals. It is particularly instructive to determine the optical properties of Ph 3 CSBFL and Ph 3 SiSBFL as neat amorphous films since this is how the materials will be applied in the semiconductor devices. The absorption and emission spectra from thin films are shown in Fig. 3.2c. The absorption spectra of both materials display features that are similar to those measured in dilute solution. Even though these two films have equivalent absorption features, their emission spectra are very different. The Ph 3 CSBFL film shows a single emission band at 342 nm, whereas the Ph 3 SiSBFL displays two separate bands at 360 nm and 430 nm. An absorption band after 340 nm, though partly due to scattering or interference effects, still led to emission from the spirobifluorene compounds upon excitation into this feature, which suggests weak absorptions are present at longer wavelength for these materials. The emission bands between 330 and 400 nm are assigned to be red-shifted singlet transitions from isolated molecules since these features have lifetimes on nanosecond timescales (Table 3.2). The emission band at 430 nm from Ph 3 SiSBFL is in the same spectral region as one seen in dimethylfluorenyl silanes and has been assigned to singlet state of the aggregates. 16 The lifetimes of these emissions are also in the nanosecond regime (Table 3.2). The red-shift could be due to weak interactions between molecules in the films. 70 1.5 1.5 1.6 5.3 5.9 6.5 BCP mCP NPD E (eV) 1.3 6.2 SBFL Host LUMO HOMO 1.9 5.7 FIrpic Alq 3 Figure 3.3 Energy diagrams of undoped devices: ITO / NPD / mCP / SBFL Host / Alq3 and FIrpic devices: ITO / NPD / mCP / FIrpic: SBFL Host (10%) / BCP. 3.3.2 OLED Studies 3.3.2.1 Undoped devices. In order to compare the charge transport properties of Ph 3 SiSBFL and Ph 3 CSBFL, OLED devices were fabricated with the following architecture: ITO/NPD (30 nm)/mCP (10 nm)/SBFL (20 nm)/Alq 3 (20 nm)/LiF (1 nm) /Al (100nm), where SBFL = Ph 3 CSBFL or Ph 3 SiSBFL, (Fig. 3.3). The electroluminescent (EL) spectrum at 9 V measured for each of these devices is shown in Fig. 3.4. The EL spectra from both devices are voltage dependent in the range of 8–15 V, with increasing NPD emission and decreasing Alq 3 emission observed at higher potentials (Fig. A2.6). However, over the whole voltage range examined, the Ph 3 CSBFL device gives rise to emission mainly from NPD, whereas 71 300 400 500 600 700 0.0 0.5 1.0 Ph 3 CSBFL Ph 3 SiSBFL Alq 3 Intensity (a.u.) Wavelength (nm) NPD Figure 3.4 Electroluminescence spectra of Ph 3 CSBFL and Ph 3 SiSBFL undoped devices: ITO / NPD / mCP / SBFL Host / Alq 3 at 9 V. the EL spectrum for the Ph 3 SiSBFL device is dominated by emission from Alq 3 . Since these two materials have comparable HOMO and LUMO energies, the charge injection barrier for holes and electrons should be similar in both devices. This suggests that the different EL spectra of these two materials may come from the difference in their morphologies. The current-voltage (J–V) characteristics of these two devices are shown in Fig. 3.5. A larger current was observed for the Ph 3 SiSBFL device, also indicating that the silyl derivative exhibits higher charge mobilities than the carbon analog. 3.3.2.2 Blue phosphorescent devices. In order to compare these two materials as host materials, PHOLED devices using a blue dopant, Firpic, were fabricated: ITO / NPD (30 nm) / mCP (15 nm) / FIrpic: SBFL (10%, 25 nm) / BCP (40 nm) / LiF (1 nm)/ Al (100 nm). The relative HOMO and LUMO 72 73 energies of the materials used in these devices are shown in Fig. 3.3, the performance is summarized in Table 3.3. NPD (30 nm) and BCP (40 nm) were used as respective hole and electron transporting layers. Considering the large difference between the HOMO energies of NPD (5.3 eV) and the spirobifluorene hosts (6.2 eV), a thin layer (15 nm) of mCP (HOMO = 5.9 eV) was inserted to facilitate the hole injection into the EML (Fig. 3.3). 32 The turn-on voltage at a luminance of 1 cd/m 2 for the Ph 3 CSBFL device (5.2 V) is higher than the Ph 3 SiSBFL device (4.6 V). Fig. 3.6a shows the J–V and luminance- voltage (L–V) characteristics of these blue devices. At every bias, devices made with Ph 3 CSBFL exhibit poorer conductivity and luminance than those using Ph 3 SiSBFL. For example, at 8 V the Ph 3 CSBFL device gives a current density of 0.9 mA/cm 2 and a luminance of 84.5 cd/m 2 , while the respective values for Ph 3 SiSBFL device are five times (4.4 mA/cm 2 ) and nine times (710 cd/m 2 ) higher. The lower current observed for the Ph 3 CSBFL device might come from the higher barrier for electron injection (0.2 eV) than Ph 3 SiSBFL or may stem from differing carrier mobilities. However, despite these difference in conductivity, charge recombination is unaffected since only FIrpic emission ( λ max = 468 nm) is observed from both devices (Fig. 7). Moreover, although the Ph 3 CSBFL device displays a higher turn-on voltage, poorer conductivity and lower brightness than the silicon analogue, the parameters are still better than those reported by Ye et. al for OLEDs made using fluorene carbon-based hosts. 17 The external quantum efficiency (EQE) for blue PHOLEDs made with the two host materials also show differences with respect to increasing bias (Fig. 3.8). The Ph 3 CSBFL device has an EQE of 6.9 % at a current density of 0.024 mA/cm 2 (at 5.8 V) that undergoes a significant drop once the current density reaches a value of 0.2 mA/cm 2 74 0246 8 10 12 14 0 200 400 600 Ph 3 CSBFL Ph 3 SiSBFL Current Density (mA/cm 2 ) Voltage (V) Figure 3.5 Current density - voltage (J-V) characteristics of Ph 3 CSBFL and Ph 3 SiSBFL undoped devices: ITO / NPD / mCP / SBFL Host / Alq 3 (at 6.9 V). On the other hand, the EQE for the Ph 3 SiSBFL device, starting from 7.4 % at a current density of 0.088 mA/cm 2 (at 5.5 V), shows no significant decrease until the current density reaches 100 mA/cm 2 at 12.3 V and then drops to 1.2 % at 15 V. The enhanced roll-off in EQE for the Ph 3 CSBFL devices could be due to triplet-triplet (T-T) annihilation caused by a hole injection barrier at the doped emissive layer. As already seen from the undoped Ph 3 CSBFL device, the charges recombine at, or near, the NPD/mCP interface. Thus, increasing current densities lead to a high concentration of excitons at the interface for the Ph 3 CSBFL device, and consequently, a high rate of T-T annihilation. 75 2468 10 12 14 20 40 60 80 100 120 140 FIrpic: Ph 3 CSBFL, 10% FIrpic: Ph 3 SiSBFL, 10% Ir(ppy) 3 : Ph 3 CSBFL, 9% Ir(ppy) 3 : Ph 3 SiSBFL, 9% Current Density (mA/cm 2 ) Voltage (V) (a) 024 68 10 12 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 FIrpic: Ph 3 CSBFL, 10% FIrpic: Ph 3 SiSBFL, 10% Ir(ppy) 3 : Ph 3 CSBFL, 9% Ir(ppy) 3 : Ph 3 SiSBFL, 9% Brightness (Cd/m 2 ) Voltage (V) (b) Figure 3.6 Comparison of (a) current density – voltage characteristic and (b) luminance – voltage characteristic of the FIrpic devices: ITO / NPD / mCP / FIrpic : SBFL Host (10%) / BCP and the Ir(ppy) 3 devices: ITO / CHATP / NPD / Ir(ppy) 3 : SBFL host (10%) / BAlq / Alq 3 . 76 3.3.2.3 Green phosphorescent devices. Devices doped with a green phosphor, Ir(ppy) 3 , were fabricated to further investigate differences in the transport properties and stability between these two materials. A different structure chosen for these devices incorporated a hole injection material hexacyano-hexaazatriphenylene (CHATP), 33 and a different hole blocking material, bis(2-methyl-8-quinolinolato-N,O)-(1,1'-biphenyl-4-olato)aluminum (BAlq): ITO / CHATP / NPD / Ir(ppy) 3 : SBFL Host / BAlq / Alq 3 . Previous work has shown that devices made using BAlq blocking layers provide longer lifetimes than those using BCP layers. 34,35 Devices were fabricated with each host material using Ir(ppy) 3 doping concentrations of 9 and 15%. EL spectra from a set of devices with 9% doping are shown in Fig. 3.7. 400 500 600 700 0.0 0.5 1.0 FIrpic: Ph 3 CSBFL, 10% FIrpic: Ph 3 SiSBFL, 10% Ir(ppy) 3 : Ph 3 CSBFL, 9% Ir(ppy) 3 : Ph 3 SiSBFL, 9% Ir(ppy) 3 Normalized Intensity (a.u.) Wavelength (nm) FIrpic Figure 3.7 Electroluminescence spectra of the FIrpic devices: ITO / NPD / mCP / FIrpic : SBFL Host (10%) / BCP and the Ir(ppy) 3 devices: ITO / CHATP / NPD / Ir(ppy) 3 : SBFL Host (10%) / BAlq / Alq 3 . 77 0.01 0.1 1 10 100 0.1 1 10 FIrpic: Ph 3 CSBFL, 10% FIrpic: Ph 3 SiSBFL, 10% Ir(ppy) 3 : Ph 3 CSBFL, 9% Ir(ppy) 3 : Ph 3 SiSBFL, 9% Quantum Efficiency (%) Current Density (mA/cm 2 ) Figure 3.8 Quantum efficiency vs. current density of the FIrpic devices: ITO / NPD / mCP / FIrpic : SBFL Host (10%) / BCP and the Ir(ppy) 3 devices: ITO / CHATP / NPD / Ir(ppy) 3 : SBFL Host (10%) / BAlq / Alq 3 . Both hosts enable good charge balance at the emissive layer leading to emission exclusively from Ir(ppy) 3 ( λ max = 511 nm). The J–L–V characteristics from devices doped at 9% are shown in Fig. 3.6, data from devices doped at 15% is shown in Fig. A2.7. Devices doped at 9% have both lower current and brightness than the 15% doped devices. OLEDs made using Ph 3 CSBFL, like those that were undoped and doped with FIrpic, give both lower current and brightness than the Ph 3 SiSBFL devices at an equivalent voltage. Correspondingly, devices made using Ph 3 CSBFL turn on at higher voltages (5.2 V, 9%; 4.0 V, 15%) than ones made with Ph 3 SiSBFL (3.9 V, 9%; 3.6 V, 15%). At 11 V and 9% doping, the Ph 3 CSBFL device only gives a current of 42.3 mA/cm 2 and a brightness of 683 cd/m 2 , but the Ph 3 SiSBFL device gives a current density of 85.7 mA/cm 2 and a brightness of 20300 cd/m 2 At 11 V and 15% doping, the Ph 3 CSBFL device only gives a 78 current of 130 mA/cm 2 and a brightness of 3800 cd/m 2 , but the Ph 3 SiSBFL device gives a current density of 190 mA/cm 2 and a brightness of 33000 cd/m 2 (Table 3.3 and Fig. A2.7). Unlike the blue PHOLEDs, these two green devices differ dramatically in their external efficiencies. The device using Ph 3 SiSBFL doped at 9% shows a maximum EQE of 9.6% (current density of 4.24 mA/cm 2 ), while the maximum EQE of the Ph 3 CSBFL device is only 0.5 % (Fig. 3.8). However, at 15% doping, the maximum EQE drops to 3.1% for Ph 3 SiSBFL device and does not significantly improve for the Ph 3 CSBFL device, giving a maximum value of 0.7% (Table 3.3 and Fig. A2.7c). The larger roll-off of the efficiency observed for the FIrpic (10% doped) device compared to the Ir(ppy) 3 (9% doped) devices could be due to a larger barrier for hole injection to FIrpic compared to that for Ir(ppy) 3 . Lifetime data for PHOLEDs using Ir(ppy) 3 has also been collected and is listed in Table 3.3. The lifetime data are given as the time (in hours) for the device to reach 80% of its initial brightness. The data for both the 9% and 15% doped devices suggest that Ph 3 SiSBFL devices have lifetimes an order of magnitude longer than the Ph 3 CSBFL devices. For example, the Ph 3 CSBFL 9% doped device has a lifetime of only 0.16 h, while that of the Ph 3 SiSBFL 9% device is 10.7 h. The data also indicate longer lifetimes for the devices with higher dopant concentrations. 3.4 Summary The two spirobifluorene compounds designed in this work have very similar chemical compositions. They also have similar crystal packing structures, electrochemical behavior and solution photophysics. Both compounds have large HOMO/LUMO gap (5.0 ± 0.2 eV) and high triplet energies (2.8 eV). However, the 79 photophysical properties of these two materials are very different in both crystalline and thin film solid state. Data from undoped devices suggest that electron transport in greater than hole transport in Ph 3 CSBFL, but the overall charge transport properties of Ph 3 SiSBFL are superior. As a result, better current conduction and higher brightness were observed for PHOLEDs using the Ph 3 SiSBFL host compared with corresponding devices utilizing Ph 3 CSBFL. Strong triplet-triplet annihilations occur in all Ph 3 CSBFL doped devices. However, a high external quantum efficiency 6.9% was still obtained for the FIrpic/Ph 3 CSBFL device at a lower voltage (5.5 V). The device lifetime studies using Ir(ppy) 3 phosphors have demonstrated that the Ph 3 SiSBFL devices have longer device lifetime than the Ph 3 CSBFL devices. Whereas dimethylfluorene materials have been reported previously to be poor hosts for blue FIrpic PHOLEDs, the spirobifluorene materials we report here have shown their properties in balancing charges in blue FIrpic devices, resulting in high external efficiencies of blue PHOLEDs. 80 Chapter 3 References 1. Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R., Journal of Applied Physics 2001, 90, 5048. 2. Kawamura, Y.; Goushi, K.; Brooks, J.; Brown, J. J.; Sasabe, H.; Adachi, C., Applied Physics Letters 2005, 86. 3. Baldo, M. A.; O'brian, D. F.; Thompson, M. E.; Forrest, S. R., Physical Review B 1999, 60, 14 422. 4. 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Lett. 2002, 81, 162. 83 Chapter 4: Synthesis and Characterization of Isomeric, Blue Iridium Complexes with Cyclometalating Rigid Ligands 4.1 Introduction A lot of researches has been focused on investigating iridium cyclometalated complexes in the last decade, due to their applications in photocatalysis, 1,2 photoelectrochemistry, sensitizations 3-7 as well as biological labeling. 8-10 Especially the intense phosphorescence from these complexes has attracted considerable attentions from the scientists, since this could be applied to phosphorescent organic light emitting diodes (PHOLEDs). 11-19 The strong spin-orbital coupling (SOC) of the heavy metal d 6 iridium complexes results in efficient intersystem crossing between the singlet excited state and the triplet excited state of these molecules, leading to strong phosphorescence. 20-23 Therefore, the iridium cyclometalated complexes can trap both singlet and triplet excitons and exhibit very high quantum efficiencies up to 100%. 24-26 A lot of iridium complexes have been discovered as emitters for PHOLEDs, with colors crossing the whole visible regions. 23,25 Nowadays, high efficiency and stable green and red PHOLED devices have been developed and are widely accepted in commercial applications. Only blue devices remain giving a poor device lifetime partly due to the unstability of the fluorinated blue phosphorescent emitters, such as iridium bis[(4,6-difluorophenyl)-pyridinato-N, C 2 ] picolinate (FIrpic). 27-30 Sivasubramaniam et al. has found fluorine cleavage of FIrpic in a degraded PHOLED device using HPLC coupled mass spectrometry. 31 Even though FIrpic could reach very high efficiency in the devices, it is still not a perfect blue emitter for 84 PHOLEDs. Therefore, researches for better Ir emitters have to be changed to investigate ligands with higher stability, such as non-fluorinated aromatic systems. Blue color tuning of iridium complexes has been found challenging when looking for blue phosphorescent emitters for PHOLEDs, mainly due to the limitation on modifying the cyclometalated ligands of these Iridium complexes, and the phosphorescent wavelength of the Iridium complexes are directly related to the phosphorescent wavelength its cyclometalated ligands. 2-Phenylpyridine (ppy) 32-35 and 1- phenylpyrazole (ppz) 15,36,37 are the most studied ligands for this purpose, because their emissions fall into the deep blue regions. Incorporating electron withdrawing groups fluoro onto the phenylpyridine (ppy) has successfully blue shifted the green emission of fac-iridium (III) tris(2-phenyl-pyridine) (Ir(ppy) 3 ) from 516 nm to 468 nm, leading to blue color of FIrpic. 27-30 fac-iridium (III) tris(1-phenyl-pyrazole) (Ir(ppz) 3 ) phosphoresces at 415 nm and show high quantum efficiency at low temperatures. 13 However, this molecule cannot be used as blue emitters for OLED, since its emissive wavelength is not totally in the visible region and it does not emit at room temperature at all. The low quantum efficiency of this molecule is from the quenching of a non-radiative state that is in a similar energy level of its radiative state. To solve this problem, there have been researches on extending conjugation system ppz ligand to slightly red shift its emission and to lower the level of its non radiative state from the radiative state. One of the examples is fac-iridium (III) tris(1-[2-(9,9’-dimethylfluorenyl)]pyrazolyl-N, C 3’ ) (fac-FlzIr), in which a dimethylfluorenyl was incorporated onto the pyrazole in the molecule. This leads to a strong red shift of the emission from near UV (415 nm) to greenish blue (480 nm). This 85 molecule has shown high quantum yields at room temperature. However, its emission could be easily quenched by oxygen and its LUMO level is too high to trap electrons in PHOLEDs. G. Treboux et. al 38 has shown that the excited Ir(ppz) 3 molecule could experience Ir-N bond breaking and this bond breaking is related to the non-radiative state of this molecule. The force of the Ir-N bond could be from the flexibility of the N-C bond in the ppz ligand which could rotate in a high energy environment. 36,38 So another approach to prevent the quenching from the non-radiative state could be increase the rigidity of the ligand and thus increase the efficiency of the Ir(ppz) 3 based complexes. The iridium complexes discussed above such as Ir(ppy) 3 , Ir(ppz) 3 and FlzIr are all six coordinated homoleptic complexes, in which all three ligands are identical, 11,13,39 except that FIrpic is a heteroleptic complex with two difluoro ppy ligands and one picolinate ligand. 27-29 Illustrations of the isomeric structures of 6-coordinated Homoleptic and heteroleptic cyclometalated Ir Complexes are shown in Scheme 4.1. To date, the synthesis and studies of homoleptic complex isomers have been well demonstrated and the facial isomer is the most common isomer used to fabricate PHOLEDs due to its high quantum efficiency. It has also been found that the facial isomer is the lowest energy isomer and it is stable in PHOLED devices without isomerization. 13 In contrast, the heteroleptic complexes for OLED applications always contain trans- geometries of the primary ligands as the structure of FIrpic. Other isomers of these heteroleptic complexes has never been reported or studied, when they were widely used to fabricate 86 Scheme 4.1 Illustration of the isomeric structures of 6-coordinated Homoleptic and heteroleptic cyclometalated Ir Complexes PHOLEDs. The relative stabilities of these isomers are still unknown. The possible isomerization of these heteroleptic complexes have never been considered until very recently, Baranoff et al. reported the isomerization of a ppy based heteroleptic complex during OLED fabrication. 40,41 In this work, we are demonstrating the synthesis of a ppz based blue ligand tpzp. This ligand shows blue phosphorescence at 433 nm and possesses a more rigid structure than ppz. Both facial homoleptic complex Ir(tpzp) 3 and picolinate heteroleptic complex Ir(tpzp) 2 pic have been synthesized and investigated. We achieved much higher quantum efficiency for the homoleptic complex Ir(tpzp) 3 than Ir(ppz) 3 . We first time report the synthesis, DFT calculation and full characterization of the three isomers of the Homoleptic Complexes N C N C N Ir C N C N N C Ir C mer- fac- Heteroleptic Complexes N O N C N Ir C N O N N C Ir C N N O C N Ir C C O N C N Ir N trans-mer cis-fac cis-mer trans-C 87 heteroleptic complex Ir(tpzp)pic. Their coordination geometries, electrochemical behaviors and photophysical properties have been well demonstrated. We also found that the blue isomer could isomerize into the two green isomers during sublimation. 4.2 Experimental 4.2.1 General. All chemicals and solvents were purchased from commercial sources without further purification except for Pd(OAc) 2 was initially recrystallized. 1 H NMR spectra were measured on Mercury 400 MHz instrument, and chemical shifts were referenced to a deuterated solvent. Elemental analyses (CHN) were performed at the Microanalysis Laboratory at the University of Illinois, Urbana- Champaign, IL. 1-(2-bromophenyl)-3-(dimethylamino)- 2-Propen-1-one (1). To a solution of 1- (2-bromo-phenyl)-ethanone (25 g, 0.126 mol) of in 250 ml toluene, DMFDMA (18.71 g , 0.157 mol) of was added dropwise under nitrogen. The reaction mixture was refluxed for 16h. The solution was allowed to cool down to the room temperature and the solvent was removed by rotary evaporation. Silica gel chromatography using hexanes and ethyl acetate (1:1) was performed to give 29.9 g pure products. Yield: 94 %. 5-(2-Bromo-phenyl)-1-(4-tert-butyl-phenyl)-1H-pyrazole (2). 1 (25 g, 0.098 mol) and Na 2 CO 3 (7.79 g, 0.074 mol) were dissolved in a mixture of 300 ml MeOH and 1.4 L of water. 4-tert-butyl-phenyl-hydrazine (23.68 g, 0.144 mol) was later added dropwise under nitrogen atmosphere. Acidity of the solution was adjusted by acetic acid to pH = 4, and the solution was heated up to 120 °C for 6 h. After the solution was cooled down to the room temperature, the solvent was reduced by reduced pressure to half volume before 88 CH 2 Cl 2 was used to extract the organics. The organic layer was washed by brine and dried over MgSO 4 , and the organic solvents were removed by rotary evaporation. Silica gel column with hexane and ethyl acetate (4:1) was used to purify the product to give 21.38 g of white solids. Yield: 86%. 9-t-butyl-pyrazolo[1,5-f]phenanthridine (tpzp) 42,43 DMF (250 ml) was bubble degassed using nitrogen for 20 mins and was added 2 (10.5 g, 0.03 mol), Pd(OAc) 2 (1.35 g, 0.006mol), K 2 CO 3 (41.46 g, 0.30 mol), LiCl (3.82 g, 0.09 mol), and nBu 4 NBr (19.34 g, 0.06 mol) under nitrogen. The reaction mixture was heated up to 120 °C and refluxed overnight under nitrogen. Water and DCM (4:1) were used to extract the organics from the mixture. The organic layer was washed by brine and dried over MgSO 4 . After the solvent was removed by reduced pressure, silica gel column chromatography with hexanes and ethyl acetate (1:15) was used to purify the product to give 3.8 g of white solids. Yield: 46%. [(tpzp) 2 Ir(µ-Cl) 2 Ir[(tpzp) 2 ] The tpzp ligand (2.274 g, 8.29 mmol) and IrCl 3 (1.125 g, 3.77 mmol) were added into a solvent mixture of 75 ml 2-ethoxyethanol and 25 ml H 2 O. The reaction mixture was refluxed at 100 °C under nitrogen overnight. The reaction mixture was allowed to cool to the room temperature and the solvents were removed by rotary evaporation. The organic layer was extracted by CH 2 Cl 2 , washed by brine and dried over MgSO 4 . The resulted solution was evaporated to dryness to obtain crude product of the Ir dichloro bridged dimer [(tpzp) 2 Ir(µ-Cl) 2 Ir[(tpzp) 2 ] . Hexane and CH 2 Cl 2 were used for recrystallization to give 2.44 g of yellow crystals. Yield: 76 %. Iridium tris[9-tbutyl-pyrazolo[1,5-f]phenanthridine] (Ir(tpzp) 3 ) [(tpzp) 2 Ir(µ- Cl) 2 Ir[(tpzp) 2 ] (0.526 g, 0.34 mmol) , tpzp (0.233 g, 0.85 mmol) and AgOTf (0.437, 1.7 89 mmol) were transferred to a 150 ml round flask containing 80 ml of o-dichlorobenzene. The solution was then heated up to 130 °C in the dark under stirring and N 2 atmosphere. The reaction mixture was refluxed for 16 h, and then allowed to cool down to room temperature. The solvent volume was reduced to about 1/8 by vacuum distillation. The crude product was suspended in hexane/DCM (1: 1) and filtered in Cefax funnel with celite, using DCM as the eluent. After the solvent was removed by rotary evaporation, the product was purified by silica gel column chromatography using DCM as eluent to give 0.688 g of yellow solids as the pure product. Yield: 50% Iridium bis[9-tbutyl-pyrazolo[1,5-f]phenanthridine]picolinate (I1, I2 and I3). [(tpzp) 2 Ir(µ-Cl) 2 Ir[(tpzp) 2 ] (1.549 g, 1 mmol) and picolinic acid (0.369 g, 3 mmol) were added into 1, 2-dichloroethane (120 ml) and refluxed overnight under nitrogen. CH 2 Cl 2 and water were used to wash the mixture, and the organics were collected and washed with brine and dried over MgSO 4 . Pure ethyl acetate was used for silica gel chromatography to give yellow solids of pure I1 (64%), I2 (7%) and I3 (9%). I3 can also be prepared from the isomerizaion of I1, described as follows: I1 (0.10 g, 0.116 mmol) was placed in a high vacuum chamber and heated for 24 h at 300 °C. After the solid mixtures were cooled down to the room temperature, pure ethyl acetate was used to isolate the product to give 35 mg of pure I3. Yield: 35%. The single crystals of I1 and I3 were grown by dissolving 20 mg of each compound into 1 ml dichloromethane and 2 ml of MeOH. The solutions were filtered and partly opened to the atmosphere for 3-7 days to obtain single crystals for the X-ray crystallography measurement. 90 The 1 H NMR, 13 C NMR, Mass spectra and the elemental analysis data of the complexes are as follows tpzp: 1 H NMR (CDCl 3 , 400Hz) δ (ppm) EI-MS(m/z): Anal. Calcd for C 19 H 18 N 2 : C, 83.18; H, 6.61; N, 10.21. Found: C, 83.92; H, 6.45; N, 10.18. (tpzp) 2 (µ-Cl)Ir(tpzp) 2 1 H NMR (CDCl 3 , 400Hz) δ ) 13 C NMR (CDCl 3 , 400 Hz) δ (ppm) Maldi-MS(m/z): Anal. Calcd for C 76 H 68 Cl 2 IrN 8 ·3H 2 O: C, 56.95; H, 4.65; N, 6.99. Found: C, 56.90; H, 4.19; N, 6.95. Ir(tpzp) 3 : 1 H NMR (CDCl 3 , 400Hz) δ (ppm) 8.50 (d, J = 8.4 Hz, 1H), 8.04 (d, J = 9.0 Hz, 1H), 7.81 (s, J =1.7 Hz, 1H),7.61(dd, J = 7.2 Hz 1H) ,7.52 (dd, J = 7.8 Hz 1H), 7.38 (d, J = 2.3 Hz, 1H), 7.20 (s, J =1.7 Hz, 1H), 6.87 (d, J = 2.3 Hz, 1H), 1.24 (s,9H) 13 C NMR (CDCl 3 , 400 Hz) δ (ppm) 148.11, 137.52, 136.67, 136.19, 135.14, 134.79, 130.76, 128.18, 126.86, 124.59, 123.56, 123.37, 117.74, 111.31, 97.83, 35.09, 31.98. Maldi-MS(m/z): 1012.9 (M+1). Anal. Calcd for C 57 H 51 IrN 6 : C, 67.74; H, 5.39; N, 8.17. Found: C, 67.74; H, 5.52; N, 7.73. I1: 1 H NMR (CDCl 3 , 400Hz) δ (ppm) 8.48 (d, J = 7.8 Hz, 1H), 8.45 (d, 7.7 Hz, 1H), 8.34 (d, 7.4 Hz, 1H), 8.19 (s, J = 2.4 Hz, 1H), 8.17 (d, J = 7.9 Hz, 1H), 8.14 (d, J = 7.9 Hz 1H), 8.00 (d, J = 5.3 Hz 1H), 7.87 (dd, J = 7.7 Hz 1H), 7.78 (s, J = 1.6 Hz 1H),7.73 (s, J = 1.6 Hz 1H), 7.63 (m, 4H), 7.28 (dd, J = 5.5 Hz 1H), 7.24 (s, J = 2.5 Hz 1H),7.12 (s, J = 2.5 Hz 1H), 7.02 (s, J = 2.5 Hz 1H),6.53 (s, J = 1.6 Hz 1H), 6.45 (s, J = 1.6 Hz 1H), 1.19 (s, 9H), 1.14 (s, 9H). 13 C NMR (CDCl 3 , 400 Hz) δ (ppm) 173.42, 154.56, 150.37, 148.84, 148.70, 139.45, 138.29, 137.08, 136.94, 136.44, 136.17, 135.30, 131.65, 130.55, 130.26, 130.03, 128.96, 128.67, 128.02, 127.72, 127.49, 127.28, 125.06, 124.78, 124.20, 123.66, 123.44, 123.17, 123.09, 118.30, 118.16, 113.22, 113.09, 98.53, 91 97.96, 35.27, 34.98, 32.00, 31.63. LC-MS (m/z): 862.5 (M+1). Anal. Calcd for C 44 H 38 IrN 5 O 2 ·1.5H 2 O: C, 59.51; H, 4.65; N, 7.89. Found: C, 59.18; H, 4.14; N, 7.79. I2: 1 H NMR (CDCl 3 , 400Hz) δ (ppm) 8.59 (d, J = 7.9 Hz, 1H), 8.46 (d, 7.4 Hz, 1H), 8.21 (d, 7.4 Hz, 1H), 8.13 (d, J = 7.6 Hz, 1H), 8.02 (d, J = 6.9 Hz, 1H), 8.00 (d, J = 1.7 Hz 1H), 7.88 (d, J =7.7 Hz 1H), 7.88 (s, J = 1.7 Hz 1H), 7.78 (d, J = 5.2 Hz 1H),7.72 (s, J = 1.7 Hz 1H), 7.68 (dd, 1H), 7.61 (m,3H), 7.40 (s, J = 2.4 Hz 1H),7.28 (s, J = 6.2 Hz 1H), 7.13 (s, J = 2.5 Hz 1H),7.03 (s, J = 2.4 Hz 1H), 6.78 (s, J = 2.5 Hz 1H), 6.67 (s, J = 1.7 Hz 1H)1.51 (s, 9H), 1.07 (s, 9H). 13 C NMR (CDCl 3 , 400 Hz) δ (ppm) 175.91, 151.26, 149.72, 147.82, 147.11, 138.12, 137.87, 137.46, 137.22, 136.64, 136.63, 136.46, 136.05, 132.31, 132.15, 130.87, 130.67, 128.97, 128.75, 127.99, 127.81, 127.40, 127.17, 126.51, 124.91, 124.75, 123.80, 123.45, 122.95, 118.18, 117.67, 113.51, 112.63, 98.45, 98.38, 35.74, 34.93, 32.20, 31.68. LC-MS(m/z): 862.5 (M+1) Anal. Calcd for C 44 H 38 IrN 5 O 2 ·H 2 O: C, 60.11; H, 4.58; N, 7.97. Found: C, 59.57; H, 4.40; N, 7.70. I3: 1 H NMR (CDCl 3 , 400Hz) δ (ppm) 8.58 (d, J = 8.0 Hz, 1H), 8.52 (d, 8.7 Hz, 1H), 8.30 (s, 2.4 Hz, 1H), 8.24 (d, J = 7.8 Hz, 1H), 8.20 (d, J = 8.0 Hz, 1H), 8.07 (d, J = 5.6 Hz 1H), 8.00 (d, J =7.8 Hz 1H), 8.00 (s, J =1.6 Hz 1H), 7.89 (dd, J = 7.7 Hz 1H), 7.81 (s, J = 1.7 Hz 1H),7.68 (m, 3H), 7.57 (dd, J=7.0 Hz 1H), 7.19 (dd, J=6.5 Hz 1H), 7.19 (s, J = 2.4 Hz 1H),7.08 (s, J = 2.5 Hz 1H), 6.76 (s, J = 1.7 Hz 1H),6.72 (s, J = 2.5 Hz 1H), 6.64 (s, J = 1.6 Hz 1H),1.38 (s, 9H), 1.14 (s, 9H). 13 C NMR (CDCl 3 , 400 Hz) δ (ppm) 173.42, 154.56, 150.37, 148.84, 148.70, 139.45, 138.29, 137.08, 136.94, 136.44, 92 136.17, 135.30, 131.65, 130.55, 130.26, 130.03, 128.96, 128.67, 128.02, 127.72, 127.49, 127.28, 125.06, 124.78, 124.20, 123.66, 123.44, 123.17, 123.09, 118.30, 118.16, 113.22, 113.09, 98.53, 97.96, 35.27, 34.98, 32.00, 31.63. LC-MS (m/z): 862.5 (M+1) Anal. Calcd for C 44 H 38 IrN 5 O 2 ·H 2 O ·1.5C 6 H 14 : C: 63.13 H 6.10 N 6.95. Found: C, 63.33; H, 5.91; N, 6.60. 4.2.2 Electrochemistry and Photophysics. Cyclic voltammetry and differential pulse voltammetry were performed using an EG&Gpotentiostat/galvanostat model 283 under N 2 atmosphere. Anhydrous DMF was used as the solvent for the scan from –3.1 to 1.8 Vs to detect both of the oxidation and reduction signals. 0.1 M Tetra(n-butyl) ammonium hexafluorophosphate (TBAH) was used as the supporting electrolyte. A glassy carbon rod, a platinum wire and a silver wire were used as the working electrode, the counter electrode and the pseudo reference electrode. The redox potentials are calculated relative to an internal reference ferrocenium/ferrocene (Cp 2 Fe+/Cp 2 Fe). The UV-visible spectra were measured in CH 2 Cl 2 by a Hewlett-Packard 4853 diode array spectrometer. Singlet and triplet emission measurements were performed on a Photon Technology International (PTI) QuantaMaster model C-60 fluorimeter at room temperature and 77 K, respectively. A dilute solution was prepared using toluene for measuring the quantum yields in solutions. All the solutions were bubble degassed for 5 min before the measurement. For the thin film quantum yield measurements, 2 mg of each compound and 100 mg of PMMA were dissolved in 1 ml of toluene and sonicated for 1 h. This solution was used for spin casting to obtain doped films of these complexes. The quantum yields were measured using an absolute method 93 on a Hamamatsu C9920 system equipped with a xenon lamp, calibrated integrating sphere and model C10027 photonic multichannel analyzer. All phosphorescent lifetimes at room temperature were measured time-correlated single-photon counting using an IBH Fluorocube instrument equipped with a 405 nm LED excitation source. Lifetimes at 77K were measured on a PTI Quantaaster model C-60 fluorimeter. 4.2.3 X-ray crystallography. X-ray quality crystals were grown as indicated in the experimental procedures for each complex, and the crystals were mounted on a glass fiber with Paratone-N oil. X-ray diffraction data were collected on a Bruker SMART APEX diffractometer using graphite- monochromated Mo K α radiation, and structures were determined using direct methods with standard Fourier techniques using the Bruker AXS software package. In some cases, Patterson maps were used in place of the direct methods procedure. Hydrogen positions were input and refined in a riding manner along with the attached carbons. The summary of the refinement details and the agreement factors for the three isomers are given in Table A3.20 and Table A3.21. 4.2.4 Sublimation. 100 mg of I1 were placed in a high vacuum gratitude sublimator and heated up for 24 hrs. The temperatures of the sublimator were set as 300 °C (Zone 1), 240 °C (Zone 2) and 220 °C (Zone 3), subsequently. The unsublimed (Zone 1) and sublimed compounds (Zone 2) were both analyzed using 1 H NMR and HPLC-MS. The ratios of the isomers were calculated by integrating the peak areas of the tert-butyl group signals from the proton NMR spectra and the peaks found from HPLC chromatogram. 94 N N N N Ir 3 N N Ir 3 N Ir F F ppz ppy Ir(ppz) 3 Ir(ppy) 3 N N 3 N N Ir O O N 2 O O N N N Ir 3 tpzp FIrpic Ir(tpzp) 3 Ir(tpzp) 2 pic Scheme 4.2 Illustration of the isomeric structure of 6-coordinated homoleptic and heteroleptic cyclometalated Ir complexes 4.3 Result and Discussion 4.3.1 Design and Synthesis of the Ligand and the complexes. The chemical structure of tpzp is shown in Scheme 4.2. It was chosen as the ligand of the iridium complexes because of following considerations: 1) tpzp is expected to be a blue emitter according to our DFT calculations and it’s a derivative of ppz and also an analogue of triphenylene. Both ppz and triphenylene show blue phosphorescent 95 emissions; 2) tpzp is a rigid ligand with a new phenyl group connected to both the pyrazole and the phenyl on the ppz. So there is no rotation allowed for the ppz C-N bond, which is expected to prevent the formation of the non-radiative state of its iridium complexes; 3) the tert- butyl group is incorporated because it could prevent the self aggregation of the molecules in the solid state or in a doped film. The picolinate was chosen as an auxiliary ligand because its triplet energy is high enough and the ligand center emission of the tpzp will not be quenched. Iridium picolinate complexes such as FIrpic have shown very high quantum yields and have been widely used as blue PHOLED emitter. tpzp was synthesized according to a literature method as shown in Scheme 4.3. 42,43 A Vilsmeier-Haack-type reagent dimethylformamide dimethyl acetal (DMFDMA) was used to transform the 2'-Bromoacetophenone into the enaminoketone (1), and the enaminoketone was latter reacted with the hydrazine to form the diarylpyrazole (2) by a tandem amine heterocyclization. A Heck reaction was used in the last step coupling reaction to afford the pure 9-tert-butyl-pyrazolo[1,5-f]phenanthridine. The overall yield of the 3 step synthesis is 37%. This synthesis of the complexes follows a standard procedure described previously. Iridium dichloro-bridged dimer [(tpzp) 2 Ir(µ- Cl) 2 Ir[(tpzp) 2 ] was first prepared from IrCl 3 and tpzp with a yield of 76%. Interestingly, the Ir chloro-bridged dimer we obtained is not the common trans N –trans N isomer. Instead, since the four tpzp ligands in this isomer are all in different chemical environment and four sets of 1 H NMR signals were observed (Fig. A3.1 in the supporting 96 O Br O Br O Br N N N N Br tpzp Pd(OAc) 2 , K 2 CO 3 LiCl, nBu 4 NBr DMF, N 2 , 120 C O Br O Br N Na 2 CO 3 DMFDMA MeOH/H 2 O NH H 2 N 1 2 ° Scheme 4.3 Synthesis of tpzp information), this isomer should have a cis –trans conformation. The homoleptic complex Ir(tpzp) 3 was made by refluxing [(tpzp) 2 Ir(µ-Cl) 2 Ir[(tpzp) 2 ] and 3 eq. tpzp with AgOTf under 150 °C to give a 50% yield of this molecule. The heteroleptic complex Ir(tpzp) 2 pic was synthesized by refluxing [(tpzp) 2 Ir(µ-Cl) 2 Ir[(tpzp) 2 ] with 3 eq. picolinic acid to afford the isomers with 65%(I1), 7% (I2) and 9%(I3) yield, respectively. I3 could be also prepared by the isomerizaiton of I1 under vacuum at 320 C for 36 h. In this isomerization reaction, most of I1 converts to I3 giving a final ratio of I1:I3 = 1:2. The isolation yield is 35%. 97 Figure 4.1 1 HNMR spectra of Ir(tpzp) 2 pic I1, I2 and I3 98 4.3.2 NMR Characterization. Fig. 4.1 shows the 1D 1 H NMR spectra of Ir(tpzp) 3 compared with the Ir(tpzp) 2 pic isomers. Fig. A3.3 and Fig. A3.4 show the 2D COSY and 2D NOESY spectra of this complex. The numbering of the protons was shown along with the 2D spectra. The two pyrazole proton signals, H a and H b , can be easily assigned from the small coupling constants of pyrazole doublets (J = 2.3 Hz), compared to the values from the phenyls (J= 7.0 -9.0 Hz). In the 2D COSY spectrum, a clear correlation between these two protons was observed. There are two singlets, H g and H h , from the phenyl group attached to the Ir center, which are located at 7.2 ppm and 7.8 ppm, respectively. Even though they are not adjacent to each other, a weak correlation was still observed from the COSY spectrum. The two triplets from the outer phenyl ring, H d and H e , are located in the range from 7.4 ppm to 7.6 ppm, while the two doublets, H c and H f , are located at 8.0 and 8.5 ppm, respectively. Strong H c -H d and H e -H f correlations were observed in the 2D COSY spectrum. Strong through space couplings H b -H c , H f -H g , H f -H i , and H g -H i were observed from the 2D NOESY spectrum as shown in Fig. A3.4. Fig. 4.1 also shows the 1D 1 H NMR spectra of the Ir(tpzp) 2 pic isomers. Fig. A3.5- A3.10 show the 2D COSY and 2D NOESY spectra of these complexes. In the heteroleptic complexes, the chemical shifts of each proton on the tpzp are in a similar region to the homoleptic complex. However, due to the incorporation of picolinate, the chemical shifts of the protons from the phenyl pyrazoles are strongly affected and two pairs of the resonances from the tpzp ligands were observed. These resonances, especially the tbutyl and the pyrazole signals, are unique and distinguishable in the spectra. This provides evidence to identify these isomers using NMR spectroscopy. Since the outer 99 phenyl rings from the tpzp ligand sit farther from the Ir center and the picolinate, its proton resonances (H c , H d , H e and H f ) are not strongly influenced. 2D COSY and 2D NOESY proton NMR experiments allow us to fully assign the proton resonances of all these complexes as show in Fig. A3.5-A3.10. Information of through bond couplings was obtained from the 2D COSY experiments, so the resonance was easily identified for each aromatic group in the molecules. 2D NOESY experiment provides through space coupling information, so it contains geometry information of molecules. Since the two tpzp ligands are trans- to each other in I1, through-space couplings between the pyrazole proton and the inner phenyl proton (H a’ -H h and H a - H h’ ) were observed (Fig. A3.6). The through-space couplings between H a’ or H h with the pyridine proton H a’’ were also observed. In contrast, the two tpzp ligands in I2 and I3 are cis- to each other, so through- space couplings were seen between the protons from the two pyrazoles (H a -H a’ ) and the two inner phenyls (H h - H h’ ). (Fig. A3.8 and A3.10) Since the orientation and of the picolinate groups are different in these two compounds. Pyrazole-pyrozole-pyridine (Ha- H a’ -H a’’ ) through space couplings were observed for I2, while phenyl-phenyl-pyridine (H h -H h’ -H a’’ ) through space coupling were observed for I3. Other through space couplings, such as H b -H c coupling, were also observed similar to the homoleptic complex, these signals allow us to fully assign all the protons for all the complexes. 100 Figure 4.2 Thermal ellipsoid (ORTEP) plots of Ir(tpzp) 3 (top), Ir(tpzp) 2 pic I1 (middle) and I3 (bottom). The atoms were colored by green (Ir), back (C), blue (N) and red (O). The hydrogen atoms were omitted for clarity 101 4.3.3 X-ray crystallography. Fig. 4.2 shows the crystal structures of I1 and I3. I1 is an isomer in which the two tpzp nitrogens are in the trans- positions. Both oxygen and nitrogen from the picolinate are in the trans-position of the phenyl groups from the tpzp ligands. Different from I1, the two tpzp ligands in I3 are in the cis-position to each other, while the pyridine nitrogen is trans- to one of the tpzp pyrazoles, making it different from I2. In I2, the pyridine nitrogen is trans- to one of the tpzp phenyls. In both of these two crystal structures, ligands that are trans- to phenyls always have a longer coordination bond to the Ir center. Table 4.3 list the distances determined by X-ray diffraction, for example, in the structure of I1, the bond length of Ir1-N3 is 2.104(6) A and that of Ir1-O1 is 2.126(6)A. These two values are significantly larger than other bond lengths which are ranging from 1.992(8) A to 2.065(8) A. Similarly, in I3, Ir1-N2 and Ir1-O1 are both trans- to phenyl groups, and their bond lengths are 2.099(10) A and 2.16 A, respectively. Results from the structure refinement for I1 and I3 are shown in Table A3.1 and Table A3.2, respectively. Due the poor morphology of I2 crystals, we were not able to obtain X-ray crystallographic data for this isomer. However, its coordination geometry has been confirmed by 2D NMR experiments. 4.3.4 DFT calculation. Density Functional Theory calculation 44,45 has been performed to predict the geometries and the energy levels of these molecules. Table 3.2 lists the relative energies of the homoleptic complexes and the four heteroleptic complexes (I1, I2, I3 and trans-C). 102 Table 4.1 Selected bond distance (Å) for Ir(tpzp) 2 pic (I1 and I3). The numbering of the atoms is shown in Fig. 4.3. Bond Distances (Å) Ir(tpzp) 3 I1 I3 bond calculated a measured b calculated a measured b calculated a measured b Ir1-N1 2.194 2.164(7) 2.06 2.065(8) 2.07 2.017(4) Ir1-N2 2.191 2.126(4) 2.06 1.992(8) 2.19 2.099(10) Ir1-N3 2.191 2.110(8) 2.19 2.104(6) 2.06 1.998(5) Ir1-C1 2.040 2.024(7) 2.04 2.046(9) 2.03 2.018(11) Ir1-C2 2.039 2.003(7) 2.05 2.034(9) 2.05 2.053(4) Ir1-O1(C3) 2.041 2.013(8) 2.16 2.126(6) 2.15 2.161(8) a DFT calculated values. b X-ray diffraction data The minimized energies of the molecules provide information of the relative stability of these molecules. As it was reported, the facial isomer of Ir(ppz) 3 is lower in energy than the meridional isomer. 13 This is because the phenyl group is a stronger trans-directing group and phenyl-phenyl trans- geometry is less stable compared to phenyl-pyrazole or pyrazole –pyrazole trans- geometries. Similarly, the trans-carbon isomer of the Ir(tpzp) 2 pic is theoretically the least stable isomer. Our DFT calculations of all these isomers also show that the energy of the trans- carbon isomer is much higher than the other three isomers, and it was not observed from either reactions or isomerizations from our studies. The minimized energies of these four isomers are given 103 in Table 4.2, showing that I3 is lowest in energy, and I1 is 0.5145 × 10 -20 J higher, I2 is 1.6132 × 10 -20 J higher and the trans- carbon isomer is 7.6823× 10 -20 J higher, giving a stability order of I3 > I1> I2 > trans-C. Table 4.2 DFT calculated energies and HOMO/LUMO levels of Ir(tpzp) 3 and the isomers of Ir(tpzp) 2 pic Energy a (10 -20 J) HOMO (eV) LUMO (eV) Ir(tpzp) 3 - -5.060 -0.935 I1 0.5145 -5.152 -1.450 I2 1.6132 -5.218 -1.535 I3 0 -5.238 -1.558 trans C 7.6823 -5.303 -1.589 a relative to I3 ig. A3.11 shows the calculated geometries and the HOMO and LUMO orbital pictures of these molecules. The LUMO orbitals of the Ir(tpzp) 3 are localized on all three tpzp ligands, while the HOMO orbitals are localized partly on the Ir center and the ligands. Similar to the homoleptic complex, the HOMO orbitals of all three Ir(tpzp) 2 pic isomers localized mainly on the Ir center and the tpzp groups, and slightly on the picolinate group. Unlike the homolepitc complex, the LUMO orbitals are only localized on the picolinate groups, which is consistent with electrochemistry results in which the first reduction of the complexes is due to the picolinate (see Fig. 4.3). The LUMO / HOMO levels calculated for these complexes are also listed in Table 4.2. The HOMO levels of Ir(tpzp) 3 and all the Ir(tpzp) 2 pic isomers are very close,ranging from -5.06- -5.30 104 eV. However, the LUMO level (– 0.936 eV) of the homoleptic complex is much higher than the heteroleptic complex, which is due to the tpzp ligand. As mentioned above the LUMO orbitals of the heteroleptic complexes are all localized on the picolinate group, which is a better electron acceptor. Therefore, the calculated LUMO levels of these complexes are all around -1.5 eV , with slight down shift from I1 to the trans-C isomer. 4.3.5 Electrochemistry. The electrochemical behaviors of the tpzp based Iridium complexes have been investigated using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The CV redox couples of tpzp, Ir(tpzp) 3 and I1 are shown in Fig. A3.12, A3.13 and A3.14. The electrochemistry data of the ligand and the DPV curves of all the complexes are shown in Fig. A3.12-3.16. The oxidations of Ir(tpzp) 3 , I1, I2 and I3 are all reversible. The reductions of the heterolepitc complexes are reversible, however the reductions of the Ir(tpzp) 3 are not reversible. The redox potentials of the ligand and the complexes are listed in Table 4.1. An oxidation wave at 0.55 V was observed for tpzp, however, the return wave was not detected by either CV or DPV. A strong reduction wave at -2.9 V was observed by both CV and DPV for this molecule and the return wave with a similar intensity was detected by DPV as shown in Fig. A3.12. 105 LUMO orbitals Ir(tpzp) 3 Ir(tpzp) 2 pic (I1) -4 -3 -2 -1 0 1 I2 I3 Voltage vs. Cp 2 Fe/Cp 2 Fe + (V) I1 Current Ferrocene -3.03 -2.97 -2.93 -2.39 -2.33 0.67 0.60 0.60 -2.41 0.34 -2.72 Ir(tpzp) 3 Figure 4.3 DFT calculated LUMO orbitals of Ir(tpzp) 3 ( top-left) and Ir(tpzp) 2 pic (I1) (top-left) and Differential pulse voltammetry (DPV) curves of Ir(tpzp) 3 and Ir(tpzp) 2 pic (I1, I2 and I3) (bottom) 106 Table 4.3 Electrochemical and photophysical properties of tpzp, Ir(tpzp) 3 and Ir(tpzp) 2 pic (I1, I2 and I3) E ox a (V) E red a (V) τ solution 77K b ( μs) Φ PL, 298 K c Φ PL,PMMA 298K d λ PL 77K b (nm) λ PL, 298 K c (nm) tpzp 0.55 -2.90 - - - 433 - Ir(tpzp) 3 0.34 -2.72, -2.90 37 0.05 0.53 452, 482 452 I1 0.60 -2.41, -2.97, -3.13 30 0.07 0.12 452, 482 522 I2 0.60 -2.33, -2.93, -3.07 11 0.005 0.02 482, 514 530 I3 0.67 -2.39, -3.03, -3.16 10 0.015 0.11 452, 520 544 a measured in DMF relative to ferrocene/ferrocium; b measured in 2-MeTHF; c measured in toluene; d 2wt% in PMMA film Multi-reductions have been observed for Iridium cyclometalated complexes. These reductions come from the individual ligands of these complexes. The fac-Ir(tpzp) 3 exhibits the first reduction at -2.7 V and the second reduction at -2.9 V (Fig. 4.3). The slightly lower reduction potential over the free ligand suggests that the metalation could lower the reduction potential of the ligand. The electrochemistry data suggest that the Ir(tpzp) 3 has lower LUMO than both of the fac-Ir(ppz) 3 and fac-FlzIr. The reduction potential of fac-FlzIr has been reported to be -3.1 V and that of fac-Ir(ppz) 3 has never been reported and is believed to be appearing beyond the solvent signal ( < -3.3 V). The reduction potential of Ir(tpzp) 3 is 0.4 V lower making it a better electron trapping molecule. Similar to the Ir(ppz) 2 pic, the Ir(tpzp) 2 pic (I1, I2 and I3) show the first reduction at -2.3 to -2.4 V. This reduction potential is attributed to the picolinate on the 107 molecule. After the first reduction happened on the picolinate, the resulted anion becomes harder to be reduced. This results in the enhancement of the reductions (-2.9 to -3.0 V) of the tpzp ligands on the heteroleptic complexes, compared to that from the homoleptic complex (-2.7 V) 4.3.6 Electronic Spectra and Emission Spectra. The absorption and emission spectra of the ligands and these complexes are shown in Fig.4.4, and A3.17. The intense bands from 230 nm to 300 nm are assigned to be the 1( π- π) transition. The features of the Ir(tpzp) 3 absorption is very similar to the ligand absorptions, while in the heterolepitc complexes, there are contributions from picolinates leading to different features in this absorption region. The bands from 320 nm to the 400 are attributed to the metal to ligand charge transfers (MLCT). The strong MLCT bands observed for all these complexes come from the strong spin orbital couplings of the Iridium center. 46 The emissions of Ir(tpzp) 3 , I1, I2 and I3 in solution are shown in Fig. 4. 4. Ir(tpzp) 3 shows blue phosphorescence at 450 nm. However, no emissions in the region was observed for the heterolepitc complexes, instead they show strong broad band emissions at about 530 nm. These emissions red shift slightly from I1 (522nm) to I2 (530nm) and I3 (544 nm). This band has been observed for picolinate or acetilacetonate heteroleptic complexes such as Ir(dfppy) 2 (pic) or Ir(dfppy) 2 (acac) 47,48 and it was assigned to the ligand to ligand charge transfer (LLCT) emissions. This could be explained from the relatively lower LUMO levels of the picolinates in the complexes which facilitate the charge transfer from the tpzp ligand to the picolinate ligand. The 108 quantum efficiencies measured for all these complexes in both solution and doped films are shown in Table 4.1. The low efficiency of Ir(tpzp) 3 in degassed dilute solution indicates that this complex behave similarly to Ir(ppz) 3 . It is very likely that the deactivated state is still formed in the excited state of this molecule. The low efficiency of this complex was improved in a 2wt% doped PMMA film reaching a peak efficiency of 53% of this molecule. This could be resulted from the rigid environment in the matrix which stabilized the triplet state of Ir(tpzp) 3 partly shuting down the deactivated pathway. However, the efficiencies of all three heterolepitc complexes are very low in both solution and doped PMMA film, due to the inefficient ligand to ligand charge transfer. The emissions of these complexes have also been measured at low temperatue (77K). Ir(tpzp) 3 show emission at the wavelength (457nm) similar to its room temperature emission. I2 and I3 still show broad band at ~ 520 nm. However, weak emissions at shorter wavelengths were also observed for I2 and I3. For example, I2 shows a weak emission at 480 nm and I3 shows a weak emission at 452 nm. This also indicates the stabilization of their triplet states in a rigid environment. The stabilization was maximized with I1 giving an intense blue shifted emission at 452 nm at 77 K. This emission is totally due to the the tpzp based triplet state ( 3 LC). This might be explained from the relatively farther position of the picolinate thus the ligand to ligand charge transfer in this molecule is relatively harder compared to I2 and I3, so its triplet state was easier to be stabilized. The lifetimes of these molecules were also measured at 77K as shown in Table 4.1. I1 shows a similar lifetime (30us) to the homoleptic complex Ir(tpzp)3 (37us), whereas I2 and I3 give lifetimes of 11 us and 10 us respectively. 109 400 450 500 550 600 650 700 0 1 Ir(tpzp) 3 , 77K I1, 77K I2, 77K I3, 77K Wavelength (nm) Intensity (a.u.) 240 300 360 420 480 540 600 660 720 0 1 0 1 PL,Ir(tpzp) 3 PL,I1 PL,I2 PL,I3 at 298K Intensity (a.u.) ABS, Ir(tpzp) 3 ABS, I1 ABS, I2 ABS, I3 Wavelength (nm) Absorbance (a.u.) Figure 4.4 Absorptions and emissions of Ir(tpzp) 3 and Ir(tpzp) 2 pic (I1, I2 and I3) at room temperature (top) and emissions at 77 K (bottom). 4.3.7 Isomerization of Ir(tpzp) 2 pic. Sublimation has been widely used to purify semiconducting materials for OLEDs. Pure I1 was sublimed in a temperature gradient vacuum sublimator under high vacuum at 300 °C for 24 hrs. When we analyzed the sublimed and unsublimed compounds, we found that I1 isomerized to the other two isomers in both samples. While it was reported 110 that isomerizaion usually happen in the solution or in the gas phase, this result indicates that isomerization of Iridium complexes could happen in the solid state. Fig. 4.5 shows the comparison the tert-butyl group 1 HNMR of the unsublimed compounds before and after sublimation. Figure 4.5 1 H NMR chemical shifts of the tert-butyl groups of the Ir(tpzp) 2 pic isomers before and after sublimation of I1 111 The comparison of the HPLC chromogram is shown in Fig. A4.18. The integration of the HPLC chromogram and the 1 HNMR spectra give the ratio of these three isomers after sublimation to be I1:I2:I3 = 0.32:0.09:0.59. This supports our calculation of the relative energy levels of these isomers, where I2 is the highest energy isomer and I3 is the lowest energy isomer among these three forms. However, we still did not observed any evidence of the formation of the trans-carbon isomer due to its unstability. It has previously been shown the dissociation of picolinate 31 from FIrpic and the Ir-(C^N) coordination has been considered as stronger bond. The isomerization from trans-tpzp isomer I1 to cis-tpzp isomer I3 indicates the re-orientation of the tpzp ligand in the complexes. This isomerization might come from the Ir-N bond breaking or strong distortion of the complexes. 4.4 Conclusion The tpzp ligand we reported in this paper shows phosphorescence at 433 nm. Using this ligand, we successfully synthesized the homoleptic complex Ir(tpzp) 3 and the three isomers of the heteroleptic complexe Ir(tpzp) 2 pic. Ir(tpzp) 3 phosphoresces at ~450 nm at both room temperature and 77K, while all the heteroleptic complexes show broad band LLCT emission at ~530 nm at room temperature. The inefficient LLCT of the heterolepitc complexes lead to the low quantum emifficiencies of these molecules in both solution (<1%) and doped films(<15%). However, the three isomers of Ir(tpzp) 2 pic exhibit different phosphorescence at 77K i.e. I1 phosphoresces at 452 nm similar to the homolepitc complexes, whereas I2 and I3 still show LLCT emission at ~520 nm. DFT calculation shows that the HOMOs of all these complexes are localized at the LC metal 112 center. Instead, the LUMO of the Ir(tpzp) 3 is localized on tpzp ligands while the LUMOs of Ir(tpzp) 2 pic isomers are all localized on the picolinate. This was reflected from the electrochemistry of these molecules. The homolepitic complex show first reduction at - 2.7 eV , corresponding to the reduction on one of the tpzp ligands. Differently, Ir(tpzp) 2 pic (I1, I2 and I3) all give first reduction at ~ -2.4 eV, corresponding to the reduction of the picolinate. The smaller reduction potential of the picolinate could be one of the reasons increasing the possibility of the ligand to ligand charge transfer leading to red shifted broad band emission of the Ir(tpzp) 2 pic. DFT calculation also shows the relative energies of the isomers of Ir(tpzp)2pic to be I3 < I1< I2 <trans-C, which was supported by our isomerization result that isomerization of I1 has lead to a ratio of the three isomers to be I1:I2:I3 = 0.32:0.09:0.59. The quantum efficiency of Ir(tpzp) 3 was improved up to 0.53 in a doped PMMA film, which is significantly higher than that of Ir(ppz) 3 ( Ф PMMA =0). This suggest that the rigidity of tpzp has partly shut down the deactivation pass way. However, the stabilization of the Ir-tpzp coordination bonds are not enough to prevent the Ir-N bond breaking in this complex. This could be indicated both from the low quantum yield of Ir(tpzp) 3 and the isomerization of I1 under higher temperature. Our studies in this paper have brought in the following concerns for applications of ppz based iridium complexes in PHOLED. 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Europian Journal of Inorganic Chemistry 2009, 16, 2407-2414. 118 Chapter 5: Heteroleptic Fluorene based Iridium Complexes as Blue Emitters for Phosphorescent Organic Light Emitting Diodes 5.1 Introduction Color tuning and color purity are important research topics when looking for good blue phosphorescent dopants. 1-5 One approach to achieve blue color emission from the Ir complexes is through the usage of cyclometalating ligands with high triplet energy as is in the case of phenylpyrazole (ppz) of Ir(ppz) 3 . 6-8 We previously reported a cyclometalated Ir complex in which 9,9-dimethylfluorene was linked to pyrazole to form a ligand flz as shown in Scheme 5.1. This ligand has phosphorescence at 445 nm and its homoleptic facial Ir complex FlzIr (Scheme 5.1) has an emission at 480 nm. 9 FlzIr phosphoreces very efficiently in solution at 77 K, and also emits at room temperature in deareated solution with 72% quantum efficiency. In the presence of oxygen, this complex could be quenched rapidly in solutions. However, it could still achieve high quantum efficiency in a rigid matrix due to the protection from the matrix in the layer. Similarly, when used as dopant in an OLED device, it is rarely quenched by the oxygen. So this material has been utilized as a dopant for blue PHOLEDs and a 6% external quantum efficiency of the device was achieved, although this value is in a moderate range compared to the EQE that has been achieved for FIrpic devices. One of the reasons leading to the moderate efficiency of FlzIr device is the high reduction potential of this complex (E red = 3.1 V). This makes it hard to be reduced and it cannot sufficiently trap electrons in its molecules. Fig. 5.1 shows the HOMO and LUMO levels of FlzIr, Ir(ppy) 3 , 10 FIrpic and PQIr. 4,10 The LUMO of FlzIr is 1.1 eV, which is 119 N N flz Ir(flz) 3 Ir N N Ir N N 3 Scheme 5.1 Structures of flz and Ir(flz) 3 0.8 eV higher than the LUMO of blue dopant FIrpic (E LUMO = 1.9 eV), and it is also much higher than those of green dopant Ir(ppy) 3 (E LUMO = 1.9 eV) and red dopant PQIr (E LUMO = 2.3 eV). As discussed in Chapter 1, charge recombination is expected to happen at the dopant molecules, since it reduces the probability of energy loss in the energy transfer processes. In order to achieve a dopant molecule charge recombination, both holes and electrons have to be trapped by the dopant. So the electron trapping at FlzIr needs to be improved to achieve better charge recombination without altering its color. A direct approach is to incorporate electron trapping moieties onto the dopant molecules, i.e. a functional group that has a lower reduction potential. Importantly, this functional group should not change the energy levels of the phosphors that provide the blue emission. It is known that introducing groups via π conjugation might lead to a significant red shift of the dopant emissions. We propose a ligand Pspiro (Scheme 5.2) which has similar photophysical properties as flz and also includes a better electron trapping functional group, the fluorene, attached to this phosphor through a sp 3 carbon. The phosphor in this ligand is very similar to flz as shown in Scheme 5.2 in red. 120 FlzIr Ir(ppy) 3 PQIr FIrpic E (eV) 1.1 1.9 1.9 2.3 5.0 5.1 5.0 5.7 Figure 5.1 HOMO and LUMO comparisons between FlzIr , Ir(ppy) 3 , PQIr and FIrpic But a fluorenyl is attached to this phosphor through a sp 3 carbon. Fluorenes have a reduction potential of 2.8 V. 11,12 This potential is 0.3 eV lower than that of FlzIr . So better electron trapping is expected with Pspiro Ir complexes. The sp 3 carbon provides a π-system in which fluorene and flz are electronically isolated, thus maintains the blue emission of the molecule. Homoleptic complexes Ir(Pspiro) 3 and heteroleptic complex Ir(ppz) 2 Pspiro were both synthesized using this ligand. Their photophysics and electrochemical behaviors were investigated. Ir(ppz) 2 flz (Scheme 5.2) was also synthesized as a reference. These two complexes were able to be used as dopants for PHOLEDs. The PHOLED performance will also be discussed in the following sections. 121 5.2 Experimental 5.2.1 Synthesis and Characterization 5.2.1.1 General. All chemicals, reagents, and solvents were received from commercial sources without further purification. 1 H NMR spectra were measured with Varian Mercury 400 NMR Spectrometer, and chemical shifts were referenced to a deuterated solvent. Elemental analyses (CHN) were performed at the Microanalysis Laboratory at the University of Illinois, Urbana- Champaign, IL. Ir(Pspiro) 3 Ir N N Ir N N 3 Pspiro N N N N Ir(ppz) 2 (Pspiro) Ir 2 N N N N Ir 2 N N N N Ir(ppz) 2 (flz) Ir 2 N N N N Ir 2 N N N N Scheme 5.2 Structures of Pspiro, Ir(pspiro) 3 , Ir(ppz) 2 flz and Ir(ppz) 2 Pspiro 122 5.2.1.2 Procedures. The 2-Bromo-9, 9-spirobifluorene (Brspiro) 13,14 and the 1-(9.9-dimethylfluoren-2- yl)-1H-pyrazole (flz) 9 were synthesized according to literature procedures. The 1-(9.9- spirobifluoren-2-yl)-1H-pyrazole (Pspiro) was synthesized by refluxing pyrazole and Brspiro overnight in a DMF solution with CuI, phenanthroline and KF/Al 2 O 3 under N 2 atmosphere. 15-17 Iridium dichloro-bridged dimer [IrCl(pspiro) 2 ] 2 was made by refluxing IrCl 3 and Pspiro in 2-ethoxyethanol for 6 hours. 18 Iridium(III) bis[1-(9.9-spirobifluoren- 2-yl)-1H-pyrazolyl- N, C2' ] acetylacetonate (Ir(Pspiro) 2 acac) were synthesized from refluxing acetylacetone and the [IrCl(pspiro) 2 ] 2 in DCE overnight. 19 The homoleptic complexes fac- Iridium(III) Tris [1- (9.9-spirobifluoren-2-yl)-1H-pyrazolyl- N, C3'] (Ir(Pspiro) 3 ) was synthesized by reacting Ir(Pspiro) 2 acac and 1.2 eq. Pspiro in glycerol overnight at 220 °C under nitrogen. Recrystallization was used to purify the complex to achieve pure Ir(Pspiro) 3 as yellow crystals. The heteroleptic complexes fac-Iridium(III) bis [1- (9,9-dimethylfluoren-2-yl)- 1H-pyrazolyl- N, C2' ] [1-Phenyl-1H-pyrazolate] (Ir(ppz) 2 flz) and fac - Iridium(III) bis [1- (9, 9 -spirobifluoren- 2 -yl)- 1H - pyrazolyl- N, C2' ] [1- Phenyl -1H -pyrazolate] (Ir(ppz) 2 Pspiro) were synthesized by refluxing [IrCl(ppz) 2 ] 2 dimer, 1.2 eq. fluorenyl pyrazole ligand and 3 eq. K 2 CO 3 overnight in 2-ethoxyethanol under nitrogen. 126 After cooling down, the organic solvents were reduced by rotary evaporation to get brown solids of crude product. Silica gel column chromatography with hexane and ethyl acetate (10:1) was performed to purify the products to give yellow solids which are a mixture of meridional and facial isomers. Photolysis in CH 3 CN was performed later to obtain pure facial isomers. 123 1-(9,9-dimethylfluoren-2-yl)-1H-pyrazole (flz): 1 H NMR (CDCl 3 , 250Hz) δ: ppm 7.90 (d, 1H, J=2.5Hz) 7.75 (d, 1H, J=2.0Hz) ppm 7.66 (m, 3H) 7.53 (m, 1H) 7.37 (m, 1H) 7.27 (m, 2H) 6.41 (m, 1H) 1.46 (m, 6H) 1-(9,9-spirobifluoren-2-yl)-1H-pyrazole (Pspiro): 1 H NMR (CDCl 3 , 250 MHz) δ: 7.86 (m, 4H) 7.71 (m, 2H) 7.58 (s, 1H) ppm 7.36 (m, 3H) ppm 7.08 (m, 4H) 6.73 (m, 2H) ppm 6.32 (m, 1H). EI-MS: 382. Tris-1-(9,9-spirobifluoren-2-yl)-1H-pyrazole iridum (III) (Ir(Pspiro) 3 ) 1H-NMR (400 MHz) ppm 7.78 (d, 6H, J=7.8Hz) 7.64 (d, 3H, J=2.5 Hz) 7.43 (d, 3H, J=7.5Hz) 7.30 (m, 6H) 7.19 (m, 6H) 7.17 (s, 3H) 6.96 (m, 9H) 6.89 (d, 3H, J=2.0Hz) 6.70 (d, 3H, J=7.5Hz) 6.58 (d, 3H, J=7.5Hz) 6.56 (s, 3H) 6.16 (m, 3H) Maldi-MS:1337.1, 955.7. Calc. for Ir(Pspiro)3 ·H 2 O: C: 74.48 H: 3.94 N: 6.20. Found: C: 74.47 H: 4.04 N: 5.94. Bis-phenylpyrazoly-(-(9,9-spirobifluoren-2-yl)-1H-pyrazoly) iridium (III) (Ir(ppz) 2 Pspiro) 1H NMR (CDCl3, 400MHz) δ ppm 7.93 (d, 1H, J=2.6Hz) 7.88 (d, 1H, J=2.2Hz) 7.77 (d, 2H, J=7.6Hz) 7.56 (d, 1H, J=2.7Hz) 7.40 (d, 1H, J=7.5Hz) 7.28 (m, 2H,) 7.19 (d, 1H, J=7.8Hz) ppm 7.19 (s, 1H) 7.10 (m, 3H) 6.99-6.72 (m, 13H) 6.64 (d, 1H, J=7.5Hz) 6.53 (d, 1H, J=7.4Hz) 6.49 (s, 1H) 6.31 (dd, 1H) 6.29 (dd, 1H) 6.12 (dd, 1H) Maldi-MS: 860.1, 717.8, 479.3 Calc. for Ir(ppz) 2 Pspiro ·2H 2 O·CH 3 CN C:61.52 H: 4.09 N:10.46. Found: C: 61.49 H: 3.86 N: 10.27. 124 Bis-phenylpyrazoly- (-(9,9-dimethylfluoren-2-yl)-1H-pyrazoly) iridium (III) (Ir(ppz) 2 flz) 1 H NMR (CDCl3, 400MHz) δ ppm 8.05 (d, 1H, J=2.7Hz) 7.96 (m, 3H) 7.28 (d, 1H, J=7.9Hz) 7.19 (d, 1H, J=7.9Hz) 7.15 (d, 1H, J=8.1Hz) 7.08 (m, 1H) 7.04 (s, 1H) 6.97 (dd, 1H, J=7.9Hz) 6.94 (s, 1H) 6.89 (dd, 1H, J=7.3Hz) 6.83 (m, 2H) 6.75 (m, 3H,) 6.65 (m, 3H) 6.37 (m, 3H) Maldi-MS: 738.2, 594.1. Calc. for Ir(ppz) 2 flz·CH 2 Cl 2 C: 54.01 H: 3.78 N: 10.21. Found: C: 54.30 H: 3.79 N: 10.49. 5.2.2 Electrochemistry. Cyclic voltammetry and differential pulse voltammetry were performed using an EG&G potentiostat/galvanostat model 283 under N 2 atmosphere. DMF was used as solvent for the scan from -0.5 to 2.0 Vs to detect the oxidation signals, and 0.1 M TBAH was used as the supporting electrolyte. A glassy carbon rod was used as the working electrode, a platinum wire was used as the counter electrode, and a silver wire was used as a pseudo reference electrode. The reversibility and the redox potentials were determined by the cyclic voltammetry and differential pulse voltammetry, respectively. The redox potentials are calculated relative to an internal reference ferrocenium/ferrocene (Cp 2 Fe + /Cp 2 Fe). 5.2.3 Photophysics. The UV-visible spectra were measured in CH 2 Cl 2 by a Hewlett-Packard 4853 diode array spectrometer. Emission measurements were performed on a Photon Technology International QuantaMaster model C-60 fluorimeter at both room temperature and 77 K. The quantum yields of the molecules were measured using an absolute method on a Hamamatsu C9920 system equipped with a xenon lamp, calibrated integrating sphere and 125 model C10027 photonic multichannel analyzer. A dilute solution was prepared using toluene for measuring the quantum yields in solutions. All the solutions were bubble degassed for 5 min before the measurement. 2 mg of each compound and 100 mg of PMMA were dissolved in 1 ml of toluene and sonicated for 1 h. This solution was used for spin casting to obtain films of 2 wt% Ir complexes in PMMA. The Ir complex/mCP films were vapor-deposited onto clean quartz substrates in a high vacuum chamber. 5.2.4 Device Fabrication and Testing. All materials used for vapor deposition were purified by temperature gradient vacuum sublimation. Indium Tin oxide (ITO) on glass was purchased from commercial resource. The ITO substrates were coated with a photo resist, baked in the oven for 20 min, cleaned by acetone, dried with nitrogen, and treated with UV ozone for 10 min. The organics were vapor-deposited onto the substrates in a high-vacuum chamber, followed by aluminum cathodes (1000 Å) deposition through a shadow mask with 2-mm wide stripes. The electrical and optical characteristics of the devices were measured with a Keithly 2400 source/meter/2000 multimeter coupled to a Newport 1835-C optical meter, equipped with a UV 818 Si photo detector. The electroluminescence (EL) spectra were measured on a Photon Technology International QuantaMaster model C-60 spectrofluorimeter. 126 5.3 Result and Discussion 5.3.1 Synthesis The synthetic procedures of Pspiro, and the complexes Ir(Pspiro) 3 , Ir(ppz) 2 (Pspiro), Ir(ppz) 2 flz are shown in Scheme 5.3 and 5.4. Ir(Pspiro) 3 was using previously reported procedures and conditions. 19,20 The Ir dichloro-bridge dimer 21-23 [Ir(Pspiro) 2 Cl] 2 was first prepared, and the acetylacetonate (acac) derivative Ir(Pspiro) 2 (acac) was prepared from the dimer under a basic condition. The facial tris complex Ir(Pspiro) 3 was synthesized by replacing acac from Ir(Pspiro) 2 (acac) by excess Pspiro under high temperature (220 °C). Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz were synthesized under lower temperatures in a 2- ethoxyethanol solution to obtain a mixture of meridional 20 and facial 20 complexes mixture. The mixture was later photolyzed in acetonitrile to obtain pure facial isomer of the heteroleptic complexes. The reason to use a lower temperature for synthesizing the heteroleptic complexes is to prevent the ligand exchange reactions which could lead to other heteroleptic complexes. For example, at higher temperature a complex Ir(Pspiro) 2 ppz may form. 127 Scheme 5.3 Synthesis of Ir(Pspiro) 3 128 N N Mixture of Fac and Mer isomers N N N N Ir Ir Cl Cl 2 2-ethoxyethanol, reflux Facial Isomer C Ir 2 N N N 2 CH 3 CN Photolysis = N N N N C Ir 2 N N N K 2 CO 3 C N or 2ethoxyethanol/H 2 O, reflux C N IrCl 3 .6H 2 O 80% 50-55% 85% Scheme 5.4 Synthesis of Ir(ppz) 2 flz and Ir(ppz) 2 Pspiro 5.3.2 Electrochemistry As discussed in Chapter 2, electrochemistry data could be used to estimate the HOMO and LUMO levels of organic semiconductors. 24,25 The HOMO and LUMO levels of materials could be calculated from the oxidation and reduction potentials, respectively. So electrochemistry was performed to evaluate the frontier orbitals, especially the LUMOs of Ir(Pspiro) 3 , Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz. The cyclic voltammetry (CV) and differential pulse voltammetry (DPV) data of Ir(Pspiro) 3 is shown in Fig. 5.2. The oxidation and reducation potentials are determined versus Cp 2 Fe + /Cp 2 Fe. Similar to most of FlzIr, Ir(Pspiro) 3 shows a reversible oxidation. The oxidation potential is determined from the DPV experiment is 0.4 V. This value is 0.1 V higher than the oxidation potential of FlzIr (E ox = 0.3 V). Two reductions were also detected for Ir(Pspiro) 3 . Since it is too 129 close to the edge of the solvent signals, the return signals are not very clear. The reduction potentials measured by DPV are -2.9 V and -3.2 V . The first reduction at -2.9 V is assigned to the isolated fluorene group shown in blue in Fig. 5.2 inset and the second reduction at -3.2 V is assigned to the Ir-pyrazole-fluorene as shown in red in Fig. 5.2 inset. The first reduction at -2.9 V is consistent with reported reduction potentials (-2.8 V) of fluorenes, while the second one is close to the reduction potential reported for FlzIr, which is -3.1 V . -4 -3 -2 -1 0 1 -0.5 0.0 0.5 Voltage (V) Current (mA) Ir(Pspiro) 3 Cp 2 Fe + /Cp 2 Fe Figure 5.2 cyclic voltammetry and differential pulse voltammetry (inset) curves of Ir(Pspiro) 3 130 -4 -3 -2 -1 0 1 -3.0 0.4 -3.1 Ir(ppz) 2 flz Ir(ppz) 2 Pspiro 0.4 -3.2 Voltage (V) Current (mA) Pspiro Ferrocene -3.1 -2.9 Figure 5.3 Differential pulse voltammetry (DPV) curves of Pspiro, Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz The DPV curves of Pspiro are shown in Fig. 5.3. Similar to Ir(Pspiro) 3 , there are two reductions detected for this molecule. The first reduction at -2.9 V is assigned to the fluorene and the second reduction at -3.1 V pyrazole related fluorene moiety. The top and bottom DPV traces of Pspiro are symmetric which suggest the reversibility of these two reductions. The electrochemistry data of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz are also shown in Fig. 5.3. A oxidation at 0.4 V was observed for both molecules and this value is identical to the measured value of Ir(Pspiro) 3 . However, these two molecules show different reduction behaviors, due to the structural difference of the two ligands. While only one reduction was observed Ir(ppz) 2 flz, there are two reductions observed for Ir(ppz) 2 Pspiro. 131 The reduction potential from the Ir-pyrazole-fluorene is – 3.1 V for Ir(ppz) 2 flz and – 3.2 V for Ir(ppz) 2 Pspiro. The other reduction potential of Ir(ppz) 2 Pspiro, which is assigned to attached fluorene on Pspiro, is – 3.0 V as shown in Fig. 5.3. The HOMO and LUMO levels of all complexes were estimated from their oxidation and reduction potentials and are as shown in Fig. 5.4. The HOMO energies of Ir(Pspiro) 3 , Ir(ppz) 2 flz and Ir(ppz) 2 Pspiro are identical, which is 5.2 eV. But the LUMO energies of Ir(Pspiro) 3 and Ir(ppz) 2 Pspiro are slightly lower than those of FlzIr and Ir(ppz) 2 flz due to the incorporated fluorene moiety. Ir(Pspiro) 3 has a LUMO of 1.3 eV, Ir(ppz) 2 Pspiro has a LUMO of 1.2 eV, and the LUMO energies of FlzIr and Ir(ppz) 2 flz are both 1.1 eV . 5.0 5.2 5.2 5.2 1.1 1.3 1.2 1.1 FlzIr Ir(ppz) 2 flz Ir(Pspiro) 3 Ir(ppz) 2 Pspiro LUMO HOMO E (eV) Figure 5.4 HOMO and LUMO levels of Ir(Pspiro) 3 , Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz 132 5.3.3 Photophysics The absorption and emission spectra of Ir(Pspiro) 3 are shown in Fig. 5.5. The absorption and emission spectra of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz are shown in Fig. 5.6. All the complexes showed similar absorption spectra, with three major absorption bands for each molecule. The band from 250 nm to 320 nm shows similar features to the ligand absorptions (Fig. 5.5), which is assigned to ligand center π- π* transition. Between 320 nm to 420 nm, the bands with lower molar absorbitivity are attributed to The metal to ligand charge transfer (MLCT) transitions. Fig. 5.6 also shows that the absorption spectrum of Ir(ppz) 2 Pspiro is very close to the Ir(Pspiro) 3 spectrum. The emission spectra of these complexes were measured in 2-MeTHF at both 77 K and room temperature. But there is no significant difference between the spectra at 77 K and at room temperature. Fig. 5.5 and Fig. 5.6 are the spectra measured at room temperature. All complexes show emissions at 476 nm, giving a greenish blue color as shown in Fig. 5.5 and Fig. 5.6. 300 400 500 600 700 0.0 0.5 1.0 0.0 0.5 1.0 360 nm Intensity (a.u.) Ir(Pspiro)3 ABS EM EX Pspiro ABS 476 nm Absorbance (a.u.) Wavelength (nm) Figure 5.5 Absorption spectrum and emission spectrum of Ir(Pspiro) 3 133 200 300 400 500 600 700 0.0 0.5 1.0 0.0 0.5 1.0 392 nm EM, Ir(ppz)2Pspiro EX, Ir(ppz)2Pspiro ABS, Ir(ppz)2Pspiro EX, Ir(ppz)2flz EM, Ir(ppz)2flz IrPspiro 358 nm Absorbance (a.u.) Intensity (a.u.) Wavelength (nm) 476 nm Ir(ppz) 2 flz Ir(ppz) 2 Pspiro Figure 5.6 Absorption and emission spectra of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz Similar to what is reported for FlzIr, all these complexes are easily quenched by oxygen. So all the solutions were bubble degassed before the measurement. The quantum yield of Ir(Pspiro) 3 in solution is 99%. This value is higher than the quantum yield (0.81%) reported for FlzIr. 26 But due to the quenching from oxygen, there is no emission from this complex in the air. Both heteroleptic complexes show similar behaviors. Even though their quantum yields are lower than Ir(Pspiro) 3 , they both show quantum yields around 50% in solutions (Table 5.1). The lifetimes of all the complexes in solution were measured at both 77K and 298 K and shown in Table5.1. The lifetime of Ir(Pspiro) 3 measured at 77K is 52 µs, while its lifetime at room temperature is slightly shorter with a value of 42 µs. These two lifetimes are very close to the values measured for FlzIr ( τ 77K = 39 µs, τ 298K = 50 µs). The lifetimes of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz at room temperature 134 are also in a similar range with values of 36 µs. But they show slightly longer lifetime at 77 K than the homoleptic complexes, with a value 62 µs for Ir(ppz) 2 Pspiro and 63 µs for Ir(ppz) 2 flz. The lifetimes of all these fluorene based complexes are longer than other ppz based Ir complexes, and the long luminescence lifetime increases the probability of oxygen quenching to their excited states, leading to an extreme low efficiency in the air. The quantum yields of these complexes were also measured in PMMA films. The rigid matrix environment provided by PMMA could stabilize the triplet state of the complexes and influence the non-radiative decay rates of the complexes. Each complex (2 wt%) in PMMA was dissolved in toluene and the solution was spin casted to make the thin films. The PMMA doped film quantum efficiencies of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz are increased, with a maximum quantum efficiency of 92% for Ir(ppz) 2 Pspiro and 78% for Ir(ppz) 2 flz. Ir(Pspiro) 3 also shows a high quantum yield 94% in PMMA that is comparable to the value measured for FlzIr (93%). Table 5.1 Summary of quantum yields and lifetimes of Ir(Pspiro) 3 , Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz. Φ PL solution 298 K Φ PL PMMA 298 K Φ PL mCP 298 K τ solution ( μs) 298 K τ solution ( μs) 77 K τ PMMA ( μs) 298 K Ir(Pspiro) 3 0.99 0.94 - 42 52 40 FlzIr 0.72 0.93 - 39 50 37 Ir(ppz) 2 Pspiro 0.49 0.92 0.15 47 62 36 Ir (ppz) 2 flz 0.56 0.78 0.38 35 63 36 135 It is also important to investigate the efficiencies of these complexes in a host material, since a host could quench the emission of its dopant. We chose to use a common blue phosphorescent host mCP, because FlzIr: mCP devices was previously optimized to achieve 6% PHOLED device efficiency. An 8 % doping percentage was used and the film was fabricated using vapor evaporation method. The quantum efficiencies of these doped films are shown in Table 5.1, which shows that Ir(ppz) 2 Pspiro:mCP has an efficiency of 15% and Ir(ppz)2flz:mCP has an efficiency of 38%. So both Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz show lower quantum efficiencies in mCP than in PMMA suggesting a possible emission quenching from the host mCP. 5.3.4 OLED studies The PHOLED device structure is shown in Fig. 5.7. An NPD layer (40 nm) was used as a HTL and a BCP layer (40 nm) was used as an ETL. In the emissive layer (25 nm), 8% of iridium fluorene dopant (FL dopant) was doped into mCP. The relative HOMO and LUMO levels of these compounds are shown in Fig. 5.7. Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz were used to fabricate the PHOLED devices and generate a comparison between these two dopants. Due to the high sublimation temperature (410 °C) of Ir(Pspiro) 3 , it could not be used for device fabrication. 136 1.5 1.6 5.3 5.9 6.5 BCP mCP NPD E (eV) LUMO HOMO Fl dopant 5.2 5.2 1.2 1.1 Ir(ppz) 2 flz Ir(ppz) 2 Pspiro Dopant Figure 5.7 Devices structure of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz devices: NPD(40 nm) /dopant: mCP(8%, 25 nm)/BCP (40 nm) Fig. 5.8 and 5.9 show the J-L-V characteristic of these three devices. The summary of the device performance is shown in Table 5.2. The performance of these two devices is very similar. At 10 V, Ir(ppz) 2 Pspiro gives a current density of 27.3 mA/cm 2 , while Ir(ppz) 2 flz device gives a current of 33.6 mA/cm 2 , which is slightly higher. Similarly these two devices exhibit similar brightness at a certain voltage, for example, at 10 V, Ir(ppz) 2 Pspiro device shows a brightness of 791.8 cd/m 2 , and Ir(ppz) 2 flz device shows a brightness of 921.7 cd/m 2 . The external quantum efficiencies both devices are low. Fig.5.10 shows the external quantum efficiencies of these two devices versus their current densities. The maximum EQE of the Ir(ppz) 2 Pspiro device reaches 2.4 % at 0.16 mA/cm 2 , and the maximum EQE of the Ir(ppz) 2 flz device is even lower which reaches 1.6 % at 4.9 mA/cm 2 . 137 Table 5.2 Performance of undoped devices I and II. Device EL peaks (nm) Turn-on Voltage a (V) Maximum Luminance (cd/m 2 ) η ext, max (%) Ir(ppz) 2 Pspiro 480 4.8 2068 (at 15 V) 2.4 Ir(ppz) 2 flz 480 4.5 2402 (at 15 V) 1.6 a Determined at a luminance of 1 cd/m 2 0.1 1 10 1E-6 1E-5 1E-4 1E-3 0.01 0.1 1 10 100 1000 Ir(ppz) 2 flz:mCP Ir(ppz) 2 Pspiro:mCP Current Density (mA/cm 2 ) Voltage (V) Figure 5.8 J-V characteristics of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz devices: ITO/NPD(40 nm)/dopant: mCP (25 nm)/ BCP (40 nm)/LiF(1 nm)/ Al (100 nm) 138 0246 8 10 12 14 16 1E-3 0.01 0.1 1 10 100 1000 10000 Ir(ppz) 2 flz Ir(ppz) 2 Pspiro Brightness (Cd/m 2 ) Voltage (V) Figure 5.9 L-V characteristics of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz devices: ITO/NPD(40 nm)/dopant: mCP (25 nm)/ BCP (40 nm)/LiF(1 nm)/ Al (100 nm) 0.01 0.1 1 10 100 0.1 1 Ir(ppz) 2 flz:mCP Ir(ppz) 2 Pspiro:mCP Quantum Efficiency (%) Current Density (mA/cm 2 ) Figure 5.10 External quantum efficiency versus current density of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz devices: ITO/NPD(40 nm)/dopant: mCP (25 nm)/ BCP (40 nm)/LiF(1 nm)/ Al (100 nm) 139 350 400 450 500 550 600 650 700 0.0 0.2 0.4 0.6 0.8 1.0 10V 11V 12V 13V 14V 15V Intensity (a.u.) Wavelength (nm) Ir(ppz) 2 flz 350 400 450 500 550 600 650 700 0.0 0.2 0.4 0.6 0.8 1.0 1.2 10V 11V 12V 13V 14V 15V Intensity (a.u.) Wavelength (nm) Ir(ppz) 2 Pspiro Figure 5.11 Voltage dependence of the electroluminescence spectra of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz devices: ITO/NPD(40 nm)/dopant: mCP (25 nm)/ BCP (40 nm)/LiF(1 nm)/ Al (100 nm) 140 Fig. 5.11 shows the EL spectra of both devices, we can see that the major emission at 480 nm is from the dopants. However, the small emission observed at 430 nm derives from the hole transporting material NPD. This indicates a failure in trapping electrons at EML or a failure of injecting holes into the EML. But since the HOMO levels of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz are similar to that of FIrpic, in a similar device structure, the hole injection performance should be similar. However, the LUMO of Ir(ppz) 2 Pspiro (E LUMO = 1.2 eV) and Ir(ppz) 2 flz (E LUMO = 1.1 eV) are much higher than that of FIrpic (E LUMO = 1.9 eV). So the charge recombination is still very likely due to weak electron trapping abilities of these two dopants. But overall speaking, the EQE of the Ir(ppz) 2 Pspiro device is slightly higher than that of Ir(ppz) 2 flz device. 5.4 Summary In the chapter, we have reported a ligand Pspiro in which a spirobifluorene is linked to the pyrazole, in order to lower LUMO level of the fluorene-pyrazole based Ir complexes and may increase electron trapping ability of the complexes. Both homoleptic and heteroleptic complexes with Pspiro were synthesized and characterized. They show very similar photophysical properties as FlzIr, but they show different electrochemistries. Both and Ir(Pspiro) 3 and Ir(ppz) 2 Pspiro have two reductions, which are from the two different fluorenes of the Pspiro ligand. PHOLED devices were fabricated using the heteroleptic complex Ir(ppz) 2 Pspiro as dopant. Ir(ppz) 2 flz device was also fabricated for comparison. The performance of Ir(ppz) 2 Pspiro device and Ir(ppz) 2 flz device are also similar, giving maximum external quantum efficiencies of 1~2%. Even though Ir(ppz) 2 Pspiro show a lower reduction at - 2.9 eV, it still cannot efficiently trap electrons 141 at the EML. So an energy leakage was seen at the NPD layer. The doped film studies also show that, when mCP used as a host, the emissions of these complexes could be quenched, leading to a low quantum efficiency of these complexes. So the reasons of the low EQEs of Ir(ppz) 2 Pspiro and Ir(ppz) 2 flz devices could be from both electron trapping and emission quenching problems. 142 Chapter 5 References 1. Tamayo, A. B.; Garon, S.; Sajoto, T.; Djurovich, P. I.; Tsyba, I. M.; Bau, R.; Thompson, M. E. Inorg Chem 2005, 44, 8723-8732. 2. Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatani, J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.; Hoshino, M.; Ueno, K. J Am Chem Soc 2003, 125, 12971-12979. 3. Li, J.; Djurovich, P. I.; Alleyne, B. D.; Yousufuddin, M.; Ho, N. N.; Thomas, J. C.; Peters, J. C.; Bau, R.; Thompson, M. E. Inorg Chem 2005, 44, 1713-1727. 4. 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, 4304-4312. 5. Yao, J. H.; Zhen, C. G.; Loh, K. P.; Chen, Z. K. Tetrahedron 2008, 64, 10814- 10820. 6. Nam, E. J.; Kim, J. H.; Kim, B. O.; Kim, S. M.; Park, N. G.; Kim, Y. S.; Kim, Y. K.; Ha, Y . B Chem Soc Jpn 2004, 77, 751-755. 7. Adamovich, V.; Brooks, J.; Tamayo, A.; Alexander, A. M.; Djurovich, P. I.; D'Andrade, B. W.; Adachi, C.; Forrest, S. R.; Thompson, M. E. New J Chem 2002, 26, 1171-1178. 8. Choi, G. C.; Lee, J. E.; Park, N. G.; Kim, Y . S. Mol Cryst Liq Cryst 2004, 424, 173. 9. Sajoto, T.; Djurovich, P. I.; Tamayo, A.; Yousufuddin, M.; Bau, R.; Thompson, M. E.; Holmes, R. J.; Forrest, S. R. Inorg Chem 2005, 44, 7992-8003. 10. Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Kwong, R.; Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E. Inorg Chem 2001, 40, 1704-1711. 11. Wei, W.; Djurovich, P. I.; Thompson, M. E. Chem Mater 2010, 22, 1724-1731. 12. Rathore, R.; Abdelwahed, S. H.; Guzei, I. A. J Am Chem Soc 2003, 125, 8712- 8713. 13. Xie, N.; Zeng, D. X.; Chen, Y . Synlett. 2006, 5, 737. 14. Zhan, X. W.; Risko, C.; Amy, F.; Chan, C.; Zhao, W.; Barlow, S.; Kahn, A.; Bredas, J. L.; Marder, S. R. J Am Chem Soc 2005, 127, 9021-9029. 143 15. Cristau, H. J.; Cellier, P. P.; Spindler, J. F.; Taillefer, M. Chem-Eur J 2004, 10, 5607-5622. 16. Hosseinzadeh, R.; Tajbakhsh, M.; Alikarami, M. Tetrahedron Lett 2006, 47, 5203- 5205. 17. Hosseinzadeh, R.; Tajbakhsh, M.; Alikarami, M. Synlett 2006, 2124-2126. 18. Nonoyama, M. Bull. Chem. Soc. Jpn. 1974, 47, 767. 19. Li, J.; Djurovich, P. I.; Alleyne, B. D.; Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. Polyhedron 2004, 23, 419-428. 20. Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. J Am Chem Soc 2003, 125, 7377-7387. 21. Sprouse, S.; King, K. A.; Spellane, P. J.; Watts, R. J. J Am Chem Soc 1984, 106, 6647-6653. 22. Garces, F. O.; King, K. A.; Watts, R. J. Inorg Chem 1988, 27, 3464-3471. 23. Garces, F. O.; King, K. A.; Craig, C. A.; Spellane, P. J.; Watts, R. J. Abstr Pap Am Chem S 1987, 193, 194. 24. D'Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.; Polikarpov, E.; Thompson, M. E. Organic Electronics 2005, 6, 11-20. 25. Djurovich, P. I.; Mayo, E. I.; Forrest, S. R.; Thompson, M. E. Organic Electronics 2009, 10, 515-520. 26. Sajoto, T.; Djurovich, P. I.; Tamayo, A. B.; Oxgaard, J.; Goddard, W. A.; Thompson, M. E. J Am Chem Soc 2009, 131, 9813-9822. 144 Chapter 6: Temperature Dependence Studies of Cyclometalated Ir Complexes with Phenylpyrazole Based Ligands 6.1 Introduction High quantum efficiency is a key requirement for an OLED emitter, as it directly related to the external quantum efficiency of an OLED device. Ir cyclometalated complexes have been utilized as PHOLED emitter, 1 since the strong spin orbital coupling (SOC) could lead to efficient intersystem crossing, resulting in efficient phosphorescence. However, when a large number of iridium complexes have been found to be highly emissive, 2,3 there are still poor iridium phosphors that are being investigated. For example, fac- Ir(ppy) 3 4-6 shows 100% quantum efficiency at both 77K and 298K, but fac- Ir(ppz) 3 7 does not emit at room temperature. 8,9 The reasons of the low quantum efficiencies of these iridium complexes are rarely known, although understanding the photophysics behind is critical to extend the applications of these iridium complexes. S 1 S 0 ISC NR E a k nr k r (T) k nr (T) T 1 Scheme 6.1 Jablonski diagram 145 Since 1970s, temperature dependence studies have been utilized to investigate the photophysical behavior of metal complexes. 10,11 Information about radiative rate, non- radiative (NR) rate and activation energy could be calculated from the temperature dependent lifetimes of the metal complexes. 12,13 An energy diagram is shown in Scheme 6.1, which shows the energy transfer pathways of these complexes, where k r is the radiative rate, k nr is the non-radiative rate independent on temperatures, k nr (T) is the non- radiative rate dependent on temperature, and ∆E is the activation energy between radiative state and non-radiative state. The energy difference between the triplet substates is defined as zero field splitting (zfs). The zfs of the complexes could be enhanced by mixing singlet and triplet states induced by SOC from the metal. 14-17 Recently T. Sajoto at el have found that the difference of the quantum efficiencies of Ir(ppy) 3 and Ir(ppz) 3 at room temperature might be due to the different activation energies between their radiative states and their non-radiative states, 9 and also due to the difference between their radiative rates and non-radiative rates. Ir(ppy) 3 has a much higher non-radiative state compared to its radiative rate. However, Ir(ppz) 3 has a much lower non-radiative state, so the energy tends to transfer from its radiative state to its non-radiative state leading to the triplet state deactivation. DFT calculation by G. Treboux et. al has revealed that the photophysical difference between Ir(ppy) 3 and Ir(ppz) 3 is related to their molecular structures. 18 The weaker Ir- pyrazole bond could be the reason of energy deactivation. Fig. 6.1 shows a calculated deactivation pathway of Ir(ppz) 3. The Ir-N bond first breaks to form a transition state in which phenyl and pyrazole are still in a same plane. But when the phenyl-pyrazole bond 146 Triplet ground state 6-coordinated Transition state 5-coordinated Lowest energy isomer 5-coordinated Bond breaking N…Ir = 2.9 Å Ring rotation N-Ir = 2.2 Å N Ir N Ir Ir N Figure 6.1 DFT calculation of deactivation pathway of Ir(ppz) 3 excited state rotates to an orthogonal position, it forms a lowest energy state. Both transition state and lowest energy state are 5-coordinated with a distorted trigonal bipyramid geometry (Fig. 6.1). This reveals a possible relationship between Ir(ppz) 3 geometry change and its photo deactivation, which provides information for approaches to improve the quantum efficiency of Ir(ppz) 3 derivatives. A more rigid structure of the Ir complexes might be needed to freeze the phenyl-pyrazole bond and prevent the formation of the deactivated state. To discover the relationship between the Ir complex structure and their phosphorescence deactivation, here we designed three molecules Ir(ppzMe 2 ) 3 , Ir(dpq) 3 and Ir(tpzp) 3 , in each of the molecules, there is different strain on the phenyl-pyrazole bond. The structures of Ir(ppz) 3 , Ir(ppzMe 2 ) 3 , Ir(dpq) 3 and Ir(tpzp) 3 are shown in Scheme 6.2. Synthesis, photophysical characterizations and temperature dependence studies will be demonstrated. 147 N Ir 3 N Ir(ppz) 3 N Ir 3 N Ir(ppzMe 2 ) 3 N Ir 3 N Ir(dpq) 3 N Ir 3 N Ir(tpzp) 3 Scheme 6.2 Structures of Ir(ppz) 3 , Ir(ppzMe 2 ) 3 , Ir(dpq) 3 , Ir(tpzp) 3 6.2 Experimental 6.2.1 General. All chemicals, reagents, and solvents were received from commercial sources without further purification. 1 H NMR spectra were measured Varian Mercury 400 NMR Spectrometer, and chemical shifts were referenced to a deuterated solvent. Elemental analyses (CHN) were performed at the Microanalysis Laboratory at the University of Illinois, Urbana- Champaign, IL. 4-(2-Iodo-phenyl)-butan-2-one (1). To a solution of 10 g 0.04 mol 1- chloromethyl-2-iodo-benzene in 100ml ethyl alcohol were added 4.5 ml 0.044mol 2,4- dione pentane and 5.5 g K 2 CO 3 . The reaction mixture was refluxed over night, worked up 148 by DCM and water, dried over brine and MgSO 4 . The solvent was removed by rotary evaporation to give the crude product as a yellow oil. Silica gel chromatography using hexane/ethyl acetate (9:1) was performed to give 16.5 g pure product as a colorless liquid. Yield: 76%. 1-dimethylamino-5-(2-iodo-phenyl)-pent-1-en-3-one (2). To 6.8 ml DMFDMA was added 7.2 g 1 and 0.28 g p-toluenesulfonic acid hydrate (PTSA·H 2 O). The reaction mixture was refluxed overnight. After cooled down to the room temperature the organic layer was extracted by DCM. The solvent was removed by rotary evaporation to give an orange oil as the crude product. The crude product was purified by silica gel chromatography using hexanes and hexanes/DCM (4:1) to the pure products. Yield: 52% 5-[2-(2-Iodo-phenyl)-ethyl]-1H-pyrazole (3). To a solution of 8.7 g 2 in 120 ml ethanol was added 2 ml hydrazine hydrate and 9 ml acetic acid. The reaction mixture was refluxed over night. The solvent was removed by rotary evaporation to give 6.7g colorless liquid and the product was not further purified. Yield: 85%. Dihydro-pyrazolo[1,5-a]quinoline (4, dpq). To a solution of 8.5 g 3 in 80 ml acetonitrile were added 0.4 g Cu and 18 g CsCO3 and the reaction mixture was refluxed overnight to give a yellow solution. The solution was cooled down to room temperature and the solvent was removed by rotary evaporation. Silica gel chromatography using Hexanes/EtOAc (20:1) was performed to give 3.4 g of the pure product. Yield: 71% fac - iridium (III) tris(Dihydro-pyrazolo[1,5-a]quinoline ) Ir(dpq) 3. To a solution of 1.70 g 4 in 60 ml 2-ethoxyethanol/H 2 O (3:1) was added 1.37 g 4.6 mmol IrCl 3 . The reaction mixture was heated up to 110 °C and stirred overnight. The white precipitate formed in the solution was filtered, washed by methanol, and dried under vacuum to give 149 2.35 g pure Iridium dichloro-bridged dimer [Ir(dpq) 2 (µ-Cl)] 2 (5). Yield: 82%. To a solution of 1.065 g 0.94 mmol 5 in 120 ml 1,2- dichloro bridged benzene was added 0.366 g 2.15 mmol dpq, 0.52 g K 2 CO 3 and 2.41 g AgOTf. The reaction mixture was refluxed for 4 days. Dichlorobenzene was removed by vacuum distillation. The resulted mixture was dissolved in DCM and the filtrate was collected and dried under reduced pressure to give a brown solid mixture. The mixture was eluted through silica gel column using hexanes/DCM 2:1 to give a clear yellow solution. A yellow solid mixture was formed after the hexanes and DCM was removed by rotary evaporation. This yellow solid mixture was used for recrystallization twice to give 50 mg product. Overall yield: 1.6%. This product contains 10% iridium complex with oxidized ligands. So the sample used for photophysical studies was further purified by reverse phase HPLC with H 2 O and actonitrile (90%). dpq: 1 H NMR (CDCl 3 , 400Hz) δ 1H-NMR (400 MHz) ppm 7.94 (m, 1H) ppm 7.63 (m, 1H) ppm 7.33 (m, 1H) ppm 7.22 (m, 1H) ppm 7.13 (m, 1H) ppm 6.15 (m, 1H) ppm 2.99 (m, 1H) ppm 2.92 (m, 1H) (ppm) EI-MS(m/z): 171. (dpq) 2 (µ-Cl) 2 Ir(dpq) 2 : 1 H NMR (CDCl 3 , 400Hz) δ 13 C NMR (CDCl 3 , 400 Hz) δ (ppm) Maldi-MS(m/z): Anal. Calcd for C 76 H 68 Cl 2 IrN 8 ·3H 2 O: C, 56.95; H, 4.65; N, 6.99. Found: C, 56.90; H, 4.19; N, 6.95. Ir(dpq) 3 : 1 H NMR (CDCl 3 , 400Hz) δ (ppm) 6.98 (d, J=1.99, 3H), 6.74 (m, 3H), 6.68 (m, 6H) , 6.05 (d, J=1.98, 3H), 3.09 (m, 6H), 2.96 (m, 6H). 13 C NMR (CDCl 3 , 400 Hz) δ (ppm) 141.1, 137.1, 136.9, 135.1, 134.6, 125.3, 120.9, 119.3, 102.7, 25.2, 21.7. HPLC-MS(m/z): 701. 150 6.2.2 X-ray crystallography. X-ray quality crystals were grown as indicated in the experimental procedures for each complex, and the crystals were mounted on a glass fiber with Paratone-N oil. X-ray diffraction data were collected on a Bruker SMART APEX diffractometer using graphite- monochromated Mo K α radiation, and structures were determined using direct methods with standard Fourier techniques using the Bruker AXS software package. In some cases, Patterson maps were used in place of the direct methods procedure. Hydrogen positions were input and refined in a riding manner along with the attached carbons. The summary of the refinement details and the agreement factors for the three isomers are given in Table A4.1. 6.2.3 Photophysics. The UV-visible spectra were measured in CH 2 Cl 2 by a Hewlett-Packard 4853 diode array spectrometer. Emission measurements were performed on a Photon Technology International (PTI) QuantaMaster model C-60 fluorimeter at room temperature and 77 K, respectively. A dilute solution was prepared using toluene for measuring the quantum yields in solutions. All the solutions were bubble degassed for 5 min before the measurement. The quantum yields were measured using an absolute method on a Hamamatsu C9920 system equipped with a xenon lamp, calibrated integrating sphere and model C10027 photonic multichannel analyzer. Samples for transient luminescent decay measurements were prepared in distilled 2-MeTHF. The samples were deaerated by bubbling with N 2 , freeze-pump-thawed three times and flame-sealed under vacuum. Phosphorescent lifetimes (77 K – 295 K) were performed using an Oxford OptistatDN-V 151 cryostat instrument equipped with an intelligent temperature controller, and measured time-correlated single-photon counting using an IBH Fluorocube instrument equipped with a 405 nm LED excitation source. Lifetimes at 77K were measured on a PTI Quantaaster model C-60 fluorimeter. 6.3 Result and Discussion 6.3.1. Design of ligands Four phenylpyrazole based ligands, ppz, ppzMe 2 , dpq and tpzp (Scheme 6.3) were chosen for this studies. The strains of these ligands come from the different substituents at a and b positions on the ppz. (Scheme 6.3) Compared to ppz, the methyl group of ppzMe 2 at the a position creates stronger repulsion to the hydrogen at the b position on the phenyl ring. This force could lead to a higher possibility of phenyl-pyrazole bond rotation. When position a and b are linked by alkyl groups, the phenyl-pyrazole bond is fixed, as seen in dpq. However, CH 2 -CH 2 bond is still of flexibility which can flip to form the diastereometers of this molecule. This could still result in certain flexibility to the phenyl-pyrazole bond. But when position a and b are linked by a phenyl group as seen in tpzp, the three aromatic group form a rigid π system, in which the phenyl and the pyrazole are lock in a plane. So the freedom of the phenyl-pyrazole bonds are in an order of ppzMe 2 > ppz> dpq > tpzp. 152 N N a b N N N N N N ppz tpzp dpq ppzMe 2 H Scheme 6.3 Structural comparison of ppz, ppzMe2, dpq and tpzp 6.3.2 Synthesis and characterization of Ir(dpq) 3 Ligand dpq was synthesized from a 4 step protocol as shown in Scheme 6.4. A Vilsmeier-Haack-type reagent dimethylformamide dimethyl acetal (DMFDMA) was used to transform the 2'-bromoacetophenone (1) into the enaminoketone (2), and the enaminoketone was later reacted with the hydrazine to form the diarylpyrazole (3) by a tandem amine heterocyclization. A Heck reaction was used in the last step coupling reaction to afford the 9-tert-Butyl-pyrazolo[1,5-f]phenanthridine (4, dpq). The overall yield of the four step synthesis is 24%. This synthesis of the complexes follows a similar procedure described previously. Iridium dichloro-bridged dimer [(dpq) 2 Ir(µ-Cl) 2 Ir[(dpq) 2 ] was first prepared from IrCl 3 and tpzp with a yield of 76%.Two typitcal reaction conditions procedures have been utilized. For a reaction temperature in the range of 120 - 130 °C, 2-ethoxyethanol was used as the solvent. However, this condition does not generate the tris-iridium dpq complex. Therefore a harsher condition using 1,2-dichloro benzene as solvent heated up to 150 °C was utilized for this reaction. Both potassium carbonate and AgOTf were utilized to dissociate the Ir-Cl bond to open coordination sites 153 I Cl I O I O N I N HN N N O O O N O Cu, CsCO 3 CH 3 CN, reflux Hydrazine EtOH, AcOH, reflux PTSA, reflux K 2 CO 3 , EtOH, reflux N N 2-ethoxyethanol/ H 2 O 110 °C IrCl 3 AgOTf, K 2 CO 3 , 1,2-dichlorobenzene 150 °C Ir 3 12 3 4 Ir(dpq) 3 Scheme 6.4 Synthesis of Ir(dpq) 3 on the Ir center. The product was detected after a reaction time of 2 days and the reaction was terminated after 4 days with a very poor isolated yield of 1.6%. The proton NMR and 2D COSY NMR spectra of this compound are shown in Fig. A4.1. The signals of the two pyrazole protons are located at 6.05 ppm and 6.98 ppm with coupling constant of 2 Hz. The coupling between these two protons is observed in 2D COSY NMR spectra (Fig. A4.1). The CH 2 proton signals are in the range from 2.90 - 3.10 ppm. Due to the possible conformation change of CH 2 -CH 2 on the ligand, two diastereomers could be formed with the ligand, so a broader band for these protons and complex coupling features are observed. 154 The single crystal of fac-Ir(dpq) 3 was obtained from hexanes and DCM and characterized by X-ray crystallography. The crystal data is shown in Table A4.1. The ORTEP drawing of fac-Ir(dpq) 3 is shown in Fig. 6.2. Similar to fac - Ir(ppz) 3 ,this complex has a pseudooctahedral geometry. All three C atoms are in a plane which is not across the Ir center. Similarly, all three N atoms are in another plane, which is not across the Ir center. This feature makes it different from its meridional isomer. The Ir-N and Ir-C bond lengths of this complex are shown in Table 6.1. The data for Ir(ppz) 3 was taken from literature for comparison. This facial complex set in a C3 point group, so all three Ir-C bonds and all three Ir-N bonds are identical, giving values of 2.024(6) Å and 2.132(5) Å respectively (Table 6.1). Due to the stronger trans- influence of carbon, Ir-C is slightly longer than the Ir-N bond. Both Ir-C and Ir-N bond lengths of fac-Ir(dpq) 3 are very close to the bond lengths reported for Ir(ppz) 3 as shown in Table 6.1. In the crystal structure, the pyrazole and the phenyl are in a plane, but the CH 2 -CH 2 is twisted due to due to the sp 3 structure of the carbon. (Fig. 6.2) 6.3.2. Absorption, Excitaiton and Emission Spectra The absorption spectra of Ir(ppzMe 2 ) 3 and Ir(dpq) 3 are shown in Fig. 6.3 compared to Ir(tpzp) 3 and Ir(ppz) 3 . Similar to Ir(tpzp) 3 and Ir(ppz) 3 . The intense absorption bands below 300 nm can also be found in ligand absorptions as shown in Fig. 6.3, so these bands are assigned to transitions to ligand centered (LC) singlet states. Strong MLCT bands between 320 nm and 400 nm have been observed for both Ir(ppzMe 2 ) 3 and Ir(dpq) 3 . Excitation spectra of these complexes were also measured at 77K as shown in Fig. 6.4, where the lowest energy excitation wavelength of Ir(ppzMe 2 ) 3 is 360 nm, and the 155 Figure 6.2 X-ray crystal structure of Ir(dpq) 3 Table 6.1 Bond lengths [Å] for Ir(dpq) 3 compared to Ir(ppz) 3 Bond type Bond distances (Å) Ir(ppz) 3 Ir(dpq) 3 Ir1-C1 2.024(6) 2.026(2) Ir1-C2 2.024(6) 2.026(2) Ir1-C3 2.024(6) 2.026(2) Ir1-N1 2.132(5) 2.124(2) Ir2-N2 2.132(5) 2.124(2) Ir3-N3 2.132(5) 2.124(2) excitation of Ir(dpq) 3 is 350 nm. These bands are consistent with the MLCT absorption of these complexes. The emission spectra of Ir(ppzMe) 3 and Ir(dpq) 3 are shown in Fig. 6.4. These two complexes have very similar emission spectra at 77K to Ir(ppz) 3 . Both Ir(ppzMe 2 ) 3 and 156 240 280 320 360 400 440 0.0 0.2 0.4 0.6 0.8 1.0 Ir(ppzMe2)3 Ir(dpq)3 Ir(tpzp)3 Absorbance (a.u.) Wavelength (nm) Figure 6.3 Absorption spectra of Ir(ppzMe 2 ) 3 , Ir(dpq) 3 compared to Ir(tpzp) 3 Ir(dpq) 3 emit at 410 nm . At room temperature the stoke shifts from the lowest energy absorption to the highest energy emission are small for both Ir(ppzMe 2 ) 3 and Ir(dpq) 3 . The emissions of these two complexes are close to the emission of Ir(ppz) 3 . This indicates very little electronic effect from the alkyl substitutions on the ligands of these complexes. What observed for Ir(tpzp) 3 is slightly different, where the extended π system has red shifted its phosphorescence to 452 nm. There is no emission observed for both Ir(ppzMe 2 ) 3 and Ir(dpq) 3 at room temperature. Their quantum yields in solution at room temperature are both lower than 0.01. Ir(tpzp) 3 also has very a low quantum efficiency of 0.05 at room temperature. The E 0-0 (cm -1 ) values of these complexes are listed in Table 6.2, which are in the range of 21000 to 25000 cm -1 . Table 6.2 summarizes the 157 photophysical parameters of these complexes compared to Ir(ppz) 3 . Similar to Ir(ppz) 3 , the lifetimes of Ir(ppzMe 2 ) 3 , Ir(dpq) 3 and Ir(tpzp) 3 at 77K are greatly different from their lifetimes at room temperature. The lifetimes measured at 77 K are 22 µs for Ir(ppzMe 2 ) 3 , 19 µs for Ir(dpq) 3 and 35 µs for Ir(tpzp) 3 . These micro second scale lifetimes are very close to the lifetime reported for Ir(ppz) 3 (14 µs at 77K). At room temperature, the lifetimes of Ir(ppzMe 2 ) 3 and Ir(dpq) 3 both drop to nano scales, giving lifetimes of 0.05 µs and 0.18 µs, respectively. The low quantum efficiency and fast decays suggest their high k nr at 298 K(<10 8 ). The lifetime of Ir(tpzp) 3 at 298 K is 1.9 µs with a quantum yield of 0.05. The calculated k nr (5.0 × 10 5 ) of this molecule is still one order of magnitude faster than its k r (2.6 × 10 4 ) as seen in Table 6.2. 6.3.3 Temperature Dependence Studies Temperature dependent lifetime measurements were performed to further investigate these complexes. Lifetimes were measured every 10 degree from 77 K to 295 K. The lifetime data could be fitted using as 1/ τ versus T using a Boltzmann model incorporating two temperature-dependent terms as shown in equation (6.1): ) / exp( ) / exp( ) / exp( ) / exp( 1 / 1 2 2 1 1 0 2 1 T k E k T k E k k T k E T k E k B a B a B a B a − + − + − + − + = = τ (6.1) Where τ is the luminescent lifetime at a certain temperature T, k B is the Boltzmann constant, E a1 and E a2 are activation energies, k 0 is the decay rate at the lowest energy triplet substate, k 1 and k 2 are decay rate constants. The parameters obtained from this model provide information about both radiative state and non radiative state of these complexes. k 1 is the rate constant of the radiative 158 Table 6.2 Photophysical parameters of Ir(ppz) 3, Ir(ppzMe 2 ) 3, Ir(dpq) 3 and Ir(tpzp) 3 E 0-0 (nm) E 0-0 (cm -1 ) τ 77K (µs) τ 298K (µs) k r298K k nr298K Φ 298K Ir(ppz) 3 412 24270 14 0.002 - > 10 8 < 0.01 Ir(ppzMe 2 ) 3 410 24630 22 0.05 - > 10 8 < 0.01 Ir(dpq) 3 410 24630 19 0.18 - > 10 8 <0.01 Ir(tpzp) 3 452 21880 35 1.9 2.6× 10 4 5.0 × 10 5 0.05 state, the decay from T 1 to S 0 shown in Scheme 6.1, while E a1 represents the activation energy from the ZFS of the triplet substates, which is usually in a range of 40 – 120 cm - 1 . 19,20 k 2 is the rate constant from the non-radiative (NR) state, and E a2 represents the activation barrier from the T 1 state to the NR state as shown in Scheme 6.1. For example, Ir(ppy) 3 only gives a radiative rate constant, which is 1.8×10 6 . But the k 1 value for Ir(ppz) 3 is 2.5×10 5 and the corresponding k 2 value obtained from the fitting is 1.2×10 11 . This reflects a much faster non-radiative decay than the radiative decay. These values explains the high quantum efficiency of Ir(ppy) 3 (0.97) and extremely low efficiency of Ir(ppz) 3 (< 0.01) at room temperature. Using the same model, the temperature dependence data were fitted for Ir(ppzMe 2 ) 3 and Ir(tpzp) 3 . The values of rate constants and activation energies are shown in Table 6.2, where 10 4 was used as a fixed value of k 0 and 20 cm -1 was used as a fixed value for E a1. These fitting data are shown in Fig. 6.5, in which 1/ τ (k observed ) was plotted as a function of temperature. The decay rate (k observed ) observed for Ir(ppzMe 2 ) 3 increases 159 200 300 400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0 Intensity (a.u.) Wavelength (nm) EX-Ir(ppzMe2)3 EM-Ir(ppzMe2)3 EX- Ir(dpq)3 EM-Ir(dpq)3 EX-Ir(tpzp)3 EM-Ir(tpzp)3 77K Figure 6.4 Excitation and emission spectra of Ir(ppzMe 2 ) 3 , Ir(dpq) 3 and Ir(tpzp) 3 at 77K much faster than Ir(tpzp) 3 as the temperature increases. At 77 K, the decay rate of both complexes are in an order of 10 4 magnitude, while at 298K, the decay rate of Ir(ppzMe 2 ) 3 reach a value of 2.3 × 10 6 , but the k observed for Ir(tpzp) 3 is one order of magnitude slower with a value of 5.3 × 10 5 . The k 1 value fitted for these two complexes are 1.54 × 10 5 and 5.23× 10 5 respectively, suggesting a faster radiative decay of Ir(tpzp) 3 than Ir(ppzMe 2 ) 3 . In contrast, the NR decay rate k 2 of Ir(tpzp) 3 is larger than that of Ir(ppzMe 2 ) 3 , suggesting a faster NR decay of Ir(ppzMe 2 ) 3 . This explained the low quantum efficiency of Ir(ppzMe 2 ) 3 at room 160 Table 6.3 Kinetic parameters for the excited-state decay of Ir(ppz) 3, Ir(ppzMe 2 ) 3, Ir(dpq) 3 and Ir(tpzp) 3 k 1 E a1 k 2 E a2 E 0-0 + E a2 Φ Ir(ppz) 3 2.5× 10 5 60 1.2× 10 12 1800 26070 < 0.01 Ir(ppzMe 2 ) 3 1.54× 10 5 20 1.6× 10 8 750 25381 < 0.01 Ir(tpzp) 3 5.23× 10 5 20 4.4× 10 9 1710 23590 0.05 temperature, where the radiative state gets quenched by the NR state due to the more efficiency NR pathway. The activation energy to the NR state E a2 obtained for Ir(ppzMe 2 ) 3 is 751 cm -1 . This value is much smaller than that of Ir(ppz) 3 . Even though the energy barrier of Ir(tpzp) 3 to the NR state is similar to Ir(ppz) 3, this compound have a slight faster radiative rate and a much slower non-radiative rate. The equilibrium between the radiative state and the NR state results in a higher quantum efficiency of this compound (0.05 in solution and 0.5 in PMMA), while no emission was observed for Ir(ppz) 3 at room temperature. The improved quantum efficiency of Ir(tpzp) 3 proves that the rigid structure of tpzp may partly shut down the NR decay pathway. Fig. 6.6 shows Temperature dependence of luminescence decay for Ir(dpq) 3 from 77 K to 150 K. The lifetime data of this complex at higher temperature were not obtained due to a complex luminescent behavior of this compound at higher temperatures. Multi- exponential lifetimes were observed. But as seen in Fig. 6.6, Ir(dpq) 3 shows a lifetime of 18.5 µs at 77 K, a dramatic drop was observed for its lifetime from 120 K to 150 K, which decreases from 13 µs to 1.3 µs. At room temperature the major component of its lifetime gives a value of 0.18 µs. 161 50 100 150 200 250 300 0.0 5.0x10 5 1.0x10 6 1.5x10 6 2.0x10 6 2.5x10 6 Ir(ppzMe 2 ) 3 Ir(tpzp) 3 1/τ (1/μs) T (K) Figure 6.5 Temperature dependence of luminescence decay of Ir(ppzMe 2 ) 3 and Ir(tpzp) 3 80 100 120 140 160 0 2 4 6 8 10 12 14 16 18 20 Ir(dpq) 3 τ (μs) T(K) Figure 6.6 Temperature dependence of luminescence decay of Ir(dpq) 3 (77 K to 150 K) 162 6.4 Summary In order to investigate the relationship been rigidity of the Ir complexes and their deactivations, we have designed and studied three Ir(ppz) 3 analogs: Ir(ppzMe 2 ) 3 , Ir(dpq) 3 and Ir(tpzp) 3 . The flexibility of the phenyl-pyrazole bond on each of the ligands have a trend: ppzMe 2 > ppz > dpq > tpzp. We have compared the photophysical properties of these Ir complexes. Since there is no conjugation introduced from the alkyl groups, ppzMe 2 and dpq maintains high energy as we seen for ppz. So Ir(ppzMe 2 ) 3 and Ir(dpq) 3 show very similar emissions to Ir(ppz) 3 which emit at 410 nm at 77K. However, these two complexes still do not emit at the room temperature, giving quantum efficiencies <0.01. Since a phenyl group was incorporated in the tpzp ligand, Ir(tpzp) 3 show red shift emissions at 452 nm. Although this complex has very low quantum efficiency in solution at room temperature, it shows 0.5 efficiency in PMMA film as discussed in Chapter 4. The radiative rate and non-radiative rates has been fitted from the temperature dependent lifetime data of Ir(ppzMe 2 ) 3 and Ir(tpzp) 3 . The activation energies (E a2 ) to the non- radiative state are both small for these two complexes, which comparable values to Ir(ppz) 3 and much smaller value than ppy based Ir complexes (E a2 = 3000 – 5000 cm -1 ). The small energy barriers could lead to thermally activated decay from the non-radiative state below room temperature. 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L.; Marder, S. R. Journal of the American Chemical Society 2005, 127, 9021-9029. 174 APPENDIX 1: Chapter 2 Supplemental Information Figure A1.1. Differential scanning calorimetric (DSC) thermograms of SiFln 0 50 100 150 200 250 -1.2 -1.0 -0.8 -0.6 -0.4 SiFl1 SiFl2 SiFl3 SiFl4 Heat Flow (W/G) Temperature (°C) Figure A1.2. Differential pulse voltammetry curves of the reductions of SiFln -4 -3 -2 -1 0 SiFl1 SiFl2 SiFl3 Current (a.u.) Voltage (V) vs. Fc/Fc + Fc/Fc + SiFl4 175 Figure A1.3. DFT Calculated HOMO (left), LUMO (middle) and the triplet (right) surfaces of SiFln 176 Figure A1.4. (a) Current density vs. voltage and (b) quantum efficiency vs. current density of devices III, IV and V (b) 0.01 0.1 1 10 100 1E-3 0.01 0.1 1 10 PQIr, device III Ir(ppy)3, device III FIrpic, device III FIrpic, device IV FIrpic, device V Quantum Efficiency (%) Current Density (mA/cm 2 ) (a) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 0 50 100 150 200 250 300 350 PQIr, device III Ir(ppy)3, device III FIrpic, device III FIrpic, device IV FIrpic, device V Current Density (mA/cm 2 ) Voltage (V) 177 APPENDIX 2: Chapter 3 Supplemental Information Figure A2.1. Packing of Ph 3 CSBFL (top) and Ph 3 SiSBFL (bottom) molecules in a unit cell. Ph 3 CSBFL Unit Cell Ph 3 SiSBFL Unit Cell 178 Figure A2.2. Calculated Hirshfeld surfaces of Ph 3 CSBFL and Ph 3 SiSBFL, which are shown in different viewing angles (front, left and back) Figure A2.3. DFT calculated HOMO (solid) and LUMO (mesh) surface and levels of Ph 3 CSBFL and Ph3SiSBFL. Calculated values are compared to the measured values. 179 Figure A2.4. Cyclic voltammetry curves of Ph 3 SiSBFL and Ph 3 CSBFL -4 -3 -2 -1 0 1 2 3 Reduction - Ph3CSBFL Oxidation - Ph3CSBFL Current (a.u.) Voltage (V) vs. Cp 2 Fe + /Cp 2 Fe Reduction - Ph3SiSBFL Oxidation - Ph3CSBFL Figure A2.5. Differential scanning calorimetric (DSC) thermograms of Ph 3 SiSBFL and Ph 3 CSBFL 72 80 88 96 104 -1.5 -1.4 -1.3 Temperature ( o C) 87 °C 50 100 150 200 250 300 -12 -8 -4 0 Ph3SiSBFL 135 °C 284 °C Heat Flow (W/G) Temperature ( o C) Ph3CSBFL Ph3SiSBFL 253 °C 180 Figure A2.6. Voltage dependence of EL spectra of the undoped devices: (a) NPD/mCP/Ph 3 CSBFL/Alq 3 and (b) NPD/mCP/Ph 3 SiSBFL/Alq 3 . As the voltage increases from 8 V to 15 V, the Alq 3 emission from the Ph 3 CSBFL device decreases, and the NPD emission from the Ph 3 SiSBFL device increases. 350 400 450 500 550 600 650 700 0.0 0.5 1.0 Alq 3 8 V 9 - 14 V 15 V Intensity (a.u.) Wavelength (nm) NPD (a) 350 400 450 500 550 600 650 700 0.0 0.5 1.0 Alq 3 8 V 9 - 14 V 15 V Intensity (a.u.) Wavelength (nm) NPD (b) 181 Figure A2.7. Device performance of Ir(ppy) 3 device with 15% doped percentage. 02 4 6 8 10 12 0 20 40 60 80 100 120 140 160 180 200 Ir(ppy)3: Ph3CSBFL, 15% Ir(ppy)3: Ph3SiSBFL, 15% Current Density (mA/cm 2 ) Voltage (V) (a) 024 68 10 10 -1 10 0 10 1 10 2 10 3 10 4 10 5 (b) Ir(ppy)3: Ph3CSBFL, 15% Ir(ppy)3: Ph3SiSBFL, 15% Brightness (Cd/m 2 ) Voltage (V) 0.01 0.1 1 10 100 1 10 (c) Ir(ppy)3: Ph3CSBFL, 15% Ir(ppy)3: Ph3SiSBFL, 15% Quantum Efficiency (%) Current Density (mA/cm 2 ) 182 APPENDIX 3: Chapter 4 Supplemental Information Figure A3.1. 1 HNMR of dichloro bridged dimer [Ir(tpzp) 2 Cl] 2 183 Figure A3.2. 13 C NMR of Ir(tpzp)3 and Ir(tpzp) 2 pic (I1, I2 and I3) 184 Figure A3.3. COSY of Ir(tpzp) 3 185 Figure A3.4. NOESY of Ir(tpzp) 3 186 Figure A3.5 COSY of Ir(tpzp) 2 pic I1 187 Figure A3.6 NOESY of Ir(tpzp) 2 pic I1 188 Figure A3.7. COSY of Ir(tpzp) 2 pic I2 189 Figure A3.8. NOESY of Ir(tpzp) 2 pic I2 190 Figure A3.9. COSY of Ir(tpzp) 2 pic I3 191 Figure A3.10 NOESY of Ir(tpzp) 2 pic I3 192 Figure A3.11. DFT calculated LUMO (top) and HOMO (bottom) orbitals of Ir(tpzp) 3 and the isomers of Ir(tpzp) 2 pic 193 Figure A3.12 Cyclic voltammetry and differential pulse voltammetry (inset) curves of tpzp versus ferrocene/ferrocenium -4 -3 0.5 1.0 1.5 -6 -4 -2 0 2 -3 0.5 1.0 Voltage vs. Cp 2 Fe/Cp 2 Fe + (V) Voltage vs. Cp 2 Fe/Cp 2 Fe + (V) Current (mA) tpzp Figure A3.13 Cyclic voltammetry curves of Ir(tpzp)3 versus ferrocene/ferrocenium -4 -3 -2 0.5 1.0 1.5 -1.5 -1.0 -0.5 0.0 0.5 Current (mA) Voltage vs. Cp 2 Fe/Cp 2 Fe + (V) Ir(tpzp) 3 194 Figure A3.14 Cyclic voltammetry curves of Ir(tpzp)2pic I1 versus ferrocene/ferrocenium -3 -2 0.5 1.0 -0.10 -0.05 0.00 0.05 0.10 Current (mA) Voltage vs. Cp 2 Fe/Cp 2 Fe + (V) Ir(tpzp) 2 pic, I1 Figure A3.15 Cyclic voltammetry curves of Ir(tpzp)2pic I2 versus ferrocene/ferrocenium -3 -2 0.5 1.0 1.5 -0.2 -0.1 0.0 0.1 Ir(tpzp) 2 pic, I2 Current (mA) Voltage vs Cp 2 Fe/Cp 2 Fe + (V) 195 Figure A3.16 Cyclic voltammetry curves of Ir(tpzp) 2 pic I3 versus ferrocene/ferrocenium -4 -3 -2 0.5 1.0 -0.4 -0.2 0.0 0.2 Current (mA) Voltage vs Cp 2 Fe/Cp 2 Fe + (V) Ir(tpzp) 2 pic, I3 196 Figure A3.17 Absorption and emission of tpzp ligand 250 300 350 400 450 500 550 600 0 1 0 1 ABS, tpzp tpzp, PL(singlet), 77K tpzp, PL(triplet), 77K Intensity (a.u.) Wavelength (nm) Absorbance (a.u.) Figure A3.18 HPLC chromatogram of the Ir(tpzp) 2 pic I1 sample before (top) and after (bottom) sublimation. The bottom spectrum shows the chromatogram of the unsublimed samples after heated for 24 h. 197 Table A3.1. Crystal data and structure refinement for Ir(tpzp) 2 pic (I1). Identification code Ir(tpzp) 2 pic (I1) Empirical formula C44 H38 Ir N5 O2 Formula weight 860.99 Temperature 293(2) K Wavelength 0.71073 Å Crystal system Monoclinic Space group P2(1) Unit cell dimensions a = 11.665(2) Å = 90°. 　 b = 11.309(2) Å = 104.26(3)°. 　 c = 16.188(3) Å = 90°. 　 Volume 2069.5(7) Å3 Z 2 Density (calculated) 1.382 Mg/m3 Absorption coefficient 3.266 mm -1 F(000) 860 Crystal size 0.36 x 0.13 x 0.10 mm 3 Theta range for data collection 1.30 to 27.51° Index ranges -14<=h<=15, -14<=k<=11, -18<=l<=20 Reflections collected 12643 Independent reflections 8061 [R(int) = 0.0240] Completeness to theta = 27.51° 97.8 % Absorption correction None Max. and min. transmission 1.000000 and 0.666043 Refinement method Full-matrix least-squares on F 2 Data / restraints / parameters 8061 / 68 / 460 Goodness-of-fit on F2 1.636 Final R indices [I>2sigma(I)] R1 = 0.0522, wR2 = 0.0844 R indices (all data) R1 = 0.0741, wR2 = 0.0868 Absolute structure parameter 0.257(11) Largest diff. peak and hole 0.946 and -0.791 e.Å -3 198 Table A3.2. Crystal data and structure refinement for Ir(tpzp) 2 pic (I3). Identification code Ir(tpzp) 2 pic (I3) Empirical formula C44 H38 Ir N5 O2 Formula weight 860.99 Temperature 148(2) K Wavelength 0.71073 Å Crystal system Monoclinic Space group C2 Unit cell dimensions a = 35.153(10) Å = 90°. 　 b = 11.085(3) Å = 100.264(4)° 　 c = 11.772(3) Å = 90°. 　 Volume 4514(2) Å3 Z 4 Density (calculated) 1.267 Mg/m 3 Absorption coefficient 2.995 mm -1 F(000) 1720 Crystal size 0.25 x 0.15 x 0.055 mm3 Theta range for data collection 1.18 to 27.55° Index ranges -41<=h<=45, -13<=k<=14, -15<=l<=9 Reflections collected 13836 Independent reflections 9394 [R(int) = 0.0219] Completeness to theta = 27.55° 96.7 % Absorption correction Semi-empirical from equivalents Max. and min. transmission 1.0000 and 0.7578 Refinement method Full-matrix least-squares on F 2 Data / restraints / parameters 9394 / 129 / 421 Goodness-of-fit on F2 1.622 Final R indices [I>2sigma(I)] R1 = 0.0524, wR2 = 0.1258 R indices (all data) R1 = 0.0606, wR2 = 0.1297 Absolute structure parameter 0.452(14) Largest diff. peak and hole 2.961 and -1.031 e.Å -3 199 APPENDIX 4: Chapter 6 Supplemental Information Figure A4.1 1D and 2D COSY NMR spectra of Ir(dpq) 3 ppm (f2) 6.00 6.50 7.00 6.00 6.50 7.00 7.50 ppm (f1 200 Table A4.1. Crystal data and structure refinement for Ir(dpq) 3 . Identification code Ir(dpq) 3 Empirical formula C33 H27 Ir N6 Formula weight 699.81 Temperature 133(2) K Wavelength 0.71073 Å Crystal system Trigonal Space group P-3c1 Unit cell dimensions a = 14.821(2) Å = 90°. 　 b = 14.821(2) Å = 90°. 　 c = 15.397(3) Å = 120°. 　 Volume 2929.2(8) Å 3 Z 4 Density (calculated) 1.587 Mg/m 3 Absorption coefficient 4.590 mm -1 F(000) 1376 Crystal size 0.29 x 0.27 x 0.23 mm 3 Theta range for data collection 1.59 to 27.49°. Index ranges -18<=h<=19, -18<=k<=19, -19<=l<=19 Reflections collected 24181 Independent reflections 2249 [R(int) = 0.0609] Completeness to theta = 25.00° 99.8 % Absorption correction Semi-empirical from equivalents Max. and min. transmission 0.7456 and 0.5996 Refinement method Full-matrix least-squares on F 2 Data / restraints / parameters 2249 / 0 / 121 Goodness-of-fit on F 2 1.094 Final R indices [I>2sigma(I)] R1 = 0.0197, wR2 = 0.0550 R indices (all data) R1 = 0.0250, wR2 = 0.0567 Largest diff. peak and hole 0.709 and -0.814 e.Å -3

Abstract (if available)

Abstract Organic light emitting diodes (OLEDs) have achieved high levels of performance including high efficiencies, color purities and device lifetimes. It has been widely utilized in commercial applications except that the blue device lifetime is still poor compared to red and green devices. The stability of high energy OLED materials is a key issue to be addressed for achieving a long life and efficient blue device. In this dissertation, a great effort has been put into developing more stable and more efficient materials that can also fulfill the band gap and photophysical requirements. The materials studied include large bad gap hosts and blue phosphorescent emitters.

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Creator Wei, Wei (author)

Core Title High energy hosts and blue emitters for phosphorescent organic light emitting diodes

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 12/15/2010

Defense Date 10/11/2010

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag emitters,fluorene host materials,iridium complexes,OAI-PMH Harvest,organic light emitting diodes

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Mark E. (committee chair), Goo, Edward K. (committee member), Prakash, G.K. Surya (committee member)

Creator Email gx_weiwei@hotmail.com,shirley.xwwei@gmail.com

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