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Synthesis and photophysical study of phosphorescent hetero-cyclometalated organometallic complexes involving phosphino-carbon ligands

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Synthesis and photophysical study of phosphorescent hetero-cyclometalated organometallic complexes involving phosphino-carbon ligands

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Content Copy Right 2014 Liu Yifei 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 2014 SYNTHESIS AND PHOTOPHYSICAL STUDY OF PHOSPHORESCENT HETERO-CYCLOMETALATED ORGANOMETALLIC COMPLEXES INVOLVING PHOSPHINO-CARBON LIGANDS By Liu Yifei ii DEDICATION Dedicated to my parents: Prof. Liu Xiaocun and Prof. Li Yanjun (ådº.Ù6ÍSXYNsY iii ACKNOWLEDGEMENTS I would like to give the deepest gratitude to my mentor, Prof. Mark E. Thompson, for the careful guidance to my pursuit of scientific knowledge in our laboratory, and for his great patience and encouragement. I sincerely thank Prof. Richard L. Brutchey, Prof. Stephan Haas, Prof. Surya Prakash, Prof. Barry C. Thompson and Prof. Martin A. Gundersen for being on my screening, qualification, and dissertation committees. I would like to thank all my former and present group colleagues for their cheerful personality, talented ideas, and many hours of discussion over research projects in the lab: especial thanks to Prof. Peter I. Djurovich for all the suggestions, conversation, ideas, arguments, and literature database; to Dr. Alberto Bossi, Dr. Wei Wei, Dr. Slava Diev, Dr. Matthew T. Whited, and Dr. Liu Zhiwei for sharing their experiences with organic and organometallic chemistry; to Dr. Valentina Krylova for important discussion over quantum mechanics calculation; to Dr. Wang Siyi, Dr. Marco Curreli, Dr. Zhang Rui, Dr. Cong Trinh, Song Yan, Patrick Erwin, Francisco Navarro, Sarah Rodney, John Facendola, Andrew Bartynski, John Chen, and Patrick Saris for their friendship, all the knowledge they share with me, and great help in the lab. I would like to thank our collaborators Prof. Robert Chow, Dr. Lin Ming-Yi, Prof. Alexander V. Benderskii, Purnim Dhar, Prof. Wolfgang Brütting and Dr. Christian Mayr for sharing fascinating projects over great ranges of research topics. I would like to thank Prof. Ralf Haiges, Prof. Travis J. Williams, and Dr. Allan Kershaw for their great help with X-ray crystallography, NMR, and many other instrumental techniques. Many iv thanks to Judy Fong, Michele Dea and Magnolia Benitez for their great help with everything in the Department of Chemistry. It is important to mention my heartily appreciation to my supervisors at France. I would like to express great thanks to Prof. Stéphane Rigaut and Prof. Pierre H. Dixneuf for introducing me to the world of chemistry and materials science, for their trust and encouragement to me all over the years. Most importantly, words cannot express how grateful I am to my parents, Prof. Liu Xiaocun and Prof. Li Yanjun; my husband and best friend, Dr. Zhang Le; my grandmother Madam Liu Shukui, for their love and greatest support to me. v Table of Contents DEDICATION.................................................................................................................... ii ACKNOWLEDGEMENTS............................................................................................... iii List of Tables ................................................................................................................... viii List of Figures…………………………………………………………………………….ix Abstract………………………………………………………………………………….xiv Chapter 1: Introduction.................................................................................................... 1 1.1. Overview of organic light emitting devices (OLEDs)................................... 1 1.2. Architecture and working principles of OLEDs. ........................................... 2 1.3. Phosphorescent OLED materials. .................................................................. 8 1.3.1. Ir(III) cyclometalated complexes................................................................... 8 1.3.2. Os(II) cyclometalated complexes. ............................................................... 15 1.4. Research focuses of this thesis..................................................................... 16 Chapter 1 References. ................................................................................................... 19 Chapter 2: Synthesis and photophysical characterization of a bis-pincer osmium complex……………......................................................................................................... 22 2.1. Introduction.................................................................................................. 22 2.2. Experimental................................................................................................ 23 2.2.1. Materials and Methods................................................................................. 23 2.2.2. Synthesis of Os(PCP) 2 (PCP = 2,6-(PPh 2 CH 2 ) 2 C 6 H 3 )................................. 24 2.2.3. X-ray Crystallography ................................................................................. 25 2.2.4. Photophysical Characterization ................................................................... 26 2.2.5. Theoretical Calculations .............................................................................. 27 2.2.6. Electrochemistry .......................................................................................... 27 2.3. Results and discussion ................................................................................. 28 2.3.1. Synthetic routes of Os(PCP) 2 ....................................................................... 28 2.3.2. X-ray crystallography .................................................................................. 29 2.3.3. NMR characterization.................................................................................. 31 2.3.4. Electrochemistry .......................................................................................... 40 2.3.5. Electronic spectroscopy ............................................................................... 41 2.3.6. DFT calculations.......................................................................................... 45 2.4. Conclusion ................................................................................................... 49 vi Chapter 2 References .................................................................................................... 50 Chapter 3: Synthesis and photophysical characterization of heteroleptic iridium complexes with phosphinoaryl cyclometalates................................................................. 52 3.1. Introduction.................................................................................................. 52 3.2. Experimental................................................................................................ 55 3.2.1. Materials and Synthesis ............................................................................... 55 3.2.2. X-ray crystallography .................................................................................. 60 3.2.3. Photophysical characterization .................................................................... 63 3.2.4. Theoretical calculations. .............................................................................. 64 3.2.5. Electrochemistry .......................................................................................... 64 3.3. Results and discussion ................................................................................. 65 3.3.1. Synthetic routes of the (ppz) 2 Ir(Cl)(C^P) complexes. ................................. 65 3.3.2. Isomerization study of the (ppz) 2 Ir(Cl)(C^P) complexes. ........................... 66 3.3.3. X-ray crystallography .................................................................................. 68 3.3.4. NMR characterization.................................................................................. 71 3.3.5. Electrochemistry .......................................................................................... 74 3.3.6. The electronic spectroscopy of complexes 1-3............................................ 78 3.3.7. Lifetime and quantum yield studies............................................................. 82 3.3.8. DFT calculations.......................................................................................... 84 3.4. Conclusion. .................................................................................................. 90 Chapter 3 References. ................................................................................................... 91 Chapter 4: Aluminum and zinc based host materials and their potential applications in OLEDs…………………………………………………………………….……………..93 4.1. Introduction.................................................................................................. 93 4.2. Experimental................................................................................................ 96 4.2.1. Materials and synthesis. ............................................................................... 96 4.2.2. X-ray crystallography. ............................................................................... 100 4.2.3. Photophysical characterization. ................................................................. 102 4.2.4. Theoretical calculations. ............................................................................ 103 4.2.5. Device Fabrication..................................................................................... 103 4.3. Results and discussion. .............................................................................. 104 4.3.1. Design and synthesis of the Ir(III) complexes. .......................................... 104 4.3.2. X-ray crystollography of Ir(III) complexes. .............................................. 107 vii 4.3.3. Electrochemistry. ....................................................................................... 109 4.3.4. Electronic Spectroscopy............................................................................. 112 4.3.5. Theoretical calculations of Ir(III) complexes. ........................................... 115 4.3.6. Design and synthesis of the Al(III) and Zn(II) complexes. ....................... 118 4.3.7. Theoretical calculations of complexes Al(PydpO) 3 and Zn(PydpO) 2 . ...... 119 4.3.8. OLEDs studies. .......................................................................................... 123 4.4. Summary.................................................................................................... 127 Chapter 4 References. ................................................................................................. 129 Appendix 1. Synthesis and characterization of [(pq) 2 Ir(bipy-(C 16 H 33 ) 2 )][PF 6 ] and [(pq) 2 Ir(bipy-(C 12 H 25 ) 2 )][PF 6 ]......................................................................................... 131 A1.1. Synthesis of the [(pq) 2 Ir(bipy-(C 16 H 33 ) 2 )][PF 6 ] and [(pq) 2 Ir(bipy-(C 12 H 25 ) 2 )][PF 6 ]..................................................................................... 131 A1.2. Characterization of the [(pq) 2 Ir(bipy-(C 16 H 33 ) 2 )][PF 6 ] and [(pq) 2 Ir(bipy-(C 12 H 25 ) 2 )][PF 6 ]..................................................................................... 132 A1.2.1. NMR characterization......................................................................... 132 A1.2.2. Photophysical characterization. .......................................................... 133 Apendix 1. Reference………...……………………………………………………...134 Appendix 2. Synthesis and characterization of [hdpIr-] ................................................. 135 A2.1. Synthesis of [hdpIr-] ......................................................................................... 135 A2.2. Characterization of [hdpIr-]. ............................................................................. 136 A2.2.1. NMR characterization................................................................................ 136 A2.2.2. Photophysical characterization. ................................................................. 137 Apendix 2. Reference………………………………………………………………...137 BIBLIOGRAPHY........................................................................................................... 138 viii List of Tables Table 2-1. Selected bond lengths (Å) and angles (degree) of Os(PCP) 2 . ......................... 31 Table 2-2. Intermediate-exchange rate constants(k) of the methylene protons in Os(PCP) 2 . ........................................................................................................................................... 39 Table 2-3. Photophysical properties of Os(PCP) 2 ............................................................. 41 Table 2-4. S 0 S n (n = 1-10) transitions, S 0 T 1 and S 0 T 2 transitions, wavelength (λ cal ), oscillator strength (f), and transition contributions (> 10%, and all transitions are listed for S 0 S 1 and S 0 S 2 ) and assignments for Os(PCP) 2 from TDDFT calculations. ....... 47 Table 3-1. Selected bond angles (degree) and distances (Å) for complexes 1-Cl, 1-mer, 1-fac. ................................................................................................................................. 69 Table 3-2. Photophysical properties of complexes 1-3..................................................... 79 Table 3-3. S 0 S 1 and S 0 →T 1 transitions, wavelength (λ cal ), oscillator strength (f), transition contributions (>10%), and assignments of complexes n(n = 1-3) from TD-DFT calculations………………………………………………………………………………87 Table 3-4. Metal and ligand contribution (%) to the major frontier orbital involved in S 0 →S 1 and S 0 →T 1 transition for the cyclometalated complexes 1-3. The ligands are defined to ppz, naphthalene/(iso)quinoline (C^P c ), and PPh 2 linkage (C^P p )………...…88 Table 4-1. Selected Bond Angles (degree) and Bond Distances (Å) of complexes (ppz) 2 Ir(PydpS), (ppz) 2 Ir(PydpO), and (ppy) 2 Ir(PydpO) (X denotes to O and S elements in different complexes). .................................................................................................. 109 Table 4-2. Photophysical properties of complexes (ppz) 2 Ir(PydpO) and (ppy) 2 Ir(PydpO). ......................................................................................................................................... 112 Table 4-3. S 0 S n (n = 1,2) transitions, S 0 T 1 and S 0 T 2 transitions, wavelength (λ cal ), oscillator strength (f), and transition contributions (> 10%) and assignments for (ppz) 2 Ir(PydpO) from TD-DFT calculations. ................................................................. 116 Table A1-1. Photophysical properties of complexes [(pq) 2 Ir(bipy-(R) 2 )][PF 6 ](R = C 16 H 33 and C 12 H 25 )……………………………………………………………………………..134 Table A2-1. Photophysical properties of complexes [hdpIr-]………………….……...136 ix List of Figures. Figure 1-1. Common heterojunction OLEDs structure....................................................... 2 Figure 1-2. Energy diagram of early OLED structures. (Left): Holes (orange circles) and electrons (green circles) generated and migrate through HOMO of HTL and LUMO of ETL materials respectively, recombine at the HTL/ETL interface, followed by exciton formation and exciton decay to the emission of photon (yellow) to achieve EL. (Right:): Holes and electrons eventually meet at the dopant doped in the ETL layer (orange square), followed by emission from the dopant................................................................................ 4 Figure 1-3. Energy diagram and working principle of an OLED with HTL, doped EML, and ETL layers.................................................................................................................... 5 Figure 1-4. Föster resonance energy transfer (FRET) and Dexter electron-exchange energy transfer (DET) from host to dopant......................................................................... 6 Figure 1-5. Jablonski diagram for general illustration of the energy transfer and states in OLEDs (abs. = absorbance, fl. = fluorescence, pl. = phosphorescence, k nr represents the non-radiative decays). The arrows in the dashed squares represent the spins at singlet ground state (S 0 ), singlet excited state (S 1 ) and triplet excited state (T 1 ). .......................... 7 Figure 1-6. Examples of some heteroleptic and homoleptic Ir(III) phosphors................. 10 Figure 1-7. Modified Jablonski diagram for illustration of the temperature dependentphosphorescence of blue emitters. Triplet spin density on six- and five- coordination forms of Ir(ppz) 3 are shown......................................................................... 11 Figure 1-8. Example of Ir(III) complexes with terdentate chelates. ................................. 13 Figure 1-9. Bis-terdentate Ir(III)-carbene complexes. ...................................................... 14 Figure 1-10. Exemples of phosphorescence Os(II) complexes. ....................................... 16 Figure 1-11. Os(PCP) 2 complex from Chapter 2. ............................................................. 16 Figure 1-12. Heteroleptic Ir(III)-C^P complexes from Chapter 2. ................................... 17 Figure 1-13. Ir(III), Al(III) and Zn(II) complexes studied in Chapter 4........................... 18 Figure 2-1. Two perspective views of Os(PCP) 2 shown in 50% probability thermal ellipsoids (left, middle). Hydrogen atoms and the phenyl rings on phosphorus (left) are omitted for clarity. Projection view down the C(31)–Os(1)–C(C47) axis of Os(PCP) 2 (right). Solid and dashed lines represent the xylyl planes. .............................................. 30 x Figure 2-2. 1 H NMR (600 MHz, 74 C and 25 C, C 6 D 6 ) spectra of Os(PCP) 2 .................. 32 Figure 2-3. 1 H NMR (600 MHz, 223 K, C 7 D 8 ) spectrum of Os(PCP) 2 ............................ 33 Figure 2-4. gCOSY NMR (600 MHz, 223 K, C 7 D 8 ) spectrum of Os(PCP) 2 . .................. 34 Figure 2-5. Stacked Spectra of 1 H NMR Inversion Recovery Data (600 MHz, 233 K, C 7 D 8 )................................................................................................................................. 35 Figure 2-6. 1 H NMR inversion recovery experiment data collected at 233 K in C 7 D 8 solution.............................................................................................................................. 35 Figure 2-7. VT- 1 H NMR(600 MHz, 233 K to 343 K, C 7 D 8 ) spectra of Os(PCP) 2 .......... 36 Figure 2-8. Variable temperature 1 H NMR spectra (600 MHz, C 7 D 8 ) for the methylene protons of Os(PCP) 2 .......................................................................................................... 37 Figure 2-9. Process proposed for the racemization of Os(PCP) 2 through a D 2d transition state. .................................................................................................................................. 38 Figure 2-10. Arrehnius plot based on intermediate-exchange rate constants(k) of the methylene protons in Os(PCP) 2 . Slope = -4320, intercept = 24. ...................................... 39 Figure 2-11. Cyclic voltammetry(at 0.1 V/s) and DPV (at 0.01 V/s) of Os(PCP) 2 (in CH 2 Cl 2 , 0.1 M [Bu 4 N][PF 6 ])............................................................................................. 40 Figure 2-12. Absorption (recorded in CH 2 Cl 2 ) and 77 K emission spectra (recorded in 2-MeTHF glass) of PCP-H ligand. ................................................................................... 42 Figure 2-13. Absorption spectrum of Os(PCP) 2 in CH 2 Cl 2 solution (black line) and the relative oscillator strengths calculated using TD-DFT (green lines) along with emission spectra recorded in 2-MeTHF glass at 77 K (blue line) and as a neat solid at room temperature (red dashed line)............................................................................................ 42 Figure 2-14. Absorption spectra of Os(PCP) 2 and [Os(PCP) 2 ] + in CH 2 Cl 2 solution........ 43 Figure 2-15. (Top) Excitation and emission spectra for 10% Os(PCP) 2 doped in PMMA, recorded at 77 K. (Bottom) Excitation and emission spectra for neat solid Os(PCP) 2 recorded at 77 K................................................................................................................ 44 Figure 2-16. Frontier orbitals involved in the S 0 S n (n = 1–6) and S 0 T 1 transitions of Os(PCP) 2 ........................................................................................................................... 46 Figure 2-17. Spin density surface calculated for the triplet state of Os(PCP) 2 ................. 48 xi Figure 2-18. Energy diagram illustrating the radiative and non-radiative decay channels involving the singlet and triplet states of Os(PCP) 2 .......................................................... 48 Figure 3-1. Traditional and modified synthetic routes. Synthetic Route i: Ag 2 O, DCE, refluxed 18 h in dark. Synthetic Route ii: Ag 2 O, KOH, DCB, heated at 120 ̊C 18 h in dark. .................................................................................................................................. 65 Figure 3-2. Photo-isomerization process of complexes 1 and 2. ...................................... 66 Figure 3-3. Absorbance of n-mern-fac under cumulative irradiation time, recorded in deaerated acetonitrile solution and the photo-isomerization rates(k) of (n-mern-fac) (n = 1-3). A 0 , A t , A f are the initial absorbance, absorbance at time t, and final absorbance of the photo-isomerization processes.. .................................................................................. 67 Figure 3-4. Perspective drawings of 1-Cl, 1-mer, and 1-fac shown in 50% probability thermal ellipsoids. The atoms are colored by green (Ir), black (C), blue (N), orange (P) and lime (Cl). The hydrogen atoms are omitted for clarity. ............................................. 68 Figure 3-5. 1 H NMR for complex 1-Cl(top), 1-mer(middle), and 1-fac(bottom). The solvent peak is marked with “x”. See SI for the corresponding gCOSY NMRs. ............. 70 Figure 3-6. gCOSY (500 MHz, CDCl 3 ) spectrum of complex 1-Cl. ............................... 71 Figure 3-7. gCOSY (500 MHz, CDCl 3 ) spectrum of complex 1-mer.............................. 72 Figure 3-8. gCOSY (500 MHz, CDCl 3 ) spectrum of complex 1-fac. .............................. 72 Figure 3-9. CV (at 0.1 V/s) and DPV (at 0.01 V/s) of PPh 2 -naphthalene, 5-PPh 2 -quinoline, and 5-PPh 2 -isoquinoline(in DMF, 0.1 M [Bu 4 N][PF 6 ])………………………………...74 Figure 3-10. CV(at 0.1 V/s) and DPV (at 0.01 V/s) of complexes 1-mer and 1-fac(in DMF, 0.1 M [Bu 4 N][PF 6 ])................................................................................................ 75 Figure 3-11. CV (at 0.1 V/s) and DPV (at 0.01 V/s) of complexes 2-mer and 2-fac(in DMF, 0.1 M [Bu 4 N][PF 6 ])................................................................................................ 76 Figure 3-12. CV (at 0.1 V/s) and DPV (at 0.01 V/s) of complexes 3-mer and 3-fac(in DMF, 0.1 M [Bu 4 N][PF 6 ])................................................................................................ 77 Figure 3-13. Left: Absorbance spectra (recorded in CH 2 Cl 2 ); Right: Emission spectra (recorded in 2-MeTHFglass at 77 K) of ligand PPh 2 -naphthalene, 5-PPh 2 -quinoline, and 5-PPh 2 -isoquinoline. ......................................................................................................... 80 Figure 3-14. Absorption and emission spectra of 1-Cl(top, left), 1-mer(top, right) and 1-fac(bottom) recorded in CH 2 Cl 2 (r.t.) and 2-MeTHF (77 K). ...................................... 81 xii Figure 3-15. Absorption and emission spectra of complexes 2 and 3 recorded in CH 2 Cl 2 (r.t.) and 2-MeTHF (77 K)................................................................................................ 82 Figure 3-16. Selected frontier orbitals and triplet spin surfaces of complexes 1-3 from DFT calculations............................................................................................................... 89 Figure 4-1. Synthetic Routes of Ir(III)-(PydpX)(X = O, S) complexes......................... 104 Figure 4-2. gCOSY(500 MHz, CD 2 Cl 2 ) of (ppz) 2 Ir(PydpO)......................................... 106 Figure 4-3. gCOSY (500 MHz, CD 2 Cl 2 ) of (ppz) 2 Ir(PydpS). ........................................ 106 Figure 4-4. gCOSY (500 MHz, CD 2 Cl 2 ) of (ppy) 2 Ir(PydpO). ...................................... 107 Figure 4-5. Perspective views of (ppz) 2 Ir(PydpO) (left), (ppy) 2 Ir(PydpO) (middle), and (ppz) 2 Ir(PydpS) (right) in 50% probability thermal ellipsoids. The methane phenyls and all hydrogen atoms are left for clarity in all complexes. The atoms are colored by green (Ir), white (C), yellow(S), red (O), and blue(N). .......................................................... 108 Figure 4-6. CV (at 0.1 V/s) and DPV(at 0.01 V/s) traces of complexes(ppz) 2 Ir(PydpO), (ppz) 2 Ir(PydpS), and (ppy) 2 Ir(PydpO)(in DMF, 0.1 M [Bu 4 N][PF 6 ])........................... 111 Figure 4-7. (top): Absorbance spectra of PydpO ligand (black dot line) and complex (ppz) 2 Ir(PydpO) (red solid line) recorded in CH 2 Cl 2 at room temperature. Emission spectra of PydpO ligand (blue dot line) and complex (ppz) 2 Ir(PydpO) (blue solid line) recorded in 2-MeTHF at 77 K. (Bottom): Absorbance spectrum of complex (ppy) 2 Ir(PydpO) (black line) recorded in CH 2 Cl 2 at room temperature. Emission spectra of complex (ppy) 2 Ir(PydpO) recorded in 2-MeTHF solution (298 K, red line) and in 2- MeTHF glass (blue line, 77 K). ...................................................................................... 113 Figure 4-8. Emission spectra of complex Ir(ppz) 3 (blue dot line) and complex (ppz) 2 Ir(PydpO) (blue solid line) recorded in 2-MeTHF at 77 K................................... 114 Figure 4-9. Frontier orbitals involved in the S 0 S n (n = 1, 2) and S 0 T 1 transitions of (ppz) 2 Ir(PydpO), (ppz) 2 Ir(PydpS), and (ppy) 2 Ir(PydpO)................................................ 117 Figure 4-10. Frontier orbitals involved in the S 0 S n (n = 1, 2) and S 0 T 1 transitions of mer/fac-Al(PydpO) 3 and Zn(PydpO) 2 ............................................................................. 120 Figure 4-11. Energy Diagrams of the fabricated OLED devices.................................... 124 Figure 4-12. Brightness vs. Voltage (top, left), current density vs. voltage (top, right), EQE vs. brightness plot (bottom, left) and electroluminescence spectra measured at 7 V for Device I and Device S............................................................................................... 124 xiii Figure 4-13. Brightness vs. Voltage (top, left), current density vs. voltage (top, right), EQE vs. brightness plot (bottom, left) and electroluminescence spectra measured at 7 V for Device II and Device S.............................................................................................. 125 Figure 4-14. Electroluminescence spectra measured at voltages of 5 V, 7 V, 11 V, and 13 V of Device III........................................................................................................... 126 Figure A1-1. (top): Absorbance and Emission spectra of [(pq) 2 Ir(bipy-(C 16 H 33 ) 2 )][PF 6 ] recorded in MeOH solution.(bottom): Absorbance and Emission spectra of [(pq) 2 Ir(bipy-(C 12 H 25 ) 2 )][PF 6 ] recorded in MeOH solution…………………………….133 Figure A2-1. Absorbance and Emission spectra of [hdpIr-] recorded in MeOH solution………………………………………………………………………………….137 xiv Abstract Organic Light Emitting Devices (OLEDs) displays and lightings are now available on the market of consumable optic electronics. The materials used in the devices must meet certain stability and energetic requirements. This dissertation takes effort in developing new generation of organometallic phosphorescent complexes with phosphino-carbon chelated ligands for potential applications in OLEDs. Chapter 1 shows an overview on the development of the OLEDs, and illustrates the mechanisms of the OLEDs from materials and device structure aspects. Chapter 2 describes the design, synthesis and photophysical characterization of an Os(II) complex with high energy bis-pincer chelated ligand. A thorough illustration is presented for the phosphorescence mechanism of the complex. Chapter 3 focuses on a series of heteroleptic Ir(III) complexes with diphenylphosphinoaryl chelated ligands. The synthesis and photophysical properties of the complexes are examined to illustrate the potential application of Ir(III)-phosphino^carbon coordinates as chromophores of the heteroleptic Ir(III) complexes. In the end, a series of high energy aluminum and zinc based complexes are studied in Chapter 4 for their potential applications as high energy host materials for blue and green dopant materials. 1 Chapter 1: Introduction 1.1. Overview of organic light emitting devices(OLEDs) OLED TVs and smart phones with OLED display are now available on market for hundreds to several thousands of dollars. It has been decades for the technique to gradually get improved, and show advantages over traditional LCD and LED techniques. For example, the OLED displays can be fabricated rather thin; has faster response time and wider viewing angles; and can achieve true black level for great color quality. The phenomenon of electroluminescence (EL) was first demonstrated by Bernanose et. al. in the 1950s. 1 Acridine orange and quinacrine EL were observed when high-voltage alternating current was applied. In 1963, Pope and co-workers reported that the EL of single crystal anthracene and single crystal anthracene with about 0.1% (mol) tetracene impurity were observed under direct current of over 400 V. 2 The attraction of the EL researches of organic materials burst out in the 1980s when Tang and VanSlyke established the pioneer of the modern electroluminescent OLEDs. 3 Two stacked organic thin film layers are sandwiched between two metal electrodes, and the OLED showed about 1% external quantum efficiency (EQE) at a low voltage. Device structure modifications and varieties of emission materials were intensively studied thereafter for reaching higher efficiency. The next big breakthrough of the OLED technique came out in late 1990s from the works of Thompson and Forrest. 4 For the first time, phosphorescent materials were introduced into the OLEDs, showing the possibility to 2 significantly increase both the internal quantum yield (IQE) and the EQE of the devices. Over the decades, researches have been greatly focused on improving the performances from the aspects of understanding the working mechanisms of the OLEDs materials, generating new families of highly efficient emitters, and improvement of the device architectures. 1.2. Architecture and working principles of OLEDs Nowadays heterojunction structures are employed in most OLEDs (Figure 1-1). As viewed through the device substrate (usually of transparent glass or polymers), Indium Tin Oxide (ITO) (In 2 O 3 :SnO 2 ) is a widely used material patterned on the substrate as the OLED anodes with important features such as transparency, high conductivity, and efficiency as a hole injector into the organic materials. 5 Several layers of organic thin films are deposited on the ITO by vapor deposition (10 -6 torr), including major layers of hole transporting layer (HTL), electron transporting layer (ETL), and emissive layer (EML). More refined devices also include hole or electron injection layers and blocking Figure 1-1. Common heterojunction OLEDs structure. 2 significantly increase both the internal quantum yield (IQE) and the EQE of the devices. Over the decades, researches have been greatly focused on improving the performances from the aspects of understanding the working mechanisms of the OLEDs materials, generating new families of highly efficient emitters, and improvement of the device architectures. 1.2. Architecture and working principles of OLEDs Nowadays heterojunction structures are employed in most OLEDs (Figure 1-1). As viewed through the device substrate (usually of transparent glass or polymers), Indium Tin Oxide (ITO) (In 2 O 3 :SnO 2 ) is a widely used material patterned on the substrate as the OLED anodes with important features such as transparency, high conductivity, and efficiency as a hole injector into the organic materials. 5 Several layers of organic thin films are deposited on the ITO by vapor deposition (10 -6 torr), including major layers of hole transporting layer (HTL), electron transporting layer (ETL), and emissive layer (EML). More refined devices also include hole or electron injection layers and blocking Figure 1-1. Common heterojunction OLEDs structure. 2 significantly increase both the internal quantum yield (IQE) and the EQE of the devices. Over the decades, researches have been greatly focused on improving the performances from the aspects of understanding the working mechanisms of the OLEDs materials, generating new families of highly efficient emitters, and improvement of the device architectures. 1.2. Architecture and working principles of OLEDs Nowadays heterojunction structures are employed in most OLEDs (Figure 1-1). As viewed through the device substrate (usually of transparent glass or polymers), Indium Tin Oxide (ITO) (In 2 O 3 :SnO 2 ) is a widely used material patterned on the substrate as the OLED anodes with important features such as transparency, high conductivity, and efficiency as a hole injector into the organic materials. 5 Several layers of organic thin films are deposited on the ITO by vapor deposition (10 -6 torr), including major layers of hole transporting layer (HTL), electron transporting layer (ETL), and emissive layer (EML). More refined devices also include hole or electron injection layers and blocking Figure 1-1. Common heterojunction OLEDs structure. 3 layers etc. for pursuing better device working function. The N,N’-di(naphthalene-1-yl)- N,N’-dipheynl-benzidine (α-NPD) and aluminum tris(8-hydroxyquinoline) (Alq 3 ) are among the most commonly used HTL and ETL materials respectively. 3, 6 The low crystal-growth velocity is necessary for all organic layer materials to prevent recrystallization in the devices within the life span of the OLEDs. 7 Sublimable materials are required for vapor deposition fabricated OLEDs, and thus the relatively high glass transition temperature (T g ) is necessary for the materials. Metal cathode is then deposited on top of the organic layers. The LiF/Al cathode is widely used in OLEDs. The aluminum as the high work function metal shows less reactivity than alkali metals such as Li, Ca and Mg. 8 The efficiency of the devices with Al cathode can be improved by a layer of LiF spacer which facilitates the electron injection efficiency. When an external driving voltage is applied to the OLEDs, the holes are generated by removing an electron from the highest occupied molecular orbital (HOMO) of the HTL material. Similar process occurs on the cathode, as electron transportation process adds an electron to the lowest unoccupied molecular orbital (LUMO) of the ETL material. Following the hole and electron injection processes, the holes and electrons migrate through the HTL and ETL layers, and then recombine and form excitons in the EML layer. The radiative decay of the excitons thus provides light from the device (EL). In the earliest work of Tang and VanSlyke, the OLEDs structure contains two organic material layers of a diamine (HTL) and Alq 3 (ETL) (Figure 1-2, left). 3 The charge recombination happens in the HTL/ETL interface, and the green fluorescence (1% EQE) 4 is from the Alq 3 of smaller HOMO-LUMO energy gap compared to the diamine. Later, they reported the OLEDs with structure of ITO/diamine/Alq 3 /doped Alq 3 /Alq 3 /Mg:Ag, where the Alq 3 takes the role of both ETL and host of the EML (Figure 1-2, right). The Alq 3 layers are used along with HTL layer to confine charge recombination and prevent leakages. The dopants (coumarin 540, DCM1, and DCM2) are of lower HOMO-LUMO energy gap than the Alq 3 , and the charges are readily transferred from Alq 3 to the dopants. The EL of the devices are purely from the dopants, and reach higher EQE (2.5%) compared to the earliest devices. 9 Further modification of the OLED structure leads to the doped tri-layer devices with HTL/EML/ETL layers (Figure 1-3). 10 Though the EML does not need to be a discrete Figure 1-2. Energy diagram of early OLED structures. (Left): Holes (orange circles) and electrons (green circles) generated and migrate through HOMO of HTL and LUMO of ETL materials respectively, recombine at the HTL/ETL interface, followed by exciton formation and exciton decay to the emission of photon (yellow) to achieve EL. (Right:): Holes and electrons eventually meet at the dopant doped in the ETL layer (orange square), followed by emission from the dopant. 4 is from the Alq 3 of smaller HOMO-LUMO energy gap compared to the diamine. Later, they reported the OLEDs with structure of ITO/diamine/Alq 3 /doped Alq 3 /Alq 3 /Mg:Ag, where the Alq 3 takes the role of both ETL and host of the EML (Figure 1-2, right). The Alq 3 layers are used along with HTL layer to confine charge recombination and prevent leakages. The dopants (coumarin 540, DCM1, and DCM2) are of lower HOMO-LUMO energy gap than the Alq 3 , and the charges are readily transferred from Alq 3 to the dopants. The EL of the devices are purely from the dopants, and reach higher EQE (2.5%) compared to the earliest devices. 9 Further modification of the OLED structure leads to the doped tri-layer devices with HTL/EML/ETL layers (Figure 1-3). 10 Though the EML does not need to be a discrete Figure 1-2. Energy diagram of early OLED structures. (Left): Holes (orange circles) and electrons (green circles) generated and migrate through HOMO of HTL and LUMO of ETL materials respectively, recombine at the HTL/ETL interface, followed by exciton formation and exciton decay to the emission of photon (yellow) to achieve EL. (Right:): Holes and electrons eventually meet at the dopant doped in the ETL layer (orange square), followed by emission from the dopant. 4 is from the Alq 3 of smaller HOMO-LUMO energy gap compared to the diamine. Later, they reported the OLEDs with structure of ITO/diamine/Alq 3 /doped Alq 3 /Alq 3 /Mg:Ag, where the Alq 3 takes the role of both ETL and host of the EML (Figure 1-2, right). The Alq 3 layers are used along with HTL layer to confine charge recombination and prevent leakages. The dopants (coumarin 540, DCM1, and DCM2) are of lower HOMO-LUMO energy gap than the Alq 3 , and the charges are readily transferred from Alq 3 to the dopants. The EL of the devices are purely from the dopants, and reach higher EQE (2.5%) compared to the earliest devices. 9 Further modification of the OLED structure leads to the doped tri-layer devices with HTL/EML/ETL layers (Figure 1-3). 10 Though the EML does not need to be a discrete Figure 1-2. Energy diagram of early OLED structures. (Left): Holes (orange circles) and electrons (green circles) generated and migrate through HOMO of HTL and LUMO of ETL materials respectively, recombine at the HTL/ETL interface, followed by exciton formation and exciton decay to the emission of photon (yellow) to achieve EL. (Right:): Holes and electrons eventually meet at the dopant doped in the ETL layer (orange square), followed by emission from the dopant. 5 layer in the device, the exciton energy of this layer can be chosen to be lower than either HTL or ETL to achieve efficient exciton confinement. The HOMO level of EML is preferred to be higher than the HOMO of the HTL. And the LUMO is preferred to be lower than the LUMO of the ETL. The energetic design can help confine the charge recombination and prevent charge leakage. In the doped EML, the excitons can be efficiently trapped on the dopant, and significantly improve the EQE and color purity of the device. The low concentration of the dopant can efficiently prevent self-quenching and thus increase the efficiency of the devices. The variation of dopant energies provides color tuning in the electroluminescence through the full-spectral range. The energy transfer process in the OLEDs involved two different mechanisms of Föster resonance energy transfer (FRET) and Dexter electron-exchange energy transfer (DET) (Figure 1-4). 10-11 The FRET is based on the electrostatic interaction between the host and dopant molecules involving the dipole-dipole coupling between the transition Figure 1-3. Energy diagram and working principle of an OLED with HTL, doped EML, and ETL layers. 5 layer in the device, the exciton energy of this layer can be chosen to be lower than either HTL or ETL to achieve efficient exciton confinement. The HOMO level of EML is preferred to be higher than the HOMO of the HTL. And the LUMO is preferred to be lower than the LUMO of the ETL. The energetic design can help confine the charge recombination and prevent charge leakage. In the doped EML, the excitons can be efficiently trapped on the dopant, and significantly improve the EQE and color purity of the device. The low concentration of the dopant can efficiently prevent self-quenching and thus increase the efficiency of the devices. The variation of dopant energies provides color tuning in the electroluminescence through the full-spectral range. The energy transfer process in the OLEDs involved two different mechanisms of Föster resonance energy transfer (FRET) and Dexter electron-exchange energy transfer (DET) (Figure 1-4). 10-11 The FRET is based on the electrostatic interaction between the host and dopant molecules involving the dipole-dipole coupling between the transition Figure 1-3. Energy diagram and working principle of an OLED with HTL, doped EML, and ETL layers. 5 layer in the device, the exciton energy of this layer can be chosen to be lower than either HTL or ETL to achieve efficient exciton confinement. The HOMO level of EML is preferred to be higher than the HOMO of the HTL. And the LUMO is preferred to be lower than the LUMO of the ETL. The energetic design can help confine the charge recombination and prevent charge leakage. In the doped EML, the excitons can be efficiently trapped on the dopant, and significantly improve the EQE and color purity of the device. The low concentration of the dopant can efficiently prevent self-quenching and thus increase the efficiency of the devices. The variation of dopant energies provides color tuning in the electroluminescence through the full-spectral range. The energy transfer process in the OLEDs involved two different mechanisms of Föster resonance energy transfer (FRET) and Dexter electron-exchange energy transfer (DET) (Figure 1-4). 10-11 The FRET is based on the electrostatic interaction between the host and dopant molecules involving the dipole-dipole coupling between the transition Figure 1-3. Energy diagram and working principle of an OLED with HTL, doped EML, and ETL layers. 6 dipole moments of the excited host and the ground state dopant. The energy from the relaxing excited host molecule is transferred through the strong Coulombic interaction to the dopant molecule, and the interaction can be very strong within up to 100 Å. The DET mechanism involves electron exchange of either simultaneous or consecutive electron transfers which bring the host to the ground state and the dopant to the electronic excited state within a much shorter distances of typically less than 20 Å. 12 The DET decreases exponentially as the distances between the host and dopant increase. The doped trilayer device architecture is especially useful with the creation of phosphorescence OLEDs. 4, 11, 13 An illustration of energy states and transfer routes for photoluminescence molecules is shown in Figure 1-5. 14 The molecules are excited to the singlet excited state (S n ) after absorbance of light. The rapid internal conversion (IC, at 10 -13 s) brings the molecules back to the lowest singlet excited state (S 1 ) via thermal Figure 1-4. Föster resonance energy transfer (FRET) and Dexter electron-exchange energy transfer (DET) from host to dopant. 6 dipole moments of the excited host and the ground state dopant. The energy from the relaxing excited host molecule is transferred through the strong Coulombic interaction to the dopant molecule, and the interaction can be very strong within up to 100 Å. The DET mechanism involves electron exchange of either simultaneous or consecutive electron transfers which bring the host to the ground state and the dopant to the electronic excited state within a much shorter distances of typically less than 20 Å. 12 The DET decreases exponentially as the distances between the host and dopant increase. The doped trilayer device architecture is especially useful with the creation of phosphorescence OLEDs. 4, 11, 13 An illustration of energy states and transfer routes for photoluminescence molecules is shown in Figure 1-5. 14 The molecules are excited to the singlet excited state (S n ) after absorbance of light. The rapid internal conversion (IC, at 10 -13 s) brings the molecules back to the lowest singlet excited state (S 1 ) via thermal Figure 1-4. Föster resonance energy transfer (FRET) and Dexter electron-exchange energy transfer (DET) from host to dopant. 6 dipole moments of the excited host and the ground state dopant. The energy from the relaxing excited host molecule is transferred through the strong Coulombic interaction to the dopant molecule, and the interaction can be very strong within up to 100 Å. The DET mechanism involves electron exchange of either simultaneous or consecutive electron transfers which bring the host to the ground state and the dopant to the electronic excited state within a much shorter distances of typically less than 20 Å. 12 The DET decreases exponentially as the distances between the host and dopant increase. The doped trilayer device architecture is especially useful with the creation of phosphorescence OLEDs. 4, 11, 13 An illustration of energy states and transfer routes for photoluminescence molecules is shown in Figure 1-5. 14 The molecules are excited to the singlet excited state (S n ) after absorbance of light. The rapid internal conversion (IC, at 10 -13 s) brings the molecules back to the lowest singlet excited state (S 1 ) via thermal Figure 1-4. Föster resonance energy transfer (FRET) and Dexter electron-exchange energy transfer (DET) from host to dopant. 7 relaxation. At S 1 state, fluorescent molecules further relax to the ground state (S 0 ) by emission of photons. The non-radiative decay pathways (k nr ) are inefficient compared to the fluorescence pathways of the molecules of fast lifetimes in the scale within several hundreds of nanoseconds (ns). However, the maximum internal quantum efficiency of OLEDs based on such fluorescence molecules is relatively low (25%). This is because the excitons in OLEDs are electronically generated in the ratio of 3 : 1 (spin-forbidden triplet : spin-allowed singlet). In many fluorescence OLEDs, only the 25% of the singlet excitons can be harvested, while the 75% of the triplet excitons are lost through non- radiative decays. A solution for reaching higher internal quantum efficiency of OLEDs is by employing phosphorescent materials that emits from the triplet state. As shown in Figure 1-5, a molecule at S 1 state can also populate to the triplet excited state (T 1 ) via inter-system crossing (ISC). In a fluorescence molecule, the ISC is quite weak, and the population to the T 1 state is inefficient. The phosphorescence is normally observed only at low Figure 1-5. Jablonski diagram for general illustration of the energy transfer and states in OLEDs (abs. = absorbance, fl. = fluorescence, pl. = phosphorescence, k nr represents the non-radiative decays). The arrows in the dashed squares represent the spins at singlet ground state (S 0 ), singlet excited state (S 1 ) and triplet excited state (T 1 ). 7 relaxation. At S 1 state, fluorescent molecules further relax to the ground state (S 0 ) by emission of photons. The non-radiative decay pathways (k nr ) are inefficient compared to the fluorescence pathways of the molecules of fast lifetimes in the scale within several hundreds of nanoseconds (ns). However, the maximum internal quantum efficiency of OLEDs based on such fluorescence molecules is relatively low (25%). This is because the excitons in OLEDs are electronically generated in the ratio of 3 : 1 (spin-forbidden triplet : spin-allowed singlet). In many fluorescence OLEDs, only the 25% of the singlet excitons can be harvested, while the 75% of the triplet excitons are lost through non- radiative decays. A solution for reaching higher internal quantum efficiency of OLEDs is by employing phosphorescent materials that emits from the triplet state. As shown in Figure 1-5, a molecule at S 1 state can also populate to the triplet excited state (T 1 ) via inter-system crossing (ISC). In a fluorescence molecule, the ISC is quite weak, and the population to the T 1 state is inefficient. The phosphorescence is normally observed only at low Figure 1-5. Jablonski diagram for general illustration of the energy transfer and states in OLEDs (abs. = absorbance, fl. = fluorescence, pl. = phosphorescence, k nr represents the non-radiative decays). The arrows in the dashed squares represent the spins at singlet ground state (S 0 ), singlet excited state (S 1 ) and triplet excited state (T 1 ). 7 relaxation. At S 1 state, fluorescent molecules further relax to the ground state (S 0 ) by emission of photons. The non-radiative decay pathways (k nr ) are inefficient compared to the fluorescence pathways of the molecules of fast lifetimes in the scale within several hundreds of nanoseconds (ns). However, the maximum internal quantum efficiency of OLEDs based on such fluorescence molecules is relatively low (25%). This is because the excitons in OLEDs are electronically generated in the ratio of 3 : 1 (spin-forbidden triplet : spin-allowed singlet). In many fluorescence OLEDs, only the 25% of the singlet excitons can be harvested, while the 75% of the triplet excitons are lost through non- radiative decays. A solution for reaching higher internal quantum efficiency of OLEDs is by employing phosphorescent materials that emits from the triplet state. As shown in Figure 1-5, a molecule at S 1 state can also populate to the triplet excited state (T 1 ) via inter-system crossing (ISC). In a fluorescence molecule, the ISC is quite weak, and the population to the T 1 state is inefficient. The phosphorescence is normally observed only at low Figure 1-5. Jablonski diagram for general illustration of the energy transfer and states in OLEDs (abs. = absorbance, fl. = fluorescence, pl. = phosphorescence, k nr represents the non-radiative decays). The arrows in the dashed squares represent the spins at singlet ground state (S 0 ), singlet excited state (S 1 ) and triplet excited state (T 1 ). 8 temperature, and of quite long lifetimes (milliseconds to seconds). However, it is discovered that heavy metals such as Ru, Os, Ir, Cu can efficiently strengthen the spin-orbit coupling (SOC) process which mixes the singlet and triplet, and significantly enhance the ISC. The heavy metal effect not only removes the spin-forbidden nature of the triplet state, but also greatly reduces the phosphorescent lifetime to microsecond (µs) scale. Carefully designed phosphorescence OLEDs can reach the maximum of almost 100% internal quantum efficiency. 13, 15 1.3. Phosphorescent OLED materials With the development of phosphorescent OLEDs, the demand of phosphorescence dopant materials dramatically increases, leading to the intensive studies on such molecules. 1.3.1. Ir(III) cyclometalated complexes 1.3.1.1. Bidentate Ir(III) cyclometalated complexes Ir(III) cycloemetalated complexes are among the most efficient phosphorescent OLEDs materials for decades (Figure 1-6). In 1999, Baldo and co-workers reported the highly efficient green OLEDs based on Ir(ppy) 3 (ppy = 2-phenylpyridine) emitter doped in 4,4’-N,N’-dicarbazole-biphenyl (CBP) host (8.0% EQE). 4b Bi-dentate Ir(III) complexes can be further defined as homoleptic and heteroleptic Ir(III) complexes depending on the identity of the chelated ligands. 16 A series of heteroleptic bis-cyclometalated Ir(III) complexes of green to red emitters were reported by Lamansky and co-workers later in 2001. 17 The iridium complexes were designed with two of a 9 cyclometalated ligand (C^N), and an ancillary ligand of β-diketonates, especially acetylacetonate ligand (acac). Light-blue phosphors with dfppy (2-(2,4- difluorophenyl)pyridine) ligand were later generated based on the bis-cyclometalated Ir(III) complexes design to expand this phosphorescent molecule family towards the full-spectrum trend. 18 Ligands such as acac and picolinic acid (pic) of high triplet energy were employed as the ancillary ligand. The phosphorescence of the complexes is originated from the mixing of metal to ligand charge transfer ( 3 MLCT) and ligand centered ( 3 LC) state of C^N ligands. The emission colors of the Ir(III) complexes can be tuned by choosing appropriate ligands of different triplet energies. For example, by the modification of the chelated ligands from ppy to 2-phenylbenzo[d]oxazole (bo), 2-(naphthalene-1-yl)benzo[d]oxazole(bon), 2-phenylbenzo[d]thiazole(bt), and 2-(naphthalene-1-yl)benzo[d]thiazole(α-bsn), the emissions are red-shifted from green to red. A series of green to blue homoleptic Ir(III) complexes with ligands of ppy, ppz, dfppy etc. were reported in 2003 by Tamayo and co-workers. 19 The homoleptic complexes showed meridional and facial isomers of significantly different photoluminescence properties. The meridional isomer of the complexes can convert to the facial isomer under irradiation. The facial isomer of the complexes Ir(ppy) 3 and Ir(dfppy) 3 show about 40% quantum yield in solution and close to unity quantum yield in doped PMMA films. The meridional isomer of the complexes showed quantum yields of only around 5%. Both facial and meridional isomers of the Ir(ppz) 3 were weak emitters at room temperature. However, the complexes showed bright blue emission at low temperature 10 (λ = 410 nm, 77 K). Similar as the bis-cyclometallated heteroleptic Ir(III) complexes, the emission of the complexes were originated from the 3 MLCT mixed with the 3 LC states. The emissions of all those complexes were highly structured especially at low temperature, indicating the high ligand contribution to the emissions. A thorough illustration about the luminescence mechanisms of green to blue Ir(III) phosphors, especially on the temperature dependent phosphorescence of the blue emitters was reported by Sajoto and co-workers in 2009. 20 The quantum yield of the green phosphor Ir(ppy) 3 (ϕ = 0.97) is not temperature dependent. The sky blue to near-UV Figure 1-6. Examples of some heteroleptic and homoleptic Ir(III) phosphors. 10 (λ = 410 nm, 77 K). Similar as the bis-cyclometallated heteroleptic Ir(III) complexes, the emission of the complexes were originated from the 3 MLCT mixed with the 3 LC states. The emissions of all those complexes were highly structured especially at low temperature, indicating the high ligand contribution to the emissions. A thorough illustration about the luminescence mechanisms of green to blue Ir(III) phosphors, especially on the temperature dependent phosphorescence of the blue emitters was reported by Sajoto and co-workers in 2009. 20 The quantum yield of the green phosphor Ir(ppy) 3 (ϕ = 0.97) is not temperature dependent. The sky blue to near-UV Figure 1-6. Examples of some heteroleptic and homoleptic Ir(III) phosphors. 10 (λ = 410 nm, 77 K). Similar as the bis-cyclometallated heteroleptic Ir(III) complexes, the emission of the complexes were originated from the 3 MLCT mixed with the 3 LC states. The emissions of all those complexes were highly structured especially at low temperature, indicating the high ligand contribution to the emissions. A thorough illustration about the luminescence mechanisms of green to blue Ir(III) phosphors, especially on the temperature dependent phosphorescence of the blue emitters was reported by Sajoto and co-workers in 2009. 20 The quantum yield of the green phosphor Ir(ppy) 3 (ϕ = 0.97) is not temperature dependent. The sky blue to near-UV Figure 1-6. Examples of some heteroleptic and homoleptic Ir(III) phosphors. 11 phosphors with the ppz based chelates mentioned in the paper all had non-radiative decays that involved bond dissociation process at the Ir-N bond(Figure 1-7). The triplet spin density of the blue phosphors is localized on the iridium center and the ppz based ligands, and consistent with the origins of the emissions observed at low temperature (77 K). However, the two-level Boltzman analysis of the varied lifetimes recorded from low to room temperatures, along with DFT calculations on Ir(ppz) 3 and ppz related Ir(III) complexes, indicates that a non-radiative decay state (NR) very likely can be thermally populated with the bond rupture and formation of five-coordinated form of the complexes, leading to much larger k nr (T) compared to the k r (T). 20-21 With this theory, it can be analyzed that the Ir(flz) 3 (ϕ = 0.81) shows much higher quantum yield at room temperature because the T 1 of the complex is relatively low (λ 0-0 = 478 nm), and thus thermally population to the NR state is more difficult compared to the Ir(ppz) 3 . The time dependent quantum yields are most obvious of complexes Ir(ppz) 3 and Ir(dfppz) 3 Figure 1-7. Modified Jablonski diagram for illustration of the temperature dependentphosphorescence of blue emitters. Triplet spin density on six- and five- coordination forms of Ir(ppz) 3 are shown. 11 phosphors with the ppz based chelates mentioned in the paper all had non-radiative decays that involved bond dissociation process at the Ir-N bond(Figure 1-7). The triplet spin density of the blue phosphors is localized on the iridium center and the ppz based ligands, and consistent with the origins of the emissions observed at low temperature (77 K). However, the two-level Boltzman analysis of the varied lifetimes recorded from low to room temperatures, along with DFT calculations on Ir(ppz) 3 and ppz related Ir(III) complexes, indicates that a non-radiative decay state (NR) very likely can be thermally populated with the bond rupture and formation of five-coordinated form of the complexes, leading to much larger k nr (T) compared to the k r (T). 20-21 With this theory, it can be analyzed that the Ir(flz) 3 (ϕ = 0.81) shows much higher quantum yield at room temperature because the T 1 of the complex is relatively low (λ 0-0 = 478 nm), and thus thermally population to the NR state is more difficult compared to the Ir(ppz) 3 . The time dependent quantum yields are most obvious of complexes Ir(ppz) 3 and Ir(dfppz) 3 Figure 1-7. Modified Jablonski diagram for illustration of the temperature dependentphosphorescence of blue emitters. Triplet spin density on six- and five- coordination forms of Ir(ppz) 3 are shown. 11 phosphors with the ppz based chelates mentioned in the paper all had non-radiative decays that involved bond dissociation process at the Ir-N bond(Figure 1-7). The triplet spin density of the blue phosphors is localized on the iridium center and the ppz based ligands, and consistent with the origins of the emissions observed at low temperature (77 K). However, the two-level Boltzman analysis of the varied lifetimes recorded from low to room temperatures, along with DFT calculations on Ir(ppz) 3 and ppz related Ir(III) complexes, indicates that a non-radiative decay state (NR) very likely can be thermally populated with the bond rupture and formation of five-coordinated form of the complexes, leading to much larger k nr (T) compared to the k r (T). 20-21 With this theory, it can be analyzed that the Ir(flz) 3 (ϕ = 0.81) shows much higher quantum yield at room temperature because the T 1 of the complex is relatively low (λ 0-0 = 478 nm), and thus thermally population to the NR state is more difficult compared to the Ir(ppz) 3 . The time dependent quantum yields are most obvious of complexes Ir(ppz) 3 and Ir(dfppz) 3 Figure 1-7. Modified Jablonski diagram for illustration of the temperature dependentphosphorescence of blue emitters. Triplet spin density on six- and five- coordination forms of Ir(ppz) 3 are shown. 12 (Ф < 0.01) with higher energy ligands. The complex Ir(pmb) 3 shows relatively high quantum yield at room temperature (Ф = 0.37), but also temperature dependent quantum yields. The strong field ligand of carbene is crucial for avoiding ligand dissociation at the thermally populated NR state. The studies on the series of blue phosphors provides important indications for design of new high energy Ir(III) emitters thereafter. 1.3.1.2. Terdentate Ir(III) cyclometalated complexes Charged bis-terdentate Ir(III) cyclometalated complexes with polypyridine based ligand (N^N^N) type of ligand were first developed for energy and electron transfer molecular array units of geometry advantages compared to their tri-bidentate cyclometallated analogues. 22 Such complexes normally show microsecond lifetime, and room temperature emission from 3 LC states. The photophysical properties of the bis- terdentate Ir(III) complexes are distinct different when metal-nitrogen bond is replaced with metal-carbon bond. The complexes with ligand containing C^N component posses stronger ligand field effect, and their lowest excited states are of MLCT character rather than LC character. Some bis-terdentate Ir(III) complexes of C^N^C, N^N^C, and N^C^N liands have been studied(Figure 1-8). 23 The charge-neutral Ir(III) complexes with C^N^C and N^C^N coordinates [Ir(dpyx)(dppy)] is of interesting with the potential applications in phosphorescence OLEDs(Ф = 0.21, τ = 3.8 μs). 23c However, photo degradation is a significant problem with the molecule. 24 Modification of the C^N^C ligand to a C^N^O ligand utilizes the design from both ppy and the high energy ancillary ligand of picolinic acid. The Ir(III) complex with C^N^C and N^C^N coordinates [Ir(dpyx)(tppic)] show much lower quantum yield in fluid solution at room temperature 13 (ϕ = 0.05, τ = 0.1 μs). 24-25 The authors claimed that dissociation from the Ir-O bond maybe a reason for the very high non-radiative rate (k nr = 8.6 x 10 6 s -1 ) of the complex. A series of neutral Ir(III) complexes with both terdentate, bidentate ligands and a coordinated chloride seem to overcome the photo-stability problem, show high quantum yield in solution, and are suitable for the application of OLEDs. 26 The phosphorescence color of the complexes can be tuned with modification on both the terdentate and the bidentate ligand. The TD-DFT calculation shows that the terdentate ligands contribute to both HOMO and LUMO of the molecules, while the bidentate ligands contribute mainly to the HOMOs. The substituted N^C^N terdentate ligands show significant influence on the excited state energies of the Ir(III) complexes. The fluorination of the ppy ligand further tunes the phosphorescence of the complexes with smaller wavelength range. Such Ir(III) complexes can take benefit from the Ir(N^C^N)(N^C)Cl structure for better stability and suppressed non-radiative channels, but also being limited in a full-spectral phosphorescent concern. It can be predicted that if the triplet energy level of the N^C^N Ir N N N N N N Ir C N N N N N N Ir N N [Ir(dpyx)(dppy)] N O O Ir N N [Ir(dpyx)(tppic)] R N N Ir Cl N N Ir Cl F F X X X X Better Stability X = CH 3 , CF 3 , F Figure 1-8. Example of Ir(III) complexes with terdentate chelates. 14 ligand is higher than the bidentate ligand, the chromophore of the complex generate at the metal-bidentate site, while the terdentate ligand becomes the ancillary ligand of the molecule system. If the triplet energy level of the terdentate ligand is similar to the bidentate ligand, the triplet spin density of the complexes most likely delocalizes on both chelated ligands. If the triplet energy level of the terdentate ligand is lower than the bidentate ligand, then the triplet spin density predominately delocalizes on the metal- terdentate site, as described in the above molecules. Another category of terdentate Ir(III) complexes catches eyes with high energy carbene ligands. Bis-terdentate Ir(III) complexes with butylimidazol-based carbene ligand are reported by De Cola and co-workers (Figure 1-9). 27 The high triplet energy of the carbene ligand, and the strong ligand field originated from the Ir(III)-carbene are essential for achieving the near UV emission (~ 400 nm) in solution with about 40% quantum yield from the complexes. The cationic form of the complexes limits their potential applications for OLEDs, though a device was fabricated and reported in the Figure 1-9. Bis-terdentate Ir(III)-carbene complexes. 15 study via spin-coating preparation. A neutral bis-terdentate Ir(III)-carbene complex would be of great interest for the pursuit of pure blue phosphorescence complexes. 1.3.2. Os(II) cyclometalated complexes Intensive studies have been focused on the synthesis and photophysical properties of the octahedral 5d 6 metal complexes with Os(II) metal center for varieties of photonic applications within photochemistry and photophysical aspects. 22a A large family of Os(II) complexes are studied by Chi and Chou over decades for OLEDs applications(Figure 1-10). 28 Neutral diphosphane based osmium derivatives with structure of Os(N^N) 2 (PR’ 3 ) 2 represent the early stable red phosphorescence Os(II) complexes. The N^N ligands are 2-pyridyl-pyrazole (pypz) or 2-pyridyl-trizaole (pytz) derivatives. The PR’ 3 ligands are free aryl or alkyl phosphine ligands. The strong spin-orbit coupling effect of the Os(II) facilitate the fast inter-system crossing from the singlet to the triplet excited states of the complexes similar as the Ir(III) metal center. The Os(II) complexes show red emissions (> 600 nm) with fair to good quantum yields (20%-76%) and fast lifetimes (~ 1 µs). The radiative rates of the complexes are comparable with Ir(III) complexes, at the level of 10 5 s -1 . The devices fabricated with those phosphorescence materials can reach up to 20% EQE. 29 Cyclometalated Os(II) complexes are further developed with linked phosphine chelates instead of the free phosphines. 30 The emission of the complexes can be tuned from green to near-IR based on design of chelated ligands. More recently, tetradentate Os(II) complexes with free phosphine chelates are reported as efficient and saturated red emitters. The OLEDs fabricated with such complexes can reach about 10% EQE. 16 1.4. Research focuses of this thesis In summary, the energetic and coordination pattern of the chelated ligands with the metal centers is crucial in determining the phosphorescence energy of the cyclometalated metal complexes. It is important to understand the phosphorescence mechanisms and energy diagrams with the metal complexes in order to design efficient OLEDs phosphors that can cover the full-spectrum from near-UV to near-IR. P' P' Os P' P' P' = PPh 2 Figure 1-11. Os(PCP) 2 complex from Chapter 2. Os N N N R N N R PR' 3 PR' 3 Os N N N N R N N N R PR' 3 PR' 3 R = CF 3 , tBu, C 3 F 7 R' = Ph 2 Me Os N N Ph 2 P P Ph 2 N N N N CF 3 Os N N Ph 2 P P Ph 2 N N N N CF 3 Os N N N N N PR' 3 PR' 3 N N F 3 C F 3 C R' = Ph 2 Me, or PhMe 2 Figure 1-10. Exemples of phosphorescence Os(II) complexes. 17 Chapter 2 focuses on the synthesis and photophysical study of a neutral bis-pincer Os(II) complex of Os(PCP) 2 (PCP = 2,6-(CH 2 PPh 2 ) 2 C 6 H 3 , Ph = C 6 H 5 )(Figure 1-11). The molecule is designed with terdentate pincer ligand of high triplet energy. However, the phosphorescence of the molecule is yellow. Thorough understanding of the phosphorescence mechanism of the molecule is presented in the chapter. Chapter 3 focuses on the design, synthesis, and photophysical characterizations of a series of heteroleptic Ir(III) complexes with C^P chelates (Figure 1-12). The C^P chelates for the Ir(III) complexes are much less studied compared to the C^N types of chelates. We design the Ir(III) complexes with two high energy ancillary ligands of ppz, and a C^P chelated ligand, in order to study exclusively the coordination conformation, synthetic details, and photophysical properties of the Ir(III)-C^P chromophore systems. Chapter 4 focuses on the synthesis, photophysical studies, and device performances of a series of aluminum and zinc based host materials with ligands of high triplet energies N N Ir Y X P 2 1-mer: X = CH, Y = CH 2-mer: X = N, Y = CH 3-mer: X = CH, Y = N Ir N N C C C P Mer, trans N-N Ir N C C C N P Fac, cis N-N Ir N C C P N C Mer, trans N-P Possible Isomers: Figure 1-12. Heteroleptic Ir(III)-C^P complexes from Chapter 2. 18 (Figure 1-13). The energetic levels of the ligand can be preliminarily determined with Ir(III) based complexes. Al X N 3 X N Zn X N N N Ir N X 2 N Ir N X 2 (ppz) 2 Ir(PydpX)(ppy) 2 Ir(PydpX) Al(PydpX) 3 Zn(PydpX) 2 X = O or S Figure 1-13. Ir(III), Al(III) and Zn(II) complexes studied in Chapter 4. 19 Chapter 1 References 1.(a) Bernanose, A.; Vouaux, P., J Chim Phys Pcb 1953, 50(4), 261; (b) Bernanose, A.; Comte, M.; Vouaux, P., J Chim Phys Pcb 1953, 50(1), 64; (c) Bernanose, A., Brit J Appl Phys 1955, S54. 2. Pope, M.; Magnante, P.; Kallmann, H. P., J Chem Phys 1963, 38(8), 2042. 3. Tang, C. W.; Vanslyke, S. A., Appl Phys Lett 1987, 51(12), 913. 4.(a) Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R., Nature 1998, 395(6698), 151; (b) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R., Appl Phys Lett 1999, 75(1), 4. 5.(a) Wu, C. C.; Wu, C. I.; Sturm, J. C.; Kahn, A., Appl Phys Lett 1997, 70(11), 1348; (b) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B., Nature 1990, 347(6293), 539; (c) Tak, Y. H.; Kim, K. B.; Park, H. G.; Lee, K. H.; Lee, J. R., Thin Solid Films 2002, 411(1), 12. 6. Giebeler, C.; Antoniadis, H.; Bradley, D. D. C.; Shirota, Y., J Appl Phys 1999, 85 (1), 608. 7. Fenter, P.; Schreiber, F.; Bulovic, V.; Forrest, S. R., Chem Phys Lett 1997, 277 (5-6), 521. 8.(a) Choong, V. E.; Shi, S.; Curless, J.; So, F., Appl Phys Lett 2000, 76(8), 958; (b) Jabbour, G. E.; Kippelen, B.; Armstrong, N. R.; Peyghambarian, N., Appl Phys Lett 1998, 73(9), 1185. 9. Tang, C. W.; Vanslyke, S. A.; Chen, C. H., J Appl Phys 1989, 65(9), 3610. 10. Shoustikov, A. A.; You, Y. J.; Thompson, M. E., Ieee J Sel Top Quant 1998, 4(1), 3. 11. Baldo, M. A.; O'Brien, D. F.; Thompson, M. E.; Forrest, S. R., Phys Rev B 1999, 60(20), 14422. 12. Baldo, M. A.; Thompson, M. E.; Forrest, S. R., Nature 2000, 403(6771), 750. 13. Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E., Appl Phys Lett 2000, 77(6), 904. 14. Minaev, B.; Baryshnikov, G.; Agren, H., Phys Chem Chem Phys 2014, 16(5), 1719. 15. Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R., J Appl Phys 2001, 90 (10), 5048. 20 16. IUPAC Compendium of Chemical Terminology. IUPAC: 2014 p675. 17.(a) 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 (18), 4304; (b) 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(7), 1704. 18. Adachi, C.; Kwong, R. C.; Djurovich, P.; Adamovich, V.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R., Appl Phys Lett 2001, 79(13), 2082. 19. 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(24), 7377. 20. Sajoto, T.; Djurovich, P. I.; Tamayo, A. B.; Oxgaard, J.; Goddard, W. A.; Thompson, M. E., J Am Chem Soc 2009, 131(28), 9813. 21. Treboux, G.; Mizukami, J.; Yabe, M.; Nakamura, S., Chem Lett 2007, 36(11), 1344. 22.(a) Sauvage, J. P.; Collin, J. P.; Chambron, J. C.; Guillerez, S.; Coudret, C.; Balzani, V.; Barigelletti, F.; Decola, L.; Flamigni, L., Chem Rev 1994, 94(4), 993; (b) Collin, J. P.; Dixon, I. M.; Sauvage, J. P.; Williams, J. A. G.; Barigelletti, F.; Flamigni, L., J Am Chem Soc 1999, 121(21), 5009. 23.(a) Polson, M.; Fracasso, S.; Bertolasi, V.; Ravaglia, M.; Scandola, F., Inorg Chem 2004, 43(6), 1950; (b) Mamo, A.; Stefio, I.; Parisi, M. F.; Credi, A.; Venturi, M.; DiPietro, C.; Campagna, S., Inorg Chem 1997, 36(25), 5947; (c) Wilkinson, A. J.; Goeta, A. E.; Foster, C. E.; Williams, J. A. G., Inorg Chem 2004, 43(21), 6513. 24. Wilkinson, A. J.; Puschmann, H.; Howard, J. A. K.; Foster, C. E.; Williams, J. A. G., Inorg Chem 2006, 45(21), 8685. 25. Williams, J. A. G., Chem Soc Rev 2009, 38(6), 1783. 26. Brulatti, P.; Gildea, R. J.; Howard, J. A. K.; Fattori, V.; Cocchi, M.; Williams, J. A. G., Inorg Chem 2012, 51(6), 3813. 27. Darmawan, N.; Yang, C. H.; Mauro, M.; Raynal, M.; Heun, S.; Pan, J. Y.; Buchholz, H.; Braunstein, P.; De Cola, L., Inorg Chem 2013, 52(19), 10756. 28. Chou, P. T.; Chi, Y., Eur J Inorg Chem 2006,(17), 3319. 29. Tung, Y. L.; Lee, S. W.; Chi, Y.; Tao, Y. T.; Chien, C. H.; Cheng, Y. M.; Chou, P. T.; Peng, S. M.; Liu, C. S., J Mater Chem 2005, 15(4), 460. 30.(a) Du, B. S.; Liao, J. L.; Huang, M. H.; Lin, C. H.; Lin, H. W.; Chi, Y.; Pan, H. A.; Fan, G. L.; Wong, K. T.; Lee, G. H.; Chou, P. T., Adv Funct Mater 2012, 22(16), 21 3491; (b) Lee, T. C.; Hung, J. Y.; Chi, Y.; Cheng, Y. M.; Lee, G. H.; Chou, P. T.; Chen, C. C.; Chang, C. H.; Wu, C. C., Adv Funct Mater 2009, 19(16), 2639. 22 Chapter 2: Synthesis and photophysical characterization of a bis-pincer osmium complex 2.1. Introduction Bis-terdentate chelated transition metal complexes such as [Ru(terpy) 2 ] 2+ , [Os(terpy) 2 ] 2+ , and [Ir(terpy) 2 ] 3+ with bis(2,2':6',2''-terpyridine)(terpy) and related complexes have been intensively studied for their interesting photophysical, photochemical, and electrochemical properties, and potential applications in molecular electronics such as photoinduced electron and energy transfer organometallic rods, molecular wires and switches, and self-assembled light-harvesting systems. 1 The terdentate chelating ligands are structurally more appealing than bidentate chelating analogues for conformation of the metal complexes for the lack of optical isomers of the bis-terdentate metal complexes, and more controllable linear arrangement for the design of functional molecular materials. 2 Terdentate carbene ligands are employed in homo- and heteroleptic ruthenium complexes for interesting photophysical, especially photoluminescence properties. 3 Neutral or ionic heteroleptic iridium complexes containing two terdentate ligands of terpyridine, N^C^N or C^N^C types of chelates also attract interest for their photophysical properties. 4 Among the array of choices for chelates of bis-terdentate transition metal complexes, a particular type of terdentate ligand, the pincer (pincer = L-X-L’-X-L, where X = CH 2 , NH, O, etc.) named after the chelating mode with the metal center are much less studied. 23 Unlike in terpy-based ligands, where the pyridyl ligands (L) are conjugated through sp 2 -sp 2 linkages, conjugation between L-moieties in pincer ligands is interrupted by the sp 3 hybridized linkages of the X-groups. Pincer ligands should therefore be poor acceptors since their unoccupied orbitals are at energies higher than that of terpy. Transition metal complexes chelated with pincer ligands were first reported in the 1970s and used diphosphine ligands of the general structure [2,6-(CH 2 PR 2 ) 2 C 6 H 3 , PCP] (R = alkyl or aryl). These ligands can be readily cyclometalated onto metals such as Ni, Ir, Rh, Pt and Pd 5 and thus organometallic complexes containing mono-pincer chelates have been extensively studied thereafter, especially in catalysis chemistry. 6 To the best of our knowledge, the only example of metal complex with bis-pincer coordination in literature is the main group metal complex Mg(P’CP’) 2 , where C is a metalated 2,6-xylyl group and P’ is a PMe 2 substituent, reported by Muller G. and co-workers in 1994. 7 Inspired by the studies on bis-terdentate transition metal complexes, and pincer type metal complexes, we are interested in preparing the neutral bis-pincer based transition metal complex of Os(PCP) 2 , with Os(II) metal center and typical PCP pincer ligand of [2,6-(CH 2 PR 2 ) 2 C 6 H 3 ] (R = C 6 H 5 ). The synthetic routes, molecule structure, and photophysical characterization of the complex is fully examined. 2.2. Experimental 2.2.1. Materials and Methods The syntheses were carried out under a nitrogen atmosphere using standard Schlenk techniques. Solvents were distilled under nitrogen from Sodium benzophenone (hexanes, 24 THF), or calcium hydride (2-propanol). The starting materials OsCl 2 (PPh 3 ) 3 and 1,3-(PPh 2 CH 2 ) 2 C 6 H 4 (PCP-H) were prepared according to literature methods. 8 Other chemicals and solvents were ordered from Sigma-Aldrich®, and used as received. 1 H NMR, 31 P NMR, gCOSY NMR at 223 K, inversion recovery NMRs at 233 K, and VT- 1 H NMR spectra were measured by Varian 600 NMR Spectrometer. The 13 C NMR cannot be resolved due to the poor solubility of Os(PCP) 2 . The chemical shifts were referenced to a deuterated solvent. Mass Spectroscopy were obtained on a Shimadzu LCMS-2020 quadrupole mass spectrometer equipped with a column oven (T = 40 °C), a PDA photodetector (200-800 nm) and an MS spectrometer (LCMS 2020; m/z range: 0- 2000; ionization modes: ESI/APCI). 2.2.2. Synthesis of Os(PCP) 2 (PCP = 2,6-(PPh 2 CH 2 ) 2 C 6 H 3 ) Route 1: OsCl 2 (PPh 3 ) 3 (240 mg, 0.23 mmol) and PCP-H(327 mg, 0.69 mmol) were evacuated in a Schlenk tube for 30 min. Then 80 ml of dry and fully degassed 2-propanol was added. The solution mixture was refluxed for 24 h, cooled to room temperature, and filtered over Al 2 O 3 plug. The filtrate solution was then evaporated over rotavap. The solid residue was washed with hexane and diethyl ether. Then the dried solid was dissolved in 15 ml THF, and precipitated with dropwise of 30 ml hexanes. The precipitate was dried under vacuum and obtained as 110 g lemon yellow solid (42%). Route 2: OsCl(PPh 3 )(PCP) was synthesized following literature process and isolated in 50% yield. The AgOTf (27 mg, 0.10 mmol) and OsCl(PPh 3 )(PCP) (100 mg, 0.10 mmol) were reacted in THF at room temperature for 1 h. The solution was filtered, and the solvent was evaporated. The in situ generated Os(OTf)(PPh 3 )(PCP)(90% yield) was directly 25 used for the next step without further purification. Os(OTf)(PPh 3 )(PCP)(100 mg, 0.09 mmol) and PCP-H(43 mg, 0.09 mmol) were evacuated in a Schlenk tube for 30 min. Then 20 ml of dry and fully degassed 2-propanol was added. The solution mixture was refluxed for 24 h, cooled to room temperature, and then followed by the same workup as in Route 1 (yield based on OsCl 2 (PPh 3 ) 3 = 31% ). 1 H NMR (600 MHz, C 6 D 6 , 343 K): δ = 7.39 (d, J = 7.2 Hz, 4H, xylyl CH), δ = 7.31 (t, J = 6.6 Hz, 2H, xylyl CH), δ = 6.92-6.73 (m, 40H, PPh 2 ), δ = 3.60 (s, 8H, CH 2 ). 1 H NMR (600 MHz, C 7 D 8 , 223 K): δ = 7.83 (m, 4H, o-Ph), δ = 7.63 (d, J = 3.6 Hz, 4H, o-Ph), δ = 7.54(d, J = 7.2 Hz, 4H, xylyl CH), δ = 7.48(t, J = 7.2 Hz, 2H, xylyl CH), δ = 7.34(t, 4H, J = 6.6 Hz, m-Ph), δ = 6.82(t, 4H, J = 7.2 Hz, m-Ph), δ = 6.67 (d, J = 6.0 Hz, 4H, o- Ph), δ = 6.41 (t, J = 6.6 Hz, 4H, p-Ph), δ = 5.76 (t, J = 7.2 Hz, 4H, m-Ph), δ = 5.49 (d, J = 6.0 Hz, 4H, o-Ph), δ = 3.62, 3.52(ABq, J AB = 14.4 Hz, 8H, CH 2 ). 31 P NMR (243 MHz, C 6 D 6 , 343 K): δ = 6.1 (s). 31 P NMR (243 MHz, C 7 D 8 , 223 K): δ = 11.2(s). LC-MS ESI (m/z): 1138.50 ([M], calcd: 1138.28). 2.2.3. X-ray Crystallography The X-ray intensity data were measured on a Bruker APEXDUO CCD system using radiation from a MoKα fine-focus tube (λ = 0.71073 Å) with a TRIUMPH monochromator. A clear yellow pear shape specimen of C 64 H 54 OsP 4 , approximate dimensions 0.16 mm x 0.09 mm x 0.04 mm, was used for the X-ray crystallographic analysis. A total of 2520 frames were collected. The frames were integrated using the Bruker SAINT V8.18C software. The integration of the data using a triclinic unit cell 26 yielded a total of 75643 reflections to a maximum θ angle of 27.51° (0.68 Å resolution), of which 11025 were independent (average redundancy 7.761, completeness = 95.5%, R int = 7.75%). The final cell constants of a = 12.6896(11) Å, b = 14.1378(12) Å, c = 14.7794(13) Å, volume = 2511.0(4) Å 3 , are based upon the refinement of the XYZ- centroids of 9013 reflections above 2θ σ(I) with 1.40° < 2θ < 27.51°. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.766. The structure was solved and refined using the Bruker SHELXTL Software Package, using the space group triclinic P1, with Z = 2 for the formula unit, C 64 H 54 OsP 4 . The final anisotropic full-matrix least-squares refinement on F 2 with 415 variables converged at R1 = 6.78%, for the observed data and wR2 = 11.25% greater than 2σ(F 2 ). The goodness-of- fit was 1.072. The largest peak in the final difference electron density synthesis was 1.352 e - /Å 3 and the largest hole was -1.041 e - /Å 3 . On the basis of the final model, the calculated density was 1.504 g/cm 3 and F(000), 1148 e - . 2.2.4. Photophysical Characterization The UV-Visible spectra were recorded on a Hewlett-Packard 4853 diode array spectrometer. Steady-state emission measurements were performed using a QuantaMaster model C-60SE spectrofluorimeter (Photon Technology International) with an excitation wavelength of 350 nm in 2-MeTHF glass(77 K) or PMMA film under N 2 protection. Phosphorescent lifetimes were measured by time-correlated single-photon counting with IBH Fluorocube instrument equipped with a 405 nm LED excitation source. Quantum yield(298 K) was measured using a Hamamatsu C9920 system equipped with a xenon 27 lamp, calibrated integrating sphere, and Model C10027 photonic multichannel analyzer. Quantum yield (77 K) was determined relative to the quantum yield of Ir(ppy) 3 at 77 K (Ф = 1). 2.2.5. Theoretical Calculations The theoretical calculations were carried out by Schrödinger 2013 Materials Science Suite, using the density function theory (DFT) with Lee-Yang-Parr correlation functional B3LYP. The basis set used was LACVP**. Time-dependent DFT (TD-DFT) calculations were performed with Gaussian 03 program to understand more about the ground state to excited states transitions of the complex. The same hybrid functional of B3LYP was employed, and the basis set was LANL2DZ/6-31G*. 2.2.6. Electrochemistry Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements were performed using an EG&G Potentiostat/Galvanostat model 283. Dry DMF was used as solvent under a N 2 atmosphere with 0.1 M tetra(n-butyl)ammonium hexafluorophosphate as the supporting electrolyte. A glassy carbon working electrode and a platinum counter electrode were used, with a silver wire as the pseudoreference electrodes. The oxidation potential was measured relative to a ferrocenium/ferrocene (Fc + /Fc) redox couple as an internal standard. Electrochemical reversibility is determined by cyclic voltammetry. 28 2.3. Results and discussion 2.3.1. Synthetic routes of Os(PCP) 2 The bis-pincer complex Os(PCP) 2 was prepared in a reaction of excess PCP-H ligand precursor(3 eq) with OsCl 2 (PPh 3 ) 3 (1 eq) in refluxing anhydrous 2-propanol for 24 hours (Scheme 2-1, Route 1). During the course of the reaction the solution changed color from green to purple. After filtration and workup, the complex is isolated as a lemon- yellow powder (42% yield) that is air-stable as a neat solid and in solution. However, the solubility of Os(PCP) 2 in general is poor, being most soluble in aromatic solvents. The complex is thermally stable as crystals can be obtained using zone sublimation at 280-240-200 °C at 1 x 10 -6 torr. The stepwise preparation of Os(PCP) 2 was also investigated. A reaction between PCP-H(1 eq) and OsCl(PPh 3 )(PCP)(1 eq) in 2-propanol, surprisingly, only results in the formation of an intractable black solution. 9 However, on the basis of previous work with Scheme 2-1. OsCl 2 (PPh 3 ) 3 1 eq. PCP-H 2-propanol reflux 24h 3 eq. PCP-H 2-propanol reflux 24h 1 eq. AgOTf THF, 1h 1 eq. PCP-H 2-propanol reflux 24h Route 1: Route 2: Total yield: Route 1: 42% Route 2: 31% OsCl(PPh 3 )(PCP) Os(OTf)(PPh 3 )(PCP) 1 eq. PCP-H 2-propanol reflux 24h X yield: 50% yield: 90% P' P' Os P' P' P' = PPh 2 29 related Ru complexes, substitution of chloride with triflate to form Os(OTf)(PPh 3 )(PCP), followed by reaction with PCP-H (Route 2) lead to Os(PCP) 2 in overall 31% yield (based on OsCl 2 (PPh 3 ) 3 ). 10 Interestingly, in the case of the Ru cogener of Os(PCP) 2 one ligand forgoes metalation and instead forms an agnostic C–H···Ru interaction, even despite several attempts at deprotonation with strong bases. 10-11 Apparently, for osmium the metal–C bond is strong enough that deprotonation of the aryl can occur without added base to form a stable dimetalated product. 2.3.2. X-ray crystallography X-ray diffraction analysis was carried out on a crystal of Os(PCP) 2 obtained from sublimation. The unit cell of Os(PCP) 2 crystal contains a pair of enantiomers in a triclinic P1 space group. Perspective drawings of the complex are depicted in Figure 2-1(left, middle). Selected bond distances and angles are given in Table 2-1. The molecular geometry of Os(PCP) 2 shows that the pincer ligands are arranged in pseudo-octahedral environment around the metal center (Figure 2-1, left). The Os(1)-C(31) and Os(1)-C(47) bond distances (2.151(7) Å and 2.149(7) Å respectively) are longer than the Os–C bond distances observed for monopincer osmium complex OsCl(PPh 3 )(PCP)(2.04(1) Å). 9 The Os–C bonds are elongated due to the trans-effect between the two opposing aryl rings. The bond lengths of the four Os–P bonds are almost identical, ranging between 2.3348(19) Å–2.3419(19) Å, and only slightly longer than the Os–P bond lengths of the OsCl(PPh 3 )(PCP) (2.313(4) Å and 2.312(4) Å). Likewise, comparison with [OsCl(=C=CHPh)(PPh 3 )(PCP)], where the vinylidene substituent (trans to Cl) is strongly electron-withdrawing, shows similar distances for the Os–C PCP (2.133(5) Å) and Os–P PCP 30 (2.3889(14) Å and 2.3547(15) Å) bonds. 9 The phenyl rings in Os(PCP) 2 are arranged around the complex into two distinct sets of pairs, designated A and B (Figure 2-1, middle). The phenyl rings in the A-type pair are slip-stacked and in close enough proximity to undergop -pinteractions (shortest C-C distance = 3.2 Å) whereas rings in the B-type pairs are further apart (shortest C-C distance 3.7 Å) and have nop -overlap. The deviation from an ideal octahedral geometry in Os(PCP) 2 is due to small chelate bite angles in the PCP ligand (average C–Os–P = 76.5°). Thus, while the axially ligated carbon atoms are near linear [C(31)–Os(1)–C(47) = 178.60(3)°] bond angles for the trans disposed P(1)-Os(1)–P(3) and P(2)–Os(1)–P(4) are 152.21(7)° and 153.75(7)°, respectively. The pseudo tetrahedral configuration of the bridging methylenes (average P–CH 2 –C angle = 105°) causes the phosphines in the PCP ligand to twist in a transoid conformation away from the plane of the xylyl group (average C Os –C–CH 2 –P dihedral angle = 24.6°). A projection view down the C(31)-Os(1)–C(47) axis (Figure 2-1, right) Figure 2-1. Two perspective views of Os(PCP) 2 shown in 50% probability thermal ellipsoids (left, middle). Hydrogen atoms and the phenyl rings on phosphorus (left) are omitted for clarity. Projection view down the C(31)–Os(1)–C(C47) axis of Os(PCP) 2 (right). Solid and dashed lines represent the xylyl planes. 30 (2.3889(14) Å and 2.3547(15) Å) bonds. 9 The phenyl rings in Os(PCP) 2 are arranged around the complex into two distinct sets of pairs, designated A and B (Figure 2-1, middle). The phenyl rings in the A-type pair are slip-stacked and in close enough proximity to undergop -pinteractions (shortest C-C distance = 3.2 Å) whereas rings in the B-type pairs are further apart (shortest C-C distance 3.7 Å) and have nop -overlap. The deviation from an ideal octahedral geometry in Os(PCP) 2 is due to small chelate bite angles in the PCP ligand (average C–Os–P = 76.5°). Thus, while the axially ligated carbon atoms are near linear [C(31)–Os(1)–C(47) = 178.60(3)°] bond angles for the trans disposed P(1)-Os(1)–P(3) and P(2)–Os(1)–P(4) are 152.21(7)° and 153.75(7)°, respectively. The pseudo tetrahedral configuration of the bridging methylenes (average P–CH 2 –C angle = 105°) causes the phosphines in the PCP ligand to twist in a transoid conformation away from the plane of the xylyl group (average C Os –C–CH 2 –P dihedral angle = 24.6°). A projection view down the C(31)-Os(1)–C(47) axis (Figure 2-1, right) Figure 2-1. Two perspective views of Os(PCP) 2 shown in 50% probability thermal ellipsoids (left, middle). Hydrogen atoms and the phenyl rings on phosphorus (left) are omitted for clarity. Projection view down the C(31)–Os(1)–C(C47) axis of Os(PCP) 2 (right). Solid and dashed lines represent the xylyl planes. 30 (2.3889(14) Å and 2.3547(15) Å) bonds. 9 The phenyl rings in Os(PCP) 2 are arranged around the complex into two distinct sets of pairs, designated A and B (Figure 2-1, middle). The phenyl rings in the A-type pair are slip-stacked and in close enough proximity to undergop -pinteractions (shortest C-C distance = 3.2 Å) whereas rings in the B-type pairs are further apart (shortest C-C distance 3.7 Å) and have nop -overlap. The deviation from an ideal octahedral geometry in Os(PCP) 2 is due to small chelate bite angles in the PCP ligand (average C–Os–P = 76.5°). Thus, while the axially ligated carbon atoms are near linear [C(31)–Os(1)–C(47) = 178.60(3)°] bond angles for the trans disposed P(1)-Os(1)–P(3) and P(2)–Os(1)–P(4) are 152.21(7)° and 153.75(7)°, respectively. The pseudo tetrahedral configuration of the bridging methylenes (average P–CH 2 –C angle = 105°) causes the phosphines in the PCP ligand to twist in a transoid conformation away from the plane of the xylyl group (average C Os –C–CH 2 –P dihedral angle = 24.6°). A projection view down the C(31)-Os(1)–C(47) axis (Figure 2-1, right) Figure 2-1. Two perspective views of Os(PCP) 2 shown in 50% probability thermal ellipsoids (left, middle). Hydrogen atoms and the phenyl rings on phosphorus (left) are omitted for clarity. Projection view down the C(31)–Os(1)–C(C47) axis of Os(PCP) 2 (right). Solid and dashed lines represent the xylyl planes. 31 illustrates the relative orientation of the xylyl moieties with respect to the phosphines. The xylyl planes are tilted 46° with respect to each other, while the projected angle of the two P–Os–P planes of the PCP ligands is perpendicular. The transoid twist in the PCP ligands leads to a pseudo-D 2 symmetry for the complex, and hence the presence ofDand L enantiomers, as opposed to the D 2d symmetry found in M(terpy) 2 n+ complexes. 1a 2.3.3. NMR characterization The Os(PCP) 2 complex undergoes a dynamic exchange process in fluid solution at ambient temperatures. The 1 H NMR spectrum measured in C 6 D 6 at 298 K shows broad and featureless resonances for the phenyl rings (δ = 6.10-7.10 ppm) and only one singlet (δ = 3.60 ppm) for the eight diastereotopic methylene protons(Figure 2-2). No signal is observed in 31 P NMR at 298 K. NMR spectra recorded in C 6 D 6 at higher temperatures are more structured. For example, the 31 P NMR at 343 K shows a single resonance (δ = 6.1 ppm) indicating that the phosphorus atoms on the pincer ligands are in equivalent Table 2-1. Selected bond lengths(Å) and angles(degree) of Os(PCP) 2 . bond lengths (Å) Os(1)–C(31) 2.151(7) Os(1)–C(47) 2.149(7) Os(1)–P(1) 2.338(2) Os(1)–P(2) 2.3348(19) Os(1)–P(3) 2.341(2) Os(1)–P(4) 2.3419(19) bond angles(degree) P(1)–Os(1)–P(3) 152.21(7) P(2)–Os(1)–P(4) 153.75(7) P(1)–Os(1)–C(47) 76.1(2) P(2)–Os(1)–C(31) 77.1(2) P(3)–Os(1)–C(47) 76.1(2) P(4)–Os(1)–C(31) 76.7(2) C(31)–Os(1)–C(47) 178.60(3) 32 environments. Sharp resonances are also observed for the xylyl protons (δ = 7.39, d, J = 7.2 Hz, 4H; δ = 7.31, t, J = 6.6 Hz, 2H) while three broad signals further upfield (δ = 6.73-6.92 ppm) are assigned to protons on freely rotating phenyl rings of the PCP ligand(Figure 2-2). Highly resolved NMR spectra are observed at low temperature (223 K) in toluene-d 8 (C 7 D 8 ) (Figure 2-3). The 31 P NMR shows a single resonance (δ = 11.2 ppm) while splitting of the diastereotopic methylenes is clearly evident in the 1 H NMR (δ = 3.62, 3.52 ppm, J AB = 14.4 Hz). Eight well-resolved resonances are observed for the protons on the PPh 2 moieties, along with two more signals obscured by residual protons on the solvent, indicating that the phenyl rings are situated in two distinct environments. Correlations between the various protons of the phenyl rings are clearly revealed in the Figure 2-2. 1 H NMR (600 MHz, 74 C and 25 C, C 6 D 6 ) spectra of Os(PCP) 2 . 2.0 4.0 8.0 36.3 Os(PCP)2inC6D6 74C ppm (t1) 4.0 5.0 6.0 7.0 8.0 8.0 37.9 4.2 2.6 25C 33 gCOSY spectrum(Figure 2-4). Resonances appear upfield for ortho (o)-, meta (m)- and para (p)-protons on one set of phenyls at δ o = 5.49 ppm, δ m = 5.76 ppm, δ p = 6.41 ppm and δ m’ = 6.82 ppm with the remaining signal at δ o’ = 7.63 ppm. Signals from the other set of phenyls appear downfield (δ o = 6.68 ppm, δ m = 6.95 ppm, δ p = 7.05 ppm, δ m’ = 7.34 ppm and δ o’ = 7.83 ppm). The NMR data is consistent with the complex being in a D 2 configuration similar to what is found in the crystal structure. The upfield resonances are tentatively assigned to protons shielded by thep -stacked phenyl rings labelled A in Figure 2-1(middle) whereas the other signals belong to the B-type phenyls. The exchange process at 233 K was Figure 2-3. 1 H NMR (600 MHz, 223 K, C 7 D 8 ) spectrum of Os(PCP) 2 . 34 probed further by performing an inverse recovery experiment(Figure 2-5 and Figure 2-6). 12 A 180° pulse applied to the ortho-proton resonance at δ = 5.49 ppm was found to transfer magnetization to all the other ortho-protons in the phenyl rings during relaxation. This behavior indicates that the exchange involves simultaneous rotation of the phenyl rings and interchange between the A- and B-sites (racemization). The attempt of measuring the NOESY spectrum to reveal the correlations between protons that are close in space did not work for this complex. The T1 relaxing time of the complex at 223 K is about 1s, while the exchanging rate at 223 K (~ 400 s -1 ) is much faster than the T1. Figure 2-4. gCOSY NMR (600 MHz, 223 K, C 7 D 8 ) spectrum of Os(PCP) 2 . 35 0 2 4 6 8 10 12 14 16 18 20 otho-H, A phenyl otho-H', A phenyl otho-H', B phenyl otho-H, B phenyl Integration (relative) Time (S) Figure 2-6. 1 H NMR inversion recovery experiment data collected at 233 K in C 7 D 8 solution. Figure 2-5. Stacked Spectra of 1 H NMR Inversion Recovery Data(600 MHz, 233 K, C 7 D 8 ). 36 Variable temperature 1 H NMR measurements in C 7 D 8 were carried out between 223 K and 343 K (Figure 2-7). The xylyl protons display sharp signals throughout the entire temperature range, undergoing only slight changes in bandwidth and a gradual shift upfield with increasing temperature. In contrast, resonances from the phenyl protons collapse into the baseline as temperature is increased from 223 K to 283 K. Upon further Figure 2-7. VT- 1 H NMR (600 MHz, 233 K to 343 K, C 7 D 8 ) spectra of Os(PCP) 2 . ppm (t1) 3.0 4.0 5.0 6.0 7.0 8.0 283K 293K 303K 313K 323K 333K 343K 273K 283K 263K 288K CH2Cl2 x 258K 253K 243K ppm (t1) 3.0 4.0 5.0 6.0 7.0 8.0 233K 278K 268K 37 warming to 343 K these aromatic signals reappear as a broad set of multiplets betweend = 6.4–6.8 ppm. In addition, the doublet pattern expected for a diastereotopic methylene protons convert to a single coalesced resonance on warming from 223 K to 273 K. This signal sharpens as temperature increases from 273 K to 343 K (Zoom in Figure 2-8). A model for the fluxional behavior leading to racemization of Os(PCP) 2 is illustrated in Figure 2-9. At 223 K, Os(PCP) 2 can be represented asD - orL -enantiomers with D 2 symmetry undergoing slow exchange of the A- and B-type phenyls on the NMR timescale. As temperature increases, the rate of exchange increases until coalescence is achieved at 273 K for the methylenes and 283 K for the phenyls (Figure 2-7). Above this temperature, the diastereotopic methylene protons undergo a fast exchange process as the phenyl rings rotate more rapidly. At 343 K, the enantiomers are in rapid exchange and display an NMR spectrum consistent with a complex in a pseudo-D 2d symmetric Figure 2-8. Variable temperature 1 H NMR spectra (600 MHz, C 7 D 8 ) for the methylene protons of Os(PCP) 2 . 38 environment. Equation(1) was introduced to calculate the rate constants(k) at each single temperature by analyzing the NMR spectra for the methylenes at temperatures between 248 K and 268 K(Table 2-2). =(1) Where: É=(½½) +J +J(2) É=½½(3) An Arrhenius plot of ln(k) vs. 1/T gives a good linear fit, with r 2 = 0.966 (Figure 2-10). 13 The line-shape analysis indicates that the activation energy for racemization of Os(PCP) 2 is 8.6 kcal/mol. P P P P P P P P P P P P E a = 8.6 kcal/mol Figure 2-9. Process proposed for the racemization of Os(PCP) 2 through a D 2d transition state. 39 Table 2-2. Intermediate-exchange rate constants(k) of the methylene protons in Os(PCP) 2 . T (K) k(s -1 ) 248.5 816.6 251.9 997.5 257.3 1302.9 262.6 1902.3 267.9 2931.5 0.0037 0.0038 0.0039 0.0040 6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0 ln(k) Linear Fit of ln(k) ln(k) 1/T Figure 2-10. Arrehnius plot based on intermediate-exchange rate constants(k) of the methylene protons in Os(PCP) 2 . Slope = -4320, intercept = 24. 40 2.3.4. Electrochemistry The electrochemical properties of Os(PCP) 2 were examined by cyclic voltammetry and differential pulse voltammetry in CH 2 Cl 2 solution(Figure 2-11). A reversible oxidation process is observed at E 1/2 (0/+) = -0.345 V (vs Fc + /Fc) followed by second quasi-reversible process at E 1/2 (+/2+) = 0.441 V (vs Fc + /Fc). Os(PCP) 2 can also be oxidized using ferrocenium hexafluorophosphate in solution to form a stable blue cation (see section 2.3.5). The metalated aryl rings destabilize the complex relative to [Os(terpy) 2 ] 2+ derivatives. 1a However, the stability of Os(PCP) 2 in the formal +2 oxidation state is in contrast to bis-cyclometalated, Os(ppy) 2 (bpy) (ppy = 2-phenylpyridyl; bpy = 2,2’-bipyridyl). 14 The oxidation potential of the latter complex, which has cis-configured phenyl rings, is ca. 400 mV more cathodic than Os(PCP) 2 and is isolated as an Os III species. Apparently, the four PPh 2 moieties impart greater stabilization to the metal center than four pyridyl groups, even in spite the unfavorable trans configuration of the metalated aryl rings in Os(PCP) 2 . -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -10.0µ 0.0 10.0µ 20.0µ 30.0µ Current (A) Potential(V) 0.066 V -0.023 V 0.493 V 0.400 V -0.377 V -0.292 V Fc + /Fc Ox 1 Ox 2 -1.0 -0.5 0.0 0.5 1.0 -2.0µ 0.0 2.0µ 4.0µ 6.0µ 8.0µ 10.0µ 12.0µ 14.0µ 16.0µ 0.441 V Fc + /Fc Current (A) Potential(Vvs Fc + /Fc) -0.345 V Figure 2-11. Cyclic voltammetry(at 0.1 V/s) and DPV(at 0.01 V/s) of Os(PCP) 2 (in CH 2 Cl 2 , 0.1 M [Bu 4 N][PF 6 ]). 41 2.3.5. Electronic spectroscopy Absorption and emission spectra for PCP-H ligand and Os(PCP) 2 are shown in Figure 2-12 and Figure 2-13, and photophysical data are listed in Table 2-3. The complex displays strong absorption at 250 nm that gradually diminishes into weaker transitions down to 450 nm (l max = 335 nm, e = 2700 L mol -1 cm -1 , and l max = 384 nm, e =1800 L mol -1 cm -1 ). Absorption from 250 to 300 nm is assigned to ligand based (π→π*) transitions on the basis of a similar intense absorbance found in the PCP-H ligand precursor. The bands at lower energy are assigned to charge transfer transitions (CT) since they are absent in the free ligand (Figure 2-12). The extinction coefficients for the CT transitions are comparable to values found in other osmium(II) complexes, but are relatively weak when compared to related Ir(III) complexes. 15 Upon oxidation with ferrocenium ion the CT transitions undergo a small blue shift that is accompanied by a concomitant appearance of broad transitions for the cation (l max = 519 nm, e = 450 L mol -1 cm -1 andl max = 591 nm,e = 480 L mol -1 cm -1 ; Figure 2-14). Table 2-3. Photophysical properties of Os(PCP) 2 . absorption a λ max (nm), (ε, 10 3 L mol -1 cm -1 ) emission at 298 K emission at 77 K λ max (nm),[τ(µs)] λ 0-0 (nm), [τ(µs)] 335 (2.7), 384 (1.8) - b 556 [8.0] b 556, [0.3 µs(63%), 2.0 µs(37%)] c 550,[2.2 (38%) 6.3(62%)] c a. Absorption spectrum was recorded in CH 2 Cl 2 at 298 K. b. Emission spectrum was recorded in 2-MeTHF glass. c. Emission spectra were recorded of the neat solid. 42 The Os(PCP) 2 complex is nonemissive in fluid solution or when doped (10%) in a PMMA matrix at room temperature. Emission of the complex can be observed when the PMMA film is pressed with 20000 psi(Figure 2-13). However, a broad, featureless 250 300 350 400 450 500 550 600 650 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 PCP-H ligand abs. Emi. Wavelength (nm) Absorbance (a.u.) 0.0 0.2 0.4 0.6 0.8 1.0 Normalized Emission Figure 2-12. Absorption (recorded in CH 2 Cl 2 ) and 77 K emission spectra (recorded in 2-MeTHF glass) of PCP-H ligand. 300 400 500 600 700 0.05 0.10 0.15 0.20 0.25 0.30 (10 4 M -1 cm -1 ) Normalized Emissions abs. tddft 77K 2MeTHF 298K solid Wavelength (nm) 0.0 0.2 0.4 0.6 0.8 1.0 Solid PMMA Figure 2-13. Absorption spectrum of Os(PCP) 2 in CH 2 Cl 2 solution(black line) and the relative oscillator strengths calculated using TD-DFT (green lines) along with emission spectra recorded in 2-MeTHF glass at 77 K(blue line) and as a neat solid at room temperature (red dashed line). 43 emission (l max = 546 nm) is observed (quantum yieldF= 0.6, measured as relative to the quantum yield of Ir(ppy) 3 ) in a dilute solution of 2-MeTHF at 77 K (Figure 2-13) with a microsecond lifetime (t = 8.0m s) indicative of phosphorescence. The radiative rate constant is of 7 x 10 4 s -1 , and the non-radiative rate constant is 5 x 10 4 s -1 . The phosphorescence is at lower energy than that observed from free PCP-H under the same conditions (l max = 426 nm,t = 1.0 s) (Figure 2-12). 16 The Os(PCP) 2 complex is also luminescent as a neat solid at room temperature (λ max = 556 nm). The quantum yield (F= 0.03) and lifetime (t = 0.3m s, 63%; 2.0m s, 37%) in the solid state lead to an estimated radiative rate constant of ca. 3 x 10 4 s -1 , while the non-radiative rate is two orders of magnitudes larger. Cooling the solid to 77 K decreases non-radiative decay(Figure 2-15). However, the measured lifetimes (t = 2.2m s, 38%; 6.3m s, 62%) indicate that the emission efficiency is less than unity(the Figure 2-14. Absorption spectra of Os(PCP) 2 and [Os(PCP) 2 ] + in CH 2 Cl 2 solution. 400 600 800 1000 0 500 1000 1500 2000 2500 ( M -1 cm -1 ) Wavelength (nm) [Os(PCP) 2 ] + Os(PCP) 2 44 lifetime time of the Os(PCP) 2 is calculated of 33m s if achieving unity quantum yield). The emission of the Os(PCP) 2 is observed in doped PMMA films at low temperature (77 K). The Discussion of the nature of the emissive state will follow from an analysis of the computational studies for Os(PCP) 2 . 300 400 500 600 700 0.2 0.4 0.6 0.8 1.0 Normalized Excitation Normalized Emission Wavelength (nm) 10% in PMMA exc. 77 K emi. 77 K 300 400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0 Normalized Emissions Normalized Excitation Wavelength (nm) Neat Solid exc, 77K emi, 77K Figure 2-15.(Top) Excitation and emission spectra for 10% Os(PCP) 2 doped in PMMA, recorded at 77 K. (Bottom) Excitation and emission spectra for neat solid Os(PCP) 2 recorded at 77 K. 45 2.3.6. DFT calculations The electronic structure of Os(PCP) 2 was investigated using density functional theory (DFT) and time-dependent (TD-DFT) methods starting from coordinates of the single crystal structure. Bond lengths within the coordination sphere calculated for Os(PCP) 2 in the gas-phase are slightly longer than values in the crystal structure. The frontier orbitals are shown in Figure 2-16. The HOMO and HOMO-1 of Os(PCP) 2 are nearly degenerate and predominantly localized on the Os center (46%, 38%) and xylyl (L, 31%, 29%), with the remaining contribution from the PPh 2 substituents (L’). The LUMO, LUMO+1 and LUMO+2 are also nearly generate and are localized on L’ with varying participation from Os. For example, the LUMO has 3% contribution from Os, whereas the amount is 14% for the LUMO+1 and 2% for LUMO+2. TD-DFT calculations were used to assign the S 0 →S n and S 0 →T n transition energies and oscillator strengths (f) of Os(PCP) 2 . The S 0 →S 1 transition calculated for Os(PCP) 2 (l = 441 nm, f = 0.0193) is close to the onset in the experimental absorption spectrum (Table 2-4). Poor overlap between the frontier orbitals of Os(PCP) 2 leads to the small oscillator strength. The S 0 →S 1 transition primarily involves HOMO→LUMO+1(94%) mixed with higher HOMO→LUMO+10(2%) and HOMO→LUMO+20(4%) configurations. The LUMO+10 and LUMO+20 are primarily metal-centered (MC) and have substantial contributions (3% and 20% respectively) from antibonding orbitals on Os. On the basis of orbital parentage, the S 0 →S 1 transition can be described as principally a metal-xylyl (ML) to PPh 2 (L’) charge transfer transition (ML-L’CT) configurationally mixed with metal centered ( 1 MC) transitions. The other lowest S 0 →S n (n = 2–10) 46 transitions are all similarly dominated by the ML-L’CT transitions mixed with varying amounts of 1 MC transitions. Likewise, the S 0 →T 1 transition (l = 491 nm) involves HOMO→LUMO+1(78%) with notable participation from HOMO→LUMO+20(16%). DFT calculation of the geometry optimized triplet state gives a self-consistent field (ΔSCF) energy separation between S 0 and T 1 that is in good agreement with the emission energy (ΔSCF = 2.42 eV, l = 512 nm). The spin density surface for this state is predominantly localized on the osmium center (65%) with an obvious contribution from the d (x2-y2) orbital (Figure 2-17). The Os–P bonds are elongated (2.53 Å) compared to values in the optimized singlet ground state (2.41 Å), whereas the Os–C bond lengths are little changed. The contribution from the PCP ligand is relatively small, which suggests that the emission from Os(PCP) 2 is dominated by the 3 MC state. LUMO+1 (-0.64 eV) LUMO+2 (-0.63 eV) LUMO+20 (1.09 eV) LUMO (-0.66 eV) HOMO (-4.25 eV) HOMO-1 (-4.30 eV) Figure 2-16. Frontier orbitals involved in the S 0 →S n (n = 1–6) and S 0 →T 1 transitions of Os(PCP) 2 . 46 transitions are all similarly dominated by the ML-L’CT transitions mixed with varying amounts of 1 MC transitions. Likewise, the S 0 →T 1 transition (l = 491 nm) involves HOMO→LUMO+1(78%) with notable participation from HOMO→LUMO+20(16%). DFT calculation of the geometry optimized triplet state gives a self-consistent field (ΔSCF) energy separation between S 0 and T 1 that is in good agreement with the emission energy (ΔSCF = 2.42 eV, l = 512 nm). The spin density surface for this state is predominantly localized on the osmium center (65%) with an obvious contribution from the d (x2-y2) orbital (Figure 2-17). The Os–P bonds are elongated (2.53 Å) compared to values in the optimized singlet ground state (2.41 Å), whereas the Os–C bond lengths are little changed. The contribution from the PCP ligand is relatively small, which suggests that the emission from Os(PCP) 2 is dominated by the 3 MC state. LUMO+1 (-0.64 eV) LUMO+2 (-0.63 eV) LUMO+20 (1.09 eV) LUMO (-0.66 eV) HOMO (-4.25 eV) HOMO-1 (-4.30 eV) Figure 2-16. Frontier orbitals involved in the S 0 →S n (n = 1–6) and S 0 →T 1 transitions of Os(PCP) 2 . 46 transitions are all similarly dominated by the ML-L’CT transitions mixed with varying amounts of 1 MC transitions. Likewise, the S 0 →T 1 transition (l = 491 nm) involves HOMO→LUMO+1(78%) with notable participation from HOMO→LUMO+20(16%). DFT calculation of the geometry optimized triplet state gives a self-consistent field (ΔSCF) energy separation between S 0 and T 1 that is in good agreement with the emission energy (ΔSCF = 2.42 eV, l = 512 nm). The spin density surface for this state is predominantly localized on the osmium center (65%) with an obvious contribution from the d (x2-y2) orbital (Figure 2-17). The Os–P bonds are elongated (2.53 Å) compared to values in the optimized singlet ground state (2.41 Å), whereas the Os–C bond lengths are little changed. The contribution from the PCP ligand is relatively small, which suggests that the emission from Os(PCP) 2 is dominated by the 3 MC state. LUMO+1 (-0.64 eV) LUMO+2 (-0.63 eV) LUMO+20 (1.09 eV) LUMO (-0.66 eV) HOMO (-4.25 eV) HOMO-1 (-4.30 eV) Figure 2-16. Frontier orbitals involved in the S 0 →S n (n = 1–6) and S 0 →T 1 transitions of Os(PCP) 2 . 47 The results from the DFT calculations can be used to construct a qualitative potential energy diagram illustrating surfaces for lowest singlet and triplet states of Os(PCP) 2 (Figure 7). The lowest singlet state (S 1 ) is ML-L’CT in character with a small contribution from the higher lying 1 MC states. The large spin-orbit coupling constant of osmium leads to effective intersystem crossing to a triplet state with an increased MC Table 2-4. S 0 →S n (n = 1-10) transitions, S 0 →T 1 and S 0 →T 2 transitions, wavelength (λ cal ), oscillator strength (f), and transition contributions (> 10%, and all transitions are listed for S 0 →S 1 and S 0 →S 2 ) and assignments for Os(PCP) 2 from TDDFT calculations. States λ cal (nm) f major contribution assignment T 1 491 0 HOMO→LUMO+1 (78%) HOMO→LUMO+20 (16%) ML-L’CT + 3 MC T 2 461 0 HOMO→LUMO+1 (81%) HOMO→LUMO+20 (11%) ML-L’CT + 3 MC S 1 441 0.0193 HOMO→LUMO+1 (94%) HOMO→LUMO+10 (2%) HOMO→LUMO+20 (4%) ML-L’CT + 1 MC S 2 424 0.0283 HOMO-1→LUMO (12%) HOMO-1→LUMO+1 (85%) HOMO→LUMO+20 (3%) ML-L’CT + 1 MC S 3 420 0.0213 HOMO→LUMO (89%) HOMO-1→LUMO+1 (11%) ML-L’CT S 4 416 0.0026 HOMO→LUMO+2 (100%) ML-L’CT S 5 409 0.0242 HOMO-1→LUMO (100%) ML-L’CT S 6 404 0.0000 HOMO-1→LUMO+2 (100%) ML-L’CT S 7 388 0.0006 HOMO→LUMO+3 (100%) ML-L’CT S 8 387 0.0000 HOMO→LUMO+4 (100%) ML-L’CT S 9 384 0.0050 HOMO→LUMO+5 (66%) HOMO→LUMO+7 (34%) ML-L’CT S 10 379 0.0000 HOMO→LUMO+3 (97%) ML-L’CT 48 character. The high triplet energy of the PCP ligand favors such mixing between the 3 ML-L’CT and 3 MC states. Any further elongation of the Os–P bond will increase participation from the 3 MC state. The T 1 state can then readily undergo non-radiative deactivation to the ground state, either vibronically or through a surface crossing, leading to the rather low quantum yield for the solid state Os(PCP) 2 at room temperature. The quantum yield of the complex is less than unity even at 77 K due to the domination from the 3 MC state, indicating that the barrier to thermal crossing into the 3 MC state is shallow. Figure 2-18. Energy diagram illustrating the radiative and non-radiative decay channels involving the singlet and triplet states of Os(PCP) 2 . Figure 2-17. Spin density surface calculated for the triplet state of Os(PCP) 2 . 48 character. The high triplet energy of the PCP ligand favors such mixing between the 3 ML-L’CT and 3 MC states. Any further elongation of the Os–P bond will increase participation from the 3 MC state. The T 1 state can then readily undergo non-radiative deactivation to the ground state, either vibronically or through a surface crossing, leading to the rather low quantum yield for the solid state Os(PCP) 2 at room temperature. The quantum yield of the complex is less than unity even at 77 K due to the domination from the 3 MC state, indicating that the barrier to thermal crossing into the 3 MC state is shallow. Figure 2-18. Energy diagram illustrating the radiative and non-radiative decay channels involving the singlet and triplet states of Os(PCP) 2 . Figure 2-17. Spin density surface calculated for the triplet state of Os(PCP) 2 . 48 character. The high triplet energy of the PCP ligand favors such mixing between the 3 ML-L’CT and 3 MC states. Any further elongation of the Os–P bond will increase participation from the 3 MC state. The T 1 state can then readily undergo non-radiative deactivation to the ground state, either vibronically or through a surface crossing, leading to the rather low quantum yield for the solid state Os(PCP) 2 at room temperature. The quantum yield of the complex is less than unity even at 77 K due to the domination from the 3 MC state, indicating that the barrier to thermal crossing into the 3 MC state is shallow. Figure 2-18. Energy diagram illustrating the radiative and non-radiative decay channels involving the singlet and triplet states of Os(PCP) 2 . Figure 2-17. Spin density surface calculated for the triplet state of Os(PCP) 2 . 49 2.4. Conclusion In summary, we have demonstrated the synthetic routes and characterization of the bis-pincer organometallic osmium complex Os(PCP) 2 . The Os(PCP) 2 was composed ofD andLenantiomers with relatively slow exchange processes that can be observed on the NMR time scale. As expected, the poorly conjugated pincer chelates greatly increased the energy of the metal and ligand based unoccupied orbitals of the complex Os(PCP) 2 . The large spin-orbit coupling constant of the Os eventually leaded to a lowest triplet state with significant amount of metal character. The solid state Os(PCP) 2 took advantage from the stable coordination pattern of PCP pincer ligands and showed 3% quantum yield dominated by the 3 MC state. However, the non-radiative rate of the complex Os(PCP) 2 was much higher than its radiative rate also due to the lowest metal centered state. The PCP ligand can be viewed as composed of the weaker field ligand diphenyl-phosphine and the stronger field ligand of xylyls. Even though the xylyl phenyls destabilized the metal centered ligand field state to some extent, the metal centered state was still accessible. Another non-radiative channel was induced by the racemization between the two Os(PCP) 2 enantiomers. The non-radiative decay was suppressed when the complex Os(PCP) 2 was in a frozen solution, and also at solid state. 50 Chapter 2 References 1.(a) Sauvage, J. P.; Collin, J. P.; Chambron, J. C.; Guillerez, S.; Coudret, C.; Balzani, V.; Barigelletti, F.; Decola, L.; Flamigni, L., Chem Rev 1994, 94(4), 993; (b) Encinas, S.; Flamigni, L.; Barigelletti, F.; Constable, E. C.; Housecroft, C. E.; Schofield, E. R.; Figgemeier, E.; Fenske, D.; Neuburger, M.; Vos, J. G.; Zehnder, M., Chem-Eur J 2002, 8(1), 137; (c) Wu, K. Q.; Guo, J.; Yan, J. F.; Xie, L. L.; Xu, F. B.; Bai, S.; Nockemann, P.; Yuan, Y. F., Organometallics 2011, 30(13), 3504; (d) Tsai, C. N.; Allard, M. M.; Lord, R. L.; Luo, D. W.; Chen, Y. J.; Schlegel, H. B.; Endicott, J. F., Inorg Chem 2011, 50(23), 11965; (e) Seo, K.; Konchenko, A. V.; Lee, J.; Bang, G. S.; Lee, H., J Am Chem Soc 2008, 130(8), 2553; (f) Benniston, A. C.; Harriman, A.; Pariani, C.; Sams, C. A., Phys Chem Chem Phys 2006, 8(17), 2051; (g) Zhong, Y. W.; Vila, N.; Henderson, J. C.; Flores-Torres, S.; Abruna, H. D., Inorg Chem 2007, 46(25), 10470; (h) Dhungana, K. B.; Mandal, S.; Pati, R., J Phys Chem C 2012, 116(32), 17268; (i) Cooke, M. W.; Hanan, G. S.; Loiseau, F.; Campagna, S.; Watanabe, M.; Tanaka, Y., J Am Chem Soc 2007, 129(34), 10479; (j) Loiseau, F.; Nastasi, F.; Stadler, A. M.; Campagna, S.; Lehn, J. M., Angew Chem Int Edit 2007, 46(32), 6144. 2. Passalacqua, R.; Loiseau, F.; Campagna, S.; Fang, Y. Q.; Hanan, G. S., Angew Chem Int Edit 2003, 42(14), 1608. 3.(a) Son, S. U.; Park, K. H.; Lee, Y. S.; Kim, B. Y.; Choi, C. H.; Lah, M. S.; Jang, Y. H.; Jang, D. J.; Chung, Y. K., Inorg Chem 2004, 43(22), 6896; (b) Dinda, J.; Liatard, S.; Chauvin, J.; Jouvenot, D.; Loiseau, F., Dalton T 2011, 40(14), 3683; (c) Schulze, B.; Escudero, D.; Friebe, C.; Siebert, R.; Gorls, H.; Kohn, U.; Altuntas, E.; Baumgaertel, A.; Hager, M. D.; Winter, A.; Dietzek, B.; Popp, J.; Gonzalez, L.; Schubert, U. S., Chem-Eur J 2011, 17(20), 5494; (d) Brown, D. G.; Sanguantrakun, N.; Schulze, B.; Schubert, U. S.; Berlinguette, C. P., J Am Chem Soc 2012, 134(30), 12354; (e) Friese, V.; Nag, S.; Wang, J. H.; Santoni, M. P.; Rodrigue-Witchel, A.; Hanan, G. S.; Schaper, F., Eur J Inorg Chem 2011,(1), 39; (f) Park, H. J.; Chung, Y. K., Dalton T 2012, 41(18), 5678; (g) Lee, C. S.; Zhuang, R. R.; Wang, J. C.; Hwang, W. S.; Lin, I. J. B., Organometallics 2012, 31(14), 4980; (h) Park, H. J.; Kim, K. H.; Choi, S. Y.; Kim, H. M.; Lee, W. I.; Kang, Y. K.; Chung, Y. K., Inorg Chem 2010, 49(16), 7340. 4. Williams, J. A. G., Chem Soc Rev 2009, 38(6), 1783. 5. Moulton, C. J.; Shaw, B. L., J Chem Soc Dalton 1976,(11), 1020. 6.(a) Albrecht, M.; van Koten, G., Angew Chem Int Edit 2001, 40(20), 3750; (b) van der Boom, M. E.; Milstein, D., Chem Rev 2003, 103(5), 1759. 7. Pape, A.; Lutz, M.; Muller, G., Angewandte Chemie-International Edition in English 1994, 33(22), 2281. 51 8.(a) Hoffman, P. R.; Caulton, K. G., J Am Chem Soc 1975, 97(15), 4221; (b) Elliott, G. P.; Mcauley, N. M.; Roper, W. R.; Shapley, P. A., Inorg Syn 1989, 26, 184; (c) Rimml, H.; Venanzi, L. M., J Organomet Chem 1983, 259(1), C6. 9. Wen, T. B.; Cheung, Y. K.; Yao, J. Z.; Wong, W. T.; Zhou, Z. Y.; Jia, G. C., Organometallics 2000, 19(19), 3803. 10. Dani, P.; Toorneman, M. A. M.; van Klink, G. P. M.; van Koten, G., Organometallics 2000, 19(25), 5287. 11. Dani, P.; Karlen, T.; Gossage, R. A.; Smeets, W. J. J.; Spek, A. L.; vanKoten, G., J Am Chem Soc 1997, 119(46), 11317. 12. Williams, T. J.; Kershaw, A. D.; Li, V.; Wu, X. P., J Chem Educ 2011, 88(5), 665. 13. Kaplan, J., NMR of Chemically Exchanging Systems. Elsevier Science: 2012. 14. Ceron-Camacho, R.; Hernandez, S.; Le Lagadec, R.; Ryabov, A. D., Chem Commun 2011, 47(10), 2823. 15.(a) Chi, Y.; Chou, P. T., Chem Soc Rev 2007, 36(9), 1421; (b) 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(24), 7377. 16. Fife, D. J.; Moore, W. M.; Morse, K. W., Inorg Chem 1984, 23(11), 1545. 52 Chapter 3: Synthesis and photophysical characterization of heteroleptic iridium complexes with phosphinoaryl cyclometalates 3.1. Introduction The energy and coordination pattern of the chelated ligand in both homoleptic and heteroleptic tris-cyclometalated Ir(III) complexes is crucial for obtaining tunable emission energies, high phosphorescence efficiencies, and long excited lifetimes; all of which are important in applications such as organic light emitting devices (OLEDs), 1 nonlinear optics, water splitting, bioimaging and biosensing, and singlet oxygen sensitization. 2 The C^N type ligands are among the best ligands for both heteroleptic and homoleptic Ir(III) complexes. The Ir(III) metal center efficiently mixes singlet and triplet excited states via strong spin-orbit coupling. The triplet spin density of a homoleptic Ir(C^N) 3 complex is most often delocalized on both the Ir(III) and the C^N ligands. 1 In heteroleptic Ir(III) complexes, the chelated ligands can be separated into chromophore (C^N) and ancillary ligands. Typical examples of these complexes have two cyclometalated C^N ligands and a higher energy single monoanionic bidentate ancillary ligand(LX) such as a β-diketonate or picolinic acid, giving the fomula(C^N) 2 Ir(LX). 2a, 2b The lowest triplet excited state of these complexes originates from the mixing of 3 MLCT and 3 LC state of the C^N chelates. The phosphorescence of(C^N) 2 Ir(LX) complexes is 53 largely generated from the [(C^N) 2 Ir-] fragments, and can be tuned from blue to red with high luminescence efficiency. 3 Heteroleptic Ir(III) complexes with C^P type of chelates are much less studied compared to the(C^N) 2 Ir(LX) complexes. Some studies by Chi, Chou, and co-workers have successfully employed the X^P (X can be C, N) chelates along with the C^N chelates into heteroleptic Ir(III) complexes structures (Scheme 1, top). 4 Such X^P chelates typically show high triplet energy (over 3.0 eV). The lowest triplet excited state of the Ir(III) complexes does not involve the X^P ligands, but are localized on the Scheme 1. Ir C C P Ph 2 N N Ir C C P Ph 2 O N N Ir C N C P Ph 2 N N N CF 3 R' R' R' = H, F N Z Ir P Ph 2 2 F N Ir P Ph 2 2 F Z = CH, N N N Ir P Ph 2 Z N N But 2 N N Ir P Ph 2 Z N F 3 C 2 54 Ir-(C^N) fragment of the complexes similar to the(C^N) 2 Ir(LX) complexes, with the X^P chelates serving as the ancillary ligand. Du, Lin, and co-workers later reported the study on Ir(III) complexes containing diphenylphosphinoaryl cyclometalates of naphthalene-PPh 2 and isoquinoline-PPh 2 ligand. 5 The triplet energies of these phosphinoaryl ligands are lower than the other X^P chelates. However, they were used with red emission C^N ligands, so the C^P ligand remain as ancillaries(Scheme 1, middle). In some complexes where C^P and C^N ligands show comparable triplet energies, the lowest excited state of the Ir(III) complexes are dominated by intra-ligand charge transfer(Scheme 1, bottom). Phosphinoaryl chelates provide 5-membered cyclometalated ring, and are alternative ligand choices to the C^N type ligands used in homoleptic and heteroleptic Ir(III) complexes. The C^P ligands can be employed as either chromophore or ancillary ligand based on their energy levels compared to the other ligands in a heteroleptic Ir(III) complex. In order to study the C^P ligands as chromophore fragments in heteroleptic Ir(III) complexes, and to understand the photoluminescence mechanism of Ir(III) complexes based on Ir-(C^P) chelated structure, a series of Ir(III) complexes with two of the high energy (C^N) ligands (ppz = 2-phenylpyrazole) and a single C^P chelate of PPh 2 -naphthalene, 5-PPh 2 -quinoline, and 5-PPh 2 -isoquinoline were prepared. In these complexes the ppz C^N ligand is not involved in the photoluminescent properties and is thus an ancillary ligand. The photophysical and photochemical properties of the complexes can be directly controlled via the aryl fragments of the C^P ligands. 55 3.2. Experimental 3.2.1. Materials and Synthesis The compounds PPh 2 -naphthalene, 5-PPh 2 -quinoline, and 5-PPh 2 -isoquinoline were prepared following literature procedures. 6 IrCl 3 •H 2 O was ordered from Next Chimica. [(ppz) 2 Ir(µ-Cl)] 2 was synthesized following previously reported procedures. 7 Other chemicals and solvents were ordered from Sigma-Aldrich®, and used as received. The syntheses were carried out under a nitrogen atmosphere using standard Schlenk techniques. 1 H NMR, 31 P NMR, 13 C NMR and gCOSY spectra were measured by Varian 400 and 500S NMR Spectrometer. The 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. General synthesis procedure for the meridional complexes (ppz) 2 Ir(PPh 2 -naphthalene) (1-mer), (ppz) 2 Ir(5-PPh 2 -quinoline) (2-mer), and (ppz) 2 Ir(5-PPh 2 -isoquinoline) (3-mer). A mixture of [(ppz) 2 Ir(µ-Cl)] 2 (1 equiv), diphenylphosphinoaryl ligand (2.4 equiv), Ag 2 O (10 equiv), and KOH pellets (20 equiv) were dissolved in 40 ml of 1,2-dichlrobenzene (DCB), fully degassed under N 2 , and then heated at 120 °C in dark for 18 h. The DCB was removed at 80 °C under low pressure. Dichloromethane was added, and the solution was filtered with a celite plug to remove the insoluble salts. The filtrate was collected and concentrated via rotavap. The crude product of n-mer(n = 1-3) was purified by silica gel column chromatography eluting 56 with hexane and ethyl acetate. The yield of compound 1-mer, 2-mer, and 3-mer were 85%, 75% and 90% respectively. Spectra data of 1-mer. 31 P NMR (202 MHz, CDCl 3 , 298 K): δ = 15.09 (s). 1 HNMR (500 MHz, CDCl 3 , 298 K): δ = 7.88-7.90 (m, 2H), 7.86 (d, J = 3 Hz, 1H), 7.77 (d, J = 2.7 Hz, 1H), 7.75 (dt, J 1 = 1.3 Hz, J 2 = 8.5 Hz, 1H), 7.49 (ddd, J 1 = 1.8 Hz, J 2 = 7.0 Hz, J 3 = 8.1 Hz, 1H), 7.34-7.40(m, 5H),7.15 (dt, J 1 = 1.2 Hz, J 2 = 7.7 Hz, 1H), 6.99 (td, J 1 = 1.2 Hz, J 2 =7.0 Hz,1H), 6.82-6.95(m, 7H), 6.72-6.76 (m, 3H), 6.51 (ddd, J 1 = 1.2 Hz, J 2 = 8.0 Hz,J 3 = 9.3 Hz, 2H), 6.40 (m, 1H), 6.17-6.20(m, 2H), 5.92 (t, J = 2.6 Hz, 1H), 5.84 (t, J = 2.7 Hz, 1H). 13 C NMR (126 MHz, 298 K): δ = 160.1, 154.6, 154.1, 153.9, 148.8, 143.8, 141.4, 139.4, 138.6, 137.6, 137.2, 135.0, 134.8, 134.4, 134.2, 133.9, 133.4, 133.1, 132.7, 132.4, 132.1, 131.2, 130.6, 129.0, 128.6, 127.9, 127.8, 126.9, 126.1, 124.9, 124.8, 121.7, 111.3, 111.2, 107.5, 106.9. LC-MS ESI (m/z): 791.25([M+1] + , calcd : 790.18). Anal. calcd. for C 40 H 30 IrN 4 P: C, 60.82; H, 3.83; N, 7.09. Found: C, 61.36; H, 3.79; N, 7.15. Spectra data of 2-mer. 31 P NMR (202 MHz, CDCl 3 , 298 K): δ = 14.43 (s). 1 HNMR (500 MHz, CDCl 3 , 298 K): δ = 8.23(d, J = 4.4 Hz, 1H), 8.17(dt, J 1 = 1.5 Hz, J 2 = 8.2 Hz, 1H), 7.89-7.95 (m, 3H), 7.87(d, J= 2.9 Hz, 1H), 7.79 (ddd, J 1 = 2.3 Hz, J 2 = 5.6 Hz, J 3 = 7.1 Hz, 1H), 7.49(d, J = 2.9 Hz, 1H), 7.41-7.47 (m, 3H), 7.24 (dt, J 1 = 1.0 Hz, J 2 = 7.9 Hz, 1H), 6.81-7.05(m, 10H), 6.57 (ddd, J 1 = 1.0 Hz, J 2 = 8.0 Hz, J 3 = 9.5 Hz, 2H), 6.57 (m, 1H), 6.24 - 6.27 (m, 2H), 6.03 (t, J = 2.6 Hz, 1H), 5.94(t, J = 2.6 Hz, 1H). 13 C NMR (126 MHz,CDCl 3 , 298 K): δ =152.3, 151.5, 151.2, 150.9, 149.8, 147.6, 147.2, 143.1, 140.9, 139.1, 138.3, 137.9, 137.6, 136.5, 133.9, 133.5, 133.0, 132.3, 132.2, 132.1, 131.9, 130.7, 57 130.5, 129.4, 128.8, 128.5, 127.9, 127.7, 126.7, 125.9, 124.9, 124.7, 121.9, 121.7, 121.7, 111.1, 110.9, 107.4, 106.8. LC-MS ESI (m/z): 791.20([M], calcd : 791.18). Anal. calcd. for C 39 H 29 IrN 5 P: C, 59.23; H, 3.7; N, 8.86. Found: C, 58.78; H, 3.51; N, 8.58. Spectra data of 3-mer. 31 P NMR (162 MHz, CDCl 3 , 298 K): δ = 14.13(s). 1 H NMR (400 MHz, CDCl 3 , 298 K):δ =8.90(s,1H), 8.02-8.08 (m, 2H), 7.89-7.95 (m, 2H), 7.85(d, J = 2.8Hz, 1H), 7.81 (s, 1H), 7.69 (J 1 = 1.5 Hz, J 2 = 8.0 Hz, 1H), 7.43-7.49 (m, 4H), 7.21 (d, J = 8.0Hz, 1H), 6.76-7.04 (m, 9H),6.51-6.57 (m, 3H),6.24-6.27 (m, 2H), 6.00 (t, J = 2.5Hz, 1H), 5.93 (t, J = 2.5Hz, 1H). 13 C NMR (101 MHz,CDCl 3 , 298 K): δ = 156.0, 155.7, 152.0, 151.2, 150.2, 148.9, 147.2, 145.5, 143.0, 140.5, 138.6, 138.0, 136.3, 135.7, 133.6, 132.8, 132.4, 132.2, 132.0, 131.8, 131.0, 130.4, 130.2, 129.1, 128.9, 128.6, 128.2, 127.4, 126.4, 125.7, 125.6, 124.5, 124.4, 121.5, 121.2, 110.8, 111.6, 106.9, 106.3. Maldi- MS(m/z): 791.86([M] + , calcd : 791.18). Anal. calcd. for C 39 H 29 IrN 5 P: C, 59.23; H, 3.7; N, 8.86. Found: C, 58.03; H, 3.71; N, 8.49. Synthesis of(ppz) 2 Ir(Cl)(PPh 2 -naphthalene)(1-Cl). The [(ppz) 2 Ir(µ-Cl)] 2 (1 equiv), Ag 2 O (10 equiv), and PPh 2 -naphthalene ligand (2.2 equiv) were refluxed under N 2 atmosphere in 40 mL of 1,2-dichlroethane(DCE) for 18 h. After the mixture was cooled to room temperature, celite plug was used to remove any insoluble salts in the solution mixture. Then the DCE was removed via rotavap. The crude product was chromatographed on silica gel with hexane and ethyl acetate (6 : 4) as the eluent to yield 70% of pure 1-Cl. Spectra data of 1-Cl. 31 P NMR (162 MHz, CD 2 Cl 2 , 298 K): δ = -7.9(s). 1 H NMR (400 MHz, CD 2 Cl 2 , 297 K): δ = 8.10 (d, J = 2.9 Hz, 1H), 7.97 (dd, J 1 = 7.2 Hz, 58 J 2 = 13.0 Hz, 1H), 7.83 (d, J = 8.2 Hz, 1H), 7.65 – 7.72 (m, 4H), 7.57 (dt, J 1 = 1.0 Hz, J 2 = 3.0 Hz, 1H), 7.37 – 7.47 (m, 3H), 7.11 – 7.31 (m, 9H), 7.05 (d, J = 8.5 Hz, 1H), 6.83 - 6.90 (m, 3H), 6.65 – 6.74 (m, 3H), 6.42 (dt, , J 1 = 1.2 Hz, J 2 = 7.5 Hz, 1H), 5.89 (t, J = 2.5 Hz, 1H), 5.72 – 5.77 (m, 2H). 13 C NMR (101 MHz,CD 2 Cl 2 , 298 K): δ =145.2, 143.7, 142.6, 142.2, 139.3, 137.2, 135.8, 134.7, 134.6, 134.5, 134.4, 134.3, 134.1, 134.0, 133.9, 133.5, 132.7, 132.1, 130.4, 130.2, 129.3, 129.2, 127.8, 127.7, 127.4, 127.0, 126.1, 125.9, 125.4, 123.6, 122.2, 111.8, 111.0, 108.4, 107.1. LC-MS ESI (m/z): 791.25([M- Cl] + , calcd : 826.16). Anal. calcd. for C 40 H 31 ClIrN 4 P: C, 58.14; H, 3.78; N, 6.78. Found: C, 55.79; H, 3.65; N, 6.39. General synthesis procedure of n-fac (n = 1-3). An acetonitrile solution of 200 mg of the meridional complex n-mer(n = 1-3) was fully degassed under N 2 , and then stirred in a UV reactor with irradiation at wavelength of 375 nm for 15 h. The solution was then dried. Chromatography was done on silica gel using hexane and ethyl acetate (6 : 4) as the eluent to yield > 95 % of the facial complex n-fac(n = 1, 2). The complex 3-fac decomposed on silica or alumina columns. Thus several precipitations were performed with layered ethyl acetate and hexane to achieve a yield of about 10%. Spectra data of 1-fac. 31 P NMR (162 MHz, CDCl 3 , 298 K): δ = 15.19 (s). 1 H NMR(500 MHz, CDCl 3 , 298 K):δ = 7.97 (d, J = 2.7Hz, 1H), 7.90 (dt, J 1 = 1.4 Hz, J 2 = 8.0 Hz, 1H), 7.71 (m, 2H), 7.67 (d, J = 2.7Hz, 1H), 7.60 (ddd, J 1 = 1.2 Hz, J 2 = 7.0 Hz, J 3 = 8.2 Hz, 1H), 7.43-7.50 (m, 4H), 7.39 (dt, J 1 = 1.3 Hz, J 2 = 8.0 Hz, 1H), 7.29 (dt, J 1 = 1.4 Hz, J 2 = 8.0 Hz, 1H), 7.08 (m, 1H), 7.00 (ddd, J 1 = 1.5 Hz, J 2 = 7.0 Hz, J 3 = 8.3 Hz, 2H), 6.93 (dd, J 1 = 7.2Hz, J 2 = 8.0Hz, 1H), 6.70-6.80 (m, 7H), 6.60 (ddd, J 1 = 1.2 Hz, J 2 = 59 8.4 Hz, J 3 = 9.8 Hz, 2H), 6.52(ddd, , J 1 = 2.7Hz, J 2 = 5.8Hz, J 3 = 7.6Hz, 1H), 6.33 (t , J = 2.5 Hz, 1H), 6.28 (m, 1H), 6.06 (t, J = 2.5 Hz, 1H), 6.02 (d, J = 2.2Hz, 1H). 13 C NMR (126 MHz, CDCl 3 , 298 K): δ = 155.5, 154.7, 152.4, 152.1, 146.7, 142.6, 142.4, 139.2, 137.9, 137.2, 136.1, 136.0, 134.8, 134.7, 134.4, 134.3, 134.0, 133.9, 132.6, 132.0, 131.7, 130.8, 130.7, 130.6, 129.4, 128.2, 127.5, 127.1, 126.8, 126.1, 125.7, 124.9, 124.2, 124.0, 121.7, 120.0, 119.8, 110.7, 106.5, 106.0. LC-MS ESI (m/z): 791.20([M+1] + , calcd : 790.18). Anal. calcd. for C 40 H 30 IrN 4 P: C, 60.82; H, 3.83; N, 7.09. Found: C 59.33; H, 3.49; N, 6.74. Spectra data of 2-fac. 31 P NMR (162 MHz, CDCl 3 , 298 K): δ = 14.25 (s). 1 H NMR(400 MHz, CDCl 3 , 298 K): δ = 8.14(d, J = 7.9 Hz, 1H), 8.06 (d, J = 4.4 Hz, 1H), 7.99 (d, J = 2.7 Hz, 1H), 7.64-7.72 (m, 5H), 7.44 - 7.48 (m, 3H), 7.31(d, J= 8.0 Hz, 1H), 7.01 – 7.06 (m, 3H), 6.66 – 6.83 (m, 7H), 6.57 – 6.62 (m, 2H), 6.54 (ddd, J 1 = 3.1 Hz, J 2 = 5.3 Hz, J 3 = 7.9 Hz, 1H), 6.35 (t, J = 2.3 Hz, 1H), 6.26(d, J = 7.5 Hz, 1H), 6.10(t, J = 2.4 Hz, 1H), 6.00(d, J = 2.0 Hz, 1H). 13 C NMR (101 MHz, 297 K): δ = 153.3, 152.5, 150.4, 150.1, 149.4, 142.4, 139.3, 137.2, 136.5, 136.0, 135.8, 135.4, 135.0, 133.7, 133.4, 132.4, 131.9, 131.4, 131.1, 130.9, 130.8, 130.6, 129.7, 128.3, 127.8, 127.5, 127.0, 126.3, 125.9, 125.2, 124.4, 122.2, 120.6, 110.9, 110.8, 106.7, 106.3. LC-MS ESI (m/z): 791.20 ([M], calcd : 791.18). Anal. calcd. for C 39 H 29 IrN 5 P: C, 59.23; H, 3.7; N, 8.86. Found: C, 59.49; H, 3.89; N, 8.09. Spectra data of 3-fac. 31 P NMR (162 MHz, CD 2 Cl 2 , 298 K): δ = 14.00 (s). 1 H NMR (400 MHz, CD 2 Cl 2 , 298 K): δ = 8.83(m, 1H), 7.99 (m, 2H), 7.82 (m, 1H) 7.59-7.69(m, 5H), 7.46-7.48 (m, 3H), 7.30(m, 1H), 6.96-7.03 (m, 3H), 6.72-6.81 (m, 5H), 6.52-6.60 60 (m, 3H), 6.36 (t, J = 2.2 Hz, 1H), 6.31(m, 1H), 6.08(t, J = 2.3 Hz, 1H), 6.03(d, J = 2.2 Hz, 1H). 13 C NMR (101 MHz, CDCl 3 , 298 K): δ = 142.5, 140.1, 139.3, 137.4, 136.5, 136.2, 132.7, 130.8, 130.5, 129.9, 128.9, 128.6, 127.3, 126.5, 125.9, 125.7, 125.3, 124.6, 122.3, 120.6, 110.0, 106.9, 106.2. Maldi-MS(m/z): 791.23([M] + , calcd : 791.18). 3.2.2. X-ray crystallography The X-ray intensity data were measured on a Bruker APEX II CCD system equipped with a TRIUMPH curve-crystal monochromator and a MoKα fine-focus tube (λ = 0.71073 Å). X-ray Crystallography of 1-Cl. A clear colorless prism specimen of C 40 H 30 ClIrN 4 P, approximate dimensions 0.170 mm × 0.110 mm × 0.090 mm, was used for the X-ray crystallographic analysis. A total of 2520 frames were collected. The frames were integrated with the Bruker SAINT software package using a SAINT V8.18C algorithm. The integration of the data using a monoclinic unit cell yielded a total of 9914 reflections to a maximum θ angle of 30.56° (0.68 Å resolution), of which 9914 were independent (completeness = 99.6%, R int = 0, R sig =3.35%) and 8310(83.82%) were greater than 2σ(F 2 ). The final cell constants of a = 10.1325(5) Å, b = 14.7127(8) Å, c = 22.0098(11) Å, volume = 3244.9(3) Å 3 , are based upon the refinement of the XYZ-centroids of 9888 reflections above 20 σ(I) with 4.65° < 2θ < 61.00°. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.746. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.5314 and 0.7122. The structure was solved and refined using the Bruker SHELXTL Software Package, 61 using the space group P12 1 /c1, with Z = 4 for the formula unit, C 40 H 30 ClIrN 4 P. The final anisotropic full-matrix least-squares refinement on F 2 with 424 variables converged at R 1 = 2.83%, for the observed data and wR 2 = 5.40% for all data. The goodness-of-fit was 1.039. The largest peak in the final difference electron density synthesis was - 0.780 e - /Å 3 and the largest hole was 2.299 e - /Å 3 with an RMS deviation of -0.780 e - /Å 3 . On the basis of the final model, the calculated density was 1.691 g/cm 3 and F(000), 1632 e - . X-ray Crystallography of 1-mer. A clear light yellow cubic like specimen of C 40 H 30 IrN 4 P, approximate dimensions 0.260 mm × 0.270 mm × 0.390 mm, was used for the X-ray crystallographic analysis. A total of 2520 frames were collected. The total exposure time was 1.40 hours. The frames were integrated with the Bruker SAINT software package using a SAINT V8.18C algorithm. The integration of the data using an orthorhombic unit cell yielded a total of 75643 reflections to a maximum θ angle of 31.36° (0.68 Å resolution), of which 9746 were independent (average redundancy 7.761, completeness = 97.1%, R int = 3.11%, R sig =2.68%) and 9478(97.25%) were greater than 2σ(F 2 ). The final cell constants of a = 17.8380(8) Å, b = 54.371(3) Å, c = 12.8861(6) Å, volume = 12497.8(10) Å 3 , are based upon the refinement of the XYZ-centroids of 9013 reflections above 20 σ(I) with 4.500° < 2θ < 62.43°. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.829. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.2817 and 0.4014. The structure was solved and refined using the Bruker SHELXTL Software Package, 62 using the space group Fdd2, with Z = 16 for the formula unit, C 40 H 30 IrN 4 P. The final anisotropic full-matrix least-squares refinement on F 2 with 415 variables converged at R1 = 1.38%, for the observed data and wR2 = 2.99% for all data. The goodness-of-fit was 1.060. The largest peak in the final difference electron density synthesis was 0.497 e - /Å 3 and the largest hole was -1.102 e - /Å 3 with an RMS deviation of 0.067 e - /Å 3 . On the basis of the final model, the calculated density was 1.679 g/cm 3 and F(000), 6240 e - . X-ray Crystallography of 1-fac. A translucent intense colorless-yellow esclipse- like specimen of C 40 H 30 IrN 4 P, approximate dimensions 0.130 mm × 0.150 mm × 0.210 mm, was used for the X-ray crystallographic analysis. A total of 2520 frames were collected. The total exposure time was 7.00 hours. The frames were integrated with the Bruker SAINT software package using a SAINT V8.27B algorithm. The integration of the data using a monoclinic unit cell yielded a total of 75346 reflections to a maximum θ angle of 31.41° (0.68 Å resolution), of which 9717 were independent (average redundancy 7.754, completeness = 95.4%, R int = 3.25%, R sig =2.03%) and 8768(90.23%) were greater than 2σ(F 2 ). The final cell constants of a = 9.4204(4) Å, b = 14.0325(5) Å, c = 23.4444(9) Å, β = 96.4970(10)°, volume = 3079.3(2) Å 3 , are based upon the refinement of the XYZ-centroids of 9356 reflections above 20 σ(I) with 5.231° < 2θ < 62.27°. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.829. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.4567 and 0.6055. The structure was solved and refined using the Bruker SHELXTL Software Package, 63 using the space group P12 1 /c1, with Z = 4 for the formula unit, C 40 H 30 IrN 4 P. The final anisotropic full-matrix least-squares refinement on F 2 with 415 variables converged at R1 = 1.98%, for the observed data and wR2 = 3.96% for all data. The goodness-of-fit was 1.082. The largest peak in the final difference electron density synthesis was0.615 e - /Å 3 and the largest hole was -0.808 e - /Å 3 with an RMS deviation of 0.092 e - /Å 3 . On the basis of the final model, the calculated density was 1.704 g/cm 3 and F(000), 1560 e - . 3.2.3. Photophysical characterization The UV-Visible spectra were recorded on a Hewlett-Packard 4853 diode array spectrometer. Steady-state emission measurements were performed using a QuantaMaster model C-60SE spectrofluorimeter (Photon Technology International) with an excitation wavelength of 350 nm in dilute (10 -5 M), N 2 -degassed CH 2 CL 2 (298 K) and 2-MeTHF (77 K) in quartz cuvette. Phosphorescent lifetimes(> 50 µs) were measured with the same Spectrofluorimeter equipped with a microsecond xenon flash lamp. The lifetimes shorter than 50 µs can be measured by time-correlated single-photon counting with IBH Fluorocube instrument equipped with a 405 nm LED excitation source. Room temperature quantum yields were measured using a Hamamatsu C9920 system equipped with a xenon lamp, calibrated integrating sphere, and Model C10027 photonic multichannel analyzer. The reproducibility in the quantum efficiency measurements is ±10% in solution, and ±5% in PMMA film. 64 3.2.4. Theoretical calculations. The theoretical calculations were carried out by Schrödinger 2013 Materials Science Suite, using the density function theory (DFT) with Lee-Yang-Parr correlation functional B3LYP. The basis set used was LACVP**. Time-dependent DFT (TDDFT) calculations were performed with Gaussian03 program to understand more about the ground state to excited states transitions of the complexes. The same hybrid functional of B3LYP was employed, and the basis set was LANL2DZ/6-31G*. 3.2.5. Electrochemistry Cyclic voltammetry(CV) and differential pulse voltammetry (DPV) measurements were performed using an EG&G Potentiostat/Galvanostat model 283. Dry DMF was used as solvent under a N 2 atmosphere with 0.1 M tetra(n-butyl)ammonium hexafluorophosphate as the supporting electrolyte. A glassy carbon working electrode and a platinum counter electrode were used, with a silver wire as the pseudoreference electrodes. The oxidation potential was measured relative to a ferrocenium/ferrocene (Fc+/Fc) redox couple as an internal standard. 65 3.3. Results and discussion 3.3.1. Synthetic routes of the (ppz) 2 Ir(Cl)(C^P) complexes. Literature methods and reaction conditions were initially used to prepare the targeting cyclometalated Ir(III) complexes (ppz) 2 Ir(PPh 2 -naphthalene) (1-mer), (ppz) 2 Ir(5-PPh 2 - quinoline) (2-mer), and (ppz) 2 Ir(5-PPh 2 -isoquinoline) (3-mer). 8 The mixture of the C^P ligand, silver oxide, and [(ppz) 2 Ir(µ-Cl)] 2 was refluxed in 1,2-dichloroethane (DCE) for 18 h in dark. The products were isolated at low yield (< 10%). The major product was (ppz) 2 Ir(Cl)(C^PH) (n-Cl, n = 1-3) (> 70%) (Figure 3-1, route i). When the mixture of the C^P ligand, silver oxide, KOH, and [(ppz) 2 Ir(µ-Cl)] 2 was heated at 120 °C in 1,2-dichlorobenzene (DCB) for 18 h, in the dark, the n-mer(n = 1-3) products were formed in good yield (up to 90%), and the(ppz) 2 Ir(Cl)(P^CH) was not observed in the crude reaction products (Figure 1, Route ii). McGee and Mann reported the synthesis of Ir(III) tris-cyclometalates involving a [(ppz) 2 Ir(µ-OH)] 2 precursors. 9 The hydroxy-bridged dimers were used for the selective synthesis of mer-Ir(C^N) 3 complexes Y X P Cl Ir(zpp) 2 Cl Ir(ppz) 2 + Route i. Route ii. N N Ir Y X P 2 1-mer: X = Y = CH 2-mer: X = N, Y = CH 3-mer: X = CH, Y = N + N N Ir Y X P 2 Cl 1-Cl: X = Y = CH 2-Cl: X = N, Y = CH 3-Cl: X = CH, Y = N Figure 3-1. Traditional and modified synthetic routes. Synthetic Route i: Ag 2 O, DCE, refluxed 18 h in dark. Synthetic Route ii: Ag 2 O, KOH, DCB, heated at 120 ̊C 18 h in dark. 66 at 100 °C in DCB. In route ii, the addition of KOH in the DCB reaction solution was crucial for the high yield of the meridional products. Adding KOH to the DCE reaction solution led to low yield of the n-mer complexes similar as in Route i. No n-mer products were observed when KOH was absent in Route ii. 3.3.2. Isomerization study of the (ppz) 2 Ir(Cl)(C^P) complexes. All meridional Ir(ppz) 2 (C^P) complexes photo-isomerize to the facial isomers in coordinated solvent such as acetonitrile and DMSO (Figure 3-2). The process is known for both homoleptic complexes, such as Ir(ppz) 3 and Ir(ppy) 3 , and heteroleptic complexes with three C^N chelates. 3g, 8a, 10 The fact that only meridional complexes are obtained during the synthesis indicate that they are kinetically favored. The complexes have three possible isomers, n-mer, n-fac, and n-c(n = 1-3). The energies of the three isomers were estimated from the DFT calculations. The complex 1-fac is the most stable one and formed with thermodynamical preference during photo-isomerization. 3i, 11 By setting the total energy of the complex 1-fac to 0 eV, the energies of the complex 1-mer and 1-c are estimated to be 0.33 eV and 0.38 eV respectively. Ir N N C C C P Ir N C C C N P Ir N C C P N C Irradiation, 375 nm ~100% conversion 1-mer, 2-mer Mer, trans N-N 1-fac, 2-fac Fac, cis N-N 1-c, 2-c(traces) Mer, trans N-P Irradiation, 375 nm + decomposition Figure 3-2. Photo-isomerization process of complexes 1 and 2. 67 Irradiation of an acetonitrile solution of 1-mer(C = 3×10 -5 M) with 375 nm light leads to complete conversion to 1-fac(Figure 3-3). The absorbance spectra of the solution are plotted with cumulative irradiation times, and show clear isosbestic points. Similar photo-isomerization processes are observed for the complexes 2-mer and 3-mer. Although isosbestic points are shown in the absorbance spectra of the dilute 3-mer solution, most of the complex 3-fac decomposes during bulk irradiation. HPLC-MS shows the traces of the third photo-isomerized compound 1-c and 2-c when complexes 200 300 400 500 600 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 300 350 400 0.1 0.2 0.3 0min errcorr Wavelength (nm) 220 240 260 1.4 2.1 2.8 0min errcorr Wavelength (nm) Absorbance (a.u.) Wavelength (nm) 0min 10min 20min 30min 40min 50min 60min 70min 80min 90min 200 300 400 500 600 0.0 0.5 1.0 1.5 2.0 2.5 3.0 60s 120 s 180s 480s 900s 1500s 2400s 3600s 240 300 360 0.0 0.4 0.8 1.2 Absorbance (a.u.) Wavelength (nm) Absorbance (a.u.) Wavelength (nm) Start with 2-mer 200 300 400 500 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 300 360 420 0.0 0.2 0.4 Absorbance (a.u.) Wavelength (nm) Absorbance (a.u.) Wavelength (nm) 0min 30min 60min 2h 5h 8h 12h 15h Start with 3-mer 0 10000 20000 30000 0 1 2 3 4 5 mer to fac: 1: k = 4.6x10 -4 s -1 2: k = 5.0x10 -3 s -1 3: k = 4.4x10 -5 s -1 ln[(A 0 -A f )/(A t -A f )] Irradiation time (s) Figure 3-3. Absorbance of n-mer→n-fac under cumulative irradiation time, recorded in deaerated acetonitrile solution and the photo-isomerization rates(k) of (n-mer→n-fac)(n = 1-3). A 0 , A t , A f are the initial absorbance, absorbance at time t, and final absorbance of the photo-isomerization processes.. 68 1-fac and 2-fac are irradiated further. However, 1-c and 2-c decomposed before reaching equilibrium. The complexes n-mer(n = 1-3) show different photo-isomerization rates (Figure 3-3) 12 . The complex 2-mer shows the fastest photo-isomerization rate (k = 5.0 x 10 -3 s -1 ), while the conversion rate of 1-mer→1-fac(k = 4.6 x 10 -4 s -1 ) is intermediate, and the 3-mer→3-fac conversion is very slow (k = 4.4 x 10 -5 s -1 ). The explanation for the order of the rate constants is associated with their excited state energies and will be discussed in later sections. Thermal isomerization is not observed for any of the meridional complexes (1-3) either by high vacuum sublimation at 250 °C-300 °C or by heating in glycerol at 200 °C. 3.3.3. X-ray crystallography X-ray diffraction analysis was carried out on single crystals of the complexes 1-Cl, 1-mer, and 1-fac grown from dichloromethane/hexane bi-layer solution. The crystals of the complexes are packed in monoclinic P2 1 /c, orthorhombic Fdd2, and monoclinic P12 1 /c1 space groups respectively. The perspective drawings of the complexes are Figure 3-4. Perspective drawings of 1-Cl, 1-mer, and 1-fac shown in 50% probability thermal ellipsoids. The atoms are colored by green (Ir), black (C), blue (N), orange(P) and lime (Cl). The hydrogen atoms are omitted for clarity. 69 shown to emphasize the conformation of the C^P chelates relative to the ppz ancillary ligands (Figure 3-4, Table 3-1). All three complexes have the chelated ligand arranged in pseudo-octahedral geometry around the metal center as indicated by the bond angles along the octahedral axes of the complexes. The trans N-Ir-N angles of the complex the complexes 1-Cl(170.94(8)°) and 1-mer(169.85(5)°) are slightly smaller than the trans C-Ir-P angles of the complexes (174.80(7)°, 176.42(5)°, and 175.08(5)° respectively for 1-Cl, 1-mer, and 1-fac). The crystal structure of the complex 1-Cl is more distorted from the pseudo-octahedral geometry compared with the cyclometalated complexes 1-mer and 1-fac. The bond angle of trans N-Ir-Cl(163.64(7)°) is about 10 degrees more compressed than the trans C-Ir-C (C^P) angle of 1-mer(172.59(6)°), and the trans N-Ir-C (C^P) angle of 1-fac (174.88(6)°). The naphthalyl fragment is flipped away from the Ir(III) center in the non- Table 3-1. Selected bond angles (degree) and distances (Å) for complexes 1-Cl, 1-mer, 1-fac. 1-Cl 1-mer 1-fac Bond Angles (Degree) trans N-Ir-N 170.94(8) 169.85(5) ---- trans C-Ir-P 174.80(7) 176.42(5) 175.08(5) trans C-Ir-Cl 163.64(7) ---- ---- trans C-Ir-C (C^P) ---- 172.59(6) ---- trans N-Ir-C (C^P) ---- ---- 174.88(6)̊ Bond Distances (Å) Ir1-P1 2.4164(6) 2.3109(4) 2.3011(5) Ir1-N1 ---- 2.0153(2) ---- Ir1-N3 2.032(2) 2.0447(2) ---- Ir1-N2 2.016(2) ---- 2.1078(2) Ir1-N4 ---- ---- 2.1202(2) Ir1-C15 ---- 2.111(2) 2.0551(2) Ir1-C27 ---- 2.090(2) ---- Ir1-C23 ---- ---- 2.0354(2) 70 cyclometalated complex. The Ir-P-C31 angle of the complex 1-Cl(112.48(8)°) increases compared to the Ir-P-C13 angles of the complexes 1-mer(103.79(6)°) and 1-fac (103.34(6)°). The Ir-P bond length of the cyclometalated complexes 1-mer(2.3109(4) Å) and 1-fac (2.3011(5) Å) are shorter compared to that of the complex 1-Cl(2.4164(6) Å) with more flexible C^P ligand. The Ir-N bond lengths in the meridional isomer 1-mer(Ir1-N1 = Figure 3-5. 1 H NMR for complex 1-Cl(top), 1-mer(middle), and 1-fac(bottom). The solvent peak is marked with “x”. See SI for the corresponding gCOSY NMRs. 70 cyclometalated complex. The Ir-P-C31 angle of the complex 1-Cl(112.48(8)°) increases compared to the Ir-P-C13 angles of the complexes 1-mer(103.79(6)°) and 1-fac (103.34(6)°). The Ir-P bond length of the cyclometalated complexes 1-mer(2.3109(4) Å) and 1-fac (2.3011(5) Å) are shorter compared to that of the complex 1-Cl(2.4164(6) Å) with more flexible C^P ligand. The Ir-N bond lengths in the meridional isomer 1-mer(Ir1-N1 = Figure 3-5. 1 H NMR for complex 1-Cl(top), 1-mer(middle), and 1-fac(bottom). The solvent peak is marked with “x”. See SI for the corresponding gCOSY NMRs. 70 cyclometalated complex. The Ir-P-C31 angle of the complex 1-Cl(112.48(8)°) increases compared to the Ir-P-C13 angles of the complexes 1-mer(103.79(6)°) and 1-fac (103.34(6)°). The Ir-P bond length of the cyclometalated complexes 1-mer(2.3109(4) Å) and 1-fac (2.3011(5) Å) are shorter compared to that of the complex 1-Cl(2.4164(6) Å) with more flexible C^P ligand. The Ir-N bond lengths in the meridional isomer 1-mer(Ir1-N1 = Figure 3-5. 1 H NMR for complex 1-Cl(top), 1-mer(middle), and 1-fac(bottom). The solvent peak is marked with “x”. See SI for the corresponding gCOSY NMRs. 71 2.0153(2) Å, Ir1-N3 = 2.0447(2) Å) are similar as those in the non-cyclometalated complex 1-Cl(Ir1-N2 = 2.016(2) Å, Ir1-N3 = 2.032(2) Å); but shorter than those in the facial complex 1-fac(Ir1-N2 = 2.1078(2) Å, Ir1-N4 = 2.1202(2) Å). Due to the trans- phenyl effect, Ir-C bond lengths of the complex 1-mer(Ir1-C15 = 2.111(2) Å, Ir1-C27 = 2.090(2) Å) are longer than those of the complex 1-fac(Ir1-C15 = 2.0551(2) Å, Ir1-C23 = 2.0354(2) Å). 8a 3.3.4. NMR characterization Figure 3-6. gCOSY (500 MHz, CDCl 3 ) spectrum of complex 1-Cl. 71 2.0153(2) Å, Ir1-N3 = 2.0447(2) Å) are similar as those in the non-cyclometalated complex 1-Cl(Ir1-N2 = 2.016(2) Å, Ir1-N3 = 2.032(2) Å); but shorter than those in the facial complex 1-fac(Ir1-N2 = 2.1078(2) Å, Ir1-N4 = 2.1202(2) Å). Due to the trans- phenyl effect, Ir-C bond lengths of the complex 1-mer(Ir1-C15 = 2.111(2) Å, Ir1-C27 = 2.090(2) Å) are longer than those of the complex 1-fac(Ir1-C15 = 2.0551(2) Å, Ir1-C23 = 2.0354(2) Å). 8a 3.3.4. NMR characterization Figure 3-6. gCOSY (500 MHz, CDCl 3 ) spectrum of complex 1-Cl. 71 2.0153(2) Å, Ir1-N3 = 2.0447(2) Å) are similar as those in the non-cyclometalated complex 1-Cl(Ir1-N2 = 2.016(2) Å, Ir1-N3 = 2.032(2) Å); but shorter than those in the facial complex 1-fac(Ir1-N2 = 2.1078(2) Å, Ir1-N4 = 2.1202(2) Å). Due to the trans- phenyl effect, Ir-C bond lengths of the complex 1-mer(Ir1-C15 = 2.111(2) Å, Ir1-C27 = 2.090(2) Å) are longer than those of the complex 1-fac(Ir1-C15 = 2.0551(2) Å, Ir1-C23 = 2.0354(2) Å). 8a 3.3.4. NMR characterization Figure 3-6. gCOSY (500 MHz, CDCl 3 ) spectrum of complex 1-Cl. 72 Figure 3-7. gCOSY (500 MHz, CDCl 3 ) spectrum of complex 1-mer. Figure 3-8. gCOSY (500 MHz, CDCl 3 ) spectrum of complex 1-fac. 72 Figure 3-7. gCOSY (500 MHz, CDCl 3 ) spectrum of complex 1-mer. Figure 3-8. gCOSY (500 MHz, CDCl 3 ) spectrum of complex 1-fac. 72 Figure 3-7. gCOSY (500 MHz, CDCl 3 ) spectrum of complex 1-mer. Figure 3-8. gCOSY (500 MHz, CDCl 3 ) spectrum of complex 1-fac. 73 The complexes 1-3 show singlets in 31 P NMRs, and are slightly shifted between different isomers. The chemical shifts of the free C^P ligands are at δH -15 ppm ( 31 P NMR). 6b The chemical shift of the complex 1-Cl is shifted downfield compared with the free PPh 2 -naphthalene ligand(δ= -7.9 ppm). Chemical shifts of the cyclometalated complexes n-mer and n-fac(n = 1-3) are shifted further downfield(δH 15 ppm). The relatively downfielded chemical shifts of the cyclometalated complexes compared to the non-cyclometalated 1-Cl, and the free ligands are related with the increased Ir-phosphine electron backdonation. The chemical shifts difference between meridional and facial isomers is less than 1 ppm, but useful for monitoring the mer→fac conversion during photo-isomerization processes. The 1 H spectra for these C1 symmetry complexes of the meridional and facial isomers can be assigned via the correlations between X-ray structures, gCOSY NMRs and the 1 H NMRs, especially of the six protons from two pyrazole rings(Figure 3-5-8). The signals of the six protons from two pyrazole rings of complexes 1 are all separately located at different chemical shifts due to the environment difference brought by the C^P ligands. The same correlation differences are found consistently in meridional and facial isomers of complexes 2 and 3. 74 3.3.5. Electrochemistry The electrochemical properties of all phosphinoaryl ligand and iridium complexes were examined by cyclic voltammetry(CV) and differential pulse voltammetry (DPV) in DMF solution, and are reported relative to an internal ferrocene reference (Fc + /Fc)(Table -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 -120.0μ -100.0μ -80.0μ -60.0μ -40.0μ -20.0μ 0.0 20.0μ Red 1 Ox 1 Fc + /Fc Current (A) Potential (V) PPh 2 -naphthalene, Ox PPh 2 -naphthalene, Red -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 -60.0μ -50.0μ -40.0μ -30.0μ -20.0μ -10.0μ 0.0 10.0μ 20.0μ 30.0μ Red 1 Ox 1 Fc + /Fc Current (A) Potential(V vs. Fc + /Fc) PPh 2 -naphthalene E ox1 = 0.72 V E red1 = -2.46 V -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 -120.0µ -100.0µ -80.0µ -60.0µ -40.0µ -20.0µ 0.0 20.0µ Red 2 Red 1 Ox 1 Fc + /Fc Current (A) Potential (V) 5-PPh 2 -quinoline Ox 5-PPh 2 -quinoline Red -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 -25.0μ -20.0μ -15.0μ -10.0μ -5.0μ 0.0 5.0μ 10.0μ 15.0μ 20.0μ 25.0μ Red 2 E ox1 = 0.76 V E red1 = -2.28 V E red2 = -2.43 V Red 1 Ox 1 Fc + /Fc Current (A) Potential(V vs. Fc + /Fc) 5-PPh 2 -quinoline -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 -120.0µ -100.0µ -80.0µ -60.0µ -40.0µ -20.0µ 0.0 20.0µ Red 1 Ox 1 Fc + /Fc Current (A) Potential (V) 5-PPh 2 -isoquinoline Ox 5-PPh 2 -isoquinoline Red -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 -30.0μ -25.0μ -20.0μ -15.0μ -10.0μ -5.0μ 0.0 5.0μ 10.0μ 15.0μ 20.0μ 25.0μ E ox1 = 0.72 V E red1 = -2.46 V Red 1 Ox 1 Fc + /Fc Current (A) Potential(V vs. Fc + /Fc) 5-PPh 2 -isoquinoline Figure 3-9. CV (at 0.1 V/s) and DPV (at 0.01 V/s) of PPh 2 -naphthalene, 5-PPh 2 - quinoline, and 5-PPh 2 -isoquinoline(in DMF, 0.1 M [Bu 4 N][PF 6 ]). 75 3-2 and Figure 3-9-12). The redox potentials are determined via DPV. 13 A first partially reversible or irreversible oxidation process is observed for all complexes(Ox 1 from 0.38 to 0.68 V). For some of the complexes, a second oxidation process is also detected. Complex 3-mer shows three irreversible oxidation processes. In general, the facial complexes are more difficult to oxidize than the meridional complexes. The oxidation potentials of 1-fac and 3-fac are 40 mV more anodic than their meridional isomers. The oxidation potential shift between 2-fac and 2-mer is even more pronounced(160 mV). The quinoline markedly affects the oxidation potentials of the -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 -30.0µ -25.0µ -20.0µ -15.0µ -10.0µ -5.0µ 0.0 5.0µ 10.0µ 15.0µ 20.0µ Red 1 Fc + /Fc Ox 1 Current (A) Potential(V) 1-mer -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 -25.0µ -20.0µ -15.0µ -10.0µ -5.0µ 0.0 5.0µ 10.0µ 15.0µ 20.0µ 25.0µ Red 1 Ox 1 Fc + /Fc Current (A) Potential(Vvs. Fc + /Fc) 1-mer -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 -30.0µ -25.0µ -20.0µ -15.0µ -10.0µ -5.0µ 0.0 5.0µ 10.0µ 15.0µ Red 1 Ox 1 Ox 2 Fc + /Fc Current (A) Potential(V) 1-fac -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 -15.0µ -10.0µ -5.0µ 0.0 5.0µ 10.0µ 15.0µ 20.0µ Red 1 Ox 1 Fc + /Fc Current (A) Potential(Vvs. Fc + /Fc) 1-fac Figure 3-10. CV (at 0.1 V/s) and DPV (at 0.01 V/s) of complexes 1-mer and 1-fac(in DMF, 0.1 M [Bu 4 N][PF 6 ]). 76 complexes 2-mer and 2-fac, due to the strong electron withdrawing property of the para-nitrogen. The oxidation potential shifts by 140 mV of the meridional isomer, and 260 mV of the facial isomer, relative to complexes 1. The meta-nitrogen of isoquinoline shows less electron withdrawing property than the para-nitrogen of quinoline. The oxidation potential of the complexes 3-mer and 3-fac shifts by 60 mV of both meridional and facial isomers, relative to complexes 1. All complexes show a reversible first reduction process (Red 1 from -2.88 to -2.60 V). A second irreversible reduction process (Red 2 from -3.18 to -2.84 V) is observed for -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 -40.0µ -30.0µ -20.0µ -10.0µ 0.0 10.0µ 20.0µ 30.0µ 40.0µ Ox 2 Ox 1 Red 2 Red 1 Fc + /Fc Current (A) Potential (V) 2-mer -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 -50.0µ -40.0µ -30.0µ -20.0µ -10.0µ 0.0 10.0µ 20.0µ 30.0µ Red 2 Red 1 Ox 2 Ox 1 Fc + /Fc Current (A) Potential(Vvs. Fc + /Fc) 2-mer -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 -30.0µ -25.0µ -20.0µ -15.0µ -10.0µ -5.0µ 0.0 5.0µ 10.0µ Red 2 Red 1 Ox 1 Fc + /Fc Current (A) Potential(Vvs. Fc + /Fc) 2-fac -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 -25.0µ -20.0µ -15.0µ -10.0µ -5.0µ 0.0 5.0µ 10.0µ 15.0µ Red 2 Red 1 Ox 1 Fc + /Fc Current (A) Potential(Vvs. Fc + /Fc) 2-fac Figure 3-11. CV (at 0.1 V/s) and DPV (at 0.01 V/s) of complexes 2-mer and 2-fac(in DMF, 0.1 M [Bu 4 N][PF 6 ]). 77 complexes 2 and 3. The facial isomer complexes are easier to be reduced by only 20 mV compared to the meridional isomers. Both the quinoline and isoquinoline based complexes 2 and 3 show markedly lower reduction potentials than naphthyl analogs, complexes 1. The ppz based reduction of the Ir(III) complexes are often beyond the measurement range of DMF solution (< -3.5 V vs. Fc + /Fc), as is seen for complexes 1. 3i And the red 1 is assigned predominantly contributed by the naphthalene, quinoline, and isoquinoline moieties. The Red 2 processes in complexes 2 and 3 are related with either the ppz or the C^P ligand, and can be observed in DMF solution due to the electron withdrawing effect of the quinoline and isoquinoline. -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 -35.0µ -30.0µ -25.0µ -20.0µ -15.0µ -10.0µ -5.0µ 0.0 5.0µ 10.0µ 15.0µ 20.0µ Ox 3 Ox 2 Ox 1 Red 2 Red 1 Fc + /Fc Current (A) Potential (V) 3-mer -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 -20.0µ -10.0µ 0.0 10.0µ 20.0µ Red 2 Red 1 Ox 2 Ox 1 Fc + /Fc Current (A) Potential(Vvs. Fc + /Fc) 3-mer -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 -80.0µ -60.0µ -40.0µ -20.0µ 0.0 20.0µ Red 2 Red 1 Ox 2 Ox 1 Fc + /Fc Current (A) Potential (V) 3-fac -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 -40.0µ -30.0µ -20.0µ -10.0µ 0.0 10.0µ 20.0µ 30.0µ Red 2 Ox 1 Ox 2 Red 1 Fc + /Fc Current (A) Potential(Vvs. Fc + /Fc) 3-fac Figure 3-12. CV (at 0.1 V/s) and DPV (at 0.01 V/s) of complexes 3-mer and 3-fac(in DMF, 0.1 M [Bu 4 N][PF 6 ]). 78 In total, the complexes 3 show much narrower redox gap (ΔE (mer) = 3.04 V and ΔE (fac) = 3.10 V vs. Fc + /Fc) compared to the complexes 1(ΔE (mer) = 3.26 V and ΔE (fac) =3.28 V vs. Fc + /Fc) and complexes 2(ΔE (mer) = 3.20 V and ΔE (fac) = 3.34 V vs. Fc + /Fc). And the correlations between the electrochemical and the photophysical studies are discussed in the following section. 3.3.6. The electronic spectroscopy of complexes 1-3 The PPh 2 -naphthalene ligand shows absorption bands in the ultraviolet (UV) region (λ max = 280 nm), and rather weak absorption in the near-UV region (from 350 nm to 400 nm) (Figure 3-13). The absorption of the 5-PPh 2 -quinoline (λ max = 319 nm) and 5-PPh 2 -isoquinoline ligand (λ max = 325 nm) are red-shifted compared to the PPh 2 -naphthalene ligand. The absorptions of both ligands are weak in the near-UV region. All the Ir(III) complexes show intense absorbance bands in the UV region of the spectra (around 300 nm, typical extinction coefficient ε at 10 4 M -1 cm -1 ), assigned to the ligand contributed (π→π*) transitions (Figure 3-14-15, Table 3-2). Broad absorption bands originated from the MLCT states are observed in the near-UV to visible region (from 300 nm to 450 nm) in the spectra of the meridional complexes. Broader and stronger absorption bands are observed for the facial complexes in the same region of the spectra especially from 350 nm to 450 nm, assigned to more efficient MLCT transitions compared to the meridional analogs. 79 Table 3-2. Photophysical properties of complexes 1-3. Redox c E red1 V ---- -2.88 -2.86 -2.68 -2.66 -2.60 -2.62 E ox1 V ---- 0.38 0.42 0.52 0.68 0.44 0.48 PL (PMMA, 5% doped) k nr 10 3 s -1 ---- 1.6 2.1 2.6 1.9 4.3 16.6 k r 10 3 s -1 ---- 0.7 1.5 0.6 2.7 1.3 4.7 Q.Y . % ---- 29 41 17 59 23 22 Emission 298 K a λ max (nm), [τ(µs)] ---- 568, [431] 580, [277] 522, [317] 524, [218] 576, [180] 596, [47] PL (Solution) k nr 10 3 s -1 ---- ---- 6.7 ---- 9.7 12.1 36.0 k r 10 3 s -1 ---- ---- 1.7 ---- 3.1 1.1 4.0 Q.Y. % ---- < 5 20 < 5 24 8 10 Emission 298 K a λ max (nm), [τ(µs)] 538 570, [171] 584, [120] 518, [153] 526, [79] 602, [76] 596, [25] Emission 77 K b λ 0-0 (nm), [τ(µs)] 488, [12300] 518, [660] 530, [350] 476, [580] 484, [380] 530, [232] 556, [144] Absorption a λ max (nm)(ε, 10 3 M -1 cm -1 ) 290 (17.0) 315 (11.5) 309 (10.5), 352 (sh, 4.1) 302 (sh, 14.4) 302 (12.9), 346 (sh, 5.9) 302 (sh, 10.2) 310 (9.4), 392 (2.6) 1-Cl 1-mer 1-fac 2-mer 2-fac 3-mer 3-fac a Absorption and 298 K PL (solution) were carried out in CH 2 Cl 2 . b The 77 K PL (solution) were recorded in 2-MeTHF glass. c Redox potentials were recorded in DMF solution and reported relative to internal Fc + /Fc reference. 80 The phosphorescence of the C^P ligands are observed in 2-methyltetrahydrofuran (2-MeTHF) glass at 77 K. The emission band of the PPh 2 -naphthalene ligand is clearly structured, while the emission bands of the quinoline and isoquinoline based C^P ligands are much less structured (Figure 3-13). The three C^P ligands give the same E 0-0 (λ 0-0 = 380 nm), indicating that the triplet energy levels of the ligands are not affected by the electron withdrawing effect of the nitrogen atoms in quinoline and isoquinoline. The phosphorescence for the complex 1-Cl and all of the cyclometalated complexes n-mer and n-fac(n = 1-3) are broad with rough vibronic features in fluid solution (298 K). The emission bands of complexes 3 are of the most broad and featureless structure. The low temperature (77 K) phosphorescence of all complexes are more structured than the emissions recorded at 298 K, and show typical features consistent with the C^P ligand phosphorescence. The complex 1-Cl(2.54 eV) has the highest triplet energy, followed by 1-mer(2.39 eV) and 1-fac(2.34 eV). The emissions of the complexes 2 are blue-shifted, while those of the complexes 3 are red-shifted compared 300 400 500 0.0 0.5 1.0 1.5 2.0 Absorbance (a.u.) Wavelength (nm) PPh 2 -naphthalene 5-PPh 2 -quinoline 5-PPh 2 -isoquinoline 400 500 600 700 800 0.0 0.2 0.4 0.6 0.8 1.0 Normalized Emissions Wavelength (nm) PPh 2 -naphthalene 5-PPh 2 -quinoline 5-PPh 2 -isoquinoline Figure 3-13. Left: Absorbance spectra (recorded in CH 2 Cl 2 ); Right: Emission spectra (recorded in 2-MeTHFglass at 77 K) of ligand PPh 2 -naphthalene, 5-PPh 2 -quinoline, and 5-PPh 2 -isoquinoline. 81 with the analog of complexes 1. All facial complexes show lower triplet energies compared to their meridional analogs. At room temperature (298 K), the emissions of the complexes 1-fac and 2-fac are red-shifted compared to the complexes 1-mer and 2-mer. The emissions of the 3-mer and 3-fac show less difference. The emission bands of the cyclometalated complexes in 5 wt% doped PMMA films are broader, but with similar λ max as in solution. 300 400 500 600 700 800 0 5000 10000 15000 20000 abs. PL r.t. PL 77K Wavelength (nm) (M -1 cm -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Normalized emissions 1-Cl 300 400 500 600 700 800 5000 10000 15000 20000 abs. PL r.t. PL 77K Wavelength (nm) (M -1 cm -1 ) 1-mer 0.0 0.2 0.4 0.6 0.8 1.0 Normalized Emissions 300 400 500 600 700 800 0 5000 10000 15000 20000 abs. PL r.t. PL 77K Wavelength (nm) (M -1 cm -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Normalized emissions Figure 3-14. Absorption and emission Spectra of 1-Cl(top, left), 1-mer(top, right) and 1-fac(bottom) recorded in CH 2 Cl 2 (r.t.) and 2-MeTHF (77 K). 82 3.3.7. Lifetime and quantum yield studies The room temperature quantum yield and lifetime of complex 1-Cl cannot be accurately determined due to its rather weak luminescence in solution. The phosphorescence quantum yields of the meridional complexes 1-mer and 2-mer are low (Ф < 0.05) at room temperature, at least partially due to the photo-isomerization to their facial isomers in the fluorimeter. The photo-isomerization rate of 3-mer → 3-fac is much slower compared to the complexes 1-mer and 2-mer, and thus the quantum yield of the complex 3-mer(Ф = 0.08) is likely closer to the luminance yield of the meridional 300 400 500 600 700 800 5000 10000 15000 20000 abs. PL 77K PL r.t. Wavelength (nm) (M -1 cm -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Normalized Emissions 2-mer 300 400 500 600 700 800 5000 10000 15000 20000 abs. PL r.t. PL 77K Wavelength (nm) (M -1 cm -1 ) 2-fac 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Normalized emissions 300 400 500 600 700 800 0 5000 10000 15000 20000 abs. PL r.t. PL 77K Wavelength (nm) (M -1 cm -1 ) 3-mer 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Normalized emissions 300 400 500 600 700 800 0 5000 10000 15000 20000 abs. PL r.t. PL 77 Wavelength (nm) (M -1 cm -1 ) 3-fac 0.2 0.4 0.6 0.8 1.0 Normalized Emissions Figure 3-15. Absorption and emission spectra of complexes 2 and 3 recorded in CH 2 Cl 2 (r.t.) and 2-MeTHF (77 K). 83 isomer. The quantum yields of the facial complexes 1-fac and 2-fac(Ф = 0.20 and 0.24 respectively) increase significantly compared with the complexes 1-mer and 2-mer. In contrast, the quantum yield of the complex 3-fac(Ф = 0.10) is comparable with that of the complex 3-mer, due to the photo-instability of the complex. Photochemical induced substitution reaction can occur at carbon C1 in the isoquinoline fragment, leading to high non-radiative rates. 14 The phosphorescence lifetimes of the free C^P ligand are at levels of seconds. The lifetime of the non-cyclometalated complex 1-Cl(τ = 12.3 ms at 77 K) is significantly decreased due to the efficient inter-system crossing via strong spin-orbit coupling of the Ir(III) center. The lifetimes of the cyclometalated complexes are further decreased to microsecond levels. All facial complexes show even shorter lifetimes than the meridional complexes at both room temperature and 77 K. The complexes 2-fac and 3-fac shows similar but larger radiative rates (k r ) compared to the complex 1-fac. It is worth noticing that the radiative rates of all the cyclometalated Ir(III)-C^P complexes are much lower than many of the cyclometalated Ir(III)-C^N complexes, but comparable with other cyclometalated complexes containing C^P chelates. 2a, 2b, 5a, 8a The non-radiative rates (k nr ) of complexes 1-fac and 2-fac are comparable, while the k nr of 3-fac is significantly larger, indicating the stability problem. The quantum yields of all meridional complexes are significantly higher in doped PMMA films due to the elimination of the non-radiative decays from photo-isomerization processes. The quantum yields of the 1-fac and 2-fac also increase, with markedly suppressed non-radiative decay in the rigid PMMA matrix. However, very little 84 difference is observed between the quantum yields of the complexes 3-mer and 3-fac. Although complex 3-fac show higher k r among all complexes, the k nr of complex 3-fac is also the highest in the PMMA film. The phosphorescence mechanism for the Ir(III)-C^P complexes are further studied with the quantum theory calculations below. 3.3.8. DFT calculations Calculations using density functional theory (DFT) were performed on all complexes (Figure 3-16). For complexes 1, the bond lengths predicted by the gas-phase DFT calculations are slightly longer than those of the crystal structures. The time-dependent DFT (TDDFT) calculations were used to identify the S 0 →S n and S 0 →T n transition energies and oscillator strength (f) of the complexes (Table 3-3). The calculated S 0 →S 1 transitions of complex 1-Cl(λ = 393 nm, f = 0.0047), 1-mer(λ = 366 nm, f = 0.0144), and 1-fac(λ = 380 nm, f = 0.0143) are close to the onsets in the absorption spectra, indicating the successful prediction of the lowest Frank-Condon excited state based on the ground state geometry of the complexes. The calculations of the complexes 2 and 3 are also consistent with the experimental absorption data. The DFT calculated triplet spin density of all cyclometalated complexes are located predominantly on the C^P ligands and the Ir(III) metal center (Table 3-4), in consistent with the photoluminescence studies. The S 0 →S 1 transition of the non-cyclometalated complex 1-Cl involves HOMO-1→LUMO (26%) and HOMO-1→LUMO+1 (74%). The HOMO(-5.11 eV) of the complex 1-Cl predominately delocalizes on the Ir(III) metal center (40%), the ppz ligand(L)(25%), and the chloride (27%). The HOMO-1 (-5.35 eV) is occupied by similar contributions from the Ir(III) metal center (39%), the ppz ligand (20%), and the 85 chloride (35%). The LUMO (-1.27 eV) of the complex 1-Cl is localized on the ppz ligand (35%) and the PPh 2 -naphthalene ligand (L’) (62%). There is very little contribution from the metal center (3%) and the chloride (1%). The LUMO+1(-1.17 eV) is degenerated from the LUMO, and predominantly localized on the ligands of PPh 2 - naphthalene (46%) and ppz (52%) with little contribution from the metal center (2%). Thus the S 0 →S 1 transition of the complex 1-Cl is assigned to the metal-ligand (ML) to ligand (L’) charge transfer (ML-L’CT). The S 0 →T 1 transition of complex 1-Cl involves frontier orbital of HOMO-3→LUMO (23%), HOMO-3→LUMO+1 (13%) and HOMO- 2→LUMO+1 (14%). The HOMO-3 is predominantly delocalized on the ligands of ppz (48%) and PPh 2 -naphthalene (41%) with little contribution from the Ir(III) (4%). The ligand contributions from ppz (40%) and PPh 2 -naphthalene (28%) are predominant along with significant contribution from the chloride (22%). The S 0 →T 1 transition is assigned to inter-ligand charge transfer (ILCT). The charge transfer assignments are performed for all the cyclometalated complexes following the above methods. Both the meridional and the facial isomer of the complexes 1-3 show significant MLCT character in the S 0 →S 1 and the S 0 →T 1 transitions. Significant contribution is also observed from Ir(III)-C^P to the C^P ligand in the S 0 →S 1 and S 0 →T 1 transitions for the complexes. The photophysical properties of the cyclometalated complexes show significant difference as the naphthalene fragment of the C^P ligands is replaced with quinoline or isoquinoline fragments. The radiative rates of the cyclometalated complexes 2-fac and 3-fac are larger compared to 1-fac. If the T 1 is only considered as originating from the mixing of 3 MLCT and 3 LC, the radiative rates are expected to be proportional to the 86 contribution from the 3 MLCT state. Since the 3 LC states of all complexes are the same, the complex 2-fac with the highest 3 MLCT transition state should lead to the slowest radiative rate. 15 However, it has been shown that the radiative rates of the iridium complexes are related with several factors, and can be expressed as in proportion to: 16 ( | | ) ×(1) Where E is the lowest excited state energy, <S|H SO |T> is the spin-orbit coupling Hamiltonian between S 1 and T 1 states, f s is the oscillator strength of the S 0 S 1 transition. The E T of the complex 2-fac is the highest among the three facial isomer complexes, and the lowest of the complex 3-fac. The E S is determined by the redox potentials from electrochemistry, with the trend of E S (2-fac) > E S (1-fac) > E S (3-fac). The (E S - E T ) is determined as E S-T (1-fac) > E S-T (3-fac) > E S-T (2-fac). The f s of complex 3-fac is significantly higher than that of complex 1-fac and 2-fac. In the sum of all factors, the radiative rates of 2-fac and 3-fac are comparable, and larger than that of 1-fac. The complexes n-mer(n = 1-3) show clean photo-isomerization conversion to facial isomers in dilute solution with significantly different conversion rates. The T 1 →S 0 energies of the meridional complexes follow the trend of 2-mer > 1-mer > 3-mer (2.95 eV, 2.94 eV, and 2.74 eV respectively), which mirrors the photo-isomerization rate sequence. The complex 2-mer with the highest T 1 →S 0 energy shows the fasted conversion rate. Similar as Ir-(C^N) 3 systems, the photo-isomerization process is suggested related with a bond rupture process occurred in the lowest triplet excited state of the complexes, and likely via Ir-N or Ir-P bonds, rather than the Ir-C bonds. 8a The 87 meridional isomers overcome the energy barriers of the non-cyclometalated transition state species, and form the facial complexes. Thus, the process is kinetically more accessible with complexes of higher T 1 energies. Table 3-3. S 0 S 1 and S 0 →T 1 transitions, wavelength (λ cal ), oscillator strength (f), transition contributions (>10%), and assignments of complexes n(n = 1-3) from TD-DFT calculations. States λ cal (nm) f major contribution assignment 1-Cl T 1 476 0 HOMO-3→LUMO(23%) HOMO-3→LUMO+1 (13%) HOMO-2→LUMO (14%) ILCT S 1 393 0.0047 HOMO-1→LUMO (26%) HOMO-1→LUMO+1 (74%) ML-L’CT 1-mer T 1 499 0 HOMO→LUMO (62%) HOMO→LUMO+3 (11%) MLCT S 1 366 0.0144 HOMO→LUMO (49%) HOMO→LUMO+3 (51%) MLCT 1-fac T 1 507 0 HOMO→LUMO (37%) HOMO→LUMO+1 (11%) HOMO→LUMO+3 (31%) MLCT S 1 380 0.0143 HOMO→LUMO (74%) HOMO→LUMO+3 (26%) MLCT 2-mer T 1 465 0 HOMO-1→LUMO (53%) HOMO-2→LUMO (11%) HOMO-6→LUMO (31%) MLCT S 1 373 0.0006 HOMO→LUMO (95%) MLCT 2-fac T 1 471 0 HOMO→LUMO (50%) HOMO-5→LUMO(13%) MLCT S 1 355 0.0134 HOMO→LUMO (51%) HOMO-5→LUMO+1 (37%) MLCT 3-mer T 1 501 0 HOMO-1→LUMO (68%) MLCT S 1 402 0.0009 HOMO-1→LUMO (98%) MLCT 3-fac T 1 511 0 HOMO→LUMO (70%) MLCT S 1 382 0.0365 HOMO→LUMO (75%) HOMO-1→LUMO (25%) MLCT 88 Table 3-4. Metal and ligand contribution (%) to the major frontier orbital involved in S 0 →S 1 and S 0 →T 1 transition for the cyclometalated complexes 1-3. The ligands are defined to ppz, naphthalene/(iso)quinoline (C^P c ), and PPh 2 linkage (C^P p ). 1-mer 2-mer 3-mer Ir ppz C^P c C^P p Ir ppz C^P c C^P p Ir ppz C^P c C^P p LUMO+3 5 11 17 67 LUMO 2 3 50 45 1 2 64 33 1 1 67 31 HOMO 31 11 54 4 36 49 14 1 31 45 23 1 HOMO-1 39 19 39 3 39 15 43 3 HOMO-2 5 40 49 6 HOMO-6 15 22 59 4 1-fac 2-fac 3-fac LUM+3 2 3 33 62 LUMO+1 1 69 9 21 2 79 9 10 LUMO 3 18 27 52 2 9 50 39 1 2 62 35 HOMO 32 9 55 4 41 16 39 4 38 13 45 4 HOMO-1 36 36 26 2 HOMO-5 25 22 42 11 89 1-Cl 1-mer 1-fac 2-mer 2-fac 3-mer 3-fac Frontier Orbital LUMO+3 -0.76 eV -0.81 eV LUMO+1 -1.67 eV -0.93 eV -1.07 eV LUMO -1.27 eV -1.04 eV -1.01 eV -1.24 eV -1.18 eV -1.36 eV -1.28 eV HOMO -5.11 eV -5.02 eV -4.88 eV -5.23 eV -5.32 eV -5.11 eV -5.14 eV HOMO-1 -5.35 eV -5.43 eV -5.28 eV -5.25 eV HOMO-2 -5.64 eV -5.74 eV HOMO-3 -5.82 eV HOMO-5 -6.26 eV HOMO-6 -6.28 eV Triplet Spin Density(ΔSCF, eV)(2.94)(2.91)(2.95)(2.96)(2.74)(2.71) Figure 3-16. Selected frontier orbitals and triplet spin surface of complexes 1-3 from DFT calculations. 90 3.4. Conclusion The synthetic reactivity of the phosphinoaryl chelated Ir(III) complexes are different compared to C^N chelated Ir(III) complexes. The C-H bond activation at the aryl fragment is less favored, and thus higher reaction temperature, more basic reaction condition, and in situ formed Ir-OH bridged metal precursor instead of Ir-Cl precursor is required. The meridional complexes are exclusively formed via the modified synthetic route, and the facial complexes are obtained from photo-isomerization. The photophysical properties of the complexes 1-3 are significantly different as controlled by the aryl fragments of naphthalene, quinoline, and isoquinoline, due to the different electron withdrawing properties of the nitrogen atom. The emission energies of the complexes are tuned within green to yellow range even though the triplet energies of the free C^P ligands are the same. 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Y., Dalton T 2009,(8), 1267; (c) Zanoni, K. P. S.; Kariyazaki, B. K.; Ito, A.; Brennaman, M. K.; Meyer, T. J.; Iha, N. Y. M., Inorg Chem 2014, 53(8), 4089. 16.(a) Obara, S.; Itabashi, M.; Okuda, F.; Tamaki, S.; Tanabe, Y.; Ishii, Y.; Nozaki, K.; Haga, M., Inorg Chem 2006, 45(22), 8907; (b) Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer, T., Coordin Chem Rev 2011, 255(21-22), 2622; (c) Glenna So Ming Tong, P. K. C., Wai-Pong To, Wai-Ming Kwok, Chi-Ming Che, Chemistry, A European Journal 2014, 20, 6433. 93 Chapter 4: Aluminum and zinc based host materials and their potential applications in OLEDs 4.1. Introduction The doping technique has been globally accepted to optimize the efficiencies of phosphorescence OLEDs since the self-quenching of the emitter molecules can be prevented by this fabrication strategy. 1 Several thin film layers are employed in the typical doped OLEDs, including: hole transport layer (HTL), emissive layer (EML), and electron transport layer (ETL). The HTL and ETL are designed in preferable energetic levels to facilitate the injection and transportation of the holes and electrons. Ideally, the holes and electrons meet in the EML followed by recombination and emission of light. The EML is commonly composed of the host materials and the dopant materials. Design and selection of the host materials is crucial for the OLEDs because the energy levels and charge mobility of the host directly determines whether the holes and electrons can recombine at the EML, and whether the emission can be confined exclusively on the dopant. 2 There are several types of host materials classified based on the charge carrier transporting feature of the materials: 1) the hole-transport-type (HT-type), 2) electron-transport-type (ET-type) and 3) bipolar transport hosts. 3 Ever since Tang and Van Slyke showed the first successful OLED composed of a diamine HTL and Alq 3 ETL bi-layer organic structure in 1987, Alq 3 have been adopted as one of the most commonly used materials in OLEDs(Scheme 4-1). 4 Viewing the 94 molecular structure of Alq 3 , three quinolate ligands arranged around the Al(III) metal center showing qseudo-octahedral geometry. The facial and meridional isomers of the Alq 3 most likely co-exist in the thin film devices. 5 Many quinolated metal complexes have been studied, with a variety of metal centers such as Rhq 3 , Irq 3 , Biq 3 , Pbq 2 , Ptq 2 , 6 while Znq 2 is one of the alternative candidate for Alq 3 in OLEDs of high efficiency and low operating voltage. 7 There is no isomer effect for the Znq 2 , but both single crystal and powder X-ray crystallography of the vacuum-deposited thin films indicates that tetramer [(Znq 2 ) 4 ] is the energetically favored configuration. The quinolated-metal complexes show specific benefits with their organometallic structures compared with many organic host materials. The 8-hydroxyquinoline ligand Scheme 4-1. N O Al O N N O N O Al N O N O N O Zn N O mer-Alq 3 fac-Alq 3 Znq 2 N X Al X N N X mer-Al(PydpX) 3 fac-Al(PydpX) 3 Zn(PydpX) 2 X = O, S N X Al N X N X N X Zn N O 95 itself is a non-sublimable small organic molecule. By forming the homogeneous aluminum metal complex, the Alq 3 is stable in solid state and sublimable with a high glass transition temperature(Tg = 172 ̊C). 8 Even for the Znq 2 of much less molecular mass, the molecule is sublimable, and the Tg is above 100 ̊C(~105 ̊C). Both Alq 3 and Znq 2 are widely used as the ETL in doped OLEDs. The excellent electron transporting property of Alq 3 also makes it useful as the ET-type host material. Unfortunately, the low triplet energy (2.0 eV) and relatively narrow HOMO-LUMO gap (4.0 eV) restricted the application of Alq 3 to be only suitable as a host for deep red to near-infrared dopants. 9 However, it is important to stress that the complexes with Al and Zn metal center can reserve the energetic levels of the ligand. New complexes with ligands of higher energies can potentially lead to high energy host materials that can be suitable for RGB full-spectrum dopant. In this study, our approach is to break down the conjugation in the 8-hydroxyquinoline ligand in order to increase the energy levels of the ligand analogs significantly(Scheme 4-1). The diphenyl(pyridine-2-yl)methanol (PydpO) and lithium diphenyl-2-pyridylmethanethiolate (PydpS) ligands are chsosen for the high energy metal chelate hosts. Iridium (III) complexes with PydpX(X = O, S) ligands are also designed in order to determine the energetic levels of the ligand versus blue and green chromophores. 96 4.2. Experimental 4.2.1. Materials and synthesis The syntheses were carried out under a nitrogen atmosphere using standard Schlenk techniques. Solvents were distilled under nitrogen from Sodium benzophenone (THF), or calcium hydride (CH 2 Cl 2 ). The starting materials [Ir(ppz) 2 (µ-Cl)] 2 , [Ir(ppy) 2 (µ-Cl)] 2 were prepared according to literature methods. 10 Other chemicals and solvents were ordered from Sigma-Aldrich®, and used as received. 1 H, gCOSY and 13 C NMR spectra were measured by Varian 500S NMR Spectrometer. The chemical shifts were referenced to a deuterated solvent. Mass Spectroscopy were obtained on a Shimadzu LCMS-2020 quadrupole mass spectrometer equipped with a column oven (T = 40 °C), a PDA photodetector (200-800 nm) and an MS spectrometer (LCMS 2020; m/z range: 0-2000; ionization modes: ESI/APCI). Synthesis of the PydpO and PydpS ligand. The diphenyl(pyridine-2-yl)methanol (PydpO) and lithium diphenyl-2-pyridylmethanethiolate (PydpS) were prepared following literature reported processes. 11 For the PydpO ligand, 2-brompyridine was treated with n-butylithium solution, and then the in situ formed 2-lithiopyrdine was treated with benzophenone, and followed by aqueous workup. For the PydpS ligand, the diphenyl-2-pyridylmethane was treated with dropwise of n-butyllithium at -60 °C. Elemental sulfur was added at -60 °C, and the reaction mixture was stirred for 1 h, and left stirred at room temperature for 24 h. to yield the lithium salt of PydpS ligand. 97 Synthesis of(ppz) 2 Ir(PydpO). The [Ir(ppz) 2 (µ-Cl)] 2 (288 mg, 0.28 mmol, 1 eq.), PydpO ligand (220 mg, 0.84 mmol 3 eq.) and K 2 CO 3 (298 mg, 2.8 mmol, 10 eq.) were dried under vacuum in a Schelenk tube. 1,2-Dichloroethane (20 mL) was added, and then the reaction mixture was refluxed at 90 °C for 18 h. The yellow reaction solution was then cooled to room temperature, passed through a celit plug, and dried over rotavap. The residue was dissolved in CH 2 Cl 2 , and layered with hexanes to yield yellow crystals of the (ppz) 2 Ir(PydpO) (50%). 1 H NMR (500 MHz, CD 2 Cl 2 , 298 K): δ = 8.05 (d, J(HH) = 2.8 Hz, 1H, ppz), δ = 7.95 (m, 2H, ppz and Py), δ = 7.60 (t, J(HH) = 7.8 Hz, 1H, Py), δ = 7.36 (m, 1H, ppz), δ = 7.00-7.25 (m, 14H, Ph and Py), δ = 6.64-6.90 (m, 4H, Ph), δ = 6.47 (m, 1H, ppz), δ = 6.27 (m, 2H, ppz and Ph), δ = 6.16 (m, 1H, ppz), δ = 6.12 (m, 1H, Ph). Synthesis of(ppz) 2 Ir(PydpS). The [Ir(ppz) 2 (µ-Cl)] 2 (200 mg, 0.19 mmol, 1 eq.), PydpS ligand (165 mg, 0.59 mmol 3 eq.), and K 2 CO 3 (202 mg, 1.9 mmol, 10 eq.) were dried under vacuum in a Schelenk tube. 1,2-Dichloroethane (20 mL) was added, and then the reaction mixture was refluxed at 140 °C for 18 h. The light yellow-green reaction solution was then cooled to room temperature, passed through a celit plug, and dried over rotavap. The residue was dissolved in CH 2 Cl 2 , and layered with hexanes to yield light yellow crystals of the(ppz) 2 Ir(PydpS) (40%). 1 H NMR(400 MHz, (CD 3 ) 2 SO, 298 K): δ = 8.23 (d, J(HH) = 5.6 Hz, 1H, Py), δ = 8.02 (d, J(HH) = 2.8 Hz, 1H, ppz), δ = 7.96 (d, J(HH) = 2.8 Hz, 1H, ppz), δ = 7.75 (d, J(HH) = 2.0 Hz, 1H, ppz), δ = 7.51 (t, J(HH) = 8.0 Hz, 1H, Py), δ = 7.44 (m, 2H, Ph), δ = 7.27 (d, J(HH) = 7.9 Hz, 2H, Ph), δ = 7.09-7.15 (m, 7H, Ph), δ = 6.95-7.00 (m, 3H, Ph), δ = 6.91 (t, J(HH) = 7.1 Hz, 1H, Py), δ 98 = 6.84 (d, J(HH) = 8.1 Hz, 1H, Py), δ = 6.79 (t, J(HH) = 7.2 Hz, 1H, Ph), δ = 6.57 (t, J(HH) = 8.2 Hz, 1H, Ph), δ = 6.36 (m, 3H, ppz+Ph), δ = 6.29 (t, J(HH) = 2.4 Hz, 1H, ppz), δ = 6.01 (t, J(HH) = 7.5 Hz, 1H, Ph). Synthesis of(ppy) 2 Ir(PydpO). The [Ir(ppy) 2 (µ-Cl)] 2 (100 mg, 0.09 mmol, 1 eq.), PydpO ligand (73 mg, 0.28 mmol 3 eq.) and K 2 CO 3 (125 mg, 0.9 mmol, 10 eq.) were dried under vacuum in a Schelenk tube. 1,2-Dichloroethane (20 mL) was added, and then the reaction mixture was refluxed at 90 °C for 18 h. The orange reaction solution was then cooled to room temperature, passed through a celit plug, and dried over rotavap. The residue was dissolved in CH 2 Cl 2 , and layered with hexanes to yield orange crystals of the (ppy) 2 Ir(PydpO) (30%). 1 H NMR (500 MHz, CD 2 Cl 2 , 298 K): δ = 9.50 (d, J(HH) = 5.5 Hz, 1H, ppy), δ = 7.83 (d, J(HH) = 8.2 Hz, 1H, ppy), δ = 7.77 (d, J(HH) = 8.2 Hz, 1H, Py), δ = 7.71 (d, J(HH) = 6.1 Hz, 1H, PydpO), δ = 7.68 (t, J(HH) = 7.3 Hz, 1H, ppy), δ = 7.65 (m, 2H, Ph), δ = 7.60 (t, J(HH) = 7.9 Hz, 1H, PydpO), δ = 7.40 (t, J(HH) = 7.5 Hz, 1H, ppy), δ = 7.32 (d, J(HH) = 7.9 Hz, 1H, PydpO), δ = 7.25 (m, 1H, Ph), δ = 7.11-7.17 (m, 5H, Ph), δ = 7.08 (d, J(HH) = 6.2 Hz, 1H, ppy), δ = 6.99 (t, J(HH) = 6.2 Hz, 1H, PydpO), δ = 6.83-6.94 (m, 7H, Ph), δ = 6.70 (m, 2H, Ph), δ = 6.25 (t, J(HH) = 6.2 Hz, 1H, ppy), δ = 6.16 (m, 2H, Ph). Synthesis of Al(PydpO) 3 . The n-butyllithnium (1.0 ml, 1.65 mmol, 3.3 eq, 2.5 M) is added to PydpO (384 mg, 1.5 mmol, 3 eq) at -78 °C in dry THF (20 ml), and stirred for 1 h. Al(OiPr) 3 (100 mg, 0.5 mmol, 1 eq) was dissolved in THF (10 ml), and transferred to the reaction mixture at -78 °C via a cannula. White precipitates were immediately formed. The reaction mixture was warmed to room temperature, and stirred for overnight. 99 The white precipitates were filtered, washed with more THF, and dried under vacuum (80% yield). The compound is unstable with moisture, and suffered from self-degrading, and decomposed over vacuum sublimation. 1 H NMR (400 MHz, CDCl 3 , 298 K): δ = 8.60 (d, J(HH) = 6.0 Hz, 3H, py), δ = 7.64 (t, J(HH) = 7.8 Hz, 3H, py), δ = 7.24-7.32 (m, 33H, Ph and Py), δ = 7.10 (d, J(HH) = 7.9 Hz, 3H, py). Synthesis of Zn(PydpO) 2 . Route 1: The ZnCl 2 (188 mg, 1.38 mmol, 1 eq), PydpO (758 mg, 2.9 mmol, 2.1 eq), Et 3 N (1.9 ml, 14 mmol, 10 eq) were dissolved in THF (20 ml), and refluxed for 24 h. The white precipitates were filtered and washed with more THF (5 ml x 3) (20% yield). Route 2: The n-butyllithnium (1.22 ml, 3.04 mmol, 2.2 eq, 2.5 M) is added to PydpO (758 mg, 2.9 mmol, 2.1 eq) at -78 °C in dry THF (20 ml), and stirred for 1 h. The reaction mixture is transferred to ZnCl 2 (188 mg, 1.38 mmol, 1 eq) at -78 °C, warmed up to room temperature, and stirred for 18 h. The white precipitates were filtered and washed with THF (5 ml x 3) (70% yield). 1 H NMR (400 MHz, CDCl 3 , 298 K): δ = 8.52 (d, J(HH) = 5.0 Hz, 2H), δ = 7.76 (t, J(HH) = 7.8 Hz, 2H), δ = 7.39 (d, J(HH) = 8.8 Hz, 2H), δ = 7.26-7.29 (m, 22H). MS (m/z): 586, calcd. for C36H28N 2 O 2 Zn: 586.01. Synthesis of Zn(PydpS) 2 . The Zn(CH 3 CO 2 ) 2 ·2H 2 O (188 mg, 1.38 mmol, 1 eq) and PydpS (856 mg, 3.02 mmol, 2.4 eq) were refluxed in 2-propanol for 24 h. The reaction mixture was cooled to room temperature. The white precipitates were washed with 2-propanol (5 ml x 2), and then with Et 2 O (5 ml x 3)(50% yield). 1 H NMR (400 MHz, CDCl 3 , 298 K): δ = 8.50(d, J(HH) = 5.0 Hz, 2H), δ = 7.77(t, J(HH) = 7.8 Hz, 2H), δ = 7.58(d, J(HH) = 8.8 Hz, 2H), δ = 7.36-7.09 (m, 22H). 100 4.2.2. X-ray crystallography X-ray crystallography of the(ppz) 2 Ir(PydpO). The X-ray intensity data were measured on a Bruker APEXDUO CCD system using radiation from a MoKα fine-focus tube (λ = 0.71073 Å) with a TRIUMPH monochromator. An opaque yellowish prism shape specimen of C 36 H 28 IrN 5 O, approximate dimensions 0.20 mm x 0.17 mm x 0.15 mm, was used for the X-ray crystallographic analysis. A total of 2520 frames were collected. The frames were integrated using the Bruker SAINT V8.18C software. The integration of the data using a triclinic unit cell yielded a total of 6899 reflections to a maximum θ angle of 27.49° (0.68 Å resolution), of which 6124 were independent (average redundancy 7.761, completeness = 90%, R int = 1.96%). The final cell constants of a = 10.0553(12) Å, b = 16.052(2) Å, c = 19.558(2) Å, volume = 3093.5(7) Å 3 , are based upon the refinement of the XYZ-centroids of 6220 reflections above 2θ σ(I) with 2.43° < 2θ < 27.49°. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.809. The structure was solved and refined using the Bruker SHELXTL Software Package, using the space group monoclinic P2 1 /n, with Z = 4 for the formula unit, C 36 H 28 IrN 5 O.CH 4 O. The final anisotropic full-matrix least-squares refinement on F 2 converged at R1 = 2.63%, for the observed data and wR2 = 5.28% greater than 2σ(F 2 ). The goodness-of-fit was 1.054. The largest peak in the final difference electron density synthesis was 1.517 e - /Å 3 and the largest hole was -0.532 e - /Å 3 . On the basis of the final model, the calculated density was 1.655 g/cm 3 and F(000), 1528 e - . X-ray crystallography of the(ppz) 2 Ir(PydpS). The X-ray intensity data were 101 measured on a Bruker APEXDUO CCD system using radiation from a MoKα fine-focus tube (λ = 0.71073 Å) with a TRIUMPH monochromator. An opaque yellow needle shape specimen of C 36 H 28 IrN 5 S, approximate dimensions 0.20 mm x 0.09 mm x 0.06 mm, was used for the X-ray crystallographic analysis. A total of 2520 frames were collected. The frames were integrated using the Bruker SAINT V8.18C software. The integration of the data using a triclinic unit cell yielded a total of 13677 reflections to a maximum θ angle of 27.53° (0.68 Å resolution), of which 9617 were independent (completeness = 70%, R int = 2.39%). The final cell constants of a = 13.821(3) Å, b = 15.971(3) Å, c = 17.523(2) Å, volume = 3218.6(10) Å 3 , are based upon the refinement of the XYZ- centroids of 8988 reflections above 2θ σ(I) with 2.45° < 2θ < 27.03°. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.680. The structure was solved and refined using the Bruker SHELXTL Software Package, using the space group Triclinic P 1 , with Z = 2 for the formula unit, 2 C 36 H 28 IrN 5 S.CH 2 Cl 2 . The final anisotropic full-matrix least-squares refinement on F 2 converged at R1 = 4.05%, for the observed data and wR2 = 8.75% greater than 2σ(F 2 ). The goodness-of-fit was 0.930. The largest peak in the final difference electron density synthesis was 2.004 e - /Å 3 and the largest hole was -1.501 e - /Å 3 . On the basis of the final model, the calculated density was 1.645 g/cm 3 and F(000), 1572 e - . X-ray crystallography of the(ppy) 2 Ir(PydpO). The X-ray intensity data were measured on a Bruker APEXDUO CCD system using radiation from a MoKα fine-focus tube (λ = 0.71073 Å) with a TRIUMPH monochromator. An opaque orange block shape 102 specimen of C 40 H 30 IrN 3 O, approximate dimensions 0.28 mm x 0.26 mm x 0.17 mm, was used for the X-ray crystallographic analysis. A total of 2520 frames were collected. The frames were integrated using the Bruker SAINT V8.18C software. The integration of the data using a triclinic unit cell yielded a total of 7587 reflections to a maximum θ angle of 27.51° (0.68 Å resolution), of which 6559 were independent (average redundancy 7.761, completeness = 87%, R int = 2.89%). The final cell constants of a = 17.353(2) Å, b = 17.353(2) Å, c = 17.353(2) Å, volume = 4996.3(10) Å 3 , are based upon the refinement of the XYZ-centroids of 8988 reflections above 2θ σ(I) with 2.20° < 2θ < 27.48°. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.795. The structure was solved and refined using the Bruker SHELXTL Software Package, using the space group Rhombohedral R 3 , with Z = 6 for the formula unit, C 40 H 30 IrN 3 O. The final anisotropic full-matrix least-squares refinement on F 2 converged at R1 = 3.25%, for the observed data and wR2 = 6.89% greater than 2σ(F 2 ). The goodness-of-fit was 1.020. The largest peak in the final difference electron density synthesis was 2.364 e - /Å 3 and the largest hole was -0.545 e - /Å 3 . On the basis of the final model, the calculated density was 1.517 g/cm 3 and F(000), 2256 e - . 4.2.3. Photophysical characterization The UV-Visible spectra were recorded on a Hewlett-Packard 4853 diode array spectrometer. Steady-state emission measurements were performed using a QuantaMaster model C-60SE spectrofluorimeter (Photon Technology International) with an excitation wavelength of 350 nm in dilute (10 -5 M), N 2 -degassed CH 2 CL 2 (298 K) and 2-MeTHF 103 (77 K) in quartz cuvette. Phosphorescent lifetimes were measured by time-correlated single-photon counting with IBH Fluorocube instrument equipped with a 405 nm LED excitation source. Room temperature quantum yields were measured using a Hamamatsu C9920 system equipped with a xenon lamp, calibrated integrating sphere, and Model C10027 photonic multichannel analyzer. 4.2.4. Theoretical calculations The theoretical calculations were carried out by Schrödinger 2013 Materials Science Suite, using the density function theory (DFT) with Lee-Yang-Parr correlation functional B3LYP. The basis set used was LACVP**. DFT calculations were also carried out with Gaussian03 program to check the consistency. The same hybrid functional of B3LYP was employed, and the basis set was LANL2DZ/6-31G*. Time-dependent DFT (TDDFT) calculations were performed with Gaussian03 program using the same basis set. Unfortunately, the TDDFT calculations did not match the experimental data. 4.2.5. Device Fabrication All materials used for the vapor deposition were purified by high vacuum sublimation (1 x 10 -6 torr). Indium tin oxide(ITO) on glass was provided by Thin Film Devices Inc. ITO substrates were cleaned with Tergitol® detergent, rinsed with deionized water, and then sonicated in acetone (HPLC grade, 3 times). The substrates were then treated with UV ozone (UVOCS T10 x 10/OES) for 10 min, and loaded into the high vacuum chamber. The organics were vapor deposited on the substrate. The masks with 2 mm stripe width were then placed on the substrates under N 2 atmosphere in the glovebox 104 attached to the high vacuum chamber. The LiF (10 Å) and aluminum cathode (1000 Å) were deposited on top of the organic layers. The electrical and optical characteristics of the devices were measured with a Keithly 2400 source/mter/2000 multimeter coupled to a New port 1835-C optical meter, equipped with a UV 818 Si photo detector. The electroluminescence spectra were recorded using a Photon Technology International QuantaMaster model C-60 fluorimeter. 4.3. Results and discussion 4.3.1. Design and synthesis of the Ir(III) complexes X = O, S Cl (ppz) 2 Ir Cl Ir(ppz) 2 + K 2 CO 3 or AgOTf DCE, reflux N N Ir N X 2 N X X = O, S Cl (ppy) 2 Ir Cl Ir(ppy) 2 + K 2 CO 3 or AgOTf DCE, reflux N Ir N X 2 N X N Br 1) nBuLi 2) O Ph Ph 3) H 2 O N HO Et 2 O/Hexane H 1) nBuLi 2) S THF/Hexane S Li Figure 4-1. Synthetic Routes of Ir(III)-(PydpX)(X = O, S) complexes. 105 The energetic levels of PydpX (X = O, S) ligands can be preliminary determined by photophysical characterization of related heteroleptic Ir(III) complexes with PydpX chelates. Previous studies have been demonstrated by former group members that fac- Ir(ppy) 3 emits at 510 nm at room temperature with unity quantum yield. 12 By replacing the pyridyl substituent with pyrazolyl substituent of higher triplet energy, the fac-Ir(ppz) 3 [ppz = 1-phenylpyrazole] shows weak emission at room temperature (ϕ < 0.01), but intense blue phosphorescent emission at low temperature (410 nm, 77 K in 2-MeTHF glass). 13 In heteroleptic Ir(III) complexes, the molecules are designed as composed of chromophore and ancillary chelates. The [(ppz) 2 -Ir] and [(ppy) 2 -Ir] chromophore are chosen for evaluating the energetic levels of the PydpX ancillary ligands. The photoluminescence properties of the complexes can provide direct suggestion about the triplet energy levels of the PydpX ligands. The Ir(III) complexes of(ppz) 2 Ir(PydpO),(ppz) 2 Ir(PydpS),(ppy) 2 Ir(PydpO), and (ppy) 2 Ir(PydpS) were synthesized following traditional synthetic routes for heteroleptic Ir(III) complexes (Figure 4-1). 14 The mixture of [Ir(ppz) 2 (µ-Cl)] 2 or [Ir(ppy) 2 (µ-Cl)] 2 dimers, along with the PydpO or PydpS ligand and K 2 CO 3 were refluxed in 1,2-dichlroethane (DCE) for 18 h to obtain the targeting complexes (30%-80% yield). For the synthesis of(ppz) 2 Ir(PydpS) and(ppy) 2 Ir(PydpS) starting with lithium salt of PydpS, the addition of the mild base of K 2 CO 3 can be omitted. The AgOTf was used as an efficient chloride remover, and the following cyclometalation went smoothly. The 1 H NMR signals are assigned based on the correlation between 1 H and gCOSY NMRs (Figure 4-2-4). 106 Figure 4-3. gCOSY (500 MHz, CD 2 Cl 2 ) of (ppz) 2 Ir(PydpS). Figure 4-2. gCOSY (500 MHz, CD 2 Cl 2 ) of (ppz) 2 Ir(PydpO). 106 Figure 4-3. gCOSY (500 MHz, CD 2 Cl 2 ) of (ppz) 2 Ir(PydpS). Figure 4-2. gCOSY (500 MHz, CD 2 Cl 2 ) of (ppz) 2 Ir(PydpO). 106 Figure 4-3. gCOSY (500 MHz, CD 2 Cl 2 ) of (ppz) 2 Ir(PydpS). Figure 4-2. gCOSY (500 MHz, CD 2 Cl 2 ) of (ppz) 2 Ir(PydpO). 107 4.3.2. X-ray crystollography of Ir(III) complexes Single crystals of (ppz) 2 Ir(PydpO), (ppz) 2 Ir(PydpS), and (ppy) 2 Ir(PydpO) were grown in hexane/dichloromethane layered solutions. The unit cell of (ppz) 2 Ir(PydpS) crystals contains a pair of enantiomers, belonged to the triclinicP1 space group. The crystals of complexe (ppz) 2 Ir(PydpO) are enantiopure, belonged to monoclinic P2 1 /n space group (Z = 4). The (ppy) 2 Ir(PydpO) belongs to rhombohedralR3 space group (Z = 6), with three pairs of enantiomers in one unit cell. The cyclometallated ligands in all three complexes are in the pseudo-octahedral geometry around the Ir(III) metal center (Figure 4-5 and Table 4-1). The Ir-N bond lengths and Ir-C bond lengths of the (ppz) 2 -Ir and (ppy) 2 -Ir chelates in all three Figure 4-4. gCOSY (500 MHz, CD 2 Cl 2 ) of(ppy) 2 Ir(PydpO). 107 4.3.2. X-ray crystollography of Ir(III) complexes Single crystals of (ppz) 2 Ir(PydpO), (ppz) 2 Ir(PydpS), and (ppy) 2 Ir(PydpO) were grown in hexane/dichloromethane layered solutions. The unit cell of (ppz) 2 Ir(PydpS) crystals contains a pair of enantiomers, belonged to the triclinicP1 space group. The crystals of complexe (ppz) 2 Ir(PydpO) are enantiopure, belonged to monoclinic P2 1 /n space group (Z = 4). The (ppy) 2 Ir(PydpO) belongs to rhombohedralR3 space group (Z = 6), with three pairs of enantiomers in one unit cell. The cyclometallated ligands in all three complexes are in the pseudo-octahedral geometry around the Ir(III) metal center (Figure 4-5 and Table 4-1). The Ir-N bond lengths and Ir-C bond lengths of the (ppz) 2 -Ir and (ppy) 2 -Ir chelates in all three Figure 4-4. gCOSY (500 MHz, CD 2 Cl 2 ) of(ppy) 2 Ir(PydpO). 107 4.3.2. X-ray crystollography of Ir(III) complexes Single crystals of (ppz) 2 Ir(PydpO), (ppz) 2 Ir(PydpS), and (ppy) 2 Ir(PydpO) were grown in hexane/dichloromethane layered solutions. The unit cell of (ppz) 2 Ir(PydpS) crystals contains a pair of enantiomers, belonged to the triclinicP1 space group. The crystals of complexe (ppz) 2 Ir(PydpO) are enantiopure, belonged to monoclinic P2 1 /n space group (Z = 4). The (ppy) 2 Ir(PydpO) belongs to rhombohedralR3 space group (Z = 6), with three pairs of enantiomers in one unit cell. The cyclometallated ligands in all three complexes are in the pseudo-octahedral geometry around the Ir(III) metal center (Figure 4-5 and Table 4-1). The Ir-N bond lengths and Ir-C bond lengths of the (ppz) 2 -Ir and (ppy) 2 -Ir chelates in all three Figure 4-4. gCOSY (500 MHz, CD 2 Cl 2 ) of(ppy) 2 Ir(PydpO). 108 complexes are within 2.0 Å, and are comparable to fac-Ir(ppy) 3 and fac-Ir(ppz) 3 complexes. The Ir-PydpX (X = O, S) chelates show longer Ir-N bond lengths(around 2.14 Å). The Ir-S bond length (2.4025(15) Å) is significantly longer than the Ir-O bond lengths (2.1258(17) Å and 2.1344(19) Å respectively. The trans N-Ir-N bond angles of all three complexes are identical (170.75(18)°, 172.52(8)°, 172.04(9)° respectively). The trans S-Ir-C bond bridge (175.42(16)°) is the least distorted, and the trans O-Ir-C bond bridge of complex(ppy) 2 Ir(PydpO)(165.68(10)°) is the most distorted. The Ir-(PydpX) chelates are significantly more distorted than heteroleptic iridium complexes with ancillary ligand of acetylacetone (acac) and picnilinic acid (pic), such as (ppy) 2 Ir(acac), and (ppz) 2 Ir(pic). The diphenyl-2-pyridyl-methane substituents of the Ir-(PydpX) chelates maintain the pseudo-tetrachedral geometry with tetrahedral angles close to 109.5°(110.6(4)°, 110.5(2)°, and 109.4(2)° respectively). The dihedral angle between the C24-S1-Ir plane and the C23-C24-Ir plane in complex(ppz) 2 Ir(PydpS) is 26°. The dihedral angle between Figure 4-5. Perspective views of (ppz) 2 Ir(PydpO)(left),(ppy) 2 Ir(PydpO)(middle), and (ppz) 2 Ir(PydpS)(right) in 50% probability thermal ellipsoids. The methane phenyls and all hydrogen atoms are left for clarity in all complexes. The atoms are colored by green (Ir), white (C), yellow(S), red (O), and blue (N). 109 the C6-O1-Ir plane and the C5-C6-Ir plane(ppy) 2 Ir(PydpO) shows larger dihedral angle of 33°. The Ir-(PydpX) chelates in the complex(ppz) 2 Ir(PydpO) is of the least distorted geometry. The dihedral angle decreases to 21°. 4.3.3. Electrochemistry The electrochemical properties of(ppz) 2 Ir(PydpO),(ppz) 2 Ir(PydpS), and (ppy) 2 Ir(PydpS) are examined by CV and DPV in DMF solution (Figure 4-6). The reduction and oxidation potentials of the complexes vs. Fc + /Fc are listed in Table 4-2. A reversible oxidation process is observed for all complexes (at E 1/2 (0/+) = 0.20, 0.14, and 0.22 V respectively), followed by a second irreversible oxidation process. The oxidation values are compared with previously reported electrochemistry results of complex Table 4-1. Selected Bond Angles (degree) and Bond Distances (Å) of complexes (ppz) 2 Ir(PydpS),(ppz) 2 Ir(PydpO), and (ppy) 2 Ir(PydpO)(X denotes to O and S elements in different complexes). (ppz) 2 Ir(PydpS)(ppz) 2 Ir(PydpO)(ppy) 2 Ir(PydpO) Bond Distances (Å) Ir1-X1 2.4025(15) 2.1258(17) 2.1344(19) Ir1-N1 2.031(4) 2.015(2) 2.144(2) Ir1-N2 ---- ---- 2.014(2) Ir1-N3 2.009(4) 2.021(2) 2.044(2) Ir1-N5 2.150(4) 2.140(2) ---- Ir1-C9 2.053(5) 2.012(2) ---- Ir1-C18 2.012(6) 2.011(3) ---- Ir1-C29 ---- ---- 2.005(3) Ir1-C40 ---- ---- 1.996(3) Bond Angles (degree) trans N-Ir-N 170.75(18) 172.52(8) 172.04(9) trans N-Ir-C 172.47(19) 170.85(9) 171.40(19) trans X-Ir-C 175.42(16) 171.01(8) 165.68(10) C24-X-Ir 100.30(18) 116.08(14) ---- C6-O1-Ir ---- ---- 113.96(15) C23-C24-X 110.6(4) 110.5(2) ---- 110 Ir(ppz) 3 . The work of Tamayo and co-workers showed that the first oxidation of mer- Ir(ppz) 3 was observed at E 1/2 (0/+) = 0.28 V vs. Fc + /Fc in DMF solution, and that fac- Ir(ppz) 3 was even more difficult to be oxidized (E 1/2 (0/+) = 0.39 V). 13b The PydpX (X = O, S) ligands make the oxidation prcesses of complexes(ppz) 2 Ir(PydpO) and(ppz) 2 Ir(PydpS) more cathodic than the complex Ir(ppz) 3 . The complex(ppz) 2 Ir(PydpS) is 0.6 V easier to be oxidized than the complex(ppz) 2 Ir(PydpO). The first reduction process for the complexes(ppz) 2 Ir(PydpS),(ppz) 2 Ir(PydpO),(ppy) 2 Ir(PydpO) are quasi-reversible. The first reduction process of complex Ir(ppz) 3 was not detectable in DMF solvent. Thus, the observed reduction processes for complexes(ppz) 2 Ir(PydpO) and(ppz) 2 Ir(PydpS) are assigned to the PydpO and PydpS ligand reduction. The complex(ppz) 2 Ir(PydpO) is 0.1 V more cathodic than the(ppz) 2 Ir(PydpS) and indicating a higher LUMO level of the (ppz) 2 Ir(PydpO). The complex(ppz) 2 Ir(PydpS) is easier to be oxidized due to the more nucleophilic and polarizable sulfur atom. 15 A second quasi-reversible reduction process is observed for the complex(ppy) 2 Ir(PydpO)(E 1/2 (-2/-1) = -3.0 V). Both the first oxidation potential and the first and second reduction potentials of the complex(ppy) 2 Ir(PydpO) are comparable with those of the complex fac-Ir(ppy) 3 (E 1/2 (-/0) = -2.7 V, E 1/2 (-2/-1) = -3.0 V). The HOMO and LUMO levels of the complexes can be estimated based on the electrochemical data. 16 The HOMO energy of the(ppz) 2 Ir(PydpO)(-4.9 eV) is consistent with the HOMO of(ppy) 2 Ir(PydpO)(-4.9 eV). The LUMO of the(ppy) 2 Ir(PydpO)(-1.7 eV) is 0.2 eV lower than the LUMO of the(ppz) 2 Ir(PydpO)(-1.5 eV), due to the easier reduced ppy ligand. The HOMO of the(ppz) 2 Ir(PydpS)(-4.8 eV) is higher than the HOMO levels of the(ppz) 2 Ir(PydpO) and the(ppy) 2 Ir(PydpO). The LUMO of 111 (ppz) 2 Ir(PydpS)(-1.6 eV) is lower than that of the(ppz) 2 Ir(PydpO). The HOMO-LUMO optical gap of complex(ppz) 2 Ir(PydpO)(3.4 eV) is wider than those of the complexes (ppz) 2 Ir(PydpS)(3.2 eV) and(ppy) 2 Ir(PydpO)(3.2 eV). -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 -50.0µ -40.0µ -30.0µ -20.0µ -10.0µ 0.0 10.0µ 20.0µ Red 1 Ox 1 Fc + /Fc Current (A) Potential (V) Oxidation Reduction (ppz) 2 Ir(PydpO) -3 -2 -1 0 1 -20.0µ -10.0µ 0.0 10.0µ 20.0µ (ppz) 2 Ir(PydpO) Red 1 Ox 1 Fc + /Fc Current (A) Potential(V vs. Fc + /Fc) dpv dpv -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 -50.0µ -40.0µ -30.0µ -20.0µ -10.0µ 0.0 10.0µ (ppz) 2 Ir(PydpS) Ox 1 Red 1 dmfc + /dmfc Current (A) Potential (V) Oxidation Reduction -3 -2 -1 0 1 -20.0µ -10.0µ 0.0 10.0µ 20.0µ(ppz) 2 Ir(PydpS) Red 1 Ox 1 dmfc + /dmfc Current (A) Potential(vs. Fc + /Fc) DPV DPV -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 -50.0µ -40.0µ -30.0µ -20.0µ -10.0µ 0.0 10.0µ (ppy) 2 Ir(PydpO) Red 2 Red 1 Ox 1 dmfc + /dmfc Current (A) Potential (V) Oxidation Reduction -3 -2 -1 0 1 -30.0µ -20.0µ -10.0µ 0.0 10.0µ 20.0µ (ppy) 2 Ir(PydpO) Red 2 Red 1 Ox 1 dmfc + /dmfc Current (A) Potential(V vs. Fc + /Fc) DPV DPV Figure 4-6. CV (at 0.1 V/s) and DPV (at 0.01 V/s) traces of complexes (ppz) 2 Ir(PydpO), (ppz) 2 Ir(PydpS), and (ppy) 2 Ir(PydpO) (in DMF, 0.1 M [Bu 4 N][PF 6 ]). 112 4.3.4. Electronic Spectroscopy The absorbance spectrum of PydpO ligand shows intense absorbance band from 250 to 300 nm(Figure 4-7 top). The absorption band between 300 nm and 400 nm is weak. The complex(ppz) 2 Ir(PydpO) shows intense absorbance band similar as the PydpO ligand in the ultraviolet region below 300 nm, assigned to the ligand based(π→π*) transitions. Weaker and broader bands are observed in the near-UV and visible region from 320 nm to 450 nm(λ = 346 nm, ε = 3633 L mol -1 cm -1 , and λ = 387 nm, ε =1693 L mol -1 cm -1 ), and are assigned to be metal-to-ligand charge transfer transitions (MLCT). The complex(ppy) 2 Ir(PydpO) shows intense absorbance band similar as the PydpO ligand in the ultraviolet region below 300 nm, assigned to the ligand based(π→π*) transitions. Weaker and broader bands are observed in the near-UV and visible region from 320 nm to 530 nm, and are assigned to be metal-to-ligand charge transfer transitions (MLCT). Table 4-2. Photophysical properties of complexes(ppz) 2 Ir(PydpO) and (ppy) 2 Ir(PydpO). Absorption a Emission 77 K b Emission 298 K a Q.Y . E ox c E red c λ (nm)(ε, 10 3 L mol -1 cm -1 ) λ 0-0 (nm) τ (µs) λ max (nm) τ(µs)(%)(V)(V)(ppz) 2 Ir(PydpO) 276 (14.3), 302 (7.8), 346 (3.7), 387 (1.7) 411 14 ---- 0.01(76%) 0.02 (24%) <0.01 0.20 -2.8 (ppy) 2 Ir(PydpO) 286 (28.8), 342 (6.2), 400 (5.2), 460 (2.8), 484 (sh) (1.8) 475 14 549 1 60 0.22 -2.6 a. Absorption spectra and 298K emission spectra, and quantum yields were recorded in CH 2 Cl 2 solution. b. 77 K emission spectra were recorded in 2-MeTHF glass. c. CV and DPV data were recorded n DMF solution. 113 300 400 500 600 700 0 1 2 Normalized Emissions Absorbance (a.u.) Wavelength (nm) PydpO ligand abs. (ppz)2Ir(PydpO) abs. PydpO ligand PL. 77K (ppz)2Ir(PydpO) PL. 77K 300 400 500 600 700 -50 0 10k 20k 30k Normalized Emissions (Lmol-1cm-1) Ir(ppy)2(PydpO) abs. Ir(ppy)2(PydpO) PL 77K Ir(ppy)2(PydpO) PL 298K Wavelength (nm) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Figure 4-7. (top): Absorbance spectra of PydpO ligand (black dot line) and complex (ppz) 2 Ir(PydpO)(red solid line) recorded in CH 2 Cl 2 at room temperature. Emission spectra of PydpO ligand (blue dot line) and complex(ppz) 2 Ir(PydpO)(blue solid line) recorded in 2-MeTHF at 77 K.(Bottom): Absorbance spectrum of complex (ppy) 2 Ir(PydpO)(black line) recorded in CH 2 Cl 2 at room temperature. Emission spectra of complex(ppy) 2 Ir(PydpO) recorded in 2-MeTHF solution (298 K, red line) and in 2-MeTHF glass(blue line, 77 K). 114 Phosphorescent emission of PydpO ligand is observed at 77 K (λ max = 390 nm, τ = 1.4 s), indicating the high triplet energy level from the ligand (in 2-MeTHF glass). The phosphorescence emission of the complex(ppz) 2 Ir(PydpO) in 2-MeTHF at 298 K is weak (Ф < 0.01, τ = 1 ns (76%), τ = 4 ns (24%)). Bright blue emission is observed at 77 K (λ 0-0 = 411 nm, τ = 14 µs). The onset of the low temperature emission is the same as that of the complex Ir(ppz) 3 (ppz = 2-phenylpyrazole) (Figure 4-8). The Ir(ppz) 3 shows sharp featured emission bands of the vibronic structure from ppz ligand, and was assigned to a ligand centered/MLCT excited state. On contrary, the vibronic structure of the emission from the complex(ppz) 2 Ir(PydpO) was broadened, and indicating greater MLCT character in the excited state. The emission character is studied in detail in the theoretical section. 400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0 absorbance (a.u.) Wavelength (nm)(ppz) 3 Ir PL. 77K (ppz) 2 Ir(PydpO) PL. 77K Figure 4-8. Emission spectra of complex Ir(ppz) 3 (blue dot line) and complex (ppz) 2 Ir(PydpO)(blue solid line) recorded in 2-MeTHF at 77 K. 115 The complex(ppy) 2 Ir(PydpO) shows broad and featureless phosphorescence emission at 298 K (λ max = 549 nm,Ф = 0.6, τ = 1 μs) (Figure 4-7 bottom). The emission is blue-shifted at 77 K (λ 0-0 = 475 nm, τ = 14 µs). The emission band is still broad, and shows a little more structure than the room temperature emission band. The low temperature emission band of the complex(ppy) 2 Ir(PydpO) shows that [(ppy) 2 Ir-] is the origin of the emission with significant MLCT character. 4.3.5. Theoretical calculations of Ir(III) complexes The electronic structures of complexes(ppz) 2 Ir(PydpO),(ppz) 2 Ir(PydpS) and (ppy) 2 Ir(PydpO) are predicted by DFT and TD-DFT calculations, and compared with the electronic structures of complexes(ppz) 2 Ir(acac) and(ppz) 2 Ir(pic) (pic = picolinic acid) in order to evaluate the energy level of PydpO for blue phosphorescent chromophore. The frontier orbitals of the complexes are shown in Figure 4-9. The HOMO and LUMO energy levels for all complexes are close to those energy levels calculated based on electrochemical data. The HOMO and HOMO-1 of all complexes are predominately localized on the metal, the ppz/ppy ligand and the O or S atom. The LUMO and LUMO+1 of the complexes are predominately localized on the phenyls of the PydpX ligands, with small contribution from the metal and ppz/ppy ligand. The TD-DFT calculations are used to assign the S 0 →S n (n = 1, 2) and S 0 →T 1 transition energies and oscillator strengths (f) of the complexes (Table 4-3). Unfortunately, TD-DFT calculations do not work well for the complexes(ppz) 2 Ir(PydpO) and(ppz) 2 Ir(PydpS). The calculations were performed with LACVP** basis set via Schrodinger 2013 Materials Science Suite, and also with LANL2DZ/6-31G* basis set via Gaussian 03 116 program. Neither calculation shows results that are consistent with the experimental results. The S 0 →S 1 transition calculated for(ppy) 2 Ir(PydpO) is close to the onset in the experimental absorption spectrum (λ = 500 nm, f = 0.0043). The S 0 →S 1 and S 0 →S 2 transitions primarily involve HOMO→LUMO+1 (89% and 92% respectively), and can be described as principally a MLCT transition. Likewise, the S 0 →T 1 transition (λ = 519 nm) involves HOMO→LUMO+1 (87%) transition. Table 4-3. S 0 →S n (n = 1,2) transitions, S 0 →T 1 and S 0 →T 2 transitions, wavelength (λ cal ), oscillator strength (f), and transition contributions (> 10%) and assignments for (ppz) 2 Ir(PydpO) from TDDFT calculations. (ppz) 2 Ir(PydpO) States λ cal (nm) f major contribution assignment T 1 504 0 HOMO→LUMO (100%) MLCT S 1 501 0.0023 HOMO→LUMO (100%) MLCT S 2 463 0.0004 HOMO-1→LUMO (100%) MLCT (ppz) 2 Ir(PydpS) T 1 576 0 HOMO→LUMO (100%) MLCT S 1 571 0.0002 HOMO→LUMO (100%) MLCT S 2 456 0.0010 HOMO-1→LUMO+1(96%) MLCT (ppy) 2 Ir(PydpO) T 1 519 0 HOMO→LUMO+1(87%) MLCT S 1 500 0.0043 HOMO→LUMO+1(89%) ML-L’CT S 2 490 0.0170 HOMO→LUMO+1(92%) ML-L’CT 117 (ppz) 2 Ir(PydpO)(ppz) 2 Ir(PydpS)(ppy) 2 Ir(PydpO) LUMO+1 (-1.04 eV) LUMO+1 (-1.09 eV) LUMO+1 (-1.09 eV) LUMO (-1.50 eV) LUMO (-1.58 eV) LUMO (-1.51 eV) HOMO (-4.77 eV) HOMO (-4.49 eV) HOMO (-4.73 eV) HOMO-1 (-4.90 eV) HOMO-1 (-5.08 eV) HOMO-1 (-5.00 eV) Figure 4-9. Frontier orbitals involved in the S 0 →S n (n = 1, 2) and S 0 →T 1 transitions of (ppz) 2 Ir(PydpO),(ppz) 2 Ir(PydpS), and (ppy) 2 Ir(PydpO). 118 4.3.6. Design and synthesis of the Al(III) and Zn(II) complexes The PydpO ligand was then introduced into the Al(III) and Zn(II) complexes for high energy host materials (Scheme 4-2). For Al(PydpO) 3 , the PydpO ligand was first treated with nBuLi at -78 ̊C for 1 h, and then the solution was transferred to dry Al(OiPr) 3 and reacted in dry THF for 4 days. Similar reaction conditions were applied for the complex Zn(PydpO) 2 , with ZnCl 2 as the metal precursor. The reaction time was shorter than that of the Al(PydpO) 3 , with only 24 h in THF solution. The Zn(PydpO) 2 can also be prepared by adding a mild base of triethylamine (Et 3 N) in the reaction of ZnCl 2 and PydpO ligand, and then followed by reflux of the reaction mixture in THF for 24 h. The complexes Al(PydpO) 3 and Zn(PydpO) 2 showed as precipitates from the reaction mixture, and can be purified with more THF solvents. Both complexes are moisture and air sensitive, and degradation with time is observed. The solubility of the complexes is quite Scheme 4-2. OH N 1) nBuLi, -78C, 1h 2) Al(OiPr) 3 THF, 4 days Al O N 3 OH N 1) nBuLi, -78C, 1h 2) ZnCl 2 , r.t. 24 h THF O N Zn O N OH N ZnCl 2 , Et 3 N reflux 24h, THF O N Zn O N 119 low. The complexes can be further purified by zone sublimation at 240-200-160 °C with 50% yield. Parts of the complexes are decomposed during sublimation. 4.3.7. Theoretical calculations of complexes Al(PydpO) 3 and Zn(PydpO) 2 The DFT and TDDFT calculations are used to investigate the electronic structure of Al(PydpO) 3 and Zn(PydpO) 2 . The frontier orbitals of the complexes are shown in Figure 4-10. Several studies have been reported about the facial and meridional isomers of Alq 3 presenting in the amorphous-like thin films. 5, 17 A detailed DFT calculation was performed by Curioni and co-workers on the structural and electronic properties of the Alq 3 in neutral and charged states. 18 In this study, the theoretical calculations are performed on the fac/mer-Al(PydpO) 3 , and compared with the calculated electronic structures of fac/mer-Alq 3 . DFT calculations are also performed for complex Zn(PydpO) 2 , and compared with Znq 2 . The trans O-Al-O (166°), N-Al-N (170°), and N-Al-O (173°) angles of the mer-Alq 3 shows that the quinolate ligands arrange around the Al atom in pseudo-octahedral geometry. The cis N-Al-N and O-Al-O angels are around 95°. The fac-Alq 3 shows identical trans O-Al-N (171°) bond angles and cis N-Al-N (90°) and O-Al-O (98°) angles. The fac-Al(PydpO) 3 also shows identical trans O-Al-N (169°) bond angles, cis N-Al-N (90°) and O-Al-O (100°) angles. The trans O-Al-O (159°, 168°) and N-Al-N (161°) angles of the mer-Al(PydpO) 3 indicates more distorted coordination geometry. The cis N-Al-N (92°, 102°) and O-Al-O (95°, 104°) angles also show much more different values. 120 The Al-N and Al-O bond lengths of fac-Alq 3 are identical (2.13 Å and 1.88 Å respectively). The Al-N bond lengths (2.11, 2.10 and 2.06 Å) in the mer-Alq 3 are shorter mer-Al(PydpO) 3 fac-Al(PydpO) 3 Zn(PydpO) 2 LUMO+1 (-1.43 eV) LUMO+1 (-1.54 eV) LUMO+1 (-1.57 eV) LUMO (-1.44 eV) LUMO (-1.62 eV) LUMO (-1.59 eV) HOMO (-5.62 eV) HOMO (-5.73 eV) HOMO (-5.76 eV) HOMO-1 (-5.91 eV) HOMO-1 (-5.76 eV) HOMO-1 (-5.88 eV) Figure 4-10. Frontier orbitals involved in the S 0 →S n (n = 1, 2) and S 0 →T 1 transitions of mer/fac-Al(PydpO) 3 and Zn(PydpO) 2 . 121 than those in the facial isomer, while the Al-O bond lengths are slightly longer (1.92, 1.91, and 1.89 Å). The calculated bond lengths are only slightly different as reported in literature due to the different basis set employed in the calculations. 18 The Al-N bond lengths (2.18, 2.17, 2.14 Å) in the fac-Al(PydpO) 3 are longer than those in the Alq 3 complexes. The Al-O bond lengths (1.84 Å) in the fac-Al(PydpO) 3 are shorter compared with the Alq 3 complexes. The Al-N bond lengths (2.18, 2.12, 2.07 Å) in the mer-Al(PydpO) 3 is due to the more distorted structure of the complex. The Al-O bond lengths are identical as those in the complexes Alq 3 (around 1.88 Å). The HOMO (-5.39 eV) and LUMO (-2.06 eV) of fac-Alq 3 are overlapped on all three of the quinolate ligands. The HOMO is more localized on the phenoxide side, and the LUMO is more localized on the pyridyl side. 18 The HOMO (-5.18 eV) and LUMO (-2.11 eV) of the mer-Alq 3 are localized on one specific ligand. The HOMO-LUMO gap (3.33 eV) of the mer-Alq 3 is narrower than the gap of the fac-Alq 3 (3.07 eV). The HOMO (-5.62 eV) of mer-Al(PydpO) 3 is localized on the diphenyl-methoxy side. The LUMO (-1.44 eV) of the mer-Al(PydpO) 3 is localized on the pyridyl side. The LUMO+1 (-1.43 eV) is degenerated from the LUMO, and also localized on the pyridyl side of the PydpO ligand. The overlap between the HOMO and LUMO is much poorer compared with the Alq 3 complexes. The HOMO and LUMO energy gap (4.18 eV) is wide as the conjugation of the PydpO ligand is broken down by the sp 3 diphenyl-methoxy substituent. The HOMO (-5.73 eV) and LUMO (-1.62 eV) levels of the fac-Al(PydpO) 3 are both deeper compared with the mer-Al(PydpO) 3 . The HOMO is also predominately localized on the diphenyl-methoxy side, and the LUMO is predominately localized on the 122 pyridyl side. The HOMO-LUMO optical gap is comparable with that of the mer-Al(PydpO ) 3 . Interestingly, even though the mer-Al(PydpO) 3 has a more distorted structure than the fac-Al(PydpO) 3 , the gas phase DFT calculations indicate that the meridional isomer of the Al(PydpO) 3 is lower in energy than the facial isomer by 5.6 kcal/mol. The result is consistent with the reported studies of the meridional and facial Alq 3 isomer, where a 4 kcal/mol energy difference is observed with the mer-Alq 3 showing a lower energy. 18 The energy differences may be different when considering the real materials prepared in the devices due to solid state molecular packing. But the small energy difference shows that a mixture of both meridional and facial isomers of the complex Al(PydpO) 3 can co-exist. The Zn(PydpO) 2 significantly decreases the steric hindrances observed in complex Al(PydpO) 3 , and avoid isomeric structures. The Zn-O (1.97 Å) bond lengths are just slightly longer than the Zn-O (1.94 Å) bond lengths of complex Znq 2 . The Zn-N (2.14 Å) bond lengths are comparable with the Zn-N (2.12 Å) bond lengths of complex Znq 2 . The N-Zn-N (118°) and O-Zn-O (141°) bond angels are also comparable with those (124° and 141° respectively) of the complex Znq 2 . The HOMO (5.76 eV) and LUMO (1.59 eV) levels of the Zn(PydpO) 2 are comparable with the fac-Al(PydpO) 3 . The LUMO+1 (-1.57 eV) is degenerated from the LUMO. Similar as the complexes Al(PydpO) 3 , the HOMO is localized on the diphenyl-methoxy side, and LUMO is localized on the pyridyls. 123 4.3.8. OLEDs studies Tri-layer OLEDs with the Al(PydpO) 3 and Zn(PydpO) 2 complexes were fabricated, and compared with the standard bi-layer devices composed of NPD/Alq 3 . The device structures used were ITO / NPD (40 nm) / Alq 3 (20 nm) / M(PydpO) n (20 nm) / LiF (1 nm) / Al (100 nm) (Device I: n = 3 when M = Al, Device II: n = 2 when M = Zn), and ITO / NPD (40 nm) / Zn(PydpO) 2 (20 nm) / Alq 3 (20 nm) / LiF (1 nm) / Al (100 nm)(device III). The standard device structure was ITO / NPD (40 nm) / Alq 3 (40 nm) / LiF (1 nm) / Al (100 nm) (device S). The relative HOMO and LUMO energy values of all materials are shown as indicated by the DFT calculations (Figure 4-11). The LUMO levels of M(PydpO) n (~1.6 eV) are higher than the LUMO of Alq 3 (2.0 eV). The HOMO levels of M(PydpO) n (~5.7 eV) lie below the LUMO levels of NPD (5.3 eV) and Alq 3 (5.2 eV). A small barrier of about 0.4 eV is presented in both the electron and hole injection processes. The turn on voltage (at 0.1 cd/m 2 ) of Device I (2.6 V) is 0.2 V lower than the Device S (2.8 V) (Figure 4-12). The current density at 10 V of Device I is 1699 mA/cm 2 , much higher than the Device S (130 mA/cm 2 ). Pure Alq 3 emission is observed for the Device I between 3 V and 11 V. The above data indicate that the electron transporting property of the Al(PydpO) 3 is excellent. The holes and electrons recombine at the Alq 3 layer. However, the EQE of Device I is 0.4% compared to the Device S of 0.9%. A crucial problem with the Al(PydpO) 3 material is that decomposition is unavoidable during the vacuum deposition process. 124 2 3 4 5 6 7 8 9 10 2000 4000 6000 8000 Device S Device I Bright,(cd/m 2 ) Voltage (V) 0 2 4 6 8 10 200 400 600 800 1000 1200 1400 1600 1800 Device S Device I C.D.,(mA/cm 2 ) Voltage (V) 1 10 100 1000 10000 0.0 0.2 0.4 0.6 0.8 1.0 Device S Device I E.Q.E. Brightness, Cd/m 2 400 450 500 550 600 650 700 0.0 0.2 0.4 0.6 0.8 1.0 Device I Device S Electroluminescence Wavelength (nm) Figure 4-12. Brightness vs. Voltage (top, left), current density vs. voltage (top, right), EQE vs. brightness plot (bottom, left) and electroluminescence spectra measured at 7 V for Device I and Device S. Figure 4-11. Energy Diagrams of the fabricated OLED devices. 124 2 3 4 5 6 7 8 9 10 2000 4000 6000 8000 Device S Device I Bright,(cd/m 2 ) Voltage (V) 0 2 4 6 8 10 200 400 600 800 1000 1200 1400 1600 1800 Device S Device I C.D.,(mA/cm 2 ) Voltage (V) 1 10 100 1000 10000 0.0 0.2 0.4 0.6 0.8 1.0 Device S Device I E.Q.E. Brightness, Cd/m 2 400 450 500 550 600 650 700 0.0 0.2 0.4 0.6 0.8 1.0 Device I Device S Electroluminescence Wavelength (nm) Figure 4-12. Brightness vs. Voltage (top, left), current density vs. voltage (top, right), EQE vs. brightness plot (bottom, left) and electroluminescence spectra measured at 7 V for Device I and Device S. Figure 4-11. Energy Diagrams of the fabricated OLED devices. 124 2 3 4 5 6 7 8 9 10 2000 4000 6000 8000 Device S Device I Bright,(cd/m 2 ) Voltage (V) 0 2 4 6 8 10 200 400 600 800 1000 1200 1400 1600 1800 Device S Device I C.D.,(mA/cm 2 ) Voltage (V) 1 10 100 1000 10000 0.0 0.2 0.4 0.6 0.8 1.0 Device S Device I E.Q.E. Brightness, Cd/m 2 400 450 500 550 600 650 700 0.0 0.2 0.4 0.6 0.8 1.0 Device I Device S Electroluminescence Wavelength (nm) Figure 4-12. Brightness vs. Voltage (top, left), current density vs. voltage (top, right), EQE vs. brightness plot (bottom, left) and electroluminescence spectra measured at 7 V for Device I and Device S. Figure 4-11. Energy Diagrams of the fabricated OLED devices. 125 Zn(PydpO) 2 is more stable than the Al(PydpO) 3 during sublimation, and the Device II is also prepared via vacuum deposition. The turn on voltage (at 0.1 cd/m 2 ) of Device II (8.1 V) is much higher than the Device S (4.6 V) (Figure 4-13). The current density at 10 V of Device II is only 1.23 mA/cm 2 compared to the current density of 192 mA/cm 2 of the Device S. Pure Alq 3 emission is also observed for the Device II from the voltage range between 3 V and 11 V. The electron transporting property of the Zn(PydpO) 2 is 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1000 2000 3000 4000 5000 Device S Device II Bright, Cd/m 2 ) Voltage (V) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 0 100 200 300 400 500 600 700 Device S Device II C.D.,(mA/cm 2 ) Voltage (V) 1 10 100 1000 10000 0.0 0.2 0.4 0.6 0.8 Device S Device II E.Q.E. Brightness(cd/m 2 ) 400 450 500 550 600 650 700 750 0.0 0.2 0.4 0.6 0.8 1.0 Device S Device II Electroluminescence Wavelength (nm) Figure 4-13. Brightness vs. Voltage (top, left), current density vs. voltage (top, right), EQE vs. brightness plot (bottom, left) and electroluminescence spectra measured at 7 V for Device II and Device S. 126 poorer than the Al(PydpO) 3 , but electron and holes still recombine in the Alq 3 layer. The EQE of Device II is 0.3% compared to the Device S of 0.7%. Wei et. al. previously reported the study of a series of fulorenylsilane host materials. 2 Different device structures were designed to examine the charge injection and transportation abilities of the hosts. Following this idea, Device III was fabricated in order to examine the hole transporting property of the Zn(PydpO) 2 . The turn on voltage of the device is 3.0 V. In general, the performance of the device is poor. The EQE of the device is less than 0.1% compared with the Device S of 0.9%, and the maximum brightness of the device is only 23 cd/m 2 . The electroluminescence spectra of the Device III changes with different working voltage (Figure 4-14). At low voltage of 5 V, the blue emission of broad feature indicates 400 500 600 700 800 0.0 0.2 0.4 0.6 0.8 1.0 Device III 5V 7V 11V 13V Electroluminescence Wavelength (nm) 445 523 489 Figure 4-14. Electroluminescence spectra measured at voltages of 5 V, 7 V, 11 V, and 13 V of Device III. 127 that the charge recombination happens on the NPD-Zn(PydpO) 2 interface. At the voltage increases to 7 V, emission from Alq 3 is shown along with the NPD luminescence bands. With even higher voltage of 11 V, the emission from Alq 3 is dominated. With voltage of 13 V, emission from the NPD side is barely observed. The phenomena indicate that Zn(PydpO) 2 shows better electron transporting over hole transporting properties. 4.4. Summary We have designed and synthesized a series of metal chelates as perspective high energy host materials for OLEDs applications. The PydpX (X = O, S) chelated ligand have electronically isolated structures, and thus show high triplet energy. The ligand are first applied as ancillary ligands in blue and green phosphorescence Iridium(III) complexes of Ir(ppz) 2 (PydpX) and Ir(ppy) 2 (PydpX). The photoluminescence properties of the Ir(ppz) 2 (PydpO) and Ir(ppy) 2 (PydpO) indicates that the PydpO ligand is of higher triplet energy than the ppz ligand. The photoluminescence property of the complex Ir(ppz) 2 (PydpS) shows that PydpS ligand is of lower triplet energy than the PydpO ligand. And thus PydpO ligand is investigated further for host materials. In order to maintain the high energy levels of the ligand, complexes with Al and Zn metal centers are prepared. The chelated ligand are isolated by the metal center. The complexes Al(PydpO) 3 and Zn(PydpO) 2 show high triplet energy levels (~3.3 eV) close to pyridine and benzene. The HOMO-LUMO gaps of the complexes are around 4.1 eV. The problems of the complexes are that: 1) not stable in air, and with moisture. Self-degrading of the complexes is also observed with time. 2) Decomposition of the complexes with sublimation is about 50%., and will cause problems in device fabrication and device 128 performance. 3) The compounds are barely soluble in solvents, and thus full characterization is tough. 4) The complexes were examined in devices. The electron transporting properties of the complexes are promising as compared with Alq 3 . The hole transporting ability of the complexes is very weak. Compared the turn on voltages of Device II and Device III, it is also noteworthy that the electron injection through the Zn(PydpO) 2 is much more difficult compared with Al(PydpO) 3 and Alq 3 . 129 Chapter 4 References 1. Tang, C. W.; Vanslyke, S. A.; Chen, C. H., J Appl Phys 1989, 65(9), 3610. 2. Wei, W.; Djurovich, P. I.; Thompson, M. E., Chem Mater 2010, 22(5), 1724. 3. Tao, Y. T.; Yang, C. L.; Qin, J. G., Chem Soc Rev 2011, 40(5), 2943. 4. Tang, C. W.; Vanslyke, S. A., Appl Phys Lett 1987, 51(12), 913. 5. Brinkmann, M.; Gadret, G.; Muccini, M.; Taliani, C.; Masciocchi, N.; Sironi, A., J Am Chem Soc 2000, 122(21), 5147. 6. Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F., Inorg Chem 1986, 25(22), 3858. 7. Sapochak, L. S.; Benincasa, F. E.; Schofield, R. S.; Baker, J. L.; Riccio, K. K. C.; Fogarty, D.; Kohlmann, H.; Ferris, K. F.; Burrows, P. E., J Am Chem Soc 2002, 124(21), 6119. 8. Papadimitrakopoulos, F.; Zhang, X. M.; Higginson, K. A., Ieee J Sel Top Quant 1998, 4(1), 49. 9. Lee, T. C.; Hung, J. Y.; Chi, Y.; Cheng, Y. M.; Lee, G. H.; Chou, P. T.; Chen, C. C.; Chang, C. H.; Wu, C. C., Adv Funct Mater 2009, 19(16), 2639. 10. Nonoyama, M., B Chem Soc Jpn 1974, 47(3), 767. 11. Kim, I.; Nishihara, Y.; Jordan, R. F.; Rogers, R. D.; Rheingold, A. L.; Yap, G. P. A., Organometallics 1997, 16(15), 3314. 12. Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R., J Appl Phys 2001, 90 (10), 5048. 13.(a) Sajoto, T.; Djurovich, P. I.; Tamayo, A. B.; Oxgaard, J.; Goddard, W. A.; Thompson, M. E., J Am Chem Soc 2009, 131(28), 9813; (b) 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(24), 7377. 14. 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(7), 1704. 15. 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(18), 4304. 130 16.(a) D'Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.; Polikarpov, E.; Thompson, M. E., Org Electron 2005, 6(1), 11; (b) Djurovich, P. I.; Mayo, E. I.; Forrest, S. R.; Thompson, M. E., Org Electron 2009, 10(3), 515. 17. Schmidbaur, H.; Lettenbauer, J.; Wilkinson, D. L.; Muller, G.; Kumberger, O., Z Naturforsch B 1991, 46(7), 901. 18. Curioni, A.; Boero, M.; Andreoni, W., Chem Phys Lett 1998, 294(4-5), 263. 131 Appendix 1. Synthesis and characterization of [(pq) 2 Ir(bipy- (C 16 H 33 ) 2 )][PF 6 ] and [(pq) 2 Ir(bipy-(C 12 H 25 ) 2 )][PF 6 ] A1.1. Synthesis of the [(pq) 2 Ir(bipy-(C 16 H 33 ) 2 )][PF 6 ] and [(pq) 2 Ir(bipy-(C 12 H 25 ) 2 )][PF 6 ] The alkyl substituted bipyridines of 4,4'-dihexadecyl-2,2'-bipyridine and 4,4'-didodecyl-2,2'-bipyridine were prepared according to literature procedure by reaction of 4,4'-dimethyl-2,2'-bipyridine with lithium diisopropylamide(LDA) at -78 ̊C, followed by condensation with 1-Bromo-R’ (R’ = C 15 H 31 and C 11 H 23 respectively)(Scheme A1-1). 1 The [(pq) 2 Ir-µ-Cl] 2 , the alkyl substituted bipyridines, and the AgPF 6 were mixed in CH 2 Cl 2 and stirred at room temperature for 3 h in dark. The solution was filtered, and the filtrates were concentrated in minimum amount of CH 2 Cl 2 solvent. A Scheme A1-1. N Ir N N PF 6 R R 2 R = C 16 H 33 , C 12 H 25 N Ir 2 N Ir Cl Cl 2 + N N R R AgPF 6 CH 2 Cl 2 r.t. N N 1) LDA, -78 C 2) Br-R' , -78 C in THF N N R R (R' = C 15 H 31 , C 11 H 23 ) 132 layer of Et2O was added above the CH 2 Cl 2 solution. The overnight precipitation at -20 ̊C yielded >70% of the [(pq) 2 Ir(bipy-(C 16 H 33 ) 2 )][PF 6 ] and th [(pq) 2 Ir(bipy-(C 12 H 25 ) 2 )][PF 6 ]. LC-MS ESI+ (m/z): 1205.60 ([(pq) 2 Ir(bipy-(C 16 H 33 ) 2 )] + ), calcd : 1205.69); 1093.48 ([(pq) 2 Ir(bipy-(C 12 H 25 ) 2 )] + ), calcd : 1093.57) The solubility of the complexes follow the trend of H 2 O<MeOH<Et 2 O<CH 2 Cl 2 , Ethyl Acetate. A1.2. Characterization of the [(pq) 2 Ir(bipy-(C 16 H 33 ) 2 )][PF 6 ] and [(pq) 2 Ir(bipy-(C 12 H 25 ) 2 )][PF 6 ] A1.2.1. NMR characterization 1 H NMR of [(pq) 2 Ir(bipy-(C 12 H 25 ) 2 )][PF 6 ](400 MHz, CDCl 3 , 298 K): δ = 8.19(m, 4H, bipy), δ = 7.95 (m, 6H, bipy and pq), δ = 7.70 (d, J(HH) = 8.2 Hz, 2H, pq), δ = 7.36 (t, J(HH) = 7.2 Hz, 2H, pq), δ = 7.29(m, 2H, pq), δ = 7.17(m, 4H, pq), δ = 6.98(t, J(HH) = 7.2 Hz, 2H, pq), δ = 6.80(t, J(HH) = 7.6 Hz, 2H, pq), δ = 6.53 (d, J(HH) = 7.6 Hz, 2H, pq), δ = 2.73(s, 4H, C 12 H 25 ), δ = 1.54(s, 8H, C 12 H 25 ), δ = 1.24(s, 32H, C 12 H 25 ), δ = 0.88 (s, 6H, C 12 H 25 ). 1 H NMR of [(pq) 2 Ir(bipy-(C 16 H 33 ) 2 )][PF 6 ](400 MHz, CDCl 3 , 298 K): δ = 8.15(m, 4H, bipy), δ = 7.96(m, 6H, bipy and pq), δ = 7.73(d, J(HH) = 8.2 Hz, 2H, pq), δ = 7.38 (t, J(HH) = 7.2 Hz, 2H, pq), δ = 7.22(m, 2H, pq), δ = 7.10(m, 4H, pq), δ = 6.97(t, J(HH) = 7.2 Hz, 2H, pq), δ = 6.76(t, J(HH) = 7.6 Hz, 2H, pq), δ = 6.55(d, J(HH) = 7.6 Hz, 2H, pq), δ = 2.71(s, 4H, C 16 H 33 ), δ = 1.67(s, 8H, C 16 H 33 ), δ = 1.25(s, 48H, C 16 H 33 ), δ = 0.87 (s, 6H, C 16 H 33 ). 133 A1.2.2. Photophysical characterization 300 400 500 600 700 800 0 10000 20000 30000 40000 50000 60000 [(pq) 2 Ir(bipy-(C 16 H 33 ) 2 )][PF 6 ] abs. 77K PL rt PL Wavelength (nm) (L mol -1 cm -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Normalized Emissions 300 400 500 600 700 800 0 10000 20000 30000 40000 50000 60000 (pq) 2 Ir(bipy-(C 12 H 25 ) 2 )][PF 6 ] abs. 77K PL rt PL Wavelength (nm) (L mol -1 cm -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Normalized Emissions Figure A1-1. (top): Absorbance and Emission spectra of [(pq) 2 Ir(bipy- (C 16 H 33 ) 2 )][PF 6 ] recorded in MeOH solution. (bottom): Absorbance and Emission spectra of [(pq) 2 Ir(bipy-(C 12 H 25 ) 2 )][PF 6 ] recorded in MeOH solution. 134 Appendix 1 Reference 1.(a) Ellison, D. K.; Iwamoto, R. T., Tetrahedron Lett 1983, 24(1), 31; (b) Garcia, P.; Marques, J.; Pereira, E.; Gameiro, P.; Salema, R.; de Castro, B., Chem Commun 2001, (14), 1298. Table A1-1. Photophysical properties of complexes [(pq) 2 Ir(bipy-(R) 2 )][PF 6 ](R = C 16 H 33 and C 12 H 25 ). Absorption (in MeOH) PL(in MeOH) Q.Y . (%) k r (x 10 5 s -1 ) k nr (x 10 5 s -1 ) λ max (nm), (ε, 10 3 M -1 cm -1 ) Emission 77 K λ 0-0 (nm), [τ (µs)] Emission 298 K λ max (nm), [τ (µs)] R = C 16 H 33 272 (57.0), 337(19.3), 440 (4.2) 546, [4.2] 562, [2.9] 80 2.7 0.7 R = C 12 H 25 270 (51.9), 337 (22.6), 438 (5.0) 540, [4.1] 562, [2.9] 85 2.9 0.5 135 Appendix 2. Synthesis and characterization of [hdpIr-] A2.1. Synthesis of [hdpIr-] The hdp ligand was prepared following literature routes by treating the 2-(p-tolyl)pyridine with LDA at -78 ̊C, followed by condensation with BrC 15 H 31 at -78 ̊C. The reaction mixture was allowed to stir at room temperature for overnight. The anion was quenched with ice, and the organic layer was extracted with diethyl ether. The crude product was recrystallized in hot MeOH to yield a white solid (81%). The hdpIr^Cl was Scheme A2- 1. C 16 H 33 N + IrCl 3 xH 2 O Ethoxyethanol /H 2 O, reflux C 16 H 33 N Ir Cl Cl Ir C 16 H 33 N 2 2 C 16 H 33 N Ir Cl Cl Ir C 16 H 33 N 2 2 N N SO 3 O 3 S C 16 H 33 N Ir 2 N N Na hdpIr^Cl [hdpIr-] C 16 H 33 N N 1) LDA, -78C 2) BrC 15 H 31 THF overnight AgPF 6 , DCM/MeOH hdp SO 3 SO 3 2Na 136 prepared via standard Nonoyama reaction, filtered from 2-ethoxyethanol solution, and used for the next step without further purification. A1 The hdpIr^Cl was mixed with bathophenanthrolinedisulfonic acid disodium salt hydrate and AgPF 6 in a solution of DCM and MeOH (1:1), and stirred at room temperature for 3h. The crude product was purified by liquid chromatography to yield 70% of the [hdpIr-] as a greenish greasy solid. A2.2. Characterization of [hdpIr-] A2.2.1. NMR characterization 1 H NMR of hdp(400 MHz, CDCl 3 , 298 K): δ = 8.68(d, 1H, py), δ = 7.91(m, 2H, ph), δ = 7.72(m, 2H, py), δ = 7.29(m, 2H, ph), δ = 7.21(d, 1H, py), δ = 2.66(t, 2H, alkyl), δ = 1.26(s, 28H, alkyl), δ = 0.88(s, 3H, Me). 1 H NMR of [hdpIr-](400 MHz, CDCl 3 , 298 K): δ = 8.48(d, 2H, bp), δ = 8.28(d, 2H, bp), δ = 8.10(m, 6H, hdp), δ = 7.90-7.60(m, 12H, bp+hdp), δ = 6.90(d, 4H, hdp), δ = 6.25(d, 2H, hdp), δ = 2.46(t, 4H, alkyl), δ = 1.35(s, 56H, alkyl), δ = 0.90(s, 6H, Me). Table A2-1. Photophysical properties of complexes [hdpIr-]. Absorption (in MeOH) PL(in MeOH) Q.Y . (%) k r (x 10 5 s -1 ) k nr (x 10 5 s -1 ) λ max (nm), (ε, 10 3 M -1 cm -1 ) Emission 77 K λ 0-0 (nm), [τ (µs)] Emission 298 K λ max (nm), [τ (µs)] [hdpIr-] 368(sh) 560, [4.1] 620, [0.5] 22 4.3 15.1 137 A2.2.2. Photophysical characterization. Appendix 2 Reference 1. M. Nonoyama, Bull. Chem. Soc. Jpn. 1974, 47, 767. 400 600 800 0.0 0.2 0.4 0.6 0.8 1.0 [hdpIr-] abs. r.t. 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Abstract (if available)

Abstract Organic Light Emitting Devices (OLEDs) displays and lightings are now available on the market of consumable optic electronics. The materials used in the devices must meet certain stability and energetic requirements. This dissertation takes effort in developing new generation of organometallic phosphorescent complexes with phosphino‐carbon chelated ligands for potential applications in OLEDs. Chapter 1 shows an overview on the development of the OLEDs, and illustrates the mechanisms of the OLEDs from materials and device structure aspects. Chapter 2 describes the design, synthesis and photophysical characterization of an Os(II) complex with high energy bis‐pincer chelated ligand. A thorough illustration is presented for the phosphorescence mechanism of the complex. Chapter 3 focuses on a series of heteroleptic Ir(III) complexes with diphenylphosphinoaryl chelated ligands. The synthesis and photophysical properties of the complexes are examined to illustrate the potential application of Ir(III)‐phosphino^carbon coordinates as chromophores of the heteroleptic Ir(III) complexes. In the end, a series of high energy aluminum and zinc based complexes are studied in Chapter 4 for their potential applications as high energy host materials for blue and green dopant materials.

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Asset Metadata

Creator Liu, Yifei (author)

Core Title Synthesis and photophysical study of phosphorescent hetero-cyclometalated organometallic complexes involving phosphino-carbon ligands

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 09/05/2016

Defense Date 07/21/2014

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

Tag OAI-PMH Harvest,OLEDs,phosphino‐carbon ligands,phosphorescent organometallic complexes

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Mark E. (committee chair), Brutchey, Richard L. (committee member), Haas, Stephan W. (committee member)

Creator Email caaus2012@gmail.com,yiifeiliu@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-468714

Unique identifier UC11286911

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Legacy Identifier etd-LiuYifei-2878.pdf

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Document Type Dissertation

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Rights Liu, Yifei

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Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

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