University of Southern California Dissertations and Theses

Synthesis, structural and photophysical characterization of phosphorescent three-coordinate Cu(I)-N-heterocyclic carbene complexes

(USC Thesis Other)

Synthesis, structural and photophysical characterization of phosphorescent three-coordinate Cu(I)-N-heterocyclic carbene complexes

PDF

Download a page range

Download transcript

Copy asset link

Request this asset

Transcript (if available)

Content SYNTHESIS, STRUCTURAL AND PHOTOPHYSICAL CHARACTERIZATION OF PHOSPHORESCENT THREE-COORDINATE CU(I)-N-HETEROCYCLIC CARBENE COMPLEXES by Valentina Krylova 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 2013 Copyright 2013 Valentina Krylova ii Dedication Dedicated to my son Victor iii Acknowledgements I would like to express my special appreciation and thanks to my advisor Professor Mark Thompson for giving me the opportunity to be a part of his research group and constant support, caring, patience and guidance throughout my graduate school career. Thank you for being a great mentor. I am fortunate to have had Prof. Richard Brutchey, Prof. Stephan Haas, Prof. Curt Wittig and Prof. Barry Thompson serve on my PhD Guidance Committee. Thank you for your time and valuable feedback. Special thanks to Prof. Peter Djurovich for motivation, encouragement and for willing to direct your insightful intellect towards my research problems. I have been lucky to work in such a dynamic and stimulating research environment. I’d like to thank all past and present Thompson group members for sharing their experience, help in the lab and friendship. I must say special thank you to Dr. Tim Stewart, Dr. Matthew Whited and Dr. Ralf Haiges for X-ray crystallography. I would like to thank Dr. Brian Conley, Prof. Travis Williams, Markus Leitl and Prof. Hartmut Yersin for fruitful collaboration. I also would like to acknowledge Jacob Aronson for participating in my research project. You have been very helpful when I most needed a research partner. iv I would like to express my greatest gratitude to Judy Fong and Michele Dea for their support and help. I’m grateful to my parents and grandparents for supporting me in various ways. Above all I would like to thank my husband, Ivan for his true love. This would not have been possible without you. v Table of Contents Dedication ...........................................................................................................................ii Acknowledgements ..........................................................................................................iii List of Tables ......................................................................................................................viii List of Figures ....................................................................................................................ix Abstract ................................................................................................................................xv CHAPTER 1. Introduction .............................................................................................1 1.1. Photophysics of phosphorescent Cu(I) complexes ....................................................1 1.1.1. Cationic Cu(I) complexes .......................................................................................1 1.1.2. Neutral Cu(I) complexes .........................................................................................5 1.1.3. Three-coordinate Cu(I) complexes .........................................................................7 1.2. Application of Cu(I) complexes in Organic Light-Emitting Diodes .........................9 1.3. N-Heterocyclic carbene ligands .................................................................................14 1.4. NHC-Cu(I) complexes ...............................................................................................18 Chapter 1 References .............................................................................................................19 CHAPTER 2. Synthesis and structural studies of three-coordinate (NHC)Cu(N^N) complexes ............................................................................................24 2.1. Introduction ................................................................................................................24 2.2. Synthesis ....................................................................................................................24 2.2.1. Synthesis of complexes ...........................................................................................24 2.2.2. Synthesis of precursors ...........................................................................................28 2.3. X-ray crystallography ................................................................................................29 2.4. NMR studies ..............................................................................................................36 2.5. Experimental section ..................................................................................................49 Chapter 2 References .............................................................................................................65 CHAPTER 3. Photophysical and computational studies of (IPr)Cu(N^N) complexes ................................................................................................68 3.1. Introduction ................................................................................................................68 3.2. Results and discussions ..............................................................................................70 3.2.1. Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) studies .....................................................................................................................70 3.2.2. Photophysical and electrochemical properties of cationic complex vi [(IPr)Cu(phen)]OTf.................................................................................................75 3.2.3. Photophysical properties of neutral complexes (IPr)Cu(N^N) ...............................78 3.2.3.1. Absorption spectra ............................................................................................78 3.2.3.2. Emission properties ...........................................................................................80 3.2.3.2.1. Photophysical properties at 77 K. Excited state distortions in three-coordinate Cu(I) complexes. ..............................................................80 3.2.3.2.2. Emission in solution at RT ..........................................................................84 3.2.3.2.3. Solid state emission at RT and 77 K. Thermally activated emission ......................................................................................................86 3.3. Conclusion .................................................................................................................90 3.4. Experimental section ..................................................................................................90 Chapter 3 References .............................................................................................................91 CHAPTER 4. Modulation of photophysical properties of (NHC)Cu(N^N) complexes through NHC ligand variation ...................................95 4.1. Introduction ................................................................................................................95 4.2. Results and discussions ..............................................................................................97 4.2.1. Photophysical and computational studies ..............................................................97 4.3. Conclusion .................................................................................................................106 4.4. Experimental section ..................................................................................................107 Chapter 4 References .............................................................................................................108 CHAPTER 5. Phosphorescent three-coordinate (NHC)Cu(N^N) complexes as host materials for organic light-emitting diodes .............................111 5.1. Introduction ................................................................................................................111 5.2. Results and discussions ..............................................................................................113 5.2.1. Electrochemistry .....................................................................................................113 5.2.2. (IPr)Cu(pybim) as host material for OLED ............................................................115 5.2.3. Lower energy emitting Cu(I)-based host for phosphorescent Ir(III) dopant ......................................................................................................................118 5.2.3.1. Quenching studies .............................................................................................120 5.2.3.2. OLED performance ...........................................................................................125 5.3. Conclusion .................................................................................................................127 5.4. Experimental section ..................................................................................................127 Chapter 5 References .............................................................................................................129 BIBLIOGRAPHY .............................................................................................................133 APPENDIX 1. Synthesis and photophysical characterization of four- coordinate Cu(I) complexes (P^P)Cu(N^N) ..............................................................143 A1.1. Introduction ................................................................................................................143 A1.2. Results and discussion ...............................................................................................144 A1.2.1....Synthesis and crystal structure .............................................................................144 vii A1.2.2. Photophysical studies ...........................................................................................146 A1.2.3. DFT calculations ..................................................................................................149 A1.3. Experimental section ..................................................................................................150 Appendix 1 References ..........................................................................................................159 APPENDIX 2. Luminescent two-coordinate NHC-Cu(I) complexes .................161 A2.1. Introduction ................................................................................................................161 A2.2. Synthesis and emission properties .............................................................................162 A2.3. Experimental section ..................................................................................................168 Appendix 2 References ..........................................................................................................172 viii List of Tables Table 1.1. Photophysical data (CH 2 Cl 2 , RT) for selected cationic Cu(I) complexes. Ligand definitions are given in Figure 1.1 ..........................................................2 Table 1.2. Photophysical properties of neutral ( R PN)Cu(phosphine) complexes .................5 Table 1.3. Average percent σ and π contributions of  oi E  and  oi E  to the orbital interaction energy, and average percent contributions due to π*-backdonation and π-donation to  oi E  . Averages are based on data for 36 M-NHC model complexes compiled in Ref. 63 ................................................................................................................18 Table 2.1. Selected bond lengths (Å) and angles (deg) for complexes 3.1, 3.2, 3.4-3.7 ....................................................................................................................................34 Table 2.2. Selected bond lengths (Å) and angles (deg) for complexes 4.1-4.4 .....................36 Table 3.1. Lowest energy transitions for complexes 3.1–3.7 determined from TD- DFT calculations ....................................................................................................................74 Table 3.2. Summary of photophysical properties of 3.1 .......................................................78 Table 3.3. Photophysical properties of complexes 3.2–3.7 in solution .................................86 Table 3.4. Photophysical properties of complexes 3.2–3.7 in solid state .............................87 Table 4.1. Photophysical properties of complexes 4.1-4.3 ...................................................100 Table 4.2. Lowest energy transitions for complexes 4.1-4.3 obtained from TD- DFT calculations ....................................................................................................................103 Table 5.1. Summary of electrochemical properties of (IPr)Cu(fppz) an (IPr)Cu(pybim). The HOMO energies calculated from the oxidation potentials are shown in parenthesis ........................................................................................................115 Table 5.2. Performance of OLED devices D5.1, D5.2, D5.3 ................................................118 Table A1.1. Selected bond lengths (Å) and angles (deg) for complexes A1.1-A1.3 ............146 Table A1.2. Summary of photophysical properties of complexes A1.1-A1.4 ......................149 Table A1.3. Crystal data and structure refinement for A1.1-A1.3 ........................................158 Table A2.1. Photophysical properties of complexes A2.4-A2.8 in the solid state ................167 ix List of Figures Figure 1.1. Structures of ligands used to prepare luminescent Cu(I) complexes ..................2 Figure 1.2. Excited state (Jablonski) diagram for four-coordinate cationic [Cu(N^N) 2 ] + complexes .........................................................................................................3 Figure 1.3. Diagram representing relative order of the lowest excited states for a series of isostructural Cu(I), Ag(I) and Au(I) complexes ......................................................7 Figure 1.4. Molecular structures for the three-coordinate Cu(I)-complexes ........................8 Figure 1.5. Molecular structure of [Cu(PNP- t Bu)] 2 complex and its emission spectra recorded at 100 K and 295 K .....................................................................................13 Figure 1.6. General structure of NHC ligands ......................................................................15 Figure 1.7. Singlet and triplet carbenes.................................................................................15 Figure 1.8. Electronic stabilization of NHC .........................................................................16 Figure 1.9. The three bonding contributions to the NHC-M bond .......................................17 Figure 1.10. Selected example of NHC-Cu(I) complexes ....................................................19 Figure 2.1. Synthesis of three-coordinate (NHC)Cu(N^N) complexes ................................25 Figure 2.2. The structures of cationic three-coordinate (NHC)Cu(N^N) complexes ..............................................................................................................................26 Figure 2.3. The structures and numbering scheme for neutral three-coordinate (NHC)Cu(N^N) complexes (N^N = azolate ligand) .............................................................26 Figure 2.4. The structures of neutral three-coordinate (NHC)Cu(N^N) complexes (N^N = bis(pyrazolyl)borate or bis(pyridyl)borate ligand) ...................................................27 Figure 2.5. Synthesis of (NHC)CuCl precursors ..................................................................28 Figure 2.6. Synthesis of (PzI-3,5Me)CuCl ...........................................................................28 Figure 2.7. ORTEP representation of (IPr)Cu(phen) + cation (3.1), (IPr)Cu(pybim) (3.6) and (IPr)Cu(beniq) (3.7). Hydrogen atoms are omitted for clarity ...............................31 Figure 2.8. ORTEP representation of (IPr)Cu(fpyro) (3.2), (IPr)Cu(fppz) (3.4) and (IPr)Cu(fpta) (3.5). Hydrogen atoms are omitted for clarity ..........................................32 x Figure 2.9. Crystal packing diagram showing π interactions in dimmers of 3.4 and 3.5 ....................................................................................................................................33 Figure 2.10. ORTEP representation of (IPr)Cu(dp-BMe 2 ) (4.1), (BzI- 3,5Me)Cu(dp-BMe 2 ) (4.2), (PzI-3,5Me)Cu(dp-BMe 2 ) (4.3) and (IPrBIAN)Cu(dp- BMe 2 ) (4.4). Hydrogen atoms are omitted for clarity ............................................................34 Figure 2.11. Schematic diagram showing the binding mode of the chelating dp-BMe 2 ligand in 4.1-4.4 .....................................................................................................35 Figure 2.12. 1 H NMR spectra of (IPr)CuCl in CDCl 3 at 25 °C ............................................37 Figure 2.13. 600 MHz 1 H NMR spectra of (IPr)Cu(fpyro) (3.2) in CDCl 3 at temperatures ranged from 25 °C to -40 °C ............................................................................38 Figure 2.14. 600 MHz 1 H NMR spectra of (IPr)Cu(ppz) (3.3) in CDCl 3 at temperatures ranged from 25 °C to -40 °C ............................................................................39 Figure 2.15. 400 MHz 1 H NMR spectra of (IPr)Cu(fppz) (3.4) in CDCl 3 at temperatures ranged from 25 °C to -50 °C ............................................................................39 Figure 2.16. 600 MHz 1 H NMR spectra (aromatic region) of complex 3.5 in CDCl 3 at temperatures ranged from 40 °C to -40 °C. At -40 °C two sets of signals correspond to the two interconverting major (closed symbols) and minor (open symbols) states .......................................................................................................................40 Figure 2.17. 600MHz 1 H NMR spectra (aliphatic region) of complex 3.5 in CDCl 3 at temperatures ranged from 40 to -40°C ...................................................................41 Figure 2.18. 1 H NMR spectra (aromatic region) of (IPr)Cu(fpta) (3.5) in CD 2 Cl 2 at 25 °C and -45 °C ................................................................................................................42 Figure 2.19. Qualitative energy diagram for rotation about C NHC –Cu bond in complexes 3.2–3.4 in CDCl 3 (red solid line) and 3.5 (blue dashed line), along with that for complex 3.6 (black dotted line). The dihedral angle between the planes is defined by the (N NHC , N NHC , C NHC ) and (Cu, N py , N az ) atoms ...............................................44 Figure 2.20. 1 H NMR spectra (aromatic region) of complex 3.6 in CDCl 3 at 25 °C and -50 °C ..............................................................................................................................44 Figure 2.21. 1 H NMR spectra of complexes 3.2-3.6 in benzene-d 6 at 25°C .........................46 Figure 2.22. 1 H NMR spectra of (IPr)Cu(fpyro) (3.2) in acetone-d 6 at 25°C and - 40°C .......................................................................................................................................46 Figure 2.23. 1 H NMR spectra of (IPr)Cu(ppz) (3.3) in acetone-d 6 at 25°C and - xi 40°C .......................................................................................................................................47 Figure 2.24. 1 H NMR spectra of (IPr)Cu(fppz) (3.4) in acetone-d 6 at 25°C and - 40°C .......................................................................................................................................47 Figure 2.25. 1 H NMR spectra of (IPr)Cu(fpta) (3.5) in acetone-d 6 at 25°C and - 40°C .......................................................................................................................................48 Figure 2.26. 1 H NMR spectra of (IPr)Cu(pybim) (3.6) in acetone-d 6 at 25°C .....................48 Figure 3.1. Molecular structures of (NHC)Cu(N^N) complexes discussed in Chapter 3 ................................................................................................................................69 Figure 3.2. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) plots and energies (eV) (A) and triplet spin density surface (B) for 3.1 ..................................................................................................................71 Figure 3.3. The frontier orbitals for 3.2 and calculated HOMO and LUMO energies (eV) for complexes 3.2–3.5 .....................................................................................72 Figure 3.4. HOMO, HOMO-1, HOMO-2 and LUMO plots and energies (eV) for 3.6 and 3.7 ..............................................................................................................................73 Figure 3.5. Optimized triplet geometries and triplet spin density contour plots (isovalue: 0.004 e a 0 -3 ) for complexes 3.1, 3.3 and 3.6. Hydrogen atoms are omitted for clarity ..................................................................................................................75 Figure 3.6. Absorption (RT, CH 2 Cl 2 ) spectrum of complex 3.1 ...........................................76 Figure 3.7. Excitation (closed symbols) and emission spectra (open symbols) of complex 3.1 ............................................................................................................................77 Figure 3.8. Cyclic voltammogram of (IPr)Cu(phen)]OTf (3.1) ............................................78 Figure 3.9. Absorption (RT, CH 2 Cl 2 ) spectrum of complexes (IPr)Cu(fpyro) (3.2), (IPr)Cu(ppz) (3.3), (IPr)Cu(fppz) (3.4) and (IPr)Cu(fpta) (3.5) (left) and ligand precursors (right) .........................................................................................................79 Figure 3.10. Absorption (RT, CH 2 Cl 2 ) spectra of complexes (IPr)Cu(pybim) (3.6) and (IPr)Cu(beniq) (3.7) ........................................................................................................80 Figure 3.11. Emission spectra of complexes 3.2-3.6 in methylcyclohexane at 77K (left) and 3.7 in 2-methyltetrahydrofuran at 77K (right) .......................................................81 Figure 3.12. Influence of the nature of ligand chromophore and excited state distortions on the emission energies in three-coordinate (IPr)Cu(N^N) complexes .............83 xii Figure 3.13. Emission spectra of (IPr)Cu(pybim) (3.6) in solution at RT ............................84 Figure 3.14. Emission spectra of complexes 3.2-3.5 in cyclohexane at RT .........................85 Figure 3.15. Solid state emission spectra of complexes 3.2-3.5 at RT (top) and at 77 K (bottom) .........................................................................................................................87 Figure 3.16. Solid state emission spectra of complexes 3.6 (left) and 3.7 (right) at RT (open symbols) and at 77 K (closed symbols) .................................................................88 Figure 3.17. Mechanism of thermally activated delayed fluorescence .................................89 Figure 4.1. Molecular structures of complexes 4.1-4.4 ........................................................96 Figure 4.2. Absorption spectra ((RT, CH 2 Cl 2 ) of complexes 4.1-4.3 and (IPr)Cu(pz 2 -BH 2 ) ...................................................................................................................97 Figure 4.3. Absorption spectra of precursors (IPr)CuCl, (BzI-3,5Me)CuCl and (PzI-3,5Me)CuCl in CH 2 Cl 2 and Na[dp-BMe 2 ] in acetonitrile .............................................97 Figure 4.4. Emission spectra of complexes 1-3 in the solid state at RT (closed symbols) and at 77 K (open symbols)....................................................................................99 Figure 4.5. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) plots and energies for 4.1-4.3. Hydrogen atoms are omitted for clarity ..................................................................................................................102 Figure 4.6. Optimized triplet geometries and triplet spin density contour plots (isovalue: 0.004 e a 0 -3 ) for complexes 4.1-4.3. Hydrogen atoms are omitted for clarity .....................................................................................................................................104 Figure 4.7. Absorption (RT, CH 2 Cl 2 ) and emission (77 K, 2-MeTHF) spectra of complex 4.4 ............................................................................................................................105 Figure 4.8. HOMO and LUMO plots and energies (A) and triplet spin density contour plots (isovalue: 0.004 e a 0 -3 ) (B) for 4.4. Hydrogen atoms are omitted for clarity .....................................................................................................................................106 Figure 5.1. Molecular structures of host and emitter materials discussed in Chapter 5 ................................................................................................................................113 Figure 5.2. CV (left) and DPV (right) traces of (IPr)Cu(fppz) .............................................114 Figure 5.3. CV (left) and DPV (right) traces of (IPr)Cu(pybim) ..........................................114 Figure 5.4. Energy level diagrams and EL spectra for device D5.1 (top) and D5.2 xiii (bottom)..................................................................................................................................116 Figure 5.5. Current density (black) and luminance (red) vs voltage plot (top) and quantum efficiency vs current density plot (bottom) of devices D5.1 and D5.2 ...................117 Figure 5.6. Schematic potential energy surfaces diagram illustrating electronic relaxation and structural distortion in Cu(I) complexes ........................................................119 Figure 5.7. Absorption (dash line) and emission (solid line) spectra of Ir(ppy) 3 and (IPr)Cu(fppz) in solution (toluene) .................................................................................121 Figure 5.8. Emission spectra of mixture of Ir(ppy) 3 (C=4.43 µM) and (IPr)Cu(fpyro) (C=7.36 mM) in toluene at RT at different excitation wavelengths .............122 Figure 5.9. Stern-Volmer plot for quenching of Ir(ppy) 3 emission by (IPr)Cu(fppz)..........................................................................................................................123 Figure 5.10. Emission spectra of doped films (300 Å, 7 wt%) of Ir(ppy) in (IPr)Cu(fppz) host and CBP host and Ir(ppy) 3 emission in solution .....................................125 Figure 5.11. Energy level diagram (A), EL spectra (B), current density (black) and luminance (red) vs voltage plot (C) and quantum efficiency vs current density plot (D) of device D5.4 ..........................................................................................................126 Figure A1.1. Structures of complexes A1.1-A1.4.................................................................144 Figure A1.2. Reaction scheme for the synthesis of A2.1-A2.4. ...........................................145 Figure A1.3. ORTEP diagram of complexes A1.1-A1.3. Hydrogen atoms and co- crystallized solvent molecule (for A1.2) have been omitted for clarity ................................145 Figure A1.4. Absorption spectra of complexes A1.1-A1.4 (left) and N^N ligand precursors (right) in CH 2 Cl 2 ...................................................................................................147 Figure A1.5. Emission spectra of complexes A1.1-A1.4 in 2-MeTHF at 77 K (left) and in doped (2 wt%) PMMA film at RT .....................................................................148 Figure A1.6. LUMO (A), HOMO (B) and triplet spin-density (C) obtained by density functional (DFT) calculations of A1.1-A1.4. Plot contours are shown at an isovalue of 0.004 electrons au -3 .........................................................................................149 Figure A2.1. Molecular structures of complexes A2.1-A2.10 ..............................................162 Figure A2.2. Synthesis of (BzI-3,5Me) 2 Cu + (A2.1) .............................................................163 Figure A2.3. Synthesis of (PzI-3,5Me) 2 Cu + (A2.2) ..............................................................163 xiv Figure A2.4. Synthesis of (IPr)Cu(BzI-3,5Me) + (A2.3) .......................................................164 Figure A2.5. Solid-state emission of A2.1 at RT and at 77 K ..............................................164 Figure A2.6. Synthesis of complexes A2.6-A2.8 .................................................................165 Figure A2.7. Solid-state emission of A2.4-A2.7 at RT.........................................................166 Figure A2.8. Solid-state emission of A2.9 and A2.10 at RT ................................................167 xv Abstract Copper(I) complexes can provide inexpensive and environmentally friendly alternative to traditionally utilized phosphorescent materials that have relied largely on heavy metal (Pt(II), Ir(III), Ru(II), Os(II)) complexes. Nonetheless the copper(I)-based materials have not been as well-developed as their third row counterparts. To date research has focused principally on four-coordinate Cu(I) complexes. A new family of phosphorescent three- coordinate (NHC)Cu(N^N) complexes, where NHC is a monodentate N-heterocyclic carbene ligand and N^N is a neutral or monoanionic chelating ligand is presented in this work. Tunable photophysical properties and broad potential for variation in design make these materials potentially useful for a variety of photophysical applications. Chapter 1 highlights the state-of-the-art in development of phosphorescent mononuclear Cu(I) complexes and their application in organic light-emitting diodes (OLED). Design and synthesis of (NHC)Cu(N^N) complexes is described in Chapter 2. The solid state structures of several examples were established by X-ray crystallography and conformational behavior in solution was investigated by variable temperature 1 H NMR spectroscopy. The geometrical preferences revealed by single crystal XRD analysis correlate well with the NMR data. Photophysical properties of (NHC)Cu(N^N) complexes are discussed in Chapters 3 and 4. Excited state properties strongly depend on electronic and steric properties of the ligands, degree of metal participation, and on experimental conditions. Wide-range xvi emission color tuning from deep blue (418 nm) to red (650 nm) can be achieved by modification of chelating (N^N) ligand or alternatively through variation of carbene ligand (NHC). Thus either NHC or N^N ligand can act as ligand chromophore. Excited state lifetimes are in the 10’s of microsecond range and quantum yields are up to 80% in solid state and up to 17% in fluid solution. Significant emission enhancement observed in rigid environment suggests the presence of non-radiative relaxation decays associated with geometrical rearrangements in the excited state. Photophysical studies performed at room temperature and at 77 K suggest participation of a higher-lying state with faster radiative rate at room temperature. In Chapter 5 (NHC)Cu(N^N) complexes are evaluated as potential hosts for OLEDs. Experimental and theoretical studies indicate that the triplet energies of these (NHC)Cu(N^N) complexes are often higher than can be predicted from emission spectra observed experimentally. A host:guest system was realized where Ir(III)-based emitter was doped into Cu(I)-based host material that phosphoresces at lower energies than the dopant, however does not quench dopant emission via triplet energy transfer. 1 CHAPTER 1. Introduction 1.1. Photophysics of phosphorescent Cu(I) complexes Phosphorescent Cu(I) complexes have garnered a great deal of attention as inexpensive and abundant alternatives to phosphorescent materials based on heavy-metal complexes. Although copper is a first row transition metal and does not possess a strong “heavy atom” effect, it has been shown that Cu(I) complexes can exhibit efficient phosphorescence at room temperature. 1.1.1. Cationic Cu(I) complexes The Cu(I) metal ion possesses a 3d 10 -electronic configuration and has a preference to form 4-coordinate tetrahedral complexes. Since the first report of room temperature phosphorescence from Cu(I) compounds by McMillin, 1 research has focused principally on cationic complexes [Cu(N^N) 2 ] + , where N^N denotes a chelating bis-imine ligand, typically a substituted phenanthroline (Figure 1.1). The photophysical properties of these Cu(I) complexes have been thoroughly investigated. 2-4 The absorption spectra of these complexes display LC bands in the UV and weaker MLCT transitions throughout the whole visible region (up to 700 nm). Low energy MLCT states occur due to low oxidation potential of Cu(I) and easily accessible low-lying empty π * orbitals of phenanthroline ligands. Consequently, [Cu(N^N) 2 ] + complexes exhibit broad orange-red MLCT emission. 2 Figure 1.1. Structures of ligands used to prepare luminescent Cu(I) complexes. Table 1.1. Photophysical data (CH 2 Cl 2 , RT) for selected cationic Cu(I) complexes. Ligand definitions are given in Figure 1.1. Entry Complex PL λ max , (nm) τ (µs) Φ (%) Ref 1 [Cu(dmp) 2 ]BF 4 730 0.09 0.04 5 2 [Cu(dbp) 2 ]BF 4 725 0.15 0.09 ″ 3 [Cu(dpp) 2 ]PF 6 690 0.24 0.087 6 4 [Cu(dtbp) 2 ][B(C 6 H 5 ) 4 ] 599 3.26 5.6 7 5 [Cu(dmp)(dtbp)]PF 6 646 0.73 1.0 8 6 [Cu(phen)(POP)]BF 4 700 0.19 0.18 9 7 [Cu(dmp)(POP)]BF 4 570 14.3 15 ″ 8 [Cu(dnbp)(POP)]BF 4 560 16.1 16 ″ 9 [Cu(dmp)(PPh 3 ) 2 ]BF 4 a 560 0.33 0.14 10 10 [Cu(mdpbq)(PPh 3 ) 2 ]BF 4 625 6.9 10 11 11 [Cu(mdpbq)(POP)]BF 4 631 0.75 0.76 ″ 12 [Cu(bimda)(POP)]BF 4 b 470 2.1 8.0 12 13 [Cu(Et-tbzimda)(POP)]BF 4 b 525 16.1 34 ″ 10 [Cu(dppb)(POP)]BF 4 494 2.44 2.0 13 a In methanol; b In PMMA doped films. 3 Low-lying metal-centered d-d exited states in d 6 (Ir(III)) or d 8 (Pt(II)) complexes can provide efficient pathways for non-radiative decay. The complete filling of d-orbitals in Cu(I) complexes eliminates the possibility of d-d electronic transitions. Yet, their emission efficiency in solution is generally low due to non-radiative deactivation pathways caused by structural reorganization and exciplex formation. PL quantum yields in dichloromethane at room temperature usually do not exceed 0.01 and emission lifetimes are in the range of 70 to 1000 ns. In coordinating solvents these complexes are effectively non-emissive. The quenching processes in 4-coordinate cationic Cu(I) complexes have been well established. 14-18 The proposed mechanism is shown in Figure 1.2. Figure 1.2. Excited state (Jablonski) diagram for four-coordinate cationic [Cu(N^N) 2 ] + complexes. Upon MLCT excitation, the tetrahedral d 10 Cu(I) center is effectively oxidized to d 9 4 Cu(II). The d 9 Cu(II) metal center undergoes a Jahn-Teller flattening distortion. This opens an additional coordination site and a 5-coordinate exciplex may be formed upon coordination of nucleophiles such as solvent molecules or counter-ions. These two processes, significant structural change between the ground and excited states and additional ligand pick up, promote non-radiative decay and significantly quench emission. The generally accepted approach to alleviate this problem and to maximize radiative decay is to increase the steric bulk of ligands, in particular, by using 2,9- substituted phenanthroline derivatives or bulky phosphine ligands. 7,9,19,20 The replacement of one bis-imine ligand in [Cu(N^N) 2 ] + with a bulky phosphine ligand led to higher luminescence efficiency, longer excited state lifetimes and also changed the emission color. 9 The most commonly used phosphines are: triphenylphosphine (PPh 3 ), 1,2-bis(diphenylphosphino)benzene (dppb) and bis[2-(diphenylphosphino)phenyl]ether (POP). 10,13,21-23 Photophysical properties for selected Cu(I)-phosphine complexes are given in Table 1.1. The choice of both bis-imine and phosphine ligands in heteroleptic [Cu(N^N)(P^P)] + complexes is crucial. The highly emissive [Cu(dmp)(POP)] + (dmp = 2,9-dimethyl-1,10-phenanthroline) shows broad emission (λ max = 570 nm) with a quantum yield of 15% and 14.3 µs excited state lifetime in degassed dichloromethane . By contrast, [Cu(phen)(POP)] + , bearing unsubstituted phenanthroline, has weak (Φ = 0.18%) red-shifted (λ max = 700 nm) emission with a shorter (0.19 µs) lifetime. A similar effect was observed for [Cu(dmp)(PPh 3 )] + where a chelating POP ligand is replaced with two PPh 3 ligands. The emitting state in these mixed-ligand Cu(I) complexes is assigned as 3 MLCT. Similar to [Cu(N^N) 2 ] + complexes, [Cu(N^N)(P^P)] + 5 complexes are also prone to solvent-induced exciplex quenching, 24 which is less pronounced for bulkier phosphine ligands, i.e., POP. 9,21 However, the combination of steric constraints and electron-withdrawing character of the phosphine ligand leads to an increase in the MLCT energy compared to [Cu(N^N) 2 ] + complexes. As a result, non- radiative decay is reduced and the emission is blue-shifted. 1.1.2. Neutral Cu(I) complexes Unlike cationic analogs, the photophysical properties of mononuclear neutral Cu(I) compounds have not been well investigated. Following the successful utilization of phosphine ligands to improve photoluminescence in cationic Cu(I) complexes, neutral analogs often possess phosphine ligands as well. Various chelating monoanionic ligands complete the tetrahedral coordination environment. 25-29 Peters and co-workers used substituted amidophosphine ligands ( R PN) and obtained neutral ( R PN)Cu(phosphine) complexes with remarkable luminescent properties (Table 1.2). 25 Table 1.2. Photophysical properties of neutral ( R PN)Cu(phosphine) complexes. These compounds have long phosphorescent lifetimes (16-150 µs) in benzene at RT and quantum efficiencies ranging from 16% to 70%, which are among the highest achieved Complex PL λ max (nm) Φ (%) τ (µs) [PN]Cu(PPh 3 ) 2 504 56 20.2(1) [PN]Cu(PMe 3 ) 2 497 21 22.3(7) [PN]Cu(dppe) 2 534 32 16.3(3) [ Me PN]Cu(PPh 3 ) 2 498 70 6.7(1) [ CF 3 PN]Cu(PPh 3 ) 2 552 16 150(3) 6 for Cu(I) complexes in solution. Their emission spectra are broad and featureless with maxima ranging from 480 to 552 nm. Interestingly, the calculated HOMO and LUMO of ( R PN)Cu(phosphine) have very little metal character and are localized on the amidophosphine ligand, suggesting the presence of an ILCT transition. The contribution from orbitals on copper appears in HOMO-1 and HOMO-2. Thus, the lowest excited state may have some mixed-in MLCT character. The importance of participation of metal d-orbitals to achieve effective SOC and to induce efficient phosphorescence in organometallic compounds was discussed earlier in this chapter. This was also demonstrated based on the combination of experimental and computational results in a recent study of isoleptic d 10 -complexes. 28 The photophysical properties of a series of neutral [NN]M(I)[PP] complexes (NN = functionalized 2-pyridyl pyrrolide; PP = POP or PPh 3 ; M(I) = Cu(I), Ag(I), Au(I)) were systematically investigated. One would expect that the radiative rates would decrease from the third row gold(I) to first row copper(I) complexes due to weakening of the “heavy-atom” effect. Interestingly, while the Cu(I) complex showed efficient phosphorescence in solution at RT with a quantum yield and excited state lifetime of 34% and 23.9 µs, respectively, an isostructural Ag(I) complex showed very weak dual emission - fluorescence (0.75%, 176 ps) and phosphorescence (0.95%, 138 µs) and the Au(I) analog displayed weak phosphorescence (3.7%, 124 µs). Based on these data, the radiative rates of these isostructural Cu(I), Ag(I) and Au(I) complexes are 1.4·10 4 , 6.88·10 1 and 2.98·10 2 s -1 respectively. Theoretical calculations reveal that the lowest excited state of the Cu(I) complex is a triplet LC with mixed in singlet and triplet MLCT character, while the lowest excited states of Ag(I) and Au(I) 7 complexes are pure triplet LC, with no contribution from MLCT (Figure 1.3). Figure 1.3. Diagram representing relative order of the lowest excited states for a series of isostructural Cu(I), Ag(I) and Au(I) complexes. [Adapted with permission from Ref. 28. Copyright 2011 American Chemical Society.] Thus, it was shown that the direct contribution from a metal d orbital in the lowest excited state is more important than the “external” heavy-atom effect in order to attain fast intersystem crossing rates and therefore high phosphorescent radiative rates. This result reveals the potential of a “light” first-row Cu(I) metal for OLED applications. 1.1.3. Three-coordinate Cu(I) complexes Although the d 10 Cu(I) center has a strong preference to form four-coordinate tetrahedral complexes, it is possible to obtain mononuclear three-coordinate Cu(I) compounds; however examples of luminescent complexes are rare. Unlike four-coordinate tetrahedral Cu(I) compounds that undergo a flattening distortion and exciplex quenching in the excited state, the three-coordinate geometry eliminates the possibility of a flattening distortion in the excited state, though exciplex formation and different type of distortion 8 are still possible. Barakat et al. have described a Jahn–Teller induced, Y- to T-shape distortion in the excited state of three-coordinate, trigonal planar Au(I) complexes. 30 A similar type of distortion may possibly occur in three-coordinate Cu(I) complexes. Recent reports of luminescent three-coordinate Cu(I) complexes suggest that these compounds can exhibit efficient phosphorescence at RT. 31,32 Figure 1.4. Molecular structures for the three-coordinate Cu(I)-complexes. The choice of ligands is crucial in order to stabilize three-coordinate Cu(I) structure. Peters and Lotito reported a series of brightly phosphorescent Cu(I) arylamidophosphine complexes [PP]Cu[N], where [PP] is a bidentate phosphine ligand or two monodentate phosphines and [N] is a terminal diphenylamido ligand (Figure 1.4, (A)). 32 Solution quantum yields in methylcyclohexane range from 11% to 24% and excited state lifetimes are short and do not vary substantially within the series (1.7 to 3.17 µs) with the 9 exception of [N] = carbazole (11.7 µs). The calculated (DFT) HOMO is predominantly localized on diphenylamide with little contribution from copper and the LUMO is phosphine-based. This suggests that emission properties can be tuned by variation of both ligands. When the two PPh 3 groups in green-emitting (521 nm) (Ph 3 P) 2 Cu(NPh 2 ) were replaced with one chelating POP ligand in (POP)Cu(NPh 2 ), a 42 nm red-shift of the spectrum (563 nm) was observed. On the other hand, replacement of NPh 2 with carbazole gave a blue-emitting (461 nm) complex (Ph 3 P) 2 Cu(cbz). Introduction of electron- donating (-CH 3 ) or electron-withdrawing substituents (-F) in the para position of the phenyl groups of the diphenylamido ligand or on the phosphine did not lead to such drastic shifts, but allowed for fine tuning of the emission color with shifts up to 25 nm. Hashimoto and co-workers isolated a series of three-coordinate complexes (dtpb)CuX (dtpb = 1,2-bis(o-ditolylphosphino)benzene, X = Cl, Br, I) (Figure 1.4, (B)) through the use of the bulky dtpb ligands. 31 These complexes phosphoresce in the green region of the spectrum (517-534 nm) and are highly emissive both in dichloromethane solution (Φ = 43-60%) and when doped into a mCP film (57-68%). Excited state lifetimes are relatively short ranging from 3.2 to 6.5 µs. Therefore, the triplet radiative rates of these complexes are up to 9·10 5 s -1 , which is comparable to the radiative rate of Ir(ppy) 3 . 1.2. Application of Cu(I) complexes in Organic Light-Emitting Diodes The advantages of Organic Light-Emitting Diode (OLED) technology over existing display and lighting technologies are apparent. However, cost is a major challenge for the current generation of OLEDs to overcome in order to become more commercially 10 viable. To date, the best emitters for OLEDs are cyclometallated complexes of expensive metals (Ir and Pt). It is desirable to reduce the cost of materials, especially for large-scale applications (e.g. lighting), without compromising device performance. A potential solution is to develop inexpensive phosphorescent materials based on abundant metals. In search for inexpensive phosphors for OLEDs a great deal of attention has focused on Cu(I)-based materials. In view of their promising photoluminescent properties, heteroleptic [Cu(N^N)(P^P)] + complexes (Table 1.1) have been used in electroluminescent devices. 11-13,33-38 These complexes exhibit much higher quantum yields in a film than in solution due to the lack of solvent-induced exciplex quenching. For example, the PL efficiency of [Cu(dnbp)(POP)]BF 4 (dnbp = 2,9-di-n-butyl-1,10-phenanthroline) at room temperature in degassed dichloromethane and 20 wt% doped PMMA film are 16% and 69% respectively. 33 The emission maximum in a film is blue-shifted (519 nm) compared to solution (560 nm). Based on its high PL quantum yield in a PMMA film, [Cu(dnbp)(POP)]BF 4 was utilized as a phosphorescent dopant in a green OLED. When [Cu(dnbp)(POP)]BF 4 was doped in commonly used host materials, the PL efficiencies of the films varied depending on the triplet energy of the host materials. 38 In TAZ (E T = 2.6 eV) the PL quantum yield was 16%, while in high triplet energy hosts, i.e. 2,6-dicarbazolo-1,5-pyridine (PYD2) (E T = 2.93 eV), it reached 56%. It was found that the triplet energy of green-emitting [Cu(dnbp)(POP)]BF 4 is 2.72 eV, which is higher than that of Ir(ppy) 3 and blue-emitting FIrpic. Thus, a high triplet energy host is required in order to achieve a high efficiency OLED with this Cu(I) complex. A device comprised of 11 [Cu(dnbp)(POP)]BF 4 doped in a PVK host at 16 wt% exhibited a current efficiency of 10.5 cd/A. 33 When PYD2 was used as the host and bis(2- (diphenylphosphino)phenyl)ether oxide (DPEPO) (E T = 3 eV) as the electron blocking layer, EQEs as high as 15% (49.5 Cd/A) were realized. 38 A non-doped single layer (ITO/[Cu(dnbp)(POP)]BF 4 /metal cathode) LEC-type device (light-emitting electrochemical cell) achieved efficiencies up to 56 cd/A, corresponding to an EQE of 16% 39 . By extending the conjugation of the bis-imine ligand of the Cu(I)-dopant complex, Zhang et al. reported the first red-emitting Cu(I)-based OLEDs. 11 Complexes [Cu(mdpbq)(PPh 3 ) 2 ]BF 4 and [Cu(mdpbq)(POP) 2 ]BF 4 (mdpbq = 3,3’-methylen-4,4’- diphenyl-2,2’-biquinoline) (Table 1.1, entries 10,11) showed efficient PL in 20 wt% doped PMMA film with quantum yields of 0.56 and 0.43 and emission maxima of 606 nm and 617 nm, respectively. After optimization, the OLED with a configuration ITO/PEDOT/[TCCz:Cu(mdpbq)(POP)](BF 4 )(15 wt%)/TPBI/LiF/Al (TCCz = N-(4- (carbazol-9-yl)phenyl)-3,6-bis(carbazol-9-yl)carbazole; TPBI = 1,3,5-tris-(N- phenylbenzimidazol-2-yl)benzene) exhibited a current efficiency of 6.4 cd/A and an external quantum efficiency of 4.5% at 1.0 mA/cm 2 . Su and co-workers were able to shift the emission of [Cu(N^N)(P^P)] + complexes into the blue-green region of the spectrum. 12 Utilization of diimine ligands with high-lying π * orbitals: 1H,1’H- [2,2’]biimidazole (bimda) and 1-ethyl-2-thiazol-4-yl-1H-benzoimidazole (Et-tbzimda) (Table 1.1, entries 12,13) selectively increased the LUMO energy level and did not affect the HOMO. The blue-emitting OLED (EL λ max = 480 nm) with [Cu(bimda)(POP)]BF 4 12 doped at 23 wt% into CBP, showed a maximum brightness of 2850 cd/m 2 and low efficiency roll-off at high current density. The efficiency was 1.47 cd/A at low current and only dropped by 10% at 100 mA/cm 2 . The green-emitting [Cu(Et-tbzimda)(POP)]BF 4 -based device (EL λ max = 532 nm) had a maximum brightness of 2320 cd/m 2 and higher efficiency of 2.35 cd/A due to higher PL quantum yield of [Cu(Et-Tbzimda)(POP)]BF 4 34% in 5 wt% doped PMMA film vs. 8% for [Cu(Bimda)(POP)](BF 4 ). However, the longer PL lifetime (16.1 vs. 2.1 µs) leads to a significant efficiency drop, up to 72% of its maximum at 100 mA/cm 2 . 12 The successful applications of cationic Cu(I) complexes as phosphorescent emitters in OLEDs stimulated recent interest in neutral Cu(I) complexes. Current results clearly indicate that the development of charge-neutral Cu(I) complexes is crucial for achieving high phosphorescent radiative rates from these materials and thus for realization of highly efficient OLEDs that would be able to compete with Ir-based devices. Several examples of efficient OLEDs containing neutral Cu(I) complexes have been reported. 38,40,41 Liu et al. demonstrated a new approach for utilizing metal-based phosphors in OLEDs, that involves in situ formation of a Cu(I) phosphor by codeposition of CuI and 3,5- bis(carbazol-9-yl)pyridine (mCPy). 40 PL quantum yields of codeposited films were as high as 64% depending on the molar ratio of CuI and mCPy. It was determined that the emitting species in CuI:mCPy thin films is the dimeric complex [CuI(mCPy) 2 ] 2 . OLEDs made using codeposited CuI:mCPy films as emissive layers exhibited green electroluminescence at 530 nm and reached maximum luminance of 9700 cd/m 2 and EQE of 4.4%. 13 A highly efficient OLED was realized by Eastman Kodak using the neutral bis(bis(diisobutylphenylphosphino)amido) dicopper(I) complex [Cu(PNP- t Bu)] 2 (Figure 1.5). 41-43 Figure 1.5. Molecular structure of [Cu(PNP- t Bu)] 2 complex and its emission spectra recorded at 100 K and 295 K. [Reprinted with permission from Ref. 41. Copyright 2010 American Chemical Society.] This complex has a very high PL quantum yield of 57% in solution at RT and lifetime of 11.5 µs. Variable temperature photophysical studies suggest that [Cu(PNP- t Bu)] 2 exhibits thermally activated (E-type) delayed fluorescence at RT. Such behavior, resulting from a small S 1 -T 1 energy gap, is quite common for Cu(I) complexes. 29,44-46 If E-type delayed fluorescence occurs in a system, then a blue shift of the emission maximum and increase in the emission lifetime is observed experimentally as temperature increases. The excited state lifetime of a [Cu(PNP- t Bu)] 2 film (doped at 1 wt% into 1,1-bis(4-(N,N-di-p-tolylamino)phenyl)cyclohexane (TAPC)) at 80K is 336 µs and corresponds to purely triplet emission. Such a long lifetime of a phosphorescent dopant is a drawback for OLED application, because it will lead to 14 efficiency roll-off at high current density due to saturation effects. At RT, the emission lifetime of the [Cu(PNP- t Bu)] 2 film is reduced to 11.5 µs due to thermal equilibration of the S 1 and T 1 states that lie approximately 786 cm -1 apart. An OLED fabricated with a luminescent layer composed of CBP/TAPC(25%)/[Cu(PNP- t Bu)] 2 (0.2%) had a maximum external quantum efficiency of 16.1% (47.5 cd/A) at 0.01 mA/cm 2 which dropped to 10.9% (32 cd/A) at 1 mA/cm 2 . This performance is among the best achieved with Cu(I)-based devices. An impressive example of utilization of three-coordinate Cu(I) complexes in OLEDs was demonstrated by Hashimoto et al. 31 The fabricated OLED with (dtpb)CuBr (Figure 1.4, (B)) doped into mCP at 10 wt% as an emitting layer showed a maximum external quantum efficiency of 21.3% and current efficiency of 65.3 cd/A, records for OLEDs fabricated with Cu(I) complexes. However significant efficiency roll-off was observed at higher current densities. Nonetheless, this example demonstrates the high potential of three-coordinate Cu(I) complexes for OLED applications. 1.3. N-Heterocyclic carbene ligands N-heterocyclic carbenes (NHC) (also known as Wanzlick-Arduengo carbenes) is a well established class of ligands in the field of organometallic chemistry. 47 Since the first report of stable crystalline NHC by Arduengo 48 in 1991 a remarkable amount of work has focused on developing new synthetic methods and understanding their properties and reactions. 49-51 This research is driven mainly by their applications in transition metal catalysis. 52 NHCs are considered as analogs of phosphine ligands, however they often 15 outperform phosphines in catalytic reactions. 49,53,54 N-heterocyclic carbene is a nitrogen-containing heterocycle that possesses a divalent carbon (Figure 1.6). The carbon atom has a sp 2 - hybridized orbital (called σ) and a p orbital (p π ) that are orthogonal to each other. The ground state of NHC is singlet with σ 2 electronic configuration. The triplet state with configuration σ 1 p π 1 is close in energy and thus singlet triplet energy gap has to be considered when evaluating stability of a free NHC (Figure 1.7). 55 The singlet-triplet gap of stable NHC is above 65 kcal/mol. 56,57 In this case the singlet state is favored. Otherwise the triplet state will be stabilized and NHC will be prone to dimerization. 58 Figure 1.7. Singlet and triplet carbenes. The singlet state of NHC is stabilized through stabilization of σ orbital by: 1)inductive electron-withdrawing effect of the electronegative nitrogen atoms and 2)N(p π )→C(p π ) donation from nitrogen to carbon (Figure 1.8). 47,59 Figure 1.6. General structure of NHC ligands. 16 Figure 1.8. Electronic stabilization of NHC. It is interesting to note that unsaturated imidazol-2-ylidene NHC are more stable that unsaturated imidazolin-2-ylidene NHC. 55,60 However annulation of imidazole ring, e.g. to form benzimidazol-2-ylidene, leads to smaller singlet-triplet gap and therefore less stable free NHC. 61 Same effect was observed when nitrogen atoms were incorporated in azabenzimidazole-2-ylidene. The nature of substituents at nitrogen atoms have little influence on the electronic properties of NHC ligands, however they are responsible for steric control. NHC are very attractive ligands for various transition metal applications because not only they are highly electronically and sterically tunable ligands, but also their sterics can be modified with or without affecting electronic properties. NHCs are known to form strong bond with transition metals 62 and often act as ancillary ligands. They are neutral two electron donors. Metal-NHC bond mainly results from a σ-donation of singlet carbene carbon electron pair into empty the metal d orbital (Figure 1.9, a). A smaller contribution (10-30%) to M-NHC bond comes from π-backdonation from filled metal d orbital into p π orbital of NHC (Figure 1.9, b). In some cases (e.g. in 17 electron deficient complexes) π-donation from NHC to metal also contribute to M-NHC bond (Figure 1.9, c). Figure 1.9. The three bonding contributions to the NHC-M bond. [Reprinted with permission from Ref. 62. Copyright 2008 Elsevier B.V.] While σ-donation is usually dominates, relative contributions of the three interactions depend on NHC electronic properties as well as the metal d-electron count. Table 1.3 shows average input of σ and π interactions to the orbital interaction energy based on computational study of 36 M-NHC complexes. 63 18 Table 1.3. Average percent σ and π contributions of  oi E  and  oi E  to the orbital interaction energy, and average percent contributions due to π*-backdonation and π- donation to  oi E  . Averages are based on data for 36 M-NHC model complexes compiled in Ref. 63. [Reprinted with permission from Ref. 62. Copyright 2008 Elsevier B.V] 1.4. NHC-Cu(I) complexes To date NHC-Cu(I) complexes have been principally developed as catalysts for a variety of key chemical transformations. 64,65 Copper forms very strong bonds with NHC ligands. Boehme and Frenking determined from theoretical calculations at CCSD(T) level that NHC-Cu(I) dissociation energy of (imidazol-2-ylidene)copper(I) chloride is 67.4 kcal/mol. 66 They concluded that electrostatic interaction between the positively charge metal ion and the lone pair electrons on the NHC ligand and covalent interaction of the σ-lone pair of the NHC ligand and a metal (d z 2 +s) hybridized orbital have main contributions to NHC-CuCl bond, while π-backdonation is small. The coordination chemistry of NHC-Cu(I) complexes is rich. 67 Four-, three-, two- coordinate mononuclear complexes as well as multinuclear clusters have been reported. These include neutral and cationic compounds. Selected examples of NHC-Cu(I) complexes are shown in Figure 1.10. 19 Figure 1.10. Selected example of NHC-Cu(I) complexes. There are only a few examples of applications of NHC-Cu(I) complexes in fields other than catalysis. 68 Teyssot et al. evaluated a series of (imidazol-2-ylidene)copper(I) chloride and (imidazolin-2-ylidene)copper(I) chloride derivatives as antitumor agents. 69 Matsumoto and co-workers reported photophysical studies of luminescent binuclear NHC-Cu(I) complex. 70 Chapter 1 References (1) Blaskie, M. W.; McMillin, D. R. Inorg. Chem. 1980, 19, 3519. (2) McMillin, D. R.; McNett, K. M. Chem. Rev. 1998, 98, 1201. (3) Scaltrito, D. V.; Thompson, D. W.; O'Callaghan, J. A.; Meyer, G. J. Coord. Chem. Rev. 2000, 208, 243. (4) Lavie-Cambot, A.; Cantuel, M.; Leydet, Y.; Jonusauskas, G.; Bassani, D. M.; McClenaghan, N. D. Coord. Chem. Rev. 2008, 252, 2572. (5) Eggleston, M. K.; McMillin, D. R.; Koenig, K. S.; Pallenberg, A. J. Inorg. 20 Chem. 1997, 36, 172. (6) Miller, M. T.; Gantzel, P. K.; Karpishin, T. B. Inorg. Chem. 1999, 38, 3414. (7) Green, O.; Gandhi, B. A.; Burstyn, J. N. Inorg. Chem. 2009, 48, 5704. (8) Miller, M. T.; Gantzel, P. K.; Karpishin, T. B. J. Am. Chem. Soc. 1999, 121, 4292. (9) Cuttell, D. G.; Kuang, S. M.; Fanwick, P. E.; McMillin, D. R.; Walton, R. A. J. Am. Chem. Soc. 2002, 124, 6. (10) Rader, R. A.; McMillin, D. R.; Buckner, M. T.; Matthews, T. G.; Casadonte, D. J.; Lengel, R. K.; Whittaker, S. B.; Darmon, L. M.; Lytle, F. E. J. Am. Chem. Soc. 1981, 103, 5906. (11) Zhang, Q.; Ding, J.; Cheng, Y.; Wang, L.; Xie, Z.; Jing, X.; Wang, F. Adv. Funct. Mater. 2007, 17, 2983. (12) Zhang, L.; Li, B.; Su, Z. J. Phys. Chem. C 2009, 113, 13968. (13) Moudam, O.; Kaeser, A.; Delavaux-Nicot, B.; Duhayon, C.; Holler, M.; Accorsi, G.; Armaroli, N.; Seguy, I.; Navarro, J.; Destruel, P.; Nierengarten, J.-F. Chem. Commun. 2007, 3077. (14) Stacy, E. M.; McMillin, D. R. Inorg. Chem. 1990, 29, 393. (15) Chen, L. X.; Jennings, G.; Liu, T.; Gosztola, D. J.; Hessler, J. P.; Scaltrito, D. V.; Meyer, G. J. J. Am. Chem. Soc. 2002, 124, 10861. (16) Chen, L. X.; Shaw, G. B.; Novozhilova, I.; Liu, T.; Jennings, G.; Attenkofer, K.; Meyer, G. J.; Coppens, P. J. Am. Chem. Soc. 2003, 125, 7022. (17) Everly, R. M.; McMillin, D. R. Photochem. Photobiol. 1989, 50, 711. (18) Vorontsov, I. I.; Graber, T.; Kovalevsky, A. Y.; Novozhilova, I. V.; Gembicky, M.; Chen, Y.-S.; Coppens, P. J. Am. Chem. Soc. 2009, 131, 6566. (19) Accorsi, G.; Armaroli, N.; Duhayon, C.; Saquet, A.; Delavaux-Nicot, B.; Welter, R.; Moudam, O.; Holler, M.; Nierengarten, J.-F. Eur. J. Inorg. Chem. 2010, 164. (20) Gothard, N. A.; Mara, M. W.; Huang, J.; Szarko, J. M.; Rolczynski, B.; 21 Lockard, J. V.; Chen, L. X. J. Phys. Chem. A 2012, 116, 1984. (21) Kuang, S. M.; Cuttell, D. G.; McMillin, D. R.; Fanwick, P. E.; Walton, R. A. Inorg. Chem. 2002, 41, 3313. (22) McCormick, T.; Jia, W. L.; Wang, S. N. Inorg. Chem. 2006, 45, 147. (23) Smith, C. S.; Branham, C. W.; Marquardt, B. J.; Mann, K. R. J. Am. Chem. Soc. 2010, 132, 14079. (24) Palmer, C. E. A.; McMillin, D. R. Inorg. Chem. 1987, 26, 3837. (25) Miller, A. J. M.; Dempsey, J. L.; Peters, J. C. Inorg. Chem. 2007, 46, 7244. (26) Manbeck, G. F.; Brennessel, W. W.; Eisenberg, R. Inorg. Chem. 2011, 50, 3431. (27) Crestani, M. G.; Manbeck, G. F.; Brennessel, W. W.; McCormick, T. M.; Eisenberg, R. Inorg. Chem. 2011, 50, 7172. (28) Hsu, C.-W.; Lin, C.-C.; Chung, M.-W.; Chi, Y.; Lee, G.-H.; Chou, P.-T.; Chang, C.-H.; Chen, P.-Y. J. Am. Chem. Soc. 2011, 133, 12085. (29) Czerwieniec, R.; Yu, J.; Yersin, H. Inorg. Chem. 2011, 50, 8293. (30) Barakat, K. A.; Cundari, T. R.; Omary, M. A. J. Am. Chem. Soc. 2003, 125, 14228. (31) Hashimoto, M.; Igawa, S.; Yashima, M.; Kawata, I.; Hoshino, M.; Osawa, M. J. Am. Chem. Soc. 2011, 133, 10348. (32) Lotito, K. J.; Peters, J. C. Chem. Commun. 2010, 46, 3690. (33) Zhang, Q. S.; Zhou, Q. G.; Cheng, Y. X.; Wang, L. X.; Ma, D. G.; Jing, X. B.; Wang, F. S. Adv. Mater. 2004, 16, 432. (34) Su, Z. S.; Che, G. B.; Li, W. L.; Su, W. M.; Li, M. T.; Chu, B.; Li, B.; Zhang, Z. Q.; Hu, Z. Z. Appl. Phys. Lett. 2006, 88. (35) Che, G.; Su, Z.; Li, W.; Chu, B.; Li, M.; Hu, Z.; Zhang, Z. Appl. Phys. Lett. 2006, 89. (36) Si, Z.; Li, J.; Li, B.; Liu, S.; Li, W. J. Lumin. 2008, 128, 1303. (37) Wada, A.; Zhang, Q.; Yasuda, T.; Takasu, I.; Enomoto, S.; Adachi, C. 22 Chem. Commun. 2012, 48, 5340. (38) Zhang, Q.; Komino, T.; Huang, S.; Matsunami, S.; Goushi, K.; Adachi, C. Adv. Funct. Mater. 2012, 22, 2327. (39) Zhang, Q. S.; Zhou, Q. G.; Cheng, Y. X.; Wang, L. X.; Ma, D. G.; Jing, X. B.; Wang, F. S. Adv. Funct. Mater. 2006, 16, 1203. (40) Liu, Z.; Qayyum, M. F.; Wu, C.; Whited, M. T.; Djurovich, P. I.; Hodgson, K. O.; Hedman, B.; Solomon, E. I.; Thompson, M. E. J. Am. Chem. Soc. 2011, 133, 3700. (41) Deaton, J. C.; Switalski, S. C.; Kondakov, D. Y.; Young, R. H.; Pawlik, T. D.; Giesen, D. J.; Harkins, S. B.; Miller, A. J. M.; Mickenberg, S. F.; Peters, J. C. J. Am. Chem. Soc. 2010, 132, 9499. (42) Harkins, S. B.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 2030. (43) Harkins, S. B.; Mankad, N. P.; Miller, A. J. M.; Szilagyi, R. K.; Peters, J. C. J. Am. Chem. Soc.y 2008, 130, 3478. (44) Blasse, G.; McMillin, D. R. Chem. Phys. Lett. 1980, 70, 1. (45) Breddels, P. A.; Berdowski, P. A. M.; Blasse, G. J. Chem. Soc. Farad T II 1982, 78, 595. (46) Kirchhoff, J. R.; Gamache, R. E.; Blaskie, M. W.; Delpaggio, A. A.; Lengel, R. K.; McMillin, D. R. Inorg. Chem. 1983, 22, 2380. (47) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39. (48) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361. (49) Peris, E. Top. Organomet. Chem. 2007, 21, 83. (50) Benhamou, L.; Chardon, E.; Lavigne, G.; Bellemin-Laponnaz, S.; Cesar, V. Chem. Rev. (Washington, DC, U. S.) 2011, 111, 2705. (51) Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.; Cavallo, L. Coord. Chem. Rev. 2009, 253, 687. (52) Glorius, F. Top. Organomet. Chem. 2007, 21, 1. (53) Poater, A.; Ragone, F.; Giudice, S.; Costabile, C.; Dorta, R.; Nolan, S. P.; 23 Cavallo, L. Organometallics 2008, 27, 2679. (54) Crabtree, R. H. J. Organomet. Chem. 2005, 690, 5451. (55) Heinemann, C.; Muller, T.; Apeloig, Y.; Schwarz, H. J. Am. Chem. Soc. 1996, 118, 2023. (56) Heinemann, C.; Thiel, W. Chem. Phys. Lett. 1994, 217, 11. (57) Dixon, D. A.; Arduengo, A. J. J. Phys. Chem. 1991, 95, 4180. (58) Hahn, F. E.; Wittenbecher, L.; Le, V. D.; Frohlich, R. Angew. Chem., Int. Ed. 2000, 39, 541. (59) Macdougall, P. J.; Bader, R. F. W. Can J Chem 1986, 64, 1496. (60) Boehme, C.; Frenking, G. J. Am. Chem. Soc. 1996, 118, 2039. (61) Ullah, F.; Bajor, G.; Veszpremi, T.; Jones, P. G.; Heinicke, J. W. Angew. Chem., Int. Ed. 2007, 46, 2697. (62) Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.; Cavallo, L. Coord. Chem. Rev. 2009, 253, 2784. (63) Jacobsen, H.; Correa, A.; Costabile, C.; Cavallo, L. J. Organomet. Chem. 2006, 691, 4350. (64) Diez-Gonzalez, S.; Nolan, S. P. Synlett 2007, 2158. (65) Diez-Gonzalez, S.; Nolan, S. P. Aldrichim. Acta 2008, 41, 43. (66) Boehme, C.; Frenking, G. Organometallics 1998, 17, 5801. (67) Lin, J. C. Y.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A.; Hwang, W. S.; Lin, I. J. B. Chem. Rev. 2009, 109, 3561. (68) Mercs, L.; Albrecht, M. Chem. Soc. Rev. 2010, 39, 1903. (69) Teyssot, M. L.; Jarrousse, A. S.; Chevry, A.; De Haze, A.; Beaudoin, C.; Manin, M.; Nolan, S. P.; Diez-Gonzalez, S.; Morel, L.; Gautier, A. Chem.- Eur. J. 2009, 15, 314. (70) Matsumoto, K.; Matsumoto, N.; Ishii, A.; Tsukuda, T.; Hasegawa, M.; Tsubomura, T. Dalton Trans. 2009, 6795. 24 CHAPTER 2. Synthesis and Structural Studies of Three-Coordinate (NHC)Cu(N^N) complexes 2.1. Introduction Due to the filled d shell (d 10 configuration) Cu(I) has a preference for tetrahedral coordination geometry in order to minimize electrostatic repulsions. As a result, to date the majority of reported luminescent Cu(I)-based materials are four-coordinate complexes bearing phosphine and/or bisimine ligands. 1-3 Nevertheless it is possible to stabilize the three-coordinate Cu(I) center through a judicious choice of ligands, in particular by using sterically bulky ligands. 4,5 Our design strategy for the three-coordinate complexes is to use N-heterocyclic carbene (NHC) ligands. It is known that NHCs can form mononuclear three-coordinate complexes with Cu(I). 6-8 However, to the best of our knowledge there were no prior reports of luminescence form these complexes. In search for efficient phosphorescent Cu(I)-based emitters we synthesized a family of (NHC)Cu(N^N) complexes, where N^N is a chelating neutral or monoanionic ligand (Figure 2.1). Synthetic routs to a large variety of NHC ligands have been reported. 9 Thus (NHC)Cu(N^N) complexes offer great potential for ligand variation. 2.2. Synthesis 2.2.1. Synthesis of complexes General scheme for the synthesis of three-coordinate (N-heterocyclic carbene)-Cu(I) 25 complexes (NHC)Cu(N^N) is shown in Figure 2.1. Figure 2.1. Synthesis of three-coordinate (NHC)Cu(N^N) complexes. All complexes were prepared from a (NHC)CuCl precursor. Reaction of (NHC)CuCl and neutral chelating ligand in the presence of silver triflate in tetrahydrofuran gave cationic complexes. Neutral complexes were obtained by deprotonation of chelating ligand with sodium hydride in THF followed by addition of a (NHC)CuCl precursor. Pure products were isolated as solids in good yields (38-86%). On the basis of the molecular structure all synthesized complexes can be divided into three classes, namely cationic complexes 3.1 and 3.10 (Figure 2.2), neutral complexes 3.2-3.9 with conjugated azolate type ligands (Figure 2.3) and complexes 4.1-4.6 bearing non-conjugated anionic borate-based ligand (Figure 2.4). 26 Figure 2.2. The structures of cationic three-coordinate (NHC)Cu(N^N) complexes. Figure 2.3. The structures and numbering scheme for neutral three-coordinate (NHC)Cu(N^N) complexes (N^N = azolate ligand). The majority of complexes discussed in this thesis bear 1,3-bis(2,6- diisopropylphenyl)imidazol-2-ylidene (IPr) as monodentate NHC ligand. This ligand is widely used to prepare catalytically active Cu(I)-based complexes. Therefore the (IPr)CuCl precursor can be easily prepared on a large scale via literature 27 procedure 10,11 or obtained from commercial sourses. Also, isopropyl groups are crucial for stability of complexes. In general, complexes discussed in this thesis that possess 2,6-diisopropylphenyl groups on NHC ligand have better stability in air compared to complexes with 3,5-dimethylphenyl substituted NHC. All complexes (with exception of 4.3) are indefinitely stable to air in the solid state. In solution IPr- based complexes are stable for several hours in air and over several days under N 2 . Complexes 4.2 and 4.3 readily decompose in solution in air, but stable for hours under N 2 . All complexes are sensitive to acid, e.g. they decompose in chloroform due to trace acid, but stable in acid-free solution. Complexes 3.2, 3.4-3.6, 4.1 and 4.2 can be sublimed at 225-265 ºC (10 -6 Torr). Figure 2.4. The structures of neutral three-coordinate (NHC)Cu(N^N) complexes (N^N = bis(pyrazolyl)borate or bis(pyridyl)borate ligand). 28 2.2.2. Synthesis of precursors (NHC)CuCl precursors (with exception of (PzI-3,5Me)CuCl) were prepared according to the procedure developed by Nolan 10,12 and Buchwald 11 groups. General scheme for the synthesis of (NHC)CuCl complexes is shown in Figure 2.5. Figure 2.5. Synthesis of (NHC)CuCl precursors. Figure 2.6. Synthesis of (PzI-3,5Me)CuCl. The synthesis of (PzI-3,5Me)CuCl is shown in Figure 2.6. To the best of our knowledge this NHC ligand has not been previously reported in peer reviewed literature. It was prepared form N 2 ,N 3 -bis(3,5-dimethylphenyl)pyrazine-2,3-diamine that was treated with 29 triethylorthoformate and HCl to give ethoxyimidazolidine (PzI-3,5Me)OEt. The latter was used to generate the carbene in situ upon heating, which was subsequently converted to (PzI-3,5Me)CuCl. 2.3. X-ray crystallography X-ray diffraction analyses were performed for complexes 3.1, 3.2, 3.4-3.7 and 4.1-4.4. Crystal structures for are shown in Figure 2.7 (3.1, 3.6 and 3.7), Figure 2.8 (3.2, 3.4, 3.5) and Figure 2.10 (4.1-4.4). Crystallographic data is given in Table 2.1 (3.1, 3.2, 3.4-3.7) and Table 2.2 (4.1-4.4). All compounds are monomeric, three-coordinate structures with the geometry around the copper center characterized as distorted trigonal planar. The values for the bond length between copper and the carbon atom of the NHC ligand are within the 1.862(2)-1.895(2) Å range that is typical for Cu(I)-NHC complexes. 12-14 Complex 3.1 has an almost ideal Y-shaped copper center. The Cu-N bond distances are nearly equal (Cu(1)-N(3) = 2.039(2) Å and Cu(1)-N(4) = 2.052(2) Å), as are the C-Cu-N bond angles (C(1)-Cu(1)-N(4) = 136.64(11) o and C(1)-Cu(1)-N(3) = 141.49(11) o ). The geometry of 3.1 is planar and the sum of the bond angles around the copper atom is 359.38 o . Surprisingly, the N^N ligand and the imidazolylidene ring of the carbene ligand are coplanar. This geometry is unusual for three-coordinate copper(I)-NHC complexes, as the ancillary ligand is usually oriented perpendicular to the copper-imidazolylidene plane. 7,8 In 3.1 the dihedral angle between the Cu-imidazolylidene plane and the N-Cu-N 30 chelate is 8º and the phenanthroline ligand is slightly tilted (9.5º) from the N-Cu-N plane. These distortions are likely due to steric constraints, since hydrogen atoms at the 2- and 9- positions of phenanthroline ligand are directed towards the center of phenyl rings of the NHC ligand. The importance of sterics is highlighted in our attempts to prepare an analog of 3.1 using bathocuproine (bcp), a ligand with methyl substituents at the 2-, 9-positions of phenanthroline. Under the reaction conditions described above only Cu(bcp) 2 + and (IPr) 2 Ag + were obtained as major products. The coordination environment around copper in 3.6 and 3.7 is distorted from a Y-shape due to the asymmetric anionic 2-(2-pyridyl)benzimidazole and 1-(1H-benzimidazol-2- yl)isoquinoline ligands. The Cu-N az (imidazolyl) bond (1.9227(18) Å (3.6); 1.9443(17) (3.7)) is shorter than the Cu-N py (pyridyl) bond (2.2907(18) Å (3.6), 2.1935(17) (3.7)), and the C NHC -Cu-N az and C NHC -Cu-N py bond angles are 154.24(8) o and 126.82(8) o , respectively for 3.6 and 151.38(8) o and 129.70(8) o for 3.7. As in 3.1, the plane of chelating ligand in 3.6 and 3.7 is oriented coplanar to the copper-imidazolylidene plane despite of the steric encumbrance between the phenyl rings of the NHC ligand and hydrogen atoms at the H6 position on the pyridyl ring and H7’ position on the benzimidazolide moiety. The dihedral angles between planes defined by the N NHC , N NHC , C NHC atoms and Cu, N py and N az atoms are close to 9º in both 3.6 and 3.7. 31 Figure 2.7. ORTEP representation of (IPr)Cu(phen) + cation (3.1), (IPr)Cu(pybim) (3.6) and (IPr)Cu(beniq) (3.7). Hydrogen atoms are omitted for clarity. Surprisingly, in contrast to complexes 3.1, 3.6 and 3.7 the pyridyl-azolate ligands in complexes 3.2, 3.4 and 3.5 have a perpendicular orientation with respect to the imidazolylidene ring of the carbene ligand. The dihedral angles between planes defined by the N NHC , N NHC and C NHC atoms and Cu, N py and N az atoms are 69.15° in 3.2, 79.98° in 3.4 and 81.73° in 3.5. There are two unique molecules in the unit cell of 3.2; however, both have similar geometric parameters. The N^N ligand is tilted in order to accommodate the CF 3 -group at the 5’-position of pyrrolide moiety, which sterically conflicts with the isopropyl groups of the IPr ligand and causes the geometry around copper to distort toward trigonal pyramidal. The copper atom is 0.121 Å above a plane defined by the coordinating C NHC , N py and N az atoms. Complex 3.2, similar to complexes 3.6 and 3.7 bearing asymmetric 32 chelating ligands, exhibits a distorted T-shaped geometry. The C NHC –Cu–N py and C NHC – Cu–N az bond angles in 3.2 are 123.93(5)° and 154.34(5)° respectively. These values are close to those found in complexes 3.6 and 3.7. A similar T-shaped structure was observed by Caulton and co-workers in a three-coordinate Cu(I) (fpyro)Cu(NCMe) complex. 15 As the NCMe ligand imparts a minimal steric constrain in this complex, the T-shaped geometry was attributed to the differing σ-donor characteristics of the pyridyl and pyrrolide moieties. In 3.4 and 3.5, however, the coordination environment around the copper atom is nominally Y-shaped. The C NHC –Cu–N py and C NHC –Cu–N az bond angles in both 3.4 (139.27(9)° and 139.29(9)°) and 3.5 (140.22(6)° and 138.44(6)°) are almost identical. The coordination geometry is planar as the sum of the bond angles around the copper atom is 359.99° for 3.4 and 359.90° for 3.5. Figure 2.8. ORTEP representation of (IPr)Cu(fpyro) (3.2), (IPr)Cu(fppz) (3.4) and (IPr)Cu(fpta) (3.5). Hydrogen atoms are omitted for clarity. 33 In the crystal lattice molecules 3.4 and 3.5 pack as dimers with two pyridyl-azolate ligands stacked above each other in a head-to-tail fashion (Figure 2.9). Figure 2.9. Crystal packing diagram showing π interactions in dimmers of 3.4 and 3.5. The intermolecular separation between the planes of pyridyl-azolate ligands in a dimer is 3.31 Å for 3.4 and 3.28 Å for 3.5, indicating the presence of weak π–π interactions. All three complexes 3.2, 3.4 and 3.5 have Cu–pyridyl distances that are longer than the corresponding, formally anionic Cu–azolate bond lengths (3.2, Cu–N py = 2.1346(11) Å, Cu–N az = 1.9552(11) Å; 3.4, Cu–N py = 2.0805(18) Å, Cu–N az = 1.9697(19) Å; 3.5, Cu– N py = 2.0979(13) Å, Cu–N az = 1.9925(14) Å). The Cu–N az bond lengths increase in the order pyrrolide (3.2) < pyrazolide (3.4) < triazolide (3.5), which correlates with the σ-donor ability of azolate ligand. 34 Table 2.1. Selected bond lengths (Å) and angles (deg) for complexes 3.1, 3.2, 3.4-3.7. C NHC -Cu Cu-N py a Cu-N az b C NHC -Cu-N py c C NHC -Cu-N az d N py -Cu-N az e 3.1 1.884(3) 2.039(2) 2.052(2) 141.49(11) 136.64(11) 81.24(11) 3.2a 1.8828(12) 2.1346(11) 1.9552(11) 123.93(5) 154.34(5) 80.14(4) 3.2b 1.8804(12) 2.1196(11) 1.9658(10) 127.23(5) 150.17(5) 79.66(4) 3.4 1.862(2) 2.0805(18) 1.9697(19) 139.27(9) 139.29(9) 81.43(8) 3.5 1.8838(14) 2.0979(13) 1.9925(14) 140.22(6) 138.44(6) 81.22(5) 3.6 1.877(2) 2.2907(18) 1.9227(18) 126.82(8) 154.24(8) 78.94(7) 3.7 1.888(2) 2.1935(17) 1.9443(17) 129.70(8) 151.38(8) 78.39(7) a Cu-N(3) for 3.1. b Cu-N(4) for 3.1. c C NHC -Cu-N(3) for 3.1. d C NHC -Cu-N(4) for 3.1. e N(3)-Cu-N(4) for 3.1. In complexes 4.1-4.4 (Figure 2.10) the chelating ligand is not planar and the Cu-N-C-B-C-N ring formed upon coordination of chelating dp-BMe 2 ligand adopts a boat-shaped conformation similar to that reported in metal complexes bearing analogous ligands (Figure 2.11). Figure 2.10. ORTEP representation of (IPr)Cu(dp-BMe 2 ) (4.1), (BzI-3,5Me)Cu(dp- BMe 2 ) (4.2), (PzI-3,5Me)Cu(dp-BMe 2 ) (4.3) and (IPrBIAN)Cu(dp-BMe 2 ) (4.4). Hydrogen atoms are omitted for clarity. The environment around copper in complexes 4.1-4.4 can be described as Y-shaped. The compounds display planar geometry with the sum of bond angles around copper close to 360º (359.98º in 4.1, 359.72º in 4.2, 358.64º in 4.3, 359.87º in 4.4). 35 Figure 2.11. Schematic diagram showing the binding mode of the chelating dp-BMe 2 ligand in 4.1-4.4. Complex 4.2 has two unique structures in the unit cell that have similar geometric parameters. Complex 4.1 has a mirror plane of symmetry that contains C NHC , Cu and B atoms. Therefore the Cu-N py bond lengths (2.0288(15) Å) and the C NHC -Cu-N py angles (132.78(4)º) are equal for both pyridyl rings. In complexes 4.2-4.4 the Cu-N py ’ and Cu-N py ’’ bond lengths are slightly different (4.2: 1.9929(16) Å and 1.9997(16) Å; 4.3: 2.010(9) Å and 2.014(9) Å; 4.4: 2.0081(9) Å and 2.0224(9) Å) and so are C NHC -Cu-N py ’ and C NHC -Cu-N py ’’ angles (4.2: 134.32(7)º and 129.27(7)º; 4.3: 135.0(6)º and 128.1(6)º; 4.4: 133.84(4)º and 133.32(4)º). The relative orientations of NHC and dimethyldi(2- pyridyl)borate ligands in crystals differ within the series. In complex 4.1 the two ligands coordinate to copper in such a way that pyridyl rings of the dp-BMe 2 ligand oppose the phenyl rings of the NHC ligand. In contrast to 4.1, in all other complexes reported here the NHC ligand is revolved about C NHC -Cu bond so that the two pyridyl rings are situated above and below the plane defined by N NHC , N NHC and C NHC atoms. These orientation is observed in crystal of 4.4 despite the presence of isopropyl groups at 2,6-positions of phenyl rings of the NHC ligand similar to complex 4.1. 36 Table 2.2. Selected bond lengths (Å) and angles (deg) for complexes 4.1-4.4. C NHC -Cu Cu-N py ’ Cu-N py ’’ C NHC -Cu-N py ’ C NHC -Cu-N py ’’ N py ’-Cu-N py ’’ 4.1 1.895(2) 2.0288(15) 2.0288(15) 132.78(4) 132.78(4) 94.43(9) 4.2a 1.8678(19) 1.9929(16) 1.9997(16) 134.32(7) 129.27(7) 96.14(6) 4.2b 1.8678(19) 1.9876(16) 1.9985(16) 133.95(7) 129.41(7) 96.28(6) 4.3 1.8808(11) 2.010(9) 2.014(9) 135.0(6) 128.1(7) 95.51(4) 4.4 1.8831(10) 2.0081(9) 2.0224(9) 133.84(4) 133.32(4) 92.71(4) 2.4. NMR Studies The conformational behavior of the complexes 3.2-3.7 was investigated by variable- temperature 1 H NMR spectroscopy. As revealed by crystal structure analysis of IPr- based derivatives, the coordination environment around copper is rather sterically crowded by the 2,6-diisopropyl phenyl groups of the NHC ligand. It has been shown in other metal complexes coordinated with bulky NHC ligands that rotation about C NHC – metal bond can be severely hindered. 16-20 Rotation around the N NHC –С aryl and aryl– CH(CH 3 ) 2 bond axes is also inhibited by the steric bulk of the isopropyl groups as evidenced by the 1 H NMR of (IPr)CuCl in CDCl 3 , where the diastereotopic methyl groups appear as two distinct doublets (Figure 2.12). Molecular models similarly show that rotation around the N NHC –С aryl bond is highly unlikely in complexes 3.2–3.6 due to steric constraints imposed by the 2,6-isopropyl groups and the N^N ligands. Yet, despite the steric crowding, resonances for the protons on the two aromatic rings of the NHC ligand, as well as the two protons of imidazolylidene ring, are equivalent for 3.2–3.6 in CDCl 3 . Such equivalence with an asymmetric N^N ligand can only occur if there is either (1) no rotation, with the N^N 37 ligand positioned in a perpendicular C s symmetric orientation with respect to the NHC imidazole plane or, (2) rapid rotation about the C NHC –Cu bond axis. However, a static structure as in case (1) would lead to four diastereotopic methyl resonances for the isopropyl groups due to the asymmetry of the N^N ligand. Figure 2.12. 1 H NMR spectra of (IPr)CuCl in CDCl 3 at 25 °C. Instead, complex 3.2 displays evidence consistent with case (2) as the methyl resonances appear as a simple doublet in CDCl 3 (Figure 2.13), a situation that can only come about by simultaneous rotation around the C NHC –Cu and all the aryl–CH(CH 3 ) 2 bond axes. Apparently, close contact between the CF 3 -group at the 5’-position of the fpta ligand and the methyl substituents in the isopropyl groups leads to correlated motion around the aryl–CH(CH 3 ) 2 bond axes during rotation about the C NHC –Cu bond. 38 Figure 2.13. 600 MHz 1 H NMR spectra of (IPr)Cu(fpyro) (3.2) in CDCl 3 at temperatures ranged from 25 °C to -40 °C. In a related manner, the methyl resonances in complexes 3.3 and 3.4 appear as a pair of doublets (Figures 2.14–2.15). This pattern, similar to what is observed for (IPr)CuCl, is likewise consistent with rapid rotation around the C NHC –Cu bond. While the 1 H NMR spectra for 3.2–3.4 are sharp in CDCl 3 at 25 °C, significant line broadening does occur at -40 °C although no coalescence is observed before freezing point of the solvent. 39 Figure 2.14. 600 MHz 1 H NMR spectra of (IPr)Cu(ppz) (3.3) in CDCl 3 at temperatures ranged from 25 °C to -40 °C. Figure 2.15. 400 MHz 1 H NMR spectra of (IPr)Cu(fppz) (3.4) in CDCl 3 at temperatures ranged from 25 °C to -50 °C. 40 The 1 H NMR spectrum for (IPr)Cu(fpta) (3.5) in CDCl 3 , in contrast, shows more complex behavior. Resonances for the fpta ligand are extremely broad at 25 °C, which indicates the presence of an additional exchange process that is slow on the NMR time scale (Figures 2.16-2.17). Figure 2.16. 600 MHz 1 H NMR spectra (aromatic region) of complex 3.5 in CDCl 3 at temperatures ranged from 40 °C to -40 °C. At -40 °C two sets of signals correspond to the two interconverting major (closed symbols) and minor (open symbols) states. 41 Figure 2.17. 600MHz 1 H NMR spectra (aliphatic region) of complex 3.5 in CDCl 3 at temperatures ranged from 40 to -40°C. Upon cooling, the signals for fpta sharpen and a second set of resonances appear. At -40 °C, two sets of sharp resonances for the fpta ligand, along with a corresponding set of aliphatic resonances for the isopropyl groups, are present in a relative population of 2:1 as determined by integration of the respective signals. The same ratio is obtained when different sources of CDCl 3 are used, and also in the presence of triethylamine. This indicates that protonation of the 3’-nitrogen by trace acid in CDCl 3 is not responsible for the observed dynamic behavior of complex 3.5. In addition, the dynamic process does not have a significant influence on the orientation of the CF 3 -group as only one resonance is observed in the 19 F NMR at -40 °C. The 1 H chemical shifts for the exchanging sites of the fpta ligand are significantly different at -40 °C, particularly for the H6 hydrogen (ortho to the pyridyl ring nitrogen, Figure 2.3) where the major and minor resonances are 42 at  = 6.89 ppm and  = 6.04 ppm, respectively. Thus, the two different sets of fpta resonances correspond to two conformers of 3.5 that interconvert slowly on the NMR time scale. The equilibrium between the two conformers is solvent dependent as the ratio of the two exchanging sites is 10:1 in CD 2 Cl 2 at -45 °C (Figure 2.18). Figure 2.18. 1 H NMR spectra (aromatic region) of (IPr)Cu(fpta) (3.5) in CD 2 Cl 2 at 25 °C and -45 °C. The nature of the exchange process that occurs in complex 3.5 in CDCl 3 can be rationalized on the basis of the differing chemical shifts for the H6 hydrogen of the fpta ligand. In 1 H NMR spectrum of free Hfpta, the H6 resonance is downfield at  = 8.85 ppm in CDCl 3 . Upon coordination to the copper, this hydrogen faces the NHC ligand and its resonance appears upfield near  = 6.95 ppm. Rotation about the C NHC –Cu bond will bring the H6 hydrogen in and out of close proximity to the phenyl rings of the 43 NHC ligand when the two ligands are in a respective coplanar and perpendicular orientation. In the coplanar orientation, the H6 proton is directed toward the center of one of phenyl rings and will be shifted upfield due to shielding from the diamagnetic ring current. On the other hand, a perpendicular orientation of ligands should lead to a relative downfield shift. Hence, the chemical shift will be strongly affected by the dihedral angles between the fpta and NHC ligands. Since the H6 resonance of the minor species appears considerably upfield, it is assigned to a conformer with a predominantly coplanar orientation of the two ligands. Conversely, the major conformer is assigned to an orthogonal arrangement of ligands, in accordance with the crystal structure data of 3.5, which shows that the fpta ligand is oriented nearly perpendicular to the imidazolylidene ring of the NHC ligand. The dynamic exchange process observed in CDCl 3 (and CD 2 Cl 2 ) thus corresponds to a hindered rotation about C NHC –Cu bond through an asymmetric energy well (Figure 2.19). Upon raising the temperature, the equilibrium shifts and the population of the coplanar conformer decreases as the rotation rate about the C NHC –Cu bond increases. As a result, only one set of resonances is observed in 1 H NMR spectrum after the fpta resonances merge above 0 °C. The similar two-state potential surface likely exists for complexes 3.2–3.4, albeit with a much shallower energy of the coplanar conformer, and thus only one set of resonances are observed for these derivatives even at low temperature. 44 Figure 2.19. Qualitative energy diagram for rotation about C NHC –Cu bond in complexes 3.2–3.4 in CDCl 3 (red solid line) and 3.5 (blue dashed line), along with that for complex 3.6 (black dotted line). The dihedral angle between the planes is defined by the (N NHC , N NHC , C NHC ) and (Cu, N py , N az ) atoms. Figure 2.20. 1 H NMR spectra (aromatic region) of complex 3.6 in CDCl 3 at 25 °C and -50 °C. As mentioned earlier, compound (IPr)Cu(pybim) 3.6, in contrast to complexes 3.2, 3.4 and 3.5, has a coplanar orientation of the two ligands in the solid state. The H6 pyridyl and H7’ benzimidazolide protons of the pybim ligand in 3.6 are thus directed toward the 45 centers of aromatic rings of the carbene ligand. This conformational preference is reflected in the 1 H NMR spectrum recorded in CDCl 3 (Figure 2.20). The resonances of H6 and H7’ protons of pybim ligand appear significantly upfield at 6.06 ppm and 5.62 ppm, respectively, indicating that they are shielded by an aromatic ring current. Upon cooling to -50 °C, the H6 and H7’ resonances of 3.6 remain sharp and are shifted further upfield. Therefore, unlike complexes 3.2–3.5, the lowest energy conformation of 3.6 has the N^N ligand oriented predominantly at a dihedral angle near 0° relative to the imidazolylidene ring of the NHC ligand (Figure 2.19). The sharp resonances in the 1 H NMR spectra at -50 °C for 3.6 indicate that rotation about C NHC –Cu bond still persists at low temperature. The principal difference between the geometry of the (IPr)Cu(N^N) complexes in solution is thus the dihedral angle of the lowest energy conformers; orthogonal in 3.2–3.5, coplanar in 3.6. To further probe the conformational behavior in solution, 1 H NMR spectra of 3.2–3.5 were also obtained in both non-polar (benzene-d 6 ) (Figure 2.21) and polar, coordinative (acetone-d 6 ) solvents (Figures 2.22-2.26). Complexes 3.2–3.6 showed only one set of sharp proton signals both in benzene-d 6 (25 °C) and acetone-d 6 (25 °C and -40 °C); there is no evidence of solvent coordination to the copper center observed in solution. Using arguments developed above for behavior in CDCl 3 , the 1 H NMR data indicate rapid rotation about C NHC –Cu bond on the NMR time scale. Downfield shifts of up to 0.45 ppm for the H6 resonances of 3.2–3.5 in acetone-d 6 are observed upon cooling to -40 °C (Figures 2.22-2.25). These shifts may in part be caused by a stabilization of the perpendicular conformation under these conditions. 46 Figure 2.21. 1 H NMR spectra of complexes 3.2-3.6 in benzene-d 6 at 25°C. Figure 2.22. 1 H NMR spectra of (IPr)Cu(fpyro) (3.2) in acetone-d 6 at 25°C and -40°C. 47 Figure 2.23. 1 H NMR spectra of (IPr)Cu(ppz) (3.3) in acetone-d 6 at 25°C and -40°C. Figure 2.24. 1 H NMR spectra of (IPr)Cu(fppz) (3.4) in acetone-d 6 at 25°C and -40°C. 48 Figure 2.25. 1 H NMR spectra of (IPr)Cu(fpta) (3.5) in acetone-d 6 at 25°C and -40°C. Figure 2.26. 1 H NMR spectra of (IPr)Cu(pybim) (3.6) in acetone-d 6 at 25°C. 49 2.5. Experimental Section Synthesis. All reactions were performed under nitrogen atmosphere in oven dried glassware. Chloro[1,3-bis(2,6-di-i-propylphenyl)imidazol-2-ylidene]copper(I) (IPr)CuCl and potassium dihydrobis(1-pyrazolyl)borate K[pz 2 -BH 2 ] were purchased from TCI America, 2-(pyridin-2-yl)-1H-benzo[d]imidazole (Hpybim) and all other commercially available reagents were purchased from Sigma-Aldrich and used as received. Solvents were obtained from commercial sources and used without further purification except for tetrahydrofuran, which was purified by Glass Contour solvent system by SG Water USA, LLC and CDCl 3 , which was passed through a plug of Al 2 O 3 . Isoquinoline-1- carbaldehyde 21 , 3,5-bis(trifluoromethyl)-2’-(2’-pyridyl)pyrrole 15,22,23 (Hfpyro), 2-(1H- pyrazol-5-yl)pyridine 24 (Hppz) 3-trifluoromethyl-5-(2-pyridyl)pyrazole 25,26 (Hfppz), 3- fluoromethyl-5-(2-pyridyl)1,2,4-triazole 27 (Hfpta), 1H,1'H-2,2'-bibenzimidazole 28 (H 2 bbim), sodium dimethyldi(2-pyridyl)borate 29,30 Na(dp 2 -BMe 2 ), chloro[1,3-bis(2,6-di- i-propylphenyl)imidazolidin-2-ylidene]copper(I) 10 (SIPr)CuCl, 1,3-bis(3,5- dimethylphenyl)-1H-benzo[d]imidazol-3-ium chloride 31 (BzI-3,5Me)Cl, 7,9-bis(2,6- diisopropylphenyl)-7H-acenaphtho[1,2-d]imidazol-9-ium chloride 32 (IPrBIAN)Cl were prepared following published procedures. 1 H, 13 C and 19 F NMR spectra were recorded on a Varian Mercury 400, Varian VNMRS 500 or a Varian VNMRS 600 spectrometer. The chemical shifts are given in units of ppm. All 1 H and 13 C chemical shifts were referenced to the residual solvent signals. All 19 F chemical shifts were referenced to CFCl 3 as an external standard at 0.0 ppm. Variable temperature (VT) NMR experiments were performed as follows. Data were collected starting from the lowest temperature and 50 then heating the sample in 10-20 °C steps, allowing 15 minutes for temperature equilibration at each set point. 1 H-NMR spectra of the sample at room temperature (25 °C) were recorded before and after VT NMR experiment and compared to ensure no decomposition occurred during VT studies. Elemental analyses were carried out by the Microanalysis Laboratory at the University of Illinois, Urbana-Champaign, IL. 2-(Isoquinolin-1-yl)benzimidazole (beniq). 33 Isoquinoline-1-carbaldehyde (315.2 mg, 2 mmol) and 1,2-phenylenediamine (216 mg, 2 mmol) were mixed in 40 ml of absolute ethanol and stirred for 2 h at RT. Iodobenzene diacetate (IBD) (966.3 mg, 3 mmol) was added and the reaction mixture was stirred for further 1 h. The volatiles were removed by rotary evaporation and the resulting brown crude mixture was purified by silica gel column chromatography (ethyl acetate:hexane 1:20). The product was obtained as light- yellow solid (160 mg, 33%). 1 H NMR (400 MHz, CDCl 3 , δ) 7.31-7.37 (m, 2H), 7.54- 7.56 (m, 1H), 7.75-7.83 (m, 3H), 7.88-7.90 (m, 1H), 7.96-7.98 (m, 1H), 8.58 (d, J = 5.5 Hz, 1H), 10.21 (d, J = 7.2 Hz, 1H), 10.96 (br s, 1H). Sodium [2,2'-bibenzimidazole]-1,1'-diide (Na 2 bbim). 1H,1'H-2,2'-bibenzimidazole (175 mg, 0.747 mmol) and sodium methoxide (110 mg, 2.04 mmol) were mixed in CH 2 Cl 2 (30 ml)/methanol (30 ml) and stirred for 2 h at RT. Then the solvent was removed by rotary evaporation and compound was used without further purification. N 2 ,N 3 -bis(3,5-dimethylphenyl)pyrazine-2,3-diamine. 2,3-Dichloropyrazine (5.58 g, 0.037 mol) and 3,5-dimethylaniline (11.21 g, 0.0925 mol) were heated neat at 140ºC overnight under nitrogen atmosphere. The resulting solid was suspended in 200 ml of 51 water and the mixture was made alkaline with 25% NaOH. The mixture was extracted with 3×75 ml of CH 2 Cl 2 and 3×75 ml of ethyl acetate. Combined organic layers were dried with Na 2 SO 4 and volatiles were removed by rotary evaporation to give brown oily residue. Addition of pentane afforded the off-white solid, which was filtered, washed with pentane and dried (7.145 g, 60%). 1 H NMR (400 MHz, CDCl 3 , δ) 2.29 (s, 12H), 6.18 (br s, 2H), 6.70 (s, 2H), 6.88 (s, 4H), 7.77 (s, 2H). 1,3-bis(3,5-dimethylphenyl)-2-ethoxy-2,3-dihydro-1H-imidazo[4,5-b]pyrazine ((PzI- 3,5Me)OEt). To N 2 ,N 3 -bis(3,5-dimethylphenyl)pyrazine-2,3-diamine (750 mg, 2.35 mmol) was added 10 ml of triethylorthoformate, followed by 0.193 ml of HCl conc . The mixture was stirred overnight in a closed vessel at 110ºC. The product was precipitated from dark yellow solution upon addition of pentane and cooling (0ºC). Beige solid was collected by filtration, washed with pentane and dried (610 mg, 69%). 1 H NMR (500 MHz, CDCl 3 , δ) 1.10 (t, J = 7.0 Hz, 3H, OCH 2 CH 3 ), 2.39 (s, 12H, ArCH 3 ), 3.33 (q, J = 7.0 Hz, 2H, OCH 2 CH 3 ), 6.84 (br s, 2H, H Ar ), 7.18 (s, 1H, NCHN), 7.52 (s, 2H, H Pz ), 7.62 (br s, 4H, H Ar ). 13 C NMR (126 MHz, CDCl 3 , δ) 14.71, 21.81, 55.29, 96.37, 116.68, 126.20, 130.62, 138.05, 139.02, 143.81. Anal. Calcd for C 23 H 26 N 4 O : C, 73.77; H, 7.0; N, 14.96. Found; C, 73.68; H, 7.02; N, 14.81. (PzI-3,5Me)CuCl. (PzI-3,5Me)OEt (300 mg, 0.8 mmol) and CuCl (72.3 mg, 0.73 mmol) were refluxed overnight in 20 ml of THF. The resulting bright-yellow suspension was filtered, the solid was collected, washed with THF and dried in vacuo (273 mg, 87%). 1 H NMR (500 MHz, CDCl 3 , δ) 2.46 (s, 12H, ArCH 3 ), 7.21 (br s, 2H, H Ar ), 7.45 (s, 4H, H Ar ), 52 8.55 (s, 2H, H Pz ). 13 C NMR (126 MHz, CDCl 3 , δ) 21.54, 123.56, 131.86, 135.59, 139.71, 140.22, 140.55, 189.04. (BzI-3,5Me)CuCl. 1,3-bis(3,5-dimethylphenyl)-1H-benzo[d]imidazol-3-ium chloride (BzI-3,5)Cl (500 mg, 1.38 mmol), CuCl (164 mg, 1.66 mmol) and NaOtBu (132.5 mg, 1.38 mmol) were mixed in 40 ml of THF and stirred at room temperature overnight. The reaction mixture was then filtered under inert atmosphere through a plug of Celite and the solvent was evaporated in vacuo. The resulting solid was exposed to air and washed with pentane (50 ml). The product was obtained as white air stable solid (466 mg, 79%). 1 H NMR (500 MHz, acetone-d 6 , δ) 2.45 (s, 12H, ArCH 3 ), 7.27 (br s, 2H, H Ar ), 7.48 ( br s, 4H, H Ar ), 7.50-7.52 (AA’BB’, 2H, H Ar ), 7.61-7.63 (AA’BB’, 2H, H Ar ). 13 C NMR (126 MHz, acetone-d 6 , δ) 21.27, 113.26, 124.62, 125.78, 131.67, 134.89, 138.50, 140.84, 184.75. Anal. Calcd for C 23 H 22 ClCuN 2 : C, 65.93; H, 5.21; N, 6.58. Found; C, 65.12; H, 5.45; N, 6.31. (IPrBIAN)CuCl. 7,9-bis(2,6-diisopropylphenyl)-7H-acenaphtho[1,2-d]imidazol-9-ium chloride (IPrBIAN)Cl (492 mg, 0.89 mmol), CuCl (105.7 mg, 1.068 mmol) and NaOtBu (85.4 mg, 0.89 mmol) were mixed in 30 ml of THF and stirred at room temperature overnight. The reaction mixture was exposed to air, filtered through a plug of Celite and solvent was removed by rotary evaporation. Toluene (30 ml) was added to the crude product and the resulting suspension was filtered. The obtained filtrate was collected and solvent was evaporated to dryness to give yellow solid (175 mg, 32%). 1 H NMR (400 MHz, CDCl 3 , ) 1.12 (d, J = 6.8 Hz, 12H, ArCHCH 3 ), 1.35 (d, J = 6.8 Hz, 12H, 53 ArCHCH 3 ), 2.84 (septet, J = 6.8 Hz, 4H, ArCHCH 3 ), 7.00 (d, J = 6.8 Hz, 2H), 7.39-7.46 (m, 6H) , 7.59 (t, J = 7.8 Hz, 2H), 7.80 (d, J = 8.3 Hz, 2H). [(IPr)Cu(phen)]OTf (3.1). (IPr)CuCl (121.9 mg, 0.25 mmol) and silver triflate (64.2 mg, 0.25 mmol) were mixed under nitrogen in a 25 mL flask and 10 mL of dry THF were added. The reaction mixture was stirred at RT for 30 min. A solution of 1,10- phenanthroline (45.05 mg, 0.25 mmol) in dry THF (5 mL) was added. The reaction mixture turned yellow and was stirred at RT for 3 h. The resulting mixture was filtered through Celite ® and solvent was removed by rotary evaporation. Recrystallization from CH 2 Cl 2 by vapor diffusion of Et 2 O gave 120mg (61%) of bright yellow crystals. 1 H NMR (400 MHz, CDCl 3 , ): 1.07 (d, J = 7.2 Hz, 12H), 1.30 (d, J = 7.2 Hz, 12H), 2.69 (sept, J = 6.8 Hz, 4H), 6.74 (dd, J = 4.8 Hz, J = 1.6 Hz, 2H), 7.40 (s, 2H), 7.51 (d, 8Hz, 4H), 7.60 (dd, J = 8.4 Hz, J = 4.8 Hz, 2H), 7.78 (t, J = 8 Hz, 2H), 7.95 (s, 2H), 8.51 (dd, J = 8 Hz, J=1.6 Hz, 2H). 13 C NMR (101 MHz, CDCl 3 , ): 23.63, 25.08, 28.87, 123.56, 124.74, 125.23, 127.09, 128.99, 130.70, 135.93, 138.99, 143.51, 146.37, 149.65, 183.09. Anal. Calcd. for C 40 H 44 CuF 3 N 4 O 3 S: C, 61.48; H, 5.68; N, 7.17; Found: C, 61.06; H, 5.61; N, 7.14. (IPr)Cu(fpyro) (3.2). 3,5-bis(trifluoromethyl)-2-(2’-pyridyl)pyrrole (300 mg, 1.1 mmol) was dissolved in 10 mL of THF under nitrogen and this solution was transferred via cannula to suspension of NaH (42.9 mg, 1.1 mmol, 60% in mineral oil) in THF (20 ml). The reaction mixture was stirred at RT until all the NaH had reacted and gas evolution stopped (30 min). The clear solution was then transferred to a flask charged with 54 chloro[1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]copper(I) ((IPr)CuCl) (523 mg, 1.1 mmol) under nitrogen. The resulting yellow mixture was stirred at RT for 3 h and then it was filtered through Celite ® and solvent was removed by rotary evaporation. The product was obtained as yellow solid by crystallization from its toluene solution by layering with n-pentane in the freezer (430 mg, 55%). 1 H NMR (500 MHz, acetone-d 6 , ) 1.21 (d, J = 6.9 Hz, 24H, ArCHCH 3 ), 3.04 (septet, J = 6.8 Hz, 4H, ArCHCH 3 ), 6.61 (s, 1H, H-4 pyrrole), 7.11 (m, 1H, H-5 py), 7.34 (d, J = 7.8 Hz, 4H, H Ar ), 7.45 (t, 2H, H Ar ), 7.63 (s, 2H, NCH=), 7.65 (d, J = 8.2 Hz, 1H, H-3 py), 7.70 (d, J = 5.0 Hz, 1H, H-6 py), 7.77 (m, 1H, H-4 py). 13 C NMR (126 MHz, acetone-d 6 , ) 23.51, 25.47, 29.38, 111.02 (m), 111.65 (q, J = 35 Hz), 120.14 (m), 121.93, 123.33 (q, J = 266 Hz, CF 3 ), 124.93 (2C), 125.15, 126.04 (q, J = 265 Hz, CF 3 ), 129.96 (q, J = 36 Hz), 130.75, 137.19, 139.02, 146.53, 149.7, 152.63, 185.63. 19 F NMR (470 MHz, acetone-d 6 , ) -59.88, -55.07. Anal. Calcd for C 38 H 41 CuF 6 N 4 : C, 62.41; H, 5.65; N, 7.66. Found: C, 62.39; H, 5.64; N, 7.54. (IPr)Cu(ppz) (3.3). Complex 3.3 was prepared following the procedure described for the preparation of 3.2 from 2-(1H-pyrazol-5-yl)pyridine (72.6 mg, 0.5 mmol) and (IPr)CuCl (243.8 mg, 0.5 mmol). The product was obtained as yellow solid by crystallization from toluene/n-pentane solvent mixture (120 mg, 40%). 1 H NMR (500 MHz, acetone-d 6 , ) 1.24 (d, J = 6.9 Hz, 12H, ArCHCH 3 ), 1.33 (d, J = 6.9 Hz, 12H, ArCHCH 3 ), 2.95 (septet, J = 6.9 Hz, 4H, ArCHCH 3 ), 6.33 (d, J = 1.6 Hz, 1H, H-4 pz), 6.91 (m, 1H, H-5 py), 7.23 (d, J = 1.6 Hz, 1H, H-3 pz), 7.33 (d, J = 7.7 Hz, 4H, H Ar ), 7.36 (d, J = 8.0 Hz, H-3 py), 7.45 (t, J = 7.8 Hz, 2H, H Ar ), 7.58 (m, 1H, H-4 py), 7.62-7.63 (m, 2H, NCH=, H-6 py). 55 13 C NMR (126 MHz, acetone-d 6 , ) 24.57, 24.65, 29.45, 100.93, 118.89, 120.74, 124.48, 124.75, 130.70, 136.92, 138.24, 141.02, 146.84, 149.18, 149.70, 154.76, 185.79. Anal. Calcd for C 35 H 42 CuN 5 : C, 70.5; H, 7.1; N, 11.74. Found: C, 70.52; H, 7.17; N, 11.51. (IPr)Cu(fppz) (3.4). Complex 3.4 was prepared following the procedure described for the preparation of 3.2 from 3-trifluoromethyl-5-(2-pyridyl)pyrazole (213.2 mg, 1.0 mmol) and (IPr)CuCl (487.6 mg, 1.0 mmol). The crude product after filtration and removal of solvent was purified by vacuum sublimation (225°C, 10 -6 torr) to give yellow solid (349 mg, 53%). 1 H NMR (500 MHz, acetone-d 6 , ) 1.24 (d, J = 6.9 Hz, 12H, ArCHCH 3 ), 1.30 (d, J = 6.9 Hz, 12H, ArCHCH 3 ), 2.96 (septet, J = 6.9 Hz, 4H, ArCHCH 3 ), 6.66 (s, 1H, H-4 pz), 7.04 (m, 1H, H-5 py), 7.34 (d, J = 7.7 Hz, 4H, H Ar ), 7.44-7.48 (m, 3H, H-3 py, H Ar ), 7.57 (d, J = 5.0 Hz, 1H, H-6 py), 7.64 (s, 2H, NCH=), 7.70 (m, 1H, H-4 py). 13 C NMR (126 MHz, acetone-d 6 , ) 24.41, 24.67, 29.44, 99.77 (q, J = 1.9 Hz, C-4 pz), 119.35, 122.11, 124.51 (q, J = 267 Hz, CF 3 pz), 124.52, 124.77, 130.67, 136.93, 139.08, 144.35 (q, J = 34 Hz, C-3 pz), 146.81, 149.32, 150.62, 153.41, 185.72. 19 F NMR (470 MHz, acetone-d 6 , ) -60.57. Anal. Calcd for C 36 H 41 CuF 3 N 5 : C, 65.09; H, 6.22; N, 10.54. Found: C, 64.74; H, 6.21; N, 10.31. (IPr)Cu(fpta) (3.5). Complex 3.5 was prepared following the procedure described for the preparation of 3.2 from 3-fluoromethyl-5-(2-pyridyl)1,2,4-triazole (107 mg, 0.5 mmol) and (IPr)CuCl (228.8 mg, 0.5 mmol). The crude product after filtration and removal of solvent was purified by vacuum sublimation (265 °C, 10 -6 torr) to give yellow solid (145 mg, 46%). 1 H NMR (500 MHz, acetone-d 6 , ) 1.25 (d, J = 6.9 Hz, 12H, 56 ArCHCH 3 ), 1.27 (d, J = 6.9 Hz, 12H, ArCHCH 3 ), 2.90 (septet, J = 6.9 Hz, 4H, ArCHCH 3 ), 7.22 (m, 1H, H-5 py), 7.37 (d, J = 7.8 Hz, 4H, H Ar ), 7.41 (d, J = 4.9 Hz, 1H, H-6 py), 7.50 (t, 2H, J = 7.8 Hz, H Ar ), 7.70 (s, 2H, NCH=), 7.80 (d, J = 7.7 Hz, 1H, H-3 py), 7.84 (m, 1H, H-4 py). 13 C NMR (126 MHz, acetone-d 6 , ) 24.45, 24.52, 29.48, 120.12, 122.29 (q, J = 268 Hz, CF 3 pz), 124.39, 124.80, 124.88, 130.89, 136.76, 139.54, 146.76, 149.68, 150.61, 156.37 (q, J = 35 Hz, C-4 pz), 163.62, 184.60. 19 F NMR (470 MHz, acetone-d 6 , ) -64.13. Anal. Calcd for C 35 H 40 CuF 3 N 6 : C, 63.19; H, 6.06; N, 12.63. Found: C, 63.24; H, 6.06; N, 12.41. (IPr)Cu(pybim) (3.6). 2-(2-Pyridyl)benzimidazole (78.1 mg, 0.4 mmol) was dissolved in 10 mL of dry THF under N 2 and this solution was transferred via cannula to suspension of sodium hydride (17.6 mg, 0.44 mmol, 60% in mineral oil) in dry THF. The reaction mixture was stirred at RT for 1 h and then chloro[1,3-bis(2,6- diisopropylphenyl)imidazol-2-ylidene]copper(I) (195.1 mg, 0.4 mmol) was added. The reaction mixture was stirred at RT for 3 h. The resulting mixture was filtered through Celite ® and solvent was removed by rotary evaporation. Recrystallization by vapor diffusion of diethyl ether into a CH 2 Cl 2 solution of product gave 154 mg (60%) of dark yellow crystals. 1 H NMR (400 MHz, CDCl 3 , ): 1.14 (d, J = 6.8 Hz, 12H), 1.25 (d, J = 6.8 Hz, 12H), 2.77 (sept, J = 6.8 Hz, 4H), 5.62 (d, J = 8 Hz, 1H), 6.06 (dd, J = 4.8 Hz, J=1.2 Hz, 1H), 6.73 (m, 2H), 6.94 (m, 1H), 7.17 (s, 2H), 7.43 (d, J = 7.6 Hz, 4H), 7.56 (m, 1H), 7.61 (d, J = 8 Hz, 1H), 7.67 (t, J = 7.6 Hz, 2H), 8.22 (d, J = 7.6 Hz, 1H). 13 C NMR (101 MHz, CDCl 3 , ): 23.55, 24.72, 28.83, 116.52, 117.80, 119.37, 119.56, 120.07, 57 121.73, 123.05, 124.49, 130.05, 136.38, 136.98, 144.62, 146.37, 147.38, 148.09, 152.54, 159.81, 184.82. Anal. Calcd for C 39 H 44 CuN 5 : C, 72.47; H, 6.86; N, 10.48; Found: C, 72.55; H, 6.94; N, 10.84. (IPr)Cu(beniq) (3.7). Complex 3.7 was prepared by following the procedure described for the preparation of 3.6 from (IPr)CuCl (92.6 mg, 0.19 mmol) and 2-(isoquinolin-1- yl)benzimidazole (46 mg, 0.19 mmol). The product was obtained as orange crystals (50 mg, 38%). 1 H NMR (500MHz, CDCl 3 , ): 1.15 (d, J = 6.9 Hz, 12H), 1.26 (d, J = 6.9 Hz, 12H), 2.81 (sept, J = 6.9 Hz, 4H), 5.61 (d, J = 7.9 Hz, 1H), 5.99 (d, J = 5.8 Hz, 1H), 6.75 (m, 1H), 6.97 (m, 1H), 7.08 (d, J = 5.8 Hz, 1H), 7.19 (s, 2H), 7.45 (d, J = 7.9 Hz, 4H), 7.63 (m, 3H), 7.71 (m, 3H), 10.55 (d, J = 8.1 Hz, 1H). (SIPr)Cu(pybim) (3.8). Complex 3.8 was prepared by following the procedure described for the preparation of 3.6 from (SIPr)CuCl (195.8 mg, 0.4 mmol) and 2-(2- pyridyl)benzimidazole (78.1 mg, 0.4 mmol). The product was obtained as yellow crystals (224 mg, 86%). 1 H NMR (400MHz, acetone-d 6 , ): 1.22 (d, J = 6.9 Hz, 12H), 1.38 (d, J = 6.8 Hz, 12H), 3.43 (sept, J = 6.9 Hz, 4H), 4.42 (s, 4H), 5.55 (d, J = 7.5 Hz, 1H), 6.13 (d, J = 4.4 Hz, 1H), 6.57-6.61 (m, 1H), 6.77 (t, J = 7.2 Hz, 1H), 6.85 (dd J = 7.3 Hz, J = 5.1 Hz, 1H), 7.34 (d, J = 7.5 Hz, 1H), 7.47 (d, J = 7.8 Hz, 4H), 7.62-7.67 (m, 3H), 8.10 (d, J = 7.2 Hz, 1H). (IPr) 2 Cu 2 (bbim) (3.9). Sodium [2,2'-bibenzimidazole]-1,1'-diide (Na 2 bbim) (278.22 mg, 0.202 mmol) and (IPr)CuCl (487.6 mg, 0.404 mmol) were mixed in 50 ml of CH 2 Cl 2 and stirred at RT 24 h. The resulting mixture was filtered through PTFE (0.2 µm) filter and 58 solvent was removed by rotary evaporation. Recrystallization by vapor diffusion of diethyl ether into a CH 2 Cl 2 solution of product over 3 days gave 118 mg (51%) of light yellow crystals. 1 H NMR (400MHz, CDCl 3 , ): 1.18 (d, J = 6.8 Hz, 24H), 1.19 (d, J = 6.8 Hz, 24H), 3.00 (sept, J = 6.8 Hz, 8H), 6.59-6.63 (AA’BB’, 4H), 6.71-6.75 (AA’BB’, 4H),7.06 (s, 4H), 7.21 (d, J = 7.8 Hz, 8H), 7.40 (t, J = 7.8 Hz, 4H). [(IPr)Cu(bpy)]OTf (3.10). Complex 3.10 was prepared by following the procedure described for the preparation of 3.1 from (IPr)CuCl (195.04 mg, 0.4 mmol) and 2,2’- bipyridine (62.4 mg, 0.4 mmol). The product was obtained as orange crystals (215 mg, 71%). 1 H-NMR (CDCl 3 , 400 MHz): δ 1.06 (d, J = 6.9 Hz, 12H), 1.27 (d, J = 6.9 Hz, 12H), 2.63 (sept, J = 6.9 Hz, 4H), 6.30 (d, J = 4.3 Hz, 2H), 7.16 (ddd, J = 7.6 Hz, J = 5.1 Hz, J = 1.0 Hz, 2H), 7.35 (s, 2H), 7.46 (d, J = 7.8 Hz, 4H), 7.71 (t, J = 7.8 Hz, 2H), 7.99 (td, J = 7.9 Hz, J = 1.7 Hz 2H), 8.31 (d, J = 8.1 Hz, 2H). (IPr)Cu(dp-BMe 2 ) (4.1). (IPr)CuCl (122.15 mg, 0.25 mmol) and sodium dimethyldi(2- pyridyl)borate (55 mg, 0.25 mmol) were mixed in 10 ml of THF and stirred at RT for 1 h under N 2 atmosphere. The resulting light-yellow solution was filtered through Celite and the solvent was evaporated under reduced pressure. The product was obtained as light yellow solid by slow crystallization from acetone/hexanes solvent mixture under N 2 atmosphere (75 mg, 46%). Alternatively, the crude product obtained after removal of solvent can be purified by zone vacuum sublimation (220 °C, 10 -6 torr). Sublimation yield 66%. 1 H NMR (500 MHz, acetone-d 6 , ) -0.15 (br s, 6H, BCH 3 ), 1.16 (d, J = 6.9 Hz, 12H, ArCHCH 3 ), 1.22 (d, J = 6.9 Hz, 12H, ArCHCH 3 ), 3.04 (septet, 59 J = 6.9 Hz, 4H, ArCHCH 3 ), 6.50-6.53 (m, 2H, H py ), 7.20 (tdd, J = 7.6 Hz, J = 1.9 Hz, J = 0.8 Hz, 2H, H py ), 7.30-7.35 (m, 8H, H Ar , H py), 7.49 (t, J = 8 Hz, 2H, H Ar ), 7.62 (s, 2H, NCH=). Anal. Calcd for C 39 H 50 BCuN 4 : C, 72.15; H, 7.76; N, 8.63; Found: C, 72.2; H, 7.8; N, 8.58. (BzI-3,5Me)Cu(dp-BMe 2 ) (4.2). (BzI-3,5Me)CuCl (319 mg, 0.75 mmol) and sodium dimethyldi(2-pyridyl)borate (165 mg, 0.75 mmol) were mixed in 30 ml of THF and stirred at RT for 1 h under N 2 atmosphere. The resulting yellow solution was filtered through a pad of Celite in glove box. The filtrate was concentrated under reduced pressure to ~5 ml. Pentane was added dropwise resulting in precipitation of yellow solid that was collected by filtration and dried. (310 mg, 70%). 1 H NMR (500 MHz, acetone-d 6 , ) -0.06 (br s, 6H, BCH 3 ), 2.27 (s, 12H, ArCH 3 ), 6.73-6.76 (m, 2H, H py ), 7.13 (s, 2H, H Ar ), 7.35 (td, J = 7.6 Hz, J = 1.8 Hz, 2H, H py ), 7.50 (d, J = 7.8 Hz, 2H, H py), 7.41- 7.44 (AA’BB’, 2H, H Ar ), 7.47 (d, J = 7.8 Hz, 2H, H py ), 7.52-7.56 (m, 6H, H Ar ), 7.97 (d, J = 5.1 Hz, 2H, H py ). (PzI-3,5Me)Cu(dp-BMe 2 ) (4.3). (PzI-3,5Me)CuCl (427.4 mg, 1 mmol) and sodium dimethyldi(2-pyridyl)borate (220.11 mg, 1 mmol) were mixed in 30 ml of THF and stirred at RT for 1 h under N 2 atmosphere. The resulting orange solution was filtered through a pad of Celite in glove box. The filtrate was concentrated under reduced pressure to ~5 ml. Pentane was added dropwise resulting in precipitation of bright orange solid that was collected by filtration and dried. (444 mg, 75%). 1 H NMR (500 MHz, acetone-d 6 , ) -0.05 (br s, 6H, BCH 3 ), 2.25 (s, 12H, ArCH 3 ), 6.77-6.80 (m, 60 2H, H py ), 7.13 (s, 2H, H Ar ), 7.39 (td, J = 7.6 Hz, J = 1.8 Hz, 2H, H py ), 7.50 (d, J = 7.8 Hz, 2H, H py), 7.67 (s, 4H, H Ar ), 8.06 (d, J = 5.3 Hz, 2H, H py ), 8.49 (s, 2H, H pz ). Anal. Calcd for C 33 H 34 BCuN 6 : C, 67.29; H, 5.82; N, 14.27; Found: C, 67.2; H, 5.86; N, 14.02. (IPrBIAN)Cu(dp-BMe 2 ) (4.4). (IPrBIAN)CuCl (170 mg, 0.28 mmol) and sodium dimethyldi(2-pyridyl)borate (61.63 mg, 0.25 mmol) were mixed in 20 ml of THF and stirred at RT for 1 h under N 2 atmosphere. The resulting light-yellow solution was filtered through Celite and the solvent was evaporated under reduced pressure. The product was obtained as dark orange solid by crystallization from its toluene solution by layering with n-pentane under N 2 atmosphere (136 mg, 63%). 1 H NMR (500 MHz, acetone-d 6 , ) -0.09 (br s, 6H, BCH 3 ), 1.09 (d, J = 6.9 Hz, 12H, ArCHCH 3 ), 1.19 (d, J = 6.9 Hz, 12H, ArCHCH 3 ), 3.29 (septet, J = 6.9 Hz, 4H, ArCHCH 3 ), 6.55-6.58 (m, 2H, H py ), 7.01 (d, J = 7.0 Hz, 2H, H Ar ), 7.24 (td, J = 7.6 Hz, J = 1.8 Hz, 2H, H py ), 7.36 (d, J = 7.7 Hz, 2H, H py ), 7.40 (d, J = 5.2 Hz, 2H, H py ), 7.50 (d, J = 7.8 Hz, 4H, H Ar ), 7.54 (dd, J = 8.3 Hz, J = 7.0 Hz, 2H, H Ar ), 7.66 (t, J = 7.8 Hz, 2H, H Ar ), 7.90 (d, J = 8.0 Hz, 2H, H Ar ). (IPr)Cu(pz 2 -BH 2 ) (4.5). (IPr)CuCl (195.04 mg, 0.4 mmol) and potassium dihydrobis(1- pyrazolyl)borate K[pz 2 -BH 2 ] (74.4 mg, 0.4 mmol) were mixed in 10 ml of THF and stirred at RT for 1 h under N 2 atmosphere. The resulting solution was filtered through a pad of Celite and the solvent was evaporated under reduced pressure. The product was obtained as white solid. (132 mg, 55%). 1 H NMR (500 MHz, acetone-d 6 , ) 1.20 (d, J = 6.9 Hz, 12H, ArCHCH 3 ), 1.23 (d, J = 6.9 Hz, 12H, ArCHCH 3 ), 2.88 (septet, 61 J = 6.9 Hz, 4H, ArCHCH 3 ), 5.72 (t, J = 1.7 Hz, 2H, H pz ), 5.91 (d, J = 1.8 Hz, 2H, H pz ), 7.24 (d, J = 1.8 Hz, 2H, H pz ), 7.38 (d, 4H, J = 7.8 Hz, H Ar ), 7.53 (t, J = 7.8 Hz, 2H, H Ar ), 7.63 (s, 2H, NCH=). (SIPr)Cu(dp-BMe 2 ) (4.6). (SIPr)CuCl (146.87 mg, 0.3 mmol) and sodium dimethyldi(2- pyridyl)borate (66.03 mg, 0.3 mmol) were mixed in 10 ml of THF and stirred at RT for 1 h under N 2 atmosphere. The resulting solution was filtered through Celite and the solvent was evaporated under reduced pressure. The product was obtained as beige solid and used without further purification. (85 mg, 44%). 1 H NMR (400 MHz, acetone-d 6 , ) -0.28 (br s, 6H, BCH 3 ), 1.14 (d, J = 6.9 Hz, 12H, ArCHCH 3 ), 1.32 (d, J = 6.9 Hz, 12H, ArCHCH 3 ), 3.56 (septet, J = 6.9 Hz, 4H, ArCHCH 3 ), 4.21 (s, 4H, NCH 2 ), 6.56-6.60 (m, 2H, H py ), 7.20 (td, J = 7.6 Hz, J = 1.7 Hz, 2H, H py ), 7.26-7.30 (m, 6H, H Ar , H py), 7.38 (t, J = 7.9 Hz, 2H, H Ar ), 7.50 (d, J = 4.5 Hz, 2H, H py ). X-ray crystallography The single crystal X-ray diffraction data for compounds 3.2, 3.5 and 4.2-4.4 were collected on a Bruker SMART APEX DUO 3-circle platform diffractometer with the - axis fixed at 54.745°, and using Mo K  radiation (  = 0.71073 Å) monochromatized by a TRIUMPH curved-crystal monochromator. 34 The diffractometer was equipped with an APEX II CCD detector and an Oxford Cryosystems Cryostream 700 apparatus for low- temperature data collection. The crystals were mounted in Cryo-Loops using Paratone oil. The frames were integrated using the SAINT algorithm 35 to give the hkl files corrected for Lp/decay. Data were corrected for absorption effects using the multi-scan 62 method (SADABS). 36 The structures were solved by the direct method and refined on F 2 using the Bruker SHELXTL Software Package. 37 All non-hydrogen atoms were refined anisotropically. ORTEP drawings were prepared using the ORTEP-3 for Windows V2.02 program. 38 Diffraction data for compounds 3.1, 3.4, 3.6, 3.7 and 4.1 were collected on a Bruker SMART APEX CCD diffractometer with graphite monochromated Mo K  radiation (  = 0.71073 Å). Crystallographic data for the complexes have been deposited at the Cambridge Crystallographic Data Centre, nos. 902418 (3.4), 902419 (3.5), 902420 (3.2), 778544 (3.1), 778545 (3.6) and can be obtained free of charge via www.ccdc.cam.ac.uk. C 38 H 41 CuF 6 N 4 (3.2) : A clear, light yellow specimen of, approximate dimensions 0.16 mm x 0.26 mm x 0.35 mm, was used for the X-ray crystallographic analysis. A complete hemisphere of data was scanned on omega (0.5°) with a run time of 30-seconds per frame at a detector distance of 50.4 mm and a resolution of 512 x 512 pixels. A total of 4320 frames were collected. The integration of the data using a monoclinic unit cell yielded a total of 156641 reflections to a maximum  angle of 30.54° (0.70 Å resolution), of which 21794 were independent (average redundancy 7.187, completeness = 98.6%, R int = 3.13%, R sig = 2.06%) and 17438 (80.01%) were greater than 2 (F 2 ). The final cell constants of a = 21.3154(8) Å, b = 18.0984(7) Å, c = 21.5042(8) Å,  = 119.4410(10)°, V = 7224.5(5) Å 3 , are based upon the refinement of the XYZ-centroids of 2097 reflections above 20 (I) with 3.127° < 2  < 61.60°. The ratio of minimum to maximum apparent transmission was 0.889. The calculated minimum and maximum transmission 63 coefficients (based on crystal size) are 0.4921 and 0.7312. The structure was solved and refined using the space group P2 1 /n, with Z = 8 for the formula unit, C 38 H 41 CuF 6 N 4 . The final anisotropic full-matrix least-squares refinement on F 2 with 899 variables converged at R 1 = 3.31%, for the observed data and wR 2 = 9.23% for all data. The goodness-of-fit was 1.011. The largest peak in the final difference electron density synthesis was 0.548 e - /Å 3 and the largest hole was -0.395 e - /Å 3 with an RMS deviation of 0.055 e - /Å 3 . On the basis of the final model, the calculated density was 1.345 g/cm 3 and F(000), 3040 e - . C 36 H 41 CuF 3 N 5 (3.4): A clear yellow prism of approximate dimensions 0.17 mm x 0.19 mm x 0.22 mm was used for the X-ray crystallographic analysis. A complete hemisphere of data was scanned on omega (0.3°) with a run time of 10 seconds per frame. The integration of the data using a monoclinic unit cell yielded a total of 20207 reflections to a maximum  angle of 27.51° (0.77 Å resolution), of which 7458 were independent (average redundancy 2.709, completeness = 99.7%, R int = 4.48%, R sig = 5.00%) and 5528 (74.12%) were greater than 2 (F 2 ). The final cell constants of a = 11.8209(11) Å, b = 14.4304(14) Å, c = 20.0438(18) Å,  = 104.402(2)°, V = 3311.6(5) Å 3 , are based upon the refinement of the XYZ-centroids of 7208 reflections with 2.31° < 2  < 27.47°. The ratio of minimum to maximum apparent transmission was 0.855. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.6374 and 0.7456. The structure was solved and refined using the space group P2 1 /n, with Z = 4 for the formula unit, C 36 H 41 CuF 3 N 5 . The final anisotropic full-matrix least-squares refinement on F 2 with 414 variables converged at R 1 = 5.00%, for the observed data and 64 wR 2 = 7.89% for all data. The goodness-of-fit was 1.447. The largest peak in the final difference electron density synthesis was 1.060 e - /Å 3 and the largest hole was -0.545 e - /Å 3 with an RMS deviation of 0.06 e - /Å 3 . On the basis of the final model, the calculated density was 1.332 g/cm 3 and F(000), 1392 e - . C 35 H 40 CuF 3 N 6 (3.5): A clear yellow prism-like specimen of approximate dimensions 0.30 mm x 0.30 mm x 0.39 mm, was used for the X-ray crystallographic analysis. A complete hemisphere of data was scanned on omega (0.5°) with a run time of 5-seconds per frame at a detector distance of 50.4 mm and a resolution of 512 x 512 pixels. A total of 4320 frames were collected. The integration of the data using a monoclinic unit cell yielded a total of 115485 reflections to a maximum  angle of 30.71° (0.70 Å resolution), of which 10151 were independent (average redundancy 11.377, completeness = 99.8%, R int = 4.69%, R sig = 2.05%) and 8352 (82.28%) were greater than 2 (F 2 ). The final cell constants of a = 11.8747(2) Å, b = 14.5369(3) Å, c = 19.6509(4) Å,  = 104.9210(10)°, V = 3277.79(11) Å 3 , are based upon the refinement of the XYZ-centroids of 132 reflections above 20 (I) with 3.525° < 2  < 36.70°. The ratio of minimum to maximum apparent transmission was 0.900. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.7672 and 0.8135. The structure was solved and refined using the space group P2 1 /n, with Z = 4 for the formula unit, C 35 H 40 CuF 3 N 6 . The final anisotropic full-matrix least-squares refinement on F 2 with 414 variables converged at R 1 = 4.04%, for the observed data and wR 2 = 10.63% for all data. The goodness-of-fit was 1.066. The largest peak in the final difference electron density synthesis was 0.667 65 e - /Å 3 and the largest hole was -0.527 e - /Å 3 with an RMS deviation of 0.074 e - /Å 3 . On the basis of the final model, the calculated density was 1.348 g/cm 3 and F(000), 1392 e - . Chapter 2 References (1) Armaroli, N.; Accorsi, G.; Cardinali, F.; Listorti, A. Top. Curr. Chem. 2007, 280, 69. (2) Scaltrito, D. V.; Thompson, D. W.; O'Callaghan, J. A.; Meyer, G. J. Coord. Chem. Rev. 2000, 208, 243. (3) Barbieri, A.; Accorsi, G.; Armaroli, N. Chem. Commun. 2008, 2185. (4) Lotito, K. J.; Peters, J. C. Chem. Commun. 2010, 46, 3690. (5) Hashimoto, M.; Igawa, S.; Yashima, M.; Kawata, I.; Hoshino, M.; Osawa, M. J. Am. Chem. Soc. 2011, 133, 10348. (6) Tulloch, A. A. D.; Danopoulos, A. A.; Kleinhenz, S.; Light, M. E.; Hursthouse, M. B.; Eastham, G. Organometallics 2001, 20, 2027. (7) Welle, A.; Diez-Gonzalez, S.; Tinant, B.; Nolan, S. P.; Riant, O. Org. Lett. 2006, 8, 6059. (8) Hsu, S. H.; Li, C. Y.; Chiu, Y. W.; Chiu, M. C.; Lien, Y. L.; Kuo, P. C.; Lee, H. M.; Huang, J. H.; Cheng, C. P. J. Organomet. Chem. 2007, 692, 5421. (9) Benhamou, L.; Chardon, E.; Lavigne, G.; Bellemin-Laponnaz, S.; Cesar, V. Chem. Rev. 2011, 111, 2705. (10) Kaur, H.; Zinn, F. K.; Stevens, E. D.; Nolan, S. P. Organometallics 2004, 23, 1157. (11) Jurkauskas, V.; Sadighi, J. P.; Buchwald, S. L. Org. Lett. 2003, 5, 2417. (12) Diez-Gonzalez, S.; Escudero-Adan, E. C.; Benet-Buchholz, J.; Stevens, E. D.; Slawin, A. M. Z.; Nolan, S. P. Dalton Trans. 2010, 39, 7595. (13) Goj, L. A.; Blue, E. D.; Delp, S. A.; Gunnoe, T. B.; Cundari, T. R.; Pierpont, A. W.; Petersen, J. L.; Boyle, P. D. Inorg. Chem. 2006, 45, 9032. (14) Teyssot, M.-L.; Chevry, A.; Traikia, M.; El-Ghozzi, M.; Avignant, D.; 66 Gautier, A. Chem.-Eur. J. 2009, 15, 6322. (15) Flores, J. A.; Andino, J. G.; Tsvetkov, N. P.; Pink, M.; Wolfe, R. J.; Head, A. R.; Lichtenberger, D. L.; Massa, J.; Caulton, K. G. Inorg. Chem. 2011, 50, 8121. (16) Shibata, T.; Ito, S.; Doe, M.; Tanaka, R.; Hashimoto, H.; Kinoshita, I.; Yano, S.; Nishioka, T. Dalton Trans. 2011, 40, 6778. (17) Herrmann, W. A.; Goossen, L. J.; Spiegler, M. J. Organomet. Chem. 1997, 547, 357. (18) Weskamp, T.; Schattenmann, W. C.; Spiegler, M.; Herrmann, W. A. Angew. Chem., Int. Ed. 1998, 37, 2490. (19) Chianese, A. R.; Li, X. W.; Janzen, M. C.; Faller, J. W.; Crabtree, R. H. Organometallics 2003, 22, 1663. (20) Dible, B. R.; Sigman, M. S. Inorg. Chem. 2006, 45, 8430. (21) Giordano, C.; Minisci, F.; Vismara, E.; Levi, S. J. Org. Chem. 1986, 51, 536. (22) Klappa, J. J.; Rich, A. E.; McNeill, K. Org. Lett. 2002, 4, 435. (23) Pucci, D.; Aiello, I.; Aprea, A.; Bellusci, A.; Crispini, A.; Ghedini, M. Chem. Commun. 2009, 1550. (24) Uber, J. S.; Vogels, Y.; van den Helder, D.; Mutikainen, I.; Turpeinen, U.; Fu, W. T.; Roubeau, O.; Gamez, P.; Reedijk, J. Eur. J. Inorg. Chem. 2007, 4197. (25) Singh, S. P.; Kumar, D.; Jones, B. G.; Threadgill, M. D. J. Fluorine Chem. 1999, 94, 199. (26) Thiel, W. R.; Eppinger, J. Chem.-Eur. J. 1997, 3, 696. (27) Funabiki, K.; Noma, N.; Kuzuya, G.; Matsui, M.; Shibata, K. J. Chem. Res., Synop. 1999, 300. (28) Yin, J.; Elsenbaumer, R. L. J. Org. Chem. 2005, 70, 9436. (29) Hodgkins, T. G.; Powell, D. R. Inorg. Chem. 1996, 35, 2140. (30) Khaskin, E.; Zavalij, P. Y.; Vedernikov, A. N. J. Am. Chem. Soc. 2006, 128, 13054. 67 (31) Chianese, A. R.; Mo, A.; Datta, D. Organometallics 2009, 28, 465. (32) Vasudevan, K. V.; Butorac, R. R.; Abernethy, C. D.; Cowley, A. H. Dalton Trans. 2010, 39, 7401. (33) Du, L. H.; Wang, Y. G. Synthesis-Stuttgart 2007, 675. (34) Bruker Instrument Service v2011.4.0.0 ed.; Bruker AXS Madison, WI: 2011. (35) SAINT; V7.68A ed.; Bruker AXS Madison, WI: 2009. (36) SADABS; V2008/1 ed.; Bruker AXS Madison, WI: 2008. (37) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112. (38) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565. 68 CHAPTER 3. Photophysical and Computational studies of (IPr)Cu(N^N) Complexes 3.1. Introduction Phosphorescent Cu(I) complexes is an emerging class of luminescent materials based on inexpensive and abundant metal. Since the first report of room temperature phosphorescence form Cu(I) complexes 1 their emission properties have been considerably improved. It was shown that Cu(I) complexes can exhibit emission properties comparable to materials using third row transition metals (Ir(III), Pt(II), Os(II)). 2-4 Successful examples of applications of luminescent Cu(I) complexes in organic electronics, 4-6 sensors 7 and biological systems 8,9 have been demonstrated. Nonetheless the copper based materials have not been as well-developed as their third row counterparts. It is important to obtain greater knowledge of their chemical and photophysical behavior in order to tune their properties in a predictable manner. Research to date has focused principally on four-coordinate tetrahedral homo- and heteroleptic Cu(I) complexes bearing bisimine and phosphine chelating ligands. 10,11 Photophysical processes that occur in luminescent four-coordinate Cu(I) complexes have been thoroughly investigated. Strong evidence supports a Jahn-Teller based distortion (flattening) in the excited state, and consequent formation of a five-coordinate exciplex, promotes non-radiative decay. 12-14 The generally accepted approach to alleviate this problem is to increase the steric bulk of ligands in order to block excited state geometrical distortion and non-emissive relaxation pathways. 15 Alternatively, Lotito et al. have recently reported several examples of emissive three-coordinate copper(I) 69 arylamidophosphine complexes. 16 Unlike four-coordinate tetrahedral Cu(I) compounds, the three-coordinate geometry eliminates the possibility of flattening distortion in the excited state, though exciplex formation is still possible. We have synthesized and characterized a series of three-coordinate (NHC)Cu(N^N) complexes, where NHC is a monodentate N-heterocyclic carbene ligand and N^N is a neutral or monoanionic chelating ligand. These complexes are rare examples of emissive Cu(I)-NHC compounds. To the best of our knowledge the only other luminescent Cu(I)-NHC derivative is a binuclear copper(I) complex reported by Matsumoto and co-workers. 17 Figure 3.1. Molecular structures of (NHC)Cu(N^N) complexes discussed in Chapter 3. A common approach to alter emission properties of transition metal complexes is through modification of the ligands. 18-20 The strategy can also be helpful in providing valuable insight into excited state properties. 70 3.2. Results and Discussions 3.2.1. Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) studies Density functional theory (DFT) calculations were carried out for complexes 3.1–3.7 using geometric parameters obtained from X-ray analyses as starting structures. A similar approach has been used to gain insight into ground and excited state properties of neutral Cu(I) complexes. 21 In general, geometrical parameters for the optimized ground state geometries are in good agreement with the corresponding values determined by X-ray crystallography. The calculated metal ligand bonds are slightly longer than experimental values for all complexes. For example, bond lengths calculated for 3.2 are C NHC –Cu = 1.931 Å, Cu–N py = 2.149 Å and Cu–N az = 2.009 Å, whereas experimental values are 1.8828(12) Å, 2.1346(11) Å and 1.9552(11) Å, respectively. The calculated bond angles are within 5° of the X-ray structures for 3.1-3.2 and 3.4-3.7. According to the structure of the N^N ligand and molecular orbital (MO) composition the complexes discussed in this chapter can be divided into three categories, namely cationic complex 3.1; pyridine-azolate derivatives 3.2-3.5 and benzimidazole derivatives 3.6-3.7. The HOMO (-8.49 eV) of 3.1 is predominantly metal-based with little contribution from the phenanthroline ligand. The LUMO (-4.63 eV) is effectively the π* orbital of phenanthroline. The molecular orbital (MO) diagram of 3.1 suggests the presence of metal-to-ligand charge transfer (MLCT) transitions in this complex (Figure 3.2). 71 Figure 3.2. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) plots and energies (eV) (A) and triplet spin density surface (B) for 3.1. Complexes 3.2–3.5 have similar frontier MO compositions, but variable MO energies. The molecular orbitals (MO) surfaces calculated for complex 3.2 are shown in Figure 3.3. The HOMOs have significant participation of metal d-orbitals, as well as contribution from orbitals on imidazolylidene ring of the carbene ligand and pyridyl-azolate ligand, whereas the LUMOs are localized primarily on pyridyl-azolate ligand. Complex 3.3 has the highest HOMO and LUMO energies among the four complexes. Addition of an electron-withdrawing CF 3 -group in complex 3.4 leads to equal stabilization both the HOMO and LUMO. Introduction of a third nitrogen in azolate moiety in 3.5 leads to further stabilization of both frontier orbitals. Thus, the HOMO and LUMO energies of complex 3.2 fall within the values for compounds 3.4 and 3.5 due to the presence of two electron-withdrawing CF 3 -groups. While the energies calculated for the HOMO and LUMO are strongly influenced by the identity of the pyridyl-azolate ligand, the HOMO- LUMO gap remains effectively constant within the series. The ability to systematically vary the HOMO and LUMO energies while keeping the energy separation between them 72 unchanged can be an advantageous feature for potential applications in organic electronics devices. Figure 3.3. The frontier orbitals for 3.2 and calculated HOMO and LUMO energies (eV) for complexes 3.2–3.5. In contrast to complexes 3.1-3.5, the calculated MOs of 3.6 and 3.7 have little metal character (Figure 3.4). The HOMO and HOMO-1 of both compounds are dominated by benzimidazole orbitals, and only a small contribution comes from the copper atom. Significant participation from orbitals on copper first appears in the HOMO-2. The LUMOs of 3.6 and 3.7 mainly consist of orbitals on pyridine and isoquinoline groups respectively, with some contribution from orbitals on NHC ligand for 3.6. Based on MO distribution modification of pyridine moiety in 3.6 to form isoquinoline in 3.7 should principally affect LUMO energies. Accordingly, extending of π-system of lowers the LUMO energy, while the HOMO energy is effectively unchanged. 73 Figure 3.4. HOMO, HOMO-1, HOMO-2 and LUMO plots and energies (eV) for 3.6 and 3.7. Time-dependent DFT (TD-DFT) calculations were used to determine the lowest energy electronic transitions in 3.1–3.7, the results are summarized in Table 3.1. The lowest lying singlet excitations for complexes 3.1-3.5 are HOMO→LUMO and can thus be ascribed to metal-to-ligand charge transfer (MLCT) transitions. The lowest triplet state for complexes 3.1, 3.2, 3.4 and 3.5 is also principally HOMO→LUMO (MLCT) in character. A second triplet state, which according to calculations lies from 400 cm -1 to 2200 cm -1 above the lowest triplet, is mainly ligand-centered (LC) in character. In complex 3.3, the position of these two triplet states are switched, such that the lowest triplet transition is principally LC in character, while the MLCT state lies 493 cm -1 higher in energy. Nevertheless, on the basis of these results we assign the lowest excited state as being mixed 3 MLCT/LC in character. 74 Table 3.1. Lowest energy transitions for complexes 3.1–3.7 determined from TD-DFT calculations. States λ (nm) f Major contribution a Assignments 3.1 S 1 T 2 T 1 426 437 445 0 0 0 HOMO→LUMO (100%) HOMO-5→LUMO+1 (52%) HOMO→LUMO (100%) MLCT LC MLCT 3.2 S 1 T 2 T 1 427 441 449 0.0002 0 0 HOMO→LUMO (100%) HOMO-1→LUMO (82%) HOMO→LUMO (92%) MLCT LC MLCT 3.3 S 1 T 2 T 1 422 446 455 0.0001 0 0 HOMO→LUMO (100%) HOMO→LUMO (100%) HOMO-1→LUMO (90%) MLCT MLCT LC 3.4 T 2 S 1 T 1 425 427 451 0 0.0001 0 HOMO-1→LUMO (77%) HOMO→LUMO (100%) HOMO→LUMO (100%) LC MLCT MLCT 3.5 T 2 S 1 T 1 401 419 442 0 0.0001 0 HOMO-2→LUMO (70%) HOMO→LUMO (100%) HOMO→LUMO (100%) LC MLCT MLCT 3.6 S 2 S 1 T 3 T 2 T 1 394 394 414 455 490 0.0344 0.007 0 0 0 HOMO→LUMO (66%) HOMO-2→LUMO (24%) HOMO-2→LUMO (100%) HOMO-2→LUMO (100%) HOMO-1→LUMO (72%) HOMO→LUMO (65%) MLCT/ILCT MLCT MLCT LC ILCT 3.7 S 2 S 1 T 3 T 2 T 1 446 460 481 540 562 0.0336 0.0006 0 0 0 HOMO→LUMO (100%) HOMO-2→LUMO (100%) HOMO-2→LUMO (100%) HOMO→LUMO (79%) HOMO-1→LUMO (75%) LC MLCT MLCT LC LC a transitions with >15% contribution For complexes 3.6 and 3.7 the three lowest energy excitations are triplet transitions. The T 1 and T 2 are mainly HOMO→LUMO and HOMO-1→LUMO transitions and can be ascribed as ILCT/LC for 3.6 and LC for 3.7. The T 3 state (HOMO-2→LUMO) which is MLCT in character lays 2176 cm -1 (3.6) and 2271 cm -1 (3.7) above the T 2 state. Therefore MLCT contribution to the lowest excited state of complexes 3.6 and 3.7 is low and it was assigned as 3 ILCT/LC for 3.6 and 3 LC for 3.7. The lowest singlet excited state 75 S 1 for both complexes is mainly associated with HOMO-2→LUMO transition and therefore is principally MLCT in character and correlates with the T 3 state. The singlet states that correlate with the T 1 and T 2 states lie at higher energies due to greater S-T energy gap between 1 LC and 3 LC states compared to S-T splitting for MLCT states. 22 The calculated triplet spin density surfaces (Figure 3.5) are localized on chelating (N^N) ligand ligand with contribution from copper in case of 3.1. For 3.2-3.5 participation of copper is minimal and it is not observed for 3.6-3.7. This result indicates that metal participation in the lowest excited state is decreasing when going from 3.1 to 3.7. Figure 3.5. Optimized triplet geometries and triplet spin density contour plots (isovalue: 0.004 e a 0 -3 ) for complexes 3.1, 3.3 and 3.6. Hydrogen atoms are omitted for clarity. 3.2.2. Photophysical and electrochemical properties of cationic complex [(IPr)Cu(phen)]OTf The photophysical properties of complex 3.1 are summarized in Table 3.2. The absorption spectrum of 3.1 recorded in CH 2 Cl 2 (Figure 3.6) shows intense bands in the region between 250–300 nm (ε = 10 000–30 000 M -1 cm -1 ) that are assigned to π → π* ligand-centered (LC) transitions. Transitions at lower energy (332 nm, ε = 3280 M -1 cm -1 ; 348 nm, ε = 2280 M -1 cm -1 ) can be ascribed to dπ →π* metal-to-ligand charge transfer 76 (MLCT) absorptions. Very weak (ε< 100 M -1 cm -1 ) bands at wavelengths between 400 and 500 nm are assigned to triplet MLCT states. The excitation spectrum (Figure 3.6) matches the absorption spectrum and supports the assignment. 250 300 350 400 450 500 550 0 5000 10000 15000 20000 25000 30000 35000 40000 x 15 Molar absorbance (M -1 cm -1 ) Wavelength (nm) Figure 3.6. Absorption (RT, CH 2 Cl 2 ) spectrum of complex 3.1. Emission spectrum recorded at 77 K in frozen 2-methyltetrahydrofuran (2-MeTHF) glass displays a broad, featureless band centered at 630 nm (Figure 3.6). The luminescence decay of 3.1 has two components with lifetimes of 1.8 µs and 4.6 µs (major component, 76%). The unstructured emission with large Stokes shift and long lifetime is indicative of luminescence from a triplet charge transfer (CT) state. 77 300 350 400 450 500 550 600 650 700 750 800 0.0 0.2 0.4 0.6 0.8 1.0 exc (2MeTHF, 77K) em (2MeTHF, 77K) em (CH 2 Cl 2 , RT) Normalized intensity (a.u.) Wavelength (nm) Figure 3.7. Excitation (closed symbols) and emission spectra (open symbols) of complex 3.1. Complex 3.1 shows weak luminescence in oxygen-free CH 2 Cl 2 solution at room temperature. Emission maximum is red shifted by 25 nm compared with that at 77 K (Figure 3.7). After degassing the solution by sparging with N 2 for 15 minutes, the quantum yield (Φ) for 3.1 is <0.1% and excited state lifetime (τ) is 0.08 µs. The latter value is comparable to the shortest lifetimes reported for Cu(I)–bisphenanthroline complexes in degassed solution. 10 Emission efficiency is slightly higher in rigid environment. Compound 3.1 emits with Φ = 2.6% as a crystalline solid and Φ = 1.5% (t = 24.7 ms) when doped (2 wt%) in a PMMA film. This result suggests that molecular distortions in the excited state may be one of the mechanisms for quenching in solution. 78 Table 3.2. Summary of photophysical properties of 3.1. CH 2 Cl 2 300K PMMA film (2 wt%) 300 K crystals 300K 2-MeTHF 77K τ (µs) 0.08 0.23(11%), 1.1 (89%); 1.2 0.23(11%), 1.1 (89%); Φ <0.001 0.015 0.026 - a a Quantum yield not measured. The electrochemical properties of 3.1 in CH 2 Cl 2 solution were investigated using cyclic voltammetry (CV) (Figure 3.8). Complex 3.1 undergoes irreversible oxidation and reduction at 1.23 V and -2.15 V vs. Fc+/Fc, respectively. -3 -2 -1 0 1 2 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Current (mA) Potential (V) Figure 3.8. Cyclic voltammogram of (IPr)Cu(phen)]OTf (3.1). 3.2.3. Photophysical properties of neutral complexes (IPr)Cu(N^N) 3.2.3.1. Absorption spectra Absorption spectra for complexes 3.2–3.5 recorded in dichloromethane solution at room temperature are shown in Figure 3.9. Intense bands (ε = 10 000–18 000 M -1 cm -1 ) 79 between 250–350 nm are assigned to spin-allowed 1 ( -  * ) ligand-centered (LC) transitions. The LC energy between 300–375 nm undergoes a hypsochromic shift in going from 3.2→3.5 that parallels trends observed in the spectra obtained from the free pyridyl-azolate ligands (Figure 3.9). Very weak transitions at lower energies (> 375 nm) are also observed in the excitation spectra obtained in methylcyclohexane at 77K. These features, assigned to triplet CT transitions, absorb down to 450 nm and are responsible for the yellow coloration of the complexes in the solid state. The solution absorption cut- offs obtained experimentally for complexes 3.2–3.5 correlate well with the small range (442–455 nm) calculated for the lowest triplet transitions. 250 300 350 400 450 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 3.2 3.3 3.4 3.5 Molar Absorbance (M -1 cm -1 ) Wavelength (nm) 240 260 280 300 320 340 0.0 0.5 1.0 1.5 2.0 2.5 (IPr)CuCl Hfpyro Hppz Hfppz Hfpta Absorbance (a.u.) Wavelength (nm) Figure 3.9. Absorption (RT, CH 2 Cl 2 ) spectrum of complexes (IPr)Cu(fpyro) (3.2), (IPr)Cu(ppz) (3.3), (IPr)Cu(fppz) (3.4) and (IPr)Cu(fpta) (3.5) (left) and ligand precursors (right). The optical spectra of 3.6 and 3.7 feature a single intense absorption band at 335 nm (ε = 16 500 M -1 cm -1 ) and at 383 nm (ε = 14 696 M -1 cm -1 ) respectively that are assigned to a singlet π → π* LC transition (Figure 3.10). No distinct triplet transitions were observed in the visible region. These observations are consistent with the results of TD-DFT calculations. They predict intense singlet transition at 365 nm (f=0.1916) for 3.6 and 80 406 nm (f=0.2916) for 3.7, which is mainly HOMO-1→LUMO excitation ( 1 LC). Bathochromic shift of absorption spectra of 3.7 is consistent with expansion of the size of π-system of a ligand chromophore and in accord with the lower HOMO/LUMO gap calculated for 3.7 relative to that of 3.6. 300 350 400 450 500 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 3.6 3.7 Wavelength (nm) Molar Absorbance (M -1 cm -1 ) Figure 3.10. Absorption (RT, CH 2 Cl 2 ) spectra of complexes (IPr)Cu(pybim) (3.6) and (IPr)Cu(beniq) (3.7). 3.2.3.2. Emission properties 3.2.3.2.1. Photophysical properties at 77K. Excited state distortions in three- coordinate Cu(I) complexes. Emission spectra recorded in frozen solvent matrix at 77 K are shown in Figure 3.11 and data are summarized in Table 3.3. Low temperature data provide information about the nature of the lowest excited state. Complexes 3.2-3.6 have similar emission properties regardless of differences in the pyridyl-azolate ligand. They exhibit broad featureless emission bands that are significantly red-shifted relative to the absorption. Emission maxima fall within a small range (555–570 nm) and excited state lifetimes vary from 58 81 s to 77 s. These results indicate that the observed emission originates from a triplet excited state that is largely charge-transfer in character. Complex 3.7, unlike 3.2-3.6 shows structured emission and very long lifetime of 2.2 ms at 77K (Figure 3.11). This behavior is indicative of ligand localized (LC) emission. 450 500 550 600 650 700 750 800 0.0 0.2 0.4 0.6 0.8 1.0 3.2 3.3 3.4 3.5 3.6 Normalized intensity (a.u.) Wavelength (nm) 550 600 650 700 750 800 0.0 0.2 0.4 0.6 0.8 1.0 3.7 Normalized intensity (a.u.) Wavelength (nm) Figure 3.11. Emission spectra of complexes 3.2-3.6 in methylcyclohexane at 77K (left) and 3.7 in 2-methyltetrahydrofuran at 77K (right). Interestingly, the emission properties of complexes 3.2–3.6 at 77K are distinctly different from other transition metal complexes bearing the same pyridyl-azolate ligands. For example, homoleptic iridium complexes mer-Ir(fppz) 3 and mer-Ir(fpta) 3 have structured emission in frozen solvent at 77K with the first vibronic peak at 430 nm. 23 In yet another example where a pyridyl-azolate is the chromophoric ligand, the four-coordinate copper complex (POP)Cu(fpyro) (POP = bis[2-(diphenylphosphino) phenyl]ether) showed structured emission spectra at 77K. 21 Emission from this complex, assigned to a mixed 3 MLCT/LC state, has a first vibronic peak (446 nm) that correlates well with the calculated lowest triplet transition (441 nm). However, complexes 3.2 –3.6 display emission at 77 K that is very broad and is red-shifted compared to values calculated for 82 the lowest triplet transition. A likely origin of this behavior is the large structural reorganization in the excited state. It is worth noting that the energies of the excited states obtained from TD-DFT calculations correspond to Franck-Condon (vertical) transitions. When a compound undergoes structural distortion after photoexcitation, its lowest emissive state will lie at lower energy than the Franck-Condon state. It is well- known that tetrahedral four-coordinate Cu(I) complexes are prone to a Jahn–Teller-type flattening distortion in the excited state that has significant impact on their photophysical properties. 13,24-26 Although the three-coordinate geometry eliminates the possibility of a similar flattening distortion, other types of distortion may still occur in the excited state. The type of distortion has not been confirmed experimentally; however, theoretical calculations of three-coordinate Au(I) and Cu(I) complexes postulate that a Y- to T-shape distortion may occur in the triplet excited state for such planar species. 27 The calculated HOMOs in 3.2-3.5 have significant contribution from copper 3d-orbital that are largely antibonding in character (Figure 3.3). Therefore, removal of an electron from the HOMO upon photoexcitation will likely induce a geometrical relaxation that can have a pronounced effect on photophysical properties, depending on the degree of metal participation in the lowest excited state. Emission from complexes 3.2–3.5 is predominantly MLCT in character (with some LC mixed-in), thus structural reorganization is likely to occur in the excited state. As a result, experimentally observed emission (λ max = 555-570 nm) is significantly red shifted relative to the calculated values of the lowest Franck-Condon state (442-455 nm). Complex 3.6 has smaller MLCT contribution in the lowest excited state according to theoretical calculations, therefore it 83 shows smaller red shift of observed emission (λ max = 555 nm) relative to the predicted lowest Franck-Condon state (490 nm). Finally, complex 3.7 displays 3 LC emission at 77 K, consequently the first vibronic peak at 564 nm correlates well with the calculated (TD-DFT) lowest triplet excitation at 561 nm. These findings, summarized in Figure 3.12 demonstrate that the emission energy of these and related Cu(I) complexes is determined not only by the energy of a ligand chromophore, but also by degree of structural relaxation, which in turn depends on the degree of metal participation in the lowest excited state. Figure 3.12. Influence of the nature of ligand chromophore and excited state distortions on the emission energies in three-coordinate (IPr)Cu(N^N) complexes. 84 3.2.3.2.2. Emission in solution at RT Complexes 3.2–3.7 are emissive in fluid solution at room temperature (Table 3.3). The emission properties in solution are solvent dependent (Figure 3.13). For example, excited state lifetime and quantum yield of complex 3.6 in cyclohexane are 13.4 µs and 1.4% respectively. In CH 2 Cl 2 τ = 0.27 µs and Φ = 0.5%. In more coordinating 2-MeTHF, the lifetime is 0.14 µs, whereas no emission can be observed in acetonitrile. These observations suggest that emission in solution is quenched to some extent by solvent exciplex formation. 400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0 c-hexane toluene 2MeTHF DCM Normalized intensity (a.u.) Wavelength (nm) Figure 3.13. Emission spectra of (IPr)Cu(pybim) (3.6) in solution at RT. For complexes 3.2-3.5 measurements were performed in a non-coordinating solvent (cyclohexane) in order to avoid possible formation of exciplexes that may lead to luminescent quenching. 28,29 Emission spectra are shown in Figure 3.14. 85 450 500 550 600 650 700 750 800 0.0 0.2 0.4 0.6 0.8 1.0 3.2 3.3 3.4 3.5 Normalized intensity (a.u.) Wavelength (nm) Figure 3.14. Emission spectra of complexes 3.2-3.5 in cyclohexane at RT. The behavior of complexes 3.3 –3.5 are similar to each other in that, relative to data recorded at 77 K, spectra (centered around 592 nm) are red-shifted 20–35 nm and have short emission lifetimes (  = 0.54–1.3 s) along with low quantum yields (  < 3%). The bathochromic shifts and low efficiencies are attributed to molecular distortions in the excited state that occur readily in fluid media. In contrast, emission from (IPr)Cu(fpyro) (3.2) undergoes a 10 nm blue-shift relative to its 77 K spectrum, and has both a longer excited state lifetime (  = 10 s) and higher quantum yield (  = 17%) than complexes 3.3–3.5. This behavior is ascribed to the smaller extent of excited state distortion in complex 3.2, which leads to an order of magnitude decrease in the nonradiative decay rate (k nr = 8.3 x 10 4 s -1 ). The smaller distortion may have its origin in differences seen in the ground state geometry of the complexes. The crystal structure for 3.2 shows the Cu coordinated in a T-like geometry, whereas complexes 3.4 and 3.5 (and presumably 3.3) have a Y-like coordination geometry around copper. The T-like structure of 3.2 is closer to the optimized triplet geometry proposed for other three-coordinate d 10 complexes 27 and 86 may predispose 3.2 to undergo less distortion in the excited state than geometric changes that occur from the Y-like structures of 3.3–3.5. Complex 3.7 shows very weak (<0.5%) emission in CH 2 Cl 2 centered at 690 nm and lifetime of 0.23 µs. Table 3.3. Photophysical properties of complexes 3.2–3.7 in solution. emission at RT a emission at 77K b  max (nm)  ( s)  PL k r (s -1 )  k nr (s -1 )    max (nm)  ( s) 3.2 560 10 0.17 1.7 x 10 4 8.3 x 10 4 570 58 3.3 594 1.3 0.032 2.5 x 10 4 7.4 x 10 5 574 68 3.4 592 0.54 0.014 2.6 x 10 4 1.8 x 10 6 560 77 3.5 590 1.1 0.024 2.2 x 10 4 8.9 x 10 5 555 61 3.6 555 13.4 0.014 1.0 x 10 4 7.3 x 10 4 555 62 3.7 690 c 0.23 c <0.005 c - - 564, 578, 610, 662 d 2.2 x 10 4 d a In cyclohexane deaerated with N 2 . b In methylcyclohexane c In CH 2 Cl 2 deaerated with N 2 d In 2- MeTHF. 3.2.3.2.3. Solid state emission at RT and 77 K. Thermally activated emission Photophysical properties from neat crystalline samples were recorded at room temperature and 77 K. Emission spectra are depicted in Figures 3.15 and 3.16 and photophysical data are summarized in Table 3.4. At room temperature complexes 3.2 and 3.4-3.6 are bright emitters with phosphorescence quantum yields ranging from 0.23 to 0.62. They have similar emission lineshapes (  max = 560–570 nm) and excited state lifetimes (  = 12–33 s). On the other hand, emission from complex 3.3 is rather weak (Φ = 0.065) with a short lifetime (  = 2 s), and its spectrum is almost 40 nm red-shifted from the other derivatives. The bathochromic shift of (IPr)Cu(ppz) (3.3) and high nonradiative decay rate (and consequent low luminescence efficiency) is likely caused by 87 a crystal packing arrangement unlike that which occurs in derivatives with bulky CF 3 - groups. The radiative rates (k r ) for compounds 3.3-3.6 at room temperature fall within a range (k r = 1.7–5.2 x 10 4 s -1 ) that is similar to values found in fluid solution. 450 500 550 600 650 700 750 800 77K 3.2 3.3 3.4 3.5 RT Normalized intensity (a.u.) Wavelength (nm) Figure 3.15. Solid state emission spectra of complexes 3.2-3.5 at RT (top) and at 77 K (bottom). Table 3.4. Photophysical properties of complexes 3.2–3.7 in solid state. emission at RT emission at 77 K  max (nm)  ( s)  PL k r (s -1 ) k nr (s -1 )   max (nm)  ( s) 3.2 570 13 0.23 1.7 x 10 4 5.9 x 10 4 592 28 3.3 605 2.0 0.065 3.3 x 10 4 4.7 x 10 5 632 17 3.4 560 12 0.62 5.2 x 10 4 3.2 x 10 4 566 74 3.5 560 12 0.48 4.0 x 10 4 4.3 x 10 4 590 35 3.6 566 33 0.58 1.7 x 10 4 1.3 x 10 4 566 55 3.7 628 73 0.13 1.8 x 10 3 1.2 x 10 4 566,578, 612,660 638 Compound 3.7 emits in orange-red part of the visible spectrum (  max = 628 nm) with Φ = 0.13 and τ = 73 µs. The radiative rate of 3.7 is an order of magnitude slower than that of complexes 3.2-3.6, which can be attributed to lower MLCT character in the 88 emissive state of 3.7. The observed enhancement of emission efficiency for all complexes discussed here, relative to values from fluid solution, is consistent with the rigid, crystalline environment lowering the nonradiative decay rates by suppressing molecular vibrations and distortion in the excited state. 400 450 500 550 600 650 700 750 800 0.0 0.2 0.4 0.6 0.8 1.0 RT 77K Normalized intensity (a.u.) Wavelength (nm) 500 550 600 650 700 750 800 0.0 0.2 0.4 0.6 0.8 1.0 RT 77K Normalized intensity (a.u.) Wavelength (nm) Figure 3.16. Solid state emission spectra of complexes 3.6 (left) and 3.7 (right) at RT (open symbols) and at 77 K (closed symbols). Upon cooling to 77 K, the emission spectra for all complexes undergo a red-shift accompanied by an increase in emission lifetime (Table 3.4). Such behavior is common for luminescent Cu(I) complexes and is usually ascribed to a decrease in thermally- activated (E-type) delayed fluorescence (Figure 3.17). 5,30-33 At low temperatures, thermally activated population of higher-lying state with faster radiative rate is suppressed, leading to red-shifted emission and longer excited state lifetimes. Complexes 3.2-3.5 show 6-30 nm red shift and a 2 to 8.5-fold increase in lifetime (  = 17–74 s). For 3.6 emission maximum does not charge upon cooling, but the low energy rise of emission spectrum undergoes bathochromic shift and lifetime becomes 2 times longer (55 µs). In contrast to room temperature spectrum, that is broad and structureless, 89 emission of complex 3.7 shows vibronic progression and relatively long lifetime of 638 µs, which is indicative of 3 LC emission. Figure 3.17. Mechanism of thermally activated delayed fluorescence. Interestingly, the TD-DFT calculations for complexes 3.2-3.5 suggest that both S 1 and T 2 states lie close enough in energy (≤ 2314 cm -1 ) to be thermally accessible from the lowest triplet state, T 1 . For complexes 3.6 and 3.7 the calculated T 2 state lie within 1570 cm -1 from T 1 state and T 3 state is separated by 3746 cm -1 (3.6) and 2997 cm -1 (3.7) from T 1 . The S 1 state is 4973 cm -1 (3.6) and 3946 cm -1 (3.7) above T 1 . Thus, it is unlikely that the singlet state is thermally populated at room temperature in case on complexes 3.6 and 3.7. While our data provide strong evidence of the participation of a higher-lying state with faster radiative rate at room temperature, the nature of this state (singlet or triplet) has not been confirmed experimentally. 90 3.3. Conclusion A new family of phosphorescent 3-coordinate mononuclear Cu(I) complexes (NHC)Cu(N^N), where NHC is a monodentate N-heterocyclic carbene ligand and N^N is a neutral or monoanionic chelating ligand has been developed. These complexes show broad-band phosphorescence in solution and the solid state at room temperature and 77K associated with chelating N^N ligand chromophore. The degree of metal participation in the lowest excited state varies within the series. Photophysical properties are strongly dependent on experimental conditions. In particular, phosphorescence in fluid solution is significantly lower than in rigid media, a phenomenon attributed to quenching by excited state distortions. In the solid state, a hypsochromic shift in emission wavelength upon going from 77K to room temperature, along with a decrease in emission lifetime, is consistent with thermal population to a higher-lying state with a faster radiative rate. These findings provide insights into structure and photophysical behavior of three- coordinate NHC–Cu(I) complexes and should aid in design of new related materials in the future. 3.4. Experimental section Synthesis. Synthetic procedures for [(IPr)Cu(phen)]OTf (3.1), (IPr)Cu(fpyro) (3.2), (IPr)Cu(ppz) (3.3), (IPr)Cu(fppz) (3.4), (IPr)Cu(fpta) (3.5), (IPr)Cu(pybim) (3.6), (IPr)Cu(beniq) (3.7) are described in Chapter 2. Density Functional Calculations. Density functional theory (DFT) calculations were 91 performed with the Gaussian03 34 software package employing the B3LYP functional 35,36 using LANL2DZ basis set 37-39 for Cu and 6-31G* for C, N, H and F. Geometric parameters obtained from XRD analyses were used as a starting point for geometry optimization in the ground state. The optimized geometries were used for time- dependent density functional calculations (TD-DFT). Photophysical Characterization. The UV-visible spectra were recorded on a Hewlett- Packard 4853 diode array spectrometer. Steady state emission measurements were performed using a Photon Technology International QuantaMaster model C-60 fluorimeter. All reported spectra are corrected for photomultiplier response. Phosphorescence lifetime measurements were performed on the same fluorimeter equipped with a microsecond Xe flash lamp or using an IBH Fluorocube instrument equipped with a 405 nm LED excitation source using time-correlated single photon counting method. Quantum yields at room temperature were measured using a Hamamatsu C9920 system equipped with a xenon lamp, calibrated integrating sphere and model C10027 photonic multichannel analyzer. All solutions were prepared in air and deaerated by sparging with nitrogen for 15 minutes prior to performing emission, lifetime and quantum yield measurements. Chapter 3 References (1) Blaskie, M. W.; McMillin, D. R. Inorg. Chem. 1980, 19, 3519. (2) Cuttell, D. G.; Kuang, S. M.; Fanwick, P. E.; McMillin, D. R.; Walton, R. A. J. Am. Chem. Soc. 2002, 124, 6. (3) Miller, A. J. M.; Dempsey, J. L.; Peters, J. C. Inorg. Chem. 2007, 46, 92 7244. (4) Hashimoto, M.; Igawa, S.; Yashima, M.; Kawata, I.; Hoshino, M.; Osawa, M. J. Am. Chem. Soc. 2011, 133, 10348. (5) Deaton, J. C.; Switalski, S. C.; Kondakov, D. Y.; Young, R. H.; Pawlik, T. D.; Giesen, D. J.; Harkins, S. B.; Miller, A. J. M.; Mickenberg, S. F.; Peters, J. C. J. Am. Chem. Soc. 2010, 132, 9499. (6) Liu, Z.; Qayyum, M. F.; Wu, C.; Whited, M. T.; Djurovich, P. I.; Hodgson, K. O.; Hedman, B.; Solomon, E. I.; Thompson, M. E. J. Am. Chem. Soc. 2011, 133, 3700. (7) Smith, C. S.; Branham, C. W.; Marquardt, B. J.; Mann, K. R. J. Am. Chem. Soc. 2010, 132, 14079. (8) Liu, F.; Meadows, K. A.; McMillin, D. R. J. Am. Chem. Soc. 1993, 115, 6699. (9) Mahadevan, S.; Palaniandavar, M. Inorg. Chem. 1998, 37, 693. (10) Armaroli, N.; Accorsi, G.; Cardinali, F.; Listorti, A. Top. Curr. Chem. 2007, 280, 69. (11) Scaltrito, D. V.; Thompson, D. W.; O'Callaghan, J. A.; Meyer, G. J. Coord. Chem. Rev. 2000, 208, 243. (12) Siddique, Z. A.; Yamamoto, Y.; Ohno, T.; Nozaki, K. Inorg. Chem. 2003, 42, 6366. (13) Chen, L. X.; Jennings, G.; Liu, T.; Gosztola, D. J.; Hessler, J. P.; Scaltrito, D. V.; Meyer, G. J. J. Am. Chem. Soc. 2002, 124, 10861. (14) Stacy, E. M.; McMillin, D. R. Inorg. Chem. 1990, 29, 393. (15) Lavie-Cambot, A.; Cantuel, M.; Leydet, Y.; Jonusauskas, G.; Bassani, D. M.; McClenaghan, N. D. Coord. Chem. Rev. 2008, 252, 2572. (16) Lotito, K. J.; Peters, J. C. Chem. Commun. 2010, 46, 3690. (17) Matsumoto, K.; Matsumoto, N.; Ishii, A.; Tsukuda, T.; Hasegawa, M.; Tsubomura, T. Dalton Trans. 2009, 6795. (18) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.; Bau, R.; Thompson, M. E. Inorg. Chem. 2002, 41, 3055. 93 (19) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H. E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304. (20) You, Y.; Park, S. Y. Dalton Trans. 2009, 1267. (21) Hsu, C.-W.; Lin, C.-C.; Chung, M.-W.; Chi, Y.; Lee, G.-H.; Chou, P.-T.; Chang, C.-H.; Chen, P.-Y. J. Am. Chem. Soc. 2011, 133, 12085. (22) Yersin, H. Proc. SPIE-Int. Soc. Opt. Eng. 2004, 5214, 124. (23) Yeh, Y.-S.; Cheng, Y.-M.; Chou, P.-T.; Lee, G.-H.; Yang, C.-H.; Chi, Y.; Shu, C.-F.; Wang, C.-H. ChemPhysChem 2006, 7, 2294. (24) Chen, L. X.; Shaw, G. B.; Novozhilova, I.; Liu, T.; Jennings, G.; Attenkofer, K.; Meyer, G. J.; Coppens, P. J. Am. Chem. Soc. 2003, 125, 7022. (25) Siddique, Z. A.; Yamamoto, Y.; Ohno, T.; Nozaki, K. Inorg. Chem. 2003, 42, 6366. (26) Gothard, N. A.; Mara, M. W.; Huang, J.; Szarko, J. M.; Rolczynski, B.; Lockard, J. V.; Chen, L. X. J. Phys. Chem. A 2012, 116, 1984. (27) Barakat, K. A.; Cundari, T. R.; Omary, M. A. J. Am. Chem. Soc. 2003, 125, 14228. (28) Palmer, C. E. A.; McMillin, D. R.; Kirmaier, C.; Holten, D. Inorg. Chem. 1987, 26, 3167. (29) Stacy, E. M.; McMillin, D. R. Inorg. Chem. 1990, 29, 393. (30) Czerwieniec, R.; Yu, J.; Yersin, H. Inorg. Chem. 2011, 50, 8293. (31) Blasse, G.; McMillin, D. R. Chem. Phys. Lett. 1980, 70, 1. (32) Breddels, P. A.; Berdowski, P. A. M.; Blasse, G. J. Chem. Soc., Faraday Trans. 1982, 78, 595. (33) Kirchhoff, J. R.; Gamache, R. E.; Blaskie, M. W.; Delpaggio, A. A.; Lengel, R. K.; McMillin, D. R. Inorg. Chem. 1983, 22, 2380. (34) Frisch, M. J. T., G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,; M. A.; Cheeseman, J. R. M., J. A., Jr.; Vreven, T.; Kudin, K. N.;; Burant, J. C. M., J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;; Mennucci, B. C., M.; Scalmani, G.; Rega, N.; Petersson, G. A.;; Nakatsuji, H. H., M.; Ehara, 94 M.; Toyota, K.; Fukuda, R.; Hasegawa,; J.; Ishida, M. N., T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,; X.; Knox, J. E. H., H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;; Gomperts, R. S., R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;; Pomelli, C. J. O., W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;; Salvador, P. D., J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,; A. D.; Strain, M. C. F., O.; D. Malick, K.; A. Rabuck, D.;; Raghavachari, K. F., J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;; Clifford, S. C., J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,; P.; Komaromi, I. M., R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;; Peng, C. Y. N., A.; Hallacombe, M.; Gill, CP. M. W.; Johnson,; B.; Chen, W. W., M. W.; Gonzalez, C.; and Pople, J. A.; Gaussian, Inc., : Wallingford, CT, 2004. (35) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (36) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (37) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (38) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (39) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. 95 CHAPTER 4. Modulation of photophysical properties of (NHC)Cu(N^N) complexes through NHC ligand variation 4.1. Introduction The ability to tune chemical and photophysical properties in a desirable and predictable way is highly important when considering potential applications of Cu(I)-based phosphors. Modulation of excited state properties in these and related luminescent materials is usually achieved through ligand variation. 1-3 To date the set of ligands that have been used to prepare phosphorescent Cu(I) complexes is somewhat limited. The most commonly used ligands on Cu(I) are phosphine and bisimine ligands. 3-8 N- heterocyclic carbene (NHC) ligands are often compared to phosphine ligands in terms of bonding properties and have been used both as chromophore ligands 9-13 and as ancillary ligands 14-16 in luminescent transition metal complexes. NHCs are an attractive ligand class as they are electronically and sterically tunable and form strong bonds with transition metals giving robust complexes. In Chapter 3 we have shown that photophysical properties of three-coordinate Cu(I) complexes (NHC)Cu(N^N) can be tuned by variation of chelating N^N ligand. We have exploited 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) as a monodentate NHC ligand, that has large ππ* energy gap and high triplet energy, and therefore emission of (IPr)Cu(N^N) complexes discussed in Chapter 3 was associated with the N^N ligand. Herein, we report a series of luminescent (NHC)Cu(N^N) complexes 4.1-4.4 (Figure 4.1), where NHC is largely responsible for emission and demonstrate wide range 96 emission color tuning through modification of carbene ligand. This was realized by elaborate complex design. In particular, we utilized a non-conjugated N^N ligand, i.e. dimethylbis(2-pyridyl)borate (dp-BMe 2 - ) that possesses high triplet energy. To the best of our knowledge this ligand has never been used to prepare luminescent complexes, unlike bis(1-pyrazolyl)borate ligands (pz 2 -BR 2 - ), that have been used as high energy ancillary ligand in phosphorescent Ir(III) 17 and Cu(I) 18 complexes. We have found that dp-BMe 2 - gives (NHC)Cu(dp-BMe 2 ) complexes that are more robust and brighter emitters than the (NHC)Cu(pz 2 -BH 2 ) derivatives. In addition we systematically lowered the energy gap of IPr ligand in 4.1 by benzannulation of imidazolylidene ring in 4.2 and 4.4 and by introducing extra nitrogens to form pyrazine in 4.3. Figure 4.1. Molecular structures of complexes 4.1-4.4. 97 4.2. Results and discussions 4.2.1. Photophysical and computational studies Photophysical data for complexes 4.1-4.3 are summarized in Table 4.1. The UV-Vis absorption spectra for complexes 4.1-4.3 are shown in Figure 4.2. 250 300 350 400 450 500 550 0 5000 10000 15000 20000 25000 4.1 4.2 4.3 (IPr)Cu(pz 2 -BH 2 ) Molar Absorbance (M -1 cm -1 ) Wavelength (nm) Figure 4.2. Absorption spectra ((RT, CH 2 Cl 2 ) of complexes 4.1-4.3 and (IPr)Cu(pz 2 - BH 2 ). 250 300 350 400 450 0 5000 10000 15000 20000 25000 (IPr)CuCl (BzI-3,5Me)CuCl (PzI-3,5Me)CuCl Na[dp 2 -BMe 2 ] Molar Absorbance (M -1 cm -1 ) Wavelength (nm) Figure 4.3. Absorption spectra of precursors (IPr)CuCl, (BzI-3,5Me)CuCl and (PzI- 3,5Me)CuCl in CH 2 Cl 2 and Na[dp-BMe 2 ] in acetonitrile. 98 High energy bands, assigned to spin-allowed singlet ligand centered (LC) transitions appear down to 290 nm in 4.1 (ε ~ 7000-14200 M -1 cm -1 ), 310 nm in 4.2 (ε ~ 6500- 19000 M -1 cm -1 ) and 340 nm in 4.3 (ε ~ 4000-13350 M -1 cm -1 ). Lower energy bands that are not observed in absorption spectra of precursors (Figure 4.3) are assigned to charge transfer (CT) transitions. Complex 4.1 shows a CT band (ε = 6100 M -1 cm -1 ) at 315 nm and a shoulder (ε ~ 1300 M -1 cm -1 ) at 360 nm. Upon expansion of π-system of NHC ligand in 4.2 the CT band shifts to lower energy (346 nm) and becomes more intense (ε = 9110 M -1 cm -1 ). Substitution of two CH groups with nitrogens in 4.3 results in additional red shift (423 nm) and increase in molar absorption (ε = 10320 M -1 cm -1 ) of the CT band. It is expected that transitions involving orbitals on borate ligand occur at similar energies for all three complexes, while those involving carbene orbitals should change within the series. Therefore NHC ligand is directly involved in the CT transitions at 315 nm (1), 346 nm (2) and 423 nm (3). The shoulder at 360 nm observed for complex 4.1 is assigned to dipyridylborate-based transition. In order to confirm this assignment we prepared complex (IPr)Cu(pz 2 -BH 2 ) and compared its absorption spectrum to that of 4.1. As can be seen from Figure 4.2, these two complexes have similar absorption profiles, with exception of the CT band at 360 nm, that is not observed for (IPr)Cu(pz 2 -BH 2 ). Thus it was attributed to d(Cu)→π*(py) transition of 4.1. This transition is also present in absorption spectra of 4.2 and 4.3 but is obscured by NHC-based CT transitions that appear at lower energies compared to that of 4.1. The observed trends in absorption spectra are consistent with the reduction of the optical gap when going from 4.1 to 4.3. 99 The emission spectra of complexes 4.1-4.3 recorded in the solid state at room temperature and at 77 K are broad and featureless (Figure 4.4). At RT complex 4.1 gives sky blue emission centered at 476 nm, complex 4.2 shows yellow emission with λ max = 570 nm and 4.2 has orange-red emission centered at 638 nm. Emission maxima shift by 70-85 nm upon NHC ligand variation clearly indicates that photophysical properties of complexes 4.1-4.3 are governed largely by the NHC ligand. Solid powders of 4.1 and 4.2 glow very brightly upon excitation with emission quantum yields of 0.80 and 0.70 respectively, while 4.3 has moderate emission efficiency of 0.26. Emission lifetimes of 7.5-15 µs indicate that observed emission is phosphorescence in nature. The radiative rates in the solid state at RT vary within the small range of values (k r = (3.3-7.2) × 10 4 s -1 ). 400 450 500 550 600 650 700 750 800 0.0 0.2 0.4 0.6 0.8 1.0 4.1 4.2 4.3 Normalized intensity (a.u.) Wavelength (nm) Figure 4.4. Emission spectra of complexes 4.1-4.3 in the solid state at RT (closed symbols) and at 77 K (open symbols). It is known that Cu(I) complexes are prone to thermally activated fluorescence at RT due 100 to close proximity of a singlet state to the lowest triplet excited state. 18-22 This behavior is characterized by a bathochromic shift of emission and increase of lifetime upon decreasing experimental temperature. Observed lifetimes and radiative rates at RT correspond to delayed emission from a higher-lying state. On the other hand, at 77K thermally activated population of higher-lying emissive states is suppressed, therefore one can observe purely prompt phosphorescence from the lowest triplet state. Table 4.1. Photophysical properties of complexes 4.1-4.3. emission at RT a emission at 77K a λ max (nm) τ (µs) Φ PL k r (s -1 ) k nr (s -1 ) λ max (nm) τ (µs) 4.1 476 11 0.8 7.2 x 10 4 1.8 x 10 4 492 36 4.2 570 15 0.7 4.7 x 10 4 2.0 x 10 4 586 17 4.3 638 7.5 0.25 3.3 x 10 4 1.0 x 10 5 650 21 a In solid state. Complexes 4.1-4.3 at 77 K (Figure 4.4) display a red shift in emission relative to RT (363-683 cm -1 estimated at λ max ; 1053-1317 cm -1 estimated at the onset (I = 0.2) of emission spectra), however they show only minimal (×1.1-3.3) increase in emission lifetimes. Emission lifetimes measured under these conditions are quite short being in the range of 17-36 µs. For comparison, four-coordinate Cu(I)-phosphine complexes usually display long-lived emission at 77 K with lifetimes of hundreds of microseconds. 5,18 For example, we have prepared four-coordinate phosphine analog of (NHC)Cu(dp 2 -BMe 2 ) complexes discussed here, where NHC is replaced with bidentate phosphine ligand POP = bis(2-diphenylphosphinophenyl)ether (POP)Cu(dp 2 -BMe 2 ). At room temperature in the solid state it has similar emission properties to complex 4.1. In particular, it gives 101 bright (Φ = 1) blue (λ max = 470 nm) emission with lifetime of 16 µs. At 77 K, however, its emission lifetime is much longer 713 µs. Assuming high quantum yields at this temperature, the estimated triplet radiative rates for complexes 4.1-4.3 are quite fast, being on the order of 10 4 s -1 . This value is order of magnitude faster than that for phosphine complex (POP)Cu(dp 2 -BMe 2 ) and comparable to triplet radiative rates often observed for Pt(II)-based complexes. 23,24 The latter, unlike materials discussed here, bear third row transition metal possessing strong heavy atom effect that induces efficient spin- orbit coupling. Computational analyses of the ground and excited state properties performed using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations compare favorably with the experimental observations. Frontier molecular orbitals (MO) diagram for 4.1-4.3 is shown in Figure 4.5. For all three complexes the calculated HOMOs have essentially identical spatial distribution consisting predominantly of d orbitals on copper (39-48%) mixed with orbitals on dipyridylborate ligand (41-45%). The LUMOs in 4.1-4.3 are localized on the NHC ligand (85-94%) with little metal character (4-6%). There is small contribution (8%) of dipyridylborate ligand orbitals in the LUMO of complex 4.1, however it is not observed for 4.2 and 4.3. Therefore in 4.1 dipyridylborate ligand may also be involved in emission process. It is worth noting substantial participation (8-23%) of carbene carbon 2p z orbital in LUMOs of all three complexes. 102 Figure 4.5. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) plots and energies for 4.1-4.3. Hydrogen atoms are omitted for clarity. Consistent with the orbital composition, variations of the carbene ligand have pronounced effect on LUMO energies. Complex 4.1 has the highest LUMO in the series; it is lower by 0.4 eV for 4.2 and by additional 1.07 eV for 4.3. The HOMO energies show similar stabilization trend, however they vary to a lesser degree. The HOMO of 4.3 is stabilized relative to that of 4.2 by 0.24 eV, which in turn is 0.11 eV lower than the HOMO of 4.1. The HOMO-LUMO gap is progressively smaller for 4.1 (4.27 eV), 4.2 (3.98 eV) and 4.3 (3.15 eV). This result qualitatively agrees with the bathochromic shift observed in absorption spectra. The observed red shift of emission spectra is consistent with expected bathochromic shift upon expanding the size of the π-system of a ligand chromophore and 103 N-substitution, which lead to decrease in separation between the HOMO and LUMO. The lowest vertical singlet and triplet excitations obtained from TD-DFT calculations are mainly HOMO→LUMO transitions (Table 4.2). Therefore based on the MO description given above the lowest lying transition for complexes 4.2 and 4.3 can be ascribed as metal-ligand to ligand charge transfer ((M+L)LCT) and for complex 4.1 as (M+L)LCT with mixed-in intra-ligand π→π* (dp 2 -BMe 2 ) charge transfer (ILCT) character. The calculated spin density surfaces for the triplet electronic configuration further support this assignment. For complexes 4.1-4.3 the surfaces are mainly localized along the C NHC -Cu bond. Both ligands also contribute to the triplet spin density, however while complex 4.1 has larger contribution from the dipyridylborate ligand, triplet spin density distribution for complexes 4.2 and 4.3 is shifted toward NHC ligand (Figure 4.6). Table 4.2. Lowest energy transitions for complexes 4.1-4.3 obtained from TD-DFT calculations. complex states vertical excitation energy (nm) oscillator strength major contribution a character 4.1 T 1 381 0 HOMO→LUMO (76%) HOMO→LUMO+4 (18%) (M+L)LCT/ILCT S 1 375 0.0028 HOMO→LUMO (81%) HOMO→LUMO+4 (8.8%) (M+L)LCT/ILCT 4.2 T 1 400 0 HOMO→LUMO (100%) (M+L)LCT T 2 367 0 HOMO-2→LUMO (61%) HOMO-1→LUMO (35%) (M+L)LCT S 1 364 0.1440 HOMO→LUMO (96%) (M+L)LCT 4.3 T 1 522 0 HOMO→LUMO (100%) (M+L)LCT S 1 451 0.1645 HOMO→LUMO (100%) (M+L)LCT a transitions with >5% contribution The calculated energies of the lowest vertical triplet excitations 381 nm (4.1), 400 nm 104 (4.2) and 522 nm (4.3) are in parallel with the onsets of excitation spectra of solid powders at 77K (Figure 4.4). Emission spectra however are significantly shifted to longer wavelengths, than the calculated vertical (Franck-Condon) lowest triplet excitations. This behavior is largely caused by structural reorganizations in the excited state. Excited state structural relaxation is a common feature of Cu(I) complexes, that has been well documented in the literature. 25-30 Our calculations also show that the optimized triplet geometry is drastically distorted from the ground state geometry (Figure 4.6). The calculated oscillator strength of the lowest singlet transitions progressively increases for 4.1 (0.0028), 4.2 (0.1440) and 4.3 (0.1645). This result is consistent with the trend observed in absorption spectra, i.e. increase of molar absorption for the CT bands when going from 4.1 to 4.3. Figure 4.6. Optimized triplet geometries and triplet spin density contour plots (isovalue: 0.004 e a 0 -3 ) for complexes 4.1-4.3. Hydrogen atoms are omitted for clarity. In contrast to 4.1-4.3, complex 4.4 is virtually non-emissive. We could not detect any emission from complex 4.4 in the solid state at room temperature and at 77 K. Very weak structured emission from 4.4 was recorded in frozen 2-methyltetrahydrofuran glass 105 at 77K (Figure 4.7). It was tentatively assigned as weak phosphorescence (Φ<0.01, τ<10 ns). 300 400 500 600 700 800 0 5000 10000 15000 20000 25000 Molar Absorbance (M -1 cm -1 ) Wavelength (nm) 0.0 0.5 1.0 Normalized Intensity (a.u.) Figure 4.7. Absorption (RT, CH 2 Cl 2 ) and emission (77 K, 2-MeTHF) spectra of complex 4.4. The calculated HOMO of 4.4 is essentially identical to that of complexes 4.1-4.3, the LUMO is largely localized on the aromatic system of NHC ligand and in contrast to complexes 4.1-4.3 has no electron density on C NHC atom (Figure 4.8(A)). Such frontier orbital distribution gives poor frontier orbitals overlap leading to low oscillator strength of the lowest lying transitions. Consistent with this result, the intensity of the lowest lying absorption bands (400-550 nm) of complex 4.4 is low (ε = 450 M -1 cm -1 ). The HOMO/LUMO gap of complex 4.4 is 2.55 eV that is lower that the optical gap of compound 4.3. This result is in accord with the bathochromic shift of absorption spectrum of 4.4 compared to 4.3. 106 Figure 4.8. HOMO and LUMO plots and energies (A) and triplet spin density contour plots (isovalue: 0.004 e a 0 -3 ) (B) for 4.4. Hydrogen atoms are omitted for clarity. The triplet spin density of 4.4 is mainly localized on the acenaphthene moiety and unlike that of complexes 4.1-4.3 has a minimum along C NHC -Cu bond (Figure 4.8(B)). This, together with structured emission spectrum, suggests that emission is largely ligand centered in character. Thus, extension of π-system of NHC ligand leads to smaller HOMO-LUMO gap, but also lowers the energy of 3 LC state, which now dominates the emission process. 4.3. Conclusion In summary we reported a series of (NHC)-Cu(I) complexes that show phosphorescence associated primarily with NHC ligand chromophore. Judicious modification of NHC 107 ligand allows for long-range emission color tuning over 200 nm from blue to orange-red while retaining good emission efficiencies. The estimated triplet radiative rates are comparable with those of third row transition metal complexes. Taking into account electronic and steric tunability of NHC ligands, our findings demonstrate a versatile tool for control of photophysical properties in these and related (NHC)-Cu(I) complexes. 4.4. Experimental section Synthesis. Synthetic procedures for (IPr)Cu(dp-BMe 2 ) (4.1), (BzI-3,5Me)Cu(dp-BMe 2 ) (4.2), (PzI-3,5Me)Cu(dp-BMe 2 ) (4.3), (IPrBIAN)Cu(dp-BMe 2 ) (4.4) and (IPr)Cu(pz 2 - BH 2 ) (4.5) are described in Chapter 2. DFT Calculations. Density functional theory (DFT) calculations were performed with the Gaussian03 31 software package employing the B3LYP functional 32,33 using LANL2DZ basis set 34-36 for Cu and 6-31G* for C, N, H and B. Geometric parameters obtained from XRD analyses were used as a starting point for geometry optimization in the ground state. The optimized geometries were used for time-dependent density functional calculations (TD-DFT). Photophysical characterization. The UV-visible spectra were recorded on a Hewlett- Packard 4853 diode array spectrometer. Steady state emission measurements were performed using a Photon Technology International QuantaMaster model C-60 fluorimeter. All reported spectra are corrected for photomultiplier response. Phosphorescence lifetime measurements were performed on the same fluorimeter 108 equipped with a microsecond Xe flash lamp or using an IBH Fluorocube instrument equipped with a 405 nm LED excitation source using time-correlated single photon counting method. Quantum yields at room temperature were measured using a Hamamatsu C9920 system equipped with a xenon lamp, calibrated integrating sphere and model C10027 photonic multichannel analyzer. Chapter 4 References (1) You, Y.; Park, S. Y. Dalton Trans. 2009, 1267. (2) Chi, Y.; Chou, P. T. Chem. Soc. Rev. 2010, 39, 638. (3) Barbieri, A.; Accorsi, G.; Armaroli, N. Chem. Commun. 2008, 2185. (4) Armaroli, N.; Accorsi, G.; Cardinali, F.; Listorti, A. Top. Curr. Chem. 2007, 280, 69. (5) Crestani, M. G.; Manbeck, G. F.; Brennessel, W. W.; McCormick, T. M.; Eisenberg, R. Inorg. Chem. 2011, 50, 7172. (6) Manbeck, G. F.; Brennessel, W. W.; Eisenberg, R. Inorg. Chem. 2011, 50, 3431. (7) Harkins, S. B.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 2030. (8) Hsu, C.-W.; Lin, C.-C.; Chung, M.-W.; Chi, Y.; Lee, G.-H.; Chou, P.-T.; Chang, C.-H.; Chen, P.-Y. J. Am. Chem. Soc. 2011, 133, 12085. (9) Sajoto, T.; Djurovich, P. I.; Tamayo, A.; Yousufuddin, M.; Bau, R.; Thompson, M. E.; Holmes, R. J.; Forrest, S. R. Inorg. Chem. 2005, 44, 7992. (10) Au, V. K. M.; Wong, K. M. C.; Zhu, N. Y.; Yam, V. W. W. J. Am. Chem. Soc. 2009, 131, 9076. (11) Unger, Y.; Zeller, A.; Ahrens, S.; Strassner, T. Chem. Commun. 2008, 3263. (12) 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, 6896. 109 (13) Unger, Y.; Meyer, D.; Molt, O.; Schildknecht, C.; Munster, I.; Wagenblast, G.; Strassner, T. Angew. Chem., Int. Ed. 2010, 49, 10214. (14) Hsieh, C. H.; Wu, F. I.; Fan, C. H.; Huang, M. J.; Lu, K. Y.; Chou, P. Y.; Yang, Y. H. O.; Wu, S. H.; Chen, I. C.; Chou, S. H.; Wong, K. T.; Cheng, C. H. Chem.-Eur. J. 2011, 17, 9180. (15) Chang, C. F.; Cheng, Y. M.; Chi, Y.; Chiu, Y. C.; Lin, C. C.; Lee, G. H.; Chou, P. T.; Chen, C. C.; Chang, C. H.; Wu, C. C. Angew. Chem., Int. Ed. 2008, 47, 4542. (16) Chung, L.-H.; Cho, K.-S.; England, J.; Chan, S.-C.; Wieghardt, K.; Wong, C.-Y. Inorg. Chem. 2013, 52, 9885. (17) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Yousufuddin, M.; Ho, N. N.; Thomas, J. C.; Peters, J. C.; Bau, R.; Thompson, M. E. Inorg. Chem. 2005, 44, 1713. (18) Czerwieniec, R.; Yu, J.; Yersin, H. Inorg. Chem. 2011, 50, 8293. (19) Deaton, J. C.; Switalski, S. C.; Kondakov, D. Y.; Young, R. H.; Pawlik, T. D.; Giesen, D. J.; Harkins, S. B.; Miller, A. J. M.; Mickenberg, S. F.; Peters, J. C. J. Am. Chem. Soc. 2010, 132, 9499. (20) Blasse, G.; McMillin, D. R. Chem. Phys. Lett. 1980, 70, 1. (21) Breddels, P. A.; Berdowski, P. A. M.; Blasse, G. J. Chem. Soc. Farad. Trans. II 1982, 78, 595. (22) Kirchhoff, J. R.; Gamache, R. E.; Blaskie, M. W.; Delpaggio, A. A.; Lengel, R. K.; McMillin, D. R. Inorg. Chem. 1983, 22, 2380. (23) Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer, T. Coord. Chem. Rev. 2011, 255, 2622. (24) Williams, J. A. G. In Photochem. Photophys. Coord. Comp. II 2007; Vol. 281, p 205. (25) Chen, L. X.; Jennings, G.; Liu, T.; Gosztola, D. J.; Hessler, J. P.; Scaltrito, D. V.; Meyer, G. J. J. Am. Chem. Soc. 2002, 124, 10861. (26) Chen, L. X.; Shaw, G. B.; Novozhilova, I.; Liu, T.; Jennings, G.; Attenkofer, K.; Meyer, G. J.; Coppens, P. J. Am. Chem. Soc. 2003, 125, 7022. (27) Iwamura, M.; Takeuchi, S.; Tahara, T. J. Am. Chem. Soc. 2007, 129, 5248. 110 (28) Shaw, G. B.; Grant, C. D.; Shirota, H.; Castner, E. W.; Meyer, G. J.; Chen, L. X. J. Am. Chem. Soc. 2007, 129, 2147. (29) Gothard, N. A.; Mara, M. W.; Huang, J.; Szarko, J. M.; Rolczynski, B.; Lockard, J. V.; Chen, L. X. J. Phys. Chem. A 2012, 116, 1984. (30) Mara, M. W.; Jackson, N. E.; Huang, J.; Stickrath, A. B.; Zhang, X. Y.; Gothard, N. A.; Ratner, M. A.; Chen, L. X. J. Phys. Chem. B 2013, 117, 1921. (31) Frisch, M. J. T., G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,; M. A.; Cheeseman, J. R. M., J. A., Jr.; Vreven, T.; Kudin, K. N.;; Burant, J. C. M., J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;; Mennucci, B. C., M.; Scalmani, G.; Rega, N.; Petersson, G. A.;; Nakatsuji, H. H., M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,; J.; Ishida, M. N., T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,; X.; Knox, J. E. H., H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;; Gomperts, R. S., R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;; Pomelli, C. J. O., W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;; Salvador, P. D., J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,; A. D.; Strain, M. C. F., O.; D. Malick, K.; A. Rabuck, D.;; Raghavachari, K. F., J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;; Clifford, S. C., J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,; P.; Komaromi, I. M., R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;; Peng, C. Y. N., A.; Hallacombe, M.; Gill, CP. M. W.; Johnson,; B.; Chen, W. W., M. W.; Gonzalez, C.; and Pople, J. A.; Gaussian, Inc., : Wallingford, CT, 2004. (32) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (33) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (34) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (35) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (36) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. 111 CHAPTER 5. Phosphorescent three-coordinate (NHC)Cu(N^N) complexes as host materials for organic light-emitting diodes 5.1. Introduction To realize high efficiency organic light-emitting devices (OLED) it is essential to use phosphorescent emitters as they enable attainment of the maximum theoretical efficiency. 1-4 Moreover, to maximize device performance, a phosphor is usually dispersed (doped) in a host matrix. 5-7 The role of the host material is to inhibit dopant self-quenching and to promote charge or exciton trapping and recombination at the dopant. It was shown that performance of OLEDs comprising of the same emissive material can vary substantially when different hosts were employed. 2,8,9 Thus, the choice of host material for an OLED is crucial for device performance. Of foremost importance when selecting a host-guest system are the triplet energies of materials. In order to confine triplet excitons at the dopant, the triplet energy of the host must be greater than the triplet energy of the dopant. If recombination occurs in the host matrix, the energy will be then exothermically transferred to the emitter. When dopant acts as charge trap and recombination occurs at the dopant, back energy transfer from guest to the host will be largely suppressed. However, if a lower energy host is used it will quench the dopant triplet. For example, thin films of the blue emitter bis[(4,6-difluorophenyl)pyridinato- N,C 2 ](picolinato)iridium(III) (FIrpic) (E T = 2.65 eV) 10 doped into the lower energy host, 4,4’-bis(N-carbazolyl)-2,2’-biphenyl (CBP) (E T =2.56 eV) 11 showed maximum photoluminescence (PL) efficiency of 78% at 15 mol%. 12 However, when FIrpic was doped in higher triplet energy host m-bis(N-carbazolyl)benzene (mCP) (E T = 2.90 eV) 10 it 112 showed efficiency as high as 99% at lower concentration of 1.2 mol%. This behavior is reflected in OLED performance. For FIrpic:CBP devices maximum external efficiency of 6.1% was achieved, while FIrpic:mCP devices showed 7.5% efficiency. 10,13,14 Most commonly used hosts for phosphorescent OLEDs (PHOLEDs) are organic materials possessing charge transporting moieties such as arylamine, 15,16 carbazolyl, 9,11,14 arylsilanyl 17-19 and phosphine oxide 20-23 groups. These materials usually have high triplet energies, large HOMO-LUMO gaps and have good thermal stability. Another class of materials that have been employed as hosts for PHOLEDs are phosphorescent metal complexes, in particular, cyclometallated Ir(III) complexes Ir(ppy) 3 , (ppy) 2 Ir(acac) and their derivatives. 24-26 These materials have relatively small energy splitting between the lowest triplet and singlet excited states ΔE ST (exchange energy) on the order of 10 3 cm -1 , compared to purely organic fluorescent hosts (ΔE ST ~10 4 cm -1 ). Using a host material with smaller ΔE ST in an OLED will reduce exchange energy losses that occur when opposite charges recombine to form singlet and triplet excitons, which in turn will lead to lower operating voltages of a device. Despite this advantage, examples of phosphorescent metal complexes employed as hosts are scarce mainly due to high cost of these materials. Recently there has been a great deal of interest in phosphorescent copper (I) complexes as inexpensive alternative to heavy-metal (Ir(III), Pt(II)) based phosphorescent materials. 27- 32 Photophysical studies show that ΔE ST in Cu(I) complexes is generally smaller than in their third row counterparts being in the range of 500-2000 cm -1 . 33-36 Efficient PHOLEDs 113 with Cu(I) complexes as emissive dopants have been demonstrated 32,33,37-40 but no phosphorescent Cu(I)-based hosts in OLEDs have been reported. To explore the potential of Cu(I) complexes as host materials in PHOLEDs we employed three- coordinate complexes (IPr)Cu(fppz) (3.4) and (IPr)Cu(pybim) (3.6) as hosts for phosphorescent Ir(III)-based dopants PQIr and Ir(ppy) 3 (Figure 5.1). Figure 5.1. Molecular structures of host and emitter materials discussed in Chapter 5. 5.2. Results and Discussions 5.2.1. Electrochemistry The electrochemical properties of (IPr)Cu(fppz) (3.4) (IPr)Cu(pybim) (3.6) in CH 2 Cl 2 solution were investigated using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The CV and DPV traces are shown in Figures 5.2 and 5.3 and data are summarized in Table 5.1. Both complexes show quasi-reversible oxidation wave, however no reduction wave is observed under experimental conditions. This suggests that these compounds have high LUMO and shallow HOMO energies. Irreversibility of the observed oxidation process can be attributed to the structural rearrangements that occur after oxidation of Cu + center to Cu 2+ . 114 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 I, (mA) E (V) -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 I (mA) E (V) Fc + /Fc Figure 5.2. CV (left) and DPV (right) traces of (IPr)Cu(fppz). -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.1 0.0 0.1 0.2 0.3 I(mA) E(V) -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 -0.10 -0.05 0.00 0.05 0.10 I(mA) E(V) Fc + /Fc Figure 5.3. CV (left) and DPV (right) traces of (IPr)Cu(pybim). There is small difference in oxidation potentials obtained from CV and DPV. For (IPr)Cu(pybim) oxidation potential obtained from CV and DFT experiments are 0.53 V and 0.67 V respectively. (IPr)Cu(fppz) shows oxidation peak at 0.43 V (CV) and 0.52 (DPV). The values of oxidation potentials (E Ox ) can be used estimate the HOMO energies (E HOMO ) from the following equation 41 : ) 08 . 0 6 . 4 ( ) 1 . 0 4 . 1 (      Ox HOMO E E The HOMO energies determined from electrochemical data are quite similar for both complexes: -5.2 eV for (IPr)Cu(fppz) and -5.3 eV for (IPr)Cu(pybim). This result 115 correlates well with the HOMO energies obtained from DFT calculations for (IPr)Cu(fppz) (-5.42 eV). However in case of (IPr)Cu(pybim) DFT calculations suggest that it should be much easier to oxidize (E HOMO = -4.86 eV) compared to fppz derivative. Table 5.1. Summary of electrochemical properties of (IPr)Cu(fppz) an (IPr)Cu(pybim). The HOMO energies calculated from the oxidation potentials are shown in parenthesis. E Ox (CV) E Ox (DPV) E HOMO (DFT) (IPr)Cu(fppz) 0.43V (-5.2 eV) 0.52 V (-5.3 eV) -5.42 eV (IPr)Cu(pybim) 0.53V (-5.3 eV) 0.67 V (-5.5 eV) -4.87 eV 5.2.2. (IPr)Cu(pybim) as host material for OLED (IPr)Cu(pybim) was investigated as host material for orange-red dopant PQIr 42 (emission λ max = 597 nm). It exhibits broad yellow-green phosphorescence in neat thin film centered at 562 nm, therefore its triplet energy is higher than the triplet energy of the chosen dopant. Also, (IPr)Cu(pybim) can be sublimed at 250ºC under high vacuum, thus it can be used for vacuum thermo-deposited devices. The fabricated OLED had the structure ITO/NPD(200 Å)/PQIr:(IPr)Cu(pybim)(10 wt%, 200Å)/BCP(400Å)/LiF(10Å)/Al(1000Å) (D5.1). The electroluminescence (EL) spectrum of this device showed emission from PQIr with small contribution from NPD emission. This indicates that some recombination also occurs on NPD. To prevent electrons form leaking into NPD we fabricated device D5.2, where a 100Å layer of electron blocking (EB) material Ir(ppz) 3 was introduced between NPD and emissive layer. The EL spectrum of D5.2 showed no NPD emission and is consistent with PL 116 spectrum of PQIr. The energy diagrams of the fabricated OLEDs together with the corresponding EL spectra are shown in Figure 5.4. Figure 5.4. Energy level diagrams and EL spectra for device D5.1 (top) and D5.2 (bottom). Addition of EB layer did not significantly affect device performances, as both OLEDs had similar device characteristics. The data are summarized in Table 5.2 and plotted in Figure 5.5. Both devices turned-on at 3.2 V (0.1 cd/m 2 ). D5.1 and D5.2 showed maximum luminance of 2640 cd/m 2 and 3326 cd/m 2 respectively at 12 V. External quantum efficiency (EQE) at low current densities (<0.4 mA/cm 2 ) was higher for D5.1, i.e. at 0.2 mA/cm 2 EQE is 7.0% (D5.1) and 5.6% (D5.2). At higher current densities D5.2 showed higher EQE, i.e. at 70 mA/cm 2 EQE is 2.3% (D5.1) and 1.8% (D5.2). 117 1 2 3 4 5 6 7 8 9 10 11 12 13 0 20 40 60 80 100 D5.1 D5.2 Current Density (mA/cm 2 ) Voltage (V) 0.01 0.1 1 10 100 1000 10000 Brightness (Cd/m 2 ) 0.01 0.1 1 10 100 0.01 0.1 1 10 D5.1 D5.2 Quantum Efficiency (%) Current Density (mA/cm 2 ) Figure 5.5. Current density (black) and luminance (red) vs voltage plot (top) and quantum efficiency vs current density plot (bottom) of devices D5.1 and D5.2. OLEDs D5.1 and D5.2, bearing phosphorescent Cu(I)-based host show comparable performance with a reported OLED of similar architecture, where emissive layer is composed of PQIr doped into purely organic fluorenylsilane host (9,9’-dimethylfluoren- 2-yl) 4 Si(phenyl) (SiFl 4 ). 17 Device with a configuration of ITO/NPD(400Å)/PQIr:SiFl 4 (6 wt%, 200Å)/ BCP(400Å)/LiF(10Å)/Al(1000Å) (D5.3) showed EQE of 8.0% at 0.01 mA/cm 2 , that reduced to ~2.5% at 70 mA/cm 2 . 118 Table 5.2. Performance of OLED devices D5.1, D5.2, D5.3. D5.1 D5.2 D5.3 b EL maxima (nm) 430; 596 596 595 Turn-on voltage a (V) 3.2 3.2 3.0 Max luminance (cd/m 2 ) 3326 (12V) 2640 (12V) 4814 (14V) η ext , (%) at 0.01 mA/cm 2 - 7.8 8.0 at 1 mA/cm 2 5.8 5.8 - a At 0.1 cd/m 2 ; b Ref. 17. 5.2.3. Lower energy emitting Cu(I)-based host for phosphorescent Ir(III) dopant To choose a host-guest system for an OLED it is essential to consider relative triplet energies of both components. For cyclometallated Ir(III) complexes the lowest triplet excited state energies (E T ) could be quite accurately estimated from the first vibronic peak of phosphorescence spectra. In contrast, Cu(I) complexes usually have broad featureless emission spectra. Moreover, due to small ΔE ST thermal population of close- lying upper states often occurs at room temperature, leading to a blue shift of emission spectra relative to pure emission from the lowest triplet state. 33-35,43,44 Therefore triplet energies of Cu(I) emitters often could not be unambiguously predicted from experimentally observed emission spectra. Adachi and co-workers investigated photophysical properties of phosphorescent Cu(I) complexes [Cu(dnbp)(DPEPhos)]BF 4 (dnbp = 2,9-di-n-butylphenanthroline, DPEPhos = bis[2- (diphenylphosphino)phenyl]ether) and [Cu(µI)dppb] 2 (dppb = 1,2- bis[diphenylphosphino]benzene). 40 These complexes are green emitters and show broad featureless emission in doped films with λ max = 509 nm for [Cu(dnbp)(DPEPhos)]BF 4 119 and λ max = 507 nm for [Cu(µI)dppb] 2 . Interestingly authors found that the energies of the lowest triplet excited states of these complexes are 2.72 eV (456 nm) and 2.76 eV (449 nm) respectively, which is higher than the triplet energy of the blue emitting iridium complex FIrpic (λ max =475 nm, E T = 2.65eV). 10 We have observed similar behavior for three-coordinate (NHC)Cu(N^N) complexes. 45 As described in Chapter 3, (IPr)Cu(fppz) (3.4) shows yellow emission in solid state at RT centered at 560 nm. However its lowest vertical triplet state obtained from TD-DFT calculations is in the blue region of the visible spectrum at 452 nm (2.74 eV), which correlates well with the solution absorption cutoff observed at ~445 nm (2.79 eV). This value is higher than the triplet energy of a well-known green dopant Ir(ppy) 3 (2.42 eV). 46 Such behavior is ascribed to excited state geometry distortions. Figure 5.6. Schematic potential energy surfaces diagram illustrating electronic relaxation and structural distortion in Cu(I) complexes. 120 After photoexcitation to a Franck-Condon state, the complex undergoes structural reorganizations and experimentally observed emission originates from a lower-lying structurally relaxed state (Figure 5.6). This state is only accessible through excitation into the Franck-Condon state and can not be populated via direct excitation. Thus, in the guest:host system Ir(ppy) 3 : (IPr)Cu(fppz) the triplet state of Ir(III) complex will not be quenched by energy transfer to the triplet Cu(I) complex. In order to probe this hypothesis we have performed quenching studies of Ir(ppy) 3 : (IPr)Cu(fppz) system in solution and doped film and explored an unconventional approach to OLED device design, where emissive layer (EML) is comprised of green emitting phosphorescent emitter Ir(ppy) 3 doped into lower energy emitting host (IPr)Cu(fppz). 5.2.3.1. Quenching studies If the energy of the lowest triplet state of Ir(ppy) 3 is lower than the energy of the triplet Franck-Condon state of (IPr)Cu(fppz), then upon excitation of the Ir(ppy) 3 :(IPr)Cu(fppz) mixture at 2.42 eV<E(λ exc )<E T1 (Cu), we should observe exclusively Ir(ppy) 3 phosphorescence. However at E(λ exc )>E T1 (Cu) we should observe emission from both complexes. Individual absorption and emission spectra of Ir(ppy) 3 and (IPr)Cu(fppz) obtained in toluene solution are depicted in Figure 5.7. 121 300 400 500 600 700 0 5000 10000 15000 Molar Absorbance (M -1 cm -1 ) Wavelength (nm) 0.0 0.5 1.0 (IPr)Cu(fppz) Ir(ppy) 3 Normalized Intensity (a.u.) Figure 5.7. Absorption (dash line) and emission (solid line) spectra of Ir(ppy) 3 and (IPr)Cu(fppz) in solution (toluene). The dependence of emission spectra of the Ir(ppy) 3 :(IPr)Cu(fppz) mixture in toluene solution (C Ir(ppy)3 ~ 10 -5 M, C (IPr)Cu(fppz) ~ 10 -2 M) on the energy of excitation light is shown in Figure 5.8. Contribution from emission from (IPr)Cu(fppz) is observed at λ exc = 445 nm and increases at higher excitation energies. This value correlates well with the solution absorption cutoff obtained experimentally and theoretically predicted lowest triplet state of (IPr)Cu(fppz). 122 400 450 500 550 600 650 700 750 0.0 0.2 0.4 0.6 0.8 1.0 365 nm 375 nm 385 nm 395 nm 405 nm 415 nm 425 nm 435 nm 445 nm 455 nm 465 nm 475 nm Ir(ppy) 3 Normalized intensity (a.u.) Wavelength (nm) Figure 5.8. Emission spectra of mixture of Ir(ppy) 3 (C=4.43 µM) and (IPr)Cu(fpyro) (C=7.36 mM) in toluene at RT at different excitation wavelengths. Emission quenching study of phosphorescence of Ir(ppy) 3 by (IPr)Cu(fppz) was carried out using Stern-Volmer analysis. The radiative lifetimes ( τ) of Ir(ppy) 3 (C = 10 -5 M) emission at 515 nm were measured at different concentrations of quencher (IPr)Cu(fppz) (C = 0 to 10 -2 M). The mixture was excited at λ exc = 405 nm. The quenching rate constant (k q ) was determined from the slope of the Stern-Volmer plot (Figure 5.9) according to the equation:   Q k q 0 0 1      where τ 0 is the radiative lifetime of Ir(ppy) 3 in the absence of quencher and [Q] is the 123 concentration of quencher. The results indicate that quenching of Ir(ppy) 3 emission by (IPr)Cu(fppz) is an endothermic process occurring with a relatively slow rate constant of 1.6 x 10 7 s -1 . Figure 5.9. Stern-Volmer plot for quenching of Ir(ppy) 3 emission by (IPr)Cu(fppz). Photophysical properties of Ir(ppy) 3 :(IPr)Cu(fppz) mixture were examined in doped amorphous thin film. Ir(ppy) 3 was co-deposited with (IPr)Cu(fppz) at concentration of 7 wt% by vacuum thermal evaporation on a quartz substrate. Doped film of Ir(ppy) 3 in common organic host CBP was also fabricated. Emission spectra and emission lifetimes of these two films were measured and compared. Figure 5.10 shows PL spectra if Ir(ppy) 3 in solution and in doped films. PL spectrum of Ir(ppy) 3 :CBP film is almost identical to solution emission spectrum of Ir(ppy) 3 . Emission spectrum of Ir(ppy) 3 :(IPr)Cu(fppz) film has similar shape, however it is slightly broader. Also, unlike solution emission of Ir(ppy) 3 :(IPr)Cu(fppz) mixture the shape of emission spectrum of 124 the film is independent of excitation wavelength. This can be attributed to more efficient quenching of Franck-Condon triplet state of (IPr)Cu(fppz) by energy transfer to a lower triplet state of Ir(ppy) 3 . At 7 wt% doping the concentration of iridium complex in film is much higher than in solution experiments described above. In addition, the rate of structural reorganization in solid film may be slower than in fluid environment. So, when (IPr)Cu(fppz) is excited into triplet state with the ground state geometry, fast energy transfer to Ir(ppy) 3 triplet quenches emission of copper complex before it undergoes structural rearrangement. Therefore only Ir(ppy) 3 emission is observed. Slight broadening of emission spectrum is possibly due to different morphology of Ir(ppy) 3 :(IPr)Cu(fppz) film compared to Ir(ppy) 3 :CBP film. PL lifetime data were collected at λ exc = 435 nm and can be approximated with two exponential decays. The obtained values are similar for both films. The major components are 1.15 µs (88%) and 1.2 µs (90%) respectively for Ir(ppy) 3 doped in (IPr)Cu(fppz) and CBP. The radiative rate of emission is independent of a host material, therefore quantum efficiency of these two films should also be comparable. The reported quantum yield of Ir(ppy) 3 :CBP film doped at 6wt% is 92%. 12 Based on this value the estimated emission efficiency of Ir(ppy) 3 :(IPr)Cu(fppz) is 88%. 125 450 500 550 600 650 700 750 0.0 0.2 0.4 0.6 0.8 1.0 in CBP  exc =405nm in (IPr)Cu(fppz)  exc =435nm in (IPr)Cu(fppz)  exc =405nm in (IPr)Cu(fppz)  exc =370nm in (IPr)Cu(fppz)  exc =350nm Ir(ppy) 3 solution Normalized intensity (a.u.) Wavelength (nm) Figure 5.10. Emission spectra of doped films (300 Å, 7 wt%) of Ir(ppy) in (IPr)Cu(fppz) host and CBP host and Ir(ppy) 3 emission in solution. 5.2.3.2. OLED performance To investigate electroluminescent performance of Ir(ppy) 3 doped in a lower energy emitting host (IPr)Cu(fppz) the OLED has been fabricated with a device structure ITO/NPD(400 Å)/Ir(ppy) 3 :(IPr)Cu(fppz)(7 wt%,300Å)/BCP(100Å)/Alq 3 (400Å)/ LiF(10Å)/Al(1000Å) (D5.4). Figure 5.11 shows energy diagram and EL performance characteristics of D5.4. EL spectrum is consistent with PL spectrum of Ir(ppy) 3 :(IPr)Cu(fppz) film with small contribution of NPD emission, that increases upon increasing of driving voltage of the device. The device turns-on at 3.5 V and reaches maximum luminance of 3732 Cd/m 2 at 9.1 V. EQE is in the range of 3.0-3.7% at current 126 densities of 0.1-10 mA/cm 2 , however substantial efficiency roll-off occurs at current densities > 10 mA/cm 2 . For comparison, OLED with a similar device configuration employing CBP host for Ir(ppy) 3 demonstrated quantum efficiency of 7.5% at doping concentration of 6 wt%. 2 Taking into account the fact that PL efficiencies of Ir(ppy) 3 :(IPr)Cu(fppz) and Ir(ppy) 3 :CBP films are comparable and high, lower EQE of (IPr)Cu(fppz)-based device can be explained by unbalanced hole and electron transport. 400 450 500 550 600 650 700 750 0.0 0.2 0.4 0.6 0.8 1.0 5V 6V 7V 8V 9V 10V Normalized intensity (a.u.) Wavelength (nm) (B) 1 2 3 4 5 6 7 8 9 10 11 12 13 100 200 300 400 500 Current density (mA/cm 2 ) Voltage (V) 0.1 1 10 100 1000 Brightness (Cd/m 2 ) (C) 0.01 0.1 1 10 100 1000 0.01 0.1 1 10 EQE (%) Current Density (mA/cm 2 ) (D) Figure 5.11. Energy level diagram (A), EL spectra (B), current density (black) and luminance (red) vs voltage plot (C) and quantum efficiency vs current density plot (D) of device D5.4. 127 5.3. Conclusion Three-coordinate (NHC)Cu(N^N) complexes have been examined as host materials for cyclometallated Ir(III)-based dopants in OLEDs. Orange-red OLED utilizing (IPr)Cu(pybim) as host showed EL performance comparable to devices based on common organic hosts. It was demonstrated that phosphorescence of Ir(ppy) 3 is not quenched by copper complex (IPr)Cu(fppz), that emits at lower energies. This indicates that the triplet energy of yellow emitting (IPr)Cu(fppz) is higher than the triplet of Ir(ppy) 3 . However, experimentally observed emission of (IPr)Cu(fppz) originates from a lower-lying state that results from excited state structural reorganization of copper complex that occurs shortly after photoexcitation. This result suggests that prediction of the lowest triplet excited state energies from experimentally observed emission spectrum is often problematic and can be misleading, because excited state distortions are common in phosphorescent Cu(I) complexes. Photophysical studies showed that (IPr)Cu(fppz) can be used as host for higher energy emitting dopant Ir(ppy) 3 without quenching its luminescence via triplet energy transfer. An OLED employing Ir(ppy) 3 doped in (IPr)Cu(fppz) (7 wt%) was fabricated and showed EL spectrum consistent with Ir(ppy) 3 emission and maximum EQE of 3.7%. 5.4. Experimental section General information. Synthesis of complexes 3.4 and 3.6 is described in Chapter 2. Facial-tris(2-phenylpyridinato-N,C 2’ )iridium(III) (Ir(ppy) 3 ), facial-tris(1- phenylpyrazolyl-N,C 2’ )iridium(III) (Ir(ppz) 3 ), iridium(III) bis(2-phenylquinolyl-N,C 2’ ) 128 acetylacetonate (PQIr), N,N’-diphenyl-N,N’-bis(1-naphthyl)benzidine (NPD), 2, 9- dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), tris(8-hydroxyquinolinato)aluminium (Alq 3 ) were received from commercial sources. Solvents were obtained from commercial sources and used without further purification. Cyclic voltammetry was performed in anhydrous CH 2 Cl 2 using an EG&G potentiostat/galvanostat model 283 under N 2 atmosphere. 0.1 M tetrabutylammonium hexafluorophosphate (TBAH) was used as the supporting electrolyte. A glassy carbon rod was used as the working electrode, a platinum wire was used as the counter electrode, and a silver wire was used as a pseudo reference electrode. The redox potentials are calculated relative to an internal reference ferrocenium/ferrocene (Fc + /Fc). The UV-visible spectra were recorded in dichloromethane at room temperature on a Hewlett-Packard 4853 diode array spectrometer. Solution samples were degassed by sparging with N 2 for 15 minutes and thin film samples were placed in a glass cell and degassed (0.1 torr) before emission and lifetime measurements. Steady state emission measurements were performed on a Photon Technology International (PTI) QuantaMaster TM model C-60 spectrofluorimeter equipped with a 820 PMT detector and corrected for detector response. Phosphorescence lifetime measurements were performed on IBH Fluorocube instrument equipped with a 405 nm LED excitation source using time-correlated single photon counting method. OLED fabrication and characterization. All metal complexes and organic materials were purified by gradient sublimation before use. Prior to device fabrication, purchased 2 mm wide striped pre-patterned ITO substrates were cleaned in Tergitol solution, rinsed with deionized water and dried with N 2 . Finally, the substrates were treated with UV 129 ozone for 10 min. After cleaning, the substrates were immediately loaded into a high vacuum chamber. All layers were deposited by vacuum thermal evaporation at a base pressure of ∼1x10-6 Torr. After organic depositions, masks with 2 mm stripe width were placed on substrates under N 2 . Cathodes consisted of a 10 Å thick layer of LiF followed by a 1000 Å thick of Al. The devices were tested in air within 1 hour of fabrication. Device current-voltage and light-intensity characteristics were measured using a LabVIEW program with a Keithley 2400 SourceMeter/2000 Multimeter coupled to a Newport 1835-C Optical Meter, equipped with a UV-818 Si photocathode. The electroluminescence spectra were measured on a PTI QuantaMaster TM model C-60 spectrofluorimeter equipped with a 820 PMT detector and corrected for detector response. Chapter 5 References (1) Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151. (2) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4. (3) Yersin, H. Transition Metal and Rare Earth Compounds III 2004, 241, 1. (4) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl. Phys. 2001, 90, 5048. (5) Tang, C. W.; Vanslyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65, 3610. (6) Shi, J. M.; Tang, C. W. Appl. Phys. Lett. 1997, 70, 1665. (7) O'Brien, D. F.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 74, 442. (8) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl. Phys. 130 Lett. 2000, 77, 904. (9) Tanaka, D.; Sasabe, H.; Li, Y. J.; Su, S. J.; Takeda, T.; Kido, J. Jpn. J. Appl. Phys. 2 2007, 46, L10. (10) Holmes, R. J.; Forrest, S. R.; Tung, Y. J.; Kwong, R. C.; Brown, J. J.; Garon, S.; Thompson, M. E. Appl. Phys. Lett. 2003, 82, 2422. (11) Baldo, M. A.; Forrest, S. R. Phys. Rev. B 2000, 62, 10958. (12) Kawamura, Y.; Goushi, K.; Brooks, J.; Brown, J. J.; Sasabe, H.; Adachi, C. Appl. Phys. Lett. 2005, 86, 071104. (13) Adachi, C.; Kwong, R. C.; Djurovich, P.; Adamovich, V.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 2001, 79, 2082. (14) Tokito, S.; Iijima, T.; Suzuri, Y.; Kita, H.; Tsuzuki, T.; Sato, F. Appl. Phys. Lett. 2003, 83, 569. (15) Watanabe, S.; Ide, N.; Kido, J. Jpn. J. Appl. Phys. 1 2007, 46, 1186. (16) Wu, C.; Djurovich, P. I.; Thompson, M. E. Adv. Funct. Mater. 2009, 19, 3157. (17) Wei, W.; Djurovich, P. I.; Thompson, M. E. Chem. Mater. 2010, 22, 1724. (18) Lin, J. J.; Liao, W. S.; Huang, H. J.; Wu, F. I.; Cheng, C. H. Adv. Funct. Mater. 2008, 18, 485. (19) Ren, X. F.; Li, J.; Holmes, R. J.; Djurovich, P. I.; Forrest, S. R.; Thompson, M. E. Chem. Mater. 2004, 16, 4743. (20) Sapochak, L. S.; Padmaperuma, A. B.; Cai, X. Y.; Male, J. L.; Burrows, P. E. J. Phys. Chem. C 2008, 112, 7989. (21) Cai, X. Y.; Padmaperuma, A. B.; Sapochak, L. S.; Vecchi, P. A.; Burrows, P. E. Appl. Phys. Lett. 2008, 92. (22) Jeon, S. O.; Yook, K. S.; Joo, C. W.; Lee, J. Y. Adv. Funct. Mater. 2009, 19, 3644. (23) Jeon, S. O.; Yook, K. S.; Joo, C. W.; Lee, J. Y. Adv. Mater. 2010, 22, 1872. (24) Kwong, R. C.; Lamansky, S.; Thompson, M. E. Adv. Mater. 2000, 12, 1134. 131 (25) Tsuzuki, T.; Tokito, S. Adv. Mater. 2007, 19, 276. (26) Tsuzuki, T.; Tokito, S. Appl. Phy.s Express 2008, 1. (27) Armaroli, N.; Accorsi, G.; Cardinali, F.; Listorti, A. Top. Curr. Chem. 2007, 280, 69. (28) Barbieri, A.; Accorsi, G.; Armaroli, N. Chem. Commun. 2008, 2185. (29) McMillin, D. R.; McNett, K. M. Chem. Rev. 1998, 98, 1201. (30) Miller, A. J. M.; Dempsey, J. L.; Peters, J. C. Inorg. Chem. 2007, 46, 7244. (31) Lotito, K. J.; Peters, J. C. Chem. Commun. 2010, 46, 3690. (32) Hsu, C.-W.; Lin, C.-C.; Chung, M.-W.; Chi, Y.; Lee, G.-H.; Chou, P.-T.; Chang, C.-H.; Chen, P.-Y. J. Am. Chem. Soc. 2011, 133, 12085. (33) Deaton, J. C.; Switalski, S. C.; Kondakov, D. Y.; Young, R. H.; Pawlik, T. D.; Giesen, D. J.; Harkins, S. B.; Miller, A. J. M.; Mickenberg, S. F.; Peters, J. C. J. Am. Chem. Soc. 2010, 132, 9499. (34) Czerwieniec, R.; Yu, J.; Yersin, H. Inorg. Chem. 2011, 50, 8293. (35) Kirchhoff, J. R.; Gamache, R. E.; Blaskie, M. W.; Delpaggio, A. A.; Lengel, R. K.; McMillin, D. R. Inorg. Chem. 1983, 22, 2380. (36) Asano, M. S.; Tomiduka, K.; Sekizawa, K.; Yamashita, K.; Sugiura, K. Chem. Lett. 2010, 39, 376. (37) Hashimoto, M.; Igawa, S.; Yashima, M.; Kawata, I.; Hoshino, M.; Osawa, M. J. Am. Chem. Soc. 2011, 133, 10348. (38) Liu, Z.; Qayyum, M. F.; Wu, C.; Whited, M. T.; Djurovich, P. I.; Hodgson, K. O.; Hedman, B.; Solomon, E. I.; Thompson, M. E. J. Am. Chem. Soc. 2011, 133, 3700. (39) Igawa, S.; Hashimoto, M.; Kawata, I.; Yashima, M.; Hoshino, M.; Osawa, M. J. Mater. Chem. C 2013, 1, 542. (40) Zhang, Q.; Komino, T.; Huang, S.; Matsunami, S.; Goushi, K.; Adachi, C. Adv. Funct. Mater. 2012, 22, 2327. (41) D'Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.; Polikarpov, E.; Thompson, M. E. Org. Electr. 2005, 6, 11. 132 (42) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Kwong, R.; Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E. Inorg. Chem. 2001, 40, 1704. (43) Breddels, P. A.; Berdowski, P. A. M.; Blasse, G. J. Chem. Soc. Farad. Trans. II 1982, 78, 595. (44) Blasse, G.; McMillin, D. R. Chem. Phys. Lett. 1980, 70, 1. (45) Krylova, V. A.; Djurovich, P. I.; Aronson, J. W.; Haiges, R.; Whited, M. T.; Thompson, M. E. Organometallics 2012, 31, 7983. (46) Goushi, K.; Kwong, R.; Brown, J. J.; Sasabe, H.; Adachi, C. J. Appl. Phys. 2004, 95, 7798. 133 Bibliography Accorsi, G.; Armaroli, N.; Duhayon, C.; Saquet, A.; Delavaux-Nicot, B.; Welter, R.; Moudam, O.; Holler, M.; Nierengarten, J.-F. Eur. J. Inorg. Chem. 2010, 164. Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl. Phys. Lett. 2000, 77, 904. Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl. Phys. 2001, 90, 5048. Adachi, C.; Kwong, R. C.; Djurovich, P.; Adamovich, V.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 2001, 79, 2082. Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361. Armaroli, N.; Accorsi, G.; Cardinali, F.; Listorti, A. Top. Curr. Chem. 2007, 280, 69. Asano, M. S.; Tomiduka, K.; Sekizawa, K.; Yamashita, K.; Sugiura, K. Chem. Lett. 2010, 39, 376. Au, V. K. M.; Wong, K. M. C.; Zhu, N. Y.; Yam, V. W. W. J. Am. Chem. Soc. 2009, 131, 9076. Baldo, M. A.; Forrest, S. R. Phys. Rev. B 2000, 62, 10958. Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett.1999, 75, 4. Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151. Barakat, K. A.; Cundari, T. R.; Omary, M. A. J. Am. Chem. Soc. 2003, 125, 14228. Barbieri, A.; Accorsi, G.; Armaroli, N. Chem. Commun. 2008, 2185. Becke, A. D. J. Chem. Phys. 1993, 98, 5648. Benhamou, L.; Chardon, E.; Lavigne, G.; Bellemin-Laponnaz, S.; Cesar, V. Chem. Rev.2011, 111, 2705. Bergmann, L.; Friedrichs, J.; Mydlak, M.; Baumann, T.; Nieger, M.; Brase, S. Chem. Commun.2013, 49, 6501. Blaskie, M. W.; McMillin, D. R. Inorg. Chem. 1980, 19, 3519. Blasse, G.; McMillin, D. R. Chem. Phys. Lett. 1980, 70, 1. 134 Blessing, R. H. Acta Crystallogr. A 1995, 51, 33. Boehme, C.; Frenking, G. J. Am. Chem. Soc. 1996, 118, 2039. Boehme, C.; Frenking, G. Organometallics 1998, 17, 5801. Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39. Breddels, P. A.; Berdowski, P. A. M.; Blasse, G. J. Chem. Soc., Faraday Trans. 1982, 78, 595. Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.; Bau, R.; Thompson, M. E. Inorg. Chem. 2002, 41, 3055. Bruker Instrument Service v2011.4.0.0 ed.; Bruker AXS Madison, WI: 2011. Cai, X. Y.; Padmaperuma, A. B.; Sapochak, L. S.; Vecchi, P. A.; Burrows, P. E. Appl. Phys. Lett. 2008, 92. Chang, C. F.; Cheng, Y. M.; Chi, Y.; Chiu, Y. C.; Lin, C. C.; Lee, G. H.; Chou, P. T.; Chen, C. C.; Chang, C. H.; Wu, C. C. Angew. Chem., Int. Ed. 2008, 47, 4542. Che, G.; Su, Z.; Li, W.; Chu, B.; Li, M.; Hu, Z.; Zhang, Z. Appl. Phys. Lett. 2006, 89. Chen, L. X.; Jennings, G.; Liu, T.; Gosztola, D. J.; Hessler, J. P.; Scaltrito, D. V.; Meyer, G. J. J. Am. Chem. Soc. 2002, 124, 10861. Chen, L. X.; Shaw, G. B.; Novozhilova, I.; Liu, T.; Jennings, G.; Attenkofer, K.; Meyer, G. J.; Coppens, P. J. Am. Chem. Soc. 2003, 125, 7022. Chi, Y.; Chou, P. T. Chem. Soc. Rev. 2010, 39, 638. Chianese, A. R.; Li, X. W.; Janzen, M. C.; Faller, J. W.; Crabtree, R. H. Organometallics 2003, 22, 1663. Chianese, A. R.; Mo, A.; Datta, D. Organometallics 2009, 28, 465. Chung, L.-H.; Cho, K.-S.; England, J.; Chan, S.-C.; Wieghardt, K.; Wong, C.-Y. Inorg. Chem. 2013, 52, 9885. Crabtree, R. H. J. Organomet. Chem. 2005, 690, 5451. Crestani, M. G.; Manbeck, G. F.; Brennessel, W. W.; McCormick, T. M.; Eisenberg, R. Inorg. Chem. 2011, 50, 7172. Cuttell, D. G.; Kuang, S. M.; Fanwick, P. E.; McMillin, D. R.; Walton, R. A. J. Am. 135 Chem. Soc. 2002, 124, 6. Czerwieniec, R.; Yu, J.; Yersin, H. Inorg. Chem. 2011, 50, 8293. D'Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.; Polikarpov, E.; Thompson, M. E. Organic Electronics2005, 6, 11. Deaton, J. C.; Switalski, S. C.; Kondakov, D. Y.; Young, R. H.; Pawlik, T. D.; Giesen, D. J.; Harkins, S. B.; Miller, A. J. M.; Mickenberg, S. F.; Peters, J. C. J. Am. Chem. Soc. 2010, 132, 9499. Dible, B. R.; Sigman, M. S. Inorg. Chem. 2006, 45, 8430. Diez-Gonzalez, S.; Escudero-Adan, E. C.; Benet-Buchholz, J.; Stevens, E. D.; Slawin, A. M. Z.; Nolan, S. P. Dalton Trans. 2010, 39, 7595. Diez-Gonzalez, S.; Nolan, S. P. Aldrichim. Acta 2008, 41, 43. Diez-Gonzalez, S.; Nolan, S. P. Synlett 2007, 2158. Diez-Gonzalez, S.; Stevens, E. D.; Scott, N. M.; Petersen, J. L.; Nolan, S. P. Chem.-Eur. J. 2008, 14, 158. Dixon, D. A.; Arduengo, A. J. J. Phys. Chem. 1991, 95, 4180. Du, L. H.; Wang, Y. G. Synthesis-Stuttgart 2007, 675. Everly, R. M.; McMillin, D. R. Photochem. Photobiol. 1989, 50, 711. Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565. Flores, J. A.; Andino, J. G.; Tsvetkov, N. P.; Pink, M.; Wolfe, R. J.; Head, A. R.; Lichtenberger, D. L.; Massa, J.; Caulton, K. G. Inorg. Chem. 2011, 50, 8121. Ford, P. C.; Cariati, E.; Bourassa, J. Chem. Rev. 1999, 99, 3625. Fortman, G. C.; Slawin, A. M. Z.; Nolan, S. P. Organometallics 2010, 29, 3966. Frisch, M. J. T., G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,; M. A.; Cheeseman, J. R. M., J. A., Jr.; Vreven, T.; Kudin, K. N.;; Burant, J. C. M., J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;; Mennucci, B. C., M.; Scalmani, G.; Rega, N.; Petersson, G. A.;; Nakatsuji, H. H., M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,; J.; Ishida, M. N., T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,; X.; Knox, J. E. H., H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;; Gomperts, R. S., R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;; Pomelli, C. J. O., W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;; Salvador, P. D., J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,; A. D.; Strain, M. C. F., O.; D. Malick, K.; A. 136 Rabuck, D.;; Raghavachari, K. F., J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;; Clifford, S. C., J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,; P.; Komaromi, I. M., R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;; Peng, C. Y. N., A.; Hallacombe, M.; Gill, CP. M. W.; Johnson,; B.; Chen, W. W., M. W.; Gonzalez, C.; and Pople, J. A.; Gaussian, Inc., : Wallingford, CT, 2004.Funabiki, K.; Noma, N.; Kuzuya, G.; Matsui, M.; Shibata, K. J. Chem. Res., Synop. 1999, 300. Giordano, C.; Minisci, F.; Vismara, E.; Levi, S. J. Org. Chem. 1986, 51, 536. Glorius, F. Top. Organomet. Chem. 2007, 21, 1. Goj, L. A.; Blue, E. D.; Delp, S. A.; Gunnoe, T. B.; Cundari, T. R.; Pierpont, A. W.; Petersen, J. L.; Boyle, P. D. Inorg. Chem. 2006, 45, 9032. Gothard, N. A.; Mara, M. W.; Huang, J.; Szarko, J. M.; Rolczynski, B.; Lockard, J. V.; Chen, L. X. J. Phys. Chem. A 2012, 116, 1984. Goushi, K.; Kwong, R.; Brown, J. J.; Sasabe, H.; Adachi, C. J. Appl. Phys. 2004, 95, 7798. Green, O.; Gandhi, B. A.; Burstyn, J. N. Inorg. Chem. 2009, 48, 5704. Hahn, F. E.; Wittenbecher, L.; Le, V. D.; Frohlich, R. Angew. Chem., Int. Ed. 2000, 39, 541. Harkins, S. B.; Mankad, N. P.; Miller, A. J. M.; Szilagyi, R. K.; Peters, J. C. J. Am. Chem. Soc. 2008, 130, 3478. Harkins, S. B.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 2030. Hashimoto, M.; Igawa, S.; Yashima, M.; Kawata, I.; Hoshino, M.; Osawa, M. J. Am. Chem. Soc. 2011, 133, 10348. Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. Heinemann, C.; Muller, T.; Apeloig, Y.; Schwarz, H. J. Am. Chem. Soc. 1996, 118, 2023. Heinemann, C.; Thiel, W. Chem. Phys. Lett. 1994, 217, 11. Herrmann, W. A.; Goossen, L. J.; Spiegler, M. J. Organomet. Chem. 1997, 547, 357. Hodgkins, T. G.; Powell, D. R. Inorg. Chem. 1996, 35, 2140. Holmes, R. J.; Forrest, S. R.; Tung, Y. J.; Kwong, R. C.; Brown, J. J.; Garon, S.; Thompson, M. E. Appl. Phys. Lett. 2003, 82, 2422. 137 Hsieh, C. H.; Wu, F. I.; Fan, C. H.; Huang, M. J.; Lu, K. Y.; Chou, P. Y.; Yang, Y. H. O.; Wu, S. H.; Chen, I. C.; Chou, S. H.; Wong, K. T.; Cheng, C. H. Chem.-Eur. J. 2011, 17, 9180. Hsu, C.-W.; Lin, C.-C.; Chung, M.-W.; Chi, Y.; Lee, G.-H.; Chou, P.-T.; Chang, C.-H.; Chen, P.-Y. J. Am. Chem. Soc. 2011, 133, 12085. Hsu, S. H.; Li, C. Y.; Chiu, Y. W.; Chiu, M. C.; Lien, Y. L.; Kuo, P. C.; Lee, H. M.; Huang, J. H.; Cheng, C. P. J. Organomet. Chem. 2007, 692, 5421. Igawa, S.; Hashimoto, M.; Kawata, I.; Yashima, M.; Hoshino, M.; Osawa, M. J. Mater. Chem. C 2013, 1, 542. Iwamura, M.; Takeuchi, S.; Tahara, T. J. Am. Chem. Soc. 2007, 129, 5248. Jacobsen, H.; Correa, A.; Costabile, C.; Cavallo, L. J. Organomet. Chem. 2006, 691, 4350. Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.; Cavallo, L. Coord. Chem. Rev. 2009, 253, 687. Jeon, S. O.; Yook, K. S.; Joo, C. W.; Lee, J. Y. Adv. Funct. Mater. 2009, 19, 3644. Jeon, S. O.; Yook, K. S.; Joo, C. W.; Lee, J. Y. Adv. Mater. 2010, 22, 1872. Jurkauskas, V.; Sadighi, J. P.; Buchwald, S. L. Org. Lett. 2003, 5, 2417. Kaur, H.; Zinn, F. K.; Stevens, E. D.; Nolan, S. P. Organometallics 2004, 23, 1157. Kawamura, Y.; Goushi, K.; Brooks, J.; Brown, J. J.; Sasabe, H.; Adachi, C. Appl. Phys. Lett. 2005, 86, 071104. Khaskin, E.; Zavalij, P. Y.; Vedernikov, A. N. J. Am. Chem. Soc. 2006, 128, 13054. Kirchhoff, J. R.; Gamache, R. E.; Blaskie, M. W.; Delpaggio, A. A.; Lengel, R. K.; McMillin, D. R. Inorg. Chem. 1983, 22, 2380. Klappa, J. J.; Rich, A. E.; McNeill, K. Org. Lett. 2002, 4, 435. Krylova, V. A.; Djurovich, P. I.; Aronson, J. W.; Haiges, R.; Whited, M. T.; Thompson, M. E. Organometallics 2012, 31, 7983. Krylova, V. A.; Djurovich, P. I.; Whited, M. T.; Thompson, M. E. Chem. Commun. 2010, 46, 6696. Kuang, S. M.; Cuttell, D. G.; McMillin, D. R.; Fanwick, P. E.; Walton, R. A. Inorg. 138 Chem.2002, 41, 3313. Kwong, R. C.; Lamansky, S.; Thompson, M. E. Adv. Mater. 2000, 12, 1134. Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Kwong, R.; Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E. Inorg. Chem. 2001, 40, 1704. Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H. E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304. Lavie-Cambot, A.; Cantuel, M.; Leydet, Y.; Jonusauskas, G.; Bassani, D. M.; McClenaghan, N. D. Coordination Chemistry Reviews 2008, 252, 2572. Lavie-Cambot, A.; Cantuel, M.; Leydet, Y.; Jonusauskas, G.; Bassani, D. M.; McClenaghan, N. D. Coord. Chem. Rev. 2008, 252, 2572. Lazreg, F.; Slawin, A. M. Z.; Cazin, C. S. J. Organometallics 2012, 31, 7969. Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785. Li, J.; Djurovich, P. I.; Alleyne, B. D.; Yousufuddin, M.; Ho, N. N.; Thomas, J. C.; Peters, J. C.; Bau, R.; Thompson, M. E. Inorg. Chem. 2005, 44, 1713. Lin, J. C. Y.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A.; Hwang, W. S.; Lin, I. J. B. Chem. Rev. 2009, 109, 3561. Lin, J. J.; Liao, W. S.; Huang, H. J.; Wu, F. I.; Cheng, C. H. Adv. Func. Mater. 2008, 18, 485. Liu, F.; Meadows, K. A.; McMillin, D. R. J. Am. Chem. Soc. 1993, 115, 6699. Liu, Z.; Qayyum, M. F.; Wu, C.; Whited, M. T.; Djurovich, P. I.; Hodgson, K. O.; Hedman, B.; Solomon, E. I.; Thompson, M. E. J. Am. Chem. Soc. 2011, 133, 3700. Liu, Z.; Qayyum, M. F.; Wu, C.; Whited, M. T.; Djurovich, P. I.; Hodgson, K. O.; Hedman, B.; Solomon, E. I.; Thompson, M. E. J. Am. Chem. Soc.2011, 133, 3700. Liu, Z.; Qayyum, M. F.; Wu, C.; Whited, M. T.; Djurovich, P. I.; Hodgson, K. O.; Hedman, B.; Solomon, E. I.; Thompson, M. E. J. Am. Chem. Soc. 2011, 133, 3700. Lotito, K. J.; Peters, J. C. Chem. Commun. 2010, 46, 3690. Macdougall, P. J.; Bader, R. F. W. Can. J. Chem. 1986, 64, 1496. Mahadevan, S.; Palaniandavar, M. Inorg. Chem. 1998, 37, 693. 139 Manbeck, G. F.; Brennessel, W. W.; Eisenberg, R. Inorg. Chem. 2011, 50, 3431. Mara, M. W.; Jackson, N. E.; Huang, J.; Stickrath, A. B.; Zhang, X. Y.; Gothard, N. A.; Ratner, M. A.; Chen, L. X. J. Phys. Chem. B 2013, 117, 1921. Matsumoto, K.; Matsumoto, N.; Ishii, A.; Tsukuda, T.; Hasegawa, M.; Tsubomura, T. Dalton Trans. 2009, 6795. McCormick, T.; Jia, W. L.; Wang, S. N. Inorg. Chem. 2006, 45, 147. McMillin, D. R.; McNett, K. M. Chem. Rev. 1998, 98, 1201. Mercs, L.; Albrecht, M. Chem. Soc. Rev. 2010, 39, 1903. Miller, A. J. M.; Dempsey, J. L.; Peters, J. C. Inorg. Chem. 2007, 46, 7244. Min, J. H.; Zhang, Q. S.; Sun, W.; Cheng, Y. X.; Wang, L. X. Dalton Trans. 2011, 40, 686. Moudam, O.; Kaeser, A.; Delavaux-Nicot, B.; Duhayon, C.; Holler, M.; Accorsi, G.; Armaroli, N.; Seguy, I.; Navarro, J.; Destruel, P.; Nierengarten, J.-F. Chem. Commun. 2007, 3077. O'Brien, D. F.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 74, 442. Opalka, S. M.; Park, J. K.; Longstreet, A. R.; McQuade, D. T. Org. Lett. 2013, 15, 996. Palmer, C. E. A.; McMillin, D. R. Inorg. Chem. 1987, 26, 3837. Palmer, C. E. A.; McMillin, D. R.; Kirmaier, C.; Holten, D. Inorg. Chem. 1987, 26, 3167. Peris, E. Top. Organomet. Chem. 2007, 21, 83. Poater, A.; Ragone, F.; Giudice, S.; Costabile, C.; Dorta, R.; Nolan, S. P.; Cavallo, L. Organometallics 2008, 27, 2679. Pucci, D.; Aiello, I.; Aprea, A.; Bellusci, A.; Crispini, A.; Ghedini, M. Chem. Commun. 2009, 1550. Rader, R. A.; McMillin, D. R.; Buckner, M. T.; Matthews, T. G.; Casadonte, D. J.; Lengel, R. K.; Whittaker, S. B.; Darmon, L. M.; Lytle, F. E. J. Am. Chem. Soc. 1981, 103, 5906. Ren, X. F.; Li, J.; Holmes, R. J.; Djurovich, P. I.; Forrest, S. R.; Thompson, M. E. Chem. Mater. 2004, 16, 4743. 140 SADABS; V2008/1 ed.; Bruker AXS Madison, WI: 2008. SAINT; V7.68A ed.; Bruker AXS Madison, WI: 2009. Sajoto, T.; Djurovich, P. I.; Tamayo, A.; Yousufuddin, M.; Bau, R.; Thompson, M. E.; Holmes, R. J.; Forrest, S. R. Inorg. Chem. 2005, 44, 7992. Sapochak, L. S.; Padmaperuma, A. B.; Cai, X. Y.; Male, J. L.; Burrows, P. E. J. Phys. Chem. C 2008, 112, 7989. Scaltrito, D. V.; Thompson, D. W.; O'Callaghan, J. A.; Meyer, G. J. Coord. Chem. Rev. 2000, 208, 243. Shaw, G. B.; Grant, C. D.; Shirota, H.; Castner, E. W.; Meyer, G. J.; Chen, L. X. J. Am. Chem. Soc. 2007, 129, 2147. Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112. Sheldrick, G. M.; SHELXTL, version 6.14; Bruker Analytical X-ray System, Inc.: Madison, WI, 1997. Shi, J. M.; Tang, C. W. Appl. Phys. Lett. 1997, 70, 1665. Shibata, T.; Ito, S.; Doe, M.; Tanaka, R.; Hashimoto, H.; Kinoshita, I.; Yano, S.; Nishioka, T. Dalton Trans. 2011, 40, 6778. Si, Z.; Li, J.; Li, B.; Liu, S.; Li, W. J. Lumin. 2008, 128, 1303. Siddique, Z. A.; Yamamoto, Y.; Ohno, T.; Nozaki, K. Inorg. Chem. 2003, 42, 6366. Singh, S. P.; Kumar, D.; Jones, B. G.; Threadgill, M. D. J. Fluorine Chem. 1999, 94, 199. Smith, C. S.; Branham, C. W.; Marquardt, B. J.; Mann, K. R. J. Am. Chem. Soc. 2010, 132, 14079. 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, 6896. Stacy, E. M.; McMillin, D. R. Inorg. Chem. 1990, 29, 393. Su, Z. S.; Che, G. B.; Li, W. L.; Su, W. M.; Li, M. T.; Chu, B.; Li, B.; Zhang, Z. Q.; Hu, Z. Z. Appl. Phys. Lett. 2006, 88. Tanaka, D.; Sasabe, H.; Li, Y. J.; Su, S. J.; Takeda, T.; Kido, J. Jpn. J. Appl. Phys., Part 2 2007, 46, L10. 141 Tang, C. W.; Vanslyke, S. A.; Chen, C. H. J .Appl. Phys. 1989, 65, 3610. Teyssot, M. L.; Jarrousse, A. S.; Chevry, A.; De Haze, A.; Beaudoin, C.; Manin, M.; Nolan, S. P.; Diez-Gonzalez, S.; Morel, L.; Gautier, A. Chem.- Eur. J. 2009, 15, 314. Teyssot, M.-L.; Chevry, A.; Traikia, M.; El-Ghozzi, M.; Avignant, D.; Gautier, A. Chem.-Eur. J. 2009, 15, 6322. Thiel, W. R.; Eppinger, J. Chem.-Eur. J. 1997, 3, 696. Tokito, S.; Iijima, T.; Suzuri, Y.; Kita, H.; Tsuzuki, T.; Sato, F. Appl. Phys. Lett. 2003, 83, 569. Tsuzuki, T.; Tokito, S. Adv. Mater. 2007, 19, 276. Tsuzuki, T.; Tokito, S. Appl. Phys. Express 2008, 1. Tulloch, A. A. D.; Danopoulos, A. A.; Kleinhenz, S.; Light, M. E.; Hursthouse, M. B.; Eastham, G. Organometallics 2001, 20, 2027. Uber, J. S.; Vogels, Y.; van den Helder, D.; Mutikainen, I.; Turpeinen, U.; Fu, W. T.; Roubeau, O.; Gamez, P.; Reedijk, J. Eur. J. Inorg. Chem. 2007, 4197. Ullah, F.; Bajor, G.; Veszpremi, T.; Jones, P. G.; Heinicke, J. W. Angew. Chem., Int. Ed. 2007, 46, 2697. Unger, Y.; Meyer, D.; Molt, O.; Schildknecht, C.; Munster, I.; Wagenblast, G.; Strassner, T. Angew. Chem., Int. Ed. 2010, 49, 10214. Unger, Y.; Zeller, A.; Ahrens, S.; Strassner, T. Chem. Commun. 2008, 3263. Vasudevan, K. V.; Butorac, R. R.; Abernethy, C. D.; Cowley, A. H. Dalton Trans. 2010, 39, 7401. Vorontsov, I. I.; Graber, T.; Kovalevsky, A. Y.; Novozhilova, I. V.; Gembicky, M.; Chen, Y.-S.; Coppens, P. J. Am. Chem. Soc. 2009, 131, 6566. Wada, A.; Zhang, Q.; Yasuda, T.; Takasu, I.; Enomoto, S.; Adachi, C. Chem. Commun. 2012, 48, 5340. Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. Watanabe, S.; Ide, N.; Kido, J. Jpn. J. Appl. Phys., Part 1 2007, 46, 1186. Wei, W.; Djurovich, P. I.; Thompson, M. E. Chem. Mater. 2010, 22, 1724. 142 Welle, A.; Diez-Gonzalez, S.; Tinant, B.; Nolan, S. P.; Riant, O. Org. Lett. 2006, 8, 6059. Weskamp, T.; Schattenmann, W. C.; Spiegler, M.; Herrmann, W. A. Angew. Chem., Int. Ed. 1998, 37, 2490. Williams, J. A. G. In Photochem. Photophys. Coord. Comp. 2007; Vol. 281, p 205. Wu, C.; Djurovich, P. I.; Thompson, M. E. Adv. Funct. Mater. 2009, 19, 3157. Yeh, Y.-S.; Cheng, Y.-M.; Chou, P.-T.; Lee, G.-H.; Yang, C.-H.; Chi, Y.; Shu, C.-F.; Wang, C.-H. ChemPhysChem 2006, 7, 2294. Yersin, H. Proc. SPIE-Int. Soc. Opt. Eng. 2004, 5214, 124. Yersin, H. Transition Metal and Rare Earth Compounds III 2004, 241, 1. Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer, T. Coord. Chem. Rev. 2011, 255, 2622. Yin, J.; Elsenbaumer, R. L. J. Org. Chem. 2005, 70, 9436. You, Y.; Park, S. Y. Dalton Trans. 2009, 1267. Zhang, L.; Li, B.; Su, Z. J. Phys. Chem. C 2009, 113, 13968. Zhang, Q. S.; Zhou, Q. G.; Cheng, Y. X.; Wang, L. X.; Ma, D. G.; Jing, X. B.; Wang, F. S. Adv. Funct. Mater. 2006, 16, 1203. Zhang, Q. S.; Zhou, Q. G.; Cheng, Y. X.; Wang, L. X.; Ma, D. G.; Jing, X. B.; Wang, F. S. Adv. Mater. 2004, 16, 432. Zhang, Q.; Ding, J.; Cheng, Y.; Wang, L.; Xie, Z.; Jing, X.; Wang, F. Adv. Funct. Mater. 2007, 17, 2983. Zhang, Q.; Komino, T.; Huang, S.; Matsunami, S.; Goushi, K.; Adachi, C. Adv. Funct. Mater. 2012, 22, 2327. 143 APPENDIX 1. Synthesis and photophysical characterization of four- coordinate Cu(I) complexes (P^P)Cu(N^N) A1.1. Introduction Copper(I) complexes is the largest class of phosphorescent materials based on abundant metal. 1-4 The most extensively studied family of luminescent Cu(I) complexes is four- coordinate cationic homo- and heteroleptic compounds, with diimine and phosphine ligands. 1,5 However, the scope of applicability for these derivatives is somewhat limited due to their charged nature. It has been shown that stable neutral Cu(I) complexes can be isolated and often have superior emission properties to those of cationic analogs. 6-8 Bergmann et al. compared photophysical properties of cationic (P^P)Cu(N^N) complexes (P^P = chelating phosphine ligand, N^N = pyridyl-tetrazole ligand) and their neutral analogs obtained by deprotonation of bisimine ligand. 9 They reported that solid-state luminescence efficiencies of neutral compounds are 2 to 23 times greater than emission efficiencies of their cationic counterparts. In a similar study Min et al. found that luminescence efficiencies of neutral complexes (P^P)Cu(N^N) (N^N = 2-(2’- quinolyl)benzimidazole) in solution is comparable or greater than their cationic analogs, while in doped polymer film they have similar PL efficiencies. 10 We synthesized and characterized a series of neutral (P^P)Cu(N^N) complexes, where P^P is a chelating bisphosphine and N^N is 2-(pyridin-2-yl)benzo[d]imidazol-1-ide (pybim) or 2-(isoquinolin-1-yl)benzo[d]imidazol-1-ide (beniq) ligands (Figure A1.1). These complexes show enhanced emission efficiencies in rigid media compared to reported charged analogs. It is now well established that photophysical properties of 144 four-coordinate Cu(I) complexes are governed by both electronic and steric properties of the ligands. 2,11 To gain insight into photophysical properties of these complexes we investigated the influence of the steric bulk of ligands on the excited state properties via modification of the P^P ligand, while keeping N^N ligand unchanged (compounds A1.1-A1.3). On the other hand we varied the energy of N^N ligand without affecting steric properties of the complex (compounds A1.2 and A1.4). Figure A1.1. Structures of complexes A1.1-A1.4. A1.2. Results and discussions A1.2.1. Synthesis and crystal structure Complexes were obtained from the corresponding phosphine ligand, deprotonated pyridine-benzimidazole or benzimidazole-isoquinoline and CuCl as copper source. The synthetic scheme is shown in Figure A1.2. 145 Figure A1.2. Reaction scheme for the synthesis of A2.1-A2.4. The crystal structures of A1.1-A1.3 were established by X-ray diffraction analysis. The crystals were obtained by vapor diffusion of ethyl ether into solution of each complex. Molecular structures of complexes A1.1-A1.3 are shown in Figure A1.3 and relevant bond lengths and angles are given in Table A1.1. Figure A1.3. ORTEP diagram of complexes A1.1-A1.3. Hydrogen atoms and co- crystallized solvent molecule (for A1.2) have been omitted for clarity. The compounds adopt a distorted tetrahedral geometry around the copper center. The dihedral angles between the planes defined by the P 1 , P 2, and Cu atoms and N py, N bim and Cu atoms are 84.59º for A1.1, 88.73º for A1.2 and 83.41º for A1.3. In complexes A1.1 and A1.2 the Cu-pyridyl distances are longer than the Cu-benzimidazole distances, while the latter is slightly greater than the former in complex A1.3. The Cu-phosphine bond 146 lengths are in the range of 2.2225(6)-2.2547(13) Å which is comparable to other neutral (P^P)Cu(N^N) derivatives. 8,12 The N py -Cu-N bim bond angles are comparable for all three compounds, while the P 1 -Cu-P 2 bond angles vary substantially in the series. In A1.1 the P 1 -Cu-P 2 bond angle is the smallest being 88.49º, in two other compounds these values are much greater (113.36(5)º (A1.2) and 115.740(8)º (A1.3)). This indicates that the POP and Xantphos ligands are sterically bulkier than the dppbz ligand. Table A1.1. Selected bond lengths (Å) and angles (deg) for complexes A1.1-A1.3. A1.1 A1.2 A1.3 Cu-N py 2.0896(19) 2.084(4) 2.0708(15) Cu-N bim 1.9791(19) 2.019(4) 2.1252(16) Cu-P 1 2.2281(6) 2.2547(13) 2.2353(5) Cu-P 2 2.2273(6) 2.2352(12) 2.2225(6) N py -Cu-N bim 81.58(8) 81.91(17) 79.19(6) P 1 -Cu-P 2 88.49(2) 113.36(5) 115.740(18) Dihedral P 1 -Cu-P 2┴ N py -Cu-N bim 84.59 88.73 83.41 A1.2.2. Photophysical studies The absorption spectra of the complexes A1.1-A1.4 and free N^N ligands are shown in Figure A1.4. Variation of the phosphine ligand does not have significant effect on absorption properties of pybim derivatives as all the complexes A1.2-A1.3 have similar absorption spectra. Their spectra are characterized by an intense absorption band at 341 nm (ε = 20000-24000 M -1 cm -1 ), which is assigned to π → π* ligand centered (LC) transitions of the pybim ligand. This band is also observed in absorption spectrum of pyridine-benzimidazolate anion and phosphine ligand LC transitions to occur at higher energies. 13 The shoulders at lower energies (λ < 385 nm, ε = 5700-7600 M -1 cm -1 ) can be 147 ascribed to charge transfer (CT) transitions. The optical spectrum of A1.4 has a similar, however it is red shifted compared to the spectra of pybim derivatives. The strong absorption band at 388 nm (ε = 15815 M -1 cm -1 ) is assigned to spin-allowed LC transition of beniq ligand. The observed trend in absorption spectra is consistent with expected bathochromic shift upon expanding the size of the π-system of a ligand chromophore. 300 325 350 375 400 425 450 475 500 0 5000 10000 15000 20000 25000 A1.1 A1.2 A1.3 A1.4 Molar Absorbance (M -1 cm -1 ) Wavelength (nm) 250 275 300 325 350 375 400 0.0 0.5 1.0 1.5 2.0 Hpybim Napybim (in THF) Hbeniq Absorbance (a.u.) Wavelength (nm) Figure A1.4. Absorption spectra of complexes A1.1-A1.4 (left) and N^N ligand precursors (right) in CH 2 Cl 2 . Luminescence spectra of complexes A1.1-A1.4 were recorded in 2- methyltetrahydrofuran at 77K and in doped (2 wt%) PMMA film (Figure A1.5). The photophysical data are summarized in Table A1.2. Emission spectra of A1.1-A1.3 similar to absorption spectra are virtually identical. At 77K the spectra show some vibronic structure while at RT they are broad and featureless. Emission spectra of A1.4 are bathochromically shifted from that of pybim derivatives A1.1-A1.3 and has more pronounced vibronic structure at 77 K that still persists at RT. Excited state lifetimes measured at 77 K are on the order of hundreds of microseconds for A1.1-A1.3 and in the millisecond range for A1.4. This indicates that the observed emission is phosphorescence 148 in nature. All complexes are bright emitters at RT in doped polymer films with emission efficiencies in the range of 0.31-0.72. 400 450 500 550 600 650 700 750 0.0 0.2 0.4 0.6 0.8 1.0 A1.1 A1.2 A1.3 A1.4 Normalized intensity (a.u.) Wavelength (nm) 400 450 500 550 600 650 700 750 0.0 0.2 0.4 0.6 0.8 1.0 A1.1 A1.2 A1.3 A1.4 Normalized intensity (a.u.) Wavelength (nm) Figure A1.5. Emission spectra of complexes A1.1-A1.4 in 2-MeTHF at 77 K (left) and in doped (2 wt%) PMMA film at RT. The luminescence decays obtained under these conditions have two components. The major component is 13 µs (94%) for A1.1, 52 µs (92%) for A1.2 and 47 µs (92%) for A1.1. The radiative rates are similar for A1.1-A1.3 (1.2-2.4x 10 4 s -1 ), while the non- radiative rates of A1.2 and A1.3 are an order of magnitude lower than that for A1.1. Thus there is a qualitative correlation between the non-radiative rates and the phosphine ligands bite angles P-Cu-P obtained from crystallographic analysis. The complexes with the larger bite angles and have smaller non-radiative rates. Emission lifetime of complex A1.4 is longer than that of A1.1-A1.3 being 1.5 x 10 3 µs (53%) and 390 µs (47%). This result together with structured emission spectra at 77 K and at RT suggests that emission of A1.4 is principally 3 LC. Broader emission spectra of A1.1-A1.3 indicate that their emission is charge transfer in character. 149 Table A1.2. Summary of photophysical properties of complexes A1.1-A1.4. em at RT in PMMA (2 wt%) em at 77K in 2-MeTHF λ max (nm) τ (µs) Φ PL k r (s -1 ) k nr (s -1 ) λ max (nm) τ (µs) A1.1 542 13 (94%) 42 (6%) 0.31 2.4 x 10 4 5.3 x 10 4 478, 506 826.5 A1.2 542 52 (92%) 231(8%) 0.62 1.2 x 10 4 7.2 x 10 3 464, 494, 526 973 (44% ) 342 (56%) A1.3 542 47 (92%) 204 (8%) 0.72 1.5 x 10 4 6.3 x 10 3 464, 494, 526 413 (45%) 946 (55%) A1.4 564, 602 1.5x10 3 (53%) 390 (47%) 0.47 - - 552, 567, 598, 650 2.2x10 3 A1.2.3. DFT calculations Density functional theory (DFT) calculations were carried out for complexes A1.1-A1.4 (Figure A1.6). Figure A1.6. LUMO (A), HOMO (B) and triplet spin-density (C) obtained by density functional (DFT) calculations of A1.1-A1.4. Plot contours are shown at an isovalue of 0.004 electrons au -3 . 150 The HOMOs have essentially identical spatial distribution and similar energies for all complexes A1.1-A1.4. It is localized on benzimidazole moiety with small contribution from orbitals on copper. For complexes A1.1 and A1.2 the LUMO is principally phosphine-based, while for A1.3 it is distributed over both ligands and for A1.4 the LUMO is localized on N^N ligand. The energy of the LUMO gradually decreases when going from A1.1 to A1.3. Expansion of π conjugation in A1.4 leads to significant stabilization of the LUMO and therefore decreases the HOMO/LUMO gap compared to that of A1.1-A1.3. This theoretical result is consistent with experimentally observed red shift of absorption and emission spectra of beniq derivative relative to pybim based complexes. The calculated triplet spin density surfaces are localized on N^N ligand. Based on theoretical calculations together with experimental observations emission of complexes A1.1-A1.3 is assigned as 3 LC/ILCT and as 3 LC for A1.4. A1.3. Experimental section The reactions were performed under nitrogen atmosphere in oven dried glassware. 1,2- Bis(diphenylphosphino)benzene (dppbz), bis(2-diphenylphosphinophenyl)ether (POP), 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos), 2-(pyridin-2- yl)benzo[d]imidazole (pybim) and CuCl were purchased and used as received. 2- (Isoquinolin-1-yl)benzo[d]imidazole (beniq) was prepared using the literature procedure. 14 Solvents were obtained from commercial sources and used without further purification except for tetrahydrofuran, which was purified by Glass Contour solvent system by SG Water USA, LLC. 1 H and 31 P NMR spectra were recorded on a Varian 151 Mercury 400. The chemical shifts are given in units of ppm. All 1 H chemical shifts were referenced to the residual solvent signals. The UV-visible spectra were recorded on a Hewlett-Packard 4853 diode array spectrometer. Steady state emission measurements were performed using a Photon Technology International QuantaMaster model C-60 fluorimeter. All reported spectra are corrected for photomultiplier response. Phosphorescence lifetime measurements were performed on the same fluorimeter equipped with a microsecond Xe flash lamp or using an IBH Fluorocube instrument equipped with a 405 nm LED excitation source using time-correlated single photon counting method. Quantum yields at room temperature were measured using a Hamamatsu C9920 system equipped with a xenon lamp, calibrated integrating sphere and model C10027 photonic multichannel analyzer. Thin film samples were placed in a glass cell and degassed (0.1 torr) before emission and lifetime measurements. Quantum yields of thin film samples were measured under N 2 atmosphere. Density functional theory (DFT) calculations were performed with the Gaussian03 15 software package employing the B3LYP functional 16,17 using LANL2DZ basis set 18-20 for Cu and 6-31G* for C, N, H and F. Geometric parameters obtained from XRD analyses were used as a starting point for geometry optimization in the ground state. 2-(Isoquinolin-1-yl)benzo[d]imidazole (beniq). 14 Isoquinoline-1-carbaldehyde 21 (315.2 mg, 2 mmol) and 1,2-phenylenediamine (216 mg, 2 mmol) were mixed in 40 ml of absolute ethanol and stirred for 2 h at RT. Iodobenzene diacetate (IBD) (966.3 mg, 152 3 mmol) was added and the reaction mixture was stirred for further 1 h. The volatiles were removed by rotary evaporation and the resulting brown crude mixture was purified by silica gel column chromatography (ethyl acetate:hexane 1:20). The product was obtained as light-yellow solid (160 mg, 33%). 1 H NMR (400 MHz, CDCl 3 , δ) 7.31-7.37 (m, 2H), 7.54-7.56 (m, 1H), 7.75-7.83 (m, 3H), 7.88-7.90 (m, 1H), 7.96-7.98 (m, 1H), 8.58 (d, J = 5.5 Hz, 1H), 10.21 (d, J = 7.2 Hz, 1H), 10.96 (br s, 1H). (dppbz)Cu(pybim) (A1.1). 2-(Pyridin-2-yl)benzo[d]imidazole (97.6 mg, 0.5 mmol) was dissolved in 10 mL of THF and this solution was transferred via cannula to suspension of sodium hydride (22 mg, 0.55 mmol, 60% in mineral oil) and bis(diphenylphosphino)benzene (223.23 mg, 0.5 mmol) in 15 ml THF. The reaction mixture was stirred at RT for 1 h and then CuCl (49.5 mg, 0.55 mmol) was added. The reaction mixture was stirred at RT for 3 h. The resulting mixture was filtered and solvent was removed by rotary evaporation. The obtained yellow solid was washed with hexane and recrystallized by vapor diffusion of diethyl ether into a CH 2 Cl 2 solution to give 120 mg (60%) of yellow crystals. 1 H NMR (CDCl 3 , 400MHz): δ 6.93 (t, 1H), 7.07-7.32 (m, 23H), 7.43 (d, J = 4.8 Hz, 1H), 7.51-7.60 (m, 4H), 7.74 (t, J = 7.7 Hz, 1H), 7.85 (d, J = 7.9 Hz, 1H), 8.51 (d, J = 7.9 Hz, 1H). 31 P NMR (CDCl 3 , 162MHz): -7.09 (br s). (POP)Cu(pybim) (A1.2). 2-(Pyridin-2-yl)benzo[d]imidazole (97.65 mg, 0.5 mmol) was dissolved in 5 mL of THF and this solution was transferred via cannula to suspension of sodium hydride (22 mg, 0.55 mmol, 60% in mineral oil) and bis(2- diphenylphosphinophenyl)ether (269 mg, 0.5 mmol) in 15 ml THF. The reaction mixture 153 was stirred at RT for 1 h and then CuCl (49.5 mg, 0.55 mmol) was added. The reaction mixture was stirred at RT for 3 h. The resulting mixture was filtered and solvent was removed by rotary evaporation. The obtained yellow solid was dissolved in small amount of CH 2 Cl 2 and left overnight. The precipitated solid was filtered and discarded. The filtrate was collected and solvent was evaporated to dryness to give 280 mg (70%) of yellow solid. Crystals suitable for X-ray analysis were grown by vapor diffusion of diethyl ether into a CH 2 Cl 2 solution of complex. 1 H NMR (CDCl 3 , 400MHz): δ 6.73- 6.77 (m, 2H), 6.82 (ddd, J = 7.5 Hz, J = 5.1 Hz, J = 1.3 Hz, 1H), 6.90 (td, J = 7.6 Hz, J = 1.1 Hz, 3H), 7.00-7.24 (m, 25H), 7.34 (d, J = 7.9 Hz, 1H), 7.64 (td, J = 7.8 Hz, J = 1.6 Hz, 1H), 7.78 (d, J = 7.9 Hz, 1H), 7.94 (d, J = 4.6 Hz, 1H), 8.45 (d, J = 8.0 Hz, 1H). 31 P NMR (CDCl 3 , 162MHz): -14.06 (br s). (Xantphos)Cu(pybim) (A1.3). 2-(Pyridin-2-yl)benzo[d]imidazole (78.1 mg, 0.4 mmol) was dissolved in 5 mL of THF and this solution was transferred via cannula to suspension of sodium hydride (17.6 mg, 0.44 mmol, 60% in mineral oil) and 4,5- bis(diphenylphosphino)-9,9-dimethylxanthene (231.44 mg, 0.4 mmol) in 15 ml THF. The reaction mixture was stirred at RT for 1 h and then CuCl (49.5 mg, 0.55 mmol) was added. The reaction mixture was stirred at RT for 3.5 h. Pale yellow solid precipitated during the course of reaction. The precipitate was collected dissolved in small amount of CH 2 Cl 2 and left overnight. The precipitated solid was filtered and discarded. The filtrate was collected and solvent was evaporated to dryness to give 100 mg (30%) of pale yellow solid. Crystals suitable for X-ray analysis were grown by vapor diffusion of diethyl ether into a CH 2 Cl 2 solution of complex. 1 H NMR (CDCl 3 , 400MHz): δ 1.71 (s, 154 3H), 1.88 (s, 3H), 6.35 (m, J = 8.0 Hz, 1H), 6.40-6.44 (m, 2H), 6.63 (t, J = 7.0 Hz, 1H), 6.91-7.22 (m, 24H), 7.58 (d d, J = 7.8 Hz, J = 1.4 Hz, 2H), 7.69 (td, J = 7.8 Hz, J = 1.5 Hz, 1H), 7.75 (d, J = 8.0 Hz, 1H), 8.02 (d, J = 4.9 Hz, 1H), 8.52 (d, J = 8.0 Hz, 1H). 31 P NMR (CDCl 3 , 162MHz): -13.76 (br s). (POP)Cu(beniq) (A1.4). 2-(Isoquinolin-1-yl)benzo[d]imidazole (122.64 mg, 0.5 mmol) was dissolved in 10 mL of THF and this solution was transferred via cannula to suspension of sodium hydride (22 mg, 0.55 mmol, 60% in mineral oil) and bis(2- diphenylphosphinophenyl)ether (269 mg, 0.5 mmol) in 10 ml THF. The reaction mixture was stirred at RT for 1 h and then CuCl (49.5 mg, 0.55 mmol) was added. The reaction mixture was stirred at RT overnight. The resulting mixture was filtered through Celite® and solvent was removed by rotary evaporation. The obtained yellow solid was dissolved in small amount of CH 2 Cl 2 and left overnight. The precipitated solid was filtered and discarded. The filtrate was collected and solvent was evaporated to dryness. Recrystallization from CH2Cl2 by vapor diffusion of diethyl ether gave 260 mg (61%) of yellow crystals. 1 H NMR (CDCl 3 , 400MHz): δ 6.76-6.80 (m, 2H), 6.89-6.93 (m, 3H), 6.98-7.26 (m, 26H), 7.39 (J = 7.9 Hz, 1H), 7.64-7.69 (m, 2H), 7.73-7.78 (m, 1H), 7.87 (d, J = 7.9 Hz, 1H), 7.93 (d, J = 5.8 Hz, 1H), 10.8 (d, J = 8.6 Hz, 1H). 31 P NMR (CDCl 3 , 162MHz): -13.60 (br s). X-ray crystallography C 42 H 32 CuN 3 P 2 (A1.1). Diffraction data were collected at 118 K on a SMART APEX CCD diffractometer with graphite fine-focused monochromatic Mo-K α radiation (λ = 155 0.71073 Ǻ). The cell parameters for (C 42 H 32 CuN 3 P 2 ) were obtained from the least- squares refinement of the spots (from 60 collected frames) using the SMART program of a yellow cubic crystal sample measuring 0.79 x 0.30 x 0.15 mm 3 in size. A hemisphere of data were collected up to a resolution of 0.77 Ǻ, the intensity data were processed using the Saint Plus program. All calculations for the structure determination were carried out using the SHELXTL package (version 6.14). 22 Initial atomic position were located by direct methods using XS, and the structure was refined by the least square methods using SHELX with 6,519 independent reflections and within the range of theta 2.07 to 27.52 o (completeness 97.3%). Absorption corrections were applied by SADABS. 23 Calculated hydrogen position were input and refined in a riding manner along with the corresponding carbons. A summary of the refinement details and the resulting factors are given in Table (C 42 H 32 CuN 3 P 2 ). Final structure refinement for C 42 H 32 CuN 3 P 2 results in a R int = 1.9%, R 1 = 3.0% and wR 2 = 7.0%. Data to parameter ratio = 15:1. The crystal system found is monoclinic and of space group Cc, Z equals 4 and dimensions: a = 15.83 Ǻ b = 13.46 Ǻ, c = 17.38 Ǻ and β = 114.23 o . C 48 H 36 CuN 3 OP 2 ×CH 2 Cl 2 (A1.2). Diffraction data were collected at 118 (2) K on a SMART APEX CCD diffractometer with graphite fine-focused monochromatic Mo-K α radiation (λ = 0.71073 Ǻ). The cell parameters for (C 48 H 36 CuN 3 OP 2 ×CH 2 Cl 2 ) were obtained from the least-squares refinement of the spots (from 60 collected frames) using the SMART program of a yellow crystal sample measuring 0.90 x 0.32 x 0.30 mm 3 in 156 size. A hemisphere of data were collected up to a resolution of 0.77 Ǻ, the intensity data were processed using the Saint Plus program. All calculations for the structure determination were carried out using the SHELXTL package (version 6.14). 22 Initial atomic position were located by direct methods using XS and the structure was refined by the least square methods using SHELX with 7,075 independent reflections and within the range of theta 1.71 to 27.49º (completeness 50.8%). Absorption corrections were applied by SADABS. 23 Calculated hydrogen position were input and refined in a riding manner along with the corresponding carbons. A summary of the refinement details and the resulting factors are given in Table (C 48 H 36 CuN 3 OP 2 ×CH 2 Cl 2 ). Final residual structure refinement for C 48 H 36 CuN 3 OP 2 ×CH 2 Cl 2 results in a R int = 2.0%, R 1 = 7.2% and wR 2 = 20.8%. Data to parameter ratio = 10:1. The crystal system found is triclinic and of space group P-1, Z equals 2 and dimensions: a = 8.45 Ǻ b = 12.39 Ǻ, c = 19.03 Ǻ, α = 84.71º, β = 79.50º and γ = 73.68º. We have located one molecule of disordered dichloromethane solvent in the asymmetric unit cell. Disorder of chlorine- containing molecules has a strong (negative) effect on the quality of the model than other solvents because of the relative high number of electrons in chlorine, which are disordered with the solvent. To add difficulty refining the disordered solvent, one of the chlorine atoms lies near a special mirror plane position. We’ve decided to exclude the chlorine atom which are located very close to the special position because the symmetry would lead to chemically unreasonable arrangements. C 51 H 40 N 3 OP 2 Cu (A1.3). Diffraction data were collected at 135(2) K on a SMART APEX 157 CCD diffractometer with graphite fine-focused monochromatic Mo-K α radiation (λ = 0.71073 Ǻ). The cell parameters for (C 51 H 40 N 3 OP 2 Cu) were obtained from the least-squares refinement of the spots (from 60 collected frames) using the SMART program of a large yellow crystal sample measuring 1.0 x 0.6 x 0.2 mm 3 in size. A hemisphere of data were collected up to a resolution of 0.77 Ǻ, the intensity data were processed using the Saint Plus program. All calculations for the structure determination were carried out using the SHELXTL package (version 6.14). 22 Initial atomic position were located by XS, and the structure was refined by the least square methods using SHELX with 9,335 independent reflections and within the range of theta 1.97 to 27.52º (completeness 97.8%). Absorption corrections were applied by SADABS. 23 Calculated hydrogen position were input and refined in a riding manner along with the corresponding carbons. A summary of the refinement details and the resulting factors are given in Table (C 51 H 40 N 3 OP 2 Cu). Final structure refinement for C 51 H 40 N 3 OP 2 Cu results in a R int = 2.1%, R 1 = 3.7% and wR 2 = 10.2%. Data to parameter ratio = 18:1. The crystal system found is monoclinic and of space group P2 1 /c, Z equals 4 and dimensions: a = 10.38 Ǻ b = 18.53 Ǻ, c = 21.61 Ǻ. 158 Table A1.3. Crystal data and structure refinement for A1.1-A1.3. A1.1 A1.2 A1.3 Empirical formula C 42 H 32 CuN 3 P 2 C 48 H 36 CuN 3 OP 2 ×CH 2 Cl 2 C 51 H 40 CuN 3 OP 2 Formula weight 704.19 881.26 836.34 Temperature 118(2) K 118(2) K 135(2) K Wavelength 0.71073 Å 0.71073 Å 0.71073 Å Crystal system Monoclinic Triclinic Monoclinic Space group Cc P-1 P2(1)/c Unit cell dimensions a = 15.8344(13) Å a = 9.4532(9) Å a = 10.3817(15) Å b = 13.4642(11) Å b = 12.3893(11) Å b = 18.533(3) Å c = 17.3815(15) Å c = 19.0309(17) Å c = 21.614(3) Å α = 90°. α = 84.7120(10)°. α = 90°. β = 114.2300(10)°. β = 79.4980(10)°. β = 93.611(2)°. γ = 90°. γ = 73.6780(10)°. γ = 90°. Volume 3379.2(5) Å 3 2101.2(3) Å 3 4150.4(10) Å 3 Z 4 2 4 Density (calculated) 1.384 Mg/m 3 1.420 Mg/m 3 1.338 Mg/m 3 Absorption coefficient 0.777 mm -1 0.797 mm -1 0.646 mm -1 F(000) 1456 924 1736 Crystal size 0.79x0.30x0.15 mm 3 0.90x0.32x0.30 mm 3 1.00 x 0.60 x 0.20 mm 3 Theta range for data collection 2.07 to 27.52°. 1.71 to 27.49°. 1.97 to 27.52°. Index ranges -19<=h<=20, -17<=k<=13, -22<=l<=20 -12<=h<=11, -16<=k<=11, -24<=l<=24 -12<=h<=13, -23<=k<=24, -21<=l<=27 Reflections collected 10130 7075 25169 Independent reflections 6519 [R(int) = 0.0190] 4907 [R(int) = 0.0199] 9335 [R(int) = 0.0206] Completeness to theta 97.3 % 50.8 % 97.8 % Absorption correction Empirical Empirical Empirical Max. and min. transmission 0.8924 and 0.5719 0.7960 and 0.5340 0.8817 and 0.5644 Refinement method Full-matrix least- squares on F 2 Full-matrix least-squares on F 2 Full-matrix least- squares on F 2 Data / restraints / parameters 6519 / 2 / 433 4907 / 29 / 524 9335 / 0 / 525 Goodness-of-fit on F 2 1.003 1.009 1.042 Final R indices [I>2sigma(I)] R1 = 0.0297, wR2 = 0.0704 R1 = 0.0718, wR2 = 0.2079 R1 = 0.0377, wR2 = 0.1019 R indices (all data) R1 = 0.0313, wR2 = 0.0712 R1 = 0.0822, wR2 = 0.2174 R1 = 0.0436, wR2 = 0.1059 Absolute structure parameter -0.020(7) - - Largest diff. peak and hole 0.597 and -0.189 e.Å -3 1.302 and -0.690 e.Å -3 0.652 and -0.499 e.Å -3 159 Appendix 1 References (1) Armaroli, N.; Accorsi, G.; Cardinali, F.; Listorti, A. Top. Curr. Chem. 2007, 280, 69. (2) Barbieri, A.; Accorsi, G.; Armaroli, N. Chem. Commun. 2008, 2185. (3) McMillin, D. R.; McNett, K. M. Chem. Rev. 1998, 98, 1201. (4) Ford, P. C.; Cariati, E.; Bourassa, J. Chem. Rev. 1999, 99, 3625. (5) Scaltrito, D. V.; Thompson, D. W.; O'Callaghan, J. A.; Meyer, G. J. Coord. Chem. Rev. 2000, 208, 243. (6) Harkins, S. B.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 2030. (7) Hsu, C.-W.; Lin, C.-C.; Chung, M.-W.; Chi, Y.; Lee, G.-H.; Chou, P.-T.; Chang, C.-H.; Chen, P.-Y. J. Am. Chem. Soc. 2011, 133, 12085. (8) Crestani, M. G.; Manbeck, G. F.; Brennessel, W. W.; McCormick, T. M.; Eisenberg, R. Inorg. Chem. 2011, 50, 7172. (9) Bergmann, L.; Friedrichs, J.; Mydlak, M.; Baumann, T.; Nieger, M.; Brase, S. Chem. Commun. 2013, 49, 6501. (10) Min, J. H.; Zhang, Q. S.; Sun, W.; Cheng, Y. X.; Wang, L. X. Dalton Trans. 2011, 40, 686. (11) Siddique, Z. A.; Yamamoto, Y.; Ohno, T.; Nozaki, K. Inorg. Chem. 2003, 42, 6366. (12) Manbeck, G. F.; Brennessel, W. W.; Eisenberg, R. Inorg. Chem. 2011, 50, 3431. (13) Czerwieniec, R.; Yu, J.; Yersin, H. Inorg. Chem. 2011, 50, 8293. (14) Du, L. H.; Wang, Y. G. Synthesis-Stuttgart 2007, 675. (15) Frisch, M. J. T., G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,; M. A.; Cheeseman, J. R. M., J. A., Jr.; Vreven, T.; Kudin, K. N.;; Burant, J. C. M., J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;; Mennucci, B. C., M.; Scalmani, G.; Rega, N.; Petersson, G. A.;; Nakatsuji, H. H., M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,; J.; Ishida, M. N., T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,; X.; Knox, J. E. H., H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;; Gomperts, R. S., R. E.; Yazyev, O.; Austin, A. 160 J.; Cammi, R.;; Pomelli, C. J. O., W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;; Salvador, P. D., J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,; A. D.; Strain, M. C. F., O.; D. Malick, K.; A. Rabuck, D.;; Raghavachari, K. F., J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;; Clifford, S. C., J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,; P.; Komaromi, I. M., R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;; Peng, C. Y. N., A.; Hallacombe, M.; Gill, CP. M. W.; Johnson,; B.; Chen, W. W., M. W.; Gonzalez, C.; and Pople, J. A.; Gaussian, Inc., : Wallingford, CT, 2004. (16) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (17) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (18) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (19) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (20) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (21) Giordano, C.; Minisci, F.; Vismara, E.; Levi, S. J. Org. Chem. 1986, 51, 536. (22) Sheldrick, G. M.; SHELXTL, version 6.14; Bruker Analytical X-ray System, Inc.: Madison, WI, 1997. (23) Blessing, R. H. Acta Crystallogr. A 1995, 51, 33. 161 APPENDIX 2. Luminescent two-coordinate NHC-Cu(I) complexes A2.1. Introduction Studies have shown that the coordination geometry has a significant impact on the photophysical properties of Cu(I) complexes. 1 To date four-coordinate Cu(I) complexes and clusters is the largest class of luminescent Cu(I)-based materials. 2-4 Recently a new family of luminescent Cu(I) complexes having three-coordinate geometry has been developed by us 5,6 and others. 7,8 In the literature there are numerous examples of mononuclear two-coordinate NHC-Cu(I) complexes that are useful in catalysis. 9,10 However, to the best of our knowledge there are no reports of luminescence form these complexes. The only example of luminescent Cu(I) complexes with two-coordinate geometry around copper center bearing a carbene ligand is a binuclear compound, which emission properties are largely determined by Cu(I)-Cu(I) interactions. 11 We have prepared a series of two-coordinate mononuclear NHC-Cu(I) complexes that show phosphorescence at room temperature. We have utilized a diverse set of ligands to prepare homoleptic and heteroleptic, cationic and neutral complexes bearing one or two carbene ligands (Figure A2.1). Emission energies of these compounds span a wide range of visible spectrum from blue to yellow. It was shown in Chapters 3 and 4 that emission properties of the three-coordinate (NHC)Cu(N^N) complexes can be modulated by NHC as well as N^N ligand. Similar, in two coordinate NHC-Cu(I) complexes discussed here either of the two ligands coordinated to copper center may act as a chromophore or as an ancillary ligand as well as modulate the steric bulk of compound. 162 Figure A2.1. Molecular structures of complexes A2.1-A2.10. A2.2. Synthesis and emission properties On the basis of the molecular structure the ten complexes shown in Figure A2.1 can be divided into three classes, namely cationic bis-NHC complexes A2.1-A2.3, neutral complexes (NHC)CuX (X = Cl, Br, I) A2.4-A2.8 and (NHC)Cu(Cbz) (Cbz = carbazol-9- ide) A2.9-A2.10. 163 Complex A2.1 was synthesized from a respective imidazolium salt according to the literature procedure for (NHC) 2 Cu + reported by Nolan and co-workers (Figure A2.2). 12 Figure A2.2. Synthesis of (BzI-3,5Me) 2 Cu + (A2.1). Imidazolium salt precursor for A2.2 was not available. In this case the NHC was generated from ethoxyimidazolidine precursor (PzI-3,5Me)OEt (Figure A2.3). Figure A2.3. Synthesis of (PzI-3,5Me) 2 Cu + (A2.2). Complex A2.3 was obtained from (IPr)CuOH starting material (Figure A2.4). This precursor is useful for preparation of manifold heteroleptic NHC-Cu(I) complexes. 13,14 164 Figure A2.4. Synthesis of (IPr)Cu(BzI-3,5Me) + (A2.3). Complex A2.1 gives bright blue phosphorescence in the solid state (Φ = 79%), while A2.2 and A2.3 are virtually non-emissive. Emission spectra of A2.1 at RT and at 77 K are depicted in Figure A2.5. Emission is broad and featureless with λ max = 462 nm (RT) and λ max = 472 nm (77 K). Solid-state excited state lifetime at RT is 11.8 µs and 13 µs at 77 K. 400 450 500 550 600 650 0.0 0.2 0.4 0.6 0.8 1.0 77K RT Normalized intensity (a.u.) Wavelength (nm) Figure A2.5. Solid-state emission spectra of A2.1 at RT and at 77 K. 165 Neutral complexes A2.4 and A2.5 were prepared following a well-established procedure for the synthesis of (NHC)CuX (X = Cl, Br, I) complexes independently developed by Nolan 15,16 and Buchwald 17 . According to the procedure imidazolium salt precursor is deprotonated with sodium tert-butoxide to generate free NHC in situ, which then reacts with CuCl. Synthetic route for complexes A2.6-A2.8 is shown in Figure A2.6. Complexes were obtained from the ethoxyimidazolidine precursor (PzI-3,5Me)OEt and corresponding CuX salt and in refluxing THF. Figure A2.6. Synthesis of complexes A2.6-A2.8. It is important to note that for (NHC)CuX (X = Cl, Br, I) complexes, mononuclear as well as polynuclear structures were reported in the solid state. 16 For example, (ICy)CuCl crystallizes as a monomer, while (ICy)CuBr and (ICy)CuI form polynuclear species with [Cu 3 X 3 ] core. Unfortunately attempts to crystallize complexes A2.4-A2.8 were unsuccessful, therefore two-coordinate geometry can not be unambiguously assigned for these compounds. Luminescent properties for A2.4-A2.8 in the solid state are summarized in Table A2.1 166 and emission spectra are shown in Figure A2.7. Complexes A2.4 and A2.5 are benzimidazolilidene derivatives, that only differ in substituents at the nitrogen atom. Substitution of xylenyl group in A2.4 to cyclohexyl group in A2.6 leads to 91 nm blue shift of emission spectrum for A.2.6. Emission lifetimes and efficiencies are comparable for the two compounds. Complexes A2.6-A2.8 possess the same NHC ligand, but X group is systematically varied. Emission efficiency for A2.6-A2.8 decreases when going from X = Cl (26%) to Br(10%) to I(<0.5%). The radiative rates calculated for A2.6 (k r = 6.2x10 4 s -1 ) and A2.7 (k r = 5.6x10 4 s -1 ) are similar, while non-radiative rates increase from A2.6 (k nr = 1.8x10 5 s -1 ) to A2.7 (k nr = 5.1x10 5 s -1 ). 400 450 500 550 600 650 700 750 800 0.0 0.2 0.4 0.6 0.8 1.0 A2.4 A2.5 A2.6 A2.7 Normalized intensity (a.u.) Wavelength (nm) Figure A2.7. Solid-state emission spectra of A2.4-A2.7 at RT. 167 Table A2.1. Photophysical properties of complexes A2.4-A2.8 in the solid state. Complex λ max , (nm) τ (µs) Φ PL A2.4 586 2.8 (11%) 10.4 (89%) 23% A2.5 495 1.3 (14%) 9.0 (86%) 38% A2.6 546 4.2 26% A2.7 524 0.39 (31%) 1.77 (69%) 10% A2.8 575 - a <0.5% a Not measured. Complexes A2.9 and A2.10 were obtained from corresponding (NHC)CuCl precursors upon reaction with sodium carbazolate. For preparation of A2.9 sodium carbazolate was generated in situ upon deprotonation of carbazole with sodium hydride in THF. A2.9 and A2.10 are weakly luminescent in the solid state having emission efficiency of 6% and 3.5% respectively. Their solid state emission spectra are shown in Figure A2.8. 400 450 500 550 600 650 700 0.0 0.2 0.4 0.6 0.8 1.0 A2.9 A2.10 Normalized intensity (a.u.) Wavelength (nm) Figure A2.8. Solid-state emission spectra of A2.9 and A2.10 at RT. 168 A2.3. Experimental section All reactions were performed under nitrogen atmosphere in oven dried glassware. Chloro[1,3-bis(2,6-di-i-propylphenyl)imidazol-2-ylidene]copper(I) (IPr)CuCl was purchased from TCI America. 1,3-dicyclohexyl-1H-benzo[d]imidazol-3-ium chloride (BzICy)Cl and all other commercially available reagents were purchased from Sigma- Aldrich and used as received. Solvents were obtained from commercial sources and used without further purification except for tetrahydrofuran, which was purified by Glass Contour solvent system by SG Water USA, LLC. 1,3-bis(3,5-dimethylphenyl)-1H- benzo[d]imidazol-3-ium chloride 18 (BzI-3,5Me)Cl and hydroxyl[1,3-bis(2,6- diisopropylphenyl)imidazol-2-ilidene]copper(I) 13 (IPr)CuOH were prepared following published procedures. Sodium carbazolate was generated upon deprotonation of carbazole by sodium hydride in tetrahydrofuran. Synthetic procedures for (PzI- 3,5Me)OEt, (BzI-3,5Me)CuCl (A2.4) and (PzI-3,5Me)CuCl (A2.6) are described in Chapter 2. 1 H, 19 F NMR and 31 P spectra were recorded on a Varian Mercury 400, Varian VNMRS 500 or a Varian VNMRS 600 spectrometer. The chemical shifts are given in units of ppm. All 1 H chemical shifts were referenced to the residual solvent signals. Steady state emission measurements were performed on a Photon Technology International (PTI) QuantaMaster TM model C-60 spectrofluorimeter equipped with a 820 PMT detector and corrected for detector response. Phosphorescence lifetime measurements were performed on IBH Fluorocube instrument equipped with a 405 nm LED excitation source using time-correlated single photon counting method. Quantum 169 yields at room temperature were measured using a Hamamatsu C9920 system equipped with a xenon lamp, calibrated integrating sphere and model C10027 photonic multichannel analyzer. 1,3-bis(3,5-dimethylphenyl)-1H-benzo[d]imidazol-3-ium tetrafluroborate ((BzI- 3,5Me)BF 4 ). 1,3-bis(3,5-dimethylphenyl)-1H-benzo[d]imidazol-3-ium chloride (440 mg, 1.21 mmol) was mixed with ammonium tetrafluoroborate (127 mg, 1.21 mmol) in 30 ml of water and stirred at room temperature for 6 h. The white product was collected by filtration, washed with water and dried in vacuo (325 mg, 65%). 1 H NMR (500 MHz, acetone-d 6 , δ) 2.48 (s, 12H), 7.42 (s, 2H), 7.62 (s, 4H), 7.85 (m, 2H), 8.04 (m, 2H), 10.15 (s, 1H). 19 F NMR (470 MHz, acetone-d 6 , δ) -151.55. [(BzI-3,5Me) 2 Cu](xPF 6 )(yBF 4 ) (A2.1). (BzI-3,5Me)BF 4 (282 mg, 0.68 mmol), NaOtBu (84.9 mg, 0.884 mmol), [Cu(CH 3 CN) 4 ]PF 6 (126.7 mg, 0.34 mmol) and 15 ml of anhydrous THF were mixed in 50 ml flask under N 2 atmosphere and stirred at room temperature overnight. The reaction mixture was filtered through Celite ® . The product was precipitated from dark yellow solution upon addition of pentane and off-white solid was collected by filtration. The product was further purified by recrystallization from THF/pentane mixture (114 mg). 1 H NMR (500 MHz, acetone-d 6 , δ) 2.35 (s, 24H), 7.17 (s, 4H), 7.34 (s, 8H), 7.54 (m, 4H), 7.68 (m, 4H). 19 F NMR (470 MHz, acetone-d 6 , δ) - 73.44, -71.94. 31 P NMR (202 MHz, acetone-d 6 , δ) -144.25. [(PzI-3,5Me) 2 Cu](PF 6 ) (A2.2). (PzI-3,5Me)OEt (400 mg, 1.07 mmol), [Cu(CH 3 CN) 4 ]PF 6 (199.4 mg, 0.535 mmol) and 30 ml of anhydrous THF were mixed in 170 50 ml flask under N 2 atmosphere and refluxed overnight. The resulting bright-yellow suspension was filtered, the solid was collected, rinsed with THF and dried in vacuo (384 mg, 83%). 1 H NMR (400 MHz, acetone-d 6 , δ) 2.42 (s, 24H), 7.27 (s, 4H), 7.54 (s, 8H), 8.65 (s, 4H). [(IPr)Cu(BzI-3,5Me)](BF 4 ) (A2.3). (BzI-3,5Me)BF 4 (106 mg, 0.26 mmol) and (IPr)СuOH (120 mg, 0.26 mmol) were mixed in 20 ml of dry THF and stirred overnight at 60ºC under nitrogen atmosphere. The reaction mixture was cooled down to RT and concentrated to ~2 ml by rotary evaporation. White solid was precipitated upon addition of ethyl ether (169 mg, 76%). The final product contained 15% of the starting material (IPr)CuOH. 1 H NMR (400 MHz, CDCl 3 , δ) 0.95 (d, 12H), 1.14 (d, 12H), 2.35-2.42 (m, 16H), 6.68 (s, 4H), 6.95 (m, 2H), 7.12-7.17 (m, 8H), 7.27 (m, 2H), 7.42 (t, 2H). (BzICy)CuCl (A2.5). 1,3-dicyclohexyl-1H-benzo[d]imidazol-3-ium chloride (BzICy)Cl (239.2 mg, 0.75 mmol), CuCl (74.2 mg, 0.75 mmol) and NaOtBu (72.1 mg, 0.75 mmol) were mixed with 10 ml of THF and stirred overnight at room temperature under nitrogen atmosphere. The reaction mixture was then filtered under inert atmosphere through a plug of Celite ® and the solvent was removed by rotary evaporation. The product was obtained as white solid. (190 mg, 66%). 1 H NMR (lit. 19 ) (400 MHz, CDCl 3 , δ) 1.38-1.57 (m, 6H), 1.81 (d, 2H), 2.00 (d, 4H), 2.09 (d, 4H), 2.43 (qd, 4H), 4.50 (tt, 2H), 7.33-7.36 (m, 2H), 7.55-7.57 (m, 2H). (PzI-3,5Me)CuBr (A2.7). (PzI-3,5Me)OEt (500 mg, 1.335 mmol) and CuBr (174 mg, 1.213 mmol) were refluxed overnight in 30 ml of anhydrous THF under N 2 atmosphere. 171 The resulting bright-yellow suspension was filtered, the solid was collected, rinsed with THF and dried in vacuo (407 mg, 71%). 1 H NMR (400 MHz, CDCl 3 , δ) 2.46 (s, 12H), 7.20 (s, 2H), 7.49 (s, 4H), 8.56 (s, 2H). (PzI-3,5Me)CuI (A2.8). 1,3-bis(3,5-dimethylphenyl)-2-ethoxy-2,3-dihydro-1H- imidazo[4,5-b]pyrazine (500 mg, 1.335 mmol) and CuI (231 mg, 1.213 mmol) were refluxed overnight in 30 ml of anhydrous THF under N 2 atmosphere. The resulting bright-yellow suspension was filtered, the solid was collected, rinsed with THF and dried in vacuo. The solid was then redissolved in 120 ml of CH 2 Cl 2 and insoluble residue was filtered off to give clear yellow solution, which was concentrated by rotary evaporation to ~30 ml. The yellow product precipitated overnight after addition of diethyl ether (40 mg, 6%). 1 H NMR (400 MHz, CDCl 3 , δ) 2.46 (s, 12H), 7.19 (s, 2H), 7.55 (s, 4H), 8.56 (s, 2H). (IPr)Cu(Cbz) (A2.9). Carbazole (83.6 mg, 0.5 mmol) and NaH (12 mg, 0.5 mmol) were mixed with 15 ml of THF and stirred at room temperature under nitrogen atmosphere until bubbling stopped (15 min). Chloro[1,3-bis(2,6-diisopropylphenyl)imidazol-2- ylidene]copper(I) (IPr)CuCl was added and the reaction mixture was stirred for 1 hour. The mixture was then filtered under inert atmosphere through a plug of Celite ® and the solvent was removed by rotary evaporation. The product was obtained as white solid (170 mg, 55%). 1 H NMR (400 MHz, acetone-d 6 , δ) 1.31 (d, 24H), 2.82 (sept, 4H), 6.34 (d, 2H), 6.75 (t, 2H), 6.87 (t, 2H), 7.55 (d, 4H), 7.73 (t, 2H), 7.80 (d, 2H), 7.87 (s, 2H). (BzICy)Cu(Cbz) (A2.10). (BzICy)CuCl (A2.5) (60 mg, 0.16 mmol) and sodium 172 carbazolate (30.3 mg, 0.16 mmol) were stirred in 15 ml of THF for 1h at room temperature under nitrogen atmosphere. The reaction mixture was filtered under inert atmosphere through a plug of Celite ® and the solvent was removed by rotary evaporation. The product, obtained as light-yellow solid, contained ~50% of starting NaCbz. (45 mg, 51%). 1 H NMR (400 MHz, acetone-d 6 , δ) 1.35-1.46 (m, 2H), 1.60-1.81 (m, 6H), 2.00 (d, 4H), 2.30 (d, 4H), 2.65-2.80 (m, 4H), 4.95 (tt, 2H), 6.96 (t, 2H), 7.25 (t, 2H), 7.45-7.48 (m, 2H), 7.67 (d, 2H), 7.93-7.95 (m, 2H), 8.04 (d, 2H). Appendix 2 References (1) Siddique, Z. A.; Yamamoto, Y.; Ohno, T.; Nozaki, K. Inorg. Chem. 2003, 42, 6366. (2) McMillin, D. R.; McNett, K. M. Chem. Rev. 1998, 98, 1201. (3) Barbieri, A.; Accorsi, G.; Armaroli, N. Chem. Commun. 2008, 2185. (4) Armaroli, N.; Accorsi, G.; Cardinali, F.; Listorti, A. Top. Curr. Chem. 2007, 280, 69. (5) Krylova, V. A.; Djurovich, P. I.; Whited, M. T.; Thompson, M. E. Chem. Commun. 2010, 46, 6696. (6) Krylova, V. A.; Djurovich, P. I.; Aronson, J. W.; Haiges, R.; Whited, M. T.; Thompson, M. E. Organometallics 2012, 31, 7983. (7) Lotito, K. J.; Peters, J. C. Chem. Commun. 2010, 46, 3690. (8) Hashimoto, M.; Igawa, S.; Yashima, M.; Kawata, I.; Hoshino, M.; Osawa, M. J. Am. Chem. Soc. 2011, 133, 10348. (9) Diez-Gonzalez, S.; Nolan, S. P. Aldrichim Acta 2008, 41, 43. (10) Lin, J. C. Y.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A.; Hwang, W. S.; Lin, I. J. B. Chem. Rev. 2009, 109, 3561. (11) Matsumoto, K.; Matsumoto, N.; Ishii, A.; Tsukuda, T.; Hasegawa, M.; 173 Tsubomura, T. Dalton Trans. 2009, 6795. (12) Diez-Gonzalez, S.; Stevens, E. D.; Scott, N. M.; Petersen, J. L.; Nolan, S. P. Chem.-Eur. J. 2008, 14, 158. (13) Fortman, G. C.; Slawin, A. M. Z.; Nolan, S. P. Organometallics 2010, 29, 3966. (14) Lazreg, F.; Slawin, A. M. Z.; Cazin, C. S. J. Organometallics 2012, 31, 7969. (15) Kaur, H.; Zinn, F. K.; Stevens, E. D.; Nolan, S. P. Organometallics 2004, 23, 1157. (16) Diez-Gonzalez, S.; Escudero-Adan, E. C.; Benet-Buchholz, J.; Stevens, E. D.; Slawin, A. M. Z.; Nolan, S. P. Dalton Trans. 2010, 39, 7595. (17) Jurkauskas, V.; Sadighi, J. P.; Buchwald, S. L. Org. Lett. 2003, 5, 2417. (18) Chianese, A. R.; Mo, A.; Datta, D. Organometallics 2009, 28, 465. (19) Opalka, S. M.; Park, J. K.; Longstreet, A. R.; McQuade, D. T. Org. Lett. 2013, 15, 996.

Abstract (if available)

Abstract Copper(I) complexes can provide inexpensive and environmentally friendly alternative to traditionally utilized phosphorescent materials that have relied largely on heavy metal (Pt(II), Ir(III), Ru(II), Os(II)) complexes. Nonetheless the copper(I)-based materials have not been as well-developed as their third row counterparts. To date research has focused principally on four-coordinate Cu(I) complexes. A new family of phosphorescent three-coordinate (NHC)Cu(N^N) complexes, where NHC is a monodentate N-heterocyclic carbene ligand and N^N is a neutral or monoanionic chelating ligand is presented in this work. Tunable photophysical properties and broad potential for variation in design make these materials potentially useful for a variety of photophysical applications. ❧ Chapter 1 highlights the state-of-the-art in development of phosphorescent mononuclear Cu(I) complexes and their application in organic light-emitting diodes (OLED). ❧ Design and synthesis of (NHC)Cu(N^N) complexes is described in Chapter 2. The solid state structures of several examples were established by X-ray crystallography and conformational behavior in solution was investigated by variable temperature 1H NMR spectroscopy. The geometrical preferences revealed by single crystal XRD analysis correlate well with the NMR data. ❧ Photophysical properties of (NHC)Cu(N^N) complexes are discussed in Chapters 3 and 4. Excited state properties strongly depend on electronic and steric properties of the ligands, degree of metal participation, and on experimental conditions. Wide-range emission color tuning from deep blue (418 nm) to red (650 nm) can be achieved by modification of chelating (N^N) ligand or alternatively through variation of carbene ligand (NHC). Thus either NHC or N^N ligand can act as ligand chromophore. Excited state lifetimes are in the 10’s of microsecond range and quantum yields are up to 80% in solid state and up to 17% in fluid solution. Significant emission enhancement observed in rigid environment suggests the presence of non-radiative relaxation decays associated with geometrical rearrangements in the excited state. Photophysical studies performed at room temperature and at 77 K suggest participation of a higher-lying state with faster radiative rate at room temperature. ❧ In Chapter 5 (NHC)Cu(N^N) complexes are evaluated as potential hosts for OLEDs. Experimental and theoretical studies indicate that the triplet energies of these (NHC)Cu(N^N) complexes are often higher than can be predicted from emission spectra observed experimentally. A host:guest system was realized where Ir(III)-based emitter was doped into Cu(I)-based host material that phosphoresces at lower energies than the dopant, however does not quench dopant emission via triplet energy transfer.

Linked assets

University of Southern California Dissertations and Theses

Conceptually similar

PDF

Synthesis, photo- and electroluminescence of three- and two-coordinate coinage metal complexes featuring non-N-heterocyclic carbene and non-conventional N-heterocyclic carbene ligands

PDF

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

PDF

Simple complexes: synthesis and photophysical studies of luminescent, monovalent, 2-coordinate carbene-coinage metal complexes and higher coordination geometries

PDF

Synthesis and photophysical characterization of phosphorescent cyclometalated iridium (III) complexes and their use in OLEDs

PDF

Molecular approaches to solve the blue problem in organic light emitting diode display and lighting applications

PDF

Synthesis, photophysical and electrochemical characterization of 1,3-bis(2-pyridylimino)isoindole derivatives

PDF

Molecular design strategies for blue organic light emitting diodes

PDF

Studies of molecular orientation using iridium phosphors and integration of corannulene into organic light emitting diodes (OLEDs)

PDF

Qualitative quantum chemical description of the structure-color relationships in phosphorescent organometallic complexes

PDF

Photophysical properties of luminescent iridium and coinage metal complexes

PDF

Two-coordinate coinage metal complexes for solar fuels and organic LED chemistry

PDF

Development of N-type chromophores for organic photovoltaics, and thermally activated delayed fluorescence NHC complexes for organic light-emitting diodes

PDF

Modeling graphene: magnetic, transport and optical properties

PDF

Synthesis and characterization of metal chalcogenide semiconductor nanocrystals using dialkyl dichalcogenide precursors

PDF

Molecular design of pyridine and quinoline based compounds for organic optoelectronics

PDF

Electronic structure analysis of challenging open-shell systems: from excited states of copper oxide anions to exchange-coupling in binuclear copper complexes

PDF

Ir(III) and Pt(II) phosphorescent emitters in organic light emitting diodes: from materials development to light out-coupling

PDF

Nanocrystal surface engineering as a route towards improved photovoltaics

PDF

Integration of cost effective luminescent materials into organic light emitting diodes (OLEDs) and molecular approaches to improve the device efficiency

PDF

Catalytic applications of palladium-NHC complexes towards hydroamination and hydrogen-deuterium exchange and development of acid-catalyzed hydrogen-deuterium exchange methods for preparative deut...

Asset Metadata

Creator Krylova, Valentina (author)

Core Title Synthesis, structural and photophysical characterization of phosphorescent three-coordinate Cu(I)-N-heterocyclic carbene complexes

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 11/26/2013

Defense Date 10/11/2013

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

Tag copper(I) complexes,N-heterocyclic carbenes,OAI-PMH Harvest,OLED,phosphorescence

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 krylova@usc.edu,krylvalent@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-350621

Unique identifier UC11296138

Identifier etd-KrylovaVal-2180.pdf (filename),usctheses-c3-350621 (legacy record id)

Legacy Identifier etd-KrylovaVal-2180.pdf

Dmrecord 350621

Document Type Dissertation

Format application/pdf (imt)

Rights Krylova, Valentina

Type texts

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

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA