Trifluoromethyl substituted cationic cyclometallated iridium(III) complexes: Photophysical properties and theoretical studies

Three new trifluoromethyl substituted (-CF3) cationic iridium(III) complexes 1-3, with 3,5-dimethyl-1-(3-(tri-fluoromethyl)phenyl)-1H-pyrazole (DTPP) as the cyclometallated ligand, namely, [Ir(DTPP)2bpy](PF6) (1, bpy = 2, 2 '-bipyridine), [Ir(DTPP)2phen](PF6) (2, phen = 1,10-phenanthroline) and [Ir(DTPP)2pphen](PF6) (3, pphen = pyrazino[2,3-f][1,10]phenanthroline) have been rationally designed and synthesized in good yields (60-77%). The photophysical, electrochemical, and electroluminescent properties were fully characterized. Complexes 1-3 show bright blue to yellow emission in the both solution and neat film states with maximum emission wavelength in the range of 484-560 nm. Photoluminescence quantum yields (PLQYs) are very high ranging from 62% to 72% in the solution state with microsecond excited-state lifetimes (0.39-0.77 mu s). More detailed photophysical and theortical calculation studies reveal that introduction of -CF3 substituent onto the cyclometallated ligand can induce a blue-shift emission compared with conventional fluorinated phenylpyridine or phenylpyrazole complexes, which can be contributed to the decline in HOMO energy level of complexes. Single-crystal structure analysis of complex 3 shows a distorted octahedral geometry with strong intermolecular pi-pi stacking (face-to-face) interactions between ancillary ligands in adjacent molecules, which might be responsible for the obviously red-shifted emission compared with complexes 1 and 2 in the aggregated state. In addition, electroluminescent properties of all three new complexes have been studied in the application of light-emitting electrochemical cells (LECs), which show green to yellow electroluminescence (lambda max, EL = 516-540 nm).