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

Abstract

Three new trifluoromethyl substituted (–CF3) cationic iridium(III) complexes 1–3, with 3,5-dimethyl-1-(3-(trifluoromethyl)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 μs). 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 π–π 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 (λmax, EL = 516–540 nm).