Blue-Green Iridium(III) Emitter and Comprehensive Photophysical Elucidation of Heteroleptic Cyclometalated Iridium(III) Complexes

Synthesis and photophysical properties of the highly emissive complex [Ir(Fppy)(2)(dmb)](+) are reported along with those of additional heteroleptic cyclometalated Ir(III) complexes, [Ir(ppy)(2)(NN)](PF6): FppyH = 2-(2,4- difluorophenyl)pyridine; ppyH = 2-phenylpyridine; NN = 4,4'-dimethyl-2,2'-bipyridine (dmb), 1,10-phenanthroline (phen), or 4,7-diphenyl-1,10-phenanthroline (Ph(2)phen). TD-DFT calculations and Franck-Condon emission spectral band shape analyses show that the broad and structureless emission from [Ir(Fppy)(2)(dmb)(+) in acetonitrile at 298 K mainly arises from a triplet metal-to-ligand charge-transfer excited state, (MLCTir(ppy)-> NN.)-M-3 The emission maximum varies systematically with variations in electron-donating or -withdrawing substituents on both the NN and the Xppy ligands, and emission efficiencies are high, with an impressive empty set approximate to 1 for [Ir(Fppy)(2)(dmb)](+). At 77 K in propionitrile/butyronitrile (4/5, v/v), emission from [Ir(Fppy)(2)(dmb)](+) is narrow and highly structured consistent with a triplet ligand-centered transition ((LCNN)-L-3) and an inversion in excited-state ordering between the (MLCTIr(ppy)-> NN)-M-3 and (LCNN)-L-3 states. In a semirigid film of the poly(ethyleneglycol)dimethacrylate with nine ethylene glycol spacers, PEG-DMA550, emission from [Ir(Fppy)(2)(dmb)](+) is MLCT-based. The thermal sensitivity of the photophysical properties of this excited state points to a possible application as a temperature sensor in addition to its more known use in light-emitting devices.