Comprehensive spectroscopic studies of cis and trans isomers of red-phosphorescent heteroleptic iridium(III) complexes

We have synthesized heteroleptic Ir(III) complexes showing red phosphorescence, which can be separated as two geometrical isomers. Each isomer showed different absorption and emission spectral behaviours. The phosphorescence of the cis isomer is observed at shorter wavelengths than the trans one. Based on the results of electrochemical experiments and theoretical calculations, the LUMOs of both isomers are at the same energy level, but the HOMO of the cis isomer is shifted to a significantly lower energy than that of the trans isomer. The red emissions originate from the triplet metal-to-ligand charge transfer (MLCT) state, which is assigned by femtosecond time-resolved transient absorption spectroscopic analysis. The MLCT emission spectra of both isomers show highly structured bands with a vibronic spacing of ca. 1470 cm(-1). Observation of vibrational fine structure in the emission spectra is attributed to the limited excited state structural change imposed by the rigidity of metal coordination. (C) 2016 Elsevier B.V. All rights reserved.