Theoretical studies on structures and spectroscopic properties of bis-cyclometalated iridium complexes [Ir(ppy)2X2]−

The electronic structures and spectroscopic properties of a series of mixed bis-cyclometalated iridium(III) complexes [Ir(ppy)2X2]− (X = CN, 1; X = NCS, 2; X = NCO, 3; ppy = 2-phenylpyridl) were investigated at the B3LYP/LANL2DZ and CIS/LANL2DZ levels. The calculated geometry parameters in the ground state are well consistent with the corresponding experimental values. The HOMO of 1 is dominantly localized on Ir atom and ppy ligand, but the HOMO of 2 and 3 have significant X ligand composition. Under the TD-DFT level with PCM model, the absorption and phosphorescence in CH2Cl2 media were calculated based on the optimized geometries in the ground and excited states, respectively. The lowest-lying absorption of 1 at 403 nm is attributed to {[dx2-y2(Ir)+dxy(Ir)+π(ppy)]→[π∗(ppy)]}{[dx2-y2(Ir)+dxy(Ir)+π(ppy)]→[π∗(ppy)]} transition with metal-to-ligand and intraligand charge transfer (MLCT/ILCT) transition characters, whereas those of 2 (449 nm) and 3 (475 nm) are related to {[dx2-y2(Ir)+dxy(Ir)+π(ppy)+π(NCS/NCO)]→[π∗(ppy)]}{[dx2-y2(Ir)+dxy(Ir)+π(ppy)+π(NCS/NCO)]→[π∗(ppy)]} transition with MLCT/ILCT and ligand-to-ligand charge transfer (LLCT) transition characters. The phosphorescence of 1 at 466 nm can be described as originating from 3{[dx2-y2(Ir)+dxy(Ir)+π(ppy)][π∗(ppy)]}3{[dx2-y2(Ir)+dxy(Ir)+π(ppy)][π∗(ppy)]} excited state, while those of 2 (487 nm) and 3 (516 nm) originate from 3{[dx2-y2(Ir)+dxy(Ir)+π(ppy)+π(NCS/NCO)][π∗(ppy)]}3{[dx2-y2(Ir)+dxy(Ir)+π(ppy)+π(NCS/NCO)][π∗(ppy)]} excited states. The calculated results showed that the transition character of the absorption and emission can be changed by adjusting the π electron-accepting abilities of the X ligands and the phosphorescent color can be tuned by altering the X ligands.