Theoretical studies on structures and spectroscopic properties of a series of heteroleptic iridium complexes based on tridentate bis(benzimidazolyl)pyridine ligand

Electronic structures and spectroscopic properties of series of novel mixed-ligand Ir(III) complexes, [Ir(Mebip)(bpy)Cl]2+ (1), [Ir(Mebip)(ppy)Cl]+ (2), [Ir(Mebip)(ppz)Cl]+ (3) and [Ir(Mebip)(ptz)Cl]+ (4) [bpy = 2,2′-bipyridine, ppy = phenylpyridine, ppz = 5-(2-pyridyl)-pyrazole and ptz = 5-(2-pyridyl)-triazole, Mebip = bis(N-methylbenzimidazolyl)pyridine] have been investigated in detail by theoretical calculation. The geometries in the ground and excited state for the cationic complexes 1–4 were optimized by density functional theory (DFT) and single excitation configuration interaction method (CIS), respectively. The absorptions and emissions of 1–4 in CH3CN solution were calculated using time-dependent density functional theory (TDDFT) associated with the polarized continuum model (PCM). With the assistance of the analysis of frontier molecular orbitals, HOMOs of 2–4 mainly comprise Ir(III) ion and different chelating ligands, but that of 1 is dominated by Ir(III) ion and Mebip. LUMO and LUMO + 1 of 1–4 predominantly reside on the Mebip ligand. The lowest-lying absorption of 1 at 460 nm (from HOMO to LOMO) is assigned to MLCT and ILCT transitions, while that of 2–4 occurs at 457, 448 and 418 nm, respectively, originating from a mixture of MLCT, LLCT and ILCT transitions. The 542 nm phosphorescence of 1 is assigned as 3MLCT and 3ILCT characters, while the emission for 2–4 at 591, 572 and 549 nm is described as 3MLCT, 3LLCT and 3ILCT transitions. Phosphorescent emissions of 1–4 mirroring their lowest-lying absorption transitions, display a red shift of 2 > 3 > 4 > 1 which is in line with the σ donor ability of bidentate ligands following an order of ppy(2) > ppz(3) > ptz(4) > bpy(1).