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).