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

Abstract

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 CH 3CN 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).