Syntheses and Properties of Emissive Iridium(III) Complexes with Tridentate Benzimidazole Derivatives

A series of novel emissive Ir(III) complexes having the coordination environments of [Ir(N N N)2]3+, [Ir(N N N)(N N)Cl]2+, and [Ir(N N N)(N C N)]2+ with 2,6-bis(1-methyl-benzimidazol-2-yl)pyridine (L1, N N N), 1,3-bis(1-methyl-benzimidazol-2-yl)benzene (L2H, N C N), 4ʹ-(4-methylphenyl)-2,2ʹ:6ʹ,2ʹ ʹterpyridine (ttpy, N N N), and 2,2ʹ-bipyridine (bpy, N N) have been synthesized and their photophysical and electrochemical properties studied. The Ir(III) complexes exhibited phosphorescent emissions in the 500-600 nm region, with lifetimes ranging from approximately 1-10 (mu)s at 295 K. Analysis of the 0-0 energies and the redox potentials indicated that the lowest excited state of [Ir(L1)(L2)]2+ possessed the highest contribution of 3MLCT (MLCT = metal-to-ligand charge transfer) among the Ir(III) complexes, reflecting the (sigma)-donating ability of the tridentate ligand, ttpy < L1 < L2. The emission quantum yields ((phi)) of the Ir(III) complexes ranged from 0.037 to 0.19, and the highest (phi)value (0.19) was obtained for [Ir(L1)(bpy)Cl]2+. Radiative rate constants (kr) were 1.2 × 10^4 s^-1 for [Ir(ttpy)2]3+, 3.7 × 10^4 s^-1 for [Ir(L1)(bpy)Cl]2+, 3.8 × 10^4 s^-1 for [Ir(ttpy)(bpy)Cl]2+, 3.9 × 10^4 s^-1 for [Ir(L1)2]3+, and 6.6 × 10^4 s^-1 for [Ir(L1)(L2)]2+. The highest radiative rate for [Ir(L1)(L2)]2+ with the highest contribution of 3MLCT could be explained in terms of the singlet-triplet mixing induced by spin-orbit coupling of 5d electrons in the MLCT electronic configurations.