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), 4‘-(4-methylphenyl)-2,2‘:6‘,2‘ ‘-terpyridine (ttpy, N∧N∧N), and
2,2‘-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 μs 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 σ-donating ability of the tridentate ligand, ttpy < L1 < L2. The emission
quantum yields (Φ) of the Ir(III) complexes ranged from 0.037 to 0.19, and the highest Φ value (0.19) was obtained
for [Ir(L1)(bpy)Cl]2+. Radiative rate constants (kr) were 1.2 × 104 s-1 for [Ir(ttpy)2]3+, 3.7 × 104 s-1 for [Ir(L1)(bpy)Cl]2+, 3.8 × 104 s-1 for [Ir(ttpy)(bpy)Cl]2+, 3.9 × 104 s-1 for [Ir(L1)2]3+, and 6.6 × 104 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.