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 (k _r) 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.