Mechanism of Ir(ppy)₂(N∧N)⁺ (N∧N = 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline) Sensor for F^-, CF₃COOH, and CH₃COO^-: Density functional theory and time-dependent density functional theory studies

The geometries, electronic structures, and spectroscopic properties of Ir(ppy)₂(N∧N)⁺ (1) (N∧N = 2-phenyl1H-imidazo[4,5- f)[1,10]phenanthroline, ppy = 2-phenylpyridine), Ir(ppy)₂(N∧N) ⁺·F^~ (2), Ir(ppy)₂(N∧N) ⁺CF₃COOH (3/3a), and Ir(ppy)₂(N∧N) ⁺·CH₃COO^- (4) were investigated theoretically. The ground and the excited state geometries of 1-4 were optimized at the B3LYP/LANL2DZ and UB3LYP/LANL2DZ levels, respectively. The optimized geometries agree well with the corresponding experimental results. The HOMOs of 1-4 and 3a are composed of π(ppy) and d(Ir), and the LUMOs of 1, 2, 3a, and 4 are contributed by π*(N∧N), whereas the LUMO of 3 is composed of π*(N∧N) and π*(CF₃COOH). Under the time-dependent density functional theory level with polarized continuum model model, the absorption and phosphorescence in CH₂Cl₂ media were calculated on the basis of the optimized ground and excited state geometries, respectively. The lowest-lying absorptions of 1 (412 nm) and 3/3a (409/419 nm) have MLCT/LLCT transition characters, and those of 2 (448 nm) and 4 (427 nm) are contributed by ILCT character. The calculated lowest-energy triplet excited states responsible for phosphorescence of 1 (519 nm) and 3/3a (661/702 nm) have mixing ³MLCT/³LLCT/³ILCT characters, but those of 2 and 4 only have ³ILCT but without ³MLCT character, which is the reason for the no-emissive character of 2 and 4. Moreover, the phosphorescence character of 3 is hardly changed by different addition sites of CF₃COOH group (3a). The calculated results also showed that complex 1 is more suitable for an F^- sensor than for CF₃COOH and CH₃COO- sensors.