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-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline, ppy = 2-phenylpyridine), Ir(ppy)2(N--N)(+) x F(-) (2), Ir(ppy)2(N--N)(+) x CF3COOH (3/3a), and Ir(ppy)2(N--N)(+) x 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 pi(ppy) and d(Ir), and the LUMOs of 1, 2, 3a, and 4 are contributed by pi*(N--N), whereas the LUMO of 3 is composed of pi*(N--N) and pi*(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 (3)MLCT/(3)LLCT/(3)ILCT characters, but those of 2 and 4 only have (3)ILCT but without (3)MLCT 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.