The geometries, electronic structures, and spectroscopic properties of Ir(ppy)(2)(N N)(+) (1) (N N = 2-phenyl-1H-imidazol[4,5-f][1,10]phenanthi-oline, ppy = 2-phenylpyridine), Ir(ppy)(2)(N N)(+)center dot F- (2), Ir(ppy)(2)(N N)(+)center dot (4) were investigated theoretically. The -round and the CF3COOH (3/3a), and Ir(ppy)(2)(N N)(+)center dot CH3COO- 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 CH,Cl, 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 mu) 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 (MLCT)-M-3/(LLCT)-L-3/(ILCT)-I-3 characters, but those of 2 and 4 only have (ILCT)-I-3 but without (MLCT)-M-3 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.