The ground- and excited-state geometries, electronic structures, absorptions, and emissions of two ruthenium(II) complexes Ru(bpy) 2 (N ^ N) [bpy = 2,2'-bipyridine, N ^ N = hydrazone (1) and azine (2)] were investigated theoretically. Their ground and the excited state geometries were fully optimized at the B3LYP/MP2/LANL2DZ and UB3LYP/UMP2/LANL2DZ levels, respectively, and the calculated geometries are consistent with the X-ray results. At the TD-DFT level with the PCM model, the absorptions and phosphorescence properties of 1 and 2 were calculated on the basis of the optimized ground- and excited-state geometries, respectively. The calculated lowest-lying absorptions of 1 (512 nm) and 2 (598 nm) are attributed to a {[d x2-y2 (Ru) + d xy (Ru) + π(N^N)] - [π*(bpy)]} transition with MLCT/LLCT transition characters and a {[d z 2(Ru) + d xy (Ru)] → [π*(N^N)]} transition with dominant MLCT transition character, respectively. The calculated phosphorescence of 1 (638 nm) and 2 (731 nm) can be described as originating from a 3 {[d x 2 -y 2(Ru) + d xy (Ru) + π(N^N)] [π*(bpy)]} excited state with 3 MLCT/ 3 LLCT character and a 3 {[d z 2(Ru) + d x 2 -y 2(Ru)] [π*(N^N)]} excited state with dominantly 3 MLCT character, respectively. The calculated results showed that the modulation of the lowest 3 MLCT excited state of this kind of Ru complexes can be achieved by changing the N^N ligand from hydrazone to azine. Moreover, the fact that 2 displays phosphorescence but 1 does not can be interpreted by the different properties of the 3 MLCT excited state: the 3 MLCT excited state of 2 is more than 60 % occupied, whereas that of 1 is less than 20% (k r1 k nr2 ).