Electronic structures of ruthenium complexes [Ru(bpy) 2 (L)] 2+ were studied by DFT calculations, where bpy is 2,2′-bipyridine and L’s are 1,10-phenanthroline (phen) derivatives. They are parent (1) and eight complexes with L=4-phenyl-phen (2), 4,7-diphenyl-phen (3), 2,9-dimethyl-4,7-diphenyl-phen (4), 2,9-dimethyl-phen (5), 3,4,7,8-tetramethyl-phen (6), 5-amino-phen (7), 5-methyl-phen (8), and 3-phenylethynyl-phen (9). Those complexes except 7, and 9 were prepared and characterized.Three triplet (called here 3 MLCT, 3 MC 1 and 3 MC 2 ) states were obtained in all the complexes. 3 MC 1 and 3 MC 2 states were found to have the elongated Ru–N bonds and large spin densities on Ru 2+ . The geometry optimization of the triplet state of ruthenium complexes leads to the elongation of two trans Ru–N bonds confirming the 3 MC character. The correlation between quantum yields and ( 3 MLCT – 3 MC 2 ) energy gaps was examined and discussed. Successive conversion paths of ( 3 MLCT→ 3 MC 1 ) and ( 3 MC 1 → 3 MC 2 ) of 1 and 5 were determined. Their activation energies were calculated to be very small.