A series of bis-cyclometalated cationic iridium (Ir) complexes were synthesized employing two coumarin 6 ligands and a 2,2'-bipyridine (bpy) with various substituents as new sensitizers, realizing both features of strong visible-light absorption and long-lived excited state. Complexes 2-4, with electron-donating methyl and methoxy groups, absorbed visible light strongly (epsilon: 126 000-132 000 M-1 cm(-1) ) and exhibited room -temperature phosphorescence with remarkably long lifetimes (21-23,mu s) in dichloromethane. In contrast, the excited state of prototype complex 1 without any substituents was short-lived, particularly in highly polar acetonitrile. Phosphorescence of complex 5 with the strong electron withdrawing CF3 groups was too weak to be detected at room temperature even in less polar dichloromethane. The triplet energies of their coumarin ligand-centered ((LC)-L-3) phosphorescent states were almost invariable, demonstrating that selective tuning of the excited-state lifetime is possible through this "simple chemical modification of the bpy ligand" (we name it the "SCMB" method). The spectroscopic and computational investigations in this study suggest that a potential source of the nonradiative deactivation is a triplet ligand-to-ligand charge-transfer state ((LLCT)-L-3 state, coumarin 6 -> bpy) and lead us to conclude that the energy level of this dark (LLCT)-L-3 state, as well as its thermal population, is largely dependent on the substituents and solvent polarity. In addition, the significant difference in excited-state lifetime was reflected in the photosensitizing ability of complexes 1-5 in visible-light driven hydrogen generation using sodium ascorbate and a cobalt(III) diglyoxime complex as an electron donor and a water reduction catalyst, respectively. This study suggests that the SCMB method should be generally effective in controlling the excited state of other bis-cyclometalated cationic Ir(III) complexes.