An NAD/NADH-functionalized ligand, benzo[b]pyrido[3,2-f][1,7]-phenanthroline (bpp), was newly synthesized. A Ru compound containing the bpp ligand, [Ru(bpp)(bpy)2]2+, underwent 2e- and 2H+ reduction, generating the NADH form of the compound, [Ru(bppHH)(bpy)2]2+, in response to visible light irradiation in CH3CN/TEA/H2O (8/1/1). The UV-vis and fluorescent spectra of both [Ru(bpp)(bpy)2]2+ and [Ru(bppHH)(bpy)2]2+ resembled the spectra of [Ru(bpy)3]2+. Both complexes exhibited strong emission, with quantum yields of 0.086 and 0.031, respectively
values that are much higher than those obtained from the NAD/NADH-functionalized complexes [Ru(pbn)(bpy)2]2+ and [Ru(pbnHH)(bpy)2]2+ (pbn = (2-(2-pyridyl)benzo[b]-1.5-naphthyridine, pbnHH = hydrogenated form of pbn). The reduction potential of the bpp ligand in [Ru(bpp)(bpy)2]2+ (-1.28 V vs SCE) is much more negative than that of the pbn ligand in [Ru(pbn)(bpy)2]2+ (-0.74 V), although the oxidation potentials of bppHH and pbnHH are essentially equal (0.95 V). These results indicate that the electrochemical oxidation of the dihydropyridine moiety in the NADH-type ligand was independent of the π system, including the Ru polypyridyl framework. [Ru(bppHH)(bpy)2]2+ allowed the photoreduction of oxygen, generating H2O2 in 92% yield based on [Ru(bppHH)(bpy)2]2+. H2O2 production took place via singlet oxygen generated by the energy transfer from excited [Ru(bppHH)(bpy)2]2+ to triplet oxygen.