Photochemical Properties and Reactivity of a Ru Compound Containing an NAD/NADH-Functionalized 1,10-Phenanthroline Ligand

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.