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, underwent 2e(-) and 2H(+) reduction, generating the NADH form of the compound, Ru(bppHH)(bpy)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 and Ru(bppHH)(bpy)2 resembled the spectra of Ru(bpy)3. 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 and Ru(pbnHH)(bpy)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 (-1.28 V vs SCE) is much more negative than that of the pbn ligand in Ru(pbn)(bpy)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 allowed the photoreduction of oxygen, generating H2O2 in 92% yield based on Ru(bppHH)(bpy)2. H2O2 production took place via singlet oxygen generated by the energy transfer from excited Ru(bppHH)(bpy)2 to triplet oxygen.