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

An NAD/NADH-functionalized ligand, benzo­[<i>b</i>]­pyrido­[3,2-<i>f</i>]­[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[<i>b</i>]-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