Novel Visible-Light-Driven Photocatalyst. Poly(p-phenylene)-Catalyzed Photoreductions of Water, Carbonyl Compounds, and Olefins

Article abstract of DOI:10.1021/j100368a063

The insoluble yellow powder of poly(p-phenylene) (PPP) prepared by nickel-catalyzed polycondensation of the Grignard reagent from 1,4-dibromobenzene shows photocatalytic activity under visible light toward water, carbonyl compounds, and olefins.Water is photoreduced to H2 in the presence of amines as sacrificial electron donors.The H2 evolution is enhanced 3-20 times by noble-metal deposition, in which Ru deposition is the most effective.Apparent quantum yields (Φ(1/2H2)) for Ru-loaded PPP-catalyzed H2 evolution depend on the irradiation wavelength, reaching a maximum value of 0.015 at 405 nm.On the other hand, nonmetallized PPP can more efficiently photocatalyze the reduction of carbonyls and electron-deficient olefins by triethylamine in methanol compared to Ru-loaded PPP, in cases where the reduction potentials of the substrates are more positive than -2.0 V vs Ag/0.01 M AgNO3.The carbonyls are reduced to the corresponding alcohols and/ or pinacols, whereas the reduction of the olefins to the dihydro compounds is accompanied by rapid cis-trans photoisomerization.From the deuterium incorporation experiments for the photocatalyzed reduction of methyl 4-cyanocinnamate, 6j, in methanol-O-d, disproportionation of one-electron-transfer reduction intermediates is suggested to be responsible for the eventual two-electron reductions and the cis-trans-photoisomerization.The physical and spectral properties of PPP's are characterized, and the mechanism is discussed in terms of the energy structure.