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 (4>('/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 AgNO}. 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-rf, 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.Since the photolysis of water on a Ti02 electrode (Honda-Fujishima effect) was reported,1 photoinduced charge separation on inorganic semiconductors has been widely investigated as a key step in the conversion of light to chemical potential energy.1 2 In order to achieve high efficiency in photochemical conversion or to utilize visible light much more effectively, integrated chemical systems using semiconductor/redox couple interfaces combined with metallic cocatalysts, electron relays, sensitizers, or polymers have been extensively studied.2'3 On the other hand, since polyacetylene film was synthesized and chemical doping effects on electric conductivity were discovered,4 a number of organic polymers with conjugated welectrons have been synthesized and some novel applications such as electrochromic polymers,5 organic batteries,6 sensing polymers,7 and solar batteries8 have been successfully proposed. Conjugated polymers called conducting polymers show electrical conductivity only when they are doped chemically with either electron donors or acceptors. Under undoped conditions, however, they are semiconducting and should be called polymeric organic semiconductors.9 Among such 7r-bond conjugated polymers, poly(pphenylene) (PPP) polymers are resistant to oxidation, thermal degradation, and radiation under aerobic conditions10 and are reported to have band gaps in the visible-light region.11 Evolution