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

Freshly prepared colloidal ZnS suspensions (ZnS-0) effectively catalyze photoreduction of C02 in water at pH 7 with NaH2P02 in the coexistence of Na2S under UV irradiation. With competitive H2 evolution, formate and a very small quantity of CO were formed with the apparent quantum yield $i/2hcoO ' 0.24 at 313 nm, where H2P02" was quantitatively photoxidized to HP032'. The efficiency strongly depends on the pH of the system, preparation methods of ZnS photocatalysts, and synergistic effects of electron donors. Quantized ZnS crystallites with low density of surface defects are indispensable for the effective C02 reduction. The synergistic effect in the use of both SH' and H2P02" ions is discussed in terms of spectral properties of the photocatalyst.Photoreduction of carbon dioxide (C02) to organic compounds has attracted substantial interest as a means for fuel production2•3 and resolving the greenhouse effect.4Electrochemical and photoelectrochemical as well as photochemical reductions of C02 have been extensively studied.2 Development of artificial photosynthetic systems is one of the ultimate goals in the reduction of C02. Photoassisted reductions of C02 in semiconductor particulate systems have been studied,1•5'10 since Inoue et al.5 first reported photoreductions of C02 with suspensions of various semiconductor powders such as W03, Ti02, ZnO, CdS, GaP, and SiC. This system is also interesting in terms of abiotic photosynthesis." However, most semiconductor-catalyzed photoreductions of C02 using water as electron donor resulted in low conversion and selectivity to reduction products.8 Recent studies have revealed that quantized semiconductor particles and their loose aggregates are effective photocatalysts since their band gap is increased and the recombination of a photoformed electron and hole pair is relatively slowed down.12'14In addition, the efficiency of the semiconductor photocatalysts is controlled by the migration of electrons and holes to the semiconductor solution interface.13•14 Metalization of semiconductor particles with noble metals is a general means to eliminate pho-

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Photoinduced processes of a series of phosphorus tetraphenylporphyrin (PTPP) derivatives ([PTPP-(NHC6H4X)2]+Cl-, X = OCH3, CH3, H, Cl, CF3, and CN) have been investigated by using femtosecond laser flash photolysis mainly. PTPP with OH as an axial ligand showed S2 fluorescence upon excitation of the Soret band. The S2 fluorescence lifetime was estimated to be 1.5 ps. On the other hand, both S2 and S1 fluorescence bands of PTPP-(NHC6H4X)2 were difficult to observe, indicating the existence of an additional deactivation process such as charge separation (CS). From MO calculation and cyclic voltammetry, PTPP and the axial ligand are expected to act as an acceptor and a donor, respectively, upon excitation of PTPP. CS via the S2 state was confirmed during the femtosecond laser flash photolysis by observing the transient absorption of radical anion of PTPP. Furthermore, CS via the S1 state of PTPP was also observed. The CS rate via the S1 state was faster than that from the S2 state. The free energy dependence of the electron-transfer rates was discussed on the basis of Marcus theory.

Colloidal CdS suspensions (CdS-0) prepared at 0 "C from methanolic Cd(CIO,), and Na,S solutions consist of quantised CdS microcrystallites (2-5 nm) and their loose aggregates, which catalyse t h e effective photoreduction of aromatic ketones and electron-deficient alkenes with triethylamine as electron donor. Under visible light induced photolysis, the methanolic CdS-0 suspension becomes brown owing to the reduction of lattice Cd2+ to Cd', leading to the effective formation of alcohols from ketones, and dihydro compounds from alkenes. With the reduction potential < -1.56 V vs. standard calomel electrode (SCE), ketones were partly photoreduced to pinacols in methanol. Compared with highly pure bulk CdS (CdS-Ald), CdS-0 is more effective for photoreduction because of the size quantisation effect. The presence of a n excess of sulfide ion ( S 2 -) in the CdS-0 system, however, suppresses the formation of lattice Cd', inducing one-electron transfer photoreductions which result in t h e exclusive formation of pinacols and 1,2,3,4-tetra(rnethoxycarbonyl)butane from t h e respective ketones and dimethyl maleate. The relationship between t h e two-electron reductions and photogenerated lattice Cdo is discussed in terms of the regulation of semiconductor photocatalysis.

Photocatalytic activity and spectroscopic properties of ZnS suspensions for the two-electron reduction of aldehydes or related compounds in aqueous medium are described. The ZnS suspension (ZnS-0) prepared by cooling from aqueous ZnSOl and Na2S solutions catalyzes photoredox reactions of acetaldehyde, giving ethanol without much H2 evolution as a two-electron-reduction product, and acetic acid, biacetyl, and acetoin as oxidation products. When the ZnS-0 suspension is refluxed (giving ZnS-100) or dried to powder, the resulting ZnS shows an increased activity for H2 evolution but a decreased activity for the two-electron reduction. The two-electron photoreduction is ascribed to the sequential transfer of active electrons in the conduction band of defect-free aggregates of ZnS microcrystallites (quantized ZnS). This mechanism is supported by product analysis, energetics at ZnS interfaces, the sharp and blue-shifted onset of absorption and excitation spectra, and the long-life band gap emission of the active ZnS-0 suspension. UV, emission, and ESR spectra, as well as the enhancement of the particle size and crystallinity, suggest that the activity change observed after heating or drying to powder is due to the formation of surface states which may trap active electrons. This interpretation is also supported by the generated activity of ZnS-100 for the H2 photoevolution under >350-nm irradiation. ZnS photocatalysis under >350-nm irradiation and relationship with spectroscopic properties are also discussed.Photoreduction of organic molecules on semiconductor particles has attracted much attention from the viewpoint of solar energy conversion? organic synthesis,3 and prebiotic chemi~try.~.~ Such irreversible photoreductions, which require at least a net twoelectron transfer, have been hitherto carried out by using noble metal deposited semiconductors.6-8 The deposited metal plays an important role as a relay of conduction band electrons to substrates, being usually believed as a requisite for efficient two-electron reductions on irradiated semiconductors. Cuendet and Gratzel recently disclosed the direct two-electron reduction of pyruvate into lactate under irradiation of aqueous suspensions of nonmetallized Ti02.9Recent studies on nonmetallized ZnS semiconductor1~13 re-(1) Part 7: Yanagida, s.; Miyake, Y.; Midori, Y.; Ishimaru, Y.; Pac, C. Azuma, T.; Kawakami, H.; Kizumoto, H.; Sakurai, H. J . Chem. Soc., Chem. Commun. 1984, 21. (c) Yanagida, S.: Kawakami, H.; Hashomoto, K.; Sakata, T.; Pac, C.; Sakurai, H. Chem. Lett. 1984, 1449. (d) Yanagida, S.; Kizumoto, H.; Ishimaru, Y.; Pac, C.; Sakurai, H. Chem. Lett. 1985, 141. (e) Yanagida, S.; Azuma, T.; Midori, Y.; Pac, C.; Sakurai, H.

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