Semiconductor Photocatalysis. Cis-Trans Photoisomerization of Simple Alkenes Induced by Trapped Holes at Surface States

Article abstract of DOI:10.1021/ja00264a014

The use of ZnS or CdS as photocatalysts induces an efficient cis-trans photoisomerization of simple alkenes, e.g., the 2-octenes, 3-hexen-1-ols, and methyl 9-octadecenoates in photostationary cis-trans ratios almost identical with the thermodynamic equilibrium ratios achieved by the phenylthio radical.Quantum yields for the cis-trans photoisomerization, Φ_c-t, exceed largely over unity.Mechanistic studies involving Stern-Volmer analyses, quenching effect of oxygen, and ESR analysis under band-gap irradiation on ZnS in methanol demonstrate that the photoisomerizations take place with high turnover numbers at active sites where trapped holes at surface states, i.e., sulfur radicals arising from Zn vacancies and/or interstitial sulfur on sulfide semiconductors, play decesive roles.A highly efficient catalysis occurs with ZnS sols prepared from polysulfide-containing Na2S solution.The trapped-hole mechanism is further supported by the enhanced effect of water acting as a good electron acceptor as well as the quenching effect of diethylamine acting as an electron donor.