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Les mecanismes de Langmuir-Hinshelwood et Eley-Rideal modifies permettent d'expliquer les resultats obtenus

Photodimerisation du bis-[methoxy-4 phenyl]-1,1 ethylene photosensibilisee par le dicarbonitrile-9,10 anthracene

The CdS dispersion mediated photoinduccd dimerization of N-vinylcarbazole (NVC) has been investigated and found to be efficient. The rate of reaction is not affected over quite a wide range of CdS masses at constant NVC concentration in solution. The reaction requires the presence of oxygen. At low light intensities the rate is linearly related thereto. A cation radical mechanism is proposed since the reaction can be efficiently quenched by electron donors. The dimerization appears to follow the Langmuir-Hinshelwood and modified Eley-Rideal mechanisms, as does the quenching. CdS of different origins with different surface areas, purities and structures were used and the rates varied by a factor of four. None of these functions appears to play a dominant role in the rate of dimerization. The variation in rates and in activation energies suggested the importance of traps located close to the conduction band.

Three reactions: the CdS-mediated valence isomerization of hexamethyldewarbenzene to hexamethylbenzene, the cycloreversion of dianthracene, and the TiO2-mediated cis–trans isomerization of stilbene are known to be suppressed when the semiconductor excitation takes place in the absence of oxygen. The oxygen, when present, is presumed to trap the electron delaying electron-hole recombination. It has been found that CBr4 may replace oxygen and functions in a similar way. In addition, the concomitant formation of oxidation products in the TiO2-mediated aerobic reaction is avoided. The mechanism is discussed. Keywords: cadmium sulphide, carbon tetrabromide, dianthracene, heterogeneous photocatalysis, hexamethyldewarbenzene, titanium oxide.

Six substituted styrenes have been shown to achieve thermodynamic equilibrium via a radical-ion mechanism when exposed to the semiconductor CdS excited by light of wavelength shorter than its band gap.