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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.