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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.

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Quadricyclane (1) and 1,8-bishomocubane (2) have been found to undergo valence isomerization to norbornadiene and tricycl0[4.2.2.0~.~]deca-3,7-diene, respectively, on illuminated CdS and ZnO. An electron transfer mechanism is proposed.Quantum yield, solvent effects, the role of oxygen, and the quenching of the reaction were investigated, and were consistent with this interpretation. The thermal reaction of 1 on CdS was also suggested to be an electron transfer process involving, in this case, defects or trapped holes on the surface of the semiconductor. An examination of a series of strained hydrocarbons structurally related to 1 (

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