Photo-oxidation of triarylmethanes sensitised by carbonium ions

Abstract

Absorption of light by stable carbonium ions (xanthydryl, triphenylmethyl, and 5H-dibenzo[a,d]cycloheptenyl cations) in solution in trifluoroacetic acid containing oxygen brings about oxidation of triarylmethanes to the corresponding triarylmethyl cation. Excess of molecular oxygen inhibits the photo-oxidation. The dependence of the quantum yield for photo-oxidation on the concentration of triarylmethane indicates that the hydrocarbon reacts directly with an excited state, believed to be triplet, of the sensitising cation. The effect of ring substituents in triphenylmethane on the partitioning of the triplet sensitiser between oxygen quenching and reaction with the hydrocarbon is interpreted as evidence that the photo-oxidation is initiated by electron transfer from triaylmethane to the excited sensitiser.