Photochemical C-C bond cleavage of 1,2-diarylcyclopropanes bearing an acetylphenyl group. Generation and observation of triplet 1,3-biradicals

The photochemical properties of 1,2-diarylcyclopropanes bearing an acetylphenyl group were studied by product analysis and laser flash photolysis. All cyclopropanes showed efficient cis-trans photoisomerization followed by inefficient isomerization to 1,3-diarylpropenes. All the products were unquenched by the addition of triplet quencher, 2-methyl-1,3-butadiene (E-T similar or equal to 60 kcal mol(-1)), whereas molecular dioxygen gave oxygenated products. Triplet 1,3-biradicals generated from a short-lived acetophenone-like triplet (less than or similar to 1 ns) were observed as intermediates in these reactions through nanosecond laser flash photolysis. Polar substituent effects on the lifetime of the 1,3-biradicals were small, except for a heavy atom effect in the case of Br. Spin-orbit coupling calculations on model 1,3-biradicals show a negligible effect on polar substituent and thus predict a negligible effect on the intersystem crossing rate. Conjugated biradicals in general now seem unlikely to show the "ionic character effect" on intersystem crossing suggested by the work of Salem and Rowland.