Gage Effects in the Photochemistry of (S)-(+)-2-Phenylpropiophenone

Irradiation of (S)-(+)-2-phenylpropiophenone (1) in benzene in the presence of dodecanethiol scavenger leads to the formation of benzaldehyde (φ = 0.44) and racemized ketone 1 (φ = 0.33). The quantum inefficiency (~20%) is attributed to cage recombination of the benzoyl-1-phenylethyl radical pair prior to loss of chirality rather than to radiationless triplet decay. It is concluded that the rate of radical pair cage reactions is comparable to the rates for diffusion or rotation of the 1-phenylethyl radical with respect to the benzoyl radical.