Photochemistry and photophysics of ketyl radicals containing the anthrone moiety

Article abstract of DOI:10.1021/ja00167a053

The photochemistry of several ketones containing an anthrone moiety has been employed to produce the corresponding ketyl radicals 1-4 by photoreduction in the presence of suitable hydrogen donors. The excited-state behavior of these radicals has been examined with use of two-laser, two-color techniques. The lifetimes for the excited ketyl radicals, ranging from 7.9 ns for 3 to 33 ns for 2, are longer than that observed for benzophenone ketyl, thus suggesting that conformational restrictions play a key role in controlling excited radical lifetimes. In the case of 3 the dominant mode of decay involves loss of a benzyl radical from the 10-position, while for 1 and 2 the process involves the loss of a hydrogen atom from the hydroxylic position; in the case of 2 this has been confirmed by Raman spectroscopy. The quantum yields of radical photobleaching are 0.20, 0.46, and 0.75 for 1, 2, and 3, respectively, while 4 is essentially photostable. Deuterium substitution at the hydroxylic position leads to a considerable lengthening of the excited-state lifetime. In the case of dibenzylanthrone we have also detected a rather unprecedented (monophotonic) process involving triplet cleavage of a C-C bond at the 10-position, leading to PhCH₂* and a 10-benzylanthranoxyl radical.