Photochemical Reactions of Bromoanthraquinones in Ethanol at Room Temperature Studied by Steady-State Photolysis and Laser Photolysis

Photolyses of α-bromoanthraquinones (1-bromo-, 1,5-dibromo-, and 1,8-dibromoanthraquinones) with 366-nm light in ethanol at room temperature gave rise to the formation of debrominated 9,10-anthracenediol as a final product. This was interpreted in terms of the sequence of the formation of α-bromo-9,10-anthracenediols followed by the photochemical dehydrobromination yielding the corresponding anthraquinones with one less bromine atom than original ones. Photolysis of 2-bromoanthraquinone gave rise to the formation of 2-bromo-9,10-anthracenediol, and no dehydrobromination was observed. Combined with the values of the quantum yields of photoreduction, the results of the laser photolyses revealed that the rate constant for the hydrogen-atom abstraction from ethanol decreased as an increase of the ππ* character of the lowest triplet states of α-bromoanthraquinones.