Photoreduction of anthraquinone sulphonates in aqueous organic media

Abstract

The kinetics of photoreduction of sodium anthraquinone sulphonates in anaerobic aqueous solutions of simple alcohols and N-alkyl amides have been investigated by spectrophotometric measurement of the anthrahydroquinone produced. At substrate concentrations below 1.5 M the rate of the non-chain radical reaction is determined by competition between transfer of a hydrogen atom to the photoexcited triplet of the anthraquinone sulphonate and its radiationless deactivation. The behaviour of the 2-, 2,6- and 2,7-sulphonates has been quantitatively compared in neutral and acidic media. Diacetyl, phenol, hydroquinone, Cl^–, Br^– and I^– retard photoreduction of the β-sulphonates. It is suggested that phenol, hydroquinone and the halide ions act by chemical quenching of the photo-excited triplet, diacetyl by triplet energy transfer. Benzoquinone inhibits temporarily the formation of anthrahydroquinone by rapid reaction with anthrasemiquinone. Velocity constant ratios provide two independent estimates of substrate reactivity towards hydrogen atom transfer and a measure of the quenching efficiency of additives.