Photodegradation of coumarin laser dyes: an unexpected singlet self-quenching mechanism

The photolysis of a series of aminocoumarin laser dyes, including 7-diethylamino-4-methylcoumarin (1), has been investigated. Dealkylation of groups at the 7-dialkylamino functionality, reduction of the lactone moiety and overall photodecomposition of the dyes have been observed. Concentration quenching of dye fluorescence is important at concentrations above 0.01 M. The concentration dependence of photoreduction is shown to be consistent with a singlet self-quenching mechanism. Dye photodegradation is not quenched on addition of trans-stilbene, although the latter is an effective quencher of the triplet of 1. Intersystem crossing yields for the coumarin dyes are very low. A mechanism for dye photodegradation is proposed involving singlet self-quenching and net hydrogen atom transfer between dye molecules, followed by disproportionation and coupling of radicals.