Photoinduced electron transfer for pyrromethene dyes

The photophysics and electrochemistry of pyrromethene dyes (PM), an important class of laser dyes, have been investigated. Selected dyes were found to be moderately good electron donors. Diffusion-limited fluorescence quenching (ca. 6 x 10(9) M-1 s(-1)) is observed for the dyes having half-wave oxidation potentials < 1.07 V (vs SCE) in the presence of an electron acceptor, such as methyl viologen or pyromellitic dianhydride. For pyrromethene 567, the dye radical cation and triplet state, along with the pyromellitic dianhydride radical anion and the methyl viologen radical cation, were identified using transient absorption spectroscopy. The dye radical cation, which displays a sharp absorption band at 410 nm, is found to be insensitive toward oxygen. Secondary electron transfer involving the reduction of oxygen and producing superoxide radical anion is suggested. Oxygen plays a critical role in PM dye photochemistry as it induces intersystem crossing to generate the dye triplet state and gives rise to products of oxidation.