Photoreactions of rhodamine dyes in basic solvents

Using a highly sensitive spectrometer we measured the absorption of transient states of different dyes in liquid solution. Analysing the temporal and spectral dependency of the absorption we determined lifetimes as well as absorption spectra of transient states. The kinetic parameters are specifically influenced by interaction with certain bases. On investigating the acid–base equilibrium of the triplet state of different rhodamine dyes we found the deprotonated triplet state to be an efficient source of a long-lived transient species. We suggest a reaction scheme which explains the dependence of the observed triplet lifetime on the concentration of amine. The possible consequences for the photostability of the dye are discussed.