Kinetic and spectral properties of rhodamine 6G free radicals: a pulse radiolysis study

Using the pulse radiolysis technique Br2− has been shown to react with the laser dye, rhodamine 6G (RhH+) to produce cation radicals. At pH 4, the species designated RhH2+ is produced which was shown to deprotonate with a pKa of 5.7 yielding Rh+. Both forms of the radical have characteristic transient difference absorption spectra with distinct maxima at 470 nm. tion radicals are relatively long-lived and can be observed for up to 1 s after the pulse. At the end of the reaction(s) of Rh+, significant increases in absorption relative to the ground state area were observed in the dye laser output region, 555–620 nm. Oxygen had no effect on the decay of these radicals; however, it was demonstrated that O2− could regenerate the dye ground state in a redox process. At high concentrations of rhodamine 6G, it appears that only monomeric cation radicals are formed on reaction with Br2−, suggesting that any dimer cation radicals formed initially are short-lived (k[dissociation]>5×106 s−1). The relevance of cation radical formation to the efficiency and photostability of rhodamine dye laser systems in both liquid and solid state is discussed. It is proposed that the formation of such radicals, particularly in the solid state, may account for the loss of efficiency and photostability and so provide an alternative mechanism to one involving the dye triplet state.