Solvent dependence of the fluorescence lifetimes of xanthene dyes

Fluorescence lifetimes of five representative xanthene dye species-the rhod amine B zwitterion (RB+/-), the rhodamine B cation (RB+), the rhodamine 6G cation (R6G(+)), the rhodamine 101 zwitterion (R101(+/-)) and the fluoresce in dianion (F2-)-were measured in H2O, D2O and in a series of alcohol solve nts ranging from methanol to octanol, The lifetimes of both RB+/- and RB+ i ncreased markedly as the solvent was varied from water to octanol, In contr ast, the lifetimes of R6G(+) and R101(+/-) decreased slightly over the alco hol series and that of F2- increased only slightly in the same series. For all the dyes studied the fluorescence lifetimes observed in D2O were slight ly longer than those in H2O. Possible causes for the variations observed ar e discussed.