Solvent Dependence of the Fluorescence Lifetimes of Xanthene Dyes

Fluorescence lifetimes of five representative xanthene dye species-the rhodamine B zwitterion (RB'), the rhodamine B cation (RB+), the rhodamine 66 cation (R6G+), the rhodamine 101 zwitterion (R101') and the fluorescein dianion (F2-)-were measured in HzO, D20 and in a series of alcohol solvents ranging from methanol to octanol. The lietimes of both RB* and RB+ increased markedly as the solvent was varied from water to actanol. In contrast, the lifetimes of R6G+ and R101' decreased slightly over the alcohol series and that of F2- increased only slightly in the same series. For all the dyes studied the fluorescence lifetimes observed in DzO were slightly longer than those in H20. Possible causes for the variations observed are discussed.