Solvent Dependence of the Fluorescence Lifetimes of Xanthene Dyes

Abstract Fluorescence lifetimes of Ave representative xanthene dye species‐the rhodamine B zwitterion (RB = ), the rhoda‐mine B cation (RB + ), the rhodamine 6G cation (R6G + ), the rhodamine 101 zwitterion (R101) and the fluorescein dianion (F 2‐ )‐were measured in H 2 O, D 2 O and in a series of alcohol solvents ranging from methanol to octanol. The lifetimes of both RB = and RB + increased markedly as the solvent was varied from water to octanol. In contrast, the lifetimes of R6G + and R101± decreased slightly over the alcohol series and that of F 2‐ increased only slightly in the same series. For all the dyes studied the fluorescence lifetimes observed in D 2 O were slightly longer than those in H 2 O. Possible causes for the variations observed are discussed.