Picosecond Fluorescence Studies of Xanthene Dyes

Subnanosecond lifetime measurements using picosecond pulses from a mode locked Nd3+/glass laser together with conventional absorption and fluorescence yield methods have been used to study the photophysics of fluorescein and three of its halogenated derivatives (eosin, erythrosin, and rose bengal) in aqueous and simple alcoholic solvents. For each of the dye molecules absorption and fluorescence maxima move towards higher energy (“blue shift”) as the solvent changes from i-PrOH to HlO. Fluorescence lifetimes and quantum yields are found to decrease markedly with this solvent change and also with increased halogenation (“heavy-atom effect”) of the fluorescein parent. Published triplet yield data confirm that the variations observed in the nonradiative part of the decay rate can be attributed almost wholly to variations in the rate of SI-TI intersystem crossing. A simple and reasonable explanation of the observed effects can be found if for these particular solvent-solute combinations stabilization energies lie in the order AE(TI) < AE(S1) < AE(S0) . This idea is consistent with both the increased SI-S0 spectral “blue shifts” and the enhanced intersystem crossing rate, arising from a smaller SI-TI energy gap, when these dye molecules are placed in a more aqueous solvent environment. The studies are relevant to the use of these dyes as fluorescent probes in biologically important molecules.