Picosecond fluorescence studies of xanthene dyes

Abstract

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 in- creased 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 intersys- tem 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. I. Introduction The fluorescence properties of the xanthene dyes have both theoretical and practical interest. The advent of the dye laser' has sparked off renewed interest in the correlation between molecular structure and fluorescence properties, while the use of fluorescent dye probes in structural studies of molecules of biological significance has become an area of considerable a~tivity.~,~ The usefulness of a fluorescent probe relies on variations in the dye's fluorescent properties with alterations in its environment, solvent polarity being an example. Despite a considerable amount of work, no detailed theory explaining the often very dramatic effects of environment and structure on fluorescence has emerged. For example, the effect of solvent and macromolecular environment on the fluorescence lifetime and quantum yield of perhaps the most widely used fluorescent probe, ANS- ( 1 -anilinonaphthalene-8-sulfonate), not well understood although various mechanisms for the fluorescence quenching in polar solvents have been pr~posed.~-~ Until a better understanding of the environmental factors affecting the fluorescence of dye molecules has been achieved, detailed conclusions based on fluorescent probe studies must be treated with caution. Fluorescein and its halogenated derivatives provide an ex- cellent model series for studies of this kind, for not only do the degree and type of halogenation greatly alter the fluorescence yield, but the photophysical properties of these dyes are also very dependent on the nature of the solvent. We have studied absorption and emission spectra, fluorescence lifetimes, and fluorescence yields of eosin, erythrosin, and rose bengal (Figure 1 j in a series of alcohols and in aqueous solution. The data provide a clear picture of the changes in nonradiative decay rate that occur in these molecules as a result of changes in the solvent environment. 11. Experimental Section (a) Chemicals. Eosin (BDH) was purified by recrystallization from acidic solution and rose bengal (BDH) was purified by chromatog- raphy on an alumina-talc column. The erythrosin sample (BDH) was used as supplied. Spectroscopic grade (Merck) methanol, ethanol, 2-propano1, and triply distilled water were used as solvents. All the aqueous solutions were buffered to pH 9.2. Dye concentrations of