Fluorescence and competing events on the picosecond time scale

Abstract

An experimental system for the study of subnanosecond fluorescence spectroscopy is described and some general classes of experiments in this research area are proposed. A special emphasis is placed on nonexponential decays, and competition between radiative and nonradiative processes in this temporal domain is described. A new type of diffusion controlled chemical reaction, a rotational diffusion controlled reaction, is proposed for study using picosecond fluorescence depolarisation methods. These methods are also useful for assessment of solvent dependent molecular structure of flexible molecules and the relationship these structural changes might have with solvent dependent chemical reaction rates. Initial experimental results show the feasibility of such studies using the existing technology. Finally, the study of fluorescence probe molecules on surfaces or in biological membranes or macromolecules is described. The picosecond emission technique extends the study of rotation of whole macromolecules to small molecules or small macromolecular segments. The importance is being assessed of photoionisation as a nonradiative path in biological fluorescence probe molecules such as tryptophan, an intrinsic probe, and ANS (1,8 anilinonaphthalene sulphonate) and TNS (2,6 toluylnaphthalene sulphonate), both extrinsic probes. Mixed solvent (water/ethanol) studies of the latter two molecules show nonexponential decays which are interpreted in terms of inhomogeneities of solute sites in the mixed solvent. © 1978.