DYNAMICS OF IONIC LIPOPHILIC PROBES IN MICELLES - PICOSECOND FLUORESCENCE DEPOLARIZATION MEASUREMENTS

Time-resolved fluorescence anisotropies of two ionic lipophilic probes , merocyanine 540 and octadecylrhodamine B, in alcohol solvents and in sodium dodecyl sulfate, dodecyltrimethylammonium bromide, and Triton X-100 micelles were measured using time-correlated single-photon count ing. In alcohols, the anisotropy decays were single exponentials. In m icelles, the anisotropy decays were biexponential, corresponding to a short and a long rotational correlation time. The results are interpre ted in terms of a two-step model consisting of fast restricted rotatio n of the probe and slow lateral diffusion of the probe in the micelle. The decrease in the residual anisotropy is caused mainly by lateral d iffusion of the probe in the micelle. Information about the restricted rotation of the probe is obtained by using the parameters in the biex ponential fit to calculate cone angles and wobbling diffusion constant s for the wobbling-in-cone model. Lateral diffusion constants are also determined.