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 counting. In alcohols, the anisotropy decays were single exponentials. In micelles, the anisotropy decays were biexponential, corresponding to a short and a long rotational correlation time. The results are interpreted in terms of a two-step model consisting of fast restricted rotation of the probe and slow lateral diffusion of the probe in the micelle. The decrease in the residual anisotropy is caused mainly by lateral diffusion of the probe in the micelle. Information about the restricted rotation of the probe is obtained by using the parameters in the biexponential fit to calculate cone angles and wobbling diffusion constants for the wobbling-in-cone model. Lateral diffusion constants are also determined.