Rotational diffusion of nondipolar probes in Triton X-100 micelles: Role of specific interactions and micelle size on probe dynamics

Temperature dependent rotational relaxation studies of two structurally similar nondipolar probes: 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP), have been carried out in Triton X-100 micelles in an attempt to explore the influence of specific interactions and micellar size on the dynamics of probe molecules. The time-resolved anisotropy in micelles, decays as a sum of two exponentials with two time constants, one corresponding to a fast reorientation time and the other to a slow one, for both the probes over the entire range of temperature studied. The results are analyzed 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 that are coupled to the rotation of the micelle as a whole. However, as the temperature is raised, the size of the Triton X-100 micelles increase significantly and the measured slow reorientation time corresponds solely to the lateral diffusion of the probe in the micelle. This is because the reorientation time for the overall rotation of the micelle becomes very long and consequently the fluorescence depolarization due to this process becomes negligible. It has also been observed that the short and long components of the anisotropy decay for DPP are found to be considerably slower than the corresponding ones for DMDPP due to the strong hydrogen bonding interactions between the ethylene oxide groups of the surfactant units and the secondary amino groups of the probe.