Fluorescence anisotropy of rotating molecules in the presence of energy migration

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Abstract
Energy migration between fluorescent molecules undergoing Brownian rotational motion has been studied by numerically solving the master equation of a many-body system. The molecules interact according to Förster’s mechanism of dipole–dipole coupling. An algorithm for solving the master equation, and for calculating the fluorescence anisotropy and the mean-square displacement of the excitation is presented. In this study the molecules rotate like spherical particles in a liquid solvent at a reduced concentration of unity. The time-dependent fluorescence anisotropy was determined and compared to an analytical theory [G. H. Fredrickson, J. Chem. Phys. 88, 5291 (1988)]. At rotational rates less than the rate of fluorescence, the agreement between our results and those predicted by the theory is not satisfactory. The decay of the fluorescence anisotropy predicted by Fredrickson’s theory is generally too fast. A modification of his model using parameters obtained in our simulations gives a better agreement.