Concentration depolarization of fluorescence in the presence of molecular rotation

Abstract

A theory is presented to describe the observables in fluorescence depolarization experiments when both rotational depolarization and concentration depolarization mechanisms are active. The latter mechanism arises from electronic excitation migration among chromophores and becomes more important as their concentration is increased. We treat the simplest case of randomly distributed chromophores, with fixed centers of mass, undergoing independent rotational Brownian motion. Our formalism reduces to existing theories if either depolarization mechanism is turned off, but makes new predictions in the situation where both are operative. Because electronic excitation transfer and molecular orientation are dynamically correlated, we find that previous ad hoc expressions that neglected such correlations are inappropriate. Approximate expressions for the fluorescence anisotropy are provided that can be tested by time-resolved or photostationary experiments on dye molecules in solution. Extensions of the theory to cases of correlated chromophore positions are straightforward and will allow interpretation of experiments on labeled macromolecular systems.