Direct and reverse energy transport in systems of monomers and imperfect traps: Monte Carlo simulations.

A Monte Carlo simulation of the concentration dependence of the fluorescence quantum yield ŋM and emission anisotropyr M of a system containing dye molecules in the form of monomers M and clusters T (statistical pairs and trimers) playing the role of the imperfect traps for nonradiative excitation energy transfer (NET) has been carried out. The simulation has been made for determined values of Förster critical distancesR 0 (MM) andR 0 (MT) and for several values ofR 0 (TM) andR 0 (TT) , assuming that the energy may be transferred from M() to T as well as from T() to M (reverse nonradiative energy transfer, RNET). It was shown that the RNET process in the range of high concentrations may strongly change the values ofr M as well as those of ŋM. For emission anisotropyr M an effect of repolarization was observed which decreases rapidly with increasingR 0 (TM) andR 0 (TT) . A very good agreement between the simulation results of ŋM and the theoretical model with no adjustable parameters was found. In the model, the RNET process and influence of correlation between active molecules on energy migration among monomers were taken into account.