Excitation energy transport and conformational-librational motion in chains

Abstract

Incoherent electronic excitation transport (EET) along chromophore-sites has been studied in the presence of conformational and librational motion by using a rotor chain model as the mobile vehicle. The time-evolution of the electronic site excitation probability vector Pexc(t) averaged by conformational-librational dynamic disorder has been calculated by a second-order cumulant expansion (CE) approach. A master equation for conformer distributions in torsional space has been used [G. J. Moro, J. Chem. Phys. 94, 8577 (1991) and J. Chem. Phys. 97, 5749 (1992)] to describe the coupled, motional stochastic dynamics of a rotor chain. Based upon this model, the correlation functions of excitation transfer rates, important for the calculation of the second cumulants of the stochastic excitation transfer rate matrices, have been constructed. A simplified procedure for evaluating the correlation functions for the L steep minima of torsional angle-dependent symmetrical potentials of nearest-neighbor rotor-pairs and for a Gaussian distribution of torsional angles displacement has been carried out. By using analytical fits to the second cumulants, the evolution of excitation energy transfer has been calculated in terms of the averaged site probabilities 〈Pexc(t)〉 for chromophores, substituted to the L-fold symmetric rotors. The calculated profiles show a pronounced dependence on the curvature ratio in the saddle point of the rotor pair potential, on the number of the pair potential minima as well as on the average of the mean-square deviation of torsional angles from the stable conformations.