EXCITATION-ENERGY TRANSPORT AND CONFORMATIONAL-LIBRATIONAL MOTION IN CHAINS

Incoherent electronic excitation transport (EET) along chromophore-sit es has been studied in the presence of conformational and librational motion by using a rotor chain model as the mobile vehicle. The time-ev olution of the electronic site excitation probability vector P-exc(t) averaged by conformational-librational dynamic disorder has been calcu lated by a second-order cumulant expansion (CE) approach. A master equ ation for conformer distributions in torsional space has been used [G. J. Moro, J. Chem. Phys. 91, 8577 (1991) and J. Chem. Phys. 97, 5749 (1 992)] 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 constr ucted. A simplified procedure for evaluating the correlation functions for the L steep minima of torsional angle-dependent symmetrical poten tials of nearest-neighbor rotor-pairs and for a Gaussian distribution of torsional angles displacement has been carried out. By using analyt ical fits to the second cumulants, the evolution of excitation energy transfer has been calculated in terms of the averaged site probabiliti es (P-exc(t)) for chromophores, substituted to the L-fold symmetric ro tors. The calculated profiles show a pronounced dependence on the curv ature ratio in the saddle point of the rotor pair potential, on the nu mber of the pair potential minima as well as on the average of the mea n-square deviation of torsional angles from the stable conformations. (C) 1998 American Institute of Physics.