The quantum master equation approach involving a weak exciton-phonon coupling is applied to the exciton migration dynamics of dendritic molecular aggregates modeled after a phenylacetylene dendrimer, D25, which exhibits an efficient light-harvesting property. The mechanism of efficient exciton migration from the periphery to the core is studied by analyzing relaxation terms among the exciton states originating in weak exciton-phonon coupling. Partial overlaps of exciton distributions between neighboring exciton states are found to be important for realizing the unique migration behavior by step-by-step transfer from the periphery to the core via multi-step exciton states. The same calculation method is applied to the exciton dynamics of a larger dendritic aggregate model, D127, modeled after the largest synthesized phenylacetylene dendrimer D127 in order to examine the dependencies of the exciton migration on the strength of the intermolecular interaction and the temperature of phonon bath. In the case of the relatively weak dipole-dipole coupling, exciton is not observed to migrate efficiently from the periphery to the core, while the largest exciton population is found to remain in the intermediate generations. This is ascribed to the fact that the thermal excitation to the higher exciton states significantly contributes to the exciton distribution in the equilibrium state when the weak intermolecular interaction reduces the energy difference between exciton states. This indicates that the intermolecular interaction is important not only for the overlaps of the exciton distributions between the exciton states, but also the relation between the exciton energy differences and thermal excitations, which spoil the distinct concentration of exciton distribution in the core generation. In the case of a low temperature, even if the intermolecular interaction is weak, the exciton population in the core region is found to be the largest in all the generations. This suggests that exciton tends to efficiently migrate from the periphery to the core when the temperature is sufficiently low. Implications of these theoretical results are discussed in relation to design of magneto-optical materials and other technological applications.
Linear Response Theory for Thermally Driven Quantum Open Systems
