Electronic excitation energy transfer (EET) is described theoretically for the chromophore complex P4 formed by a butanediamine dendrimer to which four pheophorbide-a molecules are covalently linked. To achieve a description with atomic resolution, and to account for the effect of an ethanol solvent, a mixed quantum–classical methodology is utilized. Room-temperature molecular dynamics simulations are used to describe the nuclear dynamics, and EET is accounted for in utilizing a mixed quantum–classical formulation of the transition rates. Therefore, the full quantum expression of the EET rates is given and the change to a mixed quantum–classical version is briefly explained. The description results in the calculation of transition rates which coincide rather satisfactory with available experimental data on P4. It is also shown that different assumptions of classical Förster theory are not valid for P4. The temporal behavior of EET deduced from the rate equations is confronted with that following from the solution of the time-dependent Schrödinger equation entering the mixed quantum–classical description of EET. From this we can conclude that EET in flexible chromophore complexes such as P4 can be rather satisfactory estimated by single transition rates. A correct description, however, is only achievable by using a sufficiently large set of rates that correspond to the various possible equilibrium configurations of the complex.
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