Mixed quantum-classical description of excitation energy transfer in supramolecular complexes: screening of the excitonic coupling.

A mixed quantum-classical description of excitation energy transfer (EET) in large chromophore complexes with significant conformational flexibility is improved by considering screening and local-field effects. To account for the environmentally induced modification of the excitonic coupling J(mn)(ε>1) between chromophore m and n, the Poisson-transition-charges-from-electrostatic-potential (Poisson-TrEsp) method is utilized. A parameterization scheme for the such derived screening/local field factors is introduced, which allows for their incorporation to a mixed quantum-classical description. The method is applied to the supramolecular complex P16 formed by sixteen pheophorbide-a molecules covalently linked to a butanediamine dendrimer and dissolved in ethanol. Data calculated using the novel parameterized screening method are compared to those obtained by alternative screening approaches. Averaging the screening factors in different ways may reproduce ensemble experiments on EET well, while the description of single molecule properties requires the consideration of individual screening factors.