Electronic Energy Transfer in Condensed Phase Studied by a Polarizable QM/MM Model

We present a combined quantum mechanics and molecular mechanics (QM/MM)
method to study electronic energy transfer (EET) in condensed phases. The method introduces
a quantum mechanically based linear response (LR) scheme to describe both chromophore
electronic excitations and electronic couplings, while the environment is described through a
classical polarizable force field. Explicit treatment of the solvent electronic polarization is a key
aspect of the model, as this allows account of solvent screening effects in the coupling. The
method is tested on a model perylene diimide (PDI) dimer in water solution. We find an excellent
agreement between the QM/MM method and “exact” supermolecule calculations in which the
complete solute-solvent system is described at the QM level. In addition, the estimation of the
electronic coupling is shown to be very sensitive to the quality of the parameters used to describe
solvent polarization. Finally, we compare ensemble-averaged QM/MM results to the predictions
of the PCM-LR method, which is based on a continuum dielectric description of the solvent. We
find that both continuum and atomistic solvent models behave similarly in homogeneous media
such as water. Our findings demonstrate the potential of the method to investigate the role of
complex heterogeneous environments, e.g. proteins or nanostructured host materials, on EET.