Excitation energy transfer in donor-bridge-acceptor systems: a combined quantum-mechanical/classical analysis of the role of the bridge and the solvent.

The technical application of excitation energy transfer requires a fine control of the geometry of the system. This can be achieved by introducing a chemical bridge between the donor and acceptor moieties that can be tuned in its chemical properties and its length. In such donor-bridge-acceptor systems, however, the role of the bridge in enhancing or depleting the energy transfer efficiency is not easy to predict. Here we propose a computational strategy based on the combination of time-dependent density functional theory, polarizable molecular mechanics and continuum approaches. The resulting three-layer model when applied to the study of the energy transfer process in different porphyrin-based systems, each characterized by a specific donor/acceptor pair and various types of bridges, allows us to dissect the role of through-bond and through-space mechanisms and clarify their dependence on the nature and length of the bridge as well as on the presence of a solvent.