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The mediated excitation energy transfer: Effects of bridge polarizability.

The observation of bridge-mediated excitation energy transfer (EET) has raised questions on the physical origin of such an effect. In this work, we studied the effect of bridge fragments in the Coulomb coupling, the major contribution to the electronic coupling in an EET process. For a series of ortho-phenyleneethynylene oligomers spaced donor-acceptors, we found that a large influence of the bridge fragment in EET coupling is through changes in the Coulomb couplings. Both enhancement and screening effects of the bridge were observed as the EET rates were modified by a factor of 0.3-23 with an intervening bridge in our calculations. The dependency of EET couplings on the orientation of transition dipoles of the donor and acceptor from quantum mechanical computations is very similar to that of a simple classical dielectric model. Our work shows that the bridge fragments can modify the Coulomb coupling with their polarizability by providing an optical dielectric medium between the donor and acceptor. In particular, when the transition dipoles of the donor and acceptor were longitudinal to a polarizable bridge, the EET rates were enhanced by one order of magnitude, as compared to the values of through-space models. Our results offer important insights into the design of efficient energy transfer systems.