Interplay of Direct and Indirect Charge-Transfer Pathways in Donor-Bridge-Acceptor Systems

Charge transfer in donor-bridge-acceptor (DBA) structures typically takes place through the combination of donor-bridge and bridge-acceptor overlap integrals forming an effective, indirect electronic coupling between the donor (D) and acceptor (A) moieties. Here we examine the effects of an additional direct DA electronic coupling of charge-transfer processes in DBA systems with local interaction with thermal baths. First, using the exact Nakajima-Zwanzig master equation (NZME) for the reduced density matrix, we rigorously define probability currents as the coherent part of the NZME, thereby allowing us to quantify the contribution of the different electronic pathways (direct and indirect) to the charge-transfer dynamics. Focusing on two minimal DBA systems of three sites (V and Λ models) and adopting well-developed methods, we find that the interplay between different transfer pathways can be assessed by the McConnell formula in the weak system-bath coupling regime. We then demonstrate that the combination of indirect and direct donor-acceptor coupling either enhances or leads to a destructive quantum interference effect on charge-transport processes, depending on the energy landscape of the DBA system.