Singlet Excitation Energy Transfer Mediated by Local Exciton Bridges

Singlet excitation energy transfer (EET) generally occurs via a Forster mechanism and originates from direct electronic coupling between the donor (D) and acceptor (A). However, indirect EET (the so-called superexchange mechanism) can be crucial in some situations. In this study, we have explored the contributions of various linker types, linker lengths, and tunneling energies in D-linker-A molecular systems using quantum chemical methods [Kawatsu et al. J. Phys. Chem. A 2011, 115, 10814], obtaining conditions in which the super-exchange term emerges. To analyze the results in terms of physical parameters, a model Hamiltonian was developed and found to yield results qualitatively consistent with those from the quantum chemical calculations. In a model compound, the superexchange term can be up to 4 times larger than the direct (Forster) term when the energy of the excited state at a linker unit is close to the tunneling energy and when the interactions between these excited states are strong. The ratio of the indirect term to the direct term is greatest at a certain D-A distance because the number of multistep terms increases and the sizes of multistep indirect terms decay exponentially. Single-step indirect terms, which exhibit a polynomial decay, dominate the indirect term for larger D-A distances. The decay behavior with the distance is very different from that of the McConnell type donor-bridge-acceptor superexchange mechanism proposed for the charge transfer (CT). The result of pathway analysis showed that the exciton states mediate EET. The coupling between the local excited states is driven by the pseudo-Coulombic interaction, even in the superexchange terms.