Correlated Reaction Coordinate Motion Produces Nonadditive Rate Enhancement for Electron and Energy Transfer in Multiacceptor Structures

Molecular structures with multiple donor, bridge, or acceptor units can display quantum interference effects that influence electron and energy transfer (ET and EnT) rates. Recent experiments found a 4-5-fold increase in ET rates for donor-acceptor structures with two acceptors compared to one. This result is surprising: simple classical or quantum analysis suggests a factor of 2 rate enhancement. We analyze the coupling interactions in multi-acceptor systems and find that rate enhancements beyond additive effects arise from acceptor-acceptor interactions that (1) shift the reaction free energy, (2) change the donor-acceptor couplings, and (3) alter the reaction-coordinate motion. Consideration of these effects explains the observed rates in multiacceptor systems and suggests strategies to tailor energy and electron transfer kinetics.