Abstract
We present here a formally exact model for electronic transitions between an initial (donor) and final (acceptor) states linked by an intermediate (bridge) state. Our model incorporates a common set of vibrational modes that are coupled to the donor, bridge, and acceptor states and serves as a dissipative bath that destroys quantum coherence between the donor and acceptor. Taking the memory time of the bath as a free parameter, we calculate transition rates for a heuristic 3-state/2 mode Hamiltonian system parameterized to represent the energetics and couplings in a typical organic photovoltaic system. Our results indicate that if the memory time of the bath is of the order of 10-100 fs, a two-state kinetic (i.e., incoherent hopping) model will grossly underestimate overall transition rate.
Highlights
In a multi-state system, quantum transitions from an initial to a final state are rarely direct and often involve a coherent transfer between one or more intermediate states
Two limits of approximations are the super-exchange model whereby population is transferred from an initial state to an intermediate state or bridge before being transferred to the final state and the hopping model where population is transferred in a sequence of discrete steps with no quantum coherence between each subsequent step
We set about to construct a suitable super-exchange theory that accounts for a common vibronic bath coupled to all the electronic states involved in the system and accounts for the fact that the longer the population remains in the bridging state, quantum decoherence will effectively kill the coherent transfer between D → A
Summary
In a multi-state system, quantum transitions from an initial to a final state are rarely direct and often involve a coherent transfer between one or more intermediate states. For transitions involving the electronic states of molecular systems in which there is significant coupling to a large number of molecular vibrational degrees of freedom, one needs to properly account for the the effects of dissipation, memory, and coherence [1,2,3,4]. Entropic effects and randomness would lead to localized states and an incoherent hopping mechanism With this in mind, we set about to construct a suitable super-exchange theory that accounts for a common vibronic bath coupled to all the electronic states involved in the system and accounts for the fact that the longer the population remains in the bridging state, quantum decoherence will effectively kill the coherent transfer between D → A. We consider the interstate relaxation dynamics in a model for chargeseparation in an organic heterojunction system
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