Excitation relaxation in molecular chain and energy transfer at steady state

TL;DR

This paper analyzes excitation relaxation and energy transfer in a molecular chain coupled to a phonon bath, deriving closed-form rates, exploring steady-state dynamics, and examining efficiency dependence on external parameters with applications to photosynthesis.

Contribution

It provides analytical expressions for relaxation rates and energy transfer efficiency in molecular chains, highlighting the effects of external parameters and source power.

Findings

Relaxation rates are quadratic in chain length.

Energy transfer efficiency depends on external parameters.

High source power concentrates excitations in the lowest energy mode.

Abstract

We consider the reduced dynamics of a molecular chain weakly coupled to a phonon bath. With a small and constant inhomogeneity in the coupling, the excitation relaxation rates are obtained in closed form. They are dominated by transitions between exciton modes lying next to each other in the energy spectrum. The rates are quadratic in the number of sites in a long chain. Consequently, the evolution of site occupation numbers exhibits longer coherence lifetime for short chains only. When external source and sink are added, the rate equations of exciton occupation numbers are similar to those obtained earlier by Fr\"{o}hlich to explain energy storage and energy transfer in biological systems. There is a clear separation of time scale into a faster one pertaining to internal influence of the chain and phonon bath, and a slower one determined by external influence, such as the pumping rate of the source, the absorption rate of the sink and the rate of radiation loss. The energy transfer efficiency at steady state depends strongly on these external parameters, and is robust against a change in the internal parameters, such as temperature and inhomogeneity. Excitations are predicted to concentrate to the lowest energy mode when the source power is sufficiently high. In the site basis, this implies that when sustained by a high power source, a sink positioned at the center of the chain is more efficient in trapping energy than a sink placed at its end. Analytic expressions of energy transfer efficiency are obtained in the high power and low power source limit. Parameters of a photosynthetic system are used as examples to illustrate the results.