Insights into photosynthetic energy transfer gained from free-energy structure: Coherent transport, incoherent hopping, and vibrational assistance revisited

TL;DR

This paper offers a simplified free-energy landscape perspective to understand energy transfer mechanisms in photosynthesis, clarifying the roles of coherence, incoherence, and vibrational assistance in highly efficient light-harvesting systems.

Contribution

It introduces a straightforward free-energy framework to analyze energy transfer, highlighting the near barrierless operation of photosynthetic systems and elucidating the conditions for vibrational assistance.

Findings

Photosynthetic energy transfer occurs in an almost barrierless regime.

The free-energy approach distinguishes between coherent and incoherent transfer.

Vibrational assistance is context-dependent and can be understood through energy landscape analysis.

Abstract

Giant strides in ultrashort laser pulse technology have enabled real-time observation of dynamical processes in complex molecular systems. Specifically, the discovery of oscillatory transients in the two-dimensional electronic spectra of photosynthetic systems stimulated a number of theoretical investigations exploring possible physical mechanisms of the remarkable quantum efficiency of light harvesting processes. However, the theories employed have reached a high degree of sophistication and have become complex, making it difficult to gain insights into microscopic processes and biologically significant questions. In this work, we revisit the elementary aspects of environment-induced fluctuations in the involved electronic energies and present a simple way to understand energy flow with the intuitive picture of relaxation in a funnel-type free-energy landscape. The presented free-energy description of energy transfer reveals that typical photosynthetic systems operate in an almost barrierless regime. The approach also provides insights into the distinction between coherent and incoherent energy transfer and criteria by which the necessity of the vibrational assistance is considered.