Long-range coherent energy transport in Photosystem II

TL;DR

This study uses advanced quantum simulations to analyze long-range energy transfer in Photosystem II, revealing robustness to initial conditions and the importance of vibrational effects for high efficiency.

Contribution

It introduces a non-Markovian quantum master equation approach to quantify coherence and pathways in Photosystem II energy transfer, highlighting vibrational effects and robustness.

Findings

Energy transfer is robust against initial excitation variations.

Vibrational coupling is crucial for high transfer efficiency.

Quantum and classical models show good agreement in overall dynamics.

Abstract

We simulate the long-range inter-complex electronic energy transfer in Photosystem II -- from the antenna complex, via a core complex, to the reaction center -- using a non-Markovian (ZOFE) quantum master equation description that allows us to quantify the electronic coherence involved in the energy transfer. We identify the pathways of the energy transfer in the network of coupled chromophores, using a description based on excitation probability currents. We investigate how the energy transfer depends on the initial excitation -- localized, coherent initial excitation versus delocalized, incoherent initial excitation -- and find that the energy transfer is remarkably robust with respect to such strong variations of the initial condition. To explore the importance of vibrationally enhanced transfer and to address the question of optimization in the system parameters, we vary the strength of the coupling between the electronic and the vibrational degrees of freedom. We find that the original parameters lie in a (broad) region that enables optimal transfer efficiency, and that the energy transfer appears to be very robust with respect to variations in the vibronic coupling. Nevertheless, vibrationally enhanced transfer appears to be crucial to obtain a high transfer efficiency. We compare our quantum simulation to a "classical" rate equation based on a modified-Redfield/generalized-F\"orster description that was previously used to simulate energy transfer dynamics in the entire Photosystem II complex, and find very good agreement between quantum and rate-equation simulation of the overall energy transfer dynamics.