Explaining the Efficiency of Photosynthesis: Quantum Uncertainty or Classical Vibrations?

Abstract

Photosynthetic organisms are known to use a mechanism of vibrationally assisted exciton energy transfer to efficiently harvest energy from light. The importance of quantum effects in this mechanism is a long-standing topic of debate, which has traditionally focused on the role of excitonic coherences. Here, we address another recent claim: that the efficient energy transfer in the Fenna–Matthews–Olson complex relies on nuclear quantum uncertainty and would not function if the vibrations were classical. We present a counter-example to this claim, showing by trajectory-based simulations that a description in terms of quantum electrons and classical nuclei is indeed sufficient to describe the funneling of energy to the reaction center. We analyze and compare these findings to previous classical-nuclear approximations that predicted the absence of an energy funnel and conclude that the key difference and the reason for the discrepancy is the ability of the trajectories to properly account for Newton’s third law.

Coherence and Efficient Energy Transfer in Molecular Wires: Insights from Surface Hopping Simulations

“…We limit ourselves to 3 sites in this work. We have chosen only a few representative parameters out of several parameters that are available in the literature [30,31,[65][66][67][68] for FMO. We have listed parameters in Table 1 from Ref.…”

“…[24] This suggested the role of coherences towards EET in natural molecular wires. After this experimental observation, several theoretical studies [25][26][27][28][29][30][31] have been performed to extract the origin of these oscillations (electronic or vibrational) as well as if these oscillations (or coherences) are important for efficient energy transfer. Cao's group has studied this problem deeply, [32][33][34][35][36] showing the effect of different kinds of coherences on EET, elucidating the difference between dynamic coherence (oscillations) vs. static coherence.…”

“…We investigate only three sites in this work for simplicity, and the parameters for these three sites are chosen as those that describe part of the FMO complex. [25,30,31,38] Moreover, the three-site molecular wire [13] has been recognized to be sufficient for efficient EET. Though motivated by FMO parameters, our aim is not to study FMO but the mechanism of energy transfer in a molecular wire, with particular emphasis on coherences.…”

“…We have explored different parameters describing FMO in literature. [25,30,31,38] To test the applicability of A-FSSH to these parameter regimes, we begin by first benchmarking A-FSSH for two site problem by comparing its results to that obtained using HEOM. We show that long-lived oscillations can be captured without any quantum nuclear effects and these oscillations arise due to large diabatic coupling.…”

Coupled charge and energy transfer dynamics in light harvesting complexes from a hybrid hierarchical equations of motion approach

“…Approximate theories, namely modified-Redfield/generalized Förster theory [38][39][40] have been applied to study CT processes in lightharvesting complexes, 26,41,42 but these methods do not always accurately describe the EET dynamics in the absence of CT processes. 21 Semi-classical approaches have also been used to study exciton dynamics, [43][44][45][46][47][48][49] but these methods often break down for systems with large system-bath couplings, as is encountered in CT processes. In order to facilitate the study of reaction center and CT quenching processes in photosynthesis an accurate and computationally efficient method that can describe coupled EET and CT processes is needed.…”