Influence of environment induced correlated fluctuations in electronic coupling on coherent excitation energy transfer dynamics in model photosynthetic systems

Abstract

Two-dimensional photon-echo experiments indicate that excitation energy transfer between chromophores near the reaction center of the photosynthetic purple bacterium Rhodobacter sphaeroides occurs coherently with decoherence times of hundreds of femtoseconds, comparable to the energy transfer time scale in these systems. The original explanation of this observation suggested that correlated fluctuations in chromophore excitation energies, driven by large scale protein motions could result in long lived coherent energy transfer dynamics. However, no significant site energy correlation has been found in recent molecular dynamics simulations of several model light harvesting systems. Instead, there is evidence of correlated fluctuations in site energy-electronic coupling and electronic coupling-electronic coupling. The roles of these different types of correlations in excitation energy transfer dynamics are not yet thoroughly understood, though the effects of site energy correlations have been well studied. In this paper, we introduce several general models that can realistically describe the effects of various types of correlated fluctuations in chromophore properties and systematically study the behavior of these models using general methods for treating dissipative quantum dynamics in complex multi-chromophore systems. The effects of correlation between site energy and inter-site electronic couplings are explored in a two state model of excitation energy transfer between the accessory bacteriochlorophyll and bacteriopheophytin in a reaction center system and we find that these types of correlated fluctuations can enhance or suppress coherence and transfer rate simultaneously. In contrast, models for correlated fluctuations in chromophore excitation energies show enhanced coherent dynamics but necessarily show decrease in excitation energy transfer rate accompanying such coherence enhancement. Finally, for a three state model of the Fenna-Matthews-Olsen light harvesting complex, we explore the influence of including correlations in inter-chromophore couplings between different chromophore dimers that share a common chromophore. We find that the relative sign of the different correlations can have profound influence on decoherence time and energy transfer rate and can provide sensitive control of relaxation in these complex quantum dynamical open systems.

Limits and potentials of quantum chemical methods in modelling photosynthetic antennae

“…90,91 The investigation of these possible correlations through MD-QM/MM approaches can be divided in two parts. An interesting finding resulting from the spatial analysis is the important role played by the solvent.…”

Normal Mode Analysis of the Spectral Density of the Fenna–Matthews–Olson Light-Harvesting Protein: How the Protein Dissipates the Excess Energy of Excitons

“…40−45 Whereas in the simple models, correlations prolong the quantum beating and the energy transfer times, the more general models show that, in principle, it is possible to simultaneously increase the lifetime of coherences, and to decrease the energy transfer times. 44 Thus, there is a clear need for a microscopically calculated spectral density to clarify the possible role of correlations.…”

Importance and Nature of Short-Range Excitonic Interactions in Light Harvesting Complexes and Organic Semiconductors

“…[41][42][43] Any fluctuation of the excitonic couplings due to short range interactions coupled to thermal motion is expected to decrease the coherence and affect the exciton dynamics. 39,44,45 The aim of this paper is to evaluate the excitonic coupling between pairs of chromophores in three different LHCs and in two organic semiconductors (one amorphous and one crystalline), to achieve a broader understanding of the importance of short range interactions valid for most of the problems under current consideration. This large database of structures is used to assess the role of short range interactions with respect to the Coulombic one.…”

“…[35][36][37] Furthermore, short range interactions at finite temperature cause strong fluctuations in the excitonic coupling 3,13,36 which in some instances can influence or limit the energy transfer dynamics. [37][38][39] In photosynthesis, light absorption and efficient transport of the excitation energy is achieved by clusters of chromophores in proteins commonly called light harvesting complexes (LHCs). 2 Generally, the majority of chromophore pairs are sufficiently distant that their interaction is reasonably well described by expressions like (1).…”