Time-Dependent Atomistic View on the Electronic Relaxation in Light-Harvesting System II

Abstract

Aiming at a better understanding of the molecular details in light absorption during photosynthesis, spatial and temporal correlation functions as well as spectral densities have been determined. At the focus of the present study are the light-harvesting II complexes of the purple bacterium Rhodospirillum molischianum. The calculations are based on a time-dependent combination of molecular dynamics simulations and quantum chemistry methods. Using a 12 ps long trajectory, different quantum chemical methods have been compared to each other. Furthermore, several approaches to determine the couplings between the individual chromophores have been tested. Correlations between energy gap fluctuations of different individual pigments are analyzed but found to be negligible. From the energy gap fluctuations, spectral densities are extracted which serve as input for calculations of optical properties and exciton dynamics. To this end, the spectral densities are tested by determining the linear absorption of the complete two-ring system. One important difference from earlier studies is given by the severely extended length of the trajectory along which the quantum chemical calculations have been performed. Due to this extension, more accurate and reliable data have been obtained in the low frequency regime which is important in the dynamics of electronic relaxation.

Theoretical characterization of excitation energy transfer in chlorosome light-harvesting antennae from green sulfur bacteria

“…We use MD simulations to generate R(t) and TDDFT excited-state calculations to obtain ǫ(R), which is consistent with the models proposed previously [35][36][37][38][39] Thus, in contrast to many studies based on a quantum master equation, this approach can describe the system-bath coupling in complete atomistic detail.…”

“…A similar approach has been previously applied to small lightharvesting systems such as the FMO complex [37][38][39] and light-harvesting complexes II (LHII) of purple bacteria [35,36]. For a large number of BChls present in the chlorosome, it is computationally unfeasible to calculate site energies of all pigment molecules along an MD trajectory.…”

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

“…Recent molecular dynamics (MD) simulation studies on the Fenna-Mathews-Olsen (FMO) excitation energy transfer network, 11,12 the light harvesting complex 2 (LHII) photosynthetic light harvesting complex 13 and the reaction center complex, 14 however, show no significant correlation in site energy fluctuations after averaging, and similar findings have been reported from calculations for the PE545 15 light harvesting system. However, Kleinekathöfer and co-workers, for example, have found that in FMO, 11 there is evidence of correlations in fluctuations of site energy and inter site electronic couplings and electronic coupling-electronic coupling correlations that are more significant compared to the apparently uncorrelated site energy fluctuations.…”

“…Several theoretical groups [11][12][13][14][15] have begun exploring detailed microscopic simulation models looking for different types of environment driven correlated fluctuations in chromophore electronic properties. The ubiquitous finding from the various photosynthetic energy transfer systems studied so far is that fluctuations in site energies seem to be such that on average they are uncorrelated.…”

“…To develop an understanding of the influence of correlated fluctuations in site energy and electronic couplings between states on excitation energy transfer dynamics in relevant parameter ranges we use a realistic 3 state model of the FMO photosynthetic light harvesting complex 42 where, (14) Again, the energy units here are cm −1 . In this model, 13 is 5-10 times smaller than the other off-diagonal electronic couplings so the system part of the Hamiltonian is approximately block diagonal and the two state theoretical analysis 29 In future work we plan to make detailed comparisons with this generalized HSR theory to test the effects of the underlying approximations and benchmark the analysis. For the first three-state model we consider here that includes the effects of correlation in site energy and electronic coupling terms, the harmonic bath and bi-linear system-bath interaction Hamiltonian terms,Ĥ b andĤ s−b , respectively, take the following forms:…”