From Atomistic Modeling to Excitation Transfer and Two-Dimensional Spectra of the FMO Light-Harvesting Complex

Abstract

The experimental observation of long-lived quantum coherences in the Fenna-Matthews-Olson (FMO) light-harvesting complex at low temperatures has challenged general intuition in the field of complex molecular systems and provoked considerable theoretical effort in search for explanations. Here we report on room-temperature calculations of the excited-state dynamics in FMO using a combination of molecular dynamics simulations and electronic structure calculations. Thus we obtain trajectories for the Hamiltonian of this system which contains time-dependent vertical excitation energies of the individual bacteriochlorophyll molecules and their mutual electronic couplings. The distribution of energies and couplings are analyzed together with possible spatial correlations. It is found that in contrast to frequent assumptions the site energy distribution is non-Gaussian. In a subsequent step, averaged wave packet dynamics is used to determine the exciton dynamics in the system. Finally, with the time-dependent Hamiltonian linear and two-dimensional spectra are determined. The thus obtained linear absorption lineshape agrees well with experimental observation and is largely determined by the non-Gaussian site energy distribution. The two-dimensional spectra are in line with what one would expect by extrapolation of the experimental observations at lower temperatures and indicate almost total loss of long-lived coherences.

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.…”

“…In this approach, short MD trajectories are sampled from a full MD trajectory, and the excitonic system is propagated under a unitary evolution for each short MD trajectory. Here in analogy to previously developed models [38,39], it is assumed that the excitonic system does not affect the bath dynamics. The density matrix of the excitonic system ρ is obtained as an average of these unitary evolutions:…”

“…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.…”

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

“…3,4 For the study of the quantum mechanical evolution of excited states it has become commonplace to evaluate the excitonic coupling for a large number of structures, e.g. those deriving from a molecular dynamics simulation [5][6][7][8][9][10][11][12][13] or representing the interaction between chromophores in an amorphous system. [14][15][16][17][18][19][20] To perform these large scale simulations efficiently, to rationalize the observed properties and to design new materials it is important to identify the main components of the excitonic coupling and assess their relative importance.…”

Non-Markovianity during the quantum Zeno effect

“…Due to advancement in femptosecond based spectroscopy, there is currently great interest in the quantum processes which underpins quantum coherences and the exceptionally high efficiencies of energy transfer observed in lightharvesting systems [41][42][43][44][45]. Accordingly, we examine the occurrence of an environment induced quantum Zeno effect associated with dissipative photosynthetic sinks.…”