Localization and coherent dynamics of excitons in the two-dimensional optical spectrum of molecular J-aggregates

Long-lived quantum coherence has been experimentally observed in the Fenna-Matthews-Olson (FMO) light-harvesting complex. It is much debated which role thermal effects play and if the observed low-temperature behavior arises also at physiological temperature. To contribute to this debate we use molecular dynamics simulations to study the coupling between the protein environment and the vertical excitation energies of individual bacteriochlorophyll molecules in the FMO complex of the green sulphur bacterium Chlorobaculum tepidum. The so-called spectral densities, which account for the environmental influence on the excited state dynamics, are determined from temporal autocorrelation functions of the energy gaps between ground and first excited states of the individual pigments. Although the overall shape of the spectral density is found to be rather similar for all pigments, variations in their magnitude can be seen. Differences between the spectral densities for the pigments of the FMO monomer and FMO trimer are also presented.

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“…21,22 Interference of different quantum pathways have also been proposed as the origin of the long-lived coherences. 18,23 …”

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confidence: 99%

Theory and Simulation of the Environmental Effects on FMO Electronic Transitions

Olbrich

Strümpfer²,

Schulten³

et al. 2011

J. Phys. Chem. Lett.

220

351

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“…8,17,21,22 The effects of the particular choice of the distribution of static disorder on the coherence length as well as the role of diagonal and off-diagonal disorder in one-dimensional chains and in extended tubular aggregate systems have also been explored. [23][24][25][26] However, the influence of thermal noise on the coherence length is much more difficult to treat, and is generally either completely neglected or included only approximately. 8,9,27,28 Notable exceptions include the exact calculations of Ray and Makri in which they used the size of the bead from imaginary time path integral calculations as yet another estimate for the coherence length in LH2.…”

Section: Introductionmentioning

confidence: 99%

Equilibrium-reduced density matrix formulation: Influence of noise, disorder, and temperature on localization in excitonic systems

2012

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An exact method to compute the entire equilibrium reduced density matrix for systems characterized by a system-bath Hamiltonian is presented. The approach is based upon a stochastic unraveling of the influence functional that appears in the imaginary time path integral formalism of quantum statistical mechanics. This method is then applied to study the effects of thermal noise, static disorder, and temperature on the coherence length in excitonic systems. As representative examples of biased and unbiased systems, attention is focused on the well-characterized light harvesting complexes of FMO and LH2, respectively. Due to the bias, FMO is completely localized in the site basis at low temperatures, whereas LH2 is completely delocalized. In the latter, the presence of static disorder leads to a plateau in the coherence length at low temperature that becomes increasingly pronounced with increasing strength of the disorder. The introduction of noise, however, precludes this effect. In biased systems, it is shown that the environment may increase the coherence length, but only decrease that of unbiased systems. Finally it is emphasized that for typical values of the environmental parameters in light harvesting systems, the system and bath are entangled at equilibrium in the single excitation manifold. That is, the density matrix cannot be described as a product state as is often assumed, even at room temperature. The reduced density matrix of LH2 is shown to be in precise agreement with the steady state limit of previous exact quantum dynamics calculations. * Electronic address: jianshu@mit.edu 2

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“…In particular the application to biological light harvesting complexes has become a matter of increasing interest in the recent years. [19][20][21][22][23][24] Two-dimensional electronic spectroscopy has for example also been used to study the delocalization length 25 and interactions 26 in molecular J-aggregates. Since the dimer is the smallest aggregate (and allows a fairly detailed theoretical treatment) it has been extensively investigated.…”

Section: Introductionmentioning

confidence: 99%

Intermolecular torsional motion of a π-aggregated dimer probed by two-dimensional electronic spectroscopy

Seibt

Eisfeld

2012

The Journal of Chemical Physics

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The energetic splitting of the two exciton states of a molecular dimer depends strongly on the relative orientation of the monomers with respect to each other. The curvature of the corresponding adiabatic potential energy surfaces can lead to torsional motion of the monomers. It has been suggested recently that this torsional motion could provide a possible relaxation mechanism for the upper state which proceeds via a crossing of the two singly excited state potentials. Another, competing, relaxation mechanism is provided by coupling to the environment, leading to direct exciton relaxation. Here we examine theoretically the combined dynamics of torsional motion and excitonic relaxation for a π-aggregated dimer. Using two-dimensional (2D) spectroscopy, it is shown how torsional motion through a crossing of the adiabatic excitonic potential surfaces could be distinguished from direct relaxation. For the calculations a mixed quantum/classical approach is used, where the torsional motion is treated by an Ehrenfest type of equation, while the excitonic dynamics including dephasing and direct relaxation is described by a quantum master equation.

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Localization and coherent dynamics of excitons in the two-dimensional optical spectrum of molecular J-aggregates

Cited by 44 publications

References 54 publications

Theory and Simulation of the Environmental Effects on FMO Electronic Transitions

Theory and Simulation of the Environmental Effects on FMO Electronic Transitions

Equilibrium-reduced density matrix formulation: Influence of noise, disorder, and temperature on localization in excitonic systems

Intermolecular torsional motion of a π-aggregated dimer probed by two-dimensional electronic spectroscopy

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