Origin of Long-Lived Coherences in Light-Harvesting Complexes

Abstract

A vibronic exciton model is applied to explain the long-lived oscillatory features in the two-dimensional (2D) electronic spectra of the Fenna–Matthews–Olson (FMO) complex. Using experimentally determined parameters and uncorrelated site energy fluctuations, the model predicts oscillations with dephasing times of 1.3 ps at 77 K, which is in a good agreement with the experimental results. These long-lived oscillations originate from the coherent superposition of vibronic exciton states with dominant contributions from vibrational excitations on the same pigment. The oscillations obtain a large amplitude due to excitonic intensity borrowing, which gives transitions with strong vibronic character a significant intensity despite the small Huang–Rhys factor. Purely electronic coherences are found to decay on a 200 fs time scale.

Laser-Limited Signatures of Quantum Coherence

“…Such is the case for FMO, where an underdamped bath mode with an energy of 185 cm −1 matches the splitting between the lowest-energy pigment and one of its neighbors [14]. However, all molecular systems that fulfill this resonance criterion share the problem that most excited state coherences considerably overlap with contributions to the 2D spectra arising from vibrational wavepackets.…”

“…[14] and [15], despite the differences in excitonic and vibronic coupling. Moreover, the cyanine dimer considered here forms the perfect model system to experimentally address these predictions, given that the strong couplings lead to well-resolved crosspeaks, and that states A and B dominate the nonlinear response.…”

“…Over the last year, several independent publications alluded to vibronic coupling as an explanation for long-lived spectral beatings [14][15][16]. Coupling of electronic transitions to an underdamped vibrational mode may result in inter-exciton coherences with strong intrapigment character, that is, consisting of wavepackets in different vibrational levels on the same chromophore.…”

“…Due to the inherent correlation of these levels, such coherences will experience an extended dephasing time. For FMO, the Huang-Rhys factors are very small, suggesting that the corresponding photophysics is primarily electronic in nature, yet calculations using a vibronic exciton model nevertheless predicted an inter-exciton dephasing on the order of picoseconds [14]. As such, vibronic coupling acts as a mechanism to conserve electronic coherence.…”

Scalable High-Performance Algorithm for the Simulation of Exciton Dynamics. Application to the Light-Harvesting Complex II in the Presence of Resonant Vibrational Modes

“…6,[8][9][10] The coupling to the environment determines the transfer efficiency in LHCs 11,12 through the bath-correlation time of the phonon bath, [13][14][15] the shape of the continuum part of the spectral density, 9,16 and specific structures within the spectral density. 9,17 For the FMO complex, the superohmic character of the spectral density results in long-lasting electronic coherences despite a strong coupling to the environment. 9 To investigate to what extend specific modes in the spectral density affect transfer time-scales in larger light-harvesting networks calls for efficient methods to calculate the exciton dynamics for realistic spectral densities.…”