Disentangling Dynamical Quantum Coherences in the Fenna-Matthews-Olson Complex

TL;DR

This study uses two-dimensional electronic spectroscopy and modeling to show that quantum coherence in the Fenna-Matthews-Olson complex persists only at very low temperatures (~20 K) and decays rapidly above 150 K, clarifying its role in energy transfer.

Contribution

The paper provides direct measurements and analysis demonstrating that electronic quantum coherence in the FMO complex is negligible at physiological temperatures, challenging previous assumptions.

Findings

Quantum coherence persists up to 200-500 fs at 20 K.

Coherence decays rapidly above 150 K, becoming irrelevant.

Long-lived beatings are due to vibrational ground state coherences.

Abstract

In the primary step of light-harvesting, the energy of a photon is captured in antenna chlorophyll as an exciton. Its efficient conversion to stored chemical potential occurs in the special pair reaction center, which has to be reached by down-hill ultrafast excited state energy transport. The interaction between the chromophores leads to spatial delocalization and quantum coherence effects, the importance of which depends on the coupling between the chlorophylls in relation to the intensity of the fluctuations and reorganization dynamics of the protein matrix, or bath. The latter induce uncorrelated modulations of the site energies, and thus quantum decoherence, and localization of the exciton. Current consensus is that under physiological conditions quantum decoherence occurs on the 10 fs time scale, and quantum coherence plays little role for the observed picosecond energy transfer dynamics. In this work, we reaffirm this from a different point of view by finding that the true onset of electronic quantum coherence only occurs at extremely low temperatures of ~20 K. We have directly determined the exciton coherence times by two-dimensional electronic spectroscopy of the Fenna-Matthew-Olson complex over an extensive temperature range with a supporting theoretical modelling. At 20 K, electronic coherences persist out to 200 fs (close to the antenna) and marginally up to 500 fs at the reaction-center side. It decays markedly faster with modest increases in temperature to become irrelevant above 150 K. This temperature dependence also allows disentangling the previously reported long-lived beatings. We show that they result from mixing vibrational coherences in the electronic ground state ...