The idea that excitonic (electronic) coherences are of fundamental importance to natural photosynthesis gained popularity when slowly dephasing quantum beats (QBs) were observed in the two-dimensional electronic spectra of the Fenna-Matthews-Olson (FMO) complex at 77 K. These were assigned to superpositions of excitonic states, a controversial interpretation, as the strong chromophore-environment interactions in the complex suggest fast dephasing. Although it has been pointed out that vibrational motion produces similar spectral signatures, a concrete assignment of these oscillatory signals to distinct physical processes is still lacking. Here we revisit the coherence dynamics of the FMO complex using polarization-controlled two-dimensional electronic spectroscopy, supported by theoretical modelling. We show that the long-lived QBs are exclusively vibrational in origin, whereas the dephasing of the electronic coherences is completed within 240 fs even at 77 K. We further find that specific vibrational coherences are produced via vibronically coupled excited states. The presence of such states suggests that vibronic coupling is relevant for photosynthetic energy transfer.

Biology Centre CAS and Faculty of Science University of South Bohemia České Budějovice Czech Republic

Chemical Physics Lund University Lund Sweden

Department of Chemistry Columbia University New York NY USA

Fakultät für Chemie Technische Universität München Garching Germany

TOPTEC Institute of Plasma Physics Academy of Sciences of the Czech Republic Prague Czech Republic

University of Groningen Zernike Institute for Advanced Materials Groningen the Netherlands

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Quantum biology revisited