Nonadiabatic Sunlight-harvesting

TL;DR

This paper investigates how intramolecular vibrational modes influence exciton coherence in light-harvesting complexes under natural sunlight, revealing that long-lived coherences are primarily induced by coherent laser sources rather than sunlight.

Contribution

It demonstrates that vibrational modes enhance exciton coherence but that long-lived coherences observed under laser excitation are not present under incoherent sunlight conditions.

Findings

Vibrational modes increase exciton coherence by up to tenfold.

Long-lived coherences are absent in thermal states under sunlight.

Non-classical correlations are induced only by coherent laser excitation.

Abstract

Experimental and theoretical evidence point out to the crucial role of specific resonant intramolecular vibrational modes in the interpretation of long-lived coherences observed in two-dimensional spectra of some natural and synthetic light harvesting complexes. For the natural situation of illumination by incoherent (sun)light, the relevance of these vibrations is analyzed here for light-harvesting vibronic prototype dimers. The detailed analysis of the density matrix dynamics reveals that the inclusion of the intramolecular vibrational modes reinforce up to one order of magnitude the exciton coherence and may increase the populations of lowest energy single exciton states, as well as populations and coherences in the site basis. In sharp contrast to the case of initial-state preparation by coherent (laser)light-sources, the initial thermal state of the local vibrational modes, as well as that of the anti-correlated mode, evolves devoid of non-classical correlations as confirmed by the absence of negative values of its phase-space quasi-probability distribution at all times. Therefore, not only the long-lived coherences observed in two-dimensional spectra are induced by the coherent character of pulsed laser sources, but it is unambiguously shown here that the non-classical character generally assigned to the anti-correlated vibrational mode also comes as the result of the preparation of the initial state by coherent pulsed laser sources.