Photosynthetic light harvesting: excitons and coherence

Abstract

Photosynthesis begins with light harvesting, where specialized pigmentprotein complexes transform sunlight into electronic excitations delivered to reaction centres to initiate charge separation. There is evidence that quantum coherence between electronic excited states plays a role in energy transfer. In this review, we discuss how quantum coherence manifests in photosynthetic light harvesting and its implications. We begin by examining the concept of an exciton, an excited electronic state delocalized over several spatially separated molecules, which is the most widely available signature of quantum coherence in light harvesting. We then discuss recent results concerning the possibility that quantum coherence between electronically excited states of donors and acceptors may give rise to a quantum coherent evolution of excitations, modifying the traditional incoherent picture of energy transfer. Key to this (partially) coherent energy transfer appears to be the structure of the environment, in particular the participation of non-equilibrium vibrational modes. We discuss the open questions and controversies regarding quantum coherent energy transfer and how these can be addressed using new experimental techniques.

Redesigning photosynthesis to sustainably meet global food and bioenergy demand

“…A photosynthetic membrane having this arrangement of energy-transducing complexes would have more than twice the energetic efficiency of current oxygenic photosynthesis (19,22) if the appropriate wavelengths of light were directed to the two now very distinct photosystems. Indeed, as our understanding of quantum coherence develops (22)(23)(24), it can be imagined that specific antenna could be "wired" to appropriate reaction centers. Of course other possibilities can be envisioned, but even before the powerful potential of synthetic biology is brought into play, we can initiate the research required to reengineer photosynthesis because, as illustrated in Fig.…”

Exciton Dynamics in Phthalocyanine Molecular Crystals

“…[9][10][11][12] Inspired by their photosynthetic homologous (e.g., bacteriochlorophyll chromophores), different representative examples of self-assembled molecular materials based on Pc building blocks have been synthesized to investigate their exciton transport properties. 13 In contrast to the field of photosynthesis, 2,14,15 in the field of organic electronics a hopping (incoherent) mechanism is generally assumed when the exciton/charge dynamics in Pcbased self-assembled materials is investigated. 4,16,17 However, this assumption is not completely justified in this class of Pc systems since they may present quite large excitonic couplings (20−100 meV) and small exciton-phonon couplings (reorganization energies can be as small as 37 meV).…”

The Hormesis of Thinking: A Deeper Quantum Thermodynamic Perspective?

“…Life may use thermal vibrations to pump coherence, which results in a phenomenon known as quantum beating. There is thus an optimal zone where just the right amount of "noise" can result in enhanced coherence and tunnelling due to stimulating particular vibrational modes in proteinsso called vibronic coupling [38][39][40][41]. Moreover many biomolecules exhibit quantum criticality, displaying properties somewhere between an insulator and a conductor, so potentially acting as charge carriers [42].…”

“…It is now also thought that photosynthesis utilises fundamental quantum principles, involving coherence and excitons to transfer energy efficiently [38,39,41] This is especially interesting as lysozyme appears to demonstrate a Fröhlich condensate; Fröhlich suggested that terahertz radiation could be absorbed by proteins leading to them behaving as a series of coupled oscillators generating a macroscopic quantum effect [49]. If combined with the concept that the electro-magnetic (EM) fields generated by mitochondria can lead to water order, this suggests an overall protection against decoherence within the cell [50].…”