Interplay of disorder and delocalization in photosynthetic light harvesting

Abstract

Photosystems, the machines of photosynthesis, are highly complex and energetically disordered pigment-protein structures. Yet, they perform their function, be it highly efficient energy transfer and charge separation or the ability to switch between light-harvesting and photoprotective states, extremely well. In this opinioned review we describe the interplay of disorder and exciton delocalization in photosynthetic light harvesting. By discussing recent research advances on grounds of well-established concepts, we demonstrate that not only is the excitation delocalization a robust phenomenon, but that it in fact enables the light-harvesting function in the disordered environment.

Energy cascades in donor-acceptor exciton-polaritons observed by ultrafast two-dimensional white-light spectroscopy

“…Because of the broad electronic transitions, very large light-matter couplings are needed to make the bright states energetically well-separated from the molecular states. In contrast, it is well-known from studies on photosynthetic light-harvesting proteins that the energetic disorder of the individual chromophores and the couplings among them are important factors for determining the nature of the bright and dark eigenstates as well as their corresponding dynamics 30 – 32 . Indeed, the couplings are often comparable to the energetic disorder, creating energetically overlapping eigenstates that are not spectroscopically resolved but have very different wavefunctions.…”

Dynamic Disorder Drives Exciton Transfer in Tubular Chlorosomal Assemblies

“…The survival of green sulfur bacteria in extreme low-light condition relies on the efficiency of their light-harvesting antenna, chlorosomes, which are assemblies of hundreds of thousands bacteriochlorophylls (BChls) pigments. − Elucidating the design principles behind the efficient excitation energy transfer (EET) that takes place in chlorosomes will contribute to our understanding of photosynthesis − and may help to develop artificial light-harvesting − or other optoelectronic devices. − In spite of recent progress in theoretical − and experimental − understandings of exciton dynamics in chlorosome systems, a microscopic origin of efficient EET remains unclear.…”

“…41 This head−head rotational dynamics between neighboring pigments is expected to affect their electronic couplings. 11,42 Introducing such rotational disorder in the Frenkel Hamiltonian is found to induce delocalization of exciton states. Optically active states near the bottom of the exciton band are found to be composed of scattered domains of population density that are distributed over the whole tube.…”

Molecular Wires for Efficient Long-Distance Triplet Energy Transfer

“…Implementing directed long-distance ΤΕΤ on a single-molecule scale requires building molecular linkers (bridges) that connect TE donors (D) to acceptors (A). ,− It is known that the speed of bridge-mediated D-to-A singlet-exciton transfer (SET) may be improved by enhanced π-stacking interactions between nearest-neighbor molecular bridge (B) units linking D and A. , The π-stacking amplifies the nearest-neighbor SET couplings ( V SET ), leading to delocalized bridge singlet excitons (SE’s) that channel D-to-A SET. − There are many examples of molecular assemblies with enhanced π-stacking interactions. − …”

“…Similar design principles have been discussed in the context of SET in molecular nanocrystals, e.g., refs and , using results of theoretical studies of transport efficiency (for electrons, holes, or excitons) based on tight-binding (multisite) models. − ,,,,, The important parameters for transport in a model with identical sites and independently fluctuating site energies are the root-mean-square coupling ( V rms ) between nearest-neighbor sites (localized electronic states), the site reorganization energy λ, and the standard deviation of site energy σ E induced by dynamic disorder. Several studies show that delocalized eigenstates and coherent or quasi-coherent transport are possible when V rms ≥ σ E , λ/2 (e.g., see refs , − , and ). This condition is relevant to identical nearest-neighbor sites i, i + 1 and is derived from the assumption that each site has independent energy fluctuations.…”