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Photosynthesis is a highly optimized process from which valuable lessons can be learned about the operating principles in nature. Its primary steps involve energy transport operating near theoretical quantum limits in efficiency. Recently, extensive research was motivated by the hypothesis that nature used quantum coherences to direct energy transfer. This body of work, a cornerstone for the field of quantum biology, rests on the interpretation of small-amplitude oscillations in two-dimensional electronic spectra of photosynthetic complexes. This Review discusses recent work reexamining these claims and demonstrates that interexciton coherences are too short lived to have any functional significance in photosynthetic energy transfer. Instead, the observed long-lived coherences originate from impulsively excited vibrations, generally observed in femtosecond spectroscopy. These efforts, collectively, lead to a more detailed understanding of the quantum aspects of dissipation. Nature, rather than trying to avoid dissipation, exploits it via engineering of exciton-bath interaction to create efficient energy flow., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)

Photosynthetic proteins have evolved over billions of years so as to undergo optimal energy transfer to the sites of charge separation. On the basis of spectroscopically detected quantum coherences, it has been suggested that this energy transfer is partially wavelike. This conclusion depends critically on the assignment of the coherences to the evolution of excitonic superpositions. We demonstrate that, for a bacterial reaction center protein, long-lived coherent spectroscopic oscillations, which bear canonical signatures of excitonic superpositions, are essentially vibrational excited-state coherences shifted to the ground state of the chromophores. We show that the appearance of these coherences arises from a release of electronic energy during energy transfer. Our results establish how energy migrates on vibrationally hot chromophores in the reaction center, and they call for a reexamination of claims of quantum energy transfer in photosynthesis.

We describe a method to measure ultrafast protein structural changes using time-resolved wide-angle X-ray scattering at an X-ray free-electron laser. We demonstrated this approach using multiphoton excitation of the Blastochloris viridis photosynthetic reaction center, observing an ultrafast global conformational change that arises within picoseconds and precedes the propagation of heat through the protein. This provides direct structural evidence for a 'protein quake': the hypothesis that proteins rapidly dissipate energy through quake-like structural motions.

Two-dimensional electronic spectroscopy, and especially the polarization-controlled version of it, is the cutting edge technique for disentangling various types of coherences in molecules and molecular aggregates. In order to evaluate the electronic coherences, which often decay on a 100 fs time scale, the early population times have to be included in the analysis. However, signals in this region are typically plagued by several artifacts, especially in the unavoidable pulse overlap region. In this paper, we show that, in the case of polarization-controlled two-dimensional spectroscopy experiment, the early-time dynamics can be dominated by the "incorrect" pulse ordering signals. These signals can affect kinetics at positive times well beyond the pulse overlap region, especially when the "correct" pulse ordering signals are much weaker. Moreover, the "incorrect" pulse ordering contributions are oscillatory and overlap with the spectral signatures of energy transfer, which may lead to misinterpretation of "incorrect" pulse ordering signals for fast-decaying coherences. [ABSTRACT FROM AUTHOR]

Conformational disorder of conjugated polymers is an important issue to be understood and quantified. In this paper we present a new method to assess the chain conformation of conjugated polymers based on measurements of intrachain energy transfer. The chain conformation is modeled on the basis of monomer-monomer interactions, such as torsion, bending, and stretching of the connecting bond. The latter two potentials are assumed to be harmonic, while the torsional potential was calculated by density functional theory using B3-LYP functional with the SVP basis set. The energy transfer dynamics of excitons on these chains are quantitatively simulated using Förster-type line-dipole energy transfer. This allows us to compare the simulated ground state conformation of single polymer chains to ultrafast depolarization experiments of poly [3-(2,5-dioctylphenyl)thiophene] in solution. We identify torsional rotation as the main contributor to conformational disorder and find that this disorder is mainly controlled by the energy difference between syn and anti bonds. [ABSTRACT FROM AUTHOR]

We study exciton migration in low molecular weight poly[3-(2,5-dioctylphenyl)thiophene] in dilute solution by means of ultrafast spectroscopy and Monte Carlo simulations of resonance energy transfer using the line-dipole Förster approach. The model includes the build-up of polymer chains, site-selective exciton generation, and diffusion through incoherent energy transfer. Time-resolved, ensemble-averaged experimental data are reproduced, namely photoluminescence spectral migration and stimulated emission anisotropy decays measured by streak camera and femtosecond transient absorption spectroscopy under site-selective excitation conditions. Importantly, the relatively simple line-dipole Förster-type approach beyond the point-dipole approximation reproduces both experiments quantitatively. Since explicit chain conformations are used in the model, the simulations yield a descriptive microscopic picture of exciton migration. The effective conjugation length (lseg=2.9 nm, 7.4 monomer units) and the disorder of the chains (Ω=0.8) are yielded as the only fitting parameters. We find an extra component that is not covered by our fits in anisotropy decays at early times for high excitation energies. This is interpreted within the context that the effective conjugation is limited by conformational disorder. [ABSTRACT FROM AUTHOR]