Solar fuel production often starts with the energy from light being absorbed by an assembly of molecules; this electronic excitation is subsequently transferred to a suitable acceptor. For example, in photosynthesis, antenna complexes capture sunlight and direct the energy to reaction centres that then carry out the associated chemistry. In this Review, we describe the principles learned from studies of various natural antenna complexes and suggest how to elucidate strategies for designing light-harvesting systems. We envisage that such systems will be used for solar fuel production, to direct and regulate excitation energy flow using molecular organizations that facilitate feedback and control, or to transfer excitons over long distances. Also described are the notable properties of light-harvesting chromophores, spatial-energetic landscapes, the roles of excitonic states and quantum coherence, as well as how antennas are regulated and photoprotected.

Lessons from nature about solar light harvesting

The electronic excited states of molecular aggregates and their photophysical signatures have long fascinated spectroscopists and theoreticians alike since the advent of Frenkel exciton theory almost 90 years ago. The influence of molecular packing on basic optical probes like absorption and photoluminescence was originally worked out by Kasha for aggregates dominated by Coulombic intermolecular interactions, eventually leading to the classification of J- and H-aggregates. This review outlines advances made in understanding the relationship between aggregate structure and photophysics when vibronic coupling and intermolecular charge transfer are incorporated. An assortment of packing geometries is considered from the humble molecular dimer to more exotic structures including linear and bent aggregates, two-dimensional herringbone and “HJ” aggregates, and chiral aggregates. The interplay between long-range Coulomb coupling and short-range charge-transfer-mediated coupling strongly depends on the aggregate ...

Expanded Theory of H- and J-Molecular Aggregates: The Effects of Vibronic Coupling and Intermolecular Charge Transfer.

Femtosecond visible and infrared analogues of multiple-pulse nuclear magnetic resonance techniques provide novel snapshot probes into the structure and electronic and vibrational dynamics of complex molecular assemblies such as photosynthetic antennae, proteins, and hydrogen-bonded liquids. A classical-oscillator description of these spectroscopies in terms of interacting quasiparticles (rather than transitions among global eigenstates) is developed and sets the stage for designing new pulse sequences and inverting the multidimensional signals to yield molecular structures. Considerable computational advantages and a clear physical insight into the origin of the response and the relevant coherence sizes are provided by a real-space analysis of the underlying coherence-transfer pathways in Liouville space.

Multidimensional femtosecond correlation spectroscopies of electronic and vibrational excitations.

Electron Transfer Past and Future (R Marcus) Electron Transfer Reactions in Solution: A Historical Perspective (N Sutin) Electron Transfer--From Isolated Molecules to Biomolecules (M Bixon & J Jortner) Charge Transfer in Bichromophoric Molecules in the Gas Phase (D Levy) Long--Range Charge Separation in Solvent--Free Donor--Bridge--Acceptor Systems (B Wegewijs & J Verhoeven) Electron Transfer and Charge Separation in Clusters (C Dessent, et al) Control of Electron Transfer Kinetics: Models for Medium Reorganization and Donor--Acceptor Coupling (M Newton) Theories of Structure--Function Relationships for Bridge--Mediated Electron Transfer Reactions (S Skourtis & D Beratan) Fluctuations and Coherence in Long--Range and Multicenter Electron Transfer (G Iversen, et al) Lanczos Algorithm for Electron Transfer Rates in Solvents with Complex Spectral Densities (A Okada, et al) Spectroscopic Determination of Electron Transfer Barriers and Rate Constants (K Omberg, et al) Photoinduced Electron Transfer Within Donor--Spacer--Acceptor Molecular Assemblies Studied by Time--Resolved Microwave Conductivity (J Warman, et al) From Close Contact to Long--Range Intramolecular Electron Transfer (J Verhoeven) Photoinduced Electron Transfers Through sigma Bonds in Solution (N--C Yang, et al) Indexes

Electron transfer : from isolated molecules to biomolecules

We review recent theoretical and experimental advances in the elucidation of the dynamics of light harvesting in photosynthesis, focusing on recent theoretical developments in structure-based modeling of electronic excitations in photosynthetic complexes and critically examining theoretical models for excitation energy transfer. We then briefly describe two-dimensional electronic spectroscopy and its application to the study of photosynthetic complexes, in particular the Fenna-Matthews-Olson complex from green sulfur bacteria. This review emphasizes recent experimental observations of long-lasting quantum coherence in photosynthetic systems and the implications of quantum coherence in natural photosynthesis.

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Dynamics of light harvesting in photosynthesis.

We have studied excitation energy transfer in the photosynthetic antenna systems LH1 and LH2 of purple bacteria. Femtosecond pump−probe experiments are combined with computer simulations using the recently established crystal structure of these systems to assess the nature of excitation motion. We have measured the transient absorption kinetics and spectra of the LH1 and LH2 complexes in the temperature range from 4.2 to 296 K with femtosecond time resolution. The calculations based on the Pauli master equation disagreed with experimentally measured population and anisotropy kinetics, suggesting that the simple model of excitation hopping between bacteriochlorophyll a molecules is not a proper description for energy transport in LH1 and LH2. As a next step we have used the exciton theory to reproduce the transient absorption spectra of LH2, and we found that the coherence length of the exciton in B850 of LH2 1.5 ps after excitation of B800 is 4 ± 1.

Excitons in Photosynthetic Purple Bacteria: Wavelike Motion or Incoherent Hopping?

Transient absorption anisotropy of a polythiophene polymer in a thin film was studied on a femtosecond time scale. The anisotropy has a non-exponential decay on the sub-picosecond time scale, with a fastest component characterized by an ∼40 fs time constant. To simulate the anisotropy decay an incoherent energy migration model has been used. Comparison between the simulated and experimental kinetics enabled us to estimate the nearest-neighbor pair wise hopping time (τh=1±0.1 ps), the fraction of the interchain aggregates (∼10%) and the structural disorder of the polymer. The initial ∼30 fs anisotropy decay does not originate from incoherent hopping energy transfer but from some other relaxation among electronic excited states within a spectroscopic unit. (Less)

Ultrafast excitation transfer and trapping in a thin polymer film

Vibrational dynamics in the light-harvesting complexes of the photosynthetic bacterium Rhodobacter sphaeroides

In this work, we combined femtosecond transient absorption (population and anisotropy) spectroscopy with ab initio electronic structure methods to study excited-state deactivation pathways of the pyridine molecule in liquid solutions. Studies of the effects of excitation energy, deuteration and substitution, solvent polarity and viscosity, and protonation of pyridine were performed. The experiments reveal the dynamics of S_1(nπ^*) and S_2(ππ^*) excited states of pyridine. The photoexcitation of the S_2(ππ^*) state leads to formation of the prefulvenic form of pyridine, a valence isomer, in ∼2.2 ps, while nonradiative deactivation of the S_1(nπ^*) state occurs in 9−23 ps and is to a large extent due to intersystem crossing. Using ab initio methods at the CASSCF and time-dependent DFT levels, we calculated the potential energy surfaces of the ground and S_2(ππ^*) states. A conical intersection was found responsible for the ultrafast deactivation of the pyridine molecule.

Mirianas Chachisvilis

Papers

Excitons in Photosynthetic Purple Bacteria: Wavelike Motion or Incoherent Hopping?

Ultrafast excitation transfer and trapping in a thin polymer film

Vibrational dynamics in the light-harvesting complexes of the photosynthetic bacterium Rhodobacter sphaeroides

Femtosecond dynamics of pyridine in the condensed phase: valence isomerization by conical intersections

Electronic coherence in pseudo two-colour pump-probe spectroscopy