Light-induced Electron Transfer Reactions Research Articles

Factors that Impact Photochemical Cage Escape Yields.

The utilization of visible light to mediate chemical reactions in fluid solutions has applications that range from solar fuel production to medicine and organic synthesis. These reactions are typically initiated by electron transfer between a photoexcited dye molecule (a photosensitizer) and a redox-active quencher to yield radical pairs that are intimately associated within a solvent cage. Many of these radicals undergo rapid thermodynamically favored "geminate" recombination and do not diffuse out of the solvent cage that surrounds them. Those that do escape the cage are useful reagents that may undergo subsequent reactions important to the above-mentioned applications. The cage escape process and the factors that determine the yields remain poorly understood despite decades of research motivated by their practical and fundamental importance. Herein, state-of-the-art research on light-induced electron transfer and cage escape that has appeared since the seminal 1972 review by J. P. Lorand entitled "The Cage Effect" is reviewed. This review also provides some background for those new to the field and discusses the cage escape process of both homolytic bond photodissociation and bimolecular light induced electron transfer reactions. The review concludes with some key goals and directions for future research that promise to elevate this very vibrant field to even greater heights.

Isolation and characterization of trimeric and monomeric PSI cores from Acaryochloris marina MBIC11017.

Photosystem I (PSI) catalyzes light-induced electron-transfer reactions and has been observed to exhibit various oligomeric states and different energy levels of chlorophylls (Chls) in response to oligomerization. However, the biochemical and spectroscopic properties of a PSI monomer containing Chls d are not well understood. In this study, we successfully isolated and characterized PSI monomers from the cyanobacterium Acaryochloris marina MBIC11017, and compared their properties with those of the A. marina PSI trimer. The PSI trimers and monomers were prepared using trehalose density gradient centrifugation after anion-exchange and hydrophobic interaction chromatography. The polypeptide composition of the PSI monomer was found to be consistent with that of the PSI trimer. The absorption spectrum of the PSI monomer showed the Qy band of Chl d at 704nm, which was blue-shifted from the peak at 707nm observed in the PSI-trimer spectrum. The fluorescence-emission spectrum of the PSI monomer measured at 77K exhibited a peak at 730nm without a broad shoulder in the range of 745-780nm, which was clearly observed in the PSI-trimer spectrum. These spectroscopic properties of the A. marina PSI trimer and monomer suggest different formations of low-energy Chls d between the two types of PSI cores. Based on these findings, we discuss the location of low-energy Chls d in A. marina PSIs.

Special Section on “Next Generation Solar Cells”

Molecular photovoltaic devices, in a broad sense, are devices made by using molecules or molecular materials that are capable of converting sunlight into electrical current and voltage. Since the use of low molecular weight organic molecules by Tang,[1] the sensitization of TiO2 with dyes coupled to the use of iodine/iodide red/ox couple by Grätzel and O'Regan,[2] the fabrication of bulk-heterojunction polymer/fullerene-based films by Sariciftci et al.,[3] to the latest breakthrough using hybrid lead based perovskite semiconductor materials by Miyasaka and collaborators,[4] there has been an amazing journey harnessing solar power. The dream for cheap, globally available solar energy is closer than ever with a great deal of excellent research being done in laboratories worldwide. This special collection of research articles in Solar RRL presents a flavor of the research done recently in hot research topics from dye sensitized solar cells, organic solar cells, and perovskite solar cells and serves as an example about how proactive the research in molecular photovoltaic devices is. Photochromic thin films based on Grätzel solar cells represent the ultimate frontier in the application of transparent devices that hold the promise for efficient translucid solar cells for applications in building photovoltaics. The dye design still holds the key for the optimization of the solar cell efficiency, when the primary role of the solar cell is not just the efficiency but also its transparency and its operational stability. In organic solar cells, the design of molecules and organic materials with alike goal, transparency, is also key. Moreover, taking into account the general low open circuit voltage in organic solar cells, it is also crucial to understand and reduce the possible pathways for voltage losses, which leads to better solar cell efficiencies. In fact, the efficiency of organic solar cells has exponentially increased during the last few years due to the synthesis and use of non-fullerene electron acceptor molecules and molecular materials. Further research on non-fullerene electron acceptors, as well as organic doping of semiconductors, will remain a hot topic in the near future with the challenge to match the current efficiency of their perovskite-based counterparts. Perovskite semiconductor materials for photovoltaic applications have been, without doubt, a major achievement in recent research. Applications ranging from energy conversion devices, artificial lighting, sensors and photonic applications, have rapidly appeared in the scientific literature since 2009. Indeed, scientific challenges remain, such as the passivation of defects to boost the solar cell efficiencies and the replacement of lead by a less harmful element, but the gap between laboratory and industrial deployment is closing rapidly. For their industrial application, better materials for grids, low-cost manufacture contacts, and treatments to increase the device stability are goals that will be the focus of present and future intense research. Emilio Palomares studied Biology at the UVEG. After graduating, he joined Prof. Hermenegildo García's group at the UPV where he got his PhD. In 2009, he was awarded an ERC Starting Grant to work on quantum dots for energy conversion devices and an ERC PoC in 2015. In 2019, he was awarded Energy & Environmental Solutions International Chair (E2S) by the Université de Pau et des Pays de l'Adour (UPPA). Later in 2020, he was elected ICIQ Director for the next 5 years. His research is focused on several aspects of light induced electron transfer reactions in supramolecular structures and nanostructured inorganic materials, which has evolved towards the control and improvement of the reactions that govern the efficiency on devices such as molecular solar cells, the creation of new hybrid nanomaterials for hydrogen production, and molecular based sensing devices to detect toxic substance in the environment. He is also involved in promoting science and education in society through chemistry workshops for primary and secondary schools.

The electronic structure of the metal-organic interface of isolated ligand coated gold nanoparticles.

Light induced electron transfer reactions of molecules on the surface of noble metal nanoparticles (NPs) depend significantly on the electronic properties of the metal–organic interface. Hybridized metal–molecule states and dipoles at the interface alter the work function and facilitate or hinder electron transfer between the NPs and ligand. X-ray photoelectron spectroscopy (XPS) measurements of isolated AuNPs coated with thiolated ligands in a vacuum have been performed as a function of photon energy, and the depth dependent information of the metal–organic interface has been obtained. The role of surface dipoles in the XPS measurements of isolated ligand coated NPs is discussed and the binding energy of the Au 4f states is shifted by around 0.8 eV in the outer atomic layers of 4-nitrothiophenol coated AuNPs, facilitating electron transport towards the molecules. Moreover, the influence of the interface dipole depends significantly on the adsorbed ligand molecules. The present study paves the way towards the engineering of the electronic properties of the nanoparticle surface, which is of utmost importance for the application of plasmonic nanoparticles in the fields of heterogeneous catalysis and solar energy conversion.

Structure, Function, and Variations of the Photosystem I-Antenna Supercomplex from Different Photosynthetic Organisms.

Photosystem I (PSI) is a protein complex functioning in light-induced charge separation, electron transfer, and reduction reactions of ferredoxin in photosynthesis, which finally results in the reduction of NAD(P)- to NAD(P)H required for the fixation of carbon dioxide. In eukaryotic algae, PSI is associated with light-harvesting complex I (LHCI) subunits, forming a PSI-LHCI supercomplex. LHCI harvests and transfers light energy to the PSI core, where charge separation and electron transfer reactions occur. During the course of evolution, the number and sequences of protein subunits and the pigments they bind in LHCI change dramatically depending on the species of organisms, which is a result of adaptation of organisms to various light environments. In this chapter, I will describe the structure of various PSI-LHCI supercomplexes from different organisms solved so far either by X-ray crystallography or by cryo-electron microscopy, with emphasis on the differences in the number, structures, and association patterns of LHCI subunits associated with the PSI core found in different organisms.

D‐Band EPR and ENDOR Spectroscopy of 15N‐Labeled Photosystem I

For billions of years, nature has optimized the photosynthetic machinery that converts light energy into chemical energy. Key primary reactions of photosynthesis occur in large membrane protein–cofactor complexes. The light-induced sequential electron transfer reactions occur through a chain of donor/acceptor cofactors embedded in the protein matrix resulting in a long-lived transmembrane charge-separated state. EPR is the method of choice to study electron transfer and the interaction of protein environment with redox-active cofactors. However, the spectra of organic cofactor radicals typically are not fully resolved and severely overlap at conventional X-band EPR. Even at Q-band EPR, this overlap is present and often a serious problem. As a result, there is a large variation of the reported EPR data and limited understanding of electronic structures of several redox-active cofactors. These serious problems can often be overcome by the excellent spectral resolution provided by high-frequency EPR (HF EPR). Here, we study the electronic structure of the primary electron donor P700 and the secondary electron acceptor A1 of Photosystem I (PSI) using 130 GHz (D-band) EPR and Electron–Nuclear-Double-Resonance (ENDOR) spectroscopy. PSI was isotopically labeled with 15N (I = ½) to avoid quadrupolar interactions in the most abundant nitrogen isotope 14N (I = 1) and simplify the ENDOR spectra. ENDOR spectroscopy is central for determining the hyperfine coupling of nitrogen atoms of the two chlorophyll molecules comprising oxidized P700 and the involvement of protein nitrogen atoms with reduced A1. While HF ENDOR of A1− allows identification of two nitrogen atoms, HF ENDOR of P700+ still does not permit unique assignment of the recorded hyperfine couplings.

Antagonistic Effects of Point Mutations on Charge Recombination and a New View of Primary Charge Separation in Photosynthetic Proteins

Light-induced electron-transfer reactions were investigated in wild-type and three mutant Rhodobacter sphaeroides reaction centers with the secondary electron acceptor (ubiquinone QA) either removed or permanently reduced. Under such conditions, charge separation between the primary electron donor (bacteriochlorophyll dimer, P) and the electron acceptor (bacteriopheophytin, HA) was followed by P+HA– → PHA charge recombination. Two reaction centers were used that had different single amino-acid mutations that brought about either a 3-fold acceleration in charge recombination compared to that in the wild-type protein, or a 3-fold deceleration. In a third mutant in which the two single amino-acid mutations were combined, charge recombination was similar to that in the wild type. In all cases, data from transient absorption measurements were analyzed using similar models. The modeling included the energetic relaxation of the charge-separated states caused by protein dynamics and evidenced the appearance of an intermediate charge-separated state, P+BA–, with BA being the bacteriochlorophyll located between P and HA. In all cases, mixing of the states P+BA– and P+HA– was observed and explained in terms of electron delocalization over BA and HA. This delocalization, together with picosecond protein relaxation, underlies a new view of primary charge separation in photosynthesis.

Interprotein electron transfer biohybrid system for photocatalytic H2 production.

Worldwide there is a large research investment in developing solar fuel systems as clean and sustainable sources of energy. The fundamental mechanisms of natural photosynthesis can provide a source of inspiration for these studies. Photosynthetic reaction center (RC) proteins capture and convert light energy into chemical energy that is ultimately used to drive oxygenic water-splitting and carbon fixation. For the light energy to be used, the RC communicates with other donor/acceptor components via a sophisticated electron transfer scheme that includes electron transfer reactions between soluble and membrane bound proteins. Herein, we reengineer an inherent interprotein electron transfer pathway in a natural photosynthetic system to make it photocatalytic for aqueous H2 production. The native electron shuttle protein ferredoxin (Fd) is used as a scaffold for binding of a ruthenium photosensitizer and H2 catalytic function is imparted to its partner protein, ferredoxin-NADP+-reductase (FNR), by attachment of cobaloxime molecules. We find that this 2-protein biohybrid system produces H2 in aqueous solutions via light-induced interprotein electron transfer reactions (TON > 2500 H2/FNR), providing insight about using native protein-protein interactions as a method for fuel generation.

Kinetic pathway for interfacial electron transfer from a semiconductor to a molecule.

Molecular approaches to solar-energy conversion require a kinetic optimization of light-induced electron-transfer reactions. At molecular-semiconductor interfaces, this optimization has previously been accomplished through control of the distance between the semiconductor donor and the molecular acceptor and/or the free energy that accompanies electron transfer. Here we show that a kinetic pathway for electron transfer from a semiconductor to a molecular acceptor also exists and provides an alternative method for the control of interfacial kinetics. The pathway was identified by the rational design of molecules in which the distance and the driving force were held near parity and only the geometric torsion about a xylyl- or phenylthiophene bridge was varied. Electronic coupling through the phenyl bridge was a factor of ten greater than that through the xylyl bridge. Comparative studies revealed a significant bridge dependence for electron transfer that could not be rationalized by a change in distance or driving force. Instead, the data indicate an interfacial electron-transfer pathway that utilizes the aromatic bridge orbitals.

Evaluation of bis-cyclometalated alkynylgold(iii) sensitizers for water photoreduction to hydrogen.

Well-defined gold sensitizers for hydrogen production from water remain extremely rare despite decades of interest, and are currently limited to systems based on ruthenium, iridium or platinum complexes. This report details the synthesis and characterization of a series of neutral cyclometalated gold(iii) complexes of the type [(RC^N^CR)Au(C[triple bond, length as m-dash]C-R')] (R = H or tert-butyl group; R' = aryl groups) that have been found to be good candidates to function as harvesting materials in light-induced electron transfer reactions. We established the efficacy of systems with these gold(iii) complexes as photosensitizers (PSs) in the production of renewable hydrogen in the presence of [Co(2,2'-bipyridine)3]Cl2 or [Rh(4,4'-di-tert-butyl-2,2'-bipyridine)3](PF6)3 as a H2-evolved catalyst and triethanolamine (TEOA) as a sacrificial electron donor in acetone-water solution. All complexes are active, and there is a more than threefold increase over other candidates in photocatalytic H2 generation activity. Under the optimal reaction conditions, hydrogen evolution took place through a photochemical route with the highest efficiency and with a turnover number (TON) of up to 1441.5 relative to the sensitizer over 24 hours. In the initial photochemical path, the reductive quenching of the excited gold(iii) complex by TEOA due to the latter's greater concentration in the system followed by electron transfer to the catalyst species is proposed to be the dominant mechanism. A photo-to-H2 quantum yield of approximately 13.7% was attained when illuminated with monochromatic light of 400 nm. Such gold(iii) complexes have demonstrated significant utility in solar-to-hydrogen reactions and thus represent a new effective class of light-harvesting materials. These results open possibilities for pursuing more efficient photosensitizers featuring gold(iii) complexes in photocatalytic solar energy conversion.

Ground and excited-state electron-transfer reactions of a few (anthracen-9-yl)methanamines: a comparative study

Comparative study of chemical and light induced ele ctron-transfer reactions of some (anthracen- 9-yl)methanamines is reported. Ceric ammonium nitrate was used to initiate chemically-induced electron-transfer reactions of anthracenemethanamin es. Nitration at the 2-position followed by oxidation was the major pathway observed in the rea ction with CAN while electron-transfer mediated C-N bond cleavage followed by dimerization was the major pathway in photochemical reactions.

Requirement for Asn298 on D1 protein for oxygen evolution: analyses by exhaustive amino acid substitution in the green alga Chlamydomonas reinhardtii.

PSII generates strong oxidants used for water oxidation. The secondary electron donor, Y(Z), is Tyr161 on PSII reaction center D1 protein and mediates electron transfer from the oxygen-evolving Mn(4)CaO(5) cluster to the primary electron donor, P680. The latest PSII crystal structure revealed the presence of a hydrogen bond network around Y(Z), which is anticipated to play important roles in the electron and proton transfer reactions. Y(Z) forms a hydrogen bond with His190 which in turn forms a hydrogen bond with Asn298 on D1 protein. Although functional roles of Y(Z) and His190 have already been characterized, little is known about the functional role of Asn298. Here we have generated 19 mutants from a green alga Chlamydomonas reinhardtii, in which the Asn298 has been substituted by each of the other 19 amino acid residues. All mutants showed significantly impaired or no photosynthetic growth. Seven mutants capable of photosynthetic growth showed oxygen-evolving activity although at a significantly reduced rate. Interestingly the oxygen-evolving activity of these mutants was markedly photosensitive. The 19 mutants accumulated PSII at variable levels and showed a light-induced electron transfer reaction from 1,5-diphenylcarbazide (DPC) to 2,6-dichlorophenolindophenol (DCIP), suggesting that Asn298 is important for the function and photoprotection of the Mn(4)CaO(5) cluster.

Improving the efficiency of water splitting in dye-sensitized solar cells by using a biomimetic electron transfer mediator

Photoelectrochemical water splitting directly converts solar energy to chemical energy stored in hydrogen, a high energy density fuel. Although water splitting using semiconductor photoelectrodes has been studied for more than 40 years, it has only recently been demonstrated using dye-sensitized electrodes. The quantum yield for water splitting in these dye-based systems has, so far, been very low because the charge recombination reaction is faster than the catalytic four-electron oxidation of water to oxygen. We show here that the quantum yield is more than doubled by incorporating an electron transfer mediator that is mimetic of the tyrosine-histidine mediator in Photosystem II. The mediator molecule is covalently bound to the water oxidation catalyst, a colloidal iridium oxide particle, and is coadsorbed onto a porous titanium dioxide electrode with a Ruthenium polypyridyl sensitizer. As in the natural photosynthetic system, this molecule mediates electron transfer between a relatively slow metal oxide catalyst that oxidizes water on the millisecond timescale and a dye molecule that is oxidized in a fast light-induced electron transfer reaction. The presence of the mediator molecule in the system results in photoelectrochemical water splitting with an internal quantum efficiency of approximately 2.3% using blue light.

Mechanism of Photosynthetic Water-splitting Based on the High Resolution Structure of Photosystem II

Photosystem II (PSII) is a membrane-protein complex consisting of 17 trans-membrane subunits and 3 peripheral, extrinsic subunits with a total molecular mass of 350 kDa for a monomer. PSII performs a series of light-induced electron transfer reactions, leading to the conversion of light energy into biologically useful chemical energy, coupled with this is the splitting of water and generation of molecular oxygen. Both the chemical energy converted and molecular oxygen generated by PSII are indispensible for sustaining life on the earth; thus PSII is an extremely important protein complex. We have succeeded in crystallizing the PSII complex at a resolution of 1.9 Å, and analyzed its structure. Here we describe the structure of PSII, in particular the Mn4CaO5-cluster, which is the catalytic center of water-splitting, analyzed at the atomic resolution. Based on these, we discuss the possible mechanisms of light-induced water-splitting and its implications in artificial photosynthesis.

Artificial Light-Harvesting Arrays: Electronic Energy Migration and Trapping on a Sphere and between Spheres

A sophisticated model of the natural light-harvesting antenna has been devised by decorating a C(60) hexa-adduct with ten yellow and two blue boron dipyrromethene (Bodipy) dyes in such a way that the dyes retain their individuality and assist solubility of the fullerene. Unusually, the fullerene core is a poor electron acceptor and does not enter into light-induced electron-transfer reactions with the appended dyes, but ineffective electronic energy transfer from the excited-state dye to the C(60) residue competes with fluorescence from the yellow dye. Intraparticle electronic energy transfer from yellow to blue dyes can be followed by steady-state and time-resolved fluorescence spectroscopy and by excitation spectra for isolated C(60) nanoparticles dissolved in dioxane at 293 K and at 77 K. The decorated particles can be loaded into polymer films by spin coating from solution. In the dried film, efficient energy transfer occurs such that photons absorbed by the yellow dye are emitted by the blue dye. Films can also be prepared to contain C(60) nanoparticles loaded with the yellow Bodipy dye but lacking the blue dye and, under these circumstances, electronic energy migration occurs between yellow dyes appended to the same nanoparticle and, at higher loading, to dye molecules on nearby particles. Doping these latter polymer films with the mixed-dye nanoparticle coalesces these multifarious processes in a single system. Thus, long-range energy migration occurs among yellow dyes attached to different particles before trapping at a blue dye. In this respect, the film resembles the natural photosynthetic light-harvesting complexes, albeit at much reduced efficacy. The decorated nanoparticles sensitize amorphous silicon photocells.

Kinetics of phyllosemiquinone oxidation in the Photosystem I reaction centre of Acaryochloris marina

Light-induced electron transfer reactions in the chlorophyll a/d-binding Photosystem I reaction centre of Acaryochloris marina were investigated in whole cells by pump-probe optical spectroscopy with a temporal resolution of ~5ns at room temperature. It is shown that phyllosemiquinone, the secondary electron transfer acceptor anion, is oxidised with bi-phasic kinetics characterised by lifetimes of 88±6ns and 345±10ns. These lifetimes, particularly the former, are significantly slower than those reported for chlorophyll a-binding Photosystem I, which typically range in the 5–30ns and 200–300ns intervals. The possible mechanism of electron transfer reactions in the chlorophyll a/d-binding Photosystem I and the slower oxidation kinetics of the secondary acceptors are discussed.

ChemInform Abstract: Artificial Photosynthetic Model Systems Using Light-Induced Electron Transfer Reactions in Catalytic and Biocatalytic Assemblies

Artificial photosynthetic devices provide a means for the use of solar light in generating fuel materials and valuable chemicals and for the removal of environmental pollutants. Control of photosensitized electron transfer reactions and development of catalysts for utilizations of the intermediate electron transfer products are essential aspects in designing artificial photosynthetic systems. Homogeneous and heterogeneous catalysts as well as biocatalysts (enzymes and cofactors) can be coupled to photochemically induced electron transfer reactions and effect photosynthetic transformations such as hydrogen evolution, CO2-fixation, hydrogenation, and hydroformylation processes. The progress in tailoring artificial photosynthetic devices in the context of thermodynamic and kinetic limitations of such systems is described. Integrated systems, where catalytic performance and control of electron transfer reactions which occur in organized assemblies are specifically emphasized.

Bidirectional Electron Transfer in the Reaction Centre of Photosystem I

In the past decade light-induced electron transfer reactions in photosystem I have been the subject of intensive investigations that have led to the elucidation of some unique characteristics, the most striking of which is the existence of two parallel, functional, redox active cofactors chains. This process is generally referred to as bidirectional electron transfer. Here we present a review of the principal evidences that have led to the uncovering of bidirectionality in the reaction centre of photosystem I. A special focus is dedicated to the results obtained combining time-resolved spectroscopic techniques, either difference absorption or electron paramagnetic resonance, with molecular genetics, which allows, through modification of the binding of redox active cofactors with the reaction centre subunits, an effect on their physical-chemical properties.

Computational Analysis of the Light-induced Electron Transfer Reactions in Photosynthetic Reaction Centers

Atmospheric oxygen is generated by water oxidation at the MnCa cluster of the protein-pigment complex, photosystem II located in the thylakoid membrane. At the heart of photosynthetic reaction centers (RCs) are pairs of chlorophylla, P680 in photosystem II and P700 in photosystem I of cyanobacteria, algae, or plants, and a pair of bacteriochlorophylla, P870 in purple bacterial RCs. These pairs differ greatly in their redox potentials for one-electron oxidation, Em. For P680, Em is 1100-1200 mV, but for P700 and P870, Em is only 500 mV. Calculations with the linearized Poisson-Boltzmann equation reproduce these measured Em differences successfully.

Introduction to optical methods in photosynthesis

Optical spectroscopy is widely used to study structure and function of photosynthetic systems. Due to the large variety of different methods, these studies have contributed a lot to the identification of the cofactors involved in the primary reactions of photosynthesis and to the elucidation of the kinetics of the light-induced energy and electron transfer reactions. Within other aspects of photosynthesis research as e.g. photoinhibition, these techniques play an important role as well. In this brief introduction, I will focus on the basic principles of the different methods and the information obtained by applying these various techniques. In the reviews that follow, under the section "Optical Methods", these methods are discussed in detail.