Light-induced Electron Transfer Research Articles

Photo-isomerization of the Cyanine Dye Alexa-Fluor 647 (AF-647) in the Context of dSTORM Super-Resolution Microscopy.

Cyanine dyes, as used in super-resolution fluorescence microscopy, undergo light-induced "blinking", enabling localization of fluorophores with spatial resolution beyond the optical diffraction limit. Despite a plethora of studies, the molecular origins of this blinking are not well understood. Here, we examine the photophysical properties of a bio-conjugate cyanine dye (AF-647), used extensively in dSTORM imaging. In the absence of a potent sacrificial reductant, light-induced electron transfer and intermediates formed via the metastable, triplet excited state are considered unlikely to play a significant role in the blinking events. Instead, it is found that, under conditions appropriate to dSTORM microscopy, AF-647 undergoes reversible photo-induced isomerization to at least two long-lived dark species. These photo-isomers are characterized spectroscopically and their interconversion probed by computational means. The first-formed isomer is light sensitive and transforms to a longer-lived species in modest yield that could be involved in dSTORM related blinking. Permanent photobleaching of AF-647 occurs with very low quantum yield and is partially suppressed by the anaerobic redox buffer.

Constructing a dual-mode photochromic and intrinsically electrochromic device based on organic salts prepared by acid-base neutralization of pyromellitic diimides bearing a carboxyl group with aliphatic amines

In this research, the reaction of pyromellitic diimides bearing acidic functional group(s) in the side chain with aliphatic amines was found to produce organic salts with excellent photochromic properties in polar organic solvents like dimethylformamide. Upon exposure to sunlight, the solution color changed from pale-yellow to green, showing the characteristics of pyromellitic diimide radical-anion. The study of photo-coloration mechanism using experimental and theoretical methods revealed that the acid-base reaction of the mentioned pyromellitic diimides with an aliphatic amine causes unequal increases in the HOMO and LUMO energy levels. Larger enhancement in the HOMO compared to the LUMO energy level resulted in a decrease of HOMO-LUMO energy gap. Under this condition, leading to a facilitated intramolecular electron transfer, the light-induced single electron transfer from the side chain to diimide carbonyl group produced a green radical-anion. The prepared salts could be considered as intrinsic electrochromic materials besides possessing the photochromic behavior owing to their good electrical conductivity and electrochromic properties.

Light-driven deracemization enabled by excited-state electron transfer.

Deracemization is an attractive strategy for asymmetric synthesis, but intrinsic energetic challenges have limited its development. Here, we report a deracemization method in which amine derivatives undergo spontaneous optical enrichment upon exposure to visible light in the presence of three distinct molecular catalysts. Initiated by an excited-state iridium chromophore, this reaction proceeds through a sequence of favorable electron, proton, and hydrogen-atom transfer steps that serve to break and reform a stereogenic C-H bond. The enantioselectivity in these reactions is jointly determined by two independent stereoselective steps that occur in sequence within the catalytic cycle, giving rise to a composite selectivity that is higher than that of either step individually. These reactions represent a distinct approach to creating out-of-equilibrium product distributions between substrate enantiomers using excited-state redox events.

Studying hydrogen bonding and dynamics of the acetylate groups of the Special Pair of Rhodobacter sphaeroides WT

Although the cofactors in the bacterial reaction centre of Rhodobacter sphaeroides wild type (WT) are arranged almost symmetrically in two branches, the light-induced electron transfer occurs selectively in one branch. As origin of this functional symmetry break, a hydrogen bond between the acetyl group of PL in the primary donor and His-L168 has been discussed. In this study, we investigate the existence and rigidity of this hydrogen bond with solid-state photo-CIDNP MAS NMR methods offering information on the local electronic structure due to highly sensitive and selective NMR experiments. On the time scale of the experiment, the hydrogen bond between PL and His-L168 appears to be stable and not to be affected by illumination confirming a structural asymmetry within the Special Pair.

Protein Biophotosensitizer-Based IGZO Photo-thin Film Transistors for Monitoring Harmful Ultraviolet Light.

The development of health monitoring devices to prevent skin cancers or various diseases arising from exposure to harmful light has attracted increasing scientific interest and has led to the exploration of hybrid inorganic-biological systems through the incorporation of biomolecules. Here, ultraviolet (UV) photodetectors based on transistors incorporating green fluorescent protein (GFP) molecules on multilayer-stacked indium-gallium-zinc-oxide (IGZO) thin films are studied, where the top layer of the IGZO films has different surface properties. Light-sensitive GFP can play a role as a biophotosensitizer due to light-induced electron transfer during photoexcitation. Intriguingly, the IGZO photo-thin film transistors (TFTs) with GFP molecules on a relatively more hydrophilic surface (less defective surface) have better device performance and exhibit a dramatic decrease in the photocurrent after turning the UV light off compared to the cases without GFP molecules on the more hydrophilic surface and on the less hydrophilic surface (more defective surface). A physical mechanism based on energy band diagrams is proposed, and the light-induced threshold voltage shift in the IGZO photo-TFTs is estimated and explained in terms of oxygen-related vacancy sites and trap/interface conditions in the IGZO film and light-induced electron transfer from the GFP molecules.

Protons venture into the inverted region

Physical Chemistry One of the most counterintuitive features of electron transfer kinetics is the inverted region. As Marcus theory predicts and experiments have borne out, electron transfer slows down once the driving force for it becomes especially favorable. Parada et al. now offer evidence for such inverted behavior in proton-coupled electron transfer (see the Perspective by Dempsey). Specifically, they examined a series of compounds with phenol bridging anthracene (electron acceptor) and pyridine (proton acceptor) derivatives. Time-resolved spectroscopy and accompanying theory revealed slower rates at higher driving forces in the back reaction that follows light-induced intramolecular proton and electron transfer. Science , this issue p. [471][1]; see also p. [436][2] [1]: /lookup/doi/10.1126/science.aaw4675 [2]: /lookup/doi/10.1126/science.aaw9900

Synthesis and Luminescence of Optical Memory Active Tetramethylammonium Cyanocuprate(I) 3D Networks.

The structures of three tetramethylammonium cyanocuprate(I) 3D networks [NMe4]2[Cu(CN)2]2•0.25H2O (1), [NMe4][Cu3(CN)4] (2), and [NMe4][Cu2(CN)3] (3), (Me4N = tetramethylammonium), and the photophysics of 1 and 2 are reported. These complexes are prepared by combining aqueous solutions of the simple salts tetramethylammonium chloride and potassium dicyanocuprate. Single-crystal X-ray diffraction analysis of complex 1 reveals {Cu2(CN)2(μ2-CN)4} rhomboids crosslinked by cyano ligands and D3h {Cu(CN)3} metal clusters into a 3D coordination polymer, while 2 features independent 2D layers of fused hexagonal {Cu8(CN)8} rings where two Cu(I) centers reside in a linear C∞v coordination sphere. Metallophilic interactions are observed in 1 as close Cu⋯Cu distances, but are noticeably absent in 2. Complex 3 is a simple honeycomb sheet composed of trigonal planar Cu(I) centers with no Cu…Cu interactions. Temperature and time-dependent luminescence of 1 and 2 have been performed between 298 K and 78 K and demonstrate that 1 is a dual singlet/triplet emitter at low temperatures while 2 is a triplet-only emitter. DFT and TD-DFT calculations were used to help interpret the experimental findings. Optical memory experiments show that 1 and 2 are both optical memory active. These complexes undergo a reduction of emission intensity upon laser irradiation at 255 nm although this loss is much faster in 2. The loss of emission intensity is reversible in both cases by applying heat to the sample. We propose a light-induced electron transfer mechanism for the optical memory behavior observed.

Concerted proton-electron transfer reactions in the Marcus inverted region.

Electron transfer reactions slow down when they become very thermodynamically favorable, a counterintuitive interplay of kinetics and thermodynamics termed the inverted region in Marcus theory. Here we report inverted region behavior for proton-coupled electron transfer (PCET). Photochemical studies of anthracene-phenol-pyridine triads give rate constants for PCET charge recombination that are slower for the more thermodynamically favorable reactions. Photoexcitation forms an anthracene excited state that undergoes PCET to create a charge-separated state. The rate constants for return charge recombination show an inverted dependence on the driving force upon changing pyridine substituents and the solvent. Calculations using vibronically nonadiabatic PCET theory yield rate constants for simultaneous tunneling of the electron and proton that account for the results.

A kinetic model for redox-active film based biophotoelectrodes.

Redox-active films are advantageous matrices for the immobilization of photosynthetic proteins, due to their ability to mediate electron transfer as well as to achieve high catalyst loading on an electrode for efficient generation of electricity or solar fuels. A general challenge arises from various charge recombination pathways along the light-induced electron transfer chain from the electrode to the charge carriers for electricity production or to the final electron acceptors for solar fuel formation. Experimental methods based on current measurement or product quantification are often unable to discern between the contributions from the photocatalytic process and the detrimental effect of the short-circuiting reactions. Here we report on a general electrochemical model of the reaction-diffusion processes to identify and quantify the "bottlenecks" present in the fuel or current generation. The model is able to predict photocurrent-time curves including deconvolution of the recombination contributions, and to visualize the corresponding time dependent concentration profiles of the product. Dimensionless groups are developed for straightforward identification of the limiting processes. The importance of the model for quantitative understanding of biophotoelectrochemical processes is highlighted with an example of simulation results predicting the effect of the diffusion coefficient of the charge carrier on photocurrent generation for different charge recombination kinetics.

Pyrylenes: A New Class of Tunable, Redox-Switchable, Photoexcitable Pyrylium-Carbene Hybrids with Three Stable Redox-States.

A new synthetic and modular access to a large family of redox-switchable molecules based upon the combination of pyrylium salts and carbenes is presented. The redox-properties of this new molecule class correlate very well with the π-accepting properties of the corresponding carbenes. While the pyrylium moiety acts as a chromophore, the carbene moiety can tune the redox-properties and stabilize the corresponding radicals. This leads to the isolation of the first monomeric pyranyl-radical in the solid-state. The three stable oxidation states could be cleanly accessed by chemical oxidation, characterized by NMR, EPR, UV-vis, and X-ray diffraction and supported by (TD)-DFT-calculations. The new hybrid class can be utilized as an electrochemically triggered switch and as a powerful photoexcited reductant. Importantly, the pyrylenes can be used as novel photocatalysts for the reductive activation of aryl halides and sulfonamides by consecutive visible light induced electron transfer processes.

How Trap States Affect Charge Carrier Dynamics of CdSe and InP Quantum Dots: Visualization through Complexation with Viologen

The depth of surface trap states in semiconductor quantum dots (QDs) is influenced by the degree of covalency, which in turn affects the charge recombination process in hybrid donor–acceptor systems. By taking relatively ionic cadmium selenide (CdSe) QDs with shallow trap states and covalent indium phosphide (InP) QDs having deep trap states as examples, the charge-transfer dynamics are explored using viologen derivative as an electron acceptor. Light-induced electron transfer in a 1:1 stoichiometric complex of both the donor–acceptor systems occurs in a picosecond time scale. The presence of deep hole trap states in InP QDs retards the charge recombination to a submillisecond time scale, which is 7 orders of magnitude lower than that in CdSe QDs in homogeneous solutions. The immobile quenchers in the quenching sphere of InP further stabilize the electron-transfer products to seconds through charge hopping, which extends the potential of these systems for charge-transfer and transport applications in phot...

Aerobic Allylation of Alcohols with Non-Activated Alkenes Enabled by Light-Driven Selenium-π-Acid Catalysis

A new organocatalytic protocol for the aerobic dehydrogenative allylation of alcohols using non-activated alkenes as the allylating reagent and ambient air as the terminal oxidant is established. Mechanistically, the procedure relies on the interplay of a diselane and a photo­redox catalyst by means of a light-induced electron transfer process. Under optimized conditions, a broad range of both cyclic and acyclic ethers is accessed with very high functional group tolerance and excellent regioselectivity.

Light-induced ultrafast proton-coupled electron transfer responsible for H2 evolution on silver plasmonics

Light-driven proton-coupled electron transfer (PCET) reactions on nanoplasmonics would bring temporal control of their reactive pathways, in particular, prolong their charge separation state. Using ...

Near-IR Light-Induced Electron Transfer via Dynamic Quenching

The utilization of near-infrared light is an important objective for the high-efficiency utilization of solar energy. Here, we describe a new class of a near-infrared light-induced electron transfer system using a distorted phthalocyanine as a photosensitizer. We revealed that the appropriate modification of distorted phthalocyanine affords a near-infrared absorbing dye with high photostability and long excitation lifetime, and a near-infrared light-induced electron transfer system was successfully established using the dye. The mechanistic investigation clarified that the electron transfer system works via a dynamic quenching mechanism. The system generated a long-lived anion radical species of the dye upon near-infrared light irradiation (>750 nm).

A stable metal-covalent-supramolecular organic framework hybrid: enrichment of catalysts for visible light-induced hydrogen production

Cubic metal-covalent-supramolecular organic framework (MCSOF-1) hybrid has been created from the reaction of two molecular components and subsequent co-assembly with cucurbit[8]uril (CB[8]) in water. In the presence of CB[8], [Ru(bpy)3]2+ -based acylhydrazine 1·2Cl reacted with aldehyde 2·Cl to quantitatively yield six-armed precursor 3·8Cl through the generation of MCSOF-1. MCSOF-1 combines the structural features of metal-, covalent- and supramolecular organic frameworks. Its periodicity in water and in the solid state was confirmed by synchrotron X-ray scattering and diffraction experiments. MCSOF-1 could enrich discrete anionic polyoxometalates (POMs), maintain periodicity in acidic medium, and remarkably facilitate visible light-induced electron transfer from its [Ru(bpy)3]2+ units to enriched POMs, leading to enhanced catalysis of the POMs for the reduction of proton to H2 in both aqueous (homogeneous) and organic (heterogeneous) media.

Near-Infrared Fluorescent Nanoprobes for Revealing the Role of Dopamine in Drug Addiction.

Brain imaging techniques enable visualizing the activity of central nervous system without invasive neurosurgery. Dopamine is an important neurotransmitter. Its fluctuation in brain leads to a wide range of diseases and disorders, like drug addiction, depression, and Parkinson's disease. We designed near-infrared fluorescence dopamine-responsive nanoprobes (DRNs) for brain activity imaging during drug abuse and addiction process. On the basis of light-induced electron transfer between DRNs and dopamine and molecular wire effect of the DRNs, we can track the dynamical change of the neurotransmitter level in the physiological environment and the releasing of the neurotransmitter in living dopaminergic neurons in response to nicotine stimulation. The functional near-infrared fluorescence imaging can dynamically track the dopamine level in the mice midbrain under normal or drug-activated condition and evaluate the long-term effect of addictive substances to the brain. This strategy has the potential for studying neural activity under physiological condition.

Visible light induced electron transfer from a semiconductor to an insulator enables efficient photocatalytic activity on insulator-based heterojunctions.

Photogenerated electrons play a vital role in photocatalysis as they can induce the formation of radicals participating in the reaction or recombine with holes preventing them from the subsequent redox reaction. In this work, we explore an Earth-abundant insulator coupled with a semiconductor and construct insulator-semiconductor heterojunctions to effectively realize the efficient electron transfer from the semiconductor to the insulator and thus the enhanced charge carrier separation on the semiconductor. This result will challenge the traditional opinion that free electrons cannot be transferred onto insulators. Taking the BaCO3 insulator as a case study, the combined experimental and theoretical evidence indicates that the photogenerated electrons from the BiOI semiconductor could transfer directly to the BaCO3 insulator through a preformed electron delivery channel when they are coupled to form BaCO3/BiOI heterojunctions. The potential difference between the Bi layer of BiOI (5.03 eV) and the carbonate layer of BaCO3 (12.37 eV) would drive the transfer of excited electrons from Bi atoms across the energy barrier to the adjacent carbonate layer under visible light irradiation. Consequently, the free electrons on BaCO3 can be utilized to produce the oxidative radicals (˙OH, ˙O2- and 1O2) participating in the photocatalytic oxidation reaction. The in situ FT-IR spectra illustrate that the visible light induced active species in the heterojunctions could react with NO, leading to its oxidation to high valence state intermediates (NO+ and NO2+) first and then conversion to the final product of nitrates. This research offers new perspectives to explore insulator-based photocatalysts and unravel the gas-phase photocatalytic reaction mechanism.

Capturing the Light Fantastic

Artificial photosynthesis began as a serious research topic in the late 1970s as the oil crisis reached a critical stage. Early work focussed on the identification of photochemical systems for the evolution of either hydrogen gas or oxygen gas and usually involved precious metal catalysts and respective sacrificial electron donors or acceptors. The photochemical reduction of CO2 was soon recognised as being a highly prized goal but the experimental conditions needed for an effective system were unsuited for practical exploitation. Parallel to this rather applied research ran a theme to design and synthesize highly imaginative bio-inspired mimics that resembled the structures and functions of natural photosynthesis. This work led to massive advances in our understanding of electronic energy transfer and light-induced electron transfer between weakly coupled donor–acceptor pairs. As the price of oil stabilised and then fell, research in artificial photosynthesis took on a more fundamental nature and the synthesis of evermore complicated molecular architectures replaced the optimisation of solar fuel production. Photovoltaic cells, not normally associated with artificial photosynthesis, took centre stage in terms of realistic approaches for the harvesting of sunlight. Over the intervening years, there has been a progressive growth and diversification of photovoltaic devices. We have also witnessed the rapid rise of public concerns about climate change and global warming due to greenhouse gases. There has been a growing recognition that solar electricity production is insufficient for our needs and that there must be an avenue for the sustainable production of chemical feedstocks and fuels from readily available materials driven entirely from renewable energy sources. It is hard to imagine any sustainable photochemical cycle that does not include water as the electron and proton source, such as hydrogen evolution, or reduction of CO2 as well as N2 fixation. Options exist, however, for the complementary reductive process as long as the product can be collected, stored or utilised on-site. To be competitive with existing protocols, solar fuels production has to be transformed from the test tube to an industrial plant and from the microliter to the cubic meter scale. These are critical problems that demand urgent solutions. This special issue of ChemPhotoChem represents an opportunity to establish the current state-of-the-art in the field of Artificial Photosynthesis and to identify both barriers and promising routes for future development. Contributions address each of the key subjects, including both heterogeneous (i.e., semiconductor-based) and homogeneous (i.e., photosensitizer-based) systems. The photochemical reduction of CO2 receives well-deserved attention as an alternative to hydrogen production but we still lack ideas on how to reduce nitrogen under ambient conditions. Hybrid photosystems and the application of dedicated light-harvesting machinery are advancing quickly and could offer important benefits in terms of stability and versatility. Energy storage in the form of chemical transformations, often overlooked in the quest to design innovative strategies for solar energy conversion, makes a timely entry. Water oxidation, possibly the most important single step in the overall energy transduction cycle, gets a fresh look. These contributions are a valuable resource and offer a potential stepping stone to a future fuelled by energy from the sun. Anthony Harriman started research into bioinspired artificial photosynthesis in the late 1970s, working with Lord George Porter at the Royal Institution in London. Research centred on using metalloporphyrins to sensitise water reduction or oxidation and on the development of effective catalysts for oxygen evolution. Subsequent studies were aimed at seeking correlations between rates of light-induced electron or energy transfer and molecular topology. On-going work looks at ways to construct artificial light-harvesting arrays as universal solar concentrators. Dr. Haruo Inoue is a specially appointed full professor of Applied Chemistry at Tokyo Metropolitan University and serves also as a Director of the Center for Artificial Photosynthesis. He has been serving as the Research Supervisor of Precursory Research for Embryonic Science and Technology Project (PRSETO/JST) on “Chemical Conversion of Light Energy” and has been leading the All-Nippon Artificial Photosynthesis Project for Living Earth (AnApple: MEXT/Japan). Prof. Licheng Sun received his Ph.D. in 1990 from Dalian University of Technology (DUT), and went to Germany as a postdoc at Max-Planck-Institut für Strahlenchemie with Dr. Helmut Görner (1992–1993), and then as an Alexander von Humboldt postdoc at Freie Universität Berlin (1993–1995) with Prof. Dr. Harry Kurreck. He moved to KTH Royal Institute of Technology, Stockholm in 1995 as a postdoc with Prof. Björn Åkermark, became assistant professor in 1997, associate professor in 1999 at Stockholm University and full professor in 2004 at KTH. He is presently also a distinguished professor at DUT. His research interests cover artificial photosynthesis, including dye sensitized solar cells, perovskite solar cells, bio-inspired catalysts for water oxidation and hydrogen generation, nanomaterials and photoelectrochemical cells for water splitting and CO2 reduction.

Charging through the looking glass

Organic Chemistry Asymmetric catalysis is a commonly applied technique to prepare just one of two mirror-image products in a chemical reaction. But what if you already have the compound you want, stuck in a mixture of left- and right-handed enantiomers? Shin et al. now show that light-induced electron transfer can trigger a favorable succession of proton and hydrogen-atom transfer steps, both of which are susceptible to biasing by catalysts, to preferentially convert a mixture of cyclic urea enantiomers into just one (see the Perspective by Wendlandt). Science , this issue p. [364][1]; see also p. [304][2] [1]: /lookup/doi/10.1126/science.aay2204 [2]: /lookup/doi/10.1126/science.aay6919

Mutations in algal and cyanobacterial Photosystem I that independently affect the yield of initial charge separation in the two electron transfer cofactor branches

In Photosystem I, light-induced electron transfer can occur in either of two symmetry-related branches of cofactors, each of which is composed of a pair of chlorophylls (ec2A/ec3A or ec2B/ec3B) and a phylloquinone (PhQA or PhQB). The axial ligand to the central Mg2+ of the ec2A and ec2B chlorophylls is a water molecule that is also H-bonded to a nearby Asn residue. Here, we investigate the importance of this interaction for charge separation by converting each of the Asn residues to a Leu in the green alga, Chlamydomonas reinhardtii, and the cyanobacterium, Synechocystis sp. PCC6803, and studying the energy and electron transfer using time-resolved optical and EPR spectroscopy. Nanosecond transient absorbance measurements of the PhQ to FX electron transfer show that in both species, the PsaA-N604L mutation (near ec2B) results in a ~50% reduction in the amount of electron transfer in the B-branch, while the PsaB-N591L mutation (near ec2A) results in a ~70% reduction in the amount of electron transfer in the A-branch. A diminished quantum yield of P700+PhQ− is also observed in ultrafast optical experiments, but the lower yield does not appear to be a consequence of charge recombination in the nanosecond or microsecond timescales. The most significant finding is that the yield of electron transfer in the unaffected branch did not increase to compensate for the lower yield in the affected branch. Hence, each branch of the reaction center appears to operate independently of the other in carrying out light-induced charge separation.