Proton-Coupled Electron Transfer Guidelines, Fair and Square

Bioinspired Ruthenium-Porphyrin Electrocatalysts with Atomic N4/N2 Proximal Sites for Efficient Proton-Coupled Electron Transfer in Water Electrolysis

Mimicking the proton-coupled electron transfer (PCET) pathways of natural enzymes, we engineer a porphyrin-based ruthenium coordination polymer (Ru-PCPN) with precisely positioned atomic-level N4/N2 proximal sites through molecular-scale coordination engineering. This bioinspired architecture establishes a dual-site relay mechanism where the Ru-N2 center accelerates water dissociation kinetics while the adjacent Ru-N4 site optimizes hydrogen recombination. Experimental and theoretical results reveal that the sub-nanometer-proximate N4/N2 sites function as proton donor-acceptor pairs, enabling directional proton transfer via PCET and synergistically enhancing water electrolysis. When integrated with carbon substrates, the Ru-PCPN@CB catalyst demonstrates exceptional hydrogen evolution performance in alkaline conditions, achieving a low overpotential at 10 mA cm-2 (42 mV, comparable to 44 mV of Pt/C), high mass activity and TOF of 9.02 A mg-1 and 4.73 s-1 (∼7.0 and 3.6 times of Pt/C), and good stability. This work establishes atomic-scale coordination proximity as a new paradigm for breaking scaling relationships in multistep electrocatalysis.

Operando Contactless EFISH Study of the Rate-Determining Step of Light-Driven Water Oxidation on TiO2 Photoanodes

For many slow solar-fuel-forming reactions, the accumulation of photogenerated minority carriers on the photoelectrode surface leads to light-induced band edge unpinning, affecting the junction properties by decreasing band bending in the semiconductor space charge layer and increasing the driving force of surface reactions in the electric double layer. In this study, we demonstrate a contactless operando electric field-induced second harmonic generation (EFISH) method for measuring the band bending change (δΔΦSCRL) on photoelectrodes upon photoexcitation. For n-doped rutile TiO2 water oxidation photoanodes at pH 7, δΔΦSCRL increases at more positive potentials or higher illumination power density until it reaches saturation values. We show that under fast mass transport conditions, δΔΦSCRL is exclusively attributed to the accumulated charged rate-determining species that can be regarded as temporary surface states, and the relationship between the photocurrent and δΔΦSCRL can be well modeled by assuming that hole trap states function as the reaction center. Kinetic isotope experiments identify proton-coupled electron transfer as the rate-determining step and suggest a possible chemical nature of the key intermediate. We demonstrate that light-induced band edge unpinning is a beneficial feature under high illumination conditions for oxygen evolution reaction on TiO2 because it maintains the photon-to-current conversion efficiency by enhancing the surface reaction driving force, shedding light on the actual device application.

Excited-State H-Aggregation Enables Proton Abstraction from Water in Weak Photobases

Photobasic molecules can abstract a proton from water in the picosecond range, enabling efficient photochemical reactions, especially photocatalytic water splitting. However, such molecules are rarely reported due to the challenge of achieving an excited-state pKa (pKa*) close to the pKa of water ( 15.7). Here, we demonstrate that excited-state H-aggregation can substantially enhance proton-attracting capability by raising the energy of unprotonated species. According to the Förster cycle, the thermodynamic driving force for proton attraction is determined by the energy difference between the protonated and unprotonated species. For the model weak photobasic molecule citrazinic acid, excited-state H-aggregates are formed simply by adjusting the concentration. The energy of the unprotonated species increases by 0.38 eV upon aggregation, while that of the protonated species remains unchanged. The increased energy difference significantly increases the pKa* from a ground-state value of 2.5 to 15.4. Consequently, the weak photobasic aggregates are enabled to abstract a proton from water, a capability absent in the isolated form. Meanwhile, the lower-energy state in H-aggregates functions as a rate-limiting intermediate state, delaying the proton transfer dynamics, but the delay can be modulated by the excitation wavelength. This work provides fundamental insights into H-aggregation-induced photobasicity, opening new avenues for modulating photochemical reactions simply through concentration and light.

Theoretical Study of Imide Formation in Nitrogen Fixation Catalyzed by Molybdenum Complex Bearing PCP-Type Pincer Ligand with Metallocenes

Homogeneous catalysts using a mononuclear molybdenum nitride (Mo≡N) complex bearing PCP-type pincer ligands allow nitrogen fixation under very mild conditions. The catalytic cycle involves three hydrogenation processes yielding an Mo-ammine complex [MoI(NH3)(PCP)] from the Mo-nitride complex [MoI(N)(PCP)]. We primarily focused on the first hydrogenation step, forming an Mo-imide complex [MoI(NH)(PCP)] since previous experimental and theoretical studies suggest that imide formation is the rate-limiting step in the catalytic cycle. The choice of protonating agent and reductant strongly influences the catalytic reactivity in imide formation. In this computational quantum chemical study, 2,4,6-collidinium (ColH+) was employed as the protonation agent, while metallocenes Cp2MII and decamethylmetallocenes Cp*2MII (M = V, Cr, Mn, Fe, Co, and Ni) were employed as reductants. The reaction of ColH+ with the metallocenes yields protonated metallocenes, where a cyclopentadienyl ring of the metallocenes is protonated. Protonated Cp*2CrII and Cp*2CoII are potential proton-coupled electron transfer (PCET) mediators to facilitate the imide formation of [MoI(N)(PCP)] with low activation free energies. The concerted reaction mechanism was compared with the stepwise reaction, where ColH+ directly protonates [MoI(N)(PCP)], followed by reduction with the decamethylmetallocenes. Furthermore, we analyzed how proton transfer and electron transfer are concerted in the reaction of the PCET mediators with [MoI(N)(PCP)] by tracing electronic states along the reaction coordinates.

Effective Hole Utilization for Atomically Dispersed Low-Coordination Molybdenum Accelerating Photocatalytic C─H Activation

Photocatalytic acceptorless dehydrogenation of alcohols offers a promising strategy to produce the corresponding carbonyl compounds and clean fuel H2. However, the sluggish kinetics of the alkoxy C─H bond cleavage attributes to the inefficient utilization of photogenerated holes greatly restricts the photocatalytic activity. Here we develmically dispersed low-coordination Mo on ultrathin ZnIn2S4 nanosheets that can greatly accelerate photocatalytic C─H activation. An internal quantum efficiency of 45.2% at 400 nm together with 99% benzaldehyde (BAD) selectivity is achieved using benzyl alcohol (BA) as a model substrate. Extensive experimental characterizations and theoretical calculations reveal that the low-coordination Mo tunes the local atomic configuration of highest occupied molecular orbital to trap holes produced under photoexcitation within picoseconds. Moreover, the incorporated site-specific Mo greatly improves the lifetime and diffusion length of photogenerated holes and optimizes the driving force of alkoxy C─H activation, which are responsible for the excellent performance. This work marks a significant stride to enhance the utilization efficiency of holes for promoting photocatalytic C─H activation.

Isotope Techniques in Chemical Wastewater Treatment: Opportunities and Uncertainties

A comprehensive and in-depth analysis of reaction mechanisms is essential for advancing chemical water treatment technologies. However, due to the limitations of conventional experimental and analytical methods, the types of reactive species and their generation pathways are commonly debatable in many aqueous systems. As highly sensitive diagnostic tools, isotope techniques offer deeper insights with minimal interference from reaction conditions. Nevertheless, precise interpretations of isotope results remain a significant challenge. Herein, we first scrutinized the fundamentals of isotope chemistry and highlighted key changes induced by the isotope substitution. Next, we discussed the application of isotope techniques in kinetic isotope effects, presenting a roadmap for interpreting KIE in sophisticated systems. Furthermore, we summarized the applications of isotope techniques in elemental tracing to pinpoint reaction sites and identify dominant reactive species. Lastly, we propose future research directions, highlighting critical considerations for the rational design and interpretation of isotope experiments in environmental chemistry and related fields.

Mass spectrometric monitoring of redox transformation and arylation of tryptophan

Tryptophan (Trp) is an essential amino acid obtained from human diet. It is involved not only in de novo biosynthesis of proteins but also in complex metabolic pathways. Redox transformation of tryptophan is under-explored in comparison with kynurenine, serotonin and indole pyruvate pathways. We described herein a mass spectrometric approach that can not only detect electron transfer-associated changes in masses and charges, but also identify electron-directed bond cleavages and radical-radical cross-coupling reactions in redox transformation of tryptophan. Photoactive TiO2 that is widely applied in cosmetic products is used as electron donor and receptor because of the capability to generate photoelectrons and holes. It was demonstrated tryptophan undergoes redox transformation through the removal of an electron from amino nitrogen atom by hole oxidization along with an electron capture in the indole ring. The back and forth electron-shuttle converts electric energy into chemical energy that enforces bond cleavages. Sodium-coupled electron transfer (SCET) was found in complementary with proton-coupled electron transfer in tryptophan. The movement of sodium ions avoids electric charge buildup caused by electron transfer. Various redox products were detected on both light irradiated TiO2 and skins, among which β-carboline shows extensive radical scavenging ability for diverse cross-coupling with indole derivatives. Light-independent redox products have been detected in vivo such as in mouse brain, indicating the presence of in vivo electron transfer-directed redox transformation. It has also been revealed that tryptophan can be arylated on Cα and Cβ atoms in response to the exposure of halogenated aromatics.

Dynamic proton coupled electron transfer quenching as a sensing modality in fluorescent probes

Fluorescent off-on probes based on a modular design where an analyte sensitive PET moiety is attached to a fluorophore are extremely successful. Here we report a new modular fluorescence probe design switched by dynamic quenching due to proton coupled electron transfer (PCET) mediated by collision with weak bases in solution. The fluorescence lifetime of this probe directly reports on the rate of deprotonation by the weak bases in the solution. We investigate the probe design, mechanism of response, and sensitivity to various abundant weak bases/metabolites including acetate, glutamate, phosphate, valine, and amines. We find that this modular PCET based probe design, in contrast to traditional PET probes, can work efficiently with a fluorescence lifetime readout providing a calibration free probe for weak bases. Upon further development we envision such dynamic PCET probes as sensitive tools for studies of cellular buffer systems and metabolite pools.

Effect of Ligand Backbone on the Electrochemical Hydrogen Evolution Reaction and Hydrogen-Atom-Transfer Reactivity Using a Nickel Polypyridine Quinoline Complex

Redox-active quinoline-containing [NiII(2PyN2Q) (H2O)]2+ complex (1) has been developed for the electrocatalytic (e) hydrogen evolution reaction (HER) in the presence of organic acids and water and for the hydrogen-atom-transfer (HAT) reaction with styrene in the presence of acids. Complex 1 shows promising e-HER performance in water up to pH 9. It exhibits a stepwise (E)ECEC mechanism with AcOH, while a potential-dependent bimolecular homolytic pathway and CEEC mechanism is operative with p-toluene sulfonic acid during the e-HER. The one- and two-electron-reduced species of 1 are characterized by spectro-electrochemistry, optical, and EPR studies. Moreover, the inverse kinetic isotope effect (KIE = 0.83) between AcOH and d4-AcOH during the e-HER and e-HAT with styrene for the hydro-functionalization reaction using catalyst 1 possibly suggests the involvement of nickel hydride species. The e-HER and e-HAT reactivity of 1 have been compared with redox-inactive redox-inactive [NiII(N4Py)(H2O)]2+ (2), demonstrating the prominent effect of quinoline in the e-HER and pyridine in the e-HAT. The proposed mechanism of the e-HER with AcOH is well supported by DFT studies.

Proton-coupled electron transfer controls peroxide activation initiated by a solid-water interface

Decentralized water treatment technologies, designed to align with the specific characteristics of the water source and the requirements of the user, are gaining prominence due to their cost and energy-saving advantages over traditional centralized systems. The application of chemical water treatment via heterogeneous advanced oxidation processes using peroxide (O-O) represents a potentially attractive treatment option. These processes serve to initiate redox processes at the solid-water interface. Nevertheless, the oxidation mechanism exemplified by the typical Fenton-like persulfate-based heterogeneous oxidation, in which electron transfer dominates, is almost universally accepted. Here, we present experimental results that challenge this view. At the solid-liquid interface, it is demonstrated that protons are thermodynamically coupled to electrons. In situ quantitative titration provides direct experimental evidence that the coupling ratio of protons to transferred electrons is almost 1:1. Comprehensive thermodynamic analyses further demonstrate that a net proton-coupled electron transfer occurs, with both protons and electrons entering the redox cycle. These findings will inform future developments in O-O activation technologies, enabling more efficient redox activity via the tight coupling of protons and electrons.

An excitation-wavelength-dependent organic photoluminescent molecule with high quantum yield integrating both ESIPT and PCET mechanisms

Excitation wavelength-dependent (Ex-De) chromophores, which exhibit changes in spectral composition with varying excitation wavelengths, have garnered significant interest. However, the pursuit of novel photoluminescence (PL) mechanisms and high luminescence quantum yields is facing huge challenges. Here, we discover that the introduction of a spinacine moiety to 2-(2-hydroxy-5-methylphenyl)benzothiazole, a traditional excited-state intramolecular proton transfer (ESIPT) fluorophore, results in a novel Ex-De PL molecule. The luminescent color of this compound can be effectively modulated from greenish-blue to yellow-green by adjusting either the excitation wavelength or temperature. Transient absorption and spectroelectrochemistry spectra elucidate the underlying mechanism, demonstrating the roles of ESIPT and proton-coupled electron transfer (PCET). When embedded in a poly(vinyl alcohol) film, the composite exhibits remarkable Ex-De PL behavior, achieving absolute fluorescence quantum yields of 55.6% (λ ex: 396 nm) and 69.6% (λ ex: 363 nm), as well as phosphorescence at room temperature. These properties highlight its potential for multiple encryption features, enhancing its application in anti-counterfeiting technologies.

Electron transfer engineering of artificially designed cell factory for complete biosynthesis of steroids

Biosynthesis of steroids by artificially designed cell factories often involves numerous nicotinamide adenine dinucleotide phosphate (NADPH)-dependent enzymes that mediate electron transfer reactions. However, the unclear mechanisms of electron transfer from regeneration to the final delivery to the NADPH-dependent active centers limit systematically engineering electron transfer to improve steroids production. Here, we elucidate the electron transfer mechanisms of NADPH-dependent enzymes for systematically engineer electron transfer of Saccharomyces cerevisiae, including step-by-step engineering the electron transfer residues of 7-Dehydrocholesterol reductase (DHCR7) and P450 sterol side chain cleaving enzyme (P450scc), electron transfer components for directing carbon flux, and NADPH regeneration pathways, for high-level production of the cholesterol (1.78 g/L) and pregnenolone (0.83 g/L). The electron transfer engineering (ETE) process makes the electron transfer chains shorter and more stable which significantly accelerates deprotonation and proton coupled electron transfer process. This study underscores the significance of ETE strategies in steroids biosynthesis and expands synthetic biology approaches.

Explaining Kinetic Isotope Effects in Proton-Coupled Electron Transfer Reactions

ConspectusProton-coupled electron transfer (PCET) is essential for a wide range of chemical and biological processes. Understanding the mechanism of PCET reactions is important for controlling and tuning these processes. The kinetic isotope effect (KIE), defined as the ratio of the rate constants for hydrogen and deuterium transfer, is used to probe PCET mechanisms experimentally but is often challenging to interpret. Herein, a theoretical framework is described for interpreting KIEs of concerted PCET reactions. The first step is to classify the reaction in terms of vibronic and electron-proton nonadiabaticities, which reflect the relative time scales of the electrons, protons, and environment. The second step is to select the appropriate rate constant expression based on this classification. The third step is to compute the input quantities with computational methods.Vibronically adiabatic PCET reactions occur on the electronic and vibrational ground state and can be described within the transition state theory framework. The nuclear-electronic orbital (NEO) method, which treats specified protons quantum mechanically on the same level as the electrons, can be used to generate the electron-proton vibronic free energy surface for hydrogen and deuterium and to compute the corresponding free energy barriers. Such reactions typically exhibit moderate KIEs that arise from zero-point energy and shallow tunneling effects.Vibronically nonadiabatic PCET reactions involve excited electron-proton vibronic states and can be described with a golden rule formalism corresponding to nonadiabatic transitions between pairs of reactant and product vibronic states. Such reactions can exhibit KIEs ranging from unity, or even slightly less than unity, to more than 500. These KIEs can be explained in terms of multiple, competing reaction pathways corresponding to electron and proton tunneling between different pairs of vibronic states. The tunneling probability is determined by the vibronic coupling, which can be computed using a general expression but often is proportional to the overlap between the reactant and product proton vibrational wave functions. In this regime, the KIE is influenced by the vibronic couplings, the proton donor-acceptor equilibrium distance and motion, and contributions from excited vibronic states.Three illustrative examples of vibronically nonadiabatic PCET are discussed. The unusually large KIEs in soybean lipoxygenase of ∼80 for the wild-type enzyme and ∼700 for a double mutant are explained in terms of a large equilibrium proton donor-acceptor distance and nonoptimal orientation, leading to a small overlap between vibrational wave functions and therefore a large difference in hydrogen and deuterium tunneling probabilities. The KIEs for benzimidazole-phenol molecules ranging from unity to moderate are explained in terms of the dominance of different pairs of vibronic states with different vibrational wave function overlaps. The potential-dependent KIE observed for proton discharge from triethylammonium acid to a gold surface in acetonitrile is explained in terms of different pairs of vibronic states contributing for hydrogen and deuterium, with the reaction channels exhibiting different dependencies on the applied potential. These examples show that the KIE can vary widely, depending on which pairs of vibronic states dominate and their corresponding vibronic couplings. This work has broad implications for the interpretation of experimentally measured KIEs of PCET reactions.

The interference of baicalein with uric acid detected by the enzymatic method and its correction method

In recent years, the frequency of clinical application and international recognition of Chinese herbal medicines have been increasing, but the effect of Chinese herbal medicines on common clinical biochemical tests is still unclear. This study aimed to investigate the effect of baicalein, a Chinese herbal medicine ingredient, on uric acid (UA), cholesterol, triglycerides, high-density lipoprotein cholesterol, and low-density lipoprotein cholesterol and to alleviate the interference of baicalein on these assays by improving the reagent. The interferences of baicalein during the detection of these five analytes were investigated on the Hitachi 7600 system. We prepared UA assay kit according to commercial standards to facilitate the improvement of the formulation and evaluated its performance. Tempol, which could eliminate the interference of baicalein, was found based on the chemical properties of the drug, and the optimum concentration for adding it to our UA reagent was determined. We found that the interference was concentration-dependent for five analytes, with the largest negative interference on UA determination. Self-prepared UA assay kit had a safe analysis performance. Our kit and the commercial kit showed a higher interference of - 71.75% and - 89.98% at 200 µg/mL baicalein, respectively. The addition of 5 mmol/L Tempol to the UA reagent could strongly resist the interference of baicalein. In Conclusion, baicalein has a negative interference effect on analysis based on the Trinder reaction, especially UA assay. With the increase in baicalein concentrations, the negative bias increased, and our improved UA reagent could resist the interference of baicalein on UA detection.

Liquid Metal Nanobiohybrids for High-Performance Solar-Driven Methanogenesis via Multi-Interface Engineering

Nanobiohybrids for solar-driven methanogenesis present a promising solution to the global energy crisis. However, conventional semiconductor-based nanobiohybrids face challenges such as limited tunability and poor biocompatibility, leading to undesirable spontaneous electron and proton transfer that compromise their structural stability and CH4 selectivity. Herein, we introduced eutectic gallium-indium alloys (EGaIn), featuring a self-limiting surface oxide layer surrounding the liquid metal core after sonication, integrated with Methanosarcina barkeri (M. b). The well-designed M. b-EGaIn nanobiohybrids exhibited superior performance, achieving a maximum CH4 yield of 455.64±15.99 μmol g-1, long-term stability across four successive 7-day cycles, and remarkable CH4 selectivity of >99 %. These improvements stem from enhanced proton-coupled electron transfer involving hydrogen atoms at the core-shell interface, further facilitated by the elevated expression of hydrogenases at the abiotic-biotic interface. This study provides an insightful concept for nanobiohybrid design through multi-interface engineering, advancing sustainable and scalable CO2-to-biofuel conversion under ambient conditions.

Homodimerization of 3-substituted-2-oxindoles for the construction of vicinal all-carbon quaternary centers: chemical, photochemical and electrochemical approaches

Advancements in organic synthesis are revolutionizing the synthesis of complex natural products, which are essential in biomedical research and drug discovery due to their intricate structures. Natural products such as chimonanthine, folicanthine, calycanthine, psychotriadine, etc., with vicinal all-carbon quaternary stereocenters, are particularly significant for their strong binding properties and biological activities. One common feature of these natural products is the presence of dimeric 3-substituted-2-oxindoles having vicinal all-carbon quaternary stereocenters. This review focuses on the chemical, photochemical, and electrochemical approaches for the homodimerization of 3-substituted-2-oxindoles employed by different researchers, with a strong focus on the mechanistic details of proton-coupled electron transfer (PCET). The article also demonstrates that PCET facilitates the reduction of kinetic barriers through the formation of low-energy intermediates and the expansion of synthetic possibilities. Furthermore, natural product syntheses (folicanthine and chimonanthine) from dimeric 3-substituted-2-oxindoles are discussed. Chemical syntheses are time-consuming and, even more importantly, generate significant waste due to the use of metal-based oxidants and catalysts. In this regard, electrochemical synthesis methods offer promising solutions by avoiding the use of chemical oxidants and metal catalysts, thus minimizing environmental impact. The article also outlines the advantages and disadvantages of different synthesis methods and proposes a new direction for future research in this field.

Insights into the Proton-Coupled Electron Transfer Mechanism in Fuel Cells

Proton-coupled electron transfer (PCET) is not only an important fundamental process in energy systems but also a pivotal factor in enhancing electrocatalytic functions in fuel cells (FCs). This article investigates the PCET mechanism in low-temperature (300-500 °C) protonic ceramic fuel cells, focusing on its role in catalyzing the hydrogen oxidation reaction and the oxygen reduction reaction. Our findings reveal that PCET significantly enhances the electrocatalytic activity by mitigating polarization losses, reducing charge-transfer resistance by 1 to 2 orders of magnitude, and thereby accelerating the reaction kinetics compared to scenarios without PCET. Importantly, changes in relaxation time upon proton injection evidence the robustness of PCET. The marked reduction in activation energy to 0.31 eV further illustrates how PCET overcomes energy barriers, facilitating more efficient reaction pathways. These insights highlight the critical role of PCET in optimizing the electrocatalytic performance of FCs, underscoring its significant importance in advancing FC technology.

High-pressure pump-probe experiments reveal the mechanism of excited-state proton-coupled electron transfer and a shift from stepwise to concerted pathways

Chemical energy conversion and storage in natural and artificial systems rely on proton-coupled electron transfer (PCET) processes. Concerted proton-electron transfer (CPET) can provide kinetic advantages over stepwise processes (electron transfer (ET)/proton transfer (PT) or PT/ET), so understanding how to distinguish and modulate these processes is important for their associated applications. Here, we examined PCET from the excited state of a ruthenium complex under high pressures. At lower buffer or quencher concentrations, a stepwise PT/ET mechanism was observed. With increasing pressure, PT slowed and ET sped up, indicating a merging of the two steps. In contrast, CPET at higher concentrations of buffer or quencher showed no pressure dependence of the reaction rate. This is because the simultaneous transfer of electrons and protons circumvents changes in charges and, consequently, in solvent electrostriction during the transition state. Our findings demonstrate that pressure can serve as a tool to monitor charge changes along PCET pathways, aiding in the identification of its mechanisms.

Synthesis of [Os(bpy)2(py)(OH2)](PF6)x substituted pyridine complexes; characterization of a singly bridged H3O2- ligand

Proton-coupled electron transfer (PCET) underpins energy conversion processes in biological systems and fuel-forming reactions. Interrogation of the dynamics of electron and proton transfer in PCET processes requires tunable models, with synthetic transition metal aquo complexes being particularly well-explored examples. A previous study on a PCET model, [OsII(bpy)2(py)(OH2)]2+ (bpy = 2,2'-bipyridine; py = pyridine), reported synthetic intractability which limits access to this class of models. Herein, we report an improved protocol to synthesize a family of [OsII(bpy)2(py)(OH2)]2+ complexes enabling the modular tuning of the pyridine ligand (pyL) with electron-donating or -withdrawing groups at the para-position. The modification of the electron density about the osmium center is reflected in Hammett plots of half-wave peak potential for the OsII/OsIII couples and pKa values of the coordinated water. Moreover, a hydrogen-bonded osmium dinuclear structure featuring a short, strong hydrogen bonding network in the solid state was observed; we find the dinuclear Os structure is likely not maintained in solution. Our work expands access to osmium aquo complexes and provides an avenue to understand how modification of supporting ligands can influence PCET processes.

Dual-Function Strategy for Enhanced Quercetin Detection Using Terbium(III) Ion-Bound Gold Nanoclusters

The engineering of metal nanoclusters (NCs) that exhibit bright emissions and high sensing performance under physiological conditions is still a formidable challenge. In this study, we report a novel design strategy for realizing excellent performance metal NC-based probes by leveraging both concerted proton-coupled electron transfer (PCET) and photoinduced electron transfer (PET) mechanisms, with terbium(III) (Tb3+) ions serving as a key modulator. Our findings indicate that the binding of Tb3+ ions to the 6-aza-2-thiothymidine (ATT) ligand effectively inhibits the proton-transfer step in the concerted PCET pathway of Au10(ATT)6 NCs, giving rise to over a 10-fold enhancement in fluorescence and a quantum yield of 7.2%. Moreover, the capped Tb3+ ions on the surface of Au10(ATT)6 NCs can act as a bridge to facilitate an efficient donor-linker-acceptor type PET reaction from quercetin (Que) to the excited Au10 core by specifically interacting with the bare 3-OH group. These advancements enable the Tb3+/Au10(ATT)6 NC-based probe to achieve a significantly lower limit of detection for Que, reduced by nearly 3 orders of magnitude to 2.6 nM, while also addressing the critical difficulty of selectively detecting Que in the presence of its glycosylated analogues. This work opens new opportunities for the precise control of photoluminescence in metal NC probes at the molecular level, potentially promoting the development of next-generation metal NC-based sensing technologies.

Enhanced Photocatalytic Proton-Coupled Electron Transfer by Ligand Design in a Zr Coordination Cage

Developing excited state proton and electron donors is a promising area of research that merges the benefits of proton-coupled electron transfer (PCET) with the use of light as renewable energy input. Based on the demonstrated PCET reactivity of Zr coordination cages for reductive photocatalysis, here we synthetize and characterize a new cage with enhanced photocatalytic activity. The new design targets the extended biphenyl-4,4-dicarboxylate linker with an amino group in the meta position relative to the carboxylate. Our results show that these aspects are key to increase the stability and reduction power of the excited state, features that are typically tuned by inductive effects. As a result, the new Zr-cage promotes significantly faster PCET reactions than the previous related platform, resulting in higher chemical and quantum yields. We further showcase how the solvent can impact the photophysical properties and the PCET reaction rates depending on the cage structure. These results highlight the factors that influence excited state PCET reactivity and complement similar efforts made in the realm of H2 evolution.

Electrocatalytic Ammonia Oxidation by Pyridyl-Substituted Ferrocenes

Ammonia (NH3) is a promising carbon-free fuel when prepared from sustainable resources. First-row transition metal electrocatalysts for ammonia oxidation are an enabling technology for sustainable energy production. We describe electrocatalytic ammonia oxidation using robust molecular complexes based on Earth-abundant iron. Electrochemical studies of ferrocenes with covalently attached pyridine arms reveal facile ammonia oxidation in DMSO (2.4 M NH3) with modest overpotentials (η = 770-820 mV) and turnover frequencies (125-560 h-1). Experimental and computational studies indicate that the pendant pyridyl base serves as an H-bond acceptor with an N-H bond of ammonia that transfers a proton to the pyridine following oxidation by the attached ferrocenium moiety in a proton-coupled electron transfer (PCET) step. This generates an amidyl (•NH2) radical stabilized via H-bonding to a pendant pyridinium moiety that rapidly dimerizes to hydrazine (H2N-NH2), which is easily oxidized to nitrogen (N2) at the glassy carbon working electrode. This report identifies a general strategy to oxidize ammonia via H-bonding to a base (B:), thereby activating [B···H-NH2] toward PCET by a proximal oxidant to form [BH···NH2]+/• radical cations, which are susceptible to dimerization to form easily oxidized hydrazine.

Evidence for Competing Proton-Coupled Reaction Pathways of Molecular Triads in a Low-Polarity Solvent

The temperature dependence of concerted proton-electron transfer (CPET) reactions of two anthracene-phenol-pyridine (An-PhOH-py) triads is investigated in toluene. Light excitation forms an anthracene local excited state (1*An), which undergoes CPET to form a charge separated state (CSS, An•--PhO•-pyH+), which in turn undergoes CPET charge recombination (CR). In toluene, compared with polar solvents, the CSS is energetically destabilized. First, this makes another reaction competitive with CPET, which we propose is proton-coupled energy transfer (PCEnT) from 1*An to form the short-lived excited state keto tautomer of the phenol-pyridine subunit (*[PhO═pyH]). Second, it puts CR deep into the Marcus inverted region, and CSS lifetimes therefore reach several nanoseconds at room temperature. The slow kinetics makes CR to the anthracene triplet state (3*An) competitive, as well as another reaction that is strongly activated and dominates CSS deactivation at T ≥ 240 K for one of the triads. The latter is proposed to be CR via initial formation of the same [*PhO═PyH] state as above by an unusual electron transfer (ET) from An•- to pyH+, instead of CR with the juxtaposed PhO•. The two different pathways to form *[PhO═pyH] lead to CSS yields and lifetimes that vary significantly with temperature, and in markedly different ways between the triads. This is rationalized by the differences in the energies of the states involved. The results broaden the scope and understanding of the still rare phenomena of inverted CPET and PCEnT and may aid toward their use in solar fuels and photoredox catalysis.

HOO radical scavenging activity of curcumin I and III in physiological conditions: a theoretical investigation on the influence of acid-base equilibrium and tautomerism

Curcumin possesses various effective medicinal properties, such as anti-cancer, anti-Alzheimer's, anti-inflammatory, and antioxidant effects, where its free radical scavenging activities play a crucial role in its therapeutic mechanisms. Although the antioxidant properties of curcumin and its derivatives have been previously studied, a systematic investigation of the thermodynamics and kinetics of the reaction with the hydroperoxide radical (HOO˙) - a standardized free radical - in different solvents is lacking. This study examined the HOO˙ radical scavenging activities of two curcumin derivatives, specifically curcumin I (Cur-I) and curcumin III (Cur-III), in water and pentyl ethanoate (PEA) solutions using Density Functional Theory (DFT) approaches. The antioxidant properties of the neutral and anionic forms of their tautomers, including the keto-enol and diketone forms, were explored via three standard mechanisms: hydrogen abstraction (Abs), radical addition (Add), and single electron transfer (SET). Intrinsic parameters, thermochemical parameters, and kinetics of the curcumin-HOO˙ reactions were systematically characterized. As a result, the overall rate constant for the reaction of Cur-I in the water (9.36 × 107 M-1 s-1) is approximately 3.6 times higher than that of Cur-III (2.60 × 107 M-1 s-1). Meanwhile, the rate constants in PEA solvent are less significant, being 4.02 × 101 M-1 s-1 and 8.16 × 102 M-1 s-1 for Cur-I and Cur-III, respectively. Due to the dominant molar fraction of the keto-enol form compared to the diketone, the reaction rates are primarily attributed to the keto-enol form. The SET reaction of dianionic form contributes a decisive proportion to the overall rate constants of both Cur-I and Cur-III. Finally, an analysis of the chemical nature of the Abs reactions reveals that the most predominant hydrogen transfer at the phenolic -OH groups (i.e., O22H and O23H) occurs via a proton-coupled electron transfer (PCET) mechanism.

Mechanistic Perspective on C-N and C-S Bond Construction Catalyzed by Cytochrome P450 Enzymes

Cytochrome P450 enzymes catalyze a large number of oxidative transformations that are responsible for natural product synthesis. Recent studies have revealed their unique ability to catalyze the formation of C-N and C-S bonds, broadening their biosynthetic applications. However, the enzymatic mechanisms behind these reactions are still unclear. This review focuses on theoretical insights into the mechanisms of P450-catalyzed C-N and C-S bond formation. The key roles of the conformational dynamics of substrate radicals, influenced by the enzyme environment, in modulating selectivity and reactivity are highlighted. Understanding these reaction mechanisms offers valuable guidance for P450 enzyme engineering and the design of biosynthetic applications.

Radical Brook rearrangement: past, present, and future

The Brook rearrangement has emerged as one of the most pivotal transformations in organic chemistry, with broad applications spanning organic synthesis, drug design, and materials science. Since its discovery in the 1950s, the anion-mediated Brook rearrangement has been extensively studied, laying the groundwork for the development of numerous innovative reactions. In contrast, the radical Brook rearrangement has garnered comparatively less attention, primarily due to the challenges associated with the controlled generation of alkoxyl radicals under mild conditions. However, recent advancements in visible-light catalysis and transition-metal catalysis have positioned the radical Brook rearrangement as a promising alternative synthetic strategy in organic synthesis. Despite these developments, significant limitations and challenges remain, warranting further investigation. This review provides an overview of the radical Brook rearrangement, tracing its development from past to present, and offers perspectives on future directions in the field to inspire the creation of novel synthetic tools based on this transformation.

Disentangling Driving Force Effects, Polar Effects, e-/H+ Imbalance, and Other Influences on H-Atom Transfer Reactions

Hydrogen atom transfer (HAT) reactions and their kinetic barriers ΔGHAT‡ are important in organic and inorganic chemistry. This study examines factors that influence ΔGHAT‡, reporting the kinetics and thermodynamics of HAT from various ruthenium bis(acetylacetonate) pyridine-imidazole complexes to nitroxyl radicals. Across these 36 reactions, the ΔGPT° and ΔGHAT° can be independently varied, with different sets of Ru complexes primarily tuning either their pKas or their E°s. The ΔΔGHAT‡ are analyzed using multiple linear free energy relationships (LFERs), the first largely experimental study of its kind. The barriers vary most strongly with the overall driving force, ΔΔGHAT‡ = 0.28 × ΔΔGHAT°, but are also affected by HAT intrinsic barriers (λ), sterics, and the thermochemical e-/H+ imbalance of the reactions, |ΔGPT° - ΔGET°|. The latter is a small but significant effect, revealed only by comparing LFERs. The imbalance analysis is closely related to traditional explanations of polar effects, but it is quantitative: ΔGHAT‡ shifts by ∼4% with changes in |ΔGPT° - ΔGET°|. This is the same dependence as was observed for purely organic HAT from toluenes─a remarkable result because traditional explanations of organic polar effects, e.g., using X-H bond polarities, do not apply to the Ru complexes in which the e- and H+ are spatially separated. This work demonstrates the strong similarities between different kinds of HAT reactions when viewed through the lens of H+/e- (PCET) free energies. This lens also shows that ΔGHAT‡ are ∼10-fold more sensitive to changes in ΔGHAT° and λ than to the e-/H+ free-energy imbalance.

Photocatalytic Three-Component Reductive Coupling Synthesis of gem-Difluorohomoallyl Secondary Amines

gem-Difluorohomoallyl amines, an important class of gem-difluoroalkenes, are prevalent moieties in many bioactive compounds. However, limited methods are suitable for the synthesis of this type of compound containing secondary amines. Here, we display a photocatalytic multicomponent protocol for the synthesis of gem-difluoroalkenes containing secondary amines, which makes use of readily available materials: arylamines, alkyl aldehydes, and α-trifluoromethyl alkenes. Moreover, ketones and secondary amines are also suitable substrates. Preliminary mechanistic experiments indicate that a key α-amino radical was involved, generated from the reduction of in situ-formed imines (or iminium ions) by a reduced photocatalyst. Subsequent addition of the α-amino radical to α-trifluoromethyl alkenes and β-F elimination deliver the desired products.

Elaborating H-bonding effect and excited state intramolecular proton transfer of 2-(2-hydroxyphenyl)benzothiazole based D-π-A fluorescent dye

2-(2-Hydroxyphenyl)benzothiazole (HBT) derivatives with donor-π-acceptor (D-π-A) structure have received extensive attention as a class of excited state intramolecular proton transfer (ESIPT) compounds in the fields of biochemistry and photochemistry. The effects of electron-donors (triphenylamine and anthracenyl), the position of substituents and solvent polarity on the fluorescence properties and ESIPT mechanisms of HBT derivatives were investigated through time-dependent density functional theory (TDDFT) calculations. Potential energy curves (PECs) and frontier molecular orbitals (FMOs) reveal that the introduction of the triphenylamine group on the benzene ring enhances intramolecular HB, thereby benefiting the ESIPT process. Analysis of their spectra reveals that P-TPA (para position for TPA) and M-TPA (meta position for TPA) are both excellent candidates for fluorescent dyes because of their large Stokes shifts. The PECs of four derivatives indicate that the ESIPT process of P-TPA in dimethyl sulfoxide (DMSO) solvent is the most likely to occur. The research revealed that both P-TPA and P-En (para positions for both TPA and En) can undergo a spontaneous transformation from the enol to the keto form in the S1 state. Furthermore, the ESIPT process was found to be enhanced with an increase in polarity. The energy barrier of P-TPA(N*) → P-TPA(K*) is 3.06 kcal mol-1 in the S1 state and its reversed energy barrier is 4.47 kcal mol-1. The para triphenylamine group could accelerate the ESIPT reactions, as it has a greater impact on the excited state intramolecular hydrogen bond (ESIHB) compared to meta-substitution of the triphenylamine group.

Engineering mechanisms of proton-coupled electron transfer to a titanium-substituted polyoxovanadate-alkoxide

Metal oxides are promising catalysts for small molecule hydrogen chemistries, mediated by interfacial proton-coupled electron transfer (PCET) processes. Engineering the mechanism of PCET has been shown to control the selectivity of reduced products, providing an additional route for improving reductive catalysis with metal oxides. In this work, we present kinetic resolution of the rate determining proton-transfer step of PCET to a titanium-doped POV, TiV5O6(OCH3)13 with 9,10-dihydrophenazine by monitoring the loss of the cationic radical intermediate using stopped-flow analysis. For this reductant, a 5-fold enhanced rate (k PT = 1.2 × 104 M-1 s-1) is accredited to a halved activation barrier in comparison to the homometallic analogue, [V6O7(OCH3)12]1-. By switching to hydrazobenzene as a reductant, a substrate where the electron transfer component of the PCET is thermodynamically unfavorable (ΔG ET = +11 kcal mol-1), the mechanism is found to be altered to a concerted PCET mechanism. Despite the similar mechanisms and driving forces for TiV5O6(OCH3)13 and [V6O7(OCH3)12]1-, the rate of PCET is accellerated by 3-orders of magnitude (k PCET = 0.3 M-1 s-1) by the presence of the Ti(iv) ion. Possible origins of the accelleration are considered, including the possibility of strong electronic coupling interactions between TiV5O6(OCH3)13 with hydrazobenzene. Overall, these results offer insight into the governing factors that control the mechanism of PCET in metal oxide systems.

Two-fold proton coupled electron transfer of a Ta(V) aniline complex mediated by a redox active NNN pincer ligand

We report the proton-coupled electron transfer (PCET) reactivity of an octahedral Ta(V) aniline complex supported by an acridane-derived redox active NNN pincer ligand. The reversible binding of aniline to a Ta(V) dichloride induces significant coordination-induced bond weakening (CIBW) of the aniline N-H bonds. This enables a rare two-fold hydrogen atom abstraction, resulting in a terminal imido complex and a two-electron oxidation of the NNN pincer ligand, all while maintaining the metal's oxidation state. The bond dissociation free energies (BDFEs) of the aniline and a transient radical amido complex are estimated through stoichiometric reactions with different hydrogen atom abstractors and donors, further supported by density functional theory calculations.

Proton-electron coupling and mixed conductivity in a hydrogen-bonded coordination polymer

The fundamental understanding of coupled proton-electron transport in mixed protonic-electronic conductors (MPECs) remains unexplored in materials science, despite its potential significance within the broader context of mixed ionic-electronic conductors (MIECs) and the possibility of controlled diffusion of protons using hydrogen-bond networks. To address these limitations, we present a hydrogen-bonded coordination polymer Ni-BAND ({[Ni(bpy)(H2O)2(DMF)2](NO3)2·2DMF}n), which demonstrates high mixed protonic-electronic conductivity at room temperature. Through detailed analysis, we unravel the coupled transport mechanism, offering insights for the rational design of high-performance MPECs. We demonstrate the practical implications of this mechanism by examining the humidity-dependent synaptic plasticity of Ni-BAND, showcasing how MPECs can expand into traditional MIEC applications while leveraging their unique proton-mediated advantages.

The Hidden Mechanism: Excited-State Proton-Electron Pair Transfer in Metal Nanocluster Emission

Comprehending the underlying factors that govern photoluminescence (PL) in metal nanoclusters (NCs) under physiological conditions remains a highly intriguing and unresolved challenge, particularly for their biomedical applications. In this study, we evaluate the critical role of excited-state proton-coupled electron transfer in the emission of metal NCs. Our findings demonstrate that hydronium ion (H3O+) binding can trigger a nonlinear, pH-dependent excited-state concerted electron proton transfer (CEPT) reaction. This involves simultaneous electron transfer from the Au(0) core to the Au(I)-ATT (ATT denotes 6-aza-2-thiothymidine) surface and proton transfer from H3O+ to the ATT ligand in a single step, greatly promoting vibrations and rotations of the Au(I)-ATT surface, resulting in substantial PL quenching of Au10(ATT)6 NCs. Further analyses show that the unique CEPT dynamics are strongly influenced by the opposing effects of increased reorganization energy and a larger pre-exponential factor on the electron transfer rate. Moreover, the proposed excited-state CEPT process is found to be prevalent in core-shell relaxation metal NCs, such as Au25(SR)18 (SR denotes thiolate) NCs, and serves as an important factor in limiting their PL emission. By simply controlling the pKa of the ligands, the emission performance of Au25(SR)18 can be easily regulated in physiological environments.

Silicate-Confined Hydrogen on Nanoscale Zerovalent Iron for Efficient Defluorination Reactions

Defluorination reactions are increasingly vital due to the extensive use of organofluorine compounds with robust carbon-fluorine (C-F) bonds; particularly, the efficient defluorination of widespread and persistent per- and polyfluoroalkyl substances under mild conditions is crucial due to their accumulation in the environment and human body. Herein, we demonstrate that surface-modified silicate of pronounced proton affinity can confine active hydrogen (•H) onto nanoscale zerovalent iron (nZVI) by withdrawing electrons from nZVI to react with bound protons, generating confined active hydrogen (•H*) for efficient defluorination under ambient conditions. The exposed silicon cation (Siσ+) of silicate functions as a Lewis acid site to activate the C-F bond by forming Siσ+...F--C and substantially lowers the energy barrier of nucleophilic •H* attack, thereby facilitating selective C-F hydrodefluorination and subsequent fluorine immobilization. In a column flow reactor, silicate-modified nZVI efficiently removes perfluorooctanoic acid (PFOA) of concentrations ranging from 0.24 to 24 μmol/L with 75-92% defluorination efficiencies, 8 times higher than those of nZVI, generating environmentally friendly alkyl carboxylic acids as the primary products. Besides PFOA, this novel nZVI also realizes deep defluorination of other organofluorine compounds, including perfluorooctanesulfonates and fluoroquinolones, demonstrating its superior defluorination potential.

Transforming Adsorbate Surface Dynamics in Aqueous Electrocatalysis: Pathways to Unconstrained Performance

Developing highly efficient catalysts to accelerate sluggish electrode reactions is critical for the deployment of sustainable aqueous electrochemical technologies, yet remains a great challenge. Rationally integrating functional components to tailor surface adsorption behaviors and adsorbate dynamics would divert reaction pathways and alleviate energy barriers, eliminating conventional thermodynamic constraints and ultimately optimizing energy flow within electrochemical systems. This approach has, therefore, garnered significant interest, presenting substantial potential for developing highly efficient catalysts that simultaneously enhance activity, selectivity, and stability. The immense promise and rapid evolution of this design strategy, however, do not overshadow the substantial challenges and ambiguities that persist, impeding the realization of significant breakthroughs in electrocatalyst development. This review explores the latest insights into the principles guiding the design of catalytic surfaces that enable favorable adsorbate dynamics within the contexts of hydrogen and oxygen electrochemistry. Innovative approaches for tailoring adsorbate-surface interactions are discussed, delving into underlying principles that govern these dynamics. Additionally, perspectives on the prevailing challenges are presented and future research directions are proposed. By evaluating the core principles and identifying critical research gaps, this review seeks to inspire rational electrocatalyst design, the discovery of novel reaction mechanisms and concepts, and ultimately, advance the large-scale implementation of electroconversion technologies.

Photoredox-Catalyzed Decarboxylation of Oxetane-2-Carboxylic Acids and Unique Mechanistic Insights

Oxetanes are valuable motifs in medicinal chemistry applications, with demonstrated potential to serve as bioisosteres for an array of functional groups. Through the visible-light-mediated photoredox hydrodecarboxylation of 2-aryl oxetane 2-carboxylic acids this work enables access to the products of a [2+2]-photocycloaddition between alkenes and aryl aldehydes without the challenges associated with a traditional UV-light-mediated Paternò-Büchi reaction. Investigation into the hydrodecarboxylation mechanism reveals substrate-dependent modes of initiation under the conditions reported herein. Divergence in diastereomeric outcome is observed, with mechanistic probes elucidating key hydrogen-bonding and steric interactions.

Spectroscopic investigation of proton bonding at sub-kelvin temperatures

The proton bond is a pivotal chemical motif in many areas of science and technology. Its quantum chemical description is remarkably challenged by nuclear and charge delocalization effects and the fluxional perturbation that it induces on molecular substrates. This work seeks insights into proton bonding at sub-kelvin temperatures. In this way, intrinsic features of the proton bond are exposed, essentially free from thermal fluctuations of the molecular frame. To this end, a proton is bound within the molecular ring cavity provided by the 12-crown-4 ether. The resulting ion is isolated in a He-droplet at ∼0.4 K, where it is interrogated by infrared laser spectroscopy. The recorded spectrum features narrow vibrational bands, consistent with a robust proton bond bridging ether sites across the cavity of the essentially frozen crown ether. The potential energy surface sustaining the proton bond is broad and markedly anharmonic. In consequence, common modeling methods within the harmonic approximation fail to capture the observed band positions, whose accurate description seems to be even beyond perturbative anharmonic approaches. Calculations show that at elevated temperatures, the crown ether backbone is highly fluxional and that the distance between the oxygen atoms fluctuates in time, modulating the potential that the proton or deuteron is exposed to, and yielding dynamic inhomogeneous broadening and blue shifts with respect to the cryogenic spectra. These observations call for novel computational developments, for which the vibrational signatures outlined in this work should provide a valuable benchmark.

C(sp3)-F Bond Activation by Lewis Base-Boryl Radicals via Concerted Electron-Fluoride Transfer

Selective C-F bond activation through a radical pathway in the presence of multiple C-H bonds remains a formidable challenge, owing to the extraordinarily strong bond strength of the C-F bond. By the aid of density functional theory calculations, we disclose an innovative concerted electron-fluoride transfer mechanism, harnessing the unique reactivity of Lewis base-boryl radicals to selectively activate the resilient C-F bonds in fluoroalkanes. This enables the direct abstraction of a fluorine atom and subsequent generation of an alkyl radical, thus expanding the boundaries of halogen atom transfer reactions.

Proton Tunneling Allows a Proton-Coupled Electron Transfer Process in the Cancer Cell

Proton-coupled electron transfer (PCET) is a fundamental redox process and has clear advantages in selectively activating challenging C-H bonds in many biological processes. Intrigued by this activation process, we aimed to develop a facile PCET process in cancer cells by modulating proton tunneling. This approach should lead to the design of an alternative photodynamic therapy (PDT) that depletes the mitochondrial electron transport chain (ETC), the key redox regulator in cancer cells under hypoxia. To observe this depletion process in the cancer cell, we monitored the oxidative-stress-induced depolarization of mitochondrial inner membrane potential (MMP) using fluorescence lifetime imaging microscopy (FLIM). Typically, increasing metabolic stress of cancer cells is reflected in a nontrivial change in the fluorophore's fluorescence lifetime. After 30 min of irradiation, we observed a shift in the mean lifetime value and a drastic drop in overall fluorescence signal. In addition, our PCET strategy resulted in drastic reorganization of mitochondrial morphology from tubular to vesicle-like and causing an overall depletion of intact mitochondria in the hypodermis of C. elegans. These observations confirmed that PCET promoted ROS-induced oxidative stress. Finally, we gained a clear understanding of the proton tunneling effect in the PCET process through photoluminescence experiments and DFT calculations.

Reductive Photocatalytic Proton-Coupled Electron Transfer by a Zirconium-Based Molecular Platform

Reductive proton-coupled electron transfer (PCET) has important energetic implications in numerous synthetic and natural redox processes. The development of catalytic systems that can mediate such transformations has become an attractive target, especially when light is used to generate the reactive species towards solar-to-chemicals conversion. However, such approach becomes challenged by kinetic competition with H2 evolution. Here we describe the excited state reactivity of a molecular Zr-based platform under visible light irradiation for the efficient reduction of multiple bonds. Mechanistic investigations shine light on a charge separation process that colocalizes an excited electron and an acidic proton to promote selective PCET. We further leveraged this reactivity for the photocatalytic reduction of a variety of organic substrates. Our results demonstrate the promise of this molecular platform to design strong photocatalytic PCET mediators for reductive transformations. More broadly, we also show the potential relevance of PCET mechanisms in the (photo)redox chemistry of Zr-based molecular materials.

Unraveling proton-coupled electron transfer in cofactor-free oxidase- and oxygenase-catalyzed oxygen activation: a theoretical view

Oxygen plays a crucial role in the metabolic processes of non-anaerobic organisms. However, a detailed understanding of how triplet oxygen participates in the enzymatic oxidation of organic compounds involved in life processes is still lacking. It is noteworthy that recent studies have found that cofactor-free oxidase- and oxygenase-catalyzed oxygen activation occurs through proton-coupled electron transfer (PCET), which is significantly different from the previously proposed single electron transfer (SET) mechanism. Herein, we summarize the recent advances in the general mechanism of catalytic activation reactions of triplet oxygen by these enzymes. We believe that this review not only helps in providing a deep understanding of the processes involved in oxygen metabolism in organisms but also provides valuable theoretical reference data for designing more efficient enzyme mutants for treating diseases and handling environmental pollution in the future.

Mechanistic studies of NOx reduction reactions involving copper complexes: encouragement of DFT calculations

The reduction of nitrogen oxides (NOx), which is mainly mediated by metalloenzymes and metal complexes, is a critical process in the nitrogen cycle and environmental remediation. This Frontier article highlights the importance of density functional theory (DFT) calculations to gain mechanistic insights into nitrite (NO2-) and nitric oxide (NO) reduction reactions facilitated by copper complexes by focusing on two key processes: the reduction of NO2- to NO by a monocopper complex, with special emphasis on the concerted proton-electron transfer, and the reduction of NO to N2O by a dicopper complex, which involves N-N bond formation, N2O2 isomerization, and N-O bond cleavage. These findings underscore the utility of DFT calculations in unraveling complicated reaction mechanisms and offer a foundation for future research aimed at improving the reactivity of transition metal complexes in NOx reduction reactions.

Photochemical reactions of biomass derived platform chemicals

Platform chemicals obtained from biomass will play an important role in chemical industry. Already existing compounds or not yet established chemicals are produced from this renewable feedstock. Using photochemical reactions as sustainable method for the conversion of matter furthermore permits to develop processes that are interesting from the ecological and economical point of view. Furans or levoglucosenone are thus obtained from carbohydrate containing biomass. Photochemical rearrangements, photooxygenation reactions or photocatalytic radical reactions can be carried out with such compounds. Also, sugars such pentoses or hexoses can be more easily transformed into heterocyclic target compounds when such photochemical reactions are used. Lignin is an important source for aromatic compounds such as vanillin. Photocycloaddition of these compounds with alkenes or the use light supported multicomponent reactions yield interesting target molecules. Dyes, surfactants or compounds possessing a high degree of molecular diversity and complexity have been synthesized with photochemical key steps. Alkenes as platform chemicals are also produced by fermentation processes, for example, with cyanobacteria using biological photosynthesis. Such alkenes as well as terpenes may further be transformed in photochemical reactions yielding, for example, precursors of jet fuels.

Cu(II) Stability and UV-Induced Electron Transfer in a Metal-Organic Hybrid: An EPR, DFT, and Crystallographic Characterization of Copper-Doped Zinc Creatininium Sulfate

Single-crystal X-ray diffraction and electron paramagnetic resonance (EPR) spectroscopic experiments, complemented by quantum chemical DFT calculations, were carried out on the copper-doped metal-organic hybrid and Tutton salt analogue zinc creatininium sulfate to determine its crystal structure, to characterize the electronic structure of the doped Cu(II) binding site, and to propose a pathway for an excited-state, proton-coupled electron transfer (PCET) process in UV-exposed crystals. The crystal structure is isomorphous to that of cadmium creatininium sulfate, which has the transition ion, not in direct coordination with the creatinine, but forming a hexahydrate complex, which is bridged to a creatininium through an intervening sulfate ion. The EPR g (2.446, 2.112, 2.082) and copper hyperfine (ACu: -327, -59.6, 10.8 MHz) tensor parameters are consistent with doped copper replacing host zinc in the metal-hexahydrate complex. These parameters are similar to those observed for copper hexahydrate in doped Tutton salt systems at low temperature, where the unpaired electron occupies mainly the copper 3dx2-y2 orbital. At room temperature in the Tutton systems, vibration couplings stemming from a dynamic Jahn-Teller effect cause tensor averaging which results in a reduction in their maximum g-tensor and hyperfine tensor values. However, like for the doped isomorphous Cd creatinine crystal, the Cu(II) EPR exhibits little, or no room temperature averaging compared to its low temperature pattern. Samples exposed to 254 nm UV light generate a carbon-centered free radical species, characterized by an isotropic g-tensor (g = 2.0029) and an alpha-proton hyperfine coupling (-24 -14 +4 G). These parameters identify it as a creatinine radical cation formed by the oxidative release of one of its C2 methylene hydrogens. DFT calculations confirm the unpaired electronic structures of both the Cu(II) site and free radical. The growth in radical concentration with an increase in the UV exposure time coincides with a decrease in the copper EPR signal, indicating a coupled light-induced oxidation reduction process. A comparison of the crystal structure with the EPR parameters and DFT results provides evidence for a UV-induced PCET.

Electron transfer in biological systems

Examples of how metalloproteins feature in electron transfer processes in biological systems are reviewed. Attention is focused on the electron transport chains of cellular respiration and photosynthesis, and on metalloproteins that directly couple electron transfer to a chemical reaction. Brief mention is also made of extracellular electron transport. While covering highlights of the recent and the current literature, this review is aimed primarily at introducing the senior undergraduate and the novice postgraduate student to this important aspect of bioinorganic chemistry.

Multifunctional Strategies of Advanced Electrocatalysts for Efficient Urea Synthesis

The electrochemical reduction of nitrogenous species (such as N2, NO, NO2 -, and NO3 -) for urea synthesis under ambient conditions has been extensively studied due to their potential to realize carbon/nitrogen neutrality and mitigate environmental pollution, as well as provide a means to store renewable electricity generated from intermittent sources such as wind and solar power. However, the sluggish reaction kinetics and the scarcity of active sites on electrocatalysts have significantly hindered the advancement of their practical applications. Multifunctional engineering of electrocatalysts has been rationally designed and investigated to adjust their electronic structures, increase the density of active sites, and optimize the binding energies to enhance electrocatalytic performance. Here, surface engineering, defect engineering, doping engineering, and heterostructure engineering strategies for efficient nitrogen electro-reduction are comprehensively summarized. The role of each element in engineered electrocatalysts is elucidated at the atomic level, revealing the intrinsic active site, and understanding the relationship between atomic structure and catalytic performance. This review highlights the state-of-the-art progress of electrocatalytic reactions of waste nitrogenous species into urea. Moreover, this review outlines the challenges and opportunities for urea synthesis and aims to facilitate further research into the development of advanced electrocatalysts for a sustainable future.

Optically Gated Dissociation of a Heptazinyl Radical Liberates H• through a Reactive πσ* State

Using trianisole heptazine (TAHz) as a monomeric analogue for carbon nitride, we performed ultrafast pump-photolysis-probe transient absorption (TA) spectroscopy on the intermediate TAHzH• heptazinyl radical produced from an excited state PCET reaction with 4-methoxyphenol (MeOPhOH). Our results demonstrate an optically gated photolysis that releases H• and regenerates ground state TAHz. The TAHzH• radical signature at 520 nm had a lifetime of 7.0 ps, and its photodissociation by the photolysis pulse is clearly demonstrated by the ground state bleach recovery of the closed-shell neutral TAHz. This behavior has been previously predicted as evidence of a dissociative πσ* state. For the first time, we experimentally demonstrate photolysis of the TAHzH• heptazinyl radical through a repulsive πσ* state. This is a critical feature of the proposed reaction mechanisms involving water oxidation and CO2 reduction.

Spectroscopically Elucidating the Local Proton-Coupled Electron Transfer Loop from Amino to Nitro Groups via the Au Surface in a N2 Atmosphere

Proton-coupled electron transfer (PCET) has been significant in understanding the reactions in solution. In a solid-gas interface, it remains a challenge to identify electron transfer or proton transfer intermediates. Here, in a Au/N2 interface, we regulated and characterized the PCET from p-aminothiophenol (PATP) to p-nitrothiophenol (PNTP) in the plasmon-mediated conversion to p,p'-dimercaptoazobenzene by variable-temperature surface-enhanced Raman spectroscopy. The Raman bands of PATP and PNTP characteristically blue shifted and red shifted as the laser wavelength- and power density-regulated PCET from PATP to PNTP, respectively. These characteristic Raman band shifts were well reproduced by the density functional theoretical simulations of positively charged PATP and negatively charged PNTP, which explicitly evidenced the electron transfer intermediates of PATP or PNTP on the Au surface. PCET did not occur in the temperature cycle between 100 and 370 K without laser illumination. These results demonstrated a characteristic local PCET loop composed of electron transfer between PATP/PNTP and Au followed by intermolecular proton transfer between PATP and PNTP and the significance of conducting electron transfer on Au.

Intracellular Gold Nanocluster/Organic Semiconductor Heterostructure for Enhancing Photosynthesis

Photosynthetic microorganisms, which rely on light-driven electron transfer, store solar energy in self-energy carriers and convert it into bioenergy. Although these microorganisms can operate light-induced charge separation with nearly 100 % quantum efficiency, their practical applications are inherently limited by the photosynthetic energy conversion efficiency. Artificial semiconductors can induce an electronic response to photoexcitation, providing additional excited electrons for natural photosynthesis to improve solar conversion efficiency. However, challenges remain in importing exogenous electrons across cell membranes. In this work, we have developed an engineered gold nanocluster/organic semiconductor heterostructure (AuNCs@OFTF) to couple the intracellular electron transport chain of living cyanobacteria. AuNCs@OFTF exhibits a prolonged excited state lifetime and effective charge separation. The internalized AuNCs@OFTF permits its photogenerated electrons to participate in the downstream of photosystem II and construct an oriented electronic highway, which enables a five-fold increase in photocurrent in living cyanobacteria. Moreover, the binding events of AuNCs@OFTF established an abiotic-biotic electronic interface at the thylakoid membrane to enhance electron flux and finally furnished nicotinamide adenine dinucleotide phosphate. Thus, AuNCs@OFTF can be exploited to spatiotemporally manipulate and enhance the solar conversion of living cyanobacteria in cells, providing an extended nanotechnology for re-engineering photosynthetic pathways.