Simultaneous Electron Transfer Research Articles

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.

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.

Superhydrophilic covalent organic frameworks accelerate photocatalytic production of hydrogen peroxide through proton channels

Superhydrophilic COFs with D–A structure accelerate photocatalytic synthesis of H2O2 using water as a proton supply reservoir.

Synthesis, characterization, electrochemistry and spectroelectrochemistry of new non-peripherally substituted lutetium bis phthalocyanines

The synthesis, characterization, spectral, and electrochemical properties of bis-(phthalocyanine)lutetium (III) (LuPc2) bearing halogenated (F (F-LuPc2), Cl (Cl-LuPc2) and Br (Br-LuPc2)) phenoxy groups were reported for the first time. To explore the possible application of the LuPc2 complexes, their detailed redox mechanisms were determined by voltammetric and spectroelectrochemical characterizations. One of the Pc rings exhibits radical characteristics as a result of the sandwich structures of the complexes, which led to the typical one electron reduction process between 0.20 and 0.40 V and the typical one electron oxidation process between 0.60 and 0.80 V.In addition to the characteristic Pc.-Lu3+Pc2-/Pc2-Lu3+Pc2-1- reduction and Pc.-Lu3+Pc2-/Pc.-Lu3+Pc.-1+ oxidation couple of the complexes, further two-electron reduction and oxidation processes were also observed due to the simultaneous electron transfer reactions of each Pc ring at very close potentials. Moreover, at more negative potentials sequential one electron reduction waves for each Pc ring were also observed. While the Cl-LuPc2 reduced at less positive potential and more negative potential, changing Br with Cl and F alters the ease of the electron transfer reactions of the complexes. In situ spectroelectrochemical results supported these voltammetric assignments. Moreover, distinct color changes of LuPc2 in the solution observed with in situ spectroelectrochemical measurements indicated their functionality as electrochromic materials.

Unravelling the Effect of Acid-Driven Electron Transfer in High-Valent FeIV =O/MnIV =O Species and Its Implications for Reactivity.

The electron transfer (ET) step is one of the crucial processes in biochemical redox reactions that occur in nature and has been established as a key step in dictating the reactivity of high-valent metal-oxo species. Although metalloenzymes possessing metal-oxo units at their active site are typically associated with outer-sphere electron transfer (OSET) processes, biomimetic models, in contrast, have been found to manifest either an inner-sphere electron transfer (ISET) or OSET mechanism. This distinction is clearly illustrated through the behaviour of [(N4Py)MnIV (O)]2+ (1) and [(N4Py)FeIV (O)]2+ (2) complexes, where complex 1 showcases an OSET mechanism, while complex 2 exhibits an ISET mechanism, especially evident in their reactions involving C-H bond activation and oxygen atom transfer reactions in the presence of a Lewis/Bronsted acid. However, the precise reason for this puzzling difference remains elusive. This work unveils the origin of the perplexing inner-sphere vs outer-sphere electron transfer process (ISET vs OSET) in [(N4Py)MnIV (O)]2+ (1) and [(N4Py)FeIV (O)]2+ (2) species in the presence of Bronsted acid. The calculations indicate that when the substrate (toluene) approaches both 1 and 2 that is hydrogen bonded with two HOTf molecules (denoted as 1-HOTf and 2-HOTf, respectively), proton transfer from one of the HOTf molecules to the metal-oxo unit is triggered and a simultaneous electron transfer occurs from toluene to the metal centre. Interestingly, the preference for OSET by 1-HOTf is found to originate from the choice of MnIV =O centre to abstract spin-down (β) electron from toluene to its δ(dxy ) orbital. On the other hand, in 2-HOTf, a spin state inversion from triplet to quintet state takes place during the proton (from HOTf) coupled electron transfer (from toluene) preferring a spin-up (α) electron abstraction to its σ* (dz 2 ) orbital mediated by HOTf giving rise to ISET. In addition, 2-HOTf was calculated to possess a larger reorganisation energy, which facilitates the ISET process via the acid. The absence of spin-inversion and smaller reorganisation energy switch the mechanism to OSET for 1-HOTf. Therefore, for the first time, the significance of spin-state and spin-inversion in the electron transfer process has been identified and demonstrated within the realm of high-valent metal-oxo chemistry. This discovery holds implications for the potential involvement of high-valent Mn-oxo species in performing similar transformative processes within Photosystem II.

Facilitating Proton Coupled Electron Transfer Reaction through the Interfacial Micro Electric Field with Fe─N4 ─C in FeMOFs Glass.

The proton-coupled electron transfer(PCET) reaction plays a crucial role in the chemical transformation process andhas become one of the most concerned elementary reactions. However, the complex kinetics of PCET reaction, which requires the simultaneous transfer of protons and electrons, leads to the dilemma that thermodynamics and kinetics cannot bebalanced and restricts its further development. In this, an interface micro-electric field (IMEF) basedon Fe─N4 in FeMOFs (Fe-Based Metal-Organic Frameworks) glass is designed tosynchronize proton/electron interface behavior for the first time to realizeefficient PCET reaction and optimize reaction thermodynamics and kinetics. The IMEF facilitates the separation of photogenerated electrons and holes, and accelerates Fe(III)/Fe(II) cycle. Driven by near-surface electric field force, the protons near surfacemigrate to Fe sites and participate in Fe(IV)═O formation and reaction, lowering the reaction energy barrier. Based on the interface regulation ofIMEF, a high-efficiency PCET reaction is realized, and kinetic reactionrate constant of photocatalytic oxidation of emerging contaminants is increasedby 3.7times. This study highlights a strategy for IMEFs to modulate PEC Treactions for a wide range of potential applications, including environmental and ecological applications.

The role of the electrolyte in non-conjugated radical polymers for metal-free aqueous energy storage electrodes.

Metal-free aqueous batteries can potentially address the projected shortages of strategic metals and safety issues found in lithium-ion batteries. More specifically, redox-active non-conjugated radical polymers are promising candidates for metal-free aqueous batteries because of the polymers' high discharge voltage and fast redox kinetics. However, little is known regarding the energy storage mechanism of these polymers in an aqueous environment. The reaction itself is complex and difficult to resolve because of the simultaneous transfer of electrons, ions and water molecules. Here we demonstrate the nature of the redox reaction for poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl acrylamide) by examining aqueous electrolytes of varying chao-/kosmotropic character using electrochemical quartz crystal microbalance with dissipation monitoring at a range of timescales. Surprisingly, the capacity can vary by as much as 1,000% depending on the electrolyte, in which certain ions enable better kinetics, higher capacity and higher cycling stability.

Non-conjugated polymer ligand: stimulating charge transfer towards photocatalytic selective organic transformation

Non-conjugated polymer ligand and metal NCs as simultaneous electron transfer mediators are synergistically harnessed to stimulate photocatalytic selective organic transformation.

How a Formate Dehydrogenase Responds to Oxygen: Unexpected O2 Insensitivity of an Enzyme Harboring Tungstopterin, Selenocysteine, and [4Fe–4S] Clusters

The reversible two-electron interconversion of formate and CO2 is catalyzed by both nonmetallo- and metallo-formate dehydrogenases (FDHs). The latter group comprises molybdenum- or tungsten-containing enzymes with the metal coordinated by two equivalents of a pyranopterin cofactor, a cysteinyl or selenocysteinyl (Sec) ligand supplied by the polypeptide, and a catalytically essential terminal sulfido ligand. In addition, these biocatalysts incorporate one or more [4Fe–4S] clusters for facilitating long-distance electron transfer. However, an interesting dichotomy arises when attempting to understand how the metallo-FDHs react with O2. Whereas existing scholarship portrays these enzymes as being unable to perform in air due to extreme O2 lability of their metal centers, studies dating as far back as the 1930s emphasize that some of these systems exhibit formate oxidase (FOX) activity, coupling formate oxidation to O2 reduction. Therefore, to reconcile these conflicting views, we explored context-dependent functional linkages between metallo-FDHs and their cognate electron acceptors within the same organism vis-à-vis catalysis under atmospheric O2. Here, we report the discovery and characterization of an O2-insensitive FDH2 from the sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough (DvH) that ligates tungsten, Sec, and four [4Fe–4S] clusters. By advancing a robust expression platform for its recombinant production, we eliminate both the requirement of nitrate or azide during purification and reductive activation with thiols and/or formate prior to catalysis. Because the distinctive spectral signatures of formate-reduced DvH-FDH2 remain invariant under anaerobic and aerobic conditions, we benchmarked the enzyme activity in air, identifying CO2 as the catalytic product. Full reaction progress curve analysis discloses a high catalytic efficiency when probed with a high-potential artificial electron acceptor. Furthermore, we show that DvH-FDH2 enables near-stoichiometric hydrogen peroxide production without superoxide release to achieve O2 insensitivity. Notably, simultaneous electron transfer to cytochrome c and O2 reveals that metal-based electron bifurcation is operational in this system. Taken together, our work proves the co-occurrence of redox bifurcated FDH and FOX activities within a metalloenzyme scaffold. These findings set the stage for uncovering previously unknown O2-insensitive flavin-based electron bifurcation mechanisms, as well as for developing authentic formate/air biofuel cells, engineering O2-stable FDHs and biohybrid metallocatalysts, and discerning formate bioenergetics of gut microbiota.

Evidence of the Ambipolar Behavior of Mo6 Cluster Iodides in All-Inorganic Solar Cells: A New Example of Nanoarchitectonic Concept.

Ambipolar materials such as carbon nanotubes, graphene, or 2D transition metal chalcogenides are very attractive for a large range of applications, namely, light-emitting transistors, logic circuits, gas sensors, flash memories, and solar cells. In this work, it is shown that the nanoarchitectonics of inorganic Mo6 cluster-based iodides enable to form thin films exhibiting photophysical properties that enable their classification as new members of the restricted family of ambipolar materials. Thus, the electronic properties of the ternary iodide Cs2[{Mo6I8i}I6a] and those of thin films of the aqua-complex-based compound [{Mo6I8i}I4a(H2O)2a]·xH2O were investigated through an in-depth photoelectrochemical study. Once hole/electron pairs are created, the holes and electrons turn to be transported simultaneously in opposite directions, and their lifetimes exhibit similar values. The ambipolar properties were demonstrated via the integration of [{Mo6I8i}I4a(H2O)2a]·xH2O as light harvesters in an all-solid solar cell. A significant photoresponse with a typical diode characteristic clearly provides evidence of the simultaneous transfer and transport of holes and electrons within the [{Mo6I8i}I4a(H2O)2a]·xH2O layer. The ambipolar behavior results, on the one hand, from the confinement of electrons imposed by the nanometric size of the molecular metal clusters and, on the other hand, from the poor electronic interactions between clusters in the solid state. Such molecular structure-based layers lead naturally to an intrinsic semiconducting behavior.

Insights into Proton Coupled Electron Transfer in the Field of Artificial Photosynthesis

AbstractArtificial photosynthesis with respect to water splitting is usually divided into water oxidation catalysis (WOC) and the hydrogen evolution reaction (HER). Though in the combined redox dissociation of water into oxygen and hydrogen no protons and electrons occur, both half reactions show photoinduced proton coupled electron transfer (PCET). Regarding a classical approach, photosensitizers (PS) deliver electrons and protons that are accepted by water reduction catalysts (WRC) containing suitable basic atoms like nitrogen working as proton relays. However, the mechanisms of PCET reactions differ, where concerted proton electron transfer (CPET) is an elementary step. In CPET simultaneous electron and proton transfer occurs in the femtoseconds range, being rapid when compared to the periods for coupled vibrations and solvent modes. This has to be distinguished from stepwise electron and proton transfer, leading to underlying thermodynamics of the intermediates. DFT calculations based on X‐ray diffraction (XRD) data help to specify the different reaction pathways. A plethora of experimental procedures are used in order to verify the theoretical predictions. Among them femtosecond pump‐probe spectroscopic measurements play an important role. Furthermore, cyclic voltammetry (CV) has proven to be also a powerful tool. In the case of electrochemical PCET rotating‐disk electrode voltammetry, electrochemical impedance spectroscopy and spectroelectrochemistry complete the experimental tools. In this minireview a selection of examples, where PCET occurs is discussed with respect to possible mechanisms and used methods.

Proton Transfer Management for Efficient Oxygen Evolution on α-Fe2O3 Electrode

The oxygen evolution reaction (OER, 2H2O → O2 + 4H+ + 4e-) has received considerable attention in recent years as a promising method for utilizing water as a sustainable electron source. By coupling the OER with a reduction half-reaction such as hydrogen evolution (2H+ + 2e- → H2) or carbon dioxide reduction (ex. CO2 + 8H+ + 8e- → CH4 + 2H2O), the intermittent energy from renewable sources, including solar energy or wind power, can be captured in the form of chemical fuels. However, driving the four-electron process to oxidize water to oxygen has proven difficult due to the large overpotential. Although RuO2 and IrO2 have been reported to show good catalytic performance, the scarcity and high costs associated with precious metals limit the large-scale applicability of these materials. Therefore, it is highly desirable to substitute them with catalysts derived from earth-abundant 3d metals such as Mn, Co, and Fe.This article reports the investigation of the OER mechanism of hematite (α-Fe2O3) to gain insight into the critical factors determining its electrocatalytic activity. Using spectroelectrochemimcal measurements, an intermediate species assignable to Fe4+ was identified as the precursor of OER across the entire pH range investigated (pH 4−13) [1,2]. The pH dependences of the onset potentials for OER and Fe4+ formation revealed that α-Fe2O3 showed higher catalytic activity under alkaline conditions and that there were two reaction mechanisms which switched at approximately pH 10. From the results of kinetic isotope effect experiments, it was found that the manner of proton and electron transfer during Fe4+ formation (Fe3+–OH → Fe4+=O + H+ + e-) was different between the two mechanisms. Whereas proton transfer and electron transfer proceed as discrete reaction steps at neutral pH, they proceed simultaneously at alkaline pH. Because the simultaneous transfer of proton and electron enables to avoid formation of high-energy protonated or deprotonated intermediates, the simultaneous transfer is energetically more favorable than the stepwise one, resulting in the higher activity under alkaline pH conditions. Notably, by adding appropriate pyridine derivatives as proton accepting reagents, the switching of the rate-determining step from the stepwise process to the simultaneous process was demonstrated [3]. As a result, a decrease in overpotential of 250 mV was achieved at pH 7 and the OER activity of α-Fe2O3 under neutral pH conditions became comparable to that under alkaline pH conditions. Furthermore, when the pyridine derivatives were replaced with a solid proton accepting material that is robust against anodic decomposition, the enhanced activity was maintained in the long-term operation [4]. These findings will be helpful to design active Fe-based OER catalysts and offer the opportunities to achieve high OER activity under neutral pH using 3d-metal catalysts which often show remarkable activities under alkaline pH conditions.[1] T. Takashima, K. Ishikawa, H. Irie, J. Phys. Chem. C, 120, 24827–24834 (2016).[2] T. Takashima, S. Hemmi, Q. Liu, H. Irie, Catal. Sci. Technol., in press.[3] T. Takashima, K. Ishikawa, H. Irie, Chem. Commun., 52, 14015–14018 (2016).[4] T. Takashima, K. Ishikawa, H. Irie, ACS Catal., 9, 9212–9215 (2019).

Acid Catalysis via Acid‐Promoted Electron Transfer

This feature article focuses on acid catalysis in various redox reactions of not only organic compounds but also metal complexes, in particular metal‐oxygen complexes via acid‐promoted electron transfer (APET). On the other hand, proton‐coupled electron transfer (PCET) has been extensively studied in relation with important biological PCET reactions such as O2 reduction in respiration and water oxidation in photosynthesis. Because PCET is normally defined as simultaneous electron and proton transfer from the same substrate, acid‐promoted electron transfer using external acids is called APET in this article. Various types of chemical transformations exhibiting acid catalysis via APET discussed in this article include CC bond formation, CH bond oxidation, aromatic hydroxylation, and oxygen atom transfer reactions such as sulfoxidation and epoxidation reactions.

Hybrid Multipixel Array X-Ray Detectors for Real-Time Direct Detection of Hard X-Rays

X-ray detectors currently employed in dosimetry suffer from a number of drawbacks including the inability to conform to curved surfaces and being limited to smaller dimensions due to available crystal sizes. In this study, a hybrid X-ray detector (HXD) has been developed which offers real-time response with added advantages of being highly sensitive over a broad energy range, mechanically flexible, relatively inexpensive, and able to be fabricated over large areas on the desired surface. The detector comprises an organic matrix embedded with high-atomic-number inorganic nanoparticles which increase the radiation attenuation and within the device allows for simultaneous transfer of electrons and holes. The HXD delivers a peak response of 14 nA cm−2, which corresponds to a sensitivity of $30.8~\mu \text{C}$ Gy−1 cm−2, under the exposure of 6-MV hard X-rays generated by a medical linear accelerator. The angular dependence of the HXD has been studied, which offers a maximum variation of 26% in the posterior versus lateral beam directions. The flexible HXD can be conformed to the human body shape and is expected to eliminate variations due to source-to-skin distance with reduced physical evaluation complexities.

Photocatalytic degradation and rate constant prediction of chlorophenols and bisphenols by H3PW12O40/GR/TiO2 composite membrane

Photocatalysis is a promising approach to remove highly toxic and refractory aromatics pollutants. However, developing highly active photocatalyst is a long-standing challenge for pollutant degradation. This study addressed this challenge by developing GR (graphene)/TiO2 and HPW (H3PW12O40)/GR/TiO2 membranes by sol-gel method. The removal efficiencies of HPW/GR/TiO2 (the doping of 1.0% HPW) membrane for chlorophenols (including o-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, and pentachlorophenol) and bisphenols (such as Bisphenol A, Bisphenol AP, Bisphenol AF, and Bisphenol S) were up to 97.02–82.71% and 93.28–68.63% with simulated sunlight radiation for 5 h, respectively. Compared with GR/TiO2 composite membrane, HPW/GR/TiO2 remarkably accelerated the formation rates of O2− and OH, due to the simultaneous transfer of photo-generated electrons (generated by TiO2) to GR and HPW. In addition, the activity of the HPW/GR/TiO2 membrane did not decline noticeably after 10-time recycle. Furthermore, the photocatalytic degradation reaction rate constants (k) of phenols by HPW/GR/TiO2 membrane were calculated, and those for other chlorophenols and bisphenols were predicted using a quantitative structure-activity relationship model. The HPW/GR/TiO2 membrane developed in this study poses high potential as an ideal photocatalyst for removal of phenolic pollutants in wastewater.

Electron correlation in simultaneous electron transfer and excitation: results of new 2eAOCC calculations and comparison to experiment in 50-500 keV He+ + H/H2 collisions

SynopsisNew two-electron semiclassical atomic orbital coupled channel calculations (2eAOCC) are compared to experiment and to older coupled channel calculations. Progress in these type of calculations is assessed in an effort to investigate the effect of electron correlations in this fundamental collision system.

Reagent or catalyst? – FeS as activator for persulfate in water

FeS was evaluated as heterogeneous activator for peroxydisulfate (PS) with trichloroethene (TCE) chosen as model substance representing organic water pollutants prone to fast oxidation by sulfate radicals. The TCE degradation followed in most cases pseudo-first-order kinetics with a FeS-normalized maximum rate constant of k’TCE = 140 M−1min−1 (with cFeS,0 = 0.28 mM and cPS,0 = 5 mM).Sulfate radicals were determined as predominant radical species formed during PS activation by FeS by evaluation of the kinetic isotope effect for oxidation of cyclohexanes C6H12 vs. C6D12 (kC6H12: kC6D12 = 2.22). Monitoring of sulfur and iron species during the course of the reaction revealed that sulfide is rapidly oxidized, whereas Fe(II)solid is rather stable over time.The present study describes in detail the influence of the pH value on the reaction and the long-term performance of FeS as activator for PS with an optimum pH value of 5. On the basis of the radical yield from PS of 1.6 mols SO4− per mol S2O82− and the activation energy of EA = (31 ± 1) kJ mol−1 for the heterogeneous radical generation, a surface-assisted homolytic bond cleavage without simultaneous electron transfer is proposed as activation mechanism, which is contrary to the currently prevailing opinion.

Micropollutant Oxidation Studied by Quantum Chemical Computations: Methodology and Applications to Thermodynamics, Kinetics, and Reaction Mechanisms.

The abatement of organic micropollutants during oxidation processes has become an emerging issue for various urban water systems such as drinking water, wastewater, and water reuse. Reaction kinetics and mechanisms play an important role in terms of efficiency of these processes and the formation of transformation products, which are controlled by functional groups in the micropollutants and the applied oxidants. So far, the kinetic and mechanistic information on the underlying reactions was obtained by experimental studies; additionally, predictive quantitative structure-activity relationships (QSARs) were applied to determine reaction kinetics for the oxidation of emerging compounds. Since this experimental approach is very laborious and there are tens of thousands potential contaminants, alternative strategies need to be developed to predict the fate of micropollutants during oxidative water treatment. Due to significant developments in quantum chemical (QC) computations in recent years and increased computational capacity, QC-based methods have become an alternative or a supplement to the current experimental approach. This Account provides a critical assessment of the current state-of-the-art of QC-based methods for the assessment of oxidation of micropollutants. Starting from a given input structure, QC computations need to locate energetic minima on the potential energy surface (PES). Then, useful thermodynamic and kinetic information can be estimated by different approaches: Experimentally determined reaction mechanisms can be validated by identification of transition structures on the PES, which can be obtained for addition reactions, heavy atom transfer (Cl+, Br+, O·) and H atom transfer (simultaneous proton and electron transfer) reactions. However, transition structures in the PES cannot be obtained for e--transfer reactions. Second-order rate constants k for the reactions of micropollutants with chemical oxidants can be obtained by ab initio calculations or by QSARs with various QC descriptors. It has been demonstrated that second-order rate constants from ab initio calculations are within factors 3-750 of the measured values, whereas QSAR-based methods can achieve factors 2-4 compared to the experimental data. The orbital eigenvalue of the highest occupied molecular orbital ( EHOMO) is the most commonly used descriptor for QSAR-based computations of k-values. In combination with results from experimental studies, QC computations can also be applied to investigate reaction mechanisms for verification/understanding of oxidative mechanisms, calculation of branching ratios or regioselectivity, evaluation of the experimental product distribution and assessment of substitution effects. Furthermore, other important physical-chemical constants such as unknown equilibria for species, which are not measurable due to low concentrations, or p Ka values of reactive transient species can be estimated. With further development of QC-based methods, it will become possible to implement kinetic and mechanistic information from such computations in in silico models to predict oxidative transformation of micropollutants. Such predictions can then be complemented by tailored experimental studies to confirm/falsify the computations.

Ultrafast Energy Transfer of Both Bright and Dark Excitons in 2D van der Waals Heterostructures Beyond Dipolar Coupling.

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have shown great potential in ultrathin and flexible optoelectronic and photonics devices. Besides emissive bright excitons, they also possess rich non-emissive dark excitons including momentum-forbidden indirect excitons and spin-forbidden triplet-like excitons, which could be dominant species under optical or electrical excitation in 2D optoelectronic and photonic devices. Efficient harvesting of both bright and dark excitons from TMDs and understanding the exciton-transfer mechanism consequently are not only of fundamental interest but also a technological challenge. Here, by combining steady-state photoluminescence spectroscopy and ultrafast transient reflectance spectroscopy, we show efficient exciton harvesting by ultrafast energy transfer in WSe2/MoTe2 van der Waals heterostructures, leading to the photoluminescence enhancement of MoTe2. The energy transfer occurs with near-unity yield and in an ultrafast (∼200 fs) manner for both bright and dark excitons, suggesting a dominant Dexter-type energy-transfer process consisting of simultaneous transfer of both electron and hole in van der Waals coupled 2D layers at ultimate proximity. This result is beyond the conventional dipole-dipole coupling mechanism typically assumed at 2D interfaces and offers a path to high speed and enhanced light harvesting and emission applications based on 2D heterostructures.

The Thermodynamics of Insertion Electrochemical Electrodes-A Team Play of Electrons and Ions across Two Separate Interfaces.

Insertion electrochemical electrodes exhibit simultaneous electron and ion transfer, with the two transfers proceeding across different interfaces. Herein the thermodynamics of the overall electrochemical electrode reaction is discussed with respect to the thermodynamics of these two charge-transfer equilibria. This Minireview includes insertion electrochemical systems where the redox centers are in a solid phase and the ions are transferred between that phase and a solution, and also systems where the redox centers are in a liquid phase that is immiscible with another liquid phase and ions are transferred between the two liquid phases. The Minireview is intended to spark similar studies on battery materials to improve their performance.