ET Reactions Research Articles

Variable temperature kinetic isotope effects demonstrate extensive tunnelling in the C-H activation reactivity of a transition metal-oxo complex.

There has been recent interest about how the rates of concerted proton electron transfer (CPET) are affected by the thermodynamic parameters of intermediates from stepwise PT or ET reactions. Semiclassical arguments have been used to explain these trends despite the importance of quantum mechanical tunneling in CPET reactions. Here we report variable temperature kinetic isotope effect (KIE) data for the reactivity of a terminal Co-oxo complex with C-H bonds. The KIEs for the oxidation of both 9,10-dihydroanthracene (DHA) and fluorene have significant tunneling contributions and fluorene has a largely temperature-insensitive KIE which is inconsistent with semiclassical models. These findings support recent calls for a more detailed understanding of tunneling effects in thermodynamically imbalanced CPET reactions.

Dynamics of Electron Transfers in Photosensitization Reactions of Zinc Porphyrin Derivatives

Photocatalytic systems for CO2 reduction operate via complicated multi-electron transfer (ET) processes. A complete understanding of these ET dynamics can be challenging but is key to improving the efficiency of CO2 conversion. Here, we report the ET dynamics of a series of zinc porphyrin derivatives (ZnPs) in the photosensitization reactions where sequential ET reactions of ZnPs occur with a sacrificial electron donor (SED) and then with TiO2. We employed picosecond time-resolved fluorescence spectroscopy and femtosecond transient absorption (TA) measurement to investigate the fast ET dynamics concealed in the steady-state or slow time-resolved measurements. As a result, Stern-Volmer analysis of fluorescence lifetimes evidenced that the reaction of photoexcited ZnPs with SED involves static and dynamic quenching. The global fits to the TA spectra identified much faster ET dynamics on a few nanosecond-time scales in the reactions of one-electron reduced species (ZnPs•-) with TiO2 compared to previously measured minute-scale quenching dynamics and even diffusion rates. We propose that these dynamics report the ET dynamics of ZnPs•- formed at adjacent TiO2 without involving diffusion. This study highlights the importance of ultrafast time-resolved spectroscopy for elucidating the detailed ET dynamics in photosensitization reactions.

Multiple free radical scavenging reactions of flavonoids

The multiple free radical scavenging reactions of flavonoids have been studied considering the gas, benzene, and water phases by applying density functional theory (DFT). Intramolecular hydrogen-bonds are found in all the most stable geometries of flavonoids and can reduce the antioxidant activity of hydroxyl groups, acting as hydrogen-bond donors (5−OH, 3−OH and 3’−OH), while enhancing the antioxidant activity of hydroxyl groups, acting as hydrogen-bond acceptors (4’−OH). In the gas and benzene phases, all of the flavonoids first prefer performing continuous di-hydrogen atom transfer (HAT) reaction from the B ring OH groups to trap two free radicals with the formation of stable quinones for ampelopsin, taxifolin, dihydro orobol and eriodictyol and benzodioxole for hesperetin. They would trap the third free radical via the HAT in the gas phase. In the benzene phase, ampelopsin also favours to apply HAT to trap the third free radical, and the other flavonoids would use the sequential proton loss electron transfer (SPLET) mechanism. In the water phase, the investigated flavonoids would first perform consecutive proton loss (PL) reactions from all of the OH groups with the formation of polyanions. The multiple PL reactions begin from the 7−OH group. The second PL reaction prefers performing in the OH groups on the B benzene ring due to the better delocalization of the negative charge via conjugation over the entire skeleton. The polyanions of the investigated flavonoids scavenge three free radicals via three consecutive ET reactions.

Synergistic dynamics of photoionization and photoinduced electron transfer probed by laser flash photolysis and ultrafast fluorescence spectroscopy.

Photoionization (PI) and photoinduced electron transfer (PET) dynamics of coumarin 450 (C450) in micelles were investigated in the time domains of micro to femtoseconds using steady-state and time-resolved absorption and fluorescence spectroscopy. The PI of C450 occurs inside the micelles leads to the formation of C450 cation radical (CR) and hydrated electron, which is characterized by the respective transient absorption. The PI of C450 is monophotonic in nature and the yield is dependent on the charge of the micelles. The observation of amine CR in the transient absorption confirms the PET from amine to the excited state of C450 in micelles, which results in the quenching of both fluorescence intensity and lifetime. The decrease in femtosecond fluorescent decay of C450 and the absence of transient C450 radical anion in the presence of amine implies that the concerted ultrafast PET promoted PI and PET to the C450 CR-electron pair. The decrease in the time constant for the formation of relaxed state in the presence of amines is due to the ultrafast PET to the C450 CR-electron pair, which prevents the formation of a relaxed state through recombination at a longer time scale. In the present investigation, the recombination dynamics of the CR-electron pair is justified as one of the origins of the slow solvation in micelles. The influence of amine concentration on the decay of C450 CR indicates ET reaction between C450 CR and amine, which is further confirmed by the bleach recovery of C450 ground state in the presence of amine.

(Invited) Understanding Inner-Sphere Electrochemical Reactions at the Molecular Level

Interfacial ET processes can be broadly classified as either outer-sphere or inner-sphere. For outer-sphere reactions, the electrode serves only as a chemically inert source or sink of electrons, whereas, for inner-sphere reactions, ET accompanies bond rearrangement at the electrode surface. While outer-sphere ET is well-described by Marcus-type models, there currently exists no analogous molecular-level understanding of inner-sphere ET at electrode surfaces, despite the central role of these reactions in virtually all electrochemical technologies, ranging from supercapacitors to batteries to fuel cells and electrolyzers. This persistent knowledge gap results from the inability to characterize or control the chemistry of metallic electrode surfaces at the molecular level, making it difficult, if not impossible, to unambiguously correlate the thermodynamics and kinetics of inner-sphere reactions to the local structure of the surface site undergoing bond rearrangement. This talk will highlight a strategy developed in our group for conjugating well-defined molecular active sites to graphitic electrodes. Electrochemical and spectroscopic data indicate that these graphite-conjugated molecules do not behave like their solution-phase analogues, but rather as metallic active sites that proceed exclusively through inner-sphere ET reaction mechanisms. This new class of functional interfaces enables investigations of inner-sphere interfacial ET with an unprecedented level of detail.

Exact eigenenergies of a model of vibronically coupled electron transfer reactions.

We present the first exact solution to the time-independent Schrödinger equation of a model Hamiltonian consisting of a vibrational mode coupled to three electronic states. This Hamiltonian serves as a generic model for photo-induced electronic transfer reactions. The solution is non-perturbative and can be applied to ET reactions with weak and strong electronic and vibrational coupling strengths. This work suggests a new direction towards understanding the vibronic effects in ET dynamics beyond the non-adiabatic limit and Condon approximation.

Lanthanide (III) ions as multichannel acceptors for bimolecular photoinduced electron transfer reactions with coumarin dyes

Understanding the kinetics and energetics of photoinduced electron transfer (PET) reactions is of considerable research interest in photochemical sciences. In this study, interactions between trivalent lanthanide ions (Ln(III)) as electron acceptors and coumarin dyes as electron donors have been investigated in aqueous media using ground state absorption, steady-state (SS) emission, time-resolved (TR) fluorescence and laser flash photolysis (LFP) techniques. Among different Ln(III) ions, only Eu(III), Yb(III) and Sm(III) are found to display substantial interaction with the excited singlet (S1) state of the coumarin dyes, which correspond well with the favorable free energy changes (negative ΔG0) associated with the PET reactions in these coumarin-Ln(III) systems. For other Ln(III) ions, as ΔG0 is not favorable, they do not show any interaction with the excited coumarin dyes. Among the interacting coumarin-Ln(III) systems, bimolecular quenching rate constant (kq) increases consistently with the increasing exergonicity (−ΔG0), when each of the Ln(III) ions are considered independently, except for Eu(III), for which the kq values have already reached the diffusion controlled limit. However, considering all the Ln(III) ions together, the kq values do not show any common correlation with the changing ΔG0 values. The kq values are in general much lower for Yb(III) quencher as compared to Sm(III), even though the exergonicity values for coumarin-Yb(III) systems are more favorable than the coumarin-Sm(III) systems. Correlating the observed results based on the diffusion mediated bimolecular reaction model, we infer that depending on the ΔG0 values, some Ln(III) quenchers possibly also engage their upper electronic energy states as well to assist multichannel ET with the excited coumarin donors, and thereby modulating the kq values very substantially for different coumarin-Ln(III) systems. Apparently, the involvement of multichannel ET causes the total reorganization energy (λ) for ET reactions to differ largely for different Ln(III) ions, which would otherwise be very similar for all the coumarin-Ln(III) pairs. Observed results suggest that the intramolecular reorganization energy (λi) associated with lower electronic states of the hydrated Ln(III) ions is much larger than that associated with higher electronic states of these ions. While the involvement of multichannel ET is itself an intriguing inference, modulation in the ET kinetics and total λ values for different Ln(III) quenchers through the differential participation of the multichannel PET is also expected to find potential uses in the controlled utilization of such PET reactions in many practical areas.

Dynamical effects in metalloprotein heterogeneous electron transfer

In this work we assess the influence of physiological viscosities on metalloprotein electron transfer reactions. To that end we investigated the direct electrochemistry of the copper proteins azurin and CuA adsorbed on SAM-coated electrodes. The experimental results show a change of ET regime from non-adiabatic to friction control upon shortening tunneling distances, which is paralleled by a sigmoidal increase of the apparent ET reorganization energies. These data could be accurately described using novel Matyushov’s model, thereby validating this recent theoretical development, which however did not describe viscosity effects in its original formulation. We demonstrate that ET rate constants and effective relaxation times vary with viscosity following power laws. Moreover, the crossover parameter g that determines the ET regime could be redefined in terms of the exponents of these power laws. The magnitude of g, i.e. the extent of frictional control, was found to be protein specific as it is determined by the dynamical features of the protein milieu. Interestingly, Stokes shift and diffusional relaxation times were found to be of similar magnitude, thus resulting in non-negligible frictional control even at tunneling distances as long as 19 Å, and this effect is amplified by physiologically high viscosities, thus highlighting the influence of intracellular macromolecular crowding in modulating protein ET reactions.

Hybridization Biosensors Relying on Electrical Properties of Nucleic Acids

AbstractElectrochemical nucleic acids (NA) technologies allow development of sensitive and accurate, yet simple, inexpensive and robust analytical platforms, which can successfully compete with other approaches. In this Review, I critically overview the basic trends in construction of label‐free DNA hybridization genosensors exploiting variations in electrical properties of the electrode‐immobilized NA. It will be discussed how interfacial and electronic properties of NA can be modulated to allow efficient biosensor applications, including the choice of the redox probe and use of alternative linkers and immobilization strategies These results contribute to understanding of interfacial ET reactions in DNA and advanced design of fast, reliable and cost‐effective diagnostic devices.

Kinetics of Quantized Charging of Au144 Nanoclusters

AbstractElectron transfer (ET) in monolayer‐protected gold clusters (Au MPCs) is crucial for revealing their fundamental energetic and kinetic properties and potential electrochemical and optical applications. In this Communication, we measured the kinetics of quantized charging processes of the Au144 MPCs. A detailed mathematical model was developed, including ET kinetics and diffusion in MPCs. The charging processes of controlled core states were treated as multistep ET reactions, and the corresponding rate constants were determined by steady‐state voltammetry and scanning electrochemical microscopy (SECM).

Probing Electron‐Transfer and Ion‐Transfer Coupling Processes at Liquid/Liquid Interfaces with Pipette Electrodes

AbstractElectron‐transfer (ET) reactions at a liquid/liquid (L/L) interface have attracted much attention because they are relevant to many important chemical and biological processes. At an L/L interface, the ET reaction is usually coupled with an ion‐transfer (IT) reaction, and it is difficult to discriminate between them. Despite the great progress that has been made, the nature of the coupling is still unclear and the two proposed mechanisms have been the subject of much debate over the past several decades. Herein, the coupling reactions between ferrocene (Fc) in 1,2‐dichloroethane and [Fe(CN)6]3− in water are investigated in detail by using various types of pipette electrodes, and the two mechanisms are clarified with the help of salting‐out effects. It is demonstrated that the mechanism which dominates is dependent on the concentration ratio of the redox species in the two phases.

Nucleophilic displacement versus electron transfer in the reactions of alkyl chlorosilanes with electrogenerated aromatic anion radicals

Anion radicals of a series of aromatic compounds (C6H5CN, C6H5COOEt, anthracene, 9,10-dimethyl-, 9,10-diphenyl- and 9-phenylanthracene, pyrene and naphthalene) react with trialkyl chlorosilanes R1R2R3SiCl (R1-3=Me, Et; R1,2=Me, R3=t-Bu) in multiple ways, following classical bimolecular schemes. The ratio of one-electron transfer (ET) to a two-electron process (SN2-like nucleophilic attack of the reduced form of mediator on the chlorosilane, with k2≅102-108M−1s−1) is inversely related to the steric availability of Si for nucleophilic displacement reactions. The nucleophilic substitution pathway mainly results in mono- and disilylated aromatic products. Paralleling the electrochemical data with DFT calculations, the role of silicophilic solvent (DMF) in SN process was shown to be quite complex because of its involvement into coordination extension at silicon, dynamically modifying energetics of the process along the reaction coordinate. Although 2,2'-bipyridine also forms delocalized persistent anion radicals, they do not induce neither ET nor SN reactions in the same manner as aromatic mediators. Silicophilicity of 2,2'-bipyridine being superior to that of DMF, a R3SiCl·bipy complex of hypercoordinated silicon with electroactive ligand was formed instead, whose reduction requires about 1V less negative potentials than bipyridine itself.

Inverse Kinetic Solvent Isotope Effect in TiO2 Photocatalytic Dehalogenation of Non‐adsorbable Aromatic Halides: A Proton‐Induced Pathway

AbstractAn efficient redox reaction between organic substrates in solution and photoinduced h+vb/e−cb on the surface of photocatalysts requires the substrates or solvent to be adsorbed onto the surface, and is consequentially marked by a normal kinetic solvent isotope effect (KSIE≥1). Reported herein is a universal inverse KSIE (0.6–0.8 at 298 K) for the reductive dehalogenation of aromatic halides which cannot adsorb onto TiO2 in a [D0]methanol/[D4]methanol solution. Combined with in situ ATR‐FTIR spectroscopy investigations, a previously unknown pathway for the transformation of these aromatic halides in TiO2 photocatalysis was identified: a proton adduct intermediate, induced by released H+/D+ from solvent oxidation, accompanies a change in hybridization from sp2 to sp3 at a carbon atom of the aromatic halides. The protonation event leads these aromatic halides to adsorb onto the TiO2 surface and an ET reaction to form dehalogenated products follows.

Analysis of transformations of the ultrafast electron transfer photoreaction mechanism in liquid solutions by the rate distribution approach

Representation of the experimental reaction kinetics in the form of rate distribution is shown to be an effective method for the analysis of the mechanisms of these reactions and for comparisons of the kinetics with QC calculations, as well as with the experimental data on the medium mobility. The rate constant distribution function P(k) can be obtained directly from the experimental kinetics N(t) by an inverse Laplace transform. The application of this approach to kinetic data for several excited-state electron transfer reactions reveals the transformations of their rate control factors in the time domain of 1-1000 ps. In neat electron donating solvents two components are observed. The fastest component (k > 1 ps(-1)) was found to be controlled by the fluctuations of the overall electronic coupling matrix element, involving all the reactant molecules, located inside the interior of the solvent shell, rather than for specific pairs of reactant molecules. The slower component (1 > k > 0.1 ps(-1)) is controlled by the medium reorganization (longitudinal relaxation times, τL). A substantial contribution from the non-stationary diffusion controlled reaction is observed in diluted solutions ([Q] < 1 M). No contribution from the long-distance electron transfer (electron tunneling) proposed earlier for the excited-state electron transfer between perylene and tetracyanoethylene in acetonitrile is observed. The rate distribution approach provides a simple and efficient method for the quantitative analysis of the reaction mechanism and transformation of the rate control factors in the course of the reactions.

Tryptophan-accelerated electron flow across a protein-protein interface.

We report a new metallolabeled blue copper protein, Re126W122Cu(I) Pseudomonas aeruginosa azurin, which has three redox sites at well-defined distances in the protein fold: Re(I)(CO)3(4,7-dimethyl-1,10-phenanthroline) covalently bound at H126, a Cu center, and an indole side chain W122 situated between the Re and Cu sites (Re-W122(indole) = 13.1 Å, dmp-W122(indole) = 10.0 Å, Re-Cu = 25.6 Å). Near-UV excitation of the Re chromophore leads to prompt Cu(I) oxidation (<50 ns), followed by slow back ET to regenerate Cu(I) and ground-state Re(I) with biexponential kinetics, 220 ns and 6 μs. From spectroscopic measurements of kinetics and relative ET yields at different concentrations, it is likely that the photoinduced ET reactions occur in protein dimers, (Re126W122Cu(I))2 and that the forward ET is accelerated by intermolecular electron hopping through the interfacial tryptophan: *Re//←W122←Cu(I), where // denotes a protein-protein interface. Solution mass spectrometry confirms a broad oligomer distribution with prevalent monomers and dimers, and the crystal structure of the Cu(II) form shows two Re126W122Cu(II) molecules oriented such that redox cofactors Re(dmp) and W122-indole on different protein molecules are located at the interface at much shorter intermolecular distances (Re-W122(indole) = 6.9 Å, dmp-W122(indole) = 3.5 Å, and Re-Cu = 14.0 Å) than within single protein folds. Whereas forward ET is accelerated by hopping through W122, BET is retarded by a space jump at the interface that lacks specific interactions or water molecules. These findings on interfacial electron hopping in (Re126W122Cu(I))2 shed new light on optimal redox-unit placements required for functional long-range charge separation in protein complexes.

Distance-independent charge recombination kinetics in cytochrome c-cytochrome c peroxidase complexes: compensating changes in the electronic coupling and reorganization energies.

Charge recombination rate constants vary no more than 3-fold for interprotein ET in the Zn-substituted wild type (WT) cytochrome c peroxidase (CcP):cytochrome c (Cc) complex and in complexes with four mutants of the Cc protein (i.e., F82S, F82W, F82Y, and F82I), despite large differences in the ET distance. Theoretical analysis indicates that charge recombination for all complexes involves a combination of tunneling and hopping via Trp191. For three of the five structures (WT and F82S(W)), the protein favors hopping more than that in the other two structures that have longer heme → ZnP distances (F82Y(I)). Experimentally observed biexponential ET kinetics is explained by the complex locking in alternative coupling pathways, where the acceptor hole state is either primarily localized on ZnP (slow phase) or on Trp191 (fast phase). The large conformational differences between the CcP:Cc interface for the F82Y(I) mutants compared to that the WT and F82S(W) complexes are predicted to change the reorganization energies for the CcP:Cc ET reactions because of changes in solvent exposure and interprotein ET distances. Since the recombination reaction is likely to occur in the inverted Marcus regime, an increased reorganization energy compensates the decreased role for hopping recombination (and the longer transfer distance) in the F82Y(I) mutants. Taken together, coupling pathway and reorganization energy effects for the five protein complexes explain the observed insensitivity of recombination kinetics to donor-acceptor distance and docking pose and also reveals how hopping through aromatic residues can accelerate long-range ET.

Direct electrochemistry and Os-polymer-mediated bioelectrocatalysis of NADH oxidation by Escherichia coli flavohemoglobin at graphiteelectrodes

Escherichia coli flavohemoglobin (HMP), which contains one heme and one FAD as prosthetic groups and is capable of reducing O2 by its heme at the expense of NADH oxidized at its FAD site, was electrochemically studied at graphite (Gr) electrodes. Two signals were observed in voltammograms of HMP adsorbed on Gr, at −477 and −171mV vs. Ag|AgCl, at pH 7.4, correlating with electrochemical responses from the FAD and heme domains, respectively. The electron transfer rate constant for ET reaction between FAD of HMP and the electrode was estimated to be 83s−1. Direct bioelectrocatalytic oxidation of NADH by HMP was not observed, presumably due to impeded substrate access to HMP orientated on Gr through the FAD-domain and/or partial denaturation of HMP. Bioelectrocatalysis was achieved when HMP was wired to Gr by the Os redox polymers, with the onset of NADH oxidation at the formal potential of the particular Os complex (+140mV or −195mV). Apparent Michaelis constants KMapp and jmax were determined, showing bioelectrocatalytic efficiency of NADH oxidation by HMP exceeding the one earlier shown with diaphorase, which makes HMP very attractive as a component of bioanalytical and bioenergetic devices.

Dynamics and Mechanisms of Interfacial Photoinduced Electron Transfer Processes of Third Generation Photovoltaics and Photocatalysis

Photoinduced electron transfer (PET) across molecular/bulk interfaces has gained attention only recently and is still poorly understood. These interfaces offer an excellent case study, pertinent to a variety of photovoltaic systems, photo- and electrochemistry, molecular electronics, analytical detection, photography, and quantum confinement devices. They play in particular a key role in the emerging fields of third-generation photovoltaic energy converters and artificial photosynthetic systems aimed at the production of solar fuels, creating a need for a better understanding and theoretical treatment of the dynamics and mechanisms of interfacial PET processes. We aim to achieve a fundamental understanding of these phenomena by designing experiments that can be used to test and alter modern theory and computational modeling. One example illustrating recent investigations into the details of the ultrafast processes that form the basis for photoinduced charge separation at a molecular/bulk interface relevant to dye-sensitized solar cells is briefly presented here: Kinetics of interfacial PET and charge recombination processes were measured by fs and ns transient spectroscopy in a heterogeneous donor-bridge-acceptor (D-B-A) system, where D is a Ru(II)(terpyridyl-PO3)(NCS)3 complex, B an oligo-p-phenylene bridge, and A nanocrystalline TiO2. The forward ET reaction was found to be faster than vibrational relaxation of the vibronic excited state of the donor. Instead, the back ET occurred on the micros time scale and involved fully thermalized species. The D-A distance dependence of the electron transfer rate was studied by varying the number of p-phenylene units contained in the bridge moiety. The remarkably low damping factor beta = 0.16 angstroms(-1) observed for the ultrafast charge injection from the dye excited state into the conduction band of TiO2 is attributed to the coupling of electron tunneling with nonequilibrium vibrations redistributed on the bridge, giving rise to polaronic transport of charges from the donor ligand to the acceptor solid oxide surface.

Photoinduced electron transfer in a directly linked meso-triphenylamine zinc porphyrin-quinone dyad

A multimodular donor-acceptor system composed of three triphenylamine entities at the meso-positions of a zinc porphyrin macrocycle and a quinone at the fourth meso-position was newly synthesized and characterized. The triphenylamine entities acted as energy transferring antenna units in addition of improving the electron donor ability of the zinc porphyrin. Appreciable electronic interactions of the triphenylamine and quinone entities with the porphyrin π-system were observed. In agreement with the spectral and electrochemical results, the computational studies performed by the DFT B3LYP/3-21G(*) method revealed delocalization of the frontier HOMO over the triphenylamine and the porphyrin macrocycle while the LUMO to be fully localized over the quinone entity. Free-energy calculations suggested photoinduced electron transfer from the singlet excited zinc porphyrin to the directly linked quinone to be exothermic and this was experimentally confirmed by the time-resolved pump probe and up-conversion techniques. In the investigated system, the ET reaction path was found to depend upon the excitation wavelength. That is, when Zn porphyrin was predominantly excited, a rapid charge separation followed by equally fast charge recombination was observed. However, excitation of the peripheral TPA substituents resulted in an extremely long-lived CS state with triplet spin character via the TPA triplet and Zn porphyrin triplet states.

Ultrafast Charge Separation at the CdSe/CdS Core/Shell Quantum Dot/Methylviologen Interface: Implications for Nanocrystal Solar Cells

Exciton separation dynamics in the electron transfer system containing highly photostable CdSe/CdS core/shell nanocrystal quantum dots and adsorbed methylviologen was investigated by means of femtosecond absorption spectroscopy. The experiments revealed that electron extraction from the photoexcited core is possible, and the rate of the ET reaction strongly depends on the CdS shell thickness. A CdS associated exponential decay constant β of 0.33 Å−1 was obtained reflecting the electronic barrier effect of the shell. These findings show that core/shell structures are well suited for the design of optimized QD-based solar cells.