Electron Transfer Model Research Articles

Origin of Kinetic Dispersion in Eosin-Sensitized TiO2: Insights from Single-Molecule Spectroscopy

By removing the effects of ensemble averaging and molecular aggregation, we untangle the factors that govern the dispersive electron transfer kinetics of eosin-sensitized TiO2, focusing on the impact of environmental heterogeneity versus injection from multiple excited states. The blinking dynamics of single eosin Y chromophores on nanocrystalline TiO2 films are analyzed using a change point detection algorithm for binned data. Robust statistical analysis based on maximum likelihood estimation, Kolmogorov–Smirnov tests, and log likelihood ratio tests is used to determine the functional form that best fits the resulting on- and off-time distributions and to distinguish between mechanisms for dispersive electron transfer. Using this approach, we find that the on- and off-time distributions for eosin Y on TiO2 are best fit to lognormal distributions corresponding to μon = −0.64 ± 0.04, σon = 1.52 ± 0.02, μoff = −0.23 ± 0.04, and σoff = 1.96 ± 0.03, respectively. Monte Carlo simulations based on the Albery model for dispersive electron transfer (i.e., where the median rate constant κ is modified by the exponential of a parameter, x, that is normally distributed, k = κ e–γx) successfully reproduce this behavior using a median rate constant for injection and back electron transfer of ∼1010 and ∼104 s–1, respectively, and a corresponding energetic dispersion, γ, of ∼200–350 meV. To examine how injection from both the singlet and triplet excited states contributes to this dispersion, we studied two rhodamine sensitizers, R123 and 5ROX, that inject only from their singlet excited state. Surprisingly, when access to the T1 state is minimized in going from EY to R123 and 5ROX, kinetic dispersion actually increases. Collectively, these observations support the interpretation that static and dynamic inhomogeneities at the EY–TiO2 interface govern kinetic dispersion, with dynamic fluctuations in binding configuration and/or vibrational motion playing a decisive role.

Probing Hydrogen-Bonding Interactions within Phenol-Benzimidazole Proton-Coupled Electron Transfer Model Complexes with Cryogenic Ion Vibrational Spectroscopy.

Hydrogen-bonding interactions within a series of phenol-benzimidazole model proton-coupled electron transfer (PCET) dyad complexes are characterized using cryogenic ion vibrational spectroscopy. A highly red-shifted and surprisingly broad (>1000 cm-1) transition is observed in one of the models and assigned to the phenolic OH stretch strongly H-bonded to the N(3) benzimidazole atom. The breadth is attributed to a combination of anharmonic Fermi-resonance coupling between the OH stretch and background doorway states involving OH bending modes and strong coupling of the OH stretch frequency to structural deformations along the proton-transfer coordinate accessible at the vibrational zero-point level. The other models show unexpected protonation of the benzimidazole group upon electrospray ionization instead of at more basic remote amine/amide groups. This leads to the formation of HO-+HN(3) H-bond motifs that are much weaker than the OH-N(3) H-bond arrangement. H-bonding between the N(1)H+ benzimidazole group and the carbonyl on the tyrosine backbone is the stronger and preferred interaction in these complexes. The results show that conjugation effects, secondary H-bond interactions, and H-bond soft modes strongly influence the OH-N(3) interaction and highlight the importance of the direct monitoring of proton stretch transitions in characterizing the proton-transfer reaction coordinate in PCET systems.

Molecular Dynamics Simulations of Bimolecular Electron Transfer: the Distance-Dependent Electronic Coupling.

Understanding the distance dependence of the parameters underpinning Marcus theory is imperative when interpreting the results of experiments on electron transfer (ET). Unfortunately, most of these parameters are difficult or impossible to access directly with experiments, necessitating the use of computer simulations to model them. In this work, we use molecular dynamics simulations in conjunction with constrained density functional theory calculations to study the distance dependence of the electronic coupling matrix element, |HRP|, for bimolecular ET. Contrary to what is typically assumed for such intermolecular reactions, we find that the magnitude of |HRP| does not decay exponentially with the center-of-mass separation of the reactants, rCOM. The addition of other simple measures of donor/acceptor (D/A) orientation did not improve the correlation of |HRP| with rCOM. Using the minimum distance separation, rmin, of the reactants as the structural descriptor allowed the system to be partitioned into high-coupling/close-contact and low-coupling/non-contact regimes, but large fluctuations of |HRP| were still found for the close-contact reactant pairs. Despite the persistent large fluctuations of |HRP|, its mean value was found to decay piecewise exponentially with increasing rmin, which was attributed to significant changes in the average D/A pair structure. The results herein advise one to use caution when interpreting the experimental results derived from spherical reactant models of bimolecular ET.

The reactivity of O2 with copper cluster anions Cun− (n = 7−20): Leveling effect of spin accommodation

The activation of molecular oxygen is an important step in metal-catalyzed oxidation reactions and a hot subject for the research of gas-phase metal clusters. It is known that the Ag and Au clusters readily react with O2 when they have open shell electronic structures. Distinct from this, here we observed Cun− (n = 7−20) clusters of both open and closed shells possess high reactivity with O2 with few exceptions. In a combination with ab initio calculations, we demonstrate that the activation of O2 on the even- and odd-sized Cun− clusters follows the single and double electron transfer models, respectively. Such phenomenon of metal clusters with different basicity to activate oxygen is enabled by the leveling effect of spin accommodation. The activity of Cun− clusters is correlated to the HOMO level, and for the close-shell clusters is also governed by the vertical spin excitation energy (VSE). In encountering the attack of dioxygen, the activity of the copper cluster anions not only depends on their basicity to donate electrons, but also closely associated with the cluster sizes. Small copper clusters Cun− (n = 7−13) can dissociate O2 spontaneously, while large clusters require extra energies and display close relationship between the reaction rates and electronic vertical detachment energies (VDE). Our work illuminates a novel reaction mechanism between Cun− clusters and O2, which sheds light in manipulating the activity and stability of coinage clusters by controlling the spin and charge states.

Two-Channel Model for Electron Transfer in a Dye-Catalyst-Dye Supramolecular Complex for Photocatalytic Water Splitting.

To improve the performance of dye‐sensitized photoelectrochemical cell (DS‐PEC) devices for splitting water, the tailoring of the photocatalytic four‐photon water oxidation half‐reaction represents a principle challenge of fundamental significance. In this study, a Ru‐based water oxidation catalyst (WOC) covalently bound to two 2,6‐diethoxy‐1,4,5,8‐diimide‐naphthalene (NDI) dye functionalities provides comparable driving forces and channels for electron transfer. Constrained ab initio molecular dynamics simulations are performed to investigate the photocatalytic cycle of this two‐channel model for photocatalytic water splitting. The introduction of a second light‐harvesting dye in the Ru‐based dye‐WOC‐dye supramolecular complex enables two separate parallel electron‐transfer channels, leading to a five‐step catalytic cycle with three intermediates and two doubly oxidized states. The total spin S=1 is conserved during the catalytic process and the system with opposite spin on the oxidized NDI proceeds from the Ru=O intermediate to the final Ru‐O2 intermediate with a triplet molecular 3O2 ligand that is eventually released into the environment. The in‐depth insight into the proposed photocatalytic cycle of the two‐channel model provides a strategy for the development of novel high‐efficiency supramolecular complexes for DS‐PEC devices with buildup and conservation of spin multiplicity along the reaction coordinate as a design principle.

RGO modified R-CeO2/g-C3N4 multi-interface contact S-scheme photocatalyst for efficient CO2 photoreduction

Construction of multi-interface contact step-scheme (S-scheme) photocatalyst is a promising pathway to achieve high-electron transfer efficiency for photocatalytic CO2 reduction. In this paper, g-C3N4 nanosheets were selected as the main photocatalyst, rod-like CeO2 (R-CeO2) with unique Ce4+→Ce3+ conversion property and rGO were loaded on the g-C3N4 surface to construct 2D-1D-2D sandwich photocatalyst. The yields of CO and CH4 were about 63.18 and 32.67 μmol/g after 4 h when the rGO/R-CeO2/g-C3N4 was used as catalyst, which were about 4 and 6 times higher than that of pure CN, respectively. Cyclic experiments proved that the composite had excellent photocatalytic and material stability. Photoelectrochemical tests showed that the construction of S-scheme electron transfer model and the introduction of rGO can great enhance the electron transmission and separation of photogenerated electron-hole pairs. CO2 adsorption test identified that the loading of R-CeO2 and rGO obviously enhanced the CO2 adsorption ability of pure g-C3N4. Density functional theory (DFT) calculations used to analyze the electron transfer path and the formation of the build-in electric field at the semiconductor interface. In-situ FTIR and 13CO2 element-tracer detection carried out to research the process of CO2 photoreduction. A possible multi-interface contact S-scheme electron transfer mechanism for enhanced CO2 photoreduction activity has been discussed.

Effect of Photo-Excitation on Contact Electrification at Liquid-Solid Interface.

Liquid-solid triboelectric nanogenerator (L-S TENG) is one of the major techniques to collect energy from tiny liquids, while the saturated charge density at the L-S interface is the key element to decide its performance. Here, we found that the saturated charge density of L-S contact electrification (CE) can be further increased under the illumination of an ultraviolet (UV) light. The fluorine-containing polymers and SiO2 are chosen as the electrification materials and with and without UV illumination on the L-S TENG. A series of experiments have been done to rule out the possible influences of anion generation, chemical change of solid surface, ionization of water, and so on. Therefore, we proposed that electrons belonging to water molecules can be excited to high energy states under UV illumination, which then transfer to solid surface and captured by the solid surface. Finally, a photoexcited electron transfer model is proposed to explain the enhancement of CE under the UV illumination. This work not only helps to further understand CE at L-S interface, but also offers an approach to further enhance the performance of L-S TENG, which can promote the TENG applications in the field of microfluidic systems, liquid energy harvesting, and droplet sensory.

Back-bonding driven negative thermal expansion along the one dimensional Hg[sbnd]C[tbnd]N linkage in [HgCN](NO3) probed by Raman spectroscopy

In this paper, the mechanistic aspect for anisotropic negative thermal expansion (NTE) in [Hg(CN)](NO3) is correlated to back-bonding along a one dimensional HgCN network using temperature dependent Raman spectroscopy. Signatures of back-bonding are contraction of the HgC bond (~299 cm−1), expansion of the CN bond (~2249 cm−1) and bending of the HgCN bond. These are clearly observed in the Raman spectra, correlating to a continuous increase in the HgCN bending frequency (~346 cm−1) with an increase in temperature. This bending of the HgCN bond may be responsible for an expansion of the Hg lattice along the ab-plane and a reorientation of the NO3− ions, as seen by the change in frequency variation of the 1637 cm−1 mode and the reduction of its intensity with the increase in temperature. All these effects are in agreement with the reported single crystal X-ray data. Thus, the present model of back-bonding electron transfer from the metal d-orbital to the π*-orbital of the cyanide ligand and the effect of temperature changes on it can be explored to understand the NTE behaviour in metal cyanide systems and more broadly in multifunctional materials.

Theoretical Evaluation of Flow Electronic Rate at Au/TFB Interface

This investigation presents a quantum model for electron transfer allows us to calculate the flow electronic transfer rate through metal/molecule devices. The flow electronic transfer rate at interfaces between Au metal contact with poly(9,9’-dioctylfluorene-co-bis-N, N’-(4-butylphenyl)-diphenylamine) (TFB) and solvents media with polarity is ranging from 0.315 for Diethyl ether to 0.5361 for Methanol is investigated via quantum theory of donor acceptor model. The flow rate are large with Au energy level alignment with TFB levels has been found when the potential barrier at interface is 0.066 eV with Diethyl ether solvents. Flow electronic transfer rate for Au/TFB system with methanol solvent decreased cross interface when the potential barrier is as large as 0.122 eV. This lead to produce accumulate the charges on the both side of Au and TFB when the potential at interface is large. The resulting slow transfer of electrons cross interface. Furthermore, Au metal contact on TFB molecule interface frequently with different solvents media and have large flow rate with Diethylether solvents and low flow rate with Methanol.

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.

Probing photoinduced proton coupled electron transfer process by means of two-dimensional resonant electronic-vibrational spectroscopy.

We develop a detailed theoretical model of photo-induced proton-coupled electron transfer (PPCET) processes, which are at the basis of solar energy harvesting in biological systems and photovoltaic materials. Our model enables us to analyze the dynamics and the efficiency of a PPCET reaction under the influence of a thermal environment by disentangling the contribution of the fundamental electron transfer and proton transfer steps. In order to study quantum dynamics of the PPCET process under an interaction with the non-Markovian environment, we employ the hierarchical equations of motion. We calculate transient absorption spectroscopy (TAS) and a newly defined two-dimensional resonant electronic-vibrational spectroscopy (2DREVS) signals in order to study the nonequilibrium reaction dynamics. Our results show that different transition pathways can be separated by TAS and 2DREVS.

Theoretical aspects of electrochemistry at low temperature

• Theoretical models of heterogeneous electron transfer can predict electrochemical responses at low temperature. • Kinetic and thermodynamic data can be extracted from various electrochemical methods at low temperature. • Chemically-coupled electron-transfer reactions can be better investigated by using low-temperature electrochemistry. This paper aims at providing theoretical basis on electrochemical processes performed at low temperature ( T ) according to three different aspects. First, the effect of T -decrease is treated in terms of thermodynamics, mass-transfer, and kinetics of electron transfer (ET) heterogeneous reactions. In particular, predictions of ET kinetics at low temperature are discussed in the frame of Marcus-Hush’s model. The second part is focused on the changes associated to temperature decrease for different electrochemical methods including cyclic voltammetry (CV), chronoamperometry (CA), AC Impedance (ACI) and AC voltammetry (ACV). This section gives keys to extract electrochemical data from low- T experimental curves. In the third part, theoretical aspects of low- T cyclic voltammetry for multiple or chemically-coupled electron-transfer reactions are discussed. CVs of typical molecular mechanisms (EE, EC, ECE…) are provided in order to better visualize the impact of temperature on the redox behavior.

Nonadiabatic Dynamics at Metal Surfaces: Fewest Switches Surface Hopping with Electronic Relaxation.

A new scheme is proposed for modeling molecular nonadiabatic dynamics near metal surfaces. The charge-transfer character of such dynamics is exploited to construct an efficient reduced representation for the electronic structure. In this representation, the fewest switches surface hopping (FSSH) approach can be naturally modified to include electronic relaxation (ER). The resulting FSSH-ER method is valid across a wide range of coupling strengths as supported by tests applied to the Anderson-Holstein model for electron transfer. Future work will combine this scheme with ab initio electronic structure calculations.

Interfacial Electron Transfer in Dye-Sensitized TiO2 Devices for Solar Energy Conversion

The development of cost-effective molecular devices that efficiently capture and convert sunlight into other useful forms of energy is a promising approach to meet the world’s increasing energy demands. These devices are designed through a successful combination of materials and molecules that work synergistically to promote light-driven chemical reactions. Light absorption by a surface-bound chromophore triggers a sequence of interfacial electron transfer processes. The efficiencies of the devices are governed by the dynamic balance between the electron transfer reactions that promote energy conversion and undesirable side reactions. Therefore, it is necessary to understand and control these processes to optimize the design of the components of the devices and to achieve higher energy conversion efficiencies. In this context, this review discusses general aspects of interfacial electron transfer reactions in dye-sensitized TiO2 molecular devices for solar energy conversion. A theoretical background on the Marcus-Gerischer theory for interfacial electron transfer and theoretical models for electron transport within TiO2 films are provided. An overview of dye-sensitized solar cells (DSSCs) and dye-sensitized photoelectrosynthesis cells (DSPECs) is presented, and the electron transfer and transport processes that occur in both classes of devices are emphasized and detailed. Finally, the main spectroscopic, electrochemical and photoelectrochemical experimental techniques that are employed to elucidate the kinetics of the electron transfer reactions discussed in this review are presented.

Electrolocation? The evidence for redox-mediated taxis in Shewanella oneidensis.

Shewanella oneidensis is a dissimilatory metal reducing bacterium and model for extracellular electron transfer (EET), a respiratory mechanism in which electrons are transferred out of the cell. In the last 10years, migration to insoluble electron acceptors for EET has been shown to be nonrandom and tactic, seemingly in the absence of molecular or energy gradients that typically allow for taxis. As the ability to sense, locate, and respire electrodes has applications in bioelectrochemical technology, a better understanding of taxis in S. oneidensis is needed. While the EET conduits of S. oneidensis have been studied extensively, its taxis pathways and their interplay with EET are not yet understood, making investigation into taxis phenomena nontrivial. Since S. oneidensis is a member of an EET-encoding clade, the genetic circuitry of taxis to insoluble acceptors may be conserved. We performed a bioinformatic analysis of Shewanella genomes to identify S. oneidensis chemotaxis orthologs conserved in the genus. In addition to the previously reported core chemotaxis gene cluster, we identify several other conserved proteins in the taxis signaling pathway. We present the current evidence for the two proposed models of EET taxis, "electrokinesis" and flavin-mediated taxis, and highlight key areas in need of further investigation.

Towards a Holomorphic Density Functional Theory.

Self-consistent-field (SCF) approximations formulated using Hartree-Fock (HF) or Kohn-Sham density-functional theory (KS-DFT) have the potential to yield multiple solutions. However, the formal relationship between multiple solutions identified using HF or KS-DFT remains generally unknown. We investigate the connection between multiple SCF solutions for HF or KS-DFT by introducing a parameterized functional that scales between the two representations. Using the hydrogen molecule and a model of electron transfer, we continuously map multiple solutions from the HF potential to a KS-DFT description. We discover that multiple solutions can coalesce and vanish as the functional changes, forming a direct analogy with the disappearance of real HF solutions along a change in molecular structure. To overcome this disappearance of solutions, we develop a complex-analytic extension of DFT-the "holomorphic DFT" approach-that allows every SCF stationary state to be analytically continued across all molecular structures and exchange-correlation functionals.

The Efficiency of Photoinduced Intramolecular Charge Separation from the Second Excited State: What Factors Can Control It?

The efficiency of photoinduced charge separation (CS) in electron donor-acceptor compounds is commonly limited due to fast deactivation processes, such as the excited-state internal conversion and ultrafast hot reverse electron transfer to the acceptor, charge recombination (CR). A traditional way to avoid undesired energy losses due to CR is to put the reverse electron transfer into the Marcus inverted region, thus effectively suppressing it. This method, however, is not generally applicable when considering CS from the second locally excited state because the driving force of CR to the first excited state is small, and thus charge recombination is ultrafast and efficient. In this paper, we study the kinetic features of CS/CR from the second locally excited state of the donor using a semiclassical stochastic model of electron transfer. Particular attention is paid to the CS efficiency as well as the influence of the polar environment and intramolecular high-frequency vibrational modes on the kinetics of the charge-separated state. The influence of a number of model parameters on the CS yield and the energy efficiency has been analyzed using the results of numerical simulations. Several simple practical recipes for creating molecular compounds with high CS yields have been suggested. Simulations have also revealed a strong and non-monotonous (double-humped) dependence of both the yield and energy efficiency of CS on the driving force.

Interplay between Structure and Mechanism in Reductive Dissociative Electron Transfers to α,β -Epoxyketones.

The electrochemical reduction of several α,β -epoxyketones was studied using cyclic (linear sweep) voltammetry, convolution voltammetry, and homogeneous redox catalysis. The results were reconciled to pertinent theories of electron transfer. α,β -Epoxyketones undergo dissociative electron-transfer reactions with C-O bond cleavage, via both stepwise and concerted mechanisms, depending on their structure. For aliphatic ketones, the preferred mechanism of reduction is consistent with the "sticky" concerted model for dissociative electron transfer. Bond cleavage occurs simultaneously with electron transfer, and there is a residual, electrostatic interaction in the ring-opened (distonic) radical anion. In contrast, for aromatic ketones, because the ring-closed radical anions are resonance-stabilized and exist at energy minima, a stepwise mechanism operates (electron transfer and bond cleavage occur in discrete steps). The rate constants for ring opening are on the order of 108 s-1 , and not significantly affected by substituents on the 3-membered ring (consistent with C-O bond cleavage). These results and conclusions were fully supported and augmented by molecular orbital calculations.

Radiation of Chiral Gold Nanotubes under the Influence of Alternating Electric Current

In chiral gold nanotubes, alternating electric current generates an electromagnetic field; under these conditions, the nanotubes become emitting solenoid nanoantennas. With the use of the Maxwell equations and taking into account the geometry of nanotubes, their band structure, and the ballistic model of electron transfer, we have calculated the alternating axial magnetic and azimuthal electric fields generated by helical currents in chiral gold nanotubes. The calculations demonstrate that large alternating magnetic and electric fields can be generated in nanoscale volumes of gold nanotubes and that the eigenfrequencies of the field components are in the X-ray range.

Coherence preservation and electron-phonon interaction in electron transfer in DNA.

We analyze the influence of electron-phonon (e-ph) interaction in a model for electron transfer (ET) processes in DNA in terms of the envelope function approach for spinless electrons. We are specifically concerned with the effect of e-ph interaction on the coherence of the ET process and how to model the interaction of DNA with phonon reservoirs of biological relevance. We assume that the electron bearing orbitals are half filled and derive the physics of e-ph coupling in the vicinity in reciprocal space. We find that at half filling, the acoustical modes are decoupled to ET at first order, while optical modes are predominant. The latter are associated with inter-strand vibrational modes in consistency with previous studies involving polaron models of ET. Coupling to acoustic modes depends on electron doping of DNA, while optical modes are always coupled within our model. Our results yield e-ph coupling consistent with estimates in the literature, and we conclude that large polarons are the main result of such e-ph interactions. This scenario will have strong consequences on decoherence of ET under physiological conditions due to relative isolation from thermal equilibration of the ET mechanism.