Electrochemical transistor based on bridge tunneling contact containing two redox groups

The method of kinetic equations was used to show that a bridge tunneling contact containing two redox groups in the sequential configuration immersed into a solution of electrolyte at the room temperature features pronounced transistor properties at a given set of physical system parameters. Valence electrons of redox groups interact strongly with the classical phonon subsystem of the liquid medium. Debye screening of the electric field in the tunneling gap and Coulomb repulsion between electrons in different redox groups are taken into account. The case of nonadiabatic electron transfer both between redox groups and between electrodes and redox groups is considered in the limit of infinitely high Coulomb repulsion between electrons in a redox group. For sufficiently high absolute values of difference δ between unperturbed energy levels of redox groups, the system features voltammetric characteristics typical for a transistor. The amplification effect appears due to a strong dependence of tunneling current on overpotential. The emphasis is upon the peculiaritiespeculiarities of voltammetric characteristics in the case of asymmetric tunneling contacts.     

Study on the spectroelectrochemical properties of S-benzyl-N-(ferrocenyl-1-methyl-metylidene)-dithio-carbazate nickel (II) complex

Mechanism of electron transfer of a bridge biferrocene complex, Ni(LSB)2, derived from the Schiff base ligand, HLSB, S-benzyl-N-(ferrocenyl-1-methyl-metylidene)-dithio-carbazate Ni(LSB)2, during redox processes is illuminated by in-situ Fourier transform infrared (FTIR) spectroelectrochemistry. The results indicated that two consecutive one-electron steps were involved which gives the corresponding mono- and di-ferricenium cations, respectively. This complex exhibits moderately strong degree of electronic communication between the two-ferrocene moieties, taking place through the skeleton chain of the ligand due to an extensive electron delocalization in the molecule. Possible pathways of electron transfer of Ni(LSB)2 during the redox processes are postulated.     

Theoretical analysis of the telegraph-like fluctuations in bridged electrochemical contacts

Steady-state current-voltage characteristic of an electrochemical bridged contact and the telegraph noise power are calculated in terms of the model of two redox states. It is shown that the overvoltage dependence of the tunnel current at a constant bias voltage is S-shaped, which was observed in works on the electron tunneling in similar systems. The overvoltage dependence of the contact conductance has a maximum near the equilibrium potential of the bridge-molecule redox group. The overvoltage dependence of the noise power at zero frequency has a maximum whose position is determined by the bias voltage.     

Solution phase electron transfer versus bridge mediated electron transfer across carboxylic acid terminated thiols

Self-assembled monolayers (SAMs) of thiols with carboxylic acid terminal groups were formed on gold substrates. The electron transfer characteristics of redox species on the above SAM-modified electrodes were studied in acid and neutral media with the help of voltammetry under two different conditions: (1) solution phase electron transfer and (2) bridge mediated electron transfer. Two redox systems, viz., [Fe(CN)₆]^4-/3− and Ru[(NH₃)₆]^2+/3+ were chosen for the solution phase study. Investigations of bridge mediated electron transfer were carried out by functionalising the SAM with redox moieties and then studying their redox behaviour. For this study, ferrocene carboxylic acid and 1,4-diamino anthraquinone were used and they were linked to carboxylic acid terminated thiols by covalent linkage. The voltammetric results with mercaptoundecanoic acid SAM demonstrate the difference in behaviour between solution phase and bridge mediated electron transfer processes.     

Theoretical studies of intramolecular electron transfer reactions: Distance and free energy dependences

Electron tunneling through a square potential energy barrier is used to calculate the distance-dependent factors of electron transfer (ET) processes in metal-monolayer-metal junctions, donors and acceptors dispersed in rigid organic glasses, intramolecular ET in rigid donorbridge—acceptor species in solution and redox centers attached to electrodes through adsorbed monolayers. This tunneling model of distancedependent non-adiabatic factors is incorporated in the intersecting state model (ISM). The result is a simple semiclassical theory which is used to calculate the rates of non-adiabatic ET reactions. When the electron is originally located in a π* molecular orbital of the donor and the reaction free energy is no lower than approximately −50 kJ mol⁻¹, no adjustable parameters are necessary to calculate the intramolecular ET rates from a donor, through a rigid bridge, to an acceptor. Such calculated rates are within an order of magnitude of the experimental values. The model can also account for the ET rates of more exothermic reactions provided that the value of an empirical parameter, which is constant for structurally related reactants and solvents of similar polarity, is estimated. The physical meaning of this parameter is related to the dynamics of the reactions. The profiles of the distance and free energy dependences of photoinduced ET rates are closely reproduced. The occurrence of distance-dependent non-adiabatic factors in intermolecular σ*-d ETs is rationalized.     

Studying single molecule electrochemistry with scanning tunneling microscope break-junction technique

The fundamental principle of molecular electronics is to comprehend electrical properties of single molecules connected between two probe electrodes. In recent years, substantial advances in this field have been made to underpin experimental and theoretical understanding of single molecule electrochemistry. By using scanning tunneling microscope (STM) break-junction technique, the switching events of electrical current from single molecule bridge tuning by electrochemical gating are investigated to uncover the relationship between electrochemical electron transfer and charge transport processes in chemical and biological molecule junctions. In this short review, we outline the latest works of single molecule electrochemistry studied with STM break-junction technique from Nongjian Tao's group, and share the insights on the opportunities and challenges for future research.     

Electrode grafting by oxidation of an amine catalyzed by a ferrocenyl “antenna” through intramolecular electron transfer

Two aminoferrocene complexes were studied by electrochemical techniques. Molecules retain the redox properties of both ferrocene and amine groups, but fundamentally different behaviours were observed depending on whether the linker between the two redox end groups was saturated (ethyl bridge) or not (ethynyl bridge). The possibility of an intramolecular electron transfer from the amine to the ferricenium moiety through the π-conjugated linker was demonstrated and the ethynyl bridge is expected to have a dual effect by facilitating both the oxidation of the amine into the cation radical and the production of aminyl radical, due to its strong electron withdrawing effect. Because of this synergy of properties, grafting of the conjugated aminoferrocene complex can occur just by oxidizing the ferrocene group without the presence of a base in solution.     

Electrochemistry in nanometer-wide electrochemical cells

The electrochemical properties of an electrochemical cell defined by two concentric spherical electrodes, separated by a 1 to 20-nm-wide gap, and a freely diffusing electrochemically active molecule (e.g., ferrocene) have been investigated by coupling of Brownian dynamics simulations with long-range electron-transfer probability values. The simulation creates a trajectory of a single molecule and calculates the likelihood that the molecule undergoes a redox reaction during each time interval based on a probability-distance function derived from literature first-order kinetic data for a surface-bound ferrocene. Steady-state voltammograms for the single-molecule concentric spherical electrochemical cell are computed and are used to extract a heterogeneous electron-transfer rate for the freely diffusing molecule redox reaction. The Brownian dynamics simulations also indicate that long-range electron transfer, between the redox molecule and electrode, leads to nonsigmoidal-shaped i-E characteristics when the distance between electrodes approaches the characteristic redox tunneling decay length. The long-range electron transfer generates a "tunneling depletion layer" that results in a potential-dependent diffusion-limited current.     

Fluoride binding to an organoboron wire controls photoinduced electron transfer

We demonstrate that the rates for long-range electron transfer can be controlled actively by tight anion binding to a rigid rod-like molecular bridge. Electron transfer from a triarylamine donor to a photoexcited Ru(bpy)₃²⁺ acceptor (bpy = 2,2′-bipyridine) across a 2,5-diboryl-1,4-phenylene bridge occurs within less than 10 ns in CH₂Cl₂ at 22 °C. Fluoride anions bind with high affinity to the organoboron bridge due to strong Lewis base/Lewis acid interactions, and this alters the electronic structure of the bridge drastically. Consequently, a large tunneling barrier is imposed on photoinduced electron transfer from the triarylamine to the Ru(bpy)₃²⁺ complex and hence this process occurs more than two orders of magnitude more slowly, despite the fact that its driving force is essentially unaffected by fluoride addition. Electron transfer rates in proteins could potentially be regulated via a similar fundamental principle, because interactions between charged amino acid side chains and counter-ions can modulate electronic couplings between distant redox partners. In artificial donor-bridge-acceptor compounds, external stimuli have been employed frequently to control electron transfer rates, but the approach of exploiting strong Lewis acid/Lewis base interactions to regulate the tunneling barrier height imposed by a rigid rod-like molecular bridge is conceptually novel and broadly applicable, because it is largely independent of the donor and the acceptor, and because the effect is not based on a change of the driving-force for electron transfer. The principle demonstrated here can potentially be used to switch between conducting and insulating states of molecular wires between electrodes.     

LONG RANGE ELECTRON TUNNELING IN BIOLOGICAL AND MODEL SYSTEMS

Abstract— This review paper reports the data on the role of long range electron tunneling in photosynthetic and model systems. The main papers concerned with elucidation mechanisms and kinetical peculiarities of electron transfer in reaction centers of bacteria and green plants are considered. The paper reviews the articles on long range electron transfer in reactions of metalloporphyrins—synthetic analogs of the main natural pigment of photosynthesis, chlorophyll. Liquid and solid phase redox reactions of electronically excited porphyrin molecules, intramolecular and photosensitized electron transfer processes are considered.     

The direct electron transfer of myoglobin based on the electron tunneling in proteins

The electron tunneling of the protein-polypeptide interactions was observed in the study of direct electron transfer of the myoglobin (Mb) on the electrode surface. The Mb was selected as a redox active protein and gelatine was selected to couple with Mb to form an electron tunneling. The electrochemical results indicated the presence of the electron tunneling and the direct electron transfer. The circular dichroism spectra suggested that the beta-sheet chain of gelatine could interact with alpha-helical chain to form an electron tunneling to promote the protein direct electrochemistry. The SDS-PAGE results proved that the electron tunneling between Mb and gelatine was noncovalent hydrogen bonds. The immobilized Mb showed a couple of quasi-reversible redox peaks with a formal potential of -0.37V (vs SCE) in 0.1 M pH 7.0 PBS. The modified electrodes displayed a rapid amperometric response to the reduction of oxygen, H2O2, and nitrite.     

Coherence in electron transfer pathways

Central to the view of electron-transfer reactions is the idea that nuclear motion generates a transition state geometry at which the electron/hole amplitude propagates coherently from the electron donor to the electron acceptor. In the weakly coupled or nonadiabatic regime, the electron amplitude tunnels through an electronic barrier between the donor and acceptor. The structure of the barrier is determined by the covalent and noncovalent interactions of the bridge. Because the tunneling barrier depends on the nuclear coordinates of the reactants (and on the surrounding medium), the tunneling barrier is highly anisotropic, and it is useful to identify particular routes, or pathways, along which the transmission amplitude propagates. Moreover, when more than one such pathway exists, and the paths give rise to comparable transmission amplitude magnitudes, one may expect to observe quantum interferences among pathways if the propagation remains coherent. Given that the effective tunneling barrier height and width are affected by the nuclear positions, the modulation of the nuclear coordinates will lead to a modulation of the tunneling barrier and hence of the electron flow. For long distance electron transfer in biological and biomimetic systems, nuclear fluctuations, arising from flexible protein moieties and mobile water bridges, can become quite significant. We discuss experimental and theoretical results that explore the quantum interferences among coupling pathways in electron-transfer kinetics; we emphasize recent data and theories associated with the signatures of chirality and inelastic processes, which are manifested in the tunneling pathway coherence (or absence of coherence).     

Effect of Coulomb interaction between the electrons on two-electron redox-mediated tunneling

Two-electron non-adiabatic redox-mediated tunneling through a symmetric electrochemical contact with a bridge molecule having one electron energy level participating in tunneling is considered under ambient conditions. It is shown that the current/overpotential dependence in this system can disclose two distinct or overlapping clear-cut maxima depending on the value of the effective Coulomb repulsion energy. This new effect is due to the opening of the channel for tunneling of second electron with the variation of the electrode potential. The system manifests also a rectification effect in the current/bias voltage curve which depends on the value of the effective Coulomb repulsion energy.     

Electrochemical proton relay at the single-molecule level

A scheme for the experimental study of single-proton transfer events, based on proton-coupled two-electron transfer between a proton donor and a proton acceptor molecule confined in the tunneling gap between two metal leads in electrolyte solution is suggested. Expressions for the electric current are derived and compared with formalism for electron tunneling through redox molecules. The scheme allows studying the kinetics of proton and hydrogen atom transfer as well as kinetic isotope effects at the single-molecule level under electrochemical potential control.     

Theory of the shot noise in electrochemical bridge contacts

Correlation functions of the current and the noise power for a redox-group-containing electrochemical tunnel contact are calculated under the assumption on a shot character of the electron tunneling. Weak and strong interaction between the redox group and electrodes is considered. Overvoltage dependence of the Fano factor at different bias-potentials is found. The conditions of passing of the calculated dependences into the Schottky law are formulated.     

Photo‐Induced Electron Transfer from Excited Tris[2,2′‐bipyridine‐κN1,κN1′]ruthenium(2+) Ions to Dyadic Electron Acceptors That Can Be Protected by Host‐Guest Interaction against Fast Back Electron Transfer,Characterization of the Guest Molecules

Anthraquinone and viologen moieties were combined to dyadic molecules with two redox centers. These dyades and their constituents were used as acceptors in photo‐induced electron‐transfer reactions. The experiments show that a caveat is necessary if one tries to derive the properties and the reactions of the dyades from those of their constituents: the spectral properties appear to be independent superpositions of those of the constituents. However, the redox potentials of the two redox centres in the dyades deviate from that of their constituents, the methylene bridge can not suppress a considerable interaction between the two redox centres. This is especially true when the redox potentials of the constituents are close to one another. From the quenching experiments, it can be concluded that electrolyte cations like Na⁺ are engaged in the transition states of the electron‐transfer reactions. In this way, they can control the fate of the transferred electron.     

Does the electron spin affect the rates of electron tunneling in electrochemical systems?

An effect of spin degeneracy of electron energy levels in the metal electrode on the observable characteristics of electrochemical systems in the absence of magnetic field is discussed. Single-electrode outer-sphere electron transfer reactions are considered as well as redox-mediated electron tunneling in electrochemical contacts. Particular attention is paid to the difference between the spin-less model and the limit of infinitely large Coulomb repulsion of the electrons occupying the same valence orbital in the redox group. Adiabatic and non-adiabatic regimes of the transitions are studied and the expressions for the tunnel current are obtained.     

Interplay between barrier width and height in electron tunneling: photoinduced electron transfer in porphyrin-based donor-bridge-acceptor systems

The rate of electron tunneling in molecular donor-bridge-acceptor (D-B-A) systems is determined both by the tunneling barrier width and height, that is, both by the distance between the donor and acceptor as well as by the energy gap between the donor and bridge moieties. These factors are therefore important to control when designing functional electron transfer systems, such as constructs for photovoltaics, artificial photosynthesis, and molecular scale electronics. In this paper we have investigated a set of D-B-A systems in which the distance and the energy difference between the donor and bridge states (DeltaEDB) are systematically varied. Zinc(II) and gold(III) porphyrins were chosen as electron donor and acceptor because of their suitable driving force for photoinduced electron transfer (-0.9 eV in butyronitrile) and well-characterized photophysics. We have previously shown, in accordance with the superexchange mechanism for electron transfer, that the electron transfer rate is proportional to the inverse of DeltaEDB in a series of zinc/gold porphyrin D-B-A systems with bridges of constant edge to edge distance (19.6 A) and varying DeltaEDB (3900-17 600 cm(-1)). Here, we use the same donor and acceptor but the bridge is shortened or extended giving a set of oligo-p-phenyleneethynylene bridges (OPE) with four different edge to edge distances ranging from 12.7 to 33.4 A. These two sets of D-B-A systems-ZnP-RB-AuP+ and ZnP-nB-AuP+-have one bridge in common, and hence, for the first time both the distance and DeltaEDB dependence of electron transfer can be studied simultaneously in a systematic way.     

The role of long range tunneling in electron transfer processes in condensed media

Experimental evidence is given for the importance of long range tunneling in electron transfer reactions in condensed media: the unusually weak effect of electrostatic repulsion on the rate of some electron transfer and spin exchange processes; electron transfer between distant (up to ≈ 30 A) species in solids at a rate considerably exceeding that of thermal diffusion; the unusual concentration dependence of radiation yields in the presence of scavengers, etc. The concept of long range tunneling is shown to permit quantitative explanation and correlation of experimental data on electron transfer in quite different fields.The factors determining the efficiency of tunneling, as well as some peculiar features of tunneling kinetics are considered.The role of long range tunneling in various chemical processes involving electron transfer in condensed media are discussed (ion reactions in solutions, photochemistry, radiation chemistry, reactions with polymers, some biochemical reactions).