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ABSTRACT: Os(II)/(III) and Co(II)/(III) polypyridine complexes in aqueous solution are robust molecular entities both in freely solute state and adsorbed on Au(111)- and Pt(111)-electrode surfaces. This class of robust coordination chemical compounds have recently been characterized by electrochemical scanning tunneling microscopy (in situ STM). The Os-complexes were found to display strong tunneling spectroscopic (STS) features at the level of resolution of the single molecule while STS features of the Co complexes, although clear, were much weaker. The data was framed by concise but phenomenological theory of interfacial electrochemical electron transfer extended to the electrochemical in situ STM configuration. With a view on first-principle insight into the in situ STM behavior of robust redox (as opposed to nonredox) molecules, we present in this report a density functional theory (DFT) study of the complexes in both free and adsorbate state, in either state exposed to both stoichiometric counterions and a large assembly of solvent water molecules. The oxidation states of the complexes were controlled, first by introducing chlorine counter atoms followed by spontaneous attraction of electrons from the complexes, also at first in electrostatically neutral form. Second, the solvent is found to provide strong dielectric screening of this charge transfer process and to be crucial for achieving the full chemically meaningful charge separated ionic oxidation states. The molecular charge and structure of the complexes in the presence of the solvent, are conserved upon adsorption, whereas the structural features of the different oxidation states are completely lost upon adsorption under vacuum conditions. Detailed microscopic insight such as offered by the present study will be important in molecular-based approaches to "smart" redox molecules enclosed in in situ STM or other nanoscale and single-molecules scale configurations in condensed matter environments.
The Journal of Physical Chemistry B 08/2011; 115(30):9410-6. · 3.70 Impact Factor
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Journal of Physical Organic Chemistry 06/2010; 23(7):647 - 659. · 1.96 Impact Factor
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ABSTRACT: The electrochemical behavior of small metal nanoparticles is governed by Coulomb-like charging and equally spaced charge-transfer transitions. Using electrochemical gating at constant bias voltage, we show, for the first time, that individual nanoparticles can be operated as multistate switches in condensed media at room temperature, displaying distinct peak features in the tunneling current. The tunneling conductance increases with particle charge, suggesting that solvent reorganization and dielectric saturation become increasingly important.
Journal of the American Chemical Society 08/2007; 129(29):9162-7. · 9.91 Impact Factor
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W Haiss,
T Albrecht,
H van Zalinge,
S J Higgins,
D Bethell,
H Höbenreich,
D J Schiffrin,
R J Nichols,
A M Kuznetsov,
J Zhang,
Q Chi, J Ulstrup
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ABSTRACT: Experimental data and theoretical notions are presented for 6-[1'-(6-mercapto-hexyl)-[4,4']bipyridinium]-hexane-1-thiol iodide (6V6) "wired" between a gold electrode surface and tip in an in situ scanning tunneling microscopy configuration. The viologen group can be used to "gate" charge transport across the molecular bridge through control of the electrochemical potential and consequently the redox state of the viologen moiety. This gating is theoretically considered within the framework of superexchange and coherent two-step notions for charge transport. It is shown here that the absence of a maximum in the Itunneling versus electrode potential relationship can be fitted by a "soft" gating concept. This arises from large configurational fluctuations of the molecular bridge linked to the gold contacts by flexible chains. This view is incorporated in a formalism that is well-suited for data analysis and reproduces in all important respects the 6V6 data for physically sound values of the appropriate parameters. This study demonstrates that fluctuations of isolated configurationally "soft" molecules can dominate charge transport patterns and that theoretical frameworks for compact monolayers may not be directly applied under such circumstances.
The Journal of Physical Chemistry B 07/2007; 111(24):6703-12. · 3.70 Impact Factor
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ABSTRACT: We present a density functional theory (DFT) study of an osmium polypyridyl complex adsorbed on Au(111). The
osmium polypyridyl complex [Os(bpy)2(P0P)Cl]n+ [bpy is 2,2'-bipyridine, P0P is 4,4'-bipyridine, n ) 1 for osmium-
(II), and n ) 2 for osmium(III)] is bound to the surface through the free nitrogen of the P0P ligand. The calculations
illuminate electronic properties relevant to recent comprehensive characterization of this class of osmium complexes by electrochemistry and electrochemical scanning tunneling microscopy. The optimized structures for the compounds are in close agreement with crystallographic structures reported in the literature. Oxidation of the complex has little
effect on these structural features, but there is a substantial reordering of the electronic energy levels with
corresponding changes in the electron density. Significantly, the highest occupied molecular orbital shifts from the metal center to the P0P ligand. The surface is modeled by a cluster of 28 gold atoms and gives a good description of the effect of immobilization on the electronic properties of the complexes. The results show that the coupling
between the immobilized complex and the gold surface involves electronic polarization at the adsorbate/substrate
interface rather than the formation of a covalent bond. However, the cluster is too small to fully represent bulk gold
with the result that, contrary to what is experimentally observed, the DFT calculation predicts that the gold surface is more easily oxidized than the osmium(II) complex
Inorganic Chemistry 01/2007; 47:117-124. · 4.60 Impact Factor
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ABSTRACT: A theory of electron tunneling through a single-center bridge redox group that has two electronic levels participating in
the electron transfer process is presented. The temperature is presumed to be low enough to ignore activation redox conversions
of the bridge group. Salient features of this system are due both to the presence of two electroactive states of the bridge
group and to relaxation processes along the reaction coordinate. The processes in question make the tunneling current time-dependent
at fixed potentials and can bring about hysteresis in current-voltage curves when cycling the bias potential. Effects of inelastic
tunneling with excitation of vibrations of a local quantum degree of freedom are considered.
Russian Journal of Electrochemistry 06/2006; 42(7):760-766. · 0.53 Impact Factor
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ABSTRACT: Inorganic transition metal complexes were identified as potential candidates for transistor-like behavior in an electrochemical scanning tunnelling microscope (STM) configuration at room temperature. The theoretical background has been established based on condensed matter charge transfer theory. It predicts a distinct increase of the tunnelling current close to the equilibrium potential, i.e., if molecular bridge states are tuned into resonance with the Fermi levels of the enclosing electrodes. The complexes display robust electrochemistry on Au(111) electrode surfaces. STM images at molecular resolution reveal detailed information on their surface structure and scanning tunnelling spectroscopy experiments have shown clear evidence of transistor-like behavior.
IEEE Transactions on Nanotechnology 08/2005; · 2.29 Impact Factor
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ABSTRACT: A description of the physical mechanism and operation of a novel nanometric electronic switch [D.I. Gittins et al., Nature 2000 408, 67] is presented. New options for controlling the properties of this device are suggested and analyzed.
ChemPhysChem 05/2005; 6(4):583-6. · 3.41 Impact Factor
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ABSTRACT: A new mechanism of proton transfer in donor–acceptor complexes with long hydrogen bonds is suggested. The transition is regarded as totally adiabatic. Two closest water molecules that move synchronously by hindered translation to and from the reaction complex are crucial. The water molecules induce a shift of the proton from the donor to the acceptor with simultaneous breaking/formation of hydrogen bonds between these molecules and the proton donor and acceptor. Expressions for the activation barrier and kinetic hydrogen isotope effect are derived. The general scheme is illustrated with the use of model molecular potentials, and with reference to the excess proton conductivity in aqueous solution.
Russian Journal of Electrochemistry 09/2004; 40(10):1000-1009. · 0.53 Impact Factor
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ABSTRACT: Inorganic transition metal complexes were identified as potential candidates for transistor-like behaviour in an electrochemical STM configuration at room temperature. The theoretical background has been established based on condensed matter charge transfer theory. It predicts a distinct increase of the tunnelling current close to the equilibrium potential, i.e. if molecular bridge states are tuned into resonance with the Fermi levels of the enclosing electrodes. The complexes display robust electrochemistry on Au(111) electrode surfaces. STM images at molecular resolution give detailed insight into the surface structure. STS experiments are on the way to probe putative transistor-like behaviour.
Nanotechnology, 2004. 4th IEEE Conference on; 09/2004
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ABSTRACT: A new mechanism and formalism for proton transfer in donor-acceptor complexes with long hydrogen bonds introduced recently [1], is applied to a proton transfer in liquid water. Structural diffusion of hydroxonium ions is regarded as totally adiabatic process, with synchronous hindered translation of two closest water molecules to and from the reaction complex as crucial steps. The water molecules induce a gated shift of the proton from the donor to the acceptor in the double-well potential with simultaneous breaking/formation of hydrogen bonds between these molecules and the proton donor and acceptor. The short-range and long-range proton transfer as structural diffusion of Zundel complexes is also considered. The theoretical formalism is illustrated with the use of Morse, exponential, and harmonic molecular potentials. This approach is extended to proton transfer in strongly hydrogen-bonded donor-acceptor complexes. In contrast to the above model [1], the short hydrogen bond between the donor and acceptor moieties, however, completely erodes the barrier along the proton transfer mode. This introduces some physical pattern differences from proton transfer reactions in truly double-well potentials with a finite proton transfer barrier at the transition configuration with respect to the environmental nuclear coordinates. The differences apply particularly to the origin of the kinetic isotope effect. We discuss explicitly details of the excess proton conductivity in aqueous solution, but the concepts and formalism apply broadly to acid-base reactions, proton conduction channels, and other strongly hydrogen-bonded O- and N-proton donor-acceptor systems.
Russian Journal of Electrochemistry 01/2004; 40(10):1010-1018. · 0.53 Impact Factor
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ABSTRACT: A simple theory of elementary act of interrelated reactions of electron and proton transfer is developed. Mechanisms of synchronous and multistage transfer and coherent transitions via a dynamically populated intermediate state are discussed. Formulas for rate constants of adiabatic and nonadiabatic reactions are derived.
Russian Journal of Electrochemistry 12/2002; 39(1):9-15. · 0.53 Impact Factor
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ABSTRACT: Electrochemical science and technology in the 21st century have reached high levels of sophistication. A fundamental quantum mechanical theoretical frame for interfacial electrochemical electron transfer (ET) was introduced by Revaz Dogonadze. This frame has remained for four decades as a basis for comprehensive later theoretical work and data interpretation in many areas of chemistry, electrochemistry, and biology. We discuss here some new areas of theoretical electrochemical ET science, with focus on nanoscale electrochemical and bioelectrochemical sciences. Particular attention is given to in situ scanning tunneling microscopy (STM) and single-electron tunneling (SET, or Coulomb blockade) in electrochemical. systems directly in aqueous electrolyte solution and at room temperature. We illustrate the new theoretical formalism and its perspectives by recent cases of electrochemical SET, negative differential resistance patterns, and by ET dynamics of organized assemblies of biological macromolecules, such as redox metalloproteins and oligonucleotides on single-crystal Au(111)-electrode surfaces.
Russian Journal of Electrochemistry 12/2002; 39(1):108-117. · 0.53 Impact Factor
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04/2002;
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ABSTRACT: Monolayers of molecules, which retain their function in the adsorbed state on solid surfaces, are important in materials science, analytical detection, and other technology approaching the nanoscale. Molecular monolayers, including layers of functional biological macromolecules, offer new insight in electronic properties and stochastic single-molecule features and can be probed by new methods which approach the single-molecule level. One of these is in situ scanning tunneling microscopy (STM) in which single-molecule electronic properties directly in aqueous solution are probed. In situ STM combined with physical electrochemistry, single-crystal electrodes, and spectroscopic methods is now a new dimension in interfacial bioelectrochemistry. We overview first some approaches to spectroscopic single-molecule imaging, including fluorescence spectroscopy, chemical reaction dynamics, atomic force microscopy, and electrochemical single-electron transfer. We then focus on in situ STM. In addition to high structural resolution, in situ STM offers a single-molecule spectroscopic perspective. This emerges most clearly when adsorbate molecules contain accessible redox levels, and the tunneling current decomposes into successive single-molecule interfacial electron transfer (ET) steps. Theories of electrochemical ET and in situ STM of redox molecules as well as specific cases are addressed. Two-step in situ STM represents different molecular mechanisms and even new ET phenomena, related to coherent many-electron transfer. A number of systems are noted to accord with these views. The discussion is concluded by attention to one of the still very few redox proteins addressed by in situ STM, the blue copper protein Pseudomonas aeruginosa azurin. Use of comprehensive electrochemical techniques has ascertained that well-defined protein monolayers in two opposite orientations can be formed and interfacial tunneling patterns disclosed. P. aeruginosa azurin emerges as by far the most convincing case where in situ STM of functional metalloproteins to single-molecule resolution has been achieved. This comprehensive approach holds promise for broader use of in situ STM as a single-molecule spectroscopy of metalloproteins and illuminates prerequisites and limitations of in situ STM of biological macromolecules.
The Journal of Physical Chemistry B 01/2002; 106:1131-1152. · 3.70 Impact Factor
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ABSTRACT: Single-crystal electrochemistry and scanning tunneling microscopy directly in aqueous electrolyte solution (in situ STM) are established in physical electrochemistry but new in studies of adsorption and interfacial electrochemistry of biological macromolecules. These high-resolution techniques have now been applied comprehensively to proteins and other biomolecules in recent studies, discussed in this report. Focus is on three systems. The first one is a pair of amino acids, cysteine and cystine. These are strongly adsorbed via thiolate and disulfide, respectively, with identical reductive desorption and in situ STM patterns. Long-range lateral order can be imaged to molecular resolution. The amino acids are also reference molecules for the blue single-copper protein Pseudomonas aeruginosa azurin. This protein assembles in two well-defined orientations. One applies on bare Au(111) to which the protein is linked via its surface disulfide group. This orients the copper center away from the electrode surface. The other mode is by hydrophobic interactions with variable-length alkanethiols self-assembled on Au(111). In this mode the copper center is directed towards the surface. Adsorption and long-range electron tunneling in both modes have been characterized in detail using different electrochemical and spectroscopic techniques, as well as STM. Other data show that penta-(A–T) oligonucleotide adsorbed via a covalently bound thiol linker also displays reductive desorption and in situ STM to molecular resolution. The three systems thus appear to open new perspectives for broader use of high-resolution electrochemical techniques in comprehensive investigations of large biological molecules.
Russian Journal of Electrochemistry 12/2001; 38(1):68-76. · 0.53 Impact Factor
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ABSTRACT: Intramolecular electron transfer in azurin in water and deuterium oxide has been studied over a broad temperature range. The kinetic deuterium isotope effect, k(H)/k(D), is smaller than unity (0.7 at 298 K), primarily caused by the different activation entropies in water (-56.5 J K(-1) mol(-1)) and in deuterium oxide (-35.7 J K(-1) mol(-1)). This difference suggests a role for distinct protein solvation in the two media, which is supported by the results of voltammetric measurements: the reduction potential (E(0')) of Cu(2+/+) at 298 K is 10 mV more positive in D(2)O than in H(2)O. The temperature dependence of E(0') is also different, yielding entropy changes of -57 J K(-1) mol(-1) in water and -84 J K(-1) mol(-1) in deuterium oxide. The driving force difference of 10 mV is in keeping with the kinetic isotope effect, but the contribution to DeltaS from the temperature dependence of E(0') is positive rather than negative. Isotope effects are, however, also inherent in the nuclear reorganization Gibbs free energy and in the tunneling factor for the electron transfer process. A slightly larger thermal protein expansion in H(2)O than in D(2)O (0.001 nm K(-1)) is sufficient both to account for the activation entropy difference and to compensate for the different temperature dependencies of E(0'). Thus, differences in driving force and thermal expansion appear as the most straightforward rationale for the observed isotope effect.
Proceedings of the National Academy of Sciences 05/2001; 98(8):4426-30. · 9.68 Impact Factor
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ABSTRACT: In situ scanning tunneling microscopy (STM) of redox molecules, in aqueous solution, shows interesting analogies and differences compared with interfacial electrochemical electron transfer (ET) and ET in homogeneous solution. This is because the redox level represents a deep indentation in the tunnel barrier, with possible temporary electronic population. Particular perspectives are that both the bias voltage and the overvoltage relative to a reference electrode can be controlled, reflected in spectroscopic features when the potential variation brings the redox level to cross the Fermi levels of the substrate and tip. The blue copper protein azurin adsorbs on gold(111) via a surface disulfide group. Well resolved in situ STM images show arrays of molecules on the triangular gold(111) terraces. This points to the feasibility of in situ STM of redox metalloproteins directly in their natural aqueous medium. Each structure also shows a central brighter contrast in the constant current mode, indicative of 2- to 4-fold current enhancement compared with the peripheral parts. This supports the notion of tunneling via the redox level of the copper atom and of in situ STM as a new approach to long-range electron tunneling in metalloproteins.
Proceedings of the National Academy of Sciences 03/1999; 96(4):1379-84. · 9.68 Impact Factor
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ABSTRACT: PAC spectra (perturbed angular correlation of gamma-rays) of cadmium-substituted carboxypeptidase A (CPD) show that the enzyme in solution imposes a flexible, pH- and chloride-dependent coordination structure on the metal site, in contrast to what is found in the crystalline state. A much more restricted coordination geometry occurs for the steady-state peptide intermediates of Bz-Gly-l-Phe and Bz-Gly-Gly-l-Phe in solution, suggesting that substrate binding locks the structure in a rigid conformation. The results further indicate that the peptide intermediate has a six-coordinated metal coordination geometry with an OH- ligand at the solvent site and a carbonyl oxygen at an additional ligand site. In marked contrast, conformational rigidity is not induced by the inhibitor/poor substrate Gly-L-Tyr nor by the products of high turnover substrates, Bz-Gly, Bz-Gly-Gly, and L-Phe. These results are consistent with an intact scissile peptide bond in the enzyme-substrate complex of Bz-Gly-L-Phe and Bz-Gly-Gly-L-Phe. A single nuclear quadrupole interaction (NQI) is observed for the crystalline state of the enzyme between pH 5.7 and pH 9.4. This NQI agrees with calculations based on the metal coordination geometry for cadmium in crystalline CPD derived from X-ray diffraction studies. A single broad distribution of NQIs is observed for CPD in sucrose solutions and 0.1 M NaCl at pH values below 6.5. This NQI (NQI-1') has parameters very close to those for the crystalline state. The enzyme metal site, characterized by this NQI, is converted into two new enzyme metal sites over the pH range of 6.5-8.3. The metal coordination sphere of one of these has a NQI (NQI-1) with parameters similar to those at lower pH values (NQI-1') while the other NQI (NQI-2) is characterized by markedly different NQI parameters. Angular overlap model (AOM) calculations indicate that the coordination sites giving NQI-1' and NQI-1 both have a metal-bound water molecule while the coordination site giving NQI-2 has a metal-bound hydroxide ion. PAC results at pH 8.3-10.5 indicate that in this pH range the two metal coordination geometries related to NQI-1 and NQI-2 occur in a pH independent ratio of 2:1, with the one with the water ligand being the most abundant species. The observed pH-independent equilibrium between the two different metal coordination geometries for cadmium can be explained by an equilibrium between tautomeric forms of a hydrogen bond between the Glu-270 carboxyl group and the metal-bound water (Glu-270 COO-...(HOH)M <==> Glu-270 COOH...(OH-)M) being slow on the time scale of a PAC experiment, i.e., slower than 0.5 micros. We finally suggest that NQI-1' observed at low pH reflects an enzyme species containing a metal-coordinated water molecule and the protonated carboxyl group of Glu-270.
Biochemistry 10/1997; 36(38):11514-24. · 3.42 Impact Factor
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ABSTRACT: A systematic discussion of the effect of the medium on the elementary act of electron and proton transfer reactions is given. The effects due to the softening or freezing of different discrete or continuum vibrational modes are elucidated. The role of dielectric polarizability of the environment and its spatial dispersion and the influence of the ionic atmosphere are discussed for different reaction types. The options offered by biological macromolecules as the reaction medium are considered. Focus is put on the differences of reaction rates in liquids and solids with an emphasis on some counterintuitive effects, which follow from the detailed analysis.
07/1997;
