Redox couple, electrochemical potential

While many metal centers can be reversibly cycled between two (or more) oxidation states, few organic moieties can match such reversibility especially in protic media. Nevertheless, the first supramolecular example of an electroswitch-able luminescent device involved the benzoquinone-hydroquinone couple. The luminescence of 55 " is switched off due to PET in the benzoquinone state of the redox couple. Electrochemical or chemical reduction of the benzoquinone under protic conditions to hydroquinone recovers the luminescence of the tris(2,2 -bipyridyl) Ru(II) unit. It is noted that the luminescence of tris(2,2 -bipyridyl) Ru(Il) itself is electroswitchable. Indeed tris(2,2 -bipyridyl) Ru(II) came to fame as a solar energy material from more humble beginnings as a luminescent redox indicator. However 55 achieves the same switching at a lower magnitude of reduction potential. Here lies the advantage of the supramolecular design. Like tris(2,2 -bipyridyl) Ru(II), many lumophores show electroswitchable luminescence. An... 

In order to determine the standard potentials of other redox couples, electrochemical cells are built in which one of the redox reactions corresponds to the reaction Scheme (1. III). As an example, let us consider the following electrochemical cell (see Fig. 1.4) ... 

Anion receptors incorporating cobaltocenium have been studied extensively due to the combination of an accessible redox couple and potential favorable electrostatic interactions of the cationic organometallic metallocene moiety with anions. The first anion receptors utilizing the cobaltocenium motif were reported by Beer et al. in 1989. The macrocyclic bis-cobaltocenium receptor 54 was shown to bind (via electrostatic interaction) and to electrochemically sense Br in GH3CN solution. 

For fundamental studies, the simplest possible electrochemical cell of single-pellet type was used. The behavior of the reference electrode, prepared by deposition of a gold film, was found to be quasi-reversible, /zOa/O being the potential determining redox couple. The potential distribution in the single-pellet cell was estimated to be fairly symmetrical, and the IR drop correction was shown to be negligible for low currents typical of EP experiments. 

Electrochemical Reversibility and Determination of m In deriving a relationship between 1/2 and the standard-state potential for a redox couple (11.41), we noted that the redox reaction must be reversible. How can we tell if a redox reaction is reversible from its voltammogram For a reversible reaction, equation 11.40 describes the voltammogram. 

The photoelectrolysis of H2O can be performed in cells being very similar to those applied for the production of electricity. They differ only insofar as no additional redox couple is used in a photoelectrolysis cell. The energy scheme of corresponding systems, semiconductor/liquid/Pt, is illustrated in Fig. 9, the upper scheme for an n-type, the lower for a p-type electrode. In the case of an n-type electrode the hole created by light excitation must react with H2O resulting in 02-formation whereas at the counter electrode H2 is produced. The electrolyte can be described by two redox potentials, E°(H20/H2) and E (H20/02) which differ by 1.23 eV. At equilibrium (left side of Fig. 9) the electrochemical potential (Fermi level) is constant in the whole system and it occurs in the electrolyte somewhere between the two standard energies E°(H20/H2) and E°(H20/02). The exact position depends on the relative concentrations of H2 and O2. Illuminating the n-type electrode the electrons are driven toward the bulk of the semiconductor and reach the counter electrode via the external circuit at which they are consumed for Hj-evolution whereas the holes are dir tly... 

Dong et al. have reported on crown ether-bound biferrocene, 21, in which the redox potentials are shifted by the capture of metal ions within the crown ether (55). For example, the electrochemical behavior observed for the biferrocene derivative with Ba2+ ion is fundamentally different, as new redox couples are observed for 9 with Ba2+ concentrations within the range 0 < [Ba2+] < 2 eq. The peak currents for the two new redox couples increase with concentrations of the Ba2+ ion until a full equivalent is added at this point, the original redox couples disappear and the new redox couples reach full development. 

In an ideal case the electroactive mediator is attached in a monolayer coverage to a flat surface. The immobilized redox couple shows a significantly different electrochemical behaviour in comparison with that transported to the electrode by diffusion from the electrolyte. For instance, the reversible charge transfer reaction of an immobilized mediator is characterized by a symmetrical cyclic voltammogram ( pc - Epa = 0 jpa = —jpc= /p ) depicted in Fig. 5.31. The peak current density, p, is directly proportional to the potential sweep rate, v ... 

The sensitizers display a crucial role in harvesting of sunlight. To trap solar radiation efficiently in the visible and the near IR region of the solar spectrum requires engineering of sensitizers at a molecular level (see Section 9.16.3).26 The electrochemical and photophysical properties of the ground and the excited states of the sensitizer have a significant influence on the charge transfer (CT) dynamics at the semiconductor interface (see Section 9.16.4). The open-circuit potential of the cell depends on the redox couple, which shuttles between the sensitizer and the counter electrode (for details see Section 9.16.5). 

Phase-sensitive detection is not at all specihc for EPR spectroscopy but is used in many different types of experiments. Some readers may be familiar with the electrochemical technique of differential-pulse voltammetry. Here, the potential over the working and reference electrode, E, is varied slowly enough to be considered as essentially static on a short time scale. The disturbance is a pulse of small potential difference, AE, and the in-phase, in-frequency detection of the current affords a very low noise differential of the i-E characteristic of a redox couple. 

For a metal, the negative of the work function gives the position of the Fermi level with respect to the vacuum outside the metal. Similarly, the negative of the work function of an electrochemical reaction is referred to as the Fermi level Ep (redox) of this reaction, measured with respect to the vacuum in this context Fermi level is used as a synonym for electrochemical potential. If the same reference point is used for the metal s,nd the redox couple, the equilibrium condition for the redox reaction is simply Ep (metal)= Ep(redox). So the notion of a Fermi level for a redox couple is a convenient concept however, this terminology does not imply that there are free electrons in the solution which obey Fermi-Dirac statistics, a misconception sometimes found in the literature. 

An alternative electrochemical method has recently been used to obtain the standard potentials of a series of 31 PhO /PhO- redox couples (13). This method uses conventional cyclic voltammetry, and it is based on the CV s obtained on alkaline solutions of the phenols. The observed CV s are completely irreversible and simply show a wave corresponding to the one-electron oxidation of PhO-. The irreversibility is due to the rapid homogeneous decay of the PhO radicals produced, such that no reverse wave can be detected. It is well known that PhO radicals decay with second-order kinetics and rate constants close to the diffusion-controlled limit. If the mechanism of the electrochemical oxidation of PhO- consists of diffusion-limited transfer of the electron from PhO- to the electrode and the second-order decay of the PhO radicals, the following equation describes the scan-rate dependence of the peak potential ... 

For the operation of an STM in a conventional two-electrode configuration, the presence or absence of significant faradaic current in the tip-sample circuit depends on three factors 1) the redox potential(s) of the solution species, 2) the reversibility of the electron transfer events for the dissolved redox couple(s), and, 3) the extent to which solution species are permitted access to the tunneling gap. We have identified four limiting cases of electrochemical interest, and discuss each separately below. 

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