Voltammetry irreversible redox process

Electrochemical properties were examined to gain more quantitative insight into the redox properties of this system. Cyclic voltammetry on bis(dithiazole) 23 in acetonitrile (with Pt electrodes and 0.1 M -Bu4NPF6 as supporting electrolyte) reveals a reversible oxidation wave with i/2(°x) = 0.93 V and a second, irreversible oxidation process... 

In order to understand fluorescence quenching of anthyl units in Rh(I) and Ir(I) complexes of the type shown in Fig. 26, the redox chemistry of these complexes has been investigated (91). There are two main redox processes observed with cyclic voltammetry in THF a reversible antryl centered reduction and an irreversible metal centered oxidation in all cases. The observation of irreversible oxidation waves in THF indicates that the electrode generated cationic species are not stable at room temperature. Apparently, however, the use of the solvent l,l,l,3,3,3-hexafluoropropan-2-ol (HFP) somewhat stabilizes these species. Oxidation of the M(I) species with thalium(III) trifluoroacetate in... 

Within the electrochemical framework of this classical example of a redox process whose rate is limited by the transport by diffusion, it was shown that, even for a reversible redox process, the derivation of the current response in the time domain is far from simple. In contrast, the impedance approach allows the more difficult case of an irreversible (finite reaction rate constants) redox process to be derived. Using the same approach, we will now examine the case of a multistep reaction, which is very difficult to investigate using techniques of potential step cyclic voltammetry. 

Device structure and energy level diagram of green phosphorescent OLED with mixed EML and TBADN Bphen ETL. HOMO energies were obtained as IPs by UPS spectroscopy or as electrochemical oxidation potentials by cyclic voltammetry. LUMO values were estimated from solution-determined redox data. Unless otherwise noted, redox processes were reversible. No reduction was observed for TCTA within solvent window, LUMO of TCTA is significantly higher than the shown value. Irreversible reduction was observed for Bphen LUMO may be up to 0.3 eV higher. ... 

The use of cyclic voltammetry (CV) has been mentioned earlier during the characterization of the two mononuclear Fe(III) complexes (5), and (6). ° An acetonitrile solution was used with 0.1 M TRAP as the supporting electrolyte. It was shown that (5) exhibits a one-electron redox process at Ei/ 2 = 0.316 V attributed to the Fe " Fe + e redox system, and the process is chemically as well as electrochemically irreversible as indicated by ipc/jpa 1 and an extremely high AEp value of 632 mV. In addition there is another oxidation peak at 1.072 V attributed to the irreversible ligand oxidation. Similarly complex (6) also exhibits an irreversible redox couple for Fe /Fe oxidation at E1/2 = 0.106 V with AEp = 212 mV along with the ligand oxidation peak at 1.159 V. 

Finally, other types of voltammetric experiments may be employed beneficially for the characterisation of the redox properties of redox active compounds. Figure H.l.lbd shows square wave voltammograms [72] for the oxidation and reduction of the binuclear ruthenium complex. Well-defined peak responses indicate the presence of a reversible redox process. In this situation, the peak position corresponds closely to the reversible potential for the process and the peak height is related to the number of transferred electrons. Square-wave voltammetry may be employed to enhance reversible redox processes and to discriminate against irreversible and background processes (see also Chap. II.3). 

The complexes of Cu(II) and Fe(III) with adenine and adenosine and Fe(II) with adenosine show a quasirreversible couple corresponding to the one-electron reduction of Cu(II) to Cu(I) or Fe(III) to Fe(II). All other redox processes (peaks II, II and III) are irreversible due to following chemical reactions, adsorption, and film formation. The low currents and poorly defined peaks in cyclic voltammetry for these complexes reflect steric effects imposed by the bulky biological ligands and dimeric and polymeric structures. 

The redox characteristics, using linear sweep and cyclic voltammetry, of a series of (Z)-6-arylidene-2-phenyl-2,3-dihydrothiazolo[2,3-r][l,2,4]triazol-5(6//)-ones 155 (Figure 24) have been investigated in different dry solvents (acetonitrile, 1,2-dichloroethane, tetrahydrofuran (THF), dimethyl sulfoxide (DMSO)) at platinum and gold electrodes. It was concluded that these compounds lose one electron forming the radical cation, which loses a proton to form the radical. The radical dimerizes to yield the bis-compound which is still electroactive and undergoes further oxidation in one irreversible two-electron process to form the diradical dication on the newly formed C-C bond <2001MI3>. 

The electrochemical behavior of Np ions in basic aqueous solutions has been studied by several different groups. In a recent study, cyclic voltammetry experiments were performed in alkali ([OH ] = 0.9 — 6.5 M) and mixed hydroxo-carbonate solutions to determine the redox potentials of Np(V, VI, VII) complexes [97]. As shown in Fig. 2, in 3.1 M LiOH at a Pt electrode Np(VI) displays electrode processes associated with the Np(VI)/Np(V) and Np(VII)/Np(VI) couples, in addition to a single cathodic peak corresponding to the reduction of Np(V) to Np(IV). This latter process at Ep —400 mV (versus Hg/HgO/1 M NaOH) is chemically irreversible in this medium. Analysis of the voltammetric data revealed an electrochemically reversibleNp(VI)/Np(V)... 

The application of surface-enhanced Raman spectroscopy (SERS) for monitoring redox and other processes at metal-solution interfaces is illustrated by means of some recent results obtained in our laboratory. The detection of adsorbed species present at outer- as well as inner-sphere reaction sites is noted. The influence of surface interaction effects on the SER spectra of adsorbed redox couples is discussed with a view towards utilizing the frequency-potential dependence of oxidation-state sensitive vibrational modes as a criterion of reactant-surface electronic coupling effects. Illustrative data are presented for Ru(NH3)63+/2+ adsorbed electrostatically to chloride-coated silver, and Fe(CN)63 /" bound to gold electrodes the latter couple appears to be valence delocalized under some conditions. The use of coupled SERS-rotating disk voltammetry measurements to examine the kinetics and mechanisms of irreversible and multistep electrochemical reactions is also discussed. Examples given are the outer- and inner-sphere one-electron reductions of Co(III) and Cr(III) complexes at silver, and the oxidation of carbon monoxide and iodide at gold electrodes. 

This equation is often used to determine the formal potential of a given redox system with the help of cyclic voltammetry. However, the assumption that mid-peak potential is equal to formal potential holds only for a reversible electrode reaction. The diagnostic criteria and characteristics of cyclic voltammetric responses for solution systems undergoing reversible, quasi-reversible, or irreversible heterogeneous electron-transfer process are discussed, for example in Ref [9c]. An electro-chemically reversible process implies that the anodic to cathodic peak current ratio, lpa/- pc equal to 1 and fipc — pa is 2.218RT/nF, which at 298 K is equal to 57/n mV and is independent of the scan rate. For a diffusion-controlled reduction process, Ip should be proportional to the square root of the scan rate v, according to the Randles-Sevcik equation [10] ... 

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