Thin-layer spectroelectrochemistry

A challenge in using bulk electrolysis to generate redox states for in situ spectroscopic investigations is to minimize the time for exhaustive electrolysis. The most common 

A common arranganent for thin layer geometry is to sandwich the solution between glass and an rrO electrode using a Teflon or Kapton spacer such cells are now commercially available. For a thin layer arrangement, the cell width through which the excitation beam is directed ranges between 50 and 250 pm and the electrolysis rate is controlled by finite diffusion. 

At short electrolysis times when mass transport occurs via linear diffusion, the approximate time, t, required to fully electrolyze the cell is given by 

Spectroelectrochemistry provides a convenient avenue to assess whether this conclusion is correct by allowing the isolation and spectroscopic smdy of the mixed valence state. Oxidation of the first metal center results in the formation of the mixed valence Ru(II)Ru(III) ion reflected in the grow-in of a new optical transition centered at 1700 nm (s = 2250 dm mol cm ) and identified as an intervalence charge transfer transition (IVCT). The optical characteristics of such transitions can be analyzed according to the Hush theory (21) and used to estimate the extent of electronic coupling between two metals across the intervening bridge. The full width at half maximum (FWHM) (Vj j) of the 

IVCT band reflects the extent of electronic delocalization between the metal centers and this was measured to be 1390 cm For a compound in which the electron on the reduced metal is localized on that metal, known as a class II system, can be theoretically predicted from the Hush theory and this was estimated to be 3680 cm for this complex. The considerably narrower experimental IVCT observed reflects significant delocalization of electron density across the metal centers which is further reflected in the coupling constant, H, which was estimated be 2940 cm . On the basis of spectroelectrochemistry, this complex is assigned as a class III or delocalized species in which the mixed valence state is more accurately described as Ru(2.5)-Ru(2.5). 

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Describe clearly how thin-layer spectroelectrochemistry is used for measuring the values of E° and n. 

The electrosynthesized (0EP)Ge(CgHs)C10, was characterized in situ by thin-layer spectroelectrochemistry. The final product of electrosynthesis was spectrally compared with the same compounds which were synthesized using chemical and photochemical methods(35). (0EP)Ge(C6H5)Ci and (0EP)Ge(CsHs)0H were also electrochemically generated by the use of specific solvent/supporting electrolyte systems(35). 

The oxidation of 5,6-diaminouracil provides a good example of the use of thin-layer spectroelectrochemistry to determine the rate constant of an electrogenerated intermediate [34]. The reaction sequence is as follows ... 

To a large extent, the discovery and application of adsorption phenomena for the modification of electrode surfaces has been an empirical process with few highly systematic or fundamental studies being employed until recent years. For example, successful efforts to quantitate the adsorption phenomena at electrodes have recently been published [1-3]. These efforts utilized both double potential step chronocoulometry and thin-layer spectroelectrochemistry to characterize the deposition of the product of an electrochemical reaction. For redox systems in which there is product deposition, the mathematical treatment described permits the calculation of various thermodynamic and transport properties. Of more recent origin is the approach whereby modifiers are selected on the basis of known and desired properties and deliberately immobilized on an electrode surface to convert the properties of the surface from those of the electrode material to those of the immobilized substance. 

A thorough electrochemical characterization of new metalloporphyrins is nowadays state of the art for the synthetic inorganic chemist. In many of the papers cited in Sects. 3 and 4, a characterization of the new complexes by cyclic voltametry and electrolysis at controlled potential has been done. Thin-layer spectroelectrochemistry is very fruitful [346]. Fortunately, apart from classical articles of Davis et al. [347], Felton et al. [292], Fuhrhop et al. [293], Buchler et al. [190], more recent reviews of Kadish et al. are available which systematically cover the field of general metalloporphyrins [294] or organometallic porphyrin complexes [306]. Therefore, a short, update of these articles will be given in the form of Table 7. For details, the reader is referred to the original literature. 

Durliat, H., Comtat, M., Investigation of Electron lYansfer Between Platinum and Large Biological Molecules by Thin-Layer Spectroelectrochemistry , Anal. Chem. 54 (1982) 856-861. 

Comtat M, Durliat H. Some examples of the use of thin layer spectroelectrochemistry in the study of electron transfer between metals and enzymes. Biosens Bioelectron 1994 9 663-668. 

The electrochemical behavior of the acac analogs R(TPP)(acac) (R=Eu-Lu) is remarkably different (Iwase and Igarashi 1993). As revealed by cyclic voltammetry and rotating-disk experiments as well as thin-layer coulometry, these compounds exhibit two oxidation waves (. 1/2 = 0.60-0.76, 0.92-0.94 V) and two reduction waves ( 1/2 =-1.32 to -1.37, -1.67 to -1.69 V), all of which correspond to a reversible one-electron transfer process. The second reduction may be associated with a dissociation reaction of the acac ligand which is supported by thin-layer spectroelectrochemistry. By contrast, the Sm " analog Sm(TPP)(acac) undergoes four reversible oxidations ( 1/2 = 0.41,0.59,0.80,1.03 V) and one reversible reduction ( 1/2=-1.51 V). The coulometric results suggest that all these... 

The first reduction of (TPPBrj )Co is reversible and [(TPPBrjc)Co ] is generated after the first reduction. A further reduction at the macrocycle leads to a stepwise elimination of Br , thus giving a series of [(TPPBrjc)Co ] complexes with smaller and smaller values of x and (TPP)Co is ultimately formed as the final porphyrin electroreduction product in PhCN. These results were confirmed by experiments carried out at a rotating ring disk electrode and also by thin-layer spectroelectrochemistry (vide infra). ... 

Multiple minigrids were stacked between spacers to increase the optical pathlength of the cell while retaining thin-layer diffusional distances. This cell was used to demonstrate infrared thin-layer spectroelectrochemistry with ninhydrin. This was chosen for its three carboxyl groups, the reduction of which would be easily observable in the infrared. Another thin-layer cell was constructed with quartz plates in order to illustrate applicability in the UV (40). 

It is well known that half-wave potentials can be correlated with other physicochemical characteristics of a compound such as the UV-visible data. Figure 5 shows the spectra of Cgo. Ceo and which were obtained by thin-layer spectroelectrochemistry in dichloromethane, 0.05 M TBABF4 [9]. 

Chen T, Dong S, Xie Y (1994) Influence of the ohmic polarization effect on thin-layer spectroelectrochemistry. J Electroanal Chem 379 239-245... 

ABTS + production has been described by using thin-layer spectroelectrochemistry [46]. Fifty microliters of ABTS in 0.1 M acetate buffer solution (pH 5) was oxidized in a quartz flat cell (0.1 cm width) containing an optical transparent thin-layer electrode (OTTLE system). The radical formation was measured with a potential scan from 0.65 to 0.70 V and returned to 0.65 V at the scan rate of 0.05 mV s . The reactions were monitored spectrophotometrically every 30 s. Figure 31.11 shows the 3D plots obtained for spectral changes during ABTS electrolysis at different intervals. At the beginning, with ABTS as the sole species, two peaks were observed (A = 214 nm, A.2 = 340 nm), but as the... 

Kadish KM, Rhodes RK (1981) Reactions of metalloporphyrins Jt-radicals. 2. Thin-layer spectroelectrochemistry of zinc tetraphenylporphyrin cation radicals and dications in the presence of nitrogenous bases. Inorg Chem 20 2961-2966... 

Vitamin B 2 (cyanocob(III)alamin) is an example of a quasi-reversible redox system that exhibits slow heterogeneous electron-transfer kinetics. Cyclic voltammetry alone suggests that the reduction of vitamin B 2 is a single two-electron process at = -0.93 V vs SCE to the Co(I) redox state (Figure lOA). However, thin-layer spectroelectrochemistry using a... 

UV-visible spectral measurements under electrochemical control can often be made using a conventional spectrometer. For thin-layer spectroelectrochemistry experiments, a sufficiently large sample spectrometer compartment is required to accommodate the OTTLE cell. 

The situation is more complex with the Fe(III)/(Fe II) and Fe(II)/Fe(I) waves which show coupling of the electron transfer with homogeneous ligation/deligation reactions in a number of cases. Cyclic voltammetry and thin-layer spectroelectrochemistry were then used jointly for obtaining the characteristic standard potentials and equilibrium constants as well as the rate constants for the complexation of iron(II) by chloride ions. The procedures employed in this purpose have been described in detail , requiring in several cases an extension of the available theory2 26 of... 

The information power of electrochemistry can be expanded by coupling it with methods which can determine the chemical identity of intermediates and products of electrode reactions. In-situ information can be obtained by on-line spectroscopic methods such as ultraviolet/visible (UV/vis) thin-layer spectroelectrochemistrys. Recently, effective on-line coupling of an electrochemical cell with a mass spectrometer has been demonstrated and its application to the study of biological redox reactions has been described . Electrochemistry/mass spectrometry as well as off-line methods were used in determining redox and related chemical reactivity of purine drugs. 

Oxidation pathways of the purine antimetabolite 2,6-diaminopurine and its metabolite 2,6-diamino-8-purinol have recently been reported ,i4, Electrochemistry was used to determine the number of electrons and protons involved in the oxidation as well as the sequence, rate of formation, and stability of the reaction intermediates and products. Stability and rate of formation of intermediates and products were confirmed by UV/vis thin-layer spectroelectrochemistry. The products and stable intermediates were isolated at different stages in the oxidation and were identified by gas chromatography/mass spectrometry and by FAB MS. 

On-line thin-layer spectroelectrochemistry can provide chemical information but by itself it is not sufficient to confirm chemical identity. Typically the supporting evidence has been obtained by off-line methods such as mass spectrometry, FTIR and NMR. 

Durliat H. and Comtat M. (1984) Amperometric enzyme electrode for determination of glucose based on thin layer spectroelectrochemistry of glucose oxidase. Anal. Chem., 56, 148-152. 

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