The accurate identification of redox products is an important goal in all of chemistry. Many redox intermediates have only a very limited lifetime, as they usually go on to generate the final thermodynamically stable reaction products. Thus, there has always been great interest in the creation of new methodology that allows a better determination of the identities of redox products generated during complex reactions.
Various chemical and electrochemical methods have been developed over the last few decades to allow the spectral characterization of redox intermediates. However, each method, while having its advantages, also has some disadvantages. We, in active collaboration with Michael Shaw's group at SIUE, have published new methodology that uses fiber-optic technology to characterize redox intermediates that are generated at electrode surfaces.
In summary, an IR beam is directed through a fiber-optic probe to the solution (~3 mm path) and then bounced off a Pt disk electrode back through the probe to a liquid-nitrogen cooled detector. The sketch on the left shown the relative placement of the vertical dip probe and the Pt electrode at the bottom; we have used standard 24/40 glass joints to encase the setup so that we can work anaerobically. On the right is the photo of the IR setup with electrodes attached. By applying a potential to the electrode/mirror surface, we generate small amounts of electrolysed material at and/or near the electrode surface. By combining the applied potential with concurrent IR spectroscopy, we are able to obtain spectra of the redox products generated at the electrode surface on a cyclic voltammetry time scale! We are also able to encase the electrode assembly in a low-temperature bath to perform variable-temperature IR spectroelectrochemistry experiments.
We recently extended this work to the application of simultaneous chronoamperometry and chronoabsorptometry for the determination of UV-vis spectra of unstable redox intermediates. The newly-modified next-generation spectroelectrochemical cell is shown below.
Important modifications include: (a) horizontal placement of the fiber-optic probe and electrode assembly, and (b) allowing easy stirring (in-between experiments) and placement of a cold bath for variable temperature experiments.
In the simplest case, we assume a reversible electrochemical process such as that described below (where A is reduced to B). We then apply the integrated the Cotrell equation shown, assuming that the double-layer charging and adsorption phenomena are small.
By manipulating known electrochemistry equations, we can calculate the relationship between absorbance of species B generated an electrode surface with time, assuming that the diffusion coefficients of A and B are similar. We can combine the resulting equation with a similar one for species A to give the equation below.
This latter equation describes how a change in absorbance in a set of difference spectra varies with the charge passed through the electrode; the equation is in the form of that describing a straight line. Thus, if the values of n, A, and the absorptivity of species A are known, then we should be able to extract the absorptivity of species B at each individual wavelength; combining these data over a range of wavelengths generates the UV-vis spectrum of electrogenerated B!
The power of this methodology lies in the fact that we no longer rely on manual spectral subtraction to obtain the spectra of species B that is produced in low yield. To illustrate this, we turn to the electooxidation of a cobalt nitrosyl porphyrin. The product spectrum generated (after a total of only 5 secs!) via the simultaneous chronoamperometry-chronoabsorptivity method is shown below; the visible band at 690 nm is characteristic of a pi-radical cation intermediate.
We have applied this methodology to several inorganic, organometallic, and bioinorganic model systems to generate hitherto unknown spectra of several redox intermediates. Recent results are shown in our 2010 publication on this method.