Infrared Spectroelectrochemistry

STEPHEN P. BEST, STACEY J. BORG AND KYLIE A. VINCENT

1.1 Introduction

A set of electrochemical measurements may, with the aid of simulations, provide skeletal details of the redox-coupled reactions of a system, although the extent of the detail or uniqueness of the description depends on the complexity of the system and the relative rates of reaction. Since such an approach can, at best, yield only limited insight into the structure of intermediate species there is a clear need to supplement the electrochemical measurements by spectroscopic investigations. This need has spawned a number of approaches designed to provide the spectroscopic details of the electrogenerated intermediates/products. Spectro-electrochemical (SEC) techniques allow in-situ spectroscopic interrogation of electrogenerated complexes and this may permit the study of shorter-lived species and also establish the chemical reversibility of these reactions. This allows the building, testing and refinement of the mechanism and, crucially, provides insights into the structures of the intermediates.

The structure of the intermediate implicitly encompasses molecular, electronic, and vibrational components where the molecular structure is most commonly deduced by X-ray crystallography. More limited structural data may also be obtained from solute species through analysis of the X-ray absorption fine structure (XAFS) spectra and this will be discussed briefly in Section 1.6. Clearly the electronic and vibrational structure must be obtained from analysis of the spectra. The interconnection between these aspects of the structure is reinforced by in-silico techniques, where advances in DFT (density-functional theory) have greatly expanded the range of transition-metal compounds and smaller clusters that are amenable to study. For systems of moderate complexity a combination of structural, spectroscopic and in-silico approaches is required in order to achieve a satisfactory understanding of the intermediates formed during reaction. This chapter focuses on the use of IR spectroscopy to delineate the chemistry following redox activation and the integration of these results with a range of electrochemical, spectroscopic and computational methods to characterise the charge state and structure of intermediate species. While the vibrational structure of a species would ideally be determined through the examination of both its IR and Raman spectra, in most cases the complementary nature of the physical constraints associated with the two techniques results in studies concentrating on one or the other approaches. In cases where the system under investigation incorporates strongly IR absorbing chromophores, such as CO or CN-, IR spectroscopy can be both effective and easily implemented.

Since the objective of the studies described herein is the characterisation of the solute species formed following redox reaction the very extensive research dealing with characterisation of the electrode/solute interface will not be discussed, excellent overviews of the experimental aspects of this subject are available. While this contribution focuses on applications involving IR, Raman spectroscopy has proved to be invaluable to many SEC studies where surface-enhanced Raman spectroscopy (SERS) and resonance Raman spectroscopy dominate. Reviews and recent studies attest to the value of these approaches.

In this contribution we aim to illustrate the impact of IR-SEC techniques on the elucidation of the chemistry following a redox reaction. The most effective experimental approach will depend on the stability of the redox products together with the rates or nature of the following reactions. In Section 1.5 we show the experimental results obtained from several systems chosen so as to highlight the different experimental approaches that can be applied to good effect. We have limited the discussion to studies of solute species and to concentrating on examples drawn from our own research, published and unpublished. This is driven, in large part, by the availability of the raw experimental data and the opportunity that this provides to recast the figures in a self-consistent form. As a result, there is an overrepresentation of studies conducted using external reflectance SEC cells.

1.2 Overview of IR-SEC Techniques for the Study of Solute Species

The marriage between the spectroscopic and electrochemical requirements of the SEC experiment necessarily involves compromise, the nature of which will be dictated by the objectives of the study. For thin-layer cells with large surface area electrodes uncompensated solution resistance will generally present problems and these will be accentuated for studies conducted in highly resistive solvents. In many cases it is not practicable to use a conventional reference electrode and in these instances a pseudoreference consisting of a silver or platinum wire or foil is used. While such electrodes are susceptible to a drift in potential the impact of this deficiency may be minimised if the duration of the experiment is short relative to a change in the concentration of the species near the reference electrode. Several different experimental approaches have proved to be effective for the collection of IR-SEC results from electrogenerated solute species and these may be distinguished in terms of the characteristics of the working electrode. These include (i) optically transparent electrodes, (ii) perforated electrodes and (iii) reflective electrodes. To these may be added approaches in which a probe beam is brought close to the working electrode of an electrosynthesis cell by means of a waveguide or optical fibre. The sampling element may consist either of a pair of launch and collection fibres or include an optical element that is arranged so as to give near-total internal reflection (attenuated total reflection, ATR). In the latter case the spectrum of the solution in contact with the ATR crystal is sampled through its interaction with the evanescent wave that propagates beyond the reflecting surface. Depending on the cell geometry, and volume of solution, the time required for electrosynthesis can be substantial (> 1 h) in which case the distinction between in-situ and ex-situ spectroscopic interrogation is not clear cut. With careful attention to the design it is possible to reduce the volume of solution subject to electro-synthesis and thereby reduce the response time. An ATR IR-SEC cell featuring a sample chamber with a volume of 20 µL has recently...