Spectroelectrochemistry (SEC) refers to a range of experimental techniques that combine electrochemistry and spectroscopy. While electrochemical experiments provide information on macroscopic properties, spectroscopic techniques give information on a molecular level. The combination of the two therefore provides a comprehensive picture of a given sample .
In spectroelectrochemistry, the spectroscopic response is typically observed in situ and the electrochemical reaction is carried out under controlled conditions .
The design of spectroelectrochemical cells is usually determined by the requirements of the spectrometer. The cell materials, electrodes, and electrolytes must be transparent enough to observe the analyte, but even when using photoelectrochemical cells, measurements in the UV and IR regions are often limited by the absorption properties of the cell windows or electrolyte components (e.g. the solvent and conducting salt). In particular, the strong absorption bands exhibited by many solvents limits their use in the infrared region. By comparison, the choice of solvent for UV-Vis measurements is relatively straightforward; most solvents commonly used in standard electrochemistry are suitable.
Spectroelectrochemical techniques can generally be classified based on the type of spectroscopy (UV-vis, NIR or IR, Raman spectroscopy, EPR, NMR, XAS, or luminescence) or the electrochemical method (chronopotentiometry, chronoamperometry, linear sweep voltammetry, cyclic voltammetry, differential pulse voltammetry, stripping voltammetry, or pulse techniques) being used. In practice, it is more common for spectroelectrochemical techniques to be classified according to their spectroscopic methods  than their electrochemical methods.
Spectroelectrochemistry cells have response times that range from 10 seconds to several minutes for a single electron wave, which is a bit longer than the typical sweep rate in cyclic voltammetry (CV) of about 100 mVs-1. Reactions labelled as fully reversible by cyclic voltammetry therefore become less reversible due to slow reactions of the electrogenerated products.
UV-Vis Absorption Spectroelectrochemistry
UV-vis spectroelectrochemistry (UV-vis-SEC) can be used to probe the electronic transitions of molecules that absorb light in the ultraviolet and visible regions of the electromagnetic spectrum . Both the wavelengths at which absorption occurs and degree of absorption are measured by an optical spectrometer, and the spectrum is then presented as a graph of either absorbance versus wavelength or absorbance versus time. Based on the applied electrode potential, the resulting spectrum shows only the absorptions caused by the electrochemically generated species .
With optically transparent electrodes, UV-vis spectroelectrochemistry can study molecular adsorbates, polymer films, or other modifying layers which are either attached to the electrode surface or in the phase adjacent to it.
Spectroelectrochemical methods can be used to spectroscopically detect unique chemical species that are generated in situ at electrode surfaces during redox reactions. Photoluminescence spectroelectrochemistry (PL-SEC) allows for selective investigation of the excited state properties of in situ generated chromophores .
With photoluminescence spectroelectrochemistry, properties of new excited state species can be investigated along with their reactivity.
One example of this is the Re(II) complex, [Re(dmpe)3]2+, where dmpe is 1,2-bis(dimethylphosphino)ethane. The parent Re(I) complex, [Re(dmpe)3]+, is readily synthesised and exhibits a reversible one-electron oxidation to Re(II) in acetonitrile. The d6 Re(I) form is colourless and upon excitation into the UV absorption band exhibits no luminescence. By contrast, oxidation in the luminescence spectroelectrochemical cell to the d5 Re(II) electron configuration gives a reddish-pink solution with an absorption maximum at 530nm, which is accruable to a ligand-to-metal charge-transfer (LMCT) band. Excitation at 530nm produces a strong emission at 593nm with a quantum efficiency of 0.066, significantly higher than that of the standard typically used for comparison, i.e., [Ru(bpy)3]2+ in water, which is 0.042 .
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Infrared spectroelectrochemistry (IR-SEC) has been used to investigate adsorption phenomena in electrochemical systems and to identify and unravel molecular mechanisms of biological electron transfer. It is also used in mechanistic studies in electrocatalysis and has applications in the study of conducting polymers, redox-active organic molecules in solution, and inorganic or organometallic complexes.
It is becoming increasingly common to combine IR-SEC with other physical techniques, such as scanning electrochemical microscopy , for a more complete analysis of the sample.
In Raman spectroelectrochemistry (Rama SEC), vibrational information is used to gauge the strength of chemical bonds within molecules and between molecules and electrode surfaces.
Raman spectroelectrochemistry is often the method of choice for analysing the structure of materials, including carbon materials such as graphite, graphene, carbon nanotubes (or nanostructures in general) and fullerenes. It can also be used to probe information about crystalline or amorphous structures, and crystallite size can be obtained from the shift and shape of vibration modes. In battery research, Raman SEC can be used to determine the state and quality of nanostructured materials and lithium content.
When used to analyse self-assembled monolayers, Raman spectroelectrochemistry can be used to obtain information on the orientation of molecules on the surface of electrodes .
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X-ray spectroelectrochemistry (X-ray SEC) provides information about some basic considerations and symmetry on the electronic structure of active atomic orbitals of different materials, such as bond lengths and angles, identity of neighbouring atoms, and oxidation states . X-ray SEC is useful for spectroelectrochemical studies on corrosion processes, electrodeposition, ionic adsorption and yields of fuel cell anodes .
Nuclear Magnetic Resonance Spectroelectrochemistry
Nuclear magnetic resonance spectroelectrochemistry (NMR-SEC) presents an interesting approach to monitor and understand electrochemical and electrocatalysis processes at a molecular level.
NMR-SEC can provide dynamic structural characterisation and mechanism elucidation in an electrochemical process, as in the study of the redox behaviours of quinone-based systems , electrooxidation of dopamine at various voltages and pH using Au nanoparticles modified electrode , and electrocatalysis of hydroquinone by nano-polyaniline (nano-PAn) films .
Electron Paramagnetic Resonance Spectroelectrochemistry
Electron spin/paramagnetic resonance spectroelectrochemistry (ESP/EPR-SEC) can be used to detect short-lived radicals. Since ESP/EPR can provide valuable information about the oxidation state of an atom or atoms within a (macro)molecule and its environment by using unpaired electrons as probes, ESP/EPR-SEC can be used to investigate paramagnetic species such as organic radicals, proteins, polymers, and transition-metal complexes .
In-situ ESR/EPR spectroscopy can also be combined with UV-vis spectroscopy.
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