Spectroelectrochemical analysis of bianthrone monolayers
Lucey, Darragh (2006) Spectroelectrochemical analysis of bianthrone monolayers. PhD thesis, Dublin City University.
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The redox chemistry of quinones and their analogues can be seen in virtually all living organisms and without them, important biological functions would not be possible. They are an essential component in bioenergetics and occur naturally as components in many biochemical molecules. The redox properties of quinone and its analogues, particularly species that display an observable change between their oxidised and reduced species, has also shown potential in the field of molecular electronics and sensors and also provide accessible models for the study of electron transfer. Much work has been done to develop an understanding of such molecules and further study is vital to expand this understanding and encourage the exploitation of these molecules.
In work prior to this thesis, focus has been placed on the electrochemical and photochemical analysis of bianthrone in solution. In brief, bianthrone undergoes a conformational change from a puckered form to a twisted form when undergoing a two-electron transfer in solution. This was described as an ECE mechanism.
However, no electrochemical studies had been carried out on bianthrone selfassembled monolayers immobilised on electrode surfaces and no evidence existed to suggest that the conformational behaviour of bianthrone in monolayer form would be comparable to its behaviour in solution.
Studies into the formation of self-assembled monolayers of bianthrone on mercury electrodes were carried out. General electrochemical properties suggest that bianthrone undergoes a two-electron, two-proton transfer. Bianthrone voltammetry showed evidence of current spikes that appeared in the voltammetry as the surface coverage approached full surface coverage. These spikes were indicative of intermolecular interactions between molecules on the surface. Studies into the dynamics of bianthrone monolayer formation were carried out by comparing a theoretical model, obtained from a paper by Hubbard et al. that described the influence of diffusion of molecules to the surface and the reorganization of molecules on the electrode surface, with surface coverage/time data obtained by cyclic voltammetry. . The diffusion rate constants ranged from 6.0 x 10'11 for 0.09 \xM to 1.5 x 10'12 for 0.3 j^M. The surface reorganization rate constants, K, ranged from 9.31 x 10'7 for 0.09 jiM to 9.8 x 10'8 for 0.3 jiM.
A spectroelectrochemical cell in which electrochemistry could be carried out while being incorporated into a Raman microscope was fabricated to examine bianthrone monolayers adsorbed on mercury electrodes. The potential of the cell was held at different points between where the monolayer was fully oxidised and fully reduced according to the cyclic voltammetry obtained and Raman spectra were recorded at these points. The spectra taken at these potentials were compared and contrasted to elucidate any structural differences that occurred in the reduction of the monolayer. This analysis revealed some subtle changes in intensity in the lower wavenumber region of the spectra. However, there were no changes indicative of a large structural change to the monolayer, As a result of this analysis, it is not believed that bianthrone undergoes conformational changes that would induce major structural changes to the monolayer upon reduction.
Studies into the formation of bianthrone monolayers on glassy carbon electrodes were carried out. The voltammetry is consistent with a surface adsorbed molecule undergoing a two-electron, two-proton transfer. Unlike adsorption on mercury, current spikes are not observed Studies into the dynamics of monolayer formation on GC were also carried out using the same theoretical model as published by Hubbard et al Values for the surface reorganization rate constant were obtained. This value ranged from 1 26 xlO 7 ± 0 05 cms 1 for 1 |iM to 4 36 ± 0 02 x 10 8 cms 1 for 20 |iM. A spectroelectrochemical cell similar to the mercury electrochemical cell in which electrochemistry could be carried out but that could still be incorporated into a Raman microscope was fabricated for use with a GC electrode Raman spectra of bianthrone monolayers recorded in the absence of an applied potential compared favourably with the spectra of solid bianthrone and spectra obtained in the literature. The potential of the cell was held constant at different points between where the monolayer was oxidised and reduced according to the voltammetry obtained. The spectra recorded at each potential were compared and contrasted to elucidate any structural differences that may have occurred at these potentials as the monolayer is reduced. Some changes in intensity at certain bands were observed, however, there were no large changes in the spectra associated with large structural changes arising from conformational changes in the molecule. From this, it was concluded that conformational change in the molecule did not occur when it was immobilised on the glassy carbon surface.
Electrochemistry was carried out on bianthrone in the solid state, on a glassy carbon electrode Evidence of “breaking in” phenomenon was observed from voltammetric studies of the solid layer.
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