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Our Research |
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Photochemistry in Supercritical FluidsSupercritical fluids are phases created when substances are at conditions of temperature and pressure above their critical points. A number of substances are routinely used in chromatography as supercritical fluid eluents, by far the commonest being carbon dioxide since this forms a supercritical fluid at relatively modest temperatures and pressures. Supercritical fluid carbon dioxide has a similar solubilising power to hexane, and hence polar compounds are only sparingly soluble. However, the solubilising properties of the fluid can be modified by the addition of solvents such as acetonitrile or methanol; these additives are known as "modifiers". Diffusion coefficients in supercritical fluids are higher than in conventional solvents as a result of a lower fluid density. Consequently photochemically induced reactions such as triplet-triplet energy transfer occur at faster rates than in conventional solvents. By using triplet-triplet energy transfer as a probe for diffusion rates, we have shown that not only is the diffusion rate a sensitive function of the concentration of modifier, but that at low modifier concentrations selective clustering of solutes leads to faster than expected energy transfer rates. Other supercritical fluids which lend themselves particularly to photochemical investigations include supercritical fluid Xenon, where the heavy atom effect results in efficient triplet state formation even for compounds showing almost unity fluorescence quantum yields in conventional solvents, rendering their triplet states amenable to study. Recently, we have been studying the effect of the parameters of a supercritical fluid on the lifetime, quantum yield and reactivity of singlet oxygen, a reactive intermediate of immense importance in many systems. Selected References Abdel-Shafi, A.A., Wilkinson, F. and Worrall, D.R. “Photosensitised Production of (O2*(a1Dg)) in Supercritical Fluid Xenon (SCF-Xe), a unique “heavy atom” solvent. Effect of pressure and temperature on the lifetime of (O2*(a1Dg)) and quenching by ground state oxygen in SCF-Xe” Chemical Physics Letters 343(3-4), 2001, pp. 273-280 Worrall, D.R., Abdel-Shafi, A.A. and Wilkinson, F.,''Factors affecting the Rate of Decay of the First Excited Singlet State of Molecular Oxygen O2*(a1 Δg) in Supercritical Fluid Carbon Dioxide'', Journal of Physical Chemistry A, 105, 2001, pp 1270-1276 Worrall, D.R. and Wilkinson, F., ''Photochemistry in Modified Supercritical Carbon Dioxide. Effect of Modifier Concentration on Diffusion Probed by Triplet-Triplet Energy Transfer'', Journal of the Chemical Society, Faraday Transactions, 92(9), 1996, pp 1467-1471 |
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