Use of Solvatochromism to Assay Preferential Solvation of a Prototypic Catalytic Site
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The composition of the reaction medium near photoactive catalytic sites can be inferred from the solvatochromism of the absorption and emission spectra of the wetted sites, which depend on the polarizability of the fluid. In brief, solvatochromism measures the interaction of the dipole moments of the ground and excited states with the electric field imposed by the solvent shell: a field, which does not relax on the time scale of the absorption or emission events. To establish the utility of the technique for inorganic catalysts that operate in complex reaction media, such as encountered in the upgrading of biogenic fuels, we have measured the solvatochromism of a common, structural feature of metal oxide catalysts, mono-oxide or dioxide of a transition metal prepared by incorporating the OM or O2M moiety into the framework of a polyhedral oligomeric silsesquioxane (POSS). In toluene, cyclohexene, chloroform and tetrahydrofuran, POSS-ligated oxometalates exhibit strong ligand-to-metal charge-transfer bands in their UV–visible absorption and emission spectra. From the solvatochromism of the chromophores dissolved in toluene-chloroform mixtures we inferred an unexpectedly strong, preferential solvation of the chromophore even when all three components (oxometalate and the two solvents) were highly miscible.
KeywordsLuminescence spectroscopy vanadium Chromium Molybdenum Tungsten oxides TD-DFT Ligand-to-metal charge transfer spectroscopy Mesoscale measurements
This research was supported in part by the Laboratory Directed Research & Development program at Pacific Northwest National Laboratory. PNNL is operated by Battelle for the US Department of Energy under contract DE-AC05-76RL01830. A portion of the research was performed at EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at PNNL. This research also used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
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