Intramolecular charge-transfer reactions studied in a supercritical fluid of varying densities and in a molecular beam

Abstract

In a supercritical fluid solvent, the solvent molecules are found to gather around the solute molecule, even in a low density state. By using a simple model based on the solute–solvent interaction potential, the solvation number has been derived as a function of the fluid density. The correctness of the model has been examined by a comparison between the solvent spectral shifts measured in supercritical CF₃H and those observed for van der Waals molecules in a supersonic beam. The formation of the charge-transfer (CT) state of N,N-dimethylaminobenzonitrile was then studied in supercritical CF₃H. The increase in the intensity of the CT. emission, as well as the bathochromic shift of the emission maximum, with increasing fluid density suggests that the clustering of CF₃H molecules strongly stabilizes the highly polar CT state. Thus solvation by polar molecules is shown to be an essential factor in facilitating CT state formation. However, on a microscopic level the location of the solvent molecules around the solute is also a crucial factor. This fact was demonstrated by the measurements of the emission spectra of various 9,9-bianthryl–acetone van der Waals complexes in a supersonic free jet.