Local Density Augmentation in Supercritical Solvents: Electronic Shifts of Anthracene Derivatives

Abstract

Fluorescence spectroscopy is used to measure local density augmentation in solutions of anthracene, 9-cyanoanthacene, 9,10-dichloroanthracene, and 9,10-diphenyl-anthracene in the supercritical solvents ethane, carbon dioxide, and fluoroform. For this purpose, the relationship between density and spectral shift is calibrated using dielectric continuum models of solvatochromism together with data in the gas phase and in liquid solutions. This approach, and the uncertainties inherent in it, are discussed in detail. The effective local densities (ρeff) deduced from the emission shifts in all of these solute/solvent combinations are comparable at temperatures near to the critical point (Tc+5 K or T/Tc = 1.02). Density enhancement factors ρeff/ρ increase with decreasing bulk density (ρ), reaching values of 5−6 at the lowest bulk densities observable. Density augmentation, the difference between the local and bulk densities (Δρeff ≡ ρeff − ρ) reaches a maximum at densities of ∼0.6 times the critical density (ρc)...