Fluorescence of 4-aminophthalimide in supercritical CO2–ethanol mixtures

Abstract

Steady-state and time-resolved fluorescence studies of 4-aminophthalimide (AP) in neat supercritical CO2 and supercritical CO2–ethanol mixtures at 35 and 45 °C are presented. In neat CO2, the emission maximum of AP shifts to the red upon density increase because of the increase of average number of solvent molecules interacting with the probe. In CO2–ethanol mixtures of different ethanol densities (0.025 and 0.125 M) the tendency upon CO2 density increase is opposite. In mixtures, the CO2 density increase, also increases the probability of exchange of the ethanol molecules interacting with AP by CO2 molecules. This causes a blue shift that is bigger than the red shift caused by density increase. In all the cases, emission spectra were time independent in the nanosecond time range. This allowed to take solvation effects into account using a Langmuir adsorption model, under equilibrium conditions. This is the simplest association model that can semiquantitative describe the results and can successfully explain the lack of solvation entropic effects in emission of AP in the mixtures near the critical density of CO2.