Proton Transfer from 2-Naphthol to Aliphatic Amines in Supercritical CO2

The proton transfer from 2-naphthol to aliphatic amines was studied in supercritical CO2 (scCO2) and in cyclohexane as reference solvent, by absorption and fluorescence spectroscopy and by time-resolved emission. Irradiation of 2-naphthol in scCO2 in the presence of ethyldiisopropylamine shows dynamic fluorescence quenching of the acidic form of 2-naphthol and emission from the basic form. Fluorescence excitation spectra show that the emission of the basic form is originated upon excitation of the acidic form. The interaction between 2-naphthol and the amines is described by the formation of a complex with proton donor–acceptor character in the ground and excited states of 2-naphthol. The acidity increase of 2-naphthol upon electronic excitation to the first excited singlet in scCO2 is as high as in water. Proton transfer quantum yields of 0.6 can be easily achieved in scCO2. The results have implications for carrying out acid–base catalyzed reactions in scCO2.