Proton Transfer from 2-Naphthol to Amines in Supercritical Fluids

SIMONCELLI, SABRINA; HOIJEMBERG, PABLO. A.; JAPAS, M. LAURA; ARAMENDÍA, PEDRO F.

Mendoza

Conferencia; XXI Inter-American Photochemical Society Conference; 2011

We report in this work the successful quantification of the dynamics of proton transfer from excited-state 2-naphthol to various amines in supercritical CO2 (scCO2) at 35 ºC and in the pressure range between 100 and 200 bar. 2-Naphthol is a photoacid that changes its pKa in water from 9.5 in the ground state to 2.5 in the singlet excited state. Furthermore, the absorption but notably the emission spectra of the acid form and its conjugated base are easily distinguished. As proton acceptor, N-ethyldiisopropylamine (DIEA), triethylamine (TEA) and morpholine (MP) were used. Absorption, steady state fluorescence emission and excitation spectra were measured as a function of amine concentration and pressure at 35ºC. Equilibrium constants for the proton transfer reaction in the ground state in cyclohexane and in scCO2 indicate that this reaction is less effective in supercritical fluids (SCF) than in water or, even, cyclohexane. On the other hand, the difference in pK values for the proton transfer reaction in the ground and in the excited states is nearly 8 units, indicating that the change in acidity upon excitation in scCO2 is as large as in water. Our results show that, depending on the amine used, solvent effects on the absorption spectrum of 2-naphthol, or on the emission spectrum of 2-naphtholate, static or dynamic quenching of the excited state of 2-naphthol are observed. For DIEA and TEA, proton transfer yields anion emission. Linear Stern-Volmer relations for the system 2-naphthol/DIEA were observed in scCO2 at all pressures. The decrease in singlet lifetime (ff) of 2-naphthol in the presence of amine is consistent with a dynamic quenching mechanisms. In the case of MP, emission of 2-naphtholate is not observed and ff does not change with amine concentration, indicating the formation of a complex in the ground state, with the concomitant static deactivation of the 2-naphthol. We found that, for all systems 2-naphthol/amine, fluorescence excitation spectra recorded at the emission wavelength of 2-naphthol and 2-naphtholate are coincident, thus proving that 2-naphtholate builds up in the excited state, and further emits.