INVESTIGATION OF HYDROGEN-BONDING AND PROTON-TRANSFER OF AROMATIC ALCOHOLS IN NONAQUEOUS SOLVENTS BY STEADY-STATE AND TIME-RESOLVED FLUORESCENCE

Aromatic alcohols in aqueous solution become stronger acids upon excitation from the singlet ground state to the first excited singlet state. Hydrogen-bonding and excited-state proton-transfer reactions might also occur in the nonaqueous environment of a protein steroid-binding pocket or a tyrosine residue. Using absorption, steady-state fluorescence, and time-resolved fluorescence spectroscopies, we investigated these processes by titrating the aromatic alcohols 2-naphthol, 17-beta-dihydroequilenin, and phenol with the strong proton acceptor triethylamine in cyclohexane or toluene, organic solvents of different polarity and polarizability. Analysis of the absorption and steady-state fluorescence spectra as a linear combination of spectra (LINCS) shows that in the titration with triethylamine there are two separate ground-state and two separate excited-state species. Decay associated spectra (DAS) also show that there are two separate excited-state species. The ground-state and excited-state species that are formed from the interaction between the alcohol and triethylamine are hydrogen-bonded complexes. The LINCS analysis of the steady-state fluorescence emission and the fluorescence decay kinetics of the free alcohol indicate that formation of the hydrogen-bonded complex is diffusion-limited in the excited state. Excited donors (free alcohol) that subsequently form exciplexes in either nonpolar or polar solvents have the same emissive properties as those hydrogen-bonded complexes excited directly from the ground state. In addition the degree of charge transfer in the hydrogen bond depends on solvent polarity and polarizability. On the basis of these observations, we can make predictions about the interactions of the tyrosine residue in a protein or a 2-naphthol-containing estrogen in the presence of a strong proton acceptor (i.e., an unprotonated amino group). We conclude that emission in such a nonaqueous environment will be from the excited states of the free and hydrogen-bonded species; the fully ionized alcohol does not occur in the excited state.