Solvatochromism of beta-naphthol

Excitation and emission fluorescence spectra of 2-naphthol and 2-methoxynap hthalene were measured in a series of pure solvents. The spectral shifts ar e correlated by the Kamlet-Taft parameters (pi*, beta, and alpha). As judge d from the pi* dependence, both molecules have a negligibly small dipole mo ment in their ground electronic state, which increases in the excited (S-1) state. However, the majority of the Stokes shift is due to hydrogen-bondin g rather than to dipole-dipole interactions. By comparing the shifts for th e two compounds, it is demonstrated that the beta dependence in 2-naphthol is due exclusively to a hydrogen bond donated from its hydroxylic hydrogen atom to the solvent. This bond becomes stronger upon excitation and hence p roduces a bathochromic shift. We find a dependence only in the excitation s pectrum, indicating that protic solvents stabilize the ground state by dona ting a hydrogen bond to the hydroxylic oxygen. This bond breaks following e xcitation to S-1 but re-forms following proton transfer.