Solvatochromism of β-naphthol

Excitation and emission fluorescence spectra of 2-naphthol and 2-methoxynaphthalene were measured in a series of pure solvents. The spectral shifts are correlated by the Kamlet - Taft parameters (π*, β, and α). As judged from the π* dependence, both molecules have a negligibly small dipole moment in their ground electronic state, which increases in the excited (S₁) state. However, the majority of the Stokes shift is due to hydrogen-bonding rather than to dipole - dipole interactions. By comparing the shifts for the two compounds, it is demonstrated that the β 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 produces a bathochromic shift. We find α dependence only in the excitation spectrum, indicating that protic solvents stabilize the ground state by donating a hydrogen bond to the hydroxylic oxygen. This bond breaks following excitation to S₁ but re-forms following proton transfer.