Evidence for L-1(a), L-1(b) dual state emission in 1-naphthol and 1-methoxynaphthalene fluorescence in liquid solutions

The steady-state fluorescence spectra of 1-mathoxynaphthalene and 1-naphtho l were measured in pure organic solvents and in binary mixtures of water an d several organic solvents. The 1-isomers exhibited a much larger fluoresce nce Stokes-shift than the corresponding 2-isomers. The emission spectra of 1-methoxynaphthalene and 1-naphthol in pure nonpolar organic solvents exhib ited roughly the same structured spectral features, while the fluorescence spectra in water and formamide exhibited much broader red-shifted and less structured spectral features. In accord with previous observations, we attr ibute these spectral changes to two emitting states, L-1(b) and 1L(n), whos e relative fluorescence intensities are sensitive to solvent polarity. Our analysis of the fluorescence spectra of 1-naphthol and 1-methoxynaphthalene by Pekarian functions have demonstrated that the L-1(a) state becomes the dominant emitting state in polar solvents. In addition, the L-1(a) state wa s found to be further stabilized in hydrogen-bond-donating solvents. In con trast to previous suggestions, the onset of the excited-state proton transf er reaction from 1-naphthol occurred at higher solvent polarities than thos e required for the establishment of a dominantly L-1(a) emitting state.