Vibronic coupling and microscopic solvation of 1-naphthol

Vibronic structure in the S-1 <-> S-0 absorption and fluorescence spectra of jet-cooled 1-naphthol is reexamined. Selective excitation of bands 410 or 414 cm(-1) above the S-1 ((1)L(b)) Origin leads to atypical, highly structured fluorescence, indicating that strong vibrational mixing occurs. From the intensity patterns and the appearance of otherwise dark modes, a large normal coordinate (Duschinsky) rotation is inferred. Vibronic coupling with the second electronic excited state ((1)L(a)) is proposed as the cause of the rotation, as supported by absorption/emission mirror asymmetry, rotationally resolved spectra, and calculations. Coupled molecular dynamics and semiempirical simulations suggest that this vibronic coupling plays a major role in the solvent-controlled excited state proton transfer of naphthol in water by increasing the solvent-solute interaction. With vibronic mixing, L(a)/L(b) state inversion is predicted correctly as a result of coupled evolution of the solvent and solute. The excited naphthol charge distribution and the orientation of nearby water molecules change together, leading to large naphthol dipole moments. The model results are supported by the relevant data on 1-naphthol in water and water clusters.