Journal of Physical Chemistry A, Vol.102, No.47, 9599-9606, 1998
Solvatochromism of beta-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 (pi*, beta, and alpha). As judged from the pi* dependence, both molecules have a negligibly small dipole moment in their ground electronic state, which increases in the excited (S-1) 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 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 produces a bathochromic shift. We find a 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 S-1 but re-forms following proton transfer.
Keywords:STATE PROTON-TRANSFER;ACID-BASE CHEMISTRY;EXCITED-STATE;GAS-PHASE;FLUORESCENCE-SPECTRA;SOLVATION DYNAMICS;ELECTRONICSTATES;ALKYL DERIVATIVES;METHANOL-WATER;PH JUMP