Vibrational-Energy Redistribution and Vibronic Coupling in 1-Naphthol·Water Complexes

Resonant two-photon ionization (R2PI) and fluorescence spectra of the 1-naphthol·H 2 O and 1-naphthol- d 1 ·D 2 O cluster are reported and interpreted with emphasis on intracomplex vibrational energy redistribution (IVR) and vibronic coupling. The analysis included an ab initio normal-coordinate calculation of both complexes at the 6-31G (d,p) level. In S 1 ← S 0 R2PI spectra, the intermolecular modes were found to be very weak; only one such S 1 vibration was observed at 57 cm -1 (54 cm -1 for 1-naphthol-OD·D 2 O). At excitations less than 400 cm -1 above the S 1 ← S 0 origin, the fluorescence spectra retain a simple 1-naphthol-like pattern. At higher excess energies, the fluorescence exhibits complex structure due to emission from both relaxed and unrelaxed S 1 vibrational states, showing efficient coupling through the hydrogen bond. This restricted IVR behavior changes to dissipative IVR at about 834 cm -1 above the S 1 ← S 0 origin. Exceptions to general IVR are observed. The S 1 + 403 cm -1 level is a restricted IVR case, while the nearby S 1 + 400 cm -1 level undergoes nearly dissipative IVR. On the other hand, a nearly “frozen” mode is observed at 553 cm -1 above the S 1 ← S 0 origin. By use of the ab initio vibrational analysis and lowest-order anharmonic coupling, the IVR-active vibrational “bath” states can be predicted, enabling tentative assignment of much of the relaxed fluorescence structure. Ion-dip spectroscopy shows that IVR in the electronic ground state may have a higher onset threshold than in the S 1 by ≥130 cm -1 . Exciting one quantum of the 57 cm -1 intermolecular mode results in a decreased S 0 IVR threshold. The strong vibronic S 1 −S 2 coupling earlier observed for vibrations at S 1 + 410 and S 1 + 414 cm -1 in free 1-naphthol was found to be partially inhibited in 1-naphthol·H 2 O but still significant in 1-naphthol-OD·D 2 O. This is attributed to harmonic mode mixing. In contrast, vibronic coupling becomes active in the S 1 + 292 cm -1 level of 1-naphthol·H 2 O.