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 Si 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 greater than or equal to 130 cm(-1). Exciting one quantum of the 57 cm(-1) intermolecular mode results in a decreased SO 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.