Vibronic Coupling and Microscopic Solvation of 1-Naphthol

Abstract

Vibronic structure in the S 1 T 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.

Substituent and Solvent Effects on the Nature of the Transitions of Pyrenol and Pyranine. Identification of an Intermediate in the Excited-State Proton-Transfer Reaction

“…Internal vibronic coupling could well also play an important role in this state inversion process. 14,43 The basic dynamics just described for the second step in the overall 3sPyOH ESPT in water has much in common with those found in a simulation of excitedstate 1-naphthol in water solvent by Knochenmuss et al, 14 who stressed the solvent-induced level inversion after absorption to 1 L b : one important difference is that the latter authors viewed such dynamics as those directly producing the proton-transferred products in that case. 44 The final step, in VB language, is the proton-transfer step proper and would involve conversion between CT and the PT/VB form, which in the Mulliken picture 45 recently developed and supported for ground electronic state PT reactions, 9 would involve charge transfer from the nonbonding orbital of the bases here H 2 Os into a σ* antibonding orbital of the OH-bond on the acid, inducing H motion toward the base, leading to the proton-transferred products.…”

“…As noted in the Introduction, previous discussions of 1 L b / 1 L a inversion (for other hydroxyarene acids) have identified 1 L a as the proton-transferred product state. 12, 14 A significant role in the interpretation of 1 L a of the acid as an intermediate was played by the solvatochromism study of section IV, and it is of interest to consider related studies in the literature. In particular, we can compare our data with recent solvatochromism studies of 5-cyano-2-naphthol (5CN2NpOH).…”

“…While the 1 L a state in the Franck-Condon configuration (but not accessed in the lowest energy transition) is more polar than the 1 L b state, 13 this level inversion scenario does not directly address why it is that the excited-state acidity is enhanced compared to that of the ground state. The 1 L b / 1 L a level inversion picture has been greatly enriched in important recent studies by Knochenmuss and co-workers 13,14 and in particular the contribution 14 which argues that both the surrounding solvent motion and internal vibronic coupling in 1-NpOH* are critical features in the inversion process. Nonetheless, the inversion is again associated with the ESPT process itself, and the underlying cause of the enhanced excited-state acidity is not addressed.…”

Charge Transfer in Photoacids Observed by Stark Spectroscopy

“…[4][5][6] The change in acidity between the ground and excited states of photoacids can be extraordinary, with the acidity equilibrium constant (K a ) differing by more than 10 orders of magnitude in some cases. 7 Photoacids have been studied in both gas and condensed phases, with benzene and naphthalene derivatives examined most often in the gas phase, [8][9][10][11] and naphthalene and pyrene derivatives studied more extensively in condensed phases. [12][13][14][15][16][17] However, there are many other types of aromatic molecules that also display an excited-state shift in pK a , including hydroxystilbenes, 18 triarylamines, 19 and the chromophore of Green Fluorescent Protein (GFP).…”

Solvent‐Dependent Photoacidity State of Pyranine Monitored by Transient Mid‐Infrared Spectroscopy

“…For aromatic molecular systems such as indoles, [27][28][29][30][31][32][33] tryptophane, [34] or pyrene derivatives [35] internal conversion between these 1 L b and 1 L a levels has been suggested in the interpretations. For 1-naphthol [36,37] the 1 L b ! 1 L a transition is considered to be the ESPT-determining step [9,38,39] In contrast, in a recent combined experimental/theoretical study of ESPT of pyranine [40][41][42] the rate determining step is not the conversion from the optically accessible locally excited (LE) state (or alternatively the 1 L b state), to the second photoacid electronic excited n-p* CT state (or 1 L a state), but the transition of the photoacid CT to photobase CT states.…”