Photoinduced Proton Transfer and Rotational Motion of 1-Hydroxy-2-acetonaphthone in the S₁ State:  A Theoretical Insight into Its Photophysics

The internal rotational motion, the intramolecular proton-transfer reaction, and the subsequent internal twisting process of 1-hydroxy-2-acetonaphthone (HAN) are studied in the first singlet excited electronic state (S 1 ) using the ab initio electronic-structure method at the CIS/6-31G** level. The calculations show a multiple-well potential-energy surface (PES) with a low energy barrier (5.49 kcal/mol) between the excited OH···OC enol form (E*, the most stable structure at the ground state) and its keto tautomer (K*, result of intramolecular proton-transfer reaction). An internal rotation with activation energy of 7.53 kcal/mol in the produced K* may give a more stable twisted rotamer KR*. The energy of this structure is 6.23 kcal/mol lower than that of E*. The involvement of a twisting motion in the tautomer explains the reported structured fluorescence band, the low quantum yield, and short emission lifetime. The existence of energy barriers in the PES accords with the dependence of its emission spectroscopy and dynamics on the nature of the transferred isotope (H/D) and on the excess energy of excitation. Frequency modes analysis shows that in-plane and out-of-plane motions of the H-bonded chelate ring of E* will play a crucial role in the proton-transfer dynamics and spectroscopy. These theoretical results are in full agreement with previous experimental observations in the gas phase (Douhal, A.; et al. Chem. Phys. 1993, 178, 493. Lu, C.; et al. Chem. Phys. Lett . 1999, 310, 103) and in solution (Tobita, S.; et al. J. Phys. Chem. A . 1998, 102, 5206). However, they contrast with the conclusion of Catalán et al. stating the absence of proton transfer in the S 1 state and the presence of a single minimum at the S 0 state (Catalán, J.; et al. J. Am. Chem. Soc . 1993, 115, 4321; Chem. Phys. Lett. 1997, 269, 151). Moreover, the ER−OH and ER−CO isomers having rotated the OH and C O groups, are found at 13.95 and 17.36 kcal/mol above the E* structure, respectively. As a result of the pseudoaromaticity provided by the H-bonded chelate ring in the enol form, the electronic excitation induces in this one a stabilization of ∼2 kcal/mol with respect to ER−OH and ER−CO rotamers.