The excited-state multiple proton transfer mechanism of the 7-hydroxyquinoline–(CH3OH)3 cluster

Abstract

In this paper, the excited-state proton transfer (ESPT) of 7-hydroxyquinoline (7HQ) in hydrogen-donating methanol (MeOH) was investigated theoretically. The change in the calculated primary bond lengths and the infrared spectra of the 7HQ–(CH₃OH)₃ configuration from the S₀ state to the S₁ state testify that the intermolecular hydrogen bonds were strengthened in the S₁ state. Therefore, we propose two proton transfer (PT) types in which the ESPT reaction occurs along the hydrogen-bonded bridge O₇⋯H₈–O₉ (Type-A) or N₁⋯H₂–O₃ (Type-B). The normal Stokes shift and the large Stokes shift suggest the existence of the enol form 7HQ–(CH₃OH)₃ and the keto form 7HQ–(CH₃OH)₃-PT, respectively, and thus that the proton transfer can occur in the excited-state. In order to further study the details of the complicated multiple proton transfer of the 7HQ–(CH₃OH)₃ cluster, we scanned the potential energy curves for the ground state (S₀) and the excited state (S₁) of the hydrogen-bonded 7HQ–(CH₃OH)₃ complex along with the hydrogen-bonding coordinate (distance of the H₂–O₃ and H₈–O₉ bonds). The results show the close barrier height between Type-A (5.35 kcal mol⁻¹) and Type-B (6.65 kcal mol⁻¹), which indicates that the two possible PT types may co-exist and have competition between themselves.