title: TDDFT study on the excited-state proton transfer of 8-hydroxyquinoline: key role of the excited-state hydrogen-bond strengthening.
abstract: Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been employed to study the excited-state intramolecular proton transfer (ESIPT) reaction of 8-hydroxyquinoline (8HQ). Infrared spectra of 8HQ in both the ground and the lowest singlet excited states have been calculated, revealing a red-shift of the hydroxyl group (-OH) stretching band in the excited state. Hence, the intramolecular hydrogen bond (O-H···N) in 8HQ would be significantly strengthened upon photo-excitation to the S1 state. As the intramolecular proton-transfer reaction occurs through hydrogen bonding, the ESIPT reaction of 8HQ is effectively facilitated by strengthening of the electronic excited-state hydrogen bond (O-H···N). As a result, the intramolecular proton-transfer reaction would occur on an ultrafast timescale with a negligible barrier in the calculated potential energy curve for the ESIPT reaction. Therefore, although the intramolecular proton-transfer reaction is not favorable in the ground state, the ESIPT process is feasible in the excited state. Finally, we have identified that radiationless deactivation via internal conversion (IC) becomes the main dissipative channel for 8HQ by analyzing the energy gaps between the S1 and S0 states for the enol and keto forms.
authors.affiliation: Department of Physics, College of Mathematics and Physics, Bohai University, Jinzhou 121013, China; State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China.
authors.forename: Sheng-Cheng
authors.initials: SC
authors.lastname: Lan
authors.affiliation: Department of Physics, College of Mathematics and Physics, Bohai University, Jinzhou 121013, China. Electronic address: yhliu@bhu.edu.cn.
authors.forename: Yu-Hui
authors.initials: YH
authors.lastname: Liu
mesh_terms: D003198:Computer Simulation; D004583:Electrons; D006860:Hydrogen Bonding; D008752:Methylene Chloride / Q000737:chemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D015125:Oxyquinoline / Q000737:chemistry*; D011522:Protons; D011789:Quantum Theory; D012996:Solutions; D013050:Spectrometry, Fluorescence; D013237:Stereoisomerism; D013816:Thermodynamics; D013997:Time Factors; D014732:Vibration
chemical_list: D011522:Protons; D012996:Solutions; D008752:Methylene Chloride; D015125:Oxyquinoline
keywords: Excited-state intramolecular proton transfer; Hydrogen bond; Internal conversion; Time-dependent density functional theory