Theoretical Study On Hydrogen Bond Induced Excited-State Proton Transfer Reaction

In this dissertation,the excited state proton transfer（ESPT）reaction of 8-hydroxyquinoline（8HQ）,and the hydrogen bond effect on electronic spectra of isoquinoline（IQ）in water have been studied by using density functional theory（DFT）and time-dependent density functional theory（TDDFT）methods.The main work is as follows:1、The geometries of ground state,excited state and excited state isomers of 8HQ have been optimized at CAM-B3LYP/TZVP level.The vibrational motions of H atom were analyzed to simulate the absorption,emission spectra.In addition,the potential energy curves for proton transfer were calculated and the different mechanisms of excited-state double proton transfer（ESDPT）between the 8HQ chromophore and the hydrogen-bonded chains of NH₃/H₂O/AcOH were discussed.The study indicates:（1）The strong hydrogen bonds could effectively facilitate the corresponding proton transfer first upon excitation.The stronger HB1（hydrogen bond formed between-OH group in 8HQ and NH₃/H₂O/AcOH）in 8HQ·NH₃ led to the first transfer of H1,whereas the tighter HB2（hydrogen bond formed between-N-group in 8HQ and NH₃/H₂O/AcOH）in 8HQ·AcOH led to the transfer of H2 at the beginning of the ESDPT reaction.（2）The mechanism of ESDPT reaction is mainly controlled by the hydrogen bond between the hydrogen bond bridge and the corresponding chromosphere.Therefore,we believe that hydrogen bond bridge plays an important role in the excited-state proton transfer reaction.2、The hydrogen bond complexes of IQ/IQc and H₂O were optimized by using the DFT and TDDFT methods at CAM-B3LYP/TZVP level.Moreover,the CPCM model has been employed to simulate the solvation effect.The vertical excitation energies,oscillator strengths and corresponding orbitals have been calculated to simulated the absorption and emission spectra.Hence,the hydrogen bonding effect on the electronic spectra of IQ have been revealed.The study indicates:（1）The simulated emission spectra of free IQc are in good agreement with the experimental results,since the hydrogen bond weakening and cleavage would occur in S₁state upon the photoexcitation.（2）Oppositely,with the enhanced hydrogen bond in the excited IQ:H₂O,different characteristics could be found in the emission spectra,caused by the enhanced hydrogen bonding in the excited IQ:H₂O and IQc:H₂O,thermal equilibrium favors the formation of the IQ:H2O complex in the S₁ state.Therefore,the fluorescence is mainly emitted by the hydrogen bonding complex.（3）The above two points obviously indicate that,hydrogen bond plays an important role in solution-phase electronic spectroscopy.