Theoretical Study On Excited State Proton Transfer In Organic Conjugated Molecular System Molecular System Based On Hydrogen Bond Strengthening

Molecular proton transfer is one of the most important reactions based on hydrogen bond dynamics,which plays an important role in chemistry,life sciences,biology,genetics and other disciplines.Excited state intramolecular proton transfer(ESIPT)process is widely present in photophysical and photochemical reaction processes.Molecules with ESIPT properties are often used in many aspects such as organic light-emitting materials,fluorescent probes and chemical sensors due to their high quantum yields and large Stokes shifts,which makes the study of the ESIPT process a hot topic.Based on density functional theory(DFT)and time-dependent DFT theory(TDDFT),this dissertation studies the mechanism of molecular ESIPT and explores the effect of different ?-conjugated substitutions on the ESIPT process.The mechanism of ESIPT of the probe molecule 4a-E in ethanol solution was investigated using DFT and TD-DFT methods.The optimized molecular configuration,frontier molecular orbital analysis and non-covalent interaction analysis showed that the intramolecular hydrogen bonding interaction of the 4a-E molecule in the first electronic excited state was enhanced compared with that in the ground state.The results of potential energy curve analysis show that the proton transfer process of 4a-E molecule in the ground state cannot proceed spontaneously due to the high transfer energy barrier,while the 4a-E molecule in the first electronic excited state can realize the proton transfer process.The final results show that the intramolecular hydrogen bond has a tendency to strengthen after photoexcitation,and the strengthening of the hydrogen bond can promote the transfer of protons.The effects of ?-conjugated substitution on the photophysical processes of oxazole-substituted hydroxyfluorenes(Oxa-OH),hydroxylated benzoxazoles(BO-OH)and naphthoxazoles(NO-OH)molecules were investigated using DFT and TD-DFT methods with Oxa-OH,BO-OH and NO-OH as the research models.The optimized molecular configuration and infrared vibrational spectroscopy analysis results show that the O-H distances of Oxa-OH,BO-OH and NO-OH molecules gradually shortened with the addition of ?-conjugated groups,a phenomenon that led to enhanced interactions between intramolecular hydrogen bonds.In addition,frontier molecular orbital and reduced density gradient(RDG)map analysis results indicate that the intramolecular hydrogen bonding of Oxa-OH,BO-OH and NO-OH molecules in the first electronic excited state is enhanced.After the addition of ?-conjugated substitution,the barrier height of NO-OH is higher,and the fluorescence peaks of its enol and ketone configurations are significantly shifted relative to the Oxa-OH configuration,which means that the ?-conjugated substitution can obtain wider fluorescence.