Study On The Excited State Intramolecular Proton Transfer Mechanism Of Organic Molecules For Fluorescent Probe

The development of fluorescence probe technology has been widely concerned by people.People use photogenerated reagent or strong fluorescence labeling reagent to derive or label the object to be detected,and generate a new kind of substance,which is characterized by the ability to emit fluorescence,so as to further analyze the substance to be detected^[1].The development of fluorescence probe technology has never been interrupted since human beings stepped into the modern physical period.In this paper,two kinds of molecule——Quercetin?QC?and 4'-n.n-dimethylamino-3-hydroxyflavone?DMA3HF?were studied.All the correlation calculations of the two molecules are carried out by Gauss 09 program,and the whole calculation method is based on the time-dependent density functional theory?TDDFT?.In view of Quercetin molecule,Cam-B3LYP functional and 6-311+G**group were used to simulate THF solution environment by PCM solvation model,optimize its ground state and excited state configuration,and scan the potential energy curve of Quercetin ground state and excited state.The results show that the energy barrier of Quercetin molecule in the process of proton transfer is about 6.73kcal/mol,which is smaller than17.82kcal/mol in the ground state,indicating that Quercetin molecule is excited state proton transfer reaction.In order to explain the effect of aqueous solution on the process,water molecules were introduced into the ground state configuration of Quercetin,and the ground state configuration was optimized again.The potential energy curves of ground state and excited state were scanned.The results show that there is a hydrogen bond interaction between Qquercetin and water molecules after the introduction of water molecules,which reduces the energy barrier of the excited state proton transfer process to4.93kcal/mol,promotes the ESIPT process,and enhances the fluorescence intensity of ESIPT process,which is consistent with the experimental results.For 4'-n.n-dimethylamino-3-hydroxyflavone?DMA3HF?,B3LYP functional and6-311+G**group were used to optimize the structure of DMA3HF.The potential energy curves of ground state and excited state of DMA3HF were scanned.The results showed that the energy barrier required for the process of ground state transfer was about13.08kcal/mol,which was much higher than that of excited state energy barrier5.51kcal/mol.It was proved that the excited state proton transfer reaction occurred in DMA3HF.We choose ethanol solution was selected in order to explore the influence of solution polarity on ESIPT process^[2],Ethanol molecules were introduced into theDMA3HF configuration to optimize the configuration,and the potential energy curves of ground state and excited state were scanned and the reaction path was considered.The results show that due to the addition of ethanol molecules,DMA3HF and ethanol molecules form a complex under the hydrogen bond interaction,and follow the O₂₆-H₂₇path,the ESIPT reaction energy barrier of DMA3HF is reduced from 5.51kcal/mol to2.72kcal/mol,indicating that there is a hydrogen bond interaction between DMA3HF and ethanol molecules in ethanol solution,and the excited state biproton transfer reaction occurs,reducing the reaction energy barrier After understanding the reaction properties of DMA3HF in polar solvent,the PCM solvation model was used to simulate the methanol and n-propanol solution environment.The ground state and excited state potential energy curves were also scanned and the reaction path was considered.The results show that the excited state proton transfer of DMA3HF-METH complex follows the O₄₁-H₄₂2 path,and the excited state energy barrier is 1.79kcal/mol;the excited state proton transfer of DMA3HF-PRO complex follows the O₂₆-H₂₇7 path,and the excited state energy barrier is2.74kcal/mol.Combined with the calculated data in ethanol solution,the proton transfer process of DMA3HF in different polar solution environment is compared.We found that the hydrogen bond interaction between DMA3HF single molecule and solvent molecule weakened with the decrease of solution polarity,and the energy barriers to be crossed for the excited state of the complex were 1.79kcal/mol,2.72kcal/mol and 2.74kcal/mol,respectively.The results show that the lower the polarity of the solution,the higher the energy barrier of the excited state biproton transfer,and the less the transition occurs.