Mechanism Of Excited-State Intramolecular Proton Transfer For Different Substituents

In 1956,excited-state proton transfer?ESPT?was first observed in experimental studies of salicylic acid.Subsequently,intramolecular and intermolecular photoinduced proton transfer reactions have been extensively studied by using theoretical and experimental methods.Many chemical and physicochemical phenomena are related to the weak intermolecular interactions,which hydrogen bonding is one of the earliest studied intermolecular weak interactions.During proton transfer,the molecular structure generates double fluorescence through radiative transitions.In general,double fluorescence is a sign of the proton transfer process in the excited state,ie normal and isomer structures coexist in the solvent.Different substituents have an important influence on the intramolecular proton transfer reaction barrier and the fluorescence emission process of the molecule in the excited state,which in turn changes the photochemical and photophysical properties of the molecule.In this paper,the excited state intramolecular proton transfer?ESIPT?reactions of AYAAQs and BTBs were studied using density functional theory?DFT?and time-dependent density functional theory?TDDFT?.The effect of different substituents on the ESIPT reaction of AYAAQs and BTBA_S was investigated.Hydrogen bond strength in the excited state was first reported by the influence of different substituents.Through theoretical calculations,we give visual data information,such as the bond parameters,vibration frequency,molecular frontier orbitals?MOs?,reduced density gradients?RDG?,potential energy curves,and so on related to hydrogen bonding in the structure.Based on the analysis of the calculation results,the hydrogen bond strengthening mechanism in the excited state was confirmed,which in turn provided the driving force for the ESIPT reaction of the molecule.We can explain the changes of the photophysical and photochemical properties of AYAAQs in the excited state intramolecular proton transfer process by calculating the electronic spectra and combining the potential energy curves.We reveal that with the different substituents CH₃?CH₂Cl?CHCl₂?CF₃substitutions,from AAAQ?CAAQ?DCAQ?TFAQ,the excited state intramolecular proton transfer reaction of AYAAQs derivatives is getting easier.Then we report the spectral characteristics of the benzothiazole derivatives BTBAs with different substituents.Through the analysis of the theoretical calculations,we have clarified the redistribution of electrons in the excited state resulting in the BTBAs derivatives,from BTBA?BTBA-OCH₃?BTBA-NO₂,the proton transfer process is more and more difficult to occur,and in the article we also revealed that F^-can make BTB-OCH₃ fluorescence regenerate.