Excited Proton Transfer Mechanism In Organic Molecular Systems And Its Related Applications

Hydrogen bond is a kind of intermolecular interaction dominated by electrostatic energy,which can be found everywhere in our daily life.Compared with other kinds of intermolecular interactions,hydrogen bonds are very weak,but this does not mean that it is not important,but essential.Many substances that we come into contact with every day contain hydrogen bonds.Therefore,it is essential for you and me to learn the theoretical knowledge of hydrogen bonds.For decades,people have been committed to the study of excited state intramolecular proton transfer(ESIPT).As one of the typical reaction modes of hydrogen bond dynamics,its importance is self-evident.The ESIPT process is usually accompanied by a dual fluorescence phenomenon,and its large Stokes shift(>150 nm)and its characteristics of being very sensitive to the surrounding medium paved the way for its application prospects.At present,the research results of ESIPT have important applications in physical chemistry,biochemistry,etc.,which are embodied in luminescent dyes and fluorescent probes.Proton transfer needs to occur in the excited state(S₁).Under the effect of photoexcitation,the ground state molecule(E^S0)becomes the lowest excited state by absorbing energy,and forms a stable enol formula(E^S1)in the S₁state through vibrational relaxation.It should be noted that in the S₁state,the minimum energy point is the ketone form(K^S1),so in the S₁state,the ESIPT reaction can occur spontaneously.The dual fluorescence phenomenon confirms the fact that the two structures of the luminophore can coexist in the S₁state.It is far from enough to explain the reaction mechanism of ESIPT by optical spectroscopy.Therefore,this article will study the dual fluorescence phenomenon of ESIPT reaction kinetics based on theoretical calculations and make research on its application.In this paper,we introduce a new synthesized triazole derivative BH-BA based on the mechanism of excited state intramolecular proton transfer.The relationship between the photophysical properties of the two-proton system BH-BA and the ESIPT reaction was confirmed by the simulation calculation of the molecule.We have fully optimized the geometric configuration of BH-BA,and performed theoretical calculations and analysis of infrared spectroscopy(IR),which show that the intramolecular hydrogen bonds of the two excited states have been strengthened.Later,the analysis of frontier molecular orbitals(FMOs)and Mulliken charges explained the redistribution of the charge density of molecules during the photo-induced process,and fundamentally explained the reason for the ESIPT reaction of molecules.Reduced density gradient isosurfaces(RDG)are used to distinguish complex non-covalent bond interaction forces in molecules.In addition,through the analysis of the molecular potential energy curves and transition state structures,we have obtained the most likely reaction path for proton transfer.The vertical absorption and emission energies of molecules are simulated based on density functional theory(DFT)and time-dependent density functional theory(TDDFT),which perfectly reproduces the photophysical phenomenon in the experiment.Finally,we concluded that the hydrogen bond structure that reacted in the experiment was O1-H2···N3 instead of O4-H5···N6.Thesecondworkofthisarticleintroduced2-(benzo[d]thiazol-2-yl)-6-methoxyphenyl triflate(HMBT-LW)as a fluorescent probe for detecting O₂^{·_</sup>Ion reaction mechanism.HMBT-LW reacts with O₂·_</sup>ions in the solution to form a molecule with a hydrogen bond structure,2-(benzo[d]thiazol-2-yl)-6-methoxyphenol(HMBT).By analyzing the hydrogen bond structure parameters of HMBT,frontier molecular orbital,Mulliken charge analysis,infrared spectra,potential energy curves,electronic spectra,etc.,the calculated vertical excitation energies are compared and found to be in good agreement,which prove that our calculation method is correct.Finally,we concluded that the single fluorescence we observed in the experiment was emitted by the isomer structure of HMBT.This conclusion can be used to detect the presence of O₂·_</sup>ions in living organisms,and plays an important role in the prevention of cardiomyopathy,autism,diabetes,cancer and neurodegenerative diseases.}