Theoretical Study On Excited State Intramolecular Proton Transfer Of Several Small Organic Molecules

Excited state intramolecular proton transfer（ESIPT）,a rapid photoisomerization reaction,which occurs on organic molecules with intramolecular hydrogen bonds.In general,the proton donors（-OH,-NH-）and proton receptors（-C=O,-N=）are close enough to form intramolecular hydrogen bonds in ESIPT molecules.ESIPT molecules often exist in the enol form in the ground state（S₀）,upon excited with ultraviolet or visible light,the proton can transfer within the molecule and form a tautomer（keto form）.Compared with the absorption peak,the fluorescence emission of the tautomer has a strong red shift and a large Stokes shift can be observed.Long wavelength of fluorescence for tautomers has broad application prospects in supramolecular chemistry,cell biophysics,environmental/metal ion fluorescence monitoring and so on.The ESIPT reaction of different systems is complex and changeable,which is easily affected by experimental conditions and environment.Therefore,further theoretical research is needed to reveal the specific mechanism and various factors of ESIPT process.In order to understand the mechanism of ESIPT process in detail,more and more studies have been conducted on the effects of different chromophore systems,substituents and solvents.In this thesis,the ESIPT reactions of several small organic molecules were studied theoretically from two aspects:substituent effect and solvent effect.（1）The effects of different substituents（-NH₂,-OMe,-CN and-NO₂）on the excited state proton transfer process of 4-（2-hydroxybenzylidene）-1,2-dimethyl-1H-imidazol-5（4H）-one（o-HBDI）have been studied based on the density functional theory（DFT）and time-dependent density functional theory（TDDFT）.The calculated results of the bond lengths and angles confirm that the enhanced tendency of intramolecular hydrogen bond（O1-H2···N3）in the excited singlet state（S₁）,which should provide a driving force for process.The calculated absorption and emission spectra of the compounds are in good agreement with the experimental results.According to the vertical excitation process,it can be found that the charge redistribution is attributed toπ→π*transition from highest occupied molecular orbital（HOMO）to lowest unoccupied molecular orbital（LUMO）.Further,the corresponding potential energy curves among the stable structures in S₀ and S₁ states indicate that proton transfer is difficult to occur in the S₀ state,whereas in the S₁ state,the electron-withdrawing substitutions could facilitate the ESIPT process of o-HBDI system,while the electron-donating substitutions could hinder it,and the stronger the electron donor capacity,the harder the ESIPT reaction is to occur.（2）The effects of different substituents（-Ph、-CN）on the ESIPT of ortho-hydroxylated oxazoline molecular system（DDOPs）have been systematically explored by DFT and TD-DFT methods.The results show that there are two stable configurations（enol and keto）in the S₀ states and the enol form only exists in the S₁ states when the substituent is electron-donating group（-Ph）.The proton transfer potential energy curve studies have shown that the enol forms can barely convert into keto forms in the S₀ states.However,the proton transfer of all the compounds could occur in S₁ states.Without substituent or the substituent is electron-withdrawing group（-CN）,the ESIPT reactions are barrierless process.But there is a small barrier when the substituent is electron-donating substituent.Therefore,the electron-donating group is not favorable to the ESIPT reactions of ortho-hydroxylated oxazoline molecules.（3）The dye,2-{4-[[2-（2-Methoxy-ethoxy）-ethyl]-（2-methoxymethoxy-ethyl）-amino]-phenyl}-5-（5-trifluoromethyl-benzooxazol-2-yl）-benzofuran-6-ol（MMTB）,has been theoretically investigated by us.Solvent effects on the photochemical behavior of the excited state and ESIPT mechanism for this compound have been mainly considered.Based on the changes of molecular hydrogen bond structure and infrared vibration spectrum,we can conclude that the hydrogen bond is strengthened by light-induced excitation in the S₁state.And the greater the polarity of the solvent,the stronger the intramolecular hydrogen bond is in the S₁state.The calculation results of absorption and fluorescence emission wavelengths explain the previous experimental characteristics.In addition,the simulation of potential energy curves in three solvents（Toluene,ethanol and DMSO）revealed the ultrafast ESIPT mechanism.In particular,the polarity of solvent can regulate the excited state behavior of MMTB,and the solvent with higher polarity is favorable for ESIPT reaction.