Theoretical Study On The ESIPT Mechanisms Of Several Thiazole-based Compounds And The Effects Of Structural Modification On Proton Transfer

Excited state intramolecular proton transfer（ESIPT）molecules have the characteristics of double fluorescence and large Stokes shift,which have broad application prospects in the fields of fluorescent molecular probes,organic light-emitting materials and laser dyes at the moment.As one of the typical compounds with ESIPT characteristics,benzothiazole molecules have attracted close attention of researchers and are widely applied in the field of fluorescent probes due to their simple synthesis and excellent fluorescence properties.Since it is difficult to obtain microscopic information such as the mechanism of proton transfer reaction by existing experimental methods,theoretical study of the photophysical and photochemical properties of ESIPT molecules at the molecular level can provide a solid theoretical basis for the development of ESIPT molecules in application.In this thesis,the photophysical peoperties and ESIPT mechanisms of 2-（2’-hydroxyphenyl）-4-chloromethylthiazole,2-（2’-hydroxyphenyl）-4-phenylthiazole,2-（2’-hydroxyphenyl）-4-hydroxymethyl-thiazole,3-（benzo[d]thiazol-2-yl）-2-hydroxy-5-methoxybenzaldehyde（BTHMB）,3-（benzo[d]oxazol-2-yl）-2-hydroxy-5-methoxybenzaldehyde（BOHMB）and 3-（benzo[d]selenazole-2-yl）-2-hydroxy-5-methoxybenzaldehyde（BSe HMB）molecules were studied by using density functional theory（DFT）and time-dependent density functional theory（TDDFT）,respectively.The main results obtained are listed below:1.At the computational level of TD-B3PW91/6-31+G（d,p）,the excited state intramolecular proton transfer reactions of 2-（2’-hydroxyphenyl）-4-chloromethylthiazole,2-（2’-hydroxyphenyl）-4-phenylthiazole and 2-（2’-hydroxyphenyl）-4-hydroxymethyl-thiazole in acetonitrile were investigated.From the geometric configuration,infrared spectrum and topological analysis,it can be found that the intramolecular hydrogen bond is significantly enhanced in the first singlet excited state（S₁）,which is conducive to the occurrence of ESIPT process.The absorption and fluorescence emission spectra obtained by theoretical simulation can reproce the experimental results well.According to the potential energy curve,it can be found that different groups（-CH₂Cl,-Ph,-CH₂OH）will affect the strength of intramolecular hydrogen bonds,which in turn affects the ESIPT process.Among them,the electron donating group（benzene ring）makes ESIPT have a higher potential barrier,while the electron-withdrawing groups（-CH₂Cl,-CH₂OH）will reduce the ESIPT potential barrier and make the proton transfer reaction occur easier.2.The direction of the excited state intramolecular proton transfer reaction of3-（benzo[d]thiazol-2-yl）-2-hydroxy-5-methoxybenzaldehyde（BTHMB）containing two proton acceptor was investigated by TD-CAM-B3LYP/6-311G（d,p）.By constructing the potential energy curve of the H₂-O₁-C-C dihedral angle,it is confirmed that both BTHMB-O and BTHMB-N structures can exist stably in the ground state（S₀）,and the transition between BTHMB-O and BTHMB-N can occur.Since the potential barriers of the two excited state intramolecular proton transfer paths are similar and very low,protons can transfer from-OH group to the two ESIPT directions（O and N in BTHMB）.3.By the TD-CAM-B3LYP/6-311G（d,p）method,the potential energy curves of3-（benzo[d]oxazol-2-yl）-2-hydroxy-5-methoxybenzaldehyde（BOHMB）and3-（benzo[d]selenazole-2-yl）-2-hydroxy-5-methoxybenzaldehyde（BSe HMB）were constructed.As atomic substitution changes the strength of intramolecular hydrogen bond,the ESIPT barriers of BOHMB-O,BTHMB-O and BSe HMB-O compounds with proton acceptors O increase with the gradal weakening of electron-withdrawing ability（from O to S and Se）.The ESIPT barriers of BOHMB-N and BTHMB-N molecules with proton acceptor N reduce with the gradual decrease of electron-withdrawing ability（from O to S）.For the two proton-transferring isomers of the BOHMB and BSe HMB molecules,the ESIPT process tends to transfer from-OH group to the direction where the proton acceptor is the O atom.The simulated electronic spectral data show that the electron-withdrawing ability of atom（from O to S to Se）is weakened,which makes the fluorescence peaks of normal and tautomeric forms of BOHMB-N,BTHMB-N and BSe HMB-N red-shift,and the corresponding Stokes shift increase.