Synthesis And Properties Of Symmetrical 1,3,4-Oxadiazole Derivatives With Strong Intramolecular Charge Transfer And Acid Response

D-A type organic conjugated molecules with excellent intramolecular charge transfer properties are widely used in organic light-emitting diodes,dye-sensitized solar cells,fluorescent sensors and nonlinear optical materials because of their simple and easily tunable structure,fast optical response and good nonlinear optical properties.Development of D-A type organic fluorescent materials with excellent intramolecular charge transfer strength is a goal that scientists are constantly pursuing.The intramolecular charge transfer strength of a material depends not only on the geometric configuration of the molecule,but also on the type of substituent in the molecule.Therefore,it is important to deeply understand the influence of molecular structure and substituent species on the intramolecular charge transfer properties.In D-A type organic conjugated molecules,the modulation of D and A fragments can make the molecules possess a variety of stimulus-responsive properties.Such as solvatochromic,ionic responsiveness,excited state proton transfer properties,etc.In this paper,four symmetrical bis-1,3,4 oxadiazole derivatives with excellent intramolecular charge transfer properties were designed and synthesised.By modulating the geometrical configuration of the molecules and the type of donor substituents,the intramolecular charge transfer properties and plasmonic sensing performance were optimised,which is important for the development of organic fluorescent materials with excellent performance and the exploration of their applications in organic optoelectronic devices and fluorescent sensors.The following results were obtained in this study:1.The carbazole substituted bi-1,3,4-oxadiazole derivative CZOXD was designed and synthesized,and CZOXDBEN was synthesized by introducing a benzene ring between the bi-1,3,4 oxadiazole rings of the molecule.It was found that the change in solvent polarity had a small effect on the redshift distance of the UV-vis absorption spectra of CZOXD and CZOXDBEN molecules(about 25 nm for CZOXD and 15 nm for CZOXDBEN).However,the fluorescence emission spectra of CZOXD and CZOXDBEN showed a more pronounced solvatochromic behavior.The fluorescence emission spectra of both molecules showed a red shift of 112 nm for CZOXD and 134 nm for CZOXDBEN from nonpolar cyclohexane to polar dimethylsulfoxide.Meanwhile,trifluoroacetic acid can quenched the fluorescence emission of both molecular solutions and their fluorescence emission can be restored by using triethylamine.Theoretical calculations have shown that the dipole moments of the intramolecular charge transfer excited states of CZOXD and CZOXDBEN vary by23.60 D and 27.99 D compared to those of the ground state,respectively.This indicating that both CZOXD and CZOXDBEN have good intramolecular charge transfer properties in the excited state,and the intramolecular charge transfer intensity of CZOXDBEN is higher than that of CZOXD.In addition,the ADCH charge and charge density difference indicated that the carbazole acted as electron donor and the benzene ring attached to the carbazole,the bi-1,3,4 oxadiazole ring and the central benzene ring in CZOXDBEN acted as electron acceptors during the charge transfer process of both molecules.Therefore,the high intramolecular charge transfer intensity of CZOXDBEN molecules is due to the introduction of the central benzene ring increasing the distance of charge transfer in the molecule.2.Two symmetrical dimethylamino substituted bi-1,3,4-oxadiazole derivatives DMAOXD and DMAOXDBEN were designed and synthesized,which are based on CZOXD and CZOXDBEN molecules,using dimethylamino group instead of carbazole group with stronger electron giving ability.According to the spectroscopic results of the two molecules,the UV-vis absorption peak positions of the DMAOXD and DMAOXDBEN were red-shifts by a small distance(DMAOXD ～ 25 nm,DMAOXDBEN ～ 19 nm)with the change of solvent polarity.However,the positions of their fluorescence emission peaks were red-shifts by 114 nm and 170 nm,respectively.There were larger than those of the CZOXD and CZOX DBEN molecules,which indicates that the introduction of dimethylamine with stronger electron giving ability can significantly improve the intramolecular charge transfer intensity of the molecules.Moreover,the changes in the dipole moments between the excited states of the intramolecular charge transfer and ground states of the two molecules were 18.36 D and 21.23 D,respectively.The differences in ADCH charge and charge density indicate that the dimethylamino and the adjacent benzene ring acted as electron donor during the charge transfer of the DMAOXD and DMAOXDBEN,while the bi-1,3,4-oxadiazole in the molecule and the DMAOXDBEN in the molecule acted as electron acceptors for the central benzene ring.In addition,the introduction of dimethylamino may improve the ability of the molecule to bind protons,so we explored the proton response properties of these two molecules in depth.It was found that the emission of DMAOXD and DMAOXDBEN in chloroform solution gradually quenched with increasing trifluoroacetic acid volume,and the solution was accompanied by a red-shift of fluorescence emission.Triethylamine can be used to deprotonate the molecules in solution,allowing the restoration of the emission of the solution.In addition,trifluoroacetic acid and triethylamine can also achieved the fluorescence patterns of both molecules "on-off".Based on the excellent intramolecular charge transfer properties and proton response properties,these D-A type organic conjugated molecules have potential applications in solvent polarity detection,proton detection and data encryption.These effective methods of improving intramolecular charge transfer properties can provide guidance and reference for the design of better performing D-A organic materials in the future.