Based On The Density Functional Theory To Study The Influence Of Excited State Hydrogen Bond And Intramolecular Proton Transfer Under Different Water Content

Fluorescent molecular probes are widely used in biological sciences and environmental monitoring due to their high analytical sensitivity and mild detection conditions.In the case of intensity-variable fluorescent probes,the detection of an object substance can be achieved by the degree of variation in the intensity or position of the fluorescence peak.However,the fluorescence intensity and the position of the emission peak are also affected by many environmental factors,leading to obvious limitations in the quantitative detection of guest substances.Therefore,an in-depth study of the factors affecting fluorescence variation is beneficial in finding ways to improve the sensitivity and selectivity of fluorescence analysis methods and to broaden the use of intensity variation fluorescent probes.ESIPT fluorescent probes have a large Stokes shift due to their unique enol-keto photoisomerisation process,which allows the keto structure to avoid self-absorption and internal filtering of the probe molecule,thus improving the luminescence efficiency of the fluorescent probe.But,ESIPT fluorescent probes are often inhibited by the proton transfer process within the excited state in close proximity to pure water,which prevents them from taking advantage of their large Stokes shifts,making the use of ESIPT probes extremely limited.In recent years,many studies have shown that the introduction of AIE functional groups seems to promote the ESIPT process,which manifested as the probe molecule can still emit fluorescence with a ketone structure under extremely high water content.Since it is not clear for the moment in which way the AIE functional group facilitates the ESIPT process,this paper produced USC-001 using triphenylamine as the AIE group and 2-(4,5-dihydrothiazole)phenol as the ESIPT group,measured the spectroscopic data of USC-001 at different tetrahydrofuran-water ratios,and critically discussed the changes produced in stages were discussed.Details of the work are as follows.(1)Using a combination of Density functional theory(DFT)and time-containing Density functional theory(TDDFT)as well as experimental comparisons,it was determined that USC-001 undergoes an ESIPT effect in the range of 0%-70% water content and emits fluorescence in a ketonic structure;by means of solvent dielectric constant gradient experiments,surface electrostatic type distribution,dipole moment,and leap dipole moment density isohedral variation plots together,the red shift of the fluorescence emission peak at this stage was demonstrated to be caused by the ICT effect.(2)Through the comparison and analysis of the most stable configuration of the ground state/excited state,UV absorption peak simulation and experimental data,it is proved that in the range of 80%-90%,USC-001 is formed by intermolecular hydrogen bonds with water molecules in the solvent.The USC-001-H2 O structure,which is not conducive to the ESIPT process,causes it to emit fluorescence with an enol structure,and the fluorescence peak blue shifts.Further refinement of the water content experiment also found that the suppression of the ICT effect is also an important reason for the blue shift of the fluorescence emission peak.(3)Through the study of the changes in the microstructure of USC-001,it is found that the flatter the overall structure of USC-001,the higher the proportion of intramolecular hydrogen bond structures that are conducive to the ESIPT process,and the occurrence of a large number of configurations in the solution is not conducive to The transition of the intermolecular hydrogen bond structure performed by ESIPT to the intramolecular hydrogen bond structure in favor of ESIPT results in the observation of double-peak fluorescence emission in the range of 95%-100% water content.