Theoretical Study On Sensing Mechanism Of Fluorescence Sensor Based On Excited State Proton Transfer

The excited state intramolecular proton transfer（ESIPT）is one of the most important processes in physical chemistry and biology,which exists widely in nature.In recent years,due to the rapid development of fluorescence sensing technology,ESIPT has been considered as an ideal recognition mechanism for designing ratiometric fluorescent probes with large stokes shifts.And an increasing number of researchers have designed and synthesized ESIPT-based fluorescent probes for the selective detection of various anions,metal ions and organic small molecules.However,the theoretical elaboration of the mechanism in various fluorescent probes and the design of novel probes are still in the developing stage.Therefore,we will systematically study the photophysical properties of ESIPT-based fluorescent probes and the sensing mechanism.The relationship between structure and electronic properties is also studied.The main contents are as follows.（1）Investigation on the ESIPT process of BTT probe and the mechanism of detecting Zn²⁺ions.Compared with the previously reported ratio probe for detecting Zn²⁺,the fluorescent probe BTT based on ESIPT has high sensitivity and accuracy.In our study,the ESIPT process of BTT probe and the mechanism of detecting Zn²⁺ions are studied in detail for the first time.The ESIPT process in BTT molecules is investigated by methods of density functional theory and Time-dependent density functional theory.The calculated absorption and emission spectra reproduce the dual-emission photophysical phenomena observed in experiment.The potential energy curves confirm that the BTT probe with ESIPT reaction can proceed in the first excited state（S₁）.By predicting the possible positions of coordination and comparing the binding free energies,we give a reasonable structure of the BTT-Zn²⁺complex.Finally,combining electronic spectra and energy gap between the highest occupied molecular orbital（HOMO）and the lowest unoccupied molecular orbital（LUMO）,the mechanism for the detection of Zn²⁺ions is revealed.（2）The effect of atomic substituents on the ESIPT process of fluorescent dyes.Dipendra Dahal et al.synthesized two styryl dyes with different substituents using the fluorophore of HBT（S substituent）and HBO（O substituent）.In our study,the ESIPT process and luminescence properties of the styryl dyes are systematically investigated by the quantum chemical methods.The potential energy curves show that the proton of the S substituent are more easily to transfer than the O substituent because of the lower energy barrier in the S₁state.Inspired by the photophysical properties of the S and O substituents,and considering atomic substituents generally affect the ESIPT process of the molecule,the Se atom-substituted styryl dye is introduced.The simulated electronic spectra in different solvents show that the stokes shift of the Se substituent is the largest and the solvent polarity has less effect on the emission spectra.It is not only effective in preventing the spectral overlap in imaging applications,but also contributes to the improvement of imaging sensitivity.In addition,the energy barrier in S₁state of the Se substituent is the lowest,indicating that it responds to the ESIPT reaction fastest.