Theoretical And Experimental Study On The Regulation Of Excited States Intramolecular Proton Transfer Of Fluorescent Dyes

Exploring how to regulate the ESIPT process of fluorescent dye molecules will help to design and synthesize new fluorescent materials.The theoretical calculation method in this paper combines femtosecond transient absorption spectroscopy to control the ESIPT process of dye molecules from the two aspects of solvent and structural modification.DFT and TDDFT was used to study the different emission mechanisms of isoquinoline derivative 3-Hydroxy-4-pyridylisoquinoline(2a)in tetrahydrofuran and methanol solvents.The calculation and analysis results of relevant structural parameters,infrared vibration spectrum and density reduction gradient isosurface show that the molecules exist in the form of monomers in THF solvent without excited proton transfer,while the ESIPT process occurs after the formation of 2aMe OH complex by intermolecular hydrogen bond and methanol bridging in methanol solvent.The scanning results of potential energy curves show that the energy barrier of ESIPT process is significantly reduced under the bridging of methanol molecules.In addition,the excited state proton transfer process in 2a-Me OH complex starts from2 a molecule through a stepwise mechanism.Therefore,Me OH molecule induced the ESIPT process of 2a molecule and triggered the double emission phenomenon.The effect of diethylamino group on ESIPT process of coumarin CCA molecule was studied by DFT and TDDFT theory and transient absorption spectroscopy.The results show that coumarin CCA molecule has ESIPT characteristics and double emission characteristics in different solvents.We modified the structure of CCA molecule and obtained 7-DCCA molecule substituted by diethylamino group.The results show that there is only a fast process of tens of picoseconds in different solvents.Combined with theoretical calculation,it is found that the time scale is inconsistent with ESIPT process.The de excitation mechanism of 7-DCCA molecule was clarified by scanning potential energy surface.The substituted 7-DCCA molecule experienced a low barrier TICT process,which quenched the ESIPT process of7-DCCA molecule.ESIPT process can not be observed in different solvent environments.Therefore,the substitution of diethylamino group makes the ESIPT process change from presence to absence.