Experimental And Theoretical Study On The Mechanism Of Excited State Double Proton Transfer

Molecules with excited state double proton transfer(ESDPT)properties are widely used in many fields such as biological cell coding,fluorescence sensors,biomolecular probes and luminescent dyes.The ESDPT process in different systems is complex and variable,and is susceptible to structural changes and environmental factors.The detailed study of the mechanism of the excited state double proton transfer will provide a theoretical basis for the design,synthesis and optimization of new materials and biomolecular probes.In this paper,femtosecond transient absorption spectroscopy and time-dependent density functional theory are used to investigate the mechanism of ESDPT in molecules with asymmetric structures and in different polar solvents.The main research contents can be summarized as follows:(1)We use the density functional theory and time-dependent density functional theory method to explore the ESDPT in the 1-[(2-hydroxy-3-methoxy-benzylidene)-hydrazonomethyl]-naphthalen-2-ol(HYDRAVH₂)ligand.The geometric configurations of ground state and excited state of the HYDRAVH₂in acetonitrile solvent were optimized.which provides the driving force for the ESDPT process.The asymmetric structure of molecules makes the driving force provided by two hydrogen bonds different.The results of potential energy surface analysis adequately proved that the HYDRAVH₂can undergo the ESDPT process in the S₁state and the double proton transfer process is a stepwise proton transfer mechanism.(2)The ESDPT properties of 1,5-dihydroxyanthraquinone(1,5-DHAQ)in various solvents were investigated using femtosecond transient absorption spectroscopy and the DFT/TDDFT method.The steady-state fluorescence spectra in toluene,THF and ACN solvents presented that the position of the ESDPT fluorescence emission peaks were blue shifted with the increase of solvent polarity.Ultrafast kinetic fitting results showed that the increasing polarity of the solvent accelerated the rate of excited state dynamics.In addition,the non-covalent interaction and potential energy curve obtained by theoretical calculation further revealed the ESDPT mechanism of 1,5-DHAQ system.With the enhance of solvent polarity,the intramuscular hydrogen bond interaction gradually enhanced,and the energy barrier of proton transfer gradually decreased,which can promote the occurrence of the ESDPT dynamic processes for the 1,5-DHAQ system.