Effect Of Solvent Polarity On Excited State Intramolecular Proton Transfer

Hydrogen bonding is a common and important weak interaction,which is found in many substances and molecules.The hydrogen atom is a medium between electronegative atoms X and Y to generate a special intermolecular or intramolecular interaction in the form of X–H···Y.Excited intramolecular proton transfer(ESIPT)is an important reaction mechanism.The reaction occurs mainly through hydrogen bond.The interaction force of hydrogen bonding provides the driving force for the reaction.The strength of the hydrogen bond will affect the proton transfer rate.It has been found that the polarity of the solvent has an important influence on hydrogen bond and excited state intramolecular proton transfer,which is of great significance in the fields of physics,chemistry,and biology.The fluorescence characteristics of 4'-methoxy-3-hydroxyflavone(4M3HF)and2-(4-(dimethylamino)phenyl)-3-hydroxy-6,7-dimethoxy-4H-chromen-4-one(HOF)molecules in different solvent environments are found through experimental observation.However,experimental studies have not discussed the reaction mechanism of these two molecules in detail.In this study,density functional theory(DFT)and time-density functional theory(TDDFT)methods are used to theoretically study the excited-state intramolecular proton transfer process of 4M3 HF and HOF molecule.The molecular structure in the ground state and the excited state is optimized,and the infrared vibrational spectra,the frontier molecular orbitals,the charge transfer,the potential energy curves and the transition-state structures are calculated.The calculated absorption and emission spectral values match the experimental data.Hydrogen bond lengths and angles are calculated to confirm the excited state hydrogen bond strengthening mechanism.The frontier molecular orbitals display a redistribution of electron density in different solvents,affecting the photophysical properties of this dye.The infrared vibrational spectrum,reduced density gradient(RDG)isosurfaces,and potential energy curves are analyzed.It is found that when the molecule is in the excited state,the strength of the hydrogenbond gradually decrease with the increase of the solvent polarity.The calculated results prove that the solvent polarity has a great influence on the ESIPT reaction of4M3 HF and HOF molecule.As the solvent polarity increased,the intensity of the intramolecular hydrogen bond decreased,and ESIPT is more difficult to occur.This work has studied the mechanism of the ESIPT reaction in more detail,and pave the way for future research on 4M3 HF and HOF molecules.