Application Of Excited State Hydrogen Bond Dynamics In Probe Field And Effect Of Solvent Polarity On ESIPT

In the second half of the 19 th century,researchers discovered that the physical and chemical phenomena of some substances could not be explained by the ideal gas law,and even if the researchers later proposed the more general concepts of van der Waals’ law,ideal solution vapor pressure and osmotic pressure,the physical laws of these substances were still abnormal.It has been found that these abnormal substances all have the same structure,that is,they all have hydrogenated functional groups,and these hydrogenated functional groups in the molecules will form interaction,this interaction is hydrogen bond interaction.The concept is used by researchers for a long time,until it is clearly defined in 1939,when it is understood in more detail.The formation of hydrogen bonds originates from the mutual attraction between or within molecules,which is mainly composed of electrostatic force,induction force and dispersion force,hydrogen bond is a weak bond,the strength is between weak van der Waals action and strong covalent bond.The existence of hydrogen bond not only makes the physical and chemical properties of substances abnormal,but also is closely related to some important reaction mechanisms.For example,excited state hydrogen bond kinetics plays an important role in photophysics and photochemistry.As one of the most important kinetic behaviors,excited intramolecular proton transfer(ESIPT)occurs based on hydrogen bond structure.The unique fluorescence phenomenon of ESIPT reaction has been widely used in the development and application of fluorescent materials,and the process of reaction is also affected by the polarity of solvents,we will introduce it briefly.Firstly,we give a brief introduction to photochemistry,ground and excited states,hydrogen bond,and excited state proton transfer.Secondly,This paper mainly uses density functional theory(DFT)and time-dependent density functional theory(TDDFT)methods for theoretical calculation,so we introduce the origin and development of density functional theory.Then,we mainly introduce the application of ESIPT in reactive fluorescent probe.The probe hydrolyzes esterase to form an enol structure(YE-1),and YE-1 undergoes an ESIPT reaction that releases strong green fluorescence.However,the clear mechanism of fluorescence emission is not given experimentally,so we systematically explored the fluorescence mechanism of YE-1.The first step of studying this mechanism is to optimize the structure of the molecule,and ensure that the geometric structure energy is at the local minimum through the analysis of the infrared vibration spectral frequency.Subsequently,two absorption peaks,one fluorescence peak,are found in the calculation of the electron spectrum.The absorption peak came from the enol structure of the ground state and the ketone structure,and the fluorescence peak come from the excited ketone structure,and the spectral calculation results are consistent with the experimental results.The third step,the O-H bond of the ground state and excited state structure of YE-1 is used as the independent variable to scan the potential energy curve.It is found that the ketone structure can exist in both the ground state and excited state,but ground state isomer unstable.Finally,we mainly studied the influence of solvent polarity on ESIPT.By analyzing the geometric structure of 2,4-bisimidazolphenol(2,4-b Im P)molecules in different solvents and drawing the reduced density gradient(RDG)isosurface visualization hydrogen bonds,it is found that hydrogen bonds are enhanced in excited states and the hydrogen bond strength gradually decreased with the increase of solvent polarity.By analyzing the front-line molecular orbitals,we find that the reason for the change in hydrogen bond strength is that the charge density within the molecule is redistributed under photoexcitation,which is favorable for excited hydrogen bonding.Using the Hershfield charge calculation of molecules,we understand that solvent polarity affects hydrogen bond strength and thus proton transfer difficulty,and the fundamental reason is that solvent polarity affects the charge density distribution of proton donors and acceptors.Potential energy curve proves the most powerful evidence of the difficulty of proton transfer.From the scan results,it can be seen that the solvent polarity can regulate the ESIPT process,and the ESIPT reaction gradually becomes difficult as the solvent polarity increases.