Theoretical Study Of Photophysical Properties Of Several Organic Heterocyclic Hydrogen-bonded Systems

As one of the important weak interactions,hydrogen bonds widely present in many molecular and supramolecular systems.The changes of intramolecular and intermolecular hydrogen bonds in the excited state have significant effects on the photophysical properties and photochemical reactions in those molecules.The excited-state hydrogen bonding dynamics can even modulate or change the fluorescence emission behaviors of chromophores and dye molecules in liquid phase.The study of photophysical properties of hydrogen-bonded systems is of great significance in the fields of fluorescent probes,biological imaging,organic photoelectric materials and so on.In the present work,we mainly use the density functional theory?DFT?combined with time-dependent density functional theory?TDDFT?to study the excited-state hydrogen bonding dynamics of several organic heterocyclic molecules and effects on their photophysical properties.We specifically discussed the hydrogen bonding dynamics of2-?2-Hydroxy-phenyl?-4?3H?-quinazolinone?HPQ?in different solutions,and explored the mechanism of excited state intramolecular proton transfer?ESIPT?reaction in detail.In the polar protic methanol solution,the multiple hydrogen-bonded complex composed by HPQ and two methanol molecules?HPQ-2M?could exist stably in the ground state,and its intermolecular hydrogen bonds are enhanced in the excited state.Both HPQ monomer and HPQ-2M complex undergo the ESIPT process coupled with twisted charge transfer,and their photoexcitation behaviors show differences significantly.Our theoretical investigation has shown that the solvent environment has an important impact on the charge distribution,excitation transition behavior,and steady-state and infrared spectra of molecular systems in different electronic states.We also explored the intramolecular hydrogen bonding dynamics of several derivatives of 10-Hydroxybenzo[h]quinolone?HBQ?.The effects of different substituents and?-expanded structures on the photophysical properties of HBQ are discussed.Those HBQ derivatives are photoexcited to the S₁ state,and all of them undergo the ESIPT process coupled with charge transfer.Different substituent groups acting in the same position of HBQ results in different changes of hydrogen bonds in the excited state.In steady-state absorption and fluorescence spectra,the electron-donating group substitution of HBQ produces a smaller Stokes shift than the electron-withdrawing substituted derivatives.The effect of?-expanded structure on spectra is similar to that of the electron-donating group substitution.In addition,the HBQ derivative which is substituted by electron-donating group undergoes ESIPT process more easily.Through quantum chemical calculations and analysis,it is proved that the existence and enhancement of hydrogen bonds have a great influence on the photochemical properties of organic heterocyclic compounds.The study about their spectral characteristics and photophysical processes is of great significance for the development and application of fluorescent probes and sensors.