Study On Photophysical And Photochemical Processes Based On Proton Transfer Reaction Systems

Photochemical reaction plays a very important role in nature and human production and life.Photochemical reaction is also widely used in many fields,such as environment,biology,detection,materials and information storage.The study of molecular photophysical and photochemical processes is helpful to understand the photochemical reaction mechanism and develop its potential applications.In this paper,the photophysical and photochemical processes of three different molecular systems are studied by using density functional theory(DFT),quantum mechanics/molecular mechanics(QM/MM)and diabatic molecular dynamics simulation.The main contents are as follows:(1)Combined with static potential energy surface calculation and diabatic kinetic simulation,we studied the photophysical and photochemical processes of SDA and SDB salicylaniline derivatives with aggregated fluorescence enhancement(AEE)in liquid phase and condensation direction.The results show that there are two competing excited state relaxation paths for SDA and SDB molecules excited by light in solution.The first way is that the excited state intramolecular proton transfer(ESIPT)process occurs rapidly after the two molecules are excited by light,forming isomers and emitting light.The other way is that after the two molecules are excited by light,C=N bond distortion occurs,resulting in intramolecular rotation,and return to the ground state through the conical intersection of S₁/S₀ in a non-radiative transition.In the crystal,the intramolecular rotation process of two molecules is limited because of the close packing of molecules,which inhibits the reaction path.The ESIPT process is not limited,so that the two molecules show AEE characteristics.The results of nonadiabatic kinetic simulation in gas phase show that the two isopropyl groups in SDA molecule inhibit the intramolecular rotation process to a certain extent,making the probability of ESIPT of SDA molecule higher than that of SDB molecule.Therefore,the fluorescence yield of SDA molecule is higher than that of SDA molecule.(2)Then,we studied the non radiation relaxation mechanism and fluorescence sensing mechanism of diaminobenzidine(H₂L)zinc ion probe by TD-DFT method.The calculation results show that after H₂L molecule is excited by light,due to the small energy barrier of ESIPT,the ESIPT process occurs rapidly to form intermediates,and then return to the ground state through intramolecular rotation.When zinc ion is involved,the probe molecule can form a complex with zinc ion and acetic acid molecule,which blocks the ESIPT of H₂L and the subsequent intramolecular rotation process.The formation of the complex inhibits the non radiative transition channel of H₂L and makes it emit fluorescence after absorbing light.(3)Finally,we studied the photophysical and photochemical process of artificial DNA base pair P-Z.P and Z represent 2-aminoimidazo[1,2-a]-1,3,5-triazine-4(8H)one and 6-amino-5-nitro-2(1H)-pyridinone,respectively.The static TDDFT results show that the single proton transfer process is difficult to occur due to the high excited state energy of the intermediate of single proton transfer(SPT)of P-Z base pair and the high energy barrier of SPT process.The excited state energy of the isomer formed by nitro isomerization is relatively low,and the energy difference between the first excited state and the ground state is less than 0.5 e V,indicating that the P-Z base pair may return to the ground state without radiation through nitro isomerization.The non adiabatic kinetic results show that nitro rotation and nitro out of plane bending are the main excited state inactivation modes of P-Z base pairs.In addition,it is also observed that an SPT process from Z to P occurs in a track,and then P deforms and returns to the ground state.Therefore,compared with natural base pairs,the excited state relaxation mechanism of P-Z base pairs is different.Our results show that nitroisomerization plays an important role in the photostability of P-Z base pairs.