Theoretical Study On The Mechanism Of Excited-state Proton Transfer In Several Organic Molecular Systems

Excited-state proton transfer（ESPT）reaction is one of the most basic and important reactions in organisms.ESPT refers to the isomerization process of protons from donors to adjacent heteroatoms such as N and O along intramolecular hydrogen bonds or intermolecular hydrogen bonds after molecules are excited by light.ESPT systems have the advantages of high fluorescence efficiency and large Stokes shift.It has a wide application prospect in fluorescence probes,biological imaging,DNA base pairs,organic functional materials and so on.Therefore,it is important to study the excited-state proton transfer mechanism in different organic molecular systems from the perspective of theoretical research,which can not only provide a reasonable explanation for the experiment,but also provide theoretical guidance for the design of new ESPT organic light-emitting molecules.In this work,the ESPT mechanism of three different organic molecular systems were studied,and the photophysical and photochemical processes are analyzed in detail.The specific contents are divided into the following aspects:1.The excited-state intermolecular proton transfer mechanism of 3-Me-2,6-diazindole with two or three water molecules（（2,6-aza）Ind·（H₂O）_n（n=2,2+1,3））was studied by density functional theory（DFT）and time-dependent density functional theory（TDDFT）.The results show that when two water molecules participate in the ESPT process of（2,6-aza）Ind,the water molecules are rearranged first,and then the excited-state triple proton transfer is carried out.When the third water molecule is added,the energy barrier of ESPT will be further reduced.The proton transfer of（2,6-aza）Ind·（H₂O）₃ is compared with that of（2,6-aza）Ind·（H₂O）₂ and（2,6-aza）Ind·（H₂O）₂₊₁,indicating that（2,6-aza）Ind in aqueous solution is more prone to excited-state quadruplet proton transfer with the assistance of three water molecules.2.The photophysical and photochemical properties of 3-hydroxyflavone derivatives（PPC,EPC,MNC）in 1,4-dioxane and DMSO with different polarity were studied by DFT and TDDFT,and we explore the effects of solvents and substituents on ESIPT progress.The results show that in 1,4-dioxane and DMSO solutions,the excited-state Keto configuration(Keto _S1)of PPC,EPC and MNC is more stable than the Enol configuration(Enol _S1),and the smaller the electron-donating capacity of the substituents,the greater the energy difference between Enol _S1 and Keto _S1.The potential energy curves of S₀ and S₁states along the direction of proton transfer were calculated,and the transition states of excited-state proton transfer were optimized.The electron-donating group increases the energy barrier of ESIPT.In addition,with the increase of solvent polarity,the energy barrier of ESIPT for PPC,EPC and MNC also increases.3.The different photophysical and photochemical properties of HBT derivatives（HBT-s-Ph,HBT-d-Ph）with excited-state intramolecular proton transfer（ESIPT）and aggregation induced emission（AIE）properties in toluene and solid phase were studied.The results show that the energy barrier of HBT-s-Ph and HBT-d-Ph molecules in toluene and solid phase is small,indicating that the ESIPT is easy to occur,which is consistent with the observed emission wavelengths generated by Keto _S1.There is a non-radiative transition path form Keto _S1 alongθ（N₁-C₁-C₂-C₃）in toluene,which cause the low quantum yield of the two molecules in solution.In the aggregation state,the molecule rotation along theθ（N₁-C₁-C₂-C₃）is limited due to the high potential barrier,so only the rapid ESIPT process occurs after the molecule is excited,and Keto _S1 determines the solid-phase fluorescence.The AIE phenomenon of HBT-s-Ph and HBT-d-Ph molecules is the the combination of ESIPT and the restriction of intramolecular rotation.In addition,the fluorescence quantum yield of HBT-d-Ph molecule is higher than that of HBT-s-Ph,which is related with low exciton coupling,higher HR factors in solvent and intersystem crossing between singlet and triplet states.