Theoretical Study On The Excited Proton Transfer Mechanism Of Complex Small Molecule Systems

Due to the advantages of excited state proton transfer（ESPT）systems such as high fluorescence efficiency and large Stokes shift,excited state proton transfer reactions have broad application prospects in organic functional materials,fluorescent probes,optical devices,biological imaging,and other fields.In this thesis,static calculations were conducted using density functional theory（DFT）,time-dependent density functional theory（TDDFT）,and ab initio（RICC2）methods.Combined with excited state non adiabatic dynamic simulations,the behavior of excited state proton transfer in three different molecular systems was studied to reveal the mechanism of excited state proton transfer reaction.The main research contents are as follows:1.The sensing process of Al³⁺fluorescence probes based on Schiff base derivatives（HL）（Sens.Actuator B-Chem.,2018,260,888-899）was studied using DFT,TDDFT,and RICC2 methods.The sensing mechanism of Al³⁺fluorescence sensor is of great significance to the design of high-efficiency fluorescence sensor.This probe is proposed to be a fluorescence probe based on the photoinduced electron transfer（PET）mechanism.However,the PET mechanism has not been experimentally and theoretically confirmed.In addition,the fluorescence response mechanism of the Al³⁺fluorescence probe is unclear.It was found that the quenching of the fluorescence probe was caused by the formation of a twisted intramolecular charge transfer（TICT）state induced by the excited state intramolecular proton transfer mechanism（ESIPT）.This state minimum is close to the conical intersection in gas phase,and the TICT state minimum formed in the solvent still shows a dark state,and the locally excited state minimum in the solvent could also reach the dark state nπ*state through dihedral rotation.When Al³⁺is added dropwise,the N and O atoms in HL can provide a complex site for Al³⁺,which prevents the formation of ESIPT and TICT states,ultimately causing the Al³⁺complex to emit blue fluorescence in a locally excited state.2.The excited state proton transfer processes of 3-hydroxyflavone derivatives（PPC,EPC,and MNC）in the implicit solvation model and the explicit solvation model were studied using DFT and TDDFT methods.It was found that in the implicit solvation model,the greater the polarity of the solvent or the stronger the electron donor ability of the substituent,the greater the energy barrier of ESIPT.This is consistent with the experimental phenomena of dual fluorescence emission of PPC and EPC in 1,4-dioxane and single fluorescence emission in DMSO and Me CN.In the explicit solvation model water,the ESPT kinetics of PPC and EPC are influenced by the water molecular environment.In chain clusters,with the addition of more water molecules,the ESPT energy barriers of PPC,EPC,and MNC gradually increase,and the stability of tautomers decreases;compared to chain clusters,non-chain clusters have lower energy barriers and more unstable tautomers in the ESPT process.It is worth noting that the energy barrier of MNC-（H₂O）₁₊₂ is lower than that of all 3HF derivative clusters,and even lower than that of isolated MNC.For the ESPT reaction,the IRC process was further studied,and the results showed that the tautomerism of 3HF derivative clusters was a single-step multi-proton transfer process.Compared to MNC,PPC and EPC are more sensitive to water.This theoretical result is consistent with the experimental phenomenon that the fluorescence color of PPC and EPC changes from red to yellow and cyan,respectively,and the fluorescence color of MNC remains yellow.This work provides in-depth theoretical guidance for the design of water sensitive organic fluorescence probes with excellent performance.3.The excited state proton transfer mechanism of 3-hydroxyisoquinoline dimer was studied using static calculations carried out using DFT,TDDFT,and RICC2 methods in combination with excited state non-adiabatic dynamics at RICC2 level.The direct excited state double proton transfer（ESDPT）from Enol/Enol（EE）to Keto/Keto（KK）is an exothermic process without energy barrier.For the stepwise proton transfer,the first step of single proton transfer（SPT）（from Enol/Enol to the intermediate Enol/Keto）releases heat of 0.91 e V,with very small energy barrier of 0.12 e V and the second step of single proton transfer（from Enol/Keto to the intermediate Keto/Keto）is endothermic process of0.34 e V,with an energy barrier of 0.84 e V.The results indicate that the direct ESDPT are more likely to occur,while the single proton transfer is also possible.The TDDFT predict similar results with that using RICC2 method.In addition,in non-adiabatic dynamics simulations,there are 17 trajectories starting from the S₁ state,from where SPT occurs in8 trajectories and the ESDPT occur in 9 trajectories.Among the ESDPT trajectories,there are 2 trajectories where a reverse proton transfer from Keto/Keto to Enol/Keto occurs,from where a non radiative transition happens;otherswise,ESDPT returns to the ground state through radiation transitions.There are 15 dynamic trajectories starting from the S2state,including 7 for SPT and 8 for ESDPT,respectively.Among ESDPT trajectories,there are 5 trajectories where a reverse proton transfer process occurs,from where a non radiative transition happens;otherswise,ESDPT returns to the ground state through radiation transitions.The statistical results of excited state non adiabatic dynamics indicate that both double proton transfer and single proton transfer may occur.In addition,the intermediate Enol/Keto ends its dynamic trajectory near the conical intersection,and Enol/Keto is likely to return to the ground state in a non radiative transition manner.This work resolved the controversy over the excited state double proton transfer process of3-hydroxyisoquinoline dimers.