Study On The Excited State Intramolecular Proton Transfer Of N-heterocycles

Proton transfer process is widely found in the nature,which is a kind of important reaction in chemical and biological fields.Excited state proton transfer plays an key role in both physical and biological systems.Therefore,in recent years,more and more researchers have explored the properties of excited state intramolecular proton transfer both experimental and theoretical aspects.In this paper,the mechanism and properties of the excited state intramolecular proton transfer of [2,2-Bipyridyl]-3,3-diol(Bp(OH)2)and 10-Hydroxybenzo[h]quinoline(HBQ)were studied by quantum chemical calculation methods,and it may provide a good theoretical basis for molecular design.Furthermore,the proton transfer process of10-Hydroxybenzo[h]quinoline have been studied in different solvents,temperatures and under the supramolecular interaction with cucurbit[7]uril(CB7)by theoretical and experimental methods.The main results are as follows:1.We have studied the excited state intramolecular double proton transfer(ESIDPT)mechanisms of Bp(OH)2 by using DFT and TD-DFT methods.The bond distance,angle,IR spectra,and the hydrogen bond energy analyses show that the two intramolecular hydrogen bonds in the Bp(OH)2 molecule are significantly strengthened in the S1 state.The potential energy surfaces of the ground state and excited state show that the single or double proton transfer can occur in the S0 and S1 state,the double proton transfer in the S1 state is sequential not concerted.The Stocks shift observed in the experiment can be explained more comprehensively according to the ESIDPT mechanism.2.The intramolecular proton transfer processes of HBQ compounds have been studied by using DFT and TD-DFT methods,and the substituent effects existed in the intramolecular proton transfer reactions have been explored.The calculated results show that HBQ compounds can form intramolecular hydrogen bonds,which can be significantly strengthened in the excited state.In the ground state,the enol form is the normal form,but the keto form is the normal form in the excited state.The maximum absorption peaks of each compound are mainly caused by the transition of electrons from HOMO to LUMO.Because the GSIPT(enol form→keto form)barrier is very high,the proton transfer process is hard to occur.However,in the excited state,there is only a low-barrier ESIPT curve for HBQ compounds,and the barrier decreases when the substituent is electron withdrawing,so the electron withdrawing substituent is in favor of ESIPT.Substituent effects affect the spectral properties of HBQ compounds.3.We have explored the proton transfer process of HBQ in different solvents,temperatures and under the supramolecular interaction with CB7 by theoretical and experimental methods.The results indicated that the polar solvents promoted the excited state proton transfer of HBQ.Transient fluorescence spectra shows that quantum yield of HBQ increases when CB7 was tirated.The fluorescent emission spectra of interaction between HBQ and CB7 showed that CB7 and HBQ formed the inclusion complexes with a ratio of 1:1.The effects of solvent on the potential energy curve of HBQ proton transfer was investigated by using quantum chemical calculation methods.According to the simulated potential energy curves,it was inferred that the polar solvents could promote the intramolecular excited state proton transfer.Therefore,it promoted the conversion of enol form→keto form,and enhanced fluorescence at long wavelength,this conclusion was in agreement with the experimental results.