Photoinduced Intramolecular Proton Transfer And Excited State Dynamics Of Base Derivatives

The excited state intra-molecular proton transfer(ESIPT)system has important applications in fluorescent probes,photoelectric materials,and near-infrared emitters due to its unique characteristics.7-Hydroxy-1-indanone(7HIN)and 2’-Hydroxyphenylacetone(OHPP),as the simplest and similar molecular weight ESIPT molecules,have different spectral responses to solvents.Studying its dynamic process may provide theoretical reference for practical application.The ESIPT process is a very fast process,and femtosecond time-resolved absorption spectroscopy can provide information about the excited state lifetime and excited state species relaxation.The density functional theory(DFT)method and time-dependent density functional theory(TDDFT)method can study the structure and properties of the excited state of the system at the molecular level,and the calculation accuracy is good.The UV absorption spectrum of 7HIN and OHPP has almost no dependence on the solvent,and the fluorescence is more sensitive.7HIN emits single fluorescence in aprotic solvents,double fluorescence(methanol)and even multiple fluorescence(water)in protic solvents.7HIN’s ESIPT process needs to cross the energy barrier and is an irreversible process.OHPP emits double fluorescence in different solvents.The fluorescence intensity of excited state proton transfer(PT)state is stronger than that of local excited(LE)state.The proton transfer lifetime of 7HIN in cyclohexane and acetonitrile solvents is about 3 ps,and it is 5 ps in water and methanol solvents.PT fluorescence lifetime is 884 ps(cyclohexane),634ps(acetonitrile),384 ps(water),1100 ps(deuterated water),688 ps(methanol),and 1400 ps(deuterated methanol).OHPP requires 4 life time to simulate in methanol solvents and only 3life time needed for in other solvents.The proton transfer lifetime of the excited state of OHPP is 2 ps,the internal conversion lifetime is 20-40 ps,and the lifetime of PT state is 970 ps.7HIN and OHPP both relax to the triplet state and have a long life.As part of the PT system,methanol and water solvents participate in the 7HIN PT process,which is a cooperative mechanism.7HIN forms a new excited state fluorescent species in an aqueous solvent.N6-methyladenine(N6ADE)、 N6-methyl-2’-deoxyadenosine(N6ADO)、 N6,N6-dimethyladenine(DMADE)、and N6,N6-dimethyladenosine(DMADO),which emits double fluorescence in acetonitrile and water solvents.And the intensity of the intramolecular charge transfer(ICT)state fluorescence is sensitive to the solvent.The fluorescence intensity of N6 ADE and N6 ADO in the ICT state is lower than that in the LE state,and the quenching is obvious in aqueous solvents.DMADE and DMADO in acetonitrile solvents have stronger fluorescence intensity in the ICT state than in the LE state,but the opposite is true in water.Dimethyl substitution improves the electron donating ability of the amino group,and the ICT process is a process with little or no energy barrier.Theoretical calculations indicate that methyl-substituted adenine derivatives form intermolecular hydrogen bonds with water,causing fluorescence peaks to redshift and quench.Solvents and sugar groups can change the type of transition orbits and the energy order of transition orbits in the Franck-Condon region.The UV absorption transition information of DMADO and N6 ADO was analyzed,and it was suggested that the ability and proportion of DMADO and N6 ADO in the conical intersection reaction in Franck-Condon region may be different.The formation of intermolecular hydrogen bonds between water and N6 ADE and N6 ADO may also be beneficial to the conical intersection formed by the dissociation reaction of N10H.