Study On Excited State Intramolecular Proton Transfer Of 2-ketones And GFPs

Since excited-state intramolecular proton transfer(ESIPT)theory was proposed by Weller in 1950,many fluorescent chromophores with excited-proton molecular proton transfer property have received widespread attentions due to their characteristics of emission bandwidth,emission compatibility and dual emission.Possible applications in functional materials of ESIPT chromophores depend on their fluorescence properties.However,the fluorescence quantum yield of ESIPT emission(K*)with a large stokes shift is usually very low for most of ESIPT chromophores which have been reported,due to the fact that K*configuration of ESIPT chromophore tend to non-radiation deactivation.Today,it is still an important topic to study and design ESIPT chromophores with high fluorescence quantum efficiency and long fluorescence lifetime.In this paper,the density functional theory(DFT)and time-dependent density functional theory(TDDFT)were carried out to investigate ESIPT dynamics of two molecule systems.The experimental spectral phenomenon is reasonably explained,and the effect of substituent and solvent on excited-state intramolecular proton transfer is focused.The DFT and TDDFT theoretical methods were employed to investigate the excited-state intramolecular proton transfer(ESIPT)mechanism of 1-Aryl-2-(furan/thiophen-2-yl)butane-1,3-diones(AYFBDs).A coexistence mechanism of forward and backward excited-state intramolecular proton transfer was proposed for AYFBDs.The fluorescence spectra of four kinds of AYFBDs molecules observed in a previous experiment were attributed to the emission of keto(K*)configuration after excited-state intramolecular proton transfer.And it can be speculated that the luminescence of enol(E*)configuration can be obtained after enlarging the measurement range of fluorescence spectrum.According to the different electronic properties of the four kinds of substituents,-OCH3 and-CH3 were classified as electron-donating groups,meanwhile,-CF3 and-F were classified as electron-withdrawing groups.In addition,the order in which the strengthening of excited state intra-molecular hydrogen bonds to promote the excited state intramolecular proton transfer is predicted to be AYFBD-OCH3<AYFBD-CH3<AYFBP-CF3<AYFBD-F.Combined with the analysis of the potential energy curve and transient state,it can be concluded that the electron-withdrawing groups located in the para-substitution position of AYFBDs can promote the excited-state intramolecular proton transfer better than the electron-donating groups.We have studied theoretically the double fluorescence phenomenon of green fluorescent protein(GFP)which was observed in a previous experimental work for the first time.The fluorescence spectra of three kinds of GFP chromophore derivatives with smaller Stokes shift observed in the previous experiment were classified as emission of enol(E*)configuration.And the fluorescence spectra with larger Stokes shift observed in the previous experiment were classified as emission of keto(K*)configuration formed by excited state intramolecular proton transfer.The order in which the strengthening of excited state intramolecular hydrogen bonds to promote the excited state intramolecular proton transfer is predicted to be 'strong polar solvent<medium polar solvent<weak polar solvent and OHIM<CHBDI<MHBDI.Functional substituent groups of the three kinds of GFP chromophore derivatives have the same electronic properties,which have been classified as electron-donating groups.Combined with the analysis of the potential energy curve,it can be concluded that the weakening of electron-donating activity of functional substituent groups and the reduction of the solvent polarity can promote the excited state intramolecular proton transfer,and the enhancement of the solvent polarity can promote the reverse excited state intramolecular proton transfer of the GFP derivatives.The three kinds of GFP chromophore derivatives have different fluorescence phenomena,because the amounts of E*and K*configuration molecules have changed regularly in different polar solvents under the control of functional substituent groups and the solvent effect.In general,the spectral properties of ESIPT chromophores depend on the intramolecular hydrogen bonds,the process of the twisted intramolecular isomerization,the acidity and alkalinity properties of the environmental medium,the types and positions of substituents on the donor or recipient units and so on.The fluorescence characteristics of ESIPT chromophores can be adjusted by changing these functional parameters.