Revelation Solvent Effects:Excited State Hydrogen Bond And Excited State Intramolecular Proton Transfer

Hydrogen bond,a fundamental weak interaction,is closely related to photophysics and photochemistry phenomenon in nature.The path of proton transfer is connected by hydrogen bond.Upon photo-excitation,charge distribution of the proton donor and acceptor will reorganize.Intramolecular proton transfer reaction often accompanies intramolecular charge transfer characteristic.Solvent environment has important influence on hydrogen bond strength,proton transfer barrier and charge transfer process,and then it changes the photophysical and photochemical properties of molecule.The 2-(benzo[d]thiazol-2-yl)-3-methoxynaphthalen-1-ol(MMT) and 2-(benzo[d]thiazol-2-yl)naphthalene-1,3-diol(DHT)molecules are naphthalene based derivative.Under different solvent environments photophysical phenomena of MMT and DHT were observed and reported in experiment.However,solvent effect on hydrogen bond dynamical process is failed to carefully theoretical analyzes.We give molecular-level investigation that excited-state hydrogen bond and excited state intramolecular proton transfer(ESIPT)is affected by solvent polarity based on the density functional theory and time-dependent density functional theory methods.For this purpose,the hydrogen bond parameters,infrared vibration spectra,electronic spectra,frontier molecular orbitals and potential energy curves are computed and analyzed.The computed absorption and fluorescence emission spectrum are accord with the spectrum peak values in experiment.Through theoretical analysis,the excited-state hydrogen bond enhancing mechanism is verified,and the enhanced hydrogen bond provides driving force for ESIPT reaction.Meanwhile,we also uncover perfectly that the intensity of excited-state hydrogen bond weaken with solvent polarity augment.Importantly,Hirshfeld charges essentially unveil that the excited-state hydrogen bond strength depends on solvent effects on electronegativity of proton donor and proton acceptor.Based on frontier molecular orbitals,we find that MMT and DHT exhibit intramolecular charge transfer characteristics during photo-excitation and that the degree of charge transfer is gradually increasing with the solvent polarity augment.By analyzing the potential energy curves,the effects of different solvents on the ESIPT of MMT and DHT molecules are explained.It can conclude that proton transfer reactions of MMT and DHT are endothermic in the ground state and potential barriers of reverse proton transfer are low,illustrating the tautomerism reactions are not prone to take place.In the first excited state,the ESIPT reaction is gradually inhibited as the solvent polarity augment for MMT and DHT,which more likely to occur in these solvents: DMSO ? DMF ? Acetonitrile ? MeOH ? Chloroform ? Toluene ? 1,4-Dioxane.Considering the solvent effects on ESIPT will be conducive to develop products of high Stokes shift,which will have significant applications in optoelectronics.