The Theoretical Investigation Of Excited-State Intramolecular Single Proton Transfer Mechanism

The hydrogen bond?H-bond?is an important weak interaction existing commonly in the nature,which can form within molecules as well as between molecules.Hydrogen bond's existing sustains the life-cycle in the world.Because of its unique photo-chemical and photo-physical properties,H-bond has been playing a significant role in many biological,chemical and physical processes,such as intramolecular charge transfer?ICT?,fluorescence resonance energy transfer?FRET?,photoinduced electro transfer?PET?and excited-state intramolecular proton transfer?ESIPT?,and so on.The properties of the hydrogen bond in solution is of particular interest and has been widely focused on diverse experimental and theoretical methods since solute-solvent interactions play a fundamental role in molecular nonequilibrium processes.In addition,the ESIPT based on the H-bond's weak interaction is a fundamental reaction in the fields of chemistry and biology,which contributes a lot to the designs and complications of UV filters,laser dyes and LEDs.Therefore,in the recent years,there are so many meaningful works about excited-state intramolecular proton transfer process in experimental and theoretical investigations.The present work employs density functional theory?DFT?and time-dependent DFT?TDDFT?to investigate the excited-state intramolecular proton transfer?ESIPT?mechanism of the newly synthesized 3 chromophore and 2-acetylindan-1,3-dion?AID?in the nonpolar solvent?hexane?and the polar one?acetonitrile?,respectively.The specific contents are as follows:1.The density functional theory?DFT?and time-dependent density functional theory?TDDFT?were employed to investigate the excited-state intramolecular proton transfer?ESIPT?mechanism of 3 chromophore in the aprotic DMSO solvent.The primary bond lengths,angles,and the IR vibrational spectra of the molecule in the ground?S₀?and excited states?S₁?are obtained.In addition,the potential energy curves of proton transfer of 3 chromophore in S₀and S₁ states are calculated.The evidences showing is presented as follows:?1?The intramolecular hydrogen bond?O-H???N?is proved to be strengthened in the S₁ state,which facilitated the ESIPT reaction of 3chromophore.?2?The electronic spectra observed at the previous experiment were reproduced well through the calculated vertical excited energy,indicating the rationality and correctness of the methods adopted in our calculation.The analysis of the intramolecular charge transfer via the frontier molecular orbitals predicted the reaction of the ESIPT process.As a consequence,the ESIPT mechanism of the 3 chromophore is elucidated definitely.2.The density functional theory?DFT?and time-dependent density functional theory?TDDFT?were employed to investigate the excited-state intramolecular proton transfer?ESIPT?mechanism of 2-acetylindan-1,3-dion?AID?in the nonpolar solvent?hexane?and the polar one?acetonitrile?.The primary bond lengths,angles,and the IR vibrational spectra involving H-bond of the molecule in the ground?S₀?and excited states?S₁?are obtained.In addition,the potential energy curves of proton transfer of AID chromophore in S₀ and S₁ states are calculated.The evidences showing are presented as follows:?1?The intramolecular hydrogen bond?O₂…H₅-O₃?of AID chromophore in the form of Enol-A is proved to be strengthened in the S₁ state.The evidence suggesting the proton transfer along the H-bond?O₂…H₅-O₃?is presented through the analyzing of the calculated results of the frontier molecular orbitals?MOs?.?2?The electronic spectra of AID in the polar acetonitrile solvent measured in the experiment are reproduced well by our calculation.But the ones in hexane are different from the experimental results,our theoretical investigation illustrates why the emission peak of Enol-A at about 457 nm with a weak oscillator strengthen is absent from the new experimental report[Verma et al?2016?.J.Struct.Dyn,3,023606],as the fluorescence emission from the proton transferred structure Enol-B in the first excited state.Thus,the single fluorescence mechanism is well explained.?3?Moreover,the potential energy curves of the ground state?S₀?and the first singlet excited state?S₁?are constructed.In the S₁ state,the absence of the potential barrier of AID in the polar solvent,compared with the nonpolar one?0.57 kcal/mol?,indicates that the ESIPT process of AID molecule is relatively easier to accomplish in the polar solvent,highlighting the influence of solvent polarity on ESIPT mechanism of AID chromophore.