Theoretical Study On The Mechanism Of Intramolecular Proton Transfer In Excited States Of Several Typical Organic Molecular Systems

Hydrogen bond,one of the most important weak interactions in nature,it has attracted great attention to the exploration of reactions initiated by hydrogen bonds,which have wide prospects in the fields of photophysics,photobiology and photochemistry.Especially for the excited intramolecular proton transfer?ESIPT?,one of the most important process in biological and chemical acid-base neutralization reactions,it occurs between the adjacent proton donor and proton acceptor inside the excited state molecule.It used in many applications,such as UV absorbers,fluorescent probes and molecular switches,etc.In this paper,the time-dependent density functional theory method is used to investigate the ESIPT mechanism of three different series of molecules and to detect metal Al³⁺ions by comparing fluorescence changes.Geometric configuration,infrared spectra,frontier molecular orbitals,absorption spectra,fluorescence spectra and potential energy curves were calculated separately.1.the ESIPT mechanism of o-hydroxynaphthyl phenantrimidine?HNPI?molecule with excited state proton transfer system in the acetonitrile solventFirstly,the molecular structure and infrared vibrational spectra of the S₀ state and the S₁ state show that the hydrogen bond has an enhanced tendency in the S₁ state.Analysis of the frontier molecular orbitals?HOMO-LUMO?leads to the conclusion that the form of molecular excitation is intramolecular charge transfer excitation.The vertical excitation calculated based on TDDFT can effectually recurrence the experimental electronic spectra.However,no emission spectra similar to the normal configuration were detected in the experiment.The analysis showed that the fluorescence measured in the experiment put down to the two structures.In addition,the calculation results of the potential energy curves of the S₀ state and the S₁ state indicate that the proton transfer process of the HNPI molecule occurs in the excited state.2.The different derivatives of 1-aryl-2-?furan-2-yl?butane-1,3-dione?AYFBD?in ethanol solvent to analyze the effect on the ease of proton transferFirstly,we conclude that the intramolecular hydrogen bonds are strengthened in the S₁ state by comparing the geometric configuration,the infrared spectra,and the frontier molecular orbital of S0 and S1 state.The calculated absorption spectra and emission spectra of the derivatives of AYFBD with Keto configuration are consistent with the experimental data reported previously.After contrasting the potential energy curves of four different derivatives in S₁ state and S₀ state,the sequence of molecular transfer follows:FFBD>MFBD>TFBD>OFBD.3.A detecting Al³⁺ion luminophor 2-?Anthracen-1-yliminomethyl?-phenol:Theoretical investigation on the fluorescence properties and ESIPT mechanismWe calculated the proton transfer process of 2-?indol-1-aminomethyl?-phenol?AYP?in methanol solvent,and the proton transfer process of AYP molecule occurred in the excited state and the metal Al³⁺was detected by this property.The change of phase key parameters,the charge distribution of the leading edge molecular orbital and the red shift of the infrared vibration spectrum all indicate the enhancement of hydrogen bond in S₁ molecule,and the reduced density gradient function?RDG?can also be observed obviously.The calculated difference between the AYP Enol form and the AYP Keto form is only 4nm,which explains why the AYP Enol form is not observed in the experiment.Then we scanned the potential energy curve of AYP chemical sensor.By comparing the barrier of S₀ state and S₁ state,it is confirmed that proton transfer takes place in S₁ state.In addition,the calculated fluorescence spectrum was used to explain the detection mechanism of Al³⁺,which agree well with the experimental data.