Theoretical Studies On The Excited State Intramolecular Proton Transfer In Benzotriazole Derivatives With Chromphore

The intramolecular proton transfer reactions draws peoples's more and more atention in chemistry and biochemistry. This paper presents ab initio (HF), density functional theory calculations (DFT, TDDFT) and configuration interaction including single state (CIS) on the excited state intramolecular proton transfer (ESIPT) of a chromophore linked with 2-(2-hydroxyl-phenyl)-benzotriazole: {2-hydroxyl-5- [(p-nitro-styrene)yl- oxymethylene]-phenyl}-(2H-benzotrizole) (C1), 4′-nitro- 3,4-bis- [2-hydroxyl-(2H-benzotrizole)-benzyloxy]-stilbene (C2) and C3 compound with–H, -F, -OCH3 and -N(C2H5)2 to instead -NO2.We undertook a comprehensive investigation on the ground state and excited state changes of the ESIPT tautomers including their bond lengths, bond angles, Mulliken charges, dipole moments, frontier orbitals and potential surface curves.A stable keto form (K) was not obtained for C1 and C2 in the ground state, and all need absorb a lot of energy in the isomerization process. Therefore, the ground state intramolecular proton transfer (GSIPT) was impossible. The hydrogen bond strength of the keto form (K*) was higher than that of the enol form (E*) at their excited states. As excited, the negative charge of the hydrogen donor was diminished while the negative charge of the hydrogen acceptor was enhanced (except R=-N(C2H5)2). For K*, the electron density moved from a"phenol cycle"to a protonated heterocycle because of the HOMO→LUMO electron transition (except R=-N(C2H5)2). The small energy barrier from E* to K*(ca 41 kJ·mol-1) was also observed. These results indicated that the possibility of ESIPT occurring was high for C1. While, for C3, their ESIPT reaction had to get across an energy barrier about from 32.864 kJ·mol-1 to 63.343 kJ·mol-1 for different substituents. C2 could not happen ESIPT at room temperature as the energy of the productions were slightly higher than that of the reactants in isomerization process. For C2, stable structures for the EK tautomers (including enol and keto forms simultaneously), 2K (including two keto forms simultaneously) and 2K* (the excited state of 2K) could not be obtained because of their high energies. As a consequence, the probability of intramolecular single or double proton transfer at the ground state or the excited state intramolecular double proton transfer occurring was negligible. The low transition energy for 2E*→EK*(the excited state of EK) suggested that the excited state intramolecular single proton transfer was possible for C2. Besides, further quantum chemical calculations had been done for forecasting the UV-Vis absorption and fluorescence spectra, and the ESIPT fluorescence emission with a large stokes shift was observed.