Theoretical investigation of solvent-polarity-dependent excited-state intramolecular proton transfer behavior for...

In this work, the molecular properties of the novel semi-aliphatic polyimide derivative with incorporating bulky -CF₃ side groups (3H-6F) in gas, chloroform and acetonitrile have been studied by DFT and TDDFT methods. The optimal reaction path could be found to regulate the occurrence of excited-state intramolecular proton transfer (ESIPT) reaction. In S₁ state, the strength of O1-H2···O3 hydrogen bond increases significantly and contributes to the ESIPT reaction providing the driving force. We calculated the infrared (IR) vibrational spectrum to analyze the movement of O1-H2 bond expansion vibration and then studied the variations of hydrogen bonding strength. In addition, from the rearrangement of frontier molecular orbital (MOs), the electron density distribution should be also an extremely important positive factor in ESIPT process. According to potential energy curves (PECs), ESIPT reaction occurs after the molecule absorbs the photon to reach the first excited state, and the hydrogen atom of the O1-H2 bond combines with the adjacent O3 atom to form an isomer. After the completion of the ESIPT reaction process, the S₁ state returns to S₀ state with recovering the original structure. The barrier size and photoexcitation characteristics in different surroundings are also compared, based on which we present that the increase of solvent polarity promotes occurrence of ESIPT reaction process for 3H-6F fluorophore.

All molecular structures have been optimized using DFT and TDDFT method with B3LYP/6-311 +  + G(d,p) level by Gaussian 16 software. Vertical excitation simulations were based on TDDFT method with analyzing charge redistributions. Using Multiwfn 3.8 software, the core-valence bifurcation indexes were performed. Further, potential energy surfaces have been constructed, based on which the transition state configurations were found at the same level.