Insights into solvent-polarity-dependent excited state behaviors for EDBT fluorophore: A computational study

The molecular properties of 2,2′-((1E, 1′ E)-hydrazine-1,2-diylidenebis(methanylylidene))bis(4-(1,2,2-triphenylvinyl)phenol) (EDBT) in cyclohexane (CYC), tetrahydrofuran (THF), and acetonitrile (MeCN) solvents have been studied theoretically. The optimal reaction path can be found to regulate the occurrence of excited state intramolecular proton transfer (ESIPT) reaction. In S₁ state, the strength of dual hydrogen bonds O1-H2···N3 and O4-H5···N6 increases significantly and contributes to the ESIPT reaction in a way providing the driving force. We calculated the infrared (IR) vibrational spectrum to analyze the movement of both O1-H2 and O4-H5 bond expansion vibrations and then studied the change of hydrogen bonding strength. In addition, from the rearrangement of frontier molecular orbital (FMOs), the electron density distribution is also an extremely case for predicting ESIPT tendency. According to potential energy curves, the ESIPT reaction occurs after the molecule absorbs the photon to reach the excited state, and the hydrogen atoms of the O1-H2 and O4-H5 bonds combine with the adjacent nitrogen atoms to form an isomer. After the completion of the ESIPT reaction process, the S₁ state returns to the S₀ state with recovering the original structure. The barrier size and photoexcitation characteristics in different solvents were compared, based on which we present that the increase of solvent polarity promotes the occurrence of ESIPT reaction process for EDBT fluorophore.