Computational Explorations About Photoinduced Behaviors for 2‐(2‐hydroxyphenyl)benzoxazole Derivatives by Substituted Alkyl Groups: A TDDFT Study

The molecular properties of three substituted 2‐(2‐hydroxyphenyl)benzoxazole derivatives including methyl, ethyl, and isopropyl moieties (i.e., HBO‐1, HBO‐2, and HBO‐3) have been studied theoretically. The optimal reaction path can be found to regulate the occurrence of excited state intramolecular proton transfer (ESIPT) reaction. In S 1 state, the strength of O─H⋯N hydrogen bond 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 O─H bond expansion vibration, and then studied the change of hydrogen bonding strength. In addition, from the rearrangement of frontier molecular orbital (MOs), the electron density distribution is also an extremely important positive factor in ESIPT process. According to potential energy curves, ESIPT reaction occurs after the molecule absorbs the photon to reach the excited state, and the hydrogen atom on the O─H bond combines with the adjacent nitrogen atom to form an isomer. After the completion of the ESIPT reaction process, the S 1 state returns to the S 0 state with recovering the original structure. The barrier size and photoexcitation characteristics in HBO‐1, HBO‐2, and HBO‐3 were compared, based on which we present that the effects of substituted alkyl groups play vital roles in regulating the ESIPT behaviors for HBO derivatives.