Effect of different substituted groups on excited state intramolecular proton transfer of BOHMB

The photophysical features of 3-(benzo[d]oxazol-2-yl)-2-hydroxy-5-methoxy benzaldehyde (BOHMB) were investigated through experimental (J. Phys. Chem. A 2019, 123, 10,246–10,253) and theoretical (Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2022, 266, 120,406) methods. However, the effect of substituent groups on the excited state proton transfer process has not been studied. In this work, the excited state intramolecular proton transfer (ESIPT) dynamics and photophysical properties of BOHMB with different substituent groups were investigated by density-functional theory (DFT) and time-dependent DFT (TDDFT) methods at CAM-B3LYP/6-311G(d,p) level. The primary parameters related to hydrogen bonding and infrared vibration frequency were obtained to understand the ESIPT properties of BOHMB derivatives. The results indicate that the excited-state intramolecular hydrogen bond (ESIHB) strengthening behaviors, and the intramolecular hydrogen bond O1–H2···O3 for 1a in the S₁ state is the strongest among BOHMB derivatives. From the calculated potential energy curves, it can be inferred that the substitution and position of NH₂ and NO₂ groups will regulate the excited-state energy barrier and thus affect the ESIPT process. The molecular absorption peak and fluorescence peak are affected by different substituting groups and different positions.