Theoretical study of the direction of the excited-state intramolecular proton transfer of the HBS molecule

Excited-state intramolecular proton transfer (PT), an important process in photosynthesis, has been widely available for fluorescence sensors and fluorescent probes. Previously, PT dynamics in the desired direction have been controlled by simply perturbing the external hydrogen-bonding network, which can regulate their photophysical properties. However, a deep understanding of the mechanism of the direction of PT dynamics remains lacking. In this study, the fluorescent chemosensor molecule N & PRIME;-[(1E)-[5-(2,3-dihydro-1,3-benzothiazol-2-yl)yl)-6-oxocyclohexa-1,3-dien-1-yl]methylenyl]methylene]-2-hydroxybenzohydrazine (HBS) was studied theoretically in detail with a time-dependent density functional theory method. It was found that dual PT channels of HBS molecules can occur in the S-1 state. The analysis of the hydrogen bond length, the infrared vibration spectrum, and the subsequent charge redistribution also provided distinct evidence for this viewpoint. According to the analysis results of the potential energy curves, the PT process of the HBS-N-3 and HBS-N-4 configurations readily occurs in the S-1 state, which contributes to an in-depth understanding of the HBS mechanism in different directions. This study offers new routes toward regulating and designing novel fluorescent sensors.