Mechanism of fluorescence enhancement of HClO detected by excited-state intramolecular proton transfer based HBT-OMe molecule

Abstract

The molecule with excited-state intramolecular proton transfer (ESIPT) has wide applications in fluorescent probe, biology imaging, light-emitting materials, etc. Biologically active oxygen hypochlorite (HClO) exists widely in the biological and chemical environment, which can pose a great threat to human health. Design of HClO-sensitive molecules in solvents is very important. Recently, Wu et al. [Wu L L, Yang Q Y, Liu L Y, et al. <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1039/C8CC03717E">2018 <i>Chem. Commun.</i> <b>54</b> 8522</ext-link>] designed an ESIPT-based HBT-OMe probe molecule, which can detect HClO due to its methoxy-hydroxy-benzothiazole. They found that the fluorescence intensity of the system gradually increases with HClO increasing. However, the microscopic mechanism of this highly efficient fluorescent probe is not well understood. Therefore, in this work, we theoretically investigate the ESIPT mechanism of the HBT-Ome and its product molecule by using density functional theory and time-dependent density functional theory. Based on polarizable continuum model (PCM) with the integral equation formalism variant (IEFPCM) and Becke’s three-parameter hybrid exchange function with the Lee-Yang-Parr gradient-corrected functional (B3LYP) as well as the TZVP basis, the optimized structures are obtained. The structures show that the HBT-Ome product molecules tend to undergo proton transfer in the excited state but HBT-OMe molecules cannot undergo the proton transfer process. The analysis of frontier molecular orbitals not only explains the reason why the fluorescence of the HBT-Ome product is enhanced, but also demonstrates that the HBT-Ome fluorescence intensity is diminished owing to twisted intramolecular charge transfer in the excited state. It is twisted intramolecular charge transfer that leads smaller charge density to be overlapped and the fluorescence intensity of HBT-OMe molecule to be further weakened. Infrared vibrational spectrum shows the enhancement of intramolecular hydrogen bond of O—H, which indicates the tendency of proton transfer. The molecular covalent interaction analysis shows that the intramolecular interactions of HBT-OMe remain largely unchanged clearly. The intramolecular O—H bonding interaction is weakened, and the N—H bonding interaction is increased for HBT-OMe product molecule. The enhancement of intramolecular hydrogen bond of N—H further illustrates the trend of proton transfer. The calculated potential energy curve provides direct evidence for the occurrence of ESIPT in the HBT-Ome product molecule. Our work is of great significance in designing and synthesizing the HClO fluorescent probes based on ESIPT molecules.