Fluorescence probes detecting O2•_ based on intramolecular charge transfer and excited-state intramolecular proton transfer mechanisms

Abstract

Detection of O2•_ ions in living organisms plays an important role in the prevention of cardiomyopathy, autism, diabetes, cancer and neurodegenerative diseases. Recently, Wu et al. have successfully synthesized a fluorescence probe 2-(Benzo [d] thiazol-2-yl)-6-methoxyphenyltrifluoromethanesulfonate (HMBT-LW) based on excited intramolecular proton transfer (ESIPT) mechanism [L. Wu, et al. New J. Chem., 2019, 43, 2875–2877]. In experiment, HMBT-LW reacted with O2•_ ions in solution to generate the molecule 2-(Benzo [d] thiazol-2-yl) -6-methoxyphenol (HMBT) with a hydrogen bonding structure. Because only the single fluorescence phenomenon was detected during the experiment, we could not determine whether the fluorescence was emitted by HMBT or its isomer structure. We reported the theoretical mechanism of photochemistry and photophysics in the whole reaction process at the first time. The calculated hydrogen bonding parameters indicated that the intramolecular hydrogen bonding interaction was enhanced in the excited state. The analysis of the frontier molecular orbitals (FMOs) and Mulliken charge illustrated that the intramolecular charges would be redistributed during photoexcitation process, which fundamentally explained the enhanced hydrogen bonding provided the driving force for the ESIPT processes. Infrared spectra (IR) showed that the strength of the hydrogen bond interaction would be changed in different electronic states. Afterwards, the calculations of molecular potential energy curves quantitatively explained the feasibility of the ESIPT reactions. The calculated spectral values were consistent with the absorption and emission energies in the experiment, proving the scientificity of our calculation method. The isosurfaces of reduced density gradient (RDG) was used to visually distinguish the complex non-covalent bond interactions and showed us the mechanism of excited state proton transfer intuitively. Combining the above data, we finally came to a conclusion that the single fluorescence observed in the experiment was emitted by the isomer structure of HMBT.