Manipulating the ESPT process and photophysical properties of HQCT by water-based hydrogen bond bridge

The 8-hydroxyquinoline-2-carboxaldehyde thiosemicarbazone (HQCT) is sensitive to water molecules in dimethyl sulfoxide according to the experimental phenomenon description, with a sensory performance that is dependent on the water-based hydrogen bond bridge as mediator. Here the excited state intramolecular (ESAPT) and intermolecular (ESEPT) proton transfer mechanism involved by water solvent and the optical properties associated with them are clarified in details. Along the minimum energy pathways in the scanned potential energy surfaces (PESs), we find that the ESEPT reaction induced by water molecule is a synergistic double-proton transfer process. And the results indicate that trace water molecules can promote the reaction effectively. In addition, we also investigate the photophysical properties involved with the excited state proton transfer (ESPT) reaction with the increase of water fraction in the solvent. The results show that the fluorescent oscillator strength (f) and radiative rates (kr) decrease with the increase of water fraction, which are consistent with the experimental phenomenon. We attribute the phenomenon of fluorescence quenching to the increasing transition dipole moment (μ) which accelerates the intramolecular charge transfer (ICT) process. Our work not only explain the water-detection mechanism in the organic solvent before water-sensitive reactions essentially, but also provide a theoretical basis for the synthesis of new Schiff base fluorescent chromophores for the water detection in organic solvents.