Ingeniously regulating the conformational equilibrium and ESPT mechanism of HBT-DPI by solvent environment: A novel perspective.

HBT-DPI was a single-molecule multi-conformational fluorescent material and had unique applications for hydrophobic/hydrophilic mapping on large-scale heterogeneous surfaces. In this paper, the different proton transfer processes and luminescence mechanisms of HBT-DPI in Dichloromethane (DCM, no hydrogen bond (HB) receptor) and N, N-Dimethylformamide (DMF, HB receptor) solvents were systematically studied. Using the quantum chemistry method, the stable structures of HBT-DPI in two solvents were determined based on the Boltzmann distribution. The analysis related to HBs parameters and charge distribution confirmed the HBs strength variation and ICT properties of HBT-DPI after photoexcitation. We confirmed that HBT-DPI underwent the stepwise (A → B → C) excited state double proton transfer (ESDPT) process in DCM. Whereas, HBT-DPI underwent the excited state intramolecular proton transfer (ESIPT) process in DMF, with the weakened intramolecular hydrogen bond (IAHB) after photoexcitation. By simulating the fluorescence spectra in the two solvents, we attributed the fluorescence in the experiment of DCM to the S state radiation. In DMF, the fluorescence of the S and PT states jointly contributed to the fluorescence band at 500 nm observed in the experiment. Furthermore, the hole-electron analysis indicated that the fundamental reason for fluorescence quenching of HBT-DPI in DCM was the charge decoupling effect. Our work provided a comprehensive and systematic explanation of the different proton transfer mechanism and luminescence characteristic for HBT-DPI in two kinds of solvents. On this basis, we furnished the theoretical guidance and reference for its application in hydrophobic/hydrophilic monitoring and measurement.