Study on ESIPT of salicylaldehyde derivative EQCN in DCM solvent.

The fluorescence of salicylaldehyde derivative (EQCN) as an excitation-wavelength-dependent molecule with long-persistent luminescence has been investigated experimentally and theoretically. However, the excited-state intramolecular proton transfer (ESIPT) process mechanism and optical property associated with photochemical process of EQCN molecule in dichloromethane (DCM) solvent has not been discussed in detail. In this work, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were used to investigate ESIPT process of EQCN molecule in DCM solvent. By optimizing the geometry of the EQCN molecule, the hydrogen bond interaction of EQCN molecule of Enol structure in excited state (S1 state) is strengthened. The calculated absorption peak and fluorescence peak agree well with the experimental values. Based on the optimized geometric structure, the frontier molecular orbital isosurface (FMOs) were drawn, and the redistribution of electron density in DCM solvent was depicted, which intuitively explain the changes in the photophysical properties of EQCN. Through the calculated potential energy curves (PECs) of EQCN in both DCM solvent and ethanol solvent, the ESIPT process of EQCN was found more likely to occur in ethanol solvents.