The Nitrogen Substitution Effect on ESIPT Process and ICT Characteristic as well as Aromaticity of Novel Chalcone-Based Fluorophore: A TD-DFT Study.

Recently, the fluorescent probe (E)-3-(4-(di(p-toluylamino)phenyl)-1-(2-hydroxyphenyl)prop-2-en-1-one (TAPHP) was synthesized for hydrazine detection in living cells (Sensors and Actuators B 263 (2018) 229). However, the excited-state intramolecular proton transfer (ESIPT) mechanism of TAPHP has not been experimentally elucidated. In this work, we employ density functional theory (DFT) and time-dependent DFT (TD-DFT) methods to investigate the ESIPT process, electronic spectra, and ring aromaticity of TAPHP in detail. Furthermore, four derivatives (TAPHP-1, TAPHP-2, TAPHP-3, TAPHP-4) were designed by substituting carbon atoms with nitrogen at various positions on the benzene ring to explore the effect of nitrogen substitution on TAPHP's properties. The calculated electronic spectra show good agreement with experimental data, validating the computational approach. Our results reveal that photoexcitation strengthens intramolecular hydrogen bonds, promoting the ESIPT process. Nitrogen substitution causes red-shifts in absorption and fluorescence wavelengths, modifies the ESIPT energy barrier, and reduces both intramolecular charge transfer (ICT) and aromaticity in TAPHP. These findings provide deeper insight into the structure-property relationships governing ESIPT processes and may guide the design of improved fluorescent probes.