Ring aromaticity and ESIPT behavior of 4-dimethylamino-2′-hydroxychalcone based on nitrogen substitution: A TD-DFT insight

4-dimethylamino-2'-hydroxychalcone (HCA) exhibits excited state intramolecular proton transfer (ESIPT) properties and strong fluorescence in the solid state, making its highly attractive for application in organic optoelectronics. In order to investigate the effect of nitrogen atom substitution on the fluorescent properties and ESIPT process of HCA compound, three HCA derivatives (HCA-1, HCA-2 and HCA-3) were designed by replacing the -CH groups at different positions on the benzene ring of HCA with nitrogen atoms. Using density function theory (DFT) and time-dependent DFT (TD-DFT) methods, the ESIPT processes, fluorescent properties and aromaticity of HCA and its derivatives were studied in detail. The calculation level of B3LYP/6-311+G(d,p) has been proven to be reliable since the absorption/fluorescence wavelength of HCA obtained in the experiment has been reproduced. The results showed that nitrogen substitution led to a blue-shift in the maximum absorption peak, a significant red-shift in the maximum fluorescence peak of tautomer, and an obvious increase in the Stokes shift generated by its tautomer. Nitrogen substitution also reduces the ESIPT barrier and aromaticity. Di-nitrogen substitution is more effective than mono-nitrogen substitution. • Photo-induced hydrogen bonding enhancement will promote the ESIPT process. • Nitrogen substitution significantly red-shifts the long-wavelength fluorescence, and increases the corresponding Stokes shift. • Both aromaticity and ESIPT barrier of HCA are reduced by nitrogen substitution. • Di-nitrogen substitution has more effect on HCA than mono-nitrogen substitution.