Push-pulling induces the excited-state intramolecular proton transfer in 2′-aminochalcones

Abstract

Usually, chalcones are not luminescent, although their derivatives with push-pull patterns of substitution may present fair quantum yields. Intramolecular hydrogen bonding between a hydrogen donor (–OH or –NH) and the chalcone carbonyl promotes Excited State Intramolecular Proton Transfer (ESIPT). If the dye is fluorescent, the emission can derive from one of the excited tautomers, or from both and in this case a dual emission is observed. Here, 2′-aminochalcones with a strong push-pull character, incorporating electron-donating groups on one side, and electron-withdrawing groups on the amine, present dual emission ascribed to ESIPT between the keto-enamine and the enol-imine tautomers. The hydrogen bonds strength was studied by NMR and DFT calculations. A correlation between the tautomerization energy gap and the hydrogen bond strength was observed, depending on the electron donating or electron withdrawing group on the amine. The electron withdrawing groups bonded to the proton donor have little influence on the quantum yield and emission wavelength. In contrast, the electron donating groups connected to the proton acceptor have a significant effect on the quantum yield and induce an absorption and emission red shift