Effect of stacking mode on the mechanofluorochromic properties of 3-aryl-2-cyano acrylamide derivatives

Three structurally simple 3-aryl-2-cyano acrylamide derivatives, 2-cyano-3-(2-methoxyphenyl)-2-propenamide (1), 2-cyano-3-(3-methoxyphenyl)-2-propenamide (2) and 2-cyano-3-(4-methoxyphenyl)-2-propenamide (3) were synthesized. They exhibited different optical properties due to their distinct face-to-face stacking mode. The as-prepared crystals of 1 exhibited green luminescence and the emission peak did not change after grinding treatment. However, the emission peaks of 2 (Phi(f) = 12%) and 3 (Phi(f) = 16%) exhibited an obvious red-shift upon grinding, and their corresponding quantum yields decreased to 8% and 10%, respectively. Differential scanning calorimetry and powder X-ray diffractometry data indicated that the optical properties of 2 and 3 could be attributed to the transformation from the crystalline phase to the amorphous phase. X-ray crystal structures, infrared spectroscopy and data from fluorescence lifetime experiments further validated the relationship between fluorescence switching, stacking mode and molecular interactions.