Reversible Piezochromism and Protonation Stimuli-Response of (Z)-2-Cyano-3-(3,4-dimethoxyphenyl)acrylamide

A 3-aryl-2-cyano acrylamide derivative (Z)-2-cyano-3-(3,4-dimethoxy-phenyl)acrylamide (CDMPA) was designed and synthesized, which exhibited piezochromism and acidchromism properties. Under external mechanical force stimuli, CDMPA showed a red-shift of 20 nm in its fluorescence emission and the mechanically induced luminescence color could return to the original state via heating or solvent vapor treatment. Powder X-ray diffraction (XRD) and fluorescence lifetime experiments indicated that the piezochromic luminescence could be attributed to the transformation from the crystalline to the amorphous phase. Additionally, the fluorescence color changed from blue to yellow with a red-shift of 33 nm using the stimulus of protonation. The emission color was recovered upon fuming with dimethyl formamide (DMF) vapor. Infrared (IR) spectra of CDMPA powder and theoretical calculation of the frontier molecular orbitals showed that protonation of the amino moieties in CDMPA had a significant effect on the frontier molecular orbitals and, thus, caused the acidchromism phenomenon. This study provides comprehensive insight into the stimuli-responsive luminescent mechanisms within this type of compound and the reversible switching of emission color may enable discovery of novel applications of CDMPA for detection and sensing devices.