Crystal Form Diversity of 2-(4-(Diphenylamino)benzylidene) Malononitrile

In the present work, we report the synthesis and characterization of 2-(4-(diphenylamino)benzylidene) malononitrile (DPAM) via a piperidine-catalyzed Knoevenagel condensation reaction. Two distinct crystal forms (A-1 and A-2) of this product were obtained by controlling the crystallization conditions, exhibiting orthorhombic and monoclinic crystal systems, respectively. Single-crystal X-ray diffraction revealed that both forms exhibited highly twisted benzene rings, which suppressed exciplex or excimer formation, enhancing luminescence. Crystal A-1, with a higher density, showed stronger hydrogen bonding and more rigid molecular packing, while A-2, with a lower density, exhibited weaker π–π interactions. Both crystals demonstrated high thermal stability. Notably, the A-2 crystal displayed a mechanochromic behavior: grinding or applying pressure induced a structural transformation into A-1, accompanied by a fluorescence shift from red to yellow. This transformation was attributed to increased steric hindrance and changes in molecular packing. This study highlights the relationship between crystal structure and optoelectronic properties, offering insights into the design of organic crystalline materials for applications in pressure sensing, anti-counterfeiting, and information encryption.