Polymorphism and mechanochromism of N-alkylated 1,4-dihydropyridine derivatives containing different electron-withdrawing end groups

Organic compounds exhibiting polymorphic and/or mechanochromic (MC) properties are promising for applications in multiple areas. However, the design strategy for such compounds is not very clear. Herein, several series of N-alkylated 1,4-dihydropyridine (DHP) derivatives incorporating different electron-withdrawing end groups were synthesized and compared. The electron-withdrawing groups were responsible for their polymorphic and MC properties. Additionally, a number of polymorphs of these DHP derivatives showed a decreasing trend as length of the alkyl chain increased, indicating that a longer alkyl chain was not conducive to formation of the polymorphs. Although differences in emissions of the polymorphs were mainly attributed to their different intermolecular interactions and molecular packing patterns, a subtle difference in the distances of their intermolecular interactions could also be a key factor in the formation of specific polymorphs. Different polymorphs of DHP derivatives could be interconverted by a simple recrystallization process with a specific solvent or through the application of pressure and vapor stimuli. Additionally, the MC properties of these DHP derivatives were ascribed to a phase transition between different crystalline states, instead of the more common transformation between crystalline and amorphous states.