The influence of different N-substituted groups on the mechanochromic properties of 1,4-dihydropyridine derivatives with simple structures

A twisted molecular conformation is known to be beneficial to the formation of mechanochromic (MC) phenomenon for organic fluorescent molecules, but it is sometimes not a decisive factor. Up to now, the clear design guidance for MC-active materials based on these twisted molecules is still lacking. Herein, four twisted 1,4-dihydropyridine derivatives (3a-3d) with a simple structure were synthesized to investigate the effect of N-substituted groups on their potential MC activities. Compound 3d containing 2-phenylethyl group presented reversible high-contrast MC properties, while 3a with an ethyl group and 3c with a 1-phenylethyl group were MC-inactive. Moreover, 3b with a benzyl group had two different crystalline forms, interestingly, the blue-emitting 3b-B was MC-active, whereas the green-emitting 3b-G was MC-inactive. The results indicated that N-substituted groups played a decisive role in determining whether or not these compounds had MC activities. Through the careful analyses of their X-ray diffraction and differential scanning calorimetry experiments, it was found that the phase transformation between different crystalline states was responsible for the reversible MC properties of 3b-B and 3d. Furthermore, the red shift of their fluorescence spectra was ascribed to the planarization of molecular conformation and the resultant enhancement of the degree of pi-electron conjugation. This work provides a feasible design direction for the development of efficient MC materials based on twisted fluorescent molecules.