Cocrystallization Enabling Photoinduced Rotation of an Azopyridine Crystal

It is challenging to expand the abundant photoresponse mode of photoactive functional crystals. In this study, a 2:1 cocrystal of (E)-4-((4-(propyloxy)­phenyl)­diazenyl)­pyridine (APO3C) and tetrafluoroterephthalic acid (TFTA) was designed and synthesized to adjust the robustness of APO3C and to realize new photomechanical motion. The thermal stability of APO3C was enhanced by inserting the coformer. More importantly, photoinduced rotation was achieved under the irradiation of UV light, which was unreported before. A molecular and structural analysis of crystals revealed that the photoinduced rotation can be attributed to three indispensable factors: the linear synthon induced by hydrogen bonding, unsymmetrical isomerization of the APO3C molecules, and their diagonal arrangement. The opposite photoisomerization of APO3C molecules at both ends of a synthon could create torque inside with its diagonal arrangement in three dimensions, producing the unevenness and finally driving the crystal to rotate. The results presented in this work help to enrich the strategy for designing new crystals with novel photoactive functions and expand the diversity of photomechanical molecular crystals through crystal engineering.