Photomechanical Molecular Crystals of an Azopyridine Derivative and its Zinc(II) Complex: Synthesis, Crystallization and Photoinduced Motion

In this work, photomechanical molecular crystals of 4-(4-(6-Hydroxyhexyloxy) phenylazo) pyridine (6cazpy) and its zinc(II) organic complex (complex-I) were synthesized and crystallized. DSC and TGA were used to characterize and compare properties of 6cazpy and its complex-I crystals. Photoinduced motions of 6cazpy crystals and its complex-I crystals were investigated and compared by UV/Vis irradiation. Bending away motions from the light source were observed from both 6cazpy crystals and its complex-I crystals. The bending away motion was attributed to the trans-to-cis photoisomerization of azopyridine derivatives in the crystalline phase. It is worth noting that the photomechanical properties of complex-I were enhanced by the formation of the ligand, which might be caused by the looser packing of molecules inside complex-I crystal. In addition, because of the existence of ligand, which combined two photoactive groups in each complex-I molecule, the isomerization reactions of these two photoactive groups in the molecules can increase the photomechanical movement ability of the crystal. It was also found that the crystal size and shape will affect the photoinduced movement of the crystals. PXRD and AFM were used to investigate the molecular mechanism and the surface topological change upon photoisomerization. The corresponding mechanism was proposed.