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

Abstract

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.

Light‐Switchable Metal Complexes: Introducing Photoresponsive Behaviour Through Azoheteroarenes

“…Only a few azobenzene‐based metal complexes are known for their light‐induced photomechanical changes [80,81] . Among the azoheteroarene‐based metal complexes, molecular crystals of 4‐(4‐(6‐Hydroxyhexyloxy) phenylazo) pyridine and its zinc(II) complex 42 (Figure 11) reported by Hao and co‐workers showed photoinduced motion upon irradiation [82] . They demonstrated the bending of needle‐like crystals of the complex away from the light source upon UV irradiation that returned immediately to their native states after stopping the irradiation.…”

“…[80,81] Among the azoheteroarenebased metal complexes, molecular crystals of 4-(4-(6-Hydroxyhexyloxy) phenylazo) pyridine and its zinc(II) complex 42 (Figure 11) reported by Hao and co-workers showed photoinduced motion upon irradiation. [82] They demonstrated the bending of needle-like crystals of the complex away from the light source upon UV irradiation that returned immediately to their native states after stopping the irradiation. Since the metal-based photoswitchable complexes are rarely studied, there are plenty of scope for achieving this in actuator applications with additional metal-related properties.…”

“…Photomechanical motion in molecular crystals of Zn-phenylazopyridine complex 42. [82] Figure 12. Representative examples of azoheteroarene-based metal complexes exhibiting metallogels (43 and 44) [84,88] and metallomesogen (45).…”

Recent Progress in Azopyridine-Containing Supramolecular Assembly: From Photoresponsive Liquid Crystals to Light-Driven Devices

“…The obtained crystals exhibited a fascinating photomechanical bending motion, which was related to the trans-to-cis photoisomerization of the AzPy derivatives in the crystalline phase. After forming the metal-AzPy complex, this motion was enhanced because of the looser packing of the molecules inside the complex crystal [112].…”

Cocrystallization Enabling Photoinduced Rotation of an Azopyridine Crystal

“…The representative of such reactions is the E – Z photoisomerization of azobenzene and its derivatives, which is one of the first systems found to be capable of photomechanical effects in the crystalline state . Although the combinations of azo compounds with liquid crystals, metal–organic frames, and other systems have been widely researched and some interesting photoresponsive behaviors have been found determined, − most studies of molecular crystals have been focused on a single component. The influence of multiconstituent organic molecular solid systems on the photoresponsive properties needs to be further explored.…”