Photoreactive Zn(II) Coordination Compounds: Exploring Biomimetic Mechanical Motion and Photosalient Behavior

Locomotion plays a pivotal role in the survival of most organisms, enabling essential activities such as foraging, predator evasion, and reproduction. In the realm of biomimetics, seedpod explosion and bark peeling, well-established biological mechanisms employed by various plant species for defense and reproduction, offer a fascinating avenue for exploration. In this study, we present six novel photoreactive Zn(II)-based coordination compounds capable of significant mechanical motion, including explosion and a peeling effect under UV light irradiation. These compounds were synthesized using aryl derivatives of 4-vinylpyridines, namely, 4spy (4-styrylpyridine), 3tpy (4-(3-(thiophene-3-yl)vinyl)pyridine), and 2tpy (4-(2-(thiophene-2-yl)vinyl)pyridine), in conjunction with chloride or bromide colinkers. The resulting complexes, [ZnCl2(4spy)2] (1), [ZnCl2(3tpy)2] (2), [ZnCl2(2tpy)2] (3), [ZnBr2(4spy)2] (4), [ZnBr2(3tpy)2] (5), and [ZnBr2(2tpy)2] (6), were characterized as isostructural, with slight variations observed in compound 6's structural packing. X-ray diffraction analysis confirmed the tetrahedral geometry of Zn(II) in all six complexes. Notably, compounds 1–5 exhibited coordination involving both planar and nonplanar linkers, leading to an expected 50% photoreaction. Interestingly, despite not meeting Schmidt's criteria, the nonplanar linkers also exhibited photoreaction at slower rates. Furthermore, alongside the UV-induced photoreaction, these compounds displayed intriguing and vigorous mechanical motion reminiscent of a photosalient effect, characterized by rolling, cracking, jumping, and fragmentation. In contrast, compound 6 demonstrated complete photoreaction due to both coordinated linkers adopting planar configurations. Additionally, these crystals exhibited a peeling effect under UV irradiation, akin to the natural peeling of tree bark due to aging. These findings highlight the potential of Zn(II)-based coordination compounds as promising candidates for developing metal-based photoactuators and optical switches, with biomimetic applications.