Photoreactive Zn(II)\nCoordination Compounds: Exploring\nBiomimetic Mechanical Motion and Photosalient Behavior

Abstract

Locomotion plays a pivotal role in the survival of most\norganisms,\nenabling essential activities such as foraging, predator evasion,\nand reproduction. In the realm of biomimetics, seedpod explosion and\nbark peeling, well-established biological mechanisms employed by various\nplant species for defense and reproduction, offer a fascinating avenue\nfor exploration. In this study, we present six novel photoreactive\nZn(II)-based coordination compounds capable of significant mechanical\nmotion, including explosion and a peeling effect under UV light irradiation.\nThese compounds were synthesized using aryl derivatives of 4-vinylpyridines,\nnamely, 4spy (4-styrylpyridine), 3tpy (4-(3-(thiophene-3-yl)vinyl)pyridine),\nand 2tpy (4-(2-(thiophene-2-yl)vinyl)pyridine), in conjunction with\nchloride or bromide colinkers. The resulting complexes, [ZnCl₂(4spy)₂] (<b>1</b>), [ZnCl₂(3tpy)₂] (<b>2</b>), [ZnCl₂(2tpy)₂] (<b>3</b>), [ZnBr₂(4spy)₂] (<b>4</b>),\n[ZnBr₂(3tpy)₂] (<b>5</b>), and [ZnBr₂(2tpy)₂] (<b>6</b>), were characterized as\nisostructural, with slight variations observed in compound <b>6</b>’s structural packing. X-ray diffraction analysis confirmed\nthe tetrahedral geometry of Zn(II) in all six complexes. Notably,\ncompounds <b>1</b>–<b>5</b> exhibited coordination\ninvolving both planar and nonplanar linkers, leading to an expected\n50% photoreaction. Interestingly, despite not meeting Schmidt’s\ncriteria, the nonplanar linkers also exhibited photoreaction at slower\nrates. Furthermore, alongside the UV-induced photoreaction, these\ncompounds displayed intriguing and vigorous mechanical motion reminiscent\nof a photosalient effect, characterized by rolling, cracking, jumping,\nand fragmentation. In contrast, compound <b>6</b> demonstrated\ncomplete photoreaction due to both coordinated linkers adopting planar\nconfigurations. Additionally, these crystals exhibited a peeling effect\nunder UV irradiation, akin to the natural peeling of tree bark due\nto aging. These findings highlight the potential of Zn(II)-based coordination\ncompounds as promising candidates for developing metal-based photoactuators\nand optical switches, with biomimetic applications.