Chen Xiaofeng, Xie Haomiao, Lorenzo Emmaline R, Zeman Charles J, Qi Yue, Syed Zoha H, Stone Aaron E B S, Wang Yao, Goswami Subhadip, Li Peng, Islamoglu Timur, Weiss Emily A, Hupp Joseph T, Schatz George C, Wasielewski Michael R, Farha Omar K
School of Environmental and Geographical Sciences, Shanghai Normal University, Shanghai 200234, China.
Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States.
J Am Chem Soc. 2022 Feb 16;144(6):2685-2693. doi: 10.1021/jacs.1c11417. Epub 2022 Feb 7.
Owing to their switchable spin states and dynamic electronic character, organic-based radical species have been invoked in phenomena unique to a variety of fields. When incorporated in solid state materials, generation of organic radicals proves challenging due to aggregation. Metal-organic frameworks (MOFs) are promising candidates for immobilization and stabilization of organic radicals because of the tunable spatial arrangement of organic linkers and metal nodes, which sequesters the reactive species. Herein, a flexible, redox-active tetracarboxylic acid linker bearing two imidazole units was chosen to construct a new Zr-MOF, , with topology. By exploiting the structural flexibility of , we successfully modulate the dynamics between an isolated organic radical species and an organic radical π-dimer species in the MOF system. Single-crystal X-ray diffraction analysis reveals that through solvent exchange from ,-diethylformamide to acetone, undergoes a structural contraction with interlinker distances decreasing from 8.32 Å to 3.20 Å at 100 K. Organic radical species on the bridging linkers are generated via UV light irradiation. Direct observation of temperature-induced spin switches from an isolated radical species to a magnetically silent radical π-dimer in after irradiation in the solid state was achieved via variable-temperature single-crystal X-ray diffraction and variable-temperature electron paramagnetic resonance spectroscopy. Ultraviolet-visible-near infrared spectroscopy and density functional theory calculations further substantiated the formation of a radical cation π-dimer upon irradiation. This work demonstrates the potential of using flexible MOFs as a platform to modulate radical spin states in the solid phase.
由于其可切换的自旋态和动态电子特性，有机基自由基物种已被应用于各种领域特有的现象中。当掺入固态材料时，由于聚集，有机自由基的生成具有挑战性。金属有机框架（MOF）因其有机连接体和金属节点的可调空间排列而有望成为固定和稳定有机自由基的候选材料，这种排列可以隔离反应性物种。在此，选择一种带有两个咪唑单元的柔性、氧化还原活性四羧酸连接体来构建一种具有 拓扑结构的新型Zr-MOF。通过利用 的结构灵活性，我们成功地调节了MOF系统中孤立有机自由基物种和有机自由基π-二聚体物种之间的动力学。单晶X射线衍射分析表明，通过从 ，-二乙基甲酰胺到丙酮的溶剂交换， 在100 K下经历结构收缩，连接体间距离从8.32 Å减小到3.20 Å。桥连连接体上的有机自由基物种通过紫外光照射产生。通过变温单晶X射线衍射和变温电子顺磁共振光谱，实现了对固态照射后 中温度诱导的自旋从孤立自由基物种切换到磁性沉默自由基π-二聚体的直接观察。紫外可见近红外光谱和密度泛函理论计算进一步证实了照射后自由基阳离子π-二聚体的形成。这项工作展示了使用柔性MOF作为平台在固相中调节自由基自旋态的潜力。
