Han Han, Zhang Di, Zhu Ziqi, Wei Rong, Xiao Xiao, Wang Xiaoge, Liu Yiming, Ma Yuguo, Zhao Dahui
Beijing National Laboratory for Molecular Sciences, Center for the Soft Matter Science and Engineering, the Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China.
J Am Chem Soc. 2021 Oct 27;143(42):17690-17700. doi: 10.1021/jacs.1c08262. Epub 2021 Oct 12.
To investigate the capability of π-π stacking motifs to enable spin-spin coupling, we designed and synthesized three pairs of regio-isomers featuring two radical moieties joined by a [2.2]paracyclophane (CP) unit. By fusing indeno units to CP, two partially stacked fluorene radicals are covalently linked, exhibiting evident antiferromagnetic (AFM) coupling regardless of the orientation of two spins. Remarkably, while possessing high diradical indices of 0.8 and 0.9, the two molecules demonstrate good air stability by virtue of their singlet ground state. Single crystals help unravel the structural basis of their AFM coupling behaviors. When two radical centers are arranged at the pseudometa-positions around CP, the face-to-face stacked phenylene rings intrinsically confer orbital interactions that promote AFM coupling. On the other hand, if two radicals are directed in the pseudopara-orientation, significant orbital overlapping is observed between the radical centers (i.e., C9 of fluorene) and the aromatic carbons laid on the side, rendering AFM coupling between the two spins. In contrast, when two fluorene radicals are tethered to CP via C9 through a single C-C bond, ferromagnetic (FM) coupling is manifested by both diradical isomers featuring pseudometa- and pseudopara-connectivity. With minimal spin distributed on CP and thus limited contribution from π-π stacking, their spin-spin coupling properties are more similar to a pair of nitroxide diradical analogues, in which the two spins are dominantly coupled via through-space interactions. From these results, important conclusions are elucidated such as that although through-space interactions may confer FM coupling, with weakened strength shown by PAH radicals due to their lower polarity, face-to-face stacked π-frameworks tend to induce AFM coupling, because favorable orbital interactions are readily achieved by PAH systems hosting delocalized spins that are capable of adopting varied stacking motifs.
为了研究π-π堆积基序实现自旋-自旋耦合的能力，我们设计并合成了三对区域异构体，其具有通过[2.2]对环芳烷（CP）单元连接的两个自由基部分。通过将茚并单元与CP融合，两个部分堆叠的芴自由基共价连接，无论两个自旋的取向如何，都表现出明显的反铁磁（AFM）耦合。值得注意的是，虽然这两个分子具有0.8和0.9的高双自由基指数，但由于它们的单重基态，表现出良好的空气稳定性。单晶有助于揭示其AFM耦合行为的结构基础。当两个自由基中心排列在CP周围的假间位时，面对面堆叠的亚苯基环本质上赋予促进AFM耦合的轨道相互作用。另一方面，如果两个自由基以假对位取向排列，则在自由基中心（即芴的C9）与位于侧面的芳族碳之间观察到明显的轨道重叠，从而使两个自旋之间发生AFM耦合。相比之下，当两个芴自由基通过单个C-C键经由C9连接到CP时，具有假间位和假对位连接性的双自由基异构体均表现出铁磁（FM）耦合。由于CP上分布的自旋最小，因此π-π堆积的贡献有限，它们的自旋-自旋耦合特性更类似于一对氮氧化物双自由基类似物，其中两个自旋主要通过空间相互作用耦合。从这些结果中，可以阐明一些重要的结论，例如，尽管空间相互作用可能导致FM耦合，但由于多环芳烃自由基的极性较低，其强度减弱，面对面堆叠的π框架倾向于诱导AFM耦合，因为具有离域自旋的多环芳烃系统能够采用各种堆叠基序，很容易实现有利的轨道相互作用。
