北京大学赵达慧团队报道了环烷组装二自由基中芳香族堆积介导的自旋-自旋耦合。相关研究成果于2021年10月12日发表于国际顶尖学术期刊《美国化学会杂志》。

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，其特征在于两个自由基部分由[2.2]对环烷（CP）单元连接。通过将indeno单元融合到CP，两个部分堆叠的芴自由基共价连接，无论两个自旋的方向如何，都表现出明显的反铁磁（AFM）耦合。值得注意的是，尽管这两个分子的双自由基指数分别为0.8和0.9，但由于它们的单线态基态，它们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周围的伪单孢位置时，面对面堆叠的苯烯环本质上会产生促进AFM耦合的轨道相互作用。

另一方面，如果两个自由基指向伪对位方向，则在自由基中心（即芴的C9）和侧置的芳香碳之间观察到显著的轨道重叠，从而在两个自旋之间形成AFM耦合。相反，当两个芴自由基通过C9通过一个C–C键系在CP上时，铁磁（FM）耦合表现为具有伪超配位和伪对位连接性的双自由基异构体。由于CP上的自旋分布最小，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二自由基类似物，其中两个自旋主要通过空间相互作用耦合。

这些结果阐明了重要的结论，如尽管通过空间相互作用可能导致FM耦合，但由于PAH自由基极性较低，其强度减弱，面对面堆叠的π-框架倾向于诱导AFM耦合，因为良好的轨道相互作用很容易通过多环芳烃系统实现，这些系统能够采用各种堆叠基序的离域自旋。

附：英文原文

Title: Aromatic Stacking Mediated Spin–Spin Coupling in Cyclophane-Assembled Diradicals

Author: Han Han, Di Zhang, Ziqi Zhu, Rong Wei, Xiao Xiao, Xiaoge Wang, Yiming Liu, Yuguo Ma, Dahui Zhao

Issue&Volume: October 12, 2021

Abstract: 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.

DOI: 10.1021/jacs.1c08262

Source: https://pubs.acs.org/doi/10.1021/jacs.1c08262