The synthesis of stable organic polyradicals is important for the development of magnetic materials. Herein we report the synthesis, isolation, and characterization of a series of X-shaped pyromellitimide (PI) oligomers (Xn-R, n = 2-4, R = Hex or Ph) linked together by single C-C bonds between their benzenoid cores. We hypothesize that these oligomers might form high-spin states in their reduced forms because of the nearly orthogonal conformations adopted by their PI units. 1H and 13C nuclear magnetic resonance (NMR) spectroscopies confirmed the isolation of the dimeric, trimeric, and tetrameric homologues. X-ray crystallography shows that X2-Ph crystallizes into a densely packed superstructure, despite the criss-crossed conformations adopted by the molecules. Electrochemical experiments, carried out on the oligomers Xn-Hex, reveal that the reductions of the PI units occur at multiple distinct potentials, highlighting the weak electronic coupling between the adjacent redox centers. Finally, the chemically generated radical anion and polyanion states, Xn-Hex•- and Xn-Hexn(•-), respectively, were probed extensively by UV-vis-NIR absorption, EPR, and electron nuclear double resonance (ENDOR) spectroscopies. The ENDOR spectra of the radical monoanions Xn-Hex•- reveal that the unpaired electron is largely localized on a single PI unit. Further reductions of Xn-Hex•- yield EPR signals (in frozen solutions) that can be assigned to spin-spin interactions in X2-Hex2(•-), X3-Hex3(•-), and X4-Hex4(•-). Taken together, these findings demonstrate that directly linking the benzene rings of PIs with a single C-C bond is a viable method for generating stabilized high-spin organic anionic polyradicals.

中文翻译：

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X形低聚均苯四酰亚胺多自由基

稳定的有机多自由基的合成对于磁性材料的开发具有重要意义。在本文中，我们报告了一系列 X 形均苯四甲酰亚胺 (PI) 低聚物（Xn-R，n = 2-4，R = Hex 或 Ph）的合成、分离和表征，这些低聚物通过其苯型核之间的单个 CC 键连接在一起。我们假设这些低聚物可能以其还原形式形成高自旋状态，因为它们的 PI 单元采用了几乎正交的构象。1H 和 13C 核磁共振 (NMR) 光谱证实了二聚体、三聚体和四聚体同系物的分离。X 射线晶体学表明，尽管分子采用了纵横交错的构象，但 X2-Ph 结晶成致密的超结构。在低聚物 Xn-Hex 上进行的电化学实验，揭示 PI 单元的减少发生在多个不同的电位上，突出了相邻氧化还原中心之间的弱电子耦合。最后，化学生成的自由基阴离子和聚阴离子状态，分别是 Xn-Hex•- 和 Xn-Hexn(•-)，分别通过 UV-vis-NIR 吸收、EPR 和电子核双共振 (ENDOR) 光谱进行了广泛的探测。自由基单阴离子 Xn-Hex•- 的 ENDOR 谱表明未配对电子主要位于单个 PI 单元上。进一步减少 Xn-Hex•- 产生 EPR 信号（在冷冻溶液中），这些信号可以分配给 X2-Hex2(•-)、X3-Hex3(•-) 和 X4-Hex4(•-) 中的自旋-自旋相互作用. 综合起来，