Spirobifluorene Mediating the Spin-Spin Coupling of Nitronyl Nitroxide Diradicals.
Spirobifluorene Mediating the Spin-Spin Coupling of Nitronyl Nitroxide Diradicals.
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DOI：
10.1021/acs.jpca.2c06648
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发表时间：
2023-01
期刊：
影响因子：
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通讯作者：
Zheng Yue;Jin Liu;M. Baumgarten;Di Wang
中科院分区：
文献类型：
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作者：
Zheng Yue;Jin Liu;M. Baumgarten;Di Wang
为了研究这种螺旋共轭磁性行为的机理，我们设计并合成了三个双自由基22s bf - nn、44s bf - nn和27SBF-NN。它们由螺芴和硝基氮氧化物（NN）二自由基作为自旋中心桥接。值得注意的是，通过SQUID和电子顺磁共振（EPR）零场分裂数据分析，22sbf - nn和27SBF-NN双基表现出分子内明显的反铁磁（AF）耦合，2J(22sbf - nn)/kB = -5.86 K, 2J(27SBF-NN)/kB = -24.6 K。22s bf - nn的AF与基于Hund规则的自旋密度交替规则预测的AF相反。讨论了由螺碳偶联桥接的双自由基分子内共轭偶联，其中22sbf - nn小于27SBF-NN，符合室温EPR表征。这种螺旋共轭比传统的平面共轭弱，通常导致螺旋双基分子中的自旋-自旋耦合较弱。44s bf - nn双自由基的EPR谱显示一条变形的9线曲线，表明分子内交换偶联。密度泛函理论计算得到一个非常弱的耦合常数为2Jcalc/kB = 0.06 K，以铁磁相互作用为证明，这与自旋极化预测一致。进一步的磁化率χm和VT-EPR数据分析表明，在44′位置的双自由基中确实存在极弱的FM相互作用。我们发现桥是一个44'取代的SBF结构，它阻挡了共轭作用，并且在位阻中含有较大的扭转，这也阻碍了足够的自旋密度离域，导致自旋耦合相互作用弱得多。结合分子轨道计算结果的分析，进一步解释了22s bf - nn的分子内异常AF耦合机理。
To investigate the mechanism of this spiro conjugation magnetic behavior, we designed and synthesized three diradicals─22'SBF-NN, 44'SBF-NN, and 27SBF-NN. They are bridged by spirobifluorene and nitronyl nitroxide (NN) diradicals as the spin centers. Notably, by SQUID and electron paramagnetic resonance (EPR) zero-field splitting data analyses, the 22'SBF-NN and 27SBF-NN diradicals exhibit intramolecular, distinctly antiferromagnetic (AF) coupling, with 2J(22'SBF-NN)/kB = -5.86 K and 2J(27SBF-NN)/kB = -24.6 K, respectively. The AF of 22'SBF-NN is opposite to that predicted by the spin density alternation rule based on Hund's rule. Diradical intramolecular conjugation coupling bridged by spiro-carbon conjugation is discussed, in which the 22'SBF-NN is smaller than that of 27SBF-NN, corresponding to the room-temperature EPR characterization. This spiro conjugation is weaker than the traditional planar conjugation and generally leads to a weaker spin-spin coupling in the helical biradical molecule. The EPR spectrum of the 44'SBF-NN diradical shows a deformed nine-line curve, indicating intramolecular exchange coupling. The density functional theory calculation gives a very weak coupling constant of 2Jcalc/kB = 0.06 K, with ferromagnetic (FM) interaction as the proof, which is consistent with the spin-polarized prediction. Further analysis of magnetic susceptibility χm and VT-EPR data shows that there is indeed an extremely weak FM interaction in the 44' position diradical. We find the bridge, which is a 44' substituted SBF structure, blocks the conjugation and contains a larger twist in steric hindrance, which also hampers sufficient spin density delocalization, resulting in a much weaker spin coupling interaction. Combined with the analysis of molecular orbital calculation results, the anomalous intramolecular AF coupling mechanism of 22'SBF-NN is further explained.