Exploring Applications of Covalent Organic Frameworks: Homogeneous Reticulation of Radicals for Dynamic Nuclear Polarization.
Exploring Applications of Covalent Organic Frameworks: Homogeneous Reticulation of Radicals for Dynamic Nuclear Polarization.
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探索共价有机框架的应用：动态核极化的自由基均质网状结构
DOI：
10.1021/jacs.8b02839
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发表时间：
2018-06-06
影响因子：
15
通讯作者：
Wang W
中科院分区：
文献类型：
--
作者：
Cao W;Wang WD;Xu HS;Sergeyev IV;Struppe J;Wang X;Mentink-Vigier F;Gan Z;Xiao MX;Wang LY;Chen GP;Ding SY;Bai S;Wang W
过去十年，共价有机框架（COF）和动态核极化（DNP）领域取得了快速进展。在这篇文章中，我们通过构建自由基嵌入的 COF 作为有前途的 DNP 试剂，将这两个领域联系起来。通过从不成对电子到原子核的极化转移，DNP 实现了 NMR 信号强度的显着增强。 DNP 的关键问题之一是筛选合适的自由基作为有效的极化剂，其基本标准是不成对电子的均匀分布和固定方向。因此，我们设想，COF 的晶体和多孔结构如果均匀地嵌入自由基，可能会成为 DNP 实验的新“晶体海绵”。作为概念证明，我们构建了一系列代理自由基嵌入 COF（表示为 PR(x)-COF）并成功应用它们来实现 DNP 的实质性增强。受益于自下而上和多元合成策略，代理自由基已共价网状、均匀分布，并以可调节浓度（x%）刚性嵌入晶体和介孔框架中。 PR(x)-COF 在 DNP 1H、13C 和 15N 固态 NMR 增强方面表现出优异的性能。该贡献不仅实现了自由基单体直接构建自由基COF，而且探索了COF作为DNP偏光剂的新应用。鉴于许多自由基COF因此可以合理设计并易于构建，具有明确的组成、分布和孔径，我们期望我们的努力将为在DNP NMR实验中利用自由基COF作为标准偏振剂铺平道路。
Rapid progress has been witnessed in the past decade in the fields of covalent organic frameworks (COFs) and dynamic nuclear polarization (DNP). In this contribution, we bridge these two fields by constructing radical-embedded COFs as promising DNP agents. Via polarization transfer from unpaired electrons to nuclei, DNP realizes significant enhancement of NMR signal intensities. One of the crucial issues in DNP is to screen for suitable radicals to act as efficient polarizing agents, the basic criteria for which are homogenous distribution and fixed orientation of unpaired electrons. We therefore envisioned that the crystalline and porous structures of COFs, if evenly embedded with radicals, may work as new “crystalline sponge” for DNP experiments. As a proof of concept, we constructed a series of proxyl radical-embedded COFs (denoted as PR(x)-COFs) and successfully applied them to achieve substantial DNP enhancement. Benefitting from the bottom-up and multivariate synthetic strategies, proxyl radicals have been covalently reticulated, homogenously distributed, and rigidly embedded into the crystalline and mesoporous frameworks with adjustable concentration (x%). Excellent performance of PR(x)-COFs has been observed for DNP 1H, 13C, and 15N solid-state NMR enhancements. This contribution not only realizes the direct construction of radical COFs from radical monomers, but also explores the new application of COFs as DNP polarizing agents. Given that many radical COFs can therefore be rationally designed and facilely constructed with well-defined composition, distribution, and pore size, we expect that our effort will pave the way for utilizing radical COFs as standard polarizing agents in DNP NMR experiments.
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