Guest Inclusion Modulates Concentration and Persistence of Photogenerated Radicals in Assembled Triphenylamine Macrocycles
Guest Inclusion Modulates Concentration and Persistence of Photogenerated Radicals in Assembled Triphenylamine Macrocycles
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DOI：
10.1021/jacs.9b11518
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发表时间：
2020-01-08
影响因子：
15
通讯作者：
Shimizu, Linda S.
中科院分区：
文献类型：
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作者：
Sindt, Ammon J.;DeHaven, Baillie A.;Shimizu, Linda S.
取代三苯胺（TPA）自由基阳离子在聚合物磁体中作为氧化剂和含自旋单元具有很大的潜力。它们的性质可以通过超分子组装进一步调整。在这里，我们研究了TPA大环固有的光生自由基阳离子的性质如何在它们自组装成一维柱时发生改变。这些大环由两个tpa和两个亚甲基脲组成，它们驱动组装成多孔有机材料。有利的是，在激活后，晶体可以在单晶到单晶的转变中进行客体交换，产生一系列的等骨架主客体配合物，其性质可以直接比较。利用365 nm发光二极管引发的光致电子转移，通过电子顺磁共振（EPR）光谱观察到，在室温下产生自由基。包覆客体的极性和重原子包裹可以调节EPR谱线的形状和自由基的数量。这些光产生的自由基是持久的，半衰期在1到7天之间，并且在自由基衰变时不表现出降解。样品的再辐照可使自由基浓度恢复到相似的最大浓度，这一特征可在几个循环中重现。典型光谱的EPR模拟显示两种，一种包含两个N超精细相互作用，另一种包含无可分辨超精细相互作用的附加宽信号。有趣的是，没有溴取代的TPA类似物也表现出类似数量的光生成自由基，这表明超分子策略可以使稳定的TPA自由基结构具有更大的灵活性。这些研究将有助于指导新型光活性材料的开发。
Substituted triphenylamine (TPA) radical cations show great potential as oxidants and as spin-containing units in polymer magnets. Their properties can be further tuned by supramolecular assembly. Here, we examine how the properties of photogenerated radical cations, intrinsic to TPA macrocycles, are altered upon their self-assembly into one-dimensional columns. These macrocycles consist of two TPAs and two methylene ureas, which drive the assembly into porous organic materials. Advantageously, upon activation the crystals can undergo guest exchange in a single-crystal-to-single-crystal transformation generating a series of isoskeletal host-guest complexes whose properties can be directly compared. Photoinduced electron transfer, initiated using 365 nm light-emitting diodes, affords radicals at room temperature as observed by electron paramagnetic resonance (EPR) spectroscopy. The line shape of the EPR spectra and the quantity of radicals can be modulated by both polarity and heavy atom inclusion of the encapsulated guest. These photogenerated radicals are persistent, with half-lives between 1 and 7 d and display no degradation upon radical decay. Re-irradiation of the samples can restore the radical concentration back to a similar maximum concentration, a feature that is reproducible over several cycles. EPR simulations of a representative spectrum indicate two species, one containing two N hyperfine interactions and an additional broad signal with no resolvable hyperfine interaction. Intriguingly, TPA analogues without bromine substitution also exhibit similar quantities of photogenerated radicals, suggesting that supramolecular strategies can enable more flexibility in stable TPA radical structures. These studies will help guide the development of new photoactive materials.