Shanghai Key Laboratory of Green Chemistry and Chemical Processes, College of Chemistry and Molecular Engineering, East China Normal University , Shanghai 200062, China.
ACS Appl Mater Interfaces. 2017 Feb 15;9(6):5503-5512. doi: 10.1021/acsami.6b15540. Epub 2017 Feb 6.
An electron-deficient and potentially chromic ligand has been utilized to impart redox activity, photo- and hydrochromism, and solvotomagnetism to metal-organic frameworks (MOFs). A pair of MOFs were constructed from the flexible zwitterionic viologen-tethered tetracarboxylate linker N,N'-bis(3,5-dicarboxylatobenzyl)-4,4'-bipyridinium (L): [Co(L)(N)] (1) and [Mn(L)(N)(HO)]·3HO (2). Both compounds show three-dimensional frameworks in which mixed azido- and carboxylato-bridged chains are connected through the electron-deficient viologen moieties. The chain in 1 is built from alternating bis(azide) and (azide)bis(carboxylate) bridges, while that in 2 contains uniform (azide) (carboxylate) bridges. The MOFs shows the characteristic redox properties of the viologen moieties. The redox activity affords the MOFs with different chromic properties, owing to subtle differences in chemical environments. 1 shows reversible photochromism, which is related to the radical formation through photoinduced electron transfer from azide-carboxylate to viologen according to UV-vis, X-ray photoelectron, and electron spin resonance spectroscopy and DFT calculations. 2 is nonphotochromic for lack of appropriate pathways for electron transfer. Unexpectedly, 2 shows a novel type of solid-state hydrochromism. Upon the removal and reabsorption of water, the compound shows remarkable color change because of reversible electron transfer accompanying a reversible structural transformation. The radical mechanism is distinct from those for traditional hydrochromic inorganic and organic materials. Magnetic studies indicate ferro- and antiferromagnetic coupling in 1 and 2, respectively. What's more, 2 shows marked magnetic response to the removal of water molecules owing to the formation of radicals. The compound illustrates a unique material exhibiting dual responses (color and magnetism) to water.
一种缺电子且潜在的变色配体被用于赋予金属-有机骨架（MOF）氧化还原活性、光致变色、水致变色和溶剂化磁体性质。一对 MOF 由柔性两性离子联吡啶连接的四羧酸酯配体 N,N'-双（3,5-二羧基苄基）-4,4'-联吡啶（L）：[Co（L）（N）]（1）和[Mn（L）（N）（HO）]·3HO（2）构建而成。这两个化合物都具有三维骨架，其中混合叠氮化物和羧酸酯桥接链通过缺电子联吡啶部分连接。1 中的链由交替的双（叠氮化物）和（叠氮化物）双（羧酸酯）桥组成，而 2 中的链则含有均匀的（叠氮化物）（羧酸酯）桥。MOF 表现出联吡啶部分的特征氧化还原性质。氧化还原活性赋予 MOF 不同的变色性质，这归因于化学环境的细微差异。1 表现出可逆光致变色性，这与自由基形成有关，自由基是通过光诱导电子从叠氮化物-羧酸酯向联吡啶转移产生的，根据紫外可见光谱、X 射线光电子能谱和电子自旋共振光谱以及 DFT 计算得出。2 由于缺乏合适的电子转移途径，因此不具有光致变色性。出乎意料的是，2 表现出一种新型的固态水致变色性。在去除和再吸收水时，由于伴随可逆结构转变的可逆电子转移，化合物表现出显著的颜色变化。自由基机制与传统的无机和有机水致变色材料的机制不同。磁性研究表明，1 和 2 分别表现出铁磁和反铁磁耦合。更重要的是，2 对水分子的去除表现出明显的磁响应，这是由于自由基的形成。该化合物说明了一种独特的材料，其对水表现出双重响应（颜色和磁性）。
