Three-dimensional architectures constructed via coordination of metal ions to organic linkers (broadly termed as metal-organic frameworks, MOFs), are highly interesting for many demanding applications such as gas adsorption, molecular separation, heterogeneous catalysis, molecular sensing etc. Being constructed from heterogeneous components, such framework solids show characteristic features from both of the individual components as well as framework-specific features. One such interesting physicochemical property is thermal expansion, which arises from thermal vibration from the organic linker and metal ions. Herein, we show a very unique example of thermal responsiveness for DUT-49 framework, a MOF well-known for its distinctive negative gas adsorption (NGA) property. In the guest-free form, the framework shows another counter-intuitive phenomenon of negative thermal expansion (NTE), i.e. lattice size increase with decrease of temperature. However, in the solvated state, it shows both NTE and positive thermal expansion (i.e. lattice size decreases with lowering of temperature, PTE) based on a specific temperature range. When the solvent exists in liquid form inside the MOF pore, it retains the pristine NTE nature of the bare framework. But freezing of the solvent inside the pores induces a strain, which causes a structural transformation through in-plane bending of the linker and this squeezes the framework by ~10 % of the unit cell volume. This effect has been verified using 3 different solvents where the structural contraction occurs immediately at the freezing point of individual solvent. Furthermore, studies on a series of DUT-49(M) frameworks with varying metal confirm the general applicability of this mechanism.


中文翻译：

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金属有机框架DUT-49中正负热膨胀之间的可逆切换

通过金属离子与有机连接基的配位（普遍称为金属有机骨架，MOF）构建的三维体系结构对于许多要求苛刻的应用（例如气体吸附，分子分离，多相催化，分子传感等）非常有趣。这样的框架实体是异质组件，它们既显示单个组件的特征，又显示特定于框架的特征。这种有趣的物理化学性质之一就是热膨胀，它是由有机连接基和金属离子的热振动引起的。在此，我们展示了DUT-49框架的热响应性的非常独特的示例，DUT-49框架是MOF，以其独特的负气体吸附（NGA）特性而闻名。以无访客的形式，框架显示了另一个与负热膨胀（NTE）相反的直觉现象，即晶格尺寸随温度降低而增加。但是，在溶剂化状态下，它会同时显示NTE和正热膨胀（即，晶格尺寸随温度PTE的降低而减小）。当溶剂以液态形式存在于MOF孔中时，它保留了裸框架的原始NTE性质。但是溶剂在孔内的冻结会引起应变，该应变通过接头的面内弯曲引起结构转变，这将骨架压缩了约10％的单位细胞体积。使用3种不同的溶剂已验证了这种效果，其中的结构收缩会在单个溶剂的凝固点立即发生。此外，