Tunable Anisotropic Thermal Expansion of a Porous Zinc(II) Metal–Organic Framework

A novel three-dimensional metal–organic framework (MOF) that displays anisotropic thermal expansion has been prepared and characterized by single-crystal X-ray diffraction (SCD) and thermal analysis. The as-prepared MOF has one-dimensional channels containing guest molecules that can be removed and/or exchanged for other guest molecules in a single-crystal to single-crystal fashion. When the original guest molecules are replaced there is a noticeable effect on the host mechanics, altering the thermal expansion properties of the material. This study of the thermal expansion coefficients of different inclusion complexes of the host MOF involved systematic alteration of guest size, i.e., methanol, ethanol, n-propanol, and isopropanol, showing that fine control over the thermal expansion coefficients can be achieved and that the coefficients can be correlated with the size of the guest. As a proof of concept, this study demonstrates the realizable principle that a single-crystal material with an exchangeable guest component (as opposed to a composite) may be used to achieve a tunable thermal expansion coefficient. In addition, this study demonstrates that greater variance in the absolute dimensions of a crystal can be achieved when one has two variables that affect it, i.e., the host–guest interactions and temperature.一种展现出各向异性热膨胀行为的新型三维金属有机骨架（metal–organic framework, MOF）已通过单晶X射线衍射（SCD）与热分析手段完成制备与表征。所制备的该MOF具有一维孔道，孔道内包埋客体分子；这些客体分子可通过单晶到单晶的方式被脱除，或与其他客体分子进行交换。当原始客体分子被替换后，会对骨架主体的力学性能产生显著影响，进而改变该材料的热膨胀特性。本研究针对该主体MOF的不同包合配合物的热膨胀系数开展系统探究，通过系统性调控客体分子尺寸（即甲醇、乙醇、正丙醇与异丙醇），证实可实现对热膨胀系数的精准调控，且该系数与客体分子尺寸存在明确相关性。作为概念验证，本研究证实了一项可行原理：相较于复合材料，带有可交换客体组分的单晶材料可用于实现热膨胀系数的可调谐性。此外，本研究表明，当存在两个影响晶体绝对尺寸的变量（即主客体相互作用与温度）时，可实现晶体绝对尺寸的更大幅度变化。