Functional Versatility of a Series of Zr Metal–Organic Frameworks Probed by Solid-State Photoluminescence Spectroscopy

Many
of the desirable properties of metal–organic frameworks
(MOFs) can be tuned by chemical functionalization of the organic ligands
that connect their metal clusters into multidimensional network solids.
When these linker molecules are intrinsically fluorescent, they can
pass on this property to the resultant MOF, potentially generating
solid-state sensors, as analytes can be bound within their porous
interiors. Herein, we report the synthesis of a series of 14 interpenetrated
Zr and Hf MOFs linked by functionalized 4,4′-[1,4-phenylene-bis­(ethyne-2,1-diyl)]-dibenzoate
(peb2–) ligands, and we analyze the effect of functional
group incorporation on their structures and properties. Addition of
methyl, fluoro, naphthyl, and benzo­thia­diazolyl units
does not affect the underlying topology, but induces subtle structural
changes, such as ligand rotation, and mediates host–guest interactions.
Further, we demonstrate that solid-state photoluminescence spectroscopy
can be used to probe these effects. For instance, introduction of
naphthyl and benzo­thia­diazolyl units yields MOFs that
can act as stable fluorescent water sensors, a dimethyl modified MOF
exhibits a temperature dependent phase change controlled by steric
clashes between interpenetrated nets, and a tetrafluorinated analogue
is found to be superhydrophobic despite only partial fluorination
of its organic backbone. These subtle changes in ligand structure
coupled with the consistent framework topology give rise to a series
of MOFs with a remarkable range of physical properties that are not
observed with the ligands alone.

金属有机框架（metal–organic frameworks, MOFs）的诸多优异性能可通过有机配体的化学官能化进行调控——此类配体将金属簇连接为多维网络固体。当此类有机连接配体本征具有荧光特性时，可将该荧光特性传递至所得金属有机框架中，进而有望制备固态传感器，因分析物可结合于其多孔孔道内部。在此，我们报道了一系列由14个互穿锆（Zr）、铪（Hf）基金属有机框架组成的体系，这些框架由功能化的4,4′-[1,4-亚苯基双(乙炔-2,1-二基)]-二苯甲酸根（peb2–）配体连接而成，并分析了官能团引入对其结构与性能的影响。引入甲基、氟代基、萘基及苯并噻二唑基官能团并不会改变其基本拓扑结构，但会引发配体旋转等细微结构变化，并调节主客体相互作用。进一步研究表明，固态光致发光光谱可用于表征此类结构变化。例如，引入萘基与苯并噻二唑基官能团可得到可作为稳定荧光水传感器的金属有机框架；经二甲基修饰的金属有机框架表现出由互穿网络骨架间空间位阻冲突调控的温度依赖性相变；而仅对有机主链进行部分氟代的四氟代类似物则展现出超疏水特性。配体结构的细微变化与一致的框架拓扑结构相结合，使得该系列金属有机框架具备了一系列仅靠配体本身无法实现的优异物理性能。