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，可通过将金属簇连接为多维网络固体的有机配体的化学官能化策略进行精准调控。若此类连接配体本身具有固有荧光特性，则可将该属性传递至合成得到的MOF中，进而有望制备固态传感器——因分析物可结合于MOF的多孔内部空间。本文报道了14种由官能化4,4′-[1,4-亚苯基-双(乙炔-2,1-二基)]-二苯甲酸根（peb2–）配体桥联的互穿型锆基与铪基MOF的合成工作，并分析了官能团引入对其结构与性能的影响。引入甲基、氟基、萘基与苯并噻二唑基单元，并不会改变该类MOF的基础拓扑结构，但会引发配体旋转等细微结构变化，并可调控主客体相互作用。此外，本研究证实固态光致发光光谱可用于表征此类结构与性能变化。例如，引入萘基与苯并噻二唑基单元得到的MOF可作为稳定的荧光水传感器；经二甲基修饰的MOF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；而四氟取代的衍生物尽管有机骨架仅部分氟化，却展现出超疏水性。配体结构的上述细微变化与统一的框架拓扑结构相结合，造就了一系列仅靠配体本身无法实现的、物理性能跨度极大的MOF材料。