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.