Ultraporous, Water Stable, and Breathing Zirconium-Based Metal–Organic Frameworks with ftw Topology

“Breathing”
metal–organic frameworks (MOFs)
are an emerging class of soft porous crystals (SPCs) with potential
for high working capacity for gas storage applications. However, most
breathing MOFs have low stability and/or low surface area. Here we
report a water-stable, high surface area, breathing MOF of ftw topology, NU-1105. While Zr6-oxo clusters
as nodes introduce water stability in NU-1105, its high
surface area and breathing character stem from its pyrene-based tetracarboxylate
(Py-FP) linkers, in which the fluorene units (F) in the FP “arms” play a key role in
promoting breathing behavior. During gas sorption studies, the “closed
pore” (cp) ↔ “open
pore” (op) transition of NU-1105 occurs at a propane pressure of ∼3 bar. At
1 bar, NU-1105 is in its cp form and adsorbs less propane than it would in its op form, highlighting improved working capacity. In situ powder X-ray diffraction during propane sorption
was used to track the cp ↔ op transition, and molecular modeling was used
to elucidate the structure of the op and cp forms of NU-1105. According to TD-DFT calculations, the proposed conformations of
the Py-FP linkers in the op and cp forms are consistent with the
measured excitation and emission spectra of the op and cp forms of NU-1105. Similar structural transitions are also observed in the porphyrinic
MOF NU-1104 depending on the identity of the porphyrin
core; we observed breathing behavior if the constituent Por-PTP linker is nonmetalated.