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

Abstract

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 (peb) 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.

A Chemiluminescent Metal–Organic Framework

“…The changes in the PXRD patterns upon Soxhlet extraction and during activation and sorption experiments of the material (Figure S3) may indicate that this material can occur in structural variations while keeping the connectivity within the framework unaltered. Such a case has been reported for a structurally related Zr‐MOF with a PEAEP‐dicarboxylate (E stands for the ethynylene unit) as the linker . However, it is noteworthy that a coplanarity of the anthracene and the benzene moiety in case of PAP‐UiO is rather improbable due to steric reasons, quite in contrast to the coplanarity of these units in the PEAEP linker.…”

“…Such ac ase has been reported for as tructurally relatedZ r-MOF with a PEAEP-dicarboxylate( Es tands for the ethynylene unit) as the linker. [15] However,i ti sn oteworthy that ac oplanarity of the anthracenea nd the benzene moiety in case of PAP-UiO is rather improbable due to steric reasons, quite in contrastt ot he coplanarity of these units in the PEAEP linker.…”

Correlating Pressure‐Induced Emission Modulation with Linker Rotation in a Photoluminescent MOF

“…In our previously reported structure of Hf-peb, (and the structures of the Zr analogues) [14, 19a] such disorder was not observed, but these data were collected at low temperature (150 K), where the twisted conformer was not populated. [14] On increasing pressure, the population of the twisted conformer increased, reaching 100 % occupancy at 2.1 GPa (Figure 2 a). A small expansion in volume was observed on loading the crystal into the DAC at 0.1 GPa (39 3 , 0.006 %), although the change observed was small and barely significant (3.2s).…”

“…[13] Ligandflexibility in UiO-MOFs can also be induced by guest inclusion, leading to changes in fluorescence emission spectra, with potential for use of these materials as sensors. [14] The intrinsic fluorescence of Zr-MOFs has also been demonstrated to be useful in pH sensing, [15] while changes in ligand conformation in tetraphenylethylene- [16] and porphyrinlinked [17] Zr MOFs have been shown to result in differences in their steady-state emission spectra, although there is no in situ X-ray diffraction data to provide structural mechanisms for such changes on uptake of guest molecules.…”

“…In our previous work, large changes in the fluorescence emission spectra on increasing relative humidity were observed for analogous Zr UiO-MOFs with naphthyl-and benzothiadiazolyl-modified peb 2À linkers. [14] Here, in order to directly determine structure/property relationships in this class of porous framework, we collected absorption and emission spectra of a crystal of Hf-peb as a function of increasing pressure to 2.1 GPa, using custom built UV/Vis absorption and fluorescence emission spectrometers (Figure 3 a, ESI sections 2 and 3). Both the UV/Vis absorption spectrum and the fluorescence emission spectrum shift to longer wavelengths on increasing pressure (Figure 3 b and c, Figures S5 and S9, Tables S5 and S7).…”