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

Ross J Marshall,Barry A Blight,Ross S Forgan,Sarah L Griffin,Yaroslav Kalinovskyy,Claire Wilson

doi:10.1021/jacs.7b02184

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 (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.

Highlights

Metal−organic frameworks (MOFs)[1−7] are multidimensional network materials containing both inorganic and organic constituents
The diverse choice of organic and inorganic units that can be used to construct MOFs results in a wide variety of structures.[8−10] The high porosity of MOFs has prompted investigations toward potential applications such as gas capture and storage,[11−16] catalysis,[5,17−19] sensing,[20−22] and drug delivery.[23−26] Lately, many synthetic efforts toward MOFs have focused on those containing group IV transition metal ions, especially zirconium and hafnium.[27−32] Zr and Hf MOFs linked by linear dicarboxylate ligands generally adopt the welldocumented UiO-66 topology (UiO = Universitetet i Oslo) which contains M6O4(OH)[4] clusters (M = Zr or Hf) linked in three dimensions by 12 bridging organic ligands.[27]
Careful choice of reaction parameters, such as concentration, temperature, and modulator choice/ equivalency resulted in the isolation of 14 highly crystalline interpenetrated Zr and Hf MOFs

Summary

■ INTRODUCTION

Metal−organic frameworks (MOFs)[1−7] are multidimensional network materials containing both inorganic and organic constituents. We detail how linker functionalization modulates the structural and optical properties of the resulting Zr and Hf MOFs, whose unusual structural features and host− guest behavior have been probed via solid-state emission techniques

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES