Multifunctional Zr-MOF Based on Bisimidazole Tetracarboxylic Acid for pH Sensing and Photoreduction of Cr(VI)

Herein,
a new luminescent zirconium MOF [Zr-BBI, BBI = 4,4′,4″,4‴-(1,4-phenylenebis­(1H-imidazole-2,4,5-triyl))­tetrabenzoic acid] was successfully
constructed by a rationally designed functionalized bisimidazole tetracarboxylic
acid ligand. Zr-BBI consists of eight-connected Zr6 clusters
and four-connected BBI ligands. The high connection mode must be responsible
for the high stabilities of Zr-BBI in both acidic and basic systems.
Apart from the high stability, the inherent bisimidazole units endow
Zr-BBI with the traits of intense fluorescent emission as well as
the protonation/deprotonation behavior. Therefore, Zr-BBI could display
a highly sensitive fluorescence response along with the varying pH
values in the aqueous solutions and act as a pH sensing scaffold,
especially in the pH value range from 4.6 to 7.12. Zr-BBI also shows
good fluorescence detection performance toward Cr2O72– at a low concentration with a high KSV value up to 6.49 × 104 M–1. Moreover, by the utilization of Zr-BBI as a catalyst,
Cr­(VI) could be effectively photoreduced to Cr­(III) in aqueous solution
under visible light irradiation, in which the introduction of a hole
scavenger (benzyl alcohol) could further significantly enhance the
photocatalytic efficiency. Compared to that of the recently representative
MOFs, the k value of the photocatalytic reaction
over Zr-BBI is as high as 0.073 min–1. With the
consideration of the presented results, Zr-BBI can serve as a multifunctional
platform for efficiently sensing and photoreducing Cr2O72– in an aqueous system, which fully illustrates
the feasibility that introducing specific functional groups in the
framework of MOFs would enhance the related photocatalytic activity.