Thiazole-Decorated PCN-700 Metal–Organic Frameworks for Diclofenac Luminescence Sensing and Adsorption in Wastewater

The
defective sites on the eight-connected [Zr6O4(OH)8(H2O)4]8+ nodes
of the metal–organic framework (MOF) PCN-700 are the
anchoring points for its postsynthetic decoration with extra linkers via solvent-assisted linker incorporation (SALI). The thiazolo[5,4-d]thiazole-2,5-dicarboxylic acid (H2TTz) and the newly prepared 2,2′-(1,4-phenylene)bis(4-methylthiazole-5-carboxylic
acid) (H2TzPhTzMe) have been inserted into PCN-700 to obtain
the mixed-linker MOFs (MIXMOFs) [TTz@PCN-700] (1) and [TzPhTzMe@PCN-700]
(2). The two MIXMOFs are luminescent, and they have been
exploited as sensors and adsorbents of diclofenac sodium (DCF) in
aqueous solutions at ambient temperature. The emission intensity of 2 shows an unprecedented “parabolic” trend combined
with a (λmax)em blue shift from 470 to
430 nm that induces a color change from turquoise to blue in the presence
of DCF upon irradiation with a suitable wavelength. Its DCF limit
of detection is 8.4 × 10–6 M. 1 is an excellent DCF “sponge” in water, with a maximum
adsorption capacity (Xm) of 263.2 mg/g.
Density functional theory (DFT) modeling of the electronic structure
of the empty and DCF-loaded MOFs has revealed that the highest-occupied
crystal orbital (HOCO) is mainly centered on Me2-BPDC2– in 1, while it is prevalently centered
on TzPhTzMe(2−) in 2. This may be at the origin of the different emission behavior
of the two MOFs.