Thioether Side Chains Improve the Stability, Fluorescence, and Metal Uptake of a Metal–Organic Framework

This work builds on the recently developed hard–soft approach, as is embodied in the carboxyl–thioether combination, for functionalizing metal–organic frameworks (MOFs), and it aims to further demonstrate its efficacy and generality in connection with the prototypic MOF-5 system [i.e., Zn₄O(bdc)₃, where bdc is 1,4-benzene dicarboxylate]. Specifically, the thioether side chain CH₃SCH₂CH₂S– (methylthioethylenethio, or MSES) is placed at the 2,5- positions of bdc, and the resultant molecule (L) was crystallized with Zn(II) ions into a porous, cubic network [Zn₄O(L)₃] topologically equivalent to MOF-5. Compared with the previously used methylthio (CH₃S−) group, the MSES side chain is more flexible, has more S atoms as the binding sites (per chain), and extends further into the channel region; therefore, this side chain is predisposed for more-efficient binding to soft metal species when installed in a porous MOF matrix. Here, we report the significantly improved properties, with regard to stability to moisture, fluorescence intensity, and capability of metal uptake. For example, activated solid samples of 1 feature long-term stability (more than 3 weeks) in air, have a notable sensing response to nitrobenzene (in the form of fluorescence quenching), and are capable of taking up HgCl₂ from an ethanol solution at a concentration as low as 84 mg/L.