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.