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.本研究基于近期开发的、以羧基-硫醚（carboxyl–thioether）组合为代表的硬-软功能化策略，用于金属有机框架（metal–organic frameworks, MOFs）的修饰，并旨在以典型MOF-5体系[即Zn₄O(bdc)₃，其中bdc为1,4-苯二甲酸（1,4-benzene dicarboxylate）]为对象，进一步验证该策略的有效性与普适性。具体而言，将硫醚侧链CH₃SCH₂CH₂S–（甲硫基亚乙基硫基，简称MSES）引入bdc的2,5-位，所得配体（L）与锌(II)离子结晶得到多孔立方网络结构[Zn₄O(L)₃]，其拓扑结构与MOF-5一致。相较于此前使用的甲硫基（CH₃S−）基团，MSES侧链具有更高的柔性，每条侧链包含更多硫原子作为结合位点，且能更深入地延伸至孔道区域；因此，将该侧链引入多孔MOF基质后，更易于与软金属物种实现高效结合。本文报道了该MOF材料在多项性能上的显著提升，包括湿气稳定性、荧光强度以及金属捕获能力。例如，活化后的样品1可在空气中长期稳定存放（超过3周），对硝基苯具有显著的传感响应（表现为荧光猝灭），且能够从浓度低至84 mg/L的乙醇溶液中捕获氯化汞（HgCl₂）。