SCSC Transformation and Post-Synthesis Modification of MOFs with Proton Conduction and Ratiometric Fluorescence-Sensing Properties

The modification of metal–organic framework (MOF) materials to facilitate their practical applications is an extremely challenging and meaningful topic. In this work, two stepwise modification strategies for MOFs were conducted. First, we have demonstrated a single-crystal-to-single-crystal (SCSC) transformation from a microporous three-dimensional (3D) MOF to a two-dimensional (2D) coordination polymer (CP). The centrosymmetric [Cd­(3-bpdb)­(MeO-ip)]n (1) transforms into a chiral [Cd2(3-bpdb)­(MeO-ip)2(CH3OH)2]n (2), which is triggered by the reaction time with methanol that acts as a structure-directing agent. The conversion relationship of 1 to 2 at different reaction times was studied in detail. Density functional theory (DFT) calculations clearly state that the irreversible formation of 2 is thermodynamically favorable. Intriguingly, 2 exhibits good proton conduction of 1.34 × 10–3 S cm–1 under 363 K and 98% relative humidity (RH) due to unique H-bond network characteristics. To the best of our knowledge, there are very few cases of 3D to 2D SCSC transformation stimulated by reaction time. The results have important implications for understanding the SCSC transformation mechanism and synthetic chemistry. On the other hand, the lanthanide3+-functionalized hybrids (Ln3+-MOF), Ln3+@1, were continuously prepared by incorporating luminescent Ln3+ ions into the structure of 1 through encapsulating post-synthesis modification (PSM). Tb3+@1 exhibits double emission in water and shows visual ratiometric fluorescence behavior for sensing glutamic acid (Glu), tryptophan (Trp), and Al3+, which is more reliable and accurate than single emission. Our work may not only provide new insights into the multiple modification of MOF materials but also promote the practical application of such materials.金属有机框架（metal–organic framework, MOF）材料的修饰以推动其实际应用，是一项极具挑战性且意义重大的研究课题。本研究针对MOFs开展了两种分步修饰策略。首先，本工作证实了从微孔三维（three-dimensional, 3D）MOF到二维（two-dimensional, 2D）配位聚合物（coordination polymer, CP）的单晶到单晶（single-crystal-to-single-crystal, SCSC）转变过程：中心对称的配合物[Cd(3-bpdb)(MeO-ip)]ₙ（标记为1）会转变为手性配合物[Cd₂(3-bpdb)(MeO-ip)₂(CH₃OH)₂]ₙ（标记为2），该转变由以甲醇作为结构导向剂的反应时间触发。本研究详细探究了不同反应时间下1向2的转化关系。密度泛函理论（Density functional theory, DFT）计算结果明确表明，2的不可逆生成在热力学上具备优势。有趣的是，由于独特的氢键网络特征，2在363 K、98%相对湿度（relative humidity, RH）条件下展现出1.34×10⁻³ S·cm⁻¹的优异质子传导性能。据我们所知，由反应时间触发的三维到二维SCSC转变案例极为罕见。该研究结果对于理解SCSC转变机制与合成化学具有重要意义。另一方面，本研究通过封装式后合成修饰（post-synthesis modification, PSM）方法，将发光型镧系离子引入1的结构中，连续制备得到镧系离子功能化杂化材料（Ln³⁺-MOF），即Ln³⁺@1。Tb³⁺@1在水溶液中展现出双发射特性，并可用于可视化比率型荧光传感，实现对谷氨酸（glutamic acid, Glu）、色氨酸（tryptophan, Trp）以及Al³⁺的检测，其性能相较于单发射传感更为可靠精准。本研究不仅为MOF材料的多维度修饰提供了新的研究思路，同时也推动了这类材料的实际应用进程。