Knapp Julia E, Ortín-Rubio Borja, Heck Fabian, Gjorgjevikj Kristina, Sleptsova Anastasia, Krause Simon, Bette Sebastian, Lotsch Bettina V
Max Planck Institute for Solid State Research Heisenbergstraße 1 Stuttgart 70569 Germany
Department of Chemistry, University of Stuttgart Pfaffenwaldring 55 Stuttgart 70569 Germany.
Chem Sci. 2025 Jun 25. doi: 10.1039/d5sc02585k.
Understanding the structural response of framework materials to external stimuli has been of great interest. However, little is known about the stimuli responsiveness of titanium-based metal-organic frameworks. We investigate the reproducibility of the synthesis of the two-dimensional metal-organic framework NTU-9 (NTU = Nanyang Technological University) composed of Ti cations and 2,5-dihydroxyterephtalate as the organic linker and the flexible response of the framework structure to external stimuli. Using acetic acid simultaneously as mediator and solvent leads to the reproducible formation of large NTU-9 crystallites after long reaction times. The MOF synthesis in isopropanol:acetonitrile (-PrOH : MeCN) mixtures without a modulator is significantly faster but yields smaller NTU-9 crystallites. Pure heat treatment under ambient conditions removes a significant amount of the incorporated host solvent molecules without alteration of the framework's structure. After applying additional external stimuli (, vacuum), the material exhibits a pore distortion by compression in the lateral dimension, depending on the synthetic procedure. The new, distorted, metastable form NTU-9-d shows a reduction in unit cell volume, pore size, and crystal symmetry. Under humidity/air exposure or solvent resuspension, the framework reverts into its original state. The synthesis conditions significantly affect the flexibility of the MOF structure, where samples synthesized without modulator showed a lower tendency for distortion. Our results emphasise the importance of an in-depth understanding of the structure-property relationships in flexible MOFs through a detailed characterisation of the material's stimuli responsiveness process.
了解骨架材料对外部刺激的结构响应一直备受关注。然而，关于钛基金属有机骨架的刺激响应性却知之甚少。我们研究了由Ti阳离子和作为有机连接体的2,5 - 二羟基对苯二甲酸组成的二维金属有机骨架NTU - 9（NTU = 南洋理工大学）合成的可重复性以及骨架结构对外部刺激的灵活响应。使用乙酸同时作为介质和溶剂，经过长时间反应后可重复形成大尺寸的NTU - 9微晶。在没有调节剂的异丙醇：乙腈（-PrOH : MeCN）混合物中进行MOF合成速度明显更快，但生成的NTU - 9微晶尺寸更小。在环境条件下进行纯热处理会去除大量掺入的主体溶剂分子，而不会改变骨架结构。施加额外的外部刺激（如真空）后，根据合成程序，材料在横向尺寸上会因压缩而出现孔扭曲。新的、扭曲的亚稳态形式NTU - 9 - d显示出晶胞体积、孔径和晶体对称性的减小。在湿度/空气暴露或溶剂重悬的情况下，骨架会恢复到其原始状态。合成条件显著影响MOF结构的灵活性，其中在没有调节剂的情况下合成的样品显示出较低的扭曲倾向。我们的结果强调了通过详细表征材料刺激响应过程来深入理解柔性MOF中结构 - 性能关系的重要性。
