Dynamic breathing behaviour of the titanium-based metal–organic framework NTU-9 upon adsorption of water and organic solvents

Abstract

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 Ti⁴⁺ 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 (i-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 (i.e., 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.