Correlating Pressure‐Induced Emission Modulation with Linker Rotation in a Photoluminescent MOF

Conformational changes of linker units in metal‐organic frameworks (MOFs) are often responsible for gate‐opening phenomena in selective gas adsorption and stimuli‐responsive optical and electrical sensing behaviour. Herein, we show that pressure‐induced bathochromic shifts in both fluorescence emission and UV‐Vis absorption spectra of a two‐fold interpenetrated Hf MOF, linked by 1,4‐phenylene‐bis(4‐ethynylbenzoate) ligands ( Hf‐peb ), are induced by rotation of the central phenyl ring of the linker, from a coplanar arrangement to a twisted, previously unseen conformer. Single‐crystal X‐ray diffraction, alongside in situ fluorescence and UV‐Vis absorption spectroscopies, measured up to 2.1 GPa in a diamond anvil cell on single crystals, are in excellent agreement, correlating linker rotation with modulation of emission. Topologically isolating the 1,4‐phenylene‐bis(4‐ethynylbenzoate) units within a MOF facilitates concurrent structural and spectroscopic study in the absence of intermolecular perturbation, allowing characterisation of the luminescence properties of a high‐energy, twisted conformation of the previously well‐studied chromophore. We expect the unique environment provided by network solids, and the capability of combining crystallographic and spectroscopic analysis, will greatly enhance understanding of luminescent molecules and lead to the development of novel sensors and adsorbents

Pore Shape Modification of a Microporous Metal-Organic Framework Using High Pressure:Accessing a New Phase with Oversized Guest Molecules

The authors thank the Royal Society of Edinburgh and the Scottish Government for a fellowship to S.A.M. The authors thank EPSRC (EP/J02077X/1) and Leverhulme Trust for a research project grant (RPG-209) for financial support. They also thank the UK Carr Parinello consortium for allocation of computing time on the EPSRC high performance computing resource ARCHER (managed by the Edinburgh Parallel Computing Centre, the EaSTCHEM Research Computing Facility and the University of Edinburgh ECDF facility).Pressures up to 0.8 GPa have been used to squeeze a range of sterically "oversized" C5-C8 alkane guest molecules into the cavities of a small-pore Sc-based metal?organic framework. Guest inclusion causes a pronounced reorientation of the aromatic rings of one-third of the terephthalate linkers, which act as "torsion springs", resulting in a fully reversible change in the local pore structure. The study demonstrates how pressure-induced guest uptake can be used to investigate framework flexibility relevant to "breathing" behavior and to understand the uptake of guest molecules in MOFs relevant to hydrocarbon separation.PostprintPeer reviewe

Pressure-and temperature induced phase transitions, piezochromism, NLC behaviour and pressure controlled Jahn–Teller switching in a Cu-based framework

In situ single-crystal diffraction and spectroscopic techniques have been used to study a previously unreported Cu-framework bis[1-(4-pyridyl)butane-1,3-dione]copper(II) (CuPyr-I). CuPyr-I was found to exhibit high-pressure and low-temperature phase transitions, piezochromism, negative linear compressibility, and a pressure induced Jahn?Teller switch, where the switching pressure was hydrostatic media dependent.The support by the Spanish Ministerio de Econom´ıa, Industria y Competitividad (PGC2018-101464-B-I00), and INNVAL 18/28 is also acknowledged

Opening the Gate:Framework Flexibility in ZIF-8 Explored by Experiments and Simulations

ZIF-8 is a zeolitic imidazole-based metal-organic framework with large cavities interconnected by narrow windows. Because the small size of the windows, it allows in principle for molecular sieving of gases such as H-2 and CH4. However, the unexpected adsorption of large molecules on ZIF-8 suggests the existence of structural flexibility. ZIF-8 flexibility is explored in this work combining different experimental techniques with molecular simulation. We show that the ZIF-8 structure is modified by gas adsorption uptake in the same way as it is at a very high pressure (i.e., 14 700 bar) due to a swing effect in the imidazolate linkers, giving access to the porosity. Tuning the flexibility, and so the opening of the small windows, has a further impact on the design of advanced molecular sieving membrane materials for gas separation, adjusting the access of fluids to the porous network.</p