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A large team of researchers from the University of Glasgow, UK, has created a novel inorganic material known as a ‘flexi-crystal’. This is the first material of its kind and gathers its name from the ability to switch between 8 different single-crystalline states.
Crystal engineering employs powerful techniques to create complex molecules, with highly crystalline lattices, by self-assembly processes. Many crystals employ organic-based molecules, but so far efforts have yielded structures with limited properties and/or stabilities.
The researchers have created a highly stable material that is both flexible and robust enough to withstand conformational changes and crystal metamorphisms. The material employs an inorganic based building block composed of tungsten oxide (P8W48O184), pieced together by cobalt-based linker molecules.
Whilst metal organic frameworks (MOF’s) are now well established with an efficient combination of robustness and flexibility, they are composed of both inorganic and organic linkers. The flexi crystal is the first of its kind to only employ inorganic metal oxides.
The flexi-crystal undergoes the largest single crystal-to-crystal transformation thus far known. They also show a huge crystal volume contraction and expansion change, which shows conformational flexibility whilst maintaining its robustness. This material challenges the norm that robustness and flexibility are mutually exclusive by employing both principles in a single crystal.
The shape of the building blocks is similar to that of a doughnut. The central void aligns in the assembled crystal to form extended pores that are open to guest molecule inclusion, or exclusion. It is the incorporation and removal of guest molecules that causes the rings within the lattice to transform and re-align themselves. The crystal architectures span across 0-3 dimensional architectures and can occupy 0, 1D, 2D and 3D systems.
The crystal’s transformation ability is attributed to the stability of the ring-shaped clusters, their flexibility and their ability to reorganise themselves within the crystal lattices. The reorganisation capability is due to the W-O(W) and the Co-O(W) bonds in the lattice, as they can be easily broken and reformed.
The rearrangement of the flexi-crystal is activated by the absorption/desorption of small molecules such as water, ammonia and methanol into the crystal. The crystal can undergo dehydration, rehydration and reactions under vacuum to produce the different crystal arrangements, which can be distinguished by the human eye due to the visible colour change exhibited by the crystal(s). The crystals produced by dehydration have found to occupy a smaller unit cell, and vice versa for hydrated crystals.
The original crystal formed by the synthetic process is a 1-dimension chain of tungsten building blocks. Each molecule and/or reaction produces a specific crystal that could either be a 0D molecular crystal, one of two types of 2D columnar crystals, a 2D sheet crystal, one of the two 3D network crystals or a differently orientated 1D chain.
However, once the crystal has changed it orientation into a new crystal, it can never return to its original form. It can however switch continuously between the other crystal forms, with the exception being a 3D network crystal produced by methanol vapour. The other states can switch between each other and have been found to be stable for more than 6 cycles- an indicator that this material is exceptionally robust.
The ability to switch between states, without structural degradation leans itself to many possible applications in the future. Because of the ability to uptake small molecules, and change under different humidities, it may have potential future application as small molecule, VOC and humidity sensors.
Source:
Zhan C., Cameron J. M., Gabb D., Boyd T., Winter R. S., Vila-Nadal L., Mitchell S. G., Glatzel S., Breternitz J., Gregory D. H., Long D-L., Macdonell A., Cronin L., A metamorphic inorganic framework that can be switched between eight single-crystalline states, 2017, Nature Communications, 8, 14185
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