Crystal-to-crystal transformation


The halogen-bonded cis-azo cocrystal undergoes irreversible photo-mechanical bending

Scientists in Canada have generated a new crystalline material that undergoes irreversible light-induced bending.

Azobenzenes are an important family of molecules where cistrans isomerisation causes considerable changes in molecular shape. Oleksandr Bushuyev and colleagues at McGill University used a supramolecular interaction, known as halogen bonding, to combine photo-mechanically active azobenzene units with a variety of countermolecules, to create cocrystals with a range of photo-mechanical characteristics.

The photo-mechanical cistrans isomerisation in one of the multi-component azobenzene cocrystals took place in a crystal-to-crystal fashion, giving the scientists a rare opportunity to use x-ray diffraction to study a crystal as it undergoes cistrans isomerisation. Previous work on similar reactions has involved liquid crystals, polymers and gels – environments that are unconducive for direct structural analysis.

Whereas it has long been a mystery if and how crystalline solids could accommodate sterically demanding cistrans isomerisations, the direct insight revealed that the transformation is topotactic – it proceeds with a memory of the initial crystal structure – and is mediated by an amorphous phase. The researchers say this is an exciting observation as it indicates that the halogen-bonding interactions that are found in the initial and resultant crystals are most likely conserved in the non-crystalline intermediate.

 


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