This review summarizes the recent advances in utilizing halogen bond-driven cocrystal formation as a design element in the creation of optical or photoresponsive materials, notably cocrystals and liquid crystals. The extensive work over a relatively short time has demonstrated halogen bonding as a versatile supramolecular interaction capable of creating libraries of fluorescent, phosphorescent, dichroic, photoswitchable, as well as photomechanical crystals and liquid crystals. Such rapid development, and the diversity of materials properties that have been designed or fine-tuned with success, signal the emergence of halogen-bonded cocrystallization as an exciting new playground for crystal engineers, chemists, and physicists to develop next-generation optical materials.
We describe a simple setup for in situ continuous monitoring of vapour-induced transformations of organic solids using a benchtop powder X-ray diffractometer. Proof-of-principle application of this setup to model pharmaceutical cocrystals reveals complex reactivity, transformations of reaction intermediates within minutes, and a simple route to the rarely observed monoclinic form of the pharmaceutical cocrystal of carbamazepine and saccharin.
We describe the use of dicyanoaurate ions as linear ditopic metal-organic acceptors for the halogen bond-driven assembly of a dichroic metal-organic cocrystal based on azobenzene chromophores. Structural analysis by single crystal X-ray diffraction revealed that the material is a four-component solid, consisting of anticipated anionic metal-organic halogen-bonded chains based on dicyanoaurate ions, as well as complex potassium-based cations and discrete molecules of the crown ether 15-crown-5. Importantly, the structural analysis revealed the parallel alignment of the halogen-bonded chains required for dichroic behaviour, confirming that crystal engineering principles developed for the design of halogen-bonded dichroic organic cocrystals are also applicable to metal-based structures. In the broader context of crystal engineering, the structure of the herein reported dichroic material is additionally interesting as the presence of an ion pair, a neutral azobenzene and a molecule of a room-temperature liquid make it an example of a solid that simultaneously conforms to definitions of a salt, a cocrystal, and a solvate.
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