Prochiral nanocrystals

Chiral nanocrystals have received intensive attention, leading to development of their synthetic routes and a host of applications, ranging from optics to catalysis. It seems that the prochirality of nanocrystals is a structural feature that has not received any attention so far. Prochirality in that context is a shape property by which two faces of the achiral nanocrystal - the enantiotopic faces - are enantiomorphic 2D-chiral polygons. The main feature of prochirality is that it allows conversion into chiral objects in a single step. The prochirality of crystals is particularly relevant for the nanoscale, due to the dominance of the high surface-to-volume ratio in the activity of these nanomaterials, due to the well-developed ligand/adsorbate toolkit for controlling the shape, and due to the ability to obtain a large population of crystals with practically the same habit. We provide a detailed explanation of prochirality in the context of crystals, and describe literature examples that report nanocrystal shapes that are prochiral (not recognizing that this is the case). Since achiral habits are found not only in crystals molecularly characterized by the improper-element space group symmetries but also in crystals that adopt the 65 Sohncke space groups, the interplay between the 2D-molecular level chirality of enantiotopic faces and their 2D-shape chirality is discussed as well. Finally, we propose a variety of potential applications of prochiral nanocrystals, including their use as building blocks for chiral nano-metamaterials and metasurfaces.