Strain-induced helical chirality in polyaromatic systems
Helicity in a molecule arises when the molecule contains a stereogenic axis instead of a stereogenic centre. In a molecule that is not inherently helically chiral, helicity can be induced by designing the molecule such that an unfavourable steric interaction, or strain, is present in its planar conformation. The release of this strain forces the molecule to adopt a helical twist against the cost of the torsional strain induced in the backbone, an interplay of forces, which must be balanced in favour of the helical conformation over the planar one. In this tutorial review, design principles that govern this process are analysed and the selected examples are categorised into three main (I, II and III) and two related (IV and V) classes, simply by their relation to one of the three types of helically twisted ribbons or two types of helically twisted cyclic ribbons, respectively. The presented examples were selected such that they illustrate their category in the best possible way, as well as based on availability of their solid-state structures and racemisation energy barriers. Finally, the relationship between the structure and properties is discussed, highlighting the cases in which induced helicity gave rise to unprecedented phenomena.