Helicene molecule. A helicenes is a hydrocarbon formed of linked aromatic benzene rings. The rings are flat, but the physical presence of other rings in the structure forces the molecule into a spiral shape. As the spiral is set one way, either spiralling clockwise (as here) or anticlockwise, the molecule exhibits chirality. This means that it is non-superimposable on its mirror image. Most chiral molecules achieve this by having, for instance, four different side groups bonded to a central tetrahedral carbon atom. This is the case with most biological molecules. Helicenes are notable as they achieve chirality despite lacking these chiral centres.