Dispersion-Driven Formation of Chiral Twisted PAH Double Helices [Data]

Molecular double helices are ubiquitous in nature and have also been generated artificially. These are usually based on helical ribbons. Here, a new type of double helices based on twisted ribbons is introduced. The monomeric strands are polycyclic aromatic hydrocarbons of various lengths with up to 25 linearly annulated six-membered rings. Single-crystal X-ray structure analysis of all revealed that the major driving force are multiple dispersion interactions of alkyl substituents. The thermodynamic stability and formation of the twisted double helices were studied by NMR as well as the kinetics of their inversions by circular dichroism spectroscopy. In combination with theoretical calculations, the mechanism of isomerization is suggested to depend on the rate of monomerization of double helical strands rather than the double helices racemize themselves as intact pairs. The new type of twisted nanoribbons in combination with their aromatic nature opens up new possibilities to design chiral materials.