Chiral J-aggregates formed by achiral cyanine dyes.

The self-assembly of helical supramolecular structures from chiral building units is a basic principle of biological materials. The mesoscopic structure of a chiral molecular aggregate, which formed spontaneously from a nonchiral J-aggregating cyanine dye in aqueous solution, is presented. In single crystals (as shown in the picture) a coexistance of planar molecules with left- and right-handed twisted conformers of the same dye are found; the latter may act as templates to build up the helical superstructures.