Stacking of conical molecules with a fullerene apex into polar columns in crystals and liquid crystals

Abstract

Polar liquid crystalline materials can be used in optical and electronic applications, and recent interest has turned to formation strategies that exploit the shape of polar molecules and their interactions to direct molecular alignment^1,2. For example, banana-shaped molecules align their molecular bent within smectic layers³, whereas conical molecules should form polar columnar assemblies^4,5,6,7,8,9. However, the flatness of the conical molecules used until now^4,5,6,9 and their ability to flip^7,8 have limited the success of this approach to making polar liquid crystalline materials. Here we show that the attachment of five aromatic groups to one pentagon of a C₆₀ fullerene molecule yields deeply conical molecules that stack into polar columnar assemblies. The stacking is driven by attractive interactions between the spherical fullerene moiety and the hollow cone formed by the five aromatic side groups of a neighbouring molecule in the same column. This packing pattern is maintained when we extend the aromatic groups by attaching flexible aliphatic chains, which yields compounds with thermotropic and lyotropic liquid crystalline properties. In contrast, the previously reported fullerene-containing liquid crystals^{10,11,12,13,14,15,16,17} all exhibit thermotropic properties only, and none of them contains the fullerene moiety as a functional part of its mesogen units. Our design strategy should be applicable to other molecules and yield a range of new polar liquid crystalline materials.