Molecular arrangement of alkylated fullerenes in the liquid crystalline phase studied with X-ray diffraction

Fullerenes (C(60)) with attached aliphatic chains are shown to form fluid bilayer structures that are distinguished by very characteristic and well-resolved X-ray diffraction patterns. Since, in addition, we vary systematically the number and length of the chains, detailed understanding of the structures can be achieved. To make the analysis transparent, simple boxlike electronic density profile models are proposed to explain the relative intensity of the several Bragg peaks present in the X-ray patterns. The models allow detailed characterization of the molecular organization. The molecules arrange themselves in bilayers with their long axis on average perpendicular to the plane of the layers. Considering the bilayers composed of three sections with different electronic density, the C(60) heads occupy a fixed length of approximately 17 A, corresponding to almost no interdigitation, the connector section around 4 A, and the carbon chains' perpendicular length depends on the number and length of the chains. The analysis reveals that reducing the number of carbons per chain (from 20 to 16) results in a shorter unit cell, while reducing the number of chains (from 3 to 2) results in a shorter but also slightly thinner unit cell, in agreement with known molecular packing volumes.