Anisotropic molecular packing of soluble C₆₀ fullerenes in hexagonal nanocrystals obtained by solvent vapor annealing

We describe the self-assembly of soluble, chemically modified fullerene [6,6]-phenyl C 61 butyric acid methyl ester (PCBM) into a new crystalline phase where the C 60 moieties are arranged in parallel layers. Minimum C 60 center-to-center distance is 10 within the layers, and up to 15 perpendicular to the layers. Highly anisotropic, mesoscopic hexagonal crystals of this material, with a lateral size of many microns and a thickness below 1 μm, are obtained from chloroform solution by solvent vapor annealing, and characterized by optical microscopy and X-ray diffraction. The crystalline structure is deduced combining experimental data with molecular modeling and ab initio calculations. The large difference in C 60-C 60 spacing indicates a high anisotropy in electrical and charge transport properties of this new phase.