Molecular surface structure of organic crystals observed by atomic force microscopy

The bulk molecular structure of free-standing organic crystals is well known from analytical methods such as X-ray diffraction and phonon emission detection. With the inception of the field of scanning force microscopy, it is now also feasible to investigate the surface molecular structure. Rearrangements of the molecules to produce a surface structure different from the bulk structure have been suspected in certain organic crystals, although detection in such a thin layer is difficult. We have now observed this rearranged surface structure in an atomic force microscopy study of pyrene, a four-ring aromatic compound. The bulk structure of pyrene is composed of dimer pairs, whereas only monomers are detected by AFM on its ab-surface. Preliminary molecular modeling has been performed to create, out of the bulk data, this monomeric arrangement. In contrast, in a parallel study on tetracene, another four-ring aromatic, AFM measurements confirm that intermolecular spacings on the surface correspond with those in the bulk.