A naphthalene bisimide organogelator forms different supramolecular assemblies in the gel phase and in the solid states; the structures of these different polymorphs have been uncovered by electron and X-ray diffraction, UV and FTIR spectroscopies.

An organogelator based on a N,N'-substituted H-bonding perylenebisimide (PBI-C10) self-assembles to form either a green J-type (form I) or a red H-type (form II) aggregate structure. The molecular packing of both polymorphs was determined from a combination of Transmission Electron Microscopy (TEM) (low dose electron diffraction and high resolution), Grazing incidence X-ray diffraction and polarized infrared spectroscopy. To that aim, highly oriented films have been prepared by mechanical rubbing at controlled film temperature and DFT calculations were performed to identify representative vibrational IR bands and their associated polarizations. H-Bonding between amides generates either a rectangular columnar phase (form I) in the dried gel or a hexagonal packing of supramolecular 21/1 helices with a long period of 97 Å (form II) in annealed thin films. In aligned films of form I, polarized FTIR spectroscopy helps determine the orientation of both intermolecular H-bonds and the PBI core with respect to the substrate. In form II, PBI-C10 molecules assemble into pairs to form off-centered 21/1 helices whose helical axis is made of strongly H-bonded amides. TEM investigations show that three 21/1 helices are packed in a frustrated trigonal structure formed by H-bonding. The Form I → Form II transformation implies a redistribution of a single population of strong intra-columnar H-bonds between amides in form I to a mixture of strong and weak H-bonds in the supramolecular helices, the strong H-bonds forming the spine of the helices.

A combined atomic force microscopy and theoretical study reveals preferred fracture directions in the topmost graphene sheets of mechanically exfoliated graphite. Fractures in the basal plane of single-crystal grains are found to depend on applied stress direction and crystallographic orientation. Interatomic bonds that are aligned with the applied stress direction exhibit the largest resistance against fracture. The findings are modeled trough an angle-dependent failure of sp 2 hybrid bonds which results from a larger interatomic energy dispersion of sheared C-C bonds. I. INTRODUCTIONMany solids exhibit preferential fracture directions when subjected to external loads. It is a feature that relies on defect density and symmetry of atomic bond structure 1 . Lamellar materials, however, have special mechanical responses usually at variance with continuum fracture models. One striking example is graphene, which shows a marked insensitiveness of failure strength to density, location, chemistry, and size of atomic defects 2,4,6,39 .The fracture mechanics of graphene is nowadays of great interest for the fabrication of nanoscale single-crystal nanoribbons, as they are considered key to development of many future electronic devices 5,7,8 . This is due to their unique electronic properties conferred by the reduced width and atomic structure at edges. In particular, it has been shown that graphene edges with a certain degree of electron-hole symmetry near the Fermi level support quantumconfined electronic states relevant for anisotropic electron and spin transport 9-14 . Currently, a considerable effort is engaged to develop versatile techniques for the fabrication of graphene nanostructures with controlled edge orientations 15,16 . Nanoparticle cutting 17,18 , chemical ripping 19,20 , Joule heating 21 , electron etching 22,23 , or diamond nanotomy 24 are found very promising for graphene edge engineering. At the atomic scale, however, the problem boils down to how local factors impact the stability of individual atomic bonds subjected to a mechanical stress 25 . This is a fundamental and technological relevant issue which is far from being understood in a simple and intuitive manner. The fracture mechanics of graphene layers has been studied in relation with, elastic [26][27][28][29] and inelastic [30][31][32] bending properties, inherent presence of defects 2,27,33 , and adhesive substrates 34,35,45 . There are