In the formation of chiral crystals, the tendency for twist in the orientation of neighboring molecules is incompatible with ordering into a lattice: Twist is expelled from planar layers at the expense of local strain. We report the ordered state of a neat material in which a local chiral structure is expressed as twisted layers, a state made possible by spatial limitation of layering to a periodic array of nanoscale filaments. Although made of achiral molecules, the layers in these filaments are twisted and rigorously homochiral--a broken symmetry. The precise structural definition achieved in filament self-assembly enables collective organization into arrays in which an additional broken symmetry--the appearance of macroscopic coherence of the filament twist--produces a liquid crystal phase of helically precessing layers.

The metastable phase of isotactic polypropylene (βiPP) is crystallized epitaxially on two specific nucleating agents:  γ-quinacridone and dicyclohexylterephthalamide (DCHT). The resulting thin films are investigated by electron microscopy and atomic force microscopy. Epitaxial crystallization yields a biaxially oriented sample of βiPP, an orientation which cannot be achieved by mechanical means due to the β → α phase transformation on stretching. Electron diffraction indicates that the βiPP (110) plane is the contact face involved in both epitaxies. AFM investigation with methyl group resolution reveals a lateral periodicity of 19 Å in that (110) contact face, which corresponds to the distance between three chains and is a trademark indicator of the frustrated packing of βiPP. AFM further indicates some variability of the surface pattern, suggesting that two different frustrated structures may coexist in the surface layer; this, in turn, suggests some type of surface reconstruction. Structural requirements that efficient βiPP nucleating agents must meet are analyzed.

The degree of polymerization (DP n ) of poly(p-phenylene) (PPP) 3 with appendant third-generation, Fréchet-type dendrons was determined as 110 after a chemical modification. This DP n is extraordinarily high and shows that the mechanistically complex Suzuki polycondensation can thus be employed even in the case of sterically highly loaded (dendronized) AA-type monomers, which paves the way to obtain extremely rigid nanorods. A molecular model of 3 was obtained by molecular dynamics simulations which show that the diameter of the rodlike dendrimer fluctuates between approximately 2 and 4 nm in vacuo. Scanning force microscopy (SFM) on 3 adsorbed on graphite indicates the formation of multilayer films made of densely packed nanorods. SFM with molecular resolution reveals highly ordered domains in which the molecules are packed parallel to each other, separated by a lateral periodicity of 4.8 ± 0.5 nm. Lateral and vertical spacings indicate that the (110) plane of the densely packed nanorods is exposed at the film surface. SFM further indicates that different domains exhibit three molecular orientations, reflecting the 3-fold symmetry of the graphite substrate. Chain ends can be resolved at grain boundaries. Time-dependent SFM experiments show reorientation of small domains on a time scale of minutes.