Planar Sheets

Sheets of noncovalent polymers, which would be analogous to the p-pleated sheets of silk, are commonplace on water and solid surfaces. The molecular weight and extension of these surface mono- and bilayers is unlimited. Of current interest are well-defined rigid gaps within such monolayers. They provide small reactive domains which can be manipulated on a molecular scale. It was found possible, for example, to fixate porphyrin heterodimers with a plane-to-plane distance of 10 or 20 A within such gaps and to position tyrosine monomers as electron relays between them. These sheets can then be fixated on spherical colloids (see Fig. 3.4) and we are back to spherical polymers.  

Porphyrin bolaamphiphiles have been shown to form platelets with a thickness of 8 A in bulk water. Well-defined monolayers have been obtained with bolaamphiphilic porphyrins with two positive charges on each edge. Charge repulsion obviously first prevents crystallization and second unlimited growth of the isolated sheets. Their surface area is a few [ixn. In aqueous solution lecithin bilayers of unlimited extension (myelin figures) are known to have erne s below 10 M and can thus be considered as noncovalent polymers.  

Both biopolymers and noncovalent polymers are produced at the borderline between hydrophobic membrane structures and bulk water. The hydrophobic effect enforces curvature in both, hydrogen bonds lead to linear chains, chirality produces helices, sheets are often insoluble and are applied only occasionally. All kinds of planar thin layers occur, however, on solid surfaces, e.g., bones and minerals in nature or gold, graphite and mica in model systems. 

If one leaves water and gold to organic solvents as a medium, the situation will change. Biopolymers and synthetic block polymers form ill-defined clusters, nanometer helices are replaced by micrometer twisted ribbons, and sheets immediately crystallize. In the case of micellar assemblies bilayer sheets prevail, showing a tendency to roll up and form bulky scrolls. 

Water is clearly the best medium to produce polymer assemblies with stable imprints on their surface. Small clefts do not survive in organic solvents, so swelling crystallization cannot be avoided. 

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The basic building block of carbon is a planar sheet of carbon atoms arranged in a honeycomb structure (called graphene or basal plane). These carbon sheets are stacked in an ordered or disordered manner to form crystallites. Each crystallite has two different edge sites (Fig. 2) the armchair and zig-zag sites. In graphite and other ordered carbons, these edge sites are actually the crystallite planes, while in disordered soft and hard carbons these sites, as a result of turbostratic disorder, may not... 

We can understand the differences in properties between the carbon allotropes by comparing their structures. Graphite consists of planar sheets of sp2 hybridized carbon atoms in a hexagonal network (Fig. 14.29). Electrons are free to move from one carbon atom to another through a delocalized Tr-network formed by the overlap of unhybridized p-orbitals on each carbon atom. This network spreads across the entire plane. Because of the electron delocalization, graphite is a black, lustrous, electrically conducting solid indeed, graphite is used as an electrical conductor in industry and as electrodes in electrochemical cells and batteries. Its... 

Silicates also exist in which each silicon atom bonds to one outer oxygen and to three inner oxygen atoms. The result is a linked network in which every silicon atom forms three Si—O—Si links, giving a planar, sheet-like structure. The empirical formula of this silicate is S12 O5. In many minerals, aluminum atoms replace some of the silicon atoms to give aluminosilicates. The micas—one has the chemical formula... 

In PdO, each Pd is surrounded by four oxygen atoms, but planar sheets do not exist. Explain why they are not expected. 

Figure 4. Conceptual oligomerization schemes for cyclocarbons to planar sheets (graphdiynes)11 291 and buckytubes.

Single planar sheet of sp -bonded carbon atoms corresponds to one hexagonal basal plane of graphite and is termed graphene. Recently, a successful attempt to isolate such a graphene layer has been reported [3], and apparently, it becomes possible to produce... 

In a skillfully performed reaction of pyrimidine (72) with AgNOs, [Ag(Pym-N2 N3)]4(N03)4 (88) was formed. Its X-ray crystal structure shows tetrameric cyclic self-assembly forming supramolecular 1 1 Ag pyrimidine squares [3.8 X 3.8 A] which exist in planar sheets with each square face to face stacked... 

Squaric acid (H2SQ) has been chosen as a first test compound because it has a very simple molecular structure. Planar sheets of the squarate (C4O4) groups are linked to each other in a two-dimensional network through O - H...0 bonds (Fig. 1) with weak van der Waals forces [52,53]. The protons perform an order/disorder motion above the antiferroelectric phase transi-... 

In the search for new organic conducting materials, x-ray structure determinations of fused 1,2,5-selenadiazoles and 1,2,5-thiadiazoles have been carried out. Compounds (25a) and (25b) are planar molecules possessing a two-fold axis and mirror plane. Short C=N S and C N Se contacts connect each molecule in the crystal with four neighbors, forming a virtually planar sheet network (Figure 7), The sheets stack to form infinite layers with face-to-face overlapping between... 

We determined the phase behavior of the HDPE/styrene/CC>2 using the method described by Berens et al. (1992), modeling mass uptake data as Fickian dilfusion into a planar sheet (Crank, 1975). Ethylbenzene was used as the penetrant to model styrene. 

Since their early studies Eley et al. (1959) (see Eley, 1967) have largely confined their attentions to the study of the electronic and structural properties of bipyridinium2+ (TCNQ)2" and related complexes (Ashwell et al., 1975a, b, c Ashwell et al., 1977a, b, c Eley et al., 1977). Most complexes, such as 4,4-bipyridyl (TCNQ)2, five (N,N-dialkyl-4,4 -bipyridylium)2+ (TCNQ)J+ salts (alkyl = methyl, ethyl, propyl, isopropyl and benzyl ), and l,2-di(N-ethyl-4-pyridinium) ethylene2+ (TCNQ), are low gap semiconductors except one form of the last compound which exhibited metallic behaviour. The asterisked complexes comprise planar sheets of TCNQ molecules grouped in tetrads. 

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