Columnar stacking

The systems discussed up to now all showed chiral susceptibilities that were of the same order of magnitude or smaller than the achiral susceptibility components. The system that we discuss in this section has chiral susceptibilities that dominate the nonlinear optical response.53 The material is a chiral helicenebisquinone derivative shown in Figure 9.22. In bulk samples, the nonracemic, but not the racemic, form of the material spontaneously organizes into long fibers clearly visible under an optical microscope. These fibers comprise columnar stacks of helicene molecules.54,55 Similar columnar stacks self-assemble in appropriate solvents, such as n-dodecane, when the concentration exceeds 1 mM. This association can be observed by a large increase in the circular dichroism (CD) of the solutions. 

Imagine a columnar stack of aromatic rings placed about 3.4 A apart in parallel planes (Figure 26). How well would such an array mediate ET and... 

Note 2 The adjective diseotic is also employed to describe the nematic mesophases formed by diseotic mesogens. The mesophases formed by a columnar stacking of disc-like molecules are described as columnar mesophases. 

The adduct is not luminescent, a result that is interpreted as due to quenching of the luminescence by the rupture of the columnar stack. 

In columnar stacked one-dimensional complexes, such as the tetracyanoplatinates, with relatively short intrachain Pt—Pt separations, it is possible to postulate a simple band model by considering overlap of the filled 5dzi orbitals on individual atoms in the chain to produce a full 5dp band, and overlap of the empty 6pz orbitals to produce an empty 6pz band (Figure 1). 

Figure 1 Schematic representation of the band formation in a columnar stack structure. (A) Semiconductor based on integral oxidation state complex. (B) One-dimensional metal produced by partial oxidation and resulting in a non-integral oxidation state...

Metallic properties are also observed in columnar stacked complexes in which the intrachain separation is too long for conduction to be associated with the formation of a band from overlap of the metal dz2/pz orbitals. For these complexes the conduction process involves carriers present in the delocalized MOs extending over die whole complex. The conduction process in these compounds may be described by a hopping mechanism in which the on-site Coulomb repulsion energy has been reduced by the delocalization of the charge over the whole molecule. 

The simple integral oxidation state compounds behave as either insulators or semiconductors. The tetracyanoplatinates of monovalent cations form anionic columnar stack structures with conductivities in the range 10-4—10-9 fi 1 cm-1 and a systematic variation of cry with 1 /electrical conductivity can be increased by as much as a factor of 104 by pressures up to 180 kbar.27... 

Neutral square coplanar complexes of divalent transition metal ions and monoanionic chelate or dianionic tetrachelate ligands have been widely studied. Columnar stack structures are common but electrical conductivities in the metal atom chain direction are very low and the temperature dependence is that of a semiconductor or insulator. However, many of these compounds have been shown to undergo partial oxidation when heated with iodine or sometimes bromine. The resulting crystals exhibit high conductivities occasionally with a metallic-type temperature dependence. The electron transport mechanism may be located either on predominantly metal orbitals, predominantly ligand re-orbitals and occasionally on both metal and ligand orbitals. Recent review articles deal with the structures and properties of this class of compound in detail.89 90 12... 

Very recently the crystallization of non-spherical nanoparticles into ordered arrangements has been observed that had not been seen with spherical particles. Thanks to the high density of DNA on the particle solid faces, hexagonal packing of nanorods, and columnar stacking of nanoprisms, fee crystals of rhombic dodecahedrons were observed in [152] (see Fig. 35). 

Much attention is currently devoted to the synthesis and properties of shape-persistent macrocycles[l]. Such compounds are interesting for a variety of reasons including formation of columnar stacks potentially capable of performing as nanopores for incorporation into membranes or for the generation of nanowires[2]. Furthermore, in shape-persistent macrocycles incorporating coordination units, enc/o-cyclic metal-ion coordination may be exploited to generate nanowires[3], whereas e.ro-cyclic coordination can be used to construct large arrays of polynuclear metal complexes[4]. Shape-persistent macrocycles with reactive substituents may also be linked to other units to yield multicomponent, hierarchical structures. 

Replacement of the N-methyl groups with the bulkier N-benzyl or N-isopropyl groups did not reproduce any columnar stack in the crystal structure that was found in 9, and no solvoluminescent behavior was observed [110, 111]. The closest intermolecular Au-Au contact of 3.662 and 6.417 A in the crystal packing in 10a and 10b (Fig. 14), respectively, indicated the absence of any intermolecular aurophilic interaction. 

Fig. 14.38 Two polymorphs of 6,13-bis(tert- direction, (b) tert-butyl groups point in ap-butyldimethylsilylethynyl)pentacene (52). (a) proximately the same direction, leading to a tert-butyl groups arrange pointing in opposite columnar stacking arrangement in the crystal.

Helicenes have an intrinsic, rigid helical shape and can be separated into enantiomers. Katz et al. found that the nonracemic helicene 6 with quinone residues self-assembles into one-handed helical columns in apolar solvents, where the molecules are stacked along their helix axes as shown in Fig. 4 [40-42], In this case, the 7r-donor-acceptor interactions appear to induce aggregations and stabilize the columnar stacks. Interestingly, unlike the nonracemic 6, the corresponding racemic 6 produced no such aggregation. 

Just as chiral induction can be realised in discotic liquid crystals, it can also be realised in assemblies of disc-like molecules or disc-like aggregates. As far as molecules are concerned, C3-symmetrical trisamides (Fig. 15), which actually exhibit discotic liquid crystalline phases, also form chiral columnar stacks through it-it interactions when dissolved in apolar solvents, which are depicted schematically in Fig. 15 [121]. An achiral compound of this type (15) exhibits no optical activity in dodecane, but when the compound is dissolved in the chiral CR)-(-)-2,6-dimelhyloctanc significant Cotton effects (only slightly less intense than those observed in a chiral derivative) are detected. The chiral disc-like trisamide 16 can also be used as a dopant at concentrations as low as 2.5% to induce supramolecular chirality in the stacks of achiral compound. In this case, the presence of the additional hydrogen... 

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