Helicene-like helical structure

In the previous section, we reported PMP derivatives bearing chiral side chains having a helicene-like helical structure through intrachain 7r-electron overlap interactions, which lead to a self-assembled whisker morphology due to interchain van der Waals interactions. However, each whisker of the PMP derivative is still randomly oriented, even if it has a self-assembled structure. It is therefore desirable to align the PMP derivative to construct a higher-ordered hierarchical structure and also to evolve an anisotropic nature in its potentially profound op-toelectrical properties. 

The first example of whiskers composed of helicene-like helical structures was published by Akagi et al., who used poly(m-phenylene) derivatives (Figure 5.18) with racemic or chiral side chains. ... 

Helical phases have similar symmetry properties to those of a molecule of helicene in that they are dissymmetric because they possess a twofold (infinite) axis of rotation normal to the helical axis through the midpoint of the helical structure. As helical phases are dissymmetric and do not have supeiimposable mirror images, like helicene, they are also optically active. Thus a beam of plane polarized light traversing such an arrangement of molecules will be rotated by the helical ordering [16]. However, the specific rotation (a) is usually much greater for helical phases than for the sum of the specific rotation(s) of the individual molecules. Thus, in this case, as the optical activity is due to the helical macrostructure of the phase, rather than due to the sum of the optical activities of the individual molecules, it is termed/orm optical activity. 

One core chiral system that shows dramatic amplification of its chiral structure is the substituted helicenes of Katz and coworkers [83]. In essence, this research cuts the helix into a number of six-helicene subunits that self-assemble (Figure 10). Only when these subunits, which look like lock washers, are prepared in optically pure form the material associates into supramolecular helical columns [84]. The assemblies have been synthesized with different amounts of substitution around the exterior. Depending on the helicenes substitution, the material exhibits hexagonally ordered soft-crystalline [84] or liquid-crystalline phases [85]. The liquid-crystalline versions of these molecules switch when electric fields are applied to neat and solution-phase samples and have been characterized as a dielectric response [85-87]. Upon association, these materials have enormous changes in their CD intensities and optical rotations [74]. In addition, this supramolecular chirality also significantly enhances the second-order nonlinear optical behavior of these materials in Langmuir-Blodgett films [88]. 

Screw-like structures are found above all for molecules such as helicenes, 1,3-disubstituted allenes, 2,2 -disubstituted biphenyls, etc., displaying C2 symmetry. With the aid of the 4-point figures indicated in the formulae the absolute helicities (P or M) can straightforwardly be ascertained. The currently still customary descriptors aS (or Sa) and aP (or Pa) regrettably correspond only inversly to the much more generally applicable helicity symbols P and M. 

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