Chirality suprastructural

Zhang Y, O Callaghan MJ, Baumeister U, Tschierske C (2008) Bent-core mesogens with branched carbosilane termini flipping suprastructural chirality without reversing polarity. Angew Chem Int Ed 47 6892-6896... 

For molecules the optical chirality measurements CD and ORD can be performed between vacuum ultraviolet (UV) (ECD CD of electronic transitions) and infrared (IR) (VCD CD of vibrational transitions) spectral regions in which 1 is large in comparison with the diameter of a molecule. Suprastructural chirality can be observed in a spectral region in which 1 is in the order of the periodicity of the chiral structures as can be established by the selective reflection of a cholesteric phase in the IR spectral region. 

S and D are the Saupe order parameters that describe the order of the molecules in the absorbing sample. s% (i = 1, 2, 3) are the diagonal elements of the transition moment tensor. They are proportional to the squares of the components of the transition moments (<(/< > i = 1,2,3)) given with respect to a molecule fixed-coordinate system (xj) joc(Circular dichroism of chiral anisotropic phases without suprastructural chirality). 

New and promising fields are the chiroptical analyses of chiral crystals, chiral liquid crystal phases, chiral membranes, Langmuir-Blodgett films, chiral surfaces, etc. Their chirality is caused by the molecular chirality and the suprastructural chirality of the phase. Because the spatial extension of the structures of the suprastructural chirality is often about three orders of magnitude larger than molecular diameters, the spectral regions of chiroptical phenomena for both levels of chirality are different. 

Circular Dichroism of Chiral Anisotropic Phases without Suprastructural Chirality... 

The HTP is a chirality measurement which results from the fact that the chirality of the guest molecule is mapped onto the phase. As long as a cholesteric phase is induced, the suprastructural chirality of the phase can be described as a P or M helicity. Important points are that the HTP depends... 

There is a third problem for which chirality information is of current interest anisotropic phases are often stabilized by chiral structures. Apart from chiral structures with enantiomorphic crystals of chiral compounds, suprastructural chirality exists in liquid crystal phases built up by chiral molecules as in the cholesteric phases and the smectic C phases. Even liquid crystalline phases with suprastructural chirality originating in achiral, so-called banana-shaped molecules, seem to be possible. Anisotropic polymer films with chiral structures have been found. It can be anticipated that chiroptical spectroscopy with anisotropic chiral systems will lead to new questions and answers. 

Figure 2 Schematic representation of the two eigenstates of iight (see Table 1) propagating through an anisotropic phase of chiral molecules with or without suprastructural chirality.

Phenomenology of the CD of anisotropic phases (ACD) without suprastructural chirality... 

With ACD and CD spectroscopy, information can be obtained about suprastructural chirality of isotropic phases, e.g. cubic phases with suprastructural chirality. There is no interference with linear effects and thus Ae and As can easily be measured. A well-known example is crystalline NaC103. Another known example is that of liquid crystalline blue phases (O, O ). Both with light in the region where A is of the order of the periodicity within the phase and with wavelengths very different from the periodicity, a CD spectrum is obtained from which a detailed analysis of the phase structure has been derived, e.g. by Flornreich and colleagues. 

In inorganic salts such as NaC103 discussed above, or quartz, chiral structures are built up from achiral species. An interesting field has been opened by the analysis by Takezoe and colleagues of the so-called banana shaped molecules, which build up chiral domains together with phase separation, although, as is generally believed, they consist of achiral molecules. The analysis of such chiral phases by CD or ACD spectroscopy opens up a new field for an analysis of suprastructural chirality. 

The phase chirality is given by the molecular chirality and the suprastructural chirality of the orientational or positional order. Thus, a phase has to be specified by symbols for the molecular and the suprastructural chirality. Chiral nematics as given in FIGURE 1 are then described by R or S or P or M for the molecules (first letter) and by P M fin- the helicity of the phase (second letter) RP or SP. 

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