Chiral gelators

On the macroscale gels do not flow and hence a simple test for a gel is to invert its container - if it lands on the floor with a splash it is probably not a gel On the microscale the solid-like component generally comprises fibrillar bundles of high aspect ratio and in the case of chiral gelators can adopt... 

Figure 14.21 (a) a typical gel passing the inversion test , (b) SEM micrograph of a dried gel (xerogel) made from a chiral gelator showing helical fibrils. ... 

Chiral gelators based on the trflns-l,2-diaminocyclohexane unit were successfully used as templates for the preparation of silica or transition metal oxide nanostructures hehcal fibers or tubes [24-26]. 

Fia 30 Chiral gelators 162 and 163 and representation of different packing in homochiral and heterochiral combinations of 162 and 163... 

Chiral gelator forming gels with a wide variety of org. solvs. Mp 220-221°. Md +21.6 (Py). 

The above gels of compound 25a and b do not form helical fibers that can be observed by SEM. However, several chiral gelators do selectively self-assemble into helicoidal fibers. For example, Escuder et al. have reported that (S,S)-26 gelated several organic solvents. SEM of the gel formed in benzene showed the presence of isolated right-handed twisted ribbons of several micrometers of... 

Typically, racemic mixtures of chiral gelators either do not form gels or they form unstable ones that degenerate easily into precipitates or bulk-separated crystals [69a, 70]. However, there have been some interesting exceptions in which a racemate produces stronger gels than their enantio-pure counterparts [70, 71]. 

As an example, the addition of only 0.01 mol% of chiral R-23 or S-23 to achiral 22 (Figure 1.19a) induced a remarkable Cotton effect by forming helical columnar structures R-23 or S-23, alone at the same concentration, exhibited a very weak Cotton effect [81c]. Similar chiral amplification effects were also observed from the co-assembly of OPV-based [81a] or pyrene-based [81e] achiral and chiral gelators (Figure 1.19b,c, respectively). 

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