Microscopic chirality

Recent experimental and theoretical studies on crystal growth, especially in the presence of tailor-made inhibitors, provide a link between macroscopic and microscopic chirality. We shall discuss these principles in some detail for chiral molecules. Furthermore, we shall examine whether it is indeed feasible today to establish the absolute configuration of a chiral crystal from an analysis of solvent-surface interactions. Since these analyses are based on understanding the interactions between a growing crystal and inhibitors present in solution, we shall first illustrate the general mechanism of this effect in various chiral and nonchiral systems. 

Symmetry breaking associated with chiral phenomena is a theme that recurs across the sciences—from the intricacies of the electroweak interaction and nuclear decay [1-3] to the environmentally influenced dimorphic chiral structures of microscopic planktonic foraminifera [4, 5], and the genetically controlled preferential coiling direction seen in the shells of snail populations [6, 7]. 

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. 

The simplest way to assign the absolute configuration of a chiral molecule would certainly be by direct inspection of the molecule itself. Were the technical means at hand powerful enough to allow it, no other technique could compete with one providing a direct three-dimensional photograph of the molecule in question. Since at present, electron microscopes can achieve resolutions as low as 3 A, we are indeed not far from this goal. 

Note 2 The () in SmC and analogous notations indicate, as in 3.1.5.1.2 (Note 6), that the macroscopic structure of the mesophase is chiral. However, it is also used simply to indicate that some of the constituent molecules are chiral even though the microscopic structure may not be. 

Gulikkrzywicki T, Fouquey C, Lehn JM. Electron-microscopic study of supramolecular liquid-crystalline polymers formed by molecular-recognition-directed self-assembly from complementary chiral components. Proc Natl Acad Sci USA 1993 90 163-167. 

On the other hand, a change in the configuration of a chiral center in a single molecule will be described by the term epimerization. This is a microscopic event that happens to a single molecule, and occurs prior to the amide bond formation (Scheme 2). 

Lopinski, G. P., Moffatt, D. J., Wayner, D. D. and Wolkow, R. A. Determination of the absolute chirality of individual adsorbed molecules using the scanning tunnelling microscope. Nature 392, 909 (1998). 

The history of liquid crystals started with the pioneer works of Reinitzer and Lehmann (the latter constructed a heating stage for his microscope) at the end of the nineteenth century. Reinitzer was studying cholesteryl benzoate and found that this compound has two different melting points and undergoes some unexpected color changes when it passes from one phase to another [1]. In fact, he was observing a chiral nematic liquid crystal. 

Later, Pasteur 15) had arrived at the general stereochemical criterion for a chiral or dissymmetric molecular structure. Thus, the specific rotations of the two sets of sodium ammonium tartrate crystals in solution, isolated from the racemic mixture by hand-picking, were equal in magnitude and opposite in sign, from which Pasteur inferred that enantiomorphism of the dextro- and laevorotatory crystals is reproduced in the microscopic stereochemistry of the (+)- and (—)-tartaric acid molecules. The term dissymmetry or chirality is used when there is no superimposability between the two enantiomers, as seen in Sect. 2.1. 

As described in the Introduction, Pasteur showed beautifully that racemic molecules resolve spontaneously into chiral forms when they crystallize. We call them conglomerates, in which molecules form condensates comprised of only one enantiomer. The condensation into conglomerates can now be observed not only in crystals but in monolayers, fibers, and supramolecules self-assembled in solution [35]. The researches became possible because of the development of microscopic observation techniques at the nanometer scale. However, in crystals we still do not know what kinds of molecules show spontaneous resolution. Hence, observation of chiral resolution in soft matter may provide important information on the general question. 

---