Chirality

The adjective chiral is used for objects that lack mirror symmetry. The most famous chiral object is the human hand as you can easily verify by looking at the image of your right hand in the mirror and comparing with your real left hand: they are the same. In contrast, if you try to wear a right-hand glove on your left hand it will not work, because the right and left hands are indeed different objects (the word chiral actually comes from the Greek word for hand). They are said to be enantiomers, or stereoenantiomers, of each other.

Numerous compounds that play important roles in our lives, such as sugar, DNA and many pharmaceuticals, are chiral. The molecule exists in at least two forms, one that can be referred to as left-handed and another that is the mirror image of the first and can then be considered to be right-handed (many molecules have more than one points where mirror symmetry is broken and then more than two versions are possible). The lack of mirror symmetry can have immense consequences for the properties of the chiral compound and its interaction with surrounding matter, sometimes of academic importance only, but in other cases the consequences of the wrong enantiomer can be disastrous (as in some famous pharmaceuticals).

If a liquid crystal contains chiral molecules or particles with one enantiomer being present at higher concentration than the other, the breaking of mirror symmetry is often translated from the molecular to the macroscopic scale. The long-range order of the liquid crystal phase then amplifies the chirality, for instance onto the helical superstructures that the liquid crystal may form in case of chiral nematic or chiral smectic-C-type phases. In the latter case the combination of lack of mirror symmetry with a layered molecule arrangement in which the molecules tilt away from the layer normal additionally leads to the appearance of a spontaneous polarization, and then the liquid crystal can be ferro- or antiferroelectric. Chirality thus renders the whole liquid crystal phase chiral, a phenomenon that is highlighted by adding a star to the phase short-hand in case of chiral liquid crystals, thus N*, SmA*, SmC* etc.

Chiral liquid crystals play important roles in our research, mainly as self-assembled and responsive photonic crystals of chiral nematic type, giving for instance electrospun polymer fibers new functionality. We are also intrigued by the actual mechanism of chirality transfer, which is nothing but trivial and in fact still a largely open research question. In particular in the case of chiral lyotropic liquid crystals, like in our cholesteric liquid crystalline suspensions of cellulose nanocrystal nanorods in water, we are still far from a clear understanding on how the molecular chirality of the cellulose is able to produce a chiral structure in a system that consists to some 95% of (isotropic) water.

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Cartoon of a chiral smectic-C phase formed by unichiral molecules, as illustrated by a spiral shape of the same handedness.