Liquid crystals (LCs) are organic materials exhibiting spontaneous symmetry-breaking in the liquid phase, to form optically anisotropic fluids.  Chirality has been a key aspect of LC science since its very beginning, with Friedrich Reinitzer’s discovery in the late 1880s that cholesteryl benzoate forms a birefringent liquid phase, now known as the chiral nematic phase, which exhibits a spontaneous helical variation in the molecular orientation.  This macroscopic twist represents a chirality phenomenon; a macroscopic manifestation of the molecular chirality. Until the late 1990s, the formation of all known chiral LC phases required enantiomerically enriched molecules.  However, beginning 1997, the discovery of chiral fluid phases formed from achiral or racemic molecules led to the characterization of several variations of spontaneous achiral symmetry breaking (“breaking” of achiral Schoenflies symmetry to become chiral) in fluids.  These represent fluid analogs to Pasteur’s seminal discovery of achiral symmetry breaking upon crystallization of sodium ammonium racemate. LCs showing achiral symmetry breaking are most often (perhaps always) of the “bent-core” variety – the classic example being formation of chiral smectic (layered) phases from a class of resorcinol diesters possessing two “mesogenic” wings.  In that system, four diastereomeric phases are formed, two d,l pairs, and two “meso” LC phases. Then, in 2009, two variants of “dark conglomerates” were described.  In these phases, layering is only short-range, with fluid order giving rise to chiral, optically isotropic phases (dark between polarizer and analyzer) with optical rotatory power for visible light on the order of 0.5°/ µm.  Most recently, in 2013 bent-core materials possessing fluid order in three, were proven to exist.  In these phases, known as “twist-bend nematic,” packing in the fluid phase favors a spontaneous twist, affording an optically anisotropic conglomerate lacking any layering. These “fluid” examples of conglomerate formation will be discussed.

Symmetry-breaking in organic fluids

256th National Meeting (2018)

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