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Twisted Structures Emerge from Achiral Molecules

The spontaneous formation of chiral (left- or right-handed) structures from effectively achiral molecules is at the heart of intense worldwide research. In particular, the phenomenon is thought to play a central role in the origin of biological homochirality—how certain molecules essential to life (e.g. amino acids or sugars) ended up displaying one type of chirality, even though there appears to be no chemical preference for the formation of one type over the other.

Twisted structures made from chiral building blocks are relatively common in nature, but examples of achiral molecules that assemble into helical aggregates are not. Recently, such a phenomenon was found in a liquid crystalline nematic phase (where identical molecules align their long axes but are otherwise free to move). Although built of achiral molecules, the phase exhibited structural chirality—the average molecular direction follows a short-pitch helix.

In this work, researchers studied a series of achiral asymmetric molecules consisting of two subunits attached by a spacer having an odd number of atoms (CB6OIBeOn, where n represents the number of carbon atoms in the terminal alkyl chain). These molecules form twisted structures in the nematic phase as well as in the smectic phase (the molecules are free to move only within a layer). The tight-pitch heliconical structure of the nematic (NTB) and tilted smectic C (SmCTB) phases was identified by resonant soft x-ray scattering (RSoXS) studies at ALS Beamline 11.0.1.2, performed with x-rays tuned to carbon absorption edge. The extremely strong tendency to form twisted structures was also expressed through the formation of mesoscopic helical filaments in the lowest-temperature smectic phase, the HexI phase.

Top: Schematic of the proposed SmCTB phase in which dimeric molecules form a short-pitch helix; p = pitch length. Dotted vertical lines denote smectic-layer interfaces; d = interlayer spacing. Bottom left: Two-dimensional RSoXS patterns for CB6OIBeO7 in the HexI (47.6 °C) and SmCTB (108.4 °C) phases, and in the phase coexistence range (87.2 °C). Bottom right: Temperature evolution of the RSoXS signal for CB6OIBeO7 measured on cooling; whereas in the HexI phase the signal position is practically temperature independent (it corresponds to ~100 Å), in the SmCTB phase it changes strongly owing to changes in the heliconical pitch length from 117 to ~150 Å close to the transition to the SmA phase.

J.P. Abberley, R. Killah, R. Walker, J.M.D. Storey, C.T. Imrie, M. Salamonczyk, C. Zhu, E. Gorecka, and D. Pociecha, “Heliconical smectic phases formed by achiral molecules,” Nat. Commun. 9, 228 (2018), doi:10.1038/s41467-017-02626-6.