A Correlation Between the Absolute Configuration of Alkyl Aryl Sulfoxides and their Helical Twisting Powers in Nematic Liquid Crystals

In this paper, for the first time, a systematic experimental and theoretical analysis of the cholesteric induction due to solutes whose chirality is originated only by a single stereogenic center has been carried out. The twisting power β of a series of alkyl aryl sulfoxides has been determined in several nematic solvents. The sign of β, which reflects the handedness of the induced helical arrangement of the solvent molecules, correlates with the configuration of the stereogenic sulfur in the nematic solvents E7, Phase 1083, and ZLI 2359: S-configurated dopants induce (M)-chiral nematics. (S)-Configurated cyclic sulfoxides, which are forced to adopt a different conformation with respect to the parent acyclic compounds, induce, instead, right-handed chiral nematics. The experimental data have been interpreted in the light of the surface chirality model, which allows the calculation of β in terms of the molecular properties of the dopant, namely, the anisotropy and helicity of its molecular surface. The calculations reliably reproduce the behavior experimentally observed. The more flexible, open-chain compounds investigated induce chiral nematics of opposite handedness in MBBA and Phase 1053: temperature-dependent experiments point out the importance of the conformation in determining the effective sign of β. The results have been discussed in terms of different conformation populations in these latter solvents with respect to E7, Phase 1083, and ZLI 2359