Enlarged helical twisting power in chiral cyclic dimers based on conventional chiral alkyl diols

Abstract

We prepared a homologous series of chiral cyclic liquid crystal (LC) dimers containing two biphenyl mesogens linked by two alkyl spacers, one of which was based on a R-enantiomeric chiral diol. In solution, the circular dichroism spectra of all the R-enantiomeric dimers showed the first negative and second positive Cotton effects, which indicated that the two biphenyl mesogens were located in a skewed overlapping alignment with counter-clockwise helicity. The helical twisting power (HTP) of these cyclic dimers was confirmed by the cholesteric LCs in mixtures with the nematic host. The induced cholesteric LCs in all mixtures had the same counter-clockwise helical sense as the skewed conformation of the two biphenyl mesogens. Furthermore, HTP values and Cotton effect amplitudes were linearly correlated, indicating direct induction of cholesteric twisting by the skewed biphenyl moieties. The induced HTP values were quite large, irrespective of the use of conventional chiral diols; the largest HTP value exceeded 100 μm⁻¹ and was obtained in the cyclic dimer based on the R,R-1,2-diphenylethane diol.

Keywords:

• cyclic dimer
• helical twisting power
• circular dichroism
• skewed conformation