Liquid crystal dimers derived from naturally occurring chiral moieties: synthesis and characterization

Naturally occurring cholesterol and α-chloroalkanoyl units derived from natural α-amino acids (l-valine, l-leucine, and l-isoleucine) have been utilized to prepare three different series of nonsymmetric liquid crystal dimers. Tolane (diphenylacetylene), which is known to possess several promising structural features, has been chosen as the other mesogenic segment to covalently tether with cholesterol through a flexible spacer. In each series, the terminal α-chloro ester group attached to the tolane unit is kept constant, while the length and parity of the spacer have been varied; specifically, three dimers comprising even-parity spacer of varying length, and one compound with an odd-parity spacer constituted a series. The phase behavior of these dimers has been ascertained mainly by polarizing microscopic and calorimetric studies. Except one, all the 11 dimers display enantiotropic mesomorphism. Within the series, clearing temperatures exhibit a dramatic odd–even effect wherein the even-parity dimers possess higher values. In general, the dimers comprising α-chloro ester group derived from l-valine and l-leucine stabilize chiral nematic and/or smectic phase/s, while the compounds with terminal group resulting from l-isoleucine show twist grain boundary phase additionally; this implies that the nature of the α-chloro ester group influences the phase behavior. Notably, an odd-parity dimer with an α-chloro ester group derived from l-valine exhibits a transition from an intercalated smectic A phase to a monolayer (unknown) smectic phase, as evidenced by optical, calorimetric, and X-ray diffraction studies. As representative case, a dimer has been screened for antimicrobial activity by disc diffusion method; a notable activity has been found against some microbes.