Synthesis, optical and structural properties of sanidic liquid crystal (cholesteryl)benzoate-ethynylene oligomers and polymer

Abstract

(Cholesteryl)benzoateethynylene oligomers having 3, 5 and 7 repeat units and the homologue polymer were synthesized by a divergence–convergence approach by the Sonogashira–Heck reaction. Their chemical structure was analyzed by ¹H, ¹³C NMR, UV-Vis, fluorescence spectroscopy and circular dichroism. X-Ray diffraction patterns of all the materials show only a first order peak at 2.05° in 2θ, indicative of a lamellar order within a distance of 4.3 nm. This is twice the calculated distance of a molecule in its more extended conformation, i.e. of 2.15 nm, and is consistent with a disordered Smectic A phase for the trimer and pentamer and a nematic phase for the heptamer and polymer. The most probable model of organization corresponds to a supramolecular assembly in bilayers, where the cholesteryl chains are oriented in an opposite sense, i.e. in a back-to-back “comb like” fashion, which is in agreement with AM1 semiempirical optimization and DFT theoretical calculations. Since the molecular shape of the materials is consistent with brick or board-like structures and the molecules are stacked in blocks randomly oriented as observed by High Resolution Transmission Microscopy, the (cholesteryl)benzoateethynylene materials could be described as Sanidic LCs as a general term to classify their mesomorphic behavior. Optical properties are discussed in terms of the conjugation length and the terminal groups, iodine or in one case, hydrogen. In general the results confirm that all the molecules assume in solution a more planar conformational geometry when passing from the ground to the excited state as confirmed by the theoretical calculations. Circular dichroism spectra measured were consistent with the observed structural properties.