Enrico Dalcanale

New columnar liquid crystals Correlation between molecular structure and mesomorphic behaviour

Abstract The effect of molecular structural changes on the mesomorphic nature of a new class of macrocyclic columnar liquid crystal is reported. Twenty-six new compounds with general molecular structure I were prepared, characterized and compared. Only dodecasubstituted ester derivatives 2d-h exhibit thermotropic mesophases. The design of mesogens based on these new, unusual macrocyclic cores requires the presence of the following structural elements: twelve aliphatic side chains, esters as bridging units and small R groups on the core.

Columnar mesomorphic properties of new macrocyclic mesogens

Abstract The mesomorphic properties of a new class of columnar liquid crystals, consisting of a bowl-shaped metacyclophane core surrounded by twelve flexible alkyl chains are reported. These mesogens form Dho columnar mesophases having negative optical and dielectric anisotropy. Both planar and homeotropic alignment is obtained. An antiferroelectric pairing of the molecules within the columns is observed, ruling out the possibility of a ferroelectric self-organization of the cores. For all the mesogens a complex thermal behaviour is observed. Two different solid phases C1 and C2 have been characterized and their thermal behaviour elucidated: C1 is the mesophase precursor, while C2, more ordered, melts directly to an isotropic liquid. Mesophase stability over time depends on the length of the side chains. The overall mesomorphic complexity is attributed to the combination of the different possible core packing geometries with the numerous conformations available to the alkyl chains.

Deuterium NMR investigation of a new class of macrocyclic columnar liquid crystal

Abstract Deuterium NMR spectroscopy of C6D6, added as a probe to compound 1, shows the formation of an uniaxial columnar mesophase, with domain directors perpendicular to the magnetic field. Spectral patterns at various temperatures are interpreted in terms of slow equilibrium between two solvent sites placed respectively between the cores and among the lateral chains. Deuterium NMR spectra of the deuteriated derivative 2 confirm this model, allow the evaluation of the molecular order parameter, Szz, and exclude the presence of fast rotational diffusion around the main molecular axis.