Design of conductive crown ether based columnar liquid crystals: impact of molecular flexibility and geometry

Abstract

The influence of the molecular shape on the charge transport in the columnar mesophases of bi-centred crown ether based mesogens is studied. Even though a bent shaped bridging of two aromatic cores promotes the stability and the width of the columnar liquid crystal phase, the quality of the intra-columnar packing and thus the charge carrier mobility are reduced. Keeping instead a linear arrangement of the cores and increasing the molecular flexibility by enlarging the crown is detrimental to the liquid crystal phase stability while the charge carrier mobility is only slightly affected. This leads to the conclusion that the optimal molecular geometry for crown ether based mesogens providing a stable mesophase with good charge transport properties is achieved by using small crown ether bridges connecting two π-systems as linear as possible. We used discotic mesogens based on 12-crown-4, 15-crown-5, 18-crown-6 and 21-crown-7 moieties linking two triphenylene cores. The inter- and intra-columnar structure is studied by detailed small- and wide-angle X-ray scattering experiments, respectively. To study the electronic properties temperature dependent photoconductivity and mobility measurements in the organic field effect transistor setup are carried out.