Graphene-induced bi-tilted two-component smectic CG phase with bulk ferroelectricity in hydrogen-bonded dimer liquid crystals

Abstract

Nanocomposites of the hydrogen-bonded dimeric liquid crystal heptyloxybenzoic acid (7OBA) with admixture of graphene flakes (GFs) are investigated with microtexture polarization analysis for new effects in their electro-optical behavior and characterized with Raman spectroscopy. In the nanocomposite with GF concentration of 3 × 10−4 wt% we establish as lowest-temperature LC state the triclinic smectic CG phase featuring chirality and ferroelectricity, previously found only in 7OBA nanocomposites with carbon nanotubes and in large banana-like bent-core molecules even though pure 7OBA is typically achiral. In the present study we find the CG phase manifested as two detached sub-structures with a smooth transition between them. For the appearance of the CG phase with its substructures denoted as CGcl and CGln we propose an explanation based on the π–π interaction of the 7OBA molecules with graphene and different coupling combinations of the clinic and leaning degree of freedom of the doubly tilted molecular geometry in the CG ordering. We examine the response of these structures to electric field, and demonstrate the presence in both of them of ferroelectric polarization with polar vector pointing out of the smectic layer planes. Molecular models are proposed for the 7OBA ordering in the substructures CGcl and CGln which are supported by the Raman spectroscopy results. The dispersion of the GFs in the nanocomposite bulk is confirmed by scanning electron microscopy in the solid state.