Structure and Intermolecular Interactions in Ionic Liquid Crystals Doped with Electrochromic Viologen

Abstract

The results of an investigation of structural and electrooptical properties of potassium caprylate lyotropic liquid crystals (LCs) doped with electrochromic viologen are presented. The samples are colored when an electric field is applied. The color deepens with the voltage applied, and relevant features appear in electronic absorption spectra. Two stages may be recognized in the processes that occur in a sample in the field. The first stage (at 2–2.5 V) involves the formation of radical cations (RCs) from viologen molecules, which makes the samples blue (two absorption bands appear at wavelengths of 395 and 610 nm in optical spectra). At the second stage (at 3.5–4 V), the radical cations are dimerized, and the sample becomes red (the spectra show bands in the region of 365 and 520 nm). Absorption in a lyotropic LC increases with the viologen concentration increasing from 1 to 2 to 4%. Small-angle X-ray scattering was measured on unoriented samples of pure potassium caprylate with thicknesses ranging from 30 μm to 1 mm. In all samples, a smectic order was observed in lyotropic ionic LC (LILC). From the X-ray scattering data, the bilayer thickness and the smectic correlation length were computed. It was inferred that viologen molecules, when present in low concentrations (1–4%), are organically incorporated into the matrix without spoiling the layer order intrinsic to the matrix.