Dielectric permittivity properties of a fulgide dye guest-host liquid crystal

The dielectric permittivity of the nematic liquid crystalline mixture E7, doped with a low concentration of the photochromic material thiophene fulgide, was studied and compared with the properties of the pure E7. Fulgides are a group of thermally stable photochromic materials. On irradiation with ultraviolet light, a ring-closure occurs, giving an isomer which is stable if the dye is kept in the dark. The isomerism induces changes to the steric, dipolar and electronic conjugation properties of the species. A capacitive technique was used to determine the dielectric constants of the fulgide-doped liquid crystalline mixtures. The parallel and perpendicular components of dielectric constant were measured using a single cell, in the latter case in the presence of a magnetic field. Measurements were made on the photochromic system before and after ultraviolet irradiation and a marked variation in the dielectric properties of the mixture was observed. This was shown to be entirely due to the differences in phase transition temperatures between the irradiated and non-irradiated guest-host mixtures.