Addressing fluorescence and liquid crystal behaviour in multi-mesogenic BODIPY materials

Abstract

Two series of BODIPYs (4,4′-difluoro-3a,4a-diaza-s-indacene) bearing mesogenic substituents have been synthesised and their fluorescent and self-assembly behaviour has been observed and characterised. In order to induce a strong preference for liquid crystallinity, two mesogenic units have been attached to each fluorophore. The compounds were prepared by microwave-assisted Suzuki couplings in moderate yields. Two different, but related, mesogens were employed but only one series was found to exhibit the formation of liquid crystal mesophases. Monotropic nematic phases, observed by differential scanning calorimetry (DSC) and optical polarising microscopy (OPM), were adopted over varying temperature ranges depending on the degree of alkyl substitution on the fluorophore. Increasing alkyl substitution caused a gradual increase in fluorescence quantum yield due to rotational restriction of the BODIPY 8-phenyl ring, however, this also disrupted the formation of the nematic phase causing a reduction in the temperature range over which the mesophase was exhibited. The large, branched structure of the compounds prepared caused them to have a high viscosity in both the nematic and isotropic liquid phase. Due to the high viscosity, the Schlieren texture of the nematic phase could only be observed after several hours annealing.