Highly dichroic benzo-2,1,3-thiadiazoledyes containing five linearly π-conjugated aromatic residues, with fluorescent emission ranging from green to red, in a liquid crystal guest–host system

A number of fluorescent benzothiadiazole dyes have been synthesised and their optical properties, when dispersed in a liquid crystal host phase, examined. These dyes have five linearly π-conjugated aromatic ring systems (both with and without ethene and 1,3-butadiene spacers). The liquid crystalline host medium, used was the nematic phase of MLC-2039. The emission colours observed ranged from green to red, depending on the unsaturated chains attached at the 4- and 7- positions of the bezo-2,1,3-thiadiazole core. A highly dichroic and efficient fluoresence was observed. The dye 4,7–bis[5-bis(phenylethenyl)thiophen-2-yl]-2,1,3-benzothiadiazole is of particular importance. This is the first example of a dye emitting strong red fluorescence (633 nm). When dispersed in MLC-2039, dichroic ratio values up to 12.3 were obtained.