Luminescent liquid crystals containing a sulphur-based heterocyclic core

Two series of highly pi-conjugated calamitic liquid crystals containing a sulphur-based heterocyclic core, 1,3,4-thiadiazole or 1,3-thiazole ring, were synthesised and characterised. The part containing the heterocycle was linked to a pi-extended mesogenic unit (phenyl, biphenyl, naphthyl and phenylbenzoate) through a carbon-carbon triple bond by Sonogashira's cross coupling, in order to exhibit the liquid crystalline and luminescent properties. These compounds exhibited liquid crystalline behaviour in a special SmC phase presented by all compounds, characterised by polarised optical microscopy, differential scanning calorimetry and X-ray diffraction. Some compounds in the thiazole series showed much higher ordered phases below the SmC phase characterised as tilted hexatic phases. All the compounds showed blue luminescence in the solution and a study regarding temperature-dependent emission was performed in their pure state. The phase transitions upon heating were followed by a significant decrease in the emission intensity due to an increase of self-quenching aggregates and also by non-radiative decaying processes that took place with increasing temperature. In the thiazole series, the entrance in the liquid phase was accompanied by a small increase in the luminescence, an indicator of better excimer formation in its molecular packing within the smectic phase that is lost when it enters the isotropic state.