Thermochromic luminescence in dual-dye-doped liquid crystal mixture induced by varying the energy transfer rate

Chromism in luminescent materials have attracted increasing interest for their unique feature of controllable luminescent colours and their applicability in various fields. Here we report a novel thermochromic luminescent liquid crystals (LC) system containing two luminescent dyes which do not have chromic properties themselves. The luminescent colours of the dual dye-LC mixture can be reversibly and continuously controlled between orange and blue by applying thermal stimuli; the relative intensity ratio between the two luminescent peaks is continuously controlled. This unprecedented phenomenon originates mainly from varying efficiency of the Förster energy transfer rates between two dye dopants depending on the phases of host LCs. The energy transfer rate is the highest in the crystal phase, giving rise to orange emission, and it is lowest in the isotropic state, producing blue emission. In the middle phases (nematic and smectic), mixed colours of these two colours are obtained. The proposed methodology, which takes advantage of controllable energy transfer rates, can greatly broaden the range of affordable dye materials for chromic luminescence systems because common fluorescent dyes can be used instead of special chromic compounds.