Liquid-crystalline Triplet Emitters of Iridium(III)

Flat-panel displays now outsell those based on cathode ray tube technologies and by far the most popular of these are liquid crystal displays. However, OLEDs represent a competitive technology that can have niche applications and that are very compatible with printing technologies and mechanically flexible displays. Organic LED materials (e.g. polymers LEDs) emit from singlet states created in the device by charge injection, but the triplet states produced have lifetimes that are too long (i.e. milliseconds) to be useful. As three times as many triplet state are produced compared to singlet states, efficiency in these systems is not optimised. To be useful, triplet emitters need much shorter lifetimes. This can be accomplished in metal complexes containing heavy transition elements where efficient spin-orbit coupling 'circumvents' the spin-forbibben nature of triplet decay, allowing emission from singlet and triplet states. The metal complexes currently used in devices contain iridium(III) (these are the red emitting component).A significant development in the application of triplet emitters could be realised if the complexes were prepared as liquid crystalline derivatives, as this could lead to alignment and, therefore, polarised emission. White, polarised emission would greatly improve the efficiency of backlighting for liquid crystal displays by removing the need for the back polariser, reducing absorptive losses hugely. However, liquid crystallinity is not readily compatible with the geometries of the iridium(III) complexes (octahedral).We have now demonstrated that this incompatibility can be addressed and we have the first examples of LC iridium emitters. In the proposal for development of these systems we propose:-tuning of the available liquid crystal range;-modified design to allow ready tuning of the chromophore;-evaluation of device characteristics to provide essential data to potential end users.