Long-range resonantly enhanced triplet formation in luminescent polymers doped with iridium complexes

he current drive to produce cheap flexible plastic displays(1) has led to rapid improvements in device effciency. Inclusion of highly phosphorescent heavy-metal organic complexes as dopants ensures that both singlet and triplet excitations formed on charge recombination can be used efficiently to emit light(2). However, the inclusion of these dopants affects the photophysics of the host in a surprising way, generating a ten- to twentyfold instantaneous increase in the number of host triplet states, independent of host triplet energy, quenching up to 95% of all singlet states. Once created however, these triplets are only weakly quenched by the same mechanism. We ascribe this to a resonant, remote intermolecular heavy-atom effect that greatly increases the inter-system crossing rate of the host polymer(3) arising through the strong overlap of the delocalized pi orbitals of the host and ligands. This mechanism competes effectively with Forster energy transfer, and operates over large distances.