Existence of optimum intermolecular spacing for maximum exciton diffusion length in tris(2-phenylpyridine) iridium(III)

A study of one-dimensional exciton diffusion in tris(2-phenylpyridine) iridium(III) molecules as a function of intermolecular spacing is reported. We show for the first time the existence of an optimum intermolecular spacing that gives the longest diffusion length in [Ir(ppy)3] molecules. When the intermolecular spacing is too short, the long-range excitonic quenching becomes dominant; on the other hand, when it is too long, the exciton energy transfer is miniscule. A specially-designed organic light-emitting diode with separate exciton formation and emission was constructed to measure the one-dimensional exciton diffusion length. The maximum one-dimensional exciton diffusion length for this material is around 8.5 nm for an inter-molecular separation of around 4.5 nm.