Triplet energy exchange between fluorescent and phosphorescent organic molecules in a solid state matrix

The measured lifetime of the triplet state in a phosphorescent iridium complex [Ir(ppy)(3)] lengthens as the concentration of a fluorescent diamine derivative (TPD) in their mixture in a polycarbonate (PC) matrix increases. The effect is explained by the Dexter-type triplet energy exchange between these molecules. The localization Bohr's radius is found to be 1.1 Angstrom for [Ir(ppy)(3)] and 2.4 Angstrom for TPD molecules. The long-living triplet states of TPD form a transient reservoir of excitation energy for short-living triplets of [Ir(ppy)(3)] (the excitation energy "reservoir effect"). The rates of forward and back energy transfer are interrelated through their energy, intrinsic lifetimes and intermolecular distance. The critical donor-acceptor distance at which energy transfer competes equally with the total rate of other triplet exciton decay pathways is found to be R-0=(1.05+/-0.02) nm. These results may be the first quantitative analysis of the "reservoir effect" in solid solutions.