Endothermic energy transfer: A mechanism for generating very efficient high-energy phosphorescent emission in organic materials

Intermolecular energy transfer processes typically involve an exothermic transfer of energy from a donor site to a molecule with a substantially lower-energy excited state (trap). Here, we demonstrate that an endothermic energy transfer from a molecular organic host (donor) to an organometallic phosphor (trap) can lead to highly efficient blue electroluminescence. This demonstration of endothermic transfer employs iridium(III)bis(4,6-di-fluorophenyl)-pyridinato-$N,C2′)picolinate$ as the phosphor. Due to the comparable energy of the phosphor triplet state relative to that of the 4,4^′-$N,N′$-dicarbazole-biphenyl conductive host molecule into which it is doped, the rapid exothermic transfer of energy from phosphor to host, and subsequent slow endothermic transfer from host back to phosphor, is clearly observed. Using this unique triplet energy transfer process, we force emission from the higher-energy, blue triplet state of the phosphor (peak wavelength of 470 nm), obtaining a very high maximum organic light-emitting device external quantum efficiency of (5.7±0.3)% and a luminous power efficiency of (6.3±0.3)lm/W.