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

Intermolecular energy transfer processes typically involve an exothermic tr ansfer of energy from a donor site to a molecule with a substantially lower -energy excited state (trap). Here, we demonstrate that an endothermic ener gy transfer from a molecular organic host (donor) to an organometallic phos phor (trap) can lead to highly efficient blue electroluminescence. This dem onstration of endothermic transfer employs iridium(III)bis(4,6-di-fluorophe nyl)-pyridinato-N,C-2')picolinate as the phosphor. Due to the comparable en ergy of the phosphor triplet state relative to that of the 4,4'-N,N-'-dicar bazole-biphenyl conductive host molecule into which it is doped, the rapid exothermic transfer of energy from phosphor to host, and subsequent slow en dothermic transfer from host back to phosphor, is clearly observed. Using t his unique triplet energy transfer process, we force emission from the high er-energy, blue triplet state of the phosphor (peak wavelength of 470 nm), obtaining a very high maximum organic light-emitting device external quantu m efficiency of (5.7 +/-0.3)% and a luminous power efficiency of (6.3 +/-0. 3)lm/W. (C) 2001 American Institute of Physics.