  The development of luminescent organic radicals has resulted in materials
with excellent optical properties for near-infrared (NIR) emission.
Applications of light generation in this range span from bioimaging to
surveillance. Whilst the unpaired electron arrangements of radicals enable
efficient radiative transitions within the doublet-spin manifold in organic
light-emitting diodes (OLEDs), their performance is limited by non-radiative
pathways introduced in electroluminescence. Here, we present a host:guest
design for OLEDs that exploits energy transfer with demonstration of up to 9.6%
external quantum efficiency (EQE) for 800 nm emission. The
tris(2,4,6-trichlorophenyl)methyl-triphenylamine (TTM-TPA) radical guest is
energy-matched to the triplet state in a charge-transporting
anthracene-derivative host. We show from optical spectroscopy and
quantum-chemical modelling that reversible host-guest triplet-doublet energy
transfer allows efficient harvesting of host triplet excitons.
