Doublet fluorescence from organic radicals has been suggested as a promising route to achieve high efficiency in electroluminescence (EL) with nanosecond decay lifetime, especially for deep red/near-infrared (NIR) emission. Here, a highly efficient and bright doublet emissive system is suggested by combining a thermally activated delayed fluorescence (TADF) host supporting both electron and hole transport and a tris(2,4,6-trichlorophenyl)-methyl-based radical emitter. Strong NIR steady-state photoluminescence (PL) by host photoexcitation demonstrates effective singlet-to-doublet Fӧrster resonance energy transfer. Strong temperature dependence in the delayed emission of transient PL profiles suggests additional energy transfer pathways, in particular triplet-to-doublet Dexter energy transfer. Turning to EL devices, a high maximum external quantum efficiency and radiance of 17.4% and 110,000 mW sr-1 m-2 is achieved with a peak emission wavelength of 707 nm. This new doublet EL design shows the disruptive potential of organic radicals for NIR light-emitting technologies.
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