Triplet-energy-lowered multiple-resonance organoboron emitters for highly efficient phosphor-sensitized fluorescent OLEDs with suppressed efficiency roll-off.

By adopting an asymmetric molecular design, benzo[g]benzofuro[2,3-b]carbazole and benzo[g]benzo[4,5]thieno[2,3-b]carbazole motifs were strategically incorporated into a multiple-resonance (MR) skeleton to lower its triplet-state energy. This molecular architecture suppresses the thermally activated delayed fluorescence (TADF) pathway while preserving the intrinsic narrowband emission characteristics of the MR framework, thereby realizing efficient narrowband fluorescence. In toluene, BNCz-O and BNCz-S exhibit narrowband emissions peaking at 491 and 502 nm, with full width at half maximum (FWHM) values of 22 and 20 nm, respectively, while BNCz-O shows a photoluminescence quantum yield of 82%. Phosphor-sensitized organic light-emitting diodes (OLEDs) based on BNCz-O demonstrate an electroluminescence peak at 489 nm, an FWHM of 28 nm, and a maximum external quantum efficiency (EQE) of 31.4%. The device also exhibits minimal efficiency roll-off, maintaining an EQE of 29.9% at a luminance of 1000 cd m-2. These results demonstrate the potential of BNCz-O for high-efficiency OLEDs with suppressed efficiency roll-off and provide a viable molecular design strategy for developing next-generation narrowband fluorescent emitters.