Achieving high efficiency ultrapure violet OLEDs with a CIEy coordinate of below 0.015 through precise manipulation of peripheral heavy atom sulfur

The development of efficient pure violet organic light-emitting diodes (OLEDs) featuring a low Commission Internationale de l'Éclairage (CIE) _y coordinate of below 0.02 remains a critical yet challenging objective. Herein, three ultrapure violet hot-exciton emitters, namely BO-2DBT, BO-3DBT, and BO-4DBT, have been developed through strategic integration of an oxygen-bridged cyclized boron (BO) skeleton with a dibenzothiophene (DBT) at varied substitution positions, where the effect of the regioisomerism of heavy sulfur atom was investigated. All three emitters demonstrate narrowband violet emission in toluene solution, with maxima centered at 405, 408, and 404 nm, respectively, and narrow full widths at half-maximum (FWHM) of 23, 22, and 22 nm, respectively. Theoretical analyses reveal significant high-lying reverse intersystem crossing rates (10⁶-10⁷ s^-1) across all emitters. Notably, BO-2DBT exhibits superior fluorescence efficiency, with its intersystem crossing rate from S₁ to T₁ reduced by over one order of magnitude compared to others, attributable to minimal spin-orbit coupling (0.059 cm^-1). Consequently, the optimized device employing BO-2DBT as an emitter achieves ultrapure violet electroluminescence with a peak at 405 nm, FWHM of 25 nm, and CIE coordinates of (0.166, 0.014). The device demonstrates a peak external quantum efficiency of 7.90%, retaining 7.67% at 500 cd m^-2. To the best of our knowledge, this work represents the first report of ultrapure violet OLEDs with a CIE _y coordinate <0.015 and establishes a new efficiency benchmark for this class of devices.