para-Boron Dendritic Modulation of MR-TADF Emitters Enables High-Efficiency Narrowband Solution-Processed OLEDs

The rigid planar structure of multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters delivers narrowband emission with high color purity but also induces spectral broadening and poor solution processability, hindering their application in solution-processed OLEDs. Herein, we develop a dendritic functionalization strategy at the para-position of boron in the classical MR-TADF emitter BCz-BN by tuning the electron-withdrawing characteristics of peripheral dendrons, affording two solution-processable emitters, CN-BN and Ph-BN. The cyano-containing CN-BN exhibits accelerated reverse intersystem crossing but broadened emission due to the presence of strong charge-transfer character. By removing the cyano group, Ph-BN preserves the intrinsic MR character with reduced excited-state relaxation. Combined with the steric protection of the dendritic units, Ph-BN achieves a narrow full width at half maximum (fwhm) of 28 nm and a high EQEmax of 24.9% in solution-processed OLEDs. Notably, the fwhm of Ph-BN increases by only 5 nm from photoluminescence to electroluminescence, whereas BCz-BN exhibits a much larger increase of 16 nm.