A Terminal Carbazole Fused Ladder‐Type B,N‐Doped Multiresonant Thermally Activated Delayed Fluorescence Emitter: A Strategy for Deep‐Blue Emission

Attaining precise control of emission within the blue spectral region remains inherently challenging for ladder‐type π‐extended multiresonant thermally activated delayed fluorescent (MR‐TADF) compounds, despite their many advantageous design features. Here, we aim to address the undesired redshifting of the emission in ladder‐type MR‐TADF compounds by incorporating terminal fused carbazoles. We report two MR‐TADF compounds, B2N3 and B3N3 , the latter of which is a p‐ and n‐doped heptacene polycyclic aromatic π‐hydrocarbon (PAH), both of which contain fused carbazoles at both ends of the molecule. Relative to the previously reported MR‐TADF compound α ‐3BNMes , the emission of B3N3 is blueshifted and narrower, emitting at λ _PL of 427 nm and having a full width at half maximum (FWHM) of 25 nm in toluene. In contrast, B2N3 , which is more conformationally flexible, emits at λ _PL of 441 nm and has a FWHM of 32 nm in toluene. Sensitized organic light‐emitting diodes (OLEDs) using B2N3 and B3N3 as terminal emitters showed EQE _max of 12.8% and 10.7% emitting with CIE coordinates of (0.15, 0.10) and (0.16, 0.09), respectively.