Centrosymmetric Molecular Strategy With Quadruple Boron–Nitrogen Heteroarenes Enabling Efficient Ultra‐Narrowband Emission

Realizing ultra‐narrowband multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) with long‐wavelength emission remains a formidable challenge due to the increased vibronic coupling and excited‐state relaxation with the extension of π‐conjugated framework. In this contribution, a proof‐of‐concept of centrosymmetric molecular strategy fused with quadruple boron–nitrogen heteroarenes is proposed to develop ultra‐narrowband long‐wavelength MR‐TADF emitters. Thus, two MR‐TADF emitters, called DCz‐MR and DAn‐MR, are prepared and characterized. Compounds DCz‐MR and DAn‐MR exhibit the emission peaks at 562 nm and 518 nm in toluene solution, respectively. Small full‐width at half‐maximum (FWHM) of only 16 nm (0.063 eV) for DCz‐MR and 12 nm (0.055 eV) for DAn‐MR are obtained in toluene, especially 14.2 nm (0.058 eV) for DCz‐MR and 8.2 nm (0.040 eV) for DAn‐MR in hexane. In solution‐processed phosphorescence‐sensitized organic light‐emitting diodes, DAn‐MR delivers a maximum external quantum efficiency of 24.8% with an electroluminescence FWHM of 27 nm. This research work provides a new method for designing ultra‐narrowband long‐wavelength MR‐TADF emitters.