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.