Extending Frontier Molecular Orbitals Delocalization via Donor Fusion: Construct Long-Wavelength Twisted Multiple Resonance Emitters with Efficient Narrowband Emission

Abstract

Multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters are promising candidates for ultra-high-definition organic light-emitting diodes (OLEDs) displays. Here, we propose a general strategy for post-functionalizing MR core, introducing donors on the lowest unoccupied molecular orbital (LUMO) position via Suzuki coupling and then fusing the donors to the highest occupied molecular orbital (HOMO) position via Scholl cyclization, which can extend frontier molecular orbitals delocalization to construct long-wavelength narrowband emitters. Two proof-of-concept molecules, BN-PhAzCz and BN-tCzAzPh, are constructed, achieving large spectral red-shifts while maintaining narrowband emission. The distorted configuration resulting from the hetero[6]helicene structure introduces twisted π–π* transition, which effectively enhances spinorbit coupling, ultimately enabling molecule BN-PhAzCz to achieve a fast reverse intersystem crossing rate (kRISC = 6.4 × 104 s−1). BN-PhAzCz-based single-host OLED exhibits green emission at 528 nm with Commission Internationale de L'Eclairage (CIE) coordinates of (0.26, 0.69) and maximum external quantum efficiency (EQE) of 38.2% with efficiency roll-off of 28.0% at 1000 cd m−2 under 5 wt% doping concentration. Even at 20 wt% doping concentration, the maximum EQE remains at 29.6% with little change in the spectral profile.