Breaking the Fused Ring: A Novel MR-TADF Skeleton for Solution-Processed Pure-Red OLEDs
Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters are pivotal for achieving high color purity and 100% internal quantum efficiency in organic light-emitting diodes (OLEDs). However, extending emission to the red region (>600 nm) remains challenging due to limited MR cores and the synthetic complexity of fused-ring extensions. Here, we report a novel strategy that breaks the fused ring by employing a linear conjugated diene linker between nitrogen and carbonyl moieties, enabling simultaneous introduction of three MR units in one step with high yield. Two emitters, NF-CON1 and NF-CON2, exhibit pure-red emission at 620 nm with narrow full-width at half-maximum (FWHM) below 35 nm in dilute toluene, small Stokes shifts of 26 nm, and weak solvatochromic shifts (~25 nm from toluene to ethanol), confirming the MR-characteristic short-range charge transfer. The emitters show low reorganization energies (0.209 eV for NF-CON1 and 0.194 eV for NF-CON2), singlet-triplet energy gaps of ~0.2 eV, and high reverse intersystem crossing rates of ~3.0×10^5 s^-1. In doped films with m-MTDATA, delayed lifetimes of 31–40 μs are achieved. These properties, combined with high oscillator strengths, ensure efficient RISC and high IQE. The facile synthesis and solution-processability of this MR-TADF skeleton address the bottlenecks of cost and scalability, offering a promising route for practical pure-red OLEDs.