Centrosymmetric Coupling Strategy Enabling High‐Color‐Purity Green Multi‐Resonance Thermally Activated Delayed Fluorescence Emitters With 40.84% EQE and CIE _y > 0.70

ABSTRACT

Extending the narrow emission characteristics of multi‐resonance thermally activated delayed fluorescence (MR‐TADF) to all three‐primary‐color emitters is of great significance for next‐generation ultra‐high definition (UHD) displays. Herein, we propose a novel centrosymmetric coupling strategy to construct two MR‐TADF isomeric molecules ( 2pBZT‐2BNO and 2mBZT‐2BNO ) by covalent dimerization. Both molecules achieve ultra‐narrow green emission in toluene solutions with full‐width at half‐maximum (FWHM) values of 17 nm (0.086 eV) and 18 nm (0.085 eV), respectively. By combining experimental and theoretical investigations, the spectral narrowing mechanism was elucidated based on this centrosymmetric topological structure through the symmetry dilution of charge variations upon electronic excitation. Non‐sensitized organic light‐emitting diodes (OLEDs) based on these two isomers exhibit outstanding maximum external quantum efficiencies (EQEs) of 37.90% ( 2pBZT‐2BNO ) and 38.00% ( 2mBZT‐2BNO , CIE _y  = 0.71). Notably, with the assistance of sensitizer, ternary‐doped devices of 2pBZT‐2BNO and 2mBZT‐2BNO push EQEs to nearly 40% (39.86% and 40.84%) while maintaining well‐suppressed efficiency roll‐offs. These performances represent the state of the art for narrow‐band MR‐TADF green OLEDs. Overall, this work not only provides a novel centrosymmetric coupling strategy for designing high‐performance narrowband green emitters, but also offers profound insights into intrinsic spectral narrowing mechanisms in MR‐TADF systems.