Deep-Blue Hot Exciton Emitters Approaching the BT.2020 Standard: Spiro-Fluoreno-Imidazole Acceptors with Twisted D-π-A Architectures.

Abstract

Achieving deep-blue electroluminescence with high efficiency and color purity that meets the BT.2020 standard remains a major challenge for organic light-emitting diodes. Here, we report two D-π-A emitters, SFIA-1 and SFIA-2, based on a spiro-fluorene-imidazole acceptor (A) and a 10H-spiro(acridine-9,9'-fluorene) donor (D). By strategically introducing a para-phenylene bridge, the molecules adopt highly twisted conformations. Experimental and theoretical analyses further reveal CT-dominated, hybridized local-excited and charge-transfer states. Both emitters exhibit wide optical band gaps (>3.3 eV). A negligible singlet-triplet energy gap of 0.01 eV, combined with a moderate spin-orbit coupling, activates an efficient high-lying reverse intersystem crossing "hot exciton" channel. As a result, the SFIA-1-doped device achieves a maximum external quantum efficiency (EQE) of 7.53% with an electroluminescence peak at 425 nm, while SFIA-2 delivers an electroluminescence peak at 420 nm with Commission Internationale de l'Éclairage coordinates of (0.159, 0.043), closely aligned with the BT.2020 blue light standard. Furthermore, the non-doped devices based on SFIA-1 and SFIA-2 exhibit maximum EQEs of 6.29 and 10.89%, respectively. Therefore, SFIA-2 was utilized as a host for PO-01-TB, tCzphB-Ph, and Ir(mphmq)2(tmd) emitters, yielding maximum EQEs of 30.47, 24.09, and 15.57%, respectively. These results demonstrate a molecular design strategy for ultra-deep-blue hot-exciton materials and high-performance hosts that reconciles the trade-off between exciton utilization efficiency and color purity.