Ultrafast activationless hot-exciton triplet harvesting of a hybrid local and charge-transfer emitter for blue hyperfluorescent organic light-emitting diodes.

This work reports a hybridized local and charge-transfer (HLCT) emitter (EHBIPOAz) containing a rigid 11,13-dihydro-12H-dibenzo[a,c]imidazo[4,5-i]phenazin-12-one acceptor fragment orthogonally fused with 5H-dibenzo[b,f]azepine donor moieties. Designing EHBIPOAz, we aimed to suppress vibrational reorganization while enabling ultrafast activationless hot-exciton mediated reverse intersystem crossing (hRISC). This rigid structure minimises the reorganization energy to 0.08-0.09 eV, thereby preserving a locally excited-dominated emission with a narrow bandwidth. The molecular dispersion of EHBIPOAz in inert polymer Zeonex exhibits a high photoluminescence quantum yield of 92%, with a prompt fluorescence lifetime of 2.19 ns and an ultrafast delayed component with the lifetime of 3.94 ns, consistent with activationless hRISC. Time-dependent density functional theory-derived spin-orbit couplings treated within Marcus theory and a two-level master equation support an ultrafast (khRISC ≥ 1011 s-1) activationless population transfer via multiple Tn → S1 channels. Hyperfluorescent organic light-emitting diodes fabricated using EHBIPOAz exhibit high-color-purity electroluminescence with a relatively narrow full width at half maximum of 55 nm. The devices reach a maximum external quantum efficiency of 23.4%, supported by a high horizontal dipole orientation of 93%. These results demonstrate that rigid HLCT architecture can enable hot triplet harvesting without heavy metals, providing efficient and pure-color emitters for high-speed OLEDs.