Direct Experimental Evidence of Hot-Exciton Channels via Anti-Kasha T₂ Phosphorescence in High-Efficiency Red OLEDs

High-efficiency red-emissive organic materials are crucial for full-color displays and solid-state lighting but remain fundamentally constrained by the energy-gap law, which accelerates non-radiative decay in low-bandgap systems. Herein, two asymmetric donor-acceptor-donor' (D-A-D') red emitters, PPIPP and PPIPOX, were developed by integrating a benzothiadiazole (BT/BTZ) acceptor with phenoxazine and phenanthroimidazole donors. Both emitters exhibit hybridized local and charge-transfer (HLCT) excited states and favorable high-lying triplet-state alignment, enabling hot-exciton processes. Most importantly, distinct anti-Kasha T₂ phosphorescence was directly observed at low temperature, providing direct evidence that high-lying triplet excitons can persist despite the normally rapid T₂ → T₁ internal conversion. Furthermore, pronounced negative magneto-electroluminescence (MEL) under high magnetic fields supports spin-flip processes associated with high-lying reverse intersystem crossing (hRISC). The resulting organic light-emitting diodes (OLEDs) exhibit efficient red electroluminescence with high brightness. Mechanistic investigations reveal that triplet-triplet annihilation mainly replenishes high-lying triplet populations, while efficient exciton conversion is dominated by the hot-exciton high-lying reverse intersystem crossing (hRISC) pathway. This work establishes anti-Kasha T₂ phosphorescence together with negative MEL as powerful experimental signatures for validating hot-exciton mechanisms in red OLED emitters.