Spiro‐fluoreno‐imidazole (SFI) Emitters Efficiently Harvesting Hot Excitons for Deep‐Blue Organic Light‐Emitting Diodes with CIE y Below 0.046

Abstract Hot exciton emitters have emerged as promising candidates for efficient deep‐blue fluorescent organic light‐emitting diodes (OLEDs), by enabling triplet exciton harvesting through high‐lying reverse intersystem crossing. However, limited molecular catalogues impede comprehensive mechanistic investigations and device performance elevations. Here, for the first time, we introduced a novel rigid and steric spiro‐fluoreno‐fused imidazole (SFI) skeleton to engineer high‐performance deep‐blue hot exciton emitters. Three derivatives, Cz‐SFI, tCz‐SFI, and 3Cz‐SFI, were strategically designed to modulate donor‐acceptor orbital overlap and hybridized local and charge‐transfer character. Detailed theoretical calculations and transient absorption spectra clearly revealed the dominant channel of high‐lying reverse intersystem crossing, with a rapid rate constant up to 3.36 × 10 9 s −1 . Remarkably, the tCz‐SFI‐based OLED device achieved deep‐blue electroluminescence with an exciton utilization efficiency of 83.2% and an unprecedented external quantum efficiency of 12.55% by efficiently harvesting hot triplet excitons. The device exhibited stable electroluminescence peaking at 410 nm, corresponding to Commission Internationale de l'Eclairage coordinates of (0.161, 0.043), setting a new benchmark for deep‐blue hot‐exciton OLEDs.