Spiro-fluoreno-imidazole (SFI) Emitters Efficiently Harvesting Hot Excitons for Deep-Blue Organic Light-Emitting...

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 × 10⁹ 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.