Intramolecular-locking modification enables efficient asymmetric donor-acceptor-donor' type ultraviolet emitters for high-performance OLEDs with reduced efficiency roll-off and high color purity.

Developing high-performance ultraviolet organic light-emitting diodes with low efficiency roll-off and high color purity remains challenging due to their inherent wide-bandgap characteristics. In this work, we present an intramolecular noncovalent bond locking strategy to modulate donor-acceptor-donor' (D-A-D') type ultraviolet fluorophores (mPImCZ2F, mPIoCZ2F and mPImCP2F) with a hot-exciton mechanism. Notably, these asymmetric emitters exhibit significantly enhanced bipolar transport capacity and fluorescence efficiency compared to their counterparts. Among them, mPIoCZ2F exhibits a more remarkable intramolecular locking effect due to multiple C-H⋯F interactions and -substitution-induced steric hindrance, which endows it with a higher radiation rate, narrower emission spectrum, and more balanced charge transport. Consequently, the mPIoCZ2F-based non-doped device achieves an electroluminescence (EL) peak at 393 nm with a maximum external quantum efficiency (EQE) of 6.62%. Moreover, in the doped device, mPIoCZ2F emits stable ultraviolet light with an EL peak at 391 nm and a full width at half maximum (FWHM) of 40 nm, corresponding to color coordinates of (0.167, 0.025). It also exhibits an exceptionally high EQE of 8.71% and minimal efficiency roll-off (7.95% at 1000 cd m), ranking among the best EL efficiencies reported for UV-OLEDs at high brightness levels.