Space-Crowded Donor-Acceptor-Donor Ultraviolet Emitters for Efficient Narrowband Electroluminescence with Minimal Roll-Off.

Ultraviolet (UV) organic light-emitting diodes (OLEDs) hold great promise in phototherapy, information storage, excitation light sources, chemical/biological sensors, anticounterfeiting, and other fields. However, the design and synthesis of efficient UV materials with emission peaks below 390 nm and the development of high-efficiency and low roll-off UV-OLEDs remain formidable challenges. In this work, four effective wide-bandgap emitters, namely, DMID-pPy, BFCz-pPy, DMID-mPy, and BFCz-mPy, were developed based on a space-crowded donor-acceptor-donor molecular design strategy, where two carbazole-fused donors and pyridine are ortho-linked via a benzene ring. This approach effectively limits π-conjugation to achieve short-wavelength UV emission while utilizing steric hindrance and van der Waals interactions to enhance the molecular rigidity and further suppress the spectrum broadening. The meta-substitution of pyridine endows DMID-mPy and BFCz-mPy with a weakened charge transfer state, small structural deformation, a short radiative lifetime, and multiple high-lying reverse intersystem crossing channels. As a result, DMID-mPy- and BFCz-mPy-based devices exhibit stable and narrow UV emission with peaks of 387 and 384 nm, full width at half-maximum of 39 and 41 nm, and color coordinates of (0.166, 0.031) and (0.167, 0.029), respectively. More remarkably, they achieve excellent maximum external quantum efficiencies of 7.88% and 7.90% and maintain high values of 7.66% and 7.59% at 500 cd m-2, showing a minimal efficiency roll-off. This work presents a feasible strategy for developing high-performance UV materials and devices.