Origins of Narrowband Emission in Nitrogen/Carbonyl Multiresonance Thermally Activated Delayed Fluorescence Emitters: Steric Locks and Vibrational Coupling Effects

-butyl substitution, acting as an end lock, offers minimal reorganization energy and optimal structural stability when combined with a fused lock. Furthermore, new compounds such as 1tBuCZQ and 1tBuDQAO have been identified as promising MR-TADF emitters, delivering ultranarrowband emission as high-quality organic light-emitting diodes.