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

The incorporation of -butyl groups and spiro-functionalization into C═O/N-embedded multiresonance thermally activated delayed fluorescence (MR-TADF) systems has yielded materials with superior narrowband emission and excellent color purity. To elucidate the mechanisms underlying the enhanced properties, we present a theoretical study of a series of fused nitrogen/carbonyl derivatives with narrower emission profiles. The key steric factors that contribute to narrowband emission were identified through energy decomposition analysis, induced by structural relaxation in states S and S. Additionally, we achieved potential narrower-band and deep-blue emission by targeting the suppression of vibrational coupling effects. This work provides compelling evidence that a 1--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.