Origins of Narrowband\nEmission in Nitrogen/Carbonyl\nMultiresonance Thermally Activated Delayed Fluorescence Emitters:\nSteric Locks and Vibrational Coupling Effects

The incorporation of <i>tert</i>-butyl groups\nand spiro-functionalization\ninto CO/N-embedded multiresonance thermally activated delayed\nfluorescence (MR-TADF) systems has yielded materials with superior\nnarrowband emission and excellent color purity. To elucidate the mechanisms\nunderlying the enhanced properties, we present a theoretical study\nof a series of fused nitrogen/carbonyl derivatives with narrower emission\nprofiles. The key steric factors that contribute to narrowband emission\nwere identified through energy decomposition analysis, induced by\nstructural relaxation in states S<sub>0</sub> and S<sub>1</sub>. Additionally,\nwe achieved potential narrower-band and deep-blue emission by targeting\nthe suppression of vibrational coupling effects. This work provides\ncompelling evidence that a 1-<i>tert</i>-butyl substitution,\nacting as an end lock, offers minimal reorganization energy and optimal\nstructural stability when combined with a fused lock. Furthermore,\nnew compounds such as 1tBuCZQ and 1tBuDQAO have been identified as\npromising MR-TADF emitters, delivering ultranarrowband emission as\nhigh-quality organic light-emitting diodes.