Abstract
Narrowband blue multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters are promising for high-colour-purity OLEDs, but structural modifications that preserve spectral purity can compromise delayed fluorescence. Here, two closely related tetraboron MR-TADF emitters, BO-4B and c-BO-4B, were designed to examine the effect of central-core planarisation on blue emission and triplet harvesting. The compounds were found to retain narrowband deep-blue fluorescence in dilute toluene, with emission maxima at 449 and 444 nm and full widths at half maximum of 14 and 19 nm, respectively. Quantum-chemical calculations indicate similar bright MR-type S1 states but a larger singlet-triplet gap (ΔEST) for planarised c-BO-4B. Low-temperature fluorescence and phosphorescence measurements of PMMA films containing 1 wt% emitter show that core planarisation leaves the S1 energy nearly unchanged while lowering T1, thereby increasing ΔEST from 95 to 165 meV. Time-resolved fluorescence reveals an identical prompt fluorescence quantum yield of 33%, whereas the delayed fluorescence yield decreases from 45 to 3%. Within a simplified kinetic model, this corresponds to a two-order-of-magnitude decrease in a reverse intersystem crossing rate. These findings suggest that core planarisation preserves a narrow prompt emission but strongly suppresses a delayed fluorescence by unfavourably shifting triplet-state energetics.