Peripheral Nitrogen-Embedding Strategy for Fine Tuning the Emission Peak of MR-TADF Emitters

Abstract

Various molecular design strategies have broadened the emission range of multiple resonance-induced thermally activated delayed fluorescence (MR-TADF) materials to cover the visible spectrum, yet fine-tuning the emission peak remains a formidable challenge. In this study, we finely tune the emission by subtly modulating the electronic structure at the atomic level, based on the peripheral nitrogen-embedding strategy. Two novel MR-TADF emitters, γ-Cb-tCzBN and β-Cb-tCzBN, are designed by incorporating a nitrogen atom at the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) position of the carbazole moiety in tCzBN, respectively. This site-specific peripheral heteroatom engineering induces slight electronic variations, thus enabling fine modulation of the emission peak. In particular, γ-Cb-tCzBN exhibits stronger intermolecular interactions, leading to a higher degree of horizontal dipole orientation. Organic light-emitting diodes (OLEDs) based on γ-Cb-tCzBN, tCzBN, and β-Cb-tCzBN exhibit sharp sky-blue emission at 481, 484, and 486 nm, with full-width at half-maximums (FWHMs) of 35, 27, and 28 nm, and maximum external quantum efficiencies (EQE max ) of 25.5%, 18.1%, and 22.7%, respectively. This work presents an effective approach for fine-tuning the emission peak through subtle modification of the molecular framework while retaining its intrinsic MR characteristics.