Saturated carbazole-embedded BN-aromatic systems as narrowband sky-blue emitters

Abstract

Due to their rigid structure and short-range charge transfer emissive excited states, multiresonant thermally activated delayed fluorescence (MR-TADF) emitters provide exceptional color purity due to their narrowband emission. Many examples are based on nitrogen/boron-doped polycyclic aromatic hydrocarbons. The emission color is in part modulated by the strength of the donor heterocycle. In an effort to shift the emission to the blue, we explore in this study the partial saturation of the carbazole donor. We designed four emitters-tButHCzB, DtButHCzB, SpAc-tButHCzB, and SpAc-DtButHCzB featuring tetrahydrocarbazole donor moieties, which disrupt the planarity and reduce the conjugation length. Theoretical calculations predict moderate singlet–triplet excited-state energy gaps (ΔE_ST = 250–270 meV) and low-lying T₂ and T₃ states, suggesting that these compounds should exhibit TADF. In toluene, the four compounds emit in the sky-blue region (λ_PL = 476–480 nm), with narrow full-width at half-maximum (FWHM) values (18–21 nm) and high photoluminescence quantum yields (Φ_PL) of 86–91%. In 1 wt% doped films in mCP, however, their Φ_PL drops to between 40–45% and their emission slightly red-shifts to λ_PL of 483–491 nm. Temperature-dependent time-resolved PL measurements confirm that these compounds exhibit TADF. This study offers valuable insights into a region of chemical space scantly explored in MR-TADF emitter design.