Saturated Carbazole-Embedded BN-Aromatic Systems as Narrowband Sky-Blue Emitters

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 polycy-clic aromatic hydrocarbons. The emission color is in part modulated by the strength of the do-nor hetereocycle. In an effort to shift the emission to the blue, we explore in this study the par-tial saturation of carbazole donor. We designed four emitters-tButHCzB, DtButHCzB, SpAc-tButHCzB, and SpAc-DtButHCzB featuring tetrahydrocarbazole donor moieties, which disrupt planarity and reduce conjugation length. Theoretical calculations predict moderate singlet-triplet excited-state energy gaps (EST = 250-270 meV) and low-lying T2 and T3 states, suggesting that these compounds should be TADF. In toluene, the four compounds emit in the sky-blue (λPL = 476-480 nm), with narrow full-width at half-maximum (FWHM) values (18-21 nm) and high photoluminescence quantum yields (ΦPL) of 86-91%. In 1 wt% doped films in mCP, however, their ΦPL drops to between 40-45% and their emission slightly red-shifts to λPL of 483-491 nm. Temperature-dependent time-resolved PL measurements confirm that these compounds are TADF. This study offers valuable insights into a region of chemical space scantly explored in MR-TADF emitter design.