Understanding and modulating the horizontal orientations and short‐range charge transfer excited states for high‐performance narrowband emitters

Abstract Recently, a novel paradigm of boron‐ and nitrogen‐embedded polycyclic nanographites featuring multiple resonance thermally activated delayed fluorescence (MR‐TADF) has garnered substantial interest due to their extraordinary attributes of efficient narrowband emissions with small full width at half maxima (FWHMs). Despite an array of diverse color tuning strategies, it remains elusive how to effectively manipulate device efficiencies without altering the materials' intrinsic MR‐TADF characteristics. Here, an advanced ‘non‐conjugate fusion’ design methodology was proposed, aimed at dramatically amplifying the horizontal orientations of MR‐TADF emitters while preserving the short‐range charge‐transfer properties. As envisioned, when compared to the classical BCz‐BN mother core, the proof‐of‐concept emitter mICz‐BN achieved an impressively enhanced horizontal dipole ratio (83% vs. 75%) at analogous emission wavelengths (∼486 nm), FWHMs (∼26 nm) and photoluminescence quantum yields (∼93%). Consequently, the external quantum efficiency of the optimized device yielded a performance enhancement of 1.2‐fold (30.5% vs. 25.3%) whilst keeping the spectrum almost unchanged.