Macrocyclic Covalent Encapsulation of a Multi-Resonant Emitter: Understanding and Controlling Interactions in Highly Efficient Deep-Blue OLEDs.

Multi-resonant thermally activated delayed fluorescence (MR-TADF) emitters have emerged as popular candidates for the development of blue organic light-emitting diodes (OLEDs), offering narrowband emission, high photoluminescence quantum yields (PLQYs), and the ability to upconvert dark triplet states to bright singlet states. However, their planar polycyclic structures promote detrimental intermolecular interactions in the solid-state which diminish the color purity and introduce nonradiative loss pathways. Furthermore, the intrinsic luminescence of many MR-TADF emitters fails to satisfy the stringent color purity standards required for next-generation display technologies. Here, we synthetically address these issues by covalently encapsulating a blue-shifted MR-TADF emitter within a protective macrocyclic ring. We identify a previously undiscovered utility of macrocyclic encapsulation, whereby it can shield the MR core from the surrounding environment to enhance its radiative rate, PLQY, and reverse intersystem crossing (RISC) efficiency. Only with spectrally resolved transient photoluminescence measurements were we able to identify the weakly emissive aggregate and excimer species, and definitively confirm that the macrocycle suppresses their formation in the solid-state, thereby preserving narrowband deep-blue emission and reducing nonradiative losses. Notably, these performance enhancements were achieved without compromising thermal stability or vacuum-processability. When integrated into an OLED device based on the "hyperfluorescent" strategy, this emitter delivers an exceptional combined maximum external quantum efficiency (EQE) of 33% and (0.146, 0.046) CIE coordinates with peak emission at 451 nm, satisfying BT.2020 blue color requirement, and significantly outperforming its nonencapsulated analogue. This material represents one of the highest efficiency deep-blue OLEDs to date and therefore establishes macrocyclic encapsulation as a powerful synthetic strategy for unlocking the full potential of MR-TADF materials for next-generation OLEDs.