1,4-AzaborineParticipation Enables Inaccessible Cycloarenewith Unique Photophysical Properties

Cycloarenes and heterocycloarenes, characterized by fused macrocyclic π-conjugated structures, hold significant promise in synthetic chemistry and materials science. However, their further development remains constrained by formidable synthetic challenges, particularly for those with contracted cavities. Inspired by advances in the synthesis of organoboron-based multiresonance thermally activated delayed fluorescence (TADF) emitters, we herein report the convenient access and detailed characterization of a 1,4-azaborine-embedded cycloarene that features the smallest cavity among known (hetero)cycloarenes. The contracted cavity induces a bowl-shaped molecular geometry, as confirmed by crystallographic analysis, while also triggering through-space conjugation with delocalized π-electrons at the cavity site. Comparative studies between this compound and its helical analogue reveal a substantial topological impact on photophysical properties, including a bathochromic-shifted and broadened emission band, prolonged radiative decay process, and more efficient triplet-to-singlet spin-flip. Capitalizing on its efficient TADF with a remarkably high quantum yield, we successfully fabricated the first (hetero)cycloarene-based organic light-emitting diodes, achieving over 30% external quantum efficiency and minimal efficiency roll-off. These findings offer new insights into the design of topologically distinct organic compounds with unique properties.