Boron‐Doped Azacalix[3]arene Macrocycles Enabling Narrowband Blue TADF

ABSTRACT Development of narrowband blue thermally activated delayed fluorescence (TADF) emitters remains both crucial and challenging for high‐performance organic light‐emitting diodes (OLEDs). Here, we report a modular design strategy to construct narrowband, high‐efficiency multi‐resonance (MR)‐TADF emitters, B1‐ACCz3 and B2‐ACCz3 , via controlled one‐shot borylation of a functional azacalix[3]arene ( ACCz3 ). Single‐crystal X‐ray diffraction of B2‐ACCz3 confirms a highly twisted “butterfly” macrocyclic architecture in which peripheral boron‐embedded MR wings impose conformational rigidity on the azacalix[3]arene framework. Theoretical studies reveal hybridized local and charge‐transfer character with suppressed high‐frequency vibronic coupling, rationalizing the observed narrowband emissions. B1‐ACCz3 and B2‐ACCz3 exhibit pure blue photoluminescence (PL) peaking at 457 and 473 nm, respectively, with full widths at half maximum as narrow as 22–28 nm and near‐unity PL quantum yields. Time‐resolved PL measurements indicate efficient triplet‐to‐singlet upconversion with reverse intersystem crossing rates of up to 10 5 s −1 . Vacuum‐deposited TADF‐OLED based on B1‐ACCz3 delivers ultrapure blue electroluminescence at 460 nm with CIE coordinates of (0.13, 0.08), while B2‐ACCz3 ‐based device achieves a maximum external quantum efficiency of 23.8%. These findings establish constrained peripheral MR extension of azacalix[3]arene as an effective design strategy for high‐efficiency narrowband blue emitters for ultra‐high definition display applications.