Precise Multi-Site Borylation Engineering of Indolo[2,3-c]Carbazole-Bridged Narrowband MR-TADF Emitters

Indolocarbazole-bridged double-boron multi-resonance thermally activated delayed fluorescence (MR-TADF) materials enable precise modulation of emission colors while retaining ultra-narrowband characteristics in organic light-emitting diodes (OLEDs). Herein, we first introduce indolo[2,3-c]carbazole (23cIC) into the design of MR-TADF emitters and develop an electronically and sterically controlled multi-site borylation strategy. This strategy affords the electronically favored exo-configuration for Tbu-exo, which exhibits narrowband green emission at 524 nm with a full-width at half-maximum (FWHM) of 20 nm, while yielding the steric hindrance-guided endo-configuration for Ad-tph-endo exhibiting by yellow emission at 566 nm with a FWHM of 25 nm. Notably, the endo-configuration gives rise to (P/M)-Ad-tph-endo, which exhibits significant chiral luminescence properties. Vacuum-deposited OLEDs based on Tbu-exo and Ad-tph-endo exhibit emission peaks at 530 nm (FWHM = 27 nm) and 568 nm (FWHM = 33 nm), respectively, both achieving external quantum efficiencies (EQE) exceeding 30%. Furthermore, we fabricated solution-processed top-emitting OLEDs based on Tbu-exo for the first time, which realize ultra-narrow pure-green emission with CIE coordinates of (0.21, 0.75)-closely approaching the BT.2020 green standard-and a record-high current efficiency of 200.9 cd A^-1.