Recovering the Thermally Activated Delayed Fluorescence in Aggregation-Induced Emitters of Carborane

The aggregation-induced emission (AIE) behaviors of carborane-based hybrid emitters have been extensively reported, while their combinations with the thermally activated delayed fluorescence (TADF) are still scarce. We designed and synthesized three Janus carboranes (the chemical structures resemble the double-faced god, Janus) Cb-1/2/3 with different carbazole moieties. All of the Janus carboranes exhibited quenched emission in solution with Φ_PL (quantum efficiency of photoluminescence (PL)) lower than 0.01. The PL performance was improved by proceeding to the aggregates in THF/water (Φ_PL 0.17-0.35) and further improved in the crystals or solid with Φ_PL up to 0.99 for Cb-1, 0.85 for Cb-2, and 0.61 for Cb-3, which agreed with the AIE enhancement. Although the PL of solid Cb-1/2/3 showed non-TADF properties with lifetimes only at several nanoseconds, the crystallographic studies have shown a root cause of π···π stacking that quenched the TADF, and the theoretical calculations forecasted small singlet-triplet energy gaps (ΔE_S-T) therein. According to these findings, TADF was recovered in Cb-1/2/3 by doping into 1,3-bis(carbazol-9-yl)benzene (mCP). The 10 wt % doped films of Cb-1/2/3 have achieved a trade-off of Φ_PL (0.84 in Cb-3 and 0.83 in Cb-1) and delayed lifetime (up to 8 μs). The doped devices of organic light-emitting diodes incorporating Cb-1/2/3 achieved the highest external quantum efficiency at 10.1% and the maximized luminance of 5920 cd/m² at a driving voltage of 8 V.