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 Phi(PL) (quantum efficiency of photoluminescence (PL)) lower than 0.01. The PL performance was improved by proceeding to the aggregates in THF/water (Phi(PL) 0.17-0.35) and further improved in the crystals or solid with Phi(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 pi center dot center dot center dot pi stacking that quenched the TADF, and the theoretical calculations forecasted small singlet-triplet energy gaps (Delta ES-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 Phi(PL) (0.84 in Cb-3 and 0.83 in Cb-1) and delayed lifetime (up to 8 mu s). 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/m(2) at a driving voltage of 8 V.

Automated summary

The study found that Janus carboranes have low luminescence efficiency in solutions, but the efficiency increases in aggregates and solid, showing AIE enhancement. By doping with 1,3-bis(carbazol-9-yl)benzene (mCP), the TADF properties of Cb-1/2/3 were successfully restored, achieving high external quantum efficiency and maximum brightness in organic light-emitting diodes.