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

Abstract

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 ΦPL (quantum efficiency of photoluminescence (PL)) lower than 0.01. The PL performance was improved by proceeding to the aggregates in THF/water (ΦPL 0.17–0.35) and further improved in the crystals or solid with ΦPL 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 ΦPL (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.

A simple construction of multiple highly-efficient orange-emitting carboranes based on interlocked molecular aggregations

“…As previously reported in the donor–acceptor configured carboranes, the short τ could be attributed to the extensive π⋯π interactions, which induced the triplet excited state annihilation. 42 Because these carboranes possessed the same TPA-Cb-Cz structure, it was conceivable that they all preserved the evident π⋯π interactions and this was most obvious in CB-Cz . Because of this drawback, they could behave as efficient photoluminescent materials in a white-light source by combining them with blue-LEDs (LED = light-emitting diodes) or by using molecular engineering, such as introducing branched substituents to diminish the π⋯π interactions, they could behave as electroluminescent materials because of the recovery of their potential TADF properties.…”

“…Because of this drawback, they could behave as efficient photoluminescent materials in a white-light source by combining them with blue-LEDs (LED = light-emitting diodes) or by using molecular engineering, such as introducing branched substituents to diminish the π⋯π interactions, they could behave as electroluminescent materials because of the recovery of their potential TADF properties. 42…”

“…Recently, the combination with a specific aromatic moiety, 31–35 including anthryl (An) 36–38 and fluorenyl, 39–41 endowed carboranes with efficient PL in the solid state, which verified the AIE phenomena in these species. Due to the twisted intramolecular charge-transfer (TICT) excited state, green-to-yellow PL in the solid state covering thermally activated delayed fluorescence 42–44 (TADF) with a high absolute quantum efficiency ( Φ ) has been consistently reported. As for the orange-to-red emitting carboranes, reports of examples with a large Φ are relatively less (Fig.…”

“…Thus, the pairing aryl-moiety that was attached to Cb became crucial. Secondly, it was demonstrated that linking Cb with a carbazolyl moiety at the C-3 site led to compacted stacking and excellent yellow PL, 42 but its combination with TPA has not been proven to be effective in the orange-to-red solid state PL, due to the effect of the energy-gap law. Thirdly, the expansibility of the designed carboranes should be considered energetically.…”

Rational design for thermochromic luminescence in amorphous polystyrene films with bis‐o‐carborane‐substituted enhanced conjugated molecule having aggregation‐induced luminochromism

“…Therefore, many types of the aryl-modified o-carborane dyads exhibit the aggregation-induced emission (AIE) property. [27,[44][45][46][47][48][49][50] Correspondingly, intense emission was observed around 400 nm in solution thio-CBH, meanwhile significant emission band around 650 nm was obtained in aggregation and crystal. From the solvatochromic behavior (Figures S2 and S3 and Table S2) and longer lifetimes, it was suggested that the emission band in the longer wavelength region should be obtained from the ICT state.…”