Dual Thermally Activated Delayed Fluorescence from a Single Carbazole Derivative with Multiple Charge Transfer Pathways

Two carbazole derivatives BPA‐BNC and 4BPABNC are designed and synthesized to explore the probable dual emission behavior and mechanisms. BPA‐BNC contains two arylamino donors and one B‐N multi‐resonance (MR) segment at 3,6,9‐positions of a single carbazole, while 4BPABNC has two extra arylamine donors at 1,8‐positions in addition to the above substituent groups. Two compounds in polar solvents show a clear dual emission peaked at near 490 and 540 nm with high photoluminescence quantum yields of up to 98%, short delayed lifetimes and extremely small single‐triplet splitting energies. The dual emissions originate from the MR and through‐bond charge transfer transitions, which are supported by theoretical calculation and experimental data. The solution‐processed devices based on BPA‐BNC and 4BPABNC also exhibit dual emissions and achieve the maximum external quantum efficiencies (EQEs) of 13.1% and 12.7%, whereas the model MR molecule provides an EQE of 7.3% under the same device architecture.