Multiple Donor Modification of Boron-Based Emitters to Produce Aggregation-Induced Emission and Thermally Activated Delayed Fluorescence for Blue OLEDs

Two novel multi-donor emitters, bearing 1,1,4,4,12,12,15,15-octamethyl-1,2,3,4,12,13,14,15-octahydro-6,10-dioxa-16b-boraanthra[3,2,1-de]tetracene (CyDOBNA) are presented. In 3TPA-CyDOBNA, three electron-rich triphenylamine (TPA) units are positioned on the central benzene ring of CyDOBNA to maximize distortion from an acceptor plane. This modification reduces the singlet-triplet energy gap (Delta EST) by 0.16 eV, highlighting its thermally activated delayed fluorescence (TADF) characteristics. However, congested TPA donors on the bottom side of the central benzene are found to facilitate non-radiative decay in a triplet state, indicating that Delta EST alone is insufficient for optimal device performance. Further refinement led to the creation of Ph2TPA-CyDOBNA, where two TPA donors and one phenyl group are positioned side by side to minimize non-radiative pathways while preserving the TADF behavior. This results in a reverse intersystem crossing rate of 2.30 x 105 s-1 and Delta EST of 0.15 eV. Photophysical investigations reveal that these emitters exhibit not only TADF but also aggregation-induced emission properties, maintaining a high photoluminescence quantum yield (PLQY) even in the solid state. In a doped OLED device, an external quantum efficiency (EQE) of 18.9% is achieved for 3TPA-CyDOBNA. Compared with 3TPA-CyDOBNA, the EQE of Ph2TPA-CyDOBNA improves by 21.1%, primarily because of the suppression of excessive flexibility from the three TPA donors.