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 (ΔE_ST) 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 ΔE_ST 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 × 10⁵ s⁻¹ and ΔE_ST 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.