Rational design of phenoxazine-based donor-acceptor-donor thermally activated delayed fluorescent molecules with high...

A series of donor-acceptor compounds, including asymmetric D-B-Ai-B and symmetric D-B-Ai-B-D topologies, have been designed and investigated using density functional theory and time dependent density functional theory toward highly efficient thermally activated delayed fluorescent (TADF) materials. Phenoxazine (PXZ) is adopted as a donor (D) fragment, while 1,3,4-oxadiazole (A1), benzo[c][1,2,5]thiadiazole (A2), and quinoxaline (A3) are selected as acceptor fragments. A phenyl ring (B) is connected to Ai to extend the π-conjugation, leading to strong electron-withdrawing ability. Our results indicate that the singlet-triplet energy gaps (ΔEST) of symmetric D-B-Ai-B-D compounds are smaller than those of asymmetric D-B-Ai-B ones. For the same topologic series, the ΔEST values decrease with increasing electron-withdrawing strength of B-Ai-B. The lowest ΔEST value has been obtained for D-B-A2-B-D among all these investigated compounds, which displays the most efficient up-conversion from triplet to singlet excited states. Then, the potential energy surface and normal mode analyses were applied to discuss the charge injection and transport characteristics. The designed D-B-Ai-B-D compounds exhibited more effective charge injection with a lower ionization potential and a higher electron affinity than D-B-Ai-B ones. Meanwhile, the temperature dependent mobility was predicted by Marcus theory, both hole and electron mobilities of D-B-Ai-B-D increase with increasing temperature in the range of 5-200 K. However, hole mobility slightly decreases from 200 K to 300 K. The newly designed D-B-A2-B-D compounds demonstrate higher electron and hole mobilities than D-B-A1-B-D, implying that the chemical modification of acceptors effectively improves the carrier transport ability. Our theoretical investigation might provide more chances to challenge the rational design of novel and high-performance TADF-based organic light emitting diodes.