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

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.