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

Abstract

A series of donor–acceptor compounds, including asymmetric D–B–A_i–B and symmetric D–B–A_i–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 A_i to extend the π-conjugation, leading to strong electron-withdrawing ability. Our results indicate that the singlet–triplet energy gaps (ΔE_ST) of symmetric D–B–A_i–B–D compounds are smaller than those of asymmetric D–B–A_i–B ones. For the same topologic series, the ΔE_ST values decrease with increasing electron-withdrawing strength of B–A_i–B. The lowest ΔE_ST 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–A_i–B–D compounds exhibited more effective charge injection with a lower ionization potential and a higher electron affinity than D–B–A_i–B ones. Meanwhile, the temperature dependent mobility was predicted by Marcus theory, both hole and electron mobilities of D–B–A_i–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.