Novel D-σ-A type thermally activated delayed fluorescence emitters with C–S σ bond for the orange-red OLEDs

Three D-σ-A type thermally activated delayed fluorescent materials (TADF), AQ-S-Cz, AQ-S-DPA and AQ-S-DMAC, with sulfur atom connect donors and acceptor, were designed and synthesized. All of the emitters exhibit small ΔE_ST (<0.2 eV) and the limited overlap of the molecular frontier orbitals because of the C–S σ bond and the distorted molecular structure. AQ-S-Cz and AQ-S-DMAC with rigid donor obtained higher photoluminescence quantum yields than AQ-S-DPA. The three emitters have excellent TADF performance and short delayed fluorescence lifetimes (0.32 μs for AQ-S-Cz, 0.15 μs for AQ-S-DPA, 0.28 μs for AQ-S-DMAC). The experiment results show that the three emitters have intra-molecular charge transition (intra-CT), especially inter-molecular charge transition (inter-CT) effect. The luminescence of these emitters can be efficiently regulated by intra-CT, especially inter-CT and realize prominent red shift from yellow (556 nm) to red emission (657 nm). Compared with that of AQ-S-Cz and AQ-S-DPA, the device based on AQ-S-DMAC obtained maximum external quantum efficiencies of 7.17% with turn-on voltage of 3.5 V. This work enriches the D-σ-A type orange-red TADF materials and provides a perspective approach of developing long wavelength TADF emitters.