The effect of electron donor and acceptor conformations on thermally activated delayed fluorescence

A novel organic molecule, 2-(9,9-dimethyl-9H-acridin-10-yl)-anthraquinone (DMAC-AQ), is designed and synthesized by introducing a dual-conformation electron donor 9,9-dimethyl-9,10-dihydroacridine to a typical electron acceptor anthraquinone. DMAC-AQ exhibits decent thermally activated delayed fluorescence (TADF) property. However, using DMAC-AQ as an emitter, the doped and non-doped electroluminescence devices show maximum external quantum efficiency values of 5.63% and 0.32%, respectively, which are much lower than expected. Crystallographic analysis reveals that DMAC-AQ has two nearly orthogonal conformers (C1 and C2), in which planar and non-planar DMAC moieties are both linked to the AQ group with high twisting angles. Interestingly, the non-planar conformations of donor (D) and acceptor (A) units contribute to the generation of luminescent enantiomers in C2. C1 exhibits obvious TADF while C2 shows no TADF. The results suggest that nonplanar conformation of the donor in the emitter may inhibit TADF and significantly reduce device efficiency. The present work may provide new insight into understanding the effect of D and A conformations in TADF emitters.