Performance enhancement by additional donors in D–A–D′ type thermally activated delayed fluorescence materials†

Two donor–acceptor–donor’ (D–A–D′) type thermally activated delayed fluorescence (TADF) molecules, tCz-BP-PXZ and tCz-PhBP-PXZ, comprising a benzoyl acceptor and two different donors, i.e. phenoxazine (PXZ) and 3,6-di-tert-butyl-9-methyl-9H-carbazole (tCz), were designed and prepared. The second donor tCz was linked either directly (tCz-BP-PXZ) or through a phenylene bridge (tCz-PhBP-PXZ) to the benzoyl acceptor. The quantum calculations and spectral measurements revealed that the PXZ donor preferentially underwent charge transfer interaction with the benzoyl acceptor to achieve a small ΔE_ST for the TADF feature, while the tCz donor offered twisted molecular configurations to reduce intermolecular interactions and tuned the higher triplet state's nature and alignment and enhanced the RISC process and the TADF ratio. Thus, the incorporation of the tCz donor in tCz-BP-PXZ and tCz-PhBP-PXZ increased the photoluminescence quantum yields and reverse intersystem crossing rates relative to the tCz-free reference emitter BP-PXZ. Yellow organic light-emitting diodes (OLEDs) of tCz-BP-PXZ and tCz-PhBP-PXZ exhibited high EQEs of 17.2% and 23.5% that are much higher than that of BP-PXZ (14.0%) and slow efficiency roll-offs (with EQEs of 15.9% and 16.8% at 1000 cd m⁻²) due to a suppressed quenching effect. They also exhibited acceptable performance in non-doped OLEDs. It was observed that the phenylene linkage between the tCz group and the acceptor in tCz-PhBP-PXZ is beneficial and responsible for the superior performance of tCz-BP-PXZ.