Optimizing molecular rigidity and thermally activated delayed fluorescence (TADF) behavior of phosphoryl center π-conjugated heterocycles-based emitters by tuning chemical features of the tether groups

Four novel fluorescent molecules based on pentavalent phosphoryl center π-conjugated heterocycles (PCCH) as acceptor have been constructed by ether, sulfide, and sulfonyl groups as tethering groups and phenoxazine or phenothiazine as the donor. Their molecular configurations, photophysical, electrochemical behaviors and electroluminescent (EL) performances have been studied. The molecular configuration of these PCCH-based emitters look like bowls, due to the bowl-shape PCCH-based acceptors. According to their special molecular structure and photophysical results, it can be concluded that the PCCH-based emitters with phenoxazyl as donors have efficient thermally activated delayed fluorescence (TADF) behavior. Interestingly, their molecular rigidity, photoluminescent quantum yield (PLQY) and TADF behaviors are correlate with the depth of the bowl in the case of the same donor. The sulfone group as tethering group into the acceptor unit among of these PCCH-based emitters exhibit optimal PL and EL performances. It has a highest PLQY of ca. 0.83, measured in doped CBP film, and the highest k_RISC of 10⁶ s⁻¹. Importantly, its doped OLEDs can achieve exceptional EL performances with a maximum η_ext of 28.2%, η_L of 90.8 cd A⁻¹ and η_P of 83.5 lm W⁻¹. In addition, these bowl molecules of PCCH-based emitters will enrich the structural diversity of high efficient TADF emitters.