Sulfur Lone Pairs Open Avenues for π* → n Orange-to-Red TADF and OLEDs

It is always important and fascinating to explore new organic emitters that exploit unconventional pathways to unveil their emission with unique properties, such as thermally activated delayed fluorescence (TADF). In this study, we report that the rarely explored sulfur lone pair (n) is a promising alternative, where the correlated π* → n emission can be used to attain strong TADF and thus practical OLEDs. The designed strategy incorporates several key concepts (Figure 1a), in which the persulfide aromatic spirocycle enhances spin-orbit coupling, thereby increasing the intersystem crossing rate. Next, molecules with a twisted donor-acceptor configuration bridged by spiro[4.4]nonane as well as spatially orthogonal sulfur lone pairs and π* features significantly reduce the singlet-triplet gap. Finally, the rigid spirocyclic backbone inhibits nonradiative transitions. The proof-of-concept is given by compound 1, which achieves nπ* thermally activated delayed fluorescence (TADF) maximized at 635 nm with photoluminescence quantum yields as high as 52% in CH2Cl2. Wet-processed OLEDs containing 1 achieved a maximum external quantum efficiency of 6.4% at a brightness of 189 cd m-2 (∼600 nm), opening an alternative 1nπ* route for practical π* → n TADF OLEDs.