Unraveling the Position Effect of Spiroxanthene-Based n-Type Hosts for High-Performance TADF-OLEDs

For developing high-performance organic light-emitting diodes (OLEDs) with thermally activated delayed fluorescent (TADF) emitters, the diphenyltriazine (TRZ) unit was introduced onto the 2 '- and 3 '-positions of xanthene moiety of spiro[fluorene-9,9 '-xanthene] (SFX) to construct n-type host molecules, namely 2 '-TRZSFX and 3 '-TRZSFX. The outward extension of the TRZ unit, induced by the meta-linkage, resulted in a higher planarity between the TRZ unit and xanthene moiety in the corresponding 3 '-TRZSFX. Additionally, this extension led to a perched T-1 level, as well as a lower unoccupied molecular orbital (LUMO) level when compared with 2 '-TRZSFX. Meanwhile, the 3 '-TRZSFX molecules in the crystalline state presented coherent packing along with the interaction between TRZ units; the similar packing motif was spaced apart from xanthene moieties in the 2 '-TRZSFX crystal. These endowed 3 '-TRZSFX superior electron transport capacity in single-carrier devices relative to the 2 '-TRZSFX-based device. Hence, the 3 '-TRZSFX-based TADF-OLED showed remarkable electroluminescent (EL) performance under the operating luminance from turn-on to ca. 1000 cd<middle dot>m(-2) with a maximum external quantum efficiency (EQE(max)) of 23.0%, thanks to its matched LUMO level with 4CzIPN emitter and better electron transport capacity. Interestingly, the 2 '-TRZSFX-based device, with an EQE(max) of 18.8%, possessed relatively low roll-off and higher efficiency when the operating luminance exceeded 1000 cd<middle dot>m(-2), which was attributed to the more balanced carrier transport under high operating voltage. These results were elucidated by the analysis of single-crystal structures and the measurements of single-carrier devices, combined with EL performance. The revealed position effect of the TRZ unit on xanthene moiety provides a more informed strategy to develop SFX-based hosts for highly efficient TADF-OLEDs.