Comparative Study on Two Kinds of Spiro Systems for\nConstructing Through-Space Charge-Transfer Emitters

Significant achievements have been\ndemonstrated with thermally\nactivated delayed fluorescence (TADF) emitters through a spatial charge\ntransfer (TSCT). However, due to the difficulty in synthesis, introducing\nmultiple donor/acceptor units into TSCT-TADF emitters remains a continuous\nchallenge. This work proposed a TSCT-TADF material with the acceptor/donor/acceptor\n(A/D/A) structure based on a 10,12-dihydroindeno[2,1-<i>b</i>]fluorene (indenofluorene) framework, named DSTO. We also synthesized\na molecule with an A/D structure for comparison, named STO. The introduction\nof multiple acceptors effectively modulates the excited-state properties,\nachieving a rapid reverse intersystem crossing process rate of 7.1\n× 10<sup>5</sup> s<sup>–1</sup>. The OLED device with\nDSTO as the emitter shows a maximum external quantum efficiency (EQE<sub>max</sub>) of around 20%. This study broadens the design of multiple\ndonor/acceptor TSCT-TADF materials and deepens our further understanding\nof the structure–property relationship.