Structure–property investigations of benzoylspirocyclic aromatics as small-molecule hosts for solution-processable TADF devices

Abstract

A series of benzophenone-embedded spirocyclic aromatics, namely, spiro[fluorene-9,9′-xanthene]-2′-yl(4-fluoro-phenyl)-methanone (SFX-BzF), spiro[fluorene-9,9′-xanthene]-2′-yl(4-(9H-carbazol-9-yl)-phenyl)methanone (SFX-BzCz), spiro[fluorene-9,9′-thioxanthene]-2′-yl(2-(9H-carbazol-9-yl)-phenyl)methanone (SFT-Bz(o)Cz), and 10′,10′-dioxidospiro[fluorene-9,9′-thioxanthene]-2′-yl(4-(9H-carbazol-9-yl)phenyl)methanone (SFTO₂-BzCz), were synthesized successfully using a convenient method. All of the compounds were used as host materials to fabricate TADF devices for investigating the structure–property relationship among molecular structure, material properties and device performance. The compounds SFX-BzF, SFX-BzCz, SFT-Bz(o)Cz, and SFTO₂-BzCz showed outstanding thermal stability with high decomposition temperatures of 337 °C, 424 °C, 424 °C, and 432 °C at 5% weight loss, respectively. This demonstrated that the thermal stabilities of the four compounds improved as their molecular weight increased. The HOMO and triplet energy levels of SFX-BzCz, SFT-Bz(o)Cz, and SFTO₂-BzCz were nearly unchanged, but their LUMO energy levels were selectively controlled as the electron-withdrawing abilities of the S, O, and SO₂ groups increased. This is a good strategy to design and synthesize excellent hosts for high-performance devices. Furthermore, an SFX-BzCz-hosted device with a maximum EQE of 18.8% was fabricated by solution processing. After understanding the structure–property relationship, in the next step, the state-of-the-art models were set up to accumulate experimental evidence for benzoylspirocyclic aromatic-based high-efficiency devices.