Entropy-driven charge-transfer complexation yields thermally activated delayed fluorescence and highly efficient OLEDs

Exciplex-forming systems that display thermally activated delayed fluorescence are widely used for fabricating organic light-emitting diodes. However, their further development can be hindered through a lack of structural and thermodynamic characterization. Here we report the generation of inclusion complexes between a cage-like, macrocyclic, electron-accepting host (A) and various N-methyl-indolocarbazole-based electron-donating guests (D), which exhibit exciplex-like thermally activated delayed fluorescence via a through-space electron-transfer process. The D/A cocrystals are fully resolved by X-ray analyses, and UV-visible titration data show their formation to be an endothermic and entropy-driven process. Moreover, their emission can be fine-tuned through the molecular orbitals of the donor. Organic light-emitting diodes were fabricated using one of the D/A systems, and the maximum external quantum efficiency measured was 15.2%. An external quantum efficiency of 10.3% was maintained under a luminance of 1,000 cd m-2. The results show the potential of adopting inclusion complexation to better understand the relationships between the structure, formation thermodynamics and properties of exciplexes. Although exciplex-forming systems are widely used for fabricating organic light-emitting diodes (OLEDs), their structural and thermodynamic characterization is limited. Now donor/acceptor inclusion complexes that demonstrate thermally activated delayed fluorescence have been generated. Their cocrystal structures have been resolved and the thermodynamics of exciplex formation determined, which has enabled the fabrication of efficient OLEDs.