Efficient Red Electroluminescent Copper Complexes with Fluorination-Balanced Dual Emission

Copper complexes hold a promise for electroluminescent applications, owing to their dual emissive feature based on the moderate spin-orbital coupling effect of Cu⁺ ion for controllable singlet-triplet conversion. However, efficient red dual emission from copper complexes remains an important challenge, because emission wavelengths and thermally activated delayed fluorescence (TADF)/phosphorescence (PH) ratios are simultaneously correlated to electronic effects. Herein, fluorine atoms with suitable electron-withdrawing inductive effect were introduced into a typical tridentate phosphine ligand coordinated CuI skeleton, namely, TTPPCuI, to reduce the lowest unoccupied molecular orbital (LUMO) energy levels, giving rise to narrowed energy gaps between the highest occupied molecular orbital and LUMO, corresponding to emission wavelengths red shifted from 574 to 603 nm. Fluorine atoms simultaneously enhance metal-ligand charge transfer, therefore adjusting positive and reverse intersystem crossing for dual emission balance, leading to TADF/PH ratios changing from 56/44 over 75/25 to 83/17. The devices based on these fluorinated CuI complexes realized efficient red electroluminescence with the maximum wavelength and external quantum efficiency beyond 600 nm and 20%, respectively. These results demonstrate that, based on electronic effects from functional groups, ligand engineering is a feasible way for comprehensively manipulating excited-state characteristics of dual-emissive copper complexes.