Four-coordinate Cu(I) complexes supported by N-heterocyclic carbene ligands bearing electron-donating/withdrawing groups: Synthesis, structures and photophysical properties

Abstract

Four new mononuclear cationic four-coordinate Cu(I) complexes supported by N-heterocyclic carbene (NHC) ligands bearing electron-donating/withdrawing groups and diphosphine ligand were successfully synthesized. All of these complexes have high stability with thermal decomposition temperatures (Td) > 310 °C. Single crystal X-ray diffraction analysis show that these complexes adopt the typical distorted tetrahedral configuration, and the electron-donating methyl and methoxyl groups at NHC ligands can give shorter Cu−N bond distances in comparison with the electron-withdrawing fluorine and chlorine. Theoretical calculations indicate that the lowest energy electronic transitions of these complexes are mainly from the metal-to-ligand charge transfer (MLCT) transition and ligand-to-ligand charge transfer (LLCT) transition. At room temperature, these complexes as crystal powder exhibit bright emissions with high photoluminescence (PL) quantum yields of 0.42–0.69, and emission wavelength can be tuned from 489 nm to 539 nm by the electron-donating/withdrawing groups at NHC ligands. The photophysical behaviors at 298 K and 77 K show that emissions of these complexes at room temperature contain TADF and phosphorescence.