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

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 (T-d) > 310 degrees 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.