Four-coordinated copper(I) complexes containing variably substituted N-heterocyclic carbenes (NHCs): Synthesis, photophysical properties and theoretical investigation

Abstract

Three four-coordinated N-heterocyclic carbene (NHC) copper(I) complexes, [Cu(NaphIm-Py)(POP)]PF6 (1), [Cu(AnthrIm-Py)(POP)]PF6 (2), and [Cu(PhBenIm-c-Py)(POP)]PF6 (3) (NaphIm-Py = 3-(naphthalen-2-ylmethyl-1-(pyridin-2-yl)-1H-imidazolylidene, AnthrIm-Py = 3-(anthracen-9-ylmethyl)-1-(pyridin-2-yl)-1H-imidazolylidene, PhBenIm-c-Py = 3-benzyl-1-(pyridin-2-ylmethyl)-1H-benzo[d]imidazolylidene, POP = bis[2-diphenylphosphino]-phenyl)ether) have been synthesized and characterized. The effect of varying carbene ligands with different substituents on the structural aspects and photophysical properties of the complexes is systematically investigated. The lowest absorption bands of 1 at about 350 nm are assigned as metal-to-ligand charge transfer (MLCT) character with some contribution from ligand-to-ligand charge transfer (LLCT) character, while the π-π* transition of the anthryl group are responsible for the lowest absorption band of 2. For 3, no MLCT absorption band are observed in lower energy region. The emission wavelengths of all NHC-Cu(I) complexes are in the range of 520–550 nm with the phosphorescent origin in the solid state. 1 and 2 with naphthyl and anthryl groups show the lower photoluminescence efficiency due to the strong π-π stacking interactions, whereas 3 exhibit good photoluminescence properties companying with the higher quantum yields and long excited-state lifetimes. Density functional theory (DFT) and time dependent density functional theory (TDDFT) calculations were employed to rationalize the photophysical properties of the NHC-Cu(I) complexes.