Synthesis, characterization, and photoluminescent studies of three-coordinate Cu(i)-NHC complexes bearing...

A series of heteroleptic three-coordinate Cu(i) complexes bearing monodentate N-heterocyclic carbene (NHC) ligands of the type 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene (SIPr), and bidentate N-donor ligands of the type unsymmetrically-substituted dimethyl dipyridylamine (Me₂Hdpa) and bis(mesityl)biazanaphthenequinone (mesBIAN) have been synthesized. The complexes [Cu(IPr)(3,4'-Me₂Hdpa)]PF₆, 1; [Cu(IPr)(3,5'-Me₂Hdpa)]PF₆, 2; [Cu(IPr)(3,6'-Me₂Hdpa)]PF₆, 3; [Cu(IPr)(mesBIAN)]PF₆, 6; [Cu(SIPr)(3,4'-Me₂Hdpa)]PF₆, 7; [Cu(SIPr)(3,5'-Me₂Hdpa)]PF₆, 8; and [Cu(SIPr)(3,3'-Me₂Hdpa)]PF₆, 11 have been characterized by ¹H and ¹³C NMR spectroscopies, elemental analysis, cyclic voltammetry, and photophysical studies in solid and solution phase. Single crystal X-ray structures were obtained for all complexes except 11. The crystallographic data reveal a mononuclear structure for all complexes with the copper atom ligated by one C and two N atoms. The UV-Vis absorption spectra of all dipyridylamine complexes in CH₂Cl₂ show a strong ligand-centered absorption band around 250 nm and a strong metal-to-ligand charge transfer (MLCT) band around 300 nm. When irradiated with UV light, the complexes exhibit strong emission maxima at 453-482 nm with photoluminescence quantum yields (PLQY) ranging from 0.21 to 0.87 in solid state. While the PLQY values are comparable to those of the symmetrical [Cu(IPr)(Me₂Hdpa)]PF₆ complexes, a stabilizing CH-π interaction has been reduced in the current systems. In particular, complex 3 lacks any strong CH-π interaction, but emits more efficiently than 1 and 2 wherein the interactions exist. Structural data analysis was performed to clarify the role of ligands' plane angle and the NH/CH⋯F interactions to the observed light interaction of unsymmetrical [Cu(NHC)(Me₂Hdpa)]PF₆ complexes. DFT calculations were performed to assist in the assignment of the electronic structure and excited state behavior of the complexes.