Mononuclear copper(I) bromide complexes chelated with bis(pyrazol-1-ylmethyl)-pyridine ligands: Structures, electronic properties and solid state photoluminescence

A series of mononuclear Cu(I) complexes formulated as [Cu(L)Br] were prepared by reaction of copper(I) bromide with equimolar amounts of bis(pyrazol-1-ylmethyl)- pyridine (L) in toluene solvent. The five complexes were characterized by UV/Vis, 1H NMR and MS spectroscopy. The molecular structure of complex B2 has been established by single-crystal analyses. It features a tetrahedral copper center surrounded by three nitrogen atoms from the chelating pyridine-pyrazole ligand and one halogen atom. The complexes are luminescent in the solid state at ambient temperature. Intense green or yellow emission is observed, with maxima in the range of 520-569 nm, depending on the substitution pattern of the pyridine-pyrazole ligands used for the complexation. The emission lifetimes of the complexes in powder were in a dozen microseconds, indicating that the emission may be phosphorescent in origin. Moreover, the emission energies of the complexes show a concentration-dependence in the rigid environment, probably due to the different geometry in the lowest excited states. The emission of the complexes was assigned to metal-to-ligand charge-transfer excited states with some mixing of the halide-to-ligand characters, which was further supported by means of density functional theory (DFT) calculations.