Synthesis, structure and photophysical properties of two tetranuclear copper(I) iodide complexes based on acetylpyridine and diphosphine mixed ligands.

Two copper(I) iodide tetramers, namely, [μ-1,3-bis(diphenylphosphanyl)propane-κP:P']di-μ-iodido-di-μ-iodido-[1-(pyridin-3-yl)ethan-1-one-κN]tetracopper(I) dichloromethane disolvate, [CuI(CHNO)(CHP)]·2CHCl (CuL), and [μ-1,3-bis(diphenylphosphanyl)propane-κP:P']di-μ-iodido-di-μ-iodido-[1-(pyridin-4-yl)ethan-1-one-κN]tetracopper(I), [CuI(CHNO)(CHP)] (CuL), have been synthesized from reactions of CuI, 1,3-bis(diphenylphosphanyl)propane (dppp) and 3- or 4-acetylpyridine (3/4-acepy). The complexes were characterized by elemental analysis, IR spectroscopy, single-crystal X-ray diffraction (XRD), powder XRD and photoluminescence spectroscopy. Both complexes possess a stair-step [CuI] cluster structure with a crystallographic inversion centre located in the middle of a CuI ring (Z' = 1/2). The dppp ligands each adopt a bidentate coordination mode that bridges two Cu centres on one side of the [CuI] cluster and the acepy ligands act as terminal ligands. The solid-state samples of similar complexes show highly efficiency thermally activated delayed fluorescence (TADF) at room temperature. At ambient temperature, both CuL and CuL exhibit photoluminescence, with a maximum emission in the region 560-580 nm and with short emissive decay times, but only phosphorescence was observed at 77 K. The narrow gaps between the higher lying singlet state and the triplet state, ΔE(S - T), also confirm the presence of TADF. Structure analysis and consideration of photoluminescence indicates that the position of the acetyl group on the heterocyclic ligand has an obvious influence on the structural arrangement, on intermolecular interactions and on the observed photophysical properties.