Photoluminescence of Copper(I) 4,7-Dichloroquinoline Complexes: A Combined Experimental and Theoretical Perspective

Polymeric [CuX1(DCQ)]n (DCQ = 4,7-dichloroquinoline, X1 = Cl, Br, I) and dimeric [CuX2 (DCQ)(PPh3)]2 (PPh3 = triphenylphosphine, X2 = Cl, Br) complexes were synthesized and characterized by powder X-ray diffraction (PXRD), single crystal X-ray diffraction (SCXRD), spectroscopy, and analytical methods. In the solid state, the photophysical response varied with the halide, with emission colors spanning red to yellow and showing measurable halide-dependent changes in emission energy (EPL), Stokes shift, photoluminescence quantum yield (ΦPL), and average lifetime (τ). Time-resolved measurements revealed millisecond lifetimes, suggesting the participation of long-lived excited states with charge transfer (CT) character. Density functional theory (DFT) calculations were consistent with low-energy excited states with mixed metal-to-ligand (MLCT), metal/halide-to-ligand charge-transfer ((MX)LCT) and ligand-ligand charge transfer (LLCT) characters and singlet-triplet energy state that could facilitate the intersystem crossing (ISC) within qualitative framework. Emissions of the five copper(I) complexes were observed in the solid-state, whereas no measurable luminescence was observed in common solvents. Overall, the photoluminescence reflected the combined influence of the CuI coordination environment, halide identity, and solid-state structural constraints.