Comparison of different synthesis methods for Cu(I) complexes: luminescence properties and theoretical study of the mechanical grinding method vs. the one-pot method.

This study presents a comparative investigation of the structures and luminescence properties of six novel Cu(I) complexes synthesized via mechanochemical grinding and one-pot methods. These complexes were prepared using 2,2'-bithiazole (BTZ) as the nitrogen-donor ligand, together with POP or 9,9-POP as phosphine ligands, reacting with Cu(I) salts bearing different counteranions. Single-crystal X-ray diffraction analysis reveals that all complexes are mononuclear Cu(I) complexes with distorted tetrahedral geometries. Among them, complexes P1-P4 crystallize in the monoclinic system, while P5-P6 crystallize in the triclinic system. Powder X-ray diffraction patterns of the mechanochemically ground samples show excellent agreement with those of the one-pot crystalline samples and the simulated patterns, except for broadened peaks, indicating that the grinding process disrupts long-range order but does not alter the crystalline phase. Photoluminescence spectra show broad emission peaks in the range of 563-593 nm for all complexes. Compared to the original crystals, the ground samples exhibit a blue shift of 6-13 nm, which is attributed to the disruption of weak intermolecular interactions (e.g., π-π stacking). Quantum yield measurements indicate that complexes containing 9,9-POP exhibit higher luminescence efficiency (up to 15.0%) due to the greater rigidity of the ligand. After grinding, the quantum yields generally decrease, which is ascribed to increased non-radiative transitions and local structural distortions. Density functional theory (DFT) calculations reveal that the emission primarily originates from a synergistic combination of metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT). This study demonstrates that mechanochemical grinding represents a green, efficient, solvent-free or minimal-solvent synthetic method. The resulting Cu(I) complexes exhibit structure and luminescence properties consistent with those prepared by conventional methods, showing potential for applications in mechanochromic materials and optoelectronic devices.