Quantitative Solvent-Free Thermal Synthesis of Luminescent Cu(I) Coordination Polymers

The luminescent Cu(I) coordination polymers [Cu₂I₂( m, m'-bpy)] _n (CuI- m; m, m'-bpy = m, m'-bipyridine; m = 3, 4) were successfully synthesized by the solvent-free thermal reaction of the metal salt CuI with the organic linkers m, m'-bpy. Powder X-ray diffraction analysis revealed that CuI-3 was immediately formed when a mixture of CuI and 3,3'-bpy was ground in a mortar at room temperature (20 °C). In contrast, a temperature >120 °C was required to synthesize the CuI-4 isomer, probably because of the higher melting point of the 4,4'-bpy linker. Although excess bpy linker was necessary to afford the CuI- m in high yield, the quantitative synthesis, without any purification processes, was successfully achieved by simple heating at 140 °C, whereby the excess bpy linker was thermally removed by evaporation. Single crystal X-ray structural analysis indicated that in CuI-3 the dinuclear {Cu₂I₂} rhombic cores were bridged by 3,3'-bpy linkers. A similar structure was observed for CuI-4; however, the intermolecular π-π stacking that was effective in CuI-4 was suppressed in CuI-3 because of the twisted configuration of the two pyridyl rings of the 3,3'-bpy linker. CuI-3 exhibited bright green emission with the maximum (λ_em) at 519 nm and a high emission quantum yield (Φ = 0.58) in the solid state at room temperature, in contrast to the weak red emission of CuI-4 (λ_em = 653 nm, Φ < 0.01). Emission decay analysis and density functional theory calculations suggested that the CuI- m emissions could be attributed to the delayed fluorescence from the metal-to-ligand charge-transfer excited state effectively mixed with the halide-to-ligand charge-transfer excited state.