Light-Induced Intramolecular Electron Transfer in a 1D [CuFe] Coordination Polymer Containing the [Fe(C₂O₄)₃]^3- Core

A one-dimensional (1D) ladder-like coordination polymer {NH₄[{Cu(bpy)}₂(C₂O₄)Fe(C₂O₄)₃]·H₂O} (1; bpy = 2,2'-bipyridine) containing [Cu(bpy)(μ-C₂O₄)Cu(bpy)]²⁺ cationic units linked by oxalate groups of [Fe(C₂O₄)₃]^3- building blocks was investigated as a new type of photoactive solid-state system. It exhibits a photocoloration effect when exposed to direct sunlight or UV/vis irradiation. The photochromic properties and mechanism were studied by powder and single-crystal X-ray diffraction, UV/vis diffuse reflectance, IR and electron paramagnetic resonance spectroscopy, magnetization and impedance measurements, and density functional theory calculations. The process of photochromism involves simultaneous intramolecular electron transfers from the oxalate ligand to Fe(III) and to [Cu^II(bpy)(μ-C₂O₄)Cu^II(bpy)]²⁺, leading to the reduction of the metal centers to the electronic states Fe(II) and Cu(I), accompanied by the release of gaseous CO₂.