Design of Photomagnetic Ultra-Thin Films by Electrodeposition of Switchable Cyanide-Bridged Fe₄Co₄ Complexes

Preparing thin films of molecular polymetallic materials remains one of the current limitations in the implementation of polymetallic complexes into devices. The hurdle is tackled here using an electrochemical route for depositing the cyanido-bridged polymetallic complex {Tl⊂[Fe^II(2-TPhTp)(CN)₃]₄[Co^III(Tp)]₃[Co^II(Tp)]}(1) (where 2-TPhTp = [4-(2-thienyl)Phenyl]tris(pyrazol-1-yl)borate; and Tp = hydrotris(pyrazol-1-yl)borate), functionalized with thiophene groups, on conductive surface (Pt/mica). Cyclic voltammograms show that the electrochemical pattern of the cubic units is maintained in the electropolymerized film, notably with four quasi-reversible successive Fe^III/Fe^II redox events. Assessing the morphology and the chemical composition of the resulting thin film by Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS) experiments respectively, reveal a homogenous deposition (thickness of ca. 20 nm) showing the expected metallic ratio. More importantly, X-ray Absorption Spectroscopy (XAS) and X-ray Magnetic Circular Dichroism (XMCD) measurements demonstrate that the photo-induced metal-metal electron transfer is preserved in the film. XMCD signals of Fe and Co atoms at their L_2,3 absorption edges both indicate an equivalent conversion of Fe^II─CN─Co^III diamagnetic pairs into Fe^III─CN─Co^II paramagnetic ones under laser light irradiation below 62 ± 5 K. As for the photomagnetic complex, the phenomenon is reversible: the metastable Fe^III─CN─Co^II paramagnetic pairs thermally relax to the diamagnetic ground state upon heating to room temperature.