Long-Sought Redox Isomerization of the Europium(III/II) Complex Achieved by Molecular Reorientation at the Interface

Redox-isomerism, i.e. the change of metal cation valence state in organic complexes, can find promising applications in multi-stable molecular switches for various molecular electronic devices. However, despite a large amount of studies devoted to such processes in organic complexes of multivalent lanthanides, redox-isomeric transformations were never observed for eu-ropium. In the present work, we demonstrate the unique case of redox isomerization of Eu(III)/Eu(II) complexes on the exam-ple of Eu(III) double-decker octa-n-butoxyphthalocyaninate (Eu[(BuO)8Pc]2) under ambient conditions (air, room tempera-ture). It is shown that assumption of the face-on orientation on the aqueous subphase surface, in which two of each phthalo-cyanine decks in Eu[(BuO)8Pc]2 are located in different media (air and water), leads to the intramolecular electron transfer that results in formation of divalent Eu(II) cation in the complex. Lateral compression of the thus formed monolayer brings on the reorientation of the bisphthalocyaninate to the edge-on state, in which the ligands can be considered identical, and occur-rence of the reverse redox-isomeric transformation into the complex with trivalent Eu cation. Both redox-isomeric states were directly observed by XANES spectroscopy in ultrathin films formed under different conditions.