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

Abstract

Redox-isomerism, i.e. the change of metal cation valence state in organic complexes, can find promising applications in multistable molecular switches for various 2 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 europium. In the present work, we demonstrate the unique case of redox isomerization of Eu(III)/Eu(II) complexes on the example of Eu(III) doubledecker octa-nbutoxyphthalocyaninate (Eu[(BuO)8Pc]2) under ambient conditions (air, room temperature). It is shown that assumption of the face-on orientation on the aqueous subphase surface, in which two of each phthalocyanine 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 occurrence 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.

Interface Asymmetry Induced and Surface Pressure Controlled Valence Tautomerism in Monolayers of bis-Phthalocyaninates of Lanthanides

“…Thus, in the research works devoted to the study of this process on the example of sandwich-substituted cerium bis-phthalocyaninate [22], it was revealed that intramolecular electron transfer from the ligand to the lanthanide metal center and back (Figure 2a), i.e., reversible redox isomerization of the complex involving the multivalent Ce 3+/4+ cation, occurs when its solution is deposited on the water surface (Ce 4+ → Се 3+ ), and the subsequent compression of the thus obtained monolayer results in the reverse transition [13,22,[24][25][26][27]. Similar Eu 3+ ↔ Eu 2+ transitions for monolayers of donor-substituted europium bis-phthalocyaninates were described and unambiguously proved [23] (Figure 2b). Thus, in the research works devoted to the study of this process on the example of sandwich-substituted cerium bis-phthalocyaninate [22], it was revealed that intramolecular electron transfer from the ligand to the lanthanide metal center and back (Figure 2a), i.e., reversible redox isomerization of the complex involving the multivalent Ce 3+/4+ cation, occurs when its solution is deposited on the water surface (Ce 4+ →Ce 3+ ), and the subsequent compression of the thus obtained monolayer results in the reverse transition [13,22,[24][25][26][27].…”

“…Thus, in the research works devoted to the study of this process on the example of sandwich-substituted cerium bis-phthalocyaninate [22], it was revealed that intramolecular electron transfer from the ligand to the lanthanide metal center and back (Figure 2a), i.e., reversible redox isomerization of the complex involving the multivalent Ce 3+/4+ cation, occurs when its solution is deposited on the water surface (Ce 4+ →Ce 3+ ), and the subsequent compression of the thus obtained monolayer results in the reverse transition [13,22,[24][25][26][27]. Similar Eu 3+ ↔Eu 2+ transitions for monolayers of donor-substituted europium bis-phthalocyaninates were described and unambiguously proved [23] (Figure 2b). The europium bis-phthalocyaninates, as well as most of trivalent lanthanide bisphthalocyaninates, in bulk solutions exist in neutral forms, where one macrocyclic ligand is in a formal dianionic state and another one is anion-radical-[(Pc 2− )Ln 3+ (Pc −• )] 0 .…”

“…It should be noted that, although one can find several works devoted to the phenomena of valence tautomerism at the interface [14,21], they consider only the intramolecular electron transfer induced by light irradiation in the visible range. The observation of redox-isomer transformations in two-dimensional systems at the interface under the influence of transition from homogeneous bulk solution volume to the asymmetric 2D environment of the interface, as well as lateral compression, was first demonstrated in the works of our group on examples of crown-substituted bis-phthalocyaninates of cerium Ce[(15C5) 4 Pc] 2 [22] and europium Eu[(BuO) 8 Pc] 2 [23] (Figure 1).…”

“…It should be noted that, although one can find several works devoted to the phenomena of valence tautomerism at the interface [14,21], they consider only the intramolecular electron transfer induced by light irradiation in the visible range. The observation of redox-isomer transformations in two-dimensional systems at the interface under the influence of transition from homogeneous bulk solution volume to the asymmetric 2D environment of the interface, as well as lateral compression, was first demonstrated in the works of our group on examples of crown-substituted bis-phthalocyaninates of cerium Ce[(15C5)4Pc]2 [22] and europium Eu[(BuO)8Pc]2 [23] (Figure 1). Thus, in the research works devoted to the study of this process on the example of sandwich-substituted cerium bis-phthalocyaninate [22], it was revealed that intramolecular electron transfer from the ligand to the lanthanide metal center and back (Figure 2a), i.e., reversible redox isomerization of the complex involving the multivalent Ce 3+/4+ cation, occurs when its solution is deposited on the water surface (Ce 4+ → Се 3+ ), and the subsequent compression of the thus obtained monolayer results in the reverse transition [13,22,[24][25][26][27].…”

Functionalized heterocycle-appended porphyrins: catalysis matters

“…In the framework of our ongoing projects, our investigations are focused on tetrapyrrolic compounds, namely porphyrins and phthalocyanines, and their synthetic modications and search for new types of applications. 17,[23][24][25][26][27][28][29][30][31][32][33] Recently, we developed synthetic strategies towards imidazole-and pyrazine-annulated porphyrins starting from tetrapyrrole dioxo-or diamino-derivatives and aromatic aldehydes. 28,[34][35][36][37][38] The sterically hindered tetrapyrrole was used as a synthetic platform for the development of bifunctionalized linear and angular annulated polyaromatic compounds, water soluble derivatives for biomimetic applications and macrocycles with terminal anchoring groups.…”