RUDN University chemists developed new magnetic and luminescent lanthanide-siloxane-based compounds

Complexes obtained by the team are based on the lanthanide metals, namely, terbium and europium. Lanthanide compounds are known for their unusual magnetic and optical properties: the former make them an excellent source for the production of contrast agents for medical applications, and the latter — materials for electroluminescent devices. However, until this work, these properties had never been studied in detail for lanthanide-containing cage silsesquioxanes. Compounds obtained by the team have an unusual structure that has never been observed before, with prism-shaped cage including central core with four lanthanide atoms. This central layer is coordinated by two cyclic silsesquioxane fragments, solvent molecules, and organic (phosphorus- or nitrogen-containing) cations. Notably, terbium compounds synthesized by the team were the first-ever silsesquioxanes to contain this metal.

“Until recently, only two types of lanthanide CLMSs had been known and had undergone X-ray diffraction study. The first type was cubane siloxane compounds — cube-shaped structures with a lanthanide atom in each corner. The second was the so-called sandwiches: two siloxane fragments with a layer of lanthanide ions and alkaline metals laying between them. Both types were only considered as unusual structural types and/or catalytic system models, and their optical and magnetic properties were largely understudied,” said Dr. Alexey Bilyachenko, a Deputy Head of the Joint Institute for Chemical Research at RUDN University.

To obtain new compounds, the team developed a two-step reaction. First, a reactive substance (sodium phenylsiloxanolate) was synthesized. Second, the so-called self-assembly reaction took place: sodium phenylsiloxanolate (in the presence of organic cations) formed a regular structure due to coordination to lanthanide ions. X-ray diffraction analysis allowed to establish the structure of the products and identified fourmembered siloxane cycles in their structures. Previously, such cycles have only been observed in titanium- and cobalt-based CLMSs.

Magnetic properties of lanthanide-based silsesquioxanes were investigated for the first time. The terbium-based compound demonstrated the magnetic spin flip effect (i.e. the switch from antiferromagnetic behaviour into a ferromagnetic one).

To study the optical properties of the substances, the team subjected them to photoexcitation under the influence of UV or visible blue light. The compounds demonstrated characteristic luminescence: europium-containing substances provided red emission, while terbium-containing ones provided a green one. Therefore, these compounds turned out to be the first CLMSs with magnetic and luminescent properties studied in detail.

The results of the work were published in Chemistry — A European Journal.