1. University of Sofia, Faculty of Chemistry, 1, J. Bourchier av., 1126 Sofia, Bulgaria
2. Institute of Inorganic and Structural Chemistry, Heinrich Heine University-Düsseldorf, D-40225-Düsseldorf, Germany
3. Institute of Polymers, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
4. Laboratory of Lanthanide Supramolecular Chemistry, BCH 1402, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland