1. a Institute of Polymers, Bulgarian Academy of Sciences , 1113 , Sofia , Bulgaria
2. b Department of Ecology , Forestry University , 1756 , Sofia , Bulgaria
3. c Department of Inorganic and Structural , Chemistry Heinrich-Heine-University-Düsseldorf; , D-40225 , Düsseldorf , Germany
4. d Heinrich Heine Universität Diisseldorf. Inst. Für Anorg. Chemie Universitätsstr , 10 D-40225 , Düsseldorf , Germany