1. Department of Chemistry and Institute of Nanoscale Physics and Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium, Centre Interdisciplinaire de Nanoscience de Marseille (UPR 3118, CNRS), Campus de Luminy, Case 913, F-13288 Marseille cedex 09, France, Max-Planck-Institut für Polymerforschnung, Ackermannweg 10, D-55128 Mainz, Germany, and Centre de Recherche Paul Pascal (UPR 8641, CNRS), 115 avenue du docteur Albert Schweitzer, F-33600 Pessac, France