1. Contribution from the Department of Inorganic Chemistry 1, University of Bielefeld, P.O. Box 100 131, D-33501 Bielefeld, Germany, the Max-Planck-Institute for Polymer Research, P.O. Box 3148, D-55021 Mainz, Germany, and the Max-Planck-Institute of Colloids and Interfaces, D-14424 Potsdam, Germany