1. Contribution from the Department für Chemie und Biochemie and Center for Nanoscience (CeNS), Ludwig-Maximilians-Universität, Butenandtstrasse 11, D-81377, München, Germany, Department of Physics, 1110 West Green Street, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801-3080, and Max Planck Institut für Polymerforschung, Mainz, Germany