1. Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany, Department of Chemistry, Division of Molecular and Nanomaterials, Laboratory of Photochemistry and Spectroscopy, and Institute of Nanoscale Physics and Chemistry, Katholieke Universiteit Leuven (KULeuven), Celestijnenlaan 200 F, B-3001 Leuven, Belgium, and National Center for Nanoscience and Technology of China, Zhongguancun, Beiyitiao 11, 100190 Beijing, P. R. China