1. Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium, BIOSYST - MeBioS, Katholieke Universiteit Leuven, Willem de Croylaan 42, B-3001 Heverlee, Belgium, Max-Planck-Institut für Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany, and Department of NanoBiophotonics, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, Germany