1. Contribution from the Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium, Max-Planck-Institut für Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany, Physikalisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany, and Department of Chemistry, University of Perugia, Via Elce di Sotto, 8, I-06123 Perugia, Italy.