1. Foundation for Research and Technology−Hellas (FO.R.T.H.), Institute of Electronic Structure and Laser, P.O. Box 1527, 711 10 Heraklion Crete, Greece, Department of Chemistry, University of Athens, Panepistimiopolis, Zografou 15771, Athens, Greece, and Max-Planck Institut für Polymerforschung, Postfach 3148, D-55021 Mainz, FRG