1. Contribution from the Laboratory for Chemistry of Novel Materials, Center for Research on Molecular Electronics and Photonics, University of Mons-Hainaut, Place du Parc 20, B-7000 Mons, Belgium, Department of Chemistry and Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and Institute for Polymers and Organic Solids, University of California, Santa Barbara, California 93106-5090