1. Center for Supramolecular Science, Department of Chemistry, University of Miami, 1301 Memorial Drive, Florida, 33146-0431, Dipartimento di Scienze Chimiche, Universitá di Catania, viale Andrea Doria 8, Catania, I-95125, Italy, and Department of Chemistry, Imperial College London, South Kensington, London, SW7 2AY, U.K.