1. Contribution from the Departments of Organic Chemistry C-I and Physics of Materials C-IV, Universidad Autónoma de Madrid (UAM), Cantoblanco, Madrid-28049, Spain, Institut Químic de Sarrià, Universitat Ramon Llull, Via Augusta 390, 08017-Barcelona, Spain, and Department d'Electronique Quantique et Moléculaire, France Telecom, CNET Paris-B, 196, Avenue Henri Ravera, 92225 Bagneux, France