1. Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of basic Sciences, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland, Philips Research Laboratories, High Tech Campus 4, 5656 AE Eindhoven, The Netherlands, and Istituto CNR di Scienze e Tecnologie Molecolari (ISTM-CNR), c/o Dipartimento di Chimica, Università di Perugia, I-06123 Perugia, Italy