1. Contribution from the Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of basic Sciences, Swiss Federal Institute of Technology, CH - 1015 Lausanne, Switzerland, and Istituto CNR di Scienze e Tecnologie Molecolari (ISTM-CNR), c/o Dipartimento di Chimica, Università di Perugia, via Elce di Sotto 8, I-06123, Perugia, Italy