1. Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Sciences, Ecole Polytechnique Fédérale de Lausanne, Station 6, 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-06213 Perugia, Italy