1. Department of Chemistry, LMU, Butenandtstrasse 11, 81377 Munich, Germany, Lehrstuhl für BioMolekulare Optik, LMU, Oettingenstrasse 67, 80538 Munich, Germany, and Laboratoire de Chimie Physique-UMR 8000, Université Paris Sud 11, Bat 349, 91405 Orsay, France