1. Institut Curie, CNRS UMR168, Laboratoire Physico-Chimie Curie, 26 rue d'Ulm, 75248 Paris Cedex 05, France; Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, China; and Laboratoire de Physique des Solides, CNRS UMR8502, Université Paris-Sud, 91405 Orsay Cedex, France