1. Department of Chemistry, University of Fribourg, Ch. du Musée 9, CH-1700 Fribourg, Switzerland, Institute of Microtechnology, University of Neuchâtel, Rue Emile Argand 11, CH-2009 Neuchâtel, Switzerland, and Scheikundig Laboratorium, Vrije Universiteit van Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands