1. Instituto de Investigaciones
en Materiales, Universidad Nacional Autónoma de México, Ciudad Universitaria, C.P. 04510 México
D.F., México
2. Institute
for Polymer Research,
Waterloo Institute for Nanotechnology, Department of Chemistry, University of Waterloo, Waterloo, ON N2L 3G1, Canada
3. Laboratoire de Chimie de Coordination
du CNRS, 205 route de Narbonne 31077 Toulouse Cedex 4, France