1. Contribution from the Departamento de Química Orgánica, Universidad Autónoma de Madrid, Cantoblanco 28049-Madrid, Spain, University of Erlangen, Institute of Physical and Theoretical Chemistry and Interdisciplinary Center for Molecular Materials, Egerlandstrasse 3, 91058 Erlangen, Germany, and Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco 28049-Madrid, Spain