1. Contribution from the Department of Chemistry, The University of Arizona, Tucson, Arizona 85721, Division of Chemistry and Chemical Engineering and Beckman Institute, California Institute of Technology, Pasadena, California 91125, Optical Sciences Center, The University of Arizona, Tucson, Arizona 85721, and Center for Research on Molecular Electronics and Photonics, Université de Mons-Hainaut, B-7000 Mons, Belgium