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