1. Contribution from the Institut für Organische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany, School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, 770 State Street, Atlanta, Georgia 30332-0400, and Department of Chemistry, The University of Arizona, 1306 East University Boulevard, Tucson, Arizona 85721-0041