1. Contribution from the Opto-Electronic Materials Section, DelftChemTech, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands, Afdeling Theoretische Chemie, Scheikundig Laboratorium der Vrije Universiteit, De Boelelaan 1083, NL-1081 HV Amsterdam, The Netherlands, and Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113