1. Contribution from the Department of Materials and Interfaces, The Weizmann Institute of Science, 76100 Rehovot, Israel, Organic Chemistry 1, Ruhr-University Bochum, D-44780 Bochum, Germany, Materials Research Department, Risø National Laboratory, DK 4000, Roskilde, Denmark, and Niels Bohr Institute, H. C. Ørsted Laboratory, DK 2100, Copenhagen, Denmark