1. Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland, Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Functional Polymers, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland, and Department of Chemistry, Korea University, Jochiwon, Chungnam 339-700, Korea