1. Department of Chemistry, National Research Lab for Polymer Synthesis & Physics, Pohang Accelerator Laboratory, Center for Integrated Molecular Systems, Polymer Research Institute, and BK21 Program, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea, and Department of Chemical Engineering and National Research Lab of Materials and Processes for New Memory, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea