1. Beijing National Laboratory for Molecular Sciences; Department of Applied Chemistry; Center for the Soft Matter Science and Engineering and the Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education; College of Chemistry; Peking University; Beijing 100871 China
2. Department of Chemistry; Hong Kong University of Science and Technology; Clear Water Bay Kowloon Hong Kong China
3. Department of Physics; North Carolina State University; Raleigh NC 27695 USA