1. State Key Laboratory of Polymer Physics and Chemistry; Beijing National Laboratory for Molecular Sciences; Institute of Chemistry; Chinese Academy of Sciences; Beijing 100190 P.R. China
2. Department of Physics; North Carolina State University; Raleigh NC 27695 USA
3. Key Laboratory of Photochemistry; Beijing National Laboratory for Molecular Sciences; Institute of Chemistry; Chinese Academy of Sciences; Beijing 100190 P. R. China