1. State Key Laboratory of Polymer, Physics and Chemistry; Institute of Chemistry, Chinese Academy of Sciences; Beijing 100190 P. R. China
2. University of Chinese Academy of Sciences; Beijing 100049 P. R. China
3. Department of Physics; North Carolina State University; Raleigh North Carolina 27695 USA
4. Institute of Polymer Optoelectronic Materials and Devices; State Key Laboratory of Luminescent Materials and Devices; South China University of Technology; Guangzhou 510640 P. R. China