1. Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China
2. Key Laboratory of Luminescence and Optical Information, Ministry of Education, School of Science, Beijing Jiaotong University, Beijing 100044, P. R. China
3. Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States
4. National Center for Nanoscience and Technology, Beijing 100190, P. R. China
5. Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China