1. College of Chemistry and Material Science; Hebei Normal University; Shijiazhuang 050024 P.R. China
2. Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science; Institute of Chemistry; Chinese Academy of Sciences; Beijing 100190 P.R. China
3. CAS Research/Education Center for Excellence in Molecular Sciences; University of Chinese Academy of Sciences; Beijing 100049 P.R. China
4. Department of Chemistry; University of Leicester; University Road Leicester LE1 7RH UK