1. Center
for Organic Photonics and Electronics, and School of Chemistry and
Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States
2. Department
of Materials Science and Engineering, College of Engineering, Key
Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing 100871, China
3. Department
of Physics, The Chinese University of Hong Kong, New Territories, Hong Kong, China