1. Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education Department of Chemistry Tsinghua University Beijing 100084 China
2. Department of Materials Science and Engineering University of Washington Seattle WA 98195 USA
3. Department of Chemistry The Hong Kong University of Science and Technology Clear Water Bay Kowloon Hong Kong
4. Department of Biology and Chemistry City University of Hong Kong Kowloon Hong Kong