1. Department of Chemistry City University of Hong Kong Kowloon 999077 Hong Kong
2. Department of Materials Science and Engineering University of Washington Box 352120 Seattle WA 98195‐2120 USA
3. The Institute for Advanced Studies Wuhan University Wuhan 430072 China
4. Department of Physics Chinese University of Hong Kong New Territories 999077 Hong Kong
5. Key Laboratory of Luminescence and Optical Information Ministry of Education Beijing Jiao tong University Beijing 100044 China
6. Department of Chemistry Hubei Key Lab on Organic and Polymeric Optoelectronic Materials Wuhan University Wuhan 430072 China
7. Department of Materials Science and Engineering City University of Hong Kong Kowloon 999077 Hong Kong