1. Beijing National Laboratory for Molecular Sciences National Biomedical Imaging Center College of Chemistry and Molecular Engineering Peking University Beijing 100871 P. R. China
2. Department of Basic Science Graduate School of Arts and Sciences The University of Tokyo Tokyo 153‐8902 Japan
3. Department of Chemistry Osaka Dental University Osaka 573‐1121 Japan
4. Center of Single‐Molecule Sciences Institute of Modern Optics Frontiers Science Center for New Organic Matter College of Electronic Information and Optical Engineering Nankai University 38 Tongyan Road, Jinnan District Tianjin 300350 P. R. China