1. Institute of Molecular Functional Materials (Areas of Excellence Scheme University Grants Committee, Hong Kong) and Department of Chemistry and Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Waterloo Road, Hong Kong, P. R. China, Fax: +852‐3411‐7348
2. School of Chemistry and Chemical engineering, Xi'an University of Science and Technology, Xi'an 710054, P. R. China