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
2. Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station A5300, Austin, Texas 78712‐0165, USA, Fax: +1‐852‐3411‐7348