1. Contribution from the Department of Chemistry and Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, P. R. China, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, P. R. China, and Departments of Chemistry and Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China