1. Department of Biology and Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, P. R. China, Department of Chemistry, the George Washington University, 725 21st Street NW, Washington, D.C. 20052, Department of Chemistry, Youngstown State University, One University Plaza, Youngstown, Ohio 44555, and Department of Chemistry and HKU-CAS Joint Laboratory on New Materials, The University of Hong Kong, Pokfulam Road, Hong Kong, China