1. Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, People's Republic of China, Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, People's Republic of China, and Laboratoire de Physico-Chimie des Matériaux Luminescents, Université Claude-Bernard Lyon-1, UMR 5620 du CNRS, Bâtiment A. Kastler, 69622 Villeurbanne Cedex, France