1. Department of Chemistry; National University of Singapore; 3 Science Drive 3 Singapore 117543 Singapore
2. Present address: School of Environmental and Chemical Engineering; Jiangsu University of Science and Technology; Zhenjiang Jiangsu 212003 People's Republic of China
3. Division of Chemistry and Biological Chemistry; School of Physical and Mathematical Sciences; Nanyang Technological University; 21 Nanyang Link Singapore 637371 Singapore