1. Institute of Atomic and Molecular Sciences, Academia Sinica, P. O. Box 23-166, Taipei 10617, Taiwan, Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan, Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan, Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan, and Department of Chemistry, Emory University, Atlanta, Georgia 30322