1. A. H. Flood and J. F. Stoddart are at the California NanoSystems Institute and the Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095, USA.
2. D. W. Steuerman is in the Department of Physics, University of California, Santa Barbara, CA 93106, USA.E-mail:
3. J. R. Heath is in the Division of Chemistry and Chemical Engineering, California Institute of Technology, CA 91125, USA.