1. M. R. Arkin, E. D. A. Stemp, R. E. Holmlin, J. K. Barton, Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA.
2. A. Hörmann, E. J. C. Olson, P. F. Barbara, Department of Chemistry, University of Minnesota, Minneapolis, MN 55455, USA.