1. a Materials and Chemical Sciences Division , Lawrence Berkeley Laboratory , 1 Cyclotron Road, Berkeley , California , 94720 , U.S.A.
2. b Department of Chemistry , University of California , Berkeley , California , 94720 , U.S.A.
3. c Department of Pharmaceutical Chemistry , University of California , San Francisco , California , 94143 , U.S.A.
4. d Institut für Makromolekulare Chemie, Universität Freiburg , Stefan-Meier-Strasse 31, D-7800 , Freiburg , F.R. Germany