1. Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, and Department of Chemistry, University of California, Berkeley, California 94720
2. Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003
3. MPI für Medizinische Forschung, Arbeitsgruppe Molekülkristalle, Jahnstrasse 29, D-69120 Heidelberg, Germany