1. Department of Chemical Engineering; Key Laboratory of Advanced Materials (MOE); Tsinghua University; Beijing 100084 P.R. China
2. High-Performance Materials Institute; Florida State University; 2005 Levy Ave. Tallahassee FL 32310 USA
3. Institut Curie; Centre de Recherche, CNRS UMR 168; Université Pierre et Marie Curie; 26 rue d'Ulm 75248 Paris CEDEX 05 France
4. Department of Chemistry and Biochemistry and Liquid Crystal Materials Research Center; University of Colorado; Boulder CO 80309 USA