1. School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400;
2. Department of Material Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307; and
3. Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria