1. School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, Department of Materials Science and INSTM, University of Milano-Bicocca, Via Cozzi 53, I-20125 Milano, Italy, CREOL, the College of Optics and Photonics, and Department of Physics, University of Central Florida, Orlando, Florida 32816-2700, and Key Laboratory of Organic Solids, Institute of Chemistry, The Chinese Academy of Sciences, 100080 Beijing, P. R. China