1. Department of Physics, Washington State University, 2710 University Drive, Richland, Washington 99354, Chemical & Materials Sciences Division, Pacific Northwest National Laboratory, MS K8-88, P.O. Box 999, Richland, Washington 99352, Department of Chemistry and Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Tsinghua University, Beijing 100084, China, and Department of Chemistry, Brown University, Providence, Rhode Island 02912