1. State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, No. 381 Wushan Road, Tianhe District, Guangzhou 510640, P. R. China
2. School of Physics and Optoelectronics, South China University of Technology, No. 381 Wushan Road, Tianhe District, Guangzhou 510640, P. R. China
3. Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721-0088, United States