We would like to thank the National Natural Science Foundation of China (Grant No. 51525304 & 51173096) and the National Key Basic Research and Development Program of China (Grant No. 2015CB655002) for financial support. We are grateful to Dr. Zhen Cao, Dr. Yuan Li and Prof. Jean-Luc Bredas for a critical reading of the manuscript and helpful discussions. The computation in this research was performed on the "Explorer 100" cluster system of Tsinghua National Laboratory for Information Science and Technology. This work was also supported by CAS Interdisciplinary Innovation Team.