Kong Jie, Zhang Wei, Zhang Xiaomin, Liu Bo, Li Yang, Xia Andong
School of Science, Beijing University of Posts and Telecommunications (BUPT), Beijing 100876, People's Republic of China.
Hebei Key Laboratory of Organic Functional Molecules, College of Chemistry and Material Science, Hebei Normal University, No. 20, East Road of Nan Er Huan, Shijiazhuang 050024, People's Republic of China.
J Chem Phys. 2022 May 7;156(17):174902. doi: 10.1063/5.0092880.
Understanding the excited-state charge transfer/separation (CT/CS) of donor-π-acceptor chromophores can provide guidance for designing and synthesizing advanced dyes to improve the performance of dye-sensitized solar cells (DSSCs) in practical applications. Herein, two newly synthesized electronic push-pull molecules, CS-14 and CS-15, that consist of carbazole donor and benzothiadiazole acceptor segments are chosen to explore the ultrafast dynamics of intramolecular CT/CS processes. The theoretical calculation results depict an excited-state intramolecular CT character for both dyes, while the dihedral angle between donor and acceptor of CS-14 is larger than that of CS-15, suggesting a more significant CT character of CS-14. Furthermore, compared to CS-14, the bond rotation of CS-15 between donor and π-bridge is restricted by employing the hexatomic ring, indicating the stronger molecular planarization of CS-15. Ultrafast spectroscopy clearly shows a solvent polarity-dependent excited-state species evolution from CT to CS-the CT character is observed in low-polar toluene solvent, while the feature of the CS state in polar tetrahydrofuran and acetone solvents is captured, which successfully proved a solvent polarity modulated excited-state CT/CS characters. We also found that though the generation of the CS state within CS-14 is slightly faster than that of CS-15, the charge recombination process of CS-15 with excellent planar conformation is much slower, providing enough time for a higher charge migration efficiency in DSSCs.
了解供体-π-受体发色团的激发态电荷转移/分离（CT/CS）可为设计和合成先进染料提供指导，以在实际应用中提高染料敏化太阳能电池（DSSC）的性能。在此，选择了由咔唑供体和苯并噻二唑受体片段组成的两种新合成的电子推拉分子CS-14和CS-15，以探索分子内CT/CS过程的超快动力学。理论计算结果表明两种染料均具有激发态分子内CT特征，而CS-14供体与受体之间的二面角大于CS-15，表明CS-14具有更显著的CT特征。此外，与CS-14相比，CS-15供体与π-桥之间的键旋转通过采用六元环而受到限制，这表明CS-15具有更强的分子平面性。超快光谱清楚地表明，激发态物种从CT到CS的演化与溶剂极性有关——在低极性甲苯溶剂中观察到CT特征，而在极性四氢呋喃和丙酮溶剂中捕捉到CS态的特征，这成功证明了溶剂极性调制的激发态CT/CS特征。我们还发现，尽管CS-14中CS态的生成略快于CS-15，但具有优异平面构象的CS-15的电荷复合过程要慢得多，这为DSSC中更高的电荷迁移效率提供了足够的时间。
