Liu Heyuan, Wang Xiangyang, Ma Lishuang, Wang Weijie, Liu Shanshan, Zhou Jun, Su Pengkun, Liu Zhaobin, Li Zhi, Lin Xufeng, Chen Yanli, Li Xiyou
Shandong Energy Group Co., Ltd., Jinan, Shandong 250014, People's Republic of China.
J Am Chem Soc. 2022 Aug 31;144(34):15509-15518. doi: 10.1021/jacs.2c03550. Epub 2022 Aug 5.
To harvest two triplet excitons of singlet fission (SF) via a two-electron transfer efficiently, the revelation of the key factors that influence the two-electron-transfer process is necessary. Here, by using steady-state and transient absorption/fluorescence spectroscopy, we investigated the two-electron-transfer process from the two triplet excitons of intramolecular SF (iSF) in a series of tetracene oligomers (dimer, trimer, and tetramer) with 7,7,8,8-tetracyanoquinodimethane (TCNQ) as an electron acceptor in solution. Quantitative two-electron transfer could be conducted for the trimer and tetramer, and the rate for the tetramer is faster than that for the trimer. However, the maximum efficiency of the two-electron transfer in the dimer is relatively low (∼47%). The calculation result of the free energy change (Δ) of the second-electron transfer for these three compounds (-0.024, -0.061, and -0.074 eV for the dimer, trimer, and tetramer, respectively) is consistent with the experimental observation. The much closer Δ value to zero for the dimer should be responsible for its low efficiency of the two-electron transfer. Different Δ values for these three oligomers are attributed to the different Coulomb repulsive energies between the two positive charges generated after the two-electron transfer that is caused by their various intertriplet distances. This result reveals for the first time the important effect of the Coulomb repulsive energy, which depends on the intertriplet distance, on the two-electron transfer process from the two triplet excitons of iSF.
为了通过双电子转移有效地收获两个单线态裂变（SF）的三重态激子，揭示影响双电子转移过程的关键因素是必要的。在这里，我们使用稳态和瞬态吸收/荧光光谱，研究了在一系列以7,7,8,8-四氰基对苯二醌二甲烷（TCNQ）为电子受体的并四苯低聚物（二聚体、三聚体和四聚体）中，分子内SF（iSF）的两个三重态激子的双电子转移过程。三聚体和四聚体可以进行定量双电子转移，且四聚体的转移速率比三聚体快。然而，二聚体中双电子转移的最大效率相对较低（约47%）。这三种化合物第二次电子转移的自由能变化（Δ）的计算结果（二聚体、三聚体和四聚体分别为-0.024、-0.061和-0.074 eV）与实验观察结果一致。二聚体的Δ值更接近零，这应该是其双电子转移效率低的原因。这三种低聚物的不同Δ值归因于双电子转移后产生的两个正电荷之间不同的库仑排斥能，这是由它们不同的三重态间距离引起的。该结果首次揭示了取决于三重态间距离的库仑排斥能对iSF的两个三重态激子的双电子转移过程的重要影响。
