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 (ΔG) 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 ΔG value to zero for the dimer should be responsible for its low efficiency of the two-electron transfer. Different ΔG 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.

中文翻译：

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分子内单重态裂变产生的两个三重态之间的分离距离对双电子转移过程的影响

为了通过双电子转移有效地收获两个单重态裂变（SF）的三重态激子，有必要揭示影响双电子转移过程的关键因素。在这里，通过使用稳态和瞬态吸收/荧光光谱，我们研究了一系列并四苯低聚物（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，其中 7 ,7,8,8-四氰基醌二甲烷 (TCNQ) 作为溶液中的电子受体。三聚体和四聚体可以进行定量的双电子转移，并且四聚体的速率比三聚体的快。然而，二聚体中双电子转移的最大效率相对较低（~47%）。自由能变化的计算结果（ΔG）这三种化合物的第二电子转移（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 G值更接近于零应该是其二电子转移效率低的原因。这三种低聚物的不同ΔG值归因于在两电子转移后产生的两个正电荷之间的库仑排斥能不同，这是由它们的不同三重态间距引起的。该结果首次揭示了依赖于三重态间距的库仑排斥能对 iSF 的两个三重态激子的二电子转移过程的重要影响。