高对称性蒽并噻吩分子堆积基序促进热活化单线态fission
High symmetry anthradithiophene molecular packing motifs promote thermally-activated singlet fission
摘要：
当考虑在有机光伏材料中实现电荷倍增的最佳分子堆积时，分子间电荷转移（CT）耦合的细微变化可以强烈地调制单线态裂变。为了理解为什么某些包装形态更有利于通过三重态对（TT）形成的电荷倍增，我们测量了来自氟化蒽并噻吩的四种单晶官能化衍生物的衍射限制的瞬态吸收（TA）响应：diF R-ADT（R = TES，TSBS，TDMS，TBDMS）。diF TES-ADT和diF TDMS-ADT均呈现二维砖状堆积结构，diF TSBS-ADT采用一维人字形堆积结构，diF TBDMS-ADT呈现一维扭曲柱状堆积结构。当砖砌或扭曲柱状单晶共振探测平行于他们的电荷转移（CT）轴的投影，TA信号占主导地位的皮秒时间尺度（率$k_{TT}$）归因于TT状态人口的上升分量。当垂直于CT轴探测时，我们反而恢复了以速率$k_{A}$的单重态耗尽的下降TA动力学。上升与下降速率比估计TT形成效率，$ε_{TT}$ = $k_{TT}/k_A$相对于激子自陷。$ε_{TT}$的范围从diF TES-ADT的接近1至diF TDMS-ADT的84%。有趣的是，diF TSBS-ADT晶体仅表现出CT介导的自捕获和单重态耗尽的下降动力学。diF TSBS-ADT晶体中的单线态裂变是禁止的，这是由于其较低的对称性夹层人字形堆积，这导致$S_1$到CT态的能量分离比其他堆积大约3倍。总的来说，这些结果突出了增强或完全抑制CT介导的TT对形成的最佳包装配置。
英文摘要：
When considering the optimal molecular packing to realize charge multiplication in organic photovoltaic materials, subtle changes in intermolecular charge transfer (CT) coupling can strongly modulate singlet fission. To understand why certain packing morphologies are more conducive to charge multiplication by triplet pair (TT) formation, we measure the diffraction-limited transient absorption (TA) response from four single-crystal functionalized derivatives of fluorinated anthradithiophene: diF R-ADT (R = TES, TSBS, TDMS, TBDMS). diF TES-ADT and diF TDMS-ADT both exhibit 2D brickwork packing structures, diF TSBS-ADT adopts a 1D sandwich-herringbone packing structure, and diF TBDMS-ADT exhibits a 1D twisted-columnar packing structure. When brickwork or twisted-columnar single crystals are resonantly probed parallel to their charge transfer (CT)-axis projections, the TA signal is dominated by a rising component on the picosecond timescale (rate $k_{TT}$) attributed to TT state population. When probed orthogonal to the CT-axis, we instead recover the falling TA kinetics of singlet state depletion at rate, $k_{A}$. The rising to falling rate ratio estimates the TT formation efficiency, $ε_{TT}$ = $k_{TT}/k_A$ relative to exciton self-trapping. $ε_{TT}$ ranged from near unity in diF TES-ADT to 84% in diF TDMS-ADT. Interestingly, diF TSBS-ADT crystals only manifest falling kinetics of CT-mediated self-trapping and singlet state depletion. Singlet fission is prohibitive in diF TSBS-ADT crystals owing to its lower symmetry sandwich herringbone packing that leads to $S_1$ to CT-state energy separation that is ~3x larger than in other packings. Collectively, these results highlight optimal packing configurations that either enhance or completely suppress CT-mediated TT-pair formation.
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