Conversion between triplet pair states is controlled by molecular coupling in pentadithiophene thin films,Chemical Science

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Conversion between triplet pair states is controlled by molecular coupling in pentadithiophene thin films
Chemical Science ( IF 7.6 ) Pub Date : 2020-06-22 , DOI: 10.1039/d0sc02497j
Natalie A Pace ₁ , Brandon K Rugg ₁ , Christopher H Chang ₁ , Obadiah G Reid _{1,

2} , Karl J Thorley ₃ , Sean Parkin ₃ , John E Anthony ₃ , Justin C Johnson ₁

Affiliation

In singlet fission (SF) the initially formed correlated triplet pair state, ¹(TT), may evolve toward independent triplet excitons or higher spin states of the (TT) species. The latter result is often considered undesirable from a light harvesting perspective but may be attractive for quantum information sciences (QIS) applications, as the final exciton pair can be spin-entangled and magnetically active with relatively long room temperature decoherence times. In this study we use ultrafast transient absorption (TA) and time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy to monitor SF and triplet pair evolution in a series of alkyl silyl-functionalized pentadithiophene (PDT) thin films designed with systematically varying pairwise and long-range molecular interactions between PDT chromophores. The lifetime of the (TT) species varies from 40 ns to 1.5 μs, the latter of which is associated with extremely weak intermolecular coupling, sharp optical spectroscopic features, and complex TR-EPR spectra that are composed of a mixture of triplet and quintet-like features. On the other hand, more tightly coupled films produce broader transient optical spectra but simpler TR-EPR spectra consistent with significant population in ⁵(TT)₀. These distinctions are rationalized through the role of exciton diffusion and predictions of TT state mixing with low exchange coupling J versus pure spin substate population with larger J. The connection between population evolution using electronic and spin spectroscopies enables assignments that provide a more detailed picture of triplet pair evolution than previously presented and provides critical guidance for designing molecular QIS systems based on light-induced spin coherence.

中文翻译：

三重态对状态之间的转换由五二噻吩薄膜中的分子耦合控制

在单重态裂变 (SF) 中，最初形成的相关三重态对态¹ (TT) 可能会演化为独立的三重态激子或 (TT) 物质的更高自旋态。从光捕获的角度来看，后一种结果通常被认为是不受欢迎的，但对于量子信息科学（QIS）应用来说可能很有吸引力，因为最终的激子对可以是自旋纠缠的并且具有磁活性，并且具有相对较长的室温退相干时间。在这项研究中，我们使用超快瞬态吸收（TA）和时间分辨电子顺磁共振（TR-EPR）光谱来监测一系列烷基甲硅烷基功能化五二噻吩（PDT）薄膜中的SF和三重态对演化，这些薄膜设计有系统地变化的成对以及 PDT 发色团之间的长程分子相互作用。 (TT)物种的寿命从40 ns到1.5 μs不等，后者与极弱的分子间耦合、尖锐的光学光谱特征以及由三重态和五重态混合组成的复杂TR-EPR光谱有关。喜欢特征。另一方面，更紧密耦合的薄膜产生更宽的瞬态光谱，但更简单的TR-EPR光谱与⁵ (TT) ₀中的显着分布一致。这些区别通过激子扩散的作用以及低交换耦合J 的 TT 态混合与具有较大J 的纯自旋子态群体的预测来合理化。使用电子和自旋光谱的群体进化之间的联系使得分配能够提供比以前提出的更详细的三重态对进化图片，并为设计基于光诱导自旋相干性的分子 QIS 系统提供关键指导。

更新日期：2020-07-15

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