Solvent-Controlled Branching of Localized versus Delocalized Singlet Exciton States and Equilibration with Charge Transfer in a Structurally Well-Defined Tetracene Dimer
Solvent-Controlled Branching of Localized versus Delocalized Singlet Exciton States and Equilibration with Charge Transfer in a Structurally Well-Defined Tetracene Dimer
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DOI：
10.1021/acs.jpca.7b09458
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发表时间：
2017-12-07
影响因子：
2.9
通讯作者：
Damrauer, Niels H.
中科院分区：
文献类型：
--
作者：
Cook, Jasper D.;Carey, Thomas J.;Damrauer, Niels H.
一个详细的电子物理图片阐述了一个结构良好的定义和对称的双并四苯二聚体在溶液中。该分子被设计用于询问分子内单线态裂变（SF）中的初始物理步骤（S-1 ->（TT）-T-1）。（三异丙基甲硅烷基）乙炔取代基的二聚体TIPS-BT 1以及单体模型TIPS-Tc，使比较的物理性质，包括瞬态吸收动力学，作为溶剂极性是不同的。在非极性甲苯溶液中，TIPS-BT 1通过辐射和非辐射途径衰变到基态，没有证据表明与SF初始阶段相关的动力学。这与先前报道的未取代的二聚体BT 1的行为形成对比，并且可能是由（三烷基甲硅烷基）乙炔取代基对TIPS-BT 1的单重激发态歧管的能量扰动的结果。在极性苯甲腈中，出现了两个关键发现。首先，光激发的TIPS-BTI显示出分叉成两个臂本地化（S1-loc）和二聚体离域（S1-dim）单重态激子。S1-loc衰变到基态，其发射特征的弱温度依赖性表明，一旦它形成，它是从S1-dim分离的S1-dim状态的发射特征，另一方面，是强烈的温度依赖性，瞬态吸收动力学表明，S1-dim在室温下在50 ps内与分子内电荷转移状态平衡。这种平衡衰变到基态，几乎没有证据表明形成了长寿命的三重态，也没有（TT）-T-1。这些详细的研究在光谱上表征了这类二聚体中分子内SF中的许多关键状态，但强调了调整S-1 ->（TT）-T-1光反应的电子耦合和能量学的需要。
A detailed photophysical picture is elaborated for a structurally well-defined and symmetrical bis-tetracene dimer in solution. The molecule was designed for interrogation of the initial photophysical steps (S-1 -> (TT)-T-1) in intramolecular singlet fission (SF). (Triisopropylsilyl)acetylene substituents on the dimer TIPS-BT1 as well as a monomer model TIPS-Tc enable a comparison of photophysical properties, including transient absorption dynamics, as solvent polarity is varied. In nonpolar toluene solutions, TIPS-BT1 decays via radiative and nonradiative pathways to the ground state with no evidence for dynamics related to the initial stages of SF. This contrasts with the behavior of the previously reported unsubstituted dimer BT1 and is likely a consequence of energetic perturbations to the singlet excited-state manifold of TIPS-BT1 by the (trialkylsilyl)acetylene substituents. In polar benzonitrile, two key findings emerge. First, photoexcited TIPS-BTI shows a bifurcation into both arm-localized (S1-loc) and dimer-delocalized (S1-dim) singlet exciton states. The S1-loc decays to the ground state, and weak temperature dependence of its emissive signatures suggests that once it is formed, it is isolated from S1-dim Emissive signatures of the S1-dim state, on the other hand, are strongly temperature-dependent, and transient absorption dynamics show that S1-dim equilibrates with an intramolecular charge-transfer state in 50 ps at room temperature. This equilibrium decays to the ground state with little evidence for formation of long-lived triplets nor (TT)-T-1. These detailed studies spectrally characterize many of the key states in intramolecular SF in this class of dimers but highlight the need to tune electronic coupling and energetics for the S-1 -> (TT)-T-1 photoreaction.