Polaron pair versus bipolaron on oligothiophene chains:: A theoretical study of the singlet and triplet states
Polaron pair versus bipolaron on oligothiophene chains:: A theoretical study of the singlet and triplet states
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DOI：
10.1002/cphc.200200446
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发表时间：
2003-04-14
期刊：
影响因子：
2.9
通讯作者：
Brédas, JL
中科院分区：
文献类型：
--
作者：
Geskin, VM;Brédas, JL
p共轭低聚物和聚合物的几何形状对链携带的电荷量非常敏感。考虑到这种强烈的电子±声子耦合，研究表明，多余的电荷可以集中并导致延伸到链的有限部分的结构扭曲。p-共轭链中这种缺陷的自定位已经有了充分的文献记载不仅单个电荷可以通过这种方式被捕获形成极化子（自由基离子），而且双电荷也可以形成双极化子（分裂离子）结构。[1,2]带电物种的直接结构数据很少；因此，从各种光谱和电化学实验中推导出了电荷存储的形式；这些结果有时会导致不同的解释。除了孤立链上的单极化子和双极化子外，还调用了双链上两个极化子的p二聚体[3,4]和单链上的极化子对[5]（这里可以注意到，在某些情况下，早期被认为是双极化子特征的光谱分配后来被重新分配给极化子[6]）。在双极化子的情况下，区分固有稳定性和相对稳定性是有用的。当势能面呈现与其结构相对应的最小值时，一个分子物种是本质稳定的。然而，相对于具有相同组成但具有不同几何形状和/或处于不同电子状态的物质，这样的物质可能是不稳定的。在这里，我们提出的问题是，在没有反离子的情况下，孤立的寡硫吩链上的双极化子是否本质稳定，以及它是否相对于解离的极化子对稳定；当这个问题涉及到收费的额外准备步骤时，排除反价是特别相关的。不同长度的脂肪酸（Sigma Aldrich）溶解在1-苯辛烷中，无需对分子或溶剂进行任何额外的纯化或干燥。用玻璃移液管将该溶液的液滴转移到基板上，并将尖端浸入液滴中（典型的隧道条件：I% 500 pA， Ubias% 750 mV，具体取决于所研究的系统）。在用STM检查模板层后，将同样溶解于1-苯辛烷中的尿素或醋酸钯（II） （Sigma Aldrich）等吸附剂小心地添加到液滴中以保持尖端位置。由于模板分子的浓度应保持近似恒定，因此吸附质溶液的浓度应达到饱和。为了排除不同隧道条件造成的STM伪影，在对装饰模板层成像时保持隧道参数不变。所有STM图像均以定高模式记录。作者承认与Gerhard Erker教授和J ø rg Wonnemann博士（德国M∏nster大学有机化学研究所）进行了富有成果的讨论，他们还提供了核磁共振调查和SH-C14H28-COOH材料。这项工作得到了德国研究委员会（SFB 424）的财政支持。
The geometry of p-conjugated oligomers and polymers is very sensitive to the amount of charge the chains carry. Given this strong electron±phonon coupling, it has been shown that excess charge can concentrate and lead to structural distortions extending over a limited section of the chain. Such a selflocalization of defects in p-conjugated chains has been amply documented.[1] Not only a single charge can be trapped in this way forming a polaron (radical-ion species), but also a double charge is supposed to form a bipolaron (di-ion) structure.[1, 2] Direct structural data for charged species are scarce; the form of charge storage has therefore been deduced from various spectral and electrochemical experiments; these results can sometimes lead to different interpretations. In addition to single polarons and bipolarons on isolated chains, p dimers involving two polarons on two chains [3, 4] and polaron pairs on a single chain [5] have been invoked (it can be noted here that in some instances, spectral assignments considered early on as a signature of bipolarons were later reassigned to polarons [6]). In the case of a bipolaron, it is useful to discriminate between intrinsic and relative stability. A molecular species is intrinsically stable when the potential energy surface presents a minimum corresponding to its structure. However, such a species can turn out to be unstable relative to a species of the same composition but of a different geometry and/or in a different electronic state. Herein, we ask the question of whether a bipolaron on an isolated oligothiophene chain in the absence of counterions is intrinsically stable and whether it is relatively stable versus a dissociated polaron pair; excluding counterions is especially relevant when this question is addressed to the case of charges additional preparation steps. Fatty acids of different length (Sigma Aldrich) were dissolved in 1-phenyloctane without any additional purification or drying of the molecules or the solvent. A droplet of this solution was transferred to the substrate with a glass pipette and the tip was immersed into the droplet (typical tunneling conditions: I% 500 pA, Ubias% 750 mV depending on the investigated system). After inspection of the template layer by means of STM the adsorbates such as urea or palladium (II) acetate (Sigma Aldrich), also dissolved in 1-phenyloctane, were carefully added to the droplet to maintain the tip position. The concentration of the adsorbate solution should be saturated since the concentration of the template molecules should remain approximately constant. To exclude STM artifacts due to different tunneling conditions the tunneling parameters were kept unchanged for the imaging of the decorated template layers. All STM images were recorded in constant height mode.The authors acknowledge fruitful discussions with Prof. Dr. Gerhard Erker and Dr. Jˆrg Wonnemann (Organisch Chemisches Institut, University of M∏ nster, Germany), who also provided NMR investigations and the SH-C14H28-COOH material. This work has been financially supported by the Deutsche Forschungsgemeinschaft (SFB 424).