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From π-Conjugated Rods to Shape-Persistent Rings, Wheels, and Ladders: The Question of Rigidity
Accounts of Chemical Research ( IF 16.4 ) Pub Date : 2024-08-16 , DOI: 10.1021/acs.accounts.4c00383 Sigurd Höger 1 , John M Lupton 2
Accounts of Chemical Research ( IF 16.4 ) Pub Date : 2024-08-16 , DOI: 10.1021/acs.accounts.4c00383 Sigurd Höger 1 , John M Lupton 2
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Rigid-rod oligomers and polymers are mostly based on (hetero)aromatic rings connected with each other, either directly or via ethynylene or butadiynylene linkers, or by a combination of both structural elements. Although they are much more rigid than vinyl polymers, they exhibit considerable structural flexibility, often more than would be expected merely from their chemical structure. This disparity holds for both linear as well as for cyclic structures. The flexibility of rigid-rod polymers, which is directly observable for defined oligomers of different lengths at the solid–liquid interface by means of scanning-tunneling microscopy, also impacts their optical and electronic properties. The flexibility can be used, for example, to control whether an oligomer with two different fluorescent end-groups emits from either the one or the other. The flexibility of shape-persistent macrocycles also has an impact on the overall thermal stability of mechanically interlocked molecular architectures. However, the degree of flexibility can be reduced when rigid struts are covalently mounted into the inside of the rings, leading to the formation of so-called molecular spoked wheels. The combination of these two elements─rings and rods─stiffens both of them: the ring perimeter is prevented from collapsing and the internal rods from bending. These compounds have been further developed as platform molecules, where three spokes stiffen the ring and together form a tripod-like platform, while a fourth arm points─after adsorption to a solid substrate─above the plane of the molecule. This pillar makes it possible to decouple a functional group at the end of the arm from the surface. Rigidity enhancement by the introduction of rigid spacer elements can also be applied to the case of rigid-rod polymers and is visualized by sophisticated molecular dynamics simulations. In this case, formation of single-stranded oligomers and polymers, and a subsequent zipping reaction to form ladder-like structures, directly allows, by means of single-molecule fluorescence spectroscopy, a comparison of the single- and double-stranded molecules. In particular in the case of the polymers, which can be up to 100 nm in length, the enhancement of rigidity is quite remarkable. Overall, the covalent connection of two or more rigid molecular entities has a self-reinforcing effect: all parts of the molecule gain rigidity. Since overall synthetic yields for such complex high-molecular weight covalently bound shape-persistent structures can still be low, scanning tunneling microscopy and single-molecule fluorescence spectroscopy are the methods of choice for structural analyses. Preliminary results illustrate how these compounds can serve as versatile sources of deterministic single photons on demand, since rigidity also enhances the intramolecular flow of excitation energy, and suggest a range of applications in optoelectronic devices.
中文翻译：
从 π 共轭杆到形状持久的环、轮子和梯子：刚性问题
刚性棒低聚物和聚合物大多基于直接或通过乙炔基或丁二炔基连接体或通过两种结构元素的组合彼此连接的（杂）芳环。尽管它们比乙烯基聚合物刚性得多，但它们表现出相当大的结构灵活性，通常超过仅从其化学结构预期的灵活性。这种差异对于线性结构和循环结构都成立。刚性棒聚合物的灵活性，可以通过扫描隧道显微镜在固液界面直接观察到不同长度的特定低聚物，也影响其光学和电子性能。例如，这种灵活性可用于控制具有两个不同荧光端基的低聚物是否从其中之一或另一个发出荧光。形状持久大环的灵活性也对机械联锁分子结构的整体热稳定性有影响。然而，当刚性支柱共价安装到环内部时，柔性程度会降低，从而形成所谓的分子辐条轮。这两个元件（环和杆）的组合使它们都变得更加坚固：防止环的周边塌陷，内部的杆也不会弯曲。这些化合物已被进一步开发为平台分子，其中三个辐条使环变硬并一起形成一个三脚架状平台，而第四个臂在吸附到固体基质后指向分子平面上方。该柱使得可以将臂末端的功能组与表面分离。 通过引入刚性间隔元件来增强刚性也可以应用于刚性棒聚合物的情况，并且可以通过复杂的分子动力学模拟来可视化。在这种情况下，单链低聚物和聚合物的形成，以及随后形成梯状结构的拉链反应，可以直接通过单分子荧光光谱法对单链和双链分子进行比较。特别是对于长度可达100 nm的聚合物来说，刚性的增强是相当显着的。总体而言，两个或多个刚性分子实体的共价连接具有自我强化效应：分子的所有部分都获得刚性。由于这种复杂的高分子量共价结合形状持久结构的总体合成产率仍然较低，因此扫描隧道显微镜和单分子荧光光谱是结构分析的首选方法。初步结果说明了这些化合物如何根据需要充当确定性单光子的多功能来源，因为刚性还增强了激发能量的分子内流动，并表明了在光电器件中的一系列应用。
更新日期：2024-08-16
中文翻译：
从 π 共轭杆到形状持久的环、轮子和梯子：刚性问题
刚性棒低聚物和聚合物大多基于直接或通过乙炔基或丁二炔基连接体或通过两种结构元素的组合彼此连接的（杂）芳环。尽管它们比乙烯基聚合物刚性得多，但它们表现出相当大的结构灵活性，通常超过仅从其化学结构预期的灵活性。这种差异对于线性结构和循环结构都成立。刚性棒聚合物的灵活性，可以通过扫描隧道显微镜在固液界面直接观察到不同长度的特定低聚物，也影响其光学和电子性能。例如，这种灵活性可用于控制具有两个不同荧光端基的低聚物是否从其中之一或另一个发出荧光。形状持久大环的灵活性也对机械联锁分子结构的整体热稳定性有影响。然而，当刚性支柱共价安装到环内部时，柔性程度会降低，从而形成所谓的分子辐条轮。这两个元件（环和杆）的组合使它们都变得更加坚固：防止环的周边塌陷，内部的杆也不会弯曲。这些化合物已被进一步开发为平台分子，其中三个辐条使环变硬并一起形成一个三脚架状平台，而第四个臂在吸附到固体基质后指向分子平面上方。该柱使得可以将臂末端的功能组与表面分离。 通过引入刚性间隔元件来增强刚性也可以应用于刚性棒聚合物的情况，并且可以通过复杂的分子动力学模拟来可视化。在这种情况下，单链低聚物和聚合物的形成，以及随后形成梯状结构的拉链反应，可以直接通过单分子荧光光谱法对单链和双链分子进行比较。特别是对于长度可达100 nm的聚合物来说，刚性的增强是相当显着的。总体而言，两个或多个刚性分子实体的共价连接具有自我强化效应：分子的所有部分都获得刚性。由于这种复杂的高分子量共价结合形状持久结构的总体合成产率仍然较低，因此扫描隧道显微镜和单分子荧光光谱是结构分析的首选方法。初步结果说明了这些化合物如何根据需要充当确定性单光子的多功能来源，因为刚性还增强了激发能量的分子内流动，并表明了在光电器件中的一系列应用。
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