Fluctuating exciton localization in giant π-conjugated spoked-wheel macrocycles
Fluctuating exciton localization in giant π-conjugated spoked-wheel macrocycles
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DOI：
10.1038/nchem.1758
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发表时间：
2013-11-01
期刊：
影响因子：
21.8
通讯作者：
Lupton, John M.
中科院分区：
文献类型：
--
作者：
Aggarwal, A. Vikas;Thiessen, Alexander;Lupton, John M.
共轭聚合物提供了许多不同的应用潜力，但我们仍然缺乏对其电子结构的基本微观了解。基本光激发(激子)只跨越分子的几个纳米，而分子本身可以延伸到微米以上，它们的行为如何受到分子尺寸的影响还不是很明显。例如，激子在共轭片段中形成在哪里，它总是位于相同的重复单元上吗？在这里，我们引入了直径为6 nm的结构刚性分子轮子，作为扩展pi共轭的模型。单分子荧光表现为随机激子局域化，导致发射极化随时间变化。最初，由于与温度无关的自发对称性破缺，这种随机局域化出现在每个光子吸收事件之后。在长时间光照后，这些快速波动会减慢到毫秒级。在低温光谱学中，跃迁能跳跃揭示了分子内的不均一性，但发射极化也可以在不发生光谱跳跃的情况下切换，这意味着局域介电环境中的长程均匀。
Conjugated polymers offer potential for many diverse applications, but we still lack a fundamental microscopic understanding of their electronic structure. Elementary photoexcitations (excitons) span only a few nanometres of a molecule, which itself can extend over microns, and how their behaviour is affected by molecular dimensions is not immediately obvious. For example, where is the exciton formed within a conjugated segment and is it always situated on the same repeat units? Here, we introduce structurally rigid molecular spoked wheels, 6 nm in diameter, as a model of extended pi conjugation. Single-molecule fluorescence reveals random exciton localization, which leads to temporally varying emission polarization. Initially, this random localization arises after every photon absorption event because of temperature-independent spontaneous symmetry breaking. These fast fluctuations are slowed to millisecond timescales after prolonged illumination. Intramolecular heterogeneity is revealed in cryogenic spectroscopy by jumps in transition energy, but emission polarization can also switch without a spectral jump occurring, which implies long-range homogeneity in the local dielectric environment.