Kekulé-Institut für Organische Chemie und Biochemie der Universität Bonn, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany.
Nat Chem. 2013 Nov;5(11):964-70. doi: 10.1038/nchem.1758. Epub 2013 Sep 29.
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 π 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.
共轭聚合物在许多不同的应用中具有潜力，但我们仍然缺乏对其电子结构的基本微观理解。基本的光激发（激子）仅跨越分子的几个纳米，而分子本身可以延伸到几微米，它们的行为如何受到分子尺寸的影响并不明显。例如，激子在共轭段内形成的位置在哪里，它是否总是位于相同的重复单元上？在这里，我们引入了结构刚性的分子辐条轮，直径为 6 纳米，作为扩展 π 共轭的模型。单分子荧光揭示了随机激子定位，这导致了随时间变化的发射偏振。最初，这种随机定位是在每次光子吸收事件之后出现的，因为它是与温度无关的自发对称破缺。这种快速波动在长时间的光照后会减慢到毫秒时间尺度。低温光谱学揭示了分子内异质性的跃迁能量跳跃，但发射偏振也可以在不发生光谱跳跃的情况下发生切换，这意味着局部介电环境的长程均匀性。
