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Fluctuating exciton localization in giant π-conjugated spoked-wheel macrocycles

Abstract

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.

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Figure 1: Structure and synthesis of the spokes and ring 1, and of the non-cyclic analogues 911.
Figure 2: Fluctuations in exciton localization caused by spontaneous symmetry breaking.
Figure 3: Photoinduced fluctuations in exciton localization apparent in the temporal dynamics of single-ring luminescence.
Figure 4: Low-temperature PL spectroscopy of single rings showing switching in transition energy and polarization.

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Acknowledgements

The authors are indebted to the Volkswagen Foundation for providing collaborative funding. A.V.A. and A.T. acknowledge financial support by the Fonds der Chemischen Industrie. J.M.L. is a David & Lucile Packard Foundation fellow and is grateful for a European Research Council Starting Grant (MolMesON, #305020).

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A.V.A., A.I., D.K. and S.H. designed and synthesized the compounds. A.T., D.W., T.S., F.S., J.V. and J. M. L. conceived, designed and performed the spectroscopy experiments and analysed the data. S-S.J. and S.H. performed and interpreted the STM experiments. A.T., J.V., S.H. and J.M.L. wrote the manuscript.

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Correspondence to Sigurd Höger or John M. Lupton.

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Aggarwal, A., Thiessen, A., Idelson, A. et al. Fluctuating exciton localization in giant π-conjugated spoked-wheel macrocycles. Nature Chem 5, 964–970 (2013). https://doi.org/10.1038/nchem.1758

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