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Amplification of dissymmetry factors in $\pi$-Extended [7]- and [9]helicenes

Abstract : $\pi$-Extended helicenes constitute an important class of polycyclic aromatic hydrocarbons with intrinsic chirality. Herein, we report the syntheses of $\pi$extended [7]helicene $4$ and $\pi$-extended [9]helicene $6$ through regioselective cyclodehydrogenation in high yields, where a "prefusion" strategy plays a key role in preventing undesirable aryl rearrangements. The unique helical structures are unambiguously confirmed by X-ray crystal structure analysis. Compared to the parent pristine [7]helicene and [9]helicene, these novel $\pi$-extended helicenes display significantly improved photophysical properties, with a quantum yield of 0.41 for $6$. After optical resolution by chiral high-performance liquid chromatography, the chiroptical properties of enantiomers $4$-P/M and $6$-P/M are investigated, revealing that the small variation in helical length from [7] to [9] can cause an approximately 10-fold increase in the dissymmetry factors. The circularly polarized luminescence brightness of 6 reaches 12.6 M$^{−1}$ cm$^{−1}$ as one of the highest among carbohelicenes.
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https://hal.inrae.fr/hal-03321354
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Submitted on : Tuesday, August 17, 2021 - 2:58:17 PM
Last modification on : Tuesday, August 24, 2021 - 2:55:07 PM

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Zijie Qiu, Cheng-Wei Ju, Lucas Frédéric, Yunbin Hu, Dieter Schollmeyer, et al.. Amplification of dissymmetry factors in $\pi$-Extended [7]- and [9]helicenes. Journal of the American Chemical Society, American Chemical Society, 2021, 143 (12), pp.4661 - 4667. ⟨10.1021/jacs.0c13197⟩. ⟨hal-03321354⟩

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