Amplification of dissymmetry factors in $\pi$-Extended [7]- and [9]helicenes
Résumé
$\pi$-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.
Domaines
Chimie
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2021_Qiu_jacs.pdf (3.15 Mo)
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ja0c13197_si_001.pdf (3.16 Mo)
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