Abstract
π-Extended helicenes constitute an important class of polycyclic aromatic hydrocarbons with intrinsic chirality. Herein, we report the syntheses of π-extended [7]helicene 4 and π-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 π-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.
Highlights
Carbohelicenes constitute a unique class of polycyclic aromatic hydrocarbons with benzene rings that are angularly annulated in the ortho-configuration
Theoretical studies have shown that the dissymmetry factor (g) of single-stranded [n]carbohelicenes increases with the helical length n
Tremendous efforts have been made to synthesize higher [n]helicenes since the first report of [6]helicene by Newman and Lednicer in 1956.4−7 To date, the longest carbohelicene reported is [16]helicene, which was synthesized by Fujita and co-workers in 2015.8 The low yield of the final photocyclization step (7%), hinders a further increase of the helical length by this approach
Summary
Carbohelicenes constitute a unique class of polycyclic aromatic hydrocarbons with benzene rings that are angularly annulated in the ortho-configuration. An ideal CPL emitter should possess both a high emission quantum yield (Φ) and a large luminescence dissymmetry factor (glum), but these properties are often difficult to achieve simultaneously. Illustration of the Prefusion Strategy To Prevent Aryl Rearrangement and Achieve the Desired π-Extended Helicenes 4 and 6 mechanism indicated that the rearrangement occurred in the first step of dehydrogenation and was favored over direct C−C bond formation for 4. To prevent this undesired yet highly efficient aryl rearrangement, we adopted a new strategy that employs precursors 3 and 5 by prefusing the tetraphenylbenzene moiety (Scheme 2B). The combination of the elongated helical length and extended π-conjugation empower 6 as a promising CPL emitter with a Φf of 0.41 and a glum of 7.4 × 10−3, distinguishing it from π-extended carbohelicenes in the literature
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