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.
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