Synthesis, Structures, and Chiroptical Properties of NBN-Doped Helicenes with Boron Atoms in the Inner Rims

The precise synthesis of helicenes
with topologically defined length
and specific heteroatomic perturbation in the screw-like conjugated
skeletons plays an emerging role in the manipulation of chiral materials.
Facile, selective, and programmable routes to helicenes or heterohelicenes
are highly desirable yet challenging for structure–chiroptical
property relationship studies. Herein, we report the synthesis and
characterization of NBN-doped helicenes with boron atoms in the inner
rims, enabled by the highly regioselective one-pot borylation of rationally
designed precursors with, namely, fold-in or pan-out manner. The incorporation
of nonbonded boron and nitrogen atoms resulted in narrow-band emission
and improved optical properties for the single-stranded carbon helix.
In addition, numbers and arrangement modes of fused six-membered rings
have distinct effects on configurational stability and chiroptical
properties, revealing that BN-[6]H with strong circular
dichroism is a promising candidate for chiral sensors. The combination
of experimental and theoretical studies on these helical structures
might provide insights into the design of helically chiral small-molecule-based
sensors or emitters.