Electrophilic C–H Borylation of Aza[5]helicenes Leading to Bowl-Shaped Quasi-[7]Circulenes with Switchable Dynamics

Abstract

The intrinsic relationship between helicenes and circulenes is of fundamental interest and importance in molecular engineering. Herein, electrophilic borylation of phenanthroline-derived aza [5]helicenes is presented, resulting in the incorporation of a boryl unit into two termini of helicenes to afford quasi-[7]circulenes. Their bowl-shaped structures were determined by X-ray diffraction. UV−vis absorption and fluorescence spectroscopy, as well as electrochemical measurements and DFT calculations, gave insight into their electronic properties. Variable-temperature NMR studies and DFT calculations revealed bowl-to-bowl inversion at room temperature and bowl-to-helix equilibria at elevated temperature, highlighting the important role of B ← N bond strength in tuning their dynamic properties.

Efficient Narrowband Circularly Polarized Light Emitters Based on 1,4‐B,N‐embedded Rigid Donor–Acceptor Helicenes

“…This indicates the short‐range CT characteristic of the excited states, i.e., little change in dipole moment. Notably, in 2‐methylpentane, the FWHM of the fluorescence band for all three compounds is only ≈17 nm (≈0.07–0.08 eV), which is among the smallest reported values for helicene‐containing organic emitters [13b, 16a, 18b, 23] …”

Unraveling the Aromatic Rule of Cyclic Superatomic Molecules in π-Conjugated Compounds

“…The conventional Hückel’s 4 n + 2 electron rule is applicable to monocyclic conjugated systems but not applicable to common [ n ]­circulenes with a total of 4 n π electrons. Considering that various hetero­[ n ]­circulenes ( n = 6–9) have been synthesized and proved to be stable compounds that are probably aromatic, ,− a unified electron rule applicable to these colorful and highly symmetric π-conjugated structures is much needed.…”

Synthesis of a Blue‐Emissive Azaborathia[9]helicene by Silicon‐Boron Exchange from Unusual Atropisomeric Teraryls

“…Lewis acidic boron with an empty p z -orbital can be introduced into the aromatic scaffold at strategic positions [12] and stabilized by an adjacent nitrogen atom. [13] Replacing CÀ C bonds with a BÀ N pair tremendously alters the electronic behavior of the system, hence its optical properties, while also affecting the molecular geometry. [14] This was exemplified by a hexahelicene bearing one BÀ N-motif (A, Figure 1) that displayed improved absorptivity, fluorescence efficiency and racemization half-life compared to its parent carbo [6]helicene.…”

“…This versatile range of applications is owed to the tunability of helicenes’ properties, which can be adjusted by a series of architectural acts, such as heteroatom‐doping within the helical framework, [11] e.g. Lewis acidic boron with an empty p z ‐orbital can be introduced into the aromatic scaffold at strategic positions [12] and stabilized by an adjacent nitrogen atom [13] . Replacing C−C bonds with a B−N pair tremendously alters the electronic behavior of the system, hence its optical properties, while also affecting the molecular geometry [14] .…”