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

Abstract

We report the concise synthesis and chir-(optical) properties of an azaborathia[9]helicene consisting of two thienoazaborole motifs. The key intermediate, a highly congested teraryl with nearly parallel isoquinoline moieties, was generated as a mixture of atropisomers upon fusion of the central thiophene ring of the dithienothiophene moiety. These diastereomers were characterized by single crystal X-ray analysis revealing intriguing interactions in the solid state. Subsequent insertion of boron into the aromatic scaffold via silicon-boron exchange involving triisopropylsilyl groups fixed the helical geometry, thereby establishing a novel method for the preparation of azaboroles. The ligand exchange at boron in the final step afforded the blue emitter displaying a fluorescence quantum yield of 0.17 in CH 2 Cl 2 and excellent configurational stability. Detailed structural and theoretical investigation of unusual atropisomers and the helicene provide insights into their isomerization processes.

Boron-containing helicenes as new generation of chiral materials: opportunities and challenges of leaving the flatland

“…0.9 × 10 −3 , CH 2 Cl 2 ). 63 In contrast, Φ FL of blue-emissive BO-helicene 56 is 0.06 in cycloxexane ( λ em = 392 nm) and 0.04 in THF ( λ em = 411 nm), implying that the B–O motif does not have such an impact on the electronic spectra of thiohelicenes. 76 On the other hand, the effect of B–O motifs is evident in OBO-derivatives.…”

“…Recently, we reported a third approach for the synthesis of azaborole helicenes via silicon–boron exchange. 63 This method was developed for the preparation of unprecedented azaborathia[9]helicene 30. The synthesis proceeded via highly congested atropisomeric teraryl 29, which was assembled from isoquinoline and thiophene building blocks 26 and 27 ( Scheme 5 ).…”

“…Consequently, the enantiomerization of this compound proceeds via a single TS, whereas a multi-step process is already expected for carbo[8]helicene (see above). 63 …”

Azaborahelicene fluorophores derived from four-coordinate N,C-boron chelates: synthesis, photophysical and chiroptical properties

“…While carbo-[ n ]-helicenes and their derivatives have been an archetypical class of organic chiral systems for developing efficient CPL emitters, the introduction of a B atom with an empty p z -orbital as a functional group on chiral π-conjugated systems, or within the helical molecular systems has been shown to be an ingenious way of tuning the resulting chiral optoelectornic properties. 41–53 In another approach, the B atom has also been used as an anchoring unit for a chiral ligand, such as 1,1′-bi(2-naphthol) (BINOL) and its derivatives, on boron dipyrromethene (BODIPY) dyes, affording a simple and efficient way of obtaining new CPL emitters. 54–57…”

Triple B←N Lewis Pair-Functionalized Triazatruxenes with Large Stokes Shifts

“…Introducing heteroatoms, such as B, Si, N, S, and P atoms, into polycyclic aromatic hydrocarbons (PAHs) has emerged as an efficient strategy for modulating their chemical structures and optoelectronic properties. − B←N Lewis pairs are easily constructed via electrophilic borylation, − especially for electron-rich PAHs containing nitrogen atoms as anchors. , The energy of the B←N bond is 30 kcal/mol, which is strong enough to enhance the rigidity and planarity of PAHs. Replacing C–C bonds with isoelectronic B←N coordinate bonds is a versatile and simple method for tuning molecular structures, , solid-state packings, frontier orbitals, energy levels, and optoelectronic properties. − Furthermore, strongly electron-deficient borylated N-heterocyclic PAHs can also possess lower LUMO levels and decreased HOMO–LUMO energy gaps, leading to red-shifts in absorption and emission. − Despite the remarkable achievements in the synthesis of B←N Lewis pair-functionalized PAHs (Figure a), ,,,− PAHs fused with multiple B←N Lewis pairs are still rare due to the lack of multidentate aromatic ligands capable of chelating several boron atoms in one system.…”