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

Abstract

Increased interest in chiral functional dyes has stimulated activity in the field of boron-containing helicenes over the past few years. Despite the fact that the introduction of boron endows π-conjugated scaffolds with attractive electronic and optical properties, boron helicenes have long remained underdeveloped compared to other helicenes containing main group elements. The main reason was the lack of reliable synthetic protocols to access these scaffolds. The construction of boron helicenes proceeds against steric strain, and thus the methods developed for planar systems have sometimes proven ineffective in their synthesis. Recent advances in the general boron chemistry and the synthesis of strained derivatives have opened the way to a wide variety of boron-containing helicenes. Although the number of helically chiral derivatives is still limited, these compounds are currently at the forefront of emissive materials for circularly-polarized organic light-emitting diodes (CP-OLEDs). Yet the design of good emitters is not a trivial task. In this perspective, we discuss a number of requirements that must be met to provide an excellent emissive material. These include chemical and configurational stability, emission quantum yields, luminescence dissymmetry factors, and color purity. Understanding of these parameters and some structure-property relationships should aid in the rational design of superior boron helicenes. We also present the main achievements in their synthesis and point out niches in this area, e.g. stereoselective synthesis, necessary to accelerate the development of this fascinating class of compounds and to realize their potential in OLED devices and in other fields.

Fig. 1. Types of boracycles embedded in boron-containing helicenes.

Scheme 1. The synthetic route to azaborole helicenes through the extension of carbohelicenes. Azaborole rings are formed by electrophilic borylation with BBr₃. Helicene 8 was synthesized from a dipyridyl-substituted naphthalene.

Scheme 2. Modular approach to the synthesis of azaborole helicenes. Azaborole rings are formed by electrophilic borylation with BBr₃.

Scheme 3. Stereospecific synthesis of azaborole helicenes by axial-to-helical chirality transfer for the selected enantiomer of biaryl 20.

Scheme 4. Synthetic route to azaborole helicenes with a ring attached at a sterically hindered position via Ir-catalyzed borylation.

Scheme 5. Synthesis of azaborathiahelicenes via silicon–boron exchange.

Scheme 6. Synthesis of 1,4-oxaborinine-containing helicenes.

Scheme 7. Synthesis of 1,4-azaborinine-containing helicenes. The bonds formed via borylative cyclization are colored in red.

Scheme 8. Synthetic strategy to O–B–O helicenes via demethylative cyclization. Method A: via lithium–halogen exchange; method B: via directed C–H borylation.

Scheme 9. Synthesis of four-coordinate O–B–O helicenes.

Scheme 10. Synthesis of a B–O based helicene by postfunctionalization of a boracycle.

Scheme 11. Synthesis of 1,2-azaborinine helicene 59.

Scheme 12. Synthesis of 1,2-azaborinine helicenes by electrophilic borylation.

Scheme 13. Synthesis of 1,2-azaborinine helicenes with two 1,2-azaborinine rings.

Scheme 14. Synthesis of helicenes with a N–B–N motif at the zigzag edge.

Scheme 15. Synthesis of helicenes with a N–B–N motif on the inner helicene rim.

Scheme 16. Synthesis of B-doped [4]helicenes and double [4]helicenes by Yamamoto coupling and lithium–halogen exchange/transmetalation/C–H borylation.

Scheme 17. Synthesis of boron embedded [4]helicenes by borylative cyclization of alkynes.

Fig. 2. The structure of configurationally stable helicene 103 consisting of only carbon and boron atoms.

Scheme 18. Synthesis of BODIPY–helicenes via borylation with BF₃·OEt₂ or BPh₃ in the presence of an amine in the final step.

Scheme 19. Synthesis of BINOL-BODIPYs.

Scheme 20. Synthesis of BODIPY–helicenes by nucleophilic substitution with hydroxy-substituted aryls or demethylative borylation in the key step.

Scheme 21. Synthesis of regioisomeric BODIPY–helicenes by Suzuki–Miyaura cross-coupling of a dibromo-BODIPY and 2-hydroxyphenylboronic acid.

Scheme 22. Enantiomerization of a helicene with a five-membered ring. Y and Z denote carbon or a heteroatom.

Fig. 3. Wedge angles φ of cyclopentadiene and heteroles calculated at the B3LYP/def2-TZVP level of theory.

Fig. 4. Configurational stability in kcal mol⁻¹ of various boron helicenes. The experimental ΔG^‡ value is given where available.

Fig. 5. Structures of other compounds discussed in this article.

Fig. 6. DFT-optimized structures of 18 and 19a and the spatial arrangement of the electric (μ, red) and magnetic (m, blue) dipole moments in the S1 excited state at the TD-M06-2X/def2-SVP level.

Fig. 7. (a) Frontier molecular orbitals in MR-TADF frameworks. (b) Predicted reverse intersystem crossing pathways present in MR-TADF scaffolds. (c) Correlation between the vibronic coupling and emission spectrum.

Agnieszka Nowak-Król

Patrick T. Geppert

Kenkera Rayappa Naveen