1,10a-Dihydro-1-aza-10a-boraphenanthrene and 6a,7-Dihydro-7-aza-6a-boratetraphene: Two New Fluorescent BN-PAHs

BN-embedded aromatic hydrocarbons: synthesis, functionalization and applications

Substituting CC double bonds with B-N bonds in polycyclic aromatic hydrocarbons (PAHs) has emerged as a promising approach to advance and diversify organic functional materials. This structural modification not only imparts unique electronic and optical properties, but also enhances chemical stability, thereby opening new avenues for material design and applications. However, the widespread adoption of BN-fused aromatic hydrocarbons in practical applications is still in its nascent phase. This constraint stems primarily from the challenges in precisely tailoring molecular structures to optimize photophysical and electronic properties, thereby influencing their efficacy in targeted applications. Consequently, a comprehensive evaluation of historical, current, and prospective developments in BN-fused aromatic hydrocarbons is deemed essential. This review offers an in-depth overview of recent advancements in BN-fused aromatic hydrocarbons, focusing on synthetic strategies, fundamental properties, and emerging applications. Additionally, we elucidate the pivotal role of computational chemistry in directing the design, discovery, and optimization of these materials. Our objective is to foster interdisciplinary collaboration and stimulate innovative approaches to fully harness the potential of azaborinine chemistry across various fields, including organic optoelectronics, biomedicine, and related disciplines.

Synthesis of phenanthrylboroles and formal nitrene insertion to access azaborapyrenes

Extended conjugation has been introduced into boroles but only on the 2,3- or 4,5-positions of the central BC4 core. In this work we synthesize phenanthrylboroles that also have conjugation on the 3,4-positions and demonstrate their insertion reactivity with azides to furnish B,N-analogues of pyrene.

From propenolysis to enyne metathesis: tools for expedited assembly of 4a,8a-azaboranaphthalene and extended polycycles with embedded BN

The synthesis of BN-containing molecules, which have an interesting isosteric relationship to their parent all-C cores, has drawn a great deal of attention as an avenue to alter and tune molecular function. Nevertheless, many cores with embedded BN are still hard to synthesize, and thus, further effort is required in this direction. Herein, we present an integrated approach to BN-containing polycycles rooted in an exceptionally clean B-N condensation of amines with a tri-allylborane. Having released propene as the only byproduct, the resulting BN precursors are seamlessly telescoped into BN-containing polycyclic cores via a set of additional methodologies, either developed here ad-hoc or applied for the first time for the synthesis of BN-cycles. As the "sharpening stone" of the process, BN-embedded naphthalene, which has previously only been obtained in low yield, can now be synthesized efficiently through propenolysis, ring-closing metathesis and a new high-yielding aromatization. As a more advanced application, an analogously obtained BN-containing bis-enyne is readily converted to BN-containing non-aromatic tetra-, penta- and hexacyclic structures via ring-closing enyne metathesis, followed by the Diels-Alder cycloaddition. The resulting air-sensitive structures are easily handled by preventive hydration (quaternization) of their B-N bridge; reverting this hydration restores the original Bsp2-Nsp2 structure. In the future, these structures may pave the way to BN-anthracenes and other π-extended BN-arenes.

Hyper-Cross-Linked Porous Polymer Featuring B-N Covalent Bonds (HCP-BNs): A Stable and Efficient Metal-Free Heterogeneous Photocatalyst

The first example of a porous polymer containing B-N covalent bonds, prepared from a tetraphene B-N monomer and biphenyl as a comonomer, is reported. It was prepared using the solvent knitting strategy, which allows the connection between the aromatic rings of the two monomers through methylene groups provided by an external cross-linking agent. The new polymer exhibited micromeso porosity with an S_BET of 612 m²/g, high thermal stability, and potential properties as a heterogeneous photocatalyst, since it is very active in the aza-Henry coupling reaction (>98% of conversion and selectivity). After the first run, the catalyst improves its photocatalytic activity, shortening the reaction time to only 2 h and maintaining this activity in successive runs. The presence of a radical in this structure that remains stable with successive runs makes it a new type of material with potential applications as a highly stable and efficient photocatalyst.

A Pyridyl-1,2-azaborine Ligand for Phosphorescent Neutral Iridium(III) Complexes

A novel 1,2-azaborine (i.e., 4-methyl-2-(pyridin-2-yl)-2,1-borazaronaphthalene, 1a) has been synthesized and used for the first time as a B-N alternative to common cyclometalating ligands to obtain neutral phosphorescent iridium(III) complexes (i.e., 2a, 3, and 4) of general formula [Ir(C^∧N)₂(N^∧NB)], where C^∧N indicates three different cyclometalating ligands (Hppy = 2-phenylpyridine; Hdfppy = 2-(2,4-difluoro-phenyl)pyridine; Hpqu = 2-methyl-3-phenylquinoxaline). Moreover, the azaborine-based complex 2a was compared to the isoelectronic C═C iridium(III) complex 2b, obtained using the corresponding 2-(naphthalen-2-yl)pyridine ligand 1b. Due to the dual cyclometalation mode of such C═C ligand, the isomeric complex 2c was also obtained. All new compounds have been fully characterized by NMR spectroscopy and high-resolution mass spectrometry (MS), and the X-ray structure of 2a was determined. The electronic properties of both ligands and complexes were investigated by electrochemical, density functional theory (DFT), and photophysical methods showing that, compared to the naphthalene analogues, the azaborine ligand induces a larger band gap in the corresponding complexes, resulting in increased redox gap (basically because of the highest occupied molecular orbital (HOMO) stabilization) and blue-shifted emission bands (e.g., λ_max = 523 vs 577 nm for 2a vs 2b, in acetonitrile solution at 298 K). On the other hand, the ³LC nature of the emitting state is the same in all complexes and remains centered on the pyridyl-borazaronaphthalene or its C═C pyridyl-naphthalene analogue. As a consequence, the quantum yields of such azaborine-based complexes are comparable to those of the more classical C═C counterparts (e.g., photoluminescence quantum yield (PLQY) = 16 vs 22% for 2a vs 2b, in acetonitrile solution at 298 K) but with enhanced excited-state energy. This proves that such type of azaborine ligands can be effectively used for the development of novel classes of photoactive transition-metal complexes for light-emitting devices or photocatalytic applications.

Synthetic Doping of Acenaphthylene through BN/CC Isosterism and a Direct Comparison with BN-Acenaphthene

Boron/nitrogen-doped acenaphthylenes, a new class of BN-doped cyclopenta-fused polycyclic aromatic hydrocarbons, were synthesized via indole-directed C-H borylation. The reference molecule BN-acenaphthene was also synthesized in a similar manner. Both BN-acenaphthylene and BN-acenaphthene were unequivocally characterized by single-crystal X-ray analysis. The aromaticities of each ring in BN-acenaphthylenes were quantified by experimental and theoretical methods. Moreover, doping the BN unit into acenaphthylene can increase the LUMO level and decrease the HOMO level, resulting in wider HOMO-LUMO energy gaps. Furthermore, regioselective bromination of BN-acenaphthylene (B-Mes) afforded monobrominated BN-acenaphthylene in good yield. Subsequently, cross-coupling of brominated BN-acenaphthylene gave a series of BN-acenaphthylene derivatives. In addition, the photophysical properties of these BN-acenaphthylene derivatives can be fine-tuned by the substituents on the BN-acenaphthylene scaffold.

Synthesis of BN-Polyarenes by a Mild Borylative Cyclization Cascade

Reaction of BCl₃ with suitably substituted o-alkynylanilines promotes a cascade reaction in which BN-polycyclic compounds are obtained via the formation of two new cycles and three new bonds in a single operational step. The reaction is highly efficient and takes place at room temperature, providing a very mild and straightforward strategy for the preparation of BN-aromatic compounds, which can be further transformed into a variety of BN-PAHs with different polycyclic cores and substituents.

Concise Synthesis of BN-Dibenzo[f,k]tetraphenes with Different BN Substitution Positions and Direct Comparison with Their Carbonaceous Analogue

Two types of "parental" BN-dibenzo[f,k]tetraphenes (BNDBT-1 and BNDBT-2) have been synthesized via a transition-metal-catalyzed tandem cross-coupling reaction as key steps. Both BNDBT-1 and BNDBT-2 are fully characterized; one of them is unambiguously confirmed by a single X-ray crystal structure. Compared to its all-carbon analogue DBT, BNDBT-1 and BNDBT-2 exhibit a higher highest occupied molecular orbital (HOMO) and lower lowest unoccupied molecular orbital (LUMO) energy, while the BN doping position slightly influences the HOMO and LUMO energies of BNDBT-1 and BNDBT-2. Both BNDBT-1 and BNDBT-2 exhibit red-shifted absorption and emission spectra and higher emission efficiencies, as compared to their carbonaceous analogue DBT. Moreover, organic light emitting diodes were fabricated using BNDBT-1 and BNDBT-2 as emitters, demonstrating their potential applications.