Boron-containing Z-type and bilayer benzoxene

(Press-News.org) The research group of Professor Chuandong Dou at the State Key Laboratory of Supramolecular Structure and Materials, Jilin University, recently constructed two novel boron-hexane two-dimensional benzobenzenes using a borane-controlled cyclization strategy, elucidating the importance of boron atom doping. Using conjugated boranes as precursors, the researchers synthesized boron-hexane Z-type and bilayer benzobenzenes C32B2 via FeCl3 and Bi(OTf)3-mediated intramolecular cyclization reactions, obtaining narrow-spectrum fluorescence (half-width at half-maximum as narrow as 19 nm) and amplified spontaneous emission properties, demonstrating their potential application as gain mediators. Furthermore, by studying the electronic structures of their divalent anions and all-carbon isoelectronic forms, they revealed that boron atoms can perturb the aromaticity of the conjugated framework, thereby reconstructing the spin density distribution. This research expands the topological diversity of boron-containing carbon molecules and provides a new system for studying luminescent functions and spin properties. The article was published as an open access Research Article in CCS Chemistry, the flagship journal of the Chinese Chemical Society.

Background information:

Polycyclic aromatic hydrocarbons (PAHs), a type of molecular carbon with a graphene-like fragment structure, are of significant research value in synthetic chemistry, materials science, and related interdisciplinary fields. Serrated edges can induce spin polarization states in PAHs, giving them unique open-shell electronic states and magnetic properties. Therefore, these serrated-edge PAHs (such as two-dimensional benzo[a]benzenes, Z-type benzo[a]benzenes, and multilayer benzo[a]benzenes) have attracted widespread attention. However, open-shell radical structures typically lead to high oxidative activity and low stability, posing significant challenges to the synthesis and purification of these compounds and limiting their functional exploration and materials research. To address these issues, researchers have incorporated heteroatoms (such as boron atoms) into the conjugated molecular framework to improve molecular stability, achieve diversity in elemental composition and topological morphology, and endow them with tunable electronic structures and photoelectric properties (Figure 1).

Highlights of this article:

With the support of the National Natural Science Foundation of China and Jilin University, Chuandong Dou's research group has focused on the integration of boron element chemistry and molecular carbon materials. In this work, the authors designed and synthesized a double boron-doped unit 4, which can be regarded as a lateral double fusion of boron-heteromeric unit 3, laying a key structural foundation for the construction of boron-heteromeric benzene systems (Figure 2). Based on the borane controlled cyclization strategy, two examples of boron-heteromeric two-dimensional benzobenzenes (1 and 2) were prepared by implementing FeCl3 and Bi(OTf)3-mediated intramolecular cyclization reactions. The single-crystal structures show that they have a C32B2 isomeric boron-carbon conjugated framework, with two boron atoms located on parallel serrated edges. This can be regarded as doping two boron atoms into superzethrene (SZ) and peri-pentacenopentacene (PP), respectively, forming boron-heteromeric Z-type and bilayer benzobenzenes.

The authors systematically investigated the photophysical properties and excited-state dynamics of films 1 and 2. Their toluene solutions emitted yellow-green and deep orange fluorescence, respectively. The emission peak of film 1 was located at 528 nm, while that of film 2 red-shifted to 578 nm (Figure 3), with the optical bandgap decreasing from 2.33 eV to 2.12 eV, indicating that the lateral merging mechanism favored enhancing the conjugation degree of the boron-carbon structure. In particular, both films 1 and 2 exhibited narrow-band fluorescence properties, with a full width at half maximum (FWHM) of approximately 20 nm. The excited-state dynamics were studied using femtosecond transient absorption spectroscopy . Films 1 and 2 showed negative signal peaks at 567 nm and 622 nm, respectively. These negative signal peaks did not appear in the steady-state absorption spectra but instead existed as shoulder peaks in the fluorescence spectra, indicating that films 1 and 2 possess unique stimulated emission (SE) behavior. Based on this, the authors conducted laser-pumped tests on polystyrene (PS) doped films of films 1 and2 . For the 1/PS film, when the pump intensity exceeds the threshold of 110 kW cm⁻², the emission band at 565 nm rapidly intensifies and narrows, with a full width at half maximum (FWHM) of 6.0 nm. This emission peak coincides with the stimulated emission peak, and its emission intensity and FWHM exhibit a significant nonlinear relationship with the pump intensity. Similarly, for the 2/PS film, the emission peak at 622 nm intensifies and narrows after the pump intensity exceeds 390 kW cm⁻². These results indicate that both 1 and 2 possess amplified spontaneous emission (ASE) properties. This is the first time that narrowband fluorescence and ASE properties have been observed simultaneously in a single organic molecule, demonstrating the application potential of this type of borobenzene as an organic narrowband luminescent and laser material.

Both 1 and 2 exhibit two reversible reduction processes, indicating the formation of stable reduced species. Therefore, the authors performed electrochemical reduction tests on them and used UV-Vis-NIR absorption spectroscopy to monitor the formation of divalent anionic species in situ. As shown in Figure 4, 1 and 2 exhibit two clear electron-gaining reduction processes, with a significant change in solution color. 1′ and 2′ show a weak low-energy absorption band at 800–1000 nm , a feature typically associated with radical structures, suggesting a possible open-shell structure. To further investigate the regulatory effect of boron atom introduction on electronic structure, as well as the aromaticity and open/closed-shell properties of divalent anions, the authors performed DFT calculations on divalent anions and their corresponding pure carbon isoelectronic counterparts. The results show that in divalent anions, 1) the introduction of boron atoms enhances the aromaticity of the boron-containing ring and perturbs the aromaticity of neighboring rings; 2) divalent anions and their all-carbon isoelectronic counterparts maintain similar open/closed-shell characteristics, but for the open-shell structure 1′2‒,the introduction of boron atoms causes a redistribution of spin density, with radicals mainly distributed at C1 and C2 positions, and the γ resonance structure contributing more to its ground state. Furthermore, 2′2‒ maintains a closed-shell structure, with the α resonance structure dominating in the ground state. These results indicate the important regulatory role of boron atoms in the electronic structure, aromaticity, and spin properties of the benzobenzene skeleton.

Summary and Outlook:

In summary, this work employed a borane-controlled cyclization strategy to construct boro-hexane Z-type benzobenzenes and bilayer benzobenzenes, revealing the regulatory role of boron atoms and their fusion mechanism on electronic structure and photoelectric properties. The resulting molecules exhibit both narrow-band luminescence and amplified spontaneous emission, demonstrating their potential as organic light-emitting materials and optical gain media. Furthermore, comparative studies of their divalent anions and all-carbon isoelectronic forms elucidated the ability of boron atoms to perturb the molecular aromaticity and spin density distribution. This work expands the structural types of boro-hexane systems, providing a new system and important foundation for studying luminescent functions and spin properties. The relevant results were published as a Research Article in CCS Chemistry. Zeyi Li and Xinyu Tian are co-first authors, and Professor Chuandong Dou is the corresponding author.

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About the journal: CCS Chemistry is the Chinese Chemical Society’s flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem.

About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman’s Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/.

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