A C ₅₄ B ₂ Polycyclic π-System with Bilayer Assembly and Multi-Redox Activity

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES24 May 2022A C54B2 Polycyclic π-System with Bilayer Assembly and Multi-Redox Activity Liuzhong Yuan, Jiaxiang Guo, Yue Yang, Kaiqi Ye, Chuandong Dou and Yue Wang Liuzhong Yuan State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 , Jiaxiang Guo State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 , Yue Yang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 , Kaiqi Ye State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 , Chuandong Dou *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 and Yue Wang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 https://doi.org/10.31635/ccschem.022.202101738 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Doping heteroatoms into polycyclic aromatic hydrocarbons (PAHs) is an efficient strategy to achieve fascinating electronic structures and materials. However, nanoscale B-doped PAHs remain very challenging because of the intrinsic instability of the boron atom and the lack of suitable precursors. In this study, we report a C54B2 polycyclic π-system with one embedded 1,4-diboron-substituted benzene subunit, which was successfully synthesized from doubly B-doped heptazethrene. This molecule represents not only the largest B-doped PAH by far but also an unprecedented B-doped nanographene. The fully zigzag-armchair-edged structure creates a planar conformation, thus leading to its unique bilayer assembly behavior. More importantly, it possesses intriguing electronic structure and optoelectronic properties, such as very broad light absorption that covers 350–750 nm, sharp near-infrared fluorescence with a band width of only 26 nm, and reversible five-step redox capability, all of which are rarely observed for other B-doped PAHs. In addition, this molecule displays distinctive local aromaticity that cannot be reproduced via the reductive manipulation of an all-carbon congener. Download figure Download PowerPoint Introduction Nanoscale polycyclic aromatic hydrocarbons (PAHs), namely nanographenes (NGs), are not only good models to understand the fundamental properties of graphene but also attractive materials for widespread applications in the electronic, bioimaging, and energy fields.1,2 Incorporation of heteroatoms into PAHs is a powerful strategy to alter their intrinsic structures and properties, such as reactivity, energy gaps, charge transport, and aromaticity.3–6 However, it is very difficult to precisely control the topology structures (e.g., size, shape, and edge state) and doping modes (e.g., position and number) of heteroatom-doped NGs, which dominate their chemical and physical properties to a large extent. In this regard, bottom-up strategies, including solution-phase and on-surface synthetic methodologies, provide the possibility for precise synthesis at the atomic level.7,8 Indeed, based on these efficient synthetic approaches, a variety of planar and contorted NGs and even graphene nanoribbons doped with N, O, or S atoms, as well as the B/N or B/O atoms have been successfully constructed, which possess wonderful well-defined structures and exhibit intriguing optical, electronic, and magnetic properties.9–18 However, pristine B-doped NGs are still very scarce due to their extremely challenging synthesis due to the intrinsic instability of the tricoordinate boranes toward oxygen and moisture and the lack of suitable precursors. The boron atom possesses a characteristic vacant p-orbital, and thus the substitution of a carbon atom with a boron atom may bestow PAHs with electron deficiency and Lewis acidity. In the past decade, a variety of B-doped PAHs with fascinating structures have been dramatically developed and applied as organic catalysts and optoelectronic materials.19–21 Doping two boron atoms into PAHs has received increasing attention because this strategy is very efficient not only to design large-size π-frameworks and multiple edge structures but also to modulate the positions of the boron atoms.22–29 Until now, the representative subunits for such kind of B-doped PAHs are 1,4-diborabenzene ( I) and 1,5-diboron-substituted naphthalene and anthracene ( II and III, Figure 1a). For instance, diboron-doped dibenzoteranthenes with a 1,4-diborabenzene substructure have been developed as the first example of a B-doped NG.22,23 Diboron-doped pyrene and perylene, as well as their π-extended derivatives, have also been achieved via the development of one-pot borylation methods.25–28 Hence, it remains very important and attractive to explore B-doped PAHs and NGs from both a chemical and material point of view. Figure 1 | (a) Representative diboron-doped subunits for PAHs. (b) Molecular design of B-doped heptazethrene 2 and polycyclic π-system 3 (this work). Download figure Download PowerPoint We now report the incorporation of 1,4-diboron-substituted benzene as a subunit into a conjugated π-framework to develop B-doped PAHs (Figure 1b). Heptazethrene 1 with two phenalenyl groups is a well-known molecule with an open-shell resonance form.30,31 Using it as a substructure, a large number of organic diradicaloids with interesting open-shell structures and magnetic properties have been developed.32 Here, we first synthesized doubly B-doped heptazethrene 2, which is a new building block for conjugated organoboranes. Using this key precursor, we achieved the solution-phase synthesis of a C54B2 polycyclic π-system 3 with a 1,4-diboron-substituted benzene subunit. It represents not only the largest B-doped PAH by far but also an unprecedented B-doped NG. This NG sheet possesses a planar conformation due to its fully zigzag-armchair-edged structure, leading to the unique bilayer assembly behavior. Moreover, it displays intriguing electronic structure and optoelectronic properties, such as very broad light absorption but sharp near-infrared (NIR) fluorescence, as well as sufficient Lewis acidity and multi-redox activity. Detailed aromaticity comparisons with its all-carbon congener 4 further demonstrate the remarkable electronic effects of B-doping on PAHs. Experimental Methods Syntheses and characterizations Syntheses of 2a and 2b Compound 5 (95.0 mg, 0.21 mmol) was placed in a flame-dried Schlenk tube and neat BBr3 (0.79 mL, 8.4 mmol) was added under argon. Then the mixture was heated to reflux for 12 h. Excess BBr3 was removed from the red solution under reduced pressure to obtain a reddish brown solid. Dry toluene (10 mL) was added and the resulting suspension was stirred for 1 h under reduced pressure to slowly remove residual BBr3, affording the key intermediate 2a. It is very sensitive to air and moisture, and therefore, was used for the following reaction without further purification. Dry toluene (5 mL) was added to the Schlenk tube containing 2a at 0 °C. Then a solution of mesitylmagnesium bromide (MesMgBr) in tetrahydrofuran (THF) was added dropwise at 0 °C via syringe. The reaction mixture was stirred at 25 °C for 2 h. After removing the solvents in vacuo, the crude product was purified by silica gel column chromatography with CH2Cl2 as eluent to give compound 2b (43.0 mg, 35%) as an orange-yellow solid. 1H NMR (400 MHz, CDCl3, 25 °C, δ): 9.08 (s, 2H), 8.57 (d, J = 8.0 Hz, 2H), 8.28 (d, J = 8.0 Hz, 2H), 8.20 (d, J = 8.0 Hz, 2H), 8.04 (d, J = 8.0 Hz, 2H), 7.74–7.66 (m, 4H), 7.06 (s, 4H), 2.51 (s, 6H), 2.09 (s, 12H). 13C NMR (100 MHz, CDCl3, 25 °C, δ): 142.22, 139.45, 139.17, 136.98, 136.70, 133.95, 133.07, 132.34, 131.96, 130.35, 127.20, 126.35, 126.30, 125.84, 23.66, 21.60. HR-MALDI-TOF MS (m/z): [M]+ calcd for C44H36B2, 586.3003; found, 586.3491. Syntheses of 2c To a solution of 10-bromo-1,8-bis(mesityloxy)anthracene (392.2 mg, 0.75 mmol) in ether (25 mL) at 0 °C was added n-BuLi in n-hexane (0.48 mL, 1.60 M, 0.78 mmol) dropwise. The mixture was stirred at 25 °C for 1 h and the solvent was removed in vacuo to obtain the dry intermediate 6. Dry toluene (6 mL) was added to the Schlenk tube containing 2a at 0 °C, which was prepared starting with compound 5 (132.0 mg, 0.30 mmol) and neat BBr3 (1.13 mL, 11.94 mmol). Then a solution of 6 in toluene (6 mL) was added dropwise at 0 °C via syringe. The reaction mixture was stirred at 25 °C for 16 h. After removing the solvents in vacuo, the orange-red solid was washed with water (80 mL), methanol (50 mL), and hexane (100 mL) to afford the precursor 2c (117.2 mg, 31% in three steps) as an orange solid. 1H NMR (400 MHz, CDCl3, 25 °C, δ): 10.04 (s, 2H), 8.99 (s, 2H), 8.23 (d, J = 8.0 Hz, 2H), 8.19 (d, J = 8.0 Hz, 2H), 8.07 (d, J = 8.0 Hz, 2H), 7.99 (d, J = 8.0 Hz, 2H), 7.55 (t, J = 8.0 Hz, 4H), 7.18 (d, J = 8.0 Hz, 4H), 7.00 (s, 8H), 6.94(t, J = 8.0 Hz, 4H), 6.32(d, J = 8.0 Hz, 4H), 2.29–2.36(m, 36H). The 13C NMR spectrum was not obtained due to its insufficient solubility. HR-MALDI-TOF MS (m/z): [M]+ calcd for C90H72B2O4, 1238.5641; found, 1238.5642. Syntheses of 3a FeCl3 (157.0 mg, 0.96 mmol) and CH3NO2 (2.0 mL) were placed in a two-necked flask under argon. This solution was slowly added to a solution of 2c (50.0 mg, 0.040 mmol) in dry CH2Cl2 (40 mL) at 0 °C. The mixture was stirred at 0 °C for 1 h before quenching the reaction with methanol (2 mL). All volatiles were removed under reduced pressure. After addition of methanol (50 mL), black precipitates were collected by filtration. The obtained solids were purified with silica gel column chromatography (CH2Cl2:hexane = 3:2 as eluent) to give the target compound 3a (2.0 mg, 4%) as a black-purple solid. 1H NMR (600 MHz, CD2Cl2, 25 °C, δ): 10.17 (s, 2H), 8.47 (d, J = 6.0 Hz, 2H), 8.14 (d, J = 6.0 Hz, 2H), 7.93 (d, J = 6.0 Hz, 2H), 7.83 (d, J = 6.0 Hz, 2H), 7.74 (d, J = 6.0 Hz, 2H), 7.64 (s, 2H), 7.20 (s, 8H), 6.75 (d, J = 12.0 Hz, 2H), 3.00 (s, 12H), 2.65 (s, 6H), 2.49 (s, 6H), 2.17 (s, 12H). The 13C NMR spectrum was not obtained due to its insufficient solubility. HR-MALDI-TOF MS (m/z): [M]+ calcd for C90H60B2O4, 1226.4702; found, 1226.4783. Results and Discussion In conceiving the synthesis of polycyclic π-system 3, we envisioned precise assembly by using four components, including two anthracenes, two naphthalenes, one benzene, and two boron atoms. Hence, the most rational way is to construct a doubly B-doped heptazethrene precursor bearing two anthryl groups, and subsequently to perform an intramolecular cyclization reaction for π-annulation. In addition, according to the previous reports, the anthracene groups with the bulky mesityloxy substituents at the 4,5-positions are desirable for selective cyclization and increasing solubility.22,33 As shown in Scheme 1, the target B-doped PAHs 2b and 3a were synthesized starting from the disilicon-bridged dinaphthyl-benzene 5, which was prepared in two steps based on 1,8-dibromonaphthalene ( Supporting Information). The Si–B exchange reaction was performed on 5 with neat BBr3 at 90 °C, producing a key intermediate, dibromo-containing B-doped heptazethrene 2a. B-doped acenes are widely used as the building blocks to develop functional conjugated organoboranes, such as emitting materials, magnetic materials, and organocatalysts.19 Compound 2a is a new member of the B-doped acene family and can be expected to produce a new family of PAHs bearing two boron atoms at the para-positions of one hexagonal ring. Treatment of 2a with mesitylmagnesium bromide afforded compound 2b, which was used for investigating the properties of B-doped heptazethrene. Treatment of 2a with 9-lithium-bis(mesityloxy)anthracene 6 and then performing intramolecular oxidative dehydrogenation (the Scholl reaction) on 2c using FeCl3 led to 3a as a black-purple solid with a yield of 4%. This low yield is ascribed to the challenge in simultaneously forming six C–C bonds in 3a and the formation of some complex polar by-products. 2b and 3a are stable enough to be purified by silica gel column chromatography without any precautions. In addition, the excellent stability of 3a toward air and moisture was further proved by the time-dependent UV–vis absorption spectra, in which no changes were observed within 10 days ( Supporting Information Figure S12). Their excellent stabilities are because of their different protection effects, namely the bulky groups on the boron atoms of 2b and structural constraint on the boron atoms of 3a.4 Despite the large and planar skeleton of 3a (vide infra), it is soluble in common solvents (e.g., CH2Cl2, CHCl3, and toluene), permitting its further characterization. Scheme 1 | Synthesis of 2b and 3a. Reagents and conditions: (a) BBr3, 90 °C; (b) MesMgBr, toluene, 0 °C–25 °C; (c) 5, toluene, 0 °C–25 °C; (d) FeCl3, CH3NO2, CH2Cl2, 0 °C (Mes, mesityl). Download figure Download PowerPoint The structures of 2b and 3a were unambiguously confirmed by detailed NMR analysis ( Supporting Information Figures S22–S28), high-resolution mass spectrometry (HRMS) and finally X-ray crystallography. The 1H NMR spectrum of 3a shows the complex proton signals, which were clearly assigned by performing the 1H–1H correlation spectroscopy (COSY) and multiple one-dimensional nuclear Overhauser effect (1D NOE) measurements ( Supporting Information Figures S1-S3). A broad signal band around 7.2 ppm corresponds to the Hi atoms of the Mes group, most likely because of two kinds of Mes groups linked to the skeleton and the weak aggregation of the molecules in the concentrated solution. The HRMS spectrum of 3a exhibits an experimental peak at m/z = 1226.4783 and definite isotopic distributions that are in perfect agreement with its expected form ( Supporting Information Figure S4), corroborating its molecular formula of C90H60B2O4. Single crystals of 2b and 3a suitable for X-ray crystallographic analysis were obtained by the solvent diffusion method. As shown in Figure 2a, 2b exhibits the nearly planar π-skeleton that contains seven hexagons and two boron atoms. The B–C bond lengths are 1.545(3) Å for B1–C1, 1.552(3) Å for B1–C2, and 1.580(3) Å for B1–C3 ( Supporting Information Figure S5). For 3a, the π-skeleton is composed of 54 sp2-hybridized carbon atoms and two tricoordinate boron atoms (Figure 2b). Nineteen hexagonal rings are fused together to construct this planar graphene nanoflake that possesses four zigzag edges and four armchair edges. The mean derivation from planarity for the 56 atoms of the π-skeleton is smaller than 0.2 Å. It is notable that one 1,4-diboron-substituted benzene subunit is embedded into the π-skeleton and the two boron atoms are well separated by one benzene ring. To our knowledge, 3a is the largest B-doped PAH by far and thus a new kind of B-doped nanographene.22,26,28 In addition, the B-C bond lengths of 3a (1.496(5)–1.507(5) Å) are much smaller than that of 2b and nearly the shortest for B-doped PAHs ( Supporting Information Figure S8). These short B–C bond lengths are quite close to that (1.486(5) Å) of the sp2-carbon-based C23–C41 bond in 3a, thus suggestive of its highly rigid structure. These structural features of 3a, including the large and planar configuration and distinct boron segregation within the π-skeleton, are very elusive for B-doped PAHs. Figure 2 | Single-crystal structures of (a) 2b and (b) 3a with thermal ellipsoids at 50% probability. The mesityl and mesityloxy groups, as well as the hydrogen atoms are omitted for clarity. (c) Top view of the dimer structure of 3a. The two layers are illustrated with the space-filling and ellipsoid models, respectively. (d) Side view of the dimer structure of 3a. The two molecules are colored differently. Download figure Download PowerPoint In the packing structures, while 2b has a small π-π overlap, 3a displays the unexpected bilayer offset assembly that contains two molecules with similar conformations (Figures 2c and 2d and Supporting Information Figures S6 and S7). Despite the presence of the large steric hindrance in 3a due to the bulky mesityl groups, one molecule is rotated by about 60° to stack on the other molecule with the formation of a π–π stacking dimer. Between the adjacent dimers, there are multiple weak intermolecular C–H⋯π interactions, but no other π–π interactions are observed. The two mean planes are almost parallel, and the distances between one plane and the boron atoms in the other plane are 3.397 and 3.503 Å, respectively, which could be deemed as the π–π stacking distances. The planar conformation and strong π–π interactions of 3a are the main driving force for this unique bilayer assembly. Thus, the 3a dimer may be considered a molecular cutout of layered B-doped graphene, which is desirable for understanding the structure and properties of B-doped graphene at the molecular level. The B-doped PAHs 2b and 3a exhibit obviously different photophysical properties. The toluene solutions of 2b and 3a are yellow-green and deep purple, respectively (Figure 3). In the UV–vis absorption spectra, while 2b exhibits a narrow absorption band with the main absorption peaks (λabs) at 481 and 453 nm, 3a displays broad absorption bands that cover the entire visible range of 350–750 nm, with the λabs at 725, 603, and 556 nm ( Supporting Information Figures S9–S11). According to the onset absorptions, the optical bandgap (Egopt) is calculated to be 2.48 and 1.67 eV for 2b and 3a, respectively. Most of the reported B-doped PAHs, such as B-doped bisanthene, pyrene, perylene, and zethrene, display the maximum λabs of <650 nm and the Egopt of >1.90 eV.25–28 In contrast, 3a exhibits an obviously broader absorption band and narrower bandgap. In addition, the molar absorption coefficients of the entire absorption bands of 3a are up to 17,000–90,000 M−1 cm−1. Figure 3 | UV–vis absorption (black) and fluorescence (red) spectra of 2b and 3a in toluene, along with the oscillator strengths (blue bars) calculated by TD-DFT at the B3LYP/6-311G(d) level of theory. Inset are the photographs. Download figure Download PowerPoint The fluorescence spectrum of 2b in toluene (λex = 455 nm) exhibits the emission maximum (λem) at 498 nm with a full width at half maximum (FWHM) of 27 nm (1074 cm−1) and an absolute fluorescence quantum yield (ΦF) of 0.68. In contrast, 3a shows a significantly red-shifted fluorescence (λex = 610 nm) in the NIR region. Only one sharp emission band at 729 nm is observed, accompanying a small Stokes shift of 4 nm (76 cm−1) and a ΦF of 0.15. The FWHM of this emission band is only 26 nm (498 which is the reported for organic emitting The NIR fluorescence of 3a with these remarkable features were very distinctive for B-doped PAHs and all-carbon PAHs. For a displays a broad fluorescence band with the at with a π-skeleton exhibits multiple fluorescence peaks with the at The fluorescence and absorption properties of 3a are to its large and rigid structure and efficient of diboron-doped To precisely these photophysical properties, we performed on 2b and 3a using functional at the B3LYP/6-311G(d) level of theory. Their structures the planar ( Supporting Information Figures and and and In their molecular the of the boron atom significantly to the molecular and molecular of 2b and to the and of 3a ( Supporting Information Figures and In the time-dependent their calculated and oscillator strengths reproduced well the absorption features (Figure 3). The observed absorption peak of 2b is to the For 3a, the absorption bands at 725, 603, and 556 nm are assigned to calculated electronic nm, and nm, and nm, and All these the or of 3a. the of the boron atom significantly to the absorption properties of 2b and 3a. Lewis of 2b and 3a were to further the electronic effects of the boron The were and by UV–vis absorption addition of a Lewis to the solutions of 2b and 3a in the absorption spectra exhibit with definite ( Supporting Information Figures The and absorption peaks at and nm for 2b and and nm for 3a. These changes that the and Lewis are in two different the the of 3a with the first and were to be = M−1 and = respectively, the of 2b were not due to its Lewis acidity. The of 3a is than that of and to that of conjugated thus the sufficient Lewis acidity of 3a. The Lewis acidity of 3a is by its highly rigid structure, due to the boron segregation and aromatic structure of the In addition, the resulting Lewis exhibit a absorption spectra in to that of 2b and 3a, respectively, further the of the tricoordinate boron atom to the entire of B-doped PAHs. measurements were on 2b and 3a to their properties. The measurements were performed using as and as As shown in Figure 2b has only one reversible with the of For 3a, the spectrum displays three reversible and two reversible the good stabilities of the multiple reduced and The and and are and respectively. both 2b and 3a have two tricoordinate boron atoms, the remarkable multi-redox and electron deficiency were only observed for 3a. This unique five-step redox of 3a is ascribed to the of the fully embedded 1,4-diboron-substituted benzene and large π-skeleton on the In the previous reports, B-doped PAHs have been used as materials in For 3a, its along with the molecular packing provide the for the of efficient 3a has an bandgap as small as which is in with its narrow Figure 4 | of 2b and 3a in CH2Cl2 = Download figure Download PowerPoint To further the unique properties of 3a, we in measurements (Figures a of = the first with the absorption spectrum A new absorption band at nm was observed. absorption the formation of open-shell 3a the = the of the absorption band at nm with the of the band at nm, the of 3a The = led to the of three absorption bands at and These absorption can be to the formation of open-shell 3a the proved the formation of the reduced of 3a. we to perform chemical of 3a, we could not obtain the for further structural Figure 5 | absorption spectra of 3a in CH2Cl2 in with the of (a) (b) and (c) Download figure Download PowerPoint we to the electronic effects of B-doping on PAHs in of We performed to the aromatic of 3 and 3 3 and 3 are with the all-carbon PAH 4 and its 4 we their aromaticity The chemical shift and of the were at the level of ( Figures and Supporting Information Figures and 3 has aromatic hexagons chemical shift in that are to the two boron atoms, in agreement with the bond lengths of its X-ray structure ( Supporting Information Figure S8). The of 3 exhibits along the edges of one benzene two anthracene groups, and two naphthalene The that these aromatic are and the local aromaticity is in 3 to a extent. After of 3, the resulting 3 has six rings and displays along the of the π-skeleton, thus the aromaticity of 3 there are no on the boron atoms of 3 and 3 For all-carbon PAH while 4 shows along the and the edges of rings A and 4 exhibits two along the and the benzene ring. These are with the of the aromaticity of 4 and 4 B-doped PAH 3 has distinct aromatic in to its as well as the all-carbon congener and This that doping boron atoms into polycyclic may significantly alter their which cannot be achieved by reductive manipulation of the all-carbon Figure 6 | and from of 3, 3 and 4 calculated at the level of theory. Download figure Download PowerPoint A C54B2 polycyclic π-system with one 1,4-diboron-substituted benzene subunit was successfully synthesized from doubly B-doped heptazethrene. This molecule represents not only the largest B-doped PAH by far but also an unprecedented B-doped nanographene. The fully zigzag-armchair-edged structure creates its planar conformation, thus leading to the unique bilayer assembly behavior. More importantly, it possesses intriguing electronic structure and optoelectronic properties, such as very broad light absorption but sharp NIR fluorescence, as well as reversible five-step redox capability, all of which are rarely observed for other B-doped PAHs. In addition, this molecule displays distinctive local aromaticity that cannot be reproduced via the of its all-carbon congener. this the first example of nanoscale PAHs containing the 1,4-diboron-substituted benzene subunit and the understanding of the B-doping effects on polycyclic on B-doped PAHs and their assembly and a

Topics & Concepts