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 Google Scholar More articles by this author , Jiaxiang Guo State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Yue Yang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Kaiqi Ye State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Chuandong Dou *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author and Yue Wang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author 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 cm−1), which is among the smallest values reported for organic emitting materials.34–36 The NIR fluorescence of 3a with these remarkable features were very distinctive for B-doped PAHs and all-carbon PAHs. For example, a C48B2 nanographene displays a broad fluorescence band with the λem at 679 nm.22,23 Tetrabenzoperipentacene with a C56 π-skeleton exhibits multiple fluorescence peaks with the λem at 559 nm.37 The fluorescence and absorption properties of 3a are correlated to its large and rigid structure and efficient conjugation of diboron-doped π-skeleton. To precisely elucidate these photophysical properties, we performed theoretical calculations on 2b and 3a using density functional theory (DFT) at the B3LYP/6-311G(d) level of theory. Their optimized structures adopt the perfectly planar configurations ( Supporting Information Figures S13 and S14 and Tables S1 and S2). In their molecular orbitals, the p orbital of the boron atom significantly contributes to the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of 2b and to the LUMO, HOMO-1, HOMO-2, and HOMO-3 of 3a ( Supporting Information Figures S15 and S16). In the time-dependent DFT (TD-DFT) calculations, their calculated wavelength and oscillator strengths reproduced well the absorption features (Figure 3). The observed absorption peak of 2b is attributed to the HOMO→LUMO transition. For 3a, the absorption bands at 725, 603, and 556 nm are explicitly assigned to several calculated electronic transitions (719 nm, HOMO-2→LUMO and HOMO→LUMO; 607 nm, HOMO-3→LUMO, HOMO-2→LUMO, and HOMO→LUMO; 554 nm, HOMO-3→LUMO and HOMO-2→LUMO). All these transitions involve either the LUMO, HOMO-2, or HOMO-3 of 3a. Therefore, the p orbital of the boron atom significantly contributes to the absorption properties of 2b and 3a. Lewis acid-base complexations of 2b and 3a were investigated to further reveal the electronic effects of the boron atom. The titration experiments were conducted and recorded by UV–vis absorption spectroscopy. Upon addition of a Lewis base, Tetrabutylammonium Fluoride (TABF), to the separate solutions of 2b and 3a in THF, the absorption spectra exhibit two-step conversions with definite isosbestic points ( Supporting Information Figures S17–19). The first-stage and second-stage absorption peaks appeared at 450 and 395 nm for 2b and 523 and 429 nm for 3a. These stepwise changes suggest that the mono and double Lewis acid-base adducts are formed in two different steps. By fitting the titration curves, the binding constants of 3a with the first and second fluoride ions were estimated to be K1 = (1.3 ± 0.21) × 105 M−1 and K2 = (6.8 ± 0.38) × 104 M−1, respectively, whereas the binding constants of 2b were not determined probably due to its high Lewis acidity. The K1 value of 3a is slightly lower than that of trimesitylborane (3.3 × 105 M−1) and comparable to that of conjugated planarized triarylborane (1.3 × 105 M−1),38–40 thus suggesting the sufficient Lewis acidity of 3a. The Lewis acidity of 3a is negligibly diminished by its highly rigid structure, probably due to the boron segregation and aromatic structure of the π-skeleton.41 In addition, the resulting Lewis complexes exhibit a remarkably blue-shifted absorption spectra in comparison to that of 2b and 3a, respectively, further manifesting the great contribution of the tricoordinate boron atom to the entire conjugation of B-doped PAHs. Cyclic voltammetry (CV) measurements were conducted on 2b and 3a to investigate their electrochemical properties. The measurements were performed using n-Bu4NPF6 (0.1 M) as supporting electrolyte and ferrocene (Fc) as internal standard. As shown in Figure 4, 2b has only one reversible reduction process with the half-wave reduction potential (E1/2red) of −1.64 V. For 3a, the CV spectrum displays three reversible reduction processes and two reversible oxidation processes, demonstrating the good stabilities of the generated multiple reduced and oxidized species. The half-wave reduction and oxidation potentials (E1/2red and E1/2ox) are −1.06/−1.37/−1.60 V and +0.50/+0.72 V, respectively. Although both 2b and 3a have two tricoordinate boron atoms, the remarkable multi-redox activity and electron deficiency were only observed for 3a. This unique five-step redox property of 3a is probably ascribed to the capability of the fully embedded 1,4-diboron-substituted benzene and large π-skeleton on stabilizing the electrons. In the previous reports, B-doped PAHs have been used as electrode materials in Li-ion batteries.23 For 3a, its electrochemical characteristics along with the dense molecular packing provide the potential for the preparation of efficient battery devices. Additionally, 3a has an electrochemical bandgap (EgCV) as small as 1.56 eV, which is in accordance with its narrow Egopt. Figure 4 | Cyclic voltammograms of 2b and 3a in CH2Cl2 (0.80 mM). Fc/Fc+ = ferrocene/ferrocenium. Download figure Download PowerPoint To further study the unique reduction properties of 3a, we carried out in situ vis–NIR spectroelectrochemistry measurements (Figures 5a–5c). Upon applying a voltage of Eapp = −1.23 V, the first one-electron reduction occurred with the absorption spectrum gradually changing. A new absorption band at 1460 nm was observed. Such long-wavelength absorption suggests the formation of open-shell radical anion 3a •−. At the second reduction step (Eapp = −1.53 V), the intensity of the absorption band at 550 nm decreased with the disappearance of the band at 1460 nm, indicating the generation of dianionic species 3a 2−. The third reduction process (Eapp = −1.78 V) led to the emergence of three absorption bands at 1457, 970, and 865 nm. These low-energy absorption patterns can be attributed to the formation of open-shell 3a •3−. Therefore, the three-step spectral variations proved the formation of the reduced species of 3a. Although we attempted to perform chemical reduction of 3a, we could not obtain the pure targets for further structural analysis. Figure 5 | Vis–NIR absorption spectra of 3a in CH2Cl2 (0.10 mM) measured in situ during CV with the reduction potentials of (a) −1.23 V, (b) −1.53 V, and (c) −1.78 V. Download figure Download PowerPoint Finally, we would like to elucidate the electronic effects of B-doping on PAHs in terms of aromaticity.42 We performed theoretical calculations to investigate the aromatic character of 3 and 3 2−. Since 3 2− and 3 are isoelectronic with the respective all-carbon PAH 4 and its dicationic species 4 2+, we compared their aromaticity together. The nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations were carried out at the CAM-B3LYP/6-311G(d) level of theory ( Figures 6a–6h and Supporting Information Figures S20 and S21).43,44 3 has five Clar Sextet aromatic hexagons [nucleus independent chemical shift (NICS(1)ZZ) in blue bold] that are binding to the two boron atoms, in agreement with the bond lengths of its X-ray crystal structure ( Supporting Information Figure S8). The ACID plot of 3 exhibits fragmental diatropic ring currents along the edges of one central benzene ring, two anthracene groups, and two naphthalene rings. The data suggest that these aromatic moieties are partially coupled and the local aromaticity is preserved in 3 to a certain extent. After two-electron reduction of 3, the resulting 3 2− has six Clar Sextet rings and displays clockwise ring current mainly along the periphery of the π-skeleton, thus confirming the global aromaticity of 3 2−.45,46 Notably, there are no ring current flows on the boron atoms of 3 and 3 2−. For all-carbon PAH system, while 4 shows clockwise ring currents along the periphery and bypass through the inner edges of rings A and B, 4 2+ exhibits two diatropic current flows along the periphery and the central benzene ring. These ACID plots are consistent with the NICS(1)ZZ results, indicative of the dominant global aromaticity of 4 and 4 2+.45,46 Therefore, B-doped PAH 3 has distinct aromatic character in comparison to its dianionic species, as well as the all-carbon congener and isoelectronic species. This result demonstrates that doping boron atoms into polycyclic π-systems may significantly alter their aromaticity, which cannot be facilely achieved by reductive manipulation of the all-carbon congeners. Figure 6 | (a)–(d) NICS(1)ZZ values and (e)–(h) ACID plots (contribution from π electrons only) of 3, 3 2−, 4, and 4 2+, calculated at the CAM-B3LYP/6-311G(d) level of theory. Download figure Download PowerPoint Conclusion 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 reduction of its all-carbon congener. Therefore, this study provides the first example of nanosc