Triangular BN-Embedded Molecular Carbons with Zigzag Edges and Their Dual Functionality in Fluoroanion Detection and Perovskite Solar Cells

Abstract

Atomically precise synthesis of nanographenes containing polar boron–nitrogen (B–N) units, especially in a π-extended hexabenzocoronene (HBC) skeleton, has yet to be achieved, leaving this distinctive structural motif largely unexplored. Herein, we report the synthesis and characterization of two BN-embedded molecular carbons (BN-Ph and BN–OH), featuring regioselective construction of three BN bridges at the bay positions of the HBC. Incorporating B–N units into the aromatic framework produces unique solid-state stacking, photophysical properties, and electronic structures. Interestingly, despite the partial localization of BN bonds disrupting the continuous conjugation of azaborine rings, the global aromaticity of the molecular skeleton remains preserved. Furthermore, these BN-embedded nanographenes exhibited a sensitive response to fluoride anions, good reversibility, and coordination-induced color tunability. Notably, benefiting from pronounced local dipoles, extensive π-electron surfaces, and favorable energy level alignment, BN-Ph and BN–OH were further employed as interfacial layers in perovskite solar cells (PSCs). These BN-embedded carbon nanomaterials promote more efficient hole extraction and transport, leading to a significantly enhanced power conversion efficiency of 25.24% and an open-circuit voltage of 1.187 V, while their exceptional hydrophobicity substantially improved device stability, retaining over 90% of the initial efficiency after extended ambient exposure. These multifaceted findings, along with the versatile synthetic platform presented, open new avenues for designing chemically customized BN-based functional materials.

Introduction

Polycyclic aromatic hydrocarbons (PAHs), encompassing nanographenes and heteroatom-doped counterparts, − have emerged as one of the most intensively studied classes of carbon-rich materials due to their exceptional physicochemical properties and widespread applications in modern organic optoelectronic devices. − In the pursuit of advanced functional materials, steady progress has been made in designing and synthesizing novel PAH derivatives. − Among these, two approaches have proven to be particularly powerful: (i) extending the π-conjugated scaffold by hybridizing polycyclic components, , and (ii) embedding main-group heteroatoms (such as nitrogen, boron, sulfur, or silicon) into the aromatic framework. , These structural modifications not only reshape the fundamental electronic structure of PAHs but also enable precise tuning of their (opto)­electronic properties. In recent years, the replacement of CC units with polar B–N couples in PAHs has garnered over-growing interest due to the advantages conferred by CC/BN isosterism, − which alters the π-electron structural features while maintaining the overall molecular size and topology. More importantly, BN-doping goes beyond a simple structural modification, which also acts as a rational design tool that transforms otherwise inert hydrocarbon moieties into polar, tunable, and functionally responsive materials by modulating charge distribution, dipole moments, and frontier orbital energies. − As a result, a wide variety of small BN-embedded PAHs have been successfully synthesized, including derivatives based on phenanthrene, − anthracene, − pyrene, , ullazine, , perylene, , and others.

Coronene, , often termed “superbenzene”, is a highly symmetric PAH and represents one of the smallest molecular models of graphene with a zigzag periphery (Figure a). Following the prevailing trend in BN-doping of PAHs, several coronene-based systems with peripheral B–N units have been developed by Pei, Zhang, Staubitz, Yu, and Tan, et al., including 1,5,9-triaza-2,6,10-triphenylboracoronene, di-BN-coronene diimides, di-BN-fused coronenes, and heterocoronenes containing pyridine and B–N units (Figure b). These typical heteroaromatics can be structurally traced to B–N bridging at the bay regions of small PAHs (e.g., triphenylene or perylene), and some of them can serve as key building blocks for organic functional materials with potential applications in catalysis, storage, and optoelectronics. , Moreover, Pei and coworkers recently reported a series of internally coronene derivatives, which reveal notable electronic coupling and distinctive optoelectronic behavior. Despite the diversity and fascinating properties of BN-embedded coronenes, research has predominantly focused on small- to medium-sized molecular systems, whereas the incorporation of B–N units into well-defined, nanosized conjugated systems, especially those based on the π-extended HBC skeleton, remains a significant and underexplored challenge. − This gap largely originates from the scarcity of rational molecular design strategies, which often leads to synthetic inaccessibility, chemical instability, and poor solubility.

1.

(a) Schematic illustration of transformation from coronene to TrizigzagHBC. (b) Previously reported coronene derivatives with BN units at zigzag edges. (c) BN-embedded nanographenes based on π-extended HBC skeleton (BN-Ph and BN–OH).

Given the difficulties with existing syntheses and transformations, we sought a modular, bottom-up synthetic approach to prepare missing BN-doped nanographenes and to investigate their structure–property relationships and functional potential. In this manuscript, we disclose the rational construction of a new category of BN-embedded molecular carbons (BN-Ph and BN–OH), featuring three B–N bridges regioselectively installed in the bay regions of a HBC framework (Figure c). A comprehensive investigation combining photophysical studies and theoretical calculations provides valuable insights into their optoelectronic properties, aromaticity, and electronic structures with distinct charge distribution. Capitalizing on the three-coordinate boron centers in BN-embedded systems, both compounds demonstrated special selectivity for fluoride anions and tight binding. Furthermore, substituents on the boron atoms significantly influence the coordination behavior of these molecules, thereby enabling coordination-induced color tunability. Benefiting from pronounced local dipoles, extensive π-electron surfaces, and favorable energy level alignment, two BN-embedded nanographenes were employed as interfacial layers in PSCs, resulting in significantly enhanced power conversion efficiencies and superior operational stability. It is worth noting that BN-Ph and BN–OH represent the largest BN-embedded PAHs employed to date in solution-processed PSCs, − underscoring their great potential for applications in organic photovoltaics.

Results and Discussion

Synthesis and Characterization

Incorporating B–N units into the HBC framework is expected to open the HOMO–LUMO gap, enhance electron affinity, and induce polarization, thereby transforming the typically nonemissive extended nanographene into a luminescent, functionally tunable system. Following this blueprint, we designed a rational synthetic route for the preparation of BN-Ph and BN–OH (Scheme ), which was accomplished through a sequence of key transformations: Suzuki–Miyaura coupling, Scholl oxidative cyclization, Pd-catalyzed Buchwald-Hartwig amination, and electrophilic borylation.

1. Synthetic Route of BN-Ph and BN–OH .

^a Reagents and conditions: (a) 4-chlorophenylboronic acid, Pd­(PPh₃)₄, K₃PO₄, PhMe/EtOH/H₂O, 100 °C, 12 h; (b) BBr₃, DCM, 0 °C to rt, 8 h; (c) PhNTf₂, DMAP, Et₃N, DMF, rt, 2 h; (d) MesZnCl, Pd­(PPh₃)₄, THF, 70 °C, 6 h; (e) DDQ, TfOH, DCM, 0 °C, 1.5 h; (f) 4-(tert-butyl)­aniline, t-BuONa, Pd­(dba)₂, P­( ^t Bu)₃·HBF₄, PhMe, 110 °C, 12 h; (g) PhBCl₂, Et₃N, o-DCB, 180 °C, 15 h; (h) BBr₃, o-DCB, 180 °C, 15 h.

Our synthesis began with the preparation of starting material 1 from 3-bromoanisole via a two-step sequence involving acylation, followed by acid-catalyzed condensation (see Supporting Information for details). Then, a Suzuki coupling reaction between 1 and 4-chlorophenylboronic acid afforded 2 in 65% yield. The chloride substituents introduced during this process can serve as functional handles for further modifications. Treatment of 2 with boron tribromide (BBr₃), followed by triflation using N-phenyl-bis­(trifluoromethanesulfonimide) (PhNTf₂) in the presence of 4-dimethylaminopyridine (DMAP), yielded triflate 4 quantitatively. It should be noted that the introduction of suitable solubilizing substituents is essential, as large-sized PAHs tend to aggregate in solution. Initial attempts to convert 4 into 5 via Suzuki coupling were unsuccessful due to competing detriflation. This limitation was overcome by employing Negishi coupling as a more robust alternative, which afforded the desired product after condition screening. The key building block 6 was obtained through Scholl reaction using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and trifluoromethanesulfonic acid (TfOH), delivering the fully fused HBC derivative in 54% yield. The structure was further confirmed by single-crystal X-ray diffraction analysis (Figure S12). Next, a palladium-catalyzed Buchwald-Hartwig amination of 6 with 4-(tert-butyl)­aniline proceeded smoothly to afford precursor 7 (NH-HBC). To our delight, the electrophilic borylation of NH-HBC with PhBCl₂ successfully introduced three B–N units through intramolecular cyclization, yielding the target compound BN-Ph in 32% yield. Treatment of NH-HBC with excess BBr₃ in o-dichlorobenzene (o-DCB) led to in situ formation of bromo-substituted intermediates, which were then transformed via hydrolysis of the B–Br bonds to provide hydroxyl-substituted compound BN–OH in 45% yield. Notably, such a bottom-up synthetic strategy enables precise control over molecular structures and doping patterns, providing a versatile platform for accessing BN-doped and other functionalized nanographenes with tailored edge topologies and emergent optoelectronic properties.

With the desired BN-embedded nanographenes in hand, we tackled their structural characterization by a combination of high-resolution mass spectroscopy (HR-MS), NMR spectroscopies, and X-ray crystallography. HR-MS exhibited protonated molecular ion peaks at m/z = 1576.7971 for BN-Ph and m/z = 1396.6872 for BN–OH, respectively, which are in good agreement with their simulated molecular masses and isotopic distributions (Figures S89 and S94). The ¹H NMR spectrum of BN-Ph displayed nine distinct signals in the low field region (δ = 6.57–9.80 ppm) and three sets of signals in the high field region (δ = 1.33–2.18 ppm) with an intensity ratio of 3:3:3:6:6:6:3:6:6:9:18:27 (Figures a and S85), indicating its perfect C ₃ symmetry and nonaggregated state in solution. Key assignments of the aromatic protons were readily identified through a combination of 2D NMR techniques (Figure b). Moreover, the similar ¹H, ¹³C, and ¹¹B NMR patterns observed for BN–OH suggest it shares comparable structural characteristics with BN-Ph (Figures S90–S93). The only notable difference is the presence of a characteristic signal at δ = 4.72 ppm in BN–OH, which is assigned to the protons of the hydroxyl groups attached to the boron atoms. Two BN-embedded molecules were found to be robust in most organic solvents, exhibiting no structural or chemical changes. Thermogravimetric analysis (TGA) also revealed their high thermal stability, with degradation temperatures at 5% weight loss (T _d,5%) exceeding 380 and 420 °C, respectively (Figure S1).

2.

NMR characterization of BN-Ph. (a) ¹H NMR spectrum of the aromatic region. (b) ¹H–¹H COSY NMR spectrum. (c) Density functional theory (DFT) optimized structure of BN-Ph.

X-ray Single-Crystal Analysis

Single crystals of BN-Ph were obtained as yellow grains by slow diffusion of n-hexane into its carbon disulfide solution. Unfortunately, despite numerous attempts under various cultivation conditions, we were unable to obtain single crystals of BN–OH. In the crystal of BN-Ph, there is only one type of crystallographically independent molecule, which crystallizes in the monoclinic P2₁/c space group (Table S1). The X-ray crystallographic analysis, shown in Figure a, revealed a C ₃-symmetric nanostructure and defined C–N–B–C zigzag-edged periphery of BN-Ph. The length and width of the molecular main skeleton are 11.48 Å and 11.11 Å, respectively. Due to steric interactions of the peripheral phenyl substituents, BN-Ph adopts a slightly bent backbone with a maximum torsion angle of 4.01° (Figure b). The molecular planarity parameter (MPP) of 0.116 Å further supports this observation (Figure S16). The average B–N bond length in azaborine rings is 1.411 Å (Figure a), which is longer than the localized BN double bond length of 1.403 Å but shorter than the typical B–N single bond length of 1.488 Å, indicating its partially localized double bond character. The B–C bond lengths range from 1.517 to 1.567 Å (Figure S17), similar to those in the 1,2-azaborine aromatic ring. , The calculated harmonic oscillator model of aromaticity (HOMA) based on the single-crystal structure provided the same tendency: the three azaborine rings in BN-Ph exhibit moderate aromaticity, with HOMA values ranging from 0.38 to 0.60, averaging approximately 0.49 (Figures a and S18). In general, incorporating B–N units into the aromatic framework can induce dipole–dipole interactions, thereby enhancing intermolecular π–π contacts and modulating solid-state stacking. , In contrast to other BN-embedded small PAHs, no significant face-to-face π–π stacking is observed in BN-Ph, which may be attributed to the steric hindrance imposed by bulky aryl groups. However, BN-Ph is orderly organized in a wave-like packing motif along the c-axis, driven by local dipole–dipole interactions and multiple C–H···π interactions (Figures S5, S6 and S8). Furthermore, a face-to-edge herringbone packing pattern was observed in the solid state, stabilized by strong C–H···π contacts (d _{C–H···π} = 2.55–3.24 Å) between adjacent molecules (Figures c, S9 and S10). Two layers with the same orientation exhibit a large separation distance of 15.41 Å due to the presence of additional interlayer molecule from a neighboring column (Figure c).

3.

X-ray crystal structure and DFT calculations (thermal ellipsoids are shown at 20% probability. Aryl groups and hydrogen atoms are omitted for clarity). (a) Top view showing the length, width, average bond lengths, and HOMA values of BN-Ph. (b) Side view showing the MPP and torsion angle of BN-Ph. (c) 2D herringbone packing pattern of BN-Ph. (d, g) ACID plot (isovalue: 0.03) and NICS(1) _zz values of BN-H and TrizigzagHBC. (e, h) The calculated ESP maps of BN-H and TrizigzagHBC. (f, i) LOL-π maps (isovalue: 0.5). Note: substituents on the boron atoms have a negligible effect on the calculated aromaticity and electronic structure of the molecules.

Aromatic Character and Electronic Structure

To gain deeper insight into the influence of BN doping, the aromaticity and electronic structures were evaluated using unsubstituted BN-H as a model and compared to those of its all-carbon analogue, TrizigzagHBC. As shown in Figure d, the calculated nucleus-independent chemical shift (NICS) values for the six-membered rings A, B, C, and F of BN-H were found to be −35.17, −10.67, −29.32, and −29.83 ppm, respectively, demonstrating their typical aromatic feature. In contrast, ring D on the coronene core shows less or nonaromaticity, with a NICS(1) _zz value of 1.08 ppm. These results are similar to the aromaticity of its all-carbon analogue (Figure g). However, the NICS(1) _zz value of ring E (−9.43 ppm) in BN-H is significantly less negative than that of TrizigzagHBC (−28.22 ppm), indicating that the azaborine ring possesses weaker aromaticity due to partial electron localization on the BN bond. Anisotropy of the induced current density (ACID) plots for BN-H clearly revealed clockwise (diamagnetic) 6π ring currents in rings A, C, and F (Figure d), indicating local aromaticity that aligns well with their NICS(1) _zz results. Interestingly, despite the absence of an apparent local ring current in the azaborine ring E, consecutive 30π ring currents are observed along the peripheral conjugation pathway involving the electrons on three BN bonds, suggesting its global aromaticity.

The electrostatic surface potentials (ESP) of BN-H and TrizigzagHBC were calculated at B3LYP/6-311G­(d,p) level of theory to illustrate their electronic structures. In TrizigzagHBC, the ESP map from top view revealed a uniformly negative potential distribution (Figure h), indicative of a fully delocalized and symmetric π-electron cloud above and below the molecular backbone. In stark contrast, the ESP surface of BN-H exhibited a polarized electrostatic potential distribution: positive charges (blue) are localized around the nitrogen atoms, whereas negative charges (red) are concentrated at the boron centers and the molecular skeleton of HBC (Figure e). This distinct charge separation arises from the polar nature of the B–N bonds, where boron acts as a π-acceptor and nitrogen as a π-donor, thereby perturbing the original electronic symmetry. Further insight is provided by localized orbital locator for π orbitals (LOL-π) calculations. , A common electronic structure characteristic of BN-H and TrizigzagHBC is that the π-electrons are localized on the hub ring A and the six peripheral benzene rings C and F of the HBC skeleton (Figures f and i). Upon introducing B–N units into six-membered rings, the electrons become primarily localized on the B–N bond of ring E, suggesting partial conjugation in this region. These observations are in good agreement with the results from NICS and ACID analyses, which collectively demonstrate that BN doping selectively modulates the π-topology, aromaticity, and charge distribution in the polycyclic framework.

Photophysical and Electrochemical Properties

UV–vis absorption and photoluminescence (PL) spectra of BN-embedded nanographenes were recorded in dichloromethane and compared with their CC congener, TrizigzagHBC-Ph, as well as the precursor NH-HBC. As depicted in Figure a, the absorption profiles of BN-Ph and BN–OH are nearly identical and exhibit three types of absorption bands (β, p, and α) with well-resolved vibronic transitions, which are typical features of PAH molecules. According to time-dependent density functional theory (TD-DFT) calculations, the very intense β-band and the weak α-band are respectively ascribed to symmetry-allowed S₀→S₃/S₄ transitions and symmetry-forbidden S₀→S₁ transition (Tables S5 and S6), which predominantly involve electronic transitions from the highest occupied molecular orbital (HOMO)/HOMO–1 to the lowest unoccupied molecular orbital (LUMO)/LUMO+1. The absorption maxima were observed at 396 nm (ε = 2.81 × 10⁵ M^–1 cm^–1) for BN-Ph and 394 nm (ε = 1.52 × 10⁵ M^–1 cm^–1) for BN–OH, showing a slight blueshift of 9–11 nm relative to their all-carbon analogue (405 nm, ε = 3.58 × 10⁵ M^–1 cm^–1), yet a redshift of 3–5 nm compared to NH-HBC (391 nm, ε = 0.86 × 10⁵ M^–1 cm^–1). Although BN doping extends the periphery of HBC skeleton, the partially localized BN double bond interrupts the continuous π-conjugation within the azaborine subunits, resulting in a blue-shifted absorption. This trend aligns reasonably well with their DFT calculations, which reveal a diminished π-electron delocalization in the present BN-embedded systems (Figure f). Then, the electronic effect of BN doping was further investigated by comparing the frontier molecular orbitals (FMOs) and energies of BN-H and TrizigzagHBC. The calculated FMOs at B3LYP/6-311G­(d,p) level revealed that both BN-H and its carbon analogue possess degenerate HOMOs and LUMOs, which are uniformly delocalized over the whole π-skeleton (Figures b and S28). However, their orbital energy levels are different. Isoelectronic substitution of CC units with B–N units in the bay regions of HBC resulted in a lowered HOMO level (−5.38 eV vs −5.35 eV), a raised LUMO level (−2.07 eV vs −2.17 eV), and an increased HOMO–LUMO gap (−3.31 eV vs −3.18 eV), signifying that the p orbitals of B and N atoms in 1,2-azaborine moieties contribute substantially to the molecular orbitals and subtly modulate the electronic structure. Furthermore, the HOMO energy level of BN-H was dramatically higher than those of previously reported peripheral or internal BN-embedded coronenes (Figure S29), implying that this new type of BN-embedded carbon nanomaterials could be a promising candidate for organic optoelectronic applications, such as photovoltaic cells.

4.

Photophysical properties and calculated frontier molecular orbitals. (a) UV–vis absorption (left) and PL spectra (right) of NH-HBC, BN-Ph and BN–OH in CH₂Cl₂ (5.0 × 10^–6 M for absorption, 1.0 × 10^–6 M for emission), respectively. Inset: images of the solutions under a 365 nm UV lamp. (b) Calculated HOMO and LUMO energy levels of BN-H and TrizigzagHBC.

The fluorescence spectra of BN-Ph and BN–OH exhibited three vibronic peaks in the 450–650 nm range, corresponding to green fluorescence (Figure a). The emission maxima, centered around 530 nm, were slightly blue-shifted compared to TrizigzagHBC-Ph (534 nm), while displaying a distinct red shift of ca. 20 nm relative to NH-HBC (511 nm), consistent with the trend observed in their absorption spectra. The absolute fluorescence quantum yields (Φ _f ) and fluorescence lifetimes (τ _f ) of two BN-doped molecules were measured to be 16.9–18.4% and 25.12–29.67 ns, respectively, both of which are higher than those of the NH-HBC (Φ _f = 14.5%; τ _f = 16.68 ns) (Figure S33). These findings indicate that the nonradiative transition rates (k _nr) for BN-Ph and BN–OH are slower than NH-HBC (Table S8), which has a more flexible geometry. In addition, both BN-embedded systems exhibited minimal solvent effects on their emission spectra (Figures S34 and S35), demonstrating that the luminescence of localized states.

The electrochemical properties of BN-Ph and BN–OH were elucidated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements in dry o-DCB/CH₃CN (4:1). Both BN-embedded nanographenes exhibited similar electrochemical behavior, characterized by irreversible reduction and oxidation waves (Figure S36). In the 0 to 2.5 V scan window, the onset potentials for the first oxidation wave of BN-Ph and BN–OH are 0.92 V and 0.90 V, respectively. Thus, the corresponding HOMO energy levels were estimated to be −5.30 eV and −5.28 eV (Table S11), which are qualitatively consistent with their DFT calculations. In addition, the half-wave oxidation potentials of BN-Ph exhibited more dense and well-defined oxidation peaks than those of BN–OH bearing three hydroxyl groups, resulting in narrower potential splittings between different oxidation states (Figure S37). This elucidated that substituents on the boron atoms play an important role in contributing to the half-wave oxidation potentials.

Coordination with Fluoride Anions

As the vacant 2p orbitals of boron atoms feature innate Lewis acidity, three-coordinate boron centers in organoboron compounds can respond to anions through intramolecular Lewis acid–base interactions, , especially for fluoride anion (F^–). Accordingly, the coordination behavior of BN-embedded nanographene (BN-Ph or BN–OH) toward F^– was initially investigated by in situ ¹H NMR characterization (Figures S38 and S40). Upon addition of tetrabutylammonium fluoride (TBAF) to BN-Ph or BN–OH in TCE-d ₂, the proton signals in the aromatic region shifted upfield due to axial coordination of F^–. Furthermore, the appearance of additional signals in their ¹H NMR spectra indicated the loss of the original C ₃ symmetry. To further confirm the interaction between BN-Ph (or BN–OH) and F^–, ¹⁹F NMR titration experiments were conducted (Figures S39 and S41). The addition of TBAF to TCE-d ₂ solutions of BN-Ph and BN–OH resulted in coordination-induced signals appearing at −129.05 ppm and −128.88 ppm, respectively. Further addition of TBAF (up to 5 equivs) did not induce new resonances in the ¹⁹F NMR spectra, demonstrating that the observed new signal can be exclusively ascribed to the formation of a monofluoride complex.

In line with the structural changes, the UV–vis and fluorescence spectra also exhibited obvious anion-responsive characteristics. As seen in Figure a, the intensities of the absorption peaks of BN-Ph at 377, 396, and 431 nm gradually decreased after the addition of TBAF. Meanwhile, a new broad absorption band appeared between 500 and 600 nm, with a long tail extending to 800 nm (Figure S42). It should be noted that such a red-shifted phenomenon contrasts with the blue shift generally observed in four-coordinate boron aromatics, which hampers π–π* transitions of the molecular backbone. By fitting the UV–vis titration data with a 1:1 model using the BindFit program, the binding constant (K _a ) for TBAF was determined to be (1.18 ± 0.10) × 10⁵ M^–1 (Figure S44), indicating the high binding affinity of π-extended BN-Ph to F^–. Additionally, fluorescence switching by coordination of F^– to BN-Ph was observed (Figure b). When TBAF was added to a solution of BN-Ph, a long-wavelength emission band around 621 nm appeared, accompanied by a conspicuous change in fluorescence color from bright green to orange. In contrast, although the addition of TBAF to BN–OH solution produced similar spectral changes, no distinct new peaks were observed in the absorption and emission spectra; only weak, broad bands appeared at longer wavelengths (Figures d and e). Accordingly, the K _a was calculated to be (5.51 ± 0.16) × 10⁴ M^–1 based on 1:1 binding model by fitting the regression curve to the change in absorbance (Figure S45). Moreover, the resulting fluoride adduct displayed yellow fluorescence under 365 nm UV light, which was markedly different from that of the BN-Ph adduct. The above results indicate that substituents on boron atoms modulate the coordination behavior of organoboron compounds, thereby enabling coordination-induced color tunability in π-extended BN-embedded aromatic systems.

5.

Spectroscopic titration analyses and optimized complex structures. (a, d) UV–vis absorption changes (1.0 × 10^–5 M) and curve-fitting analyses of BN-Ph and BN–OH with TBAF. Inset: images of the solutions before and after binding under room light. (b, e) Fluorescence intensity changes (5.0 × 10^–6 M) of BN-Ph and BN–OH with TBAF. Inset: images of the solutions before and after binding under a 365 nm UV lamp. (c, f) Optimized structures and stabilization energies of [BN-Ph·F]⁻ and [BN–OH·F]⁻ calculated at the B3LYP/6-311G­(d) level. (d _B–F and ρ_F represent the B–F bond lengths and NPA charges of the F atoms, respectively.)

To elucidate the differences in the coordination capacities of two BN-embedded nanographenes toward F^–, the bond lengths, charge densities, and stabilization energies of the TBA⁺ [BN-Ph·F]⁻ and TBA⁺ [BN–OH·F]⁻ were calculated and compared. As shown in Figures c and f, both optimized complexes displayed nonplanar geometries, featuring a clear B–F connection, with the boron atom involved in coordination significantly deviating from the plane of the 1,2-azaborine ring. However, the B–F bond length in the [BN-Ph·F]⁻ complex (1.484 Å) was much shorter than that in the hydroxyl-substituted counterpart, [BN–OH·F]⁻ (1.518 Å). In addition, the natural population analysis (NPA) charge of the fluorine atom (ρ_F) in [BN-Ph·F]⁻ (−0.583) exhibited a less negative charge character compared to [BN–OH·F]⁻ (ρ_F = −0.607). These results suggest that the anionic charge in [BN-Ph·F]⁻ is more effectively delocalized over the π-conjugated backbone, resulting in enhanced binding affinity for F^–. The stabilization energies of the two complexes were calculated as −20.52 kcal/mol and −15.81 kcal/mol (Table S12), respectively, further demonstrating greater stabilization between BN-Ph and TBAF upon binding.

As a key indicator for practical applications, the selectivity of the coordination process of BN-embedded nanographenes toward nine different anions (F^–, Cl^–, Br^–, NO₃ ^–, ClO₄ ^–, BF₄ ^–, PF₆ ^–, AcO^–, H₂PO₄ ^–) was investigated. In the presence of F^–, the fluorescence spectra of BN-Ph and BN–OH exhibited distinct changes, while other interfering anions caused negligible changes in fluorescence intensity (Figures S47 and S48). Quantitative histograms of the fluorescence responses of the two BN-embedded molecules to various anions clearly revealed differences between F^– and other anions (Figure S49), suggesting high F^– selectivity and strong anti-interference ability. Moreover, no significant changes were observed in the fluorescence lifetimes of free BN-Ph (or BN–OH) and the corresponding fluoride adduct (Figure S50), indicating typical static quenching characteristics, which is consistent with sensing mechanisms observed in other organoboron systems. − Reversibility is another crucial feature in the sensing process. Gratifyingly, the fluorescence color of the anionic adduct completely converted back to its original state after the addition of fluoride scavenger (BF₃·Et₂O), accompanied by the recovery of both the spectral shape and fluorescence intensity (Figures S47 and S48). This indicates that the BN-embedded molecules maintain a stable conformation without decomposing during the sensing process. Furthermore, no observable degradation in the fluorescent colors occurred after more than five repetitions of the tests (Figures b, S51 and S52), demonstrating good reversibility and potential as a fluorescent probe for detecting F^–.

6.

Reversibility of the coordination process. (a) Interconversion between BN-Ph/BN–OH and their corresponding complexes. (b) Reversibility between three-coordinate and four-coordinate boron species in BN-Ph/BN–OH upon iterative addition of TBAF and BF₃·Et₂O.

Photovoltaic Performance

The solution-processed organic–inorganic hybrid PSCs have attracted considerable interest due to their low cost, facile fabrication, and high certified power conversion efficiency (PCE) exceeding 27%. − Within PSC architectures, hole transport materials (HTMs) play a critical role in enhancing both the stability and efficiency of photovoltaic devices. Among them, 2,2′,7,7′-tetrakis­[N,N-di­(4-methoxyphenyl)-amino]-9,9′-spirobifluorene (Spiro-OMeTAD) remains the most widely used HTM in conventional n-i-p structured PSCs, attributable to its broad applicability and commercial availability. , However, the relatively low intrinsic hole mobility of Spiro-OMeTAD, coupled with interfacial energy-level misalignment between the perovskite (PVK) and HTM, often impedes sufficient carrier extraction and transport. , Therefore, further performance improvements can be pursued through interface engineering, particularly by introducing interlayer materials with suitable energy-levels and efficient charge extraction capabilities. , Our BN-embedded nanographenes exhibit pronounced local dipoles and extensive π-electron surfaces, along with proper HOMO energy levels (−5.38 eV for BN-Ph; −5.35 eV for BN–OH; Figures S53 and S54) that align well with that of Spiro-OMeTAD (−5.20 eV) and the valence band of PVK (−5.41 eV). This favorable energy-level alignment suggests that BN-embedded carbon nanomaterials could serve as interfacial layers in PSCs, facilitating more efficient hole extraction and transport (Figure b).

7.

Photovoltaic performance of the PSCs. (a) The schematic diagram of the device structure. (b) Schematic energy level alignment. (c) J–V curves of control and modified devices. (d) PCE distribution of the devices. (e) Carrier mobility and conductivity of Spiro-OMeTAD and modified Spiro-OMeTAD films, respectively. (f) Stable PL spectra of perovskite films before and after modification, excited from the Spiro-OMeTAD side. (g) The TRPL spectra of the perovskite films before and after modification. (h) The air stability of unencapsulated devices before and after modification under environmental conditions for 480 h, with a relative humidity of 30% RH.

To evaluate the impact of BN-Ph and BN–OH as interfacial materials on photovoltaic performance, the devices with the architecture FTO/TiO₂/FA_0.9Cs_0.1PbI₃/BN-Ph (or BN–OH)/Spiro-OMeTAD/Au were fabricated (Figure a). Figure c illustrated the current density–voltage (J-V) curves and corresponding performance parameters of different devices. The control device exhibited a PCE of 23.64%, an open-circuit voltage (V _OC) of 1.166 V, a short-circuit current density (J _SC) of 25.04 mA cm^–2, and a fill factor (FF) of 81.00%. Upon incorporating BN-embedded interfacial molecules into the PVK/HTM layers, the photovoltaic performance of devices demonstrated obvious improvement. Among these, the device treated with BN–OH exhibited the best performance, achieving a PCE of 25.24%, V _OC of 1.187 V, J _SC of 25.44 mA cm^–2, and FF of 83.59%, which is comparable to the reported performances of n-i-p-type devices modified with hole-transport interfacial materials (Table S14). Such distinct changes are not solely from the direct influence of energy level alignment but also from the synergistic interactions involving the anchoring groups of interfacial molecules. Peripheral hydroxyl groups in BN–OH can strongly interact with I^– anions on the perovskite surface to form hydrogen bonds, reducing the charge recombination rate and facilitating efficient carrier extraction at the interface. , Furthermore, the statistical distributions based on the photovoltaic parameters of the 20 independent devices showed good reproducibility, as shown in Figures d and S55. In comparison to the control, the modified devices consistently exhibited higher V _OC, J _SC, and FF. Two BN-modified devices achieved an average PCEs of 23.96% and 24.83%, respectively, displaying a narrower distribution than the control devices (PCE_avg = 23.10%).

To further elucidate the origin of the PCE improvement after modification, the space charge limited current (SCLC) method was employed to examine the influence of BN-Ph and BN–OH on the charge transport properties of Spiro-OMeTAD. Dark-current measurements were performed on devices with the structure FTO/BN-Ph (or BN–OH)/Spiro/Au. Based on the Mott–Gurney equation, the hole mobility (μ) of the Spiro-OMeTAD film increased significantly from 2.79 cm²·V^–1·s^–1 (pristine) to 4.22 and 4.73 cm²·V^–1·s^–1 upon modification with BN-Ph and BN–OH, respectively. Correspondingly, the conductivity (σ) also rose from 9.91 × 10^–3 mS·cm^–1 (pristine) to 1.51 × 10^–2 and 1.68 × 10^–2 mS·cm^–1 (Figure e). These results indicate more efficient hole extraction and transport, along with suppressed nonradiative recombination at the PVK/HTM interface. Steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements were carried out to probe interfacial carrier dynamics. As shown in Figure f, when the light was illuminated from Spiro-OMeTAD side, the PL intensity of perovskite films coated with BN-Ph and BN–OH was significantly quenched compared with the control, confirming enhanced hole-transfer capability. The TRPL decay curves were fitted to obtain carrier lifetimes (Table S13, see Supporting Information for details). The BN–OH-treated sample exhibited the shortest average carrier lifetime (τ_ave) of 24.99 ns, followed by BN-Ph-treated sample (τ_ave = 75.50 ns) and the control sample (τ_ave = 108.08 ns) (Figure g). The faster decay reflects more rapid carrier transport at the modified interfaces, consistent with the steady-state PL trend, and underscores the role of these BN-embedded interlayers in reducing charge recombination and improving charge-extraction efficiency.

Finally, the stability of unencapsulated PSCs was evaluated under ambient conditions at 25 °C and 30% relative humidity. The control device retained only 56.09% of its initial PCE after 480 h, in contrast, the BN–OH-treated device showed substantially improved stability, maintaining 84.32% of its initial efficiency over the same period. Notably, the BN-Ph-treated device performed even better, retaining up to 91.17% of its initial efficiency (Figure h). This enhanced stability originates mainly from the strong hydrophobicity imparted by the interfacial layers. Water contact angle measurements further support this conclusion: the films treated with BN-Ph or BN–OH exhibited larger contact angles (102.8°/87.3°) than the control film (45.8°) (Figure S56), indicating superior moisture resistance and effectively slowing humidity driven degradation. After 15 days of storage in air with 30–65% relative humidity, the control film gradually degraded, transitioning from the black perovskite phase to a yellow nonperovskite phase (Figure S57). Meanwhile, the contact angle of the control film dropped significantly from 45.8° to 18.8°, whereas the changes in films modified with BN-Ph or BN–OH were minor (Figure S56). These results clearly indicate that BN-embedded interfacial materials effectively preserve surface hydrophobicity against moisture attack. Scanning electron microscopy (SEM) images revealed that the introduction of the BN-based interlayers improved the surface morphology of the perovskite, resulting in denser surface and high-quality perovskite film (Figure S58). Atomic force microscopy (AFM) results further consolidate these observations. As shown in Figure S59, the control film displays a high root-mean-square (RMS) roughness of 30.0 nm, while films modified with BN-Ph and BN–OH exhibit reduced RMS roughness values of 18.6 and 20.3 nm, respectively, indicating the absence of large aggregates.

Conclusion

In conclusion, two C ₃-symmetric, BN-embedded molecular carbons, BN-Ph and BN–OH, were successfully synthesized via a nitrogen-directed 3-fold electrophilic borylation strategy that installed B–N bridges at the bay positions of the HBC. X-ray crystallography clearly revealed a triangular nanostructure with well-defined C–N–B–C zigzag-edged periphery. Experiments and theoretical calculations have demonstrated that incorporating polar B–N couples into the aromatic framework induces blueshifted absorption and emission, an enlarged bandgap, and unique electronic structure compared to the all-carbon analog. Although the partial localization of BN bonds diminishes the aromatic character of azaborine and its adjacent benzenoid rings, the consecutive 30π ring currents along the peripheral conjugation pathway are preserved, suggesting global aromaticity. Furthermore, these BN-embedded nanographenes have potential as fluoride-anion sensors, exhibiting sensitive response, special selectivity, and good reversibility. In perovskite photovoltaics, both BN-embedded molecules were further employed as interfacial modifiers, significantly enhancing device performance (PCE = 24.38% and 25.24%) by efficient hole extraction/transport, while also promoting operational stability due to their exceptional hydrophobicity. This work not only introduces a novel class of dual-functional, BN-embedded molecular carbons but also demonstrates a powerful synthetic strategy for accessing nanographenes with tailored edge topologies and various doping patterns. Further investigation on the development of molecular carbons containing more B–N units is currently underway in our laboratory.

Methods

Detailed information regarding the syntheses and characterizations of these compounds are provided in the Supporting Information. All general chemicals were purchased from commercial suppliers and used without further purification unless otherwise noted. The (¹H-, ¹³C-, ¹¹B-, ¹⁹F- and ¹H–¹H COSY) nuclear magnetic resonance (NMR) data were recorded on a Bruker Avance 600 MHz spectrophotometer. The high-resolution mass spectra were recorded on a Bruker Avance spectrometer (maXis). The crystal structure was recorded on a Bruker D8 Venture X-ray single crystal diffraction spectrometer. UV–vis absorption spectra were recorded on a UV–visible near-infrared spectrometer (Lambda 1050, PerkinElmer). Fluorescence spectra were performed on a Fluorolog-3 fluorescence spectrophotometer (Horiba JY, USA). The fluorescence lifetime was measured using a FLSP920 fluorescence spectrophotometer. Absolute fluorescence quantum yields were measured on a HAMAMATSU Absolute PL Quantum Yield Measurement System C9920-02G at room temperature. Cyclic voltammetry and differential pulse voltammetry were performed on a CHI 660B electrochemical analyzer. Thermogravimetric analysis measurements were conducted using a STA 409 PC instrument under a nitrogen flow. The current density–voltage curves were obtained by using a solar simulator (Enlitech, SS-F5-3A) and a Keithley 2400 SourceMeter under 100 mW cm^–2. The surface morphology of the perovskite films were characterized by using scanning electron microscopy (SU-8020, HITACHI) and atomic force microscopy (Bruker, Dimension ICON).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.6c00354.

• Details on experimental procedures; characterization data; NMR spectra; mass spectra; X-ray crystallography data; and optical spectra of all the compounds (PDF)

B.Y. completed the main synthetic experiments and performed measurements using NMR and X-ray single-crystal diffraction techniques; J.H. and Z.F. participated in partial synthesis; B.Y. performed most of the DFT calculations with help from Y.S.; B.Y. and J.H. conducted the measurement of photophysical and electrochemical properties with help from Z.F.; Y.L. and Z.L. contributed to the fabrication and characterization of PVK devices and also assisted in writing the manuscript; J.F. collected high-resolution mass spectrometry data; H.S. collected and processed single-crystal diffraction data; B.Y. and J.W. conceived the original idea and designed the whole study; B.Y. and J.L. performed the data analysis and wrote the manuscript and revised it based on feedback from Z.L. and J.W. All authors checked the manuscript and discussed the results. B.Y., Y.L., and J.H. contributed equally to this work.

The authors declare no competing financial interest.