Unlocking Red-to-NIR Luminescence via Ion-Pair Assembly in Carbodicarbene-Borenium Ions

Abstract

Achieving efficient red and near-infrared (NIR) emission in boron cation-based emitters remains challenging due to their intrinsic instability, strong electrophilicity of the boron center, and pronounced nonradiative decay governed by the energy gap law. Here we report a family of air- and moisture-stable carbodicarbene (CDC)-borabenzo[c]anthanthrenium ions exhibiting solid-state red to NIR luminescence (λ_em up to 730 nm) with competitive quantum yields. Crystallographic, photophysical, and computational analyses reveal that the CDC ligand plays a dual role by electronically stabilizing the boron center and directing ion-pair assembly via charge localization, thereby modulating exciton coupling and aggregate-state emission. These cationic π-extended boron frameworks represent rare examples of monoboron-doped luminophores displaying deep red-to-NIR emission. Our findings highlight that the combination of π-extension, a charge-directing CDC ligand, and ion-pair assembly constitutes an effective strategy to access efficient red/NIR emitters, providing guiding principles for the design of functional long-wavelength emitting main-group materials.

Graphical Abstract

Long-wavelength luminescence from boron-based cations is hindered both by intrinsic instability and by pronounced non-radiative decay. Now, a triadic design that integrates a non-Kekulé polycyclic aromatic hydrocarbon framework, a charge-localizing carbodicarbene ligand, and counterion-directed assembly enables stable borenium ions with tunable red-to-near-infrared emission in the solid state.

Introduction

Boron doping into graphene segments is a well-established strategy to tailor electronic and optical properties.^1,2 Through its strong Lewis acidity and p–π* conjugation, boron incorporation effectively tunes electron-accepting behavior and frontier orbital energies. However, despite diverse structural designs, mono-boron-doped nanographene fragments generally suffer from large energy gaps and emit predominantly in the blue to green region, due to disrupted π-conjugation or reduced radical character.³ To access red-shifted emission, multi-boron frameworks have been developed to cooperatively lower the LUMO energy.^4–7 However, deep-red or near-infrared (NIR) luminescence remains rare.^4,8–13 Such multi-boron systems typically require lengthy synthetic routes that involve low-yielding steps, and their extended π-systems frequently lead to fluorescence quenching via strong π interactions in the solid-state.⁴ Nevertheless, efficient solid-state luminescence is highly desirable for optoelectronic^14,15 and bioimaging applications.¹⁶ Consequently, a central challenge in boron-doped nanographene chemistry is to simultaneously achieve narrow energy gaps, long-wavelength emission, and high luminescence efficiency across both the solution- and solid-states.

Cationic boron frameworks present an attractive platform for accessing long-wavelength emission, as the incorporation of positive charge enhances p–π* conjugation, lowers the LUMO energy, and reduces the optical gap.^17–23 However, the strong electrophilicity of tricoordinate boron renders these species intrinsically unstable toward air, moisture.^24,25 To address this challenge, N-heterocyclic carbenes (NHCs) have been employed to kinetically and electronically stabilize low-coordinate boron centers by elevating the HOMO energy and suppressing deleterious π-stacking interactions in the solid state.^26–28 Nevertheless, such systems rarely support long-wavelength emission under ambient conditions, primarily due to insufficient charge delocalization and limited modulation of frontier orbital energies. In contrast, carbodicarbenes (CDCs)—divalent carbon(0) ligands bearing two orthogonal lone pairs—exhibit markedly enhanced σ- and π-donating ability.^{29–31,32–37} This enables both effective charge localization and robust stabilization of highly electrophilic boron centers.^38–42 Recently, we reported CDC-stabilized polycyclic borenium ions exhibiting notable air and moisture stability with tunable photophysical properties.^43–45 However, emission beyond 660 nm or in the NIR region remains difficult to achieve, owing to persistent thermodynamic instability and potential solvent activation behavior associated with frustrated Lewis pair-like interactions between the strongly donating CDC and the boron center.^46,47 Moreover, the energy gap law^48,49 imposes an inherent trade-off between emission efficiency and energy gap narrowing, rendering the design of red/NIR-emissive, air-stable boron cations an unresolved challenge in main-group luminescent materials chemistry.

In a prior report,⁵⁰ we showed that an NHC-stabilized boron-doped phenalenyl cation—the smallest non-Kekulé polycyclic aromatic hydrocarbons (PAHs) derived from graphene—exhibited a modest red-shift in the crystalline state, suggesting that supramolecular organization may mitigate energy gap limitations. However, simultaneous realization of long-wavelength emission and ambient stability remains challenging. To address this, we devised a triadic design strategy integrating three structural components. First, a π-extended, non-Kekulé nanographene scaffold—11-borabenzo[c]anthanthrene (Fig. 1a)—was selected to support mono-borenium ion formation while compressing the optical gap (Supplementary Fig. 47). Second, a sterically demanding CDC ligand was employed to provide strong σ/π donation and suppress co-facial π–π stacking interactions known to quench solid-state emission.⁵¹ Third, the counteranion was recognized as a key variable: beyond stabilizing the reactive boron center and preventing side reactions,^52–54 it exerts a profound influence on solid-state packing and excited-state dynamics.^55,56 Electrostatic potential analyses from previous studies revealed a region of positive charge density localized on the CDC ligand,⁴³ indicating that the CDC can act as an ‘electrostatic director’ to guide ion-pair alignment. We therefore hypothesized that tuning the counterion identity would enable systematic control over the anisotropic orientation of the cationic π-framework, thereby modulating exciton coupling and aggregate-state emission behavior (Fig. 1a).

Fig. 1. Design and synthesis of carbodicarbene–borabenzo[c]anthanthrenium ions.

a, Conceptual design of carbodicarbene–borabenzo[c]anthanthrenium ions (with numbered carbon atoms), illustrating the dual roles of carbodicarbene ligand coordination and counterion engineering in directing ion-pair assembly and modulating red-to-near-infrared (NIR) luminescence. b, Synthetic scheme of 11-borabenzo[c]anthanthrenium ions 1 and neutral 11-borabenzo[c]anthanthrene 10. MesMgBr, 2,4,6-trimethylphenylmagnesium bromide.

In this study, we report the synthesis, structures, and photophysical properties of air- and moisture-stable carbodicarbene-borabenzo[c]anthanthrenium ions (1). Relative to their neutral analogue, these cations exhibit markedly red-shifted luminescence in solution. Notably, exchanging the counterion allows systematic tuning of the solid-state emission from deep red to NIR wavelengths (λ_em up to 730 nm), a rare feature among π-extended boron-based frameworks, which typically suffer from aggregation-induced quenching. Single-crystal X-ray diffraction and quantum chemical calculations reveal that counterion identity governs intermolecular excitonic coupling and frontier orbital energies, thereby controlling the optical gap and solid-state emission characteristics. Collectively, these results establish a synergistic design strategy that combines a non-Kekulé π-extended PAH scaffold, a charge-localizing and sterically protective CDC ligand, and counterion-directed supramolecular assembly. This integrated approach provides a general framework for constructing stable, long-wavelength-emissive boron-based materials with tunable optoelectronic properties.

Results and Discussion

Synthesis and structural characterizations

The molecular design of a laterally π-extended zigzag-edged graphene fragment comprising seven fused rings was inspired by the efficient synthesis of a benzo[c]anthanthrenyl radical.⁵⁷ Boron was doped at the 11-position, a known spin-residing site, to suppress radical character and enable the formation of a genuine borenium ion. The rigid polycyclic framework imposes geometric constraints around the boron center, while the sterically exposed 11-position allows effective coordination by a bulky CDC ligand. To enhance solubility, tert-butyl groups were installed at the 1,9-positions of the π-scaffold. A key intermediate in the synthesis of the CDC-stabilized borabenzo[c]anthanthrenium ions 1 is 11-chloro-11-borabenzo[c]anthanthrene (11) (Fig. 1b). The synthesis commenced with a Suzuki coupling between 1,3-dibromo-5-(tert-butyl)-2-iodobenzene (2) and pyrenylboronate (3) to give compound 4. Selective lithiation and formylation of 4 yielded 5, which underwent a Wittig reaction to furnish vinyl ether 6. Without isolation, 6 was directly cyclized to 7 under Bi(OTf)₃ catalysis.⁵⁸ All intermediates (4–7) can be purified by recrystallization, enabling gram-scale preparation. A one-pot lithiation–transmetalation–electrophilic borylation sequence regioselectively installed boron at the bay region, affording 9 after aqueous work-up and chromatography. Treatment of 9 with BCl₃ furnished compound 11 in an overall linear yield of 17% from intermediate 2 and 3.Coordination of CDC ligand L⁵⁹ with 11 afforded the cationic complex 1^Cl, which underwent salt metathesis with LiSbF₆, NaBPh₄, and NaBAr^F₄ to yield 1^SbF₄, 1^BPh₄, and 1^BArF₄, respectively. The ¹H NMR spectra of 1 exhibits fluxional behavior at room temperature,^43,60 while peak coalescence at elevated temperatures indicates the presence of a single species in solution. The ¹¹B{¹H} NMR signals corresponding to the cationic component in 1 were observed at 54.0–56.8 ppm. Despite their electrophilic nature,²⁴ all derivatives of 1 are stable to air and moisture and handled outside of the glovebox without special precautions. The neutral analogue 10 was also prepared following the aforementioned procedures by treatment of a mixture containing intermediate 8 generated from 7 with MesMgBr.

Single crystals of 1^SbF₄, 1^BPh₄, and 1^BArF₄ were obtained as single polymorphs by slow evaporation from hexanes/o-difluorobenzene (o-DFB) mixtures, whereas crystals of neutral compound 10 were grown from DCM/MeOH. X-ray diffraction analyses unambiguously confirmed the molecular structures of all species except 1^Cl, which yielded crystals unsuitable for analysis due to weak diffraction (Supplementary Figs. 40 and 41). In all cases, the cationic boron centers showed no contact with their counteranions. The heptacyclic frameworks are very similar and adopt quasi-planar structures, influenced by the size of the counteranion. The calculated root-mean-square (RMS) deviation of the mean borabenzo[c]anthanthrene plane fitting displays a trend of 1^SbF₄ (0.147 Å) <1^BPh₄ (0.252 Å) <1^BArF₄ (0.353 Å), likely reflecting counteranion-dependent packing effects. In comparison, the neutral analogue 10 shows an RMS deviation of 0.097 Å, consistent with a nearly coplanar framework. Given that these distortions align with previously observed CDC-induced non-planarity,⁴⁵ detailed geometric analysis was not discussed further. Instead, subsequent sections address how crystal packing and counterion identity influence solid-state photophysical properties.

Electronic Structures

Geometry optimization of 1⁺ (excluding counterions) at the B3LYP-D3BJ/TZVP level with a PCM (CH₂Cl₂) solvent model yielded a structure consistent with X-ray data, including the orientation of the CDC ligand and planarity of the PAH core. Frontier orbital analysis reveals that 1⁺ is formally isoelectronic with the benzo[c]anthanthrenyl π-dication, as reflected in the similarity between its LUMO and the HOMO of 10 (Fig. 2a and Supplementary Fig. 48). Consistently, cyclic voltammetry demonstrates that the lower LUMO energy level of 1 (−3.07 eV) relative to 10 (−2.80 eV) (Supplementary Figs. 72 and 73; Supplementary Table 14). Bonding analyses indicate that the ^carboneC–B interaction in 1⁺ is dominated by CDC→B⁺ σ-donation, with modest π-contributions (Supplementary Figs. 49−51). Fragment-based natural population analysis (NPA) reveals that the CDC ligand bears the majority of the positive charge (+0.748 a.u.), with minor localization on peripheral PAH hydrogens. This is consistent with electrostatic potential (ESP) surface distribution analysis, which shows that the positive potential is concentrated on the CDC moiety (Fig. 2b and Supplementary Fig. 52). Consequently, counterions preferentially align near the CDC rather than the boron center, as confirmed by X-ray data (Supplementary Figs. 41 and 42). These findings underscore the CDC’s active role in directing charge distribution and supramolecular organization (vide infra).

Fig. 2 |. Electronic Structures of 1⁺.

a, LUMO of 1⁺ (left) and HOMO of 10 (right) computed at the PCM(CH₂Cl₂)-B3LYP-D3(BJ)/def2-TZVP//B3LYP-D3(BJ)/TZVP level. b, The molecular area statistics (in percentage) of ESP distribution over the surfaces of 1⁺ (counteranion excluded) across different ESP ranges, with the borabenzo[c]anthanthrene core represented by the green column and the CDC ligand by the blue column. c, 2D-NICS(1)_zz plot (in ppm) of the magnetic shielding of 1⁺ in the xy-plane situated 1 Å above the molecular plane, calculated at the GIAO-B3LYP-D3BJ/6–311G+(2d,p) level of theory; the Roman numerals denote different hexagonal rings. d, The ACID plots (π orbital contributions only) of 1⁺ (isovalue: 0.03). The magnetic field is perpendicular to the heptacyclic molecular plane (xy plane) and points out through the paper. The green band indicates the most global tropicity of the diatropic ring current; the AV1245 and AV_min values along the green (global) and red (local) π-conjugation pathways are indicated in green and red, respectively. e, The EDDB_H(r) function plot for 1⁺ (isovalue: 0.015).

To further evaluate the electronic delocalization and aromaticity in compounds 1 and 10, nucleus-independent chemical shifts (NICS) and anisotropy of the induced current density (ACID) calculations were performed. The 2D-NICS map (1 Å above the molecular plane) reveals pronounced local aromaticity in rings I, IV, and VI, moderate aromaticity in rings III, V, and VII, and significantly reduced aromaticity in the boron-containing ring II (Fig. 2c), consistent with bond length alternation in the solid-state. To quantify the π-aromaticity of ring II, ∫NICS_π,zz values^61,62 were calculated, yielding a more negative value for 1 (−33.57) than for 10 (−15.44), indicating enhanced aromaticity in 1 (Supplementary Fig. 59). ACID plots show two clockwise current circuits in 1: one traversing the periphery (green-colored band) and another circumventing the boracycle (red-colored band) (Fig. 2d). By contrast, in 10, the global ring current avoids the boron center (Supplementary Fig. 60). AV1245 and AV_min indices^63–65 further support that both current pathways are active in 1, whereas conjugation in 10 is confined to the benzenoid rings. These features suggest that 1 retains electronic characteristics reminiscent of the benzo[c]anthanthrenyl radical,⁵⁷ with π-delocalization extending along the periphery (Supplementary Fig. 61). The electron density of delocalized bonds [EDDBH(r)] analysis⁶⁶ confirms three-dimensional π-conjugation in 1, revealing continuous delocalization across the CDC moiety and boron-doped PAH via the boron p_z orbital (Fig. 2e). These theoretical insights reinforce the design rationale: the CDC ligand serves as a charge-localizing donor interacting electronically with the PAH core, while the non-Kekulé PAH core maintains extensive π-delocalization. The convergence of these features provides a coherent electronic basis for the observed photophysical properties.

Photophysical Properties in Solution

The optical properties of compound 1 was first examined in CH₂Cl₂ solution (20 μM). Regardless of the counteranions, all samples show an intense absorption peak at around 392 nm (logε ~4.8) and two weaker shoulder peaks around 500 and 538 nm (logε ~ 4.2) (Supplementary Fig. 30), indicating minimal counterion influence on solution-state absorption. Time-dependent density-functional theory (TD-DFT) calculations assign the lowest-energy absorption to the S₀ → S₁ (the first singlet excited state) (HOMO → LUMO) transitions with oscillator strength (f) of 0.1977, and the second-lowest absorption to the S₀ → S₂ (HOMO-1 → LUMO) transition with comparable oscillator strength (f = 0.1569) (Supplementary Fig. 64 and Supplementary Table 3). The optical energy gap (E_opt) is ~2.13 eV (λ_onset = 582 nm). Fluorescence spectra show broad, structureless emission [λ_{em, max} = 609–612 nm, full-width-at-half-maximum (FWHM) = ~95 nm] with comparable absolute quantum yields (QYs) (0.43 to 0.48) and lifetimes (~10–11 ns) across all samples (Supplementary Table 1), confirming negligible counterion effects in solution. Compound 10 displays pronounced vibronic structures in the absorption spectrum (450–500 nm), with well-resolved 0–0 and 0–1 bands at ~500 and 465 nm, respectively (Supplementary Fig. 31). Compound 10 emits at 519 nm with near-unity QY (0.98). The small Stokes shift (769 cm⁻¹) and vibronic structure are consistent with a rigid molecular scaffold. These results underscore the excellent fluorophore characteristics of the borabenzo[c]anthanthrene core and the significant effect of boron doping on optical properties.

Upon addition of nonpolar hexanes to a CH₂Cl₂ solution (25 μM) of 1^Cl, with a CH₂Cl₂/hexanes ratio of 5:95 (v/v, f_hex = 95%), the resulting colloidal suspension exhibits a broad, red-shifted absorption with resolved vibronic shoulder bands (Fig. 3a). This spectral evolution suggests Davydov splitting,⁶⁷ reflecting distinct ground states from weak rotational excitonic interactions and polarizability effects.^68,69 Long, level-off tails in the spectra are attributed to Mie scattering^70,71 caused by the large size of the borenium ions in the colloidal suspension. The emission profiles are broad (FWHM > 150 nm) and substantially red-shifted (λ_em = 651–677 nm) relative to solution-state fluorescence (Fig. 3b), likely due to disordered aggregates coexisting with discrete molecular species. Similar absorption trends are observed for 1^SbF₄ and 1^BPh₄, though with more pronounced scattering, presumably due to larger ion-pair sizes and greater aggregation. The counteranion significantly affects emission maxima (λ_{em, max} for 1^SbF₄ and 1^BPh₄ ranged from 642 to 668 nm and 632 to 664 nm, respectively, Supplementary Figs. 32–33). The distinct spectral differences from 1^Cl to 1^BPh₄ underscore the counterion’s role in modulating molecular organization in aggregates. By contrast, aggregates of 10 in CH₂Cl₂–hexanes (f_hex = 50–95%) show negligible spectral shift aside from a weakened absorption band, likely due to precipitation. Emission spectra evolve into broad, structureless bands (λ_em ≈ 560 nm) with reduced intensity, indicative of excimer formation driven by aggregation.

Fig. 3 |. Solution-phase and temperature-dependent photophysical properties.

a, Absorption spectra of 1^Cl (25 μM, 25 °C). b, Fluorescence spectra of 1^Cl (25 μM, 25 °C). c, Absorption spectra of 10 (25 μM, 25 °C) as a function of hexanes fraction (f_hex) in CH₂Cl₂. d, Fluorescence spectra of 10 (25 μM, 25 °C) as a function f_hex in CH₂Cl₂. e, Temperature-dependent absorption spectra of 1^BArF₄ (25 μM) in CH₂Cl₂/hexanes (1:9, v/v) binary solvent. f, Temperature-dependent fluorescence spectra of 1^BArF₄ (25 μM) in CH₂Cl₂/hexanes (1:9, v/v) binary solvent. g, Plot and non-linear curve fitting of the fluorescence intensity ratio (I_{585 nm}/I_{638 nm}) for 1^BArF₄ as a function of temperature. h, Temperature-dependent relative sensitivity (S_r, in % °C⁻¹) plot of 1^BArF₄. S_r = 100% × [∂(I_{585 nm}/I_{638 nm})/∂T]/(I_{585 nm}/I_{638 nm}). i, Reversible change in fluorescence intensity ratio (I_{585 nm}/I_{638 nm}) of 1^BArF₄ observed over four temperature cycles between 20 and −90 °C. Inset in b and d, Photographs of the respective colloidal solution under 365 nm UV light irradiation. Inset in f, Fluorescent photographs of cuvettes filled with 1^BArF₄ at 20 and −90 °C in CH₂Cl₂/Hex (1:9, v/v).

Colloidal suspensions of 1^BArF₄ could not be formed due to its high solubility in hexanes (up to 60 μM) (Supplementary Fig. 34). To assess aggregation behavior, its temperature-dependent optical properties were examined in a CH₂Cl₂/hexanes binary solvent mixture (1:9, v/v). As shown in Fig. 3e, negligible red-shifts were observed above −70 °C. At lower temperatures, a broad low-energy absorption band with a level-off tail emerged, indicating increased aggregation. The blue-shifted emission maximum at 585 nm arises from solvatochromism in the less polar medium (Supplementary Fig. 35). From 30 °C to −60 °C, emission intensity gradually weakened and red-shifted, due to the increasing dielectric constant upon cooling (Fig. 3f). At approximately −70 °C, an abrupt enhancement in intensity and red-shift to 638 nm was observed, consistent with pronounced aggregate formation. Notably, this process induced a visible color change from bright yellow to red, with CIE coordinates shifting from (0.56, 0.44) to (0.67, 0.33), corresponding to red emission at −90 °C (Fig. 3f and Supplementary Fig. 36). A strong exponential correlation (r² = 0.9988) between the intensity ratio (I_{585 nm}/I_{638 nm}) and temperature over a wide range of 20 °C to −90 °C (Fig. 4g), indicating that the fluorescence spectrum can be used for direct temperature readout. The maximum relative sensitivity (S_r)⁷² was determined to be 3.78 % °C⁻¹ at −90 °C (Fig. 3h), surpassing the sensing range and maximum S_r of previously reported small-molecule thermometers for cryogenic temperatures.^73,74 Moreover, the temperature-dependent emission is fully reversible (Fig. 3i and Supplementary Fig. 37), highlighting the system’s utility as a visual, ratiometric cryogenic sensor. In contrast, 1^BArF₄ showed weak temperature responsiveness (Δλ_em < 20 nm), as aggregation was effectively suppressed in good solvent (Supplementary Fig. 38). Similarly, in polystyrene thin film (1 wt% 1^BArF₄), where molecular motion is restricted, only minimal spectral shifts (Δλ_em < 10 nm) were observed upon cooling (Supplementary Fig. 38). Taken together, this low temperature sensitivity in 1^BArF₄ stems from enhanced aggregation-induced exciton interactions. The temperature-responsive emission behavior is enabled by weakly coordinating anions, underscoring the counterion as a critical variable for tuning excited-state interactions and luminescence output.

Fig. 4. |. Photophysical properties in the solid state.

a-d, UV−vis absorption spectra of annealed spin-coated thin films and solutions (CH₂Cl₂, 20 μM), along with normalized fluorescence spectra of 1^Cl (a), 1^SbF₄ (b), 1^BPh₄ (c), and 1^BArF₄ (d) in solutions (CH₂Cl₂, 20 μM), pristine microcrystals, annealed spin-coated thin films, and amorphous powders, as indicated in the legend. Inset in a–d: Photographs of the corresponding ground powders under ambient light. e, Microscopy images of pristine crystals of 1 and 10 under ambient light. f, Fluorescence microscopy images of the same crystals under 365 nm UV light. From left to right: 1^Cl, 1^SbF₄, 1^BPh₄, 1^BArF₄, and 10.

Photophysical Properties in the Solid-State

The high stability of the molecular solids enabled thorough photophysical studies of compound 1 under ambient conditions, including pristine microcrystals, annealed films, and ground powders. The annealed spin-coated films exhibited a characteristic ‘double-hump’ absorption profile (450–600 nm) featuring both blue- and red-shifted components, and scattering-induced tails extending around 700 nm (Figs. 4a–d), resembling the spectra of colloidal suspensions. The principal high-energy band at ~392 nm remained unchanged. The absorption edges for 1^Cl, 1^SbF₄, 1^BPh₄, and 1^BArF₄ were estimated at 631, 616, 622 and 596 nm, respectively, clearly reflecting counterion-dependent changes in the ground-state electronic structure. Accordingly, the E_opt decreases in the order: 1^Cl <1^SbF₄<1^BPh₄<1^BArF₄. Visible differences in powder and crystal color under ambient light further support this solid-state tunability (Figs. 4a–e). By contrast, the film of neutral compound 10 showed minimal absorption shifts relative to its solution spectrum, suggesting an unchanged ground-state electronic structure (Supplementary Fig. 39).

The solid-state fluorescence spectra of 1^Cl, 1^SbF₄, 1^BPh₄, and 1^BArF₄ span the red to NIR regions, whereas neutral 10 emits in the yellow region (Table 1, Fig. 4f and Supplementary Fig. 40). In contrast to their similar solution-state emissions, the powders exhibit notable bathochromic shifts: 0.33 eV, 0.23 eV, 0.16 eV, and 0.03 eV for 1^Cl, 1^SbF₄, 1^BPh₄, and 1^BArF₄, respectively. While the fluorescence of crystals and powders is largely featureless, spin-coated films display broader emission profiles with subtle shoulders (Figs. 4a–d), attributed to heterogeneous microenvironments and varying aggregation states. To probe the structural basis for these emission differences, powder X-ray diffraction (PXRD) analyses reveal differences in crystallinity after grinding, ranging from complete amorphization in 1^SbF₄, partial short-range order in 1^BPh₄, to retention of high crystallinity in 1^BArF₄ (Supplementary Figs. 45 and 46). These differences reflect the underlying structural complexity of the series and underscore the tunability of solid-state emission through morphological and supramolecular control. The absolute fluorescence QYs of 1^Cl, 1^SbF₄, and 1^BPh₄ are respectable given the intrinsic efficiency challenges in this spectral range (λ_em > 660 nm). This study presents a rare class of boron cations exhibiting deep-red solid-state luminescence (662–696 nm) with quantum yields of 16–35% and notable NIR efficiencies (~10% at λ_em > 700 nm), surpassing previously reported neutral multi-boron-doped PAHs,^4–9 which either lack emission data or exhibit drastically lower efficiencies in the solid-state. In contrast, crystals of 10 suffer from aggregation-caused quenching (QY ~8%), while ground powders exhibit partial recovery to ~50% QY with blue-shifted emission. PXRD confirms partial crystallinity retention (Supplementary Fig. 46), suggesting that grinding reduces nonradiative π–π stacking.⁵¹ While 1 shows monoexponential decays in solution, all solid samples exhibit biexponential decays (Table 1), implying multiple emissive species. The shortened fluorescence lifetimes in films likely result from Förster resonance energy transfer (FRET) to aggregated species, supported by significant UV–vis/fluorescence spectral overlap (Figs. 4a–d). These results reveal a pronounced difference in photophysical behavior between the solid- and solution-states, indicating the resulting properties predominantly arise from significant electronic or excitonic interactions strongly influenced by the aggregation states.

Table 1.

Fluorescence properties of pristine microcrystals, annealed spin-coated thin films and ground powders for 1 and 10.

Compound	λem/nm[a]	ΦF, cry[a]	τ [Rel.]/ns [%] [a]	λem/nm[b]	ΦF, film[b]	τ [Rel.]/ns [%] [b]	λem/nm[c]	ΦF, powder[c]	τ [Rel.]/ns [%] [c]
1Cl	712	0.120	3.59 [79] 5.80 [21]	701	0.099	2.05 [85] 4.42 [15]	730 d	0.071 d	2.13 d
1SbF6	696	0.188	0.52 [4] 4.22 [40] 5.31 [56]	668	0.205	1.91 [30] 4.66 [63] 41.26 [7]	687	0.164	0.49 [6] 0.24 [20] 3.60 [77]
1BPh4	664	0.331	0.66 [8] 4.61 [42] 10.16 [50]	672	0.183	3.12 [47] 6.11 [49] 50.10 [4]	662	0.352	1.03 [8] 5.51 [60] 9.30 [32]
1BArF4	624	0.114	1.29 [22] 3.40 [56] 8.83 [22]	646	0.334	2.39 [19] 6.02 [72] 11.04 [9]	618	0.193	0.43 [18] 3.31 [47] 7.68 [35]
10	579	0.078	3.01 [82] 5.64 [18]	588	0.315	2.53 [15] 6.87 [64] 12.67 [21]	569	0.481	8.62

^[a]Pristine microcrystals.

^[b]Annealed spin-coated thin films on quartz substrates from CH₂Cl₂ (0.02 M) and annealed for 30 min @150 °C under Ar.

^[c]Grinding powders of pristine microcrystals. Effective quantum yield without correction for reabsorption, error < 2%.

^[d]Amorphous powders by precipitation from o-DFB/hexanes mixture. Rel.: amplitude percentage.

Crystal Packing–Optical Property Relationships

Single-crystal analyses and quantum chemical calculations elucidate how supramolecular assembly dictates solid-state optical behavior. The packing structures of 1^SbF₄, 1^BPh₄, and 1^BArF₄ are governed by the counteranion, the steric demand of the CDC ligand, and o-DFB solvent inclusion. In 1^SbF₄, the isopropyl substituents of the CDC ligand effectively shield both π-faces of the PAH core, precluding face-to-face π-stacking and extending intermolecular π-distances beyond 7 Å. o-DFB molecules promote a herringbone-like arrangement (c1/c2 and c3/c4, Fig. 5a), resulting in a charge-segregated packing motif along the b-axis (Supplementary Fig.43)—a rare feature among ionic π-conjugated systems.^75,76 This structure is stabilized by electrostatic interactions between the cationic CDC and peripheral SbF₆⁻ counterions, as revealed by ESP mapping (Supplementary Fig. 54), and further reinforced by C–H···π contacts (~2.7 Å) between adjacent units. Along the c-axis, adjacent dimers (c1/c3 and c2/c4) exhibit parallel longitudinal slips (~14°) with interplanar distances of 4.23 Å, indicative of weak J-aggregation (Fig. 5a). Although the non-parallel orientation of the S₀→S₁ transition dipole moments (Supplementary Fig. 62) complicates direct exciton coupling analysis, red-shifted solid-state absorption spectra correlate with the observed bathochromic emission.

Fig. 5 |. Supramolecular packing, exciton coupling, and intermolecular interaction analysis.

a–c, Selected dimeric motifs of 1^SbF₄ (a), 1^BPh₄ (b) and 1^BArF₄ (c) extracted from the crystal structures (cation labels: c1-c4; anion labels: a1–a5, solvent molecules and counteranions in 1^BPh₄ were removed for clarity) showing Coulombic couplings (in blue double head arrows) and short-range coupling (in red double head arrows). The exciton coupling strength (in cm⁻¹) are shown in deep purple and red numbers, respectively. Crystal unit axes are given. ‘A⁻’ represents BAr^F₄⁻counterion. d, One-electron transition-density matrix and the corresponding isosurfaces of the hole (in green) and electron (in red) distributions for the first excited state of dimeric 1^BArF₄. ‘m₁’ and ‘m₂’ denote monomer 1 and monomer 2, respectively. Charge-transfer (CT) and participation ratio (PR) values are indicated. e, f, Interaction energy decompositions for 1^SbF₄ (e) and 1^BArF₄ (f) were determined via PIEDA (pair interaction energy decomposition analysis) performed at the FMO2-RI-MP2/ma-TZVP level of theory. Energies are denoted as follows: electrostatic (E_es), exchange-repulsion (E_rep), dispersion (E_disp), charge-transfer and higher-order mix terms (E_CT+mix), and total energy (E_total). PIEDA calculations were based on the crystal-state packing of 1^SbF₄ and 1^BArF₄, with the corresponding neighboring interaction fragment labels depicted in panels a and c, respectively.

To elucidate the packing–property correlation, discrete dimeric units from the crystal structures were analyzed. For 1^SbF₄, weak J-type Coulomb exciton coupling (J_Coulomb) was observed for both longitudinally slipped (−24.5 cm⁻¹) and edge-to-face dimers (−17.5 cm⁻¹). Contributions from charge transfer coupling (J_CT) were negligible due to minimal charge transfer integrals (hole transfer integral t_h = 180.1 cm⁻¹ and electron transfer integral t_e = 32.5 cm⁻¹)) and a large energetic gap between FE and CT states (Supplementary Table 5), consistent with a negligible J_CT value (J_CT = 0 cm⁻¹). Thus, the photoabsorption of aggregates 1^SbF₄ will exhibit a red-shifted spectrum due to the negative sign of J_Coulomb. However, it should be noted that the actual orientation of transition dipoles for the monomer is neither coplanar nor parallel within the two- and three-dimensional packing regimes. Thus, rotational or oblique interlayer interactions may cause resonant band splitting,⁶⁷ which would explain the Davydov splitting found in the colloidal aggregates and films (Fig. 3a and 4b). In 1^BPh₄, long-axis slipped dimers exhibit even weaker coupling (J_Coulomb = −5 to −8 cm⁻¹) owing to increased interplanar distances (> 4 Å), suppressing efficient exciton delocalization (Fig. 5b). In contrast, compound 1^BArF₄ features antiparallel π-stacked dimers to cancel the net permanent molecular dipole moment (c1/c2, Fig. 5c and Supplementary Fig. 62). Owing to the short interchromophoric π distances and strong π interactions (Supplementary Fig. 55), the J-type short-range coupling in the face-to-face dimer of 1^BArF₄ is −22.5 cm⁻¹, while the long-range Coulomb coupling is calculated to be −76.9 cm⁻¹. However, these analyses do not support the minor red-shifted phenomenon in the absorption spectra (Fig. 5d). It is noteworthy that a configuration mixing between the FE states (contributed by Coulombic interactions) and charge transfer states could lead to an excimer state.^77,78 The nature of different excited-states of dimeric 1^BArF₄, in both their Franck–Condon geometry and S₁-state geometry, was evaluated using fragment-based excited-state analysis (Supplementary Table 6 and 8).⁷⁹ The obtained charge-transfer value (CT) of the first singlet excited state (S₁) in the S₁ geometry of the dimer is 0.35, coupled with a participation ratio (PR) value of 1.45. These results indicate strong exciton interactions between the monomeric 1^BArF₄, consistent with excimer formation (PR > 1.25 and 0.2 < CT < 0.8).^80,81 As shown in Fig. 5d, the distributions and isosurfaces of holes and electrons clearly demonstrate that both the electron–hole and excited-electron densities of one monomer can reside either within the same unit or delocalized over two monomeric units, implying excimer character. Notably, 10 also exhibits excimeric features, with J_Coulomb and J_CT up to −192 and −173 cm⁻¹, respectively, consistent with its aggregation-induced luminescence profile (Supplementary Table 9 and Supplementary Fig. 66). These findings align with the negligible changes observed in the absorption spectra in the aggregation states of 1^BArF₄ and 10 (Figs. 3d and 4d, Supplementary Fig. 39), as excimer related transitions are primarily governed by excited-state dynamics and do not significantly alter ground-state electronic absorption features.⁸²

Coordination of the CDC ligand induces longitudinal displacements of the PAH core, promoting J-type excitonic coupling via lateral offsets (for 1^SbF₄ and 1^BPh₄) and herringbone arrangements (for 1^SbF₄), while minimizing π–π stacking. Pair interaction energy decomposition using the FMO2-RI-MP2/ma-TZVP method reveals that counterion–CDC electrostatics dominate cation–cation assembly in 1^SbF₄ and 1^BPh₄ (Fig. 5e and Supplementary Figs. 67–68). The small SbF₆⁻ counterion bridges adjacent CDC units through strong electrostatics (a5/c1 and a5/c3, Fig. 5e), shortening interchromophore distances and enhancing coupling. In 1^BPh₄, weaker electrostatics are offset by dispersion forces and repulsions from BPh₄⁻ anion (Supplementary Table 11), with resultant J-type arrangements boosting oscillator strengths (Supplementary Fig. 70) and enabling solid-state emission from deep red to NIR, overcoming energy gap law limitations.^83,84 By contrast, the bulky BAr^F₄⁻ counterion in 1^BArF₄ weakens cation–cation interaction (c1/c2, E_total = −6.8 kcal/mol, Fig. 5f) via strong dispersion (E_disp = −52.71 kcal/mol) and diminished electrostatic forces (a1/c1, a1/c2, and c2/a3, Fig. 5f), leading to antiparallel stacking and reduced QY via excimer formation. Compound 10, lacking CDC ligand shielding, forms cofacial π-dimers with strong dispersion-driven attraction (E_total = −50.44 kcal/mol). In both 1^BArF₄ and 10, excimers may either relax geometrically (Supplementary Fig. 71) or act as exciton traps, lowering emission efficiency. While exciton coupling explains spectral shifts, additional factors such as exciton–vibration coupling, energy transfer, singlet fission, triplet-state involvement, and symmetry-breaking charge transfer also influence fluorescence performance.⁸⁵ The tunable interplay between J-aggregation and excimer formation in 1 provides a mechanistic basis for optimizing solid-state photophysics. CDC ligands play a central role in governing supramolecular assembly and exciton behavior via counterion-directed interactions, enabling rational control over packing and nonradiative decay.

Conclusion

This study demonstrates that a single cationic boron center, embedded in a π-extended 11-borabenzo[c]anthanthrene scaffold and stabilized by a carbodicarbene ligand, enables efficient solid-state deep-red to NIR emission under ambient conditions—a rare achievement for boron-doped PAH systems. The resulting cationic boron species display tunable emission across solution, amorphous, crystalline, and colloidal states, while retaining exceptional thermal and oxidative stability. Structural and computational analyses reveal that the CDC ligand not only stabilizes the borenium center electronically, but also directs supramolecular assembly via localized charge and counterion-mediated electrostatic interactions. This triadic design strategy, combining a non-Kekulé π-scaffold, a charge-localizing donor, and counterion-directed packing, offers a modular blueprint for constructing long-wavelength-emissive main-group chromophores. The solid-state luminescence properties of these systems extend across powders, amorphous solids, polymer matrix-based films, and amphiphilic polymer-encapsulated colloids, highlighting potential applicability across a range of practical platforms. Future work will focus on extending π-conjugation and incorporating multi-boron motifs to further enhance excitonic coupling and expand utility in optoelectronics, bioimaging, and stimuli-responsive systems.

Methods

Details of experimental procedures, analytical data, and X-ray structure determinations as well as computational procedures are provided in the Supplementary Information.

Synthetic methods

All the experiments were carried out under dry oxygen-free nitrogen using standard Schlenk techniques or in an Argon-filled glove box (MBRAUN LABmaster) unless otherwise stated. Solvents were dried and degassed by standard methods. Bis(1-isopropyl-3-methyl-benzimidazol-2-ylidene)methane (L),⁵⁹ compound 2,⁸⁶ and 3⁸⁷ was prepared according to literature procedures. All other chemicals were purchased from Millipore Sigma or Ambeed and used as received. All the experimental protocols are provided in the Supplementary Information and NMR spectra are shown in Supplementary Figs. 1–25.

Spectroscopic methods

The NMR spectra were collected on Bruker Advance III 400 MHz, Bruker Advance III 500 MHz, spectrometers. High resolution mass spectrometry (HRMS) data were collected at Massachusetts Institute of Technology using a JEOL AccuTOF 4G LC-plus equipped with an ionSense DART (Direct Analysis in Real Time) source. Quantitative elemental analyses were carried out using a Perkin Elmer 2400 Series II Instrument equipped with a CHNS combustion analyser. UV-visible spectra were obtained using an Agilent Cary 60 UV-vis spectrometer. All the steady-state spectra were obtained using an Agilent Cary Eclipse Fluorescence spectrophotometer equipped with the photomultiplier tube (PMT) detector. Absolute fluorescence quantum yields were determined using a Hamamatsu C11347–11 Quantaurus-QY equipped with a temperature control sample holder. Time-correlated single photon counting (TCSPC) was obtained using an Edinburgh Instruments FS-5 spectrofluorometer equipped with pulsed LEDs at 340 nm. Temperature-dependent optical measurements were conducted on a UNISOKU CoolSpeK USP-203-B (for Agilent instruments) or Oxford Instruments Cyrostat (for the FS-5). Polycrystalline thin films of 1 and 10 were prepared by spin coating filtered (PTFE, 0.45 μm pore size) solutions in CH₂Cl₂ onto quartz substrates at 4500 rpm (60 s) using an Laurell WS-650–23B spin coater (Laurell Technologies). Polystyrene (PS) thin films were prepared by mixing 1 wt% sample and PS in CH₂Cl₂ solution, followed by spin-coating on a quartz substrate.

X-ray diffraction (XRD) methods

Single-crystal X-ray diffraction data were collected on a Bruker D8 Venture Photon III Kappa diffractometer equipped with a Cu Kα source (λ = 1.54178 Å) and HELIOS monochromator. Data integration and absorption correction were performed using the Bruker SAINT⁸⁸ and the Multi Scan method (SADABS),⁸⁹ respectively. Structures were solved and refined using the Bruker SHELXTL Software Package⁹⁰ within APEX4⁸⁸ and OLEX2.⁹¹ Non-hydrogen atoms were refined anisotropically; hydrogen atoms were placed in calculated positions. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre. Additional details for data processing and structure refinement are given in the Supplementary Information and ORTEP-style illustrations of 1 and 10 are shown in Supplementary Figs. 41–42. Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker Advance II diffractometer (Ni-filtered Cu Kα radiation) in θ/2θ geometry. Samples were mounted on zero-background silicon holders. PXRD plots were provided in Supplementary Figs. 45–46.

Electrochemical experiments

All cyclic voltammetry (CV) experiments were conducted using a 3-electrode geometry using a WavePico Wireless potentiostat inside the argon-filled glove box. Electrolyte solutions (0.1 M) were prepared from anhydrous, deoxygenated THF and anhydrous Bu₄NPF₆. The working electrode was a glassy carbon electrode (3-mm diameter), with a Pt-coil counter electrode and an Ag/AgCl reference. Sample concentrations were ca. 1 mM. The ferrocene/ferrocenium (Fc/Fc⁺) couple was used as an internal standard. The CV diagrams of 1 and 10 are shown in Supplementary Figs 72–73.

Computational methods

Geometry optimizations for 1⁺ and 10 were carried out using Gaussian 16⁹² at the B3LYP-D3(BJ)^93,94/TZVP level with CPCM solvation (CH₂Cl₂).⁹⁵ Vibrational frequency calculations confirmed the nature of the stationary points. Unless otherwise stated, all wavefunction analyses were carried out at the B3LYP-D3(BJ)/def2-TZVP level of theory. Electrostatic potential (ESP) surfaces were computed at the B3LYP-D3(BJ)/def2-SVP level based on crystallographic coordinates, with heavy atoms fixed and hydrogen positions optimized, using ORCA 5.0.4^96,97 with RIJCOSX approximation.⁹⁸ Natural population analyses (NPA) were performed using NBO 6.0.⁹⁹ Energy decomposition was conducted via the extended transition state-natural orbital for chemical valence-method (ETS-NOCV) method^100,101 implemented in ORCA 5.0.4.^96,97 Principal interacting orbital (PIO) analysis¹⁰² was performed using open-source code (https://github.com/jxzhangcc/PIO) in combination with NBO.⁹⁹ The averaged out-of-plane nucleus-independent chemical shift (NICS(1) and NICS(1)_zz) values computed at the GIAO-B3LYP-D3BJ/6–311G+(2d,p) level.¹⁰³ Two-dimensional (2D) NICS(1)_zz maps were generated with the aid of pyAroma 4.¹⁰⁴ ACID plots¹⁰⁵ were computed at the B3LYP-D3(BJ)/TZVP level. Integrated NICS_π,zz values (∫NICS_π,zz),^61,62 AV1245, and AV_min indices^63–65 were calculated using Multiwfn 3.8.¹⁰⁶ The electron density of delocalized double bonds [EDDBH(r)] was evaluated using RunEDDB⁶⁶ based on one-electron density matrices derived from Gaussian 16,⁹² in conjunction with the NBO 3 interfaced to Gaussian 16.

Vertical excitation energies for 1⁺ and 10 were calculated using TD-B3LYP-D3(BJ)/def2-TZVP with CPCM (CH₂Cl₂)⁹⁵ on B3LYP-D3(BJ)/TZVP optimized geometries, employing the corrected linear response (cLR) approach.¹⁰⁷ Exciton coupling was evaluated using nearest-neighbor dimers extracted from crystal structures, where all hydrogen atoms pre-optimized at the wB97X-D3(0)¹⁰⁸/def2-SVP level of theory using ORCA 5.0.4^96,97 with RIJCOSX approximation.⁹⁸ The S₀ and S₁ geometries of dimeric motifs were fully optimized at the (TD)-wB97X-D3(0)¹⁰⁸/def2-SVP level using ORCA and the RIJCOSX approximation⁹⁸ was applied. Long-range Coulombic coupling was computed using the transition charge from electrostatic potential (TrEsp) method¹⁰⁹ as implemented in Multiwfn 3.8.¹⁰⁶ To account for the electrostatic influence of non-participating counterions, fixed point charges (−1 e) were placed at the crystallographically determined center-of-mass positions of the anions. Charge-transfer integrals were computed at the PW91/TZVP level¹¹⁰ using CATNIP Tool v1.9 (https://github.com/JoshuaSBrown/QC_Tools/). Hole–electron analyses and natural transition orbital generation were conducted in Multiwfn.¹⁰⁶ The analysis of excited-state properties, including participation ratio (PR), mean position (POS), and charge-transfer character (CT) was performed using TheoDORE 3.2^79,81 based on Löwdin-orthogonalized fragment populations. The TD-DFT single-point calculations for π-dimers of 1^BArF₄ and 10 were performed at the TD-CAM-B3LYP¹¹¹-D3(BJ)/def2-SVP level. Pair interaction energy decomposition analysis (PIEDA)¹¹² was carried out for ion pairs extracted from crystal structures using GAMESS 2024 R2¹¹³ at the FMO2-RI-MP2/ma-TZVP level.^114,115 Input generation and fragment definition were completed using Facio.¹¹⁶ The optimized DFT cartesian coordinates are shown in a separated Supplementary Data file. Further details and discussion for each computational section are provided in the Supplementary Information.

Supplementary Material

Supplementary Figs. 1–73, Tables 1–14, detailed synthetic procedures and NMR spectra for all compounds, crystallographic, photophysical, spectroscopic studies and computational details.

Supplementary Data 1

Crystallographic data for compound 1^SbF₆, CCDC 2422333.

Supplementary Data 2

Crystallographic data for compound 1^BPh₄, CCDC 2422334.

Supplementary Data 3

Crystallographic data for compound 1^BArF₄, CCDC 2422335.

Supplementary Data 4

Crystallographic data for compound 10, CCDC 2422336.

Supplementary Data 5

Cartesian coordinates of optimized and calculated structures.

Source data

Numerical source data for Fig.3a

Numerical source data for Fig.3b

Numerical source data for Fig.3c

Numerical source data for Fig.3d

Numerical source data for Fig.3e

Numerical source data for Fig.3f

Numerical source data for Fig.3g

Numerical source data for Fig.3i

Numerical source data for Fig.4a

Numerical source data for Fig.4b

Numerical source data for Fig.4c

Numerical source data for Fig.4d

Data availability

All other relevant data generated and analysed during this study, which include experimental, spectroscopic, crystallographic and computational data, are included in this article and its Supplementary Information. Crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers CCDC 2422333 (1^SbF₄), 2422334 (1^BPh₄), 2422335 (1^BArF₄) and 2422336 (10). Copies of the data can be obtained free of charge at https://www.ccdc.cam.ac.uk/structures/. Source data are provided with this paper.