Regioselective –NO₂ substitution enables tunable photophysics and molecular packing in a polycyclic 1,2-BN heteroarene framework

Abstract

Heteroarenes containing boron–nitrogen (BN) bonds have attracted significant interest due to their often enhanced optoelectronic properties. The introduction of a nitro (–NO₂) group into heteroarenes is particularly appealing because its strong electron-withdrawing character lowers the LUMO energy, reducing the HOMO–LUMO gap and inducing bathochromic shifts in both absorbance and emission spectra. However, NO₂-substituted heteroarenes rarely fluoresce and typically suffer from aggregation-caused quenching (ACQ) of emission, limiting their practical applications. While strategies have been developed to counteract ACQ in NO₂-functionalized systems, little attention has been directed toward NO₂-substituted BN-heteroarenes, particularly three-coordinate boron species such as pyrrolidinone-fused-1,2-BN-heteroarenes (PBNHs). In this study, we address the ACQ issue in NO₂-substituted 1,2-BN-heteroarenes by synthesizing four PBNHs, denoted as NO₂-PBNHs 8, 9, 10, and 11, each featuring a –NO₂ group at a distinct position (C8–C11) on the common scaffold. Using comprehensive photophysical analyses and time-dependent density functional theory (TD-DFT) calculations, we systematically evaluated how the –NO₂ substituent's position influences photophysical properties and molecular packing. This work presents the first examples of electron-poor PBNHs with an identical framework exhibiting multifunctional, stimuli-responsive fluorescence properties. Modest positional changes of the –NO₂ group produced unexpected behaviors. NO₂-PBNHs 8 and 10 display both major aggregation-induced emission enhancement (AIEE) and solvatochromism within a single BN-aromatic backbone. NO₂-PBNH 9, although prone to ACQ, exhibited positive solvatochromism, robust reversible thermochromism, and a unique pitched π-stacking motif, highlighting its potential for temperature-sensing and charge-transport applications. NO₂-PBNH 11 uniquely exhibited both minor ACQ and major AIE properties in one BN-aromatic backbone. Together, these findings demonstrate that regioselective nitration is an effective strategy to tune molecular packing and fluorescence behavior in BN-heteroarenes. Furthermore, cytotoxicity assays confirmed that NO₂-PBNHs 8–11 are biocompatible, indicating potential for biological imaging. Overall, this study advances the understanding of aggregate formation in NO₂-substituted PBNHs and provides design principles for next generation NO₂-functionalized luminophores.

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Introduction

Polycyclic aromatic hydrocarbons (PAHs) have been extensively studied over the past several decades due to their unique optoelectronic properties,^1–4 making them valuable materials used in organic electronic devices.^5–8 Modification via inclusion of diverse electron-donating and -withdrawing moieties into PAH scaffolds is a common strategy for tuning their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels,⁹ facilitating performance optimization in photocages,¹⁰ and in optoelectronic devices such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic solar cells (OSCs).^11–14 To further enhance optical performance such as higher molar absorption coefficients and fluorescence quantum yields (QYs), larger Stokes shifts, and improved photochemical stability, researchers have incorporated heteroatoms, such as boron into PAH scaffolds to create polycyclic aromatic compounds (PACs) with modified photophysics.^15–19 For instance, in the 1950s and 1960s, Dewar et al. introduced the first three-coordinate boron center into the scaffold of a PAH, by substituting two sp²-hybridized carbon atoms in a benzene ring with a boron–nitrogen (BN) unit.^20,21 This seemingly modest substitution preserved the overall size, shape, and aromaticity nature of the PAC while significantly altering its reactivity and electronic transition energies.²²

Nearly 75 years after Dewar first reported singly BN-substituted PACs, research into the optical properties of BN-doped analogues known as polycyclic aromatic azaborines (PAAs) remains highly active. PAAs have drawn particular interest across materials sciences^23,24 and biomedical fields^25–28 owing to their favorable optoelectronic properties, including exceptional photostability, tunable emission and absorption spectra, large molar absorption coefficients, and high fluorescence QYs.^29–56 Recent studies have highlighted numerous BN-doped analogues as promising functional optoelectronic materials exhibiting striking optical properties.^57–75 Similar to PAHs, introducing electron-donating and electron-withdrawing moieties onto PAA scaffold provides precise control of the HOMO–LUMO energy gap, rendering them suitable candidates for electroluminescent (EL) devices.^29,74,76,77 For instance, installing a strong electron-withdrawing group (EWG) like a –NO₂ substituent directly on PAA or other PAC systems significantly lowers the LUMO energy, narrowing the HOMO–LUMO energy gap, and induces a bathochromic shift in absorption and emission wavelengths, all of which are advantageous for fluorescence sensing and biological labeling applications.^78–81

Despite significant progress in improving the photophysical properties of PAAs, a recurring dilemma persists: incorporating a –NO₂ substituent produces desirable bathochromic shifts in absorbance and emission wavelengths but also leads to detrimental fluorescence quenching.⁸² The strong electron-withdrawing nature of the –NO₂ group generates a pronounced dipole within the planar conjugated framework, promoting strong intermolecular π⋯π stacking and aggregate formation. This aggregation results in emission loss through the well-known aggregation-caused quenching (ACQ) effect.^{72,74,83–88} Additionally, the available pathway of non-radiative decay via intersystem crossing (ISC) to the triplet manifold further shortens excited state lifetimes.^89–92 Consequently, NO₂-substituted PAAs are often unsuitable for practical applications such as n-type semiconductors, photosensitization, and photooxidants.⁹³

We previously reported a NO₂-substituted PAA, NO₂-pyrrolidinone-fused-1,2-BN-heteroarene 9 (NO₂-PBNH 9, Fig. 1), featuring a three-coordinate boron center and exhibiting intramolecular charge transfer (ICT) in the excited state.⁷⁴ However, NO₂-PBNH 9 undergoes ACQ, restricting its use in practical applications such as in optoelectronic devices, imaging probes and biosensors. This drawback can be circumvented by applying the aggregation-induced emission (AIE) concept pioneered by Tang et al.^83,93–96 Hence, in our earlier work we introduced a phenyl spacer between the –NO₂ group and the PBNH core to create a twisted molecular geometry (NO₂-Ph-PBNH, Fig. 1) weakening intermolecular π⋯π stacking and enabling AIE behavior.⁷² Importantly, our previous findings indicate that introducing a cyano (–CN) group with a comparable Hammett σ-para value did not induce AIE, suggesting that the n → π* transition associated with the –NO₂ substituent may play a critical role in governing the solvent-dependent absorption and fluorescence properties.⁷²

Fig. 1 Molecular structure of previously reported PBNHs^72,73 and NO₂-PBNHs 8–11 discussed in this work.

Building on this understanding, we investigated how relocating the –NO₂ substituent to different positions on the pyrrolidinone hemisphere of the scaffold (Fig. 1, pink spheres) influences photophysical properties. In 2022, Tang et al. reported electron-rich PAAs capable of displaying both ACQ and AIE properties through diphenylamine rotors embedded in BN-heteroarenes.⁸³ In contrast, our study presents the first electron-poor PBNHs that exhibit stimuli-responsive, multifunctional fluorescent properties without rotors. These systems display not only ACQ but also aggregation-induced emission enhancement (AIEE), defined by active emission whose intensity strongly increases at the onset of aggregation.^97,98 Furthermore, the new PBNHs exhibit solvatochromism (reversible spectral shifts across solvents of varying polarity),⁹⁹ and thermochromism (reversible emission color change with temperature).⁶⁸ While minor solvatochromic effects have been reported in certain PAAs, to our knowledge no previous examples have demonstrated thermochromic emission behavior or AIEE properties.

In this work, we report the molecular design, synthesis, photophysical characterization, and quantum chemical analysis of four nitro-substituted PBNHs (NO₂-PBNHs 8–11, Fig. 1), each functionalized with a –NO₂ group at distinct position (C8–C11) on a common framework. The numbering of the final compounds is shown in red, indicating the position of the –NO₂ group on the pyrrolidinone hemisphere. For comparison, a previously reported unsubstituted PBNH⁷³ (Fig. 1) from our research group is included to highlight how regioselective nitration influences the photophysical behaviors.

The position of the –NO₂ group proved decisive in governing both photophysical response and solid-state molecular packing, yielding diverse phenomena including ACQ, AIEE, solvatochromism, thermochromism, and the emergence of a unique pitched π-stacking motif. For instance, NO₂-PBNH 8 displays major AIEE with negative solvatochromism, whereas relocating the –NO₂ substituent to C9 (NO₂-PBNH 9) affords ACQ, positive solvatochromism, reversible thermochromism, and pitched π-stacking: a crystal architecture known for exceptional charge-transport mobility (up to 40 cm² V⁻¹ s⁻¹)^100–103 but rarely realized synthetically.¹⁰³ Remarkably, NO₂-PBNH 9 is the first NO₂-substituted PAA to combine all of these features in a single BN-aromatic backbone. NO₂-PBNH 10 exhibits both major AIEE and negative solvatochromism, representing, along with 8, the first NO₂-substituted PAAs to couple these two properties. Finally, NO₂-PBNH 11 is the first to combine ACQ and AIE within one framework.

These results demonstrate that choosing nitration location on a PBNH scaffold is a powerful strategy for modulating molecular packing and excited-stated dynamic in NO₂-substituted PAAs, particularly those containing three-coordinate boron centers. Beyond advancing the understanding of aggregation phenomena, this study highlights design principles for electron-poor n-type semiconductors, photooxidants, and multifunctional fluorescent materials.⁹⁸ Notably, NO₂-PBNHs 8–11 also show biocompatibility, underscoring their potential in bioimaging, sensing, and luminescent device technologies.

Results and discussion

Syntheses and structural characterization

PBNHs are conjugated heteroaromatic chromophores containing a three-coordinate boron atom bonded to a carbon, nitrogen, and hydroxyl group (Fig. 1), and are known for their excellent photochemical stability.^72–74 The emission wavelength and QYs of PBNHs can be fine-tuned by altering the substitution at the C9 position of the pyrrolidinone hemisphere.⁷⁴ For instance, introduction of a –NO₂ group at C9 induces a pronounced Stokes shift via ICT in the excited state, but also leads to ACQ of emission.^72,74 This makes the PBNH scaffold an ideal platform to probe the effect of placing a –NO₂ group at alternative sites (C8, C10, and C11).

Building on our prior report of NO₂-PBNH 9,^72,74 we synthesized three new regioisomers (NO₂-PBNHs 8, 10, and 11) via a one-pot base-catalyzed condensation reaction of NO₂-substituted isoindolinones with o-formylphenylboronic acid (Scheme 1). The overall sequence proceeds in three steps. First, NO₂-substituted methyl 2-methyl benzoates undergo selective radical bromination with N-bromosuccinimide to yield mono- and di-brominated intermediates (1a–1d). Steric hindrance of the benzylic site in 1d exclusively favors mono-bromination in high yield, whereas 1a–1c afforded mixtures requiring purification. Second, treatment of 1a–1d with ammonia in methanol (MeOH) yields NO₂-substituted isoindolinones (2a–2d) in good yields, precipitating directly from solution without further purification. Finally, condensation of 2a–2d with o-formylphenylboronic acid affords NO₂-PBNHs 8–11 in high yield. Due to their low solubility in cold ethanol (EtOH), the products precipitated as solids and were readily isolated.

Scheme 1 Synthetic route to synthesize NO₂-PBNHs 8–11.

The previously unreported structures of NO₂-PBNHs8, 10, and 11 were confirmed by ¹H, ¹³C{¹H}, ¹¹B{¹H} NMR spectroscopy, FT-IR, and mass spectrometry. Since these compounds could not be effectively ionized under SI conditions, direct analysis in real time-mass spectrometry (DART-MS) was employed for mass analysis. In addition, singe crystal X-ray diffraction studies were performed for NO₂-PBNHs 8, 9, and 11 which confirmed their molecular structures (see SI for full detailed experimental conditions and characterization of all compounds). The ¹¹B{¹H} resonances (30.7, 30.2, 30.6, and 30.1 ppm for NO₂-PBNHs 8–11, respectively) are consistent with planar, three-coordinated boron centers. Additional assignments were supported by 2D-NMR experiments. All precursors and final products (NO₂-PBNHs 8–11) were stable under ambient conditions.

Experimental photophysical properties in solution and solid-state

Absorption spectra. UV-Vis spectroscopy was used to examine how solvent polarity and hydrogen-bonding capability influence the absorption profiles of NO₂-PBNHs 8–11, alongside unsubstituted PBNH as a reference (Table 1; spectra in Fig. S55). The unsubstituted PBNH exhibits a π–π* transition at 370–375 nm, which remains largely unaffected by solvent environment. By contrast, the analogous solvent affected absorption bands of NO₂-PBNHs 8–11 range from 340–390 nm, 385–410 nm, 405–415 nm, and 370–380 nm, respectively. Among the tested solvents, MeOH induces the largest redshift in absorbance for NO₂-PBNHs 9–11, attributed to stronger hydrogen-bonding interactions.

Table 1 Experimental UV-visible absorbance (λ_max) and molar absorption coefficient (ε) for unsubstituted PBNH and NO₂-PBNHs 8–11 in various solvents and excitation in the solid-state (λ_max). (PBNHs concentration = 10⁻⁶ M in solution)

Cmpds	Solvents	In solution λabs^a (nm)	ε (M^−1 cm^−1) (λabs)	Solid-state excitation λmax^a (nm)
a Wavelengths for intensity maxima. b Molar absorption coefficient for the lower energy λabs peak.
PBNH	CHCl3	375	(4.1 × 103)	385
	THF	370	(1.5 × 104)
	DCM	375	(2.1 × 104)
	CH3OH	375	(2.2 × 104)
	CH3CN	370	(4.8 × 103)
NO2-PBNH 8	CHCl3	350, 385	(9.3 × 103)	361, 432
	THF	380	(3.7 × 104)
	DCM	340, 390	(1.6 × 104)
	CH3OH	380	(3.1 × 104)
	CH3CN	380	(1.7 × 104)
NO2-PBNH 9	CHCl3	390, 410	(1.1 × 104)	390
	THF	385, 405	(3.3 × 104)
	DCM	385, 405	(3.0 × 104)
	CH3OH	410	(1.2 × 104)
	CH3CN	385, 400	(3.0 × 104)
NO2-PBNH 10	CHCl3	410	(1.5 × 104)	360, 432
	THF	405	(1.6 × 104)
	DCM	405	(2.6 × 103)
	CH3OH	415	(1.1 × 104)
	CH3CN	405	(8.3 × 103)
NO2-PBNH 11	CHCl3	375	(2.8 × 104)	355, 412
	THF	375	(9.0 × 103)
	DCM	375	(1.7 × 104)
	CH3OH	380	(4.3 × 104)
	CH3CN	370	(2.9 × 104)

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To further elucidate the positional effects of –NO₂ substitution, TD-DFT calculations (M06-2X/6-311++G(d,p)) were carried out on NO₂-PBNHs 8–11 and unsubstituted PBNH (Table S1). This functional/basis set combination was selected after benchmarking against other methods and found to best reproduce experimental trends (Fig. S64). Optimized ground-state geometries indicated predominate planar structures (Fig. 2), consistent with crystallographic data (Fig. 5). Notably, steric interactions enforce a twisted orientation of the –NO₂ group in NO₂-PBNHs 8 and 11, with 8 showing a nearly perpendicular twist.

Fig. 2 Frontier molecular orbitals (HOMO and LUMO, and LUMO+1) for unsubstituted PBNH and NO₂-PBNHs 8–11. Calculated at the M06-2X/6-311++G(d,p) in acetonitrile. Orbital energies are in eV.

Frontier orbital analysis (Fig. 2) revealed that the HOMOs of all NO₂-substituted PBNHs remain largely unaffected by the –NO₂ substituent, reflecting its minimal contribution to HOMO electron density. In contrast, the LUMO and LUMO+1 orbitals show significant delocalization onto the –NO₂ group, highlighting its strong electron-withdrawing character. This pronounced stabilization of the LUMO narrows the HOMO–LUMO gap and drives ICT from the boronic acid hemisphere to the pyrrolidinone hemisphere.

The calculated orbital energies follow the expected trends for –NO₂ substitution: both HOMO and LUMO energies are lowered, with a more pronounced effect on the LUMO (Fig. 2 and Table S1). NO₂-PBNH 10 has the most stabilized LUMO, producing the narrowest HOMO–LUMO energy gap (5.14 eV) and the longest λ_abs. NO₂-PBNH 9 displays a slightly larger HOMO–LUMO gap and shorter λ_abs, while NO₂-PBNH 11 has the widest HOMO–LUMO gap (5.49 eV), consistent with its shorter λ_abs in most solvents. Another identifiable trend is the reduced energy spacing between LUMO and LUMO+1 in NO₂-substituted PBNHs. In unsubstituted PBNH, this separation is ∼1 eV, but in twisted NO₂-PBNHs 8 and 11 the gap falls below 0.5 eV, potentially accounting for the double absorption peak features in some solvents.

Interestingly, the calculations revealed striking dipole moments (Fig. S65) for highly polar NO₂-PBNHs 8 and 9 of 10.48 and 8.10 Debye (D), respectively. Whereas NO₂-PBNHs 10 and 11 show much lower values of ∼2.5–2.6 D, even lower than the unsubstituted PBNH. The dipole moment values in Fig. S65 are scaled by a factor of 0.8 for NO₂-PBNHs 8 and 9, and by a factor of 2.8 for NO₂-PBNHs 10 and 11, to enable direct visual comparison. In actuality, the dipole moments of 8 and 9 are substantially larger than those for 10, and 11, making representation on the same scale impractical. The excitation of all but NO₂-PBNH 10 increases the dipole moment (Table S1). The same is true for unsubstituted PBNH. The largest change of almost 1 D is observed for NO₂-PBNH-9 and 11.

These polarity differences are expected to strongly influence supramolecular assembly, particularly the propensity to form J- versus H-type dimers.¹⁰⁴ In H-type dimers, dipole moments align favorably to maximize dipole–dipole interactions, whereas in J-type dimers they adopt a repulsive orientation. The much smaller dipole moments of NO₂-PBNHs 10 and 11 likely promote J-type aggregation, offering a plausible explanation for the aggregation-induced emission (AIE) observed in these compounds.

To further explore the differences in electron density, molecular electrostatic potential (MESP) maps were generated for NO₂-PBNHs 8–11 and the unsubstituted PBNH. These maps, shown in Fig. 3, illustrate regions of varying potential: red indicates more negative potential, blue denotes more positive potential, and green-yellow represents intermediate potential. The maps clearly highlight the strong electron-withdrawing nature of the –NO₂ group. Notably, the position of the –NO₂ substituents significantly influences the electronic distribution across the molecular core, as reflected in the distinct patterns of each map. These observations suggest that NO₂-PBNHs 8–11 are likely to exhibit different intermolecular interactions depending on the placement of the –NO₂ group.

Fig. 3 Molecular electrostatic potential (MESP) maps of the front (top) and in-plane (bottom) orientations of unsubstituted PBNH and NO₂-PBNHs 8–11. The red regions indicate a more negative potential, while the blue regions indicate a more positive potential. Calculated at the M06-2X/6-311++G(d,p) level in acetonitrile.

Emission spectra

The emission spectra for NO₂-PBNHs 8–11 were measured in solvents of varying polarity and hydrogen-bonding capability, as well as in the solid-state (Table 2; Fig. S56 and S57). Consistent with the well-known documented quenching effect of –NO₂ group, which renders most nitroaromatics non-fluorescent,⁸²NO₂-PBNHs 8–11 exhibited weak or negligible emission in solution. Additionally, their highly planar structures engage in strong π⋯π stacking interactions, which encourages ACQ. Consequently, NO₂-PBNHs 8–11 display much weaker fluorescence than unsubstituted PBNH (Table 2), consistent with the literature.^72,74,82

Table 2 Experimental emission (λ_em), QYs (Φ_F), and Stokes shift (nm and cm⁻¹) values for NO₂-PBNHs 8–11 in various solvents, with solid-state emission (λ_em) data. (PBNHs concentration = 10⁻⁵ M in solution)

Cmpds	Solvents	λ em /nm (exc)	In solution (ΦF)	Stokes^c shift/nm	Stokes^c shift/cm^−1	Solid-state λem^d/nm (ΦF)
a Fluorescence was not observed in selected solvents. b Quantum yields could not be obtained in selected solvents. c Stokes shift wavelengths for the lowest λabs and λem. d Solid-state wavelengths for the lowest λ.
PBNH	CHCl3	439 (356)	0.99	64	3888	486
	THF	434 (353)	1.00	64	3985
	DCM	436 (356)	0.96	61	3730
	CH3OH	433 (380)	0.93	58	3572
	CH3CN	436 (353)	1.00	66	4091
NO2-PBNH 8	CHCl3	466 (403)	0.004	81	4515	571
	THF	457 (401)	0.001	77	4434
	DCM	452 (402)	0.002	62	3517
	CH3OH	465 (407)		85	4810
	CH3CN	466 (400)		86	4856
NO2-PBNH 9	CHCl3	540 (416)	0.07	130	5871	593
	THF	512 (410)	0.02	107	5160
	DCM	550 (411)	0.11	145	6510
	CH3OH			n/a	n/a
	CH3CN	589 (408)	0.06	189	8022
NO2-PBNH 10	CHCl3	442 (374)	0.02	32	1765	560
	THF	455, 571 (400)	0.01	166	7180
	DCM	440 (363)	0.004	35	1965
	CH3OH	467 (383)	0.02	52	2683
	CH3CN	462 (362)	0.03	57	3050
NO2-PBNH 11	CHCl3	445, 616 (380)		241	10 432	575
	THF	587 (390)		212	9630
	DCM	624 (364)		249	10 641
	CH3OH	454 (399)		74	4290
	CH3CN	476, 650 (376)		280	11 642

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Previous studies revealed that NO₂-PBNH 9 experiences strong π⋯π stacking interactions and CT in the excited state, leading to rapid fluorescence quenching (ACQ) upon water addition to acetonitrile (MeCN) (Fig. 4B).⁷² Motivated by these findings, we examined the ACQ behavior of NO₂-PBNH 11, which shows notable parallel optical properties to NO₂-PBNH 9. The λ_em at ∼650 nm for 11, consistent with CT emission (Fig. 4D), is enhanced in more polar solvents.^72,105

Fig. 4 (left) Emission spectra for NO₂-PBNHs 8 (A), 9 (B), 10 (C) and 11 (D) in MeCN/water mixtures, with % water fraction (f_w) labeled. (right) Emission intensity monitored at the labeled wavelength as % fraction of water increases from left to right, with respective insets of cuvette photographs showing visible fluorescence. All performed under a 365 nm handheld UV-light lamp. (NO₂-PBNH concentration = 10⁻⁵ M).

Fig. 4 depicts spectral changes for NO₂-PBNHs 8–11 emission in MeCN/water mixtures as water fraction is increased (f_w = 0 to 95% at 10% increments). As hypothesized, the CT emission band for NO₂-PBNH 11 at ∼650 nm was quenched immediately between f_w = 0 and 10% (Fig. 4D), reflecting ACQ similar to NO₂-PBNH 9 (Fig. 4B). Unexpectedly, further water addition (f_w = 10–95%) results in a pronounced increase in fluorescence intensity, dominated by the emission band (yellow color) at 467 nm corresponding to the LE (locally excited) state. This effect is an example of aggregation-induced emission (AIE).^72,105

Together, these results establish NO₂-PBNH 11 as the first NO₂-substituted PAA to exhibit both ACQ and AIE properties within a single BN-aromatic backbone. Unlike previous systems that rely on bulky rotors to enforce twisted conformations, NO₂-PBNH 11 achieves this dual behavior intrinsically through its electronic structure and aggregation dynamic.

Interestingly, the presence and quenching of CT emission bands at ∼600 nm and ∼650 nm are responsible for ACQ behavior in NO₂-PBNHs 9 and 11, respectively, but these bands are absent in the emission spectra of NO₂-PBNHs 8 and 10 in MeCN. This lack of CT band means that increasing the water fraction in MeCN for NO₂-PBNHs 8 and 10 can only impact the LE states, with the observed increase in emission intensity being diagnostic of aggregation-induced emission enhancement (AIEE) behavior. Indeed, as shown in Fig. 4A and C, water addition induces major AIEE in 8 and 10. Notably, NO₂-PBNHs 8 and 10 remain emissive in both solution and in aggregate states at higher water fractions, exhibiting J-type aggregation.

Both NO₂-PBNHs 8 and 10 display red shifts of 14 and 15 nm in λ_em, respectively, accompanied by visible color change in the cuvettes. Their photophysical behavior may be explained by conjugation-induced rigidity (CIR) at lower water fractions. The planar PBNH scaffold enhances conjugation, red-shifting λ_em and promoting J-aggregation between f_w = 0–50%.^106,107 At higher water fractions, aggregate formation diminishes the CIR effect by disrupting close molecular packing, reducing rigidity, and allowing molecular motion.¹⁰⁶ Consequently, emission is dominated by AIEE, with progressively reduced rigidity resulting in gradually decreasing emission intensity for f_w > 50%.

Concentration-dependent fluorescence measurements were also performed for all NO₂-PBNHs 8–11 at 10⁻⁵, 10⁻⁴, and 10⁻³ M in pure MeCN to evaluate intermolecular interactions in solution as a possible origin of ACQ or AIE (Fig. S66). The emission spectra of NO₂-PBNH 8 displayed enhanced fluorescence intensity as the concentration increased from 10⁻⁵ M to 10⁻⁴ M, AIEE, followed by a gradually decrease at 10⁻³ M (Fig. S66A). In contrast, NO₂-PBNH 9 showed a continuous decrease in emission intensity with increasing concentration, confirming predominant ACQ behavior (Fig. S66B). NO₂-PBNH 10 exhibited a similar trend to NO₂-PBNH 8, with an increase in emission up to 10⁻⁴ M (AIEE) and a gradual decrease at 10⁻³ M (Fig. S66C). Meanwhile, NO₂-PBNH 11, like NO₂-PBNH 9, demonstrated typical ACQ characteristics. The concentration-dependent changes in the emission profiles clearly indicates that the compounds aggregate or form intermolecular associations in solution, confirming that the observed ACQ and AIEE in neat organic solvent arises from these interactions.

To gain deeper insight into the solid-state intermolecular interactions, X-ray quality single crystals of NO₂-PBNHs 8, 9, and 11, were obtained (Fig. 5). No X-ray quality crystals for 10 were obtained. All three crystal structures show planar PBNH frameworks featuring a three-coordinate boron atom bonded to a carbon, nitrogen and hydroxyl atoms (Fig. 5A, E, and G, respectively). The B–N bond lengths measure at 1.457 Å for 8 and 9 and 1.460 Å for 11, which are slightly longer but consistent with reported sp² B–N bond lengths in polycyclic 1,2-azaborine derivatives.^{72–74,108–110} Each structure exhibits intramolecular hydrogen bonding between the lactam C=O and boronic O–H group, with C=O—H distances of 2.131 Å (8), 2.148 Å (9), and 2.267 Å (11).

Fig. 5 Molecular structure of NO₂-PBNH 8 (A)–(C); NO₂-PBNH 9 (D)–(F); and NO₂-PBNH 11 (G)–(I). ORTEP thermal ellipsoids drawn at 50% position probability (gray: carbon; red: oxygen; blue: nitrogen; and pink: boron).

In NO₂-PBNH 8, the –NO₂ group is twisted out of plane with a dihedral angle of 32.59°, partially decoupling its π-system from that of the PBNH core. NO₂-PBNH 8 also shows bifurcated π-stacking with an interplanar distance of 3.576 Å between the boron atom and the π-cloud of the –NO₂-substituted benzene group (Fig. 5B), leading to a head-to-tail molecular arrangement that cancels the dipole moments. The space-filling model in Fig. 5C highlights reduced π⋯π overlap between adjacent molecules, a packing motif in which AIEE dominates and ACQ does not occur.

In contrast, the –NO₂ group of NO₂-PBNH 9 is coplanar to the PBNH core, consistent with its ground state geometry (Fig. 2). Its crystal structure reveals strong intermolecular π⋯π interactions in a pitched π-stacking motif (Fig. 5D–F), with stacking distances ranging from 3.267–3.458 Å and orbital overlap distance of 9.195 Å. The enhanced overlap in this arrangement rationalizes the observed red-shifted emission and ACQ response of 9 upon aggregation.

For NO₂-PBNH 11, the –NO₂ group is twisted out of plane relative to the PBNH core, with a dihedral angle of 35.54°. While the dihedral angle and the crystal stacking arrangement of 11 closely resemble that of 8 (Fig. 5H), there are more impactful similarities to NO₂-PBNH 9 in its closer crystal packing, with π⋯π stacking distances ranging from 3.369–3.408 Å, and greater intermolecular overlap relative to 8, confirmed by the space-filling model in Fig. 5I. These factors explain the ACQ behavior observed in its emission profile.

DFT calculations reveal that NO₂-PBNHs 8–11 can form both stable J-type and H-type dimers in solution (Fig. 6). For each compound, two distinct J-dimers (J1 and J2) and one H-dimer were successfully optimized. The existence of two stable J-dimer motifs has also been recently reported for BODIPY derivatives.¹¹¹ The calculated dimerization energies for all dimers lie within a narrow range between 0.83–0.96 eV (Fig. 6), with no significant energetic differences between J- and H-type dimers, suggesting that all forms are accessible in solution.

Fig. 6 Calculated possible stable dimers for NO₂-PBNHs 8–11. The bottom monomer retains an overall similar orientation across the row, with varied –NO₂ location altering the optimized dimer geometries. Calculated at the M06-2X/6-311++G(d,p) level in acetonitrile. Dimerization energies are in eV.

H-type dimers are primarily stabilized by dipole–dipole interactions, and are expected to be favorable for the more polar NO₂-PBNHs 8 and 9, whereas the less polar NO₂-PBNHs 10 and 11 may preferentially form J-dimers. Both J1 and J2-type dimers exhibit similar dimerization energies, though J2-dimers feature smaller angles between the two monomer dipole moments, which may reduce dipole–dipole repulsion relative to J1, at the cost of weaker π–π overlap. These results suggest that the observed AIEE behavior is likely observed from a combination of J1- and J2-type aggregates. Further DFT investigations are underway to examine additional dimeric and higher order aggregation modes of NO₂-PBNHs 8–11.

We further examined the solvatochromic responses of NO₂-PBNHs 8–11 in solvents of different polarity. Among the series, NO₂-PBNH 8 consistently displays the shortest emission wavelengths, with minimal solvent influence. The blue-shifted emission relative to the rest of the series likely arises from the twisted geometry of its –NO₂ group, which reduces conjugation with the PBNH scaffold, as confirmed by DFT calculations (Fig. 2). In contrast, NO₂-PBNH 9 shows longer emission wavelengths relative to 8 and 10, attributed to stronger coupling of the –NO₂ group with the conjugated core, resulting in CT emission. Polar solvents like MeCN further stabilize the CT state, producing a bathochromic shift in λ_em, as previously observed in NO₂-substituted PBNHs.^72,74,105

Accordingly, NO₂-PBNHs 9 and 11 display distinct behaviors in MeCN. NO₂-PBNH 9 exhibits a single red-shifted band λ_em at ∼590 nm, arising from relaxation through CT states. NO₂-PBNH 11 shows dual emission bands: an LE band at 476 nm and a broad CT band with λ_em at 650 nm, consistent with complete charge separation as predicted by frontier molecular orbital analysis (Fig. 2).

When compared in THF (low polarity) and MeCN (high polarity) (Fig. 7), NO₂-PBNH 8 exhibited negative solvatochromism (yellow → blue) due to the broad CT band ∼580 nm in THF (Fig. 7A). In contrast, NO₂-PBNH 9 uniquely exhibited positive solvatochromism, with a bathochromic shift in λ_em from 512 nm to 589 nm as solvent polarity increased, accompanied by a visible color change from green (in THF) to yellow (in MeCN) (Fig. 7B). This red shift in λ_em reflects stabilization of the CT state in polar solvents, where electron density transfers from the boronic acid ring to the –NO₂ group upon excitation.¹¹²

Fig. 7 Emission spectra of NO₂-PBNHs 8 (A), 9 (B), 10 (C), and 11 (D) in MeCN and THF illustrating solvatochromic behaviors. (NO₂-PBNHs concentration = 10⁻⁵ M).

NO₂-PBNH 10 showed negative solvatochromism, displaying a hypsochromic shift in λ_em from 571 nm to 462 nm (yellow → cyan), likely arising from reduced rigidity due to differential solvation in THF.¹¹²NO₂-PBNH 11 showed a bathochromic shift in emission λ_em from 587 nm to dual emissions 476 and 650 nm with increasing solvent polarity, but its solvatochromic response was obscured by overlap between the CT band at 650 nm and the emission band in THF. Importantly, NO₂-PBNHs 8 and 10 represent the first NO₂-substituted PAAs to combine both AIEE and solvatochromism within a single BN aromatic framework.

We next investigated the temperature-dependent emission of NO₂-PBNHs 8–11 in dichloromethane (DCM). Among them, only NO₂-PBNH 9 displayed appreciable thermochromic behavior, exhibiting an apparent temperature-response emission (Fig. 8A and C). As the temperature decreased from 295 K to 195 K in 20 K intervals, NO₂-PBNH 9 displayed a 46 nm bathochromic shift in λ_em accompanied by a marked increase in intensity. Correspondingly, the emission color shifted from green at 295 K to yellow at 195 K, (inset photographs of Fig. 8A and C). The thermochromic response was fully reversible, as confirmed by five heating–cooling cycles with no loss in emission intensity (Fig. 8B and D), underscoring the robustness of NO₂-PBNH 9.

Fig. 8 (A) Temperature-dependent emission spectra for NO₂-PBNH 9 from 295 K to 195 K. (B) Reversible modulation of emission intensity, shown by temperature cycling of NO₂-PBNH 9. (C) Temperature-dependent emission spectra for NO₂-PBNH 9 from 195 K to 295 K. (D) Reversible modulation of emission wavelength, shown by temperature cycling of NO₂-PBNH 9 (in distilled DCM, NO₂-PBNH concentration = 10⁻⁵ M, λ_ex = 380 nm).

To probe whether this behavior arose from stabilization of the CT state at low temperatures, we conducted parallel experiments on the other NO₂-PBNHs. NO₂-PBNH 8 showed no detectable changes upon cooling (Fig. S60A), while NO₂-PBNH 10 exhibited only a decreased in the LE emission band without a spectral shift (Fig. S60C). NO₂-PBNH 11, which exhibits both ACQ and AIE behaviors, behaved differently: cooling led to a slightly reduced emission intensity for the LE band and more pronounced decrease in the CT band, but without a striking difference in emission color (Fig. S60D).

Together, these results indicate that the thermochromism of NO₂-PBNH 9, specifically the red shift in emission and increase in intensity, stems from stabilization of the CT process at lower temperatures, whereas competing excited-state decay pathways suppress such behavior in the other regioisomers. The reversible thermochromic response of NO₂-PBNH 9 highlights its potential application in smart materials or passive optical sensors for real-time temperature mapping.¹¹³

Cytotoxicity

The cytotoxicity of NO₂-PBNHs 8–11 was assessed in human immortalized embryonic kidney 293 (HEK293) cells. Treatment with 200 µM for 72 hours produced no statistically significant reduction in cell viability relative to untreated reference groups (Fig. S63). Notably, none of the NO₂-PBNHs exhibited detectable cytotoxic, underscoring their high biocompatibility and supporting their potential for use in cellular imaging applications.

Conclusions

We synthesized and comprehensively characterized four NO₂-substituted PBNHs (8–11), each sharing a common framework but variably functionalized with a –NO₂ group at C8–C11. Structural and photophysical studies reveal that substitution on the pyrrolidinone hemisphere exerts a profound influence on optical properties, giving rise to phenomena including ACQ, AIE, AIEE, solvatochromism, and thermochromism. Notably, regioselective nitration proves to be an effective strategy for tuning molecular packing, with NO₂-PBNH 9 adopting a pitched π-stacking motif in the solid-state.

DFT calculations show that –NO₂ substitution has a minimal effect on HOMO energies but substantially stabilizes the LUMO and LUMO+1, bringing all three levels closer in energy. The dominant electronic transition corresponds to HOMO → LUMO, with minor HOMO → LUMO+1 contributions. Dipole moment analysis further distinguishes the series: NO₂-PBNHs 8 and 9 possess dipoles 3–4 times as strong as those of NO₂-PBNHs 10 and 11, influencing intermolecular interactions and aggregation behavior. Computational modeling indicates the feasibility of one stable H-type and two J-type dimer configurations for all four compounds.

Among the NO₂-PBNHs, NO₂-PBNHs 8, 9, and 10 display solvatochromism. NO₂-PBNH 9, in particular, exhibits positive solvatochromism with a bathochromic shift from 512 to 589 nm (green → yellow) upon increasing solvent polarity (THF to MeCN). By contrast, NO₂-PBNHs 8 and 10 represent the first examples of NO₂-functionalized PBNHs to combine solvatochromism with AIEE within a single 1,2-BN-aromatic scaffold. This dual responsiveness highlights the utility of these materials for highly sensitive and real-time optical sensing. NO₂-PBNH 9 further stands out as the only derivative to display reversible thermochromism, showing an emission red shift and intensity enhancement upon cooling from 295 K to 195 K, with a visible color change (green → yellow).

These findings demonstrate how minor positional changes in –NO₂ substitution elicit distinct and desirable photophysical responses in pyrrolidinone-fused-1,2-BN-heteroarenes. With confirmed biocompatibility from cytotoxicity assays, NO₂-PBNHs 8–11 show strong potential for applications in fluorescence imaging. This study establishes the groundwork for the rational design of 1,2-BN-containing luminophores with tunable optical properties for advanced functional materials.

Author contributions

Conceptualization and supervision – C. J. S. L.; methodology – O. A. S., L. G., L. K., M. R., Z. K., C. J. S. L.; formal analysis – L. G., B. W., O. A. S., F. J., R. P. N., and C. J. S. L. (collected absorbance, excitation, QYs and all emission data); formal analysis – C. J. S. L. and P. B. P. (computational resources); formal analysis – C. J. S. L. and S. D. (crystallography); formal analysis – M. I. and M. Y. (cytotoxicity studies); writing – original draft, C. J. S. L.; writing – review & editing, C. J. S. L., P. B. P., L. G., F. J., B. W.

Conflicts of interest

The authors declare the following competing financial interest(s): a provisional patent (no. 63/887,711) has been filed by Kennesaw State University on technology related to electron-deficient stimuli-responsive multifunctional fluorescent azaborine materials.

Data availability

CCDC 2487849 (8), 2487850 (9), 2487851 (11) contain the supplementary crystallographic data for this paper.^114a–c

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: experimental section: synthetic protocols, characterizations (¹H NMR, ¹³C{¹H} NMR, ¹¹B{¹H} NMR, DART-mass, FT-IR spectra), UV-visible spectra of NO₂-PBNHs 8–11 in various solvents, solid-state excitation spectra of NO₂-PBNHs 8–11, emission spectra of NO₂-PBNHs 8–11 in various solvents, solid-state emission spectra of NO₂-PBNHs 8–11, and X-ray crystallographic data for NO₂-PBNHs 8, 9, and 11. Computational section: comparison between experimental and theoretical photophysical properties calculated via different functions. See DOI: https://doi.org/10.1039/d5tc04130a.

Acknowledgements

We gratefully acknowledge the Howard Hughes Medical Institute for funding this project for the Fall 2023 SCM 2000 CURE course at KSU, as well as the College of Science and Mathematics (CSM) at KSU for financial support. We thank the Peach State Bridges to the Doctorate Program from Kennesaw State University (NIH-1T32GM150548-01) for supporting B. W. L. G. gratefully acknowledges support from the Office of Research at Kennesaw State University. P. B. P. acknowledges the High Performance Computing (HPC) resources provided, maintained, and supported by the Appalachian State University College of Arts and Sciences, Information Technology Services, and Research Computing. The authors also thank Dr. Thomas Hester for his valuable assistance in reviewing and editing this manuscript.

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