Benzoheterocyclic [b]-fused BODIPYs: synthesis and effects of N, O, and S on structure, photophysical properties, and aggregation

Abstract

Next-generation optoelectronic devices require organic small-molecule light-emitters with high luminous efficiency and strong resistance to non-radiative decay. BODIPY dyes fulfill these criteria, particularly when fused with heteroaromatic rings, which enhances emission efficiency and induces redshift by suppressing conformational relaxation and non-radiative pathways. Herein, we reported the synthesis of six symmetric and asymmetric benzoheterocyclic [b]-fused BODIPYs incorporating nitrogen, oxygen, or sulfur atoms, addressing the underexplored impact of heteroatom type on BODIPY optoelectronic behavior. Nitrogen- and oxygen-containing derivatives were synthesized via a one-pot protocol combining nucleophilic aromatic substitution (S_NAr) and C–H activation. A new method using ¹⁹F NMR was developed for the structural identification of [b]-fused heteroaromatic BODIPYs. Single-crystal X-ray diffraction, electrostatic potential (ESP) mapping, and NICS(0) calculations demonstrated that heteroatoms, through electronegativity and intermolecular interactions, promote the formation of one-dimensional J-type aggregates, facilitating electron delocalization and charge transfer. Photophysical studies revealed significantly increased fluorescence quantum yield (75.7%–85.4%) and enhanced resistance to non-radiative decay. Water-dependent spectroscopic measurements confirmed J-aggregation and associated spectral changes. Electrochemical analysis highlighted the effect of asymmetric heteroatoms on HOMO/LUMO energy levels, supporting a proposal of a “half-sum rule”, consistent with DFT/TD-DFT calculations. These findings provide valuable insights for the rational design of advanced optoelectronic molecular materials.

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Introduction

BODIPY dyes, known for their intrinsic fluorescence, tunable emission properties, high optical efficiency, and excellent biocompatibility, have emerged as a promising research direction in the field of optoelectronics.^1–4 These dyes find broad applications in organic light-emitting diodes (OLEDs),^5,6 photodynamic therapy (PDT),^7–9 bioimaging,^10–12 and chemical sensing.^13–15 As the demand for advanced optoelectronic devices increases, these is a growing need for organic light-emitting materials that combine high luminescence efficiency, resistance to non-radiative decay, and biocompatibility. Therefore, the design and development of efficient organic small-molecule light-emitting materials are crucial for the next-generation of optoelectronic devices.

Non-radiative decay is a major competing pathway that reduces the light-emission efficiency of organic small molecules.¹⁶ To suppress this decay process, a commonly adopted molecular design strategy involves tuning the molecular conformations, typically by incorporating either extended fused structures or π-elongated motifs.¹⁷ However, the free rotation of aromatic substituted π-elongated motifs often induces significant geometric and electronic rearrangements upon excitation, which can enhance internal conversion and promote non-radiative decay (Fig. 1a). In contrast, rigid fused-ring structures stabilize the molecular geometry during the excited-state transition, thereby effectively reducing non-radiative losses (Fig. 1b). Consequently, to achieve efficient near-infrared (NIR) emissive organic small molecules, it is essential to explore synthetic strategies that minimize molecular geometry distortion and reduce non-radiative decay.

Fig. 1 (a) Schematic representation of the chemical structure of BODIPYs and the free rotation of the aryl group leading to an increase in non-radiative decay. (b) The introduction of the benzoheterocycle restricts the aryl radical free rotation and thus increases the radiation decay; schematic representation of the H- and J-aggregates of the BODIPY molecules.

BODIPY molecules feature multiple modification sites,^18–24 and fusion at the 2,3 and 5,6 positions^25–31 ([b]-fusion, Fig. 1a) effectively locks the molecular conformation, suppressing non-radiative decay. This results in an optimal redshift and luminous efficiency, enabling efficient NIR emission from small organic molecules. Multifunctional heteroatoms such as nitrogen (N), oxygen (O) and sulfur (S) are widely employed in π-conjugated materials, particularly in organic electronic and molecular devices, where they serve as essential active components.^32–35 Previous molecular design studies have demonstrated that incorporating heteroatom-containing receptor units into aromatic frameworks can enhance NIR emission and improve photophysical performance.^36–45 For instance, Jiao,^{36–38,46–48} our group,^49–52 and others^53–56 have successfully optimized the performance of NIR dyes by introducing suitable heteroatoms at the [b]-fusion positions to fulfill specific functional requirements. However, the current literature still lacks a systematic investigation into how different heteroatoms (N, O, and S) influence the optoelectronic properties of these systems. As part of our ongoing research on aromatic molecules and their optoelectronic behavior, we have delved into the structural and functional impacts of BODIPY dyes fused with various heteroatoms. Our studies encompass single-crystal structures, photophysical characteristics, and electrochemical properties, offering new insights into the rational design of NIR-emissive materials for optoelectronic applications.

In this work, we report the synthesis, characterization, and optoelectronic properties of a series of fully π-conjugated symmetric and asymmetric benzoheterocycle-[b]-fused BODIPYs incorporating nitrogen, oxygen, or sulfur atoms. Distinct ¹⁹F NMR signals served as diagnostic indicators for heteroatom fusion, providing reliable evidence to support rational molecular design. Single-crystal X-ray diffraction analysis further revealed the three-dimensional spatial structures and aggregation behaviors resulting from heteroatom incorporation. To verify the aggregation modes, we conducted spectroscopic studies in mixed solvents with varying water content. Through combined spectral and electrochemical analyses, we systematically explore how the fusion of nitrogen, oxygen, and sulfur influenced molecular structure and optoelectronic performance. Furthermore, density functional theory (DFT) calculations, including electrostatic potential (ESP) mapping and nucleus-independent chemical shift (NICS(0)) analyses, were employed to clarify how heteroatom incorporation affects HOMO–LUMO energy levels, electron distribution, and aromaticity. These comprehensive comparative studies offer a deeper understanding of how heteroatom fusion modulates the structural and electronic properties of fused BODIPYs, establishing a robust theoretical foundation for future molecular design and functional optimization.

Results and discussion

Synthesis and ¹⁹F NMR identification

Following our original synthetic design, we selected BODIPY 1c as the starting material, featuring chlorine atoms at the 3,5-positions and iodine atoms at the 2,6-positions. Initially, we attempted a two-step procedure reported by Wim Dehaen's group,⁵⁶ involving nucleophilic aromatic substitution (S_NAr) followed by traditional palladium-catalyzed intramolecular C–H activation to achieve ring fusion. However, during the synthesis of the H-A precursor, we were unable to obtain the expected disubstituted product, likely due to the weak nucleophilicity of aniline. To overcome this limitation, we modified the protocol by employing Pd(OAc)₂ as the catalyst, BINAP as the ligand, and Cs₂CO₃ as the base. The reaction was carried out in toluene under an argon atmosphere at 110 °C. Remarkably, compound 2a was obtained within a shorter reaction time, and prolonging the reaction led to the successful formation of the diindole-[b]-fused BODIPY H-A. Based on this observation, we optimized the ratio of phenol or aniline and combined S_NAr reactions with palladium-catalyzed C–H activation to establish a one-pot method for synthesizing both mono-fused (2a and 2b) and bis-fused (H-A and H-B) products, as well as the asymmetric compound H-E (Scheme 1).

Scheme 1 Synthesis of benzoheterocycle-[b] fused BODIPYs.

We also explored the synthesis of sulfur-containing fused analogs using thiophenol. However, the attempted formation of mono- and disulfur-fused products (H-C) was unsuccessful, possibly due to the larger atomic radius of sulfur hindering cyclization. As an alternative, we employed our previously reported sulfur-fusion strategy,^49,50 which successfully yielded both symmetric (H-C) and asymmetric (H-D and H-F) sulfur-containing fused compounds.

The structures of the heteroatom-fused BODIPY molecules were confirmed through multi-nuclear NMR spectroscopy. In particular, the ¹⁹F NMR spectra not only verified the presence of the heteroatom-fused moieties but also revealed their impact on the fluorine chemical shift resulting from [b]-fusion (Fig. 2). As the electronegativity of the fused heteroatoms decreases, the fluorine shielding effect increases progressively in the order H-B, H-A, and H-C, with the corresponding chemical shifts shifting from −148.58 ppm to −150.21 ppm, and further to −154.42 ppm. Sulfur, being less electronegative than oxygen and nitrogen but possessing a larger atomic radius, exerts a more pronounced shielding effect on the fluorine nucleus. This implies that sulfur influences the electronic environment of the BODIPY core more strongly through its inductive effect and spatial characteristics.

Fig. 2 Overlay of ¹⁹F NMR spectra in CDCl₃ highlighting the chemical shift of the F nucleus in H-A–H-F.

In addition, the ¹⁹F NMR chemical shifts of benzoheterocyclic [b]-fused BODIPYs is largely unaffected by meso-position substituents. For example, dibenzothieno [b]-fused BODIPYs consistently show a fluorine chemical shift near −154 ppm, regardless of whether electron-donating (e.g., methyl and mesityl) or electron-withdrawing groups (e.g., –CN and –NO₂) are introduced at the meso position.^49,50 This stability in chemical shift offers a practical diagnostic tool for identifying dibenzothieno [b]-fused BODIPYs. Similarly, the asymmetric fused derivatives H-D, H-E, and H-F exhibit consistent ¹⁹F NMR behavior. Due to the differing electronegativity and atomic radius of nitrogen, oxygen, and sulfur, their interactions lead to fluorine chemical shifts that fall between those of BODIPYs fused with different heteroatoms.

X-ray single-crystal structure, ESP, and NICS(0) analysis

To elucidate the influence of heteroatom fusion on the molecular packing, single crystals of benzoheterocyclic [b]-fused BODIPYs were obtained using the vapor diffusion method (Tables S1–S6). Structural parameters including bond lengths, bond angles, and π–π stacking distances (d) were measured, and slip distances and tilt angles were calculated based on displacement along the molecule long axis (Fig. 3 and Table 1). Both symmetric and asymmetric heteroatom-fused BODIPY single crystals exhibit highly planar structures and adopt a characteristic slip-stacked one-dimensional packing mode. In these arrangements, approximately half of each molecule overlaps with its neighboring unit. The observed small slip angles (19.11°–24.33°) and relatively larger slip distances (8.19–9.54 Å) are indicative of J-aggregate stacking arrangement. All derivatives from H-A to H-F show significant π–π stacking interactions in their crystal lattices, with interplanar distances ranging from 3.403 to 3.704 Å. Such compact packing favors efficient charge transport. The meso-mesityl group maintains an orthogonal orientation relative to the BODIPY core, with dihedral angles between 83.82° and 89.33°, which arises from steric hindrance and helps prevent aggregation-induced quenching.^25,26 Additionally, a recurring structural feature is that the C–X bond (X = N, O, S) proximal to the pyrrole ring is consistently shorter than the corresponding bond adjacent to the benzene ring, consistent with observations reported in prior studies.^42,52,54 This suggests stronger conjugative or electronic interactions between the heteroatom and the electron-rich pyrrole moiety. After heteroatom fusion, BODIPY promotes intermolecular interactions between parallel dimers, resulting in stronger π–π stacking. However, in the case of H-A, the presence of hydrogen bonding increases the packing distance (d_H-B = 3.403 Å < d_H-C = 3.501 Å < d_H-A = 3.704 Å), which was confirmed through single crystal analysis (Fig. 3). These structural findings provide insights into how heteroatom type and fusion influence molecular aggregation and electronic properties.

Fig. 3 Front views, crystal packing structures and ESP (a), (b) and (c) for H-A, (d), (e) and (f) for H-B, (g), (h) and (i) for H-C, (j), (k) and (l) for H-D, (m), (n) and (o) for H-E, and (p), (q) and (r) for H-F, respectively. Thermal ellipsoids are depicted at the 50% probability level. Hydrogen atoms are omitted for clarity. The red numbers within the six-and five-membered rings indicate the NICS(0) values in (a), (d), (g), (j), (m) and (p). In the crystal packing structures, the positive and negative signs represent positive and negative potentials in (k), (n), and (q).

Table 1 Summary of the crystal packing characteristics in molecules

Dye	Bond lengths (Å)				Slipping angle (°)	Slipping distance (Å)	Stacking distance (Å)
The slip distance and slipping angle are calculated as per ref. 57 and 58.
H-A	C3–N1		C6–N1		24.33	8.19	3.704
	1.413(2)		1.355(2)
H-B	C1–O1		C7–O1		20.58	9.06	3.403
	1.423(2)		1.341(2)
H-C	C1–S1	C10–S1	C12–S2	C12–S5	20.16	9.54	3.501
	1.77(1)	1.720(9)	1.71(1)	1.80(1)
H-D	C4–S1	C8–S1	C4–N1	C8–N1	21.30	8.99	3.519
	1.766(4)	1.756(3)	1.42(1)	1.337(8)
H-E	C5–O1	C8–O1	C15–N1	C17–N1	19.11	9.35	3.455
	1.413(3)	1.352(3)	1.345(4)	1.419(2)
H-F	C11–O1	C12–O1	C11–S1	C12–S1	20.94	9.09	3.477
	1.343(7)	1.41(1)	1.717(4)	1.768(6)

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To further clarify how heteroatom fusion affects molecular packing, ESP maps were generated based on single-crystal structures of benzoheterocyclic [b]-fused BODIPYs using DFT calculations at the TPSSh/6-311 + G(d, p) level. In all heteroatom-fused BODIPY molecules, the fluorine atom—owing to its high Pauling electronegativity—induces a stronger negative electrostatic potential, which concentrates along the BF₂ group and extends laterally. Distinct differences in ESP distribution were observed depending on the nature of the fused heteroatom. Nitrogen- and sulfur-fused regions exhibited a relative decrease in electron density (indicated by blue and yellow regions), whereas oxygen-fused regions displayed an increase in electron density (highlighted by red areas). These differences arise from the varying electronegativities of the heteroatoms. The electronegativity of nitrogen leads to hydrogen atoms carrying a positive charge (47.1–48.5 kcal mol⁻¹), sulfur with lower electronegativity results in a weaker positive electrostatic potential (13.4–16.7 kcal mol⁻¹), and the high electronegativity of oxygen leads to a strong negative potential (−39.3 to −40.7 kcal mol⁻¹) (Fig. 3). Notably, the fluorine atom is located at the center of both the heteroatom-fused region and the BODIPY backbone, making it highly sensitive to changes in the electrostatic environment after introduction of heteroatoms. By analyzing the potential strength in the fluorine region, differences in the crystal packing of the compounds in the electrostatic potential maps of H-A, H-B, and H-C can be observed. The strong electronegativity of oxygen can induce a stronger negative potential around the fluorine atom, leading to the following electrostatic potential ranking in the center of the BF₂ group: H-B (−54.3 kcal mol⁻¹) < H-C (−48.6 kcal mol⁻¹) < H-A (−37.0 kcal mol⁻¹). In the case of asymmetric fused derivatives (H-D, H-E, and H-F), the presence of two different heteroatoms introduces more complex intermolecular interactions. Analysis of packing arrangement and ESP distribution reveals that the overlapping regions of the slip-stacked molecular skeletons (particularly in the heteroatom-fused zones) are influenced by both electrostatic attraction and repulsion. Specifically, H-D exhibits electrostatic repulsion, while H-E and H-F demonstrate electrostatic attraction. Among these, H-E shows the greatest difference between positive and negative charges, leading to stronger intermolecular attraction and thus the shortest π–π stacking distance: H-E (d_H-E = 3.455 Å) < H-F (d_H-F = 3.477 Å) < H-D (d_H-D = 3.519 Å). These findings emphasize that the identity and position of fused heteroatoms substantially modulate the electrostatic potential landscape, which in turn affects molecular packing and may critically influence optoelectronic properties.

NICS(0) analysis further confirms that the introduction of different heteroatoms significantly influences the electronic distribution within the molecules. In compound H-C, the aromaticity of the adjacent pyrrole ring (NICS(0) = −4.90), the benzene ring (−8.03), and the six-membered boron–nitrogen heterocycle (3.29) is notably weaker than in the two other symmetric analogues. However, the resonance effect of the lone pair electrons on the pyrrole nitrogen and the global ring current induced by the thiophene unit in the rigid framework cooperatively contribute to the formation of a global conjugated system,³⁶ leading to a more pronounced redshift in both the absorption (λ_max = 616 nm) and emission spectra (λ_em = 627 nm) of H-C, compared to those of H-A and H-B. The introduction of various heteroatoms in compounds H-D, H-E, and H-F allows electrons within the molecule to move freely across multiple atoms, promoting a continuous π-system along the fused ring framework and thus markedly altering the overall aromatic character. For instance, the NICS(0) values of the thiophene ring shifts from −4.68 in H-C to −5.51 in H-D, while that of the fused pyrrole ring changes from −4.81 in H-A to −3.94 in H-D. Their adjacent pyrrole ring exhibits stronger delocalization characteristics. These comparative NICS(0) values clearly highlight the crucial role of heteroatom orbital characteristics in modulating π-conjugation and aromaticity throughout the molecular backbone.

Photophysical properties

At room temperature in dichloromethane, compounds H-A–H-F exhibit strong red fluorescence. Compared to the naphtho-[b]-fused counterparts, which show a low fluorescence quantum yield (Φ_F = 0.09),²⁷ the benzoheterocyclic [b]-fused BODIPYs demonstrate remarkably high fluorescence quantum yields (Φ_F = 75.7–85.4%) and low non-radiative decay rate constants (K_nr < 0.5 × 10⁸ s⁻¹) (Fig. 4 and Table 2). These results suggest that benzoheterocyclic fusion effectively enhances the luminescence efficiency and confers strong resistance to non-radiative relaxation. Among the symmetric heteroatom-fused derivatives, both absorption and emission maxima exhibit a redshift trend that correlates with decreasing electronegativity of the fused heteroatoms: H-B (588/598 nm), H-A (609/617 nm), and H-C (616/627 nm). The most pronounced redshift observed in H-C can be attributed to the combined effects of electronegativity-induced electron distribution differences and the global conjugated system. Similarly, fusing more electronegative atoms like oxygen with less electronegative counterparts such as nitrogen or sulfur also results in progressive spectral redshifts across the asymmetric derivatives: H-E (593/604 nm), H-F (600/612 nm). Interestingly, H-C exhibits the lowest fluorescence quantum yield (75.7%) among the symmetric derivatives, potentially due to the heavy atom effect of sulfur, which enhances spin–orbit coupling and promotes intersystem crossing, thereby reducing radiative efficiency. This is further reflected in the trend of fluorescence lifetimes (τ): H-C (5.32 ns) > H-A (5.21 ns) > H-B (4.99 ns), indicating that heteroatom electronegativity influences not only emission energy but also excited-state dynamics. Specifically, more electronegative atoms may enhance vibrational or electron-vibration coupling within the molecule, intensifying non-radiative decay pathways and shortening the fluorescence lifetime. For the asymmetric derivatives (H-D to H-F), structural asymmetry introduces variations in orbital overlap, complicating the relaxation pathways from the excited state to the ground state. This results in noticeable differences in fluorescence lifetimes and highlights the role of molecular geometry and electronic distribution introduced by heteroatom fusion in tuning the photophysical behavior.

Fig. 4 Absorption (top) and normalized fluorescence (bottom) spectra of H-A–H-F in dichloromethane at room temperature.

Table 2 Photophysical and electrochemical properties of BODIPYs H-A–H-F in dichloromethane

Dye	λ max ^a (nm)	ε/10^5 ^b (M^−1 cm^−1)	λ em ^c (nm)	Φ F ^d (%)	τ ^e (ns)	K r/10^8 ^f (s^−1)	K nr/10^8 ^g (s^−1)
a Absorption maxima (λmax). b Molar extinction coefficients (at λmax). c Emission maxima (λem). d Fluorescence quantum yield (ΦF). e Lifetime (τ). f Radiation rate constant. g Non-radiation rate constant.
H-A	609	2.76	617	85.4	5.21	1.64	0.28
H-B	588	1.91	598	81.8	4.99	1.64	0.36
H-C	616	1.83	627	75.7	5.32	1.42	0.46
H-D	599	1.36	614	78.9	4.45	1.77	0.47
H-E	593	1.75	604	79.6	4.55	1.75	0.45
H-F	600	1.58	612	84.4	5.08	1.66	0.31

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The aggregation-caused quenching (ACQ) effect significantly limits the practical application of organic luminescent materials in biomedical, optical, and electronic fields.⁵⁹ In contrast, J-type aggregates can effectively suppress ACQ by enhancing exciton coupling between molecules, thereby improving luminescent efficiency and reducing the photoluminescence decay typically observed in traditional materials.⁶⁰ This makes J-type aggregates particularly attractive for advanced optoelectronic and bioimaging applications.

In this study, we investigated the aggregation behavior and spectral characteristics of heteroatom-fused BODIPY derivatives (H-A–H-F). At a fixed dye concentration, these compounds were dissolved in THF–water mixtures with varying water volume fractions (f_w), and their photophysical responses were monitored. When the water content increased from 0% to 70%, no significant changes were observed in the shape or intensity of electronic absorption bands, although a gradual decrease in fluorescence intensity was noted (Fig. S32–S55). However, beyond 70% water content, the absorption bands began to broaden and weaken significantly, accompanied by a drastic decline in fluorescence intensity. In the 80–99% f_w range, all compounds exhibited sharply reduced optical density and visible solution color changes, attributable to reduced solubility, enhanced self-aggregation induced by the poor solvent environment, and solvent polarity effects in the THF–water binary system. Controlling the water content below 80% was found to optimize dye aggregation behavior, which is crucial for potential applications such as fluorescent biomarkers and photodynamic therapeutic agents.

To further examine concentration-dependent aggregation, the absorption and fluorescence spectra of H-A–H-F were recorded at increasing concentrations (5 μM to 30 μM) in a constant 95% THF–water mixture. As the concentration (f_w = 95%) increased, the absorption spectrum of H-A–H-F restored their distinct fine structures, and the fluorescence intensity of J-type aggregation bands in H-A–H-C increased progressively (Fig. S32–S55). In contrast, H-D–H-F displayed a non-monotonic trend, with the fluorescence intensity initially rising and then decreasing, suggesting more complex aggregation dynamics.

Single-crystal stacking analysis and fluorescence measurements collectively confirmed that all six dyes form J-type aggregates in the THF–water mixtures across varying solvent ratios. Notably, in the 95% THF–water mixture, as the compound concentration was gradually increased to 20 μM, the J-type aggregates effectively suppressed ACQ and enhanced fluorescence brightness via exciton coupling between molecules. These findings highlight the importance of precisely tuning the aggregation behavior and photophysical properties of BODIPY dyes in water–organic media. Such control is vital for the rational design of high-performance near-infrared fluorophores and their integration into optoelectronic devices.

Electrochemical properties

Under identical experimental conditions, heteroatom-fused BODIPYs H-A–H-F exhibit similar electrochemical behavior, typically featuring a set of reversible oxidation–reduction peaks. Notably, the strong electron-donating ability of nitrogen introduces additional redox-active sites, evidenced by the appearance of a secondary oxidation peak in the voltammograms (Fig. 5, top). From the comparison of symmetric compounds H-A–H-C, the incorporation of oxygen or sulfur mainly influences the LUMO energy level (LUMO_H-B = −3.50 eV; LUMO_H-C = −3.64 eV), while having a minimal impact on the HOMO energy levels (HOMO_H-B = −5.45 eV; HOMO_H-C = −5.47 eV). This leads to a narrowed electrochemical bandgap. Conversely, nitrogen fusion in H-A elevates both the HOMO (HOMO_H-A = −4.98 eV) and LUMO (LUMO_H-A = −3.29 eV) energy levels, enhancing their sensitivity to redox processes. This is reflected in the noticeable shift of the first oxidation and reduction peaks to the left, particularly the first oxidation peak, which shifts more significantly. This change results in a further narrowing of the energy gap, an effect also observed in compounds H-D and H-E (Fig. 5 and Table 3).

Fig. 5 CV of H-A–H-F in dichloromethane (top). The electrochemical values of compounds H-D–H-F exhibit a pattern that aligns with the half-sum rule of the summation of different combinations of symmetric compounds H-A–H-C (bottom). The gap is derived from the “E^electro_g” column data in Table 3.

Table 3 Electrochemical properties of BODIPYs H-A–H-F in dichloromethane

Dyes	E ^onsetred (V)	E ^onsetox (V)	LUMO (eV)	HOMO (eV)	E ^electrog (eV)	E ^optg (eV)
E ^onsetred = the onset reduction potentials; E^onsetox = the onset oxidation potentials; LUMO = −(E^onsetred+ 4.8); HOMO = −(E^onsetox+ 4.8); E^electrog = LUMO − HOMO; E^electrog = bandgap, obtained from the intercept of the electrochemical data; λonset = the onset of absorption in CH2Cl2 solution of H-A–H-F; E^optg = 1240/λonsetE^optg = bandgap, obtained from the intercept of the absorption spectra.
H-A	−1.51	0.18	−3.29	−4.98	1.69	1.98
H-B	−1.30	0.65	−3.50	−5.45	1.95	2.04
H-C	−1.16	0.67	−3.64	−5.47	1.83	1.94
H-D	−1.37	0.42	−3.43	−5.22	1.79	1.99
H-E	−1.40	0.42	−3.40	−5.22	1.82	2.02
H-F	−1.23	0.67	−3.57	−5.47	1.90	1.99

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The modulation of HOMO–LUMO levels by heteroatom combinations becomes especially pronounced in asymmetric systems (Fig. 5, bottom). For example, in compound H-E, the LUMO energy level (LUMO_H-E = −3.40 eV) approximates the arithmetic mean of H-A and H-B (LUMO_H-A = −3.29 eV, LUMO_H-B = −3.50 eV). Similarly, the HOMO energy level of H-E (HOMO_H-E = −5.22 eV) shows the same characteristic, being half of the sum of the HOMO energy levels of H-A and H-B (HOMO_H-A = −4.98 eV, HOMO_H-B = −5.45 eV). A similar trend is observed for the electrochemical energy gap (E^electro_g). The energy gap of H-E (1.82 eV) is also approximately equal to the half-sum of the energy gaps of H-A and H-B (1.69 eV and 1.95 eV, respectively). We define this consistent trend as the “half-sum rule”, which also holds true for compounds H-D and H-F, with computational deviations within ±2%. This rule suggests that the electronic structure of asymmetric heteroatom-fused BODIPYs results from a linear superposition of the individual heteroatomic contributions. The averaging of orbital energies reflects a unique orbital mixing behavior, where LUMO and HOMO levels are governed by the cooperative influence of the two different heteroatoms.

This previously unreported phenomenon, termed the “half-sum rule”, arises from the asymmetric heteroatom fusion. It signifies that the impact of heteroatoms on a molecule's electronic structure extends beyond simple additive or canceling effects. Instead, it follows a distinct pattern: the heteroatoms modulate the HOMO and LUMO energy levels in a coordinated manner, resulting in a unique energy level distribution. The “half-sum rule” reveals the electronic effects introduced by different heteroatoms in a molecule, which interact in a specific way within the LUMO and HOMO energy levels, forming a new energy level distribution pattern. This concept provides a refined strategy for tuning the electronic properties of materials. The novel “half-sum rule”, based on the electronic effects of heteroatoms, not only expands our understanding of molecular electronic structure modulation but also offers new directions and insights for future research in material design, catalysis, optoelectronics, and other fields.

Theoretical calculations

To gain a deeper understanding of the electronic properties of the heteroatom-fused BODIPY dyes, we performed density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations for compounds H-A–H-F. The TD-DFT results (Table S10) indicate that the strongest absorption bands for all compounds arise from the S₀ → S₁ transition, which is caused by their corresponding HOMO → LUMO excitation. DFT analyses (Fig. 6) reveal that the HOMOs of these dyes are highly delocalized across the entire π-conjugated framework, whereas their LUMOs are mainly localized within the BODIPY core and adjacent fused regions. Notably, both the HOMO and LUMO exhibit negligible electron density on the mesityl substituents.

Fig. 6 Pictorial presentation of the LUMO, HOMO and their energy levels for H-A–H-F.

These differences in electronic distribution are consistent with the observed electrochemical behavior and arise from the nature of the fused heteroatoms. Nitrogen fusion influences both the HOMO and LUMO levels by modulating the electron density in the surrounding regions, thereby reducing the HOMO–LUMO gap, as shown in Fig. S66. In contrast, oxygen (H-B) and sulfur (H-C) fusion reduces the energy gap by predominantly affecting the LUMO distribution. Due to its higher electronegativity, oxygen leads to reduced electron delocalization, whereas sulfur, with its lower electronegativity and larger atomic radius, promotes more uniform electron density distribution and broader conjugation.²⁰ Therefore, H-C exhibits a smaller calculated energy gap (2.10 eV) compared to H-B (2.21 eV), resulting in a more significant redshift in its absorption spectrum. However, the energy gaps do not fully match the extent of the spectral redshift, suggesting that the redshift is not solely governed by the size of the energy gap but is also influenced by interactions between the heteroatoms. Interestingly, the “half-sum rule” observed in electrochemical experiments is also supported in the theoretical calculations for the asymmetric derivatives H-D–H-F, with relative deviations of less than ±1%. For instance, in compound H-E, the calculated LUMO energy level (−3.16 eV) closely approximates the average of those in H-A (−3.01 eV) and H-B (−3.31 eV). Likewise, the half-sum of the HOMO energy levels of H-A and H-B equals −5.32 eV, which exactly matches the HOMO energy level of H-E. The same trend is observed for H-D and H-F. Both theoretical and experimental results confirm the reliability of the “half-sum rule”, demonstrating that the electronic structures of asymmetric compounds can be accurately predicted from their symmetric analogues. This finding provides compelling theoretical validation for the rule and offers a rational framework for the design of advanced functional materials based on predictable electronic effects from heteroatom incorporation.

Conclusions

In this study, we have presented the complete structures and comprehensive optoelectronic data of six heteroatom-fused BODIPY chromophores, filling the gap in the existing literature regarding the systematic investigation of how different heteroatoms (nitrogen, oxygen, and sulfur) influence the optoelectronic properties of BODIPY dyes. A simple and efficient synthetic strategy was developed, enabling the preparation of mono-fused, di-fused, and asymmetric heteroatom-fused BODIPYs from tetrahalogenated BODIPY monomers via a one-pot nucleophilic aromatic substitution coupled with C–H activation. A novel method for identifying the types of heteroatom-fused groups was established through ¹⁹F NMR spectral analysis. Single-crystal X-ray diffraction revealed that different heteroatoms modulate molecular packing and electronic distribution through variations in electronegativity and intermolecular interactions. Notably, the delocalization of electrons and electron transfer phenomena were observed for the first time in asymmetric, rigid, π-conjugated heteroatom-fused structures using electrostatic potential maps and NICS(0) calculations. These heteroatom-fused BODIPYs exhibit excellent photophysical performance, with high fluorescence quantum yields (75.7–85.4%) and strong resistance to non-radiative decay. Their absorption and emission characteristics show clear promise for near-infrared dye applications. Combined analysis of fluorescence spectra and single-crystal packing confirmed the formation of J-type aggregates in THF–water mixtures, further emphasizing the role of heteroatom fusion in precisely tuning aggregation behavior and photophysical properties. Electrochemical analysis revealed the effects of asymmetric heteroatom incorporation on the HOMO and LUMO energy levels, culminating in the discovery of a previously unreported “half-sum rule”. This rule, further supported by DFT/TD-DFT calculations, allows for accurate prediction of electronic structures in asymmetric derivatives based on their symmetric counterparts, with excellent agreement between theoretical and experimental data. Overall, our research results enable more precise molecular or material design with specific properties based on the regularity of electronic structures and the mechanism of heteroatom effects in future molecular design and functionalization. The insights gained from the heteroatom effects and the “half-sum rule” open new avenues for research and development in molecular design, catalysis, optoelectronics, and beyond.

Author contributions

Limin He: synthesis, characterization, formal analysis, and writing original draft. Yanqing Li, Yongli Zhang, and Luyan Tian: synthesis and characterization. Yunxia Zhao and Xiaomao Zhou: data analysis. Shulin Gao: DFT calculations. Xiangguang Li: conceptualization, resources, supervision, and writing – review and editing. Yanhua Yang and Wei Jiang: resources, supervision and writing – review. Zhaohui Wang: supervision and writing – review.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: details of experimental methods, characterisation data, copies of NMR spectra and crystallographic data. See DOI: https://doi.org/10.1039/d5qo01187f.

CCDC 2477577–2477582 (H-A, H-B, H-C, H-D, H-E and H-F) contain the supplementary crystallographic data for this paper.^61a–f

Acknowledgements

This work was support by the National Natural Science Foundation of China (22065019 and 22122503), the Program for Young and Middle Aged Academic and Technical Leaders Reserve Talents of Yunnan Province (202105AC160043), the Kunming “Spring City Program” for Youth Top-Notch Talents (C202014001), the Scientific Research Funds of Kunming University (XPZJ2205 and XPZJ2205-2), the Research Projects of Yunnan Education Department (2025Y1072), the Yunnan Key Laboratory of Metal–Organic Molecular Materials and Device (YNMO-ZD-2402), and the Frontier Research Team of Kunming University 2023.

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