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. 2026 Jul 15;65(38):e4566077. doi: 10.1002/anie.4566077

Triply N‐Confused Hexaphyrins(1.1.1.1.1.0) Synthesized From N‐Confused Hexaphyrin(2.1.1.1.1.1): Aromaticity Modulation, Metal Coordination, and Photothermal Conversion

Gaojie Zhu 1, Zimei Ma 1, Mingbo Zhou 2, Hailong Wang 3, Shijun Li 4, Rinat Nasibullin 5, Glib Baryshnikov 5, Chengjie Li 1, Hans Ågren 6,7, Jianxin Song 2, Jianzhuang Jiang 3, Qizhao Li 1,2,✉, Yongshu Xie 1,✉
PMCID: PMC13573039  PMID: 42454768

ABSTRACT

Antiaromatic compounds exhibit intriguing structures, reactivity, and physicochemical properties. To address the challenge in developing stable antiaromatic porphyrinoids, a vinylene bridge and an α,β′‐linked pyrrole unit were used to synthesize a 28π Möbius aromatic N‐confused oxohexaphyrin(2.1.1.1.1.1) (1). Subsequent oxidative Cβ–Cβ fusion afforded a stable Hückel antiaromatic formally triply N‐confused oxohexaphyrin(1.1.1.1.1.0) (2) as a hybrid of 24 and 28π systems with two peripheral NH moieties, enabling further C–N fusion with the adjacent C6F5 moiety under basic conditions to yield the corresponding N‐confused dioxohexaphyrin(1.1.1.1.1.0) (3), which shows Hückel antiaromaticity with a modulated circuit. Coordination of various metals within the distinct coordination cavities afforded two mono‐rhenium complexes 3‐Re‐1 (NNη2‐coordinated) and 3‐Re‐2 (NNO‐coordinated) along with a mono‐Pd complex 3‐Pd‐1 (CNNO‐coordinated) with modulated antiaromaticity. On this basis, the heterobimetallic complex 3‐O‐Pd‐Re was synthesized with the conjugation and antiaromaticity disrupted by further oxygenation. Notably, these compounds display distinctive near‐infrared (NIR) absorption, enabling pronounced photothermal conversion under 1064 nm laser irradiation with efficiencies up to 61.4%. This work composes an effective post‐fusion approach for developing multiply N‐confused porphyrinoids with tunable antiaromaticity and distinctive coordination properties to achieve efficient NIR photothermal conversion behavior.

Keywords: antiaromaticity, intramolecular fusion, N‐confused hexaphyrins, porphyrinoids

Based on the synthesis of a vinylene‐bridged 28π Möbius aromatic N‐confused hexaphyrin 1, stepwise C–C and C–N fusion afforded Hückel antiaromatic hexaphyrins 2 and 3, with photothermal conversion behavior enhanced by antiaromaticity and metal coordination to achieve efficiencies up to 61.4%, thus offering a route to novel antiaromatic porphyrinoids with distinctive properties by fusion of vinylene‐bridged N‐confused porphyrinoids.

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1. Introduction

As a classical concept, antiaromaticity has been attracting continuous research interest. Antiaromatic compounds usually exhibit intriguing properties like narrow band gaps, near‐infrared (NIR) absorption, efficient nonradiative decay, and enhanced charge mobility [1, 2, 3, 4, 5, 6, 7]. Hence, they may be developed as promising materials applicable in various fields like organic semiconductors [8, 9], energy storage [10, 11], photothermal conversion [12, 13, 14], and organic field‐effect transistors (OFETs) [15, 16, 17]. However, it is challenging to construct stable antiaromatic compounds owing to their inherent low stability.

Regarded as “life pigment”, porphyrin has been known as a class of 18π aromatic macrocycles. In recent years, porphyrin analogues, termed porphyrinoids, have emerged as versatile platforms for developing antiaromatic molecules [1, 18, 19, 20]. In this regard, modulation of meso‐carbon atom numbers could achieve 4nπ Hückel antiaromatic or Möbius aromatic macrocycles depending on their conformations. Thus, 16π norcorrole and 24π hexaphyrin(1.0.1.0.1.0) exhibit roughly planar conformations and distinctive antiaromaticity [21, 22, 23], whereas 20π homoporphyrin exhibits a twisted conformation and Möbius aromaticity (Figure 1a,b) [24]. Based on this background and the most intensively investigated hexaphyrins (Figure 1b) [25, 26, 27, 28], it could be expected that 28π antiaromatic hexaphyrins may be developed by incorporating a vinylene bridge into the macrocycle while maintaining the planar conformation. As we know, the introduction of a reactive α,β′‐linked N‐confused pyrrole unit may facilitate fusion of the α position to achieve stable planar conformations (Figure 1c) [29, 30, 31, 32, 33]. Hence, we speculate that the construction of 28π antiaromatic hexaphyrins would be feasible by incorporating a vinylene bridge with the assistance of an N‐confused pyrrole unit, allowing for both 4nπ conjugation and planar conformation.

FIGURE 1.

FIGURE 1

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Chemical structures of (a) porphyrin, N‐confused porphyrin, and related tetrapyrrolic macrocycles with various numbers of meso‐C atoms and tunable aromaticity; (b) hexaphyrin and its analogues; (c) intramolecular α‐fusion involving N‐confused pyrrole; and (d) Möbius aromatic N‐confused oxohexaphyrin(2.1.1.1.1.1) (1) and antiaromatic formally triply N‐confused hexaphyrins(1.1.1.1.1.0) (2 and 3) reported in this work (unless otherwise noted, aromaticity and antiaromaticity in the figure refer to traditional Hückel systems).

We herein report the synthesis of diphenylvinylene‐bridged N‐confused oxohexaphyrin(2.1.1.1.1.1) (1), which exhibits a twisted conformation and weak 28π Möbius aromaticity with the α position of the N‐confused pyrrole oxygenated, thereby hampering α‐fusion. However, the spatial proximity of the β carbons from two pyrrole units near the vinylene bridge enables oxidative Cβ–Cβ fusion to afford a Hückel antiaromatic formally triply N‐confused oxohexaphyrin(1.1.1.1.1.0) (2) as a hybrid of 24 and 28π systems that incorporates a pyrrolo[3,2‐e]indole unit (Figure 1d). Regioselective inner chlorination of 2 afforded 2‐Cl with severe distortion and weakened antiaromaticity (Scheme 1). Notably, 2 underwent further C–N fusion and oxygenation under basic and heating conditions, leading to the formation of a [5.6.5.5.6]‐pentacyclic‐ring‐embedded Hückel antiaromatic dioxohexaphyrin (3). Although no complex could be obtained for 2 or 2‐Cl, treatment of 3 with Re2(CO)10 yielded two distorted mono‐Re complexes (3‐Re‐1 and 3‐Re‐2) with weakened antiaromaticity, with the Re(I) atoms located within the two distinct cavities, respectively (Scheme 2). On the other hand, coordination of 3 with Pd(OAc)2 only afforded one of the two possible mono‐Pd(II) complexes (3‐Pd‐1), which shows slightly enhanced antiaromaticity. On this basis, further coordination of either 3‐Pd‐1 or 3‐Re‐2 could afford a heterobimetallic complex (3‐O‐Pd‐Re) accompanied by further oxygenation at the β‐position of the original N‐confused pyrrole, which interrupts the global π‐conjugation, resulting in nonaromaticity. Notably, these compounds show excellent stability and considerable near‐infrared (NIR) absorption, enabling photothermal conversion under 1064 nm laser irradiation with efficiencies up to 61.4%. These results highlight the remarkable effectiveness of combining a vinylene bridge with the N‐confusion and fusion approach to develop stable hexaphyrins with tunable aromaticity and unique metal coordination abilities, which in turn effectively improve the photothermal conversion behavior.

SCHEME 1.

SCHEME 1

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Syntheses of N‐confused oxohexaphyrin(2.1.1.1.1.1) (1) and fused N‐confused hexaphyrins(1.1.1.1.1.0) (2 and 3). Reaction conditions: i) TFA, DDQ, CH2Cl2; ii) CF3COOAg, CH2Cl2; iii) magic blue, CH2Cl2; iv) Et3N, o‐dichlorobenzene, 130°C. Ar = C6F5.

SCHEME 2.

SCHEME 2

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Synthetic routes for metal complexes of 3. Reaction conditions: i) Re2(CO)10, o‐DCB, 160°C; ii) Pd(OAc)2, NaOAc, CH2Cl2/MeOH; iii) Pd(OAc)2, NaOAc, MeCN, 70°C; iv) Re2(CO)10, o‐DCB, 160°C, then MeCN, 70°C.

2. Results and Discussion

2.1. Synthesis and Characterization

The synthetic routes are outlined in Scheme 1. 1 was synthesized as a dark purple solid in 13% yield by condensation of N‐confused bilane dicarbinol [25] with Z‐diphenyldipyrroloethene (PPE) [34, 35] in the presence of trifluoroacetic acid, followed by oxidation with 5 eq. of 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ). The high‐resolution mass spectrometry (HRMS) of 1 shows a molecular ion peak at m/z = 1477.1772, indicative of an oxohexaphyrin structure (Figure S41). Consistently, its N‐confused oxohexaphyrin(2.1.1.1.1.1) framework was revealed by single‐crystal X‐ray diffraction analysis (Figure 2a,b) [36]. As a whole, the molecule adopts a singly twisted conformation, satisfying the criteria of a Möbius topology. All the pyrrolic N atoms and the phenyl groups are oriented inward the macrocycle, with the N‐confused pyrrole ring A existing in a lactam form, which enables the formation of intramolecular NH···N and NH···O hydrogen bonds (Figure 2a,b). Obviously, the vinylene moiety induces severe strain and thus becomes the most distorted position along the conjugated chain, showing the largest torsion angle of 55.8° (C13‐C14‐C15‐C16), which is considerably larger than the corresponding value of 26.5° observed for the typical Möbius aromatic [28]hexaphyrin [37] (Figure S50) and those of other related Möbius aromatic hexaphyrins reported by Osuka and coworkers [38, 39, 40]. Accordingly, the harmonic oscillator model for aromaticity (HOMA) [41] value of 1 (0.70) (Figure S51) is smaller than that of the [28]hexaphyrin (0.85), suggesting that the degree of delocalization of 1 is considerably weaker than that of the [28]hexaphyrin, which is consistent with the weakened Möbius aromaticity of 1 as revealed by the NMR data and DFT calculations (vide infra).

FIGURE 2.

FIGURE 2

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Complementary views of the molecular structures of 1 (a, b), 2‐Cl (c, d), and 3 (e, f). Phenyl groups in the side views, C6F5 groups, and hydrogen atoms attached to carbon atoms are omitted for clarity. The dotted cyan lines represent hydrogen bonds.

The 1H NMR spectrum of 1 in acetone‐d 6 reveals pyrrolic β‐protons in the relatively low field region of 6.37–7.76 ppm (Figures S3–S6), which are less downfield shifted relative to those of typical Möbius aromatic [28]hexaphyrin(1.1.1.1.1.1) (7.5–8.5 ppm) [37], indicating the weaker diatropic ring current of 1 induced by its highly twisted structure (vide supra). Interestingly, the phenyl signals are upfield‐shifted to the region of 5.75–6.83 ppm, which may be related to the location of the phenyl rings in the shielding facial region of the aromatic macrocycle as revealed by the crystal structure (vide supra). Meanwhile, four NH signals in the relatively low field within 6.50–9.44 ppm can be attributed to the presence of multiple intramolecular hydrogen bonds [24, 38, 42]. Given the relatively good flexibility of Möbius macrocycles, dynamic conformational interconversion may occur upon temperature changes [37]. Thus, the 1H NMR spectrum of 1 was recorded at 233 K (Figure S7), which reveals that the phenyl proton signals become better resolved than those observed at 298 K (Figure S3), indicating that rotation of the phenyl groups is restricted at low temperatures. On the other hand, the pyrrolic proton signals (6.49–7.89 ppm) show no obvious changes compared to those observed at 298 K, implying that the macrocycle conformation is largely preserved irrespective of the temperature changes, in sharp contrast to that of [28]hexaphyrin(1.1.1.1.1.1) [37]. This observation indicates that 1 possesses a more rigid conformation, which may arise from both the steric hindrance associated with the bulky diphenyl vinylene moiety and the rigidification effect of the hydrogen bonds involving the lactam‐oxygen atom of the N‐confused pyrrole unit.

Considering that the twisted conformation of 1 induces spatial proximity between the β positions (C13, C18) of two pyrrole units near the vinylene bridge, which may facilitate oxidative intramolecular fusion [43, 44, 45, 46, 47], 1 was treated with CF3COOAg in CH2Cl2, affording a red solid of 2 in 73% yield. The HRMS of 2 shows a molecular ion peak at m/z = 1475.1611 (Figure S42), 2 amu lower than that of 1 (Figure S41), suggesting intramolecular fusion by removing two hydrogen atoms. Upon treating 2 with tris(4‐bromophenyl)ammoniumyl hexachloroantimonate (magic blue) in CH2Cl2, a monochlorinated derivative 2‐Cl was obtained in 60% yield, and the HRMS of 2‐Cl shows a molecular ion peak at m/z = 1509.1230 (Figure S43), suggesting the simple chlorination of 2. Similar magic blue‐triggered chlorination reactions have been reported to proceed through a radical mechanism [48, 49, 50]. To explore the reaction pathway, a typical radical scavenger, 2,2,6,6‐tetramethylpiperidin‐1‐oxyl (TEMPO), was added to the reaction mixture, and the formation of 2‐Cl was not obviously suppressed, whereas the addition of nucleophilic reagents substantially lowered the yield of 2‐Cl (Figure S1). These results indicate an ionic chlorination mechanism, although the exact reason for the high regioselectivity remains unclear at the present stage. Consistent with the HRMS data, X‐ray diffraction analysis of 2‐Cl clearly reveals inner monochlorination at C19 with a C19–Cl1 distance of 1.711(2) Å (Figure S52), and pyrrole rings C and D are inverted and interconnected through C13 and C18 to generate a pyrrolo[3,2‐e]indole moiety [51] with the two phenyl groups and the nitrogen atoms of rings C and D lying at the peripheral positions (Figure 2c,d). As a result, pyrrole rings A, B, and F are nearly coplanar, with ring E and the fused tricyclic ring tilted from the mean plane with dihedral angles of 53.2° and 43.9°, respectively. The 1H NMR spectrum of 2‐Cl in CDCl3 reveals seven outer pyrrolic CH signals and one inner CH signal in the relatively high and low field regions at 5.04–6.68 ppm and 8.66 ppm, respectively (Figures S15–S17), indicating that 2‐Cl is weakly antiaromatic. Meanwhile, two inner NH protons appear at 10.79 and 12.58 ppm, and two outer NH signals are observed at 9.19 and 10.93 ppm. Similarly, the 1H NMR spectrum of 2 in CDCl3 exhibits seven outer pyrrolic protons at 4.01–5.83 ppm and two inner CH protons at 12.53 and 12.78 ppm (Figures S10–S12). Based on these data, the difference in the chemical shifts of the outer and inner CH protons (Δout‐in) for 2 (−7.46 ppm) is more negative than that of 2‐Cl (−2.59 ppm), confirming stronger antiaromaticity of 2. Consistently, the inner and outer NH signals of 2 appear in lower (16.81 and 16.96 ppm) and higher fields (5.96 and 6.22 ppm), respectively, compared with those of 2‐Cl. Meanwhile, the peripheral phenyl signals of 2 are slightly upfield‐shifted to 6.70–7.10 ppm relative to those of 2‐Cl (7.14–7.52 ppm). Based on these results, it can be inferred that the conformation of 2 is roughly planar in contrast to the Möbius topology of 1, while the π electron number remains unchanged, and thus the Möbius aromaticity of 1 has been converted to the Hückel antiaromaticity of 2 upon intramolecular C–C fusion, and the antiaromaticity of 2‐Cl is weaker than that of 2 because of the distortion induced by inner chlorination. To our knowledge, the transformation of 1 into 2 represents the only example of conversion from a Möbius aromatic hexaphyrin to a Hückel antiaromatic one through an intramolecular fusion reaction [52, 53].

Given the presence of two peripheral NH moieties, it is anticipated that 2 may be susceptible to C–N fusion with the adjacent C6F5 groups [54, 55]. As expected, treatment of 2 with Et3N in ortho‐dichlorobenzene (o‐DCB) at 130°C afforded a red solid of 3 in 47% yield. The HRMS analysis of 3 reveals a molecular ion peak at m/z = 1471.1481 (Figure S44), indicating that C–N fusion has occurred by removing a HF molecule and a second O atom has been attached to one of the pyrroles. The yield of 3 is barely changed under a nitrogen atmosphere, whereas the reaction performed in anhydrous o‐DCB affords a lowered yield of 3 accompanied by increased byproduct formation (Figure S2), where the corresponding hexaphyrin without attaching the additional oxygen atom could be detected by mass spectrum (Figure S45). These results indicate that the additional oxygen atom in compound 3 is likely derived from water. Consistent with the HRMS data, the 1H NMR spectrum of 3 in CDCl3 shows only one outer NH at 6.06 ppm and one inner CH at 13.10 ppm, while seven outer pyrrolic CH signals resonate in the high field region at 3.52–5.90 ppm (Figures S20–S22). Thus, the Δout‐in value was calculated to be −8.04 ppm, indicative of slightly enhanced antiaromaticity compared to that of 2 (−7.46 ppm), which arises from improved planarity via C–N fusion. Besides, three inner NH signals appear in the low field at 15.85, 16.24, and 17.16 ppm. The molecular structure of 3 was further confirmed by X‐ray diffraction analysis (Figure 2e,f). Indeed, the N4 atom of pyrrolic ring D is fused with the adjacent meso‐C6F5 to form a fused [5.6.5.5.6]‐pentacyclic moiety, and thus 3 adopts a relatively planar conformation with a root‐mean‐square deviation (RMSD) value of 0.409 Å. Consistent with the HRMS and NMR results, an O atom is attached at the inner β position of ring D to afford C19═O2 with a typical double bond length of 1.231(8) Å (Figure S53), which could possibly interrupt the conjugation circuit. Hence, the antiaromatic feature of 3 is indicative of the contribution of a dipolar resonance state of the C═O moiety [56, 57]. Regretfully, such a polarized resonance effect could not be clearly revealed by analyzing the C═O peaks in the infrared (IR) spectrum (Figure S57), even with the aid of density functional theory (DFT) calculations (Table S8), which may be related to the complicated coexistence of amide carbonyl moieties and interference of intramolecular hydrogen bonds. However, the polarized resonance could be well elucidated by magnetically induced current (GIMIC) analysis (vide infra) [58]. It is obvious that molecule 3 still possesses another peripheral NH moiety, which might undergo further fusion with the adjacent C6F5 group. However, attempts to obtain a doubly C–N fused product from compound 3 turned out to be unsuccessful even after repeated trials, which may be attributed to the considerable structural strain induced by further fusion.

2.2. Metal Coordination

As mentioned above, both hexaphyrins 2 and 3 possess a lactam form N‐confused pyrrole and two N‐confused pyrrole units generated upon fusion. Thus, they are formally triply N‐confused hexaphyrins(1.1.1.1.1.0) analogues featuring antiaromatic rectangular frameworks comprised of two cavities with potential metal coordination capability distinct from the reported 26π aromatic triangular‐shaped triply N‐confused hexaphyrin (Figure 1b) [59]. Considering the distinct skeletons of hexaphyrins 1–3, we proceeded to investigate their coordination chemistry, aiming to further modulate the aromatic properties and electronic behavior. However, no stable metal complexes could be isolated upon treating 1 or 2 with various metal salts [60]. In contrast, heating 3 with Re2(CO)10 in o‐DCB at 160°C afforded two more polar complexes 3‐Re‐1 and 3‐Re‐2 in yields of 20% and 26%, respectively. The single crystal structure of 3‐Re‐1 reveals that a rhenium center is coordinated by two nitrogen atoms (N1, N2) from rings A and B and three carbonyl ligands (Figure 3a,b), with Re─N bond lengths of 2.202 and 2.145 Å, respectively, and Re1─CO bond lengths in the range of 1.869–1.959 Å (Figure S54). These bond lengths are comparable to those of similar Re(I) complexes [61, 62, 63]. Interestingly, the Re atom also coordinates with the C═C double bond of ring C, with Re1···C11 and Re1···C12 distances of 2.887 and 2.534 Å, respectively, indicating a Re‐η 2‐CC bonding mode. Consistently, HRMS of 3‐Re‐1 shows a molecular ion peak at m/z = 1741.0827 (Figure S46). The 1H NMR spectrum of 3‐Re‐1 shows outer pyrrolic β‐protons at 3.92 and 5.20–6.69 ppm, while the inner CH appears at 10.59 ppm (Figure S25). Considering that the chemical shift of the inner CH of 3‐Re‐1 may be influenced by both the ring current and the nearby Re center, the Δout‐in value may be inappropriate to analyze the antiaromaticity character. On the other hand, the outer pyrrolic β‐protons of 3‐Re‐1 are downfield‐shifted relative to 3 (3.52–5.90 ppm), indicative of weakened antiaromaticity upon coordination, which may arise from the structural distortion induced by Re coordination, as evidenced by the considerably enhanced RMSD value of 3‐Re‐1 (0.623 Å) compared to that of 3 (0.409 Å).

FIGURE 3.

FIGURE 3

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Complementary views of the molecular structures of 3‐Re‐1 (a, b), 3‐Re‐2 (c, d), and 3‐O‐Pd‐Re (e, f). Phenyl groups in the side views, C6F5 groups, and hydrogen atoms attached to carbon atoms are omitted for clarity. The dotted cyan lines represent hydrogen bonds.

In contrast to 3‐Re‐1, single‐crystal X‐ray analysis reveals that the Re(I) center in 3‐Re‐2 is coordinated within another cavity to two nitrogen atoms (N5, N6), one oxygen atom (O1), and three carbonyl ligands (Figure 3c,d), with Re─N, Re─O1, and Re─CO bond lengths of 2.167–2.213, 2.185, and 1.894–1.925 Å, respectively (Figure S55). Consistently, HRMS of 3‐Re‐2 shows a molecular ion peak at m/z = 1741.0802 (Figure S47). This type of coordination induces even greater skeletal distortion, as revealed by the further enlarged RMSD value of 0.753 Å. Consequently, the antiaromatic character of 3‐Re‐2 is further weakened, and thus its 1H NMR spectrum displays further downfield‐shifted outer pyrrolic β‐protons (4.28 and 5.82–6.90 ppm) compared to 3‐Re‐1. Meanwhile, a further upfield‐shifted inner CH proton is observed at 9.38 ppm. In addition, one outer and two inner NH protons appear at 7.58, 9.95, and 10.03 ppm, respectively (Figures S28–S30). According to the NMR results, the antiaromaticity is elucidated to be gradually weakened from 3 to 3‐Re‐1 and 3‐Re‐2, which may be attributed to the increasing structural distortion induced by Re coordination. These antiaromatic changes align well with theoretical calculations (vide infra).

On the other hand, treatment of 3 with Pd(OAc)2 afforded a red solid of 3‐Pd‐1 in 79% yield. HRMS of 3‐Pd‐1 shows a molecular ion peak at m/z = 1573.0283 (Figure S48), indicating the formation of a monopalladium complex. Consistently, the 1H NMR spectrum of 3‐Pd‐1 shows no inner CH signal, suggesting the coordination of Pd(II) within the cavity is identical to that of 3‐Re‐1. Meanwhile, outer pyrrolic CH protons at 3.78–5.94 ppm suggest that the antiaromatic nature is still retained (Figures S33–S35).

With these monometallic complexes in hand, we further explored the possibility of preparing bimetallic complexes. Interestingly, treating 3‐Re‐2 with Pd(OAc)2 and 3‐Pd‐1 with Re2(CO)10 furnished the identical bimetallic complex 3‐O‐Pd‐Re in yields of 42% and 35%, respectively. The single crystal structure reveals the further attachment of a keto group at the outer β‐position of N‐confused pyrrolic ring A, accompanied by coordination of Re(I) and Pd(II) within the NNO and CNNO cavities, respectively (Figure 3e,f). The Pd(II) center adopts a slightly distorted square‐planar coordination geometry with the coordination bond lengths and cis‐coordination angles varying in the ranges of 1.932−2.127 Å and 86.5°−92.4°, respectively. Simultaneously, the Re(I) center is coordinated in an octahedral geometry with two nitrogen atoms (N5, N6), one oxygen atom (O1), and three carbonyl ligands, with bond distances similar to those of 3‐Re‐2 (Figure S56). Consistently, all three complexes (3‐Re‐1, 3‐Re‐2, and 3‐O‐Pd‐Re) exhibit prominent IR absorption bands spanning from 1850 to 2000 cm−1, characteristic of stretching vibrations of the coordinated CO ligands [61] (Figures S59–S61). Notably, the further attached keto group on ring A (C3─O3 = 1.216 Å) disrupts the global π‐conjugation and thus leads to nonaromaticity of the macrocycle, as evidenced by the 1H NMR spectrum (Figure S38), which shows pyrrolic CH signals in the region of 6.41–6.92 ppm.

2.3. Theoretical Studies on Aromatic Properties

As revealed by the crystal structure and 1H NMR spectra, N‐confused oxohexaphyrin(2.1.1.1.1.1) 1 exhibits Möbius aromaticity, albeit weaker than that of [28]hexaphyrin(1.1.1.1.1.1) [37]. In addition to the observed NMR data, chemical shifts for compound 1 were also estimated by DFT calculations, which reveal pyrrolic CH protons in the region of 6.13–7.63 ppm, roughly consistent with the observed values (Table S7), further confirming the weak aromaticity of 1 as inferred above. On this basis, anisotropy of the induced current density (ACID) plot [64], nucleus‐independent chemical shift (NICS) values [65], and 2D isochemical shielding surface (2D‐ICSS) map [66, 67] were also calculated for 1. As a result, the clockwise ring current in the ACID plot (Figure 4a), the shielding environment inside the macrocycle in the 2D‐ICSS map (Figure 4b), and the negative central NICS(0) value of −8.6 ppm (Table S9) all further support the weak 28π Möbius aromatic character of 1.

FIGURE 4.

FIGURE 4

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ACID plots of (a) 1, (c) 2, and (e) 3 with the isosurface of 0.05. The direction of the applied magnetic field is perpendicular to the paper and pointing outward. Calculated 2D‐ICSS maps of (b) 1, (d) 2, and (f) 3.

In contrast, 2 and 3 exhibit unambiguous antiaromaticity, as indicated by the deshielding environment inside the macrocycles in 2D ICSS maps (Figure 4d,f) and their positive NICS(0) values within the range from +8.2 to +15.3 ppm, measured from the molecular center to the proximity of the conjugation circuit (Table S9). The ACID plot of 2 shows anticlockwise ring current along both the inner and outer pathways of ring G, corresponding to 24π and 28π circuits, respectively (Figure 4c). GIMIC analysis of 2 reveals that the inner and outer currents of ring G are 4.4 nA/T and 12.0 nA/T, respectively (Figure S65), indicating that the 28π circuit is likely dominant in 2. Notably, the C─N fusion dramatically lowers the NICS(0) value of ring G from +12.8 ppm (2) to +1.07 ppm (3, Table S9), indicative of its nearly nonaromatic nature and its exclusion from the antiaromatic circuit of 3, implying that the dominant conjugation circuit has been changed in 3. Consistently, the ACID plot and GIMIC analysis of 3 also clearly show the exclusion of ring G from the anticlockwise circuits (Figures 4e and S65). On the other hand, the formation of a C═O group on ring D of 3 changes its NICS(0) from −5.0 (2) ppm to +10.0 ppm (3), indicating that the local aromaticity of pyrrolic ring D has been disrupted in 3, and the resulting lactam carbon is involved in the 24π antiaromatic circuit via the dipolar resonance state [57]. GIMIC analysis demonstrates that the anticlockwise ring current predominantly flows through the outer nitrogen atom (N4, 12.3 nA/T) of ring D rather than the inner carbonyl carbon atom (C19, 4.7 nA/T, Figure S65). Thus, the polarized resonance structure may play a minor role in the antiaromatic circuit of 3. The coexistence of multiple conjugated forms in 2 and 3 is likely attributed to the presence of multiply fused moieties [12, 68, 69]. For 2‐Cl, the ACID plot and 2D‐ICSS map also reveal antiaromaticity, and the decreased NICS values are indicative of weakened antiaromaticity compared to that of 2 because of its structural distortion (Table S9 and Figures S68, S76), which is consistent with the NMR data (vide supra).

The influence of metal coordination on the antiaromaticity of 3 could also be elucidated by theoretical calculations. The NICS(0) values from the molecular center to the proximity of the conjugated circuit for 3‐Pd‐1 (from +11.7 to +18.3 ppm) are larger than those of 3 (from +8.2 to +14.7 ppm), indicating enhanced antiaromaticity upon Pd(II) coordination (Table S9). According to the optimized structures, the C19═O2 bond length in 3‐Pd‐1 (1.253 Å) is longer than that in 3 (1.234 Å) (Figure S82), suggesting that the polarized C+─O− form could be stabilized by Pd(II)─O coordination, which can be further evidenced by the considerably enhanced ring current through the inner carbonyl carbon atom (8.3 nA/T), compared to that of 3 (4.7 nA/T, Figure S65). Meanwhile, the molecular planarity is improved upon Pd(II) coordination as evidenced by the decrease of the RMSD value from 0.381 to 0.364 Å (Figure S82). Both the stabilized C+─O− form and the improved planarity result in enhanced antiaromaticity of 3‐Pd‐1. In contrast, the inner NICS(0) values for the rhenium complexes for 3‐Re‐1 (from +5.83 to +9.88 ppm) and 3‐Re‐2 (from +2.69 to +5.73 ppm) are considerably lower than those of 3 (Table S9), consistent with their attenuated antiaromaticity resulting from severe structural distortion as revealed by the crystal structures and 1H NMR data (vide supra), as well as the ACID plots and 2D‐ICSS analysis (Figures S71, S72, S79, and S80). GIMIC analyses indicate that the ring currents passing through the carbonyl carbon atoms in these two Re complexes are much smaller (1.8 and 2.0 nA/T, respectively), compared to that of 3 (4.7 nA/T, Figure S65), revealing less contribution of polarized resonance to the antiaromaticity of these two Re complexes. Consistent with this inference, relatively short C19═O2 bond lengths are observed in 3‐Re‐1 (1.235 Å) and 3‐Re‐2 (1.225 Å, Figure S82). On the other hand, the NICS and ACID calculations confirm the nonaromaticity of complex 3‐O‐Pd‐Re (Table S9, Figure S73) arising from π‐conjugation disruption by the additional keto group (vide supra). In summary, the antiaromaticity of the formally triply N‐confused hexaphyrin framework can be effectively modulated via metal coordination, highlighting the exceptional structural adaptability and tunable electronic properties of this unique molecular platform.

2.4. Absorption Spectra and Electrochemical Properties

The absorption spectrum of 1 displays a Soret‐like band at 596 nm and three weak Q‐like bands at 772, 870, and 990 nm (Figure 5a). These peaks are slightly red‐shifted relative to those of the reported Möbius [28]hexaphyrin(1.1.1.1.1.1) [37], which may be related to the extended conjugation contributed by the vinylene bridge. However, the peak intensities of 1 are weakened, which may stem from its more distorted structure and weakened aromaticity relative to that of [28]hexaphyrin(1.1.1.1.1.1) (vide supra). On the other hand, the absorption spectrum of 1 shows only slight changes at low temperatures (Figure S62), further confirming its relatively rigid conformation as revealed by the NMR data (vide supra). Upon intramolecular C–C fusion, antiaromatic 2 shows a blue‐shifted but more intense Soret‐like band at 565 nm with weak absorption tailing to ca. 1450 nm, typical for antiaromatic systems. Compared with 1, the more intense Soret‐like band of 2 likely stems from its more planar structure. Consistently, time‐dependent (TD)‐DFT calculations reveal that the band at 588.1 nm for 2 arises predominantly from HOMO–1→LUMO (39%) and HOMO→LUMO+1 (47%) transitions, with a large oscillator strength of 1.2774. In contrast, the corresponding band of 1 centered at 611.3 nm is mainly attributed to HOMO–1→LUMO+1 (77%) and HOMO→LUMO (11%) transitions, with a considerably lower oscillator strength of 0.7628 (Tables S10 and S11). Compared to 2, 2‐Cl exhibits a weaker Soret‐like band at 533 nm and a blue‐shifted absorption band edge at ca. 1200 nm, consistent with its distorted structure and weakened antiaromaticity. The hexaphyrin 3 exhibits a further blue‐shifted Soret‐like band at 519 nm, attributed to the shortened conjugation, while the band edge is extended to ca. 1600 nm compared to 2, attributed to the enhanced antiaromaticity. Upon coordination, 3‐Pd‐1 exhibits a broad Soret‐like band at 562 nm, with the absorption edge tailing to ca. 1800 nm (Figure 5b). The red‐shifted absorption band edge can be attributed to the stronger antiaromaticity. On the other hand, complexes 3‐Re‐1, 3‐Re‐2, and 3‐O‐Pd‐Re all display blue‐shifted Soret‐like bands at 486, 494, and 508 nm, respectively, which may be induced by structural distortion upon Re(I) coordination. Interestingly, 3‐Re‐1 shows a Q‐like band centered at 1230 nm with the band edge of ca. 1600 nm, which is enhanced and broadened compared to that of 3. This observation can be rationalized by DFT calculations, which reveal a distinct distribution of the HOMO and LUMO orbitals, indicating a noticeable intramolecular charge transfer (ICT) effect (Table S5). Compared with 3‐Re‐1, the absorption spectrum of 3‐Re‐2 exhibits a blue‐shifted Q‐like band centered at 978 nm with an absorption band edge at ca. 1300 nm, which may originate from the more distorted structure, weakened antiaromaticity, and the absence of an ICT effect in 3‐Re‐2. Finally, the interrupted global π‐conjugation in 3‐O‐Pd‐Re results in a markedly narrowed absorption profile, with an onset wavelength of ca. 1040 nm. The relatively weak absorption bands of these antiaromatic hexaphyrins in the NIR region can be assigned to the forbidden HOMO–LUMO transitions with small oscillator strengths, according to the TD‐DFT calculations (Figures S84–S89, Tables S11–S16) [70, 71]. Consistent with the antiaromatic character, the fused hexaphyrins 2, 2‐Cl, and 3 exhibit markedly narrower electrochemical HOMO–LUMO gaps (0.55, 0.57, and 0.51 eV, respectively) compared to that of the Möbius aromatic hexaphyrin 1 (0.94 eV) (Figure S63, Table S3). Further narrowed gaps are observed for 3‐Pd‐1 and 3‐Re‐1 (0.32 and 0.40 eV), in line with their substantially red‐shifted absorption resulting from enhanced antiaromaticity and ICT effect, respectively. In contrast, 3‐Re‐2 and 3‐O‐Pd‐Re display larger electrochemical HOMO–LUMO gaps (0.59 and 0.91 eV), consistent with their weakened antiaromaticity and nonaromaticity, respectively. Overall, the sequence of electrochemical HOMO–LUMO gaps for compounds 1–3, 2‐Cl, and the metal complexes is roughly consistent with those of the absorption band edges as well as theoretical values obtained by DFT calculations (Table S6).

FIGURE 5.

FIGURE 5

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Absorption spectra of (a) 1–3, 2‐Cl and (b) metal complexes in CH2Cl2 with the inset photos showing the amplified absorption and colours of the corresponding solutions in CH2Cl2.

2.5. Photothermal Conversion

Antiaromatic molecules generally exhibit efficient nonradiative decay pathways, which in turn promote effective photon‐to‐thermal energy conversion [12, 13, 72]. Nevertheless, it should be noticed that antiaromatic molecules have not been fully explored as photothermal therapeutic agents due to their inadequate stability and extremely weak absorption in the NIR region [73, 74]. Notably, the antiaromatic hexaphyrins reported in this work exhibit appreciable stability and considerable NIR absorption, which is likely attributed to their large π‐conjugation, fused structure, intramolecular hydrogen bonds, as well as the presence of electron‐withdrawing meso‐C6F5 groups. Thus, we further investigate their photothermal conversion performance. Initially, the photothermal behavior of Möbius aromatic 1 and weakly antiaromatic 2‐Cl was investigated by irradiation of their 100 µM o‐DCB solutions with a 1064 nm laser at the maximum permissible exposure (MPE) power density of 1.0 W cm−2 for 8 min, which results in a gradual temperature rise to 67.4°C and 56.2°C, respectively. Based on the time constants (τs) for heat transfer of the systems [75], the photothermal conversion efficiencies (η) were calculated to be 39.4% and 44.5%, respectively (Figure S64). Under the same conditions, the temperatures were gradually enhanced to 61.2°C and 72.4°C for 2 and 3, respectively, with the conversion efficiencies of 57.9% and 42.7%, respectively (Figure 6). Among these compounds, the highest efficiency of 2 may originate from the nonradiative transition facilitated by its relatively remarkable antiaromaticity and suitable conformational flexibility. Compared to 2, 3 exhibits inferior efficiency, attributable to its highly fused structure, which induces enhanced molecular rigidity and suppressed nonradiative transitions. Upon coordination of 3, metal complexes 3‐Pd‐1, 3‐Re‐1, and 3‐Re‐2 show enhanced efficiencies of 57.7%, 60.8%, and 61.4%, respectively, along with maximum temperatures of 56.8°C, 82.7°C, and 81.1°C (Figure 6). The efficiency improvement may be attributed to metal‐facilitated nonradiative transitions [76]. Notably, all these compounds show excellent reproducibility over four laser ON/OFF cycles, revealing their high photostability (Figure 6). Given that most reports on the photothermal properties of porphyrinoids focus on the NIR‐I window [13, 77, 78, 79, 80], these hexaphyrins provide further insight into photothermal therapy in the NIR‐II region [81, 82, 83].

FIGURE 6.

FIGURE 6

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Temperature change curves of the 100 µM solutions under 1064 nm laser irradiation with various laser power densities (top), temperature changes of 100 µM solutions during laser ON/OFF cycles using 1.0 W cm−2 of 1064 nm laser (middle), and linear fitting of time versus the negative natural logarithm of the driving force temperature (bottom). (a) 2, (b) 3, (c) 3‐Pd‐1, (d) 3‐Re‐1, and (e) 3‐Re‐2.

3. Conclusions

In summary, we herein report the aromaticity modulation from 28π Möbius aromaticity to Hückel antiaromaticity by stepwise fusion of a vinylene‐bridged N‐confused hexaphyrin. Thus, a 28π Möbius aromatic N‐confused oxohexaphyrin(2.1.1.1.1.1) 1 was synthesized. Subsequent regioselective oxidative C–C fusion of 1 afforded a Hückel antiaromatic 2 as a hybrid of 24 and 28π systems by forming a pyrrolo[3,2‐e]indole unit with two NH moieties pointing outwards, and inner monochlorination of 2 resulted in severe distortion and weakened antiaromaticity of 2‐Cl. Interestingly, one of the peripheral NHs in 2 could be further fused with the adjacent C6F5 group to afford a Hückel antiaromatic species 3 with a modulated circuit, incorporating a [5.6.5.5.6]‐pentacyclic moiety. Upon coordination of 3, the mono‐palladium complex 3‐Pd‐1 adopts a CNNO coordination mode and displays enhanced antiaromatic character, which arises from a strengthened C+─O− polarization and enhanced planarity. In contrast, structural distortions induced by rhenium coordination result in weakened antiaromaticity for both 3‐Re‐1 (NNη2‐coordinated) and 3‐Re‐2 (NNO‐coordinated), while the heterobimetallic complex 3‐O‐Pd‐Re turns out to be nonaromatic as a result of π‐conjugation interruption by the attachment of an additional keto moiety. Notably, these compounds exhibit good stability and notable near‐infrared (NIR) absorption, giving rise to efficient photothermal conversion under 1064 nm laser irradiation with efficiencies up to 61.4%. Notably, the efficiencies could be enhanced by antiaromaticity as well as metal coordination. This work establishes a fusion‐based approach for constructing multiply N‐confused porphyrinoids from a vinylene‐bridged N‐confused porphyrinoid precursor to achieve tunable antiaromaticity, extended absorption, and distinctive coordination capabilities favorable for realizing efficient photothermal conversion behavior in the NIR‐II region, which may be practically applicable in noninvasive tumor treatment.

Author Contributions

Gaojie Zhu: writing – original draft, investigation, data curation, formal analysis, methodology. Zimei Ma: investigation, data curation. Mingbo Zhou: supervision. Hailong Wang: methodology. Shijun Li: methodology. Rinat Nasibullin: software, resources. Glib Baryshnikov: resources, software. Chengjie Li: supervision, methodology. Hans Ågren: software, resources. Jianxin Song: supervision, methodology. Jianzhuang Jiang: supervision, methodology. Qizhao Li: conceptualization, funding acquisition, writing – review and editing, supervision, methodology, project administration. Yongshu Xie: funding acquisition, conceptualization, writing – review and editing, supervision, methodology, project administration.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File 1: anie73651‐sup‐0001‐SuppMat.pdf.

ANIE-65-e4566077-s001.pdf (16.6MB, pdf)

Acknowledgments

This work at East China University of Science and Technology (ECUST) was financially supported by the National Natural Science Foundation of China (22131005, 22571085, W2621005), the Fundamental and Interdisciplinary Disciplines Breakthrough Plan of the Ministry of Education of China (JYB2025XDXM404), the Science and Technology Commission of Shanghai Municipality (24DX1400200), the Program of Introducing Talents of Discipline to Universities (B16017), the Shanghai Pujiang Program (24PJD024), and the Science and Technology Planning Project of Hunan Province (2018TP1017). G.B. is thankful for support from the Swedish Research Council (2020‐04600) and from the European Union (ERC, LUMOR, 101077649). Views and opinions expressed are, however, those of the authors only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them. The computations were enabled by resources provided by the National Supercomputer Centre (NSC), funded by Linköping University. The authors thank the Research Center of Analysis and Test of ECUST for compound characterization

Contributor Information

Qizhao Li, Email: zhaolq@ecust.edu.cn.

Yongshu Xie, Email: yshxie@ecust.edu.cn.

Data Availability Statement

The data that support the findings of this study are available in the Supporting Information of this article

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supporting File 1: anie73651‐sup‐0001‐SuppMat.pdf.

ANIE-65-e4566077-s001.pdf (16.6MB, pdf)

Data Availability Statement

The data that support the findings of this study are available in the Supporting Information of this article

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