Synergistic meso-β regulation of porphyrins: squeezing the band gap into the near-infrared I/II region

Abstract

The development of novel near-infrared (NIR) materials with extremely small energy gaps and high stability is highly desirable in bioimaging and phototherapy. Here we report an effective strategy for narrowing the energy gaps of porphyrins by synergistic regulation of meso/β substituents. The novel NIR absorbing/emitting meso-alkynyl naphthoporphyrins (Zn-TNP and Pt-TNP) are synthesized via the retro-Diels–Alder reaction. X-ray crystallography analysis confirms the highly distorted structures of the complexes. Both compounds exhibit intense Q bands around 800 nm, while Zn-TNP shows deep NIR fluorescence at 847 nm. Pt-TNP displays NIR-II room temperature phosphorescence peaking at 1106 nm with an extremely large Stokes shift of 314 nm, which are the longest wavelengths observed among the reported platinum porphyrinoids. Furthermore, Pt-TNP shows remarkable photostability and a notable capacity for synchronous singlet oxygen and heat generation under NIR light irradiation, demonstrating potential in combined photodynamic/photothermal therapy. A theoretical analysis reveals the progressive lifting of the HOMO by the β-fused benzene ring, the decrease of the LUMO upon meso-alkynyl substitution, and energy-releasing pathways varying with metal ions. This dual regulation approach demonstrates great promise in designing innovative multifunctional NIR porphyrin materials.

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Introduction

Organic near-infrared (NIR) materials are in high demand for bio-imaging and phototherapy due to their great advantages including low cytotoxicity, deep tissue penetration, and minimal damage to normal tissues.^1–5 As important pigments of life, porphyrinoids can be readily functionalized via inner-core coordination and peripheral modifications. The common tetrapyrrolic porphyrins are 18π aromatic conjugated molecules displaying intense Soret bands and weak Q bands in the visible region, but they lack near-infrared (NIR) absorption. Therefore, considerable effort has been devoted to obtaining NIR-absorbed porphyrinoids by modifying the structure, which narrows the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap. Among various designing strategies, such as developing π-extended,⁶ MO-mixing^7,8 and donor–acceptor⁹ systems, pyrrolic β-aromatic-fusion¹⁰ stands out as an exceptional approach for producing rigid porphyrins with intensified Q bands in NIR-absorption and emission. The Q bands of porphyrins can undergo a gradual red-shift by the tetra-fusion of benzene^11–13 (600–700 nm), naphthalene^14–16 (700–800 nm), and anthracene^17–20 (>800 nm), accompanied by a notable intensity increase.²¹ Despite having the longest NIR absorption and emission bands, the anthracene-fused (anthro)porphyrins were easily photooxidized under ambient light,²² which severely restricted their practical uses.

While considerable research has been conducted on expanding the porphyrinoid π-system via pyrrolic β-functionalization, little attention has been paid to decreasing the HOMO–LUMO gap by adjusting meso-substituents. Most porphyrins have meso-aryl substituents, resulting in a minor change in the HOMO–LUMO gap, because both electron-withdrawing and electron-donating groups in phenyl always cause a simultaneous rise or decrease in the HOMO and LUMO. Kobayashi et al. reported the design of stable, NIR-absorbing phthalocyanines by rationally controlling both the central core and the α-substituent, demonstrating the efficiency of multipart regulation in energy-gap tuning.^23–25Meso-alkynyl groups have been incorporated into porphyrinoids²⁶ to facilitate intermolecular coupling and create nanoscale tape and rings^27–29 with a high degree of conjugation. In this work, combined with π-extension at β-positions, we highlight the additional HOMO–LUMO gap narrowing ability of alkynyl groups at meso positions (Fig. 1). Upon the meso-β double regulation, our newly synthesized meso-alkynyl naphthalene-fused (naphtho)porphyrin complexes Zn-TNP and Pt-TNP demonstrate not only a more pronounced red-shift in both absorption and emission (>800 nm), but also significantly higher photostability than their NIR-absorbing meso-aryl anthroporphyrin counterparts. Notably, the platinum coordinated Pt-TNP is a rare example of a porphyrinoid emitter that displays phosphorescence in the desirable NIR-II (1000–1400 nm) window with a maximum of 1106 nm at room temperature, while presenting an ultra-large Stokes shift exceeding 300 nm. Furthermore, Pt-TNP exhibits significant singlet oxygen generation and photothermal conversion capacity under NIR laser irradiation, making it an ideal agent for multifunctional phototherapy.³⁰

Fig. 1 Molecular design of target NIR-absorbing/emitting porphyrin complexes.

Results and discussion

The synthesis route is presented in Fig. 2a. 4,9-Dihydro-4,9-ethano-2H-benz[f]isoindole 1 bearing a bicyclo[2.2.2]octadiene (BCOD) unit was synthesized via a reported method.³¹ The condensation of 1 and 3-(triisopropylsilyl)propiolaldehyde 2 (ref. ³²) was catalyzed by BF₃·OEt₂ in ultra-dry CH₂Cl₂ under an N₂ atmosphere, followed by oxidation with p-chloranil (TCQ), and afforded free-base porphyrin 3 in 9% yield.³³ Adding a methanol solution of Zn(OAc)₂ to the CHCl₃ solution of 3 afforded the zinc-coordinated porphyrin Zn-3 in 93% yield. The platinum complex Pt-3 was synthesized in 93% yield via the reaction between 3 and PtCl₂ in refluxing acetic acid. The naphthoporphyrins Zn-TNP and Pt-TNP were quantitatively obtained by employing the retro-Diels–Alder method,³⁴ heating precursors Zn-3 and Pt-3 in solid states, respectively, at 300 °C under vacuum.

Fig. 2 (a) Synthesis of BCOD and π-extended porphyrins: (i) BF₃·OEt₂, CH₂Cl₂, −78 °C, 2 h; (ii) TCQ, r.t., 30 min; (iii) Zn(OAc)₂·2H₂O, CHCl₃/MeOH, r.t., 3 h; (iv) PtCl₂, NaOAc, AcOH, 118 °C, N₂, 24 h; (v) 300 °C, 30 min, in vacuo. (b) ¹H NMR spectra of Zn-TNP: experimental data (above) measured in CDCl₃ and simulated data (below). (c) 3D ICSS map of Zn-TNP (isovalue = 1.5). The shielding areas are transparent blue and deshielding areas are solid red.

The successful synthesis of the desired fused porphyrins was confirmed by HR-ESI mass spectrometry and MALDI-TOF mass spectrometry (Fig. S1–S10†), and nuclear magnetic resonance (NMR) spectroscopy (Fig. S12–S16†). The resonance signals of protons on the fused naphthalenes of Zn-TNP (Fig. 2b) are divided into three groups: H₁ at δ = 10.9 ppm, H₂ at δ = 8.41 ppm and H₃ at δ = 7.75 ppm, which shift more downfield than those of the meso-tetraphenyl[2,3]tetranaphthoporphyrin zinc complex (Zn-TPTNP) reported by Ito et al.^14,35 The NMR computational simulation of Zn-TNP shows consistency with the experimental results, with H₁ at δ = 10.9/11.0 ppm, H₂ at δ = 8.56/8.66 ppm, and H₃ at δ = 7.95/7.97 ppm. To further explain the downfield proton signals on Zn-TNP, particularly the significantly shifted H₁, the 3D isochemical shielding surface (ICSS) calculation^36,37 was performed (Fig. 2c). The overall aromaticity of Zn-TNP is indicated by the large shielding area (blue) enclosing the entire molecule. Enhanced deshielding (red) is observed in the area between the alkynyl groups and fused naphthalenes of Zn-TNP, which mainly surrounds the H₁ protons. Consequently, the presence of alkynyl groups induces a more significant downfield shift in the signal of H₁, while the signals of H₂ and H₃ also undergo a slight downfield shift.

The solid structures of Zn-TNP and Pt-TNP were examined using single-crystal X-ray diffraction. The single crystals of Zn-TNP were obtained by slow diffusion of hexane into its CH₂Cl₂ solution containing a small amount of Et₄NCl. As a consequence of Cl⁻ axial coordination, the Zn–N bond lengths range from 2.072(5) to 2.137(6) Å, which are longer than those of common four-coordinated Zn porphyrins (2.039–2.040 Å).³⁸ The Zn(II) ion is located at 0.384 Å above the 4N plane, and the length of the Zn–Cl bond is 2.335(2) Å. The single crystals of Pt-TNP were obtained by slow evaporation of its toluene solution for a month. The bond length of Pt–N is 2.0073(14) Å, which is identical to those of the reported (5,10,15,20-tetraphenyl-21H,23H-porphinato)platinum(II).³⁹ The bond length alternations in crystal structures are analyzed by the harmonic oscillator model of aromaticity⁴⁰ (HOMA). The HOMA values of Zn-TNP and Pt-TNP for the 18π-circuit (containing 16 atoms as a macrocyclic internal cross) are 0.88 and 0.90, respectively, which are typical for aromatic porphyrinoids.⁴¹Zn-TNP and Pt-TNP have skeleton lengths of 16.957 Å and 17.179 Å, respectively, determined by the distance between the terminal carbon atoms on the fused naphthalenes (Fig. 3a). Both Zn-TNP and Pt-TNP exhibit highly distorted conformations, which are further investigated using the clothes-line diagrams (Fig. 3b) and normal-coordinate structural decomposition (NSD) method^42,43 (Tables S3 and S4†). Zn-TNP has a saddled conformation (B_2u = 1.89), while Pt-TNP has a distinct hybrid conformation that is primarily saddled (B_2u = 2.59) and partly ruffled (B_1u = 1.15). The single crystal packing structure of Zn-TNP displays a wave-shaped extension along the a and c directions. The formation of the “wave” is constructed by the face-to-face π-stacking between the fused naphthalenes in Zn-TNP molecules. And the Zn-TNP wave layers are separated by the Et₄N⁺ counterions. The packing structure of Pt-TNP is arranged in an interleaved manner, showing slipped π–π interaction of the fused naphthalenes (Fig. 3c).

Fig. 3 (a) Top views of single crystal structures of Zn-TNP and Pt-TNP using an ORTEP diagram with thermal ellipsoids drawn at the 50% probability. (b) Side views and out-of-plane distortions of Zn-TNP and Pt-TNP. (c) Packing diagrams of Zn-TNP and Pt-TNP. In the packing views, meso-substituents are omitted for clarity.

The absorption, luminescence, and magnetic circular dichroism (MCD) spectra of the new porphyrin compounds were recorded in toluene to investigate the photophysical properties (Fig. 4a and b). The precursors Zn-3 and Pt-3 display typical intense Soret bands at 487 and 466 nm, respectively, which are red-shifted in comparison to the Soret band of meso-tetraphenylporphyrin at ca. 410 nm (Fig. S21 and S22†). The Soret bands of Zn-TNP and Pt-TNP are further red-shifted to 549 nm and 520 nm, respectively. The Q bands of the Zn-3 and Pt-3 are weak and located in the visible region. After naphthalene-fusion, the Q bands of Zn-TNP and Pt-TNP significantly strengthen with more than 3-fold intensity and shift to the NIR region, peaking at 816 and 792 nm, respectively. The MCD spectrum of Zn-TNP shows derivative-shaped Faraday A₁ terms with a pair of opposite signs at the absorption of B₀₀ (549 nm) and Q₀₀ (816 nm), respectively, indicating the presence of a highly symmetrical structure and degenerate excited states.⁴⁴ The Faraday A₁ terms of Zn-TNP show a negative-to-positive sign sequence in ascending energy, indicating that the energy difference between HOMO and HOMO−1 (ΔHOMO) is larger than that between LUMO and LUMO+1 (ΔLUMO).⁴⁵ The MCD pattern of Pt-TNP is identical to that of Zn-TNP. Compounds 3 and Zn-3 show fluorescence bands at 742 nm and 679 nm (Fig. S25 and S26†), respectively, while Zn-TNP shows a red-shifted fluorescence band at 847 nm (Fig. 4b, purple line). The Pt complexes exhibit room temperature phosphorescence (RTP). Pt-3 shows phosphorescence emission at 823 nm with a shoulder at 930 nm (Fig. S27†). For Pt-TNP, the RTP was detected in the range from 1000 to 1400 nm in deaerated toluene, showing the maximum at 1106 nm (Fig. 4b, red line). The phosphorescence band of Pt-TNP is located in the NIR-II region and more red-shifted than that of any other reported Pt-coordinated porphyrins.^21,46 Moreover, Pt-TNP has a very large Stokes shift of 314 nm, making it a potential NIR-II phosphorescence probe for biomedical imaging. In air-saturated toluene, the RTP of Pt-TNP could also be observed with reduced intensity, and the singlet oxygen (¹O₂) generation capacity of Pt-TNP was revealed by the finding of a ¹O₂ phosphorescence peak at 1270 nm (Fig. 4c, black line). The ¹O₂ generation abilities of Pt-TNP and Zn-TNP were compared utilizing 1,3-diphenylisobenzofuran (DPBF) as a singlet oxygen scavenger (Fig. 4d, S34 and S35†). Under laser irradiation (808 nm, 10 mW cm⁻²), the absorbance of DPBF in toluene exhibited no significant change without the addition of porphyrin compounds, but decreased significantly in 21 s with the presence of Pt-TNP. Furthermore, Pt-TNP exhibited excellent photostability, whereas Zn-TNP was unstable under irradiation and underwent rapid photobleaching (Fig. 4d and S36†). Previous research has demonstrated that zinc porphyrins can undergo photo-oxidation upon light exposure.^47,48 The easier photodegradation of Zn-TNP may be attributed to its better oxygen affinity than Pt-TNP (Fig. S11, S18 and S19†), while the detailed mechanism still requires further investigation.^49,50 The photothermal conversion performance of Pt-TNP and Zn-TNP was investigated in toluene under 808 nm (0.35 W cm⁻²) laser irradiation. After 6 minutes of irradiation, the Pt-TNP solution had a temperature increase of over 30 °C, whereas the Zn-TNP solution only increased by around 4 °C due to photoinstability (Fig. 4e and S33†). The heating–cooling measurements of Pt-TNP demonstrated excellent repeatability after three cycles (Fig. 4f and S30†). A time constant (τ) of 120.05 was obtained by linearly fitting time (t) as a function of the negative natural logarithm of the driving force temperature (−ln θ) at the cooling stage (Fig. S32†). The calculated photothermal conversion efficiency for the Pt-TNP solution was 78%.

Fig. 4 (a and b) Absorption spectra of Zn-3, Zn-TNP, Pt-3 and Pt-TNP in toluene, and the MCD spectra of Zn-TNP and Pt-TNP in toluene. Inset: solution photographs of Zn-3, Zn-TNP, Pt-3, and Pt-TNP. (c) Fluorescence spectrum of Zn-TNP (λ_ex = 549 nm, purple line) in toluene, and phosphorescence spectra of Pt-TNP in deaerated and air-saturated toluene (λ_ex = 520 nm, red line and black line). (d) The absorption changes at 415 nm of DPBF, Zn-TNP + DPBF, and Pt-TNP + DPBF in toluene under 808 nm (10 mW cm⁻²) laser irradiation within 21 s. (e) Thermal images of Pt-TNP and Zn-TNP (30 μmol) under 808 nm laser irradiation (0.35 W cm⁻²). (f) Heating–cooling cycles for Pt-TNP (30 μmol) under 808 nm laser irradiation (0.35 W cm⁻²).

DFT calculations were performed on a series of Pt porphyrins to evaluate the effects of meso- and β-substituents on their energy gaps (Fig. 5a and S39†). The meso-phenyl, β-non-fused (N = 0) platinum porphyrin Pt-TPP has degenerate LUMO/LUMO+1 and near-degenerate HOMO/HOMO−1, showing a large HOMO–LUMO gap of 3.05 eV. The naphthalene-fusion (N = 2) of Pt-TPP considerably lifts its a_1u-type HOMO−1 but has little effect on the a_2u-type HOMO, resulting in a HOMO/LUMO switch and a huge HOMO/HOMO−1 splitting with a largely reduced HOMO–LUMO gap of 2.24 eV in Pt-TPTNP (Fig. S48†). By replacing the phenyl group with either the electron-withdrawing pentafluorophenyl group or the electron-donating 4-N,N-dimethylphenyl group, the naphthoporphyrin undergoes a simultaneous increase or decrease in both HOMO and LUMO energy levels, resulting in a very small change of the HOMO–LUMO gap. In contrast, the meso-alkynyl Pt-TNP (where triisopropylsilylethynyl is replaced by trimethylsilylethynyl) exhibits not only a significantly elevated HOMO consistent with its meso-phenyl counterpart, but also a substantially reduced LUMO due to the presence of electron-withdrawing alkynyl groups. The meso-β double regulation of Pt-TNP efficiently reduces the HOMO–LUMO gap to 1.92 eV, with the electron density extending to the fused naphthalenes on the HOMO and meso-alkynyl groups on the LUMO. It is noteworthy that the extended electron-density distributions towards alkynyl groups are exclusively observed in the LUMOs, but not in the HOMOs; therefore, conjugative extension could potentially play a substantial role in the large energy decreases of the LUMOs. Additional computational analysis reveals that replacing trimethylsilyl groups with H has a negligible impact on the HOMO/LUMO energy levels. By introducing electron-withdrawing groups like –CF₃ and –CN into the alkynyl groups, the HOMO–LUMO gaps are further reduced to 1.80 eV and 1.66 eV, respectively (Fig. S40†). This energy-gap decrease is accompanied by a further extension of the electron density towards the terminal group.

Fig. 5 (a) Frontier molecular orbital diagrams of Pt-TNP and other Pt–porphyrins for comparison. (b) Schematic diagram of the calculated energy gaps, spin–orbit coupling constants between S₁ and T_n (n = 3–4) of Pt-TNP and Zn-TNP, and the hole and electron distribution in the S₀–T₁ transition of Pt-TNP. (c) Plots of absorption and phosphorescence peaks of meso-phenyl (blue triangle) and meso-alkynyl (red circle) Pt–porphyrins versus linearly fused benzene numbers (N) in β positions ranging from 0 to 3. Stokes shifts are presented as blue and red rhombuses.

The optimized structures of Pt-TNP and Zn-TNP are distorted, but they exhibit varying degrees and patterns of distortion compared to the single-crystal structures (Fig. S53†). These distortions may be caused by steric effects and further influenced by the crystal-packing forces. We successfully optimized Zn-TNP for both saddled (sad-Zn-TNP) and ruffled (ruf-Zn-TNP) conformations. The total energies, HOMO–LUMO energy levels, and energy gaps of sad-Zn-TNP and ruf-Zn-TNP are nearly identical, suggesting that the distortion patterns have minimal impact on the electronic structures and optical properties. DFT calculations are also conducted on a variety of meso-free/β-fused and β-free/meso-substituted porphyrins, all of which exhibit completely planar conformations due to reduced steric hindrance. The significant HOMO-lifting effect of aromatic fusion and the LUMO-reducing effect of alkynyl substitution, which can be controlled by the degree of π-extension, were further validated on the planar porphyrin molecules (Fig. S38†).

Time-dependent (TD) DFT calculations were performed to further investigate the influence of excited states caused by different substituents and coordinated metal ions (Fig. 5b and c). The energy levels of the S₁ and T_n states of Pt-TNP and Zn-TNP are similar, and both compounds exhibit degenerate S₁/S₂ and T₁/T₂ states. Zn-TNP and Pt-TNP have large S₁–T₁ gaps (ΔE_S1–T1) of 0.62 eV and 0.58 eV, respectively, while their T₃ and T₄ states have small S₁–T_n gaps (ΔE_S1–Tn < 0.3 eV),⁵¹ which promote the intersystem crossing (ISC) activities. The S₁–T_n spin–orbit coupling matrix elements (SOCMEs) of Zn-TNP are smaller than 2.4 cm⁻¹. In contrast, the S₁–T₃ and S₁–T₄ SOCMEs for Pt-TNP have large values of 31.70 and 11.57 cm⁻¹, respectively. The energy can efficiently pass through the ISC process between S₁ and T₃ of Pt-TNP due to the narrow energy gap (ΔE_S1–T3 = 0.15 eV) and large SOCME. Subsequently, the energy undergoes a rapid internal conversion (IC) to T₁ and deactivates to the ground state S₀ with phosphorescence emission. The T₁ state of Pt-TNP can react with oxygen to generate singlet oxygen, and a significant amount of energy can be released as heat through internal conversion. The electron and hole distributions of Pt-TNP indicate that both S₀–S₁ and S₀–T₁ are π–π* transitions with negligible involvement of a central metal (Fig. 5b and S75†). The Sr index, which indicates the degree of electron–hole overlap, is 0.86 for S₁ and 0.79 for T₁, demonstrating that the two states are locally excited (LE).

The wavelength variation trends for the longest absorption (S₀–S₁) and phosphorescence (S₀–T₁) peaks of a series of Pt porphyrins are investigated (Fig. 5c). The molecules are divided into two groups: meso-phenyl (blue triangle) and meso-alkynyl (red circle). Each group contains four molecules, with the number of linearly fused benzenes (N) ranging from 0 to 3. In both groups, the calculated wavelengths corresponding to S₁, T₁, and Stokes shifts increase as N increases, and the wavelengths in the meso-alkynyl group are always larger than those in the meso-phenyl group with the same N value. The plots of the S₁ and T₁ wavelengths as a function of N exhibit strong linear correlations in both groups, with the slopes of the fitting lines for T₁ being larger than those for S₁. The meso-alkynyl group with a larger N yields greater calculated Stokes shifts, which is consistent with the remarkably large experimental Stokes shift of Pt-TNP (314 nm) resulting from the meso-β double regulation.

Based on the optimized structure of Pt-TNP and Zn-TNP, we evaluated the ring currents and induced magnetic shielding with a nucleus-independent chemical shift⁵² (NICS), anisotropy of the induced current density⁵³ (ACID), and the gauge-including magnetically induced current^54,55 (GIMIC) calculations (Table S7 and Fig. S76–S78†). The NICS(0) values are negative in both the fused naphthene and porphyrin core for both compounds. The diatropic ring currents of Zn-TNP and Pt-TNP continuously flow throughout the whole structure, showing the global aromaticity of these two compounds. Additionally, localized diatropic ring currents are observed on the naphthalene moieties and the inner 16-bond porphyrin core.

Conclusions

In summary, we successfully synthesized porphyrin complexes with remarkable red-shifted NIR absorption and emission (ranging from 800 to 1200 nm) by tailoring the molecular energy levels through meso- and β-substituents. The electronic structures and optical properties of target meso-alkynyl naphthoporphyrin complexes Zn-TNP and Pt-TNP were fully elucidated via spectroscopic measurements and theoretical calculations. The solid structures of the complexes were analyzed using single-crystal X-ray diffraction, revealing saddling or mixed saddling–ruffling distortions. Zn-TNP and Pt-TNP displayed Q bands at 816 nm and 792 nm, respectively. Zn-TNP exhibited a fluorescence band at a wavelength of 847 nm. Pt-TNP exhibited room-temperature NIR-II phosphorescence at 1106 nm with a huge Stokes shift, and its ability to produce singlet oxygen was directly shown by the emission at 1270 nm. Pt-TNP also demonstrated excellent NIR photothermal conversion capacity. The dual functionality of Pt-TNP in singlet oxygen and heat generation makes it a promising candidate for combined photothermal/photodynamic therapy. While aromatic-ring-fusion is a well-established approach for reducing the HOMO–LUMO gap by HOMO-lifting, the additional functionalization of alkynyl groups in meso positions causes more than 100 nm of shifting in absorption/emission bands by LUMO-reduction when compared to their aryl-substituted counterparts. The DFT calculations provided an in-depth explanation of the correlation between the decrease in S₁/T₁ energy levels and the incorporation of meso-β substituents, as well as enhanced ISC activity in the Pt complex. The meso-β dual regulation of porphyrins effectively reduces the energy gap and modulates the excited states, offering a promising strategy for developing novel stable NIR materials.

Data availability

All experimental details including synthetic procedures, spectroscopic data, and theoretical calculation results, are available in the ESI.†

Author contributions

C. Q. carried out the synthesis, characterization, manuscript writing and theoretical calculations. X. G., Y. S., H. G. and F. C. contributed to the synthesis, characterization, and data analysis. C. Q., H. G. and Y. Z. performed the single-crystal X-ray diffraction measurements and analysis. F. W. and Z. S. designed and supervised the project, and revised the manuscript. All authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grants 22271140, 22071103, and 22371118). The theoretical calculations are performed using the supercomputing resources at the High-Performance Computing Center of Nanjing University.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Materials and methods, characterization, additional spectroscopic data, theoretical analysis, X-ray crystallographic data, and simulation of magnetically induced ring currents. CCDC 2325275 and 2265412. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc01806k

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