A nonalternant azulene-embedded carbon nanohoop featuring anti-Kasha emission and tunable properties upon pH stimuli-responsiveness

Abstract

The insertion of a nonalternant π-system into the skeletons of [n]cycloparaphenylenes ([n]CPPs) can significantly alter their optoelectronic properties. We herein present a nonalternant azulene-embedded carbon nanohoop 1,3-Az[9]CPP. The investigations revealed that 1,3-Az[9]CPP demonstrates unique properties including anti-Kasha emission, reversible pH stimuli-responsiveness and tunable band gaps upon protonation and deprotonation. Importantly, 1,3-Az[9]CPP exhibits a highly selective binding for C₆₀ over C₇₀ (K_C60/K_C70 = 30) in acidic media. Moreover, the molecular structures of 1,3-Az[9]CPP and its fullerene complex C₆₀@1,3-Az[9]CPP were confirmed by single-crystal X-ray diffraction. Notably, 1,3-Az[9]CPP is the first pH-responsive nonalternant all-carbon nanohoop, which may hold significant promising applications in tunable optoelectronic materials.

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Introduction

[n]Cycloparaphenylenes ([n]CPPs), consisting of a cyclic arrangement of para-linked phenylenes, are the shortest conjugated fragment of armchair carbon nanotubes (CNTs).¹ Since the initial synthesis by Jasti et al. in 2008,² numerous CPP derivatives have been developed with various functionalities and thus boosted tremendous advances in the fields of host–guest chemistry and materials science,³ owing to their in-plane radially oriented π-orbitals and size-dependent optoelectronic properties. So far, a majority of CPP derivatives have been synthesized by embedding alternant aromatic units such as naphthalene,⁴ anthracene,⁵ pyrene,⁶ fluorene,^7,8 pyridine,⁹ anthraquinone,¹⁰ and graphenoids¹¹ into the skeletons of CPPs, thus leading to fascinating applications in organic light emitting diodes (OLEDs),⁸ molecular machines,⁹ and so on.^12,13 In contrast, incorporating nonalternant π-systems into nanohoops can not only reduce the aromaticity of nanohoops, but also lower their band gap, which significantly alters their electronic properties.^14,16 However, there are limited examples of nonalternant nanohoops from rubicene,¹⁴ fluoranthene,¹⁵ and dibenzo[a,e]pentenyl¹⁶ and a methylene-bridged non-alternant aromatic belt.¹⁷ Therefore, the developments of nonalternant aromatic carbon nanohoops would be complementary to the abovementioned alternant aromatic counterparts and thus hold great significance for their applications in materials science.

Azulene, a nonalternant isomer of naphthalene, consists of an electron-deficient seven-membered ring and an electron-rich five-membered ring, featuring a large dipole moment of 1.08D and a non-mirror symmetric frontier molecular orbital, which eventually lead to anti-Kasha's rule emission from S₂ to S₀.¹⁸ It is noteworthy that the odd positions of azulene are more electron-rich than the even ones, so protonation can readily occur at the C-1 position to form a stable 6π-azulenylium ion under acidic conditions (Fig. 1a).¹⁹ These unique properties endow azulene with great promise for developing advanced materials in organic semiconductors,²⁰ photoswitches,²¹ nonlinear optical materials,²² chemical sensing, bioimaging,²³ and nonplanar polycyclic aromatic hydrocarbons.²⁴ However, investigations associated with nonalternant aromatic all-carbon nanohoops with pH reversible stimuli-responsiveness remained largely unexplored to date because of the lack of suitable stimuli-responsive all-carbon building blocks. Most pH active materials normally require heteroatoms such as nitrogen, oxygen, and sulfur as proton acceptors. Therefore, the synthesis of pH-responsive nonalternant all-carbon nanohoops is still challenging.

Fig. 1 (a) Azulene and (b) nonalternant azulene-embedded carbon nanohoops used in this work.

The reversible changes in color and physicochemical properties upon protonation/deprotonation make azulene an excellent building block for developing pH stimulus-responsive all-carbon nanohoops. We anticipated that incorporating an azulene unit into the CPP backbone would allow us to obtain a nonalternant aromatic all-carbon nanohoop with appealing properties, such as anti-Kasha's rule fluorescence and a tunable band gap upon protonation/deprotonation. Herein, we report the synthesis and structure of novel nonalternant aromatic azulene-embedded [n]CPP (1,3-Az[9]CPP) (Fig. 1b) and its properties in the absence and presence of acid. Remarkably, the nonalternant aromatic 1,3-Az[9]CPP displays not only reversible stimuli-responsiveness but also high selectivity for binding C₆₀ over C₇₀ in acidic media.

Results and discussion

The synthetic procedure for 1,3-Az[9]CPP is shown in Scheme 1. Our synthetic strategy was to couple two cyclohexadiene fragments 1 and 2 by Suzuki–Miyaura cross-coupling to access dichloride intermediate 3, followed by Miyaura borylation to give cyclization precursor 4. In the presence of Pd(PPh₃)₄ and K₂CO₃, 1,3-dibromoazulene and C-shaped fragment 4 were subjected to Suzuki–Miyaura cross-coupling in a diluted reaction solution (3.5 mM) to provide the macrocyclic precursor 6. The final reductive aromatization of the macrocyclic precursor 6 was carried out in anhydrous tetrahydrofuran with freshly prepared H₂SnCl₄ at room temperature, and the target macrocycle 1,3-Az[9]CPP was obtained in 24% yield over two steps. The structure of 1,3-Az[9]CPP was confirmed by ¹H NMR, ¹³C NMR, ¹H-¹H COSY, ¹H-¹³C HSQC, ¹H-¹³C HMBC, MALDI-TOF MS and single-crystal X-ray analysis (Fig. 2, Fig. S1–S7 and S9 and Table S1, ESI†).

Scheme 1 Synthetic route of 1,3-Az[9]CPP. Reaction conditions: (a) Pd(PPh₃)₄, K₂CO₃, THF/H₂O, 100 °C, 18 h, 78%; (b) B₂pin₂, x-phos, Pd₂(dba)₃, KOAc, 1,4-dioxane, 115 °C, overnight, 91%; (c) Pd(PPh₃)₄, K₂CO₃, Aliquat 336, toluene/H₂O, 115 °C, 24 h; (d) SnCl₂, HCl, THF, r.t., 24 h, 24% in two steps.

Fig. 2 (a) X-ray crystal structures, (b) herringbone packing model, (c) long range channel alignment and (d) densely stacked 1,3-Az[9]CPP.

The structure of 1,3-Az[9]CPP was confirmed by single-crystal X-ray analysis (Fig. 2). Suitable crystals were obtained by slow diffusion of acetonitrile into 1,1,2,2-tetrachloroethane solution at room temperature. Crystallographic analysis reveals that the carbon nanohoop exhibits a nearly circular cavity with an estimated diameter of about 1.30 nm (Fig. 2a), which is similar to that of [10]CPP (1.4 nm) and thus enables 1,3-Az[9]CPP to be a suitable host for fullerenes as [10]CPP.²⁵ As shown in Fig. 2a, the five-membered ring of azulene points towards the interior of CPP due to the strain caused by the 1,3-Az[9]CPP nanohoop, thus leading to the formation of dihedral angles of about 45.0° between the azulene unit and the adjacent benzene molecules. 1,3-Az[9]CPP adopts a herringbone packing in the solid phase when viewed from the side (Fig. 2b and d) and thus forms channels when viewed from the top (Fig. 2c). Furthermore, the unit cell of 1,3-Az[9]CPP belongs to the P2₁ space group.

The ¹H NMR spectrum of 1,3-Az[9]CPP features a set of signals for the azulene protons with δ = 8.70, 7.68, 7.28, and 7.22 ppm for H4/H8, H6, H2, and H5/H7, respectively, and signals for the CPP segment protons with δ = 7.68–7.55 ppm in CD₂Cl₂ (Fig. 3 and Fig. S4, ESI†). Titrations of 0.1–1.5 equiv. HBArF·(Et₂O)₂ (BArF⁻: B[3,5-(CF₃)₂C₆H₃]₄⁻, very hygroscopic and sensitive to oxygen) into the solution of 1,3-Az[9]CPP caused significant changes in the chemical shifts (Fig. 3 and Fig. S11, ESI†). No further changes were observed for the titrations of HBArF·(Et₂O)₂ beyond 1.0 equiv. The remarkable downfield shifts of protons assigned to the azulene unit were observed, with the largest downfield shift being assigned to the H5 or H7, up to 1.78 ppm. These significant downfield shifts indicate strong deshielding effects due to the formation of an azulenylium cation. Moreover, the proton signals of H4/H8 appear as a doublet peak at 8.70 ppm due to the symmetric structure of the nanohoop, but split into two separate sets of peaks, a doublet peak at 9.34 ppm and a multiplet peak at 9.08 ppm in the protonated asymmetric nanohoop 1,3-Az[9]CPP-H⁺ (Fig. 3 and S8, ESI†). Similar splits in the triplet peak assigned to H5 and H7 in 1,3-Az[9]CPP were also observed. Furthermore, a new peak at 5.54 ppm can be assigned to the C–H proton of the protonated five-membered ring. Interestingly, the color of the solution also changed significantly from initially green to yellow and finally deep red with the titrations of HBArF·(Et₂O)₂ (Fig. 3 and Fig. S11, ESI†). All these observations demonstrate that the monoprotonation of 1,3-Az[9]CPP occurs at its C-1 or C-3 position, thus inducing an asymmetric structure of the protonated carbon nanohoop, consistent with the previously reported monoprotonation of azulene derivatives.²⁶

Fig. 3 ¹H-NMR spectra and color change of the nanohoop 1,3-Az[9]CPP before (top) and after protonation (bottom) with 1.0 eq. HBArF·(Et₂O)₂ in CD₂Cl₂ (600 MHz, 298 K).

Next, we set about investigating the reversible pH stimuli-responsiveness of 1,3-Az[9]CPP. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) was selected as a base. As shown in Fig. S12 (ESI†), addition of 1.0 equiv. of HBArF·(Et₂O)₂ to the 1,3-Az[9]CPP solution resulted in the disappearance of the proton signals in the azulene unit accompanied by the appearance of the proton signals of azulenylium cations, while the color of the solution turned from green to red. When 1.0 equiv. of DBU was added, the chemical shifts of 1,3-Az[9]CPP recovered, concomitantly accompanied with the color recovery from red to green (Fig. S12, ESI†). Notably, the process of protonation and deprotonation can be repeated multiple times, indicating the intriguing reversibility of pH stimuli-responsiveness for 1,3-Az[9]CPP.

The optical properties of 1,3-Az[9]CPP in the absence and presence of HBArF·(Et₂O)₂ were subsequently investigated by UV-vis absorption spectroscopy in a dichloromethane (DCM) solution (Fig. 4a). Furthermore, to gain insight into the relationship between the electronic structure and photophysical properties, time-dependent density functional theory (TD-DFT) calculations were performed at the ωB97XD/6-31g(d,p) level (Fig. S20–S22 and Table S3–S6, ESI†). 1,3-Az[9]CPP exhibited an absorption maximum (λ_max) at 334 nm, which was attributed to the combined transitions of S₀ → S₄ and S₀ → S₅ according to the TD-DFT calculations. Meanwhile, a shoulder peak appeared at about 385 nm is assigned to the S₀ → S₂ transition. Moreover, there is a weak broad band at 500–670 nm assigned to the S₀ → S₁ (HOMO → LUMO) transition, which is consistent with that of the parent azulene.²⁷ In contrast, significant changes in the UV-vis spectra of 1,3-Az[9]CPP were observed upon titrations with HBArF·(Et₂O)₂ (Fig. 4a). Upon the titrations with acid, the intensity of the peak at λ_max decreased remarkably accompanied by a blue-shift of 5 nm from 334 nm to 329 nm. Meanwhile, the intensity of the shoulder peak at 385 nm diminished while a new band at about 497 nm appeared, which is attributed to the generation of azulenium cations. Furthermore, an isosbestic point was clearly observed at 439 nm, implying the existence of two interconverted optically different species in the system. Based on the onset of the longest wavelength absorption (λ_onset), the optical gaps of 1,3-Az[9]CPP and its protonated species were estimated to be 2.74 eV, and 1.82 eV, respectively (Table 1). This tunable optical gap upon protonation/deprotonation was supported by theoretical calculations. In addition, the fluorescence spectrum reveals that 1,3-Az[9]CPP shows an emission band at around 493 nm. Surprisingly, the emission peak of its protonated species remains almost constant (Fig. 4b). No obvious redshift in emission wavelength can be accounted for by that the cyclic conjugation system of 1,3-Az[9]CPP in the presence of acid. The excited-state calculation results suggest that the S₁ → S₀ (f = 0.0043) transition of 1,3-Az[9]CPP is forbidden; hence the emission band at around 445 nm should come from S₂ → S₀ (f = 1.18960) transition of 1,3-Az[9]CPP, implying the anti-Kasha emission feature. However, in the presence of HBArF·(Et₂O)₂, the S₁ → S₀ (f = 0.3737) transition of the protonated 1,3-Az[9]CPP becomes allowed (Fig. S22 and Table S6, ESI†).

Fig. 4 (a) UV-vis spectra of 1,3-Az[9]CPP (2.0 × 10⁻⁵ M) upon addition of HBArF·(Et₂O)₂ in DCM and (b) fluorescence spectra of 1,3-Az[9]CPP in the absence and presence of 30% TFA (1.0 × 10⁻⁵ M) in 1,4-dioxane, THF, DMF, DCM and toluene.

Table 1 Photophysical and electrochemical data of 1,3-Az[9]CPP and 1,3-Az[9]CPP-H⁺

	λ abs ,^b (nm)	λ em ,^c (nm)	Φ F (%)	E g(opt) (eV)	E HOMO (eV)
a UV-vis absorption and fluorescence spectra were measured in CH2Cl2 (1 × 10^−5 M) at room temperature. b Wavelength of the maximum absorption. c Emission maximum (λex = highest intensity of absorption plus 10 nm). d Absolute fluorescence quantum yield. e Estimated from absorption onset, Eg(opt) = 1240/λonset. f E HOMO = −(4.74 + Eonset^ox) eV (calibration by ferrocene).
1,3-Az[9]CPP	334	493	1.4	2.74	−4.97
1,3-Az[9]CPP-H^+	329	493	1.0	1.82	−4.74

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Furthermore, we have also measured UV-vis and fluorescence spectra of 1,3-Az[9]CPP in the absence and presence of trifluoroacetic acid (TFA) in different solvents including DCM, toluene, N,N-dimethylformamide (DMF), 1,4-dioxane and tetrahydrofuran (THF) (Fig. 4b and Fig. S14, S15, ESI†). Upon titration with TFA, the variations in UV-vis spectra of 1,3-Az[9]CPP in different solvents were similar to those in DCM but to a different extent (Fig. 4a and Fig. S14, ESI†). Interestingly, a significant increase in fluorescence emission intensity was observed in DMF, 1,4-dioxane and THF solutions (Fig. 4b and Fig. S15, ESI†). The emission intensity in 1,4-dioxane was enhanced to be about 10 times higher than that in DCM at the same emission wavelength (λ_em). However, the fluorescence spectrum in toluene is almost the same as that in DCM. These phenomena suggest that polar solvents are more favorable for the fluorescence emission of the azulenylium cation.

The electrochemical properties of 1,3-Az[9]CPP were further evaluated by CV. In C₂H₄Cl₂-Bu₄NPF₆, 1,3-Az[9]CPP exhibited two oxidation processes. A quasi-reversible peak at 0.28V (peak anode current/peak cathode current, ip_a/ip_c = 0.3) and an irreversible peak at 0.79V were observed (Fig. 5). However, in the presence of HBArF·(Et₂O)₂, a new reduction peak at −0.58 V was detected, which further confirms the protonation of 1,3-Az[9]CPP. The CV data in combination with the results of NMR and UV-vis studies confirm that 1,3-Az[9]CPP features pH reversible stimuli-responsiveness. To the best of our knowledge, it is the first pH-responsive nonalternant all-carbon nanohoop.

Fig. 5 Cyclic voltammograms of 1,3-Az[9]CPP (1 mM) and 1,3-Az[9]CPP upon addition of 1.0 eq. of HBArF·(Et₂O)₂ in C₂H₂Cl₄ with 0.2 M Bu₄NPF₆ as the supporting electrolyte. The electrode potential was externally calibrated using an Fc/Fc⁺ couple.

The host–guest interaction behaviours between 1,3-Az[9]CPP and fullerenes C₆₀/C₇₀ were finally investigated because 1,3-Az[9]CPP has a cavity similar to that of [10]CPP.¹² Single crystals of the C₆₀@1,3-Az[9]CPP complex suitable for X-ray analysis were prepared by slow diffusion of acetonitrile into a mixed solution of chlorobenzene and CS₂ (Fig. 6). The unit cell of C₆₀@1,3-Az[9]CPP belongs to the space group C2/c. The crystal structure demonstrates that fullerene C₆₀ resides inside the cavity of 1,3-Az[9]CPP, clearly showing a 1 : 1 binding model. The crystal packing shows columnar stacks along the vertical axis (Fig. 6d).

Fig. 6 X-ray crystal structures of C₆₀@1,3-Az[9]CPP, (a) side view, (b) top view of the distance between 1,3-Az[9]CPP and C₆₀, (c) top view and (d) side view of crystal packing showing columnar stacks of C₆₀.

The binding behaviours of 1,3-Az[9]CPP with C₆₀ and C₇₀ were further characterized by UV-vis titration experiments in toluene at room temperature. By fitting the UV-vis adsorption changes at 334 nm at different fullerene concentrations, the association constants (K_a) of C₆₀ and C₇₀ with 1,3-Az[9]CPP were estimated to be 2.4 × 10⁴ M⁻¹ for C₆₀ and 1.6 × 10⁴ M⁻¹ for C₇₀ in toluene (Fig. 7a and Fig. S16a and S17, ESI†), which are smaller than those of 10[CPP] (2.7 × 10⁶ M⁻¹ for C₆₀ and 8.4 × 10⁴ M⁻¹ for C₇₀).²⁸ The phenomenon is attributed to the tilting of azulene units in the carbon nanohoop. Subsequently, the K_a values of 1,3-Az[9]CPP with C₆₀ and C₇₀ in the presence of HBArF·(Et₂O)₂ were also extracted by UV-vis titration experiments to be 7.9 × 10⁵ M⁻¹ and 2.5 × 10⁴ M⁻¹ for C₆₀ and C₇₀, respectively (Fig. 7b and S16b and S17, ESI†). Notably, K_a of 1,3-Az[9]CPP with C₆₀ in the presence of acid is about 30 times higher than that in the absence of acid, suggesting that the binding of 1,3-Az[9]CPP to C₆₀ was significantly enhanced in the presence of acid. This enhanced binding can be accounted for the cation–π interaction between the aromatic tropylium ion and the fullerene rich in π-electrons. This interpretation was verified by NMR titration (Fig. S18 and S19, ESI†). As for C₆₀@1,3-Az[9]CPP-H⁺ and C₇₀@1,3-Az[9]CPP-H⁺, due to the enhanced electronic shielding by cation–π-electron interaction, the proton signals of H4'/H8' and H6' attributed to the protonated azulene are observed to have moved to higher fields. Moreover, the perfect size match between 1,3-Az[9]CPP-H⁺ and symmetrical spherical structure C₆₀ may also make an important contribution for a favorable binding interaction. This acid-tunable selectivity of 1,3-Az[9]CPP for C₆₀ over C₇₀ may have potential application in enriching C₆₀ from fullerene C₆₀/C₇₀ mixtures.²⁹

Fig. 7 UV-vis spectra in toluene of (a) 1,3-Az[9]CPP (1.0 × 10⁻⁵ M) in the presence of C₆₀ (0–3.29 × 10⁻⁵ M), K_a = 2.4 × 10⁴ M⁻¹ and (b) 1,3-Az[9]CPP-H⁺ (1.0 × 10⁻⁵ M) in the presence of C₆₀ (0–1.63 × 10⁻⁵ M), K_a = 7.9 × 10⁵ M⁻¹.

Conclusions

In summary, we have successfully synthesized the first conjugated nonalternant all-carbon nanohoop by incorporating an azulene unit. The data demonstrated that the 1,3-azulene-embedded carbon nanohoop exhibits quite unique optical and physical properties such as anti-Kasha's rule emission and an adjustable band gap. Notably, 1,3-Az[9]CPP also shows good pH reversible stimuli-responsiveness and this process of protonation and deprotonation can be repeated multiple times. Remarkably, in an acidic environment, 1,3-Az[9]CPP shows a high selectivity (30/1) in binding of C₆₀ over C₇₀. This work can motivate further investigations of conjugated nanohoops containing nonalternant π-systems and develop their potential application in the field of host–guest chemistry as well as functional materials. Currently, incorporations of 2,6- and 4,7- connected azulene units into the CPP skeleton are ongoing projects in our laboratory.

Author contributions

X. L. performed the experiments, carried out the characterization, and collected and analyzed the data. L. J. and Y. W. performed the theoretical calculations. W. W. analyzed the data of the electrochemical section. D. S. processed the single crystal data. H. J. and X. L. wrote the manuscript. All authors participated in the discussion and revised the manuscript.

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (21971020) and the Beijing Natural Science Foundation (2212008).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2177150 and 2177151. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2tc04321a

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