Complexation study of a 1,3-phenylene-bridged cyclic hexa-naphthalene with fullerenes C₆₀ and C₇₀ in solution and 1D-alignment of fullerenes in the crystals

Abstract

To investigate the host ability of a simple macrocycle, 1,3-phenylene-bridged naphthalene hexamer N6, we evaluated the complexation of N6 with fullerenes in toluene and in the crystals. The complexes in the solid-state demonstrate the one-dimensional alignment of fullerenes. The single-crystals of the C₆₀@N6 composite have semiconductive properties revealed by photoconductivity measurements.

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Introduction

Macrocyclic arenes constitute an important class of shape-persistent host molecules.¹ This system features structural rigidness, interesting optoelectronic properties, self-assembling behavior, and host–guest chemistry.² In many cases, they form a large cavity inside to be used for fullerene recognition.^3,4 Naphthalene is one of the simplest polycyclic aromatic hydrocarbons (PAHs) with a rigid π-plane, which has been incorporated into cycloarylenes.^5–9 We reported a macrocyclic arene, 1,3-phenylene-bridged cyclic naphthalene hexamer N6, that could be synthesized by a straightforward one-pot Suzuki–Miyaura reaction (Fig. 1).¹⁰ The crystal structure showed that the N6 had a symmetric hexagonal structure and a large cavity (d = ca. 15 Å). This unique structural characteristic allows N6 to bind the spherical molecule C₆₀ by forming a one-dimensional (1D)-alignment in the solid-state.¹⁰

Fig. 1 Structures of a cyclic naphthalene hexamer N6, and fullerenes C₆₀ and C₇₀.

Here, to examine the effects of a variation in the size and shape of fullerenes,¹¹ we investigated the host–guest interactions between N6 and spherical C₆₀ and ellipsoidal C₇₀ using ¹H NMR spectroscopy in solution and single-crystal X-ray diffraction analysis in the solid state. Especially, we could determine the association constants and stoichiometry of the complexation fullerene@N6 in solutions, for the first time, based on the statistical methodology.

Results and discussion

Complexation analysis in solution

In our previous report,¹⁰ we could not observe the formation of the complexes of N6 with C₆₀ in chloroform due to the low solubility of C₆₀. In the present study, we analyzed complexation of N6 in toluene as a better solvent for fullerenes. Recently, it is noticed that Job's plot is inappropriate for estimating stoichiometries in the presence of more than one complex.¹¹ Therefore, we attempted titration experiments with three different host–guest association models; 1 : 1, 1 : 2 and 2 : 1 complexation.¹² Solutions of N6 (0.4 mM) and C₆₀ (0.4 mM) in toluene-d₈ were mixed in different ratios from 10 : 0 to 1 : 9 to prepare 10 samples. As an example, the chemical shift of the proton H^a in Fig. 1 was illustrated: the chemical shift originally observed at 7.64 ppm was shifted to down-field at 7.88 ppm due to the host–guest interactions (Fig. 2a). The spectral features are analogous to those of other naphthalene-C₆₀ supramolecular systems.¹³ This guest-binding profiles (the initial data points: 7.64, 7.53 and 7.40 ppm, 10 samples, total data points N = 30) were analyzed with the curve-fitting for 1 : 1, 1 : 2 and 2 : 1 binding systems (Fig. S2 and Table S1†).¹² All the fitted curves agreed with the observed chemical shift changes. It is difficult to compare the fitted curves of these models quantitatively so further analysis was made by investigation of the goodness-of-fit (GOF). To evaluate the GOF of these models, Akaike's information criterion (AIC) statistics were applied according to the recent exercise to a similar binding system.¹⁴

Fig. 2 (a) The chemical shift changes of inside proton resonance observed in ¹H NMR titration of N6 with C₆₀ in toluene-d₈. ([N6] : [C₆₀] = 10 : 0 to 1 : 9, the total concentration was 0.4 mM, 600 MHz, 298 K) (b) curve-fitting obtained by using the 2 : 1 binding model.

The AIC values are summarized in Table S1.† Based on the quantitative GOF analysis given by AIC calculation, the 2 : 1 additive model is the most preferred (Fig. 2b).¹⁵ We obtained K₁₁ = 1.69 (±0.06) × 10⁴ M⁻¹ and K₂₁ = 4.54 (±0.42) × 10² M⁻¹ for the first and second complexation, respectively. The smaller K₂₁ value than K₁₁ indicates that 1 : 1 complex is dominant in solution and 2 : 1 complex becomes coexistent when the concentration of the host is high (Fig. S3†).¹⁵

Previously we obtained the crystal structure of the complex of N6 and C₆₀, which allowed us to precisely determine the 1 : 1 stoichiometry of this complex in the solid state (CCDC 1838834 for N6, 1838835 for C₆₀@N6).¹⁰ The crystallographic analysis confirmed the π-stacking between N6 and encapsulated C₆₀. The dihedral angles of the facing naphthalene moieties vary to maximize the interactions between the N6 and C₆₀: upon the complexation, the dihedral angles became slightly wider from the energy minimized structure. The closest distance between the C₆₀ and naphthalene is 3.32 Å in the range of π-stacking.

Then, we performed the titration between N6 and C₇₀. The formation of the N6–C₇₀ complex in toluene-d₈ was clearly suggested using the ¹H NMR (Fig. 3a). A curve-fitting simulation of guest-binding profile also supported a 2 : 1 model (Fig. S5 and Table S2†). We obtained K₁₁ = 2.52 (±0.22) × 10⁴ M⁻¹ and K₂₁ = 2.12 (±0.80) × 10² M⁻¹ for the complexation (Fig. 3b). The estimated K₁₁ binding constant was 1.5 times larger than that with C₆₀, inevitably due to the ellipsoidal shape of C₇₀.

Fig. 3 (a) The chemical shift changes of inside proton resonance observed in ¹H NMR titration of N6 with C₇₀ in toluene-d₈. ([N6] : [C₇₀] = 10 : 0 to 1 : 9, the total concentration was 0.4 mM, 600 MHz, 298 K) (b) curve-fitting obtained by using the 2 : 1 binding model.

Interestingly, the peak of N6 at 8.38 ppm assigned to H^b exhibited down-field shift upon the addition of C₆₀, while it showed up-field shift upon the addition of C₇₀ (Fig. S7†). The peak at 7.53 ppm assigned to H^c similarly exhibited opposite peak shifts due to the addition of C₆₀ and C₇₀. These results illustrate that the angles between naphthalene and phenylene respond to the C₆₀ and C₇₀ encapsulation with smaller and larger sizes, respectively.

Single-crystal X-ray analysis

Fortunately, we obtained the composite structure of N6–C₇₀ by single-crystal X-ray diffraction analysis despite low resolution (>1.0 Å) due to very weak diffraction at the high θ angle (Fig. 4).|| Single-crystals of N6–C₇₀ composite were obtained by vapor diffusion of MeOH into a chlorobenzene solution. C₇₀ in the crystal is nicely captured within the cavity made by N6 with intermolecular distances in the range of 3.2–3.4 Å. Closer inspection of the crystal structure revealed that N6 keeps a 1 : 1 complex with C₇₀ similar to N6–C₆₀ with the dihedral angles of naphthalene toward phenylene (51° and 72°). The fullerene moiety occupied at the special position refined by applying appropriate instructions. As shown in Fig. 4c, the complex also consequently forms the directly-contacting one-dimensional C₇₀ array along the crystallographic a-axis. The long-axis of the C₇₀ is tilted to the alignment direction by 33°, which is expected to increase the contact area between two fullerenes and thus to increase the interaction strengths. The closest C–C distance between fullerenes in the array is 4.1 Å, suggesting the larger electronic interaction between fullerenes than that in the N6–C₆₀ composite (4.4 Å). In addition, the C–H⋯π interactions between the hydrogen atoms of the naphthalene units and the C₇₀ also contribute to the stabilization of the N6–C₇₀ assembly.

Fig. 4 X-ray structure of the N6–C₇₀ 1 : 1 complex, ORTEP drawing from (a) the side view and (b) top view with thermal ellipsoids scaled at 25% probability. Disordered molecules are omitted for clarity. (c) A columnar array of N6–C₇₀ along the a-axis. For clarity, C₇₀ is shown as a space-filling model.

Photoconductivity measurements

The structures of N6–C₆₀ and N6–C₇₀ are expected to have large intermolecular orbital couplings. To discuss charge transport property in detail, the charge transfer integrals of the HOMOs (V_hole) and LUMOs (V_electron) between the neighbouring C₆₀ and C₇₀ pairs were calculated based on the crystal structures using ADF program¹⁶ (Fig. S9†). Along the crystallographic a-axis, the V_hole and V_electron values for C₆₀ pairs were calculated to be 3.1 and 3.0 meV, respectively. On the other hand, the V_hole and V_electron values for C₇₀ pairs were 10.4 and 15.8 meV, respectively, higher than those of C₆₀. With these expecting charge–transport properties in mind, we conducted flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements of C₆₀@N6. This electrodeless method allows for evaluating short-range (∼10 nm) transient conductivities of materials.¹⁷ With a 355 nm laser pulse at 25 °C, the pseudo-conductivity (ϕΣμ_max in cm² V⁻¹ s⁻¹ in which ϕ is the quantum efficiency of charge generation and Σμ_max is the sum of hole and electron mobilities) of N6–C₆₀ exhibited ϕΣμ_max = 1.3 × 10⁻⁴ cm² V⁻¹ s⁻¹ (Fig. 5). This value is comparable to PC₆₁BM¹⁸ and other conjugated molecules.¹⁹ Unfortunately, for C₇₀@N6 which was expected to exhibit better charge mobilities than N6–C₆₀, we were unable to prepare single crystals of good enough quantity to measure the FP-TRMC.

Fig. 5 FP-TRMC profile of the single-crystals (C₆₀@N6·PhCl) recorded at an excitation wavelength of 355 nm with a photon density of 9.1 ×10¹⁵ photons per cm⁻².

Conclusions

In summary, we present the molecular host N6 can bind the fullerenes C₆₀ and C₇₀ in solution and in the solid-state. The NMR titration experiments and curve-fitting suggest that the binding profile between N6 and fullerenes analyzed by 2 : 1 model was most likelihood with binding constants of K₁₁ = 1.69 (±0.06) × 10⁴ M⁻¹ and K₂₁ = 4.54 (±0.42) × 10² M⁻¹ for C₆₀@N6 and K₁₁ = 2.52 (±0.22) × 10⁴ M⁻¹ and K₂₁ = 2.12 (±0.80) × 10² M⁻¹ for C₇₀@N6. In the solid-state, on the other hand, both N6–C₆₀ and N6–C₇₀ composites show 1 : 1 complex and make the 1D arrays of fullerenes with the aid of the N6 agent as confirmed by the single-crystal X-ray analysis. Among these, N6–C₆₀ exhibited the moderate ϕΣμ_max = 1.3 × 10⁻⁴ cm² V⁻¹ s⁻¹ by FP-TRMC. We are currently investigating the host–guest chemistry of N6 with larger fullerenes, expecting it to exhibit different selectivity and affinity.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partly supported by Grants-in-Aid for Scientific Research (No. JP20H02816 (HH), JP20H00379 (HY), JP20H05833 (HY), JP22K19067, and JP23H01787 (NA), JP22K05255 (KM), and JP20H05836 (AS), and JST, PRESTO (No. JPMJPR21AC (HH)). HM thanks University Fellowships for the Creation of Innovation in Science and Technology. We acknowledge Dr Mitsuaki Yamauchi (Kyoto University) for helpful discussions. This work was partially supported by ISHIZUE 2023 of Kyoto University.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Detailed experimental procedures and additional spectroscopic data. CCDC: 2296635. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3ra06526j

‡ . Current address: College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People's Republic of China.

§ . Current address: Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan.

¶ . Current address: Center for Basic Research on Materials, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan.

|| Crystallographic data for C₇₀@N6: C₉₆H₆₀ C₇₀ C₆H₅Cl, Mw = 2166.69, triclinic, space group P1̄ (#2), a = 11.75(4), b = 14.98(5), c = 15.88(5) Å α = 106.46(3), β = 101.62(3), γ = 98.45(3)°, V = 2563(14) ų, T = 103(2) K, Z = 1, reflections measured 5549, 4553 unique. The final R₁ was 0.1092 (I > 2σ(I)), and the final wR on F² was 0.4205 (all data), GOF = 1.084. CCDC: 2296635

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