Cycloparaphenylene Double Nanohoop: Structure, Lamellar Packing, and Encapsulation of C₆₀ in the Solid State

Abstract

A new member of the cycloparaphenylene double-nanohoop family was synthesized. Its π-framework features two oval cavities that display different shapes depending on the crystallization conditions. Incorporation of the peropyrene bridge within the nanoring cycles via bay-regions alleviates steric effects and thus allows 1:1 complexation with C₆₀ in the solid state. This nanocarbon adopts a lamellar packing motif, and our results suggest that the structural adjustment of this double nanohoop could enable its use in supramolecular and semiconductive materials.

Cycloparaphenylenes (CPPs) are tube-shaped radially π-conjugated molecular loops composed of distorted para-linked phenylene rings.¹ These “carbon nanohoops” are investigated as seeds for the size-controlled growth of armchair carbon nanotubes² and materials in the areas of supramolecular chemistry³ and materials science.⁴ Since the landmark synthesis of [9]-, [12]-, and [18]CPP in 2008,⁵ a diversity of [n]CPPs and more complex CPP architectures have been developed, including lemniscates,⁶ cylinders,⁷ double nanohoops,⁸ propellers,⁹ cages,¹⁰ and catenanes,¹¹ which has expedited the development of CPP chemistry. The solid-state structures of [n]CPPs were not available until the first single-crystal X-ray diffraction (SC-XRD) characterization of [12]CPP, accomplished in 2011.¹² Typically, herringbone packing motifs are observed in the solid-state structures of [n]CPPs,³ except for [6]CPP, which adopts a columnar packing structure.¹³ Notably, [6]CPP can also display herringbone packing upon adjustment of the crystallization conditions.¹⁴ Rational manipulation of the packing mode of [n]CPPs is an efficient tactic to implement functionality in the solid state. For example, columnar packing featuring porous channels is favored when [n]CPPs are fluorinated,¹⁵ carboxylated,¹⁶ heteroatom-doped,¹⁷ or reduced to their anionic forms.¹⁸ Despite the advanced synthetic methodologies that enable size-selective syntheses of [n]CPPs (n ≥ 5),¹⁹ their potential applications as solid-state materials are still rarely explored.²⁰

A unique structural feature of CPPs is their radial cavity that enables them to encapsulate fullerenes and small polycyclic aromatic hydrocarbons.³ The cavity of [10]CPP was shown to have an ideal diameter for binding C₆₀,²¹ and the solid-state structure of the 1:1 [10]CPP⊃C₆₀ complex revealed a close concave–convex π–π interaction between the host and the guest.²² In addition, it was demonstrated that [n + 5]CPPs can selectively encapsulate [n]CPPs to mimic the shortest segment of double-walled carbon nanotubes.²³ Most recently, a π-extended [12]CPP analogue was exemplified to accommodate both fullerene (C₆₀ or C₇₀) and the bowl-shaped trithiasumanene in its inner void, forming the first ternary single-nanohoop complex.²⁴ CPP-derived frameworks that feature two or more nanoring cycles are promising candidates for forming 1:n ternary complexes via the interaction of the host with n guest molecules.^6,8,9 In contrast with the single-nanohoop systems, multi-nanohoop systems can enable a stepwise complexation^8c,8d or a complexation with different guest molecules, a feature of interest in porous or sensor materials design.^6d To date, only two instances of the 1:n host–guest complex, namely, 1:2, have been reported, although the solid-state structures of these ternary complexes have not yet been validated by means of SC-XRD.^8c,8d To deepen our understanding of the 1:n complexes, which is impeded by the challenges associated with the synthesis of the host architectures, CPP-derived multi-nanohoops with improved binding abilities need to be devised.

We recently reported a peropyrene-bridged double-nanohoop architecture (CPP-PP, Figure 1) constructed through a radical dimerization of a preassembled CPP loop bearing one phenalenyl unit.²⁵ The fully π-conjugated hydrocarbon framework of CPP-PP was unambiguously validated by SC-XRD, which revealed two large circle-shaped cavities. No evidence of binding fullerenes or small polyaromatics, however, was observed, indicating that these cavities are not an ideal fit. It appeared to us that the hydrogen atoms at the two termini of the peropyrene segment (H₁, H₂, and H_2′), which point inward the cavities, might cause severe steric perturbations. To eliminate such steric effect, we designed and synthesized a new, structurally modified double-nanohoop framework, CPP-^bPP (Figure 1), where the bay-regions of the central peropyrene segment are embedded within the nanoring cycles. In this manner, the cavities have an oval shape and are relieved of significant spatial obstructions from the hydrogen atoms at the bay-regions (H₄ and H_4′), which point inward, to activate the host functions of CPP-^bPP. During our investigations of the binding ability of this new double nanohoop, we identified and herein describe its unusual behavior in the solid state: (1) In contrast with the typically observed herringbone packing of CPPs and their analogs, CPP-^bPP displays a lamellar packing mode, (2) the shape of the cavities fluctuates depending on the crystallization conditions, and (3) only the 1:1 and not the 1:2 complex with C₆₀ was observed. Intrigued by these solid-state features, we performed preliminary studies to explore the potential of CPP-^bPP as a solid material.

Figure 1.

Structures of peropyrene-bridged double nanohoops CPP-PP (R = OC₃H₇) and CPP-^bPP (R = C₆H₁₃).

Our synthetic strategy toward CPP-^bPP is based on radical dimerization and Suzuki–Miyaura macrocyclization as two pivotal steps (Scheme 1). In contrast with the synthesis we employed for CPP-PP,²⁵ the central peropyrene bridge was constructed prior to the formation of the nanoring cycles (Figure 1). 4-Iodophenyl-substituted dihydrophenalenone 4 underwent a Suzuki–Miyaura coupling to afford 3 in 60% yield, which was subjected to the reduction and dehydration to afford the 1H-phenalene intermediate. Taking advantage of the “decomposition” pathway of phenalenyl to peropyrene,²⁶ we treated the 1H-phenalene precursor with an excess of p-chloranil at room temperature to afford peropyrene building block 1 in a total isolated yield of 63%. The construction of the cyclic scaffolds is an intractable concern in CPP chemistry.¹⁹ After screening several corner units, we found U-shaped linker 2 to be ideal to unite with 1 and afford CPP-^bPP in 31% yield upon reductive aromatization. The hydrocarbon π-framework of CPP-^bPP was unambiguously confirmed by SC-XRD (Figure 2). The concise synthesis allowed the preparation of CPP-^bPP on a 50 mg scale. Benefiting from eight hexyl chains on the CPP backbone, CPP-^bPP exhibits moderate solubility in common organic solvents, such as CHCl₃ and toluene, which enables the solution-processed fabrication of thin films.

Scheme 1. Synthesis of CPP-^bPP (R = C₆H₁₃).

Reaction conditions: (a) 4-bromo-2,5-dihexylphenylboronic acid, Pd(PPh₃)₄, NaOH (2 M), 1,4-dioxane, 80 °C; (b) (i) NaBH₄, DCM/EtOH, rt, (ii) p-toluenesulfonic acid monohydrate, toluene, 90 °C, (iii) p-chloranil, DCM/toluene, rt; (c) (i) SPhos Pd G3, K₃PO₄ (2 M), 1,4-dioxane, 80 °C, (ii) sodium naphthalenide, THF, −78 °C.

Figure 2.

Solid-state structures of (a) CPP-^bPP obtained from CHCl₃, (b) CPP-^bPP obtained from toluene, and (c) the 1:1 CPP-^bPP⊃C₆₀ complex. Thermal ellipsoids are shown at the 20% probability level. Disordered solvent molecules are omitted for clarity.

The X-ray diffraction (XRD) analysis of single crystals of CPP-^bPP, obtained by the slow vapor diffusion of acetonitrile into a CHCl₃ solution at room temperature, unequivocally confirmed the structure of CPP-^bPP with a C₂ symmetry, where two CPP loops are tethered through a rigid peropyrene bridge to constitute a CPP-based double-nanohoop framework comprising 146 sp² carbon atoms. The different connection mode of CPP loops in CPP-^bPP compared with CPP-PP leads to dramatic changes in the dihedral angles and void shapes. The dihedral angles at the four linkages between the CPP loops and the peropyrene bridge of CPP-^bPP are 66, 61, 66, and 61° (Figure 2a), significantly smaller than those of CPP-PP (80° on average), indicating a stronger π-communication between the CPP loops and the central peropyrene segment in CPP-^bPP. This is in agreement with the red shift in the UV–vis absorption spectrum of CPP-^bPP compared with that of CPP-PP (Figure S10). The CPP biphenylene units of CPP-^bPP display an average dihedral angle of 41°, greater than that of CPP-PP (29° on average), possibly owing to the discrepancy in the void shape of the two double nanohoops. Unlike CPP-PP, which possesses two circle-shaped cavities, CPP-^bPP has two oval-shaped cavities with a long and short axis of 1.76 and 1.17 nm, respectively.²⁷ The overall length of the π-framework reaches 4.17 nm. Remarkably, the solid-state structure obtained from a toluene solution exhibits two shorter but wider cavities with a long and short axis of 1.70 and 1.31 nm, respectively (Figure 2b). The varied cavity size in the two solid-state structures indicates the flexible nature of the CPP loop²⁸ and is presumably the result of the crystal packing forces. On the basis of distortion analysis (Figure S8), the phenylene rings opposite the peropyrene segment are the most strained in both cases. Given the suitable size of the cavities and the alleviated steric hindrance from the hydrogen atoms of the central peropyrene segment, CPP-^bPP is a potential host toward C₆₀ or C₇₀.²⁹ However, no binding interaction between CPP-^bPP and C₆₀/C₇₀ was detected in solution by means of UV–vis absorption (Figure S15), fluorescence, or NMR spectroscopy, possibly due to the fact that the cavity-reorganization energy counteracts the host–guest complexation energy. Nevertheless, the binding behavior was clearly observed in the solid state. Slow diffusion of acetonitrile into a toluene solution of CPP-^bPP and C₆₀ at room temperature afforded single crystals of a complex. Surprisingly, a 1:1 CPP-^bPP⊃C₆₀ complex, instead of a 1:2 complex, was revealed (Figure 2c). The C₆₀ molecule resides in the center of one of the two cavities of CPP-^bPP, leading to a strong concave–convex π–π interaction with the shortest intermolecular C–C distance of 2.86 Å. In addition, an obvious quenching effect of photoluminescence was found in the spin-coated thin film of a mixture of CPP-^bPP and C₆₀ (Figure S16), indicating an interaction between the two components in the solid state.

The solid-state packing structure is one of the crucial parameters that determines charge-transport properties of organic semiconductors.³⁰ A herringbone packing structure, typical for most CPPs,³ was observed for CPP-PP (Figure 3a). Unexpectedly, CPP-^bPP adopted a lamellar packing motif (Figure 3b). To the best of our knowledge, such packing mode is unprecedented for all CPPs and architectures derived from them. Layer-to-layer, the molecules of CPP-^bPP were offset such that the CPP loops formed diagonal columns. Within one column, CH···π and CH···CH contacts were found between neighboring molecules, and the interlayer distances defined by the peropyrene planes of two CPP-^bPP molecules were 5.339 and 3.928 Å in an alternating fashion (Figure 3b). A similar packing pattern was found for CPP-^bPP obtained from toluene, but here the interlayer distance was uniform (5.347 Å, Figure S5). This unique lamellar packing mode makes CPP-^bPP a potential candidate for semiconductive materials. The CPP-^bPP⊃C₆₀ complex exhibited a packing structure (Figure 3c) that was almost identical to that of CPP-^bPP obtained from toluene, except for an increased interlayer distance of 6.111 Å. It is plausible that such a tight lamellar packing motif is the reason for the formation of the 1:1 complex and not the 1:2 complex. If a 1:2 CPP-^bPP⊃C₆₀ complex would form, significant steric repulsion would arise between the neighboring C₆₀ molecules (shortest C–C distance of 2.24 Å, Figure S7b), given that the packing and interlayer distance would stay the same.

Figure 3.

Packing in the solid-state structure of (a) CPP-PP, d = 4.995 Å, (b) CPP-^bPP obtained from CHCl₃, d₁ = 5.339 Å, d₂ = 3.928 Å, and (c) the 1:1 CPP-^bPP⊃C₆₀ complex, d = 6.111 Å. Note that the distribution of C₆₀ shown in panel c is only a representative example, and C₆₀ can randomly reside in one of the two cavities, resulting in an average ratio of 1:1. Thermal ellipsoids are shown at the 20% probability level. Disordered solvent molecules are omitted for clarity.

Inspired by the unique lamellar packing motif of CPP-^bPP and the electronic properties of CPPs and their derived structures, we measured the electrical conductivity of the single crystals of CPP-^bPP (see the Supporting Information for details). CPP-^bPP showed a conductivity value of 5.8 × 10^–3 S cm^–1, comparable to that of its structural analog CPP-PP (5.9 × 10^–3 S cm^–1) and higher than that of C₆₀ (1.7 × 10^–8 S cm^–1).³¹ The reason why CPP-^bPP does not display higher conductivity than CPP-PP is presumably the lack of π–π interactions between the peropyrene units of the neighboring CPP-^bPP molecules, where the HOMO is predominantly distributed, and the relatively large interlayer distance. Nevertheless, alteration of the crystal packing in these two regards could improve the charge-transport properties.

In summary, we synthesized a novel member of the CPP-derived double-nanohoop family, CPP-^bPP, in which the bay-regions of the peropyrene bridge are incorporated within the nanoring cycles. The skeletal modification with regards to the structural analog CPP-PP gives rise to a remarkable change in the packing motif identified in the solid state, where an unprecedented lamellar packing was observed. This packing mode is in stark contrast with the herringbone motif typically observed for CPPs and CPP-derived structures. Because of the relief of steric obstruction in CPP-^bPP compared with CPP-PP, the large voids of CPP-^bPP enable it to function as a molecular host toward C₆₀ in the solid state. Interestingly, the lamellar arrangement of CPP-^bPP molecules allows the formation of only a 1:1 CPP-^bPP⊃C₆₀ complex, which has been validated by SC-XRD. Finally, CPP-^bPP performs as a semiconductive material with a conductivity value of 5.8 × 10^–3 S cm^–1. Given the unique solid-state structure of CPP-^bPP, we envision that a fullerene C₆₀ dimer, namely, C₁₂₀,³² could be an ideal guest molecule for encapsulation by CPP-^bPP to accomplish the full occupancy of its two cavities and the formation of encapsulated zigzag C₆₀ wires. Such fullerene-based conductive supramolecular wires³³ are of interest as organic semiconductors. We believe that our results will aid the crystal engineering of CPPs and CPP-based nanohoops and promote their application in semiconductive materials and host–guest chemistry.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.orglett.1c02950.

• Experimental procedures, characterization data (¹H and ¹³C NMR and HRMS), X-ray analysis, photophysical and redox data, and density functional theory calculations (PDF)

Accession Codes

CCDC 2104607–2104609 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

The authors declare no competing financial interest.