Influence of crystallization solvents on the crystal structures and supramolecular assemblies of a [2]naphthyl-extended pillar[6]arene

Abstract

A newly synthesized [2]naphthyl-extended pillar[6]arene was crystallized from a range of solvent systems to examine the influence of crystallization conditions on its structural and supramolecular behavior. Single-crystal X-ray diffraction revealed solvent-dependent packing modes, giving rise to distinct assemblies including inclusion complexes, and supramolecular polymers driven by solvent guest molecules. The variations are governed by solvent–host interactions, emphasizing the crucial role of solvents in directing supramolecular assembly in the solid state. The resulting supramolecular assemblies were fully characterized using single-crystal X-ray diffraction and Hirshfeld surface analysis.

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Introduction

Pillar[n]arenes and their functional derivatives have emerged as an important class of macrocyclic hosts in supramolecular chemistry, owing to their well-defined cavities, highly tunable chemical functionality and selective guest binding capability.^1–6 In recent years, considerable attention has been devoted to the construction of extended-pillararenes, a rapidly expanding subclass of pillararene-based macrocycles.^7–15 In these systems, additional aromatic or heterocyclic fragments are incorporated into the parent macrocyclic skeleton, producing hosts with enlarged cavities, modified conformational behaviour, and new physicochemical properties. The larger cavities of these extended pillararenes frequently result in enhanced binding affinities towards aromatic/alicyclic guests, enabling applications in selective molecular recognition, controlled adsorption and separation processes for comparatively bigger species. Moreover, extended pillararenes could exhibit distinct optical and photophysical features, including enhanced fluorescence and improved photostability, positioning them as promising functional platforms for chemical sensing, photo- or redox-responsive host–guest systems, luminescent supramolecular materials and bioimaging probes.^7,16–23 The rapid rise of research on extended pillararenes, thus, reflects a broader shift that connects classical host–guest chemistry of pillararenes with modern supramolecular material design. Deliberately altering the macrocyclic framework of pillararenes enables new assembly modes, electronic behaviours and optoelectronic applications that conventional pillararenes cannot achieve.

Embedding larger π-conjugated units, such as naphthalene, anthracene, pyrene, perylene, or other polycyclic aromatic hydrocarbons, into the pillararene framework is expected to produce π-extended macrocycles with more electron-rich cavities and enhanced π–π stacking, CH–π interactions, and donor–acceptor complexation.^24–36 However, the synthesis and study of such PAH-based π-extended pillararenes remain very limited because of significant synthetic challenges. In this work, we report the synthesis and structural investigation of a naphthalene-embedded extended-pillararene in which two naphthalene units are introduced into the macrocyclic framework alongside four methoxy-substituted phenyl rings, generating an eight-membered, π-extended pillar[6]arene. The presence of electron-rich naphthalene units is expected to confer distinct host–guest binding features, as well as valuable photophysical properties, such as enhanced fluorescence, offering potential for future development in chemical sensing and bioimaging applications.^37–39

As part of our ongoing efforts to explore macrocyclic arenes with diverse cavity depth, size, and shape, we have recently reported the synthesis and characterization of pillar[5]arene, pagoda[4]arene, prism[5]arene, and prism[6]arene with α,ω-dibromoalkanes and their linear supramolecular polymer assembled via halogen–halogen interactions both in solution and in the solid state.^40–43 Moreover, the influence of the crystallization solvent on the host–guest inclusion complexes and the supramolecular self-assembly architectures of asymmetric A1/A2-difunctionalized pillar[5]arenes has been studied, with a direct comparison to their behavior in solution.²² In this paper, we report the crystal structures and supramolecular assemblies of a newly synthesized [2]naphthyl-extended pillar[6]arene. The effects of different crystallization solvents, including dichloromethane (DCM), toluene, ethyl acetate, and N,N-dimethylformamide (DMF) on the solid-state supramolecular architectures are examined. When crystallized from DMF, the [2]naphthyl-extended pillar[6]arene forms a linear supramolecular polymer driven by hydrogen-bonding interactions of the entrapped DMF molecules. The supramolecular interactions of all obtained crystals are characterized in detail using single-crystal X-ray diffraction and Hirshfeld surface analysis.

Experimental

The single crystal data were collected using a Rigaku Rapid II diffractometer with Mo-Kα radiation at 150 K. The data obtained were processed using the ‘Crystalclear’ software package. The structure was solved using the Bruker SHELXTL software package and refined using SHELXL-2019/21.⁴⁴ Nuclear magnetic resonance (NMR) spectroscopy was conducted using a Bruker Avance II 600 MHz (Germany) spectrometer. All non-hydrogen atoms were refined anisotropically, while hydrogen atoms were placed at calculated positions and refined using the riding model. Molecular graphics and the calculation of intermolecular interactions were conducted using Mercury (ver. 2024.3.0), while Hirshfeld surface analysis was performed using CrystalExplorer 21.5.⁴⁵ The cavity width was determined by averaging the centroid–centroid distances of opposite aromatic rings, whereas the cavity height was measured as the distance between the planes formed by the oxygen atoms at the upper and lower rims of the macrocycle.⁴⁶

Preparation of single crystals for X-ray diffraction

Single crystals of the compounds reported in this study were obtained either by slow solvent evaporation or by solvent diffusion methods. Crystals of [2]Naph-ExP6·DCM were grown by dissolving [2]Naph-ExP6 (10 mg) in dichloromethane (2 mL) and allowing the solution to evaporate slowly in a refrigerator. [2]Naph-ExP6·EtOAc crystals were obtained by dissolving [2]Naph-ExP6 (10 mg) in ethyl acetate (1 mL) and allowing diffusion with hexane. [2]Naph-ExP6·DMF crystals were grown by dissolving [2]Naph-ExP6 (10 mg) in a minimum amount of DMF (0.3 mL) and leaving the open test tube undisturbed for one week. [2]Naph-ExP6·toluene crystals were obtained by dissolving [2]Naph-ExP6 (10 mg) in a chloroform/toluene mixture (1 mL; 70 : 30 v/v) followed by slow solvent evaporation. The crystallographic data for the structures reported in this paper have been deposited at the Cambridge Crystallographic Data Centre (CCDC 2405326–2405329).

Synthesis of 2,6-bis(2,5-dimethoxybenzyl)naphthalene

1,4-Dimethoxybenzene (2.76 g, 20 mmol) was dissolved in freshly distilled dichloromethane (40 mL) and treated with aluminum chloride (585 mg, 4.4 mmol). A solution of 2,6-bis(bromomethyl)naphthalene (628 mg, 2 mmol) in dichloromethane (10 mL) was then added dropwise to the reaction mixture. The resulting solution was stirred at room temperature for 2 h and subsequently quenched with water. The organic layer was separated, washed successively with water and brine, and dried over anhydrous sodium sulfate. The solvent was removed under reduced pressure, and the residue was purified by flash column chromatography on silica gel (petroleum ether/dichloromethane = 70:30 v/v) to afford the product as a white powder (556 mg, 65%).¹H NMR (600 MHz, CDCl₃), δ: 3.70 (s, 6H), 3.77 (s, 6H), 4.09 (s, 4H), 6.68 (m, 2H), 6.72 (m, 2H), 6.80 (s, 1H), 6.81 (s, 1H), 7.31 (m, 2H), 7.58 (s, 2H), 7.65 (s, 1H), 7.67 (s, 1H). ¹³C NMR (150 MHz, CDCl₃), δ: 36.3, 55.8, 56.3, 111.5, 111.7, 117.1, 127.0, 127.7, 127.9, 131.1, 132.4, 137.8, 151.9, 153.7. HRMS 451.1885 (calc. for C₂₈H₂₈O₄Na); 451.1864 (found).

Synthesis of [2]naphthyl-extended pillar[6]arene

Paraformaldehyde (90 mg, 3.0 mmol) and 2,6-bis(2,5-dimethoxybenzyl)naphthalene (428 mg, 1.0 mmol) were dissolved in dichloromethane (75 mL) and stirred under a nitrogen atmosphere for 10 min. Boron trifluoride diethyl etherate (148 μL, 1.2 mmol) was then added, and the reaction mixture was stirred for an additional 25 min at room temperature. The reaction was quenched with aqueous sodium hydroxide solution, and the organic layer was separated, washed successively with water and brine, and dried over anhydrous sodium sulfate. The solvent was removed under reduced pressure, and the residue was purified by flash column chromatography on silica gel using a dichloromethane/ petroleum ether mixture (70 : 30 v/v) to afford the desired macrocycle as a white solid (128 mg, 29%).¹H NMR (600 MHz, CDCl₃), δ: 3.64 (s, 12H), 3.73 (s, 12H), 3.93 (s, 4H), 4.07 (s, 8H), 6.71 (s, 4H), 6.77 (s, 4H), 7.26 (s, 2H), 7.27 (s, 2H), 7.48 (s, 4H), 7.55 (s, 2H), 7.56 (s, 2H). ¹³C NMR (150 MHz, CDCl₃), δ: 30.7, 36.6, 56.3, 56.4, 114.1, 114.5, 126.4, 127.5, 127.6, 127.7, 128.5, 132.2, 138.4, 151.5, 151.7. HRMS 880.3970 (calculated for C₅₈H₅₆O₈); 880.3971 (found).

Results and discussion

The incorporation of naphthyl groups into the pillar[n]arene framework significantly enhances the structural and functional diversity of these macrocycles. The extended π-surface of the naphthalene units not only increases the overall rigidity and conjugation of the macrocyclic backbone, but also improves guest binding through enhanced π–π interactions and facilitates the formation of well-ordered supramolecular architectures in the solid state. Moreover, the introduction of naphthyl moieties imparts distinct photophysical and electronic properties, making these expanded systems promising platforms for selective molecular recognition, sensing, and optoelectronic applications.^47–52

In this context, new naphthalene-bridged expanded pillar[6]arene derivative [2]Naph-ExP6 was synthesized starting from 2,6-bis(bromomethyl)naphthalene following a procedure similar to that reported in the literature (Scheme 1).²⁴

Scheme 1 Synthesis of the [2]naphthyl-extended pillar[6]arene ([2]Naph-ExP6) macrocycle.

Macrocycle arenes connected with methylene are known to be flexible and change their geometry in solution and in the solid state when their cavity is occupied with guest molecules. To explore the effect of the crystallization solvents, suitable single crystals for X-ray diffraction analysis were grown from dichloromethane (DCM), ethyl acetate (EtOAc), toluene (PhMe) and dimethylformamide (DMF). The crystal structures of [2]Naph-ExP6 obtained from four solvent systems along with chemical structure representations are illustrated in Fig. 1. Single-crystal X-ray diffraction analysis revealed that the conformation and overall geometry of the [2]Naph-ExP6 macrocycle are highly dependent on the nature of the encapsulated guest molecules within its cavity.

Fig. 1 Crystal structures and chemical structure representations of the [2]naphthyl-extended pillar[6]arene ([2]Naph-ExP6) obtained from dichloromethane, DCM (a), ethyl acetate, EtOAc (b), toluene, PhMe (c), and dimethyl formamide, DMF (d).

Crystal structure of [2]Naph-ExP6·CH₂Cl₂

Single-crystal X-ray diffraction analysis reveals that in the [2]Naph-ExP6·CH₂Cl₂ complex, the [2]Naph-ExP6 macrocycle adopts a markedly distorted conformation, in which the dimethoxyphenyl rings deviate substantially from an idealized macrocyclic framework. The macrocycle is characterized by an average cavity width of 10.53 Å and a cavity height of approximately 4.35 Å, with a maximum vertex-to-vertex distance of 13.27 Å. The rigid naphthalene moieties are positioned on two distinct planes relative to a common molecular base, giving rise to a deformed, cage-like architecture. Notably, the centroid–centroid separation between the naphthalene rings is 9.47 Å, representing the shortest distance observed within this series.

One naphthalene ring is tilted inward toward the macrocyclic cavity by 47.2° with respect to the mean molecular plane. Its symmetry-related counterpart, oriented parallel to the first, is tilted outward from the cavity by 133.0° (180–47.0°). A similar arrangement is observed for the phenyl rings: one phenyl ring is inclined inward by 48.5°, while its symmetry-related partner is tilted outward by 131.5° (180–48.5°). The remaining pair of opposite phenyl rings is likewise parallel, with one tilted inward by 51.7° and the other outward by 128.3° (180–51.7°). The dihedral angles between adjacent phenyl rings are 85.6°, whereas those between the naphthalene and phenyl rings are 81.1° and 78.3°, respectively.

The encapsulated dichloromethane molecule in the center of the cavity stabilizes the adopted conformation through two short C–Cl⋯H–C(sp³) contacts (2.79 and 2.82 Å), both shorter than the sum of the van der Waals radii (2.95 Å). An additional C–H⋯π contact (3.45 Å) between the guest and the adjacent naphthyl rings, together with two additional CH₂Cl₂ molecules located diagonally at the outer corners of the cavity, contributes to further reinforce the locked conformation through a combination of C–Cl⋯H–C(sp³), C–H⋯O, and C–H⋯π interactions involving the perpendicularly oriented dimethoxybenzene fragments (Fig. 2).

Fig. 2 Crystal structure of [2]Naph-ExP6 obtained from dichloromethane (CH₂Cl₂), showing the side view of the non-covalent interactions that stabilize the adopted macrocyclic conformation (a) and (b) top view of the macrocycle.

The intermolecular interactions in the [2]Naph-ExP6·CH₂Cl₂ crystal were further visualized using the three-dimensional d_norm Hirshfeld surface analysis (Fig. 2c). On the d_norm surface, red regions indicate intermolecular contacts shorter than the sum of the corresponding van der Waals radii, white regions correspond to contacts close to van der Waals separations, and blue regions represent contacts longer than the van der Waals radii. The Hirshfeld surface of the [2]Naph-ExP6·CH₂Cl₂ system reveals dichloromethane–macrocycle interactions occurring both inside and outside the macrocyclic cavity. The red spots observed at the center of the cavity signify short contacts, which can be attributed to C–H⋯Cl interactions between dichloromethane and the macrocyclic host. In addition, the white regions within the cavity mainly arise from C–H⋯π interactions between CH₂Cl₂ and [2]Naph-ExP6. Interactions involving dichloromethane molecules located outside the macrocycle are also evident from the 3D d_norm of Hirshfeld surfaces.

Crystal structure of [2]Naph-ExP6·EtOAc

In the [2]Naph-ExP6·EtOAc system, the macrocycle adopts a slightly distorted hexagonal geometry, characterized by an average cavity width of 11.20 Å and cavity height of 4.28 Å. The longest distance between two opposite vertices of the macrocyclic hexagon is 13.05 Å, while the separation between oppositely positioned naphthalene units is 10.84 Å.

In this distorted hexagonal geometry, one naphthalene ring is tilted inward toward the macrocyclic cavity by 57.6° relative to the mean molecular plane. Its symmetry-related counterpart is tilted outward from the cavity by 122.4° (180–57.6°). A similar inward–outward arrangement is observed for the phenyl rings: one phenyl ring is inclined inward by 55.1°, while its symmetry-related partner is tilted outward by 124.9° (180–55.1°). The remaining pair of opposite phenyl rings is likewise parallel, with one ring tilted inward by 83.6° and the other outward by 96.4° (180–83.6°). Dihedral angles between adjacent phenyl rings are 60.6°, and between the naphthalene and phenyl rings are 89.6° and 75.8° respectively.

In the [2]Naph-ExP6·EtOAc complex, two ethyl acetate molecules are partially encapsulated within the cavity, with the ethyl moieties residing inside. The residual void space is occupied by two methoxy substituents from the adjacent [2]Naph-ExP6 unit, stabilizing the crystal packing through inter-macrocyclic interactions (Fig. 3). The methylene hydrogens of the ethyl group of ethyl acetate are stabilized by two C–H⋯π interactions (2.93 and 3.18 Å), while one methyl hydrogen engages in a C–H⋯O contact (2.59 Å) with the oxygen atom of a neighboring macrocycle methoxy group and a C–H⋯π interaction (3.68 Å) with a phenyl ring of the cavity wall. This immersed methoxy group in the cavity is further fixed by two additional C–H⋯π interactions (2.61 and 3.06 Å) involving the naphthyl ring. The second methoxy substituent forms a C–H⋯π interaction (3.02 Å) with a neighboring dimethoxybenzene fragment. An identical arrangement of interactions is observed on the opposite side of the cavity (Fig. S10).

Fig. 3 Crystal structure of [2]Naph-ExP6 obtained from ethyl acetate (EtOAc), showing the top-side non-covalent interactions involving the partially encapsulated EtOAc molecule and the adjacent macrocycle.

Hirshfeld surface analysis also revealed significant stabilization of the [2]Naph-ExP6 cavity through partial encapsulation of ethyl acetate molecules, along with additional occupancy by methoxy groups from neighboring macrocycles in the crystal lattice. These interactions appear as red spots and white regions on the macrocyclic cavity of the Hirshfeld surface (Fig. 3). Furthermore, the three-dimensional d_norm Hirshfeld surface clearly reveals interactions between an externally located ethyl acetate molecule and [2]Naph-ExP6.

Crystal structure of [2]Naph-ExP6·DMF

[2]Naph-ExP6 maintains an overall hexagonal conformation in this system. The macrocycle exhibits an average cavity width of 10.94 Å and an average cavity height of 4.10 Å, with the longest distance between two opposite vertices of the macrocyclic hexagon measuring 13.51 Å. The distance between oppositely positioned naphthalene units within the hexagon is 10.10 Å.

In the observed conformation, one naphthalene ring is tilted inward toward the macrocyclic cavity by 77.0° relative to the mean molecular plane, while its symmetry-related counterpart is tilted outward from the cavity by 103.0° (180–77.0°). For the phenyl rings, one ring is inclined inward by 45.2° relative to the mean molecular plane, whereas its symmetry-related partner is oriented outward at 134.8° (180–45.2°). About the other remaining pair of opposite phenyl rings, one tilted inward by 49.2° and the other outward by 130.8° (180–49.2°).The dihedral angles between adjacent phenyl rings are 84.2°, while those between the naphthalene and phenyl rings are 86.6° and 78.3°, respectively.

In the [2]Naph-ExP6·DMF system, the trapped DMF molecule inside the cavity, which is disordered over two positions, is oriented vertically relative to the macrocyclic plane and stabilized by CH⋯π interactions between the two N,N-dimethyl groups and the rigid naphthalene wall (3.22, 3.36, and 3.38 Å).

Moreover, adjacent DMF molecules from neighboring [2]Naph-ExP6 units are in close proximity, leading to a linear propagation along the crystal lattice through a relatively strong C=O⋯H–C(sp3) hydrogen-bonding interaction of 2.46 Å (ca. 9.6%) shorter than the sum of van der Waals radii (2.72 Å), promoting the formation of a linear supramolecular [2]naphthyl-extended pillar[6]arene polymer within the crystal network (Fig. 4a). The linear assembly was further consolidated by multiple C–H⋯π (3.20 and 3.24 Å) and C–H⋯O (2.69 Å) interactions between the adjacent macrocycle (Fig. S11). This is possible because of the slightly offset alignment of stacked macrocycles.

Fig. 4 Side view of the linear supramolecular polymer along the crystallographic c-axis, directed by DMF molecules (a), and Hirshfeld surfaces (d_norm) of [2]Naph-ExP6·DMF (b). Red spots at the cavity opening indicate strong C=O⋯H–C(sp³) interactions.

Furthermore, the intermolecular interactions in the [2]Naph-ExP6·DMF crystals obtained from the DMF-based solvent were visualized using the 3D d_norm of Hirshfeld surface analysis (Fig. 4b). The DMF molecule encapsulated inside the cavity exhibits intense and wide red spots at the center of the cavity opening, resulting from strong C=O⋯H–C(sp3) interactions which imply the dominance of these contacts in the crystal linear assembly. In addition, white regions distributed around the cavity opening reflect the presence of multiple C–H⋯π and C–H⋯O interactions that collectively contribute to stabilizing the overall supramolecular architecture.

Crystal structure of [2]Naph-ExP6·PhMe

When a toluene molecule is encapsulated within the [2]Naph-ExP6 cavity, the macrocycle assumes a well-defined hexagonal geometry, with an average cavity width of 10.87 Å and an average cavity height of approximately 4.64 Å. The longest distance between two opposite vertices of the macrocyclic hexagon is 13.80 Å. and the distance between oppositely positioned naphthalene moieties within the hexagon is 9.88 Å.

Both naphthalene rings in [2]Naph-ExP6·PhMe are oriented exactly perpendicular to the mean molecular plane (90°). For the phenyl rings, one ring is inclined inward slightly by 82.2° relative to the mean molecular plane, while its symmetry-related counterpart is tilted outward at 97.8° (180–82.2°). About the remaining pair of opposite phenyl rings, one tilted inward by 59.9° and the other outward by 120.1° (180–59.9°). The dihedral angles between adjacent phenyl rings are 65.3°, whereas those between the naphthalene and phenyl rings are 67.6° and 61.0°, respectively.

The toluene molecule, which is disordered over two sites, resides in a horizontal orientation within the walls of the hexagonal cavity and held by two relatively strong (sp2)C–H⋯π contacts with the naphthyl moiety (Fig. 5a). The planar toluene gust is involved in (sp3)C–H⋯π interactions (3.26 and 3.47 Å) with the methoxy group of an adjacent macrocycle (Fig. S9). In contrast to the fused naphthyl units that enforce a more defined and rigid-shape architecture, the biphenyl groups reported [2]biphenyl-extended pillar[6]arene ([2]Bp-ExP6) introduce conformational flexibility due to the rotatable inter-ring bond, resulting in a less defined cavity which leads to distinct structural and host–guest characteristics.²⁴ As a result, the host–guest behavior of the two expanded pillar[6]arene scaffolds differs markedly in the solid state (Fig. 5b). The naphthyl-expanded pillar[6]arene, owing to its rigid and planar fused aromatic spacers, encapsulates a single toluene molecule positioned horizontally across the center of the cavity, stabilized by multiple C–H⋯π interactions with the extended naphthyl surfaces. In contrast, the conformationally flexible biphenyl-expanded analogue accommodates two toluene molecules, each residing on opposite sides of the cavity.

Fig. 5 Crystal structure of [2]Naph-ExP6 (a) and the reported [2]Bp-ExP6 (b)²⁴ obtained from toluene (PhMe), shown from both side and top views.

The three-dimensional d_norm Hirshfeld surface of the [2]Naph-ExP6 macrocycle containing an encapsulated toluene molecule exhibits a prominent white region within the cavity, along with few faint red spots, indicating the stabilization of the cavity by the included toluene through noncovalent interactions (Fig. S10). In addition, toluene-mediated interactions occurring outside the macrocyclic framework are also clearly evident from the Hirshfeld surface analysis.

Comparison of the four crystal structures shows that solvent inclusion modulates both cavity dimensions and the relative orientation of the aromatic walls in [2]Naph-ExP6. The CH₂Cl₂ complex displays the most distorted framework (cavity width: 10.53 Å; naphthalene centroid–centroid distance: 9.47 Å) with near-orthogonal aromatic arrangement, as reflected by large dihedral angles between adjacent phenyl rings (85.6°) and between naphthyl/phenyl units (81.1° and 78.3°), consistent with a locked, cage-like conformation stabilized by CH₂Cl₂ contacts. In contrast, the EtOAc structure adopts a more open hexagonal geometry (width: 11.20 Å; naphthalene separation: 10.84 Å) accompanied by a markedly reduced phenyl–phenyl dihedral angle (60.6°), while the naphthyl–phenyl dihedral angles (89.6° and 75.8°) support partial guest inclusion and cooperative filling of residual voids by neighbouring methoxy groups. For DMF, the cavity width remains comparable (10.94 Å) but the aromatic arrangement reverts toward orthogonality (phenyl–phenyl: 84.2°; naphthyl–phenyl: 86.6° and 78.3°), and the vertically oriented DMF promotes a 1D polymer through strong intermolecular C=O⋯H–C(sp³) contacts. The toluene complex shows a more regular hexagonal framework (naphthalene separation: 9.88 Å) with smaller dihedral angles (phenyl–phenyl: 65.3°; naphthyl–phenyl: 67.6° and 61.0°), consistent with horizontal inclusion of a single toluene molecule stabilized by C–H⋯π interactions.

Solution-state behavior

The complexation behavior in solutions was investigated by ¹H NMR titration experiments in CDCl₃ at 298 K. ¹H NMR spectra (600 MHz, CDCl₃) recorded upon incremental addition of the guest DCM, EtOAc, PhMe and DMF to a fixed concentration of the [2]naphthyl-extended pillar[6]arene display guest-dependent chemical shift changes reflecting different modes of encapsulation (Fig. S23–S28). Upon encapsulation of toluene within the macrocyclic cavity, a downfield shift is observed for the macrocycle protons, while the toluene protons exhibit an upfield shift in the H NMR spectrum (Fig. S23). These shielding effects indicate a horizontal orientation of toluene inside the cavity, in good agreement with the corresponding solid-state structure. Ethyl acetate induces a similar but less pronounced shift pattern, suggesting weaker host–guest interactions (Fig. S24).

¹H NMR spectra of dichloromethane show no significant changes in the host signals; however, the upfield shift of the DCM proton resonance indicates inclusion within the cavity (Fig. S25). In contrast to toluene, which adopts a horizontal orientation within the cavity, DMF is vertically oriented, as evidenced by the distinct ¹H NMR upfield shift pattern, in line with the solid-state structure obtained by single-crystal X-ray diffraction (Fig. S26).

In addition, two-dimensional rotating-frame Overhauser effect spectroscopy (2D ROESY) experiments (Fig. S29 and S30) show clear intermolecular cross-peaks between the naphthyl cavity protons of the host and the formyl proton of the DMF guest, providing direct evidence for encapsulation with a vertical orientation (Fig. S29), whereas the absence of correlations between the aromatic protons of toluene and the naphthyl rim protons of the host is consistent with the horizontal alignment of the benzene ring within the cavity (Fig. S30), in agreement with the solid-state structure. Overall, the solution-state ¹H NMR data closely correlate with the solid-state observations, confirming that the guest-dependent binding modes are retained in solution.

Conclusions

In summary, the crystallization of the newly synthesized [2]naphthyl-extended pillar[6]arene from different solvent systems resulted in distinct solid-state architectures governed by solvent–host interactions. The solid-state geometry of the [2]Naph-ExP6 macrocycle is strongly governed by the size, shape, orientation, and interaction profile of the guest molecule. Single-crystal X-ray diffraction revealed pronounced solvent-dependent packing modes, ranging from inclusion complexes to a linear supramolecular polymer. Crystallization from CH₂Cl₂ yields the most distorted macrocyclic framework, in which the rigid naphthalene units occupy two non-coplanar planes, producing a cage-like architecture stabilized by short C–Cl⋯H–C(sp³) and C–H⋯π contacts involving both encapsulated and peripheral dichloromethane molecules. In EtOAc, the macrocycle adopts a slightly distorted hexagonal geometry, where partially encapsulated ethyl acetate molecules and inward-pointing methoxy substituents from adjacent macrocycles cooperatively reinforce the packing through a network of C–H⋯π and C–H⋯O interactions.

A different outcome is observed in DMF assembly, where the macrocycle retains a well-defined hexagonal cavity and the vertically oriented DMF guest mediates the formation of a linear supramolecular polymer through strong C=O⋯H–C(sp³) hydrogen bonding. Additional C–H⋯π and C–H⋯O contacts further consolidate the one-dimensional chain. Hirshfeld surface analysis reveals dominant, intense red regions at the cavity opening associated with these short contacts, confirming the central role of the DMF guest in mediating the linear assembly. Crystallization from toluene yields a well-organized hexagonal cavity that encapsulates a single, horizontally oriented toluene molecule stabilized by (sp²)C–H⋯π interactions with the extended naphthyl walls. Comparison with the reported biphenyl-expanded analogue shows the accommodation of two toluene molecules due to increased conformational flexibility. Overall, [2]Naph-ExP6 displays notable conformational adaptability, enabling diverse host–guest architectures driven by optimized noncovalent interactions. The solution-state ¹H NMR data closely correlates with the solid-state observations, confirming that the guest-dependent binding modes are retained in solution. Ongoing studies in our laboratories aim to explore supramolecular self-assemblies with a broader range of macrocyclic arenes and guest compounds.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI).

Supplementary information is available. See DOI: https://doi.org/10.1039/d5ce01118c.

CCDC 2405326–2405329 contain the supplementary crystallographic data for this paper.^53a–d

Acknowledgements

The support received from the Kuwait Foundation for the Advancement of Science (KFAS) was made available through Research grant no. PN23-14SC-2096, and the College Graduate Studies and the facilities of the RSPU (Grant No. GS01/01, GS01/03, and GS03/08) are gratefully acknowledged.

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