Halogen-bonded mesogens direct polymer self-assemblies up to millimetre length scale

Abstract

Aligning polymeric nanostructures up to macroscale in facile ways remains a challenge in materials science and technology. Here we show polymeric self-assemblies where nanoscale organization guides the macroscopic alignment up to millimetre scale. The concept is shown by halogen bonding mesogenic 1-iodoperfluoroalkanes to a star-shaped ethyleneglycol-based polymer, having chloride end-groups. The mesogens segregate and stack parallel into aligned domains. This leads to layers at ~10 nm periodicity. Combination of directionality of halogen bonding, mesogen parallel stacking and minimization of interfacial curvature translates into an overall alignment in bulk and films up to millimetre scale. Upon heating, novel supramolecular halogen-bonded polymeric liquid crystallinity is also shown. As many polymers present sites capable of receiving halogen bonding, we suggest generic potential of this strategy for aligning polymer self-assemblies.

Introduction

Self-assembly of polymers has received significant attention aiming at simple and low-cost methods for new technologies^1,2,3,4,5. Supramolecular principles further allow hierarchical polymeric structures, modularity, complexity and functionalities^6,7,8,9,10. Self-assembly of polymers results in microphase-separated domains at a length scale of ca. 10–100 nm. However, typically the structures are local, that is, they lack common alignment and therefore the materials are macroscopically disordered. To achieve overall alignment in bulk matter, large electric, magnetic and flow fields have been used^11,12,13,14. In films, graphoepitaxy and surface templating allow long-range order^{15,16,17,18,19}. Also extensive thermal or solvent annealing²⁰ classically improve the overall order. However, achieving aligned and globally ordered self-assembled polymer systems organized up to millimetre length scale through simple, rapid and technologically relevant ways remains a fascinating challenge, in order to exploit the properties of the nanometre structures at macroscopic scale.

Towards tackling such a problem, we designed self-assemblies involving polymers and low-molecular-weight rod-like mesogens where the latter ones would pack in strictly parallel fashion leading to an overall broken symmetry, where the interface curvature in the local and global scale between the rods and the polymer matrix is minimized, and where plasticization by supramolecular binding of low-molecular-weight units improves dynamics to reach the equilibrium. Such features may synergistically result in an overall spontaneous organization up to macroscale without the need of external stimuli. The aimed strong repulsion of the mesogenic rods from the organic polymeric domains could be achieved by the use of perfluorinated molecules. To this purpose, we used iodoperfluoroalkanes (IPFAs) that can be halogen-bonded to polymers possessing electron-donor sites^{21,22,23,24,25}. However, in spite of its strength, specificity and directionality, halogen bonding has seldom been used for directing polymer self-assembly^26,27.

Here we report, as a model material, the amine hydrochloride derivative of a 4-arm polyethylene glycol, that is, C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ (molecular weight of 5 kDa, a branched semicrystalline polymer, see Methods), halogen-bonded to mesogenic IPFAs (Fig. 1). A star-shaped polymer is selected aiming to reduce coiling and entanglement tendency, which we expect to promote ordering and to allow a dense packing of the noncovalently bonded IPFA chains. In IPFAs, the fluorine atoms inductively boost the electron accepting ability of the terminal iodine, which promotes halogen bonding. The long-chain IPFAs adopt all-trans conformations, showing rod-like behaviour and exceptionally high repulsion from hydrocarbons (fluorophobic effect)²⁸. However, selection of proper perfluoroalkyl chain lengths of IPFA turned out to be subtle: too short chains lead to excessive volatility thus reducing the stability of the complexes, whereas too long chains do not allow solution processing to prepare the complexes. Therefore, 1-iodoperfluorodecane (I-C₁₀F₂₁) and 1-iodoperfluorododecane (I-C₁₂F₂₅) are here selected. Halogen bonds are expected to occur between the chloride anions and the iodine atoms (Fig. 1). The simplicity and rapidity of the method described here demonstrate that the synergistic use of halogen bonding and fluorophobic effect constitutes a major advancement to obtain nanostructured polymeric materials aligned up to the macroscopic scale.

Figure 1: Halogen-bonded complex formation.

The supramolecular complexes are here denoted as C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄·4 I-C₁₀F₂₁ and C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄·4 I-C₁₂F₂₅ based on their nominal stoichiometric compositions.

Results

Stoichiometry of the halogen-bonded complexes

In small-molecule co-crystals, the chloride anions may behave as either mono- or bi-dentate halogen bond acceptors (see Supplementary Note 1). Therefore, to establish the nominal halogen bonding stoichiometry in the present polymeric case, we first used a facile process, where powders of the polymer and I-C₁₀F₂₁ were ground together in a mortar and pestle for 10 min using 1:1 and 1:2 molar ratios of Cl⁻:I (see Methods). Fourier transform infrared spectroscopy (FTIR) showed that the C-F stretching mode at 1,198 cm⁻¹ and the I-CF₂ deformation mode at 631 cm⁻¹ of I-C₁₀F₂₁ exhibit a blue shift of around 8 and 11 cm⁻¹ upon complexation to the polymer (Fig. 2a). Similar blue shift is also observed for the C–O–C bending at 1,100 cm⁻¹, whereas a smaller blue shift of ca. 2 cm⁻¹ involves the stretching mode of the polymer at 2,880 cm⁻¹. An intensity decrease in the complex is observed in the region 3,700–3,300 cm⁻¹, potentially associated with the modified interaction environment of the ammonium chloride group. The spectral changes support supramolecular interaction. Importantly, the 1:1 and 1:2 molar ratios of Cl⁻:I yielded almost indistinguishable FTIR spectra. Therefore, we suggest that each chloride nominally binds one IPFA molecule, and in the process of grinding the Cl⁻:I 1:2 composition, the additional IPFA sublimes off the powder owing to its high vapour pressure. Similar FTIR spectra are observed in the complex with I-C₁₂F₂₅ (see Supplementary Fig. 1).

Figure 2: Characterization of the halogen-bonded complexes.

(a) Attenuated total reflectance (ATR) FTIR spectra in the relevant regions. (b) XPS spectra showing the shifts in I 3d and (c) Cl 2p orbital binding energies upon halogen bonding. (d) DSC thermographs upon the first heating cycle at 10 °C min⁻¹ of the starting compounds and the C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄·4 I-C₁₀F₂₁ complex prepared by grinding.

Further evidence of the halogen bond between C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ and I-C₁₀F₂₁ at the nominal 1:1 Cl⁻:I molar ratio is given by X-ray photoelectron spectroscopy (XPS) (Fig. 2b,c). The complexes were prepared upon mixing the starting materials in isopropanol (see Methods). We expected that upon halogen bonding the electron density of the I atom (halogen bond donor) would increase while that of the Cl⁻ anion (halogen bond acceptor) would decrease, resulting in a downshift of the binding energies for the I 3d doublet of I-C₁₀F₂₁ and an upshift of the Cl 2p doublet in C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄. Measuring the XPS spectrum of highly volatile I-C₁₀F₂₁ posed challenges in our setup under its high-vacuum conditions and focused beam, and therefore the binding energy for the I 3d doublet was measured using the less volatile I-C₁₂F₂₅. As XPS only probes the nearest atomic neighbourhood of the iodine, the slightly different alkyl tail length is not expected to be relevant in this case. Indeed, the peaks at 620.4 and 632 eV, corresponding to I 3d, shift to 618.7 and 630.2 eV, respectively, while the Cl 2p doublet in C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ shifts to higher energies (see Supplementary Note 2). These energy shifts agree with literature values for related interactions²⁷, supporting that chloride ion and the IPFA are halogen bonded.

Liquid-crystalline behaviour

Differential scanning calorimetry (DSC) shows a single sharp endothermic peak at 67 °C for I-C₁₀F₂₁ upon heating (Fig. 2d). As polarized optical microscopy (POM) (Fig. 3b) shows transition from an optically anisotropic state to an isotropic state upon heating, we assign this as melting. C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ polymer shows the main melting at 41 °C as well as a small exothermic peak at around 33 °C (Fig. 2d) due to cold crystallization and related melting endotherm at 37 °C, signalling polymorphism. The corresponding POM images (Fig. 3a) show growth of spherulitic crystalline domains upon cooling after having been heated above its melting temperature. On the other hand, the complex C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄·4 I-C₁₀F₂₁ shows a main endothermic peak at 44 °C (Fig. 2d) corresponding to the transition from crystalline solid to the liquid-crystalline phase (smectic A phase, SmA), as supported by POM at 47 °C, Fig. 3c. POM (Fig. 3d) agrees with the DSC data as the optical anisotropy is suppressed upon heating from 48 to 54 °C, and becomes again observable upon cooling. No sign of phase-separated I-C₁₀F₂₁ or C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ polymer was observed. The present finding shows the first observation of supramolecular halogen-bonded liquid crystals involving polymers.

Figure 3: Polarized optical micrographs.

(a) C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ after heating to 38 and 47 °C, subsequent cooling and resting at room temperature for about 5 min. The growth of crystalline spherulitic domains is observed over time upon cooling below the polymer’s melting temperature. (b) I-C₁₀F_21. (c) C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄·4 I-C₁₀F₂₁ complex at 47 °C, suggesting smectic A-type liquid crystallinity (SmA), as prepared by grinding. (d) C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄·4 I-C₁₀F₂₁ complex after heating to 39, 48 and 54 °C, and subsequent cooling to 48 °C. The complex is prepared from isopropanol solution. Scale bars, 200 μm.

Halogen-bonded mesogen directed self-assembly

The structures were elucidated by using small-angle X-ray scattering (SAXS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) (Fig. 4). As a reference, the pristine C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ shows a nanoscale periodicity due to the charge clustering of the end-group ammonium salts, as SAXS shows a broad reflection at q₂=0.067 Å⁻¹ (periodicity of ca. 9.4 nm) with a faint second-order reflection (Fig. 4a, plot 2). Two-dimensional (2D) SAXS confirms that the material is macroscopically isotropic with a poor overall alignment. Peaks q₅ and q₆ (Fig. 4b, plot 2) are signatures of the crystalline structure within C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄. Pure I-C₁₀F₂₁ shows a reflection at q₁=0.38 Å⁻¹ (periodicity of ca. 1.6 nm) (see Fig. 4a, plot 1) and its higher-order peaks in the wide-angle region (Fig. 4b, plot 1). On the other hand, the complex C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄·4 I-C₁₀F_21, as prepared from isopropanol solution, shows a highly ordered lamellar structure with the primary SAXS peak at q₃=0.055 Å⁻¹ and a series of higher-order reflections 2q₃, 3q₃, 4q₃ and 5q₃ (Fig. 4a, plot 3). Compared with the pure polymer, the periodicity has increased to 11.4 nm, due to the intercalation of the halogen-bonded I-C₁₀F₂₁ mesogens (Fig. 4d). XRD shows narrow reflections at q₄=1.4 Å⁻¹, 2q₄ and 3q₄ corresponding to a well-defined spacing of 0.45 nm (Fig. 4b, plot 3), which suggests tight lateral packing driven by the perfluoroalkyl rod-like chains. Peak q₅ (Fig. 4b, plot 3) indicates crystallization within the C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ domains. 2D SAXS shows improved overall alignment in comparison with the pure polymer, which is also confirmed by TEM (Fig. 4c). As C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄·4 I-C₁₀F₂₁ shows no SAXS reflections of the starting materials, macroscopic phase separation is not suggested.

Figure 4: Self-assembly due to complexation with I-C₁₀F₂₁.

(a) SAXS patterns of I-C₁₀F₂₁ (1), C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ (2, indicating poor self-assembly by segregation of the ionic end-groups from the polyethylene glycol-core), and supramolecular complex C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄·4 I-C₁₀F₂₁ (3, showing highly ordered self-assembly). Corresponding 2D SAXS (right) shows poor overall order for C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ and high overall order for C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄·4 I-C₁₀F₂₁. (b) XRD shows (1) fluoroalkyl chain order for pure I-C₁₀F₂₁, (2) Polyethylene glycol-crystallization for pure C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ and (3) lateral packing due to the fluoroalkyl chains in the supramolecular complex C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄·4 I-C₁₀F₂₁. (c) TEM micrograph of C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄·4 I-C₁₀F₂₁, showing lamellar smectic-like order with periodicity of 11.4 nm (scale bar, 50 nm). (d) Cartoon showing the self-assembly mechanism of the halogen-bonded complex.

The high order in the complex C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄·4 I-C₁₀F₂₁ observed by X-ray scattering and TEM can be explained by the interplay between various interactions. First, I-C₁₀F₂₁ interacts through a halogen bond with the chloride ion of the ammonium chloride end-capped polyethylene glycol polymer chains. Then, the terminal perfluoroalkyl rod-like chains pack tightly into fluorous-segregated layers, resulting in a long-range lamellar order, which can even be improved by annealing at mild conditions. Indeed, in Fig. 5 we show that vacuum treatment at sufficiently low temperature does not cause removal of I-C₁₀F₂₁ from the complexes. For that, C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄·4 I-C₁₀F₂₁ was kept in high vacuum for 2 days at 33 ^oC, which is below the melting point of the crystalline domains within the C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ phase. In these conditions the complex remains stable, as seen from the C-F (1,206 cm⁻¹) stretching and I-CF₂ (643 cm⁻¹) deformation bands in Fig. 5a as compared with those shown in Fig. 2a. Such a vacuum annealing at low temperature even slightly improves the structural order and overall alignment of the system, compared with non-annealed sample (see Fig. 5b), showing primary SAXS peak at q₁=0.052 Å⁻¹ and a series of higher-order reflections, as shown in Fig. 5b.

Figure 5: Stability of the complex below the melting point.

(a) FTIR and (b) SAXS showing structural and chemical stability of the C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄·4 I-C₁₀F₂₁ complex measured after high-vacuum treatment at 33 °C.

However, I-C₁₀F₂₁ can be completely removed upon thermal treatment at sufficiently high temperatures, as demonstrated by the thermogram obtained in the second heating cycle (Supplementary Fig. 2), which coincides with that of pure C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ (Fig. 2d). It appears, therefore, that all of I-C₁₀F₂₁ has been removed during the heating to 90 °C in the first cycle. It is important to note that at mild conditions, that is, vacuum and mild temperatures, the complex remains stable.

We next aimed to further reduce the interfacial curvature and undulations. Therefore, we incorporated a longer fluorous tail by using I-C₁₂F₂₅ to increase the repulsion to the organic domains. However, since 1-iodoperfluorododecane is poorly soluble in common organic solvents, we could not use solution methods to prepare the complexes. Here we identified a vapour-phase procedure as a facile and new processing method to prepare the complex by exposing drop-cast films of C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ to vapours of I-C₁₂F₂₅ at ca. 100 °C for 3 h. This allowed the volatile I-C₁₂F₂₅ to penetrate the melt polymer and become absorbed by forming halogen bonds. The films can even be relatively thick, even a fraction of millimetre. After the reaction, the material was returned back to room temperature and analysed by XPS (Fig. 2b,c) and FTIR (Supplementary Fig. 1; Supplementary Note 3), which showed occurrence of halogen bonding of I-C₁₂F₂₅ to C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄.

Such longer perfluoroalkyl chains led to increased thickness of the fluorous layer, as SAXS showed the main reflection at q₂≈0.05 Å⁻¹ (Fig. 6a, plot 2), indicating that periodicity has increased from 9 nm (Fig. 6a, plot 1) to 12.5 nm. This agrees with the halogen bond-driven intercalation of I-C₁₂F₂₅ upon complexing with the polymer, followed by interdigitation and packing of the perfluoroalkyl chains, which result into a highly ordered and oriented lamellar structure, as demonstrated by higher-order reflections 2q₂, 3q₂ and 4q₂ (Fig. 6a, plot 2) and 2D SAXS (Fig. 6b). However, the most striking observation was based on TEM. TEM micrographs on several positions provided direct evidence that there is an overall alignment of the C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄·4 I-C₁₂F₂₅ complex, which extends to the millimetres (Fig. 6d), that is, six orders of magnitude higher than the nanometric-scale interactions and assembly, as driven by the halogen bonding and fluorophobic effect. The starting polymer (Fig. 6c) or complexes with IPFAs shorter than 10 carbon atoms do not show such an overall alignment. The I-C₁₂F₂₅ can also be completely removed upon combined vacuum/thermal treatment, as confirmed by FTIR analysis (Supplementary Fig. 3; Supplementary Note 4) and SAXS, which showed a shrinking of the periodicity from 12.5 nm to 9.5 nm (Fig. 6a,b plot 3).

Figure 6: Self-assembly due to complexation with I-C₁₂F₂₅.

(a,b) SAXS patterns showing weak nanometer-scale order of the pristine C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ (1); halogen bonding leads to an increased periodicity by complexation of I-C₁₂F₂₅ from vapours at ca. 100 °C to C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ and allows highly ordered self-assembly with equidistant reflections based on SAXS (2) and high overall alignment. By heating and vacuum treatment, the halogen-bonded I-C₁₂F₂₅ is removed and the original periodicity is approximately recovered, however, showing better residual alignment of C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ (3). (c) TEM micrographs at various points of pure C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ showing lack of macroscale alignment. (d) TEM micrographs of the complex C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄·4 I-C₁₂F₂₅, denoted as its nominal composition, after 3 h of exposure to I-C₁₂F₂₅ vapours. The micrographs were taken at spots ~0.5 mm apart from each other, and show exceptionally well-ordered lamellar nanostructures with overall macroscale order at the millimetre scale. Scale bar, 100 nm (c,d).

Discussion

In summary, the halogen bond-driven self-assembly of a model polymer with IPFAs resulted in a very efficient spontaneous organization up to millimetre length scale without applying external aligning fields. The concept is based on the guiding effect of halogen bonding, owing to its strength and directionality, coupled with the packing of perfluoroalkyl mesogenic rods, whose extremely high repulsion from the organic polymeric phase promotes straight interfaces, and high dynamics due to their low molecular weight, which promotes plasticization and favours progress towards energy minimum. We believe that this approach is broad and not limited to the materials reported here, as many polymers possess halogen bond-accepting groups. The facile reversibility of the binding to the IPFAs opens up a plethora of new possibilities for the processing, structure control and molecular imprinting of polymeric materials, as well as for the tuning of materials properties, such as directional wetting, transport, optical, electrical and ionic properties.

Methods

Materials

C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ with M_n=5,000 g mol⁻¹ was purchased from Jenkem Technology USA Inc. and used without additional purification. Iodoperfluorodecane (I-C₁₀F₂₁) and iodoperfluorododecane (I-C₁₂F₂₅) were purchased from Apollo Scientific Ltd. and used without additional purification. The solvents in this study were used as received from commercial suppliers.

FTIR

Attenuated total reflectance-FTIR spectra were recorded on a Nicolet Nexus FTIR spectrometre equipped with a UATR unit. The values were given in wavenumbers and were rounded to 1 cm⁻¹ upon automatic assignment.

DSC

DSC analyses were performed on a Mettler Toledo DSC823e differential scanning calorimeter, using aluminium 40 μl sample pans and Mettler STARe software for calculation. The measurements were carried out from 25 to 100 °C, with a heating rate of 10 °C min⁻¹ (the first cycle); after this, the sample was cooled down to −20 °C with a cooling rate of 10 °C min⁻¹ and heated again to 100 °C (the second cycle), all at 10 °C min⁻¹. Calibration was carried out using an indium standard and an empty pan, sealed in the same way as the sample. Sample weights of about 1–4 mg were used in the measurements.

POM

Liquid crystal textures (Fig. 3) were studied with an Olympus BX51 POM equipped with a Linkam Scientific LTS 350 heating stage and a Sony CCD-IRIS/RGB colour video camera connected to a Sony video monitor CMA-D2. POM images were also taken as a function of temperature (Fig. 3) using a Leica DM4500 P polarization microscope with a Leica DFC420 digital camera.

XPS

XPS spectra were recorded by Surface Science Instruments SSX-100 spectrometre using monochromated Al K_α X-rays and operated at 100 W.

SAXS

SAXS measurements were performed with a setup consisting of a Bruker Microstar microfocus X-ray source with a rotating anode (λ=1.54 Å) and Montel optics. The beam from the X-ray source was further adjusted by four sets of four-blade slits, which resulted in an ~1 × 1-mm beam at the sample position. The scattered beam was detected with the Hi-Star 2D area detector (Bruker). For measurements, the sample to detector distance was set to 0.59 m to capture the desired length scale in the measurements. The measured 2D scattering data is azimuthally averaged to obtain one-dimensional SAXS data.

XRD

XRD spectra were recorded using XRD; Panalytical X’Pert PRO MPD. CuKα1 radiation (45 kV, 40 mA, with a wavelength of 0.154060, nm) was used.

TEM

TEM specimens were prepared by cryo-microtoming samples below −80 °C with a Leica EM UC7 ultramicrotome to 70–150 nm thick sections. During preliminary TEM measurements, it was noticed that sections introduced from room temperature to the high vacuum (~10⁻⁶ bar) melted and formed micrometre-sized droplets when exposed under the beam. Therefore for the actual TEM investigations, the specimens were cooled down to −187 ^oC after sectioning and cryotransferred to the TEM device in order to keep the sections relatively intact.

Preparation of the solid mixtures by grinding

In a typical preparation procedure, C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ (20 mg, 4 × 10⁻³ mmol) and I-C₁₀F₂₁ (10 mg, 1.6 × 10⁻² mmol or 20 mg, 3.2 × 10⁻² mmol, respectively) were ground in an agate mortar for 5 min. The mixture was allowed to stand overnight at room temperature, then ground for further 5 min and finally characterized by DSC, attenuated total reflectance-FTIR and POM.

Preparation of the iodoperfluorodecane complex from solution

I-C₁₀F₂₁ allowed preparation of complexes with C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ from solution, as isopropanol was found to be a common solvent for both compounds. The complexes were prepared by dissolving I-C₁₀F₂₁ and C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ in isopropanol in a final concentration of 10 mg ml⁻¹ and stirred overnight. After stirring, the solvent was evaporated at room temperature and atmospheric pressure in a glass vial.

Preparation of the complex by exposing to iodoperfluorododecane vapours

No common solvent was found for I-C₁₂F₂₅ and C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]_4. The latter compound was dissolved in isopropanol and cast on a glass substrate to form a film of sub-millimetre thickness by solvent evaporation. Alternatively, the C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ solution was placed on a Teflon plate and inserted on a glass substrate or into a flask. In both cases the melted material was exposed to I-C₁₂F₂₅ vapours at 100 °C for 3 h. After cooling to room temperature, the material was detached from the substrate, collected and characterized. No separate I-C₁₂F₂₅ or C-[CH₂-(OCH₂CH₂)₂₉-NH₃⁺Cl⁻]₄ XPS peaks (Fig. 2b,c) were detected, which suggests that the halogen-bonded complex is stoichiometric. Note that even relatively thick samples could be complexed.