Synthesis and properties of chiral nanoparticles based on (pS)- and (pR)-decasubstituted pillar[5]arenes containing secondary amide fragments

Abstract

Employing induced asymmetric synthesis, new decasubstituted pillar[5]arenes containing (R)-(+)- or (S)-(−)-1-phenylethane-1-acetamide fragments have been obtained and characterized. Using NTA and TEM, and circular dichroism spectroscopy, it was shown that the amidopillar[5]arenes synthesized form spherical chiral nanoscale aggregates in CHCl₃. During heating, both positive and negative Cotton effects corresponding to nanoparticles in CHCl₃ reversibly decrease. Keeping the nanoparticles at room temperature results in a decrease in their size and intensification of the Cotton effect.

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Introduction

Chirality is one of the key factors in molecular recognition, which is extremely important for various medical, biological, physical applications.¹ Development of methods for the synthesis of enantiopure compounds is crucial in the development of the pharmaceutical industry where only one enantiomer is often biologically active.^1,2 In recent years, interest in chiral structures has increased due to the discovery of new applications in nanotechnology.^2–5 High interest in the study of such structures is inspired not only by their prospective technological applications (development of sensors for asymmetric biomolecules⁵ and of new generation of catalysts^6,7), but also by their possible contribution to fundamental science. Previously it was shown in the literature,^8–10 that the macrocyclic compounds functionalized with the chiral and achiral fragments can form supramolecular assemblies able to the molecular recognition toward various types of substrates. This results in a number of successes in the construction of biomimetic and supramolecular systems.

In recent years, special attention has been paid to pillar[n]arenes (n = 5–10), para-cyclophanes constructed from hydroquinone units joined by methylene bridges.^11–17 Available by one-pot synthesis, the macrocycles¹⁸ containing 1,4-dimethoxybenzene units have hydrophobic internal cavity and a plane of chirality. Planar chirality of the pillararene derivatives results from the orientation of the hydroquinone oxygen atoms. Racemization takes place if the substituent is not large enough to prevent rotation of the oxygen through the annulus. It is possible to isolate optically active pillararene macrocycles in case of the suppression of the rotation due to steric factors.^19,20 Rotation of pillar[5]arenes depends on intramolecular hydrogen bonds, substituents nature, temperature, solvent and on the presence of guest molecules. A model for description of the structure of pillar[n]arenes was proposed by K. Mislow et al.²¹ It was shown that the planes of the benzene rings are orthogonal to the plane of the macrocyclic ring. Valent angle between the sigma-bonded methylene bridge is equal to 112°. This model agrees well with X-ray analysis. In the case of bis(2-methoxyphenyl) methane two conformations are possible. In the first case, the conformation most energetically favorable assumes the methoxyl groups positioned on the opposite sides of the central plane. Second conformation involves methoxyl groups placed on the same side against central plane. The same conformations can be attributed to the compounds with different alkoxyl substituents. This model shows significant conformational constraints that limit possible number of diastereomers from 4, 8, 9, and 19 ascribed to pillar[n]arenes with n = 5, 6, 7, and 8, respectively. The above mentioned limitations make thermodynamically most favourable one chiral conformer and its enantiomer (planar pS- and pR-isomers) (Fig. 1).^22,23

Fig. 1 Planar pS- and pR-conformations of pillar[n]arene (n = 5–10).

In order to obtain optically active pillar[5]arenes, the acylation of 1-phenylethane-1-amine with decacarboxylic acid derivative based on pillar[5]arene 1 was studied.

Results and discussion

To obtain pillar[5]arenes with planar chirality, two stereoselective synthetic routes can be employed: (1) functionalization with the optically active fragments of 1,4-hydroquinone linkages and its subsequent cyclization,²⁴ and (2) functionalization with the optically active fragments of the racemic macrocyclic platform followed by separation of the resulting diastereomers.^25,26

The principal difference between the approaches to the stereoselective preparation of pillar[5]arenes consists in preparation of the macrocyclic platform. The authors of ref. 24 proposed synthetic pathway to prepare macrocyclic platform in the last step of the synthesis. It consists in the cyclization of the functionalized 1,4-hydroquinone derivatives. This approach has several drawbacks, i.e., low yields of the pillar[5]arenes due to side reactions of polymerization, relatively small variety of substituents appropriate for the further functionalization of 1,4-hydroquinone and its cyclization to pillararene. All of these disadvantages can be overcome by using of the pre-formed racemic macrocyclic platform of pillararene, which can be obtained in high yield, and following asymmetric synthesis of the mixture of stereoisomers, which can be separated using various methods. Preformed macrocyclic platform is stable in a wide range of synthetic conditions and the presence of ten free hydroxyl groups offers wide opportunities for further stereo- and regio-functionalization with good yields. We have opted currently more versatile and yet less studied second approach in relation to the pillar[5]arenes.

Although the possibility of selective induced asymmetric synthesis has been reported in literature²⁰ for chiral groups selectively introduced into one of five aromatic fragments of the macrocyclic platform, there is no literature data on the synthesis of chiral functionalized decapillar[5]arene via asymmetric induction. In this respect, we hypothesize that the introduction of sufficiently bulky chiral fragment in the structure of the pillar[5]arene can result in the stereoselective formation of macrocycles. Thus, we have specified bulky 1-phenylethane-1-acetamide fragment containing secondary amide group. It is known^27–29 that secondary amide groups can form intra- and intermolecular hydrogen bonds due to the presence of the proton donor NH– and the proton acceptor carbonyl groups in their structure. Macrocycles containing secondary amide groups in their structure are capable of self-association and aggregation.^30–33

The compound 1 was initially synthesized from commercially available reagents according to the literature methods,³⁴ and then converted in situ into the decacarboxylic acid chloride with thionyl chloride in the presence of catalytic amount of DMF.³⁵ Further acylation of (R)-(+)-1-phenylethane-1-amine or (S)-(−)-1-phenylethane-1-amine with decacarboxylic acid chloride of 1 resulted in the compounds 2 and 3 with 71 and 74% yields, respectively (Scheme 1). The reaction was carried out in anhydrous dichloromethane in the presence of triethylamine for 48 hours.

Scheme 1 Reagents and conditions: (i) SOCl₂, reflux; (ii) (R)-(+)-1-phenylethane-1-amine, Et₃N/CH₂Cl₂; (iii) (S)-(−)-1-phenylethane-1-amine, Et₃N/CH₂Cl₂.

The structure and composition of the derivatives 2 and 3 obtained were characterized by ¹H, ¹³C, ¹H–¹H NOESY NMR, IR spectroscopy, mass spectrometry (MALDI-TOF), circular dichroism spectroscopy and elemental analysis.

Stereoselectivity of the reaction was first studied using ¹H NMR spectroscopy (Fig. 2). The spectra of the products 2 and 3 were recorded in two different solvents, i.e., DMSO-d₆ and CDCl₃, which widely differ in their solvating ability. The ¹H NMR spectra of the compounds 2 and 3 are similar to each other. The ¹H NMR spectra of each compound 2 and 3 substantially differed in different solvents (ESI†). It was observed that replacing the proton acceptor, DMSO-d₆, with CDCl₃ led to significant broadening of the proton signals in the ¹H NMR spectra of the compounds 2 and 3 (Fig. 2b). This possibly indicates the formation of the colloidal system as a result of the pillararene association. As an example, Fig. 2 shows the ¹H NMR spectra of the compound 2 in DMSO-d₆ and CDCl₃. Unfortunately, it was impossible to interpret the proton signals for two planar chiral pillar[5]arenes pSR-2 and pRR-2 in the ¹H NMR spectrum recorded in CDCl₃. In the spectra obtained in DMSO-d₆ (Fig. 2a), the proton signals for two planar chiral pillar[5]arenes pSR-2 and pRR-2, were identified (Scheme 1). A similar pattern was observed for the compound 3 with a pair of planar chiral pillar[5]arenes pSS-3 and pRS-3 (Fig. 1, ESI†).

Fig. 2 (a) ¹H NMR spectrum of the compound 2 in DMSO-d₆ at 25 °C; inset – the amide fragments region in the temperature range from 30 °C to 72 °C. (b) ¹H NMR spectrum in CDCl₃ of the compound 2 at 25 °C.

As was shown earlier,^19,26 correlation between the positive and negative Cotton effect and pS-/pR-isomers ratio exists. Chiral groups are selectively introduced into one of five aromatic moieties of pillar[5]arene. The reaction mixture of diastereomers was analyzed with HPLC and ¹H NMR spectroscopy and two planar stereoisomers were indentified. The CD and UV spectra were also recorded. The ¹H NMR spectrum of the reaction mixture showed duplication of the signals, each of them related to one of planar isomers (pS- or pR-).²⁶

Characteristic signals that uniquely identify major product involve those of aromatic protons of substituents and of the aromatic protons of the macrocyclic platform. The proton signals are well resolved and do not overlap with those of other spin systems. Depending on the intensity of the signal in chromatography, the correlation with the signals of ¹H NMR spectra was found. The appropriate fraction was isolated with HPLC. Based on ref. 26, ¹H NMR analysis of the compounds 2 and 3 prepared was performed. In a similar manner, pSR-2/pRR-2 and pRS-3/pSS-3 isomers were identified. In our case, the compounds 2 and 3 also contain substituents with aromatic fragments. The proton signals are duplicated in the ¹H NMR spectra in different proportions. In case of enantio-pure (R)-(+)-1-phenylethane-1-amine as starting reagent, pSR isomer was found as a major product of the asymmetric synthesis. The signals of aromatic protons of the substituents and aromatic protons of the macrocyclic platform in pSR isomer are much more intensive against similar proton signals corresponded to the equilibrium with pRR isomer. In case of enantiopure (S)-(−)-1-phenylethane-1-amine as a starting reagent, pRS isomer becomes a major product against pSS isomer in asymmetric synthesis. This conclusions coincides well with the data previously published and semi-empirical calculations (PM6 method in MOPAC2012) performed elsewhere for similar compounds.³⁶ However, these spin systems were unresolved while the region of amide protons was clearly expressed. This facilitated further study.

The ratio of two stereoisomers of the compound 2 (pSR/pRR) at 25 °C was determined as 80/20 based on relative integrated signal intensities of the protons of amide groups (Fig. 2a). Thus, the diastereomer excess (60%) was found to be much higher in the induced asymmetric synthesis than in the previously considered example²⁴ where 1,4-hydroquinone was functionalized with optically active fragments followed by cyclization to form the macrocycle.

It is well known that the rotation of the aromatic rings in pillar[5]arenes is hampered at room temperature, because an energy barrier between the rotamers exists.³⁷ The rate of rotation increases with the temperature, and racemization can occur. Thus, we have established how the ratio of two stereoisomers (pSR/pRR) varied with the temperature increase. Temperature changes in the ratio of rotamers of the synthesized products 2 and 3 were significant. For example, the ratio of pSR/pRR was 70/30 at 50 °C and 57/43 at 72 °C. Thus, an increase in the temperature corresponds to increased concentration of the pRR-rotamer and decreased concentration of the pSR-rotamer. It should be noted that the rotamer ratio returns to 80/20 with the solution cooled to the initial temperature.

The enthalpy of transition between diastereomeric forms pRS/pSS of the compound 4 was estimated from the ¹H NMR data on the equilibrium at variable temperature (from 30 to 72 °C). The enthalpy determined from the ln K vs. 1/T plot was found to be 22.330 ± 830 kJ mol⁻¹ (R² = 0.9944).

Assuming equal surface area and volume occupied by each diastereomer, the non-specific interactions with the solvent are also similar to each other and the difference in stability (ΔH = 22.330 ± 0.830 kJ mol⁻¹) is caused by the formation of one or more additional hydrogen bonds by pSR/pRS against pRR/pSS (ESI†).³⁸

To examine the effect of the internal rotation on the optical rotation angle, the compounds 1–3 were studied in DMSO using circular dichroism spectroscopy and UV spectroscopy. Maximum absorption bands at 295 nm corresponding to π–π* transition of the aryl fragments of the compounds 2 and 3 were observed in the UV spectra (Fig. 3b). For the compounds 2 and 3, positive and negative Cotton effects were observed, respectively, in the CD spectra (Fig. 3a). No signal was found in the CD spectrum of the initial compound 1, because it was a racemic mixture. Thus, the CD spectroscopy also confirms the presence of two stereoisomers of pillar[5]arenes 2 and 3, having planar chirality.

Fig. 3 The CD (a) and UV (b) spectra of the compounds 1–3 (1 × 10⁻⁴ M in DMSO) at 25 °C.

To confirm the ¹H NMR spectroscopy data, the possibility for “thermal racemization” of the products 2 and 3 in DMSO was studied using CD spectroscopy within the temperature range from 20 °C to 90 °C (Fig. 4). The CD intensity decreased in DMSO threefold with the temperature increased in the above range. This change correlates with the data obtained by ¹H NMR spectroscopy. As in the case of the ¹H NMR spectroscopy, the CD intensity upon cooling to the initial temperature, coincides with appropriate value obtained at this temperature before heating. This fact affirms cyclic nature of the process and the system ability to return to its original state.

Fig. 4 Dependence of CD intensity on temperature for the compounds 2 (a) and 3 (b) in DMSO (1 × 10⁻⁴ M).

According to the ¹H NMR spectroscopy (Fig. 2b), pillar[5]arenes 2 and 3 form associates in CHCl₃. It should be noted that the specific rotation of the compounds in CHCl₃ ([α]²⁰_D = 47.30 for the compound 2 and −47.21 for the compound 3, 1 × 10⁻³ M) differs from their specific rotation in DMSO ([α]²⁰_D = 70.32 and −70.25 for the compounds 2 and 3, respectively, 1 × 10⁻³ M). Possibly, the association of the macrocycles 2 and 3 affects the ratio of stereoisomers, pSR and pRR (or pSS and pRS). Besides, the observed rise in the baseline in the UV spectrum normally indicates the formation of colloidal nanoaggregates.

Since chiral pillar[5]arenes 2 and 3 form associates in CHCl₃, it was interesting to determine the effect of association on their optical properties in the same solvent. For the macrocycles 2 and 3 (Fig. 5), the observed positive and negative Cotton effects in the CD spectra recorded in CHCl₃ were similar by intensity to the Cotton effects in the CD spectra recorded in DMSO. The possibility for reversible “thermal racemization” of the products 2 and 3 in CHCl₃ (in the temperature gradient from 50 °C to −10 °C) was studied by the CD spectroscopy.

Fig. 5 Dependence of the CD intensity on temperature for the compounds 2 (a) and 3 (b) in CHCl₃ (1 × 10⁻⁴ M).

The CD and UV spectra of the compounds 2 and 3 in CHCl₃ (Fig. 5, ESI†) made it possible to identify the area of the absorption bands related to macrocyclic platform (250–313 nm, pS (−) or pR (+)) and to the amide substituent (245 nm, S-(−) or R-(+)). Fig. 5 clearly shows that the major signals are referred to pSR-2 and pRS-3 forms.

Fivefold increase in intensity of CD was shown in CHCl₃ for the temperature range mentioned above. Such a change correlates with the data obtained for the non-associated molecules in DMSO as the proton acceptor solvent. However, the difference in the CD intensity shifts in DMSO and CHCl₃ cannot be explained only by the nature of a solvent. Possibly, self-association is influenced by various dynamic processes during the particle formation, which, in turn, is reflected in the CD intensity changes.

In order to investigate these processes, self-association of the macrocycles 2 and 3 in the concentration range from 1 × 10⁻³ to 1 × 10⁻⁵ M was studied by the nanoparticle tracking analysis (NTA). It was demonstrated that associates were not formed in DMSO (ESI†). This can be explained by the nature of the solvent. A proton acceptor solvent prevents the formation of intra- and intermolecular hydrogen bonds. In addition, it was observed in the proton-donor solvent, CHCl₃, that the compounds 2 and 3 at their concentration of 1 × 10⁻⁴ M formed associates (Table 1). From the analysis of the number and size of nanoparticles, it can be concluded that in the solution of the compound 3 (after one day), particles with 80–100 nm diameter (23 × 10⁶ particles per ml) predominated. Second fraction by number of particles (10 × 10⁶ particles per ml) had diameter of 130–150 nm only few particles (3 × 10⁶ particles per ml) were bigger 150 nm. The average hydrodynamic diameter of the particles in the solution of the compound 3 (after one day) was 84.9 ± 0.6. Meanwhile, in the solution of the compound 2, the 30–50 nm fraction was found to be largest (11 × 10⁶ particles per ml). However, since the solution of the compound 2 (after one day) also contained fractions of the particles with larger diameter (50, 83, 120, 160 and 338 nm), the average hydrodynamic diameter (120.9 ± 26 nm) was shifted towards the larger aggregates.

Table 1 Particle sizes of the compounds 2 and 3 (1 × 10⁻⁴ M) in CHCl₃, obtained using NTA and TEM

Compound	Time (days)	Average Dh by NTA (nm)	The main fraction^a (nm)	Average concentration (10^6 particles per ml)	Average Dh by TEM (nm)
a For distribution by fractions, see ESI.
2	1	120.9 ± 26	30–50	193 ± 21	86
	7	56.4 ± 3.6	30	1530 ± 128	46
3	1	84.9 ± 0.6	90	256 ± 13	89
	7	67.2 ± 7.4	50	1680 ± 21	50

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It should be noted that the particle size after one and seven days significantly differs (Table 1). The particle size decreases over time, while their concentration increases by eight times in the case of the compounds 2 and seven times in the case of the compound 3. The 30–50 nm (156 × 10⁶ particles per ml) fraction quantitatively predominates in the solution of the compound 3. In the solution of the compound 2, the first fraction by the number of particles is the 20–30 nm particles (197 × 10⁶ particles per ml). The 30–50 nm particles (178 × 10⁶ particles per ml) belong to the fraction which is second by the number of particles and coincides in size and concentration with the fraction of the compound 3. Thus, the average hydrodynamic diameter of the particles in the solution of the compounds 2 and 3 (after seven days) are 56.4 ± 3.6 and 67.2 ± 7.4 nm, respectively.

The obtained pillar[5]arene 2 and 3 particles were examined by transmission electron microscopy (TEM) (Fig. 6). It was shown that the compounds 2 and 3 form spherical nanoparticles.

Fig. 6 TEM – image (CHCl₃, 1 × 10⁻⁴ M): (a) the compound 2 particles after one day; (b) the compound 3 particles after one day; (c) the compound 2 particles after seven days; (d) the compound 3 particles after seven days.

Results obtained by the NTA method (particle size reduction over the time) are in good agreement with the TEM data. The average diameter of the particles resulting from the compounds 2 and 3 after one day is 86 and 89 nm, and after seven days is 46 and 50 nm, respectively (Fig. 6a–d) (see Table 1, ESI Video†). We hypothesize that the dynamic processes (reduction in size of the pillar[5]arene colloidal nanoparticles) occurring temporally in CHCl₃ result in the changes in specific rotation of the colloidal system. This hypothesis can be easily confirmed by the CD spectroscopy within a time interval. The compounds 2 and 3 were examined in CHCl₃ and in DMSO at 25 °C by the CD spectroscopy after one and seven days. It was shown that the CD intensity did not change with time in DMSO while in chloroform it increased (Fig. 7).

Fig. 7 The CD spectra of the compounds 2 and 3 (CHCl₃, 1 × 10⁻⁴ M, 25 °C) after one and seven days.

Thus, it was established using NTA, TEM and the CD spectroscopy, that the synthesized decasubstituted pillar[5]arenes containing (R)-(+)-1-phenylethane-1-acetamido or (S)-(−)-1-phenylethane-1-acetamido fragments form spherical chiral nanoscale aggregates in CHCl₃. When the nanoparticles are heated, the Cotton effect (both positive and negative) reversibly decreases, and maintaining the nanoparticles in CHCl₃ for a week results in intensifying the Cotton effect.

Conclusions

For the first time, decasubstituted pillar[5]arenes containing (R)-(+)-1-phenylethane-1-acetamide or (S)-(−)-1-phenylethane-1-acetamide fragments able to form chiral nanoparticles in CHCl₃ have been synthesized by induced asymmetric synthesis. The stereoselectivity of the reaction was found using the CD spectroscopy. By the ¹H NMR spectroscopy, the ratio of two stereoisomers pSR/pRR (80/20) in DMSO-d₆ was determined. It was established by NTA that the compounds 2 and 3 form in CHCl₃ chiral nanoparticles, the size and number of which change over time. The hydrodynamic diameter of the particles decreases while their number increases. Using TEM, the formation of spherical nanoparticles was shown: the diameter of the compound 2 particles is 86 nm (after one day) and 46 nm (after seven days), and the diameter of the compound 3 particles is 89 nm (after one day) and 50 nm (after seven days). It was demonstrated that during the decrease in size of chiral pillar[5]arene nanoparticles in CHCl₃ over time, the Cotton effect intensifies. The synthesis of spherical chiral nanoparticles based on amidopillar[5]arene provides new possibilities for the design of dynamic supramolecular systems and new routes for the development of sensors for asymmetric biomolecules.

Experimental

General

The ¹H and ¹³C NMR spectra of the compounds (3–5% solution in (DMSO-d₆)) were recorded on 400 MHz and 100 MHz Bruker Avance 400 spectrometer using DMSO-d₆ as internal standard.

The IR spectra were recorded on Spectrum 400 (Perkin-Elmer) IR spectrometer. The IR spectra from 4000 to 400 cm⁻¹ were considered in this analysis. The spectra were measured with 1 cm⁻¹ resolution and 64 scans co-addition.

Elemental analysis was performed on Perkin-Elmer 2400 Series II instruments.

Mass spectra (MALDI-TOF) were recorded on Ultraflex III mass spectrometer in the 4-nitroaniline matrix.

Melting points were determined using Boetius Block apparatus. The purity of the compounds was monitored by melting and boiling points, ¹H NMR and thin layer chromatography (TLC) at 200 μm UV 254 silica gel plate using UV light (254 nm).

The CD spectra were recorded on Jasco-1500 spectrophotometer. The spectra were measured with the scan speed of 100 nm min⁻¹, the spectral range of 240–350 nm, the slit width of 1 nm, sampling step 1 nm and 3 scans co-addition.

Pillar[5]arene 1 (ref. 34) and (S)-N-(1-phenylethyl)acetamide (4) (model compound)³⁹ was synthesized according to the literature procedure.

General procedure for the synthesis of the compounds 2 and 3. 4,8,14,18,23,26,28,31,32,35-Deca-(carboxymethoxy)pillar[5]arene 1 (0.3 g, 0.252 × 10⁻³ mol) was placed into a round-bottom flask and SOCl₂ (10 ml, 0.084 mol) and catalytic amount of DMF were added. The mixture was refluxed for 18 h after which excess of SOCl₂ was removed under reduced pressure. The remainder was dried under reduced pressure for 2 h. The obtained residue was dissolved in 10 ml of dichloromethane. The resulting solution was added to a mixture of (R)-(+)-1-phenylethane-1-amine or (S)-(−)-1-phenylethane-1-amine (7.56 × 10⁻³ mol) and 5 ml (0.036 mol) of triethylamine in 20 ml dichloromethane over a period of 20 minutes. The mixture was stirred under argon at rt for 48 h. The reaction mixture was then washed with 2 M HCl (2 × 30 ml) and water (2 × 30 ml). The organic layer was separated and evaporated in vacuo. The residue was crystallized from acetonitrile. The precipitate obtained was dried under reduced pressure over phosphorus pentoxide.
4,8,14,18,23,26,28,31,32,35-Deca-[(R)-(+)-(1′-phenylethyl-1′-amidocarbonyl)methoxy]-pillar[5]arene 2. Yield 0.40 g (71%), mp 124 °C. [α]²⁰_D = 70.32 (DMSO, 1 × 10⁻³ M). ¹H NMR (DMSO-d₆): 1.35 (30H, m, –NH–CH–Ph), 3.67 (10H, m, ––), 4.25–4.56 (20H, m, –O––C(O)–), 5.02 (20H, m, –NH–C̲H̲(CH₃)–Ph), 6.90 (10H, s, ArH), 7.33–7.17 (50H, m, –NH–CH(CH₃)–P̲h̲), 8.30–8.65 (10H, m, –N̲H̲–CH(CH₃)–Ph). ¹³C NMR (DMSO-d₆): 22.55 (–NH–CH()–Ph), 28.72 (C of methylene bridge), 47.92 (–NH–C̲H̲(CH₃)–Ph), 67.45 (–O––C(O)–), 114.42, 125.90, 126.73, 127.98, 128.27, 144.28, 148.68 (C of aryl), 167.29 (–O–CH₂––). IR (ν/cm⁻¹): 3288 (–N̲H̲–C(O)–), 3060–2889 (Ar, –CH–, –CH₃–), 1660 (–NH––), 1203 (Ar–O–CH₂–). MS (MALDI-TOF): calc. [M⁺] m/z = 2222.0, found [M]⁺ m/z = 2222.0, [M + Na]⁺ m/z = 2245.0, [M + K]⁺ m/z = 2261.0. Found (%): C, 72.23; H, 5.54; N, 5.71. Calc. for C₁₃₅H₁₄₀N₁₀O₂₀ (%): C, 72.45; H, 6.05; N, 6.03.
4,8,14,18,23,26,28,31,32,35-Deca-[(S)-(−)-(1′-phenylethyl-1′-amidocarbonyl)methoxy]-pillar[5]arene 3. Yield 0.41 g (74%), mp 124 °C. [α]²⁰_D = −70.25 (DMSO, 1 × 10⁻³ M). ¹H NMR (DMSO-d₆): 1.35 (30H, m, –NH–CH()–Ph), 3.67 (10H, m, ––), 4.25–4.56 (20H, m, –O––C(O)–), 5.02 (10H, m, –NH–C̲H̲(CH₃)–Ph), 6.90 (10H, s, ArH), 7.33–7.17 (50H, m, –NH–CH(CH₃)–P̲h̲), 8.30–8.65 (10H, m, –N̲H̲–CH(CH₃)–Ph). ¹³C NMR (DMSO-d₆): 22.55 (–NH–CH()–Ph), 28.72 (C of methylene bridge), 47.92 (–NH–C̲H̲(CH₃)–Ph), 67.45 (–O––C(O)–), 114.42, 125.90, 126.73, 127.98, 128.27, 144.28, 148.68 (C of aryl), 167.29 (–O–CH₂––). IR spectrum, ν/cm⁻¹: 3286 (–N̲H̲–C(O)–), 3059–2971 (Ar, –CH–, –CH₃–), 1658 (–NH–), 1203 (Ar–O–CH₂–). MS (MALDI-TOF): calc. [M⁺] m/z = 2222.0, found [M + H]⁺ m/z = 2223.5, [M + Na]⁺ m/z = 2244.6, [M + K]⁺ m/z = 2261.5. Found (%): C, 72.12; H, 5.74; N, 5.67. Calc. for C₁₃₅H₁₄₀N₁₀O₂₀ (%): C, 72.37; H, 6.02; N, 5.98.

Determination of the shape and size of particles by TEM

Imaging was carried out with the Carl Zeiss Merlin scanning electron microscope. Images were processed with the STEM detector on the 300 mesh copper grid coated with Formvar. Probe preparation was carried out using negative staining protocol with 2% uranyl acetate solution. The concentration of the pillar[5]arene 2 and 3 was equal to 1 × 10⁻⁴ M. Images of the solutions of pillar[5]arene were recorded an 1 hour after thermostating of the solutions at 20 °C.

Determination of the concentration, size and movement of particles by NTA

The size, concentration and movement of nanoparticles were determined using the NanoSight LM-10 instrument (Malvern Instruments Ltd, UK) equipped with a CMOS camera C11440-50B with scientific image sensor FL-280 Hamamatsu Photonics (Japan) as a detector. Measurements were carried out in a special cell for organic solvents having a modified entry angle for the laser beam into the solution, a 405 nm laser (version cd, S/N 2990491), and Kalrez sealing ring. Contact thermometer, OMEGA HH804 (Engineering, Inc/Stamford, CT USA) was used to determine the temperature in the cell through the experiment. The Nanosight NTA 2.3 software (build 0033) was used to process the results. Measurement of the samples was conducted with a time interval of seven days. Samples were dissolved in distilled chloroform and filtered through a 25 nm filter (inorganic membrane filter Anatop 25; Whatman GE) before use. Particle size measurements were performed with an interval of 0.2 ml. Particle analysis was performed post factum, with a varied shooting time from 10 to 230 seconds, depending on the concentration of particles in the solution.

NTA is a relatively new technique which combines laser light scattering microscopy and a video imaging system for sizing of particles between approximately 50 nm and 2000 nm.⁴⁰ After recording a video of the sample illuminated by a laser, the NTA software identifies and tracks the individual particles in the field of view (appearing as point-scatterers) which are moving by Brownian motion. The rate of diffusion (speed) of each individual particle captured on film is related to its size according to a modified Stokes–Einstein relationship. A syringe pump ensures continuous sample flow and improves the statistical significance especially for low concentrated samples. Using the total number of measured particles in a given size range, the concentration of particles per size bin can be calculated by extrapolation using an approximate measuring volume estimated by the manufacturer. Detection limits (smallest size) depend on the refractive index of the nanoparticles analyzed.

Acknowledgements

The work was supported by the Russian Science Foundation (No. 14-13-00058).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra25562g

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