Introduction

A calixarene/calixresorcinarene is a macrocyclic molecule composed of ‘n’ phenolic/resorcinolic units connected to each other by a methylene bridge located in the ortho position to the hydroxyl group as shown in Fig. 1. Their synthesis is based on the condensation between para-substituted phenols/resorcinols and aldehydes [1].

Fig. 1
figure 1

a Molecular structure of calix[4]resorcinarene, b molecular structure of calix[4]arene

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Calix[4]arenes/calix[4]resorcinarenes are a target for researchers in supramolecular science examinations, because of their receptor properties or host–guest behavior, due to the distinct hydrophilic lower rim and the hydrophobic upper rim. The calixarene’s/calixresorcinarene’s upper rim forms the ‘entry’ to the cavity; this cavity has a hydrophobic character. Due to these properties, those macrocycles are amphiphilic.

Generally, calix[4]arene/calix[4]resorcinarene molecules can form polymers by intermolecular hydrogen bonds between hydroxyl groups. Figure 2 concerns the conformers of calix[4]resorcinarene, such as: ‘chair,’ ‘boat’ and ‘cone’ conformations. The capacity of these molecules to entrap different molecules/ions determines the applicability calix[4]arenes/calix[4]resorcinarenes. It is inferred that they contain some amount of water or solvent traces, which are retained from the preparation process. The number of reports on calixarenes/calixresorcinarenes is enormous [2,3,4,5,6,7]; they are mainly applied as extractants [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31] and sensors [32,33,34,35,36,37,38].

Fig. 2
figure 2

Different conformations of calix[4]resorcinarene

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Characterization of calixarenes/calixresorcinarenes layers to be applied as sensors is carried out by several methods such as thermal analysis [39,40,41,42,43,44,45,46,47].

This research work deals with the synthesis and characterization of three calix[4]resorcinarene molecules, namely: C-dec-9-en-1-ylcalix[4]resorcinarene, C-trans-2, cis-6-octa-1,5-dien-1-ylcalix[4]resorcinarene and C-nonylcalix[4]resorcinarene code named as CAL 11U, CAL 9U and CAL 10, respectively. The three synthesized molecules were the subject of characterization by Attenuated total reflectance–Fourier transform infrared (ATR-FTIR), powder X-ray diffraction (XRD) and simultaneous thermogravimetry (TG), differential scanning calorimetry (DSC) and mass spectrometric evolved gas analysis (MS-EGA). The molecular structures of the compounds were confirmed using nuclear magnetic resonance spectrometry (1H NMR and 13C NMR).

The compound CAL 11U was synthesized by Azoz et al. [48], the compound CAL 10 has also been reported by Shen et al. [49], and they were characterized using IR, NMR and XRD. To the best of our knowledge, their thermal behavior was never studied and compared to the XRD analysis, while the compound CAL 9U has been synthesized for the first time and presented in this paper.

Experimental

Synthesis of calixresorcinarenes

The basic synthetic route used in our experiments is valid for almost all calix[4]resorcinarenes [48, 49], and the only difference is the aldehyde utilized; for the CAL 11U, we used the undec-9-en-1-yl aldehyde, for the CAL 9U, we utilized the trans-2, cis-6-nona-1,5-dien-1-yl aldehyde, and for the CAL 10, the decanal has been used; the structures of the three molecules are presented in Fig. 3.

Fig. 3
figure 3

Molecular structures of the synthesized calix[4]resorcinarenes: a C-nonylcalix[4]resorcinarene, b C-dec-9-en-1-ylcalix[4]resorcinarene, c C-trans-2, cis-6-octa-1,5-dien-1-ylcalix[4]resorcinarene

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The synthesis was carried out based on the following recipe: 0.23 mol of resorcinol and 0.23 mol of aldehyde were dissolved into 240 mL absolute ethanol. The solution was cooled down to 0 °C in an ice bath, and then 37 mL of concentrated hydrochloric acid was injected, and it was stirred for 1 h, then heated and refluxed for another 12 h. When the mixture was cooled down to room temperature, most of the product precipitated. Excess of water was added to completely precipitate the formed calixresorcinarene, which was filtered on a glass frit G3 type. The solid material was rinsed by distilled water and dried in vacuum over NaOH or P2O5. The recrystallization from methanol and then from an acetone/hexane 1:1 mixture gave a yield of 49%, 50% and 48.8%, respectively, for the compounds CAL 11U, CAL 9U and CAL 10.

FTIR measurements

The IR spectra were collected on a Varian 2000 (Scimitar Series) FTIR spectrometer (Varian Inc., US) equipped with an MCT (mercury–cadmium–telluride) detector and with a single reflection diamond ATR unit (Specac Ltd, UK). The resolution was 4 cm−1. For single spectra, 64 individual scans were averaged; all spectra were ATR-corrected by the instrument’s data acquisition software (ResPro 4.0).

NMR studies

The NMR measurements were performed at 600 MHz (400 MHz) in DMSO-d6 at 25 °C (40 °C, 50 °C) on Varian VNMR SYSTEM™ spectrometers. An indirect detection triple resonance 1H {13C, X} Z-gradient probe was used. And, the proton and carbon chemical shifts are referenced to the residual solvent signals (δ1H = 2.50 ppm, δ13C = 39.5 ppm).

Thermal analysis

Simultaneous thermogravimetric, differential scanning calorimetric and mass spectrometric (TG–DSC–MS) measurements were performed on a Setaram Labsys Evo thermal analyzer, in helium (6.0) atmosphere, with a flow rate of 90 mL min−1. The heating rate was 20 °C min−1, and 7–8 mg of sample was weighed into 100-µL aluminum crucibles. The measurements were recorded in the 20–500 °C temperature range. The obtained data were baseline-corrected and further evaluated by the thermoanalyzer’s processing software (Calisto Processing, ver. 1.492). Parallel with the TG–DSC measurement, the analysis of the evolved gases/decomposition products were carried out on a Pfeiffer Vacuum OmniStarTM Gas Analysis System coupled to the above-described TGA. The gas splitters and transfer lines to the spectrometer were thermostated at 230 °C. The measurements were taken in SEM Bar Graph Cycles acquisition mode, in which the total ion current (TIC), the analog bar graph spectra (for structure determination), and the separate ion current of each scanned individual mass (101 masses) were recorded. The scanned mass interval was 10–111 amu, with a scan speed of 20 ms amu−1, and the spectrometer was operated in electron impact mode.

X-ray powder diffraction

The samples subjected for analysis were in the form of finely grinded powders and suitable amounts were pressed in the diffractometer’s sample holder. The powders were characterized by X-ray diffraction using a Philips Powder Diffractometer 1810/3710 with Bragg–Brentano parafocusing geometry, by using monochromatic CuKα (l = 0.154056 nm) radiation. The diffractograms were recorded at room temperature in the reflection mode, and the data were collected in a 2θ range from 3° to 77° with a step of 0.04° and exposure time at each point of 1.00 s. The main goal of the measurement was to identify the degree of crystallinity of the samples.

Results and discussion

FTIR spectra

The evaluation of the FTIR spectra can help in the determination and confirmation of the chemical structure of the synthesized calix[4]resorcinarenes. The infrared spectra of the three compounds is shown in Fig. 4, the spectrums were recorded in the wave number range (4000–400 cm−1) to confirm the presence of the functional groups. A stretching frequency was observed at 3253 cm−1, 3364 cm−1, 3484 cm−1 for CAL 11U, CAL 9U and CAL 10 respectively, it’s due to the vibration of the OH groups of the calixresorcinarene macrocycles. The difference between the three spectra can be seen in the fingerprint region (2000–400 cm−1), this difference can be explained by the diversity of the alkyl chains in the macrocycles, thus all the functional groups describing the molecular structures were found on the spectra. The Tables 1–3 for CAL 11U, CAL 9U and CAL 10 respectively presents all the functional groups with their wave numbers and nature of vibrations.

Fig. 4
figure 4

FTIR spectra of the three calix[4]resorcinarenes

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Table 1 IR parameters of the sample CAL 11U
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Table 2 IR parameters of the sample CAL 9U
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Table 3 IR parameters of the sample CAL 10
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NMR studies

The 1H and 13C NMR spectra of the compounds CAL 10 and CAL 11U are presented in Fig. 5; they show characteristic peaks for the aromatic and the aliphatic carbons and protons, confirming the structures suggested. The NMR spectra of the CAL 9U are not presented, for the reason that the compound is not soluble in the majority of deuterated solvents available. The development of a suitable solvent mixture, which could dissolve the calixarene in a suitable concentration for the measurement, is still in process, that is why its NMR spectra will be published in the near future.

Fig. 5
figure 5figure 5

1H NMR spectra of the compound a CAL 10 and c CAL 11U, 13C NMR spectra of the compound b CAL 10 and d CAL 11U

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CAL 10

1H NMR (DMSO-d6, 400 MHz, 40 °C) δ (ppm): 8.75 (8H, s); 7.12 (4H, s); 6.13 (4H, s); 4.23 (4H, t, J = 8.0 Hz); 2.02 (8H, m); 1.47–1.00 (56H, m); 0.82 (12H, t, J = 6.3 Hz).

13C NMR (DMSO-d6, 100 MHz, 50 ° C) δ (ppm): 151.5; 124.4; 123.0; 102.3; 33.8; 32.8; 31.1; 28.9; 28.8; 28.5; 27.5; 21.8; 13.6.

CAL 11U

1H NMR (DMSO-d6, 600 MHz, 25 °C) δ (ppm): 8.85 (8H, s); 7.12 (4H, s); 6.13 (4H, s); 5.75 (4H, m); 4.98 (4H, m); 4.92 (4H, m); 4.22 (4H, t, J = 8.3 Hz); 2.07–1.97 (16H, m); 1.43–1.05 (48H, m).

13C NMR (DMSO-d6, 150 MHz, 25 °C) δ (ppm): 151.7; 138.7; 124.7; 122.9; 114.5; 102.3; 33.2; 33.0; 29.2; 29.1; 28.9; 28.5; 28.3; 27.7.

Thermal analysis by TG-DSC-MS

The aim of thermal and evolved gas measurements was to determine the thermal stability of the prepared molecules, to measure their melting points and to determine any volatile content released from the samples at lower temperature (determination of any volatile contaminant). In Fig. 6a, the TG curves are plotted against temperature, while in Fig. 6b, the corresponding heat flow curves are presented. The inset in Fig. 6a shows a magnified part of the TG curves from the beginning of the measurement up to 145 °C, on which the mass losses at lower temperatures are clearly visible. It can be seen that at lower temperatures (from 40 °C up to 120 °C), a small mass loss step (CAL 11U—1.36%, CAL 9U—0.6% and CAL 10—1.1%) is present on the TG curve of all three investigated samples.

Fig. 6
figure 6

The results of thermogravimetric (a) and differential scanning calorimetric (b) measurements (the inset in a is a magnification of the TG curve from the beginning of the measurement up to 150 °C)

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It can also be noticed that the samples are having different thermal stabilities, the less stable is the CAL 9U, which can be explained by its higher degree of unsaturation; each hydrocarbon side chain contains two carbon–carbon double bonds. The degradation of CAL 9U starts above 150 °C and accelerates above 175 °C. In contrary, the CAL 10 is having the highest thermal stability, from 120 °C no mass loss occurs up to 300 °C, above this temperature the calixresorcinarene degrades fast. This outstanding thermal stability can be the consequence of the saturated hydrocarbon side chains in the molecule. Following this idea, the thermal stability of the CAL 11U having one carbon–carbon double bond at the end of the C11 carbon chain should lie in between the two above-mentioned macrocycles, which is also observable in Fig. 6a; the material starts to degrade above 250 °C. The total mass losses determined up to 500 °C are having the following values for each calixarene: CAL 11U—89,76%, CAL 9U—66.34% and CAL 10—86.94%. In Fig. 6b, the heat flow curves of each calixresorcinarene are plotted against temperature. It can be seen that on all three heat flow curves, a small endotherm can be observed around 90 °C, accompanied by small mass losses which are the result of evaporation of physically bound water, or traces of residual solvents remained from the preparation process. Around 200 °C, a more intensive endothermic transformation is observable on the heat flow curves of all three materials. These endotherms can be the result of some internal molecular transformations in the common calixresorcinarene ring of the molecules, since not all of them are accompanied by mass loss; additionally, the peak maximum values differ only by 2–3 °C (endotherm peak maxima: CAL 11U—194.1 °C, CAL 9U—196.5 °C and CAL 10—196.9 °C). Comparing the heat flow curves of the samples between 250 and 350 °C, a very sharp endotherm can be seen in the case of CAL 10, which corresponds to the melting of the molecule (onset temp. (To) 300.75 °C, peak maximum (Tm) 304.23 °C, melting enthalpy (ΔH) 266.7 J g−1). For CAL 11U, in the above-mentioned temperature range, a much smaller and broader endotherm [onset temp. (To) 269.5 °C, peak maximum (Tm) 289.9 °C] is noticeable, which is due to the melting of the sample; however, considering the shape of the peak, the calixresorcinarene could be partly crystallized. On the heat flow curve of the CAL 9U, no thermal event is noticeable, which could mean that the sample is amorphous. The events on the heat flow curves above 350 °C are the result of the thermal degradation of the samples.

In Figs. 7–9, the mass spectrometric evolved gas analysis results are presented. The chosen spectra for each investigated compound correspond to the point, where the release of the volatiles takes place at the highest rate (determined using the DTG curve and corrected with the transfer time from the TGA to the mass spectrometer).

Fig. 7
figure 7

Mass spectra of the evolved volatiles form sample CAL 10, at 87 °C

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In Fig. 7 and 8, the mass spectra of the evolved volatiles formed from sample CAL 10 (at 87 °C) and CAL 9U (at 83 °C) are shown. On both spectra, the main component is water (m/z—18 and m/z—17), confirming the evaporation of physically bound water. The peaks with m/z—44, 32, 28 and 16—are the characteristic peaks of the components in air, which means that some residual air was still present in the transfer line, which was entrapped during the sample changing process.

Fig. 8
figure 8

Mass spectra of the evolved volatiles form sample CAL 9U, at 83 °C

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In Fig. 9, the analog mass spectra of the evolved volatiles released from sample CAL 11U (at 92 °C) are plotted. While still the major component formed at 92 °C is water, some traces of acetonitrile (m/z—41, 40, 39 and 38) and ethanol (m/z—46, 45, 43 and 31) can be detected too. These are traces of impurities, which remained in the sample possibly from the preparation and purification processes. The characteristic peaks of the residual air are also visible.

Fig. 9
figure 9

Mass spectra of the evolved volatiles form sample CAL 11U, at 92 °C

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X-ray powder diffraction studies

The aim of the XRD studies was to determine the crystalline or the amorphous character of synthesized calixresorcinarenes. The diffraction patterns of compounds CAL 9U, CAL 10 and CAL 11U are presented in Fig. 10. As it can be seen, the CAL 10 is totally crystalline, the CAL 11U is a mixture of amorphous and crystallized fractions, and the CAL 9U is practically amorphous. By the way, the obtained results are in perfect agreement with the data obtained from the thermal measurements. Based on the XRD patterns, CAL 10 is crystallized; this fact can be observed in the heat flow curve of the sample as a very sharp melting endotherm around 300 °C. CAL 9U is practically amorphous because no thermal event was seen, and CAL 11U is partly crystallized; a small, wide melting endotherm on the heat flow curve confirms this observation.

Fig. 10
figure 10

Powder X-ray diffractograms of the three calix[4]resorcinarenes

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Conclusions

A series of three calix[4]resorcinarene macromolecules were synthesized, namely C-dec-9-en-1-ylcalix[4]resorcinarene (CAL 11U), C-trans-2, cis-6-octa-1,5-dien-1-ylcalix[4]resorcinarene (CAL 9U) and C-nonylcalix[4]resorcinarene (CAL 10); their chemical structure has been confirmed by 1H NMR and 13C NMR studies and ATR-FTIR measurements. The 1H and 13C NMR spectra of the compounds CAL 10 and CAL 11U showed the characteristic peaks for the aromatic and the aliphatic carbons and protons, confirming the structures suggested. Furthermore, the FTIR spectra show similarity in the region between (4000–2000 cm−1), the difference could be seen in the fingerprint region (2000–400 cm−1), and the difference seen is explained by the variance of alkyl chains in the three macrocycles. The thermal behavior of these molecules has been studied by simultaneous thermogravimetry–differential scanning calorimetry coupled to mass spectrometric evolved gas analysis. It has been found that all three investigated samples contain some amount of physically absorbed water and some traces of residual solvents in the case of CAL 11U, which were entrapped by the calixresorcinarenes during their preparation process. The materials are showing different thermal stabilities; the most stable is the molecule having saturated hydrocarbon side chains CAL 10, while the less stable is the one which is having two unsaturated carbon–carbon double bond in the side chain CAL 9U. The crystallized samples are having sharp melting endotherms (the melting point of the samples can be determined). Based on this, they can be differentiated from the amorphous sample CAL 9U. The last step of this study was to evaluate the crystalline character by the X-ray powder diffraction. The CAL 10 is totally crystalline, the CAL 9U is practically amorphous, and the CAL 11U is a mixture of amorphous and crystalline parts. The XRD results were in good agreement with those of thermal analysis.

The synthesis of these macrocycles was the first part of a research project; the second part of this work will be dedicated to the application of these molecules as sensing agents for the detection of toxic metallic elements in the environment by complexation and some electrochemical methods.