Introduction

Supramolecular chemistry is a relatively young area of interdisciplinary field of science in which the design of receptors is a main principle for making nonbonding supramolecular interactions with substrates. In order to produce high levels of affinity and selectivity, the binding ability of receptors is related to mutual size, shape, rigidity and binding sites with analytes [1, 2].

The inevitable widespread use of cyanide ion (CN¯) in various modern industries, such as the polymer industry and gold extraction process, has polluted water resources [3,4,5]. Given that CN¯ has the capability to suppress the transportation process of oxygen in human body, the concentration control of CN¯ in various sources of water, under the WHO allowable level, is an important duty [6]. Therefore, the development of new optical chemosensors for this hypertoxic ion is a paramount challenge.

Although there are numerous efficient optical chemosensors to recognize CN¯ in various samples, the design of an accurate, cost-effective, easily synthesized and fast-response sensor for on-site CN¯ detection is still urgently needed [7,8,9]. Therefore, the monitoring of remote polluted sites can be easily achieved by portable sensitive and selective optical sensors in countering environmental crisis.

Schiff bases, shown as S1-S13 in Table 1, have been recently applied as optical chemosensors for the detection of CN¯. As clear in their structures, the imine groups are located at the ortho position of the hydroxyl groups with a possible intramolecular hydrogen bonds (O-H…N = C) [10]. According to the intrinsic nature of CN¯, both imine and hydroxyl groups of these Schiff bases are important binding site for CN¯ recognition. In this regard, the imine groups in the structures of S1-S8 have been activated through the resonance-assisted hydrogen bonds to promote the nucleophilic addition of CN¯ on imine groups [11,12,13,14,15,16,17]. In contrast, CN¯ sensing of S9-S13 is taken place via the deprotonation of hydroxyl groups in which imines are remained inert [18,19,20,21,22].

The supramolecular recognition of salophenes 1–3 and their analogues for coordination with various metal ions have been already investigated [23,24,25,26,27,28]. However, there is no report on evaluation of these dipodal Schiff bases for sensing of CN¯, whileas they have suitable coordinating sites like imine and hydroxyl moeities in their tweezer-shaped architecture (Scheme 1).

Scheme 1
scheme 1

Structure of compounds 1–4

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Table 1 Detection of CN¯ by Schiff base probes S1-S13 (N.A.= Nucleophilic addition; D.P.= Deprotonation)
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Among probes S1-S13 listed in Table 1, the dipodal probes S7-S9 have the closest architecture to 1–3. While S7 and S8 undergo deprotonation followed by nucleophilic attack of CN¯ to imine bond via Strecker’s reaction mechanism, [17] the CN¯ sensing of S9 is taken place via deprotonation followed by disaggregation [18].

For a long time, we have been interested to the design and synthesis of various supramolecular based chemosensors [29,30,31,32,33,34,35,36,37,38,39,40]. In this regards, we have designed and synthesized some optical receptors for sensing of CN¯ which are based on the different recognition mechanism, such as deprotonation, [37, 38] metal-replacement [39] and aggregation induced emission [40].

We herein report the CN¯ sensing of salophenes 1–3 by optical methods. Among them, compounds 1 and 2 recognized CN¯ which bolded the critical role of hydroxyl groups. Both sensors are applicable in buffer solutions and show highly selectivity, very low detection limit (1–10 nM) and rapid response toward CN¯. As for practical applications, the solution test kits of both probes was successful for quantitative measurment of CN¯. Moreover, the various experimental data propose that the sensing mechanism of 1 and 2 is taken place in different ways.

Results and Discussion

The compounds 1 and 2 were simply synthesized by the condensation reaction of 1,2-diaminobenzene with 2,3-dihydroxy benzaldehyde, [23, 24] or 2-hydroxy benzaldehyde [25] respectively (Schemes S1 and S2). The known compound 4 has been also prepared from the reaction of NaCN with 2 for 48–64 h stirring at room temperature under harsh reaction condition [26,27,28]. The synthesis routes are drawn in Schemes S1-S3. All products were characterized by 1H NMR measurements which are identical to those of the reported data (Figs. S1-S3).

Optical Response of 1 Toward CN¯ Ion

To investigate the optical response of 1 to CN¯ ion, a solution of NaCN was gradually added to CH3CN : H2O buffered solution (7:3, pH 7.4) of 1 and after each addition the absorption and emission spectra were recorded.

Fig. 1
figure 1

a Absorbance and b emission spectra of 1 (0.015 µM) upon addition of CN¯ ions (0–1.5 equiv.) in CH3CN : H2O buffered solution (7:3, pH 7.4)

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The UV–vis spectra of 1 exhibits two absorption peaks at 290 and 321 nm which are assigned to the π–π* and n–π* transition of the C = C and C = N bonds. When CN¯ (0–1.5 equiv.) is added into the buffer solution of 1, the intensity of original peaks gradually reduced, whereas a new peak/shoulder at 410 nm appeared (Fig. 1a). At the same time, a clear isosbestic point at 355 nm is formed to indicate a distinct interaction between 1 and CN¯. This spectral change is supported by the color change of solutions from colorless to yellow (the inset of Fig. 1a). Therefore, the observed bathochromic shift suggests that the transition of intramolecular charge transfer (ICT) band is occurred through the deprotonation of the chemosensor 1 by CN¯ [19,20,21, 41].

Compound 1 displays a single fluorescence emission band at 442 nm when excited at 400 nm. Upon the addition of CN¯ (0–1.5 equiv.) to the solution of 1, this emission peak is gradually increased with a virtually unchanged emission shift (Fig. 1b) and a more intensive green emission color which is perceptible to the naked eyes under 365 nm UV light (the inset of Fig. 1b). In contrast, other anions do not induce such changes (Fig. S4). The fluorescence enhancement might be attributed to the deprotonation of hydroxyl groups, through which an ICT process (from hydroxyl anion to the electron-deficient imine groups) can be improved [37].

The binding mode of 1 with CN¯ was measured by Job plot analysis and gave a 1:1 stoichiometry (Fig. 2a and b). The optical response of 1 fits well with the Stern-Volmer equation to confirm the strong interaction between 1 and CN¯ (Fig. 2c and d). The binding constants (Ka) were calculated using the Benesi-Hildebrand equation and found 2.8 × 103 and 1.0 × 105 M− 1 by UV-vis and Fluorescence methods, respectively. According to calculations, a high sensitivity by fluorescence (LOD = 1.2 nM) and UV–vis (LOD = 2.3 nM) methods were obtained which are much lower than the maximum allowable level of CN¯ ions in drinking water (1.9 µM) set by the WHO [6].

Fig. 2
figure 2

Job plots for the binding of 1 and CN¯ based on absorbance (a) and emission (b) titrations. The changes in absorbance (c) and emission (d) intensity of 1 upon CN¯ addition

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To evaluate the interference of CN¯ with the following sodium anions (NO2¯, NO3¯, SCN¯, HS¯, S2−, Br¯, Cl¯, F¯, I¯, H2PO4¯, IO3¯, IO4¯, ClO4¯, BrO3¯, ClO3¯, MoO42−, SO32−, S2O32−, S2O42−, S2O52−, SO42−) competitive experiments by UV-vis (Fig. 3a) and fluorescence (Fig. 3b) spectroscopy were carried out and showed no interference between CN¯ and other anions. Moreover, as the other anions, except CN¯, did not induce the solution color change, the interference between CN¯ and other anions is distinctable via solution color under naked eye (Fig. 3c and d). It means that among other anions, CN¯ has the most basic and least hydrogen bonding character which might deprotonate the protons of catechol rings, resulting in the color change of solution [37, 38].

On the other hand, it was found that addition of different chloride cations (Li+, Na+, Ca2+, Ba2+, Sr2+, K+, Mg2+, Al3+, Cu2+, Cd2+, Co2+, Fe3+, Ni2+, Hg2+, Ag+, Mn2+, Pb2+, Zr4+) to solutions of 1 did not interfer the CN¯ sensing (Fig. S5). The high selectivity of CN¯ over cations confirmed that the deprotonation process is superior to metal complexation.

Fig. 3
figure 3

Competition of CNˉ with various anions under a UV-vis measurements and b fluorescence. c visual and d fluorescence images of 1 (1 equiv.) in the presence of 10 equiv. of various sodium anions CH3CN : H2O buffered solution (7:3, pH 7.4). (From A to U: Fˉ, Clˉ, Brˉ, Iˉ, NO2ˉ, NO3ˉ, IO3ˉ, IO4ˉ, ClO3ˉ, ClO4ˉ, SCNˉ, CNˉ, H2PO4ˉ, S²ˉ, SHˉ, SO3ˉ, SO4²ˉ, S2O3²ˉ, S2O4²ˉ, S2O5²ˉ, BrO3ˉ, MoO4²ˉ)

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To figure out the interaction between 1 and CN¯, 1H NMR measurements were first performed in the presence of NaCN, as shown in Fig. 4a. When 1 equiv. of CN¯ was added to the solutions of 1 in DMSO-d6, the corresponding signals of the OH protons at 10.5 and 13.5 ppm vanished, confirming the 1H NMR result is consistent with the Job’s plot result. At the same time, the imine peak shifted upfield slightly from 8.7 to 8.5 ppm to show that this functional group is inert to CN¯ ions. Meanwhile, the peaks of aromatic rings showed upfield shield to different extent, indicating the increase of electron density through charge delocalization in the conjugated system [42]. In addition, no new peak related to formation of amine was observed to show that the nucleophilic addition of CN¯ to the imine moiety did not occur.

Fig. 4
figure 4

 a 1H NMR spectra of 1 in the presence of 1 equiv. of CN¯. b and c The emission and absorption response of 1 to OH¯ and CN¯ ions in CH3CN:H2O (7:3)

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To further support of CN¯ sensing mechanism, the interaction between 1 and alkaline base (OH¯) as a good blank sample for CN¯ was investigated. As shown in Fig. 4b, c, the addition of OH¯ or CN¯ to 1 gave the identical emission and absorption spectrum to support the deprotonation mechanism of 1 [37, 38].

Based on the experimental data, 1 can recognize CN¯ via its tetradentate site which formed between four hydroxyl groups. As depicted in Scheme 2, this binding site prohibits the nucleophilic attack of CN¯ on imine groups.

Scheme 2
scheme 2

Proposed recognition mechanism for sensor 1 and its reversibility route

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Optical Response of 2 Toward CN¯ Ion

The binding ability of 2 toward CN¯ ions was also studied in CH3CN : H2O buffered solution (7:3, pH 7.4) by UV–vis and fluorescence spectroscopy (Fig. 5).

The absorbance response of 2 to different amounts of CN¯ are depicted in Fig. 5a. When 20 equiv. of CN¯ was gradually added to 2, the intensity of the absorption band at 320 nm decreases. At the same time, a new shoulder at 417 nm together with a clear isosbestic point at 370 nm was observed. These spectral changes is consistent with a distinct color change from colorless to pale yellow, as shown in Fig. 6c.

The fluorescence response of 2 to CN¯ is more distinctable than its corresponding UV − vis spectrum. Upon addition of CN¯ ions (0–30 equiv.) to a solution of 2 (0.015 µM), the fluorescence peak at 466 nm was blue-shifted to 446 nm (Fig. 5b). This spectral changes is consistent with distinct color change of solution, as shown in inset of Fig. 5b. It is inferred that sensor 2 is converted to a new emissive product, shown as compound 4, through which the electronic property has been changed. This change is evidenced by the non-linear emission response 2 to CN¯, as shown in inset of Fig. 5b.

Fig. 5
figure 5

Absorbance (a) and emission (b) spectra of 2 (0.015µM) upon addition of CN¯ ions (0–30 equiv.) in CH3CN : H2O buffered solution (7:3, pH 7.4)

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The binding mode of 2 with CN¯ was measured by Job plot analysis and gave a 1:1 stoichiometry (Fig. S6). The calculated detection limit by fluorescence (2 µM) is remarkably lower than that of UV–vis (10 nM). However, using of emission intensity ratio (I466/I446) method improved the ratiometric detection of CN¯ (0.9 µM, Fig. S7). In contrast, a linear response was observed in the 10–30 µM range and a detection limit as low as 8.2 nM was obtained.

To examine the selectivity, probe 2 was incubated with various anion species including (CN¯, NO2−, NO3−, SCN¯, HS¯, S2−, Br¯, Cl¯, F¯, I¯, H2PO4¯, IO3−, IO4−, ClO4−, BrO3¯, ClO3−, MoO42−, SO32−, S2O32−, S2O42−, S2O52−, SO42−). Among them, only CN¯ responded to 2 (Fig. 6a and b) with a distinct color and spectral change in solution (6 C, 6D and Fig. S8). We therefore assume CN¯ selectivity of the dosimeter 2 over anions and cations (Fig. S9) is due to its unique reaction pathway, as shown in Scheme 3.

Fig. 6
figure 6

Competition of CNˉ with various anions under fluorescence (a) and UV-vis (b) measurements. Visual (c) and fluorescence (d) images of 2 (1 equiv.) in the presence of 10 equiv. of various sodium anions CH3CN : H2O buffered solution (7:3, pH 7.4). (From A to U: Fˉ, Clˉ, Brˉ, Iˉ, NO2ˉ, NO3ˉ, IO3ˉ, IO4ˉ, ClO3ˉ, ClO4ˉ, SCNˉ, CNˉ, H2PO4ˉ, S²ˉ, SHˉ, SO3ˉ, SO4²ˉ, S2O3²ˉ, S2O4²ˉ, S2O5²ˉ, BrO3ˉ, MoO4²ˉ)

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To investigate the reaction pathway of 2 with CN¯, 1H NMR measurement was carried out in DMSO-d6 (Fig. 7a). When 1 equiv. of CN¯ was added to solution of 2, the protons of hydroxyls and imines at 13 and 8.9 ppm, respectively, disappeared. Moreover, all the protons of aromatic rings exhibited an upfield shield on different levels. At the same time, a new broad peak at 4.5 ppm, related to the amine protons, appeared as attributed to the nucleophilic attack on imine moeities.

Fig. 7
figure 7

 a 1H NMR spectra of 2 in the presence of 1 equiv. of CN¯. b and c The emission and absorbance response of 2 to OH¯ and CN¯ ions in MeOH:H2O (7:3)

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Furthermore, the interaction between 2 and CN¯ was studied by the OH¯ test. As shown in Fig. 7b, the emission peaks of 2 in the presence of CN¯ and OH¯ in CH3CN:H2O (7:3), unlike those of probe 1, are differ to each other. This difference is also observable by UV-vis spectroscopy Fig. 7c, with an improvement in MeOH:H2O (7:3) to show that recognition mechanism of CN¯ by 1 and 2 is taken place in different route.

As a result, NMR and OH¯ data support the formation of a new compound 4 through the route depicted in Scheme 3.

Accordingly, an equivalent mixture of 2 and CN¯ in MeOH was reacted at room temperature for 1 h and gave quantitative yield of dihydroxyquinoxaline 4. The structure of 4 was well supported by 1H NMR and mass spectroscopy (Figs. S3 and S11) [26, 27]. As shown in Figs. 7 and 8, 1H NMR and fluorescence changes of 2 and 4 in the presence of CN¯ is identical (Fig. 8b), confirming the CN¯ sensing of 2 is happened via 4.

Fig. 8
figure 8

 a 1H NMR of 4 in the presence of 1 equiv. of CN¯. b The comparison of fluorescence response of 2 and 4 to CN¯ ions

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Compound 4 has two emission peaks at 470 and 520 nm (Fig. 9). Upon addition of CN¯ (0–1.0 equiv.) to a solution of 4, the peak at 520 nm gradually disappeared. At the same time, the peak intensity at 470 nm was gradually increased. This unique feature can be attributed to gradual breaking of intramolecular hydrogen bonds of 4 followed by the deprotonation process. This response is fast and show highly sensitivity to CN¯ (LOD = 6.5 nM) with significant color change from yellow to blue under 365 nm UV light (below inset of Fig. 9) with a binding constant of 5 × 103 M− 1 between 4 and CN¯. On the other hand, UV-vis spectroscopy was not suitable method, because no absorption changes appeared when CN¯ was added to 4 (Fig. S15).

Fig. 9
figure 9

Emission spectra of 4 (0.1 µM) upon addition of CN¯ ions (0–1 equiv.) in 30 % aqueous CH3CN

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Based on the mentioned spectral data, we depicted the sensing mechanism in Scheme 3. Therefore, CN¯ attacks to the first aldimine group to form a cyano-adduct intermediate. The suitable distance between two aldimine groups of 2 promoted the intramolecular aldimine condensation cyclization (IACC) process on this intermediate, followed by formation of product 4. On the other hand, The obtained spectral data by 4 in the presence of CN¯ is similar to those of 2 to confirm that the CN¯ sensing of 2 is occured via 4.

Taking the structures of 2 and 3, the comparison of their respective behavior to CN¯ may give clues about the proposed mechanism. In this regard, no obvious color and spectral changes for 3 was observed even in a large excess of CN¯. Therefore, the phenolic hydrogen may activate the imines groups of 2 through an intramolecular hydrogen bond [43].

Scheme 3
scheme 3

Proposed recognition mechanism for sensors 2 and 4 (the color changes are observable under UV light 365 nm)

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Effect of Solvents on CN¯Sensing

It is well known that solvents play a fundamental role in binding processes between host and guest [44]. In this study, to assess the impact of solvents on UV-vis and fluorescence spectroscopy of probes, CH3CN, MeOH and DMF together with their aqueous solutions were selected. As shown in Fig. S12, the absorption and emission spectroscopy of each compound, except absorption of 4, is almost similar in all solvents. In contrast, the response of these compounds to CN¯ is solvent dependent, as shown in Table S1 and Figs. S13-S15. It was found that DMF solutions are not suitable solvents for CN¯ sensing because of the insignificant spectral changes of probes. On the other hand, solutions of MeOH or CH3CN bold the spectral response of probes in the presence of CN¯. At the same time, the absorption and color response of MeOH solutions of 2 to CN¯ becomes more outstanding which is suitable for quantitative experiments (see Figs. 7c and 12), whileas the calculated DLs for CN¯ is above the level of WHO (2 µM as discussed in 2.4.3). Regarding the spectral response and DLs, therefore, CH3CN : H2O (7:3) is the solvent of choice for sensing tests of all probes. Notably, the addition of more amount of H2O in CH3CN led to decreased solubility of probes in solutions.

Practical Application

pH Dependence

Since the pH variation effects the charge distribution on a molecule, it can change the absorbance and fluorescence response of the probes. To check the versatility of probes 1 and 2 in different pH, the effects of pH on these probes and their complexes with CN¯ by UV-vis and fluorescence spectroscopy were also studied. As shown in Fig. 10 and Fig. S10, the detection can work well in the pH range 5.0–8.0. Accordingly, we set all measurements at buffer pH = 7.4 which is applicable for biological samples.

Fig. 10
figure 10

The effect of pH on the emission of 1 (a) and 2 (b) with and without CN¯ ions

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Reversibility Experiment

The reversibility between 1 and CN¯ or 2 and CN¯ were evaluated by the HCl addition experiments. When 1 equiv. of HCl was added to the mixture of 1 and CN¯, the solution color changed from yellow to colorless (Fig. 11a). The color change was confirmed by the disappearance of the absorbance shoulder at 417 nm, suggesting the reversible deprotonation-protonation cycle after several cycles.

In contrast, probe 2 shows the irreversible behavior under the similar experiment to suggest a nucleophilic reaction was taken place on it.

Fig. 11
figure 11

 a Reversibility of 1 through UV-vis spectra and visual color change upon alternate addition of CN¯ and HCl. b Photographs of 1 after the sequential immersion into water solution of CN¯ and HCl on test strips under visible light

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The reversibility of probe 1 was also studied on silica gel plates. As shown in Fig. 11b, a colorless test strip was prepared by immersing TLC plate into MeOH solution of 1 (10− 4 M). When this colorless paper was immersed into solution of CN¯ (10− 4 M), a yellow color appeared on it. This yellow color subsequently returned back to original color of 1 when plate immersed into solution of HCl (10− 4 M). These results showed that sensor 1 can work well in both solution and solid state as portable kits for CN¯ analysis.

Solution Kit Tests

The performance of 1 and 2 for qualitative detection of CN¯ was evaluated. As shown in Fig. 12, the gradual color change from colorless to yellow with the addition of various concentrations of CN¯ was detectable by both probes.

Fig. 12
figure 12

Absorbance peaks and colorimetric changes of 1 in 30 % aqueous CH3CN (a and c) and 2 in 30 % aqueous MeOH (b and d) upon addition of various concentrations of CN¯

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The visual color changes are accompanied with gradual increasing of absorption shoulder/peak of 1 in 30 % aqueous CH3CN (Fig. 12a and c). In contrast, the observation of gradual color change of 2 in 30 % aqueous CH3CN was almost impossible. Therefore, the measurements were carried out in 30 % aqueous MeOH which showed an improvement in gradual color and spectral changes (Fig. 12b and d). Although aqueous MeOH is suitable solvent for the qualitative study, its calculated detection limit is far above the guidelines of WHO (2.0 µM) [6].

Conclusions

The study on optical responses of salophenes 1–3 toward CN¯ showed that hydroxyl groups are playing critical roles in the recognition of this anion. Therefore, probes 1 and 2 are promising CN¯ indicators in terms of their ease-of-use, highly selectivity, very low detection limit (1–10 nM), visual and emission color change together with rapid response. Moreover, the solution test kits of 1 and 2 for quantitative measurment of CN¯ was successful. On the other hand, the recognition mechanism of CN¯ by 1 and 2 is different as evidenced by fluorescence, OH¯ test, reversibility experiment and 1H NMR measurements. Therefore, probe 1 senses CN¯ via deprotonation process, whiles 2 recognizes CN¯ via intramolecular aldimine condensation cyclization, leading to formation of dihydroxyquinoxaline 4. This distinguished response is related to the specific binding sites of 1 and 2. Finally, it was found that the calculated detection limits of these probes are significantly superior to those of all reported S1-S13 (Table 1).

Experimental

Materials and Instrumentation

All chemical reagents and solvents were of analytical grade and commercially available. The fluorescence spectra were carried out on a Jasco FP-6500 spectrofluorimeter. The absorbance spectra were recorded on an Agilent 8453 UV-Vis spectrophotometer. Mass spectra were obtained on a Fisons instrument. 1H NMR was measured by Varian 400 MHz. Infrared spectra were recorded on a Perkin Elmer 883 spectrometer.

Synthesis of 4

A mixture of compound 2 (1 mmol) and NaCN (8 mmol) in MeOH was stirred at room temperature for 60 min. After removing of MeOH, the solid was extracted with chloroform. The organic layer was dried over sodium sulfate. The orange product 4 was purified by column chromatography and gave 80 % yield.

Dihydroxyquinoxaline 4: IR (KBr, cm-1): 3400 (O-H), 2921 (C-H), 1610 (C = N), 1470 (C = C), 1226 (C-N); 1 H NMR (DMSO-d6): δ 14.92 (s, 1 H), 10.17 (s, 1 H), 7.44 (d, 1 H), 7.36–7.17 (m, 2 H), 7.06 (t, 1 H), 6.94 (m, 4 H), 6.79 (t, 1 H), 6.63 (m, 3 H), 6.23 (s, 1 H).; MS (EI), m/z (rel. intensity %) 317 (M+, 100 %), 299 (10 %), 223 (75 %), 196 (60 %).

Fluorescence and UV-vis Titration Measurements of Probes with NaCN

Caution

CN¯ solutions are also very toxic! All the experiments are strongly recommended to do with respiratory protection under good fume hood. Keep any remaining CN¯ solution in alkaline solution of ferrous sulfate (pH > 9).

Receptors 1 or 2 or 4 were dissolved in 100 mL of CH3CN : H2O buffered system (7:3, pH 7.4) to make the final concentration of 0.3 µM.

0–100 µL of the NaCN solution (0.5 µM) was transferred to the receptor 1 solution prepared above.

0–300 µL of the NaCN solution (24 µM) was transferred to the receptor 2 solution prepared above.

0–70 µL of the NaCN solution (0.2 µM) was transferred to the receptor 4 solution prepared above.

After mixing the above solutions for a few seconds, Fluorescence and UV-vis spectra of 1 or 2 or 4 were measured at room temperature.

Determination of the Detection Limit

To determine the signal-to-noise ratio (S/N), the absorbance and emission intensities of 1 or 2 or 4 in the presence of NaCN were measured 5 times and the standard deviation of the blank measurements was determined. The measurement of the absorbance was performed in the presence of NaCN ions, and the mean intensity was plotted as a concentration of NaCN to determine the slope. The detection limit (DL) was calculated using the following equation:

$$\mathrm{DL}=3\sigma/m$$

where σ is the standard deviation of the intensity of 1, 2 and 4 in the presence of NaCN and m is the slope between the intensity (A0/A) or (I0/I) and concentration (Ct).

1H NMR of Probes 1, 2 and 4 in the Presence of CN¯

Each of compounds 1, 2 and 4 was added to the NMR tube and then dissolved in DMSO-d6. Then, 1 equiv. of dissolved NaCN in DMSO-d6 was added to each solution of receptors. After shaking them for a minute, 1H NMR spectra were measured at room temperature.