Functionalized azobenzocrown ethers as sensor materials—The synthesis and ion binding properties

Sensors and Actuators B 177 (2013) 913–923

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Functionalized azobenzocrown ethers as sensor materials—The synthesis and ion binding properties Ewa Wagner-Wysiecka a,∗ , Tomasz Rzymowski a , Mirosław Szarmach a , Marina. S. Fonari b , ˙ Elzbieta Luboch a,∗ a b

Department of Chemical Technology, Faculty of Chemistry, Gdansk University of Technology, Narutowicza Street 11/12, 80-233 Gda´ nsk, Poland Institute of Applied Physics, Academy of Sciences, Chis¸in˘au MD 2028, Republic of Moldova

a r t i c l e

i n f o

Article history: Received 5 June 2012 Received in revised form 17 November 2012 Accepted 19 November 2012 Available online 5 December 2012 Keywords: Azobenzocrown ethers Chromoionophores Molecular recognition Spectroscopic methods Ion-selective electrodes Screen printed electrodes X-ray structure

a b s t r a c t New 13- and 16-membered azobenzocrown ethers with aromatic amino, amide, ether–ester or ether–amide residue in para position to an azo moiety were obtained. Acid–base properties and ion binding ability of the colored compounds were studied by spectroscopic methods: UV–vis, fluorimetry and 1 H NMR spectroscopy. Selected azobenzocrowns were tested as ionophores in ion-selective membrane electrodes (ISEs) – classic and miniature all solid state. The X-ray structure of the sodium complex of ether–ester derivative of 16-membered azobenzocrown was presented. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Supramolecular chemistry, taking its early inspiration from Nature, is nowadays one of the most active field of science [1,2]. It covers, among others, the design, synthesis and studies of the interactions between host and guest molecules for chemical sensing purposes. At the beginning, metal cation coordination chemistry has been the main stream in supramolecular science. Since that time, properties of many compounds of various structures have been studied pointing the wide field of their possible applications. Well known metal cation complexing agents are molecular receptors based on crown ethers [3]. Among them an important class is macrocyclic compounds containing an azo residue [4]. Crown ethers incorporating azobenzene moiety as a part of the macrocycle – azobenzocrown ethers – are interesting metal ion complexing compounds. In addition, these macrocycles are photo and redox active [5–7]. Numerous macrocyclic compounds with inherent 2,2 -azobenzene [8–15] or 4,4 -azobenzene [16,17] have been synthesized and exhaustively studied. Wide possibilities of azobenzocrowns functionalization result in their potential

∗ Corresponding authors. Tel.: +48 58 3471759; fax: +48 58 3411949. E-mail addresses: [email protected] (E. Wagner-Wysiecka), [email protected], [email protected] (E. Luboch). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.11.068

applications in the variety of fields, e.g. lipophilic crowns may be successfully used as ionophores, both in classic [11,12,18] and miniature all solid state ion-selective membrane electrodes (ISEs) [19]. As an azo group is an integral part of the macroring it can act as a donor place in ion binding. This was confirmed, in a solid state, by several reported X-ray structures of azobenzocrown complexes with metal cations [19–23]. Azoarylcrown ethers behave also as chromoionophores selectively binding metal cations in solution [24–26]. Cation–ligand interactions are well manifested by color and UV–vis spectral changes. Further structure modification of the azobenzocrown ethers skeleton can leads to compounds comprising the merits of chromoionophores and fluoroionophores [27]. It is worth noting that some compounds mentioned above can be concurrently used as good ionophores in membrane ion-selective electrodes. It makes them an universal, to some extend, analytical tool for eventual metal cation detection and/or determination. Anion coordination is another important field of interest in supramolecular chemistry. This is, among others, because of anions abundance in nature and their key role in many biochemical processes. The design and synthesis of anion receptors are widely presented in exhaustive review articles [28–34]. Depending on the structure of receptor, the respective ligands may be used for particular chemical sensors construction or/and as selective reagents for spectroscopic purposes [31,34]. It was found that ion-selective electrodes with complexes of azothiacrown with heavy metal cations

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(silver, mercury and copper) as ionophores show anionic response [35]. Numerous examples of both neutral crowns and their metal cation complexes of different structure have been reported up to now as anion receptors, for examples see [36–40]. Here we present new functionalized azobenzocrown ethers of different size of the macrocycle and their properties. The synthesized 13- and 16-membered crowns have aromatic amino (1, 2) or aromatic amide (3, 4), ether–ester (5–10), and ether–amide (13–16) groups as a side residue located in para position to an azo moiety (Scheme 1). For selected macrocycles acid–base properties and complexation studies were carried out by UV–vis spectrophotometry, fluorimetry and 1 H NMR spectroscopy in acetonitrile. Ion binding properties were compared with data available for reference compounds A–H (Fig. 1) [9,10,18,24]. Additionally, N H group influence on the possible anion binding was also investigated. Some more lipophilic compounds were tested as ionophores in ion-selective membrane electrodes (ISEs) – classic and all solid state. 2. Experimental 2.1. General All chemicals of highest available purity were purchased from commercial sources and used without further purification. THF for synthesis and membrane preparation was freshly distilled over LiAlH4 . TLC: aluminum sheets covered with silica gel 60F254 were purchased from Merck. For column chromatography silica gel 60 (0.063–0.200 mm) (Merck) was used. 1 H and 13 C NMR spectra were recorded at a Varian instrument (500 and 125 MHz, respectively). Chemical shifts are reported as ı [ppm] values in relation to TMS. FTIR spectra were recorded on a Mattson Genesis II instrument. Mass spectra were recorded on an AMD-604 (EI method, 70 eV) and a GCT Premier (TOF MSFD+ ) instruments. For UV–vis measurements an UNICAM UV 300 apparatus was used. Fluorescence spectra were recorded on an AMINCO-Bowman Series 2 luminescence spectrometer (flash xenon lamp). Bandpass at excitation and emission monochromators: 16 nm. Fluorescence spectra are uncorrected to instrument response. Spectroscopic measurements were carried out in acetonitrile (LiChrosolv® ) of gradient grade. Deionized water (18 M cm, Hydrolab, Poland) for water containing solvent system for UV–vis spectrophotometry and EMF measurements was used. EMF measurements were carried out using a 16-channel Lawson Lab potentiometer (USA). The screen-printed graphite electrodes were prepared in Institute of Electronic Materials Technology, Warsaw, Poland (plates of 18–15 mm with six electrodes, openings area ca. 1 mm2 ). All measurements were carried out at room temperature. 2.2. Syntheses For synthetic details and spectral data, see Supplementary data ST2. 2.3. Complexation studies Complexation studies were performed by UV–vis titration of the ligand solution in acetonitrile with the respective metal perchlorates (for metal cations) or tetra-n-butylammonium (TBA) salts (for anions). Caution! Perchlorate salts should be regarded as potentially explosive and handled with care. The stock solutions of azobenzocrowns (∼10−4 M) and metal perchlorates or TBA salts (∼10−2 M) were prepared by weighing the respective quantities of them and dissolving in acetonitrile in volumetric flasks. Titrations were carried out in a quartz cuvette with path length of 1 cm keeping

constant volume of the ligand solution (2.3 mL). The stability constant values were calculated with the use of OPIUM [41] program on the basis of titration experiment data. 2.4. Ion selective electrodes 2.4.1. Classic ISEs The membrane components (8 mg of ionophore, 50 mg of PVC, 0.1 mL of o-nitrophenyl-octyl ether (o-NPOE) and 1 mg of potassium tetrakis(4-chlorophenyl)borate (KTpClPB)) were dissolved in freshly distilled, dry THF (1.2 mL). The solution was poured into a glass ring (diameter 15 mm). After 1 day, membranes of d = 7 mm were cut out and incorporated into Ag/AgCl electrode bodies of IS type (Moeller S.A., Zurich, Switzerland). NaCl or KCl 10−2 M were used as internal electrolyte for sodium and potassium selective electrodes, respectively. The electrode was conditioned by soaking it in a 10−2 M solution of MCl (M – the main ion) for 24 h. A doublejunction Ag/AgCl, KCl 1 M reference electrode (Monokrystaly RAE 112) was used with 1 M NH4 NO3 solution in the bridge cell. The selectivity coefficients (KNa,K , KNa,H or KK,Na , KK,H ) were determined using the separate solution method (SSM) [42] at ion activities of 10−1 M in neutral and 3% TRIS (pH ∼ 9) for 13-membered ionophores and neutral pH for electrodes with 16-membered ioncarriers. 2.4.2. Screen-printed electrodes Ionophore and ca. 0.05 mg of carbon nanotubes (single wallet, Aldrich) in 1 mL of THF were sonicated for 1 h. Next, the remaining membrane constituents were added as described above. The solution (1–0.5 ␮L) was applied onto graphite screen-printed electrodes and was left to dry over 24 h at room temperature. Next electrodes were conditioned by soaking them in a 10−3 M solution of NaCl or KCl for 10 h. All the other experimental details were identical as described for classic electrodes. The measurements were carried out in accordance with procedures specified for microfabricated ion-selective electrodes [43]. The response of both classic and screen-printed electrodes toward alkali (Li+ , Na+ , K+ , Rb+ , and Cs+ ), alkaline earth (Mg2+ and Ca2+ ) and ammonium (used as chlorides) ions was studied. 2.5. X-ray crystal structure determination 2.5.1. Preparation of crystals Ethoxycarbonylbutylenoxy-16-azobenzocrown – compound 8 (16.5 mg, 0.055 mmol) and sodium iodide (22 mg, 0.055 mmol) were dissolved in methanol (5 mL) and filtered. Filtrate was evaporated under reduced pressure. To the obtained solid acetone:propan-2-ol (1:1, v/v) mixture (2 mL) was added. Very slow solvent evaporation has resulted in crystals melting at 160–166 ◦ C. 2.5.2. Determination of crystal structure The X-ray data for [Na(8trans)]I complex were collected at 150 K on a KM4CCD diffractometer using graphite-monochromated MoK␣ radiation and were corrected for Lorentz and polarization effects. The structure was solved by direct methods and refined by full-matrix least squares technique based on F2 . Analytical numeric absorption correction using a multifaceted crystal model based on expressions derived by Clark and Reid [44] was applied. Non-hydrogen atoms were refined with anisotropic displacement parameters. C-bound hydrogen atoms were placed in geometrically calculated positions and refined using temperature factors 1.2 times those of their bonded carbon atoms. Calculations were performed using SHELX-97 crystallographic software package.

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915

n OH

HO

NH2

HO

OH

HCl

OTs

N

N

tBuOK, THF

OH

NH2 N

1. n = 1 2. n = 2

+

H2O, NaOH

O

O Ts

17

NaNO2

NH2

O

O

NH2

N a)

O

n

H2N OH

N

N 18 n

n b)

O

O

O

O

O N

NH2

NH C N

THF

N

O

O

C11H23COCl, Et3N

C11H23

N

3. n = 1 4. n = 2 c)

n

n

O O

O

O OH N

n

O

N

C. n = 1 D. n = 2

O

Br(CH2)mCOOEt N

K2CO3, acetone O

5. n = 1, m = 1 6. n = 2, m = 1 7. n = 1, m = 3

ClCH2 C NHC7H15

O O O(CH2)mC OEt

N

O

KOH EtOH

8. n = 2, m = 3 9. n = 1, m = 5 10. n = 2, m = 5

O

O O(CH2)3 C

N

C7H15NH2 DCC, HONSu

OH

N

11. n = 1 12. n = 2

DMF, NEt3

K2CO3, DMF n

n O

O O

O

O N

OCH2 C N

NHC7H15

O 13. n = 1 14. n = 2

O

O N

O(CH2)3 C N

15. n = 1 16. n = 2

NHC7H15

Scheme 1. The synthetic routes for compounds 1–16.

3. Results and discussion 3.1. Synthesis Target compounds 1–10 and 13–16 were prepared as shown in Scheme 1a–c. For synthetic details see Supplementary data, ST1 and ST2. 3.2. The properties of aromatic amino- and amide-azobenzocrowns (tautomerism, solvatochromism, acid–base properties) In aromatic azo compounds substituted in ortho or para position with hydroxyl or amino group intramolecular proton transfer may occur. This phenomena was also observed for hydroxyazobenzocrowns studied earlier [18,24,47]. As amino group in 1 and 2 is located in para position to azo moiety, tautomeric equilibrium

can be taken into consideration. Moreover, possible tautomerism of N-acetyl derivatives of aminoazobenzocompounds was also taken into account [48]. Solvent influence on tautomeric equilibrium of aminoazobenzocrowns was studied by 1 H NMR spectroscopy. It was found that in aprotic solvents such as acetonitrile (Fig. 2a) and DMSO (Fig. 2b) 1 exists in aminoazoform. In 1 H NMR spectrum of 1 recorded in methanol and acetonitrile:water mixture (not shown) aromatic proton signals distribution is similar to this which was observed in DMSO. It also points that in these solvents aminoazoform of 1 is dominating. Above observations can lead to conclusion that tautomeric equilibrium of 1 is neutral solvent independent. Similar experiments for 2 confirmed that in acetonitrile, acetone and DMSO aminoazo form is dominating. This is opposite to the properties of hydroxyazobenzocrowns studied ealier. A quinone-hydrazone equilibrium of

Fig. 1. The reference azobenzocrown ethers.

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Fig. 2. The comparison of the 1 H NMR spectra (7.9–3.6 ppm) of 1 recorded in: d-acetonitrile (a) and d-DMSO (b).

O O

O OH

N

N

H N

O

O

O O

O

O O N

C

O

O NH2

N

N

H N

O NH N

1

Fig. 3. Tautomerism of hydroxy- (C) and aminoazobenzocrown (1) ethers exemplified with 13-membered azobenzocrowns.

hydroxyazobenzocrowns was found to be macrocycle size and solvent type dependent [18,24,47]. And so, an analog of 1 – 13-membered hydroxyazobenzocrown C (Fig. 1) exists mainly in quinone-hydrazone form and only in DMSO its azophenol tautomer (∼30%) was observed. This probably affects the stability constant values of hydroksyazobenzocrowns metal cation complexes. They are lower than for their unsubstituted parent compounds A and B (Fig. 1). It can be explained by tautomeric equilibrium distribution, where stabilized by intermolecular hydrogen bond quinone-hydrazone form is dominating. Aminoazobenzocrowns, similarly to simple open-chain aminoazobenzene compounds [48], preferably exist in aminoazo forms. This in turn can explain the higher values of the stability constant of their metal cation complexes. Tautomeric equilibrium of functionalized azobenzocrown ethers is exemplified in Fig. 3 with 13-membered crowns. The solvatochromism of aminoazobenzocrown 1 (see Supplementary data, Fig. S1a) was also studied. In aprotic, highly dipolar (non-hydrogen bond donor) solvents: acetonitrile, DMSO and DMF spectra are similar in shape, but differ in their intensity. In methylene chloride new band of a moderate intensity appear at about 500 nm. This band is better observable in protic (HB) methanol. Excluding solvent affected tautomeric equilibrium, it may be assumed that a band at ∼500 nm in UV–vis spectrum is an effect of interactions between ligand and solvent, namely intermolecular hydrogen bond formation resulting in possible aggregation of azo compound. Observed spectral changes upon water addition to acetonitrile solution of 1 (see Supplementary data, Fig. S1b) can point interactions between azobenzocrown and water molecules. Acid–base properties of aromatic azo compounds make them useful as potential pH indicators. Protonation of azobenzene moiety takes place at one of the two azo N-atoms [45]. Aminoazobenzenes are generally protonated both at amino and nitrogen atom of azo group [46,48]. Protonation of aminoazobenzocrown 1 was studied by spectroscopic methods in organic solvents. UV–vis titration with perchloric acid in acetonitrile results in the formation of a new, intensive band at 483 nm (see Supplementary data, Fig. S2a). Color changes from yellow to orange. Clear isosbestic point (414 nm) and molar ratio plot (not shown) suggest monoprotonation of compound 1. In 1 H NMR spectrum of 1 recorded in the presence of 1 eq. of perchloric acid

in d-acetonitrile (see Supplementary data, Fig. S2b) aromatic proton signals are shifted downfield and one N H proton signal is observed at 13.6 ppm. 1 H NMR spectrum recorded in the presence of 1 eq. of perchloric acid in d-DMSO shows two signals at ∼9.4 and one singlet at 13.4 ppm. Similar changes were observed in 1 H NMR spectrum 1 in the presence of p-toluenesulfonic acid (TosOH) and isolated adduct of 1-TosOH (1:1) (see Supplementary data, Fig. S2b). Those signals were also well observable in spectrum registered in the presence of the excess of TosOH in d-chloroform. Observed signals pattern can suggest stimulated by acids tautomeric equilibrium shifted toward protonated imino-hydrazone form of 1. The scheme of reversible protonation of 1 is shown in Fig. S2d (see Supplementary data). Protonation of aromatic amides 3 and 4 causes color change from yellow to pink-red. Color changes of 4 in the presence of TosOH are illustrated in Fig. 4. Most of azo compounds are nonfluorescent at room temperature; however, there are some exceptions reported in literature [49–53]. We found that protonated form of azobenzocrown ethers is fluorescent. The comparison of the normalized absorption and fluorescence spectra of protonated A, 1 and 3 are shown in Fig. 5. Changes in UV–vis and fluorescence spectrum upon titration of 4 with p-toluenesulfonic acid in acetonitrile are shown in Fig. 6a and c. Fig. 6b and d illustrates color change and red fluorescence of ligand 4 in the presence of 2 eqs. of TosOH. The reversible protonation of azobenzocrowns 1–4 causes red fluorescence with emission band over ∼600 nm. The highest Stokes shift is observed in a case of an unsubstituted azobenzocrown A 121 nm, then for aromatic amide 3 and aromatic amine 1: 86 and 81 nm, respectively.

Fig. 4. Color change of 4 (1.12 × 10−3 M) solution in acetonitrile in the presence of p-toluenesulfonic acid (TosOH). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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(b)

1.0

1.0 0.8

0.8 0.6

0.6

A 1 3

0.4

A 1 3

0.4 0.2

0.2 0.0 400

450

500

550

nm

550

600

650

Normalized fluorescence

Normalized absorbance

(a)

917

0.0 700

nm

Fig. 5. Normalized absorption spectra of protonated (TosOH, 2 eq.) 13-membered azobenzocrowns A, 1 and 3 (a) and normalized fluorescence spectra of A (ex = 487 nm, em = 608 nm), 1 (ex = 482 nm, em = 568 nm) and 3 (ex = 490 nm, em = 576 nm) (b) in acetonitrile.

3.3. Complexation studies in solution 3.3.1. Metal cation complexation Alkali and alkaline earth metal cations (used as perchlorates) binding was studied in acetonitrile by UV–vis spectroscopy. As a first target, 13- and 16-membered aminoderivatives 1 and 2 and 16-membered aromatic amide 4 were chosen. As can be expected, for 13-membered azobenzocrown 1 changes in absorption spectra were observed in the presence of lithium perchlorate. Changes upon titration of 1 with lithium perchlorate are shown in Fig. 7a. Sodium and potassium salts did not influence significantly the absorption spectrum. In comparison to the parent compound A spectral changes upon lithium complexation are more remarkable for 1, although the stability constants values (log K, 1:1) of their lithium complexes are comparable: 4.00 [9] for A and 4.01 for 1. Azobenzocrown 1 in its spectral behavior resembles compound E [18]. Among alkaline earth metal cations the most distinct spectral changes for 1 were observed in the presence of magnesium perchlorate. Titration with magnesium salt results in the formation of a new band at 482 nm ( =+99 nm). Absorption spectrum, typically for azo compounds, is almost completely analogous to this which is registered in acidic medium [48,54]. Moreover, longwave band is still observable not only in the presence of neutral salt but also in slightly basic solution in the presence of organic base (Et3 N) (see Supplementary data, Fig.S3). Addition of triethylamine to solution of 1 does not affect UV–vis spectrum. Our studies suggest the origin of the observed spectral behavior as ion–ligand interaction. Amino derivative 1 preferably binds magnesium cation (being a hard acid in HSAB theory) with stability constant (log K) 6.43. It

proves a beneficial electron donating resonance effect of the amine residue introduction into 13-membered azobenzocrown skeleton. Fig. 7b shows the limiting absorption spectra obtained upon titration of 1 with alkaline earth metal salts. Fig. 7c illustrates titration course for 1 and magnesium perchlorate in acetonitrile. Additionally, photos in Fig. 7 show selective color response of 1 toward magnesium cation. Alkali and alkaline earth metal cations complexation was also studied for 16-membered azobenzocrown ethers. The obtained stability constant values for 16-membered azobenzocrowns 2 (amine), 4 (aromatic amide) and 16 (ether–amide) and data available for the reference compounds are collected in Table 1. In a case of 2, typically for 16-membered azobenzocrowns, spectral changes (acetonitrile) were observed in the presence of lithium, sodium and potassium salts (see Supplementary data, Fig. S4a). Titration course of 2 with lithium and sodium salts illustrate Fig. S4b and c. Spectral changes in the presence of alkali metal perchlorates are the most distinct for lithium cation. The comparison of the stability constant values of alkali metal cation complexes with 16-membered azobenzocrown ether 2 (Table 1) with values obtained for unsubstituted reference azobenzocrown B points that amino derivative 2 forms more stable complexes with the studied metal cations. Only in the case of larger potassium cation this effect is not observed. Higher values of the respective binding constant can be explained by electron donating resonance effect of the amino group increasing electron density at nitrogen atom of an azo group participating in metal cation binding. Aminoazobenzocrown 2 forms stronger complexes with alkaline earth metal cations than with lithium, sodium and potassium.

Fig. 6. Changes in absorption spectrum upon titration of 4 (3.73 × 10−5 M) with TosOH (a), color change of 4 in the presence of 2 eq. TosOH (b), changes in fluorescence spectrum of 4 (3.73 × 10−5 M) upon titration of 4 with TosOH (ex = 510 nm, em = 598 nm) (c), fluorescence of 4 in the presence of 2 eq. TosOH (d) in acetonitrile. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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Fig. 7. Spectral changes upon UV–vis titration of 1 (3.27 × 10−5 M) with lithium perchlorate (a), the limiting spectra obtained during titration of 1 with alkaline earth metal perchlorates (b), titration course for magnesium perchlorate (c), in acetonitrile. Photos show color changes of solution of 1 in the presence of 1 eq. of metal perchlorates in acetonitrile. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Table 1 Comparison of the stability constant values of new 16-membered azobenzocrown ethers 2, 4, 6 and the respective reference compounds in acetonitrile. By bold the highest values of binding constants are marked.

a

Compound

Log KLi

Log KNa

Log KK

Log KMg

Log KCa

Log KSr

Log KBa

2 4 16 B [10] F [24] H [55]

5.33 4.82 3.58 4.00 4.28 4.18

5.04 4.60 3.61 3.69 4.75

2.90

6.06 3.47 4.99 5.00

6.40 5.30 5.96 5.15

6.13 5.87 6.32 4.91

6.32 6.00 6.26 4.61

a

a

a

a

4.02

a

a

a

a a

3.15 3.74 3.34

a

No data or changes too small to estimate the stability constant value.

It is in agreement with the general trend observed for nonsubstituted azobenzocrown ethers studied by Nakamura et al. [10], but stability constant values are higher for aminoazobenzocrown 2 than for parent compound B (Fig. 1). Fig. 8a shows the limiting spectra obtained during titration of 2 with alkaline earth metal salts. Comparison of the spectrophotometric response of 1 and 2 in acetonitrile toward alkaline earth metal cations implies that magnesium selectivity is determined by macrocyle size. Larger 16-membered aminoazobenzocrown does not show such selective changes in absorption spectrum in the presence of magnesium salt. In Fig. 8b and c changes upon spectrophotometric titration of 2 with magnesium and barium perchlorates are presented. Complexing properties of 2 were compared to the cation binding ability of the aromatic amide 4 of the same ring size. For 4, among studied alkali metal cations, changes in absorption 1.0

(b)

0.9

0.7

0.5

0.3

0.2

0.2

0.1

0.1

0.0

0.0

nm

500

550

Ba2+ = 0-8.2x10-4M

0.5

0.5

0.3

450

Mg = 0-8.3x10 M

0.6

0.4

400

0.7 0.6

-4

0.7

0.4

350

2+

0.8

A

A

0.6

(c)

0.9

free Mg Ca Sr Ba

0.8

1.0

600

A

(a)

spectrum were observed in the presence of lithium and sodium salts (see Supplementary data, Fig. S5a). In Fig. S5b the respective limiting spectra obtained for alkaline earth metal cations are shown. Opposite to 16-membered aminoazobenzocrown 2 less significant and selective spectral changes were observed for aromatic amide 4 in the presence of alkaline earth metal cations. Only in the presence of magnesium cation the band shape is slighty different than in a case of other studied metal cations. Fig. S5c is an example of spectral changes upon titration of 4 with metal cation salts in acetonitrile and shows titration with calcium perchlorate. For ether–amide 16 spectral changes caused by the presence of metal cations are similar to those, which were observed for 4. Stability constant values of its complexes are generally lower than for 16-membered aminoazobenzocrown 2. Comparing to aromatic amide 4 ether–amide 16 forms more stable complexes with

0.4 0.3 0.2 0.1 0.0

350

400

450

nm

500

550

600

350

400

450

500

550

600

nm

Fig. 8. The limiting spectra obtained during titration of 2 (5.62 × 10−5 M) with alkaline earth metal perchlorates (a), changes upon UV–vis titration with magnesium (b) and barium (c) perchlorates in acetonitrile.

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(a) 0.8

(b) 0.5

(c)

0.7

3.5

0.2

0.3

Log K

0.4

3.0

free Li Na K

0.3

A

A

0.5

4.0

0.4

free Li Na K

0.6

919

2.5 2.0 1.5

0.2

1.0

0.1 0.1

0.5 0.0

0.0

0.0 250

300

350

400

450

500

250

300

nm

350

400

450

500

A

nm

G

5

7

9

Azobenzocrown

Fig. 9. Spectral changes of azobenzocrowns: 9 (3.75 × 10−5 M) (a), G (1.84 × 10−5 M) (b) in the presence of 50-fold excess of alkali metal perchlorates, the comparison of the stability constant values of lithium complexes of azobenzocrowns A, G, 5, 7 and 9 (c) in acetonitrile.

alkaline earth metal cations, especially with strontium (Table 1). Obtained results for 1, 2, 4 and 16 allow us to conclude that the most promising sensor material for spectrophotometric detection of metal cations, mainly magnesium, is 13-membered aminoazobenzocrown 1. Metal cation recognition can be considered as an acid–base interaction of hard acid with basic donor atoms: oxygen of polyether chain and nitrogen atom of azo moiety. Ion recognition and the binding strength in studied azobenzocrowns are determined not only by the size of the macrocyle, but also by the type of substituent in para position to an azo residue. Alkali metal cations binding by selected oxyalkylenesters 5–10 was studied in acetonitrile as a solvent. For 13-membered 5, 7 and 9 differing in the length of aliphatic linker (cf. Scheme 1), typically for 13-membered azobenzocrowns, the largest spectral changes were observed for lithium. Neglible or no spectral changes were observed in the presence of sodium and potassium salts. Spectral changes caused by the presence of metal salts for oxyalkylenesters mentioned above are similar to alkoxy derivative G spectral behavior [55]. In Fig. 9a and b spectra of 9 and G in the presence of 50fold excess of lithium perchlorate in acetonitrile are compared. Fig. 9c shows stability constant values (log K) lithium complexes of 13-membered oxyalkylenesters of different length of the acid residue. Earlier we have reported fluorescence properties of pyrrole containing azobenzocrowns [27]. Our preliminary tests showed that not only acids, but also metal cations cause changes in fluorescence of some azobenzocrown ethers presented here. These experimental studies are ongoing in our laboratory. 3.3.2. Anion binding testing The presence of N H residue as amino and particularly as amide function, cause that anion–ligand interactions being a result of hydrogen bond formation between ion and ligand are probable. Thus, the possibility of anion binding by the selected macrocycles was also tested. Firstly, anion–azobenzocrown ethers interactions were studied with UV–vis spectrophotometry in acetonitrile. The studies were carried out for amino (1, 2) and aromatic amide azobenzocrown 3 and 4. Among the studied anions were halogenides, oxygen containing inorganic (hydrogen sulfate, dihydrogen phosphate, and perchlorate) and organic (acetate, benzoate, and p-toluenesulfonate) anions used as their tetra-nbutylammonium salts. Besides p-toluenesulfonates and hydrogen sulfates no or not significant spectral changes were found in the presence of investigated anions. Spectra in the presence of hydrogen sulfate were not stable within the timescale of experiment. The possible interaction with p-toluenesulfonate is hardly to evaluate due to extremely strong response of the investigated sensor materials toward even traces of acids. In basic environment, e.g. equimolar

to ligands amount of Et3 N minimal changes in absorption spectra caused by the presence of TBATos are observed. 3.4. Ion selective membrane electrodes The ionophoric properties of functionalized azobenzocrown ethers 3–10 and 13–16 were investigated in classic ion selective membrane electrodes (ISEs). It was found that, ether–ester and ether–amide, behave similarly to described earlier alkyl and dialkyl derivatives of 13- and 16-membered azobenzocrown ethers [11,12,18]. They are good ionophores in “classic” membrane electrodes. Electrodes with 13-membered crowns as ionophores show sodium selectivity. Sensors with larger 16-membered ion-carriers are potassium sensitive when ionophore concentration in a membrane is high enough. A driving force in this case seems to be “sandwich” type complexes formation with the main ions [20,22]. It was found, that aromatic amides 3 and 4, in contrast to compounds 5–10 and 13–16, are not good ionophores for ISE (long time needed for electrodes conditioning, nonlinear characteristics, etc.) Electrodes with 13-membered azobenzocrowns substituted in para position to azo group as ionophores in membrane are more pH sensitive than electrodes with membranes containing crowns substituted in meta position. Thus preferentially potentiometric measurements should be carried out in slightly basic solutions (electrodes potential is more stable comparing to measurements at neutral pH, selectivity coefficient log KNa,K is slightly better, but differences in log K generally do not differ more than 0.1). The main selectivity coefficients values (log KNa/K and log KK/Na ) for classic electrodes with PVC membranes containing ∼5% (w/w) of ionophore and o-NPOE used as a plasticizer are collected in Table 2. Selectivity coefficients log KM,H are also included. A significant change of selectivity coefficient log KM,H was found for electrodes with changing of the macrocycle size of the ionophore. Electrodes with 13-membered ion-carriers are more pH sensitive in relation to main ion than those containing their 16-membered analogs. As example, potentiometric response toward sodium, potassium and hydrogen cations for electrodes with membranes based on compounds 9 and 10 as ionophores are shown in Fig. 10. Because of the observed progress in sensor miniaturization we decided to use compounds 7–10 and 13–16 as ionophores in miniature, planar, all solid state potentiometric sensors. Previously, we have investigated the possibilities of bisazocrown ethers application as ionophores in graphite screen printed electrodes [19]. Electrodes of this type are also used in the present studies. Compounds 5 and 6 were excluded since their leakage from classic electrode membranes. Membranes of similar composition as used for classic ISE, but carbon nanotubes enriched, were poured onto graphite screen-printed electrodes. Characteristics of the obtained

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Table 2 The selectivity coefficients (log KNa/K , log KNa/H ) or (log KK/Na , log KK/H ) and slopes for classic ISE (PVC/o-NPOE) with membranes doped with functionalized azobenzocrowns and their comparison with electrodes based on ether derivatives G and H. Ionophore

Slope (mV/dec)

log KNa/K

log KNa/H

Ionophore

Slope (mV/dec)

SSM (10−1 M) G 5 7 9 13 15

−2.2 −2.2 −2.3 −2.3 −2.3 −2.5

58 57 58 56 57 58

log KK/Na

log KK/H

SSM (10−1 M) 1.8 1.1 1.5 1.5 1.2 1.6

H 6 8 10 14 16

−3.1 −3.3 −3.3 −3.2 −3.1 −3.3

59 54 58 58 55 55

−1.0 −1.8 −1.8 −1.7 −1.9 −1.4

Fig. 10. Potentiometric response toward sodium, potassium and hydrogen cations for classic membrane electrodes based on 9 (a) and 10 (b) ionophores. Table 3 Characteristics of the microfabricated membrane electrodes on screen printed graphite surface based on 7, 9, 13, 15 (for sodium) and 8, 10, 14, 16 (for potassium) ionophores. Ionophore

7

9

13

15

8

10

14

16

Linear response range [log a] Detection limit [log a] Slope (mV/dec)

−5 to −1 −5.5 58.4

−4.5 to −1 −4.8 60.2

−4.6 to −1 −5.1 58.7

−5 to −1 −5.2 57.5

−5.9 to −1 −6.2 58.9

−5.2 to −1 −5.7 57.3

−5.8 to −1 −6.2 56.4

−6 to −1 −6.3 55.9

sensors, i.e. linear response range, detection limit and slope are collected in Table 3. The obtained potentiometric selectivity coefficient values log KNa,X and log KK,X are shown in Fig. 11. Generally, the obtained results for all solid state and classic electrodes are comparable. Among 13-membered azobenzocrowns the best selectivity coefficient log KNa,K = −2.44 was found for microfabricated sensor with compound 15 (with oxybutyramide residue) as ionophore. In a case of electrodes with 16-membered azobenzocrown ethers as ion-carriers no clear relationship between the nature of a side chain and the selectivity coefficient can be drawn out. Typical value is log KK,Na = −3.30. Compounds 8 and 10 were also used as membrane components for a glassy carbon (GC) electrodes [56], but in this case bis(2-ethylhexyl)sebacate (DOS) and potassium tetrakis[3,5bis-trifluoromethyl)phenyl] borate as a lipophilic salt were components of the membrane. 3.5. X-ray structure of ligand 8 sodium complex Good quality single crystals were obtained and it was found by X-ray study, that analogously to parent compound B, azobenzocrown 8 forms with sodium iodide complex of stoichiometry 1:1. Complex of the composition [Na(8trans)]I crystallizes in the ¯ Details of data collection and structure triclinic space group P 1. refinement are given in Table 4. The content of the asymmetric unit is shown in Fig. 12. The structure is ionic. Sodium cation

centers the 16-membered macrocyclic cavity. The coordination polyhedron around the sodium ion is a pentagonal pyramid. The sodium ion is coordinated by one nitrogen and four oxygen atoms in the basal plane and an iodide anion in the apical position. Table 4 Crystal and structure solution and refinement data for [Na(8trans)]I. Compound

[Na(8trans)]I

Empirical formula Formula weight Crystal system Space group Unit cell dimensions a, Å b, Å c, Å ˛,◦ ˇ,◦ ,◦ V, Å3 Z Dcalc , Mg/m3 , mm− 1 F(0 0 0)  range for data collection,◦ Reflections collected/unique Data/restraints/parameters GOOF on F2 Final R indices [I > 2(I)], R1 , wR2 Largest diff. peak and hole, e A˚ −3

C24 H30 IN2 NaO7 608.39 Triclinic P 1¯ 8.4253(3) 10.4563(3) 15.6545(5) 88.582(3) 87.169(3) 70.496(3) 1298.35(8) 2 1.556 1.30 616 2.4–28.5 18,772/5644 5644/0/317 1.06 0.036, 0.086 1.21 and −0.52

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Fig. 11. Potentiometric selectivity coefficient values, determined by SSM method (10−1 M) obtained for microfabricated electrodes based on screen printed graphite: log KNa,X with 13-membered azobenzocrown ethers (a), log KK,X with 16-membered azobenzocrown ethers (b) as ionophores. Electrode numbers correspond to compound numbers.

Fig. 12. View of “scorpion-like” [Na(8trans)]I complex.

˚ The The Na–O distances fall within the range 2.361(2)–2.474(2) A. N(1) nitrogen atom of the macrocyclic molecule coordinates to ˚ the Na(1)–N(2) the sodium atom at the distance of 2.502(3) A, ˚ the iodide separation till the distal nitrogen atom being 3.469 A, anion which is completed the coordination polyhedron is at the ˚ Four oxygen and one nitrogen Na–I separation of 3.0143(12) A. ˚ the Na(1) atoms in the basal plane are coplanar within ±0.24 A, ˚ and N(2) atoms are displaced from this plane at 0.77 and 0.47 A, respectively. The structure of the complex is quite similar to [Na(Btrans)]I (where B = non-substituted 16-membered azocycle [23] where Na–O distances are in the range 2.390–2.464, Na–N dis˚ The torsion tances are 2.455–2.481, Na–I separation being 3.008 A). angle C(1) N(1) N(2) C(13) equal to 179.2◦ indicates the trans orientation of the aromatic substituents in 8. The ester side chain is not involved in coordination to the metal center and adopts an extended conformation. The conformations of the 16-membered macrocycle are quite similar for 8 and B as it follows from comparison of torsion angles which differ only by the C8 O2 C9 C10 and C9 C10 O3 C11 torsion angles (see Table S1)

Two complex molecules related by an inversion center form the dimeric units via weak H· · ·I contact, H(17)· · ·I(1)* = 3.11 A˚ and ␲–␲ stacking interactions between the partially overlapping aromatic ˚ In the fragments, the centroid· · ·centroid separation being of 4.20 A. crystal complexes stack along the shortest a crystallographic axis (Fig. S6). 4. Conclusions New sensor materials: 13- and 16-membered azobenzocrown ethers with aromatic amino, aromatic amide, ether–ester and ether–amide residue were obtained. Spectroscopic studies showed that not only macrocyle size, but also the nature of the substituent in benzene ring in para position to an azo group, i.e. aromatic amine (1, 2) or aromatic amide (4) strongly influences metal cation selectivity and the stability constant values of their alkali and alkaline earth metal complexes in acetonitrile. The respective values are in most cases higher than for parent compounds A and B. 13-Membered aminoazobenzocrown shows magnesium selective spectrophotometric response in acetonitrile. Magnesium

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cation recognition is manifested by color change from yellow to orange and large (∼99 nm) spectral shift of the complex band. Large spectral bands separation (∼100 nm) and color change from yellow to orange or pink-red was found to be response of 1–4 for acids in acetonitrile. Weakly fluorescent azobenzocrowns 1–4 in their protonated forms show characteristic red fluorescence with Stokes shift about 100 nm. Lariat derivatives: oxyalkylesters and oxyalkylamides are better material for potentiometric than spectrophotometric sensors. Azobenzocrowns studied here, analogously to 13-membered alkyl and ether derivatives, are sodium selective ionophores in ISE, whereas 16-membered crowns show potassium sensitivity. Mentioned ion carriers may be used in membranes of miniature all solid state type electrode. The obtained results also showed the possibilities of chemical connection of azobenzocrowns via ester or amide bond with membrane components without loss of the selectivity. 16-Membered azobenzocrown 8 forms crystalline complex with sodium iodide (1:1) – similarly to reference compound B. It explains relatively large amounts of 16-membered crowns needed for electrode membranes preparation what favors sandwich type complex formation with potassium and in a consequence good potassium selectivity of electrodes. Acknowledgments E. W-W. kindly acknowledges support from Sources for Science grant no. N N204 137438 in years 2010–2011. Financial support of this work from Gdansk University of Technology (DS grant no. 020223/003) is gratefully acknowledged. Authors thank MS students for their experimental contribution. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2012.11.068. References [1] K. Ariga, T. Kunitake, Supramolecular Chemistry – Fundamentals and Applications, Springer-Verlag, Berlin, Heidelberg, 2006. [2] D.N. Reinhoudt (Ed.), Supramolecular Materials and Technologies Perspectives in Supramolecular Chemistry, vol. 4, John Willey & Sons Ltd., Chichister, 1999. [3] E.N. Ushakov, M.V. Alfimov, S.P. Gromov, Design principles for optical molecular sensors and photocontrolled receptors based on crown ethers, Russian Chemical Reviews 77 (2008) 39–58. [4] For example see review: E. Luboch, R. Bilewicz, M. Kowalczyk, E. WagnerWysiecka, J.F. Biernat, Azo macrocyclic compounds, in: G. Gokel (Ed.), Advances in Supramolecular Chemistry, vol. 9, Cerberus Press, South Miami, USA, 2003, pp. 71–162. [5] I. Zawisza, R. Bilewicz, E. Luboch, J.F. Biernat, Voltammetric recognition of Cis (Z) and Trans (E) isomers of azobenzene and azo-crown ethers, Supramolecular Chemistry 9 (1998) 277–287. [6] I. Zawisza, R. Bilewicz, E. Luboch, J.F. Biernat, Complexation of metal ions by azocrown ethers in Langmuir–Blodgett monolayers, Journal of the Chemical Society, Dalton Transactions (2000) 499–503. [7] I. Zawisza, R. Bilewicz, K. Janus, J. Sworakowski, E. Luboch, J.F. Biernat, Comparison of ZE isomerization in Langmuir–Blodgett layers and in solution, Materials Science and Engineering C 22 (2002) 91–98. [8] M. Shiga, M. Takagi, K. Ueno, Azo-crown ethers. The dyes with azo group directly involved in the crown ether skeleton, Chemistry Letters (1980) 1021–1022. [9] M. Shiga, H. Nakamura, M. Takagi, K. Ueno, Synthesis of azobenzo-crown ethers and their complexation behavior with metal ions, Bulletin of the Chemical Society of Japan 57 (1984) 412–415. [10] R. Tahara, T. Morozumi, H. Nakamura, M. Shimomura, Photoisomerisation of azobenzocrown ethers. Effect of complexation of alkaline earth metal ions, Journal of Physical Chemistry B 101 (1997) 7736–7743. [11] E. Luboch, J.F. Biernat, E. Muszalska, R. Bilewicz, 13-Membered crown ethers with azo or azoxy unit in the macrocycle – synthesis. Membrane electrodes. Voltammetry and Langmuir monolayers, Supramolecular Chemistry 5 (1995) 201–210. [12] E. Luboch, J.F. Biernat, Yu.A. Simonov, A.A. Dvorkin, Synthesis and electrode properties of 16-membered azo- and azoxycrown ethers. Structure of tribenzo16-azocrown-6, Tetrahedron 54 (1998) 4490–4977.

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Biographies Ewa Wagner-Wysiecka obtained her PhD in Chemistry at Gdansk University of Technology (Poland) in 2002, where now is working as assistant professor. The main stream of her research is supramolecular chemistry, namely studies of the mechanisms of molecular recognition. Currently the topic of the studies is design, synthesis and studies of anion recognition by chromogenic and fluorescent receptors. Tomasz Rzymowski obtained his PhD in chemistry in 2012 after being PhD student at Chemical Faculty of Gdansk University of Technology (Poland). His PhD thesis, ˙ supervisor Prof. Elzbieta Luboch, was entitled: “Macrocycles with chromo- and/or fluoroionophoric nature.” Mirosław Szarmach (MSc) is a PhD student at Chemical Faculty of Gdansk University of Technology (Poland). The topic of his PhD thesis under supervising of Prof. ˙ Luboch covers the synthesis of macrocyclic sensor materials for potentioElzbieta metric sensors. Marina S. Fonari obtained her PhD in crystallography and crystal physics at A.V. Shubnikov Institute of Crystallography in 1992 (Moscow, Russia). Her scientific interests cover X-ray crystallography of supramolecular and coordination compounds, with an emphasis on weak interactions in molecular complexes based on the crown ethers and crownophanes; crystallography of fluorine-containing compounds and complexes. She is leading scientific worker in Institute of Applied Physics of the Academy of Sciences of Moldova. ˙ Luboch (PhD 1983, DSc 2007) is currently a professor at Gdansk University Elzbieta of Technology (Poland). The main topics of her scientific work are connected with organic synthesis, organic supramolecular chemistry, chemical potentiometric sensors and studies of correlation between structure and ionophoric properties of the sensor materials. She is also interested in pharmaceutical chemistry (identification of drug impurities, standard substances synthesis).