Fb

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b

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ARR1AA

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1

Ncaphtbtrt[tipwht

e

0h

Sensors and Actuators B 177 (2013) 913– 923

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical

journa l h o mepage: www.elsev ier .com/ locate /snb

unctionalized azobenzocrown ethers as sensor materials—The synthesis and ioninding properties

wa Wagner-Wysieckaa,∗, Tomasz Rzymowskia, Mirosław Szarmacha, Marina. S. Fonarib,lzbieta Lubocha,∗

Department of Chemical Technology, Faculty of Chemistry, Gdansk University of Technology, Narutowicza Street 11/12, 80-233 Gdansk, PolandInstitute of Applied Physics, Academy of Sciences, Chis inau MD 2028, Republic of Moldova

r t i c l e i n f o

rticle history:eceived 5 June 2012eceived in revised form7 November 2012ccepted 19 November 2012vailable online 5 December 2012

eywords:

a b s t r a c t

New 13- and 16-membered azobenzocrown ethers with aromatic amino, amide, ether–ester orether–amide residue in para position to an azo moiety were obtained. Acid–base properties and ionbinding ability of the colored compounds were studied by spectroscopic methods: UV–vis, fluorimetryand 1H NMR spectroscopy. Selected azobenzocrowns were tested as ionophores in ion-selective mem-brane electrodes (ISEs) – classic and miniature all solid state. The X-ray structure of the sodium complexof ether–ester derivative of 16-membered azobenzocrown was presented.

© 2012 Elsevier B.V. All rights reserved.

zobenzocrown ethershromoionophoresolecular recognition

pectroscopic methodson-selective electrodes

creen printed electrodes-ray structure

. Introduction

Supramolecular chemistry, taking its early inspiration fromature, is nowadays one of the most active field of science [1,2]. Itovers, among others, the design, synthesis and studies of the inter-ctions between host and guest molecules for chemical sensingurposes. At the beginning, metal cation coordination chemistryas been the main stream in supramolecular science. Since thatime, properties of many compounds of various structures haveeen studied pointing the wide field of their possible applica-ions. Well known metal cation complexing agents are moleculareceptors based on crown ethers [3]. Among them an impor-ant class is macrocyclic compounds containing an azo residue4]. Crown ethers incorporating azobenzene moiety as a part ofhe macrocycle – azobenzocrown ethers – are interesting metalon complexing compounds. In addition, these macrocycles arehoto and redox active [5–7]. Numerous macrocyclic compounds

ith inherent 2,2′-azobenzene [8–15] or 4,4′-azobenzene [16,17]ave been synthesized and exhaustively studied. Wide possibili-ies of azobenzocrowns functionalization result in their potential

∗ Corresponding authors. Tel.: +48 58 3471759; fax: +48 58 3411949.E-mail addresses: ewa.wagner-wysiecka@pg.gda.pl (E. Wagner-Wysiecka),

lzluboc@pg.gda.pl, elub@chem.pg.gda.pl (E. Luboch).

925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2012.11.068

applications in the variety of fields, e.g. lipophilic crowns maybe successfully used as ionophores, both in classic [11,12,18] andminiature all solid state ion-selective membrane electrodes (ISEs)[19]. As an azo group is an integral part of the macroring it can actas a donor place in ion binding. This was confirmed, in a solid state,by several reported X-ray structures of azobenzocrown complexeswith metal cations [19–23]. Azoarylcrown ethers behave also aschromoionophores selectively binding metal cations in solution[24–26]. Cation–ligand interactions are well manifested by colorand UV–vis spectral changes. Further structure modification of theazobenzocrown ethers skeleton can leads to compounds compris-ing the merits of chromoionophores and fluoroionophores [27]. Itis worth noting that some compounds mentioned above can beconcurrently used as good ionophores in membrane ion-selectiveelectrodes. It makes them an universal, to some extend, analyticaltool for eventual metal cation detection and/or determination.

Anion coordination is another important field of interest insupramolecular chemistry. This is, among others, because of anionsabundance in nature and their key role in many biochemical pro-cesses. The design and synthesis of anion receptors are widelypresented in exhaustive review articles [28–34]. Depending on the

structure of receptor, the respective ligands may be used for partic-ular chemical sensors construction or/and as selective reagents forspectroscopic purposes [31,34]. It was found that ion-selective elec-trodes with complexes of azothiacrown with heavy metal cations

9 s and

([cn

dsag(

aflic

it–

2

2

csLp(rCFMaartmio(fELwsct

2

S

2

lraezwdr

14 E. Wagner-Wysiecka et al. / Sensor

silver, mercury and copper) as ionophores show anionic response35]. Numerous examples of both neutral crowns and their metalation complexes of different structure have been reported up toow as anion receptors, for examples see [36–40].

Here we present new functionalized azobenzocrown ethers ofifferent size of the macrocycle and their properties. The synthe-ized 13- and 16-membered crowns have aromatic amino (1, 2) orromatic amide (3, 4), ether–ester (5–10), and ether–amide (13–16)roups as a side residue located in para position to an azo moietyScheme 1).

For selected macrocycles acid–base properties and complex-tion studies were carried out by UV–vis spectrophotometry,uorimetry and 1H NMR spectroscopy in acetonitrile. Ion bind-

ng properties were compared with data available for referenceompounds A–H (Fig. 1) [9,10,18,24].

Additionally, N H group influence on the possible anion bind-ng was also investigated. Some more lipophilic compounds wereested as ionophores in ion-selective membrane electrodes (ISEs)

classic and all solid state.

. Experimental

.1. General

All chemicals of highest available purity were purchased fromommercial sources and used without further purification. THF forynthesis and membrane preparation was freshly distilled overiAlH4. TLC: aluminum sheets covered with silica gel 60F254 wereurchased from Merck. For column chromatography silica gel 600.063–0.200 mm) (Merck) was used. 1H and 13C NMR spectra wereecorded at a Varian instrument (500 and 125 MHz, respectively).hemical shifts are reported as ı [ppm] values in relation to TMS.TIR spectra were recorded on a Mattson Genesis II instrument.ass spectra were recorded on an AMD-604 (EI method, 70 eV) and

GCT Premier (TOF MSFD+) instruments. For UV–vis measurementsn UNICAM UV 300 apparatus was used. Fluorescence spectra wereecorded on an AMINCO-Bowman Series 2 luminescence spec-rometer (flash xenon lamp). Bandpass at excitation and emission

onochromators: 16 nm. Fluorescence spectra are uncorrected tonstrument response. Spectroscopic measurements were carriedut in acetonitrile (LiChrosolv®) of gradient grade. Deionized water18 M� cm, Hydrolab, Poland) for water containing solvent systemor UV–vis spectrophotometry and EMF measurements was used.MF measurements were carried out using a 16-channel Lawsonab potentiometer (USA). The screen-printed graphite electrodesere prepared in Institute of Electronic Materials Technology, War-

aw, Poland (plates of 18–15 mm with six electrodes, openings areaa. 1 mm2). All measurements were carried out at room tempera-ure.

.2. Syntheses

For synthetic details and spectral data, see Supplementary dataT2.

.3. Complexation studies

Complexation studies were performed by UV–vis titration of theigand solution in acetonitrile with the respective metal perchlo-ates (for metal cations) or tetra-n-butylammonium (TBA) salts (fornions). Caution! Perchlorate salts should be regarded as potentiallyxplosive and handled with care. The stock solutions of azoben-

ocrowns (∼10−4 M) and metal perchlorates or TBA salts (∼10−2 M)ere prepared by weighing the respective quantities of them andissolving in acetonitrile in volumetric flasks. Titrations were car-ied out in a quartz cuvette with path length of 1 cm keeping

Actuators B 177 (2013) 913– 923

constant volume of the ligand solution (2.3 mL). The stability con-stant values were calculated with the use of OPIUM [41] programon the basis of titration experiment data.

2.4. Ion selective electrodes

2.4.1. Classic ISEsThe membrane components (8 mg of ionophore, 50 mg of PVC,

0.1 mL of o-nitrophenyl-octyl ether (o-NPOE) and 1 mg of potas-sium tetrakis(4-chlorophenyl)borate (KTpClPB)) were dissolved infreshly distilled, dry THF (1.2 mL). The solution was poured into aglass ring (diameter 15 mm). After 1 day, membranes of d = 7 mmwere cut out and incorporated into Ag/AgCl electrode bodies of IStype (Moeller S.A., Zurich, Switzerland). NaCl or KCl 10−2 M wereused as internal electrolyte for sodium and potassium selectiveelectrodes, respectively. The electrode was conditioned by soakingit in a 10−2 M solution of MCl (M – the main ion) for 24 h. A double-junction Ag/AgCl, KCl 1 M reference electrode (Monokrystaly RAE112) was used with 1 M NH4NO3 solution in the bridge cell. Theselectivity coefficients (KNa,K, KNa,H or KK,Na, KK,H) were determinedusing the separate solution method (SSM) [42] at ion activi-ties of 10−1 M in neutral and 3% TRIS (pH ∼ 9) for 13-memberedionophores and neutral pH for electrodes with 16-membered ion-carriers.

2.4.2. Screen-printed electrodesIonophore and ca. 0.05 mg of carbon nanotubes (single wal-

let, Aldrich) in 1 mL of THF were sonicated for 1 h. Next, theremaining membrane constituents were added as described above.The solution (1–0.5 �L) was applied onto graphite screen-printedelectrodes and was left to dry over 24 h at room temperature. Nextelectrodes were conditioned by soaking them in a 10−3 M solutionof NaCl or KCl for 10 h. All the other experimental details were iden-tical as described for classic electrodes. The measurements werecarried out in accordance with procedures specified for microfab-ricated ion-selective electrodes [43].

The response of both classic and screen-printed electrodestoward alkali (Li+, Na+, K+, Rb+, and Cs+), alkaline earth (Mg2+ andCa2+) and ammonium (used as chlorides) ions was studied.

2.5. X-ray crystal structure determination

2.5.1. Preparation of crystalsEthoxycarbonylbutylenoxy-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 wasevaporated under reduced pressure. To the obtained solidacetone:propan-2-ol (1:1, v/v) mixture (2 mL) was added. Veryslow solvent evaporation has resulted in crystals melting at160–166 ◦C.

2.5.2. Determination of crystal structureThe X-ray data for [Na(8trans)]I complex were collected at

150 K on a KM4CCD diffractometer using graphite-monochromatedMoK� radiation and were corrected for Lorentz and polarizationeffects. The structure was solved by direct methods and refinedby full-matrix least squares technique based on F2. Analyticalnumeric absorption correction using a multifaceted crystal modelbased 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 geometri-cally calculated positions and refined using temperature factors1.2 times those of their bonded carbon atoms. Calculations wereperformed using SHELX-97 crystallographic software package.

E. Wagner-Wysiecka et al. / Sensors and Actuators B 177 (2013) 913– 923 915

OH

NH2

O

OO

NN

NH2

OHOH

NN

NH2

O

OO

n

Ts Ts

O

OO

NN

NH2

O

OO

NN

NH C

C11

H23

O

NH2OH

O

OO

NN

OH

O

OO

NN

O(CH2) C

O

OEt

O

OO

NN

O(CH2)3

C

O

OH

O

OO

NO(CH

2)3

C

O

NHC H

NH2

OH

NN

OH

O

OO

NOCH

2

C

O

NHC7H

15

ClCH2

C

O

NHC H

NaNO2

HCl

n

tBu OK, T HF

n

C11H23COCl, Et3N

THF

n

1. n = 12. n = 2

H2O, NaOH

3. n = 14. n = 2

C. n = 1D. n = 2

n

Br(CH2)mCOOEt

K2CO3, acetonem

n n

KOH

EtOH

C7H15NH2DMF , NEt3

n

a)

b)

c)

17

5. n = 1, m = 1 6. n = 2, m = 17. n = 1, m = 3

8. n = 2, m = 3 9. n = 1, m = 510. n = 2, m = 5

11. n = 112. n = 2

15. n = 116. n = 2

+

18

DCC, HONSu

13. n = 1 14. n = 2

n

K2CO3, DMF

c rout

3

3

ia

3aa

tmzi

N 7 15

Scheme 1. The syntheti

. Results and discussion

.1. Synthesis

Target compounds 1–10 and 13–16 were prepared as shownn Scheme 1a–c. For synthetic details see Supplementary data, ST1nd ST2.

.2. The properties of aromatic amino- andmide-azobenzocrowns (tautomerism, solvatochromism,cid–base properties)

In aromatic azo compounds substituted in ortho or para posi-

ion with hydroxyl or amino group intramolecular proton transfer

ay occur. This phenomena was also observed for hydroxyazoben-ocrowns studied earlier [18,24,47]. As amino group in 1 and 2s located in para position to azo moiety, tautomeric equilibrium

Fig. 1. The reference azob

N 7 15

es for compounds 1–16.

can be taken into consideration. Moreover, possible tautomerism ofN-acetyl derivatives of aminoazobenzocompounds was also takeninto account [48].

Solvent influence on tautomeric equilibrium of aminoazoben-zocrowns was studied by 1H NMR spectroscopy. It was foundthat in aprotic solvents such as acetonitrile (Fig. 2a) and DMSO(Fig. 2b) 1 exists in aminoazoform. In 1H NMR spectrum of1 recorded in methanol and acetonitrile:water mixture (notshown) aromatic proton signals distribution is similar to thiswhich was observed in DMSO. It also points that in these sol-vents aminoazoform of 1 is dominating. Above observations canlead to conclusion that tautomeric equilibrium of 1 is neutralsolvent independent. Similar experiments for 2 confirmed that

in acetonitrile, acetone and DMSO aminoazo form is dominat-ing.

This is opposite to the properties of hydroxyazoben-zocrowns studied ealier. A quinone-hydrazone equilibrium of

enzocrown ethers.

916 E. Wagner-Wysiecka et al. / Sensors and Actuators B 177 (2013) 913– 923

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

O

OO

N

H O

N

O

OO

NN

OH

O

OO

N

H NH

N

O

OO

NN

NH2

wn (1

hs1q(vTAdqsphpe

ShDia(iiiawdw

utzostmdist

shift is observed in a case of an unsubstituted azobenzocrown A121 nm, then for aromatic amide 3 and aromatic amine 1: 86 and81 nm, respectively.

C

Fig. 3. Tautomerism of hydroxy- (C) and aminoazobenzocro

ydroxyazobenzocrowns was found to be macrocycle size andolvent type dependent [18,24,47]. And so, an analog of 1 –3-membered hydroxyazobenzocrown C (Fig. 1) exists mainly inuinone-hydrazone form and only in DMSO its azophenol tautomer∼30%) was observed. This probably affects the stability constantalues of hydroksyazobenzocrowns metal cation complexes.hey are lower than for their unsubstituted parent compounds

and B (Fig. 1). It can be explained by tautomeric equilibriumistribution, where stabilized by intermolecular hydrogen bonduinone-hydrazone form is dominating. Aminoazobenzocrowns,imilarly to simple open-chain aminoazobenzene compounds [48],referably exist in aminoazo forms. This in turn can explain theigher values of the stability constant of their metal cation com-lexes. Tautomeric equilibrium of functionalized azobenzocrownthers is exemplified in Fig. 3 with 13-membered crowns.

The solvatochromism of aminoazobenzocrown 1 (seeupplementary data, Fig. S1a) was also studied. In aprotic,ighly dipolar (non-hydrogen bond donor) solvents: acetonitrile,MSO and DMF spectra are similar in shape, but differ in their

ntensity. In methylene chloride new band of a moderate intensityppear at about 500 nm. This band is better observable in proticHB) methanol. Excluding solvent affected tautomeric equilibrium,t may be assumed that a band at ∼500 nm in UV–vis spectrums an effect of interactions between ligand and solvent, namelyntermolecular hydrogen bond formation resulting in possibleggregation of azo compound. Observed spectral changes uponater addition to acetonitrile solution of 1 (see Supplementaryata, Fig. S1b) can point interactions between azobenzocrown andater molecules.

Acid–base properties of aromatic azo compounds make themseful as potential pH indicators. Protonation of azobenzene moietyakes place at one of the two azo N-atoms [45]. Aminoazoben-enes are generally protonated both at amino and nitrogen atomf azo group [46,48]. Protonation of aminoazobenzocrown 1 wastudied by spectroscopic methods in organic solvents. UV–visitration with perchloric acid in acetonitrile results in the for-

ation of a new, intensive band at 483 nm (see Supplementary

ata, Fig. S2a). Color changes from yellow to orange. Clear

sosbestic point (414 nm) and molar ratio plot (not shown)uggest monoprotonation of compound 1. In 1H NMR spec-rum of 1 recorded in the presence of 1 eq. of perchloric acid

1

) ethers exemplified with 13-membered azobenzocrowns.

in d-acetonitrile (see Supplementary data, Fig. S2b) aromaticproton signals are shifted downfield and one N H proton signal isobserved at 13.6 ppm. 1H NMR spectrum recorded in the presenceof 1 eq. of perchloric acid in d-DMSO shows two signals at ∼9.4 andone singlet at 13.4 ppm. Similar changes were observed in 1H NMRspectrum 1 in the presence of p-toluenesulfonic acid (TosOH) andisolated adduct of 1-TosOH (1:1) (see Supplementary data, Fig. S2b).Those signals were also well observable in spectrum registered inthe presence of the excess of TosOH in d-chloroform. Observedsignals pattern can suggest stimulated by acids tautomeric equi-librium shifted toward protonated imino-hydrazone form of 1. Thescheme of reversible protonation of 1 is shown in Fig. S2d (seeSupplementary data).

Protonation of aromatic amides 3 and 4 causes color changefrom yellow to pink-red. Color changes of 4 in the presence ofTosOH are illustrated in Fig. 4.

Most of azo compounds are nonfluorescent at room temper-ature; however, there are some exceptions reported in literature[49–53]. We found that protonated form of azobenzocrown ethersis fluorescent. The comparison of the normalized absorption andfluorescence spectra of protonated A, 1 and 3 are shown inFig. 5.

Changes in UV–vis and fluorescence spectrum upon titra-tion of 4 with p-toluenesulfonic acid in acetonitrile are shownin Fig. 6a and c. Fig. 6b and d illustrates color change andred fluorescence of ligand 4 in the presence of 2 eqs. ofTosOH.

The reversible protonation of azobenzocrowns 1–4 causes redfluorescence with emission band over ∼600 nm. The highest Stokes

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

E. Wagner-Wysiecka et al. / Sensors and Actuators B 177 (2013) 913– 923 917

400 450 500 5500.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d a

bso

rba

nce

nm

A1 3

(a)

550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d flu

ore

sce

nce

nm

A13

(b)

F azobe� ) in ac

3

3

b

2etCFtsflA[ccaiwba(toAh

Fsi

ig. 5. Normalized absorption spectra of protonated (TosOH, 2 eq.) 13-membered

em = 608 nm), 1 (�ex = 482 nm, �em = 568 nm) and 3 (�ex = 490 nm, �em = 576 nm) (b

.3. Complexation studies in solution

.3.1. Metal cation complexationAlkali and alkaline earth metal cations (used as perchlorates)

inding was studied in acetonitrile by UV–vis spectroscopy.As a first target, 13- and 16-membered aminoderivatives 1 and

and 16-membered aromatic amide 4 were chosen. As can bexpected, for 13-membered azobenzocrown 1 changes in absorp-ion spectra were observed in the presence of lithium perchlorate.hanges upon titration of 1 with lithium perchlorate are shown inig. 7a. Sodium and potassium salts did not influence significantlyhe absorption spectrum. In comparison to the parent compound Apectral changes upon lithium complexation are more remarkableor 1, although the stability constants values (log K, 1:1) of theirithium complexes are comparable: 4.00 [9] for A and 4.01 for 1.zobenzocrown 1 in its spectral behavior resembles compound E

18]. Among alkaline earth metal cations the most distinct spectralhanges for 1 were observed in the presence of magnesium per-hlorate. Titration with magnesium salt results in the formation of

new band at 482 nm (�� =+99 nm). Absorption spectrum, typ-cally for azo compounds, is almost completely analogous to this

hich is registered in acidic medium [48,54]. Moreover, longwaveand is still observable not only in the presence of neutral salt butlso in slightly basic solution in the presence of organic base (Et3N)see Supplementary data, Fig.S3). Addition of triethylamine to solu-

ion of 1 does not affect UV–vis spectrum. Our studies suggest therigin of the observed spectral behavior as ion–ligand interaction.mino derivative 1 preferably binds magnesium cation (being aard acid in HSAB theory) with stability constant (log K) 6.43. It

ig. 6. Changes in absorption spectrum upon titration of 4 (3.73 × 10−5 M) with TosOH

pectrum of 4 (3.73 × 10−5 M) upon titration of 4 with TosOH (�ex = 510 nm, �em = 598

nterpretation of the references to color in this figure legend, the reader is referred to the

nzocrowns A, 1 and 3 (a) and normalized fluorescence spectra of A (�ex = 487 nm,etonitrile.

proves a beneficial electron donating resonance effect of the amineresidue introduction into 13-membered azobenzocrown skeleton.Fig. 7b shows the limiting absorption spectra obtained upon titra-tion of 1 with alkaline earth metal salts. Fig. 7c illustrates titrationcourse for 1 and magnesium perchlorate in acetonitrile. Addition-ally, photos in Fig. 7 show selective color response of 1 towardmagnesium cation.

Alkali and alkaline earth metal cations complexation was alsostudied for 16-membered azobenzocrown ethers. The obtainedstability constant values for 16-membered azobenzocrowns 2(amine), 4 (aromatic amide) and 16 (ether–amide) and data avail-able 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 oflithium, sodium and potassium salts (see Supplementary data, Fig.S4a). Titration course of 2 with lithium and sodium salts illustrateFig. S4b and c. Spectral changes in the presence of alkali metal per-chlorates are the most distinct for lithium cation. The comparisonof the stability constant values of alkali metal cation complexeswith 16-membered azobenzocrown ether 2 (Table 1) with valuesobtained for unsubstituted reference azobenzocrown B points thatamino derivative 2 forms more stable complexes with the studiedmetal cations. Only in the case of larger potassium cation this effectis not observed. Higher values of the respective binding constantcan be explained by electron donating resonance effect of the amino

group increasing electron density at nitrogen atom of an azo groupparticipating in metal cation binding.

Aminoazobenzocrown 2 forms stronger complexes with alka-line earth metal cations than with lithium, sodium and potassium.

(a), color change of 4 in the presence of 2 eq. TosOH (b), changes in fluorescencenm) (c), fluorescence of 4 in the presence of 2 eq. TosOH (d) in acetonitrile. (For

web version of the article.)

918 E. Wagner-Wysiecka et al. / Sensors and Actuators B 177 (2013) 913– 923

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 metalperchlorates (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 inacetonitrile. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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

Compound Log KLi Log KNa Log KK Log KMg Log KCa Log KSr Log KBa

2 5.33 5.04 2.90 6.06 6.40 6.13 6.324 4.82 4.60 a 3.47 5.30 5.87 6.0016 3.58 3.61 a 4.99 5.96 6.32 6.26B [10] 4.00 3.69 3.15 5.00 5.15 4.91 4.61F [24] 4.28 4.75 3.74 a a a a

a

Isb2ssim1cIm

i4

Fb

H [55] 4.18 a 3.34

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

t is in agreement with the general trend observed for non-ubstituted azobenzocrown ethers studied by Nakamura et al. [10],ut stability constant values are higher for aminoazobenzocrown

than for parent compound B (Fig. 1). Fig. 8a shows the limitingpectra obtained during titration of 2 with alkaline earth metalalts. Comparison of the spectrophotometric response of 1 and 2n acetonitrile toward alkaline earth metal cations implies that

agnesium selectivity is determined by macrocyle size. Larger6-membered aminoazobenzocrown does not show such selectivehanges in absorption spectrum in the presence of magnesium salt.n Fig. 8b and c changes upon spectrophotometric titration of 2 with

agnesium and barium perchlorates are presented.Complexing properties of 2 were compared to the cation bind-

ng ability of the aromatic amide 4 of the same ring size. For, among studied alkali metal cations, changes in absorption

350 400 450 500 550 600

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

A

nm

free

Mg

Ca

Sr

Ba

(a)

350 400 450 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

A

nm

(b)

ig. 8. The limiting spectra obtained during titration of 2 (5.62 × 10−5 M) with alkaline eaarium (c) perchlorates in acetonitrile.

4.02 a a a

spectrum were observed in the presence of lithium and sodiumsalts (see Supplementary data, Fig. S5a). In Fig. S5b the respec-tive limiting spectra obtained for alkaline earth metal cations areshown. Opposite to 16-membered aminoazobenzocrown 2 less sig-nificant and selective spectral changes were observed for aromaticamide 4 in the presence of alkaline earth metal cations. Only inthe presence of magnesium cation the band shape is slighty dif-ferent than in a case of other studied metal cations. Fig. S5c is anexample of spectral changes upon titration of 4 with metal cationsalts in acetonitrile and shows titration with calcium perchlo-rate. For ether–amide 16 spectral changes caused by the presence

of metal cations are similar to those, which were observed for4. Stability constant values of its complexes are generally lowerthan for 16-membered aminoazobenzocrown 2. Comparing to aro-matic amide 4 ether–amide 16 forms more stable complexes with

500 550 600

Mg2+ = 0-8.3x10-4

M

350 400 450 500 550 600

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Ba2+ = 0-8.2x10-4M

A

nm

(c)

rth metal perchlorates (a), changes upon UV–vis titration with magnesium (b) and

E. Wagner-Wysiecka et al. / Sensors and Actuators B 177 (2013) 913– 923 919

250 300 350 400 450 500

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8A

nm

free

Li

Na

K

(a)

250 300 350 400 450 500

0.0

0.1

0.2

0.3

0.4

0.5

A

nm

free

Li

Na

K

(b)

A G 5 7 90.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Lo

g K

Azobenzocrown

(c)

F ) (b) ins ) in a

aOmozaoams

wd1oict[fFor

tnos

3

arpmitawfbbspgpda

ig. 9. Spectral changes of azobenzocrowns: 9 (3.75 × 10−5 M) (a), G (1.84 × 10−5 Mtability constant values of lithium complexes of azobenzocrowns A, G, 5, 7 and 9 (c

lkaline earth metal cations, especially with strontium (Table 1).btained results for 1, 2, 4 and 16 allow us to conclude that theost promising sensor material for spectrophotometric detection

f metal cations, mainly magnesium, is 13-membered aminoa-obenzocrown 1. Metal cation recognition can be considered as ancid–base interaction of hard acid with basic donor atoms: oxygenf polyether chain and nitrogen atom of azo moiety. Ion recognitionnd the binding strength in studied azobenzocrowns are deter-ined not only by the size of the macrocyle, but also by the type of

ubstituent in para position to an azo residue.Alkali metal cations binding by selected oxyalkylenesters 5–10

as studied in acetonitrile as a solvent. For 13-membered 5, 7 and 9iffering in the length of aliphatic linker (cf. Scheme 1), typically for3-membered azobenzocrowns, the largest spectral changes werebserved for lithium. Neglible or no spectral changes were observedn the presence of sodium and potassium salts. Spectral changesaused by the presence of metal salts for oxyalkylenesters men-ioned above are similar to alkoxy derivative G spectral behavior55]. In Fig. 9a and b spectra of 9 and G in the presence of 50-old excess of lithium perchlorate in acetonitrile are compared.ig. 9c shows stability constant values (log K) lithium complexesf 13-membered oxyalkylenesters of different length of the acidesidue.

Earlier we have reported fluorescence properties of pyrrole con-aining azobenzocrowns [27]. Our preliminary tests showed thatot only acids, but also metal cations cause changes in fluorescencef some azobenzocrown ethers presented here. These experimentaltudies are ongoing in our laboratory.

.3.2. Anion binding testingThe presence of N H residue as amino and particularly as

mide function, cause that anion–ligand interactions being aesult of hydrogen bond formation between ion and ligand arerobable. Thus, the possibility of anion binding by the selectedacrocycles was also tested. Firstly, anion–azobenzocrown ethers

nteractions were studied with UV–vis spectrophotometry in ace-onitrile. The studies were carried out for amino (1, 2) andromatic amide azobenzocrown 3 and 4. Among the studied anionsere halogenides, oxygen containing inorganic (hydrogen sul-

ate, dihydrogen phosphate, and perchlorate) and organic (acetate,enzoate, and p-toluenesulfonate) anions used as their tetra-n-utylammonium salts. Besides p-toluenesulfonates and hydrogenulfates no or not significant spectral changes were found in theresence of investigated anions. Spectra in the presence of hydro-

en sulfate were not stable within the timescale of experiment. Theossible interaction with p-toluenesulfonate is hardly to evaluateue to extremely strong response of the investigated sensor materi-ls toward even traces of acids. In basic environment, e.g. equimolar

the presence of 50-fold excess of alkali metal perchlorates, the comparison of thecetonitrile.

to ligands amount of Et3N minimal changes in absorption spectracaused by the presence of TBATos are observed.

3.4. Ion selective membrane electrodes

The ionophoric properties of functionalized azobenzocrownethers 3–10 and 13–16 were investigated in classic ion selec-tive membrane electrodes (ISEs). It was found that, ether–esterand ether–amide, behave similarly to described earlier alkyl anddialkyl derivatives of 13- and 16-membered azobenzocrown ethers[11,12,18]. They are good ionophores in “classic” membrane elec-trodes. Electrodes with 13-membered crowns as ionophores showsodium selectivity. Sensors with larger 16-membered ion-carriersare potassium sensitive when ionophore concentration in a mem-brane is high enough. A driving force in this case seems to be“sandwich” type complexes formation with the main ions [20,22]. Itwas found, that aromatic amides 3 and 4, in contrast to compounds5–10 and 13–16, are not good ionophores for ISE (long time neededfor electrodes conditioning, nonlinear characteristics, etc.)

Electrodes with 13-membered azobenzocrowns substituted inpara position to azo group as ionophores in membrane are morepH sensitive than electrodes with membranes containing crownssubstituted in meta position. Thus preferentially potentiometricmeasurements should be carried out in slightly basic solutions(electrodes potential is more stable comparing to measurementsat neutral pH, selectivity coefficient log KNa,K is slightly better, butdifferences in log K generally do not differ more than 0.1). Themain selectivity coefficients values (log KNa/K and log KK/Na) forclassic electrodes with PVC membranes containing ∼5% (w/w) ofionophore and o-NPOE used as a plasticizer are collected in Table 2.Selectivity coefficients log KM,H are also included. A significantchange of selectivity coefficient log KM,H was found for electrodeswith changing of the macrocycle size of the ionophore. Electrodeswith 13-membered ion-carriers are more pH sensitive in relationto main ion than those containing their 16-membered analogs. Asexample, potentiometric response toward sodium, potassium andhydrogen cations for electrodes with membranes based on com-pounds 9 and 10 as ionophores are shown in Fig. 10.

Because of the observed progress in sensor miniaturization wedecided to use compounds 7–10 and 13–16 as ionophores in minia-ture, planar, all solid state potentiometric sensors. Previously, wehave investigated the possibilities of bisazocrown ethers appli-cation as ionophores in graphite screen printed electrodes [19].Electrodes of this type are also used in the present studies. Com-

pounds 5 and 6 were excluded since their leakage from classicelectrode membranes. Membranes of similar composition as usedfor classic ISE, but carbon nanotubes enriched, were poured ontographite screen-printed electrodes. Characteristics of the obtained

920 E. Wagner-Wysiecka et al. / Sensors and Actuators B 177 (2013) 913– 923

Table 2The 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 azobenzocrownsand 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) log KK/Na log KK/H

SSM (10−1 M) SSM (10−1 M)

G 58 −2.2 1.8 H 59 −3.1 −1.05 57 −2.2 1.1 6 54 −3.3 −1.87 58 −2.3 1.5 8 58 −3.3 −1.89 56 −2.3 1.5 10 58 −3.2 −1.713 57 −2.3 1.2 14 55 −3.1 −1.915 58 −2.5 1.6 16 55 −3.3 −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 3Characteristics 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

sc

l

tbfazno

pcbc

3

bz1tru

centers the 16-membered macrocyclic cavity. The coordinationpolyhedron around the sodium ion is a pentagonal pyramid. Thesodium ion is coordinated by one nitrogen and four oxygen atomsin the basal plane and an iodide anion in the apical position.

Table 4Crystal and structure solution and refinement data for [Na(8trans)]I.

Compound [Na(8trans)]I

Empirical formula C24H30IN2NaO7

Formula weight 608.39Crystal system TriclinicSpace group P1Unit cell dimensions

a, Å 8.4253(3)b, Å 10.4563(3)c, Å 15.6545(5)˛,◦ 88.582(3)ˇ,◦ 87.169(3)� ,◦ 70.496(3)V, Å3 1298.35(8)Z 2Dcalc, Mg/m3 1.556�, mm−1 1.30F(0 0 0) 616

� range for data collection,◦ 2.4–28.5Reflections collected/unique 18,772/5644

Linear response range [log a] −5 to −1 −4.5 to −1 −4.6 to −1Detection limit [log a] −5.5 −4.8 −5.1

Slope (mV/dec) 58.4 60.2 58.7

ensors, i.e. linear response range, detection limit and slope areollected in Table 3.

The obtained potentiometric selectivity coefficient valuesog KNa,X and log KK,X are shown in Fig. 11.

Generally, the obtained results for all solid state and classic elec-rodes are comparable. Among 13-membered azobenzocrowns theest selectivity coefficient log KNa,K = −2.44 was found for micro-abricated sensor with compound 15 (with oxybutyramide residue)s ionophore. In a case of electrodes with 16-membered azoben-ocrown ethers as ion-carriers no clear relationship between theature of a side chain and the selectivity coefficient can be drawnut. Typical value is log KK,Na = −3.30.

Compounds 8 and 10 were also used as membrane com-onents for a glassy carbon (GC) electrodes [56], but in thisase bis(2-ethylhexyl)sebacate (DOS) and potassium tetrakis[3,5-is-trifluoromethyl)phenyl] borate as a lipophilic salt wereomponents of the membrane.

.5. X-ray structure of ligand 8 sodium complex

Good quality single crystals were obtained and it was foundy X-ray study, that analogously to parent compound B, azoben-ocrown 8 forms with sodium iodide complex of stoichiometry

:1. Complex of the composition [Na(8trans)]I crystallizes in thericlinic space group P1. Details of data collection and structureefinement are given in Table 4. The content of the asymmetricnit is shown in Fig. 12. The structure is ionic. Sodium cation

−5 to −1 −5.9 to −1 −5.2 to −1 −5.8 to −1 −6 to −1−5.2 −6.2 −5.7 −6.2 −6.357.5 58.9 57.3 56.4 55.9

Data/restraints/parameters 5644/0/317GOOF on F2 1.06Final R indices [I > 2�(I)], R1, wR2 0.036, 0.086Largest diff. peak and hole, e A−3 1.21 and −0.52

E. Wagner-Wysiecka et al. / Sensors and Actuators B 177 (2013) 913– 923 921

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.

n-like

TNtsaNaar[[taoca1fb(

Fig. 12. View of “scorpio

he Na–O distances fall within the range 2.361(2)–2.474(2) A. The(1) nitrogen atom of the macrocyclic molecule coordinates to

he sodium atom at the distance of 2.502(3) A, the Na(1)–N(2)eparation till the distal nitrogen atom being 3.469 A, the iodidenion which is completed the coordination polyhedron is at thea–I separation of 3.0143(12) A. Four oxygen and one nitrogentoms in the basal plane are coplanar within ±0.24 A, the Na(1)nd N(2) atoms are displaced from this plane at 0.77 and 0.47 A,espectively. The structure of the complex is quite similar toNa(Btrans)]I (where B = non-substituted 16-membered azocycle23] where Na–O distances are in the range 2.390–2.464, Na–N dis-ances are 2.455–2.481, Na–I separation being 3.008 A). The torsionngle C(1) N(1) N(2) C(13) equal to 179.2◦ indicates the transrientation of the aromatic substituents in 8. The ester sidehain is not involved in coordination to the metal center anddopts an extended conformation. The conformations of the

6-membered macrocycle are quite similar for 8 and B as itollows from comparison of torsion angles which differ onlyy the C8 O2 C9 C10 and C9 C10 O3 C11 torsion anglessee Table S1)

” [Na(8trans)]I complex.

Two complex molecules related by an inversion center form thedimeric units via weak H· · ·I contact, H(17)· · ·I(1)* = 3.11 A and �–�stacking interactions between the partially overlapping aromaticfragments, the centroid· · ·centroid separation being of 4.20 A. In thecrystal complexes stack along the shortest a crystallographic axis(Fig. S6).

4. Conclusions

New sensor materials: 13- and 16-membered azobenzocrownethers with aromatic amino, aromatic amide, ether–ester andether–amide residue were obtained. Spectroscopic studies showedthat not only macrocyle size, but also the nature of the substituentin benzene ring in para position to an azo group, i.e. aromaticamine (1, 2) or aromatic amide (4) strongly influences metalcation selectivity and the stability constant values of their alkali

and alkaline earth metal complexes in acetonitrile. The respec-tive values are in most cases higher than for parent compoundsA and B. 13-Membered aminoazobenzocrown shows magnesiumselective spectrophotometric response in acetonitrile. Magnesium

9 s and

costips

tAawtsib1srtfs

A

go0d

A

t

R

[

[

[

[

[

[

[

[

[

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22 E. Wagner-Wysiecka et al. / Sensor

ation recognition is manifested by color change from yellow torange and large (∼99 nm) spectral shift of the complex band. Largepectral bands separation (∼100 nm) and color change from yellowo orange or pink-red was found to be response of 1–4 for acidsn acetonitrile. Weakly fluorescent azobenzocrowns 1–4 in theirrotonated forms show characteristic red fluorescence with Stokeshift about 100 nm.

Lariat derivatives: oxyalkylesters and oxyalkylamides are bet-er material for potentiometric than spectrophotometric sensors.zobenzocrowns studied here, analogously to 13-membered alkylnd ether derivatives, are sodium selective ionophores in ISE,hereas 16-membered crowns show potassium sensitivity. Men-

ioned ion carriers may be used in membranes of miniature all solidtate type electrode. The obtained results also showed the possibil-ties of chemical connection of azobenzocrowns via ester or amideond with membrane components without loss of the selectivity.6-Membered azobenzocrown 8 forms crystalline complex withodium iodide (1:1) – similarly to reference compound B. It explainselatively large amounts of 16-membered crowns needed for elec-rode membranes preparation what favors sandwich type complexormation with potassium and in a consequence good potassiumelectivity of electrodes.

cknowledgments

E. W-W. kindly acknowledges support from Sources for Sciencerant no. N N204 137438 in years 2010–2011. Financial supportf this work from Gdansk University of Technology (DS grant no.20223/003) is gratefully acknowledged. Authors thank MS stu-ents for their experimental contribution.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.snb.2012.11.068.

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Biographies

Ewa Wagner-Wysiecka obtained her PhD in Chemistry at Gdansk University ofTechnology (Poland) in 2002, where now is working as assistant professor. The mainstream of her research is supramolecular chemistry, namely studies of the mecha-nisms of molecular recognition. Currently the topic of the studies is design, synthesisand studies of anion recognition by chromogenic and fluorescent receptors.

Tomasz Rzymowski obtained his PhD in chemistry in 2012 after being PhD studentat Chemical Faculty of Gdansk University of Technology (Poland). His PhD thesis,supervisor Prof. Elzbieta Luboch, was entitled: “Macrocycles with chromo- and/orfluoroionophoric nature.”

Mirosław Szarmach (MSc) is a PhD student at Chemical Faculty of Gdansk Univer-sity of Technology (Poland). The topic of his PhD thesis under supervising of Prof.Elzbieta Luboch covers the synthesis of macrocyclic sensor materials for potentio-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 scientificinterests cover X-ray crystallography of supramolecular and coordination com-pounds, with an emphasis on weak interactions in molecular complexes basedon the crown ethers and crownophanes; crystallography of fluorine-containingcompounds and complexes. She is leading scientific worker in Institute of AppliedPhysics of the Academy of Sciences of Moldova.

Elzbieta Luboch (PhD 1983, DSc 2007) is currently a professor at Gdansk University

of Technology (Poland). The main topics of her scientific work are connected withorganic synthesis, organic supramolecular chemistry, chemical potentiometric sen-sors and studies of correlation between structure and ionophoric properties of thesensor materials. She is also interested in pharmaceutical chemistry (identificationof drug impurities, standard substances synthesis).