Inorganica Chimica Acta 372 (2011) 126–135

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Inorganica Chimica Acta

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Calix[4]arene based molecular sensors with pyrene as fluoregenic unit: Effectof solvent in ion selectivity and colorimetric detection of fluoride

Debdeep Maity, Ashish Chakraborty, Ravi Gunupuru, Parimal Paul ⇑Analytical Science Division, Central Salt and Marine Chemicals Research Institute (Constituents of CSIR, New Delhi), G.B. Marg, Bhavnagar 364002, India

a r t i c l e i n f o

Article history:Available online 3 February 2011

Dedicated to Prof. S.S. Krishnamurthy.

Keywords:Fluorescent sensorsCalixareneIon-bindingExcimer emissionPyrene

0020-1693/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.ica.2011.01.053

⇑ Corresponding author. Tel.: +91 278 2567760; faxE-mail address: ppaul@csmcri.org (P. Paul).

a b s t r a c t

Fluorescent molecular sensors having excimer emission property have been designed and synthesisedincorporating calix[4]arene derivatives in cone and 1,3-alternate conformation as ionophore and two pyr-ene moieties at close proximity as fluorophore. They exhibit strong excimer emission around 515 nm,which is used to monitor interaction of metal ions with the ionophores. Ion-binding study of these fluo-roionophore has been investigated in acetonitrile–chloroform and THF–H2O with a wide range of cationsand anions and the recognition process is monitored by luminescence, UV–Vis and 1H NMR (for F�) spec-tral changes. The present study demonstrated profound influence of solvent in ion selectivity, in acetoni-trile–chloroform they formed complexes with Hg2+, Pb2+, Cu2+ and Ni2+, whereas in THF–H2O they exhibitselectivity only for Cu2+. In the case of anions, selectivity for only F� with color change is observed. Com-position of the complexes formed was determined from mass spectrometry and the binding constantswere determined from fluorescence titration data. The reasons for formation of excimer emission,quenching of it in presence of certain metal ions, role of solvent in selectivity and energy/electron transferprocess involved in the ion-recognition event have been discussed on the basis of experimental data.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Development of molecular sensors for detection of specific me-tal ion is an emerging area in research because of their potentialapplications in various fields of chemistry, biology and environ-mental processes [1–5]. Among various available methods, detec-tion by fluorescent technique has been widely used because ofsome distinct advantages in terms of sensitivity, selectivity, re-sponse time, in-situ monitoring, etc. [1,5]. Fluorescent molecularsensors can be constructed by the combination of an ionophore,designed for the binding of specific incoming ion, and a fluorogenicunit whose photophysical properties perturbs during the recogni-tion process to produce a measurable output signal. In designingof fluorescent sensors, an interesting approach is to attach two aro-matic fluorophores in proximity so that they are close enough(within van der Waals contact) to make interaction such as p–pstacking. Under this condition, electronic excitation of one ringcan cause an enhanced interaction with its neighbor, leading towhat is termed as excited-state dimer or excimer [5,6]. This exci-mer typically provides a broad fluorescence band with the maximaat lower energy compared to emission from monomer and thisband can be used to read out the molecular recognition process

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more conveniently [7,8]. In general, highly p-delocalized planarsystems such as pyrene and naphthyl moieties have been usedfor this purpose, they exhibited intramolecular excimer due tostrong p–p interaction between two fluorophore units and thisexcimer emission perturbed in presence of guest ions [9,11–15].It has been reported that the excimer emission is stronglyquenched upon addition of Pb2+ and Fe3+ [9,11–15] whereas signif-icant enhancement of excimer emission intensity is reported formetal ions such as In3+, K+ and Li+ and anion like F� [10–12,14].

Ionophore is an important component in designing molecularsensors, its donor atoms, conformation, size, steric hindrance etc.determines selectivity. Among various ionophores, macrocyclicligands are noted for their remarkable selectivity towards cationsmaking them excellent choices for selective binding of metal ions[1,3,16]. In this regard, calixarenes are found to be very attractivebecause modifications of calixarene gives rise to a large varietyof derivatives with various functional groups, which provide ahighly preorganized architecture for the assembling of convergingbinding sites [5,17–19,20].

With a view to develop calixarene based fluoroionophores withexcimer emission, two compounds have been synthesised incorpo-rating pyrene moieties at close proximity and with variation inconformation in the calixarene unit. Here we report synthesisand characterization of these molecules and their ion-bindingproperty with a large number of cations and anions, effect of sol-vents and conformation of the ionophores on ion-selectivity, and

D. Maity et al. / Inorganica Chimica Acta 372 (2011) 126–135 127

binding constants determined from fluorescence titration data. Allof these results have been discussed in light of ion-selectivity and en-ergy/electron transfer process involved in the ion-recognition event.

2. Experimental

2.1. Reagents and materials

Chemicals used in this study were purchased from Aldrich andS.D. Fine Chemicals. All metal perchlorate salts were purchasedfrom Alfa Aesar (Johnson Matthey Company). All organic solventswere analytical grade and were used as received for syntheticpurpose. Solvents for spectral studies were freshly purified bystandard procedures before use. The starting compounds A and Dwere synthesized following the literature procedures [21,22].

2.2. Physical measurements

Elemental analysis (C, H, and N) was performed on a model2400 Perkin–Elmer elemental analyzer. Mass spectra wererecorded on a Q-TOF MicroTM LC–MS instrument. IR spectra wererecorded on a Perkin–Elmer spectrum GX FT-IR spectrometer asKBr pellets. NMR spectra were recorded on an Avance II 500Brucker FT-NMR instrument. The UV–Vis spectra were recordedon a CARY 500 Scan UV–Vis–NIR spectrophotometer (Varian).Luminescence spectra were recorded on a model Fluorolog HoribaJobin Yvon spectrofluorimeter at room temperature.

Caution: Perchlorate salts of metal ions are potentially explo-sive. Therefore, they should be handled with great care.

2.3. Synthesis of ionophores

2.3.1. Synthesis of the intermediate compound BIn a solution of p-tert-butylcalix[4]arene (A) (1.30 g, 2 mmol) in

freshly distilled acetone (80 mL), methylbromo acetate (0.76 g,5 mmol) and K2CO3 (0.35 g, 2.5 mmol) were added and the reactionmixture was heated at reflux for 20 h under inert atmosphere. Thesolution was then allowed to cool to room temperature and evap-orated to dryness by rotary evaporation. The residue was then trit-urated three times with methanol (25 mL each time) and filteredoff the compound as white solid, which dried in high vacuum forovernight. Yield: 1.12 g, (71 %). 1H NMR (500 MHz, CDCl3):d = 7.03 (s, 4H, Ar-Hm), 6.98 (s, 2H, Ar-OH), 6.81 (s, 4H, Ar-Hm),4.73 (s, 4H, –OCH2CO), 4.43 (d, 4H, J = 13.2 Hz, ArCH2Ar), 3.85 (s,6H, –OCH3), 3.32 (d, 4H, J = 13.2 Hz, ArCH2Ar), 1.27 (s, 18H, –C(CH3)3), 0.97 (s, 18H, –C(CH3)3) ppm. MS: m/z (%) 831.31 (100%)[B+K+]+ (calc. 832.13). IR (KBr pellet): m = 3435 (OH), 1759 cm�1

(C@O). Anal. Calc. for C50H64O8 (793.04): C, 75.72; H, 8.13. Found:C, 74.89; H, 7.95 %.

2.3.2. Synthesis of the intermediate compound CA mixture of compound B (0.79 g, 1 mmol) and 2 mL 80% hydra-

zine hydrate solution was refluxed in chloroform/methanol (1:3,60 mL) for 12 h. To this solution, upon addition of water (1 mL) awhite precipitate was formed. It was filtered-off and washed withwater and methanol. The white product was then kept in high vac-uum over night. Yeild: 0.71 g (90 %). 1H NMR (500 MHz, CDCl3):d = 9.69 (s, 2H, –OH), 7.77 (s, 2H, –CONH), 7.08 (s, 4H , Ar-Hm),6.94 (s, 4H, Ar-Hm), 4.64 (s, 4H, J = 7.0 Hz, –OCH2CO), 4.31 (s, 4H,–NH2), 4.12 (d, 4H, J = 6.4 Hz, ArCH2Ar), 3.44 (d, 4H, J = 6.4 Hz,ArCH2Ar), 1.27 (s, 18H, C(CH3)3), 1.04 (s, 18H, C(CH3)3) ppm. MS:m/z (%) 793.19 (100%) (B+H+]+ (calc. 794.04), 816.19 (60%) [B+Na+]+

(calc. 816.18). IR (KBr pellet): m = 1654 (C@O), 3310 cm�1 (NH). Anal.Calc. for C48H64N4O6 (793.04): C, 72.70; H, 8.13; N, 7.06. Found: C,72.28; H, 7.98; N, 6.92%.

2.3.3. Synthesis of 1A solution of pyrenecarboxyldehyde (0.79 g, 1 mmol) in dry tol-

uene (30 mL) was stirred at room temperature for 30 min and thencompound C was added (0.48 g, 2.1 mmol) drop wise over 15 minand stirring of the reaction mixture was continued for 24 h. Thecompound, which was precipitated during stirring, was filteredoff, washed with ethanol and water, and recrystalised fromCHCl3/CH3CN (10:1). Yield 0.61 g (50%). 1H NMR (500 MHz, CDCl3):d = 12.59 (s, 2H, -OH), 9.25 (s, 2H, –N@CH), 9.11 (s, 2H, –CONH),8.84 (d, 2H, J = 9.5 Hz, pyrene-H), 7.93 (d, 2H, J = 6.0 Hz, pyrene-H), 7.86 (d, 2H, J = 9.0 Hz, pyrene-H), 7.69 (m, 2H, pyrene-H), 7.62(d, 2H, J = 8.5 Hz, pyrene-H), 7.53 (t, 2H, J = 7.3 Hz, pyrene-H),7.41 (m, 4H, pyrene-H), 7.31 (m, 2H, pyrene-H), 7.24 (m, 6H, Ar-Hm), 6.95 (d, 2H, J = 9.5 Hz, Ar-Hm), 6.69 (d, 2H, J = 12.5 Hz, –OCH2-

CO), 5.23 (d, 2H, J = 12.5 Hz, –OCH2CO), 5.00 (d, 2H, J = 16.0 Hz,ArCH2Ar), 4.83 (d, 2H, J = 12.0 Hz, ArCH2Ar), 3.73 (d, 2H,J = 16.0 Hz, ArCH2Ar), 3.59 (d, 2H, J = 12.0 Hz, ArCH2Ar), 1.96 (s,18H, C(CH3)3), 1.35 (s, 18H, C(CH3)3) ppm. MS: m/z (%) 1218.89(30%) [1+H+]+ (calc. 1218.53), 1240.89 (100%) [1+Na+]+ (calc.1240.52), 1256.89 (12%) [1+K+]+ (calc. 1256.63). IR (KBr pellet):m = 1685 (C@O), 3407 cm�1 (NH). Anal. Calc. for C82H84N4O7

(1237.80 with 2H2O): C, 79.56; H, 6.84; N 4.52. Found: C, 78.66;H, 7.04; N, 4.38%.

2.3.4. Synthesis of the intermediate compound EA mixture of D (2.03 g, 4 mmol), and NaH (0.48 g , 20 mmol)

was stirred in THF under nitrogen atmosphere at room tempera-ture for 30 min. Ethyl bromoacetate (4 g, 24 mmol) in THF(50 mL) was then added slowly. The mixture was stirred at roomtemperature for 24 h and quenched with a small amount of water(added dropwise). The THF of the solution was removed by rotav-apour and the solid mass was extracted with CH2Cl2 (200 mL) and10% HCl (50 mL). The organic layer was separated, washed withwater twice (50 mL each time), dried over anhydrous Na2SO4 andthe solvent was evaporated by rotavapour. The compound thus ob-tained was purified by column chromatography using alumina aspacking material and ethyl acetate-hexane (1:9) as eluent. Yield:1.24 g (50 %). 1H NMR (500 MHz, CDCl3): d = 7.11 (d, 4H, J =7.5 Hz, Ar-Hm), 7.03 (d, 4H, J = 7.5 Hz, Ar-Hm), 6.76–6.72 (m, 4H,Ar-Hp), 4.14 (q, 4H, J = 7.0 Hz, –OCH2CH3), 3.92 (d, 4H, J =15.0 Hz, ArCH2Ar), 3.70 (d, 4H, J = 15.0 Hz, ArCH2Ar), 3.58 (s, 4H,–OCH2CO–), 3.54 (t, 4H, J = 7.5 Hz, –OCH2CH2CH3), 1.51–1.43 (m,4H, –OCH2CH2CH3), 1.26 (t, 6H, J = 7.25 Hz, –OCH2CH3), 0.86 (t,6H, J = 7.25 Hz, -OCH2CH2CH3). MS: m/z (%) 703.47 (100%) [E +Na+]+ (calc. 703.81), 719.45 (95%) [E+ K+]+ (calc. 719.92). IR (KBrpellet): m = 1756 cm�1 (C@O). Anal. Calc. for C42H48O8 (680.82): C,74.09; H, 7.10. Found: C, 73.57; H, 7.14%.

2.3.5. Synthesis of the intermediate compound FThis compound was synthesised following the same procedure

as described for C. Yield (80 %). 1H NMR (500 MHz, CDCl3): d = 7.06(d, 4H, J = 7.4 Hz, Ar-Hm), 7.00 (d, 4H, J = 7.4 Hz, Ar-Hm), 6.83–6.91(m, 4H, Ar-Hp), 4.12 (s, 4H, –OCH2CO), 4.06 (s, 4H, –NH2) 3.77 (m,8H, ArCH2Ar), 3.46 (t, 4H, J = 7.4 Hz, –OCH2CH2CH3), 1.34 (m, 4H,–OCH2CH2CH3), 0.72 (t, 6H, –OCH2CH2CH3). MS: m/z (%) 653.63(35%) [ F+H+]+ (calc. 653.77), 675.54 (100%) [F+ Na+]+ (calc.675.76), 691.60 (40%) [F+ K+]+ (calc. 691.88). IR (KBr pellet):m = 3387, 3310, 3224 (NH2, NH), 1654 cm�1 (C@O). Anal. Calc. forC38H44N4O6 (652.77): C, 69.70; H, 7.08; N, 8.56. Found: C, 68.93;H, 7.27; N 8.38%.

2.3.6. Synthesis of 2A solution of 1-pyrenecarboxyldehyde (0.56 g, 2.2 mmol) was

stirred at room temperature in dry ethanol (30 mL) for 30 minand then compound F (0.65 g, 1.0 mmol) was added dropwise over15 min. The reaction mixture was then refluxed for 24 h. The

128 D. Maity et al. / Inorganica Chimica Acta 372 (2011) 126–135

microcrystalline compound, which precipitated during reflux, wasfiltered off and washed with ethanol and acetonitrile and dried invacuum. Yield (65 %). 1H NMR (500 MHz, CDCl3): d = 9.73 (s, 2H, –N@CH), 9.29 (s, 2H, –CONH), 8.23 (m, 4H, pyrene-H), 7.95–7.90 (m,2H, pyrene-H) 7.78–7.82 (m, 4H, pyrne-H), 7.49–7.69 (m, 8H, pyr-ne-H), 7.23 (d, 4H, J = 7.8 Hz, Ar-Hm), 7.18 (d, 4H, J = 7.8 Hz, Ar-Hm), 6.88–7.00 (m, 4H, Ar-Hp), 4.09 (s, 4H, –OCH2CO–), 4.08–3.88(m, 8H, ArCH2Ar), 3.64 (t, 4H, J = 7.5 Hz, –OCH2CH2CH3), 1.52 (m,4H, –OCH2CH2CH3), 0.88 (t, 6H, J = 7.5 Hz, –OCH2CH2CH3). MS:m/z (%) 1099.72 (100%) [2+ Na+]+ (calc. 1099.25). IR (KBr pellet):m = 3383 (NH), 1687 cm�1 (C@O). Anal . Calc. for C72H60N6O4

(1078.47): C, 80.12; H, 5.61; N, 5.20; Found: C, 79.40; H, 5.87; N,5.23%.

2.4. Ion-binding property

2.4.1. Luminescence methodStock solutions of the ionophores 1 and 2 (2 � 10�6 M) and that

of the perchlorate salts of various cations (Li+, Na+, K+, Cs+, Mg2+,Ca2+, Ba2+, Sr2+ Zn2+, Cd2+, Hg2+, Pb2+, Cu2+, and Ni2+) were preparedin freshly purified acetonitrile–chloroform (9:1). Then 2 mL stocksolution of the ionophore and 2 mL stock solution of each anionsalt were taken in a 5 mL volumetric flask, so that the effective con-centration of the ionophore is 1 � 10�6 M and that of the anionsare 1 � 10�4 M (100-fold). The luminescence spectra of the result-ing solutions and that of the original ionophores (1 � 10�6 M) wererecorded with excitation at the absorption maxima (kmax) at 378and 362 nm for 1 and 2, respectively. The spectra of the metalion containing reaction mixtures were compared with that of theoriginal solution to ascertain the interactions of the cations withthe ionophore. These experiments were repeated using THF–water(8:2) as solvent. Similar experiments were also carried out withtetrabutyl ammonium salts (2 � 10�4 M) of various anions (F�,Cl�, Br�, I�, H2PO4

�, ClO4� , NO3

�, BF4�, CH3COO�, and HSO4

�) infreshly purified acetonitrile.

2.4.2. Emission titrationCations and anions, which exhibited substantial changes in

emission intensity, were considered for emission titration to eval-uate binding constant. For emission titration study, the same stocksolutions of the ionophores were used and the solutions of the per-chlorate salt of the cations of desired concentration were preparedby proper dilution of the stock solution. Then 2 mL of each solutionwere mixed in a 5 mL volumetric flask to prepare reaction mixturewith 0.1–100 M equivalent of the concentration of metal ion withrespect to the concentration of ionophore and the luminescencespectra of the resulting solutions were recorded. Binding constantswere calculated following the literature procedure as described be-low in the Results and Discussion Section [18,23–25].

2.4.3. Absorption methodThe UV–Vis spectra of all the solutions containing cations and

anions (100 equivalent) were recorded and the same were com-pared to that of the original solutions to ascertain the chromogenicresponse of the ionophores in presence of various ions. A few ionsexhibited significant changes in absorption spectra with detectablecolor changes.

2.4.4. NMR methodTo investigate the interaction of anions with fluoroionophores,

1H NMR spectroscopic study was carried out in chloroform. The1H NMR spectra of the solutions containing 2 mg of ionophores,dissolved in 0.5 mL of solvent (CDCl3), were recorded. The solutionsof anions were then added with increasing concentration to makeit a maximum of 6 M equivalent of the concentration of ionophoreand the corresponding 1H NMR spectra were recorded. The spectra

in presence and in absence of anions were compared to assess theinteractions between anions and the ionophores.

2.5. Mass spectrometry

Mass spectroscopic study of the complex formation was carriedout in CH3CN–CHCl3 (9:1) solution. The solution of the ionophores(3 mL, 3.5 � 10�5 M) of each compound was taken in a 5 mL volu-metric flask and 10 equivalent of metal percholorate salt wasadded into this solution and the mass spectra of the resulting solu-tions were recorded.

MS data for 1: For Hg2+, m/z 1416.97 [1�2H++Hg2++H+]+ (calc.1417.42); for Pb2+, m/z 1525.48 [1+Pb2++ClO4

�]+ (calc. 1524.51),1423.80 [1�2H++Pb2++H+]+ (calc. 1424.03), 712.51 [1+Pb2+]2+ (calc.712.50); for Cu2+, 1280.64 [1�2H++Cu2++H+]+ (calc. 1280.37),1280.64 [1�2H++Cu2++H+]+ (calc. 1280.37), 1442.10 [1�2H++2Cu2++ClO 4

�]+ (calc. 1442.41); for Ni2+, m/z 1274.90 [1�2H++Ni2++H+]+ (calc.1274.90); for F�, 1458.79 [1�2H++TBA+]- (calc. 1458.39), 1699.89 [1�3H++(TBA+)2]- (calc. 1699.56).

MS data for 2: For Hg2+, m/z 1376.46 [2+Hg2++ClO4�]+ (calc.

1377.56); for Cu2+, 1141.08 [2�2H++Cu2++H+]+ (calc. 1141.01); forNi2+, m/z 1136.00 [2�2H++Ni2++H+]+ (calc. 1136.16); for F�,1318.86 [2�2H++TBA+]- (calc. 1318.97).

3. Results and discussion

3.1. Synthesis of compounds 1 and 2

The route followed for the synthesis of 1 and 2 is shown inScheme 1 and the details of experimental procedures are given inthe Section 2. The compounds 1 and 2 were obtained from theirpreceding intermediates C and E, respectively by the reaction withpyrene carboxylaldehyde. The crude products were purified byrecrystalization from acetonitrile–chloroform for 1 and from etha-nol for 2. Elemental analysis (C, H and N), IR, mass and 1H NMRspectral data for 1 and 2 and also for all intermediate compoundsare given in the Section 2. The elemental analysis and mass dataare in excellent agreement with the calculated values. It may benoted that the m/z values of all these compounds correspond tothe H+/Na+/K+ adduct, which is a well known phenomenon whenESMS is used for the determination of mass [17–19,25]. Detailsfor characterization of 1 and 2 are given below.

The IR spectra exhibited bands of moderate intensity at 3407and 3383 cm�1 for 1 and 2, respectively, which are assigned tom(N–H) of the amide moiety. The strong bands at 1685,1687 cm�1 for 1 and 2, respectively are due to m(C@O). The 1HNMR spectrum of 1, exhibited singlets for OH, CH and NH protonsat d = 12.59, 9.25 and 9.11 ppm, respectively. In the aromatic re-gion eight doublets and a triplet appeared for the two chemicallyequivalent 1-pyrene moieties. The meta-protons (with respect tothe OH substituent) of the calixarene moiety appear as a multipletfor six protons and a doublet for two protons. For a cone conforma-tion, two singlets of equal intensity are expected. The unusualbehavior is probably due to the encapsulation of solvent mole-cule(s) in the upper rim of the calix cavity, which observed in manyoccasions [26,27]. The solvent molecule makes interaction with thebenzene rings making the protons attached with it chemically non-equivalent. Support of this statement is noted during recording of1H NMR spectra. In CDCl3, the compound was not completely sol-uble; however upon addition of a drop of CD3CN, a clear solution ofthe compound was obtained. In CD3CN, the compound is poorlysoluble, which indicates that the acetonitrile molecule interactswith the compound making an adduct, which is highly soluble inchloroform. Due to this solvent inclusion into the calix cavity, thesignal for methylene protons (ArCH2Ar), which expected to be

Scheme 1. Route followed for the synthesis of compound 1 and 2.

D. Maity et al. / Inorganica Chimica Acta 372 (2011) 126–135 129

two doublets, is further splitted and appeared like a doublet ofdoublet. The OCH2 protons are also appeared as two doublets.The tert-butyl protons of 1 appeared as singlets at d = 1.96 and1.35 ppm with equal intensity.

For compound 2, the signals due to CH and NH appeared assinglets at d = 9.73 and 9.29 ppm, respectively. The signals due topyrene moieties are similar to that of compound 1, except a fewoverlap of signals. The protons at the m-positions of the benzenerings of the calix moiety appeared as two doublets, and the protonsat the p-positions appeared as two overlapped triplets. In the ali-phatic region, the OCH2 protons appeared as a singlet atd = 4.09 ppm and the ArCH2Ar protons appeared as a multiplet,which is the characteristic signal for 1,3-alternate conformationof the calixarene unit [19,28,29]. The solvent inclusion phenome-non is not noted here possibly due to 1,3-alternate conformationof the calixarene moiety. The protons of the propyl groups ap-peared at d = 3.64 as triplet, at d = 1.52 as multiplet and atd = 0.88 ppm as triplet. The 1H NMR data for both the compoundsare, therefore, consistent to the proposed structures for 1 and 2,as shown in Scheme 1.

ig. 1. Luminescence spectra of 1 and 2 (2.8 � 10�6 M) recorded in acetonitrile–chloroform (9:1).

3.2. Absorption and luminescence

Absorption and emission spectra of 1 and 2 were recorded inboth actonitrile–chloroform (9:1) and THF–H2O (4:1) at room tem-perature. Compound 1 exhibited absorption bands at 397, 378, 366and 288 nm in actonitrile–chloroform. Compound 2 also exhibitedfour absorption bands at 397, 379, 365 and 297 nm in same sol-vent. No significant change in absorption maxima is noted whenthe spectra were recorded in THF–H2O. Among these absorptionbands, the first three are due to conjugated pyrene moiety and

the band at 288 for 1 and 297 for 2 are due to calix[4]arene moiety[10,30]. The difference of 9 nm is due to different conformation ofthe calixarene unit in 1 and 2. On excitation at 365 nm for both 1and 2 in actonitrile–chloroform, strong emission bands appearedat around 407, 427 and 517 nm for 1 and 408, 431 and 512 nmfor 2 (Fig. 1), respectively. There is no significant change in emis-sion maxima in THF–H2O (4:1) for both the compounds. The lowenergy band (517 and 512 nm) are assigned to the excimer emis-sion, which is originated because of the p-stacking of the closelyspaced two pyrene moieties with face to face orientations [9,11–

F

Fig. 3. Luminescence spectra of 2 (1 � 10�6 M) recorded in THF–H2O (4:1) inpresence of various cations (100 equivalent).

130 D. Maity et al. / Inorganica Chimica Acta 372 (2011) 126–135

15]. The high energy two bands are due to emissions from mono-mer [9,14]. The relative intensity ratio of excimer to monomer(Iex/Im) are 1:1. and 3.5:1 for 1 and 2, respectively. The higher valueof Iex/Im for 2 than that of 1 in the same solvent is presumably dueto the changes in conformation of the calixarene moiety, the 1,3-alternate conformation in 2 makes less steric crowding at the low-er rim of the calixarene unit, which allows two pyrene rings tocome closer to make strong p- p stacking interaction.

3.3. Ion-binding property

The ion-binding property of the fluoroionophores 1 and 2 havebeen investigated with a large number of cations (Li+, Na+, K+, Cs+,Mg2+, Ca2+, Sr2+, Ba2+, Zn2+, Cd2+, Hg2+, Cu2+, Ni2+, and Pb2+) as theirperchlorate salts and anions (F�, Cl�, Br�, I�, HSO4

�, ACO�, H2PO4�,

ClO4�, NO3

� and BF4�) as their tetrabutyl ammonium salts. The

ion-recognition process was monitored by luminescence, UV–Visand 1H NMR spectral changes. The composition of the complexesformed was confirmed by ESMS study and the binding constantswere calculated from the fluorescence titration data.

3.3.1. Fluorescence studyThe fluorescence spectra of the compounds were recorded in

actonitrile–chloroform (9:1) and THF–H2O (4:1) in presence of100-fold excess of various metal ions and the spectra thus obtainedwere compared with that recorded in absence of metal ions undersimilar experimental conditions to ascertain the complexing abilityof the fluoroionophores. In actonitrile–chloroform, compound 1exhibits significant quenching in emission intensity of both themonomer and excimer bands in presence of Hg2+, Pb2+, Cu2+, andNi2+ (Fig. 2), indicating strong complex formation with these metalions. Compound 2 in the same solvent also exhibits strong quench-ing in emission intensity in presence of same metal ions (Hg2+,Pb2+, Cu2+, and Ni2+), however Zn2+ and Na+ also show significantquenching. Interestingly, dramatic change in selectivity is observedwhen the same experiments were carried out in THF–H2O (4:1),under this condition, both the compounds exhibit strong complex-ation with Cu2+ (Fig. 3) and 1 also exhibits significant quenching inpresence of Ni2+. Other metal ions did not show any significantchanges in emission intensity. The decrease in intensities of theexcimer emission is because of the divergence orientation of thetwo pyrene moieties due to formation of complex with metal ions[9,11–15]. For 1, the monomer region is also quenched in presenceof metal ion not only due to the heavy metal ion effect but also dueto the reverse PET [12,31–33]. Interestingly, for Cu2+ ion after

Fig. 2. Luminescence spectra of 1 (1 � 10�6 M) recorded in acetonitrile–chloroform(9:1) in presence of various cations (100 equivalent).

almost complete quenching of both monomer and excimer emis-sions, the intensity of the monomer emission is further increasedsubstantially upon addition of increasing amount of metal ion(Fig. 4). This is probably due to formation of dinuclear complexas shown in Scheme 2, the coordination of the nitrogen atom of–C@N with metal ion blocked the intramolecular quenching pro-cess involving lone pair of nitrogen, which enhanced intensity ofthe monomer emission of the pyrene moiety. The formation ofdinuclear complex is supported by ESMS data (discussed below).Apparently other metal ions did not show any indication for theformation of dinuclear complex. In compound 2, the calixarenemoiety is in 1,3-alternate conformation and there is no OH, themass data suggested deprotonation of NH, on the basis of thesedata the only possible structure of the complexes is shown inScheme 2. In this case formation of dinuclear complex, like 1, isnot possible.

For anions, significant quenching in emission intensity isobserved for 1 only in presence of F- in acetonitrile–chloroform;compound 2 also exhibits considerable changes with F� undersimilar experimental conditions. However, in THF–H2O (4:1), nosignificant change in emission spectra is noted in presence of any

Fig. 4. Emission spectral changes of 1 (1 � 10�6 M) upon addition of increasingconcentration of Cu(ClO4)2. Excitation wavelength: 365 nm. The excimer (517 nm)and monomer bands (407 and 427 nm) quenched upon addition of 10 M equiva-lents, on further addition the intensity of the monomer emission band increasedwithout enhancement of the excimer band.

Scheme 2. Complexes formed after interaction with metal ions are shown.

Fig. 5. UV–Vis spectral change for 1 (1.83 � 10�5 M) recorded in acetonitrile–chloroform (9:1) in presence of various cations (100 equivalent).

D. Maity et al. / Inorganica Chimica Acta 372 (2011) 126–135 131

anions for both the compounds. The observation, therefore sug-gests that only F� out of all the anions used in this study formscomplex with the ionophores in acetonitrile–chloroform.

3.3.2. UV–Vis studyAbsorption spectra of 1 and 2 (1.83 � 10�5 M) were recorded in

presence of all the cations and anions (100 equivalent) mentionedabove in both acetonitrile–chloroform and THF–H2O. In acetoni-trile–chloroform, significant changes in absorption spectra wasnoted in presence of Hg2+ and Cu2+ with a color change from lightgreen to yellow for both the compounds. For compound 1, a newband at 440 nm is appeared with Hg2+, the 365 and 397 bandsdue to pyrene moiety are shifted to 380 and 403 nm, respectively(Fig. 5). For Cu2+, no shift in absorption maxima is noted, howeverdecrease in absorption intensity of the 365 nm band is observed.Similar change for 2 is also noted in presence of Hg2+ and Cu2+ un-der similar experimental conditions. In THF–H2O, no significantchange in absorption spectra is noted except for 1 with Hg2+, whichexhibits significant enhancement in absorption intensity for thebands due to pyrene moiety. The UV–Vis study, therefore, suggestcomplexation of both the compounds with Cu2+ and Hg2+. In thecase of anions, only F� showed a new band around 430 nm witha color change (Fig. 6). The new bands for all the cations around440 nm are due to ligand to metal charge transfer because of thecoordination of OH/NH [34]. For F�, the new band is due to intra-molecular charge transfer (ICT) because of the deprotonation ofOH [34].

3.3.3. 1H NMR studyThe 1H NMR study was carried out with F� ion only as some of

the metal ions are paramagnetic and also the perchlorate salts con-tain water, which creates problem in the 1H NMR spectra. Thespectra of 1 and 2 were recorded with the addition of increasingconcentration of tetrabutyl ammonium salt of F�. The changes in

Fig. 6. UV–Vis spectral change for 1 (1.83 � 10�5 M) recorded in acetonitrile–chloroform (9:1) in presence of various anions (100 equivalent). Inset: color changeof 1 upon addition of F� (TBAF). (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

132 D. Maity et al. / Inorganica Chimica Acta 372 (2011) 126–135

1H NMR spectra with the addition of increasing concentration of F�

are shown in Fig. 7. It may be noted that the signal for OH

1112

1

1+2eq F-

1+4eq F-

1+6eq F-

1+8eq F-

9.09.510.0

2

2 + 1e

2 + 2eq

2 + 4eb

a

Fig. 7. Selected portion of the 1H NMR spectra for 1 (a) and 2 (b) recorde

(d = 12.59) for 1 (Fig. 7a) disappeared and signals for NH for both1 (d = 9.11) and 2 (d = 9.29, Fig. 7b) initially desheilded and finallydisappeared, when 10-fold excess was added. The observation sug-gests that the F- makes interactions with OH and NH protons, asthe OH protons are highly acidic, it deprotonates first formingHF2

�/HF/H2F+ and the NH protons initially form complex makingN–H. . .F interactions and finally deprotonated upon addition of10-fold excess of F�. This statement is also supported by massspectral data, discussed below. The 1H NMR signal(s) for the spe-cies such HF2

�/HF/H2F+, however, could not be identified underthe experimental conditions.

3.3.4. Mass spectrometryIn order to find out the composition of the complexes formed,

mass spectra of 1 and 2 were recorded in acetonitrile–chloroformupon addition of excess amount of some selected metal ions andF�, the e/m values for the complexes formed are given in the Exper-imental Section. The relevant portions of the mass spectrum of 1,recorded in presence of Hg(ClO4)2 and tetrabutyl ammonium fluo-ride (TBAF) are shown in the Figs. 8 and 9. In the case of metal ions,the mass data suggest the formation of complex with general for-mula [1 or 2�2H++M2++H+]+, in other words deprotonation of OH/NH of the ionophores took place with the formation of 1:1 neutralcomplex, which exhibited signal in the positive mass spectra with

8910 ppm

8.08.5 ppm

q F-

F-

q F-

d in CD3CN–CHCl3 upon addition of increasing amount of F� (TBAF).

Fig. 8. Relevant portion of the mass spectra for 1 in presence of Hg(ClO4)2 (10equivalent) recorded in acetonitrile–chloroform (9:1).

Fig. 9. Relevant portion of the mass spectra for 1 in presence of F- (TBAF, 10equivalent) recorded in acetonitrile–chloroform (9:1).

Fig. 10. Emission spectral changes for 2 (1 � 10�6 M) in THF–H2O (4:1) uponaddition of increasing amount of Cu(ClO4)2 (0–100 equivalent). Excitation wave-length: 365 nm. Inset: linear regression fit (double-logarithmic plot) of the titrationdata as a function of concentration of metal ion.

Fig. 11. Emission spectral changes for 1 (1 � 10�6 M) in acetonitrile–chloroform(9:1) upon addition of increasing amount of Hg(ClO4)2 (0–100 equivalent).Excitation wavelength: 365 nm. Inset: linear regression fit (double-logarithmicplot) of the titration data as a function of concentration of metal ion.

D. Maity et al. / Inorganica Chimica Acta 372 (2011) 126–135 133

a H+. For 2 with Hg2+, however peak due to [2+Cu2++ClO4�]+ is also

observed. For 1 with Cu2+, however a signal is noted, which corre-sponds to a dinuclear complex, [1–2H++2Cu2++ClO4

�]+. In the caseof F-, the (negative) mass data for 1 correspond to the species[1�2H++TBA+]- and [1–3H++(TBA)2

2+]� and for 2 it is [2�2H++TBA+]-, which suggest deprotonation of the ionophores in presenceof excess F-, which is consistent to the observation noted in 1HNMR study. For 1, no detectable signal for the species with depro-tonation of four protons, [1�4H++(TBA)3

3+]�, is observed. The ob-served e/m values for all the complexes/compounds are inexcellent agreement with the calculated values.

3.3.5. SelectivityIon-binding study was carried out in acetonitrile–chloroform

(9:1) and THF–H2O (4:1) and it has been observed that solventhas profound influence in ion-selectivity. In the former case, boththe compounds formed strong complexes with Hg2+, Pb2+, Cu2+

and Ni2+ but in THF–H2O (4:1) they showed selectivity towardsCu2+. This is probably due to effective size of the solvated cations.The two pyrene moieties are stacked by p–p interaction and undersuch a situation the binding sites behaves like a macrocyclic iono-phore (Scheme 1). Therefore, formation of complex in THF–H2O de-pends on matching of the cavity size of the ionophore and effectivediameter of the first hydration shell of metal ion. Ionic diameter ofCu2+ and Ni2+ are close (0.72 and 0.69 Å, respectively) but for Hg2+

and Pb2+ it is significantly high (1.10 and 1.21 Å, respectively) [35].The coordination number for water molecules around all of thesefour metal ions is equal to six with octahedral geometry but forCu2+ due to Jahn–Teller distortion two axial water molecules areeasily removable [36,37]. Due to lower size of the metal ion andJahn–Teller effect, the size of the hydrated shell of the Cu2+ is prob-ably fits to enter the cavity to form complex. The ionic diameter ofthe Ni2+ is close to that of Cu2+ but due to lack of Jahn–Teller effect,the size of the octahedral hydrated shell may not be comparable tothat of Cu2+ and hence it showed a weak interaction with 1. The hy-drated shells of the Hg2+ and Pb2+ are large and could not enter intothe cavity for the formation of complex.

3.3.6. Binding constantBinding constants for the fluoroionophores 1 and 3 with

strongly interacting ions were determined using emission titrationdata following the literature procedure [18,23–25]. The titrationexperiments were carried out following the method described inthe Experimental Section. A few representative spectra showing

Fig. 12. Emission spectral changes for 1 (1 � 10�6 M) in acetonitrile–chloroform(9:1) upon addition of increasing amount of F� (TBAF, 0–100 equivalent). Excitationwavelength: 365 nm. Inset: linear regression fit (double-logarithmic plot) of thetitration data as a function of concentration of metal ion.

134 D. Maity et al. / Inorganica Chimica Acta 372 (2011) 126–135

the changes observed in emission intensities upon addition ofincreasing concentration of ions are shown in Figs. 10–12. Accord-ing to this procedure, the fluorescence intensity (F) scales with themetal ion concentration ([M]) through (F0 � F)/(F � F1) = ([M]/Kdiss)n. The binding constant (Ks) is obtained by plottinglog [(F0 � F)/(F � F1)] versus log [M], where F0 and F1 are the rela-tive fluorescence intensities of the complex without addition ofguest metal ion and with maximum concentration of metal ion(when no further change in emission intensity takes place), respec-tively. The value of log [M] at log [(F0 � F)/(F � F1)] = 0 gives thevalue of log (Kdiss), the reciprocal of which is the binding constant(Ks). The plots log [(F0 � F)/(F � F1)] versus log [M] for selectedcompounds are shown as insets in Figs. 10–12. The titration datashowed very good linear fit (R = 0.99) with the above equation.The binding constants of all complexes are summarized in Table 1.

Analysis of the data (Table 1) showed that for 1 in acetonitrile–chloroform, the binding constant for Cu2+ is significantly higherthan that of Hg2+ and Pb2+ and it is lowest for Ni2+. The Cu2+ ionis harder acid compared to Hg2+ and Pb2+ and prefers to bind oxy-gen donor atoms, which promotes formation of stable complex

Table 1Emission maxima and binding constant (Ks) for compounds 1 and 2 in CH3CN–CHCl3

(9:1) and THF–H2O (4:1).

Compound Solvent Emission maxima(kem)

Metalion

Bindingconstant (Ks)(M�1)

Excimer Monomer

1CH3CN–CHCl3 (9:1)

517 407, 427 Cu2+ 2.88 � 105

Hg2+ 5.4 � 104

Pb2+ 6.0 � 104

Ni2+ 5.6 � 103

THF–H2O(4:1)

517 407, 427 Cu2+ 3.98 � 105

2CH3CN–CHCl3 (9:1)

512 408, 431 Cu2+ 3.3 � 104

Hg2+ 1.8 � 104

Pb2+ 4.6 � 104

Ni2+ 4.9 � 104

Na+ 1.8 � 103

THF–H2O(4:1)

512 408, 431 Cu2+ 4.25 � 104

with four oxygen donor atoms, C@O and deprotonated OH (Scheme2), as evident from mass spectra and new absorption band dis-cussed above. For compound 2, the binding constants with allthe four cations, Hg2+, Pb2+, Cu2+ and Ni2+ are similar, Cu2+ has lostpreferential binding as there is no OH and complexation took placewith deprotonation of less acidic NH protons. The C@O also cannotbind with metal ion because with deprotonated NH only substi-tuted phenolic oxygen atoms can bind with cation. It is observedthat the propyl substituted 1,3-alternate conformation of calixa-rene containing amide moiety binds alkali metal ions, in this casethe metal ion probably binds at the lower rim of the calixarenemoiety with two substituted phenolic oxygen atoms, oxygenatoms of the amide moieties and also making interaction withthe p-cloud of the inverted benzene rings [38]. In THF–H2O, thebinding constant for 1 and 2 with Cu2+ is slightly higher than thatobserved in acetonitrile–chloroform, the difference is not so signif-icant to make any comment on it.

4. Conclusions

Calixarene based fluorescent receptors having excimer emissionproperty have been designed and synthesised incorporating pyreneas fluoregenic unit. These compounds exhibit strong excimer emis-sion due to face to face p–p stacking of the pyrene moieties. Ion-binding property of these receptors was studied with a wide rangeof cations and anions in different solvents. In presence of Hg2+,Pb2+, Cu2+ and Ni2+, which formed strong complex in acetoni-trile–chloroform, the intensity of the excimer emission decreaseddue to break down of the p–p stacking as the pyrene moieties sep-arated from each other because of complex formation. The excimeremission band has been used to monitor ion recognition processand for the determination of binding constant using emission titra-tion data. Deprotonation of OH (1) and NH (2) occurred duringcomplex formation. Dramatic change in selectivity is observedwhen binding constant study was carried out in THF–H2O. The me-tal ion is hydrated and the effective size of the first hydration shellof metal ion with six water molecules played crucial role in deter-mining the selectivity. The size of the hydrated shell of the Cu2+ ionprobably fits to enter the ionophore cavity formed by stacked pyr-ene moieties to form complex. The large size of the hydrated shellof the bigger cations could not enter the cavity to form complex.Moderate to strong binding constants for both the compoundswere obtained in two different solvent systems. Both the iono-phores exhibit strong interaction with F- with sharp color change,out of a large number of anions used in this study.

Acknowledgements

We are grateful to the Department of Science and Technology(DST), Government of India, for financial support. We thank CSIR,New Delhi for generous support towards infrastructures and corecompetency development. D.M. gratefully acknowledges the CSIRfor awarding Fellowship (CSIR-JRF). We thank Mr. A.K. Das, Mr.V.P. Boricha and Mr. V. Agrawal for recording mass, NMR, IR andspectra, respectively.

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