Synthesis of near-infrared absorbing pyrylium-squaraine dye forselective detection of Hg2+

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Sensors and Actuators B 188 (2013) 847– 856

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l h om epage: www.elsev ier .com/ locat e/snb

ynthesis of near-infrared absorbing pyrylium-squaraine dye forelective detection of Hg2+

oddu Ananda Rao, Hyungjoo Kim, Young-A. Son ∗

epartment of Advanced Organic Materials Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea

r t i c l e i n f o

rticle history:eceived 1 March 2013eceived in revised form 22 July 2013ccepted 22 July 2013vailable online xxx

eywords:yrylium-squaraine dye

a b s t r a c t

A chemosensor based on symmetrical squaraine dye (SQ) absorbing in the near-infrared region(690–870 nm), is synthesized with quantitative yields by a reaction between squaric acid and 4-methyl2,6-di-t-butyl pyrylium trifluoromethanesulfonate, and its sensing behavior toward various metal ionshas been investigated using UV–visible spectroscopy and fluorescence spectroscopy. SQ shows the highlyselective detection behavior toward Hg2+ in comparison with various other metal ions such as Co2+, Mg2+,Pb2+, Zn2+, Cs+, Ag+, Ni2+, Li+, K+, and Na+ due to the soft acid nature and the size of mercuric ion. Theselective detection of Hg2+ with the symmetrical-squaraine unit gave rise to a significant bathochromic

hemosensorolorimetrication sensingg2+ ion binding

shift toward the NIR region (�max 697–840 nm in CH3CN). The absorption studies also indicate a highaffinity of Hg2+ toward the formation of 2:1 complexes, which is in well understanding with the obser-vational consequences. Fluorescence studies experimentally proved that only 640 nm absorption peakis responsible for the emission peak at 683 nm. A recognition mechanism based on the binding mode isproposed by means of absorption changes, 1H NMR titration experiments, FT-IR study and theoreticalcalculations.

. Introduction

Interesting materials in near infrared absorbing (NIR) dye types,amely squaraines (SQ) are well-known functional dyes and thesere firstly reported about 30 years ago [1]. SQ dyes are havingnique optical properties such as intense light absorption, deeplyolored and efficient fluorescence emission in the visible-to-NIRegion [2,3]. SQ dyes are known to have atypical resonance beingtabilized by zwitterionic structures, which have strong absorp-ion (ε > 105 L mol−1 cm−1) in the visible region and shown to behotostable. They show sharp and intense absorption bands fromhe visible to NIR wavelengths due to the donor–acceptor–donorD–A–D) type of charge transfer system and the extensively con-ugated molecular structure: acting as electron acceptor with theentral electron deficient cyclobutene ring (C4O2) and donor withromatic/heterocyclic groups (N,N-dialkylanilines, benzothiazoles,henols, azulenes and pyrroles) at both ends [4–6]. Theoreticaltudies have proposed that SQ dyes show substantial bond delo-alization and resonance-stabilized zwitterionic structure [7] as

hown in (Scheme 1). Varying the aromatic/heterocyclic groupsllows us to tune the optical and electronic properties of SQ dyes.bsorption properties of these SQ-based NIR dyes around 700 nm

∗ Corresponding author. Tel.: +82 42 821 6620; fax: +82 42 821 8870.E-mail address: [email protected] (Y.-A. Son).

925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2013.07.073

© 2013 Elsevier B.V. All rights reserved.

have generated considerable interests and their excellent photo-physical properties make them highly suitable for a wide range ofconcerns in the fields of optochemical areas [8].

SQ dyes are extensively used in sensor design [9–13],photodynamic therapy [14–19], photovoltaics [20–23], photoconducting devices [24], imaging process [25,26], conjugatedpolymers [27–29], nonlinear optics [30–37], optical recordings[24,38,39], solar energy conversions [40–43], electrophotography[44], organic light-emitting diodes [45,46], supramolecular archi-tectures [47–50] photosensitizers in dye-sensitized solar cells[51–55] and so on. Therefore, in the past several decades, squarainechemistry has been at the central stage of research from both fun-damental and technological points of view. Previous reports onthe synthesis and properties of SQ chemosensors, sensitivity andselectivity to a specific analyst remain as a big challenge in thedevelopment of the sensor studies. Due to the significant electron-deficiency and the favorable optical property make them especiallysuitable for the design of chemosensor applications (especiallymetal ions and anions) [56–65].

Due to higher toxicity to the mercury (Hg2+) exposure andits compounds, great interest has been devoted to the develop-ment of chemosensors [66–75] for the detection toward Hg2+

and mercury salts with sufficient sensitivity and selectivity in theenvironmental circumstance. Even low concentration of Hg2+ ionscan give rise to a wide variety of diseases and consequently itleads to many health problems such as central nervous system

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Scheme 1. Possible electroni

efects, arrhythmia, cardiomyopathy, and kidney damage [76–78].n recent years, many studies and several techniques have beenntroduced to develop various probes for the detection of Hg2+ andts related compounds [79–81] using inexpensive methods. Evenhough unlimited achievements in the field of calorimetric and/orhemosensors for mercury have been obtained, still there are greatemands to design and develop new indicators, especially withigh efficiency in the visible and NIR region: having specific selec-ivity toward Hg2+ ions over other competitive metal ions [82–86].

In our earlier study, we have incessantly worked with theynthesis and determination of the optical properties of new rho-amine derivatives as chemosensors, which can potentially betilized as a useful chromogens for the selective and quantitativeetection of metal ions to the biological and environmental appli-ations [87,88]. In addition, we have recently reported the synthesis89] and binding properties of SQ dye chemosensor which exhib-ted high selectivity for cyanide anion [90] and the cation sensingroperties, especially showing highly selective absorption changesbout Hg2+ ions among the various metal ions [91] as presented inScheme 2).

The pertained requirements and potential applications with thislass of materials have promoted us to investigate the synthesisnd to validate the chemosensor applications using SQ deriva-ives. One of interesting approaches to synthesize NIR-SQ dyes to increase the electron donating ability of the donor groups.he stronger electron donating effect can cause a pronouncedathochromic shift in the absorption band. In this context, weave explored new 2 step synthesis method on the preparationf symmetrical pyrylium-squaraine dye [92–94] using the 2,6-di-ert-butyl-4-methyl pyrylium trifluoromethanesulfonate [95] andquaric acid as shown in (Scheme 3) with high yields (62%) as ear-ier reports. Synthesized SQ dye showed a promising chemosensorffect with high selectivity and sensitivity toward Hg2+ ions.

. Experimental

.1. Materials and instruments

All solvents and reagents (analytical grade and spectroscopicrade) were obtained commercially and used as received unlesstherwise mentioned. NMR spectra were recorded on a JNM-

L400 spectrometer operated at 400 (1H NMR) MHz and 100 (13CMR) MHz. Chemical shifts (ı values) were reported in ppm downeld from internal Me4Si (1H and 13C NMR). Mass spectra wereecorded on a JEOL MStation [JMS-700] mass spectrometer. FTIR

ctures of squaraine (SQ) dye.

spectra were recorded FTS-175C spectrometer (Bio-laboratories,Cambridge, USA). Melting points were recorded on a Bamsteadelectrothermal (UK) apparatus and are uncorrected. Elementalanalyses were performed on a Carlo Elba Model 1106 analyzer.UV–visible absorption spectra were recorded on Agilent 8453 spec-trophotometer. Fluorescence emission spectra were recorded onShimadzu RF-5301PC. The HOMO/LUMO calculation and modelingsimulation proceed with DMol3 of Material Studio 4.3.

The inorganic salts Hg(ClO4)2·XH2O, Mg(ClO4)2, CsClO4,AgClO4, Ni(ClO4)2·6H2O, KClO4, NaClO4 from Sigma–Aldrich andCo(ClO4)2·6H2O, Pb(ClO4)2·3H2O, Zn(ClO4)2·6H2O from Alfa-AesarChemical Reagent Co. were purchased.

2.2. General spectroscopic methods

Metal ions and SQ were dissolved in acetonitrile to obtain1 × 10−3 M stock solutions. Before spectroscopic measurements,the solution was freshly prepared by diluting the concentratedstock solution to the required concentration. All the experimentswere conducted at standard barometric pressure and room tem-perature.

2.3. Synthesis of 2,6-di-t-butyl-4-methyl pyryliumtrifluoromethanesulfonate (Pyr)

In a 500 mL three neck round bottom flask equipped with atemperature probe, magnetic stir bar and a nitrogen inlet adaptorwith the condenser and the condenser filled with acetone-dry ice,24 g (0.2 moles) of pivaloyl chloride and 3.7 g (0.05 moles) of anhy-drous t-butyl alcohol was loaded. The reaction mixture was heatedto 85 ◦C with stirring. Then 15 g (0.1 moles) of trifluoromethane-sulfonic acid was added with stirring during a period of 2–3 min.After the addition was completed, the temperature was raised upto 95–105 ◦C for 10 min. Later, the reaction mixture color has beenchanged to brown and then the reaction mixture was allowed tospontaneously cool to 50 ◦C and finally cooled to −10 ◦C with anice-salt bath. While adding the 100 mL of cold diethyl ether tothe reaction mixture, the precipitate was formed. After precipitateformation was filtered with Buchner funnel, collected solid waswashed with diethyl ether (3 × 100 mL) and dried over P2O5 to give

10.57 g of the title compound. (Yield 59%)

M.P = 163–165 ◦C; 1H NMR (400 MHz, CDCl3): ı 1.52 (s, 18H, 2C(CH3)3), 2.83 (s, 3H, CH3), 7.83 (s, 2H, Ar) ppm; 13C NMR (100 MHz,CDCl3): ı 24.5, 27.9, 38.7, 120.3, 176.6, 185.6 ppm.

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ye fo

2

a0ridrr

Scheme 2. Binding properties of SQ d

.4. Synthesis of symmetrical squaraine dye (SQ)

A mixture of 712 mg (0.002 moles) of pyrylium salt, squariccid 114 mg (0.001 moles) and quinoline 258 mg (0.236 mL,.002 moles) in n-butanol of 80 mL and toluene of 20 mL wasefluxed for 3–5 h with azeotropically removal of water: monitor-ng the reaction with UV–visible spectrophotometer up to complete

isappearance of pyrylium salt and squaric acid. After cooling tooom temperature, the solvent was removed by rotary evapo-ator. For the removal of excess quinoline, the precipitate was

Scheme 3. Synthesis of pyrylium salt, SQ dye and b

r CN− and Hg2+, respectively [87,88].

filtered out through a Buchner funnel and washed with diethylether (3 × 100 mL). The solid was purified with chloroform andmethanol (99:1/v:v) by column chromatography and the obtainedpure compound was 0.303 g for SQ dye. (Yield 62%).

M.P = 243–245 ◦C; IR. �max in cm−1 (in KBr): 2968, 2929, 2868,1641, 1602, 1570, 1489, 1450, 1348, 1271, 1211, 1111, 1078, 929;1H NMR (400 MHz, CDCl ): ı 1.27 (s, 18H, C(CH ) ), 1.34 (s, 18H,

3 3 3C(CH3)3), 5.81 (s, 2H, olefin), 6.20 (s, 2H, Ar), 8.65 (s, 2H, Ar) ppm;13C NMR (100 MHz, CDCl3): ı 27.8, 27.9, 36.3, 36.6, 104.3, 108.5,109.1, 149.5, 170.4, 171.3, 180.7 ppm; EI-MS (m/z): 490 [M]+ (100);

inding methods of SQ dye for Hg2+ complex.

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ig. 1. Addition of various metal cations (6.0 × 10−4 M) to the solutions of SQ dyenterpretation of the references to color in this figure legend, the reader is referred

lemental Analysis calcd (%) for C32H42O4: C 78.31, H 8.63; found: 77.98, H 8.76.

. Results and discussion

The sensing activity was primarily looked into by adding vari-us metal ions (Hg2+, Co2+, Mg2+, Pb2+, Zn2+, Cs1+, Ag1+, Ni2+, K1+,a1+) to the acetonitrile solution of SQ dye. When two equivalents

equiv.) of Hg2+ (6.0 × 10−4 M) with respect to SQ dye (3.0 × 10−4 M)as added to the solution, the sensor was replied with indication

f color changes from bluish green to blue as shown in Fig. 1.

.1. UV–visible selectivity and titration of Hg2+ with SQ

The absorption spectra of SQ dye (3.0 × 10−5 M) for the vari-us metal ions have been studied by UV–visible spectroscopy andiven in Fig. 2. We selected some metal ions, such as Hg2+, Co2+,g2+, Pb2+, Zn2+, Cs+, Ag+, Ni2+, K+ and Na+ with concentration

6.0 × 10−5 M) in acetonitrile. In the absorption spectra apart fromg2+, the other metal ions did not show any significant changes.ure SQ dye absorption at 641 and 697 nm correspondingly. Exhil-ratingly, only after the addition of Hg2+, the intensity of thebsorption spectra peaks were decreased along with bathochromichifts as 682 and 708 nm in addition a new peak was observed at

40 nm. This finding demonstrates the unique selectivity of SQ dyeoward Hg2+.

To evaluate the sensing behavior of SQ dye toward Hg2+, thebsorption spectral titration of SQ dye with Hg2+ in acetonitrile

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ig. 2. UV–visible absorption of SQ dye (3.0 × 10−5 M) in the presence of differentetal ions: SQ dye, Co2+, Mg2+, Pb2+, Zn2+, Cs1+, Ag1+, Ni2+, K1+, Na1+ (6.0 × 10−5 M)

n CH3CN.

10−4 M) in CH3CN: photo image of color changes observed with naked eye. (For web version of the article.)

was carried out. As shown in Fig. 3, upon addition of Hg2+ (up to 2.8equivalents), the intensity of original absorption band at 640 nmand 697 nm was progressively decreased. In addition a new peakat 840 nm is shown in Fig. 3 along with two isosbestic points are732 nm and 620 nm. The appearance of these isosbestic points pro-poses that a stable complex exists between SQ and two Hg2+ ions.When Hg2+ ion binding at the dilolate oxygen anion of the cen-tral four membered ring in SQ dye increases the intramolecularcharge-transfer (ICT) character in the molecular electronic transi-tion, this in turn results in the red shift in the absorption maxima.While increasing the concentration of Hg2+ from 0 to 2 equivalent,absorption peak intensity at 840 nm gradually increases and after 2equivalents (excess) the intensity of the peak was decreased (Fig. 3inset). Correspondingly, the color of pure SQ dye transformed frombluish green to blue with 2 equivalents of Hg2+ addition, after excess(more than 2 equivalents) blue color was changed to purple.

The proposed stoichiometric complex model of SQ dye withHg2+ has been determined by a Job’s plot in Fig. 4 [84]. This showsa maximum absorbance at 840 nm. When the mole fraction of Hg2+

was reached to 0.66, this was a signature of 1:2 binding formationbetween SQ dye and Hg2+ ions. For SQ dye, resonance-stabilizedzwitterionic structure was preferred. The positive charge on theHg2+ was attracted to the negative charge on dilolate oxygen anionsalong with central four-membered cyclobutene ring (C4O2). Inter-

nally delocalized +ve charge of the cyclobutene ring stabilized the−ve charge of dilolate oxygen anion. On the basis of these results,it can be concluded that the symmetrical squaraine dye could onlybind two Hg2+ ions on either side of dilolate oxygen anions, which

Fig. 3. Absorption spectra of SQ dye (3 × 10−5 M) upon the addition of Hg2+

(0–7.1 × 10−5) in CH3CN. Inset: the binding isotherms of SQ dye at 840 nm uponthe addition of Hg2+.

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Fig. 4. Job’s plot for the complex of SQ dye with Hg2+ in CH3CN.

as in consistent with the 1:2 stoichiometry between SQ dye andg2+ from Job’s plot analysis.

.2. Fluorescence selectivity and titration of Hg2+ with SQ

The above absorption results encouraged us to explore the emis-ion changes of SQ dye in the metal complex form. The absorptioneak at 641 nm has been considered for excitation to get emissionhanges in metal complex state. Absorption peak at 697 nm wasot shown with good emission changes for the SQ dye due to themall stroke shift in fluorescence. We chose same metal ions usedor the absorption studies, such as Hg2+, Co2+, Mg2+, Pb2+, Zn2+, Cs+,g+, Ni2+, K+ and Na+ with concentration (6.0 × 10−5 M) in acetoni-

rile. The assessment of the selectivity of SQ (3.0 × 10−5 M) towardg2+ studied by fluorescence spectroscopy has been given in Fig. 4.he observation established that SQ dye exhibited high selectivityunction for quantitative determining toward Hg2+ ions more thanarious other metal ions. After the addition of Hg2+ (6.0 × 10−5 M)o SQ dye, emission peak was observed at 683 nm. However, in thease of other metal ions detection, the emission behavior was sames SQ dye itself. This observation evidences that exclusively Hg2+ iselective for this prepared SQ dye (Fig. 5).

To evaluate the sensing behavior SQ dye with Hg2+ metal ion,he titration studies have been carried out for emission spectra.he emission spectra of SQ dye (3 × 10−5 M) in acetonitrile solu-ion upon addition of Hg2+ (0 equivalents to 2.8 equivalents) wereetermined and shown in Fig. 6. Initially, SQ dye does not show anmission peak at 683 nm. As the concentration of Hg2+ increased,he intensity in the emission peak of SQ at 683 nm graduallyncreased and when the Hg2+ concentration reached at 6 × 10−5 M,he emission peak intensity approximately increased to 400 folds.n this fluorescence studies, different excitation at 700 nm and10 nm were used, but all showed with merged absorption andmission peaks. In addition, at 840 nm excitation, no emissionignal was observed due to the nature of smaller HOMO–LUMOhighest occupied molecular orbital–lowest unoccupied molecularrbital) gap in molecular energy states.

.3. 1H NMR studies for complexation between SQ dye and Hg2+

To conclude the binding mode of complexation of Hg2+ metalons with SQ dye, 1H NMR titration studies were carried out in the

bsence and presence of different concentrations of Hg2+ in CD3CNolvent and the corresponding spectra were presented in Fig. 7 (A).MR studies of symmetrical SQ dye were earlier reported by De-in Huang et al. [91] in CDCl3 solvent. Herein, we analyzed pure

Fig. 5. Fluorescence spectra of SQ dye (3.0 × 10 M) in the presence of differentmetal ions: SQ dye, Co2+, Mg2+, Pb2+, Zn2+, Cs1+, Ag1+, Ni2+, K1+, Na1+ (6.0 × 10−5 M)in CH3CN. Conditions: �exc = 641 nm; slit: 5 nm/5 nm; T = 298 K.

symmetrical SQ dye in CD3CN solvent (Supplementary informationFig. S3). SQ dye contains terminal t-butyl group protons presenton the pyran ring showing as two singlets at ı 1.26 and ı 1.31respectively. Olefin protons (Ha) present nearer to four-memberedsquaraine ring are at (ı 5.73). For Hb protons (ı 6.47) and Hc protons(ı 8.63), Hc protons are more influenced by O (oxygen) atom (hydro-gen bonding of dilolate oxygen anion decreases the electron densityaround the proton and makes them to lower field). Most valuablestructural information could be extracted from the 1H NMR spectraacquired in CD3CN, as most signals of SQ dye remain as sharp afterthe addition of 1 and 2 equivalents of mercury perchlorate. Metalbinding produces pronounced spectral changes, especially in thedown field region: reflecting a stepwise coordination of at leastone and most probably two Hg2+ ions. When 1 equivalent of Hg2+

Fig. 6. Change in fluorescence intensity of SQ dye (3.0 × 10−5 M) with increasingHg2+ addition (0–6 × 10−5 M) in CH3CN. Inset: the binding isotherms of SQ dyeat 683 nm upon the addition of Hg2+. Conditions: �exc = 641 nm; slit: 5 nm/5 nm;T = 298 K.

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F 2+: thv

ro6gtpanatr

ig. 7. (A) 1H NMR spectra of SQ dye in CD3CN with increasing concentration of Hgalues of SQ, SQ-1equiv. of Hg2+ and SQ-2equiv. of Hg2+ in CD3CN.

ing. Hb Protons presented on the pyran ring away from dilolatexygen anion are slightly shifted to down filed from ı 6.47 to ı.40. Hc Protons are affected by dilolate oxygen anion with hydro-en bonding and shown in up filed shift from ı 8.63 to ı 8.39 andhis is attributed to the increase of electron density around thoserotons (decrease in strength of hydrogen bonding between diolatenion and Hc protons). After adding 2 equivalents of Hg2+, no sig-

ificant change was observed in the chemical shifts. But Ha protonsre further shown in down fielded to ı 8.02. This is a consequence ofhe increased electron deficiency in the four-membered squaraineing. Hb Protons exhibit the least up field shift at ı 6.44 and these

e mole ratios of [Hg2+] to [SQ] are (a) 0, (b) 1 and (c) 2 respectively. (B): 1H NMR ı

are less affected by Hg+2 ions. In the case of Hc protons, regular upfiled shift is observed at ı 8.38. This is due to the further decrease inthe strength of hydrogen bonding and increase of electron densityaround these protons. In the case of terminal t-butyl protons, theseare far away from the central four-membered squaraine ring. Thus,the changes in the chemical shift values are ı 0.13 to 0.16 only.All the protons of 1H NMR ı values in CD3CN are demonstrated in

Fig. 7(B). However, the peaks are considerably broadened due to theturbidity of the solution. By this titration study, it can be concludedthat two Hg2+ ions are attached to dilolate of oxygen anion and areresponsible for electron deficiency in the central four member ring.

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Fig. 8. FTIR spectra of (A) SQ alone, (B) SQ-1 e

.4. FTIR (Fourier transform infrared spectroscopy) studies foromplexation between SQ dye and Hg2+

FTIR spectra of SQ dye shows characteristic strong absorptioneak around 1640 cm−l, which is resulted from the cyclobutene,3-diolate anion moiety of the resonance stabilized zwitteri-nic form [9]. In the FTIR spectra of SQ dye do not show C Otretching vibrations at −1700 cm−1. Instead, C C stretching vibra-ions of four-membered ring appear as strong absorption bandst −1600 cm−1. Strong indication of extensive bond delocalizationn the four-membered ring of squaraine dye is resulted from the

onappearance of any C O stretching in the squaraine dye [95,96].ut in the case of metal bonded with SQ dye, we observed C Otretching absorption peaks as shown in Fig. 8. Interestingly, withhe addition of 1equivalent of Hg2+ to SQ dye, the strong intensive

of Hg2+, and (C) SQ-2 equiv. of Hg2+ complex.

new peak was appeared at 1716 cm−1. This finding confirms thebonding of metal ion with the one of the dilolate oxygen anion oncentral four member ring and in turn, which leads to the decreasein C C stretching vibrations noticed at 1625 cm−1. On the otherhand, non bonded dilolate oxygen anion (C O) shows more strongstretching vibration at (1716 cm−1), which results in more car-bonyl stretching character to the carbon and oxygen bond. Afterthe addition of 2 equiv of Hg2+ to SQ dye, C C stretching vibra-tions (1629 cm−1) of the four-membered ring do not change muchextent, but in the case of dilolate oxygen anion group, stretch-ing intensity of the peak decrease (1718 cm−1) due to the other

metal ion attached at second oxygen atom of dilolate anion. Theseresults assured that two Hg2+ ions were binded with the oxygenatoms of the dilolate anion groups of the squarylium moiety in SQdye.

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ergy l

3

tehqPebtdscsariretr8iSce

4

oowPet

Fig. 9. Electronic distributions and the HOMO/LUMO en

.5. Theoretical calculations

Finally, computational calculations have been carried outo investigate the binding mechanism and the HOMO/LUMOnergy levels of SQ itself and SQ-Metal complex (Fig. 9). Itas been simulated with Material Studio 4.3 suite which is auantum mechanical code using density functional theory (DFT).erdew–Burke–Ernzerhof (PBE) function of the generalized gradi-nt approximation (GGA) level with double numeric polarizationasis set have been used to calculate the energy level of the fron-ier molecular orbitals (MOs) [97–101]. Comparison of the electronistribution of the HOMO and LUMO states in SQ and SQ-complexhowed that electron density decreased in the LUMO levels withompared to the HOMO levels in cyclobutene ring structure ashown in Fig. 9(A) and (B). The energy gap between the HOMOnd LUMO levels of SQ is 0.963 eV and 0.108 eV with SQ-Complex,espectively. Therefore, the oxygen atom of dilolate oxygen anions bonded with 2 Hg2+ ions and electron density in the cyclobuteneing decreased. These calculation results shown, increase of thelectron deficiency in squarylium moiety of SQ dye and extendso enhance of acceptor capacity. According to the experimentalesults, SQ-complex showing bathochromic shift absorption at40 nm, when are compared with SQ absorption at 697 nm. This

s attributed to the decrease in HOMO–LUMO gap from SQ toQ-complex. The increase in the absorption wavelength is theonsequences of complex formation and is also evidenced by thexperiment with UV spectroscopy (Fig. 2).

. Conclusion

In conclusion, we have synthesized and demonstrated the usef symmetrical SQ dye as a dual-mode sensor for the detectionf Hg2+ in acetonitrile solutions. Optical properties of this SQ dye

ere investigated with various metal ions such as Hg2+, Co2+, Mg2+,

b2+, Zn2+, Cs+, Ag+, Ni2+, K+ and Na+. The results showed thenhanced sensitivity and the significant selectivity in the recogni-ion of Hg2+ over other metal ions. The stoichiometry of the complex

evels of (A) SQ alone and (B) SQ-2 equiv. Hg2+ complex.

between SQ and Hg2+ was revealed in 1:2 compositions and it alsonotice the color changes from bluish green to blue and later purplecolor. Fluorescence studies resolved that 640 nm absorption peakis responsible for the emission peak at 683 nm. 1H NMR titrationstudies proved that Hg2+ is bonded to the dilolate anion groups ofthe squarylium moiety in SQ dye. Further studies are in evolutionto understand the effect of various substituents in the recognitionand to develop SQ dye based sensors for important metal ions.

Supplementary data

The 1H NMR, 13C NMR and mass spectra of pyrylium salt and SQdye associated are available. Supplementary data associated withthis article can be found.

Acknowledgments

This research was supported by the Basic Science Research Pro-gram through the National Research Foundation of Korea (NRF)funded by the Ministry of Education, Science and Technology (Grantno. 20120008198). This research was supported by a grant from theFundamental R&D Program for core technology of materials fundedby the Ministry of Knowledge Economy, Republic of Korea.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.snb.2013.07.073.

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Biographies

Boddu Ananda Rao received his M.Sc. (organic chemistry) in 2002 and doctoraldegree at Osmania University, Hyderabad, India, under the supervision of Prof. Dr.V. Jayathirtha Rao, Deputy Director, CSIR-Indian Institute of Chemical Technology[IICT] in 2012. He became a postdoctoral fellow in the Department of AdvancedOrganic Materials and Textile System Engineering, Chugnam National University,South Korea in 2012–present. His research interest lies in the synthesis of nearinfrared absorbing dyes, Squaraine & Croconate dyes, Pyrylium salts, Iminium salts,Rhodamine & Rhodanine derivatives and their chemosensor applications for metalions in addition to anions.

Hyungjoo Kim received his B.Sc (Advanced Organic Materials and Textile SystemEngineering) in 2012 at Chungnam National University, Daejeon, South Korea, underthe supervision of Prof. Dr. Young-A. Son. He joined in the master degree, Depart-ment of Advanced Organic Materials and Textile System Engineering, ChungnamNational University, in 2012–present. His research interest lies in the synthesis ofPyrylium salts, Iminium salts, Rhodamine, Rhodanine, Coumarin derivatives andtheir chemosensor applications for metal cations, anions detection in addition toliving cell images.

Young-A. Son is a professor in the Department of Advanced Organic MaterialsEngineering, Chungnam National University. He received his doctor degree (colorchemistry) from Department of Color Chemistry, University of Leeds, United King-dom in 2001. His current research interests include luminescence organic materials,chemosensors, functional dye materials and bio-sensors.