A highly selective fluorescent chemosensor for Ag+ based on calix[4]arene with lower-rim proximal triazolylpyrenes

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Sensors

and

Actuators

B:

Chemical

jou rn a l h o m e pag e :w w w . e l s e v i e r . c o m / l o c a t e / s n b

A

highly

selective

fluorescent

chemosensor

for

Ag

+

based

on

calix[4]arene

with

lower-rim

proximal

triazolylpyrenes

Nae-Jen

Wang,

Chung-Ming

Sun

∗

,

Wen-Sheng

Chung

∗

DepartmentofAppliedChemistry,NationalChiaoTungUniversity,Hsinchu,30050,Taiwan,ROC

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received23March2012

Receivedinrevisedform7June2012 Accepted7June2012

Available online 16 June 2012 Keywords: Chemosensor Fluorescence Calix[4]arene Triazolylpyrene Click Excimer

a

b

s

t

r

a

c

t

Ligand3,acalix[4]arenewithlower-rimproximaltriazolylpyrenes,isaratiometricfluorescent chemosen-sor for Ag+ _{with higher sensitivity compared to ligand 5, a calix[4]arene with lower-rim distal}

triazolylpyrenes,andtheconventionalfluoroionophorewithnon-conjugatedtriazoleandpyrene.Ina polarproticsolvent,thebindingratiosfor3·Ag+_{wasdeterminedtobe1:1andtheassociationconstant}

wasfoundtobe7.0×103_M−1_{.ThedetectionofAg}+_{byligand3evenworksinaqueousmethanolsolution.}

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Silver ion hasbeen used in killingharmful bacteria [1] and itscomplexeshavebeenwidelyusedinelectricindustry, photo-graphicandimagingindustry,andpharmacy.However,silverion isalsoknowntohavenegativeimpactonenvironmentandhuman beings,forexample,silveroxidemaydestroytheenvironmental benign bacteriaby inhibiting theirgrowthand disturbing their reproductiveability,andsilverionsinactivatesulfhydrylenzymes andbindwithvariousmetabolites[2].Additionally,therearealso manyreportsonsilverbioaccumulationandtoxicity[3].Itshould benotedthatAg+_{hasmoderatecoordinationability,makingitquite} difficulttobediscriminatedfromotherchemicallysimilarheavy metalions.Therefore,thedesignandsynthesisofaselectiveand sensitivefluorescentchemosensorforAg+_{,preferablyaratiometric} one,isstillhighlydesirabletodate[4].

The 1,2,3-triazole unit, from the “clickchemistry” [5] of an azide and an alkyne, has been frequently used in ionophores [6,7]especiallythosecombiningpyrenesasfluoroionophores[8,9]. Pyrene is one of the most popular fluorophores in the design of a fluorescent chemosensor, becauseof its highfluorescence quantumyieldanditsmonomertoexcimeremissionisvery sen-sitivetomicroenvironmentalchange[10].Ourgroupwasoneof

∗ Correspondingauthors.Tel.:+88635131517;fax:+88635723764. E-mailaddresses:[email protected](C.-M.Sun),

[email protected],[email protected](W.-S.Chung).

thefirsttouseclickchemistryoncalix[4]arenetoconstruct sen-sorsformetalions[6c,7].Inpreviouscalixarenefluoroionophores, the triazole and pyrene (or other fluorophore) units are usu-allylinkedbyamethylene-bridgeandworkedinpolarnonprotic solvents because of the poor solubility of calixarenes in polar proticsolvents, furthermore,polarproticsolvents usually com-pete with the ligand making it inefficient in ion sensing [9]. Tworesearchgroups[9a,b]independentlyreportedthesynthesis ofcalix[4]arenewithlower-rimdistal1,2,3-triazole–CH2–pyrene units showingsimilarsensitivities toward Pb2+_{, Hg}2+_{,and Cu}2+ in CH3CN withquenched fluorescence. Their systemshowed a decreasedexcimerbutenhancedmonomeremissionstowardZn2+_. Recently,Yamatoandco-workersalsoreportedtheincorporation ofthetriazole–CH2–pyrenefluoroionophoresintothelower-rimof homooxacalix[3]arenes[9c]andthiacalix[4]arenes[9d]. Homoox-acalix[3]arenes,withlower-rimdistal triazole–CH2–pyrenes,led to quenched fluorescence in thepresence of Cu2+ _{and Hg}2+ _in CH3CN/H2O/DMSO(1000:50:1,v/v)cosolvent;however,itshowed adecreasedexcimerbutincreasedmonomeremissioninthe pres-enceofPb2+_{[9c].Interestingly,thethiacalix[4]arenes,appended} withlower-rimdistaltriazole–CH2–pyrenes,showedaquenched excimer but an enhanced monomer emission toward Ag+ _in CH3CN/CH2Cl2(1000:1,v/v)cosolvent[9d].

Herein,wereportthefirstsynthesisofcalix[4]areneswith con-jugatedtriazolylpyrenestostudytheirmetalionselectivityinpolar proticsolventand/oraqueousmethanolsolutions.Inaddition, lig-ands3and5,withproximalanddistaltriazolylpyrenesonthelower rim ofcalix[4]arenes,respectively,aresynthesized tostudythe 0925-4005/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved.

influenceoftheorientationofthefluoroionophoresonmetalions selectivity.UV–vis,fluorescentemissionandexcitationspectra,1_H NMR,andmassspectrometryareusedtostudythepossiblebinding modesofligands3and5towardmetalions.Molecularmodeling (DMol3)[11]isalsocarriedouttorationalizetheobservedspectral changes.

2. Experimental

2.1. General

Allreported yieldswere isolatedyields. Flashcolumn chro-matographywasperformedusingsilicagel(70–230mesh).1_HNMR spectrawererecordedina300MHzNMRspectrometer,usingthe CHCl3solventpeakasaninternalstandard.13CNMRspectrawere recordedat75.4MHz.Thefollowingabbreviationsareused:singlet (s);doublet(d);triplet(t);quartet(q);andmultiplet(m).Allthe 1_Hand13_{CNMRspectraarelistedinSupportingInformation.High} resolutionmassspectraweremeasuredatFABmodeorElectron ImpactbyJMS-700HRMS.Meltingpointsweremeasuredwitha YanacoMP500Dapparatusandwereuncorrected.UV–visspectra weremeasuredwithanHP-8453spectrophotometerandsolvents wereofHPLCgrades.Fluorescencespectraweremeasuredwith anAMINCO-BowmanSeries2orFluoroMax-3luminescence spec-trometerandsolventswereofHPLCgrades.

2.2. Synthesisofcompound2

2.2.1.

5,11,17,23-tetra-t-butyl-25,26-bis(O-propargyl)calix[4]arene(2)

A solution of 5,11,17,23-tetra-t-butylcalix[4]arene 1 (0.5g, 0.77mmol),propargylbromide(0.20g,1.70mmol)inDMF(20mL) andsodiumhydride(0.27g,1.92mmol)wasstirredat70◦C for 2h. ThereactionmixturewasextractedtwicewithCH2Cl2 and theorganicsolutionwasdriedoverMgSO4andthenevaporatedto givethestickycrudeproducts.Purificationbycolumn chromatog-raphyonsilicagelelutingwithethylacetate/hexane(v/v,2:3)gave paleyellowsolidcompound2(0.31g,56%);Rf=0.9;CH2Cl2/hexane (v/v,1:1);mp120–130◦C;1_{HNMR (CDCl3,300MHz)ı8.33(s,} 2H,OH),7.04–7.00(m,8H,ph-H),4.99–4.81(m,4H,OCH2),4.59 (d,J=12.8Hz,1H,Ph-CH2-Ph),4.52(d,J=13.1Hz,2H,Ph-CH2-Ph), 4.37(d, J=13.4Hz, 1H, Ph-CH2-Ph), 3.40 (d, J=13.2Hz, 2H, Ph-CH2-Ph),3.38(d,J=13.5Hz,2H,Ph-CH2-Ph),2.67(t,J=2.4Hz,2H, OCH2CC-H),1.24(s,18H,t-Bu),1.17(s,18H,t-Bu);13CNMR(CDCl3, 75.4MHz)ı150.8(Cq),148.8(Cq),147.3(Cq),142.7(Cq),133.8(Cq), 133.6(Cq),128.6(Cq),128.5(Cq),126.0(CH),125.9(CH),125.3(CH), 125.1(CH),79.5(Cq),76.0(CH),62.4(CH2),53.2(CH2),34.1(Cq), 33.8(Cq),32.6(CH2),32.3(CH2),31.5(CH3),31.2(CH3);EI–MSm/z 724(M+_{);HRMSm/zcalcd.forC50H60O4724.4492,found724.4498.}

2.3. Synthesisofcompounds3,5,and7

2.3.1. Generalproceduresforthesynthesisof3,5,and7

Asolutionof1-azidopyrene(0.25g,1.03mmol)andCuIabout (1mg,0.005mmol)wasaddedto2(0.37g,0.52mmol),4(0.37g, 0.52mmol),and6(0.19g,1.03mmol)inTHF/H2O(v/v,2:1,30mL), respectively,andtheheterogeneousmixturewerestirredat50◦C for1day.ThemixturewasextractedtwicewithCH2Cl2andallthe organiclayerswerecombined,driedoverMgSO4,andthen evap-oratedtogivethesolidcrudeproducts.Columnchromatography onsilicagelelutingwithethylacetateandhexanegavewhitesolid compounds3,5,and7in74%,72%,and79%yield,respectively.

2.3.2. 5,11,17,23-tetra-t-butyl-25,26-bis[(O-methyl)-2H-trizole-1-pyrene]calix[4]arene (3)

Thesolidwaselutedwithethylacetate/hexane(v/v,1:1)and gave3(0.45g,72%)asawhitesolid,mp201–202◦C;Rf=0.3,ethyl acetate/hexane(v/v,1:3);1_{HNMR(CDCl3,300MHz)ı9.17(s,2H,} OH),8.39(s,2H, CCHN),8.02–7.52(m,18H,pyrene-H),7.14(d, J=2.4Hz,2H,Ph-H),7.09(d,J=2.3Hz,2H,Ph-H),7.04(d,J=2.3Hz, 2H, Ph-H), 7.00 (d,J=2.3Hz, 2H, Ph-H), 5.74(d, J=11.5Hz, 2H, OCH2),5.16(d,J=11.5Hz,2H,OCH2),4.85(d,J=12.7Hz,1H, Ph-CH2-Ph),4.54(d,J=12.9Hz,2H,Ph-CH2-Ph),4.29(d,J=13.5Hz, 1H, Ph-CH2-Ph), 3.61 (d, J=12.7Hz, 1H, Ph-CH2-Ph), 3.45 (d, J =12.9Hz,2H,Ph-CH2-Ph),3.38(d,J=13.5Hz,1H,Ph-CH2-Ph),1.23 (s,18H,t-Bu),1.19(s,18H,t-Bu);13_{CNMR (CDCl} 3,75.4MHz)ı 151.4(Cq),149.1(Cq),147.3(Cq),144.1(Cq),142.7(Cq), 134.0 (Cq),133.2 (Cq),131.3(Cq), 130.6(Cq),130.0 (Cq),129.6(CH), 129.2(Cq),128.9(CH),128.3(CH),127.9(Cq),126.9(CH),126.4 (CH),126.3(CH),126.3(CH),126.2(CH),125.9(CH),125.6(CH), 125.5(CH),125.4(CH),124.7(CH),124.2(Cq),124.1(CH),123.4 (Cq),122.5(CH),120.5(CH),69.1(CH2),34.2(Cq),33.9(Cq),32.8 (CH2), 31.5(CH3), 31.4(CH2), 31.3(CH3), 29.7(CH2);MS (FAB, m/z)1211[M+_{];HRMSm/zcalcd.forC82H78N6O41210.6085,found} 1210.6100.

2.3.3. 5,11,17,23-tetra-t-butyl-25,27-bis[(O-methyl)-2H-trizole-1-pyrene]calix[4]arene (5)

Thesolidwaselutedwithethylacetate/hexane(v/v,1:1)and gave5(0.47g,74%)asawhitesolid,mp196–197◦C;Rf=0.3,ethyl acetate/hexane(v/v,1:3);1_HNMR(CDCl 3,300MHz)ı8.08(s,2H, CCHN),8.05–7.62(m,18H,pyrene-H),7.30(s,2H,OH),7.11(s,4H, Ph-H),6.87(s,4H,Ph-H), 5.32(s,4H,OCH2),4.40(d,J=13.1Hz, 4H,Ph-CH2-Ph),3.40(d,J=13.1Hz,4H,Ph-CH2-Ph),1.31(s,18H, t-Bu),0.99(s,18H,t-Bu);13_CNMR(CDCl 3,75.4MHz)ı150.5(Cq), 149.7(Cq),147.4(Cq),144.1(Cq),141.8(Cq),132.6(Cq),131.7(Cq), 130.7(Cq),130.3(Cq),129.8(Cq),129.3(CH),128.4(CH),127.8(Cq), 126.6(CH),126.4(CH),126.1(CH),125.9(CH),125.7(CH),125.3 (CH),125.2(CH),124.5(CH),123.7(Cq),122.7(CH),120.8(CH), 69.6(CH2),34.0(Cq),33.8(Cq),31.9(CH2),31.7(CH3),31.0(CH3); MS(FAB,m/z)1211(M+_{,0.45),1234([M+Na}+_{],0.27);HRMSm/z} calcd.forC82H78N6O41210.6085,found1210.6089.

2.3.4. t-Butylphenyl-(O-methyl)-2H-trizole-1-pyrene(7)

Thesolidwaselutedwithethylacetate/hexane(v/v,1:1)and gave7(0.35g,79%)asawhitesolid,mp170–171◦C;Rf=0.6,ethyl acetate/hexane(v/v,1:3),1_HNMR(CDCl 3,300MHz)ı8.32–8.06 (m,9H,pyrene-H),7.88(d,J=9.3Hz,1H,CCHN),7.38–7.35(m,2H, Ph-H),7.05–7.02(m,2H,Ph-H),5.43(s,2H,OCH2),1.32(s,9H, t-Bu);13_CNMR(CDCl 3,75.4MHz)ı156.0(Cq),144.7(Cq),144.1(Cq), 132.3(Cq),131.1(Cq),130.6(Cq),130.3(Cq),129.8(CH),129.0(CH), 127.0(CH),126.8(CH),126.4(CH),126.4(CH),126.2(Cq),126.1 (CH),125.8(CH),125.0(Cq),124.7(CH),124.1(Cq),123.4(CH), 121.1(CH),114.3(CH),62.3(CH2),34.1(Cq),31.5(CH3);MS(EI, m/z)431[M+_{];HRMSm/zcalcd.forC29H25N3O431.1998,found} 431.1999.

3. Resultsanddiscussion

3.1. Thesynthesisanddifferentiationofcompounds3,5,and7

Thesyntheticpathwaysforligands3,5,andacontrolcompound

7 areshown inScheme1,in whichthe mainfluoroionophores arecomposedof proximal(3) anddistal (5) triazolylpyrenesat thelowerrimofcalix[4]arenes.Itshouldbenotedthatthe 1,2,3-triazolylpyrenenotonlyfunctioningasacoordinationsitebutalso actingasasignalingunit.1-Azidopyrene9[12a]wasobtainedfrom thediazotizationof1-aminopyrene8,followedbyitsclickreaction withcompounds2,4[12b],and6[12c]toaffordtriazolylpyrene

Scheme1.Synthesisoffluorogeniccalix[4]arenes3,5,andcontrolcompound7.Reagentsandconditions:(i)propargylbromide,sodiumhydride,DMF,70◦_C,2h;(ii)

1-azidopyrene,CuI,THF/H2O,50◦C,1d;(iii)NaNO2/concdHCl,acetone,sodiumazide/H2O,rt,1day.

ligands3,5,and7in74%,72%,and79%yield,respectively.The struc-turesofallligandssynthesizedinthisworkwerefullycharacterized byNMR(1_Hand13_{C),MS,andHRMS(seeSection2).}

Thestructuresofligands3and5canbereadilydistinguishedby theirdifferentsymmetryinNMRspectra.The1_{HNMRspectraof} theproximalligand3exhibitedsixdoubletsignalsaround3–5ppm forthemethylene-bridgeprotonsbecauseofitsCssymmetry;in contrast,themethylene-bridgeprotons ofligand5 appearedas twodoubletsignalsbecauseofitsC2vsymmetry(cf.Figs.S3and S5,Supportinginformation).Furthermore,ligand3exhibitedthree methylene-bridgecarbons(29.7,31.4,and32.8ppm)butthemore symmetricligand5displayedonlyonemethylene-bridgecarbon at31.9ppm(seeSection2).Allthemethylene-bridgecarbonsof

ligands3and5exhibitedsignalsaround31ppmsupportingthat theywereinconeconformations[13].Moreover,thelower-rim O-methyleneprotonsofligand3exhibitedtwodoubletsignalsaround 5–6ppminthe1_{HNMRspectraimplyingthatthetwoO-methylene} protonsarenonequivalent(Fig.S3,Supportinginformation).

3.2. Spectralcharacteristicsofcompounds3,5,and7

TheUV–visandfluorescentemissionspectraofligands3,5,and thecontrolcompound 7inMeOH/CHCl3 (v/v, 98:2)polarprotic cosolventareshowninFig.1,wheretheconcentrationofallthree compoundswasfixedat10␮M.

600 500 400 300 0.0 0.2 0.4 0.6 free 3 free 5 free 7 Absorbance Wavelength (nm) 3 7 (a) 5 700 600 500 400 0 1 2 3 4

Fluorescence Intensity (au)

Wavelength (nm) 3 472 nm Excimer Monomer 5 476 nm 7 378, 398 nm (b)

Fig.1.(a)UV–visand(b)fluorescencespectra(excitationwavelength=342nm)of ligands(10␮M)3,5,and7inapolarproticMeOH/CHCl3(v/v,98:2)cosolventat

25◦_C.

Thetwoabsorptionbands,around276nmand342nminthe UV–visspectraof3,5,and7,arefromtriazolylpyrenes(Fig.1a)[14]. Thefluorescencespectraof3,5,and7weretakenwhenirradiating atthe342nmofthetriazolylpyreneband(Fig.1b).Compound7,

bearingonlyasingletriazolylpyrene fluoroionophore,displayed vary strongmonomer emission bands around 378and 398nm withoutanyemissionfromtheexcimer(Fig.1b)indicatingthat intermolecular␲–␲stackingoftheconjugatedtriazolylpyrene flu-oroionophoresdidnotexistunderthelowconcentration(10␮M) conditions.Incontrast,bothligands3and5,displayedveryweak monomeremissions(379and398nm)butstrongexcimer emis-sions(around476nm,Fig.1b),implyingthatstrongintramolecular ␲–␲interactionsofthetwoneighboringtriazolylpyreneunitsexist evenintheverydilutecondition.

3.3. Spectralcharacteristicsofcompounds3,5,and7forcations Thebinding properties ofligands 3, 5, and 7 (all at 10␮M) towardmetalionswereassessedbyUV-visandfluorescence spec-troscopybytheadditionof10equivofmetalperchlorates(Li+_,Na+_, K+_,Mg2+_,Ca2+_,Ba2+_,Ag+_,Cu2+_,Ni2+_,Cd2+_,Hg2+_,Zn2+_,Mn2+_,Pb2+_, andCr3+_{)inMeOH/CHCl3(v/v,98:2)cosolventandtheresultsare} depictedinFigs.2andS9(Supportinginformation).

Upon the addition of 15 different metal perchlorates, the UV–visspectraof3,5,and7didnotshowanybathochromicor hypsochromic shift(Fig. S9, Supportinginformation); therefore throughoutthework,thefluorescencespectraweretakenusing theexcitationwavelengthat342nm.Incontrast,thefluorescence

700 600 500 400 0 1 2 3 4 free3 Ag+ Ba2+ Ca2+ Cd2+ Cr3+ Cu2+ Hg2+ K+ Li+ Mg2+ Mn2+ Na+ Ni2+ Pb2+ Zn2+

Fluorescence Intensity (au)

Wavelength (nm)

3 an

d

3 with other

metal

ca

tions

472 nm

3 + Ag

+ 384, 398 nm

3 + Cu

2+ (a)

3 +

Hg

2+ 700 600 500 400 0 1 2 3 4 free 5 Ag+ Ba2+ Ca2+ Cd2+ Cr3+ Cu2+ Hg2+ K+ Li+ Mg2+ Mn2+ Na+ Ni2+ Pb2+ Zn2+

Fluorescence Intensity (au)

Wavelength (nm)

5 and

5 with other

metal catio

ns

(b)

5 + Ag

+ 398 nm

5 + Cu

2+

5 + Hg

2+ 700 600 500 400 0 1 2 3 4 free 7 Ag+ Ba2+ Cd2+ Cr3+ Cu2+ Hg2+ K+ Li+ Mg2+ Mn2+ Na+ Ni2+ Pb2+ Zn2+

Fluorescence Intensity (au)

Wavelength (nm)

7 and 7 with othe

r metal cati

ons

378, 398 nm

(c)

Fig.2. Fluorescencespectraofligands(a)3,(b)5,and(c)7(each10␮M)inthe absenceandpresenceof10equivof15metalperchlorates(Li+_,Na+_,K+_,Mg2+_,

Ca2+_,Ba2+_,Ag+_,Cu2+_,Ni2+_,Cd2+_,Hg2+_,Zn2+_,Mn2+_,Pb2+_,andCr3+_{)inpolarprotic}

MeOH/CHCl3(v/v,98:2)cosolventat25◦C(excitationwavelength=342nm).

spectraofligands3and5weresignificantlychangedwhenAg+_, Cu2+_,orHg2+_{wasadded(Fig.2aandb);whereas,thecontrol} com-pound7,bearingonlyasingletriazolylpyrene,didnotshowany obviouschangeupontheadditionof15metalions(Fig.2c).The resultsindicatethatthetwotriazolylpyrenefluoroionophoreson thelower-rimofcalix[4]arenesarenecessaryfortheselective sens-ingofAg+_,Cu2+_,andHg2+_.

Heavyatoms are wellknowntoquench the fluorescenceof nearby fluorophores via enhanced spin–orbital coupling [15a], and/orphotoinducedelectrontransfer(PET)[15b].Asanticipated, Cu2+_andHg2+_{quenchedthefluorescenceofligands3and5(Fig.2a} andb)through heavyatomeffect[9b] and/orthereversedPET [7a,9a,d,16].In thelattercase,whenCu2+_orHg2+_wasbounded

tothenitrogenatomsofthetriazoleunits,thepyreneunits prob-ablybehavedasPETdonorsand thetriazole groupsbehavedas PETacceptors.Interestingly,ligands 3and5displayed very dif-ferent fluorescent responses toward Ag+_{. Upon the addition of} 10equivofAg+_{,theexcimeremissionofligand3decreasedwiththe} concomitantenhancementonitsmonomeremission(Fig.2a).In contrast,themonomerandexcimeremissionsofligand5wereboth enhancedbyAg+_{(Fig.2b).Theverydifferentfluorescentresponses} ofligands3and5towardAg+_{implyingthattheorientationofthe} lower-rimtriazolylpyrenesofcalix[4]areneplaysanimportantrole intheirsensingtowardAg+_.

3.4. Bindingpropertiesofcompounds3,5,forAg+

The Job plot [17] experiments of ligands 3 and 5 withAg+ (Fig.S10,Supportinginformation)showedthattheexcimer emis-sionsof3and5reachedaminimumat0.4ofthemolarfractions of ligands to Ag+_{, indicating that both 1:1 and 1:2} complexa-tion modes coexisted for both 3 and 5. To clarify the binding ratio,theprecipitatesfromtherespectiveligands (10␮M)with 40equivofAg+_{inMeOH/CHCl3(v/v,98:2)cosolventwereobtained} andmeasuredbytheelectrospraymassspectrometry.Themass spectrashoweda peakat m/z=1317.5for complex3·Ag+ _{and a} peakatm/z=1317.6for complex5·Ag+ _{(Figs.S11andS12,} Sup-porting information), corresponding to the masses of [3+Ag+_] and [5+Ag+_{], respectively. No signals from the 1:2 complexes} of 3·(Ag+_{)2 and 5·(Ag}+_{)2 were observed despite our repeated} efforts.

Inthetitrationof3withAg+_inMeOH/CHCl

3(v/v,98:2) cosol-vent(Fig.3a),thefluorescenceintensityofthemonomeremission bands (at 379 and 398nm) gradually increased with the con-comitantdecreaseoftheexcimeremission(max476nm)asthe concentration of Ag+ _{increased, and an isoemissive point was} clearlyidentified at443nm.Thespectral featureofthe fluores-cencetitration of ligand3 with Ag+ _{was consistentwitha 1:1} bindingratio;furthermore,thesystemfeaturesahighlyselective ratiometricfluorescentchemosensorforAg+_{.Forcomparison,both} themonomerandexcimeremissionsofligand5weregradually enhancedastheconcentrationofAg+_{increased(Fig.3b)andthere} wasasmallblueshiftofitsexcimeremissionmaximum(maxfrom 476to474nm).Sincetheabsorptionspectraof5showedvery lit-tleintensitychangeupontheadditionofAg+_{(Fig.S13b,Supporting} information),thesmallhypsochromicshiftoftheexcimeremission wasattributedtoaslightlydifferentorientationofthelower-rim distaltriazolylpyreneunitsintheexcitedstatewhen5complexed withAg+_{.Theassociationconstantsofcomplexes3·Ag}+_and5·Ag+_, calculatedbythenonlinearleast-squarecurve-fittingmethod[18], werefoundtobe7.11±0.80×103_M−1_{and1.83±0.33×10}4_M−1_, respectively(Fig.S14,Supportinginformation).

Upon the addition of 60equiv of Ag+_{, the intensity of the} monomer emission of 3 at 398nm increased by 14-fold while that for theexcimer emission at 476nm decreased by0.5-fold (insetofFig.3a). Ontheotherhand,theintensityof monomer emissionat398nmincreasedby8-foldbutthatforexcimer emis-sionat476nmincreasedbyonly0.2-fold,when40equivofAg+ was added to ligand 5 (inset of Fig. 3b). For comparison, the reportedfluorescenceenhancementfactorsfor calixarenes with non-conjugatedtriazole–CH2–pyreneupon theadditionofmetal ionswerearound0.5–9-fold[9].Ourresultsdemonstratedthat con-jugatedtriazolylpyreneisamoresensitivefluoroionophorethan conventionaltriazole–CH2–pyreneinmetalionsensing; further-more,therelativeorientationofthetwotriazolylpyrenesonthe lower-rimofcalix[4]areneplaysanimportantroleintheirunique fluorescentsensingtowardAg+_.

700 600 500 400 0 2 4 6 8 0 20 40 60 80 100 -4 0 4 8 Δ

F

[Ag

+

]/[3]

398 nm 476 nm

Fluorescence Intensity (au)

Wavelength (nm) 443 nm 476 nm 379, 398 nm 0 eq. 10 eq. 40 eq. 80 eq. 60 eq. (a) 700 600 500 400 0 1 2 3 4 5 0 20 40 60 80 0.0 0.5 1.0 1.5

Δ

F

[Ag

+

]/[

5]

398 nm 476 nm 40 eq

Fluorescence Intensity (au)

Wavelength (nm) 398 nm

476-474 nm

0 eq

(b)

Fig.3. Fluorescencetitrationspectra upontheadditionof variousamountof AgClO4toligands(a)3and(b)5(10␮M)inapolarproticMeOH/CHCl3(v/v,98:2)

cosolvent(excitationwavelength=342nm).TheinsetshowstheplotofF/Fovs.

[Ag+_{]/[ligand]basedonthefluorescencetitrationofligands(a)3and(b)5with}

variousamountofAgClO4.

3.5. Excitationspectraofcompounds3and5forAg+

Inprinciple,therearetwo typesofexcimerforpyrene com-pounds:dynamicandstatic[10a].Theformerresultsfromapyrene dimerformedintheexcitedstate,whereasthelatterarisesfrom a pyrene dimer exists in the ground state.One of the power-ful methodstodistinguishadynamicexcimer fromastaticone is bycomparingtheirexcitation spectra.Theexcitation spectra of ligands 3 and 5 and complexes 3·Ag+ _{and 5·Ag}+ _were mea-sured in MeOH/CHCl3 (v/v, 98:2)cosolvent,and we foundthat thosemonitoredatthemonomer(∼398nm)andexcimer emis-sions(∼475±1nm)werenotsuperimposable(Fig.S15,Supporting information).Alltheexcitationspectramonitoredattheexcimer emissionwerered-shiftedbyattheleast1–5nmcomparedtothose monitoredatthemonomeremission,indicatingthestaticnature oftheexcimeremissions[10a],irrespectiveoftherelative orienta-tionsofthetwotriazolylpyrenefluoroionophoresonligands3or5

andtheirmetalcomplexes.

3.6. 1_{HNMRcharacterizedtherecognitionbehaviorforAg}+

Inordertogaininsightintothestructuresofthetwocomplexes,

3·Ag+_and5·Ag+_,1_{HNMRtitrationexperimentsonligands3and}

5withvariousamountofAg+ _{werecarriedoutinCD3OD/CDCl3} (v/v, 3:2) cosolvent (Figs. 4 and 5) and in CD3OH/CDCl3 (v/v, 3:2) cosolvent to observe the chemical shift changes of the hydroxygroups oncalix[4]arene ofcomplexes 3·Ag+ _{and 5·Ag}+

Fig.4. 1_{HNMRspectraof3(2.5mM)inCD}

3OD/CDCl3(v/v,3:2)cosolventandCDCl3asanexternalstandardinthepresenceofdifferentamountofAgClO4:(a)0mM,(b)

1.25mM(0.5equiv),(c)2mM(0.8equiv),and(d)2.5mM(1equiv).♦:CHD2OD,:CD3OH,:H2O,•:internalCHCl3,and*:externalCHCl3.

(Figs.S16,andS17,Supportinginformation).Toavoidthe inter-ference of AgClO4 on the chemical shifts of d-solvents, all 1H NMR titration experimentswere carried outusing CDCl3 as an externalstandard.Significantdifferenceinspectralchangeswere observedwhenligands3and 5wereeach titratedwithAg+_(cf. Figs.4and5),inparticular,thechemicalshiftsofOCH2-triazole protons (He1 and He2), triazolyl protons (Hf), and pyrene pro-tons.

3.6.1. 1_{HNMRtitrationofcompound3forAg}+

TheprotonsofOCH2(He1andHe2)andpyrenegroupsshowed moresignificantchemicalshiftchangesinthe1:1complexof3·Ag+ (Fig.4d)comparedtothoseof5·Ag+_{(Fig.5d).Thechemicalshift} dif-ference(ı)betweenthetwosetsofthelower-rimOCH2protons (He1andHe2)reducedfrom0.65ppm(Fig.4a)to0.38ppmandmost ofthepyreneprotonsaround8–8.5ppmweredownfieldshifted by0.2ppm(Fig.4d).However,thetriazolylprotonsHfwerebarely

1 2 3 4 5 6 7 8 9 ppm 1.6 ppm

a a’

a, a’

c1

f

b, b’

c2

e

pyrene

(a)

b, b’

a, a’

pyrene

f

(d)

(b)

(c)

e

_c2

c1

Fig.5. 1_{HNMRspectraof5(2.5mM)inaCD}

3OD/CDCl3(v/v,3:2)cosolventandCDCl3asanexternalstandardinthepresenceofdifferentamountofAgClO4:(a)0mM,(b)

Scheme2.Apossiblebindingmodeof3·Ag+_inMeOH/CHCl

3(v/v,98/2)cosolvent.

shifted(Fig.4d)possiblyduetothemagneticanisotropyeffectfrom themovingpyreneunits.Althoughthefoursetsofthephenyl pro-tonsonthecalix[4]areneH_b1−4becameoverlappedinthepresence of1equivofAg+_{(Fig.4d),thestructureofthecomplex3·Ag}+_was assumedtobestillsymmetricalbecausethesplittingpatternsof theprotonsontriazoleunits(Hf),OCH2(He1,He2),andthet-butyl groups(H_a,a)ontheupperrimofcalix[4]arenewereunchanged. Basedonall1_{HNMRspectralfeaturesof3withAg}+_,wepropose thattheproximalpyreneunitsmovedapartfromeachotherin thecomplex3·Ag+_{(Scheme2)[9d]causingsubstantialmagnetic} anisotropicshiftsofthepyreneprotonsandtheseveredecreaseof theexcimeremission(videsupra).

3.6.2. 1_{HNMRtitrationofcompound5forAg}+

Inthetitrationofligand5withAg+_{inCD3OD/CDCl3(v/v,3:2)} cosolvent(Fig.5d),thetriazolylprotonHfwasdownfieldshifted by0.34ppmat1equivofAg+_{,whichwasthemostaffectedproton} amongalltheprotonsofligand5.Thechemicalshiftsof methy-leneprotonsoftheOCH2(He)andthepyreneprotonswerelittle affected. Furthermore, thehydroxyl proton of phenol, Hd, was foundtobeburiedintheprotonsofthepyrenesintherangeof 8.52–7.94ppmandthechemicalshiftsdidnotseemtohave obvi-ouschange when5 (2.5mM)with1equivof Ag+ _{wastaken in} CD3OH/CDCl3(v/v,3:2)cosolvent(Fig.S17,Supporting informa-tion).ThesespectralfeaturesimpliedthatAg+_{wasboundtothe} distaltriazoleunitsofligand5withthetwodistal pyreneunits remainedinsimilarorientationsasthatinafreeligand5.The chem-icalshiftdifference(ı)betweenthetwosetsofmethylene-bridge protonsHc1andHc2ofligand5decreasedfrom0.97ppm(Fig.5a) to0.66ppminthe1:1complexof5·Ag+_{(Fig.5d),indicatingthatthe} calix[4]areneincomplex5·Ag+_{wasinaflattenedcone} conforma-tion[19].AstheconcentrationofAg+_{increased(Figs.5andS17),} signalsofthetwomethylprotons(HaandHa)oftheupper-rim t-butylandtheO–CH2protons(He)remainedtobesingletsindicating thatthestructureofcomplex5·Ag+_{wassymmetrical.}

Basedonthespectralfeaturesof 5·Ag+_{,weproposethatthe} complex5·Ag+_{isinasymmetricalflattenedconeconformationand} thetwodistalpyreneunitskeepitsface-to-faceconformationas thatinafreeligand5(Scheme3).Theenhancementofboththe monomerandexcimeremissionsofligand5inthepresenceofAg+ (Fig.3b)wasattributedtotheCHEF(chelation-enhanced fluores-cence)effect[20]andthe_␲–␲stackingofthedistalpyreneunits. Thatis,whenAg+_{wasbondedto5,ithelpedtolockthetwo} lower-rimtriazolylpyreneunits(Scheme3),hence,enhancedtheoverall fluorescenceintensityofthetriazolylpyrenes(videsupra). 3.7. DMol3characterizedtherecognitionbehaviorforAg+

Inordertorationalizethequitedifferentfluorescentresponses ofthetwostructurallyrelatedligandsonAg+_{,wecalculatedthe}

Scheme3.Apossiblebindingmodeof5·Ag+_inMeOH/CHCl

3(v/v,98:2)cosolvent.

optimizedgeometriesofligands3and5andtheircomplexes3·Ag+ and5·Ag+_{bythemolecularmodelingDMol3[11]methodand} sim-ulatedtheminMeOHenvironment.WeusedaB3LYPfunctionwith thedoublenumericplusd-functions(DND)basisset[21].Thesize oftheDNDbasissetisequivalenttotheGaussian6-31+G**[21a]. Theoptimizedgeometriesofligands3and5andcomplexes3·Ag+ and5·Ag+ _{aredisplayedinFig.6,andrelated dataofthose} cal-culationbyDMol3aresummarizedinTablesS5–S16(Supporting information).

BasedontheresultsfromtheDMol3calculations,thedistance betweenthetwotriazoleunitsisabout3.7–3.9 ˚Ainligand3(Fig.6a) andisabout7.2–7.4 ˚Ainligand5(Fig.6b).Inthecomplexation ofligands3and5withAg+_{,therearetwopossiblebindingsites} foreachofthetwotriazoleunits:oneisviatheuppernitrogen atoms(N3)andtheotherisviathelowernitrogenatoms(N2)of thetriazoleunits.Thetwopossiblesituationsforcomplexes3·Ag+ and5·Ag+_{wereoptimizedandareshowninFig.S18(Supporting} information).WhenAg+_{wasboundtoN3atomsoftriazoleunits,} thetwo pyreneunits ofcomplex would moveapart fromeach other(Figs.S18a andc,Supportinginformation).The conforma-tionalchangeofcomplex3·Ag+_{comparedtofreeligand3explains} whytheexcimeremissiondecreasedwhilethemonomer emis-sionincreasedwhichisconsistentwiththeobservedfluorescent changesandtheproposedbindingmodeofcomplex(cf.Figs.3a and6candScheme2).Ontheotherhand,thecomplexof5·Ag+ revealedthatAg+_{preferredtobeboundtoN2atomsofthetriazole} unitsandthetwopyreneunitsremainedtheirface-to-face confor-mationsasthatinfreeligand5(Fig.6bandd).Suchconformation wouldshowstrongerexcimerthanmonomeremissionandisin accordwiththefluorescenceresultsdescribedabove(cf.Figs.3b and6dandScheme3).

3.8. Propertiesofcomplex3·Ag+ 3.8.1. Reversibilityofcomplex3·Ag+

Thecomplexationofligand3withAg+_inaMeOH/CHCl 3(v/v, 98:2)cosolventwassetfreebytheadditionofH2Ofollowedby extractionwithdichloromethane(Fig. S19,Supporting informa-tion).

3.8.2. CompetitiverecognitionofAg+_{withothermetalcations} Thefluorescenceofligand3(10␮M)inthepresenceof10equiv of Ag+ _{and in the presence of 14 other competing metal ions} (10equiv,Li+_,Na+_,K+_,Mg2+_,Ca2+_,Ba2+_,Cu2+_,Ni2+_,Cd2+_,Hg2+_,Zn2+_, Mn2+_,Pb2+_,andCr3+_)inMeOH/CHCl

3(v/v,98:2)cosolventwere studiedandtheresultsareshowninFig.7.Ascanbeseen,the enhancedmonomeremission(398nm)ofcomplex3_·Ag+_remained thesameupontheadditionof13competitivemetalionsexcept Cu2+_{,inwhicha20%decreaseinthefluorescenceenhancement}

Fig.6. Optimizedgeometriesforligands(a)3;(b)5,andcomplexes(c)3·Ag+_{(N3);(d)5·Ag}+_{(N2)inMeOHbyDMol3calculation.Where“N3”meansthebindingsitehappened}

attheuppernitrogenatom(N3)oftriazoleunitsonligands,and“N2”meansthebindingsitehappenedatthelowernitrogenatom(N2)oftriazoleunitsonligands. wasobserved (Fig.7h).Accordingly, ligand3 canbeclaimedas

ahighlyselective,ratiometricfluorescentchemosensorforAg+_in thepresenceofmostothercompetingmetalionsinapolarprotic solvent. 0 2 4 6 8 10 (o) (n) (m) (l) (k) (j) (i) (h) (g) (f) (e) (c) (d) I/Io at 398 nm Metal ions Ag+ Li+ Na+K+Mg2+Ca2+Ba2+Cu2+Ni2+Cd2+Hg2+Zn2+Mn2+Pb2+Cr3+ (a) (b)

Fig.7. Fluorescenceintensityratioofligand3(10␮M)inthepresenceof10equiv (a)Ag+_and(b–o)Ag+_{plus14othermetalperchlorates(Li}+_,Na+_,K+_,Mg2+_,Ca2+_,Ba2+_,

Cu2+_,Ni2+_,Cd2+_,Hg2+_,Zn2+_,Mn2+_,Pb2+_,andCr3+_{)versusthatofafreeligand3in}

MeOH/CHCl3(v/v,98:2)cosolventat25◦C(excitationwavelength=342nm).

3.8.3. Recognitionbehaviorofcomplex3·Ag+_inaqueous methanolsolution

Furthermore,wealsofoundthattheuniqueratiometric fluo-rescentsensingof Ag+ _{byligand3still workedin10%aqueous} methanol solution, although the enhancement factor of the monomeremission of3dropped from9.2in MeOH/CHCl3 (v/v,

-1 0 1 2 3 4 (I - I o )/Io Cations 398 nm 472 nm 3 Ag+ Ba2+ Ca2+ Cd2+ Cr3+ Cu2+ Hg2+ K+ Li+ Mg2+ Mn2+ Na+ Ni2+ Pb2+ Zn2+

Fig.8. Fluorescenceintensitychangesofligand3(10␮M),upontheadditionof15 metalperchlorates(10equiv,Li+_,Na+_,K+_,Mg2+_,Ca2+_,Ba2+_,Ag+_,Cu2+_,Ni2+_,Cd2+_,Hg2+_,

Zn2+_,Mn2+_,Pb2+_,andCr3+_)inaqueousH

2O/MeOH/CHCl3(v/v,10:88:2)cosolventat

98:2)cosolvent(Fig.2a)tothatof3.1in10%aqueousmethanol solution(Fig.8)when10equivofAg+_wasadded.

4. Conclusion

Aconjugatedtriazolylpyrenefluoroionophorewassynthesized and introduced into the lower-rim of a calix[4]arene scaffold withproximaldisubstitutioninligand3anddistaldisubstitution in ligand 5. Both ligands 3 and 5 possessed a high selectivity toward Ag+ _{relative to most other metal ions in MeOH/CHCl3} (v/v, 98:2) polar protic cosolvent where the conjugated tria-zolylpyrenefluoroionophoreisprovedtobenotonlyanionophore but also a more sensitive signaling unit than the conventional triazolyl–CH2–pyrenefluoroionophore.Inaddition,theorientation ofthe fluoroionophore wasprovedtoinfluence thefluorescent responseintheeventofmetalionbindingwheretheproximal ori-entationofthelower-rimfluoroionophoresofcalix[4]arene,ligand

3,showedaratiometricfluorescentresponseforAg+_;whereas,the lower-rimdistalorientationoffluoroionophoresofcalix[4]arene, ligand5,didnotshowratiometric responsetoAg+_.Finally,the highlyselectiveandratiometricdetectionofAg+_by3isnot inter-feredbymostothermetalionsinpolarproticsolventMeOH/CHCl3 (v/v,98:2)anditworksevenin10%aqueousmethanolsolution.In addition,theoptimizedgeometriesofcomplexes3·Ag+_and5·Ag+ werecalculatedbyDMol3whichprovidedrationalexplanationsfor theobservedfluorescenceand1_{HNMRspectralchanges.}

Acknowledgments

WethanktheNationalScienceCouncil(NSC),Taiwanandthe MinistryofEducation (MOE)Approaching TopUniversity (ATU) ProgramoftheMinistryofEducation,Taiwan,RepublicofChina forfinancialsupport.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.snb.2012.06.014.

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Biographies

Nae-JenWangisastudentinPh.D.programattheDepartmentofAppliedChemistry, NationalChiaoTungUniversity,Taiwan.Herresearchinterestisinsupramolecular chemistry.

Chung-MingSunreceivedhisPh.D.degreein1994fromStateUniversityofNew YorkatStonyBrook,U.S.A.Currently,heisaProfessorintheDepartmentofApplied

Chemistry,NationalChiaoTungUniversity,Taiwan.Hisresearchareasarein com-binatorychemistryandsyntheticorganicchemistry.

Wen-ShengChungreceivedhisPh.D.degreein1990fromColumbiaUniversity, U.S.A.Currently,heisaProfessorintheDepartmentofAppliedChemistry,National ChiaoTungUniversity,Taiwan.Hisresearchareasareinphysicalorganicchemistry andsupramolecularchemistry.