Showcasing research from Fang lab, Shaanxi Normal University, China.Title: Calix[4]arene-based supramolecular gels with unprecedented rheological propertiesA mixture of n-decane and acetonitrile can be effi ciently gelled by introduction of a small amount of a specifi c calix[4]arene-based dimeric-cholesteryl derivative as designed and synthesized in the present work. The gel exhibits super-smart thixotropic property and adopts an unusual O/O gel-emulsion structure.As featured in:See Yu Fang et al., Soft Matter, 2012, 8, 3756.www.rsc.org/softmatterRegistered Charity Number 207890SoftMatter
Citethis:SoftMatter,2012,8,3756www.rsc.org/softmatter
DynamicArticleLinks PAPER Calix[4]arene-basedsupramoleculargelswithunprecedentedrheologicalproperties? XiuqinCai,abKaiqiangLiu,aJunlinYan,aHelanZhang,aXiaoyuHou,aZhangLiuaandYuFang*aPublished on 15 February 2012. Downloaded on 08/07/2013 08:27:33. Received25thNovember2011,Accepted11thJanuary2012DOI:10.1039/c2sm07251c Anovelcalix[4]arene-baseddimeric-cholesterylderivativewassynthesized,anditsgelationbehaviourinthirtyorganicsolventswasinvestigated.Ithasbeenshownthatthecompoundcannotgelanyofthepuresolventstested.However,itgelsamixtureofsolventsn-decaneandacetonitrileef?ciently,providedthevolumeratioofthetwosolventsinamixtureiswithin9:1and3:2.AFMandSEMmeasurementsrevealedthatthemoleculesofthecompoundaggregateintomicro-/nano-rods?rst,then?ne?bers,andthenthick?bers,and?nallynetworkedstructuresinthemixturesolvents.Interestingly,thegelwithacompositionof1to1(Vn-Decane:VAcetonitrile)and2.5%(w/v)ofthecompoundexhibitssuper-smartandfullyreversiblethixotropicproperties,aphenomenonneverreportedbefore. Furthermore,themechanicalstrengthofthegelcouldbeeasilyadjustedbyalteringtheconcentrationofthegelatorandthecompositionofthemixturesolvents.Furtherinterrogationofthegelrevealedthatstructurallythegelisagel-emulsionwithacetonitriledispersedinn-decane,ararelyfoundO/O(oilinoil)gel-emulsionwhichmay?ndusesinthetemplatedpreparationoflow-densitymaterialswithcomplicatedinternalstructures. Introduction Supramoleculargelsbasedonlow-molecular-massgelators(LMMGs)haveattractedgreatattentionduringthelasttwodecades.1–6Thisisbecause,ononehand,itoffersuniquefeaturestocreatevarioussuperstructureswithrespecttoself-assemblyphenomena,7–12andontheotherhandithaspotentialforcreatingnewsoftmaterials,whichmay?nduseinoilrecovery,controlledrelease,bioactivitymaintenance,proteinseparation,regenerativemedicine,tissueengineering,andtemplateprepa-rationofmicro-/nano-materialsetc.13–19Todate,severalclassesoflow-molecularmasscompoundsfromsimplealkanestocrownethers,cyclodextrinsandcholesterylderivativeshavebeenshowntobeef?cientLMMGsforwateror/andorganicsolvents.20–26However,fromtheviewpointofpracticalusesofLMMGs-basedsupramoleculargelstherehaven’tbeenmanyreported.Inparticular,LMMGscontainingmacrocycliccomponents,suchascrown-ethers,cyclodextrins,calixarenesandcucurbiturilsareverylimited.20–26Thesecomponentsareimportantbecausetheyarerichinhost–guestandsupramolecularchemistry,andthereby KeyLaboratoryofAppliedSurfaceandColloidChemistryofMinistryofEducation,SchoolofChemistryandChemicalEngineering,ShaanxiNormalUniversity,Xi’an710062,P.R.China.E-mail:yfang@snnu.edu.cn;Fax:+0086-29-85310081;Tel:+0086-29-81530788bCollegeofChemistryandLifeScience,WeinanNormalUniversity,Weinan714000,P.R.China ?Electronicsupplementaryinformation(ESI)available:Gelationbehavioursofthecompound4;Microscopyimages;Molecularstructureofthe?uorescentprobeandrheologicalstudies.SeeDOI:10.1039/c2sm07251c aitisanticipatedthatcombiningthemintoLMMGsmaybringmoleculargelsnewfunctionalities. Amongthemacrocycliccompoundsknowntillnow,calixar-enesareuniquebecauseoftheirabilitytobemodi?ed,andtheir‘‘unlimitedpossibilities’’.27Itistheseadvantagesthatmakethemhaveawidevarietyofapplicationsinmolecularrecognition,sensing,selectivebindingandcarrying,etc.28However,calixar-eneshavebeenrarelystudiedasacomponentofLMMGs.Tothebestofourknowledge,only7reportshavebeenpublishedtillnow,ofwhich5werepublishedrecently.27,29–35In1993,Shinkaiandcoworkerspresentedthe?rstreportonthegelationprop-ertiesofcalix[4]arenederivatives,whichbearlongalkylgroupsattheirlowerrims.Itwasdemonstratedthatthecalix[4]arenederivativescouldserveasagelatorforsomeapolarsolvents,includingcyclohexane,hexane,andcarbondisul?de.29,30Tomimictheuptakeofalkanesbybacteriafromtheaqueousphase,XuandcoworkerscreatedastableDMSOgelbyreacting3-pyridine-azo-calix[4]arenewith[Pd(en)(H2O)2](NO3)2(en?ethylenediamine)insitu.Modelsystemstudiesdemon-stratedthattheorganogel‘‘uptakes’’toluenefromanaqueoussolutionselectivelyandcompletely,andtheabsorbingprocessfollows?rstorderkinetics.31,32Ogdenandcoworkersdesignedandpreparedaproline-functionalizedcalix[4]arene,whichformshydrogelswhentriggeredbythepresenceofspeci?canions.Thepropertiesofthegelscouldbemodi?edbytheassociatedcations,andfurthermore,thegelscouldbedisassembledbyincreasingthepHabove7.ItwasbelievedthatanewkindofLMMGs-basedstimulus-responsivehydrogelwasfound.33Zhengandcoworkersreportedaseriesofcalix[4]arenederivatives,which ThisjournalisaTheRoyalSocietyofChemistry2012 3756|SoftMatter,2012,8,3756–3761 bearlongtertiaryalkylgroupsattheupperrimandS-1-phenylethylaminegroupsatthelowerrim.34GelationandaggregationstudiesshowedthatinthepresenceofD-2,3-dibenzoyltartaricacid,thecompoundsgelatecyclohexanewhenheatedoraggre-gateintoegg-likevesicles,whichisthe?rstexampleofheat-setgelsresultingfromadifferenceininteractionsbetweentwocomponentgelators.Recently,thesamegrouppreparedanothercalix[4]arenederivative,whichbearsL-2,3-dibenzoyltartaricacidgroupsatthelowerrim.35Thiscompoundcouldenantio-selec-tivelyself-assembleandresultinagelorsuspensiononlywithoneenantiomerofanumberofchiralamines,asrevealedbySEM.TheresultsfromZhengandcoworkers’researchdemon-stratedclearlythatmolecularrecognitioncouldbeutilized,viacarefulselectionoftworecognitionunits,fortheconstructionofsupramoleculargels,astrategyrarelyemployedandreportedintheliterature.Veryrecently,Liuandcoworkersemployedasimilarstrategytocreateaseriesofsupramolecularbinaryhydrogels.27Thiscalix[4]arenederivativewaspreparedbybindingfourprolinestructuresattheupperrimofthemacro-cycliccompound.Acombinationofthiscalix[4]arenederivativewitharginine,histidineorlysinegelswateref?ciently.Thestructuresofthegelnetworksaredependentuponthenatureofaminoacidemployed. Asiswellknown,calixarenesbearmultiplebindingsitesthatpromotesself-assemblyinto3-Dstructuresviaspeci?cweakinteractions.36–38Furthermore,thecavityofcalixarenecouldholdsolventmoleculesorotherguestmolecules.39,40Similarly,cholesterolisawellstudiedfragmentforbuildingupLMMGsduringthelastfewdecades.Ithasbeenshownthatcholesterolself-assemblesinadirectionalwayviavanderWaalsinteractionsandformsaggregateswithspeci?cstructures.41–45Itshouldbeofinteresttoseewhatthegelationbehavioursofcompounds,whichcontainthetwocomponents,are.Drivingbythecuriositydiscussedabove,calix[4]areneandcholesterolwerechosenaskeycomponentsforthe?rsttimetoconstructanovelLMMGs.Intheeffort,twocholesterylresiduesweresuccessfullyattachedtothelowerrimofcalix[4]arenevia?exiblelinkers.Gelationtestsrevealedthatthecompoundisanef?cientgelatorformixedsolventsofn-decaneandacetonitrileprovidedthevolumeratiosofthemarewithintherange9:1to3:2(Vdecane:Vacetonitril).Interestingly,thegelofamixedsolvent(1:1)exhibitsaverysmartthixotropicpropertywherebythesol–gelphasetransitioncouldbereversiblycontrolledbyimposingorremovingasimpleshakeorshearstress. equilibrateforsometime.Finally,thetesttubewasinvertedtoobserveifthesolutioninsidecouldstill?ow.Thesystemswhichlose?uidityaredenotedas‘‘G’’.Thosethatremainedassolutionsaredenoted‘‘S’’.However,systemswithprecipitatesorcrystalsaredenotedas‘‘P’’. SEMpicturesofxerogelsweretakenonaQuanta200scanningelectronmicroscopyspectrometer(Philops-FEI).Theacceler-atingvoltagewas20kV,andtheemissionwas10mA.Thexerogelwaspreparedbyfreeze-dryingagelofthemixturesolventatagelatorconcentrationof2.5%(w/v).Priortoexam-ination,thexerogelwasattachedtoacopperholderbyusingconductiveadhesivetape,andthenitwascoatedwithathinlayerofgold. AFMmeasurementswereconductedonaSOLVERP47PROsystem.Thesamplewaspreparedbydropcastingthedilutesolutionofthecompoundontoafreshlycleavedmicasurface.AllFTIRmeasurementswereperformedonaBrucherEQUINX55spectrometerinanattenuatedtotalre?ection(ATR)modewithZnSeasasampleslot.TheKBrpelletsmixedwithsamplesweremeasuredintransmittancemode. Rheologicalmeasurementswerecarriedoutwithastress-controlledrheometer(TAinstrumentAR-G2)equippedwithsteel-coatedparallel-plategeometry(20mmdiameter).Thegapdistancewas?xedat1000mm.Asolventtrappingdevicewasplacedabovetheplateandmeasurementtemperaturewassetat15??Cinordertoavoidsolventevaporation. Forrheologicalmeasurements,astresssweepmeasurementat?xedfrequencywas?rstlyconducted,whichprovidesinforma-tionaboutthemechanicalstrengthofthegelsample.Andthen,atimesweepwasmadetoobservetherecoverypropertyofthegel.Forthismeasurement,aconstantoscillatoryshearstresswasappliedtodestroythesample,andthen,asmallconstantshearstresswasapplied,andthestoragemodulusG0andthelossmodulusG00ofthesampleweremonitoredasfunctionsoftime.1HNMRandMSmeasurements:1HNMRdataofsampleswerecollectedonBrukerAVANCF300MHzspectrometer.MALDI-TOFmassspectrawererecordedinaKratos’AXIMA-CFRplusinstrumentbyusing2,5-dihydroxybenzoicacid(DHB)asamatrix.Syntheticprocedure Synthesisofthecholesterylderivativeofcalix[4]areneisschematicallyshowninScheme1,andthedetailsofthesynthesisaredescribedbelow. Preparationofcompound1.ReferencingtoaworkreportedbyGutsche,46compound1wassynthesizedbythefollowingmethod.20.5g(140mmol)ofp-tertbutyphnol,14.5mLof37%formalin(200.2mmol),and0.5mLof12.5mmolmLà1NaOHsolutionweremixedtogetherina500mLfour-neck?ask.Then,themixturewasstirred,andheatedunderanitrogenatmospherefor2hinanoilbathat110–120??C.Themixturebecameviscousandtookanorangecolor.Furthermore,waterinthesystemwasalmostdistilledoutcompletelyatthistemperature.Thesystemwasthencooleddowntoroomtemperature,resultinginasolid-likemixture.Themixturewassuspendedbystirringin200mLofdiphenyletherundernitrogenatmosphere.Thesuspensionwasre?uxedagainfor2h,anditscolorbecamedarkredeventually. SoftMatter,2012,8,3756–3761|3757 Published on 15 February 2012. Downloaded on 08/07/2013 08:27:33. Experimentalsection Reagentsandmaterials p-Tertbutyphnol(AladdinChemistryCo.Ltd.,98%)useddirectlywithoutfurtherpuri?cation.THFwasdistilledoversodiuminthepresenceofbenzophenoneunderN2atmospherebeforeuse.Allotherreagentswereanalyticallypure.Generalmethods Preparationofgels:inatypicalgelationtest,aweighedamountofthepotentialgelatorandameasuredvolumeofthesolventmixturewereplacedinasealedtesttube,followedbyshakingorsonicatingforawhile.Andthen,thesystemwasleftto ThisjournalisaTheRoyalSocietyofChemistry2012 Aftercoolingthesystemdowntoroomtemperature,200mLofethylacetatewasadded.Thenthesystemwasstirredforanother30min,andthenitwasallowedtostandforatleast30min.Via?ltration,aprecipitatethatwasformedinthemixturewascollectedandwashedtwicewithethylacetateandaceticacid,separately.Thecrudeproductwaspuri?edbyre-crystallizationfromtoluenetogiveawhiteproduct(compound1)in50%yield.Forcompound1:1HNMR(CDCl3/Me4Si,300MHz):d?(ppm)1.21(s,36H,–C(CH2)3),3.47(d,J?12.0,4H,–ArCH2Ar–),4.23(d,J?12.0,4H,–ArCH2Ar–),7.04(s,8H,–ArH),10.34(s,4H,–OH);MS:m/zcalcdfor[(M+Na)+]:671.41,found:671.41. Published on 15 February 2012. Downloaded on 08/07/2013 08:27:33. twice,anddriedundervacuum.Inthisway,awhitepowder(compound3)wasobtained(yield:91%)For3:1HNMR(CDCl3/Me4Si,300MHz):d(ppm)0.95(bs,4H,–NH2),1.04(s,18H,-C(CH2)3),1.27(s,18H,–C(CH2)3),3.43(d,J?15.0,4H,–ArCH2Ar–),4.10(d,J?15.0,4H,–ArCH2Ar–),4.64(s,4H,–ArOCH2–),6.92(s,4H,–ArH),7.04(s,4H,–ArH),7.76(bs,2H,–NHCO–),9.67(s,2H,–OH);MS:m/zcalcdfor[(M+Na)+]:814.02,found:814.02.Anal.calcdforC48H64N4O6:C72.70,H8.14,N7.06;found:C72.61,H8.17,N7.13.Preparationofcompound4.Compound3(0.790g,1mmol)andcholesterylcholoformate(0.898g,2mmol)weredissolvedin60mLoftetrahydrofuran(THF),andthemixturewasre?uxedfor5h.Afterthereaction,themixturewasevaporatedtodryness.Theprecursorwasre-crystallizedwithethanol,andthendriedinvacuumasawhitepowder(yield:92%).For4:1HNMR(CDCl3/Me4Si,300MHz)d(ppm):0.68–1.84(m,132H,–CH2CH2–,–C(CH3)3,methyl,cholesterylprotons)3.39(d,J?18.0,4H,–ArCH2Ar–),4.10(d,J?18.0,4H,–ArCH2Ar–),4.58–4.67(s,6H,ArOCH2,oxcyclohexyl),5.39(s,2H,–OH),5.40(s,2H,alkenyl),6.87(s,4H,–ArH),7.08(d,4H,–ArH),7.41(s,2H,–NHCO–),10.35(s,2H,–NHCO-);FTIR,nmax/cmà1:3417(NH),2955(CH),1715(C]O,–O),1687(C]O,NH),1529(NH,bendingvibration),and1327(–C–O).MS:m/zcalcdfor[(M+Na)+]:1640.14,found:1640.14.Anal.calcdforC104H152N4O10:C77.18,H9.47,N3.46;foundC76.74,H9.75,N3.09. Preparationofcompound2.47Compound1(0.648g,1mmol),potassiumcarbonate(0.166g,1.2mmol)andpotassiumiodide(0.017g,0.1mmol)weredissolvedin80mLofacetone,andthenethylchloroacetate(0.294g,2mmol)wasaddedslowlywithstirring.Themixturewasre?uxedfor10hundernitrogenatmosphere.Thesolventwasevaporated,andtheresidueobtainedwasdried.Then,itwasdissolvedorsuspendedinchloroformandthenextracted(washed)with10%hydrochloricacidforthreetimes.Theorganicphasewascollectedanddriedwithanhydrousmagnesiumsulfate,andfurtherevaporatedtodryness.Inthisway,acrudeproductwasprepared. Thecrudeproductwas?rstlypuri?edbycolumnchromatog-raphy(silicone,200–300mesh;acetone/petroleumether,v:v?1:9),andthenbyre-crystallizationfrompetroleumetherthreetimestogivecompound2aswhitecrystalsinayieldof51%.Forcompound2:1HNMR(CDCl3/Me4Si,300MHz):d(ppm)1.04(s,18H,–C(CH2)3),1.25(s,18H,–C(CH2)3),1.31(t,6H,–CH3),3.29(d,J?15.0,4H,–ArCH2Ar–),4.26(q,4H,–CH2CH3),4.10(d,J?15.0,4H,–ArCH2Ar–),4.72(s,4H,–ArOCH2),6.81(s,4H,–ArH),7.01(s,4H,–ArH),7.09(s,2H,–OH);MS:m/zcalcdfor[(M+Na)+]:844.08,found:844.08. Preparationofcompound3.Compound2(0.820g,1.0mmol)andhydrazinehydrate(1.0mL)weredissolvedinamixedsolventoftolueneandmethanol(20mL,v:v?1:1)undernitrogenatmosphere.Themixturewasre?uxedunderstirringfor10h.Then,thesolventwasevaporatedundervacuum,andthenaprecipitatewasproducedbyadditionofasmallamountofwaterandcollectedby?ltration.Finally,thecrudeproductobtainedwaswashed,respectively,withmethanolandwater Resultsanddiscussion Gelationbehaviours Thegelationbehavioursofthecompoundweretestedin30differentorganicsolventsincludingprotic/aproticandpolar/apolarsolventsinaconcentrationof2.5%(w/n)(c.f.TableS1,ESI?).Itwasfoundthatthecompounddissolvesinmostofthesolventstestedexcludingmethanol,acetonitrile,andtriethyl-amine(TEA).However,thecompoundaggregatesslowlyinn-decaneanditwasfound,experimentally,thatsome?occulantsappear12hlaterafterthecompoundwasdissolvedinthesolventatroomtemperature.Forthissystem,gelationoccursatroomtemperaturewhentheconcentrationofthecompoundexceeds7%(w/n),thecriticalgelationconcentration(CGC)ofthepuresolvent.TodecreasetheCGC,apoorsolvent,acetonitrilewasintroduced.Itwasbelievedthatintroductionofthispoorsolventmustpromotetheaggregationofthecompound,whichmayenhancetheestablishmentofgelnetworks,anecessityforgelation.Inthisway,theadditionof1.0%(w/n)ofthecompoundatroomtemperatureresultsininstantgelationina1:1mixedsolventsystemofn-decaneandacetonitrile(involume)providedsimpleshakingorsonicationwasgiven,whichmustbring Table1Gelationbehavioursofthecompoundinn-decaneandaceto-nitrilemixture(2.5%,w/v)VDecane:VAcetonitrileResult VDecane:VAcetonitrileResult 10:0S4:6G 9:1G*3:7P 8:2G*2:8P 7:3G1:9P 6:4G0:10P 5:5G Scheme1Synthesisofadimeric-cholesterylderivativebridgedbyacalix[4]areneunit. 3758|SoftMatter,2012,8,3756–3761ThisjournalisaTheRoyalSocietyofChemistry2012
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