Synthesis, characterization and photophysical properties of

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Synthesis,characterizationandphotophysicalpropertiesofaSWNT-phthalocyaninehybrid{

BeatrizBallesteros,aSte´phaneCampidelli,bcGemadelaTorre,aChristianEhli,dDirkM.Guldi,*dMaurizioPrato*bandTomasTorres*aReceived(inCambridge,UK)23rdFebruary2007,Accepted17thApril2007FirstpublishedasanAdvanceArticleontheweb10thMay2007DOI:10.1039/b702819a

Downloaded on 15 September 2011Published on 10 May 2007 on http://pubs.rsc.org | doi:10.1039/B702819AWereportthesynthesis,characterizationandphotophysicalfeaturesofanewnanometerscalecarbonnanostructure,thatis,asingle-wallcarbonnanotubebearingphthalocyaninechromophores.

Carbon-basednanomaterialsarecurrentlyunderactiveinvestiga-tionforproducinginnovativematerials,compositesandelectronicdevicesofgreatlyreducedsize.1Inparticular,single-wallcarbonnanotubes(SWNTs)areone-dimensionalnanowiresthatareeithermetallicorsemiconducting.Theyreadilyacceptcharges,whichcanthenbetransportedundernearlyidealconditionsalongthetubularSWNTaxis.2Theelectricalconductivity,morphologyandgoodchemicalstabilityofSWNTsarepromisingfeaturesthatstimulatetheirintegrationintoelectronicdevices.

ThecombinationofSWNTswithelectrondonorssuchasferrocene,3porphyrins4orTTF5hasbeenachieved,andhasledtonewmaterialsgivingrise—uponphotoexcitation—tointraensem-bleelectrontransferprocesses.Amongthewidevarietyofelectrondonormoieties,phthalocyanines(Pcs),6whicharesyntheticporphyrinanalogues,exhibitparticularlyintenseabsorptioncharacteristicsinthered/near-infraredspectralregion,whereporphyrinsfailtoabsorbappreciably.Pcshavebeenextensivelyusedasdyes,andmorerecentlyhavealsobeenintegratedasexcitedelectrondonorsintodonor–acceptorsystems,incombina-tionwithelectronacceptingfullerenes.7Here,wereportthesynthesis,characterizationandphotophy-sicalpropertiesofSWNTsthatweredecoratedwithfreebasephthalocyanines(SWNT-H2Pc,1).CovalentfunctionalizationofcarbonnanotubeswithPcsusingamidolinkageshasalsobeenreportedbybothusandothers.8However,theresultingmaterialsprovedtobenearlyinsolubleincommonorganicsolvents.

Scheme1summarisesthesynthesisoftheSWNT-H2Pcnanoconjugate(1).Thefunctionalisedcarbonnanotubes2werepreparedby1,3-dipolarcycloadditionbetweenaminoacid3,9DepartamentodeQuı´micaOrga´nica,UniversidadAuto´nomadeMadrid,Cantoblanco,E-28049Madrid,Spain.E-mail:tomas.torres@uam.esbINSTM,UnitofTrieste,DipartimentodiScienzeFarmaceutiche,Universita`diTrieste,PiazzaleEuropa,1,I-34127Trieste,Italy.E-mail:prato@units.itcLaboratoired’ElectroniqueMole´culaire,ServicedePhysiquedel’EtatCondense´(CNRSURA24),CEASaclay,F-91191

GifsurYvetteCedex,France.E-mail:stephane.campidelli@cea.frdInstituteofPhysicalChemistry,Friedrich-Alexander-Universita¨tErlangen-Nu¨rnberg,Egerlandstraße3,D-91058Erlangen,Germany.E-mail:guldi@chemie.uni-erlangen.de

{Electronicsupplementaryinformation(ESI)available:Techniques,synthesisandanalyticaldata.SeeDOI:10.1039/b702819a

aScheme1Reagentsandconditions:(i)Aminoacid3,paraformaldehyde,DMF,120uC,5d;(ii)HClgas,DMF,rt,1h;(iii)H2Pc4,EDC,HOBt,ethyldiisopropylamine,DMF,rt,5d.

paraformaldehydeandpurifiedHiPCOSWNTs10inDMF.After5d,theDMF-solublematerialwasseparatedfromthereactionmixturebyseveralsonication,centrifugationandfiltrationprocesses(seetheESIforexperimentdetails{).Thenanotubeswerere-suspendedinDMF,andremovaloftheN-tertbutoxy-carbonylprotectinggroups(Boc)wasachievedbybubblinggaseousHClthroughthesuspension.Thecorrespondingammo-niumchloridesaltprecipitatedduringtheacidtreatment,followedbyfiltration.Synthesisof1involvedreactionoftheammoniumchloridesaltwithphthalocyanine4inthepresenceofN-(3-dimethylaminopropyl)-N0-ethylcarbodiimide(EDC),1-hydroxy-benzotriazole(HOBt)andethyldiisopropylamine.

Thecarboxyphenyl-substitutedphthalocyanine4,ontheotherhand,wassynthesizedinfourstepsstartingfrom4-(hydroxy-methyl)phenylphthalonitrile(5)(seeESI{)and4,5-tert-butylphe-noxyphthalonitrile11(Scheme2).Thestatisticaltetramerizationreactionof5and4,5-tert-butylphenoxyphthalonitrileinthepresenceoflithiumledtothe(hydroxymethyl)phenyl-substitutedphthalocyanine6.ThelatterwasthenoxidisedwithMnO2toyieldtheformylphenylphthalocyanine7.Inthefinalstep,phthalocy-anine4wasobtainedbytheoxidationof7usingsulfamicacidandsodiumchlorite.AllPcs(4,6and7)arenewandwerefullycharacterisedbyUV-vis,1HNMRandIRspectroscopies,

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Downloaded on 15 September 2011Published on 10 May 2007 on http://pubs.rsc.org | doi:10.1039/B702819AScheme2Reagentsandconditions:(i)Pd(PPh3)4,K2CO3,THF,H2O,reflux,24h;(ii)Li,1-pentanol,150uC,18h;(iii)MnO2,CHCl3,60uC,5h;(iv)NH2SO3H,NaClO2,THF,H2O,rt,3h.

MALDI-TOFmassspectrometryandelementalanalysis(seeESI{).

Thenanotubederivatives1and2werecharacterizedbystandardanalyticaltechniques,suchasUV-vis-NIR,thermogravi-metricanalysis(TGA),transmissionelectronmicroscopy(TEM)andatomicforcemicroscopy(AFM).Thephotophysicalproper-tiesof2andreferencephthalocyanine4havebeenalsoinvestigatedbysteady-stateandtimeresolvedfluorescence,aswellasbyfemtosecondtransientabsorption.

TGAofcompounds1and4areshowninFig.S1{.SWNT-H2Pcnanoconjugate1givesrisetoaweightlossofabout27%at600uC.However,atthistemperature,thephthalocyanineisnotcompletelydecomposed(seeFig.S1{),with38%ofitsoriginalmassstillremaining.Ifweassumeasimilarbehaviourin1,weroughlyapproximatetherealamountoffunctionalgroupstobearound43%;thiswouldcorrespondto1functionalgroupper190carbonatoms.

TEMconfirmedthepresenceofSWNTinoursamples.Tworepresentativeimagesofnanoconjugates1,showninFig.S2{,revealhighaspectratioobjectsthatappearthroughoutthescannedregions.Themeanlengthoftheseobjectsistypicallyintheorderofseveralmicrons,andtheirdiametersrangebetweenafewnanometersandseveraltensofnanometers.1hasalsobeen

investigatedbyAFM.ThesampleswerepreparedbyspincoatingonasiliconwaferfromaDMFsolution.ThepicturesinFig.1revealthepresenceofthinbundlesofnanotubes,welldispersedonthesubstrate,andterminatedbyindividualSWNTs.Thelineprofilebelowtheright-handpictureshowsthetypicalheightoftheobjectsonthesurface,withbundlesofnanotubeshavingdiametersrangingfrom3to20nm.

Next,theSWNT-H2Pcnanoconjugate1wasprobedinanumberofphotophysicalexperiments.Inthiscontext,animportanttestemergesarounditsabsorptioncharacteristics.Inparticular,theabsorptionspectrainTHFrevealonlyfeaturesthatcorrelatewithSWNT,despitethestrongabsorptioncross-sectionsofH2Pcinthevisibleregionofthesolarspectrum.Extinctioncoefficientsashighas26105M21cm21arefoundinthe700nmregion(Fig.2,top).Itisreassuringthat,uponphotoexcitationat680nm,asteady-statefluorescencespectrumevolvesthatisreminiscentofthatseenforH2Pc(4),namely,afluorescencemaximumat706nm.ThisconfirmsunmistakablythepresenceofH2PcinSWNT-H2Pc(Fig.2,bottom).WhenrelatingthefluorescenceintensityofSWNT-H2Pc(1)tothatofH2Pc(4)—withidenticalgroundstateabsorptionattheexcitationwave-length—asignificantquenchingofthefluorescenceisderived(y100).AnobviousrationaleforthistrendimpliesthepresenceofelectroactiveSWNT.

Time-resolvedfluorescencemeasurementsfurtherconfirmedthetrendofelectronicinteractionsbetweenphotoexcitedH2PcandSWNT.Whilealonglivedfluorescence—withalifetimeof4.3ns—wasseenfor4intherangeofthe700nmfluorescencemaximum,inSWNT-H2Pc,themajorcomponent(78%)offluorescencedecaywas57ps.{

Fig.1AFMimagesofSWNT-H2Pcnanoconjugate1preparedbyspincoatingonasiliconwaferfromaDMFsolution.

Fig.2Top:AbsorptionspectraofthepristineSWNT(palegrey),SWNT-H2Pc(1)(darkgrey)andH2Pc(4)(black).Bottom:Steady-statefluorescencespectraof1and4inTHFatroomtemperaturewithmatchingabsorptionsattheexcitationwavelength(i.e.,680nm).

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SWNTs.ThepropertiesoftheSWNT-H2Pcnanoconjugate1havebeenstudied,andwedemonstratedthatphthalocyaninecaninteractphotophysicallywiththenanotubesidewalls.

ThisworkwascarriedoutwithpartialsupportfromtheUniversityofTrieste,MUR(prinprot.2006034372andFirbRBNE033KMA),EU(RTNnetwork‘‘WONDERFULL’’),SFB583,DFG(GU517/4-1),FCIandtheOfficeofBasicEnergySciencesoftheU.S.DepartmentofEnergy.FundingfromMEC(CTQ2005-033/BQU)andComunidaddeMadrid(S-0505/PPQ/000225)isalsoacknowledged.

Notesandreferences

Downloaded on 15 September 2011Published on 10 May 2007 on http://pubs.rsc.org | doi:10.1039/B702819A{Theminorcomponent(22%),ontheotherhand,hada4.3nslifetime.

Fig.3Differentialabsorptionspectrum(NIR)obtaineduponfemtose-condflashphotolysis(660nm)ofSWNT-H2Pc(1)innitrogen-saturatedTHFwithtimedelaysbetween0and50psatroomtemperature.

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Decisiveinsightsintothenatureoftheelectronicinteractionscamefromtransientabsorptionmeasurements,whichwereperformedwithH2Pc(4)andSWNT-H2Pc(1)followingphotoirradiationoftheH2Pcgroundstatefeaturesaround680nm.IntheH2Pcreference,wesawimmediately,withtheconclusionoftheexcitation,thefollowingspectroscopicfeatures:maximaat485/800nmandminimaaround700nm.However,thesingletexcitedstatetransientwassurprisinglystable.Infact,withinthetimewindowofourexperimentalset-up(1.6ns),itbarelyshowedanydecayatall.Fromthefluorescencelifetimedata,wemustinferanintersystemcrossingtothetripletmanifoldsomewherearound4to5ns.

Whenlookingat1,thetransientabsorptioncharacteristicsthatwereseenduringtheinitialstage(i.e.,upto1ps)wereidenticaltothosesummarizedabovefor4(Fig.3).Thisobservationiscritical,sinceitcorroboratestheselectiveformationoftheH2Pcsingletexcitedstate.However,afterthisinitialstage,thespectroscopicandkineticfeaturesdiffersubstantiallyfrom4,whichlackanySWNT.Kinetically,forexample,amuchfastersingletdecayevolves.Infact,45psisagoodreflectionofthefluorescencelifetime.Spectroscopically,thetransients—withmaximaat845and920nm,andminimaat2,650and688nm—bearnoresemblancetoanyH2Pcexcitedstate(i.e.,singletortriplet).Particularlyimportantisthemaximumat920nm,whichcorrespondstothesignatureoftheone-electronoxidizedH2Pcradicalcation.

Inthecontextofradicalionpairformation,weturnedtotheNIRregion,wherethemostprominentelectrontransferchangesareknowntooccurforSWNT.Here,thefollowingfeatures—amaximumat995nm,andminimaat1080,1136,1204,1290and1395nm—promptthereductivechargingofSWNT.Importantinthiscontextistheblue-shiftoftheminimarelativetotheirgroundstatemaximaandexcitedstateminimaat1150,1215,1315and1435nm.Implicitarenewconductionbandelectrons—injectedfromphotoexcitedH2Pc.4b,12Extendingthemeasurementstonanosecondtimescalesalloweddeterminationoftheradicalionpairstability,withamajorcomponentof305ns.

Inconclusion,wehavedescribedthesynthesisofanewseriesoffreebasephthalocyaninesandtheattachmentofoneofthemto

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