TheEmergenceofCovalentOn-Surface Polymerization
ChristopheNacci,StefanHechtandLeonhardGrill
Abstract Thecovalentlinkingofmolecularbuildingblocksdirectlyinthe two-dimensionalconfinementofasurface,theso-calledon-surfacepolymerization, hasdevelopedrapidlyinthelastyearssinceitrepresentsareliablestrategytogrow functionalmolecularnanostructuresinacontrolledfashion.Here,wereviewthe growthofsuchstructuresviaon-surfaceUllmanncouplingandhighlightthemajor chemicalandphysicalaspects.Thesesystemsaretypicallystudiedbyscanning tunnelingmicroscopythatallowsexplorationoftheinitialmonomerspecies, intermediateproductsand fi nalnanostructureswithsub-molecularspatialresolution.Inthisway,thechemicalstructuresoftheexsitusynthesizedmolecular buildingblocksaredirectlycorrelatedwiththeoutcomeofthechemicalreaction. Wealsopresentexampleswithdifferentmonomerspeciesinviewofgrowing heterogeneousmolecularstructuresaswellastheimportanceofthemolecular interactionwiththetemplatesurfaceasafurtherkeyparametertocontrolthe moleculardiffusionandtunethe finalmoleculararchitecture.
1Introduction
Assemblingfunctionalmolecularbuildingblocksonasurfaceisapromisingroute towardcentralobjectivesofnanotechnologyandinparticularmolecularelectronics sinceitmightallowthegrowthofelectroniccircuitsbasedonthefunctionalitiesof individualmolecularspecies[1, 2].Otherbottom-upstrategiesleadtothegrowthof
C.Nacci L.Grill(&) DepartmentofPhysicalChemistry,Fritz-Haber-InstituteoftheMax-Planck-Society, 14195Berlin,Germany e-mail:leonhard.grill@uni-graz.at
C.Nacci L.Grill DepartmentofPhysicalChemistry,UniversityofGraz,8010Graz,Austria
S.Hecht(&) DepartmentofChemistry,Humboldt-Universit ätzuBerlin,12489Berlin,Germany e-mail:sh@chemie.hu-berlin.de
© SpringerInternationalPublishingSwitzerland2016
A.Gourdon(ed.), On-SurfaceSynthesis,AdvancesinAtomandSingle MoleculeMachines,DOI10.1007/978-3-319-26600-8_1
extendedsurfacesupportedtwo-dimensionalnetworkswithoutstandingtechnologicalrelevance[3, 4].Thus,althoughtheprecursormoleculesdonotcontaina functioninthesecases,theassemblyofmoleculesinthetwo-dimensionalconfinementofasurfacecanbeveryefficient.Inthe fi eldofweakerintermolecular interactions,manysuccessfulattemptsofgrowingsupramolecularpatternsatsurfaces[5–8]havebeenachieved.However,theuseofcovalentlinkingtostabilize moleculararrangementsatsurfacesattractedconsiderableattentioninthelastyears [9–27],becomingnowadaysawell-establishedtechnique.Thisapproachresultsin thepresenceofmolecularpolymersonsurfacesthatcouldhardlybedepositedonto thesurfaceundercleanconditionsbyusingconventionaltechniquesandpreventing anydefragmentationprocess[28].Thenatureofthecovalentbondprovideshigh stabilityandrobustnesstotheresultingnanostructuresandallowsforefficient “throughbond” chargetransport[29–31].
Inthischapter,wereviewthedevelopmentandconceptualfoundationofthe covalenton-surfacepolymerizationtechnique.Asourandmanyothers’ workis basedontheUllmannreaction[32],wefocusonthearyl–arylhomocouplingof halogenatedmonomerbuildingblockstypicallyperformedoncoinagemetalsurfaces.We firstprovidechemicalconsiderationsregardingthereactionmechanism andderivecriticalparametersforsuccessfullycarryingouton-surfacepolymerizations.Usingthisapproachcovalentlyboundmolecularassemblieswithapredefinedshapeandsizeareproducedunderultrahighvacuum(UHV)conditions.We showhowthe finaltopologyofthedesiredmolecularaggregatesisintimately connectedtothedesignofthesingle-moleculebuildingconstituents.Different growthstrategies,e.g.,one-stepversustwo-step(hierarchical)processes,can eventuallyleadtothesame finalmoleculararchitecture:themajordifferences betweenthetwocasesarehighlighted.Thesubstratesurfacecorrugationcanbe furthermoreexploitedtodriveon-surfacesynthesisprocessesalongcertaindirectionsandpromotethegrowthofnanostructureswithprede finedorientations.Inthis regard,theimportanceofthesurfaceanisotropyisdiscussed.
2ResultsandDiscussions
2.1On-SurfacePolymerizationTechnique
Ingeneral,theon-surfaceassemblyofmolecularbuildingblocksintolargeand extendedstructuresaccordingtoabottom-upschemecanbeachievedbydifferent strategies.Ifstabilizedbyratherweaknon-covalentintermolecularinteractions[5–8, 33],thesenanostructuresbelongtothe fieldofsupramolecularchemistry[34].For instancedipole–dipoleinteractionshavebeenusedtogovernthemolecularaggregationofporphyrinderivatives,carryingtwo trans-positionedcyanophenylgroups, intolonglinearchainsonaAu(111)surface(Fig. 1a)[6].Twoopposingcyanophenyl groupscanengageinaself-complementarydipolarinteraction(hydrogenbond) therebydrivinganddirectingtheself-assemblyintoelongatedporphyrinchains.
Fig.1 Supramolecularself-assembledmolecularstructures. a STMimageat63Kof trans-BCTBPPwiresholdtogetherviadipole–dipoleinteractionsonAu(111)(Reproducedfrom [6],withpermission). b Two-dimensionalnetworksofPTCDIandmelaminemoleculesstabilized viaH-bonding(modelstructuresofthemoleculesandnetworkintheupperpanel).Inthelower panel,anSTMimageofthenetwork( 2V,0.1nA).Inthe inset,ahigh-resolutionviewofthe Ag/Si(111)-√3 × √3R30° isshown(ReprintedbypermissionfromMacmillanPublishersLtd: Nature[7],copyright2003). c STMtopographicimageofanextendedandhighlyregularnetworks formedbyCodirectedassemblyofNC–Ph3–CNlinkers.Inthe inset,thestructureofthemolecule includingitslengthandSTMtopographyofthethreefold Co–carbonitrilecoordinationmotifwith modelstructureisshown(AdaptedwithpermissionfromSchlickumetal.[8].Copyright2007 AmericanChemicalSociety)
Amoreconventionalandstrongermultiplehydrogenbondingmotifwasusedto stabilizeatwo-componentmixtureof3,4,9,10-perylenetetracarboxylicdiimide (PTCDI)andmelaminemoleculesintoahoneycombpatternonametalsurface (Fig. 1b)[7].Thethreefoldsymmetricalmelaminemoleculesrepresentthebranch pointsofthehexagonalnetwork,whilethePTCDImoleculesserveasstraight connectors(Fig. 1b).Theassemblygeometryallowsforthelocalformationofthree hydrogenbondsforeachcomplementarymelamine–PTCDIconnectionandthis ratherstrongnon-covalentinteractionplaysthekeyroleinguidingthementioned speciesintolargelyextendedsupramolecularnetworks.Moreover,manyexamples oftwo-dimensionalmolecularassemblieshavebeenreportedinthe fieldof metallo-supramolecularchemistrywheremetalatomsareusedtobridgesuitably functionalizedmolecularunits(ligands).Themetal–ligandbondistypicallystronger ascomparedtohydrogenbondingandthisallowstheformationofmorerobust networks[33].Suchmetal–ligandinteractionshave,forexample,beenexploitedto fabricatetwo-dimensionalarchitecturesbasedonthecoordinationofrod-likedinitrilemolecules(NC–Phn–CN)tocobaltcenters(Fig. 1c)[8].
Inadditiontotheseinteractions,theformationofevenstrongercovalentcarbon–carbonbondsbetweenmoleculesonthesurfacegainedlargeattentioninthelast years[9–18, 30, 31, 35].Thenatureofthecovalentbondallowstoconferhigh stabilityanddurabilitytothemolecularstructures,incontrasttonon-covalent intermolecularbonds-basedstructures.Thispropertyisakeywhenthinkingof potentialuseinfutureapplications[2].Inanalogytotheapproachhere,the
Fig.2 Covalentlylinkedmoleculararchitecturesbyon-surfacepolymerization.Singlebuilding blocksaresynthesizedexsituwithhalogensubstituents.Afterbeingthermallyactivated,the speciesdiffuseacrossasurface,interacttoeachotherandtheformationofnewcarbon–carbon covalentbondstakeplaceattheactivatedsitespositions[9]
bottom-upgrowthoflargenetworksasgraphene[3]andboronnitride[4]sheets alsoledtohighlystablestructures,becauseofthecovalentnatureoftheirlinks.
Theconceptoftheon-surfacepolymerizationtechniqueisillustratedinFig. 2. Eachsinglebuildingblockisbasedonachemicallystablemolecularunitcarryinga certainnumberofpotentiallyreactivesitesatspeci ficpositions.Thesesitesare representedbyacarbon–halogenbondthathasabonddissociationenergylower thanallotherbondsinthemolecularframework.Afterdepositingthemoleculeson asurface,thehalogensubstituentsareactivated(i.e.,halogen–carbonbonddissociation)thermally,leavingthechemicalstructureofthemolecularbuildingblocks intact.Atthesametime,thenewspeciesthermallydiffuseoverthesurfaceand formnewcovalentC–Cbondsattheactivatedsitepositionswhentheygetcloseto eachother.
Thedesignandexsitusynthesisofmoleculeswithdifferentnumbersand arrangementsofinterconnectionpointsopensupthepossibilitytopreciselytunethe topologyofthe finalmoleculararchitecture.Beforedetailingthearchitectural controlachievableusingtheon-surfacepolymerizationapproach,afewaspects regardingthechemistryofboththemonomersaswellasthesurfacesneedtobe considered.Duetoitsdominantuseinthe fieldanditsimportanceforourown work,welimitthefollowingdiscussiontotheUllmannreaction.
3ChemicalConsiderations
WhenconsideringanUllmanncouplingreaction[32]astheconnectionsequence foranon-surfacepolymerization,severalkeycriteriahavetobemet.First,one needstodesignmonomers,whichontheonehandhavetobereactiveatthedesired connectionsitestoallowforregioselectiveactivation,forexample,bycarrying labilehalogensubstituents,yetotherwiseneedtobestableatthedepositionand reactionconditions.Inaddition,theactivatedmonomersalsohavetobemobileon thesurfacetodiffusetoothermonomersandthegrowingpolymer.Thelatterpoint inevitablyalsodependsonthesurface,whichneedstostabilizetheformedaryl
Fig.3 Possibleactivationmechanismsforarylhalidestoinitiatecovalenton-surface polymerization
radicalintermediates,yetalsohastoprovidemobilityandideallyfacilitateboththe activationandconnectionsteps,i.e.,actastemplateandcatalyst.
Whiletheseaspectsgenerallyapplytomoston-surfacepolymerizationreactions, therearesomespecificaspectswhenfocussingontheUllmannreaction.The reactioncanbeinitiatedbyseveraldifferentdissociationmechanismscausedsimply byheat(inabsenceorpresenceofametalcatalyst),electrons(fromthetipofan STM,andelectrodeorareducingagent)orphotons(Fig. 3).
Whileinallcasesthearyl–halogensinglebondisbroken,thetechnique/stimulus usedforactivationpotentiallyprovidescontroloverwheretheUllmannreaction andhencepolymerizationistakingplace.Incontrasttothepioneeringworkofthe RiedergrouponthedimerizationofiodobenzeneinducedwiththeSTMtipatthe stepedgeofaCu(111)surface[36]themajorityofthereportedworkhasbeen exploringthermalactivationmostlyinconjunctionwithcoinagemetalsubstrates. Hereby,thetemperaturerequiredfordissociationofthehalogensubstituentcruciallydependsonthetypeofhalogen(andpotentiallyalsoonthetypeof(het)aryl moiety)aswellasthetypeofsubstrate.The firstaspecthasbeenexploitedbyusfor thehierarchicalgrowthoftwo-dimensionalpolyporphyrinnetworks(seebelow), whereweutilizesequentialactivationof firstiodineandthenbrominesubstituents toseparatethetwoorthogonalgrowthdirections[35].WhiletheC–Ibondsare cleavedat120 °C,theC–Brbondscleaveat250 °ContheemployedAu(111) surface.Ofcourse,thelatterisimportantaswellsincesimilarC–Ibondscleaveat muchhighertemperaturesintheabsenceofacoinagemetalasshownbythework ofGourdon,Kühnle,andcoworkersoncalcite(CaCO3),wheretemperatureabove 300 °Carenecessaryforactivation[37].
ClearlyandnotsurprisinglyinthecontextoftheclassicUllmannworkusing copperspecies[32],coinagemetalsfacilitateactivationandaryl–arylcoupling[38]. However,therearetwoopposingeffectswhencomparingthecoinagemetalswith regardtotheirabilitytoaidon-surfaceUllmanntypepolymerization:Ontheone handthehigherreactivityoflessnoblecoppersurfacesaidsboththeinitialhalide dissociationaswellasthecouplingoftheactivatedarylmonomersbutalsosignificantlylowersthemobilityandhencediffusionofthemonomersandgrowing
polymers,therebyinhibitinggrowth.Faselandcoworkershaveactuallyengagedin adetailedcomparativestudyshowingtheseopposingeffectsfortheCu(111),Ag (111),andAu(111)surfaces[39].Theauthorsfoundtheonsetofnetworkformation fromhexa(meta-phenylene)macrocyclichexaiodidemonomerstooccurat200 °C forCu(111),whileonAu(111)250 °CandonAg(111)300 °Cwererequired. However,themorphologyoftheobtainedpoly(1,3,5-phenylene)sdifferssignificantlyastheCu(111)grownstructuresarehighlybranchedfractal-likewhileinthe caseofAg(111)extendedhigh-quality2Dnetworkswereformed.Basedontheir experimental findingsaswellastheoreticalinvestigations,theyconcludethatthe loweractivityofAg(111)inthearyl–arylcouplingcombinedwiththehigher monomermobility(diffusion)onthissurface,bothcomparedtoCu(111),leadto betternetworkformation.Inourworkwehavebeenmostlyfocussingongold surfacesthatprovideagoodcompromisebetweenthesefeatures.Notethateven withoneandthesamemetalitssurfacereconstructionplaysanimportantroleas shownbyourownwork(seebelow)aswellasothers[40].
Inaddition,defects,stepedges,andadatomsareofutmostimportanceasthey canfacilitateactivation(seebelow)[41],stabilizeintermediates,andeveninhibit theircoupling.Thisisnicelyillustratedbythefactthatactivatedarylmonomers cannotbeconsideredastruly “freeradicals” butarestronglystabilizedbethemetal surface[39].Thisalsopreventsskeletalrearrangementstotakeplaceandthereby assuresregioselectivecouplingattheinitiallyhalide-substitutedpositions(Fig. 4). Dependingonthepresenceofadatoms,analternativecouplingmechanism involvestheformationofanaryl–metal–arylintermediate,whichcanreductively eliminatetoformthedesiredaryl–arylconnection(Fig. 4).Whilethissequencehas infactsuccessfullybeenobservedbyLinandcoworkerstotakeplaceinthe polymerizationof4,4″-dibromoterphenylonaCu(111)surface[42],inmanycases theintermediatelyformedcoppercomplexesareratherstableandcannotbeforced toeliminatethedesiredproducts[43, 44].Forexample,usinghexabenzo-coronene (HBC)dibromidemonomersonCu(111)gaveCu-bridgedHBCchains;however, onaAu(111)surfacethecorrespondinggoldcomplexeswerenotobserved andhencecovalentaryl–arylconnectionscouldsuccessfullybeobtained(Fig. 5)
Fig.4 Possiblecouplingmechanismsforarylhalidestoinitiatecovalenton-surfacepolymerization:regioselectivecoupling(a)andaryl–arylconnectionviaanintermediateformationofan aryl–metal–arylintermediate(b)
Fig.5a ChemicalstructureofBr2–HBC. b Cu-bridgedHBCchainonCu(111)(5.5 × 2.0nm2, 300mV,0.3nA). c HBCchainonAu(111)(5.5 × 2.0nm2, 300mV,0.1nA). d Heightprofiles inSTMimagesalongaHBCtrimeronCu(111)andAu(111)[43]
[43].Therefore,notonlythetypeofsurfacebutalsotheavailabilityofadatoms seemstohaveamarkedeffectonthepolymerizationoutcome.
Ingeneral,wenotethatusingtheUllmannreactionposestwoinherentlimitationstotheon-surfacepolymerizationprocess.Firstandforemost,thereactionis irreversibleundertheemployedconditions,i.e.,formeddefectscannotbehealed. Therefore,theoutcomeofthereactionsolelyreliesonkineticcontrolandequilibrationtotheglobalthermodynamicminimumstructurescannotbeusedasoften thecasefornon-covalentself-assemblyordynamiccovalentchemistry[45].Using otherconnectionssuchasboronicestersoriminesthisdrawbackcanbeovercome, however,atthecostofstability(towardhydrolysis)andfunctionality(inan optoelectroniccontext).Second,theemployedpolymerizationapproachisthatofa stepgrowth,morepreciselyapolycondensation,andthereforeintrinsiclimitation withregardtopolymerizationefficiencyandcontroloverthepolymerizationoutcomeexist.Aftersketchingthechemicalbasisformakingaryl–arylconnections, wewillnowdetailthemethodofcovalenton-surfacepolymerizationandhighlight themeansofcontrollingtheformedpolymerstructures.
4On-SurfaceSynthesisofCovalentlyBound Nanostructures
Twoalternativemethodscanbeusedfortheactivationofmolecularbuildingblocks (methodsIandII)andthegrowthofcovalentlyboundnanoarchitectures,leadingto similarresults[9].InmethodI,intactmoleculesare firstdepositedontoasurface andsubsequentlythermallyactivated.Conversely,inmethodII,theactivationof molecularspeciestakesplacealreadyintotheevaporatorcellandtheyaredeposited ontothesurface.
Inbothcases,thecovalentlinkingtakesplaceonthesupportingsurfaceupon thermaldiffusion.Asa firstcandidateforon-surfacesynthesis,aporphyrinbuilding blockwithfourbrominesubstituents(Br4TPP)hasbeenused(insetofFig. 6a).If theevaporatortemperaturewas550Korlowerduringdeposition,largeandordered islandsofintactBr4TPPwerefoundasaresultofmoleculardiffusionatthesurface
Fig.6 Molecularnanostructuresformedbydifferentapproaches(methodsIandII). a STMimage (20 × 20nm2)ofaBr4TPPmolecularislandonAu(111)afterdepositionatlowevaporator temperatureof550Kontothesubstratesurfacekeptatroomtemperature.Moleculesaredeposited intactontothesurface.The inset showsthechemicalstructureofBr4TPP. b STMimage (41nm × 41nm2)fordepositionatelevatedevaporatortemperatureof610K.Thiscausesthe activationofthemolecularspeciesintotheevaporatorandsubsequentlytheformationof covalentlyboundstructuresontothesurface.TheAu(111)samplewascleanedbyrepeatedNeion sputtering(E =1.5keV)andsubsequentannealingupto720K.Measurementswereperformed underUHVconditionswithalow-temperatureSTMoperatedatatemperatureof10K.STM imageswererecordedinconstantcurrentmodewiththebiasvoltagereferringtothesamplewith respecttotheSTMtip[9]
(methodI,Fig. 6a).Acarefulanalysisoftheouterborderofthemolecularisland revealsthatmanymoleculeshaveonlythreeBratomsconnectedwhilethereare fourontheintactmolecules.Thissuggeststhattheusedevaporatortemperatureis enoughtoinitiatetheBrdissociationofasmallamountofmolecules(morethan 90%ofthemoleculesremainintact).
Athigherevaporatortemperatures(Fig. 6b)mostofthemoleculesareactivated withthelossofseveralBrsubstituentsintheevaporator(methodII).Theactivated speciescanreactwitheachotheronthesurfaceandformnewintermolecularbonds uponthermaldiffusion,leadingtotheformationofcovalentlyboundstructureswith differentsizesandshapes(Fig. 6b).
Toinvestigatetheabilitytocontrolthearchitectureofthe finalmolecular nanostructures,differentTPP-basedmonomerbuildingblockshavebeensynthesizedwithone,two,andfourBrsubstituents(Fig. 7a–c).Intactmoleculeshave beenidentifi edbyusinglowevaporatortemperatures:theSTMimagesafterthe preparationshowclearlytheexpecteddifferentstructures(Fig. 7d–f).Allspecies havebeendepositedontoaAu(111)surfacekeptatlowtemperature(tosuppress anycarbon–halogenbonddissociation)andafterwardsannealedtothermally activatetheBrdissociation.Thus,thetopologyofthemoleculararchitecturesis intrinsicallyencodedinthedesignofthesinglemonomerbuildingblock(cf. fi rst andthirdrowsofFig. 7).
Ifthemonomerbuildingblockprovidesjustonereactiveside(BrTPP,Fig. 7a),the onlypossibleresultisadimer.Porphyrinbuildingblockscarryingtworeactivesides
Fig.7 Buildingnanoarchitecturesusingdifferentmonomerbuildingblockscarryingone(left column,preparedbymethodI),two(middlecolumn,preparedbymethodII)andfour(right column,preparedbymethodI)Brsubstituents(a–c).STMimages(3.5 × 3.5nm2)ofthesingle intactmolecules(d–f).OverviewSTMimages(30 × 30nm2)ofthenanostructuresafteractivation andconnection(g–i).DetailedSTMimagesoftheresultingnanoarchitectures(j 5 × 5nm2; k 10 × 10nm2; l 8.5 × 8.5nm2).Correspondingchemicalstructuresofthenanostructures(m–o). Measurementswereperformedunderultrahighvacuum(UHV)conditionswithalow-temperature scanningtunnelingmicroscope(STM)operatedatatemperatureof10K.Covalentlylinked molecularstructureswereproducedincaseofmethodIfrommolecularbuildingblocksvia on-surfacepolymerization[9],i.e.,dehalogenationatatypicaltemperatureof523K(bromine dissociation)for10minandsubsequentcovalentlinkingofthemolecules[9]
astrans-Br2TPP(Fig. 7b)allowsaccordinglytheformationoflongandlinearchainsas showninFig. 7h,k.WhenallfourporphyrinunitlegscarryBrsubstituents(Fig. 7c), theconstructionoftwo-dimensionalmolecularnetworkisenabled(Fig. 7i,l).This provesthatacarefulchoiceofthemoleculardesign,i.e.,thearrangementoftheactive endgroupswithinthemolecularframeworkofthesinglebuildingblock,andasuccessfulexsituorganicsynthesisoftheinitialbuildingblocksgivehighcontroloverthe finalarchitectureofthemolecularstructures.
Animportantissueistheprecisechemicalnatureofthenewlyformedintermolecularbonds(orintramolecularbondsinthe finalpolymer,respectively).The firstevidencecomesfromthedistancesbetweenthebuildingblocks,whichis characteristicforsuchabond.Thereisagoodagreementbetweentheexperimentallymeasuredneighboringporphyrincoresinterdistance(17.2 ± 0.3 Å)andthe DFT-calculateddistance(17.1 Å)calculatedforacovalentlyboundporphyrins dimer(Fig. 8d).Furthermore,thecovalentnatureoftheintermolecularbondscan beinvestigatedbySTMsingle-moleculemanipulation.Molecularislandsmadeof intactBr4TPP(Fig. 6a)areeasilydisassembledbySTM-basedlateralmanipulation [9].Incontrast,dimers,chains,andmolecularnetworks(Fig. 7)canfollowthe STMtippathwayduringapullingexperiment[30, 31]withoutundergoingfragmentationprocesses.Thisisaclearsignaturefortherobustnessoftheintermolecularbondswithinthemolecularstructuresaftertheend-grouplegsactivation. Consequently,theinterpretationasacovalentbondseemsreasonable.Other
Fig.8 Thecovalentnatureofintermolecularbonds.STMimages(5 × 5nm2)ofaTPPdimerat 0.5V(a)and3.0V(b).Thebrightprotrusioninthemiddleofthedimer(b)isasignaturerelatedto anelectronicfeatureclearlyvisibleinthedI/dV curvemarkedbya cross inpanel(c).ThelowerdI/ dV curve(markedbya circle)takenontopofaporphyrinlegisfeatureless.DFTcalculationsreveal theformationofacovalentbondbetweenthetwoneighboringphenyllegs,withcorrespondingC–C bonding(s)andantibonding(s*)orbitals. d Calculatedgeometricstructureoftheisolateddimer. e Calculatedcontributiontothelocaldensityofstatesduetothestateatabout2.8eVabovethe HOMO(at7 Å fromtheporphyrinplane). f Sideviewofathree-dimensionalcontourplotofthe orbitaldensityofthisstateatamuchhigherdensity.Scanningtunnelingspectroscopy(STS)was performedat10Kwithalock-inamplifierwith20mVpeak-to-peakmodulationamplitudeat 640Hz(frequency)(seecaptionFig. 7 forfurtherexperimentaldetails)[9]
options,i.e.,chemicalbondsasHormetal-ligandbondingand π–π stacking,canbe ruledoutbecauseofthemolecularstructureandadsorptiongeometry,andadditionallytheycouldhardlyexplainwhythenanostructureremainsstablewhenbeing pulledbyanSTMtip.
Aclearsignatureforthecovalentnatureoftheintramolecularbondwithinthe dimerisprovidedbyspectroscopyofsinglemolecules(byscanningtunneling spectroscopy,STS).Theinterconnectionsitewithinthedimerappearshomogenously atlowbiasvoltages,whileitappearsasabrightprotrusionwhenimagedat+3.0V (Fig. 8a,b).Thisprotrusionisindeedrelatedtoanelectronicbroadfeatureslocalized ataround+3.0V(upperSTScurveinFig. 8c)suggestingthepresenceofalocalized orbital[9].DFTcalculationsprovethelocalformationofacovalentC–Cintermolecularbondinfullagreementwiththeexperimentallymeasuredporphyrincores interdistance.Speci fically,thecalculationsrevealedtheformationofC–Cbonding (σ)andantibonding(σ*)orbitalsthatgiverisetothesignalinthedI/dV spectra. Hence,thepeakataround3eVisadirect fingerprintofthechemicalnatureofthe covalentbond.Itiscausedbythestronginteractionwiththetwonon-occupied antibonding π orbitalsassociatedtothetwolegs,resultinginanin-phaseandan out-of-phasecombination,whicharesplitby1.3eV.Thein-phasecombinationis responsiblefortheincreaseofthecalculatedlocaldensityofstatespreciselylocated inbetweentheporphyrincoresatabout2.8eV(Fig. 8e,f)abovethehighestoccupied molecularorbital(HOMO).Thiscalculatedelectronicfeatureisassociatedtothe experimentallyprobedelectronicfeatureatabout3.0eVshowninFig. 8 [9].
AnotherclearexampleofUllmanndehalogenationreactionthatresultsinpolymerizationonthesurfacewasreportedbythegroupofRosei[19].Theydeposited diiodobenzenemolecularspecies(1 and 2 inFig. 9a)onCu(110)andfoundat fi rst theformationofCuboundphenyleneintermediates,i.e.,notyetlinkedbyC–C
Fig.9 Formationofpolyphenylene-basedpolymersbyon-surfacepolymerization. a Ullmann couplingofdiiodobenzenemolecules. b STMimage(T =115K,19 × 19nm2, V = 1.93V, I=1.06nA)ofPPP-basedolygomers.1,3-diodobenzene(1 inpanel a)weredosedonCu(110) keptatroomtemperatureandafterwardsannealedto500K. c 0.2Lof1,3-diiodobenzenedosed ontoCu(110)heldat500K.STMtopography(11.3 × 11.3nm2, Vs = 0.57V, It =0.82nA)of oligomerbranches.AmodelofPMPisoverlaidononeoftheoligomers. Top-rightinset aforce fieldrelaxedmodelofPMPchainintheiodinematrixonaslabofCucorrespondingtothemarked regionintheSTMimage. Bottomrightinset ascaleportionoftheRTdepositedsurface,showinga protopolymerofmolecule 2 inpanel a.Reproducedfrom[19]. © 2009Wiley-VCHVerlagGmbH &Co.KGaA,Weinheim.doi:10.1002/smll.200801943
covalentbondswhendepositingmoleculesonthesurfacekeptatroomtemperature. Heatingthesampleto500Kfor5–10minisneededtoinducetheformationof straightconjugatedPPPolygomers(Fig. 9b).TheformationofzigzagPMPwires andmacrocyclesaswellwerepromotedandobservedwhenusing 1,3-diiodobenzene(Fig. 9c,kinksareascribedtothemolecularsymmetry)[19].
5ControllingNanostructuresbyHierarchicalGrowth
Theresultspresentedsofararerelatedtothegrowthofsimplehomogeneous moleculararchitectures,becausetheyarebasedonaone-stepprocess.Growing complexnanostructures,e.g.,morecomplexmolecularaggregates,requiresa fine andaccuratecontrolofthereactionpathwaythatleadstothe finalmolecular architecture.Thiscanbeachievedsplittingthereactionpathwayintoindividual connectionstepsandcontrollingtheiractivationsequence,thusrealizinga “programmedreactivity” ofthemoleculesthatallows selective activationoftheir reactivityatdifferentsites.Asequentialgrowthfashioncanbeimplementedby designingsinglemolecularbuildingblocksthatcarrydifferenttypesofhalogen substituents.Thesampletemperaturecanbeusedasan “externalknob” thatallows toenableorsuppressspecifi edhalogendissociations,i.e.,on-surfacepolymerizationprocessescanbeinitiatedandsystematicallycontrolledviathesampletemperature.ThetemperatureneededtobreakC–halogenbondsismainlydefinedby thehalogenspeciesandthecatalyticactivityofthesurface.Thecarbon–halogen bonddissociationisactivatedattemperaturesthatdecreasewiththehalogenatomic number.Inotherwords,thebindingenergytothecarbonatomcanbetunedviathe typeofhalogenatom.Iodinedissociationfrommoleculescanbeinitiatedalreadyat roomtemperatureandcompletedataround120 °C,whilethistemperaturerange goesfrom100to250 °CforBrsubstituents[35, 46].Aproperchoiceofthesurface iscrucialasithasbeenshownthattheon-surfacecovalentlinkingoccursat differenttemperaturesfordifferentnoblemetalsurfaces[39],orcanevenbesuppressedforothersurfaces.Gutzleretal.[16]reportedonthegrowthof two-dimensionalcovalentboundnetworksbyusingpolyaromaticmoleculescarryinghalogensubstituents.TheydepositedthesemolecularspeciesonCu(111)and Ag(110)andindeedveri fiedthepresenceofactivatedspeciesalreadyatroom temperature,i.e.,withouttheneedofadditionalactivationenergy.Thesameprocedurerepeatedongraphite(001)resultedintheformationofwell-ordered non-covalentlyboundnetworksstabilizedbyhalogen–hydrogenbonding.This provestheimportanceofthesurfaceinpromotingthecarbon–halogendissociation atroomtemperatureandthesubsequentmolecularassembly.
Asmentionedabove,thearchitectureofthe finalstructuresisencodedinthe singlemonomerbuildingblockbyincorporatingdistinctcarbon–halogenbonds thatdissociateandcreateactivesitesatthehalogensites.Withthispurpose,a porphyrin trans-Br2I2TPPunithasbeendesignedandsynthesizedinordertocarry twodifferenttypesofhalogen–phenylsidegroups(Fig. 10a). Trans-Br2I2TPP
Fig.10 SinglemonomerbuildingblockscarryingBrandIsubstituentsforsequentialactivation. a Chemicalstructureofthe trans-Br2I2TPP.BrandIchemicalgroupshavedifferentchemical activationtemperatures. b STMimage(0.5V,0.1nA)ofasingleintact trans-Br2I2TPPonAu (111).IsubstituentsappearbrighterthanBronesbecauseoftheirdifferentchemicalstructure. MeasurementswereperformedunderUHVconditionswithalow-temperatureSTMoperatedata temperatureof10K.Moleculesweresublimatedat593KontoAu(111)keptatroom temperature[35]
moleculeshavetwopairsofhalogensubstituents(BrandI)eachofthemina trans configurationonoppositesidesoftheporphyrinunit(seeFig. 10a).Thischemical structureintrinsicallyencodestwodifferentgrowthdirections.Asthetwosubstituentshaveapronounceddifferenceintermsofbonddissociationenergy(the bindingenergyofiodine–carbonislowerthanthatofbromine–carbon)[35],this allowstocreateactivesitesinthemoleculestep-by-step.Inthisway,newcovalent intermolecularC–Cbondsareformedwithgeometriccontrol(viathetemperature) andconsequentlysequentialgrowthofnanostructuresisachieved(seegrowth schemeinFig. 10a).Low-temperatureSTMimagingallowstoresolvewith sub-molecularresolutionthefeaturesofintact trans-Br2I2TPPmoleculesdeposited ontopofAu(111):thetypicalfour-legsstructureoftheporphyrinunitisrecognized andsubstituenthalogenscanbechemicallydistinguishedbecauseoftheirdifferent appearanceinSTM(Fig. 10b):IandBrsubstituentshavespecifi capparentheights andtheformerlookbrighterindependentofthebiasvoltageovertheinvestigated range( 1V,+1V)[35].Thus,bycomparisonwithotherporphyrinderivativesthat containeitheronlyBroronlyIsubstituents,itispossibletoassignthecharacteristic apparentheightstoiodineandbrominesubstituents.Thispreciseknowledgeofthe chemicalcompositioninanSTMimage(Fig. 10b)isimportantinthenextstepto identifywhichsubstituentsremainafteraheatingstepandwhichonesare dissociated.
Trans-Br2I2TPPmoleculeshavebeendepositedontoAu(111)whilekeepingthe substrateatatemperatureof80Ktosuppresscatalyticallydriveniodinedissociationfromthemoleculesthatoccursathighertemperatures,thustokeepthe moleculesintactwithallfourhalogensubstituents[35].Undertheseconditions, molecularunitsarepreferentiallyfoundinclose-packedarrangements(Fig. 11b).
Fig.11 Hierarchicalgrowthofhomogeneousmolecularstructures. a Schemeofthesequential activationmechanism(from left to right).Inthe firstactivationstep,Isubstituentsaredissociated andactivesitesina trans geometry(firstgrowthdirection)arecreatedenablingtheformationof linearstructures(from b to c).Inthesecondstep,Braredissociatedbyannealingathigher temperatures.Thisfurtherstepallowstocreatelateralactivesitesthatenablethegrowthalongthe secondgrowthdirection,i.e.,theformationof2Dnetworks(from c to d).STMimages(8 × 8nm2, b)of trans-Br2I2TPPmoleculesonAu(111),afterheatingupto120K(step1,8 × 8nm2, c),and afterfurtherannealingupto250K(step2,10 × 10nm2, d). e STMimage(10 × 10nm2)of close-packedporphyrinchainsafterthe firstactivationstep.Furtherexperimentaldetailsarein captionFig. 9 andRef.[35]
AnnealingofthesampleuptoroomtemperatureinducesapartialIdissociation, whileannealingupto120 °Cenablesanefficientpolymerizationacrossthe transiodinedirection(firstgrowthdirectioninpanelFig. 11a)duringthe fi rst step.Accordingtothe trans-arrangementofhalogensubstituentswithinthesingle monomer,linearchainsofporphyrinunitsaregrown(Fig. 11c).Thisisinanalogy tothe trans-Br2TPPmolecules(Fig. 7h)butatlowertemperaturesbecauseiodineis involvedhere.Therearetwoimportantcharacteristicsoftheseintermediateproducts(showninFig. 11c):(1)Thesechainsalwayshaveabrightlovesattheirend, whichreflectsaniodineatom(asinFig. 10b).Hence,allnewlyformedbondsare locatedatformeriodinesites,whichconfi rmsthesuccessfulselectiveactivationin this firststep.(2)TheBrsubstituents,whichappeardarkerthantheiodines,canbe clearlyseensidewaysatthepolymerchainandarethereforestillpresent.However, theyhavenotbeenactivatedyetandarethereforedormant,waitingtobeactivated atasuitabletemperature.
Furthermore,covalentlylinkedporphyrinchainsarrangethemselvesparallelto eachotherintoclose-packedislands(Fig. 11e).Inthenextgrowthstep,Brsubstituentsareeffi cientlydissociatedbythermalannealingupto250 °Cenablingthe polymerizationprocessalongthesecondgrowthdirection(asindicatedinFig. 8a) andtriggeringtheformationofTPP-basedtwo-dimensionalnetworks(Fig. 11d). Thisrepresentsanelegantwaytogrowtwo-dimensionalnetworksinasequential manner,anditisworthtocompareitwiththesamestructureobtainedbythe one-stepgrowthprocess(TPP-basednetworksinFig. 7i,l).Ananalysisofthe
regularityoftheTPP-basednetworksgrownfollowingbothmethodssuggeststhat thehierarchicalgrowthallowstoprepare2Darchitectureswithlessincorrectly connectedbuildingblocks,i.e.defects,andlargerspatiallyextentregularnetworks (adetailedanalysisispresentin[35]).
Heterogeneousmoleculararchitecturesmightbegrownaccordingtoahierarchicalgrowthscheme.Covalentlylinkedtwo-componentstructuresonmetalsurfacesunderUHVconditionhavealreadybeenachieved[10],althoughinaone-step growthprocessandthuslimitedcontrol.Thecapabilitytoactivatedifferentreaction pathwaysstep-by-stepallowsabettertuningofthegrowthprocess.Whilethe formationoftwo-dimensionalTPPnetworkscouldalsobeachievedinaone-step process(Fig. 7l),themixtureoftwomolecularspeciesinadditiontotheselective activationmechanismleadstomolecularnanostructuresthatcannotbeformedin onestep.Whencombining trans-Br2I2TPPandDBTFmolecules(Fig. 12a)onaAu (111)surfacethetwogrowthstepsaresequentiallyactivatedwhenheatingthe sampleat250 °C.First,iodinesof trans-Br2I2TPPmoleculesaredissociatedand linearporphyrinchainsarecreatedwhileBr-phenylgroupsremainintact(Fig. 11c). Second,BrsitesaredissociatedandDBTFmoleculesformlinearchainsthat connecttotheformerBrsiteofporphyrinbuildingblocks(Fig. 12b).Inthiswaya ladder-typestructureisformedthatcouldnotbeachievedinonestep.
Adetailedanalysisofthecovalentlinksattheactivatedphenylgroupsof porphyrinbuildingblocks(showninFig. 12c)revealsthehighselectivityofthe process:98%oftheformerIsitesof trans-Br2I2TPPmoleculesareconnection pointsforfurtherporphyrinunitsasdesiredfromthemoleculardesign.Only2%of
Fig.12 Hierarchicalgrowthofheterogenousarchitectures. a ChemicalstructureofDBTF molecules. b STMimage(T =10K,18 × 13nm2)ofheterogenousnetworksbasedonDBTFand trans-Br2I2TPPonAu(111)byhierarchicalgrowthafterheatingupto250 °C. c Statisticalanalysis ofporphyrinand fluorineattachmenttotheporphyrin trans-Br2I2TPPmonomeratformerbromine andiodinesites(numberofevaluatedsites: nI =489, nBr =269).Measurementswereperformed underUHVconditionswithalow-temperatureSTMoperatedatatemperatureof10K.AKnudsen cellwasusedtoevaporateBr4TPPmoleculesat550KandDBTFmoleculesat503KontoAu (111).Theon-surfacesynthesiswasachievedraisingthesampletemperatureto250 °C[35]
thesesitesareincorrectlyusedfor fluorineconnections.Thesecondgrowthstep determinesapronouncedoccupationoftheremainingtwoBrsitesby fluorene molecules(70%,seeFig. 11c).Thenumbersarelessimpressiveinthissecond case,becauseattheformerBrsitesalsotwoporphyrinchainscouldbelinkedside toside,whichrepresentacompetitionprocessfortheplannedladderstructure.This provesthatthehierarchicalgrowthleadstotheformationofcopolymersassistedby aremarkabledegreeofselectivityofthechemicalspeciesinvolvedintheprocess.
6Substrate-DirectedGrowthbyOn-SurfaceSynthesis
Theon-surfacesynthesisconsistsoftwoprocessesatwork:activationanddiffusion ofthesinglemonomerbuildingblocksacrossthesurface.Elevatedtemperaturesare requiredtoenabletheseprocesses,butthisalsofavorsdisorderintothemolecular assemblyandcanthereforereducetheefficiencyofthepolymerizationprocess.It shouldbenoted,however,thatthesubstratesurfaceisnotapassivesupportfor chemicalspecies[16]butcanplayanactiveroleintermsofactivationofthe molecularspeciesinvirtueofitscatalyticproperties[16, 41].
Anycrystallinesurfaceexhibitsacertaincorrugation,dependingonthecrystal structureandthesurfaceorientation,whichplaysacrucialroleformoleculardiffusion.Thisfeaturecanbeusedinordertointroduceafurtherdegreeoffreedom, thusimprovingthecovalentlinkingandvaryingthe finalorientationofananostructurecomparedtotheunderlyingsubstratesurface.Bychoosingproperlythe surfaceitispossibletorestrictthemoleculardiffusionalongthelowestcorrugation directionsandfavortheformationofspeci ficmoleculararchitectureswithapredefinedorientation.Forinstance,theAu(110)-1 × 3surfacehasbeenusedto constrainthediffusionandsubsequentpolymerizationofalkylchainsalongits missingrows[40].Theconfinementofmoleculardiffusiontoonedimension (Fig. 13)leadstointermolecularinteractionsbetweenneighboringmoleculesthat resultintheformationoflinearmolecularchains[40].
Theeffectofsurfaceanisotropyonthegrowthoftwo-dimensionalnetworkshas beenstudiedwithanAu(100)single-crystalsurface.Thereconstructedsurface showsaquasi-hexagonal(5 × 20)superstructurewithstraightrowsofvertically displacedatoms[47],asshowninFig. 14a[35]. Trans-Br2I2TPPmoleculeshave beendepositedonAu(100)atlowtemperatureinordertokeepallhalogen–phenyl groupsintact.Afterwardsthesamplewasannealedto120 °C.Afterthisprocedure, covalentlyboundporphyrinchainswithapreferentialorientationarefoundas illustratedinFig. 14a.Hence,thesurfacereconstructiondeterminestheorientation ofthe finalnanostructure.
Fig.13 Polymerizationofhydrocarbonsonananisotropicgoldsurfaces. a STMtopographic imageofDEBmolecules(12 × 12nm2, 0.5V,0.5nA)onAu(110)-1 × 2at300K. b STM topographicimage(17.5 × 6nm2, 1V,2nA)polymerizedDEBchainslocatedinthemissing rowsofAu(110)-1 × 3afterheatingat420Kfor10h. Circles denotethephenylenegroups; arrows denotethemethylsidegroups. c AsectionofDEBpolymerchainandsuperimposedthe molecularstructure(14.4 × 1.6nm2, 1V,2nA).ThenewlyformedC–Cbondsareshownin red From[40].ReprintedwithpermissionfromAmericanAssociationfortheAdvancementofScience
Ananalysisofthechainsangulardistributionrevealsapreferredangleat51° betweenchainsandatomicrows(Fig. 14b).This findingcanbeeasilyrationalized bygeometricargumentssinceallporphyrinunitsareadsorbedonequivalentsites (Fig. 14c),thusreducingthetotalenergybythisparticularangle[35].Thisisin contrasttotherather flatAu(111)surfacewheretheangulardistributionofchainsis lessdefi ned(Fig. 7 andRef.[1])andunderlinestheimportanceofthesurface corrugation.Afterheatingto250 °C(thesecondactivationstep),formationof rectangularnetworksisagainfound(Fig. 14d)withaclearorientationofftheAu (100)atomicrowsorientation[35].Smallnetworksrevealdeviationsfromthe rectangularshape(angle β =101 ± 3° insteadof90°)asshowninFig. 14d.This effectismostlikelyascribedtothereducedrelativecontributionofintermolecular bondenergycomparedtotheinteractionofthemolecularassemblywiththesurface [35]forsmallnetworks.Furthermore,alargeraveragesizeofnetworksisachieved ascomparedtotheAu(111)surface[35],whichcandirectlybeassignedtothe surfaceanisotropy.Thecorrugationrowsleadtoaparallelarrangementofthe intermediateproducts(asillustratedinFig. 14a)thatresultsinasortofzipping mechanismforthetwo-dimensionallinkinginthesecondstep:Ifthe firstlink betweentwochainsisestablished,allotherporphyrinunitsareinaperfect arrangementwithrespecttoeachotherandaratherefficientlinkingoflongchain segmentscanoccur.Asaconsequence,the finalnanostructuresarelargerfor hierarchicalgrowthonacorrugatedsurfacethaninasingle-stepprocess.
Fig.14 Substrate-directedgrowthofnetworks. a STMimage(42 × 42nm2)of trans-Br2I2TPP chainsgrownonAu(100)afterthe firstactivationprocess. b Angulardistributionforchainsshown inpanel a c AdsorptiongeometryschemeofpolymericchainonAu(100)surfacewithanangleof 55° forequivalentadsorptionsitesforallporphyrins(a0 =1.44nmand d0 =1.76nm). d STM image(20 × 20nm2)ofanapproximatelysquaredcovalentlylinkedmolecularnetworkafterthe secondactivationprocess.MeasurementswereperformedunderUHVconditionswitha low-temperaturescanningtunnelingmicroscope(STM)operatedatatemperatureof10K. AKnudsencellwasusedtoevaporateBr4TPPmoleculesat550KontoAu(100).The firstand secondactivationstepswereinducedbyheatingto120and250 °C,respectively[35]
7Summary
ThegrowthofmolecularnanostructuresonsurfacesviaUllmanncouplingcanbe controlledbyboththechemicalstructureoftheinitialbuildingblocks,whichis preciselyreflectedinthe finalproducts,aswellasthesurfaceunderneath,inparticularthepresenceofdefects,stepedges,andadatoms.Diffusionoftheactivated monomersandintermediateoligomersisanotherkeyissuesinceitdefinestherate ofpolymerizationandthepossibilityofsubstrate-directedgrowththatallows improvedlinkingreactions.Variousmoleculeshavebeenusedinthelastyearsand itturnsoutthaton-surfacepolymerizationrepresentsaveryfeasiblemethodto createstablecovalent1Dand2Dpolymersonasurfaceandtoimagethemby scanningprobemicroscopyinrealspaceassuccessfullydemonstratedinmany cases.Thecovalentnatureofthenewlycreatedbondisnotonlyevidentfromthe realspacedistancesandorientations,butcouldadditionallybeprovenbyspectroscopicdetectionofcharacteristicelectronicstates.Whenusingdifferenthalogen substituents,ahierarchicalgrowthschemecouldberealizedsinceselectiveand sequentialactivationofthedifferentsubstituentsresultsinaprogrammedreactivity ofthemolecules.Basedonthegatheredmechanisticinsightandwiththeabilityto directreactivitybydesigningpropermonomerbuildingblocksaswellasusingthe surfaceasatemplate,1Dand2Dpolymersofincreasingstructuralandcompositionalcomplexitywillemerge.Besidesthiscontinuedexplorationofon-surface polymerizationasanewmethodforgeneratingdefinednanostructures,their resultingpropertiesandfunctionswillbecomeincreasinglyimportantinthefuture.
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