Homocoupling of terminal alkynes on calcite (10.4)

Homocoupling of terminal alkynes on calcite (10.4)

Antje Richter

, Manuel Vilas-Varela

, Diego Peña

, Ralf Bechstein

, Angelika Kühnle

aInstitute of Physical Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10-14, Mainz 55099, Germany

bCentro de Investigación en Química Biolóxica e Materiais Moleculares (CIQUS) and Departamento de Química Orgánica, Universidade de Santiago de Compostela, Santiago de Compostela 15782, Spain

a r t i c le i n f o

Keywords:

On-surface synthesis Glaser coupling Terminal alkynes Atomic force microscopy Bulk insulator Molecular electronics

a b s t r a ct

On-surfacesynthesishasbeenidentifiedashighlyversatilestrategytopreparemolecularstructuresonsurfaces withsingle-atomprecision.InspiredbytheclassicalGlasercoupling,homocouplingofterminalalkyneshasat- tractedgreatattentionforon-surfacesynthesis.Thiscouplingisknownforprovidingarigidandlinearlinkage, whichishighlyinterestingforthesynthesisofmolecularwires.Formolecularwireformation,non-conductive substratesareneededforelectronicdecoupling.Sofar,however,couplingofterminalalkyneshasnotbeen performedonabulkinsulatorsurface.Here,wepresentanatomicforcemicroscopystudy,indicatingthat4,4″- diethynyl-[1,1′:4′,1″-terphenyl]-2′,5′-dicarboxylicacidundergoesdimerizationbyhomocouplingoftheterminal alkynemoietyonthe(10.4)surfaceofthebulkinsulatorcalcite.Ourresultssuggestthatuponlow-temperature depositionmonomersarefoundonthesurface,whichcanbedimerizedbyannealingthesample.Whenfollowing ahigh-temperaturedepositionprotocol,thedimerizationappearstohappenalreadyinthecrucibleanddimers aredepositeddirectly.Ourwork,thus,indicatesthathomocouplingofterminalalkynescanalsobeperformed onanon-metallicsurface,inlinewitharecenttheoreticalstudythatsuggesttheroleofthesurfacebeingthe constrainofthechemicalmotionratherthancontributingintermsofelectrontransfer.

© 2017ElsevierB.V.Allrightsreserved.

1. Introduction

Thebottom-upsynthesisoffunctionalmolecularstructurehasat- tractedgreatattentioneversinceRichardFeynmangavehisinspiring speechin1959,claimingthat“Thereisplentyofroomatthebottom” [1]. Inthespirit of thisidea, itwasKarl-Heinz Riederandcowork- erswhohavedemonstrated thatallstepsof achemicalreaction can beperformedusingthetipofascanningtunnelingmicroscope[2].In thispivotalwork,theclassicalUllmann couplingof iodobenzenehas beeninducedonacopper(111)surface,demonstratingmolecularengi- neeringatthesinglemoleculelevel.Nowadays,thefieldofon-surface synthesisisahighlyactiveresearcharea,whichholdsgreatpromises forthefabricationoftailor-madefunctionaldevicesatsurfaces[3–6]. Ullmann-likecouplinghasbeenusedasapopularlinkingmotifonvar- iousmetalsurfaces[7–12].Toenlargethepossiblelinkingschemesfor on-surfacereactions,Glaser-inspiredhomocoupling[13]hasbeenex- ploredasaroutetocoupleterminalalkynesandconstructrigidlin- earlinkers[14,15].Thisreactionhasfirstbeenpresentedusing1,3,5- triethynyl-benzeneonasilver(111)surface[16],butisnowwellstudied onawiderangeofconductivesurfaces[17–20].Interestingly,inoneof thesestudies,thedimerizationofthe1,4-diethynylbenzeneprecursor moleculesinthecruciblehasbeenreported,whichhasbeeninduced

∗Corresponding authors.

E-mail addresses: diego.pena@usc.es (D. Peña), angelika.kuehnle@uni-bielefeld.de , kuehnle@uni-mainz.de (A. Kühnle).

byannealingthecrucibleto105°Cduringmoleculedeposition[17]. Moreover,theformationofunwantedsideproductsuponlinkageofa lineardiethynylterphenylonflatterraceshasbeenaddressedbyusing thealignmentofthemoleculesatthestepedgesonavicinalAg(877) surface[21].Also,photochemicalactivationasanalternativerouteto annealinghasbeensuccessfullypresented[19].Adetailedtheoretical studyhasbeendevotedtounravelthemolecularreactionmechanism of1,3,5-triethynyl-benzenelinkingonAg(111)andtouncovertherole ofthesurface,whichisexplainedbyconstrainingthemolecularmotion ratherthancontributingchemicallyintermsofelectrontransfer[22].

Intheviewofthisgrowingbodyofresearch onmetalsurfaces it isinterestingtoraisethequestionastowhetherhomocouplingofter- minal alkynescan also beachieved on abulk insulatorsurface. For thispurpose,weinvestigate4,4″-diethynyl-[1,1′:4′,1″-terphenyl]−2′,5′- dicarboxylicacid(DETDCA,Fig.1a)onthe(10.4)cleavageplaneofcal- cite(Fig.1b)usingdynamicatomicforcemicroscopy(AFM)operated inultrahighvacuum(UHV).

Themoleculeisequippedwithtwoethynylgroups,whicharein- tendedforon-surfacehomocoupling.Additionally,themoleculeisalso equippedwithtwocarboxylicacidfunctionalities,whichhaveprevi- ouslyproventoactassuitableanchorsonthecalcite(10.4)surfaceand preventthedesorptionatelevatedtemperatures.Density-functionalthe- orycalculationshavesuggestedaninteractionofthecarbonylgroups

https://doi.org/10.1016/j.susc.2017.12.012

Received 8 September 2017; Received in revised form 5 December 2017; Accepted 22 December 2017 Available online 27 December 2017

0039-6028/© 2017 Elsevier B.V. All rights reserved.

A. Richter et al. Surface Science 678 (2018) 106–111

Fig. 1. (a) Model of 4,4 ″ -diethynyl-[1,1 ′ :4 ′ ,1 ″ -terphenyl] − 2 ′ ,5 ′ -dicarboxylic acid (DETDCA) molecule studied here. (b) Model of the calcite (10.4) surface with the crystallographic axes.

The surface unit cell is marked by the black rectangle. The scale bar applies to both subsets.

Scheme 1. Synthesis of diyne DETDCA.

withthecalciumionsviaelectrostatics,whilethehydroxylgroupsform hydrogenbondswiththecalcitecarbonategroupoxygenatoms[23,24]. Furthermore,adetailedinvestigationtowardsapossibledeprotonation hasbeenperformed,suggestingatransitionofthehydrogenfromthe hydroxylgrouptowardstheoxygenofthecarbonategroupunderfor- mationofahydrogenbond[23,24].Twoanchorgroupswerechosen hereforsymmetryreasonsandtoalsopromotemolecule-moleculein- teractionsthatcanbebeneficial fortheformationofanorderedself- assembledstructureascomparedtomobilespecies.

OurexperimentalresultsindicatethatDETDCAdimerizeinthecru- ciblewhenchoosingahigh-temperaturedepositionroute.Whenfollow- ingalow-temperaturedepositionroute,themonomersaredepositedon thesurface.Uponannealingthesampleaftermonomerdepositionre- sultsinAFMimagesthatcanbereadilyexplainedbythedimerization ofthemoleculesonthecalcite(10.4)surface.Thus,ourworkprovides experimentalindicationforextendingtheconceptofon-surfacehomo- couplingofterminalalkynestoabulkinsulatorsurface.

2. Experimentalsection 2.1. Moleculesynthesis

ThesynthesisofDETDCAwasperformedinfivestepsfollowingthe routeshowninScheme1.First,commerciallyavailablediester1was treatedwith Tf₂Otoobtainbistriflate2in 91%yield. Then,double Pd-catalyzedSuzuki couplingofbistriflate2withtwoequivalentsof boronicester3ledtotheisolationofdiyne4in78%yield.Finally,se- quentialtreatmentofcompound4withK₂CO₃,KOHandHClafforded DETDCAin69%yield(seesupportinginformationfordetails).

2.2. Surfacepreparationandmoleculedeposition

CalcitecrystalswerepurchasedatKorthKristalle(Altenholz,Kiel, Germany).Beforemountinginthesampleholder,thecrystallographic directionswereidentifiedbasedonthecrystal’sbirefringence[25].Prior tomoleculedeposition,thesamplewascleavedinsituandannealedat 625K.Atomicallyresolvedimageswereobtainedtocheckthecleanli- nessofthesurfaceandtoconfirmthecrystallographicdirections.The DETDCAmoleculesweredepositedfromahome-builtKnudsencell.Two differentdepositionprotocolswereused.Forthelow-temperaturedepo- sitionroute,acrucibletemperatureof355Kandadepositiontimeof about12–15hwasused.Forthehigh-temperaturedepositionroute,a crucibletemperatureof445Kandadepositiontimeof45mintofew hourswasused.

2.3. Atomicforcemicroscopyimaging

Forimagingthemolecularstructuresonthebulkinsulatorsurface, weusedynamicAFMimagingoperatedunderUHVconditions.Avari- abletemperature,beamdeflectionAFM(VTAFM)fromScientaOmicron (Taunusstein,Germany)wasusedintheso-calledfrequencymodulation mode.Siliconcantilevers fromNanosensors (Neuchâtel,Switzerland) witheigenfrequenciesintheorderof300kHzandforceconstantsof about40N/mwereused.Theimagechannelsaswellastheslowand fastscandirectionsaregivenintheupperrightcorneroftheimages.

Duringimagingthesamplewaskeptatroomtemperature.

Fig. 2. AFM images acquired after low-temperature deposition before annealing the sample. Depending on the deposition protocol, low (a–c) and high (d–f) molecular coverages can be achieved.

Fig. 3. High-resolution AFM image of the (5 ×1) structure with a superimposed model of a possible DETDCA monomer structure. The surface directions and the supercell are marked.

3. Resultsanddiscussion 3.1. Low-temperaturedeposition

Aftermoleculedepositionusingthelow-temperaturedepositionpro- tocol(sublimationtemperatureofabout355Kfor12–15h),molecu- larislandsarefoundonthesurfaceasshowninFig.2a–c.Inthelow- coverageregime,theseislandsexhibitfuzzyedges,whichareindicative ofhighmoleculemobility,resultingincharacteristicsteakyfeatures.A zoomontoanislandinthelow-coverageregime(Fig.2c)doesnotre- vealaninnerstructure.Whenincreasingthecoverage(Fig.2d–f),the islandstructuresbecomemorestable,whichisexplainedbyareduced

moleculemobility.Azoomontoanislandinthehigh-coverageregime unravelsahighlyorderedinnerstructure(Fig.2f).

Tofurtheranalyzethisstructure,weperformedahigh-resolutionex- perimentasshowninFig.3.Inthisimage,the(5×1)superstructureof themolecularislandisindicatedbyawhiterectangle.Wesuperimpose amodelofthemoleculestotheimagetoprovideanimpressionofthe moleculardimensions.Thissuperimposedmodelsuggeststhattheunit cellcontainsoneDETDCAmonomer.Wealsoprovideamodelforthe underlyingcalcitecrystaltoillustratethearrangementofthemonomers onthecalcitesurface.Westress,however,whilethedimensionsandori- entationofthemoleculesarewelldefinedfromtheexperiment,theab- solutepositionofthemoleculesonthesurfaceisunknown.Inthemodel,

A. Richter et al. Surface Science 678 (2018) 106–111

Fig. 4. AFM images acquired after low-temperature deposition and annealing the sample to 555 K for about one hour. A model illustrating the proposed alignment of the dimers within the stripes is given in the inset in (c).

Scheme 2. Expected on-surface coupling of DETDCA.

weassumethatthemoleculeisnotflat,giventhatthesidegroupsinthe 2-position,namelyhydrogenandcarboxylicacidgroup,wouldoverlap.

Tocircumventthisoverlap,arotationofthebenzeneringstransverse themolecularaxesisexpected.Hence,onlyonecarboxylicacidgroup wouldanchortothesurfacewhilethesecondoneispointingupwards andperhapsformsahydrogenbondswiththeacidgroupoftheneigh- boringmolecule.

Next,weannealedthesampleafterlow-coveragemoleculedeposi- tionasshowninFig.2a–cto555Kforaboutonehour.Theresultof suchanannealingexperimentisshowninFig.4.Theoverallappear- anceoftheislandshasremainedunchangeduponannealing,ascanbe seenbycomparingthetwooverviewimagesshowninFigs.2aand4a, respectively.However,theinnerstructureoftheislandshaschanged significantlyasdemonstrateinthezoomimagegiveninFig.4c.After annealing,astripe-likestructureisobserved,exhibitingsomewhatir- regularstripeswithawidthofabout4.5nm.Thiswidthfitsinlength withdimerizedDETDCAmoleculesthatarelinkedviahomocoupling oftheterminalalkynes,asillustratedintheinsetinFig.4c.Moreover, theislandedgesappearwell-definedanddonolongerpresentthefuzzy structurethatwasobservedfortheas-depositedmolecularislands.As- sumingthatthedimerspossessareduceddiffusivityonthesurfaceas comparedtothemonomers,thisobservationisinlinewiththeabove madeinterpretationofDETDCAdimerization(Scheme2).

3.2. High-temperaturedeposition

Wealsoperformeddepositionexperiments ontoasampleheld at roomtemperaturewithincreasedsublimationtemperature(445K)in an attempted to reduce the long sublimation times needed for the

above-mentionedlow-temperaturedepositionprotocol.Whensublimat- ingwithasublimationtemperatureof445Kforabout15h,molecular islands asshown inFig.5a–careobtained.These islandsexhibitthe characteristicstripe-likeinnerstructurethatwasreportedaboveforthe annealedsampleobtainedafterthelow-temperaturedepositionproto- col.However,acleardifferencecanberecognizedwhencomparingthe islandstructuresinFig.4(low-temperaturedepositionafterannealing) andFig.5a–c(high-temperaturedepositionwithoutannealing).Forthe high-temperature depositionprotocolwithout annealing,fuzzy edges andstreakyfeaturesinbetweenthemolecularislandsareobserved,in- dicativeofdiffusingmolecularspecies. Thesehigh-temperaturedepo- sition resultscan be explainedbythedirectsublimationof DETDCA dimers,i.e.,wesuggestthatthehighsublimationtemperatureinduces dimerizationdirectlyinthecrucibleashasbeenreportedbeforefor1,4- diethynylbenzene[17].Besidesthedimers,fewmonomersmightbede- positedsimultaneously.Thesemonomersmightdiffuseinbetweenthe island,resultinginthestreakystructuresobservedintheimagesshown inFig.5a–c.

Next,weexaminedtheeffectofannealingandirradiatingthestruc- turesasobtainedafterhigh-temperaturedeposition.Wefoundthatboth, annealingthesampletoabout555Kandirradiatingthesamplewitha mercurylamp(wavelength220–590nm),resultedinthesamefinalcon- ditions.Thisobservationagreeswithpreviousresultspresentedinthe literaturethathavedemonstratethatcouplingofterminalalkynescan beinducedbyboth,annealingorirradiation[19].Thechangesinduced areillustratedinFig.5d–f,whichshowasampleafterirradiationfor about12h.Ascan beseen,theinnerstructureislargelyunchanged.

However,theedges appearmuchmore welldefined andthestreaky featuresinbetweentheislandsarevanished.Thisresultindicatesthat

Fig. 5. AFM images acquired after high-temperature deposition before (a–c) and after (d–f) irradiating the sample.

remainingmonomersreactuponannealingorirradiation,thus,thedif- fusingspeciesareremoved.

Tosummarize,wesuggestthatboth,annealingto555Kandirradiat- ingwithmercurylamp(wavelength220–590nm)caninducehomocou- plingoftheterminalalkynes.Thefactthatthisispossibleintheabsence ofametalsurfaceindicatesthatelectrontransferfromthesupportsur- faceisnotrequiredtoinducethereaction.Thisisinlinewitharecent theoreticalstudyoncouplingofterminalalkynesonAg(111)[22].In thelatterstudy,itwasconcludedthatthesurfaceplaysacentralrole forthereactioninconstrainingthemolecularmovementaswellassta- bilizingreactionintermediates.Itis,however,notchemicallyactivein asensethatit,e.g.,donateselectrons.Forthepresentcase,wespeculate thatthecarboxylicacidgroupsatthemoleculecorearecrucialforan- choringthemoleculetothesubstrateandconstrainthemobilityonthe surface.Thelimitedmobilitymayfurthermoreexplainwhyonlydimers andnotrimersorlargeroligomersareformed.Thisinterpretationisin agreementwithpreviousobservationsinliteratureonAg(111)[19].

4. Conclusion

Inconclusion, we presented an experimental AFM study investi- gatingthestructureformationandreactionofDETDCAmoleculeson the(10.4)surfaceofthebulk insulatorcalcite.Whendepositingthe moleculesfollowingalow-temperaturedepositionroute,highlyordered islandsarefoundonthesurfacethatexhibita(5×1)innerstructure.

Thisstructurecanbereadilyexplainedbyanorderedarrangementofthe DETDCAmonomers.Thecharacteristicstreakyedgesoftheseislandsare explainedbydiffusingmonomerspecies.Uponannealing,theseislands drasticallychangetheirstructure,asisexpectedwhenthemonomers dimerize.Now,islandswithastripedinnerstructurearefoundonthe surface.Interestingly,thewidthofthestripesfitsinsizewiththelength ofadimer.Moreover,thefuzzyedgesarevanished,givingfurtherev- idenceforanon-surfacedimerization.Forthehigh-temperaturedepo- sitionprotocol,thestripedislandsareobtaineddirectly,indicativeofa directdepositionofthedimers.Thisresultcanbeeasilyexplainedby adimerizationinthecrucible,whichhasbeenobservedbeforeforan- otherdiynemolecule.Alsohere,fuzzyedgesvanishafterannealingor

irradiation,indicativeoffewremainingmonomersthataredeposited duringthehigh-temperatureroute.

AuthorContributions

Themanuscriptwaswrittenthroughcontributionsofallauthors.All authorshavegivenapprovaltothefinalversionofthemanuscript.

Acknowledgments

WegratefullyacknowledgefinancialsupportfromtheEUthrough grantPAMS(seventhframeworkprogramGA610446),theAgenciaEs- tataldeInvestigación(MAT2016-78293-C6-3-R),theXuntadeGalicia (CentrosingulardeinvestigacióndeGaliciaaccreditation2016-2019, ED431G/09)andtheEuropeanRegionalDevelopmentFund(ERDF).We thankRobertLindnerforhelpinthelaboratory.

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