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SyntheticMetals
journalhomepage:www.elsevier.com/locate/synmet
Electrochemical,opticalandelectrochromicpropertiesofiminepolymerscontainingthiopheneandcarbazoleunits
˙smetKaya,FatmaBaycanKoyuncu,EyupOzdemir∗SermetKoyuncu∗,I
DepartmentofChemistry,FacultyofSciencesandArts,C¸anakkaleOnsekizMartUniversity,TR-17020C¸anakkale,Turkey
articleinfoabstract
Newiminepolymerscontainingthiopheneandcarbazolemoietyweresynthesizedbyusingbothchem-icallyoxidativeinthepresenceofFeCl3andelectrochemicallyoxidativeprocess.WhileUV–vis,FT-IR,
1
H-and13CNMRtechniqueswereusedforthestructuralcharacterization,thenumber-averagemolec-ularweight(Mn),weight-averagemolecularweight(Mw)andpolydispersityindex(PDI)valuesweredeterminedbysizeexclusionchromatography(SEC).Thehighestoccupiedmolecularorbital(HOMO),
thelowestunoccupiedmolecularorbital(LUMO)energylevels,electrochemical(Eg)andopticalbandgap(Eg)valueswerecalculatedbyusingtheresultsofcyclicvoltammetryandUV–visabsorptionspec-troscopymeasurements,respectively.Spectroelectrochemicalmeasurementswerecarriedoutagainstthechangesintheopticalpropertiesofconductingpolymersuponvoltagechanged.Alsotheyellowcoloroffilmchangedtogreencolorwithappliedpotential.Conductivitiesofthesepolymersweredeterminedbyusingfour-pointprobetechnique.Itwasseenthattheconductivitieswereincreasedbyiodinedoping.
©2009ElsevierB.V.Allrightsreserved.
Articlehistory:
Received27October2008
Receivedinrevisedform5January2009Accepted15January2009
Availableonline20February2009Keywords:
ConductingpolymersIminepolymersCarbazoleThiophene
1.Introduction
Iminepolymers,including–CHNlinkageinthebackbone,areusedinvariousfieldsbecauseoftheirelectronstructureproper-ties[1].–CHN–groupisisoelectronicwiththe–CHCH–groupinthesepolymersandtheincorporationofnitrogenatomsintotheconjugatedsystemisanotherapproachtoformclassesofmaterialswithequallyinterestingelectronicandopticalproper-ties[2].Recently,conductingiminepolymershavebeenusedinwiderangeofpracticalapplicationsinseveralareassuchaslightemittingdiodes,thinfilmtransistors,electrochromicdevicesandphotovoltaiccells[3–5].Also,theywereusedtopreparecom-positeswithresistancetohightemperature,thermostabilizations,graphitematerials,epoxyoligomer,andblockcopolymers,photre-sistantandantistaticmaterials[6,7].Electricalandelectrochemicalpropertiesandthermalstabilityofthesepolymerswillestablishtheindustrialapplicabilityofthesematerials.Also,manyofthesepolymersformmesophasesonheating[8].Thistypeofmate-rialhasdrawbackssuchaslowsolubilityincommonorganicsolventsandhigh-meltingtemperature,whichmakesitsprocess-ingdifficult.Howeverseveralapproacheshavebeenundertakentoimprovetheprocessabilityofconjugatedpolyazomethinesbyintroducingvariousalkylsubstitutedaromaticringsinthemain
∗Correspondingauthor.Tel.:+902862180018;fax:+902862180533.E-mailaddresses:sermetkoyuncu@hotmail.com(S.Koyuncu),eozdemir@comu.edu.tr(E.Ozdemir).
0379-6779/$–seefrontmatter©2009ElsevierB.V.Allrightsreserved.doi:10.1016/j.synthmet.2009.01.024
chain[9,10],byusingmonomerscontainingcertainheterocyclicunitssuchasthiophene[11–14],selenophene[15],oxadiazole,thiadiazole[16],pyridine[17],diphenylfluorene[18]andothers.Copolymerswithphenyl-substitutedquinoxalineringsprovedtobeverybeneficial[19,20].Dendriticphenyl-azomethinessupplyagoodsolution,becauseoftheirframeworksaroundvariousfunc-tionalcores[21,22].
Polythiophenesareimportantrepresentativeclassofconju-gatedpolymersthatformsomeofthemostenvironmentallyandthermallystablematerialsthatcanbeusedaselectricalconduc-tors,nonlinearopticaldevices,polymerLEDs,electrochromicorsmartwindows,photoresistant,antistaticcoatings,sensors,bat-teries,electromagneticshieldingmaterials,artificialnosesandmuscles,solarcells,electrodes,microwaveabsorbingmaterials,newtypesofmemorydevices,nanoswitches,opticalmodulatorsandvalves,imagingmaterials,polymerelectronicinterconnects,nanoelectronicandopticaldevices,andtransistors[23–28].Alotofinterestincarbazolecontainingpolymerswascausedbythediscoveryofphotoconductivityinpoly(N-vinylcarbazole)(PVK)byHoegl[29].Thesepolymershavevarioususefulproperties;suchaseasilyformsrelativelystableradicalcations(holes),high-chargecarriermobilities,andhighthermalandphotochemicalstability.Furthermorethesepolymersconstituteanimportantpartofphoto-conductivepolymersmuchattentionispaidtothecarbazole-basedcompoundsinthestudiesreviewingphotoconductivepolymersandorganicphotoreceptors[30–34].
Althoughdirectly[35]andvinyl[36]coupledcarbazole-thiophenepolymerswerealsosynthesizedearlier,thisreport
S.Koyuncuetal./SyntheticMetals159(2009)1034–10421035
highlightsonlythesecondiminecoupledcarbazole-thiophenepolymers[37].Thesepolymersweresynthesizedfrombothoxida-tivepolymerizationbyusingFeCl3asanoxidizingagentandelectrochemicalpolymerizationbygalvanostictechniques.Elec-trochromicswitchingofpolymerfilmpreparedviaelectrochemicalpolymerizationwereinvestigatedbydoublestepchronoamperom-etryandUV–visspectroscopy.Alsoelectrochemical(Eg
)andopticalbandgaps(Eg)ofmonomersandpolymerswerecalculatedfromtheresultofcyclicvoltammetryandUV–visspectroscopy,respectively.Conductivitymeasurementswerecarriedoutbyelectrometerandcalculatedbyusingfourpointprobetechnique.Sizeexclusionchro-matography(SEC)wasusedformolecularweightdeterminationofthesepolymers.2.Experimental2.1.Materials
Carbazole,2-thiophenecarboxaldehyde,bromoethane,1-bro-mobutane,1-bromohexane,1-bromododecane,tetrabutylam-moniumhexafluorophosphatewaspurchasedfromAldrich.Hidraziniumhydroxide,aceticanhydride,aceticacid,dimethyl-formamide,dimethylsulfoxide,methanol,chloroform,acetonitrile,toluene,tetrahydrofuranpurchasedfromMerck.Potassiumhydroxide,copper(II)nitrate-hemipentahydrate,18-Crown-6,andpalladiumonactivatedcarbon(10%,w/w)werepurchasedfromFluka.Wholechemicalsusedwithoutfurtherpurification.
Althoughtherearesomereferencesaboutthesynthesisof9H-N-alkylcarbazole(9-NAC),3,6-dinitro-9H-N-alkylcarbazole(3,6-DN-9-NAC)and3,6-diamino-9H-N-alkylcarbazole(3,6-DA-9-NAC),theproceduresusedforthesesynthesesareslightlydifferent(byusingdifferentreagentorcondition)[38–41].2.2.Synthesisof9H-N-alkylcarbazole(9-NAC)
Carbazole(10mmol),KOH(15mmol)and18-Crown-6(10mg)asphasetransfercatalysiswereaddedto20mloftolueneandrefluxedfor30min.Thesolutionof1-bromoalcane(bromoethane,1-bromobutane,1-bromohexane,1-bromododecane)(15mmol)compoundsin10mltoluenewereslowlyaddedtothereactionmixture.Themixturewasstirredat110◦Cfor6handthencooledtotheroomtemperature.Then,thereactionmixturepouredintothe150mlofethanolandstrippedoffbyarotaryevaporator.Theproductwasrecrystallizedfrommethanolanddriedinvacuumdesiccators.Yield:9-N-ethyl-Carb:88%;9-N-butyl-Carb:84%;9-N-hexyl-Carb:76%;9-N-dodecyl-Carb:54%.
UV–vis(max)(MeOH):236,261,233and330nm.FT-IR(cm−1):(C–Haromatic)3048;(C–Haliphatic)2911,2845;(CCphenyl)1593,1484,1451;1HNMR(CHCl3-d):ıppm,8.16(s,d,2H,Ar-Haa);7.65(d,2H,Ar-Hdd);7.55(t,2H,Ar-Hbb);7.28(s,2H,Hcc);4.38(t,2H,–N–CH2–);1.94–1.27(CHaliphatic;forN-butyl-Carb,N-hexyl-Carb,N-dodecyl-Carb),0.92(d,3H,R–CH3).13CNMR(CHCl3-d):ıppm,141(C1);108(C2);126(C3);117(C4);119(C5);123(C6);44(N–CH2);34–21(N–CH2–(CH2)n);14(CH2–CH3)(Scheme1).
2.3.Synthesisof3,6-dinitro-9H-N-alkylcarbazole(3,6-DN-9-NAC)
Cu(NO3)2·2.5H2O(20mmol)wasaddedintoamixtureofaceticacid(15ml)andaceticanhydride(30ml)atroomtemperature.Themixturewasstirredfor10min,andthenN-alkylcarbazole(18mmol)wasslowlyaddedtothissolution.After5min,10mlofaceticacidwasaddedtothereactionmixture.Themix-turewasfurtherstirredatthistemperaturefor45minandthenpouredintodistilledwater(500ml).Theyellowprecipitate
Scheme1.
wascollectedbyfiltration,washedwithwater(300ml×3)anddriedat60◦Cundervacuum.Yield:3,6-dinitro-N-ethyl-Carb:94%;3,6-dinitro-N-butyl-Carb:96%;3,6-dinitro-9-N-hexyl-Carb:82%;3,6-dinitro-N-dodecyl-Carb:78%.
UV–vis(max)(MeOH):230,265,292and365nm.FT-IR(cm−1):(C–Haromatic)3090–3063;(C–Haliphatic)2920,2850;(–NO2),1512,1312;(CCphenyl)1601,1473,436;1HNMR(CHCl3-d):ıppm,9.18(s,2H,Ar–Hcc);8.59(d,2H,Ar–Hbb);7.61(d,2H,Ar–Haa);4.41(t,2H,–N–CH2–);1.94–1.22(CHaliphatic;forbutyl-Carb,N-hexyl-Carb,N-dodecyl-Carb),0.92(d,3H,R-CH3).13CNMR(CHCl3-d):ıppm,147(C1);109(C2);127(C3);144(C4);116(C5);124(C6);43(N–CH2);34–21(N–CH2–(CH2)n);14(–CH2–CH3)(Scheme2).
2.4.Synthesisof3,6-diamino-9H-N-alkylcarbazole(3,6-DA-9-NAC)
Pd/C(10%,w/w)(0.01g)wasaddedtothesolutionof3-6-dinitro-9-N-alkylcarbazole(15mmol)in200mlofethanolatroomtemperature.Thenthemixturewasheatedtorefluxtempera-turefor10min.10mlofhydraziniumhydroxidein30mlethanolwasthenaddedtothesolutionasdropwisefor45min.Themix-turewasstirredat40◦Cfor24handthenthePd/Cwasfilteredoff.Ethanolwasstripedbyrotaryevaporatorandremainingsolidwascrystallizedontoluenetwotimes.Thesolidwasdriedat50◦Cundervacuum.Yield:3,6-diamino-N-ethyl-Carb:68%;3,6-diamino-N-butyl-Carb:66%;3,6-diamino-9-N-hexyl-Carb:62%;3,6-diamino-N-dodecyl-Carb:58%.
UV–vis(max)(MeOH):232,273,288and317nm.FT-IR(cm−1):(–NH2)3387,3291;(C–Haromatic)3049,3018;(C–Haliphatic)2918,2849;(CCphenyl)1579,1496,1474;1HNMR(CHCl3-d):ıppm,7.41(s,2H,Ar–Hcc);6.(d,2H,Ar–Hbb);7.18(d,2H,Ar–Haa);4.38(t,2H,–N–CH2–);3.71(broad,2H,–NH2)1.96–1.21(C–Haliphatic;forbutyl-Carb,N-hexyl-Carb,N-dodecyl-Carb),0.92(d,3H,R–CH3).13CNMR(CHCl3-d):ıppm,147(C1);109(C2);127(C3);144(C4);116(C5);124(C6);43(N–CH2);34–21(N–CH2–(CH2)n);14(CH2–CH3)(Scheme3).
Scheme2.
1036S.Koyuncuetal./SyntheticMetals159(2009)1034–1042
Scheme3.
2.5.Generalprocedureforsynthesisof
9-alkyl-N,N-bis-(2-thienylmethylene)-9H-carbazole-3,6-diamine(TCs)
9-Alkyl-N,N-bis-(2-thienylmethylene)-9H-carbazole-3,6-diamine(TCs)werepreparedfromthecondensationof2-thiophenecarboxaldehyde(20mmol)with3,6-diamino-9-N-alkylcarbazole(10mmol)inmethanol(25ml)achievedbyboilingthemixtureunderrefluxfor3hat70◦C.Theprecipitatewascollectedbyfiltration,re-crystallizedfromn-heptaneandthendriedinvacuumdesiccator.Yield:TC-1:88%;TC-2:82%;TC-3:75%;TC-4:69%.
UV–vis(max)(CHCl3):232,292and366nm.FT-IR(cm−1):(C–Haromatic)3098,3068;(C–Haliphatic)2921,2840;(–CHN–,imine),1611;(CCphenyl)1570,1479,1423;1HNMR(CHCl3-d):ıppm,8.(s,2H,–CHN–);8.02(d,2H,Ar-Haa);7.68(d,2H,Ar–Hcc);7.62(d,2H,Ar–Hff);7.58(t,2H,Ar–Hbb);7.41(d,2H,Hdd);7.18(t,2H,Hee);4.38(t,2H,–N–CH2–);1.96–1.21(C–Haliphatic;butyl-Carb(TC-2),N-hexyl-Carb(TC-3),N-dodecyl-Carb(TC-4));0.92(d,3H,R–CH3).13CNMR(CHCl3-d):ıppm,151(–CHN–),144(C1);108(C2);116(C3);139(C4);121(C5);114(C6);123(C7);132(C8);127(C9);130(C-10);43(N–CH2);34–21(N–CH2–(CH2)n);14(CH2–CH3)(Scheme4).
2.6.Generalprocedureforoxidativepolymerizationof
9-alkyl-N,N-bis-(2-thienylmethylene)-9H-carbazole-3,6-diamine(poly-TCs)
AnhydrousFeCl3(0.4g,2.5mmol)issuspendedin50mlofdryCHCl3underargonatmosphere,towhichsolutionofiminemonomers(TC)(5mmol)in20mlofdryCHCl3wasaddeddropwiseunderfaststirring.Themixturestirredfor24hatroomtemperatureandthen50mlCHCl3wasaddedtothissolutionandpouredinto200mlmethanolandtheblackprecipitatewasfiltered.Theprecipi-tatewasthenimmersedintoa1%(v/v)solutionofthehidraziniumhydroxideinmethanolandstirredfor12h.Thepolymerwasfil-
Scheme4.
tered,washedwithmethanoltreetimesanddriedundervacuumat70◦Covernight.Yield:poly-TC-1:68%;poly-TC-2:66%;TC-3:58%;TC-4:49%.
UV–vis(max)(CHCl3):248,274and386nm(forTC-1).FT-IR(cm−1):(C–Haromatic)3104,3042;(C–Haliphatic)2921,2840;(–CHN–,imine)1618;(CCphenyl)1599,1467,1386;1HNMR(CHCl3-d):ıppm,8.86(s,2H,–CHN–);8.02–6.88(m,10H,C–Haromatic),4.34(br,2H,–N–CH2–),1.96–1.21(C–Haliphatic;N-butyl-Carb(poly-TC-2),N-hexyl-Carb(poly-TC-3),N-dodecyl-Carb(poly-TC-4)),0.92(d,3H,R–CH3).13CNMR(CHCl3-d):ıppm,154(–CHN–);144–112(C,aromatic);45(N–CH2);36–19(N–CH2–(CH2)n);14(CH2–CH3)(Scheme5).2.7.Electrochemicalmeasurements
ElectrochemicalpropertiesofTCmonomersandpolymersweredeterminedbyCHinstruments660Bcyclicvoltammetry.Theelec-trochemicalcellconsistofanAgwireasreferenceelectrode(RE),Ptwireascounterelectrode(CE)andglassycarbonworkingelectrode(WE)immersedin0.1MTBAPF6assupportingelec-trolyte.Theexperimentswerecarriedunderargonatmosphere.Thepotentialswerecalibratedaccordingtotheferroceneredoxcou-pleE◦(Fc/Fc+)=+0.41Vvs.Ag/Ag+.TheelectrochemicalHOMOandLUMOenergygaps(Eg)ofthesecompoundswerecalculatedfromtheiroxidationandreductiononsetvalues[26].2.8.Electricalmeasurements
ConductivitymeasurementswerecarriedoutbyaKeithley2400electrometer.Thepelletswerepressedonhydraulicpressdevelop-inguptoabout1.70×103kg/cm2.Iodinedopingwascarriedoutbyexposureofthepelletstoiodinevaporatatmosphericpressureandroomtemperatureinadesiccator[15].2.9.Opticalmeasurements
UV–visabsorptionspectraweremeasuredbyPerkinElmerLambda25.TheabsorptionspectraofthesecompoundswererecordedinCHCl3(liquidphase)orITO/glasstransparentfilm(solidphase).Theopticalbandgaps(Eg)ofmonomericandpolymericcompoundswerecalculatedfromtheirabsorptionedges[27].
WeusedthedataobtainedfromUV–visspectraandcyclicvoltammetryforspectroelectrochemicalmeasurementsofpoly-TC-1moleculesonITO/glasstransparentfilm[28].Thesemea-surementswerecarriedouttoconsiderabsorptionspectraofthispolymerfilmunderappliedvoltage.Thespectroelectrochemicalcellincludesaquartzcuvette,anAgwire(RE),Ptwirecounterelec-trode(CE)andITO/glassastransparentworkingelectrode(WE).Thesemeasurementswerecarriedoutinthe0.1MTBAPF6assup-portingelectrolyteinCH3CN.
2.10.Solubilityandcharacterizationtechniques
SolubilitytestresultsofsynthesizedcompoundsareshowninTable1.FT-IRspectrawererecordedbyaPerkinElmerFT-IRSpec-trumOnebyusingATRsystem(4000–650cm−1).1H-and13CNMR(BrukerAvanceDPX-400)datarecordedat25◦CbyusingCHCl3-dassolventandTMSasinternalstandard.ThermaldatawereobtainedbyusingPerkinElmerDiamondThermalAnalysisinstrument.TheTGAmeasurementswereperformedbetween20and1000◦C(inN2,rate10◦C/min).Thenumber-averagemolecularweight(Mn),weight-averagemolecularweight(Mw)andpoly-dispersityindex(PDI)valuesweredeterminedbysizeexclusionchromatography(SEC)techniqueoftheShimadzuCo.ForSECinvestigationsweusedanSGX(100Åand7nmdiameterload-ingmaterial)3.3mmi.d.×300mmcolumn;eluent:DMF/methanol
S.Koyuncuetal./SyntheticMetals159(2009)1034–10421037
Scheme5.
Table1
Solubilityofsynthesizedcompounds(0.05gin10ml).a.CompoundsTC-1TC-2TC-3TC-4
Poly-TC-1Poly-TC-2Poly-TC-3Poly-TC-4
a
NMP+/++/++/++/++/++/++/++/+
DMSO+/++/++/++/++/++/++/++/+
DMF+/++/++/++/++/++/++/++/+
Ethylacetate+/++/−+/++/+−/+−/++/++/+
CHCl3+/++/++/++/++/++/++/++/+
Acetone+/++/++/++/+−/+−/++/++/+
CH3OH+/++/−+/−+/+−/−−/−−/+−/+
CH3CN+/++/++/++/++/++/++/++/+
Toluene−/+−/++/++/+−/−−/−−/+−/+
Hexane−/−−/++/++/+−/−−/−−/−−/+
+/+:solubleatroomtemperature;−/+:solubleatheating;−/−:insoluble.
(v/v,4/1,0.4ml/min),polystyrenestandardsandarefractiveindexdetectorat25◦C.
3.Resultsanddiscussion3.1.Synthesisandcharacterization
Iminemonomerscontaining9-N-alkylcarbazole(9-NAC)andthiophenemoiety(TCs)weresynthesizedinfoursteps(Scheme1).First,9-NACderivativeswerepreparedbyalkylationofcarbazoleintoluenesolutionusingn-alkylbromidecompoundsinthepresenceofKOHand18-Crown-6.ThesecompoundswerethendinitratedusingCu(NO2)2·2.5H2Oinamixtureofaceticacidandaceticanhy-dride(v/v,1:2).Inthethirdstep,dinitrocompoundswerereducedbyusingPdonactivatedcarbon(Pd/C)andhydraziniumhydroxide(N2H4OH).Finally,iminemonomerscontainingthiopheneandN-
alkylcarbazole(TC-1,TC-2,TC-3,TC-4)derivativesweresynthesizedbythecondensationreactionof3,6-diamino-9-N-alkylcarbazole(3,6-DA-9-NAC)compoundsand2-thiophenecarboxaldehyde.Synthesizedcompoundswerepurifiedbycolumnchromatographyandre-crystallizationfromn-heptane.OxidativepolymerizationoftheseproductswascarriedoutbyusinganhydrousFeCl3indrychloroformunderargonatmosphereatroomtemperaturefor48h.ForthereductionofexcessofFe3+ionstoFe2+thatwouldbeeliminatedinafurtherstep,theprecipitateimmersedintoa1%(v/v)solutionofthehydrazineinmethanolwasstirredfor12h.Theyieldsofpolymerswerefoundtobebetween68%,66%,58%and49%,respectively.Accordingtothesolubilitytestresults,itwasseenthatwholepolymersweresolubleinpolarsolvents,suchasNMP,DMSO,DMF,CHCl3andCH3CN.Thesepolymerswerepartlysolubleinacetone,ethylacetate,tolueneandmethanolbyincreasingofthealkyllengthonthecarbozylnitrogen.Thismeans
1038S.Koyuncuetal./SyntheticMetals159(2009)1034–1042
Fig.1.FT-IRspectraof(a)TC-4,(b)poly-TC-4.
thatthedistancebetweenpolymerchainswillincreasebecauseofstericeffectandthebiggerflexibilityofthealiphaticchainsarisingfromtheincreasingofthealkyllengthandsolubilityofthepolymerswillimprove.Finally,poly-TC-3andpoly-TC-4aremoresolublethanothersynthesizedpolymers.
Aftercompletionofthesyntheticworks,allcompoundswerecharacterizedbyFT-IR,1H-and13CNMR.Therearesignificantchangesinthespectralpropertiesoftheinitialcompoundsandtheproducts.Whilesomeofthesignalsweredisappeared,somenewoneswereappeared.InFT-IRspectrumof9-NACderiva-tives,aliphaticC–Hvibrationwasobservedat2911and2845cm−1.Furthermore,characteristicN–Hvibrationat3412cm−1forsec-ondaryamineswasremovedbysubstitutedwithalkylgroups.InFT-IRspectraofthe3,6-DN-9-NAC,occurred–NO2boundsvibra-tionswereobservedat1512and1312cm−1.Alsocharacteristic–NH2vibrationsobtainedfromreductionof3,6-DN-9-NACwasclearlyobservedat3387and3291cm−1inthesespectra.Finally,inFT-IRspectraofTC-1,TC-2,TC-3andTC-4synthesizedfromthecondensationreactionof2-thiophenecarboxaldehydewith3,6-DA-9-NACderivatives,presentedtheiminebond(–CHN–)vibrationat1607cm−1.Also–NH2boundsattributedto3,6-DA-9-NACcom-poundsweredisappeared(Fig.1).
MolecularstructureofTCmonomersandpolymerswerealsoidentifiedfromtheir1H-and13CNMRspectrarecordedinCHCl3-d.1HNMRspectraofTC-2andpoly-TC-2wereshowninFig.2.Whilecharacteristicimineprotonssignalwereobservedat8.ppm,aro-maticprotonssignalswereobservedbetween8.02and7.18ppm.Sincenitrogenisanelectronegativeatom,N–CH2protonswereobservedathigherppmthanotheraliphaticprotons.Furthermore,–CH3endgroupwereobservedat0.92ppmasexpected.Attheendofthepolymerization,wholeprotonssignalsbroadenedandslightlyshifted.Finally,theseresultsindicatethatallreactionswerecompletedsuccessfully.
SECanalysesofpoly-TC-1,poly-TC-2,poly-TC-3andpoly-TC-4synthesizedfromoxidativepolymerizationwereperformedat30◦CusingDMF/MeOH(v/v,4/1)aseluentataflowrateof0.4ml/min.Mn,Mw,andPDIvalueswerecalculatedaccordingtoapolystyrenestandardcalibrationcurveandaregiveninTable2.
Table2
Thenumberaveragemolecularweight(Mn),massaveragemolecularweight(Mw),polydispersityindex(PDI)valuesofTCpolymers.CompoundsMnMwPDIPoly-TC-19,65011,2001.161Poly-TC-211,10012,5501.131Poly-TC-312,20014,4001.180Poly-TC-4
11,700
15,100
1.291
Fig.2.
1
HNMRspectraof(a)TC-2,(b)poly-TC-2.
AccordingtoSECanalysis,wholeTCpolymersshowonlyonepeakinthesechromatograms.Thesepolymerscontainabout20–25repeat-ingunitsandshownarrowmolecularweightdistribution.Thus,PDIvaluesofpoly-TC-1,poly-TC-2,poy-TC-3andpoly-TC-4werefoundtobe1.161,1.131,1.180and1.261,respectively.3.2.Opticalandelectrochemicalproperties
TheUV–visspectrawererecordedinliquid(solution:CHCl3)andsolidphase(ITO/glasssurface)(Fig.3).ElectronicabsorptionandopticalbandgapdataofTCmonomersandpolymerswereshowninTable3.TCmonomersandpolymersshowedmulti-absorptionpeakattributedto→*andn→*transitionofthethiophene,carbazoleandiminegroups.Fig.3showsthattheabsorptionmaximum(max)ofpoly-TC-1exhibitsa20nmredshiftascomparedtoTC-1.Thisshiftwasalsoobservedforthebandgap.Whilethelow-energyedgeoftheabsorptionspectrumofTC-1isat438nm,whichcorrespondstobandgapof2.83eV,forpoly-TC-1theabsorptiononsetis465nmcorrespondingtoabandgap(Eg)of2.67eV.Themaxandonsetvaluesarelowerthaninsolidphase,becauseoftheabsenceofstarkingofpoly-TC-1insolution.Due
Fig.3.AbsorptionspectraofTC-1[(a)inchloroformsolution]andpoly-TC-1[(b)inchloroformsolutionand(c)onITO/glasssurface].
S.Koyuncuetal./SyntheticMetals159(2009)1034–1042
Table3
)andopticalbandgaps(Eg)valuesofTCmonomersandpolymers.HOMOandLUMOenergylevels,electrochemical(EgCompounds
Reducedgroupsandpeak
potentials(V)–HCN–(imine)Oxidizedgroupsandpeakpotentials(V)Thiophene(ring)Carbazole(ring)HOMO(eV)
LUMO(eV)
Electrochemical
)(eV)bandgap(Eg
1039
Opticalband
gap(Eg)(eV)
TC-1−1.96+1.68+1.17TC-2−1.99+1.68+1.18TC-3−2.02+1.69+1.21TC-4
−2.16+1.71+1.24Poly-TC-1−1.81+1.54+0.94Poly-TC-2−1.82+1.55+0.95Poly-TC-3−1.84+1.56+0.96Poly-TC-3
−1.90
+1.58
+0.99
Fig.4.Cyclicvoltammogramof(a)TC-1and(b)poly-TC-1intheTBAPF6-acetonitrile,scanrate100mV/s,Ag-AgCl.
tosameaffect,alsowholeabsorptionbandsofthesolidphasebroadened.Finally,thebandgapwascalculatedas2.54eVforsolidphase.Basedontheintermolecularelectrontransfer,increasingofthelengthofalkylgroupsonthecarbozylnitrogen,theopticalbandgapchangedintherangeofbetween2.67and2.74eV.Incompari-sonwithelectrochemicalbandgaps,thevaluesarenotagreement.Itisshowsthattheelectroactivecentersandphotoactivecentersbehaveindependentfromeachothersforsynthesizedpolymers.
TheelectrochemicalpropertiesofTCmonomersandpolymers(TC-1,TC-2,TC-3andTC-4)wereinvestigatedbycyclicvoltam-metryandFig.4showstheCVvoltammogramoftheTC-1andpoly-TC-1.ThecyclicvoltammetrymeasurementswerecarriedoutinCH3CNsolutionatroomtemperatureand0.1MTBAPF6wereusedassupportingelectrolyte.Inthecathodicscanregion,TCandpoly-TCmoleculesshowonlyoneirreversiblereductionpotentials[39],whichreflectthereductionofimine(–CHN–)(Scheme6).Sincethereductionpotentialvaluesaresensitivetointramolecularandintermolecularelectrontransfer,theiminereductionpeakwasbroadenedandshiftedtohigherpotentialattheendofthepoly-merization.Becauseofthesameeffect,iminereductionpeakwerealsoinfluencedbyalkylgrouponthenitrogenatom,thereduc-tionpeakwasshiftedtolowerpotentialbyincreasinglengthofthe
Scheme6.
−5.33−2.592.742.83−5.36−2.602.762.85−5.41−2.622.792.87−5.56−2.2.922.94−4.99−2.812.182.67−4.99−2.812.182.67−5.01−2.812.202.69−5.08
−2.84
2.24
2.74
alkylchain.ThereactionmechanismcanbeproposedasshowninScheme6.
DuringanodicscanregionofwholeTC’s,twoirreversibleoxi-dationwaveswereobserved(Fig.4).ForTC-1molecule,thefirstoxidationpeakat+1.17Vcanbeattributedtocarbazoleoxidation,andthesecondoneat+1.68Vcanbeattributedtothiopheneoxi-dation[14].AlsotheothermoleculeshavesimilaroxidationwavesasshowninTable3.ThereactionmechanismcanbeproposedasshowninScheme7.
3.3.Electrochemicalpolymerizationandelectrochromicproperties
Heteroatomcontainingaromaticcompoundssuchaspyrrole,thiophene,furane,etc.canbeeasilyoxidizedanddepositedaselec-troactivepolymerfilmsontotheITO/glassWEsurfacebyrepeatedanodicscans[28].Sincepyrrole-iminecompoundshavealow-oxidationpotential(≈0.8eVagainstAg/Ag+electrode),theycanbeeasilypolymerizedbytheelectrochemicalprocess.Ontheotherhand,thiophene-iminederivativeshavinghigheroxidationpotential(≈1.6eV)needaco-momomersuchasthiopheneorpyr-role[42].Becauseofthisproperty,thiophenewasusedastheco-monomerforelectropolymerizationofTC-1.Thefirstandthesecondpeakattributedtooxidationofthecarbazylandthiophenemoietiesrespectivelyincreasewithrepeatinganodicscan.Thus,thesegroupsbecomemorerepeatedinthepolymerchains.Duetoaromaticconjugationbecomesmorepronouncedinthepoly-merbackbone,thesepeaksbroadenedandslightlyshiftedtolowerpotential.ElectropolymerizationofTC-1inthepresenceofthio-phenewasshowninFig.5andScheme8.
Fig.5.RepeatedpotentialscanofTC-1inthepresenceofthiopheneand0.1MLiClO4-NaClO4-acetonitrile,scanrate250mV/s.
1040S.Koyuncuetal./SyntheticMetals159(2009)1034–1042
Scheme7.
Scheme8.
Spectroelectrochemicalmeasurementinvestigatesthechangesinopticalpropertiesofconductingpolymersuponvoltagechange.Thesemeasurementsshowingtheformationofradicalcation(polaron)ofpoly-TC-1moleculeontheITO/glasspolymerfilmwerestudiedbyapplyingpotentialsrangingbetween0and+1.6Vinmonomerfreeacetonitrile/TBAHF6(0.1M)medium.Theappear-anceofbroadpeaksaround950nmcouldbeattributedtotheevolutionofpolaronbands.AscanbeseenfromFig.6,theyellowcoloroffilminneutralstatehaschangedtogreencolorbyappliedpotential.
Weextractedtheelectrochromicparametersofthepoly-TC-1filmsbyanalysisoftransmittancechangeatincreasingtheabsorp-tionbandat950nmwithrespecttotimewhilethepotentialbetween0and+1.6Vwasstepwiseswitchedbetweentheneutral
Fig.6.Spectroelectrochemicalmeasurementsofpoly-TC-1film.
S.Koyuncuetal./SyntheticMetals159(2009)1034–10421041
Fig.7.Electrochromicswitching,opticalabsorbancemonitoredforpoly-TC-1filmat950nm(0–1.6V).
andoxidizedstateswitharesidencetimeof10s.Doublepotentialstepchronoamperometrywascarriedouttoestimatetheresponsetimeofthepolymerfilm.Duringtheexperiment,the%transmit-tanceatthewavelengthofmaximumcontrastwasmeasuredbyusingaUV–visspectrophotometer.Theopticalcontrastmeasuredasthedifferencebetween%Tintheneutralandoxidizedforms(%T)werefoundtobe24%forpoly-TC-1(Fig.7).Furthermoretheswitchingtimewerecalculatedas3.4sforthispolymer.Takingtogethertheseresultssuggestthatpoly-TC-1filmsexhibitenoughpromisingelectrochromiccharacteristicunderaerobicconditions.3.4.Conductivity
Conductivityisrelatedtobothintermolecularandintramolec-ularelectrontransferonthepolymerbackbone.Thereforetheseresultshaveshownthatthedopantinteractswiththedoublebondinthepolymerbackboneandformsapolaronicstate(radicalcation)asanelectronistransferredfromthedoublebondtothedopantcre-atingaholeorapositivecarrieratthedoublebondsite.Theseholesorpositivecarriersareresponsiblefortheelectricalconductioninthesematerials[43].Conductivitymeasurementsofpoly-TC’swerecarriedoutwithanelectrometerusingafourpointprobetechnique.Fig.8showstheelectricalconductivitybehaviorresultsofpoly-TC-1,poly-TC-2andpoly-TC-3moleculesunderiodinedopinginvaryingperiodsat25◦C.Firstofall,conductivitiesofallTCpolymerswerefoundtobeapproximately10−10S/cmandthenanincrease
Fig.8.ChangeintheelectricalconductivitiesofTCpolymersduringtheprocessofiodinedoping(doneatatmosphericpressureand25◦C).
wasobservedafteriodinedoping.Althoughthesemoleculesaresimilartoeachother,conductivitymeasurementscarriedoutattheendofeachdopingperiod,poly-TC-3andpoly-TC-4showedlowerconductivitythanpoly-TC-1andpoly-TC-2.Thesedifferencesareassumedtobecausedbythedifferentalkylgroupswhichsubsti-tutedonthecarbozylnitrogen.Lengthofthesealkylgroupsaffecttointermolecularelectrontransfer.Sopoly-TC-3andpoly-TC-4having
Scheme9.
1042S.Koyuncuetal./SyntheticMetals159(2009)1034–1042
longeralkylchainintheirstructureshowlowerconductivitiesthanpoly-TC-1andpoly-TC-2.Also,thesealkylgroupshinderthecoor-dinationofI3−withnitrogen,becauseofthestericeffectexertedbythem.Sincenitrogenisaveryelectronegativeelementandiscapa-bleofcoordinatingwiththeI3−,Diazetal.suggestedaconductivitymechanismofimine(–CHN–)polymerswhendopedwithiodine[15].Furthermore,itisbelievedthatiodinedopingoccursnotonlyonthenitrogenatomsbutalsoonthethiopheneandcarbozylrings.TheconductivitymechanismoverthepolymerbackbonecanbeproposedasScheme9.4.Conclusion
ThesynthesisandpolymerizationofaseriesofiminecoupledTCmonomersbothoxidativepolymerizationusingFeCl3ascatalystandalsotheelectrochemicaloxidativepolymerizationingalvanos-taticconditionsisreported.Thesemoleculeswhichhavesimilarelectrochemicalandopticalpropertiesshoweddifferentelectri-calandthermalproperties.Poly-TC-3andpoly-TC-4didnotformstrongcoordinationwithiodineatomsbecauseofthepresenceoflongalkylgroupsoncarbozylnitrogen.Thus,afteriodinedoping,theconductivityofpoly-TC-3andpoly-TC-4wasnothigherthanofpoly-TC-1andpoly-TC-2.Thesealkylchainalsoinfluencedtheoxi-dationandreductionpotentialoftheTCmonomersandpolymers.Theseresultsmaypointthatallmoleculescanbeexperimentedinphoto-electronictechnologiesasholetransportmaterials(HTMs).Furthermore,theyellowcoloroffilmpreparedviaelectroxida-tiveprocessontheITO–glasssurfacehaschangedtogreencolorbyappliedpotential.Inaddition,spectroelectrochemicalmeasure-mentsshowedthatTP-1moleculeswereelectrochromicallystableontransparentITO–glasssurface.Thestableelectrochromismmaybeanadvantagefordesignofmolecularlyelectrochromicdevices.Acknowledgement
WegratefullyacknowledgethesupportsfromCanakkaleOnsekizMartUniversityGrantsCommission(ProjectNumber:2007/28).References
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