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Structureandinfrared(IR)assignmentsfortheOLEDmaterial:N,N-diphenyl-N,N-bis(1-naphthyl)-1,1-biphenyl-4,4/-diamine(NPB)¤MathewD.
Halls,aCarlP.
Tripp*bandH.
BernhardSchlegel*aaDepartmentofChemistry,WayneStateUniversity,DetroitMI48202,USA.
E-mail:hbs=chem.
wayne.
edubDepartmentofChemistryandLASST,UniversityofMaine,OronoME04469,USA.
E-mail:ctripp=maine.
maine.
eduReceived19thFebruary2001,Accepted12thApril2001FirstpublishedasanAdvanceArticleontheweb10thMay2001Organiclight-emittingdiodes(OLEDs)arecurrentlyunderintenseinvestigationforuseinnext-generationdisplaytechnologies.
ResearchintothefundamentalpropertiesofthematerialsusedinOLEDs,suchasstructureandvibrationalmodes,willhelpprovideexperimentalprobeswhicharerequiredtogaininsightintotheprocessesleadingtodevicedegradationandfailure.
CalculationsusingthehybridB3LYPfunctionalandthesplit-valencepolarized6-31G(d)basissethavebeencarriedouttoassigntheIRbandsoftheOLEDholetransportmaterialN,N@-diphenyl-N,N@-bis(1-naphthyl)-1,1@-biphenyl-4,4A-diamine(NPB).
ExcellentagreementwasfoundbetweenthecomputedandexperimentalwavenumbersallowingthereliableassignmentofmajorIRbands.
ComparisonofthereectionabsorptionIR(RAIRS)spectraobtainedfromroomtemperatureandthermallyannealedNPBthinlmsindicatesthat,uponannealing,structuralchangesoccurandtheaverageorientationoftheNPBnaphthylgroupsbecomepredominatelyatwithrespecttothesurface.
IntroductionFollowingtheinitialreport,byTangandVanSlyke,1organiclight-emittingdiodes(OLEDs)havereceivedwidespreadattentionfortheirpotentialuseinnext-generationdisplaytechnologies.
2,3OLEDsaretypicallyamorphousthinsolidlmheterojunctiondevicesconstructedbyvacuumevapo-rationofthetransportlayersontoasupportingelectrode.
Theorganicmaterialscomposingtheactivelayersarechosenwithcloseregardtotheirrelativeorbitalenergyosets,usuallysuchthatexcitonformationandrecombinationoccursintheelectrontransportlayerofthedevice.
MaterialsdevelopmentforelectrontransportandemissioninOLEDshaslargelyfocusedonmetallo-quinolates,4,5withtris(8-hydroxy-quinoline)-aluminium(III)(Alq3)beingthemostoftenused.
AromaticaminesareoftenusedasholetransportmaterialsinOLEDdevicesandhavehadgoodsuccess.
Inparticular,thenaphthyldiamineN,N@-diphenyl-N,N@-bis(1-naphthyl)-1,1@-biphenyl-4,4A-diamine(NPB)wasshown,byTangandco-workers,6toaordimprovedstabilityoverpreviouslyusedamines.
Althoughshowingexcellentdevicecharacteristics,OLEDsstillsuerfromalackoflong-termdevicestability.
Numerouscausesofdevicedegradationhavebeenproposedintheliter-ature,includingthedelaminationofelectrodes,7cathodeoxi-dation,8electrochemicalreactionsatelectrode/organicinterfaces,9hydrolysisofthemetallo-quinolatelayer,10andanintrinsicinstabilityofthemetallo-quinolatecation.
11Devicefailurehasalsobeenattributedtocrystallizationoftheactivelayers,especiallytheholetransportlayer.
12DespitetheimportanceofthearchetypeholetransportmoleculeNPB,relativelyfewstudiesofitsfundamentalmolec-¤ElectronicSupplementaryInformationavailable.
Seehttp://www.
rsc.
org/suppdata/cp/b1/b101619i/ularpropertieshaveappearedintheliterature.
TheoreticalinvestigationsintotheelectronicdensityofstatesandtheeectofchargingontheelectronicstructureofNPBwerereportedbyLeeandco-workers.
13,14TheholetransportmobilityofNPBwasmeasuredbyDengetal.
15usingthetimeofighttechnique.
AlsoTangandco-workers16havestudiedthegrowthmodelsofNPBonITOsubstratesusingAFM.
Infrared(IR)spectroscopyisastandardtoolforstructuralcharacterizationandfollowingtheevolutionanddynamicsofchemicalsystems.
TheinfraredassignmentsofNPBhavenotyetbeenreported.
Forlargemolecules,quantumchemicalcal-culationspredictingharmonicfrequenciesandspectralinten-sitiesareessentialwheninterpretingexperimentalIRspectra,wherethehighdensityofstatesresultsinspectralcomplexityintheregionbelowca.
1700cm~1.
Theavailabilityofanalyti-calgeometricandelectriceldderivatives,17coupledwithadvancesincomputerperformancehasextendedtheapplica-bilityofelectronicstructuremethodstosystemsaslargeasNPB.
Inthetheoreticalpredictionofmolecularvibrationalproperties,densityfunctionaltheory(DFT)hasbeendemon-stratedtobeacost-eectivealternativetoconventionalabinitioapproaches,signicantlyoutperformingmethodssuchasHartreeFockandsecond-orderperturbationMllerPlessettheory(MP2).
18h20Inthepresentwork,theequilibriumgeometry,vibrationalfrequenciesandIRintensitiesforNPBarecomputedusinghybridDFTandamediumsizedsplit-valencebasissettoenabletheassignmentofmajorbandsintheexperimentalpelletIRspectrum.
TheobservedIRbandsareassignedonthebasisofthefrequencyagreementandIRintensitypatternsbetweenthetheoreticalandobservedspectraandvisual-izationofcomputernormalmodedisplacementvectors.
UsingtheNPBIRassignments,acomparisonofthesurfaceIRspectra,obtainedfromroomtemperatureandthermallyDOI:10.
1039/b101619iPhys.
Chem.
Chem.
Phys.
,2001,3,213121362131ThisjournalisTheOwnerSocieties2001(annealedNPBthinlms,providesinsightintotheconforma-tionalchangesarisinguponannealing.
MethodsN,N@-diphenyl-N,N@-bis(1-naphthyl)-1,1@-biphenyl-4,4A-diamine(NPB)wasobtainedfromtheXeroxResearchCenterCanada(XRCC).
ThetransmissionIRspectrumwasrecordedfromisotropicallydispersedNPBinKBr.
TheIRspectrumwascollectedoverthespectralregion400cm~1to4000cm~1onaBomem102FT-IRequippedwithaCsIbeamsplitterandaDTGSdetectorwith4cm~1resolution.
Toexaminethespec-tralchangesinthinsolidlmsuponthermalannealing,athinsolidlmofNPBwasdepositedontoasilvermirror.
Thesilverwasthermallyonaglassslidetoatotalevaporatedmassthicknessof1000usingastandardvacuumsystemAevaporatoroperatingatabackgroundpressureof10~6Torr.
TheNPBwasdepositedontotheAg/glasssubstratetoatotalmassthicknessof200usingasecondvacuumsystemevapo-Arator.
ReectionabsorptionIRspectra(RAIRS)werecol-lectedfromthethinsolidlmusingtheBomem102FT-IRequippedwithaSpectra-TechFT-80grazingangleaccessoryxedat80.
TheRAIRSspectrumoftheNPBthinlmwasrecordedatroomtemperatureandafterannealingat125Cfor30min,forcomparison.
ThetheoreticalresultsreportedherewereobtainedusingtheGAUSSIAN98suiteofprograms.
21ThegeometryofNPBwasoptimizedandharmonicfrequenciesandIRinten-sitieswerecomputedusingthehybridB3LYPdensityfunc-tional,correspondingtothecombinationoftheBeckesthree-parameterexchangefunctional(B3)22withtheLeeYangParrtforelectroncorrelation(LYP),23alongwiththepolarizedsplit-valencebasisset6-31G(d)(whichprovidesatotalof754basisfunctionsforNPB).
24ResultsanddiscussionStructureandmolecularvibrationsofNPBThemolecularstructureofNPBalongwiththepelletIRspec-trumisshowninFig.
1.
NPBiscomposedofterminalphenylamineswithnaphthylmoietiesjoinedbyabridgingbiphenylgroup.
ThenaphthylgroupsinNPBgiverisetoanumberofstructuralconformations,whichmaybepresentinthesolidstate.
ThegasphasegeometryofNPBwasoptimizedattheB3LYP/6-31G(d)leveloftheorywithoutsymmetryconstraintsstartingfromastructurethatcorrespondstotheglobal(C1)minimum,ascalculatedbyasemiempiricalPM3molecularorbitalstudybyotherauthors.
14TheoptimizedstructureofNPBisdeterminedtohaveapointofsymmetryatthecentralCCbondinthebiphenylbridgeinagreementwiththepre-vioussemiempiricalcalculations.
AtableofcalculatedheavyatombondlengthsofNPBalongwithexperimentaldataforsmallermoleculesrepresentativeofthefragmentscomposingNPB(1-aminonaphthalene,anilineand1,1@-biphenyl-4,4@-diamine(benzidine))isavailable(seeESITableS1).
¤Fig.
1MolecularstructureofNPBandtheIRspectrumfromiso-tropicallydispersedNPBinKBr.
CharacteristicIRbandsarelabelled(ag).
NPBiscomposedof78atomsgivingriseto228vibrationaldegreesoffreedom.
Toallowinterpretationoftheexperimen-talpelletIRspectrum,harmonicvibrationalfrequencies,cor-respondingnormalmodesandIRintensitiescomputedinthedoubleharmonicapproximationwerecalcu-(IIRPodk/dQko2)lated.
WorkinourlaboratoryhasdemonstratedthatthehybridB3LYPfunctionalpredictsIRintensitiesincloseagreementwiththosecalculatedwiththeconventionalhighlycorrelatedabinitiomethodquadraticcongurationinter-actionincludingsinglyanddoublyexciteddeterminants(QCISD).
20Inparticular,theB3LYP/6-31G(d)leveloftheoryrepresentsacosteectivechoiceforthecalculationoftheo-reticalIRspectra,particularlyforlargemoleculessuchasNPB.
Thecalculatedvibrationalfrequenciesareallreal,veri-fyingthattheoptimizedgeometryisatrueminimumonthepotentialenergysurface.
Acompletetableoftheoreticalhar-monicfrequenciesandIRintensitiesforNPBisavailable(seeESITableS2).
¤Theoreticalharmonicfrequenciestypicallyoverestimateobservedfundamentalsduetotheneglectofmechanicalanharmonicity,electroncorrelationandbasisseteects.
Tocompensate,variousscalingstrategiesexisttobringthecom-putedharmonicsintogreateragreementwithexperi-ment.
18,19,25h28StudiesbyScottandRadom,18andWong19haveshownthatB3LYPcalculationsemployingthe6-31G(d)basissetprovidesharmonicfrequenciesthatcanbeeectivelyscaledforcomparisonwithexperimentalwavenumbers.
Inthiswork,toimprovetheagreementwithexperiment,theB3LYP/6-31G(d)harmonicfrequencieswerescaledbyafactorof0.
97asdiscussedbelow.
Tocomparewiththeexperimentalresults,asimulatedIRspectrumwasconstructedusingthescaledtheoreticalvibra-tionalfrequenciesandcomputedintensitiesbyrepresentingtheIRbandsbyGaussianlineshapeswithafullwidthathalfmaximum(FWHM)of4cm~1.
Thevibrationalspectraofcomplexmoleculesareusuallydiscussedintermsofdierentwavenumberregionsknowntogenerallycorrespondtodier-enttypesofvibrationalmodes.
Theupperwavenumberregion(ca.
3600cm~1to1700cm~1)containsvibrationscomposedlargelyoflocalisedhydrogenstretches,whereasthemid-wavenumberregion(ca.
1700cm~1to1000cm~1)containsheavyatomin-planestretchesandbends,andthelow-wavenumberregion(below1000cm~1),theout-of-planeandtorsionalmodes.
Itisinthelattertworegions,below1700cm~1(thengerprintregion),wherequantumchemicalpre-dictioncanbethemostusefulinmakingvibrationalbandassignmentsthatmaynototherwisebeinterpretable.
TheexperimentalpelletIRspectrumforNPBconsistsoftwogroupsofbandshavingsubstantialintensityasseeninFig.
1.
ThesimulatedandtheexperimentalIRspectraforthesetworegionsareexpandedandcomparedinFig.
2.
TheagreementbetweenthesimulatedandexperimentalIRspectraisexcel-lent,allowingreliablecorrelationbetweentheoreticallypre-dictedandexperimentallyobservedbands.
In-planeregionassignmentsThetoppanelinFig.
2presentstherstspectralrangeofsubstantialintensity,fromca.
1150cm~1to1650cm~1,whichgenerallycontainsheavyatomin-planestretchesandbends.
TheexperimentalandtheoreticalfrequenciesandgeneralmodeassignmentsforobservedIRbandsinthein-planeregionaregiveninTable1.
Thebandassignmentsweremadeonthebasisoffrequencyandintensitypatternagreementandthedescriptionfromvisualisationoftheatomicdisplacementvectors.
ThemostintensebandsinthisregionaredenotedinFig.
1andTable1withlettersa,c,dande.
Thebandmarkedaat1592cm~1intheexperimentalspectrumisassignedtoaCCstretchingvibrationlargelyinvolvingtheterminalphenylgroups(t-phenyl),predictedtohaveafrequencyof1606cm~1.
2132Phys.
Chem.
Chem.
Phys.
,2001,3,21313136Fig.
2ExperimentalIRspectrumandtheB3LYP/6-31G(d)simu-latedIRspectrumofthein-planeregion(toppanel)andtheout-of-planeregion(bottompanel)forcomparison.
ThisiscomparabletotheCCstretchingvibrationofanilineobservedat1604cm~1intheliquidphaseandcalculatedtobe1608cm~1usingscaledB3LYP/6-31G(d).
29Althoughitislessintensethantheotherbandsdiscussedhere,thevibrationmarkedbinFig.
1andTable1at1573cm~1isnotable,sinceitisassignedasanaphthylCCstretchingmodecomputedat1578cm~1.
Thecbandobservedat1491cm~1ispredictedat1494cm~1andcorrespondstoaCC/CNstretching]CHbendingvibrationassociatedwithboththeterminalandbridgingphenylgroupsinNPB.
Thedvibrationat1392cm~1ispredictedtooccurat1393cm~1andinvolvesCC/CNstretching]CHbendingofthenaphthylmoietiesofNPB.
AnenvelopeofoverlappingbandsisobservedintheexperimentalIRwithanobviouspeakwithmaximumintensityat1293cm~1,labellede.
Theebandiscomputedat1279cm~1andisattributedtoaCH/CCNbending]CNstretchingvibrationinvolvingtheterminalandbridgingphenylgroups.
Out-of-planeregionassignmentsThebottompanelofFig.
2showsthesecondintenseregion,fromca.
860cm~1to400cm~1,whichgenerallycontainsout-of-planevibrationalmodes.
Theexperimentalandtheo-reticalwavenumbersandgeneralmodeassignmentsforobservedIRbandsintheout-of-planeregionaregiveninTable2.
ThemostintenseabsorptioninthiswavenumberregionisdenotedfinFig.
1andTable2andisobservedat772cm~1.
Thisbandiscomputedtohaveafrequencyof769cm~1andisassignedtotheout-of-planeCHwagofthenaph-thylgroupsofNPB.
Thisbandiscomparabletothatcom-putedat767cm~1for1-aminonaphthaleneusingthescaledB3LYP/4-31Gleveloftheory,asreportedrecentlybyBausch-licher.
30Thebandmarkedg,observedat424cm~1intheexperimentalspectrum,correspondstoanaphthylCCtorsionvibration,predictedat424cm~1.
AgreementbetweenscaledharmonicfrequenciesandexperimentTheoreticalharmonicfrequenciesareoftenscaledtocomparewithexperimentalwavenumbers.
Thescalingfactoremployedinthepresentstudyof0.
97iscomparabletothescalingfactorsuggestedbyScottandRadomof0.
9614.
18Theaverageabsolutedierence,averagedierenceandstandarddeviationbetweenthetheoreticalandexperimentalfrequenciesfortheassignmentspresentedhereareca.
25cm~1,25cm~1and14cm~1,respectively.
Afterscaling,theagreementimprovessig-nicantly,givinganaverageabsolutedierence,averagedier-enceandstandarddeviationofca.
6cm~1,[4cm~1and6cm~1.
UnscaledB3LYP/6-31G(d)harmonicfrequenciesshowatendencytooverestimateexperimentalfundamentals,withalargenumberoffrequenciesoverestimatingtheexperimentaldatabymorethan50cm~1.
Afterscaling,theerrordistribu-tionismuchmorefavourable,beingpeakedatzerodierence(seeESI¤Fig.
S1forhistogram).
TherawB3LYP/lcalc[lexpt6-31G(d)frequenciesareincludedinTables1and2forindi-vidualcomparisonwiththescaledandexperimentallyobservedwavenumbers.
Table1ExperimentalandB3LYP/6-31G(d)calculatedfrequencies,andgeneralmodeassignmentsforobservedIRbandsinthein-planeregionofthespectrumB3LYP/6-31G(d)/cm~1Scaleda/cm~1Observed/cm~1Assignmentb167116211610CCstretch(biphenyl)a165616061592CCstretch(t-phenyl)b162715781573CCstretch(naphthyl)154514981504shCCstretch]CHbend(phenyl)c154014941491CCstretch]CHbend]CNstretch(phenyl)150814631463CCstretch]CHbend]CNstretch(naphthyl)148714431434CCstretch]CHbend(naphthyl)d143613931392CCstretch]CHbend]CNstretch(naphthyl)138513431343CCbend(naphthyl)133912991310CHbend]CHstretch]CCNbend(phenyl)e131912791293CHbend]CNstretch]CCNbend(phenyl)130312641274CCstretch]CHbend]CNstretch128412451251CCstretch]CHbend]CNstretch(naphthyl)121711811183CHbend]CNstretch(biphenyl)119311571156CHbend]CCstretch(naphthyl)111910861087CHbend]CCstretch(t-phenyl]naphthyl)111010771074CHbend]CCstretch(t-phenyl]naphthyl)107810461051CHbend]CCdeformation105610251028Ringdeformation105010191015Ringdeformation10179861001Ringdeformation(biphenyl)aHarmonicfrequencieswerescaledby0.
97.
bTerminalphenylgroupsareindicatedby"t-phenyl.
Phys.
Chem.
Chem.
Phys.
,2001,3,213121362133Table2ExperimentalandB3LYP/6-31G(d)calculatedfrequencies,andgeneralmodeassignmentsforobservedIRbandsintheout-of-planeregionofthespectrumB3LYP/6-31G(d)/cm~1Scaleda/cm~1Observed/cm~1Assignment975946966CHwag(naphthyl)962933953CHwag914887896CHwag(naphthyl)881854861CHwag(naphthyl)866840848CHwag(biphenyl)840,842,843815,816,818821CHwag(phenyl)818793798CHwag(naphthyl)804780789CHwag]CCdeformation(naphthyl]t-phenyl)f793769772CHwag(naphthyl)767744751CHwag(t-phenyl)751,755729,733742CHwag(naphthyl)]CHwag(t-phenyl)730708717CCtorsion(biphenyl)711690697CCtorsion(t-phenyl)706685691shCCtorsion(t-phenyl)]CCdeformation679658662CCtorsion(naphthyl)]CCdeformation661641644CCdeformation]CCtorsion(biphenyl)640621624CCtorsion]CCdeformation632614616CCtorsion]CCdeformation(t-phenyl)619601606CCtorsion]CCdeformation575558558CCtorsion(biphenyl]naphthyl)550534538CCtorsion(biphenyl]naphthyl)533517521CCtorsion]CCdeformation528512508CCtorsion(biphenyl)508493497CCtorsion(phenyl)]CCdeformation(naphthyl)477,483463,469467CCtorsion]CCdeformation]CCNwag443430439CCtorsion]CCbend(biphenyl]naphthyl)g437424424CCtorsion(naphthyl)aHarmonicfrequencieswerescaledby0.
97.
AnnealingofNPBthinlmsReectionabsorptioninfraredspectroscopy(RAIRS)iscom-monlyusedtostudytheorientationofnanometriclmsdepositedonareectingmetalsubstrate.
InRAIRS,theelec-triceldcouplingtothevibrationalmodesofthematerialisnormaltothesurface,allowingthedeterminationofaveragemolecularorientationthroughcomparisonofrelativeexperi-mentalbandintensities.
RAIRSisawellestablishedtechniqueandhasbeenusedtoinvestigatetheeectsofthermalanneal-ingfororganicsemiconductormaterials,suchasperylenebasedphotoconductors31h33andtheOLEDelectrontrans-portmaterialAlq3.
34,35Recently,Popovicetal.
36usedRAIRStomonitortheeectofdopantmoleculesonthestruc-turalchangesoccurringinNPBthinsolidlmsuponthermalannealing,howeverdetaileddiscussionwasnotgiven.
Inthepresentwork,withtheIRassignmentsofNPBestablishedbycomparisonwiththeDFTcalculations,wewilldiscusstheeectofannealingonNPBthinlmsisgreaterdetail.
TheRAIRSspectrumofa200NPBlmonasilvermirrorwasAcollectedatroomtemperatureandthenagainafterannealingat125Cfor30min.
Fig.
3ExperimentalpelletIRandthinlmrefectionabsorptionIR(RAIRS)spectraofNPBforcomparison.
Notethedierenceinrela-tiveintensitiesbetweenthetwo.
ThepelletIRspectrumandtheroomtemperaturethinlmRAIRSspectrumareshowninFig.
3.
Comparisonoftherela-tiveintensitiesoftheout-of-planenaphthylCHwagatca.
772cm~1andthein-planenaphthylCCstretchingvibrationobservedatca.
1392cm~1,withtransitiondipolesperpen-dicularandparalleltothenaphthylgroupplanerespectively,indicatesthatonaveragethenaphthylgroupsofNPBinthethinlmadoptapartiallyatorientationrelativetotheFig.
4TheRAIRSspectraforaNPBthinlmatroomtemperatureandafterannealingfor30minat125Cforthein-planeregion(toppanel)andtheout-of-planeregion(bottompanel).
TheIRdierencespectrumisshownindicatingthebandsofsignicant(IR125]C[IRRT)intensitychangediscussedinthetext.
Theordinatescalesofthespectrahavebeenexpandedforclarity.
2134Phys.
Chem.
Chem.
Phys.
,2001,3,21313136surface.
TheroomtemperatureandtheannealedRAIRSspectra,andtheIRdierencespectrumfor(IR125C[IRRT)theNPBthinlmareshowninFig.
4.
Theordinatescalesofthespectrahavebeenexpandedforclarity.
TheIRdierencespectrumintheout-of-planeregion(ca.
860cm~1to400cm~1)(lowerpanel))indicateskeydierencesbetweentheroomtemperatureandannealedspectra.
MostsignicantisthemarkedincreaseinintensityofthenaphthylCHwagatca.
772cm~1andthedecreaseinintensityofvibrationsatca.
821cm~1and751cm~1,assignedtoCHwagingvibrationsinvolvingthephenylgroups,withthelatterbeingmainlylocalisedontheterminalphenylgroupsofNPB.
Theseinten-sitychangessuggestthat,uponannealing,thenaphthylgroupsofNPBrelaxfurtherintoaataverageorientationandthephenylgroupstendtopreferaperpendicularconfor-mationwithrespecttothesurface.
Lookingtothein-planeregion(ca.
1150cm~1to1650cm~1(Fig.
4,toppanel))forindicationsoforientationalchangesshowsthatthebandatca.
1491cm~1,assignedtoaphenylCC/CNstretching]CHbendingvibration,gainssig-nicantintensityuponannealing.
OtherbandsthatgainintensityaretheCCstretchlargelyinvolvingtheterminalphenylgroupsandthephenylCHbendingvibration,atca.
1592cm~1and1293cm~1,respectively.
Theatomicdisplace-mentvectorsofthevibrationshavingthelargestincreaseinintensityuponannealingassignedtothecandfIRbandsareshowninFig.
5.
ThereectionabsorptionIRspectraindicatethatuponannealingtheNPBlmsundergoorientationalchangesconsistentwiththenaphthylgroupsbeinglargelyparalleltothesurface.
ApotentialconformationofNPBonthesurfaceinwhichthenaphthylgroupscouldbepredomi-nantlyatisthatwherethenaphthylgroupsarecistoeachother,asopposedtothegasphaseglobalminimumtranscon-formation.
Insuchaconformationtheterminalphenylgroupscouldbedirectedupfromthesurface,whichwouldcauseanincreaseintheintensityofthephenylCCstretchingbandsandadecreaseinthephenylCHout-of-planewags,asisobservedintheannealedIR.
ConclusionThemajorIRmodesoftheOLEDmaterialNPBhavebeenassignedusingtheB3LYP/6-31G(d)leveloftheory.
ExcellentagreementwasfoundbetweentheexperimentalIRspectrumandthesimulatedDFTspectrum,allowingthereliableassign-mentofobservedbands.
ReectionabsorptionIRspectros-copy(RAIRS)wasusedtoinvestigateorientationalchangesinNPBthinlmsuponthermalannealing.
Withannealing,thenaphthylgroupsofNPBarefoundtoadoptapredominatelyatorientationwithrespecttothesurface.
Fig.
5Normalmodeatomicdisplacementvectorsforthecandfvibrations,whichshowthelargestincreaseinintensityuponannealing.
Theexperimentalfrequenciesareindicated.
Phys.
Chem.
Chem.
Phys.
,2001,3,213121362135AcknowledgementsHBSandMDHgratefullyacknowledgenancialsupportfromtheNationalScienceFoundation(GrantNo.
CHE9874005)andagrantforcomputingresourcesfromNCSA(GrantNo.
CHE980042N).
MDHwouldalsoliketothanktheDepartmentofChemistry,WayneStateUniversityfornancialsupportprovidedbyaWilfredHellerGraduateFellowship.
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