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StrongEnhancementofNonlinearOpticalPropertiesThroughSupramolecularChiralityThierryVerbiest,*SvenVanElshocht,MarttiKauranen,LouisHellemans,JohanSnauwaert,ColinNuckolls,ThomasJ.
Katz,AndrePersoonsAnewapproachtosecond-ordernonlinearoptical(NLO)materialsisreported,inwhichchiralityandsupramolecularorganizationplaykeyroles.
Langmuir-Blodgettlmsofachiralhelicenearecomposedofsupramoleculararraysofthemolecules.
Thechiralsupramolecularorganizationmakesthesecond-orderNLOsusceptibilityabout30timeslargerforthenonracemicmaterialthanfortheracemicmaterialwiththesamechemicalstructure.
Thesusceptibilityofthenonracemiclmsisarespectable50picometerspervolt,eventhoughthehelicenestructurelacksfeaturescommonlyassociatedwithhighnonlinearity.
Susceptibilitycomponentsthatareallowedonlybychiralitydominatethesecond-orderNLOresponse.
Second-orderNLOeffectsareusuallyob-servedonlyfromnoncentrosymmetricmate-rials(1).
Commonly,suchmaterialsarecon-structedbyincorporatingdonor-acceptor–substitutedmoleculesthathaveanonvanish-ingmolecularhyperpolarizability(2)intononcentrosymmetricstructuressuchaspoledpolymerfilms,Langmuir-Blodgett(LB)films,self-assembledfilms,orcrystals(3,4).
Thenonlinearityofsuchmaterialshasbeenimprovedbyoptimizingthemicroscopicre-sponseofthemolecules(5)orbyimprovingtheiralignmentororientationinthemacro-scopicstructure(6).
Anotherwaytoachievenoncentrosymme-tryistousechiralmolecules.
Suchmoleculesarenecessarilynoncentrosymmetric,andtheirsecond-orderNLOresponseisthereforenonzero(7).
Evensuchhighlysymmetricmacroscopicassembliesasisotropicsolutionsofasingleenantiomer(asinglemirror-imageform)ofachiralmoleculearenoncentrosym-metricandcanbeusedforsecond-ordernon-linearoptics,asshownbysum-frequencygenerationinsugarsolutions(8,9).
Thein-trinsicvalueofthenonlinearsusceptibilityofsuchmaterialscanbequitehigh(0.
4pm/V).
However,theprocessisnotphasematch-able,anditsoverallefficiencyisthereforelow.
Amoreefficientapproachhasbeentousechiralityindirectly,toensuremolecularcrystallizationinanoncentrosymmetricgroup(10).
Recently,nonlinearopticshasalsobeenusedasasensitivetooltostudychiralsurfaces(11,12).
Weshowthathighnonlinearitycanbeachievedbyassemblingthemoleculesofanenantiomericallypurehelicene(13)intosu-pramoleculararrays.
TheNLOresponseoftheirfilmsismuchhigherthanthatoffilmsofthecorrespondingracemic(50/50)mixtureoftheenantiomers,eventhoughtheconstit-uentmoleculesinbothfilmshavethesamechemicalstructure.
ThemoleculeswestudiedwerethoseofthetetrasubstitutedhelicenebisquinoneshowninFig.
1(13,14).
Inbulksamples,thenonrace-mic,butnottheracemic,formofthematerialspontaneouslyorganizes(15)intolongfibersclearlyvisibleunderanopticalmicroscope.
Thesefiberscomprisecolumnarstacksofheli-cenemolecules(15).
Similarcolumnarstacksself-assembleinappropriatesolvents,suchasn-dodecane,whentheconcentrationsaregreat-erthan1mM,andwhentheyassemble,thecirculardichroisms(CD)ofthesolutionsin-creasesignificantly(13).
WepreparedLBfilms(16)oftheheli-cenebyspreadingadilutechloroformsolu-tion(210–4M)ontothepurewatersub-phaseofanLBtrough.
Afterthesolventhadbeenevaporatedat20°C,thefilmswereslowlycompressedtoasurfacepressureof20mN/m.
Afterstabilizingfor30min,thefilmsweredepositedbyhorizontaldippingontohydrophobicglass[forsecond-harmonicgen-eration(SHG)measurements],fusedquartz[forultraviolet(UV)–visibleabsorptionandatomicforcemicroscopy(AFM)measure-ments],orsilicon(forAFMmeasurements).
Theopticalqualityofthefilmswasexcellent.
Although60isthelargestnumberoflayersdepositedtodate,thereisnoindicationthatthequalityoffilmswithmorelayerswouldbelower.
Opticalmicroscopydetectednofibersorothermacroscopicfeaturesinthefilms.
ThismeansthatintheLBfilmsofeventhenonracemicmaterial,anysupramolecularorganizationextendsonlytosubmicrometerlengths.
Thesampleswereirradiatedata45°angleofincidencewithafundamentalbeamfromaNd–yttrium-aluminum-garnet(Nd:YAG)la-ser(1064nm,50Hz,8ns),andtheSHGsignals(532nm)weredetectedinthetrans-mitteddirection.
Half-andquarterwave-plateswereusedtocontrolthepolarizationoftheirradiatingbeam,andthesecond-harmon-iclightwasresolvedintop-(intheplaneofincidence)ands-(outoftheplaneofinci-dence)polarizedcomponents.
TheSHGsignalsmeasuredarisefromthequadraticresponseofthematerialtothefun-damentalbeam.
ThisresponseisrepresentedbytheNLOpolarization(1)Pi(2)j,kijkEj()Ek()whereijkrefertothecartesiancoordinates,Ej()andEk()arecomponentsoftheelec-tric-fieldamplitudeatthefundamentalfre-quency,Pi(2)isacomponentofthenon-linearsourcepolarizationatthesecond-har-monicfrequency2,andijkisacomponentofthesecond-ordersusceptibilitytensor.
Forsufficientlythinsamples,thepolarizationleadstotheamplitudeofthesecond-harmon-icfieldE(2)growinglinearlywiththickness(1).
Consequently,theintensityofthesec-ond-harmonicfield,whichisproportionaltothesquareoftheamplitude,shouldincreasequadraticallywithboththethicknessofthefilmandthevalueofthesusceptibility.
Thefilmsofthenonracemichelicenegen-eratedstrongSHGsignalswhoseintensityincreasedquadraticallyasafunctionofthenumberofdepositedlayers(Fig.
2A),whichconfirmsthegoodstructuralandopticalqual-ityofthefilms.
ThestrongestSHGsignalfromaone-layernonracemicfilmwasap-proximately1000timesasintenseasthatfromasimilarracemicfilm,correspondingtoa30-foldenhancementinthevalueofthesusceptibility.
Thisenhancementisextraordi-nary,becausetheindividualmoleculesinbothfilmshavethesamechemicalstructures.
Forthenonracemicsample,theSHGsig-nalwasstrongestwhentheincidentbeamwasp-polarizedandtheSHGbeamwass-polarized(p-in–s-outsignal),whereas,fortheracemicsample,thesignalwasstrongestwhenbothbeamswerep-polarized(p-in–p-outsignal).
Forisotropicsurfacesandthinfilms(17),thep-in–s-outsignalisduetothecomponentsofthesecond-ordersusceptibili-T.
Verbiest,S.
VanElshocht,M.
Kauranen,L.
Helle-mans,J.
Snauwaert,A.
Persoons,LaboratoryofChem-icalandBiologicalDynamics,KatholiekeUniversiteitLeuven,Celestijnenlaan200D,B-3001Heverlee,Bel-gium.
C.
NuckollsandT.
J.
Katz,DepartmentofChem-istry,ColumbiaUniversity,NewYork,NY10027,USA.
*Towhomcorrespondenceshouldbeaddressed.
E-mail:thierry@lcbdiris.
fys.
kuleuven.
ac.
beREPORTSwww.
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orgSCIENCEVOL28230OCTOBER1998913tytensorthatarenonvanishingonlyinthepresenceofchirality(chiralcomponents).
Thep-in–p-outsignal,however,isallowedforallsurfacesandthusisduetotheachiralcomponentsofthesusceptibilitytensor.
Theseresultssuggestthatinthenonracemicsamples,thedominantpartofthesusceptibil-itytensoristhatassociatedwithchirality.
Intheracemicsamples,thispartmustcancel.
Analternativeexplanationforthedifferentbehaviorofthetwotypesofsamplesisthatthehelicenepacksdifferentlyintheracemicandnonracemicfilms.
Toanalyzethesepossibilities,wefirstinvestigatedwhetherthedominantcompo-nentsofthesecond-ordersusceptibilityten-sorareallowedonlybychirality.
Althoughseparatingthechiralandachiraltensorcom-ponentsisstraightforwardforthinfilmsthatareisotropic,in-planeanisotropysignificant-lycomplicatessuchseparation(18).
There-fore,insteadofrelyingontypicalchiralef-fects(19),wedeterminedthesymmetryandnonvanishingtensorcomponentsofthesam-ples,usingseveraldifferentmeasurements.
WefirstmeasuredSHGsignalswhilero-tatingthesamplesabouttheirsurfacenormal(20).
Filmsoftheracemicmaterialwereiso-tropic(Cvsymmetry).
Ontheotherhand,therotationpatternobtainedforafive-layernonracemicfilmandp-in–s-outsignal(Fig.
2B)isanisotropicandsuggeststhatthenon-racemicfilmpossessesC2symmetry.
Ifsuchafilmwereachiral,thatis,ifithadC2vsymmetry,thesignalshowninFig.
2Bwouldvanishatsomerotationalangle.
Becauseitdoesnot,theremustbeafinitechiralsuscep-tibilitycomponent.
Moreover,thiscompo-nentprobablyplaysanimportantrole,be-causetheaveragesignalisverylarge.
ToseparatetheeffectsofchiralityandanisotropyontheNLOresponseofthenon-racemicfilms,wedeterminedthecompo-nentsofthenonlinearsusceptibilitytensorofafive-layerfilm.
Weuseacoordinatesystemwithzalongthesurfacenormalandxandyintheplaneofthesubstrate.
ThenonvanishingcomponentsofthesusceptibilitytensoroftheC2grouparezzz,zxx,zyy,xxzxzx,yyzyzy,xyzxzy,yxzyzx,andzxyzyx.
Forahypotheticalachiralsam-plewithtwofoldsymmetry(C2v),amirrorplanecontainingthesurfacenormalfixesthedirectionsofthein-planecoordinatesxandy.
Furthermore,reflectioninthatplaneimpliesthatthecomponentsxyz,yxz,andzxyareallowedonlybychirality.
However,ourchiralC2samplehasnomirrorplane,andthereforethechoiceofxandyisarbitrary.
Inaddition,rotationofthein-planecoordinatesmixestheachiralandchiraltensorcompo-nents.
Forexample,whenrotated,zxycon-tributestozxx,zyy,zxy,andzyx.
Hence,noneofthetensorcomponentscanuniquelybeassociatedwithchirality.
Tocircumventthisarbitrariness,itisnecessarytoconsiderlinearcombinationsofthetensorcomponentsthatdonotdependonthein-planerotationalangle.
Suchisotropiccombinationsare:zzz,zxxzyy,xxzyyz,andxyz–yxz.
Thelastofthesecombinationscanbeuniquelyassociatedwithchirality.
Todeterminethecomponentsofthesus-ceptibilitytensor,weusedthepolarizationtechniqueof(21)tomeasuretheSHGsignalsfromsamplesatseveralin-planeazimuthalrotationalangles.
Wealsoverifiedthattheisotropiccombinationsareindependentoftheassumed0°orientationofthesample.
Themagnitudesdeterminedforthesecombina-tions,referencedtoaquartzwedge(d110.
3pm/V)(22),areshowninTable1.
Theachiralcombinationsareseentobeatleast10timessmallerthanthechiralone.
Thenonlinearityofthenonracemicsamplesisthereforedom-inatedbythechiraltensorcomponents.
Thus,thechiralityofthenonracemicsamplecon-tributessignificantlytothedifferentNLOresponsesoftheracemicandnonracemicsamples.
Thatitisessentiallythesolefactorresponsibleforthedifferencewasverifiedbyobservingthattheabsolutelevelsofthep-in–p-outsignals,averagedoverthein-planero-tationalangle,were,within20%error,equalfortheracemicandnonracemicsamples.
Hence,theachiralpartsofthenonlinearityareessentiallythesamefortheracemicandnonracemicsamples.
Whythechiralsuscep-tibilitycomponentsarelargeis,however,notexplainedbytheseresults.
TheUV-visibleabsorptionspectraofLBfilmsoftheracemicandnonracemicsamplesareidenticalandsimilartothoseofnonrace-micsolutionsinwhichthemoleculesareaggregated,notisolated(13).
Thissuggeststhatthemoleculesoftheracemicandnonra-cemicmaterialsareorganizedintosimilaraggregatesonasmallscale.
AFMimages(Fig.
3)supportthissuggestion,fortheyshowthattheaggregatedmoleculesinbothfilmsarefurtherassembledintofibrousstruc-tures.
However,theorganizationisgreaterinthefilmsofthenonracemicmaterial(Fig.
3,AandC).
Thestructuresintheseare5nmwide[aboutthewidth(4.
1nm)ofthecol-umnsintowhichthemoleculesassembleinbulk(14)],severaltensofnanometerslong,andarrangedinbundles.
However,thescaleoftheorganizationintheLBfilmsissuffi-cientlysmallthatthestructurescannotbeseenoptically,althoughthealignmentofthebundlesintheplaneofthesubstrateprobablyaccountsfortheC2symmetryevidencedinFig.
2B.
Intheracemicmaterial(Fig.
3B),thefibrousstructuresareshorterandnotaswellFig.
1.
(Top)Chemicalstructureoftheheliceneand(bottom)schematicrepresentationofcol-umnsofstackedhelicenemoleculesasob-servedinsolidbulksamples(thesidechainshavebeendeletedforclarity,andthersthelicenesarearbitrarilyshowntobeinthesamerotationalphase).
Fig.
2.
Second-harmonicsignalsfromthenon-racemicLBlms.
(A)Thesignalasafunctionofthenumberofdepositedlayers.
(B)Thes-polarizedsecond-harmonicsignalforp-polar-izedfundamentalbeamasafunctionoftheazimuthal(in-plane)rotationangleofave-layersample.
The0°azimuthalorientationischosenarbitrarily.
Table1.
Absolutevaluesofchiralandachiralisotropiccombinationsofsusceptibilitycompo-nentsofthenonracemicLBlms.
IsotropiccombinationAbsolutevalue(pm/V)Chiralityzzz6Achiralxxzyyz2Achiralzxxzyy4Achiralxyz–yxz50ChiralREPORTS30OCTOBER1998VOL282SCIENCEwww.
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Thissuggeststhattheinteractionsbetweentheenantiomersarenotfavorableforlarge-scaleorganization,whichmayrelatetowhytheracematedoesnotformfibrousstructuresinbulk(15).
Thelesserdegreeoforganizationintheracemicmate-rialalsoconformswiththefailuretoseex-raydiffractionfromthesefilms,whereasitisseenfromfilmsofthenonracemicmaterial.
Forthelatter,x-raydiffractionshowstheinterlayerdistancestobesimilartothediam-etersofthehelicenemoleculesandtothecolumnsobservedinbulk(23).
Theaboveresultssuggestthattheprimaryexplanationforthehighsecond-orderNLOresponseofthehelicenesistheaggregationintocolumnarstackswithlargechiral(xyz-type)nonlineartensorcomponents.
Sponta-neouschiralsegregation(24)probablyoccursintheracemicfilms,witheachenantiomeraggregatingwithitselfandmaintainingthelargechiralcomponents.
Intheracemicsam-pleshowever,thesedominantcomponentsofthetwoenantiomersareequalinmagnitudebutoppositeinsign,whichlowerstheoverallNLOresponse.
FurtherorganizationoftheaggregatesintobundlesappearstobelessimportantfortheNLOproperties,althoughitaccountsforthedifferentrotationalsymme-tryoftheracemicandnonracemicfilms.
The50pm/Vvalueofthedominantpartofthenonlinearity(xyz–yxz;Table1)issufficientlyhighthatthematerialcouldbeuseful(25).
Thelinearabsorptionspectrum,however,suggeststhattheNLOresponsemayberesonantlyenhanced.
Nevertheless,itiswithinoneorderofmagnitudeofthehigh-estvaluesreported(26).
Wefindtheresultremarkablebecause(i)thenonlinearityisdominatedbythechiraltensorcomponents,whichsuggeststhatthetraditionalrequire-mentsfornonlinearmoleculestobeorientedoutoftheplane(1)arenotimportantforstructuresbasedonthehelicenederivative,and(ii)althoughthechemicalstructuredoesnotfulfillclassicalrequirementsforhighmo-lecularnonlinearity(2),theNLOsusceptibil-ityisneverthelessrelativelyhigh,whichsug-geststhatfurtherimprovementsmightbeachievablethroughsynthesis.
Thefilmsofthenonracemichelicenehaveothervirtues.
Afive-layerfilmshowedim-pressivethermalstability.
Whenstoredfor250hoursinairat150°C,itlostnoSHGefficiency,andevenshortexcursionsabove200°ChadnodetrimentaleffectontheNLOresponse.
Moreover,topreparethickfilms,theverticaldippingprocedurecouldbeused,whichisdesirablebecauseitislesstime-consumingthanhorizontaldipping.
Althoughthetypeofdepositionachievedwhenthehelicenewasdippedverticallydependedonthehumidity,thetemperature,andthewaythesubstratewasprepared,wewereabletomakeY-typeLBfilms(thosedepositeddur-ingbothupanddownstrokes)whoseopticalqualitiesandnonlinearefficienciesequaledthoseoffilmsformedbyhorizontaldipping.
Thehelicenefilmsalsosuggestanewwaytoachievephasematching.
Phasematchingisanimportantconsiderationinfrequencycon-version,inwhichthephaserelationbetweenthedrivingnonlinearpolarizationandthegeneratedfieldcanusuallybemaintainedonlyoverthedistanceofcoherencelength(27).
However,phasematchingoverarbitrarydistancescanbeachievedbyusingmultilayerstructuresinwhichthesignofthenonlinearityisreversedaftereverycoherencelength(quasi–phasematching)(28).
Thesignofthechiralpartofthenonlinearitycanbereversedsimplybychang-ingtheenantiomerofthemoleculewithnoneedforadditionaldomainreversal.
Theuseofthetwoenantiomersinaquasi–phase-matchedstructurealsohastheadditionalbenefitthatnonetpolarizationrotationduetolinearop-ticalactivitywilloccur.
Asapreliminarytestofthemutualcompatibilityofthetwoenan-tiomersofthehelicene,wepreparedfourlayersofoneenantiomerbyverticaldippingandcoveredthemwithfourlayersoftheother.
Forsuchthinfilms,thenonlinearitiesofthetwoenantiomersshouldcancel.
TheSHGsignalfromthesamplevanished,whichsuggeststhatitmightbepossibletousehelicenestopreparequasi–phase-matchedstructures.
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WeacknowledgenancialsupportfromtheBelgiangovernment,theBelgianNationalScienceFounda-tion,andtheKatholiekeUniversiteitLeuven.
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31July1998;accepted29September1998Fig.
3.
AFMimages(80nmby80nm)offour-layerLBlmsofthehelicenemeasuredinthetappingmode.
(A)Phase-contrastimageofanonracemiclmonsilanizedsilicon.
(B)Phase-contrastimageofaracemiclmonsilanizedsilicon.
(C)Topographicimageofanonracemiclmonsilanizedglass.
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