attention Theoretical Insight into the Origin of Large Stokes Shift and

TheoreticalInsightintotheOriginofLargeStokesShiftandPhotophysicalPropertiesofAnilido-PyridineBoronDifluorideDyes

Jun-LingJin,Hai-BinLi,YunGeng,YongWu,Yu-AiDuan,andZhong-MinSu*[a]

Thegeometricandelectronicstructuresandphotophysicalpropertiesofanilido-pyridineborondifluoridedyes1–4,aseriesofscarce4,4-difluoro-4-bora-3a,4a-diaza-s-indacene(BODIPY)derivativeswithlargeStokesshift,areinvestigatedbyemployingdensityfunctionaltheory(DFT)andtime-depen-dentDFT(TD-DFT)calculationstoshedlightontheoriginoftheirlargeStokesshifts.Tothisend,asuitablefunctionalisfirstdeterminedbasedonfunctionaltestsandarecentlypro-posedindex—thecharge-transferdistance.ItisfoundthatPBE0providessatisfactoryoverallresults.Anin-depthinsightintoHuang–Rhys(HR)factors,Wibergbondindices,andtransi-tiondensitymatricesisprovidedtoscrutinizethegeometricdistortionsandthecharacterofexcitedstatespertainingtoab-sorptionandemission.TheresultsshowthatthepronouncedgeometricdistortionduetotherotationofunlockedphenylgroupsandintramolecularchargetransferareresponsibleforthelargeStokesshiftof1and2,while3showsarelativelyblue-shiftedemissionwavelengthduetoitsmildgeometricdistortionuponphotoemission,althoughithasacomparableenergygapto1.Finally,compound4,whichisdesignedtore-alizetherareredemissioninBODIPYderivatives,showsdesira-bleandexpectedproperties,suchashighStokesshift(4847cmÀ1),redemissionat660nm,andreasonablefluores-cenceefficiency.Thesepropertiesgiveitgreatpotentialasanidealemitterinorganiclight-emittingdiodes.ThetheoreticalresultscouldcomplementandassistinthedevelopmentofBODIPY-baseddyeswithbothlargeStokesshiftandhighquantumefficiency.1.Introduction

Luminescentorganoboroncompoundsareofgreatimportanceinapplicationsasdiverseasorganiclight-emittingdiodes(OLEDs),nonlinearoptics,fluorescentmolecularprobes,andin-tracellularfluorescenceimaging.[1]Amongthem,four-coordi-natedchelateboroncomplexeshaveattractedmuchattentionduetotheireasyavailability,goodphotostability,andhighchemicalandthermalstability[1a,b,j–n,2]relativetothree-coordi-natedboroncomplexes,whichneedbulkygroupstostabilizethem.Inparticular,4,4-difluoro-4-bora-3a,4a-diaza-s-indacene(BODIPY)anditsderivativeshavebeeninthespotlightofsci-entificinterestbecauseoftheirexcellentcharacteristicssuchashighfluorescencequantumyields,andsharpandtunableabsorption/emissionprofiles,[3]whichareinhighdemandforidealoptoelectronicmaterials.Moreover,goodtransportprop-ertiesenablethemtobeusedasorganicsemiconductorsinor-ganicfield-effecttransistors.[1c]Forexample,Zhaoetal.[4]re-portednovelBODIPYderivativeswithelectronmobilityupto0.291cmÀ2VÀ1sÀ1,whichcanfunctionasscarcen-typeorganicsemiconductors.[5]

ItisgenerallyrecognizedthattheremarkabledisadvantageofBODIPYdyesistheirverysmallStokesshifts,whichareaslowasapproximately10nm.Asaresult,thisextremelysmallStokesshift,whichcouldreducetheemissionintensityviaself-absorption,hasimpededtheirwideapplicationinOLEDsandasfluorescentmolecularprobes.Therefore,itisaninterestingissuetoenlargetheStokesshiftofBODIPYdyes.Uptonow,thestrategybasedonFçrsterresonanceenergytransfer,thatis,introducingotherfluorophoresatthe1,1’,2,3,4,5,6,7,8-posi-tionsoftheBODIPYcore,tendstoresultinthemoleculeoccu-pyingalargervolumeofspace.[6]However,suchanapproachusuallyrequiresacomplexsynthesisprocessandisrathercom-plicatedduetotheintricatemechanismoftheenergy-transfercassette;moreimportantly,thismethodcannotcompletelyeliminatethesmallStokesshiftofBODIPYs.[6c,e,f]Inthisrespect,itisstillagreatchallengetoincorporatealargeStokesshiftandnon-cassettesystemintoasingleBODIPYdye.Unfortu-nately,fewresearchstudieshavebeendevotedtothisaspect.Toourknowledge,onlyafewexamplesofsuchchromophores,suchasbisBODIPY[7]and2-thienyland2,6-bisthienylBODIPYderivatives,[8]exhibitalargeStokesshift.Recently,aseriesofBODIPYderivatives,namelyanilido-pyridineborondifluoridedyeswithbothlargeStokesshifts(80–114nm)andhighfluo-rescencequantumyields(0.33–0.75),werereportedbyPiersandco-workers;[9]somerepresentativecompounds1–3arede-pictedinFigure1.

Herein,ourinitialinterestinanilido-pyridineborondifluoridedyes1–3wascatalyzedbytheoriginoftheirlargeStokes

[a]J.-L.Jin,H.-B.Li,Dr.Y.Geng,Y.Wu,Y.-A.Duan,Prof.Dr.Z.-M.Su

InstituteofFunctionalMaterialChemistryFacultyofChemistry

NortheastNormalUniversityChangchun130024(China)Fax:(+86)431-85684009E-mail:zmsu@nenu.edu.cn

SupportinginformationforthisarticleisavailableontheWWWunderhttp://dx.doi.org/10.1002/cphc.201200384.

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experimentaldataareacceptable,thusindicatingthatthePBE0/6-31+G(d)levelcanprovidequiteaccuratepredictionofthegroundgeometricstructuresforthestudiedcompounds.Asforthebasisset,Jacqueminetal.[11d]foundthat6-311G(2d,p)iscrucialtoobtaintheconvergedgeometriesforaza-BODIPY.Thus,comparativemoleculargeometricalcalcula-tionsatthePBE0/6-311G(2d,p)andPBE0/6-31+G(d)levelshavebeencarriedoutforcompound1(TableS2,SupportingInformation).Itisfoundthatthelargestdifferencebetweenthebondlengthsofthetwolevelsislessthan0.005󰀃.More-over,theresultsfordihedralanglescomputedatthePBE0/6-31+G(d)levelareclosertotheexperimentaldata.FromtheS0toS1state,thevariationsofthebondlengthsanddihedralanglescalculatedbythetwolevelsarequitesimilar.Thesere-sultsdemonstratethatthe6-31+G(d)basissetusedinthisworkissuitableenoughandthusitisusedforgeometryopti-mizationoftherestofthecompounds.Additionally,thesol-venteffects(CH2Cl2)onthemoleculargeometrieswerealsotestedandareshowninTableS3(SupportingInformation).Theresultsdemonstratethatthesolventeffectsalsohavelittleinfluenceonthegeometriesofthestudiedcompounds.

FromTableS1intheSupportingInformationandFigure2itisimportanttonotethattheground-stateconfigurationsofthemainaromaticskeletonareslightlydistortedin1and2withanunlockedphenylgroup.TheBF2subunitexhibitsade-

Figure1.Structuresofcompounds1–4andtheirnumberingscheme.

shiftsandhighquantumyields,becausethegeometricstruc-turesofthesedyesaresimpleandverysimilartothetradition-alBODIPYframework.Tothisend,thegeometricandelectron-icstructures,absorptionandemissionspectra,Wibergbondin-dices,andHuang–Rhys(HR)factorswereexploredusingdensi-tyfunctionaltheory(DFT)calculationstoprovideinsightintothestructure–propertyrelationshipsandtounderstandtheoriginoftheirlargeStokesshiftsandhighquantumyields.Ontheotherhand,BODIPYdyeshaverelativelyshortfluorescenceemissionmaxima.ReportsonBODIPYderivativeswithemissionwavelengthlongerthan600nmarerelativelyrare.[10]Recently,someBODIPYmoleculeswithlongemissionwavelengthhavebeenreportedinexperimentalandtheoreticalworks,[11]buttheirStokesshiftsarerelativelysmall,whichhamperstheminbeingidealemitters.Therefore,thesecondobjectiveistodesignBODIPY-basedchromophoreswithbothlongeremis-sionbandsandlargeStokesshifts.Compound4,quitesimilarto3,wasdesignedinanattempttoobtainagooddyewithredemission.Theresultsrevealthat4hasared-shiftedemissionwavelength,largeStokesshift,andcomparablefluorescenceef-ficiency.

2.ResultsandDiscussion

2.1.Ground-StateGeometriesTheselectedgeometricalpara-metersoptimizedatthePBE0/6-31+G(d)leveltogetherwiththeavailableX-raycrystaldiffrac-tiondataof1–3aresummarizedinTableS1intheSupportingIn-formation.TheatomiclabelingschemeisshowninFigure1andtheoptimizedgeometriesof1–4invacuumatthegroundstate(S0)andthelowestsingletexcit-edstate(S1)arepresentedinFigure2.Therelativeerrorsbe-tweentheoptimizedbondlengthsof1–3andthecorre-spondingexperimentaldataarenomorethan1.3,1.4,and0.9%,respectively.Suchdiscrepanciesbetweenthecalculatedandthe

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Figure2.OptimizedS0andS1geometries(topandsideviews)ofcompounds1–4invacuumatthePBE0/6-31+G(d)level.

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viationanddisplacementoutwardsfromthemeanC3N2planeby0.566and0.433󰀃in1and2,respectively.ThisparticularconfigurationiscausedbyCÀH···FÀBinteraction.[12]ThephenylgroupattheN2siteistwistedrelativetothemainplaneowingtostericinteraction,thuspullingoutwardtheBF2subu-nitandcreatingaslightlydistortedmainaromaticskeleton.Incontrast,for3and4withlockedphenylgroup,theground-stategeometriesarestrikinglydifferentfromthoseof1and2.Theypossessplanarconfigurationsowingtotheabsenceofsterichindrance.Therefore,fromtheground-statestructuresof1–4,theunlockedphenylgroupsgreatlydisturbtheplanarityofthebackbonesof1and2,whichisexpectedtoexertadirecteffectonthephotophysicalperformance.2.2.FrontierMolecularOrbitalsintheGroundStateFrontiermolecularorbitals(FMOs)areincloseconnectionwiththeelectronicexcitationandtransitioncharacters.ToevaluatepreliminarilytheeffectofaromaticsubstitutionontheelectrondistributioninFMOs,Figure3showsthehighestoccupiedmo-tiononthebondC4ÀC6intheHOMOandLUMOof3.Thus,compound3exhibitssimultaneouslyloweredHOMOandLUMOlevelsandacomparableenergygaprelativeto1.Inter-estingly,incontrastto1–3,compound4possessesasomewhathigherHOMO,amuchlowerLUMO,andconsequentlyanarrowenergygapalthoughithassimilarconjugationwith3,whichisascribedtothesignificantlydifferentdistributionofFMOs.AscanbeenseenfromtheFMOsinFigure3,thepelec-tronoftheN2atomof3showsantibondinginteractionwiththepelectron,whileastrongbondinginteractionisfoundin4.Hence,theeffectiveconjugationofp–pelectronsseriouslylowerstheLUMOandraisestheHOMO,whichresultsinanar-rowerenergygap.Therefore,aredshiftcanbeexpectedintheabsorptionandemissionofcompound4incomparisonwith1–3.

2.3.OpticalPropertiesandTransitionDensityMatrix2.3.1.TheChoiceofFunctional

Itisgenerallyrecognizedthatpurefunctionalsandsomehybridfunctionalsunderestimatethetransitionenergiesofex-citedstateswithcharge-transfercharacter.[13]Therefore,tochooseanappropriatefunctionalforcompounds1–4,wefirstselected1asanexampletocheckthecharge-transferproper-tiesofthiscompoundusingcodeMultiwfn2.3(thecalculationmodelisdescribedintheSupportingInformation).[14,25]Thecentroidsofcharge(C+/CÀ)andthecharge-transferdistance(DCT)betweenthebarycentersofdensitydepletionanddensityincrementzone[14]computedforthefirstelectronictransitionatthePBE0/6-31+G(d)levelaredepictedgraphicallyinFigure4.DCTisveryshortwiththevalue2.07󰀃,anditcanbe

Figure3.Molecularorbitalplots,HOMOandLUMOenergylevels,and

energygapsfor1–4attheirS0optimizedgeometriesobtainedinvacuumatthePBE0/6-31+G(d)level(themolecularorbitalplotswereobtainedwithanisodensitysurfaceof0.0004a.u.).

lecularorbital(HOMO)andlowestunoccupiedmolecularorbi-tal(LUMO)levelsandtheenergygapsfor1–4attheirS0opti-mizedgeometriescalculatedatthePBE0/6-31+G(d)level.

AscanbeseeninFigure3,HOMOsaregenerallydelocalizedoverthewholemolecularskeletonofthefourcompounds,whileLUMOsaresomewhatlocalized.Simultaneously,theFMOenergylevelsaresignificantlyaffectedbythedifferentarrange-mentofthewholemolecularskeleton.ByinspectingtheHOMOandLUMOenergylevelsof1and2,itisfoundthat2hasalowerLUMOandhigherHOMOlevelandthusnarrowerenergygap,whichisascribedtotheenlargementofpconju-gationbyincorporationofthevinylbridgeinthemainplaneof2.Asfor3,thephenylgroupislockedbybondC4ÀC6.Al-thoughaplanarskeletonbenefitstheextensionofconjuga-tion,thedistributionofFMOsof3closelyresemblesthatof1.Theplanareffectonlybringsabouttheextensionofdelocaliza-

Figure4.GraphicalrepresentationofDCT(A)andcentroidsofchargeC+/CÀ(B)computedatthePBE0/6-31+G(d)levelwithisocontourvalueof

0.0004a.u.Thegrayanddarkgrayzonescorrespondtodensityincrementanddecrement,respectively.

seenthatthedensitydepletionzoneanddensityincrementzonearealmostsuperposed.Therefore,thisBODIPYderivativeexhibitsaveryweakcharge-transfercharacteristic,andhenceconventionalfunctionalscanbefullysuitableforthedescrip-tionofexcitedstatesofthesecompounds.

Toobtainmoreaccurateresults,agroupofrepresentativefunctionalsincludingB3LYP,PBE0,BHandHLYP,andCAM-B3LYPwastestedontheabsorptionandemissionenergyandStokesshiftof1.ThecalculateddatacollectedinTableS4(Supporting

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Information)showthatallthefunctionalsexceptforB3LYPprovidecomparablepredictionofStokesshiftcomparedtotheexperimentalvalue.Importantly,PBE0suppliesrelativelymoreaccurateresultsforabsorptionandemissionenergy.Therefore,PBE0[15]wasadoptedforthestudiedcompoundsconsideringthefocusofthisworkistostudytheStokesshiftanddesignnewemitterswithlongeremissionwavelength.

Fortheemissionspectra,theemissionpeaksfor1–4allarisefromtheS1state,whichconsistsofaLUMO!HOMOtransi-tion.Itshouldbepointedoutthatalthough4hasasimilarconfigurationto3,theiremissionwavelengthsareremarkablydifferent.Compound4exhibitsared-shiftedemissionwave-length.Moreimportantly,thisamazingwavelengthlocatesintheredregion,whichisscarceforBODIPYderivatives.Thisfea-tureallows4tohavegreatpotentialasanimportantredemit-ter.Furthermore,5showsquiteasmallStokesshift,whereas

2.3.2.OpticalProperties

1–4alldisplaymuchlargerStokesshiftsthanthatof5(1509cmÀ1),especially1,2,and4.AnenhancedStokesshiftisThetheoreticalresultsfortheabsorptionandemissionspectra

necessaryforanemitterinOLEDapplications,sinceasmallof1–4calculatedatthePCM(CH2Cl2)-TD-PBE0/6-31+G(d)level

onewillresultinthereabsorptionoftheemittedphotons.arelistedinTable1andTable2,respectively.Moreover,toper-Therefore,thisexcellentfeatureenables1–4tohavebetterpo-tentialapplications.

Table1.Calculatedabsorptionwavelengthslabs[nm],excitationenergiesItisnoteworthythatthecalculatedemissionwavelengthofEx[eV],oscillatorstrengthsf,anddominantexcitationcharacterofcom-1isslightlyoverestimatedcomparedwiththeexperimental

pounds1–4andparentBODIPYcompound5togetherwithexperimentalone(598.5nmfromDFTand531nmfromexperiment)withan

absorptionresultslabsexp[nm].CalculationswereperformedatthePCM-errorofabout0.15eV.Moreover,byinspectingtheconfigura-(CH2Cl2)-TD-PBE0/6-31+G(d)//PBE0/6-31+G(d)level.tionsofexcitedstatesandthecharacterofFMOsforthefour

ExfComposition[a]labsexpCompoundTransitionlabscompounds,anintramolecularchargetransfer(ICT)process[b]1S0!S1399.63.1030.277H!L(99%)417stillexistsuponexcitation.Tocrosscheckthattheobtained2S0!S1437.32.8350.215H!L(98%)465[b]spectralresultsarereliable,adiagnostictestbasedonthet[14]

[b]3S0!S1399.33.1050.309H!L(99%)416andL[17]factorwascarriedout.Anegativevalueoftsignifies4S0!S1500.22.4790.175H!L(99%)–aspatialoverlapbetweencentroidsofcharges,andwhentis409.03.0310.580H!L(96%)505[c]5S0!S1largerthan1.6󰀃,thetransitionsprovidedbypureorhybrid

[a]HdenotesHOMOandLdenotesLUMO.[b]Measuredindichlorome-functionalsareproblematic.[14]Lrepresentstheoverlapbe-thane,ref.[9].[c]Measuredinchloroform,ref.[16].tweenoccupiedandvirtualmolecularorbitalsinvolvedinexci-tation;theLparametercanbeemployedtojudgethereliability

Table2.Calculatedemissionwavelengthslem[nm],emissionenergiesEm[eV],oscillatorstrengthsf,dominantofthegeneralexcitationsfromexcitationcharacter,andStokesshifts(SS)[cmÀ1]ofcompounds1–4andparentBODIPYcompound5togetherfunctionalsPBEandB3LYP.ThatwithexperimentalSSvalues(SSexp)[cmÀ1]andemissionlemexp[nm]data.Calculationswereperformedattheis,whenL<0.4forPBEorL<

PCM(CH2Cl2)-TD-PBE0/6-31+G(d)//TD-PBE0/6-31+G(d)level.0.3forB3LYP,theexcitationisexp[a]EmfNatureSSSSexplemCompoundTransitionlemlikelytobeinverysignificant

error.[17]ThetandLparameters1S1!S0598.52.0710.069H!L(99%)83175148531[b]2S1!S0551.12.2500.147H!L(99%)47224382584[b]havebeencalculatedusing3S1!S0471.42.6300.188H!L(99%)38303877496[b]codesMultiwfn[25]andQ-Chem

4S1!S0660.31.8780.079H!L(100%)4847––package,[18]respectively.Thecor-435.92.8440.406H!L(92%)1509791526[c]5S1!S0respondingtvalues(tx,x-axis

[a]HdenotesHOMOandLdenotesLUMO.[b]Measuredindichloromethane,ref.[9].[c]Measuredinchloro-component)computedatthe

form,ref.[16].PBE0/6-31+G(d)levelareÀ0.36,À0.45,À0.23,and0.07for1–4,

formasystematiccomparisonbetweenourstudiedcom-respectively(TableS5,SupportingInformation),whilethecor-poundsandtheBODIPYs,theabsorptionandemissionspectrarespondingLvaluescalculatedbytheB3LYPfunctionalfor1–

[16]

oftheparentborondipyrromethenecompound,BODIPY4are0.56,0.57,0.52,and0.49,respectively.Theseresults

stronglydemonstratethatthespatialoverlapbetweentheoc-(hereafterlabeled5;thestructureispresentedinFigure5and

cupiedandvirtualmolecularorbitalsinvolvedinexcitationisFigureS1intheSupportingInformation),werealsocomputed.

largeenough.Therefore,theexcitationenergiesofthestudiedThecalculatedwavelengthsandthevariationtrendsof1–3are

compoundscomputedbyPBE0arereliable.ingoodagreementwithexperimentalresults,thusconfirming

thattheadoptedmethodsaresuitable.Theelectronictransi-tionsof1–5areofp!p*typefromFigure3andFigureS1.

2.3.3.TransitionDensityMatrix

Theabsorptionpeaksof1–5allcorrespondtotheS0!S1tran-Torevealunambiguouslythetransitionnature,thetransitionsition,whichismainlydescribedbyasingleone-electronexci-densitymatrixes(TDMs)of1–4andtheparentBODIPYcom-tationfromHOMOtoLUMO.Thus,4showsared-shiftedab-pound5werecomputed.Itshouldbepointedoutthatinthissorptionwavelengthcomparedwith1–3,whichisingood

partofthecalculations,6-31G(d)wasusedtoavoidthelinearagreementwiththetrendoftheenergygapsdiscussedabove.

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Therefore,thetransitionnatureofabsorptionandemissionfor1–4isthemixingoflocalizedtransitionandICTtransition.ICTnormallydissipatestheexcita-tionenergyofamoleculeandrendersared-shiftedemissionband.Therefore,ICTcharacterplaysacrucialroleintheen-hancedStokesshiftsof1–4.

Furthermore,fromabsorptiontoemission,1and2undergoasomewhatrearrangedelectrondensity,especiallyinthediago-nalboxes,whichindicatesgeo-metricrelaxationfollowingtheelectronicexcitations.Thereis,however,almostnorearrange-mentseenin5.Thisfeatureim-pliesthatthestrikinglylargerStokesshiftsof1–4haveanon-negligiblecontributionfromthegeometricrelaxation.Toobtainafirmassessmentofthegeo-metricrelaxationintheS1state,weturnnexttothediscussionofHRfactorsandtheinvestiga-tionofexcited-statestructuresofthefourcompounds.2.4.RationalizationofLargeStokesShift

TodisclosetheunderlyingoriginofthelargeStokesshift,theHRparameter,whichhasbeenprovedbyprevioustheoreticalstudiestobeausefulparameter

Figure5.Transitiondensitymatrixofthehotexciton(absorption,firstcolumn)andcoldexciton(emission,

tomeasuretheextentofthesecondcolumn)forcompounds1–5calculatedbyPBE0/6-31G(d)invacuum.Thenumberingofeachcompound

geometricdistortion,[19]iscalcu-ispresentedinthethirdcolumn(thehydrogenatomshavebeenomitted).

lated.TheHRparameterdirectlyconnectswiththeshiftDQof

theequilibriumpositionsoftheinvolvedelectronicstates,thatdependenceofthebasissetforthecompoundsunderstudy.

is,theS0andS1statesinvolvedinthephotoexcitationandTheTDMisusefultovisualizethesizeandlocationofexcitons

foroptoelectronicmolecules.TheTDMforabsorptionandfluorescenceemission.AccordingtotheFranck–Condonprinci-emissionassociatedwiththeS1statewascalculatedinvacuumple,nogeometricchangetakesplaceduringphotoexcitation.

Therefore,theHRparametercharacterizestheextentofgeo-atthePBE0/6-31G(d)levelandisdepictedinFigure5.Thehor-metricrelaxationattheS1statebeforefluorescenceemission.izontalandverticallinesseparatethepyridineparts,BF2sub-units,andbenzeneparts(parta,BF2,andpartb,respectively)Asdiscussedabove,theinfluencesofsolventeffectsonthegeometryarenegligible.Asaresult,theresultsforHRfactorsof1–4.Notethatthehydrogenatomshavebeenomittedin

depictedinFigure6werecomputedatthePBE0/6-31+G(d)viewoftheirverysmallcontribution.ByinspectingFigure5,

levelinvacuum(valuesofHRfactorsof1–5arecollectedinTa-therearealmostnoexcitonslocalizedonBF2subunitsof1–5.

blesS6–S10intheSupportingInformation).AsshowninImportantly,excitonsmainlydistributeoverthemainconjuga-Figure6,exceptforonemodeat38.78cmÀ1thathasamoder-tionplaneof5,thusdemonstratingthat5favorslocalized

transition.However,for1–4,excitonsprimarilylocalizeintheateHRfactor(7.60),alltheHRfactorsfor5aresparseandtrivi-diagonalboxes(a–a)and(b–b),andoff-diagonalboxes(a–b)alinthevibration-accessibleenergyregion.Insharpcontrast,(or(b–a))forphotoexcitationandfluorescenceemission.1and2displayevidentlylargerHRfactors,especiallyinthe

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citationarestrikinglydifferent(seeFigure2andTableS1intheSupportingInformation).FromS0toS1,thewholemolecularskeletonofcompound1un-dergoesasignificantcoplanareffect,withthedihe-dralanglesC7-N1-N2-BandC3-N2-C5-C6decreasingby26.24and29.438,respectively.Thiscoplanareffectisevidentlybeneficialtotheenlargementofconjuga-tion(FigureS3intheSupportingInformation).TheHOMOisgreatlyraisedandtheLUMOisgreatlylow-eredfor1intheS1statecomparedwiththoseintheS0state.However,onlyslightchangeoccursforthegeometryof3fromS0toS1becauseoftheconstrain-edstructure.Hence,theFMOenergylevelandenergygapexhibitlittlevariationintheS1state.Ontheotherhand,thedifferentHRfactorsof1and3manifestthattheirgeometricrelaxationsaregreatlydifferent.Thesignificantgeometricdistortiondissi-patestheexcitationenergyof1,thusitsred-shiftedemissioncanberationalized,whiletheslightgeo-metricdistortionof3accessesahigh-energyemis-sion.Therefore,3showsablue-shiftedemissionincomparisonto1.Thisfactalsodemonstratesthatthephenylgroupplaysanimportantroleintuningtheemissioncolorbyadjustingthegeometricdistortion.TofurtherevaluatetheeffectsoftheunlockedorlockedphenylgroupsonthechangeofStokesshifts,thegeometriesinthegroundandexcitedstateswereanalyzedindetail.Fromtheenergy-minimizedS0toS1state(Figure2andTableS1intheSupportingInformation),remarkablechangestakeplaceinthegeometriesof1and2,whichcanbeviewedfrom

Figure6.CalculatedHRfactorsandthenormal-modewavenumbersfor1–5.Theinsets

twoperspectives:1)asignificantdecreaseofthedi-showthecorrespondingvibrationalmodesofnormal-modedisplacementswithrelatively

hedralanglebetweenthephenylgroupandC3N2BhugeHRfactor;thevaluesinparenthesesarethecorrespondingHRfactors.

meanplane(by29.438)andaconsequentlyevidentcoplanartrendareobserved;and2)thebenzeneringwithC3andC4atomsexhibitsanoutwarddisplace-À1

mentfromthemainplane.Thesecharacteristicsconfirmagainhigh-frequencyregionof500–1500cm,whichdemonstrates

thattheunlockedphenylgrouphasimportanteffectsonthethattheirS1statesundergoastronggeometricdistortion

geometricaldistortionof1and2.Interestingly,bondN2ÀC5inuponexcitation.Theexcitationenergiesarethuspartlydissi-1and2showsantibondingcharacterinboththeHOMOandpated,whichresultsinalargeStokesshiftfor1and2.Onthe

theLUMO.However,thebondlengthsareclearlylongerintheotherhand,for3and4,alltheHRfactorsarelessthan0.9,

groundstate(1.419󰀃for1and1.423󰀃for2)thanthoseinthusindicatingthatthegeometricdistortionsaremild.There-theS1state(1.364󰀃for1and1.370󰀃for2).Thebondlengthsfore,3hasasomewhatlowerStokesshiftthan1and2.How-ofN2ÀC5intheS1statearewithintherangeofaC=Ndoubleever,4stillhasahighStokesshift.Thiscanbeattributedto

bond(1.34󰀃forpyridineand1.37󰀃forporphyrin),[20]whilethefactthat4ischaracterizedbyICT,whichcanbeseenfrom

thoseofthegroundstatearefoundbetweenthetypicalsingletheTDMshowninFigure5.

(1.47󰀃formethylamine)[20]anddoubleC=Nbondlength.ThevibrationalmodeswithrelativelylargeHRfactorin1–4

areplottedinFigure6(insets).For1and2withanunlockedThentheWibergbondindexforN2ÀC5(WBIN2ÀC5)of1–4phenylgroup,thesevibrationalmodesareout-of-planemo-wascalculated(Table3).InthegroundstateWBIN2ÀC5values

tionsofthemainskeletonandtherotationsofthephenylare0.99and0.98for1and2,respectively,whileintheS1stateunit,whilethevibrationalmodesof3and4aremainlyin-theyare1.05and1.01,respectively.Generally,thesmallerthe

planemotions.AllthesefeaturesindicatethatwhentheWBIvalue,thelongerthebondlength.Thus,wecanconcludephenylgroupswitchesfromunlockedtolocked,thegeometricthatN2ÀC5showssingle-bondcharacterinthegroundstate.distortionisverydifferentandleadstodifferentStokesshifts.Thisfeatureenablescompounds1and2tohavetwistedgeo-Asdiscussedabove,asignificantlyblue-shiftedemissionwasmetries.WhenturningtotheS1state,N2ÀC5displaysdouble-observedin3,althoughithasasimilargeometryto1.Onthebondcharacter.Therefore,thecoplanartendencyoftheonehand,thegeometricrearrangementsof1and3uponex-ChemPhysChem2012,13,3714–3722

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Table3.ComputedWibergbondindexofbondN2ÀC5(WBIN2ÀC5),thesumofHRfactors(Sum(HR)),andexperimentalluminescencequantumyields(FF).Compound12345WBIN2ÀC5S00.990.981.091.15–Sum(HR)S11.051.011.181.09–97.4050.813.703.9611.470.33[a]0.60[a]0.75[a]–0.76[b]FF3.Conclusions

Systematictheoreticalcalculationshavebeenperformedtoin-vestigatethephotophysicalpropertiesoffourBODIPYderiva-tives.Thecharge-transferpropertiesofthestudiedcompoundsandthefunctionaltestswerefirstdeterminedwiththeaimofselectingasuitablefunctional.Thecalculatedresultscanwellreproducetheexperimentalmeasurements.Itisfoundthatwiththephenylgrouplockedorunlocked,theelectronicstruc-ture,excitonicbehavior,emissionwavelength,Stokesshift,andfluorescenceefficiencyaresignificantlydifferent.TheICTchar-acterandpronouncedgeometricdistortionduetotherotationoftheunlockedphenylgroupfor1and2bothcontributetoanenhancedStokesshift.Forplanarcompound3,themildgeometricdistortionleadstoablue-shiftedemissiondespiteithavingacomparableenergygaptothatof1.Thedesignedcompound4alsodisplaysacommendablylargeStokesshiftarisingmainlyfromitsintrinsicICTcharacter.Impressively,4showsfascinatingproperties,suchaslargeStokesshift(4847cmÀ1),redemission,andreasonablefluorescenceeffi-ciency,whichallowittohavegreatpotentialapplicationinOLEDs.Finally,weareconvincedthatourworkshouldcontrib-utetofocusingthesynthesiseffortsandwillhelpunderstand-ingofthestructure–propertyrelationshipsofthiskindofcom-pound.

[a]Measuredindichloromethane,ref.[9].[b]Measuredinchloroform,ref.[16].phenylgroupwithC3N2Bmeanplanecausesasignificantgeo-metricdistortion,whichisinfavorofalargeStokesshift.

Thephenyl-locked3and4maintaintheplanargeometryfromtheS0toS1state.TheWBIN2ÀC5valueof3exhibitsthesametendencyasthoseof1and2,whichcanbeattributedtothefactthatbondN2ÀC5intheHOMOshowsanantibondinginteractionandintheLUMOrepresentsthebondinginterac-tion(Figure3).Forcompound4,theWBIN2ÀC5valuesare1.15and1.09fortheS0andS1states,respectively,becausebondN2ÀC5exhibitsbondingcharacterintheHOMOandantibond-ingcharacterintheLUMO.

TheoreticalMethods

2.5.FluorescenceEfficiency

Inrealisticapplications,thefluorescenceefficiencyofanemit-terisveryimportantfortheperformanceofOLEDs.ItiswellknownthatBODIPYderivativesalwayshavehighfluorescenceefficienciesbothinsolutionandthesolidstate.Thefluores-cenceefficienciesofthefourcompoundswerequalitativelyan-alyzedbycomparingtheircorrespondingHRparameters.

Toperformamoredirectcomparisonoffluorescencequan-tumefficiencyforthefourcompounds,theHRfactorsofeachcompoundareaddedtogether,thetotalvaluesbeing11.47,94.7,50.81,3.70,and3.96for5and1–4,respectively.Thenon-radiativedecayrateisproportionaltotheHRfactor;thesmall-ertheHRfactor,thesmallerthenonradiativedecayrateandthehigherthefluorescencedecayrate.Therefore,theorderoffluorescencedecayratesforcompounds1–3is3>2>1,whichisingoodagreementwiththeexperimentallymeasuredluminescencequantumyieldsinCH2Cl2.ItisseenthatthesumofHRfactorsfor4issmallerthanthatfor5andcomparablewiththatfor3.Thus,itisreasonabletobelievethat4canactasahighlyefficientredemitter.Moreover,uponinspectionofFigure6,itisreadilyseenthatthevibrationalmodesof1and2withlargeHRfactorlocateinthelow-frequencyregion(<100cmÀ1),whicharemainlyout-of-planemotionsofthephenylgroupandmainplane.Previousexperimentalandthe-oreticalreportshavesuggestedthatout-of-planevibrationalmodeswillberestrictedinaggregationstatessuchasfilm;[19,21]therefore,weconceivethatcompounds1and2mayactasaggregation-inducedenhancedemissionmaterials.

TheoreticalBackground

Thephotoluminescenceefficiencyhcanbeexpressedas[Eq.(1)]:h¼

krkrþknr

ð1Þ

wherekristheradiativedecayrateandknristhenonradiativedecayrate.Ingeneral,thenonradiativedecayratecomprisesinter-nalconversion(kIC)andintersystemcrossingprocesses(kISC).Gener-ally,foraflexibleorganicmoleculewithouttransition-metalatom,thenonradiativedecayispredominantlygovernedbyinternalcon-version.Therefore,thenonradiativedecayrateknrarisingfromin-ternalconversioncanbeexpressedas[Eqs.(2)and(3)]:knr¼

X1󰀂wl

l

hh2\"\"󰀃jRlðfiÞj2NFC

ð2Þ

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi\"#P0

2

ðwþwþSwÞ2pfiljjj

NFC¼P0expÀP022\"jþ1Þ\"jþ1Þ2jSjwjð2njSjwjð2n

ð3Þ

whereimeanstheinitialstateSnandfmeansfinalstateS0,NFC

referstothedensity-weightedFranck–Condon(FC)factor,andSjistheHRfactorforthejthmode.TheNFCfactorisexponentiallypro-portionaltotheHRfactorsofasumofallthemodesexceptthatofthepromotingmode,andthemostimportantcontributioncomesfromthedenominatorintheexponentialfunction.Thus,theHRfactorcanqualitativelyevaluatethenonradiativedecayrate;[19a,21]thatis,vibrationalmodesshowappreciableHRfactors,therebyresultinginalargeFCfactorandalargenonradiativedecayrate.

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TheOriginofLargeStokesShift

TheHRparameterdirectlyconnectswiththeshiftDQbetweentheequilibriumpositionsoftheinvolvedelectronicstates.Itsformulais[Eq.(4)]:wjÁDQ2j

Sj¼

2\"hð4Þ

ROCS,andtheScienceandTechnologyDevelopmentPlanningof

JilinProvince(201201071).WearealsogratefultoMissSoberevaattheUniversityofScienceandTechnologyBeijingfordiscussionandkindhelpandtoPatrikCallis(MSU)forsupplyingtheBoze-suiteprogram.

Keywords:densityfunctionalcalculations·dyes/pigments·electronicstructure·photophysics·Stokesshift

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wherewjrepresentsthevibrationalfrequencyforthej-thmodeandDQjisthenormal-modedisplacement.

Thetransitiondensitymatrix(TDM)betweengroundandexcitedstates,PGE,canbeobtainedfromtheirelectronicwavefunctionsyGandyE.Basedontheexpansionbyatomic-centerbasisfunc-tions,theTDMPGEcanbecontractedintonewmatrixPtomakeitrepresentedbyatomcenters[Eq.(5)]:PGEA;B¼

XX

a2Ab2B

2

ðPGEa;bÞ

ð5Þ

whereaandbdenotethebasisfunctionscenteredonatomsA

andB,respectively.Diagonalterms,forexamplePA,A,representthemagnitudeofinducedchargeonatomAwhenthesystemunder-goeselectrontransitionfromgroundstatetoexcitedstate.Off-di-agonalterms,forexamplePA,B,displaythestrengthofelectron–holecoherencebetweenatomsAandBwhenanelectrontransits.Therefore,TDMvisualizesthechangesintheelectronicdensityduringphotoexcitationandfluorescenceemission.

ComputationalDetails

CalculationswereperformedusingtheGaussian09package,[22]unlessotherwisestated.DFTcalculationsusingthePBE0functionaltogetherwiththe6-31+G(d)basissetwerecarriedouttoopti-mizetheground-stategeometricalstructuresof1–4invacuum.ThegeometriesoftheS1stateinvacuumwerecalculatedbytime-dependentDFT(TD-DFT)atthePBE0/6-31+G(d)levelbasedontheoptimizedground-stategeometries.Frequencycalculationsfol-lowingtheoptimizationswereperformedtoensurethatrealminimawereobtained.AllthecalculationswerecarriedoutusingtheCssymmetry.Basedontheoptimizedmoleculargeometries,absorptionandemissionspectraweresystematicallyinvestigatedbytheTD-DFTmethodwithinthenonequilibriumpolarizablecon-tinuummodel(PCM)approach[23]simulatingthesolventeffects(CH2Cl2).Additionally,asystematiccomparisonofthespectralre-sultsobtainedfromequilibriumPCMandnonequilibriumPCMwasperformed(TableS11,SupportingInformation).Evidently,bothequilibriumandnonequilibriumPCMresultsreproducethevaria-tiontrendoftheexperimentalvalues.Theabsorptionandemissionspectraof1–4withinequilibriumPCMexhibitaslightredshift(ca.10nm)comparedwiththoseinnonequilibriumPCM.Importantly,theequilibriumandnonequilibriumPCMsproducethesamevaria-tionsforabsorptionandemissionspectra,aswellasStokesshifts.Thus,onlythespectraresultswithinthenonequilibriumPCMap-proachwerepresentedanddiscussedinthiswork.Togainfurtherinsightsintothebondingsituation,Wibergbondindex[24]calcula-tionsusingthenaturalbondorbital(NBO)approachwereem-ployed.Thenatureoftheexcitedstateswascharacterizedbycal-culatingtheTDMthroughcodeMultiwfn2.3.[25]

Acknowledgements

WegratefullyacknowledgefinancialsupportfromtheNSFC(20903020and21131001),973Program(2009CB623605),SRFfor

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Received:May10,2012Revised:July16,2012

PublishedonlineonAugust16,2012

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