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Luminescent Properties of Mercury-taining Diethynylfluorene Derivatives

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Luminescent Properties of Mercury-taining Diethynylfluorene Derivativesof

    Luminescent Properties of

    Mercury-taining Diethynylfluorene

    Derivatives

    CHEM.RES.CHINESEU.

    2007,23(1),9295

    ArticleID1005-9040(2007)4)14)92-04

    Availableonlineatwww.sciencedirect.com

    .

    1黼一

    

    

    :ScienceDirect

    LuminescentPropertiesofMercury-taining

    DiethynylfluoreneDerivatives

    LIAOYit?,SHILili,FENGJi.kang_.YANGLiandRENAi.min

    1?StateKeyLaboratoryofTheoreticalandComputationalChemistry,InstituteofTheoreticalChemistry,

    2.TheCollegeofChemistry,JilinUniversity,Changchun130021,P.R.China; 3.InstituteofFunctionalMaterialChemistry,FacultyofChemistry, NortheastNormalUniversity,Changchun130024,P.R.China

    ReceivedMar.13,2006

    Excitedstatestmeturesandspectroscopicpropertiesofmercury(1I)complexes,Hg-TFT(1)anditselectron.

    withdrawingsubstituentsHg?TFOT(2),Hg?TFCNT(3),whereTFI'=diethynylfluoreny1.TFOT=diethyny1.

    fluorenone,andTFcNT=diethynyl-[9?(dicyanomethylene)fluorene],werestudiedusingsi

ngletexcitationconfigu.

    rationinteraction(CIS)andtime-dependentdensityfunetionaltheory(TDDFT)methods.T

    heresultsofthetheores.

    calcalculationsindicatethattheelectron

    withdrawingsubstitutionsleadtoasignificantdecreaseintheenergygapbe.

    tweenthegroundstateandthefirstexcitedstates.Inthecaa,eofHg-TFCNT,thesecondsinglet

    excitedstate()

    maycontributetotheluminescencebecauseofitslargeS1?S2separation. KeywordsTD.DFT;CIS;Luminescence

    Introduction

    Therehasbeenanongoinginterestinthedesignof

    alkynylmetalcomplexesoverthepastfewdecadesbe

    causeofthepotentialapplicationsofthesecompounds tothediverseareas,suchasorganicandorganometallic syntheses[.

    homoandheterogeneouscatalysis[]

    andmaterialsciences[---6]

    alkynylmetalcomplexes,

    systemsistherigidrod

    tion.Theseresultssuggestthatthephotophysical propertiesofHgTFTaremainlybasedontheTFTfrag-

    ment.Thus,itisreasonabletoassumethatthemodifi

    cationsofthemolecularbackbonewithelectron.defi.. cientcarbonyl?-ordicyano-substitutedelectronacceptors

    inthesidechain(atC9position)mayeffectivelytune Amongavarietyofthebandgapsandemissionwavelengthofthisclassof oneofthemostpromising

    metalacetylideofthetype

    [PH3MCCRCCMPH,]whereM=Au(I),

Hg(?),Pt(?);R=phenyl,pyridyl,oligothienyl,

    fluorenyl,etc..Theopticalandelectronicpropertiesof thesematerialscanbeadjustedeitherbyvaryingorby modifyingthespacerunits.Itisvisualizedthat

    theyareexcellentmodelsystemstostudythetriplet excitedstateandtoprovideimportantinformationon thephotophysicalprocessesthatoccurinconjugatedor- ganometalliccomplexes.Mostrecendy,Wongand Liu[jhavereportedthefirstexamplesofsolublemer- cury(?)polyyneswith9,9-dialkylfluorenespacer (HgTFT)andtheelectronicstructuresandoptical propertiesofHg-TFTmonomerandthepolymerhave beensystematicallyinvestigatedusingquantumcalcula

    materialsorofsimilarcompoundswhilemaintaining theirsolubilityandprocessability.

    Thisstudyprovidesomeinsightintotherelaxation processesduringexcitationbyacomparativestudyon Hg-TFT(1)anditselectron-deficiencyderivatives Hg-TFOT(2)andHgTFCNT(3)whereFO=fluoren

    9-one-2,7-diyl;FCN=9(dicyanomethylene)fluorine

    2.7diy1.Theaimofthisstudyistodeterminetheex

    tentoftheelectron.withdrawingsubstituentsthataffect thenatureofluminescence.

    ComputationalMethods

    Theexcitedstatesstructuresofcomplexes1-3

    werefullyoptimizedusingsingleexcitationcorrfigura- tioninteraction(cIs)method?.TheStu

    BonneffectivecorepotentialECP6OMWB[]wasused formercury,whichwastreatedupuntilthe4pshellas Supposedbythe"973"ProgramofChina(No.2002CB613406)andYouthScienceFoundatio

nofNortheastNormalUniversity

    (No.111494018).

    Towhomcorrespondenceshouldbeaddressed.E-mail:Jikangf@yahoo.com

No.1UAOYi.

    thec0re,withthe55p65do6selectr0nsasvalence. ForC.H.OandN,al1.electron6.31Gbasissetwas adopted[I9j.

    Startingfromtheoptimizedexcited--stategeome-- tries,time.dependentDVI"(TDDFT)calculations[?l21]

    withtheB3LYPfuncti0nalE22,231werepel0ed.All

    thecalculationsdescribedinthisarticlewereperformed attheSGIOrigin2000serverwithGaussian03 f24]program?

    ResultsandDiscussion

    1ExcitedStateGeometry

    Toexploretheemissionspectraofmercurydiethy- nylfluorenylderivatives,thegeometryoftheexcited statemustbeoptimizedbecausethestructuralrelaxa- tionafterphotoexcitationcannotbereproduced byground-stateequilibriumstructure.CIShasbeen extensivelyusedtostudythepropertiesoftheexcited states[25,26]

    .Itdescribestheexcitedstatewavefluncti0n atalevelcomparabletoHartreeFockwithsingleexcita- tionsfromtheHFdeterminant.

    Theoptimizedgeometricalparametersofthe mercurycomplexesHgrFT(1),Hg-TFOT(2),and

    HgTFCNT(3)inthelowestsinglet(SI)andtriplet (T1)excitedstatesarepresentedinTable1(SeeFig.1

forthenumberingofatoms).

    H3C_HgRHg--CH,

    R(1)H

    gTFr

    (2)HgTFOT

    (3)HgTFCNT

    Fig.1Theschematicrepresentationofthe

    alkynylmercury(II)complexes

    Table1Bondlengths(nin)ofHFgroundstateandCISexcitedstateforHg-rI.1,Hg-.I.FOTand

    Hg-TFCNT

    Forcomparison,HFgroundstate(S0)geometries arealsolisted.Comparisonbetweentheoptimizedex

    cited--stateandtheground--stategeometrieswillprovide anindicationoftheoverallgeometryrelaxationsthat occurintheexcitedstateinthepossibleemission processes?

    First,foreachmolecule,thepronouncedgeomet- ricalmodificationismanifestedinthediethynylfluorene fragmentratherthaninthemetalfragment.Thelatter appearstobeextremelyrigidwithrespecttoelectron excitation.Furthermore,moreprominentmodifications arefoundtooccurinthecentralpartonthefluorene ringinthediethynylfluoreneligand,notmuchonthe ethynylpart.

    Second.theenhancementofbond.1engthalterna

    tionfromSotoSIandtoinallthesystemsleadsto theappearanceofaquinoidicgeometriccharacterin theexcitedstatesofthefluorenering.Forexample,Ar [definedasthedifferencebetweentheaveragelength 0fthe"single"and"double"bonds;Ar=(Rc1+

c2c3+c4

    cll+c1o

    cl1+c9c1o)/5(Rc3c4+

    c1cl0+c11

    c12)/3]f0rHg-TFOTincreasesfrom

    ca.0.0003nminthegroundstatetoca.0.0052nm inthesingletexcitedstate.andtoca.0.00l57nmin thetripletstate.Thehigherbondlengthalternationin T1withrespecttoS1isinaccordancewiththehigher relaxationenergiesinTi(0.49eV)thanthoseinSl (0.35eV)uponverticalexcitation.Alsointhiscase. therelaxationofthestructuresduringtheexcitationcan beinterpretedintermsoftheantibondingandbonding characteristicsofthehighestoccupiedmolecularorbital (HOMO)andthatofthelowestunoccupiedmolecular orbital(LUMO),respectively.

    Itisunderstoodthatifthereisnonodalacrossthe bondintheHOMO.whilethereisanodalacrossthe samebondintheLUM0.thenthisbondwillbe

    一一一鬈

CHEM.RES.CHINESEUVn1.23

    elongatedwhenthecomplexchangesfromtheground statetotheexcitedstate.

    Anotherinterestingaspectofthesebondparame- tersconcernsthedecreasinguendinthebondlength alternation(Ar)fromHg-TFTrtoHg-TFOT,andto Hg-TFCNT,theirelectronicconfigurationsbeingthe same.ItcanbeseenfromTable1that,becauseofthe electron-withdrawingeffectofthesubstituents,the

    bondlengthalternation(Ar)inS1andstatesdecrea sesmarkedly(aromaticityincrease)whengoingfrom Hg-TFT(0.068,0.073)tocarbonylsubstituted

    Hg-TFOT(0.052,0.057)andtothehighlyelectro negativeHg-TFCNT(0.046,0.055).Asisknown, theopticalbandgapinconjugatedmoleculesoriginates mainlyfromdistortionbecauseofsingle/doublebond lengthalternation.Therefore,thereductioninbond lengthalternationalongthebackboneoftheconjugated mercurycomplexesprovidesanindicationofthe decreaseinthebandgap.

    2EmissionSpectra

    OnthebasisoftheCISoptimizedexcitedstates geometries,thefluorescenceandphosphoresceemis

    sionenergiesofHg-TFTr,HgTFOT,andHg-TFCNT

    werecalculatedatTDB3LYPlevelandtheresultsare

    showninTable2.

    Table2Calculatedfluorescenceandphosphorescence wavelengths(A)andtheirtransitionnaturefor mercurydiethynyifhorenylderivativesat TD.B3LYPlevel

    Thecontourplotsfortheelectrondensitymodi. ficationdistributionuponS/TnS0emissionare presentedinFig.2.

    Ascanbeseen,althoughtheemissionsfrom

    thelowestexcitedstatesinmercurycomplexesare mainlyassociatedwiththetransitionfromUIMOt0 HOMO.theirnatureconsiderablydiffer.Whenthe electron?withdrawinggroups,carbonyl(2)andcya- no(3),wereintroducedintothefluorenylside

chain,anobviousredshifts[by182nmforHg

    TFOT(2)and407nmf0rHg-TFCNT(3)inSlstate andby148nmfor2and345nmfor3inTstate1

    couldbeobservedintheemissionspectraascom. t

    ??!i}??硇毫s,s

    ;

    ?-.tSI

    ;..?i二五

    7

    Fig.2Changeofelectrondensitydistributionuponthe S_/_+S0dectronictransitionofsubstituted mercuryethynylfluorenecompiexes

    Greyandblackcolorscorrespondtodecreaseandincreaseofelectron

    density,respectively.

    paredwiththoseofHg-TFTr(1).Thisisattributed tothepresenceoftheelectron--withdrawingsubstitu?- ents,COandCNgroups,whichareelectmnegative andgiveamoredelocalized7r-systemandhence. 1eadtoasignificantloweringofthefirstexcitedstate energy.Furthermore,thered-shiftedlowestenergy emissionsforcarbonyl2andcyano3exhibitadiffer- entLMCT[(fluoreny1)d(Hg)](MLCTwas

    assignedinS1emissionf0r1)inadditiontoIL[17" (fluoreny1)7r(fluoreny1)]andLLCT

    [17-'(fluoreny1)(CC)].

    Itshouldbenotedthatamongallthemercury complexes,Hg-TFCNTpossessesthelowestSO-S1/ T1energygap.TheSo-gap(1.26eV)ofHg-TF- CNTis1.59eV,whichismuchhighercompared

    withthatofTI-(0.79eV),whiletheSo-S1gapof Hg-TFCNTis1.57eV,whichisnearlyequaltothe S1-S2gapof1.52eV.

    Generally,ahigherenergygapbetweenthe

    lowestandthenextlowestexcitedstatestendtore. ducetheinternalconversionandtherebyenhances theemissionfromthehigherelectronicstate[.

    Therefore,thesecondsingletexcitedstate()'of HgTFCNTshouldbetakenintoaccountwhenthe luminescenceisduetointerpretedmercury(I)di- ethynylfluorenylcomplexes.Accordingtothesecal- culations,theS2ofHgrrFCNT,predictedat403

    nm.isdescribedbytl1eH1}Lexcitationandthis

    mainlycorrespondstoLLCT(fluorenyl--~CEC)and LMCT[7r(fluoreny1)d(Hg)].

    Thesecalculationsdemonstratethatthepres

No.1UAOYien2.95

    enceofelectronwithdrawingcarbonylanddicyano

    substituentsinthesidechaincandecreasetheener

    giesofthelowestunoccupiedmolecularorbitalrela- tivet0thatoftheunsubstitutedmolecule,allowing rationaldesignofthemolecular"bandgap".There

    fore,themodificationoftheco~ugatedmolecular backbonewithelectron.deficientacceptorswouldbe veryeffectivefortuningtheoptoelectronicproperties 0fthisclassofmaterials.

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