光谱学分析-chapter04-荧光-03-CTC.ppt
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1、光谱学分析光谱学分析-chapter04-荧光荧光-03-CTCMLCT:可变价过度金属离子可向配体转移电子或电荷可变价过度金属离子可向配体转移电子或电荷2TheFBandFAemissionbandsdifferintheirnature,FBcorrespondingtoatypicalpi,pi*transition,delocalizedoverthewholeconjugatedpi-electronicsystem,141andFArepresentingatypicalCTfluorescence,aradiativeback-ETtransition.Forsuchdeloca
2、lized excitation,however,generallyseveninthisreviewstheparadoxicaltermlocal excitation(LE)istraditionallyused,inoppositiontotheCTstate.33.CriterionofaPureETState?WhichcriterioncanweapplytoassignsfromexperimentsapureETstate,i.e.,astatewithalocalizationoffullchargesintheD+andA-unitsinthecaseofaD-Acomp
3、ound?HowcanwediscriminateitfromaCTstate,inwhichtherespectiveorbitalsextendbeyondthesestructuralunits,toencompassinparttheotherone?Thereisobviouslynoclear-cutanswer.Perhapsmostusefulwouldbethefollowingthreecriteria.(i)Theexcited-statedipolemomentshouldcorrespondtoafullseparationofcharges.(ii)Thetrans
4、itiondipoleshouldbeverysmall,correspondingtoanoverlap-forbiddentransitioninfluorescence.(iii)TheabsorptionspectrumoftheexcitedstateshouldmatchthesumofthespectraofthecationradicalD+andoftheanionradicalA-.Thefirsttwocriteriaareself-explanatory.Letusconsiderthelastone.Iftheabsorptionbandscharacteristic
5、oftheD+andA-radicalionsappearattheproperwavelengths(e.g.,inthecaseof11246or1134)andnootherabsorptionfeaturesarepresent,thenthereislittledoubtaboutthefullchargeseparation.If,alongwiththebandsoftheradicalionsD+andA-,newmaximaorotherfeaturesappear,analternativecanbeimplied:eitheracoexistence,attheinsta
6、ntofmeasurement,oftheETstatewithsomeotherstate(whichisoftentheprecursoroftheETreaction;sometimestheyareinequilibrium),oraconsiderableoverlapoftheD+andA-orbitalsandacoupling,whichusuallymeansamarkeddeviationfromtheorthogonaltowardaquinoidplanarconformation.StructuralChangesAccompanyingIntramolecularE
7、lectronTransfer:FocusonTwistedIntramolecularCharge-TransferStatesandStructuresZbigniewR.Grabowski*,andKrystynaRotkiewicz,WolfgangRettig*Chem.Rev.2003,103,3899-4031StructuralChangesAccompanyingIntramolecularElectronTransfer:FocusonTwistedIntramolecularCharge-TransferStatesandStructuresZbigniewR.Grabo
8、wski*,andKrystynaRotkiewicz,WolfgangRettig*Chem.Rev.2003,103,3899-403145ChargetransfercomplexFromWikipedia,thefreeencyclopediaAcharge-transfercomplex(orCTcomplex,electron-donor-acceptor-complex)isachemicalassociationoftwoormoremolecules,orofdifferentpartsofoneverylargemolecule,inwhichtheattractionbe
9、tweenthemolecules(orparts)iscreatedbyanelectronictransitionintoanexcitedelectronicstate,suchthatafractionofelectronicchargeistransferredbetweenthemolecules.Theresultingelectrostaticattractionprovidesastabilizingforceforthemolecularcomplex.Thesourcemoleculefromwhichthechargeistransferrediscalledtheel
10、ectrondonor,andthereceivingmoleculeiscalledtheelectronacceptor,hencethealternatename,electron-donor-acceptor-complex.Thenatureoftheattractioninacharge-transfercomplexisnotastablechemicalbondandismuchweakerthancovalentforces,ratheritisbettercharacterizedasaweakelectronresonance.Asaresult,theexcitatio
11、nenergyofthisresonanceoccursveryfrequentlyinthevisibleregionoftheelectro-magneticspectrum.Thisproducestheusuallyintensecolorscharacteristicforthesecomplexes.Theseopticalabsorptionbandsareoftenreferredtoascharge-transferbands,orCTbands.Opticalspectroscopyisapowerfultechniquetocharacterizecharge-trans
12、ferbands.Charge-transfercomplexesexistinmanytypesofmolecules,inorganicaswellasorganic,andinallphasesofmatter,i.e.insolids,liquids,andevengases.Ininorganicchemistry,mostcharge-transfercomplexesinvolveelectrontransferbetweenmetalatomsandligands.Thecharge-transferbandsintransitionmetalcomplexesresultfr
13、ommovementofelectronsbetweenmolecularorbitals(MO)thatarepredominantlymetalincharacterandthosethatarepredominantlyligandincharacter.IftheelectronmovesfromtheMOwithligandlikecharactertothemetallikeone,thecomplexiscalledligand-to-metalcharge-transfer(LMCT)complex.IftheelectronmovesfromtheMOwithmetallik
14、echaractertotheligand-likeone,thecomplexesiscalledmetal-to-ligandcharge-transfer(MLCT)complex.Thus,aMLCTresultsinoxidationofthemetalcenterwhereasaLMCTresultsinthereductionofthemetalcenter.ResonanceRamanSpectroscopy1isalsoapowerfultechniquetoassignandcharacterizechargetransferbandsinthesecomplexes.。6
15、Contents1IdentificationofCTbands12Ligandtometalchargetransfercomplexeso2.1TrendofLMCTenergies3MetaltoLigandChargeTransferComplex(MLCT)12o3.1PhotoreactivityofMLCTexcitedstates4Chargetransfercomplexesandcolor5History6Otherexamples7Chargetransfercomplexesanddisease8Seealso9ReferencesIdentificationofCTb
16、ands1Chargetransfercomplexesareidentifiedby1)Intensity:CTabsorptionsbandsarehighlyintenseandoftenlieintheultravioletorvisibleportionofthespectrum.Thetypicalmolarabsorptivities,ofchargetransfercomplexesareabout50000Lmol-1cm-1,thatarethreeordersofmagnitudehigherthantypicalof20Lmol-1cm-1orlower,ford-dt
17、ransitions(transitionfromt2gtoeg).ThisisbecausetheCTtransitionsarenotspinorLaporteforbiddenasd-dtransitions.2)Solvatochromism:Thetransitionfrequencyvarieswithvariationinsolventpermittivity(介电常数,电容率介电常数,电容率)(dielectricconstant),indicatingalargeshiftinelectrondensityasaresultofthetransition.Thisdistin
18、guishesitfromthe*transitionsontheligand.7LigandtometalchargetransfercomplexesLMCTcomplexesarisefromtransferofelectronsfromMOwithligandlikecharactertothosewithmetallikecharacter.Thistypeoftransferispredominantifcomplexeshaveligandswithrelativelyhighenergylonepairs(exampleSorSe)orifthemetalhaslowlying
19、emptyorbitals.Manysuchcomplexeshavemetalsinhighoxidationstates(evend0).Theseconditionsimplythattheacceptorlevelisavailableandlowinenergy.Considerad6octahedralcomplex(exampleIrBr63-).Thet2glevelsarefilledasshowninFigure1.Consequentlyanintenseabsorptionisobservedaround250nmcorrespondingtoatransitionfr
20、omligandMOtotheemptyegMO.However,inIrBr62-thatisad5complextwoabsorptions,onenear600nmandanothernear270nm,areobserved.Thisisbecausetwotransitionsarepossible,onetot2g(thatcannowaccommodateonemoreelectron)andanothertoeg.The600nmbandcorrespondstotransitiontothet2gMOandthe270nmbandtotheegMO.Figure1.nextp
21、age8Figure1.MOdiagramshowingLigandtoMetalChargeTransferforad6octahedralcomplexAnotherthingtonoteisthatCTbandsmightalsoarisefromtransferofelectronsfromnonbondingorbitalsoftheligandtotheegMO.TrendofLMCTenergiesOxidationNumber+7MnO4-TcO4-ReO4-+6CrO42-MoO42-WO42-+5VO43-NbO43-TaO43-Theenergiesoftransitio
22、nscorrelatewiththeorderoftheelectrochemicalseries.Themetalionsthataremosteasilyreducedcorrespondtothelowestenergytransitions.Theabovetrendisconsistentwithtransferofelectronsfromtheligandtothemetal,thusresultinginareductionofmetalionsbytheligand.Examplesinclude:MnO4-:Thepermanganateionhavingtetrahedr
23、algeometryisintenselypurpleduetostrongabsorptioninvolvingchargetransferfromMOderivedprimarilyfromfilledoxygenporbitalstoemptyMOderivedfrommanganese(VII).CdS:ThecolorofartistspigmentcadmiumyellowisduetotransitionfromCd2+(5s)S2-().HgS:itisredduetoHg2+(6s)S2-()transition.FeOxides:theyareredandyellowdue
24、totransitionfromFe(3d)O2-().9MetaltoLigandChargeTransferComplex(MLCT)12MLCTcomplexesarisefromtransferofelectronsfromMOwithmetallikecharactertothosewithligandlikecharacter.Thisismostcommonlyobservedincomplexeswithligandshavinglow-lying*orbitalsespeciallyaromaticligands.Thetransitionwilloccuratlowener
25、gyifthemetalionhasalowoxidationnumberforitsdorbitalswillrelativelybehighinenergy.ExamplesofsuchligandstakingpartinMLCTinclude2,2-bipyridine(bipy),1,10-phenanthroline(phen),CO,CN-andSCN-.Examplesofthesecomplexesinclude:Tris(2,2-bipyridyl)ruthenium(II):Thisorangecoloredcomplexisbeingstudied3astheexcit
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