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ThePhotophysicalPropertiesofaSymmetricallySubstituted2,5–
DiarylideneCyclopentanoneDye:(2E,5E)‐2,5‐bis(4‐
methoxycinnamylidene)‐cyclopentanone
AMajorQualifyingProjectReport
SubmittedtotheFacultyof
WORCESTERPOLYTECHNICINSTITUTE
Inpartialfulfillmentoftherequirementsforthe
DegreeofBachelorofScience
By:
‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐KatarinaLopezApril28,2011
Approvedby:
‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐Prof.RobertE.Connors,Ph.DProjectAdvisor
2
TableofContents
Abstract ........................................................................................................................................................................ 5Acknowledgements ................................................................................................................................................. 6
Introduction................................................................................................................................................................ 7
ExperimentalProcedures .................................................................................................................................. 101.Synthesisof(2E,5E)‐2,5‐bis(4‐methoxycinnamylidene)‐cyclopentanone(2dbmxcp) 102.SpectrophotometricAnalysis–AbsorbanceandFluorescenceSpectra.............................. 143.FluorescenceQuantumYieldDetermination .................................................................................. 144.Fluorescencelifetimedetermination.................................................................................................. 15
ResultsandDiscussion........................................................................................................................................ 171.Introduction ................................................................................................................................................... 172.AbsorptionandFluorescenceProperties.......................................................................................... 183.QuantumChemicalCalculations ........................................................................................................... 32
Conclusions .............................................................................................................................................................. 34
References ................................................................................................................................................................ 35AppendixA:FluorescenceQuantumYieldCalculation......................................................................... 36
AppendixB:FluorescenceLifetimeSampleCalculation ...................................................................... 41
3
ListofFigures
Figure1.GeneralStructureof2,5‐diarylidenecyclopentanones. ............................................8Figure2.Structureof(2E,5E)‐2,5‐bis(4‐methoxycinnamylidene)‐cyclopentanone
(2dbmxcp). ............................................................................................................................8Figure3.Jablonskidiagramshowingfluorescencewithsolventrelaxation10. ........................9Figure4.Reactionschemeforthesynthesisof2dbmxcp. .......................................................10Figure5:1HNMR2dbmxcpd2.5‐8.0ppm................................................................................11Figure6:1HNMR2dbmxcpd6.0‐8.05ppm .............................................................................12Figure7:1HNMR2dbmxcpd1.95‐4.05ppm ...........................................................................13Figure8.ChemicalStructureof2dbma .....................................................................................17Figure9.Absorbanceandemissionspectraof2dbmxcpinvarioussolvents. .......................19Figure10.Plotofmaximumabsorptionandfluorescentwavenumbersif2dbmxcpagainst
Δfinpolarprotic,polaraprotic,andnonpolarsolvents.Alcoholsarerepresentedasthecleardiamondandsquareshapes......................................................................................21
Figure11.Plotofmaximumabsorptionandfluorescentwavenumbersof2dbmxcpagainstET(30)invarioussolvents.Alcoholsarerepresentedascleardiamondandsquareshapes. .................................................................................................................................22
Figure12.Plotofmaximumabsorptionandfluorescentwavenumbersof2dbmaagainstΔfinvariousprotic,aprotic,andnonpolarsolvents.Alcoholsaredesignatedbytheredshapeddiamondsandsquares. ..........................................................................................22
Figure13.Plotofmaximumabsorptionandfluorescentwavenumbersof2dbmaagainstET(30)invarioustypesofsolvents.Alcoholsarerepresentedbytheredshapeddiamondsandsquares. .......................................................................................................23
Figure14.Plotofthefluorescencequantumyieldof2dbmxcpagainstνflinallsolvents. ....25Figure15.Solventcalculationsof2dbmxcpinTolueneandEthanol......................................27Figure16.Computedmolecularorbitalsof2dbmxcpand2dbma. .........................................29Figure17.Lippert‐MatagaPlotofStoke'sshift(Δν)againstΔfof2dbmxcpinallsolvents. .30Figure18.Lippert‐MatagaPlotofStoke'sshift(Δν)againstΔfof2dbmxcpinpolarprotic
andnonpolarsolvents. .......................................................................................................30Figure19.Lippert‐MatagaPlotofStoke'sshift(Δν)againstΔfof2dbmxcpinallsolvents.
Alcoholsaredesignatedastheredsquares. .....................................................................31Figure20.ComputedMolecularOrbitalsof2dbmxcp..............................................................32
4
ListofTables
Table1.Spectroscopicandphotophysicalcharacteristicsof2dbmxcpinvarioussolvents. 20Table2.Spectroscopicandphotophysicalpropertiesofboth2dbmxcpand2dbmainMeOH
andEtOH. .............................................................................................................................27Table3.ComputedMolecularOrbitalCalculationsof2dbmxcp. ............................................32Table4.2dbmxcpTD‐DFTSpectralCalculationsinEthanolandToluene .............................33
5
Abstract
Thisprojectextendsinterestintothephotophysicalpropertiesofsymmetrically
substituted2,5–diarylidenecyclopentanonedyes.Thefocusforthisresearchisonthe
compound(2E,5E)‐2,5‐bis(4‐methoxycinnamylidene)‐cyclopentanone(2dbmxcp).This
compoundwassynthesizedviaacrossedaldolcondensationreactionbetween
cyclopentanonewith(E)‐4–methoxycinnamaldehydeinthepresenceofNaOH.The
electronicabsorptionandfluorescencepropertieswereinvestigatedinavarietyof
nonpolar,polarprotic,andaproticsolvents.Solvatochromicshifts,specifically
bathochromic(red)shiftswereobservedinboththeabsorptionandfluorescencespectra
asaresultofsolventpolarity.Inadditiontothespectroscopicproperties,thephotophysical
propertieswerealsoinvestigated.Thisinvolvedcalculatingthefluorescencequantum
yields,andmeasuringthefluorescencelifetimeparameters.Thefluorescencequantum
yieldsof2dbmxcprangedfrom0.0012incarbontetrachloride,to0.17in2‐propanol.This
rangewasofparticularinterestbecauseahigherquantumyieldinalcoholsdiffersfrom
analogouscompounds.First‐orderradiativeandnonradiativeratesofdecaywere
determined.Quantumchemicalcalculationswereperformedon2dbmxcpattheDFT
B3LYP/6‐31G(d)leveloftheoryforgeometryoptimizationandTD‐DFTspectral
calculations.
6
AcknowledgementsIwouldliketothankProfessorRobertE.Connorsforadvisingmeduringthecourseofthe
projectandfortheuseofhistime,laboratory,andequipment.Iwouldalsoliketothank
ChristopherZotoforallhisguidance,patience,andtime.
7
Introduction
Conjugatedcompoundscontaincarbon‐carbondoublebondsina1,3–
conformation;aconjugatedsystemisdependentontheoverlapofpatomicorbitals.The
classoforganicconjugatedcompoundsthatareofinterestforthisstudyisthe2,5–
diarylidenecyclopentanonedyes.Thesehighlyconjugatedfluorescentdyeshavereceived
attentionfortheirlargescopeofapplications.Thesecompoundshavebeenutilizedas
photosensitizers3,fluorescentsolventpolarityprobes4,5,fluoroionophores6,andnonlinear
opticalmaterials7.
Arecentapplicationofthesefluorescentcompoundsisatechnologythathasbeen
developedbyConstellation3Dknownasthefluorescentmultilayerdisc(FMD).Thisisan
opticaldiscthatusesfluorescentcompoundsasdigitalreceptorsinsteadofthenormal
digitalreflectionusedintraditionalopticaldiscstostoredata.Fluorescentcompoundsare
filledintothepitsofanFMD,andlightisabletotravelthroughtheclearFMDdiscs
unimpeded.Thisenablesthedisctocarryuptoapproximately100datalayers,muchlarger
thanthetraditional2layersonanormaldisc.Asaresultoftheabilityofthefluorescent
compoundstoachievepossibleexcitationtohigherorderenergystates,theFMD
technologycanhavecapacitiesofuptoaterabytewhilemaintainingthesamephysicalsize
ofatraditionalopticaldisc8.
Previousresearchon2,5–diarylidenecyclopentanonessuggestthattheycould
drawattentionforfeasibleapplicationsinseveralnewtechnologies.Theresearch
conductedbyConnorsandUcak‐Astarlioglu1reporttheelectronicstructureand
spectroscopicpropertiesforasetofunsubstituted2,5–diarylidenecyclopentanonesina
varietyofsolvents;thegenericchemicalstructureofthedyeisshowninFigure1.The
researchhasaprimaryfocusthatinvolvesstudyingtheelectronicabsorptionand
fluorescencepropertiesofallthetransconfigurationsofunsubstituted2,5–diarylidene
cyclopentanones(R=H)withn=1,2,and31.
8
Figure1.GeneralStructureof2,5‐diarylidenecyclopentanones.
Thisprojectservesasanextensionofpreviousresearch,anditinvolvesstudyinga
derivativewithmethoxysubstitutedgroups(‐OCH3)(thatserveaselectrondonating
groups)bondedattheparapositionsonthephenylrings.Thespectroscopicand
photophysicalspectroscopicpropertiesof(2E,5E)‐2,5‐bis(4‐methoxycinnamylidene)‐
cyclopentanone(2dbmxcp)havebeenstudiedinavarietyoffourteennonpolar,polar
aprotic,andproticsolvents.Thechemicalstructureof2dbmxcpisshownbelowinFigure2.
Investigationofthespectroscopiccharacteristicsof2dbmxcpprovidevaluableinsightinto
thesolvatochromicandphotophysicalpropertiesofthecompoundinvarioussolvent
systems.
Figure2.Structureof(2E,5E)‐2,5‐bis(4‐methoxycinnamylidene)‐cyclopentanone(2dbmxcp).
Solvatochromismistheabilityofasubstancetochangecolorwithrespecttosolvent
polarity2.Inabsorptionandfluorescencespectra,amoleculethatexhibitssolvatochromic
propertiesundergoeseitherbathochromic(red)shiftsorhypsochromic(blue)shifts
dependingonthesolventpolarity.Pastsolvatochromicresearchofanalogousdyesshowa
bathochromicshiftwhentestingfromnonpolartopolaraproticsolvents1.AJablonskistate
energy‐leveldiagramcanbeusedtoexplainthespectralshifts.Thisdiagram(Figure3)
illustratestheelectronicenergystatesofamoleculeandthetransitionsbetweenthem.
9
Morespecifically,thediagramisusedtoshowtheradiativeprocessofabsorptionand
fluorescencewithnonradiativesolventrelaxation.Investigationofthesolvatochromic
propertiesof2dbmxcpshowthatthereisabathochromic(red)shiftasthesolventpolarity
increases.
Figure3.Jablonskidiagramshowingfluorescencewithsolventrelaxation10.
Theprimaryobjectiveforthisprojectistoinvestigateandanalyzethespectroscopic
andphotophysicalpropertiesof2dbmxcpinvarioussolvents,andexplainthetrends
observed.Examinationofthephotophysicalpropertiesinvolveexperimentallydetermining
theabsorptionandfluorescencespectra,fluorescencequantumyields(Фf),and
fluorescencelifetimes(τf).
Quantumchemicalcalculationswereperformedon2dbmxcpattheDFTB3LYP/6‐
31G(d)leveloftheory.Theoreticalcalculationsconsistedofrunninggeometry
optimization,TD‐DFTspectralcalculation,andsolventcalculationsusingtheself‐
consistentreactionfieldpolarizablecontinuummodel(SCRFPCM).
10
ExperimentalProcedures
1.Synthesisof(2E,5E)2,5bis(4methoxycinnamylidene)cyclopentanone(2dbmxcp)
2dbmxcpwassynthesizedpreviouslyandreadyfortestingatthestartofthe
project.Althoughthesynthesiswasnotconductedinthisproject,alookathowthe
moleculewassynthesizedprovidesvaluableinsighttothestructureandcompositionof
2dbmxcp.
Thecompound2dbmxcpwassynthesizedviaacrossedaldolcondensationreaction
betweencyclopentanone(1moleq)with(E)‐4‐methoxycinnamaldehyde(2moleq)inthe
presenceofNaOH(seeFigure4).Anorange‐coloredsolidprecipitatedoutofsolution.The
crudematerialwascollectedbyvacuumfiltrationandrecrystallizedfromtoluene,yielding
lustrousorangecrystals.1HNMRspectroscopywasusedforstructuralidentificationof
2dbmxcp.Both1HNMRinCDCl3andIRspectraldataarepresentedinFigures5‐7.Purity
wasconfirmedbyTLC(showingonespotupondevelopment).
Figure4.Reactionschemeforthesynthesisof2dbmxcp.
11
1HNMRSpectraof2dbmxcp(forstructuralconfirmation)
Figure5:1HNMR2dbmxcpd2.5‐8.0ppm
12
Figure6:1HNMR2dbmxcpd6.0‐8.05ppm
13
Figure7:1HNMR2dbmxcpd1.95‐4.05ppm
14
2.SpectrophotometricAnalysis–AbsorbanceandFluorescenceSpectraTheUV/VISabsorptionspectraweremeasuredwithaPerkinElmer®Lambda35
UV/VISspectrometer(2nmband‐pass).Thefluorescenceemissionspectrawerecollected
usingaPerkinElmer®LS50BluminescencespectrophotometerequippedwithanR928
phototubedetector.
3.FluorescenceQuantumYieldDeterminationThefluorescenceyieldofacompound(Φf)isdefinedastheratioofphotonsemitted
tothenumberofphotonsabsorbedbythecompound,andcanbecalculatedbythe
followingequation:
(Eq.1)
Inthisequation,Φsisthefluorescencequantumyieldoftheknownstandard
(obtainedfromliterature),Aistheabsorbancevalueatafixedwavelengthofexcitation,n
istherefractiveindexofthesolventsused,andDisthecalculatedareaunderthecorrected
emissionspectrum.Thesubscriptsreferstothestandard,andthesubscriptcreferstothe
compoundbeinginvestigated.
Thefluorescencequantumyieldsof2dbmxcpwerecalculatedbypreparingtwo
stocksolutionsofthecompoundandasolventwithamaximumabsorbanceof0.5.Two
stocksolutionsweremadeinorderfortheproceduretobeperformedtwicefor
reproducibility.Absorptionwasmeasuredusingsolventsofdifferingpolaritiesranging
frompolarprotic,polaraprotic,andnonpolarsolvents.Allofthesolventswere
commerciallyavailableandspectrophotometricgrade.Thesolventsusedwere:methanol,
ethanol,n‐butanol,1‐propanol,2‐propanol,chloroform,dimethylsulfoxide,acetonitrile,
acetone,ethylacetate,dichloromethane,toluene,benzene,andcarbontetrachloride.
Thestocksolutionswerethendilutedtenfoldandtheopticalabsorptionspectraof
boththestocksolutionsandthedilutedsolutionswerecollected.Thefluorescence
emissionspectrumofthetenfolddilutionswererecorded,fixingtheexcitationwavelength
15
atλ=450nm.Absorptionandfluorescenceemissionspectrawereobtainedforallfourteen
standardstocksolutions,andfluoresceinin0.1MNaOH(Φf=0.95)9.Thediluted
fluoresceinsolutionwasre‐measuredforeachsolventinordertokeepthedataconsistent,
andtoaccountforinstrumentresponse.MicrosoftExcel®wasusedtoconvertspectraldata
fromwavelengthunitstowavenumbers.ThedatawasthenimportedintoMathcad®,which
wasusedtocorrectthefluorescenceemissionspectraandcomputethefluorescence
quantumyields.AppendixAillustratesanexampleofafluorescencequantumyieldcalculationfor2dbmxcpinchloroform.
Inordertocorrectthefluorescenceemissionspectraforinstrumentresponse,the
literatureemissionspectrumofN,N‐dimethylamino‐3‐nitrobenzene(N,N‐DMANB)was
comparedtotheexperimentalemissionspectrumofN,N‐DMANBmeasuredusingtheLS‐
50Bstatus.Scalefactorsweredeterminedevery50cm‐1between12,500and22,200cm‐1.9
4.Fluorescencelifetimedetermination
Thefluorescencelifetimeofacompound(τf)isdefinedastheinverseofthesumof
thefirst‐orderradiativeandnonradiativeratesofdecay:
τf=1/(kf+knr) (Eq.2)
whereknr=kic+kisc.Thefluorescencelifetimes(τf)of2dbmxcpweremeasuredinthe
followingfivesolvents:ethanol,1‐propanol,acetone,chloroform,andtoluene.
Fluorescencelifetimesof2dbmxcpweremeasuredusingaPhotonTechnology
InternationalfluorescencelifetimespectrometerequippedwithaGL‐3300nitrogenlaser
andGL‐302dyelasercompartments.Inordertopreventfluorescencequenchingby
oxygen,thesolutionswereproperlydegassedbypurgingwithnitrogenpriortomeasuring
thefluorescencedecaycurves.FeliX32computersoftwarewasusedtogeneratethetime‐
dependentfluorescencedecayspectra.Thefluorescencedecayprofileoftheinstrument
responsefunction(IRF)wasgeneratedatthesamemaximumintensityasthedecaycurve
ofthecompoundbeinginvestigated.Ludox(Aldrich),acolloidalsuspensionofsilica,was
usedastheIRFtoscattertheexcitationbeam.Neutraldensityfilterswereusedtoadjust
16
thefluorescenceintensityoftheIRFprofile.ThefluorescencedecayandIRFscatterdata
wereanalyzedusingacurve‐fittingprocedure.Thebest‐fitcurvesweredeterminedby
statisticallyanalyzinghowwellthefieldfitcurvefittedthedecaysamplecurve.Anexample
worksheetforthefluorescencelifetimedeterminationcanbefoundinAppendixB.
17
ResultsandDiscussion
1.Introduction
Theelectronicabsorptionandfluorescencepropertiesof2dbmxcpwerestudiedina
varietyoffourteenpolarprotic,polaraprotic,andnonpolarsolvents.Experimentaldata
showthatthesolvatochromicpropertiesexhibitabathochromic(red)shiftincolorwhen
goingfromnonpolartopolarsolvents.ThisshiftinspectraisillustratedinFigure9.The
resultingsolutionsdifferedincolor,rangingfromlimegreen,tolightgreen,toyellow,and
thentoorangewithrespecttoanincreaseinsolventpolarity;illustratingthesolvent’s
influenceonlightabsorption.Theabsorptionandfluorescencepropertiesweremeasured,
andphotophysicalpropertieswereinvestigated,whichinvolvedmeasuringfluorescence
quantumyieldsandfluorescencelifetimesinvarioussolvents.Boththefirst‐order
radiativeandnonradiativeratesofdecaywerecalculatedfromthequantumyieldand
lifetimedatainethanol,1‐propanol,toluene,acetone,andchloroform.Finally,quantum
chemicalcalculationswereperformedon2dbmxcpattheDFTB3LYP/6‐31G(d)levelof
theory,whichinvolvedcarryingoutgeometryoptimizationinthegasphase,alongwithTD‐
DFTspectralcalculationsmodeledinthegasphase,andinethanolandtoluene
environments.
Inordertogainabetterunderstandingoftheexperimentalresultsand
characteristicsfoundfrom2dbmxcp,thissectionwilldoacomparisonstudywithan
analogouscompound,2,5‐bis(p‐dimethylaminocinnamylidene)‐cyclopentanone(2dbma).
2dbmaisalsoa2,5–diarylidenecyclopentanonedye,anditalsocontainselectron
donatinggroups(dimethylamino)substitutedonthearylmoieties.Figure8showsthe
chemicalstructureof2dbma.
Figure8.ChemicalStructureof2dbma
18
2.AbsorptionandFluorescenceProperties
Theabsorptionandfluorescencespectraof2dbmxcpinsixsolventsareshownin
Figure9.Thisfigureillustratesthesolvatochromicpropertiesofthecompound,andshows
thatthecompoundundergoesbathochromicshiftswhengoingfromnonpolar,topolar
aproticandproticsolvents.Figure9showsthatthereisamorepronouncedredshiftinthe
fluorescencespectrathanthatintheabsorbtionspectra.2dbmxcpshiftsinthe
fluorescencespectrafrom518nminCCl4to620nminMeOH.Aninterestingexperimental
findingwasthatthecompoundhashighfluorescencequantumyieldinthepolarprotic
solvents,whichwasuncharacteristicofanalogousdyes.Typicallyfluorescencesignal
intensityislowerinpolarproticsolventsduetothequenchingofthecompoundinthe
solvent.
Thespectroscopicandphotophysicalcharacteristicsof2dbmxcparepresentedin
Table1.Alsoincludedinthetablearethesolventpolarityfunction(∆f)andtheempirical
scaleofsolventpolarity(ET(30))ofeachsolvent,whichareempiricalvaluesbasedon
literature2.Thesolventpolarityfunction(∆f)isdependentonboththedielectricconstant
(ε)andtherefractiveindex(n)ofthesolvent.Thisrelationshipisgivenby:
€
Δf =ε −12ε +1
−n2 −12n2 +1 (Eq.3)
TheET(30)empiricalsolventpolarityscaleisbasedonthechargetransfershiftofthefirst
maximumofabetaninedye2.
19
Figure9.Absorbanceandemissionspectraof2dbmxcpinvarioussolvents.
20
Table1.Spectroscopicandphotophysicalcharacteristicsof2dbmxcpinvarioussolvents.
Solvent νabs(cm‐1)
νflu(cm‐1) Δf* ET(30)*
(kcalmol‐1) ϕfτf(ns)
kf(s‐1)
knr(s‐1)
MeOH22831(438nm)
16119(620nm) 0.3093 55.4 0.15 ‐‐‐ ‐‐‐ ‐‐‐
EtOH22831(438nm)
16689(599nm) 0.2887 51.9 0.15 0.71 2.08x108 1.20x109
1‐PrOH22779(439nm)
16813(595nm) 0.2746 50.7 0.15 0.72 2.08x108 1.18x109
1‐BuOH22727(440nm)
18620(537nm) 0.2642 50.2 0.08 ‐‐‐ ‐‐‐ ‐‐‐
2‐PrOH23202(431nm)
17098(585nm) 0.2769 48.4 0.17 ‐‐‐ ‐‐‐ ‐‐‐
DMSO22779(439nm)
17392(575nm) 0.2637 45.1 0.16 ‐‐‐ ‐‐‐ ‐‐‐
ACN23419(427nm)
17558(570nm)
0.3054 45.6 0.07 ‐‐‐ ‐‐‐ ‐‐‐
Acetone23529(425nm)
17696(565nm) 0.2843 42.2 0.04 0.46 1.00x108 2.07x109
DCM23095(433nm)
18580(538nm) 0.2171 40.7 0.07 ‐‐‐ ‐‐‐ ‐‐‐
Chloroform22989(435nm)
18561(539nm) 0.1491 39.1 0.09 0.53 1.74x108 1.71x109
EtOAc23923(418nm)
18903(529nm) 0.1996 38.1 0.02 ‐‐‐ ‐‐‐ ‐‐‐
Benzene23641(423nm)
18979(527nm) 0.0031 34.3 0.005 ‐‐‐ ‐‐‐ ‐‐‐
Toluene23753(421nm)
18979(527nm) 0.0131 33.9 0.003 0.27 1.18x107 3.69x109
CCl423810(420nm)
19302(518nm) 0.0119 32.4 0.001 ‐‐‐ ‐‐‐ ‐‐‐
*2BothΔfandET(30)valuesweretakenfromSuppan,P.andGhonheim,N.,inSolvatochromism,TheRoyalSocietyofChemistry,Cambridge,1997.
21
Theabsorptionandfluorescencecharacteristicswereplottedagainstboththe∆f
andtheET(30)empiricalsolventpolarityscales,asillustratedinFigures10and11.The
figuresshowthatsolvatochromicpropertiesareobserved,forboththe∆fandtheET(30),
havingboththeabsorptionandfluorescentwavenumbersdecreasingwithrespectto
solventpolarity.ThedecreaseismoreprevalentintheET(30)scale,whereasthe∆fFigure
showsonlyaveryslightdecrease,signifyingonlyminorsolvatochromism.
Figure10.Plotofmaximumabsorptionandfluorescentwavenumbersif2dbmxcpagainstΔfinpolarprotic,polaraprotic,andnonpolarsolvents.Alcoholsarerepresentedasthecleardiamondandsquareshapes.
Inthecaseof2dbma,thecompoundexhibitedstrongersolvatochromicproperties,
resultinginalinewithgreaterslope.Theobservedsolvatochromicpropertiesin2dbma
wereconsistentwithachargetransferelectronictransition.
y=‐0.2414x+2.373R²=0.4047
y=‐0.701x+1.9382R²=0.6137
0
0.5
1
1.5
2
2.5
3
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
(νab
s and
νfl)
/104
(cm
-1)
Δf
Absorption
Fluorescence
22
Figure11.Plotofmaximumabsorptionandfluorescentwavenumbersof2dbmxcpagainstET(30)invarious
solvents.Alcoholsarerepresentedascleardiamondandsquareshapes.
Figure12.Plotofmaximumabsorptionandfluorescentwavenumbersof2dbmaagainstΔfinvariousprotic,
aprotic,andnonpolarsolvents.Alcoholsaredesignatedbytheredshapeddiamondsandsquares.
y=‐0.0047x+2.5296R²=0.64344
y=‐0.0123x+2.3276R²=0.77393
0
0.5
1
1.5
2
2.5
3
30 35 40 45 50 55 60 65
(νab
s and
νfl)
/104
(cm
-1)
ET(30) (kcal mol-1)
Absorbance
Fluorescence
23
Figure13.Plotofmaximumabsorptionandfluorescentwavenumbersof2dbmaagainstET(30)invarious
typesofsolvents.Alcoholsarerepresentedbytheredshapeddiamondsandsquares.
Experimentalresultsfromthefluorescencelifetimes(τf)andfluorescencequantum
yields(Φf)showasolventdependenceforthe2dbmxcp.Anobservationthatwasmadeis
thatthelifetimesandquantumyieldschangedproportionallyfromonesolventtoanother.
Thealcoholshadthehighestquantumyieldsoutoftheentiresolventset,andwere
experimentallydetermined(outofthefivesolventstested)tohavethehighestlifetimes
values.ThelifetimevaluesarelistedinTable1,andtherangeoflifetimeswentfrom0.27
nsintolueneto0.72nsin1‐propanol.
TheΦfandτfparameterswereusedtocalculatethefirst‐orderradiative(kf)and
nonradiativedecayconstantsofthefirstexcitedsingletstateof2dbmxcp.Thefirst‐order
radiativedecayconstantcanbecalculatedbythefollowingequation:
(Eq.4)
24
whereΦfisthefluorescencequantumyieldandτfisthefluorescencelifetimeofthe
decayingsample.Thefirstorder‐nonradiativedecayconstant(knr),canbecalculatedfrom
thefollowingequation:
€
knr =1Φ f
−1
k f (Eq.5)
Althoughonlyfivefluorescencelifetimesweremeasured,theknrappearedtogenerally
decreaseasthesolventpolarityincreased.AsshowninTable1,theknrwentfrom3.69x109
s‐1intolueneto1.20x109s‐1inethanol.Theenerygaplawisusedtopredictthe
exponentialdependenceofkic(internalconversion)onΔE,theenergygapbetweenS0and
S1.Thislawisexpressedbythefollowingequation:
€
kic = Ce−αΔE ,ΔE = ES1− ES0
(Eq.6)
whereCandαareconstants.Inaccordancewiththeenergygaplawofinternalconversion
forexcitedstates,knrisexpectedtoincreaseastheevergygapbetweenS0andS1decreases
duetovibrationaloverlapbetweentheS0andS1states.WhentheS0toS1energygap
increases,thevibrationaloverlapbetweentheenergylevelsofS0andS1decreases,thus
yieldingadecreaseintherateofinternalconversion.Essentially,thisequationstatesthat
askicincreasesasΔEdecreases,andthusΦfdecreases,assumingkiscandkfareconstant.
With2dbmathebehaviorofknrcanbeexplainedbytheenergygaplaw,butwith2dbmxcp,
thehigherquantumyieldsshowanti‐energygapbehavior.
25
Thefluorescencequantumyieldswereplottedagainstthemaximumfluorescent
wavenumbers(νfl)inavarietyofpolarprotic,polaraprotic,andnonpolarsolvnets,shown
inFigure14.
Figure14.Plotofthefluorescencequantumyieldof2dbmxcpagainstνflinallsolvents.
Figure14illustratesthatthequantumyieldsincreaseforsolventswiththestrongest
polarity.Thisisalsotrueforthefluorecencelifetimes,thesovlentswiththelargerpolarity
wereexperimentallyproventohavelongerfluorscencelifetimes.
Both2dbmxcpand2dbmageneratedsimilarquantumyieldsinthehigh
wavenumbersregions,althoughtheydidnothavethesamequantumyieldsinthealcohols,
2dbmxcpgeneratedhigherquantumyieldsinthealcoholsthan2dbma.
For2dbmatheplotofthefluorescencequantumyieldagainstνflillustratesaparabolic
relationship.Itisobservedthatthequantumyieldreachesamaximumforsolventsof
0.0000
0.0200
0.0400
0.0600
0.0800
0.1000
0.1200
0.1400
0.1600
0.1800
0.2000
12000 14000 16000 18000 20000
Φf
νfl (cm-1)
2‐PrOH
DMSOMeOHEtOH 1‐PrOH
ACN1‐BuOHDCM
Acetone
EtOAc
CCl4Benzene
Toluene
Chloroform
26
moderatepolarity,andthisobservationisconsistentwiththeinterpretationofthetrendin
knr.Rearrangmentofequations4and5withτf=1/(kf+knr),yieldsanexpressionforthe
fluorescencequantumyieldofanelectronicsystem:
€
Φ f =k f
k f + knr (Eq.7)
€
Φ f =k f
k f + kic + kisc
whereknr=kic+kisc.For2dbma,thequantumyieldislowatlowνflvalues.Whenthe
polarityofthesolventincreasesΦfincreasesduetothedecreasingrateinintersystem
crossing.Whentheνflincreasesinmorepolarsolvents,therateofinternalconverstion
increasesandproceedstodominateoverthecompetingdecreasingrateofintersystem
crossing,resultinginalowerquantumyield,andthusaparabolictrendwhenplottingΦf
vs.νfl.For2dbma,thistrendissupportedbytheenergygaplaw.
With2dbmxcp,thefluorescencequantumyieldagainstνfldoesnotillustratea
parabolicrelationship,asseeninFigure14.Inthiscasetherateofinternalconversion
doesnotproceedtodominateoverthecompetingrateofinterststemcrossinginmore
polarsolvents.Duetothisanti‐energygapbehavior,theplotofΦfvs.νflonlyillustratesthe
decreasingtrendinkisc,andthushigherquantumyieldsareobservedinthemorepolar
solvents.Thisexplanationillustratesthatthequantumyielddependsonthecompitition
betweenkf,kic,andkisc.
Acomparisonof2dbmaand2dbmxcpinsolventsofhighpolaritysuchasethanol
andmethanolcanbeusedtosupporttheexplanationoftheobservedhigherquantum
yieldsseenin2dbmxcp,andalsohelpinexplainingtheobservedanti‐energygaplaw
behavior.
In2dbmxcp,theνflinbothMeOHandEtOHissignificantlylargerthan2dbma,as
seeninTable2.ThismeansthattheenergygapbetweentheS0andS1statesislarger
whichmeansthattheΔEislarger.Whenusingtheenerygaplawequation,havingalarger
ΔEyieldsalessdominantrateofkic,andthusalargerquantumyield.Since2dbmahad
27
smallerνflinbothMeOHandEtOH,thisleadstoasmallerΔE,largerkic,andthusasmaller
quantumyield.
Table2.Spectroscopicandphotophysicalpropertiesofboth2dbmxcpand2dbmainMeOHandEtOH.
2dbmxcp(vfl,cm‐1) 2dbma(vfl,cm
‐1)MeOH:16119(620nm) MeOH:13605(735nm)EtOH:16689(599nm) EtOH:13445(744nm)
Thesolventcalculationscomputationallyconductedinethanolandtoluenehelp
explainthelowquantumyieldandhighknrobservedfor2dbmxcpintoluene.Thisbehavior
canbeattributedtothelocationofthe(n,π*)orbitaltypeanditsinfluenceonintersystem
crossing.AsshowninFigure15,thekiscfor(π,π*)(n,π*)issignificantlylargerthanthe
kiscfor(π,π*)(π,π*).ThisisdeterminedbyElSayed’srule12,whichstatesthattherateof
intersystemcrossingisfasterandmoreefficientbetweentwodifferentorbitaltypesthan
twoofthesameorbitaltype.Theoveralleffectisthatkiscandknrdecreaseassolvent
polarityincreases,whichisworkinginoppositionofthetrendobservedinkiccausedbythe
energygaplaw.Thuswithmorepolarsolvents,suchasethanol,slowkiscoccurswhich
resultsinalargerquantumyield.
Figure15.Solventcalculationsof2dbmxcpinTolueneandEthanol.
!"#$%&%''
!
()*+&"#'
!"##$%& '!&(n,!*)
!"()$%& '!(n,!*)
!"*($%& '+&(!,!*)
!"**$%&
+"#,$%&
+"-.$%&
(n,!*)
(!,!*)
(!,!*)
/.&
/!&
/+&
!"*-$%&
/.&
/!&
/+&
(n,!*)
(!,!*)
(!,!*)
',&',&
!"--$%&
+"#.$%&
+"-+$%&
!0&1&,",,.& !0&1&,"+*&
,-'
,.-'
,.-'
,-'
,.-'
23456&7849&1&:3;<$&
':=>6&7849&1&4?3::&'+&(!,!*)
28
Lippert‐MatagaPlotswerecreatedfor2dbmxcpfromthespectroscopicdataandare
showninFigures17,18,and19.Figure17wasconstructedwithallofthesolvents,
whereasFigure18hasjustthepolarandnonpolarsolvents,whichyieldedalargerR2
value.Lippert‐MatagaplotsareusedtodirectlyrelatetheStoke’sshiftforamoleculein
differentsolventstothesolventpolarityfunction.TheStokesshift(Δν),istheenergy
differenceinwavenumbersbetweentheabsorptionandfluorescencemaxima,related
linearlytoΔfbytheLippert‐Matagaequation11:
€
Δν =2Δµ2
hca3Δf + Δν 0 (Eq.8)
where∆µ=µe‐µgisthedifferencebetweentheexcited‐stateandground‐statedipole
moments,hisPlank’sconstant(6.626x10‐34J),cisthespeedoflightinavacuum
(2.998x108ms‐1)andaistheOnsagercavityradiusforthesphericalinteractionofthe
dipoleinasolvent.
Lippert‐Matagaplotscreatealinearrelationshipbetweenthestokesshift(Δν)and
Δf,yieldingastraightlinewithaslopethatisequalto2Δμ2/hca3.Bothaandμgare
calculatedcomputationally,andareputintotheequationtocalculatetheexcitedstate
dipolemoment.ComputedresultsyieldanOnsagercavityradiusequalto5.86Åanda
groundstatedipolemoment(µg)of2.04D.UtilizingtheLippert‐Matagacalculation
method,theexcitedstatedipolemomentsof2dbmxcpwerecalculatedtobe10.3D(all
solvents),and11.62D(non‐alcohols).
2dbmxcpyieldsasmallerdipolemomentcontraryto2dbma,whichgivesan
excited‐statedipolemomentof22.23D.Theelectronicdistributionisinternallytransferred
toalargerdegreefor2dbmathan2dbmxcpingoingfromthegroundstatetotheexcited
singletstate.Thisobservationistakenfromcomputingthemolecularorbitalsandthe∆µ
magnitudebetweenthegroundandexcitedstates.Soconceptually,itappearsthat
2dbmxcpwouldhaveasmallerchargetransfer,duetothesmallincreaseindipolemoment.
29
Additionally,lookingatthecomputedmolecularorbitalsof2dbmxcpincomparison
to2dbmashowsthatthereisaclearchargetransferfromthehomotothelumostagein
2dbma.With2dbmxcp,thechargeistransferredtoalesserdegree,asshowninFigure16.
AccordingtoresearchconductedbyMorimoito13,fluorescenceofanexcitedmoleculewith
asmallerchargetransfercannotbequenchedwellbyanalcoholbecauseoftheweak
interactiononthecarbonyloxygen,whichisconsistentwiththeresultsreportedhere.
Figure16.Computedmolecularorbitalsof2dbmxcpand2dbma.
TheLippert‐Matagacalculationcanbereasonedinamorequantitativewayby
calculatingtheunit‐chargeseparation.AccordingtoLacowicz9,4.8Distheelectronicdipole
momentthatresultsfromachargeseparationofoneunitchargeby1angstromoflength.
Withthatinformation,ifthe11.62D(non‐alcohols)calculationisused,itiscomparableto
aunit‐chargeseparationof2.4angstroms.For2dbma,thedipolemomentof22.23Dis
comparabletoaunitchargeseparationof4.6angstroms.
S1 (!,!*) S1 (CT, !,!*)
!"#$%&'( !"#$)(
30
Figure17.Lippert‐MatagaPlotofStoke'sshift(Δν)againstΔfof2dbmxcpinallsolvents.
Figure18.Lippert‐MatagaPlotofStoke'sshift(Δν)againstΔfof2dbmxcpinpolarproticandnonpolarsolvents.
y=4596.4x+4347.8R²=0.41668
0
1000
2000
3000
4000
5000
6000
7000
8000
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
Δν
(cm
-1)
Δf
y=3384.2x+4454.4R²=0.53919
0
1000
2000
3000
4000
5000
6000
7000
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
Δν
(cm
-1)
Δf
31
Figure19.Lippert‐MatagaPlotofStoke'sshift(Δν)againstΔfof2dbmxcpinallsolvents.Alcoholsare
designatedastheredsquares.
y=3384.2x+4454.4R²=0.53919
y=46006x‐7201.5R²=0.6376
0
1000
2000
3000
4000
5000
6000
7000
8000
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
∆ υ
(cm‐1)
∆f
PolarProticandNonpolarPolarAprotic
32
3.QuantumChemicalCalculations DFTB3LYP/6‐31G(d)geometryoptimizationaswellasTD‐DFTspectralcalculations
wereperformedon2dbmxcp.MolecularorbitalsofthiscompoundareshowninFigure20.
CalculationsshowthattheS0→S1transitionwasfoundtobe(π,π*)andtheS0→S2
transitionwaspredictedtobe(n,π*)Electrondensityisdistributedevenlyacrossthe
conjugatedπsysteminthecomputedHOMO,butgetstransferredtoaminordegree
closertothecarbonylcenterinthecomputedLUMOasillustratedinFigure20.Table3
showsthemolecularorbitalcalculationsof2dbmxcpaswellasthecorresponding
oscillatorstrengths(f)foreachtransition.Table4showstheTD‐DFTspectralcalculations
attheB3LYP/6‐31G(d)leveloftheoryinethanolandToluene.
Table3.ComputedMolecularOrbitalCalculationsof2dbmxcp.
Figure20.ComputedMolecularOrbitalsof2dbmxcp.
HOMOLUMO S1(π,π*)λ=449.72nm f=1.9045
HOMO‐2LUMO S2(n,π*)λ=445.41nm f=0.0000
33
Table4.2dbmxcpTD‐DFTSpectralCalculationsinEthanolandToluene
LevelofTheory:DFTB3LYP/6‐31G(d)
Solvent MolecularOrbitalCalculations Experimentalλmax
PercentError(%)
DipoleMoment(µg)
EtOH HOMOLUMO S1(π,π*) λ=483.76nm f=2.1424 9.45 HOMO‐2LUMO S2(n,π*) λ=417.49nm f=0.0050
438nm
4.91
2.78D
Toluene HOMOLUMO S1(π,π*) λ=478.89nm f=2.1470 12.08 HOMO‐2LUMO S2(n,π*) λ=430.84nm f=0.0004
421nm
2.28
2.38D
34
Conclusions
Theexperimentalresultsgeneratedfromthemeasuredphotophysicaland
spectroscopicpropertiesindicatethat2dbmxcpexhibitssolvatochromicpropertieswhen
testedfromnonpolartopolarprotictopolaraproticsolvents.2dbmxcpshows
bathochromic(red)shifts,andlessred‐shiftingoccurredinthealcoholswhencomparedto
ananalogouscompound2dbma,signifyinglesssolvatochromism.Thespectroscopic
characteristicsshowalinearcorrelationinboththeΔfandET(30)scale.2dbmxcp
generatedhigherquantumyieldsinthealcoholsthan2dbma.Thiscanbeattributedto
lowersolvatochromism,higher∆E,andlowerkic.Thesefactorscombinedwillproduce
higherquantumyields.Whenanalyzingthecalculatedmolecularorbital’sof2dbmxcpand
2dbma,therewasaclearindicationthattherewasasmallerchargetransferin2dbmxcp
than2dbmawhentransitioningfromthehomotothelumo.Thequantumchemical
calculationsof2dbmxcpindicatedthatS1is(π,π*)andS2is(n,π*).Additionallyitwas
determinedthatthequantumyieldsandfluorescencelifetimesvaryuponthenatureofthe
solvent.Themostpolarsolventsexhibitedthelargestquantumyieldsandlongestlifetimes.
35
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36
AppendixA:FluorescenceQuantumYieldCalculation
37
38
39
40
41
AppendixB:FluorescenceLifetimeSampleCalculationFluorescencelifetimecalculationof2dbmxcpinEthanol2dbmxcpinEthanol(sample1)*************************************************AnalysisFunction: SatApr022011at14:26******one‐to‐fourexponentials***********InputValues*****Decaycurve :A1430:578_s1IRFcurve :normalize(A1430:430_irfs2)StartTime :40.91EndTime :55.06OffsetwillbecalculatedShiftwillbecalculatedPre‐exp.1 :1Lifetime1 :1*****Statistics*****Jobdoneafter4iterationsin0.063sec.Fittedcurve :FLDFit(2)Residuals :FLDResiduals(2)Autocorrelation :FLDAutocorrelation(2)DeconvolvedFit :FLDDeconvoluted(2)Chi2 :2.419DurbinWatson :0.5903Z :‐0.1885Pre‐exp.1 :1.94 ±2.365e‐002(100 ±1.219%)Lifetime1 :0.6975 ±7.571e‐003F1 :1Tau‐av1 :0.6975Tau‐av2 :0.6975
42
Offset :16.63Shift :‐0.184*************************************************