The Photophysical Properties of a Symmetrically ... · This project extends interest into the...

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*************************************************