Heavy-atom quenching of 9-ethylcarbazole in heterogeneous matrice

Rochester Institute of TechnologyRIT Scholar Works 8-1-2001 Heavy-atom quenching of 9-ethylcarbazole in heterogeneous matrices: Micelles and latexes Paul Conrow Follow this and additiona

Rochester Institute of Technology

RIT Scholar Works

8-1-2001

Heavy-atom quenching of 9-ethylcarbazole in

heterogeneous matrices: Micelles and latexes

Paul Conrow

Follow this and additional works at: http://scholarworks.rit.edu/theses

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Conrow, Paul, "Heavy-atom quenching of 9-ethylcarbazole in heterogeneous matrices: Micelles and latexes" (2001) Thesis Rochester Institute of Technology Accessed from

Heavy-atom Quenching of 9-Ethylcarbazole

Copyright Release Form

Heavy-atom Quenching of 9-Ethylcarbazole

in Heterogeneous Matrices: Micelles and Latexes

I, Paul D Conrow, hereby grant permission to Wallace Memorial Library of the Rochester Institute of Technology, to reproduce my thesis in whole or in part Any reproduction will not be for commercial use or profit.

The fluorescence quenchingof9-ethylcarbazolewas studiedinsolutionand

heterogeneous dispersions A heavy-atom quencher, 4-iodotoluene, was usedtoquenchthe fluorescenceof9-ethylcarbazole Polymeranalogs of9-ethylcarbazoleand

4-iodotoluene wereincorporated intoseparatelatex solutions, andquenchingexperimentswere carried out with non-polymer andlatex-boundanalytes Quenching in micellar

solutions andlatex dispersions was20-1000 timesmore efficientthan in homogeneous

solutions The diffusion ofanalytesinto the dispersedphase ofheterogeneous matricescreatedhigh localconcentrations ofthe fluorescing species andquencher, whilethe

global concentrations remainedrelatively low The fluorescence quenching in latex

solutions of poly[(methylmethacrylate)-co-vinylcarbazole] wasexceptionallyefficient

At SDS concentrations of0.008 M, aStern-Volmerslope of80,000wasobtained A

quencher concentration of1 x 10"6Mwas sufficienttocause significantquenching in latex solutionsthat hada carbazole concentration of5.5 x

10"5M.

Thisworkis dedicated to

my parents, Gary andCarolyn Conrow.

The best teachers I everhad kissedme goodnighteverynight

and gaveme everyspare moment oftheir time.

Thank You.

Acknowledgements Manypeoplehaveputin countless hoursandgiven me alotof guidance andsupportto helpmereachmygoals and aspirations Iwouldlike to thank:

-Mr. Stevenson You introducedmeto Chemistry in 10thgrade and plantedthe

seeds thatgrewintoalove forphotochemistry Thanks foryourexample You inspired

meto become ahigh schoolchemistry teacher.

-ThefacultyatSUNY Geneseo You gave measolidfoundation in the finer

points ofchemistry in my fouryears(and many late nights) in the halls ofGreene.

-DaveGeiger Ofallmy professors, you, in particular, helpedme realizethat

chemistry is best learned inalabandthrough discussingresearch results Ialsolearned fromyouthatcollege professorscanbe greatfriends.

-TheChemistry DepartmentatRIT Thanksgo outto theall themembers of

the stockroom, Tom Allston, theassistants in the Chemistry office,and all theprofessorswhomI interactedwithin class, or as ateachingassistant ThepeopleIworkedwith

every dayenrichedmyexperience atRIT in small andlargeways

-My committee: Dr Kotlarchyk, Dr Morrill,andDr Worman I thankeachmemberforyouradvice, suggestions, andhelp Youall made surethat my very best

madeitonto the followingpages

-MyfriendsatRIT: Pranita, Subu,andJulie filled theresearchlab with

laughterandjoy Ahmad, Akshy, Anvar, Wei, Dana, Sangita, Xuan, andYanirawerealways readywitha welcome storyor conversation

-Chrisand Rohini D'Souza You two have been atRITwith me from dayone

to the very last We have doneso muchtogetherat school and off campus. Icannot

overstatehow happy Iamto havemet such greatfriendsatRIT.

-Elizabeth Conrow During twoyears of constantchemistry from 7:30 am

-5:30 pm, I loved coming home, sharingourdays, and neverreally getting too deep into

chemicalprinciples Thankyoufor remindingmehowunimportantchemistry really is.

-Myadvisor: Dr Langner Ofallthe hands that have helpedmeshapethese

pages,none were as instrumentalas yours Your patience, yourinsight,your continuedencouragementliftedmeup whentheresearchstruggled I thankyouforyourdirection, advice, andfriendshipoverthe last two years

1.7 Deviations from Classical Quenching Models:

Quenching in Heterogeneous Media 20 1.8 Fluorescence Quenchingof9-Ethylcarbazole

3.1 Preliminary Photophysical Results of

3.2 Quenching in Solution 51 3.3 Quenching 9-Ethylcarbazole in SDS Solutions

3.8 Comparing Quenching Efficiencies Across Matrices 82

Quenching Mechanisms andInteraction Distances 18

Intensityof9-Ethylcarbazole Fluorescence from

1 x 10"7Mto 1 x

10"2

AbsorptionandFluorescence Data for

EffectofNitrogen Purgingonthe Fluorescence Intensityof

Raw Data from Quenching Experiments of

9-Ethylcarbazoleand4-Iodotoluene in 2-Propanol 51 Intermediate Results from Quenching Experimentsof

9-Ethylcarbazoleand4-Iodotoluene in 2-Propanol 52 Final Results from Quenching Experiments of

9-Ethylcarbazoleand4-Iodotoluene in 2-Propanol 53 Raw Data from Quenching Experiments of

9-Ethylcarbazoleand4-Iodotoluene in Hexanes 57 Intermediate Results from Quenching Experiments of

9-Ethylcarbazoleand4-Iodotoluene in Hexanes 57 Final Results from Quenching Experimentsof

9-Ethylcarbazoleand4-Iodotoluene in Hexanes 57 Raw Data from Quenching Experiments of

9-Ethylcarbazoleand4-Iodotoluene in Toluene 58 Intermediate Results from Quenching Experimentsof 9-

Ethylcarbazole and4-Iodotoluene in Toluene 58 Final Results from Quenching Experiments of

9-Ethylcarbazoleand4-Iodotoluene in Toluene 59 Raw Data from Quenching Experimentsof

9-Ethylcarbazoleand4-Iodotoluene in 0.0091 M SDS 63

Final Results from Quenching Experiments of

9-Ethylcarbazoleand4-Iodotoluene in 0.009 1 M SDS 64

Raw Data from Quenching Experiments of

9-Ethylcarbazoleand4-Iodotoluene in 0.0144 M SDS 64

Final Results from Quenching Experimentsof

9-Ethylcarbazoleand4-Iodotoluene in 0.0144 M SDS 64

Table 3.17 Raw Data from Quenching Experiments of

9-Ethylcarbazoleand4-Iodotoluene in 0.0213 M SDS 65 Table 3.18 Final Results from Quenching Experiments of

9-Ethylcarbazoleand4-Iodotoluene in 0.0213 M SDS 65 Table 3.19 Raw Data from Quenching Experimentsof

9-Ethylcarbazoleand4-Iodotoluene in 0.0278 M SDS 65 Table 3.20 Final Results from Quenching Experiments of

9-Ethylcarbazoleand4-Iodotoluene in 0.0278 M SDS 65 Table 3.21 Raw Data from Quenching Experimentsof

9-EthylcarbazoleandP(S-co-NIPMI) in 0.0091 M SDS 69 Table 3.22 Final Results from Quenching Experiments of

9-EthylcarbazoleandP(S-co-NIPMI) in 0.0091 M SDS 69 Table 3.23 Raw Data from Quenching Experimentsof

9-EthylcarbazoleandP(S-co-NIPMI) in 0.0103 M SDS 70 Table 3.24 Final Results from Quenching Experiments of

9-EthylcarbazoleandP(S-co-NIPMI) in 0.0103 M SDS 70 Table 3.25 Raw Data from Quenching Experimentsof

9-Ethylcarbazole andP(S-co-NIPMI) in 0.0171 M SDS 70 Table 3.26 Final Results from Quenching Experiments of

9-Ethylcarbazole andP(S-co-NIPMI) in 0.0171 M SDS 70 Table 3.27 Raw Data from Quenching Experimentsof

9-Ethylcarbazole andP(S-co-NIPMI) in 0.0281 M SDS 71 Table 3.28 Final Results from Quenching Experimentsof

9-EthylcarbazoleandP(S-co-MPMI) in 0.0281 M SDS 71 Table 3.29 Raw Data from Quenching Experimentsof

P(S-co-VnCz)and4-Iodotoluene in 0.0087 M SDS 73 Table 3.30 Final resultsfrom Quenching Experimentsof

P(S-co-VnCz)and4-Iodotoluene in 0.0087 M SDS 73 Table 3.31 Raw Data from Quenching Experimentsof

P(S-co-VnCz)and4-Iodotoluene in 0.0149 M SDS 74 Table 3.32 Final resultsfrom Quenching Experiments of

P(S-co-VnCz)and4-Iodotoluene in 0.0149 M SDS 74 Table 3.33 Raw Data from Quenching Experiments of

P(S-co-VnCz)and4-Iodotoluene in 0.0204 M SDS 74 Table 3.34 Final resultsfrom Quenching Experiments of

P(S-co-VnCz)and4-Iodotoluene in 0.0204 M SDS 74

Table 3.35 Raw Data from Quenching Experimentsof

P(S-co-VnCz) and4-Iodotoluene in 0.0263 M SDS 75 Table 3.36 Final resultsfrom Quenching Experimentsof

P(S-co-VnCz)and4-Iodotoluene in 0.0263 M SDS 75 Table 3.37 Results from Quenching Experimentsof

P(MMA-co-VnCz)and4-Iodotoluene in 0.008 1 M SDS 77 Table 3.38 Results from Quenching Experiments of

P(MMA-co-VnCz) and4-Iodotoluene in 0.0141 M SDS 77 Table 3.39 Results from Quenching Experiments of

P(MMA-co-VnCz)and4-Iodotoluene in 0.0164 M SDS 77 Table 3.40 Results from Quenching Experiments of

P(MMA-co-VnCz) and4-Iodotoluene in 0.0199 M SDS 77 Table 3.41 Results from Quenching Experiments of

P(MMA-co-VnCz)and4-Iodotoluene in 0.0257 M SDS 78 Table 3.42 Raw Data from Quenching Experiments of

P(MMA-co-VnCz) Copolymerand4-Iodotoluene in THF 80 Table 3.43 Final Results from Quenching Experiments of

P(MMA-co-VnCz) Copolymerand4-Iodotoluene in THF 81

List of Figures

Figure 1.2 Emulsion Polymerization: Chain Growth 5

Figure 1.4 Molecular StructureofSodium Dodecyl Sulfate 8 Figure 1.5 Conductivity MeasurementsofSDS Solutions 10 Figure 1.6 Jablonski DiagramofRelaxation Pathways 14 Figure 1.7 Favorable Exciplex Geometries in Aromatic Compounds 20 Figure 1.8 Chemical Structures ofFluorophore andQuencher 27 Figure 2 1 *H NMRofN-(4-iodophenyl)maleamic acid 32

Figure 3.4 Beer's Law Plotof4-Iodotoluene in 2-propanol 44

Figure 3.5 Beer's Law Plotof4-Iodotoluene ina5.0x

10"5M 9-Ethylcarbazole Solution in 2-Propanol 44 Figure 3.6 Fluorescence Spectraof5.6x

Figure 3.9 Fluorescence Spectraof-5.5 x

Figure 3.11 Perrin Plotof9-Ethylcarbazole Quenched by 4-Iodotoluene in

Figure 3.12 EffectofNitrogen Purgeonthe Absorptionof

Figure 3.13 Stern-Volmer Plotof9-Ethylcarbazole Quenched by

4-Iodotoluene in 2-Propanol, Hexanes, andToluene 60 Figure 3.14 Perrin Plotof9-Ethylcarbazole Quenched by 4-Iodotoluene in2-

Propanol, Hexanes,andToluene 61 Figure 3.15 EffectofNitrogen Purgingonthe Fluorescenceof

9-Ethylcarbazole in SDS Micelle Solutions 62 Figure 3.16 Stern-Volmer Plotof9-Ethylcarbazole Quenched by

Figure 3.17 Perrin Plotof9-Ethylcarbazole Quenched by 4-Iodotoluene in

Figure 3.18 EffectofSDSon theStern-Volmer Slope in Micellar Solutionsof

9-Ethylcarbazole Quenched by 4-Iodotoluene 68 Figure 3.19 Stern-Volmer Plotof9-Ethylcarbazole Quenched byP(S-co-

co-VnCz) Quenched by 4-Iodotoluene 80 Figure 3.25 Stern-Volmer PlotofP(MMA-co-VnCz) Quenched by

1 Introduction

Fluorescence quenching has beenusedto studyavarietyof chemical

environments and matrices Quenching techniques are among themostsensitive and

affordable waystoextractinformationonintermolecular interactions at molecular

distances Such methodology has been in voguesincethe 1970's to study, amongother

things,protein folding1'2andDNAinteractions3'4in aqueous solution,molecular

requiresthe interactionoftwoseparate molecules over adistanceof angstromsto tens of

nanometers, the lengthscalebeing dependentonthepair of molecules that isused The

multi-phase nature of micelles andlatexes canbemanipulatedto carryoutinteresting

studies If thepair of molecules usedcanbe confinedto the interiorof micelles orlatex

particles, theoverall concentration ofthe analyte canbe kept low, whilethe local

concentration of analyte withintheparticles canbeenhancedbyseveral orders of

magnitude

1.1 Fundamentals of Emulsion Polymerizations

A latex is a suspension of polymer particlesin an aqueous mediumthat is

stabilizedbysurfactant Thesynthesis ofalatexsolution is a complexbalanceofkinetics

andthermodynamics, though experimentally theprocedureis rather simple The

reactants requiredto make alatex include: a polar solvent(generally water),asurfactant

at aconcentration well-abovethe critical micelle concentration (CMC),ahydrophobic

monomer, initiator, andin somecases a co-surfactant Emulsionpolymerization

occursin two distinctsteps: initiation and chain growth(see Figures 1.1 and 1.2).

Assuming waterto be thepolarsolvent, during the initiation step,monomeris addedto

anexistingsurfactant solutionthatcontains an aqueous soluble initiator Thesurfactant

assemblesin small clusters calledmicelles Monomer is driven toward the hydrocarbon interiorofthesemicelles Theaddition of monomer creates athree-phasesystem The

continuousphaseis the aqueousphase,whichcontains averysmallamount of monomerand most ofthe initiator There isa phase of swollen micelles several nanometers in diameter The thirdphaseis the dropletphase The dropletphase consists ofrelatively large pools of monomer separatedfrom the bulkphasebyalayerof surfactant molecules

The droplets may beontheorder of micrometers tomillimetersin diameter Thereare

far fewer droplets than swollenmicelles, but themass of monomerin the two dispersed

phasesis approximatelyequal Though the droplets holdas much monomeras the

micelles, the totalsurface area oftheswollen micelles greatlyexceedsthe total surfacearea ofthe larger droplets Thesurfacearea of each dispersedphaseis dependentupon

theamount of monomerinsolution andthemethod of agitation usedtostirthereactants

Initiation beginswhen aninitiatorreacts witha monomerin theaqueous phase

The initiator is a water soluble saltthatreactsin aqueous solutionto generate a radicalspecies In thecase of sodium persulfate(Na2S20g), the followingreaction occursto

generatethe anionic sulfate radicalinitiator:

Na2S208(S)+H20(i) -> 2Na+(aq) + S2082"(aq)

S2O8 (aq) > 2S04 *(aq).

Anactivatedinitiator, *,will attackthe double-bondofthe monomerand create a

radicalmonomer-initiatorcomplexin theaqueous phase Theactivatedinitiatorcomplex is driven to the hydrocarbon interiorofthemicellarphase Thegreater

monomer-surface area oftheswollen micellesleads toahigher probability that theactivatedmonomer will diffuse intoaswollenmicelle,ratherthan into adroplet Once in the

swollenmicelle, the activatedmonomerinitiates a radical polymerization withinthe

micelle Thisoccursthroughout thereaction medium Therearein essencetrillionsofswollen micellar polymerization vesselsdispersed by theaqueous solution

During thechain growth regime ofpolymerization, the dropletphasebecomes important At this stage, thepolymerizationis occurring in the swollen micelles andthe

concentration of monomer within each swollen micelleis decreasingasthepolymergrows Thepolymerthat ismade withinthemicellesis held there by kinetic and

thermodynamic factors The monomer, however, isstill able to diffuse through the

surfactant-waterinterface Aconcentration gradientdevelops between the dropletandmicelle phase The largepools of monomerin the droplets feed the ever-swelling

micellestomaintain abalancedmonomerconcentrationthroughout the dispersedphase

This diffusion drivenprocessresultsinagrowing latexparticle Monomercontinuesto

be convertedintopolymeruntil either allinitiatorand activated polymer chains are

deactivated,or until thepools of monomerin the dropletphasedry up

1.2 Fundamentals ofMicelles

Micellesaredynamicmolecularassemblies,composedofsurfactantmolecules

that have hydrophobic tales andhydrophilic head groups, dispersed ina polar solution

The hydrophobic tales aretypically long hydrocarbonchains thatare not

thermodynamicallystableinsolution Thenonpolar segments of surfactant molecules are

driven toward each otherinpolarsolvents, particularly inwater The hydrophilic, or

"water-loving",chargedheadgroups arestablein solution When a critical concentration

ofsurfactantis reached, themolecules self-assemblesothat the hydrophobicends ofthe

molecules clustertogether, andthe hydrophilicends remainincontact with solvent This

processis entropy driven The entropyofthesurfactantdecreases, but this decrease is

overshadowedby the entropygains ofthesolvent Surfactantassembliesin nonpolarsolvents are referredtoasreversemicelles Reversemicelles are organized sothat the

chargedheadgroups are directed to thecore ofthemicelleandhydrocarbon tails form

theouter shell This formofself-assembly is enthalpy driven.

Anionic surfactants contain anionic headgroupsbalanced bysimple cations(K+, Nil/,etc.) Thesurfactantdissociates inpolar solvents and eachioniccomponentis driven toaspecific region ofthe solution(see Figure 1.3) [Cationicmicellesassemble

in the samefashion, but thecharges arereversed.] The anionicheadgroups arrangeinto

sphericalmicelles, surroundedbya region of solvated cationsheldatthemicellesurface

byelectrostaticinteractions Thisorganized arrangement ofionswithin asolventis knownastheelectric doublelayer.18 The typical diameterofthemicelle andsurrounding double layer ison theorder of 10-25nm The formationofthis double layerandthe

micellarassembly significantlychanges the conductivityofthesolution Before this

formation, surfactant ionsarerandomly distributed throughout the bulkphase andare

concentratedat the air-solvent interfaceas amonolayer An increase in theconcentration

ofions insolution gives riseto an

increasingconductivity Upon reaching acritical

concentrationofsurfactant, the surfactantmoleculesspontaneously assembleinto

micelles The conductivityofthesolution increases less rapidlyas a result ofthis

organization Thepointat whichthere isanoticeablechangein slope ofconductivity

versusconcentration is knownasthecritical micelle concentration(CMC).

10-25 nm

Figure 1.3 Cross-sectional Viewof anAnionic Micelle in Aqueous Solution

Micellarassemblies are not static bunchesof molecules Rather, there isa

constant dynamicexchangeof surfactant moleculesbetween thesolvent and micellar

Despite their dynamic nature,micelles exhibit anarrowrange of sizesdepending

onthe typeofsurfactant used and concentration of solutes within thesolution matrix

oTypicalradii for micelles, excluding the double layer region, are ontheorder of25 A Sodium dodecylsulfate(see Figure 1.4), abbreviatedSDS, isone ofthemostcommonandwell-characterizedsurfactant molecules that formsmicelles Thesodium cations are

held in the double layerofthe dodecylsulfate anionic micelles In solutions ofSDS and

water, spherical micellesformat aconcentrationofapproximately 0.00810

M12

andthe

aggregation number atthe CMC is approximately 50.19 The aggregation number

indicates how manysurfactantmolecules, onaverage,makeup a single micelle At the

CMC, theconcentration of micelles is 1.6x

10"

M Thevalue ofthe aggregation numbervariesdependingon theexperimentaltechniqueusedtomeasurethis quantity

Figure 1.4 Molecular StructureofSodium Dodecyl Sulfate (SDS)

As the SDS concentration israisedabovethe CMC, theaggregation number(n)

alsoincreases Ranganathan,et

al.20

usedtime-resolved fluorescence quenching to

develop aformula to determine the aggregation number ofSDS as afunctionof

surfactant concentration The increase is afunctionofthesodiumion concentration

dissolved in theaqueous phase andK2,a complexterm dependentonthe CMCand

several physical constants ofSDS:

n = K2([Na+]aq)a23

(1).

The lonesource of sodiumions in the simplest possible systemis from the SDS If this is

the only sourceofsodiumthe formula becomes:

n = K2([SDS]aq)023

(2).

A three-fold increase in SDS concentrationabovethe CMCresultsin a2.3-fold increase

in theconcentrationofmicelles within a solution Theaggregation numberatthree times

the CMC is 66andthemicelleconcentrationis 3.7x 10"4M.

Theproperties ofamicellar solution are altered when another species or analyteis introduced Co-surfactants,such as alcohols, arecommonly addedto SDS solutions to

help drive the formationof micelles Co-surfactantsprovide afurther thermodynamic

driving force to accommodatetheorganization of micelles Theeffectof an alcohol can

beseen in thecomparative results ofadding two percent, by volume,of 1,4-dioxane and

2-propanol toaqueous solutions ofSDS As theconcentration ofSDS is increased, the

slopeofthe conductivitychanges at a concentration of0.0072 M for thesolution

containing 2-propanol 1,4-dioxane alsolowers the CMC, however, with aless drastic

effect(see Figure 1.5).

0.0 300 0.0025 0.0050 0.0075 0.0100 0.0125 0.0150 0.0175

MolarityofSDS

Figure 1.5 Conductivity MeasurementsofAqueous Solutionswith2%, by Volume,of aDelivery Solvent.

SDS micellesinwater are spherical at concentrationsbetween the CMC andten

times the CMC Organic moleculesandions canbe solubilizedby the interioror onthe

surface ofthese particles This tends to swellthe micelle, potentially changingsuchparametersastheaggregationnumber, the CMC,andthe shape ofthemicelle If the

analyte concentrationis kept low, theseeffects areless noticeable The behaviorofanalytes within micelles and emulsion particles can bestudied with sensitivetechniques

such as fluorescencespectroscopy

1.3 Fundamentals of Photochemistry:

When discussingphotochemicalphenomena, like fluorescence, somefundamental

principles of atomic and molecularenergystates mustbe addressed Everyatom or set ofatomsinamoleculehas specific electronic states that haveassociated quantized energies

The ground electronic state of an atom or molecule(So)correspondsto themoststable

state possible

-theonethat has the leastchemical potentialenergy Withina given

molecule, there is aseries of vibrational states (S0,) thatcorrespond

to different stretchingorbendingmodesofatoms withinthemolecule Thesevibrational

statesarisefrom themotion ofthenuclei ofthemolecule

Whenaquantum ofenergy is absorbed, specific toa given atom ormolecule, an

excited electronic state(Sn), orvibrational state(S0,n)occurs In such an excitedstate, the

entire electronic configurationof an atomor molecule rearrangesto accommodatethe

absorbedlightorkinetic energy, giving rise toanincrease inchemical potential energy

Like the groundstate, there is a series of vibrational states withinthe excited electronicstate of a molecule (Sn,n) Theexcitation froma groundtoan excited electronic statemay

alsobe accompaniedbyachangein thevibrational state inmolecules Forexample:

S0,i + energy -> Si,2.

Theprocess of electronic excitation occurs on atime scale

occurs, there isnot enough time for thenuclei of atoms within a moleculetochange

significantly from the original ground state configuration The only"allowed"

or

probable electronic excitationsinclude transitions that havesimilar nuclear

configurations This general principle of molecularphotochemistry is calledthe

Frank-Condonprinciple22

In additionto the Frank-Condon principle, transitionprobabilities mayalsobe influenced by thespin state In accordance withthe Pauliexclusionprinciple, theground

electronic state of most atoms or non-radical molecules musthaveall electronspairedin

orbitals such that there isnonet spin Inotherwords, thespins ofthe twoelectronsin agiven occupied orbital mustbe oppositeto one another When energy is absorbedand an

molecule is promoted to anexcitedstate, thenew electron configuration retains

its originalnetspinofzero Once in theexcitedstate,one of several processes maycauseone oftheelectrons toreverseitsspin Thenew stateformed is atripletstate(T) In thisstatetherearetwo electrons in differentmolecular orbitals withthe same spin Anytime

thatan atom or moleculehasno net spinit is saidto be in the singlet state (S).

1.4 Fundamentals of Fluorescence:

Everygroundstate molecule can be irradiatedwitha characteristic wavelength of

light, X, thatpromotesthemoleculeintoan excited electronic state The energy

associatedwith an irradiatingphoton isafunctionofthewavelength (E=hc/A).

S0 + hc/X - Si.

When light is absorbed, the moleculetakeson alessstable configuration Theexcitedmolecule spontaneouslyrelaxesback to themore stable groundstate, releasing the

absorbed energy The majorityof molecules releasethe excessenergy asheat However,

thereare several classes of moleculesthatwill release some oftheabsorbedenergyas

light,ratherthan heat, whenrelaxing to the ground state:

Si -4 S0 + hcA,.

Therelease oflight, uponirradiation, is luminescence Luminescencecan be divided into twosub-groups: fluorescence and phosphorescence By definition,

fluorescence is theemission of absorbedlightwhen an excited statemolecule relaxes toa

lower energyelectronicstate withthesame spin Anymoleculethat fluoresces is

referredto geneticallyas afluorophore Phosphorescence is theemissionoflight

accompaniedbyatransitionof an excitedtripletstate moleculeto alower energysinglet

In some cases offluorescence, the energyof excitation and emission areequal, but

inmostcases, the energyof emissionis lower than thatof excitation This phenomenon, known as aStokes shift, occursbecausean excitedstate molecule releases someenergy

asheat whenitrelaxesfrom onevibrationalstateto alower energyvibrationalstate The energy thatremainsfor luminescence is then less than theoriginalenergyof excitation

Phosphorescence is always moreStokes-shifted than is fluorescence.

Thereareseveralcompetingrelaxation pathways that an excited moleculemay

take toreachtheground state Thesepathways can be broken down into two groups,

luminescentand non-radiative(see Figure 1.6) Thenon-radiativetransitions include:

vibrationalrelaxation, internalconversion (IC), external conversion(EC), intersystem crossing (ISC), andin some casesdissociation (D).23

(Si) doesnot preclude a radiative processfrom the Si stateto theground electronicstate,

So- External conversion, or collisionalquenching, isanintermolecularprocess where anexcited state moleculetransfers kinetic energy toeithera solvent or another solute

molecule, heating up the surroundingmedium Externalconversionisreducedwhenthe

temperature ofa matrixis loweredorthe viscosityoftheenvironmentis increased Lowertemperaturesand greaterviscosityreduce thenumber ofpossible collisions in

solution, decreasing the probability thatexternalconversion willoccur Intersystem

crossing is flipping the spinofa moleculebetween

states The flipping betweensinglet andtripletstates iscausedby the mixingof spin andorbitalmomentumofmolecules Similar to internal conversion, intersystem crossingmost often occurs whenvibrationalenergy levels of a molecule overlap In thecase of

intersystem crossing, however, theelectronic state changeis between a singlet and a

tripletstate Dissociation isa unique case where a photochemical excitation results inapermanent cleavage of a chemicalbond, and no relaxation to theoriginalground stateis

Triplet Excited State

Figure 1.6 JablonskiDiagram: LuminescentandNon-Radiative ModesofRelaxation foranExcited StateMolecule

Kinetics determinewhethera moleculeluminescesor relaxesnon-radiatively In

allrelaxation pathways, the energy difference betweenan excited andgroundstatemoleculeis thermodynamically equivalent regardless ofthepathway Eachrelaxation

pathwaycan becharacterizedbya rateconstant, k It is onlywhentherateconstantof

fluorescence (kF) is significantly larger than that forall non-radiativetransitions

(kic, kEc, kisc, ko) thata moleculenoticeably fluoresces It is also usefulto deal with

lifetimes offluorescence in some casesratherthanrate constants A lifetime, xF, is the

reciprocaloftherate constantfor fluorescence The lifetime describes how longanexcited state moleculetakes to fluoresce to the groundstate Typical fluorescence

lifetimesrange from 10"5

secondsto 10"8 seconds

The intensityoffluorescenceof a sampledependsonthearea ofthesamplethat is

irradiated, thearea ofthe detectorexposedto the fluorescence radiation, anddaily

fluctuations in thepower oftheexcitationlamp Therefore, it isnotgenerallymeasuredabsolutely Accounting for instrumental factors, the fluorescence intensitycanberelated

toa quantumefficiency (<E>o) This efficiency isa measure ofthe likelihood thatanexcited state species will relax viafluorescence Thequantumefficiency isatheratioof

therateoffluorescence to thesummation oftherates of all possible relaxation

pathways22:

kF+kIC+kEC+kISC+kD k2

1.5 Classic Models of Fluorescence Quenching

The intensityoffluorescencecanbe reducedifa speciesis addedthat introducesanew non-radiative relaxationpathway for the fluorophore. Thisnewpathway is

characterizedbya rate constant, kq Sucha speciesthatreduces fluorescence is referred

to as aquencher, Q Theeffectofthe quencheris to increase thesummationoftherate

fornon-radiativetransitions Thequantumefficiency in thepresence of

quencher, Oq, becomes afunctionofthequenchermolarity, [Q]:

*< =

(4)-When theratio ofthequantumefficiency in the absence and presence of quencheris

taken (eq 3/eq 4) theresultis linearly dependenton quencher concentration:

<5-This formulation is furtherrefinedby defining l/(ks)asthe fluorescence lifetime in the

absence ofquencher, To, andby combining therate constantfor quenching withthe

fluorescence lifetime intoa singleterm Ksv:

with an associated rate constantkq equalto therate constantfor diffusion, kd Essentially,

diffusion occurs on atimescale faster than the lifetimeoftheexcitedfluorophore Asa

result, there is achancethata quencher and an excited statefluorophorewill mixto form

abimolecularcomplexbefore fluorescence takesplace Ifsuch acomplexforms,

quenching As the concentrationofquencheris increased, there is alinear

increase in the likelihood thataquencherwilldiffuseclose enough toan excited

fluorophore tocausequenching.

Deviations from the Stern-Volmermodel areobserved, especially in systems ofpolymerfilms andlatexes Upwardcurvatureis observed,ratherthana simple linear relationship betweenconcentration andtheratio of quantum efficiencies The Perrin

model, developed in the 1920's21, accountsfor the curvature, observedinmorerigid

matrices In this model, the quencher andfluorophoreare unabletomovein spaceduring

the lifetime oftheexcited state Therate offluorescence ismuch faster than therate of

diffusion Thismodel is appliedtosystems where fluorophoreand quencher arelocked

in arigid position,such as polymerfilms,orinconfined spaces wherediffusion is

difficult, such aslatex particles Quenching onlyoccursif thequencheris within aneffective radiusfrom a ground state fluorophore during themoment of excitation If the

quencheriswithin this 'quenching sphere', defined by the effective radius (Reff),a

ground statefluorophore that is excited willbequenched 100%ofthe time If the

quencherisoutsideofthis sphere, there isnochance offluorescencequenching The Perrin modelpredictsthe following linear relationship betweenquantumefficiencyandquencher concentration:

where V is thevolume ofthe quenching sphere equalto 4/37t(Reff)3, andNA is

Avogadro's number

The Stern-Volmermodel canbe thoughtof as puredynamic quenching, whereas

the Perrinmodel appliesto totallystatic quenching Theexact mathematicalfunction that

relates the concentrationof quencherto the decrease in fluorescence efficiency yieldsagreatdeal ofinformationontheexact mechanism ofquenching, andtherelativelocations

ofquencher andfluorophore Theadherencetoeithera strictStern-VolmerorPerrin

typemodel provides insightonthe viscosityoftheenvironmentofthe fluorophoreand

quencher However, amixingofthese two models may beneededto best describe some

results

1.6Mechanisms of Quenching

In orderfora quenchertoreducethe fluorescence intensity, itmustbewithin an

interaction distanceofthe fluorophore Thereare several mechanisms bywhich a

quenchermayact and eachis associated with a range ofinteraction distances Themorecommonmechanisms,andtherelateddistances areitemized in Table l.l.2

Table 1.1 Quenching MechanismsandMaximum Interactive Distances

24

Interaction Mechanism Interaction Distance Dipole Coupling 10-100 A Electron Exchange 4-15 A

Reabsorption As longas emission reaches

Regardless of whetherthe Stern-VolmerorPerrinmodelis usedto interpret data, the interaction distancerepresentsthemaximumintermolecular distance between

quencher andfluorophore for quenching tooccur Dependingonthe type ofstudy that is

conducted, it ispossible topickaquencher-fluorophorepairtomeasureinterparticle interactions from 4A to 100A Ofparticularinterestareheavy atom quenchers Such

molecules acttoquenchfluorescenceover arelatively shortdistance, 4 A Theyare

commerciallyavailableina widevarietyof chemicaltypes, including: cations (Tl3+)25,

anions (Br") , and organic molecules(dibromomethane).27 Assuming the heavy atomis

withinthe interaction distance, itpromotes electron spin/orbital flipswithinthe

fluorophore Therate ofintersystem crossing is increasedand excited state singlet

fluorophoresare convertedinto the tripletstate:

Intersystem crossing is accompaniedbya reductionin the intensityoffluorescence Non-radiative relaxations or phosphorescenceincrease inprobability

In aromatic systems,exciplexformation becomes important Exciplex formation

occurs when a molecular orbital of an excited statefluorophore (F ) interactswiththe

LUMO (lowestunoccupied molecularorbital) of another ground state species (E) to from

a short-lived complex(if theground statemoleculeis thesame asthe fluorophore, the

complexis referredto as an excimer):

F*

+ E -> (F E)*.

Whensuch acomplex, orexciplex, forms, there is atransferof charge from theexcited

statefluorophore to the LUMOofthe ground state species This transfer is accompanied

byacleavingofthecomplexand anew, lower energy, luminescentmode ofrelaxation:

(F - F + E + Energyreiax.

Thenewluminescence pathway may be adifferentmodeoffluorescence oranew

phosphorescent radiation Regardlessoftherelaxationpathway, the fluorescenceoftheoriginal excited statefluorophore isreduced The specific mode of relaxationdepends on

thephotophysical propertiesofthegroundstatespecies, E Aromatic compounds, in

particular, formexciplexes21

because theenergiesofthe molecular orbitalsbetween twoaromaticrings often overlap The aromaticrings also provide afavorable geometry (see Figure 1.7)neededforexciplexformation An exciplex canbe identified bychangesin a

fluorescencespectrum In general, an exciplex spectrumis red-shifted, less intense,

peak-broadened, andstructureless,relativeto thespectrum of alonefluorophore.21'22

Figure 1.7Optimal GeometryofAromatic Compounds intheFormationofExciplexes

1.7Deviationsfrom Classical Quenching Models: Quenching in Heterogeneous Media

The Stern-VolmerandPerrinmodelsneedto be amendedsomewhatwhendealing

withfluorescence in heterogeneous media,such as latexes andmicelles Fluorescence quenching becomesmore complex when atwophase systemis considered Inatwo

phase systemit becomes important to characterizeboth phases, the partitioningof

analytesbetween the phases, theconcentrationof analytes withinthe dispersed phase, the

accessibility fluorophore between phases,andthe diffusivityorpacking

ofanalytes withinagiven phase

Incases whereStern-Volmerplots give non-exponential upwardcurvature, a

mixingofthe linear Stern-VolmerandtheexponentialPerrinmodelhas been proposed29:

*- = l+Ksv[Q]eV[Q]NA (8).

^q Here the quantity Vrepresentsthe volume element ofthe Perrin model andthe

exponential termaccounts for the totalstatic contribution At lowquencher

concentrationstheexponentialtermcanbeexpressed as (1 +Ka[Q])whereKa isa

constant dependentonthe quenchingvolume Theresult ofthis substitutionis:

<D0

I

=(l+Ksv[Q])(l+Ka[Q])=l + (Ka+Ksv)[Q] +

KaKsv[Q]2

(9).

^q

A revised explanation of upward curvaturehas beenproposedby Ware,etal, for

solutions ofrelatively highviscosity In this formulation, quenching is diffusion

controlled However, atransient term is included in therate constantfor thecollision ofquencherand excitedfluorophore:

Borrowing from the Perrin model, quenching will occurifa quencheris withinthe

effective radius of an excitedfluorophore In addition,unquenchedexcited state

molecules will besusceptibleto diffusional quenching ThemodifiedStern-Volmer

31

<J>0 /

whereI(A,), derived in Ware's work,29

is a complexfunctionofthe time dependent

variable, kd(t).

A second considerationwhen developingaquenchingmodel concernsthe

quenching efficiency,y If this efficiency is unity,non-radiative relaxation occursevery

timea quencherencounters an excitedfluorophore In such caseskd, the diffusionalrate

constant, is equalto the quenching rate, kq However, many fluorophore-quencher

complexes donotalwaysresultin deactivation oftheexcitedstate:

F + Q

k-d

There isakinetically limited deactivation ofthequencher-fluorophore complexthat

reducesthe efficacyofthe quencher,y Inall cases, therate constants andquenching efficiency are related asfollows:

kq =

The quenching efficiency, derived by Eftinkand

Ghiron,32

is dependentonthe

diffusion-limitedrate constants (kdandk.d), therate constant ofquenching (kO, thequencher

concentration, andthe fluorescence lifetime asfollows:

ki

Y =

]q+kd +x0 +kd[Q]

When kj is muchlarger thanall otherterms in the denominator, the quenching efficiency

willequal one However, if anyoftheotherterms in the denominator havemagnitudes

onthesame order aski? the quenching efficiency falls belowone If the kd[Q] term, in

particular, is nearin magnitude to ki; the efficiencywillbeafunctionoftheconcentration

ofquencher,suchthatadownward Stern-Volmercurve wouldbeobserved with

increasingquencherconcentration.

DownwardcurvatureofStern-Volmerplotsis commonly observedin latex

solutions ' ' ' In suchcases, asaturation ofquenching isobserved Thesaturationisa

consequence ofthe inaccessibilityoffluorophore boundwithinthe latexpolymer

Quenching is veryefficientatlowquencherconcentration As theconcentrationis

increased, <&o/frqreaches a maximum becauseall accessible fluorophoremolecules

interactwith atleastonequencher The remaining fluorophores areinaccessible to the quencher, and are still abletoabsorb and emitlight Thevariation in the accessibilityof

fluorophorescreates aheterogeneously emittingsystem within ahomogeneous local

environment

Eftinkand

Ghiron2

proposed a combinedStern-Volmer andPerrin formula that

describes the fluorescence quenching in aheterogeneously emittingsystem wheretwo

regimes aredefined, quenchable(a) and unquenchable(b):

proposedthe followingsimplificationthatassumes noquenching in the 'b' regime, and

that there is noPerrin-type quenching in the 'a' regimesothat Va=Vb=

In this formulation, the fractionoffluorophoreaccessible to quenching, fa,andthe

Volmer constant, Ka, is determined fromalinearplot ofthereciprocal quencher

concentration andfluorescenceefficiencies

Nakashima, et

al.,29

usedthe hinderedaccessmodeltocompare fluorescence quenching of 1-pyrenemethanol in homogeneous aqueous solutions and polystyrenelatex dispersions Tryptamineandtryptophan were usedasquenchersthatactthroughanelectrontransfermechanism Quenching was observedin bothsolutionandin latexes Classic Stern-Volmer quenchingwas observedin aqueous solutionforquencherup toamillimolarconcentration In the latex dispersion, saturation ofquenchingwas observed

formicromolar concentrations of quencher The efficiencyofquenchingwasnearly

1000 timesgreaterin thepolystyrenelatex, but theextent ofquenchingwas limited by

the accessibilityof quencherto the 1-pyrenemethanol molecules The highlypolarquenchers and some ofthe hydrophobic 1-pyrenemethanol were adsorbed ontothe latex

surface The remaining fluorophorewasdriven to the hydrophobiccore ofthe latex

particles Thecore ofthe latex wasinaccessible to the quencher,andthus quenching did

not occur atthesesites.Usingequation 15, the fractionof accessiblefluorophoresiteswithinthe latex system was determined to be 0.61.

Miyashita,et

al.,36

studiedaquenchingmatrixthatalsooperated under electron

transfer In thissystemthe fluorophorewas an anionic vinylcarbazoleterpolymer latex The fluorescence was quenched with methyl viologen cationsthatwereattractedto the

negativelycharged groups withinthe latex Estimated latexparticle diameterswere75

nm,measuredfroman electron micrograph Saturation was observedfor theassociated

Stern-Volmerplots Theeffect ofvarying the latexconcentration wasstudied asthe

concentration range of quencherwaskeptconstant As theconcentration offluorophore labeled latexwas decreased, the quenching efficiency increased because therewas a

constant numberofquenchers associatingwith an everdecreasingnumberoffluorophore labeled latexparticles

Theupward curvature, associatedwith mixedStern-VolmerandPerrin models,

combinedwithdownward curvature, due to quenching efficiencyconsiderations and

fluorophore accessibility, resultina wide range of possible Stern-Volmercurvesin heterogeneousmedia Quenching in simple solution matrices and micelle matrices must

beexaminedfirst, in orderto best interpret quenching behaviorwithin alatex.

Quenchingresultsfrom simpler media give crucialinformation regarding

fluorophore-quencherinteractions These interactionsare independentoffluorophore accessibility,

whichbecomes importantwhenlatexes are considered

1.8 Fluorescence Quenching of 9-Ethylcarbazole Species by 4-Iodotoluene Derivatives

Initially, carbontetrabromide, aheavyatomquencher, was usedto quenchthe

fluorescenceof9-ethylcarbazole However, itsoonbecameapparentthat thisquencherundergoesphoto-decompositionwhen exposedtoultra-violet

light37

in 1,4-dioxane Additionally, there isno polymeranalog for thismolecule 4-Iodoanilinewas alsotried

as aquencher, but thepresence oftheelectron pair ontheaminegroup complicatedthe

fluorescencequenching Theaminegroupquenches fluorescence byelectrontransfer,38

in additionto the heavy-atom iodo groupquenching This mixingofquenchingmodes

introducedunwantedcomplexity to thestudy

Basedonthose preliminary experiments, this study focusesonthe heavy

atomquencher, 4-iodotoluene, toquenchthe fluorescence of9-ethylcarbazole inseveral

environments, ranging from singlephase solutions, micelles, latexes, and polymer

solutions Polymeranalogsofboth the quencherandfluorophorewere synthesizedfor

thisstudy Thepolymer analogs (see Figure 1.8) included thecopolymers:

poly(styrene-co-vinylcarbazole), poly [(methylmethacrylate)-co-vinylcarbazole],iodophenylmaleimide)], and poly[(methyl methacrylate)-co-(N-iodophenylmaleimide)].

poly[styrene-co-(N-These copolymerscontainedless than tenmole percent ofthe analytes ofinterest The

major component was either styrene or methyl methacrylate

polymerizedquencher: Poly[styrene-co-(N-iodophenyl)maleimide]

Figure 1.8 DiagramofFree 9-Ethylcarbazoleand4-Iodotolueneand aPolymer AnalogofEach

Analyte.

2 Experimental

Solvents Deionized, distilled water, filtered througha

Barnstead

Nanopure

ultrapure watersystem was usedinall syntheses andanalyses Organic solvents,

including chloroform, hexanes, 2-propanol, tetrahydrofuran andtoluenewere used as

receivedfrom Aldrich andBaker.

Reagents Allreagents (9-ethylcarbazole, 4-iodotoluene, sodiumpersulfate,

sodiumacetate, maleicanhydride, and acetic anhydride)werepurchasedfrom Aldrich

andused asreceivedincluding Sodium dodecyl sulfate was obtainedfrom ABNPrime

Research reagents

Monomers N-Vinylcarbazolewas used as receivedfrom Aldrich Styreneandmethylmethacrylate, purchasedfrom Aldrich, were run through inhibitorremoval

columns onthe dayof use Thecolumns removefree-radical trappingmoleculesthat

prevent spontaneous polymerization The styrene was runthrougha 10-15ppm

4-tert-butylcatechol column Themethyl methacrylate was passedthrougha 10-100ppmmonomethyletherhydroquinonecolumn

UV-visabsorbance spectrometer A Hewlett Packard 8453spectrophotometer

was usedto carryouttheearliest absorbancemeasurements Unwantedphotochemical

side-reactions were observed withthis instrument inseveral matrices Themodeofexcitation ofthis single-beam, diode array detection instrument interactedwiththe

fluorescing analyte,andthereforewasnot usedfurther A Shimadzu CPS-240A

spectrophotometergave reliable and consistentmeasurements, withoutaltering the

analyte orthe matrix This instrumentusesadouble-beam detectionmethod which

requires alower intensityexcitationbeam.