Effects on charge recombination of an electron accepting group in a Ru-dye for p-type Dye-Sensitized-Solar-Cells.

Fa oltà di S ienze Matemati he, Fisi he e Naturali

Corso di Laurea Spe ialisti a in Chimi a

Tesi di Laurea Spe ialisti a

Ee ts of a Ru-Based Donor-A eptor Photosensitizer on the Charge Re ombination and E ien y of a pType

Dye-Sensitized-Solar-Cell

Candidato:

Lu a D'Amario

Relatori: Controrelatore:

Prof. Maurizio Persi o Prof. Fabio Mar hetti

Prof. Leif Hammarström

1 Introdu tion:

Dye Sensitized Solar Cells 5

2 Experimental se tionand te hniques 13

2.1 Synthesis . . . 13

2.1.1 Compounds sour e . . . 13

2.1.2 Synthesisof[Ru(bipy)

2

(Phen-NMI)℄(PF

6

)

2

. . . 14

2.1.3 Synthesisof[Ru(d b)

2

(Phen-NMI)℄(PF

6

)

2

. . . 14

2.2 Devi eassembly . . . 19 2.3 Spe tros opi te hniques . . . 20 2.3.1 LASER-ash-photolysis . . . 20 2.3.2 Spe tro-ele tro- hemistry . . . 28 2.4 Spe tros opi method . . . 29 2.4.1 Instrumentation . . . 29

2.4.2 Laserashphotolysisset-up. . . 29

3 Resultsand dis ussion 31 3.1 Measurementsin solution . . . 32

3.1.1 Chara terizationofthedyes: UV-Visspe tra . . . 32

3.1.2 Chara terizationofthedyes: ele tro hemi almeasurements . . . 35

3.1.3 TransientSpe traof[Ru(bipy)

2

(Phen-NMI)℄(PF

6

)

2

. . . 41

3.1.4 Photo-redu tionof[Ru(bipy)

2

(Phen-NMI)℄(PF

6

)

2

: introdu tion. . . . 44

3.1.5 Photo-redu tionof[Ru(bipy)

2

(Phen-NMI)℄(PF

6

)

2

withferro ene. . . 46

3.1.6 ConsiderationsaboutthenatureofMLCTstate . . . 47

3.1.7 Photo-redu tionof[Ru(bipy)

2

(Phen-NMI)℄(PF

6

)

2

withCo(II) . . . . 56

3.1.8 Ru(bipy)

2+

3

omparison . . . 57

3.1.9 Pseudorst-orderexperiments . . . 60

3.2 Measurementsin surfa e . . . 64

3.2.1 Measurementswithoutele trolyte . . . 64

3.2.2 Measurementswithele trolyte . . . 70

3.3 Measurementsofe ien y. . . 73

ACN A etonitrile DSSC Dye-Sensitized-Solar-Cell; LFP LASER-Flash-Photolysis SEC Spe tro-ele tro- hemistry CV Cy li -Voltammetry bipy 2,2'-bipyridine; d b 4,4'-di arboxy-2,2'-bipyridine; phen 1,10-phenanthroline dtb di-tertbutyl-bipyridine

NMI naphtalenemonoimide

FeCp

2

ferro ene

Phen-NMI Ele trona eptingligand. N-(1,10-phenanthroline)-4-nitronaphthalene-1,8-di arboximide;

RuB

2

PN Oursolutiondye: [Ru(bipy)

2

(Phen-NMI)℄(PF

6

)

2

;

RuB

2

PN Oursurfa edye: [Ru(d b)

2

(Phen-NMI)℄(PF

6

)

2

;

RuB [Ru(bipy)

3

℄(PF

6

)

2

;

Introdu tion:

Dye Sensitized Solar Cells

Oneofthemostimportantglobalproblemsofourtimeis thefuture energysupply.

Atthemomentmostofthe energyusedby Humanityis omingfromfossil hydro arbons [40℄. Even dis ounting its limati ee ts, on whi h the s ienti ommunity has an open dis ussion, we are sure that this kind of resour e is nite and in some future we shall need otherenergysour es.

Looking at renewable alternatives (geothermal, wind, tidal, Sun), we see that the most powerfuland abundant is the light of Sun. This last armation is easy to prove. We an takeasreferen ethedatadis ussedin theBasi Energy S ien esWorkshoponSolar Energy Utilization [1℄. Energy powerused by theglobeisabout15TW (TW=

10

12

W), aboutthe 86%ofthe energyisused asfuels(dire tly) andmost oftherest asele tri ity. Howeverthis al ulation is notso simplebe ausenot all the ountries a ount for the energy, so we an assumean ex essvalueof 20TW forthe totalenergy onsumption. Theenergy ofSun that rea hesthesurfa eof Earthis about1000 W/m

2

asameanon ayearand in latitude. This poweristhat rea hesperpendi ularly thelandsurfa e. Hen e tomakeagood al ulationwe haveto onsidertheperiodi ityofnightanddayandthegeometryofEarth. Forthealternation of the daywedivide for 2. Forgeometry, theEarth at hestheSun lightasa ir le of

πR

2

areawhere

R

istheEarthradius,thenthispowerisspreadsonahalf-spheresurfa e(

2πR

2

). Thus the surfa e average of powerfor square meter is about 250W/m

2

. Obviously at the Equatoritrea hthemaximumandatpolewillbeaboutzero. Thus,forthe al ulation,ifwe divide thepowerrequested, 20 TW, by the energy ux of 0.25 kW/m

2

, we obtaina surfa e of 80000 km

2

. This means that to get enough energyfor theentire world we ould over a surfa eof80000km

2

(1/4ofsurfa eofItaly)withsolar ellsof100%ofe ien yorasurfa e of800000km

2

(lessthan1/10ofsurfa eofEuropeor1/6ofglobalurbanisedsurfa e[2℄)with solar ellsof10%ofe ien y.

Theamountofsurfa eisnotsurprisingly small. Howeverwearetalking abouttheenergy requestof entireworldandthesolutionisreasonablypossible. Of ourse,theSun energyhas

somebigproblems,forexampleu tuationsintime(duringthedayortheyear)and inhomo-geneityonthesurfa eoftheworld,but theabundan eofthiskindofenergygiveshopefora solution.

Therearemanywaysto takeenergyfromtheSun(see[3℄). Obviouslyallofthem involve the at hingof light,andabasi me hanismamongothersisthephotovoltai ee t.

Thisphenomenon isnormallyobservedinsemi ondu tors(see[4℄). Whenaphoton(with enoughenergy)isabsorbedbyasemi ondu tor,anele tronfrom thevalen eband isex ited to the ondu tion band,this produ esapairof hargeswithoppositesigns alledanex iton pair. Ifthisex itonpairispla edinanele tri eldthetwo harges,so reated, ansplitand followtheeld inoppositedire tions. Inthiswayitispossibletogenerateanele tri urrent. Usuallyagoodsemi ondu torforthisuseisSili ondopedwithelementsofthethirdandfth grouptomaintainapermanentele tri eld. Thismakesaso allednpjun tion thatallows thephotovoltai event,asdes ribedbefore.

The photovoltai panels made of doped Sili on are rather expensive, be ause they need rystalline Si whi h is very expensive to produ e. The e ien y of these panels is not so ex iting,the ommer ialversionrea hesalmost17%(whiletheestimatedtheoreti almaximum is 34%, Sho kley-Queisser limit, see [5℄) and at the end of their life y le they should be disposed inaproperwayifthey ontaintoxi elementssu hasArseni . Thesereasonsindu e thes ienti ommunitytolookforotherwaysto at henergyfromtheSun.

Inthelasttwentyyearsalotofnewsystemsto onvertsolarlightinele tri ityhavebeen developed. Intheeldofsemi ondu torpanels,improvedwiththemultijun tionte hnique,the e ien yhasin reasedupto40%;biodegradableorgani semi ondu torshavebeenintrodu ed [6, 7℄. Re ent work on entrateson miming thephotosynthesis, using some hromophore to absorblightandthen onvertingthesolarenergyin hemi alenergy[8℄. The on eptofDye SensitizedSolarCells(DSSCs)belongstothiskindofapproa hes.

Dye SensitizedSolar Cells: me hanism.

Dye Sensitized Solar Cells (DSSCs) are the new hallenge in the sustainabledevelopment: takingenergyfromSunwitha heapande o-friendlyte hnology.

Sin e1991whenMi haelGrätzelbuilttherstsolar ellofthiskind [39℄,manyresear hers arespendingtheirtime andfor esto enhan eits e ien y. This kindofsolar ell onsistsof twotransparentele trodes(usuallymadeofITO,IndiumTin Oxide),one oated withathin lm of semi ondu torand theother with aplatinumlayer(see g. 2.6). Then the semi on-du tor issensitized withadyeadsorbedonitssurfa e. Theele trodes,sotreated,are pla ed fa e to fa e asa apa itor, at adistan eof about50

µ

m. Betweentheele trodesthere isan ele trolyte solution to allow the ondu tion (for example Iodine-Iodide in ACN). With this te hnique, when the dye absorbsaphoton an ex itonpair is generated. Theex iton pair is thenbrokenbytheinje tionofoneofthetwo hargesinthesemi ondu tor. Theother harge isthentransferredtotheele trolytethatbringsba kthedyetoitsinitialstate. Thenalstep is thedeliveryofthe hargenow arriedby theele trolyteto theotherele trode. Depending onwhetheranegativeorapositive hargeisinje tedwetalkaboutntype orptypeDye

SensitizedSolarCells, respe tively.

Nowwe anseewhi harethesinglepassagesof theme hanismofthiskindofsolar ells, namelythe ptypeDSSCs. Tobetter explainthestepsthese aredrawnin twodiagrams(g. 1.1), the rst showing the spatial path of the harges and the se ond the energeti s of the system(ea harrow orrespondsto anele trontransfer):

Figure1.1: Me hanismofaptypeDSSC.Thephotonisnotshown. Intheleft,thespatials heme; intherightthelevels heme.

1. Absorptionofaphoton. Whenthephoton(h

ν

)isabsorbedfromthedye(

D/D

−

ingure 1.1),thelatterisex itedtoa harge-separated-state(

D

∗

_/D

−

);usuallythisbehaviouris favouredwhenthedyemole ule ontainsadonorandana eptorgroup,thatwill host apositiveand anegative harge,respe tively,in theex ited state[7,9,10, 11℄.

2. Chargeinje tion. The harge(hole)isinje tedintothevalen ebandofthesemi ondu tor (ni keloxide)from the ex ited-stateof the dye. Inthe other ase, ntypeDSSCs, the hargeisanele tronandthepreferredsemi ondu toristitanium dioxide. Normallythe inje tionisaveryfastpro ess,onthepi ose ondtimes ale[12,13℄.

3. Regeneration of the dye. The redu ed dye is brought ba k to its initial state by the oxydized member of the ele trolyti ouple (

E/E

−

, ele tron arrier). This pro ess is limitedby diusion, then the hoi e ofan ele trolyti ouple ofgood dimension(small enough)and asolventof ni evis osity(lowenough) is ru ial foragood e ien y. As otherdiusionlimitedpro esses,theregenerationofthedyetypi allyo ursonthes ale ofnano-mi rose onds[14,15℄.

4. E-transferto theele trode. The arriertransfers theele tron at the ounter-ele trode losingthe ir uit;

5. Re ombination. Thiskindofpro essesisthemainlimitingfa torthatnegativelyae ts thee ien y [16, 17, 18, 19℄ . It is notreferable to asingle phenomenon but it ould

be dened as any way to re ombine the hole and the ele tron after their separation. Therearethreemainwaystomakeare ombination: rst,thenegative hargeinthedye re ombineswiththeholeinthesemi ondu torbeforebeingtransferredtotheele trolyte; se ond,theholeinthesemi ondu torre ombineswiththeredu edele trolyteinsolution; third,theholeintheele trode(ITO)re ombineswiththeredu edele trolyteinsolution.

E ien y

What wewantisto in reasethee ien yof SolarCells, whi h meansin reasingthe urrent andthevoltagebetweentheele trodes. Thetotale ien yofasolar ellisgivenby:

η =

P

max

out

P

sun

≃

V

OC

× I

SC

P

sun

where

V

OC

istheopen ir uitvoltageand

I

SC

istheintensityof urrentatshort ir uit. The dieren ebetweentheRedOxpotentialsoftheele trolyte oupleandthevalen ebandofthe semi ondu torgivestheopen ir uitvoltage(

V

OC

). The urrentdependsonabalan eofthe kineti pro essesof hargeseparationand annihilation. We havefew hoi es for the RedOx ouple, witha limitedrangeof

V

OC

, so wefo us onin reasing the urrent. Parti ularlywe wanttomaketheregeneration(pro ess3ing. 1.1)fasterandthere ombination(pro ess5) slower. Inthissensealotofresear hhasbeenmade(seereviews[20,21,22℄).

Theregenerationpro essis limitedby diusion. Whenthe dyeis in theredu ed stateit waitsfortheOx-formof theRedOx oupleto beregenerated. Thiseventismoreprobableif thediusion ofthe ele trolyteis fast,i.e. ifthesolventvis osityislow andthesize andthe mass of the ele trolyte are small. Obviously the Ox-form of the ele trolytemust be agood oxidantfortheredu edstateofthedye.

There ombinationmaydependondiusionornot,be ause theele trondonormaybethe dyeortheele trolyte,andonlyin thelatter asethediusionisneeded. Ifthere ombination involvesthe dyethe pro ess is slowerwhen the harge (negativein our ase)is stabilized in someway,forexampleifthedyemole ule ontainsanele trona eptor. Moreoverthe re om-bination rate isa fun tion of thedistan e betweenthesemi ondu tor surfa eand the group wheretheextraele tronofthedyeis lo alized: thelargerthedistan e, theslowerthe re om-bination [23, 24℄. In the ase of re ombination from the ele trolyte a fast diusion towards thesurfa ea eleratesthepro ess. A waytosolvethisproblemisto forbidtheele trolyteto rea hthesurfa e[14,15,25,26℄. Ifthegapsbetweendyemole ulesaresmallenough,this an be doneby using alarge ele trolyte (as we madewith our Co omplexes, see se tion3.1.4). These onsiderationsarevalidforboth(pandn)typesofDSSC.

Fromathermodynami pointofviewthemaximume ien y isrea hedwhentheenergy ofphotonistotally onvertedintoele tri energy. Thankto thisobservation we anestimate thewavelengthoflighttohaveamaximume ien ystartingfromtheteoreti alV

teor

oc

1 . Ina hypoteti alsystemareasonablevalueof V

teor

oc

is800mV. An ele tronof that ellhasenergy 1

V

teor

even at 0.8 eV that orrespondsto light of sameenergyof wavelength 1550nm (

E =

h

c\λ

). Inthissystem,aninje tionprodu edbyaphotonofshorterwavelengthwilllosetheex essof lightenergy(inheat).

However,buildinga ellwithaperfe tequivalen e ofenergygap betweenV

teor

oc

and light ouldprodu eaveryslowinje tionbe ausetheex itedstatewouldhavenoinje tiondriving for e[27℄. Thebalan eofthese requirementsis ru ial inasolar ell.

Consequentlyevery ellbuiltwith adyeabsorbinginawiderangeofspe trumwilllose a ertainamountof energy. Apartialsolutiontothisproblemarethetandem ells,[28℄.

Atandemsolar ellis aDSSCbuiltwith bothof oxidetypes,ntypeandptype. Inthis kindofdevi ethePtele trodeissubstitutedwithafurthersensitizedele trodeofpropersign

2 . Thissettinggivethepossibilitytoin reasethee ien ysin ethespe tralabsorbedregion ouldbedistributed betweenthetwodyes. Inotherwords,thesettingofea hsideoftandem ell ouldbeoptimizedforamoretightanddierentspe tralregion,asdepi tedin gure1.2.

Figure 1.2: Tandem DSSC.We represented the entering light as two omponents, blue and red; thersten ounteredntypeabsorbsonlytheredportion,thentheptypeabsorbtheremainingblue portion.

Inthiskindof elltheV

oc

isduetothedieren ebetweentheRedOxpotentialsof semi on-du tor bands, then ethe ele trolyte oupleworksonly asele tron arrier. Consequentlythe frequen yof hargeonpsideanddis hargeonnsideshouldbethesame. Inother wordsto haveagoodtandem ellthee ien yofthe ellsmustbe omparable. OnntypeDSSCalot ofresear hwasmadeandthemaximume ien yrea hesabout12%[29℄. Inotherwords,for ptypethedevelopingisabit slowerandthepresentbest e ien yisaround7%. Improving thee ien yofptypetobuildagoodtandem ellisthemainreasontomakeresear hinthis eld.

2

Ruthenium omplexes

IntheDSSCresear htheRuthenium(II) omplexes playanimportantrole[30℄, [31℄. Then typesolar ellwiththebeste ien yhasbeenmadewithRu-C104(a omplexofRuthenium linkedtoabipyridine,athiopheneandathio yanate,[29℄). Theimportan eofRu omplexes isduetoitsni eRedOxproperties,andtoitslonglivedex itedstatethatallowstheinje tion pro ess,inparti ularwitharomati aminesasligands. Forourtypeofsolar ellsweneedadye that produ esa harge separatedstatewhen is ex ited,whi h is essentialforthe working of thephotovoltai ee t. Inthe aseofRu- omplexesthisismadebya Metal-to-Ligand-Charge-Transfer(MLTC) transition,[30℄, [32℄. Inthis transitiononeele tronis transferredfrom the

d

orbitalofthemetalto theorbitalsoftheligand. TheRuthenium bipyridine omplexes(g. 1.3)haveaMLCTabsorptionband inthevisibleregionofthespe trum, orrespondingtothe transferofanele trontoabipyridineligand.

InaDSSCthe hargeseparationshouldbefollowedbytheinje tionin thesemi ondu tor, therefore the harge-separatedstateshould beredu ent enough(oroxidantin the ase ofp typeDSSCs). TheRedOxpotentialoftheex ited state anbeestimated onthebasisofthe RedOxpotentialsofthedyeandoftheenergyofex itedstate,andmustbe omparedwiththe RedOx potentialof the semi ondu tor[22℄(weshall explain this model in se tion 3.1.2). In general,thedrivingfor eforinje tioninntypeandptypeDSSCswithRuthenium omplexes islargeenough. UntilthismomentRu-dyeshavebeenusedonlywiththentypeDSSCsand the inje tion has been found quite e ient [29℄. Often in ntype dyes one or moreligands with ele trondonorgrouptakepla e tobipyridineligand. Thisis madetobring thepositive hargefarfromthesurfa etoenhan eitsre ombinationinertiawhenthedyeisoxidized(after inje tion)[21℄.

InntypeDSSCswithRuthenium omplexes, ifthephotoindu ed hargeseparationputs anegative harge onthebipyridinethat is linked to thesemi ondu tor, theinje tion anbe veryfastandthepositive harge anbetransferred toanele trondonorligandinafollowing step(seeg. 1.3).

Figure 1.3: Ru(bipy ) inje tion. The arrows show the ele tron tharfer that o ur in the follow rea tion.

Thepro essesinvolvedherehavebeendeeplyexaminedintheoreti alandexperimentalway [33,34,35,36,37℄. Alotofquantum al ulationshavebeenmadetounderstandthenatureof

inje tion. In many worksthe main fa torforane ientinje tionis theoverlapbetweenthe linkinggrouporbitalsandthe ondu tionbandandthepresen eofalinkinggroup omponent on theLUMOs. This hasbeen onrmedbytheexperimentthat showsafaster inje tionfor dyesthatrespe tstheformer hara teristi s.

Another interesting step for understanding the me hanism of inje tion was given by De Angelisgroup[38℄. The ombinationofRu-dyeorbitalsandsemi ondu tororbitalswas inves-tigated fordierent protonationof arboxyli a idgroup. Whenthe Ru-dyeis ableto make three bonds with the semi ondu tor, anew LUMO of the system ompares and has a om-ponent into the semi ondu tor. The apa ityto make three bonds in rease the e ien y of aboutoneorderinmagnitude. This onrmstheimportan eofagoodoverlapamongthedye andsemi ondu tororbitals.

For ptype DSSCs theme hanismof ex itation, inje tion and de ay to the groundstate of the redu edRu dye hasneverbeen examinedin de tail. It is reasonableto think that in thepresen eof anele trona eptorligand, inthe hargeseparatedstate,thenegativeoneis lo atedontheele trona eptinggroup(push-pull ee t), andthepositiveoneshould have enough energy to be inje ted into the semi ondu tor passingthrough the bipyridine linking ligand. P-typeDSSC with Ru-dyewith noa eptingligand were alreadybuilt, then e there is the proof that inje tion ano ur. As already said adyewith agroup that stabilize and keepawayfrom thesurfa ethe ex eeding harge in reasethe e ien y. This on ept isnot demonstrateforRu-dyeinptypeDSSC.So,ourstepwouldbetheevaluationoftheproperty ofaRu-dyewithalonga eptingligand.

To this aim we hose a bipyridine- omplex of Ru with one ele tron a epting ligand to enhan e the harge separation. We hose a nitro-naphthalene-1,8-di arboximide (NMI) as ele trona eptinggroupandweatta hedittoaphenanthroline,theresultingele tron a ept-ingligandistheN-(1,10-phenanthroline)-4-nitronaphthalene-1,8-di arboximide, alled Phen-NMI (seeg. 1.4).

OurhypothesisisthattheMLTCtransition onsistsintransferringananele tronfromthe Ru orbitalstothedelo alizedNMI orbitals,that aremorestable thenthoseofbipirydine. In this waythenegative hargeispla edin amorestable position and theex ited stateshould be longerlived. Sin e we havepla edthe NMI a eptinggroupfar from thelinking groups, thenegative hargeoftheredu eddyewillbefarfrom thesurfa e. Boththe longlifetime of theex itedstateandthelargedistan eofthenegative hargefromthesurfa eshouldin rease thee ien yoftheDSSC.

The ompoundswehavesynthesizedareshowningure1.4: the[Ru(bipy)

2

(Phen-NMI)℄(PF

6

)

2

( alledRuB

2

PN)and[Ru(d b)

2

(Phen-NMI)℄(PF

6

)

2

( alledRuB

2

PN),wherebipyisthe 2,2'-bipyridineligandandd b isthe4,4'-(COOH)(2)-2,2'-bipyridineligand.

Onlythe mole uleRuB

2

PN is ableto sti k to aNiOsurfa e, thanksto the arboxyli groups. Toprovethat after inje tion theele tron ispla ed in theNMI it issimplerto work onasolutionofthedyeratherthanonthesurfa eadsorbeddye. Ourassumptionisthatthe essentialpropertiesarenotalteredsigni antlyuponabsorptiononNiO.Sin etheRuB

2

PN mole ulehadbeenalreadysynthesizedinasimpleway[41℄,whileRuB

2

PNwasnotdes ribed in the literature, we de idedto perform ourrst measurementson RuB

2

PN before looking

(a)RuB

2

PN (b)RuB

2

PN

Figure 1.4: Dyesused in this work. RuB

2

PN the Ru omplex with ana epting groupligand, Phen-NMI,andwithoutan horinggroups. RuB

2

PNtheRu omplexwitha eptinggroupligand andlinkinggroup.

forawaytosynthesizeRuB

2

PN.

Whatwedidisndingaspe tros opi proofthatthenegative hargeofredu edRuB

2

PN in solutionislo ated ontheele trona eptingligand. Then we he kedthat thesameholds for the dye adsorbed on a NiO surfa eand we measured the regenerationtime. Finally we measuredthee ien y oftheDSSCbuiltwith thisdye. Inallof theseexperimentswe om-paredourdyewithaRu omplexeswithoutanya eptinggroup: with[Ru(bipy)

3

℄(PF

6

)

2

for theexperimentsinsolutionand[Ru(d b)

3

℄(PF

6

)

2

forthose insurfa e.

Theexperimental work of this thesiswasperformed in the Departement of Photo hem-istryandMole ularS ien e atUppsalaUniversity(Sweden)in thegroupofProf. Leif Ham-marström(externalsupervisor),underthedire t areofJamesM.Gardnerforthespe tros opy partandJonathanFreysforthesynthesispart.

Experimental se tion and

te hniques

In this se tion we des ribe the synthesis pro edure, the assembly of the ell, and the other experimental te hniquesusedinthiswork.

2.1 Synthesis

The synthesis of the ele tron a epting ligand ontained in our dye is already des ribed in the literature[41℄. The authorsbuilt ann-typeDSSC andtheir dye ontainedaligandwith almost the same stru ture as ours, i.e. naphthalenimide linked to phenanthroline but with a donor substituent (piperidine) in pla e of the nitro group (we will all this other ligand as Phen-PMI). In that synthesis our nitro substituted ligand was an intermediate, so we followed the same pro edure ex ept for the last steps, that repla e -NO

2

with piperidine. We shallonly des ribethesynthesisof the[Ru(bipy)

2

(Phen-NMI)℄(PF

6

)

2

(RuB

2

PN) and [Ru(d b)

2

(Phen-NMI)℄(PF

6

)

2

(RuB

2

PN) omplexes, thatarenotfoundintheliterature.

2.1.1 Compounds sour e

All the ne essary ompounds wereordered from the atalogueof Sigma Aldri h (Sto kholm, Sweden). WeusedRutheniumtri hloridehydrate(with35-40% Ru),2,2'-bipyridine(wewill allthisasbipy )95%pure,4-Nitronaphthalene-1,8-di arboxyanhydride( allednnanhy )95%, 5-amino-1,10-phenanthroline( alledphenam)97%,a eti a id99%pure(newbottle),ethanol 99.5% pure, methanol(MeOH) HPLC grade 99.9%, a etonitrile (ACN)HPLC grade 99.9%, N,N-dimethylformamide 99.9% pure. Preparative olumn hromatography was arried out using sili agel (Mer kKieselgel,sili agel60,0.063-0.200mm). Weusedstandardglassware. Forrea tions ondu ted underMi roWavesweusedaBiotageInitiatorM.W.apparatus.

2.1.2 Synthesis of [Ru(bipy)

2

(Phen-NMI)℄(PF

6

)

2

InCastellano'swork[41℄theyprodu etheanalogue ompoundto[Ru(bipy)

2

(Phen-PMI)℄(PF

6

)

2

byreuxinginmethanolforthreehoursRu(bipy)

2

Cl

2

andPhen-PMIinstoi hiometri ratio. Then afterthepreparationofallthereagentsasdes ribedin theCastellano'swork wemixed Phen-NMI(64,2mg 0,15mmol)and Ru(bipy)

2

Cl

2

(58,9mg0,12mmol) in a100mLask in 50mLMeOHandweput itat stirredreuxfor8hours,asshowedin gure2.1.

Figure2.1: RuB

2

PNsynthesisrea tion.

Afterthe rea tionwe ooleddownthe systemand weadded 10mLof water andthen we ltered the solution. Wefurther added 10mL of water to thesupernatant fra tion and also a su ient amount of NH

4

PF

6

, so that a brown pre ipitated wasformed. At this point we made an HPLC-MS analysis of thesolid fra tion in ACN assolvent,but thepurity wasnot found satisfa tory. Forthis reasonwemadeaash olumn separationin gradientofpolarity of ourprodu t. We hose sili a-gel assolidphase be ause in theTLC it gaveagood result. Thus westarted with pureACN, then weadded somewater(ratio5.5:1 ACN:H

2

O v:v)and attheendweaddedKNO

3

(ratio0,83:1:0.005ACN:H

2

O:KNO

3

v:v:w). Afterevaporatingthe solventwesubmittedtoHPLC-MSthesamplesandtheresultsofthebestfra tion(thoseused su essively for spe tros opy)are reported in g. 2.2. As we ansee, there are twopeaks that show thesame mass417m/z. This massmat hes pretty well themole ular RuB

2

PN mass: m/z=417from MSandmass(RuB

2

PN)/2=417.5u. ThetwoLCpeaks anbedue to adierent onformation ofthe omplexorapresen eof adimer,howeverwesuppose that is thesame omplex.

Afterthe evaporationofthe solventwehad thesolid dyemixed with theKNO

3

salt. To separate these twosolids we dissolved thedyewith averylittleamountof ACN leavingthe nitrateonthebottombe auseitisnotsosolubleinACN.Atthispointweobtainedabout15 mgofsubstan e. Theyieldwasapproximately12%.

2.1.3 Synthesis of [Ru(d b)

2

(Phen-NMI)℄(PF

6

)

2

The synthesis of [Ru(d b)

2

(Phen-NMI)℄(PF

6

)

2

, i.e. RuB

2

PN, was not so simple asthe previous one. The dieren e between RuB

2

PN and RuB

2

PN is, obviously, the presen e in RuB

2

PN of the arboxyli groupsonthe bipy ligands. This hanges the onditionsof the addition rea tion of Phen-NMI on theRu omplex. The reuxed rea tion annot be madeinMeOHbe ausetheal ohol anrea twiththe arboxyli groupyieldinganester. We

Figure 2.2: Liquid Chromatogram of RuB

2

PNprodu t after puri ation; in the inset the Mass spe trum.

veriedthatforourdye,asin most ases,anesterisnotagoodan horinggroupfortheNiO surfa e. A rst solution to this problem is making the rea tionin ethylenegly ol at reux (about200

◦

C)foralongtimeunderinertatmosphere(Ar-purge). Unfortunately,su hdrasti onditions(hightemperature,longtime)leadtothedyedegradation. Infa tafterpuri ation we obtained a mixture of small mole ules or Ruthenium omplexes with toolow mole ular mass(HPLC-MSanalysis).

Wealso triedwater assolventbut verylongrea tiontimeswererequired, be ause ofthe lowsolubilityofthestartingmaterials. Hen etheresultafter longreuxingistheadditionof Phen-NMItoRu butalsothe ompletehydrolysisoftheimidebond.

Therefore welookedfor an other method to synthesise RuB

2

PN. Inliterature there is a pro edure to prepare Ru omplexes qui kly and with good yield using Mi ro Waves [42℄ (againfromCastellano). Thispro edureisalsosuitabletoprepare omplexeswith arboxyli groups. The authors also des ribe the preparation of a Ru(bipy )

2

(d b) (where d b is the 4,4'-di arboxylatebipyridine)startingwith Ru(bipy )

2

Cl

2

andd bin a eti a id/water(4:1). We assumed that thepresen e of the arboxyli groups onthe starting omplexrather than on the entering ligand would not hange the rea tionme hanism. So we adopted the same onditionsfortherea tionofRu(d b)

2

Cl

2

withPhen-NMI.

Pro edure

Following that pro edure we put in a 25 mL tube amixture of Ru(d b)

2

Cl

2

((200 mg, 0,3 mmol) and Phen-NMI(127 mg, 0,3mmol) in 20 mLof A eti a idand water (in ratio4:1 respe tively). Thesolutionlookeddark olouredandhomogeneous. Webubbledthatsolution withArgonfor10min. Thenweput thetubesopreparedintheM.W.apparatussettingthe polarityofthe solventat 3(high)in as aleof4,the temperatureat 100

◦

Candthe timeof rea tionat1,5hours

1

. S hemati allythisstepisshowedin passage1in g. 2.3. 1

Figure2.3: RuB

2

PNsynthesispro edure.

Therea tionprodu ts wereanalysed byHPLC-MS takingadropofsolutiondilutedwith 1 mL of ACN. The masses present in theHPLC-MS peaks didnot mat h with the mass of RuB

2

PN. Wefound amass at 561m/zand anotherat 621m/z insteadthe wantedmass was505m/z(the masof RuB

2

PNdividedby2,theRu harges).

Aftertohavedriedtheprodu tinrotavaporwemadeanNMRspe truminACN-deuterate. Thespe trumshowedthetypi alpeaksofRu-bypiridine omplexandthepeaksofthe Phen-NMI. Thepeaksshapewasnotsodened. This anbeduetothepresen eofadimerofthe Ru- omplex. In that aseit would notbeaproblem be ausea dimer is in equilibrium with relativemonomerthat ansti konthesurfa e.

Furthermore, the NMR spe trum showed also other peaks in the aliphati region that should notbepresentin aRuB

2

PNspe trum. Thehydrogen ountandthemultipli ityof thesignals anberelatedtoethyl groupspresen e,in parti ularsomeethylestergroups. An estimationoftheRuB

2

PNmasswiththeethyl-esteried arboxyli groupsgivetheresultof 561m/z( harge2+). Thismat hesperfe tlywiththatintheMSspe trummentionedbefore. Theotherfoundpeakat621m/z ouldbethedoublesubstituted,inotherwordstheRuwith oned bandtwoPhen-NMI(thatwewill allRuB PN

2

),stillwiththeesteried arboxyli groups.

Resuming, we had a mixture of our ompound and its double substitute with their ar-boxyli groupsesteried. Thiswasabigproblem,be ausewedidnotndanotherwaytoadd thePhen-NMItoRu(d b)

2

Cl

2

. Thus wefoundawayto hydrolysethepresentesters.

Hydrolysis

Thehydrolysisofanesterneedsa idorbasi atalysisto be ondu edinatimeofaday. As well-known the basi atalysed is faster then the a id one

2

. As already mentioned also the imidebond ansuerthehydrolysisprodu ingthedestru tionoftheligand. Somedataabout hydrolysisof theimide group an befound in literature[43℄. It is provedthat for the y li imides,likeours,thebased atalysedhydrolysiso urs atpH upperthan3,insteadtheygive the a id atalysed oneat pH lowerthan2units,in water. Thismeansthat between2and 3 pH units weshouldhaveagoodstabilityoftheimidebond. That levelofpH shouldbehigh enoughtopermitthehydrolysisoftheesters.

Wetried this way, step 2of s heme 2.3. Weput 1gof esteried ompound in a150ml askwith50mlofwaterandtherightamountofHydro hlori a idtopla etheentirebulkat pH 2,5(we hooseHClinsteadofabuersolutionbe ausewedidnotwantintrodu efurther salts). Thesystem wasreuxed in inert atmosphere. We he kedthe amount of hydrolysed esterandtheamountofeventualhydrolysedimidebyHPLC-MSon ourseanalysis. Wemade therstanalysishalfanhourafterthebeginningbutnodieren efrom thestartingmaterial wasnoted. The rst dieren e appeared after a day and the MS-spe trumis plotted in g. 2.5(a). TheMSdatashowagoodamountofmonohydrolysedoftheRuB

2

PN,547=561-14 (half mass ofethylradi al), and alittleamountof monohydrolysedof RuB PN

2

, however noprodu toftheimidehydrolysisisnoted. Thismeansthattherea tionofhydrolysisofthe estersisslowbutsu ientlyfasterthanthat forimides.

Ifwe onsiderthestatisti softherea tionandthefa tthatweneedtwo arboxyli groups into abipyridine topermitthelinking ofthedyeonthesurfa ewemustwaitalot oftimeto haveagoodamountofopportunelyhydrolysed ompound. Inotherwordswewouldbesureto havetheentire ompound ableto link inthesurfa eonlywhenea hmole ulewillhavethree hydrolysedgroups(be ausewith threehydrolysedgroupsthere isat leastone d bgroupfor ea h RuB

2

PN mole ule). Thetime to havethe entire produ tin that statewastoolong, then wede idedto stopthe rea tionat themomenttohavethemaximumamountofdouble hydrolysedandtheminimumofthemonohydrolysed ompound. Inthisway,attheendofthe rea tion,weshould haveaboutthehalf of theprodu t abletolink on thesurfa e. However, sin etheamountofthe ompoundthatdoesnotsti konsurfa ewill bewashedout(seese . 2.2)wehaveonthesurfa eonlythe ompound withoutimpurities.

Then,afteralmostaweekofreuxina idwaterwe olle tedthesample. Atthispointwe hadamixtureof RuB

2

PNandRuB PN

2

almostfull-hydrolysed. Weneededtohavethem separatedbe ausetheirsolar ells ould havedierentproperties. Thesetwo ompounds an have dierentsolubilityin waterbe ausethey haveadierentamountof arboxyli groups, in parti ular 4groups (in the best ase) for RuB

2

PN and 2 groups(in the best ase)for RuB PN

2

. ThereforeweaddedanamountofNaOHto deprotonatethe arboxyli groupto in rease thedieren e of solubility. The resultwasabrown solutionwith apre ipitate. We ltered thesolutionandonthe supernatantpart(with moreRuB

2

PN) we putan amount

2

Thisbe ause,inthebasi atalysis,theatta hoftheesterisdire tlymadewiththe base,insteadinthe otherwayismadeatthea id-a tivateesterbyaweakbaselikewater.

Figure 2.4: LiquidChromatogramof RuB

2

PNafterhydrolysis.

(a)1day,inset:isotopeproleof561m/zpeak (b)3days ( ) 1week(nalprod.)

Figure 2.5: Mass spe traof RuB

2

PN at various times duringthe hydrolysis, 2.5(a) 1day after start,2.5(b)3daysafter,2.5( )1weekafter(thenalprodu t).

ofNH

4

PF

6

toex hangethe ounterionsandpre ipitateRuB

2

PN. Thisamountofprodu t waslteredand washed with hilledwater andthen analysed at HPLC-MS.The resultis in g. 2.5( ),that showsthe greaterquantity of RuB

2

PN with respe t to RuB PN

2

and a su ient amountof hydrolysedgroups. The nal yield isnot sohigh, about 10%(30mg of nal produ t). NMR data Phen-NMI

1

HNMR(CDCl

3

,400MHz,298K):

δ

,9.25(m,2H),8.95(d,1H),8.80(t,2H),8.47(d,1H),8.29 (dd, 1H),7.97(m,2H) 8.12(m, 2H),7.88(s,1H),7.70(dd, 1H),7.58(dd,1H). RuB

2

PN RuB

2

PN

1

H NMR (DMSO,400 MHz,298K):

δ

, 9.10(m, 2H),8.80(m, 7H),8.50(m, 5H),8.29 (dd, 1H),7.97(m, 6H)8.12(m,2H),7.91(s,1H),7.60(dd, 2H).

2.2 Devi e assembly

Thedyewaslinkedonsurfa eandpla edinadevi ebyastandardmethodfortheDSSCs,of whi hwegiveashort des ription.

Thedevi ewas omposed bytwotransparentele trodesmadeofIndiumTinOxide(ITO) orFluorine-dopedTinOxide(FTO),15mm*30mm,pla edfa e-to-fa easina apa itorwith adistan eofabout50

µ

m. As hemati pi tureofthedevi eisplotted ing. 2.6.

Figure2.6: S hemeofdevi eassembling.

AmesoporouslmofNiOwasdeposedononeoftheseele trodes,bythestandardmethod for the solar ells [44℄. The Sol-Gelsolution

3

of Ni wasspread on the ondu ting ele trode (FTO) masked with adhesive tape. After 5 min the ele trode was sintered in a furna e at 440

◦

Cfor35minutes. Onthe ounterele trodetherewasdeposed aplatinumlmtopermit thedis hargingoftheRedOx ouple. Further,thisele trodeboretwoholestoenablethelling oftheRedOx ouplesolution.

ThedyewaslinkedontheNiO surfa ebyastandardmethod forGrätzel ells. The ele -trode withthe NiO lmwasdipped in asaturatedsolution ofdyein ethanolfor about 16h. Only in the aseof Ru(d b) we hanged the pro edure. We made asolution of dye with a mixture of ethanol and water 4:1and wekepttheele trode in the solutionfor2 days. This dierentwaywasadopted sin etheinertiaof RuB tobelinkedonNiO.

Thesomadeele trodeswereassembledbyinterposingbetweenthemaplasti foil(Surlyn) 3

Sol-Gelpro edure: 1gNiCl

2

anhydrideand1gF108polymerweredissolvedinamixtureof3gofdeionized waterand6gof99.5%ofethanol. Theresultinggreenvis oussolutionwassoni atedovernight. Themixture was entrifugedfor3hourstosedimentoutundissolvedNiCl

2

andamorphousNiO.After entrifugation,the solutionwasseparatedintothreelayers,whi hwere olle tedasdierentbat hes.

withasquarehole 4

ofexa tly oin idingwiththeNiOlm(seethepi ture2.6). Theassembling was ompletedbyheatandpressure. Thesomadedevi ePt.ele trode/foil/NiO.ele trodewas put onaheaterplate (150

◦

C)withthefa eof thePtele trodeonthehotplate,andaslight pressurewasmanuallyapplied. Thedevi ewaskeptontheplateuntiltheplasti foilistotally transparent,but notmorethan 30se onds to avoiddamagingthe dye. Afterthis operation, thedistan ebetweenthetwoele trodesturnedouttobe

∼ 50µ

m.

AtthispointthesolutionoftheRedOx ouplewasspilledintothedevi e, arefullyexpelling all theairfrom the ell. Theholeswere losed with anotherplasti foil and apie e of over glassformi ros opy,atta hingea hotherbyheatingwithasolderingiron.

2.3 Spe tros opi te hniques

In this work we used some standard te hniques of physi al hemistry, su h as NMR, UV-Vis spe tros opy, LC-MS and Cy li -Voltammetry. Weshall notgivea des ription of these te hniques,be ausetheyarewell-knownandof ommonuseinresear h.

Wealsomadeamassiveuseoftwolessstandardte hniques: Laser-Flash-Photolysis(LFP) andthespe tro-ele tro- hemistry(spe e hem). These twote hniquesarenotso ommonand they are seldomtreated in degree ourses. Neverthelessto omprehend

5

thedata showed in thisworkweneedtointrodu esomedetailsaboutthete hniques.

2.3.1 LASER-ash-photolysis

Introdu tion

Generally the term photolysis means the breaking, or more generally a hemi al transfor-mation, of a substan e through light absorption. By ash-photolysis one indi ates a time resolved spe tros opi experiment, whereby the pro ess is initiated by a short ash of light [45℄. When both the initial ex itation and the dete tion of the transients are operated by LASERpulses, wetalkaboutLASER-ash-photolysis

Usually in a spe tros opi experiment the absorption of a substan e is measured. This means measuring the ux of a beam of light before and after going through asample. The uxesarestationaryandthemeasurementisnottime-resolved,i.e. it orrespondstoanaverage overa verylong time if ompared with the mole ular response. Basi ally, one measures the rate ofphotonabsorption, i.e. the numberofmole ules that makeatransition to anex ited stateperunittime. Usuallyanex itedmole ulegoesba ktothegroundstateinaveryshort time, of theorder of nanose ondsoreven less(the ex ited statelifetime). Therefore when a samplestarts toabsorblight,afterseverallifetimestheamountofmole ulesin ex itedstate and in ground state will be onstant be ause the ex itation and de-ex itation rates will be equal,andasteady-state isestablished.

4

18mm*9mmindevi eforspe tros opy,6mm*6mminrealsolar elldevi e. 5

Toquoteoneofmybesttea hersofinstrumental hemistry:You an'tevaluatewhatyoumeasuredifyou don'tknowhowyoumadethemeasurement.

In a hypotheti al measurement where the ux of light is measured instantaneously the absorptionwouldbeafun tionoftime,in therstpla ebe ausetheinitialpopulationof ab-sorbingmole ulesinthegroundstateislargerthanthesteady-stateone. Moreover,amole ule in an ex ited stateis also able to absorb light, but with a dierentwavelength dependen e, andit analsoemitphotons,byspontaneousorstimulatedemission. Alsothe ontributionof theex itedstatestotheabsorption/emissionspe trumdoes hangeintime. This ontribution isalsopresentinsteady-statespe tra,butusuallyitisnegligiblebe auseofthe omparatively small populationoftheex itedstates.

Atimeresolvedspe tros opi measurementaimsatmonitoringtheex itedstatedynami s and de ay. This an be made via absorption measurement and via emission measurement. However the latter is simpler be ause require less instrumental requirements (no light for absorption measurement). It is lear that the output of this kind of measure is fun tion of absorbed light therefore, prin ipally to the power of ex iting light. This is one reason why this timeresolved measurement usethe LASER assour eof light. Howeverin the real te hniquethemeasuredlightis notthat fromtheex iting light,this be ause isverydi ult nd a light dete tor that at h very little absorption dieren e in a huge amount of light su hthat ofex itation. Thustheused ongurationforthis kindofexperimentisthat alled pump&probe.

Inapump&probe te hniqueweex ite(pump in the ex itedlevel)themole ules with a pulsed beam of LASER lightand then wemeasure (probe) thedieren e in absorption by another pulsed beamthat is orthogonal to the rst one, oronly the emission of the sample (without probelight). Inthis wayweusethepowerofaLASERto ex itealargefra tion of mole ulesandwepreservethedete torfromthebeamsho k. Usuallyatunableash-LASER isemployed. Tunablemeansthatthewavelengthoftheprodu edlight anbemodied.

A ash-LASERhas apeakpowermu h higher thana ontinuous oneand also be ause with aash-LASER wehaveapulse ofex itation thatprodu eaquitepre isestartingpoint fortheprobemeasure.

Inthefollowingwedes ribethevarious ontributionstothetransientspe trum. Takingthe totalpopulationofmole ulesnormalizedto1,whentheLASERpulseex itesafra tion

f

ofthe mole ularsample,thepopulationofthegroundstatede reasesto

1 − f

. Therefore,if

A

eq

(λ, t)

is the absorption of the sample at equilibrium (no pump), after ex itation the ontribution

f A

eq

(λ, t)

will be missing. In its pla e, we shall nd the ontribution

f A

exc

(λ, t)

, whi h is the absorption of the ex ited states, minus the orresponding stimulated emission (whi h is negligibleinour onditions). Theintensity olle tedbythedete torin theabsen eofapump pulse is

I

eq

(λ, t) = I

probe

(λ, t) − A

eq

(λ, t)

where

I

probe

(λ, t)

istheprobebeamintensity. AfteraLASERpulse, thedete torwill see theintensity

I

exc

(λ, t) = I

probe

(λ, t) − (1 − f )A

eq

(λ, t) − f A

exc

(λ, t) + I

L

(λ, t)

dieren ebetweentheno-pumpandthepump ase,weget

I

eq

(λ, t) − I

exc

(λ, t) = f [A

exc

(λ, t) − A

eq

(λ, t)] − I

L

(λ, t)

TheTransientDierentialAbsorptionis denedas

∆

abs

= f [A

exc

(λ, t) − A

eq

(λ, t)]

Figure 2.7: TransientAbsorptiondieren e. Leftred plot: the absorptionof groundstatewithout ex itation. Leftbla kplot:dashedline,theresidualabsorptionofgroundstateafterex itation;lled line,theabs. ofex itedstate. Rightplot:thedieren eabsorption,

∆

abs

.

Figure 2.7 illustrates the ontributions

A

eq

(λ, t)

,

(1 − f )A

eq

(λ, t)

(the residual absorp-tionof thegroundstateafter pump),and

f A

exc

(λ, t)]

(the ontributionoftheex itedstate).

∆

abs

(λ, t)

anbe evaluated by three measurements, respe tivelyperformedwith probeonly,

withpump&probe,andwith pumponly:

∆

abs

(λ, t) = I

eq

(λ, t) − I

exc

(λ, t) + I

L

(λ, t)

Inother words, thesimpledieren e

I

eq

(λ, t) − I

exc

(λ, t)

mustbe orre ted forthe lumi-nes en e ontribution

I

L

(λ, t)

, to getthepure absorption transient

∆

abs

, asshownin Figure 2.8.

Figure 2.8: Pro edurefor uores en e orre tion. Left:

∆

abs

withoutuores en e orre tion (red, blea hing; green, uores en e; blue, ex . absorption spe trum). Center: measurementof Transient Fluores en e. Right:

∆

abs

withaddition( orre tion)ofFluores en e.

one anmeasureabsorptionandemissionspe tra,i.e. the

∆

abs

(λ, t)

and

I

L

(λ, t)

quantities at xed

t

, or the kineti tra es, i.e. the samequantities at xed wavelength as fun tions of time.

Dete ted mole ular pro esses

When the a mole ule absorbslight it is promoted to an ex ited state. If the light belongs to the UV-Visspe trumtheinvolved transitionis ele troni . At roomtemperature onlythe groundele troni stateisnormallypopulated. Hen ethetransitionleadsthemole ulefromthe groundtotherstorahigherex itedstate. A ordingto Kasha'srule,spontaneousemission and photo hemi alrea tionsinvolvetherstex ited stateofthe samespin-symmetryasthe ground state, be ause higher states de ays to the rst one in averyshort (sub-pi ose ond) times ale.

Our te hnique explores a range of times going from nanose onds to millise onds, hen e the pro esseswe monitororiginatefrom therst ex ited state. Basi allysu h pro essesare: InternalConversion(de aytothegroundstatewithoutemission,thermalization);Fluores en e (de aywithemissionoflight);Inter-System-Crossing(spin- hangingradiationlesstransitions), andpossiblyPhosphores en e;rea tionsinvolvingtheex itedstates.

The instrumentation

Therearefourkindsofmeasurementsthat anbedonewithourLFPapparatus. Theydepend onwhi ha tionismadeafterlaserex itation. Infa t,we anmeasuretheTransientAbsorption spe trum,theTransientFluores en espe trum,theAbsorptionKineti sandtheFluores en e Kineti s.

How to distinguish between Dierential Absorption and Fluores en e has been already laried. The dieren e between Transientspe trum and the Kineti s is the way to olle t thesignal. Inthespe trummodewe olle tthewholeDierentialAbsorptionorFluores en e spe trum at xed delay time, while in the kineti mode we olle t the signal of a spe i wavelength ontinuouslyin arangeoftime.

Our apparatus for LFP is a Quanta-Rayfrom Edimburg Instruments. A s heme of the instrumentisreporteding. ,althoughitisaratherstandardLFPapparatus.

Figure 2.9: LASER-ash-Photolysiss heme.

After thatwehavethemono hromati lightatthewanted wavelength(inour ase480nm). This light isused forex itation and enters into the sample hamber(see g. 2.9)thanks to ashutterthat opensforfewmillise onds. Thenthelightgoesthroughthepolarizerandnext throughthesample.

Inthe ase of DierentialAbsorption, or

∆

abs

, we needof a light sour e to measure the absorption, the probelight. Normallyto this aim weuse apulsedXe ar -lamp(the pulse is longabout100

µ

s). Thislampprodu esaveryhigh intensitywhitespe trum. Thelightfrom Xe lamp is ontrolled by a further shutter to. Both the shutters, of the LASER and of the Xe-lamp, workstopreservethesamplefromtheex essivelightabsorption. After rossingthe sampletheprobelight,togetherwiththeemitteduores en e,goesthroughamono hromator andtheismeasuredbyadete tor. Thesameholdswhenuores en ealoneismeasured. Ifwe want to makeTransient spe trum measurements,weuse asdete tor aCCD ( harge- oupled devi e) amera,whileforKineti smeasurementsweuseaphotomultipliertube(PMT).

ns-LASER

WedonotgiveaexplanationofLASERtheorybutonlyalittledes riptionofhowtheLASER lightis used. Basi allyaLASER(LightAmpli ation byStimulatedEmissionofRadiation) isamono hromati ,time-spa e oherent,highpoweredsour eoflight. Itisduetostimulated emissionfrom ahighlypopulatedex itedstatedueto aninversionofpopulation.

The oreofaLASERisaresonator. Theresonatoris omposedbytwomirrors(withxed ree tingproperties)pla edoneinfronttheother. Inthemiddlethereisamediumlledwith appropriatemole ulesfortheworkingoftheLASER.Usuallytheinversionofstatepopulations is produ ed with a ash lamp. Whenthe mole ules are ex ited they start to emit in every dire tion. The photonsthat are ree ted between the two mirrors resonate in the so alled  avity. The rossinglightprodu esanampli ationof intensityby stimulated emission. In the ontinuousLASERs oneof thetwomirrorsis totallyree ting andtheother is partially ree ting, thus apartoftheampliedlightpass ontinuouslythroughonesideof the avity. In this way the populationwill be never totally inverted be ause a fra tion is ontinuously emitting. In pulsed lasers the two mirrors are both totally ree ting and an almost total populationinversionis indu ed(almost be ause there are alwayssmall losses). Then oneof the two mirror is qui kly taken out (ele troni ally, it being not a simple mirror) and the LASERemission ano ur. Herethepeakpowerisveryhigh, inoursystemit anrea h4W in light-spotofabout0.5 m

2

.

Frequen ytuning

The light produ ed by LASER is very mono hromati . This is a quality be ause it is di- ult produ e alightsopure and sostrong but it is alsoa problem whenoneneeds dierent wavelengths.

The ommon way to manipulate the frequen y of LASER light is by non linear opti s. Whenaele tromagneti wavepassesthroughamediumit ausesanos illationofpositiveand negative hargesinoppositedire tions,andthereforea ertaindipolemoment

∆µ

inavolume

toele tri eld,

P = χ·

E.

χ

istheele tri sus eptibility,apropertyofthematerial. Hen ethis phenomenonisin ludedintheMaxwell'sequationforthewavesandleadstorefra tiveindex:

∇

2

E

−

1

c

2

·

∂

2

∂

2

_t

P

= 0 =⇒ ∇

2

E

−

n

2

c

2

·

∂

2

∂

2

_t

E

= 0

Where

χ

1/2

be ames

n

therefra tiveindex.

Thelinear relationbetween Pand E is only an approximation, in fa t when the ele tri eldisverystrongthisapproximationfails. Thedependen eof

P

on

E

anbemorea urately representedasaTaylor'sseries:

P

nl

= χ

1

E

+ χ

2

E

2

_{+ χ}

3

E

3

_{+ . . .}

Again,iftheeld isnottoolarge,we antrun atetheseries. Ifwestopatse ond order, Maxwell'sequationbe omes:

∇

2

E

−

1

c

2

·

∂

2

∂

2

_t

P

nl

= 0 =⇒ ∇

2

E

−

n

2

c

2

·

∂

2

∂

2

_t

E

=

χ

2

c

2

·

∂

2

∂

2

_t

E

2

The last term

χ

2

c

2

·

∂

2

∂

2

_t

E

2

it also alled wave mixing be ause it is at the origin of the frequen ymanipulation. Infa t ifwesupposethatE is omposedbytwofrequen ies

ω

1

and

ω

2

we ouldwrite: E

(t) =

E

1

e

−iω

1

t

₊

E

2

e

−iω

2

t

_{+ compl.conj.}

andtherefore E

2

(t) =

E

2

1

e

−i2ω

1

t

+

E

2

2

e

−i2ω

2

t

+ 2

E

1

E

2

e

−i(ω

1

+ω

2

)t

_{+ 2}

E

1

E

∗

2

e

−i(ω

1

−ω

2

)t

+ 2

E

1

+ 2

E

2

Thisgeneratesasetofnewfrequen ies,orwavelengths:

ˆ The term E

2

1

e

−i2ω

1

t

auses the Se ond harmoni generation and for example in our Nd:YAGLASERprodu esa532nmwavefromthe1064nmfundamental wave;

ˆ The terms

2

E

1

E

2

e

−i(ω

1

±ω

2

)t

ause the Sum/dieren e frequen y generation,that is themostimportantbe auseitisusedintheOPOsystem(optoparametri os illator)to tunethewantedwavelength;

ˆ

2

E

1

+ 2

E

2

thisis alledOpti alre ti ation anditisnotusedinoursystem. Inour systemthe 1064 nmlight produ ed by the Nd:YAG LASER generatesthe se ond harmoni at532nm, andthese twofrequen iessum uptogenerate the355nmwave. These three wavesareusedin anOpto-Parametri -Os illatorsystem(OPO)to generatethewanted wavelengthlight. Oursystemisabletoprodu eamono hromati lightofanywavelengthfrom 410nmto 780nm. Thenon-linearopti almaterialused tomakethewavemanipulationis a parti ularkindof rystals. Thegeneratedlightbeamstravelindierentdire tions, so anbe

Theproblem of this method are thelosses. Everynon linear opti passagede reasesthe power.Thepowerredu ingitisnotproportionaltotheinitialpowerbut,aslogi ,itissquared proportional. Thusthenalpowervariesfrom50mWto300mWifthestartingpower hanges from 3Wto 4Wrespe tively.

Dete tors

Before starting to talk aboutspe trum interpretation, we want to saysomething about the used dete tors. Thedete tors are themostimportant partofthe system,through whi h we getalltheinformation. Hen e itisimportantto understandhowtheywork.

Asalreadysaid wemakeuse oftwodete tors, theCCD ameraand thePMT.TheCCD ameraisusedforregisteringaspe truminaveryfastway. It anbeseenasa2Dplatewith anarrayofdete tinglightpixels. Thelightfromthesample(for

∆

-absorptionoruores en e) isnotmono hromati . Beforerea hingthe ameraithitsagratingthat splitsthewaveswith dierentwavelength. Thewavessoseparatedgotothepixelplateofthe ameratoberevealed. Inthiswayin thesamemomentwe olle ttheentirespe trum.

ThePMTworksdierently. It ountsphotonswithhighersensibilityandforawiderange oftimes(fromnstos),butitis(almost)insensitivetowavelength. Herethelightissele tedby agrating-mono hromatorbeforerea hingthePMT.Withthisdete torthequalityofmeasure dependsontwofa tors,thee ien yofthegratingandthee ien yofPMT-photo athode.

Thequalityof themeasurementsdepends onthee ien y ofthegrating,whi his shown in g. 2.10. The e ien y is dened asthe ratio betweenthe ux of light s attered by the gratingandin identone,anddependsonthewavelength.

Figure 2.10: Gratinge ien yforCCD amera. 150gmm,blazedat500nm.

Thee ien y prole shows how the CCD-gratingworks well in theVis rangebut has a lowe ien y under 400 nmand over800 nm. Hen e in our measure we haveto ount this behaviourevaluating withtherightimportan edatainthoseregionofspe trum.

Inour system we havethree gratings. Oneis optimized for the CCD ameraand works quitegoodin awidespe tralregion,g. 2.10. Twomoregratingsarein themono hromator forthePMT2.11. Oneoftheseisadequateforshortwavelengths(200-600nm),theotherfor thelongones (600-1200nm). Overall,wehaveagoodgratingperforman einall ases.

The other fa tor ae ting the a ura y is the e ien y of photo athode. This part of the PMT is a athode generally made with alkali metals (one or a mixture of them). By

(a) Grating e . for visible range. 1800gmm, blazedat500nm.

(b) Grating e . for near infrared range. 600gmm,blazedat1,0

µ

m.

Figure2.11: E ien yofPMTgratings.

photoele tri ee t thephoto athode emits an ele tronwhen aphoton hits itssurfa e. The amount of ele trons is measured in the PMT and it is proportional to the light intensity. However, the e ien y of the photo athode is a fun tion of the wavelength, in parti ular it doesnotworkunderthephotonenergythreshold. Asplotteding. 2.12thee ien yde reases by about twoorders in magnitude from 400nm to 800 nm. Hen e at longwavelengths the qualityof measurementis worse.

Figure2.12: Photo athodee ien yforPMT.

Somewordsshouldbesaidaboutproblemsinre ordingthekineti tra esat ertain wave-lengths. As already dis ussed, the photo athode e ien y limits the possibility to perform measurementsattoolongwavelengths(more thanabout850nm). Furthermore theposition respe t toex itation wavelength isrelevant. Infa t theLASERlightissostrongto passthe mono hromatorsele tion. Obviouslythisdrawba kismoreimportantthe loserthere ording and the pump wavelengths. At about 20 nm from ex itation wavelength the LASER light ould hit thedete tor with an intensitytens to hundredstimes higherthan

∆

abs

(λ, t)

. This is espe ially dangerous with diusing sample su h as a mesoporous lm of DSC.The PMT is a quitesensitivedete tor and its response ould be semi-permanentlyaltered by too high intensities. Afterastrongpulse likeas atteredLASER light itneedsof alongequilibration time to re over thenormal sensitivity. In other words, re ording thekineti tra eat 20 nm fromex itinglightwouldbelikemeasuringtheweightofabis uitwithakit henbalan eafter

ex iting wavelengthwas480nmandsometimes we olle tedatra eat500-510nm. Inthose asesweuseda495nmpass-bandlter(pass lightwithwavelengthupperthan495nm).

Asalreadysaidin aTransientAbsorptionorEmissionexperimentweregisteraspe trum a ertaintimeaftertheex itation. Soin ea hTransientexperimentwemustspe ifythetime (

t

) at whi h was registeredand also how longthe light was olle ted(the time window,

w

). Usuallyin ourexperimentweusedatimewindowofabout10-20%ofthe olle tiontime: for

t

100ns,

w

20ns;

t

200ns,

w

20ns;

t

1000ns,

w

50ns;

t

2000 ns,

w

100ns; e t. Wewillno morespe ifythetimewindowin thepresenteddata.

Forkineti tra es,wesetthewavelengthandtherangeof olle tiontime. Thisrangewillbe spe ifyatea hexperiment. Theshowedtra esaretheexperimental datewitha ompression of1:5(one pointea h5realexperimentalone).

2.3.2 Spe tro-ele tro- hemistry

TheSpe tro-ele tro- hemistryte hniqueisusedtore ordtheabsorptionspe trumofredu ed or oxidised spe ies. The prin iple is similar to that of Transient Spe tros opy: we olle t two spe tra, one before and another after the redu tion/oxidation, and then we take their dieren e.

Theapparatusforthisexperiment omprehends:

ˆ aparti ular1mm uvette ontainingaPtgrid(working)ele trode,a ounterele trode andareferen eele trode(as showninpi ture2.13);

ˆ agalvanometer;

ˆ adiode-arrayspe trometer.

Figure 2.13: Cuvetteapparatus for Spe tro-ele tro- hemistry. W is the working ele trode, R the referen eoneandCthe ounter-ele trode.

After having an ideaof theRedOx properties of the system (forexample by meansof a Cy li Voltammetryexperiment),aparti ularredu tionoroxidationstepis hosenforfurther analysis. For simpli ity from now on we will talk aboutredu tion only. The voltage of the working ele trode is set at thewanted redu tionpotential. Spe traare olle tedbefore and after thisredu tion.

appliesthe hosenpotentialandspe traare olle tedatregulartimeintervals,thezeroofthe times alebeingtheinstantwhenthepotentialhasbeenswit hedon. Inthedatapro essing, thetimezerospe trumissubtra tedfromeveryotherspe trum. Thisqui kwaytoregisterthe spe trarequiresadiode-arrayspe trometerthat an olle tthespe trainfra tionsofase ond.

Thus, in analogywith TA spe tros opy, also here we have positive and negative bands. The spe ies weprodu e byredu tionnormallyhaveadierentspe trumwith respe tto the initial state,afa t that an beinterpretedby onsidering the hanges in theorbital energies and o upations. Wehavenegativebands where theredu ed mole uleabsorbslessthanthe starting one,and positivebands where it absorbs more. In this way we an identify the ab-sorption bandsof aredu edspe ies(positivebands)andalso whi h transitionsareweakened orsuppressedbyredu tion(negativebands).

Obviouslyifaspe ieshasmorethanoneredu tionstep,itispossibletoisolatetherstone, butthespe trarelativetotheothersteps(withhigherRedOxpotentials)wouldbealteredby thepreviousone.Forexample,ifwehaveaspe ieswithtworedu tionpotentials,withtherst oneweonlyobtainthespe ieswithoneextraele tronandwe andete tisosbesti points. On the ontrary,withthese ond potentialweobtainamixture ofone-andtwo-ele tronredu ed spe ies,andnoisosbesti points an bepresent. Furthermoretheese ondredu tion ano ur only ifenoughamountofrst spe ies ispresent. Then e agoodwaytoobservethepresen e ofasubsequentredu tionisthede reasingofbandofpreviousredu tion.

2.4 Spe tros opi method

2.4.1 Instrumentation

Forspe tros opi measurementsweused:

ˆ anAgilent8453(arraydiodesasdete tor),forUV-Vismeasurements;

ˆ a Quanta-Ray/MOPO laser pump&probe system of Spe tra-Physi s for transient ab-sorption measurements(CCD amera asdete tor) and for the kineti tra es(PMT as dete tor);

ˆ aBrilliantlasersystemforsensitizedsurfa ekineti tra es.

Forele tro hemistryande ien y ofsolar ellsweused:

ˆ apotenziostat-galvanometerMa LabmodelML160;

ˆ thee ien yandtheIPCEaremadewithinstrumentofHagfeldtLab. inUppsalaUniv.;

2.4.2 Laser ash photolysis set-up

ThesampleinthesemeasurementswasthesameoftheAbsorptionmeasurements,inthesame uvette. The uvettewaspla edat the rossingof thepump and probebeam. Theangle of

thefa eof the uvettewiththepump beamis hosentooptimizetheilluminatesolutionand toavoidtheba kree tionofthepumpbeam

6

. WhenrequiredthesamplewasArgonpurged. The on entrationof the ompounds isspe iedforea h experiment.

Inexperimentswiththedyedeposedonsurfa eweuseda495nmpassbandltertoredu e thes atteringfromthesurfa eoftheLASERlight(480nm).

IneveryLFPexperimenttheex itinglightwassettedat480nm. Therangeofabsorption measure fortheCCD amerawas400-800nm.

6

In1 m uvette asethe fa e wasalmost perpendi ularto the beam, in1mm uvetteand devi e-type

Results and dis ussion

Intheintrodu tionweanalysedthefa torsthatae tthee ien yofaDSSC.Themost im-portantoneisthere ombinationrate. Inmost asesthispro essisinhibitedbyin reasingthe distan e betweenthetwo hargesgenerated byinje tion. Weadoptedthestrategyof pla ing the ele tron in the redu ed dye far from the NiO surfa e. Tothis aim we used an ele tron a eptingligand(Phen-NMI) inourRu omplex. Oneoftheaimsof thisworkisto under-standif thepresen e ofthea eptingligand(Phen-NMI) inRu-dyeimprovesthee ien y withrespe ttothewidelyusedRu(d b)

2+

3

. Therefore,weshall omparethee ien y of ells builtwithRuB

2

PNandRu(d b)

2+

3

dyes,aftermakingsurethatthenegative hargea tually lo alizes in the Phen-NMI ligandafter inje tion (pro ess 2 of s heme 1.1). In as hemati waythemain stepsofthisworkare:

1. ndingawaytodierentiatethepositionsoftheminus hargeintheex itedandredu ed statesofthedyemole uleinsolution;

2. extendingthesameinvestigationtothesupporteddyeafterinje tionfromNiO;

3. measuringthee ien iesto showwhetherthedyewith hargelo alisationworksbetter thatstandardRu(d b)

2+

3

.

Atypi alwayto examinetheele troni stru tureofamole uleisopti alspe tros opy. In our ase we ompared the spe tra of the mole ules in initial oxidation state and in redu ed one,to havesomeinformationaboutthe hargedistribution. Westudy ele troni transition, thereforetheregionoftheele tromagneti spe trumweareinterestedinistheUV-Visregion. By omparing the UV-Vis spe tra of the redu ed RuB

2

PN and the redu ed Phen-NMI and Ru(d b)

2+

3

weshould understandifthenegative hargeispla edintheNMI-grouporin thebipy-group. The riterionisquitesimple: iftheredu edRuB

2

PNspe trumissimilarto theredu edRu(d b)

2+

3

one,thenegative hargewill beinthe bipyridine-group;otherwise,if theredu edRuB

2

PNspe trum ontainsabandtypi alofredu edPhen-NMI,the harge will beintheNMI-group.

Themostimportantte hniquesto olle tthiskindofspe traaretheLaserFlash Photol-ysis andtheSpe tro-ele tro- hemistrydes ribedinChapter2. Asalreadysaidbefore,with theLaserFlashPhotolysisweareabletoseethespe trumoftheprodu tofaphoto-ex itation andtheSpe tro-ele tro- hemistryyieldsinformationaboutthespe trumofaredu edspe ies. We apply these te hniques and others of moreroutinely use in a physi o- hemi alstudy to analysethedyeanditsredu edstate.

Weareespe iallyinterestedinourdyeafterinje tionofthepositive hargeonthe semi on-du tor. Thisimpliesinvestigatingthedyesupportedonthesurfa e,whi hismore ompli ated thanworkinginsolutionphase,forexampleduetothelargerlights atteringfromthesurfa e. Moreoverthe dye with arboxyli linking groupswas not available at the beginning of this work. Therefore we started with aset of experimentswith the dye in solution. Weassume that,inour omplexes,thelinkwiththesemi ondu torandthepresen eofthelinkinggroups themselvesdonot hangesigni ativelythe hargedistribution. Thismeansthatthe measure-mentsandtheiranalysis anbemadewiththeRuB

2

PNandRu(bipy)

2+

3

dyes(fromnowon weshall allRu(bipy)

2+

3

asRuB).

3.1 Measurements in solution

Firstofall,we hara terizedRuB

2

PNbyUV-Visspe tros opyandCi li Voltammetry mea-surements. Then we made measurement with Spe tro-ele tro- hemistry to investigate the redu ed state of RuB

2

PN. Finally, by Laser FlashPhotolysis, we tried to see whether the negative hargeafter aphoto-redu tion isin asimilar position asin theredu edstate pro-du edbyele tro hemi alredu tion. Theideaisthatthephoto-redu tionemulatesmore losely theDSSCpro esses.

3.1.1 Chara terization of the dyes: UV-Vis spe tra

TheUV-Visspe trumoftheRuB

2

PNwasre ordedinastandard(1 m) uvette.

Intheliterature[41℄asimilar ompound wasstudied,theonlydieren ebeinganele tron donorligand(apiperidine)repla ingthenitrogroup. Forthat ompound,agoodaddivityof thespe traofits omponentswasnoted: the lassi albandofRuB(theMLCT metal-ligand- harge-transferband),plusthebandof thephenathrolinebasedligand. Our omplexhasthe samekindofmetal-ligandbonds, sowe anexpe t asimilaradditivityofthespe tra.

TheUV-Visspe traofPhen-NMI,RuB,andRuB

2

PNareshowedingures 3.1(a),3.1(b) and3.1( ) respe tively:

ThePhen-NMI spe trumshowsthree important bands, one at 230nm assigned to the

π

-

π∗

phenantroline transition,another at 265 nmnot assigned and nally the wide band at 350nmwhi histypi alofNMI[41℄. BothRuthenium-based omplexes(RuB

2

PNandRuB) showastrongfeature near295nm thatwasassigned asthebipy ligand entered(LC)

π

-

π∗

(a)Phen-NMIabsorption. (b)Ru(bipy)

3

absorption.

( )RuB

2

PNabsorption.

Figure3.1: UV-VisSpe tra

Transfertransition (MLCT) [32℄. MoreoverRuB

2

PN showsaband around350nm, that is absent in RuB and anbe assigned to the transition of the NMI moiety mentioned above. Thisband isnotexploitableto triggerthephotovoltai ee t,sin eitisnotanMLCTband.

TheMLCT(Metal-to-LigandChargeTransfer)transitionisakindofabsorption frequent in Ru- omplexes. This transition involvesthe jump of an ele tronfrom ametal orbital to a ligandorbital. S hemati allythe hargetransfertransitionisrepresentedin gure:

WeknowthatinRuBthe hargeistransferredto(oneof)thebipyligands. InRuB

2

PN we ansuppose thattheele trongoesto thePhen-NMIligandandmorespe i ally tothe ele tron-attra tingNMIgroup.

In the hypothesis that the LC and MLCT bands of RuB

2

PN involving the bipyridines aresimilarto thoseof RuBandthattheLCbandsof Phen-NMIarenotdeeplyalteredby omplexation,we an tryto reprodu etheRuB

2

PNspe trumbysummingup ontributions ofthespe traof Phen-NMIandRuB.

Ingure3.3weplottheabsorptionspe traofRuB

2

PN,of Phen-NMI,ofRuBandtwo ombinationsofthoseof Phen-NMIandRuB.Namely,thespe trumof RuBwasaddedto thatof Phen-NMIwithtwodierentfa tors. Thefa tor1seemsmoresuitableforthemetal

(a)S hemeofjump. (b)Hypotheti alRuB

2

PNex itedstate.

Figure 3.2: Ontheleft thequalitatives hemeof hargejump inaMLCTtransition. Ontheright the hargestru tureofMLCTstateoftheRuB

2

PN.

basedand theMLTCbands,while thefa tor2/3was hosento a ountfor theLCbands of the2bipy's. Allthespe traareinarbitraryunits,withas alingtomakethem on entration onsistent.

E

x

t

i

n

c

t

i

o

n

c

o

e

f

f

i

c

i

e

n

t

(

M

-1

c

m

-1

)

0

2e+04

4e+04

6e+04

8e+04

1e+05

1,2e+05

1,4e+05

W.length (nm)

200

300

400

500

600

Abs spectrum of RuB2PN

Sum spec. PhenNMI + (2/3)Ru(bipy)

Abs spec. of RuB2PN norm. to black one

Sum spec. PhenNMI + Ru(bipy)

Figure 3.3: Comparisonofreal LD1spe trumand LDlig, RuBsum-spe travariouslynormalized: red,abs-spe trumof RuB

2

PN;bla k,sum-spe trumof Phen-NMIand2/3of RuB;blue, sum-spe trumofPhen-NMIand2/3of RuB; yan,sum-spe trumofPhen-NMIandRuB

Wenotethatalmosttheentirespe trumof RuB

2

PNisdownshiftedwithrespe ttothe ombinations, parti ularlyfor the350 nmband andthe other features regardingthe Phen-NMI ligandat 230nm and 265nm. A smaller dis repan y, still in thesame dire tion also on ernsthe280nmband,whi hisassignedto aLCbipytransition[32℄.

The MLCT bands in the RuB

2

PN spe trum and in the ombinations are very similar. Apartfromasmalls aling,theyhaveexa tlythesameshapeandnolongerwavelengthband or shoulder is present. So, apparently the NMI moiety does not play a role in the MLCT

spe trum. Thisdoesnotmeanthatthepresen eofanele tron-attra tinggroup annotdeeply ae t theele tro hemistryandphoto hemistryof RuB

2

PN.

3.1.2 Chara terization of the dyes: ele tro hemi al measurements

As said in the introdu tion, anestimation of the driving for e of the ele tron inje tion and re ombinationpro esses anbemadeon thebasisof somespe tros opi andele tro hemi al parametersofthedyeandofthesemi ondu tor.

Ifwe onsiderpro esses1,2and5(ex itation, inje tion andre ombination, respe tively) ofthes heme1.1,weseetheyforma y leandthereforetherelativefreeenergiesarerelated:

1) Ex itationbyaphoton(h

ν

)

RuB

2

PN+h

ν −→

RuB

2

PN*

Inthis step

∆E

ex

is the energy dieren ebetweenthe ele troni ally and vibrationally relaxedex itedstateRuB

2

PN*andthegroundstate.

2) Inje tionofanele tronfrom VBofNiO

RuB

2

PN*+NiO

−→

RuB

2

PN

−

+NiO

+

We all

∆G

inj

thefreeenergydieren eforthisRedOxrea tion. 5) Re ombination RuB

2

PN

−

+NiO

+

_−→

RuB

2

PN +NiO

We all thisfreeenergydieren e

∆G

rec

. All

∆G

'saremeantpermole.

Wehavedatafortherst andthelast pro esses. Inprin iple,wewould needfreeenergy dieren esinallthree ases,butfortherstonewerelyonspe tros opi data,i.e. onenergy dieren es. We anassume,however,thattheentropi ontributioninthis aseissmall,sin e thereareno hemi al hanges. Weshallestimate

∆E

ex

fromtheabsorptionandlumines en e spe tra(seese tion3.1.3). Soweshallevaluatetheinje tionfreeenergyas

∆G

inj

= −∆E

ex

− ∆G

rec

∆G

rec

anbe al ulatedasthealgebrai sumofthehalfredu tionpotentialsoftherea tants RuB

2

PN

−

and NiO

+

. In fa t the re ombination an beseenlike thesum ofRedOx half-rea tionsofNiO

+

redu tion(

∆G

N iO

+

Red

)andRuB

2

PN

−

oxidation(

∆G

RuB

2

P N

−

Ox

):

Foranele tro hemi alhalf-rea tionthefreeenergyis

∆G = −F E

◦

,where

F

istheFaraday onstant and

E

◦

is the standardRedOxpotentials of thehalf-rea tion. Thus for

∆G

rec

we have:

∆G

rec

= F E

RuB

◦

2

P N/RuB

2

P N

−

− F E

◦

N iO

+

_{/N iO}

Here

E

◦

RuB

2

P N/RuB

2

P N

−

istakenwithoppositesignbe ausethehalf-rea tionweare inter-ested inisanoxidation.

E

◦

N iO

+

_{/N iO}

isknownfromliterature[25℄andis-0.12V(vsF /F

+

). For

E

◦

RuB

2

P N/RuB

2

P N

−

weperformedaCy li Voltammetrymeasurement.

TheCy li Voltammetry potentials anbe veryuseful also for other purposes. We shall ompare thoseof RuB

2

PN andPhen-NMIto evaluatethealteration of the ele tro hem-i al properties of the ligand when it is inserted in the omplex. Furthermore, the Cy li VoltammetrypotentialsareneededtoperformtheSpe tro-ele tro- hemistryexperiments. We also measured the RedOx potential of RuB

2

PN with Cy li Voltammetry, besides those of RuB

2

PN and of Phen-NMI, to show that the arboxyli groups do not introdu e an importantperturbation.

Cy li Voltammetry

The ele tro hemi al experiment was performed with ACN ondu tive solutions (0,1 M of

t−

butylammonium hloride) of the ompounds: RuB

2

PN, RuB

2

PN, and Phen-NMI. Theresultedvoltammogramsarereporteding. 3.4.

(a)CVspe trumofPhen-NMI.Inset:ferro ene stan-dard.

C

u

r

r

e

n

t

(

A

)

-6e-05

-4e-05

-2e-05

0

2e-05

4e-05

6e-05

8e-05

Potential (V)

-3

-2

-1

0

1

2

(b)CVspe truaofRuB

2

PN(bla k),andRuB

2

PN (red)

Figure 3.4: Ci li Voltammetryspe tra of RuB

2

PN, RuB

2

PN, Phen-NMIand the referen e (ferro ene)

InPhen-NMI voltammogram (g3.4(a)) we note tworeversibleand resolved redu tion peaks, the rstat -0.90V and these ond at -1.18 V. Theyare assigned tothe rst andthe se ond redu tionof nitrogroupin NMI [41℄. InRuB

2

PN ase (3.4(b))we an see6peaks. Two of them at about -0.8 V (reversible) and -1.15 V are quite similar to the Phen-NMI ones,and an be assignedto theredu tionsof NMI.With useofliterature data[32℄,we an