Maniga Sumida Stone Moore Moore Gust J Porphyr Phthalocyan 3 1999 32

Increasing the Yield of Photoinduced Charge Separa-tion through Parallel Electron Transfer Pathways

NYANGENYA I. MANIGA, JOHN P. SUMIDA, SIMON STONE, ANA L. MOORE*, THOMAS A. MOORE* andDEVENS GUST*

Center for the Study of Early Events in Photosynthesis, Department of Chemistry and Biochemistry, Arizona State University,Tempe, AZ 85287-1604, USA

Received 29 January 1998Accepted 6 March 1998

ABSTRACT: A strategy for increasing the yield of long-lived photoinduced charge separation in artificialphotosynthetic reaction centers which is based on multiple electron transfer pathways operating in parallel hasbeen investigated. Excitation of the porphyrin moiety of a carotenoid (C)–porphyrin (P)–naphthoquinone (Q)molecular triad leads to the formation of a charge-separated state C

.�–P–Q.ÿ with an overall quantum yield of

0.044 in benzonitrile solution. Photoinduced electron transfer from the porphyrin first excited singlet state givesC–P

.�–Q.ÿ with a quantum yield of�1.0. However, electron transfer from the carotenoid to the porphyrin

radical cation to form the final state does not compete well with charge recombination of C–P.�–Q

.ÿ, reducingthe yield. The related pentad C3–P–Q features carotenoid, porphyrin and quinone moieties closely related tothose in the triad. Excitation of this molecule gives a C

.�–P(C2)–Q.ÿ state with a quantum yield of 0.073. The

enhanced yield is ascribed to the fact that three electron donation pathways operating in parallel compete withcharge recombination. The yield does not increase by the statistically predicted factor of three owing to smalldifferences in thermodynamic driving force between the two compounds.# 1999 John Wiley & Sons, Ltd.

KEYWORDS: porphyrin; photoinduced electron transfer; spectroscopy; synthesis; quinone; carotenoid

INTRODUCTION

A large number of model photosynthetic reactioncenters based on porphyrins or related chromophorescovalently linked to electron donor and/or acceptormoieties have been described in the last two decades[1–6]. Although dyads consisting of porphyrins linkedto quinones or other electron acceptors can undergophotoinduced electron transfer to produce charge-separated states in high yield, generating such stateswherein charge recombination is slow, and thusfacilitating the harvesting of the stored energy bydiffusional or other processes, has in general requiredmolecular triads or more complex supermolecularconstructs which employ multistep electron transferpathways conceptually related to those found innatural reaction centers. Carotene (C)–porphyrin(P)–quinone (Q) triad1 exemplifies this strategy

[7, 8]. Excitation of the porphyrin moiety generatesC–1P–Q, which decays mainly by photoinducedelectron transfer to the quinone, giving C–P

.�–Q.ÿ

with a quantum yield of 0.95 in benzonitrile (step 2 inFig. 1). This state recombines to the ground statewithin a few picoseconds (step 3), but competing withcharge recombination is electron donation from thecarotene to the porphyrin radical cation to produce theC

.�–P–Q.ÿ charge-separated state (step 4). Owing to

the large spatial separation of the charges in thisspecies and the weak electronic coupling between theanion and cation radicals, charge recombination (step5) requires 67 ns. The two-step electron transferprocess has tremendously increased the lifetime ofthe charge separation. Photosynthetic reaction centersalso use a multistep electron transfer cascade toseparate charge across the thickness of a lipid bilayermembrane, generating long-lived charge separation.

The yield of C.�–P–Q

.ÿ for triad1 in benzonitrile isonly 0.044 [8]. The low yield results from inefficientcompetition of step 4 with charge recombination bystep 3. In our laboratories we have used a variety ofstrategies to increase the yield of the final, long-livedcharge-separated state in artificial reaction centers [2].

Journal of Porphyrins and PhthalocyaninesJ. Porphyrins Phthalocyanines3, 32–44 (1999)

CCC 1088–4246/99/010032–13 $17.50# 1999 John Wiley & Sons, Ltd.

———————Correspondence to: D. Gust, Center for the Study of Early Events inPhotosynthesis, Department of Chemistry and Biochemistry,Arizona State University, Tempe, AZ 85287-1604, USA.

Theenergiesof thevariousspecieshavebeenadjustedin order to increasethe rates of favorable electrontransferstepsoccurring in the normal region of theMarcusrelationshipandslow down chargerecombi-

nationreactionsin theMarcusinvertedregion[9, 10].The electroniccoupling betweendonorand acceptormoieties has been tuned in order to alter electrontransferrateconstantsin favorabledirections[8, 11–13]. Theeffectsof medium(solvent)andtemperaturehave also been investigated[14,15]. Although theyieldsof long-livedchargeseparationcanbeincreasedsubstantiallyusingtheseapproaches,oneof the mostsuccessfulhas been the incorporationof additionalsecondaryelectrondonorsandacceptors[12,13,16–19]. For example, we have reported a carotene–diporphyrin–diquinone molecularpentadin which thefinal C

.�–PZn–P–Q–Q.ÿ charge-separatedstate is

generatedwith anoverall yield of 0.83anda lifetimeof severalhundredmicroseconds[12,19].

It wasproposedthatonereasonfor thehighyield ofchargeseparationin the pentadswas the use of aparallel multistep electron transfer strategy. Twoelectrontransferstepscompetewith chargerecombi-nationof the initially formedC–PZn–P

.�–Q.ÿ–Q and

lead eventually to the final state.Thus competitionwith charge recombination is more efficient thanwould have been the case if either pathway wereoperating alone (as in the triad discussedabove).Subsequently,this strategy was investigated in aporphyrin–diquinone triad, where two photoinducedelectron transfer steps compete with decay of anexcited singlet state [20]. In the presentwork theparallelmultistepelectrontransferapproachhasbeeninvestigatedquantitatively through the preparationand study of C3–P–Q pentad2 and related modelcompounds3–5. This pentad is structurally verysimilar to triad 1. However, it was expectedthatchargerecombinationof theC3–P

.�–Q.ÿ state,formed

by photoinducedelectrontransferfrom the porphyrin

Structures

Fig. 1. Transientstatesand relevantinterconversionpath-waysfor C–P–Qtriad 1.# 1999JohnWiley & Sons,Ltd. J. PorphyrinsPhthalocyanines 3, 32–44(1999)

INCREASINGTHE YIELD OF PHOTOINDUCEDCHARGESEPARATION 33

first excitedsingletstate,would haveto competewithelectrondonationfrom all threecarotenoidmoietiestothe porphyrinradicalcation.Statistically,this shouldincreasetheyield of a final C

.�–P(C2)–Q.ÿ stateby a

factorof threeif theparallelstrategyweresuccessful.

RESULTS

Synthesis

Thepreparationof triad1 hasbeenreportedpreviously[21]. Although several general approachesto thepreparationof pentad2 were investigated,the mostsuccessfulinvolvedattachmentof thethreecarotenoidmoietiesto thetetraarylporphyrin, followedby linkageof thequinone.Basicpartialhydrolysisof 5,10,15,20-tetrakis(4-trifluoroacetamidophenyl)porphyrinyieldedthe porphyrinbearinga singletrifluoroacetylprotect-ing group. This porphyrin was linked to threecarotenoidmoietiesvia amidebonds.The remainingtrifluoroacetyl group was then removed and theresultingaminoporphyrinwascoupledto thenaphtho-quinone derivative via the quinone acid anhydride.Thesyntheticdetailsandcharacterizationdataappearin theExperimentalsection.

Electrochemistry

Cyclic voltammetric studies of model porphyrins,quinonesandcarotenoidswereperformedin ordertoobtainestimatesof theenergiesof thevariouscharge-separatedstatesin the triad and pentad.The firstoxidation potential of a model for the carotenoidmoiety commonto 1 and2 is �0.65V vs SCE[22].Thefirst oxidationpotentialin benzonitrilesolutionofa model for the porphyrin moiety of 1, 5,15-bis(4-acetamidophenyl)-10,20-bis(4-methylphenyl)por-phyrin, is�0.93V,[8] whereasthatfor a modelof theporphyrin in 2, 5,10,15,20-tetrakis(4-acetamidophe-nyl)porphyrin (3), is �0.91V. The first reductionpotential of a model for the quinone moiety, 6-phenylcarbamyl-1,4-naphthoquinone, is ÿ0.58V,measuredunderthesameconditions[8].

Absorption Spectra

Figure2 showstheabsorptionspectrumof pentad2 inbenzonitrile solution. The maxima at 430, 590 and653nm are characteristicof porphyrin absorption,whereastheshoulderat460nmandthemaximaat488and 517nm signify carotenoidabsorption.Similar

featuresareapparentin the spectraof triad 1 andofmodelcarotenoporphyrin4 andmodeltricarotenopor-phyrin 5 (Fig. 2). The spectraof 1 and4 arecloselyapproximatedin the 400–800nm region by a linearcombinationof thespectraof modelporphyrin3 andamodel carotenoid(7'-apo-7'-(4-methoxycarbonylphe-nyl)-b-carotene). The spectra of pentad 2 andtricarotenoporphyrin 5 areapproximatedby a similarlinear combinationemploying three times as muchcarotenoidabsorption.Thusthe absorptionspectraofthedyad,triad andpentadarenot indicativeof strongelectronicinteractionsamongthe chromophores.Thechromophoresin the multicomponentmoleculeswilltherefore be treated as essentiallyseparateentitieswith weakelectronicinteractions,ratherthanassinglespecieswith extendedconjugation.In this connection,the electronicinteractionamongthe chromophoresislimited in part by the fact that the aryl rings on theporphyrinmoiety resideat steep(45°–90°) anglestotheplaneof themacrocycle,owingto stericrepulsions[8].

EmissionSpectra

Theemissionspectrumof porphyrin3 in benzonitrileis typical of thoseof tetraarylporphyrinsandfeaturesmaximaat 665and730nm in a ratio of intensitiesof2.3:1. The emissionspectraof pentad2 and tricar-otenoporphyrin5, with excitation at 590nm wheremostof theabsorptionis dueto theporphyrinmoiety,are virtually identical to that of 3 in shape,but thequantumyield is substantiallyreduced.No emissiondue to the carotenoid moiety was observed, asexpectedsince the fluorescencequantumyields ofcarotenesare infinitesimal. The fluorescenceexcita-tion spectrumfor porphyrin emissionin 5 indicated

Fig. 2. Absorption spectra of C3–P–Q pentad 2 (—),tricarotenoporphyrin5 (…) andcarotenoporphyrin4 (- - -)in benzonitrilesolution.# 1999JohnWiley & Sons,Ltd. J. PorphyrinsPhthalocyanines3, 32–44(1999)

34 N. I. MANIGA ET AL.

that the efficiency of singlet–singletenergytransferfrom the carotenoidmoieties to the porphyrin wasnegligible(< 10%).

More information concerning the fluorescencequenching in 2 and 5 was obtained using time-resolvedfluorescencespectroscopy.The samplesinbenzonitrilesolution were excitedat 590nm with �10 ps laserpulsesand the fluorescencedecaysweremeasuredusing the single-photon-timingtechnique(seeExperimentalsection).In the caseof 2, decaysweremeasuredatninewavelengthsin the640–755nmregionandthe decayprofileswereanalyzedgloballyasthe sumof threeexponentialprocesses(w2 = 1.13)to yield the decay-associatedspectrain Fig. 3. Themajor component consists of a decay with theemissionbandshapeof the porphyrin and a lifetimeof 0.092ns.Theonly othersignificantcomponenthasa lifetime of 0.83ns and likely representsa smallamountof material in which the quinonehas beenreduced(see below). The fluorescencelifetime ofmodel porphyrin 3, measuredunder similar condi-tions, is 8.6ns (w2 = 1.16).The fluorescencedecayofmodel tricarotenoporphyrin 5, measuredat sevenwavelengthsin the650–750nm region,wasanalyzedglobally to yield a major (� 92%) componentwith alifetime of 0.99ns and a minor decay with a timeconstantof 2.74ns(w2 = 1.14).Themajor componentis assignedto thedecayof C3–

1Pandtheminoronetoan impurity or conceivablya minor conformation.

Thus the fluorescencelifetimes of C3–1P and

C3–1P–Q are strongly quenchedcomparedwith that

of model porphyrin 3, as expectedfrom the steadystateemissionresults.Thequenchingof theporphyrinfirst excitedsinglet stateby attachedcarotenoidshasbeenpreviouslyreportedanddiscussed[23–28]. It isdue to some form of enhancedinternal conversion

resultingfrom electrontransfer,singletenergytransferor perturbationof theporphyrinp-electronsystembythe carotenoid.Much strongerquenchingis observedin the quinone-bearingpentad 2. This increaseisattributedto photoinducedelectrontransferby step2in Fig. 4. The C3–

1P–Q excited state donatesanelectron to the quinone, yielding a C3–P

.�–Q.ÿ

charge-separatedstate. Time-resolved fluorescenceand absorptionexperimentson triad 1 and relatedporphyrin–quinonespecies have documented thiselectrontransferbehavior.For example,the C–1P–Qstateof 1 in benzonitrilesolutionhasa lifetime of 110ps and decaysby photoinducedelectron transfer toyield C–P

.�–Q.ÿ, whereasa comparablecarotenopor-

phyrin model systemhas a fluorescencelifetime of2.2ns [7, 8,21].

Transient Absorption Spectroscopy

By analogywith 1, theC3–P.�–Q

.ÿ speciesmayeitherundergochargerecombinationto the groundstatebystep3 in Fig. 4 or experienceelectrondonationfromoneof thecarotenoidpolyenesto generateafinal C

.�–P(C2)–Q

.ÿ state (steps 4). Transient absorptionspectroscopyon the nanosecondtime scalewasusedto investigatethe fate of the initial charge-separatedstate. A sample of 2 in benzonitrile solution(�4� 10ÿ5 M) was excited at 650nm (where onlytheporphyrinabsorbs)with a 5 ns laserpulseandtheabsorbanceof anytransientsproducedwasmonitoredby a steady state probe beam. A strong transientabsorption was observed in the long-wavelengthregion, with a maximum at 945nm (Fig. 5). Thiswas assignedto the carotenoidradical cation of theC

.�–P(C2)–Q.ÿ charge-separatedstate[22].

Excitation of triad 1 under similar conditionsyielded a similar carotenoidradical cation transient

Fig. 3. Decay-associatedspectraobtainedafterexcitationofa�1� 10ÿ5 M solutionof pentad2 in benzonitrilewith a590nm laser pulse.The lifetimes of the componentsare0.092(*), 0.83(~) and2.88ns(&).

Fig. 4. Transientstatesand relevantinterconversionpath-waysfor C3–P–Qpentad2.# 1999JohnWiley & Sons,Ltd. J. PorphyrinsPhthalocyanines 3, 32–44(1999)


absorptiondueto formationof C.�–P–Q

.ÿ by steps2and 4 of Fig. 1. Quantitativecomparisonof the twocompoundsrevealedthat the quantumyield of C

.�–P(C2)–Q

.ÿ in 2 was1.7 timeslargerthanthatof C.�–

P–Q.ÿ in 1. This determinationwas madeby taking

theratio of theslopesof thelineardependencesof thetransientabsorbanceof the carotenoidradical cationimmediately after the laser pulse on laser power.Given the reportedquantumyield for 1 of 0.044,thequantumyield of C

.�–P(C2)–Q.ÿ in benzonitrile is

0.075.The decayof the carotenoidradicalcationof C

.�–P(C2)–Q

.ÿ wasalsoobserved(Fig. 6). Thedecaywasexponentialwith atimeconstantof 67ns.An identicallifetime wasmeasuredfor theC

.�–P–Q.ÿ stateof triad

1.

DISCUSSION

As themainpointof thiswork is thecomparisonof thephotochemistryof pentad2 andtriad 1, thepropertiesof thesetwo moleculeswill be discussedbelow intandem.

Energetics

The wavenumberaverageof the long-wavelengthabsorptionmaximum and short-wavelengthfluores-cence maximum of pentad 2 yields an energy of1.88eV for C3–

1P–Q. The correspondingspectraoftriad 1 yield an energyof 1.89eV for C–1P–Q.Theelectrochemicalresults for the model compoundsdiscussedaboveallow estimationof the energiesofthe initial C3–P

.�–Q.ÿ and C–P

.�–Q.ÿ charge-sepa-

rated statesas 1.49 and 1.51eV respectively.The

energiesof the final C.�–P(C2)–Q

.ÿ and C.�–P–Q

.ÿstatesareboth1.23eV.Theseestimatesdonotattemptto correctfor the influenceof coulombicstabilizationof eachion by its counterpartin theradicalpairsor forany effects due to differencesin solvent dielectricproperties.Any sucheffectswould be expectedto besmall andessentiallyidentical for 1 and2 in a polarsolvent such as benzonitrile. The energiesof therelevant transientstatesof the triad and pentadareshownschematicallyin Figs1 and4.

PhotoinducedElectron Transfer

Theyield andlifetime datareportedabovecanbeusedto evaluaterateconstantsfor thephotochemicalstepsshown in Figs 1 and 4. The rate constant forphotoinducedelectron transfer step 2 may be esti-matedfrom

k2 � 1�sÿ k1 �1�

wherets is the lifetime of the porphyrinfirst excitedsinglet state as derived from the time-resolvedfluorescencestudies.For triad 1, ts is 110 ps and k1

maybeestimatedas4.5� 108 sÿ1, asdeterminedfromthe 2.2ns porphyrin singlet state lifetime of acarotenoporphyrinmodel compound. Thus k2 is8.6� 109 sÿ1. The quantumyield of C–P

.�–Q.ÿ is

givenby

�2 � k2

k1� k2�2�

and equals0.95. In the caseof pentad2 the sameequationsyield a valuefor k2 of 9.9� 109 sÿ1 (basedonak1 valueof 1.0� 109 sÿ1 from the0.99nslifetimeof the porphyrinfirst excitedsingletstatein 5) andaΦ2 valueof 0.91.

Fig. 5. Transientabsorptionspectrumof C.�–P(C2)–Q

.ÿtaken20nsafterlaserexcitationof a�4� 10ÿ5 M solutionof pentad2 with a 5 ns laserpulseat 650nm. The transientabsorptionis dueto the carotenoidradicalcation.

Fig. 6.Decayof transientin Fig.5 measuredat940nm(*).Data were taken every 5 ns. The full line is a single-exponentialfit to the decaywith a time constantof 67 ns.# 1999JohnWiley & Sons,Ltd. J. PorphyrinsPhthalocyanines3, 32–44(1999)


Secondary Electron Transfer

In 1 and2 theinitial charge-separatedstateshavetwopossiblefates. Chargerecombinationto the groundstate(step3 in Figs 1 and4) competeswith forwardelectrontransferby step4, which leadsto the final,long-lived charge-separatedspecies.For triad 1 therateconstantfor chargerecombinationby step3 hasbeenestimatedas 4.2� 1012 sÿ1, basedon subpico-secondtransientabsorptionstudiesof acloselyrelatedporphyrin–quinonedyad[8]. Thequantumyield of thefinal C

.�–P–Q.ÿ speciesin triad 1 was previously

foundto be0.044in benzonitrile,asdeterminedby thecomparative method using the triplet state of5,10,15,20-tetraphenylporphyrin as a standard [8].The rateconstantfor step4 maybecalculatedfrom

�fin � �2k4

k3� k4�3�

whereΦfin is the overall quantumyield of the finalcharge-separatedstate.Substitutingthevaluesfor Φfin,Φ2 andk3 givenaboveyieldsak4 of 2.0� 1011 sÿ1 fortriad 1.

In the caseof pentad2 thereare threecarotenoidscapableof electrondonationto the porphyrinradicalcationof theC3–P

.�–Q.ÿ state,andthreecorrespond-

ing electrontransfersteps,4a,4band4c,asperFig. 4.In the kinetic analysisof the results for 2 we willassumethat the rate constantsfor all three of thesestepsare essentiallyidentical. Of course,the carote-noidat the15-positionof theporphyrinhasadifferentrelationshipto thequinonefrom thatof thecarotenoidsat the10-and20-positions.However,electrontransferin step4 involvesonly the carotenoidandporphyrinmoieties and would be expected to be relativelyinsensitiveto this structuraldifference.In fact, in aprevious study of two carotenoporphyrin–quinonetriads differing only in the relationship of thecarotenoidandquinone(5,10vs5,15ontheporphyrinskeleton),it wasfoundthatthetwo triadshadidenticalquantumyieldsof theC

.�–P–Q.ÿ stateatboth295and

220K [11,29]. This suggeststhat the rate constantsfor the relevantelectrontransferstepsareinsensitiveto thestructuraldifference.

As a first approximation, we make the furtherassumptionthat the rate constantsfor steps3 and 4determinedfor triad 1 arealsoappropriatefor pentad2. This might seemreasonable,asthe two moleculesdiffer very little both structurally and energetically,with the exception of the number of carotenoidmoieties.The quantumyield of the C

.�–P(C2)–Q.ÿ

speciesin pentad2 is givenby

�fin � �2k4a� k4b� k4c

k3� k4a� k4b� k4c�4�

Assuming that k4a= k4b = k4c = 2.0� 1011 sÿ1,k3 = 4.2� 1012 and Φ2 = 0.91, the quantumyield ofC

.�–P(C2)–Q.ÿ should be 0.11, or 2.5 times larger

thanthatfor triad1. In fact,theexperimentalyield in 2is only 1.7 times that in 1. As discussedin the nextsubsection,this apparentdiscrepancycanbe rationa-lized.

Effect of Drivi ng Force on Quantum Yield

From the abovediscussionit is clearthat the parallelmultistep electron transfer strategy employed inpentad2 doesincreasethe quantumyield of the finalcharge-separatedstaterelative to triad 1, but that theincreaseis lessthanthestatisticallyexpectedfactorofthree.A smallpartof theeffect is dueto theincreasednon-productive quenching of the porphyrin firstexcitedsingletstatein 2 by theadditionalcarotenoids,relativeto that in 1. Thelifetime of theporphyrinfirstexcitedsingletstatein a carotenoporphyrin modelfortriad 1 is 2.2ns in benzonitrile,whereasthe corre-spondinglifetime in tricarotenoporphyrin5 is 0.99ns.Even though the rate constantsk2 for photoinducedelectron transfer in 1 and 2 are very similar, theincreasein the rate constantof the competingnon-productiveprocessesdecreasesthe quantumyield oftheinitial charge-separatedstatefrom 0.95in 1 to 0.91in 2.

Themajority of thedifferencein themeasuredandexpectedquantumyields in pentad2 must lie in theyield of electrontransferfrom the carotenoidto theporphyrin radical cation to producethe final charge-separatedstate(steps4 in Figs 1 and 4). This yielddependsupontheratio of therateconstantsfor steps3and4.Therateconstantsfor electrontransferreactions(ket) areconvenientlydiscussedin termsof

ket � ��

�h2�kBT

r jVj2 expÿ��G0� ��2

4�kBT

! �5�which hasbeendevelopedfor non-adiabatictransfers[30,31]. The pre-exponential factor includes theelectronic matrix element V that describes thecouplingof thereactantstatewith thatof theproduct.It also includes Planck’s constant h

_, Boltzmann’s

constant kB, the absolute temperatureT and thereorganization energy for the reaction, �. Thereorganizationenergyis associatedwith the nuclearmotions necessaryto carry the molecule from theinitial to thefinal state.It is convenientto express� as# 1999JohnWiley & Sons,Ltd. J. PorphyrinsPhthalocyanines 3, 32–44(1999)


the sum of a solvent-independentterm �i, whichoriginatesfrom internal molecular structural differ-encesbetweenthe reactantand product,and �s, thesolvent reorganization energy, which is due todifferences in the orientation and polarization ofsolventmoleculesaroundthe initial and final states.The exponential term includes the standard freeenergychangefor the reaction,DG0, aswell as�.

In triad 1 andpentad2 the similarity in molecularstructuresuggeststhattheelectroniccouplingtermsinthevariouselectrontransferstepswouldbeessentiallyidentical, since the stepsin questionoccur betweendirectly joined chromophoresand the linkages areidentical in the two cases.Similarly, the internalandsolvent reorganizationenergiesare expectedto benearly the same. Given these assumptions,anydifferencesin electrontransferrate constantsfor thetwo moleculesmustarisefrom differencesin thermo-dynamic driving force. At first glance,thesediffer-enceswould beexpectedto besmall.TheC–P

.�–Q.ÿ

stateof triad 1 lies 1.51eV abovethe groundstate,whereasthecorrespondingC3–P

.�–Q.ÿ stateof 2 is at

1.49eV. The energiesof the final C.�–P–Q

.ÿ andC

.�–P(C2)–Q.ÿ statesshould be essentiallyequiva-

lent. Can this small difference in DG0 lead to theobserveddifferencein quantumyields?

Equation (5) may be used to investigate thisquestion.The approachis to usetheknownvaluesofk3 and k4 for triad 1 to calculate values for theelectroniccouplingV in equation(5), thento usethesesamecouplingvaluesto calculatecorrespondingrateconstantsfor pentad2 andthusthetheoreticalratio ofthequantumyieldsof thefinal charge-separatedstates.

The calculation requiresan estimateof the totalreorganizationenergy �. A study of photoinducedelectrontransferin porphyrin–quinonesystemscarriedout in butyronitrile solution yielded a value for � of1.25eV [32]. Using this value andvaluesfor k4 andDG0 of 2.0� 1011 sÿ1 andÿ0.28eV respectivelyfortriad1 at298K, equation(5) yieldsV = 1150cmÿ1 forstep 4 in triad 1. A similar calculation employingk3 = 4.2� 1012 sÿ1 and DG0 =ÿ1.51eV yieldsV = 175cmÿ1 for chargerecombinationstep3 in thetriad.(It shouldbenotedthatthenumericalvaluesof Vcalculatedin thiswaydependstronglyonthechoiceof� and should not be taken as true estimatesofelectronic coupling. However, the choice of � hasrelatively little effect on the calculatedrateconstantsfor 2 and quantum yield ratio for 2 relative to 1discussedbelow.)

Assumingthat thesesamevaluesof � andV applyequally to pentad2, equation (5) and the thermo-

dynamicdriving force valuesfor 2 discussedabovemay be used to calculate values for k3 andk4a= k4b = k4c, which may be associatedwith thecorrespondingsteps in Fig. 4 for pentad 2. Thecalculations yield k3 = 4.5� 1012 sÿ1 and a rateconstantof 1.5� 1011 sÿ1 for eachof steps4. Withthese estimates,equation (4) gives a theoreticalquantumyield for C

.�–P(C2)–Q.ÿ of 0.083. This is

closeto themeasuredyield of 0.075andgivesa ratioof quantumyield for pentad2 to thatfor triad 1 of 1.9,ascomparedwith the experimentalratio of 1.7. Thusthe apparentdiscrepancybetween the statisticallyexpectedandobservedquantumyieldsis explainedbythesmall differencesin thermodynamicdriving forceresultingfrom differencesin theoxidationpotentialoftheporphyrinmoietiesinvolved.

CONCLUSIONS

Comparisonof the results for triad 1 and pentad2showsthat the three forward electrontransferpath-ways 4 (Fig. 4) operatingin parallel and competingwith chargerecombinationby step3 enhancetheyieldof the final, long-lived charge-separatedstateby thestatisticallyexpectedfactor of three,after adjustmentfor changesin thermodynamicdriving forceandin therateconstantsfor steps1 (Figs1 and4). Therelativelysmall change in thermodynamic driving force(0.02eV) hasa rather large effect on quantumyieldbecauseit affectstwo setsof rateconstants.Forwardelectrontransferby steps4 occursin thenormalregionof the Marcusrelationship,equation(5), andthusthereduction in driving force for this step in pentad2relative to triad 1 leads to a reduction in the rateconstantfor steps4. This in turn reducesthe yield ofthefinal C

.�–P(C2)–Q.ÿ staterelativeto thatexpected

statistically.On theotherhand,chargerecombinationof C3–P

.�–Q.ÿ by step3 occursin theinvertedregion

of the Marcus relationship.A reduction in drivingforcethusresultsin anincreasein therateconstantforcharge recombination, and this also reduces thequantumyield of C

.�–P(C2)–Q.ÿ relative to that of

thecorrespondingstatein 1. Thetwo effectsoperatingtogether account for the relatively large effect onquantumyield.

Most photosyntheticreaction centers of knownstructure approachC2 symmetry. Electron transfergenerallyproceedsdownonly oneof thetwo possibleelectron transfer pathways, although there is thesuggestionthat in somereactioncenterstransfermayoccurvia bothpaths[33–37].Parallelelectrontransfer# 1999JohnWiley & Sons,Ltd. J. PorphyrinsPhthalocyanines3, 32–44(1999)


down both branchescould makeuseof the principlediscussedaboveto competewith other pathwaysfordeactivationof chlorophyll excitedstatesandthusinprinciple enhance the quantum yield of chargeseparation. In most modern reaction centers theelectrontransferprocessis so efficient that any suchincreasewould be of little significance.On the otherhand,thisstrategycouldhavebeenemployedby moreprimitive organisms. In fact, if modern reactioncentersevolved from homologousdimeric proteins,then the original dimeric proteins, with strict C2

symmetry,would havedemonstratedparallelelectrontransferpathwaysby necessity.As mentionedearlier,the parallel multistep electrontransferstrategydoesaccount for enhanced quantum yields in somecomplexartificial reactioncentersandcanbeappliedto other multicomponent molecular systems thatdemonstrateelectron,protonand/orenergytransfer.

EXPERIMENTAL

Synthesis

The preparationof triad 1 has beenreportedin theliterature[21].

5,10,15,20-Tetrakis(4-aminophenyl)porphyrin (6). A1.96g portion (2.33mmol) of 5,10,15,20-tetrakis(4-acetamidophenyl)porphyrin3 (preparedby themethodof Adler et al. [38]) was dissolved in 50mL oftrifluoroaceticacid and the mixture was placedin a500mL flask.A 300mL portionof 12N hydrochloricacid wasaddedanda reflux condenserwasattached.Themixturewasheatedat reflux for 24h, cooledandneutralizedwith 10%aqueoussodiumhydroxide.Thepurple crystals that formed on neutralizationwereremovedby filtration. The crystalsweredissolvedin300mL of chloroformandthe mixture washeatedtoreflux for 1 h. Undissolvedmaterialwasremovedbyfiltration andthesolventwasremovedfrom thefiltrateby distillation under reducedpressure.The residuewas recrystallized from methanol to yield 1.64g(98%) of 6 as needle-likepurple crystals;1H NMR(300MHz, CDCl3) � ÿ2.72 (2H, brs, pyrrole-NH),4.01 (8H, brs, ArNH2), 7.07 (8H, d, J = 8 Hz, Ar3,5-H), 7.99 (8H, d, J = 8 Hz, Ar2,6-H), 8.90 (8H, s,pyrrole-H);MS m/z674(M�).

5,10,15,20-Tetrakis(4-trifluoroacetamidophenyl)-porphyrin (7). A 1.0mL portion of trifluoroaceticanhydridewasaddeddropwiseto a solutionof 0.13g

(0.19mmol) of 6, 1.5mL of pyridine and100mL ofdichloromethane.Theresultantmixturewasstirredatroomtemperaturefor 6 h.Themixturewasthenmixedwith 200mL of 10%aqueoussodiumbicarbonateandextracted with four 100mL portions ofdichloromethane.The organic extractswere furtherwashed with two 100mL portions of water. Theorganic phasewas dried over sodium sulfate andfiltered. The filtrate was distilled under reducedpressureto remove the solvent. The resultantsolidresiduewaspurifiedby chromatographyonasilicagelcolumnelutedwith a dichloromethane/methanol(9:1)mixture to yield 0.15g (74%) of pure 7; 1H NMR(300MHz, CDCl3/traceof CD3OD) � 7.77–7.80(8H,d, J = 8 Hz, Ar3,5-H), 8.07–8.10(8H, d, J = 8 Hz,Ar2,6-H),8.80(8H, s,pyrrole-H);MS m/z1058(M�).

5-(4-Trifluoroacetamidophenyl)-10,15,20-tris(4-amino-phenyl)porphyrin (8). Porphyrin7 (0.20g, 0.19mmol)was dissolved in 200mL of a 3:1 mixture oftetrahydrofuranand methanol in a 500mL round-bottomflask.Then3 mL of 10% aqueousKOH wereaddedand the mixture was stirred undera nitrogenatmosphereat room temperature for 72h. Thehydrolysis reaction was quenchedby dilution with100mL of waterandextractedwith 100mL portionsof chloroform/methanol (4:1) until the aqueouslayerwas clear. The combined organic extracts werewashedwith 20mL of 5% sodiumbicarbonate,driedover sodium sulfate and filtered. The solvent wasdistilled from the filtrate underreducedpressureandtheresiduewaspurifiedby chromatographyonasilicagel columnelutedwith dichloromethane/hexane/ethylacetate(55:25:20)to yield pure 8 (0.05g, 34%); 1HNMR (300MHz, CDCl3) �ÿ2.74(2H,s,pyrrole-NH),4.03 (6H, brs, ArNH2), 7.07 (6H, d, J = 8 Hz,10,15,20Ar3,5-H),7.98 (8H, m, 10,15,20Ar2,6-H,5Ar3,5-H), 8.24–8.27(3H, m, 5Ar2,6-H, ArNHCO),8.77 (2H, d, J = 5 Hz, 3,7-H), 8.92–8.94 (6H, m,pyrrole-H);MS m/z770(M�).

Tricarotenoporphyrin 9. In a 100mL round-bottomflask was placed0.13g (0.24mmol) of 7'-apo-7'-(4-carboxyphenyl)-b-carotene[39] dissolvedin 20mL oftoluene.Pyridine (0.24mL, 3 mmol) was addedandthe mixture was stirred underan argonatmosphere.Then 8.8mL (1.22mmol) of thionyl chloride wereaddeddropwise.The mixture was stirred for 20minand the solvent and excessthionyl chloride wereremovedby distillation underreducedpressure.Theresulting acid chloride was dissolved in 15mL ofdichloromethane, and 0.12mL (0.15mmol) of# 1999JohnWiley & Sons,Ltd. J. PorphyrinsPhthalocyanines 3, 32–44(1999)


pyridine was added.The flask was connectedto adroppingfunnel that contained0.040g (0.052mmol)of porphyrin 8 in 10mL of dichloromethaneand0.12mL (0.15mmol) of pyridine. The porphyrinsolution was addedslowly over 5 min. The mixturewasstirredunderan argonatmospherefor 12h, afterwhich time the reactionwasquenchedwith 50mL of10% aqueoussodiumbicarbonate.The solution wasextractedwith three100mL portionsof a 9:1 mixtureof chloroformandmethanol.Theorganicextractwaswashed twice with 100mL of water, dried oversodium sulfate and filtered and the solvent wasdistilled under reducedpressure.The residue waspurified by chromatographyon a silica gel columneluted with dichloromethane/hexane/ethyl acetate(65:25:10) to yield 0.025g (21%) of tricaro-tenoporphyrin 9; 1H NMR (500MHz, CDCl3) �ÿ2.76 (2H, brs, pyrrole-NH), 1.04 (18H, s, 16C-CH3, 17C-CH3), 1.45–1.50(6H, m, 2C-CH2), 1.58–1.66 (6H, m, 3C-CH2), 1.73 (9H, s, 18C-CH3), 1.98(9H, s,19C-CH3), 2.00(9H, s,20C-CH3), 2.01(9H, s,20'C-CH3), 2.03(6H, m, 4C-CH2), 2.10(9H, s, 19'C-CH3), 6.1–6.2 (9H, m, 7,8,10C-H), 6.27 (3H, d,J = 12Hz, 14C-H), 6.34 (3H, d, J = 12Hz, 14'C-H),6.37(3H,d,J = 16Hz,12C-H),6.45(3H,d,J = 12Hz,10'C-H), 6.48 (3H, d, J = 16Hz, 12'C-H), 6.60–6.72(15H, m, 7'C-H, 11C-H, 11'C-H, 15C-H, 15'C-H),7.07(3H, d, J = 16Hz, 8'C-H), 7.62(6H, d, J = 8 Hz,1'C-H, 5'C-H), 7.99 (6H, d, J = 8 Hz, 2'C-H, 4'C-H),7.99(2H, m, 5Ar3,5-H),8.05(6H, m, 10,15,20Ar3,5-H), 8.16 (4H, s, ArNHCO), 8.21 (8H, m,5,10,15,20Ar2,6-H),8.75–8.94(8H, m, pyrrole-H);MS m/z2321(M�).

Tricarotenoporphyrin 10. Tricarotenoporphyrin 9(0.05g, 0.022mmol) was dissolved in 30mL oftetrahydrofuran/methanol (3:1), and 4 mL of 10%aqueous potassium hydroxide were added. Themixture was stirred under nitrogen at roomtemperaturefor 36h. The mixture was then dilutedwith 100mL of waterandextractedwith three100mLportionsof a 9:1 chloroform/methanolmixture. Thecombinedorganic extractswere washedwith three100mL portions of water. The organic phasewasremoved,dried over sodiumsulfateand filtered andthe solventwasdistilled underreducedpressure.Theresiduewaspurifiedby chromatographyonasilicagelcolumnelutedwith dichloromethane/methanol(97:3),yielding 0.04g (83.5%) of 10; 1H NMR (500MHz,CDCl3) � ÿ2.77(2H, brs,pyrrole-NH),1.03(18H, s,16C-CH3, 17C-CH3), 1.45–1.50(6H, m, 2C-CH2),1.60–1.65(6H, m, 3C-CH2), 1.72 (9H, s, 18C-CH3),

1.98 (9H, s, 19C-CH3), 2.00 (9H, s, 20C-CH3), 2.01(9H, s,20'C-CH3), 2.03(6H, m, 4C-CH2), 2.10(9H, s,19'C-CH3), 6.12–6.20(9H, m, 7,8,10C-H),6.27 (3H,d, J = 11Hz, 14C-H),6.34(3H, d, J = 12Hz, 14'C-H),6.37(3H,d,J = 16Hz,12C-H),6.45(3H,d,J = 12Hz,10'C-H), 6.49 (3H, d, J = 16Hz, 12'C-H), 6.60–6.72(15H, m, 7'C-H, 11C-H, 11'C-H, 15C-H, 15'C-H),7.07(3H,d,J = 16Hz,8'C-H),7.08(2H,d,J = 7.0Hz,5Ar3,5-H),7.63(6H, d, J = 8 Hz, 1',5'C-H), 8.00(8H,d, J = 8 Hz, 5Ar2,6-H, 2'C-H, 4'C-H), 8.05 (6H, d,J = 8 Hz, 10,15,20Ar3,5-H), 8.15 (3H, ArNHCO),8.23 (6H, d, J = 8 Hz, 10 15,20Ar2,6-H),8.86–8.98(8H, m, pyrrole-H);MS m/z2225(M�).

6-Carboxy-1,4-dimethoxynaphthalene. A portion of 6-carbomethoxy-1,4-dimethoxynaphthalene [18](0.1739g, 0.71mmol) was dissolvedin 25mL of atetrahydrofuran/methanol mixture (1:4), and4 mL of10% aqueouspotassiumhydroxidewere added.Themixture wasstirredat room temperaturefor 48h andthen diluted with 150mL of chloroform andtransferred into a separatory funnel. A 100mLportion of 6N HCl was added, the whole wasthoroughly mixed and let standuntil separatedintotwo layers,and the organicphasewasremoved.Theorganic extract was washedwith 100mL of water,driedoversodiumsulfateandfiltered.Thesolventwasremovedfrom thefiltrateby distillation underreducedpressureto yield the desiredacid (0.149g, 91%); 1HNMR (300MHz, CDCl3) � 3.98(3H,s,1-OCH3), 3.99(3H, s, 4-OCH3), 6.76 (1H, d, J = 8.4Hz, 3-H), 6.85(1H, d, J = 8.4Hz, 2-H), 8.1 (1H, dd, J = 8 Hz, 8-H),8.23(1H, d, J = 8 Hz, 7-H), 9.0 (1H, s, 5-H); MS m/z231(M�-1).

6-Carboxy-1,4-naphthoquinone. A portion of 6-carboxy-1,4-dimethoxynaphthalene(0.042g, 0.181mmol) wasdissolvedin 15mL of acetonitrile.Cericammoniumnitrate (0.288g, 0.525mmol) in 2 mL ofwater was slowly added dropwise. The resultantmixture was stirred for 8 min. The reaction wasquenchedby addition of 30mL of 6N hydrochloricacid. The resultant mixture was extracted withchloroform/methanol (93:7).The organicextractwasdriedoversodiumsulfateandfilteredandthesolventwas removedby distillation underreducedpressure.The yield of quinonewas 0.035g (96%); 1H NMR(300MHz, CDCl3/traceof CD3OD) � 7.06(2H,s,2-H,3-H), 8.17 (1H, d, J = 8 Hz, 8-H), 8.42 (1H, dd,J = 8 Hz, 1.5Hz, 7-H), 8.75 (1H, d, J = 1.5Hz, 5-H);MS m/z202(M�).# 1999JohnWiley & Sons,Ltd. J. PorphyrinsPhthalocyanines3, 32–44(1999)


Pentad 2. A 0.036g (0.176mmol) portion of 6-carboxy-1,4-naphthoquinonewasdissolvedin 10mLof dichloromethane, 25mL (0.176mmol) oftriethylamine were added and the mixture wasstirred. To the solution was added 0.03g(0.112mmol) of phenyl N-phenylphosphoamido-chloridate. The mixture was stirred at roomtemperature for 20min. A solution oftricarotenoporphyrin 10 (0.044g, 0.02mmol) in4 mL of dichloromethaneand25mL (0.176mmol) oftriethylaminewasaddedand the mixture wasstirredunderargonfor 20h,whereuponTLC analysisshowedthat the starting material had been completelyconsumed.The mixture was diluted with 100mL ofchloroformandwashedtwicewith 100mL portionsofwater. The organic phasewas dried over sodiumsulfateandfilteredandthesolventwasdistilled underreducedpressure.The crudeproductwaspurified bychromatographyon a silica gel columnelutedwith adichloromethane/methanol (97:3) mixture. The yieldof 2 was 0.045g (94.5%); 1H NMR (500MHz,CDCl3) � ÿ2.76(2H, brs,pyrrole-NH),1.04(18H, s,16C-CH3, 17C-CH3), 1.45–1.50(6H, m, 2C-CH2),1.58–1.65(6H, m, 3C-CH2), 1.72 (9H, s, 18C-CH3),1.98 (9H, s, 19C-CH3), 2.00 (9H, s, 20C-CH3), 2.01(9H, s,20'C-CH3), 2.03(6H, m, 4C-CH2), 2.09(9H, s,19'C-CH3), 6.12–6.20(9H, m, 7C-H, 8C-H, 10C-H),6.27(3H,d,J = 11Hz,14C-H),6.34(3H,d,J = 12Hz,14'C-H), 6.37(3H, d, J = 15Hz, 12C-H),6.45(3H, d,J = 11Hz, 10'C-H), 6.48 (3H, d, J = 16Hz, 12'C-H),6.58–6.72(15H, m, 7'C-H, 11C-H, 11'C-H, 15C-H,15'C-H),7.02–7.18(5H,m,Q2,3-H,8'C-H),7.62(6H,brs,1'C-H, 5'C-H), 8.00(6H, d, J = 8 Hz, 2'C-H, 4'C-H), 8.07 (8H, brs, 10,15,20Ar3,5-H,5Ar3,5-H), 8.17(4H,s,ArNHCO),7.95–8.35(3H,m,Q5,7,8-H),8.20–8.38 (8H, m, 10,15,20Ar2,6-H),5Ar2,6-H, 8.92 (8H,brs,pyrrole-H);MS m/z2409(M�).

5-(4-Aminophenyl)-10,15,20-tris(4-trifluoroacetamido-phenyl)porphyrin (11). Porphyrin 7 (0.30g,0.28mmol) wasdissolvedin 150mL of a mixture oftetrahydrofuran and methanol (3:2), and 2.0mL of20% aqueouspotassiumhydroxidewere added.Themixture was stirred at room temperatureunder anitrogenatmosphere.Theprogressof theformationof11 was monitoredby TLC and was found to havepeakedafter 17h. The reactionmixture was dilutedwith 100mL of waterandextractedwith three100mLportionsof chloroform.The organic layer was driedover sodiumsulfateand filtered and the solventwasdistilled under reducedpressure.The residue waspurified by chromatographyon a silica gel column

eluted with a dichloromethane/hexane/ethyl acetate(65:25:10) mixture. The leading fraction wasunreacted7 and the secondfraction was the desiredcompound11 (0.12g, 44%); 1H NMR (500MHz,CDCl3) � ÿ2.79(2H, brs,pyrrole-NH),4.02(2H, brs,20ArNH2), 7.07 (2H, d, J = 8.5Hz, 20Ar3,5-H),7.99(10H,m, J = 8.5Hz, 5,10,15Ar3,5-H,20Ar2,6-H),8.2(3H, brs, ArCONH), 8.25 (6H, d, J = 8 Hz,5,10,15Ar2,6-H),8.8 (6H, m, pyrrole-H), 8.96 (2H,d, pyrrole3-H and7-H); MS m/z962(M�).

5-(4-Benzamidophenyl)-10,15,20-tris(4-trifluoroacet-amidophenyl)porphyrin (12). In a50mL round-bottomflaskwasplaced0.044g (0.045mmol)of porphyrin11dissolvedin 20mL of toluene.A 2.5mL portion ofpyridinewasaddedandthemixturewasstirredundernitrogen. Benzoyl chloride (0.030 g) was addeddropwise.The mixture was stirred and the progressof the reactionwasmonitoredby TLC. The reactionwascompletein 6 h andthemixturewasdilutedwith100mL of water and transferredinto a separatoryfunnel. The organic phase was removed and theaqueousphasewaswashedwith two 100mL portionsof chloroform. The combinedorganic extractswerewashedwith 100mL of saturatedsodiumbicarbonateand two 100mL portions of water and dried oversodiumsulfate.After filtration anddistillation of thesolventat reducedpressuretheresiduewaspurifiedbychromatographyon silica gel elutedwith a mixtureofdichloromethane,hexaneandethyl acetate(65:25:10)to yield 0.047g (97%) of 12; 1H NMR (300MHz,CDCl3) � ÿ2.82 (2H, s, pyrrole-NH), 7.6 (3H, m,3,4,5Bn-H),7.97 (6H, d, J = 8 Hz, 5,10,15Ar3,5-H),8.05 (5H, m, ArCONH, Bn2,6-H,20Ar3,5-H),8.15–8.2 (5H, m, CF3CONH, 20Ar2,6-H), 8.24 (8H, d,J = 8 Hz, 5,10,15Ar2,6-H),8.8–8.9(8H, m, pyrrole-H); MS m/z1066(M�).

5-(4-Benzamidophenyl)-10,15,20-tris(4-aminophenyl)-porphyrin (13).Porphyrin12(0.140g,0.13mmol)wasdissolvedin 50mL of amixtureof tetrahydrofuranandmethanol(1:1),and5 mL of 10%aqueousKOH wereadded.The mixture was stirred at room temperaturefor 36h, after which time there was no trace ofporphyrin12. Thesolutionwasdilutedwith 100mL ofwater and extractedwith three 100mL portions ofchloroform. The organic extract was dried oversodium sulfate and filtered and the solvent wasdistilled under reducedpressure.The residue waspurified by chromatographyon a silica gel columneluted with dichloromethane/hexane/ethyl acetate(65:25:20) to give 0.098g (97%) of 13; 1H NMR# 1999JohnWiley & Sons,Ltd. J. PorphyrinsPhthalocyanines 3, 32–44(1999)


(300MHz, CDCl3) � ÿ2.8 (2H, s, pyrrole-NH), 4.01(6H, brs, Ar NH2), 7.05 (6H, d, J = 8 Hz,10,15,20Ar3,5-H),7.56–7.62(1H, m, Bn4-H), 7.59(2H, d, J = 8 Hz, 5ArH-3,5), 7.98 (6H, d, J = 8 Hz,10,15,20Ar2,6-H),8.01–8.03(4H, m, Bn2,3,5,6-H),8.14 (1H, brs, ArNHCO), 8.22 (2H, d, J = 8 Hz,5Ar2,6-H),8.83–8.93(8H, m, pyrrole-H);MS m/z778(M�).

Tricarotenoporphyrin 5. A portion (0.30g,0.654mmol) of 7'-apo-7'-(4-carboxyphenyl)-b-carotene[39] wasdissolvedin 70mL of toluene,and0.65mL (7.8mmol) of pyridine was added. Themixture was stirred and 0.2mL (3.27mmol) ofthionyl chloride was addeddropwise.The mixturewas stirred undera nitrogenatmospherefor 20min.Thesolventandexcessthionyl chloridewereremovedby distillation underreducedpressure.To the residuewereadded20mL of tolueneand0.1mL (1.2mmol)of pyridine.Thesolventwasagaindistilled at reducedpressureto ensurecompleteremovalof excessthionylchloride. The residue was dissolved in 40mL ofdichloromethane,and0.45mL (5.4mmol) of pyridinewas added. To this mixture was added dropwise0.099g (0.116mmol) of porphyrin 13 dissolvedin10mL of dichloromethane.Themixturewasstirredatroom temperaturefor 16h and then quenchedbyadditionof 100mL of 10% sodiumbicarbonate.Themixture was extractedwith four 100mL portionsofchloroform. The organic extract was washedwith100mL of water, dried over sodium sulfate andfiltered and the solvent was distilled under reducedpressure.Theresiduewaspurifiedby chromatographyon silica gel. Dichloromethane(100%) was usedtoelutea leadfraction that wasnot fluorescent.Thenadichloromethane/hexane/ethyl acetate (67:25:8)mixture was usedto elute the desiredcompound5(0.06g, 20%); 1H NMR (500MHz, CDCl3) � ÿ2.74(2H, brs, pyrrole-NH), 1.03 (18H, s, 16C-CH3, 17C-CH3), 1.45–1.50(6H, m, 2C-CH2), 1.60–1.65(6H, m,3C-CH2), 1.97 (9H, s, 19C-CH3), 1.99 (9H, s, 20C-CH3), 2.00(9H, s,20'C-CH3), 2.02(6H, m, 4C-CH2),2.09(9H, s, 19'C-CH3), 6.0–6.5(42H, m, vinylic-H),7.05(3H,d, J = 16Hz, 8'C-H),7.6(5H,m, 5Ar3,5-H),7.6(1H,m, Bn3-H),7.62(6H,d, J = 8 Hz, 1'C-H, 5'C-H), 7.98 (6H, d, J = 8 Hz, 2'C-H, 4'C-H), 7.94–8.00(10H, m, 10,15,20Ar3,5-H,Bn2,3,5,6-H),8.14 (4H,brs, ArNHCO), 8.20 (8H, m, 5,10,15,20Ar2,6-H,),8.80–8.96(8H, m, pyrrole-H);MS m/z2329(M�).

Carotenoporphyrin 14.To a50mL round-bottomflaskwas added 0.048g (0.09mmol) of 7'-apo-7'-(4-

carboxyphenyl)-b-carotene dissolved in 20mL oftoluene. Pyridine (0.11mL, 1.35mmol) was addedand the mixture was stirred under an argonatmosphere.Thionyl chloride (34mL, 0.45mmol)was added dropwise. The mixture was stirred for15min and the excessthionyl chloridewasremovedby distillation underreducedpressure.Thecrudeacidchloridewasdissolvedin 15mL of dichloromethane,and0.11mL (1.35mmol) of pyridinewasadded.Themixture was stirred under an argon atmosphereas0.030g (0.030mmol) of porphyrin 11 in 10mL ofdichloromethanewasaddedover a 5 min period.Themixturewasstirredunderanargonatmospherefor 6 hand the reaction was quenchedwith 50mL of 5%aqueous sodium bicarbonate. The solution wasextracted with three 100mL portions of a 9:1mixture of chloroform and methanol.The organicextract was washedwith three 50mL portions ofwater,dried over sodiumsulfateandfiltered and thesolvent was removedby distillation under reducedpressure.Chromatographyon a silica gel columneluted with dichloromethane/hexane/ethyl acetate(65:25:10) gave 0.032g (70%) of 14; NMR(300MHz, CDCl3) � ÿ2.81 (2H, brs, pyrrole-NH),1.04(6H,s,CH3-16,17C-CH3), 1.45–1.50(2H,m, 2C-CH2), 1.60–1.66(2H, m, 3C-CH2), 1.73(3H, s, 18C-CH3), 1.99(3H, s, 19C-CH3), 2.00(3H, s, 20C-CH3),2.01 (3H, s, 20'C-CH3), 2.04 (2H, m, 4C-CH2), 2.10(3H, s, 19'C-CH3), 6.10–6.72(13H, m, vinylic-H),7.07(1H, d, J = 16Hz, 8'C-H), 7.62(2H, d, J = 8 Hz,1',5'C-H), 7.99 (2H, m, 2',4'C-H), 8.00 (6H, d,J = 8 Hz, 10,15,20Ar3,5-H),8.05 (2H, d, J = 8 Hz,5Ar3,5-H), 8.19 (4H, brs, ArNHCO), 8.25 (6H, d,J = 8 Hz, 10,15,20Ar2,6-H),8.30 (2H, m, 5Ar2,6-H),8.83–8.93(8H, m, pyrrole-H);MS m/z1479(M�).

Carotenoporphyrin 15.To a50mL round-bottomflaskwasadded0.032g (0.022mmol)of carotenoporphyrin14 dissolved in 20mL of a 3:1 mixture oftetrahydrofuranand methanol. A solution of 10%aqueouspotassiumhydroxide(2 mL) wasaddedandthemixturewasstirredunderanargonatmospherefor17h. The mixture was diluted with 100mL ofchloroformandextractedwith three100mL portionsof water. The organic layer was dried over sodiumsulfateand filtered and the solventwas removedbydistillation under reducedpressure.The yield of 15obtainedwas0.023g (89%);NMR (300MHz, CDCl3)� ÿ2.72(2H, brs,pyrrole-NH),1.04(6H, s,16C-CH3,17C-CH3), 1.45–1.50(2H, m, 2C-CH2), 1.58–1.66(2H, m, 3C-CH2), 1.73(3H, s,18C-CH3), 1.98(3H, s,19C-CH3), 2.00(3H, s, 20C-CH3), 2.02(3H, s, 20'C-# 1999JohnWiley & Sons,Ltd. J. PorphyrinsPhthalocyanines3, 32–44(1999)


CH3), 2.04(2H, m, 4C-CH2), 2.10(3H, s,19'C-CH3),4.10(6H,brs,ArNH2), 6.10–6.72(13H,m,vinylic-H),7.07 (H, m, 8'C-H), 7.07 (6H, d, J = 8 Hz,10,15,20Ar3,5-H), 7.62 (2H, d, J = 9 Hz, 1',5'C-H),7.99 (2H, m, 2',4'C-H), 8.00 (6H, d, J = 8 Hz,10,15,20Ar2,6-H), 8.03 (2H, d, J = 9 Hz, 5Ar3,5-H),8.16 (1H, brs, ArNHCO), 8.22 (2H, d, J = 9 Hz,5Ar2,6-H), 8.85–8.94(8H, m, pyrrole-H); MS m/z1191(M�).

Carotenoporphyrin 4. In a 50mL round-bottomflaskwas placed0.023g (0.019mmol) of 15 dissolvedin2 mL of pyridine.To this mixturewereadded2 mL ofaceticanhydrideandthemixturewasstirredunderanargonatmospherefor 10h. The reactionmixture wasdilutedwith 100mL of chloroformandextractedwith100mL of saturatedsodium bicarbonateand three100mL portions of water. The organic extract wasdriedoversodiumsulfateandfilteredandthesolventwas removedby distillation underreducedpressure.The residuewas purified by chromatographyon asilica gel column eluted with a 9:1 mixture ofchloroform and methanolto yield 0.025g (97%) ofcarotenoporphyrin4; NMR (500MHz, CDCl3/5%CD3OD) � ÿ2.80(2H, brs,pyrrole-NH),1.04(6H, s,16C-CH3, 17C-CH3), 1.45–1.50(2H, m, 2C-CH2),1.60–1.65(2H, m, 3C-CH2), 1.73 (3H, s, 18C-CH3),1.99 (3H, s, 19C-CH3), 2.00 (3H, s, 20C-CH3), 2.02(3H, s,20'C-CH3), 2.04(2H, m, 4C-CH2), 2.10(3H, s,19'C-CH3), 2.34 (9H, s, acetamido-CH3), 6.12–6.21(3H, m, 7,8,10C-H),6.28 (H, d, J = 10Hz, 14C-H),6.34(H, d, J = 13Hz, 14'C-H), 6.38(H, d, J = 16Hz,12C-H), 6.45 (H, d, J = 12Hz, 10'C-H), 6.48 (H, d,J = 15Hz, 12'C-H), 6.60–6.72 (5H, m,11,11',15,15',7'C-H), 7.07 (H, d, J = 16Hz, 8'C-H),7.62(2H, d, J = 8 Hz, 1',5'C-H), 7.93(6H, d, J = 9 Hz,10,15,20Ar3,5-H), 8.02 (2H, d, J = 8 Hz, 2',4'C-H),8.07 (2H, d, J = 8 Hz, 5Ar3,5-H), 8.15 (6H, d,J = 8 Hz, 10,15,20Ar2,6-H),8.21 (2H, d, J = 8 Hz,5Ar3,5-H), 8.81–8.90(8H, m, pyrrole-H); MS m/z1317(M�).

Instrumental Techniques

The 1H NMR spectrawererecordedon Varian Unityspectrometersat 300 or 500MHz. Unlessotherwisespecified,sampleswere dissolvedin deuteriochloro-form with tetramethylsilaneas an internal reference.Mass spectrawere obtainedon a Varian MAT 311spectrometeroperatingin EI modeor amatrix-assistedlaserdesorption/ionizationtime-of-flight spectrometer(MALDI-TOF). UV-vis spectrawere measuredon a

ShimadzuUV2100UUV-vis spectrometer,andfluor-escencespectraweremeasuredon a SPEXFluorologusingoptically dilute samplesandcorrected.

Cyclic voltammetric measurementswere carriedout with a PineInstrumentCompanyModel AFRDE4potentiostat.Theelectrochemicalmeasurementswereperformed in benzonitrile at ambient temperatureswith a glassycarbonworking electrode,an Ag/Ag�reference electrode and a platinum wire counterelectrode.Theelectrolytewas0.1 M tetra-n-butylam-monium hexafluorophosphate, and ferrocene wasemployedasan internalreferenceredoxsystem.

Fluorescencedecaymeasurementswereperformedon �1� 10ÿ5 M solutions by the time-correlatedsingle-photon-countingmethod.Theexcitationsourcewasa cavity-dumpedCoherent700dye laserpumpedby a frequency-doubled Coherent Antares 76sNd:YAG laser[40]. Theinstrumentresponsefunctionwas35 ps, asmeasuredat the excitationwavelengthfor eachdecayexperimentwith Ludox AS-40.

Nanosecondtransient absorption measurementswere made with excitation from an Opotek opticalparametricoscillatorpumpedby thethird harmonicofa ContinuumSureliteNd:YAG laser.Thepulsewidthwas �5 ns and the repetition rate was 10Hz. Thedetection portion of the spectrometerhas beendescribedelsewhere[41].

Acknowledgements

This work was supportedby a grant from the USDepartmentof Energy(DE-FG03-93ER14404). Thisis publication350from theASU Centerfor theStudyof Early Eventsin Photosynthesis.

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