A SIMPLEMICROCELLFORCYCLICVOLTAMMETRY: APPLICATIONSIN FLILLERENEELECTROCHEMICALINVESTIGATIONS MatthiasEiermarur"'1, Robin G. Hicks",2, and Brian W. Knightl, HelmutNeugebauerb,* FredWudl"e slnstitute Polymers for and OrganicSolids,Departrnents ChemistryandMaterials, of Universityof Califomia,Santa Barbara, CA 93106-5090 . 'LIOS, PhysicalChemisky, Johannes KeplerUniversityLinz, Altenbergerstr. 69, 4040Lm2,Austria of "ExoticMatedalsInstituteandDepartment ChemistryandBiochemistry, University California, Charles YoungDr. East,LosAngeles, 90095-1569 of 607 D. CA Abstract Due to high expenditure associated with fullerenechemistrythe amountof availablematerial for characterization usually quite small. This paper is describesa simple experimentalsetup for executingaqueousand nonaqueouscyclic voltammetric measurements small solution volumes on (around pl). This "microcell"hasbeenextensively 50 employed study to the solutionelectrochemistry buckminsterfullerene its derivatives. of and A significant dependence ofthe redoxbehaviour the carboncage's on n electronnumberand on the nature ofthe substituents found. Selected is resultsof cyclic voltammetricinvestigations tabulated. are INTRODUCTION Sincethe discovery(l) and isolation(2) of the carboncagemoleculeC60, an immense amountof research beencarriedout on the fullerenesand their derivatives has (3,4). From the beginning,the electronicand in particularthe redoxproperties havebeen central manystudies to ofC60; as such,electrochemical studies havebeenandcontinue to be an importantsubset firllerene (5-10).We (11-15) and others(16-20) of research have also made extensiveuse of solution electrochemistry probe the electronic to structureof chemicalderivativesof C6g. Knowledgeof the redoxbehaviorof fullerene compounds also crucial in light of the outstanding is solid stateelectronicpropertiesof alkali metal salts of COO(21) and the photophysical behaviorof C6g/conjugated polymercomposites (22,23)andfi.rllerene (24). containingdyadsystems The high costsassociated with performingfullerenechemisbyoftennecessitates very small scalereactions. The amountsof availablematerialsfor characterization purposes are therefore usually quite small. Conventional electrochemicalcells for cyclic voltammetrywould require severalmg of C6g or its derivatives. We were therefore
' Current address:BASF-AG, ZAGIE, 67056 Ludwigshafen, Germany ' Current Address: Departmentof Chemistry, University of Victoria, Victoria, BC, Canada Conesponding author,Tel+43-732-2468-8766,f:rx+43-732-2468-8770, e-mail helmut.neugebauer@j ku.at
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motivatedto design a microscaleelectrochemical cell, the details of which we now nlovide in this publication. The straightforwarddesign makes this an attractive and simplemeans studyingsolution electrochemistry of whän sampleamountand simplicify ofdesignis an issue.
E)(PERIMENTAL in cell - The cell setupis depictedschematically Figure l. The three-electrode is housedin a pyrex "cross" (A) which is contained an argon-filled dry box (or in the in absence thereo{ a home-bujltinert atmosphere chamber).Tf,e bottomopeningorthe ceil holder(B) is a femalel4l20 ground-glais joint; the otirerthreeopenings aräsimply 11 s{alght_glasstubing. All three electrodesare of standard-size fpurchasedfrom T;n Bioanalyticalsystems, west Lafayette,Indiana)andas suchcanbe interöhangeably used with conventional cells. The platinumdisk working elechode(c) is inseied upsidesize downinto the bottomopeningand is held in place with a l4l20 bevelsealuniversaiglass adapter (Fisherscientifrc). A platinumwirJauxiliary electrode is housedin the left @) compartment suchthat the wire sits over the working electrode, is secured and with a cut piece of hard plastic tubing (e.g., Nalgene) (C;. fne non-aqueous Ag/AgCl quasi reference electrode Aq wire coveredwith Agcl in the respeciive (!; electäfie) resides rn thetop openingwith a similarplastictubinginsert(G) anda rubberO_ring(f0.
Figure 1. schematic drawing of the electrochemical microcell assemblv. A. pvrex cross;.B,_Female l4l20 ground glassjoint; C, pt disk working electrodä;O, i't wlre counterelectrode; E, Non-aqueous AglAgcl quasi referenceelectrode; n, slvet sea universaladapter;G, Plasticadapters;H, rubberO-ring.
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electrode in therange between working electrode thereference the and is The distance (1 0.1 mM in and | -2 mm.Thecell solution mM in analyte, M in electrolyte, 0.5-1.0 of is as ferrocene internalreference) introducedvia syringethroughthe fourth openingon a the right hand side. Two drops (ca. 50 pl) of solution are sufficient to generute which adequately coversthe threeelechodes.The remainingcell openingis meniscus then capped with the underside of a 24140rubber septum and electrochemical experimentsare performed via a computer (using a Bioanalytical Systems 100W are to and workstation)outsidethe drybox. The computer potentiostat connected the cell in built-in throughputs the wall ofthe dry box. via standard RESULTSAND DISCUSSION Low-volume electrochemicalcells have been reported (25) and indeed several designs are commercially available. The cell design describedhere has several over existing setups: (1) The commercial"microcells"areusually designed advantages electrochemistry this cell can operatein either non-aqueous aqueous or for aqueous solutions. (2) The existing non-aqueouscells for low-volume work still require larger solution sizes(- 0.3 mL) and thereforelargeranalye quantities than substantially can the presentsystem,in which 1 mg of organofullerene be used for as many as 20 experiments.(3) The cell components all "standard" are size and aretherefore separate with a conventionalcell. The pyrex "cell holder" is inexpensiveand interchangeable easily made from shaight tubing. (4) The simple setup allows for extremelyfacile cleaning and changing of the cell solution, which is important when, for example, otrtothe electrodes necessitates re-assembly. cell analyteadsorption Control experiments have been carried out to determine the extent of solvent from the cell. Voltammograms either C66 or ferrocene of evaporation wererecorded in toluene:DMF, benzonitrile, and acetonitrile.The variationin 1,2-dichloroberuene,3:2 the peak currents with time for either the fust reduction of C6g or the oxidation of ferrocenewas a57o with no systematicincreasewith time. We can thereforerule out effectson the electrochemical datagenerated usingthis assembly. solventevaporation
Figure 2 shows a cyclic voltammogramof C6g run with the microcell setupwith a solventmixture of 3:2 toluene:DMF. Within the solventpotentialwindow threequas! in reversiblereduction peaksare seen,the values for which arepresented Table L The table also contains the correspondingvalues in l,2-dichlorobenzenefor C6g and a number of derivatives (structures1 - 9 in Figure 3), By convention,the values are to redox couple. As is evident from the valuesin referenced the ferrocene-fermeenium potentials the table, chemicalmodification of C6g clear\ has an effect on the reduction of and,by inference,the electronic structureof the moleculein question, Comparison L (58nelectrons)and2(60nelectrons)aswellas3(58nelectrons)and4(60fielectron points clearly to the effect of fullerene zt electron count on the carbon cage's as electronegativity.tn addition, electron withdrawing substituents, cyanogroups(7 and I in Figure 3), shift the reductionpotentialssignificantly in a more positivedirection.
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-2000
-15ü) -1000 -500 potential mV Electrode /
0
Figure 2. Cyclic voltammograrn of C6g generated with the microcell in 3:2 toluene:DMF at room temperature. Scan rate : 100 mVls. The potential values refer to the peak maximum of the ferrocenereduction.
Table I Electrochemical data generated with the microcell of Figure l.
Compound
c6o" c60
1b
1 -827 -1123
ReductionPotentials mV vs. FclFc* / 2 3 -1329 -1915 -1450 -1913 -t549 -1525 -1554 -1531 -1554 -1531
Ref.
4
2 3 4
5
6 7 8 I
-2050 -2000 -2064 -2001 -2044 -2031 -1458 -1943 -935 -1330 -1800 -997/-1071 -t424t-1485 -1979t-2089
-1169 -1135 -1164 -1141 -1164 -1151 -L073
Q6) Q6) {27) (27) (13) ( 13) (15)
-2504 -2410 -2225
(ls) (28)
All experiments performed l,2-dichlorobenzene in solventexceptasnotedotherwise. 'in 3:2 toluene:DMF. bcompound structures shownin Figure3 are
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Figure 3. Structures the firllerenederivatives. of Donor-acceptor systems with fullerenes as electron acceptorsare iryp66as1 ln _ photovoltaicdevice applications.In thesesystems,photoexcitation usually ofthe dono, component followed by an elechontransfer to the fullerenic acceptor, is which mav be either covalently attached the donor (24), or mixed with a donor forming a *o to 'lbulk heterojunction" structure (23). Important for the photovoltaic propinies "u["d is the question, whether charge transfer occurs already in the ground state oi onlv in the photoexitedstateofthe donor-acceptor system.A convenientway to study ground state interactionsis by cyclic voltammetry,since chargetransferis reflectedby changes the in redoxpotentialscompared with the redox potentialsof the individual components. with the microcell setup, severalpossibly photoactivedonor-acceptor systemswith covalent linkage betweena fullerene and a n conjugatedsystem(structures10 - 12 in Figure 3) were studied.In Figure 4, cyclic voltammogramsof the compounds shown arc and,compared with the referencecompound13. As can be seen,all reductionpeaksare shifted to more negativepotortial valuescomparedwith c6g (Figure2), due to the lower numberof n electrons.However,no signifrcant differenceswithin the sedeswere found. showing the absence ground stateinteractions in these compounds, of only the pyrene containing system (11) has additional small redox feaürresat more negativepoöntials than the main redox peaks, which are most likely due to the presengeof di- or multisubstituted fullerenes (remaining from the synthesis procedure)and the fu*her reduced numberofr electrons.
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v
i l a i .
i
i t i i
-2000
-'t500 -1000 -500 potential Electrode / mV
0
Figure 4. Cyclic voltammograms fullerenederivativesgenerated with the microcell in of 3:2 toluene:DMF roomtemperature. rate: 100mV/s.Solid:hydrogen at Scan substituted (13! dashs:phenyl substituted (10); dash-dots: pyrene substituted(11); dots: perylene substituted(12). The potential values refer to the peak maximum of the ferrocene reduction. CONCLUSIONS Giventhe intense interestin the electrochemical behaviorof the fullerenes, feel the we small-scaleelectrochemical method describedhere is a simple and useful meansof obtaining data with extremelysmall quantitiesof material. There is, or course,good reason believethe setupshouldbe usefulfor othercompounds to whenanalytequantities arelimited.
ACKNOWLEDGMENTS Financial Support from the national ScienceFoundationthrough the MRSEC and throughgrant DMR-95-00888 gratefully acknowledged.R. G. H. thanksthe Natural is Sciencesand EngineeringResearch Council for a Postdoctoral Fellowship. M. E. is indebtedto the Alexandervon Humboldt-Stiftung,Bonn, Germany,and to Prof. P. C. Ford, UCSB, for their supportwithin a Feodor-Lynen-fellowship. N. thanksfor the H. süpport by the Austrian Ministry of Economic Affaiß within the Christian Doppler Society's dedicated Laboratory ofPlasticSolarCells.
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