Electrochemicalandspectroscopiccharacterization

Electrochemicalandspectroscopiccharacterization - Journal of

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Unformatted text preview: Journal of Electrotmtilyttcal Chemistry All) ( I996) leis. [it Electrochemical and spectroscopic characterization of anodically formed nickel salen polymer films on glassy carbon, platinum, and optically transparent tin oxide electrodes in acetonitrile containing tetramethylammonium tetrafluoroborate Christopher E. Dahrn “, Dennis G. Peters Jacques Simonet b" : .l'Jcpttrtme'rtt ril'thht'ttns'tr). htdiwm l.-'m't-i.-r.ttt_r. Bloommymn. iN-I 23-105. f .5531 b Loom-worry it"Etrctrrit'itontc Urguttiqm'. Furor-rsttr: tic Hermes l', Cmnpus rt'r' Brrtulrcn. 35043 Kcrtttcs IL'tiiJ't't. France Received ll December I‘J95 A hstraL-t Anodicolly polymerized films of nickel salcn formed on glassy carbon. optically transparent t'tn UKitic‘ and platinum electrodes in :tcetonitrile containing tetramethylammonium tetrafluoroboratc have been examined by means of cyclic voltammetw. thin—layer voltammetry. spectroelectrochemistry. and scanning electron microscopy. With the aid of thin-layer volmmnictry, it has been confirmed that the global oxidative polymerization of nickclllll salcn involves three electrons per monomer. Polymerization proceeds through two distinct phases. the formation of which depend on the potential. Once the polymer film has been formed. the anodic process consists of the r-cyctsible one-electron nickellllll/“nickcllill redos couple. Cyclic voltarnmetry along with spectroclectrochernistry has been employed to probe the roles of the nickelllili/rtickellllt and nickclllll/nickeliIl redox couples in the electrochemical response of the polymer film as well as the interconversion of the different oxidation states of nickel. Ki.v_t-n-ur.t.-.: Nielth salt‘n'. Antttlic polymerization: Polymeric materials: Cyclic voltammetry; Spectt'oeleetrochemistry; thin-layer vollamnieti'y I. Introduction There has been abundant interest in the preparation and use of chemically modified solid electrodes [I]. including those coated with anodically polymerized films of metal salen cotttplcttcs. l-lorwitz and Murray [2| synthesized the Al.4’«dil :V-niethyle-phcnylniethyllsalcn complexes of nickelill). cobaltilll. anti manganeselilll and oxnlalivcly polymerized these compounds onto the surfaces of plat- intttn electrodes. Almost simultaneously. Goldsby and co— workers l3-—5I electropolymerized niekelllll salen onto hoth platinum and optically transparent tin oxide elec— trodes; the resulting polymer films were examined by means of cyclic voltammetry and scanning electron mi- croscopy. More recently, Bedioui et til. [6] investigated the anodtc polymerization of nickeltll)‘ cohallllll. anti man- gartesellll} salon onto glassy can-hon electrodes. Andcbert and ctr-workers [7—l0] have puhlislted :1 series of papers dealing with the electrochemical polymerization of salen ' Cottesprirtding authors. tiltZE-liTJH/‘io/Slill'l) 55‘; WW: EISCHCI' Science SA. All rights reserved 55!)! 0032 lll'EHirij lilLl-h' Ell-l complexes of coppertlll. nickelllll. cohaltllli. and zinctlll onto platinum anodes; these workers have probed the propagation of charge through these films as vvell as the lifetimes of reactive intermediates generated by electron transfer at the anode. [it work from one of our laboratories [1 ill]. efforts have been directed at the use of carlion electrodes coated with anodically formed films of poly- meric rtichelllli salcn for the preparative—scale catalytic reduction of iodocthanc. 2-iodopropane. and onto-Lil- haloalkancs via a polymer—hound clectrogenertttetl nickcllll salon species. This paper offers a contribution to the further tinder- Stttnding ol' the clectrocltcnticttl attd spectrochcmical prop- erties of anodieally formed nickel salen polymer films on glassy carbon. optically transparent tin oxide. and platinum electrodes in acetonitrile containing tetramethylammoninm tetrafluoroborale (TMABFfl. In the course of corroborat- ing some earlier findings concerning the properties of these films. Wt: have employed the method of thin-layer electrochemistry with a glassy carbon electrode to show that the osidative polymerization of nickelllll salcn in- volvcs thr- loss of three electrons per monomer. used IM ('15. Union at at. ./ Joan-ml oft-ileumutirtlynmi (.‘t'icmt's-rry «f it) H996! Mi- I?! spectroelectrochctnislry to examine the interconversion of the several oxidation states of nickel. and utilized cyclic voltamtnetry and spectroelectrochernistry to demonstrate that a nickel saten polymer filth exhibits electrochemical behavior that is strongly dependent on the thickness of the layer and on the mode of its electrol'ormation. 2. Experimental 2. 1. Reagents Acetonitrilc (Burtlick and Jackson high-purity ‘LJV‘ grade). which was continuously refluxed over calciunt hydride was employed as solvent for all electrochemical experiments. 'l'l‘v‘lt-‘tBF,I (Aldrich. 97%) used as supporting electrolyte was stored in a vacuum Oven at 65°C. Commer- cially available nickclllll salen (Aldrich. 98%). or “2.2”— [I ,2—ethanetliylhistnitritomethylidyneflhislphenolatoll-N.N'. 0.0”]nickellll). was used without further purification. All tleaeration procedures were carried out with Air Products UHF-grade argon. 2.2. Cells. electrodes. insmmtcntntiott and procedures A description and picture of the cell used for cyclic voltammetry were presented in an earlier publication [13]. A glassy carbon rod (Tokai Electrode Manufacturing Com— pany. Toltyo. Japan. Grade (EC—'20) was press-fitted into Kel-F to provide a planar. circular working electrode with a geometric area of 0.077 crnl. All potentials are quoted with respect to a reference electrode consisting of a cad— mium-saturated mercury amalgam which is in cont:th with dimediylformamidc saturated with both cadmium chloride and sodium chloride: this electrode has a potential of --U.76 V vs. the aqueous saturated calotncl electrode at 25°C l|4.15]. To achieve conditions suitable for thin-layer cyclic voltarnmetry. the above—mentioned glassy carbon electrode was carefully polished in the plane perpendicular to its axis anti was positioned [lush against the flat bottom of the cell. in a manner similar to that reported previously [1(1I; under these conditions. diffusion of electroactive species into or out of thc thin-layer region was insignificant. Proper thin—layer behavior was verified with a solution of a known concentration of ferrocene in acetonitrile containing [Ltlfit] M "fMABlfi. [rt thin—layer experiments involving nickeltlll salen. a known concentration of ferrocene was present to scrvc as an internal reference for the calculation of n values for the oxidation and reduction of nickellli) salett. To perform the spectroelectrocltetttieal studies, we constructed a matched pair of cylindrical three—electrode cells. each of which had on one of its ends an optically transparent. indium—doped, tin oxide—coated quartz window (Pan No. CQ—QOIN-(tl It}. Delta Technologies. Ltd. Still- water. MN}; using these cells we were able to form nickel salen polymer films directly on the surface of a conducting tin oxide window and to monitor and record in situ the ultraviolet—visible spectrum of the polymer a function of potential with the aid of a Hewlett—Packard Model llPSdelA diode—array spectrophol0meter. ln perforating the scanning electron microscopy esami— nations of nickel salon polymer films. we employed thin platinum foils us anodes. After the polymer films had been formed on the platinum electrodes. the films were washed thoroughly with acetonitrile and were air dried before being sputter coated with gold. Scanning electron micro- graphs were obtained with a Cambridge Stereoscan 250 Mk ll instrument operated at. an electron-accelerating volt— age of 2” keV. Some polymer films examined by means of scanning electron microscopy were fomted from a 2.0 mM solution of monomeric nickellll'] salen in acetonitrile containing 0.050 M TMABEl by means of controlled—potential osida~ tion at +2.25 V for a prescribed period of time. theorem. the usual procedure employed to form polymeric nickel salen films entailed a cyclic voltamtnctric method. A 2.0 mM solution of tuonon'teric nickellll} salon in acetonitrile containing (1.050 M 'I'MABH, was transferred into an electrochemical cell and the potential of the electrode {glassy carbon. tin oxide. or platinum) was scanned once from {I to +2.25 to it V at a chosen scan rate (specified later at appropriate places in the tcxtl: hereafter. at various places in this paper. we refer to lltis general procedure as ‘one polymerization scan‘. After the polymerization step. tlte electrode was rinsed with acetonitrile to remove the solution—soluble monomeric nickelflll. To remove a poly- mer film from the glassy carbon electrode fttllou'ing a set of cyclic voltammetric experiments. the electrode was cleaned with an aqueous suspension of 0.05 not alumina on a Master—Tex (Beuhlerl polishing pad. 3. Results 3. l. C yct’t‘ r‘ ttolratttmcrry Fig. 1. curve {A}. shows the first cyclic vottammogrant recorded at lttf) mV s‘E for the anodic polymerization of 2.0 mM nickelfill salen onto a freshly polished glassy carbon electrode in acctonitrile containing 0.050 M TMABE. As the potential is scanned in the positive direction front +0.50 to + 2.50 V. two overlapping waves (ED. = + |.?4 V and EM = +1.85 V) are seen. and it was found that this feature does not depend on the concentrat- tion of nicl-tel(ll) salcn. Such behavior appears to he in accord with that described in earlier investigations [3-— 6.0.| I]. On the basis of our previous work [ll] involving a study of the anodic behavior of separate solutions of salen and of nickelllll salen. the first step is assigned to oxida- tion of nick-elm) to nickelflll) and the second step is attributed to oxidation of the salon ligand. Furthermore. by cxatttitting the. etectrochemistry of a series of sytttrnclrtc and unsyntntctric nickelllll Schiff base complexes. Goldshy [3] concluded that nickeillll salen is initially oxidized to its Cal-f. Hahn: rt rtt'. (J .J'rltrnrol of Elll't rr't-rmttlytit ttl t.'ln:rrtt.trr'_v 4M} “9933? .lt‘t.l— I?! |fij CURRENT .' “A o c: 2.5 2 D l 5 1.0 O 5 POTENTIAL H! I In :IL't’lttttllrilt'. containing 0.050 M 'l'MAlle. IA! First positive—negativi: sweep ill positive-potential “1310!! with .I freshly polished glass}..- em-lton clearlle- in solvent-supporting electrolyte containing 3.0 ntM nicL't-lllll salt-n. ill}. (Cl. ll)! First three posilivt' neg-alive potential sweeps in positive-polen- Fig. I Cyclic voltanimogratns recorded at lllt‘t nt‘v' n; ttztl region With the precoated electrode of curve (Al in St'tlvent-sttppotling electrolyte. free of ntckelilll salt'n. nickeltllll counterpart which {because it is poorly stabi— lized by molecules of acclonitrile. a weak donor solvent. at its axial coordination sitesl undergoes an intramolccular electron—transfer reaction to give a ligand—oxidized species which forms; a polymer l'ilnt on the anode. On the nega— tive—going scan from +2.30 to +0.50 V in Fig. I. curve (A). there is a single wave assigned to the reduction ol' nickel‘llll} to nickellll} within the polymer film. When a precoated electrode obtained via a so-called one [allyrnerization scan at 100 th s'" (Fig. I. carve {All was rentoved from and washed free of the monomeric nickelilll salen solution and placed into aeetonilrile con- taining only 0.050 M Th-lABFy. and the potential was scanned at ltll) mV s ' l'ront +0.50 to +250 to +0.50 V. the cyclic voltarnntograms depicted in curves {B} and {C} of Fig. l were recorded. Cutve (B) of Fig. l is typical of the first positive—negative potential sweep after the deposit has been formed; characteristically. one observes lht'ec poorly-resolved anodic waves anti :1 single cathodic wave at potentials which differ somewhat front those seen in curve (A) of Fig. 1. Evidently. during this first positive—negative potential sweep. lhe l'reshly‘ lot-tiled polymer film undergoes some l‘unher and permanent elec— trochemical ntodificalion. littlerltttntp and Goldsby at- lrihtttcd such behavior to the oxidation ot‘ traces ol‘ residual monomer and ttnpolymerized oligonters trapped within the polymer l'ilm, liut later discussion in this paper will ot‘t‘er a somewhat different view. Curve {(7) of Fig. l is the positive—negative potential sweep recorded immediately after curve (13) was obtained. Once the condition that gives rise to curve (C) is established. at least ten more virtually identical positive—negative potential sweeps (Fig. 1. curve (Dll can he recorded in succession. Fig. 2 shows the behavior of a precoated electrode in the region of negative potentials from U to -- 1.30 V as well as a new feature in the region of positive potentials. To obtain these cyclic voltatnmograms. we performed one polyttterizatiott scan at 100 mV s 'I' lcondilions corre- sponding to the acquisition of Fig. 1. curve loll. washed the polymer-coated electrode tree ol' monomeric nickeltll) saten. transferred the filmed electrode into air—tree aeetoni— trile containing only [1.050 M 'l'MAlllil. and recorded curves {A}. (Bl. (Cl. and (D) of Fig. 2 in succession. Sweep 1 of curve. (fit) (the first cyclic voltatntnogram recorded in the absence of solution—phase niekelilll salenl shows the reversible one-electron redaction ol‘ nickelllll to niekeltll in the polymer film. and a prcpcak is always seen on the rising portion of the cathodic wave on the first sweep. ltt contrast. subsequent sweeps in curve {A} show no cathodic pt‘epeak. Curve (8) of Fig. 2 illustrates what happens when. immediately after cltrvc (A) was recorded. the potential of the same polymer-coated electrode was swept first in the positive direction front +0.10 to l- 1.04 V before being swept in tlte negative direction (0 to — 1.30 V}; on anodic prepeak is seen at approximately +1.0 V. and the l'tehavior observed at negative potentials mimics that of curve (Al. Curve if) of Fig. 2 was recorded immediately after curve (B). and. involves scanning the potential in the positive region ( +0.25 to -t- 2.I5 V]; once CURRENT ." tfi :: j} I l Em I _ _. c: 2.5 2.0 1 5 H: 0.5 0D 43.5 -‘| U -1 5 POTENTIAL HI" Fig. 2. Cyclic \ttltanintograms retorded at 1011 ntVs tor a glassy carbon electrode precoaled as itt Fig. HM ttt :tt‘etottitrile containing 0.050 M 'l'MABl", without solution-phase nickelllll salen. {Al Potential was cycled in negative potential region only. “3) At'tm (Al. the potential was scanned first in the positive direction jtlil to approximately I H! V and then cycled exclusively in the negative-potential region. {C} Al'tcrtB}. the potential was cycled it] the positive-potential regtott only {D} After {C}. the potential was cycled Ill the negative-potential region only. loft ('Jz'. Urrrltm rt tit/Journal rtt‘ Efct'trrimmlyrir rrl [.‘hrrnrt-rry 4' m l i996} it'll—EYE again. the anodic prepeak appears at + |.U V and. at more positive potentials. the response previously described for curves (ill and {C} of Fig. l is observed. Finally, curve (D) of Fig. 2. which was obtained immediately after curve {(7), again shows a step assigned to the reversible nickeHlll/nicltellll cottple as well as a prepeak on sweep I [at a potential that is more negative than those for the cathodic prepeaks in curves {A} and (Bi of l-‘tg. 2). For sweeps 2 and 3 of curve {D}. there is no cathodic prep-311k and the cathodic and unodic waves for the polymer-hound nickeltlll/nickelfll redos couple are smaller than for sweep I. Similar prepeaks l'or electronically conducting polymers have been previously observed and esplained for a family of polylluorenes [l7]. Moreover. a comparison of curve (D) with curves (A) and (13} reveals a striking and reproducible feature. namely that the sizes of the cathodic and anodic peaks in curve (D) are significantly smaller thart those for curves (A) and (B). Apparently. when a nickel salen—coated electrode obtained by means of one polymerization scan (Fig. 1. curve (All is subjected to additional electrochemical oxidation (Fig. I. curves (Bi and (C) or Fig. 2. curve (Cl) before being.I used to explore the nickclilll—Itickclill regime. less of the polymer-hound nickeltlll can be convened into nickeltll. We attribute this last t'thservation to the fact that. when the filtti is in its fully oxidized (reticulated and tttore compact] state. the those:- tttettt of charge compensating tetrantethyIainntonium cations within the polymer is slower. Ihcrclty preventing all of the nickel sites in the polymer from being accessed electrochemically. 3 . 2‘. Thin slayer L'Offcmmlc'flj.’ To gain more information about the number ot'electrons involved in the formation and reduction of nickel salen cunneurran I 50 as 2.5 2]] 1.5 1.0 POTENTJALFV Fig. Li. 'lltin-layer cyclic voltammograms recorded at Ill mV s" In acetoniuile containing 0.050 M TMABF, and 1.0 mM nickeltlll salon. to) First positive negative cycle obtained with a freshly polished glassy carbon electrode. {5). {Cl Nexl two positive negative cycles obtained after curve (A). 030- —- — — — 025 fl 0.20 l I 0.15 -i ate; ABSORBANCE 005 ‘ 0.00 - 200 300 4GB 500 EDD N30 8013 900 W'AVELENGTH t' rim Hg. :1. lillrax'iolcl--\'isthlc absorption spectra for different oxidalion states of :I nickel salctt polymer film on an optically transparent. tndiuhtnlopcd, tin oxide-coated. tluartx-plrttc clcctt'otlc. {A} nickelilil salcn polymer: [Bi nickeltll salen polymer: {(‘l nickeltlll] salett polymer. polymer lilms, a set of thin-layer cyclic voltamntetric experiments was carried out with a 2.0 mM solution of monomeric mckeiill) salon in acetonitrile containing 0.050 M 'l‘lvif‘tljff1 itt contact with the freshly polished glassy cru‘bon electrode. llrttlcr tltc conditions of the thin—layer technique. the el't‘ect of diffusion is strongly diminished. so that the total area of a peak is proportional to the number of electrons involved in the particular electrochemical process. We determined the area under the pair of poorly resolved vollarttmetrie peaks on the first positive sweep front +1.10 to -l— 2.25 V (Fig. 3. curve {All and compared it with the area under identical conditions for the peak for the reversible one—electron oxidation of l'errocene: these measurements revealed that three electrons per monomer are involved in the anodic polymerization of nickel(lll salon. For curve (A1 of Fig. 3. the ratio of the combined areas for the two attodic peaks on the positive-going sweep to the area of the corresponding reduction peak on the negative-going sweep is three. Moreover. one electron per monomer is transferred during the first (and each subse- quent.) negative scan front +2.25 to +1.!O V and one electron per monomer is transferred in the second (and each subsequent} positive scan front + |.l(l to +2.1? V (Fig. 3. curves (B) and (CH. Therefore. once the polymer film has been produced. its electrochemical behavior in the region of positive potentials is defined by a reversible one—electron process assigned to the nickelfllll/nickeitlll redos couple. As further proof of the reliability of the thin-layer results, we observed no evidence for any signifi— cant diffusion of nickelflll salen front the adjacent bulk solution into the thin-layer region. since no increase in current for the nickeltllll/nickelllll redo): couple was observed on remtitive scans. L'JL'. Uri‘hm If! all {'errrlm’ “If Hit-'t-n'mmu(Him! (‘hmojurr'r 4 H} r #9965- .I'J‘iJ—J H Its? Fig. i. Stiinmng t-lttctron micrograph ol'u relatively Ihiclt film nl'polymcrit‘ ItiL‘Lcl snlcn on plutinum. 3.3. Spa-(.‘rroclecrrm'l'murstry {B} is the spectrum for :1 nickel“) salon polymer l‘iim obmint‘tl by rttlhorlic polarization of the electrode used lo Shown it: Fig. 4 art: ultruviolcl—visihlc absorption spun acquire vurve (A) to — I30 V at IOU mV’ 3 1. Cum: {Cl Ira for nickel salen films on optically transparent. intlium~ is lhc spectrum for u nickellllll salcn polymerfiim recorded doped, tin oxide—comet! quartz electrodesr Curve (A) is the after the first half of one polymerization scan (0 lo +2.25 spectrum for a nickclllll salon polymer film formed by V) at :00 mV 5 '. [-1ch it should be mentioned [hill the mums of one polymerization scan at 100 m‘V H_ '. Curve electrochemical characteristics of the nick-cl ‘sulcn system Fig. (1. Scanning t'luctt'on micrograph ofu rulalivt’ly thin film [Formal by Lhch polymerization RLILlIIh' .11 5f] nt'ler x ' lo1 polymutic [th'Lt‘l «11ch on platinum. loh' (Li-f. Drrftrrr r'l' (ti. 3’ i’tirrr'rrnlqtfflr-r-rronrmhrtrrt}t't.-rttttitr_1thl t mslm mi. N; liig. ?. Lou-er ntagaifimtion of scanning electron micrograph shown In Fig. ti. on an optically transparent electrode are indistinguishable from those described previously for glassy carhon. When the potential ofa polymer—coated. optically trans parent electrode was scanned from It to t 2.25 \r' aan ultraviolet—visible spectra were simultaneously sampled and displayed at different points during this positive going potential sweep. we observed the appearance and growth of the nickeltllll salen spectrum along with the fall and disappearance of the nickeltlll spectrum. In the stthsequent negative—going scan from +2.25 to t) V, the opposite heltavior was seen. .34. Scanning: electron micr'osr'opt' In preparing to examine polymer-coated platinum foils by means ol‘ scanning electron ttticroscopy. we confirmed that such electrodes show the same cyclic voltaannctric hchavior as polymer—coated glassy carhon. Fig. 5 is a scanning electron micrograph at low magnification of :1 relatively thick film of polymeric nickel salen on a plat— inutn surface; to form the polymer film. we held the potential oi the platinum electrode at £31.25 V for l min In a 2.0 n1M solution of monomeric nickclllll salcn in acetonitrilc containing 0.050 M TMABFA. Along its right side. this picture shows a darker area‘ of platinum that rats not exposed to the solution and that was not coaterl with a polymer film. Seen it] the Center and left portions of Fig, 53' is the polymer film which appears to consist of a relatively compact and continuous (gray) layer on top of which are hranch‘d. irregular patches where tlte polymer has grown outward from the surl'ace ot’ the continuous film heltm'. At higher untgnit'ication. for both the above—described thick film as well as for a thinner film obtained via three polymerization scans at it} myr s ' (Figs. 6 and ? respec- tively). the compact and continuous layer actually eshibtts some definite contour, and the outgrowths are clearly discernible. These observations are similar to those dc; scribed by l-tot'erltamp and Goldshy Li]. who published scanning electron tnicrographs for nickel salen anodically polymerized onto platinum from acetonitrile and methyl— ene chloride. containing tetra—n-hutylannnoniutn hesafluo— rophosphate. 4. Discussion i'tnotiic oxidation of nickeltlll salen exhibits two steps. We have clearly shown that the occurrence of these ttvo steps is not an artifact owing to some kind of self—inhibi- tion because this feature is independent of the concentra- tion of monomeric niekeltill salen irt the original solution. It is now important to stress that glottal polymerization of nickel( ll) salen can he achieved only when potentials more positive than that of the second anodic peak are reached. As a matter of fact. a potential sweep that does not reach Lhc second anodic step fails to produce. a polymer film similar to those already described in previous publications. r\s revealed in Fig. X, if. during the first positive—going potential sweep for the cyclic voltantmett‘ic oxidation of nickelllt) salen at a freshly polished glassy carbon elec— trode (Fig. READ. the potential is held at a value slightly less positive than that corresponding to the first anodic peak (without ever reaching the second peak). a deposit is formed that, when washed free of the monomeric nickeltlll til-‘1'. Doha: ct tit/Jitttnad of'li'lt't'tt'mmolvrttatl t 'Itmritrri- Jill HOW?! to; I?! Int} salett solution and placed in pure supporting electrolyte— solvent. exhibits a well defined. nicely reversible nickclllll/nickeltl} redox system which is sotttewhat dif- fusion—like in appearance (Fig. 8(8)). This cathodic re- sponse was found to be reasonably stable unless rather positive potentials were reached in the course of repetitive sweeps. [t‘ we subject tltis deposit to more positive poten- tials by usingT cyclic voltanitnetry to reach the potential of the second peak for the oxidation of nickclllll salen. an intense and narrow anodic response is observed (Fig. Rich. and this second stage of oxidation leads to dramatic changes in the electrochemical behavior of tire film. First. in the region of positive potentials (second scan of Fig. BlCl). one now sees the voltammetric features that are characteristic of the fully polymerized nickel salen as previously deseribed. Second. in the region of negative potentials. the nickelllll/rtickcllll redo-x maple is much less reversible and the currents are smaller in magnitude (Fig. till)”. In the course of the present work. it was established by means of thin-layer cyclic voltammetry that complete an— otlic polymerization of nickel salen involves three elec— trons per monomer. Moreover. the doping—undoping pro- cess corresponding to charging anti discharging1 of nickel centers in the polymer matrth involves one electron per monomer unit. However. if the potential corresponding to the second step for oxidation of nickelllll salen is not reached, fewer than three electrons per monottter are in— volved in building a deposit. although we have not estab— A . Dfi tantra ,_ o B E U | I o *2 l LIJ C . g a — + l- o :3 | o . D n Flifi — o I _ __ I —" —l_" | T 2’5 20 15 1-D 05 E10 435 —10 4.5 POTENTIALIU big. it. Cyclic Hillammtlgrams recorded at ill-ll rnVs ' in acetonitnle containing (infill M 'l'MABFy. [Al Building at the film on :t lrcslily polished glassy carhon electrode in solvent-supporting electrolyte contain- ing '2.“ mini nickellll) salon by means ot a positive-going sweep stopped and held at - lti't' V for If 3. [Bl Alter (Al, the electrode was washed free of nickclllll salon and was placed Into pure solvent-suppontng electrolyte, and Ihe potential was cycled in the negativppotcntjal region, It?! Alter ill). tht' potential was cycled twice in the [Ktsitlvc-potel‘ltittl region. [D] Alter (C). the potential was cycled first in Ihc. negative direction and then in the positive direction. lishcd this number precisely. II is proposed (Reaction (ll) that the first step in the oxidation ot‘ nickeltlll salcn is Ni(ll}sa|en:NiUlllsalen +e' {ll al‘ter which the oxidized species can react rapidly with each other to form a stacked acceptor— donor deposit (with interstitial charge-cotnpensaling tctrttlluciroliorale anions not shown): Such a stacked structure should he l‘avoretl because nickel salon species are planar or nearly planar [IS]. In its red duced ot' nickelUI} form. the deposit appears to be reason— ably stable and is only slowly dissolved by aeetonitrile. As the initially formed acceptor-donor deposit is taken to potentials more positive than + LE V. two more electrons per monomer unit should be lost in an irreversible step to give a fully polymerized (reticulated) film with phenyl— pltenyl linkages. Reticulalion would necessarily result in a more compact layer for which the anodic response (nickellllllhnickelllll interconversion) is now greater than the cathodic response lnickelllll- nickelU) intercon— version). ll, can be reasonably argued that the movement ol' charge-compensating ions is less facile for the nickel(lll- ntcltcltll process {involving tetrantethylammonium cotto— let-cations) than for the nickellllll—ttickelllll process (in- volving Ietratllutirobortttc counter—anions}. As shown in Fig. 1 (curves (B) and (CH. the reticulation process — which is a crucial step in the establishment of a complete polymer film —— can also be seen when an electrode hearing a film that is freshly formed in just one polymerization scan is then anodically polarized in the absentee ol‘ monomeric nickelllll salen. Although the pre— sumany one—electron reduction peak is unchanged. at pt'e wave and a postwave are observed only during the course of the first positive—going sweep (Fig. I. curve (Bil in supporting electrolyte—solvent that is tree of monomeric nickelllll salon. We can explain the prcwave as being due to the oxidation of residual nickellll) sites that are acti— vated by the presence of axially disposer! phenyl rings. whereas the postwave is obviously attributable to the already mentioned process ol‘ reticulation. To a certain | 't‘[l (.‘J’I. :‘Jttli‘tt: t'I at./ Journal oft-ft t m rtrttufyt't'rrtt' L‘hg-tnt'.\'try «it'll H99ol Edi — l 71" extent. the proposal ol‘ Hoferkamp and Goldsby [5] that the behavior just described catt be attributed to oxidation of traces of residual monomer and unpolymerized oligomers trapped within an initially formed polymer film is reason— able. it‘ it is stressed that those monottters are part of an organized prehuilt layer. We have not been able to extract any monomeric tticltcllll) salen front a freshly fortttcd polymer film that is immersed in acetonitrile for a long period of time. Therefore. the second peak for oxidation of nickeltlll salcn should be reticulation {Reaction (2)}. in— volving tlte polymerization of monomeric nickellllll moi— eties already present at the electrode surface: tt[Ni(_Il[} salon] nEtt L' -.!tt Il' t2} ;I As pointed out in our presentation of experimental results. the compact character of the completely formed polymer gives rise to the appearance of twin anodic and cathodic prcpealts (Fig. 2). the anodic prepeak is seen on the first positive—going sweep only after the main cathodic step is reached. Similarly. the cathodic prepeak appears during the first negative-going sweep only after the nickelUlll nickeltll'l rcdox process has been explored. Our explanation of these phenomena lies in the [act that reduc— tion of nickclllll to nickeltll or oxidation of nickeltlll to nickcllllll each involves fast electron movement witltin die whole polymer layer. However. within the large region of potentials between these two retiox procusses. the polyitter tilm is electronically weakly conducting — in other words. poly[nickelt[ll salcn] is an nndoped polymer. When dis- charge or undoping processes occur, a non-conducting zone is created at the glassy carbon—polymer t‘ilnt inter face. If movement of the change-compensating ions in- volved in the doping—undoptng process is sluggish. islands ol‘ nickel centers which are not involved in the undoping processes may remain. Thus. such isolated nickeltlll} sites can undergo reduction {causing a cathodic prepeah) only when nickelllll starts to be reduced to nickeltll and the polymer film becomes conducting once again. Similarly. nickeltl) centers trapped in the polymer are rapidly oxi- dized [causing an anodic prepeakl only when the oxidation of niekedlll to nickeltlII} begins to occur. To explain more fully the origin of die anodic prepeak. let us consider tlte schematic cyclic voltatnmogram shown in Fig. 9. At the completion of a negative—going potential sweep with a polymer—coated electrode. point I is reached at which potential all of the polynter~bound nickeltll} has been converted to nickel“). Now, as the potential is swept itt the positive direction from point i. the potential reaches the formal potential E: for the oxidation of nielcelti) to nickeltlll. As the positive—going potential sweep continues. a broad region of potentials ET is entered within which the polymer film is virtually noneonducting and some nickelll) centers remain uttosidized. However. as the for- tttal potential Elf lot the oxidation of oickelllll to tttckcltlll) in the polymer is approached. the polymer becomes electronically conducting again and the residual nickeltl) centers can he oxidized to nickeltil}. giving rise to the anodic prepeal-t. and then nickeltlll is oxidized to tticltcltllll. At poittt 2 in Fig. 9. the potential is cycled just in the positive region of potentials: the anodic prepeak is now absent. and only the normal peak for oxidation ot‘ polymer—bound nickellll) lo tiicl-tellIlIl is observed. A similar picture can be constructed to depict dte processes responsible for the cathodic prcpeak. To substantiate the preceding hypothesis that the anodic prepeak appearing at + I.” V is due. to polymer—bound nickelll) which remains unoxitiizcd as the film becomes poorly conducting. We examined the ultraviolet—visible spectrum for the polymer film on an optically transparent electrode at various potentials during a complete potential sweep from t] to — 1.30 to +3.25 to t] V. At tt V“ on the positive-going scan from — |,3tt to +2.25 V. the spectrum revealed the presence of a small quantity of nickcltll. as shown by an enhanced absorbance in the vicinity of 380 not and a broadened absorption band at 350 am. When the potential reached approximately +1.0 V on the same positive-going scan, the spectrttrtt showed the absence of this residual nickelll). These observations can be repro- duced on every complete potential sweep. In some experiments. it was found that the presence of an unusually high concentration of residual water in the acetonitrile used as solvent caused the cathodic response of ED - .. _ .._ __. 40— 20 CURRENT .I' 11A to r'x: U | 40 60 - E" t . "—T—t 2‘ D 1 5 ‘I D 0.5 0.0 -D.5 -1.U v1.5 POTENTMLIV Fig. 9. Schematic cyclic voltat'ttrnogrant to explain the origin of the anotltc prepertk: see text for discussion. (7.5. Dollar: ct ttl./Juarmti oftiter'Nrtttttrtft'hrof Charmin-y 4!!) travel int—r7; I‘H the polymer film to be more diffusion—like; that is. the peak separation between the cathodic and anodic peaks for the nickel(iI)/nickclfl) redott couple was increased and the peak currents were relatively smaller. For these some experiments. it was noted that dioxygen (which gives rise to a distinct reduction peak) was always present within the fully formed polymer film and that diosygen was difficult to remove totally. We suspect that dioxygen arises from the oxidation of" water and that this process occurs con- comitantly with anodic fonnation ot‘ the polymer film. We have confirmed that dioxygen. when deliberately intro— duced into the electrolysis cell. reacts rapidly with the polymer film in its nickeltl) state. Most likely. tlioxygen is reduced to superoxidc. Which conceivably attacks the poly- mer film to form Ni—O—O—Ni bridges; this reaction would contribute greatly to the aging of the polymer by changing its structure and by contributing to a much slower move- ment of charge through the polymer. Probably, the affinity of potyInickelfI) salcrtl toward superoxitle (or its reduced form. 0% _ ) lies in the fact that nickelll) salen is a iii—elec- tron complex which is strongly stabilized by the reduced forms of dioxygcn. Several papers [l9—2l] have been published that deal with the interaction between dioxygen tttttl cobalttll) salcn. 5. Conclusions This paper has shown (a) that the oxidation of nickeltll) salt-n produces a polymer film via transfer of three elec— trons per monomer unit and (b) that the charging—dis— charging of nickel centers within the film takes place via a one—electron transfer per monomer unit. in addition. using ultraviolet visible spectrophotometry with optically trans— parent electrodes. we have seen that the active redox centers within the polymer are indeed essentially nickel sites. Another important contribution of the present study is the proposal that establishment of the final polymer film occurs in two stages: (a) the first step corresponds to formation of a molecular acceptor—donor assetttbly in which the nickeltltl site in each monomer unit is oxidized to a nickeiUII) site and in which the individual oxidized monomer units are arranged to give a relatively stable deposit: (bl the second step involves irreversible osidative coupling of the aromatic rings. A polymer exhibiting three retiott states cart be formed. It is proposed that poly[nickeltttl salen] is not activated and is therefore weakly clectroni ‘ally conducting. In contrast. and espe— cially because of the appearance of anodic and cathodic prepeaks in the cyclic voltammograms. the oxidized and reduced forms of the polymer (corresponding to poly[nicltel[l|1) saiett] and polylttickelll) salen] respec— tively) can also be viewed as p—doped and n—doped. in order to understand better the factors which goveni the formation and behavior of polymer films derived from the salcn complexes of nickel and other transition metals. studies focusing. on the donor characteristiCs of a variety ot’ solvents and electrolytes are in progress; in addition. non— plartar analogues of nickel salert will also be investigated. Acknowledgements This research was perfomted while one of us (1.3.) held an appointment as a Fellow of the institute for Advanced Study at lndituta University. in addition. we thank Dr. Rudy Turner of the Department of Biology. Indiana tini- versity. for carrying out the scanning electron microscopy experiments and Mr. Michael J. Samitle of the Department of Chemistry, tntliana University. for his assistance in preparing the samples for those experiments. References Ill R.W. Murray (Ed). Molecular Design of Electroth Surfaces. Wi- ley—interscieuce. New York. I992. [1] (LP. Hort-vita and. KW. Murray. Mol. Cryst. Liq, Cryst., lot) ( I938) 33‘). [3] KA. Goldsby, J. Coord. Cltcmn 19 [1938) 83. [ll] RA. Goldslty. J.K. Blaho and LA. l-toferkamp. Polyhedron. R {1989) | l}. [5) LA. Hoferkamp and KJL Gotdshy. Cheat. Mitten. l (3989) 3:15. [6) F. Bcdioui. E. Labbe. S. Gutiertcz-Grnrtados and J. Devyrtt‘k. .l. Elec1roanat. Chem. 31'Jl {199:} 26?, [T] P. Audehert. l". Capdeviclic and M. Maurny. New J. Chem“ 15 {'l99l1235. [Kl P. Audebert. t’. Cripdeviellc and M. Maurny. Synlli. McL, dtl USN!) 30-49. {9| P. Audchert. P. Capdcvicllc and M. Maurny. New J. Chem. 16 t. [993) 69?. Hot P. Attaches. P. Inime P. Capdet'iclle and M. Mammy. r. Elec troanal, Chum. 333 {1992) 369. _l 1] CE. Dahttt and [3.0. Peters, Anal. Chem. 6tili99-1} 3H7". El 2| CE. Delta: and DIG. Peters. I. Electroanat. Chem. 406 t 19951 ltfl. 1.1] K.I.. Victor and 0.0. Peters. 1. Electroanal. Chem. 1% t |985l 93. H] L,\‘v'. Marple. Anal. Cheni,. 39 ([9673 Hart. lSI CW. Manning and W.(.‘. Purdy. Anal. Chim. Acta. .3! (Hit!) l24. to] R. Cartier and J. Simonet. Bull. Soc. (.‘ltim. Fr. [1988) it} I. IT] J. Rault-Bertlu‘lot, L. Angely. I. Dclaunay and J. Simonet. New J. (hear. I l H98?) 48?. lit] H.L. Chen. Y.|I. Pan. 3. Grott. TE. Hogan and DP. Ridge. J. Am. Chem. Soc. 1l3tl‘t9l} ares. lEiI D. Chen and AI; Martell. Inorg. Chem, 36 figs?) 1026. EN D. Chen. i\.E. Mattel] and ‘r’. Sun. Inorg. Chem, 38 (I939) 264'}. El] M. ‘v’allto. R. Klement. P. Petikrin. R. Boat. L. Dihfiit. A. Biittchcr. l-l. Elias and L. Mi'tllct. J. Phys. Chem. 9‘? l l‘fiifil Hi. ...
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