Synthesis, Characterization and Electrochemical Polymerization of a Ru2+ Functionalized Pyrrole...

Full Paper

Synthesis, Characterization and Electrochemical Polymerization ofa Ru2þ Functionalized Pyrrole Monomer

Kevin Foster, Timothy McCormac*

Centre for Research in Electroanalytical Technology (CREATE), Department of Science, Institute of Technology Tallaght, Dublin,Ireland*e-mail: [email protected]

Received: November 27, 2006Accepted: April 19, 2007

AbstractA ruthenium(II) functionalized pyrrole, Ruthenium-bis-N,N’-(2,2’-bipyridyl)-N-(pyridine-4-ylmethyl-(8-pyrrole-1-yl-octyl)amine)chloride, 1, has been synthesized and characterized by spectroscopic (UV/vis, 1H NMR) techniques andcyclic voltammetry. In solution 1 gave a redox couple associated with the Ru3þ/2þ redox system and an irreversiblewave associated with the oxidation of the covalently linked pyrrole moiety. What is believed to be homopolymeriza-tion, redox active surface films of 1 have been prepared by electrooxidation of the monomeric solution. The resultingpolymeric film exhibited clear redox activity associated with the incorporated Ru2þ redox centre, which is covalentlylinked ruthenium centre to the pyrrole moiety in 1. The effect of surface coverage upon the electrochemical behaviorof the deposited films has been undertaken. Preliminary investigations into the homogeneous charge transportdynamics of the polymeric film deposited onto platinum microelectrodes has been undertaken. Copolymerizationwith 3-methylthiophene was carried out and a clear ruthenium response was seen. Films were formed by both cyclicvoltammetry and chronocoulometry and studied. Various ratios were attempted but the ideal was found to be52 :5 mmol (3-methylthiophene: 1).

Keywords: Ruthenium, Pyrrole, Copolymerization, 3-Methylthiophene, Immobilization

DOI: 10.1002/elan.200603884

1. Introduction

Electrode surfaces that have been modified by conductingpolymer films is a major field of research and has been forpast number of decades. The main impetus for this is thewide ranging applications that such conducting polymerfilms possess [1 – 12]. The generation of a new generation ofhybrid materials based upon the incorporation of transitionmetal complexes into these polymeric systems remains ofintense interest. The ability of these metallic centers tointeract strongly with the p-conjugated backbone has beenstudied [13 – 16]. Several systems have been explored, suchas where the metallic centre is covalently grafted off themain polymer chain with alkyl spacer groups of varyinglengths being utilized to distance the metal centre from thechain. An example of this type of system is the one studiedby Zotti et al. [17], that is, ferrocene substituted polythio-phenes. In general, researchers have designed three maintypes of these hybrid systems, which are distinguished by thelocation of the transition metal to the conducting polymericbackbone. The first type of these is where a so called linkermoiety is employed which covalently grafts the metal to thepolymer backbone [18, 19]. The second type is where thepolymer backbone and the metal itself are electronicallycoupled. This is normally achieved by the employment of aconjugated linker or by direct coordination to designated

sites on the polymer chain [20, 21]Type 3 polymers consist ofthe usual conjugated backbone but the metal site isincorporated within the chain itself. This latter type haslead to strong electronic interactions taking place betweenthe metal centre and the conducting polymer backbone[22 – 24]. Various transition metals have been linked toconducting polymeric backbones, with ruthenium function-alized polypyrrole films having received a great deal ofinterest [25 – 28]. The main impetus for this has been thepossible uses of such ruthenium hybrid systems for artificialphotosynthetic systems [29], photoelectrochemical immu-nosensors [30, 31], solar cell development [32] and electro-catalysis [25]. In this contribution we report on the synthesis,characterization and polymerization of a new ruthenium(II)functionalized type 1 pyrrole monomer, namely, ruthenium-bis-N,N’-(2,2’-bipyridyl)-N-(pyridine-4-ylmethyl-(8-pyrrole-1-yl-octyl)amine)chloride, 1. It is a follow up study to ourprevious work based on the osmium(II) version of 1 [33, 34].

2. Experimental

2.1. Materials

Compound 1 was synthesized as described below and thencharacterized by both spectroscopic (1H and 13C NMR, IR,

1509

Electroanalysis 19, 2007, No. 14, 1509 – 1517 C 2007 WILEY-VCH Verlag GmbH& Co. KGaA, Weinheim

UV/vis) and electrochemical techniques (i.e., cyclic voltam-metry). The remaining chemicals employed were all ofstandard reagent grade and they were used as received.Acetonitrile (HPLC grade water content 0.005%), wasdried by storing over anhydrous calcium chloride for 24hours and then over 4 J́ molecular sieves, with the latterhaving been previously activated at 523 K.

2.2. Apparatus and Procedures

Electronic absorption spectrawere recorded inHPLCgradedichloromethane on a Shimadzu UV-160A UV-visiblerecording Spectrophotometer using quartz glass cells.NMR spectra were recorded at room temperature on aJNM-LA FT NMR system Joel 300 MHZ apparatus.Chemical shifts d(H) in ppm are relative to the solventsCD3 CN. Electrochemical experiments were performed in asingle compartment three-electrode cell. The referenceelectrode employed was a silver wire in an acetonitrilesolution of AgNO3 (0.01 M) and 0.1 M of the samesupporting electrolyte as employed in the cell. The workingmacroelectrode, platinum (d¼ 2 mm), was polished with0.05 mm alumina, sonicated in deionized water for 5 min,after which it was washed with deionized water and acetone.The working microelectrode, platinum (d¼ 50 mm) waspolished with 1.0, 0.3 and 0.05 mm grade alumina, sonicatedand washed in deionized water. The auxiliary electrodematerial was a platinum wire. A CH 660A potentiostat wasemployed for all electrochemical experiments. All solutionswere prepared with dry HPLC grade acetonitrile anddegassed with pure argon for 15 min prior to electrochem-ical experiments. All voltammetric experiments were car-ried out at room temperature.

2.3. Monomer Synthesis

[Ruthenium-bis-N,N’-(2,2’-bipyridyl)-chloro]-pyridin-4-yl-methyl-(8-pyrrol-1-yl-octyl)amine) hexafluorophosphate,1, was synthesized by the following method [33]: Into a25 mL round bottom flask with a reflux condenser, 31.39 mg(0.11 mmol) of Ru(bpy)2Cl2 and 48.43 mg (0.1 mmol) ofpyridin-4-yl-methyl-(8-pyrrol-1-yl-octyl)amine were dis-solved in 5 mL ethylene glycol and heated under reflux for30 minutes. When the reaction mixture was cooled down toroom temperature, 10 mL of deionized H2O was added. Theproduct was precipitated by addition of solid NH4PF6

yielding a brown solid which was filtered off, washed withwater and dried. The product was then dissolved in CH2Cl2and dried over Na2SO4. After filtration and solvent evap-oration the product was dried under vacuum resulting inblack shiny crystals which were then characterized by 1HNMR spectroscopy and UV/vis spectroscopy. UV-vis inHPLC dichloromethane of 1 gave a lmax 302 nm (p!p*bpy, e¼ 13,236 mol�1 L cm�1) bands at 233 (p!p* bpy, e¼3.601 mol�1 L cm�1), 358 (d! d, e¼ 11.246 mol�1 L cm�1)and 501 nm (d! d, e¼ 8.529 mol�1 L cm�1). NMR 1H

spectra results in reference to Figure 1: aromatic pyrroleregion; 6.6277 – 6.6515 ppm (pyrrole HA), 5.9845 –6.0144 ppm (pyrrole HB); aliphatic region; 3.8714 ppm (C),3.7933 ppm (D), 2.6119 ppm (E), 1.7838 ppm (F),1.4896 ppm (G), 1.2858 ppm (H, I, J, K); pyridine moiety;8.457 – 8.5299 ppm (A), 7.587 – 7.6053 ppm (B); bipyridinemoieties; 9.9152 (HL), 7.7347 – 7.7048 ppm (Ho), 7.2056 –7.1220 ppm (HN), 7.7927 – 7.7646 ppm (HM): 8.3596 –8.3193 ppm (HP), 8.1137 – 8.0362 ppm (HQ), 8.2772 –8.2467 ppm (HR), 7.8165 ppm (HS). It should be stressedthat complete NMR assignment is extremely difficult inthese types of systems, however this coupled with CV andUV/vis did indicate that the desired compound 1 wassuccessfully synthesized. Pickup et al. [35] have previouslysynthesized a related compound, [Ru(bpy)2(pmp)Cl]PF6 ·H2O which exhibited similar UV/vis and electrochemicalbehavior, where the pmp is 3-(pyrrol-1-yl-methyl)pyridine.Also its UV/vis and redox behavior was similar to the Os(II)analogue of 1 except the Os(III/II) redox potential iscentered around 0.0 V [33].

Fig. 1. [Ruthenium-bis-N,N’-(2,2’-bipyridyl)-chloro]-pyridin-4-yl-methyl-(8-pyrrol-1-yl-octyl)amine.

1510 K. Foster, T. McCormac

Electroanalysis 19, 2007, No. 14, 1509 – 1517 www.electroanalysis.wiley-vch.de C 2007 WILEY-VCH Verlag GmbH& Co. KGaA, Weinheim

www.electroanalysis.wiley-vch.de

3. Results and Discussion

3.1. Electrochemical Solution Behavior andHomopolymerization

Initially we investigated the redox properties of theuncomplexed ligand, pyridin-4-yl-methyl-(8-pyrrol-1-yl-oc-tyl)amine. The ligand was seen to exhibit an irreversiblepyrrole oxidation wave at approximately þ1.0 V. Uponcontinuous scanning no polymer deposition onto theelectrode was in evidence, this probably being due to thelone pair on the nitrogen pyridine destabilizing any pyrrolecations generated upon the monomer oxidation, this hasbeen stated previously for a similar pyridine terminatedpyrrole ligand [36]. In attempt to effect polymerization ofthe ligand, 10 mM of HClO4 acid was added which wouldensure protonation of both the �NH in the chain and thepyridine nitrogen [36]. Figure 2a illustrates the cyclicvoltammogram of this ligand in 0.1 M TBAP in the presenceof 10 mM HClO4. What can be readily seen is the depositionof a conducting polymer onto the electrode surface and thegrowth of the polymerQs redox chemistry upon continuousscanning through the pyrrole oxidation wave. The film wasthen removed, rinsed with background electrolyte andcycled in 0.1 M TBAP, the resulting cyclic voltammogram isseen in Figure 2b. The typical conducting polymer electro-chemical signature is readily observed with an Epc ofþ0.311 V and a corresponding reoxidation wave, Epa, ofþ0.356 V (vs. Ag/AgCl). This proves that is possible toelectrochemically polymerize pyridin-4-yl-methyl-(8-pyr-rol-1-yl-octyl)amine, under acidic conditions.

The next step was to investigate if 1 could undergohomopolymerization. The electrochemical behavior of5 mM of 1 in a 0.1 M n-Bu4NClO4 CH3CN solution, at abare platinum electrode, is illustrated in Figure 3a. What isobserved is the presence of a reversible one-electron redoxwave with an E1/2 of þ0.51 V, and an irreversible anodicwave at Epa¼þ1.034 V. The former is associated with theRu(III/II) redox system whilst the latter is associated withthe oxidation of the pyrrole ring of 1. It was thought that theemployment of a long octyl amine spacer arm shouldfacilitate the homopolymerization of 1without the need forany copolymerization. Figure 3b illustrates the evolution ofthe cyclic voltammogram during repetitive voltammetriccycling between 0.0 to þ1.4 V. It can be seen that this leadsto significant and surprising changes in the cyclic voltammo-gram of 1. There is a continual decrease in the Ru(III/II)couple with an associated gradual growth of a new redoxcouple centered around þ1.0 V. This was somewhat sur-prising, as what was expected was the growth of the Ru(III/II) signal indicating polymeric deposition of a Ru(II)polypyrrole film. It is difficult to confirm exactly what ishappening during this evolution of the cyclic voltammo-gram. For certain there is a deposition process occurring aswhen the electrode, after continual redox cycling, is rinsedwith background electrolyte and cycled in monomer freeelectrolyte, a clear one-electron redox couple is observed, asseen inFigure 3c.A smooth orange transparent film can also

be observed on the electrode surface. This well-definedreversible couple, with anE1/2 of þ1.055 V, is believed to bedue to surface immobilized Ru(III/II) redox sites Also it isthought that the film is polymeric in nature even though noregular polypyrrole electroactivity is observed in thevoltammogram. This absence of the latter is probably dueto the high potential employed for the suspected electro-polymerization thereby causing the possible destruction ofthe polypyrrole conductivity by the process of over oxida-tion. This has previously been observed for other polymericsystems [36].

The major question is why is there a such a dramaticchange in the redox behavior of the coordinated Ru(III/II)moiety upon redox cycling to positive potentials. The mostlikely explanation is that the ligand environment around theRu(II) centre has been altered causing a positive shift in theE1/2 value for Ru(III/II) redox system. The chloride ligandon the Ru(II) is the most labile and we believe that a ligandexchange mechanism between this coordinated chlorideand CH3CN from the solvent electrolyte system is takingplace. The driving force behind this process is more difficultto ascertain. In our minds there are two possibilities, firstly

Fig. 2. a) Repetitive cyclic voltammogram of 5 mM pyrroleligand with a 10 mM addition of HClO4 in MeCN at a bare carbonelectrode (A¼ 0.0314 cm2). Scan rate¼ 100 mV s�1. b) Cyclicvoltammogram of polymer film from (a) in 0.1 M TEAP MeCN ata carbon electrode (A¼ 0.0314 cm2). Scan rate¼ 30 mV s�1.

1511Ru2þ Functionalized Pyrrole Monomer



would be a photosubstitution effect as has previously beenobserved for polymer-bound ruthenium complexes by Voset al. [37, 38] is occurring and secondly through theoxidation of the pyrrole monomer there is a release of aHþ which would abstract the chloride from the Ru(II).However we have not conducted any detailed irradiationexperiments so as to ascertain the presence of any photo-substitution effect. However it should be stressed that Voset al. [36] polymer-bound ruthenium complex, [Ru(bpy)2

(CH3CN)PVP]2þ, where CH3CN is the coordinated solventand PVP is poly(4-vinylpyridine), formed during the

abstraction of the Cl and solvent substitution, possesses aRu(III/II) E1/2 of þ1.13 V (vs. SCE), with the original[Ru(bpy)2(Cl)PVP]þ polymer possessing an Ru(III/II) E1/2

of þ0.64 V (vs. SCE). Our results and observations agreewell with these. Interestingly when we restrict the cycling tojust the Ru(III/II) couple a relatively stable one electronreversible redox wave is attained. It is only when thepotential is continually swept to further positive potentials,beyond the pyrrole wave, that the dramatic changes in thevoltammogram discussed above are observed. This points tothe role of Hþ in the coordination of the solvent onto theRu(II) centre. We have also conducted some preliminaryexperiments upon the monomer in solution in the presenceof HClO4. When the potential is swept towards the pyrrolewave in the presence of 10 mM HClO4 acid there is adramatic decrease in the Ru(III/II) couple and the subse-quent growth of the new couple around þ1.0 V. This filmwas then cycled in blank electrolyte yielding a similar cyclicvoltammogram to that in Figure 3c. However when theexperiment was repeated with limiting the potential scanjust to include the Ru(III/II) couple of 1, a resultant growthin this couple was observed upon continuous scanning. Thecyclic voltammogram of the resulting immobilized filmshowed a redox wave atþ0.4 V, but it is unclear if this film ispolymeric in nature. Further work is currently underwaywithin our group to expand upon this work.

Whilst the change in theE1/2 value of theRu(III/II) systemcan be explained it does not explain why a deposition of afilm should occur onto the electrode surface Previouslyauthors have shown thatwhen theRu(III/II) redoxpotentialis situated at the same potential as the irreversible pyrroleoxidation wave, a electrocatalytic oxidative effect betweenthe Ru(III) and the pyrrole unit occurs leading to polymericgrowth [39, 40] We believe this to be occurring with oursystem. Our group has also varied the nature of the solventwhilst keeping the electrolyte constant. What was found wasthatwhen the solventwas a typical non-coordinating solventlikeDMF, the changes observed in the cyclic voltammogramof 1 as seen in Figure 3b do not occur and no electrodeposition process is evident. This can be explained by thepoor labile nature of DMF thereby not allowing it to bindinto the Ru(II) centre. Hence no change in the redoxpotential of the Ru(III/II) process is observed and thereforeno electrocatalytic reaction can occur. Attempts to effectthe ligand substitution effect and subsequent homopolyme-rization of 1 in CH2Cl2 proved unsuccessful. When thecomplex was oxidized through the Ru(III/II) couple in thissolvent a process of precipitation occurred onto theelectrode surface whereby the Ru(III) form of 1 precipitatesonto the electrode surface in the form of a mixed anion salt.The main impetus for this is the inherent insolubility of theRu(III) form of 1 in the solvent system employed due to thelow dielectric constant of the system. This has been seenpreviously by us for the Os(II) analogue of 1 [33].

Table 1 summarizes the variation in peak splitting andanodic peak fwhm with sweep rate for films of varyingsurface coverages. For a surface confined species, a zeropeak-to-peak separation and a fwhm of 90.6 mV are

Fig. 3. a) Cyclic voltammogram of a 1 mM 0.1 M TBAP MeCNsolution of 1 at a bare platinum electrode (A¼ 0.0314 cm2). Scanrate¼ 100 mV s�1. b) Repetitive cyclic voltammogram of a 1 mM0.1 M TBAP MeCN solution of 1 at a bare platinum electrode(A¼ 0.0314 cm2). Scan rate¼ 100 mV s�1. c) Cyclic voltammo-gram of a surface immobilized film of 6 in 0.1 M TBAP MeCNsolution at a bare platinum electrode (A¼ 0.0314 cm2). Scanrate¼ 40 mV s�1.




expected for a one electron transfer process. Due to the factthat the peaks associated with our polymerQs redox activityare slightly broader than that expected for an idealNernstian system may indicate that there are weak desta-bilizing interactions between the Ru2þ metal centers. As canbe seen from the Table 1 four films of varied thickness wereformed with scan rate studies applied to each. The expectedtrend in results is observed, with peak splitting increasingfrom the slowest scan rate to the fastest, as expected. Thistrend is again seen for the anodic and cathodic fwhm withincreasing scan rate.

The stability of a polymer film towards redox cycling in0.1 M TBAP MeCN was investigated. It was found that for afilm of surface coverage (G) 1.36� 10�9 mol cm�2 after 25, 50and 100 cycles in background electrolyte the global loss ofelectroactivity was found to be 36, 58 and 79% respectively.The effect of the nature of the electrolyte upon the redoxactivity of the film was also undertaken. TheE1/2 value of theRu(III/II) signal for the polymer was found to be þ1.01,þ1.03 V and þ1.05 V (vs Ag/AgCl) in 0.1 M TBAP,TBABF4 and TBAPF6, respectively. The nature of theelectrolyte anion therefore affects the redox switching of thepolymeric film. This can be explained by the fact that as thecouple aroundþ1.0 V represents the redox switching of theRu(III/II) attached to the polymer chain, that anioninsertion and expulsion must accompany the oxidationand reduction of the film, respectively, for reasons for chargeneutrality. The shift in E1/2 with electrolyte nature points tothe fact that it is easier to incorporate the perchlorate, ClO�

4 ,than the hexafluorophosphate, PF�

6 . The electrochemicalbehavior was also examined in a supporting electrolyteconsisting of a larger anion namely 0.1 M TBABPh4. Whencycled it could be seen that no redox activity associated withthe Ru(III/II) species could be seen, this being due the largesize of the BPh�

4 anion and the inability of the film to allowits incorporation when being cycled.

Figures 4a, b exhibit the plots of anodic and cathodiccurrents against sweep rate for two films with different

surface coverages. Plots of current vs. scan rate indicatedthin film behavior for films (G¼ 1.34� 10�9 mol cm�2) up toscan rates of 500 mV s�1. Such behavior is expected for asurface-immobilized electroactive film which is thin enough

Table 1. Electrochemical parameters of homopolymerized films of 1.

Surface coverage G(� 10�9 mol cm�2)

Scan rate(mV s�1)

Peak splitting(mV)

Anodic peak fwhm(mV)

Cathodic peak fwhm(mV)

1.34 (�0.02) 10 5 120 12040 6 120 122

100 19 116 124400 59 152 128

4.68 (�0.02) 10 12 134 12640 13 140 114

100 22 128 130400 68 156 146

13.9 (�0.02) 10 15 150 15040 35 152 120

100 53 178 134400 75 156 146

18.4 (�0.02) 10 17 140 11040 50 154 128

100 81 162 146400 254 146 176

Fig. 4. a) Plots of ipa and ipc vs. v for a film of surface coverage1.34� 10�9. b) Plots of ipa and ipc vs. v for a film of surface coverage1.45� 10�8




to ensure that all redox active sites within the film undergocomplete oxidation and reductionduring thepotential cycle.For thicker films however (G¼ 1.449� 10�8 mol cm�2) linearrelationships were only obtained up to scan rates of 100 mVs�1, with an increase in peak splitting and a loss of peaksymmetry at a specified scan rate. Preliminary scanningelectron micrographs (SEM) along with energy dispersiveX-ray (EDX) images were carried out on the polymer afterformation. Figure 5 illustrates the resulting SEM imageobtained of a film after formation. What can be seen is that asmooth uniform polymer film is formed onto the surface ofthe electrode. The corresponding EDX images also showedthe presence of the elements C, N, O, Ru and Cl as expected.

3.2. Polymeric Homogeneous Charge Transport Dynamics

Previously, Forster et al. [41] employed cyclic voltammetry,in conjunction with microelectrodes, to elucidate thehomogeneous charge transport dynamics of a conductingthiophene polymer which contained an in-chain metalcentre. We have in this contribution taken the sameapproach as these authors so as to calculate a value for theproductDCT

1/2C*, whereDCTand C* represent the homoge-neous charge transport diffusion coefficient and the effectiveredox site concentration, respectively [41]. To elucidate thisproduct cyclic voltammetry is performed upon the conduct-ing polymer film which has been deposited upon a Ptmicroelectrode during the electrodeposition step. Figure 6ashows the resulting cyclic voltammogram for our Ru(II)polypyrrole film surface immobilized on a 50 m radius Ptmicroelectrode at scan rates of 30, 40, 50, 60, 70, 80, 90 mVs�1. What is clearly seen is the expected one electron redoxcouple associated with the Ru(III/II) system. In additionover this scan rate range the individual peak potentials (Epa,Epc) and hence theE1/2 (�þ 1.05 V) are found to be virtuallyindependent of scan rate. The peak-to-peak separation

(DEp) of 120 mV is higher than the expected theoreticalvalue of 59 mV, this being probably due to electrostaticrepulsions within the film, as observed by previous authors[41, 42]. Since theEpa,Epc andE1/2 appear to be independentof scan rate it is possible for us to employ theRandles-Sevcikequation for a reversible redox system, so as to allow thedetermination of the DCT

1/2 C* product. This involvesplotting the resulting peak currents (ip) versus square rootof scan rate, from the data in Figure 6a, as directed by theRandles-Sevcik equation:

IP¼ (2.69� 105) n3/2 A (DappCEFF)1/2 v1/2

Fig. 5. SEM image of immobilized polymeric film of 1 depositedonto a platinum electrode (A¼ 0.0314 cm2) immediately afterformation and drying.

Fig. 6. a) Cyclic voltammetric response for an immobilized filmof 1 on a 50 mm radius platinum electrode in 0.1 M TBAP inMeCN. Scan rate increases from 100 – 350 mV s�1. b) Plots of Ipa

and Ipc vs. n1/2 for the film from (a).




where C* is the concentration of the Ru(II) redox siteswithin the film, A is the area of the microelectrode, n is theequal to 1 in this case, and DCT is the homogeneous chargetransport diffusion coefficient [41]. Figure 7b shows theresulting plot. From the slopes of the plots, values of 1.92�10�9 and 1.88� 10�9 mol cm�2 s�1/2 were obtained for theDCT

1/2C*product, for both the anodic and cathodic branches.Interestingly these values are in good agreement with eachother, this implies that the electrolyte anion can easilydiffuse in and out of the polymer film upon redox switching.As expected, theDCT

1/2C* values are an order of magnitudelower than that reported for a conducting polymer contain-ing in-chain metal centers [41], due to the fact that theRu(II) centre is not electronically coupled to the polymerbackbone. For diffusion controlled processes if a plot of logip vs. log v is constructed, theoretically the slope should be0.5. When this was undertaken for both the anodic andcathodic branches from Figure 7c the slope values of theseplots were 0.594 and 0.552, respectively.

3.3. Copolymerization with 3-Methyl Thiophene

Copolymerization of 1 with 3-methylthiophene has alsobeen undertaken so as to attain conducting polymeric films.

Recently thiophenes and its derivatives have been exploredas possible matrixes for copolymerization [43 – 45]. It hasbeen suggested that 3-methylthiophene reduces the b

coupling in the polymer thereby resulting in better polymerfilms. One advantage in employing this technique is that thecopolymers obtained should possess higher electronic con-ductivities as compared to the films of 1 produced byhomopolymerization, this has previously been undertakenby Pickup et al [46]. Figure 7a illustrates the cyclic voltam-mogram obtained during the deposition of a typicalcopolymer film composed of 1 and 3-methylthiophene.What is clearly seen is the continual growth of the signal forthe Ru(III/II) redox couple upon continuous cycling. Thisindicates the electrodepostion of 1 within the surfaceimmobilized copolymer film. Figure 7b indicates the result-ing copolymer film, with a surface coverage of 7.537�10�9 mol cm�2 in a monomer free acetonitrile þ0.1 MTEAP solution. The resulting Ru(III/II) redox responseoccurs at a potential of þ0.528 V. When compared to theoriginal compound before homopolymerization it can beseen that the formal potential between the two is slightlydifferent with a shift of approximately 20 mV. This partic-ular system is behaving in a similar way to that reported byPickup et al. [44] whereby the shift was attributed to anelectrostatic effect due to the cationic polypyrrole linkages.Several copolymer films were producedwith various surfacecoverages by two different methods namely, cyclic voltam-metry and chronocoulometry. It was found by scanningbetween 2 and 20 cycles using cyclic voltammetry as themethod for deposition, films with surface coverages be-tween 10�9 and 10�8 mol cm �2 couldbe attained.Whereasbyusing chronocoulometry, films of similar surface coveragescould be achieved by allowing a deposition charge between2 to 10 mC.

For thicker films (� 10�8 mol cm�2), increasing the scanrate causes an associated increase in the DEp. For examplefor films of surface coverages ranging between 2.7 and 3.5�10�8 mol cm�2, there is a large increase in the DEp fromapproximately 30 mV at 10 mVs�1 to 320 – 420 mV at400 mVs�1. The latter peak to peak separations are higherthan the expected 59 mV for diffusion controlled one-electron processes. Table 2 summarizes the variation in peaksplitting and anodic and cathodic peak fwhm with sweeprate for all films, formed by cyclic voltammetry. Althoughthe expected zero peak to peak separation and a fwhm of90.6 mV for a one electron process are not seen the typicaltrend of peak splitting and fwhm increasing with increasingscan rate are seen throughout for each film. Figures 8a and bexhibits plots of anodic and cathodic currents against scanrate for two films with different surface coverages. Plots ofcurrent against scan rate for a relatively thin film (G¼4.74� 10�9 mol cm�2) indicated that thin film behavior waspresent up to 200 mV s�1 ensuring that all the redox siteswith the film undergo complete oxidation and reduction.Whereas for a thicker film (G¼ 3.57� 10�8 mol cm�2) thinfilm behavior only exists up to 70 mV s�1.

Fig. 7. a) Repetitive cyclic voltammogram of 0.1 mmol TEAPMeCN solution of 52/5 mmol of 3-methylthiophene/Rutheniumcomplex 1 at a bare platinum electrode (A¼ 0.0314 cm2). Scanrate¼ 100 mV s�1. b) Cyclic voltammogram of a copolymerizedfilm from (a) in 0.1 M TEAP MeCN at a bare platinum electrode(A¼ 0.0314 cm2). Scan rate¼ 100 mV s�1.




4. Conclusions

There are several important conclusions from this contri-bution which can be summarized as follows:

1) A new Ru(II) functionalized pyrrole monomer, 1, hasbeen synthesized and characterized

2) Homopolymerization of this Ru(II) pyrrole unit waspossible through a ligand-solvent exchange processfollowed by an electrocatalytic reaction between theRu(III) form and the pyrrole unit.

3) Polymeric films of varying thicknesses were investigatedwhich all exhibited a clear one electron redox coupleassociated with the Ru(III/II) system

4) An average value of 1.9� 10�9 mol cm�2 s�1/2 for theproduct DCT

1/2C* was attained for the film when itshomogenous charge transport dynamics were investigat-ed. This being typical for a transition metal functional-ized conducting polymer where the metal centre iscoordinated to the monomer unit through a longsaturated spacer arm.

5) The technique of copolymerization of 1 with 3-methyl-thiophene was also successfully applied yielding filmsthat exhibited clear redox activity for the Ru(III/II)system.

The main impetus for this work has been the aim to developa polymerizable pyrrole monomer unit that would containseveral covalently grafted transition metal species. Theapplication of such systems for mediating properties to-wards analytes of interest such as ascorbic acid shall be fullyexplored. Work is currently underway within our group toremove the Cl from 1 for the covalent grafting of a secondtransition metal (i.e., Os, Fe) through the employment of abridging ligand, such as, 4, 4’-bypyridine. This work andfuture sensor applications of such polymeric films shall bethe subject of future publications.

5. Acknowledgements

Financial support obtained through the Irish PostgraduateResearch andDevelopment of Skills Programme (GrantTA

Fig. 8. a) Plots of ipa and ipc vs. v for a copolymerized film ofsurface coverage 4.74� 10�9. b) Plots of ipa and ipc vs. v for acopolymerized film of surface coverage 3.57� 10�8.

Table 2. Electrochemical parameters of copolymerized films of 1 with 3-methylthiophene formed by CV.

Surface coverage G(� 10�9 mol cm�2)

Scan rate(mV s�1)

Peak splitting(mV)

Anodic peak fwhm(mV)

Cathodic peak fwhm(mV)

4.74 (�0.02) 10 12 120 10840 18 114 112

100 34 108 120400 87 126 140

7.54 (�0.02) 10 19 122 11440 41 120 126

100 80 126 140400 199 156 174

27.0 (�0.03) 10 30 130 12040 70 132 132

100 132 146 154400 318 176 194

35.1 (�0.02) 10 35 120 11840 92 134 140

100 165 156 162400 418 192 214




08 2002) and the ITT Dublin PhD Continuance Fund isgratefully acknowledged. Also thanks to Dr Aine Allen ofITT Dublin for running of the SEM images.

6. References

[1] M.-N. Collomb-Dunand-Sauthier, A. Deronzier, R. Ziessel,J. Phys. Chem. 1993, 97, 13.

[2] M. Kaneko, S. Moriya, A. Yamamda, H. Tamamoto, N.Oyama, Electrochim. Acta 1984, 29, 115.

[3] P. R. Samani, S. Radhakrishnan,Mater. Chem. Phys.2002, 77,117.

[4] M. E. Nicho, H. Hu, C. Lopez-Mata, J. Escalante, SolarEnergy Mater. Solar Cells 2004, 82, 105.

[5] R. Prakash, Somani, S. Radhakrishnan, Mater. Chem. Phys.2003, 77, 117.

[6] C. Arbizzani, M. G. Cerroni, M. Mastragostino, Solar EnergyMater. Solar Cells 1999, 56, 205.

[7] P. Topart, P. Hourquebie, Thin Solid Films 1999, 352, 243.[8] A. Malinauskas, Synth. Met. 1999, 107, 75.[9] Handbook of Conducting Polymers (Eds: A. Guiseppi-Elie,

G. G. Wallace, T. Matsue, T. A. Skothrim), Marcel Dekker,New York 1998, p. 963.

[10] M. S. Freund, N. S. Lewis. Proc. Natl. 1995, 92, 2652.[11] K. Nigorikawa,Y. Kunugi,Y. Harima, K.Yamashita, J. Elec-

troanal. Chem. 1995, 396, 563.[12] W. Schuhmann, S. Reiter, K. Habermuller. Sens. Actuators B.

2001, 79, 150.[13] W. B. Euler, Polyhedron. 1991, 10, 859.[14] K. D. Ley, K. A. Walters, K. S. Schanze, Synth. Met. 1999,

102, 1585.[15] W. Y. Ng, W. K. Chan, Adv. Mater. 1997, 9, 716.[16] S. S. Zhu, T. M. Swager, J. Am. Chem. Soc. 1997, 119, 12568.[17] G. Zotti, G. Schiavon, S. Zecchin, A. Berlin, G. Pagani, A.

Canavesi, Synth. Met. 1996, 76, 255.[18] J. Chen. C. O. Too, G. G. Wallace, G. F. Swiegers. Electro-

chim. Acta 2004, 49, 691.[19] J. Chen, C. O. Too, G. G. Wallace, G. F. Swiegers, B. W.

Skelton, A. H. White, Electrochim. Acta. 2002, 47, 4227.[20] S. S. Zhu, T. M. Swagger, Adv. Mater. 1996, 8, 497.[21] P.-L.Vidal, B. Divisia-Biohorn, G. Bidan, J.-L. Hazemann, J-

M. Kern, J.-P. Sauvage, Chem. Eur. J. 2000, 6, 1526.[22] Y. Zhu, M. O. Wolf, Chem. Mater. 1999, 11, 2995.

[23] S. J. Higgins, C. L. Jones, S. M. Francis, Synth. Met. 1999, 98,211.

[24] J. Hjelm, R. W. Handel, A. Hagfeldt, E. C. Constable, C. E.Housecroft, R. J. Forster, Electrochem. Commun. 2004, 6,193.

[25] M. Rodriguez, I. Romero, C. Sens, A. Llobet, A. Deronzier.Electrochim. Acta. 2003, 48, 1047.

[26] M. Rodriguez, I. Romero, A. Llobet, A. Deronzier, T.Parella, H. Stoeckli-Evans, Inorg.Chem.2001, 40, 4150.

[27] A. Deronzier, D. Eloy, P. Jardon, A. Martre, J. C. Moutet, J.Electroanal. Chem. 1998, 453, 179.

[28] M. Navarro, M. N. Collomb, A. Deronzier, J. Electroanal.Chem. 2002, 520, 150.

[29] S. Charbon-Noblat, A. Pellissier, G. Cripps, A. Deronzier, J.Electroanal. Chem. 2006, 597, 28.

[30] N. Haddour, J. Chauvin, C. Goudran, S. Cosnier, J. Am.Chem. Soc. 2006, 128, 9693.

[31] N. Haddour, S. Cosnier, C. Gondran, Chem. Commun. 2004,21, 2472.

[32] Y. Saito, T. Azechi, T. Kitamura, Y. Hasegawa, Y. Wada, S.Yanagida, Coord. Chem. Rev. 2004, 248, 1469.

[33] K. Foster, A. Allen, T. McCormac, J. Electroanal.Chem. 2004,573, 203.

[34] K. Foster, T. McCormac, Electroanalysis 2006, 18, 1097.[35] A. Deronzier, J-C. Moutet, Coord. Chem. Rev. 1996, 147,

339.[36] O. Haas, M. Kriens, J. G. Vos, J. Am. Chem. Soc. 1981, 103,

1318.[37] O. Haas, H. R. Zumbrunnen, J. G. Vos, Electrochim. Acta.

1985, 30, 1551.[38] A. Deronzier, J. C. Moutet, D. Zsoldos, J. Phys. Chem. 1994,

98, 3086.[39] S. Cosnier, A. Deronzier, J-C. Moutet, J. Electroanal. Chem.

1985, 193, 193.[40] J. Hjelm, R. W. Handel, A. Hagfeldt, E. C. Constable, C. E.

Housecroft, R. J. Forster, J. Phys. Chem. B. 2003, 107, 10431.[41] T. McCormac, J. F. Cassidy, K. Crowley, L. Trouillet, F.

Lafolet, S. Guillerez, Electrochim. Acta. 2006, 51, 3484.[42] H. P. Welzel, G. Kossmehl, G. Engelmann, W. D. Hunnius, W.

Plieth, Electrochim. Acta. 1999, 44, 1827.[43] J. Ochmanska, P. G. Pickup, J. Electroanal. Chem. 1991, 297,

197.[44] J. Roncali, Chem. Rev. 1992, 92, 711.




Synthesis, Characterization and Electrochemical Polymerization of a Ru2+ Functionalized Pyrrole...

Documents

Transcript of Synthesis, Characterization and Electrochemical Polymerization of a Ru2+ Functionalized Pyrrole...