Synthetic Metals 158 (2008) 704–711

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Synthetic Metals

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Vapour phase polymerisation of pyrrole induced by iron(III)alkylbenzenesulfonate salt oxidising agents

Priya Subramaniana, Noel B. Clarkb, Leone Spicciaa,∗, Douglas R. MacFarlanea,Bjorn Winther-Jensenc, Craig Forsytha

a School of Chemistry, Monash University, Wellington Road, Clayton, Vic. 3800, Australiab CSIRO Forest Biosciences, Private Bag 10, Clayton South, Vic. 3169, Australiac School of Materials Engineering, Monash University, Wellington Road, Clayton, Vic. 3800, Australia

a r t i c l e i n f o

Article history:Received 13 June 2007Received in revised form 2 April 2008Accepted 22 April 2008Available online 9 June 2008

Keywords:Bulk polymerisationConducting polymersIron(III) alkylbenzenesulfonatesPolypyrroleVapour phase polymerisationX-ray crystallography

a b s t r a c t

A series of iron(III) alkylbenzenesulfonate (ABS) salts were prepared by the reaction of ferric hydroxide,Fe(OH)3·xH2O, with a variety of aromatic sulfonic acids. The products were characterised by microanal-ysis and FTIR spectroscopy. The microanalysis data generated indicated that three of the salts werenot mononuclear iron(III) compounds and were of the formula [(OH2)5Fe–O–Fe(OH2)5][C2H5C6H4SO3]4,[(OH2)5Fe–O–Fe(OH2)5][CH3C6H4SO3]4·2H2O and [(OH2)5Fe–O–Fe(OH2)5][CH3C6H4SO3]4. Some degreeof condensation or cross-linking of Fe(III) centres into hydrolytic oligomers had occurred. The X-raycrystal structure of the Fe(III) toluenesulfonate salt established the formula of an oxo-centred, binu-clear complex [(H2O)5Fe(�-O)Fe(OH2)5]4+. Vapour phase polymerisation (VPP) of pyrrole monomer wascarried out using iron(III) benzenesulfonate, p-ethylbenzenesulfonate, dodecylbenzenesulfonate and p-toluenesulfonate. As the chain length of the Fe(III) alkylbenzenesulfonates increased it was found that thefilm forming ability of the polypyrrole was enhanced, probably as a result of a decrease in polymer chaininteraction resulting from increased free volume between polymer chains. Variations in the conductiv-

ity of the polypyrrole films was observed when Fe(III) p-toluenesulfonate salts obtained from differentsources (two commercial samples and one synthesised in our laboratories) were used as the oxidant.Films deposited using these oxidants generally exhibited higher conductivity than those formed using

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. Introduction

Adding electronic functionality to non-conducting flexible sub-trates is a challenging but productive field of research. Potentialpplications include organic light emitting diodes, display devices,emiconductors, printed electronics, antistatic coatings and smartackaging products [1]. Smart packaging products incorporat-

ng low cost electronics can be produced by printing conductivenks onto different substrates. In our work, inkjet printing haseen used to apply oxidant solutions as a template, followed byapour phase polymerisation of the conducting polymer onto theatterned substrate. However, commercially available oxidant solu-

ions are expensive and not suitable for some printing technologies.cope exists for improvements in performance and cost reductions,rovided cheaper oxidants can be found that possess similar or

mproved functionality for a conductive polymer application. We

∗ Corresponding author. Tel.: +61 3 9905 4526; fax: +61 3 9905 4597.E-mail address: Leone.Spiccia@sci.monash.edu.au (L. Spiccia).

oicpoost

379-6779/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rioi:10.1016/j.synthmet.2008.04.021

nate and Fe(III) p-dodecylbenzenesulfonate salts.Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.

eport studies aimed at synthesising alternative oxidants for thisurpose and comparing the films obtained from the oxidants withommercial oxidants.

Spin coating, solvent casting or printing are the most commonechniques available for depositing thin, even, conducting polymeroatings. Unfortunately, polypyrrole is difficult to process in thisay, as it has the major disadvantage of being insoluble in most sol-

ents [2]. Even dispersions stabilized sterically or by manipulatingharge, can be difficult to form because PPy readily agglomeratesue to strong interactions between polymer chains. In chemicallyynthesised polyaniline dispersions, Shin et al. have reported thatopants with long chains such as alkylbenzenesulfonic acids couldvercome these interactions [3]. As the non-polar alkyl chain lengthncreased, it began to act as a surfactant between the polyanilinehain and the solvent, resulting in increased solubility and better

roperties. Whilst the work by Shin and co-workers was focusedn producing polyaniline dispersions by chemical polymerisation,ur research interest is in the vapour deposition technique; a two-tep process that allows the growth of a uniform film of PPy on aemplate.

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The first studies of polymerisation of conducting polymers in theapour phase were conducted by Mohammadi et al. at low pressure,ut studies using iron(III) chloride as an oxidant at normal pressuresvapour phase polymerisation) have been reported subsequently bynumber of researchers [4–6]. The use of ferric p-toluenesulfonates an oxidant was initially reported by de Leeuw et al. for thehemical polymerisation of 3,4-ethylenedioxythiophene [7]. Sub-equently, Fu et al. reported the use of Fe(III) p-toluenesulfonate asn oxidant for vapour phase polymerisation on polyurethane foams8]. Winther-Jensen et al. used this same salt extensively as an oxi-ant for growing PPy and poly(3,4-ethylenedioxythiophene) filmsy vapour phase polymerisation [9]. In contrast to FeCl3, Fe(III) p-oluenesulfonate does not crystallise as the solvent evaporates andhis represents a significant advantage as it is necessary to suppressrystallite formation in the dried oxidant in order to obtain smoothPy films.

Since previous work on forming PPy films by vapour phase poly-erisation has been mainly limited to the use of FeCl3 it would

e informative to determine if a wider range of iron(III) alkylben-enesulfonate salts, with long, alkyl chains on the hydrophobicomponents can act as an anionic surfactant to improve thelectrical and surface properties of PPy [10,11]. These iron(III) alkyl-enzenesulfonate salts (see Fig. 1 for the parent sulfonic acidtructure used in this work) are not commercially available andere synthesised, characterised and then used in this work, for the

apour phase polymerisation of PPy and compared to the Fe(III)-toluenesulfonate salt obtained from two commercial suppliers.

. Experimental

Benzenesulfonic acid, p-dodecylbenzenesulfonic acid, p-oluenesulfonic acid, p-ethylbenzenesulfonic acid and Fe(III)-toluenesulfonate salt were obtained from Sigma–Aldrich,ustralia, and were used as received. Fe(III) p-toluenesulfonate

n 1-butanol (Baytron CB40) was used as received from H.C.tarck. Iron(III) alkylbenzenesulfonate complexes were synthe-ised by reaction of Fe(OH)3 with three molar equivalents of theorresponding sulfonic acid as shown in the Scheme 1.

Scheme 1. Reaction scheme for synthesis of Fe(III) alkylbenzenesulfonates.

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etals 158 (2008) 704–711 705

.1. Synthesis of Fe(III) alkylbenzenesulfonates

.1.1. Preparation of Fe(III) benzenesulfonateFerric hydroxide was prepared by slowly adding 11.1 g

0.28 moles) of NaOH to 75 mL water and then adding the NaOHolution to a solution of 25.0 g (0.09 moles) FeCl3·6H2O dissolved in50 mL of water while stirring vigorously. After 1 h the entire reac-ion mixture was filtered through a medium porosity glass sinteredunnel. The amorphous brown solid was rinsed several times withL of water. The freshly prepared ferric hydroxide was suspended in75 mL of methanol and 41.5 g (0.27 moles) of benzenesulfonic acidissolved in 75 mL of methanol was added slowly over a period ofmin. The reaction mixture was gently warmed to 50 ◦C, and stirredigorously for 3 h to complete the dissolution of the Fe(OH)3 ando ensure completion of the reaction. The orange-red solution washen cooled at room temperature and filtered to remove any insol-ble material. The filtrate was evaporated and the remaining oilroduct was placed in a crystallising dish. The reaction product wasurther dried in a vacuum oven at 50 ◦C and 100 mm Hg pressureor 72 h. The dried products was then further washed with waternd kept in a vacuum desiccator for drying.

The Fe(III) p-dodecylbenzenesulfonate, p-thylbenzenesulfonate and p-toluenesulfonate salts wereynthesised similarly by reaction of 1 mole of Fe(OH)3·xH2Ond 2.8 moles of the respective sulfonic acids.

Crystals of Fe(III) p-toluenesulfonate suitable for X-ray crystaltructure determination were grown by recrystallising the Fe(III)-toluenesulfonate from concentrated ethanol solution.

.1.2. Yields

Fe(III) benzenesulfonate: [Fe(OH2)6][C6H5SO3]3; (35.62 g, 73%).Fe(III) ethylbenzenesulfonate: [(OH2)5Fe–O–Fe(OH2)5][C2H5C6H4SO3]4; (42.62 g, 75%).Fe(III) p-toluenesulfonate (laboratory synthesised):[(OH2)5Fe–O–Fe(OH2)5][CH3C6H4SO3]4·2H2O; (10.38 g, 79%).

.2. Characterisation of Fe(III) alkylbenzenesulfonates

.2.1. FTIR analysis of Fe(III) alkylbenzenesulfonatesFTIR analysis of the Fe(III) alkylbenzenesulfonate salts were

arried out on KBr pellets of each salt using a PerkinElmer Spec-rum 2000 (Beaconsfield, Berkshire, UK). The spectra were acquirednder an absorption mode using a resolution of 4 cm−1 using 32cans over a range of 4500–370 cm−1.

.2.2. Determination of crystal structure of Fe(III)-toluenesulfonate

Crystal structure determination was carried out on a yellowlock (0.13 mm × 0.10 mm × 0.08 mm) cut from a larger crystal. Therystal was mounted under oil on a glass fiber on a Enraf NoniusAPPA CCD and cooled to 123 K. A sphere of reflection data (2�max,5◦) was collected (MoKa, l 0.71073 A, phi and omega scans, 1.0◦

rames) and processed (COLLECT, Nonius BV, 1998, DENZO-SMN, Z.twinowski and W. Minor, 1997) giving 20391 reflections of which0308 were unique. The structure was solved by direct methodsnd refined by full matrix least squares (SHELX-97, G. Sheldrick,997). Non-hydrogen atoms were refined with anisotropic thermalarameters. Hydrogen atoms on the p-toluenesulfonate moieties

ere placed in calculated positions using a riding model witheq 1.2–1.5 that of the parent C atom. Hydrogen atoms on theater molecules were located in the difference Fourier map and

efined with restrained geometries. At convergence, R1 = 0.046 andR2 = 0.103 for I > 2s(I) (R1 = 0.079 and wR2 = 0.117 all data).

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.3. Synthesis of polypyrrole (PPy) by vapour phaseolymerisation

Eight grams of each Fe(III) alkylbenzenesulfonate salts were dis-olved in 12 mL of 2-butanol to form a solution at 40 wt%. Thexidant was coated on a non-conducting overhead transparencypolyethylene terephthalate film) and various other substrates likeilicon wafers using a spin coater operating at a speed of 2000 rpmor 2 min. The oxidant-coated substrate was heated on a hot plate at0 ◦C for 90 s until the solvent evaporated. The substrate was thenxposed to pyrrole vapour in a sealed reaction vessel and poly-erisation was detected via the colour change of the oxidant. The

ubstrate was kept in the chamber for 10 min, air-dried to removenreacted monomer, and then washed in ethanol to remove unre-cted Fe3+, Fe2+. The washed PPy films were then stored in plasticatch glass containers and then further characterised by Raman

pectroscopy and XPS analysis.

.4. Characterisation of PPy films

The PPy films produced on Fe(III) alkylbenzenesulfonate coatedverhead transparencies were characterised using Raman spec-roscopy (JOBIN Yvon Horiba Raman spectromodel HR800) and-ray photon spectroscopy (Kratos XPS). Film thickness was mea-ured along the cross-section of the films using Philips XL 30 Fieldmission SEM (scanning electron microscope) and surface con-uctivity was measured using the four-point probe technique. TheV–vis spectra of the PPy films were measured under absorptionode from 320 to 1100 nm using a Schimadzu 1601 UV–vis spec-

rophotometer.

.4.1. Raman spectroscopyRaman spectra were obtained with a JOBIN Yvon Horiba Raman

pectromodel HR800. The spectra were collected with a spectralesolution of 1.5 cm-1 in the backscattering mode with 632.8 nmine from a helium/neon laser. A Gaussian–Lorentzian fitting func-ion was used to obtain band position and intensity. The incidentaser beam was focused on the substrate surface through a 100×bject lens. Samples showing fluorescence were analysed for 30 sime intervals in a dark room with a D2 filter and others werenalysed for 10 s time intervals with a D1 filter.

.4.2. XPS analysisXPS analyses were performed with a KRATOS Analytical AXIS-

Si spectroscope using monochromated Al K� X-ray sourceperated at 12 kV/12 mA emission current. The emission was nom-nally 0◦ with respect to the surface normal and the chamberressure was maintained between 2 and 8 × 10−8 mbar. Surveypectra were acquired to determine all elements present on the sur-ace of the materials. High resolution spectra were then recordedor selected elements (C, O, N) in order to obtain more specificata regarding chemical structure (e.g. oxidation state). Spectraere obtained at two different locations on each sample (at 0◦

mission). The mean atomic ratios were calculated for each sam-le/emission angle and the standard deviation determined as aeasure of compositional variation. The systematic error is esti-ated to be between 5 and 10%. The Relative Sensitivity Factorsere C1s 0.250, O1s 0.660, N1s 0.420 and Si2p 0.180.

.4.3. ConductivityThe conductivities of the polypyrrole films were measured using

he four-point probe technique. The conductivities of the films werealculated using the sheet resistance and the thickness measuredsing SEM.

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etals 158 (2008) 704–711

. Results and discussion

.1. Synthesis of Fe(III) alkylbenzenesulfonates

Iron(III) sulfonates were prepared by the reaction of hydratedron(III) hydroxide and the sulfonic acid in a ratio of 1:2.8, ashown in the reaction Scheme 1. The products were obtained aspowder following drying under vacuum. The dried powders were

paringly soluble in water and were difficult to crystallise. Theried powders were further washed with water and dried in aacuum desiccator to obtain a pure dry sample of Fe(III) alkyl-enzenesulfonates for microanalysis. Microanalysis showed thathe compounds were hydrated and polymeric, with two molesf sulfonate per mole of iron(III) corresponding to the composi-ion [(OH2)5Fe–O–Fe(OH2)5][ABS]4·xH2O except for Fe(III) BSA thatontained 3 moles of sulfonate per mole of Fe(III), i.e. with molec-lar formula [Fe(OH2)6][ABS]3.

.1.1. Elemental analysisMicroanalysis was conducted to determine the C, H and S con-

ent of the iron(III) salts and the iron content was determined byDTA titration. The results are as follows: Fe(III) benzenesulfonatefound C, 34.4%; H, 4.0%; S, 15.8%; Fe, 8.3%; Fe1C18H27S3O15 requires, 34.0%; H, 4.3%; S, 15.0%; Fe, 8.8%). Fe(III) ethylbenzenesulfonatefound C, 37.4%; H, 5.9%; S, 12.1%; Fe, 11.3%; Fe2C32H56S4O23 requires, 36.7%; H, 5.4%; S, 12.2%; Fe, 10.7%). Fe(III) p-toluenesulfonateLaboratory Synthesised) (found C, 33.6%; H, 5.2%; S, 12.4%; Fe,1.2%; Fe2C28H48S4O23 requires C, 32.7%; H, 5.1%; S, 12.5%; Fe, 10.9%)e(III) p-toluenesulfonate (Sigma–Aldrich) (found C, 34.9%; H, 5.2%;, 13.1%; Fe, 11.9%; Fe2C28H36S4O17 requires C, 34.0%; H, 4.9%; S,2.9%; Fe, 11.2%).

The elemental analysis of iron(III) dodecylbenzenesulfonateould not be done because the reaction of Fe(OH)3 with 3 moles ofodecylbenzenesulfonic acid produced a gel like substance whichas difficult to purify.

.2. FTIR analysis

The FTIR spectra of the Fe(III) alkylbenzenesulfonate sampleshown in Fig. 2 reveal similar vibrations at about 1100–1200 cm−1

hich are due to asymmetric sulfonate stretches and close to040 cm−1 which are due to symmetric sulfonate stretches. Thee(III) alkylbenzenesulfonate salts also show a broad absorptionand close to 3400 cm−1 attributed to the �OH stretch of water. Thisas confirmed by the crystal structure of Fe(III) p-toluenesulfonate,hich showed that each iron(III) centre in the complex has five

oordinated water and a complex hydrogen bonded networkormed by the water molecule of crystallisation. Most importantlyhe FTIR spectra showed the presence of the oxo-bridged dinu-lear Fe(III) units in Fe(III) p-ethylbenzenesulfonate and the Fe(III)-toluenesulfonate salts but no such bands for Fe(III) benzenesul-onate salts. The IR spectra showed an Fe–O–Fe stretch at 833 cm−1

or Fe(III) p-ethylbenzenesulfonate and at 815 cm−1 and 814 cm−1

or Fe(III) p-toluenesulfonate (Laboratory synthesised) and Fe(III)-toluenesulfonate (Sigma–Aldrich), respectively. These values aren good agreement with those reported for similar compounds byunk et al. [12]. Fe–O–Fe stretches were absent in Fe(III) benzene-ulfonate salts and this is in accordance with the elemental analysisesults which indicated this complex to be mononuclear.

.3. X-ray structure determination of the p-toluenesulfonate salt

Crystals of the iron(III) p-toluenesulfonate salt were obtainedy recrystallisation of the salt from a concentrated ethanol solu-ion. In agreement with the elemental analysis, the X-ray crystal

P. Subramanian et al. / Synthetic Metals 158 (2008) 704–711 707

(III) alkylbenzenesulfonates.

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Table 1Crystal data and structure refinement for [(H2O)5Fe–O Fe(H2O)5][SO3]4·2H2O

Empirical formula C28H56Fe2O27S4

Formula weight 1064.67Temperature (Kelvin) 123� (A) 0.71073Crystal system, space group Triclinic, P−1

Volume (A3) 2317.89(9)Z 2Calculated density (g/cm3) 1.525Absorption coefficient (mm−1) 0.892Crystal size 0.13 mm × 0.10 mm × 0.08 mm� range for data collection 1.47–27.50◦

Reflections collected/unique 20391/10308 [R(int) = 0.0590]Limiting indices −11 ≤ h ≤ 11, −12 ≤ k ≤ 12, −25 ≤ l ≤ 36Completeness to � = 27.50 (%) 97.0Absorption correction NoneUnit cell dimension, a (A) 9.0833(2)Unit cell dimension, b (A) 9.2977(2)Unit cell dimension, c (A) 27.7782(6)Unit cell dimension, ˛ (◦) 89.724(2)Unit cell dimension, ˇ (◦) 89.563(2)Unit cell dimension, � (◦) 81.1380(10)Refinement method Full-matrix least-squares on F2Data/restraints/parameters 10308/8/669Goodness of fit on F2 1.017Final R indices [I > 2�(I)] R1 = 0.0465, wR2 = 0.1029R indices (all data) R1 = 0.0788, wR2 = 0.1168Largest diff. peak and hole, A − 3 0.547 and −0.482

Table 2Selected bond lengths (A) for [(H2O)5Fe(�-O)Fe(OH2)5][O3SC6H4CH3]4·2H2O

Fe(1)–O(1) 2.144(2)Fe(1)–O(2) 1.7872(4)Fe(1)–O(3) 2.043(2)

Fig. 2. FTIR spectra of Fe

tructure of the salt confirmed that the product had the compo-ition [(H2O)5Fe–O–Fe(OH2)5][CH3C6H4SO3]4·2H2O. Furthermore,he determination revealed that the complex is a rare example ofn oxo-bridged dinuclear iron(III) aqua ion, as shown in Fig. 3. Thetructures of two other salts of this aqua ion have been publishedery recently by Junk et al. and solved a long standing debate onhether the two iron(III) centres in this dinuclear aqua ion were

inked via two hydroxo- or one oxo-bridging group [12,13].Crystal data, structure refinement details and unit cell dimen-

ions for [(H2O)5Fe–O–Fe(H2O)5][O3SC6H4CH3]4·2H2O are given inhe Table 1.

Selected bond lengths (A) and angles (◦) for [(H2O)5Fe(�-)Fe(OH2)5][O3SC6H4CH3]4·2H2O are given in Tables 2 and 3.

The [(H2O)5Fe(�-O)Fe(OH2)5][O3SC6H4CH3]4·2H2O structureonsists of two crystallographically independent, centro-ymmetric [(H2O)5Fe(�-O)Fe(OH2)5]4+ units. Each of thesere linked by a network that is extensively hydrogen-bondednvolving the coordinated H2O, lattice CH3C6H4SO3

− and lattice2O molecules in a 2D sheet lying parallel to the a*b face. The

ndividual 2D sheets, as shown in Fig. 4 have the aromatic ringsf the tosylate groups extending out of the plane and interleav-ng with the neighboring 2D sheet, but are not �-stacked. Therrangement of the aqua ions, sulfonate counterion and water ofrystallisation is similar to that found in related salts of Cr(III)nd Rh(III) complexes as reported by Spiccia et al. [14,15]. Thenteractions of the stacked hydrophobic ends of the sulfonates andydrophilic ends of water encourage crystallisation.

The iron centres in the structure are coordinated to six oxygentoms forming a distorted octahedron. The five oxygen atomsriginate from the water molecules whereas the sixth oxygen atomrises from the oxo-bridge which connects the two Fe3+ centers.

he bond length of Fe(I)–O(I)–Fe(I)# is 180(2) A and this lies inhe range found in similar related complexes reported by Junkt al. [12]. The distance between the two iron centers connectedy an oxo-bridge is 3.574(4) A which is typical for complexesith the singly bridged Fe–O–Fe cores. [13,16,17]. The waters of

Fe(1)–O(4) 2.047(2)Fe(1)–O(5) 2.040(2)Fe(1)–O(6) 2.044(2)Fe(1). . .. . .Fe(1′)# 3.574(4)

# -x+2, -y+1, -z

708 P. Subramanian et al. / Synthetic Metals 158 (2008) 704–711

Fig. 3. ORTEP plot of [(H2O)5Fe(�-O)Fe(OH2)5][O3SC6H4CH3]4·2H2O (Fe(III) p-toluenesulfonate) showing the arrangement of the complex and counter ions.

Table 3Selected bond angles (◦) for [(H2O)5Fe(�-O)Fe(OH2)5][O3SC6H4CH3]4·2H2O

O(1)–Fe(1)–O(2) 178.47(6)O(1)–Fe(1)–O(3) 81.47(9)O(1)–Fe(1)–O(4) 81.97(9)O(1)–Fe(1)–O(5) 84.05(9)O(1)–Fe(1)–O(6) 85.16(9)O(2)–Fe(1)–O(3) 100.03(8)O(2)–Fe(1)–O(4) 97.70(7)O(2)–Fe(1)–O(5) 94.46(7)O(2)–Fe(1)–O(6) 95.20(7)O(3)–Fe(1)–O(4) 90.95(10)O(3)–Fe(1)–O(5) 165.34(10)O(3)–Fe(1)–O(6) 86.15(10)O(4)–Fe(1)–O(5) 89.25(10)O(4)–Fe(1)–O(6) 167.09(10)O(5)–Fe(1)–O(6) 90.40(10)Fe(1)–O(2)–Fe(1′)# 180.0

Symmetry transformations used to generate equivalent atoms: # −x + 2, −y + 1, −z

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ings and cations and interdigitation between adjacent aromatic rings.

rystallisation connect, via hydrogen bonding, the aqua ligandsn the dinuclear cation with the sulfonate anions, forming aandwich structure with the oxo-bridged dimer in the centre. Thexo-bridge is not involved in the hydrogen bonding. The Fe(I)-�(O)istance for [(H2O)5Fe(�-O)Fe(OH2)5][O3SC6H4CH3]4·2H2O is.7842(4) A and is similar to [(H2O)5Fe(�-O)Fe(OH2)5](ClO4)4[18-rown-6]2·2H2O at 1.7752(3) A as reported by Junk et al. [12].owever, the Fe-(�-O) distance reported for [(H2O)5Fe(�-)Fe(OH2)5](NO3)10[Fe(H2O)6]2[15-crown-5]4·(H2O)6 is.136(18) A and is much higher than for the [(H2O)5Fe(�-)Fe(OH2)5][O3SC6H4CH3]4·2H2O reported here suggestingeaker oxo-bridging in the nitrate salt.

.4. Synthesis and characterisation of PPy films

The Fe(III) alkylbenzenesulfonates were spin coated on the over-ead transparencies and then the oxidant coated substrates werexposed to pyrrole monomer in a closed chamber until a uniformlack film of PPy deposited on the surface. The PPy films were thenashed with ethanol to remove excess oxidant and further char-

cterised by Raman spectroscopy and XPS techniques. The dopingatio of PPy films oxidised with Fe(III) alkylbenzenesulfonates wereetermined using XPS analysis. The PPy structure was deduced byorrelating the results of XPS and Raman spectroscopy. The resultsbtained from Raman spectroscopy were combined with the dop-ng ratios and spectral results of the XPS analysis to explain theonductivity behavior of PPy films synthesised with the differente(III) alkylbenzenesulfonate oxidants.

.4.1. Raman spectroscopyThe Raman spectra of the PPy films (Fig. 5) show absorption

t 1581 cm−1, which is characteristic of C C stretching associ-ted with the presence of the (polaron) cation. Crowley et al.ave compared PPy doped with dodecylbenzenesulfonate anionnd PPy doped with perchlorate anion and have found no dif-erence in their Raman spectra and reported that the bands inhe spectra were solely due to the polymer and not the anion18]. Similarly in this study we inferred that the bands wereolely due to the polymer and not due to the presence of theifferent alkylbenzenesulfonate anions. The asymmetric stretchf sulfonate group is generally in the region between 1333 and420 cm−1 region and the symmetric sulfonate stretches in the

−1

egion 1050–1080 cm . In the PPy films, however, the band dueo C–N stretching in PPy dominates the former region and C–H inlane bending dominates the later. As a consequence bands due tohe sulfonate stretches of the anion are not readily observed. PPybtained from Fe(III) p-toluenesulfonate (laboratory synthesised,

P. Subramanian et al. / Synthetic M

Fig. 5. Raman spectra of PPy films prepared using (a) Fe(III) benzenesulfonate,(b) Fe(III) ethylbenzenesulfonate, (c) Fe(III) p-toluenesulfonate (laboratory synthe-s(

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ised), (d) Fe(III) p-toluenesulfonate (Sigma–Aldrich), (e) Fe(III) p-toluenesulfonateBaytron CB 40) and (f) Fe(III) dodecylbenzenesulfonate.

aytron CB 40 and Sigma–Aldrich samples) shows a pronouncedhoulder at 1610 cm−1 that is associated with the presence of di-ation species (bipolaron activity). This peak is not as significantn the lower conductivity PPy film samples obtained from Fe(III)odecylbenzenesulfonate and Fe(III) ethylbenzenesulfonate. Thewo well-resolved bands at 1378 and 1333 cm−1 seen in all casesre due to the anti-symmetrical C–N stretching of the oxidisedlms. In the case of Fe(III) ethylbenzenesulfonates the absorp-ion at 1333 cm−1 is replaced by a band at 1311 cm−1 which ishought to be associated with the neutral species in the polymer18].

A broader Raman band of PPy appears in the range of000–1150 cm−1 which is assigned to the C–H in-plane deforma-ion, and absorption in this range has been studied by severalesearchers [19]. The absorptions at 1054 and 1083 cm−1 are dueo the C–H in plane bending of oxidised PPy [20–22]. Theseands can be seen as two well-resolved peaks in the casef PPy films obtained from Fe(III) benzenesulfonate and Fe(III)-toluenesulfonate (Baytron, Sigma–Aldrich and laboratory syn-

hesized). The presence of a well-resolved absorption band at083 cm−1 is assigned to oxidised PPy as reported in the litera-ure [23–25]. This band is present in lower conductivity PPy filmsbtained from Fe(III) ethylbenzenesulfonate and Fe(III) dodecyl-

able 4PS atomic concentration percentages of C, O, N and S of PPy films vapour phaseolymerised with Fe(III) alkylbenzenesulfonates

xidant % C1s % O1s % N1s % S2p

e(III) benzenesulfonate 72.00 14.01 10.89 3.11e(III) ethylbenzenesulfonate 73.13 15.53 8.83 2.51e(III) dodecylbenzenesulfonate 78.68 10.50 8.33 2.48e(III) p-toluenesulfonate (Baytron) 72.79 13.19 11.05 2.98e(III) p-toluenesulfonate (Laboratorysynthesised)

73.53 16.79 8.02 1.66

e(III) p-toluenesulfonate(Sigma–Aldrich)

71.05 20.40 7.48 1.07

bobhcid

ipadoddot(

etals 158 (2008) 704–711 709

enzenesulfonate but is not as distinct when compared with otherPy films, as shown in Fig. 5.

.4.2. XPS analysisThe concentration of S, O, C, N in the polypyrrole film was

btained by XPS analysis and is given in Table 4. The XPS results for1s spectra are shown in Fig. 6. The C1s spectra for PPy films dopedith Fe(III) alkylbenzenesulfonates are similar. Several researchersave used XPS and UV–vis spectroscopy to study the electronictructure of �-conjugated polymers and provide assignments ofhe XPS peaks [26–30].

The peak at 284.7 eV, i.e. CI in the C1s spectrum, is attributed tohe aromatic carbons in the alkylbenzenesulfonates and �-carbontoms in the polymer. The peak at 285.6 eV is attributed to theontribution of �-carbon atoms in the pyrrole units involved inlectrostatic interaction with the anion and also to the C–N interac-ions. The peak at 286.3 eV is indicative of the presence of C–O/C–Sonds [31]. The CIV peak at 288.5–289.1 eV is due to the presence of

O or O–C O bonds. Dilks et al. reported a large shift of 4 eV withespect to the �-carbon atom for carbon atoms bound to more thanne oxygen, such as in a carboxyl group [32]. This was supported bytanasoska et al. who have reported that the CIV peak may be due

o introduction of carboxylate groups at terminal positions withinhe PPy chain or at the points of pyrrole ring cleavage [31]. A sim-lar peak that is assigned to the C O bonds is observed in the XPSpectra of the PPy films synthesised here.

The presence of the C O group in the XPS results at 288.5 eVnd in Raman spectra at 1730 cm−1 suggested that either there is aormation of carbonyl bands at the �-position of the pyrrole ringsndicating over-oxidation, or that a carbonyl group is present inhe PPy film. The PPy films did not show any of the other char-cteristic peaks of over-oxidised PPy in XPS analysis and Ramanpectroscopy and the conductivities of the PPy films were high, sohe possibility of formation of carbonyl bonds at the �-position wasiscounted. The C O stretch seen at 1731 cm−1 in the Raman spec-ra and corresponding signals in XPS spectra are due to carboxylateroducts formed from the oxidation of 2-butanol [33], which issed as a solvent to dissolve the iron(III) alkylbenzenesulfonatexidants. Oxidation of 2-butanol by the strong Fe3+ oxidants dur-ng the heating process may lead to the formation of the butanoatenion and thus the presence of this additional dopant anion in thePy produced. This is consistent with known oxidation reactions of-butanol [33]. The doping levels of the PPy films (Table 5) were cal-ulated considering the presence of butanoate species in addition tohe sulfonate species. However no correlation was found betweenhe doping ratios and conductivity. The PPy film doped with BaytronB40, which is a 40% solution of Fe(III) p-toluenesulfonate in 1-utanol, did not show a significant carbonyl peak, indicating littler no butanoate species. Thus it appears that the 1-butanol is sta-le against these oxidants whereas 2-butanol to an extent is not. Inindsight therefore 1-butanol is the preferred solvent for this pro-ess; however, the replacement of some of the sulfonate counterons in the film does not appear to have a negative effect on con-uctivity.

The conductivities and doping ratios of the PPy films are shownn Table 5. The highest ever conductivity reported for vapour phaseolymerised PPy is 50 S/cm, using Fe(III) p-toluenesulfonate asn oxidant [5]. The results presented here show that the con-uctivity of PPy did not increase with longer alkyl chain lengthn the Fe(III) alkylbenzenesulfonate. For example, the PPy film

oped with Fe(III) dodecylbenzenesulfonate was not highly con-uctive, possibly due to the branching of C12H25 alkyl chain in thexidant (see Electronic Supplementary data for the 1H NMR spec-rum of dodecylbenzenesulfonic acid). The highest conductivity40 (±2) S/cm) was achieved for PPy film produced by poly-

710 P. Subramanian et al. / Synthetic Metals 158 (2008) 704–711

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ig. 6. C1s spectra of PPy films synthesised using (a) Fe(III) benzenesulfonateoluenesulfonate (laboratory), (e) Fe(III) p-toluenesulfonate (Baytron) and (f) Fe(III)

erisation with Fe(III) p-toluenesulfonate (commercially producedaytron C-B 40) followed closely by the PPy film produced withe(III) p-toluenesulfonate synthesised in laboratory (25 (±1) S/cm).he conductivity values obtained for PPy film with iron(III) p-oluenesulfonate (Baytron CB-40) oxidant is comparable to the

onductivity obtained by Winther-Jensen et al. for the same oxi-ant [5]. The other three oxidants produced PPy films of loweronductivity.

Tni

able 5onductivity of PPy films synthesised on overhead transparency using iron(III) alkylbenze

xidant Conductivity in S/cm (standard er

e(III) benzenesulfonate 6 ( ± 1)e(III) ethylbenzenesulfonate 5 ( ± 1)e(III) dodecylbenzenesulfonate 3.0 ( ± 0.5)e(III) p-toluenebenzenesulfonate (laboratory) 25 ( ± 1)e(III) p-toluenebenzenesulfonate (Baytron) 40 ( ± 2)e(III) p-toluenebenzenesulfonate (Sigma–Aldrich) 4 ( ± 1)

Fe(III) ethylbenzenesulfonate, (c) Fe(III) dodecylbenzenesulfonate, (d) Fe(III) p-enesulfonate (Sigma–Aldrich).

.4.3. UV–vis spectroscopyThe UV–vis spectroscopy was carried out in absorption mode.

he UV–vis spectra of the PPy films show a characteristic absorptioneak at 461 nm and increases gradually beyond 600 nm, as shown inig. 7 indicating the existence of bipolarons in the polymer [34,35].

he PPy films prepared from the different oxidants showed no sig-ificant difference in the spectra that could explain the differences

n conductivity.

nesulfonates

ror) Doping ratio (S/N) Doping ratio (butanoate/N) Film thickness (�m)

0.29 0.05 2.00.28 0.11 1.00.30 0.10 0.60.21 0.11 0.90.14 0.11 0.70.27 0.05 0.2

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ig. 7. UV–vis spectroscopy of PPy films synthesised using Fe(III) alkylbenzenesul-onate.

. Conclusions

A series of Fe(III) alkylbenzenesulfonates salts have beenrepared, by the reaction of Fe(OH)3·xH2O and four aromatic sul-onic acids (viz., benzenesulfonic acid, ethylbenzenesulfonic acid,-toluenesulfonic acid and dodecylbenzenesulfonic acid), charac-erised and then applied as oxidants in the synthesis of polypyrrolelms by the vapour deposition technique. Elemental analysis andpectroscopic characterisation of the oxidants revealed that threef the complexes have been isolated as oxo-bridged dinuclear ions(H2O)5Fe–O–Fe(OH2)5]4+. In case of the Fe(III) p-tolenesulfonatealt the formation of this binuclear complex was confirmed by X-rayrystallography. The Fe(III) benzenesulfonate, Fe(III) ethylbenzene-ulfonate and Fe(III) p-dodecylbenzenesulfonate, once isolatedroved difficult to recrystallise. PPy films oxidised with Fe(III)-toluenesulfonate synthesised in our laboratory showed conduc-ivity of 25 (±1) S/cm, comparable to that of the PPy films oxidisedith the Baytron CB40 (Fe(III) p-toluenesulfonate marketed by

AYER) which showed the highest conductivity (40 (±2) S/cm). Thelms doped with Fe(III) p-toluenesulfonates showed an enhancedhoulder peak at 1613 cm−1 associated with the presence ofipolaron, which has been proposed to produce PPy of higher con-uctivity.

The XPS results showed no significant differences in the PPylms doped with different Fe(III) alkylbenzenesulfonates. TheV–vis spectra of PPy films showed that the PPy films are in thexidised state. It is postulated that the C O absorption bands inoth the XPS and Raman spectroscopy are due to the presence ofarboxylates formed at elevated temperature from the oxidation of

-butanol solvent, which was used to dissolve iron(III) alkylben-enesulfonate oxidants for film deposition [33]. This potentiallyave rise to an additional dopant in the PPy films, increasing thencertainty in the calculation of the doping levels. It is clear how-ver that this does not appear to have adverse effect on the film

[

[[[

etals 158 (2008) 704–711 711

onductivity. The Fe(III) salts with CH3 group in the para-positionave the highest conductivities in PPy films which is likely due tohe presence of an electron withdrawing group (CH3) improving theegularity of the PPy backbone and preventing the side reactionshich in turn improves the conductivity of the polymer. Further

tudies with different types of Fe(III) alkylbenzenesulfonates are inrogress.

cknowledgements

This research was supported by the Smartprint Cooperativeesearch Centre (CRC). P.S. acknowledges the support of Monashniversity through a Postgraduate Publication Award.

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at doi:10.1016/j.synthmet.2008.04.021.

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