EPR study of regioregular poly(3-hexylthiophene)

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Synthetic Metals 101 (1999) 358

EPR study of regioregular poly(3-heq4thiophene) M.Trznadel”; 0. Chauvetb;M. Lapkowski”;A. Prond ’ Laboratoim de Physique des Metalrx Synthetiques, DRlWC, me des Martyrs, 38 OS4, CEA-Grenoble, Cede.x 9, France ’ Laboratoire de Physique Cristalline, Universitd de Nantes, 2 rue de la Houssiniere, 44072 Nantes, C&dex 03, France ’ Deparhnentof Chemistry, Silesian Technical University Ks. M. Strzody 9, 44-l 00 Gliwice, Poland dFacd@ of Chemi&y, Warsaw University ofTechnology, Noakowskiego 3, 00 664 rVarszawu, Polartd

Abstract Cyclic voltammetry and EPR studieshave shownthat oxidative dopingof regioregularpoly(3-hexylthiophene) is a two-step processin which polaronsare tirst createdand then they recombineto bipolarons.Both spindensity andspin/chargeratio reachthe maximumnearthe first peakobservedin cyclic voltammetry. Keywords: polythiopheneandderivatives,regioregularpolymer, electronspinresonance, electrochemical

Among conjugatedpolymerssolubleandprocessable poly(3alkylthiophenes)(PAT) exhibit unusualelectronic and eledrochemicalproperties.Thesefeaturesare very sensitivetowards structuralhomogeneityof polymer chains.The regioregularH-T (headto tail) PAT [ 1.21exhibit two stepoxidative dopingas evidenced by spectroelectrochemical and cyclic voltammetry studies [3]. The main aim of this work was to investigate polaronscreationduringelectrochemicaldopingof HI’-PHT. Regioregular poly(3-hexylthiophene)(HT-PHI’) wasobtained asdescribedelsewhere[3]. The percentageof HIT-HT couplings in the polymer were estimatedfrom the NMR integration as almost 100%. The GPC molecularweight (PS standards)was M,=17,700 and M&&=1.45. Thin layers of HT-PHT were depositedon a platine wire by castingfrom CHClasolution.The shapeof electrochemicalcell was fitted to the resonancecavity of the EPR spectrometer.Voltammogramandspin responses for different potentials were registeredin a 0.1 M solution of Bu&BF4 in acetonitrilewith a Pt foil as a counterelectrodeand an Ag/AgCl reference electrode. The measurementswere performedon PAR 273 Potentiostatand Bruker 200DSRC EPR SpectrometerusingcommerciallyavailableBruker standards. As it can be seenin Fig 1. the numberof spinsgenerated during the doping processreachesthe maximum at 0.6V for oxidationcycle and0.55V for reductioncycle. Eachvalue of the

spinnumberwasdivided by the total charge.which transferedby the cell until correspondingpotential was applied. The dependenceobtainedis presentedtogether with voltammogram registeredin the samecell in Fig. 2. Both spin density and spin/chargeratio (N,M,) reachthe maximumnear the frostpeak observedin cyclic voltammetry.The maximumvalue of N,/N, is equal 0.2 and the bipolaronhas a charge 2 times grater than polaron. Thus it can be concluded,that at this potential one polaron is created per 2 bipolarons. However the polaron contribution must have been underestimatedsince impedance spectroscopystudiesof polyalkylthiophenes[4] have shownthat the total electrochemicalcharge can include even 50% of capacitive component.Thereforein the secondstep of doping, corresponding to the maxima of the first CV peak and the spin responce,evenonepolaroncould becreatedper one bipofaron.

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Fig. 1. Spinnumbersin HT-PHT measuredduring electrochemicalcycling in 0.1 M Bu$rTBFd/MeCN electrolyte

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Fig. 2. Cyclic voltammogram(fOmV/s, 0. 1M Bu&IBF&leCN) andspin/chargeratio vs. appliedpotential for W-PHI References [l] RD.McCullough,RD,Lowe,J. Chem.Sot., Chem.Commun. 1992,70. [2] T.A.Chen,R. D&eke, J. Am. Chem.Sot. 114(1992) 10087 [3] M.Trmadelet al., J. Chem.Sot., FaradayTrans.92 (1996)1387 [4] J.Tanguyet al., Synth. M.et. 45 (1991)81

0379-6779199/$ - seefrontmatter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)013

doping