ELSEVIER Synthetic Metals 101 (1999) 118-119 Preparation and properties of fractionated regioregular poly(3-alkylthiophenes) M.Trznadel’, A. Pron’,** and M.Zagorska’ ‘Luboratoire de Physique des Me’taux S?~nthetiques/U.M.R. 5819 (CEA-CNRS-UniversirP J. Fonrier)/DRFMC/DSM CEA-Grenoble, 17 Rue des Martyrs, 38054 Grenoble Cedes 9, France. 2Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00664 Warszann, Poland. Abstract In order to establish the influence of end groups on the spectroscopic and electrochemical properties of regioregularpoly(3- alkylthiophenes) we havefractionated poly(3-hexylthiophene) into four fractions differing in their molecular weight (M,) andexhibiting relatively low polydispersity coefficients. Spectroscopic andspectroelectrochemical properties of the obtained fractions depend strongly on M,. In particular,the solvatochromic effect observed in UV-vis-NIR increases with the increase of M, whereas the potentialof the onset of the oxidative doping decreases with the increaseof M, as evidenced by cyclic voltammetry and UV-vis-NIR spectoelectrochemistry. Keywords: Poly(3-alkylthiophenes), regioregularity, fractionation, spectroelectrochemistry 1. Introduction Regioregular poly(3-alkylthiophenes) (R-PATS) prepared by Grignard type polycondensation [l-2] canbe considered as much better defined materials as compared to non-regioregular PATS synthesized by chemical or electrochemical oxidation of 3- alkylthiophenes. However, R-PATS exhibit relatively low molecular weight and for this reason their spectroscopic and spectroelectrochemical properties may dependon M, and the polydispersity coefficient. The simplest method to distinguish between the spectroscopic features originating from the-presence of chainends and those characteristic of the polymer backbone is to fractionate the polymer. Thus, we haveundertaken a taskof fractionation of R-PATS and to study the effect of this fractionation of the spectroscopic properties of R-poly(3- hexylthiophene) (R-P3HT). 2. Experimental R-PHT wasprepared using a modification of the method of McCullough et al. [l]. Its fractionation can be briefly described as follows. R-PSHT obtained by polycondensation was precipitated with methanol, filtered andwashed with a mixtureof water and methanol.The crude polymer was then dried to constant mass. Four fractions of increasing M, wereobtained by consecutive extractions with acetone, hexane,dichloromethane and THF in a Soxhlet apparatus. The separated fractions abbreviated as R-P3HT/acetone, R-P3HTJhexane, R- P3HT/CH2C12 and R-P3HTiTHF were then subjected to gel permeation chromatography (GPC), IJV-vis-NIR spectroscopy, cyclic voltammetry and spectroelectrochemical studies * Corresponding author, e-mail : apron@cea.fr 3. Results and Discussion In Fig. 1 GP chromatograms of four obtainedfractionsare shown. For each consecutive fraction the value of -M, approximately doubles and the polydispersity coefficients ineach case are below 1.5. The macromolecular parameters of the fractionsstudiedare as follows : R-P3HT/acetone (M,= 2280, Pi=l.38, 6.5% of the total mass);R-P3HTlhexane (M,= 4380, Piz1.22,9.7% of the total mass) ; R-P3HTKH2Clz (M,= 8370, P,=1.33, 33.1% of the total mass) ; R-P3HTfTHF (M,= 17700, P,=1.45, 49.2%of the total mass). In Figs 2 and 3 solution (recorded in THF) and solid state spectra of the four fractionsstudied are shown respectively. In both sets of spectra an increasing bathochromic shift of A,,, can be observed with increasing M,. This efFectis however much more pronounced in the case of thesolidfilms. In other words the solvatochromic effect usuaIly observed for PolYO- alkylthiophenes) is amplified from 25 nm for the R- P3HT/acetone fraction to 113 nm for the R-P3PHTfTHF one. The behaviour of the acetone fraction closely resembles that of poly(alkylthiophenes)with a low value of the average conjugation length.In such cases either non solvatochromic $fect is observed or its value is very low [3-41. The hexanefraction gives in turn UV-vis spectrasimilar to those recorded for nonregioregular poly(3-alkylthiophene) prepared by an oxidative route whose polymerization degree is roughly 8 times higher 1.51. This means that despite large differences in the molecular weight, the average conjugation length is comparable in both cases. The highest molecularweight fractions (R-P3HT/CH2C12 and R- P3HTITHF) exhibit clear vibrational structure. The spacing between the maxima of the individual peaks corresponds to the C=C stretching transition observed in the Raman spectra of R- P3ATs 161. 0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: s0379-6779(98)01313-7