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J. Phys. Chem. B 2008, 112, 10922–10926
Polaron Dynamics of Heavily Doped Regioregular and Regiorandom Poly(3-alkylthiophenes) Revealed by Electron Spin Resonance Spectroscopy Katsuichi Kanemoto,*,† Kazumi Muramatsu,‡ Masaaki Baba,§ and Jun Yamauchi*,‡,§ Department of Physics, Graduate School of Science, Osaka City UniVersity, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan, Graduate School of Human and EnVironmental Studies, Kyoto UniVersity, Kyoto 606-8501, Japan, and Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Oiwake-cho, Kitashirakawa, Kyoto 606-8502, Japan ReceiVed: March 12, 2008
Electron spin resonance (ESR) features in heavily doped conjugated polymers are investigated through the comparison of temperature dependences of ESR spectra between head-to-tail coupled regioregular (RR) and regiorandom (RRa) poly(3-octylthiophenes) (P3OTs). RR-P3OT, used as a model of having crystalline grains in the solid film, is found to exhibit anisotropic ESR spectra, whereas RRa-P3OT gives almost isotropic ESR spectra similar to those of usual heavily doped conjugated polymers. This difference in the degree of spectral anisotropy primarily arises from a difference in their film morphology. Spectral simulations show the anisotropy observed in RR-P3OT to be caused by g-anisotropy. The presence of the g-anisotropy in RR-P3OT indicates that its polarons spend most of the time within a single crystalline grain that has some domains with a common direction of the g-tensor. The g-anisotropy turns out to decrease with increasing temperature. This result is explained by thermally activated hopping motions between crystalline grains. We emphasize that the decrease in the g-anisotropy with temperature should be associated with its activated type of temperature dependence of conductivity. In RRa-P3OT, its isotropic ESR spectra are suggested to be caused by the interchain motion as well as the intrachain one. I. Introduction High conductivity achieved in heavily doped conjugated polymers has attracted much attention due to their potential application to semiconductor devices. Indeed, a field effect transistor (FET) using conjugated polymers has been realized by the use of the conductive carriers that have similar electronic structures to the doped states.1 In the conducting event, polaron species with a 1/2 electron spin have been considered to be major carriers.1,2 The dynamics of the polaron has thus been often studied on the basis of electron spin resonance (ESR) techniques and much significant information on the polaron has been derived.3,4 Parameters obtained from ESR techniques are sorted into two categories. One is static parameters such as hyperfine coupling constants and spectral anisotropic constants that originate from a set of components with different Larmor frequencies. The other is dynamical parameters such as a spectral line width and relaxation times that are often determined by an average among several components when spin motions exist. In the case of conjugated polymers, the interpretation of such ESR parameters is usually complicated because several morphologies differing in the strength of interchain couplings can coexist in their solid film. In order to precisely describe polaron features in conjugated polymers, therefore, a model system to distinguish ESR properties of different morphologies is required. We here pay attention to two representative morphologies present in a usual polymer film. One is a crystalline morphology that consists of oriented chain molecules with strong interchain couplings. A prototype * To whom correspondence should be addressed. E-mail: kkane@ sci.osaka-cu.ac.jp. (K.K.);
[email protected] (J.Y.). † Osaka City University. ‡ Graduate School of Human and Environmental Studies, Kyoto University. § Graduate School of Science, Kyoto University.
with such a morphology is regioregular (RR) poly(3-alkylthiophenes) (P3ATs) with head-to-tail coupled substituents.5 The other type is an amorphous morphology consisting primarily of chains with weak interchain couplings. Most of the films of conjugatedpolymersaredominatedbythisamorphousmorphology. In this article, polaron states in heavily doped RR and regiorandom (RRa) types of P3ATs are investigated by ESR spectroscopy. RRa-P3AT, with 50% of head-to-tail coupling ratio of substituents, is known to have features of the amorphous type in the solid state5 and used here as its prototype. RR- and RRa-P3ATs have been compared by optical spectroscopy to characterize the interchain and intrachain electronic states in the neutral states of conjugated polymers.6–9 The aim of the present work is to elucidate ESR features of polaron states existing in a crystalline and an amorphous morphology. An similar attempt has recently been made through the comparison of the X- and Q-band ESR features in long oligothiophenes with crystalline and amorphous phases.10 The results in this work reveal that ESR features of doped conjugated polymers are different depending on the morphology, similar to the report on the oligothiophenes.10 It is shown that some differences are found in the g-anisotropy and the line width. We reveal that detailed analyses on the two parameters enable us to extract significant information on the dynamics of polarons working as carriers in the film. II. Experimental Section P3ATs used in this work are RR- and RRa- poly(3octylthiophenes) (P3OTs) that were purchased from Aldrich. The ESR samples were prepared by exposing pristine films cast from their chloroform solutions to excess iodine vapor in 0.5 h. The casting films were formed at the inside bottom of an ESR
10.1021/jp804065k CCC: $40.75 2008 American Chemical Society Published on Web 08/13/2008
Polaron Dynamics of RR- and RRa-P3OT
J. Phys. Chem. B, Vol. 112, No. 35, 2008 10923
Figure 1. Temperature dependences of χT for regioregular (RR) and regiorandom (RRa) poly(3-octylthiophenes) (P3OTs) calculated from the product of the intensity of double-integrated differential ESR spectra and measuring temperature T.
tube (4 or 5 mm diameter) for good heat contact with cooling He gas flowing outside the tube for measurements at various temperatures. The tubes were evacuated (
