Structure and electrical transport property of poly(3-octylthiophene)

ELSEVIER

Synthetic Metals 10 1 (1999) 594-595

Structure and electrical transport property of poly(3-octylthiophene) S. Masubuchi”, R. Imai”, K. Yamazaki”, S. Kazama”, J. Takadah, T. Matsuyamah gDepartment of Physics, Chuo University, Kasuga, Bunkyo-ku, Tokyo, 112-8551 Japa3z bReserch Reactor Institute, Kyoto University, Kumatori, Osaka, 590-0494 Japarl

Abstract The relationship between electrical transport property and microscopic molecular motion in poly(3-octylthiojhene) (POT) was investigated. We observed the temperature gradient of thermoelectric power S(n changes at arou!< 2205 and 250K in POTS doped with ClO; or FeCl;. NMR results show that the observed anomaly in S(T) is closely related to the molecular motion of side chain, which might affect the scattering process of conduction electrons. Keywords: Polythiophene and derivatives, Transport tniasurements, Thermopower, Nuclear 3nagnetic resonance spectroscopy, Neutron activation a3lalysis -= 1. Introduction Most conducting polymers such as polyacetylene (PA), poly(p-phenylene) (PPP). polythiophene (PT), polypyrrole (PPy), etc., are insoluble to any solvent and infusible by heating. Poly(3-alkylthiophene) (P3AT), in contrast, are peculiar in solubility and fusibility when its alkyl-side chains (-C,H211+1) are sufficiently long, i.e., n>4 [1,2]. Poly(3-octylthiophene) (POT) with octyl (n=8) chain has been extensively studied through optical, magnetic and transport experiments [3]. The purpose of this study is to elucidate the relationship between the electrical transport property and microscopic molecular motion in PSAT. We deal with POT doped with CIO; or FeCI;. A series of experiments such as the ordinary fourterminal resistivity, thermoelectric power and nuclear magnetic resonance were made as a function of temperature. To determine the dopant concentration and/or chemical composition, the neutron activation analysis was also carried out for “8C1and ssFe. 2. Experimental Poly(3-octylthiophene) was polymerized by chemically oxidizing 3-octylthiophene (OT) with anhydrous Ferric(I11) chloride in an anhydrous chloroform. POT film was prepared by casting a POT/chloroform solution onto a glass substrate. Fe(ClO&-Hz0 and FeCl,-6H,O were used as doping agent. POT films were immersed into a solution dissolving the corresponding agent in nitromethane. Concentrations of the dopants were determined by weight-uptake and the neutral activation analysis (NAA). The dc resistivity ~(7’) and the absolute thermoelectric power S(T) were measured as a function of temperature between 1.5 and 300K for p(T), and between 10 and 300K for S(7J, respectively. The details of the measuring apparatus and conditions were reported previously [4]. The spin-lattice relaxation time T, of protons was measured with the conventional

inversion recovery or the saturation recovery pulse sequence at the NMR frequency of 5OMHz between 77 and 300K using DSX100 spectrometer (Bruker). 3. Results and Discussion The dopant concentrations of POTS doped with Fe(CICIJ),H,O were determined in two different methods; weight-uptake and NAA. The respective determination gave different molar ratios of [Cl]/[OT] as 0.32 and 0.29, presumably owing to the contamination of dopant with HZ0 as a ligand. In FeC1,-6i-ILFm doping, several solutions with different concentration of the agent were used to determine the most suitable condition for the doping. We noticed that POT film dissolves into the doping solution. Thus we determined the concentration of Fe and Cl only by NAA. A remarkable result is that the atomic ratio ofC1 to Fe is 4.0i0.2 even when the concentration of Fe changes from 0.08 to 0.247 This implies that the dopant species in POT doped with FeCl,: 6Hz0 is FeCI;, consistent with the results of the Moessbaur measurement [5]. We will report the details of NAA results elsewhere [6]. In this report we show the results on a typical sample of POT doped with Feel,-6H,O. The concentration of dopant and conductivity at room temperature Bre summarized in Table 1. The composition of POT-A indicates that the sample was fully doped. Table 1 Compositions and conductivity at room temeprature for POT doped with Fe(ClO,),-H,O or F&l,-6H,O. sample name POT-A

composition [tC1~H1sS)(ClO~o.~,(H2o)oIljlx

0 (S/cm) 12

POT-B

~(C~ZHI~~~(F~~~)O.I~I~

19

POT-C

[(CIZHI~S)(F~CL)O.ORJ~,

8.0

0379-6779/99/$ - seefront matter 0 1999 Elsevier ScienceS.A. All rights reserved. PII: SO379-6779(98)0 1156-4

S. Masubuchi

et al. I Synthetic

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101 (1999) 594-595

I

0.30

I ‘H-NMR

25 0.25-

\ 0.20-

0.10’

50

Fig. 1 Temperature dependence of s(7’) for doped POT. The temperature dependence of resistivity (TDR) for all the samples behave in a semiconducting manner throughout the whole temperatures, i.e., dpldT 0 as T-> 0. It is noteworthy that anomalies in S(T) at T, = 220 and 250K appears only in POT, but not in PT [4,9]. For intermediately doped POT (POT-B), S(7’) are larger than that in heavily doped one throughout the whole temperatures. The same anomaly as observed in POT-A also appears nearly at the same temperatures, but the variation of the anomaly at T, is weaker. For lightly doped POT (POT-C), we can not recognize any indication of such kind of anomaly. From these observations two general points become clear: (i) The anomaly in S(Q appears only in the heavily and intermediately doped POT at temperatures between 220 - 250K regardless of the dopant species. This implies that the phenomenon is originated from a motion of alkyl-side chain in POT. (ii) The phenomenon becomes clearer with the increase of dopant concentration,

IT *

I 100

undoped POT

I

I

150

200

Fig. 2 Temperature dependence undoped and CIO; doped POT.

I 250

of ‘H-NMR

300

T, for

indicating that it comes from the intrinsically metallic regions. Figure 2 depicts the temperature dependence of proton relaxation time T, for POT-A (i.e., CIO; doped). The results for undoped POT is also plotted. For undoped POT the minimum in T,, Tlmin, is observed at 150 and 220K. It implies that the molecular motion of side chains, i.e., octyl, begins at these two temperatures. We consider that 7’,“‘” at 150K corresponds to a begining temperature of the molecular motion of chain’s end groups such as methyl, ethyl or propyl group, while T,“‘” at 220K is for the entire motion of octyl group. For POT-A, Timinwere found out to be vague, presumably owing to a scatter of the characteristic time rc for molecular motion. The motion is strongly affected by the existence of dopant ions. We consider the anomalies in S(r) reflects the restricted molecular motion. Acknowledgements We are grateful for fruitful discussion with Prof. K. Mizoguchi of Tokyo Metropolitan University. References [l] D. L. Elsenbaumer et al., J. Chem. Sot., Chem. Commun. 1986, 1346. [2] M. Sato et al., Synth. Met. 18 (1987) 229. [3] Handbook of Conducting Polymers, edited by T. Skotheim (Dekker, New York, 1986) [4] S. Masubuchi and S. Kazama, Synth. Met. 74 (1995) 151. [5] S. Kitao et al.,Synth. Met. 69 (1995) 371. [6] S. Masubuchi and S. Kazama, in preparation. [7] Z.H. Wang et al., Phys. Rev. B45 (1992) 4190. [8] A. B. Kaiser, Phys. Rev. B40 (1989) 2806. [9] S. Masubuchi et al., Synth. Met. 55-57 (1993) 4962.