A novel type of semiconducting polymers: Synthesis and properties of electrochemically-doped polymers containing pendant oligothiophenes

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Synthetic Metals, 55-57 (1993) 1176-1181

A NOVEL TYPE OF SEMICONDUCTING POLYMERS: SYNTHESIS AND PROPERTIES OF ELECTROCHEMICALLY-DOPED POLYMERS CONTAINING PENDANT OLIGOTHIOPHENES Kazunari NAWAt, Kenji MIYAWAKI, Ichiro IMAE, Naoki NOMA, and Yasuhiko SHIROTA* Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka, Suita, Osaka 565 (Japan)

ABSTRACT For the purpose of developing a novel type of electrically conducting polymers, synthesis and properties of electrochemically-doped, non-conjugated vinyl type polymers containing oligothiophenes as pendant groups have been studied. New vinyl monomers, 2-[4-(2-thienyl)phenyl]-5-vinylthiophene and 5-vinyl-2,2':5',2":5",2'"-quaterthiophene, have been synthesized, and electrolytic polymerization carried out by controlled potential anodic oxidation. The resulting polymers were identified as partially oxidized radical cation salts with CIO4- as a dopant, having partially cross-linked structures. The polymers undergo reversible color changes. Electrochemicallydoped poly(5-vinyl-2,2':5',2":5",2"'-quaterthiophene)exhibits room-temperature conductivities of ca. 10-7 S cm-1 with activation energies of ca. 0.5 eV. KEYWORDS: 5-vinyl-2,2':5',2":5",2"'-quaterthiophene, 2-[4-(2-thienyl)phenyl]-5-vinylthiophene, electrolytic polymerization, electrochemically-doped polymer, electrical conductivity.

INTRODUCTION Wholly n-conjugated linear and planar polymers have received attention as electrically conducting polymers [ 1,2]. We have studied the synthesis, properties and applications of electrochemicallydoped, non-conjugated polymers containing pendant n-electron systems [3]. Non-conjugated pendant polymers are of interest for the following reasons; the variety of possible pendant molecules, chemical stability, ease of processability, photoconducting properties, and invariance of the standard oxidation/reduction potential irrespective of the doping degree. The polymers studied include poly(N-vinylcarbazole)[4-6], poly(vinylferrocene) [7], poly(3-vinylperylene)[8-10], etc. t On leave from Sumitomo Metal Industries, Ltd., Amagasaki, Hyogo 660, Japan 0379-6779/93/$6.00

© 1993- Elsevier Sequoia. All rights reserved

1177

In order to develop further new types of electrically conducting polymers, we have been studying non-conjugated vinyl polymers containing ~t-conjugated linear oligomers as pendant groups. We have chosen oligothiophenes as pendant n-conjugated linear oligomers, since oligothiophenes have recently been studied in relation to the properties of polythiophene. These new polymers are expected to have unique properties characteristic of both the wholly rt-conjugated linear oligomers and the non-conjugated backbone with regard to electrical conduction, electrochromic behavior, photoelectrical properties, etc. It is of interest to examine the correlation between the length and structure of the pendant rt-conjugated linear oligomers and the properties of the resulting electrochemically-doped polymers. We have reported the synthesis and properties of an electrochemically-doped polymer containing pendant terthiophene, poly(5-vinyl-2,2':5',2"terthiophene) (PV3T) [11,12]. We report here the synthesis of electrochemically-doped poly[2-[4-(2thienyl)phenyl]-5-vinylthiophene] (PVTPT) and poly(5-vinyl-2,2':5',2":5",2"'-quaterthiophene) (PV4T) by electrolytic polymerization of the new monomers, 2-[4-(2-thienyl)phenyl]-5vinylthiophene (VTPT) and 5-vinyl-2,2':5',2":5",2"'-quaterthiophene (V4T), respectively. The properties of resulting polymers are compared with those of PV3T. CH=CH~

VTPT

CH=CH 2

V4T

EXPERIMENTAL Materials 1,4-Dithienylbenzene and 2,2':5',2":5",2"'-quaterthiophene were prepared by Grignard coupling reaction of 1,4-dibromobenzene and 2-bromothiophene, and of 5,5'-dibromo-2,2'-bithiophene and 2-bromothiophene, respectively [13,14]. The new monomers, VTPT and V4T, were synthesized from 1,4-dithienylbenzene and 2,2':5',2":5",2"'-quaterthiophene, respectively, by the Vilsmeier reaction, followed by the Wittig reaction. They were purified by silica gel column chromatography (eluent: n-hexane/benzene), followed by recrystallization from ethanol. V4T: m.p. 189-190 °C. MS: m/z = 356 (M+). IR (KBr): v [cm -1] = 1620 (vc=c vinyl), 980 and 890 (SC-H vinyl), 795 (6C-H 2,5substituted thiophene), 695 (6C-H 2-substituted thiophene), tH NMR (CDCI3): 6 [ppm] = 7.26-6.87 (m, 9H, aromatic), 6.76 (dd, 1H, vinyl), 5.54 (d, 1H, vinyl), 5.16 (d, IH, vinyl). UV (benzene): kmax [nm] = 407. Anal. Calc. for C18H12S4: C,60.67; H, 3.37; S, 35.96. Found: C, 60.80; H, 3.58, S, 35.37%. VTPT: MS: m/z = 268 (M+). IR (KBr): v [cm -t] = 1610 (vc=c vinyl), 980 and 890 (SC-H vinyl), 820 (Sc-n 1,4-substituted benzene), 805 (6C-H 2,5-substituted thiophene), 690

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(6C-H 2-substituted thiophene). UV (THF): kmax [nm] = 355. Anal. Calc. for C16H12S2: C,71.60; H, 4.51; S, 23.89. Found: C, 71.39; H, 4.66, S, 23.52%. Electrochemical Oxidation Cyclic voltammetry was carried out in a dichloromethane solution of VTPT or V4T ( 1.0 x 10-3 mol dm -3) containing tetra-n-butylammonium perchlorate (1.0 x 10-1 mol dm -3) as a supporting electrolyte with Ag/Ag + (0.01 mol dm -3) reference electrode, respectively. Electrolytic polymerization of VTPT and of V4T was carried out potentiostatically in a twocompartment cell with three electrodes at 0.95 and 0.65 V vs. Ag/Ag + (0.01 mol dm -3) reference electrode, respectively, for dichloromethane solutions of the monomers (I.0 x 10-3 mol dm -3) containing tetra-n-butylammonium perchlorate (1.0 x 10-1 mol dm -3) as a supporting electrolyte. Platinum plates were used as the working and counter electrodes. Conductivity Measurement Electrical conductivity was measured by a two-probe d.c. method for several samples of films which were peeled off from the working electrode, on to which gold was vacuum-deposited to make electrical contact. The activation energy for electrical conduction was determined from Arrhenius plots of electrical conductivities measured in a temperature range from 20 to 80 *C. RESULTS AND DISCUSSION Figure 1 shows cyclic voltammograms for the anodic oxidation of 2-[4-(2-thienyl)-phenyl]-5vinylthiophene (VTPT) and 5-vinyl-2,2':5',2":5",2'"-quaterthiophene (V4T) in dichloromethane solutions. The anodic oxidation processes of the two monomers are irreversible. In the case of VTPT (Fig. l(a)), an anodic wave due to the oxidation of the vinyl monomer was observed at ca. Epa/2 = 0.87 V (Epa = ca. 0.95 V) vs. Ag/Ag + (0.01 mol dm -3) reference electrode in the first sweep, but the corresponding cathodic wave was negligibly small. When the sweep was repeated, the anodic wave shifted slightly to a more positive potential, and cathodic waves corresponding to the reduction of the pendant dithienylbenzene radical cation of the resulting polymer began to appear at ca. 0.90 and 0.74 V, and in addition, another new anodic wave began to be observed at a potential more negative than 0.7 V. The current intensity of these waves gradually increased with repetition of sweep. The new anodic wave at a potential more negative than 0.70 V may be ascribed to the oxidation of the dimerized chromophore of the pendant dithienylbenzene generated by the coupling reaction of the pendant dithienylbenzene radical cation. The monomer V4T also shows similar anodic oxidation behavior as observed for VTPT and for 5-vinyl-2,2':5',2"-terthiophene (V3T). That is, the oxidation wave of the vinyl monomer was observed at Epa/2 = 0.53 V (Eoa = 0.60 V) vs. Ag/Ag+ (0.01 mol dm -3) reference electrode in the first sweep. When the sweep was repeated, the anodic wave shifted slightly to a more positive potential, and the cathodic waves due to the reduction of pendant quaterthiophene radical cation began to be observed at ca. 0.6 and 0.45 V. The two cathodic waves may be due to different rates of electron transfer of the radical cation species. In addition, a new anodic wave assignable to the oxidation of the octithiophene chromophore, generated by the coupling reaction of the pendant

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quaterthiophene radical cation, and the corresponding cathodic wave were observed at a potential more negative than 0.5 V. It is indicated that the oxidation process of V4T is accompanied not only by polymerization to yield electroactive PV4T but also the coupling reaction of the pendant quaterthiophene radical cation to give an octithiophene moiety. The results are compared with those of V3T. The oxidation potential of VTPT is much higher than that of V3T (Epa/2 = 0.67 V vs. Ag/Ag + (0.01 mol dm-3)), while V4T has a lower oxidation potential than V3T. The results indicate that V4T is more conjugated than V3T.

~[1}

/

(b)

I lO1.~

6

015

1'.0

¢.5

EAt vs. Ag/Ag+(0.01 mol dm 3)

0

0.5

E./V vs. Ag/Ag+(0.01 mol dm 3)

Fig. 1. Cyclic voltammograms of (a) VTPT and (b) V4T (1.0 x 10-3 mol dm -3) in dichloromethane containing n-Bu4NCIO4 (0.1 mol din-3). 1: First sweep. Sweep rate: 10 mV s-1. Considering the results of cyclic voltammetry for VTI:W and V4T, electrolytic polymerization of the two monomers was carried out by controlled-potential anodic oxidation at 0.95 or 0.65 V vs. Ag/Ag+ (0.01 mol dm -3) reference electrode, respectively, in dichloromethane solutions. Dark reddish purple-colored, lustrous films were deposited on to the surface of the working electrode in the case of VTPT, while dark greenish-colored films were deposited in the case of V4T. The electrochemically-doped polymers were characterized by spectroscopic methods and elemental analysis. The characteristic infrared absorption bands at 980 and 890 cm-1 observed for the two monomers, VTPT and V4T,which are due to the C-H out-of-plane deformation vibrations of the vinyl group, are absent in the spectra of the resulting polymers. This indicates that the polymerization proceeded at the vinyl group. The electrochemically-doped polymers show strong infrared absorption bands due to CIO4- in a range from 1090 to 1140 cm -1 . Thus, the electrochemicallydoped polymers contain CIO4- as a dopant. Figure 2 (a) shows the electronic absorption spectra of t,'ansparent thin films of electrochemicallydoped poly[2-[4-(2-thienyl)phenyl]-5-vinylthiophene] (PVTPT), which were deposited on to the ITO glass by the electrolytic polymerization of VTPT for two minutes, and its dedoped polymer. The thin film of the electrochemically-doped polymer shows a new absorption band with )~max at 570 nm and a band in the longer wavelength, along with the absorption band with )'.max at 330 nm due to the

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pendant dithienylbenzene chromophore. In the electronic absorption spectrum of the dedoped polymer, obtained by electrochemical reduction of the doped polymer, the characteristic absorption band with 3+maxat 570 nm, observed for the electrochemically-doped polymer, disappeared, and the intensity of the absorption at ca. 400 nm increased slightly. Thus, the absorption band with kmax at 570 nm observed for electrochemically-doped PVTPT is ascribed to the pendant dithienylbenzene radical cation. The electronic absorption spectrum of electrochemically-doped PV4T shows the absorption band with kmax at 686 nm, which is assignable to the pendant quaterthiophene radical cation (Fig. 2 (b)). The absorption band due to octithiophene, generated by the coupling reaction of the pendant quaterthiophene radical cation, was observed for the dedoped polymer, which was obtained by electrochemical reduction of doped PV4T, in the wavelength region from 500 to 600 nm. The absorption band of the dithienylbenzene radical cation is at a shorter wavelength relative to that of the terthiophene radical cation (~,maxat 585 nm) [8,9]. In contrast, the absorption band of the quaterthiophene radical cation is at a longer wavelength than that of the terthiophene radical cation.

a)

/ I

!

~"++ X

x

%

F,

+ .8 < 300

400

500

600

700

800

900

Wavelength (nm)

300

4;0

500

800

7;0

800

900

Wavelength (nm)

Fig. 2. Electronic absorption spectra of (a) electrochemically-doped PVTPT and dedoped PVTPT, and (b) electrochemically-doped PV4T and dedoped PV4T in thin films. 1: doped polymer, 2: dedoped polymer. Based on the results obtained from the cyclic voltammograms, and infrared and electronic absorption spectra, the electrochemically-doped PVTPT and PV4T obtained by electrolytic polymerization of VTPT and V4T, respectively, are identified as partially oxidized radical-cation salts with CIO4" as a dopant, and have partially cross-linked structures due to the coupling reaction of the pendant radical cations. The polymer films obtained by electrolytic polymerization of both VTPT and V4T exhibit electrochromism, undergoing reversible, clear color changes from purple to orange, and from green to yellow, respectively, on electrochemical dedoping. Table 1 lists room-temperature conductivities and activation energies for electrical conduction for several samples of electrochemically-dopedPVTPT and PV4T obtained by electrolytic polymerization of the corresponding monomers. The results for poly(5-vinylterthiophene) (PV3T) are also given for reference. The electrochemically-dopedPVTPT shows much lower conductivities

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than the electrochemically-doped PV3T. Electrochemically-doped PV4T exhibits a room-temperature conductivity up to ca. 10.7 S cm-I with an activation energy of ca. 0.5 eV. The results show that electrical conductivity tends to increase with increasing length of pendant oligothiophenes. It is expected that electrical conductivity will be increased by the incorporation of longer oligothiophenes as pendant groups. Such work is in progress. TABLE I Room-temperature conductivities and activation energies of electrochemically-doped polymers Degree of doping (%)a)

Conductivity (S cm-I)bf

Activation energy (eV)

PVTPT

39 50

3xl 0! 1 6x10 -10

0.40 0.34

PV4T

68 ....

2x 10-8 2x10 -7

0.48 0.52

PV3T[II,12]

36 45

7x10 -9 2x 10-8

0.49 0.50

a) Determined from chlorine content b) Measured by a two-probe d.c. method

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