electrosynthesis and spectroelectrochemical characterization of poly(3,4-dimethoxy-thiophene),...

Electrosynthesis and spectroelectrochemical characterization ofpoly(3,4-dimethoxy-thiophene), poly(3,4-dipropyloxythiophene) and

poly(3,4-dioctyloxythiophene) films

Artur Szkurlat, Barbara Palys 1, Jozef Mieczkowski, Magdalena Skompska1 *

Department of Chemistry, Warsaw University, ul. Pasteura 1, 02 093 Warsaw, Poland

Received 4 June 2003; received in revised form 4 June 2003

Electrochimica Acta 48 (2003) 3665�/3676

www.elsevier.com/locate/electacta

Abstract

Poly(3,4-dialkoxythiophene) films with different length of alkyl chain (1,3 and 8 carbon atoms) were obtained on Pt and ITO

electrodes from the monomer solutions in acetonitrile by cyclic voltammetry (CV). The properties of the resulting films were studied

by electrochemical methods, UV�/Vis, FTIR and NMR spectra. The CVs were correlated with differential cyclic voltabsorptograms

(DCVA) recorded at the absorption maxima to explain the shape of the voltammograms of the polymers studied, dependent on the

alkyl-chain length in alkoxy group. The presence of the zones of different crystallinity in the polymer film was postulated. Significant

influence of the type of the solvent on asymmetry of the cyclic voltammograms for the polymer doping�/undoping has been

discussed in terms of the solvent interaction with radical cation (polaron) delocalized on the alkoxy side groups. The polaron

delocalization was proved by 1H-NMR spectra. Appearance of infrared activated vibrations (IRAVs) in the range 1500�/600 cm�1

and a characteristic electronic band at 3300 cm�1 at the polarization potential �/0.25 V versus Ag/Ag� and their gradual changes

upon further polymer oxidation were interpreted in terms of evolution of different charge carriers in lightly and heavily doped

polymer.

# 2003 Elsevier Ltd. All rights reserved.

Keywords: Poly(3,4-dialkoxythiophene); Electropolymerization; Redox behavior; NMR; UV�/Vis; IR spectra; Infrared activated vibration bands

1. Introduction

Polythiophene and its derivatives belong to attractive

group of polymers due to their high conductivity in the

oxidized form and electrochemical stability in doped

and undoped states. Substitution of the polythiophene

ring at b position with flexible alkyl, or alkyl containing

one or more ether groups leads to the conducting

polymers of unusual properties such as processability

or stereo- and ionoselectivity [1�/3]. Introduction of

electron-donating alkoxy substituent into the thiophene

ring results also in diminution of polymerization poten-

tial and significant increase of the polymer electroactiv-

ity in aqueous solution, in comparison with that of

poly(3-alkylthiophene) analogues [4,5]. The electroche-

mical properties of substituted thiophene depend on the

number of oxygen atoms in the side chain and their

position. If the oxygen is directly attached to the ring,

the conducting p-doping state of the polymer is sub-

stantially stabilized by stabilization of the positive

charge in the polymer backbone [4,6�/8].

Incorporation of the second alkoxy substituent into

the thiophene ring or cyclization between the 3 and 4

positions of the thiophene ring is a convenient way for

preparing the perfectly stereoregular, long conjugated

polymers by elimination of 2,4? couplings, [9�/11].

Poly(3,4-ethylenedioxythiophene) (PEDOT) is an excel-

lent example of highly conducting polymer (of conduc-

tivity ca. 500 S cm�1), very stable and optical

transparent in the oxidized state [12]. These properties

make the polymer attractive as an electrode material in

rechargeable polymer batteries, capacitors, electrochro-

mic devices and suitable as antistatic coatings [13,14].

* Corresponding author. Tel.: �/48-22-822-0211; fax: �/48-22-822-

5996.

E-mail address: [email protected] (M. Skompska1).1 ISE member.

0013-4686/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0013-4686(03)00504-8

mailto:[email protected]

However, a low oxidation potential of PEDOT is

responsible for instability of the polymer in the neutral

state at ambient conditions. Another disadvantage of

PEDOT is its insolubility in organic solvents. Someimprovement of the polymer properties may be achieved

by the change of substituents in the monomer molecule.

The conductive and magnetic properties of PEDOT and

poly(3,4-dimethoxythiophene) have been recently com-

pared by Zotti et al. [15]. The authors found rather small

influence of the dimethoxy- or ethylenedioxy-substituent

on the electrochemical properties of these two polymers.

Nevertheless, one may expect that the increase of thealkyl chain length in the alkoxy group will lead to the

polymer soluble in aprotic organic solvents. On the

other hand, this may also result in some difficulties in

electrosynthesis and shortening of conjugation length of

the resultant polymers due to the steric hindrances

[10,11,16]. However, the results discussed in the litera-

ture concern the properties of the polymers obtained in

different ways (by electrochemical or chemical synthesis)which may strongly influence on the polymer character-

istics. Therefore, it was reasonable to make a complex

comparative studies on electropolymerization and spec-

troelectrochemical properties of a series of poly(3,4-

dialkoxythiophenes) with different alkyl-chain length

obtained from the monomers synthesized by the same

procedure and electropolymerized in the same condi-

tions. In this work we compare the results of threepolymers of this group, poly(3,4-dimethoxythiophene)

(PDMT), poly(3,4-dipropyloxy-thiophene) (PDPT) and

poly(3,4-dioctyloxythiophene) (PDOT), electrodepos-

ited by cyclic voltammetry from acetonitrile solutions

of the monomers. We also discuss some advantages of

these polymers with respect to PEDOT and monosub-

stituted poly(3-alkylthiophenes). The electrochemical

and spectroscopic properties of poly(3,4-dialkoxythio-phenes) were investigated by cyclic voltammetry, in-situ

UV�/Vis absorption spectra, differential cyclic voltab-

sorptommetry (DCVA), NMR and ex-situ FTIR reflec-

tance spectroscopies.

2. Experimental

2.1. Synthesis of 3,4-dialkoxythiophenes

Synthesis of 3,4-dialkoxythiophenes has been de-

scribed in the literature [11,16�/18]. In our procedure,

presented in Scheme 1, the starting compound was

disodium 2,3-di(ethoxy-carbonyl)-3,4-thiophenediolate

(1) obtained according to Hinsberg [19]. A direct

alkylation of disodium salt leads to high synthesis yield

of about 30%.Characterization of the product: m.p.: 29.5�/30.5 8C.

Elemental Anal. C20H36O2S: Calc.: C, 70.58; H, 10.58.

Found: C, 70.39; H, 10.78%. IR (KBr, cm�1): 2925,

2850, 1580, 1510, 1470, 1380, 1210, 1155, 750; 1H-NMR

(CDCl3, d ): 6.15 (s, 2H), 3.96 (t, 4H), 1.81 (m, 4H),

1.51}/1.20 (m, 10H), 0.88 (t, 6H). 13C-NMR (CDCl3,

d ): 147.6, 96.8, 70.6, 31.8, 29.4, 29.3, 29.0, 26.9, 26.0,22.7, 14.1.

The same procedure was used for synthesis of two

other monomers of this group, 3,4-dimethoxythiophene

and 3,4-dipropyloxythiophene.

2.2. Spectroscopic and electrochemical measurements

Infrared (IR) spectra of byproducts and final product

of the synthesis were obtained using a Nicolet Magna IR500 spectrophotometer. NMR spectra were recorded on

a Varian Unity Plus spectrometer operating at 200 MHz

for 1H-NMR and at 125 MHz for 13C-NMR. Tetra-

methylsilane was used as an internal standard. Chemical

shifts are reported in ppm. TLC analyses were per-

formed on a Merck 60 silica gel glass plated and

visualised using iodine vapour. Column chromatogra-

phy was carried out at atmospheric pressure using silicagel (100�/200 mesh, Merck). Elemental analyses were

performed in a Microanalytical Laboratory of the

Institute of Organic Chemistry, Polish Academy of

Science, Warsaw. The determined elemental composi-

tion of the compounds synthesised confirmed the

expected molecular structure.

All electrochemical experiments were done in a

conventional, single compartment cell with a platinumgauze counter electrode and Ag/(0.1M AgNO3 in

CH3CN) double junction reference electrode, using an

AUTOLAB potentiostat (Ecochemie, The Netherlands).

Electrodeposition of poly(3,4-dimethoxythiophene)

(PDMT), poly(3,4-dipropyloxy-thiophene) (PDPT) and

poly(3,4-dioctyloxythiophene) (PDOT) was carried out

by cyclic voltammetry on a Pt disc with the surface area

of 0.02 cm2 or ITO (indium-tin oxide) electrodes fromdeaerated (with Ar) 0.05 M monomer solutions in

acetonitrile (AN) containing 0.1 M LiClO4 as the

supporting electrolyte. After deposition, the polymer

films were rinsed with acetonitrile and then studied by

cyclic voltammetry, chronopotentiometry or chronoam-

perometry in deaerated AN, propylene carbonate (PC)

or aqueous monomer-free solutions of 0.1 M LiClO4.

The absorption spectra of the polymer films weretaken using a double-beam UV/Vis spectrometer

(Lambda 12, Perkin�/Elmer). The electrochemical cell

containing the ITO/polymer working electrode, Pt gauze

counter electrode and a miniature Ag/Ag� reference

electrode in the supporting electrolyte was mounted in

the spectrophotometer sample compartment. The refer-

ence cell contained an identical uncoated ITO electrode

in the same electrolyte. Infrared reflectance spectra ofthe polymer layers were recorded with a FTIR 8400

Shimadzu spectrometer equipped with a specular reflec-

tance accessory (Spectra Tech) at 808 angle of incidence.

A. Szkurlat et al. / Electrochimica Acta 48 (2003) 3665�/36763666

3. Results and discussion

3.1. Electrodeposition of PDMT, PDPT and PDOT

Fig. 1a�/c show the cyclic voltammograms for electro-

deposition of PDMT, PDPT and PDOT on Pt electro-

des from 0.05 M monomer solutions in acetonitrilecontaining 0.1 LiClO4 as the supporting electrolyte. The

oxidation potentials of the monomers, determined from

oxidation peaks, decrease slightly with the length of the

alkyl chain, from 1.15 V for DMT to 1.12 V for DPT

and 1.09 V for DOT. In order to avoid degradation of

the polymers during deposition, the films were obtained

by cycling in the limited potential range, to 1.05 V for

PDMT, 1.03 V for PDPT and 1.0 V for PDOT at the

scan rate of 40 mV s�1.

As visible in Fig. 1, a character of the redox peaks,

developed gradually in the subsequent polymerisation

Scheme 1. Synthesis route of 3,4-dioctyloxythiophene.

Fig. 1. Cyclic voltammograms for electrodeposition of PDMT (a), PDPT (b) and PDOT (c) films on Pt electrodes from acetonitrile containing 0.05

M monomer�/0.1 M LiClO4, at the scan rate 40 mV s�1.

A. Szkurlat et al. / Electrochimica Acta 48 (2003) 3665�/3676 3667

scans, depends markedly on the side chain length of the

monomer. The longer chain, the sharper and more

symmetrical are the redox peaks. To compare the

behavior of the three polymers upon doping�/undoping

in the monomer-free solution, one should provide the

films of similar thickness. However, a gradual colour

change of the solutions in the vicinity of the working

electrode during polymerization suggest that any side

reaction, for example formation of soluble oligomers,

occurs. Therefore, the charge passed during polymeriza-

tion could not be used to control the film thickness.

Another reason of the charge loses may be a slow film

growth in comparison with the coupling process of

radical cations. In effect, the polymer would be formed

not only on the electrode but also in the solution. The

presence of oligomers in the solutions of 3,4-DMT and

3,4-DPT after three polymerisation cycles was con-

firmed by UV�/Vis spectra, presented in Fig. 2 (spectra

1 and 2, respectively). The absorption peaks are located

at 460 nm i.e. at the wavelength markedly lower than

that of the peak for PDOT dissolved in chloroform (536

nm, spectrum 4), as expected for short chain oligomers.

The two other polymers are not soluble in CHCl3 due to

insufficient length of alkyl chain. No oligomers were

detected in the solution after three polymerisation cycles

in 3,4-DOT (spectrum 3).

For comparison, according to the literature, the

absorption peak for poly(3,4-dibutyloxythiophene)

polymerized chemically (by means of FeCl3) and dis-

solved in chloroform is located at 480 nm [16]. Since the

absorption peaks of the monomers are located at the

same wavelength, 262 nm (see Table 1), one may

conclude that the electrochemical polymerisation of

poly(3,4-dialkoxythiophenes) leads to the polymers

with markedly longer conjugation than these obtained

by the chemical synthesis (according to the procedure

described by Daoust and Leclerc [16]).

Oligomerization in the solution is responsible for

relatively low polymerisation efficiency, h , determinedfrom EQCM experiments. The value of h for PDMT,

derived from the graph of Df versus Qdep, where Df is

the change of the resonant frequency of the quartz

resonator upon the passage of the deposition charge

Qdep, is 16%, being lower than that reported by Zotti et

al. (23%) [15].

Another factor, useful in estimation of the charge

loses during polymerisation is a charge yield, i.e. a ratioof the redox and deposition charges of the polymer, y�/

Qred/Qdep. These two charges are defined as Qred�/

zredFN and Qdep�/(2�/zred)FN(h)�1, where F is the

Faraday constant, zred is a number of electrons involved

in redox reaction of one monomer unit, N is a number

of monomer units deposited on the electrode and h is

polymerisation efficiency reflecting various loses of

electricity during deposition. According to the literature,zred for polythiophene derivatives usually ranges from

0.25 to 0.3 [20�/22]. This leads to the theoretical values

of y ranging from 11 to 13%, provided that there are no

charge loses during polymerization. The y values

obtained for the three polymer studied, listed in Table

1, are much lower than the theoretical ones. The

polymerisation efficiencies derived from these values

are 18�/21% for PDMT, 14�/16% for PDPT and 54�/63%for PDOT.

Because of the charge loses during polymerisation, the

amount of the film deposited on the electrode was

controlled not by the deposition charge but the value of

the surface coverage, G (mol cm�2). The values of G

were determined coulometrically, using the expression

G�/Qred/zredFS, where Qred is the charge passed during

reduction of the polymer in acetonitrile monomer-freesolution, S is the surface area of the working electrode

(0.02 cm2) and zred�/0.3.

3.2. Electrochemical and spectroscopic characterization

of PDMT, PDPT and PDOT films in acetonitrile solution

of LiClO4

The redox behaviors of the PDMT, PDPT and PDOT

films of G:/26�/10�8 mol cm�2 in acetonitrile solu-tion of 0.1 M LiClO4 are compared in Fig. 3. The cyclic

voltammogram of PDMT, with a broad redox peaks, is

similar to that presented by Zotti et al. [15] and

resembles the behaviour of PEDOT.

The increase of the number of carbon atoms in the

side groups gives rise to formation of a narrow oxida-

tion peak, preceded by a broad shoulder and followed

by a capacitive current. At the same time, electroactivityof the polymer shifts towards the more positive poten-

tials. Namely, the redox potentials of the polymers,

defined as Eredox�

/�/(Epa�/Epc)/2, increase from �/0.05 V

Fig. 2. Comparison of the spectra of the electrodeposition baths after

polymerisation of 3,4-DMT (1), 3,4-DPT (2) and 3,4-DOT (3) with the

spectrum of PDOT solution obtained by dissolution of the polymer

film in CHCl3 (4).


for PDMT, to 0.26 V for PDPT and to 0.39 V for

PDOT. For comparison, the redox potential for

PEDOT is �/0.54 V. On the base of the monomer

spectra, the electronic effect of the substituents may be

excluded, as in the case of ethylenedioxy- and di-

methoxy-derivatives of polypyrrole and polythiophene

[15]. Therefore, the most probable reason of the anodic

shift of the redox potential is some distortion from the

planarity of the conjugated system due to the increasing

length of the side groups or/and decrease of the

polymerisation degree. Distortion from planarity leads

to diminution of donating effect of alkoxy group due to

less efficient overlapping between the p-orbital of

oxygen atom and the conjugated system [23]. According

to the literature, no disturbance from the planarity of

the main chain was found for 3,3?-bipentoxy-2,2?-bithienyl [24]. However, in the presence of two alkoxy

groups the steric hindrances may cause some distortion

in the backbone and this effect may be enhanced with

the increase of the alkyl chain length. Unfortunately, we

are not able to verify the second possibility, i.e. lowering

the polymerisation degree with the increase of the alkyl

chain length for the three polymers studied, due to

insolubility of PDMT and PDPT in organic solvents.

The next question to answer is the origin of the

shoulder and narrow redox peak in the cyclic voltam-

mograms of PDPT and PDOT, in contrast to broad

voltammograms for PDMT and PEDOT. A very useful

tool in explanation of the character of the redox

processes in the polymers is UV�/Vis spectroelectro-

chemistry. The spectra of PDMT and PDPT recorded at

various polarization potentials are presented in Fig. 4

(the spectroelectrochemical behavior of PDOT is similar

to that of PDPT). In the neutral state, under polariza-

tion of the films at �/0.6 V, the spectra reveal a very well

developed vibronic structure, characteristic of the rigid

polymers containing crystalline zones in amorphous

phase [25,26]. The distances between the vibronic peaks,

listed in Table 1, correspond to 0.16�/0.17 eV ascribed in

the literature to C�/C stretching mode in the thiophene

ring which confirms high regularity along the polymer

backbone [25,27,28]. The vibronic peaks of poly(3,4-

dialkoxythiophenes) are much sharper and better re-

solved than those obtained for poly(3-alkylthiophene)

and monosubstituted polyalkoxythiophenes [29]. In

spite of some distortion from planarity suggested above,

the alkoxy substituents hinder a free rotation of the

neutral polymer chains around the bonds between the

adjacent monomer units making undoped poly(3,4-

dialkoxythiophene) backbone more rigid than those of

the monosubstituted polythiophenes. Oxidation of the

polymer films results in the decrease of p0/p* transition

peaks intensity with simultaneous appearance of new

bands at l�/680 nm. A comparison of the maximum

intensity of the bands in this range shows that at high

doping levels PDPT is much more transparent than

PDMT.

It is also worth noting that evolution of the spectra at

the wavelength above 680 nm is different for PDMT

than for the two other polymers studied, PDPT and

PDOT. In the former case one can observe a develop-

ment of only one broad absorption peak at about 820

nm, as it has been reported for other electrosynthesised

polythiophene derivatives [20,30,31]. In contrast, two

separate peaks are formed for PDPT and PDOT*/one

at about 800 nm and the second at 980 nm. The first

band is less resolved due to overlapping with the region

of p0/p* transition peak. These two bands develop in

the spectra at relatively low oxidation potentials (for

PDPT in the range �/0.5}/0.16 V, corresponding to the

shoulder in the cyclic voltammogram) and overlap upon

Table 1

Maximum absorption (lmax) of the monomers and the polymers in the neutral state, oxidation potentials of the monomers (Ep(ox)), redox potentials

of the polymers in acetonitrile (Eredox� ) and the charge yields (y ) for the polymers studied

Polymer lmax/nm monomer Ep(ox)/V (monomer) /y�Qred=Qdep /Eredox�

//V a (polymer) lmax/nm polymer

PDMT 262 1.15 2.4% �/0.05 520, 558, 607

PDPT 262 1.12 1.8% 0.26 522, 560, 608

PDOT 264 1.09 7% 0.39 524, 565, 612

PEDOT 253[15] 1.09 13% �/0.54 610

a/Eredox

� ; is assumed as (Epa�/Epc)/2.

Fig. 3. Cyclic voltammograms of PEDOT, PDMT, PDPT and PDOT

films in the solution of 0.1 M LiClO4 in acetonitrile.


gradual polymer oxidation. This gives a rather compli-

cated picture of spectroelectrochemical behavior of

poly(3,4-dialkoxythiophenes). Thus, in order to gain

more insight into the redox processes in the polymers

studied, we also made a series of experiments in which

the optical changes upon doping�/undoping were re-

corded simultaneously with the cyclic voltammograms.

The measurements were performed at several wave-

lengths, 560, 800, 980 nm for PDPT and at 560, 820 nm

for PDMT, corresponding to the maxima in the

absorbance spectra, and at 1100 nm. The obtained

absorbance�/time responses were differentiated, plotted

in the form of dA /dt versus E profiles (differential cyclic

voltabsorptograms, DCVA) and compared with the

cyclic voltammograms.

As visible in Fig. 5b, the shoulder and the main

oxidation peak in the voltammogram of PDPT have

their well developed counterparts in dA /dt �/E plots

obtained at 560 nm. The similar behavior was also

obtained for the spectra recorded at two other vibronic

peaks, at 522 and 608 nm. The shoulder in CV has also a

counterpart in a form of a small peak in the voltab-

sorptograms recorded at 800 and 980 nm. In contrast,

the maximum related to the main anodic peak nearly

disappeared in the voltabsorptograms recorded at these

two wavelengths. Some traces of this peak are visible

only in the curve obtained at 980 nm, probably due to

remarkable overlapping with the region of increased

absorbance above 1000 nm at E �/0.2 V. In turn, the

peak corresponding to the main current maximum is

very well developed in the voltabsorptogram obtained at

1100 nm. These observations and analysis of the spectra

presented in Fig. 4b allow us to conclude that the bands

at 800 and 980 nm correspond to the species created at

the beginning of oxidation, i.e. at relatively low doping

levels. According to the literature, in the polythiophene

Fig. 4. Evolution of UV�/Vis in-situ spectra of PDMT (a) and PDPT (b) upon gradual polymer oxidation in 0.1 M LiClO4 in AN.

Fig. 5. Cyclic voltammograms (dotted line) and voltabsorptograms (DCVAs) obtained for doping�/undoping of PDMT (a) and PDPT (b) at the

scan rate 10 mV s�1 in AN at the wavelengths: (a) 560, 820 and 1100 nm; (b) 560, 800, 980 and 1100 nm.


derivatives the radical cations are created even at

extremely low doping level, below 0.1% [32]. The main

oxidation peak for PDPT located at 0.3 V corresponds

to the species created also directly from the neutralpolymer. At the more positive potentials, these species

are transformed into an another type of the charge

carriers, which are responsible for the increase of

absorbance at the wavelength above 1000 nm. This is

confirmed by appearance of isosbestic point at 912 nm.

Highly interesting voltabsorptograms were obtained

for PDMT. As visible in Fig. 5a, one can distinguish two

different redox couples in the dA /dt profiles recorded at820 and 1100 nm but only one in the cyclic voltammo-

gram. The oxidation potentials of these two waves are

located very close each other, at 0.03 and 0.18 V. Their

cathodic counterparts, respectively at �/0.23 and 0.18 V,

are better separated. These two peaks correspond

exactly to two reduction waves in the cyclic voltammo-

gram. This suggests that the anodic peak in the

voltammogram of PDMT is, in fact, a superpositionof two distinct peaks, as it has been postulated for

poly(3-methylthiopene) [33]. In the case of PDPT and

PDOT a distance between the two oxidation peaks is

much higher than for PDMT and additionally, the

second (more anodic) peak is sharper than the first one.

In consequence, one can distinguish two distinct redox

processes. The presence of overlapping redox peaks was

also observed for other poly(3-alkylthiophenes) andexplained as a result of oxidation-reduction of the zones

of different crystallinity and conjugation length [31,34].

In our opinion the redox behaviour of poly(3,4-dialkox-

ythiophenes) may be also elucidate by co-existence in the

polymer film of the regions of different ordering. The X-

ray diffraction data presented in the literature proved

the presence of the crystalline zones in poly(3-alkox-

ythiophene) films [29]. Moreover, it was found that thecrystallinity of the polymers increased with the increase

of the alkoxy side-chain length.

3.3. Stability of PDMT, PDPT and PDOT in the neutral

and oxidized states

A shift of the redox potential of poly(3,4-dialkox-

ythiophenes) with increasing side chain length is favour-

able because it allows for modification of the polymerproperties. For example, PEDOT, often used as a

reference polymer due to its unique properties, is very

stable in the doped state but not in the neutral form due

to very low oxidation potential. Therefore, in the neutral

state it should be handled only in oxygen-free condi-

tions. This limits the possibilities of practical applica-

tions of PEDOT. From this point of view, poly(3,4-

dialkoxythiophenes), whose redox potentials are morepositive, seem to be the promising materials. Thus, the

next step of the studies was directed to examination of

the stability of PDMT, PDPT and PDOT in the neutral

state. The films were deposited electrochemically on

ITO, then polarized for 180 s at �/0.8 V to transform the

polymer into the neutral state and finally, after inter-

ruption the external polarization, the relaxation of theopen circuit potential, Eocp, and absorbance at two

wavelengths, 560 and 820 nm, in non-deoxygenated

solution were monitored. These two parameters changed

very slowly and within 20 min the absorbance decreased

only slightly, from 0.75 to 0.73, and the open circuit

potential stabilized at the values within the region of the

neutral polymer.

Electrochemical stability of the polymers in theiroxidized state was examined by the potential cycling

from �/0.6 V to various anodic potentials: 0.8, 0.9, 1.0

and 1.1 V at the scan rate of 40 mV s�1. A comparison

of the first and fourteenth scans for PDOT in each

potential range, (Fig. 6), indicates that that gradual

degradation of the film electroactivity in AN takes place

above 0.9 V. The similar stability reveal also two other

polymers from this group, PDMT and PDPT. Thus, onemay conclude that poly(3,4-dialkoxythiophenes) are

very stable both in the neutral and oxidized states.

3.4. Influence of the solvent on electrochemical behavior

of PDMT, PDPT and PDOT

The studies of the redox behavior of poly(3,4-dia-

lkoxythiophenes) in various solvents (AN, PC and

water) revealed an interesting feature*/a strong depen-dence of a position of the reduction peak on the type of

the solvent. This effect is especially remarkable for

Fig. 6. Cyclic voltammograms (1st and 14th scans) for doping�/

undoping of PDOT film in AN in the extended potential ranges. First

scans are denoted by the solid lines whereas 14th scans by: dashed lines

(to final potential 0.9 V), dotted line (to 1.0 V) and dash-dotted line (to

1.1 V). The inset plots show the decrease of the reduction charge in the

subsequent scans performed to various final potentials: 0.9 (m); 1.0

('); and 1.1 V (%).


PDPT. In the case of PDMT, the cyclic voltammograms

are very broad and therefore, the solvent effect is rather

difficult to analyse. In turn, the long hydrophobic alkyl

chains in PDOT likely decrease the polymer swelling.

This is probably a reason of the lowering of redox

charge (from 140 to 93 mC) after the transfer of the film

from AN to the aqueous solution. In effect of dimin-

ished swelling, the resistance of the film in aqueous

solution increases and the redox peaks are shifted (Fig.

7b). In the case of shorter alkyl chain (propyl) the

hydrophilicity of the oxygen competes with hydropho-

bicity of alkyl and therefore, no change of the polymer

electroactivity was detected after the transfer of PDPT

from AN to aqueous solution. As visible in Fig. 7a, the

main oxidation peak for PDPT is located at about 0.3 V

irrespectively of the solvent, whereas the reduction peak

shifts from 0.27 V in aqueous solution to 0.2 V in AN

and to 0.02 V in PC, at the scan rate of 40 mV s�1.

Decrease of the scan rate to 1 mV s�1 only slightly

diminished the separation between oxidation and reduc-

tion peaks in PC. Thus, this fact excludes the possibility

that the strong asymmetry between the peaks in this

solvent is due to a slow diffusion of the counter ions to/

from the polymer film upon doping/undoping. There-

fore, the presence of alkoxy substituents seems to be

crucial for explanation of this asymmetry. Thus, let us

consider the phenomena occurring upon the polymer

oxidation and reduction. In general, the first stage of

oxidation is formation of the radical cations along the

polymer chain with simultaneous incorporation of the

counter anions into the polymer matrix to neutralize the

positive charge. In polyalkoxythiophenes the radical

cation created upon polymer oxidation may be deloca-

lized on the alkoxy group, according to Scheme 2, as has

been postulated in the literature [4,6,18,35].To specify the potential range in which delocalization

of the radical cation occurs, we registered the 1H-NMR

spectra of PDOT in the neutral state and after oxidation

for 5 min at two different potentials, 0.15 V (at the onset

of main oxidation peak) and 0.35 V (after oxidation

peak). No shift of the signal was found for polymer

oxidized at 0.15 V, whereas the polarization to 0.35 V

led to broadening and a profound shift of the proton

peak of �/OCH2�/ group from 4.11 to 4.4 ppm (see Fig.

8).

Fig. 7. Influence of the type of solvent on the redox behavior of PDPT (a) and PDOT (b) films in the solution of 0.1 M LiClO4. Scan rate 40 mV s�1.

Scheme 2. Formation of radical cation in poly(3-alkoxythiophene)

and its delocalization on the alkoxy group.

Fig. 8. 1H-NMR spectra of PDOT in the neutral state (A) and

oxidized for 300 s at and at 0.35 V (B).


Based on the voltammograms carried out at very low

scan rate and 1H-NMR results presented above it is

reasonable to speculate that a remarkable shift of the

reduction peak in PC towards lower potentials is due tostrong interactions between the radical cations localized

on the alkoxy groups and the solvent molecules and/or

the anions accommodated by the oxidized polymer film.

It is highly plausible because PC has a very high dipole

moment (4.98 D) and the perchlorates are not solvated

in PC [36]. Since the perchlorates are also poorly

solvated in AN but a dipole moment of AN is much

lower (3.4 D) we suggest that the interactions of theradical cations with the solvent are of the main

importance. The dipole moment of the water molecule

is the lowest among the three solvent studied (1.76 D)

and probably therefore the reduction peak is the most

symmetrical with respect to the oxidation one. This

leads to the conclusion that poly(3,4-dialkoxythio-

phenes) are highly electroactive and stable both in the

presence of oxygen and the moisture. This is a greatadvantage, considering the possible practical application

of these polymers as, for example, chemical and

biosensors.

3.5. Infrared spectra

Fig. 9 presents the IR spectrum of pristine PDOT and

two monomer spectra: in a solid form (in KBr) and in

acetonitrile solution. The peak positions and proposed

assignments are listed in Table 2. A list of differences

between the polymer and monomer spectra starts from

the Ca�/H stretching band observed at 3105 cm�1 for

the monomer but missing in the spectrum of the

polymer. Two other bands, at 1205 and 870 cm�1,involving mainly the Ca�/H in-plane bending modes

(n15 and n17 in Refs. [37,38]) are strong in the monomer

spectrum but consequently not observed in the spectrum

of PDOT. In contrast, a new band at 1170 cm�1

attributable to the Ca�/Ca inter-ring stretching between

the 3,4-DOT units appears only in the polymer spec-

trum. The thiophene ring-stretching modes occur at

1565 and 1504 cm�1 in the monomer spectrum.

According to IR data for oligo- and polythiophenes,

these modes are expected to shift downwards upon

increasing conjugation length [39,41,42]. This is prob-

ably a reason of the shift of the two bands to 1437 cm�1

in the spectrum of PDOT.

Differences between the spectra of the solid monomer

and monomer in the solution, are visible mainly in the

range 1200�/600 cm�1 (see Fig. 9a and b) because it

comprises the conformation dependent bands. The C�/

O�/C motions usually mix with aromatic ring deforma-

tions involving C�/H or C-substituent bending modes

[43]. Frequencies of C�/O�/C vibrations are correlated

with the C�/O�/C angle. Therefore, in the monomer

solution, where the oxy-alkyl chain can rotate around

C�/O bond, the considered spectral region contains

broad overlapping bands. In contrast, relatively sharp

and well defined bands occur in the spectrum of a solid

3,4-DOT indicating that in the solid state a single

conformation is enforced. Moreover, in this wavenum-

ber range the polymer spectrum reveals numerous bands

forming quite different pattern than that in the mono-

mer spectrum. These differences likely result from the

changes in the thiophene ring deformation modes.

The bands due to the octyl chains are observed at the

similar positions in all spectra studied i.e. of the solid

monomer, monomer in the solution and the polymer

(see Table 2).

Fig. 9. Infrared spectra of 3,4-DOT in the acetonitrile solution (a) solid 3,4-DOT*/in KBr (b) and PDOT in KBr (c).


The effect of electrochemical oxidation on the infra-

red spectra of PDOT is presented in Fig. 10. The spectra

were recorded ex-situ after conditioning the polymer

film for 1 min at the selected potentials. A bare platinum

electrode was used to record the background spectrum.

As it results from UV�/Vis data of Fig. 4, the polymer

conditioned at �/0.3 V is in the neutral form and

therefore, the corresponding infrared spectrum repro-

duces most features characteristic of undoped polythio-

phenes [44,47]. Polarization of PDOT film at 0.25 V,

located within the current shoulder (cf. Fig. 7b), leads to

development of a broad absorption with a maximum at

3300 cm�1, feature typical of conducting polymers due

to the lowest-energy electronic transition [48,49], (Fig.

10a). At the same time, some bands visible for the

neutral polymer within the range 1600�/600 cm�1 are

preserved, in particular the ring stretching mode at 1437

cm�1, although its relative intensity (with respect to the

band of CH2 bending at 1468 cm�1) is altered (Fig.

10b).New strong and very broad bands appear at 1326,

1180 and 1042 cm�1. Their intensity, shapes and

positions are similar to the p-doping induced bands,

so-called infrared activated vibration (IRAV) bands,

observed for several polythiophene derivatives. These

strong bands appear owing to the movement of the

electronic charge carriers along the polymer chains,

giving rise to large dipole changes in the vibrations [45�/

47,50�/52]. A new band appears also at 1491 cm�1 but

its origin is not obvious. It could be assigned to another

doping induced band, but its intensity and width is very

small in comparison to typical doping induced band.

Another possible explanation for the appearance of the

band at 1491 cm�1 is the conformation or orientation

change in the polymer film. The orientation or con-

formational changes could also explain the altered

relative intensity of ring stretching mode at 1437 cm�1.

When the potential value matches the main oxidation

peak in the cyclic voltammogram of PDOT (�/0.4 V),

the doping induced bands become broader and acquire

clear new components on their low wavenumber sides.

At higher doping levels, after the main oxidation peak at

0.5 V, these ‘new’ components become stronger than the

‘old’ ones and in effect, the bands at 1326, 1180 and

1042 cm�1 (observed at �/0.25 V) are replaced by the

bands at 1282, 1130 and 1001 cm�1 at 0.5 V. Such

strong changes in the doping induced bands cannot be

explained by simple increase of the charge carriers

concentration, but rather by a formation of an another

type of charge carrier. This behavior is similar to that

reported by Jones et al. [52] for polybenzo[c ]thiophene

but different from the spectral changes observed for

many polythiophene derivatives, where the increase of

the anodic polarization potential causes merely the

increase of the IRAV band intensities without clear

shift in their position, or development of new bands

[45�/51].

Polarization of PDOT above 0.25 V also gives rise to

the shift of the electronic absorption maximum from

3300 cm�1 towards lower wavenumbers, up to about

2000 cm�1 at �/0.7 V (Fig. 10a), with simultaneous

gradual increase of its intensity. In consequence, already

at �/0.4 V the electronic absorption covers the C�/H

stretching bands (around 2900 cm�1). Since the max-

Table 2

Infrared bands of 3,4-DOT and PDOT

Mon. sol. In acetonitrile Monomer solid Polymer (neutral) Assignment a

3105 3105 �/ Ca�/H stretching (thiophene ring)

2959 2953 2954 CH3 asymmetric stretching

2932 2920/2936 b 2918 CH2 asymmetric stretching

2875 2868 2873 CH3 symmetric stretching

2859 2855 2851 CH2 symmetric stretching

1565 1565 �/ Thiophene ring deformation

1500 1504/1495 b �/ Thiophene ring asymmetric stretching (Ca�/Cb)

1470 1470 1468 CH2 bending

1437 Thiophene ring deformation

1374 1396/1374 b 1374 CH2 wagging�/Cb�/Cb stretching

1269 Thiophene ring deformation

1205 1205 Ca�/H bending

1170 Ca�/Ca interring stretching

1154 1154/1126 b �/ C�/O�/C coupled with Ca�/H bending

1038, 961 1042, 1038, 990 1100, 1076, 1055, 1025, 1010, 990, 946 C(ring)�/O�/C bending modes

910 C�/S stretching

870 870 876 (weak) Ca�/H�/C�/S stretching

760 751/742 b �/ Ca�/H out-of-plane

723 723 723 CH2 out-of-plane deformations�/C�/S�/C deformation

a On the basis of Refs. [37�/42,44].b Crystal field splitting.


imum of the electronic absorption and the dopinginduced bands for the second type of carrier (possibly

bipolarons) are located at the lower wavenumbers, these

species are characterized by lower ‘pinning’ and stronger

charge delocalization, as compared to the charge

carriers formed at the initial oxidation stage [53].

4. Conclusions

Our studies on electrosynthesis of poly(3,4-dialkox-

ythiophene) films have shown that electrochemically

stable, rigid and long conjugated polymer may be

obtained from acetonitrile solutions of the monomerin LiClO4 supporting electrolyte. Low polymerisation

efficiency results from formation of soluble oligomers in

the vicinity of the electrode.

By the increase of the alkyl-chain length in the alkyl

group one may modify the solubility of the resultant

polymer and its electroactivity in the aqueous solution.

The shift of the redox potential of poly(3,4-dialkox-

ythiophenes) to the more positive potentials with the

increase of the alkyl chain length of the alkoxy group

may result from some distortion of the polymer back-

bone from planarity or/and decreasing degree of poly-

merisation.

UV�/Vis in-situ spectra and DCVA profiles indicated

that the oxidation branch in the cyclic voltammograms

of poly(3,4-dialkoxythiophenes) consists of two over-

lapping oxidation peaks. The origin of the two peak may

be the same as in the case of regioregular poly(3-

alkylthiophenes) i.e. co-existence in the polymer film

of the zones of different crystallinity. The separation

between these two peaks is very small for PDMT and

Fig. 10. Infrared external reflectance spectra of PDOT film conditioned for 1 min at various potentials in the range 4000�/500 (a) and 1600�/600

cm�1 (b).


increases with the increase of the alkyl chain length of

alkoxy group.

The shape of the cyclic voltammograms for the

polymer doping�/undoping depends markedly on thesolvent used. In our opinion a remarkable shift of the

reduction peak of PDPT in PC towards less positive

potentials in comparison to the peak obtained in AN

and in aqueous solution is due to strong interactions of

the solvent with the radical cations delocalised on the

alkoxy groups. Delocalization of the radical cations has

been proved by NMR spectra.

The changes in location and intensity of the infraredactivated vibrations (IRAVs) evidenced the changes in

the charge carrier identity during gradual polymer

oxidation.

Acknowledgements

This work was financially supported by grantsGR1686 and BST 761/16/2002.

References

[1] M. Lemaire, D. Delabouglise, R. Garreau, A. Guy, J. Roncali, J.

Chem. Soc. Chem. Commun. (1988) 658.

[2] J. Roncali, R. Garreau, D. Delabouglise, F. Garnier, M. Lemaire,

J. Chem. Soc. Chem. Commun. (1989) 679.

[3] R.D. McCullough, Adv. Mater. 10 (1998) 93.

[4] T. Yamamoto, A. Kashiwazaki, K. Kato, Macromol. Chem. 190

(1989) 1654.

[5] J. Roncali, L.H. Shi, R. Garreau, F. Garnier, M. Lemaire, Synth.

Met. 36 (1990) 267.

[6] S.-A. Chen, C.-C. Tsai, Macromolecules 26 (1993) 2234.

[7] S. Tanaka, M.A. Sato, K. Kaeriyama, Synth. Met. 25 (1988) 277.

[8] M. Feldheus, G. Kampf, H. Litterer, T. Mechlenburg, P. Wegner,

Synth. Met. 28 (1989) C487.

[9] L.B. Groenendaal, F. Jonas, D. Feitag, H. Pielartzik, J.R.

Reynolds, Adv. Mater. 12 (2000) 481.

[10] J. Roncali, Chem. Rev. 92 (1992) 711.

[11] M. Leclerc, G. Daoust, J. Chem. Soc. Chem. Commun. 3 (1990)

273.

[12] A.G. Bayer, European Patent, 339 340.

[13] M. Dietrich, J. Heinze, G. Heywang, F. Jonas, J. Ekectroanal.

Chem. 369 (1994) 87.

[14] F. Jonas, J.T. Morrison, Synth. Met. 85 (1997) 1397.

[15] G. Zotti, S. Zecchin, G. Schiavon, L.B. Groenendaal, Chem.

Mater. 12 (2000) 2996.

[16] G. Daoust, M. Leclerc, Macromolecules 24 (1991) 455.

[17] Q.T. Zhang, J.M. Tour, J. Am. Chem. Soc. 120 (1998) 5355.

[18] M. Fall, L. Assogba, J.-J. Aaron, M.M. Dieng, Synth. Met. 123

(2001) 365.

[19] O. Hinsberg, Ber. 43 (1910) 901.

[20] A.O. Patil, A.J. Heeger, F. Wudl, Chem. Rev. 88 (1988) 183.

[21] R.H.J. Schmitz, A.S. Hudson, K. Juttner, Synth. Met. 101 (1999)

102.

[22] C. Arbizzani, M. Catellani, M. Mastragostino, M.G. Cerroni, J.

Electroanal. Chem. 423 (1997) 23.

[23] E.E. Havinga, C.M.J. Mutsaers, L.W. Jenneskens, Chem. Mater.

8 (1996) 769.

[24] S.V. Meille, A. Farina, F. Bezziccheri, M.C. Gallazzi, Adv.

Mater. 6 (1994) 848.

[25] S.D.D. Rughooputh, S. Hotta, A.J. Heeger, F. Wudl, J. Pol. Sci.

B: Polym. Phys. 25 (1987) 1071.

[26] M. Sundberg, O. Inganas, S. Stafstrom, G. Gustafsson, B.

Sjogren, Solid State. Commun. 71 (1989) 435.

[27] R.D. McCullough, R.D. Lowe, M. Jayaraman, D.L. Anderson, J.

Org. Chem. 58 (1993) 904.

[28] R.D. McCullough, R.D. Lowe, J. Chem. Soc. Chem. Commun.

(1992) 70.

[29] X. Hu, L. Xu, Polymer 41 (2000) 9147.

[30] J. Lukkari, J. Kankare, C. Visy, Synth. Met. 48 (1992) 181.

[31] M. Skompska, A. Szkurlat, Electrochim. Acta 46 (2001) 4007.

[32] Y. Harima, T. Eguchi, K. Yamashita, K. Kojima, M. Shiotani,

Synth. Met. 105 (1999) 121.

[33] M. Skompska, Electrochim. Acta 44 (1998) 357.

[34] X. Jiang, Y. Harima, K. Yamashita, Y. Tada, J. Ohshita, A.

Kunai, Chem. Phys. Lett. 364 (2002) 616.

[35] T. Yamamoto, M. Omote, Y. Miyazaki, A. Kashiwazaki, B.-L.

Lee, T. Kanbara, K. Osakada, T. Inoue, K. Kubota, Macro-

molecules 30 (1997) 7158.

[36] H.A. Berman, T.R. Stengle, J. Phys. Chem. 79 (1975) 1001.

[37] M. Rico, J.M. Orza, J. Morcillo, Spectrochim. Acta 21 (1965) 689.

[38] P.A. Christensen, A. Hamnett, A.R. Hillman, J. Electroanal.

Chem. 242 (1988) 47.

[39] V. Hernandez, J. Casado, F.J. Ramirez, G. Zotti, S. Hotta, J.T.

Lopez-Navarrete, J. Chem. Phys. 104 (1996) 9271.

[40] E. Agosti, M. Rivola, V. Hernandez, M. Del Zoppo, G. Zerbi,

Synth. Met. 100 (1999) 101.

[41] V. Hernandez, Y. Kanemitsu, J.T. Lopez-Navarrete, J. Raman

Spectrosc. 29 (1998) 617.

[42] G. Louarn, M. Trznadel, J.P. Buisson, J. Laska, A. Pron, M.

Lapkowski, S. Lefrant, J. Phys. Chem. 100 (1996) 12532.

[43] L.J. Bellamy, The Infrared Spectra of Complex Molecules,

Chapman A. Hall, London, 1978.

[44] B.C. Uff, in: A.R. Katritzky, C.W. Rees (Eds.), Comprehensive

Heterocyclic Chemistry, vol. 2A, Pergamon Press, Oxford, 1886.

[45] C. Kvarnstrom, H. Neugebauer, A. Ivaska, N.S. Sariciftci, J.

Molec. Struct. 521 (2000) 271.

[46] A. Cravino, H. Neugebauer, S. Luzzati, M. Catellani, N.S.

Sariciftci, J. Phys. Chem. B 105 (2001) 46.

[47] M. Pohjakallio, G. Sundholm, P. Talonen, J. Electroanal. Chem.

406 (1996) 165.

[48] Y.H. Khim, S. Hotta, A.J. Heeger, Phys. Rev. B 36 (1987) 7486.

[49] H. Kuzmany, N.S. Sariciftci, H. Neugebauer, A. Neckel, Phys.

Rev. Lett. 60 (1988) 212.

[50] H. Neugebauer, C. Kvarnstrom, A. Cravino, T. Yohannes, N.S.

Sariciftci, Synth. Met. 116 (2001) 115.

[51] A. Cravino, H. Neugebauer, S. Luzzati, M. Catellani, A. Petr, L.

Dunsch, N.S. Sariciftci, J. Phys. Chem. B 106 (2002) 3583.

[52] C.L. Jones, S.J. Higgins, P.A. Christensen, J. Mater. Chem. 12

(2002) 758.

[53] Comprehensive review can be found in M. Del Zoppo, C.

Castiglioni, P. Zuliani, G. Zerbi, in: T.A. Skotheim, R.L. Else-

nbaumer, J.R. Reynolds (Eds.), Handbook of Conducting Poly-

mers, 2nd ed. (Chapter 28), New York, Marcel Dekker, 1988

(Chapter 28).


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