Electrochemical synthesis and electronic properties of ...chemgroups.northwestern.edu/.../Poly(3,4-Dibutyl-a-Terthiophene.pdf · port processes of the polymer and discuss its relationship

Synthetic Metals 75 (1995) 213-221

Electrochemical synthesis and electronic properties of poly ( 3,4-dibutyl- cx-terthiophene)

S. Glenis ‘, M. Benz a, E. LeGoff a, M.G. Kanatzidis ay*, D.C. DeGroot b, J.L. Schindler b, C.R. Kannewurf b

a Department of Chemistry and Centerfor Fundamental Materials Research, Michigan State University, East Lansing, MI 48824, USA h Department of Electrical Engineering und Computer Science, Northwestern Universiry, Evanston, IL 60208, USA

Received 3 1 July 1995

Abstract

Electrochemically prepared poly( 3,4-dibutyl-a-terthiophene) has been investigated and the relationship between the electronic properties, the chemical structure of the monomer and the electrolyte used during the electropolymerization was studied. Doping studies were carried out with various electrolytes. We have electrochemically oxidized (p-doping) and reduced (n-doping) thin films of this polymer on platinum electrode under the same electrochemical conditions. We find that the films show a reversible oxidation wave yielding an electrically conducting polymer and a reversible reduction wave which is sensitive to the nature of the electrolyte. A value for the band gap derived electrochemically compares well with that obtained by optical absorption measurements and X-ray photoelectron spectroscopy. The electrical conductivity varies by lo’* S cm-’ between the doped and undoped states of the polymer. The temperature-independent magnetic susceptibility above 100 K is consistent with the conducting properties of this polymer. The electrochemically prepared material is compared with the chemically prepared one using the same monomer.

Keywords: Synthesis; Electronic properties; Poly( 3,4-dibutyl-u-terthiophene)

1. Introduction

Five-membered heterocyclic ring polymers have attracted much attention from both scientific and technological per- spectives [ l-31 due to their long-term stability [4,5]. How- ever, these materials are neither fusible nor soluble in common solvents, making processability difficult. Recently, the chemical and electrochemical syntheses of a series of poly( 3-alkylthiophenes) processed from a solution of organic solvents have been achieved [6,7]. Although the addition of alkyl side chains to the polymer backbone has enabled preparation of the first soluble and fusible conducting polymers, the added chains result in lower electrical conductivity in the doped state than the unsubstituted polymer. An explanation for the reduction in conductivity with side chain length is steric hindrance which disrupts the coplanarity of thiophene rings. Another cause of structural irregularity orig- inates from the fact that in 3-alkylthiophene monomers the 2- and 5-positions are geometrically nonequivalent, making possible two types of ol,a-couplings (see Scheme 1). Although the head-to-tail couplings are favored due to steric

* Corresponding author.

0379-6779/95/$09,50 0 1995 Elsevier Science S.A. All rights reserved SSDIO379-6779(95)03485-3

d d Scheme 1.

considerations, 13C NMR spectra of poly( 3-alkylthiophenes) reveal the presence of about lO-20% coupling defects involving head-to-head couplings [ 81. Several studies on alkyl-substituted thiophene monomers and dimers and their corresponding polymers aimed at reducing the a,@coupling and producing stereoregular polymers have been reported. However, other effects such as intrachain sulfur-alkyl steric repulsions became surprisingly dominant, forcing the backbone out of coplanarity and n-conjugation [8a]. Further studies of the electronic properties of the polymer, as a function of not only side-chain length but also of position in the backbone, are needed. Recently, significant progress has been reported in the regiospecific synthesis of about 100% head- to-tail polyalkylthiophenes which exhibit significantly higher conductivities [ 91.

214 S. Glenis et al. /Synthetic Metals 75 (1995) 213-221

We chose the monomer shown below for polymerization because it represents a significant departure from the known monoalkyl-substituted thiophenes. It contains strategically placed alkyl groups on certain thiophene units designed to avoid the problem of head-to-head coupling and give polymers in which regular cY,cY-coupling is ensured. Furthermore, the substitution of the two butyl groups on the 3,4-position of the center ring of terthiophene not only reduces steric hindrance between alkyl chains in adjacent thiophene rings, but also improves polymer solubility and could give rise to improved electrochemical and electrical properties. A common characteristic in the previous studies is that in all cases each thiophene ring is substituted by an alkyl chain. The 3,4- dibutyl-a-terthiophene (DBTT) gives a polymer in which only one-third of the thiophene units is substituted and this considerably minimizes intrachain sulfur-alkyl steric repulsions [ lo]. In this work, we used the 3,4-dibutyl-o-terthiophene as starting material for electropolymerization.

We report the electrochemical synthesis, physicochemical characterization and properties of poly( 3,4-dibutyl-a-terthiophene). In order to distinguish this material from that prepared with chemical oxidation of DBTT [ lo] we adopt the following abbreviation: the electrochemically prepared polymer will be referred to as e-poly (DBTT) while the other will be called c-poly( DBTT) . Both versions of the polymer can be either oxidized or reduced (by insertion of anions or cations) to become a p- or n-type conductor, respectively, and possess electrochromic properties. We have character- ized this polymer by a variety of techniques, such as IR and UV-Vis spectroscopy, scanning electron microscopy (SEM) , X-ray photoelectron spectroscopy (XPS), d.c. conductivity, electron spin resonance (ESR) spectroscopy, magnetic susceptibility and charge transport measurements. We report the influence of the electrolyte and the structure of the monomer on the composition, electronic properties and transport processes of the polymer and discuss its relationship to other polyalkylthiophenes.

2. Experimental

DBTT was prepared as detailed elsewhere [ 111. Poly( 3,4- dibutyl-cY-terthiophene) films were prepared by electrochemical oxidation of the monomer in a three-electrode one- compartment cell by potential sweeps between 0 and 1.0 V

(versus saturated calomel electrode ( SCE) ) . The films were grown either on platinum or on optically transparent tin- doped indium oxide electrodes using a platinum counter electrode and a reference SCE. The solutions used for the electropolymerization contained 0.02 M DBTT with 0.02 M electrolyte (tetrabutylammonium (TBA) + salts of ClO, , CF,SO,-, BF,, PF, or LiClO,-) in distilled acetonitrile that was deaerated by argon bubbling. Cyclic voltammetric experiments were performed with a Princeton Applied Research model 273 potentiostat/galvanostat.

The polymer is soluble in common organic solvents such as nitrobenzene, propylene carbonate, chloroform and tetra- hydrofuran. The oxidized doped e-poly (DBTT) coated on the Pt electrode was undoped by reducing the film to its neutral state. After undoping, the film-covered electrode was rinsed repeatedly with the solvent and then dried under vac- uum. The molecular weight of e-poly (DBTT) was determined with a Shimadzu high-pressure liquid chromatography (HPLC) system operating in gel permeation chromatography mode, as described elsewhere [ lo]. The thickness of the e- poly(DBTT) film was estimated from the relationship 7.8kO.4 nm mC_i cm-* as determined from three linear plots of charge passed during electrosynthesis versus film thickness. A Dektak surface profile measurement system was used. Thickness measurements were performed on dry oxidized films containing ClO, , having a thickness between 50 and 500 nm. UV-Vis spectra were recorded on Hitachi U- 2000 and Shimadzu UV-3 1OlPC double beam, double-mon- ochromator UV-Vis-NIR spectrophotometers. FT-IR spectroscopy was performed with a Nicolet 740 FT-IR spectrometer. The samples were prepared by grinding the polymer film with KBr and then pressing the mixture into a pellet.

The SEM micrographs were obtained using a JEOL JSM- 35CF scanning electron microscope. XPS was conducted with a Perkin-Elmer/PhysicalElectronics model 5400 ESCA system having a base pressure of 5 X lO-‘O Torr. XPS data were obtained with a Mg or Al radiation anode which pro- duces a pass energy setting of 44.75 eV for the survey and 35.75 eV for multiplexes, giving an overall energy resolution of 0.8 eV. ESR spectra were obtained with a Varian E-4 spectrometer. Variable-temperature magnetic susceptibility data were collected on a Quantum Design SQUID system at various magnetic fields. (500-1500 G) . D.c. electrical conductivity measurements of the e-poly (DBTT) powder (pressed pellet) were obtained from 100 to 320 K using a data acquisition and analysis system described elsewhere

1121.

3. Results and discussion

3.1. Electrochemistry

The polymer was electrochemically synthesized in acetonitrile at room temperature from DB’IT according to Eq. ( 1) :

S. Glenis et al. /Synthetic Metals 75 (1995) 213-221 215

r

I I I I I

0.0 0.2 0.4 0.6 0.6 1.0 Potential (v) vs. SCE

Fig. 1. Current-voltage curve for the synthesis of an e-poly( DBTT) film on a F’t electrode in acetonitrile containing 0.02 M DBTT and 0.02 M LiCIO,, Scan rate 50 mV s - ’

electrochemical oxidation

* in (BudN)Sw3 in CH3CN

4&v-& (SqCF3)0.8 + 2nH++ (2n+O.W

(1)

Fig. 1 shows the current-voltage curve obtained during the synthesis of an e-poly( DBTI’) film on a Pt electrode in CH,CN containing 0.02 M DBTTand 0.02 M LiC104. Simul- taneous with anodic current production, a blue adhesive film forms on the surface of the Pt electrode. Shown in Fig. 2 are the SEM micrographs of undoped and ClO,-doped e- poly (DB’IT) films. The smooth and continuous surface morphology shows little variation between the undoped and doped states, contrary to the wide differences observed in other polythiophene films [ 131. At electrode potentials above 0.82 V the anodic current increases sharply and stabilizes after several seconds. The oxidation potential is more than a volt lower ( 1.35 V( SCE) ) than that necessary for the polymerization of thiophene. Other polythiophenes synthesized by electropolymerization of bithiophenes and terthiophenes also showed low oxidation potential relative to thiophene and the produced polymers contained relatively short conjugated chains and had lower conductivities than materials from

thiophenes. This effect was attributed to the difference in electron density at the a and B positions of the oligomers used as starting materials [ 141. This difference decreases and the radical cation intermediates are better stabilized slowing down the polymerization kinetics, leading to shorter polymer lengths. Upon reduction of the oxidized blue film to the neutral form, the color changes to red. The reduction, or undoping, of the oxidized polymer extends over a potential range of + 0.82 to 0.0 V( SCE).

The cyclic voltammogram of a (50 nm) film of e- poly( DBTT) on a platinum electrode immersed in acetonitrile solution containing 0.02 M LiClO, is shown in Fig. 3. The oxidation is accompanied by a color change from red to blue which is completely reversed upon reduction to the neutral state. The redox process is found to be reversible as confirmed by coulometric measurements of the amount of charge exchanged during the redox process. These films can be cycled repeatedly between the oxidized and the neutral state. Interestingly, the cyclic voltammograms display two anodic waves at 0.72 and 0.74 V with a peak separation of 20 mV. Splitting of the oxidation peak is also observed in poly (3_alkylthiophenes), in varying magnitude depending on the length of the alkyl chain and the film thickness [ 151.

Fig. 2. Scanning electron micrographs of e-poly(DBTT) film surfaces: (a) undoped; (b) C104--doped.


I 0.5 mA-cd2

conduction band with concomitant insertion of cations. The average potential for the formation of the reduced-doped state is E, ,,(red) = - 1.56 V. The film becomes blue-black upon reductive doping and it is very air sensitive. The redox reac- tions are reversible and occur in the same voltage region. In this case, the cation insertion and expulsion are manifested by the well-defined and symmetric reduction and re-oxidation waves, reflecting a strong association between the negatively charged sites in the reduced polymer and the electrolyte. From the cyclic voltammetric data we can determine a value for the band gap of 2.02 eV which is defined as the difference between onset potentials for the oxidation and reduction. This value is in good agreement with that derived from the optical data (see below) and with that of the c-poly( DBTT) [ lo]. Considering that the electron affinity (EA) and the ionization

I I I I I

0.0 0.2 0.4 0.6 0.6 1

Potential (v) vs. SCE

;Jo 12 I I I I I .

Fig. 3. Cyclic voltamogram of (50 nm) e-poly (DBTT) film on a Pt electrode in acetonitrile with 0.02 M LiClO,. Scan rate 50 mV s- ‘.

ESR studies showed that the first peak is associated with a one-electron oxidation to produce the polaron state and the second peak corresponds to the formation of bipolaron states [ 161. Thiophene oligomers also display a peak separation upon oxidative scanning of potential which was attributed to the formation of the radical cation and dication forms [ 171. The reduction of the polymer to the neutral state displays two cathodic waves: one appearing as a shoulder at 0.67 V and one as a peak with a maximum at 0.53 V. The difference between the main anodic peak potential and cathodic peak potential is 0.21 V. This difference in peak potentials is com- monly observed in the electrochemistry of conducting polymers and is attributable to structural reorganization processes within the film [ 181. The anodic peak is noticeably sharper with a peak width at half-height of 10 mV, suggesting homo- geneous distribution in conjugation length along the chains. From the anodic and cathodic peak potentials we can determine the average potential necessary for the formation of the oxidized-doped state (I?,,,) of the polymer which is 0.62 V. Fig. 4 shows that the peak current ii, scales linearly with sweep rate, as is expected from a surface-bonded species and has been observed in many polythiophenes, where diffusion phe- nomena do not control the current.

The cyclic voltammetric response of a (50 nm) film of e- poly (DBTT) on a platinum surface, measured in acetonitrile containing 0.02 M TBA+PF, as the supporting electrolyte is shown in Fig. 5. All experiments were performed under the same electrochemical conditions (solvent, electrolyte and reference electrode). In the oxidation process (p-type doping) charges are removed from the valence band of the polymer as the applied voltage reaches 0.60 V( SCE), which is the onset potential for the oxidation of the polymer, while charge-balancing anions insert in the polymer lattice. Cathodic peaks for the reduction are observed in the potential region from 0 to - 2.0 V where electrons are added into the

I 00

I I 1 I I I 20 40 60 80 100 120

Scan Rate (mV/s)

Fig. 4. Plot of peak current density vs. scan rate for a 50 nm thick film of e- poly( DBTT) on a Pt electrode in acetonitrile with 0.02 M LiClO,.

l-

I I I I

I

I 1 I I I I I 1 I -2.0 -1.5 -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2

Potential (V) vs. SCE

Fig. 5. Oxidation and reduction curves for e-poly(DBTT) on a Pt electrode in acetonitrile containing 0.02 M TBA+PF,. Potentials are referenced to SCE. The same electrochemical conditions are used for both p- and n-doping. Scan rate 50 mV s- ‘.

S. Glenis et al. /Synthetic Metals 75 (1995) 213-221 217

potential (IP), respectively, are related to E,,,( red) and El ,*( ox) we can calculate the EA of the polymer to be 3.1 eV and the IP 5.5 eV. These values are similar to those reported for the unsubstituted polythiophene [ 191.

The stoichiometry for the oxidation or reduction process was estimated based on the comparison of the coulombic charge associated with the oxidation or reduction waves in the voltammogram to the weight of the film. A stoichiometry of 0.3-0.4 charges per DBTT unit in the oxidation as well as in the reduction process was evaluated. Chronoamperometry measurements made on e-poly( DBTT) films in acetonitrile solution containing 0.05 M TBAPF6, by stepping the potential from 0 to 0.9 V( SCE) for the oxidation and 0 to - 2.0 V(SCE) for the reduction, displayed a linear i versus t”’ dependence. Using the integrated form of the Cottrell equa- tion we determine the diffusion coefficients D, and D,, to be 4.62 x lo-” and 5.75 X lo-’ cm2 s- ‘, respectively.

3.2. Structural and optical properties

The IR spectrum of the PF,-doped e-poly(DBTI’) is shown in Fig. 6. The spectrum exhibits several bands characteristic of the monomer DBTT vibrational modes (3135, 2965,2934,2872,1658,1539,1473,1461,1385,1297,1222, 1138, 1035,920,878,791 cm-‘). Thesemonomerbands are retained in the spectrum of the PF,-doped polymer. The 1385, 1297 and 1222 cm- ’ bands are assigned to the vibrational modes of a single thiophene ring. The bands at 1138, 1035 and 920 cm _ ’ are ascribed to C-H bending modes. The C-H out-of-plane bending vibrations, appear in the 900-600 cm-’ region. The band at 791 cm-’ arises from the (Y,cL’- coupling of the carbon backbone and the band at 696 cm- ’ from the 2-substituted thiophene [ 201. The two bands at 1461 and 1473 cm-’ are attributed to the C=C symmetric stretch and that at 1539 cm- ’ to C=C antisymmetric stretch of the thiophene ring. The intensity ratio of these two bands is a clue of the length of the conjugated chain in the polymer [ 2 11. The bands at 2965,2934 and 2872 cm ’ are attributed to the aliphatic C-H stretching of the butyl group of the

65.0 3 I

WAVENUMBER (cm-‘)

Fig. 6. IR transmission spectrum of PF,-doped e-poly(DBTT) (pressed KBr pellet)

0.0 ’ I I I I I 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Energy (eV)

Fig. 7. UV-Vis spectra of e-poly( DBTT) coated on tin-doped indium oxide electrode: -, undoped; - - -, ClO,--doped.

polymer backbone and the band at 3 135 cm - ’ is assigned to C,-H stretching modes. The bands of PF, occur at 838 and

557 cm-‘. These bands are replaced by new ones at 1100 and 625 cm - ’ when PF, is replaced by ClO,-. In the neutral

undoped state the intensity of the bands due to counterions disappears. The IR spectral characteristics are similar to those reported for the c-poly( DBTT) [ IO].

The UV-Vis absorption spectra of the neutral undoped, and oxidized ClO,-doped e-poly( DB’IT) films are shown

in Fig. 7. The neutral undoped polymer exhibits a broad absorption maximum at A,,, = 470 nm similar to that of polythiophene. The origin of the broad absorption band is due to intrinsic energetic disorder which is characteristic of all conjugated polymers. This disorder is produced by the variation of conjugation length in polymer segments and the interaction between the delocalized charge carriers with the polymer backbone. The invariance of h,,, indicates that the substitution of the two butyl chains does not significantly influence the v electronic structure of the aromatic backbone. The band gap, E,, for a direct interband transition may be deduced from the energy absorption edge of the spectrum and was estimated to be 2.03 eV, in good agreement with the electrochemically derived band gap.

As was found for c-poly (DBTI), the optical absorptions of the oxidized polymer are very broad and extend into the near-IR region. The absorption maximum appears reduced in intensity and red shifted (A,,, = 490 nm) with respect to that of the neutral state. Similar behavior was observed in polythiophene [22]. We also observe the appearance of two absorption bands at energy below the interband transition at 750 nm ( 1.65 eV) and 2500 nm (0.49 eV) arising from two doping-induced gap states almost symmetrically spaced with respect to the valence and conduction band edges. These results were interpreted in terms of charge storage and charge transport via charged bipolarons involving local distortions of the polymer chain structure [ 231.

218

Table 1

S. Glenis et ul. /Synthetic Metals 75 (1995) 213-221

S/C atomic ratios of DBTT and PDBTT, and doping levels of e-poly( DBTT) determined by XPS

Polymer S/C undoped SIC doped Doping level/DBTT unit (doped polymer)

DBTT monomer 0.15 PDBTT (ClO,- ) 0.13 0.14 0.4 PDBTT ( PF, ) 0.14 0.14 0.4 PDBTT (SO,CF, ) 0.15 0.15 0.8

Fig. 8. C(ls) and S(2p) core level spectra of (a) undoped and (b) SO,CF,--doped polymer films. The features through the data correspond to the fitting with Gaussian functions.

3.3. XPS

In order to obtain better understanding of the electronic properties of e-poly(DBTI’) and further insight into the chemical structure of the polymer backbone, we investigated this polymer with XPS. The S/C atomic ratios estimated from the C( Is) and S(2p) core levels of ClO,-, PF,-, SO,CF,--doped and undoped e-poly(DBTT), and the S/C atomic ratio of DBTT are displayed in Table 1. The highest doping level of 0.8 charges per monomer unit was obtained in the SO&F,--doped PDB’lT. The values of S/C (0.13- 0.15) are very close to the value of the ideal chemical structure (0.15) shown in Eq. (1) and suggest that the DBTT structure is preserved in the polymer.

aliphatic chains. The weak features at 286.60 and 287.70 eV with an energy separation of 2.0 and 3.10 eV from the main C( 1 s) feature correspond to ‘shake-up’ satellite peaks which involve 7r-rr * transitions between the highest occupied valence levels and the lowest unoccupied bands [ 251. The lowest shake-up of 2.0 eV corresponds to band-gap photoex- citation and is in good agreement with the band gap deduced from the electrochemical and absorption measurements described above. In the SO,CF,--doped state the additional feature at 29 1.78 eV is due to C-F bonds present in the dopant. The S (2~) core level line exhibits two resolved components at 163.60 and 164.80 eV, corresponding to the spin-orbit splitting and a shake-up transition at 166.60 and 168.10 eV. In the doped state the appearance of two additional resolved components at 167.55 and 168.80 eV are due to the S(2p) contribution from the SO,CF,- dopant. In the PDB’IT doped state, the C( 1 s) and S( 2p) core level lines are found to be broader compared to the neutral state (see Table 2). This broadening is due to modifications of the electronic structure of the polymer during the doping process.

3.4. Electrical conductivity

The C( 1 s) and S( 2p) lineshapes of a 200 nm thick e- poly (DBTT) film in the undoped and SO,CF,- -doped state are shown in Fig. 8. In the undoped form the C ( 1 s) peak can be decomposed into four features, respectively, at 284.60, 285.30, 286.60 and 287.70 eV binding energy (BE). The feature at 284.60 eV is attributed to (Y and p carbons of the thiophene rings. The BE difference of 0.34 eV [ 241 between the (Y and B carbon atoms is not apparent in the spectrum due to limited spectrometer resolution (0.8 eV) . The feature at 285.30 eV is attributed to photoelectrons emitted from the

The room-temperature electrical conductivity rr of e- poly(DBTT) films varies over a range of 10” S cm-’ between the undoped and the CF,SO,--doped states. In the undoped state the conductivity was about lo- I’ S cm - ‘. The highest electrical conductivity obtained in the doped state of the polymer in film sample at room temperature was 20 S cm-‘. Conductivities of 0.8 S cm-’ were measured in pressed pellet form. A wide range of conductivity values of (l-20 S cm-‘) was also reported for other polyalkylthiophenes containing linear and branched alkyl chains but they remain relatively low. Several explanations have been given for this effect. In the case of poly (3-alkylthiophene) the side chains influence the r electronic structure through modifi- cation of the orientation of the adjacent planar rings [ 261.

The temperature dependence of the electrical conductivity of pressed pellets over the temperature range from 130 to 320

Table 2 Full width at half-maximum (FWHM) (eV) of C( 1s) and S( 2p) features for the corresponding peak areas

PDBTT C( 1s) (eV) FWHM S(Zp,J (eV) FwHM

Undoped 1.20 I .09 SO&F;-doped 1.31 1.12

S(~P,,,) (eV) F%wM

I .02 1.10

S. Glenis et al. /Synthetic Metals 7.5 (1995) 213-221 219

Fig. 9. Temperature dependence of (a) electrical conductivity and (b) thermoelectric power of SO&F; -doped e-poly (DBTT)

K is illustrated in Fig. 9. The strong temperature dependence of the electrical conductivity indicates a thermally activated process. The d.c. conductivities are proportional to inverse of temperature and follow the expression:

The slope of the linear plot of log fl versus 1 /T gives an activation energy of 0.2 eV. The thermal activation energy is the average ionization energy required to promote a trapped carrier to the extended state and corresponds to the trap depth. Trapping levels are caused by the presence of disorder which causes localized states originating from defects. Charge carriers are able to be trapped and released by thermal emission from the localized states to delocalized states to support the electrical conduction. The origin of the localized states, which are responsible for the thermally activated transport process, is associated with either thermal emission from structural defects and/or traps [ 271, by hopping between the domain- like sub-organization of the polymer chains [ 281 and grain boundaries. We note that the conductivity of e-poly( DBTT) is similar to that of the soluble fraction of c-poly( DBTT) and lower than that of the corresponding high molecular weight fraction. We attribute this to the similar molecular weight

of the electrochemically prepared polymer and that of the soluble fraction of the chemically prepared material (M,-4300).

Thermoelectric power (TP) measurements are typically much less susceptible to artifacts arising from the resistive domain boundaries in the material because they are essen- tially zero-current measurements and temperature drops across such boundaries are much less significant than voltage drops. The thermopower data of the corresponding SO&Y--doped sample (Fig. 9(B)) show a positive See- beck coefficient at 300 K which decreases at low temperatures. This is characteristic of a metal-like system in which hole conductivity (p-type) is dominant, as expected from this partially oxidized polymer. Similar behavior was observed in c-poly( DBTT) [ lo].

3.5. Magnetic properties

The e-poly (DBTT) was studied by magnetic susceptibility measurements and ESR in the doped and undoped state. Fig. 10 displays magnetic susceptibility data versus temperature for SO&F--doped e-poly (DBTT) . The susceptibility can be decomposed into the temperature-independent Pauli susceptibility for temperatures above 100 K and a Curie- Weiss contribution which becomes dominant at lower temperatures. At room temperature the magnetic susceptibility is 4.4X 10e5 e.m.u. mol-’ (for one DBlT unit). The temperature-dependent contribution, which is due to free radicals on the backbone acting as defects, is smaller than previously obtained on other polythiophenes [ 291 and this is consistent with a longer range of order in e-poly( DBTT) [ 301.

The room-temperature ESR spectra of the undoped and the CF,SO;-doped e-poly(DBTT) are shown in Fig. 11. The

100 .

P

SO,CF,’ -doped

B 80- 3

k

3 - 60-

s!

s ‘; E 2 40 .

Q

20- ??

. ?? . . . .

OO I I I I I

100 200 3 IO

T(K) Fig. 10. Temperature dependence of magnetic susceptibility of SO,CF;- doped e-poly( DBTT).


(a) Undoped I’DSIT

(b) SC$CFjdo~ed PDBT?

I I I I

D 3470 3490 3510 3530

Gauss Fig. 11. Room-temperature ESR spectra of e-poly( DBTT) : (a) undoped; (b) CF,SO,--doped.

spin concentrations calculated from the data are similar to those found for c-poly( DBTT) . The lineshapes of the undoped and doped samples are comparable and intermediate between Gaussian and Lorentzian. The peak-to-peak line- widths AH,, are 6.2 G for the undoped and 6.3 G for the doped polymer. These A Hpp values are comparable to those obtained from undoped polythiophene at 5 G [ 3 I]. The undoped and doped e-poly( DB’IT) films have g values of 2.0010 and 2.0012, respectively, indicating that the species responsible for the ESR signal originate principally from unpaired electrons in the conjugated n-electron system of the carbon backbone. Furthermore, the absence of a g-value anisotropy indicates that charge transport occurs along the n-conjugated system of the carbon backbone and that the sulfur atom con- tributes negligibly to the spin resonance [ 321. The fact that the ESR signals of the undoped and doped e-poly(DBTT) are almost identical, and that the charge transport measurements of doped e-poly(DBTT) have shown an increase of 12 orders of magnitude in electrical conductivity over that of the neutral polymer, supports the formation of polaron and bipolaron states with localized intragap levels. In the neutral or low doped state, polaron (spin l/2) energy states arise in intragap localized levels. At higher doping levels the polaron states combine with each other to form spinless bipolaron states localized again within the gap. This explanation is in good agreement with theoretical calculations [ 23,261 which predict the formation of bonding and antibonding bipolaron bands within the band gap of the polymer.

4. Concluding remarks

The novel monomer 3,4-dibutyl-cy-terthiophene allows the electrochemical and chemical formation of a soluble and geometrically symmetric polymer. The appropriate electrochemical conditions which allow both oxidation (p-doping) and reduction (n-doping) were determined. The basic thiophene rings of the monomer and alkyl groups are retained within the polymeric structure. The surface morphology of the deposited films is independent of the doping state of the polymer. The building block DBTI sub-structures are found not to be affected by structural disorder within the polymer. Optical absorption, IR, XPS and ESR spectroscopy were employed to investigate the chemical structure and electronic properties of this polymer. Electrochemical, optical and XPS studies show that the band gap of the undoped polymer is 2.03 eV. The ionization potential and electron affinity are similar to those of the unsubstituted polythiophenes. The electrical conductivity varies by a factor 10” between the neutral-undoped and the oxidized-doped states. Charge transport is interpretedin terms of bipolaron states localized within the band gap.

Acknowledgements

Financial support from the National Science Foundation (DMR-93-06385) is gratefully acknowledged. This work made use of the SEM facilities of the Center for Electron Optics at Michigan State University. At Northwestern Uni- versity this work made use of Central Facilities supported by the NSF through the Materials Research Center (DMR-91- 20521). M.G.K. thanks the A.P. Sloan Foundation for a Fel- lowship; M.G.K. is a Camille and Henry Dreyfus Teacher Scholar 1993-95.

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