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Makromol. Chern. 188,1833 - I839 (1987) 1833

Electrochemical synthesis of poly(1 ,I-phenylene) films

Jean-Francob Fauvarque), Michel-Alain Petit &), Abdelouafi Digua

Universitt Paris-Nord, Laboratoire Recherche Macromolecules, UA 502, Avenue J. B. Cltment, 93430 Villetaneuse, France

Gerard Froyer

Centre National d'Etudes des Ttlkommunications, LAB-ROWTIC, B.P. 40, 22301 Lannion, Cedex, France

@ate of receipt: November 21, 1986)

SUMMARY: Homogeneous films of poly(l,4phenylene) (PPP) can be deposited on various materials by

electro-reduction of 4,4'-dibromobiphenyl in presence of a nickel complex catalyst. UV and IR spectra indicate that the polymer has a regular structure (exclusively 1,4phenylene units) and an unusual average molecular weight. The nascent PPP films are obtained in their neutral form and can be either reduced or oxidized.

Introduction

Conjugated polymers are generally insoluble materials owing to their cristallinity and backbone rigidity. Therefore, chemical techniques are inefficient to produce films, except when soluble polymeric precursors' -3) or conducting polymer solutions4) can be prepared. Since films are more suitable than powders for both basic studies and technical applications, there have been recently many reports on films deposits obtained by electropolymerization of various aromatic monomers. Among the most frequently used monomers are pyrrole'*@, thi~phene'.~) and aniline9), but many others have been eiectropoiymerized such as N-vinylcarbazolelo), furan"), pyridazine"), ann~lene'~), azulene13), ~yridine'~), isothianaphthene''), thieno[3,2-b]- pyrrolei6).

Only a few similar attempts to prepare poly(l,4phenylene) (PPP) films have been described though this polymer is attractive for several reasons, such as stability of the neutral polymer, high conductivities of p- or n-doped samples") and high potentials in batteries using this polymer as an electroactive materialt8). Polyphenylene deposits have been obtained by electrochemical oxidation of benzene or biphenyl in acetonitrile"), in HF/benzeneZO) or HF/benzene/SbCJ 21)

systems, in liquid Sq") and in nitrobenzeneZ3). However, the films obtained had irregular structures and sometimes contained oxygen atoms in their chemical formulae. It has also been shown that the electrochemical reduction of an arylnickel derivative of 1,4dibromobenzene results in a poly(l,4phenylene) coating which unfortunately contained nickel according to the formula AU). Recently, it has been claimed that a flexible PPP film is obtained by electro-

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1834 J.-F. Fauvarque, M.-A. Petit, A. Digua, G. Froyer

chemical polymerization of benzene in the system benzene/nitrobenzene/CuC& /L~ASF~’~). Thin films of PPP can also be prepared by the method of Kovacic using a special reaction appara- tusZ6) or by evaporation of PPP powder”).

Previously, we reportedB) on the catalytic electrosynthesis of PPP powder by electro-reduction of 4,4’-dibromobiphenyl (Br-(SH&-Br) in the presence of NiCI, (dppe) (dppe = bis( 1,2-diphenylphosphinoethane)). We now report that this techni- que allows to produce homogeneous films of pure PPP having a large average chain length and a regular structure, i.e., a strictly linear polymer containing exclusively 1,4phenylene repeating units.

The present paper reports on the experimental conditions for PPP deposition in our system and gives a brief description of the film properties.

Experimental part

Reagents: All solvents were distilled prior to use, except acetonitrile (“Car10 Erba, polaro- grafia grade”) which could be used as received. Tetrahydrofuran (THF) was distilled under argon from naphthalene-sodium, and hexamethylphosphoric triamide (HMPT) was distilled twice under vacuum from lithium aluminium hydride. Lithium perchlorate (LiClO,) and lithium tetrafluoroborate (LiBF,) were dried overnight under vacuum at 160°C. Tetrabutyl- ammonium tetrafluoroborate (nBu.,NBF ) and the catalyst NiCl,(dppe) were prepared following the procedures previously described&). The monomer, 4,4’dibromobiphenyl, was a com- mercial product (from Janssen Chimica).

Electrodes: Foils or rods of lithium (from Johnson Matthey S.A.) were generally used as “sacrificial” counter-electrodes though anodes in magnesium, zinc or even platinum gave similar results. The cathode (10 mm x 30 mm) could be either a sheet of gold, zinc, nickel, copper, glassy carbon or a conducting glass (glass coated with IT0 or SNQ). The reference electrode was a silver wire, plunging in a 0,l M AgClO, solution in THF.

Cell and instrumentation: The electrochemical polymerization was wried out at room temperature, in an undivided three-electrode cell under argon. The tip of the reference electrode was placed near by the working electrode which was facing the counter-electrode at a distance of about 10 mm. The same type of cell was used for the study of the electrochemical properties of the PPP films in either pure THF or acetonitrile.

UV-VIS and IR spectra were run using respectively a “Varian 118” and a “Bruker IFS 114” spectrometer.

Micrographs of the films surface were obtained with a “Cambridge S 100” scanning electron- microscope.

Electrochemical experiments were conducted using a “Tacussel” instrumentation which consisted of a “PRT 20-2 2” potentiostat and a “IGSN” coulometer.

Polymerization procedure: THF/HMPT (vol. ratio 7 : 3) and LiClO, were chosen as solvents and electrolyte, since we had previously shownB) that these conditions gave the best results for the preparation of PPP powders.

The 100 ml typical solution to be electrolyzed was a mixture of THF/HMPT (vol. ratio 7 : 3) containing 30 mmol of LiClO,, 1 mmol of 4,4‘-dibromobiphenyl and 0,25 mmol of NiCl,(dppe). A potential of -2,5 V was applied to the working electrode. The current decreased sharply and then fell down slowly giving an average current density value of about 1 mA . cm-2. The electrolysis was stopped after the integrated charge (Q) passed was about 0,l C . cm-*. The films were thoroughly rinsed by THF and then dried at 60°C.

Because of limited amounts of material, the elemental analysis of homogeneous films could not be carried out. Nevertheless, the nickel content was determined by spark ion mass spectro- metry using a “CAMECA-IMS 3 F”, and was found lower than 0,1%. The spectroscopic properties (UV and IR spectra) of the films agree well with the expected PPP structure which was previously established for the powders obtained under closely related conditions2*).

Electrochemical synthesis of poly(1 ,rl-phenylene) films

Results and discussion

Poly(l,l-phenylene) deposition

1835

An analytical current-potential curve for a glassy carbon disk (diameter: 3 mm) in the reaction mixture, is shown in Fig. 1 (curve (a)). There is a sharp rise of the current at ca. -2,5V which is due to the reduction of an aryl nickel complex supposedly Br-(GH, h-Ni(dppe)Br. As soon as produced, the reduced form undergoes coupl- ing reactions leading eventually to the polymera). The height of the cathodic peak at -2,5V is linearly dependent on both the square root of the scan rate (0,Ol-0,s V - s-1 ) and the dibromide concentration. It is also linearly dependent on the catalyst concentration in so far as the ratio R of concentrations of catalyst to dibromide is lower than =0,25. This is suggestive of a catalytic process which is controlled by the dibromide diffusion.

Similar current-potential curves (Fig. 1 b) were obtained for large glassy carbon or IT0 electrodes at a low scan rate. At the end of the negative step, a pale-yellow deposit covered the electrode surface.

Several electrolyses were performed by applying to the working electrode a potential ranging from - 2,2 to - 2,8V. It was found that the electrode surface remained unchanged when operating at too high (- 2,2V) or too low (- 2,8V) potential. On the other hand, regular films were prepared at cathodic potentials between -2,4 and - 2,6V, i. e., at a potential corresponding to the reduction of the supposed arylnickel complex. Therefore, all the following experiments were carried out at - 2,5 V.

The value of R (ratio of concentrations of catalyst to dibromide) was varied over 0 to 0,5. No polymer deposit could be formed when the catalyst was absent from the electrolyzed solution. With R values lower than 0,15, yellow deposits were obtained

a E c LO

2.4 Fig. 1. Cyclic voltammo- grams of the mixture: 100 ml of THF/HMPT (vol. ratio 7: 3), 30 mmol of LiCIO, , 1 mmol of Br-(GH,h-Br, 0,25 mmol of catalyst NiC12(dppe). (- - -) (a): Glassy carbon disk (7 mm2), (0,l V . S - ' ); (-) (b): glassy carbon plate (300 mm2), ( 0 , 0 1 o V ~ s - ' )

.i' 0

0 -1 -2 -3 € i n V


showing a rough surface and a powder-like aspect. On the other hand, true films were prepared with the R parameter set at 0,5. However, under these conditions, we could observe occasionally a few dark spots on the film surface which we attributed to some nickel deposits. Therefore, electrolyses were performed with an intermediate R value of 0,25. It was, thus, possible to prepare pale yellow fims with regular smooth surfaces.

The morphology of the PPP deposit was also largely influenced by the charge passed during the experiment. We found that when increasing the charge Q, the PPP layer changed progressively from a regular film to a powder-like deposit, covering the initial film. Accordingly, the charge Q was limited to =O,l -0,2 C *cm-2 and films could be prepared with thickness in the range 0,l to 1 pm.

Other factors, such as temperature (0 - 60 "C) and cathode roughness (surfaces of glassy carbon electrodes were ground flat with 180 - 1 200 A grit carborundum) were found of minor influence on the morphology of the PPP film.

Attempts to prepare homogeneous PPP films galvanostatically at 1 and 0,2 mA * cm-2 failed surprisingly. Even at the lowest current density, the cathodic potential shifted rapidly to potentials lower than - 3,OV which do not allow the deposition of PPP.

Properties of PPP f i l m

As expected for a polymer with a high backbone rigidity, the film could not be peeled off without damage. The surface of the film was investigated using a scanning electron microscope. The film appeared to result from a compact collection of small clusters several thousands A in diameter. The micrographs closely resemble previously reported micrographs of electrochemically prepared polythiophene') and polypyr- rolez9). More details on the morphology and crystallinity of the films will be published later.

The UV-visible absorption spectrum of an as-grown PPP film on an I T 0 substrate is shown in Fig. 2. A sharp rise of the absorption is observed above 2,6 eV, indicating a high regularity of the chemical structure of the PPP film. Since the maximum of absorption occurs at = 3,2 eV, the presumption is that the polymer has a high average molecular weight. The presence of a shoulder at 3 eV has been discussed elsewhere30) and may arise from the contribution of the longest chains.

The reflectance Fourier Transform IR spectrum of a poly(1 &phenylene) film deposited on an IT0 substrate is shown in Fig. 3. The main features are the position (805 cm-' ) of the C-H out-of-plane vibration, characteristic of para-substituted phenyl, and the weak intensities of the absorptions at 768 cm-1 (C-H out-of-plane) and at 690 cm-I (ring deformation) of monosubstituted phenyl. Since the electrochemically prepared PPP has a low bromine content2*), the IR spectrum is also indi- cative of high chain length.

The electrochemical properties of the PPP films were investigated by cyclic voltam- metry. Freshly prepared films were copiously rinsed by dry THF and then dipped in various media. For example, a THF solution containing 0,3 mol/l of BkNBF, was used for the electrochemical reduction, whereas the electrochemical oxidation was

Electrochemical synthesis of poly(1 J-phenylene) films 1837

A in nrn 620 113 310

Fig. 2. UV-VIS spectrum of an as- grown film

2 3 L Energy in eV

805

I I I I I I I 1200 1100 1000 900 800 , 700 600

Wave number in cm-l

Fig. 3. Reflectance FTIR spectrum of an as-grown film

conducted in an acetonitrile solution with 0,3 mol/l of LiBF, . A typical reduction profie of the PPP film is shown in Fig. 4@). When using scan rates in the range 20 - 200 mV * s - l , a reduction peak is not found indicating that some of the reducible sites of the polymer remain unchanged. Since at low scan rate (1 - 5 mV * s - l ) a peak


Fig. 4. Electrochemical properties of a PPP film. (a): Oxidation in CH3CN/0,3 M LiBF4 ; @): reduction in THF/0,3 M BqNBF4

was indeed observed, this behaviour is suggestive of kinetic limitations arising probably from the slow diffusion of ions in the material. In the backward scan, and whatever the scan rate is, a peak is detected with a maximum at - 2,6V. The cyclic voltammogram in Fig. 4(a) indicates that p-type doping on the PPP film can also be performed. The oxidation of the polymer begins at 0,SV and is not achieved when the potential scan is reversed. A backward peak is observed at 0,SV.

The pale-yellow colour of the neutral PPP film changes to purple during n-type doping and to blood-red during p-type doping. The evolution of the absorption spectra upon applied voltages is described elsewhere”). The results offer an insight into the electronic band structure of pure poly(1,4phenylene) and its reduced and oxidized forms.

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