Spectroscopic studies of the novel conducting polymer polytetrafluorobenzo-c-thiophene

Synthetic Metals, 55-57 (1993) 281-286

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SPECTROSCOPIC STUDIES OF THE NOVEL CONDUCTING POLYMER PQLYTETRAFLUQROBENZO-c-THIOPHENE M. J. SWANN, G. BROOKE* and D. BLOOR Dept. of Physics, Chemistry, Durham University, South Road, Durham DH1 3LE, England. J. MAHER Dept. of Chemistry, University of Bdstol, Cantocks Close, Bdstol BS8 1TS, England.

ABSTRACT Tetra-fluorinated benzo-c-thiophene (or Isothianaphthalene) polymer of length distributed about 40 monomer units was studied over a wide range of oxidation levels using UV-visible-NIR and ESR spectroscopy. This soluble polymer proves to be a model system for the study of charged species on conducting polymers, free from the complications of branching, bulk confinement or inter chain interaction. Charging the oligomers can be seen to take place with carders ranging between isolated radical cations (polarons) and interacting dicationic species (bipolarons). INTRODUCTION Over the last decade considerable work has been done in the field of conducting polymers. One of the lowest bandgap materials so far produced has been polybenzo-c-thiophene (aka. polyisothionaphthalene PITN)(1). The properties of this material have been studied using a wide variety of techniques (2), as well as being the subject of theoretical work dealing with it and related compounds (3). This paper deals with the PBcT analogue PTFBcT. PTFBcT is surprisingly soluble in chlorinated as well as other organic solvents and the solution oxidation has been followed using UV-visible-NIR and ESR spectroscopies. The degree of doping has been ascertained using electrochemistry. EXPERIMENTAL PTFBcT was synthesised chemically and the synthesis is described

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elsewhere. The polymedsation has also been achieved electrochemically (4), though the compound Q was not initially identified as PTFBcT on account of its great solubility. The PTFBcT used had a broad distribution of chain lengths with molecular weight values varying according to column used, but in the region 40-100 units long. Acetonitdle and Dichloromethane were HPLC grade,

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PTFBcT

© 1993- Elsevier Sequoia. All rights reserved

282 distilled under nitrogen from calcium hydride and phosphorous pentoxide respectively. Tetraethylammonlum perchlorate (TEAP) was recrystallised to purify and vacuum oven dried at 80°(3.

Ferric chloride (anhydrous) was used as supplied by Alddch. All solutions were

deoxygenated with nitrogen or argon before use. Electrochemical experiments were done using films cast from dichloromethane solution onto ITO glass electrodes. The counter electrode was platinum/rhodium wire, and the reference a silver/silverchloride electrode (-0.04V vs. SCE). A PAR 273 potentiostat was used. The electrolyte was acetonitrile / 0.1M TEAP. UV-visible-NIR spectra were obtained using a Perkin-Elmer Lambda 19 spectrometer and lcm pathlength glass cell with dichloromethane reference. Each sample was made with the cell in place under a blanket of nitrogen in such a way that the polymer concentration was constant throughout. 9.1"10"4M monomer units in this case. ESR experiments used a Brucker ESP300E spectrometer (spectra measured at X-band 9.SGHz at 3480G approx.) and flat sealable Quartz ESR cell. This enabled UV-visible spectra to be taken of each sample. The solutions and samples were made-up in a glovebox and cell sealed to ensure sample stability and reproducibility. The ESR power used was 101mW or 20mW. The spin density was calibrated using DPPH solutions in the same cell. RESULTS Uv-vislble-NIR soectroscoov of the P-dooino of PTFBcT. The UV-visible spectrum of neutral PTFBcT is the 0% doped line shown in figure1 and shows an absorbtion maximum at 485nm(2.7eV) with extinction coef. of 4.56"106(mol monomer)'lcm 2 and bandgap of 2.1eV. This is quite disparate from PBcT (bandgap 1.2eV(2)) for which substituents on the benzene ring have been calculated to not greatly affect the bandgap; the effect of four

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283

fluorines is however more extreme than the substituents used in the calculations(3).

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bandgap may also be a result of increased sterdc hindrance. Oxidation of PTFBcT in dichloromethane solution was achieved with anhydrous ferric chloride, which is soluble to a low level in dichloromethane. The general oxidation reaction is given by: 2FeCI3 + Polymer = Polymer + + FeCI4" + FeCI2

(from ref.5)

The electrochemically estimated formal oxidation potential of PTFBcT E0/+=l.17V, thus it is surprising that FeCI3 can oxidise PTFBcT to the high levels achieved. This is a result of the nonsolvated, non-hydrated ferric ion having a significantly higher redox potential than the hydrated form. Indeed the oxidising power of the ferric solution was strongly dependent on its water content. This was the primary reason for the measures to exclude moisture from the samples. Oxidation using acetonitdle as a solvent was unsuccessful also due to its interaction with ferric chloride. Due to the problems of totally excluding water, the amount of ferric chloride added could only be used as a semi-quantitative measure of extent of oxidation. The oxidation level was obtained using in-situ electrochemical UV-visible spectroscopy on a known quantity of polymer cast into a film. The absorption at 800nm was followed as the film was partially oxidised, by ramping the electrode potential, and the charge measured. Using this method the highest doping level achieved using FeCI3 was estimated to be 100%, or one charge per thiophene unit. Figure 1 shows the evolution of the UV-visible-NIR spectnJm on oxidation. Figure 2 also shows the change of absorbtion at selected wavelengths with amount of ferdc chlodde added (oxidation state). The peak at 485nm corresponding to the ~-~" transition of the neutral polymer can be

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Between 7 and 20% oxidation the absorption at 500nm

284

decreases (and that at 800nm Increases) more slowly. On the spectra the two lower energy peaks are shifting to higher energy. This is most dramatic for the lower energy peak which starts at above 1600nm and finishes at 950nm. Above 20% doping the neutral PTFBcT peak has completely disappeared and the absorbtion at 800nm also starts to decrease despite the continuing increase at 950nm.The increase in absorbance below 500nm is attributable to FeCI3 absorbtion as well as small amounts of scattering at high oxidation levels. These changes can be viewed on a spectral level more clearly by subtracting successive spectra. This is shown in figure 3. Between 0 and 2% and 2 and 7% doping a loss of absorption at 500nm and corresponding

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increase of the two lower energy peaks can be seen. Between 7 and 20% there is a small loss at 500nm but also a loss at 1700nm, and the increase of two peaks at higher energy than before. Above 20% doped there is an increase of only one peak at 1000-950nm and a loss at both lower and higher energies. The loss at low energies with increasing oxidation would indicate that the lower energy peak is shifting to higher energy (as opposed to just more peaks appearing at higher energy). Plotting the peak positions obtained by successive subtractions with oxidation level (not shown) indicates that the single peak at high oxidation levels is a result of the continuation of that same process (le. Is a result of the same transition as the lowest energy peak at lower oxidation levels). The higher energy peak (800-900nm) would also appear to shift to higher energies, but decreases in intensity above 20% doped. It does not however completely disappear even when the polymer Is approximately 100% doped. There is possibly a small broad peak at 550nm formed upto 20% doping, visible by its loss in the difference spectra, though if present it is masked by the stronger absorptions to either side.

285

ESR of o-dooed PTFBcT. The ESR spectrum of PTFBcT at 2% doping level is shown in figure 4b. The signal is centred at 2.0044g with Ilnewidth 3.2G and is typical of an organic free radical.

There is a broad

background signal (150G) attributed to paramagnetic FeCI2. Figure 4a shows the change of spin intensity and linewidth with oxidation state. The maximun spin intensity occurs at approximately 2% doping and decays with a l/oxidation state dependence The intensity of 4.6"10 -6 molar equivalents of DPPH(about 1 spin per 200 monomer units) is a quarter of the number of charges put on the polymer. The broad distribution of chain lengths will mean that shorter chains may not be charged and longer ones may be multiply charged. As a result this value would imply initial oxidation takes place largely via the single charging of polymer chains with the formation of polarons. The linewidth remains at between 3 and 4G upto about 4% doping and then increases significantly reaching 11G at 20% doping. At this level and beyond the low and decreasing signal intensity coupled with line broadening makes measurement unreliable.

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Field / Gauss 4 b: ESR signal for PTFBcT at 2% doped

DISCUSSION The decrease of spin intensity from the maximum at about 2% can be attributable to the formation of spinless bipolarons or coupling of polarons as could be envisaged from the polymer becoming more metallic in nature (6). The extent to which the spin intensity decreases, ie. to about 1/5 of its original level is not initially predicted by either model however. The reason must be a result of increasing levels of oxidation changing the relative stability of the possible charge storage states. That this may be taking place is demonstrated by the marked shift in the optical bands with increasing doping. In terms of the bipolaron model which would seem more likely: initially the

286 formation of polarons on separate chains takes place. As more charges are put on each chain the formation of bipolarons becomes more favourable and bipolarons are formed on single chains rather than polarons on pairs of chains. Thus the number of polarons decreases to the extent observed. The presence of polarons on multiply charged chains is indicated by the broadening of the ESR signal (observed previously to be dependent on oxidation level (6)). It is not clear from our data if the broad signal is present throughout (due to discontinuities in the chain) or represents evolution of the ESR signal due to increasing interaction of the spins with the bipolarons present, though the latter is suggested by (6). No feature observed during the evolution of the optical spectra are obviously correlatable with the intensity of spins observed in the ESR spectra. This may be due to the double peaks being due to coupled polarons which form bipolarons above 20% doping with only one transition, this may be expected from data in (5), but not the expected case however. More likely, the expected transitions are too weak or absent. Thus the use of the UV-visible spectra to differentiate polarons and bipolarons would seem to be difficult, and probably the cause of the wide discrepancies found in the literature (6,8). The shift of the UV-vis. peaks from the very onset indicates that the charge carriers can "see" the whole extent of the chains and start interacting as soon as two charges are present on a chain. This must account for the rapid fall-away of the spin intensity with oxidation state and suggests that the polymer is able to extend fully when doped in solution. In a solid film or under constrained conditions this would not be the case, as interactions and bent sections of chain would stabilise polarons to much higher levels. Solution data presented in (7) agrees with this, with very low spin intensities at low concentration and a manyfold increase at intermediate concentrations where polymer extension is probably inhibited. Spectra for the low concentration region were not shown however. The reason for the single peak at high doping levels, which has been attributed to metalic "free carriers" is not entirely clear at the moment, though it has been shown to be a continuation of a transition already present. ACKNOWLEDGEMENTS We thank the SERC and universities of Bristol and Bath for funds to purchase the ESR equipment. Thanks also to A. P. Monkman for helpful discussions. REFERENCES 1 M. Kobayashi, N. Colaneri, M. Boysel, F. Wudl and A.J. Heeger, J. Chem. Phvs. 82 (1985) 5717. 2 S. M. Dale, A. Glidle and A. R. Hillman, J,~tg£J~t£[1,~2 (1992) 3 J. L. Bredas, A. J. Heeger and F. Wudl, ~1.Cherq, phys. 85 (1986) 4673. 4 G. M. Brooke and S. D. Mawson, J. Chem. Soc. Perkin Trans.. 1 (1990) 1919. 5 D. Fichou, G. Horowitz, B. Xu and F. Gamier, Svnth. Met.. 39 (1990) 243. 6 M. Schaerli, H. Kiess, G. Harbeke, W. Berlinger, K. W. Blazey and K. A. Mueller, Svnth. Met.. 22 (1988) 317. 7 M. J. Nowak, S. D. D. V. Rughooputh, S. Hotta and A. J. Heeger, Macromol.. 20 (1987) 965. 8 A. O. Patil, A. J. Heeger and F. Wudl, Chem. Rev.. 88 (1988) 183.