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Radiochromic properties of α–terthiophene–cellulose triacetate films
Radiation Physics and Chemistry 57 (2000) 707±710
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Radiochromic properties of a±terthiophene±cellulose triacetate ®lms S.S. Emmi a,*, P. Ceroni a, M. Lavalle a, P.G. Fuochi a, M. D'Angelantonio a, A. Alberti b, D. Macciantelli b, A. KovaÂcs c, E. TakaÂcs c a
Istituto di Fotochimica e Radiazioni di Alta Energia (FRAE), CNR, Via P. Gobetti 101, 40129, Bologna, Italy b Istituto dei Composti Contenenti Eteroatomi (ICoCEA), CNR, Via P Gobetti 101, 40129, Bologna, Italy c Institute of Isotopes and Surface Chemistry (IISC), HAS, H-1525, Budapest, P.O. Box 77, Hungary
Abstract An a-terthiophene/cellulose triacetate ®lm system exposed to g-rays and electron beam shows two new bands in the visible region of the spectrum. The absorbance at 490 nm is almost stable for a long time and responds linearly to the dose applied up to 400 kGy. A second band developed at 700 nm depends on dose rate and may indicate a chain-growth. The main species responsible for the color change and EPR signal is likely to be an enlarged oligomer radical cation stabilized by a semiquinoid structure. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Oligothiophenes; Radiochromic ®lms; Solid state dosimeters; Persistent radical cations
1. Introduction Organic conjugated oligomers and polymers of acetylene, pyrrole, aniline and thiophene have found applications in a number of electronic and optoelectronic devices (Friend, 1996; Garnier, 1997 and references therein reported). Substituted diacetylenes have also found applications in commercial radiochromic dosimeters (GafChromicTM) and the species responsible for the color response to irradiation have been characterized as polyconjugated linear molecules formed through radiation-induced solid state polymerization (McLaughlin et al., 1996). In this respect oligothiophenes are of a remarkable
interest because of the dramatic change in their electrical conductivity and color brought about by oxidation to radical cations or dications. Despite the substantial body of work in the ®eld of thiophene derivatives by means of various methods, few papers deal with the eect of ionizing radiation on polythiophene ®lms (Hayashi et al., 1986; Lavalle et al., 1999). This work aims to describe the conditions under which ionizing radiation can carry out a simultaneous oxidative process and chain-growth of terthiophene in cellulose triacetate ®lms. Further to that, the relationship between new induced absorbance bands and irradiation dose is also described. 2. Experimental methods
* Corresponding author. Tel.: +39-051-6399774; fax: +39051-6399848. E-mail address: [email protected] (S.S. Emmi).
Films of 20±40 mm thickness were prepared from a CH2Cl2 (Merck pro analysi) solution of cellulose tria-
0969-806X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 9 9 ) 0 0 4 3 7 - 5
708
S.S. Emmi et al. / Radiation Physics and Chemistry 57 (2000) 707±710
cetate (TA) (Fluka purum, MW 72.000±74.000) containing a-terthiophene (2,2 ':5 ',20-terthiophene)(3T) (Aldrich, 99%). The material obtained is hereafter referred to as 3T±TA. Temperature and humidity were controlled in the range 18±228C and 40±50%, respectively. All preparations and experiments have been carried out in air. The UV-vis absorption spectra of non irradiated ®lms maintained the characteristic bands typical of the isolated oligothiophene molecules as shown in a CH2Cl2 solution, i.e. with maximum at l=355 nm. EPR spectra were recorded on an upgraded Bruker ER200D X-band spectrometer at the ICoCEA Institute. Irradiations were performed with the 60Co g-cell and with the 12 MeV electron linear accelerator at the FRAE Institute. Dose rates were changed by a factor of 1000, in the range of 30.7 Gy/ min (gamma irradiation) to 49.6 kGy/min (electron irradiations). In the latter case 2 msec pulses were used (an 0.5 mm Al foil was interposed in front of the accelerator), while the dose rate was changed by changing the pulse frequency from 2 to 50 pulses per sec. The dose per pulse was measured with the standard Fricke or super-Fricke solution, in the same conditions and geometry used for the irradiation of the 3T±TA ®lms. After 710 kGy gamma and 400 kGy electron irradiation TA remained fully transparent beyond 340 nm, so that absorbance variations in the visible region are entirely due to the transformation of 3T. Actually, TA remains mostly transparent in the UV region too, so that it constitutes a suitable support to monitor the bleaching of the typical absorption of 3T during its consumption. Films become hard and fragile at doses higher than 700 kGy. Irradiated ®lms were stored in the dark. Where not explicitly stated, the ®rst spectral measurements were done about 5 min after irradiation. Monomer concentrations are reported in % weight with respect to TA.
appearing both after g- and electron-irradiation and peaking around 490 nm (Fig. 1). The eciency of color production, evaluated through its absorbance growth rate, increases approximately ®ve times by passing from the 0.5% to the 6% ®lms. A consistent concentration dependence has also been veri®ed for the 12% 3T±TA system; however, in this case non-homogeneous color areas are formed on the ®lm, so that these measurements are less reliable. Therefore, in the following discussion we will focus on 6% ®lms. Fig. 2 exempli®es that the absorption grows linearly with doses up to 400 kGy. The same ®gure also shows that absorbance values depend on the dose rate employed; in fact for g-irradiation (dose rate 30.7 Gy/min) the absorbance change, between the irradiated sample (A ) and the pre-irradiated one (A0), is A ÿ A0 g 1:6820:03 10ÿ3 D while for electron beam treatment (dose rate 39.6 kGy/ min)
3. Results and discussion 3.1. Radiation response At 0.5% concentration, the consumption of 3T can be monitored at 355 nm, and is found to be linear with doses up to 150 kGy. In the same range two isosbestic points are distinguishable at 310 and 400 nm, indicating a simple conversion of 3T into products absorbing at lower energies. However, the build-up of new absorptions in the visible is very weak at low doses. The eciency of the radiochromic process is substantially improved by increasing the concentration of 3T to 6 and 12%. Although the consumption of the monomer cannot be monitored due to its own huge absorbance, the presence of a product or products absorbing at lower energy is con®rmed by a new band
Fig. 1. New absorption bands generated on a 6% terthiophene/cellulose triacetate (3T±TA) ®lm. (a) After g-irradiation and (b) after electron irradiation. Dose rate 30.7 Gy/min for g-rays and 39.6 kGy/min for electron beams. Doses as reported in the inset.
S.S. Emmi et al. / Radiation Physics and Chemistry 57 (2000) 707±710
Fig. 2. Radiochromic response of a 6% 3T±TA ®lm to g-rays and electron beams. Symbols: absorbances measured at 490 nm. Full lines: best linear ®tting. Dose rate 30.7 kGy/min for g-rays and 39.6 kGy/min for electron beams.
A ÿ A0 e 5:520:1 10ÿ4 D where D is the dose in kGy. This trend has been con®rmed by measurements at intermediate dose rates. The absorption intensity at 490 nm only undergoes minor changes after 1 month independently of the dose rate and radiation used. Applying electron pulses of high dose rate (39.6 kGy/min) a further wide band around 700 nm is produced immediately (Fig. 1b) but a threshold dose of 100 kGy is required. After the immediate growth, the intensity at 700 nm remains almost stable for a month. The formation of the band in this region is slowed down if the dose rate is diminished. And in fact, immediately after g-irradiation (lowest dose rate) it is not detected (Fig. 1a), but develops after some days and continues to grow for over a month. This growth follows a reasonable ®rst order kinetics for about three half-lives with a rate constant k = 3.0 10ÿ3 hÿ1, as shown in Fig. 3.
709
Fig. 3. Post-g-irradiation eect on a 6% 3T±TA ®lm. Black circles: absorbance values read at 700 nm. Full line: best linear ®tting. Dose=460 kGy. Dose rate=30.7 Gy/min. Inset: selected spectra around 700 nm from 3 to 57 days after irradiation.
ment. Moreover, two previous studies (Emmi et al., 1998, 1999) in the liquid phase showed that ionizing radiation induces oxidation of oligothiophenes to radical cations, the absorptions of which lie at lower energies than those of the parent compounds. The present data, and speci®cally the absorbance band centered at 490 nm and the EPR signal, indicate that radical cations are produced in the solid state as well. Radical cations then seem to undergo polymerization Ð or at least oligomerization Ð in the way proposed for solidstate linear systems (McLaughlin et al., 1996 and refer-
3.2. Nature of color centers EPR spectroscopy reveals the presence of a single and symmetrical line having a gfactor=2.0034 (Fig. 4), independently of the nature of radiation used. However, the signal on the ®lms irradiated with an electron beam is somewhat weaker than that pertaining to g-irradiation. The line persists for a long time after irradiation (over a month), and can be reasonably attributed to a large resonance stabilization of the unpaired electron along a conjugated chain. The gfactor corresponds well to that of a polymer obtained chemically by oxidative condensation of unsubstituted thiophene (Tormo et al., 1997) where charged species were stabilized by an acid environ-
Fig. 4. EPR spectra of 6% 3T±TA ®lms. (a) After g-irradiation and (b) after electron beam irradiation. Dose rate 30.7 Gy/min for g-rays and 39.6 kGy/min for electron beams. gfactor=2.0034 in both cases.
710
S.S. Emmi et al. / Radiation Physics and Chemistry 57 (2000) 707±710
ences therein reported), i.e. through a simple ®rst-order kinetics. The growth of the post-g-irradiation absorption, in the 700 nm region, may very well represent such a process. However, it should dierentiate from the one observed at the high dose rate, which is faster and produces a band the intensity of which is constant in the long run. In this latter case in fact, a radical± radical combination may take place, leading to the production of a dimeric dication. Although the nature of the species produced is not clear at the present stage, it can be remarked that dimerization cannot be faster than propagation in the experimental conditions used, otherwise no EPR signal should be detected at high dose rates. Further work has to be done to clarify this, nevertheless we observed that radiation products show a weak spin-orbit coupling with the sulfur atom, as has been detected for oligomer radical cations produced with chemical methods (Tormo et al., 1997). This provides an indication that the p electron system is mainly delocalized along an enlarged carbon chain, reinforcing a semiquinoid structure (Horowitz et al., 1994). Furthermore, the good linear dose response of the absorption generated at 490 nm up to 500 kGy encourages the investigation of this kind of material with regard to their application as high dose radiochromic dosimeters.
Acknowledgements A fellowship granted to P. Ceroni by the Innovative Material and Related Technologies Institute (MITER) of CNR is gratefully acknowledged. Work performed in the frame of the CNR-HAS Agreement Joint Projects No. 03/2 and 03/3.
References Emmi, S.S., D'Angelantonio, M., Poggi, G., Beggiato, G., Camaioni, N., Geri, A., Martelli, A., Pietropaolo, D., Zotti, G., 1998. The spectral characterization of thiophene radical cation generated by pulse radiolysis. Res. Chem. Intermed. 24, 1±14. Emmi, S.S., D'Angelantonio, M., Beggiato, G., Poggi, G., Geri, A., Pietropaolo, D., Zotti, G., 1999. The generation and spectral characterization of oligothiophenes radical cations. A pulse radiolysis investigation. Radiat. Phys. Chem. 54, 263±270. Friend, R.H., 1996. Semiconductor device physics with conjugated polymers. Phys. Scripta T66, 9±15. Garnier, F., 1997. Scope and limits of organic-based thin-®lm transistors. Phil. Trans. R. Soc. London Ser. A 355, 815± 827. Hayashi, S., Takeda, S., Kaneto, K., Yoshino, K., Matsuyama, T., 1986. Electrical and ESR studies on polythiophene by radiation-induced doping eect. Jpn. J. Appl. Phys. 25, 1529±1532. Horowitz, G., Yassar, A., von Bardeleben, H.J., 1994. ESR and optical spectroscopy evidence for a chain-length dependence of the charged states of thiophene oligomers. Extrapolation to polythiophene. Synth. Met. 62, 245±252. Lavalle, M., Camaioni, N., Casalbore-Miceli, G., Fuochi, P.G., Geri, A., 1999. Eect of the ionizing radiation on poly(4,4'-dipentoxy-2,2'-bithiophene) ®lms electrosynthesised on indium±tin-oxide plates, Radiat. Phys. Chem. 56, 303±308. McLaughlin, W.L., Al-Sheikhly, M., Lewis, D.F., KovaÂcs, A., WojnaÂrovits, L., 1996. Radiochromic solid-state polymerization reaction. In: Clough, R.L., Shalaby, S.W. (Eds.), Irradiation of Polymers. Fundamentals and Technological Applications. ACS Symposium Series, pp. 152±166. Tormo, J., Moreno, F.J., Ruiz, J., FajarõÂ , L., JuliaÂ, L., 1997. Thallium (III) tri¯uoroacetate-tri¯uoroacetic acid in the chemistry of polythiophenes. 2. Treatment of 3-alkylthiophenes and electron paramagnetic resonance results. J. Org. Chem. 62, 878±884.
www.elsevier.com/locate/radphyschem
Radiochromic properties of a±terthiophene±cellulose triacetate ®lms S.S. Emmi a,*, P. Ceroni a, M. Lavalle a, P.G. Fuochi a, M. D'Angelantonio a, A. Alberti b, D. Macciantelli b, A. KovaÂcs c, E. TakaÂcs c a
Istituto di Fotochimica e Radiazioni di Alta Energia (FRAE), CNR, Via P. Gobetti 101, 40129, Bologna, Italy b Istituto dei Composti Contenenti Eteroatomi (ICoCEA), CNR, Via P Gobetti 101, 40129, Bologna, Italy c Institute of Isotopes and Surface Chemistry (IISC), HAS, H-1525, Budapest, P.O. Box 77, Hungary
Abstract An a-terthiophene/cellulose triacetate ®lm system exposed to g-rays and electron beam shows two new bands in the visible region of the spectrum. The absorbance at 490 nm is almost stable for a long time and responds linearly to the dose applied up to 400 kGy. A second band developed at 700 nm depends on dose rate and may indicate a chain-growth. The main species responsible for the color change and EPR signal is likely to be an enlarged oligomer radical cation stabilized by a semiquinoid structure. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Oligothiophenes; Radiochromic ®lms; Solid state dosimeters; Persistent radical cations
1. Introduction Organic conjugated oligomers and polymers of acetylene, pyrrole, aniline and thiophene have found applications in a number of electronic and optoelectronic devices (Friend, 1996; Garnier, 1997 and references therein reported). Substituted diacetylenes have also found applications in commercial radiochromic dosimeters (GafChromicTM) and the species responsible for the color response to irradiation have been characterized as polyconjugated linear molecules formed through radiation-induced solid state polymerization (McLaughlin et al., 1996). In this respect oligothiophenes are of a remarkable
interest because of the dramatic change in their electrical conductivity and color brought about by oxidation to radical cations or dications. Despite the substantial body of work in the ®eld of thiophene derivatives by means of various methods, few papers deal with the eect of ionizing radiation on polythiophene ®lms (Hayashi et al., 1986; Lavalle et al., 1999). This work aims to describe the conditions under which ionizing radiation can carry out a simultaneous oxidative process and chain-growth of terthiophene in cellulose triacetate ®lms. Further to that, the relationship between new induced absorbance bands and irradiation dose is also described. 2. Experimental methods
* Corresponding author. Tel.: +39-051-6399774; fax: +39051-6399848. E-mail address: [email protected] (S.S. Emmi).
Films of 20±40 mm thickness were prepared from a CH2Cl2 (Merck pro analysi) solution of cellulose tria-
0969-806X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 9 9 ) 0 0 4 3 7 - 5
708
S.S. Emmi et al. / Radiation Physics and Chemistry 57 (2000) 707±710
cetate (TA) (Fluka purum, MW 72.000±74.000) containing a-terthiophene (2,2 ':5 ',20-terthiophene)(3T) (Aldrich, 99%). The material obtained is hereafter referred to as 3T±TA. Temperature and humidity were controlled in the range 18±228C and 40±50%, respectively. All preparations and experiments have been carried out in air. The UV-vis absorption spectra of non irradiated ®lms maintained the characteristic bands typical of the isolated oligothiophene molecules as shown in a CH2Cl2 solution, i.e. with maximum at l=355 nm. EPR spectra were recorded on an upgraded Bruker ER200D X-band spectrometer at the ICoCEA Institute. Irradiations were performed with the 60Co g-cell and with the 12 MeV electron linear accelerator at the FRAE Institute. Dose rates were changed by a factor of 1000, in the range of 30.7 Gy/ min (gamma irradiation) to 49.6 kGy/min (electron irradiations). In the latter case 2 msec pulses were used (an 0.5 mm Al foil was interposed in front of the accelerator), while the dose rate was changed by changing the pulse frequency from 2 to 50 pulses per sec. The dose per pulse was measured with the standard Fricke or super-Fricke solution, in the same conditions and geometry used for the irradiation of the 3T±TA ®lms. After 710 kGy gamma and 400 kGy electron irradiation TA remained fully transparent beyond 340 nm, so that absorbance variations in the visible region are entirely due to the transformation of 3T. Actually, TA remains mostly transparent in the UV region too, so that it constitutes a suitable support to monitor the bleaching of the typical absorption of 3T during its consumption. Films become hard and fragile at doses higher than 700 kGy. Irradiated ®lms were stored in the dark. Where not explicitly stated, the ®rst spectral measurements were done about 5 min after irradiation. Monomer concentrations are reported in % weight with respect to TA.
appearing both after g- and electron-irradiation and peaking around 490 nm (Fig. 1). The eciency of color production, evaluated through its absorbance growth rate, increases approximately ®ve times by passing from the 0.5% to the 6% ®lms. A consistent concentration dependence has also been veri®ed for the 12% 3T±TA system; however, in this case non-homogeneous color areas are formed on the ®lm, so that these measurements are less reliable. Therefore, in the following discussion we will focus on 6% ®lms. Fig. 2 exempli®es that the absorption grows linearly with doses up to 400 kGy. The same ®gure also shows that absorbance values depend on the dose rate employed; in fact for g-irradiation (dose rate 30.7 Gy/min) the absorbance change, between the irradiated sample (A ) and the pre-irradiated one (A0), is A ÿ A0 g 1:6820:03 10ÿ3 D while for electron beam treatment (dose rate 39.6 kGy/ min)
3. Results and discussion 3.1. Radiation response At 0.5% concentration, the consumption of 3T can be monitored at 355 nm, and is found to be linear with doses up to 150 kGy. In the same range two isosbestic points are distinguishable at 310 and 400 nm, indicating a simple conversion of 3T into products absorbing at lower energies. However, the build-up of new absorptions in the visible is very weak at low doses. The eciency of the radiochromic process is substantially improved by increasing the concentration of 3T to 6 and 12%. Although the consumption of the monomer cannot be monitored due to its own huge absorbance, the presence of a product or products absorbing at lower energy is con®rmed by a new band
Fig. 1. New absorption bands generated on a 6% terthiophene/cellulose triacetate (3T±TA) ®lm. (a) After g-irradiation and (b) after electron irradiation. Dose rate 30.7 Gy/min for g-rays and 39.6 kGy/min for electron beams. Doses as reported in the inset.
S.S. Emmi et al. / Radiation Physics and Chemistry 57 (2000) 707±710
Fig. 2. Radiochromic response of a 6% 3T±TA ®lm to g-rays and electron beams. Symbols: absorbances measured at 490 nm. Full lines: best linear ®tting. Dose rate 30.7 kGy/min for g-rays and 39.6 kGy/min for electron beams.
A ÿ A0 e 5:520:1 10ÿ4 D where D is the dose in kGy. This trend has been con®rmed by measurements at intermediate dose rates. The absorption intensity at 490 nm only undergoes minor changes after 1 month independently of the dose rate and radiation used. Applying electron pulses of high dose rate (39.6 kGy/min) a further wide band around 700 nm is produced immediately (Fig. 1b) but a threshold dose of 100 kGy is required. After the immediate growth, the intensity at 700 nm remains almost stable for a month. The formation of the band in this region is slowed down if the dose rate is diminished. And in fact, immediately after g-irradiation (lowest dose rate) it is not detected (Fig. 1a), but develops after some days and continues to grow for over a month. This growth follows a reasonable ®rst order kinetics for about three half-lives with a rate constant k = 3.0 10ÿ3 hÿ1, as shown in Fig. 3.
709
Fig. 3. Post-g-irradiation eect on a 6% 3T±TA ®lm. Black circles: absorbance values read at 700 nm. Full line: best linear ®tting. Dose=460 kGy. Dose rate=30.7 Gy/min. Inset: selected spectra around 700 nm from 3 to 57 days after irradiation.
ment. Moreover, two previous studies (Emmi et al., 1998, 1999) in the liquid phase showed that ionizing radiation induces oxidation of oligothiophenes to radical cations, the absorptions of which lie at lower energies than those of the parent compounds. The present data, and speci®cally the absorbance band centered at 490 nm and the EPR signal, indicate that radical cations are produced in the solid state as well. Radical cations then seem to undergo polymerization Ð or at least oligomerization Ð in the way proposed for solidstate linear systems (McLaughlin et al., 1996 and refer-
3.2. Nature of color centers EPR spectroscopy reveals the presence of a single and symmetrical line having a gfactor=2.0034 (Fig. 4), independently of the nature of radiation used. However, the signal on the ®lms irradiated with an electron beam is somewhat weaker than that pertaining to g-irradiation. The line persists for a long time after irradiation (over a month), and can be reasonably attributed to a large resonance stabilization of the unpaired electron along a conjugated chain. The gfactor corresponds well to that of a polymer obtained chemically by oxidative condensation of unsubstituted thiophene (Tormo et al., 1997) where charged species were stabilized by an acid environ-
Fig. 4. EPR spectra of 6% 3T±TA ®lms. (a) After g-irradiation and (b) after electron beam irradiation. Dose rate 30.7 Gy/min for g-rays and 39.6 kGy/min for electron beams. gfactor=2.0034 in both cases.
710
S.S. Emmi et al. / Radiation Physics and Chemistry 57 (2000) 707±710
ences therein reported), i.e. through a simple ®rst-order kinetics. The growth of the post-g-irradiation absorption, in the 700 nm region, may very well represent such a process. However, it should dierentiate from the one observed at the high dose rate, which is faster and produces a band the intensity of which is constant in the long run. In this latter case in fact, a radical± radical combination may take place, leading to the production of a dimeric dication. Although the nature of the species produced is not clear at the present stage, it can be remarked that dimerization cannot be faster than propagation in the experimental conditions used, otherwise no EPR signal should be detected at high dose rates. Further work has to be done to clarify this, nevertheless we observed that radiation products show a weak spin-orbit coupling with the sulfur atom, as has been detected for oligomer radical cations produced with chemical methods (Tormo et al., 1997). This provides an indication that the p electron system is mainly delocalized along an enlarged carbon chain, reinforcing a semiquinoid structure (Horowitz et al., 1994). Furthermore, the good linear dose response of the absorption generated at 490 nm up to 500 kGy encourages the investigation of this kind of material with regard to their application as high dose radiochromic dosimeters.
Acknowledgements A fellowship granted to P. Ceroni by the Innovative Material and Related Technologies Institute (MITER) of CNR is gratefully acknowledged. Work performed in the frame of the CNR-HAS Agreement Joint Projects No. 03/2 and 03/3.
References Emmi, S.S., D'Angelantonio, M., Poggi, G., Beggiato, G., Camaioni, N., Geri, A., Martelli, A., Pietropaolo, D., Zotti, G., 1998. The spectral characterization of thiophene radical cation generated by pulse radiolysis. Res. Chem. Intermed. 24, 1±14. Emmi, S.S., D'Angelantonio, M., Beggiato, G., Poggi, G., Geri, A., Pietropaolo, D., Zotti, G., 1999. The generation and spectral characterization of oligothiophenes radical cations. A pulse radiolysis investigation. Radiat. Phys. Chem. 54, 263±270. Friend, R.H., 1996. Semiconductor device physics with conjugated polymers. Phys. Scripta T66, 9±15. Garnier, F., 1997. Scope and limits of organic-based thin-®lm transistors. Phil. Trans. R. Soc. London Ser. A 355, 815± 827. Hayashi, S., Takeda, S., Kaneto, K., Yoshino, K., Matsuyama, T., 1986. Electrical and ESR studies on polythiophene by radiation-induced doping eect. Jpn. J. Appl. Phys. 25, 1529±1532. Horowitz, G., Yassar, A., von Bardeleben, H.J., 1994. ESR and optical spectroscopy evidence for a chain-length dependence of the charged states of thiophene oligomers. Extrapolation to polythiophene. Synth. Met. 62, 245±252. Lavalle, M., Camaioni, N., Casalbore-Miceli, G., Fuochi, P.G., Geri, A., 1999. Eect of the ionizing radiation on poly(4,4'-dipentoxy-2,2'-bithiophene) ®lms electrosynthesised on indium±tin-oxide plates, Radiat. Phys. Chem. 56, 303±308. McLaughlin, W.L., Al-Sheikhly, M., Lewis, D.F., KovaÂcs, A., WojnaÂrovits, L., 1996. Radiochromic solid-state polymerization reaction. In: Clough, R.L., Shalaby, S.W. (Eds.), Irradiation of Polymers. Fundamentals and Technological Applications. ACS Symposium Series, pp. 152±166. Tormo, J., Moreno, F.J., Ruiz, J., FajarõÂ , L., JuliaÂ, L., 1997. Thallium (III) tri¯uoroacetate-tri¯uoroacetic acid in the chemistry of polythiophenes. 2. Treatment of 3-alkylthiophenes and electron paramagnetic resonance results. J. Org. Chem. 62, 878±884.