- Email: [email protected]
Optical absorption and vibrational spectroscopy of conducting polypyrrole under pressure
Synthetic Metals 116 (2001) 167±170
Optical absorption and vibrational spectroscopy of conducting polypyrrole under pressure J. Mikata,*, I. Orgzallb, H.D. Hochheimerc a
UniversitaÈt Potsdam, Hochdrucklabor, Institut fuÈr Physik Ð Physik kondensierter Materie, Am Neuen Palais 10, 14469 Potsdam, Germany b UniversitaÈt Potsdam, InterdisziplinaÈres Forschungszentrum fuÈr DuÈnne Organische und Biochemische Schichten, Am Neuen Palais 10, 14469 Potsdam, Germany c Department of Physics, Colorado State University, Fort Collins, CO 80523, USA
Abstract Polypyrrole in its conducting form was chemically synthesized using p-toluenesulfonic acid as doping agent. UV±Vis absorption and vibrational spectroscopy was used for characterization of the electronic properties. The measurements were conducted under high pressure up to 4 GPa which allows for observation of changes of the structural parameters without changing the chemical nature. The results are interpreted in terms of the stability of charge carriers. A change of the character of the charge carriers from bipolarons to polarons is suggested. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Conducting polymers; Polypyrrole; High pressure; Vibrational spectroscopy; Optical absorption; Charge carrier
1. Introduction Conducting polymers still achieve great interest from the possibilities of their use in technological applications. Among those, doped polypyrrole exhibits high conductivity combined with a long term stability in different environments [1±3]. Easy chemical or electrochemical synthesis with different dopants allows variation of the electrical and structural properties [4±6]. Due to the doping process electrons or holes are removed from the polymer chain. The interaction of the injected charge with the chain leads to a distortion of the chain. The charge carriers become localized forming polarons. By increasing the amount of injected charges the spin concentration will reach a maximum. With further charge injection the concentration of polarons will decrease by pairing to bipolarons as could be proved by ESR measurements [7]. Since the transport properties are strongly dependent on the type of charge carriers, investigations concerning the existence of polarons and bipolarons are conducted. Theoretical calculations of the band structure of polypyrrole in different doping states propose the incidence of both charge carrier types in dependence on the doping level [8]. From statistical considerations of the chain *
Corresponding author. Tel.: 49-331-977-2915; fax: 49-331-977-2985. E-mail address: [email protected] (J. Mikat).
lengths a coexistence of both charge carriers was proposed [9]. Also, the differences in formation energy for bipolarons and polarons should vanish at room temperature [10]. Different models were suggested for the transport behavior of the charge carriers [11]. Pressure dependent measurements show a decreasing electrical resistance with increasing pressure [12±14]. Today, a correlation of this behavior with the charge carrier species has not yet been ®nally successful. ESR measurements under pressure have been proved to be very dif®cult. This work will contribute to this ®eld by using indirect methods of determining the relative amounts of polarons and bipolarons in polypyrrole. Changes of the type of charge carriers result in a modi®cation of the band structure re¯ected in the UV±Vis absorption spectrum. From the observation of the relative intensities of Raman bands assigned to the dication (bipolaron) or radical cation (polaron), respectively, further hints for the existence of polarons and bipolarons can be obtained. Additionally, IR active modes may contribute to the interpretation. The in¯uence of pressure on optical and vibrational properties can be derived regarding the pressure in¯uence on the structural properties. On the microscopic level, applying pressure leads to a restriction of vibrational and rotational motions as well as to a shortening of bonds. These changes involve changes of the electronic system along the polymer backbone in¯uencing the charge carrier±phonon interaction. Therefore, the properties of the charge carriers
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 4 7 9 - 3
168
J. Mikat et al. / Synthetic Metals 116 (2001) 167±170
are directly in¯uenced by pressure. This concerns localization strength and size of the charge carrier and also the ability for pairing of polarons to bipolarons. For the interpretation of the results we refer to works dealing with the development of optical and vibrational properties under reduction of the polymer. In these investigations a change of the electron density along the backbone occurs. At low doping levels ®rst formation of polarons and with further doping pairing of polarons into bipolarons occurs as expressed in the optical and vibrational spectra. Analogous features are observed during pressure application. Therefore, changes of the spectra under pressure should exhibit similar behavior as under reduction. 2. Experimental A thin polypyrrole ®lm was chemically synthesized on a polycarbonate membrane. The substrate was chosen for comparison with other experiments [15]. Equal volumes of an aqueous solution 0.2 M in pyrrole and an aqueous solution 0.5 M in ferric chloride and 0.5 M in p-toluenesulphonic acid were mixed. The temperature was kept constant at 08C and the synthesis process allowed to proceed for 6 h resulting in a ®lm thickness of 300 nm as determined by AFM. For pressure investigations a lever arm Piermarini-Block diamond anvil cell (DAC) [16] was used with a copper gasket with 200 mm hole and a preindented thickness of 70 mm. The pressure was determined in situ by the shift of the R1 ruby line [17]. Paraf®n oil was used as pressure transmitting medium. Raman spectra of the ®lm were recorded with a triple Raman Jobin-Yvon T64000 spectrometer equipped with a CCD detector. For excitation a He±Ne-Laser with l0 632:8 nm was used. A CCD camera system with monitor was used to choose the investigated sample spot. Optical absorption spectra were collected with a modular spectrometer (Alphascan, Photon Technologies Industries) equipped with a photomultiplier connected to the DAC via single mode optical ®ber. The illuminating light source (Xenon short arc lamp) was connected with the DAC by another optical ®ber. IR spectra were recorded with Perkin±Elmer System 2000 FTIR with microscope and CCD detector. 3. Results and discussion 3.1. Optical absorption Fig. 1 shows the development of the optical absorbance spectrum under pressure. At ambient pressure the spectrum is dominated by a broad absorption with a maximum at 2.6 eV, in accordance with other works [18,19]. The band can be assigned to the transition between the valence band
Fig. 1. Optical absorbance of chemically prepared polypyrrole under pressure. Pressure increases from lower to upper graph.
and the anti-bonding bipolaron band [8]. Under pressure, the maximum shifts to lower energies and decreases in intensity. At a medium pressure level of 1.8 GPa the spectrums shows no distinct maximum. With further pressure increase a band at 2.2 GPa evolves and increases in intensity. A new band raises at about 3.7 eV. Under reduction a similar shift of the absorption band was observed [8]. The ambient pressure spectrum is comparable to the spectrum of the highest doped sample in this work. The band structure is assumed to be determined by bipolarons leading to a three-level energy system with absorptions at 1.0, 2.7 and 3.6 eV. Comparing the spectrum at highest pressure with spectra obtained by reduction this would correspond to a medium reduction level with absorptions at approximately 0.7, 2.4 and 3.4 eV. At this reduction level an increased number of polarons can be expected. The similar values for the absorption maxima indicate a transition from a bipolaron dominated structure to a more polaronic structure. The spectrum at medium pressure of 1.8 GPa may be a hint for the coexistence of both charge carriers. 3.2. Raman spectroscopy Raman spectroscopy is a sensitive tool for the investigation of structural changes of polymers. The Raman spectrum of polypyrrole is mainly determined by resonance absorption of bands belonging to the Ag modes. Therefore, different doping levels affect the intensities of bands by in¯uencing the electronic con®guration. The two possible charge carriers, polarons and bipolarons (radical cations and dications, respectively) yield also different electronic con®gurations. A splitting of bands due to these effects can occur. Raman bands are more sensitive to the changes than the optical absorption. As stated by Paschen et al. [20] and Blackwood et al. [21] the optical absorption spectrum is nearly independent of the chemical nature of the dopant, despite of their different sizes or organic or inorganic character. Raman spectra taken from samples with different dopants on the other hand can exhibit large differences [5,15,22].
J. Mikat et al. / Synthetic Metals 116 (2001) 167±170
169
Fig. 2. Raman spectrum under pressure. RC and DC denote bands assigned to the radical cation and dication species of charge carriers, respectively. The C=C(n) (stretching mode) band contains contributions from both.
Fig. 3. Infrared spectrum of polypyrrole under pressure. Pressure increases from lower to upper graph. The different underground for the pressure spectra is due to the diamond anvil cell.
Fig. 2 shows the Raman spectrum at three different pressures. The spectra can be subdivided into four groups belonging to different modes containing contributions from the radical cation (RC) and dication (DC) species of charge carriers. At ambient pressure the ring deformation mode exhibits a strong band accompanied by a high frequency shoulder. The symmetric CH-in-plane bending mode shows two clearly distinguishable maxima. The assignment of these maxima to the type of charge carrier follows from Furukawa et al. [22] who investigated the development of the Raman spectrum of doped polypyrrole under reduction. Under pressure the DC bands of both groups decrease in intensity. Whereas the absolute intensities of the RC and DC band in both groups is very different, the changes in relative intensities is similar for both groups. This behavior is analogous to the development of the Raman spectrum under reduction. Thus, a decrease of the relative amount of dications with respect to radical cations under pressure is suggested. The anti-symmetric CH-in-plane bending mode shows a strong decrease in intensity. With the results from the other groups it may be concluded that contributions to this band mainly arise from dications. In the ambient pressure spectrum a high frequency shoulder of the C=C(n) band is observed. Under pressure this shoulder diminishes and at the highest applied pressure it almost cannot be resolved in the slope. In accordance with the results from above we believe that this shoulder is due to the dication amount contained in the band. A more detailed analysis of the Raman spectrum of polypyrrole under pressure is in preparation [25].
pTS-doped samples [23]. The group at 1480/1540 cmÿ1, assigned to the symmetric and anti-symmetric ring stretching mode, respectively, is used as a measure for the conjugation [24]. The ratio of the intensities A(1480)/A(1540) determines the extent of the conjugation. The higher the ratio the better the conjugation. Under pressure, this group experiences drastic changes concerning the low frequency part of the group. At highest pressure a clear structure of the band at 1480 cmÿ1 is not observable. The band broadens strongly and merges with the underground. The band at 1540 cmÿ1 remains almost constant in intensity and position. From this development a clear picture of the pressure dependence of the conjugation is not possible. It is well known that pressure enhances the conjugation along the polymer backbone. Also, pressure usually induces sharpening of vibrational bands due to elimination of conformational defects. The broadening of the band at 1480 cmÿ1 would imply a strong pressure induced distortion of the symmetric vibration whereas the anti-symmetric vibration would be unaffected. Assuming that pressure shortens the bond lengths along the backbone the symmetric vibration would be more distorted. A broadening can be expected. Under pressure, a better conjugation is expected simultaneously. Therefore, the observed changes of the 1480 cmÿ1 band may indicate a better conjugation. The usual method for the determination of the band areas by subtracting a linear background is not applicable for the pressure spectra. A change of the ratio A(1480)/A(1540) can therefore, not be con®rmed. These bands at 1175 and 900 cmÿ1 show a strong pressure shift of 6 cmÿ1/GPa. The bands at 1090 and 1300 cmÿ1 keep their position while the remaining bands show a shift of about 3 cmÿ1/GPa. Major changes occur also for the region below 900 cmÿ1. The maximum at 800 cmÿ1 decreases under pressure, turns into a shoulder at 1.0 GPa and is almost not distinguishable from the slope at the highest
3.3. Infrared spectroscopy In Fig. 3 infrared spectra of polypyrrole at three different pressures are presented. The spectrum at ambient pressure is comparable to those obtained earlier by other groups for
170
J. Mikat et al. / Synthetic Metals 116 (2001) 167±170
pressure. A similar behavior is reported in the work of Ogasawara et al. [3] who stated a decrease of the 770 cmÿ1 band by decreasing the synthesis temperature. It is well known that lower synthesis temperatures enhance the conduction, presumably due to an increase of the conjugation length. 4. Summary Polypyrrole in its conducting form was investigated. Pressure induced changes of the optical properties were observed using optical absorbance, Raman and infrared spectroscopy. The absorption from valence band to the anti-bonding bipolaron band shifts to lower energies. New bands typical for a polaronic band structure evolve. The intensities of Raman bands belonging to the same vibrational mode but related to different types of charge carriers were studied. A decrease of the relative intensity on expense of the dication band was noticed. These effects imply changes of the charge carrier±phonon interaction. A transition from bipolaronic to a more polaronic character of the charge carriers may be derived from the comparison with spectra obtained under reduction of the polymer. The conjugation relevant IR band at 1480 cmÿ1 exhibits strong broadening preventing the usual evaluation of the conjugation. From pressure arguments, a better conjugation can be expected allowing to correlate the observed changes of the IR spectrum with changes of the conjugation. References [1] M. Satoh, H. Ishikawa, H. Yageta, K. Amano, E. Hasegawa, Synth. Met. 84 (1997) 167.
[2] S. Sakkopoulos, E. Vitoratos, E. Dalas, Synth. Met. 92 (1998) 63. [3] M. Ogasawara, K. Funahashi, T. Demura, T. Hagiwara, K. Iwata, Synth. Met. 14 (1986) 61. [4] G. Zotti, Synth. Met. 97 (1998) 267. [5] C.H. Olk, C.P. Beetz, J. Heremans, J. Mater. Res. 3 (1988) 984. [6] R. Turcu, M. Brie, G. Leising, A. Niko, V. Tosa, A. Mihut, A. Bot, Synth. Met. 84 (1997) 825. [7] J.C. Scott, P. Pfluger, M.T. Krounbi, G.B. Street, Phys. Rev. B 28 (1983) 2140. [8] J.L. BreÂdas, J.C. Scott, K. Yakushi, G.B. Street, Phys. Rev. B 30 (1984) 1023. [9] F. Genoud, M. Guglielmi, M. Nechtschein, E. Genies, M. Salmon, Phys. Rev. Lett. 55 (1985) 118. [10] E.M. Conwell, H.A. Mizes, Phys. Rev. B 44 (1991) 937. [11] J.P. Travers, J. Chim. Phys. Phys. Chim. Biol. 95 (1998) 1427. [12] B. Lundberg, B. Sundquist, O. InganaÈs, I. LundstroÈm, W.R. Salaneck, Mol. Cryst. Liq. Cryst. 118 (1985) 155. [13] M. Reghu, C.O. Yoon, D. Moses, A.J. Heeger, Synth. Met. 64 (1994) 53. [14] I. Orgzall, B. Lorenz, L. Dunsch, A. Bartl, S.T. Ting, P.H. Hor, H.D. Hochheimer, Synth. Met. 81 (1996) 59. [15] J. Mikat, I. Orgzall, S. Sapp, C.R. Martin, H.D. Hochheimer, in: Proceedings of the EHPRG 1999, High Pressure Research 2000, in press. [16] G.J. Piermarini, S. Block, Rev. Sci. Instrum. 46 (1975) 973. [17] H.K. Mao, P.M. Bell, K.J. Dunn, R.M. Chrenko, R.C. Devries, Rev. Sci. Instrum. 50 (1979) 1002. [18] K.K. Kanazawa, A.F. Diaz, W.D. Gill, P.M. Grant, G.B. Street, G.P. Gardini, J.F. Kwak, Synth. Met. 1 (1979) 329. [19] G.B. Street, T.C. Clarke, M. Krounbi, K.K. Kanazawa, V. Lee, P. Pfluger, J.C. Scott, G. Weiser, Mol. Cryst. Liq. Cryst. 83 (1982) 1285. [20] S. Paschen, M. Carrard, B. Senior, F. Chao, M. Costa, L. Zuppiroli, Acta Polym. 47 (1996) 511. [21] D. Blackwood, M. Josowicz, J. Phys. Chem. 95 (1991) 493. [22] Y. Furukawa, S. Tazawa, Y. Fujii, I. Harada, Synth. Met. 24 (1988) 329. [23] R. Turcu, M. Brie, G. Leising, V. Tosa, A. Mihut, A. Niko, A. Bot, Appl. Phys. A 67 (1998) 283. [24] J.T. Lei, Z. Cai, C.R. Martin, Synth. Met. 46 (1992) 53. [25] J. Mikat, to be submitted.
Optical absorption and vibrational spectroscopy of conducting polypyrrole under pressure J. Mikata,*, I. Orgzallb, H.D. Hochheimerc a
UniversitaÈt Potsdam, Hochdrucklabor, Institut fuÈr Physik Ð Physik kondensierter Materie, Am Neuen Palais 10, 14469 Potsdam, Germany b UniversitaÈt Potsdam, InterdisziplinaÈres Forschungszentrum fuÈr DuÈnne Organische und Biochemische Schichten, Am Neuen Palais 10, 14469 Potsdam, Germany c Department of Physics, Colorado State University, Fort Collins, CO 80523, USA
Abstract Polypyrrole in its conducting form was chemically synthesized using p-toluenesulfonic acid as doping agent. UV±Vis absorption and vibrational spectroscopy was used for characterization of the electronic properties. The measurements were conducted under high pressure up to 4 GPa which allows for observation of changes of the structural parameters without changing the chemical nature. The results are interpreted in terms of the stability of charge carriers. A change of the character of the charge carriers from bipolarons to polarons is suggested. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Conducting polymers; Polypyrrole; High pressure; Vibrational spectroscopy; Optical absorption; Charge carrier
1. Introduction Conducting polymers still achieve great interest from the possibilities of their use in technological applications. Among those, doped polypyrrole exhibits high conductivity combined with a long term stability in different environments [1±3]. Easy chemical or electrochemical synthesis with different dopants allows variation of the electrical and structural properties [4±6]. Due to the doping process electrons or holes are removed from the polymer chain. The interaction of the injected charge with the chain leads to a distortion of the chain. The charge carriers become localized forming polarons. By increasing the amount of injected charges the spin concentration will reach a maximum. With further charge injection the concentration of polarons will decrease by pairing to bipolarons as could be proved by ESR measurements [7]. Since the transport properties are strongly dependent on the type of charge carriers, investigations concerning the existence of polarons and bipolarons are conducted. Theoretical calculations of the band structure of polypyrrole in different doping states propose the incidence of both charge carrier types in dependence on the doping level [8]. From statistical considerations of the chain *
Corresponding author. Tel.: 49-331-977-2915; fax: 49-331-977-2985. E-mail address: [email protected] (J. Mikat).
lengths a coexistence of both charge carriers was proposed [9]. Also, the differences in formation energy for bipolarons and polarons should vanish at room temperature [10]. Different models were suggested for the transport behavior of the charge carriers [11]. Pressure dependent measurements show a decreasing electrical resistance with increasing pressure [12±14]. Today, a correlation of this behavior with the charge carrier species has not yet been ®nally successful. ESR measurements under pressure have been proved to be very dif®cult. This work will contribute to this ®eld by using indirect methods of determining the relative amounts of polarons and bipolarons in polypyrrole. Changes of the type of charge carriers result in a modi®cation of the band structure re¯ected in the UV±Vis absorption spectrum. From the observation of the relative intensities of Raman bands assigned to the dication (bipolaron) or radical cation (polaron), respectively, further hints for the existence of polarons and bipolarons can be obtained. Additionally, IR active modes may contribute to the interpretation. The in¯uence of pressure on optical and vibrational properties can be derived regarding the pressure in¯uence on the structural properties. On the microscopic level, applying pressure leads to a restriction of vibrational and rotational motions as well as to a shortening of bonds. These changes involve changes of the electronic system along the polymer backbone in¯uencing the charge carrier±phonon interaction. Therefore, the properties of the charge carriers
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 4 7 9 - 3
168
J. Mikat et al. / Synthetic Metals 116 (2001) 167±170
are directly in¯uenced by pressure. This concerns localization strength and size of the charge carrier and also the ability for pairing of polarons to bipolarons. For the interpretation of the results we refer to works dealing with the development of optical and vibrational properties under reduction of the polymer. In these investigations a change of the electron density along the backbone occurs. At low doping levels ®rst formation of polarons and with further doping pairing of polarons into bipolarons occurs as expressed in the optical and vibrational spectra. Analogous features are observed during pressure application. Therefore, changes of the spectra under pressure should exhibit similar behavior as under reduction. 2. Experimental A thin polypyrrole ®lm was chemically synthesized on a polycarbonate membrane. The substrate was chosen for comparison with other experiments [15]. Equal volumes of an aqueous solution 0.2 M in pyrrole and an aqueous solution 0.5 M in ferric chloride and 0.5 M in p-toluenesulphonic acid were mixed. The temperature was kept constant at 08C and the synthesis process allowed to proceed for 6 h resulting in a ®lm thickness of 300 nm as determined by AFM. For pressure investigations a lever arm Piermarini-Block diamond anvil cell (DAC) [16] was used with a copper gasket with 200 mm hole and a preindented thickness of 70 mm. The pressure was determined in situ by the shift of the R1 ruby line [17]. Paraf®n oil was used as pressure transmitting medium. Raman spectra of the ®lm were recorded with a triple Raman Jobin-Yvon T64000 spectrometer equipped with a CCD detector. For excitation a He±Ne-Laser with l0 632:8 nm was used. A CCD camera system with monitor was used to choose the investigated sample spot. Optical absorption spectra were collected with a modular spectrometer (Alphascan, Photon Technologies Industries) equipped with a photomultiplier connected to the DAC via single mode optical ®ber. The illuminating light source (Xenon short arc lamp) was connected with the DAC by another optical ®ber. IR spectra were recorded with Perkin±Elmer System 2000 FTIR with microscope and CCD detector. 3. Results and discussion 3.1. Optical absorption Fig. 1 shows the development of the optical absorbance spectrum under pressure. At ambient pressure the spectrum is dominated by a broad absorption with a maximum at 2.6 eV, in accordance with other works [18,19]. The band can be assigned to the transition between the valence band
Fig. 1. Optical absorbance of chemically prepared polypyrrole under pressure. Pressure increases from lower to upper graph.
and the anti-bonding bipolaron band [8]. Under pressure, the maximum shifts to lower energies and decreases in intensity. At a medium pressure level of 1.8 GPa the spectrums shows no distinct maximum. With further pressure increase a band at 2.2 GPa evolves and increases in intensity. A new band raises at about 3.7 eV. Under reduction a similar shift of the absorption band was observed [8]. The ambient pressure spectrum is comparable to the spectrum of the highest doped sample in this work. The band structure is assumed to be determined by bipolarons leading to a three-level energy system with absorptions at 1.0, 2.7 and 3.6 eV. Comparing the spectrum at highest pressure with spectra obtained by reduction this would correspond to a medium reduction level with absorptions at approximately 0.7, 2.4 and 3.4 eV. At this reduction level an increased number of polarons can be expected. The similar values for the absorption maxima indicate a transition from a bipolaron dominated structure to a more polaronic structure. The spectrum at medium pressure of 1.8 GPa may be a hint for the coexistence of both charge carriers. 3.2. Raman spectroscopy Raman spectroscopy is a sensitive tool for the investigation of structural changes of polymers. The Raman spectrum of polypyrrole is mainly determined by resonance absorption of bands belonging to the Ag modes. Therefore, different doping levels affect the intensities of bands by in¯uencing the electronic con®guration. The two possible charge carriers, polarons and bipolarons (radical cations and dications, respectively) yield also different electronic con®gurations. A splitting of bands due to these effects can occur. Raman bands are more sensitive to the changes than the optical absorption. As stated by Paschen et al. [20] and Blackwood et al. [21] the optical absorption spectrum is nearly independent of the chemical nature of the dopant, despite of their different sizes or organic or inorganic character. Raman spectra taken from samples with different dopants on the other hand can exhibit large differences [5,15,22].
J. Mikat et al. / Synthetic Metals 116 (2001) 167±170
169
Fig. 2. Raman spectrum under pressure. RC and DC denote bands assigned to the radical cation and dication species of charge carriers, respectively. The C=C(n) (stretching mode) band contains contributions from both.
Fig. 3. Infrared spectrum of polypyrrole under pressure. Pressure increases from lower to upper graph. The different underground for the pressure spectra is due to the diamond anvil cell.
Fig. 2 shows the Raman spectrum at three different pressures. The spectra can be subdivided into four groups belonging to different modes containing contributions from the radical cation (RC) and dication (DC) species of charge carriers. At ambient pressure the ring deformation mode exhibits a strong band accompanied by a high frequency shoulder. The symmetric CH-in-plane bending mode shows two clearly distinguishable maxima. The assignment of these maxima to the type of charge carrier follows from Furukawa et al. [22] who investigated the development of the Raman spectrum of doped polypyrrole under reduction. Under pressure the DC bands of both groups decrease in intensity. Whereas the absolute intensities of the RC and DC band in both groups is very different, the changes in relative intensities is similar for both groups. This behavior is analogous to the development of the Raman spectrum under reduction. Thus, a decrease of the relative amount of dications with respect to radical cations under pressure is suggested. The anti-symmetric CH-in-plane bending mode shows a strong decrease in intensity. With the results from the other groups it may be concluded that contributions to this band mainly arise from dications. In the ambient pressure spectrum a high frequency shoulder of the C=C(n) band is observed. Under pressure this shoulder diminishes and at the highest applied pressure it almost cannot be resolved in the slope. In accordance with the results from above we believe that this shoulder is due to the dication amount contained in the band. A more detailed analysis of the Raman spectrum of polypyrrole under pressure is in preparation [25].
pTS-doped samples [23]. The group at 1480/1540 cmÿ1, assigned to the symmetric and anti-symmetric ring stretching mode, respectively, is used as a measure for the conjugation [24]. The ratio of the intensities A(1480)/A(1540) determines the extent of the conjugation. The higher the ratio the better the conjugation. Under pressure, this group experiences drastic changes concerning the low frequency part of the group. At highest pressure a clear structure of the band at 1480 cmÿ1 is not observable. The band broadens strongly and merges with the underground. The band at 1540 cmÿ1 remains almost constant in intensity and position. From this development a clear picture of the pressure dependence of the conjugation is not possible. It is well known that pressure enhances the conjugation along the polymer backbone. Also, pressure usually induces sharpening of vibrational bands due to elimination of conformational defects. The broadening of the band at 1480 cmÿ1 would imply a strong pressure induced distortion of the symmetric vibration whereas the anti-symmetric vibration would be unaffected. Assuming that pressure shortens the bond lengths along the backbone the symmetric vibration would be more distorted. A broadening can be expected. Under pressure, a better conjugation is expected simultaneously. Therefore, the observed changes of the 1480 cmÿ1 band may indicate a better conjugation. The usual method for the determination of the band areas by subtracting a linear background is not applicable for the pressure spectra. A change of the ratio A(1480)/A(1540) can therefore, not be con®rmed. These bands at 1175 and 900 cmÿ1 show a strong pressure shift of 6 cmÿ1/GPa. The bands at 1090 and 1300 cmÿ1 keep their position while the remaining bands show a shift of about 3 cmÿ1/GPa. Major changes occur also for the region below 900 cmÿ1. The maximum at 800 cmÿ1 decreases under pressure, turns into a shoulder at 1.0 GPa and is almost not distinguishable from the slope at the highest
3.3. Infrared spectroscopy In Fig. 3 infrared spectra of polypyrrole at three different pressures are presented. The spectrum at ambient pressure is comparable to those obtained earlier by other groups for
170
J. Mikat et al. / Synthetic Metals 116 (2001) 167±170
pressure. A similar behavior is reported in the work of Ogasawara et al. [3] who stated a decrease of the 770 cmÿ1 band by decreasing the synthesis temperature. It is well known that lower synthesis temperatures enhance the conduction, presumably due to an increase of the conjugation length. 4. Summary Polypyrrole in its conducting form was investigated. Pressure induced changes of the optical properties were observed using optical absorbance, Raman and infrared spectroscopy. The absorption from valence band to the anti-bonding bipolaron band shifts to lower energies. New bands typical for a polaronic band structure evolve. The intensities of Raman bands belonging to the same vibrational mode but related to different types of charge carriers were studied. A decrease of the relative intensity on expense of the dication band was noticed. These effects imply changes of the charge carrier±phonon interaction. A transition from bipolaronic to a more polaronic character of the charge carriers may be derived from the comparison with spectra obtained under reduction of the polymer. The conjugation relevant IR band at 1480 cmÿ1 exhibits strong broadening preventing the usual evaluation of the conjugation. From pressure arguments, a better conjugation can be expected allowing to correlate the observed changes of the IR spectrum with changes of the conjugation. References [1] M. Satoh, H. Ishikawa, H. Yageta, K. Amano, E. Hasegawa, Synth. Met. 84 (1997) 167.
[2] S. Sakkopoulos, E. Vitoratos, E. Dalas, Synth. Met. 92 (1998) 63. [3] M. Ogasawara, K. Funahashi, T. Demura, T. Hagiwara, K. Iwata, Synth. Met. 14 (1986) 61. [4] G. Zotti, Synth. Met. 97 (1998) 267. [5] C.H. Olk, C.P. Beetz, J. Heremans, J. Mater. Res. 3 (1988) 984. [6] R. Turcu, M. Brie, G. Leising, A. Niko, V. Tosa, A. Mihut, A. Bot, Synth. Met. 84 (1997) 825. [7] J.C. Scott, P. Pfluger, M.T. Krounbi, G.B. Street, Phys. Rev. B 28 (1983) 2140. [8] J.L. BreÂdas, J.C. Scott, K. Yakushi, G.B. Street, Phys. Rev. B 30 (1984) 1023. [9] F. Genoud, M. Guglielmi, M. Nechtschein, E. Genies, M. Salmon, Phys. Rev. Lett. 55 (1985) 118. [10] E.M. Conwell, H.A. Mizes, Phys. Rev. B 44 (1991) 937. [11] J.P. Travers, J. Chim. Phys. Phys. Chim. Biol. 95 (1998) 1427. [12] B. Lundberg, B. Sundquist, O. InganaÈs, I. LundstroÈm, W.R. Salaneck, Mol. Cryst. Liq. Cryst. 118 (1985) 155. [13] M. Reghu, C.O. Yoon, D. Moses, A.J. Heeger, Synth. Met. 64 (1994) 53. [14] I. Orgzall, B. Lorenz, L. Dunsch, A. Bartl, S.T. Ting, P.H. Hor, H.D. Hochheimer, Synth. Met. 81 (1996) 59. [15] J. Mikat, I. Orgzall, S. Sapp, C.R. Martin, H.D. Hochheimer, in: Proceedings of the EHPRG 1999, High Pressure Research 2000, in press. [16] G.J. Piermarini, S. Block, Rev. Sci. Instrum. 46 (1975) 973. [17] H.K. Mao, P.M. Bell, K.J. Dunn, R.M. Chrenko, R.C. Devries, Rev. Sci. Instrum. 50 (1979) 1002. [18] K.K. Kanazawa, A.F. Diaz, W.D. Gill, P.M. Grant, G.B. Street, G.P. Gardini, J.F. Kwak, Synth. Met. 1 (1979) 329. [19] G.B. Street, T.C. Clarke, M. Krounbi, K.K. Kanazawa, V. Lee, P. Pfluger, J.C. Scott, G. Weiser, Mol. Cryst. Liq. Cryst. 83 (1982) 1285. [20] S. Paschen, M. Carrard, B. Senior, F. Chao, M. Costa, L. Zuppiroli, Acta Polym. 47 (1996) 511. [21] D. Blackwood, M. Josowicz, J. Phys. Chem. 95 (1991) 493. [22] Y. Furukawa, S. Tazawa, Y. Fujii, I. Harada, Synth. Met. 24 (1988) 329. [23] R. Turcu, M. Brie, G. Leising, V. Tosa, A. Mihut, A. Niko, A. Bot, Appl. Phys. A 67 (1998) 283. [24] J.T. Lei, Z. Cai, C.R. Martin, Synth. Met. 46 (1992) 53. [25] J. Mikat, to be submitted.