- Email: [email protected]
1064-nanometer-excited Fourier transform Raman spectroscopy of conducting polymers
05w39/91 mm+o.oo @ 1991 Fwgamon Press plc
Spdmhhica Acta, Vol. 4lA, No. 9/10, pp. 1367-1373, 1991 Rintcd in Great Britain
lOGhmometer=excited Fourier transform Raman spectroscopy of conducting polymers YUKIO FURUKAWA,* HROSHI OHTA, AKIRA SAKAM~TO and Mrrsuo TASUMI Department of Chemistry, Faculty of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan (Received 25 March 1991)
Ah&act-Application of 1064-urn-excitedFourier transform(FT)-Ramanspectroscopy to the characterixation of conducting polymers is described. 1064-nm-excitedFT-Ramanspectrawith high signal-to-noise ratios are obtained from polyacetylene (PA), poly(l,Cphenylene) (PPP), poly(l+phenylene vinylene) (PPV) and poly(2,5-thienylene vinylene) (PTV) in their neutral (insulating) state. The resonant Raman spectra of acceptor- or donor-doped (conducting) PA and PPV are also obtained wih 1064-nmexcitation. The resonant Raman spectra of Na-doped PA change in two stages with increasing dopant concentration, the first change corresponding to the increase in electrical conductivity and the second to the appearance of a Pauli susceptibility. The 1064-nmexcited FT-Raman spectrum of Na-doped PPV indicates existence of negative bipolaronswhich are equivalent to divalent anions extending over a few repeatingunits in the polymer chains. IN-I-R~DUCTION
Fourier transform (FT)-Raman spectroscopy is now the most widely used method for overcoming the fluorescence problem in Raman spectroscopy. It should be noted that this method has another advantage of obtaining resonant Raman spectra of materials having NIR absorptions, such as conducting polymers, chargetransfer complexes, biological pigments, etc. It is the purpose of this paper to demonstrate the potential of 1064-rim-excited Raman spectroscopy in the characterization of conducting polymers. Since organic polymers are typical insulators, their inability to carry electricity has been utilized in their various applications. However, a new class of organic polymers with the ability to conduct electricity has been developed for the past 15 years. These conducting polymers have conjugated n-electrons in common. They show high electrical conductivity (1 - lo5 S cm-‘, S=S2-‘) when doped with electron acceptors such as iodine, AsFs, H.$O.,, etc. or electron donors such as alkali metals, although they are insulators or semiconductors in the undoped (neutral) state [l]. The maximum conductivity reported so far for organic polymers at room temperature is more than lti S cm-’ [2,3], which is comparable with the conductivity of copper (6 x 10s S cm-‘). When electrons are removed from (or added to) n-conjugated polymer chains, structural changes supposedly extending over several repeating units occur. These doped-domain structures are classified into polarons, bipolarons, and solitons according to their types, and have been extensively discussed in relation to their roles in electric conduction [4]. Electronic absorptions associated with the doped domains often appear in the region from visible to NIR. Accordingly, NIR resonant Raman spectroscopy may give a clue to structural studies of the doped domains. In this paper, we report and discuss the 1064-nm-excited IT-Raman spectra of trunspolyacetylene (PA), poly( 1 ,Cphenylene) (PPP) , poly(1 ,Cphenylene vinylene) (PPV) and poly(2,5_thienylene vinylene) (PTV) in both the neutral (insulating) and doped (conducting) states. Their structures are shown in Fig. 1. NEAR-IN FRARBD (NIR)
EXPERIMENTAL
Samples &Rich PA films were synthesized according to SHIRAKAWA’S method [S] at -78°C and thermally isomerized to truns-PA at 180°C for 60 min in vacua. PPP was prepared by polycondensation of the Grignard reagent from l&dibromobenzene with an NiClz catalyst 163. PPV and PTV were synthesized via soluble precursors according to the methods in the literature [7,8]. * Author to whom correspondenceshould be addressed. 1367
Y. FurwKnw.4 et
1368
al.
(d)
04
Fig. 1. Structures of neutral polymers: (a) tram-polyacetylene (PA); (b) poly(l,Cphenylene) (PPP); (c) poly(l,Cphenylene vinylene) (PPV); (d) poly(2,Whienylene vinylene) @TV).
PA and PPV films were doped with Na by treating them with a THF solution of sodium naphthalide in a completely sealed ampoule. The dopant content was controlled by the concentration of sodium naphthalide and the period of immersing the film into the solution. Light doping of PA films was performed by immersing them into a 0.01 mol dme3 solution for 10-30 min. Heavy doping of PA and PPV films was performed in the same way by using a more concentrated solution (0.1 mol dmT3) and a longer time (45-60 min). After washing the film with fresh THF, the ampoule was sealed again. Raman measurments were made for such films in sealed ampoules. The dopant (Na) contents were estimated from the relationship between their 632.8-nm-excited Raman spectra and the Na contents [9]. Full doping of a PA film with K was made in a similar manner by using a 0.1 mol dmm3 THF solution of potassium naphthalide. Films of PA were fully doped with iodine by exposing them to the iodine vapor. The electrical conductivity measured by a d.c. four-probe technique was 44 S cm-‘. The iodine content was 22 mol% per CH unit (CHI,& by weight measurement. H,SOrdoping of PPV was carried out by immersing PPV films in concentrated sulphuric acid.
Measurements 1064-nm-Excited FT-Raman spectra were measured on a JEOL JIR-5500 Fourier transform spectrophotometer modified for Raman measurements. A continuous-wave Nd : YAG laser (CVI YAGMAX C-92) was operated at 1064nm for Raman excitation. The laser beam was passed through an interference filter to remove spontaneous emissions, and was led to the sample without focusing to avoid its thermal degradation. Raman-scattered radiation was collected with a 90” offaxis parabolic mirror in a backscattering configuration. Collected radiation was passed through three long-wavelength-pass dielectric filters (Microcoatings) to reject the Rayleigh scattering. An InGaAs detector (Epitaxx) operated at liquid nitrogen temperature was used. A long-wavelengthpass filter (Corion LL-950) was inserted before the detector to cut visible light. Spectral resolution was 4 cm-‘. A triangular apodization function was applied. 632.8-nm-Excited Raman spectra were measured at room temperature on a Raman spectrophotometer consisting of a Spex 1877 Triplemate and an EG & G PARC 1421 intensified photodiode array detector.
RESULTSAND
DISCUSSION
Neutral polymers The FT-Raman spectra of neutral shown in Fig. 2. The Raman spectra
PPP, PPV and PTV with 1064-nm excitation are obtained have high signal-to-noise ratios in accord
with the general trend that molecules with conjugated n-electrons exhibit large Raman scattering cross-sections. PPP shows intense “fluorescence” backgrounds when visible laser lines are used for Raman excitation; no Raman bands were observed with 488.0-nm excitation due to a strong background, and only a few bands were observed with 632.8-nm excitation [lo]. However, in the lO&nm-excited FT-Raman spectrum (Fig. 2a), no background is observed and the signal-to-noise ratio is high. It has been reported [lo] that the ratio of the intensity of the 1223 cm-’ band to that of the 1281 cm-’ band increases with the
FT-Raman spectroscopy of conducting polymers
1369
h RAMAN SHIFT I cm-’
Fig. 2. 1064~nm-Excited FT-Raman spectra of neutral polymers: (a) poly(l+phenylene) (PPP); (b) poly(l&phenylene vinylene) (PPV); (c) poly(2,5-thienyiene vinylene) (WV).
number of consecutive phenyl rings. Judging from the observed relative intensity in Fig. 2a, the chain length in our sample is rather short. In the Raman spectrum of PPP prepared electrochemically the 1223 cm-’ band is much stronger than the 1281 cm-’ band [ll]. The high quality of the FT-Raman spectrum in Fig. 2a enables us to observe several weak bands at 1485, 999, 796, 623 and 409 cm-‘. On the basis of normal coordinate calculations [12-141, the following tentative assignments may be made on the assumption that the PPP molecule has L&, symmetry. The bands at 1485, 623 and 409 cm-’ are attributed to b,, the 796 cm-’ band to elE and the 999 cm-’ band to 6%. The strong bands at 1593, 1281 and 1223 cm-’ have already been assigned to alg [12-141. A netural ,PPV film shows a broad it* + it electronic absorption band centered at about 4OUnm [15]. The wavelength of 1064 nm used in the present ET-Raman measurements is far from this absorption. Thus, the 1064~nm-excited IT-Raman spectrum in Fig. 2b represents an off-resonant case. The Raman shifts observed with laser lines of various wavelengths from near-ultraviolet to red show little changes in wavenumber [16,17]. The Raman shifts observed in the present study are almost the same as those observed with red-light excitation, which also correspond to an off-resonant case. In the Raman spectra of paru-substituted benzenes, strong bands (a doublet) are observed in the region of 860-810 cm-’ [18]. The 796cm-’ band of PPP probably corresponds to this mode, although its intensity is very weak. In the case of PPV no band is observed in this region [16]. This is the case with poly(paru-phenylene sulphide) [19] and polyaniline 1A [poly(imino-1,4-phenylene)] [20]. This absence of strong bands in this region is characteristic of the phenylene ring in conducting polymers. According to normal coordinate calculations of PPP [ 121 and PPV [ll], both the 1223 cm-’ band of PPP and the 1172 cm-’ band of PPV are assignable to mixtures of the symmetric C,=C, stretching and CH in-plane bending vibrations. A neutral PTV film has a broad Z* t x absorption centered at about 540 nm [21]. It has been reported that some Raman bands in PTV exhibit moderate shifts, depending on the exciting wavelength [16,22], i.e. the bands at 1596, 1424, 1294 and 937 cm-’ shift, respectively, to 1584, 1409, 1285 and 929 cm-’ with the shift in the exciting wavelength from 441.6 to 632.8 nm [16]. The Raman shifts observed with KM-rim excitation (Fig. 2c) are almost the same as those with 632.8~nm excitation. The 1064-rim-excited spectrum represents an off-resonant case. A neutral PA film has a broad x* t x visible absorption centered at about 670 nm [23]. In the 632.8~nm-excited resonant Raman spectrum (Fig. 3a), two strong bands are observed at 1461 and 1069cm-’ and a weak band is observed at 1291 cm-‘. The
1370
Y. FvavrtAw~ et al.
1500 1000 RAMAN SHIFT I cm’ Fig. 3. Dependence of the 632.gnm-excited Raman spectra of tranr-polyacetylene upon dopant (Na) concentration: (a) neutral; (b)-(d) Na-doped. Estimated dopant concentrations are 8.5,9.5 and 14% (full doping) for (b), (c) and (d), respectively.
1461 cm-’ band is assigned to a mode mainly consisting of the C=C stretching vibration. The 1291 and 1069 cm-’ bands are assigned to mixtures of C-C stretching and CH in-plane bending vibrations. It has been reported that the positions of the 1461 and 1069 cm-’ bands are sensitive to the number of truns-conjugated C=C bonds [23]. With increasing conjugation length these bands shift to lower wavenumbers. In the 1064~nmexcited Raman spectrum (Fig. 4a), two strong bands are found at 1458 and 1066 cm-‘. These wavenumbers are slightly lower than those observed with 632.8~nm excitation,
1500 1000 RAMAN SHFT / cm“
4
Fig. 4. Dependence of the 1064-nm-excited FT-Raman spectra of tranr-polyacetylene upon dopant (Na) concentration. See Fig. 3 for other information.
FT-Ramanspectrosmpy of conducting polymers
1371
downshifts are explained by increased contributions from very long transconjugated segments. These
Doped polymers
upon It is well known that electronic absorptions of conducting polymers change _ _ doping. As Na-doping proceeds in PA, the visible absorption becomes weak and a very broad absorption appears in the entire NIR region [9,24]. The maximum absorption region is located between 8000 and 6OOOcm-‘. Thus, the excitation wavelength of 1064nm is resonant with this NIR absorption. On the other hand, the excitation wavelength of 632.8 nm is resonant with both absorptions. Dependences of the 632.g nm- and 1064~rim-excitedRaman spectra upon the Na content are shown in Figs 3 and 4, respectively. In the 632.8~nm-excited Raman spectra of doped PA, small changes from the spectrum of the neutral PA (Fig. 3a) are observed at low’Na-doping levels (Fig. 3b and c). The Raman spectrum at full Na-doping (Fig. 3d) is greatly different from that of the neutral PA. This is consistent with the results reported previously [9]. However, weak bands are observed at 1274-1273 cm-’ at low doping levels, which were not present in the previous report [9]. In the Raman spectrum at full doping level (Fig. 3d), a band with a medium intensity is observed at 1261 cm-‘, which is characteristic of donor-doping [25]. The strong bands in the 1473-1461 cm-’ region disappear and a new band appears at 1552 cm-‘. In the case of 1064-nm-excitation, even at low doping levels significant spectral changes are observed (Fig. 4b and c). The two strong bands become very broad, and both of these bands shift to higher wavenumbers as the dopant concentration increases. Weak bands are observed at 1277-1273 cm-‘. At full doping level, the spectral pattern (Fig. 4d) is quite different from those of the neutral and lightly doped samples. A band characteristic of donor-doping is observed at 1250 cm-’ with a weak intensity. The two strong bands in the 1475-1458 and 1095-1066 cm-’ regions in Fig. 4a-c disappear, and instead a moderately broad band appears at 1505cm-‘. The position of this band is 47 cm-’ lower than the 1552 cm-’ band observed with 632.gnm excitation (Fig. 3d). Although further studies are needed to establish the assignments of the bands observed in Figs 3d and 4d, our present interpretation may be summarized as follows. (1) The 1064~nm-excited J?I’-Raman spectrum (Fig. 4d), and probably the 632.8~nmexcited Raman spectrum (Fig. 3d) as well, arises from the doped domains. (2) Comparison between Figs 3d and 4d shows that the spectral patterns are similar but the band positions are different. This indicates that the doped domains are not uniform; i.e. the intensities of Raman bands arising from domains of a certain type are resonantly enhanced with 632.8~nm excitation, and so are those arising from domains of another type with 1064-nm excitation. (3) The strongest bands in Figs 3d and 4d are found at 1552 and 1505 cm-‘, respectively. These relatively high wavenumbers mean that these bands should be assigned to a mode mainly associated with the stretches of the CC bonds having double bond characters. Therefore, the bond alternation remains in the doped domains, as is the case with neutral PA. It has been reported [24] that an abrupt increase of electrical conductivity occurs at a low dopant concentration (~0.5%) and that a Pauli susceptibility appears at a higher dopant concentration. It is possible that these changes in electrical conductivity and magnetic susceptibility are associated with the two-stage changes in the 1064~rim-excited Raman spectra, viz. the first one occurring between Fig. 4a and b and the second one between Fig. 4c and d. With 632.~nm excitation the first changes are not clearly observed, because at low doping levels the 632.8~nm line is primarily resonant with the visible absorption. Structural implications in the two-stage changes in the Raman spectra of PA upon Na-doping should be elucidated in future. The FT-Raman spectra of fully K-doped and iodine-doped PAS are shown in Fig. 5a and b, respectively. The spectrum of the fully K-doped PA is similar to those of PA
1372
Y.FURU~L~WA
etal.
RAMAN SHIFT /
cm’
Fig. 5. 1064-nm-Excited FT-Raman spectra of fully doped conducting polymers: (a) K-doped trawpolyacetylene (PA); (b) iodine-doped PA; (c) Na-doped poly(l,Cphenylene vinylene) (PPV); (d) H,SO,-doped PPV.
lightly doped with Na (Fig. 4b and c). Thus, structures of the donor-doped domains depend on the donor species. Some similarities exist at first glance between the spectral pattern of the fully iodine-doped PA and that of the fully Na-doped PA. However, they are different from each other in many respects if examined in detail. It is noted that the iodine-doped PA has a band characteristic of acceptor. doping at 1296 cm-‘, whereas the donor-doped PAS show a characteristic band in a lower-wavenumber region of 1277-1250 cm-‘. The FT-Raman spectra of heavily Na- and H,SO,-doped PPV films are shown in Fig. 5c and d, respectively. They are quite different from the spectrum of the neutral film (Fig. 2b), and are attributed to doped domains. These Raman bands can be assigned on the basis of the data obtained for radical ions and divalent ions of appropriate oligomers which represent the spectroscopic properties of the relevant polymers, because radical ions and divalent ions correspond, respectively, to polarons and bipolarons in the polymer chains [26]. We call this methodology of assignments the “oligomer approach”. In line with this methodology, we have observed the Raman spectrum of a THF solution of divalent anion of an oligomer, CH,(C,I-QCH=CH(C,I-IJCH=CH(C,&) CH=CH(C&)CH3 where C&I, stands for the 1,4-phenylene ring. The Raman spectrum of the divalent anion of this molecule shows the bands at 1591 (strong), 1566 (weak), 1506 (strong), 1319 (medium), 1296 (medium), 1217 (weak) and 1165 (strong) cm-‘. They have corresponding bands in the FT-Raman spectrum of Na-doped PPV (Fig. 5c), although small differences in wavenumber are observed. Therefore, the FT-Raman spectrum of Na-doped PPV is attributed to negative bipolarons which extend over a few repeating units. By using various laser lines between 488.0 and 740 nm, negative polarons localized over one, two and three repeating units can be identified. Details will be published elsewhere [27].
CONCLUSION
It is relatively easy to measure 1064-nm-excited FT-Raman spectra with high signal-tonoise ratios from a class of polymers which become electrical conductors upon doping, not only from their neutral (insulating) state but also from their doped (conducting) state. NIR IT-Raman spectroscopy seems to have great potentialities as a tool for
FT-Raman spectroscopy of conducting polymers
1373
detailed structrual studies of this class of polymers-doped polymers in particularbecause they usually exhibit absorptions in the NIR region. The rigorously resonant or preresonant Raman effect between the NIR absorptions and NIR laser lines is expected to selectively enhance the intensities of Raman bands due to vibrations localized in the doped domains. Therefore, Raman spectra observed in this way can be a rich source of information on the doped domains. In the present study, examples of such cases have been reported for polyacetylene and poly( 1 ,Cphenylene vinylene) . Acknowledgements-The present work was supported in part by a Grant-in-Aid for Scientific Research No. 01430004 and a Grant-in-Aid for Developmental Scientific Research No. 63840014 from the Ministry of Education, Science and Culture.
REFERENCES [l] H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang and A. J. Heeger, J. Chem. Sot., Chem. Commun. 578 (1977). [2] H. Naarmann and N. Theophilou, Synth. Met. 22, 1 (1987). [3] J. Tsukamoto, A., Takahashi and K. Kawasaki, Jap. J. Appl. Phys. 29, 125 (1990). [4] A. J. Heeger, S. Kivelson, J. R. Schrieffer and W.-P. Su, Rev. Mod. Phys. 60,781 (1988). [5] T. Ito, H. Shirakawa and S. Ikeda, J. Polym. Sci., Polym. Chem. Ed. 12, 11 (1974). [6] T. Yamamoto and A. Yamamoto, Chem. Lett. 353 (1977). [7] I. Murase, T. Ohnishi, T. Noguchi and M. Hirooka, Polym. Commun. 25,327 (1984). [8] I. Murase, T. Ohnishi, T. Noguchi and M. Hirooka, Polym. Commun. 28, 229 (1987). [9] J. Tanaka, Y. Saito, M. Shimizu, C. Tanaka and M. Tanaka, Bull. Chem. Sot. Jpn. 60,1595 (1987). [lo] S. Krichene, S. Lefrant, G. Froyer, F. Maurice and Y. Pelous, J. Phys. Colloq. 44, C3-733 (1983). [ll] S. Lefrant, J. P. Buisson and H. Eckardt, Synrh. Met. 37, 91 (1990). [12] G. Zannoni and G. Zerbi, J. Chem. Phys. 82,31 (1985). [13] I. BoZovic and D. RakoviC, Phys. Rev. B32,4235 (1985). [14] S. Krichene, J. P. Buisson and S. Lefrant, Synth. Met. 17,589 (1987). [15] D. R. Gagnon, J. D. Capistran, F. E. Karasz, R. W. Lenz and S. Antoun, Polymer 28, 567 (1987). [16] Y. Furukawa, A. Sakamoto and M. Tasumi, J. Phys. Chem. 93,5354 (1989). [17] A. Sakamoto, Y. Furukawa and M. Tasumi, Macromolecules, to be submitted. [18] M. M. Siamwiza, R. C. Lord, M. C. Chen, T. Takamatsu, I. Harada, H. Matsuura and T. Shimanouchi, Biochemfstry 14,487O (1975). [19] P. Piaggio, C. Cuniberti, G. Dellepiane, E. Campani, G. Gorini, G. Massetti, M. Novi and G. Petrillo, Specrrochim. Acra 4SA, 347 (1989). [20] Y. Furukawa, F. Ueda, Y. Hyodo, I. Harada, T. Nakajima and T. Kawagoe, Macromolecules 21, 1297 (1988). [21] S. Yamada, S. Tokito, T. Tsutsui and S. Saito, J. Chem. Sot., Chem. Commun. 1448 (1987). [22] A. J. Brasset, N. F. Colaneri, D. D. C. Bradley, R. A. Lawrence, R. H. Friend, H. Murata, S. Tokito, T. Tsutsui and S. Saito, Phys. Rev. B41, 10586 (1990). [23] I. Harada, Y. Furukawa, M. Tasumi, H. Shirakawa and S. Ikeda, J. Chem. Phys. 73,4746 (1980). [24] T.-C. Chung, F. Moraes, J. D. Flood and A. J. Heeger, Phys. Rev. B29, 2341 (1984). [25] Y. Furukawa, I. Harada, M. Tasumi, H. Shirakawa and S. Ikeda, C/rem. Len. 1489 (1981). [26] J. L. Bredas, Mol. Cryst. Liq. Cryst. 118, 49 (1985). [27] A. Sakamoto, Y. Furukawa and M. Tasumi, manuscript in preparation.
Spdmhhica Acta, Vol. 4lA, No. 9/10, pp. 1367-1373, 1991 Rintcd in Great Britain
lOGhmometer=excited Fourier transform Raman spectroscopy of conducting polymers YUKIO FURUKAWA,* HROSHI OHTA, AKIRA SAKAM~TO and Mrrsuo TASUMI Department of Chemistry, Faculty of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan (Received 25 March 1991)
Ah&act-Application of 1064-urn-excitedFourier transform(FT)-Ramanspectroscopy to the characterixation of conducting polymers is described. 1064-nm-excitedFT-Ramanspectrawith high signal-to-noise ratios are obtained from polyacetylene (PA), poly(l,Cphenylene) (PPP), poly(l+phenylene vinylene) (PPV) and poly(2,5-thienylene vinylene) (PTV) in their neutral (insulating) state. The resonant Raman spectra of acceptor- or donor-doped (conducting) PA and PPV are also obtained wih 1064-nmexcitation. The resonant Raman spectra of Na-doped PA change in two stages with increasing dopant concentration, the first change corresponding to the increase in electrical conductivity and the second to the appearance of a Pauli susceptibility. The 1064-nmexcited FT-Raman spectrum of Na-doped PPV indicates existence of negative bipolaronswhich are equivalent to divalent anions extending over a few repeatingunits in the polymer chains. IN-I-R~DUCTION
Fourier transform (FT)-Raman spectroscopy is now the most widely used method for overcoming the fluorescence problem in Raman spectroscopy. It should be noted that this method has another advantage of obtaining resonant Raman spectra of materials having NIR absorptions, such as conducting polymers, chargetransfer complexes, biological pigments, etc. It is the purpose of this paper to demonstrate the potential of 1064-rim-excited Raman spectroscopy in the characterization of conducting polymers. Since organic polymers are typical insulators, their inability to carry electricity has been utilized in their various applications. However, a new class of organic polymers with the ability to conduct electricity has been developed for the past 15 years. These conducting polymers have conjugated n-electrons in common. They show high electrical conductivity (1 - lo5 S cm-‘, S=S2-‘) when doped with electron acceptors such as iodine, AsFs, H.$O.,, etc. or electron donors such as alkali metals, although they are insulators or semiconductors in the undoped (neutral) state [l]. The maximum conductivity reported so far for organic polymers at room temperature is more than lti S cm-’ [2,3], which is comparable with the conductivity of copper (6 x 10s S cm-‘). When electrons are removed from (or added to) n-conjugated polymer chains, structural changes supposedly extending over several repeating units occur. These doped-domain structures are classified into polarons, bipolarons, and solitons according to their types, and have been extensively discussed in relation to their roles in electric conduction [4]. Electronic absorptions associated with the doped domains often appear in the region from visible to NIR. Accordingly, NIR resonant Raman spectroscopy may give a clue to structural studies of the doped domains. In this paper, we report and discuss the 1064-nm-excited IT-Raman spectra of trunspolyacetylene (PA), poly( 1 ,Cphenylene) (PPP) , poly(1 ,Cphenylene vinylene) (PPV) and poly(2,5_thienylene vinylene) (PTV) in both the neutral (insulating) and doped (conducting) states. Their structures are shown in Fig. 1. NEAR-IN FRARBD (NIR)
EXPERIMENTAL
Samples &Rich PA films were synthesized according to SHIRAKAWA’S method [S] at -78°C and thermally isomerized to truns-PA at 180°C for 60 min in vacua. PPP was prepared by polycondensation of the Grignard reagent from l&dibromobenzene with an NiClz catalyst 163. PPV and PTV were synthesized via soluble precursors according to the methods in the literature [7,8]. * Author to whom correspondenceshould be addressed. 1367
Y. FurwKnw.4 et
1368
al.
(d)
04
Fig. 1. Structures of neutral polymers: (a) tram-polyacetylene (PA); (b) poly(l,Cphenylene) (PPP); (c) poly(l,Cphenylene vinylene) (PPV); (d) poly(2,Whienylene vinylene) @TV).
PA and PPV films were doped with Na by treating them with a THF solution of sodium naphthalide in a completely sealed ampoule. The dopant content was controlled by the concentration of sodium naphthalide and the period of immersing the film into the solution. Light doping of PA films was performed by immersing them into a 0.01 mol dme3 solution for 10-30 min. Heavy doping of PA and PPV films was performed in the same way by using a more concentrated solution (0.1 mol dmT3) and a longer time (45-60 min). After washing the film with fresh THF, the ampoule was sealed again. Raman measurments were made for such films in sealed ampoules. The dopant (Na) contents were estimated from the relationship between their 632.8-nm-excited Raman spectra and the Na contents [9]. Full doping of a PA film with K was made in a similar manner by using a 0.1 mol dmm3 THF solution of potassium naphthalide. Films of PA were fully doped with iodine by exposing them to the iodine vapor. The electrical conductivity measured by a d.c. four-probe technique was 44 S cm-‘. The iodine content was 22 mol% per CH unit (CHI,& by weight measurement. H,SOrdoping of PPV was carried out by immersing PPV films in concentrated sulphuric acid.
Measurements 1064-nm-Excited FT-Raman spectra were measured on a JEOL JIR-5500 Fourier transform spectrophotometer modified for Raman measurements. A continuous-wave Nd : YAG laser (CVI YAGMAX C-92) was operated at 1064nm for Raman excitation. The laser beam was passed through an interference filter to remove spontaneous emissions, and was led to the sample without focusing to avoid its thermal degradation. Raman-scattered radiation was collected with a 90” offaxis parabolic mirror in a backscattering configuration. Collected radiation was passed through three long-wavelength-pass dielectric filters (Microcoatings) to reject the Rayleigh scattering. An InGaAs detector (Epitaxx) operated at liquid nitrogen temperature was used. A long-wavelengthpass filter (Corion LL-950) was inserted before the detector to cut visible light. Spectral resolution was 4 cm-‘. A triangular apodization function was applied. 632.8-nm-Excited Raman spectra were measured at room temperature on a Raman spectrophotometer consisting of a Spex 1877 Triplemate and an EG & G PARC 1421 intensified photodiode array detector.
RESULTSAND
DISCUSSION
Neutral polymers The FT-Raman spectra of neutral shown in Fig. 2. The Raman spectra
PPP, PPV and PTV with 1064-nm excitation are obtained have high signal-to-noise ratios in accord
with the general trend that molecules with conjugated n-electrons exhibit large Raman scattering cross-sections. PPP shows intense “fluorescence” backgrounds when visible laser lines are used for Raman excitation; no Raman bands were observed with 488.0-nm excitation due to a strong background, and only a few bands were observed with 632.8-nm excitation [lo]. However, in the lO&nm-excited FT-Raman spectrum (Fig. 2a), no background is observed and the signal-to-noise ratio is high. It has been reported [lo] that the ratio of the intensity of the 1223 cm-’ band to that of the 1281 cm-’ band increases with the
FT-Raman spectroscopy of conducting polymers
1369
h RAMAN SHIFT I cm-’
Fig. 2. 1064~nm-Excited FT-Raman spectra of neutral polymers: (a) poly(l+phenylene) (PPP); (b) poly(l&phenylene vinylene) (PPV); (c) poly(2,5-thienyiene vinylene) (WV).
number of consecutive phenyl rings. Judging from the observed relative intensity in Fig. 2a, the chain length in our sample is rather short. In the Raman spectrum of PPP prepared electrochemically the 1223 cm-’ band is much stronger than the 1281 cm-’ band [ll]. The high quality of the FT-Raman spectrum in Fig. 2a enables us to observe several weak bands at 1485, 999, 796, 623 and 409 cm-‘. On the basis of normal coordinate calculations [12-141, the following tentative assignments may be made on the assumption that the PPP molecule has L&, symmetry. The bands at 1485, 623 and 409 cm-’ are attributed to b,, the 796 cm-’ band to elE and the 999 cm-’ band to 6%. The strong bands at 1593, 1281 and 1223 cm-’ have already been assigned to alg [12-141. A netural ,PPV film shows a broad it* + it electronic absorption band centered at about 4OUnm [15]. The wavelength of 1064 nm used in the present ET-Raman measurements is far from this absorption. Thus, the 1064~nm-excited IT-Raman spectrum in Fig. 2b represents an off-resonant case. The Raman shifts observed with laser lines of various wavelengths from near-ultraviolet to red show little changes in wavenumber [16,17]. The Raman shifts observed in the present study are almost the same as those observed with red-light excitation, which also correspond to an off-resonant case. In the Raman spectra of paru-substituted benzenes, strong bands (a doublet) are observed in the region of 860-810 cm-’ [18]. The 796cm-’ band of PPP probably corresponds to this mode, although its intensity is very weak. In the case of PPV no band is observed in this region [16]. This is the case with poly(paru-phenylene sulphide) [19] and polyaniline 1A [poly(imino-1,4-phenylene)] [20]. This absence of strong bands in this region is characteristic of the phenylene ring in conducting polymers. According to normal coordinate calculations of PPP [ 121 and PPV [ll], both the 1223 cm-’ band of PPP and the 1172 cm-’ band of PPV are assignable to mixtures of the symmetric C,=C, stretching and CH in-plane bending vibrations. A neutral PTV film has a broad Z* t x absorption centered at about 540 nm [21]. It has been reported that some Raman bands in PTV exhibit moderate shifts, depending on the exciting wavelength [16,22], i.e. the bands at 1596, 1424, 1294 and 937 cm-’ shift, respectively, to 1584, 1409, 1285 and 929 cm-’ with the shift in the exciting wavelength from 441.6 to 632.8 nm [16]. The Raman shifts observed with KM-rim excitation (Fig. 2c) are almost the same as those with 632.8~nm excitation. The 1064-rim-excited spectrum represents an off-resonant case. A neutral PA film has a broad x* t x visible absorption centered at about 670 nm [23]. In the 632.8~nm-excited resonant Raman spectrum (Fig. 3a), two strong bands are observed at 1461 and 1069cm-’ and a weak band is observed at 1291 cm-‘. The
1370
Y. FvavrtAw~ et al.
1500 1000 RAMAN SHIFT I cm’ Fig. 3. Dependence of the 632.gnm-excited Raman spectra of tranr-polyacetylene upon dopant (Na) concentration: (a) neutral; (b)-(d) Na-doped. Estimated dopant concentrations are 8.5,9.5 and 14% (full doping) for (b), (c) and (d), respectively.
1461 cm-’ band is assigned to a mode mainly consisting of the C=C stretching vibration. The 1291 and 1069 cm-’ bands are assigned to mixtures of C-C stretching and CH in-plane bending vibrations. It has been reported that the positions of the 1461 and 1069 cm-’ bands are sensitive to the number of truns-conjugated C=C bonds [23]. With increasing conjugation length these bands shift to lower wavenumbers. In the 1064~nmexcited Raman spectrum (Fig. 4a), two strong bands are found at 1458 and 1066 cm-‘. These wavenumbers are slightly lower than those observed with 632.8~nm excitation,
1500 1000 RAMAN SHFT / cm“
4
Fig. 4. Dependence of the 1064-nm-excited FT-Raman spectra of tranr-polyacetylene upon dopant (Na) concentration. See Fig. 3 for other information.
FT-Ramanspectrosmpy of conducting polymers
1371
downshifts are explained by increased contributions from very long transconjugated segments. These
Doped polymers
upon It is well known that electronic absorptions of conducting polymers change _ _ doping. As Na-doping proceeds in PA, the visible absorption becomes weak and a very broad absorption appears in the entire NIR region [9,24]. The maximum absorption region is located between 8000 and 6OOOcm-‘. Thus, the excitation wavelength of 1064nm is resonant with this NIR absorption. On the other hand, the excitation wavelength of 632.8 nm is resonant with both absorptions. Dependences of the 632.g nm- and 1064~rim-excitedRaman spectra upon the Na content are shown in Figs 3 and 4, respectively. In the 632.8~nm-excited Raman spectra of doped PA, small changes from the spectrum of the neutral PA (Fig. 3a) are observed at low’Na-doping levels (Fig. 3b and c). The Raman spectrum at full Na-doping (Fig. 3d) is greatly different from that of the neutral PA. This is consistent with the results reported previously [9]. However, weak bands are observed at 1274-1273 cm-’ at low doping levels, which were not present in the previous report [9]. In the Raman spectrum at full doping level (Fig. 3d), a band with a medium intensity is observed at 1261 cm-‘, which is characteristic of donor-doping [25]. The strong bands in the 1473-1461 cm-’ region disappear and a new band appears at 1552 cm-‘. In the case of 1064-nm-excitation, even at low doping levels significant spectral changes are observed (Fig. 4b and c). The two strong bands become very broad, and both of these bands shift to higher wavenumbers as the dopant concentration increases. Weak bands are observed at 1277-1273 cm-‘. At full doping level, the spectral pattern (Fig. 4d) is quite different from those of the neutral and lightly doped samples. A band characteristic of donor-doping is observed at 1250 cm-’ with a weak intensity. The two strong bands in the 1475-1458 and 1095-1066 cm-’ regions in Fig. 4a-c disappear, and instead a moderately broad band appears at 1505cm-‘. The position of this band is 47 cm-’ lower than the 1552 cm-’ band observed with 632.gnm excitation (Fig. 3d). Although further studies are needed to establish the assignments of the bands observed in Figs 3d and 4d, our present interpretation may be summarized as follows. (1) The 1064~nm-excited J?I’-Raman spectrum (Fig. 4d), and probably the 632.8~nmexcited Raman spectrum (Fig. 3d) as well, arises from the doped domains. (2) Comparison between Figs 3d and 4d shows that the spectral patterns are similar but the band positions are different. This indicates that the doped domains are not uniform; i.e. the intensities of Raman bands arising from domains of a certain type are resonantly enhanced with 632.8~nm excitation, and so are those arising from domains of another type with 1064-nm excitation. (3) The strongest bands in Figs 3d and 4d are found at 1552 and 1505 cm-‘, respectively. These relatively high wavenumbers mean that these bands should be assigned to a mode mainly associated with the stretches of the CC bonds having double bond characters. Therefore, the bond alternation remains in the doped domains, as is the case with neutral PA. It has been reported [24] that an abrupt increase of electrical conductivity occurs at a low dopant concentration (~0.5%) and that a Pauli susceptibility appears at a higher dopant concentration. It is possible that these changes in electrical conductivity and magnetic susceptibility are associated with the two-stage changes in the 1064~rim-excited Raman spectra, viz. the first one occurring between Fig. 4a and b and the second one between Fig. 4c and d. With 632.~nm excitation the first changes are not clearly observed, because at low doping levels the 632.8~nm line is primarily resonant with the visible absorption. Structural implications in the two-stage changes in the Raman spectra of PA upon Na-doping should be elucidated in future. The FT-Raman spectra of fully K-doped and iodine-doped PAS are shown in Fig. 5a and b, respectively. The spectrum of the fully K-doped PA is similar to those of PA
1372
Y.FURU~L~WA
etal.
RAMAN SHIFT /
cm’
Fig. 5. 1064-nm-Excited FT-Raman spectra of fully doped conducting polymers: (a) K-doped trawpolyacetylene (PA); (b) iodine-doped PA; (c) Na-doped poly(l,Cphenylene vinylene) (PPV); (d) H,SO,-doped PPV.
lightly doped with Na (Fig. 4b and c). Thus, structures of the donor-doped domains depend on the donor species. Some similarities exist at first glance between the spectral pattern of the fully iodine-doped PA and that of the fully Na-doped PA. However, they are different from each other in many respects if examined in detail. It is noted that the iodine-doped PA has a band characteristic of acceptor. doping at 1296 cm-‘, whereas the donor-doped PAS show a characteristic band in a lower-wavenumber region of 1277-1250 cm-‘. The FT-Raman spectra of heavily Na- and H,SO,-doped PPV films are shown in Fig. 5c and d, respectively. They are quite different from the spectrum of the neutral film (Fig. 2b), and are attributed to doped domains. These Raman bands can be assigned on the basis of the data obtained for radical ions and divalent ions of appropriate oligomers which represent the spectroscopic properties of the relevant polymers, because radical ions and divalent ions correspond, respectively, to polarons and bipolarons in the polymer chains [26]. We call this methodology of assignments the “oligomer approach”. In line with this methodology, we have observed the Raman spectrum of a THF solution of divalent anion of an oligomer, CH,(C,I-QCH=CH(C,I-IJCH=CH(C,&) CH=CH(C&)CH3 where C&I, stands for the 1,4-phenylene ring. The Raman spectrum of the divalent anion of this molecule shows the bands at 1591 (strong), 1566 (weak), 1506 (strong), 1319 (medium), 1296 (medium), 1217 (weak) and 1165 (strong) cm-‘. They have corresponding bands in the FT-Raman spectrum of Na-doped PPV (Fig. 5c), although small differences in wavenumber are observed. Therefore, the FT-Raman spectrum of Na-doped PPV is attributed to negative bipolarons which extend over a few repeating units. By using various laser lines between 488.0 and 740 nm, negative polarons localized over one, two and three repeating units can be identified. Details will be published elsewhere [27].
CONCLUSION
It is relatively easy to measure 1064-nm-excited FT-Raman spectra with high signal-tonoise ratios from a class of polymers which become electrical conductors upon doping, not only from their neutral (insulating) state but also from their doped (conducting) state. NIR IT-Raman spectroscopy seems to have great potentialities as a tool for
FT-Raman spectroscopy of conducting polymers
1373
detailed structrual studies of this class of polymers-doped polymers in particularbecause they usually exhibit absorptions in the NIR region. The rigorously resonant or preresonant Raman effect between the NIR absorptions and NIR laser lines is expected to selectively enhance the intensities of Raman bands due to vibrations localized in the doped domains. Therefore, Raman spectra observed in this way can be a rich source of information on the doped domains. In the present study, examples of such cases have been reported for polyacetylene and poly( 1 ,Cphenylene vinylene) . Acknowledgements-The present work was supported in part by a Grant-in-Aid for Scientific Research No. 01430004 and a Grant-in-Aid for Developmental Scientific Research No. 63840014 from the Ministry of Education, Science and Culture.
REFERENCES [l] H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang and A. J. Heeger, J. Chem. Sot., Chem. Commun. 578 (1977). [2] H. Naarmann and N. Theophilou, Synth. Met. 22, 1 (1987). [3] J. Tsukamoto, A., Takahashi and K. Kawasaki, Jap. J. Appl. Phys. 29, 125 (1990). [4] A. J. Heeger, S. Kivelson, J. R. Schrieffer and W.-P. Su, Rev. Mod. Phys. 60,781 (1988). [5] T. Ito, H. Shirakawa and S. Ikeda, J. Polym. Sci., Polym. Chem. Ed. 12, 11 (1974). [6] T. Yamamoto and A. Yamamoto, Chem. Lett. 353 (1977). [7] I. Murase, T. Ohnishi, T. Noguchi and M. Hirooka, Polym. Commun. 25,327 (1984). [8] I. Murase, T. Ohnishi, T. Noguchi and M. Hirooka, Polym. Commun. 28, 229 (1987). [9] J. Tanaka, Y. Saito, M. Shimizu, C. Tanaka and M. Tanaka, Bull. Chem. Sot. Jpn. 60,1595 (1987). [lo] S. Krichene, S. Lefrant, G. Froyer, F. Maurice and Y. Pelous, J. Phys. Colloq. 44, C3-733 (1983). [ll] S. Lefrant, J. P. Buisson and H. Eckardt, Synrh. Met. 37, 91 (1990). [12] G. Zannoni and G. Zerbi, J. Chem. Phys. 82,31 (1985). [13] I. BoZovic and D. RakoviC, Phys. Rev. B32,4235 (1985). [14] S. Krichene, J. P. Buisson and S. Lefrant, Synth. Met. 17,589 (1987). [15] D. R. Gagnon, J. D. Capistran, F. E. Karasz, R. W. Lenz and S. Antoun, Polymer 28, 567 (1987). [16] Y. Furukawa, A. Sakamoto and M. Tasumi, J. Phys. Chem. 93,5354 (1989). [17] A. Sakamoto, Y. Furukawa and M. Tasumi, Macromolecules, to be submitted. [18] M. M. Siamwiza, R. C. Lord, M. C. Chen, T. Takamatsu, I. Harada, H. Matsuura and T. Shimanouchi, Biochemfstry 14,487O (1975). [19] P. Piaggio, C. Cuniberti, G. Dellepiane, E. Campani, G. Gorini, G. Massetti, M. Novi and G. Petrillo, Specrrochim. Acra 4SA, 347 (1989). [20] Y. Furukawa, F. Ueda, Y. Hyodo, I. Harada, T. Nakajima and T. Kawagoe, Macromolecules 21, 1297 (1988). [21] S. Yamada, S. Tokito, T. Tsutsui and S. Saito, J. Chem. Sot., Chem. Commun. 1448 (1987). [22] A. J. Brasset, N. F. Colaneri, D. D. C. Bradley, R. A. Lawrence, R. H. Friend, H. Murata, S. Tokito, T. Tsutsui and S. Saito, Phys. Rev. B41, 10586 (1990). [23] I. Harada, Y. Furukawa, M. Tasumi, H. Shirakawa and S. Ikeda, J. Chem. Phys. 73,4746 (1980). [24] T.-C. Chung, F. Moraes, J. D. Flood and A. J. Heeger, Phys. Rev. B29, 2341 (1984). [25] Y. Furukawa, I. Harada, M. Tasumi, H. Shirakawa and S. Ikeda, C/rem. Len. 1489 (1981). [26] J. L. Bredas, Mol. Cryst. Liq. Cryst. 118, 49 (1985). [27] A. Sakamoto, Y. Furukawa and M. Tasumi, manuscript in preparation.