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J. Phys. Chem. 1989, 93, 4994-4998
Conclusion The fluorescence spectra of 7-azaindole show a dramatic red shift during the gel-xerogel stages, revealing that 7-azaindole is an excellent photophysical probe for gel-xerogel transition. This large shift in fluorescence is due to stabilization in the excited state, which is probably induced by a specific interaction between 7-azaindole and silanol groups, which are surrounded by a large
number of polar silanol groups. Tautomeric fluorescence, due to double proton transfer, is thought to occur in the xerogel as well as the starting solution and the gel. Further study of this material system will clarify the microstructure of silica cage, the interaction between 7AI and silanol groups, and the mechanism of double Proton transfer. Registry No. 7A1, 271-63-6; C2H50H,64-17-5; silica, 763 1-86-9.
Infrared Dichroic Studies of Poiythiophenes Shu Hotta,* Mamoru Soga, and Nobuo Sonoda Central Research Laboratories, Matsushita Electric Industrial Co., Ltd.. Moriguchi. Osaka 570, Japan (Received: September 22, 1988; In Final Form: January 27, 1989)
Infrared dichroic spectra of the uniaxially drawn films of polythiophene and its 3-hexyl derivative, poly(3-hexylthiophene), are presented. The dichroic measurements were carried out using probe rays (incident perpendicular to the film plane) whose polarization plane lies either parallel or perpendicular to the drawn direction. For the neutral films of polythiophene and poly(3-hexylthiophene) the ring stretching mode, v(ring), with the symmetric species of bl is strongly polarized along the drawn direction of the film, while the CH out-of-plane deformation mode, y(CH), is highly polarized perpendicular to the drawn direction. The results of the dichroic measurements on the drawn films of partially oxidized polythiophene and poly(3-hexylthiophene) indicate that the modes induced in the mid-infrared region (ca. 135C-950 cm-I) by their partial oxidation (doping) are highly polarized along the drawn direction. Furthermore, the broad absorption mode occurring in the near-infrared region (around 4000 cm-l) is also found to be polarized along the drawn direction. On the basis of the symmetry analyses of the infrared modes on the polymers, we show that the polymer chains are highly aligned along the drawn direction of the films. This is confirmed by enhancement of electrical conductivity along the drawn direction observed for the uniaxially drawn films of the polymers.
Introduction Conducting polymers constitute a growing field of the polymer science in the 10 years since polyacetylene was discovered to be highly conducting upon partial oxidation (doping) with, for example, iodine and arsenic pentaflu0ride.l After this discovery a large set of conducting polymers has been synthesized by using various methods. These also exhibit relatively high conductivity after being doped. Such potentially high conductivity which rivals or even exceeds that of metal2 would be one of their peculiar physical properties that are brought about from the one-dimensional (or quasi-one-dimensional) electronic system. That is, these conducting polymers are characterized by their linear r-conjugated system ranging over many repeated constituent units and consequently being allowed to possess the electronic one-dimensionality and anisotropy. The investigations of anisotropic properties are, therefore, of particular interest and importance in the field of conducting polymer researches. Although single crystals of the conducting polymers are ideally needed for these investigations, it is very difficult to obtain single crystals large enough to study the anisotropy. When the conducting polymers are obtained in film form, however, the anisotropic properties can be examined by using their drawn films; polymer chains are expected to be aligned along the drawn direction. From this point of view, in fact, polyacetylene (PA) has been widely and successfully investigated among various conducting polymers. For instance, X-ray diffraction studies have demonstrated that the polymer chains in crystalline PA film are highly aligned with the c axis (fiber axis) parallel to the drawn directions3 Drawn films of partially oxidized PA exhibit enhanced electrical conductivity along the drawn direction compared with an undrawn film.3*4 ( I ) Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J. J. Chem. Sor., Chem. Commun. 1977, 578. (2) Basescu, N.; Liu, 2.-X.; Moses, D.; Heeger, A. J.; Naarmann, H.; Theophilou, N. Nature 1987, 327, 403. (3) Kahlert, H.; Leising, G. Mol. Cryst. Liq.Cryst. 1985, 117, 1. (4) MacDiarmid, A. G . ; Heeger, A. J. Synrh. Mer. 1979/80, 1 , 101.
0022-3654 I89 12093-4994$01.50/0 , I
,
Amongst the studies of the anisotropy of conducting polymers, infrared (IR) analyses are powerful and often utilized; the vibrational modes are usually observed as very sharp bands in the IR region and their absorption intensity is sensitively enhanced or reduced along the specific polarization direction by the polarized IR measurements on the basis of the requirement of the symmetry of these modes. For example, Castiglioni et al.’ have concluded from the IR dichroic measurements that the trans-PA segments do not belong to the D2hsymmetry and, hence, that the trans-PA is a Peierls distorted system. It is well-known, furthermore, that the conducting polymers in general exhibit several vibrational bands that are induced upon doping in the IR region at the positions specific to each polymer species. In the case of PA, these modes are intensified by the polarized measurement using a probe ray whose polarization plane is parallel to the drawn direction.6 Meanwhile, another class of conducting polymers composed of aromatic rings are attracting a growing attention in the conducting polymer studies at present; they are characterized by excellent thermal and environmental ~tability.’.~ Poly@phenylene), polypyrrole, and polythiophene are typical illustrations. Since the hydrogens on the aromatic rings (such as benzene, pyrrole, and thiophene) can be easily substituted with other chemical groups like alkyl, benzyl, and alkoxy, a variety of derivatives of such conducting polymers have been synthesized by using various methods. In particular, the polymers with long chains or bulky side groups are dissolution and/or fusion processible; they are readily dissolved in various kinds of organic solvents or fused by moderate heat. Thus, these polymers are subsequently formed into films by casting their solution or directly melting them.9J0 (5) Castiglioni, C.; Zerbi, G.;Gussoni, M. Solid Stare Commun.1985.56, 863. ( 6 ) Piaggio, P.; Dellepiane, G.; Piseri, L.; Tubino, R.; Taliani, C. Solid State Commun. 1984, 50, 947. ( 7 ) Waltman, R. J.; Bargon, J.; Diaz, A. F. J . Phys. Chem. 1983.87, 1459. (8) Tourillon, G.;Gamier, F. J . Electrochem. SOC.1983, 130, 2042. (9) Elsenbaumer, R. L.; Jen, K. Y . ;Oboodi, R. Synth. Mer. 1986, 15, 169.
0 1989 American Chemical Society
Infrared Dichroic Studies of Polythiophenes
The Journal of Physical Chemistry, Vol. 93, No. 12, 1989
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Figure 1. Chemical structures for polythiophene (a) and poly(3-hexylthiophene) (b). In the case of (b) the hexyl group (C6HI3)is bonded to the thiophene ring at the 3-position.
Even though poly@-phenylene), polypyrrole, and polythiophene themselves are neither dissolution nor fusion processible, they can be synthesized directly into the film form by electrochemically polymerizing corresponding monomers, i.e., benzene, pyrrole, and thiophene, re~pectively."-'~ Once these polymers are allowed to take film forms, it is quite attractive to investigate anisotropic properties using drawn films as in the case of PA. As compared with PA, however, the investigation of the anisotropy on these polymers has yet been limited,14 mainly because of difficulty in obtaining their polymer films that are durable for drawing to a sufficient extent. In this context we have successfully obtained tough and uniform films of polythiophene (PT) and its derivatives with good drawor chemical16syntheses. In particular, ability via electrochemi~al'~ poly( 3-alkylthiophenes) (P3ATs) having long alkyl side groups can be drawn up to 5 times the initial length. This excellent drawability is ascribed to mitigation of the strong interaction between polythiophene backbone through introduction of the long alkyl side groups and has the same origin as the good solubility and fusibility. These P3ATs as well as PT show high conductivity upon partial oxidation (doping).17 With these circumstances for the background, we have carried out the IR dichroic measurements on the uniaxially drawn films of PT and its 3-hexyl derivative of poly(3-hexylthiophene), P3HT (for the chemical structures, see Figure l), both in the neutral and in the partially oxidized states. On the basis of these IR results, we show that the polymer chains of PT and P3HT are aligned along the drawn direction, as in the case with PA.3 This is confirmed by enhanced electrical conductivity along the drawn direction observed for the drawn films of the partially oxidized polymers.
Experimental Section PT was directly prepared electrochemically into film form (ca. 20-km thickness) and thoroughly undoped with electrochemical reduction and methanol wash ensuring rigorous undoping according to the previously reported manner.l8 P3HT was prepared organochemically and undoped with methanol wash followed by reprecipitation from 2-methyltetrahydrofuran solution into methanol.16 The P3HT films (ca.20-km thickness) were obtained by casting a thick chloroform solution of the reprecipitated material onto flat substrates like glass slides.'' These P3HT films (as cast) were further aged at room temperature in vacuo for 2 days. The PT and P3HT films were subsequently uniaxially drawn manually by using a stretcher up to 5 times the initial length. The polymer films (both drawn and undrawn) were partially oxidized successively with iodine at various doping levels in its acetonitrile solution with Iz concentration of lo4, or M (in which P3HT was insoluble). The IR spectra were recorded with an FT/IR-5000 spectrophotometer (Japan Spectroscopic Co., Ltd. (Jasco)) at a resolution (10) Yoshino, K.; Nakajima, S.; Fujii, M.; Sugimoto, R. Polym. Commun. 1987, 28, 309. (1 1) Satoh, M.; Tabata, M.; Kaneto, K.; Yoshino, K. Polym. Commun. 1985, 26, 356. (12) Diaz, A. F.; Kanazawa, K. K.; Gardini, G. P. J . Chem. Soc., Chem. Commun. 1979, 635. (1 3) Tourillon, G.; Gamier, F. J . Electroanal. Chem. Interfacial Electrochem. 1982, l 3 5 , 173.
(14) Satoh, M.; Yamasaki, H.; Aoki, S.; Yoshino, K. Polym. Commun. 1987, 28, 144. ( 1 5 ) Hotta, S.; Hosaka, T.; Shimotsuma, W. Synth. Met. 1983, 6 , 69. (16) Hotta, S.; Soga, M.; Sonoda, N. Synth. Met. 1988, 26, 267. (17) Hotta, S.Synth. Met. 1987, 22, 103. (18) Hotta, S.; Shimotsuma, W.; Taketani, M. Synth. Met. 1984/85, 10, 85.
W 0
WAVE NUMBER ( x 1 0 3 C " ' )
Figure 2. IR dichroic spectra of the drawn film of polythiophene, PT. The drawn ratio of the film was 1.5. The bands at 3066 and 1493 cm-I are highly polarized in parallel with the drawn direction. Doubly split peaks are observed at 791 and 785 cm-I from the E , measurement. Of these, the latter is polarized perpendicular to the drawn direction. The inset shows an enlarged profile of these split modes. z
Figure 3. Geometry of the polarized probe rays whose polarization plane is either on the x-y plane (Ell) or on the z-x plane ( E , ) . The drawn direction of the film is supposed to coincide with t h e y axis and the film plane to be parallel to the y-z plane. The inset shows a polythiophene segment (thienylene) lying on the x-y plane whose symmetry axis (shown with a broken line) coincides with the x axis.
of 4 cm-' on the neutral and partially oxidized polymer films with light doping levels treated in the 104-10-3 M I2 solutions. The dichroic measurements were carried out using probe IR rays which were polarized with a KRS-5 wire grid polarizer (Jasco) and incident perpendicularly onto the film plane with their polarization plane either parallel or perpendicular to the drawn direction of the polymer films. Electrical conductivities of both the drawn and undrawn films partially oxidized with iodine at a heavier doping level in the M I, solution were measured by a four-probe method at room temperature; those of the drawn films were measured in the drawn direction.
Results and Discussion 1 . Neutral Polythiophenes. Figure 2 shows IR dichroic spectra of a film of PT of drawn ratio 1.5. As can be seen in the figure, the absorption bands at 3066 and 1493 cm-l show outstanding dichroism. These bands are highly polarized in parallel with the drawn direction. Doubly split peaks are observed at 791 and 785 cm-' in the spectrum obtained with a probe ray whose polarization plane is perpendicular ( E , ) to the drawn direction of the PT film (see the inset). On the other hand, only one intense peak is noticed
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The Journal of Physical Chemistry, Vol. 93, No. 12, 1989
Hotta et al.
I
I
I / I l 1 , , l l , ~I I, I~ I I / , , / l , , l , , , , ! , , !, ,, , 4 . 0 3 . 0 2 . 0 1.6 1.2 0.8 0.4 I
'
WAVE NUMBER (x103,,-1)
Figure 4. Nonpolarized IR spectra of the as cast and aged films (undrawn) of P3HT. Since the bands due to the hexyl group (at 2960-2860 cm-I) exhibit nearly the same absorbance for the two films, those for the aged film are illustrated for short. at 791 cm-I from E,, measurement (Le., IR measurement with a probe ray polarized parallel to the drawn direction). This difference results from occurrence of the band at 785 cm-' that is highly polarized perpendicularly to the drawn direction. Contrary to this, dichroism of the band at 791 cm-' is inconspicuous. It is noteworthy that only one peak is noticeable at the center (788 cm-') of the two wave numbers for an undrawn film or from nonpolarized measurements of the drawn film.'* Since the PT segment, a thienylene (Figure 3) belongs to the same symmetry as thiophene (monomer); i.e., ,C , each vibrational mode should also have the same symmetry species as thiophene. Following the assignments for thiophene by Dollish et al.,19 the bands at 1493 and 785 cm-' are due to the ring stretching mode, v(ring), with the symmetry species of b, and the C H out-of-plane deformation mode, y(CH), with b2 symmetry, respectively. It is fully established that the mode with bl symmetry for the thiophene is polarized along the direction which is on the thiophene ring plane and is perpendicular to the symmetry axis of the thiophene molecule, and that the mode with b2 symmetry is polarized along the direction which is perpendicular to the thiophene ring plane.20 Connecting this to the dichroic observations, we infer that the symmetry axis of the thiophene ring (or thienylene) lies perpendicular to the drawn direction (y axis); that is, the symmetry axis lies on z-x plane (see Figure 3). In Figure 3 we show the geometry of the polarized probe rays whose polarization plane is either on x-y plane (Ell)or on z-x plane (EL).We assume that the drawn direction of the film coincides with the y axis and that the film plane is parallel to y-z plane. Moreover, this figure displays the special case where the thiophene ring lies on the x-y plane and its symmetry axis coincides with the x axis. Note that the mode with b2 symmetry is not observed when the thiophene ring lies on the y-z plane and the symmetry axis coincides with the z axis. Only when the symmetry axis vector has an x component is, the b2 mode observable from the E , measurement. Consequently, the IR dichroic results imply that PT chains; Le., a sequence of thienylenes is aligned along the drawn direction with the symmetry axis of each thienylene perpendicular to the drawn direction. In contrast to these two modes, the assignment of the C H stretching mode, v(CH), at 3066 cm-' is less clear. Seeing that this band is strongly polarized in the drawn direction, it might be natural that this band also belongs to bl symmetry like the v(ring) mode at 1493 cm-'. In this case, however, the mode with a, (total symmetry) and that with bl symmetry would be located at very close po~iti0ns.l~ As previously pointed out for trans-PA,S.6 (19) Ddlish, F. R.; Fateley, W. G.; Bentley, F. F. Characteristic Raman Frequemes of Organic Compounds; Wiley: New York, 1974; p 220. (20) Herzberg, G. Molecular Spectra and Molecular Structure 11. Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand: Princeton, NJ, 1945; p 252.
WAVE NUMBER
(xlo3cm-I)
Figure 5. IR dichroic spectra of the drawn film of poly(3-hexylthiophene), P3HT. The drawn ratio of the film was 5.0. The bands at 3058 and 1512 cm-' are highly polarized along the drawn direction, whereas the band at 824 cm-I is polarized perpendicular to the drawn direction. Since the bands due to the hexyl group (at 2960-2860 cm-I) exhibit nearly the same intensity with weak dichroism in the two spectra, they are illustrated for short in the E , spectrum. The inset indicates the presence of two lines that are observed as an absorption maximum at 830 cm-' and as a shoulder at 824 cm-I (both indicated with an arrow) in the Ellspectrum. in addition, one has to consider that large fluxes of electronic charge may take place along the PT chains while the C H stretching vibration is occurring. This is probably the case with *-conjugated conducting polymers in general. Figure 4 shows spectra, which were taken by a nonpolarized ray, for the undrawn films of P3HT both as cast and aged (after casting). The IR bands from the solvent (chloroform) are absent or insignificant in the spectra for both of the films. The aged film as a whole exhibits less intense absorption bands relative to the bands at 2960-2860 cm-' due to the hexyl group than the as cast film. We used the aged films for the dichroic measurements, since the intensities of the secondary bands are lessened enough and the dichroic properties are easier to examine. Although the symmetry of C, has been broken because of introduction of the hexyl group on 3-position, we can deal with the ring-associated modes as if the symmetry of these modes were still retained. In fact, for instance, the dichroic behavior of the band at 1512 cm-' is well related to that of the v(ring) mode with b, symmetry for PT. The remainder are assigned to CH-associated modes; Le., v(CH), y(CH), and 6(CH), the C H in-plane deformation mode. In this case, the bond direction of the C H should be considered for these CH-associated modes. Even in such a case, however, the C H out-of-plane mode is expected to be strongly polarized along the direction perpendicular to the thiophene ring plane like the r ( C H ) mode with b2 symmetry for PT. The dichroic spectra of the film of P3HT of drawn ratio 5 are compared in Figure 5. Corresponding to the PT data (Figure 2), the dichroic features are again evident and can be seen even more dramatically than those of PT, because the drawn ratio of the P3HT film is larger than that (1.5 times) of PT. The dichroic features are well related to those of PT besides the absorption modes due to the hexyl group. The major vibrational modes of the hexyl group include the symmetric modes of v(CH3) and v(CH2) observed at 2960-2860 cm-I, the antisymmetric 6(CH3) and symmetric 6(CH2) modes at 1465-1450 cm-I, the symmetric 6(CH3) at 1379 cm-', and the p(CH2), the in-plane rocking deformation mode at 725 cm-l. The strong bands occurring at 1465-1450 cm-' are superposed on the v(ring) mode. Dichroism of these modes due to the hexyl group is very weak or unnoticed. The bands observed at 3058 and 1512 cm-I are highly polarized along the drawn direction; the band at 824 cm-I is polarized perpendicular to the drawn direction. The bands at 3058 and 1512 cm-I are due to the aromatic v(CH) mode and v(ring) mode with b, symmetry species, respectively, and that at 824 cm-' is at-
The Journal of Physical Chemistry, Vol. 93, No. 12, 1989 4997
Infrared Dichroic Studies of Polythiophenes TABLE I: Electrical Conductivity (S/cm) of the Polythiophene (PT) and Poly(3-hexylthiophene)(P3HT) Films Doped with Iodine as a Function of the Drawn Ratio drawn ratio sample 1.0" 1.5 2.5 3.0 5.0 PT 2.3 X 10' 3.0 X 10' b b b P3HT 2.7 X 10' c 9.5 x 10' 1.2 x 102 2.0 x 102 "Undrawn. bundrawable. CNodata.
m
1309 1300 1319
1205 1203 1197
1123 1158 1156
-a U
TABLE II: Doping-Induced Modes (cm-') for Undrawn Films of Polythiophene (PT), Poly( 3methylthiophene) (PJMT), and Poly(3-hexylthiophene)(P3HT) Partially Oxidized with Iodine sample doping-induced mode PT P3MT" P3HT
a k U
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975 967
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"See ref 25. tributed to the y(CH) mode. Although the bond direction of the aromatic C H bond must be considered for the v(CH), the dichroic characteristics resemble those of the corresponding u(CH) mode of PT. Thus, the dichroic features well related to those of PT again indicate that the P3HT chains are highly aligned along the drawn direction. It is worthwhile to compare the dichroic behavior of the CHassociated modes for PT and P3HT in spite of some ambiguity in their assignments. For PT two modes are expected to be observed for each type of CH-associated mode. Of the two y(CH) modes at 791 and 785 cm-I, the latter belongs to b2 symmetry, the former probably to a2 ~ y m m e t r y . ' ~Only . ~ ~ one mode, on the other hand, is supposed to be observed for each type of CH-associated mode of P3HT. The experimental results, however, demonstrate the presence of two lines a t 830 and 824 cm-l also for the y(CH) mode of P3HT (see the inset of Figure 5). Analogously to PT, the mode at 824 cm-' is intensified by the E , measurement, whereas the mode at 830 cm-I appears to show even less intensity and dichroism than the other mode (at 824 cm-'). In our present studies, there are no grounds for believing that the symmetry axis of each thienylene is preferentially oriented in the specific direction on the z--x plane (in Figure 3). In other words, the PT chains are probably uniaxially oriented only along the drawn direction. Two groups currently present the different conformational models for PT chains independently, on the basis of the crystallographic data.21v22 In this connection, further investigations including X-ray analyses would be required in order to determine polymer chain conformation of our PT and P3HT definitively. We emphasize, nevertheless, that the PT chains are highly aligned along the drawn direction, no matter what conformation they take. 2. Partially Oxidized Polythiophenes. Enhanced electrical conductivity along the drawn direction in the drawn films has been definitely observed, as in the case of PA.3v4 Table I shows the conductivity of the drawn films of partially oxidized PT and P3HT with heavy doping levels as a function of the drawn ratio. Monotonous conductivity enhancement along the drawn direction can be seen with the increasing drawn ratio. This observation confirms that the polymer chains of PT and P3HT are highly aligned along the drawn direction. Many researchers reported the presence of the doping-induced or doping-intensified IR modes specific to each conducting polymer independent of the chemical species of d ~ p a n t ' * . ' ~and - ~ ~related (21) Mo, Z.; Lee, K.-B.; Moon, Y. B.; Kobayashi, M.;Heeger, A. J.; Wudl, F. Macromolecules 1985, 18, 1972. (22) Garnier, F.; Tourillon, G.;Barraud, J. Y.; Dexpert, H. J . Mater. Sei. 1985, 20, 2687. (23) Fincher, C. R. Jr.; Ozaki, M.; Heeger, A. J.; MacDiarmid, A. G. Phys. Rev. B 1979, 19, 4140. (24) Street, G. B.; Clarke, T. C.; Krounbi, M.; Kanazawa, K. K.; Lee, V.; Pfluger, P.; Scott, J. C.; Weiser, G.Mol. Cryst. Liq. Cryst. 1982, 83, 253. (25) Hotta, S.; Shimotsuma, W.; Taketani, M.; Kohiki, S . Synth. Met. 1985, 11, 139.
WAVE NUMBER (x103cm-' 1
Figure 6. IR dichroic spectra of the drawn film of partially oxidized PT (drawn ratio 1.5). The PT film was doped with iodine in an acetonitrile solution containing lo-) M 12. The bands marked with asterisks are doping-induced or intensified modes.
these modes to the charged species introduced through the partial oxidation (doping). The presence of these modes has also been confirmed by the photoexcitation experiments which also introduce the charged species to the conducting polymer^.^' In Table I1 we summarize the major doping-induced (or doping-intensified) modes observed for undrawn films of partially oxidized PT, its 3-methyl derivative, Le., poly(3-methylthiophene) (P3MT),25and the 3-hexyl derivative (P3HT) partially oxidized with iodine. The modes that are invoked upon doping would be Raman active and IR inactive in the neutral polymer; such modes are probably rendered IR active through introduction of the dopant which breaks the local symmetry in the neutral polymer.23 As can be seen in Table 11, there are four major doping-induced modes for PT and five modes for P3MT and P3HT. For PT two modes are raised at 1310 and 1120 cm-l upon doping. For P3MT and P3HT, meanwhile, three modes are raised around 1310, 1160, and 970 cm-I. The origin of the other two modes for PT, P3MT, and P3HT appears to be associated with the intrinsic modes of the neutral polymers with one of the following possibilities: (1) These modes are raised very closely to the intrinsic modes. (2) These modes are identical with the intrinsic modes; in other words, such modes would be both IR and Raman active.26 Considering that partially oxidized P3MT and P3HT exhibit well-related IR characteristics, these features would be common to 3-alkyl substituted polythiophenes; introduction of the alkyl substituents does not affect the electronic structure and properties of the polythiophene backbone so much. Figure 6 shows IR dichroic spectra of the partially oxidized PT film (drawn ratio of 1.5); the film was doped with iodine in its solution with I, concentration of M. As shown in the figure, the doping-induced modes are pretty strongly polarized along the drawn direction; these bands are marked with asterisks in Figure 6. A broad band is more clearly observable around 4000 cm-I in the E,, spectrum than in the E , spectrum. This broad band probably results from the electronic transition relevant to the charged species such as polarons and bipolar on^.^^-^^ This trend is more noticeable for the P3HT films with larger drawn ratio as in the case for the neutral polymers. Figures 7 (26) Kim, Y. H.; Hotta, S.; Heeger, A. J. Phys. Rev. B 1987, 36, 7486. (27) Friend, R. H.; Schaffer, H. E.; Heeger, A. J.; Bott, C. D. J . Phys. C: Solid State Phys. 1987, 20, 6013. (28) Bredas, J. L.; Themans, B.; Andre, J. M. Phys. Rev. B 1983, 27, 7827. (29) Harbeke, G.; Meier, E.; Kobel, W.; Egli, M.; Kiess, H.; Tosatti, E. Solid State Commun. 1985, 55, 419. (30) Colaneri, N.; Nowak, M. J.; Spiegel, D.; Hotta, S.; Heeger, A. J. Phys. Rev. B 1987, 36, 7964.
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The Journal of Physical Chemistry, Vol. 93, No. 12, 1989
WAVE NUMBER
(~103cm-~I
Figure 7. IR dichroic spectra of the drawn film of partially oxidized P3HT (drawn ratio 5.0). The P 3 H T film was doped with iodine in an acetonitrile solution containing lo4 M I*. The bands marked with asterisks are doping-induced or intensified modes.
l
/
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,
,
,
l
,
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WAVE NUMBER
,
,
/
/
,
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0.8
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,
,
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(x103cm-')
Figure 8. 1R dichroic spectra of the drawn film of P3HT more heavily doped than that appearing in Figure 7. The P3HT film was doped with iodine in an acetonitrile solution containing lo-) M 12.
and 8 show IR dichroic spectra of the partially oxidized P3HT films (drawn ratio of 5) at different doping levels; they were doped with iodine in its solutions with I2 concentration of lo4 and M, respectively. While the intensity of the doping-induced modes
Hotta et al. for the more lightly doped sample is very weak or unnoticed in the E , spectrum, these modes (marked with asterisks in Figure 7) obviously dominate in the E,,spectrum. A broad band arising from the electronic transition is again observable around 4000 cm-I. The dichroic feature of this band is even more prominently observed as compared with that of the corresponding band for PT (see Figure 6). The contrast between the two dichroic spectra is more dramatic for the more heavily doped sample. In the E,! spectrum (Figure 8), the broad absorption band covers near-IR and mid-IR regions and hides both the intrinsic modes of the neutral polymer and the doping-induced modes occurring around 1350-950 cm-I. Thus, the IR dichroic characteristics of the doping-induced modes have been definitively demonstrated. These dichroic characteristics are well related to the features which the PA shows either upon doping or photoex~itation.~*~' Although the origin of the doping-induced modes is not interpreted in the present studies, any physical model concerned with these modes should properly explain their dichroic characteristics.
Conclusion IR dichroic spectra have been recorded on the drawn films of polythiophene (PT) and poly( 3-hexylthiophene) (P3HT). The dichroic features of PT and P3HT are well related to each other; the v(ring) mode around 1500 cm-I with the symmetry species of bl and the y(CH) mode around 800 cm-I are highly polarized in parallel with and perpendicular to the drawn direction of the film, respectively. On the basis of the symmetry analyses of the IR modes on these polymers, we have demonstrated that the polymer chains of PT and P3HT are highly aligned along the drawn direction. This is confirmed by the enhanced electrical conductivity along the drawn direction for the drawn films of the partially oxidized polymers. For the partially oxidized polymers, several doping-induced modes are observed in the mid-IR region (ca. 1350-950 cm-I) at the positions specific to PT or P3HT. These modes, together with the broad band around 4000 cm-I which probably results from the electronic transition, are strongly polarized along the drawn direction. Acknowledgment. We thank Dr. Takayoshi Morimoto of Matsushita Technoresearch Inc. and Mr. Hirofumi Wakemoto for their helpful discussions and suggestions. Registry No. Polythiophene, 25233-34-5; poly(3-hexylthiophene), 104934-50-1.
