J. Phys. Chem. 1994,98, 4635-4640
4635
Resonance Raman and Ultraviolet to Infrared Absorption Studies of Positive Polarons and Bipolarons in Sulfuric-Acid-TreatedPoly(pphenyleneviny1ene) Akira Sakamoto, Yukio Furukawa, and Mitsuo Tasumi’ Department of Chemistry, School of Science, The University of Tokyo, Bunkyo- ku, Tokyo I1 3, Japan Received: August 23, 1993; I n Final Form: December 2, 1993”
The resonance Raman and infrared spectra of sulfuric-acid-treated poly@-phenylenevinylene) (PPV) have been observed and analyzed on the basis of the resonance Raman spectra of the radical-cation and dication species of the model compounds CH3(CaH&H=CH),CaH&H3 (PVn, n = 1-3), which correspond, respectively, to positive polarons and bipolarons in PPV. Sulfuric-acid-treated PPV is considered to contain a considerable amount of positive polarons having a more or less uniform length and a relatively small amount of positive bipolarons. Of the two electronic absorption bands a t 2.25 and 1.OO eV of sulfuric-acid-treated PPV, the former absorption band is due to the positive polarons, whereas the latter has overlapping components arising from the positive polarons and bipolarons. It is suggested that the high electrical conductivity of sulfuric-acid-treated PPV may be due to formation of a polaron lattice, a regular array of polarons.
Introduction A class of organic polymers having conjugated a-electron systems exhibit high electrical conductivities when suitably doped with electrondonors or acceptors.’ Theultimategoalof structural studies on doped conducting polymers is to elucidate the relationship between electrical conduction and their molecular and electronic structures in the doped state. Poly@-phenylenevinylene)(Figure 1a, abbreviated as PPV) is a polymer with a prototypical nondegenerate ground state, which shows nonlinear optical properties2 and high electrical conductivities upon doping.3-’ The electrical conductivity of a highly stretched film of PPV after treatment with concentrated sulfuric acid (p-type doping) has been reported6to become as high as 1.12 X 104 S cm-1. The self-localized excitations (such as solitons, polarons, and bipolarons) created upon doping are considered to be the charge carriers in doped conducting polymers.8 Since PPV has a nondegenerate ground-state structure, only polarons and bipolarons are conceivable for this polymer. Polarons and bipolarons correspond to radical ions and divalent ions, respectively,which arecreated in conjpgated polymer chains upon doping. Schematic diagrams of a positive polaron and a positive bipolaron in doped PPV are depicted in Figure 2. They extend over a certain number of repeating units and have structures different from the regular conjugated polymer chains. It is important to elucidate the types and regions of the self-localized excitations for understanding the mechanism of electrical conduction. Recently, we have demonstrated the usefulness of resonance Raman spectroscopy in the characterization of self-localized excitationsexisting in the sodium-doped PPV (n-typedoping).ell The electronic absorption of doped PPV is observed in the region from visible to near-infrared. Accordingly, resonance Raman spectroscopy with visible and near-infrared excitations gives structural information on the self-localized excitations. The resonance Raman spectra of a heavily sodium-doped PPV film excited with laser lines between 488.0 and 1064 nm have shown marked changes with the exciting laser wavelength^.^ These spectra have been analyzed on the basis of the resonance Raman spectra of the radical anions and dianions of the model compounds C H S ( C ~ H & H = C H ) , C ~ H ~ C(PVn, H ~ n = 1-3, shown in Figure lb-d). The radical anions and dianions of PVn’s correspond, respectively, to the negative polarons (abbreviated as NPn) and negative bipolarons (abbreviated as NBPn) localized over almost
* Abstract published in Aduance ACS Absrracts, April 1, 1994. 0022-3654/94/2098-4635%04.50/0
Figure 1. (a) Repeating unit of poly@-phenylenevinylene) (PPV); (b) 4,4’-dimethyl-trans-stilbene(PVl ); (c) 1,4-bis(4-methylstyryl)benzene (PV2); (d) 4,4’-bis(4-methylstyryl)-trans-stilbene(PV3).
Figure 2. Schematicstructuresof (a) a positive polaron and (b) a positive bipolaron in oxidatively doped poly@-phenylenevinylene).
the same chain lengths as PVn. Our studyg has shown that three kinds of negative polarons (NPl, NP2, and NP3) and a negative bipolaron (NBP3) exist in the heavily sodium-doped PPV film. Other authors have also studied the radical ions and divalent ions of model compounds to clarify the spectral characteristics of polarons and bipolarons in the doped PPV.12-’8 In the present paper, we extend our study to PPV treated with concentrated sulfuric acid (p-type doping) and the radical-cation and dication species of the model compounds. Experimental Section
1. Materials. Unstretched films of PPV were prepared according to the paper by Murase et al.3 The model compounds, viz., 4,4’-dimethyl-trans-stilbene(Figure 1b, PVl), 1,4-bis(4methylstyry1)benzene (Figure IC, PV2), and 4,4’-bis(4-methylstyry1)-trans-stilbene (Figure Id, PV3), were synthesized in the manner reported by Drefahl and P16tner.19920 P-type doping was carried out by immersing the PPV film into concentrated sulfuric acid in a glass ampul under an argon 0 1994 American Chemical Society
4636
Sakamoto et al.
The Journal of Physical Chemistry, Vol. 98. No. 17, 1994
atmosphere. The glass ampul was then sealed. Raman measurements were made for such a film in a sealed glass ampul. Infrared measurements were made for a free-standing dopedPPV film exposed to the air. The radical cations of the model compounds were prepared by adding dichloromethane (CHzClz) solutions of iron trichloride (FeC13,lW mol dm-3) to the model compounds in CHzClz (1W10-5 mol dm-3) in a glass apparatus, which had been evacuated and sealed before the two solutions were mixed. This method was similar to that reported by D. Fichou et a1.21J2The progress of the oxidative reaction was monitored by measuring electronic absorption spectra. Further oxidative reactions to produce dications of the model compounds did not occur by addition of excess amounts of FeC13 in CH&. A similar observation was reported by Deussen and Bassler.18 The Raman spectra of the radical cations of the model compounds were obtained from their CHzClz solutions, immediately after the formation of each radical cation species was confirmed by its electronic absorption. The dication of PV2 was prepared by exposing its very thin layer deposited on a quartz plate to antimony pentachloride (SbCl5) vapor in a sealed glass cell. The layer of PV2 in contact with SbCl5 vapor showed two-stage changes. The first-stage species was formed immediately after exposing the layer to SbC15 vapor. The formation of the second-stage species followed, when the exposure of the layer to SbC15vapor was continued. As discussed later, the first-stage and second-stage species were considered to correspond to the radical cation and dication, respectively. The Raman spectrum of the dication of PV2 was obtained from such a doped layer on a quartz plate. 2. Measurements. Raman spectra excited with laser lines in the 441.6-740.0-nm region were measured at room temperature on a Raman spectrometer consisting of a Spex 1877 Triplemate and an EG&G PARC 1421 intensified photodiode array detector. Several lines from an NEC GLG 2018 He-Cd laser (441.6 nm), a Coherent Radiation Innova 90 Ar ion laser (514.5 nm), an NEC GLG 108 He-Ne laser (632.8 nm), and a Coherent Radiation Model 890 Ti:sapphire laser (711.0 and 740.0 nm) pumped with an Ar ion laser were used for Raman excitation. Raman spectra excited at 1064 nm were measured at room temperature on a JEOL JIR-5500 Fourier transform spectrophotometer modified for Raman measurement^.^^ The 1064-nm laser line was provided from a CVI YAG-MAX C-92 Nd:YAG laser. In order to avoid possible damage due to laser irradiation, each laser beam was loosely focused on the sample and its power was kept at a level lower than 20 mW. Infrared spectra were observed at room temperature on a JEOL JIR-100 Fourier transform infrared spectrophotometer. Electronic absorption spectra in the ultraviolet to near-infrared region were measured on a Hitachi U-3500 spectrophotometer.
Results and Discussion
1. Electronic Absorption Spectra of Neutral and Doped PPV. The electronic absorption spectra of neutral and sulfuric-acidtreated PPV films are shown in Figure 3. Upon doping, the electronic absorption due to neutral PPV (Figure 3a) disappears and the new absorptions due to the self-localized excitations, Le. polarons and/or bipolarons in the case of PPV, appear a t 2.25 and 1.00 eV (Figure 3b). Voss et al.24 have also reported for a sulfuric-acid-treated PPV film similar electronic absorptions a t 2.1 and 0.7 eV. In addition, other methods of p-type doping have given similar electronic absorption bands (by electrochemical doping at 2.3 and 0.9 eV and by arsenic pentafluoride doping at 2.1 and 0.9 eV).z5 Electronic absorption spectra of conducting polymers are generally explained in terms of a simple Huckel approximation (one-electron p i c t ~ r e ) . ~In~ this , ~ ~approximation, oxidation of conducting polymers results in the appearance of two dopinginduced energy levels in the band gap, which are symmetric with
ENERGY / eV 3.5
3.0
400
2.5
2.0
1.5
1.0
0.5
500
600
800
1200
2400
WAVELENGTH / nm Figure 3. Electronic absorption spectra of (a) neutral and (b) sulfuricacid-treated poly@-phenylenevinylene) films.
I
C. B.
I- - - - - 1- -4-
I
C.B.
I- - - - - - - -A-
Figure 4. Energy-level diagrams for (a) a positive polaron and (b) a positive bipolaron. C.B., conduction band; V.B., valence band.
respect to the gap center. Energy-level diagrams for a positive polaron and a positive bipolaron are shown in Figure 4. According to this picture, a polaron is expected to give rise to three subgap transitions w1,w2, and w3 in Figure 4a, while a bipolaron should have two transitions WI’ and w3/ in Figure 4b. In the previous studies on p-type doped PPV,z4q25formation of bipolarons with optical transitions from the valence band to two unoccupied bipolaron levels (WI’and w3’) has been discussed. As will be discussed in the following subsection, however, polarons and bipolarons are detected in sulfuric-acid-treated PPV by resonance Raman spectroscopy. Assignments of the electronic absorption spectra observed for sulfuric-acid-treated PPV will be discussed later on the basis of the resonance Raman rqults. 2. Resonance Raman Spectra of Doped PPV and the RadicalCation and Dication Species of the Model Compounds. The resonance Raman spectra of a sulfuric-acid-treated PPV film are shown in Figure 5. Raman measurements were made with various excitation wavelengths fromvisible tonear-infrared (441.6,514.5, 632.8,711.0,740.0,and 1064nm),whichareindicatedbyarrows in the electronic absorption spectra (Figure 3). These spectra do not change with sulfuric-acid-dz treatment of PPV (spectra not shown) and resemble the Raman spectrum of FeC13-doped PPV reported by Lefrant et aLZ8 Therefore, sulfuric-acid treatment of PPV causes oxidative doping but not protonic (nonoxidative) doping. This observation is consistent with the results obtained by Han and ElsenbaumerZ9for various acid dopants that sulfuric acid can function as a redox dopant, whereas CH3S03H, CF3COOH, etc. are nonoxidizing protonic dopants. The observed Raman spectra show only small changes with the exciting laser wavelengths, except for the spectra obtained with excitations at 441.6 and 1064 nm. This result is in marked contrast with the case of sodium-doped PPVS9 By analogy with the case of sodium-doped PPV,9-1’ it is expected that the observed Raman spectra can be analyzed on the basis of the resonance Raman spectra of the radical cations and dications of the model compounds. The model compounds well represent the spectroscopic properties of neutral PPV.30 The radical cations and
The Journal of Physical Chemistry, Vol. 98, No. 17, 1994 4631
Positive Polarons and Bipolarons in PPV
TABLE 1: Main Electronic Absorption Maxima of the Radical-Cation and Dication Species of the Model Compounds ,A,, nm (ev) model compd radical cation' dicationb PV 1 PV2 PV3 I
tI
A
r
r
.
1500
.
.
990 (1.25)
Generated by oxidation with FeC13 in CH2CIz. Generated by exposing a solid layer to gaseous SbC15.
7-
\-I
507 (2.45), 804 (1.54) 636 (1.95), 1200 (1.03) 713 (1.74), 1550 (0.80)
.
1000
RAMAN SHIFT/ cm-l Figure 5. Resonance Raman spectra of a sulfuric-acid-treated poly@phenylenevinylene)film. Excitationwavelengthsare441.6,514.5,632.8, 71 1 .O, 740.0, and 1064 nm for a-f, respectively. Fluorescence backgrounds are subtracted in all the spectra. Asterisks indicate the bands due to sulfuric acid.
dications of the model compounds correspond; respectively, to positive polarons and positive bipolarons in the relevant polymer. The CHzClz solution of PV1 became dark brown, immediately after the addition of a CH& solution of FeC13. The mixed solution showed main electronic absorption bands at 507 nm (2.45 eV) and 804 nm (1.54 eV). This electronic absorption spectrum is similar to that of the radical cation derived from trans-stilbene in carbon tetrachloride at 77 K by y-irradiation31 This similarity indicates that the dark brown solution contains the radical cation of PVl (PVl*+). When FeC13 in CHzC12 was added to a CHzCl2 solution of PV2, the mixed solution became blue. The main electronic absorption bands of the mixed solution were located a t 636 nm (1.95 eV) and 1200 nm (1 -03eV). These bands closely resemble those of the radical cations derived from 1,Cdistyrylbenzene (C6H5CH=CHC6H4CH=CHC~H5) by three different methods (oxidation with SbC15,Igpulse radiolysis,32 and photoionization33). These similarities indicate that the blue solution contains the radical cation of PV2 (PV2*+). On the other hand, Spangler et al.I4.'5 studied the oxidative reactions with SbCl5 of PV2-like model compounds of poly(dialkoxyphenyleneviny1ene) and concluded that dications were the dominant products. Our assignment of the observed absorption bands to PV2*+ rather than to PV22+ seemed to be inconsistent with the conclusion derived by Spangler et al., although our model compound was not exactly
the same as theirs. We therefore made an electron spin resonance (ESR) measurement to confirm our assignment and observed an intense ESR signal with a gvalue of 2.003 at room temperature. The CH2Clz solution of PV3 became light blue after the addition of an FeC13 solution, and the absorption maxima of this mixed solution were located at 713 nm (1.74 eV) and 1550 nm (0.80 eV). This electronic absorption spectrum resembles that of the radical cation derived from 4,4'-distyryl-trans-stilbene (C&CH=CHC6H4CH=CHC6H4CH=CHC6H~) in CH2C12 by oxidation with SbC15.13J8By analogy with the cases of PVl and PV2, it seems reasonable to consider that the light blue solution contains the radical cation of PV3 (PV3*+). The electronic absorption maxima of the radical cations of PV1, PV2, and PV3 are close to those of the radical anions of PVl, PV2, and PV3, respectively.9 The dications of the model compounds were not produced in CHzClz solutions by addition of excess amounts of FeC13, probably because of the decomposition of the radical cations. On the other hand, a thin layer of PV2 deposited on a quartz plate became blue at first and then almost colorless, after it was exposed to SbC15vapor. The second-stage colorless sample showed a main electronic absorption band at 990 nm (1.25 eV), which would be assignable to the dication of PV2 (PV22+). A similar attempt to produce PV32+ was not successful. The main electronic absorption maxima of the radical cations and dication of the model compounds are summarized in Table 1. The resonance Raman spectra of the radical cations of the model compounds and the dication of PV2 are shown in Figure 6. The excitation wavelengths used for Raman measurements coincide with the electronic absorptions of the radical-cation and dication species of the model compounds (Table 1). The bands due to the solvent (CHzClz) are already subtracted in Figure 6a-c. The Raman spectrum of PV1'+ (Figure 6a) considerably differs from either of the Raman spectra of PV2'+ and PV3'+ (Figure 6b,c), but the latter two resemble each other. Although assignments of the observed bands in Figure 6 are yet to be studied further, theobserved spectral patterns, particularly those of PVP+ and PV3*+,are useful for the following discussion on the Raman spectra of sulfuric-acid-treated PPV. The resonance Raman spectra of sulfuric-acid-treated PPV obtained with excitations at 514.5, 632.8, 711.0, and 740.0 nm (Figure 5b-e) are essentially the same, except for the fluorescence backgrounds which are subtracted in all the spectra. It is noted that these four spectra are very similar to those of PV2'+ and PV3*+ (Figure 6b,c), although some differences are observed in the wavenumbers of the bands in the 1590- 1560-cm-l region and in the intensity patterns in the 1340-1280-cm-1 region. These spectral differences between the doped polymer and the model species may be attributed to polymer-chain effects and/or intermolecular interactions. More important are the overall similarities, because they indicate the existence of the positive polarons localized over the regions (lengths) close to PV2 or PV3 in sulfuric-acid-treated PPV. We cannot further specify the lengths of the positive polarons, because the resonance Raman spectra of PV2'+ and PV3*+ are similar to each other. These positive polarons are called PP in the following part of this paper. The 676.4-nm-excited resonance Raman spectrum of lightly
Sakamoto et al.
4638 The Journal of Physical Chemistry, Vol. 98, No. 17, 1994
TABLE 2 Species Found in the Resonance Raman Spectra of the Doped PPV Observed with Various Excitation Waveleneths species excitation sodium-doped sulfuric-acid-treated ~
wavelength, nm 441.6 488.0 514.5 632.8 711.0 740.0 1064 a
1500
1000
RAMAN SHIFT / cm-l Figure 6. Resonance Raman spectra of the radical cations and dication of the model compounds: (a) PVl'+; (b) PVz'+; (c) PV3'+; (d) PVZZ+. Excitation wavelengths are 514.5, 632.8, 711.0, and 1064 nm for a-d, respectively. Fluorescence backgrounds are subtracted in all the spectra.
FeC13-doped PPV reported by Lefrant et aLZ8resembles the resonance Raman spectra of sulfuric-acid-treated PPV excited at 5 14.5-740.0 nm (Figure 5b-e). It is therefore considered that PP was also created upon FeCl3 doping. The resonance Raman spectrum excited at 441.6 nm (Figure 5a) is also similar to those of PV2*+ and PV3*+ (Figure 6b,c). However, extra bands are observed at 1626 and 1591 cm-' in Figure 5a. These bands of sulfuric-acid-treated PPV correspond to the bands observed at about 1626 and 1582 cm-1 for neutral PPV.28JOJ4,3s The strong band at about 1172 cm-1 of neutral PPV must be overlapped on the low-wavenumberside of the 1179cm-1 bandof sulfuric-acid-treatedPPV. Inother words, the441 -6nm-excited resonance Raman spectrum of sulfuric-acid-treated PPV is explained by superpositionof the spectra of PP and neutral PPV. This indicates that, in addition to PP, the neutral (undoped) parts exist in the sulfuric-acid-treated PPV film. However, the amount of these neutral parts would not be large, and their lengths are probably shortened in comparison with the intact polymer chain, since the electronic absorption spectrum of sulfuric-acidtreated PPV in Figure 3 does not show an appreciable absorption in the region around 3 eV where intact PPV has an intense absorption band. The 1064-nm-excited resonance Raman spectrum (Figure 5f) is explained by superposition of the spectra due to PP and PV22+. The bands at 1557, 1331, 1282, and 1178 cm-1 correspond to those of PP at 1566, 1331, 1295, and 1179 cm-l (Figure 5b-e), respectively, while the bands at 1595, 1557, 1520, 1263, and 1178 cm-I have the corresponding bands at 1596, 1577, 1541, 1284, and 1178 cm-I in the spectrum of PV22+ (Figure 6d). Therefore, sulfuric-acid-treated PPV contains the positive bipolaron (abbreviated as PBP) in addition to PP. The species observed in the Raman spectra of sulfuric-acidtreated PPV with various excitation wavelengths are listed in Table 2, together with those found for sodium-doped PPV in our previous study.g It is concluded that positive polarons (PP) are detected with all excitation wavelengths from visible to nearinfrared and positive bipolarons (PBP) with near-infrared
~
~~
PPV"
neutral PPV, NP1 neutral PPV, N P l NP2 NP3 NP3 NP3, NBP3
PPV
neutral PPV, PP PP PP PP PP PP, PBP
Reference 9.
excitation. It is generally believed that a bipolaron is more stable than two polarons in an idealized infinite polymer chain.z7 Moreover, recent calculations of the three-dimensional band structureof PPV have shown that stable polarons arenot expected to exist in a perfect PPV crystal without defects.362' Therefore, it is surprising that the positive polarons are clearly detected. The observed distribution of polarons and bipolarons in the doped PPV is influenced by the dopant species (counterions) and the type of doping (oxidation or reduction), because upon doping of PPV with sodium (n-type doping) three kinds of negative polarons with different localized lengths (NP1, NP2, and NP3) and a negative bipolaron (NBP3) have been created (see Table 2). 3. Assignmentsof the Electronic Absorption Spectraof Doped PPV. As shown in Figure 3, upon doping of PPV with concentrated sulfuric acid, new electronic absorptions appear in the region from visible to near-infrared with maxima at 2.25 eV (552 nm) and 1.00 eV (1 239 nm). On the basis of the resonance Raman results described above, the former absorption band is due to the positive polarons, whereas the latter has overlapping components arising from the positive polarons and bipolarons. The dependence of the Raman spectra on excitation wavelengths can be explained by the resonance effect. When the excitation wavelength is in resonance with the absorption arising from one of the neutral PPV part, the positive polaron PP, and the positive bipolaron PBP, the intensities of the Raman bands of the species in resonance are selectively enhanced. Since the 2.25- and 1.00eV bands give rise to the resonance enhancement of the Raman bands due to PP, both of these absorption bands can be assigned to PP. The 1.OO-eV band has also a contribution of the absorption due to PBP, because the Raman spectrum of PBP is observed with 1064-nm excitation. According to the calculation of the absorption intensities of subgap transitions due to self-localized excitation^,^^ the w1 and 0 2 transitions are strong and the w3 transition is weak for a polaron, and the w1' transition is strong and the w j l transition is weak for a bipolaron. Therefore, the 2.25-eV band is assigned to thewz transition ofthe positive polaron PP (Figure 4a), and the 1.00-eV band is attributed to the superposition of the w1 transition of the positive polaron PP (Figure 4a) and the WI' transition of the positive bipolaron PBP (Figure 4b). As shown in Figure 7, the radical cations of two model compounds (PV2*+ and PV3*+), which correspond to positive polarons in PPV, also have two electronic absorption bands; i.e., PV2*+has two bands at 1.95 and 1.03 eV and PV3'+ at 1.74 and 0.80 eV. The bands at 1.95 and 1.74 eV correspond to the 0 2 transition of positive polarons, and the bands at 1.03 and 0.80 eV to thew1 transition. By contrast, thedicationofamodelcompound (PV22+), which corresponds to a positive bipolaron in PPV, has a broad electronic absorption band at 1.25 eV (Figure 7). This band is considered to correspond to the wl' transition of a positive bipolaron. The positions of the two electronic absorption bands due to PP and PBP in the doped PPV are not in exact agreement with those of the corresponding radical cations and dication of the model compounds. These differencescould result from intra-
The Journal of Physical Chemistry, Vol. 98, No. 17, 1994 4639
Positive Polarons and Bipolarons in PPV ENERGY I eV 3.5
3.0
2.5
2.0
1.5
3 ?
Lu
0.5
1.0
E E E
E :
g g g
0
z
3 tt
53m a
A
0 400
500
600
800
1200
Figure 8. Energy-level diagrams for (a) a single positive polaron and (b) a polaron lattice (p-type doping). C.B., conduction band; V.B.,valence band.
2400
WAVELENGTH / nm Figure 7. Electronicabsorptionspectra of the radical cationsand dication of model compounds PV2 and PV3: (a) PV2'+; (b) PV3'+; (e) PV22+. The absorption intensities are given in arbitrary units. and interchain interactions of polarons and bipolarons in sulfuricacid-treated PPV. At this point, it should be noticed that when polarons and bipolarons coexist in the doped conducting polymers, electronic absorptions due to coexisting polarons and bipolarons are overlapped and it is difficult to identify polarons and/or bipolarons by electronic absorption spectroscopy alone.39 It has been reported that the electronic absorptions of electrochemically doped (oxidative) and arsenic-pentafluoridedoped PPV's appear in the region from visible to near-infrared with maxima at about 2.1 and 0.9 eV, and theseabsorptions have been assigned to positive bipolar on^.^^ However, these electronic absorptions are very similar to those of sulfuric-acid-treated PPV (Figure 3). Therefore, in the cases of other p-type doping, dominant self-localized excitations are considered to be almost the same as those in sulfuric-acid-treated PPV. It is difficult to evaluate by the resonance Raman spectra in Figure 5 the relative amounts (not to speak of the absolute ones) of polarons and bipolarons in sulfuric-acid-treated PPV, because the degrees of resonance enhancement of the Raman bands due to polarons and bipolarons are not determined. However, some information on this issue is obtainable from the electronic absorption spectra in Figure 3. The intensities of the 01 and w2 transitions calculated for a positive polaron are nearly equal to each other.38 The relative intensity of the two electronic absorption bands observed for each of PV2'+ and PV3'+ (Figure 7) is consistent with the calculation. Although no direct information is available as to the relative intensity of the 0 1 transition of a polaron and the w1' transition of a bipolaron from either calculation or experiment, it would not be too unreasonable to consider that this relative intensity is greatly deviated from unity. Then, the intensity ratio of the 2.25-eV ( 0 2 ) and 1.00-eV (01) bands in the electronic absorption spectrum of sulfuric-acid-treated PPV (Figure 3b) seems to indicate that the amount of positive polarons is much larger than that of positive bipolarons. If the amount of positive polarons in sulfuric-acid-treated PPV is nearly equal to that of bipolarons, the intensity of the 1.OO-eV band would be almost twice as strong as that of the 2.25-eV band. 4. Intra- and Interchain Interactions of Polarons. The current understanding of doped conducting polymers is dominated by the theory that polarons and bipolarons are formed along a single conjugated polymer chain. However, intra- and interchain interactions of polarons and bipolarons must be important in actual doped polymers. Recently, it has been reported that the radical cations of terthiophenes reversibly dimerize even at low concentrations (for example, 4 X 1 V mol dm-3).40941 The formation of a *-dimer at such low concentrations has important implications for doped conducting polymers, in which extensive interchain interactions would take place. This *-dimer created on neighboring polymer chains is considered to be a good model of interchain interaction of polarons. Such a polaron-dimer would
J
I
I
I
I
I
I
1500
I
1000
WAVENUMBER/cm-l Figure 9. (a) Infrared difference spectrum and (b) resonance Raman spectrum excited at 1064 nm of sulfuric-acid-treated poly@-phenylenevinylene) films exposed to the air.
be a reasonable alternative to a bipolaron. This kind of interchain interaction may be one of the reasons for the stability of polarons in sulfuric-acid-treated PPV. In sulfuric-acid-treated PPV, a considerable amount of positive polarons with a more or less uniform length are created. In such a system, a polaron lattice (a regular array of polarons), which has been proposed by Kivelson and Heeger for a metallic state in doped polya~etylene,~*3~3 may be formed. Energy-level diagrams for a positive polaron and a polaron lattice (p-type doping) are depicted in Figure 8. Intrachain interaction of polarons, which are created on the same polymer chain, results in a polaron lattice. The formation of a polaron lattice would be accompanied by the appearance of the half-filled polaron subband within the band gap (Figure 8b),4*-44and the metallic behavior would appear accordingly. In contrast with sulfuric-acid-treated PPV, sodium-doped PPV contains polarons and bipolarons of various lengths (see Table 2). Such a situation would make the formation of a polaron lattice impossible. The electrical conductivity of a highly stretched film of PPV has been reported to become as high as 1.12 X 104 S cm-' when treated with concentrated sulfuric acid (p-type doping)6and 2.0 X 10' S cm-' when doped with sodium (n-type d ~ p i n g ) .The ~ formation of a polaron lattice in the former may contribute to its higher electrical conductivity. 5. Infrared Spectra of Doped PPV. The infrared difference spectrum of a sulfuric-acid-treated PPV film, obtained by subtracting the spectrum of the undoped sample from that of the doped sample, is shown in Figure 9a. The doping-induced bands clearlyappearat 1535,1478,1416,1318,1271,1205,1119,1053, 1001, and 945 cm-1. These bands are in good agreement with those observed in the previous studies From the resonance Raman and electronic absorption studies described above, the sulfuric-acid-treated PPV film is considered to contain a considerable amount of the positive polaron PP and a relatively small amount of the positive bipolaron PBP. However, the resonance Raman measurements were made for the doped PPV .24945
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The Journal of Physical Chemistry, Vol. 98, No. 17, I994
under an argon atmosphere, while the infrared measurements were made for the doped sample in the air. Figure 9b shows the 1064-nm-excited resonance Raman spectrum of the sulfuric-acidtreated PPV after exposure to the air. The Raman bands due to PBP, which are observed for the sample under an argon atmosphere (Figure 50,arenot found in Figure9b. This indicates that the amount of PBP has decreased by exposure to the air, whereas PP still persists. Therefore, most of the infrared bands observed in this difference spectrum (Figure 9a) should be assigned to PP. The infrared difference spectrum of iodine-doped PPV and that of arsenic-pentafluoride-dopedPPV25 are very similar to that of sulfuric-acid-treated PPV. These similarities indicate that PP has been created upon doping of PPV with either iodine or arsenic pentafluoride.
Conclusion
In the present study, we have demonstrated that resonance Raman spectroscopy with visible and near-infrared excitations is useful for characterizing the self-localized excitations formed in doped conducting polymers. A considerable amount of the positive polarons, which are considered to have a more or less uniform length, are created upon treatment with concentrated sulfuric acid. The positive bipolarons as well as neutral parts also exist in sulfuric-acid-treated PPV. These results are significantlydifferent from those of sodium-doped PPV, in which three kinds of negative polarons with different localized lengths and a bipolaron have been created. The electronic absorption spectra of sulfuric-acid-treatedPPV have contributionsfrom both the positive polarons and bipolarons. Acknowledgment. We thank Dr. N. Koga and Professor H. Iwamura for their cooperation in the measurement and analysis of the ESR spectra. We are grateful to Dr. T. Ohnishi of Sumitomo Chemical Co. Ltd. for providing us with some of the PPV films used in the early stage of this work. The present work was supported in part by a Grant-in-Aid for Scientific Research (A) No. 01430004 and a Grant-in-Aid for Developmental Scientific Research No. 63840014 from the Ministryof Education, Science, and Culture. References and Notes (1) Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Dekker: New York, 1986; Vol. 1 and 2. (2) Kaino, T.; Kubodera, K.;Tomaru,S.; Kurihara,T.; Saito, S.;Tsutsui, T.: Tokito. S.Electron. Lett. 1987. 23. 1095. ' (3) Murase, I.; 0hnishi.T.; Noguchi, T.; Hirooka, M. Polym. Commun. 1984, 25, 327. (4) Gagnon, D. R.; Capistran, J. D.; Karasz, F. E.; Lenz, R. W. Polym. Bull. 1984, 12, 293. ( 5 ) Murase, I.; Ohnishi, T.; Noguchi, T.; Hirooka, M. Synth. Met. 1987, 17, 639. (6) Hirooka, M.; Murase, I.; Ohnishi, T.; Noguchi, T. In Frontiers of
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