Reexamination of the assignments of electronic absorption bands of polarons and bipolarons in conducting polymers

ELSEVIER

Synthetic

Reexamination

of the assignments

Metals

69 (1995)

629-632

of electronic absorption bands of polarons and bipolarons in conducting polymers Y. Furukawa

Department

of Chemistry,

School of Science,

The University

of Tokyo, Bunkyo-ku,

Tokyo

113, Japan

Abstract Electronic absorptions due to polarons and bipolarons in nondegenerate polymers have been reexamined on the basis of the data of the radical ions (models of polarons) and divalent ions (models of bipolarons) of a-oligothiophenes and p-oligophenyls. From the electronic absorption spectra of these model species, it has been concluded that two transitions are strong among the three expected for a polaron, whereas one transition is strong among the two expected for a bipolaron. Two bands observed in the electronic absorption spectra of doped polythiophene, poly@-phenylene) and poly@-phenylenevinylene), which were previously attributed to bipolarons, have been reassigned to polarons. These assignments are consistent with the results of Raman studies reported recently. The confinement parameters of these polymers have been estimated on the basis of the new assignments.

1. INTRODUCTION A group of conjugated polymers show quasimetallic electrical conductivities when doped with electron acceptors or donors [l]. It is considered that high conductivities are associated with formation of self-localized excitations such as solitons [2], polarons [3-51 and bipolarons [4-6]. The ground states of conjugated polymers can be classified into degenerate and nondegenerate types. In nondegenerate polymers such as polypyrrole, polythiophene, poly@-phenylene). poly@phenylenevinylene), etc., polarons or bipolarons can be generated by chemical doping. Optical absorption and electron-spinresonance (ESR) spectra of p-type doped polypyrrole were interpreted in terms of polarons and bipolarons. Electronic absorption spectra of doped polypyrrole in the range from visible to near-infrared show three bands below its band edge at low dopant contents and two bands at high contents [7]. The three bands were attributed to polarons and the two bands to bipolarons [8], because a positive polaron was expected to have three intraband transitions (WI, w2 and 03 transitions in Fig. la) and a positive bipolaron was expected to have two transitions (~1’ and

w3’ transitions in Fig. lc). Fesser, Bishop and Campbell [9] calculated absorption spectra of polarons and bipolarons within a continuum electron-phonon-coupled model (hereafter called the FBC model). A quantitative ESR study on polypyrrole [lo] seemed to confirm the interpretation of the optical absorption experiment. Thus, two bands in the electronic absorption spectra of doped polythiophene [ll, 121, poly@-phenylene) [13-G], poly@-phenylenevinylene) [16, 171, etc. were attributed to bipolarons. These criteria have been applied to assignments of photoinduced electronic absorptions of conjugated polymers. It has been recently demonstrated that Raman spectroscopy is a powerful tool for identifying self-localized excitations in doped polymers [18-271. Polarons or bipolarons in doped poly@phenylenevinylene) [21, 23, 251, poly@-phenylene) [22, 241 and polythiophene [26, 271 have been identified on the basis of the data on radical ions (polarons) and divalent ions (bipolarons) of oligomers. In addition, the electronic states have been studied by resonance Raman spectroscopy [22, 25-271. These Raman experiments have indicated the existence of polarons even at heavily doped polymers. The results of Raman studies seem to be inconsistent with those of electronic absorption experiments, which indicated the existence of bipolarons. In this paper, we will reexamine electronic absorption spectra of doped polymers by referring to those of radical ions and divalent ions of oligomers, and propose new assignments that are consistent with the results of Raman spectroscopic studies. 2. ELECTRONIC RADICAL IONS OLIGOMERS

Figure polaron;

1.

Electronic

energy

(b) a negative

(d) a negative band; black

level diagrams:

polaron;

bipolaron.

(a) a positive

(c) a positive

CB, conduction

bipolaron;

band; VB, valence

dot, electron.

0379-6779/95/$09.50 Q 199.5 Elsevier SSDI 0379-6779(94)02596-Q

Science

S.A. All rights reserved

ABSORPTION SPECTRA OF AND DIVALENT IONS OF

Radical ions and divalent ions of oligomers can be viewed, respectively, as polarons and bipolarons confined to finite sequences of repeating units. Thus, electronic transitions of polarons and bipolarons can be elucidated from the study on the dependence of electronic absorptions of the charged oligomers on chain length. We will discuss electronic absorption spectra of the radical cations and dications of a-oligothiophenes [26-28) and the radical anions and dianions of p-oligophenyls [22, 291, because reliable spectra have been reported for these charged oligomers.

630

Y. Fudawa

I Synthetic Metals 69 (1995) 629432

The positions (transition energies) of the electronic absorption bands of a-oligothiophenes from bithiophene to sexithiophene at neutral and ionized states are plotted in Fig. 2. One strong band is observed in the range between ultraviolet and visible for each oligothiophene [30]. The transition energies of neutral oligothiophenes decrease with increasing chain length (curve a in Fig. 2). Two strong bands are observed in the region between visible and near-infrared for the radical cations of quaterthiophene, quinquethiophene and sexithiophene [26-281. The lower and higher energy bands are called band I (curve b in Fig. 2) and band II (curve c in Fig. 2), respectively. On the other hand, one strong band is observed for the dications of the oligomers [26-281, and is called band I’ (curve d in Fig. 2). The transition energies of bands I, II and I’ all decrease with increasing chain length.

Similar plots for p-oligophenyls from biphenyl to sexiphenyl at neutral and ionized states are shown in Fig. 3. One band is observed in the ultraviolet region for each p-oligophenyl (curve a in Fig. 3) [31]. Two bands are observed in the region from visible to near-infrared for each radical anion, whereas one band for each dianion [22, 291. According to the nomenclature used for oligothiophenes, the lower and higher energy bands of the radical anions are called bands I and II (curves b and c in Fig. 3). respectively, and the bands of the dianions are called band I (curve d in Fig. 3). The transition energies of bands I, II and I decrease with increasing chain length. From the absorption spectra of a-oligothiophenes and poligophenyls, it can be concluded that two absorption bands are observed for a radical ion, whereas one band for a divalent ion. 3. ELECTRONIC ABSORPTIONS AND BIPOLARONS

p

(c)

2

(d) k Z Z

(W \ ’ 0

t 2

3

4

5

6

NUMBER OF RINGS Figure 2. Observed electronic transition energies of a-oligothiophenes: (a) neutral species [30]; (b) band I of radical cations [26-281; (c) band II of radical cations [26-281; (d) band I of dications [26-281.

o2

3

4

5

6

NUMBER OF RINGS Figure 3. Observed electronic transition energies of p-oligophenyls: (a) neutral species [31]; (b) band I of radical anions [22,29]; (c) band II of radical anions [22,29]; (d) band I of dianions [22,29].

DUE TO POLARONS

According to the FBC model [9], a polaron has two localized electronic levels that appear at +00 symmetrically with respect to the gap center (Fig. 1) due to the charge-conjugation symmetry, and so does a bipolaron. The positions of these electronic levels depend on the confinement parameter 7 of each conjugated polymer and the difference between the numbers of electrons occupying the levels at +wa and -we (n + and n_). The confinement parameter means the degree of nondegeneracy; y= 0 for trans-polyacetylene and y> 0 for nondegenerate polymers. For a positive polaron, n+ and n_ are 0 and 1, respectively. Thus, a positive polaron is expected to have the following three intraband transitions (Fig. la). (1) wt : the level at - wo t the valence band (2) Wz: the level at +Wg t the level at - wa (3) 03: the level at +w,-, t the valence band, and the conduction band t the level at - wa. Since n + and n_ in a negative polaron are 1 and 2, respectively, three transitions are also expected (Fig. lb). For a positive bipolaron, both n+ and n_ are 0. Thus, a positive bipolaron is expected to have the following two intraband transitions (Fig. lc). (1) wt’: the level at - wu t the valence band (2) 03’: the level at +WO t the valence band Since both n+ and n_ in a negative bipolaron are 2, two transitions are again expected (Fig. Id). Radical ions and divalent ions of oligomers correspond to polarons and bipolarons, respectively. In our recent paper [22], bands I and II of the radical anions of p-oligophenyls have been correlated to the wt and w2 transitions of a negative polaron, respectively, and band I’ of the dianions to the ~1’ transition of a negative bipolaron. Similarly, bands I and II of the radical cations of a-oligothiophenes correspond to the 01 and w2 transitions of a positive polaron, and band I’ of the dications to the wt’ transition of a positive bipolaron [26, 271. In a semiempirical molecular orbital calculation for the radical cation of sexithiophene [32], bands I and II correspond to the wt and y transitions, respectively. In the finite systems such as radical ions and divalent ions of oligomers, the ~3 and ~3’ transitions are symmetry forbidden [22, 261. According to the FBC model [9]. the absorption coefficient of each intraband transition is a function of a parameter, WO/AO (0 S w o/A0 5 1). where 240 means the band gap and 2~0 the separation between two localized levels (see Fig. 1). For a polaron, the wt and ~2 transitions are overwhelmingly dominant. For a bipolaron, the ot’ transition is dominant for large q/A0 values, whereas the w3’ transition becomes strong with decreasing WO/AO value. In two the electronic absorption spectra of radical ions (polarons),

Y Fulukawa

Table 1 Observed

data and calculated Polymer

parameters

/ Synthetic Metal

relating to the electronic

Doping

Obsd. transition

69 (1995) 629-632

absorptions

of conjugated

Assignmentb

energya / eV Polythiophene Poly@-phenylene) Poly@-phenylenevinylene)

P-type n-type p-type

from the references

in brackets.

bThe

0.65 [ll]

wt (polaron)

1.5 [ll]

w2 (p&iron)

0.7 [ 141

Wr (polaron)

2.4 [ 141

02 (polaron)

0.7

wt (polaron)

[ 171

W2 transition

energy is 2mu.

mold o

Confinement parameter

2.0 [33]

0.75

0.07

3.0

0.8

0.19

0.87

0.48

[ 141

2.38 [17]

y

CTaken from the references

in brackets.

The band gap is

Z(w,‘)/l(w,‘) value of 9 predicted by the FBC model from Fig. 6 in FBC’s paper [9]. In the case of photoexcitation, a similar intensity discrepancy was reported for two photoinduced absorptions (about 0.45 and 1.25 eV) attributed to bipolarons [34]. On the other hand, under the new assignments proposed here, the FIX model [9] predicts that 1( 01 )/I(~2) is about 1. This is in good agreement with the observed value of 2.

ABSORPTION

The reported optical absorption spectra of doped polymers will be reexamined on the basis of the results obtained above. The assignments proposed and several parameters relating to the electronic transitions of polymers are listed in Table 1. The confinement parameter y for a polaron is evaluated by the following equation from experimental values of A o and w, [9].

4.1.

Obsd. band

W2 (polaron)

bands (bands I and II) are observed. These observations are consistent with the intensity predictions. In the spectra of divalent ions (bipolarons), only one band (band I’) is observed. This suggests that the q/A0 values are large for polythiophene and poly@-phenylene). Thus, it is clear that a polaron has two strong transitions and a bipolaron has one strong transition. 4. REASSIGNMENTS OF ELECTRONIC SPECTRA OF DOPED POLYMERS

polymers

gap’ I eV

2.07 [17] aTaken 2Ao.

631

Polythiophene

Polythiophene shows a broad electronic absorption in the visible region and the band gap has been estimated to be 2.0 eV [33]. According to the paper by Chung et al. [11], two bands appear upon p-type doping at about 0.65 and 1.5 eV at dopant contents between 2.8 and 20 mol% per thiophene ring. These two bands were assigned to the or’ and 03’ transitions of bipolarons. Kaneto et al. [ 121 reported similar absorption spectra attributed to bipolarons at the dopant contents between 3 and 41 mol%. However, at dopant contents lower than 3 mol%, they observed additional peaks at 1.3 and 1.96 eV. and assigned them to the % transitions of polarons, respectively. On the other hand, the discussion in Section 3 leads us to new assignments: the two bands at 0.65 and 1.5 eV indicate the existence of polarons. The bands at 0.65 and 1.5 eV can be correlated to bands I and II of the radical cations, and are assignable to the 01 and 02 transitions of polarons. These assignments are consistent with Raman results [26, 271 of aspolymerized p-type doped polythiophene. From the values of the band gap, 2.0 eV (2Ac), and the ~2 transition energy, 1.5 eV (2%). the ~JCJAO value is determined to be 0.75. By using Eq. (1). y is calculated to be 0.07. Chung et al. [ 111 obtained WO/AO z 0.35 and y z 0.1-0.2 on the basis of their assignments. They noted the discrepancy between the observed 1(0.65)/1(1.5) value of about 2 and the

4.2.

Poly(p -phenylene)

The electronic absorption spectra of undoped and electrochemically Bu4N+-doped poly(p-phenylene) have been reported [ 141. The band gap of poly@-phenylene) is estimated to be 3.0 eV [ 141. Upon doping, new bands appear at about 0.7 and 2.4 eV [14]. Bands I and II of the radical anions of poligophenyls can be correlated to these two absorptions, respectively. Thus, these two bands are assignable to the wt and w2 transitions of negative polarons. These assignments are consistent with the results of resonance Raman studies [22, 241. By using the band gap of 3.0 eV and the w2 transition energy of 2.4 eV, we can obtain WO/AO = 0.8 and 7 = 0.19. The observed intensities of the bands at 0.7 and 2.4 eV are almost identical with each other [14]. These observed intensities are consistent with those predicted from the FBC model, as discussed for polythiophene. 4.3.

Poly@

-phenylenevinylene)

The band gap of poly(p-phenylenevinylene) is estimated to be 2.38 eV [ 171. Upon p-type doping, two doping-induced bands appear in the ranges of 0.7-1.00 eV and 2.07-2.25 eV [16, 17, 251. These two bands were attributed to the wt’ and ~1’ transitions of bipolarons, respectively [16, 171. Bradley et al. [35.36] evaluated 7 = 0.18 by considering that the photoinduced absorptions at 0.6 and 1.6 eV were due to the ~1’ and ~3’ transitions of bipolarons. They stated that the approximately equal intensities of the photoinduced absorptions were inconsistent with the predictions of the FBC model [9] and discussed the effects of inclusion of the electron-electron interactions reported by Campbell et al. [37] and Sum et al. [38]. A similar intensity discrepancy was also observed for the dopinginduced absorptions. A resonance Raman study [25] has recently indicated that the two doping-induced absorptions at 1 .OO and 2.25 eV arise mainly from the ~1 and ~2 transitions of polarons. The observed twoband-pattern can be reasonably explained by the existence of polarons, as described in Section 3. By using the band gap of 2.38 eV [ 171 and the wz transition energy of 2.07 eV [17], we can

632

Y Furukawa

I Synthetic Metals 69 (1995) 629-632

obtain OO/AO = 0.87 and y = 0.48. Under the assignments proposed here, the FBC model [9] predicts that I(wt) is almost the same as I(wz). These predicted intensities agree with the observed ones. Thus, the new assignments of the bands at 0.70.9 eV and 2.07-2.25 eV to the wt and u2 transitions, respectively, are more reasonable than the previous ones.

9. 10. Il. 12.

5. SINGLET

POLARON

PAIRS 13.

The results of optical absorption and ESR spectroscopies were previously explained by the existence of bipolarons. However, we have demonstrated that the reported optical absorption spectra can be explained by the existence of polarons. These assignments are consistent with the results of Raman spectroscopic studies. Recently, Hill et al. [39] have proposed spinless radical-cation dimers as an alternative to spinless bipolarons to explain the weak ESR signals from doped polymers. This means that whether or not bipolarons exist in doped polymers cannot be simply judged by the nonexistence or existence of ESR signals. It is considered that singlet intrachain polaron pairs and interchain polaron pairs (not bipolarons) would give no ESR signals.

14. 15. 16. 17.

18. 19. 20.

6. CONCLUSION 21. Jn light of the electronic absorption spectra of the radical ions and divalent ions of oligomers (models of polarons and bipolarons, respectively), we have proposed new criteria for the assignments of doping-induced optical absorptions of polymers; two bands to polarons and one band to bipolarons. The two absorption bands of doped polythiophene, poly@-phenylene) and poly@-phenylenevinylene), previously attributed to bipolarons, have been reassigned to polarons. The assignments proposed here are consistent with the results of Raman studies reported recently. According to the new assignments, the observed intensities can be explained by a continuum electron-phononcoupled model. The confinement parameters of polythiophene. poly@-phenylene) and poly(p-phenylenevinylene) have been evaluated to be 0.07.0.19 and 0.48, respectively.

22. 23. 24.

25. 26. 27. 28.

ACKNOWLEDGMENTS 29. The author would like to thank Professor Mitsuo Tasumi for his valuable discussions. This work was supported in part by a Grant-in-Aid for Encouragement of Young Scientists (No. 03854050) from the Ministry of Education, Science and Culture.

30. 31. 32.

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