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POLARIZED EXAFS AND NEAR-EDGE SPECTRA OF HALOGEN-DOPED POLYACETYLENE
H. Oyanagi, M. Tokumoto, T. Ishiguro, H. Shirakawa, H. Nemoto, T.
Matsushita, H. Kuroda
To cite this version:
H. Oyanagi, M. Tokumoto, T. Ishiguro, H. Shirakawa, H. Nemoto, et al.. POLARIZED EXAFS
AND NEAR-EDGE SPECTRA OF HALOGEN-DOPED POLYACETYLENE. Journal de Physique
Colloques, 1986, 47 (C8), pp.C8-615-C8-618. �10.1051/jphyscol:19868115�. �jpa-00226013�
POLARIZED EXAFS AND NEAR-EDGE SPECTRA OF HALOGEN-DOPED POLYACETYLENE
H. OYANAGI, M. TOKUMOTO, T. ISHIGURO, H. S H I R A K A W A *
,
H. N E M O T O * , T. M A T S U S H I T A ~ " and H. KURODA**'Electrochemical Laboratory, Sakuramura, Niiharigun, Ibaraki 305, Japan
'~nstitute of Materials Science, University of Tsukuba, Sakuramura, Niiharigun, Ibaraki 305, Japan
* * National Laboratory for High Energy Physics, Ohomachi, Tsukubagun, Ibaraki 305, Japan
* X *
Department of Chemistry and Research Center for
Spectrochemistry, The University of Tokyo, Hongo, Bunkyoku, Tokyo 113, Japan
ABSTRACT
Polarized EXAFS and near-edge s p e c t r a w e r e measured for (CHBry)x and (CHIy)x on t h e Br K- and I L-edges over a wide range of dopant concentration. The analysis of EXAFS and near-edge s p e c t r a indicates t h a t , in t h e lightly doped region (0.01<y<0.2), polybronine ions highly oriented along t h e c-axis a r e formed while bromines also form covalent bonds with polyacetylene chain which increases in number on going t o heavily doped region (y>0.2). Bromines a r e incorporated a s isolated ions in t h e early s t a g e of doping (y<0.01). In (CHIy)x, iodines form linear polyions which a r e chemically less reactive. The difference in t h e magnitude and dopant concentration dependence of electrical conductivity between bromine and iodine doping is ascribed t o t h e degree of termination of conjugated chains by a n addition reaction of halogens.
INTRODUCTION
Polyacetylene, (CH)x is t h e simplest conjugated system which has been extensively studied because of i t s unique properties as a conducting polymer.
The electrical conductivity of pristine (CH)x increases by many orders of magnitude upon doping with electron acceptors such a s bromine or iodine [I].
Previous polarized EXAFS [2,3] and near-edge 121 s p e c t r a of bromine-doped trans-polyacetylene, t-(CHBry), have been interpreted a s a n evidence for polybromine ions highly oriented along t h e c-axis. Two s t r u c t u r e models have been proposed based on t h e different interpretations of a short Br-C distance observed in t h e Fourier transform of EXAFS oscillations. Oyanagi e t al.
attributed this distance t o t h e Br-C bonds originating from bromines which a r e covalently bonded with polyacetylene by a substitution o r addition reaction [ 2 ] whereas Krone e t al. interpreted this peak a s t h e distance between t h e polybromine ions and polyacetylene chains [3] assuming t h a t bromines exist only a s polybromine ions. In this paper, t h e two s t r u c t u r e models a r e examined on t h e basis of t h e dopant-concentration dependence of polarized EXAFS and nea- e d g e dpectra.
EXPERIMENTAL
Trans-rich (CH), films w e r e prepared a s previously described by Shirakawa e t
al. [ 4 ] and stretch-oriented by uniaxial drawing. Halogen doping was carried
out by exposing trans-(CHIx films t o bromine o r iodine vapor a t room temperature. Halogen content was determined by a weight uptake immediately
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868115
C8-616 JOURNAL
DE
PHYSIQUEa f t e r t h e doping. Polarized X-ray absorption experiments w e r e performed with the EXAFS spectrometer a t t h e Photon Factory [5]. Polarized Br K-EXAFS and near-edge s p e c t r a w e r e measured for (CHBry)x samples with y between 0.007 and 0.6 whereas I L-edge d a t a w e r e collected f o r (CHIy), with y betvieen 0.02 and 0.2. For the measurement of Br K-edge absorption spectra, a silicon (311) channel-cut monochromator was used while for I L-edge data, a silicon (111) double crystal monochromator was detuned by 90 % t o remove t h e higher harmonics.
RESULTS AND DISCUSSION
As previously reported (21, anisotropic Br K-EXAFS oscillations of (CHBry), with respect t o t h e polarization direction shown ir? Fig. 1 indicate t h a t polybromine ions a r e highly oriented along t h e c-axis. The results of Fourier transform of Br K-EXAFS oscillations times k3for (CHBry)x with y=0.02, 0.08, and 0.179 a r e shown in Fig. 2 with E parallel ,,to t h e c-axls.
The most prominent peak observed a t 2.2 A is due ,to t h e first nearest Br-Br distance and t h e less intense peak located a t 4.8
4
is due t o t h e second nearest Br-Br distance. They a r e 2.55A
and 5.12 A, respectively, if the phase shift is corrected. Since these peaks appear only in t h e direction of c-axis and t h e second nearest peak should b e observed when bromines form linear polyions with larger number of a t o m s than three, t h e results shown in Fig. 2 demonstrate that polybromine ions a r e highly oriented along t h e c-axis in trans-(CHBrylx over a wide range of bromine concentration.Although t h e overall features a r e similar for t h r e e samples, t h e magnitude of Br-Br peak decreases with increasing dopant oconcentration whereas t h e intensity of a short Br-C distance observed a t 1.1 A increases. This short Br-C distance i s a t t r i b u t e d t o bromines covalently bonded t o polyacetylene chain by e i t h e r substitution or addition reaction [2,6].
Br K - edge
l a 1 y = 0 0 2 0 0 0 0 5
RADIAL D I S X N C E [ A ]
Fig. 1 Polarized Br K-EXAFS oscillations of Fig. 2 Fourier transform of Br K-EXAFS for (CHBry)x with y=0.02. (CHBry)x with parallel orientation.
The Fourier transform in Fig. 2 emphasizes Br-Br correlation suppressing Br- C contribution since t h e backscattering amplitude of Br extends t o a large k- region whereas t h a t of C damps rapidly with k; The contribution
of
Br-Br bondb ' I I I S I l
-40 -20 0 20 40 60 80
ENERGY ( e V 1 t.
Z
Fig. 3 Polarized Br K-EXAFS oscillations of Fig. 4 Polarized Br K-XANES s p e c t r a of
(CHBry)x with y=0.007. (CHBry)x.
(CHBry l x
is estimated t o b e less than 30% for (CHBry)X with y=0.02 from t h e coordination number of bromine determined for the Br K-EXAFS with E parallel t o t h e c-axis [2]. The r a t i o of bromines in t h e form of polybromine ion further decreases in t h e more heavily doped region. In Fig. 2, a short Br-C distance observed for (CHBry)x with y=0.179 is 1.92 A a f t e r the phase shift correction, which is quite close t o t h e Br-C covalent bondlength.
If there is only polybromine ions intercalated between polyacetylene chains a s proposed by Krone e t al. [3], t h e local s t r u c t u r e of bromines is expected t o b e independent on t h e dopant concentration. Oyanagi e t al. have attributed longer Br-C distance (3.4-3.8
A)
t o t h e correlation between polybromine ion and t h e polyacetylene chain, which is roughly t h e sum of van der Waals radii of bromine and carbon [2].Figure. 3 shows the EXAFS oscillations for lightly doped (CHBry)? On going t o t h e lightly doped region, t h e Br-Br peak intensity decreases i t s intensity and for (CHBry)x with y<0.01, only Br-C peak is observed, indicating t h a t t h e r a t i o of Br-Br bond t o t h e Br-C bond i s small in this region. These results imply t h a t bromines a r e doped a s isolated ions in t h e early s t a g e of doping.
Summarizing t h e EXAFS results for (CHBr lx, bromines a r e doped a s ions in t h e lightly doped region with y<0.01 (Region
6,
which forms polybromine ions with further doping in t h e more heavily doped region for 0.01<y<0.2 (Region 11). Throughout these two regions, a part of bromines a r e substituted with hydrogens whereas in t h e most heavily doped region for y>0.2 (Region III), most of bromines form Br-C bonds by an addition reaction [2,6,71.In Fig. 4, t h e near-edge s p e c t r a of (CHBry), a r e shown with E parallel and perpendicular t o t h e c-axis. A sharp peak observed 9-10 eV below t h e threshold in all s p e c t r a is due t o t h e transition from Br I s s t a t e t o the unfilled 4p bound states. In Region 11, this peak decreases i t s intensity and broadens for (CHBry), with E parallel t o t h e c axis as shown in Fig. 4 (c). These result indicate that t h e r e is two species of bromine, i.e. Br-Br bond and Br-C bond.
The broadening of this peak results from t h e increased higher energy component which is clearly observed with E perpendicular t o t h e c-axis a s shown in Fig. 4 (b). T h e increased intensity of higher energy peak in Fig. 4 (d) for (CHBr
Ix
with E perpendicular t o t h e c-axis indicates t h a t t h e r a t i o of Br-C gond increases on going t o heavily doped region, which is consistent with t h e EXAFS results and two-speicies model [2].
2 (a I y=0.020 Euc
(d I y =0179 ELC
,
C8-618 JOURNAL DE PHYSIQUE
Fig. 5 Polarized Br K-XANES spectra of (CHBry)x.
-40 -20 0 20 40 60 80
ENERGY ( e V
The near-edge s p e c t r a of (CHBryIx in Region I is characterized by a small anisotropy with respect t o polarization as indicated in Fig. 5. The near-edge structures within 40 eV above t h e threshold observed in (CHBry), a r e similar t o those of bromobenzene, which suggests t h a t most of bromines form Br-C bonds in Region I. In this region
,
t h e dip between t h e sharp spike and the threshold diminishes a s indicated in Fig. 5 (b) and (c). These results a r e consistent with t h e assumption t h a t bromines a r e incorporated a s isolated ions in t h e early s t a g e of doping since t h e threshold of t h e sharp resonance peak observed for bromine ions is located between the 4p peak and continuum threshold of (CHBry)k a s indicated in Fig. 6 (d).The Fourier transform of podarized I L-EXAFS shows t h a t iodines a r e separatgd with e a c h o t h e r by 2.88 A, which is t h e intermediate value between I;
(2.76 A) and I; (2.92 A). T h e radial distribution around iodine is characterized by a long I-C distance (4.06-4.68 A) which might be t h e distance between t h e polyiodine ion and polyacetylene. Since t h e contribution of I-C bond is e s t i m a t e d t o b e very small, most of iodines form linear polyions which a r e oriented along t h e c-axis with a less degree of orientation than t h a t of (CHBryIx, partly due t o t h e incommensurate geometry of polyiodine ions with respect t o polymer backbone. The Fourier transform results for (CHIy)x a r e not sensitive t o t h e dopant concentration, indicating t h a t most of iodines a r e intercalated with a linear chain structure.
These results a r e consistent with the f a c t t h a t electrical conductivity of (CHIy), is always g r e a t e i than t h a t of (CHBryIx in magnitude for t h e s a m e amount of dopant and gradually increases with further doping in t h e heavily doped region.
ACKNOWLEDGEMENT
T h e authors wish t o express their thanks t o Dr. T. Fujikawa for valuable discussions.
REFERENCES
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