Local-structure determination in interhalogen-doped polyacetylene by X-ray absorption spectroscopy

Synthetic Metals, 17(1987) 479 484

z~79

LOCAL-STRUCTURE DETERMINATION IN INTERHALOGEN-DOPED

POLY-

ACETYLENE BY X-RAY ABSORPTION SPECTROSCOPY

W. KRONE, G. WORTMANN, V. BIEBESHEIMER

and G. KAINDL

Freie Universit/it Berlin, Fachbereich Physik, D-1000 Berlin 33 (Germany) S. ROTH Max-Planck-Institut for Festk6rperforschung, D-7000 Stuttgart 80 (F.R.G.)

ABSTRACT X=ray absorption (XA) spectroscopy was applied to investigate the chemical nature and local structure of the halogen dopants in IBr- and ICl-doped polyacetylene (PA). Both the NEXAFS (near-edge XA fine structure) and EXAFS (extended XA fine structure) of the C1-K, Br-K, and I-LI,II I thresholds provide detailed information on the formed halogen acceptor molecular anions as well as on substitution/addition reactions with the (CH) x chains. The orientation of the halogen species relative to the (CH) x chains is inferred from the use of stretch-oriented PA in combination with the linear polarization of the synchrotron radiation. The results are discussed in relation to the situations in 12- and Br2-doped PA, studied previously by XA; in addition, a comparison with halogen-intercalated graphite is given.

INTRODUCTION The structural properties of acceptor-doped polyacetylene (PA) are again of considerable present interest [1,2]. Most previous pertinent studies were performed on unoriented PA synthesized by the Shirakawa method [3]. The recent progress in preparing highly oriented, quasi-single-crystalline PA has raised renewed interest in refined structural studies [4]. But even with this strongly improved pristine PA, the structural properties of conducting doped PA, particularly of acceptor-doped materials, are difficult to resolve. On the one hand, this is due

to

rather

complex structures

[1,2,5],

and

on

the

other

hand

to considerable

structural transformations induced by inserting the large acceptor anions that cause a partial loss of long-range order. Recently, it was demonstrated for 12- and Br2-doped PA that X-ray absorption (XA) spectroscopy is capable of providing detailed information on chemical nature and local structure of the inserted species 1"6-8].

In the present contribution, these investigations are

extended to IBr- and ICl-doped PA.

The reported XA studies make use of both the

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© Elsevier Sequoia/Printed in The Netherlands

1.80

information contained in the near-edge

structure (NEXAFS)

and

in the extended fine

structure (EXAFS). From the use of stretch-oriented PA samples, in combination with linearly polarized synchrotron radiation light, the orientation of the inserted halogen species with respect to the (CH) x chains as well as relevant bond lengths are inferred.

EXPERIMENTAL PA was prepared by the Shirakawa method stretching at 130 °C in an inert-gas atmosphere.

1,3]. Some of the foils were oriented by This procedure (stretching factor: 1.8 -

2.8) leads to a preferred alignment of the (CH) x fibrils (,,500 ~k in diameter) parallel to the axis of stretching ~'. Halogen doping of oriented and n o n - o r i e n t e d ' P A foils, which were transformed to trans-PA by the stretching procedure or by heat treatment, was performed from the gas phase. The dopant concentration was determined from weight uptake. The doped samples were encapsulated in vacuum-tight stainless-steel containers equipped with thin Be windows and filled with Ar gas. The XA measurements were performed at various temperatures at either the EXAFS-II or the ROMO beamline of HASYLAB/DESY in Hamburg, using S i ( l l l ) or Si(311) double-crystal monochromators. For comparison, various reference compounds were investigated additionally, e.g. gas-phase C6H5Br, C6H5I, IC1, and IBr. Further experimental details are given elsewhere 1,,7,8,10]. It should be mentioned in this context that iodine-L I near-edge spectra, especially when obtained on oriented samples, directly display the direction of molecular bonds, since the XA process promotes an I-2s electron into empty molecular orbitals with I-Sp symmetry. Analogous information is contained in the bromine-K near-edge XA spectra (ls-,4p*) 1,6] and in the chlorine-K near-edge XA spectra (ls-~3p*) [10].

RESULTS AND DISCUSSIONS (a) IBr-doped polyacetylene Non-oriented as well as stretch-oriented PA samples, doped with IBr, were investigated by XA both at the iodine-Ll,ii I and the bromine-K thresholds. Fig. la shows the iodineL1, near-edge structure of an oriented PA foil doped with IBr for parallel and perpendicular orientation of the synchrotron-radiation polarization E with respect to ~'. For comparison, the corresponding spectra of I2-doped PA are given in Fig. lb 1,'8]. The close similarity in the observed near-edge structures, particularly of the peaks A and B including their polarization dependences, shows that approximately linear and oriented polyhalogen accepter molecules are present in the IBr-doped sample. A closer inspection of the data shows that the separation between the pre-edge peak A and the peak B (or between peak A and the Li-threshold, simulated by an arctan function in the least-squares fits of the spectra) is larger by ,.1 eV in the IBr-doped case than in I2-doped PA; this strongly points to shorter bondlengths in IBr-PA as compared to the I2-doped sample 1"9,10]. This conclusion is supported by the corresponding I-LII I XA spectra of IBr-PA (not shown here)• Quantitative information on the different bondlengths may be obtained from the I-LII I EXAFS spectra, shown in Fig. 2 after Fourier transformation (FT) from k-space into real space r'. The main peaks in Fig. 2 correspond to distances r to nearest-neighbor atoms, after proper correction for phase shifts 1,,8]. For the I2-doped sample, the main peak

481 i

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I-L[edge

"~

A

'

B

'

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b) (CHI~lx

I

/

I

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l-Lx EXAFS

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5.18

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0 /

I

5.2/,

YI

0

I

2

I

I

/'

"1

r' 1]~}

m

6

~

~

8

m

10

Fig. 1 (left figure). Near-edge XA structures at I-L I threshold of (a) IBr-(CH) x with /~[l~ (solid spectrum) and /~.l.s-' (dotted spectrum). For comparison, the corresponding spectra of (CHIo.oe)x are given in (b), taken from Ref 8. Fig. 2 (right figure). Fourier-transform (FT) of I-Lin EXAFS spectra of (a) IBr-(CH) x and (b) (CHIooa) x (from Ref. 8) for comparison. Note the shift of the main peak in a) to smaller r' "values.

exhibits a strong polarization dependence corresponding to the intramolecular distance r = 2.92 ~, of the inserted symmetric I~ acceptor molecules. The EXAFS-FT of the IBr-doped sample (Fig. 2a) exhibits a double-peaked structure with the main peak A shifted to a shorter distance, which is assigned to the I-I distance in linear IxBr- acceptor molecules, tentatively assigned to 12Br-. The weaker peak B then derives from the larger I-Br distance in such molecules. As discussed above on the basis of the I-L I near-edge structures, the presence of a considerable fraction of I~ molecules may be excluded. Both the near-edge structure and EXAFS spectra at the Br-K threshold (not shown here) are typical for Br-C bonds. The observed near-edge stucture is almost identical with that of C6H5Br, but quite different from the one found for IBr and CBr 4 [10]. This shows that most of the inserted Br reacts with the (CH) x chains substituting H. This argumentation is supported by the Br-K EXAFS, which exhibits a main nearest-neighbor (n.n.) peak in agreement with the Br-C distance observed in Br2-doped PA 1"7]. A n.n. polyhalide distance, expected from the presence of I2Br- species, is only observed in the Br-K EXAFS taken at 77 K. In summary, the XA results show that a considerable part of the Br atoms reacts substitutionally with the (CH) x chains, while the rest is forming interhalogen anions with iodine, particularly 12Br-, which are oriented with respect to the (CH) x chains. The strong bromination of the (CH) x chains explains the loss of long-range order observed in IBr-doped polyacetylene by a recent neutron diffraction study [11]. (b) ICl-doped polyacetylene Both CI-K edge and I-LI,II I edge XA spectra were recorded from oriented PA foils doped with ICI. The I-L I near-edge structure (see Fig. 3a) exhibits the same spectral features and polarization dependence, as previously observed for I2-doped PA (plotted for comparison in Fig. 3b), where the dominant intercalation of linear I~ molecules was found.

482 i

i

l

I

I- LI edge -"A .

I

I

I

i

I

I

b) {CHIao+]x ."

I

I

I

I

I

A __

6

i i

I

b) I-LxEXAFS (CHlooB)x

/

.-

.O

8ID

~0

r~


J I

5.16

I

5.1B

I

I

A

u_

..LS--E .L s

I

5.20

Energy(keY)

I

5.22

I

5.24

0

0

2

o) Lcl~jCH]x

t.

6 r'

8

{ 10

1•}

Fig. 3 (left figure). Near-edge XA structures at the I-L I threshold of a) ICI-(CH) x with EII~' (solid spectrum) and t~-" (dotted spectrum). In b) corresponding spectra of gas-phase IC1 (dotted) and of (CHip.f6) x (from Ref. 8) are also given. Fig. 4 (right figure). Fourier-transforms (FT) of the I-L m EXAFS spectra of (a) ICI-(CH)x with ]~lg* and (b) of gas-phase ICI (dotted spectrum) and of (CHip.pc) x with I~ll~ (solid line), for comparison.

This already points to a similar intercalated species as in the present ICl-doped PA. The uptake of ICI molecules may be excluded from a comparison with the spectrum of gas-phase ICI shown by the dotted curve in Fig. 3b; note the considerable shift of the main pre-edge peak in IC1 to lower energies reflecting a shorter bond length in ICI as compared to the intercalated species [9,10]. In addition, near-edge structure spectra were taken at the I-LII I threshold (not shown here), which fully support the data and conclusions from the I-L I threshold. The above arguments are further supported by I-LII I EXAFS measurements presented in Fig. 4a with 1~ II i'. The main peak in the radial distribution function coincides in r'-space with the one found for I2-doped PA; a corresponding spectrum for (CHI0.06) x (from Ref. 8) is given in Fig. 4b. On the other hand, the I-CI intramolecular distance is much shorter, as is obvious from an inspection of the I-LII I EXAFS-FT obtained for gas-phase IC1 (Fig. 4b, dotted spectrum). This excludes the presence of an appreciable amount of ICI molecules in the present system. We have also studied near-edge XA spectra at the C1-K threshold around 2.82 keV. The spectra obtained in absorption for ICI-(CH)x and for gaseous C6H5C1, measured for reasons of comparison, are presented in Fig. 5. It is obvious that the spectral features in both spectra, particularly the positions of the dominant pre-edge peaks (A), are identical for the ICl-doped PA and the gas-phase C6H5C1. This indicates a chlorination of the polymer by substitution of H atoms, leading to C-C! bonds as present in the model compound C6H5C1. Furthermore, the polarization dependence of peak A is opposite to the one observed for the pre-edge peak in the I-L I near-edge structure (see Fig. 3). This shows that the CI o-bonds (as monitored by the o*-orbitals, which give rise to the strong ore-edge peak A, are directed approximately perpendicular to the (CH)x chains. This C-C1 bond direction is in

483 i ~

!

I

i

i

CI-K edge~ b~CsHsCl,gas-phose" t

c-

~2

2

oc

/~

gxsJ

a} ICl-(CHlx

s ,~

.Q

2.80

I

I

I

I

2.82 2.8/--, Energy{keY)

I

2.86

Fig. 5. Near-edge XA structures at the CI-K threshold of (a) ICI-(CH) x with l~llg' (dotted spectrum) and with I~.l.s* (solid spectrum), In (b) the corresponding spectrum of gas-phase C6H5C1 is given for comparison.

agreement with the assumption of a substitution reaction of CI with the polymer. It should also be mentioned here that gas-phase ICI exhibits a CI-K near-edge structure (not shown here) that is quite different from those observed for ICI-PA and C6H5C1. In summary, the present XA studies on ICI-PA show that the ICI dopant molecules decompose in the polymer, most probably already during the doping process. The chlorine atoms react with the (CH) x chains forming CI-C bonds, while the iodine is present in form of linear I~ (I~) acceptor molecules.

(c) Halogen-doped polyacetylene in comparison with halogen-intercalated graphite The present XA study of IBr- and ICl-doped PA as well as previous work on 12- and Br2-doped PA

[6-8]

show that only I2-doped PA is suitable to fulfill the quality and

stability criteria postulated for a useful synthetic metal. A complete transformation of the halogen dopant atoms into acceptor anions is only found for (CHIy) x. In all other cases studied, a disturbing reaction of bromine or chlorine with the (CH)x chains takes place, which strongly reduces the electrical conductivity along the polymer chains and furthermore destroys the long range crystalline order of the doped systems. The present study does not give an answer to the question if and to what extent the bromination or chlorination reactions occur already during the doping process or with some time delay afterwards, since the XA studies were usually performed 2 days after sample preparation.

There are experimental observations that indicate -

in case of IBr- and

Br2-doped PA - that bromination of the (CH)x chains increases with time at the cost of the intercalated acceptor anions. A recent neutron-diffraction study of IBr~doped PA noted a loss of long-range coherence within 30 minutes after doping [11]. We have also reinvestigated three-months old Br2-doped PA samples used in our previous studies [71, and found no Br~ acceptor molecules [10]. On the other hand, I2-doped PA samples are found to show no changes in the I-LI,II I XA spectra even after 1.5 years, indicating a high stability of the inserted I~ (I~) molecules [8,10].

484 At this point, a comparison of halogen-doped PA with halogen-intercalated graphite is useful. Many similarities between intercalated graphite and conducting PA have been discussed, especially with respect to structural properties I1,2]. The intercalation behavior of PA seems to be just opposite to that known for graphite: while Br2-graphite and ICl-graphite are well defined systems, and also IBr-graphite is known to exist, no intercalation of 12 into graphite seems to occur [121. XA studies of Br2- and ICl-graphite [13,14] show the presence of intercalated Br 2 and ICI molecules. The basic difference between halogen-doped PA and graphite lies in the amount of charge transfer (or, from a chemical point of view, in the amount of oxidation of the respective matrix). Graphite is capable of intercalating large amounts of basically neutral halogen molecules (like e.g. Br2, IC1, IBr), provided that an initial weak chlorination or bromination reaction opens the graphite layers. On the other hand, conducting polyacetylene is made up of positively charged (CH)x chains and negative acceptor anions, analogous to the situation found in cation-radical salts [5].

ACKNOWLEDGEMENT The work was supported by the Bundesminister fiir Forschung und Technologic, project No. 05-256 KA and 05-313 AXB/C3-06.

REFERENCES 1

For a recent review, see: J.P. Pouget, in: Electronic Properties of Polymers and Related Compounds, Springer Series of Solid States Sciences. 63 (1985) 26.

2

R.H. Baughman, N.S. Murphy, G.G. Miller and L.W. Shaklette, J. Chem. Phys., 79 (1983) 1065. T. Ito, H. Shirakawa and S. Ikeda, J. Polvm. ~ci. Polvm. Chem. 12 (1974) 11; H. Shirakawa and S. Ikeda, Polvm. J.. 2 (1971) 231. J.H. Edwards and W.J. Feast, Polymer. 21 (1980) 595; W.J. Feast et al., in Ref. 1, p. 45 and references therein. W. Wieners et al., Makromol. Chem. Ravid Commun.. 6 (1985) 425 ; G. Wegner, in Ref. 1, p. 18. H. Oyanagi et al., J. Phvs. Soe. Javan. 5~ (1984) 4044. W. Krone et al., Solid State Commun.. 52 (1984) 253. G. Wortmann et al., Z. Phvsik (submitted); G. Wortmann et al., in Ref. 1, p. 41. For a recent study of the correlation between molecular bondlengths and the near-edge structures, see: J. St6hr, F. Sette, and A.L. Johnson, Phys. Rev. Lett.. 53 (1984) 1684. W. Krone e t a ! . , Proc. IVth Int. EXAFS Conf. (1986), to be published in J. de Phys. C. Riekel, H.W. H~slin, K. Menke and S. Roth, Svnth. Met.. 10 (1984/85) 31; C. Riekel, in Ref. 1, p. 35. M.S. Dresselhaus and G. Dresselhaus, Advance~ in Physics. 30 (1981) 139. S.M. Heald and E.A. Stern, Phys. R e v . . B l l (1975) 4836; and Svnth. Met.. 1 (1979/80) 249. G. Wortmann, W. Krone, G. Kaindl and R. Schl6gl, to be published.

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