Mőssbauer spectroscopy studies of polyacetylene doped with iron chloride complexes

Mat. Res. BuU., Vol. 17, pp. 1505-1510, 1982. Printed in the USA. 0025-5408/82/121505-06503.00/0 Copyright (e) 1982 Pergamon Press Ltd.

MOSSBAUER

SPECTROSCOPY DOPED

WITH

STUDIES

IRON CHLORIDE

A.Pro•,

OF POLYACETYLENE COMPLEXES

M. Zag6rska

Department of Chemistry, Technical U n i v e r s i t y of Warsaw

00-664 Warsaw, Noakowskiego 3, Poland Z.Kucharski,

M.l.ukasiak, J. Suwalski

Institute of Nuclear Research, 05-4.00 Otwock,

Swierk, Poland

(Received August 16, 1982; Refereed) A B ST R A C T

57 The

,, Fe Mossbauer resonance has been applied to study

polyacetylene films doped chemically and electrochemically to different doping levels. Both methc>Rs lead to the insertion of a high spin Fe III complex with M~ssbauer

para-

m e t e r s c h a r a c t e r i s t i c of FeCl~.Degradation of[CH(FeCl~)y]x in a i r leads to gradual t r a n s f o r m a t i o n of Fe III into a high spin Fe ![

INTRODUCTION In recent years several electron accepting and- electron donating compounds have been found to convert polyacetylene into an "organic metal" /I/, /2/, /3/, /4/. Although in many cases the chemical nature of the dopant and its structural arrangement after the doping reaction has been elucidated there exist systems in which the chemical form of the dopant is still unclear /5/. Recently 1291 M~ssbauer resonance has been successfully applied to iodine doped polyacetylene proving i the existance of linear 13- and 15- ions in this compound

161, /7/. 1505

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Vol. 17, No. 12

The purpose of this paper is to clarify the nature of the anion introduced to polyacetylene upon its chemical and electrochemical doping with iron chloride complexes. EXP ERIMENTAL The polyacetylene used in all experiments was prepared using a m o d i f i c a t i o n of t h e m e t h o d of Ito e t . a l

/8/.

It h a d a c i s :

trans

ratio of approximately i :i. FeCI 3 and LiCI were vacuum dried at 100°C and 150°C respectively for one hour. Nitromethane was kept over calcium chloride and then v a c u u m distilled. T w o methods of doping were applied: chemical oxidation of ( C H ) x in FeCl 3 /nitromethane solution/ and electrochemical oxidation in FeCI3/LiCl / nitromethane solutions. The details of chemical oxidation can be found elsewhere /3/. The electrochemical oxidation experiments were carried out in a dry argon atmosphere or using high v a c u u m line technique. For the preparation of LiFeCI£ solution, equimolar amounts of FeCI 3 and LiCl were mixed in the reactor in a dry argon atmosphere. T h e n the calculated amount for the preparation of saturated solution of nitromethane was transferred into the reactor by distillation under vacuum. The exact concentrations of FeCI 3 and LiCI were determined by spectrophotometry. It was found that the concentrations of FeCl 3 and LiCI were 0.25 I%4 in all experiment s. In a two - electrode configuration a piece of ( C H ~ was attached to a strip of platinum sheet ca 0.25 c m 2 serving as the anode. A 3 c m 2 piece of platinum was used as the cathode. T h e anode and the cathode compartments in the glass reactor were separated with a f r i t ." A constant voltage of 3,5 - £ , 5 V was typically applied. The amount of the charge that passed the system was calculated by the integration of i=f(t) curve. After the reaction had been stopped the polyacetylene strip was repeatedly washed with dry nitromethane and then pumped for at least 1 hour in order to remove the solvent. For all samples doped chemically and electrochemically the m a s s uptake was measured and selected strips were subjected to elemental analysis. If For M o s s b a u e r effect M E studies samples were transfered to the cryostat chamber in dry argon and then the cryostat was evacuated. The absorbes thickness was 0.2 +_ 0.05 m g / c m 2 of 57Fe isotope. The M E measurements were carried out at the temperatures of 298,77 and f+.2 K in the standard transmission geometry using a constant acceleration velocity spectrometer coupled to 5 7 C o / C r source. The spectra were recorded with a statistics of several million counts per channel to become reliable even in the case of low absorption coefficient, and then fitted with Lorentz shape lines by the least squares method with a computer.

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POLYACETYLENE

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R E SU L T S

Elemental analysis results show a C = H ratio equal to i:i, Fe:Cl ratio very close to I:Z~ /from 1:3.76 to 1:3.95/ and no oxygen in the sample independently on the doping level and doping procedure /chemical or electrochemical/. Therefore the general formula of the obtained compounds can be expressed as follows: [CH(FeCIK)y] x M~ssbauer spectra of chemically and electrochemically oxidized polyacetylene do not differ significantly. Their M6ssbauer parameters are presented in Table I and a typical spectrum obtained at /+.2 K is depicted in Fig. I TABLE

I

M~ssbauer parameters of 57Fe nuclei in electrochemically and chemically doped polyacetylene

[CH(FeCI6)0.05]x

Tempe ratu re

(El

e2qQ

electrochemically

~f

IS

(mm/s)

doped

r/2

(mm/s)

ab so r p t i o n

(mm/s)

298

0.2

77

o. 36 +_0 . 0 2

0.31 +- 0.01

O. Z~99 + 0.017

z~ +_o.5

~-.2

0.32 +- 0.01

0.35 + 0.03

0.508 + 0.08

11+.0.5

[CH (FeCI~ 0.09] x

chemically doped

298 ~.2

x

Isomer

0.15 0.32 +_0.01

shifts

0.31 + 0.01

IS relative

to natural

0.~65 + 0.01

iron

at 3 0 0 ° K

I0.1 + 0.2

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4 Z

_o O_ C~ O

0

Fig.l. Mossbauer spectrum of electro. chemically doped polyacetylene obtained at ~.2 K

i

-8

I0

VELOCITY [rnm/s]

W e were unable to record M~ssbauer spectra of freshly prepared samples at R T due to their surprizingly low absorption coefficie~. M~ssbauer parameters given in Table I are characteristic of Fe "~ high spin complexes and very similar to the values typically obtained for FeCI 4" /9/. All these halide complexes exhibit the quadrupole splitting in the range of 0.30 - 0.~0 mm/s. Small variations in the Q.S. value of the particular species can be attributed to slight differencies in the local symmetry of the anion /the higher the asymmetry the higher the Q.S. value/. The chemical isomer shifts are in the range of 0.25 - 0.35 m m / s depending on the covalency of the Fe-CI bond. These M~ssbau~r parameters could be also interpreted with some restrictions as FeAAbut low spin iron /i0/, however this is highly unlikely since no !ow spin iron-chloride complexes have been reported to date /ii/. W e have also undertaken the study of the stability of the doped system with time. [CH(FeCl~ly]x kept in a sealed polyetylene bag filled with argon undergoes slow degradation manifested by significant changes in its M~ssbauer spectrum. In lIT spectrum a doublet, /IS=l.15mm/s Q S = 2 . 4 ram/s/characteristic of a high spin F~|appears. At the same time in 4.2 K spectrum we can observe gradual decrease of the Fe III peak intensity with simultaneous increase of the Fe ll doublet intensity. DISCUSSION

AND

CONCLUSIONS ##

Elemental analysis and Mossbauer spectroscopy data indicate that the dopant molecule is significantly changed upon insertion to polyacetylene. Both isomer shift and stoichiometry of the compound#Q..qbrained definitly exclude the fo-mulation of the type[CH÷
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POLYACETYLENE

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with partial charge transfer from the polymer chain to the dopant molecule. T h e isomer shift expected for such a formulation should be higher that the one of pure FeCI 3 due to the transformation of a fraction of Fe III into Fe II. Instead we observe IS lower than in the case of pure FeCl 3 and very similar to the one typically obtained for FeCIz~-. If we assume that the excess of chlorine above CI:Fe=3 ratio is responsible for the charge of the dopant molecule we can calculate the amount of charge transfer based on elemental analysis / C T =(~e1 - 3).100%/. T h e v a l u e s obtained v a r y from 76% to 95%. B a s e d on the above r e s u l t s we c a n c o n c l u d e that (CH) x doped with i r o n c h l o r i d e s c a n be f o r m a l l y t r e a t e d as [ C H Y + ( F e C 1 4 - ) y ] x . It should be n o t e d that i n d e p e n d e n t l y on the method of doping applied we obtain the same r e s u l t s . In the c a s e of e l e c t r o c h e m i c a l doping the following e q u i l i b r i u m of a L e w i s acid / F e C 1 3 / with LiC1, in an a p r o t i c s o l v e n t , is the s o u r c e of F e C 1 4 - in the s y s t e m : LiC1 + F e C 1 3 ~ - ~ L i + + FeC14 In the case of chemical doping a partial reduction reaction, similar to that given by Clarke for A s F 5 doping /12/, should be postulated ie. 2FeCI 3 + le - ~ F e C I 2 + FeCI 4d

In view of our recent results our interpretation of M o s s b a u e r R T spectra given in paper /3/ should be radically verified. The dopant molecules inserted to polyacetylene are undetectable by M ~ s s b a u e r spectroscopy at R T due to their extremely low absorption coefficient caused in turn by small probability of recoiless absorption. Degradation of the sample causes the reduction of Fe Ill to Fe II /probably via chlorination of the double bond of the polymer chain/ and the formation of a n e w phase that can be detected by M 6 s s b a u e r spectroscopy at R T . k should be noted that the iron of degradation product is the only species "visible" at R T temperature even if it constitutes only a small fraction of the total iron of the sample. Therefore any interpretation based exclusively on R T spectra can be very misleading.

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