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Esr of undoped polyacetylene [(CH)x and (CD)x] at 1.5 ⩽ T(K) ⩽ 300
Volume 69, number 2
CHEMICAL. PHYSICS
ESR OF UNDOPED POLYACETYLENE
M. SCHWOERER, Ph)srkahsches Instmt.
[(CH),
LETTERS
AND (CD),]
15 January
1980
AT 1.5 G T(K) G 300
U. LAIJTERBACH Univers~tatBayreuth. D-8580 Bayreuth. Germany
and W. MijLLER and G. WEGNER ZnsmtutfCr Makromolekulare Cheme - Hemmnn Staudmger Haus, D-7800 Frerburg. Germany Received
16 October
1979
Undoped trans-Isomers of (CH),, of (CD)x and of (95% CzD2 + 5% C~HZ)~ show S = l/2 ESR sgnak wthout hyperfie structure down to T= 1.5 K. No deviations from a Curie susc+ptib~I~tyare observed. The Ime broadening at low temperatures suggests a thermally activated mobfity of the defect, the structure of wluch has not yet been Identified.
I_ Lntroduction
In vrew of the Hugh electrical conductivity of doped It seemed interestmg to perform additionaI ESR experiments in undoped polyacetylene samples in order to specify theu defect structures more precisely than has been done so far. We were especially interested in (1) whether coohng below 9 K causes any structure of the ESR hne or any deviation from the Curie behavior; (2) whether perdeuteration causes a line narrowing; (3) whether in partially deuterated samples the signals show hfs due to a localization of the defects; and (4) whether there exists any further evidence that the observed ESR signals really are due to the bond alterna(CH),,
Electron spms wth a concentration of about one unpaired electron per 3000 carbon atoms have been observed 111undoped trans-(CH), by several authors [ 121. The almost free electron g-value, the nearly Iorentzum ESR line shape, the rmssing hyperfiie structure (hfs), and the Curie susceptlblllty down to T= 9 K [I] suggested the existence of highly mobile defects of the type =CH-CH=CH-CH-CH=CH-CH=CH-CH=CH
defect m long polyene molecules was exammed almost 20 years ago by Pople and Wabnsley [3,4] _ They calculated an actlvatlon energy for the h@ly mobde defect of the order of 0 2 eV, and they claimed that bond altematlon defects are one possible explanation for the observed sharp ESR lmes m sohd samples of polyenes. Recently Su et al. [S] presented a theoretlcal study where it was claimed that the bond alternation defect in polyacetylene (w2uch IS a Iong polyene) should be dlstnbuted over several C-C bond lengths. They descnbe the defect m the form of a topoiogicaI sohton or movmg domain wall and they calculated an activation energy for the neutral sohton motion of about 2 meV, correspondmg to T-20 K.
tion defect and not to impurities (for example catalyst moieties or oxidation products)_
residual
2. sampIes Three different samples, A, B and C, were investi(CD),;C: (95%C2Dz f S%C2H2jX. They were polymerized by the usual catalytic procedure as follows: acetylene was synthesized from calcium carbide; it was purified by washing with concentrated sulfuric acid and dried over P205. Deuterated acetylene was synthesized from D20 and CaC2, and a mixture of CzH2 and CzD, was prepared in order to synthesize the partially deuterated polymer. For polymerization, gated. A: (CH),;B:
359
Volume 69, number
CHEMICAL
2
PHYSICS
acetylene was drssolved at room temperature in dry toluene to a concentratron of 5 g/Q. The catalyst solutton [20 ml of a mrxture of 60 mmol Q-l AlEt and 15 mm01 Q-l TI(OBU)~] was added. The polymerrzatron started unmedrateIy after the addrtron of the catalyst. and the polymer product separated as a finely drspersed black powder. After 15 mm reactron tlnle the product was collected on a filter plate under a stream of nitrogen It was washed wrth dry toluene and transferred to a Soxlet apparatus where it was extracted wrth glacial acetrc acid over a penod of 24 h It was then drred at 50°C m vacua, and the resultant black powder was compressed to a solid disc wtth a standard KBr press. All procedures were carried out under a mtrogen atmosphere Elemental analysrs of this maternal showed the expected C/H values The IR spectra mdrcated that a pure transpolyacetylene was obtamed after the work-up procedure_ The measurements were carrred out after the samples had been stored at about 0°C for several weeks
3. Experimental The ESR measurements were carrted out wrth a Bruker spectrometer (9 4 GHz. 100 kHz field modulatlon) At all temperatures the mIcrowave power level was far away from saturatron of the spur system.
Coohng between 300 K and 4 K was performed by a gas flow cryostat (Oxford) and between 4 2 K and 1.5 K by a hehum bath cryostat. descrrbed elsewhere [9]. SampIes A and B were m a quartz glass tube which was sealed off under hehum gas and immersed into the cold gas flow or mto the liquid helmm respectrvely. Sample C was cemented onto the end of a teflon rod and thus was m direct contact wrth the cooled hehum The spin concentration
was deternuned
by comparison
with a ruby standard sample (NBS)
1.7&K
At all temperatures between 1 5 K and 300 K and for all samples A, B and C, the ESR spectrum consrsts of one single hne at g = 2.003 f 0.002 The concentration of the spins represents one free electron per 6000 C atoms in samples A and B. The lme IS almost 360
uo
-
Fig I- ESR hne of uotoprcalty mLxed trans-potyacetylene (95% CzDz + 5% C~HZ)~ at 7 = 1.47 K
lorentztan-shaped in the center but falls off more raprdly m the wmgs. Frg. 1 shows as an example the hne of sample C at T = 1.74 K. Thrs devration from the lorentzran shape 1s typrcal of both an exchangenarrowed ESR hne [lo] and of an mcoherent motion of spins between sates of two different local fields [ 1 I] which, for example, can be due to two drfferent proton spur onentations. The lure wrdth ,I, defmed as the full drstance between the maxima of the ESR line (dx”(~oYdBo). IS shown in table 1. Deuteratton causes a line narrowmg. The wrdth of the rsotoprcally muled samples has a value III between those of the pure samples. Coolmg to 4 2 K causes a line broadening by a factor 1.6 in samples A and B and by a factor of 1.5 m sample C. The mtegral suscepttbrhty, JX”dSO, mcreases as the reciprocal temperature. Frg. 2 shows as an example the susceptibhty
of sample C versus T-l
for 42
<
T(K) < 300 (a) and for 1 5 < T(K) d 3.0 (b). Table
4. Results
15 January 1980
LETTERS
1
Sample
A. (CH), B (CD), c. (95%C2D2 + SZC2H2),
Lmewldcb a(G) 5 0 2 300 K
4.2 K
41 19 27
6.7 3.1 4.0
Volume 69, number 2
CHEhiICAL
PHYSICS
15 January 1980
LETTERS
netlsm. Especially the effect ofdeuteration on the line width supports thrs interpretation, since the width ofa motionally narrowed hne is determined by both the jump probabihty and the hyperfine coupling. The jump probabdity must be of the order of lo* s-l or higher_ The line narrowing with increasing crystal temperature is an mdrcation of a thermally activated jump probabdrty for the defect. A very rough estimate leads to an actrvatron energy of a few meV. Unfortunately, the localizatron at low temperatures 1snot strong enough to cause a resolution of hfs which definitely could soIve the structure of the defects m undoped polyacetylene.
(a)
(b) Acknowledgement This work was supported by the Deutsche Forschungsgememschaft (DFG). We thank Drs. Eichele and G&tier for helpful discussions and G. Fenn, R. Hubar and W. Hart1 for assistance in the ESR experiments_
References
[ 11 I B. Goldberg. H.R. Crowe. P.R. [2] Rg 2 Integral susceptlblty of sample C in the temperature ranges (a) between 300 K and 4 2 K, (b) between 3 K and 1.5 K
5. Discussion
[6j
The Curie susceptlbtity down to 1 5 K shows that an activation energy for a transltion to the paramagnetICstate from a nonparamagnetlc state (or vice versa), If existmg, 1s extremely small. This means that the defects withS = i/2 arc produced during the sample pr+ paratlon.
It cannot
[3] [41 [S]
be excluded
that
the highly
mobtle
bond-alternation defect is responsible for the paramag-
[7] [8] 191 [lo]
Newman, AJ. Heeger and A.G. MacDnrmld, J. Chem. Phys. 70 (1979) 1132. P. Bemler, hl. RoUand, hf. GaltIer. A. hfontaner. LI. Regis. hl. CanddIe, C. Benoit, hl. AIdti. C. Lmaya. F. Schui. J Sled& J-hi. Fabre and L. Giral. J. Phys. (Paris) 40 (1979) L-297. J-A. Pople and S-H. Walmsley, Mol. Phys. 5 (I96L) IS. J.A. Pople and S.H. Walmsley, Mol. Phys_ 5 (I96I) 44I. W P. Su, J.R. Schrieffer and AJ. Heeger. Phys_ Rev_ Letters 42 (1979) 1698. G. Natta, G. Mazzan h and P. Cooradini. Cbem. Abstracts 53 (1959) 13985. hf. Hatano. J. Polymer Sci. 51 (1961) 526. T- Ito. H. Sbirakawa and S. Ikeda, J. Polymer Sci. Chem. Ed. 12 (1974) 11. hf. Schwoerer and H.C. WoIf, Mol. Cryst. 3 (L977) 177. P-W. Anderson and P.R. Weiss, Rev. hlod- Phys. 25 (1953)
269.
[ 111 P. Remeker
and H. Haken. Z. Physik 250 (1972) 300.
CHEMICAL. PHYSICS
ESR OF UNDOPED POLYACETYLENE
M. SCHWOERER, Ph)srkahsches Instmt.
[(CH),
LETTERS
AND (CD),]
15 January
1980
AT 1.5 G T(K) G 300
U. LAIJTERBACH Univers~tatBayreuth. D-8580 Bayreuth. Germany
and W. MijLLER and G. WEGNER ZnsmtutfCr Makromolekulare Cheme - Hemmnn Staudmger Haus, D-7800 Frerburg. Germany Received
16 October
1979
Undoped trans-Isomers of (CH),, of (CD)x and of (95% CzD2 + 5% C~HZ)~ show S = l/2 ESR sgnak wthout hyperfie structure down to T= 1.5 K. No deviations from a Curie susc+ptib~I~tyare observed. The Ime broadening at low temperatures suggests a thermally activated mobfity of the defect, the structure of wluch has not yet been Identified.
I_ Lntroduction
In vrew of the Hugh electrical conductivity of doped It seemed interestmg to perform additionaI ESR experiments in undoped polyacetylene samples in order to specify theu defect structures more precisely than has been done so far. We were especially interested in (1) whether coohng below 9 K causes any structure of the ESR hne or any deviation from the Curie behavior; (2) whether perdeuteration causes a line narrowing; (3) whether in partially deuterated samples the signals show hfs due to a localization of the defects; and (4) whether there exists any further evidence that the observed ESR signals really are due to the bond alterna(CH),,
Electron spms wth a concentration of about one unpaired electron per 3000 carbon atoms have been observed 111undoped trans-(CH), by several authors [ 121. The almost free electron g-value, the nearly Iorentzum ESR line shape, the rmssing hyperfiie structure (hfs), and the Curie susceptlblllty down to T= 9 K [I] suggested the existence of highly mobile defects of the type =CH-CH=CH-CH-CH=CH-CH=CH-CH=CH
defect m long polyene molecules was exammed almost 20 years ago by Pople and Wabnsley [3,4] _ They calculated an actlvatlon energy for the h@ly mobde defect of the order of 0 2 eV, and they claimed that bond altematlon defects are one possible explanation for the observed sharp ESR lmes m sohd samples of polyenes. Recently Su et al. [S] presented a theoretlcal study where it was claimed that the bond alternation defect in polyacetylene (w2uch IS a Iong polyene) should be dlstnbuted over several C-C bond lengths. They descnbe the defect m the form of a topoiogicaI sohton or movmg domain wall and they calculated an activation energy for the neutral sohton motion of about 2 meV, correspondmg to T-20 K.
tion defect and not to impurities (for example catalyst moieties or oxidation products)_
residual
2. sampIes Three different samples, A, B and C, were investi(CD),;C: (95%C2Dz f S%C2H2jX. They were polymerized by the usual catalytic procedure as follows: acetylene was synthesized from calcium carbide; it was purified by washing with concentrated sulfuric acid and dried over P205. Deuterated acetylene was synthesized from D20 and CaC2, and a mixture of CzH2 and CzD, was prepared in order to synthesize the partially deuterated polymer. For polymerization, gated. A: (CH),;B:
359
Volume 69, number
CHEMICAL
2
PHYSICS
acetylene was drssolved at room temperature in dry toluene to a concentratron of 5 g/Q. The catalyst solutton [20 ml of a mrxture of 60 mmol Q-l AlEt and 15 mm01 Q-l TI(OBU)~] was added. The polymerrzatron started unmedrateIy after the addrtron of the catalyst. and the polymer product separated as a finely drspersed black powder. After 15 mm reactron tlnle the product was collected on a filter plate under a stream of nitrogen It was washed wrth dry toluene and transferred to a Soxlet apparatus where it was extracted wrth glacial acetrc acid over a penod of 24 h It was then drred at 50°C m vacua, and the resultant black powder was compressed to a solid disc wtth a standard KBr press. All procedures were carried out under a mtrogen atmosphere Elemental analysrs of this maternal showed the expected C/H values The IR spectra mdrcated that a pure transpolyacetylene was obtamed after the work-up procedure_ The measurements were carrred out after the samples had been stored at about 0°C for several weeks
3. Experimental The ESR measurements were carrted out wrth a Bruker spectrometer (9 4 GHz. 100 kHz field modulatlon) At all temperatures the mIcrowave power level was far away from saturatron of the spur system.
Coohng between 300 K and 4 K was performed by a gas flow cryostat (Oxford) and between 4 2 K and 1.5 K by a hehum bath cryostat. descrrbed elsewhere [9]. SampIes A and B were m a quartz glass tube which was sealed off under hehum gas and immersed into the cold gas flow or mto the liquid helmm respectrvely. Sample C was cemented onto the end of a teflon rod and thus was m direct contact wrth the cooled hehum The spin concentration
was deternuned
by comparison
with a ruby standard sample (NBS)
1.7&K
At all temperatures between 1 5 K and 300 K and for all samples A, B and C, the ESR spectrum consrsts of one single hne at g = 2.003 f 0.002 The concentration of the spins represents one free electron per 6000 C atoms in samples A and B. The lme IS almost 360
uo
-
Fig I- ESR hne of uotoprcalty mLxed trans-potyacetylene (95% CzDz + 5% C~HZ)~ at 7 = 1.47 K
lorentztan-shaped in the center but falls off more raprdly m the wmgs. Frg. 1 shows as an example the hne of sample C at T = 1.74 K. Thrs devration from the lorentzran shape 1s typrcal of both an exchangenarrowed ESR hne [lo] and of an mcoherent motion of spins between sates of two different local fields [ 1 I] which, for example, can be due to two drfferent proton spur onentations. The lure wrdth ,I, defmed as the full drstance between the maxima of the ESR line (dx”(~oYdBo). IS shown in table 1. Deuteratton causes a line narrowmg. The wrdth of the rsotoprcally muled samples has a value III between those of the pure samples. Coolmg to 4 2 K causes a line broadening by a factor 1.6 in samples A and B and by a factor of 1.5 m sample C. The mtegral suscepttbrhty, JX”dSO, mcreases as the reciprocal temperature. Frg. 2 shows as an example the susceptibhty
of sample C versus T-l
for 42
<
T(K) < 300 (a) and for 1 5 < T(K) d 3.0 (b). Table
4. Results
15 January 1980
LETTERS
1
Sample
A. (CH), B (CD), c. (95%C2D2 + SZC2H2),
Lmewldcb a(G) 5 0 2 300 K
4.2 K
41 19 27
6.7 3.1 4.0
Volume 69, number 2
CHEhiICAL
PHYSICS
15 January 1980
LETTERS
netlsm. Especially the effect ofdeuteration on the line width supports thrs interpretation, since the width ofa motionally narrowed hne is determined by both the jump probabihty and the hyperfine coupling. The jump probabdity must be of the order of lo* s-l or higher_ The line narrowing with increasing crystal temperature is an mdrcation of a thermally activated jump probabdrty for the defect. A very rough estimate leads to an actrvatron energy of a few meV. Unfortunately, the localizatron at low temperatures 1snot strong enough to cause a resolution of hfs which definitely could soIve the structure of the defects m undoped polyacetylene.
(a)
(b) Acknowledgement This work was supported by the Deutsche Forschungsgememschaft (DFG). We thank Drs. Eichele and G&tier for helpful discussions and G. Fenn, R. Hubar and W. Hart1 for assistance in the ESR experiments_
References
[ 11 I B. Goldberg. H.R. Crowe. P.R. [2] Rg 2 Integral susceptlblty of sample C in the temperature ranges (a) between 300 K and 4 2 K, (b) between 3 K and 1.5 K
5. Discussion
[6j
The Curie susceptlbtity down to 1 5 K shows that an activation energy for a transltion to the paramagnetICstate from a nonparamagnetlc state (or vice versa), If existmg, 1s extremely small. This means that the defects withS = i/2 arc produced during the sample pr+ paratlon.
It cannot
[3] [41 [S]
be excluded
that
the highly
mobtle
bond-alternation defect is responsible for the paramag-
[7] [8] 191 [lo]
Newman, AJ. Heeger and A.G. MacDnrmld, J. Chem. Phys. 70 (1979) 1132. P. Bemler, hl. RoUand, hf. GaltIer. A. hfontaner. LI. Regis. hl. CanddIe, C. Benoit, hl. AIdti. C. Lmaya. F. Schui. J Sled& J-hi. Fabre and L. Giral. J. Phys. (Paris) 40 (1979) L-297. J-A. Pople and S-H. Walmsley, Mol. Phys. 5 (I96L) IS. J.A. Pople and S.H. Walmsley, Mol. Phys_ 5 (I96I) 44I. W P. Su, J.R. Schrieffer and AJ. Heeger. Phys_ Rev_ Letters 42 (1979) 1698. G. Natta, G. Mazzan h and P. Cooradini. Cbem. Abstracts 53 (1959) 13985. hf. Hatano. J. Polymer Sci. 51 (1961) 526. T- Ito. H. Sbirakawa and S. Ikeda, J. Polymer Sci. Chem. Ed. 12 (1974) 11. hf. Schwoerer and H.C. WoIf, Mol. Cryst. 3 (L977) 177. P-W. Anderson and P.R. Weiss, Rev. hlod- Phys. 25 (1953)
269.
[ 111 P. Remeker
and H. Haken. Z. Physik 250 (1972) 300.