13C NMR STUDIES OF IODINE DOPED POLYACETYLENE

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13C NMR STUDIES OF IODINE DOPED POLYACETYLENE

I. Heinmaa, M. Alla, A. Vainrub, E. Lippmaa, M. Khidekel, A. Kotov, G.

Kozub

To cite this version:

I. Heinmaa, M. Alla, A. Vainrub, E. Lippmaa, M. Khidekel, et al.. 13C NMR STUDIES OF IO-

DINE DOPED POLYACETYLENE. Journal de Physique Colloques, 1983, 44 (C3), pp.C3-357-C3-

360. �10.1051/jphyscol:1983372�. �jpa-00223033�

1 3 C NMR STUDIES OF IODINE DOPED POLYACETYLENE

I . Heinmaa*, M. A l i a * , A. V a i n r u b * , E. Lippmaa*, M.L. K h i d e k e l * * , A . I . Kotov**

and G . I . Kozub**

'Institute of Chemical Physics and Biophysics of the Academy of Sciences of the Estonian SSR, Lenini pst. 10, 200001 Tallinn, USSR

**Institute of Chemical Physics of the Academy of Sciences of the USSR, 1424Z2 Che-mogolovka, USSR

13

Résume. - Nous décrivons des expériences de RMN sur le C, utilisant la polarisation croisée et la rotation à l'angle magique, sur des échantillons de cis et trans (CH) en fonction du taux de dopage avec l'iode. Deux signaux nouveaux apparaissant au cours du dopage sont attribués à la formation de solitons chargés sans spins et à l'addi- tion chimique d'iode sur les doubles liaisons. L'isomérisation cis- trans n'apparaît pas même pour des taux de dopants élevés.

Abstract. - Natural abundance 1 3 C NMR experiments using cross-polariza- tion and magic angle spinning are reported for cis- and trans-poly- acetylene as a function of iodine doping level. Two new signals appear- ing on doping are attributed to the formation of spinless charged so- litons and the chemical addition of iodine to double bonds. The sub- stantial cis/trans isomerization does not occur even at high iodine doping levels.

High resolution 1 3 C NMR spectra of the polyacetylene (CH) X were studied for the first time in [1]. Then the method was used to study the ther- mal cis/trans isomerization [2,3]. For high-conductive AsF 5 doped poly- acetylene the appearance of Knight shift for doping levels higher than 7 mol % was reported [4]. However, the reexamination of the results showed that the shift is primarily a chemical shift appearing from the polymer oxidization on doping [5].

Experiment. The cis-(CH) x film was synthesized at -78°C using the Sira- kawa technique [6]. The product was carefully washed in toluene, THF and methanol and dried in vacuum. The trans-polyacetylene was achieved by heating the cis-isomer at 130°C during two hours in vacuum. Doping with iodine was carried out at ambient temperature by exposure the polyacetylene samples to iodine vapor carried by argon gas flow. The exposition times up to 30 hours were used to obtain heavily doped sam- ples. The doping was followed by the vacuum pumping of the sample during two hours in order to remove any residual iodine absorbed on the surface. The iodine content was subsequently determined by weight up- take. The powdered sample was packed into the cylindrical NMR rotor and closed airtight with a plastic cap. The 9 mm O.D. rotor has about 0.5cc for the sample and is mad from glass ceramics to avoid any background carbon signal. All manipulations with the polymer have been done in an inert atmosphere dry box. The NMR spectra were recorded at room temper- ature on a Bruker CXP-200 spectrometer (50.3 MHz for 1 3 C) using proton decoupling, cross polarization and magic angle spinning of the sample.

Shifts are given with respect to the external TMS.

1 3 C NMR spectra of (CHI ^y ) ^x . Dependence of spectra on iodine content is given in Fig. 1 and 2 for cis- and trans-isomer, respectively. The starting spectra (y = O, bottom curves) show a single narrow line of the principal isomer together with the weaker one representing the pres- ence of the small amount of the other isomer. The chemical shifts were found to be 127 and 136 ppra for cis- and trans-(CH) X , respectively, and

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983372

C3- 358 JOURNAL DE PHYSIQUE

a g r e e w i t h p r e v i o u s d a t a [ I - 5 1 . Achieved h i g h r e s o l u t i o n ( 5 ppm FWHH) and t h e absence of a d d i t i o n a l l i n e s i n s p e c t r a , i n d i c a t e r e l a t i v e l y h i g h chemical homogeneity o f o u r ( C H ) x f i l m s .

carbocations cis.(CHly)x /\ , 'charged solitons,

sidebands

Y = O _niL ^{5 - I}

LO0 300 200 100 0 -100 - 2 0 0 ~ o m . ^,

shift from TMS

x-spinning sidebands

-..

_' I

I I 7

LOO 300 200 100 0 -100 -200 pprn shift from TMS

r i g . 1 . ^{F i g . 2 .}

The i o d i n e doping l e a e s t o t h e f o r m a t i o n of two new broad r e s o n a n c e s a t a b o u t 150 and 50 ppm o f i n c r e a s i n g i n t e n s i t y towards h i g h e r i o d i n e con- t e n t ( u p p e r c u r v e s i n P i g . 1 and 2 ) . The f i r s t band i s s i m i l a r t o t h a t r e p o r t e d f o r AsF5 doped (CH)x [ 5 ] and i t s p o s i t i o n i s q u i t e i n s e n s i t i v e t o iodine/AsF, d o p i n g l e v e l . The doping l e a d s t o t h e n - e l e c t r o n t r a n s - f e r from p o l y a c e t y l e n e t o i o d i n e and t h u s t o t h e f o r m a t i o n o f t h e o x i d i z e d polymer c h a i n r e g i o n s , i.e. p o l y c a r b o c a t i o n s . E o s t p r o b a b l y , t h i s band i s due t o t h e s i g n a l from p o l y c a r b o c a t i o n s and s i n c e it l i e s i n t h e t y p i c a l r e g i o n o f t h e c h e m i c a l s h i f t s o f a r o m a t i c c a r b o c a t i o n s

[ 7 ] t h e r e i s no remarkable p a r a m a g n e t i c o r Knight s h i f t one may e x p e c t f o r t h e h i g h l y c o n d u c t i n g doped p o l y a c e t y l e n e , T h i s r e s u l t s u p p o r t s t h e s p i n l e s s c h a r a c t e r o f c h a r g e c a r r i e r s i n doped (CH)x an& t h e s o l i t o n c o n c e p t i o n o f t h e c h a r g e t r a n s f e r [ 8 , 9 ] .

The r e l a t i v e w i d t h of t h e band i s a b o u t 100 ppm and does n o t depend r e - markably on t h e p o l a r i z i n g magnetic f i e l d H ( H = 4 7 kOe i n o u r e x p e r i - ments and i n [41 and H = 15 kOe i n [ 5 ] ) n o r on t h e dopant t y p e . T h i s s u g g e s t s t h a t t h e w i d t h i s p r i m a r i l y d e t e r m i n e d by t h e wide r a n g e of chemical s h i f t s o f u n e q u i v a l e n t c a r b o n s i n a p o l y c a r b o c a t i o n ( c h a r g e d s o l i t o n ) c o n s i s t i n g of a b o u t 15 =CH- groups [ 8 ] . S i m i l a r chemical s h i f t d i s t r i b u t i o n i s known f o r t h e a r o m a t i c c a r b o c a t i o n s w i t h d e l o c a l i z e d T - e l e c t r o n s [ 7 1.

Comparison o f s p e c t r a i n F i g . 1 and 2 shows t h a t t h e i o d i n e doping o f t r a n s - (CK) x i s l e s s homogeneous t h a n t h a t o f c i s - ( C H ) x. I n t r a n s - (CHIy)x, a s h a r p l i n e a t 136 ppm due t o undoped r ~ g i o n s o f t h e sample i s ob- s e r v e d f o r a l l doping l e v e l s y 2 0.15. For c i s - ( C h I y ) , t h e s h a r p com- ponent d i s a p p e a r s a t i o d i n e c o n c e n t r a t i o n y = 0.13.

The band o b s e r v e d a t a b o u t 50 ppn i n t h e doped p o l y a c e t y l e n e i s connect-

[ l o ] . For example, i n C H I ( C H 2 C H 3 ) 2 t h e s h i f t s a r e 43.9 and 33.2 ppm f o r - C H I - and -CH2- c a r b o n s , r e s p e c t i v e l y . The i o d i n e a d d i t i o n i n t e r r u p t s t h e c o n j u g a t e d polymer c h a i n t h a t p r o b a b l y l e a d s t o s e v e r e d e c r e a s e of t h e c o n d u c t i v i t y o f t h e i o d i n e doped p o l y a c e t y l e n e a s compared w i t h AsFs doped m a t e r i a l [ I l l .

Undoping e x p e r i m e n t s . I o d i n e doping i s p a r t i a l l y r e v e r s i b l e . Vacuum t r e a t m e n t o f t h e h e a v i l y doped m a t e r i a l l e a d s t o s u b s t a n t i a l removal of i o d i n e . Even more complete undoping can b e a c h i e v e d by washing o f

( C H I y ) , f i l m s i n a c e t o n e . F i g . 3 shows t h e s p e c t r a of t h e same sam- p l e b e f o r e doping, a f t e r doping

and t h a t o f washed i n a c e t o n e a f t e r (CHlylx doping. The i n i t i a l c o n f i g u r a t i o n

o f t h e sample by t h e NMR spectrum

was 60% c i s - f o r m . The spectrum r e - ^{trans- ,CIS}

corded a f t e r t.he i o d i n e doping up after lodine

t o t h e l e v e l of y = 0.17 c o n s i s t s of two broad l i n e s a t 150 and

50 ppm a s d e s c r i b e d above. The AL

s h a r p l i n e s d i s a p p e a r e d c o m p l e t e l y i n d i c a t i n g t h e l a c k of undoped r e g i o n s i n t h e sample. The a c e t o n e washing o f t h e sample reduced t h e

i o d i n e c o n c e n t r a t i o n t o t h e l e v e l ~ - 0 , 1 7

of y = 0.06 d e t e r m i n e d by t h e l o s t w e i g h t and t h e c o r r e s p o n d i n g NMR spectrum i n d i c a t e s t h a t t h e o x i -

d i z i n g i o d i n e i s mostly removed x- spanning

s i n c e t h e c o n t r i b u t i o n o f - C H I - ^s~debands

l i n e a t 50 ppm i n t o t h e t o t a l i n - t e n s i t y o f t h e spectrum i n c r e s e s

w i t h undoping. F i n a l l y , t h e L O O 300 200 loo o -100 -2ooppm

c i s / t r a n s r a t i o was d e t e r m i n e d s h ~ f t from TMS

u s i n g t h e i n t e n s i t i e s o f s h a r p

l i n e s i n t h e y = 0.06 spectrum. F i g . 3.

E l i m i n a t i n g t h e broad component i n

t h e v i c i n i t y o f narrow l i n e s u s i n g t h e s u i t a b l y s c a l e a y = 0.17 spec- trum we a r r i v e t h a t t h e c i s j t r a n s r a t i o i n t h e undoped m a t e r i a l i s 2:3 ( i n s t a r t i n g m a t e r i a l 3 : 2 ) . The p r e s e n c e o f t h e s t r o n g c i s - l i n e i n t h e undoped sample spectrum c a n n o t b e e x p l a i n e d assuming n e a r l y com- p l e t e c i s - t r a n s i s o m e r i z a t i o n i n d u c e d by i o d i n e doping a s d e t e c t e d i n t h e c a s e o f AsFs doped c i s - p o l y a c e t y l e n e where t h e polymer was con- s i d e r e d c o n v e r t e d i n t o t h e t r a n s - f o r m [ 1 2 ] . A s a p o s s i b l e e x p l a n a t i o n f o r t h i s e f f e c t w e s u g g e s t t h e c h e m i c a l a d d i t i o n of t h e dopant ( i o d i n e ) t o t h e polymer c h a i n s , t h a t can make t h e i s o m e r i z a t i o n mechanism pro- posed i n [ I 3 I i n e f f i c i e n t .

Summary. The r e s u l t s o f o u r e x p e r i m e n t s a r e c o n s i s t e n t w i t h t h e s p i n - l e s s c h a r g e d s o l i t o n s model and show t h e chemical a d d i t i o n of i o d i n e t o t h e polymer on doping. No complete c i s / t r a n s i s o m e r i z a t i o n was o b s e r v e d even f o r h e a v i l y doped samples.

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