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DOPING OF POLYPARAPHENYLENE - A SPECTROSCOPIC STUDY OF THIN
TRANSPARENT FILMS
B. Tieke, Ch. Bubeck, G. Lieser
To cite this version:
B. Tieke, Ch. Bubeck, G. Lieser. DOPING OF POLYPARAPHENYLENE - A SPECTROSCOPIC STUDY OF THIN TRANSPARENT FILMS. Journal de Physique Colloques, 1983, 44 (C3), pp.C3- 753-C3-756. �10.1051/jphyscol:19833149�. �jpa-00222663�
JOURNAL DE PHYSIQUE
Colloque C3, supplement au n°6, Tome 44, juin 1983 page C3-753
DOPING OF POLYPARAPHENYLENE - A SPECTROSCOPIC STUDY OF THIN TRANSPARENT FILMS
B. Tieke , Ch. Bubeck and G. L i e s e r
Institut fur Makromolekulare Chemie der Universitat Freiburg, Hermann Staud-ingev Haus, Stefan Meier Str. SI, D-7800 Freiburg, F.R.G.
Résumé - Les dopages de types n et p du poly-1,4-phênylëne ont fait l'objet d'une étude approfondie. Le comportement de ce polymère dopé, en présence de l'humidité de l'air ambiant, a également été examine. Ces études, effectuées par spectroscopie d'absorption, ont nécessité la mise au point préalable d'une technique spéciale de préparation de films minces de polymère.
Abstract - p- and n-type doping of poly-l,4-phenylene as well as the reaction of the doped polymer with moisture were studied by absorption spectroscopy, using thin transparent films of the polymer.
I - INTRODUCTION
Poly-para-phenylene (PPP) recently gained new interest, when it was observed that it becomes highly conductive, if doped by electron donors or acceptors /l/. However, the morphology of PPP is badly defined. Usually it is obtained as a semicrystalline powder, which can be increased in its crystallinity only slightly by annealing / 2 / . Recently we reported the formation of thin films of PPP /3/. Their transparency allows a detailed spectroscopic study of the polymer and its reactions upon doping.
In the following the film formation and a transmission spectroscopic study of the p- and n-type doping and the reaction of the doped polymer with moisture are briefly reported.
II - EXPERIMENTAL PART
PPP was prepared according to the method of Kovacic / 4 / . 9.4 g of dry benzene, 4 g AlCl and 2 g CuCl were filled into a cuvette-type apparatus, which was equipped with a specially shaped stirrer /3/. The reaction mixture was stirred (2,500 r.p.ra.) at a temperature of 31-32 C for two hours. Then the stirrer was removed and the cuvette washed several times, in order to remove the excess of the catalyst. Finally, a dark red film of PPP was obtained, attached to the inner wall of the reaction ves- sel. This film is very sensitive to moisture, rapidly turning to a pale-brown colour.
l.PPP-filmon 2. coating with a 3. evaporation of benzene 9 ' " " solution of poly- 4. PPP-film attached to sfyrene in ben- polystyrene support zene{10g/U
— — _ ppp _ ^-polystyrene- ^ /
^ ^ ^ - g l a s s - ^ ^ ^ ^ ^
Fig. 1 - Schematic representation of the removal of a PPP-film from the quartz or glass support onto a polymeric support.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19833149
C3-754 JOURNAL DE PHYSIQUE
F r e e s t a n d i n g polymer f i l m s of PPP cannot b e o b t a i n e d by t h i s method, because t h e f i l m s break a p a r t , a s soon a s they a r e removed from t h e r e a c t i o n v e s s e l . Thus t h e s p e c t r o s c o p i c s t u d i e s d e s c r i b e d below were c a r r i e d o u t u s i n g PPP f i l m s s t i l l a t t a c h e d t o t h e s u r f a c e of t h e q u a r t z v e s s e l .
However, t h e f i l m s can be handled e a s i l y , i f t h e y a r e t r a n s f e r r e d o n t o polymeric s u p p o r t s a s d e s c r i b e d i n Fig. 1.
I11 - RESULTS
A s demonstrated by e l e c t r o n microscopy and d i f f r a c t i o n / 3 / , t h e f i l m s c o n s i s t o f small c r y s t a l l i t e s of d i a m e t e r s below 100 nm, which a r e weakly o r i e n t e d by t h e rapid- l y t u r n i n g s t i r r e r . The f i l m t h i c k n e s s i s about 100 nm on average. E l e c t r o n d i f f r a c - t i o n p a t t e r n s i n d i c a t e a s t r u c t u r e o f the polymer i d e n t i c a l w i t h t h a t d e s c r i b e d f o r t h e b u l k m a t e r i a l / 5 / .
Fig. 2 shows t h e r e s u l t s of a t r a n s m i s s i o n s p e c t r o s c o p i c s t u d y of t h e chemical r e - a c t i o n s o c c u r i n g upon t h e doping procedures and t h e exposure of t h e doped polymer t o moisture. Fig. 2A shows t h a t t h e f r e s h l y formed PPP 1s r e d i n t r a n s m i t t e d l i g h t , and changes i t s c o l o u r t o pale-brown, i f exposed t o m o ' s t u r e , a s i n d i c a t e d by t h e s h i f t
-t
of t h e a b s ~ r p t i o n maxima from 20,000 t o 26,400 cm , i n c l u d i n g an i s o s b e s t i c p o i n t a t
24,000 cm .
The pale-brown PPP i s h i g h l y s t a b l e t o a i r . However, i t s c o l o u r changes immediately.
i f t h e f i l m s a r e exposed t o s t r o n g e l e c t r o n a c c e p t o r s ("p-type-doping") , o r donors ("n-type d o p i n g " ) . p-type dopants a s AsF o r SbF i n d r y p e n t a n e , o r AlCl i n dry nltromethane cause t h e formation of r e d ? i l m s ( s i g . 2B), whereas n-type $opants a s potassium n a p h t h a l i d e i n dry THF l e a d t o t h e formation of d a r k green f i l m s , appearing m e t a l l i c golden i n r e f l e c t i o n ( F i g . 2D, spectrum a ) .
These c o l o u r changes a r e completely r e v e r s l b e l , a s soon a s t h e doped f i l m s a r e ex- posed t o m o i s t u r e ( F i g s . 2C and D l . I t i s obvious from t h e s p e c t r a t h a t t h e n-type doped f i l m s a r e much f a s t e r r e a c t i n g with moisture t h a n t h e p-type d o p e d f i l m s a r e . A 1 1 c o l o u r changes i n v o l v e an i s o s b e s t i c p o i n t a t 24,000 c m .
I V - DISCUSSION
Colour changes s i m i l a r t o t h o s e of t h e PPP f i l m s (Fig. 2) a r e observed, f o r example, i f oligophenylenes a r e a n o d i c a l l y oxydized / 6 / , o r s o l u b l e polyphenylenes a r e t r e a t e d with donor o r a c c e p t o r a g e n t s i n s o l u t i o n / 7 / . They can e a s i l y be e x p l a i n e d by redox r e a c t i o n s of t h e polymer, e s p e c i a l l y because t h e A1C13 - nitromethane - dopant is
WAVELENGTH (nm) WAVELENGTH (nm)
700 600 500 400 300 700 600 500
0.8
E~
W y 0.6 Q m
8 0.4
I/) m a
Q
0'2 15 20 25 30 0.2
15 20 25 30
WAVENUMBER ( I O ~ C ~ - ~ ) WAVENUMBER (lo3 ern-')
F i g . 2 - Absorption s p e c t r a of poly-l,4-phenylene f i l m s ; A) a s polymerized ( a ) , a f t e r exposure t o moisture f o r 8 s e c ( b ) , 15 s e c ( c ) , and 60 s e c ( d ) ; B) undoped i n p u r e pentane ( a ) , exposed t o a s o l u t i o n of 0.01 w t . - % SbF5 i n pentane ( b ) , exposed t o 0.03 w t . - % SbF i n pentane ( c ) 5 .
WAVELENGTH (nml
700 600 500 LOO 300 1.2
15 20 25 30
WAVENUMBER ( l o 3 cm-'1
WAVELENGTH (nm) 700600 500 LOO 300
WAVENUMBER I l ~ ~ c r n - ' )
F i g . 2 ( c o n t i n u e d ) - Reactions of doped PPP with moisture; C) PPP doped with SbF and exposed t o moisture f o r 5 s e c ( a ) , 15 s e c ( b ) , and 60 s e c ( c ) ; D) PPP doped w i t 2 a 0.1 M s o l u t i o n of potassium n a p h t h a l i d e i n THF ( a ) , a f t e r a d d i t i o n a l exposure t o moisture f o r 3 s e c ( b ) , 5 s e c ( c ) , and 10 s e c ( d ) .
known t o b e an e x c e l l e n t a g e n t f o r t h e formation of r a d i c a l c a t i o n s of a r o m a t i c hydro- carbons / 8 / . I n t h e s e redox r e a c t i o n s p o s i t i v e l y o r n e g a t i v e l y charged polymer c h a i n s , pppn+ o r pppn-, r e s p e c t i v e l y , a r e c r e a t e d , where t h e number of charges n of t h e poly- mer chain i s unknown. However, v a l u e s of n > 1 a r e very l i k e l y , s i n c e t h e o x i d a t i o n o r r e d u c t i o n of oligophenylenes t o h i g h e r charged s t a t e s ( n > 1 ) becomes p r o g r e s s i v e - l y e a s i e r , i f t h e number of phenylene u n i t s i n c r e a s e s /7/.
Conclusively, t h e a b i l i t i e s of v a r i o u s e l e c t r o n donors o r a c c e p t o r s t o a c t a s dopants of PPP must b e c o n t r o l l e d by t h e i r o x i d a t i o n and r e d u c t i o n p o t e n t i a l s . This h a s r e - c e n t l y been d e s c r i b e d by t h e f o l l o w i n g experiment /3/. PPP f i l m s were exposed t o v a r i o u s e l e c t r o n donors, d i f f e r i n g i n t h e i r r e d u c t i o n p o t e n t i a l s Er d . While donors w i t h E -< -2 - 6 0 V ( v s . s t a n d a r d calomel e l e c t r o d e , SCE) a r e a b l e Eo dope PPP, e . g . naphthgygne r a d l c a l a n i o n s (-2.60 V) / 9 / , o r chrysene d i a n i o n s (-2.73 V) / l o / , no do- p i n g was observed with donors having r e d u c t i o n p o t e n t i a l s E > -2 -23 V , e . g . r a d i c a l anions of chrysene (-2 - 2 3 V) , pyrene (-2.10 V) , f luoranthenged(-1 - 7 7 V ) , o r p e r y l e n e
(-1.67 V) /9,10/. This allowed t o e s t i m a t e t h e r e d u c t i o n p o t e n t i a l of PPP t o be
-2.23V Ered(PPP) -2.60 V ( V S . SCE) .
Describing t h e doping r e a c t i o n s i n t h e form of redox e q u a t i o n s , t h e f i r s t o x i d a t i o n and r e d u c t i o n s t e p s can b e formulated a s f o l l o w s :
SbF5 - a d d i t i o n : 3 SbF5 + 2 PPP ---) 2 PPP'. + 2 SbF - + SbF3, 6
potassium n a p h t h a l i d e (Kfnp-) a d d i t i o n :
I n c a s e of doping with A1C13 i n nitromethane, NO + - i o n s a r e t h e a c t u a l dopant, which a r e formed i n a s i d e r e a c t i o n of A l C l w i t 2 t h e s o l v e n t / 8 / :
3
A s F i g s . 2A and B i n d i c a t e , t h e p r i s t i n e and t h e doped PPP a r e s p e c t r o s c o p i c a l l y i d e n t i c a l . Thus t h e p r i s t i n e PPP i s a l r e a d y p r e s e n t i n a doped s t a t e , which may o r i g i n a t e from t h e o x i d a n t - c a t a l y s t system CuCl /A1C13, p r e s e n t i n t h e r e a c t i o n mixture d u r i n g t h e polymerization p r o c e s s /11/: 2
CuC12 + A 1 C 1 3 + PPP 3 PPP+- + ~ 1 ~ 1 ~ - + CuC1.
The r e a c t i o n s of t h e doped PPP w i t h m o i s t u r e , a s shown i n F i g s . 2C and D , can be d e s c r i b e d a s an o x i d a t i o n o r r e d u c t i o n of water according t o t h e redox e q u a t i o n s
JOURNAL DE PHYSIQUE
4 p p p f . + 2 H 2 0 --j 4 P P P + 0 2 + 4 H + , 2 PPP-' + 2 H 2 0 4 2 P P P + H2 + 2 OH-
V - CONCLUSIONS
T h e p r e p a r a t i o n of t h i n t r a n s p a r e n t f i l m s of P P P o f f e r s the p o s s i b i l i t y t o s t u d y t h e p h y s i c a l b a c k g r o u n d of t h e d o p i n g processes of P P P m o r e i n d e t a i l , e s p e c i a l l y s i n c e t h e f i l m s c a n e a s i l y be h a n d l e d a f t e r a t r a n s f e r o n t o p o l y m e r i c s u p p o r t s . T h e spec- troscopic r e s u l t s c l e a r l y s h o w t h a t t h e p r i s t i n e P P P , i f prepared by t h e m e t h o d of K o v a c i c , i s already p r e s e n t i n a doped s t a t e . T h i s d o p a n t i s r e m o v e d as s o o n a s t h e p r i s t i n e p o l y m e r i s t r e a t e d w i t h w a t e r . S i n c e t h e d o p a n t i s u s u a , l l y i n t e r c a l a t e d b e t w e e n the p o l y m e r c h a i n s / 1 2 , 1 3 / , t h e r e m o v a l of t h e d o p a n t w i l l very l i k e l y cause
a disorder f o r m a t i o n i n t h e p o l y m e r a n d h e n c e m a y e x p l a i n t h e l o w c r y s t a l l i n i t y of t h e u n d o p e d P P P / 2 / .
A s t h e o c c u r a n c e of t h e i s o s b e s t i c p o i n t I n a l l d o p i n g e x p e r i m e n t s i n d i c a t e s , t h e d o p i n g c a n be best e x p l a i n e d by a t w o - p h a s e m o d e l , o n e phase c o n s i s t i n g of t h e u n - doped a n d t h e s e c o n d of t h e doped m a t e r i a l . T h i s w o u l d be c o n s i s t e n t w i t h a perco- l a t i o n m o d e l as w e l l as a m o d e l of i n h o m o g e n e o u s doping.
V I - ACKNOWLEDGEMENT
T h e a u t h o r s t h a n k P r o f . G. W e g n e r f o r h e l p f u l d i s c u s s i o n s . F i n a n c i a l s u p p o r t by t h e S t i f t u n g V o l k s w a g e n w e r k a n d by C i b a - G e i g y AG, B a s e l , are g r a t e f u l l y a c k n o w l e d g e d .
V I I - REFERENCES
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/ 3 / EEKE B . , BUBECK C . , L I E S E R G . , M a k r o m o l . C h e m . R a p i d Comm. 3 - ( 1 9 8 2 ) 2 6 1 / 4 / KOVACIC P . , OZIOMEK J . , ~ a c r o m o l e c u l a r S y n t h . 2 ( 1 9 6 6 ) 2 3
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/9/ H O I J T I N K G . J . , DE BOER E . , VON DER M E I J P . H . , WEFLAND W.P., R e c . T r a v . C h i m . P a y s - B a s ( 1 9 5 6 ) 4 8 7
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