Electrochemical Synthesis and Characterization of New

Chapter

47

Electrochemical Synthesis and Characterization of New Polyheterocycles M.

Aldissi 1

Downloaded by FUDAN UNIV on February 13, 2017 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch047

2

1

and A. M. Nyitray

2

Los Alamos National Laboratory, Los Alamos, N M 87545

Department of Chemistry, Colorado State University, Fort Collins, CO 80523

The d i r e c t synthesis by anodic o x i d a t i o n of a new s e r i e s of electrically conducting polymers i s described. Our p o l y mers derive from sulfur and/or nitrogen containing heterocycles such as: 2-(2-thienyl)pyrrole, thiazole, indole, and phthalazine. The anodic oxidation of these monomers i s c a r r i e d out i n a c e t o n i t r i l e s o l u t i o n s containing tetrabutylammonium s a l t s (TBA X ) w i t h X = BF4 , ASF6 , and the tetraethylammonium salt, TEA H C H -SO . Characteriza t i o n of the materials by electrical conductivity, electron s p i n resonance, u v - v i s i b l e spectroscopy, and c y c l i c voltammetry i s discussed. +

-

-

-

-

+

3

6

4

3

New c o n d u c t i n g polymers w i t h conjugated double bonds, l i n e a r or c y c l i c , have a t t r a c t e d a wide i n t e r e s t due t o the v a r i o u s i n t e r e s t i n g p h y s i c a l phenomena t h a t they e x h i b i t and t o the imminent p o s s i b i l i t y o f t h e i r use i n t e c h n o l o g i c a l a p p l i c a t i o n s . The e l e c t r o c h e m i c a l s y n t h e s i s o f c o n d u c t i n g p o l y a r o m a t i c s proved t o be a c o n v e n i e n t and e f f i c i e n t method w h i c h r e s u l t s u s u a l l y i n polymers i n t h e i r c o n d u c t i n g form as f i l m s o r powders on the s u r f a c e o f the anode. T y p i c a l examples w h i c h have been w i d e l y s t u d i e d are p o l y ( p y r r o l e ) (1) and p o l y ( t h i o p h e n e ) ( 2 - 4 ) . The o x i d a t i o n of o t h e r a r o m a t i c h e t e r o c y c l e s f o l l o w e d , a d d i n g a l a r g e number o f m a t e r i a l s to the l o n g l i s t o f c o n d u c t i n g p o l y m e r s . Examples o f such com pounds are d e r i v a t i v e s o f p o l y ( p y r r o l e ) and p o l y ( t h i o p h e n e ) ( 5 - 1 0 ) , p o l y ( p y r i d a z i n e ) (11) , and the b i c y c l i c p o l y m e r , p o l y ( t h i e n o [ 3 , 2 b]pyrrole) (12). I t has been found, however, t h a t t h e s e polymers are m o s t l y amorphous or have d i s o r d e r e d s t r u c t u r e s ( 2 , 1 3 ) . C NMR (14) and XPS (15) s t u d i e s have shown t h a t p o l y p y r r o l e c o n s i s t s o f a , a * - b o n d i n g and α , β - b o n d i n g o f the p y r r o l e r i n g , w h i c h have been suggested to be the o r i g i n o f the d i s o r d e r e d s t r u c t u r e s i n the e l e c t r o c h e m i c a l l y prepared polymers.

0097-6156/87/0346-0559$06.00/0 © 1987 A m e r i c a n C h e m i c a l Society

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

POLYMERS FOR HIGH T E C H N O L O G Y

560

We have focused i n o u r study on newly s y n t h e s i z e d p o l y h e t e r o c y c l e s i n which the r e p e a t i n g u n i t ( F i g u r e 1) c o n s i s t s o f t h i a z o l e ( 1 ) , i n d o l e ( 2 ) , p h t h a l a z i n e ( 3 ) , and t h i e n y l p y r r o l e (4) f o r t h e f o l l o w i n g reasons: 1) The decrease i n d i s o r d e r by r e d u c i n g the number o f c o u p l i n g s i t e s (except i n compound 4 ) : α,a c o u p l i n g r e l a t i v e t o t h e s u l f u r atom i n t h i a z o l e i s the o n l y type o f c o u p l i n g a l l o w e d ; i n p h t h a l a z i n e , c o u p l i n g o c c u r s on the α p o s i t i o n s r e l a t i v e t o b o t h n i t r o g e n atoms, s i m i l a r l y t o p y r i d a z i n e ( 1 1 ) ; and i n i n d o l e , the most l i k e l y c o u p l i n g t o o c c u r i s α, 3 (on the p y r r o l e and f u s e d p h e n y l r i n g s r e s p e c t i v e l y ) . I t s h o u l d be noted t h a t p o l y ( i n d o l e ) has been p r e v i o u s l y prepared ( 1 6 ) . A l s o , a d e c r e a s e i n d i s o r d e r might u l t i m a t e l y l e a d t o d e c r e a s e i n the e x t e n t o f the amorphous c h a r a c ter. For example, when t h i a z o l e i s s y n t h e s i z e d by p o l y c o n d e n s a t i o n r e a c t i o n s the r e s u l t i n g m a t e r i a l i s h i g h l y c r y s t a l l i n e w i t h d e s i r a b l e m e c h a n i c a l p r o p e r t i e s . T h e r e f o r e , the e l e c t r o c h e m i c a l t e c h nique could probably lead t o a m a t e r i a l w i t h a c r y s t a l l i n e charac ter. 2) Compound 4 c o u l d , i n p r i n c i p l e , be p o l y m e r i z e d t o y i e l d p o l y ( t h i e n y l p y r r o l e ) c o n s i s t i n g o f a l t e r n a t i n g p y r r o l e and thiophene u n i t s . Such a polymer i s analogous t o c o n j u g a t e d d i a t o m i c polymers w h i c h have been p r e d i c t e d (17) t o g i v e r i s e t o e x c i t a t i o n s c o n s i s t i n g o f s o l i t o n p a i r s h a v i n g e i t h e r s p i n 0 o r s p i n 1/2. We d e s c r i b e i n t h i s paper the s y n t h e s i s o f the v a r i o u s m a t e r i a l s and some o f t h e i r p r o p e r t i e s w i t h an emphasis on p o l y ( t h i e n y l p y r r o l e ) . A d e t a i l e d study o f the v a r i o u s m a t e r i a l s w i l l be p u b l i s h e d i n a f u t u r e paper.


1

EXPERIMENTAL SECTION S y n t h e s i s : Monomers(1-3), s o l v e n t s , and e l e c t r o l y t e s , s u p p l i e d by A l d r i c h , were p u r i f i e d b e f o r e use by d i s t i l l a t i o n o r r e c r y s t a l l i z a t i o n . Monomer 4 was s y n t h e s i z e d by r e a c t i n g m e t h y l a z i d o a c e t a t e w i t h 8 - t h i e n y l a c r o l e i n , f o l l o w e d by c y c l i z a t i o n then d e c a r b o x y l a t i o n o f the o b t a i n e d d e r i v a t i v e ( 1 8 ) , w h i c h i s a g e n e r a l method employed f o r t h e s y n t h e s i s o f h e t e r o a r y l p y r r o l e s ( 1 9 ) . The chemi c a l s t r u c t u r e o f the f o u r monomers i s shown i n F i g u r e 1. Our polymers were s y n t h e s i z e d i n a t h r e e compartment e l e c t r o c h e m i c a l c e l l w h i c h c o n t a i n s In-Sn o x i d e c o n d u c t i n g g l a s s (ITO) o r a p l a t i num f o i l as the w o r k i n g e l e c t r o d e , a p l a t i n u m c o u n t e r e l e c t r o d e ( w i r e , f o i l , o r mesh), and SCE o r an Ag/AgNO^ r e f e r e n c e e l e c t r o d e . The s o l u t i o n s from w h i c h the polymers were prepared c o n s i s t e d o f a c e t o n i t r i l e t o w h i c h were added the s u p p o r t i n g e l e c t r o l y t e and t h e monomer t o y i e l d a c o n c e n t r a t i o n o f 0.1 mole/1 o f b o t h . The polymers were grown on the p l a t i n u m o r the c o n d u c t i n g g l a s s e l e c t r o d e by a p p l y i n g a c o n s t a n t a n o d i c c u r r e n t f o r p e r i o d s o f time e x t e n d i n g from a few minutes t o s e v e r a l hours depending upon the_ n a t u r e o f the monomer and the e l e c t r o l y t e . TBA A S F ^ _ and __TEA +

H

C

C

H

S 0

w

e

r

e

U

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e

d

f

o

r

m

o

n

o

m

e

r

s

3 ~ 6 4~ 3~ and TBA BF^~ was u s e d f o r monomer 4. Ttje c u r r e n t d e n s i t i e s were v a r i e d between 0.1 mA/cm and 0.5 mA/cm . The a n o d i c p o t e n t i a l s d u r i n g the s y n t h e s i s were i n the range 0.25 V - 0.5 V v s . Ag/AgNO^ r e f e r e n c e e l e c t r o d e . The r e a c t i o n s were c a r r i e d out under argon a t room temperature. The 2


47.

ALDISSI AND NYITRAY

Electrochemical

Synthesis and

Characterization

s y n t h e s i z e d m a t e r i a l s were r i n s e d w i t h a c e t o n i t r i l e to e l i m i n a t e the s u p p o r t i n g e l e c t r o l y t e from the i n t e r s t i c e s o f the f i l m s o r powders, then d r i e d under vacuum. Methods o f C h a r a c t e r i z a t i o n : The polymers were c h a r a c t e r i z e d by f o u r - p r o b e e l e c t r i c a l c o n d u c t i v i t y measurements between room temperature and l i q u i d n i t r o g e n , e l e c t r o n s p i n resonance ( V a r i a n Ε-line s e r i e s ) , s c a n n i n g e l e c t r o n m i c r o s c o p y ( H i t a c h i 520), c y c l i c voltammetry ( P r i n c e t o n A p p l i e d Research I n s t r u m e n t s ) , and u v - v i s i b l e s p e c t r o s c o p y ( P e r k i n Elmer 330).


RESULTS AND

DISCUSSION

The v a r i o u s monomers underwent o x i d a t i o n a t the anode, y i e l d i n g c o n d u c t i n g f i l m s or powders w i t h c o l o r s t h a t depend on the monomer and w h i c h r e s u l t from plasma r e f l e c t i o n . I n the o x i d i z e d form, the c o l o r s observed a r e dark b l u e and dark r e d , f o r p o l y ( t h i e n y l p y r r o l e ) and p o l y ( t h i a z o l e ) r e s p e c t i v e l y , and dark brown f o r p o l y ( p h t h a l a z i n e ) and p o l y ( i n d o l e ) . In the reduced form, w h i c h i s a c c o m p l i s h e d by the r e v e r s e c a t h o d i c d i s c h a r g e , the c o l o r s are b r o w n - y e l l o w t o y e l l o w and r e s u l t from the i n t r i n s i c i n t e r b a n d a b s o r p t i o n . The e x t e n t of r e v e r s i b i l i t y o f the o x i d a t i o n / r e d u c t i o n p r o c e s s e s depends upon the n a t u r e o f the monomer o r r e p e a t i n g u n i t i n the c h a i n f o r m a t e r i a l s w i t h comparable t h i c k n e s s e s . I n gener a l , an i n c o m p l e t e r e v e r s i b i l i t y i n d i c a t e s t h a t d u r i n g the p o l y m e r i z a t i o n s i d e r e a c t i o n s might have o c c u r r e d . For example, c r o s s l i n k i n g c o u l d take p l a c e p a r t i c u l a r l y i n the case of ct-3 c o u p l i n g . A l s o , the f o r m a t i o n o f a s l i g h t l y c o l o r e d s o l u t i o n d u r i n g the a n o d i c o x i d a t i o n a c c o u n t s f o r the i n c o m p l e t e r e v e r s i b i l i t y . Such an o b s e r v a t i o n i s supported by the f a c t t h a t the coulombic e f f i c i e n c y of the v a r i o u s c h a r g e / d i s c h a r g e c y c l e s , f o r compounds 1-3, i s lower than what i s u s u a l l y observed f o r p o l y h e t e r o c y c l e s , such as p o l y ( p y r r o l e ) and p o l y ( t h i o p h e n e ) , except i n the case of few hundreds Â-thick f i l m s o f p o l y ( t h i e n y l p y r r o l e ) f o r w h i c h the coulombic e f f i c i e n c y i s comparable t o t h a t of the two l a t t e r compounds. By t a k i n g t h i s assumption i n t o a c c o u n t , an e s t i m a t e d c o m p o s i t i o n of the v a r i o u s doped m a t e r i a l s c o n s i s t s of 1 a n i o n f o r every 3-4 r e p e a t i n g u n i t s . However, when the p o l y m e r i z a t i o n o f t h i e n y l p y r r o l e i s a l l o w e d t o proceed f o r l o n g e r times a d a r k polymer s o l u t i o n i s o b t a i n e d s i m u l t a n e o u s l y w i t h an i n c r e a s e i n the t h i c k n e s s of the f i l m a t the anode. The c o l o r of the s o l u t i o n or the f i l m depends upon the e l e c t r o l y t e used. As f o r most e l e c t r o c h e m i c a l l y formed p o l y h e t e r o c y c l e s , the morphology of the r e s u l t i n g m a t e r i a l s e x h i b i t s , i n the case o f p o l y ( t h i e n y l p y r r o l e ) i n i t s o x i d i z e d form ( F i g u r e 2 ) , a f i b r i l l a r s t r u c t u r e . The morphology of the o t h e r m a t e r i a l s w i l l be d e t e r m i n e d as w e l l , i n c l u d i n g the e f f e c t of the n a t u r e of the e l e c t r o l y t e , s p e c i f i c a l l y , the i n s e r t e d i o n on i t . The n a t u r e of the i n s e r t e d i o n a f f e c t s the m e c h a n i c a l p r o p e r t i e s and i s a l s o known t o a f f e c t e l e c t r i c a l c o n d u c t i v i t y of the o x i d i z e d p o l y m e r . For example, when p o l y ( t h i e n y l p y r r o l e ) i s s y n t h e s i z e d u s i n g TEA CH^-C^H^-SO " as e l e c t r o l y t e , the m a t e r i a l i s more f l e x i b l e than when s y n t h e s i z e d u s i n g TBA BF, , which i s i n agreement w i t h what was observed i n the case of p o l y t p y r r o l e ) ( 2 0 ) . The c y c l i c voltammograms shown i n F i g u r e 3 were o b t a i n e d f o r +




562

Fig.

2

Scanning electron micrograph poly(thienylpyrrole) film.

of

the

surface

of


a

47.

ALDISSI A N D NYITRAY

Electrochemical

Synthesis and Characterization

563


poly (thienylpyrrole) f o r m e d o n a p l a t i n u m e l e c t r o d e u s i n g T B A ' BF^, i n a c e t o n i t r i l e ( 0 . 1 m o l e / 1 ) a n d a n SCE a s a r e f e r e n c e electrode. The p r o p o r t i o n a l i t y o f t h e p e a k c u r r e n t f o r two s w e e p r a t e s , 50 m V / s e c a n d 100 m V / s e c , i s i n d i c a t i v e , a s f o r a l l c o n j u g a t e d p o l y h e t e r o c y c l e s , of a charge i n j e c t i o n r e a c t i o n confined to the volume of the f i l m and governed m o s t l y by d i f f u s i o n . In the c o n d i t i o n s mentioned a b o v e , the o x i d a t i o n and r e d u c t i o n peaks are o b s e r v e d a t a p p r o x i m a t e l y 0 . 5 V and 0 . 2 5 V v s . SCE. I t i s i m p o r t a n t to note t h a t the o x i d a t i o n peak l i e s between those measured f o r p o l y ( p y r r o l e ) ( 2 1 ) ( - 0 . 1 V) a n d p o l y ( t h i o p h e n e ) (22) ( 1 . 1 V ) . The v i s i b l e a b s o r p t i o n s p e c t r u m o f a p o l y ( t h i e n y l p y r r o l e ) film w h i c h i s a p p r o x i m a t e l y 1000 Â t h i c k o n c o n d u c t i n g g l a s s ( F i g u r e 4) i s c h a r a c t e r i z e d b y a n i n t e n s e b a n d w i t h a maximum a t 2 . 8 e V . S i m i l a r a b s o r p t i o n c h a r a c t e r i s t i c s are observed f o r the polymer solution. T h i s v a l u e , w h i c h i s t h e b a n d gap o f t h e c o n j u g a t e d p o l y m e r , l i e s b e t w e e n t h e p u b l i s h e d v a l u e s (23) f o r p o l y ( p y r r o l e ) ( 2 . 9 eV) a n d f o r p o l y ( t h i o p h e n e ) (2.6 eV). The a b s o r p t i o n m a x i m a are i n the range which i s expected f o r conjugated polyheterocycles (24). The i n t e r m e d i a t e maximum f o r p o l y ( t h i e n y l p y r r o l e ) suggests t h a t t h e a b s o r p t i o n i s due t o t h e t r a n s i t i o n π-π* o f t h e c o n j u g a t e d system which c o n s i s t s of the t h i e n y l p y r r o l e r e p e a t i n g u n i t r a t h e r t h a n a t h i o p h e n e u n i t and a p y r r o l e u n i t . Such an a s s u m p t i o n i s s u p p o r t e d by the f a c t the the o x i d a t i o n / r e d u c t i o n p r o c e s s e s of the p o l y m e r o c c u r a t d i f f e r e n t p o t e n t i a l s t h a n f o r t h e two p o l y m e r s taken separately. T h i s a l s o i n d i c a t e s t h a t an a l t e r n a t e d s t r u c t u r e m i g h t have been formed. A d e t a i l e d s t u d y w i l l be c a r r i e d o u t t o determine the e x a c t s t r u c t u r e . H o w e v e r , i t s h o u l d be m e n t i o n e d t h a t o n one h a n d , t h e o r e t i c a l c a l c u l a t i o n s i n c o n j u n c t i o n w i t h d i p o l e moment d a t a ( 2 5 ) s h o w e d t h a t a p l a n a r c i s c o n f o r m a t i o n o f t h e t h i e n y l p y r r o l e monomer i s o b t a i n e d . On t h e o t h e r h a n d , t h e p r o p o s e d s t r u c t u r e f o r i . e . , p o l y ( p y r r o l e ) (26) c o n s i s t s o f a t r a n s conformation for the p y r r o l e u n i t s . T h i s leads to the p o s s i b l e p l a n a r s t r u c t u r e f o r t h e t h i e n y l p y r r o l e p o l y m e r shown i n F i g u r e 5 . The w i d t h o f t h e a b s o r p t i o n b a n d seems t o b e c o m p a r a b l e t o t h o s e o f poly(thiophene) and p o l y ( p y r r o l e ) w h i c h s u g g e s t s a comparable d i s t r i b u t i o n f o r the l e n g t h of the conjugated sequences i n the three materials. The u v - v i s i b l e a b s o r p t i o n o f t h e o x i d i z e d p o l y ( t h i e n y l p y r r o l e ) w i l l be m e a s u r e d t o d e t e c t t h e p o s s i b l e e l e c t r o n i c states i n t h e g a p a n d w h e t h e r t h e y c o r r e s p o n d t o w h a t was s u g g e s t e d f o r a n AB p o l y m e r . E l e c t r o n s p i n resonance measurements poly(thienylpyrrole) showed an a s y m m e t r i c conduction e l e c t r o n s , w i t h a g value near i n d i c a t i n g that the s p i n resonance i s not nitrogen moieties.

a t room t e m p e r a t u r e on l i n e c h a r a c t e r i s t i c of that of the free e l e c t r o n due t o t h e s u l f u r a n d / o r

The t e m p e r a t u r e d e p e n d e n c e o f c o n d u c t i v i t y o f poly(thienylpyr r o l e ) i s shown i n F i g u r e 6. The n o n - l i n e a r c h a r a c t e r o f t h e c o n d u c t i v i t y throughout the whole range of temperatures s t u d i e d here (room temperature to l i q u i d n i t r o g e n ) i n d i c a t e s t h a t the c o n d u c t i o n m e c h a n i s m c o u l d be d i f f e r e n t i n d i f f e r e n t temperature



564

|100mV/sec


50mV/sec

-0.50

0

0.50

E(volts) vs. SCE Fig.

3

C y c l i c voltammograms o f p o l y ( t h i e n y l p y r r o l e ) TBA BF."/acetonitrile.

i n 0.1M

ENERGY ( e V ) Fig.

4

O p t i c a l a b s o r p t i o n s p e c t r a o f the bleached form of thiophene, p y r r o l e , and t h i e n y l p y r r o l e polymers.




Fig.

5

Electrochemical

P o s s i b l e conformation of

io i 1

Fig.

6

Synthesis and

1

1

Characterization

poly(thienylpyrrole).

Γ

Temperature dependence poly(thienylpyrrole).

of

c o n d u c t i v i t y of

as-grown


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POLYMERS FOR HIGH TECHNOLOGY

ranges, i . e . , the relative importance of the intra-chain and inter chain conduction processes is different^a| different temperatures. When the conductivity is plotted vs. T~ or Τ , a straight line is obtained indicating the possibility of two or three dimen sional variable range hopping conduction as predicted by MotÇ (27). Although, the conductivity of poly(thiazole) is low, 1 X 10 S/cm, the values measured along and perpendicular to the directions of the surface indicate that a higher anisotropy than in most polyheterocycles is obtained. Such a property will be further detailed in a future publication.


CONCLUSIONS We have shown for the first time that heterocycles such as thiazole and phthalazine, undergo electrochemical polymerization providing a new and perhaps the only synthetic route of their corresponding polymers. This work could lead to the synthesis of conducting polymers that are obtained conventionally as crystalline or perhaps liquid crystalline polymers. Such a possibility is currently under investigation. The polymerization product of thienylpyrrole may provide a material for testing predictions of particle excitations in conjugated AB polymers: solitons in the bleached material, and polarons and/or bipolarons in the oxidized material. This synthesis route will be extended to 2-heteroarylpyrroles which are precursors for the synthesis of chemotherapeutic compounds (28), such as 2-(2-furyl)pyrrole. Some of the materials synthesized in this work, e.g., poly(thienylpyrrole), could be good candidates for electro-optical switching elements with reasonable switching times, due to the fast oxidation/reduction cycles accompanied by a color change when thin films (up to 1000A thick) ar used. ACKNOWLEDGMENTS The authors would like to thank Dr. S. Gottesfeld for helpful discussions, and Dr. D. Wrobleski for synthesizing the thienylpyrrole monomer. This work is supported by the Office of Basic Energy Sciences (OBES/DOE) and by the Centers for Materials Science and for Non-Linear Studies of Los Alamos National Laboratory. LITERATURE CITED 1. 2. 3. 4. 5. 6.

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Synthesis and Characterization

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RECEIVED April 8, 1987


Electrochemical Synthesis and Characterization of New

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