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Electropolymerization of methoxyaniline: experimental results and frontier orbital interpretation
S y n t h e t i c Metals, 44 (1991) 117-132
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Electropolymerization of methoxyaniline: experimental results and frontier orbital interpretation J e a n C h r i s t o p h e Lacroix, P e d r o Garcia, J e a n Paul Audi6re, Ren6 C l d m e n t and Olivier Kahn Labo'ratoire de C h i m i e I n o r g a n i q u e , URA 420, Universitd d e P a r i s - S u d , P1405 Orsay (France)
(Received November 15, 1990; accepted March 8, 1991)
Abstract This report describes the electropolymerization reactions of ortho- and meta-methoxyanilines on a gold electrode and on a gold/polyaniline electrode. It is shown that the outcome of the electrooxidation process ks sensitive to the methoxy group position but not to the nature of the electrode which, nevertheless, exerts a catalytic effect. These effects are discussed in terms of frontier orbital interactions. This approach suggests that film growth involves the coupling of a nonoxidized monomeric unit via one of its carbon atoms onto a deprotonated NH2 group of the oxidized polymeric chain.
Introduction S i n c e t h e first r e p o r t o n t h e a v a i l a b i l i t y o f p o l y p y r r o l e a n d p o l y a n i l i n e as films, a s t r o n g i n t e r e s t h a s b e e n m a i n t a i n e d in t h e u s e of t h e s e m a t e r i a l s as e l e c t r o d e s in 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 s u c h as d i s p l a y s , s e n s o r s or b a t t e r i e s [ll. A new area yet to be developed and possibly leading to important 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 is t h e d e s i g n o f e l e c t r o a c t i v e c o n d u c t i v e films incorporating other active centers which could be addressed through the c o n d u c t i v e b a c k b o n e . S u c h s y s t e m s p o t e n t i a l l y offer t h e u n i q u e p o s s i b i l i t y of controlling the electronic structure of the active centers through the e l e c t r o n i c s t a t e o f t h e c o n d u c t i n g p o l y m e r . In t h i s a p p r o a c h , t h e c o n d u c t i v e p o l y m e r c o u l d u l t i m a t e l y a c t a s a m o l e c u l a r w i r e a b l e to i n p u t o r o u t p u t an a c t i v e c e n t e r . T h e r e f o r e , t h e first s t e p in t h i s field is t h e r e a l i z a t i o n of an interface between the conducting polymer and the active center. Some papers using these new concepts have already been published. T h e u s e o f s u b s t i t u t e d p y r r o l e in o r d e r t o a d d r e s s n i t r o x i d e [2] or v i o l o g e n g r o u p [3] h a s b e e n r e p o r t e d . P o l y m e r i z a t i o n o f t r a n s i t i o n m e t a l c o m p l e x e s c o n t a i n i n g p y r r o l e [ 4 - 6 ] o r a n i l i n e [7] h a s p r o d u c e d t h i n films b u t t h e polymer backbone was poorly conductive and not electroactive. Substituted aromatic amines might prove to be useful here because they could interface a p o l y a n i l i n e film t h r o u g h t h e a m i n o g r o u p a n d a n a c t i v e c e n t e r t h r o u g h t h e s u b s t i t u e n t w h i l e r e t a i n i n g c e r t a i n s e m i c o n d u c t i n g p r o p e r t i e s . If so, t h i s
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c o u l d a l l o w the active c e n t e r to b e s e n s i t i v e to the a d j u s t a b l e e l e c t r o n i c s t r u c t u r e o f the polyaniline film. T h e wide v a r i e t y o f s u b s t i t u t e d a r o m a t i c a m i n e s c o m m e r c i a l l y available w o u l d t h e n p r o v i d e a l m o s t u n l i m i t e d possibilities in the design o f t h e s e n e w m a t e r i a l s . T h e p o l y m e r i z a t i o n of s e v e r a l s u b s t i t u t e d anilines h a s a l r e a d y b e e n a c h i e v e d in the p a s t few years. S h e n g l o n g et al. s h o w e d t h a t m e t a - t o l u i d i n e a n d o r t h o - t o l u i d i n e c o u l d p o l y m e r i z e , b u t n o t nitroaniline [8]. M o r e r e c e n t l y , o r t h o - m e t h o x y a n i l i n e w a s s h o w n to yield a n e w soluble p o l y m e r [9]. The m o r e e x t e n s i v e w o r k of Cattarin et al. [10] o n a s e r i e s o f ortho- and m e t a s u b s t i t u t e d anilines clearly indicated t h a t e l e c t r o p o l y m e r i z a t i o n w a s f a v o r e d b y e l e c t r o n - d o n a t i n g s u b s t i t u e n t s a n d also t h a t it w a s m o r e effective on polyaniline-modified electrode. W e also s t a r t e d to i n v e s t i g a t e t h e e l e c t r o p o l y m e r i z a t i o n o f a m i n o a r o m a t i c c o m p o u n d s o n a gold e l e c t r o d e p r e c o a t e d with a thin p o l y a n i l i n e film to d e t e r m i n e w h e t h e r o r n o t the o u t c o m e of t h e r e a c t i o n is s e n s i t i v e to t h e polyaniline film a n d to d e t e r m i n e the p o s s i b i l i t y o f i n t e r f a c i n g films of different m o l e c u l a r s t r u c t u r e . This p a p e r r e p o r t s r e s u l t s o b t a i n e d with ortho- a n d m e t a - m e t h o x y a n i l i n e s , p a r a - M e t h o x y a n i l i n e is n o t c o n s i d e r e d s i n c e Geni~s et al. [11] h a v e s h o w n t h a t its e l e c t r o p o l y m e r i z a t i o n l e a d s to n o n p o l y m e r i c material.
Experimental All t h e c h e m i c a l s u s e d in this s t u d y w e r e r e a g e n t g r a d e a n d w e r e u s e d w i t h o u t f u r t h e r purification. T h e e l e c t r o d e s w e r e p r e p a r e d b y v a p o r d e p o s i t i o n o f 3 0 0 0 /~ of gold o n a g l a s s p l a t e w h i c h w a s p r e c o a t e d with 2 0 0 /~ of c h r o m i u m as an a d h e s i v e layer. T h e m e t a l d e p o s i t i o n w a s m a d e t h r o u g h a m a s k p r o d u c i n g a m e t a l s u r f a c e a r e a e q u a l to 0.5 c m 2. T h e g o l d / p o l y a n i l i n e e l e c t r o d e s w e r e 2 0 0 0 ~ thick. T h e films w e r e p r e p a r e d b y s w e e p i n g the v o l t a g e a p p l i e d to the e l e c t r o d e u s i n g a s w e e p r a t e o f 100 m V / s . All t h e electrochemical experiments were made using a calomel reference electrode a n d a g o l d wire c o u n t e r e l e c t r o d e . T h e a n a l y s e s w e r e p e r f o r m e d with IR c o m p e n s a t i o n a n d u s i n g a PAR 3 7 2 p o t e n t i o s t a t . A 18C NMR e x p e r i m e n t w a s p e r f o r m e d o n a CXP 3 0 0 B r u c k e r i n s t r u m e n t at 7 5 . 4 7 MHz. F o r this c h a r a c t e r i z a t i o n , t h i c k films o f p o l y a n i l i n e w e r e g e n e r a t e d o n a 10 c m 2 e l e c t r o d e u s i n g a 2 M HCI solution. T h e film w a s t h e n p o l a r i z e d f o r h a l f a n h o u r at - 2 0 0 m V in o r d e r to b e r e d u c e d as c o m p l e t e l y a s p o s s i b l e . It w a s t h e n p e e l e d off a n d dried u n d e r v a c u u m . C o n d u c t i v i t y m e a s u r e m e n t s w e r e c a r r i e d o u t u s i n g t w o parallel g o l d e l e c t r o d e s o n w h i c h p o l y m e r s w e r e d e p o s i t e d . T h e i r d i s t a n c e (typically 60 /zm, m e a s u r e d with a n o p t i c a l mic r o s c o p e ) is s m a l l e n o u g h to allow o v e r l a p p i n g o f t h e t w o p o l y m e r films. T h e t h i c k n e s s o f t h e film w a s m e a s u r e d b y M i n o t ' s m e t h o d u s i n g a n interf e r o m e t e r a n d a n i n t e r f e r e n c e filter ( 5 4 6 n m ) e q u i p p e d w i t h a c a m e r a . A c u r r e n t t h r o u g h t h e s a m p l e w a s d e l i v e r e d b y a stabilized c u r r e n t g e n e r a t o r . The potential difference between the two electrodes was m e a s u r e d using a
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Keithley 155 m i c r o v o l t m e t e r null d e t e c t o r . I n f r a r e d s p e c t r a w e r e r e c o r d e d with a P e r k i n - E l m e r 5 5 0 s p e c t r o m e t e r .
Results
Monomc~ oxidation T h e o x i d a t i o n p o t e n t i a l s of meta- a n d o r t h o - m e t h o x y a n i l i n e s on a gold e l e c t r o d e m e a s u r e d b y cyclic v o l t a m m e t r y (CV) u s i n g a 2 M H2SO4/0.01 M m o n o m e r solution, are 1 0 0 0 and 1 0 7 0 mV, r e s p e c t i v e l y . No film g r o w t h is detected under these electrochemical conditions. On a g o l d / p o l y a n i l i n e e l e c t r o d e film g r o w t h o c c u r s . T h e m o n o m e r oxidation signal w a s n o t a b l e to b e m e a s u r e d due to t h e s e c o n d o x i d a t i o n signal of the polyaniline film (Ep, is 9 0 0 mV) w h i c h c h a r a c t e r i z e s t h e s w i t c h i n g of polyaniline b e t w e e n its s e c o n d a n d third s t a t e s [12]. The f o r m e r state is stable u n d e r the e l e c t r o c h e m i c a l c o n d i t i o n s u s e d here. The latter one, w h i c h is d e s c r i b e d either as a p o l y i m i n e [ 13 ] or a p o l y p h e n y l n i t r e n i u m [ 14 ] material, is v e r y r e a c t i v e t o w a r d s v a r i o u s c h e m i c a l s . If t h e r e are no aniline units in t h e cell, d e g r a d a t i o n o f the p o l y m e r o c c u r s w h e n polyaniline is cycled t h r o u g h this state. T h e d e g r a d a t i o n p r o c e s s h a s b e e n d e s c r i b e d e i t h e r as a r e a c t i o n with w a t e r leading to b e n z o q u i n o n e [15, 16] or an internal c o u p l i n g r e a c t i o n b e t w e e n t w o a d j a c e n t p o l y m e r i c chains leading to a r e t i c u l a t e d p o l y m e r i c m a t e r i a l [17]. W i t h aniline units in the cell, film g r o w t h o c c u r s as s o o n as s o m e polyaniline is oxidized in its third s t a t e [12] a n d the d e g r a d a t i o n p r o c e s s can b e minimized. Stilwell a n d P a r k Su-Moon [18] h a v e s h o w n t h a t the o x i d a t i o n p r o d u c t r e s p o n s i b l e for g r o w t h is a s u r f a c e - b o u n d s p e c i e s and t h a t the r e a c t i o n is n o t limited b y m a s s t r a n s f e r of aniline units t o w a r d the electrode. W h e n m e t h o x y a n i l i n e units are in the cell instead of aniline units, a similar b e h a v i o r is o b s e r v e d . In fact, p o l y ( m e t h o x y a n i l i n e ) is g e n e r a t e d w h e n the e l e c t r o d e p o t e n t i a l r e a c h e s t h e f o o t of the s e c o n d o x i d a t i o n signal of the p r e c o a t e d p o l y a n i l i n e film. T h e r e f o r e , the polyaniline film c a t a l y z e s the e l e c t r o p o l y m e r i z a t i o n o f m e t h o x y a n i l i n e in a similar w a y to its catalysis of t h e e l e c t r o p o l y m e r i z a t i o n o f aniline. It is i n t e r e s t i n g to n o t e t h a t o x i d a t i o n of t h e m o n o m e r d o e s n o t s e e m to b e n e c e s s a r y f o r film growth.
Electropolymerization a n d f i l m properties W e t h e n tried to g r o w a film u n d e r t h e following e x p e r i m e n t a l conditions: 2 M H e S O J 0 . 1 M m o n o m e r , a p o t e n t i a l s w e e p f r o m - 2 0 0 mV to a n i n v e r s i o n p o t e n t i a l d e p e n d i n g o n t h e e l e c t r o d e u s e d (gold or g o l d / p o l y a n i l i n e e l e c t r o d e ) . The i n v e r s i o n p o t e n t i a l w a s c h o s e n to k e e p the o x i d a t i o n c u r r e n t d e n s i t y at a low v a l u e [12]. W e c h o s e t h e s e c o n d i t i o n s b e c a u s e t h e y p r o v e d to b e a d e q u a t e f o r the p o l y m e r i z a t i o n o f aniline on g o l d e l e c t r o d e s , l e a d i n g to v e r y clean p o l y a n i l i n e films [12]. T h e e l e c t r o o x i d a t i o n w a s also p e r f o r m e d in 2 M H2SO4/0.3 M m o n o m e r solution on t h e p u r e gold e l e c t r o d e . T h e
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(b) ~rav Fig. 1. Electropolymerization of methoxyaniline in a 2 M H2SO4/0.1 M monomer solution on a gold/polyaniline electrode: (a) ortho-methoxyaniline; (b) rneta-methoxyaniline. E/mV
g r o w i n g p r o c e s s t a k i n g p l a c e o n a g o l d / p o l y a n i l i n e e l e c t r o d e is s h o w n in Fig. l ( a ) f o r o r t h o - m e t h o x y a n i l i n e a n d Fig. 1 (b) f o r m e t a - m e t h o x y a n i l i n e . The CV r e s p o n s e s o f t h e films are r e p o r t e d in Figs. ( 2 ) - ( 5 ) . T h e s e v o l t a m m o g r a m s clearly s h o w t h a t the n a t u r e o f the e l e c t r o d e and the m e t h o x y g r o u p p o s i t i o n h a v e a d r a s t i c effect on t h e g r o w i n g p r o c e s s e s a n d on the o u t c o m e of t h e r e a c t i o n .
Poly (o-methoxyaniline) films F i l m s g e n e r a t e d with t h e o r t h o - m e t h o x y a n i l i n e b e h a v e in the s a m e w a y as polyaniline. In fact, t h e i r CV r e s p o n s e s , s h o w n in Figs. (2) a n d ( 3 ) , are s o s i m i l a r to t h o s e of p o l y a n i l i n e films t h a t t h e p r e s e n c e of OCH3 g r o u p s in the m a t e r i a l c a n b e q u e s t i o n e d . W e t h e r e f o r e u s e d n o n e l e c t r o c h e m i c a l t e c h n i q u e s to a s c e r t a i n this point. IR s p e c t r o s c o p y o f a p r e s s e d p e l l e t s h o w s C - O a n d CH3 a b s o r p t i o n b a n d s at 1 0 2 0 a n d 2 9 2 0 c m - 1. T h i s clearly i n d i c a t e s t h a t s o m e OCH3 g r o u p s a r e still p r e s e n t in t h e m a t e r i a l b u t d o e s n o t give a n y q u a n t i t a t i v e m e a s u r e o f the r a t i o o f OCH3 g r o u p s still p r e s e n t . W e t h e r e f o r e c a r r i e d o u t a 13C h i g h - r e s o l u t i o n NMR c h a r a c t e r i z a t i o n o f this material. CH3 g r o u p signal is p r e s e n t a n d t h e 'shift' is t h e s a m e as t h e o n e o b s e r v e d f o r anisole CH3 g r o u p s , i.e. 55 p p m / T M S . T h e b e n z e n i c c a r b o n s a p p e a r a s a b r o a d signal b e t w e e n 100 a n d 190 p p m f r M S a n d t h e ratio o f aliphatic to a r o m a t i c c a r b o n s is f o u n d to b e 1:6. This clearly s h o w s t h a t no ( o r m i n i m a l ) OCH3 g r o u p e l i m i n a t i o n t a k e s p l a c e d u r i n g t h e e l e c t r o p o l y merization. A significant d i f f e r e n c e b e t w e e n this s p e c t r u m a n d the p r e v i o u s l y p u b l i s h e d s p e c t r a o f polyaniline [19, 20] c a n b e o b s e r v e d . A l t h o u g h w e t o o k e x t e n s i v e c a r e to o b t a i n t h e m a t e r i a l in t h e r e d u c e d s t a t e ( s e e E x p e r i m e n t a l Section) t h e b e n z e n i c c a r b o n signals a r e v e r y b r o a d a n d n o t a s well defined as is
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Fig. 2. CV r e s p o n s e in 2 M H2SO 4 s o l u t i o n of a film g e n e r a t e d on a gold e l e c t r o d e with o r t h o m e t h o x y a n i l i n e in a 2 M H~SO4/0.3 M m o n o m e r solution. Fig. 3. CV r e s p o n s e in 2 M H2SO4 s o l u t i o n of a film g e n e r a t e d o n a g o l d / p o l y a n i l i n e electrode w i t h o r t h o - m e t h o x y a n i l i n e in a 2 M H2SO,/0.1 M m o n o m e r solution.
r e p o r t e d f o r p o l y a n i l i n e w h e n fully r e d u c e d . This r e s u l t i n d i c a t e s t h a t s o m e e l e c t r o n i c c o n d u c t i v i t y is still p r e s e n t in o u r s a m p l e . This m i g h t r e v e a l e i t h e r t h a t p o l y ( o r t h o - m e t h o x y a n i l i n e ) is c o n d u c t i v e in t h e r e d u c e d s t a t e as r e p o r t e d b y M a c i n n e s a n d F u n t [9] or t h a t o u r v e r y t h i c k s a m p l e w a s n o t fully r e d u c e d . C o n d u c t i v i t y m e a s u r e m e n t s w e r e n o t p e r f o r m e d on r e d u c e d p o l y ( o r t h o methoxyaniline). M o n o m e r c o n c e n t r a t i o n h a s a g r e a t influence on t h e o u t c o m e o f t h e r e a c t i o n w h e n t h e g o l d e l e c t r o d e is used. T h e o x i d a t i o n of o r t h o - m e t h o x y a n i l i n e b y s w e e p i n g t h e p o t e n t i a l a p p l i e d to t h e e l e c t r o d e b e t w e e n - 100 a n d 1 0 0 0 m V with a 0.1 M m o n o m e r solution yields a red, soluble e l e c t r o a c t i v e s p e c i e s w h i c h o x i d i z e s at 3 8 0 m V a n d r e d u c e s at 3 4 0 mV. T h i s p r o d u c t m i g h t be b e n z i d i n e w h i c h is k n o w n to b e r e d in its oxidized f o r m . On the o t h e r h a n d , t h e s a m e p r o c e d u r e u s i n g a 0.3 M m o n o m e r solution l e a d s t o a p o l y a n i l i n e analog material. W h e n a g o l d / p o l y a n i l i n e e l e c t r o d e is used, m o n o m e r c o n c e n t r a t i o n d o e s n o t s e e m to influence t h e o u t c o m e o f the r e a c t i o n so drastically. W i t h a 0.1 M m o n o m e r solution p o l y ( o r t h o - m e t h o x y a n i l i n e ) g r o w t h is easily o b s e r v e d .
122 T h e s a m e o b s e r v a t i o n h o l d s with 0.01 M m o n o m e r solution as a l r e a d y m e n t i o n e d . T h e r e f o r e , polyaniline s e e m s to a c t as a l a y e r c a p a b l e of i n c r e a s i n g the local m o n o m e r c o n c e n t r a t i o n o n t o the e l e c t r o d e . This effect will be d i s c u s s e d b e l o w in t e r m s o f specific a d s o r p t i o n in the light of frontier orbital interactions. It t u r n s o u t to b e i m p o s s i b l e , u n d e r o u r e x p e r i m e n t a l conditions, to e l e c t r o s y n t h e s i z e a p o l y ( o r t h o - m e t h o x y a n i l i n e ) film with no m i d d l e p e a k in the 4 5 0 m V region, e v e n if t h e c u r r e n t density u s e d during e l e c t r o p o l y m e r i z a t i o n is c a r e f u l l y c o n t r o l l e d . This o b s e r v a t i o n is in c o n t r a s t to polyanlline e l e c t r o p o l y m e r i z a t i o n a n d d o e s n o t d e p e n d on the n a t u r e o f the e l e c t r o d e . A l t h o u g h the origin o f this p e a k is n o t clearly defined, o n e i n t e r p r e t a t i o n p r o p o s e d is b a s e d on ortho c o u p l i n g s [21 ]; t h e r e f o r e the difference o b s e r v e d h e r e b e t w e e n p o l y a n i l i n e a n d poly(ortho-methoxyaniline) m i g h t reflect a g r e a t e r ratio on non-para c o u p l i n g s v e r s u s para c o u p l i n g s with the latter. This m i d d l e p e a k is l o c a t e d at 4 0 0 mV f o r the o x i d a t i o n p r o c e s s a n d at 3 8 0 mV f o r the r e d u c t i o n p r o c e s s w h e t h e r is s t a n d s at 540 o r 5 2 0 m V f o r polyaniline. T h i s m a y indicate t h a t t h e p e a k o b s e r v e d with p o l y ( o r t h o m e t h o x y a n i l i n e ) is n o t c o m p l e t e l y similar to t h a t of polyaniline. T h e s e d i f f e r e n c e s will be d i s c u s s e d b e l o w in t e r m s of f r o n t i e r orbital interactions. T h e influence o f p H o n t h e e l e c t r o c h e m i c a l signal is similar to the o n e o b s e r v e d with p o l y a n i l i n e the films o f w h i c h are e l e c t r o i n a c t i v e in n e u t r a l solution, w h i c h i n d i c a t e s that, like polyaniline, the p r o t o n i c d o p i n g p r o c e s s is p r e s e n t with t h e s e m a t e r i a l s . T h e s e m a t e r i a l s e x h i b i t e l e c t r o c h r o m i s m . T h e y are t r a n s p a r e n t y e l l o w in the r e d u c e d state, g r e e n in the first oxidized s t a t e a n d b l u e - b l a c k in t h e s e c o n d o x i d a t i o n state. C o n d u c t i v i t y m e a s u r e m e n t s p e r f o r m e d with a film in its first o x i d a t i o n s t a t e indicate a c o n d u c t i v i t y v a l u e o f 10 -2 ~ - ~ c m - 1 w h i c h is similar to p o l y a n i l i n e p r e p a r e d u n d e r the s a m e c o n d i t i o n s . T h e r e f o r e , the OCH3 g r o u p d o e s n o t influence drastically the film conductivity. T e m p e r a t u r e d e p e n d e n c e s h o w s a s e m i c o n d u c t i n g b e h a v i o r with a v e r y low t h e r m a l a c t i v a t i o n e n e r g y of 0 . 0 6 eV w h i c h is a g a i n similar to polyaniline. In c o n c l u s i o n , t h e t w o films g e n e r a t e d with o r t h o - m e t h o x y a n i l i n e are similar to polyaniline. T h e p r e c o a t e d polyaniline film m a k e s the e l e c t r o s y n t h e s i s easier, this effect b e i n g similar to t h e o n e o b s e r v e d on i n c r e a s i n g t h e m o n o m e r c o n c e n t r a t i o n in t h e solution.
Poly(m-methoxyaniline) films T h e CV r e s p o n s e s o f t h e m a t e r i a l s o b t a i n e d f r o m the e l e c t r o p o l y m e r i z a t i o n of t h e m e t a - m e t h o x y a n i l i n e a r e s h o w n in Figs. (4) a n d (5). T h e y a r e v e r y different f r o m t h o s e o b t a i n e d with t h e o r t h o m o n o m e r . On a g o l d e l e c t r o d e , the o x i d a t i o n o f m e t a - m e t h o x y a n i l i n e b y s w e e p i n g t h e p o t e n t i a l a p p l i e d to t h e e l e c t r o d e b e t w e e n - 1 0 0 a n d 1 0 0 0 mV, w i t h a 0.1 M m o n o m e r solution, yields a v e r y thin film w h i c h h a s its m a i n o x i d a t i o n signal in t h e 4 5 0 m V region. T h e e l e c t r o d e p a s s i v a t e s partially a n d t h i c k
123
400
300
200
100
<
b: 100 mV/s
40 -
I O0
< -200
~o
-300
-40 l
l
I
0
400
800
E/mY
0
400
800
E1mV
Fig. 4. CV r e s p o n s e in 2 M H2SO 4 s o l u t i o n o f a film g e n e r a t e d o n a gold electrode with metam e t h o x y a n i l i n e in a 2 M H2S04/0.3 M m o n o m e r solution. Fig. 5. CV r e s p o n s e in 2 M H2SO4 s o l u t i o n of a film g e n e r a t e d on a g o l d / p o l y a n i l i n e electrode w i t h meta-methoxyaniline in a 2 M H2SO4/0.1 M m o n o m e r solution.
films a r e n o t g e n e r a t e d . T h e s a m e b e h a v i o r is o b s e r v e d w h e n a 0.3 M m o n o m e r s o l u t i o n is u s e d . On a g o l d / p o l y a n i l i n e e l e c t r o d e , t h e m o n o m e r o x i d a t i o n p r o c e s s g e n e r a t e s , in a first stage, the g r o w t h o f an e l e c t r o c h e m i c a l signal at 4 7 0 mV f o r the o x i d a t i o n p e a k a n d 4 3 0 m V f o r t h e r e d u c t i o n p e a k ( s e e Fig. l ( b ) ) . T h e s e p e a k s s t a n d at the s a m e p o s i t i o n as t h o s e o b s e r v e d w h e n a g o l d e l e c t r o d e is used. H o w e v e r , it is n o w p o s s i b l e to g e n e r a t e t h i c k e r films. D u r i n g this first s t e p no (or m i n i m a l ) g r o w t h of t h e 2 5 0 - 1 5 0 mV p e a k s , d u e to the p r e s e n c e o f t h e p r e c o a t e d polyanfline film, is o b s e r v e d . This i n d i c a t e s t h a t s o m e s o r t o f m a t e r i a l d e p o s i t s o n t o t h e e l e c t r o d e b u t t h a t this m a t e r i a l is n e i t h e r a k i n to p o l y a n i l i n e n o r to p o l y ( o r t h o - m e t h o x y a n i l i n e ) . In a s e c o n d state, p o l y a n i l i n e signal a l t e r a t i o n t a k e s p l a c e a n d its c h a r a c t e r i s t i c p e a k s a r e shifted anodically. This i n d i c a t e s s o m e s o r t o f m o l e c u l a r i n t e r a c t i o n b e t w e e n t h e p r e c o a t e d p o l y a n i l i n e film a n d t h e d e p o s i t i n g material. T h e p H influence on t h e e l e c t r o c h e m i c a l r e s p o n s e is similar to t h e o n e o b s e r v e d with polyaniline; this film is e l e c t r o i n a c t i v e in n e u t r a l solution, w h i c h i n d i c a t e s t h a t t h e p r o t o n i c d o p i n g p r o c e s s is p r e s e n t a s in polyaniline. This s h o w s e i t h e r t h a t t h e d e p o s i t i n g m a t e r i a l g r o w s o n t o p of t h e p r e c o a t e d
124
polyaniline film or that it grows within the p r e c o a t e d polyaniline film retaining the same type of molecular structure. No additional electrochromism is visually detected when the film is cycled through the 4 7 0 - 4 3 0 mV peaks. Solubility test shows that the film is slightly soluble in ethanol and in acetonitrile but it has been impossible to solubilize the deposited material to the point where the electrochemical signal of the p r e c o a t e d polyaniline film is restored. Alteration of this signal is permanent, indicating that a strong interaction takes place between polyaniline and the deposited material. This tends to indicate that the 'poly(meta-methoxyaniline)' is chemically bonded to polyaniline.
Discussion At this point, several conclusions can be drawn. (i) The o r t h o and the m e t a m o n o m e r s do not behave in a similar way toward the chemical reaction following the m o n o m e r electrochemical oxidation. This clearly shows that the whole process is under kinetic control since a th er mo d ynam i c control of the electropolymerization would lead to the same material, as shown in Scheme 1, and should t herefore be independent of the substituent position. (ii) The controlling step of the reaction is the chemical coupling between m o n o m e r units which follows the electrochemical oxidation. (iii) The polyaniline film plays a key role in the process. It has an important catalytic effect, already mentioned by several publications [10, 12, 15], which acts on two different levels. First, the polyaniline film acts as a concentration layer through specific adsorption. Secondly, polyaniline in its second oxidation form (third state) seems capable of reacting on an adsorbed m o n o m e r i c species. However, the polyaniline film has no influence on the regioselectivity of the coupling reaction since materials obtained with the same m o n o m e r have identical electrochemical characteristics. This means that the regioselectivity of the coupling reaction is controlled by the nature of the m o n o m e r used.
NH
~
X
X
NH
S c h e m e 1.
125
This, t o g e t h e r with the n a t u r e o f t h e s p e c i e s involved h e r e , i.e. soft e l e c t r o p h i l e or n u c l e o p h i l e , l e a d s us to think t h a t an u n d e r s t a n d i n g of the p o l y m e r i z a t i o n p r o c e s s c o u l d b e f o u n d in r a d i c a l - c a t i o n f r o n t i e r orbital interactions. E v e n if this a p p r o a c h really a p p e a l s to us, we h a v e to p o i n t out t h a t it will only p r o v i d e qualitative g u i d e l i n e s since s e v e r a l effects a r e n o t c o n s i d e r e d in the a p p r o a c h s u c h as c o u l o m b i c i n t e r a c t i o n s o r s o l v e n t effect. W e n e v e r t h e l e s s p e r f o r m e d an e x t e n d e d Hiickel c a l c u l a t i o n to o b t a i n t h e m o l e c u l a r orbital (MO) s c h e m e of t h e m o n o m e r s u s e d in this study. W e also c o m p u t e d the MO o f aniline f o r c o m p a r i s o n p u r p o s e s a n d t h e MO of polyaniline o l i g o m e r s to o b t a i n a m o l e c u l a r orbital s c h e m e d e s c r i b i n g to s o m e e x t e n t the c h e m i c a l m o d i f i c a t i o n i n d u c e d on the e l e c t r o d e s u r f a c e b y t h e p r e c o a t e d polyaniline film. S t a n d a r d b o n d l e n g t h s a n d a n g l e s w e r e u s e for this c o m p u t a t i o n [22]. T h e t o r s i o n a n g l e b e t w e e n a d j a c e n t r i n g s w a s c o n s i d e r e d to b e zero. H o w e v e r , w e h a v e allowed for g e o m e t r y r e l a x a t i o n upon protonation. In a r e c e n t p u b l i c a t i o n [23], w e s h o w e d , using the f r o n t i e r orbital a p p r o a c h , t h a t the p o l y m e r i z a t i o n p r o c e s s of aniline w a s c o n t r o l l e d by t h e HOMO i n t e r a c t i o n s o f the s p e c i e s i n v o l v e d in the c o u p l i n g r e a c t i o n w h i c h follows the m o n o m e r o x i d a t i o n . In o r d e r t o e x p l a i n the head-to-tail c o u p l i n g t h a t is d o m i n a n t in the p r o c e s s , w e h a v e s u g g e s t e d t h a t the c o u p l i n g r e a c t i o n is not of a radical c o u p l i n g t y p e a n d t h a t it involves the two following species: H2
+
H
Nbk3
This is, u p to now, t h e o n l y d e s c r i p t i o n o f t h e p o l y m e r i z a t i o n p r o c e s s w h i c h is c o m p a t i b l e with t h e r e g i o s p e c i f i e i t y o f the c o u p l i n g r e a c t i o n b e t w e e n m o n o m e r i c units. This d o e s n o t m e a n t h a t the d e s c r i p t i o n is c o r r e c t b u t we will t a k e it as a w o r k i n g h y p o t h e s i s to e x a m i n e p o s s i b l e s u b s t i t u e n t effects. T h e following orbital s c h e m e s will t h e r e f o r e p r e s e n t the H O M O of t h e s e t w o s p e c i e s with m e t h o x y s u b s t i t u e n t . T h e HOMO o f o r t h o - a n d m e t a - m e t h o x y a n i l i n e s f o r t h e p r o t o n a t e d a n d the n o n p r o t o n a t e d f o r m s a r e s h o w n in S c h e m e 2. S c h e m e 3 illustrates the H O M O o f aniline a n d the H O M O o f t h e d i m e r a n d trimer, w h i c h i n d i c a t e s the c h e m i c a l m o d i f i c a t i o n t r e n d o f t h e e l e c t r o d e i n d u c e d b y t h e p r e c o a t e d polyaniline film. As e x p e c t e d , t h e m e t h o x y g r o u p s t r o n g l y m o d i f i e s t h e H O M O s of the m o n o m e r w h e n c o m p a r e d to aniline. T h i s effect is p a r t i c u l a r l y p r o n o u n c e with t h e p r o t o n a t e d f o r m o f the m o n o m e r w h e r e t h e p a r a c a r b o n of the a m i n o g r o u p l o s e s a lot o f e l e c t r o n d e n s i t y a n d is n o l o n g e r t h e c a r b o n with the h i g h e s t coefficient. N e w sites a r e a c t i v a t e d a n d this p h e n o m e n o n is highly d e p e n d e n t o n t h e p o s i t i o n o f t h e m e t h o x y g r o u p as s e e n in S c h e m e 2. This a c t i v a t i o n is localized o n t h e c a r b o n in p a r a o f t h e m e t h o x y g r o u p . T h e r e f o r e , w h e n in o r t h o of t h e a m i n o g r o u p , t h e m e t h o x y g r o u p a c t i v a t e s a c a r b o n in m e t a o f the a m i n o g r o u p a n d t h u s l e a d s to a m o n o m e r with t w o a d j a c e n t
126
Deprotonated form
Protonated form
METAMETHOXYANILINE
o 36
fill3
N 0.06
°,, o
\ 0.09
Q
o.s3
o.o,
-0.40
E=-12.09 eY
0.33
0.40
E=-12.39 eV o, 37
ORTHOMETHOXYANILINE
/o,o
0.34 ~
0.29
o.s,
-0.22
E=-11.95 eV
0
0.43
0
E=-12.39 eV
Scheme 2. Frontier orbital approach and methoxyaniline electropolymerization on a gold electrode.
carbons which can easily react with radicals. When in m e t a of the amino group, the m e t h o x y group activates a carbon in o r t h o of the amino group and thus leads to a m o n o m e r with activated carbon similar to aniline but with a stronger activation on one of the o r t h o carbons instead of the p a r a carbon. In o th er words, frontier orbital considerations indicate that the o r t h o and the m e t a m o n o m e r s should not behave identically toward the chemical reaction which follows m o n o m e r electrooxidation. This is what is observed experimentally. With m e t a substituent o r t h o coupling should be dominant, whereas p a r a and m e t a couplings should be p r e p o n d e r a n t for the o r t h o isomer. This is consistent with the CV r es po nses since ortho-methoxyaniline oxidation generates a polyaniline-like material w h e r e a s m et a-m et hoxyani l i ne oxidation does not. The 470--420 peaks observed in the CV response of the thin film o f 'poly(meta-methoxyaniline)' a p p e a r to be the ort ho coupling signature. The 2 5 0 - 1 5 0 mV signal seems to express p a r a coupling and the 4 0 0 - 3 8 0 mV couple might be due to m e t a coupling. This last point should be taken carefully; indeed, no experimental p r o o f of the presence in poly(ortho-
127
Deprotonated form
Protonated form
Monomer
0.15 -0,36
0.21
0.48
-0.32
0.52 0.57
0.13
0,15 .0.36
0.25
Dimer 0.26 -0.22
-
0
.0.30
0.25 -0.12 ~ N
-0.03
o ~
-~. 7
2--~
o-~
,jo. .0.29
.o.,
0.07
""
0~9
~.,~
~
.0.33
0.06
o.~ ~.,~
.0.12
Trimer -0.48
~ 0.05 -0.14
o.35
~
N
xr-O" 0.01 -0.17
.0.42
0.43
Scheme 3. Frontier orbital approach and methoxyaniline electropolymerization on a gold/ polyaniline electrode. m e t h o x y a n i l i n e ) o f m o n o m e r c o u p l e d t h r o u g h m e t a c o u p l i n g is available (IR a n d 13C NMR p r o v e d to be i n a d e q u a t e to a s c e r t a i n this point). W e t h e r e f o r e only s u g g e s t this last p o i n t o n the basis o f the f r o n t i e r orbital a p p r o a c h w h i c h s e e m s to rationalize fairly well s u b s t i t u e n t effects on the electrop o l y m e r i z a t i o n o f s u b s t i t u t e d anilines. The c a l c u l a t e d HOMO a n d LUMO of the d i m e r a n d the t r i m e r s h o w n in S c h e m e 3 give s o m e indication o f the e l e c t r o n i c c h a n g e s i n d u c e d by the p r e s e n c e on the gold s u b s t r a t e of a p r e c o a t e d polyaniline film. T h r e e features are i n t e r e s t i n g t o n o t e w h e n the chain length increases:
128 (1) The carbon atoms become equivalent. The para carbon reactivity slowly becomes identical to that of the other carbons, especially ortho carbons. (2) H O M O e n e r g y i n c r e a s e s and LUMO e n e r g y d e c r e a s e s . (3) The e l e c t r o n i c d en s i t y on the t e r m i n a l n i t r o g e n of the d e p r o t o n a t e d f o r m s is l o w e r e d . T h e s e r e s u l t s a r e g i v e n i n T a b l e s 1 a n d 2. T h e first p o i n t i n d i c a t e s t h a t the m o n o m e r i c unit m u s t r e a c t t h r o u g h its c a r b o n a t o m s r a t h e r t h a n i ts n i t r o g e n a t o m , p r o v i d e d t h a t f i l m g r o w t h involves coupling between a monomeric unit and a chain of several aniline units bonded to the electrode, and provided that frontier orbital interactions c o n t r o l the c o u p l i n g r e a c t i o n s . If not, t h e n i t r o g e n could r e a c t on any c a r b o n atom of the terminal aniline unit of the chain. Therefore, a progressive loss in t h e r e g i o s e l e c t i v i t y o f t h e c o u p l i n g r e a c t i o n s h o u l d b e n o t i c e d , w h i c h is n o t t h e c a s e . F u r t h e r m o r e , if t h e c o u p l i n g r e a c t i o n i n v o l v e s t h e n i t r o g e n TABLE 1 Terminal N coefficients, p a r a carbon coefficients, and HOMO and LUMO energies of protonated and deprotonated forms Terminal N coeff, in HOMO
Deprotonated f o r m Monomer Dimer Trimer
0.57 0.39 0.28
Protonated f o r m Monomer Dimer Trimer
0.02 0.07 0.04
para Carbon coeff, in HOMO
HOMO energy (eV)
LUMO energy (eV)
0.46 0.24 0.14
- 12.13 - 11.59 - 11.44
- 8.36 - 8.24 -8.22
0.52 - 0.33 0.18
- 12.60 - 11.98 - 11.52
- 8.35 - 8.25 - 8.20
-
TABLE 2 Compared reactivity of the carbon atoms Cpara coeff.
Co,~o coeff.
C,~t~ coeff.
Protonated f o r m Monomer Dimer Trimer
0.52 - 0.33 0.18
- 0.30 0.26 -0.17
0.22 0.08 0.03
Deprotonated form Monomer Dimer Trimer
0.46 - 0.24 0.14
- 0.36 0.20 -0.14
0.15 0.05 0.03
I29 a t o m o f t h e m o n o m e r i c unit, t h e n e l e c t r o p o l y m e r i z a t i o n o f m e t a - a n d o r t h o m e t h o x y a n i l i n e s on the p r e c o a t e d p o l y a n i l i n e film s h o u l d give the s a m e p r o d u c t , s i n c e the r e a c t i v i t y o f t h e n i t r o g e n a t o m o f the m o n o m e r i c unit a p p e a r s i n d e p e n d e n t of t h e m e t h o x y position. This is n o t o b s e r v e d e x p e r imentally. It is w o r t h n o t i n g t h a t this d e d u c t i o n is strictly valid for t h e first l a y e r s o f p o l y ( m e t h o x y a n i l i n e ) b u t n o t for the following layers. T h e catalytic effect c a n b e u n d e r s t o o d t h r o u g h the influence o f t h e chain length on the H O M O e n e r g y o f the o l i g o m e r . It s e e m s t h a t t h e r e is a specific a d s o r p t i o n o f m o n o m e r i c units o n t o t h e p o l y m e r i c m a t e r i a l . This specific a d s o r p t i o n c a n b e d e s c r i b e d in t e r m s o f w e a k f r o n t i e r orbital i n t e r a c t i o n s b e t w e e n t h e f r o n t i e r orbitals of the m o n o m e r i c unit a n d t h o s e o f the p o l y m e r i c chain. Let us m o d e l the p o l y m e r i c c h a i n b y the trimer. In its first oxidized s t a t e ( b e t w e e n 2 5 0 a n d 7 0 0 mV) t h e orbital s h o w n in S c h e m e 3 c o n t a i n s o n e or zero e l e c t r o n s . C o n s i d e r i n g t h e e n e r g y v a l u e s o f the f r o n t i e r orbitals o f the different s p e c i e s i n v o l v e d in t h e p r o c e s s , the m a i n i n t e r a c t i o n t a k e s p l a c e b e t w e e n t h e H O M O of the m o n o m e r i c unit w h i c h h a s n o t y e t b e e n o x i d i z e d a n d the SOMO or LUMO ( d e p e n d i n g on its e l e c t r o n p o p u l a t i o n ) of t h e trimer. O t h e r i n t e r a c t i o n s b e t w e e n f r o n t i e r orbitals h a v e to b e elimhaated b e c a u s e o f t h e h i g h e r e n e r g y gap. S o m e o v e r l a p b e t w e e n t h e two orbitals l e a d s to a w e a k c h e m i c a l b o n d . Since the e n e r g y of the t r i m e r orbital is h i g h e r t h a n t h e e n e r g y o f t h e m o n o m e r orbital, this w e a k i n t e r a c t i o n l o w e r s the e n e r g y o f t h e m o n o m e r orbital w h i c h is still d o u b l y o c c u p i e d as long as n o e l e c t r o n i c t r a n s f e r t a k e s place. In the m e a n t i m e , the e n e r g y level of t h e o l i g o m e r orbital is i n c r e a s e d a n d in t h e a b s e n c e of e l e c t r o n i c t r a n s f e r this orbital is e i t h e r singly o c c u p i e d or e m p t y . A n e w s p e c i e s ' c h a i n + a d s o r b e d m o n o m e r ' is t h e r e f o r e g e n e r a t e d , w h i c h is t h e r m o d y n a m i c a l l y m o r e stable t h a n the ' c h a i n + f r e e m o n o m e r ' s y s t e m a n d this stabilization m a i n l y c o n c e r n s t h e m o n o m e r i c p a r t o f the s y s t e m . T h e r e f o r e , f r o m a t h e r m o d y n a m i c p o i n t of v i e w it is e a s i e r to oxidize this s y s t e m t h a n the m o n o m e r i c unit. This d e s c r i p t i o n a p p e a r s to d e s c r i b e c o r r e c t l y the catalytic effect o b s e r v e d during the e l e c t r o p o l y m e r i z a t i o n of aniline a n d m e t h o x y a n i l i n e on a polyaniline electrode. T h e third p o i n t p r e d i c t s a loss o f r e g i o s p e c i f i c i t y o f t h e c o u p l i n g r e a c t i o n s w h e n the c h a i n l e n g t h i n c r e a s e s . T h i s is n o t o b s e r v e d e x p e r i m e n t a l l y since t h i c k p o l y a n i l i n e films w i t h n o m i d d l e p e a k (i.e. n o n o n - p a r a c o u p l i n g ) c a n b e g e n e r a t e d . T h e f r o n t i e r orbital m o d e l s e e m s , on t h a t point, i n a d e q u a t e . H o w e v e r , this i n a d e q u a c y m a y arise f r o m t h e i n h e r e n t limits i n t r o d u c e d b y t h e g e o m e t r i c d a t a w e u s e d to d e s c r i b e t h e t r i m e r . I n d e e d , polyaniline, in its third state, is d e s c r i b e d e i t h e r as a p o l y i m i n e [ 13 [ or a p o l y p h e n y l n i t r e n i u m [14] m a t e r i a l . T h e s e s p e c i e s h a v e g e o m e t r i c a l c h a r a c t e r i s t i c s w h i c h are different f r o m t h o s e t h a t w e used. W e t h e r e f o r e c o m p u t e d the m o l e c u l a r orbitals o f t h e t w o s p e c i e s s h o w n in S c h e m e 4 in o r d e r to o b t a i n a b e t t e r d e s c r i p t i o n o f p o l y a n i l i n e in its a c t i v e s t a t e t o w a r d s m o n o m e r s . T h e first s p e c i e s localizes the q u i n o n i c d e f o r m a t i o n o n t o t h e cycle with t h e t e r m i n a l n i t r o g e n . This is n o t c o m p l e t e l y satisfying since this quinonic d e f o r m a t i o n m u s t travel f r o m o n e cycle to a n o t h e r a l o n g the chain. T h e s e c o n d one,
130 H
\
-0.28 0 . ~1 - 0~. 4 8 ~~.~!i:~~i!~i 0.44 -0.07
-0.08~
~)0.18 0.28 -0.28
(~/'-0.18-0.08 0.14 -0.07
E = -10.84
eV H
\
d
With quinonic perturbation on each cycle
. o 0.25~-0.18 "~C ~ 0,20 @
0.20:9.0.3 0 -0.17 -0.17
Scheme4.
E = -10.64 eV
131 w h i c h d e s c r i b e s a t r i m e r with q u i n o n i c d e f o r m a t i o n on e a c h cycle, is n o t c h e m i c a l l y realistic. In a c e r t a i n sense, t h e H O M O of this h y p o t h e t i c a l s p e c i e s t e n d s to r e p r e s e n t the a v e r a g e r e a c t i v i t y o f t h e c y c l e s i n d u c e d by the d e f o r m a t i o n delocalization. The r e a c t i n g orbitals of t h e s e s p e c i e s a r e s h o w n in S c h e m e 4. T h r e e f e a t u r e s are i n t e r e s t i n g to n o t e w h e n c o m p a r i n g t h e s e orbitals with t h a t o f our previous trimeric species: (1) The c a r b o n a t o m s r e m a i n e q u i v a l e n t t o w a r d m o n o m e r i c attack. (2) The e n e r g i e s of t h e s e orbitals r e m a i n h i g h e r t h a n the H O M O e n e r g y of t h e m o n o m e r i c units. (3) The t e r m i n a l n i t r o g e n n o w h a s a n e l e c t r o n i c d e n s i t y w h i c h is c l o s e r to t h e o n e o f the n i t r o g e n in a m o n o m e r i c unit. T h e r e f o r e , it a p p e a r s t h a t t h e c o n c l u s i o n s d r a w n a b o v e s t a n d a n d t h a t the p r o g r e s s i v e loss o f regiospecificity, w h i c h t h e third p o i n t p r e d i c t e d , m a y b e due to a n i n a d e q u a t e m o d e l of p o l y a n i l i n e in its r e a c t i v e state. H o w e v e r , it s h o u l d p o i n t e d out t h a t t h e a p p r o a c h is not c o m p l e t e l y satisfying. A b e t t e r o n e w o u l d m o d e l t h e c h a i n a s a m i x e d v a l e n c e material. In s u c h a d e s c r i p t i o n the delocalization of the q u i n o n i c d e f o r m a t i o n o n the chain w o u l d lead to s e v e r a l s t a t e s c l o s e in e n e r g y in o r d e r t o allow the d e f o r m a t i o n to travel a l o n g the chain. T h e relative value o f t h e e n e r g y levels a s s o c i a t e d with t h e s e s t a t e s should give t h e i r relative p o p u l a t i o n s . T h e s t a t e c o r r e s p o n d i n g to the q u i n o n i c d e f o r m a t i o n localized o n t o the t e r m i n a l cycle with free n i t r o g e n is quite likely to b e statistically o c c u p i e d a n d m i g h t be r e s p o n s i b l e for r e a c t i o n s with m o n o m e r i c unit. W e h a v e p r o b a b l y r e a c h e d a limit h e r e for t h e f r o n t i e r orbital a p p r o a c h a n d e v e n if t h e r e is s o m e e v i d e n c e t h a t m o n o m e r i c units r e a c t via t h e i r c a r b o n a t o m s , the i n a d e q u a c y r e p o r t e d a b o v e relativizes this s t a t e m e n t . Conclusions This p a p e r s h o w s t h a t a w e a l t h o f n o v e l c h e m i s t r y can be d e v e l o p e d in the field o f s u b s t i t u t e d aniline p o l y m e r i z a t i o n . It also s u g g e s t s t h a t the f r o n t i e r orbital c o n c e p t is a useful t o o l to u n d e r s t a n d in a qualitative way, a n d in s o m e c a s e s p r e d i c t s t h e p r o c e s s e s i n v o l v e d in the e l e c t r o s y n t h e s e s . This a p p r o a c h p r o v e s to b e a d e q u a t e in o r d e r t o p r e d i c t m e t h o x y p o s i t i o n effects a n d t e n d s to i n d i c a t e t h a t t h e f i l m - g r o w i n g p r o c e s s involves the c o u p l i n g o f a m o n o m e r i c unit via o n e o f its c a r b o n a t o m s o n t o a chain. H o w e v e r , Hiickel c a l c u l a t i o n is n o t c o m p l e t e l y satisfying in o r d e r to m o d e l t h e c h a i n a n d we believe t h a t a b e t t e r d e s c r i p t i o n of polyaniline c a n b e found using a mixed valence approach. References 1 A. F. Diaz, J. F. Rubinson and H. Mark, Adv. Polym. Sci., 84 (1988) 113. 2 G. Bidan and D. Limosin, Ann. Phys. Colloq. I, Suppl. 1, 11 (1986) 5.
132 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
G. Bidan, A. Deronzier and J. C. Moutet, J. Chem. Soc., Chem. C o m m u n . , (1984) 1185. J. G. Eaves, H. S. Munro and D. Parker, J. Chem. Soc., Chem. Commun., (1985) 684. S. Cosnier, A. Deronzier and J. C. Moutet, J. Electroanal. Chem., 193 (1985) 193. F. Bediou, A. Merino, J. Devinck, C. E. Mestres and C. Bied-Charreton, J. Electroanal. Chem., 239 (1988) 433. C. D. Ellis, L. D. Margerum, R. W. Murray and T. J. Meyer, lnorg. Chem., 22 (1983) 1283. W. Shenglong, W. Fosong and G. Xiaohui, Synth. Met., 16 ( 1 9 8 6 ) 99. D. MacInnes, Jr. and B. L. Funt, Synth. Met., 25 (1988) 235. S. Cattarin, L. Doubova, G. Mengoli and G. Zotti, Electrochim. Acta, 33 (1988) 1077. E. M. Geni~s, J. F. Penneau and M. Lapkowski, N e w J. Chem., 12 ( 1 9 8 8 ) 765. J. C. Lacroix and A. F. Diaz, Makromol. Chem. Macromol. Symp., 8 (1987) 17; J. Electrochem. Soc., 135 (1988) 1457. J. C. Chiang and A. G. MacDiarmid, Synth. Met., 13 (1986) 193. E. M. Geni~s and M. Lapkowski, J. Electroanal. Chem., 220 ( 1 9 8 7 ) 67. T. Kobayashi, H. Yoneyama and H. Tamura, J. Electroanal. Chem., 161 (1984) 419. D. E. Stilwell and Park Su-Moon, J. Electrochem. Soc., 135 (1988) 2447. E. M. Geni&s, M. Lapkowski and C. Tsintavis, N e w J. Chem., 12 (1988) 181. D. E. Stilwell and Park Su-Moon, J. Electrochem. Soc., 135 (1988) 2254. F. Devreux, G. Bidan, A. A. Syed and C. Tsintavis, J. Phys. (Paris), 40 (1985) 1595. T. Hjertberg, W. R. Salaneck, I. Lundstr6m, N. L. D. Somasiri and A. G. MacDiarmid, J. Polym. Sci., Polym. Lett. Ed., 23 ( 1 9 8 5 ) 503. E. M. Geni~s and C. Tsintavis, J. Electroanal. Chem., 195 (1985) 109. J. A. Pople and D. L. Beveridge, in A p p r o x i m a t e Molecular Orbital Theory, McGrawHill, New York, 1970. J. C. Lacroix, P. Garcia, J. P. Audi~re, R. Clement and O. Kahn, N e w J. Chem., 14 (1991) 87.
117
Electropolymerization of methoxyaniline: experimental results and frontier orbital interpretation J e a n C h r i s t o p h e Lacroix, P e d r o Garcia, J e a n Paul Audi6re, Ren6 C l d m e n t and Olivier Kahn Labo'ratoire de C h i m i e I n o r g a n i q u e , URA 420, Universitd d e P a r i s - S u d , P1405 Orsay (France)
(Received November 15, 1990; accepted March 8, 1991)
Abstract This report describes the electropolymerization reactions of ortho- and meta-methoxyanilines on a gold electrode and on a gold/polyaniline electrode. It is shown that the outcome of the electrooxidation process ks sensitive to the methoxy group position but not to the nature of the electrode which, nevertheless, exerts a catalytic effect. These effects are discussed in terms of frontier orbital interactions. This approach suggests that film growth involves the coupling of a nonoxidized monomeric unit via one of its carbon atoms onto a deprotonated NH2 group of the oxidized polymeric chain.
Introduction S i n c e t h e first r e p o r t o n t h e a v a i l a b i l i t y o f p o l y p y r r o l e a n d p o l y a n i l i n e as films, a s t r o n g i n t e r e s t h a s b e e n m a i n t a i n e d in t h e u s e of t h e s e m a t e r i a l s as e l e c t r o d e s in 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 s u c h as d i s p l a y s , s e n s o r s or b a t t e r i e s [ll. A new area yet to be developed and possibly leading to important 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 is t h e d e s i g n o f e l e c t r o a c t i v e c o n d u c t i v e films incorporating other active centers which could be addressed through the c o n d u c t i v e b a c k b o n e . S u c h s y s t e m s p o t e n t i a l l y offer t h e u n i q u e p o s s i b i l i t y of controlling the electronic structure of the active centers through the e l e c t r o n i c s t a t e o f t h e c o n d u c t i n g p o l y m e r . In t h i s a p p r o a c h , t h e c o n d u c t i v e p o l y m e r c o u l d u l t i m a t e l y a c t a s a m o l e c u l a r w i r e a b l e to i n p u t o r o u t p u t an a c t i v e c e n t e r . T h e r e f o r e , t h e first s t e p in t h i s field is t h e r e a l i z a t i o n of an interface between the conducting polymer and the active center. Some papers using these new concepts have already been published. T h e u s e o f s u b s t i t u t e d p y r r o l e in o r d e r t o a d d r e s s n i t r o x i d e [2] or v i o l o g e n g r o u p [3] h a s b e e n r e p o r t e d . P o l y m e r i z a t i o n o f t r a n s i t i o n m e t a l c o m p l e x e s c o n t a i n i n g p y r r o l e [ 4 - 6 ] o r a n i l i n e [7] h a s p r o d u c e d t h i n films b u t t h e polymer backbone was poorly conductive and not electroactive. Substituted aromatic amines might prove to be useful here because they could interface a p o l y a n i l i n e film t h r o u g h t h e a m i n o g r o u p a n d a n a c t i v e c e n t e r t h r o u g h t h e s u b s t i t u e n t w h i l e r e t a i n i n g c e r t a i n s e m i c o n d u c t i n g p r o p e r t i e s . If so, t h i s
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c o u l d a l l o w the active c e n t e r to b e s e n s i t i v e to the a d j u s t a b l e e l e c t r o n i c s t r u c t u r e o f the polyaniline film. T h e wide v a r i e t y o f s u b s t i t u t e d a r o m a t i c a m i n e s c o m m e r c i a l l y available w o u l d t h e n p r o v i d e a l m o s t u n l i m i t e d possibilities in the design o f t h e s e n e w m a t e r i a l s . T h e p o l y m e r i z a t i o n of s e v e r a l s u b s t i t u t e d anilines h a s a l r e a d y b e e n a c h i e v e d in the p a s t few years. S h e n g l o n g et al. s h o w e d t h a t m e t a - t o l u i d i n e a n d o r t h o - t o l u i d i n e c o u l d p o l y m e r i z e , b u t n o t nitroaniline [8]. M o r e r e c e n t l y , o r t h o - m e t h o x y a n i l i n e w a s s h o w n to yield a n e w soluble p o l y m e r [9]. The m o r e e x t e n s i v e w o r k of Cattarin et al. [10] o n a s e r i e s o f ortho- and m e t a s u b s t i t u t e d anilines clearly indicated t h a t e l e c t r o p o l y m e r i z a t i o n w a s f a v o r e d b y e l e c t r o n - d o n a t i n g s u b s t i t u e n t s a n d also t h a t it w a s m o r e effective on polyaniline-modified electrode. W e also s t a r t e d to i n v e s t i g a t e t h e e l e c t r o p o l y m e r i z a t i o n o f a m i n o a r o m a t i c c o m p o u n d s o n a gold e l e c t r o d e p r e c o a t e d with a thin p o l y a n i l i n e film to d e t e r m i n e w h e t h e r o r n o t the o u t c o m e of t h e r e a c t i o n is s e n s i t i v e to t h e polyaniline film a n d to d e t e r m i n e the p o s s i b i l i t y o f i n t e r f a c i n g films of different m o l e c u l a r s t r u c t u r e . This p a p e r r e p o r t s r e s u l t s o b t a i n e d with ortho- a n d m e t a - m e t h o x y a n i l i n e s , p a r a - M e t h o x y a n i l i n e is n o t c o n s i d e r e d s i n c e Geni~s et al. [11] h a v e s h o w n t h a t its e l e c t r o p o l y m e r i z a t i o n l e a d s to n o n p o l y m e r i c material.
Experimental All t h e c h e m i c a l s u s e d in this s t u d y w e r e r e a g e n t g r a d e a n d w e r e u s e d w i t h o u t f u r t h e r purification. T h e e l e c t r o d e s w e r e p r e p a r e d b y v a p o r d e p o s i t i o n o f 3 0 0 0 /~ of gold o n a g l a s s p l a t e w h i c h w a s p r e c o a t e d with 2 0 0 /~ of c h r o m i u m as an a d h e s i v e layer. T h e m e t a l d e p o s i t i o n w a s m a d e t h r o u g h a m a s k p r o d u c i n g a m e t a l s u r f a c e a r e a e q u a l to 0.5 c m 2. T h e g o l d / p o l y a n i l i n e e l e c t r o d e s w e r e 2 0 0 0 ~ thick. T h e films w e r e p r e p a r e d b y s w e e p i n g the v o l t a g e a p p l i e d to the e l e c t r o d e u s i n g a s w e e p r a t e o f 100 m V / s . All t h e electrochemical experiments were made using a calomel reference electrode a n d a g o l d wire c o u n t e r e l e c t r o d e . T h e a n a l y s e s w e r e p e r f o r m e d with IR c o m p e n s a t i o n a n d u s i n g a PAR 3 7 2 p o t e n t i o s t a t . A 18C NMR e x p e r i m e n t w a s p e r f o r m e d o n a CXP 3 0 0 B r u c k e r i n s t r u m e n t at 7 5 . 4 7 MHz. F o r this c h a r a c t e r i z a t i o n , t h i c k films o f p o l y a n i l i n e w e r e g e n e r a t e d o n a 10 c m 2 e l e c t r o d e u s i n g a 2 M HCI solution. T h e film w a s t h e n p o l a r i z e d f o r h a l f a n h o u r at - 2 0 0 m V in o r d e r to b e r e d u c e d as c o m p l e t e l y a s p o s s i b l e . It w a s t h e n p e e l e d off a n d dried u n d e r v a c u u m . C o n d u c t i v i t y m e a s u r e m e n t s w e r e c a r r i e d o u t u s i n g t w o parallel g o l d e l e c t r o d e s o n w h i c h p o l y m e r s w e r e d e p o s i t e d . T h e i r d i s t a n c e (typically 60 /zm, m e a s u r e d with a n o p t i c a l mic r o s c o p e ) is s m a l l e n o u g h to allow o v e r l a p p i n g o f t h e t w o p o l y m e r films. T h e t h i c k n e s s o f t h e film w a s m e a s u r e d b y M i n o t ' s m e t h o d u s i n g a n interf e r o m e t e r a n d a n i n t e r f e r e n c e filter ( 5 4 6 n m ) e q u i p p e d w i t h a c a m e r a . A c u r r e n t t h r o u g h t h e s a m p l e w a s d e l i v e r e d b y a stabilized c u r r e n t g e n e r a t o r . The potential difference between the two electrodes was m e a s u r e d using a
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Keithley 155 m i c r o v o l t m e t e r null d e t e c t o r . I n f r a r e d s p e c t r a w e r e r e c o r d e d with a P e r k i n - E l m e r 5 5 0 s p e c t r o m e t e r .
Results
Monomc~ oxidation T h e o x i d a t i o n p o t e n t i a l s of meta- a n d o r t h o - m e t h o x y a n i l i n e s on a gold e l e c t r o d e m e a s u r e d b y cyclic v o l t a m m e t r y (CV) u s i n g a 2 M H2SO4/0.01 M m o n o m e r solution, are 1 0 0 0 and 1 0 7 0 mV, r e s p e c t i v e l y . No film g r o w t h is detected under these electrochemical conditions. On a g o l d / p o l y a n i l i n e e l e c t r o d e film g r o w t h o c c u r s . T h e m o n o m e r oxidation signal w a s n o t a b l e to b e m e a s u r e d due to t h e s e c o n d o x i d a t i o n signal of the polyaniline film (Ep, is 9 0 0 mV) w h i c h c h a r a c t e r i z e s t h e s w i t c h i n g of polyaniline b e t w e e n its s e c o n d a n d third s t a t e s [12]. The f o r m e r state is stable u n d e r the e l e c t r o c h e m i c a l c o n d i t i o n s u s e d here. The latter one, w h i c h is d e s c r i b e d either as a p o l y i m i n e [ 13 ] or a p o l y p h e n y l n i t r e n i u m [ 14 ] material, is v e r y r e a c t i v e t o w a r d s v a r i o u s c h e m i c a l s . If t h e r e are no aniline units in t h e cell, d e g r a d a t i o n o f the p o l y m e r o c c u r s w h e n polyaniline is cycled t h r o u g h this state. T h e d e g r a d a t i o n p r o c e s s h a s b e e n d e s c r i b e d e i t h e r as a r e a c t i o n with w a t e r leading to b e n z o q u i n o n e [15, 16] or an internal c o u p l i n g r e a c t i o n b e t w e e n t w o a d j a c e n t p o l y m e r i c chains leading to a r e t i c u l a t e d p o l y m e r i c m a t e r i a l [17]. W i t h aniline units in the cell, film g r o w t h o c c u r s as s o o n as s o m e polyaniline is oxidized in its third s t a t e [12] a n d the d e g r a d a t i o n p r o c e s s can b e minimized. Stilwell a n d P a r k Su-Moon [18] h a v e s h o w n t h a t the o x i d a t i o n p r o d u c t r e s p o n s i b l e for g r o w t h is a s u r f a c e - b o u n d s p e c i e s and t h a t the r e a c t i o n is n o t limited b y m a s s t r a n s f e r of aniline units t o w a r d the electrode. W h e n m e t h o x y a n i l i n e units are in the cell instead of aniline units, a similar b e h a v i o r is o b s e r v e d . In fact, p o l y ( m e t h o x y a n i l i n e ) is g e n e r a t e d w h e n the e l e c t r o d e p o t e n t i a l r e a c h e s t h e f o o t of the s e c o n d o x i d a t i o n signal of the p r e c o a t e d p o l y a n i l i n e film. T h e r e f o r e , the polyaniline film c a t a l y z e s the e l e c t r o p o l y m e r i z a t i o n o f m e t h o x y a n i l i n e in a similar w a y to its catalysis of t h e e l e c t r o p o l y m e r i z a t i o n o f aniline. It is i n t e r e s t i n g to n o t e t h a t o x i d a t i o n of t h e m o n o m e r d o e s n o t s e e m to b e n e c e s s a r y f o r film growth.
Electropolymerization a n d f i l m properties W e t h e n tried to g r o w a film u n d e r t h e following e x p e r i m e n t a l conditions: 2 M H e S O J 0 . 1 M m o n o m e r , a p o t e n t i a l s w e e p f r o m - 2 0 0 mV to a n i n v e r s i o n p o t e n t i a l d e p e n d i n g o n t h e e l e c t r o d e u s e d (gold or g o l d / p o l y a n i l i n e e l e c t r o d e ) . The i n v e r s i o n p o t e n t i a l w a s c h o s e n to k e e p the o x i d a t i o n c u r r e n t d e n s i t y at a low v a l u e [12]. W e c h o s e t h e s e c o n d i t i o n s b e c a u s e t h e y p r o v e d to b e a d e q u a t e f o r the p o l y m e r i z a t i o n o f aniline on g o l d e l e c t r o d e s , l e a d i n g to v e r y clean p o l y a n i l i n e films [12]. T h e e l e c t r o o x i d a t i o n w a s also p e r f o r m e d in 2 M H2SO4/0.3 M m o n o m e r solution on t h e p u r e gold e l e c t r o d e . T h e
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(b) ~rav Fig. 1. Electropolymerization of methoxyaniline in a 2 M H2SO4/0.1 M monomer solution on a gold/polyaniline electrode: (a) ortho-methoxyaniline; (b) rneta-methoxyaniline. E/mV
g r o w i n g p r o c e s s t a k i n g p l a c e o n a g o l d / p o l y a n i l i n e e l e c t r o d e is s h o w n in Fig. l ( a ) f o r o r t h o - m e t h o x y a n i l i n e a n d Fig. 1 (b) f o r m e t a - m e t h o x y a n i l i n e . The CV r e s p o n s e s o f t h e films are r e p o r t e d in Figs. ( 2 ) - ( 5 ) . T h e s e v o l t a m m o g r a m s clearly s h o w t h a t the n a t u r e o f the e l e c t r o d e and the m e t h o x y g r o u p p o s i t i o n h a v e a d r a s t i c effect on t h e g r o w i n g p r o c e s s e s a n d on the o u t c o m e of t h e r e a c t i o n .
Poly (o-methoxyaniline) films F i l m s g e n e r a t e d with t h e o r t h o - m e t h o x y a n i l i n e b e h a v e in the s a m e w a y as polyaniline. In fact, t h e i r CV r e s p o n s e s , s h o w n in Figs. (2) a n d ( 3 ) , are s o s i m i l a r to t h o s e of p o l y a n i l i n e films t h a t t h e p r e s e n c e of OCH3 g r o u p s in the m a t e r i a l c a n b e q u e s t i o n e d . W e t h e r e f o r e u s e d n o n e l e c t r o c h e m i c a l t e c h n i q u e s to a s c e r t a i n this point. IR s p e c t r o s c o p y o f a p r e s s e d p e l l e t s h o w s C - O a n d CH3 a b s o r p t i o n b a n d s at 1 0 2 0 a n d 2 9 2 0 c m - 1. T h i s clearly i n d i c a t e s t h a t s o m e OCH3 g r o u p s a r e still p r e s e n t in t h e m a t e r i a l b u t d o e s n o t give a n y q u a n t i t a t i v e m e a s u r e o f the r a t i o o f OCH3 g r o u p s still p r e s e n t . W e t h e r e f o r e c a r r i e d o u t a 13C h i g h - r e s o l u t i o n NMR c h a r a c t e r i z a t i o n o f this material. CH3 g r o u p signal is p r e s e n t a n d t h e 'shift' is t h e s a m e as t h e o n e o b s e r v e d f o r anisole CH3 g r o u p s , i.e. 55 p p m / T M S . T h e b e n z e n i c c a r b o n s a p p e a r a s a b r o a d signal b e t w e e n 100 a n d 190 p p m f r M S a n d t h e ratio o f aliphatic to a r o m a t i c c a r b o n s is f o u n d to b e 1:6. This clearly s h o w s t h a t no ( o r m i n i m a l ) OCH3 g r o u p e l i m i n a t i o n t a k e s p l a c e d u r i n g t h e e l e c t r o p o l y merization. A significant d i f f e r e n c e b e t w e e n this s p e c t r u m a n d the p r e v i o u s l y p u b l i s h e d s p e c t r a o f polyaniline [19, 20] c a n b e o b s e r v e d . A l t h o u g h w e t o o k e x t e n s i v e c a r e to o b t a i n t h e m a t e r i a l in t h e r e d u c e d s t a t e ( s e e E x p e r i m e n t a l Section) t h e b e n z e n i c c a r b o n signals a r e v e r y b r o a d a n d n o t a s well defined as is
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Fig. 2. CV r e s p o n s e in 2 M H2SO 4 s o l u t i o n of a film g e n e r a t e d on a gold e l e c t r o d e with o r t h o m e t h o x y a n i l i n e in a 2 M H~SO4/0.3 M m o n o m e r solution. Fig. 3. CV r e s p o n s e in 2 M H2SO4 s o l u t i o n of a film g e n e r a t e d o n a g o l d / p o l y a n i l i n e electrode w i t h o r t h o - m e t h o x y a n i l i n e in a 2 M H2SO,/0.1 M m o n o m e r solution.
r e p o r t e d f o r p o l y a n i l i n e w h e n fully r e d u c e d . This r e s u l t i n d i c a t e s t h a t s o m e e l e c t r o n i c c o n d u c t i v i t y is still p r e s e n t in o u r s a m p l e . This m i g h t r e v e a l e i t h e r t h a t p o l y ( o r t h o - m e t h o x y a n i l i n e ) is c o n d u c t i v e in t h e r e d u c e d s t a t e as r e p o r t e d b y M a c i n n e s a n d F u n t [9] or t h a t o u r v e r y t h i c k s a m p l e w a s n o t fully r e d u c e d . C o n d u c t i v i t y m e a s u r e m e n t s w e r e n o t p e r f o r m e d on r e d u c e d p o l y ( o r t h o methoxyaniline). M o n o m e r c o n c e n t r a t i o n h a s a g r e a t influence on t h e o u t c o m e o f t h e r e a c t i o n w h e n t h e g o l d e l e c t r o d e is used. T h e o x i d a t i o n of o r t h o - m e t h o x y a n i l i n e b y s w e e p i n g t h e p o t e n t i a l a p p l i e d to t h e e l e c t r o d e b e t w e e n - 100 a n d 1 0 0 0 m V with a 0.1 M m o n o m e r solution yields a red, soluble e l e c t r o a c t i v e s p e c i e s w h i c h o x i d i z e s at 3 8 0 m V a n d r e d u c e s at 3 4 0 mV. T h i s p r o d u c t m i g h t be b e n z i d i n e w h i c h is k n o w n to b e r e d in its oxidized f o r m . On the o t h e r h a n d , t h e s a m e p r o c e d u r e u s i n g a 0.3 M m o n o m e r solution l e a d s t o a p o l y a n i l i n e analog material. W h e n a g o l d / p o l y a n i l i n e e l e c t r o d e is used, m o n o m e r c o n c e n t r a t i o n d o e s n o t s e e m to influence t h e o u t c o m e o f the r e a c t i o n so drastically. W i t h a 0.1 M m o n o m e r solution p o l y ( o r t h o - m e t h o x y a n i l i n e ) g r o w t h is easily o b s e r v e d .
122 T h e s a m e o b s e r v a t i o n h o l d s with 0.01 M m o n o m e r solution as a l r e a d y m e n t i o n e d . T h e r e f o r e , polyaniline s e e m s to a c t as a l a y e r c a p a b l e of i n c r e a s i n g the local m o n o m e r c o n c e n t r a t i o n o n t o the e l e c t r o d e . This effect will be d i s c u s s e d b e l o w in t e r m s o f specific a d s o r p t i o n in the light of frontier orbital interactions. It t u r n s o u t to b e i m p o s s i b l e , u n d e r o u r e x p e r i m e n t a l conditions, to e l e c t r o s y n t h e s i z e a p o l y ( o r t h o - m e t h o x y a n i l i n e ) film with no m i d d l e p e a k in the 4 5 0 m V region, e v e n if t h e c u r r e n t density u s e d during e l e c t r o p o l y m e r i z a t i o n is c a r e f u l l y c o n t r o l l e d . This o b s e r v a t i o n is in c o n t r a s t to polyanlline e l e c t r o p o l y m e r i z a t i o n a n d d o e s n o t d e p e n d on the n a t u r e o f the e l e c t r o d e . A l t h o u g h the origin o f this p e a k is n o t clearly defined, o n e i n t e r p r e t a t i o n p r o p o s e d is b a s e d on ortho c o u p l i n g s [21 ]; t h e r e f o r e the difference o b s e r v e d h e r e b e t w e e n p o l y a n i l i n e a n d poly(ortho-methoxyaniline) m i g h t reflect a g r e a t e r ratio on non-para c o u p l i n g s v e r s u s para c o u p l i n g s with the latter. This m i d d l e p e a k is l o c a t e d at 4 0 0 mV f o r the o x i d a t i o n p r o c e s s a n d at 3 8 0 mV f o r the r e d u c t i o n p r o c e s s w h e t h e r is s t a n d s at 540 o r 5 2 0 m V f o r polyaniline. T h i s m a y indicate t h a t t h e p e a k o b s e r v e d with p o l y ( o r t h o m e t h o x y a n i l i n e ) is n o t c o m p l e t e l y similar to t h a t of polyaniline. T h e s e d i f f e r e n c e s will be d i s c u s s e d b e l o w in t e r m s of f r o n t i e r orbital interactions. T h e influence o f p H o n t h e e l e c t r o c h e m i c a l signal is similar to the o n e o b s e r v e d with p o l y a n i l i n e the films o f w h i c h are e l e c t r o i n a c t i v e in n e u t r a l solution, w h i c h i n d i c a t e s that, like polyaniline, the p r o t o n i c d o p i n g p r o c e s s is p r e s e n t with t h e s e m a t e r i a l s . T h e s e m a t e r i a l s e x h i b i t e l e c t r o c h r o m i s m . T h e y are t r a n s p a r e n t y e l l o w in the r e d u c e d state, g r e e n in the first oxidized s t a t e a n d b l u e - b l a c k in t h e s e c o n d o x i d a t i o n state. C o n d u c t i v i t y m e a s u r e m e n t s p e r f o r m e d with a film in its first o x i d a t i o n s t a t e indicate a c o n d u c t i v i t y v a l u e o f 10 -2 ~ - ~ c m - 1 w h i c h is similar to p o l y a n i l i n e p r e p a r e d u n d e r the s a m e c o n d i t i o n s . T h e r e f o r e , the OCH3 g r o u p d o e s n o t influence drastically the film conductivity. T e m p e r a t u r e d e p e n d e n c e s h o w s a s e m i c o n d u c t i n g b e h a v i o r with a v e r y low t h e r m a l a c t i v a t i o n e n e r g y of 0 . 0 6 eV w h i c h is a g a i n similar to polyaniline. In c o n c l u s i o n , t h e t w o films g e n e r a t e d with o r t h o - m e t h o x y a n i l i n e are similar to polyaniline. T h e p r e c o a t e d polyaniline film m a k e s the e l e c t r o s y n t h e s i s easier, this effect b e i n g similar to t h e o n e o b s e r v e d on i n c r e a s i n g t h e m o n o m e r c o n c e n t r a t i o n in t h e solution.
Poly(m-methoxyaniline) films T h e CV r e s p o n s e s o f t h e m a t e r i a l s o b t a i n e d f r o m the e l e c t r o p o l y m e r i z a t i o n of t h e m e t a - m e t h o x y a n i l i n e a r e s h o w n in Figs. (4) a n d (5). T h e y a r e v e r y different f r o m t h o s e o b t a i n e d with t h e o r t h o m o n o m e r . On a g o l d e l e c t r o d e , the o x i d a t i o n o f m e t a - m e t h o x y a n i l i n e b y s w e e p i n g t h e p o t e n t i a l a p p l i e d to t h e e l e c t r o d e b e t w e e n - 1 0 0 a n d 1 0 0 0 mV, w i t h a 0.1 M m o n o m e r solution, yields a v e r y thin film w h i c h h a s its m a i n o x i d a t i o n signal in t h e 4 5 0 m V region. T h e e l e c t r o d e p a s s i v a t e s partially a n d t h i c k
123
400
300
200
100
<
b: 100 mV/s
40 -
I O0
< -200
~o
-300
-40 l
l
I
0
400
800
E/mY
0
400
800
E1mV
Fig. 4. CV r e s p o n s e in 2 M H2SO 4 s o l u t i o n o f a film g e n e r a t e d o n a gold electrode with metam e t h o x y a n i l i n e in a 2 M H2S04/0.3 M m o n o m e r solution. Fig. 5. CV r e s p o n s e in 2 M H2SO4 s o l u t i o n of a film g e n e r a t e d on a g o l d / p o l y a n i l i n e electrode w i t h meta-methoxyaniline in a 2 M H2SO4/0.1 M m o n o m e r solution.
films a r e n o t g e n e r a t e d . T h e s a m e b e h a v i o r is o b s e r v e d w h e n a 0.3 M m o n o m e r s o l u t i o n is u s e d . On a g o l d / p o l y a n i l i n e e l e c t r o d e , t h e m o n o m e r o x i d a t i o n p r o c e s s g e n e r a t e s , in a first stage, the g r o w t h o f an e l e c t r o c h e m i c a l signal at 4 7 0 mV f o r the o x i d a t i o n p e a k a n d 4 3 0 m V f o r t h e r e d u c t i o n p e a k ( s e e Fig. l ( b ) ) . T h e s e p e a k s s t a n d at the s a m e p o s i t i o n as t h o s e o b s e r v e d w h e n a g o l d e l e c t r o d e is used. H o w e v e r , it is n o w p o s s i b l e to g e n e r a t e t h i c k e r films. D u r i n g this first s t e p no (or m i n i m a l ) g r o w t h of t h e 2 5 0 - 1 5 0 mV p e a k s , d u e to the p r e s e n c e o f t h e p r e c o a t e d polyanfline film, is o b s e r v e d . This i n d i c a t e s t h a t s o m e s o r t o f m a t e r i a l d e p o s i t s o n t o t h e e l e c t r o d e b u t t h a t this m a t e r i a l is n e i t h e r a k i n to p o l y a n i l i n e n o r to p o l y ( o r t h o - m e t h o x y a n i l i n e ) . In a s e c o n d state, p o l y a n i l i n e signal a l t e r a t i o n t a k e s p l a c e a n d its c h a r a c t e r i s t i c p e a k s a r e shifted anodically. This i n d i c a t e s s o m e s o r t o f m o l e c u l a r i n t e r a c t i o n b e t w e e n t h e p r e c o a t e d p o l y a n i l i n e film a n d t h e d e p o s i t i n g material. T h e p H influence on t h e e l e c t r o c h e m i c a l r e s p o n s e is similar to t h e o n e o b s e r v e d with polyaniline; this film is e l e c t r o i n a c t i v e in n e u t r a l solution, w h i c h i n d i c a t e s t h a t t h e p r o t o n i c d o p i n g p r o c e s s is p r e s e n t a s in polyaniline. This s h o w s e i t h e r t h a t t h e d e p o s i t i n g m a t e r i a l g r o w s o n t o p of t h e p r e c o a t e d
124
polyaniline film or that it grows within the p r e c o a t e d polyaniline film retaining the same type of molecular structure. No additional electrochromism is visually detected when the film is cycled through the 4 7 0 - 4 3 0 mV peaks. Solubility test shows that the film is slightly soluble in ethanol and in acetonitrile but it has been impossible to solubilize the deposited material to the point where the electrochemical signal of the p r e c o a t e d polyaniline film is restored. Alteration of this signal is permanent, indicating that a strong interaction takes place between polyaniline and the deposited material. This tends to indicate that the 'poly(meta-methoxyaniline)' is chemically bonded to polyaniline.
Discussion At this point, several conclusions can be drawn. (i) The o r t h o and the m e t a m o n o m e r s do not behave in a similar way toward the chemical reaction following the m o n o m e r electrochemical oxidation. This clearly shows that the whole process is under kinetic control since a th er mo d ynam i c control of the electropolymerization would lead to the same material, as shown in Scheme 1, and should t herefore be independent of the substituent position. (ii) The controlling step of the reaction is the chemical coupling between m o n o m e r units which follows the electrochemical oxidation. (iii) The polyaniline film plays a key role in the process. It has an important catalytic effect, already mentioned by several publications [10, 12, 15], which acts on two different levels. First, the polyaniline film acts as a concentration layer through specific adsorption. Secondly, polyaniline in its second oxidation form (third state) seems capable of reacting on an adsorbed m o n o m e r i c species. However, the polyaniline film has no influence on the regioselectivity of the coupling reaction since materials obtained with the same m o n o m e r have identical electrochemical characteristics. This means that the regioselectivity of the coupling reaction is controlled by the nature of the m o n o m e r used.
NH
~
X
X
NH
S c h e m e 1.
125
This, t o g e t h e r with the n a t u r e o f t h e s p e c i e s involved h e r e , i.e. soft e l e c t r o p h i l e or n u c l e o p h i l e , l e a d s us to think t h a t an u n d e r s t a n d i n g of the p o l y m e r i z a t i o n p r o c e s s c o u l d b e f o u n d in r a d i c a l - c a t i o n f r o n t i e r orbital interactions. E v e n if this a p p r o a c h really a p p e a l s to us, we h a v e to p o i n t out t h a t it will only p r o v i d e qualitative g u i d e l i n e s since s e v e r a l effects a r e n o t c o n s i d e r e d in the a p p r o a c h s u c h as c o u l o m b i c i n t e r a c t i o n s o r s o l v e n t effect. W e n e v e r t h e l e s s p e r f o r m e d an e x t e n d e d Hiickel c a l c u l a t i o n to o b t a i n t h e m o l e c u l a r orbital (MO) s c h e m e of t h e m o n o m e r s u s e d in this study. W e also c o m p u t e d the MO o f aniline f o r c o m p a r i s o n p u r p o s e s a n d t h e MO of polyaniline o l i g o m e r s to o b t a i n a m o l e c u l a r orbital s c h e m e d e s c r i b i n g to s o m e e x t e n t the c h e m i c a l m o d i f i c a t i o n i n d u c e d on the e l e c t r o d e s u r f a c e b y t h e p r e c o a t e d polyaniline film. S t a n d a r d b o n d l e n g t h s a n d a n g l e s w e r e u s e for this c o m p u t a t i o n [22]. T h e t o r s i o n a n g l e b e t w e e n a d j a c e n t r i n g s w a s c o n s i d e r e d to b e zero. H o w e v e r , w e h a v e allowed for g e o m e t r y r e l a x a t i o n upon protonation. In a r e c e n t p u b l i c a t i o n [23], w e s h o w e d , using the f r o n t i e r orbital a p p r o a c h , t h a t the p o l y m e r i z a t i o n p r o c e s s of aniline w a s c o n t r o l l e d by t h e HOMO i n t e r a c t i o n s o f the s p e c i e s i n v o l v e d in the c o u p l i n g r e a c t i o n w h i c h follows the m o n o m e r o x i d a t i o n . In o r d e r t o e x p l a i n the head-to-tail c o u p l i n g t h a t is d o m i n a n t in the p r o c e s s , w e h a v e s u g g e s t e d t h a t the c o u p l i n g r e a c t i o n is not of a radical c o u p l i n g t y p e a n d t h a t it involves the two following species: H2
+
H
Nbk3
This is, u p to now, t h e o n l y d e s c r i p t i o n o f t h e p o l y m e r i z a t i o n p r o c e s s w h i c h is c o m p a t i b l e with t h e r e g i o s p e c i f i e i t y o f the c o u p l i n g r e a c t i o n b e t w e e n m o n o m e r i c units. This d o e s n o t m e a n t h a t the d e s c r i p t i o n is c o r r e c t b u t we will t a k e it as a w o r k i n g h y p o t h e s i s to e x a m i n e p o s s i b l e s u b s t i t u e n t effects. T h e following orbital s c h e m e s will t h e r e f o r e p r e s e n t the H O M O of t h e s e t w o s p e c i e s with m e t h o x y s u b s t i t u e n t . T h e HOMO o f o r t h o - a n d m e t a - m e t h o x y a n i l i n e s f o r t h e p r o t o n a t e d a n d the n o n p r o t o n a t e d f o r m s a r e s h o w n in S c h e m e 2. S c h e m e 3 illustrates the H O M O o f aniline a n d the H O M O o f t h e d i m e r a n d trimer, w h i c h i n d i c a t e s the c h e m i c a l m o d i f i c a t i o n t r e n d o f t h e e l e c t r o d e i n d u c e d b y t h e p r e c o a t e d polyaniline film. As e x p e c t e d , t h e m e t h o x y g r o u p s t r o n g l y m o d i f i e s t h e H O M O s of the m o n o m e r w h e n c o m p a r e d to aniline. T h i s effect is p a r t i c u l a r l y p r o n o u n c e with t h e p r o t o n a t e d f o r m o f the m o n o m e r w h e r e t h e p a r a c a r b o n of the a m i n o g r o u p l o s e s a lot o f e l e c t r o n d e n s i t y a n d is n o l o n g e r t h e c a r b o n with the h i g h e s t coefficient. N e w sites a r e a c t i v a t e d a n d this p h e n o m e n o n is highly d e p e n d e n t o n t h e p o s i t i o n o f t h e m e t h o x y g r o u p as s e e n in S c h e m e 2. This a c t i v a t i o n is localized o n t h e c a r b o n in p a r a o f t h e m e t h o x y g r o u p . T h e r e f o r e , w h e n in o r t h o of t h e a m i n o g r o u p , t h e m e t h o x y g r o u p a c t i v a t e s a c a r b o n in m e t a o f the a m i n o g r o u p a n d t h u s l e a d s to a m o n o m e r with t w o a d j a c e n t
126
Deprotonated form
Protonated form
METAMETHOXYANILINE
o 36
fill3
N 0.06
°,, o
\ 0.09
Q
o.s3
o.o,
-0.40
E=-12.09 eY
0.33
0.40
E=-12.39 eV o, 37
ORTHOMETHOXYANILINE
/o,o
0.34 ~
0.29
o.s,
-0.22
E=-11.95 eV
0
0.43
0
E=-12.39 eV
Scheme 2. Frontier orbital approach and methoxyaniline electropolymerization on a gold electrode.
carbons which can easily react with radicals. When in m e t a of the amino group, the m e t h o x y group activates a carbon in o r t h o of the amino group and thus leads to a m o n o m e r with activated carbon similar to aniline but with a stronger activation on one of the o r t h o carbons instead of the p a r a carbon. In o th er words, frontier orbital considerations indicate that the o r t h o and the m e t a m o n o m e r s should not behave identically toward the chemical reaction which follows m o n o m e r electrooxidation. This is what is observed experimentally. With m e t a substituent o r t h o coupling should be dominant, whereas p a r a and m e t a couplings should be p r e p o n d e r a n t for the o r t h o isomer. This is consistent with the CV r es po nses since ortho-methoxyaniline oxidation generates a polyaniline-like material w h e r e a s m et a-m et hoxyani l i ne oxidation does not. The 470--420 peaks observed in the CV response of the thin film o f 'poly(meta-methoxyaniline)' a p p e a r to be the ort ho coupling signature. The 2 5 0 - 1 5 0 mV signal seems to express p a r a coupling and the 4 0 0 - 3 8 0 mV couple might be due to m e t a coupling. This last point should be taken carefully; indeed, no experimental p r o o f of the presence in poly(ortho-
127
Deprotonated form
Protonated form
Monomer
0.15 -0,36
0.21
0.48
-0.32
0.52 0.57
0.13
0,15 .0.36
0.25
Dimer 0.26 -0.22
-
0
.0.30
0.25 -0.12 ~ N
-0.03
o ~
-~. 7
2--~
o-~
,jo. .0.29
.o.,
0.07
""
0~9
~.,~
~
.0.33
0.06
o.~ ~.,~
.0.12
Trimer -0.48
~ 0.05 -0.14
o.35
~
N
xr-O" 0.01 -0.17
.0.42
0.43
Scheme 3. Frontier orbital approach and methoxyaniline electropolymerization on a gold/ polyaniline electrode. m e t h o x y a n i l i n e ) o f m o n o m e r c o u p l e d t h r o u g h m e t a c o u p l i n g is available (IR a n d 13C NMR p r o v e d to be i n a d e q u a t e to a s c e r t a i n this point). W e t h e r e f o r e only s u g g e s t this last p o i n t o n the basis o f the f r o n t i e r orbital a p p r o a c h w h i c h s e e m s to rationalize fairly well s u b s t i t u e n t effects on the electrop o l y m e r i z a t i o n o f s u b s t i t u t e d anilines. The c a l c u l a t e d HOMO a n d LUMO of the d i m e r a n d the t r i m e r s h o w n in S c h e m e 3 give s o m e indication o f the e l e c t r o n i c c h a n g e s i n d u c e d by the p r e s e n c e on the gold s u b s t r a t e of a p r e c o a t e d polyaniline film. T h r e e features are i n t e r e s t i n g t o n o t e w h e n the chain length increases:
128 (1) The carbon atoms become equivalent. The para carbon reactivity slowly becomes identical to that of the other carbons, especially ortho carbons. (2) H O M O e n e r g y i n c r e a s e s and LUMO e n e r g y d e c r e a s e s . (3) The e l e c t r o n i c d en s i t y on the t e r m i n a l n i t r o g e n of the d e p r o t o n a t e d f o r m s is l o w e r e d . T h e s e r e s u l t s a r e g i v e n i n T a b l e s 1 a n d 2. T h e first p o i n t i n d i c a t e s t h a t the m o n o m e r i c unit m u s t r e a c t t h r o u g h its c a r b o n a t o m s r a t h e r t h a n i ts n i t r o g e n a t o m , p r o v i d e d t h a t f i l m g r o w t h involves coupling between a monomeric unit and a chain of several aniline units bonded to the electrode, and provided that frontier orbital interactions c o n t r o l the c o u p l i n g r e a c t i o n s . If not, t h e n i t r o g e n could r e a c t on any c a r b o n atom of the terminal aniline unit of the chain. Therefore, a progressive loss in t h e r e g i o s e l e c t i v i t y o f t h e c o u p l i n g r e a c t i o n s h o u l d b e n o t i c e d , w h i c h is n o t t h e c a s e . F u r t h e r m o r e , if t h e c o u p l i n g r e a c t i o n i n v o l v e s t h e n i t r o g e n TABLE 1 Terminal N coefficients, p a r a carbon coefficients, and HOMO and LUMO energies of protonated and deprotonated forms Terminal N coeff, in HOMO
Deprotonated f o r m Monomer Dimer Trimer
0.57 0.39 0.28
Protonated f o r m Monomer Dimer Trimer
0.02 0.07 0.04
para Carbon coeff, in HOMO
HOMO energy (eV)
LUMO energy (eV)
0.46 0.24 0.14
- 12.13 - 11.59 - 11.44
- 8.36 - 8.24 -8.22
0.52 - 0.33 0.18
- 12.60 - 11.98 - 11.52
- 8.35 - 8.25 - 8.20
-
TABLE 2 Compared reactivity of the carbon atoms Cpara coeff.
Co,~o coeff.
C,~t~ coeff.
Protonated f o r m Monomer Dimer Trimer
0.52 - 0.33 0.18
- 0.30 0.26 -0.17
0.22 0.08 0.03
Deprotonated form Monomer Dimer Trimer
0.46 - 0.24 0.14
- 0.36 0.20 -0.14
0.15 0.05 0.03
I29 a t o m o f t h e m o n o m e r i c unit, t h e n e l e c t r o p o l y m e r i z a t i o n o f m e t a - a n d o r t h o m e t h o x y a n i l i n e s on the p r e c o a t e d p o l y a n i l i n e film s h o u l d give the s a m e p r o d u c t , s i n c e the r e a c t i v i t y o f t h e n i t r o g e n a t o m o f the m o n o m e r i c unit a p p e a r s i n d e p e n d e n t of t h e m e t h o x y position. This is n o t o b s e r v e d e x p e r imentally. It is w o r t h n o t i n g t h a t this d e d u c t i o n is strictly valid for t h e first l a y e r s o f p o l y ( m e t h o x y a n i l i n e ) b u t n o t for the following layers. T h e catalytic effect c a n b e u n d e r s t o o d t h r o u g h the influence o f t h e chain length on the H O M O e n e r g y o f the o l i g o m e r . It s e e m s t h a t t h e r e is a specific a d s o r p t i o n o f m o n o m e r i c units o n t o t h e p o l y m e r i c m a t e r i a l . This specific a d s o r p t i o n c a n b e d e s c r i b e d in t e r m s o f w e a k f r o n t i e r orbital i n t e r a c t i o n s b e t w e e n t h e f r o n t i e r orbitals of the m o n o m e r i c unit a n d t h o s e o f the p o l y m e r i c chain. Let us m o d e l the p o l y m e r i c c h a i n b y the trimer. In its first oxidized s t a t e ( b e t w e e n 2 5 0 a n d 7 0 0 mV) t h e orbital s h o w n in S c h e m e 3 c o n t a i n s o n e or zero e l e c t r o n s . C o n s i d e r i n g t h e e n e r g y v a l u e s o f the f r o n t i e r orbitals o f the different s p e c i e s i n v o l v e d in t h e p r o c e s s , the m a i n i n t e r a c t i o n t a k e s p l a c e b e t w e e n t h e H O M O of the m o n o m e r i c unit w h i c h h a s n o t y e t b e e n o x i d i z e d a n d the SOMO or LUMO ( d e p e n d i n g on its e l e c t r o n p o p u l a t i o n ) of t h e trimer. O t h e r i n t e r a c t i o n s b e t w e e n f r o n t i e r orbitals h a v e to b e elimhaated b e c a u s e o f t h e h i g h e r e n e r g y gap. S o m e o v e r l a p b e t w e e n t h e two orbitals l e a d s to a w e a k c h e m i c a l b o n d . Since the e n e r g y of the t r i m e r orbital is h i g h e r t h a n t h e e n e r g y o f t h e m o n o m e r orbital, this w e a k i n t e r a c t i o n l o w e r s the e n e r g y o f t h e m o n o m e r orbital w h i c h is still d o u b l y o c c u p i e d as long as n o e l e c t r o n i c t r a n s f e r t a k e s place. In the m e a n t i m e , the e n e r g y level of t h e o l i g o m e r orbital is i n c r e a s e d a n d in t h e a b s e n c e of e l e c t r o n i c t r a n s f e r this orbital is e i t h e r singly o c c u p i e d or e m p t y . A n e w s p e c i e s ' c h a i n + a d s o r b e d m o n o m e r ' is t h e r e f o r e g e n e r a t e d , w h i c h is t h e r m o d y n a m i c a l l y m o r e stable t h a n the ' c h a i n + f r e e m o n o m e r ' s y s t e m a n d this stabilization m a i n l y c o n c e r n s t h e m o n o m e r i c p a r t o f the s y s t e m . T h e r e f o r e , f r o m a t h e r m o d y n a m i c p o i n t of v i e w it is e a s i e r to oxidize this s y s t e m t h a n the m o n o m e r i c unit. This d e s c r i p t i o n a p p e a r s to d e s c r i b e c o r r e c t l y the catalytic effect o b s e r v e d during the e l e c t r o p o l y m e r i z a t i o n of aniline a n d m e t h o x y a n i l i n e on a polyaniline electrode. T h e third p o i n t p r e d i c t s a loss o f r e g i o s p e c i f i c i t y o f t h e c o u p l i n g r e a c t i o n s w h e n the c h a i n l e n g t h i n c r e a s e s . T h i s is n o t o b s e r v e d e x p e r i m e n t a l l y since t h i c k p o l y a n i l i n e films w i t h n o m i d d l e p e a k (i.e. n o n o n - p a r a c o u p l i n g ) c a n b e g e n e r a t e d . T h e f r o n t i e r orbital m o d e l s e e m s , on t h a t point, i n a d e q u a t e . H o w e v e r , this i n a d e q u a c y m a y arise f r o m t h e i n h e r e n t limits i n t r o d u c e d b y t h e g e o m e t r i c d a t a w e u s e d to d e s c r i b e t h e t r i m e r . I n d e e d , polyaniline, in its third state, is d e s c r i b e d e i t h e r as a p o l y i m i n e [ 13 [ or a p o l y p h e n y l n i t r e n i u m [14] m a t e r i a l . T h e s e s p e c i e s h a v e g e o m e t r i c a l c h a r a c t e r i s t i c s w h i c h are different f r o m t h o s e t h a t w e used. W e t h e r e f o r e c o m p u t e d the m o l e c u l a r orbitals o f t h e t w o s p e c i e s s h o w n in S c h e m e 4 in o r d e r to o b t a i n a b e t t e r d e s c r i p t i o n o f p o l y a n i l i n e in its a c t i v e s t a t e t o w a r d s m o n o m e r s . T h e first s p e c i e s localizes the q u i n o n i c d e f o r m a t i o n o n t o t h e cycle with t h e t e r m i n a l n i t r o g e n . This is n o t c o m p l e t e l y satisfying since this quinonic d e f o r m a t i o n m u s t travel f r o m o n e cycle to a n o t h e r a l o n g the chain. T h e s e c o n d one,
130 H
\
-0.28 0 . ~1 - 0~. 4 8 ~~.~!i:~~i!~i 0.44 -0.07
-0.08~
~)0.18 0.28 -0.28
(~/'-0.18-0.08 0.14 -0.07
E = -10.84
eV H
\
d
With quinonic perturbation on each cycle
. o 0.25~-0.18 "~C ~ 0,20 @
0.20:9.0.3 0 -0.17 -0.17
Scheme4.
E = -10.64 eV
131 w h i c h d e s c r i b e s a t r i m e r with q u i n o n i c d e f o r m a t i o n on e a c h cycle, is n o t c h e m i c a l l y realistic. In a c e r t a i n sense, t h e H O M O of this h y p o t h e t i c a l s p e c i e s t e n d s to r e p r e s e n t the a v e r a g e r e a c t i v i t y o f t h e c y c l e s i n d u c e d by the d e f o r m a t i o n delocalization. The r e a c t i n g orbitals of t h e s e s p e c i e s a r e s h o w n in S c h e m e 4. T h r e e f e a t u r e s are i n t e r e s t i n g to n o t e w h e n c o m p a r i n g t h e s e orbitals with t h a t o f our previous trimeric species: (1) The c a r b o n a t o m s r e m a i n e q u i v a l e n t t o w a r d m o n o m e r i c attack. (2) The e n e r g i e s of t h e s e orbitals r e m a i n h i g h e r t h a n the H O M O e n e r g y of t h e m o n o m e r i c units. (3) The t e r m i n a l n i t r o g e n n o w h a s a n e l e c t r o n i c d e n s i t y w h i c h is c l o s e r to t h e o n e o f the n i t r o g e n in a m o n o m e r i c unit. T h e r e f o r e , it a p p e a r s t h a t t h e c o n c l u s i o n s d r a w n a b o v e s t a n d a n d t h a t the p r o g r e s s i v e loss o f regiospecificity, w h i c h t h e third p o i n t p r e d i c t e d , m a y b e due to a n i n a d e q u a t e m o d e l of p o l y a n i l i n e in its r e a c t i v e state. H o w e v e r , it s h o u l d p o i n t e d out t h a t t h e a p p r o a c h is not c o m p l e t e l y satisfying. A b e t t e r o n e w o u l d m o d e l t h e c h a i n a s a m i x e d v a l e n c e material. In s u c h a d e s c r i p t i o n the delocalization of the q u i n o n i c d e f o r m a t i o n o n the chain w o u l d lead to s e v e r a l s t a t e s c l o s e in e n e r g y in o r d e r t o allow the d e f o r m a t i o n to travel a l o n g the chain. T h e relative value o f t h e e n e r g y levels a s s o c i a t e d with t h e s e s t a t e s should give t h e i r relative p o p u l a t i o n s . T h e s t a t e c o r r e s p o n d i n g to the q u i n o n i c d e f o r m a t i o n localized o n t o the t e r m i n a l cycle with free n i t r o g e n is quite likely to b e statistically o c c u p i e d a n d m i g h t be r e s p o n s i b l e for r e a c t i o n s with m o n o m e r i c unit. W e h a v e p r o b a b l y r e a c h e d a limit h e r e for t h e f r o n t i e r orbital a p p r o a c h a n d e v e n if t h e r e is s o m e e v i d e n c e t h a t m o n o m e r i c units r e a c t via t h e i r c a r b o n a t o m s , the i n a d e q u a c y r e p o r t e d a b o v e relativizes this s t a t e m e n t . Conclusions This p a p e r s h o w s t h a t a w e a l t h o f n o v e l c h e m i s t r y can be d e v e l o p e d in the field o f s u b s t i t u t e d aniline p o l y m e r i z a t i o n . It also s u g g e s t s t h a t the f r o n t i e r orbital c o n c e p t is a useful t o o l to u n d e r s t a n d in a qualitative way, a n d in s o m e c a s e s p r e d i c t s t h e p r o c e s s e s i n v o l v e d in the e l e c t r o s y n t h e s e s . This a p p r o a c h p r o v e s to b e a d e q u a t e in o r d e r t o p r e d i c t m e t h o x y p o s i t i o n effects a n d t e n d s to i n d i c a t e t h a t t h e f i l m - g r o w i n g p r o c e s s involves the c o u p l i n g o f a m o n o m e r i c unit via o n e o f its c a r b o n a t o m s o n t o a chain. H o w e v e r , Hiickel c a l c u l a t i o n is n o t c o m p l e t e l y satisfying in o r d e r to m o d e l t h e c h a i n a n d we believe t h a t a b e t t e r d e s c r i p t i o n of polyaniline c a n b e found using a mixed valence approach. References 1 A. F. Diaz, J. F. Rubinson and H. Mark, Adv. Polym. Sci., 84 (1988) 113. 2 G. Bidan and D. Limosin, Ann. Phys. Colloq. I, Suppl. 1, 11 (1986) 5.
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