Chiral metals: amino acid-substituted conducting polypyrroles

Synthetic Metals, 39 (1990) 117-120

117

Short Communication

Chiral metals: D. D e l a b o u g l i s e

amino

acid-substituted

conducting

polypyrroles

a n d F. G a m i e r *

Laboratoire des Matdriaux Moldculaires, CNRS, 2 rue Dunant, 94320 Thiais (France)

(Received May 11, 1990; accepted July 3, 1990)

Abstract Chiral amino acid groups are substituted on the 3 position of pyrrole. These functionalized monomers are electropolymerized, the high conductivity obtained for the polymers confirming the existence of long conjugated chains. The circular dichroism spectra of these polypyrroles exhibit a large positive Cotton effect, centred on the absorption band associated with the intrachain bipolaronic transition. These results reveal that the conjugated chains of conducting polypyrroles themselves have become chiral, because of an induction effect originating from the chiral substituents.

Introduction T h e d e v e l o p m e n t o f chirality in o r g a n i c c o n d u c t i n g p o l y m e r s is p r o m i s i n g in v a r i o u s fields, s u c h as s t e r e o s e l e c t i v e e l e c t r o c a t a l y s i s [1] a n d specific e l e c t r o m a g n e t i c p r o p e r t i e s [2]. Several a t t e m p t s h a v e b e e n d e s c r i b e d in the literature: a c o n c e p t u a l o n e d e s c r i b i n g v a r i o u s s y n t h e t i c r o u t e s f o r realizing chiral m e t a l s [3], a n d s o m e e x p e r i m e n t a l o n e s b a s e d m a i n l y o n the g r a f t i n g of a n optically active s u b s t i t u e n t o n t o the m o n o m e r [ 3 - 7 ] . H o w e v e r , n o e x p e r i m e n t a l e v i d e n c e o f p o l y m e r chirality h a s b e e n g i v e n f o r p o l y h e t e r o c y c l e s 3 - s u b s t i t u t e d w i t h s h o r t chiral alkyl chains [4] and, in t h e c a s e o f p o l y p y r r o l e N - s u b s t i t u t e d with a n optically active c a m p h o r s u l f o n a t e g r o u p , t h e o p t i c a l activity o b s e r v e d b y circular d i c h r o i s m on the p o l y m e r o r i g i n a t e d only f r o m t h a t o f the c a m p h o r s u l f o n a t e g r o u p [5]. Recently, we h a v e s h o w n t h a t p o l y t h i o p h e n e 3 - s u b s t i t u t e d with optically active p h e n y l d e r i v a t i v e s [6] or t e r p e n e s [7] p o s s e s s t h e m s e l v e s an i m p o r t a n t o p t i c a l activity, c h a r a c t e r i z e d b y a specific r o t a t i o n aeD2Of a b o u t 3 0 0 0 ° [6]. This last result, a l t h o u g h interesting, d o e s n o t give a s t r a i g h t f o r w a r d e x p l a n a t i o n as to w h e t h e r it is p o s s i b l e to i n d u c e o p t i c a l activity on the c o n j u g a t e d c h a i n o f a c o n d u c t i n g p o l y m e r itself b y m e a n s of a chiral s u b s t i t u e n t . W i t h this aim, w e c o n c e n t r a t e d o n the f u n c t i o n a l i z a t i o n o f p o l y h e t e r o c y c l e s in t h e i r 3 position, a n d f o c u s e d o u r w o r k o n t h e p o l y p y r r o l e series. In fact, t h e i r low e l e c t r o p o l y m e r i z a t i o n *Author to whom correspondence should be addressed.

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118 p o t e n t i a l , E p o l y m = 0.8 V versus SCE, allows the use of amino acid derivatives as substituents, keeping in mind that the polar groups of peptide chains are responsible for the interactions leading to the well-known helical structure in proteins.

Experimental,

results

and discussion

Three 0-methylated chiral amino acids were condensed with (3-pyrryl) acetic acid. The carboxylic group in these functionalized pyrroles remains in a suitable position, far enough from the pyrrole nucleus to avoid any inhibition by electronic or steric effects of the conjugation and electric properties in the resulting polymer (Fig. 1). The acid derivatives were obtained following a known p r o c e d u r e [8]. These chiral pyrrole m o n o m e r s were electropolymerized on platinum or indium-tin-oxide-coated (ITO-coated) glass electrodes in propylene carbonate/LiC104 1 M medium, under an applied potential of 0.5 V versus Ag/Ag + 0.1 M CHsCN. The corresponding polypyrroles were obtained in their doped conducting state, either as thin films on the electrode or as thick free-standing films. Their conductivity, m easured by the four-probe technique, is quite large when considering the bulkiness of the substituents; or= 1.2, 1.4 and 0.2 S cm -1 respectively for polymers 1, 2 and 3. This result confirms the existence of long conjugated chains in these polymers and it also emphasizes the interest in the functionalization of polypyrrole in its 3 position. The absorption s pe c t r a of these doped conducting polymers, deposited as thin films on ITO-coated glass, were r e c o r d e d on a CARY 2415 spectrop h o t o m e t e r and e xpr es s ed in molar absorptivity, e, related to the concentration of m o n o m e r units in the polymer given by the electropolymerization charge. The visible absorption band obtained (Fig. 2), with a maximum at 465 urn, c o r r es p o n d s to the well-established bipolaronic transition of the conjugated conducting polypyrrole backbone [9]. Because of their very low oxidation potential, the u n d o p e d states of these polymers were very unstable in air and unfit for spectroscopic characterization. The same doped pol ym er films, as characterized in Fig. 2, were analysed by circular dichroism s p e c t r o s c o p y using a JOBIN YVON Mark 5 spectrometer. The spectra exhibit a well-defined

",__~coocH3

H

Fig. 1. The structure of polymers 1, 2 and 3. Polymer

R

Amino acid

[a]~5 monomer

1 2

CH2OH

3

phenyl

L-serine L-valine D-2-phenylglycine

+2.2 (c=0.14 CH3OH) + 11.4 (c = 0.07 CHCIs) -- 90.0 (c ffi0.04 CHCI3)

CH(CH3) 2

119 1.8

1.4 -7

t.o

I

I

400

500

I

I

600

700

Wavelength (nm)

Fig. 2. The visible absorption band of the doped conducting polymer 2 expressed in molar absorptivity, • (see text).

0.75

="

0.50

" 0.25

b

a

0.0

[ 400

I 500

600 Wavelength (nm)

700

Fig. 3. Circular dichroism spectra of (a) the same doped polymer film as in Fig. 2 and Co) the corresponding functionalized monomer. p o s i t i v e C o t t o n effect c e n t r e d at the s a m e w a v e l e n g t h as the p r e v i o u s a b s o r p t i o n s p e c t r m n (Fig. 3(a)). T h e circular d i c h r o i s m s p e c t r a of the f u n c t i o n a l i z e d m o n o m e r s w e r e also r e c o r d e d u s i n g the s a m e c o n c e n t r a t i o n o f m o n o m e r units as in t h e p o l y m e r film (Fig. 3(b)). T h e a b s e n c e of a n y C o t t o n effect in t h e c a s e of t h e s e f u n c t i o n a l i z e d p y r r o l e s is due to the f a c t t h a t t h e s e c o m p o u n d s do n o t a b s o r b in the s p e c t r a l r a n g e u n d e r o b s e r v a t i o n h e r e ( c o v e r i n g m a i n l y t h e visible part). W e finally c o n f i r m e d t h a t u n d e r the s a m e e x p e r i m e n t a l conditions, a n u n s u b s t i t u t e d p o l y p y r r o l e film d o e s n o t s h o w a n y c i r c u l a r d i c h r o i s m f e a t u r e s . T h e s e r e s u l t s t h u s lead to t h e c o n c l u s i o n t h a t t h e l a r g e C o t t o n effect o b s e r v e d f o r t h e f u n c t i o n a l i z e d p o l y p y r r o l e (Fig. 3 ( a ) ) is n o t r e l a t e d to the o p t i c a l l y active a m i n o acid s u b s t i t u e n t , b u t is associated with the conjugated polymeric backbone on which the presence o f o p t i c a l l y active s u b s t i t u e n t s h a s i n d u c e d a p a r t i c u l a r o r d e r i n g d u r i n g t h e e l e c t r o p o l y m e r i z a t i o n r e a c t i o n . T h e circular d i c h r o i s m o f p o l y m e r 2, A~L.R,

120 e x p r e s s e d as a f u n c t i o n o f m o n o m e r unit c o n c e n t r a t i o n c a l c u l a t e d as p r e v i o u s l y described, is quite high, i.e. of the o r d e r o f unity (Fig. 3(a)). This value, w h i c h is c o m p a r a b l e to that o b s e r v e d with p o l y p e p t i d e chains [10], p r o v i d e s the first e v i d e n c e that the c o n j u g a t e d c o n d u c t i n g p o l y p y r r o l e chains t h e m s e l v e s have b e c o m e chiral, o w i n g to the effect o f optically active a m i n o acid g r o u p s s u b s t i t u t e d o n t o the p y r r o l e m o n o m e r s . A m o n g the possible p o l y m e r chain c o n f o r m a t i o n s , it m u s t be p o i n t e d o u t t h a t p r e v i o u s w o r k on p o l y h e t e r o c y c l e s h a s a l r e a d y s h o w n the existence o f helixes as preferential c o n f o r m a t i o n in e l e c t r o p o l y m e r i z e d p o l y h e t e r o c y c l e s . Thus X-ray a n d e l e c t r o n diffraction data have led us to p r o p o s e s u c h a s t r u c t u r e for p o l y ( 3 - m e t h y l t h i o p h e n e ) [ 11 ], this result b e i n g r e c e n t l y s t r o n g l y s u p p o r t e d b y e x p e r i m e n t a l [12] as well as t h e o r e t i c a l [13] studies. Furt h e r m o r e , a r e c e n t s c a n n i n g tunnelling m i c r o s c o p e analysis o f the first m o n o l a y e r s o f e l e c t r o c h e m i c a l l y g r o w n p o l y p y r r o l e h a s also c o n f i r m e d the p r e s e n c e o f helixes [14], a l t h o u g h this s t r u c t u r a l o r d e r is limited to s o m e p o l y m e r a r e a s in t h e s e examples. The results p r e s e n t e d here c o n f i r m that the functionalization o f c o n d u c t i n g p o l y m e r s is an efficient w a y to i m p o s e a long r a n g e order, and, as in the case of b i o p o l y m e r s , the p r e s e n c e o f a m i n o acid derivatives is able to i n d u c e a large chirality a l o n g the c o n j u g a t e d c o n d u c t i n g chains.

Acknowledgements W e t h a n k P r o f e s s o r M. L. Lemaire, L a b o r a t o i r e de Catalyse et Synth~se Organique, Universit6 C. Bernard, Lyon, for fruitful d i s c u s s i o n s a n d P r o f e s s o r A. Suillerot-Garnier, L a b o r a t o i r e de Chimie Bioinorganique, Universit6 Paris Nord, for r e c o r d i n g the circular d i c h r o i s m spectra.

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