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Structure-property relationships in polyaniline derivatives
Synthetic Metals, 55-57 (1993) 1527-1532
1527
STRI,ICTURE-PROPERTY RELATIONSHIPS IN POLYANILINE DERIVATIVES
M. LECLERC '), G. D'APRANO a) and G. ZOTI'Ib) •)Dtpt. de Chimie, Universit6 de Montrt..al, Montreal (Qc), Canada, H3C 3J7 b)Consiglio Nazionale delle Ricerche, I.P.EL.P., C.o Stati Uniti 4, 35020 Padova, Italy
ABSTRACT Different alkyl and alkoxy-substituted polyanilines were electropolymerized in acidic conditions.
The steric and electronic effects of the substituents on the electrical and optical
properties of the resulting polymers, were principally investigated. It has been found that alkylsubstituted anilines give less planar polymers than the alkoxy derivatives. However, alkoxy mono-substituted anilines exhibit lower conductivities than those obtained with the alkyl derivatives, because the selectivity of the head-to-tail couplings is reduced by the incorporation of a methoxy group at the ortho position. Nevertheless, more regular and processable materials have been obtained with poly(2,5-dialkoxyanilines): for instance, poly(2,5-dimethoxyaniline) exhibits a high conjugation length and a good conductivity (ca. 0.3 S/cm).
INTRODUCTION In the last fifteen years, many studies have been devoted to the development of processable conducting polymers. Among these materials, polyaniline (PAN1) has received great attention owing to its good stability and its unusual electrical and optical properties [1-4]. Moreover, this material exhibits interesting nonlinear optical properties which can be useful for the development of optical switches [5]. In order to obtain new materials with enhanced electrical and optical properties, we have proceeded to the study of the steric and electronic effects of some substituents on the physical properties of polyanilines, by varying their nature and their position on the ring (Figure 1). 0379-6779/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
1528
R R = H, C H 3 , CH~OH, O C H 3
--N R '= H, C H 3 , OCH~
R' Figure 1.
Repeat unit of substituted polyanilines.
EXPERIMENTAL Polyaniline (PANI), poly(2-methylaniline) (PMA), poly(2-aminobenzylalcohol) (PABA), poly(2-methoxyaniline) (PMOA) and poly(2,5-dimethoxyaniline) (PDMOA) have been obtained by electrochemical polymerization [4, 6-9]. The electrolyte solution used to obtain poly(2methoxy-5-methylaniline) (PMOMA) was an aqueous solution of 0.5 M of the monomer in 2 M HCIO4. All polymers were characterized by cyclic voltammetry, spectroelectrochemistry, steric exclusion chromatography and ex-situ conductivity measurements,
ln-situ conductivity
measurements [10] were performed in order to clearly measure the maximum conductivity allowed by the polymers and thus, to make consistent comparisons between these materials.
RESULTS AND DISCUSSION Electrochemical polymerization of alkyl and alkoxy-substituted anilines gave a dark-green doped polymers. The stability of the corresponding base form, as well as its solubility allowed the characterization of the polymers. As reported in Table I, high molecular weight materials were obtained.
The absorption maxima of these polymers are located at relatively short
wavelengths for conjugated materials. This is due to the non-planar conformation of the polymer backbone. Indeed, it has been reported that there is a torsion angle between the phenyl ring and the nitrogen atom [12] along the polyaniline backbone. From our results (Table I), it is clear that both the conjugation length and the redox potential are affected by the nature and the position of the substituents on the ring. For instance, PMA exhibits an absorption maximum lower than PANI (310 nm vs 315 nm), whereas PMOA exhibits a red-shifted absorption (325 nm vs 315 nm).
Moreover, with the introduction of electron-donating substituents, the
monomers are more easily oxidized and a similar behaviour should be expected for the polymers. This is found for PMOA (Table I), but a positive shift is observed for the first redox reaction of PMA. The electronic effect of the methyl substituent cannot explain the positive shift of the first oxidation reaction and the blue shift of the absorption maximum.
1529
Table I. Physical properties of alkyl and alkoxy mono and di-substituted polyanilines. Polymers
Xm.x
Ell/~")
0~)
O~)'c)
(rim)
(V/SCE)
(S/cm)
(S/cm)
PANI
315
0.12
4.4
4.9
80
PMA
310
0.20
0.26
0.42
90
PABA
308
0.22
1.7 x 10"3
2.5 x 10-3
PMOA
325
0.08
0.14
0.14
80
PMOMA
308
0.16
5.8 x 10.3
5.8 x 10-3
70
PDMOA
350
0.03
0.22
0.32
50
0.23
3x10 ~
P D M A *J
DP °)
a)electrolyte: 1 M HC1 b)four-probe technic ue (ex-situ) c)maxlmum conductivit' , obtained by in-
situ measurements d)in NMP+LiC1 (0.5 %) e)from ref [11]
Hence, the steric effects as well as the electronic effects of the substituents must be taken into account. Br6das et al [13] have reported that the presence of some substituents can induce some non-planar conformations that decrease the conjugation length (viz. decrease of the absorption maximum) along the polymer backbone and thus giving higher redox potentials. Therefore, this decrease of conjugation length and the positive shift of the first redox process can be related to higher torsion angles in reduced poly(alkylanilines) compared to PANI. On the other hand, the red shift of the absorption maximum and the lower redox potential of PMOA can be related to the possibility for this polymer to adopt a more planar conformation than that of PMA. This behaviour can be explained by the smaller van der Waals radius of an oxygen atom (1.40 A) than that of a methyl group (2.00 A). Similar results were recently observed for alkyl and alkoxy-substituted polythiophenes [14].
Despite the enhancement of
conjugation in PMOA, this polymer displays a conductivity value slightly lower than PMA. This was also found in polythiophene derivatives and was explained in terms of structural defects (an irregular chemical structure) [15]. In the case of polyaniline, such defects may be detected by cyclic voltammetry measurements. Indeed, as previously observed with the unsubstituted PANI, the cyclic voltammograms exhibit two oxidation peaks in addition to an anodic peak at ca. 0.45 V vs SCE (Figure 2). This third anodic peak was related to degradation products or
1530
side-couplings [16,17]. Figure 2, clearly shows that alkoxy mono-substituted polyanilines have a relatively high amount of defects (side-couplings).
This can explain why PMA is a more
conductive material than PMOA. 1.6
'
'
,
,
,
\/
xj
i
I
I
I
I
l
I
-0.2
0.0
0.2
0.4
0.6
0.8
1.~.
0.8 0.4 0 -0.4 -0.8 -1.2 -0.4
1.0
E (vwsez) Figure 2.
Cyclic voltammograms of poly(2-methoxyaniline) (--) and poly(2-methoxy-5methylaniline) (---) in 1 M HC1.
ABA was polymerized as a model compound, since it is a isomer of MOA. In the former case, the substituent is linked to the ring by the carbon atom, whereas in the latter case, the substituent is linked via the oxygen atom. From the results reported in Table I, it seems that the CH2OH- group act as a bulky substituent since it give rise to a twisted polymer: low conjugation length, low conductivity and high redox potential. This polymer clearly shows that the formation of a highly conjugated polymer is directly related to the nature of the atom of the substituent which is linked to the ring. Indeed, an atom having a small van der Waals radius (e.g. oxygen atom) gives rise to a polymer having good electrical and optical properties. 2,5-Disubstituted materials give materials with a more regular head-to-tail couplings (Figure 2). The presence of two methoxy groups (PDMOA) gives a material with the lowest oxidation potential for the first redox process [8]. The electron-donating properties of the methoxy groups account for the negative shift of the first oxidation reaction. Moreover, this new derivative shows a good conjugation length and high electrical conductivity (Table I).
However, the
addition of a methyl at C-5 (MOMA) gives a polymer with a reduced conjugation length which causes a reduction of the conductivity by two orders of magnitude, in relation to PMOA (Figure 3).
This should be explained by the steric effect of the methyl at the C-5 position which
increases the torsion along the polymer backbone.
1531
0,15
%
, q
0,10
<
4
Oo o
o
>
rd~ O
o
v b
0.05
•
o
o
z
•
7:, J
o o 0,00:
-0.2
-0.1
o
.~
•
0.0
0.1
,
,
,
0.2
0.3
0.4
• Qo,
! ,~
0.5
,-
0.6
,-
-±0 0,7
Eox (V vs SCE)
Figure 3.
In-situ
conductivity measurements of poly(2-methoxyaniline) (a) and poly(2-
methoxy-5-methylaniline) (o) in 1 M HC1, during the oxidation process.
The replacement of both methoxy by two methyl groups have led to a more twisted polymer (PDMA). This assumption is extrapolated by the high potential value for the first redox process and by its very low conductivity [11]. Thus, it appears that steric hindrance in the vicinity of the polymer bakbone can be decreased with alkoxy substituents. This is consistent with the results found with the mono-substituted polyanilines. Although PDMOA is more conjugated than PANI, it is less conducting by one order of magnitude. This may be explained by a more resistive interchain contacts caused by the presence of an increasing proportion of insulating material (methoxy group) in the polymer.
Similar
arguments were recently developed for poly(3-alkylthiophenes) [18].
CONCLUSIONS From this study, it is clear that both the electronic and the steric effects of any substituents must be taken into account to understand the structure-property relationships in this class of materials. We showed that steric hindrance induced by the alkoxy groups is smaller than that induced by the alkyl groups. Consequently, the torsion angle between the repeat units is greater in alkyl-substituted polyanilines; this explains the noticed decrease conductivity: e.g. PMOMA vs
PMOA and PDMA
vs
PDMOA.
Moreover, we showed that incorporation of a second
substituent at C-5 have drastic consequences, following the substituents. Indeed, side-couplings are reduced and more regular and highly conducting materials are then obtained (e.g. PDMOA),
1532
while bulkier substituents (e.g. methyl group) induce additional deformation along the polymer backbone, owing to an increase of the steric hindrance. This result in a decrease of the degree of conjugation and hence, a dramatic decrease of the conductivity (e.g. PDMA).
ACKNOWLEDGEMENTS This work was supported by the Natural Sciences and Engineering Council of Canada (NSERC). G.D.A. is grateful to NSERC for a graduate fellowship. The authors also thank Dr J.Y. Bergeron and Dr K. Faid for helpful discussions and technical assistance.
REFERENCES 1.
A . F . Diaz and A. J. Logan, J. Electroanal. Chem.. 111 (1980) 111.
2.
R. Noufi, A. J. Nozik, J. White and L. Warren, J. Electrochem. Soc., 129 (1982) 2261.
3.
E . W . Paul, A. J. Ricco, M. S. Wrighton, J. Phys. Chem., 89 (1985) 1441.
4.
W . S . Huang, B.D. Humphrey and A. G. MacDiarmid, Faraday Discuss. Chem. Soc., 82 (1986) 2385.
5.
J . A . Osaheni, S. A. Jenekhe, H. Vanherzeele and J. S. Meth, Chem. Mater.. 3 (1991) 218.
6.
M. Leclerc, J. Electroanal. Chem., 296 (1990) 93.
7.
D. Maclnnis Jr and B. L. Funt, Synth. Met.. 25 (1988) 235.
8.
G. Zotti, N. Comisso, G. D'Aprano and M. Leclerc, su.bmitted to Adv. Mater.
9.
J . C . Lacroix, P. Garcia, J. P. Audi~re, R. C16ment and O. Khan, New J. Chem., 14 (1990) 87.
10.
G. Schiavon, S. Sitran and G. Zotti, Synth. Met., 32 (1989) 209.
11.
E . M . Geni~s and P. No~l, J. Electroanal. Chem.. 296 (1990) 473.
12.
J . M . Ginder, A. J. Epstein and A. G. MacDiarmid, Synth. Met., 37 (1990) 45.
13.
J . L . Br&las, G. B. Street, B. Th6mans and J. M. Andr6, J. Chem. Phys.. 83 (1985). 1323.
14.
G. Daoust and M. Leclerc, Macromolecules. 24 (1991) 455.
15.
A . C . Chang, R. L. Elsenbaumer and L. L. Miller, J. Electro~al, Chem.. 236 (1987). 239.
16.
T. Kobayashi, H. Yoneyma and H. Tamara, J. Electroanal. Chem., 177 (1984) 293.
17.
E . M . Geni~s, J. F. Penneau and M. Lapkowski, J. Electroanal. Chem.. 249 (1988) 97.
18.
J. Roncali, R. Garreau, A. Yassar, P. Marque, F. Gamier and M. Lemaire, J. Phys. Chem., 91 (1987) 6706.
1527
STRI,ICTURE-PROPERTY RELATIONSHIPS IN POLYANILINE DERIVATIVES
M. LECLERC '), G. D'APRANO a) and G. ZOTI'Ib) •)Dtpt. de Chimie, Universit6 de Montrt..al, Montreal (Qc), Canada, H3C 3J7 b)Consiglio Nazionale delle Ricerche, I.P.EL.P., C.o Stati Uniti 4, 35020 Padova, Italy
ABSTRACT Different alkyl and alkoxy-substituted polyanilines were electropolymerized in acidic conditions.
The steric and electronic effects of the substituents on the electrical and optical
properties of the resulting polymers, were principally investigated. It has been found that alkylsubstituted anilines give less planar polymers than the alkoxy derivatives. However, alkoxy mono-substituted anilines exhibit lower conductivities than those obtained with the alkyl derivatives, because the selectivity of the head-to-tail couplings is reduced by the incorporation of a methoxy group at the ortho position. Nevertheless, more regular and processable materials have been obtained with poly(2,5-dialkoxyanilines): for instance, poly(2,5-dimethoxyaniline) exhibits a high conjugation length and a good conductivity (ca. 0.3 S/cm).
INTRODUCTION In the last fifteen years, many studies have been devoted to the development of processable conducting polymers. Among these materials, polyaniline (PAN1) has received great attention owing to its good stability and its unusual electrical and optical properties [1-4]. Moreover, this material exhibits interesting nonlinear optical properties which can be useful for the development of optical switches [5]. In order to obtain new materials with enhanced electrical and optical properties, we have proceeded to the study of the steric and electronic effects of some substituents on the physical properties of polyanilines, by varying their nature and their position on the ring (Figure 1). 0379-6779/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
1528
R R = H, C H 3 , CH~OH, O C H 3
--N R '= H, C H 3 , OCH~
R' Figure 1.
Repeat unit of substituted polyanilines.
EXPERIMENTAL Polyaniline (PANI), poly(2-methylaniline) (PMA), poly(2-aminobenzylalcohol) (PABA), poly(2-methoxyaniline) (PMOA) and poly(2,5-dimethoxyaniline) (PDMOA) have been obtained by electrochemical polymerization [4, 6-9]. The electrolyte solution used to obtain poly(2methoxy-5-methylaniline) (PMOMA) was an aqueous solution of 0.5 M of the monomer in 2 M HCIO4. All polymers were characterized by cyclic voltammetry, spectroelectrochemistry, steric exclusion chromatography and ex-situ conductivity measurements,
ln-situ conductivity
measurements [10] were performed in order to clearly measure the maximum conductivity allowed by the polymers and thus, to make consistent comparisons between these materials.
RESULTS AND DISCUSSION Electrochemical polymerization of alkyl and alkoxy-substituted anilines gave a dark-green doped polymers. The stability of the corresponding base form, as well as its solubility allowed the characterization of the polymers. As reported in Table I, high molecular weight materials were obtained.
The absorption maxima of these polymers are located at relatively short
wavelengths for conjugated materials. This is due to the non-planar conformation of the polymer backbone. Indeed, it has been reported that there is a torsion angle between the phenyl ring and the nitrogen atom [12] along the polyaniline backbone. From our results (Table I), it is clear that both the conjugation length and the redox potential are affected by the nature and the position of the substituents on the ring. For instance, PMA exhibits an absorption maximum lower than PANI (310 nm vs 315 nm), whereas PMOA exhibits a red-shifted absorption (325 nm vs 315 nm).
Moreover, with the introduction of electron-donating substituents, the
monomers are more easily oxidized and a similar behaviour should be expected for the polymers. This is found for PMOA (Table I), but a positive shift is observed for the first redox reaction of PMA. The electronic effect of the methyl substituent cannot explain the positive shift of the first oxidation reaction and the blue shift of the absorption maximum.
1529
Table I. Physical properties of alkyl and alkoxy mono and di-substituted polyanilines. Polymers
Xm.x
Ell/~")
0~)
O~)'c)
(rim)
(V/SCE)
(S/cm)
(S/cm)
PANI
315
0.12
4.4
4.9
80
PMA
310
0.20
0.26
0.42
90
PABA
308
0.22
1.7 x 10"3
2.5 x 10-3
PMOA
325
0.08
0.14
0.14
80
PMOMA
308
0.16
5.8 x 10.3
5.8 x 10-3
70
PDMOA
350
0.03
0.22
0.32
50
0.23
3x10 ~
P D M A *J
DP °)
a)electrolyte: 1 M HC1 b)four-probe technic ue (ex-situ) c)maxlmum conductivit' , obtained by in-
situ measurements d)in NMP+LiC1 (0.5 %) e)from ref [11]
Hence, the steric effects as well as the electronic effects of the substituents must be taken into account. Br6das et al [13] have reported that the presence of some substituents can induce some non-planar conformations that decrease the conjugation length (viz. decrease of the absorption maximum) along the polymer backbone and thus giving higher redox potentials. Therefore, this decrease of conjugation length and the positive shift of the first redox process can be related to higher torsion angles in reduced poly(alkylanilines) compared to PANI. On the other hand, the red shift of the absorption maximum and the lower redox potential of PMOA can be related to the possibility for this polymer to adopt a more planar conformation than that of PMA. This behaviour can be explained by the smaller van der Waals radius of an oxygen atom (1.40 A) than that of a methyl group (2.00 A). Similar results were recently observed for alkyl and alkoxy-substituted polythiophenes [14].
Despite the enhancement of
conjugation in PMOA, this polymer displays a conductivity value slightly lower than PMA. This was also found in polythiophene derivatives and was explained in terms of structural defects (an irregular chemical structure) [15]. In the case of polyaniline, such defects may be detected by cyclic voltammetry measurements. Indeed, as previously observed with the unsubstituted PANI, the cyclic voltammograms exhibit two oxidation peaks in addition to an anodic peak at ca. 0.45 V vs SCE (Figure 2). This third anodic peak was related to degradation products or
1530
side-couplings [16,17]. Figure 2, clearly shows that alkoxy mono-substituted polyanilines have a relatively high amount of defects (side-couplings).
This can explain why PMA is a more
conductive material than PMOA. 1.6
'
'
,
,
,
\/
xj
i
I
I
I
I
l
I
-0.2
0.0
0.2
0.4
0.6
0.8
1.~.
0.8 0.4 0 -0.4 -0.8 -1.2 -0.4
1.0
E (vwsez) Figure 2.
Cyclic voltammograms of poly(2-methoxyaniline) (--) and poly(2-methoxy-5methylaniline) (---) in 1 M HC1.
ABA was polymerized as a model compound, since it is a isomer of MOA. In the former case, the substituent is linked to the ring by the carbon atom, whereas in the latter case, the substituent is linked via the oxygen atom. From the results reported in Table I, it seems that the CH2OH- group act as a bulky substituent since it give rise to a twisted polymer: low conjugation length, low conductivity and high redox potential. This polymer clearly shows that the formation of a highly conjugated polymer is directly related to the nature of the atom of the substituent which is linked to the ring. Indeed, an atom having a small van der Waals radius (e.g. oxygen atom) gives rise to a polymer having good electrical and optical properties. 2,5-Disubstituted materials give materials with a more regular head-to-tail couplings (Figure 2). The presence of two methoxy groups (PDMOA) gives a material with the lowest oxidation potential for the first redox process [8]. The electron-donating properties of the methoxy groups account for the negative shift of the first oxidation reaction. Moreover, this new derivative shows a good conjugation length and high electrical conductivity (Table I).
However, the
addition of a methyl at C-5 (MOMA) gives a polymer with a reduced conjugation length which causes a reduction of the conductivity by two orders of magnitude, in relation to PMOA (Figure 3).
This should be explained by the steric effect of the methyl at the C-5 position which
increases the torsion along the polymer backbone.
1531
0,15
%
, q
0,10
<
4
Oo o
o
>
rd~ O
o
v b
0.05
•
o
o
z
•
7:, J
o o 0,00:
-0.2
-0.1
o
.~
•
0.0
0.1
,
,
,
0.2
0.3
0.4
• Qo,
! ,~
0.5
,-
0.6
,-
-±0 0,7
Eox (V vs SCE)
Figure 3.
In-situ
conductivity measurements of poly(2-methoxyaniline) (a) and poly(2-
methoxy-5-methylaniline) (o) in 1 M HC1, during the oxidation process.
The replacement of both methoxy by two methyl groups have led to a more twisted polymer (PDMA). This assumption is extrapolated by the high potential value for the first redox process and by its very low conductivity [11]. Thus, it appears that steric hindrance in the vicinity of the polymer bakbone can be decreased with alkoxy substituents. This is consistent with the results found with the mono-substituted polyanilines. Although PDMOA is more conjugated than PANI, it is less conducting by one order of magnitude. This may be explained by a more resistive interchain contacts caused by the presence of an increasing proportion of insulating material (methoxy group) in the polymer.
Similar
arguments were recently developed for poly(3-alkylthiophenes) [18].
CONCLUSIONS From this study, it is clear that both the electronic and the steric effects of any substituents must be taken into account to understand the structure-property relationships in this class of materials. We showed that steric hindrance induced by the alkoxy groups is smaller than that induced by the alkyl groups. Consequently, the torsion angle between the repeat units is greater in alkyl-substituted polyanilines; this explains the noticed decrease conductivity: e.g. PMOMA vs
PMOA and PDMA
vs
PDMOA.
Moreover, we showed that incorporation of a second
substituent at C-5 have drastic consequences, following the substituents. Indeed, side-couplings are reduced and more regular and highly conducting materials are then obtained (e.g. PDMOA),
1532
while bulkier substituents (e.g. methyl group) induce additional deformation along the polymer backbone, owing to an increase of the steric hindrance. This result in a decrease of the degree of conjugation and hence, a dramatic decrease of the conductivity (e.g. PDMA).
ACKNOWLEDGEMENTS This work was supported by the Natural Sciences and Engineering Council of Canada (NSERC). G.D.A. is grateful to NSERC for a graduate fellowship. The authors also thank Dr J.Y. Bergeron and Dr K. Faid for helpful discussions and technical assistance.
REFERENCES 1.
A . F . Diaz and A. J. Logan, J. Electroanal. Chem.. 111 (1980) 111.
2.
R. Noufi, A. J. Nozik, J. White and L. Warren, J. Electrochem. Soc., 129 (1982) 2261.
3.
E . W . Paul, A. J. Ricco, M. S. Wrighton, J. Phys. Chem., 89 (1985) 1441.
4.
W . S . Huang, B.D. Humphrey and A. G. MacDiarmid, Faraday Discuss. Chem. Soc., 82 (1986) 2385.
5.
J . A . Osaheni, S. A. Jenekhe, H. Vanherzeele and J. S. Meth, Chem. Mater.. 3 (1991) 218.
6.
M. Leclerc, J. Electroanal. Chem., 296 (1990) 93.
7.
D. Maclnnis Jr and B. L. Funt, Synth. Met.. 25 (1988) 235.
8.
G. Zotti, N. Comisso, G. D'Aprano and M. Leclerc, su.bmitted to Adv. Mater.
9.
J . C . Lacroix, P. Garcia, J. P. Audi~re, R. C16ment and O. Khan, New J. Chem., 14 (1990) 87.
10.
G. Schiavon, S. Sitran and G. Zotti, Synth. Met., 32 (1989) 209.
11.
E . M . Geni~s and P. No~l, J. Electroanal. Chem.. 296 (1990) 473.
12.
J . M . Ginder, A. J. Epstein and A. G. MacDiarmid, Synth. Met., 37 (1990) 45.
13.
J . L . Br&las, G. B. Street, B. Th6mans and J. M. Andr6, J. Chem. Phys.. 83 (1985). 1323.
14.
G. Daoust and M. Leclerc, Macromolecules. 24 (1991) 455.
15.
A . C . Chang, R. L. Elsenbaumer and L. L. Miller, J. Electro~al, Chem.. 236 (1987). 239.
16.
T. Kobayashi, H. Yoneyma and H. Tamara, J. Electroanal. Chem., 177 (1984) 293.
17.
E . M . Geni~s, J. F. Penneau and M. Lapkowski, J. Electroanal. Chem.. 249 (1988) 97.
18.
J. Roncali, R. Garreau, A. Yassar, P. Marque, F. Gamier and M. Lemaire, J. Phys. Chem., 91 (1987) 6706.