- Email: [email protected]
Conformational study on alkyl-substituted thiophene oligomers
Volume 184, number 5,6
CHEMICAL PHYSICS LETTERS
4 October 199 1
Conformational study on alkyl-substituted thiophene oligomers D.A. dos Santos, D.S. Galvgo, B. Laks Unirersidade Estadual de Campinas, Departamento de Fisica Aplicada, 13081 Campinas, SP, Bras2
and M.C. dos Santos Universidade Federal de Pernambuco. Departamento de Quimica Fundamental, 50739 Recije, PE, Brasil Received 1 June 1991
A series of Austin-method 1 geometry optimizations on alkyl-substituted thiophene small oligomers has been carried out. Torsion potential curves far dimers are obtained as a function of the length of the alkylxide group and the substitution position. Ground-state geometries are predicted to be quite independent of the radical length. On the other hand, different side-chain couplings give rise to distinct torsion potential curves. A planar ground-state geometry is obtained for 4,4’-dialkyl-bithiophene while in 3,3’ and 3.4’ coupled isomers, thiophenes are twisted by 90” but have distinct torsion barriers. The importance of the present results to the structural features of alkyl-substituted polythiophenes is discussed.
1. Introduction An important step towards processible conjugated polymers has been attained with the synthesis of soluble polymeric systems. Among the most studied soluble polymers are those belonging to the polythiophene family [ 11. Polythiophene becomes soluble in common organic solvents by the addition to its structure of long alkyl radicals which are connected to thiophenes in the p position. Thus, the resulting chains have a structure similar to that of polythiophene, with lateral saturated tails playing the role of fixed “solvent” molecules. Poly(alkyl-thiophenes) have slightly smaller electrical conductivities than the unsubstituted polymer upon doping [2]. On the other hand, the alkyl derivatives present a number of special features including [ 3,4]: (i) solubility allows the preparation of high molecular-weight chains, containing up to 800- 1000 aromatic rings; (ii) they have better crystallinity; (iii) their synthesis has opened up the possibility of preparing Langmuir-Blodgett films of conducting polymers; (iv) they show new interest0009-2614/91/$
ing effects like thermochromism and solvatochromism. Thermochromism has been investigated by many authors [2,4-g]. Although the exact mechanism responsible for the thermochromic transition remains unclear, several studies have demonstrated that conformational changes do occur upon heating and play a role in the transition. The first optical transition continuously and reversibly blue-shifts with increasing temperature. This behavior has been attributed to conformational changes involving out-of-plane twisting of thiophene rings on the otherwise planar chain. Those defects would occur in the substituted polymer (and not in pure polythiophene) due to the presence of the aliphatic tails which soften the interchain interactions. It is also believed that the torsion energies around thiophene-thiophene bonds may be affected by the presence of large lateral groups, since they may lead to steric hindrance. Thus, it is of interest to know what the torsion barriers are, and how they depend on the alkyl group, in terms of its length and substitution position. In this work, we present AM1 (Austin-method 1)
03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.
579
Volume 184, number 5,6
4 OctoberI991
CHEMICAL PHYSICS LETTERS
[ 91 geometry optimizations on alkyl-substituted thiophene oligomers. The length and position of the alkyl radical have been varied in order to investigate their influence on the torsion potential curve. The theory used in this work is presented in section 2. Results are discussed in section 3 and the concluding remarks appear in section 4.
R \
s (alhead-to-head S
*
R
2. Methodology Most geometry optimizations on small oligomers which are representative of conjugated polymers have been performed using MNDO (modified neglect of differential overlap) [ 6, lo]. Those ground-state geometries are assumed to be appropriate as the unit cell for polymer electronic-structure calculations. Our interest in the present study is to investigate possible modifications on geometry induced by the presence of lateral groups in planar thiophene oligomers, in which distortions involving out-of-plane twisting of thiophenes may be an important effect. It is well known that the MNDO technique is not able to reproduce dihedral angles in conjugated systems, so that we have chosen AM1 - an improved version of MNDO which has been developed to correct some of its weaknesses. Reports in the literature [ 91 seem to demonstrate that the main weaknesses have been largely corrected. For example, AM1 predicts dihedral angles very well, but the rotational barriers are underestimated [ 11,12 1, although general trends within a series of structurally related compounds seem to be correctly reproduced. We have considered three distinct couplings of the alkyl groups in dimers, as shown in fig. 1. Calculations on bithiophene have also been carried out for comparison. In all cases, a full geometry optimization has been performed #I.
3. Results In fig. 2 are shown the optimized geometries for (dihexyl)bithiophene in the three possible couplings @’All the calculationswereperformed on a vectorized IBM 3090 computerwiththe MOPAC program, version 5.0 (QCEP 560) using the option PARASOK to treat sulfur.
580
lblhead-to-toll
R
s
d
(c)tail-to-tail
q
s
R’
Fig. 1. The three different couplings on dimers we have investigated here: (a) head-to-head; (b) head-to-tall; and (c) tail-totail. Alkyl groups (R) were ethyl, butyl and hexyl.
and for the bithiophene molecule. We have obtained essentially the same geometries for the corresponding ethyl- and butyl-substituted species. Bond lengths and bond angles are quite insensitive to the presence of lateral groups, in agreement with the MNDO calculations by Thbmans and collaborators [ 131. However. the optimum torsion angle between the aromatic rings is drastically affected by the regiochemistry of substitution, as can be seen in figs. 2 and 3. Bithiophene and tail-to-tail coupled (dialkyl)bithiophene are both planar molecules, with potential barriers of 0.63 and 0.62 kcal, respectively. On the other hand, the head-to-head coupled isomer has a minimum-energy twist angle of 90”, with a barrier against the planar geometry of 2.65 kcal, while for the head-to-tail conformer, these values are 100.6” and 0.66 kcal. As the barriers could be sensitive to end effects and to o-n: interactions that arise in nonplanar systems, we have performed calculations on trimers and pentamers in all head-to-tail configurations, with ethyl radical as the substituent. The
Volume 184, number 5,6
CHEMICAL PHYSICS LETTERS
4 October 199 I
i 0
24
48
72
96
I20
4NGLElde grees)
(b)
(c)
(d)
Fig. 2. AMI optimized geometries of dimers for differentcouplings: (a) head-to-head; (b) head-to-tail; (c) tail-to-tail. The geometry of bithiophene molecule is also indicated in (d) for comparison. The lateral group is hexyl in all figures.
Fig. 3. Torsion potential curves for the three different couplings on dimers. In the inset, from top to bottom: (I) head-to-head, (0) head-to-tail and (A) tail-to-tail coupled (dialkyl)bithiw phenes.
ground-state geometries obtained are fully consistent with previous results: the aromatic rings are twisted by 90” relative to each other and the associated torsion energy is 1.2 kcal for the trimer, namely, 0,6 kcal per couple of rings. We recall that those values obtained for the torsion barriers may not be realistic, even though we believe they can be used to compare different conformers. The ionization potentials obtained in our calculations are 8.74 and 8.59 eV for bithienyl and tail-to-tail coupled conformer, and 9.16 and 9.25 eV for head-to-tail and head-to-head conformers. This sequence of ionization potential values is a direct consequence of their geometries, since in non-planar species, x orbitals tend to be more localized, leading to higher ionization potentials. Several theoretical investigations of poly (alkylthiophenes) have been reported in the literature [ 491, most of them using the torsion potential curve appropriate to the bithiophene molecule as a model for the torsion barrier in the substituted polymer. Our results have shown that torsion barriers may very much depend on the regiochemistry of substitution. Thermochromism is observed in substituted polythiophenes which are not regiospecific, the repeat unit being mostly like that of fig. la, or the head-to-tail substitution. A non-planar configuration is predieted for this substitution pattern, although a planar conformation of the chains is experimentally ob581
Volume 184,number $6
CHEMICALPHYSICSLETTERS
served [ 81. Thus, in the solid-state material, the packing energy as well as the energy associated to the n-electron conjugation (hereafter referred to as solidstate potential) should be large enough to compensate the energy barrier against planarity. The strength of the solid-state potential is expected to weaken with the P-substitution by longer side chains, which tends to reduce the minimum energy to induce the out-ofplane twisting. This is in agreement with the experimental observation of a decrease in the thermochromic transition temperature in a series of polythiophenes when substituted by longer side chains. We have investigated the effect of assuming a solidstate potential to be added to the molecular torsion barriers. The potential curves of fig. 3 were fitted to the analytical form I7(0)=A+Bcos(B)+Ccos(20))
(1)
and, for the solid-state potential, we have assumed the expression )
V,,(@=H[ 1 -cos(20)]
(2)
where H is half the height of the barrier. Thus, in a substituted polythiophene chain, the torsion potential around a given bond connecting rings is the summation of both contributions ( 1) and (2). Typical results are as shown in fig. 4 [ 141. We can interpret several experimental data on thermochromism by admitting that the ground-state
/
\
*/ 1’ , 0.
30
60
\\
\ >90
,/’
‘\\, \
_‘, 120 150 B(daQrser)
190
Fig. 4. Typical torsion potential curve, calculated following expressions ( I ) and (2) (see text): (a) molecular potential as obtained from AM1 calculations; (b) phenomenological soiidstate potential; (c) total potential curve.
582
4 October 1991
structure is trapped in the well around O”, namely, around the planar chain. Raising the temperature allows the rings to rotate and eventually overcome the barrier at 90”. These rings are now trapped in the well around 180” and are not likely to return to the original position on cooling. This effect might explain irreversible changes on the infrared spectrum observed on heating-cooling cycles performed on poly(alkylthiophenes) [ 41, since the rings which would be trapped in large torsion angles should give rise to specific infrared activity, not present in samples which have not been thermally treated. Our results seem to be in agreement with the experimental observation that a crystalline state is present in the material composed of head-to-tail dimers while head-to-head coupled molecules do not crystallize and form glasses at low temperature [ 3 1. We see from fig. 3 that the head-to-head isomer has a barrier against planarity which is about four times stronger than the corresponding one for head-to-tail isomer. Steric hindrance is, thus, maximized in the former case and should weaken intermolecular interactions. Moreover, the regiospecific head-to-head coupled poly (alkylthiophene) does not present solvatochromism [ 3,8], which implies a rigid backbone structure, and is consistent with a high torsion barrier.
4. Concluding remarks We have presented AM1 calculations for the investigation of torsion potential curves in alkyl-substituted thiophene oligomers as a function of substitution position and radical length. The results have shown significant differences among torsion potentials depending on the substitution pattern. They are important to interpret the properties of poly( alkylthiophenes), where thermochromism has been observed. A non-planar ground state is predicted for the headto-head substituted dimer, with a strong barrier against the planar configuration. This result appears to be in agreement with the fact that the material formed by those dimers is not able to crystallize. The polymer synthesized from those dimers is fully regiospecific and does not show solvatochromism, though it can become conductive upon doping [ 2,3].
Volume184,number5,6
CHEMICALPHYSICSLETTERS
This is a very intriguing point since one should expect poor conduction properties from a material formed by rigid, non-planar (and non-conjugated) chains. Work is in progress to investigate the relationship between structure and physical properties of poly ( alkylthiophenes ) .
Acknowledgement This work has been supported in part by the Brazilian Agencies Financiadora de Estudos e Projetos (FINEP), Conselho National de Desenvolvimento Cientifico e Tecnol6gico (CNPq), and FundaCZo de Amparo a Pesquisa do Estado de %o Paulo (FAPESP ). The authors are indebted to Prof. Mozart N. Ramos for the critical reading of the manuscript.
References [ 1] K.Jen. R. Obodi and R. Elsenbaumer, Polym. Mat. Sex.Eng.
4 October1991
[2] R. Lazzaroni, M. Liigdlung, S. StafstrGm, W.R. Salaneck and J.L. Brt!das, J. Chem. Phys. 93 (1990) 4433. [31 R.M. Souto Malor, K. Hinkelmann, H. Eckert and F. Wudl, Macromolecules 23 ( 1990) 1268. [4] 0. Inganis, G. Gustafsson and W.R.Salaneck, Synth. Metals 28 (1989) C377. [5] W.R. Salaneck, 0. Inganls, B. Thtmans, J.O. Nilsson, B. Sjiigren, J.-E. &terholm, J.L. Brkdas and S. Svensson, J. Chem. Phys. 89 (1988) 4613. [ 61 B. Thtmans, W.R. Salaneck and J.L. BrCdas, Synth. Metals 28 (1989) C359. [ 71 M.J. Winokur, D. Spiegel, Y. Kim, S. Hotta and A.J. Heeger, Synth. Metals 28 (1989) C419. [S] W.R. Salaneck, 0. InganXs, J.-O. Nilsson, J.-E. ijsterholm, B. Thtmans and J.L. Brkdas, Synth. Metals 28 (1989) C45 1. [9] M.J.S. Dewar, E.G. Zoebisch, E.F. Healy and J.J.P. Stewart, J. Am. Chem. Sot. 107 (1985) 4433. [lo] M.J.S. Dewar and W. Thiel, J. Am. Chem. Sot. 99 (1977) 4899,4907. [I l] W.M.F. Fabian. J. Comput. Chem. 9 (1989) 369. [ 121 D.A. dos Santos, D.S. GalvHo, B. Laks, M.W.C. Dezotti and M.-A. De Paoli, Chem. Phys. 144 ( 1990) 103. [ 131 B. ThCmans, J.M. Andre and J.L. Bredas, Synth. Metals 21 (1987) 149. [ 141 D.S. Galvio, D.A. dos Santos, B. Laks and MC. dos Santos, Proceedings of the International Conference on Synthetic Metals. Tiibingen, W. Germany, Sept. 1989, to appear.
53 (1985) 79; R.L. Eksenbaumer,KY. Jen and R. Obodi,Synth.Metals 15 (1986) 169.
583
CHEMICAL PHYSICS LETTERS
4 October 199 1
Conformational study on alkyl-substituted thiophene oligomers D.A. dos Santos, D.S. Galvgo, B. Laks Unirersidade Estadual de Campinas, Departamento de Fisica Aplicada, 13081 Campinas, SP, Bras2
and M.C. dos Santos Universidade Federal de Pernambuco. Departamento de Quimica Fundamental, 50739 Recije, PE, Brasil Received 1 June 1991
A series of Austin-method 1 geometry optimizations on alkyl-substituted thiophene small oligomers has been carried out. Torsion potential curves far dimers are obtained as a function of the length of the alkylxide group and the substitution position. Ground-state geometries are predicted to be quite independent of the radical length. On the other hand, different side-chain couplings give rise to distinct torsion potential curves. A planar ground-state geometry is obtained for 4,4’-dialkyl-bithiophene while in 3,3’ and 3.4’ coupled isomers, thiophenes are twisted by 90” but have distinct torsion barriers. The importance of the present results to the structural features of alkyl-substituted polythiophenes is discussed.
1. Introduction An important step towards processible conjugated polymers has been attained with the synthesis of soluble polymeric systems. Among the most studied soluble polymers are those belonging to the polythiophene family [ 11. Polythiophene becomes soluble in common organic solvents by the addition to its structure of long alkyl radicals which are connected to thiophenes in the p position. Thus, the resulting chains have a structure similar to that of polythiophene, with lateral saturated tails playing the role of fixed “solvent” molecules. Poly(alkyl-thiophenes) have slightly smaller electrical conductivities than the unsubstituted polymer upon doping [2]. On the other hand, the alkyl derivatives present a number of special features including [ 3,4]: (i) solubility allows the preparation of high molecular-weight chains, containing up to 800- 1000 aromatic rings; (ii) they have better crystallinity; (iii) their synthesis has opened up the possibility of preparing Langmuir-Blodgett films of conducting polymers; (iv) they show new interest0009-2614/91/$
ing effects like thermochromism and solvatochromism. Thermochromism has been investigated by many authors [2,4-g]. Although the exact mechanism responsible for the thermochromic transition remains unclear, several studies have demonstrated that conformational changes do occur upon heating and play a role in the transition. The first optical transition continuously and reversibly blue-shifts with increasing temperature. This behavior has been attributed to conformational changes involving out-of-plane twisting of thiophene rings on the otherwise planar chain. Those defects would occur in the substituted polymer (and not in pure polythiophene) due to the presence of the aliphatic tails which soften the interchain interactions. It is also believed that the torsion energies around thiophene-thiophene bonds may be affected by the presence of large lateral groups, since they may lead to steric hindrance. Thus, it is of interest to know what the torsion barriers are, and how they depend on the alkyl group, in terms of its length and substitution position. In this work, we present AM1 (Austin-method 1)
03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.
579
Volume 184, number 5,6
4 OctoberI991
CHEMICAL PHYSICS LETTERS
[ 91 geometry optimizations on alkyl-substituted thiophene oligomers. The length and position of the alkyl radical have been varied in order to investigate their influence on the torsion potential curve. The theory used in this work is presented in section 2. Results are discussed in section 3 and the concluding remarks appear in section 4.
R \
s (alhead-to-head S
*
R
2. Methodology Most geometry optimizations on small oligomers which are representative of conjugated polymers have been performed using MNDO (modified neglect of differential overlap) [ 6, lo]. Those ground-state geometries are assumed to be appropriate as the unit cell for polymer electronic-structure calculations. Our interest in the present study is to investigate possible modifications on geometry induced by the presence of lateral groups in planar thiophene oligomers, in which distortions involving out-of-plane twisting of thiophenes may be an important effect. It is well known that the MNDO technique is not able to reproduce dihedral angles in conjugated systems, so that we have chosen AM1 - an improved version of MNDO which has been developed to correct some of its weaknesses. Reports in the literature [ 91 seem to demonstrate that the main weaknesses have been largely corrected. For example, AM1 predicts dihedral angles very well, but the rotational barriers are underestimated [ 11,12 1, although general trends within a series of structurally related compounds seem to be correctly reproduced. We have considered three distinct couplings of the alkyl groups in dimers, as shown in fig. 1. Calculations on bithiophene have also been carried out for comparison. In all cases, a full geometry optimization has been performed #I.
3. Results In fig. 2 are shown the optimized geometries for (dihexyl)bithiophene in the three possible couplings @’All the calculationswereperformed on a vectorized IBM 3090 computerwiththe MOPAC program, version 5.0 (QCEP 560) using the option PARASOK to treat sulfur.
580
lblhead-to-toll
R
s
d
(c)tail-to-tail
q
s
R’
Fig. 1. The three different couplings on dimers we have investigated here: (a) head-to-head; (b) head-to-tall; and (c) tail-totail. Alkyl groups (R) were ethyl, butyl and hexyl.
and for the bithiophene molecule. We have obtained essentially the same geometries for the corresponding ethyl- and butyl-substituted species. Bond lengths and bond angles are quite insensitive to the presence of lateral groups, in agreement with the MNDO calculations by Thbmans and collaborators [ 131. However. the optimum torsion angle between the aromatic rings is drastically affected by the regiochemistry of substitution, as can be seen in figs. 2 and 3. Bithiophene and tail-to-tail coupled (dialkyl)bithiophene are both planar molecules, with potential barriers of 0.63 and 0.62 kcal, respectively. On the other hand, the head-to-head coupled isomer has a minimum-energy twist angle of 90”, with a barrier against the planar geometry of 2.65 kcal, while for the head-to-tail conformer, these values are 100.6” and 0.66 kcal. As the barriers could be sensitive to end effects and to o-n: interactions that arise in nonplanar systems, we have performed calculations on trimers and pentamers in all head-to-tail configurations, with ethyl radical as the substituent. The
Volume 184, number 5,6
CHEMICAL PHYSICS LETTERS
4 October 199 I
i 0
24
48
72
96
I20
4NGLElde grees)
(b)
(c)
(d)
Fig. 2. AMI optimized geometries of dimers for differentcouplings: (a) head-to-head; (b) head-to-tail; (c) tail-to-tail. The geometry of bithiophene molecule is also indicated in (d) for comparison. The lateral group is hexyl in all figures.
Fig. 3. Torsion potential curves for the three different couplings on dimers. In the inset, from top to bottom: (I) head-to-head, (0) head-to-tail and (A) tail-to-tail coupled (dialkyl)bithiw phenes.
ground-state geometries obtained are fully consistent with previous results: the aromatic rings are twisted by 90” relative to each other and the associated torsion energy is 1.2 kcal for the trimer, namely, 0,6 kcal per couple of rings. We recall that those values obtained for the torsion barriers may not be realistic, even though we believe they can be used to compare different conformers. The ionization potentials obtained in our calculations are 8.74 and 8.59 eV for bithienyl and tail-to-tail coupled conformer, and 9.16 and 9.25 eV for head-to-tail and head-to-head conformers. This sequence of ionization potential values is a direct consequence of their geometries, since in non-planar species, x orbitals tend to be more localized, leading to higher ionization potentials. Several theoretical investigations of poly (alkylthiophenes) have been reported in the literature [ 491, most of them using the torsion potential curve appropriate to the bithiophene molecule as a model for the torsion barrier in the substituted polymer. Our results have shown that torsion barriers may very much depend on the regiochemistry of substitution. Thermochromism is observed in substituted polythiophenes which are not regiospecific, the repeat unit being mostly like that of fig. la, or the head-to-tail substitution. A non-planar configuration is predieted for this substitution pattern, although a planar conformation of the chains is experimentally ob581
Volume 184,number $6
CHEMICALPHYSICSLETTERS
served [ 81. Thus, in the solid-state material, the packing energy as well as the energy associated to the n-electron conjugation (hereafter referred to as solidstate potential) should be large enough to compensate the energy barrier against planarity. The strength of the solid-state potential is expected to weaken with the P-substitution by longer side chains, which tends to reduce the minimum energy to induce the out-ofplane twisting. This is in agreement with the experimental observation of a decrease in the thermochromic transition temperature in a series of polythiophenes when substituted by longer side chains. We have investigated the effect of assuming a solidstate potential to be added to the molecular torsion barriers. The potential curves of fig. 3 were fitted to the analytical form I7(0)=A+Bcos(B)+Ccos(20))
(1)
and, for the solid-state potential, we have assumed the expression )
V,,(@=H[ 1 -cos(20)]
(2)
where H is half the height of the barrier. Thus, in a substituted polythiophene chain, the torsion potential around a given bond connecting rings is the summation of both contributions ( 1) and (2). Typical results are as shown in fig. 4 [ 141. We can interpret several experimental data on thermochromism by admitting that the ground-state
/
\
*/ 1’ , 0.
30
60
\\
\ >90
,/’
‘\\, \
_‘, 120 150 B(daQrser)
190
Fig. 4. Typical torsion potential curve, calculated following expressions ( I ) and (2) (see text): (a) molecular potential as obtained from AM1 calculations; (b) phenomenological soiidstate potential; (c) total potential curve.
582
4 October 1991
structure is trapped in the well around O”, namely, around the planar chain. Raising the temperature allows the rings to rotate and eventually overcome the barrier at 90”. These rings are now trapped in the well around 180” and are not likely to return to the original position on cooling. This effect might explain irreversible changes on the infrared spectrum observed on heating-cooling cycles performed on poly(alkylthiophenes) [ 41, since the rings which would be trapped in large torsion angles should give rise to specific infrared activity, not present in samples which have not been thermally treated. Our results seem to be in agreement with the experimental observation that a crystalline state is present in the material composed of head-to-tail dimers while head-to-head coupled molecules do not crystallize and form glasses at low temperature [ 3 1. We see from fig. 3 that the head-to-head isomer has a barrier against planarity which is about four times stronger than the corresponding one for head-to-tail isomer. Steric hindrance is, thus, maximized in the former case and should weaken intermolecular interactions. Moreover, the regiospecific head-to-head coupled poly (alkylthiophene) does not present solvatochromism [ 3,8], which implies a rigid backbone structure, and is consistent with a high torsion barrier.
4. Concluding remarks We have presented AM1 calculations for the investigation of torsion potential curves in alkyl-substituted thiophene oligomers as a function of substitution position and radical length. The results have shown significant differences among torsion potentials depending on the substitution pattern. They are important to interpret the properties of poly( alkylthiophenes), where thermochromism has been observed. A non-planar ground state is predicted for the headto-head substituted dimer, with a strong barrier against the planar configuration. This result appears to be in agreement with the fact that the material formed by those dimers is not able to crystallize. The polymer synthesized from those dimers is fully regiospecific and does not show solvatochromism, though it can become conductive upon doping [ 2,3].
Volume184,number5,6
CHEMICALPHYSICSLETTERS
This is a very intriguing point since one should expect poor conduction properties from a material formed by rigid, non-planar (and non-conjugated) chains. Work is in progress to investigate the relationship between structure and physical properties of poly ( alkylthiophenes ) .
Acknowledgement This work has been supported in part by the Brazilian Agencies Financiadora de Estudos e Projetos (FINEP), Conselho National de Desenvolvimento Cientifico e Tecnol6gico (CNPq), and FundaCZo de Amparo a Pesquisa do Estado de %o Paulo (FAPESP ). The authors are indebted to Prof. Mozart N. Ramos for the critical reading of the manuscript.
References [ 1] K.Jen. R. Obodi and R. Elsenbaumer, Polym. Mat. Sex.Eng.
4 October1991
[2] R. Lazzaroni, M. Liigdlung, S. StafstrGm, W.R. Salaneck and J.L. Brt!das, J. Chem. Phys. 93 (1990) 4433. [31 R.M. Souto Malor, K. Hinkelmann, H. Eckert and F. Wudl, Macromolecules 23 ( 1990) 1268. [4] 0. Inganis, G. Gustafsson and W.R.Salaneck, Synth. Metals 28 (1989) C377. [5] W.R. Salaneck, 0. Inganls, B. Thtmans, J.O. Nilsson, B. Sjiigren, J.-E. &terholm, J.L. Brkdas and S. Svensson, J. Chem. Phys. 89 (1988) 4613. [ 61 B. Thtmans, W.R. Salaneck and J.L. BrCdas, Synth. Metals 28 (1989) C359. [ 71 M.J. Winokur, D. Spiegel, Y. Kim, S. Hotta and A.J. Heeger, Synth. Metals 28 (1989) C419. [S] W.R. Salaneck, 0. InganXs, J.-O. Nilsson, J.-E. ijsterholm, B. Thtmans and J.L. Brkdas, Synth. Metals 28 (1989) C45 1. [9] M.J.S. Dewar, E.G. Zoebisch, E.F. Healy and J.J.P. Stewart, J. Am. Chem. Sot. 107 (1985) 4433. [lo] M.J.S. Dewar and W. Thiel, J. Am. Chem. Sot. 99 (1977) 4899,4907. [I l] W.M.F. Fabian. J. Comput. Chem. 9 (1989) 369. [ 121 D.A. dos Santos, D.S. GalvHo, B. Laks, M.W.C. Dezotti and M.-A. De Paoli, Chem. Phys. 144 ( 1990) 103. [ 131 B. ThCmans, J.M. Andre and J.L. Bredas, Synth. Metals 21 (1987) 149. [ 141 D.S. Galvio, D.A. dos Santos, B. Laks and MC. dos Santos, Proceedings of the International Conference on Synthetic Metals. Tiibingen, W. Germany, Sept. 1989, to appear.
53 (1985) 79; R.L. Eksenbaumer,KY. Jen and R. Obodi,Synth.Metals 15 (1986) 169.
583