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Solvent effect on the electronic absorption spectra of polyaniline
Synthetic Metals, 33 (1989) 11 - 17
11
SOLVENT E F F E C T ON THE ELECTRONIC ABSORPTION SPECTRA OF POLYANILINE SOUMYADEB GHOSH and V. KALPAGAM* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore (India)
(Received May 23, 1989; in revised form June 27, 1989; accepted July 5, 1989)
Abstract The electronic absorption peak at around 2 eV of polyaniline (in the emeraldine base form) solution is found to be highly sensitive to the dielectric constant of the solvent, showing a bathochromic shift. An increase in electron density on the imine nitrogen of the polymer, on '2 eV' excitation, has been concluded.
Introduction Polyaniline, although known since the last century under different names, is only recently arousing great interest among scientists in the field of conducting polymers. Apart from its unique properties, such as switching to a conducting state on exposure to acids with a simultaneous change in colour [1], electro-optic properties [2], etc., it has a high potential for technological applications because of its stability, high conductivity and solution processibility. The solubility of this material has opened up the scope for its detailed characterization. Since the recent discovery of solvents for polyaniline, reports on its molecular weight studies [3], vibrational [4] and electronic absorption properties [5], etc., have started to appear in large numbers. Electronic absorption spectra of polyaniline in its different forms have been taken in the solid state [6] and, more recently, in solution [5]. In the UV-Vis region, for the polyaniline base (PEB), there appear two peaks at around 2 and 4 eV. The electronic properties of polyaniline have been studied theoretically by many workers using techniques such as VEH [7] and various semi-empirical SCF methods [8]. It has been possible to assign the electronic absorption peaks to corresponding electronic transitions and the theoretical ~maxvalues match the experimental values quite accurately. Though the higher energy peak at about 4 eV has been assigned to a u-~* transition [9], there is controversy over the assignment of the lower * Author to whom correspondence should be addressed. 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
12 energy peak in the visible region. Duke e t al. [9] proposed the formation of a molecular exciton for interpretation of the '2 eV' peak. According to Stafstr6m e t al. [8], the peak is due to transitions of electrons from an electronic state centred on the benzenoid rings to another centred on the quinonoid rings. Kim e t al. [10], on the other hand, suggested a very different kind of hypothesis based on experimental results and assigned the peak to an n-Tr* transition from a non-bonding lone-pair state, created by broken conjugation at the imine nitrogen, to the conduction band. In this paper, we present some interesting results from studies of solvent effects on the electronic absorption spectra of polyaniline, in the polyemeraldine base (PEB) form. This might be important in solving the abovementioned, recent controversy.
Experimental Polyaniline was prepared by redox polymerization of aniline using (NHg)ES2Os [1]. Purified aniline (3.1 ml) was dissolved in 260 ml of a 7% aqueous solution of HC1. (NH4)2S2Os (9.4 g), dissolved in 50 ml water, was added dropwise for about ~A h with constant stirring, maintaining the temperature at 0 °C. The reaction was allowed to continue for another 12 h at the low temperature. The green precipitate was filtered, treated with a strong aqueous solution of ammonia, and thoroughly washed with water until a negative test for chloride ions was obtained. The deep blue coloured powder of PEB was dried under vacuum at 50 °C for several hours. The solvents were purified according to standard procedures. PEB was not fully soluble in all the solvents. Therefore, in order to use the same molecular weight fraction for all the studies, PEB was dissolved in chloroform, the poorest solvent for PEB among all the solvents used. The solution of PEB was filtered and the solvent was evaporated. The dried powder was again dissolved in different solvents and filtered, to make solutions in the concentration range 0.05 - 0.1%. Also, to study a higher molecular weight fraction of polymer, solid PEB was washed with 1,4-dioxane to remove the lower molecular weight oligomers and a higher molecular weight fraction was extracted with THF, dried and dissolved in different solvents. The polymer was characterized by IR spectroscopy using a PerkinElmer Model 597 spectrometer. The UV-Vis spectra were taken in the range 1000 - 300 nm scale using a UV-Vis spectrometer manufactured by Hitachi.
Results
The UV-Vis spectra of the PEB solution in different solvents is presented in Fig. 1. It was observed that the kmax values of the '2 eV' peak were dependent on the solvent used.
13
E
u c
i....(:".
' "..
b nm
"~,
\ ........ ,..-. /
',k
\ x( . . . . . . .
(a)
~00 600 800 Wave length (nm)
... \
;
'
..%
I
&O0
(b)
I
I
600
I
"-'1
800
Wave length (rim)
Fig. 1. U V Vis s p e c t r a o f (a) c h l o r o f o r m - s o l u b l e , ( b ) T H F - s o l u b l e f r a c t i o n o f P E B in different solvents: DMSO ( ), D M F ( ....... ), T H F ( - - - - - - ) , 1 , 4 - d i o x a n e ( . . . . . ) a n d CHC13 ( . . . . . . ).
It is well known t hat the shift in the electronic transition peak is connected with the dielectric constant (e) of the solvent and can be related by the equation [ 11 ] - - A E = / 2 g ( p e - - pg)f(e)/a 3 where AE is the energy of transition, pg and Pe are the dipole m o m e n t s of the solute in the ground and excited states respectively, a is the radius of the volume (spherical) occupied by a solute molecule and f(e) is a function of dielectric constant given by f(e)= 2 ( e - - 1 ) / ( 2 e + 1). The effect of the refractive index is neglected. The Xmax values from the spectra (Fig. l (a)) of the chloroform-soluble part of PEB in different solvents, along with the corresponding dielectric constants of the solvents, are given in Table 1. The observed Xmax values of the electronic transitions at ~ 2 eV fit well to the straight line corresponding to the above-mentioned equation (Fig. 2), except for the cases of 1,4-dioxane and dichloromethane. Macroscopic parameters, such as the dielectric constant, refractive index, etc., cannot always fully explain the solvatochromism, as it is d e p e n d e n t on m any ot her factors like the geometric as well as the electronic structure of bot h solute and solvent [ 1 2 ] , etc. However, it is quite clear that the '2 eV' peak of PEB shows a b a t h o c h r o m i c shift with an increase in the dielectric constant of the
14 TABLE 1 Values o f ~max for U V - V i s s p e c t r a o f PEB in d i f f e r e n t solvents w i t h c o r r e s p o n d i n g dielectric c o n s t a n t s Solvent
Dichloromethane Chloroform 1,4-Dioxane THF Pyridine DMF DMSO
Dielectric constant
Wavelength ( n m ) Peak I
Peak II
9.08 4.81 2.21 7.4 12.3 36.7 46.7
315.7 --318.2 -323.3 321.8
545.8 550.5 560.0 574.2 586.6 605.2 612.8
7 x
®
1 o
w
1
1(
i
04
i
06
i
1
08
J
i
)
1.0
f (C) --->
Fig. 2. Plot o f )kmax o f t h e '2 e V ' peak (in wave n u m b e r s ) against a f u n c t i o n o f t h e dielectric c o n s t a n t s o f t h e solvents f(e) = 2(e - - 1)/(2e + 1).
solvent. A similar shift was observed for the THF-soluble fraction of PEB, showing peaks at 617.0, 608.3 and 579.2 nm in the solvents DMSO, DMF and THF respectively (Fig. l(b)). THF is known to dissolve PEB chains with molecular weights as high as 104 [3a], hence the observed phenomenon can also be considered true for high molecular weight PEB.
Discussion The relation given above between the solvent shift and the dielectric constant implies that the direction of the shift (i.e. red or blue shift) depends on the relative magnitude of the dipole m o m e n t s of the solute molecules in
15 the excited and ground state respectively. Qualitatively, the phenomenon can be explained by the fact that interaction between dipole moments of solvent and solute molecules lowers the energy of the ground state of the solute. When the solute molecules are excited, they find themselves in the environment of solvent molecules oriented according to the ground state condition. If the dipole m o m e n t in the excited state, Pc, is greater than that in ground state, p~ (in the same direction), then the stabilization of excited state will be more than it were in the ground state, causing a red shift. If, on the other hand, Pc < P~, the effect will be a blue shift. A diagrammatic representation of energy levels, including a qualitative representation of their corresponding wave functions in an LCAO picture is reproduced in Fig. 3 from the paper of StafstrSm e t al. [8]. The '2 eV' peak is assigned to the nB-~Q transition, where ~B, the HOMO, is the electronic state extended over the three benzenoid rings and ~q is strongly localized to the quinonoid ring and two neighbouring imine nitrogen atoms of each repeat unit. In the ground state the dipole m o m e n t vectors are directed towards the nitrogen atoms because of their higher electronegativity. On electronic excitation of about 2 eV, the electron density on the imine nitrogen increases, enhancing the dipole m o m e n t in the same direction. So a red shift is expected, in full agreement with the experimental result obtained. In contrast, if the model proposed by Kim e t al. [10] is considered, the electron density on the imine nitrogen atoms should decrease due to the n - r * transition causing transfer of the lone pair of electrons on nitrogen to 14
I
N
E
ii
n
I
e
B-~
r
g
TEq--
\
\
Fig. 3. E n e r g y level d i a g r a m ( l e f t ) and a schematic p i c t u r e o f t h e i r respective o r b i t a l s (right) for PEB.
16
the quinonoid ring of the PEB unit. This depletion of electron density, decreasing the dipole m o m e n t , should produce a blue shift in our experiment. Further, the electron density on the nitrogen atoms does not change much due to the ~r-Tr* transition, hence the solvent effect, as observed, is small for the '4 eV' transition. The ~maxvalues for the lower energy peak of PEB in the visible region has been reported to be about 620 nm in solvents like DMSO and NMP [5], and a similar value has been obtained from solid state spectra [6]. Comparision of this value of kma x for PEB in DMSO with the values obtained from the chloroform-soluble and the THF-soluble fractions of PEB in the same solvent, indicates a dependence of the km~x value on the molecular weight of the polymer. From the observed solvatochromic effect on PEB, it seems that the true km~x value for this transition for isolated (i.e. non-interacting chains) should be much lower (higher energy) than that reported. The theoretical value predicted for this transition is 2.3 eV [8]. The higher value in the case of solid state spectroscopy may be due to interchain interaction which is considered to be significant in all conducting polymers.
Conclusions The '2 eV' peak of the electronic absorption spectra of PEB is highly sensitive to the solvent shift and, from the direction of the shift, it is assigned to the transition of electrons from the benzenoid rings to the quinonoid rings. Further, the kmax value for this transition in non-interacting PEB chains should be much lower than the reported values, which are, most probably, red shifted values due to the well-known solvatochromism. More detailed studies on this p h e n o m e n o n are expected to give a clearer picture of the excited state of polyaniline.
Acknowledgments We are grateful to R. Sumathi, N. Narendran, Vishalakshi and N. Balwalli for valuable discussions in connection with this work.
References 1 W. S. Huang, B. D. Humphrey and A. G. MacDiarmid, J. Chem. Soc., Faraday Trans. I, 82 (1986) 2385. 2 M. Kaneko and H. Nakamura, J. Chem. Soc., Chem. Commun., (1985) 346. 3 (a) A. Watanabe, K. Mori, Y. Iwasaki and Y. Nakamura, J. Chem. Soc., Chem. Commun., (1987) 3; (b) X. Tangi, Y. Sun and Y. Weuj, Makromol. Chem. Rapid. Commun., 9 (1988) 829. 4 J. Tang, X. Jing, B. Wang and F. Wang, Synth. Met., 24 (1988) 231. 5 (a) R. Jiang and S. Dong, Synth. Met., 24 (1988) 255; (b) F. Zuo, R. P. McCall,
17
6 7 8 9 10
11 12
J. M. Ginder, M. G. Roe, J. M. Leng, A. J. Epstein, G. E. Asturias, S. P. Ermer, A. Ray and A. G. MacDiarmid, Synth. Met., 29 (1989) E445. A. P. Monkman, D. Bloor, G. C. Stevens and J. C. H. Stevens, J. Phys. D: Appl. Phys., 20 (1987) 1337. S. Stafstr6m and J. L. Br~das, Synth. Met., 14 (1986) 297. S. StafstrSm, B. SjSgren and J. L. Br~das, Synth. Met., 29 (1989) E219, and refs. therein. C. B. Duke, E. M. Conwell and A. Paton, Chem. Phys. Lett., 131 (1986) 82. (a) Y. H. Kim, C. Foster, J. Chiang and A. J. Heeger, Synth. Met., 29 (1989) E285; (b) Y. H. Kim, S. D. Phillips, M. J. Nowak, D. Speigel, C. Foster, G. Yu, J. Chiang and A. J. Heeger, Synth. Met., 29 (1989) E291. D. Grasso and E. Bello, Chem. Phys. Lett., 30 (1975) 241. C. Reichardt, Angew. Chem., Int. Ed. Engl., 4 (1965) 29.
11
SOLVENT E F F E C T ON THE ELECTRONIC ABSORPTION SPECTRA OF POLYANILINE SOUMYADEB GHOSH and V. KALPAGAM* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore (India)
(Received May 23, 1989; in revised form June 27, 1989; accepted July 5, 1989)
Abstract The electronic absorption peak at around 2 eV of polyaniline (in the emeraldine base form) solution is found to be highly sensitive to the dielectric constant of the solvent, showing a bathochromic shift. An increase in electron density on the imine nitrogen of the polymer, on '2 eV' excitation, has been concluded.
Introduction Polyaniline, although known since the last century under different names, is only recently arousing great interest among scientists in the field of conducting polymers. Apart from its unique properties, such as switching to a conducting state on exposure to acids with a simultaneous change in colour [1], electro-optic properties [2], etc., it has a high potential for technological applications because of its stability, high conductivity and solution processibility. The solubility of this material has opened up the scope for its detailed characterization. Since the recent discovery of solvents for polyaniline, reports on its molecular weight studies [3], vibrational [4] and electronic absorption properties [5], etc., have started to appear in large numbers. Electronic absorption spectra of polyaniline in its different forms have been taken in the solid state [6] and, more recently, in solution [5]. In the UV-Vis region, for the polyaniline base (PEB), there appear two peaks at around 2 and 4 eV. The electronic properties of polyaniline have been studied theoretically by many workers using techniques such as VEH [7] and various semi-empirical SCF methods [8]. It has been possible to assign the electronic absorption peaks to corresponding electronic transitions and the theoretical ~maxvalues match the experimental values quite accurately. Though the higher energy peak at about 4 eV has been assigned to a u-~* transition [9], there is controversy over the assignment of the lower * Author to whom correspondence should be addressed. 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
12 energy peak in the visible region. Duke e t al. [9] proposed the formation of a molecular exciton for interpretation of the '2 eV' peak. According to Stafstr6m e t al. [8], the peak is due to transitions of electrons from an electronic state centred on the benzenoid rings to another centred on the quinonoid rings. Kim e t al. [10], on the other hand, suggested a very different kind of hypothesis based on experimental results and assigned the peak to an n-Tr* transition from a non-bonding lone-pair state, created by broken conjugation at the imine nitrogen, to the conduction band. In this paper, we present some interesting results from studies of solvent effects on the electronic absorption spectra of polyaniline, in the polyemeraldine base (PEB) form. This might be important in solving the abovementioned, recent controversy.
Experimental Polyaniline was prepared by redox polymerization of aniline using (NHg)ES2Os [1]. Purified aniline (3.1 ml) was dissolved in 260 ml of a 7% aqueous solution of HC1. (NH4)2S2Os (9.4 g), dissolved in 50 ml water, was added dropwise for about ~A h with constant stirring, maintaining the temperature at 0 °C. The reaction was allowed to continue for another 12 h at the low temperature. The green precipitate was filtered, treated with a strong aqueous solution of ammonia, and thoroughly washed with water until a negative test for chloride ions was obtained. The deep blue coloured powder of PEB was dried under vacuum at 50 °C for several hours. The solvents were purified according to standard procedures. PEB was not fully soluble in all the solvents. Therefore, in order to use the same molecular weight fraction for all the studies, PEB was dissolved in chloroform, the poorest solvent for PEB among all the solvents used. The solution of PEB was filtered and the solvent was evaporated. The dried powder was again dissolved in different solvents and filtered, to make solutions in the concentration range 0.05 - 0.1%. Also, to study a higher molecular weight fraction of polymer, solid PEB was washed with 1,4-dioxane to remove the lower molecular weight oligomers and a higher molecular weight fraction was extracted with THF, dried and dissolved in different solvents. The polymer was characterized by IR spectroscopy using a PerkinElmer Model 597 spectrometer. The UV-Vis spectra were taken in the range 1000 - 300 nm scale using a UV-Vis spectrometer manufactured by Hitachi.
Results
The UV-Vis spectra of the PEB solution in different solvents is presented in Fig. 1. It was observed that the kmax values of the '2 eV' peak were dependent on the solvent used.
13
E
u c
i....(:".
' "..
b nm
"~,
\ ........ ,..-. /
',k
\ x( . . . . . . .
(a)
~00 600 800 Wave length (nm)
... \
;
'
..%
I
&O0
(b)
I
I
600
I
"-'1
800
Wave length (rim)
Fig. 1. U V Vis s p e c t r a o f (a) c h l o r o f o r m - s o l u b l e , ( b ) T H F - s o l u b l e f r a c t i o n o f P E B in different solvents: DMSO ( ), D M F ( ....... ), T H F ( - - - - - - ) , 1 , 4 - d i o x a n e ( . . . . . ) a n d CHC13 ( . . . . . . ).
It is well known t hat the shift in the electronic transition peak is connected with the dielectric constant (e) of the solvent and can be related by the equation [ 11 ] - - A E = / 2 g ( p e - - pg)f(e)/a 3 where AE is the energy of transition, pg and Pe are the dipole m o m e n t s of the solute in the ground and excited states respectively, a is the radius of the volume (spherical) occupied by a solute molecule and f(e) is a function of dielectric constant given by f(e)= 2 ( e - - 1 ) / ( 2 e + 1). The effect of the refractive index is neglected. The Xmax values from the spectra (Fig. l (a)) of the chloroform-soluble part of PEB in different solvents, along with the corresponding dielectric constants of the solvents, are given in Table 1. The observed Xmax values of the electronic transitions at ~ 2 eV fit well to the straight line corresponding to the above-mentioned equation (Fig. 2), except for the cases of 1,4-dioxane and dichloromethane. Macroscopic parameters, such as the dielectric constant, refractive index, etc., cannot always fully explain the solvatochromism, as it is d e p e n d e n t on m any ot her factors like the geometric as well as the electronic structure of bot h solute and solvent [ 1 2 ] , etc. However, it is quite clear that the '2 eV' peak of PEB shows a b a t h o c h r o m i c shift with an increase in the dielectric constant of the
14 TABLE 1 Values o f ~max for U V - V i s s p e c t r a o f PEB in d i f f e r e n t solvents w i t h c o r r e s p o n d i n g dielectric c o n s t a n t s Solvent
Dichloromethane Chloroform 1,4-Dioxane THF Pyridine DMF DMSO
Dielectric constant
Wavelength ( n m ) Peak I
Peak II
9.08 4.81 2.21 7.4 12.3 36.7 46.7
315.7 --318.2 -323.3 321.8
545.8 550.5 560.0 574.2 586.6 605.2 612.8
7 x
®
1 o
w
1
1(
i
04
i
06
i
1
08
J
i
)
1.0
f (C) --->
Fig. 2. Plot o f )kmax o f t h e '2 e V ' peak (in wave n u m b e r s ) against a f u n c t i o n o f t h e dielectric c o n s t a n t s o f t h e solvents f(e) = 2(e - - 1)/(2e + 1).
solvent. A similar shift was observed for the THF-soluble fraction of PEB, showing peaks at 617.0, 608.3 and 579.2 nm in the solvents DMSO, DMF and THF respectively (Fig. l(b)). THF is known to dissolve PEB chains with molecular weights as high as 104 [3a], hence the observed phenomenon can also be considered true for high molecular weight PEB.
Discussion The relation given above between the solvent shift and the dielectric constant implies that the direction of the shift (i.e. red or blue shift) depends on the relative magnitude of the dipole m o m e n t s of the solute molecules in
15 the excited and ground state respectively. Qualitatively, the phenomenon can be explained by the fact that interaction between dipole moments of solvent and solute molecules lowers the energy of the ground state of the solute. When the solute molecules are excited, they find themselves in the environment of solvent molecules oriented according to the ground state condition. If the dipole m o m e n t in the excited state, Pc, is greater than that in ground state, p~ (in the same direction), then the stabilization of excited state will be more than it were in the ground state, causing a red shift. If, on the other hand, Pc < P~, the effect will be a blue shift. A diagrammatic representation of energy levels, including a qualitative representation of their corresponding wave functions in an LCAO picture is reproduced in Fig. 3 from the paper of StafstrSm e t al. [8]. The '2 eV' peak is assigned to the nB-~Q transition, where ~B, the HOMO, is the electronic state extended over the three benzenoid rings and ~q is strongly localized to the quinonoid ring and two neighbouring imine nitrogen atoms of each repeat unit. In the ground state the dipole m o m e n t vectors are directed towards the nitrogen atoms because of their higher electronegativity. On electronic excitation of about 2 eV, the electron density on the imine nitrogen increases, enhancing the dipole m o m e n t in the same direction. So a red shift is expected, in full agreement with the experimental result obtained. In contrast, if the model proposed by Kim e t al. [10] is considered, the electron density on the imine nitrogen atoms should decrease due to the n - r * transition causing transfer of the lone pair of electrons on nitrogen to 14
I
N
E
ii
n
I
e
B-~
r
g
TEq--
\
\
Fig. 3. E n e r g y level d i a g r a m ( l e f t ) and a schematic p i c t u r e o f t h e i r respective o r b i t a l s (right) for PEB.
16
the quinonoid ring of the PEB unit. This depletion of electron density, decreasing the dipole m o m e n t , should produce a blue shift in our experiment. Further, the electron density on the nitrogen atoms does not change much due to the ~r-Tr* transition, hence the solvent effect, as observed, is small for the '4 eV' transition. The ~maxvalues for the lower energy peak of PEB in the visible region has been reported to be about 620 nm in solvents like DMSO and NMP [5], and a similar value has been obtained from solid state spectra [6]. Comparision of this value of kma x for PEB in DMSO with the values obtained from the chloroform-soluble and the THF-soluble fractions of PEB in the same solvent, indicates a dependence of the km~x value on the molecular weight of the polymer. From the observed solvatochromic effect on PEB, it seems that the true km~x value for this transition for isolated (i.e. non-interacting chains) should be much lower (higher energy) than that reported. The theoretical value predicted for this transition is 2.3 eV [8]. The higher value in the case of solid state spectroscopy may be due to interchain interaction which is considered to be significant in all conducting polymers.
Conclusions The '2 eV' peak of the electronic absorption spectra of PEB is highly sensitive to the solvent shift and, from the direction of the shift, it is assigned to the transition of electrons from the benzenoid rings to the quinonoid rings. Further, the kmax value for this transition in non-interacting PEB chains should be much lower than the reported values, which are, most probably, red shifted values due to the well-known solvatochromism. More detailed studies on this p h e n o m e n o n are expected to give a clearer picture of the excited state of polyaniline.
Acknowledgments We are grateful to R. Sumathi, N. Narendran, Vishalakshi and N. Balwalli for valuable discussions in connection with this work.
References 1 W. S. Huang, B. D. Humphrey and A. G. MacDiarmid, J. Chem. Soc., Faraday Trans. I, 82 (1986) 2385. 2 M. Kaneko and H. Nakamura, J. Chem. Soc., Chem. Commun., (1985) 346. 3 (a) A. Watanabe, K. Mori, Y. Iwasaki and Y. Nakamura, J. Chem. Soc., Chem. Commun., (1987) 3; (b) X. Tangi, Y. Sun and Y. Weuj, Makromol. Chem. Rapid. Commun., 9 (1988) 829. 4 J. Tang, X. Jing, B. Wang and F. Wang, Synth. Met., 24 (1988) 231. 5 (a) R. Jiang and S. Dong, Synth. Met., 24 (1988) 255; (b) F. Zuo, R. P. McCall,
17
6 7 8 9 10
11 12
J. M. Ginder, M. G. Roe, J. M. Leng, A. J. Epstein, G. E. Asturias, S. P. Ermer, A. Ray and A. G. MacDiarmid, Synth. Met., 29 (1989) E445. A. P. Monkman, D. Bloor, G. C. Stevens and J. C. H. Stevens, J. Phys. D: Appl. Phys., 20 (1987) 1337. S. Stafstr6m and J. L. Br~das, Synth. Met., 14 (1986) 297. S. StafstrSm, B. SjSgren and J. L. Br~das, Synth. Met., 29 (1989) E219, and refs. therein. C. B. Duke, E. M. Conwell and A. Paton, Chem. Phys. Lett., 131 (1986) 82. (a) Y. H. Kim, C. Foster, J. Chiang and A. J. Heeger, Synth. Met., 29 (1989) E285; (b) Y. H. Kim, S. D. Phillips, M. J. Nowak, D. Speigel, C. Foster, G. Yu, J. Chiang and A. J. Heeger, Synth. Met., 29 (1989) E291. D. Grasso and E. Bello, Chem. Phys. Lett., 30 (1975) 241. C. Reichardt, Angew. Chem., Int. Ed. Engl., 4 (1965) 29.