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Simultaneous EPR electrochemical measurements on polyfluorene in ambient temperature ionic liquids
Synthetic Metals, 22 (1988) 407 - 414
407
SIMULTANEOUS EPR ELECTROCHEMICAL MEASUREMENTS ON P O L Y F L U O R E N E I N AMBIENT T E M P E R A T U R E IONIC LIQUIDS J. F. OUDARD, R. D. ALLENDOERFER and R. A. OSTERYOUNG
Department of Chemistry, State University of New York, Buffalo, N Y 14214 (U.S.A.) (Received July 29, 1987; accepted in revised form September 29, 1987)
Abstract Simultaneous electron paramagnetic resonance (EPR) and electrochemical measurements have been carried out on polyfluorene prepared by m o n o m e r oxidation and utilized in an ambient temperature ionic liquid consisting of 1-methyl-3-ethylimidazolium chloride-aluminum chloride. A considerable EPR signal is observed in both the reduced and oxidized states; the EPR signal achieves a maximum value coincident with the peak current during a cyclic voltammogram. Assuming that the process is two oneelectron steps, initially forming a radical cation, the difference between the two E ° values is estimated as less than 70 mV.
Introduction Unlike other conducting polymers such as polypyrrole [1, 2] or polythiophene [3], polyfluorene has been studied infrequently and to our knowledge no previous EPR measurements have been carried out on this material. The structure of fluorene (C13H10) is:
In this work, the EPR signal and the current or the charge have been measured simultaneously v e r s u s t h e potential in linear scan experiments or v e r s u s the time in chronoamperometric experiments. The solvent chosen here is AIC13:l-methyl-3-ethylimidazolium chloride (ImC1), an ambient temperature ionic liquid; it is viewed as a potential electrolyte for batteries and has been shown to be superior to classical organic solvents for electrochemical studies of polypyrrole [ 1, 4]. We have recently carried o u t an electrochemical characterization of polyfluorene in the A1C13:ImCl solvent [5]. The polymer film, formed by electropolymerization of the monomer, was shown to be conductive in the 0379-6779/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
408 oxidized state and non-conductive when reduced. It was shown that two protons are released per fluorene m o n o m e r during the electropolymerization process [5]. The polyfluorene films formed were more stable and their electrochemical behavior appeared less complicated than those prepared in acetonitrile [6 - 8]. Characterization of polymer films grown in these melts by other than electrochemical means is difficult and was not attempted here, since the infrared spectral background of the solvent [9] interferes with a direct, in situ infrared characterization, and efforts to separate and wash the polymer films w i t h o u t destroying them have been unsuccessful. Experimental section
EPR and electrochemical equipment An IBM/Bruker X-band EPR spectrometer model ER 200D-SRC with a 12" magnet and a dual channel Nicolet 2090 digital oscilloscope was used for recording both EPR and electrochemical data. An EG&G Parc 173 potentiostat equipped with a Parc 179 coulometer was used; it was controlled by a Parc 175 Universal Programmer. A Houston model 200 X Y recorder was used for recording some electrochemical data. EPR: electrochemical cell and calibration The cell employed and the calibration of the spectrometer have been described previously [ 10]. Solvent, materials and electroactive polymer film preparation The preparation of the melt used here has also been described elsewhere [11- 12]. Fluorene (Kodak) was used as received. Film preparation was carried out in a Vacuum Atmospheres Drybox under a purified, circulating helium atmosphere. Polyfluorene films were prepared by oxidation of a 0.1 M solution of fluorene at a constant current density of 0.055 mA cm -2 in a neutral (1.00: 1.00) AIC13:ImC1 melt in a cell similar to the simultaneous electrochemistry and EPR (SEEPR) cell described in ref. 10. Film thickness was roughly estimated from the charge used in the preparation assuming, as it is assumed for polypyrrole [1], that 240 mC cm -2 produces a 1 pm film. After polymerization by passing 1 C, resulting in a 0.2 #m film if uniformly deposited, the film was washed with a fresh monomer-free solution of the neutral melt, and then placed in a neutral A1C13:ImCI melt in the SEEPR cell. The cell was then sealed, removed from the drybox and placed in the EPR cavity.
Results and discussion
By applying a constant magnetic field, set to the value where the first derivative EPR absorption is a maximum, the EPR absorption was recorded
409 as a function of potential for cyclic voltammetric/linear scan experiments or as a function of time for double potential step experiments. Linear scan experiments The cyclic voltammograms obtained in the SEEPR cell for a polyfluorene-coated tungsten electrode prepared as described above, and the EPR signal (dotted line) recorded simultaneously with the current (full line) are shown in Fig. 1. The peak current does n o t vary linearly either with the scan rate or with the square root of the scan rate, as shown in Fig. 2. The EPR signal shows a background spin level in the neutral state as well as in the fully oxidized state. The intensity of this background absorption in the neutral state is about 2 X 1016 spins, which corresponds to about 1 spin per 140 monomers (-+30%). The existence of paramagnetic species in the neutral state has already been reported in the case of polyazulene (1 spin per 43 monomers) [13] and in the case of trans-polyacetylene (1 spin per 300 CH units) [14, 15]. The maximum EPR absorption decreases when the scan rate increases, which is consistent with the behavior of the charge for a process involving a mass transport phenomenon. At the lower scan rate (20 mV/s), the intensity of the EPR absorption at the maximum is 9 X 1016 spins (1 spin per 30 monomers {-+30%)). The shape of the EPR absorption versus potential can be interpreted as a process with two one-electron steps. The first step creates a paramagnetic species (polaron or radical cation}, which gives rise to the EPR absorption and the second step transforms this polaron (or radical cation} into a spinless species, thereby destroying the EPR absorption. Similar behavior has already been observed for other conducting polymers, as for example polypyrrole [10- 16]. Figure 3 shows the ratio of moles of spin to moles of electrons passed versus the potential for the anodic scan at 20 mV/s. The maximum of this curve, which occurs at the same potential as the maximum EPR absorption, is 0.3 + 30%. Using a Nernstian model for a process involving two oneelectron steps as previously described [10], we can estimate the equilibrium constant K, the disproportionation constant for the radical cation and the difference E ° - - E ° between the two formal potentials of the two electrochemical steps. This determination leads to: 0.25
--35mV < E ° - E °< +35mY This equilibrium constant is higher than that found in the case of polypyrrole (0.05 < K < 0.1) [10] in the same solvent, which indicates a greater stability for the dictation (or bipolaron) relative to the radical cation(or polaron) in the case of polyfluorene than in the case of polypyrrole. Double potential step experiments Two sets of double potential step experiments were performed on the polyfluorene-coated tungsten electrode in the SEEPR cell. The charge and
410
l
ic
3mAI
v=2OmV/s
(a)
. . . . . . .
~
. . . . . .
. . . .
E(V)vs A1/AI(III)in 1.5:1melt
(b)
..~_1 E(V) vs A 1 / A I { I I I )
0 in 1.5:1 melt
(c) Fig. 1. E P R signal ( . . . . ) and c u r r e n t ( ) recorded simultaneously on a polyfluorenec o a t e d (0.2 pro) t u n g s t e n e l e c t r o d e in neutral ( 1 . 0 0 : 1 . 0 0 ) AICl3:ImC1 melt. Scan rates were 100 m V s -1 for (a), 50 m V s-1 for (b) and 20 m V s -1 for (c). The m a g n e t i c field was set at t h e first derivative m a x i m u m EPR a b s o r p t i o n . The E P R a b s o r p t i o n is inverted for t h e a n o d i c scan.
411
1.5
o
o .5 _.
I 1.5 Log(Scan rate (mV/s))
Fig. 2. Logarithm o f peak current as a f u n c t i o n o f logarithm o f scan rate for a tungsten p o l y f l u o r e n e - c o a t e d e l e c t r o d e (0.2 p m ) in neutral (1.00:1.00) A1C13:ImCl melt. 0, Cathodic peak, [], anodic peak.
[
.25
I 0
I i 1.5
I
I
E(v) vs A t / A t ( I l l )
T in ~.5:1 melt
Fig. 3. Ratio o f m o l e s o f spin per m o l e s o f electron injected v s . p o t e n t i a l for a cyclic v o l t a m m e t r y e x p e r i m e n t o n a tungsten p o l y f l u o r e n e - c o a t e d e l e c t r o d e at 2 0 m V s -1 in neutral (1.00:1.00) A1C13:ImCl melt.
the EPR signal are both followed simultaneously v e r s u s time with the magnetic field fixed at the first derivative maximum of the EPR absorption. For the potential step experiment shown in Fig. 4, the initial and final potentials were 0.0 V (film in the neutral state) and the potential was stepped to 1.3 V, which is the region where the EPR absorption is maximum for the cyclic voltammetry at 20 mV/s. The EPR absorption reaches a steady-state value after about 10 s, whereas the charge is still increasing. The time response for the EPR signal is as fast for the anodic process as for the cathodic one, which is not true when the film is switched from neutral to
412
lO0
2O o LL z
50 lO
I
I
I
I0
20
30
(a)
Time (s)
1
--g
I 0
(b) Fig. 4. (a) Charge e x p e r i m e n t on a ImCl melt. Initial tuned to the first moles of electrons
!
I
I
lO
20
30 Time (s)
) and EPR a b s o r p t i o n ( . . . . ) vs. time for a d o u b l e p o t e n t i a l step ( tungsten p o l y f l u o r e n e - c o a t e d e l e c t r o d e in neutral ( 1 . 0 0 : 1 . 0 0 ) AICl3: p o t e n t i a l = 0.0 V and step p o t e n t i a l = 1.3 V. The magnetic field was derivative m a x i m u m EPR absorption. (b) Ratio o f m o l e s o f spin per injected vs. time for the e x p e r i m e n t described in (a).
fully oxidized (see below). The ratio of moles of spin to moles of electrons injected is almost constant at 0.4 + 30% during the entire step, which is in agreement with the value found by the cyclic voltammetry experiment (see above). Figure 5 shows the charge and the EPR signal for two potential step experiments when the film is switched from neutral to fully oxidized (Fig. 5(a), step from 0.0 V to 1.8 V) or switched from fully oxidized to neutral {Fig. 5(b), step from 1.8 V to 0.0 V). In Fig. 5(a) the film shows a background spin level in both the neutral and fully oxidized states. The shape of the EPR absorption after applying the anodic potential of 1.8 V clearly shows that substantially more spin than the equilibrium amount is created in
413
130
I00
Z 65 ~2 i
I 10
I 20
(a)
! 30
Time(s)
lOD
% 30
%
.E 65
%
J
I0
(b)
20
30
Ti~e(s)
Fig. 5. Charge ( ) and EPR a b s o r p t i o n ( . . . . ) vs. time for a d o u b l e p o t e n t i a l step e x p e r i m e n t on a tungsten p o l y f l u o r e n e - c o a t e d e l e c t r o d e in neutral ( 1 . 0 0 : 1 . 0 0 ) A1Cl3: ImC1 melt. The magnetic field was tuned to the first derivative m a x i m u m EPR absorption. (a) Initial p o t e n t i a l = 0.0 V and step p o t e n t i a l = 1.8 V. (b) Initial p o t e n t i a l = 1.8 V and step p o t e n t i a l = 0.0 V.
the process of reaching equilibrium. When the film is switched back to 0.0 V the EPR absorption increases, showing again the formation of a substantial non-equilibrium amount of spin, before decreasing very slowly to the background spin level. The comparison of the response of the EPR absorption after the two steps in Fig. 5(a) or in Fig. 5(b) leads to the conclusion that it is much faster to oxidize the radical cation (or polaron) to a doubly charged spinless species than to reduce it to a neutral species.
414
Conclusion
The simultaneous EPR and electrochemical measurements carried out on a polyfluorene film show the existence of a substantial spin level in both the neutral and fully oxidized states and allowed us to determine the disproportionation equilibrium constant of the intermediate radical cation (or polaron). The potential step experiments show very different behavior for the oxidation of the radical cation (or polaron) compared to its reduction. The behavior is also markedly different from that found for polypyrrole in the same solvent [ 10].
Acknowledgement This work was supported in part by the Air Force Office of Scientific Research.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
P. G. Pickup and R. A. Osteryoung, J. Am. Chem. Soc., 106 (1984) 2294. S. W. Feldberg, J. Am. Chem. Soc., 106 (1984) 4671. L. Janiszewska and R. A. Osteryoung, J. Electrovhem. Soc., 134 (1987) 2787. P. G. Pickup and R. A. Osteryoung, J. Electroanal. Chem., 195 (1985) 271. L. Janiszewska and R. A. Osteryoung, J. Electrochem. Soc., in press. G. Schiavon, G. Zotti and G. Bontempelli, J. Electroanal. Chem., 186 (1985) 191. R. J. Waltman and J. Bargon, J. Electroanal. Chem., 194 (1985) 49. J. Rault-Bethelot and J. Simonet, J. Electroanal. Chem., 182 (1985) 187. S. Tait and R. A. Osteryoung, Inorg. Chem., 23 (1984) 4352. J. F. Oudard, R. D. Allendoerfer and R. A. Osteryoung, J. Electroanal. Chem., in press. J. S. Wilkes, J. A. Levisky, R. A. Wilson and C. L. Hussey, Inorg. Chem., 21 (1982) 1213. A. A. Fannin, Jr., D. A. Floreani, L. A. King, J. S. Landers, B. J. Piersma, D. J. Stech. R. L. Vaughn, J. S. Wilkes and J. L. Williams, J. Phys. Chem., 88 (1984) 2614. S. Hayashi, S. Nakajima, K. Kaneto and K. Yoshino, Solid State Commun., 60 (1986) 545. I. B. Goldberg, H. R. Crowe, P. R. Newman, A. J. Heeger and A. G. MacDiarmid, J. Chem. Phys., 70 (1979) 1132. T. C. Chung, F. Moreas, J. D. Flood and A. J. Heeger, Phys. Rev. B, 29 (1984) 2341. M. Nechtschein, F. Devreux, F. Genoud, E. Vieil, J. M. Pernaut and E. Genies, Synth. Met., 15 {1986) 59.
407
SIMULTANEOUS EPR ELECTROCHEMICAL MEASUREMENTS ON P O L Y F L U O R E N E I N AMBIENT T E M P E R A T U R E IONIC LIQUIDS J. F. OUDARD, R. D. ALLENDOERFER and R. A. OSTERYOUNG
Department of Chemistry, State University of New York, Buffalo, N Y 14214 (U.S.A.) (Received July 29, 1987; accepted in revised form September 29, 1987)
Abstract Simultaneous electron paramagnetic resonance (EPR) and electrochemical measurements have been carried out on polyfluorene prepared by m o n o m e r oxidation and utilized in an ambient temperature ionic liquid consisting of 1-methyl-3-ethylimidazolium chloride-aluminum chloride. A considerable EPR signal is observed in both the reduced and oxidized states; the EPR signal achieves a maximum value coincident with the peak current during a cyclic voltammogram. Assuming that the process is two oneelectron steps, initially forming a radical cation, the difference between the two E ° values is estimated as less than 70 mV.
Introduction Unlike other conducting polymers such as polypyrrole [1, 2] or polythiophene [3], polyfluorene has been studied infrequently and to our knowledge no previous EPR measurements have been carried out on this material. The structure of fluorene (C13H10) is:
In this work, the EPR signal and the current or the charge have been measured simultaneously v e r s u s t h e potential in linear scan experiments or v e r s u s the time in chronoamperometric experiments. The solvent chosen here is AIC13:l-methyl-3-ethylimidazolium chloride (ImC1), an ambient temperature ionic liquid; it is viewed as a potential electrolyte for batteries and has been shown to be superior to classical organic solvents for electrochemical studies of polypyrrole [ 1, 4]. We have recently carried o u t an electrochemical characterization of polyfluorene in the A1C13:ImCl solvent [5]. The polymer film, formed by electropolymerization of the monomer, was shown to be conductive in the 0379-6779/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
408 oxidized state and non-conductive when reduced. It was shown that two protons are released per fluorene m o n o m e r during the electropolymerization process [5]. The polyfluorene films formed were more stable and their electrochemical behavior appeared less complicated than those prepared in acetonitrile [6 - 8]. Characterization of polymer films grown in these melts by other than electrochemical means is difficult and was not attempted here, since the infrared spectral background of the solvent [9] interferes with a direct, in situ infrared characterization, and efforts to separate and wash the polymer films w i t h o u t destroying them have been unsuccessful. Experimental section
EPR and electrochemical equipment An IBM/Bruker X-band EPR spectrometer model ER 200D-SRC with a 12" magnet and a dual channel Nicolet 2090 digital oscilloscope was used for recording both EPR and electrochemical data. An EG&G Parc 173 potentiostat equipped with a Parc 179 coulometer was used; it was controlled by a Parc 175 Universal Programmer. A Houston model 200 X Y recorder was used for recording some electrochemical data. EPR: electrochemical cell and calibration The cell employed and the calibration of the spectrometer have been described previously [ 10]. Solvent, materials and electroactive polymer film preparation The preparation of the melt used here has also been described elsewhere [11- 12]. Fluorene (Kodak) was used as received. Film preparation was carried out in a Vacuum Atmospheres Drybox under a purified, circulating helium atmosphere. Polyfluorene films were prepared by oxidation of a 0.1 M solution of fluorene at a constant current density of 0.055 mA cm -2 in a neutral (1.00: 1.00) AIC13:ImC1 melt in a cell similar to the simultaneous electrochemistry and EPR (SEEPR) cell described in ref. 10. Film thickness was roughly estimated from the charge used in the preparation assuming, as it is assumed for polypyrrole [1], that 240 mC cm -2 produces a 1 pm film. After polymerization by passing 1 C, resulting in a 0.2 #m film if uniformly deposited, the film was washed with a fresh monomer-free solution of the neutral melt, and then placed in a neutral A1C13:ImCI melt in the SEEPR cell. The cell was then sealed, removed from the drybox and placed in the EPR cavity.
Results and discussion
By applying a constant magnetic field, set to the value where the first derivative EPR absorption is a maximum, the EPR absorption was recorded
409 as a function of potential for cyclic voltammetric/linear scan experiments or as a function of time for double potential step experiments. Linear scan experiments The cyclic voltammograms obtained in the SEEPR cell for a polyfluorene-coated tungsten electrode prepared as described above, and the EPR signal (dotted line) recorded simultaneously with the current (full line) are shown in Fig. 1. The peak current does n o t vary linearly either with the scan rate or with the square root of the scan rate, as shown in Fig. 2. The EPR signal shows a background spin level in the neutral state as well as in the fully oxidized state. The intensity of this background absorption in the neutral state is about 2 X 1016 spins, which corresponds to about 1 spin per 140 monomers (-+30%). The existence of paramagnetic species in the neutral state has already been reported in the case of polyazulene (1 spin per 43 monomers) [13] and in the case of trans-polyacetylene (1 spin per 300 CH units) [14, 15]. The maximum EPR absorption decreases when the scan rate increases, which is consistent with the behavior of the charge for a process involving a mass transport phenomenon. At the lower scan rate (20 mV/s), the intensity of the EPR absorption at the maximum is 9 X 1016 spins (1 spin per 30 monomers {-+30%)). The shape of the EPR absorption versus potential can be interpreted as a process with two one-electron steps. The first step creates a paramagnetic species (polaron or radical cation}, which gives rise to the EPR absorption and the second step transforms this polaron (or radical cation} into a spinless species, thereby destroying the EPR absorption. Similar behavior has already been observed for other conducting polymers, as for example polypyrrole [10- 16]. Figure 3 shows the ratio of moles of spin to moles of electrons passed versus the potential for the anodic scan at 20 mV/s. The maximum of this curve, which occurs at the same potential as the maximum EPR absorption, is 0.3 + 30%. Using a Nernstian model for a process involving two oneelectron steps as previously described [10], we can estimate the equilibrium constant K, the disproportionation constant for the radical cation and the difference E ° - - E ° between the two formal potentials of the two electrochemical steps. This determination leads to: 0.25
--35mV < E ° - E °< +35mY This equilibrium constant is higher than that found in the case of polypyrrole (0.05 < K < 0.1) [10] in the same solvent, which indicates a greater stability for the dictation (or bipolaron) relative to the radical cation(or polaron) in the case of polyfluorene than in the case of polypyrrole. Double potential step experiments Two sets of double potential step experiments were performed on the polyfluorene-coated tungsten electrode in the SEEPR cell. The charge and
410
l
ic
3mAI
v=2OmV/s
(a)
. . . . . . .
~
. . . . . .
. . . .
E(V)vs A1/AI(III)in 1.5:1melt
(b)
..~_1 E(V) vs A 1 / A I { I I I )
0 in 1.5:1 melt
(c) Fig. 1. E P R signal ( . . . . ) and c u r r e n t ( ) recorded simultaneously on a polyfluorenec o a t e d (0.2 pro) t u n g s t e n e l e c t r o d e in neutral ( 1 . 0 0 : 1 . 0 0 ) AICl3:ImC1 melt. Scan rates were 100 m V s -1 for (a), 50 m V s-1 for (b) and 20 m V s -1 for (c). The m a g n e t i c field was set at t h e first derivative m a x i m u m EPR a b s o r p t i o n . The E P R a b s o r p t i o n is inverted for t h e a n o d i c scan.
411
1.5
o
o .5 _.
I 1.5 Log(Scan rate (mV/s))
Fig. 2. Logarithm o f peak current as a f u n c t i o n o f logarithm o f scan rate for a tungsten p o l y f l u o r e n e - c o a t e d e l e c t r o d e (0.2 p m ) in neutral (1.00:1.00) A1C13:ImCl melt. 0, Cathodic peak, [], anodic peak.
[
.25
I 0
I i 1.5
I
I
E(v) vs A t / A t ( I l l )
T in ~.5:1 melt
Fig. 3. Ratio o f m o l e s o f spin per m o l e s o f electron injected v s . p o t e n t i a l for a cyclic v o l t a m m e t r y e x p e r i m e n t o n a tungsten p o l y f l u o r e n e - c o a t e d e l e c t r o d e at 2 0 m V s -1 in neutral (1.00:1.00) A1C13:ImCl melt.
the EPR signal are both followed simultaneously v e r s u s time with the magnetic field fixed at the first derivative maximum of the EPR absorption. For the potential step experiment shown in Fig. 4, the initial and final potentials were 0.0 V (film in the neutral state) and the potential was stepped to 1.3 V, which is the region where the EPR absorption is maximum for the cyclic voltammetry at 20 mV/s. The EPR absorption reaches a steady-state value after about 10 s, whereas the charge is still increasing. The time response for the EPR signal is as fast for the anodic process as for the cathodic one, which is not true when the film is switched from neutral to
412
lO0
2O o LL z
50 lO
I
I
I
I0
20
30
(a)
Time (s)
1
--g
I 0
(b) Fig. 4. (a) Charge e x p e r i m e n t on a ImCl melt. Initial tuned to the first moles of electrons
!
I
I
lO
20
30 Time (s)
) and EPR a b s o r p t i o n ( . . . . ) vs. time for a d o u b l e p o t e n t i a l step ( tungsten p o l y f l u o r e n e - c o a t e d e l e c t r o d e in neutral ( 1 . 0 0 : 1 . 0 0 ) AICl3: p o t e n t i a l = 0.0 V and step p o t e n t i a l = 1.3 V. The magnetic field was derivative m a x i m u m EPR absorption. (b) Ratio o f m o l e s o f spin per injected vs. time for the e x p e r i m e n t described in (a).
fully oxidized (see below). The ratio of moles of spin to moles of electrons injected is almost constant at 0.4 + 30% during the entire step, which is in agreement with the value found by the cyclic voltammetry experiment (see above). Figure 5 shows the charge and the EPR signal for two potential step experiments when the film is switched from neutral to fully oxidized (Fig. 5(a), step from 0.0 V to 1.8 V) or switched from fully oxidized to neutral {Fig. 5(b), step from 1.8 V to 0.0 V). In Fig. 5(a) the film shows a background spin level in both the neutral and fully oxidized states. The shape of the EPR absorption after applying the anodic potential of 1.8 V clearly shows that substantially more spin than the equilibrium amount is created in
413
130
I00
Z 65 ~2 i
I 10
I 20
(a)
! 30
Time(s)
lOD
% 30
%
.E 65
%
J
I0
(b)
20
30
Ti~e(s)
Fig. 5. Charge ( ) and EPR a b s o r p t i o n ( . . . . ) vs. time for a d o u b l e p o t e n t i a l step e x p e r i m e n t on a tungsten p o l y f l u o r e n e - c o a t e d e l e c t r o d e in neutral ( 1 . 0 0 : 1 . 0 0 ) A1Cl3: ImC1 melt. The magnetic field was tuned to the first derivative m a x i m u m EPR absorption. (a) Initial p o t e n t i a l = 0.0 V and step p o t e n t i a l = 1.8 V. (b) Initial p o t e n t i a l = 1.8 V and step p o t e n t i a l = 0.0 V.
the process of reaching equilibrium. When the film is switched back to 0.0 V the EPR absorption increases, showing again the formation of a substantial non-equilibrium amount of spin, before decreasing very slowly to the background spin level. The comparison of the response of the EPR absorption after the two steps in Fig. 5(a) or in Fig. 5(b) leads to the conclusion that it is much faster to oxidize the radical cation (or polaron) to a doubly charged spinless species than to reduce it to a neutral species.
414
Conclusion
The simultaneous EPR and electrochemical measurements carried out on a polyfluorene film show the existence of a substantial spin level in both the neutral and fully oxidized states and allowed us to determine the disproportionation equilibrium constant of the intermediate radical cation (or polaron). The potential step experiments show very different behavior for the oxidation of the radical cation (or polaron) compared to its reduction. The behavior is also markedly different from that found for polypyrrole in the same solvent [ 10].
Acknowledgement This work was supported in part by the Air Force Office of Scientific Research.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
P. G. Pickup and R. A. Osteryoung, J. Am. Chem. Soc., 106 (1984) 2294. S. W. Feldberg, J. Am. Chem. Soc., 106 (1984) 4671. L. Janiszewska and R. A. Osteryoung, J. Electrovhem. Soc., 134 (1987) 2787. P. G. Pickup and R. A. Osteryoung, J. Electroanal. Chem., 195 (1985) 271. L. Janiszewska and R. A. Osteryoung, J. Electrochem. Soc., in press. G. Schiavon, G. Zotti and G. Bontempelli, J. Electroanal. Chem., 186 (1985) 191. R. J. Waltman and J. Bargon, J. Electroanal. Chem., 194 (1985) 49. J. Rault-Bethelot and J. Simonet, J. Electroanal. Chem., 182 (1985) 187. S. Tait and R. A. Osteryoung, Inorg. Chem., 23 (1984) 4352. J. F. Oudard, R. D. Allendoerfer and R. A. Osteryoung, J. Electroanal. Chem., in press. J. S. Wilkes, J. A. Levisky, R. A. Wilson and C. L. Hussey, Inorg. Chem., 21 (1982) 1213. A. A. Fannin, Jr., D. A. Floreani, L. A. King, J. S. Landers, B. J. Piersma, D. J. Stech. R. L. Vaughn, J. S. Wilkes and J. L. Williams, J. Phys. Chem., 88 (1984) 2614. S. Hayashi, S. Nakajima, K. Kaneto and K. Yoshino, Solid State Commun., 60 (1986) 545. I. B. Goldberg, H. R. Crowe, P. R. Newman, A. J. Heeger and A. G. MacDiarmid, J. Chem. Phys., 70 (1979) 1132. T. C. Chung, F. Moreas, J. D. Flood and A. J. Heeger, Phys. Rev. B, 29 (1984) 2341. M. Nechtschein, F. Devreux, F. Genoud, E. Vieil, J. M. Pernaut and E. Genies, Synth. Met., 15 {1986) 59.