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Vibrational analysis of reduced and oxidized forms of polyaniline
Synthetic Metals, 55-57 (1993) 475-480
475
VIBRATIONAL ANALYSIS OF REDUCED AND OXIDIZED FORMS OF POLYANILINE
S. QUILLARD, G. LOUARN, J.P. BUISSON and S. LEFRANT Laboratoire de Physique Cristalline, I.M.N., 2 rue de la Houssini~re, 44072 Nantes Cedex 03 (France) J. MASTERS and A.G. MACDIARMID Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323 (USA)
ABSTRACT In this paper, we present dynamic calculations of vibrational modes of two forms of polyaniline : the fully reduced form (Leucoemeraldine Base) and the fully oxidized form (Pernigraniline Base). Our model is based on the use of force constants to provide an interpretation of experimental frequencies obtained by Resonant Raman Scattering and infrared absorption. In order to obtain the best set of parameters, we have performed calculations on both polymers and model compounds. Our model leads to a good fit between experimental and calculated frequencies, also we present the main force constants which are in agreement with previous works performed on similar aromatic structures.
INTRODUCTION The Polyanilines relate to a large class of polymers since several forms of these compounds can be obtained. These different forms of polyaniline are defined by two parameters : the average oxidation state [1,2] and the degree of protonation [3]. In this work, we have focused our attention on the two forms of polyaniline which are the fully reduced non protonated form (Leucoemeraldine Base : LB) and the fully oxidized non protonated form (Pemigraniline Base : PB). Our purpose is to obtain information on the structure of the polymers via their vibrational properties. This vibrational study starts by the use of standard spectroscopies such as Resonance Raman Scattering (RRS) and infrared absorption. The experimental part is followed by dynamical calculations based on a valence-force-field model. In order to improve the set of force constants, these calculations are also performed on some model compounds of the polymers. Diphenylamine (DPA) and N,N'-diphenyl-l,4-phenylenediamine (PCD) were chosen as model compounds for the Leucoemeraldine Base and N,N'-diphenyl-l,4phenylenediimine (OPCD) for the Pernigraniline Base. We have obtained a good fit between experimental and calculated frequencies for all compounds. The set of force constants that we obtained is consistent with the geometrical parameters of the compounds and with values found in other aromatic compounds. 0379-6779/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
476 EXPERIMENTAL RESULTS Raman spectra of all compounds were performed on a double monochromator Jobin Yvon (Ramanor HG2S). All samples were in powder forms, pressed in pellets for Raman spectroscopy. The Raman signal was collected at 90" and the laser power on the samples never exceeded 100 mW. The infrared absorption was performed with a Fourier Transform Infrared Spectrophotometer (Nicolet 20SXC). All compounds were pressed in KBr pellets. Raman spectra of benzoquinone, OPCD and pemigraniline
base are presented figure 1. Other
spectra of both polymers and their model compounds have been published elsewhere [4,5]. Two different excitation wavelengths have been used for the Raman spectra of the fully oxidized polymer (PB) which exhibits two bands on its UV-VIS optical absorption spectrum : ko = 457,9 nm is expected to enhance "reduced units" and ~.o = 647,1 nm "oxidized units".
o~
o
~
~a~
Jl
~: lo
= 647,1
nm
to
= 514,5
nm
r~
~,
i
,
A
eq
¢~
jc) .
h/~
,~
~,o = 4 5 7 , 9
nm
~
_
p
:/| ~o
700
= 647,1
I|-
I
nm
1000 to(cm
1300 -1)
1600 _-.--
Figure 1 : Resonant Raman Spectra of : a) benzoquinone, b) OPCD, c)and d) pemigraniline base (~,o = 457.9 nm and 647.1 nm)
477
VIBRATIONAL ANALYSIS In order to interpret Raman and infrared bands observed in our compounds, we have performed dynamic calculations based on the use of a valence-force-field model. Using this model, we made the hypothesis of planar molecules and polymers. Then, one can separate in-plane and out-of-plane vibrations. According to the above assumption, we can obtain the point group of each compound and the active modes in each symmetry. Table 1 presents these different point groups for all the molecules and polymers studied. Our study concern only the region between 300 and 1700 cm -1, which means that we have not taken into account the modes around 3000 cm -1 and 3400 cm -1 corresponding respectively to the C-H stretching and the N-H stretching vibrations.
DPA (C2v)
23 A1 (R,IR) + 22 B2 (R, IR)
PCD (C2h)
35 Ag (R) + 34 Bu (IR)
Leucoemeraldine (D2h) Base
12 Ag (R) + 12 B3g (R) + 11 Blu (IR) + 11 B2u (IR)
OPCD (C2h)
33 Ag (R) + 32 Bu (IR)
Pemigraniline (C2h) Base
22 Ag (R) + 20 Bu (IR)
Table 1: Different point groups and number of predicted modes (and their activity) for each compound
We have performed two different calculations : one for the compounds DPA, PCD and LB ("reduced structure") and the other for OPCD and PB ("oxidized structure"). For each one, we have chosen a set of force constants defined by : 32 0
where (~ is the potential energy and r, r' are two internal coordinates. The unit cell of the polymers are proposed in figure 2 with the definition of the internal coordinates. The geometrical parameters have been obtained by MNDO computations [6]. In order to insure the values of our parameters, we performed dynamic calculations simultaneously on the model molecules and on the corresponding polymer.
reduced unit
oxidized unit
Figure 2 : definition of internal coordinates of the reduced and oxidized units
478
We have at our disposal 34 parameters to interpret 73 experimental frequencies of the "reduced structure" and 56 parameters to interpret 52 experimental frequencies of the "oxidized structure". However, only a few of these force constants are of real importance for the calculations. Both set of parameters are separated in three groups as follows : non perturbed ring (benzene or quinoid), perturbed ring, and intercycle group (amine -NH- or imine -N= group). The force constants related to the non perturbed ring are kept close to those used in previous works on benzene [7] and on p-benzoquinone [8]. That leaves us in fact 23 parameters for the "reduced part" and 45 for the "oxidized part". Our main force constants are reported in table 2. Experimental and calculated frequencies of both forms of polymers studied are presented on table 3. We have also reported the assignments of all the vibrational modes. The origin (B : benzene or Q : quinoid) of the vibrations is reported and the modes of the benzene ring are correlated to the Wilson notation. Keeping in mind that the number of
predicted bands is quite large, one can expect some
uncertainties still to remain in our interpretation, especially concerning non observed modes. However, the main bands of the spectra are rather well interpreted and we have obtained a good agreement between experimental and calculated frequencies.
a) non perturbed and perturbed benzene ring Fs 2
Ft 2
Fe~2
F¢2
Fst
5,08
6,32
0,99
0,50
Fltt
Ftt~
Ft¢
Ft'2
Fct'2
F¢'2
F t ' ¢ ' Ft'ct' F a t ~ '
-0,03 0,87
0,17
0,16
6,32
0,82
1,07
0,31
0,37
0,13
b) amine group FR 2
F s '2
F~ 2
F 8 '2
ERR
FR8
FRS'
5,31
6,21
0,63
1,45
0,60
0,21
0,37
c) non perturbed and perturbed benzene ring FD2
FQt 2
FQtx 2
8,19
4,62
1,13
FQ~ 2 FQlO 2 FDQct F1DQt F1QtQt 0,30
0,60
0.50
0,40
0,23
FQt 2 FQct'2 FQtQ~' F Q 0 '2 4,62
0,55
d) imine group FQR 2
F Q R '2
5,00
9,19
F Q 8 '2 F Q R Q R ' FQRtx' 1,97
0,49
-0,32
FQRQS' F Q R ' Q S ' F Q R ' Q t t ' 0,59
0,59
Table 2 : values of main force constants used in the valence-force-fields
-1,04
0,24
1,02
479
0
.=_
Z ©
+
o+~ .°
O0 Cq
t~-
i
o
c~
H o
J
<
Z
480 The main point to emphasize in the study on the "reduced structure" is the determination of the force constants related to the amine group, in addition to those of the benzene ring. Ft 2 and Ft '2 related to the carbon-carbon bonds inside the benzene ring are equal to 6.32 mdyn/A. FR 2 corresponds to the carbon-nitrogen bond of the amine group. It is found at 5.31 mdyn/A which is consistent with other values obtained for similar single bonds. Comparable values are found at 4.80 mdyn/A in the PPV [9] and at 5.31 mdyn/A in PTV [10] (both for a intercycle C-C stretching vibration). Concerning the "oxidized part", the dynamical calculations lead to parameters of the quinoid ring such as FD 2 = 8.19 mdyn/A for the carbon-carbon double bond and FQt 2 = 4.62 mdyn/A for the single bond of the ring, These values are very similar to those obtained for the benzoquinone and Ft 2 = 6.32 mdyn/A calculated for the benzene ring is situated between these two values. The main force constant corresponding to C=N and referred as FQR '2 is obtained at 9.19 mdyn/A. The parameter FQR 2 is equal to 5.00 mdyn/A. The quinoid ring is also characterized by an important decrease of coupling intracycle constants : from Fltt = 0.87 in the benzene ring, we pass to F1QtQt = 0.23 and F1DQt = 0.40 in the quinoid ring. We can already notice a good agreement between these force constants and the geometrical parameters [6] and our model leads to a good interpretation of the observed modes coming from the quinoid ring or the benzene ring. CONCLUSIONS We have made a complete interpretation of the Raman and infrared vibrational modes of two forms of polyaniline : the Leucoemeraldine Base and the Pernigraniline Base. By the use of a valence-force-field model, we obtain two sets of force constants concerning the reduced structure and the oxidized structure. These parameters interpret the quinoid or the benzenoid character as well of the imine or amine group. This demonstrates the great importance played by the vibrational spectroscopies for a comprehensive investigation of the electronic properties in this type of polymer. REFERENCES 1
A.G. Green and A.E. Woodhead, ,l, Chgm. Soc. Trans. 97. (1910) 2388.
2
A.G. Green and A.E. Woodhead, J. Chem. Soc. Trans. 101. (1912) 1117.
3
A. Ray, A.F. Richter, A.G. MacDiarmid and A.J. Epstein, $ynth. Met.. 29, (1989) 151.
4
S. Quillard, G. Louarn, J. P. Buisson, S. Lefrant, J. Masters and A. G. MacDiarmid, Sorin~er Series in Solid-State Sciences. 107. (1992) 271.
5
S. Quillard, G. Louam, J. P. Buisson, S. Lefrant, J. Masters and A. G. MacDiarmid, Svnth. Met..
6
M.C. Dos Santos and J.L. Br6das, Svnth. Met.. 29, (1989) 321.
(1992) in press. 7
J.Y. M6vellec, J. P. Buisson, S. Lefrant and H. Eckhardt, Synth. Met.. 41, (1991) 283.
8
L.-O Pi6til/t, K. Palm0 and B. Mannfors, Journal of Molecular Soectroscoov. 116, (1986) 1.
9
H. Eckhardt, K.Y. Jen and L.W. Shacklette, Conjugated Polvmeric Materials. 305. (1990).
475
VIBRATIONAL ANALYSIS OF REDUCED AND OXIDIZED FORMS OF POLYANILINE
S. QUILLARD, G. LOUARN, J.P. BUISSON and S. LEFRANT Laboratoire de Physique Cristalline, I.M.N., 2 rue de la Houssini~re, 44072 Nantes Cedex 03 (France) J. MASTERS and A.G. MACDIARMID Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323 (USA)
ABSTRACT In this paper, we present dynamic calculations of vibrational modes of two forms of polyaniline : the fully reduced form (Leucoemeraldine Base) and the fully oxidized form (Pernigraniline Base). Our model is based on the use of force constants to provide an interpretation of experimental frequencies obtained by Resonant Raman Scattering and infrared absorption. In order to obtain the best set of parameters, we have performed calculations on both polymers and model compounds. Our model leads to a good fit between experimental and calculated frequencies, also we present the main force constants which are in agreement with previous works performed on similar aromatic structures.
INTRODUCTION The Polyanilines relate to a large class of polymers since several forms of these compounds can be obtained. These different forms of polyaniline are defined by two parameters : the average oxidation state [1,2] and the degree of protonation [3]. In this work, we have focused our attention on the two forms of polyaniline which are the fully reduced non protonated form (Leucoemeraldine Base : LB) and the fully oxidized non protonated form (Pemigraniline Base : PB). Our purpose is to obtain information on the structure of the polymers via their vibrational properties. This vibrational study starts by the use of standard spectroscopies such as Resonance Raman Scattering (RRS) and infrared absorption. The experimental part is followed by dynamical calculations based on a valence-force-field model. In order to improve the set of force constants, these calculations are also performed on some model compounds of the polymers. Diphenylamine (DPA) and N,N'-diphenyl-l,4-phenylenediamine (PCD) were chosen as model compounds for the Leucoemeraldine Base and N,N'-diphenyl-l,4phenylenediimine (OPCD) for the Pernigraniline Base. We have obtained a good fit between experimental and calculated frequencies for all compounds. The set of force constants that we obtained is consistent with the geometrical parameters of the compounds and with values found in other aromatic compounds. 0379-6779/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
476 EXPERIMENTAL RESULTS Raman spectra of all compounds were performed on a double monochromator Jobin Yvon (Ramanor HG2S). All samples were in powder forms, pressed in pellets for Raman spectroscopy. The Raman signal was collected at 90" and the laser power on the samples never exceeded 100 mW. The infrared absorption was performed with a Fourier Transform Infrared Spectrophotometer (Nicolet 20SXC). All compounds were pressed in KBr pellets. Raman spectra of benzoquinone, OPCD and pemigraniline
base are presented figure 1. Other
spectra of both polymers and their model compounds have been published elsewhere [4,5]. Two different excitation wavelengths have been used for the Raman spectra of the fully oxidized polymer (PB) which exhibits two bands on its UV-VIS optical absorption spectrum : ko = 457,9 nm is expected to enhance "reduced units" and ~.o = 647,1 nm "oxidized units".
o~
o
~
~a~
Jl
~: lo
= 647,1
nm
to
= 514,5
nm
r~
~,
i
,
A
eq
¢~
jc) .
h/~
,~
~,o = 4 5 7 , 9
nm
~
_
p
:/| ~o
700
= 647,1
I|-
I
nm
1000 to(cm
1300 -1)
1600 _-.--
Figure 1 : Resonant Raman Spectra of : a) benzoquinone, b) OPCD, c)and d) pemigraniline base (~,o = 457.9 nm and 647.1 nm)
477
VIBRATIONAL ANALYSIS In order to interpret Raman and infrared bands observed in our compounds, we have performed dynamic calculations based on the use of a valence-force-field model. Using this model, we made the hypothesis of planar molecules and polymers. Then, one can separate in-plane and out-of-plane vibrations. According to the above assumption, we can obtain the point group of each compound and the active modes in each symmetry. Table 1 presents these different point groups for all the molecules and polymers studied. Our study concern only the region between 300 and 1700 cm -1, which means that we have not taken into account the modes around 3000 cm -1 and 3400 cm -1 corresponding respectively to the C-H stretching and the N-H stretching vibrations.
DPA (C2v)
23 A1 (R,IR) + 22 B2 (R, IR)
PCD (C2h)
35 Ag (R) + 34 Bu (IR)
Leucoemeraldine (D2h) Base
12 Ag (R) + 12 B3g (R) + 11 Blu (IR) + 11 B2u (IR)
OPCD (C2h)
33 Ag (R) + 32 Bu (IR)
Pemigraniline (C2h) Base
22 Ag (R) + 20 Bu (IR)
Table 1: Different point groups and number of predicted modes (and their activity) for each compound
We have performed two different calculations : one for the compounds DPA, PCD and LB ("reduced structure") and the other for OPCD and PB ("oxidized structure"). For each one, we have chosen a set of force constants defined by : 32 0
where (~ is the potential energy and r, r' are two internal coordinates. The unit cell of the polymers are proposed in figure 2 with the definition of the internal coordinates. The geometrical parameters have been obtained by MNDO computations [6]. In order to insure the values of our parameters, we performed dynamic calculations simultaneously on the model molecules and on the corresponding polymer.
reduced unit
oxidized unit
Figure 2 : definition of internal coordinates of the reduced and oxidized units
478
We have at our disposal 34 parameters to interpret 73 experimental frequencies of the "reduced structure" and 56 parameters to interpret 52 experimental frequencies of the "oxidized structure". However, only a few of these force constants are of real importance for the calculations. Both set of parameters are separated in three groups as follows : non perturbed ring (benzene or quinoid), perturbed ring, and intercycle group (amine -NH- or imine -N= group). The force constants related to the non perturbed ring are kept close to those used in previous works on benzene [7] and on p-benzoquinone [8]. That leaves us in fact 23 parameters for the "reduced part" and 45 for the "oxidized part". Our main force constants are reported in table 2. Experimental and calculated frequencies of both forms of polymers studied are presented on table 3. We have also reported the assignments of all the vibrational modes. The origin (B : benzene or Q : quinoid) of the vibrations is reported and the modes of the benzene ring are correlated to the Wilson notation. Keeping in mind that the number of
predicted bands is quite large, one can expect some
uncertainties still to remain in our interpretation, especially concerning non observed modes. However, the main bands of the spectra are rather well interpreted and we have obtained a good agreement between experimental and calculated frequencies.
a) non perturbed and perturbed benzene ring Fs 2
Ft 2
Fe~2
F¢2
Fst
5,08
6,32
0,99
0,50
Fltt
Ftt~
Ft¢
Ft'2
Fct'2
F¢'2
F t ' ¢ ' Ft'ct' F a t ~ '
-0,03 0,87
0,17
0,16
6,32
0,82
1,07
0,31
0,37
0,13
b) amine group FR 2
F s '2
F~ 2
F 8 '2
ERR
FR8
FRS'
5,31
6,21
0,63
1,45
0,60
0,21
0,37
c) non perturbed and perturbed benzene ring FD2
FQt 2
FQtx 2
8,19
4,62
1,13
FQ~ 2 FQlO 2 FDQct F1DQt F1QtQt 0,30
0,60
0.50
0,40
0,23
FQt 2 FQct'2 FQtQ~' F Q 0 '2 4,62
0,55
d) imine group FQR 2
F Q R '2
5,00
9,19
F Q 8 '2 F Q R Q R ' FQRtx' 1,97
0,49
-0,32
FQRQS' F Q R ' Q S ' F Q R ' Q t t ' 0,59
0,59
Table 2 : values of main force constants used in the valence-force-fields
-1,04
0,24
1,02
479
0
.=_
Z ©
+
o+~ .°
O0 Cq
t~-
i
o
c~
H o
J
<
Z
480 The main point to emphasize in the study on the "reduced structure" is the determination of the force constants related to the amine group, in addition to those of the benzene ring. Ft 2 and Ft '2 related to the carbon-carbon bonds inside the benzene ring are equal to 6.32 mdyn/A. FR 2 corresponds to the carbon-nitrogen bond of the amine group. It is found at 5.31 mdyn/A which is consistent with other values obtained for similar single bonds. Comparable values are found at 4.80 mdyn/A in the PPV [9] and at 5.31 mdyn/A in PTV [10] (both for a intercycle C-C stretching vibration). Concerning the "oxidized part", the dynamical calculations lead to parameters of the quinoid ring such as FD 2 = 8.19 mdyn/A for the carbon-carbon double bond and FQt 2 = 4.62 mdyn/A for the single bond of the ring, These values are very similar to those obtained for the benzoquinone and Ft 2 = 6.32 mdyn/A calculated for the benzene ring is situated between these two values. The main force constant corresponding to C=N and referred as FQR '2 is obtained at 9.19 mdyn/A. The parameter FQR 2 is equal to 5.00 mdyn/A. The quinoid ring is also characterized by an important decrease of coupling intracycle constants : from Fltt = 0.87 in the benzene ring, we pass to F1QtQt = 0.23 and F1DQt = 0.40 in the quinoid ring. We can already notice a good agreement between these force constants and the geometrical parameters [6] and our model leads to a good interpretation of the observed modes coming from the quinoid ring or the benzene ring. CONCLUSIONS We have made a complete interpretation of the Raman and infrared vibrational modes of two forms of polyaniline : the Leucoemeraldine Base and the Pernigraniline Base. By the use of a valence-force-field model, we obtain two sets of force constants concerning the reduced structure and the oxidized structure. These parameters interpret the quinoid or the benzenoid character as well of the imine or amine group. This demonstrates the great importance played by the vibrational spectroscopies for a comprehensive investigation of the electronic properties in this type of polymer. REFERENCES 1
A.G. Green and A.E. Woodhead, ,l, Chgm. Soc. Trans. 97. (1910) 2388.
2
A.G. Green and A.E. Woodhead, J. Chem. Soc. Trans. 101. (1912) 1117.
3
A. Ray, A.F. Richter, A.G. MacDiarmid and A.J. Epstein, $ynth. Met.. 29, (1989) 151.
4
S. Quillard, G. Louarn, J. P. Buisson, S. Lefrant, J. Masters and A. G. MacDiarmid, Sorin~er Series in Solid-State Sciences. 107. (1992) 271.
5
S. Quillard, G. Louam, J. P. Buisson, S. Lefrant, J. Masters and A. G. MacDiarmid, Svnth. Met..
6
M.C. Dos Santos and J.L. Br6das, Svnth. Met.. 29, (1989) 321.
(1992) in press. 7
J.Y. M6vellec, J. P. Buisson, S. Lefrant and H. Eckhardt, Synth. Met.. 41, (1991) 283.
8
L.-O Pi6til/t, K. Palm0 and B. Mannfors, Journal of Molecular Soectroscoov. 116, (1986) 1.
9
H. Eckhardt, K.Y. Jen and L.W. Shacklette, Conjugated Polvmeric Materials. 305. (1990).