6998
J. Phys. Chem. 1996, 100, 6998-7006
Vibrational Properties of PolyanilinesIsotope Effects G. Louarn,*,† M. Lapkowski,‡,§ S. Quillard,† A. Pron,|,⊥ J. P. Buisson,† and S. Lefrant† Laboratoire de Physique Cristalline, IMN, UniVersite´ de Nantes, UMR 110, 2, rue de la Houssinie` re, 44072 Nantes Cedex 03, France, Department of Chemistry, Silesian Technical UniVersity, 44 100 Gliwice, Poland, Department of Textile Engineering and EnVironmental Sciences, Technical UniVersity of Lo´ dz, Bielsko-Biala Campus, 43 300 Bielsko-Biala, Plac Fabryczny 1, Poland, Department of Materials Science and Ceramics, Academy of Mining and Metallurgy, 30 059 Krako´ w, Mickiewicza 30, Poland, and Department of Chemistry, Technical UniVersity of Warsaw, 00 664 Warszawa, Noakowskiego 3, Poland ReceiVed: NoVember 17, 1995; In Final Form: February 7, 1996X
The effect of polyaniline deuteration on the Raman spectra of this polymer has been studied in detail. Four types of samples have been prepared: nondeuterated polyaniline, polyaniline deuterated selectively on the ring, polyaniline deuterated selectively on nitrogen, and polyaniline deuterated both on the ring and on the nitrogen. Selective deuteration allowed for experimental verification of the Raman modes assignment proposed for different types of polyaniline. In particular, the contribution of C-H bending to C-C stretchings in benzoid and quinoid rings of the polyaniline chain has been confirmed. In addition, selective deuteration enabled us to identify, for the first time in polyaniline, the band associated with N-H bending deformations. Raman spectroelectrochemical investigations of the four above-mentioned types of polyaniline samples gave evidence that in protonated polyemeraldine two types of structures coexist, namely, semiquinone radical cation (polaronic lattice) and quinoid dication (bipolaronic). The vibrations of the former are in resonance with the energy of the blue (457.9 nm) excitation line, whereas the vibrations of the latter undergo resonance enhancement if the red (676.4 nm) line is used. The calculated and experimentally observed Raman modes for both polaronic and bipolaron structures are in very good agreement in all four cases of differently deuterated polyaniline.
I. Introduction Polyaniline is, in recent years, the most extensively studied conducting polymer. This interest is stimulated by environmental stability and easy processing of this polymer, which, in turn, make it an excellent candidate for various industrial applications.1 On the other hand, polyaniline exhibits rich and diversified chemistry. It can exist in a variety of oxidation and protonation states. Mutual interconversion between these states can be conveniently studied by Raman spectroelectrochemistry. Thus “in situ” Raman studies of electrochemical oxidation of polyaniline have been carried out in both aqueous2-6 and nonaqueous electrolytes,7,8 providing important information concerning the chemical nature of the changes imposed on the polyaniline chain during oxidation. Parallel to spectroelectrochemical studies, significant research effort has been focused on vibrational analysis of various basic forms of polyaniline including polyleucoemeraldine,9,10 polyemeraldine, and polypernigraniline.10 In particular, general vibrational calculations carried out for the three abovementioned forms of polyaniline led to a good agreement between the calculated and experimentally observed Raman bands. In addition, sets of force constants derived from these calculations showed good consistency with geometric parameters of other ring-containing polymers. Isotopic effects studies constitute the most reliable method for experimental verification of any proposed vibrational model. Therefore we have undertaken the task of the preparation of * E-mail: LOUARN@CNRS-IMN. FR. † Universite ´ de Nantes. ‡ Silesian Technical University. § Technical University of Lo ´ dz. | Academy of Mining and Metallurgy. ⊥ Technical University of Warsaw. X Abstract published in AdVance ACS Abstracts, April 1, 1996.
0022-3654/96/20100-6998$12.00/0
three types of differently deuterated polyaniline. The idea of these syntheses is founded on the following assumptions: (i) In all redox and acid-base reactions of polyaniline, the stoichiometry of the ring remains unchanged. The validity of this assumption has been verified by numerous experiments.11,12 (ii) No hydrogen (deuterium) exchange occurs between the ring and nitrogen in the polyaniline chain.13 The above two features associated with polyaniline chemistry enabled us to synthesize four types of polyaniline samples: (i) polyaniline with hydrogens on the ring and on nitrogen PANI(H,H)
(ii) polyaniline with hydrogens on the ring and deuterium atoms on nitrogen PANI(H,D)sshown schematically in leucoemeraldine form as
(iii) polyaniline with deuterium atoms on the ring and hydrogens on nitrogen PANI(D,H)
(iv) polyaniline with deuterium atoms on the ring and on nitrogen, i.e., fully deuterated polymer PANI(D,D) © 1996 American Chemical Society
Vibrational Properties of Polyaniline
The above four types of polyaniline samples were prepared chemically using appropriate modifications of standard polyaniline preparation procedures.14 Their Raman spectra were recorded. Another set of samples was prepared electrochemically by voltammetric polymerization.15 These samples were then studied by in situ spectroelectrochemistry using different excitation lines. In the final step of this research, we performed vibrational calculations following the method described by Quillard et al.10 Using a symmetrized dynamical matrix, we determined the force constants, the potential-energy distribution, and the Cartesian displacements for all four types of differently deuterated polyaniline. The calculated band shifts caused by deuteration were found to be in a good agreement with experimentally observed Raman lines. II. Experimental Section II.1. Chemical preparation of Differently Deuterated Polyaniline. (a) Polyaniline Deuterated on the Ring and on NitrogensPANI(D,D). PANI(D,D) was prepared from anilined5 (C6D5NH2) using chemical oxidation with (NH4)2S2O8.14 In a typical preparation, 0.02 M aniline-d5 (Aldrich, 99 at % D) was dissolved in 25 mL of 1 M DCl solution in D2O. Similarly, 0.015 M previously dried (NH4)2S2O8 was dissolved in 25 mL of 1 M DCl/D2O solution. The solution of the oxidant was then added drop by drop to previously cooled aniline solution slowly enough to maintain the temperature of the reaction mixture at 0 °C. The reaction mixture was then vigorously stirred for 2 h. Then, the precipitated polymer was separated from the solution and washed five times with small (ca. 5 mL) portions of 1 M DCl/D2O solution. Finally, the obtained powder was vacuum dried to constant mass. All operations were carried out in a nitrogen atmosphere. Special care was taken to exclude any traces of H2O from the system. A small deficit of the oxidant with respect to aniline was used in order to obtain the polymer of better quality.14 Polyaniline deuterochloride obtained via the above-described procedure contains some amount of hydrogen atoms originating from the substrate (C6D5NH2). Totally deuterated polyaniline was then prepared by removal of protons using Na2CO3/D2O solution. Na2CO3/D2O, being the salt of a strong base and a weak acid, gives the solution of high pH capable of transformation of polyemeraldine salt into polyemeraldine base. It then was transformed into polyemeraldine deuterochloride in 1M DCl solution in D2O. This sequence of reactions, repeated five times, leads to effective removal of all hydrogens from the system. (b) Polyaniline Deuterated on the Ring and Protonated on NitrogendsPANI(D,H). PANI (D,H) was prepared from C6D5NH2 in a process similar to that used for PANI (D,D), but with HCl/H2O as the reaction medium. All concentrations of reagents and solutions were exactly the same as in the case of the preparation of PANI(D,D) (vide supra). The resulting polyemeraldine hydrochloride was deprotonated in excess of 2 wt % NH3 aqueous solution. (c) Polyaniline Protonated on the Ring and Deuterated on NitrogensPANI(H,D). PANI (H,D) was prepared from C6H5NH2 using the same procedure as in the case of PANI(D,D). HCl/H2O solution was used as the reaction medium. The obtained polyemeraldine hydrochloride was then treated with Na2CO3/D2O solution followed by the treatment with 1M DCl/
J. Phys. Chem., Vol. 100, No. 17, 1996 6999 H2O. Five reaction sequencies resulted in a complete deuteration of nitrogen atoms. (d) Polyaniline Protonated on the Ring and Protonated on NitrogensPANI(H,H). PANI(H,H) was prepared from C6H5NH2 using protonated solvents and reagents. All concentrations were the same as in the preparation of PANI(D,D). After the reaction the polymer was deprotonated in excess of 2 wt % NH3 aqueous solution. II.2. Electrochemical Synthesis of Differently Deuterated Polyaniline. (a) Polyaniline Deuterated on the Ring and on NitrogensPANI(D,D). Polyaniline film was synthesized on a platinum electrode using a classical three-electrode cell with a platinum counter electrode and an Ag/AgCl reference. Ringdeuterated aniline was dissolved in a 1 M solution of DCl in D2O to give a 0.2 M concentration with respect to C6D5NH2. The polymerization was carried out voltammetrically by potential scanning between -0.2 and 0.7 V vs Ag/AgCl with a scan rate of 50 mV/s. The obtained film was then repeatedly (five times) deprotonated in Na2CO3/D2O solution and reprotonated in DCl/D2O solution to assure complete deuteration of nitrogen atoms. (b) Polyaniline Deuterated on the Ring and Protonated on NitrogensPANI(D,H). In the first step of the preparation of PANI(D,H), polymer films were deposited on the electrode by voltammetric polymerization of C6D5NH2 carried out in exactly the same manner as in section II.2.a. Then the deposited film was repeatedly deprotonated in a 2 wt % NH3 aqueous solution and reprotonated in 1.0 M HCl aqueous solution. Consecutive deprotonation-reprotonation sequences resulted in a complete protonation of nitrogen. (c) Polyaniline Protonated on the Ring and Deuterated on NitrogensPANI(H,D). PANI(H,D) was synthesized from C6H5NH2 in HCl/H2O electrolyte using voltammetric polymerization as in section II.2.a. It was then deuterated on nitrogen by repeated (five times) deprotonation in Na2CO3/D2O solution and reprotonation with 1 M DCl/D2O. (d) Polyaniline Protonated on the Ring and on Nitrogens PANI(H,H). PANI(H,H) was synthesized from C6H5NH2 in HCl/H2O electrolyte by voltammetric polymerization as described in section II.2.a. It was deprotonated in a 2 wt % aqueous solution of NH3. II.3. Raman Studies. Raman spectra obtained with the excitation lines from the visible range (λexc ) 457.9, 514.5, and 676.4 nm) were recorded on a multichannel Jobin-Yvon T64000 type spectrometer connected to a CCD detector. FT Raman spectra (λexc ) 1064 nm) were registered on a FT Raman Bruker RFS 100 spectrometer. The scattering signal was collected at 90, except for FT Raman spectra where a backscattering configuration was used. For Raman in situ spectroelectrochemical studies, PANI deposited on Pt electrode via voltammetric polymerization (see section II.2) was placed in a specially designed three-electrode spectroelectrochemical cell equipped with a Pt counter electrode and a Ag/AgCl reference electrode. Depending on the type of PANI used, either 1 M HCl in H2O or 1 M DCl in D2O served as the electrolyte. Raman spectroelectrochemical studies were correlated with cyclic voltammetry covering the potential range from -150 to 700 mV, i.e., the range of the first redox couple of polyaniline (oxidation of leucoemeraldine to emeraldine). Raman spectroelectrochemical experiments carried out at fixed potential were in each case preceded and followed by a cyclic voltammetry scan in order to eliminate the so-called “memory effect” observed in the electrochemistry of polyaniline. Since this effect
7000 J. Phys. Chem., Vol. 100, No. 17, 1996
Figure 1. Raman spectra of polyemeraldine base: (a) and d) nondeuterated polyemeraldine base PANI(H,H) prepared chemically; (b) and (e) fully deuterated polyemeraldine base PANI(D,D) prepared chemically; (c)and (f) fully deuterated polyemeraldine base prepared electrochemically.
results in a shift in the position of the oxidation peak, it may influence the spectroelectrochemical experiments.16 III. Results Strong dependence of the observed features, in the Raman spectra of polyconjugated molecules, on the excitation line wavelength is an intrinsic property of these macromolecular systems.17 This phenomenon significantly complicates the Raman spectroscopy based vibrational analysis of conjugated polymers. Therefore a detailed Raman study of polyaniline requires the registration of the spectra not only for different oxidation states of this polymer but also using different excitation wavelengths. Raman spectra of principal forms of polyaniline (polyleucoemeraldine, polyemeraldine, and polypernigraniline) have already been published.2,10 The assignments of the observed bands have been proposed on the basis of 15Nsubstituted compounds2 and by comparison with model short chain compounds.10 Figure 1 a-c show Raman spectra (λexc ) 457.9 nm) of chemically prepared polyemeraldine base and its deuterated analogues prepared chemically and electrochemically. The spectra recorded for λexc ) 676.4 nm are presented in Figure 1 d-f. Before focus is placed on the origin of the observed Raman lines, it is instructive to present principal forms of polyaniline. Polyaniline, in its basic (nonconductive) state, can be described by the following general formula
where y represents the fraction of the reduced units in the chain. The polyemeraldine oxidation state is defined as that in which y ) 0.5, i.e., the number of reduced units is equal to the number of oxidized units. The coexistence of the oxidized and reduced units in polyemeraldine base is clearly manifested in the Raman spectra. For λexc ) 457.9 nm, two lines due to C-C stretching deformations are present at 1617 and 1585 cm-1. The first is ascribed to the stretchings in the benzoid type ring, whereas the secondis ascribed to the same deformations in the quinoid rings.6,10 The lines are downshifted to 1588 and 1556 cm-1 in the deuterated samples.
Louarn et al.
Figure 2. FTIR spectra of (a) nondeuterated polyemeraldine base PANI(H,H) and (b) fully deuterated polyemeraldine base PANI(D,D).
The presence of two types of carbon-nitrogen bonds in polyemeraldine base is corroborated by the appearance of two bands: at 1480 cm-1 (due to CdN stretchings in quinoid segments) and at 1220 cm-1 (ascribed to C-N stretchings in benzoid segments2). The former is unchanged by deuteration, whereas the latter is slightly downshifted to 1207 cm-1. Finally, the peaks ascribed to C-H bending deformations in quinoid and benzoid rings at 1160 and 1180 cm-1, respectively, appear after deuteration at ca. 880 cm-1. The red (676.4 nm) excitation line tends to enhance the bands originating from the deformations of the oxidized segments. Indeed in the spectra of both chemically and electrochemically prepared polyemeraldine base, bands due to the presence of quinoid segments dominate if λexc ) 676.4 nm is used. As a result the spectra look essentially the same (Figure 1 e,f). In reality both samples differ in their degree of oxidation. In the spectra recorded with λexc ) 457.9 nm, the band at 1480 cm-1 (CdN stretching) is more intense with respect to the band at 1588 cm-1 (C-C stretching in deuterated benzoid ring) as compared to the same bands in the electrochemically prepared sample (compare parts b and c of Figure 1). Thus the chemical sample is more oxidized. FTIR spectra of polyemeraldine base PANI(H,H) and of its analogue deuterated on the ring and on nitrogen PANI(D,D) are shown in Figure 2 a,b. The band at 1592 cm-1 in PANI(H,H) which is shifted to 1570 cm-1 in PANI(D,D) can be, without any ambiguity, attributed to the presence of the oxidized (quinoid) segments of the chain since it is absent in the totally reduced form of polyaniline (polyleucoemeraldine) and its intensity increases with the increase of the oxidation degree of the polymer.10,18 This peak was assigned to the CdC vibrational mode in the quinoid ring. Similarly, the peak at 1501 cm-1 which is downshifted to 1413 cm-1 upon deuteration can be assigned to C-C stretching + C-H mixed vibrational mode in the benzoid ring. The CdN stretching peak which is essentially unaffected by deuteration in PANI(H,H) appears only as a shoulder of the 1501 cm-1 peak. After the deuteration it is more clearly seen at 1460 cm-1. The band due to benzoid ring deformation at 1310 cm-1 in PANI(H,H) is shifted after the deuteration to 1261 cm-1 in PANI(D,D). The modes involving C-H and N-H bonds are significantly shifted upon deuteration. The C-H bending deformation in the quinoid ring, appearing at 1165 cm-1 in PANI(H,H), is shifted to 823 cm-1 in PANI(D,D), whereas the N-H deforma-
Vibrational Properties of Polyaniline tion band, originating from the reduced units at 827 cm-1 in PANI(H,H), is found at 735 cm-1 in PANI(D,D). The spectrum of nondeuterated PANI(H,H) is very similar to that reported by Harada et al.19 Additional information concerning the band assignment via isotope effect analysis can be derived from carefully designed Raman spectroelectrochemical experiments. We have carried out such investigation with four types of samples using three different excitation lines (457.9, 676.4, and 1064 nm). In particular: (i) Totally deuterated polyaniline PANI(D,D) deposited on a platinum electrode was polarized in DCl/D2O electrolyte at different potentials covering the first redox couple of polyaniline. (ii) Identical experiments were carried out for ring-deuterated, N-protonated polyaniline PANI(D,H); however, HCl/H2O electrolyte was used in this case. (iii) For ring-protonated, N-deuterated polyaniline PANI(H,D), Raman spectroelectrochemical investigations were carried out in DCl/D2O electrolyte. The same polymerization potentials were used as in the two previously described cases. (iv) Finally, for reference, Raman spectroelectrochemical studies of PANI(H,H) were carried out in HCl/H2O electrolyte. The necessity of the use of different excitation lines in Raman spectroelectrochemistry is associated with the existence of chromophore groups at practically each oxidation state of polyaniline. This, in turn, results in resonance effects that are manifested by a strong dependence of the registered spectra on the position of the Raman excitation line with respect to the observed electronic transitions. In particular, it has been demonstrated in previous Raman spectroelectrochemical studies carried out for nondeuterated polyaniline PANI(H,H)6 that in the potential range of the first polyaniline redox couple (from -150 to 650 mV vs SCE) the blue (457.9 nm) and red (676.4 nm) excitation lines give strikingly different Raman spectra. This is an obvious consequence of a resonant enhancement of bands originating from different vibrations in both cases. Raman spectra recorded for PANI(D,D) at different electrode potentials, using λexc ) 457.9 nm, are presented in Figure 3. PANI(D,H), PANI(H,D), and PANI(H,H) behave in the same manner; however, the position of selected peaks are different due to different isotopic substitution. The spectra show little dependence on the electrode potential since unprotonated polyleucoemeraldine (the dominant species at lower potentials) and protonated polyemeraldine (created upon oxidation of polyleucoemeraldine in acidic media) give very similar Raman spectra with λexc ) 457.4 nm. Deuteration-induced shifts are clearly seen in Figure 4, where Raman spectra of all four types of PANI studied, registered at the same potential (E ) -150 mV), are compared. At this potential PANI exists in the most reduced form of polyleucoemeraldine. In particular, two significant shifts due to ring deuteration are observed: (i) The C-C stretching mode in the benzoid ring is shifted from 1625 cm-1 in PANI(H,H) and PANI(H,D) to 1594 cm-1 in PANI(D,H) and PANI(D,D). (ii) The C-H bending mode is shifted from 1189 cm-1 in PANI(H,H) and PANI(H,D) to 874 cm-1 in PANI(D,H) and PANI(D,D). Small downshifts are also registered for C-N stretching bands as a result of both ring and nitrogen deuteration (from 1253 cm-1 in PANI(H,H) to 1240, 1227, and 1226 cm-1 in PANI(H,D), PANI(D,H), and PANI(D,D), respectively). Different spectra are obtained with λexc ) 676.4 nm due to resonant enhancement of bands originating from different vibrations of the chain. In Figure 5 Raman spectra of PANI(D,D) are collected for increasing electrode potentials. Similarly
J. Phys. Chem., Vol. 100, No. 17, 1996 7001
Figure 3. Raman spectra of ring- and nitrogen-deuterated polyaniline PANI(D,D) recorded for different potentials (vs Ag/AgCl); electrolytic solution DCl/D2O, λexc ) 457.9 nm.
Figure 4. Raman spectra of differently deuterated polyaniline registered for the polymer polarized at -150 mV vs Ag/AgCl, λexc ) 457.9 nm: (a) PANI(H,H) in HCl/H2O electrolyte, (b) PANI(H,D) in DCl/D2O electrolyte, (c) PANI(D,H) in HCl/H2O electrolyte, and (d) PANI(D,D) in DCl/D2O electrolyte.
to the case of λexc ) 457.9 nm, a rather weak dependence of the spectra on the oxidation state of PANI can be noticed. Isotopic shifts due to ring and nitrogen deuteration can be derived from Figure 6, where Raman spectra of PANI(H,H),
7002 J. Phys. Chem., Vol. 100, No. 17, 1996
Figure 5. Raman spectra of ring- and nitrogen-deuterated polyaniline PANI(D,D) recorded for different potentials (vs Ag/AgCl); electrolytic solution DCl/D2O, λexc ) 676.4 nm.
Louarn et al. The red excitation line gives richer spectra than does the blue one. In addition to the C-C stretching deformation band of the benzoid ring, the corresponding bands originating from the quinoid segments are present and undergo a similar shift upon ring deuteration. In particular, the CdC stretching deformation band is shifted from 1581 to 1560 cm-1 upon deuteration, whereas the C-C stretching deformation band undergoes a shift from 1484 cm-1 in PANI(H,H) to 1428 cm-1 in PANI(D,D). Similarly, the C-H bending deformation band is shifted from 1166 to 856 cm-1 upon isotopic substitution. The band at 1515 cm-1 in PANI(H,H) is shifted to 1062 cm-1 (ca. x2 factor) upon nitrogen deuteration, PANI(H,D); then it returns to 1487 cm-1 in ring-deuterated, nitrogen-protonated polyaniline PANI(D,H), being again shifted to 1085 cm-1 for totally deuterated polymer PANI(D,D). The sequence of the peak position changes unequivocally, indicating that this band should be ascribed to N-H bending deformations. The set of spectra presented in Figure 6 reveals also the presence of the bands at ca. 1310-1330 cm-1, which were ascribed by Furukawa et al.20 to polysemiquinone radical structures created upon the protonation of polyemeraldine. The origin of the splitting of this “protonation band” is unclear and is still the subject of significant controversy.7,20 In the final stage of the experimental part of this research, we used the near-IR excitation line (1064 nm). The use of this line leads to the spectra that show general features similar to those observed in the spectra obtained with λexc ) 676.4 nm. Spectral lines are, however, broader and less resolved. A complete set of experimental Raman data for polyleucoemeraldine, polypernigraniline, polyemeraldine base, and deuterated polyemeraldine base is presented in Table 1. IV. Normal Coordinate Analysis IV.1. Methodology. The calculations of the force field and frequencies of the vibrational modes for the protonated and deuterated polyaniline chain are performed with the use of Fourier’s dynamic matrix. Assuming translational symmetry of the polymeric chain, one can restrict the calculations to one repeat unit provided that the chains do not contain structural defects and are long enough to neglect the end groups. The calculations require the knowledge of interatomic forces (force constants) and the configuration (bond lengths and angles) in the polymer chain. The dynamic matrix [D] is obtained from
[D] ) [M-1/2][B]+[F][B][M-1/2]
Figure 6. Raman spectra of differently deuterated polyaniline registered for the polymer polarized at 500 mV vs Ag/AgCl, λexc ) 676.4 nm: (a) PANI(H,H) in HCl/H2O electrolyte, (b) PANI(H,D) in DCl/D2O electrolyte, (c) PANI(D,H) in HCl/H2O electrolyte, and (d) PANI(D,D) in DCl/D2O electrolyte.
PANI(H,D), PANI(D,H), and PANI(D,D) are collected. The spectra were recorded at E ) 500 mV. At such polarization of the electrode, polyaniline exists in the form of polyemeraldine salt.
(1)
where [F] is the matrix of potentials written in terms of the internal coordinates (the force constants have the most direct physical meaning in terms of internal coordinates). [B] is the transformation matrix between the symmetrized internal and symmetrized Cartesian coordinates, and [B]+ is the transposition of the B matrix. Finally, [M-1/2] is the diagonal matrix with elements equal to the inverse of the square root of the atomic masses. In this work, we limit our attention to the “in plane modes” as justified below. Then two types of internal coordinates are defined: (1) the bond stretching coordinates that describe the displacement from equilibrium positions of two atoms forming a chemical bond, and (2) the bond bending coordinates that describe the changes of the interbond angle, i.e., the angle at which two bonds meet an atom.
Vibrational Properties of Polyaniline
J. Phys. Chem., Vol. 100, No. 17, 1996 7003
TABLE 1: Experimental Frequencies of Raman Bands Registered for Polyleucoemeraldine, Polypernigraniline, Nondeuterated Polyemeraldine Base PANI(H,H), and Fully Deuterated Polyemeraldine Base PANI(D,D)a polyleucoemeraldine λexc ) 457.9 nm
polypernigraniline λexc ) 676.4 nm
polyemeraldine λexc ) 676.4 nm
deuterated polyemeraldine λexc ) 676.4 nm
description of vibrations
1612 1553 1579 1480 1418 1215
1620 1551 1587 1492 1414 1220
1586
1157
1162 880 828
C-C stretching (B) C-C stretching (C) CdC stretching (Q) CdN stretching C-C stretching (Q) C-N stretching C-H or C-D bonding (B) C-H or C-D bending (Q) ring deformation (B) amine deformation (X-sens.) ring deformation (Q) imine deformation
Raman 1618 1597
1219 1181 868 820
788 749
720
1496
1571 1480
1592 1501
1282 1218 1167 814
1315 1211 1158 845
1310 1215 1165 827
1553 1462 1210 865 848 805 745
IR
a
1570 1413 1460 1261 1188 823 735
C-C stretching (B) CdC stretching (Q) CdN stretching ring deformation C-N stretching C-H or C-D bending (B) amine deformation
(B) denotes benzoid ring; (Q) denotes quinoid ring.
We start from a minimal basis set of force constants expressed in terms of the internal coordinates of the chain and defined by
FRR′ )
(∂R∂Φ∂R′)
(2)
o
where Φ, R, and R′ are the harmonic potential energy and two internal coordinates, respectively. Since the notation we have used is directly related to these internal coordinates, they can be expressed as follows: (1) the diagonal interactions refer to stretching and bending vibrations; (2) the first neighbors off-diagonal interactions refer to stretching-stretching, bending-stretching, and bending-bending couplings. Let us mention that in our model, force constants are assumed to be fairly local (only dependent on the position and the type of the few nearest neighbors), and hence they can be transferred from other compounds of appropriate stereochemical structure. Thus, a probable set of force constants is used to calculate approximate frequencies, and then the assumed parameters are progressively refined to make the frequency fit as closely as possible. The optimization method used to refine these parameters is the usual least-squares fitting procedure where we minimize the standard deviation X defined by
X)
[
1
N
i 2 (ωiexp - ωcal ) ∑ N i)1
]
Figure 7. Definition of principal internal coordinates used in frequency and potential energy distribution calculations for differently deuteriated polyaniline.
calculations and experimental data was obtained.10 The geometrical parameters of the molecules (bond lengths and angle values) were obtained from Baird and Wang.21 The set of internal coordinates used in the calculations is defined in Figure 7, whereas the principal force constants are schematically depicted in Figure 8. Depending on the oxidation and protonation state polyaniline segments, the following structures are expected: (a) unprotonated (basic) reduced unit
(b) unprotonated (basic) oxidized unit
1/2
(3)
where ωiexp and ωical are the experimental and calculated frequencies of the vibration i. The iterative process is stopped when the deviation between calculated and observed mode wavenumbers reaches a minimum. Changes of the force constants must be limited in order to control the wrong evolution, which may lead to nonphysical values at the end of iterations. In order to calculate the vibrational frequencies, it was necessary to find geometrical values and force constants for a similar local structure as justified above. We have utilized the valence force field determined for the polyleucoemeraldine and polypernigraniline base, where a fairly good agreement between
(c) protonated oxidized unit (polaronic type)
(d) protonated oxidized unit (bipolaronic type)
High electrical conductivity of polyaniline is directly related to the presence of the above charged protonated structure, created upon a so-called “doping reaction”.
7004 J. Phys. Chem., Vol. 100, No. 17, 1996
Figure 8. Principal force constants associated with polyaniline unit involving one ring and one nitrogen.
If one or two electrons are subtracted from a neutral chain, the molecular structure is changed, and a positive polaron or positive bipolaron is formed. Similarly, positive polaron or bipolaron can be formed by addition of protons to the neutral chain of a polymer exhibiting basic properties. Both types of dopings can take place in the case of polyaniline. The redox doping involves the oxidation of polyleucoemeraldine, whereas the acid-base doping involves the protonation of polyemeraldine. These chemical changes are accompanied by a local relaxation of the lattice geometric and electronic structures, supposedly extending over several repeating unit rings, which is energetically favorable. This property constitutes the basis of very interesting physical phenomena occurring upon doping of conjugated polymers which, among others, are reflected by the appearance of doping-induced Raman bands. Their peak positions can be explained by following the perturbed force constant method, previously applied successfully to poly(pphenylene) and poly(p-phenylene vinylene) dopings.22,23 It seems, therefore, very important to develop a detailed understanding of these modifications exploiting both experimentally determined Raman bands and the calculation methods described above. If we choose appropriate force constants representing the electronic properties of the relevant polymer, the evolution induced by doping can be identified on the basis of the Raman spectra and of the modification of this set of force constants. The four parameters that reflect the electronic structure of the polymer backbone are schematically shown in Figure 8 and are defined as Ft2, the force constant relative to the C2-C3 bond in the ring, Ft′2, the force constant relative to the C1-C2 bond in the ring, FR2, the force constant relative to the C1-N bond, and FR′′2, the force constant relative to the N-C1′ (adjacent ring) bond. The final set of force constants leads to a good agreement between calculated and observed frequencies for protonated polyaniline within a reasonable limit, taking into account experimental errors and anharmonicity effects. We do not present here the complete set of these parameters since it will be the subject of a separate publication in a more specialized journal.24 Here we present only the principal force constants (vide infra).
Louarn et al. quinoid rings is manifested by a ca. 30 cm-1 downshift of the corresponding bands in the deuterated samples. As expected, the band due to CdN stretchings in quinoid segments (1480 cm-1) is essentially unaffected by deuteration, whereas the corresponding band associated with C-N stretchings in benzoid segments is slightly downshifted (from 1220 to 1207 cm-1). In addition, selective deuteration enabled us to identify, for the first time in polyaniline, the band originating from N-H bending deformations (at 1515 cm-1). This band remains at essentially the same position upon ring deuteration whereas it shifts down by to 1090 cm-1 (ca x2 ratio) upon nitrogen deuteration.26,27 It should be stressed here that the observed shifts in peak positions are fully consistent with the detailed interpretation of the Raman and IR vibrational modes presented in ref 10 for nondeuterated samples. As has been already stated, the process of polyaniline doping can be conveniently studied by Raman spectroelectrochemistry. From the experimental results of this research, it seems clear that in protonated polyemeraldine (the only conducting form of polyaniline) structurally nonequivalent segments coexist. This conclusion is based on two observations indicating that strong resonance effects complicate the interpretation of Raman spectroelectrochemical data. (i) For the same electrode potential that implies the same chemical nature of the polymer, significantly different Raman spectra are obtained with λexc ) 457.9 nm and ) 676.4 nm. Evidently different chain segments are in resonance in each case. (ii) For both excitation lines mentioned above, the resulting spectra are very weakly dependent on the electrode potentials, which means that Raman spectroscopy registers mainly the vibrations from the resonating species, even if they are in a strong minority. The formation of different protonated structures for polyemeraldine can be schematically represented by the following sequence of reactions:
V. Discussion The research presented in this paper had two main goals: (1) The verification of the previously proposed2,4,6,7,20,25 band assignments via the analysis of the shifts of Raman and IR bands observed upon deuteration. (2) The elucidation of the nature of the structures created during the protonation of polyemeraldine base. The application of selective deuteration methods resulted in the preparation of three types of differently deuterated polyaniline in addition to the nondeuterated one. One can easily notice the advantage of such approach since it allows for an easy differentiation between the vibrations due to the ring deformations and those involving interring deformations. Significant contribution of C-H bending deformations to the mode originating from C-C stretching in both benzoid and
Electrochemical oxidation of the most reduced form of polyanilinespolyleucoemeraldine (I)sleads to the formation of protonated polyemeraldine with polysemiquinone radical cation chain structure (II) which is frequently called “a polaron lattice”. The polaron created upon oxidation can be transformed into spinless bipolarons (III), which upon deprotonation can give the oxidized basic unit of polyaniline (IV). The above sequence
Vibrational Properties of Polyaniline
J. Phys. Chem., Vol. 100, No. 17, 1996 7005 quinoid type segments should be observed, as shown experimentally. However, any interpretation of the spectra obtained with λexc ) 676.4 nm requires the assumption that the quinoid type structures (species III) must be present in addition to polysemiquinone radical cations (species II), since peaks characteristic of the quinoid sequence of bands dominate the spectrum. Therefore, an apparent inconsistency between the spectra recorded at the same potential with λexc ) 457.9 676.4 nm can be explained by the coexistence of both (II and III) protonated structures whose vibrations are selectively enhanced due to resonant behavior. In order to elucidate this problem, we have calculated the force constants for the four structures I-II. The results are collected in Table 2. It is clear that the formation of the polaron lattice structure results in a rather small change in the force constants values as compared to the polyleucoemeraldine unit. To the contrary, we have obtained a significant increase in Ft2 and FR2 values for the bipolaronic mode (species III) with a simultaneous decrease in Ft′2, the value and FR’2 remaining essentially unchanged. The calculated force constants for this structure are intermediate between those of unprotonated polyleucoemeraldine and unprotonated polypernigraniline consistent with the proposed mechanism of oxidation reaction. We have also carried out the calculations of the frequencies of the expected Raman modes for all four combinations of isotopic substitution studied, i.e., PANI(H,H), PANI(H,D), PANI(D,H), and PANI(D,D). In Tables 3 and 4 the calculated frequencies are collected together with the experimentally observed ones for both the polaron lattice structure, which is probed by the blue (457.9 nm) excitation line, and the bipolaron structure, whose vibrations are in resonance with the energy of the red (676.4 nm) excitation line. Taking into account the experimental errors and anharmonicity effects, a very good agreement
TABLE 2: Principal Force Constants for Reduced, Oxidized, and Two Types of Protonated Polyaniline Units
of chemical and structural changes occurring during electrochemical oxidation had been confirmed by in situ EPR spectroelectrochemical studies in which a strong increase of the EPR signal is first observed upon polyleucoemeraldine oxidation followed by its almost total annihilation at the potentials corresponding to the “plateau” between two oxidation peaks of polyaniline. The above behavior was interpreted in terms of the polysemiquinone radicals (polaron lattice) formation followed by the transformation of the created polarons into spinless bipolarons.28-30 Raman spectra obtained for λexc ) 457.9 nm can be interpreted assuming that vibrations originating from the polaron lattice structure are in strong resonance with this excitation line energy. The polaronic lattice model renders all ring and nitrogen equivalent provided that the delocalization of the charge takes place. Thus the polysemiquinone radical-cation chain can be considered as “electron deficient polyleucoemeraldine” from the structural point of view. It is therefore expected that the Raman spectrum of the polaron lattice structure will be similar to that of polyleucoemeraldine in the sense that no vibrations due to
TABLE 3: Observed and Calculated Frequencies for Polaron Lattice Model of Protonation base structure
polaron lattice structure
leucoe´me´raldine
PANI-HH
PANI-HD
PANI-DH
PANI-DD
Wilson N°
exptl
calcd
exptl
calcd
exptl
calcd
exptl
calcd
exptl
calcd
assignments
8a 8b amine gr. 9a 9a 1 6a 6b
1618 1597 1219 1181
1620 1588 1217 1168
1626
1626
1253 1192
1633 1608 1250 1190
1240 1189
1627 1606 1237 1188
1594 1566 1227
1598 1575 1229
1599 1566 1226
1591 1573 1214
867 667 603
878 669 610
886 680 608
889 676 616
858 664 603
860 669 613
874 820 634 610
869 830 648 595
876 810 632 608
864 824 641 592
C-C stretching C-C stretching C-N•+ deformation C-H bending C-D bending ring deformation ring deformation ring deformation
TABLE 4: Observed and Calculated Frequencies for Bipolaron Model of Protonation bipolaron structure pernigraniline base exptl
calcd
1606 1555 1579
1614 1591 1581
1480 1418
1496 1417
1215
1207 1171 1155
1157
PANI-HH exptl 1621
PANI-HD
calcd
exptl 1622
1581 1515
1625 1600 1586 1505
1484 1311/1332 1253 1188 1166
1457 1312 1263 1185 1183
1487 1342/1371 1249 1175
1588
1062
PANI-DH
calcd 1616 1598 1583 1476 1357 1256 1185 1182 1072
exptl
880 800
872 810
896 828
874 811
882 813
calcd
1593
1597 1563
1487
1494
1399 1313/1334 1252
1425 1312 1241
exptl 1595 1560
856 856 788
PANI-DD
820
853 868 857 796
calcd
assignments
1582 1462 1545
C-C stretching (Benz 8a) C-C stretching (Benz 8b) CdC stretching (Q) N-H bending CdN stretching C-C stretching (Q) + CH bending (Q) C-N+ stretching C-N stretching C-H bending (Benz 9a) C-H bending (Q) N-D bending C-D bending (Benz 9a) C-D bending (Q) ring deformation (Benz 1) ring deformation (B)
1428 1308/1334
1442 1346 1232
1085 856 856
1076 854 866 847 784
806
7006 J. Phys. Chem., Vol. 100, No. 17, 1996 between the experimentally determined and calculated frequencies has been achieved in all cases. The main point that still remains unclear is the existence of a doublet in the region characteristic of the C-N stretching in protonated bipolaronic structures (ca. 1320-1350 cm-1) instead of one band predicted by the calculations. This may be associated with the coexistence of two different polyaniline chain conformations which locally changes the stereochemistry of the protonated segments as previously proposed by Furukawa et al.2,19 The possibility of the coexistence of two different chain conformations in protonated polyaniline has been recently discussed by Xia et al.31 on the absis of the evolution of UV-vis-near-IR spectra upon so-called “secondary doping”. To summarize, Raman studies of polyaniline selectively deuterated on the ring or on nitrogen (or on both) enabled us to verify experimentally the vibrational model proposed previously10 on the basis of the nondeuterated polymer and some model low-molecular-weight compound. The observed shifts were fully consistent with the proposed assignments. In addition, we have carried out the calculations for two possible structures of protonated polyemeraldine, i.e., the polaron lattice structure and bipolaron structure, obtaining a very good agreement between the calculated frequencies and the experimentally determined ones of all four combinations of isotopic substitutions tested. Acknowledgment. Partial financial support of French-Polish Scientific Collaboration Program (project 5213) is greatly appreciated. Additional founding has been provided through KBN grants 3T09B00508 and PBU-31/RCh4/94. References and Notes (1) See, for example, the proceedings of ICSM Seoul (Korea, July 2429, 1994): Synth. Met. 1995, 69. (2) Furukawa, Y.; Hana, T.; Hyodo, Y.; Harada, I. Synth. Met. 1986, 16, 189. (3) Kuzmany, H.; Sariciftci, N. S. Synth. Met. 1987, 18, 353. (4) Monkman, A. P. Conjugated Polymeric Materials; NATO Advanced Study Institute Series 182; Kluwer: Boston, 1990; p 273.
Louarn et al. (5) Bernard, M. C.; Cordoba-Torresi, S.; Hugot-Le Goff, A. Sol. Energy Mater. Sol. Cells 1992, 25, 225. (6) Quillard, S.; Berrada, K.; Louarn, G.; Lefrant, S.; Lapkowski, M.; Pron, A. New J. Chem. 1995, 19, 365. (7) Ueda, F.; Mukai, K.; Harada, I.; Nakajima, T.; Kawagoe, T. Macromolecules 1990, 23, 4925. (8) Lapkowski, L.; Berrada, K.; Quillard, S.; Louarn, G.; Lefrant, S.; Pron, A. Macromolecules 1990, 23, 4925. (9) Kostic, R.; Rakovic, D.; Davidova, I. E.; Gribow, L. A. Phys. ReV. B 1992, 45, 728. (10) Quillard, S.; Louarn, G.; Lefrant, S.; MacDiarmid, A. G. Phys. ReV. B 1994, 50, 12497. (11) Genies, E. M.; Lapkowski, M. J. Electroanal. Chem. 1987, 220, 67. (12) Huang, W. S.; Humphrey, B. D.; MacDiarmid, A. G. J. Chem. Soc., Faraday Trans. 1986, 82, 2385. (13) Colomban, P.; Gruger, A.; Novak, A.; Regis, A. J. Mol. Structure 1994, 317, 261. (14) Pron, A.; Genoud, F.; Menardo, C.; Nechtschein, M. Synth. Met. 1988, 24, 194. (15) Lapkowski, M. Synth. Met. 1990, 35, 169. (16) Odin, C.; Nechtschein, M. Electronic Properties of Polymers; Kuzmany, H., Mehring, M., Roth, S., Eds.; Springer: Berlin, 1992; p 285. (17) Castiglioni, C.; Del Zoppo, M.; Zerbi, G. J. Raman Spectrosc. 1993, 24, 485. (18) Asturias, G. E.; MacDiarmid, A. G.; McCall, R. P.; Epstein, A. J. Synth. Met. 1989, 29, E157. (19) Harada, I.; Furukawa, Y.; Ueda, F. Synth. Met. 1989, 29, E303. (20) Furukawa, Y.; Ueda, F.; Hyodo, Y.; Harada, I.; Nakajima, I.; Kawagoe, T. Macromolecules 1988, 21, 1927. (21) Baird, N. C.; Wang, H. Chem. Phys. Lett. 1993, 202, 501. (22) Buisson, J. P.; Krichene, S.; Lefrant, S. Synth. Met. 1987, 21, 229. (23) Lefrant, S.; Buisson, J. P.; Eckhardt, H. Synth. Met. 1990, 37, 91. (24) Quillard, S.; Louarn, G.; Buisson, J. P.; Lapkowski, M.; Pron, A.; Lefrant, S. Submitted for publication in J. Raman Spectrosc. (25) Gruger, A.; Novak, A.; Regis, A.; Colomban, P. J. Mol. Struct. 1994, 328, 153. (26) Hadzi, D.; Skrbljak, M. J. Chem. Soc. 1957, 843. (27) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: San Diego, CA, 1990. (28) Genies, E. M.; Lapkowski, M. J. Electroanal. Chem. 1987, 236, 199. (29) Kuzmany, H.; Bartonek, M. Europhys. Lett. 1990, 12, 167. (30) Genoud, F.; Kruszka, J.; Nechtschein, M.; Santier, C.; Davied, S.; Nicolau, Y. Synth. Met. 1991, 41-43, 2887. (31) Xia, Y.; Wiesinger, J. M.; MacDiarmid, A. G. Chem. Mater. 1995, 7, 443.
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