J. Phys. Chem. 1992,96,6711-6183
6777
Structural Investigations of Aromatic Amine Polymers K. G. Neob,* E. T. Kang, Department of Chemical Engineering, National University of Singapore, Kent Ridge, Singapore 0511
and K. L. Tan Department of Physics, National University of Singapore, Kent Ridge, Singapore 0511 (Received: November 21, 1991)
The polymerization of pphenylenediamine,diphenylamine, phenyl-p-phenylenediamine, N,N’-diphenyl-p-phenylenediamine, and N,”-diphenylbenzidine by either CU(C~O,)~.~H~O in acetonitrile or (NH4)2S208 in dilute HClO, or HCI was investigated. Elemental analysis, thennogravimetric analysis, infrared and ultraviolet-visible absorption spectroscopy, and X-ray photoelectron spectroscopy (XPS) were used to elucidate the chemical structures of the resulting products. The degree of polymerization and the protonation behavior depend on both the monomer and the polymerization method. In general, of the five aromatic amines studied, the polymers from phenyl-p-phenylenediamine and N,N’-diphenyl-pphenylenediamineare found to be more similar to polyaniline. The polymerization of diphenylamine proceeds via phenyl-phenyl coupling in the para positions, while the polymer from pphenylenediamineconsists of N-N couplings in addition to the C-N bonds. The degree of polymerization of N,N’-diphenylbenzidine is very low. The XPS results show that in the as-synthesized salts, protonation may occur at either the imine or amine units depending on the monomer used. The aromatic amine polymers are susceptible to forming covalent bonds with chlorine when HCl is used as the protonic acid during synthesis.
Introduction
Polyaniline is basically poly@-phenyleneamineimine), in which the oxidation state can be varied from the fully reduced poly@phenyleneamine) or leucomeraldine to the fully oxidized poly(p-phenyleneimine) or pemigraniline.’g2 The chemical synthesis of polyaniline is usually carried out via the oxidative polymerization of aniline by (NH4)2Sz08in a protonic acid An alternative method of using copper perchlorate in acetonitrile has also been reported to result in the predominantly head-to-tail polymerization of aniline.s Aniline polymerization can also be achieved electrochemically from aqueous or organic solutions.+* A number of studies on the polymerization of ring-substituted and N-substituted aniline derivatives by chemical as well as e l a h e m i d methods has also been conducted.”6 The presence of alkyl or halogen ring substituents was found to affect both the polymer yield and conductivity.+l2 Similarly, the polymerization of the various aromatic amines results in polymers which may possess physicochemical properties which are distinctly different from polyanilin~.’~’~ The intrinsic oxidation state and protonation characteristics of polyaniline can be readily elucidated using X-ray photoelectron spectroscopy (XPS).8J7-20 Although this technique has also been uscd in some of the studies of polyaniline derivatives, few of thew provide a detailed analysis of the XPS data. In the present article, we report on the chemical synthesis and characterization of the polymers from diphenylamine (DPA), NJVdiphenylbenzidine (DPB), phenyl-p-phenylenediamine (PPD), N,”-diphenyl-p-phenylenediamine (DPPD), and p-phenylenediamine (PDA). Two different polymerization methods were used. The structures of these polymers, in particular, the intrinsic oxidation states and protonation behavior, were compared with those of polyaniline in order to gain some insight into the differences between these polymers. Experimental Section ckmiclllp. The monomers DPA and DPB (Sigma), PPD and DPPD (Aldrich), PDA (Tokyo Kasei), and aniline (Merck) were used without further purification. The oxidants (NH4)2S208 (Merck, >9896) and Cu(C104),6H20 (fluka, >9896), and HPLC grade acetonitrile (Aldrich) were also used as received. cbcmlcrlPdyodmtioll The chemical polymerization of the aromatic amines wa# performed with either C U ( C ~ O , ) ~ . ~ H in ~ O acetonitrileS (method 1) or (NH4)2S20sin protonic acids3*, (method 2), similar to the methods used for the polymerization of aniline. Since these aromatic amines are toxic, they were 0022-3654/92/2096-6777$03.00/0
handled using standard precautionary measures such as rubber gloves and the polymerization experiments were carried out in a fume cupboard. In method 1, the oxidant/monomer mole ratio was 3:l and the reaction was allowed to proceed for 18 h. The product was filtered and washed with copious amount of acetonitrile and dried under vacuum. In method 2, two different acids, 2 M HClO, (method 2a) and 2 M HCl (method 2b) were used. The oxidant/monomer ratio was also 3:l and the polymerization was carried out at 0-5 OC for 5 h. Some runs were also carried out with an oxidant/monomer ratio of 0.3. The product was washed with the respective acid of 0.1 M concentration and then dried under vacuum. The base polymers were obtained by treating the as-synthesized polymer salts with 0.5 M NaOH for 18 h, followed by washing with deionized water until neutral and then dried under reduced pressure. Instrumentation. Elemental analyses of the polymers were performed using a Perkin-Elmer 2400 CHN elemental analyzer. Since the aromatic &a, with the exception of aniline and PDA, do not dissolve in aqueous acid solutions (for method 2), thermogravimetric (TG) analysis of the product was carried out to determine if any untreated monomer remained. The differences in the thermal stability and degradation behavior of the polymers would also give an indication of their structural differences. Infrared (IR) absorption spectra of the samples dispersed in KBr were measured on a Perkin-Elmer Model 682 spectrophotometer. The ultraviolet (UV)-visible absorption spectra of solutions of the polymers were determined using a Shimadzu UV-260 spectrophotometer. The XPS measurements of the powder samples were made on a VG ESCALAB MkII spectrometer with a Mg Kar X-ray source (1253.64 photons). The samples were mounted on the standard sample studs by means of doublasided adhesive tape. A takaoff angle of 7 5 O was used in all XPS runs. The X-ray source was run at 12 kV and 10 mA. The pressure in the analysis chamber was maintained at lo-* mbar or lower during measurements. To compensate for surface charging effects, all binding energies were referenced to the C 1s neutral carbon peak at 284.6 eV. In spectral deconvolution, the full width at half maximum (fwhm) of the Gaussian peak components was kept constant in a particular spectrum. The peak area ratios for various elements were corrected by experimentally determined instrumental sensitivity factors and may be subject to a maximum of &lo%error. The electrical conductivities ( u ) of the samples were measured using both the standard collinear four-probe and two-probe techniques. 0 1992 American Chemical Society
6778 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
Neoh et al.
TABLE I: Elemental Composition and Yield of Products from Different Polvmerization Methods
polymerization product wt/ base polymer method monomer wt C H N 1 0.4 6 4.9 1 s o
monomer PDA (NHK&WHA DPA (C~HSNHC~HS) PPD
1 2a 1
2a 2b DPPD 1 (C~HSNHC~H~NHC~HS)2a 2b DPB 1 (C~HSNH(C~H~)~NHC~HS) 2a 2b
1.3 0.9 0.6 0.5 0.6
6 6 6 6 6
0.2
6
0.6 0.4 1.4
6 6 6 6 6
1.5 1 .o
4.1 4.1 4.6 4.1 4.1 4.5 4.1 4.1 5.0 4.6 4.3
0.50 0.50 0.98 0.95 0.88 0.90 0.18 0.17
0.50 0.49 0.41
:I 0
0.1
0.2 0'
u 1W
200
LOO
300
500
600
700
TEMPERATURE JC'( Figure 2. Thermogravimetric scans of polyaniline bases from methods 1, 2a, and 2b. WRTis the weight at room temperature. (a1
061
\
ibl
+'?3!-2~.Q=(-J-n* u
1
0.2
I :
I
I
1
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I
1
01 0
I
100
2W
I
300
:
I
100
I
I
I
500
600
7w1
TEMPERATURE ('CJ Figure 1. Thermogravimetric scans of (a) DPA monomer, DPAl, and
DPA2a bases and (b)DPPD monomer, DPPDl, DPPDZa, and DPPD2b bases. WRTis the weight at room temperature.
Results and Discussion Polymer Yiekl ud Composition. The product to monomer ratios (by weight) achievable from the simultaneous polymerization and oxidation of the aromatic amines by the two different methods are tabulated in Table I. In the subsequent discussion, the product from either method will be indicated by the abbreviation for the monomer followed by the method number, e.g., DPAl and DPA2a. Since the monomers DPA, PPD, DPPD, and DPB do not dissolve in the aqueous acids, the products from methods 2a and 2b may contain unreacted monomers. The monomer DPB is also not soluble in acetonitrile and hence the product from method 1 may also be contaminated with monomer in this case. On the other hand, the addition of (NH4)2S208to a suspension of DPA in HCl results in a dark solution and only a very small amount of solid product can be collected after 5 h. The reactions of PDA with (N&)&O8 in HCI and HClO, at 0-5 "C also result in no substantial solid products after 5 h. The as-synthesized polymers would be in the salt form, Udopedl to various extents by the respective counterions. In order to estimate the amount of unreacted monomer in the product, the as-synthesized products were converted to the base form by compensation with NaOH before being subjected to thermogravimetric (TG) analysis. The TG scans of two monomers, DPA
I-v
V n
Figure 3. Possible structures of aromatic amine polymers having C/N ratio of (a) 6 and (b) 12.
and DPPD, and the corresponding products from methods 1 and 2 are shown in parts a and b of Figure 1, respectively. The comparison of the scans of the DPA monomer and products (Figure la) indicates that the latter contain no substantial unreacted monomer. A similar conclusion can be drawn for the products from PPD polymerization by both methods. As expected, the products from the polymerization of PDA and DPPD by method 1 (Figure lb) also show weight loss behavior entirely different from those of the monomers. In contrast, the TG scans of the bases of DPPD2a and DPPD2b (Figure lb) and DPB1, DPB2a, and DPB2b indicate that the onset of the first major weight loss step occurs at about the same temperature as that of the monomer. The weight loss at this temperature constitutes about 20-4096 of the initial weight, depending on the sample, and is attributed to the unreacted monomer and also possibly shortchain oligomers. The subsequent weight loss occurs either gradually or as a second weight loss step at about 500 "C. The differences in the structures of the polymers from the different synthesis methods are readily evident from the TG scans of the DPA (Figure la) and PPD base polymers. In these two cases, the polymers from method 1 retain a significantly higher proportion of weight for temperatures higher than 500 OC than the corresponding polymers from method 2. This difference in degradation behavior is not as evident in the TG scans of polyaniline base obtained by the different methods (Figure 2). The onset of the major weight loss step of the DPAl base polymer is about 100 "C higher than those of the polyaniline bases and the weight retained at 700 "C is also slightly higher. In the case of PPDl base polymer, the weight loss is slow and gradual with no distinct steps, and the weight retained at 700 "C is comparable to that of DPA1. In view of the presence of unreacted monomers in some of the reaction products, the elemental analysis results in Table I should be interpreted with care. With the exception of DPBl, the composition of the products from method 1 should give the most accurate indication of the molecular structure. The C/N ratios of the as-synthesized and base polymers are almost identical. There appears to be three classes of polymers, with C/N ratios of approximately 4 (PDAl), 6 (PPD1 and DPPDl), and 12 (DPAl). The polymers with C/N ratio of 6 would have a structure consistent with that of polyaniline (Figure 3a). The
Structural Investigations of Aromatic Amine Polymers oxidation state (Y)of the polymers is best deduced from the XPS data (seebelow) as the proportions of amine and imine nitrogens can be resolved from the properly deconvoluted N 1s core-level The corresponding structure for the polymer with C/N ratio of 12 is likely to have resulted from the coupling of the phenyl rings of the monomer, as shown in Figure 3b. The structures shown in Figure 3 have been assumed to be polymerized in a head-to-tail manner, similar to that of polyaniline. IR absorption spectroscopy results will be presented in the next section to support this assumption. The C/N ratio of about 6 obtained for DPPDl implies that an aromatic ring is split off from the monomer molecule upon polymerization. This was not observed with DPAl or DPA2a. The C/N ratios of the PPD polymers from method 2 are quite similar to that of PPD1, whereas for the reaction products DPPD2a and DPPD2b which contained unreacted monomer or short chain oligomers, the C/N ratio is substantially higher than 6. The C/N ratio of the higher molecular weight base polymer component in these two cases can be obtained by dissolving away the monomer with acetonitrile and conducting the elemental analysis on the insoluble fraction. For example, for DPPD2a, the acetonitrile-insoluble fraction constitutes about 70% by weight of the total sample and possesses a C/N ratio of 6/0.9, close to that of DPPD1. The weight of the fraction which is soluble in acetonitrile also corresponds quite closely to the first weight loss step observed in the TG scan (Figure lb). The TG and IR absorption spectroscopy (see later section) results of DPB indicate a low degree of polymerization. The C/N ratio of 4 for PDAl implies the presence of three nitrogens for every two aromatic rings. Thus this polymer will involve N-N couplings in addition to the C-N bonds. The elemental analyses of the base samples indicate that C, H, and N generally constitute 90% or more of the total weight, with the notable exception of DPB2b. For this sample, the C, H, and N constitute less than 80% of the total weight and the remaining is largely attributed to C1 (see section on XPS measurements). It is interesting to compare the yields of the polymers from method 1 (DPB1 excluded). For polyaniline, the yield based on polymer/monomer weight ratio is 0.90, Although the oxidation potentials of PDA and PPD are lower than that of aniline and the polymerization rate of the latter is greatly enhanced by the addition of small amounts of either PDA or PPD?' Table I shows that the polymer yields from these two amines and DPPD are actually significantly lower than that of aniline. In the case of DPPD, the loss of an aromatic ring from the monomer upon polymerization may partially account for the low polymer yield obtained. In contrast, addition of DPA to the polymerization of aniline has been shown to reduce the rate of polymer but from Table I, the polymer yield from DPA is higher than that of aniline. The higher yield of DPAl may be in part due to the reduced solubility of the polymer structure shown in Figure 3b as evidenced by the insolubility of DPB, which is structurally equivalent to a DPA dimer, in acetonitrile. S ithe product from PPD polymerization by method 2 contains no substantial unreacted monomer, the product yields with different oxidant/monomer ratios can be compared. For this monomer as well as aniline, the reduction of the oxidant/monomer ratio from 3:l to 0.3:l in method 2b results in a substantial reduction in the product yield. Some differences in the structure of the polymers were also observed (see later section on XPS measurements). IR and W-Visible Absorption Spectra. In addition to the similarity in the C/N ratios of PPDl and DPPDl base polymers with that of polyaniline base, the IR absorption spectra of these as-synthesized polymers are also similar. This is illustrated by the two spectra in Figure 4a,b. Both spectra possess the long absorption tail between 4000 and 1650 cm-I, the intense broad band at 1140 cm-I which has been considered to be an electronic band,22and the 630-cm-l band due to perchlorate ions.23 The former two features are associated with doped and conductive polyanilines. The PPD2a and PPD2b as-synthesized polymers also have similar IR absorption spectra as PPDl except that for PDD2b, the 630-cm-I band is absent and the intensity of the 114Ocm-' band is reduced. The latter is an indication that a lower
The Journal of Physical Chemistry, V O ~96, . No. 16, 1992 6119 MICRONS i
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1
6
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12
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3000
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IO
WAVENUMBER (cm") Figure 4. IR absorption spectra of (a) polyaniline from method 1, (b) DPPDl,(c) DPA1,and (d) PDAl.
level of doping is achieved with HCl as the protonating acid. In the spectra of the DPPD polymers from method 2, the absorption bands in the 650-900-cm1 region, characteristic of the substitution pattern on the aromatic ring, are different from those of polyaniline. These differences are also noticeable in the spectrum of DPAl (Figure 4c) but is especially obvious in the spectra of DPA2a, DPB1, DPB2a, and DPB2b. For PDAl, although the 630-cm-l pcak is strongly defined in its spectrum, the lack of a long absorption tail (Figure 4d) indicates that this polymer is probably not conductive, which is confirmed by the four-probe measurements. In this spectrum, the broad band in the 300035Wcm-' region is attributed to the N-H stretching.% This band is masked by the intense absorption tail in the other spectra shown in Figure 4. In the IR absorption spectrum of the DPA monomer, the two bands at 690 and 740 cm-I are attributed to the C-H out-of-plane bending vibrations of a monosubstituted benzene ring.% The other monomers, PPD, DPPD, and DPB possess an additional band at 810 cm-I, assigned to the corresponding vibration of the 1,4-disubstituted ring. The differences in the substitution pattern of the polymers can be more clearly seen in the spectra of the base polymers (Figure 5 ) . For polymers obtained via method 1, with the exception of DPBl, the 81O-cm-' band is the most prominent of the three bands, indicating a predominantly para-disubstituted chain. This observation also holds true for the PPD polymers from method 2 (Figure 5a), but for the DPPD polymers from method 2, the intensities of the three peaks are comparable (Figure 5b). With DPB, the 810-~m-~/690-cm-Iintensity ratio is actually highest for DPB2b (comparing Figure Sc and Figure 5d). However, due to the substantial degree of covalent bonding with C1 in this sample (see later section), the substitution pattern may not be an accurate indication of the extent of polymerization. From the 810-cm-'/69O-cm-l intensity ratio, the degree of polymerization for poly(DPA) prepared electrochemically's has been determined to be 16, which appears to be higher than that of DPA 1. On the basis of the TG scans of the DPA polymers (Figure la), the DPA2a polymer probably consists of even shorter chains than DPA1. This is substantiated by the higher 810-cm-I/690-
6780 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 i
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MICRONS 6
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Neoh et al. PHOTON ENERGY (eV1
B
12
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3 A
2
1.5
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1
3 I
(a1
2 I
1.5 I
(Cl
WAVELENGTH (nml Figure 6. UV-visible absorption spectra of (a) leucoemeraldine, (b)
polyaniline (emeraldine) base, (c) DPA2a base, and (d) PPD2a base.
IO
1
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I
3000
2000
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WAVENUMBER (cm-'l Figure 5. IR absorption spectra of base polymers of (a) PPDZa, (b)
DPPD2a, (c) DPBZa, and (d) DPBl. cm-l intensity ratio of the latter. Since the 690-cm-l band may be due to the end groups of the polymer or unreacted monomer, this intensity ratio would also not give a good indication of the degree of polymerization in those ~ 8 8 where ~ 8 the latter is present (e.g. DPPD2a and DPPD2b). In some other cases, e.g. PPD2a (Figure Sa), the intensity of the 690-cm-' band is also too low to be determined accurately. Although the IR absorption spectra have been useful in establishing the para-disubstituted nature of the polymers, other important information such as the oxidation state or doping level of the polymers is not available. For example, the two spectra in parts a and c of F w e 5 are quite similar,except for the bands attributable to the substitution pattern. However the PPD2a base gives a dark blue solution in either THF or NMP while the DPB2b base gives a pinkish-violet solution in the same solvents. The UV-visible absorption spectra can provide some indication of the differenas in the oxidation states of the polymers. The Uv-visible spectra of the fuly reduced form of polyaniline, leucoemeraldine (all amine units, Y = 0), and the approximately 5096 oxidized (Y 3 0.5) polyaniline,laemeraldhe base, in NMP are shown in parts a and b of Figure 6, respectively. The spectrum in Figure 6a reveals a strong absorption band centered at 340 nm (3.6 eV) and a very weak residual absorption at 635 nm ("2 eV). The former band has been assigned to the T-T* t r a n ~ i t i o n . 2The ~ ~ ~absorption spectrum of emeraldine base (Figure 6b) has a strong band at 635 nm in addition to the T-T* transition at 330 nm. The former is attributed to a charge-transfer exciton-like transition from the highest occupied level (centered on the benzenoid rings) to the lowest unoccupied energy level (catered on the quinoid Thus the absorption spectrum of freshly prepared leucoemeraldine solution indicates that only a very small fraction of quinoid groups is present. The exciton band can then be used as a measure of the oxidation state of pol~aniline.~~ The optical spectra of DPA2a and PPD2a bases in NMP are compared in parts c and d of Figure 6. In Figure 6c there is only one aborption band in the 300-nm region. This is also the case for the bases of DPBl and DPB2b. The DPAl and DPB2a bases are only slightly soluble in THF and NMP while the PDAl base
is almost completely insoluble in these two solvents. In Figure 6d, the ?M* transition band is at 320 nm and the second band is at 600 nm. The spectroscopic study of aniline oligomers of different number of aniline units (n) and of different degrees of oxidation2ashows a red-shift of the T-T* transition peak from 288 nm for n = 2, to 309 nm for n = 4, and to 326 nm for EM base cast from DMF. The position of the exciton band is also dependent on the chain length, as well as on the distribution of benzenoid and quinoid rings. For a tetramer with one quinoid ring, the band appears at 589 nm and for oligomers with alternating benzenoid and quinoid chain structure, it is shifted to the higher energy side at 412 nm for n = 2 and 476 nm for n = 4. The spectrum in Figure 6d suggests a shorter chain length for PPD2a as compared with the polyaniline sample. This is also consistent with the IR absorption spectra which shows the 690cm-' band to be very small in the former but not discernible in the latter. The oxidation state of PPD2a is also probably lower since the intensity of the exciton band relative to that of the T-T* transition band is lower as compared with that of emeraldhe base. With the exception of PPDl base, which is only slightly soluble in NMP or THF, the other PPD and DPPD base polymers also give a blue solution in both solvents, and the optical spectra show two absorption bands as in Figure 6d. However, for the various spectra, some differences in the relative intensities of these two bands are observed due to the differences in the oxidation level of the polymer as well as in the amount of unreacted monomer in the DPPD polymers. XPS M e r s ~ ~ t The s . XPS N 1s core-level spectra of assynthesized polyaniline salts have been reported earlier to have a prominent amine (-NH-) peak at 399.4 f 0.1 eV and a high binding energy (BE) tail attributable to positively charged nit r ~ g e n . ' ~ .The ~ * N 1s core-level spectrum of the polyaniline salt from method 1 is shown in Figure 7a. On the basis of the fmed fwhm approach in peak synthesis used in the present work, the high BE tail has been resolved as two peaks separated by 1.5 eV and 3 eV from the amine peak, respectively. These two peaks have been attributed to the polaron-type and bipolaron-type s t r u ~ t u r e s ~although ~ ~ * ~ it is also probable that the positively charged nitrogen species (N+) have a continuous BE distribution as a result of charge nonuniformity. The proportions of the different nitrogen species in this polyaniline sample and in the other polymer salts which do not contain substantial amounts of unreacted monomer (as indicated by the TG scans) are presented in Table 11. The N 1s core-level spectra of the DPPDl (Figure 7b) and PPDl and PPD2a (Figure 7c) samples also indicate a high proportion of N+ species similar to that of the polyaniline sample, but the distribution of the N+species may not be identical
Structural Investigations of Aromatic Amine Polymers
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6781
TABLE Ik Coaductivity pad Doping Level of Polymer Salts and Intrinsic Structure of Base Polymers
polymer salt base polymer a(S/cm) -N=/Na -NH-/N N+/N CIO,-/Nb CI-/Nb -N=/N -NH-/N N+/N sample 3 0.56 0.44 0.55 0.39 0.5 1 0.10 polyaniline (method 1) DPA2a, DPB1, and DPB2a). However, the proportion of N+ species in DPB2b is low and the nitrogen exist predominantly as the amine species (Figure 8d). For charge neutrality, the proportion of N+ species must be balanced by an equivalent amount of counterions. These counterions would be C104- when methods 1 and 2a are used and C1when method 2b is used. In the C12p core-level spectrum, the C10,- anions would have a characteristic BE in the 207-eV re@on.” This is illustrated by the C12p core-level spectrum of PPDl in Figure 9a. This spectrum can be resolved into two spin-orbit
split doublets (Cl 2pIl2and C 1 2 ~ ~with / ~ )the C12p312 peaks at 207.4 and 208.9 eV. These peaks are attributed to the C10,anions and the presence of the C104- anions in a more positive environment (ClO,*), respectively. The C104* component is similar to an intermediate C1’ species reported earlier for HCI protonated polyaniline30and also for polypyrrole-chloridecomple~es.~’ The intermediate C1* species at a BE of 198.6 eV (Cl 2~312peak) probably arises from the charge transfer interaction between the chlorine dopant and the conductive polymer chain. The C12p corelevel spectrum of PPD2b (Figure 9b) shows the presence of a small amount of this C1* species. For this sample, the proportions of chloride anions, Cl-, (Cl 2~312peak at 197.1 eV) and covalent chlorine, -C1 (C1 2p3/, peak at 200.1 eV), are almost identical. An earlier study on p ~ l y a n i l i n ehas ~ ~reported that the fraction of chlorine existing as C1- species is dependent on the concentration of HCl used. The extent of formation of Cl- species is also dependent on the monomer used, as illustrated by the comparison of Figure 9b with the C12p corelevel spectrum of DPB2b in Figure 9c, which shows an almost complete absence of chloride anions. This is consistent with the lack of a high BE tail in the N 1s core-level spectrum (Figure 8d). A comparison of the N 1s and C1 2p spectral areas of this sample indicates extensive bonding of chloride has occurred (-Cl/N = 0.9, on a mole basis). The covalent chlorine content remains largely unaffected by the base treatment of the as-synthesized product and accounts for the lack of closure of the mass balance based on C, H, and N. For PPD2b and polyaniline from method 2b, the proportion of chlorine that is covalently bonded to the polymer
6782 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
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201
BlNOlNG ENERGY (eV1 Figure 9. XPS CI 2p core-level spectra of (a) PPDl, (b) PPDZb, and (c) DPB2b.
is reduced when the oxidant/monomer ratio is decreased from 3:l to 0.3:l. In Table 11, the N+/N and ClO,-/N or Cl-/N ratios of the polymers are compared. In general, good agreement between the N+/N and counterions/N ratio is obtained, although the ClOc/N ratio tends to be slightly higher than the N+/N ratio due to the nonvolatile nature of HClO, (in method 2a) and possible incomplete removal of CU(CIO,)~(in method 1). It should be pointed out that although the use of the total chlorine/nitrogen ratio to estimate the doping level may not result in significant errors for 1 M HCl-protonated polyaniline,4 Figure 9 shows that for the other aromatic amine polymers the doping level may be grossly overestimated by the use of this ratio. The u values of the polymer salts are also tabulated in Table 11. The u of DPPDl is slightly lower than that of perchlorate-doped polyaniline while the perchlorate-doped PPD polymers are substantially less conductive even though the protonation or doping level (N+/N or ClO,/N) of these polymers are rather similar (Table 11). The lack of correlation between u and the doping level is even more clearly illustrated by the PDA and DPA polymers. In the case of PDAl, the N 1s core-level spectrum indicates that in addition to the amine and N+ components, there is a third component at a BE of 1.2 eV below that of the amine peak. This is ascribed to the neutral imine (-N=) species. The PDAl sample is rather different from the other polymer salts from method 1 in that a substantial amount of imine species remains unprotonated. The u values of the DPA polymers are much less than that of the electrochemically synthesized DPA polymer (u = 2 S/cm).15 The N 1s core-level spectrum of the latter shows a continuous high BE tail,somewhat similar to that shown in Figure 7a. On the basis of the IR absorption spectra, it has been earlier established that the electrochemically polymerized sample possesses a substantially higher degree of polymerization. The chemically synthesized DPB salts also possess a low degree of polymerization and are much less conductive than polyaniline. Moreover, the N 1s corelevel spectra of the various aromatic amine polymers have indicated differences in the nature of the N+ species which may in turn reflect differences in the intrinsic structure of the polymers. The N 1s core-level spectra of the base samples are compared
Neoh et al. with those of the salts in Figures 7 and 8. It can be seen from Figure 7 that deprotonation of the polyaniline, DPPD1, and PPD salts result in the recovery of the imine peak component (BE of 1.2 eV lower that the amine peak component). In these four cases the proportion of imine units is substantial, consistent with the postulate that protonation in the salts occurs preferentially at the imine units.4 However, deprotonation of the DPA and DPB salts results in the recovery of mostly amine units (Figure 8eg). Thus it can be concluded that in the DPA and DPB salts, the positively charged nitrogen species are mainly of the form of protonated amine or -N+H2- units. The protonation of amine units in addition to the imine units has been reported for emeraldine base films treated with 3 M HC104.33In this case the N+/N ratio is 0.85 and the N 1s core-level spectrum also shows the second N+ component to be the predominant peak. Subsequent deprotonation of the salt film results in the recovery of almost equal proportions of imine and amine units. The u of the 85% protonated film is not increased over that of the 50% protonated (i.e. protonation of imine units only) film since the conversion of -NH- groups to -N+Hz- groups is likely to interfere with the order of the polaron lattice. In the perchlorate-doped DPA and DPB salts, the predominance of -N+Hz- groups is also consistent with the low u values obtained. The intrinsic oxidation state, i.e. the proportion of the imine and amine units,of the aromatic amine base polymers is dependent on the starting monomer as well as the method of synthesis (Table 11). The N 1s core-level spectra of the polymer bases also show the presence of a residual high BE tail,attributed partly to surface oxidation. Although the as-synthesized DPB2b sample has no significant amounts of imine and N+ species (Figure 8d), after the treatment with NaOH and storage in air, the presence of small amounts of these species is established from the XPS results (Figure 8h). For the polyaniline (method l), PDA1, and PPDl bases, the imine to amine ratio is close to that expected of the 50% oxidized emeraldine base. However the BE separation of the imine and amine component peaks in the PDAl base is 1.35 eV rather than 1.2 eV. Furthermore, this sample is almost totally insoluble in NMP or THF. The PPDl base is also different from the other PPD base polymers in its thermal decompositionbehavior and solubility. The almost complete absence of imine units in the DPA2a and the lower imine/amine ratio of the other PPD base polymers as compared to that of polyaniline are consistent with their UV-visible absorption spectra discussed earlier. In summary, we have investigated the polymerization of PDA, DPA, PPD, DPPD, and DPB by either Cu(ClO4),.6H20 in acetonitrile or (NH4)2S208in HClO, or HCl. The first method is generally preferred for the polymerization of these amines since, in the second method, the products may contain substantial amount of acted monomer or low molecular weight oligomers (in the case of DPPD) or no solid products are obtained (PDA, and DPA if HC1 is used). Although both methods result in the head-tetail polymerization of PPD, the base polymers show some dissimilarities in the thermal decomposition behavior, solubility, and intrinsic oxidation state. Upon the polymerization of DPPD, an aromatic ring is split off from the monomer molecule resulting in a structure somewhat similar to polyaniline except for a lower intrinsic oxidation state in the former. In contrast, the polymerization of DPA proceeds via phenyl-phenyl coupling in the para positions while the degree of polymerization of DPB is very low. The products from the reactions of DPA and DPB with Cu(C104)2.6H20in acetonitrile or (NH4)2S208in HC104 have a high degree of protonation as indicated by the ClO,/N or N+/N ratio. However, the XPS results suggest that in these samples the positively charged N+ species are different from those in polyaniline, with protonation occurring mainly at the amine units. Upon deprotonation of these samples, the bases do not contain substantial imine units. When HC1 is used as the protonic acid during synthesis, the aromatic amine polymers are susceptible to the formation of malent bonds with chlorine. The polymerization and oxidation of PDA result in a low conductivity complex even though it is doped with C104- anions. The C/N ratio and the distribution of imine, amine, and positively charged nitrogen
J. Phys. Chem. 1992,96,6783-6790 species of this complex indicate a structure which is distinctly different from polyaniline. R e h y NO. PDA, 25168-37-0; DPA, 25656-57-9; PPD, 89230-95-5; DPPD, 102771-69-7; DPB, 108443-85-2; (NH,)$208,7727-54-0; CU(C10,)2, 15061-57-1.
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Photovoltage Generation in Bilayer Lipid Membrane-Cadmium Sulfide Junctions R. Rolandi,**tD. Ricci,t and 0. Brandts Department of Physics, University of Genoa, 16146 Genova, Italy (Received: July 22, 1991; In Final Form: March 12, 1992)
Flat lipid films of bimolecular thickness separating two aqueous solutions, known as black lipid membranes (BLM), provide matrices for formation of semiconductor particles and films. The formation process of cadmium sulfide particulate films on BLM of glycerol monooleate is quantitativelydescribed by measuring potential differences across the membrane, membrane capacitance, and pH variations of the solutions. The values of trans-membrane potential differences induced by hydrogen sulfide gradient suggest that H2S,used to form CdS, makes the BLM selectively permeable to protons. This selectivity is responsible for the dark voltage established across the membrane after the CdS formation. Biphasic photovoltages are induced by step illumination. Control experiments have been performed to check the influence of dark potentials, temperature gradients, and proton permeability on the photopotential. On the basis of the results of these experiments and those reported by other authors, a quantitative model which describes the kinetics of the photopotential is proposed. According to this model the light creates nonlocalized electron/hole pairs in the CdS which become localized when electrons interact with O2in the solution and its corresponding hole interacts with H I S in the membrane. The charged species move through the membrane under electric fields and concentration gradients. Four differential equations are used to describe the charge separation and motion and one other equation links charges and voltage. The equations have been numerically solved and the kinetics parameters determined by fitting the experimental results.
Introduction Living systems utilize visible light as a source of energy and information via physical and chemical processes occurring in photosynthetic centers formed by specialized membranes. The light-transducing membranes, both in specialized cells of superior living systems and unicellular algae and bacteria, have structures based on bimolecular lipid layers in which photoactive proteins Correspondence should be addressed to: Dr. R. Rolandi, Department of Physics, University of Genoa, Via Dodecaneso 33, 16146 Genova GE, Italy. *Present address: Department of Biophysics and Electronics Engineering, University of Genoa, via Opera Pia l l a , 16145 Genova GE, Italy. Present address: Department of Physics, Saint Lawrence University, Canton, NY, 13617.
and pigments are densely packed. These structures can be reproduced by using membrane mimetic systems such as liposomes, unilamellar vesicles, planar monolayers, bilayers, and multilayers so that quasi biological photoactive systems can be The need of well-controllable experimental systems to investigate photoprocesses occurring in living systems and the searching for new methods to transduce light into electrical and chemical energy prompt the study of photoactive membrane mimetic systems. Out of the membrane mimetic systems, the black lipid membranes (BLMs), planar bilayers separating two aqueous solutions, are particularly suitable for electrical measurements since the two sides of the membrane are easily accessible to electrodes.2 Natural pigments, synthetic dyes, and inorganic semiconductors have been
0022-3654/92/2096-6783$03.00/00 1 9 9 2 American Chemical Society
