J. Phys. Chem. 1995, 99, 12971 - 12974
12971
Mdssbauer Spectroscopy of FeCL-Doped~Poly@-phenylenevinylene)and of 12-Doped Poly(2,S-thienylenevinylene) Joris Briers, Walter Eevers? Micha'el De Wit, and Herman J. Geise* Department of Chemistry, University of Antwerp (UIA), Universiteitsplein I , B-2610 Wilrijk, Belgium
Willem D'Olieslager Radiochemistry Laboratory, Catholic University of Leuven, Celestynenlaan 200G, B-3001 Leuven, Belgium
Jan Wauters, Anne-Marie Van Bavel, Hilde Bemelmans, and Guido Langouche Institute of Nuclear Physics, Catholic University of Leuven, Celestijnenlaan 200 D,B-3001 Leuven, Belgium Received: April 13, 199P Mossbauer spectra of FeCh-doped poly(phenyleneviny1ene) with composition [ C ~ H ~ ( F ~ C ~and ) O of . ~ '2912O]~ doped poly(2,5-thienylenevinylene) with composition [C6H4sIO,SO]n showed FeC4- and symmetrical IS-, respectively, to constitute the dopant ions. The above doping processes are in these aspects equal to the doping processes in polyacetylene, while the corresponding processes in polyphenylene (FeCl3 doping) and polythiophene (I2 doping) show a slightly different behavior.
Introduction Conjugated polymers have attracted much interest in recent years because of their potential applications in electronics and semiconductor industries. Poly@-phenylenevinylene),PPV, and poly(2,5-thienylenevinylene),PTV, have been shown to be good electrical conductors after being reacted with a strong oxidizing doping agent. For example, it has been shown' that for PPV high electrical conductivity (230 S cm-I) can be obtained upon reaction with FeCl3. PTV, in comparison with PPV, has the higher stability when doped and can be doped by less strong doping agents, because of its lower ionization potential. A dc conductivity of 10 S cm-' was obtained2 upon doping with 12. Knowledge about the mechanism of the doping process is necessary for an understanding of the transport properties of the electrical conductivity. Thus, to determine the nature of the iron and iodine dopant species, we measured the 57Fe Mossbauer spectra of FeCl3-doped PPV as well as the 1291 Mossbauer spectra and mass spectra of 12-doped PTV. Since PPV can be considered as a copolymer of polyacetylene, PA, and polyphenylene, PP, the PPV results will be compared to those of FeC13-doped PA3 and those of FeCl3-doped PPe4 Similarly, the results of PTV will be compared to those of 12doped PA5 and those of 12-doped polythiophene, pT.6 The structures of the polymers mentioned are presented in Figure 1.
Experimental Section Films of PPV were synthesized via pyrolysis of a watersoluble sulfonium polyelectrolyte, derived from p-xylenebis(tetrahydrothiophenium chloride), according to the Kanbe and Wessling procedure, later modified by Lenz et aL7 After drying in air, the free-standing yellowish-green transparent polymer films were converted to PPV by eliminating the tetrahydrothiophenium moiety by heat treatment of 300 "C for 3 h under rigorous nitrogen conditions. Doping of the PPV films was accomplished by immersing them for 10 s in a saturated 0.62 M solution of FeC13 in anhydrous nitromethane and then washing them with pure anhydrous nitromethane. Taken into
* Author to whom correspondence should be addressed.
' Present address:
NITTO N.V., Eikelaarstraat 22, B-3600 Genk, Belgium. Abstract published in Advance ACS Abstracts, August 1, 1995. @
0022-3654/95/2099-12971$09.00/0
PPV
PTV
+CH=CH%
PP
PA
p7
Figure 1. Chemical structures of the polymers mentioned.
account the natural abundance of 57Feof 2.25% and the iron contents in doped PPV films as determined previously by X-ray fluorescence,8 about 20 films were stacked upon each other, resulting in a measurable iron concentration for 57FeMossbauer spectroscopy. The measurements were performed using a 57C0(Rh) source; the velocity scale was calibrated with the hyperfine lines of a-Fe. The chemical shift values are expressed with respect to a-Fe. Spectra were taken at 300 (RT), 100, and 4.2
K. PTV was synthesized via the modified precursor r ~ u t e . ~ - ' ~ The starting product thiophene was bis-chloromethylated and the product converted into the bis(tetrahydrothiophenium)salt. The latter was polymerized in methanoVwater (2/1 v/v) solution for about 20 h at -45 "C by addition of an equimolar amount of NaOH. The resulting sulfonium precursor polymer was allowed to warm to room temperature, upon which the sulfonium groups are substituted by methoxy groups. The formed methoxy-PTV precursor precipitated and was dissolved in chloroform for further Complete elimination of methoxy groups was then achieved by treating the chloroform solution of the precursor with concentrated hydrochloric acid, followed by casting a film of about 50 p m thickness and heating the film at 120 "C in vacuo for 3 h. Doping of the PTV films was achieved using a pentane solution of '2912. The latter was prepared by oxidation of commercial Na'291with 6 N H2S04 and a 10% H202 solution, followed by extraction of the reaction mixture with pure pentane. The actual doping was performed by soaking the films with a diameter of 20 mm for 24 h in a 0.031 M pentane solution of 1291(films I), for 36 h in a 0.031 M solution (films 11), or for 72 h in a 0.015 M solution (films 111). All these actions including the wash of the doped films and the reduction of the remaining 12912to its nonvolatile form 0 1995 American Chemical Society
Briers et al.
12972 J. Phys. Chem., Vol. 99, No. 34, 1995 76
’
1
75 ad
2 74
-z u
73 72
. i
71 70
69
-6
-4
-2
0
2
4
6
Velocity (mm/s)
Figure 2. Mossbauer spectrum of FeC13-doped PPV taken at 4.2 K. TABLE 1: 57FeMirssbauer Parameters (Isomer Shift, S, and Quadrupole Coupling, A, Both in d s ) of PA, PPV, and PP doped with FeC13“ site A site B T (K) PAb PPV PF PAb PPV P F 300 6 0.23(6) 0.22(2) 1.16(3) 2.10(2) A 0.24(6) 0.16(5) 2.35(3) 1.26(5) 78-100 6 0.33(3) 0.24(2) 0.30(1) 1.28(3) 2.09(2) 0.4 A 0.30(3) 0.16(5) 0.35(1) 2.61(3) 1.27(5) 1.0 4.2 6 0.31(3) 0.21(2) 1.26(3) 1.98(2) A 0.31(3) 0.24(5) 2.52(3) 1.28(5) a Estimated standard deviations on the last significant digit are given in parentheses. * Values from Sakai et al.) Values from Pron et al.4
Na1291were executed in a sealed vessel. After drying, the doped PTV samples were transferred into a poly(methy1 methacrylate) absorber holder for Mossbauer spectroscopy and sealed hermetically. The 1291Mossbauer spectra with the PTV absorbers in transmission geometry were made at 4.2 K. A Mg3’29mTe06 Mossbauer source was used. 129mTe was produced in a highflux reactor by neutron irradiation of ‘28Te. Calibration of the Mossbauer drive was performed with a 57C0(Rh) source and an iron foil. After the Mossbauer experiments the sample holders were opened and the conductivity was measured using the four-probe method. Afterwards, the samples were inserted into the ion source of the mass spectrometer (VG 70 SEQ tandem mass spectrometer) and held at 30 OC for 17 min, followed by heating to 330 OC at a rate of 50 “C/min. The upper temperature is below the decomposition temperature of PTV. An electron impact (70 eV) mass spectrum was acquired every 5 s during the heating period. These spectra were used collectively to obtain profiles of ion intensities as a function of time. In addition, single-ion monitoring was performed on the ions associated with the halogen and halogen products: I+, HI+, and I2+. Results and Discussion The Mossbauer spectra of the heavily doped PPV sample, made at temperatures of 300, 100, and 4.2 K, consist of a superposition of two quadrupole-split doublets: a strong (A) and very weak (B) signal. The spectrum of 4.2 K (Figure 2) shows that doublet A remains paramagnetic at 4.2 K. The Mossbauer absorption of this doublet increases remarkably with lowering of the temperature. This phenomenon is characteristic of compounds having a low Deybe temperature. From the values of the quadrupole splitting (A) and the isomer shift (d), doublet A is identified as a high-spin Fe(II1) compound and doublet B as a Fe(II) compound.I3 The Mossbauer data obtained for the doped PPV sample are summarized in Table 1, together with the results of analogous experiments on PA and PP.
Possible candidates for the Fe(II1) compound in PPV are unreacted anhydrous FeC13, FeCly6Hz0, FeCk3-, or FeC4-. Anhydrous FeCl3 is reportedI4 to have a higher 6 value (0.55 “/s at 78 K) and to become antiferromagnetic below 15 K, which is at variance with the behavior of the Fe(II1) compound in our spectra. The 6 and A values published3 for the hydrated FeC136H20 (A = 0.91 “/s and 6 = 0.60 “/s at 78 K) are considerably larger than our experimental values. Therefore, the presence of the two neutral compounds FeCl3 and FeC136H20 is ruled out. The Mossbauer parameters for the FeCk3- ions are reported3 as 6 = 0.48-0.52 “/s and A = 0 mm/s (78 K), while for FeC4-, 6 values are found3 to be about 0.30 “/s at 77 K, with a A value larger than 0 indicating a distortion of the tetrahedral FeC4- ion. Only the latter values are in agreement with our experimental values. The very weak intensity of the doublet B, corresponding to a Fe(I1) compound, does not allow an accurate determination of the 6 and A values. Therefore, the data presented in Table 1 are estimates. However, a less careful wash of another FeCl3-doped sample showed a splitting of doublet B into six lines in the spectrum at 4.2 K. This suggests that the Fe(I1) compound is FeClznH20 (n = 1 and/or 2) as a remainder of the doping and wash p r o c e ~ s .FeC12 ~ is soluble in nitromethane and thus can be washed from the system together with unreacted FeC13, but the presence of moisture can result in the insoluble FeClynH20 (n = 1 and/or 2). From the conclusion that the dopant ion is a distorted FeC4tetrahedron, combined with the iron content from X-ray fluorescence,8we arrive at [CsH6(FeC4)0,3], for the composition of heavily doped PPV. The results show that the mechanism of the doping process of PPV is identical to that of PA, proposed by Sakai et al.? FeCl, -teFeC1, FeCl, 4-nH,O
-
-
FeC1,
+ C1-
+ C1- -FeC1,FeCl,*nH,O (n = 1 and/or 2)
Only the value of A is slightly larger for PA than for PPV, indicating a larger distortion of the tetrahedral FeC4- ion. In FeC13-doped PP, the majority of the iron is also present in the form of FeC4-, with 6 and A values similar to those in PA and PPV! The Mossbauer parameters, however, of the lowtensity second iron site do not correspond to the second site in PA and PPV. They rather represent a second Fe(II1) site, possibly a highly distorted FeC4- ion.4 1291 Mossbauer spectra were obtained from PTV films doped with iodine for 24,36, and 72 h (see the Experimental Section). The three groups of spectra (films I, 11, and III) are qualitatively equal but differ in the relative amounts of the dopant species (see below). We start the discussion with the spectrum (Figure 3) of films I, which contain approximately 0.48 iodine atoms per monomer unit.2 The spectrum is interpreted as the superposition of three electronic quadrupole splitted subspectra, i.e. the components A, B, and C. The corresponding isomer shifts (6) and quadrupole splittings (A = e2Qq/h),evaluated while assuming asymmetry parameters to be 0, are given in Table 2. We take the different sites in linear 13-and 15- anions, on the analogy of the 1291 Mossbauer experiments on iodinedoped pT6 and PA.I5 The presence of I- can be ruled out: it would have given a single line at 6 = -0.4 mm/s. A symmetric linear 13-, as in [Ru(cp)21]13,has 6 = 1.48 mm/s and A = -2460 MHz for the central iodine atom and 6 = 0.20 mm/s and A = -1 152 MHz for the terminal iodine atoms.I6 The experimental values of components A and C agree with the central and
FeC13-Doped PPV and 12-Doped PTV
J. Phys. Chem., Vol. 99, No. 34, 1995 12973 TABLE 3: Charges of the Sites A, B, and C Derived from Isomer Shifts and Quadrupole Coupling Constants, with h, the Number of p Electron Holes in the 5sz5p6Configuration, Up the Number of Unbalanced p Electrons, qa = h, - 1, and a* = u, - 1 A
B C
-20
-10
0
IO 20 Velocity (nmlsl
Figure 3. Mossbauer spectrum of 12912-doped PTV taken at 4.2K. TABLE 2: lZ9I Mdssbauer Parameters (Chemical Shift,6, and Quadrupole Splitting, A) of Iodine-Doped FTV Film9 film component relative intensity 6 (mm/s)b A (MHz)C I A l.oo(3) 1.30(1) -2492(20) B 0.90(3) 0.65(1) -1837(32) C 0.52(2) 0.25(1) -1013(32) I1 A l.OO(3) 1.30(1) -2492(20) B 0.88(3) 0.65(1) -1837(32) C 0.34(2) 0.25(1) -1013(32) I11 A l.OO(3) 1.30(1) -2492(20) B 0.80(3) 0.65(1) -1842(32) C 0.26(2) 0.25(1) -1015(32) Estimated standard deviations are given in parentheses and refer to the last significant digit. Isomer shifts (6) are relative to a ZnTe source. Quadrupole coupling constants converted to 1271.Conversion factor: 1 mm/s 46.457(16)MHz for 1271and 1 mm/s 22.390(16) MHz for 1291.
-
-
terminal atomic sites, respectively, in a linear 13- ion. The experimental values do not agree with 6 and A values reported for asymmetric linear 13- anions." Molecular orbital calculationsI8 gave for the linear polyiodine ions the following electronic charge distributions over the atomic sites:
13-: a(-0.5) -P(O)-a( -0.5) 15-: a(O to -0.33)-b(O)-c(-l.O
to -0.67)-b(O)a(O to -0.33)
Following the reasoning of Kaindl et al.,I5 we assign component C to sites a and c, component A to sites P and b, and component B to site a. Now that the components are assigned, one can extract the relative abundance of the dopant ions from the intensities. In 13-; the ideal intensity ratio is 112, while in Is- the ideal intensity ratio IAIIBIIc is 21211. From Table 2 it is clear that films I almost exclusively contain linear 15- ions. Films I1 and I11 have I& > 2; that is, they seem to have an excess intensity of component A. This suggests that component A corresponds also to a third, yet unassigned iodine atom with a charge density similar to the iodine sites p and b. The low charge density points to neutral 12, weakly interacting with the linear iodine ions. Hyperfine parameters of pure 1 2 and reportedI9 as 6 = 0.98 " i s and A = -2263 MHz. Weak binding of 12 molecules to the anions is expected to increase the hyperfine parameters of the I2 atoms, resulting in a fourth component in the Mossbauer spectrum, quasicoinciding with component A. Then, the combination of intensity ratios (Table 2) with gravimetric analytical data revealed that films 1-111 contain less than 5% 13-, films I contains almost 100% Is-, films I1 contain a mixture of 90(3)% Is- and 10(3)% 12, and films I11 contain a mixture of 85(5)% Is- and 15(5)% I?..
1.23 0.79 0.53
1.08 0.80 0.44
0.23 -0.21 -0.47
0.08 -0.20 -0.56
From the values of A (=Amo]) and 6, the charge density localized on each iodine atom may be calculated from relations proposed by Dailey:20
6 ("/s)
= -9.2hS
+ 1.50hP- 0.54
where A,, is the atomic quadrupole coupling constant equal to 2297 MHz for 1291 (converted to I2'I), Up is the number of unbalanced p electrons, and h, (h,) is the number of s (p) electron holes in the 5s25p6 configuration. For the pure, not hybridized 0 bonding in polyiodine ions, i.e. using only pz orbitals, h, = 0 and h, = hpz. Table 3 gives the resulting h, and Up values averaged over the films I-III (Table 2), as well as the resulting charges evaluated as qa = h, - 1 and q A = Up - 1. One notes a reasonable agreement between the results obtained from 6 values or from A constants (Le. h, = Up and q6 = qA). Since any small hybridization will give a large contribution to the isomer shift, the charges, q ~ obtained , from the isomer shift are not as reliable as the charges, qA, obtained from the quadrupole splitting constants. Using the latter, we find the total charges of 13- -1.04 (electron) and of Is- -0.80 (electron). Thus, charge transfer from the polymer chains to the iodine acceptor atoms has taken place, and the obtained charge densities of the linear anions Is- and 13- in PTV are in agreement with those obtained in the 1291 Mossbauer experiment on iodine-doped PA.I5 This result supports the picture of the charge transfer dependence on the doping concentration. In the study of iodinedoped PA by Matsuyama et a1.,2',22a small concentration of Iions is observed only in samples with very small doping concentration (below 1%). With increased doping level, Idisappears and the number of 13- and 15- increase; above 10% doping concentration the number of 13-anions decrease in favor of 15-. Both 13- and 15- ions in doped PA were found to have symmetrical charge populations. Appearance of 12 was not reported. This observation reflects the fact that charge transfer takes place only in the early stage of doping, while increased doping levels give rise to the formation of 13- and 15-, respectively, without electron transfer.2'-22 The rather heavily doped PTV samples studied in this work confirm this picture of the doping process, as we observe almost only 15- anions. In contrast to PA and PTV, the iodine doping of PT leads to unsymmetrical 13- ions and to 1 2 molecules coordinated to the 13- ions, as resulted from Raman and Mossbauer experiments by Sakai et aL6 No 15- was observed. Additional evidence for the doping process was found in a de-doping experiment. 1291-dopedPTV was brought in the direct probe of a mass spectrometer, kept there in vacuo at 30 "C for 17 min, and then heated to 330 "C at the rate of 50 "Clmin. De-doping reactions took place, and the products liberated were monitored every 5 s by EI-MS (70 eV). Figure 4 shows the evolution of the signals at mlz = 127 (If), mlz = 254 (I*+), and mlz = 128 (HIf) with time. Careful washing of the samples had ensured that none of the signals are due to iodine adsorbed to the surface. Hence, the signals at mlz = 127 and mlz = 254
12974 J. Phys. Chem., Vol. 99, No. 34, 1995
Briers et al. 13- exists at lower doping levels, while 15- is the major compound at high doping levels. This is analogous to 12 doping of PA. In contrast, PT shows unsymmetrical 13- ions and IZ molecules coordinated to the 13- ions.
m/z=127
240
535
822
11:09
16:43
1356
1950
22:17
2S:04
2751
Time (min)
__...
_ , _ ,
_ * .f i . .
.
_ ,
0 2:40
S3S
822
Il:W
la56
16:43
19:30
2217
25:04
2751
Acknowledgment. The authors wish to thank Prof. Dr. M. Claeys (UIA) for help in the mass spectrometry measurements. A.M.V.B. acknowledges support of a Research Assistant by the Belgian National Science Foundation (NFWO). This text presents research results of the Belgian Programme on Interuniversity Attraction Poles initiated by the Belgian State (Prime Minister’s Office), Science Policy Programming. Scientific responsibility, however, is assumed by the authors.
Time (min)
References and Notes
2:40
5:3S
821
11:09
1356
16:43
1930
22:I’I
25:04
272Sl
Time (min)
Figure 4. Evolution with time of the signals at m/z = 127 (I+) (top), m/z = 254 (Iz+) (middle), and mlz = 128 (HI+) (bottom) in a de-doping experiment on Iz9I-doped PTV.
both occurring in two distinct timeltemperature ranges prove the liberation of bounded iodine molecules into two distinct steps. It shows the de-doping process with increasing temperature together with the different stabilities of 13- and 15- anions.
As the temperaturekime increases, a part of the iodine becomes covalently bound to the polymer, which leads to the elimination of the HI at high temperature, as seen in the evolution of the signal at mlz = 128. The formation of covalent C-I bonds is observed in an infrared spectrum as a growing absorption peak at 550 cm-’ when an iodine-doped PTV sample is heated under helium atmosphere in an infrared sample holder.
Conclusions Mossbauer spectra of FeC13-doped PPV show that the doping counterion is FeCL-. The doping mechanism is identical to the FeC13 doping of PA. PPV and PA show a second Fe(I1) site in the spectra with a very low intensity due to the FeClynH20 (n = 1 andor 2) as a rest product, while PP shows a second Fe(II1) site in the spectra. Mossbauer spectra of FTV show three 1291 sites due to the different positions in 13- and 15- ions with symmetrical charge distributions. Also, some I:! was found in the PTV samples.
(1) Mertens, R.; Nagels, P.; Callaerts, R.; Briers, J.; Geise, H. J. Synth. Met. 1993, 55-57, 3538. (2) Eevers, W.; De Wit, M.; Briers, J.; Geise, H. J.; Mertens, R.; Nagels, P.; Callaerts, R.; Herrebaut, W.; Van der Weken, B. Polymer 1994,35 (21), 4573. (3) Sakai, H.; Maeda, Y.; Kobayashi, T.; Shirakawa, H. Bull. Chem. SOC. Jpn. 1983, 56, 1616. (4) Pron, A.; Billaud, D.; Kulszewicz, I.; Budowski, C.; Pryluski, J.; Suwalski, J. J. Muter. Res. Bull. 1981, 16, 1229. (5) Abdel-Hamied, M.; Wortmann, G.; Naarmann, H. Synth. Mer. 1991, 41-43, 175. (6) Sakai, H.; Mizota, M.; Maeda, J.; Yamamoto, T.; Yamamoto, A. Y. Bull. Chem. SOC.Jp. 1985, 58, 926. (7) Lenz, R. W.; Han, C. C.; Stenger-Smith, J.; Karasz, F. E. J. Polym. Sci., Polym. Chem. Ed. 1988, 26, 3241. (8) Mertens, R.; Nagels, P.; Callaerts, R.; Van Roy, M.; Briers, J.; Geise, H. J. Synth. Mer. 1992, 51, 55. (9) Jen, K. Y.; Maxfield, M.; Shacklette, L. W.; Elsenbaumer, R. L. J. Chem. SOC., Chem. Commun. 1987, 309. (10) Murata, H.; Tokito, S.; Tsutsui, T.; Saito, S. Synth. Met. 1990,36, 95. (1 1) Eevers, W.; De Schrijver, D.; Dierick, T.; Peten, C.; Van Der Looy, J.; Geise, H. J. Synrh. Met. 1992, 51, 329. (12) Eevers, W. Ph.D. dissertation, Univ. Antwerp, Belgium, 1993 (in dutch). (13) Greenwood, N. N.; Gibb, T. C. Mossbauer Spectroscopy; Chapman and Hall Ltd.: London, U.K., 1971; p 91. (14) Pfletschinger, E. Z. Phys. 1968, 209, 119. (15) Kaindl, G.; Wortmann, G.; Roth, S.; Menke, K. Solid State Commun. 1982, 41, 75. (16) Potasek, M. J.; Debrunner, P. G.; Momson, W. H., Jr.; Hendrickson, D. N. J . Chem. Phys. 1974, 60, 2203. (17) Ehrlich, B. S.; Kaplan, M. J . Chem. Phys. 1969, 51, 603. (18) Browmaker, G. A. Austr. J. Chem. 1978, 31, 2713. (19) Bukhspan, S.; Goldstein, C.; Sonnino, T. J. Chem. Phys. 1968,49, 5417. (20) Dailey, B. P.; Townes, Ch. J . Chem. Phys. 1949, 17, 782. (21) Matsuyama, T.; Sakai, H.; Yamoka, H.; Maeda, J.; Shirakawa, H. J. Phys. SOC.Jpn. 1983, 52, 2238. (22) Matsuyama, T.; Seto, M.; Maeda, Y.; Yamaoka, H.; Sakai, H.; Masubuchi, S.; Kazama, S.; Mizoguchi, K.; Kume, K. Synth. Met. 1993, 55-57, 690. Jp95 10655
