- Email: [email protected]
Electron localization in poly(alkoxyanilines)
Solid State Communications, Vol. 108, No. 11, pp. 817–822, 1998 䉷 1998 Elsevier Science Ltd. All rights reserved 0038–1098/98 $ - see front matter
Pergamon
PII: S0038–1098(98)00452-9
ELECTRON LOCALIZATION IN POLY(ALKOXYANILINES) A. Raghunathan, a P.K. Kahol a ,* and B.J. McCormick b a
Department of Physics and bDepartment of Chemistry, Wichita State University, Wichita, KS 67260, U.S.A. (Received 5 June 1998; accepted 10 September 1998 by J. Kuhl) Electrical conductivity, magnetic susceptibility and electron spin resonance results are reported in poly(o-methoxyaniline), poly(o-ethoxyaniline) and an equimolar blend of polyaniline and poly(o-methoxyaniline) for different protonic acids. Electron localization length is found to be much larger in poly(alkoxyanilines) compared with their respective poly(alkylanilines). The larger localization length in poly(o-alkoxyaniline) materials is thought to arise from decreased Coulomb interaction between the positive charge on the polymer chain and the anion; this interaction is reduced by the electron donating nature of the alkoxy groups. 䉷 1998 Elsevier Science Ltd. All rights reserved Keywords: A. polymers, D. electronic transport.
Alkyl substituted polyanilines have recently been investigated in detail by a.c. and d.c. conductivity, dielectric constant, thermoelectric power, magnetic susceptibility, electron spin resonance (ESR) and electrochemical methods [1–4]. In particular, poly(o-methylaniline) was shown to have greater electron localization compared to that for polyaniline (PAN), although their electronic band structures are essentially the same [1]. This increased localization is believed to arise from a decreased interchain diffusion rate and reduced interchain coherence that results from substitution of CH 3 groups for H on the phenyl rings, noting especially the random location of the substituent on the rings. This localization decreases conductivity by nearly three orders of magnitude, decreases Pauli susceptibility (x P) and increases the number of Curie spins (n C). A comparison study of PAN, poly(o-methylaniline), poly(o-ethylaniline) and poly(o-propylaniline) has also demonstrated increased electron localization along the chain with increased size of the alkyl group on the phenyl rings [2]. A systematic study involving ESR, room temperature conductivity, microgravimetry and steric exclusion chromatography measurements has been recently reported on poly(2-alkoxyanilines) and poly(2,5-dialkoxyanilines),
* Corresponding author.
with alkoxy chains containing up to six carbon atoms [3]. The conductivity in the disubstituted materials was around 0.1 S cm ¹1 for poly(2,5-dimethoxyaniline) and poly(2,5-diethoxyaniline), but for materials with longer alkoxy groups the conductivity decreased by 3–5 orders of magnitude. These observations on the effect of substituent size are qualitatively similar to those from studies on poly(alkylaniline) systems [2]. However, it is found that polyanilines substituted with dialkoxy groups give conductivities higher than that for corresponding poly(dialkylanilines) [3]. Since a number of variables determine the absolute value of conductivity, electron localization length seems to us to be the best physical quantity for comparing two systems, especially for ones with similar charge transport mechanisms. A concerted study involving a combination of techniques such as magnetic susceptibility, d.c. conductivity and ESR measurements can provide tremendously useful information about the nature of charge localization, since for most of the polyaniline based materials conductivity is found to show a temperature dependence of the type: j ¼ j0 exp½ ¹ ðT0 =TÞ1=2 ÿ;
(1)
which is typical of quasi-one-dimensional variable range hopping between nearest neighbors [1, 5]. Here, T0 ¼ 16=½a ¹ 1 NðEF ÞzkB ÿ, a ¹1 is the decay length of the localized electron state or electron localization length,
817
818
ELECTRON LOCALIZATION IN POLY(ALKOXYANILINES)
Vol. 108, No. 11
N(E F) is the density of states at the Fermi level, z is the number of nearest neighbor chains (which is assumed to be four for polyaniline [1]) and k B is the Boltzmann constant. A fit of the experimental data to equation (1) gives T 0, but a ¹1 can only be determined if N(E F) is experimentally obtained, say, from magnetic susceptibility measurements. The Pauli susceptibility, which is related to N(E F) as xP ¼ m2B NðEF Þ, can be obtained from the linear part of xT vs T plots, because xT exhibits the following temperature dependence for polyaniline and related systems xT ¼ xP T þ C;
(2)
where C is the Curie constant. The purpose of the present paper is to report results on poly(o-methoxyaniline) (POMA), poly(o-ethoxyaniline) (POEA) and POMA–PANI blends (equimolar) with the following objectives: (i) to investigate localization effects as a function of doping by different functionalized protonic acids and to compare these results with those obtained for poly(alkylanilines); and (ii) to investigate localization effects due to concentration of the dopant. POMA doped with HCl was synthesized chemically as described in the literature [6, 7]. All the polymers were synthesized, with an oxidant/monomer ratio of 1.35, by slow addition of the oxidant over a period of eight hours. The as-prepared salt was converted to the emeraldine base form by treatment with NH 4OH and the conducting POMA–ES was then prepared by the usual doping procedure with the following protonic acids: para-toluene sulfonic acid (PTSA), sulfosalicylic acid (SSA), trifluoroacetic acid (TFA) and HCl. The samples were doped at the following protonation levels (x ¼ ½Hþ ÿ=½Nÿ): 0.10, 0.25, 0.38 and 0.50. POEA doped with HCl was also prepared in a similar way. Blends of polyaniline and poly(o-methoxylaniline) protonated with HCl, [(PAN–POMA)–HCl] 0.5, were synthesized chemically in 1 M HCl by the slow dropwise addition of precooled oxidant to an equimolar mixture of aniline and o-methoxyaniline under vigorous stirring in an ice bath. Powders pressed into round pellets of thickness ⬃0.05 cm were used for conductivity measurements. Samples were mounted on the cold finger of a Janis closed cycle refrigerator for j measurements. Powdered samples of approximately 25 mg, loaded in a quartz bucket, were used for susceptibility measurements. x was measured using a ‘‘Force Magnetometer’’ down to 77 K in a magnetic field of 5000 G. A computercontrolled X-band Bruker EMX 6/1 spectrometer was used for ESR experiments. Figure 1 shows measured conductivity, plotted as log j vs T ¹1/2, of (POMA–ES) x for the dopant acids: PTSA, SSA, TFA and HCl. Data for (POMA–HCl) 0.38
Fig. 1. Temperature dependence of conductivity, plotted as log j vs T ¹1/2, of POMA–ES for various protonic acids. The values of T 0 (K) and room temperature conductivity (in S cm ¹1) are given in parentheses for each dopant acid: PTSA (17 000; 8:2 ⫻ 10 ¹ 3 ), SSA (9500; 1:5 ⫻ 10 ¹ 2 ), TFA (11 000; 4 ⫻ 10 ¹ 2 ), 0.5 HCl (9500; 4:4 ⫻ 10 ¹ 2 ), 0.38 HCl (12 000; 4 ⫻ 10 ¹ 2 ). are also shown in Fig. 1. It can be seen that j for (POMA–HCl) 0.5, (POMA–HCl) 0.38 and (POMA–TFA) 0.5 is almost the same, while j for (POMA–SSA) 0.5 and (POMA–PTSA) 0.5 is smaller than that for (POMA–HCl) 0.5 over the temperature range studied. The magnetic susceptibilities, plotted as xT vs T, of the above materials are shown in Fig. 2. Also shown in Fig. 2 are the data fitted to equation (2) as continuous lines. In general, for all these samples the localization length a ¹1 ˚ , the Pauli susceptibility is is found to be (21 ⫾ 6) A ¹1 (67 ⫾ 10) emu mol of 2-rings (except for the one doped with SSA) and the room temperature conductivity is of the order of 2 ⫻ 10 ¹ 2 S cm ¹1. The values of room temperature conductivity, T 0, density of states at the Fermi level and number of Curie spins are given in the captions of Figs 1 and 2. The ESR lineshape is nearly Lorentzian with peak-to-peak linewidth of (1:2 ⫾ 0:1) G for all the samples except (POMA–PTSA) 0.5, for which it is Gaussian with a linewidth of 2.6 G. Substitution of a methyl group for a hydrogen atom on each phenyl ring of PAN yields poly(o-methylaniline), whose room temperature conductivity is of the order of 0.02 S cm ¹1, with T 0 approximately 30 000 K and a ¹1 ˚ [1, 2]. Although the substituent approximately 10 A OCH 3 in POMA is larger than the methyl group in poly(o-methylaniline), the room temperature conductivity for POMA–ES, irrespective of the dopant, is a little ˚ ) than that higher, T 0 is smaller and a ¹1 is larger (25 A of the methyl derivative. Moreover, the localization
Vol. 108, No. 11
ELECTRON LOCALIZATION IN POLY(ALKOXYANILINES)
Fig. 2. Temperature dependence of magnetic susceptibility, plotted as xT vs T, of POMA–ES for various protonic acids. The values of N(E F) (in states eV ¹1 2-rings) and number of Curie spins per 2-rings are given in parentheses for each dopant acid: PTSA (1.8; 0.0043), SSA (3.2; 0.0037), TFA (2.0; 0.018), 0.5 HCl (1.8; 0.0035) and 0.38 HCl (2.4; 0.0035). ˚ , which in length for (POEA–HCl) 0.5 is about 30 A comparison with poly(o-ethylaniline) is six times larger [8]. The much longer localization lengths in (POMA– HCl 0.5) and (POEA–HCl) 0.5 as compared to the corresponding alkyl systems may arise from steric effects, electronic effects or both. The increased electron localization in poly(alkylanilines) relative to polyaniline itself has been attributed to [1] (i) random placement of alkyl groups on ring sites along the chain axis which reduces interchain coherence, (ii) reduced interchain bandwidth arising from increased interchain separation owing to the size of the alkyl groups and (iii) possible ring twistings due to steric effects of the alkyl groups. These same factors were expected to lead to a much smaller localization length in POMA and POEA– unlike what is experimentally observed ¹ compared to poly(o-methylaniline) and poly(o-ethylaniline). This is consistent with the results of D’Aprano et al. [3] on dialkyl- and dialkoxy-substituted polyanilines, which show higher conductivities for poly(dialkoxyanilines) compared with poly(dialkylanilines) [3]. On the basis of the observed red shift of the absorption maximum in poly(o-alkoxyanilines), these authors have argued that the steric effect of alkoxy groups is small; one of these authors has also reported the insignificance of steric effects of alkoxy groups in polythiophenes [9]. On the other hand, alkoxy groups are electron rich and thus can show electron donating effects. These effects
819
have been observed through decreased redox potentials in alkoxyanilines. In contrast, we reported ESR results on poly(o-chloroaniline), which has an electron withdrawing substituent and this material was found to be an insulator with a very small localization length [10]. To obtain further understanding of the nature of the mechanism of electron localization, blends of POMA with PAN (in equimolar amount) were synthesized. Due to sufficiently large distance between the methoxy groups, the steric effect of the methoxy group in these blends is not expected to be significant and reduction in localization length compared to polyaniline will then be due to the lack of interchain coherence. The results for conductivity, plotted as log j vs T ¹1/2, for (POMA–PAN) x, are shown in Fig. 3 for the following dopant acids: PTSA, SSA and HCl. The results on thin films of [(POMA–PAN)–CSA] 0.5 cast from m-cresol are also shown in Fig. 3. The values of j at room temperature and T 0 are given in the figure caption. It can be seen that j at room temperature for all the four polymers is of the same order of magnitude. However there is a noticeably less rapid decrease in j with decreasing temperature for the material cast from m-cresol than for the other three materials. j for [(POMA–PAN)–HCl] 0.5 decreases the most compared with the other corresponding polymers. Figure 4 shows magnetic measurements, plotted as xT vs T, for these blends. The values of N(E F) and n C, calculated from the magnetic and conductivity measurements, are given in the caption to Fig. 3. Figure 4 also shows the fitted data in the form of continuous lines. From the above, [(POMA–PAN)–PTSA] 0.5 is found ˚ to have the largest electron localization length of 38 A ˚ ˚ compared to 22 A for [(POMA–PAN)–SSA] 0.5 and 26 A for [(POMA–PAN)–HCl] 0.5. These localization lengths are a little larger than those for doped POMA samples. The localization length, a ¹1, of [(POMA–PAN)–CSA] 0.5 ˚ with a T 0 of cast from m-cresol (28% by weight) is 440 A ¹1 300 K and N(E F) of 3.5 states eV 2-ring unit. This resembles a ‘‘disordered metal’’ close to the metal insulator transition state, similar to (PAN–CSA) 0.5 obtained from m-cresol [11]. These results may be rationalized by assuming that polymerization of equimolar aniline and o-methoxyaniline provides a material having a structure very similar to that of polyaniline itself. The methoxy groups appear to be well separated and uniformly distributed along the chain. It is thus strongly implied that the mere presence of a methoxy group does not lead to any additional localization. In the light of the above interpretation for the (POMA–PAN) blends, we also expected to observe a localization to delocalization type transition as a function of the doping level x. In Fig. 5, j(T), plotted as log j vs T ¹1/2, of [(POMA–PAN)–HCl] x is shown as a function of the protonation level, x; 0.10, 0.25, 0.38 and 0.50. As
820
ELECTRON LOCALIZATION IN POLY(ALKOXYANILINES)
Vol. 108, No. 11
Fig. 3. Temperature dependence of conductivity, plotted as log j vs T ¹1/2, of (POMA–PANI)–ES for various protonic acids. The values of T 0 (in K) and room temperature conductivity (in S cm ¹1) are given in parentheses for each dopant acid: CSA (300; 0.4), PTSA (6000; 0.26), SSA (9000; 0.29) and HCl (11 000; 0.16). for earlier samples, the synthesis here was also done with slow addition of oxidant such that oxidant/molar ratio was 1.35. From the room temperature conductivity of 0.0002, 0.13, 0.20 and 0.16 corresponding to x of 0.10, 0.25, 0.38 and 0.50, respectively, it is clear that j for x ¼ 0:10 is smaller by 3 orders of magnitude from that
for the other polymers with higher protonation level. j at room temperature for [(POMA–PAN)–HCl] 0.38 and [(POMA–PAN)–HCl] 0.50 are almost the same, while that for [(POMA–PAN)–HCl] 0.25 is a little smaller than that for the other two polymers with larger x. To obtain localization lengths, magnetic susceptibility
Fig. 4. Temperature dependence of magnetic susceptibility of (POMA–PANI)–ES for various protonic acids. The values of N(E F) (in states eV ¹1 2-rings) and n C per 2-rings are given in parentheses for each dopant acid: CSA (3.5; 0.034), PTSA (2.1; 0.036), SSA (2.4, 0.042) and HCl (1.6; 0.046).
Vol. 108, No. 11
ELECTRON LOCALIZATION IN POLY(ALKOXYANILINES)
821
Fig. 5. Log j vs T ¹1/2 for (POMA–PANI)–HCl as a function of dopant concentration. The values of T 0 (in K) and room temperature conductivity (in S cm ¹1) are given in parentheses for each dopant acid concentration: 0.5 HCl (11 000; 0.16), 0.38 HCl (11 000; 0.20), 0.25 HCl (11 000; 0.13) and 0.10 HCl (550 000; 1:8 ⫻ 10 ¹ 4 ). measurements as a function of temperature were carried out on all the above samples. Figure 6 shows the results of these measurements, plotted as xT vs T, along with the fitted data in the form of continuous lines. The values of T 0, x P, n C, NðEF Þ and a ¹1 are given in the figure captions. It can be seen that x P, N(E F), a ¹1 and room
temperature conductivity are small for [(POMA–PAN)– HCl] 0.10 compared with other samples corresponding to x ¼ 0:25, 0.38 and 0.50. In conclusion, magnetic susceptibility and d.c. electrical conductivity measurements on samples of poly(o-methoxyaniline) doped with various protonic
Fig. 6. Temperature dependence of magnetic susceptibility, plotted as xT vs T, of (POMA–PANI)–HCl as a function of dopant concentration, x. The values of N(E F) (in states eV ¹1 2-rings) and number of Curie spins per 2-rings are given in parentheses for each acid concentration: 0.5 HCl (1.6; 0.046), 0.38 HCl (1.4; 0.042), 0.25 HCl (1.7; 0.038) and 0.10 HCl (0.5; 0.029).
822
ELECTRON LOCALIZATION IN POLY(ALKOXYANILINES)
acids reveal a localization length which is nearly twice of that for poly(o-methylaniline). This increased localization length in poly(o-methoxyaniline) compared to poly(o-methylaniline) is thought to arise from electronic rather than steric effects. Our experimental data on blends of polyaniline and poly(o-methoxyaniline) also show that electronic effects more than compensate for increased localization effects resulting from the reduced interchain bandwidth and reduced interchain coherence. These electronic effects, arising from the electron donating ability of the alkoxy group, reduce Coulomb interaction between the positive charge on the polymer backbone and the dopant anions and hence lead to increased delocalization and larger localization lengths. Acknowledgements—Thanks are due to the National Science Foundation (Grant No. EPS-9550487) and the state of Kansas for a matching support. REFERENCES 1. Wang, Z.H., Ray, A., MacDiarmid, A.G. and Epstein, A.J., Phys. Rev., B43, 1991, 4373; Wang, Z.H., Scherr, E.M., MacDiarmid, A.G. and Epstein, A.J., Phys. Rev., B45, 1992, 4190.
Vol. 108, No. 11
2. Pinto, N.J., Kahol, P.K., McCormick, B.J., Dalal, N.S. and Wan, H., Phys. Rev., B49, 1994, 13983; Kahol, P.K., Pinto, N.J. and McCormick, B.J., Solid State Commun., 91, 1994, 21; Pinto, N.J., Shah, P.D., Kahol, P.K. and McCormick, B.J., Solid State Commun., 97, 1996, 1029. 3. Aprano, G.D., Leclerc, M., Zotti, G. and Schiavon, G., Chem. Mater., 7, 1995, 33; Zotti, G., Comisso, N., D’Aprano and Leclerc, M., Adv. Mater., 4, 1992, 749. 4. Lian, A., Besner, S. and Dao, Le H., Synth. Metals, 74, 1995, 21. 5. Nakhmedov, E.P., Prigodin, V.N. and Samukhin, A.N., Sov. Phys. Solid State, 31, 1989, 368. 6. Maccines, D. and Funt, L.B., Synth. Metals, 25, 1988, 235. 7. Gazotti W.A. Jr. and Paoli, Marco - A.De., Synth. Metals, 80, 1996, 263 (and references therein). 8. Kahol, P.K., Raghunathan, A. and McCormick, B.J., Synth. Metals, 1998 (submitted for publication). 9. Daoust, G. and Leclerc, M., Macromolecules, 24, 1991, 455. 10. Kahol, P.K., Dyakonov, A.J. and McCormick, B.J., Synth. Metals, 89, 1997, 17. 11. Cao, Y., Smith, P. and Heeger, A.J., Synth. Metals, 48, 1992, 91.
Pergamon
PII: S0038–1098(98)00452-9
ELECTRON LOCALIZATION IN POLY(ALKOXYANILINES) A. Raghunathan, a P.K. Kahol a ,* and B.J. McCormick b a
Department of Physics and bDepartment of Chemistry, Wichita State University, Wichita, KS 67260, U.S.A. (Received 5 June 1998; accepted 10 September 1998 by J. Kuhl) Electrical conductivity, magnetic susceptibility and electron spin resonance results are reported in poly(o-methoxyaniline), poly(o-ethoxyaniline) and an equimolar blend of polyaniline and poly(o-methoxyaniline) for different protonic acids. Electron localization length is found to be much larger in poly(alkoxyanilines) compared with their respective poly(alkylanilines). The larger localization length in poly(o-alkoxyaniline) materials is thought to arise from decreased Coulomb interaction between the positive charge on the polymer chain and the anion; this interaction is reduced by the electron donating nature of the alkoxy groups. 䉷 1998 Elsevier Science Ltd. All rights reserved Keywords: A. polymers, D. electronic transport.
Alkyl substituted polyanilines have recently been investigated in detail by a.c. and d.c. conductivity, dielectric constant, thermoelectric power, magnetic susceptibility, electron spin resonance (ESR) and electrochemical methods [1–4]. In particular, poly(o-methylaniline) was shown to have greater electron localization compared to that for polyaniline (PAN), although their electronic band structures are essentially the same [1]. This increased localization is believed to arise from a decreased interchain diffusion rate and reduced interchain coherence that results from substitution of CH 3 groups for H on the phenyl rings, noting especially the random location of the substituent on the rings. This localization decreases conductivity by nearly three orders of magnitude, decreases Pauli susceptibility (x P) and increases the number of Curie spins (n C). A comparison study of PAN, poly(o-methylaniline), poly(o-ethylaniline) and poly(o-propylaniline) has also demonstrated increased electron localization along the chain with increased size of the alkyl group on the phenyl rings [2]. A systematic study involving ESR, room temperature conductivity, microgravimetry and steric exclusion chromatography measurements has been recently reported on poly(2-alkoxyanilines) and poly(2,5-dialkoxyanilines),
* Corresponding author.
with alkoxy chains containing up to six carbon atoms [3]. The conductivity in the disubstituted materials was around 0.1 S cm ¹1 for poly(2,5-dimethoxyaniline) and poly(2,5-diethoxyaniline), but for materials with longer alkoxy groups the conductivity decreased by 3–5 orders of magnitude. These observations on the effect of substituent size are qualitatively similar to those from studies on poly(alkylaniline) systems [2]. However, it is found that polyanilines substituted with dialkoxy groups give conductivities higher than that for corresponding poly(dialkylanilines) [3]. Since a number of variables determine the absolute value of conductivity, electron localization length seems to us to be the best physical quantity for comparing two systems, especially for ones with similar charge transport mechanisms. A concerted study involving a combination of techniques such as magnetic susceptibility, d.c. conductivity and ESR measurements can provide tremendously useful information about the nature of charge localization, since for most of the polyaniline based materials conductivity is found to show a temperature dependence of the type: j ¼ j0 exp½ ¹ ðT0 =TÞ1=2 ÿ;
(1)
which is typical of quasi-one-dimensional variable range hopping between nearest neighbors [1, 5]. Here, T0 ¼ 16=½a ¹ 1 NðEF ÞzkB ÿ, a ¹1 is the decay length of the localized electron state or electron localization length,
817
818
ELECTRON LOCALIZATION IN POLY(ALKOXYANILINES)
Vol. 108, No. 11
N(E F) is the density of states at the Fermi level, z is the number of nearest neighbor chains (which is assumed to be four for polyaniline [1]) and k B is the Boltzmann constant. A fit of the experimental data to equation (1) gives T 0, but a ¹1 can only be determined if N(E F) is experimentally obtained, say, from magnetic susceptibility measurements. The Pauli susceptibility, which is related to N(E F) as xP ¼ m2B NðEF Þ, can be obtained from the linear part of xT vs T plots, because xT exhibits the following temperature dependence for polyaniline and related systems xT ¼ xP T þ C;
(2)
where C is the Curie constant. The purpose of the present paper is to report results on poly(o-methoxyaniline) (POMA), poly(o-ethoxyaniline) (POEA) and POMA–PANI blends (equimolar) with the following objectives: (i) to investigate localization effects as a function of doping by different functionalized protonic acids and to compare these results with those obtained for poly(alkylanilines); and (ii) to investigate localization effects due to concentration of the dopant. POMA doped with HCl was synthesized chemically as described in the literature [6, 7]. All the polymers were synthesized, with an oxidant/monomer ratio of 1.35, by slow addition of the oxidant over a period of eight hours. The as-prepared salt was converted to the emeraldine base form by treatment with NH 4OH and the conducting POMA–ES was then prepared by the usual doping procedure with the following protonic acids: para-toluene sulfonic acid (PTSA), sulfosalicylic acid (SSA), trifluoroacetic acid (TFA) and HCl. The samples were doped at the following protonation levels (x ¼ ½Hþ ÿ=½Nÿ): 0.10, 0.25, 0.38 and 0.50. POEA doped with HCl was also prepared in a similar way. Blends of polyaniline and poly(o-methoxylaniline) protonated with HCl, [(PAN–POMA)–HCl] 0.5, were synthesized chemically in 1 M HCl by the slow dropwise addition of precooled oxidant to an equimolar mixture of aniline and o-methoxyaniline under vigorous stirring in an ice bath. Powders pressed into round pellets of thickness ⬃0.05 cm were used for conductivity measurements. Samples were mounted on the cold finger of a Janis closed cycle refrigerator for j measurements. Powdered samples of approximately 25 mg, loaded in a quartz bucket, were used for susceptibility measurements. x was measured using a ‘‘Force Magnetometer’’ down to 77 K in a magnetic field of 5000 G. A computercontrolled X-band Bruker EMX 6/1 spectrometer was used for ESR experiments. Figure 1 shows measured conductivity, plotted as log j vs T ¹1/2, of (POMA–ES) x for the dopant acids: PTSA, SSA, TFA and HCl. Data for (POMA–HCl) 0.38
Fig. 1. Temperature dependence of conductivity, plotted as log j vs T ¹1/2, of POMA–ES for various protonic acids. The values of T 0 (K) and room temperature conductivity (in S cm ¹1) are given in parentheses for each dopant acid: PTSA (17 000; 8:2 ⫻ 10 ¹ 3 ), SSA (9500; 1:5 ⫻ 10 ¹ 2 ), TFA (11 000; 4 ⫻ 10 ¹ 2 ), 0.5 HCl (9500; 4:4 ⫻ 10 ¹ 2 ), 0.38 HCl (12 000; 4 ⫻ 10 ¹ 2 ). are also shown in Fig. 1. It can be seen that j for (POMA–HCl) 0.5, (POMA–HCl) 0.38 and (POMA–TFA) 0.5 is almost the same, while j for (POMA–SSA) 0.5 and (POMA–PTSA) 0.5 is smaller than that for (POMA–HCl) 0.5 over the temperature range studied. The magnetic susceptibilities, plotted as xT vs T, of the above materials are shown in Fig. 2. Also shown in Fig. 2 are the data fitted to equation (2) as continuous lines. In general, for all these samples the localization length a ¹1 ˚ , the Pauli susceptibility is is found to be (21 ⫾ 6) A ¹1 (67 ⫾ 10) emu mol of 2-rings (except for the one doped with SSA) and the room temperature conductivity is of the order of 2 ⫻ 10 ¹ 2 S cm ¹1. The values of room temperature conductivity, T 0, density of states at the Fermi level and number of Curie spins are given in the captions of Figs 1 and 2. The ESR lineshape is nearly Lorentzian with peak-to-peak linewidth of (1:2 ⫾ 0:1) G for all the samples except (POMA–PTSA) 0.5, for which it is Gaussian with a linewidth of 2.6 G. Substitution of a methyl group for a hydrogen atom on each phenyl ring of PAN yields poly(o-methylaniline), whose room temperature conductivity is of the order of 0.02 S cm ¹1, with T 0 approximately 30 000 K and a ¹1 ˚ [1, 2]. Although the substituent approximately 10 A OCH 3 in POMA is larger than the methyl group in poly(o-methylaniline), the room temperature conductivity for POMA–ES, irrespective of the dopant, is a little ˚ ) than that higher, T 0 is smaller and a ¹1 is larger (25 A of the methyl derivative. Moreover, the localization
Vol. 108, No. 11
ELECTRON LOCALIZATION IN POLY(ALKOXYANILINES)
Fig. 2. Temperature dependence of magnetic susceptibility, plotted as xT vs T, of POMA–ES for various protonic acids. The values of N(E F) (in states eV ¹1 2-rings) and number of Curie spins per 2-rings are given in parentheses for each dopant acid: PTSA (1.8; 0.0043), SSA (3.2; 0.0037), TFA (2.0; 0.018), 0.5 HCl (1.8; 0.0035) and 0.38 HCl (2.4; 0.0035). ˚ , which in length for (POEA–HCl) 0.5 is about 30 A comparison with poly(o-ethylaniline) is six times larger [8]. The much longer localization lengths in (POMA– HCl 0.5) and (POEA–HCl) 0.5 as compared to the corresponding alkyl systems may arise from steric effects, electronic effects or both. The increased electron localization in poly(alkylanilines) relative to polyaniline itself has been attributed to [1] (i) random placement of alkyl groups on ring sites along the chain axis which reduces interchain coherence, (ii) reduced interchain bandwidth arising from increased interchain separation owing to the size of the alkyl groups and (iii) possible ring twistings due to steric effects of the alkyl groups. These same factors were expected to lead to a much smaller localization length in POMA and POEA– unlike what is experimentally observed ¹ compared to poly(o-methylaniline) and poly(o-ethylaniline). This is consistent with the results of D’Aprano et al. [3] on dialkyl- and dialkoxy-substituted polyanilines, which show higher conductivities for poly(dialkoxyanilines) compared with poly(dialkylanilines) [3]. On the basis of the observed red shift of the absorption maximum in poly(o-alkoxyanilines), these authors have argued that the steric effect of alkoxy groups is small; one of these authors has also reported the insignificance of steric effects of alkoxy groups in polythiophenes [9]. On the other hand, alkoxy groups are electron rich and thus can show electron donating effects. These effects
819
have been observed through decreased redox potentials in alkoxyanilines. In contrast, we reported ESR results on poly(o-chloroaniline), which has an electron withdrawing substituent and this material was found to be an insulator with a very small localization length [10]. To obtain further understanding of the nature of the mechanism of electron localization, blends of POMA with PAN (in equimolar amount) were synthesized. Due to sufficiently large distance between the methoxy groups, the steric effect of the methoxy group in these blends is not expected to be significant and reduction in localization length compared to polyaniline will then be due to the lack of interchain coherence. The results for conductivity, plotted as log j vs T ¹1/2, for (POMA–PAN) x, are shown in Fig. 3 for the following dopant acids: PTSA, SSA and HCl. The results on thin films of [(POMA–PAN)–CSA] 0.5 cast from m-cresol are also shown in Fig. 3. The values of j at room temperature and T 0 are given in the figure caption. It can be seen that j at room temperature for all the four polymers is of the same order of magnitude. However there is a noticeably less rapid decrease in j with decreasing temperature for the material cast from m-cresol than for the other three materials. j for [(POMA–PAN)–HCl] 0.5 decreases the most compared with the other corresponding polymers. Figure 4 shows magnetic measurements, plotted as xT vs T, for these blends. The values of N(E F) and n C, calculated from the magnetic and conductivity measurements, are given in the caption to Fig. 3. Figure 4 also shows the fitted data in the form of continuous lines. From the above, [(POMA–PAN)–PTSA] 0.5 is found ˚ to have the largest electron localization length of 38 A ˚ ˚ compared to 22 A for [(POMA–PAN)–SSA] 0.5 and 26 A for [(POMA–PAN)–HCl] 0.5. These localization lengths are a little larger than those for doped POMA samples. The localization length, a ¹1, of [(POMA–PAN)–CSA] 0.5 ˚ with a T 0 of cast from m-cresol (28% by weight) is 440 A ¹1 300 K and N(E F) of 3.5 states eV 2-ring unit. This resembles a ‘‘disordered metal’’ close to the metal insulator transition state, similar to (PAN–CSA) 0.5 obtained from m-cresol [11]. These results may be rationalized by assuming that polymerization of equimolar aniline and o-methoxyaniline provides a material having a structure very similar to that of polyaniline itself. The methoxy groups appear to be well separated and uniformly distributed along the chain. It is thus strongly implied that the mere presence of a methoxy group does not lead to any additional localization. In the light of the above interpretation for the (POMA–PAN) blends, we also expected to observe a localization to delocalization type transition as a function of the doping level x. In Fig. 5, j(T), plotted as log j vs T ¹1/2, of [(POMA–PAN)–HCl] x is shown as a function of the protonation level, x; 0.10, 0.25, 0.38 and 0.50. As
820
ELECTRON LOCALIZATION IN POLY(ALKOXYANILINES)
Vol. 108, No. 11
Fig. 3. Temperature dependence of conductivity, plotted as log j vs T ¹1/2, of (POMA–PANI)–ES for various protonic acids. The values of T 0 (in K) and room temperature conductivity (in S cm ¹1) are given in parentheses for each dopant acid: CSA (300; 0.4), PTSA (6000; 0.26), SSA (9000; 0.29) and HCl (11 000; 0.16). for earlier samples, the synthesis here was also done with slow addition of oxidant such that oxidant/molar ratio was 1.35. From the room temperature conductivity of 0.0002, 0.13, 0.20 and 0.16 corresponding to x of 0.10, 0.25, 0.38 and 0.50, respectively, it is clear that j for x ¼ 0:10 is smaller by 3 orders of magnitude from that
for the other polymers with higher protonation level. j at room temperature for [(POMA–PAN)–HCl] 0.38 and [(POMA–PAN)–HCl] 0.50 are almost the same, while that for [(POMA–PAN)–HCl] 0.25 is a little smaller than that for the other two polymers with larger x. To obtain localization lengths, magnetic susceptibility
Fig. 4. Temperature dependence of magnetic susceptibility of (POMA–PANI)–ES for various protonic acids. The values of N(E F) (in states eV ¹1 2-rings) and n C per 2-rings are given in parentheses for each dopant acid: CSA (3.5; 0.034), PTSA (2.1; 0.036), SSA (2.4, 0.042) and HCl (1.6; 0.046).
Vol. 108, No. 11
ELECTRON LOCALIZATION IN POLY(ALKOXYANILINES)
821
Fig. 5. Log j vs T ¹1/2 for (POMA–PANI)–HCl as a function of dopant concentration. The values of T 0 (in K) and room temperature conductivity (in S cm ¹1) are given in parentheses for each dopant acid concentration: 0.5 HCl (11 000; 0.16), 0.38 HCl (11 000; 0.20), 0.25 HCl (11 000; 0.13) and 0.10 HCl (550 000; 1:8 ⫻ 10 ¹ 4 ). measurements as a function of temperature were carried out on all the above samples. Figure 6 shows the results of these measurements, plotted as xT vs T, along with the fitted data in the form of continuous lines. The values of T 0, x P, n C, NðEF Þ and a ¹1 are given in the figure captions. It can be seen that x P, N(E F), a ¹1 and room
temperature conductivity are small for [(POMA–PAN)– HCl] 0.10 compared with other samples corresponding to x ¼ 0:25, 0.38 and 0.50. In conclusion, magnetic susceptibility and d.c. electrical conductivity measurements on samples of poly(o-methoxyaniline) doped with various protonic
Fig. 6. Temperature dependence of magnetic susceptibility, plotted as xT vs T, of (POMA–PANI)–HCl as a function of dopant concentration, x. The values of N(E F) (in states eV ¹1 2-rings) and number of Curie spins per 2-rings are given in parentheses for each acid concentration: 0.5 HCl (1.6; 0.046), 0.38 HCl (1.4; 0.042), 0.25 HCl (1.7; 0.038) and 0.10 HCl (0.5; 0.029).
822
ELECTRON LOCALIZATION IN POLY(ALKOXYANILINES)
acids reveal a localization length which is nearly twice of that for poly(o-methylaniline). This increased localization length in poly(o-methoxyaniline) compared to poly(o-methylaniline) is thought to arise from electronic rather than steric effects. Our experimental data on blends of polyaniline and poly(o-methoxyaniline) also show that electronic effects more than compensate for increased localization effects resulting from the reduced interchain bandwidth and reduced interchain coherence. These electronic effects, arising from the electron donating ability of the alkoxy group, reduce Coulomb interaction between the positive charge on the polymer backbone and the dopant anions and hence lead to increased delocalization and larger localization lengths. Acknowledgements—Thanks are due to the National Science Foundation (Grant No. EPS-9550487) and the state of Kansas for a matching support. REFERENCES 1. Wang, Z.H., Ray, A., MacDiarmid, A.G. and Epstein, A.J., Phys. Rev., B43, 1991, 4373; Wang, Z.H., Scherr, E.M., MacDiarmid, A.G. and Epstein, A.J., Phys. Rev., B45, 1992, 4190.
Vol. 108, No. 11
2. Pinto, N.J., Kahol, P.K., McCormick, B.J., Dalal, N.S. and Wan, H., Phys. Rev., B49, 1994, 13983; Kahol, P.K., Pinto, N.J. and McCormick, B.J., Solid State Commun., 91, 1994, 21; Pinto, N.J., Shah, P.D., Kahol, P.K. and McCormick, B.J., Solid State Commun., 97, 1996, 1029. 3. Aprano, G.D., Leclerc, M., Zotti, G. and Schiavon, G., Chem. Mater., 7, 1995, 33; Zotti, G., Comisso, N., D’Aprano and Leclerc, M., Adv. Mater., 4, 1992, 749. 4. Lian, A., Besner, S. and Dao, Le H., Synth. Metals, 74, 1995, 21. 5. Nakhmedov, E.P., Prigodin, V.N. and Samukhin, A.N., Sov. Phys. Solid State, 31, 1989, 368. 6. Maccines, D. and Funt, L.B., Synth. Metals, 25, 1988, 235. 7. Gazotti W.A. Jr. and Paoli, Marco - A.De., Synth. Metals, 80, 1996, 263 (and references therein). 8. Kahol, P.K., Raghunathan, A. and McCormick, B.J., Synth. Metals, 1998 (submitted for publication). 9. Daoust, G. and Leclerc, M., Macromolecules, 24, 1991, 455. 10. Kahol, P.K., Dyakonov, A.J. and McCormick, B.J., Synth. Metals, 89, 1997, 17. 11. Cao, Y., Smith, P. and Heeger, A.J., Synth. Metals, 48, 1992, 91.