9421
2007, 111, 9421-9423 Published on Web 07/26/2007
Ferromagnetic Spins Interaction in Poly(m-p-aniline) I. Kulszewicz-Bajer,*,† J. Gosk,‡,§ M. Pawłowski,£ S. Gambarelli,# D. Djurado,# and A. Twardowski‡ Faculty of Chemistry, Warsaw UniVersity of Technology, Noakowskiego 3, 00-664 Warsaw, Poland, Institute of Experimental Physics, UniVersity of Warsaw, Hoz˘ a 69, 00-681 Warsaw, Poland, Faculty of Physics, Warsaw UniVersity of Technology, Koszykowa 75, 00-662 Warsaw, Poland, Institute of Electronic Materials Technology, Wo´ lczyn´ ska 133, 01-919 Warsaw, Poland, and DRFMC/SCIB and SPrAM, CEA Grenoble, 17 rue des Martyrs, 38054 Grenoble cedex9, France ReceiVed: June 12, 2007; In Final Form: July 13, 2007
Alternating poly(m-p-aniline) can be oxidized to radical cations with spins S ) 1/2. For high spin density, ferromagnetic spin interaction is observed, and some of the generated radical cations are found to be in a ground triplet state.
Introduction In the past decade, the interest in purely organic magnetic materials has attracted considerable attention. Various efforts have been focused on preparing organic materials with radical species, which could be coupled in a ferromagnetic fashion. It has been reported that polycarbenes,1 phenoxy radicals,2 and aryl nitroxides3 exhibited a large magnetic moment and magnetic ordering at low temperatures. Another attempt has been devoted to oligoanilines, which can form radical cations that are relatively stable even at room temperature.4-7 The simplest model compound was a derivative of Wu¨rster’s blue coupled to a 1,3-benzene ring studied by Ito et al.4 Diradical cations obtained after partial oxidation of this compound showed a triplet ground state. Similarly, a triplet state was detected in oligoanilines linked to 2,7-naphthalenediyl5 or alternating aniline tetramers containing a 1,3-benzenediyl coupler.6,7 Considering spin interaction observed for oligoanilines, it seems very interesting to extend these studies by preparation of corresponding macromolecular compounds. Recently, we have synthesized a series of alternating poly(m-p-anilines) containing secondary and tertiary amine groups.8 EPR studies confirmed the formation of a high-spin state at low temperature. However, the magnetization measurements showed that these polymers oxidized to radical cations revealed the paramagnetic-type behavior with weak antiferromagnetic interactions. In this Letter, we have focused on the interactions of spins in linear poly(m-p-aniline) with a 1,3-benzenediyl coupler and 4-butylphenyl rings facilitating the polymer solubility. The spin states were studied for two oxidant/polymer ratios using continuous-wave (CW) and pulsed EPR spectroscopies. Moreover, magnetic properties measured by the SQUID technique are presented as well. * To whom correspondence should be addressed. E-mail: ikulsz@ ch.pw.edu.pl. † Faculty of Chemistry, Warsaw University of Technology. ‡ University of Warsaw. § Faculty of Physics, Warsaw University of Technology. £ Institute of Electronic Materials Technology. # DRFMC/SCIB and SPrAM.
10.1021/jp074540v CCC: $37.00
Experimental Section Poly(m-p-aniline) (PA) has been synthesized in its neutral, spinless form, as reported previously9 (the preparation procedure is described in the Supporting Information). The polymer was oxidized chemically using tris(4-bromophenyl)aminium hexachloroantimonate (TBA‚SbCl6) as an oxidant. The oxidation reaction was carried out in an argon atmosphere. In a typical procedure, 0.02 mmol of polymer was dissolved in 0.5 mL of dry CH2Cl2 and oxidized with corresponding amounts of TBA‚ SbCl6 in 0.5 mL of butyoronitrile (oxidant/PA ratios were equal to 1 and 1.5, where PA denotes a polymer unit, that is, (C32H34N2)n). The samples were marked as PA(1) and PA(1.5), respectively. These solutions of oxidized polymers were used directly for EPR measurements. Similar solutions after the evaporation of solvents at room temperature in a vacuum were used for magnetization measurements. The 2D electron spin transient nutation (ESTN) measurements were performed as well. The longitudinal magnetization after the nutation pulse was recorded using a π/2 - τ - π sequence, as described in Chapter 14.2.2 in ref 10. Results and Discussion The chemical oxidation of poly(m-p-aniline), PA, led to the formation of radical cations (Scheme 1), the presence of which was manifested by the appearance of new bands in the UVvis-NIR spectra, similar to those reported previously.8,9 Two new bands attributed to radical cations were located at 409 and 900 nm. The oxidation of PA with more than one equivalent of an oxidant (Ox/PA > 1) caused the appearance of a third new band at 735 nm. This band can be related to the formation of dication radicals in a p-phenylenediamine unit. A weak oxidation of PA should lead to the creation of mostly isolated radical cations with S ) 1/2, which are, however, partially delocalized on adjacent aromatic rings. According to theoretical calculations,11 the spin densities on carbon atoms in the ortho and para positions to amine groups are comparable to that on nitrogen atoms. Thus, one can suppose that spin interaction significantly depends on spin concentration along the polymer chain. © 2007 American Chemical Society
9422 J. Phys. Chem. B, Vol. 111, No. 32, 2007
Letters
Figure 1. The EPR spectra of PA oxidized to radical cations - frozen solution recorded at 5 K for the ∆Ms ) (2 (left) and for the ∆Ms ) (1 (right); (a) PA(1), (b) PA(1.5).
SCHEME 1: The Oxidation of Poly(m-p-aniline), PA
The CW-EPR spectra of PA oxidized in solution with 1 or 1.5 equiv of the oxidant, that is, PA(1) and PA(1.5) are presented in Figure 1. Both spectra are dominated by a single line with g ) 2.003, which can be attributed mainly to transitions within the S ) 1/2 doublet. This line is much stronger for PA(1.5), which is in agreement with a larger concentration of radical cations resulting from stronger oxidation of the polymer. The weak line with g ) 4.005(6) could be described to the ∆Sz ) (2 transitions and thus would suggest the presence of a highspin state (S ) 1). However, the intensity of such transitions should be much stronger for PA(1.5) than that for PA(1), which seems not to be the case. The main differences observed in these spectra are the narrowing of both lines, that is, from ∆Bpp ) 0.83 mT (∆Sz ) (1) and ∆Bpp ) 1.08 mT (∆Sz ) (2) for PA(1) to ∆Bpp ) 0.26 mT (∆Sz ) (1) and ∆Bpp ) 0.56 mT (∆Sz ) (2) for PA(1.5). We also used a pulsed EPR method in order to perform transient nutation experiments on a liquid PA(1) sample at 10 K. By such a way, it is possible to quantify the strength of the interaction between the spin and the microwave radiation. In a 2D experiment, when a transient nutation is performed as a function of the static magnetic field value, overlapping species with different spin number can be discriminated.10,12 As shown in Figure 2, in PA(1), we observed at least two species with a nutation frequency of 7.3 and 10.6 MHz. They can be attributed
to S ) 1/2 and 1 spin systems, respectively. Indeed, we verified that the frequencies ratio (1.45) is very close to the x2 theoretical value expected for pure S ) 1/2 and 1 spins. The magnetization data for PA(1), namely, magnetization as a function of magnetic field and magnetic susceptibility as a function of temperature, are shown in Figure 3a and b,
Figure 2. Transient nutations as measured by pulsed EPR spectrometry. The two spin contributions are clearly separated in the nutation frequency domain, and their positions are consistent with S ) 1/2 and 1 spin state assignments.
Letters
Figure 3. (a) Magnetization of PA(1) versus the magnetic field at different temperatures. (b) Inverse susceptibility of PA(1) measured at B ) 1.0 and 0.5 T as a function of the temperature. The straight line represents the Curie-Weiss law with θ ) -0.7 K.
Figure 4. (a) Comparison of the magnetization measured as a function of the magnetic field at T ) 2 K for PA(1) and PA(1.5). Solid lines represent Brillouin function fits to the experimental data. (b) The same experimental data but represented in reduced magnetization M(B)/M(B ) 6 T) versus the magnetic field axes.
respectively. Magnetization reveals an overall Brillouin-type paramagnetic behavior; at low temperatures (T ) 2 K), it tends to saturate at the highest experimental fields, while at higher temperatures (T > 30 K), it is practically linear with a magnetic field. Inverse susceptibility (measured as M/B) as a function of temperature shows linear dependence with temperature, that is, it obeys the Curie-Weiss law. Moreover, the extrapolated Curie-Weiss temperature θ is zero, within experimental accuracy (θ ) -0.7 ( 2.0 K, Figure 3b), which suggests that interaction between magnetic moments is negligible. Assuming that magnetic moments result mainly from spins S ) 1/2, one should be able to describe magnetization by a standard Brillouin function M ) xNgµBSBS(B,T), where spin S was chosen to be 1/ , g is g-factor, µ denotes the Bohr magneton, and x is the 2 B spin concentration, that is, the number of unpaired electrons (radical cations) per polymer unit. Fitting the magnetization data with this formula, with x as the only adjustable parameter (S was fixed to 1/2), provided a reasonable fit for x ) 0.45 (solid line in Figure 3a). This means that magnetization data can be satisfactorily interpreted as resulting from noninteracting spins S ) 1/2. The contribution of clusters with S ) 1 (observed in EPR) to magnetization data is apparently negligible (spins can be viewed as magnetically uncoupled). This effect can be related to low spin density along the chain for oxidation state of PA(1). Thus, one should not expect strong magnetic interactions, which is in agreement with the observed negligible Curie-Weiss temperature (Figure 3b). On the other hand, stronger oxidation of PA, as in the case of PA(1.5), apparently results in an increase of magnetization (Figure 4a), which can be interpreted as the effect of increased spin density. Attempts to fit the magnetization with the Brillouin function showed the increase of spin concentration by about 50% (x ) 0.74), but the overall magnetization shape deviated from the Brillouin function (solid line in Figure 4a). This is visualized in a more pronounced way in Figure 4b, where reduced magnetization M(B)/M(B ) 6 T) is depicted. More
J. Phys. Chem. B, Vol. 111, No. 32, 2007 9423 readily seen, magnetization of PA(1.5) saturates faster with an increasing magnetic field than magnetization of PA(1). Faster saturation than Brillouin-type saturation usually results from ferromagnetic (FM) coupling between magnetic species; FM coupling enhances the effect of the external magnetic field, and the moments saturate more easily. It is known13 that for an interacting magnetic system, the standard Brillouin function needs to be replaced by an effective one, where magnetic interaction yields an effective temperature Teff ) T + T0 instead of a real experimental temperature T. In the case of FM coupling, Teff is smaller than T (T0 < 0, whereas T0 > 0 for AFM interaction). The fitting procedure applied to PA(1.5) magnetization data yielded T0 ) -0.66 K and x ) 0.67 (Figure 4b), which suggests rather weak FM interactions between spins for PA(1.5). It should be mentioned here that a fit of PA(1) magnetization data with an effective Brillouin function returned Teff ≈ 1.91 K, which means that Teff ≈ T, within the accuracy of the fit. The estimated concentration of radical cations per polymer unit for the PA(1.5) x ) ∼0.7 indicates the increase of spin density and means that about 20% of the spins have nearest magnetic neighbors; therefore, even weak magnetic coupling should show up in magnetization. This may be the reason why coupling is visible for PA(1.5) but not for PA(1). Conclusions The chemical oxidation of linear poly(m-p-aniline) leads to the formation of radical cations. Spins of radical cations can be coupled to the high-spin state; however, low spin density along the polymer chain results in nearly purely paramagnetic behavior of the system, whereas high spin density causes the appearance of ferromagnetic interaction. Although the microscopic mechanism of this interaction is not known at the moment, the ferromagnetic character of the coupling is very promising for fabrication of ferromagnetic polyaniline. Acknowledgment. We wish to acknowledge financial support from the Committee of Scientific Research in Poland (KBN, Grant No. 3 T09A 100 29). Supporting Information Available: Detailed experimental procedure of polymer polycondensation and the UV-vis spectra of the chemical oxidation. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Rajca, A.; Lu, K.; Rajca, S. J. Am. Chem. Soc. 1997, 119, 1033510345. (2) Nishide, H.; Kaneko, T.; Nii, T.; Katoh, K.; Tsuchida, E.; Lahti, P. M. J. Am. Chem. Soc. 1996, 118, 9695-9704. (3) Ishida, T.; Iwamura, H. J. Am. Chem. Soc. 1991, 113, 4238-4241. (4) Ito, A.; Taniguchi, A.; Yamabe, T.; Tanaka, K. Org. Lett. 1999, 1, 741-743. (5) Selby, T. D.; Stickley, K. R.; Blackstok, S. C. Org. Lett. 2000, 2, 171-174. (6) Wienk, M. M.; Janssen, R. A. J. J. Am. Chem. Soc. 1997, 119, 4492-4501. (7) Struijk, M. P.; Janssen, R. A. J. Synth. Met. 1999, 103, 22872290. (8) Kulszewicz-Bajer, I.; Zago´rska, M.; Wielgus, I.; Pawłowski, M.; Gosk, J.; Twardowski, A. J. Phys. Chem. B 2007, 111, 34-40. (9) Gałecka, M.; Wielgus, I.; Zago´rska, M.; Pawłowski, M.; Kulszewicz-Bajer, I. Macromolecules 2007, 40, 4924-4932. (10) Schweiger, A.; Jeschke, G. Principles of Pulse Electron Paramagnetic Resonance; Oxford University Press: Oxford, U.K., 2001. (11) Ito, A.; Miyajima, H.; Yoshizawa, K.; Tanaka, K.; Yamabe, T. J. Org. Chem. 1997, 62, 38-43. (12) Hirao, Y.; Ino, H.; Ito, A.; Tanaka, K.; Kato, T. J. Phys. Chem. A 2006, 110, 4866-4872. (13) Gaj, J. A.; Planel, R.; Fishman, G. Solid State Commun. 1979, 29, 435-438.
