High-Spin Radical Cations of Alternating Poly(mp-anilines) - American

Three alternating poly(m-p-anilines) have been synthesized via palladium-catalyzed amination reactions. Polymers were oxidized to radical cations by t...
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J. Phys. Chem. B 2007, 111, 34-40

High-Spin Radical Cations of Alternating Poly(m-p-anilines) Irena Kulszewicz-Bajer,*,† Małgorzata Zago´ rska,† Ireneusz Wielgus,† Mariusz Pawłowski,‡ Jacek Gosk,§ and Andrzej Twardowski§ Faculty of Chemistry, Warsaw UniVersity of Technology, Noakowskiego 3, 00-664 Warsaw, Poland, Institute of Electronic Materials Technology, Wo´ lczyn´ ska 133, 01-919 Warsaw, Poland, and Institute of Experimental Physics, Warsaw UniVersity, Hoz˘ a 69, 00-681 Warsaw, Poland ReceiVed: June 30, 2006; In Final Form: October 27, 2006

Three alternating poly(m-p-anilines) have been synthesized via palladium-catalyzed amination reactions. Polymers were oxidized to radical cations by the use of chemical and electrochemical methods. The presence of radical cations was manifested by the appearance of two new bands in UV-vis spectra and a strong EPR signal. Moreover, EPR spectra at low temperatures confirmed the formation of a high-spin state. The magnetization measurements of polymers oxidized to radical cations revealed the paramagnetic-type behavior with weak antiferromagnetic interactions. Radical cations underwent the degradation processes in the presence of air, which led to the decrease of spin concentration.

Introduction Within the last 20 years, one could observe the spectacular progress in the synthesis of, and the knowledge about, physical properties of conjugated oligomers and polymers. They not only show high electrical conductivity, but can exhibit electroluminescent or photovoltaic properties. The search for organic materials with ferromagnetic order has attracted considerable attention. However, the results obtained to date are less promising than the achievements in the field of conducting polymers. The modification of chemical structure allows tuning of the electronic properties of conjugated compounds from typical delocalized electronic systems1 to π-conjugated systems with localized spins2,3 showing ferromagnetic coupling. Significant progress in the design and synthesis of potential organic ferromagnets has been made by Rajca and co-workers. They prepared polyradicals based on polycarbenes and more recently on macrocyclic calix[4]arene or calix[3]arene rings.4,5 These polyradicals showed high-spin ground states with strong ferromagnetic coupling (S ) 6.2).5 Another attempt to achieve highspin material was poly(phenylenevinylene) with attached phenoxy radicals.6 The strongest ferromagnetic behavior was observed for o-polyradicals with S ) 5/2. Some attention was given to diaryl nitroxides, which can show quartet ground state in the case of dinitroxide.7 An interesting approach to highspin materials was the concept advanced theoretically by Fukutome et al.,8 who proposed the use of conjugated segments bearing delocalized spins alternated by organic units (for example, m-phenylene ring), which couple spins in a ferromagnetic fashion. Following this concept, Bushby et al. have synthesized polaronic polymers containing triarylamine units linked through m-coupled benzene.9 In the best case, S ) 5/2 was achieved. However, the oxidation of these polyarylamines resulted in a low doping level, ca. 15%, and thus the concentration of spins was insufficient to sustain a ferromagnetic structure domain. It was established that one reason for low doping level * Corresponding author. E-mail: [email protected]. † Warsaw University of Technology. ‡ Institute of Electronic Materials Technology. § Warsaw University.

was the difficulty to accommodate large counter-ions within rigid polymer networks. Thus, the nature of this limitation was rather more steric than electrostatic. A closer attempt (than polyarylamines prepared by Bushby) to Fukutome’s concept is the use of oligoaniline segments as the spin-containing units. Oligoanilines seem to be very promising compounds and are closely related to polyaniline, which forms radical cations stable at room temperature. Alternating m-p-oligoanilines were synthesized and studied by Janssen et al.10,11 Oligoanilines were oxidized using thianthrenium perchlorate or phenyliodine(III) bis(trifluoroacetate), and high-spin ground states were evidenced by ESR measurements. However, the stability of radical cations was not studied. Recently, Goodson et al. have obtained a series of poly(m-arylanilines) and poly(m-p-arylanilines).12 These oxidized polymers exhibited a strong ESR signal typical of S ) 1/2 and magnetic susceptibility of 2.1 × 10-6 cm3/g, which, however, decayed with time. The instability of radical cations can be the main reason for the lack of high-spin state in this case. Effective ferromagnetic coupling was, however, observed in the cases of cyclophane containing two m-phenylenediamines13 or dendritic oligoarylamines.14 Intramolecular spin interaction was also studied for polymeric systems such as networked aromatic polyamines15 and poly(phenylenevinyleneanysilamine).16 Recently, we have prepared alternating m-p-aniline tetramers,17 which can be oxidized to form radical cations using NOBF4 or m-chloroperbenzoic acid. However, we have observed a decrease of radical cations concentration after exposition to laboratory air. In this paper, we report the synthesis of alternating poly(mp-anilines) containing secondary and tertiary amine groups. Particularly, polymer P1 contained tertiary amine groups linked to three phenylene rings and, considering the stability of small molecules of the same type, should form stable radical cations. Polymer P2 contained tertiary amine linked to two phenyl rings, and n-hexyl chain (facilitating the polymer solubility) was prepared to tune the electronic properties. We have also synthesized polymer P3 containing secondary amine groups to verify the stability of radical cations. The polymers were

10.1021/jp0641076 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/14/2006

High-Spin Radical Cations of Poly(m-p-anilines)

J. Phys. Chem. B, Vol. 111, No. 1, 2007 35

SCHEME 1: Syntheses of N,N′-Dihexyl-1,4-phenylenediamine (2) and Poly(m-p-anilines) P1, P2, and P3a

a

(a) C5H11COCl, Et3N, DMAP, toluene, (b) LiAlH4, THF, reflux, (c) Pd(OAc)2, tert-Bu, tert-BuONa, toluene, 90 °C.

Figure 1. The UV-vis-NIR spectra of P1 in CH2Cl2/TFA (9:1) solution oxidized with m-chloroperbenzoic acid.

synthesized by the use of palladium-catalyzed amination developed by Hartwig’s12 and Buchwald’s18 groups. We focused on the chemical and electrochemical oxidation of polymers to radical cations, the presence of which was monitored using UVvis-NIR, IR, and EPR spectroscopies. Moreover, we have studied the stability of radical cations, which is crucial for potential applications. Magnetic properties measured by the SQUID technique are presented as well. Results and Discussion Polycondensation was carried out between diamines and 1,3dibromobenzene by the use of palladium-catalyzed amination reaction to form polymers P1, P2, and P3 (Scheme 1). The polymers can be oxidized to radical cations via chemical or electrochemical methods. The chemical oxidation was studied with the use of different oxidants such as m-chloroperbenzoic acid, NOBF4, Pb(OAc)4, and SOCl2 and was monitored by UVvis-NIR spectroscopy. High-molecular weight fraction of polymer P1 is not soluble in standard organic solvents due to the absence of solubilizing groups; however, it becomes soluble in CH2Cl2/TFA (9:1) solution. The UV-vis spectrum of P1 in the mentioned solution shows two bands: one at 305 nm, which can be attributed to π-π* transition, and another at ca. 420 nm, which can be related to the protonated form of polyamine. Upon stepwise addition of an oxidant, the band at 305 nm decreases in its intensity, and new bands appear in the UV-vis spectrum at 417 and 858 nm (Figure 1). Further oxidation causes the displacement of the 858 nm band to 940 nm, whereas the first one does not change its initial position. These two bands are characteristic of radical cations of P1 regardless of the oxidant used. The highest intensity of these bands corresponds to the theoretical oxidation of a half-amount of nitrogens to

Figure 2. The UV-vis-NIR spectra of P2 in CH2Cl2/TFA (9:1) solution: (a) P2 oxidized to radical cations, (b) P2 oxidized to pernigraniline state, and (c) P2 pernigraniline mixed with pristine P2 and 2× diluted.

radical cations. The chemical oxidation of P2 and P3 leads to similar changes in the UV-vis-NIR spectra. In the case of the UV-vis spectra of P2, the new additional bands are registered at 382 and 620 nm, whereas in the case of the spectra of P3 the new bands appear at 394 and 735 nm. It is evident that the positions of the bands characteristic of radical cations depend on the chemical structure of polymers and especially on geometrical distortions of neighboring aromatic rings. All three polymers can also be oxidized to pernigraniline state, in which 1,4-phenylenediamine units contain imine nitrogens separated by a quinoid ring. The pernigraniline form can serve as an oxidant. The mixing of equal amounts of pernigraniline form with pristine polymer leads to the formation of radical cations (Figure 2). This effect was observed by Wienk and Janssen19 in the case of m-p-aniline tetramer containing secondary amine groups. Here, we observe the possibility of selfoxidation-reduction also for polymers containing tertiary amine groups (i.e., P1 and P2). The EPR spectra of polymers P1, P2, and P3 oxidized in CH2Cl2/TFA (9:1) solution to radical cations show at room temperature one line with g ) 2.0023 and ∆Bpp ) 0.55 mT. However, the spectra of frozen solutions reveal two signals, which could be interpreted in terms of high-spin molecule. In the case of the spectrum of P1 at 6 K, one line is observed with g ) 2.0024 and ∆Bpp ) 0.58 mT and the second line (much weaker) at half-field with g ) 4.0045 and ∆Bpp ) 1.12 mT (Figure 3). The presence of the half-field transition gives unambiguous proof for a high-spin state with S g 1. The stronger line is interpreted as resulting from the ∆Ms ) (1 transition within S ) 1 triplet, while the weak line is from the ∆Ms ) (2 transition. Because the latter transition is normally

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Figure 3. The EPR spectrum of P1 oxidized to radical cations in CH2Cl2/TFA (9:1) solution: frozen solution recorded for the ∆Ms ) (1 (left) and for the ∆Ms ) (2 (right) at (a) 6 K and (b) 13 K.

Figure 4. Cyclic voltammograms (four consecutive scans) obtained for P1 thin film cast on platinum electrode (an electrolyte, 0.1 M Bu4NBF4 in acetonitrile; reference electrode, Ag/0.1 M AgNO3 in acetonitrile).

forbidden, the resulting signal should be relatively weak, which is the case (Figure 3). Obviously, if there are centers with spin S ) 1/2, they would also contribute to ∆Ms ) (1 transition and the signal with g-factor 2. The fine structure of the signals would be helpful in more precise identification of spin states. Unfortunately, fine structure was not clearly visible in our spectra. However, some deformation of the spectrum, observed at 6 K, may reflect the fine structure or/and spin-spin interaction. At temperature higher than 6 K, the spectra become narrower and less intense. The deformation of the signal in the ∆Ms ) (1 region disappears, which may suggest the decrease of spin interactions. Similar spectra were registered for P2 and P3. Polymers P1, P2, and P3 can be oxidized electrochemically. The cyclic voltammogram of polymer P1 cast on platinum plate (Figure 4) is typical of a thin film of electrode material adsorbed on the electrode surface.20 Four consecutive electrode oxidation processes can be distinguished, where redox potential E10 at 0.29 V is very close to E20 at 0.42 V and E30 is practically impossible to separate from E40 at 0.79 V. According to Goodson et al.12 who have studied poly(m-p-anilines) of the structure similar to P1, the first pair of the waves (at 0.29 V) is due to the removal of one electron from neighboring pphenylene moieties. The unusually sharp first oxidation peak is probably connected with the rearrangement of a rather compact polymer structure to incorporate the doping species at the very beginning of the oxidation reaction. The broad two-

electron peak at ca. 0.79 V results from the oxidation of all nitrogens to imines (dication state). The evolution of the UV-vis-NIR solid-state spectrum of P1 cast on ITO, imposed by increasing working electrode potential (Figure 5), closely resembles that observed during chemical oxidation. Up to 0.65 V, two new bands at 410 and 880 nm grow in intensity, whereas the band at 315 nm decreases at the same time. It can be rationalized, as before, by the oxidation of neutral polymer to a radical cation state. Further oxidation leads to the slow disappearance of the radical cation bands with simultaneous growth of the band at 620 nm characteristic of dication state.10 Three distinctive isosbestic points can be observed: one at 372 nm for the oxidation of polymer to radical cations in the potential range -0.3 ÷ 0.65 V (Figure 5a) and two points at 453 and 709 nm (Figure 5b) for the oxidation of radical cations to dications in the potential range of 0.65 ÷ 1.0 V. The presence of isosbestic points is evidence of the coexistence of nonequivalent phases, which interconvert upon oxidation. For better correlation between the applied potential and optical transitions, voltabsorptiometric measurements have been performed. Four spectral lines were selected for these investigations: (i) one line corresponding to the π-π* transition (315 nm), two lines from the radical cation characteristic bands (410 and 880 nm), and one line at the maximum of the band characteristic of the dication (620 nm). The obtained absorbance-time responses were differentiated in the form of -dA/ dt for the line at 315 nm and as dA/dt for remaining lines. Their plots versus E are shown in Figure 6. The results are consistent with postulated radical cations formation in the potential range of 0.1-0.64 V and then dications at higher potentials of 1.0 V. Because both forms strongly absorb in the spectral visible range, the polymer film adopts two very distinctive colors during oxidation. In the radical cation range, it is bright green, whereas further oxidation leads to blue marine. We have compared the spectroelectrochemical results with the UV-vis spectra of chemically oxidized polymers. The evaporation of solvents from the solutions of P1 or P2 oxidized to radical cations leads to the formation of polymer films. The UV-vis spectra of these films are typical of radical cations; that is, the spectrum of the P1 film shows the bands at 417 and 890 nm (Figure 7) similar to those presented in Figure 5, whereas the spectrum of P2 film has the bands at 382 and 625 nm. However, the evaporation of solvents from the solution of P3 oxidized to radical cations does not lead to the formation of a film with a UV-vis spectrum similar to that of the solution.

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Figure 5. The UV-vis-NIR spectra of P1 thin film recorded for increasing electrode potential, E, in a solution of 0.1 M Bu4NBF4 in acetonitrile with Ag/0.1 M AgNO3 in acetonitrile as a reference electrode: (a) for E of -0.3 ÷ 0.65 V, and (b) for E of 0.65 ÷ 1.0 V.

Figure 6. Voltabsorptograms obtained for P1 in a solution of 0.1 M Bu4NBF4 in acetonitrile: -dA/dt at 315 nm and dA/dt at 410, 620, and 880 nm.

Figure 7. The UV-vis-NIR spectra of P1 film oxidized with m-chloroperbenzoic acid to radical cations: (a) freshly prepared film, (b) after 1 day of exposition to air, and (c) after 2 days of exposition to air.

In this case, the film becomes yellow with bands at 308 and 458 nm in the UV-vis spectrum. This observation confirmed the previous prediction that the stability of radical cations of secondary amine groups was lower than that of tertiary amines. However, the film of P3 containing radical cations can be prepared by evaporation of P3 in THF solution and subsequent oxidation of the film with (NH4)2S2O8 in 1 M HCl solution. The UV-vis spectrum of this film shows the bands at 397 and 802 nm, typical of radical cations. We have also verified that radical cations of P1, P2, and P3 are not stable, and after exposition of the films to laboratory air one can observe a gradual decrease of the intensities of the bands characteristic of radical cations (see Figure 7). After 1 day, the intensity of the 890 nm peak of the film of P1 decreases to ca. 50%, and

that of the 417 nm peak to 68% of their initial values. In the case of the film of P2, the changes are more drastic and correspond to 30% and 52%, respectively. A similar effect was registered in the case of m-p-aniline tetramers.17 The film of P3 oxidized in aqueous HCl solution is more stable, and after 1 day the intensity of the 802 nm peak decreases to 91% and that of the 397 nm peak to 83% of their initial values. Unfortunately, the oxidation in this solution is not homogeneous. Films of polymers P1, P2, and P3 oxidized to radical cations were studied using IR spectroscopy. IR spectra of oxidized polymers show new intensive bands, which can be diagnostic for the radical cationic state similar to that observed for protonated polyaniline.21 In the case of the P1 IR spectrum, a new band appears at ca. 1160 cm-1, whereas in the case of the P3 spectrum, a new band is registered at 1152 cm-1 (Figure 8). As opposed to protonated polyaniline, the spectra of polymers P1, P2, and P3 oxidized to radical cations do not show the displacement of the bands associated with the vibrations of benzoid rings at 1586, 1503, and 1484 cm-1 (P1), the band associated with C-N vibration at 1268 cm-1, or the modes characteristic of C-H deformation of substituted benzene rings. It means that the oxidation does not change significantly the aromatic character of benzoid rings in polymer chains. In the case of the oxidized P1 spectrum (Figure 8b), one can observe additional bands at 1777 and 1679 cm-1 related to CdO and COO- vibrations of TFA anions neutralizing positive charges of radical cations of the polymer. We have studied the degradation process of the polymer films oxidized to radical cations. Figure 9 shows IR spectra of oxidized P1 (freshly prepared sample Figure 9a) and degraded in water and laboratory air. One can observe that the interaction with water or air induces significant intensity decrease of the band associated with radical cations at 1160 cm-1. It can also be noticed that the IR spectrum presented in Figure 9d is exactly the same as that of pristine polymer (Figure 8a). We propose the set of reactions, which can lead to annihilation of radical cations (Scheme 2). One can suppose that oxygen reacts with radical cations of polyamines. One of the possible routes is the creation of amine oxide, but we have not observed the band typical of this compound. The protection of oxidized polymers in argon atmosphere or in vacuum does not stop the degradation. It should be mentioned that the degradation was detected for all samples independently of chemical oxidant used. Moreover, the change of solvent from dichloromethane to benzene does not improve the stability of films. Thus, we conclude that the main degradation process is related to radicals, which can be generated in the course of the oxidation. Interactions of different radicals with radical cations of polyamines lead to an annihilation of

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Figure 8. The IR spectra of polymers: (a) P1, (b) P1 oxidized to radical cations with m-chloroperbenzoic acid, (c) P3, and (d) P3 oxidized to radical cations with (NH4)2S2O8.

polymer spins and reformation of polyamines. The degradation is accelerated in the presence of water and at high temperature. However, the oxidized films are relatively stable at low temperature. The concentrations of spins of the films stored at liquid nitrogen for 10 days drop by ca. 10% from its initial value (estimated from UV-vis and magnetization measurements). Unpaired localized spins (ULS) of radical cations present in polymer chains should manifest themselves in the presence of magnetic field as a paramagnetic (PM) response. Indeed, magnetization of P1 oxidized to radical cations with CPB measured as a function of the magnetic field up to 6 T at different temperatures showed a typical paramagnetic behavior. Magnetization curves are depicted in Figure 10. At low temperature (2 K), magnetization shows pronounced tendency to saturation with increasing field, while at high temperatures the magnetization is, practically, a linear function of magnetic field. However, a close inspection of the lowest temperature curve (2 K) indicates that the magnetization saturates with magnetic field slightly slower than expected for non-interacting spins. The magnetization of non-interacting magnetic moments should be described by a standard Brillouin function in the following way:

M(B,T) ) N‚x‚g‚µB‚S‚BS(B,T)

(1)

where BS(B,T) is the Brillouin function for spin S (spin of unpaired electrons), N is the number of radical cations per mass unit, g is g-factor, and µB denotes Bohr magneton. The spin

Figure 9. The IR spectra of P1: (a) oxidized to radical cations, (b) sample washed with water for 0.5 h, (c) sample exposed to air at 50 °C for 2 h, and (d) sample exposed to air for 1 week and then washed with dichloromethane and methanol.

SCHEME 2: Degradation Processes of Radical Cations

concentration, x, is expressed as a number of nitrogen’s radical cations with unpaired electrons to the half-number of total N-atoms. It turned out that the best fit was obtained for S ) 1/2, which is equivalent to the single unpaired electron residing on nitrogen’s radical. The solid lines in Figure 10 represent Brillouin function fit (BFF) with S ) 1/2. BFF returns also another adjustable parameter, x. This way, BFF (eq 1) can be used for evaluation of spin concentration x (we denote it xBFF). Typical temperature dependence of magnetization is shown in Figure 11. A Curie-Weiss (C-W)-like contribution dominates the data. The diamagnetic contribution to total magnetization was determined from infinite temperature extrapolation of magnetization data (magnetization vs inverse temperature, magnetization time temperature (M‚T) vs temperature) and subtracted from the total magnetization. Inverse susceptibility versus temperature was then plotted. As can be seen from the inset in Figure 11, the experimental points are well described by a linear function intersecting the temperature axis at temperature T ) Θ (C-W temp.) equal to -3.1 ( 0.5 K. Negative C-W temperature

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Figure 10. Magnetization of P1 oxidized to radical cations with CPB (xBFF(2 K) ) 0.22) plotted as a function of magnetic field for several temperatures. The solid lines represent fits of the Brillouin function to experimental points calculated for S ) 1/2. Empty squares are for sample measured after being kept 10 days in helium at a temperature of ca. 100 K. Magnetization was corrected for diamagnetic contribution, as described in the text.

decreases with decreasing temperature at which magnetization was measured: xBFF(50 K) ) 0.47, xBFF(30 K) ) 0.46, xBFF(10 K) ) 0.39, and xBFF(2 K) ) 0.33. We believe that this behavior reflects AFM interaction between ULS. Roughly speaking, this interaction “locks” spins in clusters (pairs, triples...), and thus their response to the magnetic field is less effective, which can be visible as effective reduction of the number of spins, that is, smaller xBFF. Obviously, the rise of temperature “unlocks” spins and xBFF increases. One of the magnetization measurement results, spin value S ) 1/2, may seem to contradict the finding of EPR experiment, strongly suggesting spin S ) 1. However, both EPR and magnetic data can be consistently interpreted assuming simultaneous presence of S ) 1/2 and S ) 1 centers. Both centers yield EPR signal with g-factor g ) 2, while the S ) 1 center additionally produces the signal with doubled g-factor. On the other hand, magnetization data suggest that the S ) 1/2 center dominates the other. We note that, although magnetization data should be described by spin S ) 1/2, a small, additional contribution of spin S ) 1 also provides a reasonable fit. Moreover, as discussed above, our samples undergo the degradation with time, which can lead to annihilation of some spins. Therefore, we cannot exclude a possibility that the concentration of various spin centers is different in EPR and magnetic experiments. Conclusions

Figure 11. Magnetization of P1 (xBFF ) 0.33) plotted as a function of temperature at B ) 1 T. Inset shows plot of inverse susceptibility (χ ) M/B) (corrected for diamagnetic contribution χo) versus temperature.

indicates antiferromagnetic (AFM) interaction between magnetic moments. To analyze the data quantitatively, the following formula was used to describe the measured susceptibility χ ) M/B:

χ(T) ) C/(T - Θ) + χo

(2)

where the first term in the formula represents paramagnetic contribution attributed to ULS described by the Curie-Weiss law and χo represents diamagnetic contribution of the sample. The temperature Θ ) -3.2 ( 0.3 K from the fit with the use of eq 2 is consistent with the Θ-value obtained from the inverse susceptibility versus temperature plot, as described above. The ULS-concentration x can be estimated from the C-W constant C, which is given by the formula:

C ) N‚x‚(g‚µB)2‚S‚(S + 1)/3kB

(3)

where N is the number of radical cations per mass unit, kB is the Boltzmann constant, and S ) 1/2. The concentration, x, determined this way will be denoted by xC-W. Analyzing the concentrations x obtained by BFF and C-W, one finds that the x value strongly depends on the history of the sample. For the samples stored in liquid nitrogen before the measurements, we obtained xC-W equal to ca. 0.5 and xBFF changing from 0.33 to 0.35 (2 K), whereas for the samples stored in air at room temperature the spin concentration was significantly lower and varied in the range 0.13-0.16 for xC-W and 0.10-0.22 for xBFF. The spin concentration is also temperature dependent and increases with increasing temperature from, for example, xBFF ) 0.34 at 2 K to xBFF ) 0.53 at 50 K. The estimated xC-W ) 0.50 corroborates reasonably well with hightemperature BFF, xBFF(100 K) ) 0.47. We note that xBFF

To summarize, we have shown that alternating poly(m-panilines) can be oxidized to radical cations via chemical or electrochemical methods. The presence of radical cations was proven by the appearance of two new bands in UV-vis spectra, the positions of which depend on chemical structure of polymers. Radical cations form a high-spin state as confirmed by EPR measurements. The stability of radical cations is limited, and oxidized polymers undergo the degradation processes, which lead to the decrease of spin concentration. It, in turn, causes the formation of spin defects. Magnetization measurements indicate paramagnetic-type behavior with weak antiferromagnetic interactions. 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 procedures and characterization of all compounds. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Patil, A. O.; Heeger, A. J.; Wudl, F. Chem. ReV. 1988, 88, 183207. (2) Yoshizawa, K.; Takata, A.; Tanaka, K.; Yamabe, T. Polym. J. 1992, 24, 857-864. (3) Tyutyulkov, N.; Baumgarten, M.; Dietz, F. Chem. Phys. Lett. 2002, 353, 231-238 and references therin. (4) Rajca, A.; Rajca, S.; Desai, S. R. J. Am. Chem. Soc. 1995, 117, 806-816. (5) Rajca, A.; Lu, K.; Rajca, S. J. Am. Chem. Soc. 1997, 119, 1033510345. (6) Nishide, H.; Kaneko, T.; Nii, T.; Katoh, K.; Tsuchida, E.; Lahti, P. M. J. Am. Chem. Soc. 1996, 118, 9695-9704. (7) Ishida, T.; Iwamura, H. J. Am. Chem. Soc. 1991, 113, 4238-4241. (8) Fukutome, H.; Takahashi, A.; Ozaki, M. Chem. Phys. Lett. 1987, 133, 34. (9) Bushby, R. J.; McGill, D. R.; Ng, K. M.; Taylor, N. J. Mater. Chem. 1997, 7, 2343-2354. (10) Wienk, M. W.; Janssen, R. A. J. J. Am. Chem. Soc. 1997, 119, 4492-4501.

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