Synthesis of a Pendant Polyradical with a New π-Conjugated Polymer

polyradical were measured using a SQUID magnetometer, and the results .... Zhang Xueheng, Li Wen, Li Xiaojing, Liu Tongliang, Fan Zhaobo, Yang Wen...
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Chem. Mater. 2002, 14, 3898-3906

Synthesis of a Pendant Polyradical with a New π-Conjugated Polymer Backbone Containing an Anthracene Skeleton and Its Ferromagnetic Spin Coupling Takashi Kaneko,* Takayuki Matsubara, and Toshiki Aoki Department of Chemistry and Chemical Engineering, Faculty of Engineering, Niigata University, Ikarashi 2-8050, Niigata 950-2181, Japan Received April 2, 2002

A poly(9,10-anthryleneethynylene)-based polyradical with pendant stable phenoxyls was newly synthesized via polymerization of the corresponding bromoethynylanthracene monomer using Pd(0) catalysts. The average molecular weight of the polymer reached M h n ) 5 × 103 by the polymerization reaction for a few hours, and the polymer was soluble in common organic solvents. The polyradical was prepared from the corresponding hydroxyl precursor polymer and was appropriately stable at room temperature. The ESR spectrum of the polyradical suggested an effectively delocalized spin density distribution on the backbone anthracene. The magnetization and the static magnetic susceptibility of the polyradical were measured using a SQUID magnetometer, and the results revealed moderately strong ferromagnetic spin coupling (2J h ) 39 ( 3 cm-1) through the π-conjugated chain, depending on the degree of spin density distribution on the anthracene backbone.

Introduction The π-conjugated polymers have attracted much attention as an organic material with various electronic properties since last century. Especially, the magnetic properties of π-conjugated polyradicals is one of the most attractive research fields of this century about the new functional polymer materials.1,2 Numerous π-conjugated polymers substituted with pendant radicals have been synthesized and characterized,3 since ferromagnetic through-bond interaction between the pendant spins was theoretically predicted for regioregular head-to-tail π-conjugated macromolecules possessing conjugated pendant radicals by using simple polyene models and other π-conjugated polymers.4-6 However, the expected ferromagnetic behavior has been observed for only a few examples, which are poly(phenylenevinylene)-, 7-10 poly(phenyleneethynylene)-,11,12 and polythiophene-based13 * Corresponding author. E-mail: [email protected]. (1) Rajca, A. Chem. Rev. 1994, 94, 871. (2) (a) Lahti, P. M., Ed. Magnetic Properties of Organic Materials; Marcel Dekker, Inc.: New York, 1999. (b) Itoh, K.; Kinoshita, M., Ed. Molecular Magnetism: New Magnetic Materials; Kodansha and Gordon & Breach: Tokyo and Amsterdam, 2000. (3) Nishide, H.; Kaneko, T. In Magnetic Properties of Organic Materials; Lahti, P. M., Ed.; Marcel Dekker, Inc.: New York, 1999; Vol. 279, pp 285, and references therein. (4) Ovchinnikov, A. A. Theor. Chim. Acta 1978, 47, 297. (5) Yamaguchi, K.; Toyoda, Y.; Fueno, T. Synth. Met. 1987, 19, 81. (6) Lahti, P. M.; Ichimura, A. S. J. Org. Chem. 1991, 56, 3030. (7) (a) Kaneko, T.; Toriu, S.; Kuzumaki, Y.; Nishide, H.; Tsuchida, E. Chem. Lett. 1994, 2135. (b) Nishide, H.; Kaneko, T.; Toriu, S.; Kuzumaki, Y.; Tsuchida, E. Bull. Chem. Soc. Jpn. 1996, 69, 499. (8) Nishide, H.; Kaneko, T.; Nii, T.; Katoh, K.; Tsuchida, E.; Yamaguchi, K. J. Am. Chem. Soc. 1995, 117, 548. (9) Nishide, H.; Kaneko, T.; Nii, T.; Katoh, K.; Tsuchida, E.; Lahti, P. M. J. Am. Chem. Soc. 1996, 118, 9695. (10) Takahashi, M.; Nakazawa, T.; Tsuchida, E.; Nishide, H. Macromolecules 1999, 32, 6383.

polyradicals. For the ferromagnetic interaction among side chain radicals through the π-conjugated backbone, sufficient spin polarization for the main chain is required, and the coplanarity of π-conjugated pendant polyradicals throughout the backbone chain and pendant side chain is significantly effective in the magnitude of the spin polarization and the spin-exchange coupling constant. Polycyclic ladder compounds such as naphthalene and anthracene have a rigid and coplanar structure, and the incorporation of the polycyclic units to the backbone chain should reinforce the spin coupling through the π-conjugated chain. Therefore, we selected a poly(9,10-anthryleneethynylene) with side-chain radicals at the β-position (1), which probably receive less steric hindrance from the poly(9,10-anthryleneethynylene) backbone. In this study, we synthesized a poly(9,10anthryleneethynylene)-based polyradical (2b) with pendant phenoxyls and discussed its magnetic interaction in connection with the electronic structure of the π-conjugated chain. Results and Discussion Monomer Synthesis. The general synthetic method for poly(aryleneethynylene) is the coupling reaction of a dibromoarene and a diethynylarene by using palladium complex catalysts.14,15 However, head-to-tail (11) Nishide, H.; Takahashi, M.; Takashima, J.; Tsuchida, E. Polym. J. 1999, 31, 1171. (12) Nishide, H.; Maeda, T.; Oyaizu, K.; Tsuchida, E. J. Org. Chem. 1999, 64, 7129. (13) Miyasaka, M.; Yamazaki, T.; Tsuchida, E.; Nishide, H. Macromolecules 2000, 33, 8211. (14) Giesa, R. J. Macromol. Sci. Rev. Macromol. Chem. Phys. 1996, C36, 631. (15) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605.

10.1021/cm020317t CCC: $22.00 © 2002 American Chemical Society Published on Web 08/20/2002

a Polyradical with a π-Conjugated Polymer Backbone

linkage cannot necessarily be regulated, if a substituent is introduced into an asymmetrical position. The headto-tail linkage is essential for ferromagnetic interaction, since the theoretical magnetic interaction between the contiguous radicals through the π-conjugation system in polyradical 2b is considered as shown in Figure 1. Therefore, we synthesized the corresponding monomers 3, 3′, 4, and 4′, respectively, as shown in Scheme 1, which had ethynyl and bromo groups to be linked with head-to-tail bonds by self-condensation. 4-(2-Anthryl)-2,6-di-tert-butylphenol 5a was newly synthesized from 2-bromoanthracene via cross coupling reaction with (3,5-di-tert-butyl-4-trimethylsiloxyphenyl)magnesium bromide using a nickel complex catalyst.9 A corresponding 9,10-dibromoanthracene 6, which was synthesized from 5a by protection of the hydroxyl group followed by bromination at the 9- and 10-positions, was treated with equal amount of 2-methyl-3-butyn-2-ol in the presence of a palladium complex catalyst. In this cross-coupling reaction, since 6 had two reactive bromo groups, two isomers, i.e., 3 and 3′, were obtained. The

Chem. Mater., Vol. 14, No. 9, 2002 3899

isomers could be isolated by silica gel column separation with hexane/ethyl acetate (4/1 v/v) as an eluent to give 3 and 3′ with 22% and 27% yield, respectively. The monomers 3 and 3′ were converted to 4 and 4′, respectively, by the elimination reaction of the terminal acetylene-protecting group. However, the structure of isomers could not be distinguished by only using IR, 1H NMR, and 13C NMR. Therefore, we synthesized compound 7, which was derived from 4′ by the elimination of bromo group. NOESY spectrum of 7 indicates the correlation peak between H-10 and H-4, while the correlation peak between H-10 and H-1 is not observed, as shown in Figure 2. This NOESY spectrum proves that 4′ has an ethynyl group at the 9-position. Synthesis and Characterization of Polymers. As shown in Scheme 2, the monomers 3 and 4 were polymerized in the presence of the Pd(0) complex catalysts, and a purple solid polymer 2c was obtained by precipitation from the polymerization mixtures into hexane. A polymer 2′c was obtained from isomeric monomers 3′ or 4′ as well. The polymers 2c and 2′c were converted to the corresponding hydroxyl polymers 2a and 2′a as dark purple solid, respectively, after complete elimination of the protecting acetyl group by treatment with alkaline solution followed by precipitation into methanol. The polymerization and deprotection data for these resultant polymers are summarized in Table 1. The average molecular weight of each polymer reaches M h n ) 3-5 × 103 by the polymerization reaction for a few hours. It seems that the polymerization activity is not different between two isomers, i.e., 4 and 4′. When triethylamine was used as a solvent for the polymerization of 4, the polymer deposited during polymerization. Even if prolonged polymerization is carried out using other good solvents for the polymer, the yield and molecular weight of polymer did not improve. On the other hand, the prolonged polymerization of 3 raises the molecular weight of the polymer a little, although the molecular weight distribution spreads. This result is probably due to the inhibition of diyne coupling, as described in the following paragraph. The polymers were soluble in chloroform, tetrahydrofuran, and especially in aromatic solvents such as benzene and toluene, but insoluble in alcohols and aliphatic hydrocarbons. It was confirmed by IR, 1H NMR, and 13C NMR spectra and elemental analysis of the polymers that polymeri-

Figure 1. The theoretical magnetic interaction between the contiguous radicals through the π-conjugation system in polyradical 2b with three different regiochemical diad linkages.

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zation proceeded by cross-coupling reaction between a terminal acetylene and bromo group of monomers, i.e., the reduction of the peaks assignable to the ethynyl group of monomer at 3264 cm-1 (νtC-H) in IR and δ 4.06 ppm (s, 1H, tC-H) in 1H NMR, the appearance of a peak assignable to the internal acetylene at δ 99.88 ppm for 13C NMR, and the reduction of Br content compared from the monomers in Br analysis. In the palladium-catalyzed coupling of terminal alkynes to aromatic halides, there is an oxidative coupling side reaction to produce diyne linkages, and especially in polymerization, the diyne defect will necessarily decrease the degree of polymerization and disturb the regioregularity of the pendant group.15 Heitz et al. reported that the deprotection of ethynyl group and the cross-coupling to aromatic halide were combined to circumvent the diyne defect.16,17 Since the concentration of the acetylenic proton was reduced by this method, the oxidative coupling side reaction was able to be suppressed. For the polymer obtained by the polymerization of 3 for 3 h (Table 1, no. 1), the degree of polymerization calculated from the integration ratio of the acetylenic proton peak at 4.15 ppm in 1H NMR (DPn ) 5.9) agreed with that from Br analysis (DPn ≈ 4), compared with the polymers obtained from 4 and 4′ (Table 1, nos. 4 and 8; DPn ) 9.6 and 12 from 1H NMR, respectively). We succeeded in obtaining the polymer with almost no diyne defect by the short-time poly(16) Solomin, V. A.; Heitz, W. Macromol. Chem. Phys. 1994, 195, 303. (17) Ha¨ger, H.; Heitz, W. Macromol. Chem. Phys. 1998, 199, 1821.

merization of the monomer protected in the ethynyl group, although the polymerization of 3 and 3′ for 48 h (Table 1, nos. 2 and 3) resulted in producing the polymers without the acetylenic proton peak at 4.15 ppm in 1H NMR. The molecular weight of 2a slightly increases compared with 2c, as shown in Table 1, since a low molecular weight fraction of the polymer 2a, which had a hydroxyl group, was probably removed by precipitation from the chloroform solution into methanol. The complete elimination of the protecting acetyl group was confirmed by the thorough disappearance of the peak at 1768 cm-1 attributed to νCdO of the acetyl group and by the appearance of the peak at 3648 cm-1 attributed to νO-H of the sterically hindered phenolic hydroxyl group in IR spectrum of 2a. The polymers 2a and 2c (no. 1 in Table 1) were used for the following formation of polyradical 2b and spectroscopic and magnetic measurement of the polymers, since the polymers of no. 1 had the better head-to-tail regioregularity than other polymers. Electronic Structure and Formation of Polyradical. The UV-vis spectrum of 2c shows an absorption maximum (λmax) at 550 nm (chloroform,  ) 5.0 × 103 cm-1 M-1), suggesting a developed π-conjugation compared with oligomers 8 (436 nm) and 9a (477 nm), while the monomer 4 has three characteristic absorption peaks at 379, 400, and 424 nm attributed to the π-π* transition of the anthracene ring (Figure 3). The λmax of 2c shifts to longer wavelength compared with the previously reported λmax for poly(1,4-phenyleneethynylene)s (360-450 nm) and poly[(9,10-anthrylene-

a Polyradical with a π-Conjugated Polymer Backbone

Chem. Mater., Vol. 14, No. 9, 2002 3901 Scheme 2

Figure 2. NOESY spectrum of 7 (CDCl3, 500 MHz, 25 °C) showing NOE cross-peaks between H-10 and H-4, and between H-10 and H-5.

ethynylene)-alt-(1,4-phenyleneethynylene)]s (450-540 nm),14,15 which suggests that the electronic structure of the planar and π-conjugated anthracene skeleton contributes to developing the π-conjugation system throughout the backbone.

The polyradical 2b was obtained by oxidizing the polymer 2a by treatment of the polymer solution in a degassed benzene or toluene with fresh PbO2 or aqueous alkaline K3Fe(CN)6 solution. The formation of the polyradical was supported by the appearance of the ESR signal accompanying with the decrease of the peak at 3648 cm-1 attributed to νO-H of the sterically hindered phenolic hydroxyl group in the IR spectrum of 2a. The UV-vis spectrum of 2b shows a broad shoulder absorption at 650-800 nm, which is probably caused by electronic interactions between side-chain radicals and the poly(9,10-anthryleneethynylene) backbone, while the λmax of 2b hypsochromically shifts to 540 nm compared with that of 2c, as shown in Figure 3. There is almost no change in the GPC elution curves of the polymer before and after the radical generation (i.e., 2a and 2b), which suggests that the oxidation does not cause oxidative degradation or cross-linking of the main chain. The spin concentration of 2b was determined both by doubly integrating the ESR signal in comparison with that of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) solution as a standard and by analyzing the saturated magnetization at 2 K using a SQUID magnetometer. The spin concentration of 2b reached ca. 0.4 spin/unit by selecting the oxidative conditions. The polyradical 2b was appropriately stable for maintaining the initial spin concentration under the ESR and SQUID measurement conditions, and the half-life of the polyradical was 16 h at room temperature in the toluene solution. It is known that anthracenes and poly(aryleneethynylene)s have a highly fluorescent nature. For the poly(phenyleneethynylene)s incorporating anthracene units into the backbones, the emission was red-shifted depending on the concentration of the anthracene units; the maximum was at 590 nm for the alternating copolymer.18 The polymer 2a also shows the fluorescence at 580 nm by the excitation light at 550 nm corresponding to the visible absorption maximum (550 nm) attributed to the π-conjugated backbone, as shown in Figure 4. The emission intensity of 2a was sensitive to concentration quenching for concentrations >10-4 M, while the corresponding poly(phenyleneethynylene) with (18) Swager, T. M.; Gil, C. J.; Wrighton, M. S. J. Phys. Chem. 1995, 99, 4886.

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Table 1. Polymerization of the Bromoethynylanthracene Monomers 3, 3′, 4, and 4′ Using Palladium Complex Catalysts and Deprotection of the Acetyl Group for the Resulting Polymers 2c and 2′c polymerizationa of 3, 3′, 4, and 4′ no.

monomer

solvent

additive

time (h)

1 2 3 4 5 6 7 8

3

toluene toluene toluene NEt3 toluene pyridin e DMF NEt3

KOH KOH KOH

3 48 48 1 18 12 12 1

3′ 4

4′

NEt3 NEt3 NEt3 -

yieldc

(%)

65 44 45 88 1.4 31 23 91

deprotectionb of 2c and 2′c M hn

d

(×103) 4.7 5.2 5.7 3.8 3.6 3.4 3.0 3.4

M h w/M hn

yieldc (%)

M h nd (×103)

M h w/M h nd

1.5 4.6 1.9 2.1 1.8 1.6 1.5 1.5

64

4.8

1.4

89

3.9

2.1

49

3.2

1.2

d

[M]0 ) 0.1 mol/L, [Pd(PPh3)4]/[M]0 ) 0.01, [CuI]/[Pd(PPh3)4] ) 4, 5 M KOH in methanol, [KOH]/[M]0 ) 1, 110 °C, precipitated with hexane, for no. 1. [M]0 ) 0.1 mol/L, [Pd(PPh3)2Cl2]/[M]0 ) 0.01, [PPh3]/[Pd(PPh3)2Cl2]0 ) 5, [CuI]/ [Pd(PPh3)2Cl2]0 ) 4, 5 N KOHaq, [KOH]/ [M]0 ) 20, [[C6H5CH2(NEt3)]Cl]/[M]0 ) 10, 110 °C, precipitated with hexane, for nos. 2 and 3. [M]0 ) 0.1 mol/L, [Pd(PPh3)4]/[M]0 ) 0.01, [PPh3]/[Pd(PPh3)4]0 ) 5, [CuI]/[Pd(PPh3)4]0 ) 4, [additive]/[M]0 ) 20, 90-110 °C, precipitated with hexane, for nos. 4-8. b [Polymer] ) 6.7 g/L in THF, DMSO/THF ) 3 v/v, 2.5 N KOHaq/THF ) 0.17 v/v, 50 °C, 12 h, precipitated with methanol. c Calculated by neglecting the end groups of polymers. d Determined using GPC calibrated relative to polystyrene standards. a

Figure 3. UV-vis absorption spectra of the monomer 4, oligomers 8 and 9a, polymer 2c in chloroform, and the polyradical 2b (spin concentration ) 0.25 spin/unit) in benzene.

Figure 4. Excitation (dotted line, λem ) 580) and emission (solid line, λex ) 550) spectra of the polymer 2a in toluene (0.05 mM).

the same substituent did not undergo the concentration quenching up to 0.03 M.12 It was found that the intermolecular interaction of 2a was enhanced by the anthracene units compared with that of the poly(phenyleneethynylene) analogue. ESR Spectra. The ESR spectrum of 2b shows a unimodal broad signal at g ) 2.0044, indicating the formation of phenoxyl radical (Figure 5a). A broad hyperfine structure due to unresolved coupling of the protons of the phenoxyl ring and the anthracene skeleton appears by preparing the isolated radical units at low spin concentration (Figure 5b). The spin density

Figure 5. ESR spectra of the polyradical 2b (a, 0.5 mM, spin concentration ) 0.40 spin/unit; b, 2.5 mM, spin concentration ) 0.02 spin/unit) in toluene, and (c) 4-(2-anthryl)-2,6-di-tertbutylphenoxyl (5b) (0.5 mM) in benzene at room temperature. Dotted line: simulation spectrum for 5b using the parameters as follows: aH (mT) ) 0.346 (1H), 0.143 (1H), 0.142 (2H), 0.104 (1H), 0.095 (1H), 0.084 (1H), 0.072 (1H), 0.058 (1H), 0.058 (1H), 0.057 (1H); line width 0.039 mT.

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Figure 7. ESR spectra of the polyradical 2b in frozen toluene glass at 77 K: (a) at g ) 2 and (b) at g ) 4.

Figure 6. Estimation of the hyperfine coupling constants (mT) for 4-(2-anthryl)phenoxyls (a) from the ESR spectrum simulation and (b) the semiempirical ZINDO calculation, compared with the hyperfine coupling constants of 4-phenylphenoxyls from ESR experiment22 and the semiempirical ZINDO calculation.

distribution over the anthracene unit in the polyradical is further supported by the clearer hyperfine structure of the corresponding monomeric radical, 4-(2-anthryl)2,6-di-tert-butylphenoxyl (5b) (Figure 5c), whose hyperfine coupling constants (mT) are estimated by spectral simulation19,20 as follows: anthracene aH ) 0.346 (1H), 0.143 (1H), 0.104 (1H), 0.095 (1H), 0.084 (1H), 0.072 (1H), 0.058 (2H), 0.057 (1H), phenoxyl aH ) 0.142 (2H). The large aH value of H-1 for the spectral simulation does not conflict with the semiempirical ZINDO calculation,21 as shown in Figure 6.22 The ∆ms ) (2 forbidden transition ascribed to the triplet species is clearly observed at g ) 4 in frozen toluene glass of 2b at 77 K, although no fine structure that gives zero-field splitting parameters D or E is detected at g ) 2, because of the presence of several conformers and/or the long distance between unpaired electrons in the polyradical (Figure 7). (19) ESR spectrum simulation was carried out with PEST WinSim program, version 0.96 (Duling, D. R.; Laboratory of Molecular Biophysics, Public EPR Software Tools (PEST), National Institute of Environmental Health Sciences (NIEHS), 1996). Optimization of approximate parameters was carried out using the LMB1 algorithm: (a) Duling, D. R. J. Magn. Reson., Ser. B 1994, 104, 105. (b) Duling, D. R.; Motten, A. G.; Mason, R. P. J. Magn. Reson. 1988, 77, 504. As the starting set of parameters, those in ref 20 were used. (20) Kaneko, T.; Matsubara, T.; Aoki, T.; Oikawa, E. Mol. Cryst. Liq. Cryst. 1999, 334, 221. (21) ZINDO calculations were performed on a Macintosh computer with the ZINDO Hamiltonian as implemented by Cache Release 3.7. All molecular models used in the semiempirical calculations were based on SCF/PM3-optimized ground-state geometries by using the MOPAC (version 94.10) suite of programs as implemented under Cache Release 3.7. (a) Zerner, M. C.; Loew, G. H.; Kirchner, R. F.; Mueller-Westerhoff, U. T. J. Am. Chem. Soc. 1980, 102, 589. (b) Cache Version 3.7; Oxford Molecular Group, Inc.; Campbell, CA, 95008. (22) Rieker, A.; Scheffler, K. Justus Liebigs Ann. Chem. 1965, 689, 78.

Figure 8. Normalized plots of magnetization (M/Ms) vs the ratio of magnetic field and the effective temperature (H/(T θ)) for the polyradical 2b (spin concentration ) 0.36 spin/unit) in frozen toluene glass at T ) 1.8 (O), 2 (b), 2.25 (0), 2.5 (9), 3 (]), 5 ([), 7.5 (4), 10 (2), 15 (O) K and the theoretical curves corresponding to the S ) 1/2, 1, 3/2, and 2 Brillouin functions, where θ is a weak antiferromagnetic term and was determined to be -0.3 K from the χmolT vs T plots in Figure 9.

Magnetic Interaction. Static magnetic susceptibility (2-100 K at 0.5 T) and magnetization (0-7 T) of the polyradical 2b were measured using a SQUID magnetometer. The sample was prepared as the frozen glass diluted in diamagnetic toluene to minimize intermolecular interactions. The magnetization (M) normalized by saturated magnetization (Ms), M/Ms, of 2b is plotted versus the ratio of magnetic field (H) and the effective temperature (T - θ), and compared with the theoretical Brillouin curves (Figure 8). θ is a coefficient of the weak (antiferro)magnetic interaction between radicals, such as the intermolecular or through-space interaction, and is determined from curve fitting using the following χmolT vs T data.1,8,9 Despite a spin concentration of 0.36 spin/unit, the plots of 2b are located almost on the theoretical Brillouin curve of S ) 2/2 at 1.8-15 K, indicating the ferromagnetic coupling. The susceptibility data are frequently plotted as χmolT vs T in order to analyze the magnetic interaction, where χmol is the molar paramagnetic susceptibility. The χmolT vs T plots of 2b are shown in Figure 9. The χmolT plots deviate upward from the theoretical value (χmolT ) 0.375) for S ) 1/2 with a decrease in temperature at 10-100 K, but the χmolT values are reduced below ca. 10 K. This behavior indicates a relatively strong through-

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ladder-like spin coupling network, which has both the very strong ferromagnetic coupling through the anthracene unit and the moderately strong ferromagnetic coupling through the anthryleneethynylene unit, and this ladder-like spin coupling network will be insensitive to spin-defect. The synthesis and magnetic characterization of the polyradical with the ladder-like spin coupling network will be discussed in more detail in a subsequent paper. Experimental Section

Figure 9. χmolT vs T plots (E) of the polyradical 2b (spin concentration ) 0.36 spin/unit) in frozen toluene glass. Solid line: Theoretical curve calculated using the equation in ref 23 with 2J h ) 39 cm-1, θ ) -0.31 K, x1 ) 0.34, x2 ) 0.28, and x3 ) 0.38.

bond and intrachain ferromagnetic interaction and a weak through-space and interchain antiferromagnetic interaction, probably due to the slightly aggregated chains. The average spin coupling constant was approximately determined as an average value (J h ; positive for ferromagnetic) by curve fitting of the χmolT vs T data in consideration of spin exchange coupling only between neighboring units.7-9 The curve fitting of the χmolT vs T data was performed using a linear triradical system including diradical and monoradical contamination, because the average spin quantum number determined from magnetization plots was S h ) 2/2, and the spin species ratio of S e 3/2 was probably enough high.23 The 2J h value of 2b was 39 ( 3 cm-1, which agreed with the 2J value of the corresponding oligomeric diradical 9b (2J ) 31 ( 3 cm-1)24 and with the degree of spin density distribution on the anthracene backbone, as previously discussed. The 2J h value of 2b was larger than that of the corresponding poly(1,4-phenyleneethynylene)-based polyradical (2J h ) 18 cm-1).12 This large 2J h value of 2b can be explained from less steric hindrance between the side chain radicals and the poly(9,10-anthryleneethynylene) backbone. Conclusion We have shown that the poly(9,10-anthryleneethynylene)-based polyradical 2b causes moderately strong ferromagnetic spin coupling through the π-conjugated chain, and the poly(9,10-anthryleneethynylene) skeleton can be an effective backbone chain for magnetic polymer materials. The poly(9,10-anthryleneethynylene)based polyradical 1 has another site for the side-chain radical compared with 2b. The average ground-state spin quantum number of the polyradical, which has two pendant phenoxyls in one anthracene unit, will be larger than that of 2b, because the polyradical consists of a (23) The expression for the curve fitting of the χmolT vs T data is χmolT ) [NAg2µΒ2T/k(T - θ)]{x3[1 + exp(-2J h /kT) + 10 exp(J h /kT)]/12[1 + exp(-2J h /kT) + 2 exp(J h /kT)] + x2/[3 + exp(-2J h /kT)] + x1/4}, as described in ref 9, where J h , θ, x1, x2, and x3 are the average value of the exchange coupling constants, the Weiss constant for a weak intermolecular magnetic interaction, and the fractions of the doublet, the triplet, and the quartet, respectively. (24) Kaneko, T.; Makino, T.; Sato, G.; Aoki, T. Polyhedron 2001, 20, 1291.

Materials. 2-Bromoanthracene was synthesized from 2aminoanthraquinone according to the literature method.25 Dichloro[1,3-bis(diphenylphosphino)propane]nickel(II) was prepared according to the literature method.26 Tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) (Aldrich Co.), bis(triphenylphosphine)palladium(II) chloride (Pd(PPh3)2Cl2) (Aldrich Co.), n-butyllithium (Kanto Chemical Co., Inc., 1.6 or 3 M in hexane), and tert-butyllithium (Kanto Chemical Co., Inc., 1.5 M in pentane) were used without further purification. Other conventional reagents were used as received or purified by conventional methods. 2-(3,5-Di-tert-butyl-4-hydroxyphenyl)anthracene (5a). 3,5-Di-tert-butyl-4-(trimethylsiloxy)phenylmagnesium bromide prepared from 4-bromo-2,6-di-tert-butyl-1-(trimethylsiloxy)benzene (39 g, 0.11 mol) in dry THF (250 mL) was added in portions, with stirring, to a dry THF (200 mL) solution of 2-bromoanthracene (7.0 g, 0.027 mol) with dichloro[1,3-bis(diphenylphosphino)propane]nickel(II) (0.7 g, 1.3 mmol). The mixture was stirred and refluxed overnight and then treated with aqueous 3 N HCl (250 mL), extracted with chloroform, and washed with water. The chloroform layer was dried over anhydrous sodium sulfate. After evaporation, the crude product was dissolved in THF (175 mL), and methanol (350 mL) was added to this solution. The solution was acidified with aqueous 10 N HCl (50 mL) under a nitrogen atmosphere. After stirring overnight at room temperature, the mixture was evaporated to remove methanol, extracted with chloroform, and washed with water. The chloroform layer was dried over anhydrous sodium sulfate and evaporated. The crude product was washed with hot hexane to give 5a (7.8 g, 0.020 mol): yield 75%; mp 215-216 °C; IR (KBr, cm-1) 3644 (νO-H), 2968-2876 (νC-H, tert-butyl); 1H NMR (CDCl3, 500 MHz; ppm) δ 1.54 (s, 18H, tert-butyl), 5.32 (s, 1H, OH), 7.42-7.47 (m, 2H, ArH), 7.59 (s, 2H, PhH), 7.73 (dd, 1H, J ) 8.8, 1.7 Hz, ArH), 8.008.01 (m, 2H, ArH), 8.05 (d, 1H, J ) 8.8 Hz, ArH), 8.12 (d, 1H, J ) 1.7 Hz, ArH), 8.42 (s, 1H, ArH), 8.47 (s, 1H, ArH); 13C NMR (CDCl3; ppm) δ 30.38, 34.55, 124.22, 124.69, 125.12, 125.35, 125.93, 125.96, 126.19, 128.08, 128.21, 128.51, 130.63, 131.54, 132.02, 132.26, 136.35, 138.75, 153.71. 2-(3,5-Di-tert-butyl-4-acetoxyphenyl)anthracene (5c). A hexane solution of n-butyllithium (5.5 mL, 16.5 mmol) was added dropwise at -70 °C to a THF solution (160 mL) of 5a (4.2 g, 11 mmol). After stirring for 15 min, acetyl chloride (1.6 mL, 22 mmol) was added dropwise, and stirring continued overnight at room temperature. The reaction mixture was washed with water, extracted with chloroform, and dried over anhydrous sodium sulfate. The solvent was evaporated, and the crude product was purified by silica gel column separation with hexane/chloroform (1/2 v/v) as an eluent to give 5c (3.8 g, 9.0 mmol): yield 82%; mp 210-211 °C; TLC (hexane/ chloroform ) 1/2 v/v) Rf ) 0.63; IR (KBr, cm-1) 2972-2810 (νC-H, tert-butyl), 1760 (νCdO); 1H NMR (CDCl3, 500 MHz; ppm) δ 1.45 (s, 18H, tert-butyl), 2.40 (s, 3H, COCH3), 7.45-7.49 (m, 2H, ArH), 7.70 (s, 2H, PhH), 7.73 (dd, 1H, J ) 8.8, 1.7 Hz, ArH), 8.00-8.03 (m, 2H, ArH), 8.07 (d, 1H, J ) 8.8 Hz, ArH), 8.15 (d, 1H, J ) 1.7 Hz, ArH), 8.44 (s, 1H, ArH), 8.50 (s, 1H, (25) Hodge, P.; Power, G. A.; Rabjohns, M. A. Chem. Commun. 1997, 1997, 73. (26) Van Hecke, G. R.; Horrocks, W. D., Jr. Inorg. Chem. 1966, 5, 1968.

a Polyradical with a π-Conjugated Polymer Backbone ArH); 13C NMR (CDCl3; ppm) δ 22.73, 31.54, 35.64, 125.31, 125.44, 125.61, 125.63, 125.86, 125.98, 126.48, 128.12, 128.21, 128.61, 130.77, 131.72, 131.81, 132.03, 138.08, 138.19, 142.81, 147.68, 171.17. 9,10-Dibromo-2-(3,5-di-tert-butyl-4-acetoxyphenyl)anthracene (6). Bromine (1.1 mL, 22 mmol) in 1,4-dioxane (32 mL) was added to a 1,4-dioxane solution (160 mL) of 5c (3.8 g, 9.0 mmol). After stirring overnight at room temperature, aqueous sodium sulfite was added to the 1,4-dioxane solution. The reaction mixture was extracted with chloroform, washed with water, and dried over anhydrous sodium sulfate. The solvent was evaporated, and the crude product was recrystallized from hexane to give 6 (4.3 g, 7.4 mmol): yield 83%; mp 204-205 °C; IR (KBr, cm-1) 2968-2810 (νC-H, tert-butyl), 1760 (νCdO); 1H NMR (CDCl3, 500 MHz; ppm) δ 1.46 (s, 18H, tertbutyl), 2.41 (s, 3H, COCH3), 7.59-7.64 (m, 2H, ArH), 7.74 (s, 2H, PhH), 7.87 (dd, 1H, J ) 9.1, 1.7 Hz, ArH), 8.55-8.59 (m, 2H, ArH), 8.63 (d, 1H, J ) 9.1 Hz, ArH), 8.73 (d, 1H, J ) 1.7 Hz, ArH); 13C NMR (CDCl3; ppm) δ 22.74, 31.52, 35.68, 123.35, 123.63, 125.80, 125.85, 127.36, 127.54, 127.82, 128.25, 128.31, 128.93, 130.21, 130.97, 131.17, 131.37, 137.27, 140.25, 143.13, 148.19, 171.15. 9-Bromo-2-(3,5-di-tert-butyl-4-acetoxyphenyl)-10-(3-hydroxy-3-methyl-1-butynyl)anthracene (3) and 10-Bromo2-(3,5-di-tert-butyl-4-acetoxyphenyl)-9-(3-hydroxy-3-methyl-1-butynyl)anthracene (3′). 2-Methyl-3-butyn-2-ol (0.68 mL, 7.0 mmol) was added to a triethylamine solution (54 mL) of 6 (4.1 g, 7.0 mmol), bis(triphenylphosphine)palladium(II) chloride (49 mg, 0.070 mmol), triphenylphosphine (92 mg, 0.35 mmol), and copper(I) iodide (48 mg, 0.25 mmol) under a nitrogen atmosphere. The solution was stirred and refluxed for 4 h. After cooling, the solution was treated with aqueous 3 N HCl, extracted with chloroform, washed with water, and dried over anhydrous sodium sulfate. The solvent was evaporated, and the crude product was purified by silica gel column separation with chloroform and hexane/ethyl acetate (4/1 v/v) as an eluent to give 3 (0.91 g, 1.6 mmol) and 3′ (1.1 g, 1.8 mmol), respectively. 3: yield 22%; mp 109-111 °C; TLC (hexane/ethyl acetate ) 4/1 v/v) Rf ) 0.41; IR (KBr, cm-1) 3460 (νO-H), 2968-2860 (νC-H, tert-butyl), 1768 (νCdO); 1H NMR (CDCl3, 500 MHz; ppm) δ 1.46 (s, 18H, tert-butyl), 1.86 (s, 6H, CH3), 2.41 (s, 3H, COCH3), 7.58-7.64 (m, 2H, ArH), 7.74 (s, 2H, PhH), 7.85 (dd, 1H, J ) 8.9, 1.8 Hz, ArH), 8.52-8.56 (m, 2H, ArH), 8.59 (d, 1H, J ) 8.9 Hz, ArH), 8.70 (d, 1H, J ) 1.8 Hz, ArH); 13C NMR (CDCl3; ppm) δ 22.72, 31.51, 31.76, 35.66, 66.27, 78.58, 106.16, 117.41, 124.31, 125.81, 126.76, 127.06, 127.23, 127.47, 127.66, 128.18, 130.27, 130.50, 132.16, 132.92, 137.61, 140.20, 143.05, 148.06, 171.17. Anal. Calcd for C35H37BrO3: C, 71.8; H, 6.4; Br, 13.7. Found C, 71.9; H, 6.5; Br, 13.6. 3′: yield 27%; mp 109-111 °C; TLC (hexane/ethyl acetate ) 4/1 v/v) Rf ) 0.48; IR (KBr, cm-1) 3436 (νO-H), 2968-2850 (νC-H, tert-butyl), 1766 (νCdO); 1H NMR (CDCl3, 200 MHz; ppm) δ 1.46 (s, 18H, tert-butyl), 1.83 (s, 6H, CH3), 2.41 (s, 3H, COCH3), 7.54-7.63 (m, 2H, ArH), 7.77 (s, 2H, PhH), 7.88 (dd, 1H, J ) 9.2, 1.8 Hz, ArH), 8.48-8.55 (m, 2H, ArH), 8.58 (d, 1H, J ) 9.2 Hz, ArH), 8.70 (d, 1H, J ) 1.8 Hz, ArH); 13C NMR (CDCl3; ppm) δ 22.72, 31.54, 31.78, 35.70, 66.19, 78.64, 106.53, 117.56, 123.99, 124.40, 125.63, 126.90, 127.00, 127.28, 127.45, 128.19, 128.79, 129.30, 130.06, 133.18, 133.25, 137.25, 139.24, 143.13, 148.08, 171.15. 9-Bromo-2-(3,5-di-tert-butyl-4-acetoxyphenyl)-10-ethynylanthracene (4). Sodium hydride (0.26 g, 6.5 mmol) was added to a toluene solution (20 mL) of 3 (0.38 g, 0.65 mmol). The solution was heated to 80 °C and stirred for 0.5 h under a nitrogen flow. After cooling, the solution was treated with water, extracted with chloroform, and dried over anhydrous sodium sulfate. The solvent was evaporated, and the crude product was purified by silica gel column separation with chloroform as an eluent to give 4 (0.29 g, 0.56 mmol): yield 86%, mp 165-167 °C; TLC (chloroform) Rf ) 0.83; IR (KBr, cm-1) 3264 (νtC-H), 2968-2800 (νC-H, tert-butyl), 1766 (νCdO); 1H NMR (CDCl , 500 MHz; ppm) δ 1.46 (s, 18H, tert-butyl), 3 2.41 (s, 3H, COCH3), 4.06 (s, 1H, CtC-H), 7.60-7.65 (m, 2H, ArH), 7.74 (s, 2H, PhH), 7.86 (dd, 1H, J ) 9.0, 1.7 Hz, ArH),

Chem. Mater., Vol. 14, No. 9, 2002 3905 8.55-8.63 (m, 2H, ArH), 8.68 (d, 1H, J ) 9.0 Hz, ArH), 8.71 (d, 1H, J ) 1.7 Hz, ArH); 13C NMR (CDCl3; ppm) δ 22.72, 31.51, 35.65, 79.98, 89.21, 116.77, 124.98, 125.83, 126.97, 127.07, 127.47, 127.52, 127.64, 128.21, 130.24, 130.46, 132.70, 133.45, 137.60, 140.31, 143.04, 148.08, 171.15. Anal. Calcd for C 32H31BrO2: C, 72.9; H, 5.9; Br, 15.2. Found C, 72.6; H, 6.1; Br, 14.6. 10-Bromo-2-(3,5-di-tert-butyl-4-acetoxyphenyl)-9-ethynylanthracene (4′). The compound 3′ was allowed to react with sodium hydride in the same manner as described above. The crude product was recrystallized from hexane to give 4′: yield 79%; mp 166-167 °C; IR (KBr, cm-1) 3316 (νtC-H), 29682800 (νC-H, tert-butyl), 1756 (νCdO); 1H NMR (CDCl3, 500 MHz; ppm) δ 1.45 (s, 18H, tert-butyl), 2.41 (s, 3H, COCH3), 4.07 (s, 1H, CtC-H), 7.59-7.64 (m, 2H, ArH), 7.74 (s, 2H, PhH), 7.88 (dd, 1H, J ) 9.2, 1.8 Hz, ArH), 8.53-8.56 (m, 1H, ArH), 8.61 (d, 1H, J ) 9.2 Hz, ArH), 8.60-8.63 (m, 1H, ArH), 8.78 (d, 1H, J ) 1.8 Hz, ArH); 13C NMR (CDCl3; ppm) δ 22.71, 31.51, 35.65, 80.04, 89.52, 116.97, 124.56, 124.69, 125.82, 127.01, 127.13, 127.34, 127.73, 128.25, 128.76, 129.28, 130.05, 133.68, 133.85, 137.35, 139.82, 143.02, 148.09, 171.15. 2-(3,5-Di-tert-butyl-4-hydroxyphenyl)-9-ethynylanthracene (7). A pentane solution of tert-butyllithium (0.83 mL, 1.25 mmol) was added dropwise at -70 °C to a THF solution (40 mL) of 4′ (0.26 g, 0.5 mmol). After stirring for 10 h, the reaction mixture was quenched with water, treated with aqueous 3 N HCl, extracted with chloroform, and dried over anhydrous sodium sulfate. The solvent was evaporated, and the crude product was purified by silica gel column separation with hexane/dichloromethane (3/1 v/v) as an eluent to give 7 (0.049 g, 0.12 mmol): yield 24%; IR (KBr, cm-1) 3636 (νO-H), 3316 (νtC-H), 2986-2850 (νC-H, tert-butyl); 1H NMR (CDCl3, 500 MHz; ppm) δ 1.57 (s, 18H, tert-butyl), 4.02 (s, 1H, CtC-H), 5.37 (s, 1H, OH), 7.50 (ddd, 1H, J ) 8.3, 6.6, 1.2 Hz, ArH), 7.60 (ddd, 1H, J ) 8.8, 6.6, 1.2 Hz, ArH), 7.65 (s, 2H, PhH), 7.79 (dd, 1H, J ) 8.8, 1.7 Hz, ArH), 8.01 (d, 1H, J ) 8.6 Hz, ArH), 8.07 (d, 1H, J ) 8.8 Hz, ArH), 8.46 (s, 1H, ArH), 8.59 (d, 1H, J ) 8.8 Hz, ArH), 8.73 (d, 1H, J ) 1.7 Hz, ArH); 13C NMR (CDCl3; ppm) δ 30.35, 34.53, 80.46, 88.29, 115.77, 123.31, 124.44, 125.43, 126.19, 126.47, 126.82, 127.92, 128.71, 129.00, 129.97, 130.82, 132.21, 133.47, 133.50, 136.38, 140.26, 153.92. Polymerization. Polymerization was carried out by modifying the conditions described in the literature.16,17 An appropriate amount of monomers (typically, 0.2-0.5 g), palladium complex catalysts, triphenylphosphine, and copper iodide were placed in an Schlenk tube equipped with a three-way stopcock, a reflux condenser, a rubber septum, and a Tefloncoated magnetic stirring bar. The tube was placed under vacuum, followed by a nitrogen backflush. Freshly distilled solvent was transferred to the tube, and the monomers were dissolved with stirring. An appropriate basic additive was added to the monomer solution, and the mixture was heated at 90-110 °C for 1-48 h. The polymerization conditions are listed in Table 1. After cooling, the solution was treated with aqueous 3 N HCl, extracted with chloroform, washed with water, and dried over anhydrous sodium sulfate. The solvent was evaporated, and the crude product was purified by reprecipitation from chloroform into hexane to yield the polymer as purple powder. The yield and the molecular weight are also given in Table 1. Poly[2-(3,5-di-tert-butyl-4-acetoxyphenyl)-9,10-anthryleneethynylene] (2c) (Table 1, no. 1): IR (KBr, cm-1) 2997-2850 (νC-H, tert-butyl), 1768 (νCdO); 1H NMR (CDCl3, 500 MHz; ppm) δ 1.35 (s, 18H, tert-butyl), 2.37 (s, 3H, COCH3), 4.15 (s, 1/nH, CtC-H), 7.4-8.2 (br, 6H, ArH), 8.69.3 (br, 3H, ArH); DPn ) 5.9 (from 1H NMR); 13C NMR (CDCl3; ppm) δ 22.69, 30.28, 31.48, 34.50, 35.61, 99.88, 118.97, 124.26, 124.98, 125.64, 125.86, 127.50 (br), 131.68, 132.76 (br), 136.54, 137.41, 139.87, 143.06, 148.11, 171.15. Anal. Calcd for C32nH31n+1BrO2n (n ) 4): Br, 4.3. Found: Br, 4.8. Poly[2-(3,5-di-tert-butyl-4-hydroxyphenyl)-9,10-anthryleneethynylene] (2a). Poly[2-(3,5-di-tert-butyl-4-acetoxyphenyl)-9,10-anthryleneethynylene] (2c) (0.1 g) was dissolved in THF (15 mL) under a nitrogen atmosphere. DMSO (45 mL) was added to the solution, then aqueous 2.5 N KOH (2.5 mL) was added to its suspension. The mixture was stirred at 50

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°C for 12 h, cooled to room temperature, and neutralized with aqueous 3 N HCl. The organic product was extracted with chloroform, washed with water, and dried over anhydrous sodium sulfate. The solvent was evaporated, and the crude product was purified by reprecipitation from chloroform into methanol to yield the polymer as a dark purple powder. The yield and the molecular weight are given in Table 1. Poly[2-(3,5-di-tert-butyl-4-hydroxyphenyl)-9,10-anthryleneethynylene] (2a) (Table 1, no. 1): IR (KBr, cm-1) 3644 (νO-H), 2968-2800 (νC-H, tert-butyl). Anal. Calcd for C30nH28n+1BrOn (n)4): Br, 4.7. Found: Br, 4.6. Another hydroxyl polymer 2′a was prepared analogously from the corresponding acetoxy polymer 2′c. Oxidation. The monomeric radical and polyradical were prepared by chemical oxidation of the corresponding hydroxyl precursors with PbO2 or K3Fe(CN)6 under nitrogen in a glovebox as follows. Oxidation using PbO2. A degassed toluene or benzene solution of the hydroxyl precursor (0.5-10 mM per phenol unit) was treated with 0.1-10 equiv of recently prepared PbO2 and was vigorously stirred for 0.5-2 h. After filtration the solution was used for spectroscopic and magnetic measurement. Oxidation using K3Fe(CN)6. A degassed toluene solution of hydroxyl precursor (0.5 mM per phenol unit) was treated with an alkaline solution (1N NaOH aq) containing 10 equiv of K3Fe(CN)6. After vigorously stirring for 0.5 h, the organic layer was washed with water thoroughly and dried over anhydrous sodium sulfate. After filtration the toluene solution was used for ESR measurement. ESR Spectroscopic Measurement. Solutions for ESR experiments were prepared under nitrogen in a glovebox and placed in quartz tubes sealed with septa and Parafilm. ESR spectra were taken on a JEOL JES-2XG ESR spectrometer with 100 kHz field modulation in the X-band frequency region. The spin concentrations of each sample were determined by careful double integration of the ESR signal calibrated with

Kaneko et al. that of the 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) standard solution. Signal positions were calibrated against an external standard of Mn2+/ MgO (g ) 1.981). The 77 K ESR spectra were measured for the toluene glass sample using a small Dewar flask containing the liquid nitrogen, which was inserted into the cavity of the spectrometer. Magnetic Measurement. The solution of polyradical immediately after oxidation was used to give the sample diluted with diamagnetic toluene. The sample solution was contained in a diamagnetic capsule. Magnetization and static magnetic susceptibility were measured using a Quantum Design MPMS-7 SQUID magnetometer. The magnetization was measured from 0.5 to 7 T at 1.8, 2, 2.25, 2.5, 3, 5, 7.5, 10, and 15 K. The static magnetic susceptibility was measured from 2 to 150 K in a field of 0.5 T. Other Measurements. IR spectra were obtained using a Hitachi IR 270-30 spectrometer. NMR (1H, 13C) spectra were measured using a Varian Unity 500SW (500 MHz) or a Varian Gemini 200H (200 MHz) spectrometer. Average molecular weights (M h n and M h w) were evaluated by GPC using Hitachi 655A-11 liquid chromatograph instruments [polystyrene gel columns (Shodex KF-806L), THF eluent, polystyrene calibration]. UV-vis absorption spectra were recorded on a Shimadzu UV-160 or a JASCO Ubest V-550DS UV-vis spectrometer. Fluorescence spectra were measured using a Shimadzu RF5000 spectrometer.

Acknowledgment. This work was partially supported by a Grant-in-Aid for Encouragement of Young Scientists (No. 12750775), and for Scientific Research (No. 11050906) from JSPS. We thank for Prof. H. Nishide (Waseda University) for the use of their SQUID magnetometer. CM020317T