Folding-induced through-space magnetic interaction of poly(1,3- phenyleneethynylene)-based polyradicals

Article abstract of DOI:10.1021/ma302314n

We synthesized poly(1,3-phenyleneethynylene)s bearing galvinoxyl moieties. The absorption ratio of the anion form in 1 M KOH methanol solution between 309 and 294 nm (A₃₀₉/A₂₉₄) decreased with increasing degree of polymerization. The wide-angle X-ray scattering of the powder, which was prepared by precipitation in dilute hydrochloric acid solution from the anion form in 1 M KOH methanol solution, showed a crystalline peak at 2θ = 28. Polymers in which the chiral diethynyl-1,1′-binaphthyl moiety was inserted into the poly(1,3-phenyleneethynylene) chain were synthesized, and clear Cotton effects were observed in the absorption region of the galvinoxyl anion chromophore in the CD spectra taken in 1 M KOH-MeOH solution, indicating an excess of one-handed folded helical conformation. A relatively strong antiferromagnetic interaction was observed for the polyradicals prepared by precipitating the anionic form from alkaline methanol solution accompanied with oxidation using aqueous K₃Fe(CN)₆ solution. These observations suggest that the relatively strong antiferromagnetic interaction of the polyradicals was caused by the close packing between galvinoxyl radicals induced by the formation of the folded helical structure.

Full text of DOI:10.1021/ma302314n

Article
pubs.acs.org/Macromolecules
Folding-Induced Through-Space Magnetic Interaction of Poly(1,3-
phenyleneethynylene)-Based Polyradicals
Takashi Kaneko,*,†,‡,§ Hiromasa Abe,† Masahiro Teraguchi,†,‡,§ and Toshiki Aoki†,‡,§,∥
†Graduate School of Science and Technology, Niigata University, Ikarashi 2-8050, Niigata 950-2181, Japan
‡Center for Education and Research on Environmental Technology, Materials Engineering, Nanochemistry, Institute of Science and
Technology, Niigata University, Ikarashi 2-8050, Niigata 950-2181, Japan
§Center for Transdisciplinary Research, Niigata University, Ikarashi 2-8050, Niigata 950-2181, Japan
∥Venture Business Laboratory, Niigata University, Ikarashi 2-8050, Niigata 950-2181, Japan
S
* Supporting Information
ABSTRACT: We synthesized poly(1,3-phenyleneethynylene)s bearing galvi-
noxyl moieties. The absorption ratio of the anion form in 1 M KOH methanol
solution between 309 and 294 nm (A309/A294) decreased with increasing
degree of polymerization. The wide-angle X-ray scattering of the powder,
which was prepared by precipitation in dilute hydrochloric acid solution from
the anion form in 1 M KOH methanol solution, showed a crystalline peak at
2θ = 28°. Polymers in which the chiral diethynyl-1,1′-binaphthyl moiety was
inserted into the poly(1,3-phenyleneethynylene) chain were synthesized, and
clear Cotton effects were observed in the absorption region of the galvinoxyl
anion chromophore in the CD spectra taken in 1 M KOH−MeOH solution,
indicating an excess of one-handed folded helical conformation. A relatively
strong antiferromagnetic interaction was observed for the polyradicals
prepared by precipitating the anionic form from alkaline methanol solution accompanied with oxidation using aqueous
K3Fe(CN)6 solution. These observations suggest that the relatively strong antiferromagnetic interaction of the polyradicals was
caused by the close packing between galvinoxyl radicals induced by the formation of the folded helical structure.
cross-conjugated system, and the corresponding polyradicals
bearing pendant spins at the 5-position have a disjoint
nonbonding molecular orbital based on the theoretical
prediction by Borden and Davidson.39 This feature leads to
minimized through-bond magnetic interaction between pend-
ant spins, and in fact most of 1,3-PPE-based polyradicals have
not shown strong magnetic properties.31−33 However, it is well-
known that some 1,3-PPEs can form folded helical con-
formations depending on their side groups and/or solvent.40−48
Some theoretical calculations were performed for the magnetic
property related to helical conformation,49,50 and it was
reported that electron spin resonance (ESR) line broadening
was observed by forming a folded helical conformation for
oligo(1,3-phenyleneethynylene) bearing TEMPO radicals.51 In
this study, we found a moderately strong through-space
magnetic interaction, which was induced by forming the folded
helical conformation of 1,3-PPE-based polyradicals bearing
galvinoxyl side groups.
INTRODUCTION
■
The π-conjugated polymers have attracted much attention as an
organic material with various electronic properties. In
particular, the magnetic property of π-conjugated polyradicals
is one of the most attractive research fields concerning new
functional polymer materials.1−4 Numerous π-conjugated
polymers substituted with pendant radicals have been
synthesized and characterized,5,6 since ferromagnetic through-
bond interaction between the pendant spins was theoretically
predicted for regioregular head-to-tail π-conjugated macro-
molecules possessing conjugated pendant radicals by using
simple polyene models and by analogy with other π-conjugated
polymers.7−9 Some of them did exhibit the expected
ferromagnetic behavior through their π-conjugated back-
bone,10−27 but the through-space magnetic interaction based
on their steric conformation has not been investigated well,
except for a few examples.28,29 Poly(phenyleneethynylene)
(PPE) is one of the most promising backbone structures for
magnetic π-conjugated polyradicals, and various PPE-based
polyradicals have been investigated.30−38 On the basis of
molecular topology and spin polarization, regioregular head-to-
tail linked poly(1,4- and 1,2-phenyleneethynylene) (1,4- and
1,2-PPE) structures are required for a ferromagnetic through-
bond interaction between the pendant spins. On the other
hand, a poly(1,3-phenyleneethynylene) (1,3-PPE) structure is a
Received: November 7, 2012
Revised: March 4, 2013
Published: March 21, 2013
© 2013 American Chemical Society
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RESULTS AND DISCUSSION
Synthesis of Polymer 3. As shown in Scheme 1, the
diiodobenzene 1 and diethynylbenzene 2 were polymerized in
■
Scheme 1. Polymerization of 1 and 2 and Formation of
Polyradical 3b and Polyanion 3c
Figure 1. UV−vis absorption spectra of 3a(19mer) in benzene (dotted
line) and chloroform (broken line), and 3c(19mer) (solid line) in 1 M
KOH methanol at 20 °C (2.5 × 10−5 M based on the galvinoxyl unit).
conformation was preferred in acetonitrile and was accom-
panied by a decrease of the absorption at 305 nm.40,41 The
absorption ratio between 309 and 294 nm (A309/A294) of 3c in
1 M KOH methanol solution decreases with increasing DPn,
while A309/A294 of 3a in chloroform and benzene are
comparable independent of the chain length (Figure 2). It
the presence of the Pd(PPh3)4 complex catalysts, and a yellow
solid polymer 3a(19mer) was obtained by precipitation from
the polymerization mixtures into methanol. The polymerization
data for these resultant polymers are summarized in Table 1.
Table 1. Polymerization of 1 and 2 using Pd(PPh3)4
a
catalyst.
b
b
no. [2]/[1] fraction yield (%) Mn ( × 103) Mw/Mn
DPn
c
f
1
2
0.9
0.5
1
1
2
95
17
9
10
7.0
2.5
1.8
1.2
1.1
19
d
e
g
8
g
3
a
[1]0 = 0.25 mol/L, [1]0/[Pd(PPh3)4] = 20, [CuI]/[Pd(PPh3)4] = 4,
b
THF/NEt3 = 1:1 (v/v), r.t., 24 h. Measured by GPC calibrated with
polystyrene standard. Methanol-insoluble fraction. Methanol-in-
soluble fraction followed by preparative GPC. Methanol-soluble
fraction followed by preparative GPC. Based on the monomer unit
and estimated from Mn. Based on the monomer unit and estimated
from integration ratio of H NMR.
c
d
e
f
g
1
Polymerization under the initial molar ratio of monomers [2]/
[1] = 0.9 gave 3a(19mer) with the average degree of
polymerization (DPn) = 19 as predicted theoretically (DPn =
(1 + [2]/[1])/ (1 − [2]/[1])). The polymer 3a(19mer) was
soluble in tetrahydrofuran (THF), chloroform, and benzene,
but insoluble in methanol. On the other hand, polymerization
under the initial molar ratio of monomers [2]/[1] = 0.5 gave
oligomer 3a(8mer) with DPn = 8 and 3a(3mer) with DPn = 3
from methanol insoluble and soluble fraction, respectively,
through separation by preparative GPC.
Helical Folding of Anion Form. UV−vis spectra of 3a in
chloroform and benzene show absorption maxima at 420 nm
attributed to the side chain hydrogalvinoxyl chromophore52 and
multiple absorption bands at 250−350 nm attributed to the
backbone chromophore (Figure 1). Polymer 3a was converted
to the corresponding polyanion 3c by treatment with an alkali
methanol solution and exhibited absorption maximum at 604
nm attributed to the galvinoxyl anion chromophore.52 Moore
and co-workers previously reported that the solvophobically
driven folding reaction of oligo(1,3-phenyleneethynylene)s
bearing polar tri(ethylene glycol) side chains was confirmed
by UV−vis studies; that is, the oligomers in chloroform were
present in a random conformation, and the folded helical
Figure 2. UV absorption ratio at 294 and 309 nm (A309/A294) versus
chain length for 3a in benzene (open square) and chloroform (open
triangle), and 3c (filled circle) in 1 M KOH methanol at 20 °C (2.5 ×
10−5 M based on the galvinoxyl unit).
was confirmed that the polymer 3c(19mer) in 1 M KOH
methanol solution preferred to form the folded helical
conformation, while the chain lengths of 3c(8mer) and
3c(3mer) were not long enough to form the folded helical
conformation. The population of the folded helical conforma-
tion increased at lower temperature, which was supported by
the downward deviation of A309/A294 values for 3c(19mer) in 1
M KOH methanol solution from trend in A309/A294 versus
temperature for 3c(8mer) and 3c(3mer) (Figure 3).
The wide-angle X-ray scattering (WAXS) of 3a(19mer)
powder which was prepared by evaporation from the benzene
solution of 3a(19mer) showed no crystalline peaks (Figure 4a).
On the other hand, the WAXS of 3a(19mer) powder, which
was prepared by precipitation in dilute hydrochloric acid
solution from the anion form 3c(19mer) in 1 M KOH
methanol solution, showed a crystalline peak at 2θ = 28°, and
the d spacing was calculated using the Bragg equation to be 3.2
Å, which would correspond to a distance consistent with
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Scheme 2. Postpolymerization of 3a with 4 and Formation of
Polyradical 5b and Polyanion 5c
Table 2. Postpolymerization of 3a(19mer) with 4 Using
a
Pd(PPh3)4 Catalyst
Figure 3. UV absorption ratio at 294 and 309 nm (A309/A294) versus
temperature for 3c(19mer) (filled circle), 3c(8mer) (open square) and
3c(3mer) (open triangle) in 1 M KOH methanol (2.5 × 10−5 M based
on the galvinoxyl unit).
b
c
c
4
Mn of 3a ( × 104)
yield (%)
Mn ( × 104)
Mw/Mn
(R)
(S)
rac
1.0
1.0
1.0
94
90
97
2.4
2.2
3.6
2.4
2.4
.
a
[3a]0 = [4]0 = 25 mmol/L, [4]0/[Pd(PPh3)4] = 1, [CuI]/
[Pd(PPh3)4] = 4, THF/NEt3 = 1:1 (v/v), room temperature, 24 h.
b
c
Methanol insoluble fraction. Measured by GPC calibrated with
polystyrene standard.
were connected with the binaphthyl unit. The polymer 5a was
soluble in THF, chloroform, and benzene but insoluble in
methanol.
The CD spectra of enantiomeric 5a obtained by polymer-
ization using (R)-4 (abbreviated as (R)-5a) were measured in
chloroform and benzene solution. Clear Cotton effects were
observed for (R)-5a in the absorption region of the backbone
binaphthyl chromophore (300−350 nm), although no induced
CD signal appeared in the absorption region of the side chain
hydrogalvinoxyl chromophore (420 nm) (Figure 5a). This
behavior supports the random conformation of 5a in
chloroform and benzene as well as 3a.
The polyradical 5b was obtained by oxidizing the polymer 5a
by treatment of the polymer solution in degassed benzene with
fresh PbO2. The formation of the polyradical was supported by
the appearance of the ESR signal at g = 2.0048 accompanied by
the decrease of the peak at 420 nm attributed to the
hydrogalvinoxyl chromophore and appearance of the peak at
470 nm attributed to the galvinoxyl radical chromophore in
UV−vis spectra. However, the folded helical conformation was
not observed for (R)-5b as well as (R)-5a in benzene because
no induced CD signal appeared besides the backbone
binaphthyl chromophore (Figure 5b).
On the other hand, the polyanion 5c obtained by dissolution
of 5a in 1 M KOH methanol solution showed absorption
maximum at 604 nm attributed to the galvinoxyl anion
chromophore, and bisignated Cotton effects were observed
for (R)-5c and (S)-5c, which were mirror images of each other,
in the absorption region of the galvinoxyl anion chromophore
(604 nm) (Figure 5c). The dynamic character of the folded
helical conformation for 5c was confirmed by the CD
intensities depending on temperature as shown in Figure 6.
While (R)-5c in methanol with tetrabutylammonium hydroxide
showed the bisignated Cotton effects as well as in 1 M KOH
methanol solution, addition of benzene to the solution
diminished the Cotton effects (Figure 7). This behavior
Figure 4. WAXS patterns of 3a(19mer) powder (a) prepared by
evaporation from the benzene solution and (b) prepared by
precipitation in dilute hydrochloric acid solution from the anion
form 3c(19mer) in 1 M KOH methanol solution.
aromatic face-to-face stacking (Figure 4b).53 This fact suggested
that the folded helical conformation of 3c(19mer) formed in 1
M KOH methanol solution remained for 3a(19mer) powder
prepared by precipitation in dilute hydrochloric acid solution.
Postpolymerization of 3a with Chiral Monomer and
Formation of One-Handed Helical Conformation. The
helical conformation essentially has an asymmetric nature, and
the optical activity is induced by an excess of one-handed
helical conformation. Moore and co-workers previously
reported that an optically active binaphthol derivative inserted
into the oligo(1,3-phenyleneethynylene) chain imparted a bias
in the twist sense of the helical structure.54 As shown in Scheme
2, the diiodo polymer 3a(19mer) and diethynyl binaphthol
monomer 4 were polymerized in the presence of the Pd(PPh3)4
complex catalysts, and a yellow solid polymer 5a was obtained
by precipitation from the polymerization mixtures into
methanol. The polymerization data for these resultant polymers
are summarized in Table 2. Because the polymer 5a had more
than twice the molecular weight of the polymer 3a(19mer), 5a
mostly had the structure in which at least two 3a(19mer) units
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Figure 7. CD and UV−vis absorption spectra of (R)-5c in 10%
tetrabutylammonium hydroxide methanol/benzene solution at 20 °C
(0.25 mM based on the galvinoxyl unit).
3c(19mer) in alkaline methanol solution as well, while
3a(19mer) in benzene was oxidized to give 3br(19mer),
which probably had the random conformation in benzene. The
insolubility of 3bh(19mer) and 5bh in organic solvents was
probably caused by predominant formation of the dense and
rigid helical structure through the oxidation process and/or the
entanglement of the polymer chain induced by the folding
reaction.
The static magnetic susceptibility (2−100 K at 0.5 T) of
3b(19mer) and 5b powders were measured using a SQUID
magnetometer. The χmolT versus T plots of 3br(19mer) and 5br
powders are shown in Figure 8, where χmol is normalized to
Figure 5. CD and UV−vis absorption spectra of (a) (R)-5a in
chloroform, (b) (R)-5b (spin concentration = 0.58 spin/galvinoxyl
unit) in benzene, and (c) (R)- and (S)-5c in 1 M KOH methanol
solution at 20 °C (0.25 mM based on the galvinoxyl unit).
Figure 8. χmolT versus T plots of 3br(19mer) (open circle; spin
concentration = 0.57 spin/galvinoxyl unit), (R)-5br (open square; 0.53
spin/galvinoxyl unit) and rac-5br (open triangle; 0.47 spin/galvinoxyl
unit) prepared by evaporation from the benzene solution. Solid line:
The Curie−Weiss fitting using Θw = −0.89,−0.78, and −0.78 K for
3br(19mer), (R)-5br, and rac-5br, respectively.
Figure 6. CD and UV−vis absorption spectra of (R)-5c at −10, +20,
and +50 °C in 1 M KOH methanol solution (0.25 mM based on the
galvinoxyl unit).
supports the solvophobically driven folding reaction of 5c
because benzene and methanol were good and poor solvents
for 5a, respectively.
1
0.375 (S = /2) at higher temperature by spin concentration.
Formation of Folded Helical Polyradical and Magnetic
Property. As mentioned above, the polyradical 5b obtained by
oxidizing the polymer 5a in benzene, where the suffix “r” was
added to the thus obtained polyradical, i.e., as 5br, showed no
characteristics for the folded helical conformation. On the other
hand, 5b powder was obtained by precipitation from the
alkaline methanol solution of 5c accompanied with oxidation
using aqueous K3Fe(CN)6 solution, where the suffix “h” was
added to the thus obtained polyradical, i.e., as 5bh.
Unfortunately, the obtained 5bh powder was practically
insoluble in organic solvents, but the formation of polyradical
was confirmed by the appearance of the ESR signal at g =
2.0048, indicating the formation of the galvinoxyl radical. The
organic solvent-insoluble 3bh(19mer) was obtained from
The χmolT versus T data fitted the Curie−Weiss law (χmol =
0.375/(T − Θw)) with a small Weiss temperature (Θw),
indicating weak antiferromagnetic interaction as shown in
Figure 8. Figure 9 shows the χmolT versus T plots of
3bh(19mer) and 5bh powders. Linear fitting of the 1/χmol
versus T data at higher temperature range (100−300 K) gave
Θw = ca. −13 K (see Figure S16 in the Supporting Information)
indicating relatively strong antiferromagnetic interaction, while
the χmolT versus T plots deviate upward at a lower temperature
from the Curie−Weiss relationship. This deviation from the
Curie−Weiss law indicates that the magnitude of magnetic
interaction between the radicals broadly varied owing to the
heterogeneous morphology probably mixing the random
conformation, the perturbed helical conformation and the
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interaction, and the corresponding polyradical precipitates
were obtained by oxidation using aqueous K3Fe(CN)6 solution.
The static magnetic susceptibility of the polyradical thus
obtained exhibited a relatively strong antiferromagnetic
behavior compared with the weak antiferromagnetic behavior
of the polyradical, which was prepared by evaporating solvent
from the benzene solution of the polyradicals followed by
drying in vacuum. This fact suggests that the relatively strong
antiferromagnetic interaction of the polyradicals was caused by
close packing between galvinoxyl radicals induced by the
formation of the folded helical structure. This finding will lead
to the development of new polyradicals with electronic,
magnetic, and chiroptical properties through the fusion with
optically active helical polymers and the control of hierarchical
structures.
Figure 9. χmolT versus T plots of 3bh(19mer) (open circle; spin
concentration = 0.54 spin/galvinoxyl unit), (R)-5bh (open square;
0.40 spin/galvinoxyl unit) and rac-5bh (open triangle; 0.54 spin/
galvinoxyl unit) prepared by precipitating from alkaline methanol
solution of anion form accompanied with oxidation using aqueous
K3Fe(CN)6 solution. Solid line: The Curie−Weiss relation with Θw =
−13.
EXPERIMENTAL SECTION
■
Materials. The monomer 237 and 456,57 were synthesized
according to the literature procedures. Tetrakis(triphenylphosphine)-
palladium (0) (Pd(PPh3)4) (Aldrich Co.) was used without further
purification. Other conventional reagents were used as-received or
purified by conventional methods.
folded helical conformation of polyradicals and the various
magnetic coupling sets included the partially strong anti-
ferromagnetic coupling.
(3,5-Diiodophenyl)hydrogalvinoxyl (1). To a solution of 1,3,5-
triiodobenzene (12 g, 27 mmol) in ether (180 mL) was added n-butyl
lithium in hexane (17.5 mL, 28 mmol, 1.6 M) under nitrogen at −70
°C. The mixture was stirred for 30 min, and a solution of 4,4′-
diacetoxy-3,3′,5′,5-tetra-tert-butylbenzophenone (14 g, 27 mmol) in
ether (180 mL) were added. The mixture was stirred at −70 °C for 2 h
and at room temperature for another 1 h. It was neutralized with dilute
aqueous hydrochloric acid, and extracted with ether. The extract was
dried over anhydrous sodium sulfate, solvent was evaporated, and then
the residue was recrystallized from hexane to give α,α-bis(3,5-di-tert-
butyl-4-acetoxyphenyl)-3,5-diiodobenzyl alcohol (9.3 g, 11 mmol) as
colorless crystals. Yield: 41%. Mp 209−210 °C. 1H NMR (CDCl3, 270
MHz; ppm): δ 7.97 (t, 1H, J = 1.4 Hz, ArH), 7.70 (d, 2H, J = 1.4 Hz,
ArH), 7.22 (d, 2H, J = 2.4 Hz, ArH), 6.99 (d, 2H, J = 2.4 Hz, ArH),
2.68 (s, 1H, OH), 2.33 (s, 6H, −CH3), 1.28 (s, 18H, C(CH3)3), 1.23
(s, 18H, C(CH3)3). 13C NMR (CDCl3; ppm): δ 170.68, 150.90,
147.23, 143.51, 142.13, 141.82, 141.64, 136.24, 126.25, 125.86, 94.08,
81.27, 35.71, 35.62, 31.50, 31.42, 22.73. IR (KBr pellet; cm−1): 3582
(νO−H), 1763 (νCO). The obtained compound (8.5 g, 10 mmol) was
dissolved in DMSO (100 mL) under a nitrogen atmosphere, and then
aqueous 5 M KOH (60 mL) was added to the solution. The mixture
was stirred at 80 °C for 12 h, cooled to room temperature, and
neutralized with aqueous 4 N hydrochloric acid. The organic product
was extracted with chloroform, washed with water, and dried over
anhydrous sodium sulfate. The solvent was evaporated, and then the
residue was recrystallized from hexane to give (3,5-diiodophenyl)-
hydrogalvinoxyl (1) (6.1 g, 8.2 mmol) as reddish orange crystals. Yield
82%, mp 223−224 °C. 1H NMR (CDCl3, 270 MHz; ppm): δ 8.11 (t,
1H, J = 1.0 Hz, ArH), 7.59 (d, 2H, J = 1.0 Hz, ArH), 7.18 (d, 1H, 1.8
Hz, ArH (quinoid)), 7.02 (d, 1H, 1.8 Hz, ArH (quinoid)), 6.99 (s, 2H,
ArH), 5.57 (s, 1H, OH), 1.43 (s, 18H, C(CH3)3), 1.27 (s, 18H,
C(CH3)3). 13C NMR (CDCl3; ppm): δ 185.86, 155.65, 147.38,
147.29, 145.09, 144.56, 140.08, 135.48, 132.11, 131.33, 130.49, 129.84,
129.69, 93.87, 35.43, 35.42, 34.50, 30.36, 29.72, 29.59. IR (KBr pellet;
cm−1): 3630 (νO−H), 1584 (νquinoid). Anal. Calcd for C35H44I2O2: C,
56.0; H, 5.9; I, 33.8. Found: C, 56.3; H, 5.9; I, 33.6.
On the basis of theoretical prediction,39 the through-bond
magnetic interaction of 3b(19mer) and 5b would be negligible
or weak antiferromagnetic as shown in Figure 10a. In fact, the
Figure 10. (a) Connectivity pattern for 3b(19mer) and 5b, and (b)
plausible through-space magnetic interaction between galvinoxyl
radials in close proximity induced by the formation of the folded
helical structure.
magnetic behavior of 3br(19mer) and 5br, which had the
random conformation in Figure 8, is weak antiferromagnetic, as
is that of poly(binaphthyl-6,6′-diylethynylene-1,3-phenylenee-
thynylene) bearing galvinoxyl side groups, which did not form
the folded helical conformation.55 That is, the relatively strong
antiferromagnetic interaction of 3bh(19mer) and 5bh as shown
in Figure 9 is consequently caused by close packing between
galvinoxyl radicals induced by the formation of the folded
helical structure as shown in Figure 10b.
Polymerization. Typical procedure was described as follows. The
monomer 1 (1.5 g, 2.0 mmol), 2 (0.98 g, 1.8 mmol), tetrakis-
(triphenylphosphine)palladium (0) (0.12 g, 0.10 mmol), and copper
iodide (0.076 g, 0.40 mmol) were placed in a Schlenk tube equipped
with a three-way stopcock, a rubber septum, and a Teflon-coated
magnetic stirring bar. The tube was placed under vacuum, followed by
a nitrogen backflush. THF (4.0 mL) and triethylamine (4.0 mL) were
transferred to the tube, and the solution was stirred at room
temperature for 24 h. The solution was treated with aqueous 3 N
hydrochloric acid, extracted with chloroform, washed with saturated
CONCLUSION
■
We synthesized 1,3-PPEs bearing galvinoxyl moieties. The
folded helical conformation is preferred for the anion form in
alkaline methanol solution owing to the solvophobic
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saline, and dried over anhydrous sodium sulfate. The solvent was
evaporated, and the crude product was purified by precipitation from
chloroform into methanol to yield the polymer 3a as yellow powder.
UV2070, CD-2095, and polystyrene gel columns (Shodex KF-807L).
CD and UV−vis absorption spectra were recorded using a Jasco J-
720WI Spectropolarimeter with a peltier controller for temperatures at
−10 to +50 °C (a quartz cell of 1 mm path length; sample
concentration = 0.1−1 mM based on the galvinoxyl unit), and were
analyzed using the associated J-700 software. UV−vis absorption
spectra were recorded on a Jasco Ubest V-550 UV−vis spectrometer
with a peltier controller for temperatures at −10−50 °C (a quartz cell
of 10 mm path length; sample concentration = 2.5 × 10−5 M based on
the galvinoxyl unit). The wide angle X-ray scattering measurements
were performed using a Rigaku Geigerflex with a graphitemonochrom-
atized Cu Kα radiation, which was supplied at 40 kV and 20 mA.
1
The yield and the molecular weight are given in Table 1, no. 1. H
NMR (CDCl3, 270 MHz; ppm) δ 7.91 (br, 2H, ArH), 7.68 (br, (m-
2)H, ArH), 7.62 (br, 2H, ArH), 7.38 (br, (2m-2)H, ArH), 7.20 (br,
mH, ArH), 7.00 (br, 3mH, ArH), 5.53 (s, mH, OH), 1.39 (s, 18mH,
tert-butyl), 1.26 (s, 9mH, tert-butyl), 1.21 (s, 9mH, tert-butyl). IR (KBr,
cm−1): 3630 (νO−H), 2957−2870 (νC−H), 1610 (νquinoid).
Postpolymerization. Typical procedure was described as follows.
The monomer 3a (0.40 g, 0.040 mmol), 4 (0.020 g, 0.040 mmol),
tetrakis(triphenylphosphine)palladium(0) (0.046 g, 0.040 mmol), and
copper iodide (0.030 g, 0.16 mmol) were placed in a Schlenk tube
equipped with a three-way stopcock, a rubber septum, and a Teflon-
coated magnetic stirring bar. The tube was placed under vacuum,
followed by a nitrogen backflush. THF (1.6 mL) and triethylamine
(1.6 mL) were transferred to the tube, and the solution was stirred at
room temperature for 24 h. The solution was treated with aqueous 3
N hydrochloric acid, extracted with chloroform, washed with saturated
saline, and dried over anhydrous sodium sulfate. The solvent was
evaporated, and the crude product was purified by precipitation from
chloroform into methanol to yield the polymer as yellow powder. The
yield and the molecular weight are given in Table 2. 1H NMR (CDCl3,
270 MHz; ppm) δ 8.04 (br, 2H, ArH in 4), 7.85 (br, 2H, ArH in 4),
7.68 (br, mH, ArH in 3a), 7.38 (br, 2mH, ArH in 3a), 7.21 (br, mH,
ArH in 3a), 7.00 (br, 3mH, ArH in 3a), 5.53 (s, mH, OH), 3.75−3.94
(m, 4H, CH2 in hexyloxy), 1.39 (m, 18mH, tert-butyl), 1.26 (s, 9mH,
tert-butyl), 1.20 (s, 9mH, tert-butyl), 0.88 (br, 12H, CH2 in hexyloxy),
0.72 (t, 6H, J = 6.9 Hz, CH3 in hexyloxy). IR (KBr, cm−1): 3630
(νO−H), 2957−2870 (νC−H), 1609 (νquinoid).
Oxidation. The polyradicals 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 benzene solution of the
hydroxyl precursor (1 mM per galvinoxyl unit) was treated with 20
equiv of recently prepared PbO2 and was vigorously stirred for 1 h.
After filtration, the solution was used for spectroscopic measurement.
The powder sample was prepared by evaporating the solvent and by
drying in vacuo, and was used for spectroscopic and magnetic
measurement.
Oxidation Using K3Fe(CN)6. A degassed 1 M KOH methanol
solution of the anion form (1 mM per galvinoxyl unit) was treated
with 100 equiv of aqueous K3Fe(CN)6 solution (0.1M) to form
precipitates. After vigorously stirring for 0.5 h, the precipitates were
washed with water thoroughly and then with methanol. The obtained
precipitates were dried in vacuo, and were used for ESR and magnetic
measurement.
ESR Spectroscopic Measurement. Solutions for ESR experi-
ments 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. Signal positions were calibrated against
an external standard of Mn2+/MgO (g = 1.981). The spin
concentrations of each sample were determined by careful double
integration of the ESR signal calibrated with that of the 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO) standard solution.
ASSOCIATED CONTENT
* Supporting Information
■
S
1H NMR spectra of 1, 3a(19mer), 3a(8mer), 3a(3mer), (R)-5a
and (S)-5a; UV−vis absorption spectra of 3a and 3c for Figure
2 and 3; CD and UV−vis absorption spectra of (R)-5a in
benzene; ESR spectra of (R)-5b; molecular mechanics
modeling of 3c; and 1/χmol versus T plots of 3bh(19mer),
(R)-5bh and rac-5bh. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: kanetaka@gs.niigata-u.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by a Grant-in-Aid for
Scientific Research (B) (No. 24310081) from JSPS and by a
Grant for the Promotion of Niigata University Research
Projects.
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