Helical Spin Polymer with Magneto-Electro-Optical Activity

Article abstract of DOI:10.1021/acs.macromol.9b00274

A chiral π-conjugated polymer bearing a stable radical side group was synthesized by electrochemical polymerization in a cholesteric liquid crystal. The polymer thus obtained shows paramagnetism as well as electro-and optical activity. The polymer exhibit

Full text of DOI:10.1021/acs.macromol.9b00274

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Helical Spin Polymer with Magneto-Electro-Optical Activity
Masashi Otaki and Hiromasa Goto*
Department of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
*
S Supporting Information
ABSTRACT: A chiral π-conjugated polymer bearing a stable
radical side group was synthesized by electrochemical
polymerization in a cholesteric liquid crystal. The polymer
thus obtained shows paramagnetism as well as electro- and
optical activity. The polymer exhibits a one-handed helical
spin order owing to the one-handed helix of the main chain
and the stable radical side groups. This helical spin order
endows the polymer with magneto-optical activity. In situ
optical absorption and circular dichroism (CD) measurements
during cyclic voltammetry analysis were used to investigate the
electroactive chirality of the polymer. Optically active polarons
(
radical cations) as charge carriers were observed by CD and electron-spin resonance (ESR) measurements. Furthermore, the
optical ellipticity of the polarons was found to be tunable using an electric field. The ESR analysis showed that the g-value of the
phenoxy radicals on the polymer is 2.00403, whereas the g-value of the polarons in the main chain is 2.0025, indicating that the
phenoxy radical side groups are independent of the polarons in terms of magnetic behavior. CD spectroscopy measurements
revealed that the phenoxy radicals exist in a chiral environment. Thus, we successfully obtained a chiral magnetic conjugated
polymer using liquid crystal imprinting polymerization. This method is a new approach for the preparation of chiral organic
magnets.
INTRODUCTION
Accordingly, research into metal-based antiferromagnetic
materials, paramagnetic materials, and topological magnets as
alternatives to ferromagnetic materials in the development of
high-performance magnetic devices has increased.
■
Conjugated polymers currently receive considerable research
interest owing to their characteristic optical, electrochemical,
and magnetic properties, all of which can be modulated by
rational design and control of the polymer structure. The
application of these materials in organic transistors, sensors,
electrochromic devices,
Liquid crystals (LCs) are self-organizing materials and
cholesteric liquid crystals (CLCs) are chiral LCs that have
helical structures. Our group has developed a method for
electrochemical polymerization in LCs. Electrochemical
polymerization in CLCs affords π-conjugated polymer films
with CLC-like helical morphologies and optical activity owing
1
2
3
,4
5
and organic solar cells has
promoted the development of new polymer synthesis and
processing methods. In particular, following the development
of organic conductors, organic magnets consisting of π-
conjugated materials have been proposed as the next
generation of synthetic metals. Several strategies for the
synthesis of ferromagnetic polymers have been explored. For
example, Nishide et al. have reported extensive research on
16,17
to chirality transfer from the CLC.
This method allows the
synthesis of chiral polymers from achiral monomers. Optically
active chiral polymers with no stereogenic centers are obtained
as atropisomers from achiral monomers via asymmetric
electrochemical or chemical LC polymerization, as developed
6
,7
8−10
polyacetylene-based
and polyphenylenevinylene-based
18
by our group. Furthermore, the electrochemical behavior of
polyradicals, which were prepared by exploiting the high-spin
states available via intramolecular through-bond magnetic
ordering upon π-conjugation. According to these reports, the
formation of planar and cross-linked polyradical frameworks on
π-conjugated backbones are important factors for spin parallel
polarons (radical cations) and bipolarons (dications) as charge
19,20
carriers in helical π-stacking backbones has been evaluated.
In the present study, the synthesis of a chiral π-conjugated
polymer from an achiral monomer bearing a stable organic
radical was performed using electrochemical polymerization in
a CLC. Introduction of chiral substituents to the side chains of
the conjugated polymer leads to chiroptical activity and the
formation of chiral J-aggregates owing to π-stacking of the
1
1
alignment. Furthermore, it has been reported that head-to-
tail-linked polythiophene moieties having phenoxy radical side
12,13
chains exhibit ferromagnetic interactions.
Recently, Swager
et al. reported the preparation of 1,3-bisdiphenylene-2-
phenylallyl-based π-conjugated radical polymers as magneto-
optic materials that show intense Faraday rotations as a new
strategy for the development and application of organic
21−23
main chain.
Furthermore, electrochemical polymerization
Received: February 7, 2019
Revised: March 24, 2019
1
4
magnets. Furthermore, spin arrangement by the macroscopic
1
5
ordering of molecular aggregates has been reported.
©
XXXX American Chemical Society
A
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a
Scheme 1. Synthetic Routes to the Monomers Employed in This Study
a
n-BuLi: n-butyllithium, THF: tetrahydrofuran, Ni(dppp)Cl : dichloro[1,3-bis(diphenylphosphino)propane]nickel(II), NBS: n-bromosuccinimide,
2
DMF: N,N-dimethylformamide, Pd(PPh ) : tetrakis(triphenylphosphine)palladium(0), TBAF: tetrabutylammonium fluoride, TMS: trimethylsilyl
3
4
group.
Table 1. Constituents of the Electrolyte CLC Solutions
in CLCs requires no chiral modification of the monomer
substituents. Thus, chiral polymers can be obtained from
monomers with simple structures using polymerization in LC.
First, a thiophene pentamer bearing a 2,6-di-tert-butyl
phenol side group as a precursor of the stable radical was
synthesized. Next, optically active polymer films were prepared
in a CLC. The resultant polymers show magneto-electro-
optical activity.
The redox properties of the polymer films were investigated
by optical and electrochemical analysis, and the magnetic
properties of the polymer were investigated by superconduct-
ing quantum interference device (SQUID) magnetometry. The
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Figure 1. Possible process for electrochemical polymerization in a CLC medium showing the resultant transcription of its helical structure. White
ellipsoids are CLC matrix molecules. Orange ellipsoids are monomers. Orange sticks are the resultant polymer.
a
Scheme 2. Synthetic Routes to p5TP1
, p5TB , and p5TB
.
2‑red
‑radical
1‑red
a
Reduced (dedoped) polymers are obtained by treatment with hydrazine vapor. K Fe(CN) : potassium ferricyanide; TBA(OH):
3
6
tetrabutylammonium hydroxide.
−
1
transfer of chirality from the CLC matrix to the phenoxy
radicals in the side chain was confirmed. The circular
dichroism (CD) absorption spectroscopy and the SQUID
results indicated that the chiral magnetic polymers obtained in
this study exhibit chiral helical spins with magneto-optical
activity.
derived from the methyl substituent is observed at 2956 cm .
As expected, 5TB shows no signal for OH stretching. Thus, the
IR data confirm the successful syntheses of 5TP and 5TB.
Polymer Synthesis. 4-Cyano-4′-pentylbiphenyl (5CB),
which is a nematic LC at room temperature, was used as an
achiral LC matrix (i.e., the host LC). The addition of chiral
compounds to such matrices produces CLCs, which are chiral
versions of nematic LCs. Cholesteryl pelargonate (CP) is used
as a chiral dopant to induce a helical structure, and the
previously synthesized (R)-4′-(4-hexyloxybenzoyloxy)-
biphenyl-4-carboxylic acid 1-methylheptyl ester ((R)-8BpB6)
is used as a chiral dopant to induce the corresponding
opposite-handed helical structure. We performed electro-
chemical polymerization in the CLC. The constituents of the
electrolytic CLC solution for electrochemical polymerization
of 5TP and 5TB are summarized in Table 1. The electrolytic
CLC solution was prepared by adding the monomer (5TP or
RESULTS AND DISCUSSION
■
Synthesis of Monomers. 3″-(3,5-Di-tert-butyl-4-phe-
noxy)-2,2′:5′,2″:5″,2‴:5‴,2⁗-quinquethiophene (5TP) and
3
″-(3,5-di-tert-butyl-phenyl)-2,2′:5′,2″:5″,2‴:5‴,2⁗-quinque-
thiophene (5TB) were used as monomers for electrochemical
polymerization in a CLC. The synthetic routes to the
monomers are illustrated in Scheme 1. 5TP is a thiophene
pentamer bearing a 2,6-di-tert-butyl phenol substituent as a
side group and adopts a rodlike structure suitable for
electrochemical polymerization in LCs. In addition, polymer-
ization of a monomer having a bulky nonradical side group was
5
TB) and tetrabutylammonium perchlorate (TBAP) as a
supporting salt to the CLC solution. This electrolytic CLC
solution was injected between two indium−tin oxide (ITO)
coated glass electrodes as an anode and a cathode separated by
a poly(tetrafluoroethylene) spacer of 200 μm thickness.
First, the polymerization cell is heated to form an isotropic
electrolyte solution. The cell is then cooled and kept at room
temperature to allow the formation of the CLC phase. In the
polymerization cell, the helical axes of the CLC are arranged
2
4
also performed for comparison. 5TB has a structure
2
5
analogous to that of 5TP but without the phenoxy radical.
1
H NMR and infrared (IR) absorption spectrometry were used
for identification of the monomers (Figures S1−S9, Supporting
Information, SI). In the IR spectrum of 5TP, the signal derived
from the OH substituent is observed at 3618 cm− as a sharp
peak due to the absence of hydrogen bonding, and the signal
1
C
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parallel to the substrate electrodes to form fingerprint textures.
Then, a direct current of 4.0 V is applied across the ITO
electrodes and the polymerization reaction is performed for 30
min. After polymerization, the remaining CLC solution is
removed by washing with n-hexane, affording the desired
polymeric thin film on the anodic surface.
ment between the CD and the UV−vis results indicates that
the main chain forms a helical π-stacking structure. The linear
dichroism spectra have very small value and are negligible
compared to the CD signals.
Electrochemical Properties of the Polymer Films. The
electrochemical properties of the polymer films are investigated
using cyclic voltammetry (CV), as shown in Figure 3. The CV
Figure 1 illustrates the CLC polymerization process. First,
the monomer molecules align between the interhelical
directors. Then, the polymer grows along the direction of
the CLC, forming a helical aggregation structure. Simulta-
neously, phase separation between the matrix CLC and the
resultant polymer occurs. Finally, washing with an organic
solvent yields the helical polymer as the final product. In this
case, the CLC matrix functions as a helical guide for
polymerization. The reduced (dedoped) polymer can be
subsequently obtained by exposure to hydrazine vapor,
yielding p5TP1‑red and p5TB1‑red used CP as the chiral dopant
and p5TP2‑red and p5TB2‑red used (R)-8BpB6 as the different
type of chiral dopant (Scheme 2). The polymers contain no
matrix LC fractions due to phase separation between the
resultant polymer and the matrix LC during electrochemical
polymerization.
Figure 3. CV results for a p5TP film on ITO glass using 0.1 M
1
TBAP−acetonitrile solution at the scan rates of 10, 20, 30, 40, 50, 60,
−
1
70, 80, 90, 100, 200, 300, 400, and 500 mV s .
The surface structures of the resultant polymeric thin films
were observed using circular polarized differential interference
contrast optical microscopy (C-DIM). The polymer exhibits a
fingerprint-like texture with labyrinth-like stripes, indicating the
transcription of the helical continuum of the CLC to the
polymer (Figure 2). CD and ultraviolet−visible (UV−vis)
measurements are carried out in a monomer-free 0.1 M
TBAP−acetonitrile solution at the scan rates of 10−500 mV
−1
s . The potentials are measured relative to a silver−silver ion
+
(
Ag/Ag ) reference electrode. During the oxidation (doping)
process for the p5TP film, an oxidation signal at 0.596 V (scan
1
−1
rate: 10 mV s ) is observed, and the potential of the oxidation
signal gradually increases with scan rate. The oxidation signal is
−
1
shifted to 0.847 V at a scan rate of 500 mV s . During the
reduction (dedoping) process for the p5TP film, a reduction
1
−
1
signal at 0.517 V (scan rate: 10 mV s ) is observed, and the
potential of the reduction signal is largely constant up to 500
−
1
mV s .
In addition, the redox properties of the p5TP film are
1
evaluated by plotting the intensity of the signal for each scan
rate (Figure S12, Supporting Information, SI). In the oxidation
and reduction processes, the p5TP film exhibits good redox
1
properties, since the intensity change of the signal is linear.
Similar electrochemical properties are observed for the p5TB1
film (Figure S13, Supporting Information, SI), indicating that
both these polymers are electroactive.
The in situ optical absorption spectra for the p5TP film are
1
obtained at various voltages in monomer-free 0.1 M TBAP−
Figure 2. Circular polarized differential interference contrast optical
microscopy (C-DIM) image of the surface structure of a polymer film
at room temperature.
acetonitrile solution (Figure 4). Absorption bands at 488 nm
(
(
derived from π−π* transition of the main chain), 777 nm
due to polarons (radical cations) of the doping band), and
>
1000 nm (due to bipolarons (dications) of the doping band)
optical absorption spectroscopy measurements of the polymer
films reduced by hydrazine vapor confirm the transcription of
the helical structure to the polymer. For the reduced
are observed (Figure 4a). Electrochemical redox processes
change these absorption bands. The band at 488 nm decreases
in intensity with increasing voltage, and the polaron and
bipolaron bands appear at longer wavelengths. The absorption
band at 488 nm increases and the doping bands decrease for
the reduction process. The reduction process is equivalent to
electrochemical dedoping. Furthermore, this electrochemical
doping−dedoping is repeatable. The color of the p5TP film
changes from red (reduced state) to black (oxidized state)
during the electrochemical processes.
(
dedoped) polymers p5TP1‑red and p5TB1‑red, a negative signal
is observed at long wavelengths and a positive signal is
observed at short wavelengths in the CD (Figure S10,
Supporting Information, SI). In the case of the reduced
polymers p5TP2‑red and p5TB2‑red, a positive signal is observed
at long wavelengths and a negative signal is observed at short
wavelengths in the CD (Figure S11, Supporting Information,
SI). Therefore, p5TP and p5TB with opposite helical
structures are synthesized using CP and (R)-8BpB6 as chiral
dopants that induce corresponding opposite helical structures.
The inflection point of the CD is located at the maximum
wavelength of the UV−vis absorption spectrum. The agree-
1
The in situ CD optical absorption of the p5TP film is also
1
examined during the electrochemical redox process (Figure
4b). A negative signal at 571 nm and a positive signal at 438
nm are observed for the reduced state at 0 V, and the inflection
point of the spectral change in the oxidation (doping) process
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Figure 4. In situ optical absorption spectra for a p5TP film in 0.1 M TBAP−acetonitrile solution at different voltages. (a) UV−vis optical
1
absorption spectra, and (b) CD spectra of a p5TP film during oxidation (upper) and reduction (bottom) between 1.0 and 0 V at 0.1 V steps.
1
+
Figure 5. ESR data for a p5TP film during electrochemical oxidation (a) and reduction (b) vs Ag/Ag at 0.05 V steps. (c) Peak height (ΔI ) and
1
pp
peak width (ΔH ). (d) The g-value and spin concentration.
pp
is observed at 486 nm. The p5TP film exhibits a Cotton effect
electronic state of the main chain by doping. Similar redox-
driven optical behavior is observed for the p5TB1 film,
indicating that the polymer is electro-optically active (Figure
S14, Supporting Information, SI).
The electrochemical properties of p5TP2 and p5TB2
synthesized using (R)-8BpB6, which is a different type chiral
dopant, are similarly measured by using optical absorption and
1
associated with the π−π* transition of the main chain. This is
reversible in the redox process (i.e., electrochemical doping−
dedoping). In addition, this repeatable inversion of the Cotton
effect at 486 nm upon changes in the electrochemical state of
the polymer indicates that the polymer has a one-handed chiral
structure and that the chirality can be tuned by changing the
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CD spectroscopy measurements (Figure S15, Supporting
Information, SI). The UV−vis absorption spectra by electro-
chemical redox process show the same behavior as p5TP and
1
p5TB . However, the CD of p5TP with electrochemical redox
1
1
process shows opposite behavior to that of p5TB , indicating
1
the formation of p5TP and p5TB having opposite helical
2
2
structures using (R)-8BpB6, which is a chiral dopant different
from CP.
Electron-Spin Resonance (ESR) Analysis. The gener-
ation of polarons and bipolarons by the p5TP1 film is
confirmed using in situ optical absorption spectra recorded
during electrochemical processes at different voltages. In the
polaron state, radical, and cation pairs are generated along the
π-conjugated polymer backbone. The ESR spectra for the
Figure 6. CV trace for a p5TP film on an ITO glass substrate
1
obtained using a 0.1 M TBAP/TBA(OH)/acetonitrile solution at a
−
1
scan rate of 10 mV s .
p5TP film on an ITO glass substrate are measured at different
1
and 0.1 V (after the generation of phenoxy radicals). Figure 7c
displays the optical absorption spectra of the polymer before
and after the generation of polarons. The polymer exhibits CD
signals owing to its one-handed helical structure. The sign of
the split-type CD signals changes from negative to positive
voltages. In the oxidation process (Figure 5a), the intensity of
the ESR signal increases from 0 to 0.5 V with the generation of
polarons and then gradually decreases with the generation of
bipolarons (dications) from 0.5 to 0.85 V. This behavior
occurs reversibly in the reduction process (Figure 5b). Figure
(
negative first and then positive at long wavelengths) as a form
5
c,d shows the plots of ESR line widths (ΔH ), g-values, peak
pp
of Davydov splitting, indicating that the main chain forms a
left-handed helical structure (according to previous research by
heights (ΔI ), and spin concentrations at different voltages. A
pp
30
decrease in ΔH is observed with the generation of polarons
pp
Berova and Nakanishi). The generation of phenoxy radicals
side groups causes a slight change in the CD and the UV−vis
spectra. The CD signal of the spins overlaps with the entire
absorption spectrum due to the helical structure of the main
chain. Difference spectrum can extract the CD signal of the
magnetic spins. Figure 7b,d shows the difference spectra for
before and after the generation of phenoxy radicals. Optical
absorption due to the phenoxy radicals is observed at 532 nm,
which is in good agreement with the results of a previous
up to 0.5 V. Further voltage increase leads to an increase in
ΔH with the generation of bipolarons. The spin concen-
pp
tration also increases with the generation of polarons in the
main chain. In addition, the increase of polarons in the main
chain between 0.25 and 0.35 V decreases the g-value. Further
application of voltage (>0.4 V) for the polymer results in a
constant g-value (2.0025). The g-value of 2.0025 for the
polymer confirms the formation of polarons as charge carriers
in the main chain. The ESR results are consistent with the
UV−vis absorption spectra, which reveal the generation of the
polaron band at 777 nm with increasing voltage. The CD
results are also consistent with the ESR results. For the p5TB1
film, the g-value is constant at 2.0025 due to the formation of
polarons, and the spin concentration increases with electro-
chemical doping (i.e., increase of voltage), as shown in Figure
S16, Supporting Information (SI).
13
study. In the CD spectra, the peak of the negative signal is
observed at around 600 nm and that of the positive signal at
around 440 nm. Note that the phenoxy radical for p5TP1
exhibits a Cotton effect, indicating that the phenoxy radicals
possess chirality due to the twist of the main chain, which is
produced by the CLC transcription polymerization. Thus, the
radicals form helical structures based on the helical chirality of
the main chain. In other words, a helical spin state is realized
through the one-handed helical arrangement of the organic
polymer. The split-type difference spectrum shows changes in
the CD sign with wavelength, indicating that the radicals as
chromophores form a left-handed helical structure. p5TB1
Chirality of the Phenoxy Radical Side Groups. The
phenoxy radical side group could potentially exists in a chiral
environment because the main chain of the polymer exhibits
one-handed chirality endowed by chirality transcription upon
electrochemical polymerization in a CLC. Accordingly, we
performed electrochemical oxidation to generate radicals at the
shows a different tendency than p5TP in the CD results
1
(Figure S17, Supporting Information, SI). No spectral changes
due to the absence of phenoxy radical are observed in this
potential range. These results strongly indicate that the
oxygen atoms of the p5TP side groups. CV measurements
1
were performed in 0.1 M TBAP and a small amount of
tetrabutylammonium hydroxide (TBA(OH)) in acetonitrile at
a scan rate of 10 mV s− (Figure 6). In the oxidation process,
an oxidation signal derived from the generation of phenoxy
radicals is observed at −0.110 V. This potential is different
from that of the signals derived from the polarons in the main
chain. A previous study reported that the oxidation potential of
the phenoxy radical in the side chain of polythiophene is −0.10
application of potentials to p5TP generates phenoxy radicals
1
1
with left-handed helical spins.
Magnetic Properties of the Polymer. The different
oxidation potentials for the generation of phenoxy radicals and
polarons in the polymer allow the selective chemical oxidation
of OH to generate phenoxy radicals in the side groups. A
previous study reported the selective generation of phenoxy
13
13
V. This value is in good agreement with that in this study.
radicals using K Fe(CN) as an oxidizing agent. Thus, we
3
6
Selective electrochemical oxidation and generation of phenoxy
radicals is possible because the oxidation potential for the
generation of phenoxy radicals in the side groups is
independent of the generation of polarons in the main chain.
Furthermore, optical measurements confirmed the generation
of phenoxy radicals in the polymer upon application of
different potentials. Figure 7a shows the CD traces for the
polymer at −0.2 V (before the generation of phenoxy radicals)
performed selective chemical oxidation for the present
polymers. First, a small amount of TBA(OH) was added to
p5TP1‑red in toluene (2 mL) and the solution was stirred for 6
h under a nitrogen atmosphere. Then, K Fe(CN) (0.2 g) in
water (1 mL) was added to this solution and stirred for 2 h at
room temperature. The resultant compound was washed with
water three times and dried under reduced pressure to give the
desired product (p5TP1‑radical) bearing phenoxy radicals
3
6
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Figure 7. In situ CD spectra (a) and UV−vis absorption spectra (c) for a p5TP film at −0.2 V (before the generation of phenoxy radicals) and 0.1
1
V (after the generation of phenoxy radicals) in 0.1 M TBAP and TBA(OH) in acetonitrile. (b) CD difference spectrum. (d) UV−vis difference
spectrum.
Figure 8. (a) ESR spectra for p5TP1‑radical (red line) and p5TP1‑polaron (blue line). (b) χ vs T plots for p5TP1‑radical, p5TP1‑polaron, and p5TB ′. (c) χT
1
vs T plots for the polymers (Curie−Weiss plot, C = χT). (d) 1/χ vs T for p5TP1‑radical. χ: Magnetic susceptibility.
(
Scheme 2). As a control, p5TP1‑polaron in the polaron state,
Table 2. ESR Results for p5TP
, p5TP
, and
1‑radical
1‑polaron
synthesized by electrochemical polymerization in a CLC, was
examined for comparison. Radical species were identified for
p5TP1
p5TB
1
′
spin conc. (spins g−1
ΔHpp (mT)
g-value
)
by ESR measurements (Figure 8a). In the ESR
‑radical
1.53 × 1019
p5TP1‑radical
p5TP1‑polaron
p5TB1′
0.569
0.286
not detected
2.00403
2.00250
spectrum of p5TP1‑radical, the g-value of the phenoxy radical is
.00403, whereas that of the polarons in p5TP1‑polaron is 2.0025,
19
0.355 × 10
2
indicating that the radical species in p5TP1‑polaron are not
phenoxy radicals. In addition, the spin concentration of
method as that for p5TP1‑radical. No ESR signals for p5TB ′ are
1
is 1.53 × 1019 spin g , which is higher than that
−1
p5TP1
‑radical
detected, indicating that the oxidizing agent (K Fe(CN) )
3
6
of p5TP1‑polaron (Table 2). p5TB ′ as a control polymer for
generates no polarons as charge carriers in the main chain.
This result demonstrates that K Fe(CN) can be used to
1
comparison with p5TB1‑red was synthesized using the same
3
6
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Figure 9. Plausible helical structure of the polymer. (a) Intrahelical spin structure of the polymer. (b) Interhelical spin structure produced by LC
imprinting. (c) C-DIM image of the polymer showing helical stripes. The distance between the stripes of the optical structure corresponds to the
helical half-pitch.
generate phenoxy radicals on the side groups without
generating polarons in the main chain.
The magnetic susceptibility of the polymers was measured
using SQUID magnetometry, as shown in Figures
boundary was observed between the LC electrolyte solution
region and the cholesteryl oleyl carbonate region on a glass
cell, indicating the LC electrolyte solution forms an
anticlockwise (left handed) helical structure. This demon-
strates that the resultant LC-imprinted polymer forms a left-
handed helix.
The p5TP1‑radical thus obtained from the chiral LC forms π-
stacking with an anticlockwise rotation to form a left-handed
chiral aggregate as a result of imprinting by the LC matrix
during polymerization. Thus, we conclude that the CD and
SQUID results confirm that p5TP1‑radical exhibits a main chain
anticlockwise helical spin order and an anticlockwise helical
aggregation of the main chains. In other words, p5TP1‑radical
with magneto-optical activity comes from the hierarchical
intramolecular helical spin order and the CLC-like intermo-
lecular three-dimensional (3D) helical structure. The M−H
curve (magnetization curve) of the polymer shows no
hysteresis loop, confirming this polymer is a paramagnetic
material. The helical structure is stabilized by intermolecular π-
stacking, resulting in a solid polymer film that exhibits
magnetism.
8
b. p5TP1‑radical exhibits intense magnetic susceptibility
compared to p5TP1‑polaron and p5TB ′. The residual oxygen
in the samples presents weak signals at around 40 K. Figure 8c
1
shows the χT vs T (i.e., Curie−Weiss plots) for the polymers.
1
/χ plotted as a function of temperature indicates the
paramagnetism of p5TP1‑radical (Figure 8d). The χT curve of
p5TP1‑radical slightly increases from low to high temperature
owing due to interspin interactions dominating thermal
vibrations at low temperature, although no clear inflection
point at low temperature as antiferromagnetic behavior
appeared.
p5TP1‑radical exhibits intense magnetic susceptibility com-
pared to p5TP1‑polaron and p5TB ′. The χT values for
1
p5TP1‑polaron are constant with temperature because the
paramagnetic character of a small portion of polarons overlaps
with the diamagnetism as the major magnetic behavior of the
2
3
polymer, which is based on sp - and sp -hybridized chemical
bonding, as shown in Figure 8c. p5TB ′, which contains
Plausible Structure. Figure 9 shows a plausible structure
for the polymer prepared in a CLC. The main chain of the
polymer thus prepared in the twisted reaction field of the chiral
LC forms a twisted helical structure. This main chain forms an
intrahelical spin structure accompanied by helical twisting of
the main chain, as shown in Figure 9a. In Figure 9b, the
cylinder form represents an individual main chain. The main
chains undergo one-handed 3D helical aggregation upon the
LC imprinting reaction, as shown in Figure 9c. The helical
spins in the main chain form an interhelical structure. The
distance between the helical stripes of the fingerprint texture
corresponds to the helical half-pitch (1.9 μm). Therefore, one
rotation of the helical spin is 3.8 μm, which is accompanied by
the helical twist of the aggregated structure. The polymer
1
neither polarons nor phenoxy radicals, shows negative
magnetism (i.e., diamagnetic behavior), as shown in Figure
8
c (green triangles). Helical magnets can show paramagnetism
31
or antiferromagnetism. The CD results indicate that the
magnetic spins in p5TP1‑radical rotate to form an anticlockwise
helix, and the SQUID results demonstrate that the polymer is
magnetically active.
Furthermore, the LC matrix forms intermolecular anticlock-
wise helical aggregates, confirmed through miscibility test for
the LC electrolyte solution. Cholesteryl oleyl carbonate with
an anticlockwise helical architecture was employed as a
standard cholesteric LC for determining the helical direction
of the LC electrolyte solution containing monomer. No
H
DOI: 10.1021/acs.macromol.9b00274
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shows no ferromagnetic spin alignment because the polymer
thus synthesized in this study satisfies none of the conditions of
topological spin parallel alignment required to obtain
CDCl
3
; δ from TMS, ppm), 0.511 (s, 9H), 1.511 (s, 18H), 7.099 (m,
1
H), 7.49 (s, 2H).
Synthesis of 5-Trimethyltin-2,2′-bithiophene. This synthesis
29
32,33
was carried out according to a previously reported method.
ferromagnetic order.
The polymer forms a one-handed
Quantities used: bithiophene (4.5 g, 27.07 mmol), n-butyllithium
17 mL, 27.07 mmol), trimethyltin chloride (5.39 g, 27.07 mmol),
chiral continuum from the molecular level to that of
macroscopic chiral aggregation with hierarchical intrahelical
and interhelical spin orders.
(
and THF (50 mL). The crude product was used in the next step
without purification.
CLCs form a quasi-layered structure similar to the smectic
Synthesis of 3″-(3,5-Di-tert-butyl-4-trimethylsiloxyphenyl)-
2,2′:5′,2″:5″,2‴:5‴,2⁗-quinquethiophene. Under argon atmos-
phere, 2,5-dibromo-3-(3,5-di-tert-butyl-4-trimethylsiloxyphenyl)-
thiophene (162 mg, 0.313 mmol) and 5-trimethyltin-2,2′-bithiophene
(226 mg, 0.688 mmol) in toluene (3 mL) were added to an overdried
34
state observed for the formation of fan-shaped textures. The
polymer prepared in the CLC exhibits a similar pattern of
helical magnetic domains to that observed for inorganic
3
5
magnets. Although the CLC forms no crystal-like domain
structures, p5TP1‑radical exhibits a magnetic-domain-like struc-
ture produced by imprinting in the CLC.
50 mL round-bottom flask equipped with a stir bar and stirred for 0.5
h. Then, tetrakis(triphenylphosphine)palladium(0) (7.23 mg, 0.00626
mmol) was added to this solution and stirred under reflux at 75 °C for
24 h. After that, the reaction mixture was poured into water and
CONCLUSIONS
extracted with ethyl acetate. The organic layer was washed with water
■
and dried over MgSO . The solvent was evaporated. The product was
4
Organic polymers generally form amorphous solids having
partly crystalline regions. LC polymers have crystal-like
molecular order. However, conjugated LC polymers with
side groups require the addition of a bulky mesogen (an LC
generator) to the skeleton, resulting in the depression of
electronic performance. Polymerization in LC avoids this
drawback because the resultant polymers form LC structures
via imprinting from the matrix LC without the need for a
mesogen in the polymer. The LC transcription method we
propose here allows further chirality imprinting from the LC
matrix to the resultant polymer. The combination of π-stacking
and the rigid form of the conjugated main chain, its chirality,
and the stable spin state in the side group affords magneto-
electro-optically active polymers. The spin chirality of the
polymer is derived from the intramain chain and the intermain
chain helicity. Helical spins have been directly observed
previously for Fe Co Si as an inorganic magnet with Lorentz
purified by column chromatography (silica gel, eluent: hexane) to
1
afford the desired product (22 mg, 0.0319 mmol, yield: 10%). H
NMR (400 MHz; CDCl ; δ from TMS, ppm), 0.454 (s, 9H), 1.407
3
(s, 18H), 6.944 (d, 1H, J = 4.0 Hz), 6.973 (dd, 1H, J = 1.6 Hz), 6.996
(d, 1H, J = 4.0 Hz), 7.036 (m, 2H), 7.116 (dd, 2H, J = 3.33 Hz),
7
2
.163 (s, 1H), 7.187 (m, 2H), 7.235 (dd, 1H, J = 1.87 Hz), 7.301 (s,
H).
Synthesis of 3″-(3,5-Di-tert-butyl-4-phenoxy)-
2
,2′:5′,2″:5″,2‴:5‴,2⁗-quinquethiophene (5TP). Under argon
atmosphere, 3″-(3,5-di-tert-butyl-4-trimethylsiloxyphenyl)-
2,2′:5′,2″:5″,2‴:5‴,2⁗-quinquethiophene (22 mg, 0.0319 mmol)
and tetrabutylammonium fluoride (1 M in THF, 0.1 mL, 0.1
mmol) in THF (3 mL) were added to an overdried 50 mL round-
bottom flask equipped with a stir bar and stirred for 4 h at room
temperature. After that, the reaction mixture was poured into water
and extracted with ethyl acetate. The organic layer was washed with
water and dried over MgSO . The solvent was evaporated. The
4
product was purified by column chromatography (silica gel, eluent:
0
.5
0.5
hexane) to afford the desired product (15.7 mg, 0.0255 mmol, yield:
3
5
electron microscopy. In the case of the polymer synthesized
in this study, optical microscopy conveniently reveals the spin
helical order of the paramagnetism through the observation of
the LC-like texture of the polymer. This is the first example of
an optically active helical spins constructed from an organic
polymer.
1
8
1
0%). H NMR (400 MHz; CDCl ; δ from TMS, ppm), 1.445 (s,
3
8H), 5.304 (s, 1H), 6.952 (d, 1H, J = 3.6 Hz), 6.989 (dd, 1H, J = 2.9
Hz), 7.006 (d, 1H, J = 4 Hz), 7.037 (dd, 1H, J = 2.9 Hz), 7.061 (dd,
1H, J = 1.5 Hz), 7.116 (dd, 2H, J = 3.1 Hz), 7.144 (s, 1H), 7.192 (m,
1
3
2H), 7.236 (m, 1H), 7.242 (s, 2H). C NMR (CDCl
ppm), 30.4, 34.5, 123.4, 123.5, 123.8, 124.4, 124.5, 124.6, 126.2,
26.4, 126.8, 127.1, 127.9, 128.0, 135.1, 135.2, 135.9, 137.1. MALDI-
; δ from TMS,
3
1
+
TOF MS (m/z): [M + 2H] calcd for C H OS , 618.1213; found,
3
4
34
5
EXPERIMENTAL SECTION
■
618.1201. Mp 196.6 °C.
Synthesis of 3-(3,5-Di-tert-butyl-phenyl)thiophene. Under
argon atmosphere, Mg (0.542 g, 22.3 mmol) in THF (5 mL) was
added to an overdried 100 mL round-bottom flask equipped with a
stir bar. Then, this solution was slowly added dropwise to 1-bromo-
3,5-di-tert-butyl-benzene (5.00 g, 18.6 mmol) in THF (5 mL) and
stirred for 6 h at room temperature. After the disappearance of Mg,
this Grignard suspension was added to a mixture of 3-
bromothiophene (3.03 g, 18.6 mmol) and [1,3-bis-
Synthesis of (2,6-Di-tert-butyl-4-bromophenoxy)-
trimethylsilane. This synthesis was carried out according to a
previously reported method. Quantities used: 2,6-di-tert-butyl-4-
bromophenol (5.01 g, 17.6 mmol), n-butyllithium (16.5 mL, 26.3
mmol, 1.6 M in hexane), chlorotrimethylsilane (3.55 g, 32.7 mmol),
26
1
and THF (70 mL). Yield = 92% (5.77 g, 16.1 mmol). H NMR (400
MHz; CDCl ; δ from TMS, ppm), 0.418 (s, 9H), 1.394 (s, 18H),
3
7
.324 (s, 2H).
Synthesis of 3-(3,5-Di-tert-butyl-4-trimethylsiloxyphenyl)-
(diphenylphosphino)propane]nickel(II) chloride (NiCl (dppp),
2
thiophene. This synthesis was carried out according to a previously
0.065 g, 0.120 mmol) in THF (10 mL) and stirred under reflux for
12 h. When the reaction is completed, the reaction mixture poured
27,28
reported method.
Quantities used: Mg (0.31 g, 12.8 mmol), (2,6-
di-tert-butyl-4-bromophenoxy)trimethylsilane (3.50 g, 9.8 mmol), 3-
bromothiophene (1.95 g, 11.9 mmol), [1,3-bis(diphenylphosphino)-
into an aqueous sodium bicarbonate (NaHCO ) and extracted with
ethyl acetate. The organic layer was washed with water and dried over
3
propane]nickel(II) chloride (NiCl (dppp), 0.044 g, 0.081 mmol), and
THF (30 mL). Yield = 45% (2.07 g, 5.75 mmol). H NMR (400
MgSO . The solvent was evaporated. The product was purified by
2
4
1
column chromatography (silica gel, eluent: hexane) to afford the
1
MHz; CDCl ; δ from TMS, ppm), 0.453 (s, 9H), 1.47 (s, 18H), 7.36
desired product (3.54 g, 13.0 mmol, yield: 70%). H NMR (400
3
(
m, 3H), 7.493 (s, 2H).
Synthesis of 2,5-Dibromo-3-(3,5-di-tert-butyl-4-
MHz; CDCl ; δ from TMS), 1.409 (s, 18H), 7.394−7.433 (m, 3H,
3
2,4,5-(thiophene)), 7.449−7.467 (m, 3H, 2,4,6-(benzene)).
trimethylsiloxyphenyl)thiophene. This synthesis was carried out
Synthesis of 2,5-Dibromo-3-(3,5-di-tert-butyl-phenyl)-
thiophene. Under argon atmosphere, 3-(3,5-di-tert-butyl-phenyl)-
thiophene (3.50 g, 12.9 mmol) in DMF (15 mL) was added to an
overdried 100 mL round-bottom flask equipped with a stir bar. After
that, NBS (5.71 g, 32.1 mmol) was slowly added to the solution at 0
2
7,28
according to a previously reported method.
3,5-di-tert-butyl-4-trimethylsiloxyphenyl)thiophene (2.73 g, 7.58
mmol), n-bromosuccinimide (NBS, 2.97 g, 16.68 mmol), and DMF
Quantities used: 3-
(
1
(
50 mL). Yield = 38% (1.51 g, 2.91 mmol). H NMR (400 MHz;
I
DOI: 10.1021/acs.macromol.9b00274
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
°
C and stirred 24 h at room temperature. Then, the mixture was
Tsukuba, for using NMR, and Glass Work Shop of University
of Tsukuba. This research was supported by KAKENHI (No.
poured into an aqueous sodium bicarbonate (NaHCO ) and
3
extracted with ethyl acetate. The organic layer was washed with
water and dried over MgSO . The solvent was evaporated. The
17K05985).
4
product was purified by column chromatography (silica gel, eluent:
hexane) to afford the desired product (3.58 g, 8.32 mmol, yield: 65%).
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3
(
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(
2
4 h. After that, the reaction mixture was poured into water and
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4
(
purified by column chromatography (silica gel, eluent: hexane) to
1
afford the desired product (145 mg, 0.241 mmol, yield: 10%). H
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3
(
3
7
d, 1H, J = 3.6 Hz), 6.982 (m, 2H), 7.041 (m, 2H), 7.122 (dd, 2H, J =
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1
1
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40.8, 150.9. MALDI-TOF MS (m/z): [M + 2H] calcd for
+
C H S , 602.1264; found, 602.1309. Mp 192.9 °C.
34
34 5
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AUTHOR INFORMATION
Corresponding Author
E-mail: gotoh@ims.tsukuba.ac.jp.
ORCID
Hiromasa Goto: 0000-0003-4276-735X
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
We would like to thank the OPEN FACILITY, Research
Facility Center for Science and Technology, University of
■
J
DOI: 10.1021/acs.macromol.9b00274
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K
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