Structural chirality of cholesteric liquid crystal produces atropisomerism: Chiroptical polyisocyanides from achiral monomer in cholesteric liquid crystal matrix

Article abstract of DOI:10.1016/j.polymer.2011.03.009

1-Naphthyl isocyanide was polymerized with Ni(II) catalyst in a cholesteric matrix at the liquid crystal (LC) temperature range. The resultant polymers showed optical activity. In this reaction, the structural chirality of cholesteric LC effectively functions to impart one-handed helicity on the corresponding polymers as an optically active atropisomer.

Full text of DOI:10.1016/j.polymer.2011.03.009

Polymer 52 (2011) 1932e1937
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Structural chirality of cholesteric liquid crystal produces atropisomerism:
Chiroptical polyisocyanides from achiral monomer in cholesteric liquid
crystal matrix
*
Hiromasa Goto , Satoshi Ohkawa, Reina Ohta
Graduate School of Pure and Applied Sciences, Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
1-Naphthyl isocyanide was polymerized with Ni(II) catalyst in a cholesteric matrix at the liquid crystal
(LC) temperature range. The resultant polymers showed optical activity. In this reaction, the structural
chirality of cholesteric LC effectively functions to impart one-handed helicity on the corresponding
polymers as an optically active atropisomer.
Received 31 December 2010
Received in revised form
28 February 2011
Accepted 6 March 2011
Available online 16 March 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Cholesteric liquid crystal
Helical
Isocyanide
1. Introduction
The helical structure has been directly observed using atomic force
microscopy (AFM) for polyisocyanides prepared by helical sense-
Synthetic chiral polymers [1], such as polyaniline bearing
camphor sulfonic acid [2], polysilanes [3], and polythiophene [4]
with chiroptical activity, have been synthesized for the develop-
ment of novel functions and biomimetic technology. Induction of
optical activity for a polymer in solution by the external environ-
ment has been demonstrated, such as chirality transfer from
a chiroptical solvent to poly(n-hexyl isocyanate) of an optically
inactive polymer [5].
Polyisocyanides have been investigated in the chiral polymer
research; due to their unique main chain helical structure [6].
Polyisocyanides form a stable 41 helical conformations in a solution,
and the introduction of an appropriate chiral side chain maintains
the one-handed helical structure. Atropisomerism in polymers has
been demonstrated for polyisocyanides [7]. Chiroptically active
polyisocyanides having a side chain, such as polyisocyanides with
an alanine-based substituent [8], polyisocyanopeptides [9], and
poly(zwitterionic isocyanides) with birefringence [10], and liquid
crystalline polyisocyanide [11], have been studied. Helical sense-
selective polymerization initiated by arylerhodium complexes has
been developed [12], and the synthesis and doping effect of tetra-
thiafulvalene-substituted polyisocyanide has been performed [13].
selective living block copolymerization, which provided evidence
that polyisocyanides clearly form a helical structure [14]. Poly-
merizations of chiral isocyanides as monomers in isotropic solvents
were performed. This produces right- and left-handed helical poly
isocyanides whose helical sense can be controlled by the poly-
merization solvents and temperature. The resultant polymers show
lyotropic liquid crystallinity [15].
Liquid crystal (LC) science and technology have been developed
for materials science such as synthesis of new functional LC
materials [16], and preparation of uniaxial alignment of the nano-
tubes by using lyotropic LC [17]. Chiral LC, especially cholesteric LC
shows characteristic properties. The individual molecules of
cholesteric LCs aggregate in a three-dimensional (3-D) one-handed
helical structure for the formation of structural chirality.
Cholesteric LCs can play a role of chiral matrix for chemical
reactions under appropriate conditions. A cholesteric LC matrix was
employed to obtain optically active polythiophene derivatives from
optically inactive monomers [4]. The polythiophenes displayed
chiroptical activity based on chiral aggregation derived from
a cholesteric LC-like order. Furthermore, electrochemical driven
control of chiroptical activity was developed [18], and charge career
(polarons) in helical form has been proposed for polymers prepared
in cholesteric LC [19].
In this study, optically active poly(1-naphtyl isocyanide) is
synthesized in a cholesteric LC matrix with the aid of a NiCl2
* Corresponding author.
E-mail address: gotoh@ims.tsukuba.ac.jp (H. Goto).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.03.009

H. Goto et al. / Polymer 52 (2011) 1932e1937
1933
Fig. 1. 13C NMR of 1-naphtyl isocyanide. Inset shows high magnetic region of distortion enhance polarized transfer (DEPT) and normal 13C NMR spectra. Arrows show signals of
carbons having no protons revealed with DEPT.
catalyst. After removal of the cholesteric matrix, the resultant was
dried in vacuum to give polymer in powder. The polymers thus
obtained exhibit the Cotton effect in achiral solution at the corre-
tert-butanol 24 mL, (CH3)3COK (1.8 g, 16 mmol) in three necked
round bottom flask was stirred for 2 days at 45 ꢀC under argon
flow. This solution was added to another solution of 1-naph-
thylformamide (1 g, 8.2 mmol) in tert-butanol (4 mL) very slowly.
After 24 h, POCl3 (0.54 g, 35 mmol) was added to the mixture and
stirred for 40 min at 10ꢁ20 ꢀC. Further, the reaction mixture was
stirred for 1 h at room temperature. The mixture was poured into
a large volume of aqueous NaHCO3 (10 wt%, 50 mL) solution, the
organic layer was extracted with ether. The ether solution was
dried with MgSO4. After filtration, the crude product was dissolved
in n-hexane (20 mL) and cooled. The precipitation was removed by
sponding wavelength of the
pep
* transition of the side chain and
the nep
* transition of the main chain, which indicates that both
the side chain and the main chain form stable one-handed helical
structures. Thus, synthesis of the one-handed helicoidal polymer as
an optically active atropisomer was performed with 3-D structural
chiral matrix of cholesteric liquid crystal. The helicity is maintained
by inter-lock function between side chains through p-stacking.
The chiroptical activity of the polymers in this study may orig-
inate from atropisomerism for the polyisocyanides produced by the
cholesteric matrix. This can be referred to as a chiral field effective
reaction in a cholesteric matrix.
2. Experimental
2.1. Syntheis of cholesteric liquid crystal medium
Cholesteric (Ch*) LC compounds having a three ring system
with terminal alkyl groups that have (R) and (S) configurations (4-
ethoxy-benzoic acid 40-[(R)-1-methyl-heptyloxy]biphenyl-4-yl
ester and 4-ethoxy-benzoic acid 40-[(S)-1-methyl-heptyloxy]
biphenyl-4-yl ester), abbreviated as (R)-Ch*LC and (S)-Ch*LC, were
prepared as cholesteric matrices using a previously reported
method [20]
2.2. Monomer synthesis
Basically synthesis of the monomer was performed according
to the method reported in the literature [21]. A solution of
Fig. 2. 1H NMR of 1-naphtyl isocyanide.

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H. Goto et al. / Polymer 52 (2011) 1932e1937
filtration. The filtrate was purified by column chromatography
(silica gel, CH2Cl2) followed by evaporation to afford desired
monomer (pale yellow liquid, 0.42 g, 2.7 mmol, Y ¼ 33%). All of the
procedures were carried out in the hood except recrystallization
(draft chamber). The unfavorable odor of the equipment used in
this preparation due to the isocyanides can be suppressed by
(prepared in (R)-Ch*LC) and PNI[(S)-Ch*LC] (prepared in (S)-Ch*LC).
The spring form in Scheme 1 is an ideal architecture.
2.3.1. PNI[(R)-Ch*LC]
(R)-CLC (0.5 g, 1.00 mmol) was placed in
4
¼ 1 cm a small
Schlenk flask with stirring bar. The flask was heated at 140 ꢀC to
show isotropic phase of the CLC solution. Then, the temperature
was gradually decreased to 97 ꢀC and stirred at an exact speed of
75 rpm. Then, 1-naphthyl isocyanide (17.6 mg, 0.115 mmol) as
a monomer was added to the cholesteric liquid crystal solvent.
Visual inspection confirmed the solution showed cholesteric phase
with rainbow color. NiCl2 (0.96 mg, 7.5 ꢂ 10ꢁ3 mmol) as a catalyst
was added to the cholesteric liquid crystal mixture to initiate
polymerization. The reaction mixture was stirred at a rate of 75 rpm
at 97 ꢀC. After 24 h, a small amount of acetone (ca. 2 mL) was added
washing with 1e5% methanolic sulfuric acid [21]. 1H NMR and 13
C
NMR measurements confirm the chemical structure of the
monomer, as shown in Fig. 1 (13C NMR) and Fig. 2 (1H NMR).
2.3. Polymerization
Polymerization was performed in the cholesteric LC matrix
(Scheme 1). The resultantpolymers areabbreviatedasPNI[(R)-Ch*LC]
Scheme 1. Polymerization in cholesteric liquid crystal medium.

H. Goto et al. / Polymer 52 (2011) 1932e1937
1935
to the mixture, and 60 mL of acetone was added to the solution and
stirred for 4 h. The supernatant solution was removed with pipette.
And 70 mL of acetone was added to the solution and stirred. This
procedure was repeated. The polymer was collected and dried
under vacuum to yield 3.7 mg (2.4 ꢂ 10ꢁ2 mmol) of yellow powder.
Y ¼ 21%.
material. GPC vs. polystyrene standard, Mn ¼ 1500; Mw ¼ 2600;
MWD ¼ 2.1 (THF soluble part).
4.2. IR
Fig. 3 shows IR spectra of the cholesteric LC solvent ((S)-Ch*LC),
the precursor of the monomer, the monomer, and the polymer (PNI
[(S)-Ch*LC]). The vibration ascribed to CH out-of-plane stretching
was observed in the high frequency region (w3000 cmꢁ1) for the
cholesteric LC solvent. In the monomer spectrum, signals at
2.3.2. PNI[(S)-Ch*LC]
PNI[(S)-CLC] was synthesized by the similar method to PNI[(R)-
CLC]. Quantity used: (S)-CLC (0.5 g, 1.00 mmol), 2-naphthyl iso-
cyanide (17.6 mg, 0.115 mmol). Y ¼ 54%, 9.5 mg (6.2 ꢂ 10ꢁ2 mmol),
yellow powder. Note that the difference of polymerization yield
between PNI[(R)-Ch*LC] and PNI[(S)-Ch*LC] is not derived from
monomer reactivity in the LC solvents because the monomer can
be polymerized in both (R)-Ch*LC and (R)-Ch*LC with the same
condition except chirality. This micro-scale polymerization
experiment may result in low polymerization yield for the poly-
mer PNI[(R)-Ch*LC] with a technical reason in the practical
experiment.
3224 cmꢁ1
(
(
nNeH) and 1655 cmꢁ1
(
nC¼O, amide), and 1550 cmꢁ1
nC¼O, ester) are no
dNeH) due to amide group and 1730 cmꢁ1
(
observed [22]. The monomer had a sharp signal at 2120 cmꢁ1
ascribed to ChN stretching, while the corresponding polymer
shows no absorption band related with the ChN stretching. This
result indicated that opening of the isocyanide ChN bond in the
monomer was catalyzed by NiCl2 to give a polymer with C¼N
bonds. The polymers display no C¼O stretching vibration at
1730 cmꢁ1 due to the cholesteric LC matrix. On the other hand, the
IR spectra of the polymers show CeH out-of-plane vibration of
naphthalene ring at 760 cmꢁ1. A set of these results indicates that
the polymers were successfully synthesized in the cholesteric
matrix.
3. Measurements
Infrared (IR) spectra were obtained with a JASCO FT/IR 550
spectrometer. Number-average molecular weight (Mn), weight-
average molecular weight (Mw), and molecular weight distribution
(MWD) of the polymers were estimated with gel permeation
4.3. Optical activity
chromatography (GPC) (PLgel 5
mm MIXED-D columns, Agilent
Fig. S1 (Supplementary content) displays optical absorption
spectrum of the monomer in chloroform solution. An absorption
technologies) eluted with tetrahydrofuran (THF) by polystyrene
standard calibration. Circular dichroism (CD) spectra were recorded
on a JASCO J-720 spectrometer. Ultra visible (UVevis) spectra were
recorded on a JASCO U-3500 spectrophotometer. CD and UVevis
spectra of the polymers were obtained at room temperature in
chloroform solution calculated from molecular weight of monomer
repeat unit, or cast film of the polymers from chloroform solution.
Dynamic light scattering (DLS) measurements for the polymer
were carried out with Otsuka electronics FDLS-3000 (
4. Results and discussion
4.1. GPC
band at 289 nm is observed due to
pep
* transition of the naph-
thalene rings. The monomer shows no absorption in the circular
dichroism (CD). Fig. 4 shows the CD and optical absorption spectra
of
the
polymers
(PNI[(S)-Ch*LC]
and
PNI[(R)-Ch*LC)
(c ¼ 5 ꢂ10ꢁ5 mol/L) in chloroform solution, calculated by monomer
repeat unit, molar extinction coefficient for the polymers was
determined. Both polymers display an optical absorption signal at
l
¼ 532 nm).
440 nm due to the nep
* transition of the main chain. Although
polyisocyanides are not universally helical [7b], in this case the CD
results suggest helical form of the main chain. The polymers
prepared in the cholesteric matrix with S-configuration and
R-configuration display a complementary mirror image Cotton
effect. The CD signal of the polymer (PNI[(R)-Ch*LC]) prepared in
The number-average molecular weight (Mn) and weight-
average molecular weight (Mw) were estimated using gel perme-
ation chromatography (GPC).
The polymers are partially soluble in THF and chloroform. The
insoluble fraction in THF is considered to consist of high molecular
weight chains. Furthermore, thermal induced scission reaction of
the main chain might occur at the high polymerization temperature
range, yielding the low molecular weight fractions. However, the
insolubility of the polymers in acetone indicates that the molecular
weight values are satisfactory (the THF soluble fractions are insol-
uble in acetone). DLS measurements evaluated that the average
aggregation size of PNI[(R)-CLC] is 406 nm in N-methylpyrrolidone
(NMP) solution (the signal is a normal distribution). The molecular
weight measurements can be performed for the aggregation
particles less than 0.1
mm (fractions passed through 0.1 mm
membrane filter).
4.1.1. PNI[(R)-Ch*LC]
Mn ¼ 1000; Mw ¼ 2100; MWD (molecular weight distribution,
Mw/Mn) ¼ 2.1 (THF soluble part).
4.1.2. PNI[(S)-Ch*LC]
Samples for GPC were prepared by filtration in tetrahydrofuran
Fig. 3. IR absorption spectra of (S)-Ch*LC (a), the precursor (b), monomer (c), and PNI
(THF) solution using a 0.1 mm membrane filter to exclude insoluble
[(S)-Ch
*LC] (d).

1936
H. Goto et al. / Polymer 52 (2011) 1932e1937
medium can control helical direction and optical activity of the
resultant polymer.
Note that CD signals at around 300 nm correspond to
pep
*
transition of the side chain naphthyl group. The CD results for the
polymers suggest that the side chains form an inter-molecular
chiral p-stacking and main chain helicoidal structure.
4.4. Plausible mechanism
From these results, the conformation of the polymers is
concluded to be produced by the transcription of chirality from the
cholesteric matrix during the polymerization reaction. The main
chains are arranged in
a predominantly one-handed helical
manner, induced by the cholesteric matrix as a helical vector
(directors) matrix. Subsequent inter-chain interaction in the
p-electron system of the side chain locks the excess helical sense
(p-stacking inter-lock function), thereby preserving the formation
even after dissolution. The Cotton effect observed in the CD spectra
is not due to the cholesteric matrix, because the optical absorption
and CD signals of the cholesteric compounds are at short wave-
lengths [22].
In addition, reaction temperature dependence on optical
activity of the resultant polymer can be expected because chole-
steric pitch length depends on temperature. Therefore, precise
control of the reaction temperature for the polymerization in LC
could allow control of optical activity of the polymers.
Fig. 4. (a) CD and (b) optical absorption spectra of the PNI[(S)-Ch*LC] (solid lines) and
PNI[(R)-Ch*LC] (dashed lines) polymers in chloroform solution.
(R)-Ch*LC displays negative first and negative second Cotton
effects. On the other hand, the polymer (PNI[(S)-Ch*LC]) prepared
in (S)-Ch*LC exhibits an opposite change in the Cotton effect. The
result further demonstrates that helical direction of the cholesteric
Fig. 5 shows a plausible polymerization mechanism for the
isocyanide monomer in a cholesteric matrix. Polymerization gives
rise to phase separation between the resultant polymer and the
cholesteric matrix during the reaction. The cholesteric LC matrix
Fig. 5. Plausible structure of poly(1-naphtyl isocyanide). Background shows polarizing optical microscopic image of the cholesteric matrix, employed for the polymerization.

H. Goto et al. / Polymer 52 (2011) 1932e1937
1937
acts as a “one-handed chiral organized matrix” consisting of chiral
directors, which effectively imparts chiral conformation to the
polymer during the polymerization reaction. The main chain grows
with a twist in one sense during the polymerization process for
formation of a predominantly one-handed helical isocyanide.
Kawashima (U. Tsukuba) drew computer graphic art in Scheme 1
and Fig. 5.
Appendix. Supplementary material
Simultaneously,
p-stacking occurs between the pendant naphtha-
Supplementary data related to this article can be found online at
doi:10.1016/j.polymer.2011.03.009.
lene groups. It should be noted that the polymerization mechanism
in a cholesteric matrix differs from that for polymerization using
a
chiral catalyst, because the external asymmetric physical
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insolubility of PNI[(R)-Ch*LC] and PNI[(S)-Ch*LC] in methanol can be
derived from not only the production of the main chain but also the
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The authors thank the Engineering Workshop and the Chem-
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