Synthesis of a pyrimidine-based new chiral inducer for construction of cholesteric liquid crystal electrolyte solution and its electrochemical polymerization, and stimulated emission like interference

Article abstract of DOI:10.1016/j.polymer.2013.04.053

Pyrimidine-based chiral compounds were successfully synthesized via Mitsunobu reaction. The chiral molecules with rigid molecular shape can function as chiral inducers (chiral dopant) to prepare cholesteric liquid crystal (CLC) electrolyte solutions. We carried out electrochemical polymerization in the CLC electrolyte solution thus prepared with the chiral inducer. The polymerization in the CLC afforded chiroptically active polymer films. The synthesis of the pyrimidine-based chiral inducer, electrochemical polymerization, and observation of a surface image of the polymer were carried out. It is noted that the polymer shows laser diffraction in the visible range due to a periodic pattern produced by imprinting of structural chirality of the CLC electrolyte solution. The laser diffraction patterns are multiple circles, implying occurrence of stimulated emission via periodic patterns of the cholesteric order.

Full text of DOI:10.1016/j.polymer.2013.04.053

Polymer 54 (2013) 3821e3827
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis of a pyrimidine-based new chiral inducer for construction
of cholesteric liquid crystal electrolyte solution and its
electrochemical polymerization, and stimulated emission
like interference
*
Aohan Wang, Kohsuke Kawabata, Hirotsugu Kawashima, Hiromasa Goto
Faculty of Pure and Applied Sciences, Division 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:
Pyrimidine-based chiral compounds were successfully synthesized via Mitsunobu reaction. The chiral
molecules with rigid molecular shape can function as chiral inducers (chiral dopant) to prepare chole-
steric liquid crystal (CLC) electrolyte solutions. We carried out electrochemical polymerization in the CLC
electrolyte solution thus prepared with the chiral inducer. The polymerization in the CLC afforded chi-
roptically active polymer films. The synthesis of the pyrimidine-based chiral inducer, electrochemical
polymerization, and observation of a surface image of the polymer were carried out. It is noted that the
polymer shows laser diffraction in the visible range due to a periodic pattern produced by imprinting of
structural chirality of the CLC electrolyte solution. The laser diffraction patterns are multiple circles,
implying occurrence of stimulated emission via periodic patterns of the cholesteric order.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 25 February 2013
Received in revised form
20 April 2013
Accepted 22 April 2013
Available online 1 May 2013
Keywords:
Chiral inducer
Laser diffraction
Pyrimidine
1. Introduction
molecule can be expected to function as a chiral inducer because of
rigid molecular shape (indicative of good compatibility in nematic
Liquid crystals (LCs) have both crystal order and fluidity similar
to liquids [1e10]. Nematic LCs have especially low viscosity. They
can be used as a reaction solvent [11,12]. In this case, the addition of
non-LC compounds to LC materials destroys its liquid crystallinity,
resulting in a transformation into isotropic liquid or solid, whereas
the addition of a small amount of optically active molecules to non-
chiroptically active nematic LC produces cholesteric liquid crystal
(CLC) with helical aggregation structure [13,14].
CLC is a twisted nematic phase of liquid crystal in which the
director orientation is rotated progressively to form a helical
structure [15e17]. The helical aggregation of rod-like molecules in
cholesteric LC results in periodicity and the three-dimensional (3D)
molecular arrangement produces structural chirality.
The additives to the nematic LC are referred to as chiral inducers
(chiral dopants). Chiral inducers require 1) good solubility in nematic
LCs, 2) chiral molecular structure for the induction of cholesteric
phase, and 3) compatibility with nematic LC to maintain liquid
crystallinityafter the addition of theinducer into matrix nematic LCs.
To prepare a compound which covers these requirement as
chiral inducers, we introduced a chiral moiety into the terminal of a
rigid molecule having fluorine atom. A rigid chiral mesogenic
LC consisting of rigid molecules), good chiral induction property
with the aid of polar fluorine atom directed perpendicular to the
molecular axis, and compatibility with nematic LC.
Electrochemical polymerization has been carried out to prepare
p-conjugated polymers with electro-activity [18e21]. Electro-
chemical polymerization in CLC is possible by using indium-tin-
oxide (ITO) coated sandwich polymerization method [22,23]. In the
present study, the pyrimidine-type chiral compounds are employed
as a mesogen for the construction of CLC from nematic LC. Then,
electrochemical polymerization in the CLC containing a monomer,
electrolyte, and chiral inducer was carried out. The synthesis of the
pyrimidine-based chiral inducers, the preparation of CLC electrolyte
solution, its electrochemical polymerization, surface observation,
and measurements of optical properties are reported in this study.
2. Experimental
2.1. Synthesis of chiral inducers
Pyrimidine derivatives having chiral center in its terminal alkyl
group, 2-(2-fluoro-4-undecyloxy-phenyl)-5-[4-(1-methyl-hepty-
loxy)-phenyl]-pyrimidine with R or S configuration, abbreviated as
R- or S-UFP*, were synthesized (Scheme 1). 4-[2-(2-Fluoro-4-
undecyloxy-phenyl)-pyrimidin-5-yl]-phenol (UFPeOH) and (R)-
* Corresponding author.
E-mail address: gotoh@ims.tsukuba.ac.jp (H. Goto).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.04.053

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A. Wang et al. / Polymer 54 (2013) 3821e3827
Scheme 1. Synthesis of chiral inducers.
or (S)-octanol were coupled using diethyl azodicarboxylate (DEAD)
and triphenyl phosphine (TPP) in tetrahydrofuran (THF) solution
for 24 h in argon atmosphere at room temperature, according to the
Mitsunobu reaction. After the reaction, the solvent was evaporated.
The crude product was purified by column chromatography (silica
gel, CH2Cl2) followed by recrystallization from ethanol to afford the
target material as a white solid.
The absolute configuration of the chiral center is inverted
during this SN2 reaction, according to the Mitsunobu inversion.
The chemical structure of the inducers was confirmed by1H NMR
(Fig. 1). The proton at the chiral center showed a sextet signal at
around 5 ppm, confirming the chiral fraction was introduced into
the mesogen. 19F NMR of S-UPF showed a signal at 113 ppm
against tetrafluorotoluene as an internal standard, confirming that
this molecule contains a fluorine atom. The UVevis optical ab-
sorption spectroscopy confirmed that the inducers showed lmax at
texture of the CLC was observable depending on the surface contact
state to the substrate: rubbing process on the substrate allows
appearance of a fingerprint texture. Green colored Schlieren texture
typical of the SmC* phase was observed during the cooling process
(Fig. 3b). Both of them are chiral LC phases, indicating that the in-
ducers are chiral and have good compatibility with rigid nematic
molecules. Molecular orientation in the Ch* and SmC* is shown in
Fig. 3c, d. Both chiral LC phases have a helical axis, while the di-
rections of the molecular orientation are different. It should be
noted that the Ch* phase is not layered, while the SmC* has a
layered structure.
3.2. Helical direction of the inducers
Helical direction of the inducers was determined using a
miscibility test by observing boundaries between the inducer and
cholesteryl oleyl carbonate as a standard CLC material having an
anticlockwise helical direction. When both compared compounds
have opposite directions, the helicity could be unwound, resulting
in a Schlieren texture of the nematic phase. Therefore, the misci-
bility test between opposite helical directions of the sample and the
standard CLC materials showed a nematic phase boundary between
the two compounds.
(R)-UFP*(R-configuration at the chiral center) showed no
boundary with cholesteryl oleyl carbonate because of the anti-
clockwise helical direction. On the other hand, (S)-UFP*(S-config-
uration at the chiral center) showed a boundary against cholesteryl
oleyl carbonate because of its clockwise direction. These results
indicated that (R)-UFP* is a left-handed helical structure and (S)-
UFP* shows right-handed helicity, indicating that the chiral in-
ducers with opposite helical directions were successfully synthe-
sized by the Mitsunobu reaction.
303 nm due to
mesogen.
pep* transition of the aromatic rings of the
3. Results and discussion
3.1. Liquid crystallinity of the chiral inducers
The chiral inducers show thermotropic liquid crystallinity. Fig. 2
displays dynamic scanning calorimetry (DSC) curves of S-UFP*
during heating and cooling process (10 ꢀC/min). S-UFP* showed
two exothermic peaks corresponding to phase transition from
chiral crystal (Cr*) to cholesteric phase (Ch*) (Cr*eCh*) and Ch* to
isotropic phase (Iso*) (Ch*eIso*), while the cooling process dis-
played three endothermic troughs. These signals correspond to
phase transition of Iso*eCh*, Ch*esmectic C (Ch*eSmC*), and
SmC*eCr*. In the temperature range of the Ch* phase, typical
Grandjean texture of CLC was observed (Fig. 3a). Also, a fingerprint
Fig. 1. 1H NMR analysis result of R-UFP*.

A. Wang et al. / Polymer 54 (2013) 3821e3827
3823
into the Cano-wedge cell. The helical pitch of the cholesteric LC was
visually observed under the polarizing optical microscope by
measuring Grandjean lines (Fig. 4), and the helical twisting powers
(b
) were calculated by the following equation:
b
¼ ðp$cÞꢁ1
Ch*
The line width according to this equation corresponds to the
Heating
Iso*
helical half-pitch. The
b values were 0.48 for (R)-UFP* and 0.47 for
(S) -UFP* (p ¼ helical pitch, c ¼ conc), respectively.
3.2.2. Electrochemical polymerization in CLC
Electrolyte solution containing 6CB, the chiral inducer, monomer
(BEDOT-Fu (EDOT-fluorene-EDOT), EDOT
Iso*
¼
ethylenedioxythio-
Ch*
60
Cooling
SmC*
phene), and tetrabutylammonium perchlorate (TBAP) was injected
between two transparent ITO coated glass substrates sandwiched by
a 0.20 mm-thick Teflon sheet. The molecular structure of the
monomer (BEDOT-Fu) is indicated as follows. The synthesis of the
BEDOT-Fu was carried out according to the previously reported
method [24]. The electrolyte solution was heated up to w80 ꢀC to
completely dissolve the supporting salt, monomer, andchiral inducer
in the 6CB, and cooled down to room temperature (21 ꢀC). The in-
jection process produces orientation of LC in the cell. This heating
process after completion of the LC injection before polymerization
allows homogenous distribution of the LC fingerprint texture over
the entire sample in the cell. The polarizing optical microscopy image
confirmed the cholesteric phase of the electrolyte solution at room
temperature.
40
80
100
120
Temperature /oC
Fig. 2. Differential scanning calorimetry (DSC) curves of S-UFP* (scan rate ¼ 10 ꢀC/min).
3.2.1. Helical twisting power
Helical twisting power of the chiral inducer was estimated by
the Cano-wedge method. First, the chiral inducers were mixed with
nematic LC (n-hexyl cyanobiphenyl, 6CB). The mixture was injected
Fig. 3. (a,b): Polarizing optical microscopy images of S-UFP*. (a): Oily streaks texture of cholesteric phase (Ch*) at 83 ꢀC. (b) Schlieren texture of chiral smectic C phase (SmC*) at
61 ꢀC. (c,d): Molecular orientation in liquid crystal phase. (c): Ch* phase. (d): SmC* phase.

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A. Wang et al. / Polymer 54 (2013) 3821e3827
orientation of the CLC from the Grandjean structure (i.e., homo-
geneous orientation, where helical axis are normal to the substrate)
to the fingerprint structure (i.e., homeotropic orientation, where
helical axis is parallel to the substrate) because the electric field
induced the re-orientation. In this case, the helical structure of the
CLC was maintained even after application of the low electric field
(20 V/mm) while electrochemical polymerization of the monomer
took place. The color of the electrolyte solution changed to dark
blue during the electrochemical process. The polymer film thus
obtained was washed with hexane and tetrahydrofuran (THF) to
yield a dark blue polymer film deposited on an ITO glass substrate.
3.3. Surface structure
The surface structure of the polymers was observed with
polarizing differential interference contrast optical microscopy (P-
DIM) and scanning electrons microscopy (SEM) shown in Fig. 5. The
polymer showed the fingerprint structure, which resembled the
Fig. 4. Polarizing optical microscopy images of the chiral inducer S-UFP* in Cano’s
wedge cell.
O
O
O
O
S
S
BEDOT-Fu
Then, DC voltage (4 V) was applied across the sandwiched cell
for 30 min in order to obtain an insoluble and infusible polymer
film on the anode. The application of the electric field changed the
Fig. 5. Polarizing differential interference contrast optical microscopy (P-DIM) and
scanning electron microscopy (SEM) images of the polymer prepared by electro-
chemical method in cholesteric electrolyte solution induced by S-UFP*.
Fig. 6. Electron probe micro analyzer (EPMA) measurements results for polymer. (a)
Scanning electron microscopy (SEM) image. (b) Distribution of sulfur atom image. (c)
Distribution of carbon atom image.

A. Wang et al. / Polymer 54 (2013) 3821e3827
3825
optical texture of the CLC electrolyte solution. This suggested that
the polymer grew epitaxially from the ITO substrate (anode) in the
CLC with imprinting of the aggregation form of the CLC during the
polymerization process.
3.4. EPMA
Electron probe micro-analysis (EPMA) of the polymer was
conducted to confirm deposition of the polymer film on the ITO.
The EPMA results indicated that the polymer consisted of S and C
atoms (Fig. 6).
3.5. Electrochemical properties
The electrochemical measurements of the polymer film were
carried out by cyclic voltammetry. The polymer film was examined
at scan rates of 10, 20, 60, 80, 100 mV/s in a monomer-free 0.1 M
TBAP/acetonitrile solution. The polymer showed signals at 0.33 V
and 0.75 V in the oxidation process, and 0.66 V and 0.23 V in the
reduction process, respectively, as shown in Fig. 7. This result sug-
gested that an electrochemical two-electron reaction occurred
during the redox process. The good electrochemical switching
indicated that the polymer adhered well to the electrode.
Fig. 8. Spectroelectrochemistry of optical absorption spectra of the film prepared in
the CLC electrolyte solution containing S-UPF*as a chiral inducer.
band. Note that the macroscopic fingerprint texture has no relation
to electronic transition of the polymer.
3.7. ORD
Asymmetric materials show optical rotation. The plane of the
polarization of linearly polarized light is rotated in chiral media.
This is one of the most interesting physical properties of chiropti-
cally active thin films. Fig. 9 shows optical rotatory dispersion
(ORD) for the polymer film at 0.3 V vs. Ag/Agþ. This result indicated
that the polymer thus prepared in the CLC was optically active even
though the polymer had no chiral centers. This can be due to the
fact that the polymer forms a helical structure with structural
chirality produced by the electrochemical polymerization in the
CLC. The helical structure of the polymer is similar to the helical
aggregation structure of the CLC. The polymer imprints the 3-D
helical structure of the CLC electrolyte solution in the polymeriza-
tion process. Therefore, the polymer displayed liquid crystal-like
helical aggregation high-ordered structure. The CLC electrolyte
solution separated from the polymer during a phase separation
process [25], yielding pure polymer film with chiroptical activity.
3.6. UVevis
UVevis optical absorption spectra showed characteristic ab-
sorption bands of pep* transition at short wavelengths (470 nm),
as shown in Fig. 8. The absorption band at long wavelengths
(w570 nm) can be due to generation of polarons (radical cations) in
the electrochemical oxidation process. The absorption band at
470 nm decreased accompanied by the appearance of the doping
3.8. Repeating redox properties
The electro-switching behavior of polymer was examined
through observation of changes in the ORD spectra upon repeated
Fig. 7. CV measurements of the film prepared in CLC electrolyte containing S-UFP* at
Fig. 9. ORD spectra of the film prepared from CLC electrolyte electrolyte containing
several scan rates.
S-UPF*as a chiral inducer.

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A. Wang et al. / Polymer 54 (2013) 3821e3827
changes in applied voltage (Fig. 10). Electrochemical repeating
voltage scans displayed a repeatable change in optical rotation of
the polymer. This result confirmed that the polymer had chiroptical
properties. Furthermore, the chiroptical activity could be repeat-
edly controlled through application of the external electric field.
3.9. Laser diffraction
The polymer showed a diffraction pattern upon irradiation of
visible light due to periodic structure of the polymer produced by
transcription of structural chiral order of the CLC electrolyte solu-
tion. Fig. 11 shows laser diffraction pattern obtained upon the
irradiation of red laser light. The diffraction occurred with reflected
light and transmitted light (Fig. 11a). Concentric circle pattern
involved the first-order and second-order diffraction with fine
several multiplex circles. The first order (m ¼ 1) and second order
(m ¼ 2) diffractions were due to the periodic pattern of the
fingerprint texture (Fig. 11b). The fingerprint lines correspond to
helical half-pitch of the helical aggregation structure derived from
the CLC electrolyte solution. Fine concentric lines may come from
occurrence of multiplex diffraction by the periodicity of fingerprint
lines (inter-main chain periodicity) or diffraction of intra-main
chain periodicity. The intermolecular helical structure of the poly-
mer provided clear first-order and second-order diffraction, and
the periodicity of the intra-helical structure (twisting of individual
main chain) may also function as a diffraction grating for producing
multiplex circular diffraction pattern. Furthermore, the fine strips
may be derived from stimulated emission produced by periodic
structure of cholesteric order of the polymer.
Fig. 10. Repeating changes in ellipticity of polymer prepared in CLC with R-UPF*.
(a)
(b)
Reflection
Transmission
with diffraction
with diffraction
Incidence of
LASER
These results demonstrate that the polymer film prepared in
this study had diffraction properties due to the periodic dielectric
microstructure.
4. Conclusions
Pyrimidine mesogen core-based chiroptically active chiral in-
ducers were synthesized for the production of a CLC electrolyte
solution. The electrochemical polymerization afforded chiroptically
active polymer films. This result demonstrated that the chirality of
the chiral inducer is transferred to resultant polymer via induction
of a cholesteric phase of a matrix LC. In other words, chiral
amplification via induction of cholesteric phase with structural
chirality occurred in the electrochemical polymerization. The
highly ordered microstructure was produced by transcription from
the three-dimensional helical continuum of the CLC environment.
Performance of the chiral induction depends on molecular
shape, affinity, and chirality of the chiral inducers. The develop-
ment of new chiral inducers opens further possibilities for chiral LC
materials.
5. Techniques
The UVevis absorption spectra were obtained using a Hitachi
U-2000 spectrophotometer, and circular dichroism and optical
rotation measurements were performed using a Jasco J-720 spec-
trometer with an ORDE-307W ORD unit.
6. Experimental
Optical texture observations were carried out using
a
Nikon ECLIPS LV-100 high resolution polarizing microscope. The
morphology of the polymer film was studied by scanning electron
microscopy (SEM) using Hitachi S-5500 microscope. Optical ab-
sorption spectra of the polymers were measured using a Jasco V-640
spectrometer equipped with a quartz cell. ORD measurements were
Fig. 11. Laser diffraction patterns of polymer prepared in CLC with S-UPF*. (a) Sche-
matic illustration of laser diffraction. (b) Diffraction and stimulated emission like
interference pattern upon incident of red laser.

A. Wang et al. / Polymer 54 (2013) 3821e3827
3827
performed with a Jasco J-720 spectrometer equipped with ORD unit.
Cyclic voltammetry (CV) was measured with Auto Lab III poten-
Acknowledgments
m
tiostat (the Netherlands) at various scan rates between ꢁ1.0 V
and 1.0 V vs. Ag/Agþ reference electrode. Custom-made function
generator and temperature control stage were used for electro-
chemical polymerization.
This research was supported by Japan Society for the Promotion
of Science (JSPS), Grant-in-Aid for Scientific Research, 22550161.
The pyrimidine based LC precursor materials were generously
donated by Midori Chemical (Midori Kagaku), Japan.
7. Chemicals
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loxy)-phenyl]-pyrimidine (R-UFP*)
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