Tuning helical twisting power of isosorbide-based chiral dopants by chemical modifications

Article abstract of DOI:10.1080/15421406.2010.526523

Isosorbide-based chiral dopants (ICD) with various substituent groups were newly synthesized to control their helical twisting powers (HTP). Phase transition behaviors of ICD molecules were first investigated by combined techniques of differential scanning calorimetry, wide-angle X-ray diffraction, and cross-polarized optical microscopy. ICD with n-hexyloxy end groups formed the multiple ordered phases, and those with methoxy or acetoxy end groups exhibited a simple crystal-to-isotropic transition. Energy-minimized chemical conformations of ICD molecules revealed that all the ICDs had twisted conformations and that the extension of the benzoyl ester moiety induced a higher twisted conformation. By varying substitution groups, HTPs of ICDs were controlled from 26.6 to 80.μm^-1. Particularly, ICD with an acetoxy end group (ICD-2) showed the largest HTP. It was also realized that by controlling the content of ICD-2 from 3.0 to 4.5mol%, the helical pitch length of cholesteric LC mixture was adjusted to reflect a specific visible light. Copyright Taylor & Francis Group, LLC.

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Tuning Helical Twisting Power of
Isosorbide-Based Chiral Dopants by
Chemical Modifications
Seunghan Shin a , Minsu Park a , Jin Ku Cho a , Jaeryung Char b ,
Myoungseon Gong b & Kwang-Un Jeong c
a Cheonan R&D Center, Korea Institute of Industrial Technology
(KITECH) , Cheonan, Chungnam, South Korea
b Department of Chemistry , Dankook University , Cheonan, South
Korea
c Department of Polymer-Nano Science and Technology , Chonbuk
National University , Jeonju, South Korea
Published online: 18 Jan 2011.
To cite this article: Seunghan Shin , Minsu Park , Jin Ku Cho , Jaeryung Char , Myoungseon
Gong & Kwang-Un Jeong (2011) Tuning Helical Twisting Power of Isosorbide-Based Chiral
Dopants by Chemical Modifications, Molecular Crystals and Liquid Crystals, 534:1, 19-31, DOI:
10.1080/15421406.2010.526523
To link to this article: http://dx.doi.org/10.1080/15421406.2010.526523
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Mol. Cryst. Liq. Cryst., Vol. 534: pp. 19–31, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.526523
Tuning Helical Twisting Power of Isosorbide-
Based Chiral Dopants by Chemical Modifications
SEUNGHAN SHIN,1 MINSU PARK,1 JIN KU CHO,1
JAERYUNG CHAR,2 MYOUNGSEON GONG,2 AND
KWANG-UN JEONG3
1Cheonan R&D Center, Korea Institute of Industrial Technology
(KITECH), Cheonan, Chungnam, South Korea
2Department of Chemistry, Dankook University, Cheonan, South Korea
3Department of Polymer-Nano Science and Technology, Chonbuk
National University, Jeonju, South Korea
Isosorbide-based chiral dopants (ICD) with various substituent groups were newly
synthesized to control their helical twisting powers (HTP). Phase transition beha-
viors of ICD molecules were first investigated by combined techniques of differential
scanning calorimetry, wide-angle X-ray diffraction, and cross-polarized optical
microscopy. ICD with n-hexyloxy end groups formed the multiple ordered phases,
and those with methoxy or acetoxy end groups exhibited a simple crystal-to-
isotropic transition. Energy-minimized chemical conformations of ICD molecules
revealed that all the ICDs had twisted conformations and that the extension of
the benzoyl ester moiety induced a higher twisted conformation. By varying substitu-
tion groups, HTPs of ICDs were controlled from 26.6 to 80.2 lmꢀ1. Particularly,
ICD with an acetoxy end group (ICD-2) showed the largest HTP. It was also
realized that by controlling the content of ICD-2 from 3.0 to 4.5 mol%, the helical
pitch length of cholesteric LC mixture was adjusted to reflect a specific visible light.
Keywords Chiral dopants; cholesteric liquid crystal; helical twisting power;
isosorbide
Introduction
Cholesteric liquid crystals (CLCs) are optically active variants of nematic (N) LC
compounds, which are also called the chiral nematic (Nꢁ) LCs. The cholesteric phase
is readily developed by the addition of a chiral compound as a dopant into the
achiral N LC host. By incorporating a chiral moiety to LC molecules, LC twists per-
pendicular to its director and forms the Nꢁ phase. If the helical pitch, the distance
over which the LC molecules undergo a full 360ꢂ twist, is the same order of magni-
tude as the wavelength of visible light, CLCs can reflect a corresponding color [1,2].
Address correspondence to Seunghan Shin, Cheonan R&D Center, Korea Institute of
Industrial Technology (KITECH), 35-3, Hongcheon-ri, Ipjang-Myeon, Cheonan-City,
Chungnam 330-825, South Korea. E-mail: shshin@kitech.re.kr
19

20
S. Shin et al.
Therefore, they can be used in LCDs and other related applications such as reflect-
able polarizes, color filters, and chiral polymer films to improve viewing angles [3–7].
Because the cholesteric phase can be induced simply by adding a chiral dopant
into an N LC host, the helical pitch length of the CLC can be adjusted by the con-
centration of chiral dopant and by the helical twisting power (HTP) change of the
chiral dopant itself [8]. However, an excess amount of chiral dopant can decrease
the isotropic (I)-to-N phase transition temperature and a highly viscous LC mixture
and undesirable precipitation may occur because the solubility chiral dopant in the
LC host is limited [9–11]. Therefore, it is essential that a high degree of twist should
be obtained from a limited amount of chiral dopant to avoid perturbing the original
properties of LC host materials. There have been many efforts to develop a chiral
dopant with a high HTP and a good solubility and without any changes of the prop-
erties of host materials [12,13]. According to the chemical structures, the chiral
dopants can be classified to three different types [14]. The first type of chiral dopant
contains chiral centers at its alkyl chain, which makes it possible to introduce two or
more chiral centers to a single molecule. Secondly, a chiral center can be located in
the flexible spacer chain between two mesogen core units to form the structure like a
dimer or a twinned molecule. Finally, a chiral center can be incorporated in the
mesogen core unit. This final type of chiral dopant shows the greatest coupling effect
between the chiral center and core, which results in a high HTP [14–16]. Isosorbide
(1,4:3,6-dianhydro-D-sorbitol) is the most well-known chiral compound for the third
type of chiral dopants. Generally, isosorbide-based chiral dopants (ICD) are able to
generate large HTPs, however, their poor solubility in LC hosts is a drawback for
their application as chiral dopants [17–19]. In this articles, we newly designed and
synthesized a series of ICDs with various substituent groups into the 2-OH and
5-OH positions of isosorbide. Amplification of HTPs and phase transition behaviors
of ICDs were also investigated.
Experiments
Materials
1,4:3,6-Dianhydro-D-sorbitol (D-isosorbide, 98%, Aldrich) was used as a core mate-
rial. Hydroxybenzoic acid (99%, Aldrich, Milwaukee, WI, USA), acetic anhydride
(99%, Aldrich, Milwaukee, WI, USA), thionyl chloride (99%, Aldrich, Milwaukee,
WI, USA), 4-methoxy benzoylchloride (99%, Aldrich), and 4-(hexyloxy)
benzoic acid (98%, Aldrich, Milwaukee, WI, USA) were used for introducing differ-
ent substituent groups. Tetrahydrofuran (THF), pyridine, toluene, and dichloro-
methane (DCM) were used as solvents. All reagents and solvents were reagent
grade and were used without further purification. LC242 (BASF, Ludwigshafen,
Germany) was used as a host LC for preparing a CLC film.
Synthetic Procedure of Chiral Dopants (ICD)
1,4:3,6-Dianhydro-D-sorbitol-2,5-bis(4-(4-methoxybenzoyloxy) Benzoate) (ICD-1).
4-Hydroxybenzoic acid (2.72 g; 19.7 mmol) and acetic anhydride (5.10 g; 50 mmol)
were dissolved in THF with pyridine. The reaction mixture was stirred at room
temperature for 24 h. After reaction, pure water and 1 N HCl were added to form
a precipitate. The resulting suspension was filtered and the resulting solid was

Isosorbide-Based Chiral Dopants
21
recrystallized with deionized water and ethanol to produce acetoxybenzoic acid (1) in
80% yield. Prepared 1 (10 g; 55.51 mmol) and thionyl chloride (60 mL; 0.83 mol) with
small drops of toluene were refluxed. After 3 h, thionyl chloride was distilled under
reduced pressure and the remaining thionyl chloride was completely removed by
azeotropic distillation with toluene to afford the desired compound 2 in 75% yield.
Then, 2 (11.02 g; 55.49 mmol) was coupled with D-isosorbide (3.86 g; 26.41 mmol)
in THF with pyridine. After stirring at room temperature for 10 h, the resulting
solid was reprecipitated with deionized water and methanol in 85% yield. Finally
this compound (10 g; 21.26 mmol) was dissolved in ethanol and stirred for 2 h with
sodium hydroxide solution (1.7 g; 42.5 mmol). After a dilute HCl solution was
added, compound 3 was obtained in 87% yield by recrystallization of the crude
product with deionized water and methanol. Compound 3 (5 g; 12.9 mmol) and
4-methoxy benzoylchloride (4.84 g; 28.28 mmol) were dissolved in THF with
pyridine (2.46 g; 31 mmol) and stirred at room temperature for 12 h. The resulting
solid was reprecipitated with deionized water and methanol to give ICD-1 in 70%
1
yield. H NMR (600 MHz, CDCl3): d 8.18–8.15 (m, 6H), 8.11 (d, J ¼ 8.5 Hz, 2H),
7.32 (dd, J1 ¼ 11.4 Hz, J2 ¼ 8.7 Hz, 4H), 7.01 (dd, J1 ¼ 8.8 Hz, J2 ¼ 2.3 Hz, 4H),
5.52 (s, 1H), 5.45 (q, J ¼ 5.3 Hz, 1H), 5.09 (t, J ¼ 5.0 Hz, 1H), 4.71 (d, J ¼ 4.7 Hz,
1H), 4.14 (s, 2H), 4.12–4.06 (m, 2H), 2.36 (s, 6H); Anal. Calc. for C36H30O12:
654.2; Found: ESI-MS, m=z, positive [M þ H]þ: 665.
1,4:3,6-Dianhydro-D-sorbitol-2,5-bis(4-(4-acetoxybenzoyloxy) benzoate) (ICD-2).
Compound 3 (5 g; 12.9 mmol) and 4-(chlorocarbonyl)phenyl acetate (5.46 g;
27.49 mmol) were dissolved in THF with pyridine. The reaction mixture was
stirred at room temperature for 12 h. The resulting crude product was purified by
1
recrystallization with deionized water and ethanol to give ICD-2 in 70% yield. H
NMR (600 MHz, CDCl3): d 8.25 (dd, J1 ¼ 8.6 Hz, J2 ¼ 5.4 Hz, 4H), 8.19 (d,
J ¼ 8.6 Hz, 2H), 8.12 (d, J ¼ 8.6 Hz, 2H), 7.33 (dd, J1 ¼ 11.6 Hz, J2 ¼ 8.7 Hz, 4H),
7.28 (d, J ¼ 2.2 Hz, 2H), 5.52 (s, 1H), 5.46 (q, J ¼ 5.3 Hz, 1H), 5.09 (t, J ¼ 5.0 Hz,
1H), 4.71 (d, J ¼ 4.6 Hz, 1H), 4.14 (s, 2H), 4.12–4.06 (m, 2H), 3.91 (s, 6H); Anal.
Calc.for C38H30O14: 710.2; Found: ESI-MS, m=z, positive [M þ H]þ: 711.
1,4:3,6-Dianhydro-D-sorbitol-2,5-bis(4-(4-(Hexyloxy)benzoyloxy) benzoate) (ICD-3).
4-(Hexyloxy)benzoyl chloride was prepared in advance of ICD-3 synthesis.
4-(Hexyloxy) benzoic acid (5 g; 225 mmol) and thionyl chloride (50 mL; 0.69 mol)
with small drops of toluene were refluxed. After 3 h, thionyl chloride was distilled
under reduced pressure and the remaining thionyl chloride was completely
removed by azeotropic distillation with toluene. Compound 3 (5 g; 12.9 mmol) and
pyridine (2.46 g; 31 mmol) were dissolved in THF and then 4-(hexyloxy)benzoyl
chloride (6.83 g; 28.38 mmol) was added drop wise. The reaction mixture was
stirred at room temperature for 12 h. The resulting crude product was purified by
1
recrystallization with deionized water and ethanol to give ICD-3 in 70% yield. H
NMR (600 MHz, CDCl3): d 8.18–8.14 (m, 6H), 8.11 (d, J ¼ 8.7 Hz, 2H), 7.32 (dd,
J1 ¼ 11.5 Hz, J2 ¼ 8.8 Hz, 4H), 6.99 (dd, J1 ¼ 9.0 Hz, J2 ¼ 2.5 Hz, 4H), 5.52 (s, 1H),
5.45 (q, J ¼ 5.2 Hz, 1H), 5.09 (t, J ¼ 5.0 Hz, 1H), 4.71 (d, J ¼ 4.7 Hz, 1H),
4.14–4.05 (m, 8H), 1.84 (p, J ¼ 6.8 Hz, 4H), 1.49 (p, J ¼ 7.0 Hz, 4H), 1.37–1–36
(m, 8H), 0.94–0.92 (m, 6H); Anal. Calc. for C46H50O12: 794.3; Found: ESI-MS,
m=z, positive [M þ H]þ: 794.

22
S. Shin et al.
1,4:3,6-Dianhydro-D-sorbitol-2,5-bis(4-acetoxybenzoate) (ICD-4). D-Isosorbide (5 g;
34.2 mmol) and pyridine (5.95 g; 75.24 mmol) were dissolved in THF.
4-Chlorocarbonylphenyl acetate (14.3 g; 71.82 mmol) was added drop wise to the
reaction mixture and stirred at room temperature for 12 h. The resulting crude
product was purified by recrystallization with methanol to give ICD-4 in 70%
1
yield. H NMR (600 MHz, CDCl3): d 8.12 (d, J ¼ 8.6 Hz, 2H), 8.06 (d, J ¼ 8.6 Hz,
2H), 7.19 (dd, J1 ¼ 11.8 Hz, J2 ¼ 8.6 Hz, 4H), 5.49 (s, 1H), 5.43 (q, J ¼ 5.3 Hz, 1H),
5.06 (t, J ¼ 5.0 Hz, 1H), 4.67 (d, J ¼ 4.7 Hz, 1H), 4.10–4.03 (m, 4H), 2.33 (d,
J ¼ 16.6 Hz, 6H); Anal. Calc. for C24H22O10: 470.1; Found: ESI-MS, m=z, positive
[M þ H]þ: 471.
1,4:3,6-Dianhydro-D-sorbitol-2,5-bis(4-hexyloxybenzoate) (ICD-5). D-Isosorbide (5 g;
34.2 mmol) and pyridine (5.95 g; 75.24 mmol) were dissolved in THF. 4-
Hexyloxybenzoly chloride (17.3 g; 71.82 mmol) was added drop wise to the
reaction mixture and stirred at room temperature for 12 h. The resulting crude
product was purified by recrystallization with methanol to give ICD-5 in 70%
1
yield. H NMR (600 MHz, CDCl3): d 8.03 (d, J ¼ 8.8 Hz, 2H), 7.96 (d, J ¼ 8.8 Hz,
2H), 6.91 (dd, J1 ¼ 13.4 Hz, J2 ¼ 8.8 Hz, 4H), 5.46 (d, J ¼ 3.0 Hz, 1H), 5.40 (q,
J ¼ 5.3 Hz, 1H), 5.04 (t, J ¼ 5.0 Hz, 1H), 4.68 (d, J ¼ 4.6 Hz, 1H), 4.14–4.07 (m,
3H), 4.03–4.00 (m, 5H), 1.83–1.78 (m, 4H), 1.47–1.46 (m, 4H), 1.36–1.35 (m, 8H),
0.93–0.91 (m, 6H); Anal. Calc. for C32H42O8: 554.3; Found: ESI-MS, m=z,
positive [M þ H]þ: 555.
Fabrication of CLC Cells
Glass substrates were cleaned with a display-grade detergent solution. They were
rinsed with acetone and isopropyl alcohol using an ultrasonic cleaner for 15 and
20 min, respectively. LC alignment layer (Nissan SE6414, Tokyo, Japan) was intro-
duced on the glass substrate by spin-coating and was thermally cured at 225ꢂC. Its
surface was rubbed with a velvet cloth. The CLC solution including LC242, chiral
dopant, photoinitiator (Irgacure 907, Ciba, Basel, Switzerland), and solvent was
spin-coated on the alignment layer at 450 rpm for 20 s and then at 2,800 rpm for
40 s. A stable CLC film was obtained by ultraviolet (UV) curing after solvent eva-
poration. UV curing was performed at an irradiation intensity of 20 mW=cm2 for 60.
Measurements
All newly synthesized compounds were identified by nuclear magnetic resonance
1
(NMR) and mass spectrometers. The H-NMR spectra were measured using a
Bruker-600 (MHz) spectrometer with a tetramethylsilane (TMS), peak as a refer-
ence. Mass spectra were recorded on a JEOL JMS-600 W spectrometer. The
thermal transition of products was investigated with a Shimadzu D-60 differen-
tial scanning calorimeter (DSC) at a heating and cooling rate of 5 K minꢀ1. The
phase transitions were examined by a Nikon 100POL polarized optical micro-
scope (POM) equipped with Mettler Toledo hot-stage FP-82 heating and cooling
stage with temperature controller. Wide-angle X-ray diffraction analyses were
performed with a Bruker GADDS instrument with 2 theta range of 0–40 and
exposure time of 1,200 s.

Isosorbide-Based Chiral Dopants
23
Results and Discussion
Preparation of Chiral Dopants
Five isosorbide-based chiral dopants (ICD-1, ICD-2, ICD-3, ICD-4, ICD-5) were
designed and synthesized. Isosorbide itself and isosorbide derivative (3) extended
with a 4-hydroxybenzoyl group at the 2-OH and 5-OH positions of isosorbide were
used as chiral mesogen core units. In order to prepare the isosorbide derivative (3),
1,4:3,6-dianhydro-D-sorbitol-2,5-bis(4-acetoxybenzoate) (2) was synthesized from
4-hydroxybenzoic acid via a two-step reaction including a protection reaction of
4-hydroxy group with acetyl group and a acid chloride formation reaction, followed
by coupling with isosorbide to afford compound 3 (Scheme 1a). The synthetic isosor-
bide derivative (3) was attached three different substituent groups corresponding to
4-methoxybenzoyl, 4-acetylbenzoyl, and 4-hexyloxybenzoyl groups to obtain ICD-1,
Scheme 1. Synthetic routes for (a) 1,4:3,6-dianhydro-D-sorbitol-2,5-bis(4-hydroxybenzoate)
(3) and (b) chiral dopants carrying different substituent groups: (i) acetic anhydride, pyridine,
THF, room temperature, 24 h, 80%; (ii) SOCl2, toluene, reflux, 3 h, 75%; (iii) pyridine, THF,
room temperature, 10 h, 85%; (iv) NaOH, ethanol, room temperature, 2 h, 87%; (v) pyridine,
THF, room temperature, 12 h.

24
S. Shin et al.
ICD-2, and ICD-3, respectively. ICD-4 and ICD-5 were synthesized by the direct
attachment of 4-acetylbenzoyl and 4-hexyloxybenzoyl groups onto isosorbide
(Scheme 1b).
Phase Transition of Chiral Dopants
Phase transition behaviors of synthetic chiral dopants were characterized with DSC
and POM equipped with a heat processor. DSC thermograms were obtained during
the second heating and cooling cycles (Fig. 1). They showed that only ICD-3 showed
multiple transition peaks in DSC thermograms. Their phase transition temperatures
were as follows: Cr ! 103ꢂC ! LC1ꢁ ! 110ꢂC ! LC2ꢁ ! 146ꢂC ! I and I !
137ꢂC ! LC2ꢁ ! 107ꢂC ! LC1ꢁ ! 96ꢂC ! Cr in the heating and cooling pro-
cesses, respectively (Fig. 1c). In this notation, Cr, LCꢁ, and I stand for crystal, liquid
crystal, and isotropic phases, respectively. POM photographs of ICD-3 at 110ꢂC
during the cooling process showed that ICD-3 seemed to have a mosaic texture of
Figure 1. Differential scanning calorimetric (DSC) heating (upper) and cooling (bottom) scans
of (a) ICD-1, (b) ICD-2, (c) ICD-3, (d) ICD-4, and (e) ICD-5 at 5ꢂC=min heating rate. A DSC
thermogram of ICD-4 was obtained in the first cooling and second heating process.

Isosorbide-Based Chiral Dopants
25
soft crystal (Fig. 2c). In order to confirm the existence of LC phases of ICD-3, XRD
analysis was performed. XRD spectra of ICD-3 showed that very strong diffraction
peaks at 3.4ꢂ, 4.7ꢂ and 7.5ꢂ were observed during the cooling process. Especially, the
peaks at 3.4ꢂ and 19.4ꢂ increased at 110ꢂC and then decreased with cooling (Fig. 3).
Based on the experimental results of DSC and POM combined with those of XRD, it
was realized that the LC phase in ICD-3 is not a simple low-ordered LC phase, such
as N or smectic A but a highly ordered LC phases. This means that there are no
long-range positional orders in three dimensions like a true crystal. This argument
Figure 2. POM microphotographs of (a) ICD-1 taken at 80ꢂC in the cooling process, (b)
ICD-2 taken at 90ꢂC in the cooling process, (c) ICD-3 taken at 110ꢂC in the cooling process,
(d) ICD-4 taken at 55ꢂC in the heating process, (e) ICD-5 taken at 65ꢂC in the cooling process.
Magnification of all POM microphotographs is 100ꢃ.

26
S. Shin et al.
Figure 3. XRD patterns of ICD-3 measured at 140ꢂC, 110ꢂC, and 80ꢂC in the cooling process.
was also supported by DSC experiments with different cooling and subsequent
heating rates. The transition temperature and associated enthalpy change, measured
by DSC, between I and LC2ꢁ and those between LC2ꢁ and LC1ꢁ were not highly
dependent on the cooling rate (Fig. 4). This supports that ICD-3 had highly ordered
LC phases. In contrast, ICD-1, ICD-2, ICD-4, and ICD-5 showed a simple Cr-to-I
transition. ICD-1 showed phase transitions as follows: Cr ! 138ꢂC ! I in the heat-
ing process and I ! 104ꢂC ! Cr in cooling process (Fig. 1a). POM texture of ICD-1
taken at 80ꢂC in the first cooling process suggested that ICD-1 had a commonly
observed crystalline phase (Fig. 2a) [20]. ICD-2 had Cr ! 157ꢂC ! I and I !
125ꢂC ! Cr transitions in the heating and cooling processes, respectively (Fig. 1b).
The POM photographic image of ICD-2 taken at 90ꢂC in the cooling process showed
a mosaic phase pattern, which was commonly observed as a soft crystal phase
(Fig. 2b). The phase transition temperatures of ICD-5 were as follows: Cr ! 89ꢂC
! I and I ! 74ꢂC ! Cr in the heating and cooling process, respectively (Fig. 1e).
The POM photograph of ICD-5 showed needle-like structures growing from the I
melt when it was cooled to lower than 70ꢂC (Fig. 2e). In case of ICD-4, it showed
a Cr ! 121ꢂC ! I and no thermal transition in the cooling process (Fig. 1d). Crys-
tallization transition of ICD-4 was not observed in our experimental condition.
However, it showed a melting transition in the following heating process with a stay
at room temperature. This indicates that ICD-4 was very slowly crystallized; there-
fore, an annealing process was required to obtain the crystalline phase. The forma-
tion of crystal of ICD-4 was confirmed by the POM morphology at 55ꢂC, exhibiting
needle-like crystals (Fig. 2d).

Isosorbide-Based Chiral Dopants
27
Figure 4. DSC thermograms of ICD-3 obtained in the first cooling process with different
cooling rates: 2.5ꢂC =min, 5.0ꢂC=min, and 10.0ꢂC=min.
Determining HTP of Chiral Dopants
HTPs of chiral dopants were determined by a selective light reflection method that is
suitable for measuring HTP of a chiral dopant when the helical pitch of CLC film is
smaller than 1 mm [21, 22]. From the selective light reflection method, the helical
pitch of CLC film (p) is obtained by following equation
kmax ¼ n ꢃ p
ð1Þ
Figure 5. Helical pitch length of CLC films fabricated with chiral dopants as a function of
their concentration (ICD-1: square, ICD-2: circle, ICD-3: triangle up, ICD-4: triangle down,
ICD-5: diamond).

28
S. Shin et al.
where kmax is the maximum wavelength for the reflected light and n is the mean
refractive index of CLC film. The average refractive indices of CLC films fabricated
with each chiral dopant were measured with a prism coupler. The kmax of a reflected
light was measured by a UV-Vis spectrometer.
According to De Gennes and Prost [23], the pitch of CLC (p) is related to the
macroscopic HTP and the concentration of a chiral dopant (Xc) by
1=p ¼ HTP ꢃ Xc
ð2Þ
The inverse of pitch (1=p) was first plotted as a function of the mole fraction of
five synthetic chiral dopants (ICD-1, ICD-2, ICD-3, ICD-4, ICD-5). According to
Eq. (2), HTPs of synthetic chiral dopants were obtained from a slope of the linear
relationship between 1=p and Xc in the low concentration regime (Fig. 5). The mea-
sured HTPs are listed in Table 1. HTPs are variable from 26.6 to 80.2 mmꢀ1 depend-
ing on the substituent groups. ICD-1, ICD-2, and ICD-3 synthesized from extended
isosorbide showed relatively higher HTPs than ICD-4 and ICD-5 synthesized
directly from isosorbide. Comparing the HTPs of chiral dopants having the same tail
Table 1. Helical twisting powers (HTP) of synthetic chiral dopants
ID
Chemical structure
HTP (mmꢀ1
)
ICD-1
50.0
ICD-2
ICD-3
ICD-4
ICD-5
80.2
69.6
26.6
42.1

Isosorbide-Based Chiral Dopants
29
Figure 6. Molecular structures of (a) ICD-3 and (b) ICD-5 obtained by energy minimization
iteration.
parts (ICD-2 vs. ICD-4 or ICD-3 vs. ICD-5), it is clear that the extension of the
aromatic part increased the HTP. This seems to be due to the highly twisted confor-
mation by the additional benzoyl ester moieties, which could be preliminarily con-
firmed with molecular structures of ICD-3 and ICD-5 obtained by energy
minimization iteration (Fig. 6). This result suggests that more twisted chiral dopants
are preferable to get a higher HTP. Likewise, the tail parts of chiral dopants are also
able to affect the HTPs when they could twist themselves or cooperatively twist with
their neighbor molecules. ICD-1, ICD-2, and ICD-3 have the same core unit struc-
ture but their HTP values were 50.0 mmꢀ1 for ICD-1, 80.2 mmꢀ1 for ICD-2, and
69.9 mmꢀ1 for ICD-3. From these results, it was found that the acetoxy group is more
effective than the alkoxy group for transferring a twisting torque of an chiral
compound to an LC host molecule.
Interestingly, CLC film fabricated with ICD-2 was ca. 30% thicker than CLC
films fabricated with ICD-1 and ICD-2. When identical amounts (4 mol%) of
ICD-1, ICD-2, and ICD-3 were added under the same processing conditions, the
average film thicknesses measured by the prism coupler were 1.81 mm for ICD-1,
2.41 mm for ICD-2, and 1.87 mm for ICD-3. This result implies that ICD-2 induced
a stronger interaction with the host LC molecule than the other two chiral dopants.
In addition, hexyloxy-substituted ICD-3 showed a little bit larger HTP than that of
methoxy-substituted ICD-1. We believe that the relatively longer alkyl chain caused
a more twisted structure in the equilibrium state.
CLC Film Properties
The CLC films that can possess pitches comparable to the wavelength of visible light
were prepared by adding a low percentage of ICD-2. CLC films selectively reflecting
the blue color (kmax ¼ 462 nm) were fabricated by mixing 4.0 mol% of ICD-2 with a

30
S. Shin et al.
LC242. In contrast, 5.2 mol% of ICD-1 and 4.5 mol% of ICD-3 were required to
make CLC films whose kmax were 470 nm. All of the chiral dopants did not cause
a solubility problem in making CLC films within this amount.
ICD-4, however, induced a CLC film reflecting the green color (kmax ¼ 568 nm)
by adding 10 mol% to LC242. More than 10 mol% of ICD-4 was necessary to fabri-
cate a CLC film that reflects a blue light. When the amount of ICD-4 exceeded
10 mol%, ICD-4 began to be crystallized. This crystallization may be due to poor
solubility with LC242.
Conclusion
A series of isosorbide-based chiral dopants (ICD-n, n ¼ 1–5) was newly designed and
synthesized, and then CLC phases were obtained by mixing them with a host N LC
(LC242). Depending on their substituent groups, ICD-n showed various phase tran-
sition behaviors. ICD with a hexyloxy end group induced multiple phase transitions,
whereas ICD with a methoxy and acetoxy end groups exhibited a simple Cr-to-I
transition. From the evaluated HTP results combined with energy-minimized chemi-
cal conformations, it was realized that HTPs of ICDs were closely related to the sub-
stituent groups. When the substituent caused a higher twisted structure and a good
interaction with a host nematic LC molecule, HTP of ICDs became larger. In a series
of ICDs, ICD-2 wtih acetoxy as an end group showed the highest HTP value of
80.2 mmꢀ1, although it had a less twisted structure than ICD-3. This means that acet-
oxy is more effective than alkyloxy groups for transferring a twisting torque of a
chiral dopant to an LC host molecule. Furthermore, the helical pitches of the CLCs
were tuned to reflect a specific visible light by varying the content of ICD-2 from 3.0
to 4.5 mol%.
Acknowledgment
This research was supported by a grant from the Fundamental R&D Program for
Core Technology of Materials funded by the Ministry of Knowledge Economy,
Republic of Korea.
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