Bio-based chiral dopants having an isohexide skeleton for cholesteric liquid crystal materials

Article abstract of DOI:10.1039/c2gc16261j

Chiral dopants were synthesized from bio-based epimeric isohexides (glucose-derived isosorbide and mannose-derived isomannide) and their phase transition behaviors and abilities for developing cholesteric liquid crystal (CLC) films were examined with a consideration of the core structure. In spite of lower reactivity of the endo hydroxy group of isomannide caused by the steric hindrance and intermolecular hydrogen bonding, final synthetic yields of chiral dopants bearing an isomannide core (64.5% for IH-2 and 65.0% for IH-4) did not show conspicuous difference compared with chiral dopants bearing isosorbide (68.4% for IH-1 and 74.0% for IH-3). On the other hand, in phase transition behaviors, chiral dopants bearing an isomannide core showed lower crystalline and melting temperatures than IH-1, IH-3 despite of the same substituents. The helical twisting power (HTP) of chiral dopants bearing isosorbide (IH-1 and IH-3) was higher than that of chiral dopants bearing isomannide (IH-2 and IH-4). The calculated HTPs of IH-1 and IH-3 were 26.6 and 42.1 μm^-1, respectively. In the case of IH-3, the helical pitch length of CLC could be adjusted to reflect visible light by controlling its amount and showed best performance in the range 5.0 to 7.0 mol%. In contrast to IH-1 and IH-3, it was found that IH-2 and IH-4 could not induce CLC films that reflect visible light.

Full text of DOI:10.1039/c2gc16261j

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PAPER
Bio-based chiral dopants having an isohexide skeleton for cholesteric liquid
crystal materials†
Seunghan Shin,*a Jae Won Seo,a Jin Ku Cho,a Sangyong Kim,a Jaeryung Chab and Myoung Seon Gongb
Received 11th October 2011, Accepted 31st January 2012
DOI: 10.1039/c2gc16261j
Chiral dopants were synthesized from bio-based epimeric isohexides (glucose-derived isosorbide and
mannose-derived isomannide) and their phase transition behaviors and abilities for developing cholesteric
liquid crystal (CLC) films were examined with a consideration of the core structure. In spite of lower
reactivity of the endo hydroxy group of isomannide caused by the steric hindrance and intermolecular
hydrogen bonding, final synthetic yields of chiral dopants bearing an isomannide core (64.5% for IH-2
and 65.0% for IH-4) did not show conspicuous difference compared with chiral dopants bearing
isosorbide (68.4% for IH-1 and 74.0% for IH-3). On the other hand, in phase transition behaviors, chiral
dopants bearing an isomannide core showed lower crystalline and melting temperatures than IH-1, IH-3
despite of the same substituents. The helical twisting power (HTP) of chiral dopants bearing isosorbide
(IH-1 and IH-3) was higher than that of chiral dopants bearing isomannide (IH-2 and IH-4). The
calculated HTPs of IH-1 and IH-3 were 26.6 and 42.1 μm−1, respectively. In the case of IH-3, the helical
pitch length of CLC could be adjusted to reflect visible light by controlling its amount and showed best
performance in the range 5.0 to 7.0 mol%. In contrast to IH-1 and IH-3, it was found that IH-2 and IH-4
could not induce CLC films that reflect visible light.
attached to a mesogen core, which allows two or more chiral
centers to be introduced onto one molecule. Second, a chiral
1. Introduction
Cholesteric liquid crystals (CLCs) have many applications as
liquid crystal displays, reflective polarizers, color filters, and
chiral polymer films for improving viewing angle.1–5 This is
because they can selectively reflect light if their helical pitch
lengths are the same order of magnitude as the wavelength of the
light.6,7 Common methods for modulating the helical pitch
lengths of CLCs are changing the concentration of a chiral
dopant and the helical twisting power (HTP) of a chiral dopant
itself.8 However, an excess amount of chiral dopant can decrease
the isotropic (I)-to-nematic (N) phase transition temperature and
a highly viscous LC mixture and undesirable precipitation may
occur due to its low solubility in the LC host.9–11 Therefore,
development of a chiral dopant with a large HTP and a good
solubility, and without any changes of the properties of the host
LC has become a very important issue.12,13
center can be trapped in a flexible spacer chain between two
mesogen core units to form a structure like a dimer or a twinned
molecule. Third, a chiral center can be incorporated into the
mesogen core unit. According to the references, the chiral
dopant having a chiral center incorporated into a mesogen unit
is the most promising one among the three types of chiral
dopants.14 This is because they can produce a strong twisting
power by the coupling effect between chiral center and mesogen
core.
As a chiral core for the third type of chiral dopant, isohexides
(1,4 : 3,6-dianhydrohexitols) can be employed due to their chiral-
ity and intrinsic rigidity. In particular, in an effort to reduce
dependency on the petroleum-based chemical industry due to the
issues of oil depletion and global warming, isohexides have
recently attracted much attention because they are bio-based
chemicals derived from renewable and sustainable resources.15
In our previous study, we showed the feasibility that isosor-
bide (1,4 : 3,6-dianhydroglucitol) obtained from sorbitol, which
is formed by reduction of glucose, could be used as a core com-
pound of chiral dopants.16 In this paper, we report the bio-based
chiral dopants consisting of an isohexide (isosorbide or isoman-
nide) core and O-substituted 4-hydroxybenzoyl extension. Iso-
sorbide and isomannide are epimeric compounds differing only
in configuration at the C2 position. Particularly, they are prepared
from glucose and fructose in industry, respectively, via a
reduction and dehydration process. We synthesized several kinds
The chiral dopants are generally classified into three types.
First, the chiral center can be situated at a terminal alkyl chain
aGreen Process and Materials Research Group, Korea Institute of
Industrial Technology (KITECH), 35-3 Hongcheon-ri, Cheonan, 330-
825, Republic of Korea. E-mail: shshin@kitech.re.kr; Fax: +82 41-589-
8580
bDepartment of Nanobiomedical Science and WCU Research Center of
Nanobiomedical Science, Dankook University, 201 Manghyang-ro,
Cheonan, 330-714, Republic of Korea
†Electronic supplementary information (ESI) available: See DOI:
10.1039/c2gc16261j
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Synthetic procedure of 1,4 : 3,6-dianhydro-D-mannitol-2,5-bis(4-
acetoxybenzoate) (IH-2)
of isohexide-based chiral dopants (IHs) from isosorbide or iso-
mannide. Their phase transition behaviors, miscibility with host
LC, HTPs, and abilities for developing CLC films were exam-
ined with a consideration of the chiral core molecules.
D-Isomannide (5 g; 34.2 mmol) and pyridine (5.95 g;
75.24 mmol) were dissolved in THF. Compound 1 (14.3 g;
71.82 mmol) was added dropwise to the reaction mixture with
stirring at room temperature for 12 h. The usual work-up pro-
cedure as mentioned previously gave IH-2 in 64.5% yield.
2. Experiments
Materials
Synthetic procedure of 1,4 : 3,6-dianhydro-D-sorbitol-2,5-bis(4-
hexyloxybenzoate) (IH-3)
1,4 : 3,6-Dianhydro-D-sorbitol (D-isosorbide, 98%) and 1,4 : 3,6-
dianhydro-D-mannitol (D-isomannide, 98%) were used as chiral
core materials. 4-Acetoxybenzoic acid (95%, TCI, Tokyo,
Japan), acetic anhydride (99%), 4-(hexyloxy)benzoic acid
(98%), thionyl chloride (99%) and pyridine (99%) were used for
introducing substituents. All reagents except 4-acetoxybenzoic
acid were supplied by Sigma-Aldrich Milwaukee USA. Toluene,
tetrahydrofuran (THF), and methanol were used as solvents. All
reagents and solvents were reagent grade and were used without
further purification. LC242 (BASF SE, Ludwigshafen,
Germany) was used as achiral nematic host LC for preparing a
CLC film.
4-(Hexyloxy)benzoic acid (5 g; 225 mmol) was dissolved in
thionyl chloride (50 ml; 0.69 mol) with stirring at room tempera-
ture. After 3 h, thionyl chloride was removed using a rotary
evaporator and the remaining thionyl chloride was completely
removed by azeotropic distillation with toluene to afford
the desired compound 2 in 75% yield. D-Isosorbide (5 g;
34.2 mmol) and pyridine (5.95 g; 75.24 mmol) were dissolved
in THF. 4-(Hexyloxy)benzoylchloride (17.3 g; 71.82 mmol) was
added dropwise to the reaction mixture with stirring at room
temperature for 12 h. The usual work-up procedure as mentioned
previously gave IH-3 in 74.0% yield.
Measurements
Synthetic procedure of 1,4 : 3,6-dianhydro-D-mannitol-2,5-bis(4-
hexyloxybenzoate) (IH-4)
All synthesized compounds were identified by NMR and mass
spectrometry. The 1H and 13C-NMR spectra were measured
using a Bruker-600 (MHz) spectrometer with trimethylsilane
(TMS) peak as a reference. Mass spectra were recorded on a
JEOL, JMS-600W spectrometer. The thermal transition of pro-
ducts was investigated with Perkin Elmer, Diamond differential
scanning calorimeter (DSC) at a heating and cooling rate of
5 K min−1. The phase transitions were examined by a Nikon
100POL polarized optical microscope (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 an exposure time of 1200 s. The chirality of
compounds were measured using a JASC J-815 circular dichro-
ism (CD) spectrometer. Chloroform was used as a solvent and
sample concentration was adjusted to 0.05 mmol.
The synthetic procedure of IH-4 was similar to that of IH-3.
D-Isomannide (5 g; 34.2 mmol) and pyridine (5.95 g;
75.24 mmol) were dissolved in THF. Compound 2 (17.3 g;
71.82 mmol) was added dropwise to the reaction mixture with
stirring at room temperature for 12 h. The usual work-up pro-
cedure as mentioned previously gave IH-4 in 65.0% yield.
Fabrication of CLC cells
Glass substrates were cleaned with a display grade detergent sol-
ution. They were rinsed with acetone and isopropyl alcohol
using an ultrasonic cleaner for 15 min and 20 min, respectively.
An LC alignment layer (Nissan SE6414) was introduced on the
glass substrate by spin-coating and was thermally cured at
225 °C for 20 min. Its surface was rubbed with a velvet cloth. A
CLC solution including LC242, chiral dopant, photoinitiator
(Irgacure 907, Ciba), and solvent, was spin-coated onto the
alignment layer at 450 rpm for 20 s and then at 2800 rpm for 40
s. A stable CLC film was obtained by UV curing after solvent
evaporation. UV curing was performed at an irradiation intensity
of 20 mW cm−2 for 90 s.
Synthetic procedure of 1,4 : 3,6-dianhydro-D-sorbitol-2,5-bis(4-
acetoxybenzoate) (IH-1)
4-Acetoxybenzoic acid (10 g; 55.51 mmol) and thionyl chloride
(60 ml; 0.83 mol) with a small drop of toluene were refluxed for
3 h, and then thionyl chloride was distilled under reduced
pressure and completely removed by azeotropic distillation to
afford compound 1 in 75% yield. D-Isosorbide (5 g; 34.2 mmol)
and pyridine (5.95 g; 75.24 mmol) were dissolved in THF. Com-
pound 1 (14.3 g; 71.82 mmol) was added dropwise to the reac-
tion mixture and stirred at room temperature for 12 h. The
resulting product was purified by recrystallization with methanol
and followed by flash chromatography with ethyl acetate: hexane
(1 : 1 to 3 : 1 by vol.) cosolvent on silica to give IH-1 in 68.4%
yield.
3. Results and discussion
Preparation of chiral dopants
Chiral dopants bearing isosorbide or isomannide as a chiral core
were synthesized according to Scheme 1.
Isosorbide 3 and isomannide 4 were extended with 4-acetoxy-
benzoyl groups (IH-1, IH-2) and 4-(hexyloxy)benzoyl groups
(IH-3, IH-4) at the 2-OH and 5-OH positions, respectively.
1164 | Green Chem., 2012, 14, 1163–1167
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Scheme 1 Synthetic routes for isohexide based chiral dopants (IH-n,
n = 1–4): (i) SOCl2, toluene, reflux, 3 h; (ii) pyridine, THF, room temp-
erature, 12 h.
Fig. 1 Molecular structures of isosorbide 3 and isomannide 4.
Fig. 2 Differential scanning calorimetric (DSC) first cooling (blue)
and second heating (red) scans of (a) IH-1, (b) IH-2, (c) IH-3, and (d)
IH-4 at 5 K min−1 of heating and cooling rate.
Compound 1 was prepared by reaction of 4-acetoxybenzoic acid
and thionyl chloride, followed by azeotropic distillation with
toluene.
discovered that IH-1 vs. IH-2 and IH-3 vs. IH-4 showed different
transition behaviors depending on their chiral cores and substitu-
ent groups. IH-1 showed no thermal transition in a cooling
process. However it showed a melting transition at 121 °C in the
following heating process with a stay at room temperature. This
means that IH-1 was very slowly crystallized and required an
annealing process to obtain a crystalline phase. On the other
hand, IH-2 showed no crystallization or melting transition in the
following heating process if it was not cooled slowly. When it
was cooled at a rate of 0.1 K min−1, it showed a small melting
transition at 39 °C in a 2nd heating process (see Fig. S9 in the
ESI†).
DSC thermograms of IH-3 showed typical crystal-to-isotropic
transition. IH-3 was crystallized at 74 °C and melted at 89 °C.
IH-4 however was crystallized at 40 °C and melted at 69 °C.
IH-4 showed relatively low melting and crystallization tempera-
tures compared with IH-3. It also showed an additional exother-
mic peak22 at 13 °C in the 2nd heating DSC curve, which varied
with the 1st cooling rate and disappeared at a rate lower than
2.5 K min−1 (see Fig. S10 in the ESI†). This means that a fast
cooling rate suppressed crystallization of IH-4. IH-3 however
showed the similar crystallization tendency irrespective of
cooling rate (see Fig. S11 in the ESI†).
4-Acetoxybenzoic acid and 4-(hexyloxy)benzoic acid could
be readily prepared from 4-hydroxybenzoic acid17 as previously
reported.16 Crude products were purified by precipitation with
cold methanol and further purified by flash chromatography
(ethyl acetate : hexane = 1 : 1 to 3 : 1) on silica.
As shown in Fig. 1, hydroxy groups at C2 and C5 are posi-
tioned either inside or outside the V-shaped molecule. They are
designated as endo or exo, respectively. Isosorbide has one exo
and one endo hydroxy groups, whereas isomannide has two
endo hydroxy groups. Previous studies showed that exo and
endo hydroxy groups exhibit different reactivities because the
endo hydroxy group is sterically hindered and it forms an inter-
molecular hydrogen bond with the oxygen in the ring struc-
ture.20,21 However final yields of synthesized chiral dopants
(IH-2 (64.5%) vs. IH-1 (68.4%), IH-4 (65.0%) vs. IH-3 (74.0%))
were almost the same in this work in spite of the hydroxy
groups’ different reactivities.
Phase transition of chiral dopants
Phase transitions of chiral dopants were analyzed by DSC and
POM equipped with a heat processor. DSC thermograms were
recorded during the 1st cooling and 2nd heating processes at
a rate of 5 K min−1. From the DSC thermograms (Fig. 2), we
Based on the DSC results, we found that chiral dopants
bearing isomannide (IH-2, IH-4) tend to be slowly crystallized
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Fig. 3 Molecular structures of IH-n (n = 1–4) obtained by energy
minimization iteration; (a) IH-1, (b) IH-2, (c) IH-3, and (d) IH-4.
compared with chiral dopants bearing isosorbide (IH-1, IH-3)
even though they have the same substituent groups. This can be
explained by molecular structure analysis. Molecular structures
of IH-n (n = 1–4) determined by energy minimization iteration23
showed that IH-1 and IH-3 have a bent shape due to the isosor-
bide configuration (see Fig. 3). In addition, this bent shape
makes free rotation of substituents linked to IH-1 and
IH-3 more difficult than for linear shaped IH-2 and IH-4. Con-
sidering the fact that molecular geometry is related to crystal
packing,24 molecules with less conformational diversity will
be more easily crystallized. As a result, chiral dopants bearing
isosorbide are crystallized faster.
Fig. 4 CD spectra of IH-3 (blue line) and IH-4 (red line) in chloroform
at T = 20 °C; it indicates that the two compounds showed different
handedness.
POM analysis showed that small molecules like IH-1 and
IH-2 had crystal textures with low crystallinity and IH-3 and
IH-4 had typical spherulite structures by direct crystallization
from the isotropic melt without mesogenic behavior (see
Fig. S12 in the ESI†). Crystal textures of IH-3 and IH-4
were also examined by 1D WXRD analysis (see Fig. S13 in the
ESI†). Diffraction peaks of IH-3 decreased monotonously with
increasing temperatures. However IH-4 underwent an annealing
process when it was heated to 42 °C (this is coincident with the
previous DSC result).
Fig. 5 UV-visible spectra and polarized optical microscopy images of
cholesteric films prepared with different amounts of IH-3; (a) 7.0 mol%,
(b) 6.0 mol%, (c) 5.0 mol%.
CD spectrometer measurements
When a molecule is optically active, it absorbs right- and left-
handed circularly polarized light to different extents. This is
called circular dichroism (CD) and a plot of its wavelength
dependence is referred to as a CD curve. This phenomenon is
exhibited in the absorption band of an optically active molecule.
CD spectra of IH-3 and IH-4 showed they are virtually mirror
images of each other (see Fig. 4). IH-3 showed a positive Cotton
effect at 250 nm and a negative Cotton effect at 275 nm.
However IH-4 showed a negative Cotton effect at 250 nm and a
positive Cotton effect at 275 nm. This means that isosorbide and
isomannide core molecules induced oppositely handed helixes.
Considering the fact that a cholesteric polymeric film contain-
ing isosorbide bearing a chiral dopant reflects the right-handed
circularly polarized light,25 it could be known that isomannide
induces a left-handed helix.
were prepared by mixing with an achiral nematic host LC
(LC242)26 and UV curing. By incorporating a chiral dopant to
the host LC, LC twists perpendicularly to its director and forms
a cholesteric phase. This reflects light, the wavelength of which
is proportional to the size of the helix. Fig. 5 shows UV-visible
spectra and texture images of CLC films prepared with different
amounts of IH-3 and host LC. It shows that CLC films contain-
ing IH-3 at 5.0–7.0 mol% were selectively reflecting visible light
from red light (λmax = 725 nm) to blue green light (λmax
=
519 nm) and they showed oily streak textures that are typical of
cholesteric liquid crystals. On the other hand, CLC films pre-
pared with IH-1 required 8.5 mol% of IH-1 to reflect red light
(λmax = 712 nm) (see Fig. S14 in the ESI†).
According to the selective light reflection method described in
reference 16, helical twisting powers (HTPs) of IH-n (n = 1–4)
were calculated. HTPs of IH-1 and IH-3 were 26.6 μm−1
and 42.1 μm−1 respectively, whereas HTPs of IH-2 and IH-4
were not obtained. This is because they couldn’t induce CLC
films that reflect visible light. Furthermore, they caused phase
CLC film fabrication
In order to know whether isohexide-based chiral dopants are
applicable to a visible light reflective polarizer film, CLC films
1166 | Green Chem., 2012, 14, 1163–1167
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separation and precipitation problems in making CLC films
when their content exceeded 11.0 mol%.
Considering that handedness and pitch of the induced choles-
teric phase are determined by the coupling of the chirality and
orientational behavior of the dopants,27 the above results should
be interpreted with the difference in molecular structures of
chiral dopants. According to the molecular structure analysis
(see Fig. 3), the bent shape of isosorbide provided a higher
power to twist a host LC than the linear shape of isomannide
and this was considered as the main reason for the fact that only
IH-3 and IH-1 functioned as a chiral dopant.
6 P. J. Collings and M. Hird, Introduction to Liquid Crystal, Chemistry and
Physics, Taylor & Francis, London, 1997, pp. 223–244.
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22 By varying the cooling rate of IH-4, we found that the enthalpy of the
peak at 13 °C was related to the crystallization enthalpy. As the cooling
rate decreased, the crystallization enthalpies increased from −14.0 to
−35.7 J g−1 meanwhile, the enthalpies of the peak at 13 °C decreased
from −16.2 to 0 J g−1. In addition, we found that the sum of the above
two enthalpies showed almost similar values (−30.2 to −35.7 J g−1). It
was believed that the IH-4 molecules were not completely crystallized
during fast cooling, instead they possibly underwent thermally induced
ordering during a subsequent heating step, resulting in an exothermic
peak at 13 °C.
4. Conclusions
Chiral dopants with stereochemically different core structures
were synthesized from bio-based epimeric compounds, isosor-
bide and isomannide. They showed different phase transitions
and optical properties depending on their core structures. In spite
of the same substituent groups, isosorbide (IH-1 and IH-3)
induced a highly crystalline structure and helical twisting power
compared with isomannide (IH-2 and IH-4). In addition, isosor-
bide bearing chiral dopants (IH-1 and IH-3) successfully
induced CLC phases with LC242, isomannide chiral dopants
(1H-2 and IH-4) however couldn’t induce CLC phases that can
reflect visible light. These are mainly because of configurational
differences caused by exo and endo hydroxy groups of
isohexide.
Acknowledgements
23 Energy minimization was performed with the ChemBio3D Ultra 11.0
software (ChemOffice CambridgeSoft Corp.) using MM2 molecular
mechanics routine toward a 0.001 rms gradient.
24 J. J. Wolff, Angew. Chem., Int. Ed. Engl., 1996, 35 (19), 2195.
25 J. Lub, W. P. M. Nijssen, R. T. Wegh, J. P. A. Vogels and A. Ferrer, Adv.
Funct. Mater., 2005, 15, 1961.
This research was supported by a grant from the Cooperative
R&D Program funded by the Ministry of Knowledge Economy,
Republic of Korea.
26 LC242, an achiral nematic LC molecule carrying acrylates as functional
end groups, is commonly used as a host LC for visible light reflective
cholesteric film fabrication. Its phase transition was observed at 63–70 °C
(Cr to N) and at 120–125 °C (N to I), respectively.
27 S. Pieraccini, A. Ferrarini, G. Gottarelli, S. Lena, S. Masiero and
G. P. Spada, J. Org. Chem., 2005, 70, 8009.
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