Liquid crystal dimers having vary oxyethylene flexible spacers

Article abstract of DOI:10.1080/15421406.2016.1263102

In this article, we are prepared that the liquid crystal dimers have aromatic-ester type mesogenic units or aromatic-Schiff base type mesogonic units and confirmed by ¹H-NMR spectrometry. The mesomorphic and optical properties of the resultant dimers were studied by differential scanning calorimetry and polarizing optical microscopy.

Full text of DOI:10.1080/15421406.2016.1263102

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Liquid crystal dimers having vary oxyethylene
flexible spacers
Joo Hoon Park, Bal Sydulu Singu, Ok Byung Choi, Hwan Myung Lee, Jin Young
Lee, Sung Jo Kim, Eun Hee Cha, Seon Nam Park, Myeong Heon Kwak, Bong
Keun So, Ran Hee Kim, Soo Min Lee & Kuk Ro Yoon
To cite this article: Joo Hoon Park, Bal Sydulu Singu, Ok Byung Choi, Hwan Myung Lee,
Jin Young Lee, Sung Jo Kim, Eun Hee Cha, Seon Nam Park, Myeong Heon Kwak, Bong
Keun So, Ran Hee Kim, Soo Min Lee & Kuk Ro Yoon (2017) Liquid crystal dimers having
vary oxyethylene flexible spacers, Molecular Crystals and Liquid Crystals, 650:1, 1-6, DOI:
10.1080/15421406.2016.1263102
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1263102
Published online: 23 Aug 2017.
Submit your article to this journal
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
http://www.tandfonline.com/action/journalInformation?journalCode=gmcl20
Download by: [Australian Catholic University]
Date: 24 August 2017, At: 01:54

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Liquid crystal dimers having vary oxyethylene flexible spacers
Joo Hoon Parka, Bal Sydulu Singub, Ok Byung Choia, Hwan Myung Leea, Jin Young Leea,
Sung Jo Kima, Eun Hee Chaa, Seon Nam Parka, Myeong Heon Kwaka, Bong Keun Sob,
Ran Hee Kimb, Soo Min Leeb, and Kuk Ro Yoonb
aCollege of Natural Science, Hoseo University, Choongnam, Republic of Korea; bDepartment of Chemistry,
Organic Nanaomaterials Lab, Hannam University, Daejeon, Republic of Korea
ABSTRACT
KEYWORDS
Aromatic-Schiff base type
mesogonic unitsl; liquid
crystal dimers;
aromatic-ester
typemesogenic units
In this article, we are prepared that the liquid crystal dimers have
aromatic-ester type mesogenic units or aromatic-Schiff base type meso-
gonic units and confirmed by 1H-NMR spectrometry. The mesomorphic
and optical properties of the resultant dimers were studied by differen-
tial scanning calorimetry and polarizing optical microscopy.
1. Introduction
The overwhelming majority of dimers reported in the literature consists of molecules com-
prising two rod-like mesogenic units linked via a flexible alkyl or alkoxy chain normally con-
taining between 1 and 12 methylene groups [1–14]. Liquid crystalline properties of an organic
compound are basically dependent on its molecular structure in which a slight change in the
molecular geometry brings about considerable change in its mesomorphic properties. These
types of compounds are important not only as a new class of liquid crystalline compositions,
but also as models for the corresponding main chain liquid crystalline polymers containing
flexible spacers [1–19]. Recently, we studied the preparation and liquid crystalline properties
of various liquid crystal dimers [3–14, 16–19]. These liquid crystal dimers have aromatic-
ester-type mesogenic units or aromatic-Schiff base type mesogonic units bracketing a central
polymethylene spacer. The liquid crystal-line properties of the dimers were strongly depen-
dent on the nature of the length of the spacer and the terminal substituents. In the present
study, in order to obtain further understanding of the structure–property relationship of liq-
uid crystalline dimers having aromatic-ester type and aromatic-schiff base-type mesogenic
units, we have prepared homologous HBPE-n series liquid crystal dimers shown in Scheme 1.
2. Experimental
2.1. Materials
Ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1-fluoro-4-
nitrobenzene, 1-bromohexane, 4-hydroxybenzoic acid, hydrazine monohydrate, palladium
on activated carbon (Pd/C, 10%), 4-hydroxybenzaldehyde, 1,3- dicyclohexylcarbodiimide
CONTACT Kuk Ro Yoon
kryoon@hnu.kr
Daejeon -, Republic of Korea.
Department of Chemistry, Organic Nanaomaterials Lab, Hannam University,
©  Taylor & Francis Group, LLC

2
J. H. PARK ET AL.
Scheme . The molecular structure of the HBPE-n series.
(DCC), tetrahydrofuran (THF), p-toluenesulfonic acid (PTSA), anhydrous EtOH were pur-
chased from Aldrich Chemical Co. and used as received without further purification.
Preparation of HBPE-n (n = ,,,) series
1,2-Bis(4-(4-(4-hexyloxybenzoyloxy)benzylideneamino)phenoxy)ethane (HBPE-1), Bis(2-
(4-(4-(4-hexyloxybenzoyloxy)benzylideneamino)phenoxy)ethyl)ether (HBPE-2), 1,2-Bis(2-
(4-(4-(4-hexyloxybenzoyloxy)benzylideneamino)phenoxy)ethoxy)ethane (HBPE-3) and Bis
(2-(2-(4-(4-(4-hexyloxybenzoyloxy)benzylideneamino)phenoxy)ethoxy)ethyl)ether (HBPE-
4), a series of dimers were prepared and the preparation process for the designed molecules
of HBPE-n series dimers involved in several steps and described in Scheme 2.
Scheme . Preparation method of HBPE-n series of dimers.
Preparation of HBPE-
The reatcion of 4-(4-hexyloxybenzoyloxy) benzaldehyde (22 mmol) in anhydrous EtOH
(50 mL) with [1,2-bis(4-aminophenoxy)ethane (11 mmol) in the presence of PTSA
[3, 21, 22]. The reaction mixture was stirred for 16 hr at the room temperature. The
crude precipitate was collected by filtration and purified by the recrystallization of both
1
chloroform and ethyl acetate solvents. The prepared HBPE-1 was confirmed by H-NMR.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
3
1
–
–
–
H-NMR (CDCl3, ppm): δ 0.94-0.85 (t, 6H, CH3), 1.50-1.24 (m, 12H, (CH2)3 ), 1.84-1.79
–
–
–
–
–
–
(m, 4H, CH2CH2O ), 4.07-4.02 (t, 4H, CH2CH2O ), 4.37 (s, 4H, OCH2 ), 7.02-6.96 (t,
8H, Ar-H), 7.33-7.24 (d, 8H, Ar-H), 7.97-7.94 (d, 4H, Ar-H), 8.16-8.13 (d, 4H, Ar-H), 8.49
–
(s, 2H, CH = N).
Preparation of HBPE-
The HBPE-2 was prepared by the similar method described in the HBPE-1 and confirmed by
1
1
–
–
–
–
–
H-NMR. H-NMR(CDCl3, ppm): δ 0.94-0.90 (t, 6H, CH3), 1.57-1.25 (m, 12H, (CH2)3 ),
–
–
–
–
1.90-1.77 (m, 4H, CH2CH2O ), 3.98-3.95 (t, 4H, CH2CH2O ), 4.06-4.01 (t, 4H, OCH2 ),
4.21-4.18 (t, 4H, OCH2 ), 6.99-6.94 (t, 8H, Ar-H), 7.32-7.29 (d, 8H, Ar-H), 7.96-7.93 (d, 4H,
Ar-H), 8.15-8.12 (d, 4H, Ar-H), 8.47 (s, 2H, CH = N).
–
–
–
Preparation of HBPE-
The HBPE-3 was prepared by similar method described in the HBPE-1 and confirmed by
1
1
–
–
–
H-NMR. H-NMR(CDCl3, ppm): δ 0.94-0.91 (t, 6H, CH3), 1.57-1.24 (m, 12H, (CH2)3 ),
–
–
–
–
–
–
1.88-1.78 (m, 4H, CH2CH2O ), 3.75 (s, 4H, CH2CH2O ), 3.88-3.86 (t, 4H, OCH2 ), 4.06-
4.01 (t, 4H, OCH2 ), 4.17-4.14 (t, 4H, OCH2 ), 6.98-6.93 (t, 8H, Ar-H), 7.31-7.20 (d, 8H,
Ar-H), 7.95-7.92 (d, 4H, Ar-H), 8.16-8.12 (d, 4H, Ar-H), 8.46 (s, 2H, CH = N ).
–
–
–
–
–
–
Preparation of HBPE-
The HBPE-4 was prepared by similar method described in the HBPE-1 and confirmed
1
1
–
–
by H-NMR. H-NMR (CDCl3, ppm): δ 0.93-0.89 (t, 6H, CH3), 1.58-1.26 (m, 12H,
–
–
–
–
–
(CH2)3 ), 1.84-1.77 (m, 4H, CH2CH2O ), 3.74-3.71 (m, 8H, CH2CH2O ), 3.89-3.85 (t,
–
–
–
–
–
–
4H, OCH2 ), 4.59-4.00 (t, 4H, OCH2 ), 4.16-4.12 (t, 4H, OCH2 ), 6.97-6.92 (t, 8H, Ar-
H), 7.29-7.19 (d, 8H, Ar-H), 7.95-7.90 (d, 4H, Ar-H), 8.14-8.11 (d, 4H, Ar-H), 8.45 (s, 2H,
–
CH = N).
Characterization
1H-NMR spectra of the prepared intermediates and final dimers were recorded with a Varian
300 MHz NMR spectrometer. Thermal history of the dimers was obtained under a N2 atmo-
sphere on a Dupont 2100 thermal analyzer having a 910S-DSC module (differential scanning
calorimetry) with a heating/cooling rate of 10°C/min and indium was used as a reference for
Table . Thermal transition behaviors and thermodynamic data for the phase transition of HBPE-n series.
Thermal transition, °C [ꢀH, J/g]
Heating
Cooling
I
n
Cr
N

. [.]
—
. [.]
—
. [.]
−b



. [.]
. [.]
. [.]
. [.]
. [.]
. [.]
. [.]
. [.]
. [.]
. [.]
. [.]
. [.]
. [.]
. [.]
. [.]
. [.]
. [.]
. [.]
a Cr = crystalline, I = isotropic states, N = nematic phases, all data were obtained from first heating and cooling scans at
°C/min.
bReliable peaks were not obtained on cooling scans because the material was decomposed at isotropic state.
c Values from Ref. [].

4
J. H. PARK ET AL.
Figure . DSC thermograms of HBPE-n series (n = , , , ) of dimers.
temperature calibration and estimation of thermodynamic parameter for the phase transi-
tions. The optical textures of the melts were carried out using a Nikon Labophot-2 polarizing
microscope.
3. Results and discussion
All the prepared HBPE-n (n = 1–4) series dimers found to be exhibit liquid crystalline proper-
ties. The thermal transition behaviors and thermodynamic data of HBPE-n series dimers are
summarized in Table 1. Figure 1 represents the DSC thermograms of HBPE-n series obtained
for both heating and cooling curves. All of the prepared HBPE-n series dimers are enan-
tiotropic. Optical properties of prepared materials were studied using optical microscope
at hot stage. Optical micrographs of HBPE-4 obtained during the cooling cycle at different
temperatures are shown in Fig. 3. The dimers HBPE-4 show an enantiotropic nematic state
(Fig. 3(a)) brush Schlieren texture) and smectic C state (Fig. 3(b)) [24, 25].
Figure . Dependence of transition temperatures of HBPE-n on the length (n) of central flexible spacer on
heating run.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
5
Figure . Optical photomicrographs of HBPE- on cooling (°C/min, ×): (a) nematic (°C) and (b)
smectic C (°C).
DSC of HBPE-n series of materials are analyzed and illustrated in Fig. 1. DSC thermo-
grams show a three-step transition of crystalline → smectic C → nematic → isotropic state,
but reliable peaks are not obtained in case of HBPE-1 material on cooling scan because the
dimmer was decomposed at isotropic state. In the first heating scan of HBPE-2, it represents
a crystalline to smectic C state at 167.2°C, a smectic C to a nematic state at 286.6°C and a
nematic to isotropic liquid state at 291.4°C. In the first cooling scan of HBPE-2, it represents
an isotropic liquid state to a nematic state at 281.4°C, a nematic to a smectic C state at 275.4°C,
and a smectic C to a crystalline state at 136.8°C.
Figure 2 presents the dependence of the phase transition temperature on the central flexible
spacer chain length (n). This figure clearly shows the decrease in phase window of crystalline,
smectic C, and nematic with increasing the central flexible spacer chain length (n = 1–4). The
thermal behaviors of HBPE-1, 3, and 4 series of dimers and its liquid crystal properties are
similar to that seen in HBPE-2.
From Table 1, the crystalline to smectic C state change at 186.6°C (HBPE-1) decreased to
114.7°C (HBPE-4), the smectic C state to nematic state change at 351.4°C (HBPE-1) decreased
to 251.6°C (HBPE-4) and the nematic state to the isotropic liquid state change at 354.0°C
(HBPE-1) decreased to 254.2°C (HBPE-4) with increase in central flexible spacer chain length
indicating the reduction in crystallinity and an increase in flexibility of the dimers. Similar
phenomena were previously reported by us for dimesogenic compounds having azo type [23]
and schiff base type mesogenic groups [6, 7] attached terminal alkoxy chain. With increase
in central flexible spacer chain length, the decrease of the transition temperature of the crys-
talline, smectic C, and nematic phase observed in this series are in accordance with those
observed in our early reports [6, 7, 23]. However, the odd–even effect observed for similar
compounds and not observed in HBPE-n series of dimers [15–19].
4. Conclusions
We have prepared a new series of aromatic-ester type mesogenic units or aromatic-Schiff base
type mesogonic units. The chemical structures of the final products were investigated by 1H
NMR spectroscopy. The mesomorphic and optical properties of the prepared dimers were
characterized by DSC and OM. The presence of smectic A phase transition was confirmed
by the observation in optical microscopy, when the prepared materials were heated from the
crystalline phase.

6
J. H. PARK ET AL.
Funding
This research was supported by the Academic Research Fund of Hoseo Unicersity in 2013 (2013-0366).
References
[1] Demus, D., Godby, J., Gray, G. W., Spiess, H. W., & Vill, V. (1998). Handbook of Liquid Crystals.
Vol. 2B; Wiley-VCH: New York.
[2] Demus, D. & Zaschke, H. (1984). Flüssige Kristalle in Tabllen II. VEB Deutscher Verlag für Grund-
stoffindustrie: Leipzig.
[3] Park, J. H., Choi, O. B., Lee, H. M., et al. (2014). Mol. Cryst. Liq. Cryst., 599, 96.
[4] Park, J. H., Choi, O. B., Lee, H. M., et al. (2012). Bull. Korean. Chem. Soc., 33(5), 1647.
[5] Yoon, K. R., So, B. K., Kim, K. H., et al. (2012). J. Ind. Eng. Chem., 18, 6.
[6] Yoon, K. R., So, B. K., Cho, H. J., et al. (2010). Korean. Chem. Soc., 54(5), 659.
[7] So, B. K., Song, J. Y., Lee, S. M., et al. (2010). J. Korean. Chem. Soc., 54(1), 169.
[8] So, B. K., Lee, S. M., Choi, O. B., et al. (2009). Bull. Korean. Chem. Soc., 30(7), 1634.
[9] Park, J. H., Choi, O. B., Lee, H. M., et al. (2008). J. Korean. Chem. Soc., 52(5), 583.
[10] So, B. K., Kim, W. J., Lee, S. M., et al. (2007). Dyes Pigments, 75, 619.
[11] Choi, O. B., Park, J. H., Lee, Y. S., et al. (2007). Korea. Chem. Eng. Res., 45(2), 583.
[12] So, B. K., Kim, H. J., Song, H. H., et al. (2006). Dyes Pigments, 70, 38–42.
[13] So, B. K., Kim, Y. S., Choi, M. M., et al. (2004). Liq. Cryst., 31, 169.
[14] So, B. K., Jang, M. C., Park, J. H., et al. (2002). Opt. Mater., 21, 685.
[15] Griffin, A. C. & Britt, T. R. (1981). J. Am. Chem. Soc., 103, 4957.
[16] Jin, J. I. & Park, J. H. (1987). Eur. Polym. J., 23(12), 973.
[17] Jin, J. J., Oh, H. T., & Park, J. H. (1986). J. Chem. Soc. Perkin. Trans, II, 2, 343–347.
[18] Park, J. H., Choi, O. B., Lee, J. S., et al. (2005). Korean. Chem. Eng. Res., 43(1), 176.
[19] Jin, J. J., Park, J. H. (1984). Mol. Cryst. Liq. Cryst., 110, 293.
[20] Armarego, W. L. F. & Peron, D. D. (1996). Purification of Laboratory Chemicals, 4th ed.,
Butterworth-Heinmann: Oxford.
[21] Weissflog, W., Lischka, C. H., Diele, S., et al. (2000). Liquid Crystal, 27, 43.
[22] Lodewyckx, K., Vandeun, R., & Binnemans, K. (2001). Mater. Sci. Eng. C, 18, 217.
[23] Kim, T. H., Lee, C. S., & Ramaraj, B., et al. (2008). Mol. Cryst. Liq. Cryst., 492, 102.
[24] Gray, G. W. & Gooby, J. W. G. (1984). Smectic Liquid Crystal Textures and Structure. Leonard Hill:
Philadelphia.
[25] Dierking, I. T. (2003). Textures of Liquid Crystals. Wiley-VCH: Weinheim.