Synthesis and mesomorphic properties of bent-shaped molecule with low bent-angle central core and long alkylthio tail

Article abstract of DOI:10.1016/j.cclet.2011.04.008

Two homologous series of bent-shaped molecules composed of low bent-angle naphthalene central core and long alkoxy tail or alkylthio tail, N(1,7)-n-OPIMB and N(1,7)-n-SPIMB (n = 20, 22), respectively, were synthesized. The mesomorphic properties were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD). Compared to the direct transition from the isotropic phase to the chiral B4 phase examined in N(1,7)-n-OPIMB, N(1,7)-n-SPIMB exhibited a rare Iso-N_col-Col _h-B4 phase sequence. The combination of the asymmetric low bent-angle 1,7-naphathalene central core and the flexible long alkylthio tail is considered to play an important role in the formation of such a novel phase sequence.

Full text of DOI:10.1016/j.cclet.2011.04.008

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 1111–1114
www.elsevier.com/locate/cclet
Synthesis and mesomorphic properties of bent-shaped molecule
with low bent-angle central core and long alkylthio tail
*
Xiao Dong Li, Mao Sheng Zhan , Kai Wang
School of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100191, China
Received 27 December 2010
Available online 25 June 2011
Abstract
Two homologous series of bent-shaped molecules composed of low bent-angle naphthalene central core and long alkoxy tail or
alkylthio tail, N(1,7)-n-OPIMB and N(1,7)-n-SPIMB (n = 20, 22), respectively, were synthesized. The mesomorphic properties
were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD).
Compared to the direct transition from the isotropic phase to the chiral B4 phase examined in N(1,7)-n-OPIMB, N(1,7)-n-SPIMB
exhibited a rare Iso–Ncol–Colh–B4 phase sequence. The combination of the asymmetric low bent-angle 1,7-naphathalene central
core and the flexible long alkylthio tail is considered to play an important role in the formation of such a novel phase sequence.
# 2011 Mao Sheng Zhan. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Bent-shaped; Liquid crystal; Alkylthio tail; Mesomorphic behaviour
Since the spontaneous formation of smectic phases with unique polarity and microscopic chirality in bent-shaped
liquid crystals was discovered [1,2], extensive attention has been attracted in this intriguing area for the last decades.
To clarify the structure-property relationship is one of the major research targets in this area. Though a variety of bent-
shaped molecules have been synthesized, the alkoxy tail and a central core such as 1,3-dioxybenzene or 2,7-
dioxynaphthalene unit are mostly often used [3,4]. In these molecules, the bent-angle is approximately 1208 although
some angular variation is allowed through the rotation of the linkage group to the side wings. Li et al. prepared liquid
crystalline copolymers based on 1,4-, 1,5- and 2,7-dioxynaphthalene rings and discussed their morphological features
[5–7]. The introduction of side wings at different positions of the central dioxynaphthalene core have opened up new
areas of molecular structure design in bent-shaped liquid crystal field.
Lee et al. prepared molecules composed of a 1,7-naphthalene central core and schiff-base side chain with alkoxy
tail, N(1,7)-n-OPIMB (n = 4, 8, 12, 16), which forms chiral B4 phase irrespective of their low bent angle of
approximately 608. And no switchable banana phases have been found [8]. Recently, we have synthesized bent-shaped
molecules with 1,7-naphthalene central core and alkylthio tail, N(1,7)-n-SPIMB (n = 8, 12, 14, 16). Interestingly, the
introduction of the alkylthio tail in place of the alkoxy tail induces a new type of switchable hexagonal columnar phase
(Colh), with a phase sequence of Iso–Colh–B4 [9]. However, none of the previous research touches upon the
compounds with longer tail (n ꢀ 20), which might cause considerable molecular interlayer interactions. Therefore, a
* Corresponding author.
E-mail address: zhanms@buaa.edu.cn (M.S. Zhan).
1001-8417/$ – see front matter # 2011 Mao Sheng Zhan. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2011.04.008

1[(Scheme_1)TD$FIG] 112
X.D. Li et al. / Chinese Chemical Letters 22 (2011) 1111–1114
O
O
HCl
C H
Br
n
2n+1
H N
2
SC H
n
2n+1
HN
SH
HN
SC H
EtOH
n
2n+1
K CO , acetone
2
3
O
O
O
O
O
CHCl
O
O
3
H N
SC H
O
2
n
2n+1
N
N
O
H
O
N(1,7)-n-SPIMB
H
H
C S
SC H
n 2n+1
2n+1
n
n=20,22
Scheme 1. Synthetic route of 1,7-naphthalene-bis[4-(4-alkylthiophenyliminomethyl)]benzoate, N(1,7)-n-SPIMB.
continuing effort has been carried out which focused on the synthesis and characterization of bent-shaped molecules
with low bent-angle 1,7-naphthalene central core and longer tail, N(1,7)-n-SPIMB and N(1,7)-n-OPIMB (n = 20, 22).
N(1,7)-n-SPIMB was prepared according to the synthetic route shown in Scheme 1. A solution of 4-alkylthioanline
and bisaldehyde in chloroform was heated under reflux for 6 h. The reaction mixture was concentrated and
recrystallized from chloroform–ethanol to give yellow crystals. The preparation of the bisaldehyde was performed by
using procedures described in our previous report [9]. N(1,7)-n-OPIMB was prepared with similar methods. All
compounds were characterized by NMR and elemental analysis [10].
Mesomorphic properties of N(1,7)-n-OPIMB and N(1,7)-n-SPIMB (n = 20, 22) were examined under a polarizing
optical microscope(POM) equipped with a hot stage. Transition temperatures and associated enthalpy changes
obtained by DSC are listed in Table 1. N(1,7)-20-OPIMB and N(1,7)-22-OPIMB exhibited a direct transition from the
isotropic phase to the chiral B4 phase, which is the same with compounds composed of shorter tail (n = 4, 8, 12, 16).
However, N(1,7)-n-SPIMB (n = 20, 22) showed a rare Iso–Ncol–Colh–B4 phase sequence. The DSC thermo-gram of
N(1,7)-20-SPIMB is shown in Fig. 1. On cooling from the isotropic liquid between glass plates, a marble texture
appears from the isotropic melt, which gradually changes to the dark homeotropic texture (Fig. 2a). Then Colh phase
appears as a smooth fan-shaped texture, characteristic of hexagonal molecular arrangement. In some areas, the
textures also include optically isotropic zone which indicates columns align perpendicularly to the substrate (Fig. 2b).
With further cooling to the B4 phase, all areas change to a dark blue texture with a low birefringence (Fig. 2c). There
exist two types of optically active domain with opposite chirality. And the brightness of each domain is interchanged
under the positive or negative rotation of the polarizer from the cross position (Fig. 2d).
To elucidate more detailed mesomorphic structure for N(1,7)-n-SPIMB (n = 20, 22), powder X-ray investigations
were carried out with samples kept in glass capillary tubes. For N(1,7)-20-SPIMB, the X-ray diffraction shows only
˚
one inner reflection in Ncol phase with a layer spacing of 65.6 A (Fig. 3). The outer broad diffuse scattering of 4.6 A
˚
corresponds to the liquid-like lateral packing of the molecules. In Colh phase, the inner three diffraction peaks reflect
˚
˚
layer spacings of 65.1 A, 37.2 A and 32.5 A, which can be nicely indexed as the (1 0 0), (1 1 0), and (2 0 0) reflections
˚
˚
of a hexagonal columnar phase [11]. The hexagonal lattice edge was calculated as 75.2 A. The lowest-temperature B4
˚
phase shows only one inner layer reflection, which is 44.9 A. It should be noted that the layer spacing of Ncol phase,
˚
˚
65.6 A, is close to the mean distance between columns in Colh phase, 65.1 A, and larger than the molecular length
˚
(51.7 A calculated by Spartan 08 with the most stable conformation). Thus, the molecules in Ncol phase are assumed to
Table 1
Transition temperatures and enthalpies of N(1,7)-n-SPIMB and N(1,7)-n-OPIMB (n = 20, 22).
a
Compound
Transition temperature (8C) (enthalpy/kJ molÀ1
)
N(1,7)-20-SPIMB
N(1,7)-22-SPIMB
N(1,7)-20-OPIMB
N(1,7)-22-OPIMB
B4 118.6 (38.4) Colh 143.5 (2.4) Ncol 150.1 (5.2) Iso
B4 117.9 (39.8) Colh 146.0 (2.1) Ncol 151.8 (6.5) Iso
B4 147.5 (31.2) Iso
B4 146.4 (28.6) Iso
a
Based on cooling data of DSC.

[(Fig._1)TD$FIG]
X.D. Li et al. / Chinese Chemical Letters 22 (2011) 1111–1114
1113
35
30
25
20
15
Iso
N
Col
B4
col
h
10
5
0
-5
100
120
140
160
Temperature / ºC
Fig. 1. DSC thermo grams of N(1,7)-20-SPIMB.
[(Fig._2)TD$FIG]
Fig. 2. Photomicrographs of (a) Ncol phase (b) Colh phase (c) (d) B4 phase for N(1,7)-20-SPIMB (arrows indicate directions of polarizers).
[(Fig._3)TD$FIG]
Fig. 3. X-ray diffraction patterns of (a) Ncol phase (b) Colh phase and (c) B4 phase for N(1,7)-20-SPIMB.
stack in columns that possess no long range lateral positional order. Consequently, the Colh to Ncol transition could be
interpreted as the melting of the 2D hexagonal lattice, without complete destruction of the columns [12]. The
particularly low barriers of long alkylthio terminal chain might enable the compound to change conformations more
easily, which would affect the mesophase properties [13].
In conclusion, all the target bent-shaped molecules with low bent-angle central core and long tail exhibit
enantiotropic transitions. N(1,7)-n-OPIMB (n = 20, 22) exhibited an Iso-B4 phase transition behavior. In contrast,
N(1,7)-n-SPIMB (n = 20, 22) showed a rare Iso–Ncol–Colh–B4 phase sequence. The combination of the asymmetric
low bent-angle 1,7-naphathalene central core and the flexible long alkylthio tail is considered to play an important role
in the formation of such a novel phase sequence.

1114
X.D. Li et al. / Chinese Chemical Letters 22 (2011) 1111–1114
Acknowledgments
This research was supported by National Natural Science Foundation of China (No. 51073007). We thank Prof.
Junji Watanabe in Tokyo Institute of Technology for the stay in his lab to do parts of the experiment.
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[10] Analytical and secroscopic data for the target compounds. N(1,7)-20-SPIMB, yield 51.8%, 1H NMR (400 MHz, CDCl3): d 0.88 (t, 6H,
J = 6.8 Hz), 1.25–1.75 (m, 72H), 2.94 (t, 4H, J = 6.4 Hz), 7.19–8.09 (m, 18H), 8.29 (d, 2H, J = 8.4 Hz), 8.40 (d, 2H, J = 8.4 Hz), 8.56 (s, 1H),
8.58 (s, 1H). Elemental analysis: calcd. for C78H106N2O4S2: C 78.08, H 8.90, N 2.33, O 5.33, S 5.34; found: C 77.87, H 8.72, N 2.31, O(+S)
11.10. N(1,7)-22-SPIMB, yield 56.2%, 1H NMR (400 MHz, CDCl3): d 0.88 (t, 6H, J = 6.8 Hz), 1.25–1.75 (m, 80H), 2.94 (t, 4H, J = 6.4 Hz),
7.19–8.09 (m, 18H), 8.29 (d, 2H, J = 8.4 Hz), 8.40 (d, 2H, J = 8.4 Hz), 8.56 (s, 1H), 8.58 (s, 1H). Elemental analysis: calcd. for C82H114N2O4S2:
C 78.42, H 9.15, N 2.23, O 5.10, S 5.11; found: C 78.27, H 9.07, N 2.34, O(+S) 10.32. N(1,7)-20-OPIMB, yield 68.3%, 1H NMR (400 MHz,
CDCl3): d 0.87 (t, 6H, J = 7.2 Hz), 1.25–1.75 (m, 72H), 3.98 (t, 4H, J = 7.3 Hz), 7.19–8.09 (m, 18H), 8.29 (d, 2H, J = 8.4 Hz), 8.42 (d, 2H,
J = 8.4 Hz), 8.55 (s, 1H), 8.57 (s, 1H). Elemental analysis: calcd. for C78H106N2O6: C 80.23, H 9.15, N 2.40, O 8.22; found: C 80.37, H 9.11, N
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J = 7.3 Hz), 7.19–8.09 (m, 18H), 8.29 (d, 2H, J = 8.4 Hz), 8.42 (d, 2H, J = 8.4 Hz), 8.55 (s, 1H), 8.57 (s, 1H). Elemental analysis: calcd. for
C82H114N2O6: C 80.48, H 9.39, N 2.29, O 7.84; found: C 80.29, H 9.43, N 2.17, O 8.11.
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