Effect of alkylthio tail on phase behaviors of bent-shaped molecules based on naphthalene core

Article abstract of DOI:10.1246/cl.2009.424

New bent-shaped molecules with naphthalene central core and alkylthio tail are prepared, and their banana phase structures and properties are discussed in comparison with those of conventional alkoxy molecules. Copyright

Full text of DOI:10.1246/cl.2009.424

424
Chemistry Letters Vol.38, No.5 (2009)
Effect of Alkylthio Tail on Phase Behaviors of Bent-shaped Molecules
Based on Naphthalene Core
Xiaodong Li,1;2 Seng Kue Lee,ꢀ2 Sungmin Kang,2 Masatoshi Tokita,2 Susumu Kawauchi,2 and Junji Watanabe2
1School of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, P. R. China
2Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552
(Received February 12, 2009; CL-090155; E-mail: sklee@polymer.titech.ac.jp)
Table 1. Transition temperatures and enthalpies based on DSC cool-
ing data, in alkylthio and alkoxy compounds
New bent-shaped molecules with naphthalene central core
and alkylthio tail are prepared, and their banana phase structures
and properties are discussed in comparison with those of conven-
tional alkoxy molecules.
Transition temperature/ꢁC (enthalpy/kJ molꢂ1
N(1,7)-S12 B4 118.3 (22.8) B1 131.4 (3.2) Iso
N(1,7)-O12 B4 145.7 (27.4) Iso
)
N(1,6)-S12 Cr2 81.7 (3.5) Cr1 110.7 (10.2) B2 (SmCSPA) 171.2 (24.0) Iso
N(1,6)-O12 Cr2 95.8 (2.8) Cr1 130.1 (9.9) B2 (SmCSPA) 193.5 (25.4) Iso
N(2,7)-S12 B4 170.7 (22.8) B2 (SmCSPF) 200.4 (11.6) Iso
Since the discovery that bent-shaped (or banana) molecules
can form banana phases with unique polarity1 and chirality,2 a
new area in the field of liquid crystal research has opened. A
number of bent-shaped molecules have been synthesized and
characterized3–5 to clarify the structure–property relationships
and the source of polarity and chirality.
Almost all bent-shaped molecules are designed by using
alkoxy or alkyl groups as a terminal chain as the terminal chain
plays an important role in determining the phase structure. In this
study, we synthesized a new homologous series of bent-shaped
molecules with dodecylthio tail and naphthalene central core
where typical Schiff-based side wings are substituted on 1,7-,
1,6-, and 2,7-positions.9 The compounds designated N(n,m)-
S12 are shown in Scheme 1. The effect of the alkylthio tail on
their mesomorphic behaviors and phase structures is discussed.
All the compounds show enantiotropic transitions. In
Table 1, their transition temperatures and associated enthalpy
changes are listed and compared with those of conventional
alkoxy compounds, namely, N(n,m)-O12.6,7
N(2,7)-O12 B4 170.0 (8.1) B4f 196.9 (27.7) B2 (SmCAPA) 224.0 (18.9) Iso
ing a rectangular-wave field, yellow domains with an extinction
direction parallel and perpendicular to the layer normal are ob-
served, whereas dark blue domains with fine stripes are observed
in the field-off state (Figure 1a). On applying a triangular-wave
electric field, the B2 phase exhibits double switching current
peaks in half a circle (Figure 1b), indicating that it has a
racemic SmCSPA structure as a ground state and that electro-
optical switching takes place between SmCSPA and SmCAPF
(Figure 1c). On further cooling from B2 phase, two crystal phas-
es are observed with clear transition peaks on the DSC thermo-
gram. These results are very similar to those of N(1,6)-O127
(refer to Table 1).
The B2 phase is also formed from N(2,7)-S12, but its elec-
trooptic switching behavior is different from that of N(1,6)-S12.
On applying an electric field for example, the extinction direc-
tion of fan-shaped domains rotates in opposite direction on
reversing the polarity. On terminating the field, the position of
extinction direction is retained (Figure 2a). Moreover, a single
switching current peak indicating ferroelectric switching appears
in the triangular-wave field (Figure 2b). Thus, a bistable switch-
ing occurs between two homochiral ferroelectric SmCSPF states
(Figure 2c). This homochiral SmCSPF in ground state is different
from the homochiral antiferroelectric SmCAPA in N(2,7)-O12.
On further cooling to the lower temperature region, typical B4
phase with two opposite chiral domains is observed.
As mentioned above, variation of the chemical structure in
bent-shaped molecules is very important for interlayer interac-
tion, influencing the formation of banana phases. In this study,
we used an alkylthio terminal chain that is flexible than conven-
tional alkoxy terminal chains. The flexibility of the alkylthio tail
can be understood from Figure 3 which shows the relative ener-
gy diagram as a function of dihedral angle around the C–S bond
in Phenyl–S and C–O bond in Phenyl–O. Here, dodecylsulfanyl-
benzene and dodecyloxybenzene were used for simple calcula-
tion. Two stable conformations are found in both compounds
(dihedral angle: 0 or 180ꢁ). But the path from one stable con-
former to the other is different. The path for C–O is obstructed
by high energy barrier (13.2 kJ molꢂ1, in position of dihedral
angle: 90ꢁ). In contrast, the potential barrier for C–S is lower
(1.88 kJ molꢂ1, in position of dihedral angle: 50 and 130ꢁ)
N(1,7)-S12 shows the B1 phase as well as the solid B4 phase
irrespective of its low bent angle of around 60ꢁ. When it is
cooled from isotropic phase, mosaic texture of the B1 phase is
observed (Figure S1a). This phase does not switch under appli-
cation of an electric field. In X-ray diffraction measurement, two
˚
sharp inner reflections with spacings of 56.3 and 33.2 A and a
˚
broad outer reflection of 4.5 A are observed, indicating a two-
dimensional (2D) frustrated structure with a unit lattice of
8
˚
˚
a ¼ 106:2 A and c ¼ 66:4 A. On further cooling to the B4
phase, two optically active domains with opposite chiralities
are observed, in which the brightness is interchanged by positive
and negative rotations of polarizer from the cross polarization
position (Figure S1b).
In N(1,6)-S12, typical B2 phase is observed as high birefrin-
gent circular domains on cooling from isotropic phase. By apply-
O
O
O
O
N
N
N(2,7)-S12
N(1,6)-S12
N(1,7)-S12
C12H25
S
SC12H25
Scheme 1.
Copyright ꢀ 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.5 (2009)
425
(a)
(b)
+10 V/µm
0 V/µm
-10 V/µm
(c)
Figure 3. Internal rotational energy against the dihedral angles of
phenyl–S (open circles) and phenyl–O (closed circles) bonds (see
the text).
Figure 1. (a) Photomicrographs observed on switching, (b) polariza-
tion reversal current in the SmCSPA phase of N(1,6)-S12 under the ap-
plication of a 240 Vpp triangular wave voltage of 3 Hz, and (c) sche-
matic representation of molecular rearrangement on antiferro-switch-
ing (3.0 mm thick cell).
˚
Table 2. Layer spacings (A) of alkylthio and alkoxy compounds
Cry
B4
B2
B1
N(1,7)-S12
N(1,7)-O12
N(1,6)-S12
N(1,6)-O12
N(2,7)-S12
N(2,7)-O12
33.4
33.2
33.2
(a)
(b)
49.7
50.0
47.4
44.8
48.6
44.8
54.6
57.3
in N(2,7)-O12 is altered to the SmCSPF in N(2,7)-S12. Both
the phases are homochiral, but the same directionality of polar-
ization becomes maintained through the layers in N(2,7)-S12.
Although more comprehensive study will be required, the pres-
ent result shows the importance of the flexibility of terminal
chains on the phase structure and opens up prospects for new
molecular designs for bent-shaped molecules.
+8.5 V/µm
0 V/µm
-8.5 V/µm
(c)
In summary, the mesophase behaviors of new bent-shaped
molecules with naphthalene central core and alkylthio tail have
been studied. All compounds showed the well-defined fluid
smectic mesophase. However, the phase structure is different
from the conventional alkoxy tail compounds. This difference
may be caused by the flexibility of alkylthio terminal chain.
Figure 2. (a) Photomicrographs observed on switching, (b) polariza-
tion reversal current in the SmCSPF phase of N(2,7)-S12 under the
application of 84 Vpp triangular wave voltage of 1.4 Hz, and (c) sche-
matic representation of molecular rearrangement on ferro-switching
(2.2 mm thick cell).
References and Notes
T. Niori, T. Sekine, J. Watanabe, T. Furukawa, H. Takezoe, J.
Mater. Chem. 1996, 6, 1231.
1
although metastable conformer exists at 90ꢁ. These particularly
low barriers of the alkylthio terminal chain enable the compound
to change the conformation more easily and affect the meso-
phase structure and properties as follows.
First, the transition temperatures are remarkably affected.
All alkylthio compounds show transition temperatures lower by
15–25 ꢁC than alkoxy compounds (Table 1). Second, although
the layer spacings of the B4 and crystal are similar, the spacings
of the B2 phase in alkylthio compounds are larger than those of
alkoxy ones (Table 2). Thus, the molecular lengths are similar,
but the tilt angle in the B2 phase of alkylthio compounds is
smaller. Since the molecules lie perpendicular to the layer in
the B4 phase,6 for example, tilt angle is calculated as 38ꢁ for
N(2,7)-O12, while it is 27ꢁ for N(2,7)-S12. Finally, SmCAPA
2
D. R. Link, G. Natale, R. Shao, J. E. Maclennan, N. A. Clark, E.
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Supporting Information is also available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/index.
html.
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Published on the web (Advance View) March 28, 2009; doi:10.1246/cl.2009.424