Aromatic fluorination effect on the mesomorphic properties of discotic liquid crystal of alkoxybenzoyloxytriphenylene

Article abstract of DOI:10.1080/15421400903065630

2,3,6,7,10,11-hexakis (4-alkyloxy-2,3,5,6-tetrafluorobenzoyloxy) triphenylenes (CnF4; n = 6, 7, 8, 9, 10, 12, 14, 16) and the branched peripheral chain derivative [C6(2C2)F4, C8(3,7C1)F4 and C7(1C1)F4] were synthesized to study the mesomorphic transition

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Aromatic Fluorination Effect
on the Mesomorphic Properties
of Discotic Liquid Crystal of
Alkoxybenzoyloxytriphenylene
Yasuyuki Sasada a b c , Hirosato Monobe a , Yasukiyo
Ueda b & Yo Shimizu a
a National Institute of Advanced Industrial Science
and Technology , Kansai Center Nanotechnology
Research Institute , Ikeda, Osaka, Japan
b Kobe University, Graduate School of Engineering,
Faculty of Engineering , Nada, Kobe, Japan
c Research Laboratory I, Goi Research Center ,
Chisso Petrochemical Corporation , Ichihara, Chiba,
Japan
Published online: 05 Oct 2009.
To cite this article: Yasuyuki Sasada , Hirosato Monobe , Yasukiyo Ueda & Yo Shimizu
(2009) Aromatic Fluorination Effect on the Mesomorphic Properties of Discotic Liquid
Crystal of Alkoxybenzoyloxytriphenylene, Molecular Crystals and Liquid Crystals,
509:1, 186/[928]-194/[936], DOI: 10.1080/15421400903065630
To link to this article: http://dx.doi.org/10.1080/15421400903065630
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Mol. Cryst. Liq. Cryst., Vol. 509, pp. 186=[928]–194=[936], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903065630
Aromatic Fluorination Effect on the Mesomorphic
Properties of Discotic Liquid Crystal of
Alkoxybenzoyloxytriphenylene
Yasuyuki Sasada1,2,3, Hirosato Monobe1,
Yasukiyo Ueda2, and Yo Shimizu1
1National Institute of Advanced Industrial Science and Technology,
Kansai Center Nanotechnology Research Institute, Ikeda,
Osaka, Japan
2Kobe University, Graduate School of Engineering, Faculty of
Engineering, Nada, Kobe, Japan
3Research Laboratory I, Goi Research Center, Chisso Petrochemical
Corporation, Ichihara, Chiba, Japan
2,3,6,7,10,11-hexakis (4-alkyloxy-2,3,5,6-tetrafluorobenzoyloxy) triphenylenes
(CnF4; n ¼ 6, 7, 8, 9, 10, 12, 14, 16) and the branched peripheral chain derivative
[C6(2C2)F4, C8(3,7C1)F4 and C7(1C1)F4] were synthesized to study the meso-
morphic transition behavior by polarized optical microscope, DSC and XRD tech-
niques. It was found that CnF4s exhibit Colh phase having a wide range of
temperature, and the clearing points get slightly lowered as the peripheral chains
are elongated. On the other hand, the stabilities of mesomorphism for CnF4s hav-
ing the branched peripheral chains without C7(1C1)F4 larger then corresponding
non-branched peripheral chain homologues. It was shown that the elongation of
the peripheral chains and the introduction of branched structure into the chains
give no change in the type of mesomorphism, implying a strong attractive interac-
tion among the fluorinated phenyl groups.
Keyword: discotic liquid crystal; hexagonal columnar phase; tetrafluorobenzene
derivatives; attractive interaction
1. INTRODUCTION
The fluorinated compounds have attracted much interest because of
their unique properties in chemistry and physics [1]. Especially in
research field of LCDs technology, liquid crystal materials possessing
Address correspondence to Yasuyuki Sasada, National Institute of Advanced
Industrial Science and Technology Kansai Center, Nanotechnology Research Institute,
1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan. E-mail: y.sasada@chisso.co.jp
186=[928]

Aromatic Fluorination Effect
187/[929]
the fluorinated aromatic rings or linkage groups have been synthesized
and reported their properties such as dielectric anisotropy, rotational
viscosity, etc. [2], because the fluorination gives large dielectric aniso-
tropy without the depression of the mesomorphic properties. On the
other hand, only a few reports have been seen for fluorinated discotic
liquid crystals to show that the thermal stability of hexagonal colum-
nar (Colh) phase is enhanced by introducing perfluoroalkyl structures
to peripheral chains of the central core part, probably due to the fluor-
ophilic and fluorophobic interactions [3].
Recently, we reported that hexakis (4-alkoxybenzoyloxy) tripheny-
lene derivatives of which phenyl rings were fully fluorinated exhibit
a hexagonal columnar (Colh) mesophase with high thermal stability,
while the corresponding non-fluorinated homologues predominately
show a discotic nematic (ND) phase [4]. In addition, we reported that
in the derivatives of hexakis(4-alkoxybenzoyloxy)triphenylene, the
alternative positions of fluorinated sites of the peripheral phenyl
rings lead to a diversity of the mesomorphism involving the thermal
stability [5].
Here an interesting result is shown in the aromatic fluorination
effect on the mesomorphic behavior where the tetrafluoro-substitution
of the phenyl rings in the peripheral positions to the central tripheny-
lene core induces a drastic change of mesomorphism (namatic to
columnar phases) with an enhanced thermal stability.
Some homologues, 2,3,6,7,10,11-tetrakis (4-alkoxy-2,3,5,6-tetra-
fluorobenzoyloxy) triphenylene derivatives having non-branched
(CnF4s; n ¼ 6, 7, 8, 9, 10, 12, 14, 16) and branched (C7(1C1)F4,
C6(2C2)F4, C8(3,7C1)F4) peripheral chains (as shown in Fig. 1) were
synthesized to study the mesomorphic behavior and the corresponding
FIGURE 1 Chemical structures of CnF4s and Cn(nCn)F4s.

188/[930]
Y. Sasada et al.
non-fluorinated (hydrocarbon) homologues having non-branched per-
ipheral chains (CnH4s; n ¼ 6, 7, 8, 9, 10, 12) [6] were compared.
2. RESULTS AND DISCUSSION
2.1. Synthesis
Discotic liquid crystal compounds CnF4s (n ¼ 6, 7, 8, 9, 10, 12, 14, 16)
and branched peripheral chain derivatives (C7(1C1)F4, C6(2C2)F4,
C8(3,7C1)F4) were synthesized as shown in Scheme 1. These com-
pound were purified silica-gel column chromatography followed by
the recrystallization from toluene=EtOH. The identification of these
1
compounds was carried out by H-NMR, 19F-NMR, FT-IR, elemental
analyses and TOF-MS.
2.2. Mesomorphic Properties
The DSC curve of C6F4 on heating, a non-branched peripheral chain
homologue is shown in Figure 2. C6F4 exhibits an enantiotropic meso-
phase, and three endothermic peaks were observed at 130ꢀC, 157ꢀC
and 301ꢀC with the phase transition enthalpies (DH), 1.9 kJ=mol,
3.6 kJ=mol, and 25.4 kJ=mol, respectively. The microscopic texture
observation revealed the phase transition at 130ꢀC is melting and
one at 301ꢀC clearing. The textures of these mesophases were of
typical for columnar mesophase.
On the other hand, the DSC curve of C6(2C2)F4 having branched
peripheral is shown in Figure 3. C6(2C2)F4 also exhibits enantiotro-
pic mesomorphism. Two endothermic peaks were observed at 166 and
347ꢀC (DH ¼ 27.8 and 32.5 kJ=mol, respectively). The former is
assigned to the melting point and the latter is to the clearing one by
the microscopic texture observation.
The polarized optical microscope images of C6F4 and C6(2C2)F4
are shown in Figure 4. Both compounds exhibit a dendric texture con-
taining with dark domains in part, indicating these compounds exhibit
a mesophase with optical uniaxiality.
SCHEME 1 Synthetic routes of CnF4s and Cn(nCn)F4s.

Aromatic Fluorination Effect
189/[931]
FIGURE 2 A DSC curve of C6F4 (heating rate: 5ꢀC=min) Cr: crystal, Colh:
hexagonal columnar phase, Iso: isotropic liquid.
FIGURE 3 A DSC curve of C6(2C2)F4 (heating rate: 5ꢀC=min) Cr: crystal,
Colh: hexagonal columnar phase, Iso: isotropic liquid.

190/[932]
Y. Sasada et al.
FIGURE 4 Optical texture of mesophase for C6F4 (left at 170ꢀC) and
C6(2C2)F4 (right at 290ꢀC).
The X-ray diffraction patterns of this mesophases for C6F4 at
180ꢀC (non-aligned samples) show a small number of reflections as
shown in Figure 5. The reflections observed in the small angle region
p
show a spacing ratio of 1:1= 3:1=2, which is an evidence of a hexago-
nal arrangement of columns. The lattice constant ahex is calculated to
˚
be 30.4 A. As the molecular diameters evaluated by a fully extended
˚
model is 38 A for C6F4, the observed ahex values show a similar corre-
lation with those of other triphenylene hexagonal columnar mesogens
that the observed ahex is almost 80% of the molecular diameters eval-
uated by the molecular model. Furthermore, the halos having the spa-
˚
cings of 4.7 and 3.5 A were observed in the wide-angle region. The halo
˚
centered at 4.7 A could be assigned to the averaged molecular width of
a tetrafluorophenylene moiety, according to the estimated molecular
FIGURE 5 A X-ray diffraction pattern of C6F4 at 180ꢀC.

Aromatic Fluorination Effect
191/[933]
˚
˚
width of the molecular model (4.7–5.0 A), and the halos at 3.5 A could
be related to the averaged p ꢁ p staking distance of triphenylenes.
The X-ray diffraction pattern of C6(2C2)F4 having branched per-
ipheral chains at 180ꢀC (for the non-aligned samples) shows a small
number of reflections as shown in Figure 6, where the reflections
p
in the small angle region have a spacing ratio of 1:1= 3:1=2 :1=
p
7:1=3. . ., which is an evidence of a hexagonal arrangement of mole-
cularly stacking columns. Furthermore, two halos spacing of 4.8 and
˚
3.5 A were observed in the wide-angle region. The former could be
assigned to the averaged molecular width of a tetrafluorophenylene
moiety, according to the estimated molecular diameter of the molecu-
˚
lar model (4.7–5.0 A) and the p ꢁ p staking distance of triphenylene
˚
˚
cores in a column is indicated at 3.5 A. The lattice constant ahex of
C6(2C2)F4 is determined to be 31.3 A. This is the similar value to
the corresponding non-branched peripheral chain homologues.
The X-ray diffraction parameters of C6F4 and C6(2C2)F4 are sum-
marized in Table 1.
The phase transition parameters of CnF4s (n ¼ 6, 7, 8, 9, 10, 12, 14,
16) having non-branched peripheral chains are summarized in Table 2
with those of the corresponding non-fluorinated homologues (CnH4s:
n ¼ 6, 7, 8, 9, 10, 12). All homologues of CnF4 exhibit only a Colh
mesophase, though discotic nematic (ND) phase accompanied with
Colr=Colt phase in the lower temperature ranges is formed for the
corresponding non-fluorinated homologues.
FIGURE 6 X-ray diffraction patterns of C6(2C2)F4 at 180ꢀC.

192/[934]
Y. Sasada et al.
TABLE 1 Observed Parameters of X-ray Diffractions for C6F4 and
C6(2C2)F4
˚
d
hkl=A
˚
Compound
aHex.=A
hkl
Observed
Calculated
C6F4
30.4
100
110
200
26.8
15.2
13.1
26.8
15.5
13.4
4.7 (br)
3.5 (br)
C6(2C2)F4
31.3
100
110
200
210
300
310
400
230=320
330
27.1
15.6
13.6
10.2
9.30
7.52
6.76
6.22
5.23
27.1
15.6
13.6
10.2
9.00
7.52
6.78
6.22
5.22
4.8 (br.)
3.5 (br.)
br ¼ Broad.
TABLE 2 Phase Transition Temperature of CnF4s, C6(2C2)F4,
C8(3,7C1)F4, C7(1C1)F4 and CnH4s
Compound
Phase transition temperature=ꢀC (DH: kJ=mol)
C6F4
C7F4
C8F4
C9F4
C10F4
C12F4
C14F4
C16F4
C6(2C2)F4
C8(3,7C1)F4
C7(1C1)F4
C6H4[6]
C7H4[6]
C8H4[6]
C9H4[6]
C10H4[6]
C12H4[6]
Cr1 130 (1.9) Cr2 157 (3.6) Colh 301 (25.4) Iso
Cr 144 (5.7) Colh 308 (26.4) Iso
Cr 133 (6.2) Colh 308 (28.4) Iso
Cr 122 (5.2) Colh 306 (24.6) Iso
Cr 109 (5.1) Colh 302 (26.2) Iso
Cr 6.4 (50.7) Colh 288 (23.7) Iso
Cr 32.6 (93.0) Colh 281(18.0) Iso
Cr 48.2 (141.4) Colh 266 (14.2) Iso
Cr 166 (27.8) Colh 347 (32.5) Iso
Cr 41.4 (5.6) Colh 311 (30.3) Iso
Colh 287 (29.3) Iso
a
Cr (9.5) Colta 193 ND 274 Iso
a
Cr 168 (9.4) ND 253 Iso
a
a
Cr 152 (19.2) Colr 168 ND 244 Iso
a
a
Cr 154 (13.6) Colr 183 ND 227 Iso
a
a
Cr 142 (34.9) Colr 191 ND 212 Iso
a
Cr (6.11) 146 Colr 174 Iso
aColt: tetragonal columnar phase, Colr: rectangular columnar phase, ND: discotic
nematic phase.

Aromatic Fluorination Effect
193/[935]
The clearing points of CnF4s were slightly decreased with elonga-
tion of the peripheral chain, while those of CnH4s become lower as
the increase of chain length. In addition, the enthalpies of the clearing
points for CnF4s are much larger than the usual ones of Colh phase
shown by other triphenylene mesogens (<10 kJ=mol)[7]. Those results
strongly indicate that a certain attractive force works around the
fluorinated phenylene moieties.
On the other hand, the phase transition parameters of C6(2C2)F4,
C8(3,7C1)F4 and C7(1C1)F4 having branched peripheral chains also
are summarized in Table 2. The introduction of branched structures into
the chains give no change in the type of mesomorphism, all compounds
exhibit only a Colh mesophase. In comparison with the corresponding
non-branched peripheral chain homologues, thermal stabilities of the
Colh phase for C6(2C2)F4 and C8(3,7C1)F4 were increased.
Especially clearing points of C6(2C2)F4 was about 50ꢀC higher
than C6F4. For the introduction of branched structures to the periph-
eral chains in discotic liquid crystals, it was reported that thermal sta-
bilities of mesomorphism in phthalocyanine mesogens having
branched peripheral chains were much lower than those for the corre-
sponding non-branched peripheral chain homologues owing to the
excluded volume effect of branched chains [8]. These enhancements
of mesophase stability are quite new and interesting. The enthalpy
changes for the clearing points of C6(2C2)F4 and C8(3,7C1)F4 also
are much larger than those for the corresponding non-branched per-
ipheral chain homologues. These results strongly indicate that the
mesomorphism of C6(2C2)F4 and C8(3,7C1)F4 were stabilized by
introduction of branched chains to peripheral chains.
3. CONCLUSIONS
The novel discotic liquid crystal compounds, hexakis (4-alkyloxy-2,3,
5,6-tetrafluorobenzoyloxy) triphenylenes having the non-branched
(CnF4; n ¼ 6, 7, 8, 9, 10, 12, 14, 16) and branched [C6(2C2)F4,
C8(3,7C1)F4 and C7(1C1)F4] peripheral chains were synthesized to
investigate on the mesomorphic behavior. All of these exhibit Colh
phase, and this mesomorphic behavior is quite different from that of
the corresponding non-fluorinated homologues, predominantly show-
ing a ND phase. On the other hand, it was found that C6(2C2)F4
and C8(3,7C1)F4 having branched peripheral chains exhibit the
higher stabilities of Colh phase as compared to the corresponding
non-branched peripheral chain derivatives, and this enhancement of
mesophase stability is quite different from the common case of discotic
liquid crystals forming a Colh phase.

194/[936]
Y. Sasada et al.
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