Drastic enhancement of discotic mesomorphism induced by fluorination of the peripheral phenyl groups in triphenylene mesogens

Article abstract of DOI:10.1039/b716920e

Four derivatives of hexakis(4-alkoxybenzoyloxy)triphenylene, for which peripheral phenylene groups are fluorinated at the inner and the outer positions were studied for their mesomorphic behaviour to reveal that the alteration of fluorinated positions in

Full text of DOI:10.1039/b716920e

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Drastic enhancement of discotic mesomorphism induced by fluorination
of the peripheral phenyl groups in triphenylene mesogensw
Yasuyuki Sasada,*abc Hirosato Monobe,a Yasukiyo Ueda*b and Yo Shimizu*a
Received (in Cambridge, UK) 1st November 2007, Accepted 4th January 2008
First published as an Advance Article on the web 23rd January 2008
DOI: 10.1039/b716920e
Four derivatives of hexakis(4-alkoxybenzoyloxy)triphenylene,
for which peripheral phenylene groups are fluorinated at the
inner and the outer positions were studied for their mesomorphic
behaviour to reveal that the alteration of fluorinated positions in
the phenyl rings leads to a drastic change of the mesomorphism
involving the thermal stability.
mesomorphism induced by the fluorination of the phenyl rings
is reported.
These fluorinated discotic liquid crystaline compounds were
synthesised by the esterification of 2,3,6,7,10,11-hexahydroxy-
triphenylene and fluorinated 4-octyloxybenzoic acid, using
N,N-dicyclohexylcarbodiimide (DCC) and 4-dimethylamino-
pyridine (DMAP) in CH2Cl2, and purified by silica-gel column
chromatography followed by recrystallization from tolue-
ne–EtOH. The identification of these compounds was carried
Fluorinated liquid crystalline compounds have attracted much
interest because of their unique properties in chemistry and
physics.1 Especially in the research field of LCDs technology,
calamitic liquid crystal materials possessing fluorinated aro-
matic rings or linkage groups have been synthesised and
reported for their interesting physical properties.2 On the other
hand, in fluorinated discotic liquid crystals, only a few reports
have shown that the thermal stability of the hexagonal co-
lumnar (Colh) phase is enhanced by introducing perfluoroalkyl
tails to the peripheral chains of the central core part and is
probably due to fluorophilic and fluorophobic interactions.3
However, for the fluorination of aromatic rings in discotic
compounds, it has been reported that dodecafluorotripheny-
lene and triphenylene derivatives form complexes in the
mixture that have a higher melting point.4
1
out by H and 19F NMR, elemental analyses and TOF-MS.z
The mesomorphic behaviour was studied by polarised op-
tical microscopy (POM), DSC and X-ray diffraction techni-
ques in comparison with those of the corresponding non-
fluorinated (hydrocarbon) homologue H4 and the fully fluori-
nated homologue F4.
Recently, we reported that hexakis(4-alkoxybenzoyloxy)tri-
phenylene derivatives having tetrafluorophenylene moieties on
the peripheral positions exhibit a hexagonal columnar (Colh)
mesophase with high thermal stability, while the correspond-
ing non-fluorinated homologues predominately exhibit a dis-
cotic nematic (ND) phase.5
The DSC curves on heating of the 2F and 2F6F are shown in
Fig. 1. These exhibit enantiotropic mesomorphism. For 2F,
four endothermic peaks were observed at 82.3, 120, 150 and
213 1C with phase transition enthalpies (DH) of 12.5, 27.3, 28.3
For further studies, hexakis(4-octyloxybenzoyloxy)triphe-
nylene derivatives with mono- or di-fluorinated at the inner
(2- and/or 6- [2F, 2F6F]) or outer (3- and/or 5- [3F, 3F5F])
positions in the peripheral aromatic rings were synthesised.
Here in this communication, an extraordinary behaviour of
a Synthetic Nano-Function Materials Group, Nanotechnology Research
Institute, Advanced Industrial Science and Technology (AIST),
Kansai Center, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan.
E-mail: yo-shimizu@aist.go.jp; Fax: +81-72-751-9628; Tel: +81-
72-751-9525
b Department of Chemical Science and Engineering, Graduate School
of Engineering, Kobe University, 1-1 Rokkodai-cho, Nada, Kobe
657-8501, Japan. E-mail: yueda@kobe-u.ac.jp; Fax: +81-78-803-
6205; Tel: +81-78-803-6182
c Research Laboratory I, Goi Research Center, Chisso Petrochemical
Corporation, 5-1 Goikaigan, Ichihara, Chiba, 290-8551, Japan.
E-mail: y.sasada@chisso.co.jp; Fax: +81-436-23-1570; Tel: +81-
436-23-1960
w Electronic supplementary information (ESI) available: X-Ray dif-
fraction patterns of 2F6F at 140 1C and of 3F at 220 and 250 1C. See
DOI: 10.1039/b716920e
Fig. 1 DSC curves of 2F and 2F6F (heating rate 5 1C minꢀ1). Cr:
Crystal, ND: discotic nematic phase, M: highly ordered mesophase
(the packing manner of column is unknown) Iso: isotropic liquid.
ꢁc
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Fig. 2 Optical textures of mesophases for 2F (left; at 200 1C) and
2F6F (right; at 155 1C).
and 0.83 kJ molꢀ1, respectively, and 2F6F shows three peaks
at 53.2, 116 and 176 1C (DH: 18.1, 60.9 and 23.0 kJ molꢀ1
,
respectively). The microscopic texture observation of 2F re-
vealed the former two are crystal–crystal transitions, and the
latter two are melting and clearing points, respectively. A
typical Schlieren texture was observed for the mesophase of
2F (as shown in Fig. 2). 2F6F shows a solid like texture in the
mesophase and it is a highly viscous state to indicate this phase
is a highly ordered mesophase, which is supported by the XRD
results (see ESIw).
Fig. 4 X-Ray diffraction patterns of 3F at 205 1C.
mesophases above 196 1C, indicating these three mesophases
are classified to be Colr phases with a small change of the
molecular order for each with temperature (see ESIw).
The X-ray diffraction pattern of 3F5F at 160 1C (for non-
aligned samples) shows a small number of reflections as shown
in Fig. 5, where the reflections in the small angle region have a
spacing ratio of 1 : 1/O3 : 1/2, which provides evidence of a
hexagonal arrangement of molecularly stacking columns.
Furthermore, two halo spacings of 4.5 and 3.5 A were
observed in the wide-angle region. The halo centered at
4.5 A could be assigned to the averaged diameter of molten
alkyl chains6 and the p–p stacking distance of triphenylene
cores in a column is indicated at 3.5 A. The X-ray diffraction
parameters of 3F and 3F5F are summarised in Table 1.
The phase transition parameters of 2F, 2F6F, 3F and 3F5F are
summarized in Table 2, along with those of H4 and F4. These
results show the variation of the position and the number of
fluorine atoms on the peripheral phenyl rings leads to a drastic
change of mesomorphism. Especially, the thermal stability of the
columnar mesophases are extremely stabilized (over 400 1C) in
3F and 3F5F. Those results strongly indicate that a certain
attractive force between the fluorinated phenylene moieties.
On the other hand, the thermal stabilities of the mesophases
for 2F and 2F6F are decreased in comparison with H4 and F4.
This lower thermal stability may be due to the non-coplanarity
On the other hand, the DSC curves of 3F and 3F5F, in
which the outer positions (3/3 and 5 sites) of the phenyl rings
are fluorinated are shown in Fig. 3. These also exhibit en-
antiotropic mesomorphism. Four endothermic peaks were
observed for 3F at 56.1, 196, 210 and 237 1C with phase
transition enthalpies (DH) of 3.6, 3.1, 0.61 and 0.67 kJ molꢀ1
,
respectively. 3F5F shows only two peaks at 14.7 and 140 1C
(DH: 5.3 and 11.1 kJ molꢀ1, respectively).
In the microscopic texture observation up to 400 1C, the
clearing points of 3F and 3F5F could not be definitively
detected and both compounds decompose below the isotropi-
sation temperature. It is noteworthy that fluorination at the
outer positions of the peripheral groups (semi-flexible/semi-
rigid core part) drastically enhance the thermal stability of the
columnar mesophases.
X-Ray diffraction patterns of the mesophase of 3F at 205 1C
(for non-aligned samples) shows several reflection peaks hav-
ing spacings of 26.4, 24.5, 15.6, 13.3 and 9.9 A (Fig. 4), and
this mesophase was assigned to be a Colr phase. Almost no
change of the diffraction patterns was observed for the three
Fig. 3 DSC curves of 3F and 3F5F (heating rate 5 1C minꢀ1). Cr:
Crystal, Colr: rectangular columnar phase, Colh: hexagonal columnar
phase. Iso: isotropic liquid.
Fig. 5 X-Ray diffraction patterns of 3F5F at 160 1C.
Chem. Commun., 2008, 1452–1454 | 1453
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(quintet, J = 8.0 Hz, 12H), 3.89 (t, J = 6.6 Hz, 12H), 6.42 (d, J = 12.5
Hz, 6H), 6.49 (d, J = 9.0 Hz, 6H), 7.90 (t, J = 8.7 Hz, 6H), 8.33 (s,
6H); 19F NMR (CDCl3, CFCl3, 470.0 MHz), d ꢀ104.9 (d, J = 10.9
Hz, 6F); MS m/z 1981.8 (calc. 1981.7 for C96H84F24O18); FTIR (KBr,
cmꢀ1) 2929, 2856, 1750, 1622, 1511, 1468, 1422, 1343, 1243, 1179,
1123, 1043, 1022, 961, 905, 839. Anal. Calc. for C96H84F24O18: C,
71.03; H, 6.95; F, 6.24. Found: C, 70.94; H, 6.91; F, 6.25%. (b) 2F6F:
1H NMR (CDCl3, TMS, 500.0 MHz) d 0.90 (t, J = 7.2 Hz, 18H),
1.30–1.35 (m, 48H), 1.45 (quintet, J = 8.0 Hz, 12H), 1.80 (quintet, J =
7.9 Hz, 12H), 4.00 (t, J = 6.6 Hz, 12H), 6.50 (d, J = 10.6 Hz, 12H),
8.52 (s, 6H); 19F NMR (CDCl3, CFCl3, 470.0 MHz), d ꢀ105.5 (d, J =
10.9 Hz, 12F); MS m/z 2318.0 (calc. 2318.3 for C120H132F24O18); FTIR
(KBr, cmꢀ1) 2928, 2856, 1756, 1636, 1578, 1511, 1469, 1449, 1421,
1356, 1313, 1249, 1199, 1164, 1124, 1079, 1049, 1023, 949, 904, 839,
775. Anal. Calc. for C120H132F24O18: C, 67.07; H, 6.25; F, 11.79.
Found: C, 67.10; H, 6.27; F, 11.88%. (c) 3F: 1H NMR (CDCl3, TMS,
500.0 MHz) d 0.89 (t, J = 7.1 Hz, 18H), 1.30–1.34 (m, 48H), 1.45
(quintet, J = 7.9 Hz, 12H), 1.81 (quintet, J = 8.0 Hz, 12H), 3.96 (t, J
= 6.8 Hz, 12H), 6.65 (t, J = 8.0 Hz, 6H), 7.48 (dd, J = 11.5 Hz, J =
1.8 Hz, 6H), 7.65 (d, J = 8.8 Hz, 6H), 8.21 (s, 6H); 19F (CDCl3, CFCl3,
470.0 MHz) d ꢀ134.5 (d, J = 8.1 Hz 6F); MS m/z 2318.0 (calc. 2318.3
for C120H132F24O18); FTIR (KBr, cmꢀ1) 2955, 2927, 2857, 1741, 1617,
1519, 1469, 1437, 1425, 1286, 1253, 1193, 1141, 1123, 1071, 931, 893,
750. Anal. Calc. for C120H132F24O18: C, 71.03; H, 6.95; F, 6.24. Found:
C, 71.09; H, 6.93; F, 6.31%. (d) 3F5F: 1H NMR (CDCl3, TMS, 500.0
MHz) d 0.90 (t, J = 7.0 Hz, 18H), 1.30–1.34 (m, 48H), 1.46 (quintet, J
= 8.0 Hz, 12H), 1.76 (quintet, J = 8.0 Hz, 12H), 4.19 (t, J = 6.6 Hz,
12H), 7.38 (d, J = 7.8 Hz, 12H), 8.06 (s, 6H); 19F (CDCl3, CFCl3,
470.0 MHz), d ꢀ127.0 (s, 12F), MS m/z 1934.4 (calc. 1934.1 for
C108H120F12O18); FTIR (KBr, cmꢀ1) 2957, 2928, 2857, 1744, 1622,
1583, 1516, 1436, 1350, 1254, 1205, 1123, 1035, 1002, 947, 890, 751.
Anal. Calc. for C120H132F24O18: C, 67.07; H, 6.25; F, 11.79. Found: C,
67.15; H, 6.34; F, 11.87%.
Table 1 X-Ray diffraction parameters for the mesophases of 3F and
3F5F
dhkl/A
Compound
Lattice const./A
hk
Observed
Calculated
3F
(205 1C)
Colr
20
11
31
40
51
26.4
24.5
15.6
13.3
9.9
4.5 (br)b
3.5 (br)
26.4
24.5
14.9
13.2
9.9
(c2mm)
a = 52.9
b = 27.7
Z = 2a
3F5F
(160 1C)
Colh
a = 31.1
10
11
20
21
30
22
26.9
15.5
13.6
10.1
9.1
7.8
26.9
15.5
13.5
10.2
9.0
7.8
4.5 (br)
3.5 (br)
a
Calculated from the lattice constants a and b, correlation length
along the c-axis (3.5 A) and the postulated density r (1.0 g cmꢀ3).
br = Broad.
b
Table 2 Phase transition temperatures of 2F, 2F6F, 3F, 3F5F, H4
and F4
Compound
Phase transition temperatures/1C (DH/kJ molꢀ1
)
2F
Cr1 82.3 (12.6) Cr2 120 (15.0) Cr3 150 (28.3)
ND 213 (0.8) Iso
Cr1 53.2 (18.1) Cr2 116 (60.9) M 176 (23.0) Iso
1 Organofluorine Chemistry, Principles and Commercial Applications,
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2F6F
3F
Cr1 56.1 (3.6) Cr2 196 (3.1) Colr1 210 (0.6) Colr2
237 (0.7) Colr3 400 o Dec.a
3F5F
H47
F48
a
Cr1 14.7 (5.3) Cr2 140 (11.1) Colh 400 o Dec.a
Cr 149 (21.7) Colr 170 (7.2) ND 242 (0.3) Iso
Cr 133 (6.2) Colh 308 (28.4) Iso
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Dec.: Decomposition.
of the phenyl ring with the carbonyl sp2 plane probably
derived from an electrostatic repulsive force between the
strongly electronegative fluorine and carbonyl oxygen atoms.
Thermal stability of mesophases above 400 1C has been
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systems, such as hexabenzocoronenes;9 carbonaceous meso-
phase;10 metallophthalocyanines.11 However, in calamitic li-
quid crystals, no report has been seen such that fluorination of
aromatic rings so drastically enhances the mesomorphic ther-
mal stability.12 Therefore, these results indicate other factors
affected by such fluorination should be considered rather than
the change of electronic state.
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¨
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Simon, Liq. Cryst., 1993, 15, 709.
12 (a) A. S. Matharu, S. J. Cowling and G. Wright, Liq. Cryst., 2007,
34, 489; (b) S. J. Cowling, K. J. Toyne and J. W. Goodby, J. Mater.
Chem., 2001, 11, 1590.
Notes and references
1
z (a) 2F: H NMR (CDCl3, TMS, 500.0 MHz) d 0.90 (t, J = 7.0 Hz,
18H), 1.30–1.34 (m, 48H), 1.44 (quintet, J = 7.9 Hz, 12H), 1.76
ꢁc
This journal is The Royal Society of Chemistry 2008
1454 | Chem. Commun., 2008, 1452–1454