7.08 (d, 2H, Ar), 6.95 (d, 2H, Ar), 4.05 (t, 2H, –O–CH2–),
1.71 (qv, 2H, –CH2–), 1.28 (m, 16H, –CH2–), 0.81 (t, 3H,
–CH3),13C NMR dC (DMSO): 164.99, 164.43, 164.12, 163.50,
161.55, 153.97, 132.69, 129.26, 128.58, 123.48, 121.85, 121.19,
116.76, 115.40, 114.85, 68.82, 31.78, 29.57, 29.14, 15.93, 22.52,
14.37. Anal. Calcd for C32H36N2O5: C 72.7, H 6.86, N 5.30;
found C 72.9, H 6.85, N 5.32.
2-{4-[4-Octyloxy-2-(pent-4-enyloxy)carbonyloxyphenyl}phenyl-
5-[4-(4-undecyloxy)carbonyloxyphenyl]phenyl-[1,3,4]-
oxadiazole (6)
Quantities: derivative 5 (0.2 g, 0.3 mmol), 4-octyloxy-2-pent-4-
enyloxybenzoic acid (0.15 g, 0.4 mmol), DIPC (0.05 g,
0.4 mmol), DMAP (catalytic), 40 ml CH2Cl2, purification on
silica (DCM containing 10% ethyl acetate), white product,
1
yield 0.22 g (68.81%), H NMR dH (CDCl3): 8.21 (d, 2H, Ar),
8.19 (d, 2H, Ar), 8.15 (d, 2H, Ar), 8.04 (d, 1H, Ar), 7.41 (d, 2H,
Ar), 7.38 (d, 2H, Ar), 6.98 (d, 2H, Ar), 6.53 (dd, 2H, Ar), 5.82
(m, 1H, –CHL), 4.99 (m, 2H, LCH2), 4.04 (m, 6H,–O–CH2–),
2.28 (q, 2H, –CH2–), 1.95 (qv, 2H, –CH2–), 1,80 (m, 4H,
–CH2–), 1.33 (m, 26 H, –CH2–), 0.88 (t, 6H, –CH3). Anal.
Calcd for C52H64N2O8: C 73.91, H 7.63, N 3.31; found C
73.90, H 7.65, N 3.3.
Fig. 1 Textures of 6 (top) and 7 (bottom) in the N-phase.
is reduced by the different lengths of the terminal alkyl chains,
containing eight and eleven methylene groups and most
importantly by the lateral alkyl chain (pentenyloxy group)
in 6 and further by the attached pentamethyldisiloxane
group in 7.
2-{4-[4-Octyloxy-2-(5-(1,1,3,3,3-pentamethyldisiloxanyl)pentyl-
oxyphenylcarbonyloxy]}phenyl-5-[4-(4-undecyloxy)carbonyl-
oxyphenyl]phenyl-[1,3,4]-oxadiazole (7)
The transition temperatures and enthalpies are summarised
in Table 1.
A mixture of (1 equiv.) and 5 equiv. of 1,1,1,3,3-pentamethyl-
disiloxane was dissolved in dried DCM, Karstedt catalyst was
added dropwise, and the mixture was stirred at room tem-
perature for 12 h. Quantities: derivative 6 (0.05 g, 0.05 mmol),
1,1,1,3,3-pentamethyldisiloxane (0.08 g, 0.59 mmol), 15 drops
of Karstedt catalyst, 35 ml DCM, purification on silica (DCM)
and precipitation with hexane, white–yellowish product, yield
Compared to symmetrical oxadiazole derivatives1,9,10,15–17
as well as recently reported non-symmetrical systems,18,19
which do not contain lateral chains, the melting points of 6 and
7 are relatively low, with 108.3 uC for 6 and only 65.4 uC for 7,
the siloxane group being responsible for the reduction of the
melting point by about 43 uC. In other words, the effect of
lateral substitution on the transition temperatures is signifi-
cant. The materials do not show enantiotropic LC behaviour,
but supercooling of the isotropic melt leads to the formation of
a nematic phase at 104.3 uC for 6 and 63.1 uC for 7 with an
onset for the crystallisation at 62.5 uC for 6 and 52.4 uC for 7,
resulting in a nematic phase range of 41.8 uC for 6 and 10.7 uC
for 7. The reduction of the stability of the nematic phase by
about 40 uC by the attachment of pentamethylsiloxane to a
short lateral chain, is in line with behaviour observed for
laterally substituted linear mesogens where reductions between
30 to 50 uC have been detected when short siloxane groups
have been attached.20 Fig. 1 shows formation of the nematic
textures for 6 and a fully formed Schlieren texture for 7, shown
at the bottom of Fig. 1. The texture at the isotropic to nematic
texture is grainier and the fully formed texture is more
threadlike. Fig. 2 shows the DSC traces for 6 and 7. The
transition enthalpies for the isotropic to nematic transition are
rather low, at 0.8 J g21 and 0.13 J g21, with the siloxane
substituted system 7 exhibiting the lower value. This suggests a
much smaller increase of ordering going from the isotropic to
the nematic phase compared to system 6. However the
enthalpy values are quite similar to related linear systems
Typical nematic Schlieren textures can be observed for both
systems and notable is the large number of 2-brush defects.
1
51 mg (88%), H NMR dH (CDCl3): 8.18 (d, 2H, Ar), 8.16 (d,
2H, Ar), 8.14 (d, 2H, Ar), 8.11 (d, 2H, Ar), 8.01 (d, 1H, Ar),
7.38 (d, 2H, Ar), 7.36 (d, 2H, Ar), 6.95 (d, 2H, Ar), 6.50 (dd,
2H, Ar), 4.01 (q, 6H, –O–CH2–), 1.79 (m, 6H, –CH2–), 1.36 (m,
24H, –CH2–), 0.85 (t, 6H, –CH3), 0.46 (m, 2H, –CH2–Si), 0.00
(m, 15H, –CH3),13C NMR dC (CDCl3): 164.86, 164.55, 164.31,
164.14, 163.92, 163.50, 162.01, 154.14, 153.89, 134.66, 132.51,
128.43, 128.36, 122.98, 122.82, 121.46, 121.09, 121.04, 114.51,
110.62, 105.48, 100.24, 69.01, 68.48, 32.00, 31.90, 29.70, 29.68,
29.64, 29.45, 29.43, 29.32, 29.22, 29.17, 28.94, 26.07, 23.11,
22.78, 22.75, 18.36, 14.22. Anal. Calcd for C68H78N2O9Si2: C
68.67, H 8.03, N 2.86; found C 68.7, H 7.98, N 2.9.
Results and discussion
The investigation of the liquid-crystal phase behaviour of the
systems 3, 4, 6 and 7 showed that only materials 6 and 7 exhibit
mesomorphic behaviour. 3 melts at 99.0 uC and 4 at 83.0 uC
from the crystalline state to an isotropic liquid. The samples
crystallise on cooling. Increasing the size of the mesogenic
moiety by one aromatic group leads to compounds 6 and 7
(Fig. 1 and 2).
The aromatic core system, though tilted, due to the presence
of the oxadiazole group is essentially symmetric, the symmetry
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 4711–4715 | 4713