exclusively form smectic C (SmC) phases when a fluorine atom
is introduced at this position because fluoro substituents in the
outer-edge positions fill any empty space with their polar C-F
units.9 On the basis of our previous studies, we assumed that for
(F)-DPDA-OCm, the dumbbell-shaped mesogen would prevent
microphase separation.6a According to previous studies, (F)-
DPDA-OCm derivatives with m ² 8 exhibit SmC behavior.10
As expected, the transition temperatures of the (F)-DPDA-
OCm derivatives oscillated according to an odd-even effect
with respect to the number of alkoxy tail carbons; that is, even-m
derivatives have higher transition temperatures than odd-m
derivatives. Both the transition temperatures, TI-N and Tc, of
the series of (F)-DPDA-OCm derivatives were found to be
significantly lower than those of the series of DPDA-OCm
derivatives with the same alkoxy tails. For example, the Tc and
(a)
TI-N values of DPDA-OC5 decreased from 110.1 to 34.8 °C, and
from 153.1 to 80.2 °C, respectively, on fluorine substitution.
However, the magnitude of the temperature range for the
N phase of each compound is unchanged, with the exception
of (F)-DPDA-OC1 and (F)-DPDA-OC2, which, unlike their
DPDA-OCm analogs, exhibit no liquid crystallinity. Com-
pounds with shorter alkoxy tails are close to a crystal phase,
while those with longer tails are close to a smectic phase. As
a result, (F)-DPDA-OC5 and (F)-DPDA-OC6 would have the
widest N width.
The enthalpy changes (¦H) for (F)-DPDA-OCm and
DPDA-OCm at TI-N and Tc are shown in Table 1. In all cases,
the ¦H values for the (F)-DPDA-OCm derivatives are smaller
than those for the DPDA-OCm derivatives. This reduction in
¦H values occurs because fluoro substitution at the lateral
positions interferes with crystal packing and decreases intermo-
lecular attractive interaction. Importantly, such reduced ¦H
values may lead to good miscibility with other LC materials in
eutectic mixtures. The total ¦H at TI-N and Tc follows an odd-
even trend; that is, ¦H values are smaller for odd-m derivatives.
In addition, the larger ¦H value at Tc for (F)-DPDA-OC1 than
for DPDA-OC1 occurs because (F)-DPDA-OC1 does not have
a mesophase, unlike DPDA-OC1; the ¦H value for the
mesophase-to-crystal phase transition is typically larger than
that of the isotropic-to-mesophase transition.
(b)
Figure 1. (a) Temperature dependence of the extraordinary
(ne) and ordinary refractive index (no) of (F)-DPDA-OC6
(square plot) and DPDA-OC6 (circle plot) at 550 nm. (b)
Temperature dependence of birefringence (¦n) of (F)-DPDA-
OC6 (square plot) and DPDA-OC6 (circle plot) at 550 nm.
the introduction of fluorine atoms leads to a decrease of the
mean refractive index (©nª) from 1.64 for DPDA-OC6 to 1.60
for (F)-DPDA-OC6 in their mesophases. This decrease may be
caused by the lower molecular density of (F)-DPDA-OC6
owing to its expanded rotational volume, which is caused by
the introduction of the fluorine atom. In contrast, the ¦n values
did not change upon the introduction of fluorine at a reduced
temperature of 28 °C, and the ¦n value for both compounds was
0.31 at 550 nm.
Next, to examine the effect of introducing fluorine atoms in
the outer-edge position on refractive index, we measured the
refractive index values of extraordinary (ne) and ordinary rays
(no) for fluoro-substituted and nonsubstituted compounds with
the same tail length, namely (F)-DPDA-OC6 and DPDA-OC6,
using microscopic spectroscopy with the multiple-beam inter-
ference (MBI) theory, as described in our previous study;11 the
gained transmitted light and fitting curve for (F)-DPDA-OC6
are shown in Figure S4.
The dependence of the ne, no, and ¦n (ne ¹ no) values of
(F)-DPDA-OC6 on the wavelength is shown in Figure S5.
These results show a positive wavelength dispersion, which
is typical of rod-like molecules. In addition, the temperature
dependence of the ne, no, and ¦n values at 550 nm at a reduced
temperature Ti ¹ T is shown in Figure 1, which reveals that
the ne value increases, whereas the no value decreases when
temperature is decreased, and the ¦n value also increases with
a decrease in temperature. As shown in Figure 1a, interestingly,
the ne and no values of (F)-DPDA-OC6 are lower than those of
DPDA-OC6 over the entire N phase. This result suggests that
Finally, the temperature dependence of ¦n for (F)-DPDA-
OC3-7 was measured using microscopic spectroscopy, as shown
in Figure S6. The ¦n values were compared at the lowest
temperature measured immediately before crystallization: 72 °C
for OC3, 72 °C for OC4, 39 °C for OC5, 58 °C for OC6, and
43 °C for OC7. At these temperatures, the ¦n values were the
highest for each compound. All the compounds exhibited very
high ¦n values (over 0.3), and the ¦n values decreased in the
following order: OC4 > OC3 > OC6 > OC5 > OC7, probably
depending on each N phase range. In addition, the ¦n0 values
(i.e., the associated ¦n values for the perfectly aligned
condition; order parameter (S) = 1) were estimated by fitting
the curve to the following equations: ¦n = ¦n0S and S =
(1 ¹ T/TI-N)¢, where T and Ti are the measurement temperature
(T) and clearing temperature (TI-N), respectively, for which the
method has been described in previous reports.6b The exponent
¢ depends on the molecular structure, and its value is close to
0.2. All estimated values are shown in Table 2 for (F)-DPDA-
OC3-7. The ¦n0 values are very high and nearly unchanged
compared with those previously reported for DPDA.6b
© 2014 The Chemical Society of Japan | 1859
Arakawa, Yuki
