Odd-even effect of dopant molecules on clearing temperatures of nematic liquid-crystal phases

Article abstract of DOI:10.1246/cl.2012.1465

Nematic liquid-crystal phases doped with the novel dopant compounds 1 (C₆F₅COO(CH)_nOCOC₆F₅, n = 17) and 2 (C₆H₅-COO(CH)_nOCOC₆H ₅, n = 17) showed an "opposite odd-even effect" on clearing temperatures. The tendency was exactly opposite to that reported for liquid-crystal dimers. A mechanism is postulated on the basis of the difference in the formation of molecular aggregations of even- and odd-number-dopant molecules in the host liquid-crystal molecules.

Full text of DOI:10.1246/cl.2012.1465

doi:10.1246/cl.2012.1465
Published on the web November 3, 2012
1465
Odd-Even Effect of Dopant Molecules on Clearing Temperatures
of Nematic Liquid-crystal Phases
Keiki Kishikawa,* Yuri Haga, Takahiro Inoue, Tomohiro Watanabe, Masahiro Takahashi, and Shigeo Kohmoto
Department of Applied Chemistry and Biotechnology, Faculty of Engineering, Chiba University,
1-33 Yayoi-cho, Inage-ku, Chiba 263-8522
(Received July 19, 2012; CL-120771; E-mail: kishikawa@faculty.chiba-u.jp)
Nematic liquid-crystal phases doped with the novel dopant
compounds 1 (C6F5COO(CH)nOCOC6F5, n = 1-7) and 2 (C6H5-
COO(CH)nOCOC6H5, n = 1-7) showed an “opposite odd-even
effect” on clearing temperatures. The tendency was exactly
opposite to that reported for liquid-crystal dimers. A mechanism
is postulated on the basis of the difference in the formation of
molecular aggregations of even- and odd-number-dopant mole-
cules in the host liquid-crystal molecules.
examined. From the results, it is difficult to argue an odd-even
effect because of the lack of a reasonable number of examples.
Thus, a systematic study on the odd-even effect of dopant
molecules for LC phases is highly desired. For this purpose, we
developed the efficient dopant compounds 1 and 2 (Scheme 1)
possessing two pentafluorobenzoyloxy and benzoyloxy groups
linked with an alkyl spacer, respectively. A large odd-even effect
was shown on the clearing temperatures of the nematic phases of
N-(4-ethoxybenzylidene)-4-butylaniline (EBBA) and 4-octyloxy-
phenyl 4-octyloxybenzoate (OPOB). An odd-even effect opposite
to that reported for LC dimers was observed. The clearing
temperatures of the rod-like LC compounds doped with the even-
number dopants were much lower than of those doped with the
odd-number dopants.
Liquid-crystal (LC) dimers, which have two rod-like meso-
genic cores linked by an alkyl spacer, have attracted considerable
attention in recent years, and many novel superstructures and
specific properties have been found in the mesophases of those
compounds.1-27 In these molecules, the odd-even effect is a
characteristic and important phenomenon in which the physical
properties depend strongly on the number of carbon atoms (n) in
the alkyl spacer.27 For example, the clearing points of LC dimer
compounds with even numbers are much higher than of those with
odd numbers, and the effect is explained theoretically and
experimentally by the difference in the relative arrangements of
their two rod-like mesogenic units.7,9,15,23,24,26,27 As shown in
Figure 1a, the rod-like units of even-number LC dimer molecules
can align in one direction to generate stable nematic phases, but
those of odd-number LC dimer molecules (Figure 1b) cannot align
in one direction, and stable phases are not generated. If odd-even
effects occur in dimer-type dopant molecules, it is interesting to
investigate how their doping influences the properties of the host
LC compounds. However, to the best of our knowledge, there
has been no systematic study on the odd-even effect of dopant
molecules. Only one example of an odd-even effect of dopant
molecules has been reported on the helical twisting power in host
LC molecules.28 In this example, only two dopant molecules, with
an odd and even number of alkyl spacers, respectively, were
Compounds 1a-1f (n = 2-7) and 2a-2f (n = 2-7)29 were
synthesized through the condensation of pentafluorobenzoic acid
and benzoic acid with the corresponding alkanediols, respec-
tively.34
Plots of the clearing temperatures of the nematic phase of
EBBA doped with 1 (dopant ratios of 1: 0.01, 0.50, 1.00, 2.00, and
5.00 wt %) against the number of carbon atoms (n = 2-7) in the
alkyl chains of 1 are shown in Figure 2. The samples of EBBA
doped with 0.01 wt % of 1 hardly show odd-even effects, whereas
those doped with 0.5-5 wt % of 1 show clear odd-even effects. For
example, the clearing temperatures of EBBA doped with 2 wt % of
X
O
O
X
X
X
X
X
O
CH2
O
n
X
X
1 (X = F)
2 (X = H)
X
X
a: n = 2, b: n = 3, c: n = 4, d: n = 5,
e: n = 6, f: n = 7
OC2H5
H9C4
N
EBBA
O
OC8H17
H17C8O
O
OPOB
Figure 1. Schematic representations of LC dimer molecules possessing
alkyl spacers with (a) n = 2 and (b) n = 3.
Scheme 1. Molecular structures of 1, 2, EBBA, and OPOB.
Chem. Lett. 2012, 41, 1465-1467
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1466
Figure 2. Plots of the clearing temperatures of the nematic phases of
EBBA doped with 1 against the number of carbon atoms in the alkyl chains
of 1 on heating (dopant ratio of 1: 0.01, 0.50, 1.00, 2.00, and 5.00 wt %).
Figure 4. Plots of the clearing temperatures of the nematic phases of
EBBA doped with 1 and 2 against the number of carbon atoms in their
alkyl chains on heating (the contents of 1 and 2: 2 wt %).
Figure 5. Net atomic charges of (a) 1a (n = 2) and (b) 1b (n = 3)
calculated by AM1 method.33 The atoms charged negatively and positively
are indicated with blue and red circles, respectively. The dipole direction of
the carbonyl group is indicated with an arrow.
A mechanism for the generation of the odd-even effect is
proposed as follows. From the comparison of the effects of
dopants 1 and 2, it was clarified that the effect could not be
explained by the perfluoroarene-arene interaction, as we had
expected at first.30-32 Therefore, we focused on the odd-even effect
on the melting points of 1 and 2 themselves. At room temperature,
compounds 1 and 2 with n = 2, 4, and 6 are crystals, whereas
those with n = 3, 5, and 7 are oils. This difference strongly
suggests that the even-number molecules are highly self-organized
by intermolecular interactions to give the crystal phases, but those
with the odd-number molecules are not organized rigidly even at
room temperature. For the investigation of the origin of this
difference, the net atomic charges of molecules 1a (n = 2) and 1b
(n = 3) were calculated by a semiempirical method (AM133), and
the results are shown in Figure 5. A remarkable difference
between the odd- and even-number dopant molecules was found
in the relative arrangements of the dipoles of their two carbonyl
groups. Molecules 1a (n = 2) possessing the two dipoles aligned
in an antiparallel manner are suitable for the generation of linear
molecular networks with the intermolecular dipole-dipole inter-
actions. On the other hand, molecules 1b (n = 3) possessing the
two dipoles aligned in a parallel manner are suitable for
dimerization rather than for the formation of a molecular network.
This should be the origin of the difference in self-organization
abilities between 1a and 1b. This also suggests that this difference
affects the self-organization of these dopant molecules in the
nematic phases of the host LC compounds.
Figure 3. Plots of the transition temperatures of the nematic-isotropic
(N ¼ Iso) and nematic-smectic C (N ¼ SmC) phases of OPOB doped
with 1 against the number of carbon atoms in the alkyl chains of 1 on
heating (dopant ratio of 1: 1 wt %).
1a (n = 2), 1c (n = 4), and 1e (n = 6) are lower than those of
EBBA doped with 2 wt % of 1b (n = 3), 1d (n = 5), and 1f
(n = 7). Similarly, the transition temperatures of SmC-N and N-
Iso of OPOB doped with 1 (dopant ratio of 1: 1 wt %) are plotted
against n of 1 in Figure 3. Figure 3 indicates a similar odd-even
effect on the N-Iso transition temperatures. An effect on the SmC-
N transition temperatures is also observed, but it is weaker than
that for the N-Iso transition temperatures.
For the determination of the influence of the pentafluoro-
phenyl groups in 1, the effects of the nonfluorinated compounds 2
were investigated. The clearing temperatures of EBBA doped with
2 wt % of 2a-2f were plotted against n. The plot in Figure 4 shows
a similar odd-even effect. This suggests that the odd-even effect
does not originate from the intermolecular interactions of the
pentafluorophenyl groups of 1. Only the clearing temperatures of
EBBA doped with 1a (n = 2) and 1c (n = 4) are slightly lower
than those doped with 2a (n = 2) and 2c (n = 4). The nematic
phases of EBBA doped with 1 and 2 possessing alkyl spacers with
n = 3, 5, 6, and 7 gave almost the same respective clearing
temperatures.
For the investigation of the difference in the superstructures of
the nondoped and doped LC compounds, the 2D XRD profiles of
pure EBBA, EBBA doped with 1b (n = 3), and EBBA doped with
1c (n = 4) (dopant ratio of 1: 5 wt %) in the nematic phases were
measured. However, no clear difference was observed between
these samples.
We postulated the following mechanism for the generation
of the odd-even effect of the dopant molecules. As shown in
Figure 6, the even-number molecules generate molecular networks
in the LC phases by using the intermolecular dipole-dipole
interactions between their carbonyl groups. The linear molecular
networks with a periodicity destabilize the one-directional and
Chem. Lett. 2012, 41, 1465-1467
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Chem. Lett. 2012, 41, 1465-1467
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/