Kinetically induced intermolecular association: unusual enthalpy changes
in the nematic phase of a novel dimeric liquid-crystalline molecule†
Atsushi Yoshizawa* and Akihisa Yamaguchi
Department of Materials Science and Technology, Faculty of Science and Technology, Hirosaki University,
3 Bunkyo-cho, Hirosaki 036-8561, Japan. E-mail: ayoshiza@cc.hirosaki-u.ac.jp; Fax: +81-172-39-3558;
Tel: +81-172-39-3558
Received (in Cambridge, UK) 20th May 2002, Accepted 5th August 2002
First published as an Advance Article on the web 15th August 2002
A novel dimeric liquid-crystalline molecule in which two
mesogenic groups are connected via catechol was found to
have smectic-like layer ordering in the nematic phase, and
unusual enthalpy changes were observed in the nematic
phase on heating from the monotropic smectic C phase.
miscible with that of 8-PYP-6O. The SmA phase of 8-PYP-6O
was found to disappear as the percentage of 8-PYP-6O rose
above 40 wt.%.
In a photomicrograph of BOPPHB in the N phase at 50 °C
director fluctuation was observed in the homogeneous align-
ment, on the other hand, the texture of the homeotropic
alignment was found to become slight white, indicating that the
director tilts (Fig. S1, ESI†). By conoscopic observations of a
homeotropic aligned sample of BOPPHB in the nematic phase,
the interference pattern of the conoscopic texture changed to be
split with decreasing temperature, suggesting that the nematic
phase has biaxiality.9 Further investigations will be necessary to
prove the biaxiality. In the small angle region of the X-ray
diffraction profile in the N phase, a peak around 2q = 3.2°
became sharp with decreasing temperature, indicating the
formation of smectic-like layer structure in the nematic phase
(Fig. S2, ESI†). Such a sharp peak was not detected in the N
phase of the corresponding monomeric molecule 8-PYP-6O. A
XRD profile at 41 °C in the SmC phase gives a layer spacing of
d = 29 Å, whereas the length of BOPPHB is estimated to be 31
Å from a MM2 model.
DSC measurements were carried out at a scanning rate of 5
°C min21 for both cooling and heating cycles, and the results are
shown in Fig. 2. Fig. 2(c) shows a thermogram on heating from
crystal to isotropic liquid and then cooling from isotropic liquid.
Fig. 2(a) shows a thermogram on cooling from Iso Liq to N and
then heating from N to Iso Liq. The transition behaviour in Fig.
2(a) did not depend on the rate of cooling and heating. Cooling
from Iso Liq and holding the sample at 55 °C in the N phase for
1 h and then heating to Iso Liq, a reversible profile between the
cooling and heating processes was obtained. Fig. 2(b) shows a
thermogram observed on cooling from Iso Liq to SmC and then
heating from the monotropic SmC to Iso Liq. Marked hysteresis
in DSC thermograms between the cooling and heating processes
was observed. In the heating process the SmC to N transition
peak at 44.2 °C (0.23 kJ mol21) was detected reversibly,
however three new peaks at 66.4 °C (6.2 kJ mol21), 71.3 °C
(0.43 kJ mol21) and 77.0 °C (0.33 kJ mol21) were observed.
Then the N-I transition at 84.3 °C (2.5 kJ mol21) was detected.
The driving force of mesophase formation is a fundamental
topic in the investigation of molecular assembly systems. A
primary factor in thermotropic liquid crystal phases is the gross
molecular shape of a compound.1,2 Recently microsegregation
and molecular topology have attracted much attention as novel
self-organizing systems.1,3 In addition to structural studies,
molecular motion is also important to understand the micro-
scopic behaviour of liquid-crystalline molecules. Our 13C NMR
NMR study reveals that (1) cooperative motion for the core
parts contributes to the orientational order of the molecules in
each layer and (2) inter-layer permeation of tails causes
correlation between cores in adjacent layers.4
In the present study, we have designed a compound in which
two liquid-crystalline molecules are connected via catechol.
Strong correlation of rotation around the long axis of each
mesogenic group is expected. On the other hand, U-shaped
dimeric liquid crystals have been investigated by several
research groups.5–8 Attard and Douglass reported property–
structure correlations of the dimeric liquid crystals derived from
phthalic acid, providing important understanding about the U-
shaped liquid crystal system.8
Preparation of the compound, 1,2-bis{6-[4-(5-octylpyrimi-
dine-2-yl)phenyloxy]hexyloxy}benzene (BOPPHB), was ob-
tained from two successive alkylations. 5-Octyl-2-(4-hydroxy-
phenyl)pyrimidine was treated with 1,6-dibromohexane in the
presence of potassium carbonate and then catechol was
alkylated with the obtained 5-octyl-2-[4-(6-bromohexyl)phe-
nyl]pyrimidine. Purification of the material was carried out
using column chromatography and then recrystallization from
ethanol. The purity was determined to be 99.9% by HPLC
1
analysis. The structure was elucidated by IR and H NMR
studies.
The transition temperatures determined by optical micros-
copy were Iso Liq 83.3 °C N 43.4 °C SmC on cooling and the
melting point was 76.0 °C. Transition temperatures for the
corresponding monomeric molecule, 5-octyl-2-(4-hexyloxy-
phenyl)pyrimidine (8-PYP-6O), were Iso Liq 64.0 °C N 56.0
°C SmA 45.0 °C SmC. A phase diagram between BOPPHB and
8-PYP-6O is shown in Fig. 1. The N phase of BOPPHB was
† Electronic supplementary information (ESI) available: photomicrographs
of the nematic phase formed by BOPPHB on cooling, X-ray diffraction
b204901p/
Fig. 1 Binary phase diagram for BOPPHB with 8-PYP-6O.
2060
CHEM. COMMUN., 2002, 2060–2061
This journal is © The Royal Society of Chemistry 2002
Yoshizawa, Atsushi
