Fig. 3 Molecular shapes of (a) CN6E.11OMe and (b) CN6E.10OMe.
the basis of the characteristic Schlieren optical texture
containing two- and four-point singularities observed when
viewed under the polarized light microscope; this texture also
flashed when subjected to mechanical stress. The values of the
entropy changes are consistent with this assignment.
decyl or dodecyl spacer gives solely nematic materials.9 The
nematic phases of these compounds can also be supercooled to
temperatures lower than the smectic–nematic transition tem-
peratures seen for MeO6E.11CN and CN6E.11OMe. Thus, the
formation of the smectic phase is strongly dependent on the
length and parity of the flexible spacer. Given that the principal
difference between the trimers on varying spacer length is their
average molecular shape, it seems reasonable to assume that
shape also play an important role in smectic phase formation.
Specifically, MeO6E.11CN and CN6E.11OMe have stretched
S-shaped structures in which all three mesogenic units are
inclined with respect to each other, whereas the corresponding
decyl or dodecyl compounds have hockey-stick-like shapes in
which two mesogenic units are coparallel while the third is
inclined to the other two, see Fig. 3. It appears that only the
S-shaped molecules exhibit this phase.
Fig. 4 shows a highly schematic representation of the local
molecular arrangement within an alternating smectic C phase
for which d/l is approximately 0.33. Within this structure the
S-shaped molecules can pack efficiently and a microphase
separation occurs to give two types of domains, one consisting
of alkyl chains and the other of mesogenic units. Each layer of
the mesogenic units contains an equal number of the three
different moieties which is both entropically favorable and also
maximizes the favorable electrostatic interactions between the
electron deficient cyano-substituted units and the electron rich
central and methoxy-substituted groups. The alternation in the
sense of the tilt angle requires a correlation of the mesogenic
groups on passing through the phase and this is provided by
a combination of the specific interaction between the electron
rich and electron deficient units and the flexible spacers
providing the oligomeric nature of the system.
There are few examples of smectic liquid crystal trimers in
the literature10–18 and the only layer spacings reported for these
imply monoloayer arrangements.10,16,17 One of these had an
alternating tilt angle between the layers and was termed as
anticlinic,16 which may be a more appropriate term than alter-
nating SmC. A liquid crystal tetramer consisting of molecules
containing four mesogenic units and three flexible spacers has
been reported which shows an orthogonal smectic phase for
which d/l was approximately 0.25,19 while for non-symmetric
liquid crystal dimers intercalated phases are characterized
by values of d/l of approximately 0.5.5,8 Thus, the triply-
intercalated arrangement reported here is a new smectic
modification but is strongly reminiscent of phases seen in
semi-flexible main chain liquid crystal polymers.20 Further
studies are now required to establish the molecular features
required for its formation.
On cooling the nematic phases exhibited by MeO6E.11CN
and CN6E.11OMe under the polarized light microscope a well-
defined focal conic fan texture is developed, see Fig. 1(a), which
on shearing gives a Schlieren texture. On cooling a home-
otropically aligned nematic sample, a texture consisting of
regions of well-defined focal conic fans and Schlieren texture
containing both two- and four-point brush disclinations was
obtained, see Fig. 1(b). A Schlieren texture containing both
types of point singularity would normally be thought of to
imply a nematic phase but such an assignment is in conflict with
the presence of focal conic defects which indicate a layered
structure. This texture is characteristic, however, of a particular
modification of the smectic C phase in which the sense of the tilt
angle alternates between adjacent layers such that the global tilt
angle is zero.
The X-ray diffraction pattern of the smectic mesophase
exhibited by CN6E.11OMe contains a single peak in the low
angle region implying a layered structure with a sinusoidal
density wave along the layer normal, and a broad peak in the
˚
wide angle region centered at a spacing of 4.38 ¡ 0.05 A,
indicating a liquid-like arrangement of the molecules within the
˚
layers, see Fig. 2. The layer spacing d is 19.0 ¡ 0.2 A, which is
close to one-third that of the estimated all-trans molecular
˚
length, l, of the most extended conformation of 60.4 A. The
layer spacing has only a small temperature dependence, and has
the same value on heating and on cooling. The monotropic
nature of the smectic phase shown by MeO6E.11CN precluded
its study using X-ray diffraction. The molecular arrangement
within the smectic phase will be discussed later.
The observation of smectic behavior for MeO6E.11CN and
CN6E.11OMe but not for MeO6E.11OMe and CN6E.11CN is
not simply a consequence of lower melting points for the
former and, in fact, of the four trimers MeO6E.11CN exhibits
the highest melting point. Indeed, it is important to note that
the nematic phases of both MeO6E.11OMe and CN6E.11CN
can be supercooled to temperatures lower than the smectic–
nematic transition temperatures observed for the other two
trimers. This strongly suggests that interactions involving
the unlike terminal mesogenic units play an important role in
the formation of the smectic phase. To test this suggestion, the
phase behavior of an equimolar mixture of MeO6E.11OMe
and CN6E.11CN was determined and this did indeed show
an induced smectic phase in addition to a nematic phase.
Unfortunately, the monotropic nature of this smectic phase
precluded its study using X-ray diffraction. Replacing the
undecyl spacer in MeO6E.11CN and CN6E.11OMe by either a
Acknowledgements
We would like to thank Mr Nick Brooks for his help with the
X-ray measurements.
References
1
C. T. Imrie and P. A. Henderson, Curr. Opin. Colloid Interface
Sci., 2002, 7, 298.
2
C. V. Yelamaggad, S. Anitha Nagamani, U. S. Hiremath,
D. S. Shankar Rao and S. Krishna Prasad, Liq. Cryst., 2002,
29(2), 231.
Fig. 4 Schematic representation of the local molecular order within a
triply-intercalated alternating smectic C structure of CN6E.11OMe.
3
K. Praefcke and D. Singer, ‘Charge-transfer systems’, in Hand-
book of Liquid Crystals, D. Demus, J. W. Goodby, G. W. Gray,
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 4 8 6 – 2 4 8 8
2 4 8 7
Imrie, Corrie T.
