Liquid crystal tetramers

Article abstract of DOI:10.1039/a901899i

The first homologous series of liquid crystal tetramers in which four mesogenic groups are linked via three alkyl spacers has been synthesised and characterised. The length of the outer two spacers is varied from 3 to 12 methylene units while the central

Full text of DOI:10.1039/a901899i

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J O U R N A L O F
Liquid crystal tetramers
Corrie T. Imrie,a David Stewart,a Catherine ReÂmy,a David W. Christie,a I. W. Hamleyb and
R. Hardingb
aDepartment of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen, Scotland
AB24 3UE. E-mail: c.t.imrie@abdn.ac.uk
C H E M I S T R Y
bSchool of Chemistry, University of Leeds, Leeds, UK LS2 9JT
Received 10th March 1999, Accepted 25th June 1999
The ®rst homologous series of liquid crystal tetramers in which four mesogenic groups are linked via
three alkyl spacers has been synthesised and characterised. The length of the outer two spacers is varied
from 3 to 12 methylene units while the central spacer is held at 8 methylene units. All the members of
the series exhibit enantiotropic nematic behaviour except for the decyl and dodecyl homologues which
exhibit exclusively smectic A behaviour. In addition, smectic A behavior is observed for the hexyl to
dodecyl homologues. The transitional properties, namely the clearing temperatures and associated
entropy changes, exhibit a dramatic odd±even effect as the length and parity of the outer spacers are
varied. This behaviour is interpreted in terms of how the spacers control the average molecular shape.
The ratio of the smectic periodicity to the estimated all trans molecular length for the nonyl
homologue is just 0.27. Three differing local packing arrangements are suggested to account for this
surprising result.
Introduction
Experimental
Semi-¯exible main chain liquid crystal polymers consist of
alternating mesogenic groups and ¯exible spacers, most
commonly alkyl chains.1,2 These materials have attracted
much interest and not only as a result of their considerable
application potential in a range of advanced technologies but
also because they provide a demanding challenge to our
understanding of self-assembly in polymeric systems. A
particular feature of the transitional behaviour of this class
of materials is the dramatic odd±even effect exhibited by the
clearing temperatures and the associated entropy changes of a
homologous series of polymers on varying the length and
parity of the spacer.3 Speci®cally, if an odd number of atoms
link the mesogenic groups then a considerably lower clearing
temperature is observed compared to that seen for the
corresponding polymer containing either one fewer or one
greater number of atoms in the spacer. Such behaviour is
mimicked by liquid crystal dimers in which just two mesogenic
units are connected via a single ¯exible spacer4,5 although the
alternation in the transitional properties is not quite so large. In
consequence liquid crystal dimers have been used widely as
model compounds for the polymeric systems.6 In order to
investigate how the transitional behaviour evolves from the
dimers to the polymers a small number of trimers have been
characterised containing three mesogenic groups and two
¯exible spacers.7±14 We have now extended this approach to
include linear liquid crystal tetramers in which four mesogenic
units are connected via three ¯exible alkyl spacers, see Fig. 1.
The acronym used to refer to the tetrameric series is
CBOnO.O8O.OnOCB where n denotes the number of methy-
lene groups in the outer two ¯exible spacers. There are very few
examples of liquid crystal tetramers described in the literature15
and this is the ®rst report in which the transitional behaviour of
a complete homologous series is discussed.
Synthesis
The tetrameric CBOnO.O8O.OnOCB series was prepared using
the synthetic route shown in Scheme 1. The syntheses of the a-
bromo-v-(4-cyanobiphenyl-4'-yloxy)alkanes, 2, and the a-(4-
cyanobiphenyl-4'-yloxy)-v-(4-formylphenyl-4'-oxy)alkanes 3,
have been described in detail elsewhere.16
1,8-Bis(4-nitrophenyloxy)octane, 4. A mixture containing 4-
nitrophenol (10.0 g, 71.9 mmol), 1,8-dibromooctane (9.3 g,
34.3 mmol) and potassium carbonate (16.5 g, 120 mmol) in
DMF (100 mL) was re¯uxed with stirring overnight. The
reaction mixture was allowed to cool and poured into ice-cold
water (1.5 L). The resulting yellow precipitate was collected by
®ltration, recrystallised with hot ®ltration from ethanol and
dried under vacuum.
Yield 71%. dH (CDCl3) 8.2 (d, aromatic ortho to NO2, 2H, J
9.2); 6.9 (d, aromatic meta to NO2, 2H, J 9.2); 4.0 (t, OCH2,
2H, J 6.4); 1.8 (m, OCH2CH2, 2H); 1.4 (m, CH2, 4H). IR (KBr)
n
max: 1336 cm21 (N±O).
1,8-Bis(4-aminophenyloxy)octane, 5. 4 (2.86 g, 7.4 mmol)
was dissolved in boiling ethanol (100 mL) and a solution of
sodium sul®de nonahydrate (14.7 g, 61.3 mmol) in water
(50 mL) was added. The resulting mixture was re¯uxed with
stirring overnight. The reaction mixture was allowed to cool
and the precipitate collected by ®ltration. The crude product
was recrystallised from ethanol and dried under vacuum.
Yield 35%. dH 6.7, 6.6 (m, aromatic, 4H); 3.9 (t, OCH2, 2H, J
6.6); 3.3 (bs, NH2, 2H); 1.7 (m, OCH2CH2, 2H); 1.4 (m, CH2,
4H). IR (KBr) nmax: 3400, 3315 (N±H stretch); 1633 (N±H
bend).
Fig. 1 The molecular structure of the tetrameric CBOnO.O8O.OnOCB series.
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Scheme 1
Tetramers, 1. A solution containing 3 (2.8 mmol), 5 (0.4 g,
1.3 mmol) and a trace of toluene-p-sulfonic acid in ethanol
(50 mL) was re¯uxed with stirring overnight. The reaction
mixture was allowed to cool and the precipitate obtained was
puri®ed by recrystallisation from DMF. The yields in all cases
were in excess of 60%. The tetramers were too insoluble in any
appropriate solvent to allow for structural characterisation
using NMR spectroscopy. Their IR spectra, however, were
consistent with the proposed structures. Speci®cally, the bands
associated with the stretch and bend deformations of the N±H
bond in 5 and that associated with the carbonyl bond in 3 are
absent in the spectra of the tetramers. The products' spectra do
contain, however, a band corresponding to the cyano stretch at
ca. 2235 cm21 and one at ca. 1620 cm21 corresponding to the
imine stretch.
Genesis Series FTIR spectrometer. The purities of all the
intermediates were veri®ed using thin layer chromatography.
Thermal properties were determined by differential scanning
calorimetry (DSC) using a Mettler-Toledo DSC 820 system
equipped with an intracooler accessory and calibrated using an
indium standard. The heating and cooling rates in all cases
were 10 ³C min21. Phase identi®cation was performed by
polarised light microscopy using an Olympus BH-2 optical
microscope equipped with a Linkam THMS 600 heating stage
and a TMS 91 control unit.
Results and discussion
The transition temperatures and associated entropy changes
exhibited by the tetrameric series CBOnO.O8O.OnOCB are
listed in Table 1. All ten members of the series exhibit liquid
crystalline behaviour. The propyl, butyl and pentyl homo-
logues show only nematic behaviour. The hexyl, heptyl, octyl,
nonyl and undecyl homologues exhibit both smectic A and
nematic phases while the decyl and dodecyl members show
exclusively smectic A behaviour. Nematic phases were assigned
on the basis of the schlieren optical texture containing both two
Characterisation
The proposed structures of all the intermediates were veri®ed
1
1
using H NMR and IR spectroscopy. H NMR spectra were
measured in CDCl3 on a Bruker AC-F 250 MHz NMR
spectrometer. IR spectra were recorded using an ATI Mattson
Table 1 The transition temperatures and associated entropy changes for the tetrameric CBOnO.O8O.OnOCB series
TNI/³C
(DSNI/R)
n
Tm/³C
TSmAN/³C
TSmAI/³Ca
(DSm/R)
(DSSmAN/R)
(DSSmAI/R)b
3
4
5
6
7
8
9
10
11
12
192
221
161
202
179
192
152
176
150
174
Ð
Ð
Ð
220
174
243
217
Ð
246
307
250
279
241
258
235
238a
210
220a
22.4
17.5
23.5
14.8
11.6
28.2
22.3
26.0
19.0
18.0
Ð
Ð
Ð
0
2.16
5.08
2.47
5.39
2.85
5.34
3.30
5.06
3.05
5.18
0
0.41
0.63
Ð
Ð
Ð
b
190
Ð
b
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and four brush point singularities when viewed through a
polarised light microscope. For the smectic A phases focal
conic fan textures were observed in co-existence with regions of
homeotropic alignment.
whereas for odd members it is less sensitive to changes in spacer
length and indeed passes through a weak maximum on
increasing n. The dependence of the smectic A±nematic and
smectic A±isotropic transition temperatures on n is, however,
quite different. The thermal stability of the SmA phase is
critically dependent on the parity of the outer spacers.
Speci®cally for the even members the thermal stability increases
on passing from the hexyl to the octyl and decyl homologues
but subsequently decreases for the dodecyl member. By
comparison there is a much larger increase in TSmAN on
passing from the heptyl to the nonyl homologue but also a
larger subsequent decrease to the undecyl homologue. Thus,
the stability of the phase is more susceptible to changes in the
length of odd- rather than even-membered spacers. This is
highlighted by the observation of smectic A±isotropic transi-
tions for even- but not for odd-membered homologues. Thus
the smectic tendencies of the tetramers do not simply increase
on increasing spacer length as is observed for semi-¯exible main
chain liquid crystal polymers1,2 nor do they simply decrease as
for symmetric liquid crystal dimers18 or for liquid crystal
trimers.14
The dependence of the transition temperatures on the
number of methylene units, n, in the outer ¯exible alkyl
spacers for the tetrameric CBOnO.O8O.OnOCB series is
shown in Fig. 2. The melting points exhibit a pronounced
alternation which is only weakly attenuated on increasing n.
Similar behaviour has been observed for liquid crystal
trimers14 and for nematic dimers17 but not for smectic
dimers.18 This may indicate that the change in the
conformation statistical weights of the spacer on melting
into a nematic phase is small for even-membered spacers but
large for odd-membered spacers. Alternatively, this pro-
nounced alternation in the melting points re¯ects the
dif®culty that the bent conformations of the odd-membered
compounds experience in packing ef®ciently into a crystal
lattice compared with the more elongated all even-membered
tetramers, see Fig. 3. We will return to a more detailed
discussion of molecular shape later.
In order to understand the unusual dependence of smectic
phase stability on spacer length seen in Fig. 2, the smectic
periodicity, d, was measured using X-ray diffraction for
Ê
CBO9O.O8O.O9OCB and found to be 19.5 A. The estimated
Ê
all-trans molecular length, l, for this homologue is 71 A and
The clearing temperatures also show a dramatic odd±even
effect as the length and parity of the outer spacers are varied
although this attenuates on increasing n, see Fig. 2. For even
members the clearing temperature decreases on increasing n
thus, the layer thickness is considerably smaller than the
molecular length. It should be noted that the observation of
regions of homeotropic alignment excludes the possibility that
this is a highly tilted phase.
In considering possible driving forces for smectic phase
formation by these tetramers we note that they contain two
electron de®cient moieties, the cyanobiphenyl groups, and two
electron rich benzylideneaniline units. Smectic phase formation
both in binary mixtures of low molar mass mesogens contain-
ing similar groups19±26 and in non-symmetric liquid crystal
dimers27±29 in which similar units are linked via an alkyl spacer,
is attributed to a speci®c interaction between the unlike
mesogenic groups. The precise nature of this interaction is
somewhat ambiguous but is most commonly thought to be a
charge transfer interaction. More recently, however, it has been
suggested that the liquid crystalline behaviour is controlled by
electrostatic quadrupoles between groups with quadrupole
moments which are opposite in sign.30 It seems reasonable to
suggest that this speci®c interaction between the unlike
mesogenic groups is also, at least in part, the driving force
for smectic phase formation by the tetramers. Fig. 4(a) shows a
schematic representation of the local molecular arrangement
within a smectic phase composed of tetramers in which the
interactions between the unlike mesogenic groups are max-
imised. At ®rst sight this packing arrangement is analogous to
Fig. 2 The dependence of the transition temperatures on the number
of methylene units (n) in the outer ¯exible alkyl spacers for the
tetrameric CBO.nO.O8O.OnOCB series: (#) melting points, ($)
nematic±isotropic transitions, (%) smectic A±nematic transitions and
(&) smectic A±isotropic transitions.
Fig. 3 The molecular shapes of CBO3O.O8O.O3OCB (upper structure) and CBO4O.O8O.O4OCB (lower structure) with the spacers in the all-trans
conformation.
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periodicity would indeed approximate to the experimentally
observed value.
Two alternative smectic phase structures exist which are
consistent with the observation d : l#0.27. In the ®rst of these
the spacers as well as the mesogenic cores are mixed giving a
smectic phase composed of two domains, one containing
mesogenic units and the other alkyl spacers, see Fig. 4(b). The
driving force here may not only be entropic but also arises from
the formation of anti-parallel cyanobiphenyl dimers which in
turn interact with the Schiff bases. The other possible smectic
phase structure is composed of concertinaed tetramers, see
Fig. 4(c). This arrangement while consistent with the X-ray
data is the least plausible on energetic grounds of the three
suggested. In addition, it is unclear how such an arrangement is
consistent with the dramatic dependence of the transition
temperatures on spacer length. In order to remove the
ambiguity in the structure of the phase a greater range of
tetramers must now be studied and in addition further X-ray
studies are necessary in order to determine whether these
smectic phases have modulated structures. Unfortunately,
miscibility studies cannot be used as an aid to phase
characterisation because there are no appropriate standard
materials.
The molecular arrangements shown in Fig. 4(a) and (b) do,
however, allow us to rationalise the unusual dependence of
smectic tendency on spacer length. The more linear all-even-
membered tetramers will pack more ef®ciently into either
structure than the bent tetramers containing odd-membered
spacers and hence the higher transition temperatures are
observed for the former. In order to maximise the interactions
between mesogenic groups, the arrangement shown in Fig. 4(b)
requires the inner and outer spacers to be of comparable length
and is presumably most stable when the spacers are of equal
length. This is consistent with the dependence of the smectic
transition temperatures on n, see Fig. 2. The stability of the
arrangement shown in Fig. 4(a), however, should be relatively
insensitive to differences in the outer spacer lengths if the
principal driving force for smectic phase formation is the mixed
interaction and thus, quite unlike the behaviour seen in Fig. 2.
This suggests instead that the phase may form as a result of
entropically driven microphase separation.
Fig. 5 shows the dependence of the entropy change
associated with the clearing transition, expressed as the
dimensionless quantity DS/R, on the number of methylene
units in the outer ¯exible spacers. A pronounced odd±even
Fig. 4 Sketches of possible molecular arrangements of the tetramers in
the smectic A phase.
that seen in the intercalated smectic phases exhibited by
nonsymmetric liquid crystal dimers.4,5 The term `intercalated',
however, was used to indicate that differing parts of the
molecules overlapped. In the arrangement shown in Fig. 4(a)
differing mesogenic groups interact but it is similar spacers
which overlap giving a phase consisting of three regions: one
domain composed of mesogenic groups, one containing the
outer ¯exible spacers and the other the inner spacers. Thus this
should not be termed an intercalated arrangement. The
periodicity of this arrangement does not in fact correspond
to the observed d : l ratio of 0.27 but instead is approximately
two thirds. If we assume, however, that the scattering powers of
the two differing alkyl regions are similar then the apparent
Fig. 5 The dependence of the entropy change associated with the
clearing transition on the number of methylene units, n, in the outer
¯exible alkyl spacers for the tetrameric CBOnO.O8O.OnOCB series:
($) nematic±isotropic transitions and (&) smectic A±isotropic
transitions.
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effect can be clearly seen in which the values of DS/R for all-
even members are typically more than twice those of the odd
members. The values of DSNI/R for the even members are
greater than 5 and such high values are without precedent for
low molar mass mesogens. These values do, however, ®t the
emerging trend that DSNI/R increases on passing from
monomer to dimer to trimer14 and now, to tetramer. It is
not possible to comment further on this because in these
tetramers the inner spacer has a different length to that of the
outer spacers. Similarly, it is not possible to compare the
behaviour of the odd-membered dimers and trimers with that
of the tetramers as these have an even-membered inner spacer.
It may be argued that the exceptionally high values of DSNI/R
seen in Fig. 5 should be scaled according to the number of
mesogenic units and such a scaling is, in essence, routinely and
without question performed for semi-¯exible main chain
polymers. It is not possible to compare, however, the scaled
values of DSNI/R for the tetramers with those of analogous
trimers and dimers because the inner spacer is different in
length to the outer spacers. The only example for which a valid
comparison can be made is CBO8O.O8O.O8OCB and for this
the scaled DSNI/R (i.e.[DSNI/R]/4~1.34) is equal to the scaled
value for the analogous trimer (i.e. [DSNI/R]/3~1.36) but
greater than that of the dimer (i.e. [DSNI/R]/2~1.01). The
scaled DSNI/R exhibited by the dimer is considerably larger
than that of the analogous monomer (i.e. [DSNI/R]~0.25).
Thus it would appear that linking two mesogenic groups
together via an even-membered spacer has a dramatic effect on
the scaled DSNI/R but adding a further mesogenic unit via an
additional spacer leads to a much smaller increase in the scaled
DSNI/R which is subsequently insensitive to further additions.
This observation has clear implications in the manner in which
we interpret thermodynamic data for semi-¯exible main chain
liquid crystal polymers but further speculation must await the
characterisation of a greater number of trimeric and tetrameric
series in order to test the generality of these data.
for a monomer. The situation for odd-membered dimers is
quite different because the difference in free energy between the
bent and linear conformers is such that the orientational order
of the nematic phase is insuf®cient to convert bent into linear
conformers. Hence, DSNI/R exhibited by odd-membered
dimers is small. It would seem reasonable that this approach
would also successfully predict the properties of trimers and
tetramers although this assumption must now be tested.
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