Thermal behaviour and mesophase structure of cyclic oligosiloxanes with chiral mesogenic groups

Article abstract of DOI:10.1080/15421400490435206

Chiral, smectogenic pendant groups have been covalently attached to cyclic oligosiloxanes by hydrosilylation. Differential scanning calorimetry, optical polarising microscopy, and X-ray diffraction measurements reveal liquid crystalline properties for all

Full text of DOI:10.1080/15421400490435206

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Thermal Behaviour and
Mesophase Structure of Cyclic
Oligosiloxanes With Chiral
Mesogenic Groups
Joanna Sołtysiak a , Ewa Białecka-Florjańczyk b
,
Ewa Kowalczyk b & Jan Przedmojski c
a Industrial Chemistry Research Institute , Warsaw,
Poland
b Institute of Chemistry , Agriculture University ,
Warsaw, Poland
c Warsaw University of Technology , Warsaw, Poland
Published online: 18 Oct 2010.
To cite this article: Joanna Sołtysiak , Ewa Białecka-Florjańczyk , Ewa Kowalczyk
& Jan Przedmojski (2004) Thermal Behaviour and Mesophase Structure of Cyclic
Oligosiloxanes With Chiral Mesogenic Groups, Molecular Crystals and Liquid Crystals,
411:1, 217-230, DOI: 10.1080/15421400490435206
To link to this article: http://dx.doi.org/10.1080/15421400490435206
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Mol. Cryst. Liq. Cryst., Vol. 411, pp. 217=[1259]–230=[1272], 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490435206
THERMAL BEHAVIOUR AND MESOPHASE
STRUCTURE OF CYCLIC OLIGOSILOXANES WITH
CHIRAL MESOGENIC GROUPS
Joanna T. Sołtysiak
Industrial Chemistry Research Institute, Rydygiera 8,
01-793 Warsaw, Poland
´
Ewa Białecka-Florjanczyk and Ewa Kowalczyk
Agriculture University, Institute of Chemistry, Nowoursynowska159c,
02-787 Warsaw, Poland
Jan Przedmojski
Warsaw University of Technology, Faculty of Physics, Koszykowa 75,
00-662 Warsaw, Poland
Chiral, smectogenic pendant groups have been covalently attached to cyclic oli-
gosiloxanes by hydrosilylation. Differential scanning calorimetry, optical
polarising microscopy, and X-ray diffraction measurements reveal liquid crys-
talline properties for all synthesized monomers and oligomers. The monomers,
which differ by the nature of the rigid mesogenic core or end groups, showed
enantiotropic smectic A phases. Almost all of cyclic oligosiloxanes present
enantiotropic chiral smectic C phases. The mesomorphic range of these oligo-
mers is greatly enhanced relative to the type of monomers and the ring size
as well.
Keywords: cyclic oligosiloxanes; liquid crystals; smectic mesophase
INTRODUCTION
The design and synthesis of chiral thermotropic side-chain liquid crystal-
line polymers (SCLCPs) are currently the focus of intense research [1,2].
The ability of these polymers to generate chiral nematic and chiral smectic
The reported research was supported by State Committee of Scientific Research of the
Polish Government.
Address correspondence to Joanna T. Soltysiak, Industrial Chemistry Research Institute,
Rydygiera 8, Warsaw, 01-793, Poland.
217=[1259]

218=[1260]
J. T. Sołtysiak et al.
supramolecular assembles endowed with a macroscopic twist can result in
the availability of new materials with promising optical and electro-optical
properties for various potential high-tech applications. The advantage of
liquid crystalline polysiloxanes, compared to other SCLCPs, lies in their
high backbone flexibility, due to the presence of siloxane units, which
results in relatively low glass and mesophase transitions. Therefore one
limitation of SCLC polymers, particularly with respect to their use for op-
tical data storage, is their high viscosity. Within the group of SCLC polysi-
loxanes, a significant interest concentrates recently on small ring systems
with mesogenic units as substituents at silicon atoms. The development of
cyclic oligosiloxanes with mesogenic groups has shown that these materials
exhibit properties intermediate to low molecular weight liquid crystals and
polymeric ones. This class of materials combines the low melt viscosity of
conventional low molar weight LC with the glass-forming ability (above
ambient temperature) of conventional polymer structures. Moreover, the
absence of any polydispersity in some oligomers, which is by contrast an
inherent property in polymers, facilitates the reproducibility of liquid crys-
tal properties [3–7].
Until now the esters of cinnamic acid were not attached as pendant
groups to cyclic oligosiloxanes. In order to obtain new materials with ferro-
electric properties and also to study the influence of a cinnamate function
on the existence and stability of SmCꢀ phase, we have synthesised a series
of oligocyclosiloxanes with a mesogenic unit of general formula 1:
They were compared with biphenyl and benzoyloxyphenyl mesogenic
rigid cores; (S)-2-methylbutanol, (2S)-2-chloro-4-methylpentanoic acid
or (2S,3S)-2-chloro-3-methylpentanoic acid were used as chiral substitu-
ents.
A schematic representation of all of the synthesized compounds is
shown in Figure 1.
EXPERIMENTAL
General
All chemicals were of commercial grade quality and used without further
purification. Solvents were distilled by conventional methods prior to
use. The hydrosilylation catalyst -platinum tetramethyldivinyl disiloxane
complex in xylene (11% wt) was obtained from GE Silicon.

Thermal Behaviour of Cyclic Oligosiloxanes
219=[1261]
FIGURE 1 General structure of cyclic oligosiloxanes with pendant groups.
1H NMR were collected on a VARIAN INOVA (500 MHz) spectrometer,
using deuterated chloroform as a solvent and tetramethylsilane as the
internal standard. FTIR spectra were obtained using a Perkin-Elmer SPEC-
TRUM 2000 spectrometer. Monomer and oligomer samples were prepared
as either potassium bromide discs, or if liquid, were run on single crystal
sodium chloride discs.
Differential Scanning Calorimetry (DSC) thermograms were obtained
using a Perkin ELMER DSC-7, with A TAC 7=PC interface and a controlled
cooling accessory. The transition temperatures were taken from DSC traces
of samples annealed by cooling from the isotropic melt as corresponding to
the maxima and the onset points of the enthalpic peaks for polymers and
low molecular weight samples, respectively, at a heating rate of
10ꢁC=minꢂ1. Optical microscopy was performed using a Biolar polarising
microscope, fitted with a Mettler FP82HT heating stage and a Mettler
FP-90 processor. Samples were prepared as thin films between glass slide
and a glass cover slip.
X-ray diffraction measurements have been performed using a Philips
X’Pert diffractometer as well as a Guinier symmetrical focusing trans-
mission photographic camera. In the case of diffractometer copper radiation
filtered by nickel absorption filter was applied, while Guinier camera
worked with cobalt radiation. Diffractometer smectic phase measurements
were carried out using the reflecting method from flat sample aligned by
glass substrate. The chosen temperature was maintained within ꢃ0.1ꢁC.
Synthesis
The alkene mesogenic side chain precursors used in the hydrosilylation
reactions were prepared by standard methods. (S)-2-methylbutyl,

220=[1262]
J. T. Sołtysiak et al.
(S)-1-chloro-3-methylbutanocarbonyl and (1S, 2S)1-chloro-2-methylbutano-
carbonyl were used as chiral substituents.
The trans-alkenyloxy cinnamic acids were obtained starting from the
4-hydroxybenzaldehyde in a series of following reactions: etherification
of the 4-hydroxybenzaldehyde with an appropriate bromoalkene or tosy-
late and subsequent Knoevenagel condensation between the aldehyde
and the malonic acid in the presence of piperidine and pyridine. The
(S)-2-methylbutyl 4-hydroxybenzoate was obtained in the direct esterifi-
cation of 4-hydroxybenzoic acid with (S)-2-methylbutanol in the presence
of p-toluenesulfonic acid.
The final compounds M1–M3 were obtained by esterification of the
cinnamic acid and (S)-2-methylbutyl 4-hydroxybenzoate in the presence
of DCC and DMAP (Scheme 1).
They were purified by chromatography on silica gel with chloroform as
eluent. Monomers M5 and M6 were also obtained by similar procedure.
The mesogene M4 was prepared starting from the 4-hydroxybenzoic
acid methyl ester in a series of following reactions: alkylation of 4-hydroxy-
benzoic acid methyl ester with undec-10-enyl tosylate (KOH, ethanol),
subsequent hydrolysis of methyl ester and final esterification of
4-(undec-10-enyloxy)benzoic acid and (S)-2-methylbutyl 4-hydroxybenzo-
ate in the presence of DCC and DMAP.
The synthesis of 4-(40-undec-10-enyloxy)biphenyl esters of (2S)-2-chloro-
4-methylpentanoic (M7) or (2S,3S)-2-chloro-3-methylpentanoic acid (M8) is
outlined at the Scheme 2:
4-hydroxy-(40-undec-10-enyloxy)biphenyl was prepared following a
method reported in the literature [8].
a-Chloroacids were prepared from the corresponding a-aminoacids
(L-leucine and L-isoleucine) according to the procedure of Schacht [9]
(Scheme 3).
We have found only an inconsiderable degree of racemisation
[a] ¼ ꢂ2,8ꢁ for (2S,3S)-2-chloro-3-methylpentanoic acid ([a]lit. ꢂ2,9ꢁ
[10]). The structures of final products (new compounds except M4) were
1
confirmed by means of FTIR, H NMR. Spectral data of M1–M3 (Table 1)
SCHEME 1

Thermal Behaviour of Cyclic Oligosiloxanes
221=[1263]
SCHEME 2
and M7 are given as an example; values of other homologous compounds
were similar.
Monomer 7: 1H NMR (ppm CDCl3): d 1.01(d, 3H, CH(CH3)), 1.04(d, 3H,
CH(CH3)), 1.31–2.02 (m, 22H), 3.99 (t, 2H OCH2), 4,55 (t, 1H, CHCl),
4.90–5.05 (m, 2H, CH2 ¼ C), 5.72–5.92 (m, 1H, CH2 ¼ CH), 6.94–7.53
(m, 8H, Ar-H).
The thermal properties of monomers are summarized in Table 2.
General procedure for hydrosilylation of cyclic oligosiloxanes with olefin
terminated mesogens:
In a typical hydrosilylation reaction (Fig. 2), 0.52 g (1.03 mmol) (E)-3-
[4-undec-10-enyloxy)phenyl]-cinnamic acid 4[(S)-2-methylbutoxy)car-
bonyl]phenyl ester was dissolved under nitrogen in dry (sodium mirror)
toluene and then 56 mg (0.93 mmol of SiH–units) of 2,4,6,8-tetramethyl-
cyclotetrasiloxane was added under a nitrogen into a Schlenk tube con-
taining the solution. After addition of a catalyst (ꢂ4 ml of 11% PTDD
solution in xylene 2 ꢄ 10ꢂ4 mol Pt=molꢂ1 SiH) – the reaction was carried
out for 24 h, with magnetic stirring, at room temperature. During that time
SCHEME 3

222=[1264]
J. T. Sołtysiak et al.
1
TABLE 1 The H NMR Chemical Shifts (d ppm) of the Mesogens M1-M3
Proton
numbers
Mesogens
M2 (n ¼ 5)
M1 (n ¼ 2)
M3 (n ¼ 8)
1
2
5.07 (m)
5.85 (m)
1.91–2.25 (m, 4H)
4.02 (t)
5.02 (m)
5.82 (m)
1.42–1.62 (m, 10H)
4.01 (t)
4.96 (m)
5.82 (m)
1.29–1.54 (m, 16H)
4.05 (t)
3
4
5
6.93 (d)
7.53 (d)
6.92 (d)
7.53 (d)
6.97 (d)
7.54 (d)
6
7
7.84 (d)
6.48 (d)
7.84 (d)
6.48 (d)
7.39 (d)
6.58 (d)
8
9
7.25 (d)
8.10 (d)
7.25 (d)
8.10 (d)
7.25 (d)
8.10 (d)
10
11
12
13
14
15
4.22 (m)
1.87 (m)
1.02 (d)
4.21 (m)
2.10 (m)
1.02 (d)
4.21 (m)
2.05 (m)
1.02 (d)
1.29 (m)
0.97 (t)
1.28 (m)
0.97 (t)
1.28 (m)
0.97 (t)
the colourless solution turned pale brown due to the formation of col-
loidal platinum. The temperature was then raised and process was con-
tinued for another 48 h at 55–60ꢁC. FTIR analysis of the solution (NaCl
cell windows) showed the absence of a SiH band at 2165 cmꢂ1, indicat-
ing the hydrosilylation was complete. The SCLC cyclic tetramer was
separated, purified by several reprecipitations from toluene solution
into methanol and filtered through a disposable syringe filter (PTFE
membrane 0,45 mm) to remove any particulates. The solvent was care-
fully removed by rotary evaporation and the tetramer dried in vacuum.
The structures of oligomers were confirmed by FTIR and 1H NMR. Spec-
tral data of D4M3 are given as an example.
1H NMR (ppm CDCl3): d 0.11(s, 12H, SiCH3), 0.59(m, 8H, SiCH2),
0.95(m, 12H CH3), 1.28–1.82 (m, 104H, CH2 and CH3), 1.87 (m, 4H,
CH), 4.05 (m, 8H, OCH2), 6.48 (d, 4H, ¼CHCOO), 6.93 (d, 8H Ar-H),
7.25 (d, 8H, Ar-H), 7.53 (d, 8H, Ar-H), 7.84 (d, 4H, Ar-CH¼), 8.10 (d,
8H, Ar-H).
FTIR (cmꢂ1, KBr)cmax 3000–2840, 1726, 1633, 1605, 1560, 1280, 760,
absence of 2165 (SiH).

223=[1265]

224=[1266]
J. T. Sołtysiak et al.
FIGURE 2 The hydrosilylation reaction.
The other oligomers were prepared according to the procedure
described above. The thermal properties of oligomers are summarized in
Table 3.
RESULTS AND DISCUSSION
As can be seen from the data listed in Table 2, monomers M1–M3 with
cinnamic moiety in mesogenic core exhibit smectic A phases, within a dif-
ferentiate temperature range. The thermal stabilities of the smectic A
phases were increased with increasing the length of spacer from 5 (M1)
to 11 (M3) carbon atoms.
Moreover, from the transitions on heating for M3 and M4 (Table 2) we
could clearly observe that the monomer M3 exhibited a wider smectic
phase than the M4 compound. The difference between these monomers
is the nature of the mesogenic core. The benzoiloxyphenyl core (M4) leads
to narrow smectic phase only within 7 degrees , while cinnamic ester core
leads to smectic phase observable within 47 degrees. These results suggest
that monomer M3 posseses greater propensity to give rise to stable smectic
mesophase than do the monomer M4, thus indicating that the replacement
of benzoiloxyphenyl core by the cinnamoiloxy phenyl group strongly
enhances the smectogenic character of the monomer. In addition, the
monomer M5, which incorporates three phenyl rings in its structure exhi-
bits, the widest range of the chiral smectic mesophase. The change of the
position of the cinnamic substituent (M6) does not influence the transition
temperatures greatly.
M7 and M8 monomers are alkenyloxybiphenyl esters of chiral 2-chloro-
acids. Only one of them (2-chloro-3-methylpentanoic acid ester) exhibits
LC properties with narrow range of SmA phase.
All oligomers showed liquid crystalline behaviours under observation
with polarising microscope. In the case of cinnamic acid derivatives it
was SmCꢀ phase. The transition to the chiral smectic C phase was evident
by the formation of pitch lines (arcs) in the broken focal conic texture,
which breaks apart to form the broken focal conic characteristic for tilted
smectics. The DSC analyses of the liquid crystalline properties of the all of
the oligomers were measured by a glass transition or a melting transition

225=[1267]

226=[1268]
J. T. Sołtysiak et al.
FIGURE 3 DSC thermogram of cyclic oligosiloxanes O6M8 (10ꢁC minꢂ1): heating
scan.
and one or two endotherm corresponding to an isotropic transition. The
DSC thermograms of oligomer O6M8 are presented in Figure 3.
The characterization of the smectic mesophases was also carried out by
temperature-controlled X-ray diffraction of powder samples. In general, the
X-ray diagrams of the smectic phase exhibited two or three sharp reflec-
tions with Bragg molar spacing typical of layered structures in the low-
angle region. The interlayer distance d exceeded a calculated molecular
length l for almost all monomeric compounds (Table 2). This indicates
the presence of the smectic A structure and the microscopic observations
confirmed this assumption.
In the case of oligomers with phenyl cinnamate mesogenic core d=l
ratios are between 0.85–0.95 and indicate the tilted smectic phase-SmC.
The relatively higher d=l values for D6 ring is probably connected with
the mobility of the greater ring (Table 3). These values closely agree with
earlier data on chiral polysiloxanes with a a three ring mesogenic rigid
core and chiral 2-methylbutyl chain attached via sulfur atom [11]. There-
fore, the mesogenic side groups are tilted to the layer normal in the smectic
C mesophase [12] by an angle h ꢅ 20–30ꢁ. These X-ray data were sup-
ported by microscopic observations i.e. the formation of banded focal conic
textures, characteristic for chiral smectic C phase.

Thermal Behaviour of Cyclic Oligosiloxanes
227=[1269]
In the homologous series O4M4–O6M6 (phenyl benzoate mesogenic
core) the values of d were in close agreement with the lengths l of their
respective side chains. In all the above samples, the smectic interlayer spa-
cing d was practically constant ꢁfrom room temperature up to the isotropi-
ꢁ
˚
zation point (d¼32.7 A at 20C and 32.8 at 80C). This suggests the
presence of an orthogonal disordered smectic, such as a smectic A1 phase.
These results demonstrate the superiority of phenyl cinnamate mesogenic
unit to benzoiloxyphenyl in promoting LC properties.
The oligomers O6M7 and O6M8 exhibit two LC phases SmCꢀ and SmA.
Oligomer O6M8 exhibits a melting transition at 68.5ꢁC followed by a chiral
smectic C to smectic A phase transition at 77.8ꢁC and the smectic A to iso-
tropic phase transitionat 128.5ꢁC.
Figure 4 shows the temperature dependent d-spacing of the both smec-
tic layers obtained from powder samples of O6M8. At first, d-spacing
increases with increasing temperature what is characteristic for smectic
C phase, and than is almost stable (in the range about 50 degrees). Such
behaviour indicates the existence of SmC phase at first and the SmA
between 77.8. and 128.5ꢁC. These results were confirmed by microscopic
observations.
The influence of the ring size on the phase behaviour of cyclic oligo-
siloxanes was studied earlier [6,13,14]. Our own observations concerning
siloxane rings up to six Si atoms indicate that the ring size should be con-
sidered in connection with the nature of mesogenic groups. Increasing or
FIGURE 4 Temperature dependence of layer spacing for cyclic oligosiloxanes
O6M8.

228=[1270]
J. T. Sołtysiak et al.
~
FIGURE 5 Influence of ring size on ꢀphase transition temperatures; ( )-Glass
&
transition temperatures, ( ) Smectic C -isotropic.
~
FIGURE 6 Influence of ring size on phase transition temperatures; ( )-Glass
transition temperatures, (ꢆ) Smectic A-isotropic.

Thermal Behaviour of Cyclic Oligosiloxanes
229=[1271]
decreasing glass transition temperatures depend on the size of ring of course,
but also on the molecular weight of the mesogenic groups (Figs. 5–6).
So, in the case of cinnamic esters in mesogenic core, the glass transition
temperatures increase with increasing size of the siloxane backbone.
By linking the siloxane ring to the mesogenic group with phenyl
benzoate in core, decreasing glass transition temperatures are observed
with increasing number of Si atoms in the siloxane ring.
A similar result was also observed with the changes of spacer length. The
increasing number of C-atoms in the spacer length yields an increase in
glass transition temperatures (O4M1–O4M3 or O6M1–O6M3) in contrast
with the linear LC siloxane polymers [15].
The influence of the size of the siloxane ring on the distances of SmA or
SmCꢀ layers is relatively weak.
CONCLUSIONS
Three series of new side chain cyclic oligosiloxanes based on different
mesogenic rigid cores with chiral end group have been synthesized and
characterized. All of them exhibited enantiotropic smectic mesophases
with monolayer structure. In particular, cyclic oligosiloxanes with cinna-
moiloxyphenyl rigid core and (S)-2-methylbutyl chiral moiety display chiral
smectic C mesophase, whereas the series with 4-benzoiloxyphenyl in meso-
genic core form monolayer smectic A mesophase. Oligomers substituted by
biphenyl of chiral chloroacids exhibit two liquid crystalline phases: chiral
smectic C and smectic A.
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