Design of siloxane liquid crystals forming a de vries SmA* phase

Article abstract of DOI:10.1080/15421400590958089

We designed and synthesized new SmA* liquid crystals with different molecular architectures based on the chiral mesogen MR11, which was unsubstituted, or attached at one end and at both ends of a trisiloxane segment in the monosubstituted MSi3-MR11 and disubstituted DSi3-MR11 siloxanes, respectively. We studied the critical divergence of the magnetic susceptibility by measurements of the induced tilt angle θ_ind at the SmA*-SmC* transition. While all three SmA* liquid crystals presented a de Vries character, this was quite distinctive for DSi3-MR11 for which the critical exponent γ = 1.87 was maximal. X-ray diffraction experiments confirmed that the SmA*-SmC* transition occurred with essentially no layer shrinkage.

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Design of Siloxane Liquid
Crystals Forming a De Vries
SmA* Phase
G. Galli a , M. Reihmann a , A. Crudeli a , E. Chiellini
a , Yu. Panarin b c , J. Vij b , C. Blanc d , V. Lorman d
& N. Olsson e
a Dipartimento di Chimica e Chimica Industriale and
INSTM – UdR Pisa , Università di Pisa , Pisa, Italy
b Department of Electronic and Electrical
Engineering , Trinity College , Dublin, Ireland
c School of Electronics and Communication
Engineering , Dublin Institute of Technology , Dublin,
Ireland
d GDPC, UMR and LPMT, UMR , Université Montpellier
II , France
e Department of Materials and Surface Chemistry ,
Chalmers University of Technology , Göteborg,
Sweden
Published online: 31 Aug 2006.
To cite this article: G. Galli , M. Reihmann , A. Crudeli , E. Chiellini , Yu. Panarin ,
J. Vij , C. Blanc , V. Lorman & N. Olsson (2005) Design of Siloxane Liquid Crystals
Forming a De Vries SmA* Phase, Molecular Crystals and Liquid Crystals, 439:1, 245/
[2111]-257/[2123], DOI: 10.1080/15421400590958089
To link to this article: http://dx.doi.org/10.1080/15421400590958089
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Mol. Cryst. Liq. Cryst., Vol. 439, pp. 245=[2111]–257=[2123], 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590958089
Design of Siloxane Liquid Crystals Forming
a De Vries SmA* Phase
G. Galli
M. Reihmann
A. Crudeli
E. Chiellini
Dipartimento di Chimica e Chimica Industriale and INSTM – UdR Pisa,
`
Universita di Pisa, Pisa, Italy
Yu. Panarin
Department of Electronic and Electrical Engineering, Trinity College,
Dublin, Ireland and School of Electronics and Communication
Engineering, Dublin Institute of Technology, Dublin, Ireland
J. Vij
Department of Electronic and Electrical Engineering,
Trinity College, Dublin, Ireland
C. Blanc
V. Lorman
´
GDPC, UMR and LPMT, UMR, Universite Montpellier II, France
N. Olsson
Department of Materials and Surface Chemistry, Chalmers University
¨
of Technology, Goteborg, Sweden
We designed and synthesized new SmAꢀ liquid crystals with different molecular
architectures based on the chiral mesogen MR11, which was unsubstituted, or
attached at one end and at both ends of a trisiloxane segment in the monosubsti-
tuted MSi3-MR11 and disubstituted DSi3-MR11 siloxanes, respectively. We stud-
ied the critical divergence of the magnetic susceptibility by measurements of the
induced tilt angle hind at the SmAꢀ–SmCꢀ transition. While all three SmAꢀ liquid
crystals presented a de Vries character, this was quite distinctive for DSi3-MR11
for which the critical exponent c ¼ 1.87 was maximal. X-ray diffraction experiments
This work was supported by the EU Contract SAMPA (HPRN-CT-2002–00202).
Address correspondence to G. Galli, Dipartimento di Chimica e Chimica Industriale
`
and INSTM–UdR Pisa, Universita di Pisa, 56126 Pisa, Italy. E-mail: gallig@dcci.unipi.it
245=[2111]

246=[2112]
G. Galli et al.
confirmed that the SmAꢀ–SmCꢀ transition occurred with essentially no layer
shrinkage.
Keywords: de Vries smectic; electrooptics; molecular design; siloxane liquid crystal
INTRODUCTION
The liquid crystalline SmA phase is described as a 2D layered struc-
ture where the molecular director is perpendicular to the smectic layer
plane. On cooling down to the SmC phase, the molecular director
becomes tilted with respect to the smectic layer normal which leads
to a layer shrinkage. In real SmA liquid crystals the molecules are
not exactly perpendicular to the smectic layer planes due to (at least)
non-perfect molecular shape. Nevertheless, most SmA liquid crystals
are consistent with such ‘‘rigid-rod’’ microscopic molecular model
and undergo a smectic layer shrinkage at the SmA–SmC transition.
On the other hand, in a de Vries SmA phase the molecules are
assumed to be tilted similar to the SmC phase in two possible struc-
tures [1] in which the tilted layers are stacked in a random fashion,
or alternatively the molecules are tilted, but without long-range order
in the tilt direction. While the molecular tilt direction is randomly dis-
tributed in the SmA state, it becomes ordered below the SmA–SmC
transition. In both suggested structures the transition to the SmC
phase therefore occurs through an ordering of tilt directions, which
would produce no significant shrinkage of the smectic layer spacing.
Recently, there have been several reports dealing with the syn-
thesis and some physical properties, such as birefringence and electro-
clinic effect, of de Vries SmAꢀ liquid crystals [2–9]. Among them, there
are different materials like compounds including organosiloxanes,
fluoroethers and chiral lactic esters in the tail. However, up to now
there has been no really clarifying explanation reported on why such
materials should exhibit de Vries properties. Moreover, there is no
general guideline available on how to design a de Vries material start-
ing from typical molecular components of conventional chiral liquid
crystals.
In this work we synthesized new SmAꢀ organosiloxanes with varied
chemical structure (Scheme 1) and investigated their de Vries charac-
ter by electrooptical measurements and X-ray diffraction analysis. By
comparison with previous results on similar de Vries organosiloxanes
[6], we propose a molecular architecture of a disubstituted trisiloxane
which seems to be of general advantage to develop de Vries SmAꢀ
phases.

Design of Siloxane de Vries SmAꢀ
247=[2113]
SCHEME 1 Synthesis and transition temperatures of the mesogen MR11 and
the trisiloxanes MSi3-MR11 and DSi3-MR11.

248=[2114]
G. Galli et al.
EXPERIMENTAL
Materials
All solvents were distilled prior to use. All commercially avail-
able chemicals (Sigma-Aldrich, ABCR, Fluorochem) were used as
received.
40-(10-Undecenyl-1-oxy)-4-biphenol (3)
20.00 g (0.11 mol) of 4,40-biphenol (1) was dissolved with 3.5 g
(0.62 mol) of KOH and 0.1 g KI in 300 ml methanol. The mixture was
heated to 60ꢁC and ethanol was added until a clear solution was
obtained. 12.52 g (54 mmol) of 11-bromoundecene (2) was then added
dropwise and the solution was refluxed for 2 days under a nitrogen
atmosphere. The heterogeneous reaction mixture was then poured
into water and the precipitate was filtered and dried under vacuum.
After recrystallisation from methanol, 12.87 g (70% yield) of pure pro-
duct was obtained. White crystals, mp 128ꢁC; Rf ¼ 0.50 (ethyl acetate=
ꢂ1
~
petroleum ether 1:1); FT-IR (KBr) n=cm : 3400–3100 (OH), 3075
(vinyl CH), 2962–2840 (aliphatic CH2), 1643 (vinyl C¼C), 1610,
1
1503, 1462 (aromatic C¼C), 1251 (C–OH), 810 (aromatic); H NMR
(200 MHz, CDCl3) d=ppm: 7.46 (d, 2H, H30, H50-biphenyl, J ¼ 8.78 Hz),
Hz), 7.44 (d, 2H, H3, H5-biphenyl, J ¼ 8.68 Hz), 6.95 (d, 2H, H2, H6-
biphenyl, J ¼ 8.68 Hz), 6.89 (d, 2H, H20, H60-biphenyl, J ¼ 8.78 Hz),
5.93 (m, 1H, CH2¼CH), 5.81 (2d, 2H, CH2¼CH), 3.99 (t, 2h, CH2O),
2.07 (m, 2H, CH2¼CHCH2), 1.81 (m, 2H, CH2CH2O), 1.32 (12H,
aliphatic CH2).
(S,S)-2-chloro-3-methylpentanoic acid (5)
10.00 g (76.24 mmol) of (L)-isoleucine (4) was dissolved in 100 ml of
6 N HCL at 0ꢁC and a solution of 6.48 g (76.24 mmol) of NaNO2 in 30 ml
water was added dropwise. The solution was then stirred for 4 h at 0ꢁC
and then for 15 min at room temperature. The solution was extracted
with ethyl acetate and the organic phase was dried with MgSO4 and
evaporated under vacuum. The remaining oil was distilled to give
10.2 g (87% yield) of pure product. Colourless oil (90ꢁC=0.02 mbaꢂr1),
~
Rf ¼ 0.88 (ethyl acetate=petroleum ether 1:1); FT-IR (NaCl) n=cm :
3500–2880 (OH, CH2, CH3), 1723 (C¼O), 1450–1395 (CH3) 1290–
1
1203 (C–OH), 695 (CCl); H NMR (200 MHz, CDCl3) d=ppm: 11.60–
10.20 (1H, COOH), 4.24 (d, 1H, ClCH, J ¼ 6.70 Hz), 2.12 (m, 1H,
CHCH3), 1.63 (m, 1H, CH3CH2), 1.36 (m, 1H, CH3CH2), 1.07 (d, 3H,
CH3CH, J ¼ 6.70 Hz), 0.94 (t, 3H, CH3CH2).

Design of Siloxane de Vries SmAꢀ
249=[2115]
40-(10-Undecenyl-1-oxy)-4-biphenyl(S,S)-2-chloro-3-
methylpentanoate (MR11)
12.00 g (35.4 mmol) of 3, 5.34 g (35.4 mmol) of 5 and 0.5 g (3.1 mmol)
of 4-pyrrolidinopyridine (PPy) were dissolved in 200 mL of dry dichlor-
omethane under a nitrogen atmosphere and cooled to 0ꢁC. 7.31 g
(35.4 mmol) of dicyclohexylcarbodiimide (DCC) in 15 mL of dry dichlor-
omethane was added in 30 min and the mixture was then stirred for 2
days at room temperature. The precipitate was filtered and the sol-
ution evaporated under vacuum. The solid residue was purified by
flash chromatography (silica gel 240–400 mesh) using ethyl acetate=
petroleum ether 1:1 as eluent. The resulting solid was further puri-
fied by dissolving it in hot petroleum ether, cooling, filtration and
evaporation of the petroleum ether under vacuum to give 12.8 g
(77% yield) of pure product. White wax, Rf ¼ 0.82 (ethyl acetate=
ꢂ1
~
petroleum ether 1:4); FT-IR (KBr) n= cm : 3075 (vinyl CH), 2962–2840
(aliphatic CH), 1745 (C¼O), 1650 (vinyl C¼C), 1606, 1504, 1465
(aromatic C¼C), 1190–1140 (C–O), 950 (vinyl C¼C), 820, 800
(aromatic), 695 (CCl).1H NMR (200 MHz, CDCl3) d=ppm: 7.57 (d, 2H,
H3, H5-biphenyl, J ¼ 8.96 Hz), 7.50 (d, 2H, H30, H50-biphenyl,
J ¼ 8.78 Hz), 7.18 (d, 2H, H2, H6-biphenyl, J ¼ 8.96 Hz), 6.98 (d, 2H,
H20, H60-biphenyl, J ¼ 8.78 Hz), 5.83 (m, 1H, CH2¼CH), 4.99 (2d,
2H, CH2¼CH), 4.42 (d, 1H, ClCH, J ¼ 6.70 Hz), 4.01 (t, 2H, CH2O),
2.26 (m, 1H, CHCH3), 2.07 (m, 2H, CH2¼CHCH2), 1.81 (m, 4H,
CH2CH2O, CH3CH2), 1.50–1.34 (12H, aliphatic CH2), 1.17 (d, 3H,
CHCH3, J ¼ 6.70 Hz), 1.01 (t, 3H, CH3CH2).
General Procedure for the Preparation of the Siloxanes
by Hydrosilylation
250 mg of MR11 was dissolved in 5 mL of dry toluene in a pyrex
tube. The solution was degassed and sealed with a septum under a
nitrogen atmosphere. An 1.15 molar amount of 1,1,3,3,5,5,5-hepta-
methyl-1-hydrotrisiloxane (6) was added for the synthesis of the
monosubstituted trisiloxane, or a 1.8 molar amount of 1,1,3,3,5,5-
hexamethyl-1,5-dihydrotrisiloxane (7) for the synthesis of the disubsti-
tuted trisiloxane. After addition of 6 mL of a 2 wt% xylene solution of
platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex (PtDVDS),
The tube was placed in a sonicator for 2–5 h at 30ꢁC. The conversion
was followed by FT-IR detecting the progressive decrease of the
absorption bands at 2160 cmꢂ1 (Si–H) and 950 cmꢂ1 (vinyl C¼C).
Upon completion of the reaction, the solvent was evaporated and
the residue was dissolved in dichloromethane and precipitated in
cold methanol. The final product was further purified as specified
below.

250=[2116]
G. Galli et al.
1,1,3,3,5,5,5-Heptamethyltrisiloxane 1-[40-(undecyl-1-oxy)-4-
biphenyl(S,S)-2-chloro-3-methylpentanoate] (MSi3-MR11)
Flash chromatography (Rf ¼ 0.86 ethyl acetate=petroleum ether
ꢂ1
~
1:8), 38% yield; FT-IR (KBr) n=cm : 3100–3000 (aromatic CH),
2980–2880 (aliphatic CH), 1766 (C¼O), 1607, 1524, 1469 (aromatic
C¼C), 1290 (O–CO), 1256 (C–OH), 1168 (C–O), 800 (aromatic); 1H
NMR (200 MHz, CDCl3) d=ppm: 7.58 (d, 2H, H3, H5-biphenyl,
J ¼ 8.96 Hz), 7.51 (d, 2H, H30, H50-biphenyl, J ¼ 8.78 Hz), 7.18
(d, 2H, H2, H6-biphenyl, J ¼ 8.96 Hz), 6.98 (d, 2H, H20, H60-biphenyl,
J ¼ 8.78 Hz), 4.42 (d, 1H, CLCH, J ¼ 6.70 Hz), 4.02 (t, 2H, CH2O), 2.26
(m, 1H, CHCH3), 1.83 (m, 4H, CH2CH2O, CH3CH2), 1.42–1.24 (16H,
aliphatic CH2), 1.17 (d, 3H, CHCH3, J ¼ 6.70 Hz), 1.02 (t, 3H,
CH3CH2), 0.55 (m, 2H, SiCH2), 0.11, 0.08, 0.05 (21H, SiCH3).
1,1,3,3,5,5-Hexamethyltrisiloxane 1,5-bis-[40-(undecyl-1-oxy)-4-
biphenyl(S,S)-2-chloro-3-methylpentanoate] (DSi3-MR11)
Repeated precipitations from chloroform solutions into methanol;
ꢂ1
~
67% yield; FT-IR (KBr) n=cm : 3100–3000 (aromatic CH), 2980–
2880 (aliphatic CH), 1760 (C¼O), 1607, 1527, 1475 (aromatic C¼C),
1285 (O–CO), 1252 (C–OH), 1172 (C–O), 829 (aromatic); 1H NMR
(200 MHz, CDCl3) d=ppm: 7.58 (d, 4H, H3, H5-biphenyl, J ¼ 8.96 Hz),
Hz), 7.51 (d, 4H, H30, H50-biphenyl, J ¼ 8.78 Hz), 7.18 (d, 4H, H2, H6-
biphenyl, J ¼ 8.96 Hz), 6.99 (d, 4H, H20, H60-biphenyl, J ¼ 8.78 Hz),
4.42 (d, 2H, ClCH, J ¼ 6.70 Hz), 4.02 (t, 4H, CH2O), 2.27 (m, 2H,
CHCH3), 1.81 (m, 8H, CH2CH2O, CH3CH2), 1.48–1.22 (32H, aliphatic
CH2), 1.18 (d, 6H, CHCH3, J ¼ 6.70 Hz), 1.02 (t, 6H, CH3CH2), 0.56 (m,
4H, SiCH2), 0.09–0.05 (18H, SiCH3).
Characterization
The electrooptical measurements were performed in a conventional
cell (thickness either 15 mm or 9 mm), In which both ITO layers were
coated with Nissan 1266 polymer to ensure planar alignment. The cell
was filled with the liquid crystal in its isotropic state and then cooled
down very slowly to the SmAꢀ phase. An AC field (U ¼ 10 V, f ¼ 20 Hz)
was applied perpendicular to the cell confining plates. The signal from
a photodetector was measured by a lock-in amplifier ensuring high
signal to noise ratio. The measured voltage was converted to the
induced tilt angle hind by comparing with a switching angle measured
at the same applied voltage and temperature. Then, the temperature
dependence of hind was fitted to a power law to evaluate the critical
exponent c, as discussed below.

Design of Siloxane de Vries SmAꢀ
251=[2117]
X-Ray diffraction spectra were recorded on cooling the liquid crystal
samples from the isotropic phase in Lindman capillaries. The Cu-Ka
˚
radiation (k ¼ 1.54 A) (rotating anode) was used. The detector was a
2D plate (Imageplate) which allowed 2D powder spectra to be
obtained. After angular integration, the X-ray diffraction spectra pre-
sented well pronounced smectic peaks. The peak position was pre-
cisely located by fitting locally the peak with a Lorentzian and a
base line. The relative position of the peak for each sample was quite
precise, and the error bars in the presented graphs correspond to this
relative error.
RESULTS AND DISCUSSION
We synthesized three chiral liquid crystals MR11, MSi3-MR11, and
DSi3-MR11 viable to form an SmAꢀ phase at reasonably low tempera-
tures (Scheme 1). The basic mesogenic core of MR11 consisted of a
biphenyl 2-chloro-3-methylpropionate unit, that is also known to favor
the SmC–SmA phase sequence [10]. Two different chemical struc-
tures of organosiloxanes were derived from MR11 by a platinum(0)-
catalyzed hydrosilylation reaction [10,11]: a chiral mesogen was linked
to one end of a trisiloxane segment in MSi3-MR11 (monosubstituted)
or at both ends in DSi3-MR11 (disubstituted). Thus, the effect of
the flexible siloxane component on the SmAꢀ behavior of the varied
molecular architectures could be assessed.
The three molecules all displayed an SmAꢀ phase, but linking the
mesogen to either trisiloxane segment in MSi3-MR11 and DSi3-
MR11 changed the thermal characteristics of the materials notably.
In both siloxanes there existed an SmCꢀ phase (Fig. 1), which was
not present in MR11, and the SmAꢀ–isotropic phase transition rose
to higher temperatures. This was due to a stabilization of the smectic
phase probably by an intramolecular phase segregation of the incom-
patible siloxane segment. Conversely, the onset temperature of the
SmAꢀ phase was not affected significantly (Scheme 1). The siloxane
liquid crystals appeared, therefore, more suitable for characterization
and application.
The potential de Vries character of liquid crystals may be studied
conveniently by investigating the temperature dependence of the sus-
ceptibility in the SmAꢀ phase. This method is based on the concept
that, at the SmAꢀ-SmCꢀ transition temperature TC [12,13], the corre-
lation between the tilt directions diverges by different laws nꢃ(T–TC)–n
for orthogonal and de Vries SmAꢀ phases, although the order para-
meter symmetry is the same in both cases. Under idealized condi-
tions of two extreme options of SmAꢀ liquid crystals, the transition

252=[2118]
G. Galli et al.
FIGURE 1 Angular integration of the 2D powder X-ray diffraction spectrum
ꢁ
ꢀ
˚
(inset) of the SmC phase of DSi3-MR11 at 50 C (d ¼ 34.3 A).
mechanism in an orthogonal SmAꢀ is due to an induced molecular
tilt on the one hand, whereas in the de Vries phase it is due to the
rotation of molecules around a cone on the other hand. Consequently,
the respective phase transitions belong to two different universality
classes and should show quite different critical indices. In particular,
the susceptibility diverges by a power law vꢃ(T–TC)–c, in which the
critical exponent is 1.33 for an orthogonal SmAꢀ or 2 for a de Vries
SmAꢀ. However, in real SmAꢀ phases the transition can take place
as a combination of both mechanisms. The value of c can then mani-
fest a more or less pronounced degree of de Vries character for a
given mesogen [4].
Following this approach, we measured the electrooptical response,
h
ind, and monitored its temperature dependence in the SmAꢀ phase
to evaluate c. The electrooptical response for MR11 is shown in
Figure 2. This was fitted with a power law with a critical exponent
c ¼ 1.53, although the extrapolated critical temperature TC of 34ꢁC
occurred in the crystalline phase. Thus, the SmAꢀ–SmCꢀ transition
was a virtual one in MR11.

Design of Siloxane de Vries SmAꢀ
253=[2119]
FIGURE 2 Electrooptical response in the SmAꢀ phase of the mesogen MR11
(E ¼ 1 V=mm).
Interestingly, the electrooptical response of MSi3-MR11 and DSi3-
MR11 revealed different c-values (Fig. 3), and accordingly different
de Vries properties, despite the fact that both materials are very
similar from a chemical viewpoint. For the trisiloxane MSi3-MR11,
carrying one mesogen, c was 1.53. The values of c for both MR11
and MSi3-MR11 are comparable to those found for other de Vries
SmAꢀ molecules that contain one or no siloxane segment [4,6].
By contrast, the trisiloxane DSi3-MR11, having two mesogens
attached, displayed a much greater c of 1.87 (Fig. 3). This value indi-
cates that DSi3-MR11 should possess a quite distinct de Vries SmAꢀ
phase. Such a conclusion was also supported by our X-ray diffraction
results. Figure 4 illustrates the change of the smectic layer periodicity
d with temperature for DSi3-MR11. The SmAꢀ-SmCꢀ transition was
sluggish with a small shortening of d, which varied by only 3% over
the entire smectic range from 80ꢁC down to 30ꢁC.
We very recently found [6] analogous electrooptical and X-ray
diffraction behaviors at the SmAꢀ–SmCꢀ transition in trisiloxane
DSi3-AC11 (Fig. 5). The determined value of c was as high as 1.96,
indicating a practically pure de Vries scenario [6]. In complete con-
trast, the unsubstituted mesogen AC11 and the monosubstituted
disiloxane MSi2-AC11 presented c of 1.66 and 1.49, respectively [6].

254=[2120]
G. Galli et al.
FIGURE 3 Electrooptical response in the SmAꢀ phase of the trisiloxanes
MSi3-MR11 and DSi3-MR11 (E ¼ 1 V=mm).
FIGURE 4 Variation of the smectic layer periodicity with temperature for
DSi3-MR11.

Design of Siloxane de Vries SmAꢀ
255=[2121]
FIGURE 5 Structure and phase transitions of the trisiloxane DSi3-AC11 [6].
From these results we conclude that the molecular architecture of
the siloxanes DSi3-MR11 and DSi3-AC11, both consisting of two
lateral mesogenic cores interconnected by a central trisiloxane spacer
segment, is of significant advantage in realizing an SmA phase of the
de Vries type. While this propensity was inherent in the mesogens
MR11 and AC11, it was optimized in the disubstituted siloxanes. In
fact, simple incorporation of a peripheral siloxane segment in MSi3-
MR11 or MSi2-AC11 did not result in an enhancement of c.
These findings may be of general relevance and require further dis-
˚
cussion. The measured smectic layer periodicity d of around 34 A for
DSi3-MR11 corresponds roughly to half length of the (isolated) mol-
˚
ecule (L=2 ꢄ 30 A) evaluated by an AM1 energy-minimized model
˚
(Fig. 6). Similar results were previously found for DSi3-AC11 (d¼35 A,
˚
L=2 ꢄ 35 A). This suggests that the trisiloxane molecules are almost
completely interdigitated in a monolayer structure in both the SmAꢀ
and SmCꢀ phases, which arrangement possibly better accommodates
the siloxane segments between the smectic layers. A similar structural
model supporting this intramolecular segregation was recently pro-
posed for different disubstituted siloxane liquid crystals [14].
FIGURE 6 Energy-minimized AM1 model of the trisiloxane DSi3-MR11.

256=[2122]
G. Galli et al.
As suggested by the energy-minimized AM1 model of DSi3-MR11
(and DSi3-AC11, not shown here), the trisiloxane spacer facilitates
an angled disposition of the two mesogens with respect to each other.
This might induce a tilted position of the molecules varying from layer
to layer and depress the tendency of the material to form an orthog-
onal SmA phase. The siloxane segment is sufficiently flexible to allow
the molecules to adopt easily different positions on their cone. Since
the siloxane separates the molecules from each other in a way of
microphase segregation, the smectic layers are decoupled from each
other. In addition, the tilt direction is randomly distributed within
each layer. The decoupling of the layers seems to be a key point in rea-
lizing a SmA phase of the de Vries type, since by this means it is poss-
ible to eliminate effectively the long-range interlayer correlations.
One should expect that such a structure also favors the formation of
an antiferroelectric SmCꢀ phase [14,15]. This is consistent with our
observations of this phase for DSi3-MR11 below 36ꢁC. However, we
cannot exclude that the siloxane spacer segment can allow the com-
plete folding of the side mesogenic side arms so that they pack nearly
parallel to each other in a monolayer smectic structure.
CONCLUSIONS
The new liquid crystals form a de Vries SmAꢀ phase, as suggested by
electrooptical and X-ray diffraction measurements. The de Vries
character is maximized when two mesogens are interconnected by a
sufficiently long and flexible siloxane segment, such as in DSi3-
MR11 (c ¼ 1.87) and DSi3-AC11 (c ¼ 1.96). In this scenario, the siloxane
segment favors angled intralayer positions of the mesogens and
decouples the smectic layers from each other. The overall result should
be that long-range interlayer correlations are effectively eliminated, so
that the tilt angle directions adopt a random distribution in passing from
layer to layer.
REFERENCES
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¨
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883.

Design of Siloxane de Vries SmAꢀ
257=[2123]
[6] Reihmann, M., Crudeli, A., Blanc, C., Lorman, V., Panarin, Yu., Vij, J., Olsson, N., &
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