Design of liquid crystals with 'de Vries-like' properties: Carbosilane-terminated 5-phenylpyrimidine mesogens suitable for chevron-free FLC formulations

Article abstract of DOI:10.1039/c4tc00393d

Smectic liquid crystals with 'de Vries-like' properties are characterized by a maximum layer contraction of ≤1% upon transition from the orthogonal SmA phase to the tilted SmC phase. In an effort to expand the library of 'de Vries-like' liquid crystals required for the formulation of chevron-free ferroelectric liquid crystal mixtures, we report the synthesis of a homologous series of tricarbosilane 5-phenylpyrimidine liquid crystals QL16-n using an improved synthetic route, and the characterization of their liquid crystalline and 'de Vries-like' properties. Measurements of orientational order parameters S₂ and effective molecular lengths L_eff by monodomain 2D X-ray scattering suggest that 'de Vries-like' behavior in series QL16-n is due to the combined effect of an increase in S₂ and a decrease in bilayer interdigitation, thus causing a smectic layer expansion that compensates for the molecular tilt in the SmC phase. We also show how the optical tilt angle in the SmC phase may be optimized for SSFLC displays - without compromising 'de Vries-like' properties - by shortening the tricarbosilane end-group to a dicarbosilane. Two of the new materials reported herein, 5-[4-(12,12,14,14,16, 16-hexamethyl-12,14,16-trisilaheptadecyloxy)phenyl]-2-hexyloxypyrimidine (QL16-6) and 2-hexyloxy-5-[4-(12,12,14,14-tetramethyl-12,14-disilapentadecyloxy) phenyl]pyrimidine (QL24-6) rank among the best 'de Vries-like' materials reported heretofore, with broad SmC phases and reduction factors R of 0.17 and 0.18, respectively, at a reduced temperature T - T_AC = -10 K. We also show that inverting the orientation of the 5-phenylpyrimidine core in the homologous series QL17-n causes a suppression of 'de Vries-like' properties. These results suggest that non-covalent core-core interactions in the intercalated smectic bilayers formed by these mesogens may influence 'de Vries-like' behavior.

Full text of DOI:10.1039/c4tc00393d

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Materials Chemistry C
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Design of liquid crystals with ‘de Vries-like’
properties: carbosilane-terminated
5-phenylpyrimidine mesogens suitable for
chevron-free FLC formulations†
Cite this: J. Mater. Chem. C, 2014, 2,
4581
Christopher P. J. Schubert,a Andreas Bogner,b Jan H. Porada,b Khurshid Ayub,a
a
b
a
*
Tamer Andrea, Frank Giesselmann and Robert P. Lemieux
Smecticliquidcrystalswith‘deVries-like’properties arecharacterizedbya maximumlayercontractionof#1%
upontransitionfromtheorthogonal SmAphase tothetiltedSmCphase. Inan efforttoexpandthelibraryof ‘de
Vries-like’ liquid crystals required for the formulation of chevron-free ferroelectric liquid crystal mixtures, we
report the synthesis of a homologous series of tricarbosilane 5-phenylpyrimidine liquid crystals QL16-n using
an improved synthetic route, and the characterization of their liquid crystalline and ‘de Vries-like’ properties.
Measurements of orientational order parameters S2 and effective molecular lengths Leff by monodomain 2D
X-ray scattering suggest that ‘de Vries-like’ behavior in series QL16-n is due to the combined effect of an
increase in S2 and a decrease in bilayer interdigitation, thus causing a smectic layer expansion that
compensates for the molecular tilt in the SmC phase. We also show how the optical tilt angle in the SmC
phase may be optimized for SSFLC displays—without compromising ‘de Vries-like’ properties—by
shortening the tricarbosilane end-group to a dicarbosilane. Two of the new materials reported herein, 5-
[4-(12,12,14,14,16,16-hexamethyl-12,14,16-trisilaheptadecyloxy)phenyl]-2-hexyloxypyrimidine
(QL16-6)
and 2-hexyloxy-5-[4-(12,12,14,14-tetramethyl-12,14-disilapentadecyloxy)phenyl]pyrimidine (QL24-6) rank
among the best ‘de Vries-like’ materials reported heretofore, with broad SmC phases and reduction factors
R of 0.17 and 0.18, respectively, at a reduced temperature T ꢀ TAC ¼ ꢀ10 K. We also show that inverting
the orientation of the 5-phenylpyrimidine core in the homologous series QL17-n causes a suppression of
‘de Vries-like’ properties. These results suggest that non-covalent core–core interactions in the
intercalated smectic bilayers formed by these mesogens may influence ‘de Vries-like’ behavior.
Received 27th February 2014
Accepted 15th April 2014
DOI: 10.1039/c4tc00393d
www.rsc.org/MaterialsC
crystal (SSFLC) lm can be switched between opposite tilt
orientations to give an electro-optical shutter between crossed
Introduction
polarizers that is currently used in high-resolution reective
color microdisplays.
Ferroelectric liquid crystals are macroscopically polar uids
formed by rod-shaped mesogens that self-organize into a chiral
smectic C* (SmC*) mesophase, which is characterized by a
diffuse layer structure with a period d corresponding to the layer
spacing, a uniform tilt of the director n with respect to the layer
normal z, and a helical structure caused by the precession of n
about z from layer to layer (Fig. 1). Alignment of a SmC* liquid
crystal between glass slides with a rubbed alignment substrate
(e.g., polyimide) causes the unwinding of the SmC* helix into a
surface-stabilized conguration with a spontaneous polariza-
tion PS oriented perpendicular to the glass slides.1–3 By coupling
PS to an electric eld, this surface-stabilized ferroelectric liquid
aChemistry Department, Queen's University, Kingston, Ontario, Canada. E-mail:
lemieux@chem.queensu.ca
b
¨
Fig. 1 Schematic representation of hard spherocylinders forming a
SmC* phase in the absence of boundary conditions (left), and as a
Institute of Physical Chemistry, Universitat Stuttgart, Pfaffenwaldring 55, D-70569
Stuttgart, Germany
surface-stabilized ferroelectric liquid crystal film in
geometry (right).
a bookshelf
† Electronic supplementary information (ESI) available: Detailed experimental
and synthetic procedures. See DOI: 10.1039/c4tc00393d
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The formulation of liquid crystal mixtures for SSFLC display For example, the organosiloxane and carbosilane 2-phenyl-
applications requires diverse libraries of chiral and achiral thiadiazole derivatives QL5-6 and QL6-6, and the organo-
materials in order to optimize a wide range of physical parame- siloxane 5-phenylpyrimidine derivative QL15-5 undergo a SmA–
ters, including temperature range, Goldstone mode viscosity, SmC transition with a maximum layer contraction of only 0.4–
optical tilt angle, birefringence, helical pitch and spontaneous 0.5%.11,12 These three materials have reduction factors R
polarization.3 Another key requirement that has yet to be ach- ranging from 0.20 to 0.24 at 10 K below the SmA–SmC transition
ieved in commercial mixtures, and which has limited the temperature TAC, which are comparable to those calculated for
commercialization of SSFLC technology beyond microdisplay the best ‘de Vries-like’ materials reported previously.13,14 The
applications, is the formation of a chevron-free bookshelf reduction factor R (eqn (1)) is a measure of ‘de Vries-like’
geometry in the SmC* phase. In conventional FLC mixtures, a character dened as the ratio of the tilt angle d(T) required to
high quality alignment in ITO glass cells with rubbed polyimide give a layer contraction dC(T)/dAC at a temperature T below TAC
,
layers is achieved by cooling from the chiral nematic (N*) phase assuming a conventional model of hard spherocylinders, over
to the SmC* phase via the orthogonal SmA* phase. However, the the tilt angle qopt measured experimentally by polarized optical
layer contraction of ꢁ7–10% that typically occurs upon cooling microscopy.15 According to eqn (1), a SmA–SmC transition
from the SmA* to the SmC* phase results in buckling of the would approach that described by the diffuse cone model of de
smectic layers from a bistable bookshelf geometry into a chevron Vries as R approaches 0.
geometry because of surface anchoring;4 the formation of chev-
R ¼ d(T)/qopt(T) ¼ cosꢀ1[dC(T)/dAC]/qopt(T)
(1)
rons with opposite fold directions results in zigzag line defects
that severely degrade the optical quality of the SSFLC lm.
‘De Vries-like’ liquid crystals present a unique solution to the
chevron geometry problem.5 These are smectic liquid crystals
characterized by a maximum layer contraction of #1% on
cooling from the orthogonal SmA phase to the tilted SmC phase.
This unusual behavior was rst explained by de Vries using a
‘diffuse cone’ model in which mesogens in the SmA phase have
a tilted orientation and a degenerate azimuthal distribution,6
although more recent theoretical work suggests that ‘de Vries-
like’ behavior may result from an unusual combination of high
lamellar order and low orientational order.5,7–9 This is consis-
tent with the fact that most ‘de Vries-like’ mesogens feature
nanosegregating elements such as a trisiloxane end-group or a
peruorinated side-chain that strongly promote lamellar
ordering. Chiral ‘de Vries-like’ liquid crystals tend to have
unusually large electroclinic tilt susceptibilities, with the eld-
induced electroclinic tilt accompanied by a signicant increase
in birefringence and minimal contraction of the smectic layer
spacing.5 The electro-optical behavior of two such materials
with rst-order SmA*–SmC* transitions was recently explained
using a generalized Langevin-Debye model, which assumes a
random azimuthal distribution of molecules on a xed tilt cone
of angle q, with an orientational distribution in which the tilt q
is allowed to vary with the applied eld over a prescribed
range.10
The proles of relative layer spacing d/dAC vs. reduced
temperature T ꢀ TAC exhibited by these materials typically show
a pronounced negative thermal expansion in the SmA phase
that persists on cooling into the SmC phase and counteracts the
layer contraction caused by tilting to such an extent that the
layer spacing at the SmA–SmC transition is eventually restored,
as shown in Fig. 2. Recent analyses by 2D X-ray scattering of
smectic monodomains formed by ‘de Vries-like’ organosiloxane
liquid crystals similar to QL5-6 have shown that they exhibit
unusually large molecular tilt uctuations, and that the layer
contraction due to molecular tilt upon SmA–SmC transition is
almost fully compensated by an increase in the orientational
order parameter S2 as tilt uctuations decrease with decreasing
temperature.16 On the other hand, similar analyses of mono-
domains formed by the 5-phenylpyrimidine derivatives QL15-4
and QL15-9 show that S2 is almost invariant of temperature in
both SmA and SmC phases, and that the layer contraction due
to molecular tilt is compensated primarily by an increase in
17
effective molecular length Leff
.
This may be interpreted as a
change in the degree of interdigitation in smectic bilayers
formed by these materials (vide infra).
Trisiloxane end-groups are known to promote the lamellar
ordering of calamitic mesogens due to the tendency of
siloxane and hydrocarbon segments to nanosegregate into
To expand the library of ‘de Vries-like’ liquid crystals for the
formulation of chevron-free FLC mixtures, we developed a
strategy based on designing mesogenic structures that combine
Fig. 2 Relative smectic layer spacing d/dAC vs. reduced temperature T
ꢀ TAC for compounds QL5-6 (O), QL6-6 (,) and QL15-5 (C). From
a nanosegregating end-group with a SmA-promoting element. ref. 11 and 12.
4582 | J. Mater. Chem. C, 2014, 2, 4581–4589
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distinct sub-layers, which results in the formation of partially obtained by alkylation of 4(n) with 1-bromo-12,12,14,14,16,16-
intercalated smectic bilayers.18 Organosiloxane mesogens are hexamethyl-12,14,16-trisilaheptadecane. The mesogen QL24-6 is
known to form very stable SmC phases, which may be due to a similarly obtained by alkylation with 1-bromo-12,12,14,14-tet-
suppression of out-of-layer uctuations that reduces the ramethyl-12,14-disilapentadecane. The inverted mesogens
entropic cost of molecular tilt. In the design of mesogens such QL17-n are obtained by deprotection of 3 and alkylation with the
as QL5-6, ‘de Vries-like’ behavior was achieved by combining a appropriate alkyl bromide to give 6(n), followed by nucleophilic
trisiloxane end-group as SmC-promoting element with a chloro- aromatic substitution with 12,12,14,14,16,16-hexamethyl-
terminated alkyl chain as SmA-promoting element.12,19 Like- 12,14,16-trisilaheptadecan-1-ol and NaH.
wise, in the design of mesogens such as QL15-5, ‘de Vries-like’
behavior was achieved by combining a trisiloxane end-group
with a 5-phenylpyrimidine core, which is also a strong SmA-
promoting element.11,20,21
Mesogenic properties
The mesophases formed by compounds QL16-n and QL17-n
(Table 1) were characterized by polarized optical microscopy
(POM) and differential scanning calorimetry (DSC). With one
exception (QL16-9), all compounds form enantiotropic SmA and
SmC phases, as shown by the characteristic fan and homeo-
tropic textures of the SmA phase that turn into broken fan and
Schlieren textures, respectively, upon transition to the SmC
phase. Substituting the tricarbosilane end-group in QL16-6 with
a dicarbosilane end-group to give QL24-6 resulted in a 17 K
increase in melting point, but the SmC and SmA temperature
ranges remained approximately the same on heating. A higher
order smectic phase appeared on cooling below the SmC phase,
which was identied as a SmF phase based on the characteristic
change of the Schlieren texture into a mosaic texture, as shown
in Fig. 3.23
To address the hydrolytic instability of siloxanes, which
precludes their use in liquid crystal display applications, Reddy
et al. showed that a chemically inert tricarbosilane can be
substituted for a trisiloxane as the nanosegregating element of a
smectic mesogen.22 We recently carried out a similar substitu-
tion in mesogens such as QL5-n to give QL6-n without signi-
cantly affecting their mesogenic or ‘de Vries-like’ properties.12
In this paper, we report the synthesis of a homologous series of
carbosilane 5-phenylpyrimidine derivatives QL16-n using an
improved synthetic route, and the characterization of their
mesogenic and ‘de Vries-like’ properties. We also report on the
effect of inverting the orientation of the 5-phenylpyrimidine
core in the homologous series QL17-n (hereinaer called the
‘inverted’ series), which suggests that non-covalent core–core
interactions in the intercalated bilayers formed by these
mesogens inuence ‘de Vries-like’ behavior. Finally, we show
how the optical tilt angle in these materials may be optimized
for use in SSFLC devices—without compromising ‘de Vries-like’
properties—by shortening the tricarbosilane end-group to a
dicarbosilane, as in QL24-6.
In all compounds investigated, texture analyses revealed a
signicant change in interference color on cooling from the
isotropic phase to the SmC phase under polarized optical
microscopy (see the fan textures in Fig. 3a and b, for example).
We recorded the interference color change in the fan textures
formed by a 10 mm lm of QL16-6, which showed a blue-green
color in the SmA phase that is almost invariant of temperature,
but then sharply changed to a green-yellow color upon transi-
tion to the SmC phase and gradually turned to orange at 10 K
below TAC (see Fig. S1a in ESI†). The overall color change
corresponds to an increase in birefringence DDn of ca. 0.02,
according to a Michel-Levy chart, which is consistent with that
previously observed for the ‘de Vries-like’ material 3M 8422,5
and is indicative of an increase in orientational order.5,12 The
apparent discontinuity in birefringence at the SmA–SmC tran-
sition is also consistent with discontinuities in S2(T) observed at
the SmA–SmC transition of ‘de Vries-like’ materials.16,17
A
similar change in birefringence was observed with a 10 mm lm
of QL17-6 (see Fig. S1b in ESI†).
As shown in Fig. 4, increasing the length of the alkoxy chain
in the two homologous series results in a gradual decrease in
Results and discussion
Synthesis
The synthetic route originally reported for the organosiloxanes the temperature range of the SmA phase, which is consistent
QL15-n includes a rather inefficient deprotection step in which a with empirical trends. A comparison of phase transition
methoxy group is selectively demethylated in the presence of the temperatures for QL15-n and QL16-n shows that substituting
n-alkoxy chain using NaSEt.11 In the improved synthetic route the trisiloxane end-group with a tricarbosilane causes a
shown in Scheme 1, the THP-protected phenylboronic acid 1 is decrease in both clearing and SmA–SmC transition tempera-
coupled to 5-bromo-2-chloropyrimidine (2) under Suzuki– tures, whereas the variations in melting temperatures are more
Miyaura conditions to give 3, which is subjected to nucleophilic random; the homologue QL16-6 forms a particularly broad SmC
aromatic substitution withthe appropriate sodiumalkoxide, and phase, with a melting point approaching room temperature.
deprotected upon acidic workup to give the 2-alkoxy-5-(4- Furthermore, shortening the tricarbosilane end-group in QL16-
hydroxyphenyl)pyrimidine 4(n). The mesogens QL16-n are then 6 to a dicarbosilane causes the melting point to rise by 17 K,
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Scheme 1 Reagents and conditions: (a) Pd2(dba)3, Cy3P, K3PO4, 1,4-dioxane, 76%; (b) (i) CnH2n+1OH, NaH, THF; (ii) H3O+, 70–80%; (c) 1-bromo-
12,12,14,14,16,16-hexamethyl-12,14,16-trisilaheptadecane, Cs2CO3, acetone, 85–95%; (d) TsOH, 1 : 1 CH2Cl2–EtOH, 99%; (e) CnH2n+1Br,
Cs2CO3, acetone, 75–90%; (f) 12,12,14,14,16,16-hexamethyl-12,14,16-trisilaheptadecan-1-ol, NaH, THF, 60–70%.
Table 1 Transition temperatures (ꢂC) and enthalpies of transitions (kJ molꢀ1, in parentheses) for compounds QL16-n, QL17-n and QL24-6 on
heating
Compound
Cr
Cr0
SmF
SmC
SmA
I
QL16-4
QL16-5
QL16-6
QL16-7
QL16-8
QL16-9
QL17-4
QL17-5
QL17-6
QL17-7
QL17-8
QL17-9
QL24-6
ꢃ 43 (20)
ꢃ 34
ꢃ 44 (<0.1)a
ꢃ 55 (<0.1)a
ꢃ 64 (<0.1)a
ꢃ 65c
ꢃ 68 (8.2)
ꢃ 66 (7.5)
ꢃ 69 (7.8)
ꢃ 67 (3.7)
ꢃ 68 (6.6)
(ꢃ 66 (6.0))d
ꢃ 68 (7.6)
ꢃ 66 (6.2)
ꢃ 72 (9.4)
ꢃ 70 (8.0)
ꢃ 70 (8.8)
ꢃ 69 (8.5)
ꢃ 81 (11)
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ 40 (21)b
ꢃ 33 (22)
ꢃ 64 (33)
ꢃ 58 (37)
ꢃ 67 (66)
ꢃ 39 (20)
ꢃ 44 (22)
ꢃ 48 (23)
ꢃ 51 (26)
ꢃ 56 (27)
ꢃ 57
ꢃ 66 (0.9)
(ꢃ 65 (1.2))d
ꢃ 42 (<0.1)a
ꢃ 48 (<0.1)a
ꢃ 59 (<0.1)a
ꢃ 61 (<0.1)a
ꢃ 63 (0.2)
ꢃ 60 (30)b
ꢃ 63 (0.3)
ꢃ 50 (31)
(ꢃ 42 (1.9))d
ꢃ 78 (<0.1)a
Transition temperature measured by polarized microscopy. b Total enthalpy for both transitions due to partial resolution of the peaks. c Cr–SmC
a
and SmC–SmA peaks overlap. d Monotropic mesophase.
Fig. 3 Polarized photomicrographs of QL24-6 (a) at 80 ꢂC in the SmA phase, (b) at 77 ꢂC in the SmC phase and (c) at 43 ꢂC in the SmF phase.
Fig. 4 Phase transition temperatures as a function of the chain length n in the homologous series QL15-n (from ref. 11a), QL16-n and QL17-n:
melting point (C), SmC–SmA transition (>), SmA–isotropic transition (:).
4584 | J. Mater. Chem. C, 2014, 2, 4581–4589
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trisiloxane end-group with a tricarbosilane has relatively little
effect on tilt angle (e.g., 31ꢂ vs. 30ꢂ at saturation for QL15-6 and
QL16-6, respectively).11 From a formulation perspective, the tilt
angle of 23ꢂ formed by QL24-6 at saturation is near the ideal qopt
value of 22.5ꢂ required for maximum contrast in a SSFLC elec-
tro-optical device.3
Small angle X-ray scattering
Accurate measurements of the smectic layer spacing d as a
function of temperature were carried out by small angle X-ray
scattering (SAXS). The measurements were performed on heat-
ing from the crystalline phase except for compound QL16-7,
which was analyzed on cooling from the isotropic liquid phase.
As shown in Fig. 6a, the proles of relative layer spacing d/dAC
vs. reduced temperature T ꢀ TAC for selected homologues in
series QL16-n and for QL24-6 show the characteristics of ‘de
Vries-like’ materials, i.e., a negative thermal expansion of d in
the SmA phase that persists in the SmC phase and compensates
for the layer contraction due to tilt. The shorter homologues
QL16-5, QL16-6 and QL24-6 show maximum layer contractions
%lcmax of only 0.4–0.5%; the longer homologues QL16-7 and
QL16-8 have %lcmax of 0.8–1.0%, which may be explained by the
higher tilt angles formed in the SmC phase. Indeed, the calcu-
lated reduction factors R at T ꢀ TAC ¼ ꢀ10 K show relatively
little difference in ‘de Vries-like’ properties between these ve
Fig.
5
Optical tilt angle qopt vs. reduced temperature T ꢀ TAC
measured on cooling for selected homologues in series QL16-n,
QL17-n, and for compound QL24-6; the filled symbols correspond to
the three homologues with n ¼ 6.
which is consistent with a previous report on organosiloxane
liquid crystals by Naciri et al.18d Inverting the core orientation in
QL17-n has little effect on the clearing temperature, which is
more or less invariant of chain length, or the SmA–SmC tran-
sition temperature. However, the melting temperature shows a
more regular increase with the alkoxy chain length, with no
evidence of any odd-even effect.
Optical tilt angles (qopt) were measured by POM as a function
of reduced temperature T ꢀ TAC in the absence of an electric
eld for those materials that form a SmC phase with a relatively
broad temperature range on heating and/or cooling.24 As shown
in Fig. 5, the tilt angle increases with increasing alkyl chain
length in both homologous series, which is consistent with
empirical trends.11 A comparison of tilt angles formed by the
three homologues with n ¼ 6, shown as lled symbols in Fig. 5,
highlights two signicant structure–property relationships: (i)
shortening the tricarbosilane end-group to a dicarbosilane
Table 2 Maximum layer contraction %lcmax, optical tilt angle qopt and
reduction factor R at T ꢀ TAC ¼ ꢀ10 K for compounds QL16-n, QL17-n
and QL24-6
ꢂ
Compound
%lcmax
qopt
/
R
QL16-5
QL16-6
QL16-7
QL16-8
0.4
0.5
1.0
0.8
1.5
1.9
1.4
0.4
20
27
26
26
20
20
22
22
0.21
0.17
0.24
0.20a
0.49
0.56
0.44b
0.18
causes a reduction in qopt, which may be due to a weakening of QL17-6
the nanosegregation imposed by the carbosilane end-group;
and (ii) inverting the core orientation also causes a reduction in
qopt that is even more substantial than that caused by a short-
QL17-7
QL17-8
QL24-6
a
At T ꢀ TAC ¼ ꢀ7 K. b At T ꢀ TAC ¼ ꢀ6 K.
ening of the carbosilane end-group. However, substituting the
Fig. 6 Relative smectic layer spacing d/dAC vs. reduced temperature T ꢀ TAC for (a) selected homologues in series QL16-n and for QL24-6, and
(b) for selected homologues in series QL17-n; the filled symbols correspond to the three homologues with n ¼ 6. The data for QL16-7 were
acquired on cooling from the isotropic liquid phase.
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S2 ¼ 0.5h3 cos2 b ꢀ 1i
(2)
(3)
compounds; overall, the compounds QL16-6 and QL24-6, rank
among the best ‘de Vries-like’ materials reported heretofore
(Table 2).11–14
d(T) ¼ 1/3Leff(S2(T) + 2)cos q(T)
Remarkably, the inversion of core orientation in series QL17-
n has a detrimental effect on ‘de Vries-like’ properties. As shown
in Fig. 6b, the d/dAC vs. T ꢀ TAC proles for the inverted
homologues QL17-6, QL17-7 and QL17-8 show the expected
negative thermal expansion of d in the SmA phase, but it does
not persist in the SmC phase to the extent that the layer
contraction due to molecular tilt is compensated. Maximum
layer contractions for these compounds range from 1.5% to
1.9%, which is still well below the 7–10% range exhibited by
conventional smectogens. However, with optical tilt angles that
are signicantly smaller than those formed by the QL16-n
isomers, the R values for the inverted isomers QL17-n turn out
to be about twice as large, ranging from 0.44 to 0.56. It is also
worth noting that the layer spacing dAC for the material with the
The inner small-angle scattering corresponding to the peri-
odicity along the layer normal z shows a sharp fundamental and
two higher-order Bragg peaks, which is indicative of a high
lamellar order. There are two wide-angle scattering maxima
with varying intensities over the azimuthal angle c, as shown by
the decomposition into two I(q) proles in Fig. 8a, which are
consistent with a nanosegregation of the carbosilane segments
from the hydrocarbon segments. The assignment of the inner
wide-angle scattering to the carbosilane sub-layer was
conrmed by a 2D X-ray scattering analysis of 2,2,4,4,6-pen-
tamethyl-2,4,6-trisilaheptane,25 which produced an I(q) prole
of the isotropic scattering that matches perfectly the rst tted
peak of QL16-6 (see Fig. 8b). The inner wide-angle scattering is
much broader in c than the outer wide-angle scattering corre-
sponding to the hydrocarbon sub-layer, which indicates that the
carbosilane end-groups have a much lower orientational order;
a similar observation was made with respect to the siloxane end-
groups in QL15-4 and QL15-9.17 In the SmA phase, the diffuse
wide-angle maxima are perpendicular to the layer peaks and
rotate in the same direction by an angle q upon cooling into the
SmC phase; the absence of bimodal broadening of the wide-
angle intensity prole along c at the SmA–SmC transition
indicates that the incident X-ray beam is probing a well-aligned
smectic monodomain.
˚
best R value, QL16-6, is ca. 2.2 A shorter than for the inverted
isomer QL17-6, which may be ascribed to a difference in S2, but
also to a difference in degree of interdigitation in the bilayer
structure (vide infra).
Monodomain 2D X-ray scattering
In order to understand the difference in ‘de Vries-like’ proper-
ties between the two isomeric series QL16-n and QL17-n, we
carried out X-ray scattering analyses on smectic monodomains
formed by the representative isomers QL16-6 and QL17-6. Two-
dimensional X-ray scattering patterns were obtained as a func-
tion of temperature from smectic monodomains, which allowed
simultaneous measurements of the director tilt q, the layer
spacing d, and the orientational order parameter S2.16,17 The
latter is a measure of the orientational order about the director
n according to eqn (2), where b is the angle formed by the long
axis of a single molecule and the director n, which represents
the average direction of all long molecular axes. The effective
molecular length Leff is derived from S2, d and q according to
eqn (3).16 X-ray scattering patterns obtained with QL16-6 in the
SmA and SmC phases are shown in Fig. 7 as representative
examples.
To measure S2 and q, the wide-angle intensity prole I(c) was
obtained by integrating the intensity I scattered along the
Fig. 7 2D X-ray scattering patterns from a smectic monodomain Fig. 8 Intensity profiles of the X-ray scattering vector q for (a) QL16-6
formed by QL16-6 in the SmA phase at 72 ꢂC and in the SmC phase at in the SmC phase at 42 ꢂC, and (b) for 2,2,4,4,6-pentamethyl-2,4,6-
42 ꢂC. The director n is orthogonal to the dotted line joining the trisilaheptane in the isotropic liquid phase at 40 ꢂC. The dotted curves
centers of the diffuse wide angle scattering and makes an angle q with represent the Lorentz fits to the measured data for the tricarbosilane
layer normal z in the SmC phase.
scattering (TriSi) and the hydrocarbon scattering (HC).
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Journal of Materials Chemistry C
azimuthal angle c over the range of q, including both the inner both compounds. The signicant difference between the two
carbosilane and the outer hydrocarbon rings. Values of q were isomers lies in the effective length proles Leff(T). In the case of
obtained from the shi in the I(c) maxima in c (see Fig. 7); QL16-6, Leff increases with decreasing temperature, from 51.6 to
˚
values of S2 were obtained by analyzing the I(c) prole accord- 54.0 A, which is consistent with a decrease in bilayer interdig-
ing to the method of Davidson et al.26,27 As I(c) contains all itation; in the case of QL17-6, Leff shows relatively little variance
˚
contributions from the carbosilane and hydrocarbon segments, with temperature, ranging from 54.1 to 54.8 A. In both cases, Leff
˚
the resulting values of q and S2 represent averages taken over values exceed the length of 41.3 A for a fully extended molecular
the entire molecule.
As shown in Fig. 9c, S2 for QL16-6 is relatively invariant of ESI†), which is consistent with an intercalated bilayer structure.
temperature in the SmA phase, and the approximate value of We illustrate the relative contributions of changes in S2 and
model minimized at the B3LYP/6-31G* level (see Fig. S2 in
0.48 is signicantly lower than S2 values typical of a conven- Leff to the d(T) proles of QL16-6 and QL17-6 by deconstructing
tional SmA phase (0.7–0.8);28 S2 increases with decreasing the proles using eqn (3), as shown in Fig. 10. Assuming, in the
temperature in the SmC phase to a maximum value of 0.63. The borderline case of a conventional SmA–SmC transition, that S2
S2(T) prole for QL17-6 is not signicantly different, although S2 and Leff remain constant,5,9 one can calculate the d(T) proles
values are slightly higher and increase with decreasing due only to changes in molecular tilt q by inserting into eqn (3)
temperature through both the SmA and SmC phases, from 0.42 the values of S2 and Leff at the SmA–SmC transition point TAC as
to 0.63. Both S2(T) proles show a discontinuity at the SmA– constants (see the lower curves (B) in Fig. 10). By inserting the
SmC transition, which is consistent with the corresponding variable S2(T) into eqn (3) and keeping Leff constant, one obtains
interference color change observed in the texture analysis of the middle curves (O), which both show a partial compensation
of layer contraction due to the increase in orientational
ordering. The top curves (C) correspond to the experimental
d(T) proles and include contributions from both S2(T) and
Leff(T). Hence, this analysis suggests that ‘de Vries-like’ behavior
in QL16-6 is due, at a rst approximation, to the combined
effect of an increase in orientational order and a decrease in
bilayer interdigitation on a nearly equal basis. In the case of
QL17-6, the analysis suggests that the increase in orientational
order does counteract to some extent the layer contraction due
to molecular tilt, but is not enough to produce the effect
observed with QL16-6 due to the relatively small variance in
bilayer interdigitation.
Fig. 9 Temperature dependence of (a) the director tilt angle q, (b) the Fig. 10 Smectic layer spacing d vs. reduced temperature T ꢀ TAC
layer spacing d, (c) the orientational order parameter S2, and (d) the derived from eqn (3) with experimental values of q for (a) QL16-6 and
effective molecular length Leff for QL16-6 (filled symbols) and QL17-6 (b) QL17-6: (B) including S2 and Leff ¼ constants at TAC; (O) including
(open symbols).
S2(T), Leff ¼ constant at TAC; (C) including S2(T), Leff(T).
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Paper
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The results presented herein are consistent with our previous
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de ‘de Vries-like’ mesogens with broad SmC phases that may
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factor R in the homologous series QL17-n provides new insight
in our investigation of the origins of ‘de Vries-like’ properties in
these materials. The S2(T) proles resulting from the mono-
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sion has relatively little effect on orientational order in both
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However, the results of this study suggest that ‘de Vries-like’
behavior is only achieved when the increase in S2 is combined
with a concomitant decrease in bilayer interdigitation. These
ndings differ from previous X-ray diffraction studies of
siloxane-terminated mesogens suggesting that ‘de Vries-like’
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variance in bilayer interdigitation.16,17 Indeed, one can conclude
from this and previous studies that there may be a continuum
of complementary effects giving rise to ‘de Vries-like’ behavior.
The difference in Leff(T) between QL16-6 and the inverted
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tation on cooling QL16-6 from the SmA to the SmC phase may
be driven by a minimization of antiparallel core–core interac-
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bilayer formed by QL17-6 in the SmA phase. To test this
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bilayer structure formed by these mesogens; the results of this
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We thank the Natural Sciences and Engineering Research
Council of Canada (Discovery and CREATE grants) and the
Deutsche Forchungsgemeinscha (NSF/DFG Materials World
Network program DFG Gi 243/6) for support of this work.
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