Elucidating the smectic A-promoting effect of halogen end-groups in calamitic liquid crystals

Article abstract of DOI:10.1039/c3tc30534a

Two isometric series of chloro-terminated 5-alkoxy-2-(4-alkoxyphenyl) pyrimidine mesogens QL8-m/n and QL9-m/n (m + n = 16), with the chloro-terminated alkoxy chain tethered to either the pyrimidine ring or the phenyl ring, were synthesized and their liquid crystalline properties characterized by polarized optical microscopy and differential scanning calorimetry. Based on the analysis of mesogenic properties and correlations to electrostatic potential isosurfaces calculated at the B3LYP/6-31G* level, we present evidence suggesting that the SmA-promoting effect of chloro end-groups is not due to strong polar interactions at the layer interfaces, as previously postulated in the literature. Instead, the evidence suggests that the SmA-promoting effect is due to the electron-withdrawing effect of the chloro end-group, which should reduce electrostatic repulsion between alkoxy chains and, consequently, increase van der Waals interactions between aromatic cores in the orthogonal SmA phase.

Full text of DOI:10.1039/c3tc30534a

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Materials Chemistry C
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PAPER
Elucidating the smectic A-promoting effect of halogen
end-groups in calamitic liquid crystals†
Cite this: J. Mater. Chem. C, 2013, 1,
3729
Ian Rupar,a Kirk M. Mulligan,a Jeffrey C. Roberts,a Dorothee Nonnenmacher,b
Frank Giesselmann and Robert P. Lemieux
b
a
*
Two isometric series of chloro-terminated 5-alkoxy-2-(4-alkoxyphenyl)pyrimidine mesogens QL8-m/n and
QL9-m/n (m + n ¼ 16), with the chloro-terminated alkoxy chain tethered to either the pyrimidine ring or
the phenyl ring, were synthesized and their liquid crystalline properties characterized by polarized optical
microscopy and differential scanning calorimetry. Based on the analysis of mesogenic properties and
correlations to electrostatic potential isosurfaces calculated at the B3LYP/6-31G* level, we present
evidence suggesting that the SmA-promoting effect of chloro end-groups is not due to strong polar
interactions at the layer interfaces, as previously postulated in the literature. Instead, the evidence
suggests that the SmA-promoting effect is due to the electron-withdrawing effect of the chloro end-
group, which should reduce electrostatic repulsion between alkoxy chains and, consequently, increase
van der Waals interactions between aromatic cores in the orthogonal SmA phase.
Received 21st March 2013
Accepted 2nd May 2013
DOI: 10.1039/c3tc30534a
www.rsc.org/MaterialsC
et al. later reported that homologous series of chloro- and
bromo-terminated 5-phenylpyrimidine mesogens also form
exclusively SmA phases.9
Introduction
Tuning the mesogenic properties of thermotropic liquid crys-
tals that form lamellar (smectic) phases has focused primarily
on structural modications of the rigid aromatic core based on
the assumption that lamellar ordering is driven by the amphi-
philicity of smectogenic materials,1 and that variations in van
der Waals interactions between aromatic cores should have a
pronounced effect on mesogenic properties.2–4 However, studies
have shown that the introduction of various end-groups on one
aliphatic chain can also have signicant effects on mesogenic
properties, although the exploitation of such an approach in the
tuning of mesogenic properties has been narrower in scope.5,6
For example, siloxane end-groups are known to promote the
formation of smectic liquid crystal phases by nanosegregating
from hydrocarbon segments, forming a so-called “virtual
siloxane backbone” in an intercalated bilayer structure.7 More
recently, Goodby et al. have shown that bulky segments such as
cycloalkyl, bicycloalkyl and adamantyl end-groups suppress the
formation of anticlinic smectic phases in antiferroelectric
liquid crystals such as MHPOBC.8 They have also shown that the
introduction of a halogen end-group promotes the formation of
an orthogonal smectic A (SmA) phase at the expense of nematic
(N) and tilted smectic C (SmC) phases in a homologous series of
diuoroterphenyl mesogens (e.g., 1 and 2 in Fig. 1);5 Laschat
We have exploited this unusual SmA-promoting effect in the
design of liquid crystals with ‘de Vries-like’ properties, which
undergo a SmA–SmC phase transition with a maximum layer
contraction of #1%.10,11 The design of these new materials is
based on a concept of frustration between two structural
aChemistry Department, Queen's University, Kingston, Ontario, Canada. E-mail:
lemieux@chem.queensu.ca
b
¨
Institute of Physical Chemistry, Universitat Stuttgart, Pfaffenwaldring 55, D-70569
Stuttgart, Germany
† Electronic supplementary information (ESI) available: Synthetic procedures and
structural characterization data. See DOI: 10.1039/c3tc30534a
Fig. 1 The effect of a chloro end-group on the mesogenic properties of four
different materials; the phase transition temperatures are in ꢀC.5,10,12
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elements, one promoting the formation of a SmA phase and
another promoting the formation of a SmC phase. The design of
our rst generation of ‘de Vries-like’ liquid crystals was
informed by the results of a model study based on a 5-alkoxy-2-
(4-alkoxyphenyl)pyrimidine scaffold, which showed that a
Results and discussion
Synthesis and characterization
Compounds QL8-m/n, QL9-m/n and QL10-X were prepared by
sequential alkylations of 2-(4-hydroxyphenyl)pyrimidin-5-ol
with the appropriate alkan-1-ol and halogen-terminated alkan-
1-ol via Mitsunobu reactions (see ESI† for detailed synthetic
procedures), except for QL10-I, which was derived from QL10-Br
via a substitution reaction with NaI in acetone.14 The new
compounds were recrystallized from acetonitrile and hexanes,
and analyzed by polarized optical microscopy (POM) and
differential scanning calorimetry (DSC). As shown in Table 1,
the SmA-promoting effect of the chloro end-group is evident in
both isometric series. Compounds in series QL8-m/n form an
enantiotropic SmA phase except for QL8-5/11, which forms a
monotropic SmA phase. The SmA phase formed by these
compounds was identied by the formation of characteristic
fan textures and dark homeotropic domains on cooling from
the isotropic liquid phase, as shown in Fig. 2a. Compounds in
series QL9-m/n from n ¼ 4 to n ¼ 8 form an enantiotropic SmA
phase, whereas QL9-7/9 and QL9-6/10 form enantiotropic SmA
and nematic (N) phases, and QL9-5/11 forms an enantiotropic
nematic phase and a monotropic SmA phase. The nematic
phase was identied in all three compounds by the formation of
a characteristic Schlieren texture, as shown in Fig. 2b. All three
halogen-terminated compounds in series QL10-X form an
enantiotropic SmA phase, thus demonstrating the generality of
the SmA-promoting effect across the entire halogen series. For
the purpose of comparison, phase transition temperatures for
the corresponding isometric series 2PhP-m/n (m + n ¼ 17), in
which the chloro end-group is substituted with a methyl group,
siloxane end-group acts as
a strong SmC-promoter and
conrmed the nding of Goodby et al. that a chloro end-group
acts as a strong SmA-promoter (see 3 and QL8-8/8 in Fig. 1). By
combining these two elements in the same 2-phenylpyrimidine
scaffold, one can obtain mesogens such as 4, which forms both
SmA and SmC phases and undergoes a SmA–SmC phase tran-
sition with a maximum layer contraction of 1.6%.10,12 Interest-
ingly, we also found in this rst generation of mesogens that the
effect of the chloro end-group is suppressed if the orientation of
the 2-phenylpyrimidine core is inverted (e.g., 6 in Fig. 1).
The SmC-promoting effect of the siloxane end-group may be
understood in terms of nanosegregation, and the correspond-
ing suppression of out-of-layer uctuations that reduces the
entropic cost of molecular tilt in a supramolecular lamellar
structure.6 However, the origin of the SmA-promoting effect of
the chloro end-group is still not well understood. The only
explanation presented thus far suggests that this effect “may be
due to strong polar interactions at the smectic layer interfaces
caused by the terminal halogen units”.5 In this paper, we
present evidence that is inconsistent with this explanation
based on the mesogenic properties of two isometric series of
chloro-terminated 2-phenylpyrimidine liquid crystals QL8-m/n
and QL9-m/n (m + n ¼ 16). Unlike the more common homolo-
gous series, in which the length of a mesogen is gradually
increased by adding methylene units to one or both aliphatic
chain(s), mesogens in isometric series have the same molecular
length but vary in terms of the relative lengths of the aliphatic
chains.13 This experimental approach revealed an apparent
synergy between the electron-withdrawing properties of the
pyrimidine ring and the chloro end-group in series QL8-m/n
that provides the basis of a rationale for the SmA-promoting
effect in terms of an increase in electrostatic potential of the
chloro-terminated alkoxy chain. We also show the scope of the
SmA-promoting effect of halogen end-groups by characteriza-
tion of the uoro-, bromo- and iodo-terminated mesogens
QL10-X.
Table 1 Transition temperatures (ꢀC) and enthalpies of transitions (kJ molꢁ1, in
parentheses) for compounds QL8-m/n, QL9-m/n and QL10-X on heating
Compound
Cr
SmA
N
I
QL8-12/4
QL8-11/5
QL8-10/6
QL8-9/7
QL8-8/8
QL8-7/9
QL8-6/10
QL8-5/11
QL9-12/4
QL9-11/5
QL9-10/6
QL9-9/7
QL9-8/8
QL9-7/9
QL9-6/10
QL9-5/11
QL10-F
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
72 (41)
73 (44)
56 (49)
51 (25)a
67 (39)
48 (10)b
67 (46)
93 (57)
59 (37)
65 (42)
61 (45)
66 (43)
76 (28)d
66 (38)
72 (39)
65 (45)
47 (9.1)e
52 (27)
49 (29)
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
(ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
(ꢂ
ꢂ
ꢂ
ꢂ
111 (6.2)
112 (8.8)
103 (8.1)
107 (10)
105 (9.2)
98 (9.1)
95 (10)
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
85 (9.5))c
100 (8.3)
101 (9.3)
97 (8.4)
94 (7.5)
95 (6.4)
85 (1.0)
85 (1.2)
76 (1.2)
101 (10)
100 (7.7)
97 (2.8)
82 (1.7)
76 (0.3)
61 (0.6))c
ꢂ
ꢂ
ꢂ
QL10-Br
QL10-I
a
b
Preceded by a Cr–Cr transition at 48 ꢀC (19 kJ molꢁ1). Preceded by
a Cr–Cr transition at 38 ꢀC (15 kJ molꢁ1). Monotropic mesophase.
c
d
e
Preceded by a Cr–Cr transition at 72 ꢀC (34 kJ molꢁ1). Preceded by
a Cr–SmX transition at 34 ꢀC (6.8 kJ molꢁ1).
3730 | J. Mater. Chem. C, 2013, 1, 3729–3735
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were obtained from the patent literature except for compound
2PhP-12/5, which was synthesized and characterized indepen-
dently (see ESI†).15
Measurements of smectic layer spacings d as a function of
temperature were carried out by small-angle X-ray scattering
(SAXS) for the chloro-terminated compound QL8-8/8 and the
non-chloro analogue 2PhP-9/8. As shown in Fig. 3, the layer
spacing of both compounds exhibits the same degree of nega-
˚
tive thermal expansion in the SmA phase, from 30.5 to 31.0 A for
˚
QL8-8/8 and from 30.7 to 30.9 A for 2PhP-9/8, which may be
attributed to an increase in the orientational order parameter
(S2).16 The decrease in layer spacing upon transition to the SmC
phase results in a maximum layer contraction of 6.4% for 2PhP-
9/8, which is consistent with the behavior of conventional
calamitic smectogens.17 The d values in the SmA phase are near
˚
the molecular lengths calculated for QL8-8/8 (31.8 A) and 2PhP-
˚
9/8 (32.4 A) as fully extended equilibrium structures minimized
at the B3LYP/6-31G* level using Spartan'10 (Wavefunction, Inc.,
Irvine, CA). This is consistent with a monolayer organization
that does not appear to be perturbed by the chloro end-group.
A graphical comparison of the mesogenic properties of
QL8-m/n and QL9-m/n isomers (Fig. 4) reveals diagnostic struc-
ture–property relationships: (i) the mesogenic properties of
QL8-m/n andQL9-m/n aremostsimilar withthe chloro end-group
positioned in close proximity to the core (viz., QL8-12/4 and
QL9-12/4), and (ii) are most dissimilar with the chloro end-group
positioned far away from the core (viz., QL8-5/11 and QL9-5/11);
(iii) the SmA-promoting effect is evident throughout the QL8-m/n
series, in which the chloro-terminated alkoxy chain is tethered to
the pyrimidine ring, whereas (iv) the SmA-promoting effect
gradually vanishes in the QL9-m/n series, in which the chloro-
terminated alkoxy chain is tethered to the phenyl ring, as the
distance between the chloro end-group and the core increases.
Interestingly, the QL9-m/n and 2PhP-m/n series show similar
structure-property trends, if one makes no distinction between
SmA and SmC phases; in the extreme cases of QL9-6/10 and QL9-
5/11, the mesogenic properties are more or less the same as those
of the non-halogenated analogues 2PhP-11/6 and 2PhP-12/5.
The difference in mesogenic properties between QL8-m/n
and QL9-m/n is inconsistent with the hypothesis that strong
polar interactions between chloro end-groups at the smectic
layer interface are responsible for the SmA-promoting effect
since any such interaction should be invariant of the
Fig. 2 Polarized photomicrographs of (a) QL8-9/7 in the SmA phase at 96 ꢀC
(top) and (b) QL9-7/9 in the N phase at 85 ꢀC (bottom) on cooling from the
isotropic liquid phase.
Fig. 3 Smectic layer spacing d versus temperature measured on heating from
the crystalline phase for compounds QL8-8/8 (B) and 2PhP-9/8 (C).
Fig. 4 Mesophases formed by compounds QL8-m/n, QL9-m/n and 2PhP-m/n on heating.
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orientation of the 2-phenylpyrimidine core in relation to the
chloro-terminated alkoxy chain. This is particularly evident
when one considers that the two isomers with the most
dissimilar mesogenic properties are those with the chloro end-
group positioned furthest from the core. On the other hand, the
fact that the SmA-promoting effect is more consistent in the
QL8-n/m series suggests that an effect resulting from a synergy
between the pyrimidine ring and the chloro end-group
contributes to the stabilization of the SmA phase.
Molecular dipole moments
In analyzing structure–property relationships, we rst sought to
measure molecular dipole moments m for QL8-m/n and QL9-m/n
to determine if any correlation exists between m and the meso-
genic properties. Fully extended equilibrium structures were
minimized at the B3LYP/6-31G* level using Spartan'10, and
molecular dipole moments were calculated at the same level of
theory. As shown in Fig. 5, molecular dipole moments in series
QL8-m/n are signicantly smaller than in series QL9-m/n due to
the difference in orientation of the 2-phenylpyrimidine core
relative to the terminal C–Cl bond. The dipole moment in each
series also shows an odd-even effect consistent with the alter-
nating orientation of the C–Cl bond relative to the core dipole
moment, as shown in Fig. 6. However, for a given chain length
parity, the value of m remains approximately constant in each
series.
Fig. 6 Molecular structures and dipole moment vectors m (in yellow) for the
isomeric pairs QL8/QL9-7/9 and QL8/QL9-8/8 showing the alternation of m with
the parity of the chloro-terminated alkoxy chain length.
isosurfaces for the extreme isomers QL8-12/4 and QL8-5/11 and
the intermediate isomer QL8-8/8, and by comparing their
isosurfaces to those calculated for the corresponding isomers in
series QL9-m/n and 2PhP-m/n. The electrostatic potential
isosurface of a molecule displays the electrostatic potential
energy (U) of a positive charge along an electron density iso-
surface according to a spectral color scale ranging from red
(negative U) to blue (positive U), and thus gives a three-dimen-
sional representation of molecular charge distribution.
Neither the difference in m between QL8-m/n and QL9-m/n
nor the invariance in m for a given parity in each series correlate
with the observed differences in mesogenic properties, as
shown in Table 1 and Fig. 4. For example, the isomeric pairs
QL8/QL9-11/5 and QL8/QL9-5/11 are very different in terms of
their relative mesogenic properties, but the two pairs show
approximately the same difference in dipole moment (Dm ¼ 3.49
D vs. 3.43 D, respectively).
Fully extended equilibrium structures were minimized at the
B3LYP/6-31G* level using Spartan'10, and electrostatic potential
isosurfaces were calculated at the same level of theory. The
isosurfaces shown in Fig. 7 for the three intermediate isomers
QL8-8/8, QL9-8/8 and 2PhP-8/9 display electrostatic potentials
over the same dynamic range using a symmetrical color scale
from red (ꢁ155 kJ molꢁ1) to blue (+155 kJ molꢁ1), which is
based on the highest absolute value of U calculated for these
three molecules. The isosurfaces reveal the strong electron-
withdrawing effect of the pyrimidine ring—as evidenced by the
light blue shading of the methylene group proximal to each
pyrimidine ring—and the non-polar character of unsubstituted
Electrostatic potential isosurfaces
Given that the pyrimidine ring and the chloro end-group are
both electron-withdrawing, we then sought to measure their
combined effect on the electrostatic potential of the alkoxy
chains in series QL8-m/n by calculating electrostatic potential
Fig. 5 Molecular dipole moment m calculated for QL8-m/n (B) and QL9-m/n
(C) at the B3LYP/6-31G* level as a function of the chloro-terminated alkoxy chain
length n.
Fig. 7 Electrostatic potential isosurfaces calculated at the B3LYP/6-31G* level
for QL8-8/8 (top), QL9-8/8 (middle) and 2PhP-8/9 (bottom). The color scale is
displayed to the left and ranges from red (ꢁ155 kJ molꢁ1) to blue (+155 kJ molꢁ1).
3732 | J. Mater. Chem. C, 2013, 1, 3729–3735
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Fig. 8 Electrostatic potential isosurfaces calculated at the B3LYP/6-31G* level for (a) QL8-5/11 (top), QL8-8/8 (middle) and QL8-12/4 (bottom); and (b) QL9-5/11
(top), QL9-8/8 (middle) and QL9-12/4 (bottom). The color scale is displayed to the left and ranges from red (ꢁ155 kJ molꢁ1) to blue (+105 kJ molꢁ1).
alkoxy chains tethered to a phenyl ring. The isosurfaces also
reveal the combined effect of the pyrimidine ring and chloro
end-group, which results in higher (more positive) electrostatic
potentials in the substituted alkoxy chain of QL8-8/8; the effect
of the chloro end-group on the substituted alkoxy chain of QL9-
8/8 is also evident, albeit less pronounced given that it is teth-
ered to the phenyl ring.
In order to highlight the electron-withdrawing effect of the
pyrimidine ring and chloro end-group in the electrostatic
potential isosurfaces of QL8-m/n and QL9-m/n in relation to
those of the unsubstituted 2PhP-m/n, we moved the positive end
of the color scale in Fig. 8 and 9 to the highest positive U value
calculated in the threeseries, which decreasesthe dynamic range
of U (ꢁ155 kJ molꢁ1 to +105 kJ molꢁ1) and offsets the color scale.
Fig. 9 Electrostatic potential isosurfaces calculated at the B3LYP/6-31G* level
for 2PhP-5/12 (top), 2PhP-8/9 (middle) and 2PhP-12/5 (bottom). The color
Once again, the combined effect of the pyrimidine ring and
scale is displayed to the left and ranges from red (ꢁ155 kJ molꢁ1) to blue
chloro end-group in series QL8-m/n can be seen in Fig. 8,
resulting in higher electrostatic potentials than those calculated
(+105 kJ molꢁ1).
for the corresponding isomers in series QL9-m/n. The effect of
the chloro end-group on U becomes more pronounced as the
conformational degrees of freedom than shorter ones and
distance between the core and chloro-end group decreases,
therefore occupy more conformational space, thus forcing rigid
which correlates qualitatively with the SmA-promoting effect of
aromatic cores farther apart in the diffuse lamellar structure of
the latter in both series QL8-m/n and QL9-m/n. Notwithstanding
the apparent synergy between the pyrimidine ring and chloro
end-group, the calculations do reveal a measurable shi towards
higher electrostatic potentials in the chloro-terminated alkoxy
chains of both QL8-m/n and QL9-m/n relative to the non-chloro
analogues 2PhP-m/n (see Fig. 9), except perhaps in the case of
QL9-5/11, which displays an electrostatic potential isosurface
that is qualitatively very similar to that of 2PhP-12/5. This simi-
larity is consistent with the fact that both compounds exhibit
very similar mesogenic properties.
Assuming an antiparallel organization of mesogens in the
smectic layers, the effect of a more positive electrostatic
potential in one of the alkoxy chains should be to reduce elec-
trostatic repulsion between side-chains. But how can this favor
the orthogonal SmA phase over the tilted SmC phase? In
homologous mesogenic series, the lengthening of alkyl chains
normally favors the SmC phase over the SmA phase by virtue of
Fig. 10 Graphical representation of the interplay between van der Waals forces
an increase in ‘entropic pressure’ associated with the confor-
and entropic pressure in the SmA phase. The conical volumes approximate the
mational disorder of alkyl chains.12 Longer chains have more conformational space occupied by the aliphatic chains.
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an orthogonal SmA phase.18 According to this model, free (NSF/DFG Materials World Network program DFG Gi 243/6) for
energy is minimized upon tilting in the SmC phase by achieving support of this work.
a balance between the entropic pressure caused by conforma-
tional disorder of the alkyl chains and the attractive van der
Waals interactions of the aromatic cores (Fig. 10). Tilting also
References
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Summary
Based on our analysis of structure–property relationships in two
isometric series of chloro-terminated 2-phenylpyrimidine 11 (a) J. C. Roberts, N. Kapernaum, Q. Song, D. Nonnenmacher,
mesogens, and correlations to electrostatic potential iso-
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that the SmA-promoting effect of chloro end-groups is not due
to strong polar interactions at the layer interfaces, as previously
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end-group, which should reduce electrostatic repulsion 12 The parent compound 2-PhP8 undergoes a SmA–SmC phase
between the alkoxy chains and increase attractive van der Waals
interactions between aromatic cores in the SmA phase.
transition with a maximum layer contraction of 7.1%.
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Acknowledgements
We thank the Natural Sciences and Engineering Research
Council of Canada and the Deutsche Forchungsgemeinscha
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