Ring effect on helical twisting power of optically active mesogenic esters derived from benzene, bicyclo[2.2.2]octane and p-carborane carboxylic acids

Article abstract of DOI:10.1039/b512319d

Three structurally similar esters containing carborane (1A), bicyclo[2.2.2]octane (1B), and benzene (1C) were prepared and their mesogenic properties investigated. All esters exhibited chiral nematic phases, and only 1B showed rich smectic behavior. The e

Full text of DOI:10.1039/b512319d

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Ring effect on helical twisting power of optically active mesogenic esters
derived from benzene, bicyclo[2.2.2]octane and p-carborane carboxylic
acids{
Adam Januszko,a Piotr Kaszynski*a and Witold Drzewinskib
Received 31st August 2005, Accepted 7th October 2005
First published as an Advance Article on the web 17th November 2005
DOI: 10.1039/b512319d
Three structurally similar esters containing carborane (1A), bicyclo[2.2.2]octane (1B), and benzene
(1C) were prepared and their mesogenic properties investigated. All esters exhibited chiral nematic
phases, and only 1B showed rich smectic behavior. The esters were used as additives to three
structurally different nematic hosts. The resulting cholesteric pitch p was measured as a function
of concentration and temperature. The calculated helical twisting power bM followed the order 1A
, 1B ¡ 1C, and the temperature dependence of bM was ¢0 for all mixtures except for 1C in host
I. These results are discussed in terms of size and conformational properties of each ring system
A–C and the role of their ‘‘biaxiality’’ in chirality transfer. The observed trends in bM are
consistent with non-specific solute–solvent interactions in which the chiral additive breaks the
uniaxial symmetry imposed by the phase and differentiates energy of the chiral conformers of
the host.
increasing the conformational flexibility of the molecule and
Introduction
replacement of the aromatic system with aliphatic components
Chirality is an integral part of the liquid crystal phenomenon,
leads to a lower HTP. Indeed, it was found that HTP is weakly
research, and technology,1 and the question of how molecular
chirality is translated into bulk chirality of the liquid crystal-
line phase is one of the fundamental issues in experimental and
theoretical physics of liquid crystals.2 A macroscopic measure
of the extent of this chirality transfer in a single-component
phase is the helical pitch p. For multi-component mixtures, the
ability of a chiral additive to generate a helical phase is
defined as helical twisting power (HTP) or bM (yp21) and is
characteristic for the additive–mesogenic host system.
Accumulated experimental data3–9 and theoretical
results10–12 show that the molecular shape of the additive
and its conformational properties are the dominant factors
that affect the value of HTP. The emerging picture suggests
that the most effective dopants are those with strong
coupling between the stereogenic element and the extended
rigid aromatic fragment. Examples are compounds based on
atropisomeric binaphthyls,13 biphenyls,14 and certain metal
complexes.15 These findings are consistent with theoretical
results showing that the chiral additive must be ‘‘biaxial’’ for
induction of a helical structure, and that higher ‘‘biaxiality’’
leads to a higher HTP.11 Further development of the shape-
dependent HTP theory led to quantitative prediction of HTP
based on anisometry and molecular surface helicity combined
with conformational analysis of the chiral additive.10,16
In accord with this theory, diminishing the ‘‘biaxiality’’ by
dependent on the nature of the aliphatic chiral center,5,17,18
but strongly depends on the structure of the aryl component
and its anisometry in particular,5,13,19,20 as well as on the
distance of the chiral center from the rigid aromatic core.21
Small, purely aliphatic chiral dopants show very small values
of HTP.22–24
Most recent studies have concentrated on atropisomers as
chiral dopants, while there has been much less focus on
additives with a single chiral center.4,7,9,20 Such systems are
particularly well-suited for investigations of intra- and inter-
molecular chirality transfer processes,4,14,25 and their depen-
dence on the symmetry and conformational properties of
molecular components.4,5,7,9 To our knowledge, there have
been no systematic studies of the variation of the rigid core
structure and its impact on the HTP, aside from a comparison
of the pitch for some structurally similar compounds.26 Such
an opportunity is offered by properly designed derivatives
of rings A–C (Fig. 1). The 12-vertex p-carborane (A) and
aOrganic Materials Research Group, Department of Chemistry,
Vanderbilt University, Box 1822 Station B, Nashville, TN 37235, USA.
E-mail: piotr.kaszynski@vanderbilt.edu; Fax: +1 (615) 343-1234;
Tel: +1 (615) 322-3458
Fig. 1 Compound 1 and structures of ring : 1,12-dicarba-closo-
dodecaborane (p-carborane, A), bicyclo[2.2.2]octane (B), and benzene
(C). In A each vertex corresponds to a BH fragment and the sphere
represents a carbon atom.
bMilitary Technical University, Warsaw, Poland
{ Electronic supplementary information (ESI) available: computa-
tional details. See DOI: 10.1039/b512319d
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Table 1 Transition temperatures/uC and enthalpies/kJ mol21 for 1a
bicyclo[2.2.2]octane (BCO, B) rings have approximately
cylindrical topology and can be considered as ‘‘uniaxial’’,
while the benzene ring (C) can be described as a prototypical
‘‘biaxial’’ structural element (Fig. 1).
In an attempt to test the effect of ring topology and
geometry on HTP, and to gain a better qualitative under-
standing of chirality transfer mechanisms, we focused on three
specifically designed chiral mesogenic esters 1A–1C containing
the (S)-2-methyl-1-butyl substituent. Here we report the
synthesis and characterization of the new mesogens 1 and
their HTP in three nematic hosts. The discussion of experi-
mental results is supported by conformational analysis of
relevant molecular fragments.
A
Cr
G
SmB
—
SmC
—
SmA
—
N*
I
N 65
(16.1)
—
N 125
(0.6)
N
N 85
(N 84)
N
91
(1.2)
N
97
(0.04)
N
133 N 169
(0.4) (0.9)
N
N
(17.9)b
(2.6)
N 93c
(19.7)
—
—
—
—
N 148
(1.0)
Results
a
b
Cr: crystal, Sm: smectic, N: nematic, I: isotropic. Sum of
c
enthalpies for Cr–SmB and SmB–SmC transitions. Cr–Cr transition
Synthesis
at 81 uC (3.5 kJ mol21).
Esters 1A–1C were prepared by esterification of 49-pentyl-
biphenyl-4-ol with acid chlorides in the presence (1A and 1C)
or absence (1B) of triethylamine. The acid chlorides were
obtained from the appropriate carboxylic acids and PCl5 (2A
and 2B) or SOCl2 (2C) as shown in Scheme 1.
The preparation of acid 2A was reported before.27 Acid 2B28
was obtained according to a general literature procedure.29
Benzoic acid 2C was synthesized from (S)-2-methylbutyl-
benzene as described in the literature.21,30 The hydrocarbon
was conveniently prepared by Ni-catalyzed C–C coupling of
(S)-2-methylbutylmagnesium bromide31 and chlorobenzene
according to a general method.32
Thermal properties
Transition temperatures and enthalpies of the esters were
determined by differential scanning calorimetry (DSC) and the
results are shown in Table 1. All three esters form enantio-
tropic cholesteric phases. The highest clearing point is
observed for the BCO derivative 1B at 169 uC and the lowest
for the carborane ester 1A at 125 uC. This order is consistent
with the general trend in ring effectiveness in the stabilization
of a nematic phase.33–36
Fig. 2 Partial heating and cooling DSC curves for 1B recorded at a
rate of 5 uC min21
.
impose a planar orientation of the sample. The resulting
textures were compared with those published for reference
compounds.37–39 The high temperature smectic grown from
the cholesteric phase showed a fan-shaped texture typical for
smectic A (Fig. 3a). Upon cooling, a narrow temperature
range SmC phase was formed as evident from the appearance
of a birefringent texture in a homeotropic region of the sample.
Further cooling resulted in the formation of a smectic B
characterized by a mosaic texture with circular domains and
four extinction brushes (Fig. 3b). The monotropic phase
formed from the SmB phase was identified as a G phase and its
texture is shown in Fig. 3c.
Among the three esters, only the BCO derivative 1B
exhibits smectic polymorphism (Fig. 2). Two out of three
enantiotropic smectic phases had a tendency to form the
homeotropic alignment suggesting an orthogonal phase
structure. The monotropic phase formed upon cooling of the
homeotropic phase showed a schlieren-type texture.
For better identification of the orthogonal phase structure,
the bottom surface was covered with rubbed polyimide to
A comparison of the chiral esters 1 with their isomeric
n-C5H11 chain analogs35 shows that the largest depression of
the clearing temperature is observed for the BCO derivative 1B
(249 uC), moderate for carborane 1A (234 uC), and smallest
for the benzoate 1C (227 uC). The sequence of the
enantiotropic smectic phases A and B is the same in 1B and
its achiral analog,35 albeit with the appearance of a narrow
range SmC phase and lower transition temperatures in the
chiral derivative. In the pair of benzoates, the narrow
Scheme 1
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Fig. 4 Partial optical rotatory dispersion curves obtained for 1.1%
solution of 1 in CHCl3.
Chart 1
while the [W] values for the BCO ester, 1B, are 3–4 times
smaller (Fig. 4). This reflects the differences in electronic
polarizability between the aromatic systems (sigma for A and
pi for C) in the former and aliphatic skeleton of BCO in the
latter esters.
Helical twisting power
Fig. 3 Natural textures observed in polarized light for 1B in the same
sample region for a) SmA phase, b) SmB phase, and c) G phase in the
middle of the phase range. Magnification 606.
The helical twisting power (HTP) bM for esters 1 was measured
in three nematic hosts I–III of diverse chemical structures
(Chart 1; Table 2) at the same reduced temperature of 12 uC
below the TNI of the pure hosts. Results show that the
benzoate 1C exhibits the largest absolute twisting power40 bM
in the series in all three hosts. The BCO ester 1B has smaller
twisting power in 6-CHBT (I) and ester (III) hosts (about 75%
of 1C) but equal to that of 1C in the pyrimidine host II. The
lowest effectiveness in generation of a cholesteric phase is
exhibited by the carborane 1A which is only about 20%–26%
of that shown by 1C.
temperature range monotropic phases A, C, and G observed in
the n-pentyl derivative35 are absent in the chiral analog 1C.
Chirooptical characterization
Dilute solutions of 1 in CHCl3 exhibit positive optical
rotations measured for several wavelengths in the range of
365–633 nm. Dispersion curves for molar rotation [W] of
carborane and benzene derivatives, 1A and 1C, are similar,
Table 2 Absolute twisting power bM for esters 1 in nematic hostsa
1A
1B
1C
Host
|bM|/mm21
(dbM/dT)/mm21 K21
|bM|/mm21
(dbM/dT)/mm21 K21
|bM|/mm21
(dbM/dT)/mm21 K21
6-CHBT, I
Pyrimidine, II
Ester, III
a
1.60 ¡ 0.03
0.71 ¡ 0.02
1.60 ¡ 0.02
+0.017
+0.017
+0.019
5.7 ¡ 0.1
3.5 ¡ 0.1
4.9 ¡ 0.1
0.00
+0.068
+0.01
7.7 ¡ 0.2
3.5 ¡ 0.1
6.2 ¡ 0.2
20.086
+0.040
0.0
Absolute value for twisting power bM measured at 0.5 mol%–4 mol% of the dopant at T 2 TNI = 212. Structures of hosts are shown in
Chart 1.
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The most susceptible host for induction of a cholesteric
phase by 1 is 6-CHBT (I), while the least susceptible is
pyrimidine II. 6-CHBT and ester are equally effective hosts
for 1A. Interestingly, the most effective host in this study is
also the most aliphatic.
for 3B and 3C is higher by about 1.5 kcal mol21 (calculated
and experimental43 torsional potentials for ethane are
2.9 kcal mol21). The barrier to internal rotation around the
carborane–COOH bond has a very low energy and the PES
for 4A is essentially flat (Fig. 5b). The same barrier for 4B
has only a modest height (0.9 kcal mol21), while the barrier to
rotation around the Ph–COOH bond is substantial and over
twice that of ethane.
Measurements of HTP as a function of temperature showed
that thermal coefficients dbM/dT for all mixtures are ¢0
except for 1C in 6-CHBT, for which the pitch increases with
increasing temperature. The carborane ester 1A shows an
approximately constant and modest value dbM/dT of about
+0.02 mm21 per 1 uC in each host. The tightening of the helix
with increasing temperature is strongest for 1B in pyrimidine II
(about +0.07 mm21 per 1 uC). Cholesteric phases induced by
BCO 1B or benzoate 1C in ester III and also by 1B in 6-CHBT
appear to be virtually temperature-insensitive in the investi-
gated temperature range.
The conformation of the isobutyl group in derivatives 3 can
be described as a distorted all-trans-propyl chain with a Me
group appended in the gauche position. In the ground state of
all three derivatives, the propyl chain adopts an orientation
near its optimum position in the GS of unbranched analogs
(Fig. 6). Thus, for 3A the Me group forces the propyl chain
only 6u off the eclipsed conformation,44 in 3B the chain is 20u
off the staggered orientation and in benzene derivative 3C the
difference is nearly 14u. The computational result for 3C is in
good agreement with experimental solid-state structure for a
close analog45 of 1C and other derivatives of (S)-2-methyl-
butylbenzene.46,47 In the rotational transition state the propyl
group is rotated by 41u, 79u, and 60u relative to the GS of 3A,
3B, and 3C, respectively.
The generally observed positive values of dbM/dT are
expected for highly anisometric dopants such as 1 which do
not significantly perturb the local order, and the change in the
helix pitch is mainly due to lowering the nematic K22 elastic
constant with increasing temperature.41
The carboxyl group in the carborane derivative 4A adopts
the eclipsed orientation (Cs symmetry) with the CLO group
antiperiplanar to the skeletal C–B bond in the GS and
synperiplanar in the rotational TS, according to the MP2
results. The DFT results disagree on the GS structure of 4A
and predict a low energy rotational TS for the eclipsed form,
while the conformational minimum is found for the orienta-
Conformational analysis
For better understanding of the variations in HTP exhibited by
compounds 1 in nematic hosts, we conducted conformational
analyses of monosubstituted derivatives of ring
and also
molecular fragments of the hosts. We chose the isobutyl in 3 as
a model for the 2-methylbutyl group, and a carboxyl group
in 4 as a model for the ester link. Results of gas phase
calculations at the MP2 level of theory42 are represented in
Fig. 5 and a table with complete DFT and MP2 results is
provided in the ESI.{
Calculations at the MP2 level of theory show that potential
energy surfaces (PESs) for rotation around the ring–alkyl bond
have shallow five-fold minima for carborane derivatives 3A,
and nearly equally energetic three- and two-fold minima for
BCO derivative 3B and benzene 3C, respectively (Fig. 5a).
The barrier to internal rotation for 3A is lower than that in
ethane by about 0.8 kcal mol21, while the analogous DE{
…
tion with the dihedral angle C–OH C–B of nearly 9u. This
discrepancy between the two sets of calculations is related to
the flat PES for internal rotation. The orientations of the
carboxyl group in the BCO derivative 4B in the ground and
transition states are opposite. Lower steric requirements for
the CLO than for the C–OH group prefer it in the eclipsed
orientation in the GS and staggered in the TS. In both forms
the BCO ring remains twisted by about 16u, according to the
MP2 calculations. In contrast, the DFT results show smaller
twisting for the GS and nearly Cs-symmetric structure for the
TS of 4B. The orientation of the carboxyl group in benzoic
Fig. 5 Approximate profiles for conformational energy change in 3 (a) and 4 (b) obtained from the calculated (MP2/6-31G(d) with B3LYP/6-
31G(d)ZPE correction) energy of activation for internal rotation around the –R bond at 0 K. The angle is defined by two adjacent bonds: C–C or
CLO of the substituent and C–C or C–B of the ring. The dashed line represents the activation energy for rotation in ethane.
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Fig. 6 Two projections of optimized (MP2/6-31G(d)) global conformational minima for 3.
acid (4C) is as expected: coplanar with the ring in the GS and
orthogonal in the TS. Both forms have the Cs molecular
symmetry.
are orthogonal to each other and the molecule is Cs-symmetric.
This is consistent with solid-state results48 for a derivative
of 5, a close analog of 6-CHBT, and a number of other
mesogenic derivatives in which the two rings are close to
orthogonality.49 The barrier to conformer interconversion
through a Cs-symmetric conformational transition state is
moderate and about DE{ = 3.0 kcal mol21 at the DFT level of
theory and 0.5 kcal mol21 higher according to the MP2
energies. Results for the acetate 7 show that the GS conformer
is chiral and interconverts to its mirror image through
either planar or orthogonal conformational TS. The DE{
for the former TS is 1.2 kcal mol21, while the barrier to
Conformational properties of the nematic hosts were
assessed by analysis of three molecular fragments: phenyl-
cyclohexane (5) as a model for 6-CHBT (I), 2-phenylpyrimi-
dine (6) as a model for host II, and phenyl acetate (7) as a
model for ester host III.
Results collected in Table 3 show that conformational
ground states for cyclohexane 5 and acetate 7 are non-planar,
while the two rings in pyrimidine 6 are coplanar resulting in
the C2v point group symmetry. In the cyclohexane 5 two rings
Table 3 Conformational ground and transition states and calculated differences in energies at T = 298 K
GS
TS
Methoda
DE{/kcal mol21
DH{/kcal mol21
DG{/kcal mol21
B3LYP
MP2
3.0
3.5
2.4
2.9
4.3
4.7
B3LYP
MP2
7.7
5.5
7.3
5.1
8.3
6.1
B3LYP
MP2
B3LYP
MP2
0.3
1.2
0.3
0.0
20.3
0.7
20.2
20.5
1.4
2.4
1.4
1.1
a
Calculations at the B3LYP/6-31G(d) or MP2/6-31G(d) (with B3LYP/6-31G(d) thermodynamic corrections) level of theory. DE{ is a difference
in SCF energies corrected for ZPE. Ea = DH{ + RT.
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rotation through the orthogonal TS is practically non-existent,
according to MP2 calculations.50 The CLO–O–CPh–CPh
dihedral angle in phenyl acetate (7) is calculated to be 43u
at the DFT and 65u at the MP2 level. The latter value is
consistent with the 67u angle observed in a solid-state structure
of phenyl benzoate,51,52 and also with the typical 45u–70u angle
in its mesogenic derivatives.49
Both computational methods predict a planar GS structure
for pyrimidine 6, and a relatively high energy rotational TS in
which both rings are mutually orthogonal. This preference for
planarity of 6 is featured in solid-state structures of all its
mesogenic derivatives in which the angle between the phenyl
and pyrimidine rings is typically ,15u.49 For instance, the
angle between the phenyl and pyrimidinyl rings is about 12u in
a close analog of host II.53
Fig. 8 Correlation of the bM values with order of rotational
symmetry axes n (triangles) and effective van der Waals radius of ring
(circles).
Discussion
The molecular structure of esters 1 can be described as a
would be related to the shielding of the chiral center in the
(S)-2-methylbutyl group by the adjacent ring as illustrated
for 3 in Fig. 6. Thus, the larger the ring , the more shielding,
the less intermolecular contact and consequently the less
intermolecular chirality transfer. Indeed, Fig. 8 shows a trend
combination of the three main structural elements: a biphenyl
unit, ring , and the chiral group (Fig. 7). Ring
can be
considered of either a ‘‘uniaxial’’ (A and B) or ‘‘biaxial’’ (C)
topology. It is connected with the chiral group by a direct bond
and with the biphenyl core through an ester group. Both of
these linking groups have a ring -specific distribution of
conformational minima which differentiate compounds with
respect to their conformational flexibility.
in decreasing bM with increasing size of ring
.
54 This trend is
in agreement with the generally observed lower HTP for
mesogens containing the BCO rings (B) than their benzene (C)
analogs.26 It is also consistent with our recent results for
another series of compounds in which an increase in the
ring size well correlates with decrease in destabilization of a
nematic phase, DTNI, upon lateral fluorination.36,55 However,
modifications of the chiral aliphatic part typically have little
effect on HTP.5,17,41 Consequently, the variation of inter-
molecular interactions of the (S)-2-methylbutyl group is
unlikely to be a decisive factor in modifying the HTP.
Initially, the analysis of the observed order of HTP in series
1 focused on the shape (topology) and size of ring . Thus, in
all three structures 1, the biphenyl group is the primary
biaxial structural element required11 for effective intermole-
cular chirality transfer. However, the benzoate group is also
biaxial and significantly enhances the overall molecular
‘‘biaxiality’’ of 1C relative to 1A and 1B with ‘‘uniaxial’’ rings
. Thus, based on the qualitative ‘‘biaxiality’’ argument, the
carborane and BCO esters 1A and 1B should have a similar
and small HTP compared to that of the benzoate 1C.
Experiments showed however, that while the benzoate 1C
has generally the highest HTP in the series, the value for bM of
BCO ester is >3 times larger than that of the carborane 1A and
is close to (about 3/4 in hosts I and III) or practically the same
(in host II) as that for 1C. This indicates that the shape
(apparent ‘‘axiality’’) of the ring by itself is not the primary
factor directly impacting the HTP.
A much more satisfying explanation of the observed trends
in HTP is based on solute–solvent interactions through chiral
conformers.56 In this context, it useful to consider interactions
of the chiral dopant’s molecule with a nematic mean-field
potential or a uniaxial ‘‘cavity‘‘ in analogy to discussion of
chirality transfer mechanisms in smectic liquid crystals.6 Thus,
the ‘‘cavity’’ alters the equilibrium between various diaster-
eomeric conformers of the chiral dopant57 and favors
those with most elongated molecular shapes, best fitting the
‘‘cavity’’. At the same time, the diastereomers cause an
asymmetric distortion of the uniaxial ‘‘cavity’’ which affects
the distribution of chiral conformers of the host and
consequently leads to the formation of a helical phase. This
mutual interaction of the chiral dopant and the dynamically
achiral molecules of the host called chirality transfer feedback
A possible factor affecting the magnitude of HTP of a
dopant is the ability of the chiral center to interact directly
with the neighboring molecules. In this scenario, the HTP
2
or CTF6 has been recently supported by an elegant H NMR
experiment.58 This model is in agreement with the empirical
correlation between the dopant’s molecular structure and the
helical twist sense of the resulting cholesteric phase.59 The
observed trends in the handedness of the generated helix are
general and are explained with the conformational properties
of the chiral dopant.60 It was also suggested that the decrease
of the HTP with increasing distance of the chiral center
from the rigid core is related to the increasing number of
Fig. 7 Schematic representation of molecules 1A and 1B (a), and 1C
(b). The asterisk marks the chiral center and the arrows show
intramolecular rotations.
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conformational minima of the chiral fragment in relation to
the rigid core.60
pairs (8A-g1 and 8A-g2, Fig. 9). The latter become two
diastereomeric pairs upon substitution of the n-alkyl group
with a Me.
According to the above model, the number and distribution
of conformational minima is expected to have the major effect
on the HTP of the additive. Thus, in case of additives 1 the
relative effectiveness of the overall chirality transfer and hence
the magnitude of the HTP should be closely related to the
symmetry and conformational properties of the ring . This
mechanism is essentially the same as a two-stage mechanism
for chirality transfer which involves intramolecular transfer
from the stereogenic center to the rigid core, and intermole-
cular transfer to the host from the rigid core.4,14,25
Considering the number and the depth of available con-
formational minima for derivatives 8, it is apparent that
benzene derivatives with only two (both diastereomeric)
minima should exert the strongest effect on the shape of the
‘‘cavity’’. A weaker effect is expected for BCO derivatives
with three minima. Carborane derivatives have five shallow
minima and are most adaptable to the geometry of the
‘‘cavity’’. This degree of shape adaptability combined with
efficiency of shielding of the stereogenic center by ring
defines the overall chirality transfer to the host and the
resulting HTP of the dopant. Thus, the expected trend in HTP,
1C > 1B > 1A, is in agreement with experimental observations
and is shown in Fig. 8.
Analysis of the three rings shows 2-fold (C), 3-fold (B),
and 5-fold (A) rotational axes, which dictate the number of
iso-energetic conformational minima for mono-substituted
derivatives such as 3 and 4 (Fig. 5). They also define stereo-
chemical properties of simple di-substituted derivatives such as
8 containing an n-alkyl chain and a carboxyl group (Fig. 9).
Thus, both conformers of 4-n-alkylbenzoic acid (8C) are chiral,
forming an enantiomeric pair. Addition of a Me substituent
in the 2-position of the chain transforms the enantiomers to a
diastereomeric pair of conformers which are close in energy in
gas phase but differentiated energetically in the nematic
‘‘cavity’’.
The asymmetric distortion of the nematic ‘‘cavity’’ caused
by additive 1 dissymmetrizes equilibrium between rapidly
interconverting chiral conformers of the solvent. This slight
preference for one conformer over others is propagated
through elastic properties of the nematic phase and results in
the macroscopic helical twist. Thus, the HTP depends also on
the ability of the host to adopt a thermodynamically stable
chiral conformation.62
Table 2 shows that the most susceptible to helical phase
generation is 6-CHBT (I), the least aromatic of the three hosts,
while the most ineffective is pyrimidine (II). This observation is
consistent with other findings,7 including independence of
HTP on the aromatic content of the host’s core,63 and also
with the mechanism outlined above that involves non-specific
non-covalent solute–solvent interactions. Thus, the two most
effective hosts in this work, 6-CHBT and ester, have non-
planar cores, while the core of the least effective pyrimidine is
planar. Out of the former two, 6-CHBT forms two pairs of
relatively rigid enantiomeric conformers (Fig. 10), while ester
(III) is significantly more conformationally flexible.
The same analysis for the bicyclo[2.2.2]octane analog 8B
shows that it has three possible conformers, if the shallow
conformational minima with
a twisted BCO ring are
neglected.61 One of these conformations, 8B-a with the
substituents in the anti orientation, has practically the Cs
symmetry, while the two gauche conformers 8B-g form an
enantiomeric pair (Fig. 9). Introduction of a Me group in the
n-alkyl chain transforms the two conformers 8B-g into a
diastereomeric pair with differentiated thermodynamic
stability in the nematic phase. The actual approximate 15u
twist of the BCO ring provides an additional break in
symmetry and enhances the molecular chirality. The carborane
ring A has C5h molecular symmetry, and consequently
carboxylic acid 8A has one Cs-symmetric conformational
minimum (8A-s) and two conformational enatiomeric
The observed significantly inferior performance of pyrimi-
dine (II) as a host for additives 1 is in contrast with the
relatively high HTP exhibited by atropisomeric biphenyls in a
completely planar phenylpyrimidine SmC host.14 This can be
rationalized by assuming non-specific solute–solvent interac-
tions for 1 and other dopants containing stereogenic centers,7
while the induction of a helix by axially chiral dopants in an
inherently achiral host most likely involves specific solute–
solvent interactions. The helix formation in the former system
presumably relies mostly on energy differentiation of existing
Fig. 10 Newman projection of a pair of enantiomeric conformers of
6-CHBT (I) formed by breaking the Cs symmetry of phenylcyclo-
hexane (5) upon the introduction of the alkyl group R. The vertical bar
represents the plane of the phenyl ring at C(1) and the gray line the
plane of the alkyl substituent at the C(4) atom of cyclohexane. The
NCS substituent is linear and projects forward.
Fig. 9 Extended Newman projections for selected ground state
conformers of n-alkyl carboxylic acids 8. The bars represent the
COOH (black) and the plane of the alkyl chain (gray). For simplicity
the BCO ring is presented in its D3h symmetry.
458 | J. Mater. Chem., 2006, 16, 452–461
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chiral conformers of the solvent and weakly, if at all, depends
on the aromatic content of the core. In contrast, the latter is
expected to be heavily dependent on pi–pi and electrostatic
interactions between aromatic planes.8,63
uniaxial ‘‘cavity’’ result in preferential stabilization of some
diastereomeric conformers and simultaneous chiral distortion
of the ‘‘cavity’’. This chiral distortion results in energetic
differentiation of rapidly interconverting chiral conformers of
the host, and formation of a torque propagated through elastic
properties of the nematic phase. Thus lower conformational
freedom and higher structural rigidity of both the chiral
additive and the host lead to higher HTP values.
Finally, let’s return to the issue of ‘‘axiality’’ of ring and
its role in chirality transfer. At first glance, both rings A and B
appear ‘‘uniaxial’’ and play no direct role in intermolecular
chirality transfer. On a molecular level, however, the rings are
neither ‘‘uniaxial’’ nor ‘‘biaxial’’ structural elements, but in
between. The ideal biaxial element has C2 rotational axes (e.g.
benzene) and an ideal uniaxial element has C‘ axes. Since the
rotational axes for A and B are finite, they are still partially
‘‘biaxial’’. Their degree of ‘‘biaxiality’’ can be defined by the
order of the symmetry axis Cn, and it increases in the order A
(n = 5) , B (n = 3) , C (n = 2) in agreement with the observed
trends in the HPT for additives 1 (Fig. 8). Thus, the
‘‘biaxiality’’ of ring plays a role in intramolecular chirality
transfer (through defining the conformational space), which, in
turn, modulates the inter-molecular chirality transfer. The
former is manifested in conformational diastereomers energe-
tically-differentiated in a nematic phase, while the latter in the
observed HTP.
Experimental
1H NMR spectra were obtained at 400 MHz in CDCl3 and
referenced to TMS. Elemental analysis was provided by
Atlantic Microlab, Norcross, Georgia.
Optical microscopy and phase identification was performed
using a PZO ‘‘Biolar’’ polarized microscope equipped with a
HCS250 Instec hot stage. Transition temperatures (onset)
and enthalpies for pure compounds were obtained on a TA
Instruments 2920 DSC using 1–2 mg samples and a heating
rate of 5 uC min21 under a flow of nitrogen gas.
Each compound was additionally purified by dissolving
in CH2Cl2, filtering to remove particles, evaporating, and
recrystallizing from hexanes (1B and 1C) or EtOH (1A). The
resulting crystals were dried in vacuum overnight at ambient
temperature before measurements. A commercial sample of
6-CHBT (I, Aldrich) was filtered and vacuum distilled before
use. The pyrimidine65 (II) and ester66 (III) hosts were prepared
according to literature procedures.
The results presented here suggest directions for further
investigation of the chirality transfer mechanism and struc-
ture–HTP relationships. In this context, studies of analogs of 1
in which the ester group is replaced with other linkers would be
particularly instructive. Thus, if the postulate of the role of
ring ‘‘biaxiality’’ is correct, then replacing the ‘‘biaxial’’ ester
group in 1 with a ‘‘uniaxial’’ acetylene linker CMC– should
dramatically reduce the HTP of analogs 1A and 1B, but not
necessarily for 1C. These expectations are based on our results
for another series of compounds in which the introduction of
an acetylene linking group mechanically decouples two
terminal alkyl tails and results in the depression of the TNI
by over 150 uC.44 It would also be instructive to study the
10-vertex p-carborane analog of 1A, since this smaller cage has
C4 rotational axes and all conformers are chiral.44 Another
direction of studies should involve a series of nematic hosts in
which the number of chiral conformations vary. For instance,
least susceptible for chiral induction should be a series of
nematic 2-(4-alkoxyphenyl)-5-cyanopyrimidines64 or 5-alkoxy-
2-(4-alkoxyphenyl)pyrimidines for which all ground state
conformations are planar and therefore achiral.
Carboxylic acids 2B and 2C were prepared from S-(2)-2-
methyl-1-butanol containing up to 5% of 3-methylbutanol.
GC-MS analysis of their methyl esters showed y2% of the
3-methylbutyl isomer. The carborane carboxylic acid 2A was
prepared from the 99% pure alcohol. Both starting alcohols
were obtained from Aldrich.
Cholesteric pitch measurement
The cholesteric pitch p was measured using the Candau droplet
method67 for mixtures prepared from a nematic host and chiral
additives 1A–1C in five concentrations ranging from 0.5 to
4 mol%. In the case of 1A the highest concentration was
4.7 mol%. The binary mixtures were heated at the isotropic
phase for at least 1 h, cooled and dispersed in glycerol. Two
cover slides (0.22 mm thick) were placed on a microscope slide
about 5 mm apart. A drop of the micro-droplet suspension was
placed on the microscope slide between cover slides and was
covered with another cover slide. The slide was placed on a hot
stage and stabilized for 1–2 h to develop a fingerprint texture.
A selected droplet with a single spiral of disclination lines or a
set of concentric circles was photographed (magnification
3006) and compared with a photograph of a micro-ruler
(smallest division 10 mm) at the same magnification. The
twisting power bM was obtained as a slope of a linear plot of
1/p versus mole fraction c (bMc = p21). The temperature
dependence of the twisting power, dbM/dT, was obtained
by measuring bM for mixtures of a typical concentration
of about 3 mol% in a range of about 15 uC every 1 uC. Before
taking each measurement, the mixture was stabilized for at
least 15 min.
Conclusions
Investigations of chiral mesogens 1 showed that their HTP in
three nematic hosts follows the order 1A , 1B ¡ 1C. This
trend can be correlated with the size and conformational
properties of the ring . Thus, the decrease in HTP is
paralleled by increasing ring size and decreasing ‘‘biaxiality’’
defined by the order of rotational axes Cn. Both effects are
synergistic. They define interactions of conformational
diastereoisomers with the nematic cavity and consequently
chirality transfer from the stereogenic center in 1 to the host.
The analysis of host-dependent HTP suggests that the helix
formation induced by additives with a stereogenic center
involves non-specific intermolecular interactions. It is believed
that interactions of the chiral additive with a nematic
This journal is ß The Royal Society of Chemistry 2006
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49-Pentylbiphenyl-4-yl 12-((S)-2-methylbutyl)-p-carborane-1-
(S)-4-(2-Methylbutyl)bicyclo[2.2.2]octane-1-carboxylic acid28
(2B)
carboxylate (1A)
The acid was obtained from 95% pure S-(2)-2-methyl-1-
butanol according to a general literature procedure:29 mp 131–
A mixture of (S)-12-(2-methylbutyl)-p-carborane-1-carboxylic
acid27 (2A, 65 mg, 0.25 mmol) and PCl5 (56 mg, 0.27 mmol) in
dry benzene (3 mL) was stirred for 30 minutes at 40 uC.
Benzene and the resulting POCl3 were removed under reduced
pressure. The resulting colorless oil of crude acid chloride
and 49-pentylbiphenyl-4-ol (60 mg, 0.25 mmol) were dissolved
in dry benzene (2 mL) and dry Et3N (25 mg, 0.25 mmol)
was slowly added. This mixture was stirred for several hours
at rt. The solvent was evaporated, the residue was passed
through a silica gel plug (CH2Cl2–hexanes, 1 : 4) and solvent
was removed. The resulting product (110 mg, 92% yield)
was recrystallized three times from EtOH to give 1A (70 mg)
as white needles: [a]633 = + 5.3u, [a]D = +6.6u, [a]546 = +8.2u,
[a]436 = +14.5u, [a]405 = +17.5u, [a]365 = +23.4u (c = 1.1%
in CHCl3 at 25 uC); 1H NMR d 0.78 (t, J = 7.0 Hz, 3H),
0.80 (d, J = 6.4 Hz, 3H), 0.90 (t, J = 6.8 Hz, 3H), 1.15–1.28
1
132 uC (lit.28 mp 117–118 uC); H NMR d 0.79 (t, J = 7.3 Hz,
3H), 0.82 (d, J = 6.5 Hz, 3H), 0.90 (dd, J1 = 14.2 Hz, J2 =
6.4 Hz, 1H), 1.01–1.33 (m, 3H), 1.31–1.43 (m, 7H), 1.71–1.77
(m, 6H). Anal. Calcd. for C14H24O2: C, 74.95; H, 10.78.
Found: C, 74.58; H, 10.82%.
Acknowledgements
This project was supported by the NSF grant (DMR-0111657).
We are grateful to Dr Eike Poetsch (Merck, Darmstadt) for
a gift of 49-pentylbiphenyl-4-ol and to Professor Robert P.
Lemieux for helpful discussions.
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from hexanes to give 1B (99 mg) as white needles: [a]633
=
=
+1.4u, [a]D = +1.8u, [a]546 = +2.45u, [a]436 = +5.4u, [a]405
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460 | J. Mater. Chem., 2006, 16, 452–461
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