Mesogenic, optical, and dielectric properties of 5-substituted 2-[12-(4-pentyloxyphenyl)-p-carboran-1-yl] [1,3]dioxanes

Article abstract of DOI:10.1039/b608012j

Two homologous series of carborane-containing dioxanes 1[n] and 2[n] (n = 1-10) were prepared and their mesogenic properties investigated. All compounds exhibit nematic behavior and three members of series 2[n] show an E phase. Numerical analysis of the clearing temperatures gave a limiting value T _NI(∞) of 89 °C for series 2[n] and indicated conformational flexibility of the dioxane ring. Investigations of three-ring derivative 1[4] gave Δn = 0.17, S = 0.53, and Δε = +0.4 ± 0.1 at 85 °C. Extrapolation of dielectric data for dilute solutions of 1[4] in 6-CHBT gave Δε = +0.4 ± 0.25 at 24 °C. Modelling of dielectric results with the Maier-Meier equation demonstrated that conformers with a higher β angle are preferred, which is consistent with conformational selection for the most elongated conformers. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b608012j

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Mesogenic, optical, and dielectric properties of 5-substituted 2-[12-(4-
pentyloxyphenyl)-p-carboran-1-yl] [1,3]dioxanes{
Takashi Nagamine,a Adam Januszko,b Piotr Kaszynski,*b Kiminori Ohtaa and Yasuyuki Endoa
Received 6th June 2006, Accepted 18th July 2006
First published as an Advance Article on the web 24th August 2006
DOI: 10.1039/b608012j
Two homologous series of carborane-containing dioxanes 1[n] and 2[n] (n = 1–10) were prepared
and their mesogenic properties investigated. All compounds exhibit nematic behavior and three
members of series 2[n] show an E phase. Numerical analysis of the clearing temperatures gave a
limiting value TNI(‘) of 89 uC for series 2[n] and indicated conformational flexibility of
the dioxane ring. Investigations of three-ring derivative 1[4] gave Dn = 0.17, S = 0.53, and
De = +0.4 ¡ 0.1 at 85 uC. Extrapolation of dielectric data for dilute solutions of 1[4] in 6-CHBT
gave De = +0.4 ¡ 0.25 at 24 uC. Modelling of dielectric results with the Maier–Meier equation
demonstrated that conformers with a higher b angle are preferred, which is consistent with
conformational selection for the most elongated conformers.
the pure state and in nematic solutions. A combination of
Introduction
conformational analysis at the HF and DFT levels with
The low polarizability anisotropy and a moderate molecular
dipole moment1 of 2.13 ¡ 0.03 D, oriented at 47u relative to
the C2–C5 axes, make [1,3]dioxane an attractive structural
analysis of the dielectric results using the Maier–Meier
relationship yielded information about the origin of the
observed small positive dielectric anisotropy and conforma-
element for the preparation of liquid crystals with positive
tional properties of the compound.
dielectric anisotropy, low birefringence, and minimal absorp-
tion in the UV region.2–6 Analysis of the available data7 shows
Results
that the dioxane ring has a smectogenic character and even
two-ring compounds often exhibit smectic phases.2 While this
Synthesis
property of [1,3]dioxane is useful for the development of SmC*
materials,6,8,9 it is undesirable for formulation of nematic
All three series of dioxanes 1[n]–3[n] were prepared from
carborane-1-carboxaldehyde 4 and the appropriate 2-sub-
materials. Our studies of liquid crystalline derivatives of
stituted propane-1,3-diols 5[n]–7[n], according to a general
literature method2 (Scheme 1).
p-carborane demonstrated their highly nematogenic beha-
vior10–12 and low birefringence.13 Therefore, we were interested
in combining these two structural elements, [1,3]dioxane and
p-carborane, and investigating the properties of such new
materials.
The required diols 5[n]–7[n] were obtained from the corres-
ponding malonates 8[n]–10[n] by reduction with LiAlH4
(Scheme 2). Monoalkyl malonates 8[n] were prepared by
alkylation of diethyl malonate under phase-transfer-catalysis
Here, we present the mesogenic properties of two homo-
logous series of three- and four-ring compounds, 1[n] and 2[n],
respectively (Fig. 1). In addition, we prepared several members
of a third series 3[n] to establish the effect of the linking oxygen
atom on mesogenic behavior. We also performed detailed
dielectric and optical studies of one of the compounds in
conditions according to
a
general literature method.14
Arylation of diethyl malonate with 4-substituted aryl iodides,
11[n] and 12[n], and preparation of 9[n] and 10[n] was
accomplished using CuI according to a general method.15 All
intermediates 5[n]–10[n] used in this work have been reported
in the literature16–20 but many lack analysis. For completeness
of the work, 1H and 13C NMR data for all intermediates is
reported in the ESI.
Aldehyde 4 was obtained in 74% yield by formylation of
1-(4-pentyloxyphenyl)-p-carborane21 (13) with ethyl formate
Fig. 1 Mesogenic derivatives 1[n]–3[n]. In the carborane cage each
vertex represents a BH fragment and each sphere is a carbon atom.
aTohoku Pharmaceutical University, 4-4-1, Komatsushima, Aoba-ku,
Sendai, 981-8558, Japan
bOrganic Materials Research Group, Department of Chemistry,
Vanderbilt University, Nashville, TN, 37235
{ Electronic supplementary information (ESI) available: Analytical
data for derivatives 1–3 and intermediates 4–10, complete optical and
dielectric data, and also quantum-mechanical and Maier–Meier
computational details. See DOI: 10.1039/b608012j
Scheme 1
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Scheme 2
Fig. 2 Nematic–isotropic transition temperatures TNI for 1[n] as a
function of the chain length n. Lines are guide for the eye.
Mesogenic properties
Scheme 3
All investigated compounds in series 1[n]–3[n] form nematic
phases. Lamellar phases are observed only for higher members
of series 2[n] (n ¢ 8), which exhibit relatively narrow range E
phases. Most compounds in this series also show rich
crystalline polymorphism. Phase transition temperatures and
enthalpies are collected in Table 1.
(Scheme 3). 4-Alkoxy-1-iodobenzenes22 (11[n]) were prepared
according to
a
literature procedure23 by alkylation of
4-iodophenol. The three 4-alkyl-1-iodobenzenes 12[5],24
12[6],25 and 12[7]26 were obtained from the commercially
available anilines according to a literature procedure for
12[6].25
The effect of substitution of one of the linking oxygen atoms
in 2[n] with a methylene group was briefly investigated, and
three members of a homologous series 3[n] with n = 5–7 were
prepared. All three compounds show nematic phases destabi-
lized relative to the alkoxy analogs 2[n] by an average of 34 uC,
which is a typical value for such a substitution in other
mesogenic series.7
Table 1 Transition temperatures (uC) and enthalpies (kJ mol21 a
)
The clearing temperatures in both series 1[n] and 2[n] exhibit
the typical ‘‘odd–even’’ effect. The TNI values in the first series
have maxima at n = 5 and n = 6 for homologs with an odd and
even number of carbon atoms in the chain, respectively (Fig. 2).
Such behavior is typically observed for mesogenic series
containing alkylcyclohexyl groups.7 In contrast, the second
series, 2[n], exhibits a monotonous exponential decay of the
TNI values with increasing chain-length (Fig. 3). The observed
behavior of the series is typical for mesogens with the
alkoxyphenyl group.10,11
n
1
2
3
4
5
6
7
8
9
1[n]
2[n]
3[n]
—
Crb 139 (N 101)c I Cr1 130 Cr2 137 N 257.1 I
(27.7) (15.2) (12.2) (2.5)
Cr 116 (N 81.7) I Cr1 88 Cr2 171 N 264.0 I
(28.3) (1.6)
Cr 90 N 105.8 I
(21.1) (1.6)
Cr 74 N 101.0 I
(36.5) (1.5)
—
(14.2) (21.4) (1.8)
Cr1 121 Cr2 161 N 247.1 I
(8.2) (19.5) (2.5)
Cr 140 N 237.5 I
(22.7) (2.6)
—
—
Cr 92 N 111.0 I
(22.8) (1.9)
Cr 143 N 217.5 I
(25.7) (1.9)
Cr 144 N 201.2 I
(26.7) (2.0)
Crd 66 N 105.0 I Cr1 117 Cr2 144 N 210.1 I Cr 147 N 183.7 I
(21.1) (1.7)
Cr 75 N 109.6 I
(27.8) (1.9)
Cr 64 N 101.9 I
(25.0) (1.6)
(10.1) (19.4) (2.3)
Cr1 103 Cr2 145 N 197.2 I Cr 155 N 177.7 I
(6.8) (19.4) (1.9)
Cr 127 E 131 N 190.1 I
(15.7) (15.3) (2.3)
(18.9) (1.7)
(23.5) (1.8)
—
Cr 59 N 103.3 I
(23.5) (1.8)
10 Cr 58 N 96.6 I
Cr 113 E 127 N 182.5 I
(15.4) (17.1) (1.8)
—
—
b
Cre 125 E 127 N 177.4 I
(3.7) (8.7) (1.9)
(40.2) (1.6)
a
Cr: crystal, E: soft crystal E, N: nematic, I: isotropic. Cr–Cr
transition at 87 uC (3.0 kJ mol21). Microscopic observations. Cr–
c
d
Cr transition at 49 uC (19.0 kJ mol21). Cr–Cr transition at 112 uC
e
Fig. 3 Nematic–isotropic transition temperatures TNI for 2[n] as a
function of the chain length n. Best fit functions are listed in the ESI.
(7.4 kJ mol21) and 120 uC (4.1 kJ mol21).
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Numerical analysis of series 2[n] revealed that the TNI data
can be approximated by empirical three-parameter exponential
function (F1)10 and also by homographic function (F2).27
Initial fitting of all five datapoints for either an odd or even
series showed an unacceptably large error on all three para-
meters in both functions. Therefore, the first two homologs
2[1] and 2[2] were excluded from the analysis.
À
Á
pffiffi
TNIðnÞ~azexp b{c.
n
(F1)
(F2)
nzb
TNIðnÞ~a
nzc
The limiting values a lim TNIðnÞ~a in function (F1)
n??
obtained for both odd and even n-series of 2[n] were 86 ¡ 6
and 90 ¡ 37 uC, respectively, with the overall high correlation
factor r2 of .0.999 for each fit. Since both values should be the
same for infinite n, the limit a in 2[n] was constrained at 86 uC
without significant change in r2. Analogous analysis of series 2[n]
using function (F2), gave similar limiting values a with a
significantly lower error (n = odd, a = 89 ¡ 0.5 uC, r2 = 1; n =
even, a = 88 ¡ 16 uC, r2 = 0.9996).
Fig. 5 Plot of dielectric parameters vs. concentration for 1[4] in
6-CHBT at 24 uC. Correlation parameters for eI and De are r2 . 0.98.
Table 2 Dielectric parameters for 1[4] extrapolated from 6-CHBT
solutions at 24 uC and measured for pure compound at 85 uC
Dielectric measurements
In 6-CHBT
Neat at T = 85 uC
To assess the electro-optical properties of the carborane
mesogens 1[n], the butyl derivative 1[4] was selected as a
representative for the series and investigated in the pure state
and also as a low concentration additive to 6-CHBT (trans-4-
(4-hexylcyclohexyl)phenylisothiocyanate), a nematic host with
positive dielectric anisotropy (De . 0).
eI
eH
De
4.5 ¡ 0.3
4.1 ¡ 0.2
0.4 ¡ 0.25
3.45 ¡ 0.1
3.05 ¡ 0.1
0.4 ¡ 0.01
difference in the temperature and dielectric strength of the
medium.
The measurements for the pure compound were performed
in 3 cells and each was measured 3 times over the temperature
range of 75–105 uC. The resulting values were averaged and
are shown in Fig. 4. Dielectric constants for 1[4] in 6-CHBT
were obtained by linear extrapolation of data for three
solutions and the pure host to mole fraction 1.0 (Fig. 5), and
results are collected in Table 2. Each datapoint in Fig. 5
represents an average of 5 repetitive measurements in a single
cell and has an associated error of ,0.03. To lower the error
for the extrapolated values, the intercept in the fitting
functions was set at the appropriate value for the pure host.
The extrapolated dielectric anisotropy De has a small positive
value of +0.4 ¡ 0.25 which is practically the same as that
obtained for pure 1[4] at 85 uC. The individual components eI
and eH are larger than those for pure 1[4] which reflects the
Computation of molecular parameters
To better understand the experimental results, the molecular
structure for each of the principal conformers of 1[4] was
optimized, and dipole moments m and electronic polarizabi-
lities a were calculated at Gaussian standard orientation
(in which the molecular axes are defined by the nuclear
charge distribution) using the HF and the B3LYP methods,
respectively.
Conformational analysis. The HF calculations show that the
dioxane and benzene rings prefer conformations in which
the ring’s C–H bond eclipses the carborane B–H bond. In
compounds 1[n], this conformational preference results in
three relative orientations of the two rings. The presence of the
polar pentyloxy group results in six orientations of its dipole
moment with respect to the dipole moment of the dioxane ring
(A–F, Table 3). Finally, the butyl chain on the dioxane ring
may assume either a syn or anti position relative to the
pentyloxy group. This gives rise to 10 distinct conformers (and
10 of its enantiomers) as shown in Table 3.
Inspection of the conformers revealed that the relative
orientation of the alkyl chains varies significantly from roughly
co-planar in E(anti) and C(anti), F(syn), and D(syn) (h = 16–
20u) to nearly orthogonal in D(anti) and C(syn) (h = 88u). The
former conformers are expected to be favored by the nematic
phase, particularly the most extended C(anti) and E(anti). Less
compatible with the nematic phase are expected to be F(anti)
Fig. 4 Dielectric parameters for 1[4] as a function of temperature.
Estimated standard deviation for eI and eH is 0.1, and for De is 0.01.
3838 | J. Mater. Chem., 2006, 6, 3836–3843
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Table 3 Calculated molecular parameters for the conformers of 1[4]a
C
D
E
F
A
B
Conformer
syn
anti
syn
anti
syn
anti
syn
anti
b
h
(deg)
56
1.55
1.83
2.39
56
1.77
0.78
1.93
88
1.60
1.58
2.25
45
19
1.54
1.59
2.21
45
20
1.82
1.07
2.11
30
88
1.76
1.09
2.07
32
53
1.69
1.58
2.31
43
16
1.60
1.60
2.26
16
1.74
1.14
2.08
53
1.66
1.17
2.03
35
mIc/D
mH/D
m/D
bd (deg)
a
50
24
45
33
Dipole moments were obtained with the HF/6-31G(d) method. Extended Newman-type projections along the long molecular axes show the
relative orientation of the dioxane (in back) and the 4-pentyloxyphenyl ring (in front). The benzene ring is represented by the black bar and is
coplanar with the pentyloxy substituent. In the syn conformer, the butyl chain (not shown) propagates on the same side as C5, and in the anti
b
form the two alkyl chains are on the opposite sides of the molecule. The dihedral angle between the pentyl and butyl chains is measured as
…
c
C1–C2–C3 C1–C2–C3. Dipole moment vector is oriented from the phenyl (positive) to the dioxane ring (negative). Angle between the net
d
dipole vector m and long molecular axes calculated from the vector components.
and A, B and E(syn), with an intermediate dihedral angle h of
about 55u. The least favorable conformers are predicted to be
C(syn) and D(anti) for which the dihedral angle h between the
alkyl group planes is about 88u.
parameters S obtained using the Vuks model29 were about 15%
higher (e.g., 0.60 and 0.62 at 85 uC) presumably due to an
underestimation of the calculated polarizability by about 10%
and assumption of density 1.0 g cm23 for 1[4]. Experimental
optical values and computational details are listed in the ESI.
Dipole moment. The HF calculation showed that the total
molecular dipole moment varies in the series of conformers of
1[4] from 1.93 D in B, in which the dioxane oxygen atoms and
the pentyloxy group point in opposite directions, to 2.39 D in
A, in which the two local dipole moments are parallel. These
two conformers also define the extreme orientations of the
dipole moment with respect to the long molecular rotational
axes, and hence the range of the angle b (24–50u). In two
groups of conformers, D and F, the longitudinal component
of the dipole moment mI is substantially larger than the
transverse mH, which results in a relatively low b (y33u). In
contrast, the two dipole moment components are comparable
in conformers C and E, and the resulting b is about 45u.
a
n2e{n2o
S~
.
(1)
Da n2avrg{1
Dielectric data analysis
The experimental dielectric data were analyzed quantitatively
using the Maier–Meier relationship30 which relates the
dielectric anisotropy of the nematic phase and molecular
parameters [eqn (2)].31 Using this equation, the dielectric
behavior of compound 1[4] was evaluated in the pure state and
also in 6-CHBT solutions in a similar way to the recently
described analysis for another series of mesogens.12
In all calculations, 1[4] was treated as a mixture of individual
conformers shown in Table 3. For this purpose, the Maier–
Meier equation [eqn (2)] was simplified, and De was expressed
Electronic polarizability. Calculations of the electronic
polarizability components for 6 selected conformers showed
3
that the average polarizability aavrg is about 49.7 A . The
˚
anisotropy of polarizability Da also varies little between
3
3
˚
˚
conformers and approximately is 35.5 A for anti and 34.5 A
for the syn conformers.
Orientational order parameter from optical data
Refractive indices for 1[4] were measured at 589 nm as a
function of temperature using an Abbe-type refractometer and
the results are shown in Fig. 6. The resulting experimental
birefringence and calculated electronic polarizabilities were
used to derive order parameter S according to a transformed
Lorenz–Lorentz equation [eqn (1)].28 The resulting order
parameter was 0.52 for anti conformers and 0.54 for the syn
conformers at 85 uC. Similar values (0.53 and 0.55) were
obtained at 71.5 uC suggesting that the order parameter
changes little in the investigated temperature range. The order
Fig. 6 Refractive indices and birefringence measured for 1[4] at
589 nm as a function of temperature.
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as a product of the molecular parameter C and order
parameter S. Consequently, the ideal additivity of De for a
mixture’s components can be presented as eqn (3), in which xi
represents the mole fraction and Si is the individual order
parameter. In a homogenous mixture, all individual order
parameters Si have close values, and the assumption of their
equality leads to eqn (4). Thus, for the purpose of this work, it
was assumed that all the conformers considered have the same
order parameter.
Analysis of the dielectric results was conducted either for all
conformers A–F, which were assumed to be present in equal
concentration (xi = 0.1), or only for the most anisometric
C(anti) and E(anti) for which xi = 0.5. Exclusion of the least
anisometric conformers C(syn) and D(anti) from the calcula-
tions had marginal effect on Sapp and g. For each group of
conformers the angle b between m and mI was either taken as
calculated (Table 3) or adjusted for all considered conformers
to fit the experimental data. The resulting g, Sapp and
computed permittivity values are listed in Table 4, and details
of calculations are described in the ESI.
(
)
2
Fmeff
À
De = SDeixi = SCiSixi
Á
NFh
De~
Da{
1{3 cos2 b
S
(2)
e0
2kBT
Results in Table 4 for pure 1[4] show that the dielectric
anisotropy De calculated from molecular parameters and the
experimental order parameter is about twice larger than the
experimental value when all conformers A–F are included.
Calculations of the experimentally established value of eI
require the g parameter to be set at 0.51. Increasing the value
of angle b decreases the computed De, and the experimental
dielectric values are reproduced for b + 19u and g = 0.79. When
only the most anisometric conformers C(anti) and E(anti) are
considered, the computed De is lowered by 0.1. Upon
increasing the b by 13u and setting g = 0.74 the resulting De
is equal to the experimental value (0.40).
(3)
(4)
De = SSCixi where Sxi = 1
For calculations involving pure 1[4] the order parameter S
was set at the experimentally established average value of 0.53.
The reaction field factor F and the cavity factor h in eqn (2)
were obtained from the experimental average refractive index
navrg = 1.561 and permittivity eavrg = 3.18 at 85 uC. For
calculations involving binary mixtures of 1[4] in 6-CHBT, the
medium was assumed to be the pure host, and the effect of the
additive was ignored. Therefore, parameters F and h were
calculated for pure 6-CHBT using experimental optical and
dielectric data.32 With these assumptions, the dielectric
properties of pure 1[4] were modelled at 85 uC by varying
the Kirkwood factor33 g, which relates m2 and m2eff , in such a
way that the calculated eI was consistent with the experimental
value of 3.45. In the analysis of dielectric data extrapolated
from binary mixtures, the g factor and also the overall
apparent order parameter34 Sapp were varied systematically in
such a way that the reverse calculations reproduced the
extrapolated dielectric parameters eI and eH for 1[4]. Changes
in the Kirkwood factor g mostly affected the eI and eH values,
while the variation of Sapp mainly controlled the magnitude of
the dielectric anisotropy De.
Similar analysis was performed for the binary mixtures of
1[4] in 6-CHBT using extrapolated dielectric parameters. In
general, the resulting trends are similar to those obtained for
neat 1[4] (Table 4). Thus, the required value of parameter g is
generally about 1.0 and the apparent order parameter34 Sapp is
low, ,0.2, even when only the most extended conformers
C(anti) and E(anti) are considered. If all conformers assumed
the matrix’s order parameter S = 0.67, then the value of De
for 1[4] would be 2.0. Increasing the angle b by 19u for all
conformers A–F had practically no effect on g, but signifi-
cantly improved the Sapp to a reasonable value of 0.54. The
experimental dielectric values for 1[4] in 6-CHBT with Sapp
=
0.67 can be reproduced if b is increased by 20u for all
conformers or by 14u for an equimolar mixture of C(anti) and
Table 4 Bulk parameters for 1[4] calculated using the Maier–Meier equationa
Computed valuesa for
All conformers
All conformers b + 19u
C(anti) + E(anti)
C(anti) + E(anti) b + 13u
Pure compound
g
eI
eH
De
For S = 0.53, es = 3.18, T = 358 K
0.51 ¡ 0.07
3.45 ¡ 0.1
2.61 ¡ 0.06
0.84 ¡ 0.04
0.78 ¡ 0.1
3.45 ¡ 0.1
3.04 ¡ 0.1
0.41 ¡ 0.01
T = 297 K
1.03
0.54 ¡ 0.07
0.74 ¡ 0.1
3.45 ¡ 0.1
3.05 ¡ 0.1
0.40 ¡ 0.02
3.45 ¡ 0.1
2.71 ¡ 0.07
0.74 ¡ 0.03
6-CHBT solution
g
1.04
0.97
0.97
Sapp
eI
eH
De
0.13 ¡ 0.09
4.51 ¡ 0.2
4.11 ¡ 0.1
0.40 ¡ 0.28
0.54 ¡ 0.15
4.50 ¡ 0.08
4.10 ¡ 0.04
0.40 ¡ 0.10
0.17 ¡ 0.1
4.51 ¡ 0.15
4.10 ¡ 0.1
0.39 ¡ 0.25
0.60 ¡ 0.15
4.50 ¡ 0.06
4.10 ¡ 0.04
0.40 ¡ 0.10
For Sapp = 0.67, g = 1.0, T = 297 K
eI
eH
De
a
5.5
3.5
2.0
4.5
4.0
0.5
5.4
3.8
1.6
4.6
4.15
0.45
Computed using molecular parameters listed in Table 3 and experimental De in Table 2. For details see text and the ESI.
3840 | J. Mater. Chem., 2006, 6, 3836–3843
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E(anti). Increase of the angle b in the latter pair by +13u, the
same value as obtained for the pure material, gives Sapp = 0.60.
conformer (in addition to the mesogenic diequatorial con-
former) at higher temperatures.11
Among the three series of compounds, only in series 1[n] are
transition temperatures low enough to conduct optical and
dielectric studies of neat substances. For detailed investiga-
tions, the butyl derivative 1[4] was selected as a compromise of
temperature range (74–101 uC) and molecular size (a smaller
molecular size saves computation time). Optical measurements
for the three-ring mesogen 1[4] showed that the average
refractive index and the birefringence are relatively low and
similar to those found in biphenyls (e.g., nCB). This, in part, is
due to the lack of electronic conjugation between the rings and
also to the relatively low orientational order parameter S.
Analysis of the dielectric results was significantly compli-
cated by the complex conformational properties of the dioxane
derivatives. Even without various conformational forms of the
alkyl chains, the relative orientation of the three rings and
two substituents in 1[4] results in 10 pairs of chiral conformers
A–F. Their overall shapes vary significantly from the most
extended in C(anti) and E(anti), through bow-type shapes of
the syn forms, to C(syn) and D(anti) in which the alkyl chains
propagate in nearly orthogonal planes. Therefore, their
compatibility with the nematic phase varies significantly. It
can be anticipated that the conformers are differentiated
energetically in the nematic phase, and the more anisometric
forms, such as C(anti) with low angle h, are more populated in
the nematic phase. It is possible that this multitude of available
conformers and shapes is responsible for the lack of the
lamellar phases in carborane derivatives and the relatively low
TNI values observed in this work.
Discussion
All compounds in series 1[n] and 3[n], and most in series 2[n]
exhibit only nematic phases. This observed highly nematogenic
behavior is consistent with our findings for other series of
compounds containing p-carborane including an analogous
series of Schiff bases10 and diesters.11 Typically, the mesophase
stability of the p-carborane derivatives is lower than that of
their benzene analogs. Therefore, it can be expected that
clearing temperatures in series 14[n] would be significantly
higher than those in 1[n]. A literature search revealed that
direct analogs for these series of carborane derivatives are not
known, and homologous series6 15[n] is structurally closest to
the p-carborane series 1[n].
A comparison of 1[n] with 15[n] showed that nematic–
isotropic transition temperatures TNI for the p-carborane
derivatives are lower by an average of 56 uC (¡4 uC) than for
the benzene analogs. Considering that incorporation of a
linking oxygen atom generally increases the TNI by about
30 uC,7 the destabilization of the nematic phase by the
replacement of the central benzene ring in 14[n] by carborane
can be estimated as nearly 90 uC. This significant destabiliza-
tion of the mesophase is also consistent with the lack of
smectic behavior in series 1[n]. In comparison, a SmA phase
appears in 15[5] and completely replaces the nematic phase in
15[10]. Also the Bcr phase is present in all members of the
series.
Molecular geometry is not directly recognized in the Maier–
Meier relationship. Instead, it is expressed indirectly by
polarizability anisotropy Da and order parameter S. For 1[4]
however, all conformers have very similar values of Da and
individual order parameters, Si, are unattainable. The only
quantity that differentiates the conformers to some degree is
the angle b which defines the orientation of the net dipole
moment with respect to the main rotational axes. However, the
trend in b does not correspond to the trend in the conformer’s
anisometry defined by the dihedral angle h between the planes
of the two terminal alkyl chains (Table 3). Therefore, our
rational choice of the conformers to be included in calculations
is based solely on the type of conformer (syn or anti) and the
value of angle h: it is assumed that the anti conformers are
preferred over the syn, and the smaller the angle h the more
compatible conformer is with the nematic phase. Thus, the
most favorable conformers are C(anti) and E(anti), which also
have a relatively large value for the angle b.
Numerical analysis of the TNI in dioxanes 2[n] allows for
assessment of transition temperatures for the higher homologs,
provides insight into conformational behavior of the com-
pounds, and also is a useful means of verification of sample
purity and quality of DSC analysis.11 As we demonstrated
elsewhere, both empirical fitting functions (F1) and (F2) well
describe the TNI transitions of a long homologous series up to
at least n = 22.11 The limit of the functions for n = ‘ has
limited physical meaning since for an infinitely long alkyl
chain, the compounds would not be mesogenic. However, the
TNI(‘) provides a measure of series mesogenicity and allows
for the comparison of closely related series of compounds.
Thus, the TNI(‘) of approximately 90 uC obtained for series
2[n] indicates a rather low mesophase stability as compared to
TNI(‘) = 155 uC calculated for another four-benzene-ring
homologous series.35 The TNI values for the first two
homologs, 2[1] and 2[2], that were not used in the analysis,
can be estimated from the fitting functions. Thus, the
extrapolated value for 2[1] is 321 uC [function (F1)] or 315 uC
[function (F2)], while the observed value is 257 uC. For 2[2]
both functions predict 268 uC which is only 4 uC higher that
the experimental TNI. This discrepancy for 2[1] and 2[2] is
presumably related to the conformational flexibility of the
dioxane ring and the presence of the non-mesogenic diaxial
Analysis shows that the calculated angle
b has an
insufficient value to reproduce the experimental dielectric
parameters neither for pure 1[4] nor for solutions in 6-CHBT.
It was found that the angle b must be increased by 19u, when
all conformers are considered, or by 13u when only C(anti) and
E(anti) are used for analysis of the pure material. Using similar
values for analysis of the extrapolated dielectric parameters
leads to a reasonably high apparent order parameter Sapp
.
0.5. Without this correction of b, the calculated Sapp is very
small ,0.2, which implies a significant destabilization of the
binary mixture. The difference of y13u between the value of
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the angle b obtained from the fit to the experimental data and
the calculated value is reasonable. The disagreement may be
due, in part, to the computed dioxane ring geometry and dipole
moment, and the different orientations of the main molecular
rotational axis in the condensed and gas phases. However, a
difference of nearly 20u is more difficult to account for. Thus,
the expectation that the calculated b should be close to the
experimental value indicates that conformers with higher value
of angle b are required. Among five conformers with b ¢ 43u
(Table 3) two, C(anti) and E(anti), are the most anisometric
with the smallest dihedral angle h of ,20u. This is strongly
suggestive that conformational selection takes place in the
nematic phase and those with higher anisometry are preferred.
VTH = 1.55 ¡ 0.02 V; eI = 12.0 ¡ 0.1; eH = 4.0 ¡ 0.1; De =
8.0 ¡ 0.1. Literature values:36 VTH = 1.63; eI = 12.0; eH = 4.0;
De = 8.0 at 25 uC.
Dielectric measurements
The properties of dioxane 1[4] were measured by a liquid
crystal analytical system (LCAS - Series II, LC Vision1) using
GLCAS software version 0.59 which implements literature
procedures for dielectric constants.37
Solutions of 5.5, 10.7, and 13.8 mol% of 1[4] in 6-CHBT
were prepared and conditioned for 1–2 h at 50 uC. The
mixtures were loaded into ITO electro-optical cells by capillary
forces at ambient temperature. The cells (about 5 mm thick,
electrode area of 0.28 cm2 and antiparallel SiO2 with the pretilt
y0u) were obtained from LC Vision1, and their precise
thicknesses (¡0.05 mm) was measured by optical methods. The
filled cells were heated to an isotropic phase and were left for
an hour at room temperature before measurement.
Summary and conclusions
Results demonstrate that a combination of carborane and
dioxane rings give rise to a highly nematogenic materials
with relatively low transition temperatures, low Dn, and small
positive De. The clearing temperatures of series 2[n] can be
fitted with two asymptotic functions that predict the TNI for
higher members of the series, and give the limiting TNI value
for n = ‘ about 90 uC.
Conformational analysis of 1[4] showed 10 principal con-
formers, which differ in shape and in the orientation of the
molecular dipole moment with respect to the main rotational
axes. Analysis of dielectric results demonstrated that the
experimental values require higher orthogonality of the dipole
moment, which is consistent with higher fraction of the more
anisometric conformers. This suggests that conformational
selection may be taking place in the nematic phase in
preference for those with most elongated shapes. These results
indicate that the Maier–Meier relationship can provide insight
into conformational properties of liquid crystals, if sufficient
difference in Da or b exists.
Default parameters were used for measuring dielectric
constants of the mixtures: triangular shaped voltage bias
ranging from 0.1–20 V at 1 kHz frequency. The threshold
voltage Vth was measured as a 10% of change. Each mixture
was measured at least 5 times in a single cell, and results were
averaged giving the dielectric values with an associated error of
,0.03. Since cell parameters vary slightly, reproducibility of
the measurement from cell-to-cell and hence uncertainty of a
single cell measurement is about 0.08. This relatively high
uncertainty for the measured values is partially eliminated in
the fitting process of three datapoints from three different cells.
Values for dielectric permittivity e were plotted as a function of
concentration and extrapolated to pure 1[4]. The intercept in
the fitting functions was fixed at the appropriate value for the
pure host (vide supra). The results are presented in Table 2.
Temperature-dependent dielectric permittivity of neat 1[4]
was measured in three SiO2 cells. Measurements were repeated
three times for each cell and averaged. Estimated standard
deviation for the single cell measurement was about 0.01. The
resulting mean values for each of the three cells were averaged
to give the final dielectric permittivities. Standard deviation
for eI and eH was approximately 0.1, while De differed little
between the cells (std 0.01).
Experimental
Synthesis of liquid crystals and their analytical data are
provided in the ESI. Optical microscopy and phase identifica-
tion was performed using a PZO ‘‘Biolar’’ polarized micro-
scope equipped with a HCS250 Instec hot stage. Thermal
analysis was obtained using a TA Instruments 2920 DSC.
Transition temperatures (onset) and enthalpies were obtained
using small samples (1–2 mg) and a heating rate of 5 uC min21
under a flow of nitrogen gas. Enthalpies for isotropic transi-
tions were obtained by integration of the zoomed-in DSC
transition peak in the range ¡2.5 uC (for 1[n]) or ¡5 uC
(or 2[n] and 3[n]) from the peak. For DSC and microscopic
analyses, each compound was rigorously purified by dissolving
in CH2Cl2, filtering to remove particles, evaporating and
repeated recrystallization (typically EtOH and pentane for
1[n], and i-octane for 2[n] and 3[n]) till constant transition
temperature. The resulting crystals were dried in vacuum
overnight at ambient temperature. For such purified samples,
the clearing transitions were typically less than 0.3 uC wide.
trans-4-(4-Hexylcyclohexyl)phenylisothiocyanate (6-CHBT)
was purified by vacuum distillation before use. The measure-
ments for the pure host in three SiO2 planar cells at 24 uC:
Acknowledgements
This project was supported in part by the NSF grant (DMR-
0111657) and by Grant-in-Aid for Scientific Research (B) No.
13470468, the Ministry of Education, Culture, Sports, Science
and Technology, Japan.
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