Investigation of high Δε derivatives of the [closo-1-CB 9H10]- anion for liquid crystal display applications

Article abstract of DOI:10.1039/c4tc00230j

Two series of polar compounds 3[n] and 4[n] with longitudinal dipole moments ranging from 10 to 17 D were synthesized and investigated as additives to two nematic hosts, ClEster and CinnCN. Compounds 3[n] do not exhibit liquid crystalline behavior and hav

Full text of DOI:10.1039/c4tc00230j

Journal of
Materials Chemistry C
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PAPER
Investigation of high D3 derivatives of the [closo-1-
CB9H10]ꢀ anion for liquid crystal display
applications†
Cite this: J. Mater. Chem. C, 2014, 2,
2956
a
ab
Bryan Ringstranda and Matthias Bremerc
*
Jacek Pecyna, Piotr Kaszynski,
´
Two series of polar compounds 3[n] and 4[n] with longitudinal dipole moments ranging from 10 to 17 D
were synthesized and investigated as additives to two nematic hosts, ClEster and CinnCN. Compounds 3
[n] do not exhibit liquid crystalline behavior and have limited compatibility with nematic hosts, but still
are effective additives for increasing dielectric anisotropy, D3, of liquid crystalline host materials. On the
other hand, esters 4[n] typically exhibit nematic behavior and form stable solutions in ClEster up to 5
mol%. In this concentration range, the binary solutions exhibit a linear dependence of dielectric
parameters and the extrapolated D3 values range from <18 (4[3]l, m ¼ 11.3 D) to 70 (4[3]c, m ¼ 14.1 D and
4[3]k, m ¼ 17.2 D). Analysis of the dielectric data with the Maier–Meier formalism using DFT-calculated a
and m parameters gave apparent order parameter values Sapp in a range of 0.51–0.68 and Kirkwood
parameters g in a range 0.55–0.78 for esters 4[n].
Received 4th February 2014
Accepted 4th March 2014
DOI: 10.1039/c4tc00230j
www.rsc.org/MaterialsC
Introduction
Polar liquid crystals and polar compounds compatible with
nematic materials are important for adjusting dielectric
anisotropy, D3, and hence modifying electrooptical properties
Fig. 1 The structure of the [closo-1-CB9H10]
ꢀ cluster (A) and its polar
of materials used in liquid crystal display (LCD) technology.1,2 In derivatives IA and IIA. Each vertex represents a BH fragment, the
sphere is a carbon atom, and Q+ stands for an onium group such as an
ammonium, sulfonium or pyridinium.
this context, we have been investigating zwitterionic derivatives
of the [closo-1-CB9H10]
ꢀ cluster (A, Fig. 1) as potential additives
to liquid crystalline hosts. Recently, we demonstrated that polar
derivatives, 1[6], 2[n] and 3[6]a (Chart 1), of type IA (Fig. 1)
signicantly increase D3 of a nematic host and have high virtual
N–I transitions, [TNI], although they themselves rarely are
mesogenic.3,4 Unfortunately, these compounds have limited
solubility in nematic materials, and their high D3 values were
extrapolated from innite dilutions. Subsequent investigation
of zwitterionic esters of type IIA (Fig. 1), containing a sulfonium
group (4[5]a and 4[5]b, Chart 1) or a pyridinium fragment (5,5,6
Chart 1) revealed that they form nematic phases, have satis-
factory solubility in nematic hosts, and possess D3 between 30
and 40. For the 4-cyanophenol ester in series 5 a record high D3
of 113 was measured in a nematic host.5,6
In continuation of our search for new polar compounds with
improved mesogenic and dielectric properties, we investigated
derivatives 3[n] and esters 4[n] (Chart 1). Here, we report the
synthesis and thermal and dielectric characterization of the two
series of compounds in the pure form as well as in binary
mixtures. Dielectric data is analyzed with the Maier–Meier
relationship and augmented with DFT computational results.
Results
Synthesis
Compounds 3[3]b, 3[5]b, and 3[6]c were prepared by Negishi
coupling of the appropriate organozinc reagent with iodide 6
(ref. 7) in the presence of Pd2dba3 and [HPCy3]+[BF4]ꢀ in a THF–
NMP mixture (Scheme 1).3 The iodides 6 were obtained from
protected mercaptans 7 (ref. 3) upon reactions with an appro-
priate dibromides 8 (ref. 7) under hydrolytic conditions as
described before.3
Aromatic esters 4[n]a–4[n]l were prepared by reacting acid
chlorides of sulfonium acids 9[n] with appropriate phenols
10 in the presence of NEt3 (Scheme 2). The two aliphatic esters,
aDepartment of Chemistry, Vanderbilt University, Nashville, TN 37235, USA. E-mail:
piotr.kaszynski@vanderbilt.edu; Tel: +1-615-322-3458
b
´ ´
´ ´
Faculty of Chemistry, University of Łodz, Tamka 12, 91403 Łodz, Poland
cMerck KGaA, Frankfurter Strasse 250, Darmstadt, Germany
† Electronic supplementary information (ESI) available: Additional synthetic and
characterization details for 3[n], 4[n], 8[7], 9[7], 10, 11, 13–19, solubility data,
thermal and dielectric data for solutions, Maier–Meier analysis details, DFT
results, and archive of calculated equilibrium geometries for 3[n], 4[n] and 20.
See DOI: 10.1039/c4tc00230j
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Chart 1
4[3]e and 4[3]f, were obtained from the corresponding
acid chloride and excess cyclohexanols 11 in neat pyridine.
The sulfonium acids 9[3],8 9[5],5 and 9[7] were prepared by
thermolysis of the methyl ester of diazonium acid 12 (ref. 8
and 9) in the presence of the appropriate thiane 13[n] (ref. 8)
followed by hydrolysis of the resulting methyl ester 14[n]
(Scheme 2).
Thiane 13[7] was prepared in reaction of dibromide 8[7] with
Na2S in EtOH–H2O (Scheme 3).8 Dibromide 8[7] (ref. 10) was
obtained following a previously established procedure for
dibromide 8[5].7 Thus, octanal and dimethyl malonate were
Scheme
1 Synthesis of series 3[n]. Reagents and conditions: (i)
RCH(CH2CH2Br)2 (8), [NMe4]+[OH]ꢀ or Cs2CO3, MeCN ref. 3; (ii)
CnH2n+1ZnCl, Pd2dba3, [HPCy3]+[BF4]ꢀ, THF–NMP, rt, 12 h, ꢁ90%.
Scheme 2 Synthesis of esters 4[n]. Reagents and conditions: (i) 120
ꢂC, 2 h; (ii) NaOH or KOH, MeOH, reflux; (iii) HCl dil.; (iv) (COCl)2, DMF
(cat), CH2Cl2; (v) ROH (10), NEt3, CH2Cl2, rt, 12 h or ROH (11), pyridine,
90 ꢂC.
Scheme 3 Synthesis of thiane 13[7]. Reagents and conditions: (i)
piperidine (cat), benzene, reflux; (ii) HCl conc. reflux; (iii) SOCl2, reflux;
(iv) MeOH, reflux; (v) LiAlH4, THF; (vi) HBr conc., H2SO4, 120 ꢂC; (vii)
Na2S$9H2O, EtOH, 50 ꢂC, 1 h, 89%.
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converted to dimethyl 3-heptylglutarate (15[7]) in 4 steps and
65% overall yield (Scheme 3). Subsequently, the ester was
reduced to diol 16[7] and converted to dibromide 8[7].
Phenols 10j and 10k were prepared from the corresponding
4-benzyloxybenzoic acids 17. The acids were converted into
esters 18 aer which the protecting benzyl group was removed
under reductive conditions (Scheme 4).
Scheme 5 Synthesis of phenol 10g. Reagents and conditions: (i)
PdCl2, K2CO3, EtOH–H2O (1 : 1), rt, 1 h, 79%.
Phenol 10g (ref. 11) was obtained using a ligand-free Suzuki
coupling reaction12 (Scheme 5). Phenols 10h,13 10i,14,15 and 10l
(ref. 16) were obtained as reported in the literature.
trans-4-Alkylcyclohexanols 11 (ref. 17) were isolated as 19
from a mixture of stereoisomers by recrystallization of their 4-
bromobenzoate esters followed by hydrolysis.
Table 1 Transition temperatures (ꢂC) and enthalpies (kJ molꢀ1, in
italics) for 3[n]a
n
R
3
5
b
b
Cr1 185 (3.0) Cr2 224 (20.5) I
Cr1 189 (5.2) Cr2 199 (13.9) I
Thermal analysis
6
6
a
c
–CH2CH3
Cr1b 66 (21.9) Cr2 209 (10.9) Ic
Cr1 230 (9.8) Cr2 253 (11.6) I
Transition temperatures and enthalpies of compounds 3[n] and
4[n] were determined by differential scanning calorimetry
(DSC). Phase structures were assigned by optical microscopy in
polarized light, and the results are shown in Tables 1–3.
Compounds in series 3[n] display only crystalline poly-
morphism and melt at or above 200 ꢂC, which is consistent with
behavior of the previously reported derivative 3[6]a.3 Extension
of the sulfonium substituent in 3[6]a by the cyclohexylethyl
fragment in 3[6]c increased the melting point by 44 K. Analo-
gous comparison of 3[3]b and 3[5]b shows that extension of the
alkyl group at the B(10) position by two methylene groups
lowered the melting point by 25 K. Thus, in contrary to expec-
tations, elongation of the core in 3[6]a did not induce meso-
genic behavior or reduce the melting point.
a
Determined by DSC (5 K minꢀ1) on heating: Cr – crystal, N – nematic,
I – isotropic. b Additional Cr–Cr transition at 133 ꢂC (14.6). Ref. 3.
c
3,4,5-triuorophenol derivative 4[3]d, in which the addition of
the benzene ring does increase the melting point by 85 K in 4
[3]g but fails to induce a mesophase. However, another biaryl
derivative, phenylpyrimidinol ester 4[3]h with a terminal hexyl
group, does exhibit a 44 K wide nematic phase. Insertion of a
–C6H4COO– fragment into the 4-triuoromethoxyphenyl ester
4[3]c only moderately increases the melting point (by 25 K) in
4[3]j and induces a wide-range enantiotropic nematic phase
(TNI ¼ 244 ꢂC) along with rich crystalline polymorphism.
Substitution of a lateral uorine into the benzoate fragment
of 4[3]j lowered the nematic phase stability by 10 K, and,
contrary to expectations, markedly increased the melting
temperature in 4[3]k. Finally, insertion of a uorophenylethyl
fragment into the cyclohexyl ester 4[3]f increased the
melting point by 38 K and induced a 26 K wide nematic phase
in 4[3]i. Similar insertion of a uorinated biphenylethyl
fragment into 4[3]f resulted in appearance of a nematic phase
Esters in series 4[3] generally have lower melting points than
compounds in series 3[n] and some exhibit nematic behavior.
Among the single-ring phenol and alcohol derivatives,
4[3]b–4[3]f, only the 4-butoxyphenol ester 4[3]b displays a
monotropic nematic phase and has the lowest melting point
ꢂ
in the entire series (111 C, Table 2). Extension of the phenol
core by another ring generally increases the melting point,
and also induces nematic behavior. The only exception is the
ꢂ
in 4[3]l (TNI ¼ 278 C).
The effect of alkyl chain extension at the thiane ring on
thermal properties was investigated for select esters 4[3]
(Table 3). Thus, extending the C3H7 chain in 4[3]b to C5H11 in
4[5]b lowered the melting point by 10 K, and had no impact on
nematic phase stability. Further extension of the terminal chain
to C7H15 lowered TNI by 4 K in 4[7]b. The same alkyl chain
extension in the phenylpyrimidinol ester 4[3]h had little effect
on the melting temperature, however, it lowered TNI by 10 K in
the pentyl analogue 4[5]h and by an additional 22 K in the
heptyl derivative 4[7]h. More signicant melting point reduc-
tion, by about 25 K, is observed in derivatives 4[3]c and 4[3]j
Scheme 4 Synthesis of phenols 10j and 10k. Reagents and conditions:
(i) (COCl)2, DMF (cat), CH2Cl2; (ii) 4-HOC6H4OCF3, NEt3, CH2Cl2; (iii)
H2, Pd/C, EtOH/THF or AcOEt/EtOH, rt, 12 h, >90%.
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Table 2 Transition temperatures (ꢂC) and enthalpies (kJ molꢀ1, in
italics) for 4[3]a
R
Cr 111 (29.3) (N 96 (1.1)) Ib
b
Analysis of ClEster solutions demonstrated that most deriva-
tives 3[n] and 4[n] dissolve in the isotropic phase in concentra-
tions up to about 10 mol%. However, solutions stable at ambient
temperature for at least 24 h are limited to about 4–5 mol%. For
instance, compound 4[3]c forms stable 5.5 mol% solutions in
ClEster. On the other hand, compound 3[5]b and ester 4[3]g were
found to be least soluble in ClEster, and the latter precipitates
even from a 1.3 mol% solution at ambient temperature aer
several hours. In contrast, 4[3]d, the shorter analogue of 4[3]g
containing only one benzene ring, is soluble at a concentration of
3.0 mol%. Extending the alkyl chain at the thiane ring does not
signicantly improve solubility of the esters 4[n] in ClEster. As
might be expected, the most soluble compounds are those with
more exible fragments such as 4[3]i, 4[3]j, and 4[7]j.
[N 19]c
c
Cr 135 (31.6) I
Cr1 102 (6.3) Cr2 140 (21.0) I
[N ꢀ34]c [N 21]d
d
e
f
Cr1 140 (3.2) Cr2 158 (19.9) I
[N ꢀ1]c [N ꢀ4]d
Cr1 101 (2.6) Cr2 137 (15.7) I
g
Cr 225 (40.8) I
Thermal analysis of the binary mixtures established virtual
N–I transition temperatures [TNI] in both ClEster and CinnCN
hosts by extrapolation of the mixture's N–I transition peak
temperatures to pure additive (Fig. 2). Analysis of results in Table
2 demonstrates that extrapolated [TNI] values are typically lower
than those measured for pure compounds. This suggests phase
stabilization in pure 4[n] by dipole–dipole interactions. For
instance, [TNI] for pyrimidine derivative 4[3]h is 70 K lower in
ClEster, while for butoxyphenol 4[3]b, [TNI] is 77 K lower in
CinnCN. The only exception is the ve-ring mesogen 4[3]l for
which the extrapolated [TNI] is 53 K higher than measured for the
pure compound. Further analysis of the data demonstrates
signicant dependence of [TNI] on the host for triuorophenol
derivative 4[3]d, while for cyclohexanol ester 4[3]e such depen-
dence essentially is not observed.
Cr 187 (36.4) N 231 (0.7) I
h
i
[N 161]d
Cr 175 (27.7) N 201 (3.8) I
Cre 160 (23.7) N 244 (1.3) I
Crf 183 (29.9) N 234 (1.7) I
j
k
Cr 183 (36.1) N 278 (1.5) I
l
[N 331]d
In general, three-ring esters destabilize the host's nematic
phase, whereas 4- and 5-ring derivatives stabilize the host's
nematic phase.
a
Determined by DSC (5 K minꢀ1) on heating: Cr – crystal, N – nematic,
b
c
I – isotropic. Ref. 8. Virtual [TNI] obtained in CinnCN typical error
d
about ꢃ5 K. Virtual [TNI] obtained in ClEster typical error about
e
ꢃ2 K. Cr–Cr transition at 137 ꢂC (27.6); another crystalline
polymorph melts at 165 ꢂC. f Cr–Cr transitions at 81 ꢂC (18.2 kJ molꢀ1).
Dielectric measurements
Analysis of selected binary mixtures in ClEster revealed linear
dependence of dielectric parameters on concentration, which,
aer extrapolation, established dielectric values for pure additives
(Fig. 3). Analysis of the data in Table 4 demonstrates that for esters
4[5]a and 4[3]e, having no additional polar groups, extrapolated
dielectric anisotropy, D3, values are about 23. A lower D3 value,
less than 18, was estimated for 4[3]l on the basis of D3 < 0 for its
3 mol% solution in ClEster. Substituting a polar group, such as
upon extension of C3H7 to C7H15 in 4[7]c and 4[7]j, respectively.
In addition, the chain extension in 4[3]j lowered the TNI by 18 K
to 226 ꢂC in 4[7]j.
Binary mixtures
To assess the new materials for formulation of LCD mixtures, OCF3 (in 4[3]c) or three uorine atoms (in 4[3]d) into the benzene
selected compounds were investigated as low concentration ring substantially increases the extrapolated D3 value to 70 and 60,
additives to nematic host ClEster, which is an ambient respectively. Extension of 4[3]c by insertion of a –C6H4COO–
temperature nematic characterized by a small negative D3 of fragment (in 4[3]j) or a –C6H3FCOO– fragment (in 4[3]k) essentially
ꢀ0.56. In addition, solutions of 3 compounds in CinnCN were does not affect the dielectric parameters of the material. Inter-
prepared to establish their virtual clearing temperatures [TNI].
estingly, by extending the length of the alkyl chain at the thiane
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Table 3 Transition temperatures (ꢂC) and enthalpies (kJ molꢀ1, in italics) for 4[n]a
R
n ¼ 3
n ¼ 5
n ¼ 7
a
—
Cr 97 (29.5) Ib
—
b
Cr 111 (29.3) (N 96 (1.1)) Ic
Cr 101 (28.6) (N 97 (0.6)) Ib
Cr 101 (52.2) (N 93 (1.8)) I
Cr 111 (24.6) I
c
Cr 135 (34.7) I
—
h
Cr 187 (36.4) N 231 (0.7) I
Cr 187 (57.6) N 221 (2.0) I
Cr 182 (56.8) N 199 (1.3) I
Cr 132 (36.6) N 226 (1.2) I
j
Crc 160 (23.7) N 244 (1.3) I
—
a
Determined by DSC (5 K minꢀ1) on heating: Cr – crystal, N – nematic, I – isotropic. b Ref. 5. c Cr–Cr transition at 137 ꢂC (27.6); another crystalline
polymorph melts at 165 ꢂC.
ring in the former diester (4[3]j) by –(CH2)4–, D3 increases in 4[7]j
by 6%. Incorporation of the pyrimidinyl substituent as a second
polar group in 4[3]h has a modest effect on dielectric anisotropy,
and an extrapolated D3 value of 50 was obtained. It appears,
however, that unlike for other esters, dielectric parameters for
solutions of 4[3]h deviate from linearity at higher concentrations.
Dielectric values for sulfonium 3[5]b extrapolated from two
concentrations (2.5 mol% and 3.7 mol%) in ClEster are modest
and about half of those previously obtained at innite dilution
for the 3[6]a analogue, which indicates signicant aggregation
of the additive in solutions.
Analysis of dielectric data
Dielectric parameters extrapolated for pure additives were
analyzed using the Maier–Meier relationship (eqn (1)),18,19 which
includes molecular and phase parameters.20 Using experimental
3k and D3 values and DFT-calculated parameters m, a, and b
(Table 5), eqn (2) and (3) are used to calculate the apparent order
Fig. 2 Plot of peak temperatures of the N–I transition vs. concen-
tration in ClEster.
parameter, Sapp,
20 and Kirkwood factor, g (Table 4). The effect of
the additive was ignored in the determination of eld parameters
F and h in eqn (2) and (3); F and h were calculated using the
experimental dielectric and optical data for pure ClEster host.21,22
(
)
2
ꢀ
ꢁ
NFh
Fmeff
D3 ¼
Da ꢀ
1 ꢀ 3 cos2 b
S
(1)
(2)
30
2kBT
2D330
NFh½2Da þ 3að1 ꢀ 3 cos2 bÞꢄ ꢀ 3ð3 ꢀ 1Þ30ð1 ꢀ 3 cos2 bÞ
S ¼
ꢂ
ꢃ
ꢀ
ꢁ
2
3 ꢀ 1 30 ꢀ aNFh ꢀ DaNFhS 3kBT
k
3
g ¼
(3)
NF2hm2½1 ꢀ ð1 ꢀ 3 cos2 bÞSꢄ
Fig. 3 Dielectric parameters of binary mixtures of 4[3]e (black) and
4[3]c (red) in ClEster as a function of concentration.
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Table 4 Extrapolated experimental (upper) and predicted (lower in
italics) dielectric data and results of Maier–Meier analysis for selected
compoundsa
conformationally stable with a strong preference for the trans
isomer in the diequatorial form,5 sulfonium esters 4[n] exist as a
dynamic mixture of interconverting stereoisomers trans and cis
in about 4 : 1 ratio (Fig. 4).5 Therefore, their molecular param-
eters were obtained as a weighted sum of values calculated for
the two stereoisomers 4[n]-trans and 4[n]-cis and the composite
numbers for 4[n] are shown in Table 5.22
Compound
3[5]b
3[6]ac
4[5]ad
4[3]c
3k
3t
D3
Sapp
g
46
84.9
84
95.2
35.0
32.6
87
58.5
78
9
15.9
23
18.2
9.7
8.2
17
11.3
18
37
69.0
61
77.0
25.3
24.4
70
47.2
60
0.69
0.65b
0.52
0.65b
0.60
0.65b
0.62
0.65b
0.58
0.65b
0.51
0.65b
—
0.25
0.50b
0.49
0.50b
0.57
0.50b
0.77
0.50b
0.61
0.50b
0.55
0.50b
—
Results in Table 5 demonstrate that replacement of the
pentyl chain in 3[6]a with the 4-propylphenethyl group in 3[5]b
essentially has no effect on the molecular dipole moment (m z
16.5 D), however it increases anisotropy of polarizability by
3
3
˚
˚
about 50% from Da ¼ 24.8 A in 3[6]a to Da ¼ 36.2 A in 3[5]b.
The dipole moment of esters 4[n] with a non-polar alcohol
and phenol (e.g. 4[3]a and 4[3]e, Table 5) is about 6 D lower than
for compounds in series 3[n]. It can be increased by introduc-
tion of additional polar groups into the molecular structure of
4[n]. Hence, replacement of the pentyl chain in 4[3]a with a
polar group, such as OCF3 (4[3]c), 3 uorine atoms (4[3]d), or
CN (4[3]m, Chart 1) increases the longitudinal dipole moments
by 3.5 D, 4.2 D and 6.7 D, respectively. Extending the molecular
core in ester 4[3]d by another benzene ring in 4[3]g has negli-
gible effect on the dipole moment, but it does increase anisot-
4[3]d
4[3]e
68.2
32
13.0
11
55.2
21
32.6
8.1
24.5
e
e
e
4[3]g
59.5
61
44.1
86
61.8
89
56.8
87
11.4
11
9.3
17
11.5
15
10.9
16
48.1
50
34.8
69
50.3
74
45.9
71
0.65b
0.68
0.65b
0.62
0.65b
0.67
0.65b
0.63
0.65b
—
0.50b
0.69
0.50b
0.73
0.50b
0.78
0.50b
0.67
0.50b
—
4[3]h
4[3]j
4[7]j
4[3]k
4[3]l
3
˚
ropy of polarizability by about 50% from Da ¼ 30.4 A in the
67.4
12.0
55.4
3
˚
f
f
<18f
former to Da ¼ 47.6 A in the latter with a modest increase in
0.65b
—
0.50b
—
average polarizability a (ꢁ25%). Similar extension of 4[3]c with
a weakly polar –C6H4COO– fragment in 4[3]j increases the
longitudinal dipole moment by 2.2 D and signicantly increases
Da (53%) and a (28%). Placement of a uorine atom on the
–C6H4COO– group in 4[3]j further increases the dipole moment
in 4[3]k by 1 D, with a minimal impact on polarizability.
The longitudinal dipole moment in esters 4[n] was also
25.4
7.6
17.8
e
e
e
4[3]m
99.3
17.9
81.4
0.65b
0.50b
a
Values predicted for assumed Sapp ¼ 0.65 and g ¼ 0.5. For details see
text and the ESI. Typical error of experimental extrapolated dielectric
b
c
parameters ꢃ1. Assumed value. Experimental data from ref. 3.
d
e
f
Experimental data from ref. 5. Not measured; see text. D3 < 0 for
a 3.0 mol% mixture.
increased by incorporation of
a pyrimidine fragment;
compounds possessing such a fragment are known to exhibit
substantial dielectric anisotropies.23 Thus, the ester of 2-(4-
hexylphenyl)pyrimidin-5-ol, derivative 4[3]h, has a calculated
dipole moment m ¼ 13.14 D, which is about 2.5 D higher than
that of 4-pentylphenol 4[3]a.
Lateral uorination has no effect on the magnitude of the
longitudinal molecular dipole moment component, mrr. Thus,
results for 4[3]l show that mrr remains nearly the same as in the
4-pentylphenol ester 4[3]a. However, the transverse component,
mt, of the molecular dipole moment increases by 1.7 D,
changing the orientation of the net dipole moment vector with
respect to the main molecular axis from b ¼ 13.1ꢂ in 4[3]a to b ¼
21.3ꢂ in 4[3]l.
Table 5 Calculated molecular parameters for selected compoundsa
3
3
˚
˚
Compound
mk/D
mt/D
m/D
bb/ꢂ
Da/A
aavrg./A
3[5]b
3[6]a
4[3]a
4[5]a
4[3]c
4[3]d
4[3]e
4[3]g
4[3]h
4[3]j
16.39
16.10
10.39
10.30
13.99
14.66
9.77
14.77
12.94
16.15
16.16
17.16
10.56
17.18
2.38
2.95
2.42
2.96
1.95
2.16
2.64
1.90
2.33
1.68
2.05
0.67
4.13
2.25
16.57
16.37
10.66
10.72
14.12
14.81
10.12
14.89
13.14
16.24
16.29
17.18
11.34
17.33
8.3
10.4
13.2
16.1
7.9
8.4
15.1
7.1
10.2
5.9
7.2
2.2
21.4
7.5
36.18
24.84
36.45
37.50
32.91
30.49
27.02
47.60
57.74
50.23
52.12
52.73
61.47
39.14
62.40
53.34
61.32
65.05
53.92
51.54
56.83
64.19
75.86
69.08
76.59
69.77
89.21
54.86
4[7]j
4[3]k
4[3]l
Analysis of the computational results for the 4[3]-trans
isomers shows that, with the exception of 4[3]l, the net dipole
moment is nearly parallel with the long molecular axis, and the
4[3]m
a
Obtained at the B3LYP/6-31G(d,p) level of theory in ClEster dielectric
medium. For esters 4[n] calculated for an average molecule at the
b
equilibrium ([cis] ¼ 21 mol%). For details see text and the ESI. Angle
between the net dipole vector m and mk.
The molecular electric dipole moment, m, and polarizability,
a, required for the Maier–Meier analysis were obtained at the
B3LYP/6-31G(d,p) level of theory in the dielectric medium of
ClEster.22 While molecules in series 3[n] are essentially
Fig. 4 Interconversion of the trans and cis isomers of ester 4[n].
J. Mater. Chem. C, 2014, 2, 2956–2964 | 2961
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Paper
angle b ranges from 2ꢂ in 4[3]k to 14ꢂ in 4[3]e (avrg. 7.8ꢂ ꢃ3.7ꢂ). their low solubility and non-linear dependence of dielectric
In the 4[3]-cis isomers the angle b is larger by an average of 4.6ꢂ parameters versus concentration.
ꢃ1.7ꢂ relative to the trans analogues. The molecular shape also
affects anisotropy of polarizability Da, which is larger for the
Discussion
linear 4[3]-trans molecules than for the bent 4[n]-cis analogues
3
˚
by an average of 4.6 ꢃ 0.5 A .
The centerpiece of polar materials presented here is the sulfo-
nium zwitterion 20 of the [closo-1-CB9H10]ꢀ cluster with calcu-
The effectiveness of these compounds as high D3 additives to
ClEster was investigated using the Maier–Meier formalism.
lated ground-state electric dipoles of 16.3 D and 9.9 D for
zwitterions 20-I and 20-II, respectively, in ClEster dielectric
medium. The observed difference in the dipole moments orig-
inates from the strong polarization of electron density towards
Following a frequently used approach in designing of polar
liquid crystals, the analysis initially assumed the order param-
eter of the additives to be the same as for the ClEster host (S ¼
0.65), and Kirkwood factor (g) was set at 0.5.24,25 Results in Table
the carbon atom in the boron cluster. Elongation of the
4 demonstrate that esters 4[n] of non-polar phenols or alcohols
molecular core by substitution in the antipodal positions of the
ꢀ
exhibit expected D3 values of about 24 (4[5]a and 4[3]e). The
lowest D3 value of 17.8 is predicted for 4[3]l, which is the largest
molecule investigated in this series. This low value is due, in
part, to the low number of molecules in the unit volume (low N
number).
Esters of phenols with polar substituents are expected to
have higher D3 values. Thus, OCF3, F, and additional COO
groups enhance the longitudinal dipole moment, which results
in D3 of about 50 (e.g. 4[3]c, 4[3]d, and 4[3]j). A particularly large
D3 of 81 is predicted for ester 4[3]m containing a CN group
(Chart 1). Surprisingly, the least effective dipole moment
booster is the pyrimidine fragment in 4[3]h, with a predicted
relatively low D3 of 35.
[closo-1-CB9H10
] cluster and the thiane ring in 20-I and 20-II
helps to induce liquid crystalline behavior and increases
compatibility with nematic hosts. In general, compounds with a
total of 2 or 3 rings in both series 3[n] and 4[n] do not form
liquid crystalline phases; the only exception thus far is the 4-
butoxyphenol ester 4[n]b.
The calculated longitudinal dipole moment, mrr, in
compounds 3[n] is about 16 D and originates solely from 20-I.
Esters 4[n] can achieve the same magnitude of mrr by combining
the moderate dipole moment of 20-II with that of a polar
substituent. Examples include diesters 4[n]j and benzonitrile
4[3]m, in which the net dipole moments are calculated to be
16.2 and 17.3 D, respectively (Table 5). In contrast to 3[n],
compounds 4[n] with several polar groups exhibit lower melting
points, higher solubility in nematic hosts, and display meso-
genic behavior. These qualities are quantied in the Maier–
Meier analysis and reected in high order, Sapp, and Kirkwood,
g, parameters, as shown for diesters 4[n]j and 4[3]k (Table 4).
Results in Table 4 indicate that small polar compounds are
more effective additives due to their higher density of dipoles in
the unit volume (larger number density N). For instance, ester
4[3]c and its “extended” analogue diester 4[3]j have essentially
the same experimental dielectric parameters (D3 z 70) in spite
of a larger dipole moment in the latter by about 2 D (Table 5).
Benzonitrile derivative 4[3]m, although not prepared in this
investigation, is expected to exhibit a relatively large dielectric
anisotropy, on the basis of its small size and large dipole
moment.
Compounds in series 3[n] have predicted higher D3 values
(ꢁ70) than those for esters 4[n]. However, these values are
observed only at innitely low concentrations. At higher
concentration (ꢁ2 mol%) molecular aggregation signicantly
reduces D3.
Experimental D3 for 4[n] are in general agreement with
theoretical predictions, mainly due to fairly high and uniform
Sapp values. Analysis of data in Table 4 demonstrates that
experimental Sapp of 0.61 ꢃ 0.02 for the compounds are
comparable with the order parameter of ClEster (S ¼ 0.65). The
only exceptions are 4[3]e (Sapp ¼ 0.51), 4[3]h (Sapp ¼ 0.68), and
4[7]j (Sapp ¼ 0.67). These outlying Sapp values for the rst two
compounds are consistent with the extreme virtual clearing
temperatures, [TNI] ¼ ꢀ4 ꢂC for 4[3]e and [TNI] ¼ 161 ꢂC for
4[3]h, and demonstrate low compatibility of the former (4[3]e)
and higher compatibility of the latter (4[3]h) with the host.
The Kirkwood parameter, g, has a broader range for esters
4[n] between 0.55 for 4[3]e and 0.78 for 4[7]j and reects
different degrees of molecular association of the additive in
solutions. In general, the observed values for g are higher than
that initially assumed (g ¼ 0.5). Perhaps most gratifying is that
compounds 4[3]j, 4[7]j, and 4[3]k with the highest values of mrr
show little association (g ¼ 0.73, 0.78, and 0.67, respectively).
Particularly interesting is the observed decreased association
(increased g) upon alkyl chain extension in 4[n]j. This demon-
strates that molecular structure containing several polar
groups placed in the semi-rigid core provide a successful design
for preparation of high D3 materials. On the other hand, anal-
ysis of compounds in series 3[n] gives low g values (e.g. g ¼ 0.25
Finally, it should be emphasized that analysis of experi-
mental dielectric data using the Maier–Meier formalism
provides informative insight into the behavior of additives in
nematic solutions and has become an important tool in our
investigation of polar compounds.20
Conclusions
for 3[5]b), which shows that these materials are prone to We have reported a diverse library of two series of polar
excessive aggregation in solutions. This is also consistent with compounds derived from an inorganic boron cluster, which act
2962 | J. Mater. Chem. C, 2014, 2, 2956–2964
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Journal of Materials Chemistry C
as effective additives to nematic materials for modulating
dielectric anisotropy, D3, and hence electrooptical properties.
Compounds 3[n] do not display liquid crystalline behavior or
enhanced solubility even with elongated molecular cores up to 3
rings. However, they have high extrapolated D3 values, but are
hampered by limited solubility in nematic materials. On the
other hand, esters 4[n] have signicantly improved compati-
bility with nematic hosts and exhibit liquid crystalline behavior.
However, compounds with a total of 2 or 3 rings in 4[n] generally
do not form liquid crystalline phases. Esters 4[n] have more
modest extrapolated D3 values due to smaller zwitterion's dipole
moment, but incorporation of additional polar substituents
into the molecular structure can increase D3 values, while
maintaining solubility on the order of several mol%. The most
effective additives for increasing D3 of ClEster appear to be 4[3]c
and 4[n]j. These materials exhibit good compatibility with the
host (reasonable solubility, high g and Sapp values) and a large
D3 of about 70.
Preparation of 3[n]
A solution of anhydrous ZnCl2 (12 eq.) in dry THF (10 mL), was
treated with a solution of CnH2n+1MgBr (12 eq., 2 M in Et2O or
freshly prepared from CnH2n+1Br in THF) at 0 ꢂC under N2
atmosphere. The mixture was stirred for 15 min at rt, and NMP
was added (5 mL) followed by Pd2dba3 (2 mol%), [HPCy3]+[BF4]ꢀ
(8 mol%), and iodide 6 (ref. 7) (1.0 mmol). The mixture was
stirred overnight at rt, 10% HCl was added, and the mixture was
extracted with Et2O (3ꢅ). The combined extracts were dried
(Na2SO4), and the solvent was evaporated. The resulting residue
was puried by passage through a silica gel plug (hexane–
CH2Cl2, 1 : 1). The eluent was ltered through a cotton plug,
and the solvent evaporated to give the desired product in about
90% yield. Pure product for analysis was obtained by triple
recrystallization (toluene–iso-octane) as white crystals.
Analytical data for compounds 3[n] are provided in the ESI.†
Preparation of esters 4[n]
Additional structure–property relationship studies are
needed to further increase compatibility of these polar
compounds with nematic hosts. Enhanced solubility would
make these classes of compounds, especially esters 4[n], more
useful as additives in formulation of LCD mixtures.
Method A. A suspension of sulfonium acid 9[n] (0.16 mmol)
in CH2Cl2 (1 mL) was treated with (COCl)2 (3 eq.) and anhydrous
DMF (cat. amount). The suspension was stirred vigorously at rt
until it became homogeneous (ꢁ30 min). The light yellow solu-
tion was then evaporated to dryness, and the residue was redis-
solved in anhydrous CH2Cl2 (1 mL). Phenol 10 (1.1 eq.) and
freshly distilled NEt3 (3 eq.) were added, and the mixture was
stirred overnight at rt. The reaction mixture was washed with 5%
HCl (3ꢅ), and the organic layer was dried (Na2SO4) and solvent
removed. The product was puried by passage through a silica
gel plug (CH2Cl2). The eluent was ltered through a cotton plug,
and the solvent evaporated to give the desired ester in about 60%
yield. The resulting ester was puried further by repeated
recrystallization typically from an iso-octane–toluene mixture.
Method B. The crude acid chloride (generated from 9[3] as in
Method A), excess alcohol 11 (5 eq.), and freshly distilled pyri-
dine (5 eq.) were stirred and heated for 3 days at 90 ꢂC, with
protection from moisture. At times, the reaction was cooled to
rt, and minimal amount of anhydrous CH2Cl2 was added to
wash the sides of the ask. The product was puried as in
Method A.
Computational details
Quantum-mechanical calculations were carried out using
Gaussian 09 suite of programs.26 Geometry optimizations for
unconstrained conformers of 3[5]b, 3[6]a, and 4[n] with the
most extended molecular shapes were undertaken at the B3LYP/
6-31G(d,p) level of theory using default convergence limits.
Dipole moments and exact electronic polarizabilities for 3[5]b,
3[6]a, and 4[n] for analysis with the Maier–Meier relationship
were obtained in ClEster dielectric medium using the B3LYP/6-
31G(d,p)//B3LYP/6-31G(d,p) method and the PCM solvation
model27 requested with the SCRF (solvent ¼ generic, read)
keyword and “eps ¼ 3.07” and “epsinf ¼ 2.286” parameters
(single point calculations). The reported values for dipole
moment components and dielectric permittivity tensors are at
Gaussian standard orientation of each molecule (charge based),
which is close to the principal moment of inertia coordinates
(mass based).
Analytical data for esters 4[n] and synthetic procedures for
intermediates are provided in the ESI.†
Binary mixtures
Experimental part
General
Preparation and analysis. Solutions of compounds 3[n] or
4[n] and host ClEster (10–15 mg of the host) in dry CH2Cl2 (ꢁ0.5
NMR spectra were obtained at 128 MHz (11B), 100 MHz (13C), mL) were heated at ꢁ60 ꢂC for 2 h in an open vial to assure
and 400 MHz (1H) in CDCl3 or CD3CN. Chemical shis were homogeneity of the sample. The sample was degassed under
referenced to the solvent (1H, 13C) or to an external sample of vacuum (0.2 mmHg), le at ambient temperature for 2 h, and
B(OH)3 in MeOH (11B, d ¼ 18.1 ppm). Optical microscopy and analyzed by polarized optical microscopy (POM). Homogeneous
phase identication were performed using a PZO “Biopolar” samples were used for thermal and dielectric measurements.
polarized microscope equipped with a HCS400 Instec hot Long-term stability of the solutions was determined by
stage. Thermal analysis was obtained using a TA Instruments analyzing the samples by POM aer at least 20 h at ambient
2920 DSC. Transition temperatures and enthalpies were typi- temperature.
cally obtained using small samples (ꢁ0.5 mg) and a heating rate
The clearing temperature for each homogeneous mixture
was determined by DSC as the peak of the transition using small
of 5 K minꢀ1
.
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Journal of Materials Chemistry C
Paper
samples (ꢁ0.5 mg) and a heating rate of 5 K minꢀ1. The results 11 N. Aziz, S. M. Kelly, W. Duffy and M. Goulding, Liq. Cryst.,
are provided in the ESI. The virtual N–I transition temperatures,
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´
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´
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23 A. I. Pavluchenko, N. I. Smirnova, V. F. Petrov, Y. A. Fialkov,
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Acknowledgements
This work was supported by NSF grant DMR-1207585. We are
grateful to Professor Roman D˛abrowski of the Military Univer-
sity of Technology (Warsaw, Poland) for the gi of ClEster.
˙
25 P. K˛edziora and J. Jadzyn, Acta Phys. Pol., A, 1990, 77, 605–
610.
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