The synthesis of new liquid-crystalline mesogens containing bicyclohexane units

Article abstract of DOI:10.1016/j.tetlet.2006.12.094

The synthesis and mesomorphism characterization of new thermotropic derivatives of cis,trans and trans,trans-4,4′-bicyclohexyl-bis[4-(6-hydroxyhexan-1-oxy)benzoate] are described. This last intermediate produces a liquid crystalline diacrylate, with lower refractive indices than the well-known aromatic analogues, that allows to obtain polymeric network via UV photopolymerization. The acrylate is very useful to optimize the contrast of polymer dispersed liquid crystals (PDLCs). Moreover, the acylation of the intermediates previously mentioned, with alkanoic or perfluoro alkanoic acids give low-molecular liquid crystals that exhibit a broad temperature range of mesophases.

Full text of DOI:10.1016/j.tetlet.2006.12.094

Tetrahedron Letters 48 (2007) 1447–1450
The synthesis of new liquid–crystalline mesogens containing
bicyclohexane units
R. Cassano,a,* R. Da˛browski,b J. Dziaduszek,b N. Picci,a G. Chidichimo,c G. De Filpo,c
R. Muzzalupoa and F. Puocia
aDepartment of Pharmaceutical Sciences, University of Calabria, Rende, CS, Italy
bInstitute of Chemistry, Military University of Technology, Warsaw, Poland
cDepartment of Chemistry, University of Calabria, Rende, CS, Italy
Received 20 October 2006; revised 14 December 2006; accepted 15 December 2006
Available online 20 December 2006
Abstract—The synthesis and mesomorphism characterization of new thermotropic derivatives of cis,trans and trans,trans-4,40-bi-
cyclohexyl-bis[4-(6-hydroxyhexan-1-oxy)benzoate] are described. This last intermediate produces a liquid crystalline diacrylate, with
lower refractive indices than the well-known aromatic analogues, that allows to obtain polymeric network via UV photopolymer-
ization. The acrylate is very useful to optimize the contrast of polymer dispersed liquid crystals (PDLCs). Moreover, the acylation of
the intermediates previously mentioned, with alkanoic or perfluoro alkanoic acids give low-molecular liquid crystals that exhibit a
broad temperature range of mesophases.
Ó 2006 Elsevier Ltd. All rights reserved.
Liquid crystal monomers with difunctional polymeriz-
able groups are of great interest as starting materials
for the production of anisotropic networks.1–4 These
materials, with anisotropic optical and mechanical prop-
erties, represent an area of great scientific research and
technological potential. In particular, anisotropic net-
works have lots of possible uses in passive solid state
optical devices such as colour filters and polarizer
retarders.5,6 They are highly promising for usage in
microelectronics and especially for applications requir-
ing light modulation, as in electro-optics, and non-linear
optics.7–10 Bireactive monomers can be employed for the
production of these devices by in situ photopolymeriza-
tion.11–14 This technique involves the macroscopic orien-
tation of molecules containing reactive end groups, in
liquid crystalline phase, followed by UV irradiation.15,16
As a result, the molecular orientation within the system
is frozen by the formation of a three-dimensional aniso-
tropic network. Broer and co-workers1,2 initiated the
investigation of anisotropic networks obtained in situ
by means of photopolymerization of liquid crystal
monomers with ultraviolet light. Because of their liquid
crystal behaviour, the monomers can be uniaxially ori-
ented by external forces, such as magnetic and electrical
fields or surface orientation. One advantage of photo-
polymerization process, when compared with thermally
activated polymerization, is its temperature indepen-
dence. The temperature of polymerization can be chosen
to be in mesomorphic state and, after that the macro-
scopic orientation has been obtained, the polymeriza-
tion proceeds with a high degree of conversion. In this
way the degree of order and consequently the aniso-
tropic properties of the network can be adjusted as de-
sired. The resulting networks behave anisotropic in a
number of properties, such as thermal expansion coeffi-
cient and refractive index. In the production of aniso-
tropic networks, various reactive liquid crystal
molecules were synthesized.3 Such molecules, schemati-
cally represented in Figure 1, consist of three parts: (1)
central mesogenic core, (2) reactive end groups and (3)
flexible spacers. Each one of these groups plays an
important role in determining the thermal properties,
the liquid crystalline phase and the optical properties
spacer
mesogenic group photoreactive group
*
Corresponding author. Tel.: +39 0984493296; fax: +39
0984493163; e-mail: roberta.cassano@unical.it
Figure 1.
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.12.094

1448
R. Cassano et al. / Tetrahedron Letters 48 (2007) 1447–1450
in the monomeric state. They also influence the polymer-
ization behaviour leading to the formation of an aniso-
tropic network. The crosslinked polymer network is
formed by the polymerization of each of the photoreac-
tive end groups. In the past years, many scientific groups
have developed different synthetic procedures in order to
obtain the polymerizable liquid crystalline molecules
useful in the realization of composite films (polymer dis-
persed liquid crystals—PDLCs) that have attracted a lot
of interest as electrically controlled light shutters.17,18
PDLCs can exhibit either a droplet morphology,19 in
which the liquid crystal is separated as distinct micron-
sized droplets in a continuous polymer matrix, or a re-
verse morphology,20 in which the liquid crystal fills the
voids and crevices in a network structure formed by
small polymer balls. PDLCs are generally characterized
by an opaque OFF state, which can be turned into a
transparent ON state by the application of a suitable
electric field (normal-mode operation film). On the con-
trary, reverse-mode PDLCs exhibit a transparent OFF
state and an opaque ON state. They have been the sub-
ject of a great deal of research. Reverse-mode shutters
have been obtained by using dual frequency addressable
liquid crystals,21,22 by polymerizing nematic emulsion,23
by functionalizing the liquid/polymer matrix interface24
and by using a larger liquid crystal loading.25–27 In the
present work are shown (Scheme 1) (i) the synthesis of
new polymerizable low birefrigence mesogen 1, useful
in the production of PDLC operating in ‘reverse mode’,
with a mesogenic bicyclohexyl unit functionalized on
each side with an acrylate group connected by a flexible
spacer; (ii) the synthesis of analogous mesogens without
polymerizable groups 2 and (iii) the characterization of
liquid crystalline behaviour of the newly formed com-
pounds 1 and 2. The polymerization behaviour of com-
pound 1 and the properties of the prepared polymers
will be described in the following publications. The syn-
thesis of mesogens 1 and 2 is outlined in Scheme 2 and
intermediate 12 in Scheme 3. This preparative procedure
was recently adopted28 for antiferroelectric ester
realization.
Diol 12 was prepared as follows. The commercial 1,6-
hexanediol (5) was treated with sodium in equimolar
ratio to avoid disubstitution and then with benzyl chlo-
ride (6). The monosubstituted derivative [6-benzyloxy-
hexan-1-ol (4)] was separated from the raw product by
distillation and then coupled with benzenesulfonyl chlo-
ride in dichloromethane in the presence of pyridine (Py)
to quench HCl. The crude oil product (7) was submitted
to a reaction with ethyl 4-hydroxybenzoate in 2-buta-
none as the solvent and potassium carbonate (K2CO3)
as the base. The ethyl p-(6-benzyloxyhexan-1-oxy)ben-
zoate (8), a yellow oil sufficiently pure, was obtained.
It was hydrolyzed by boiling with potassium hydroxide
in ethylene glycol for 7 h. Afterwards, the reaction
mixture was acidified and the resulting p-(6-benzyloxy-
hexan-1-oxy)benzoic acid (9) was purified by recrystalli-
zation from ethanol. It was then treated by boiling with
oxalyl chloride using dry toluene as the solvent and a
few drops of dry N,N-dimethylformamide (DMF). Ex-
cess of oxalyl chloride and most of the toluene were dis-
tilled off and trans,trans-bicyclohexanediol (10) or the
commercial isomer mixture of bicyclohexanediol and
pyridine as the base were added. Compound 10 was ob-
tained by treating the commercial 4,40-bicyclohexane-
diol (cis,trans isomers mixture with composition
90 wt % of trans producer Valiant, China) with acetyl
chloride in the presence of pyridine (Py) and dichloro-
methane (CH2Cl2). The trans,trans isomer of the acetyl
derivative was separated by recrystallization from meth-
anol, purity 99.7 wt %. In the second step it was hydro-
lyzed, under reflux, with NaOH/H2O. After cooling, the
mixture was acidified and the white precipitate was fil-
tered off. The resulting crude product contains the
pure trans,trans-bicyclohexanediol (mp 141–142.5 °C).
RCOO(CH2)6O
COO
OCO
O(CH2)6OCOR
Scheme 1. (1) R = –CH@CH2, (2a) R = –OCH3, (2b) R = –C3H7, (2c) R = –C3F7, (2d) R = –C3F7 (mixture cis–trans).
HO(CH2)6O
OCO
O(CH2)6OH
12
COO
i
ii
1
CH2=CHCOO(CH2)6O
OCO
O(CH2)6OCOCH=CH2
COO
iii
iiii
2a
CH3OCOO(CH2)6O
OCO
O(CH2)6OCOOCH3
COO
2b
C3H7COO(CH2)6O
OCO
O(CH2)6OCOC3H7
COO
2c, 2d **
C3F7COO(CH2)6O
OCO
O(CH2)6OCOC3F7
COO
Scheme 2. Reagents: (i) CH2@CHCOCl, Et3N, THF; (ii) CH3OCOCl, Py, CH2Cl2; (iii) C3H7COCl, Py, CH2Cl2; (iv) C3F7COOH, THF, DEAD,
P(Ph)3 **prepared from cis, trans mixture.

R. Cassano et al. / Tetrahedron Letters 48 (2007) 1447–1450
1449
3
SO2Cl
CH2O(CH2)6OH
1. Na
4
2.
CH2Cl
6
pyridine, CH2Cl2
HO(CH2)6OH 5
CH2O(CH2)6OSO2
7
COOEt
HO
2- butanone, K2CO3
CH2O(CH2)6O
COOH
CH2O(CH2)6O
COOEt
8
9
1.
KOH / H2O
2.HCl
1.
(COCl)2
HO
2.
OH, pyridine, toluene
10
11
CH2O(CH2)6O
OCO
OCO
O(CH2)6OCH2
COO
H2, Pd/C, THF
HO(CH2)6O
O(CH2)6OH
COO
12
Scheme 3.
ded 1§ while methylchloroformate, butyryl chloride
and heptafluorobutyric acid afforded compounds
2a–c.– The liquid crystalline properties of the target mol-
ecules were investigated primarily by optical polarizing
microscopy (OPM) and differential scanning calorimetry
(DSC). The results are summarized in Table 1. The
phases were identified through the comparison of the
observed texture with reference textures.29 Final com-
pounds 1, 2a–d and intermediates 11 and 12 are meso-
genic compounds. Compound 11 shows smectic C and
nematic phases (Fig. 2). Acrylate 1 melts into smectic
A phase, then rapidly polymerizes on a rigid solid and
consequently at higher temperatures no phases were
observed. Compounds 2a and 2b show nematic and
smectic A phases (Fig. 3). Similarly, compounds 2c
Table 1. Phase sequence, transition temperatures (°C) of mesogenic
compounds from microscopic observation
Compound
Heating
1
Cr 103.7 SmA polymerizes
2a
2b
Cr 138–140 SmA 224.2–225.6 N 241.8–244.5 I
Cr 94.7–95.4 SmC 131.13 SmA 201.6–203.1
N 210.7–212 I
2c
2d
11
12
Cr 96–98 SmC 207–209 SmA 238.7–242.1 I
Cr 79–80 SmC 184 SmA 229 I
Cr 76.9–77.2 SmC 139.4 N 198.9 I
Cr 154–155 N 275.7 I
Cr = crystal; N = nematic; SmA = smectic A; SmC = smectic C;
I = isotropic phase.
trans,trans-4,40-Bicyclohexyl-bis{4-[6-(benzyloxy)hexan-
1-oxy]benzoate} 11 or the isomeric mixtures of 4,40-
bicyclohexyl-bis{4-[6-(benzyloxy)hexan-1-oxy]benzoate}
11a was formed. Hydrogenation of 11 or 11a led to
compounds 12à or 12a, respectively. This latter treated
with acroyl chloride, in the presence of pyridine, affor-
§ Compound 1: white powder, yield 3 g (52%). Cr 103.7 SmA
decomposes. IR (KBr) m in cmꢀ1: 3040, 2943, 2852, 1703, 1699,
1604, 1273. 1H NMR (CDCl3) d (ppm): 1.17–2.17 (m, 36H), 4.01 (t,
4H), 4.18 (t, 4H), 5.82 (dd, 2H), 6.12 (dd, 2H), 6.40 (dd, 2H), 6.89 (m,
4H), 7.98 (m, 4H).
– Compound 2a: white powder, yield 2.2 g (94%). Cr 138–140 SmA
224.2–225.6 N 241.8–244.5 I. IR (KBr) m in cmꢀ1: 3030, 2942, 2851,
1745, 1698, 1242, 1047. 1H NMR (CDCl3) d (ppm): 1.12–2.16 (m,
36H), 3.78 (s, 6H), 4.0 (t, 4H), 4.16 (t, 4H), 6.89 (d, 4H), 7.98 (d, 4H).
Compound 2b: white powder, yield 2.2 g (83%). Cr 94.7–95.4 SmC
131.13 SmA 201.6–203.1 N 210.7–212 I. IR (KBr) m in cmꢀ1: 3030,
2930, 2849, 1732, 1698, 1186, 1076. 1H NMR (CDCl3) d (ppm): 1.49–
2.29 (m, 50H), 4.0 (t, 4H), 4.07 (t, 4H), 6.88 (d, 4H), 7.96 (d, 4H).
Compound 2c: white powder, yield 2.7 g (83.3%). Cr 96–98 SmC 207–
209 SmA 238.7–242.1 I. IR (KBr) m in cmꢀ1: 3040, 3025, 2938, 2862,
1770, 1699, 1078. 1H NMR (CDCl3) d (ppm): 1.49–2.12 (m, 36H),
4.12 (m, 4H), 4.4 (m, 4H), 6.88 (m, 4H), 7.98 (m, 4H).
Compound 11: white powder, yield 17 g (43%). Heating: Cr 98.8 SmC
139.4 N 198.9 I. IR (KBr) m in cmꢀ1: 3076, 3031, 2941, 2851, 1699,
1
1275, 1020, 973, 942. H NMR (CDCl3) d (ppm): 1.1 (m, 10H), 1.41
(m, 8H), 1.59–1.72 (m, 16H), 2.1 (m, 2H), 3.42 (t, 4H), 3.93 (t, 4H),
4.8 (m, 2H), 4.44 (s, 4H), 6.82 (m, 4H), 7.23 (m, 10H), 7.9 (m, 4H).
à Compound 12: white powder, yield 20 g (96%). Cr 151.7 N 273.6 I. IR
(KBr) m in cmꢀ1: 3300, 3040, 3020, 2938, 2860, 1697, 1263. 1H NMR
(CD6SO) d (ppm): 1.1–1.95 (m, 34H), 2.43 (m, 2H), 3.34 (m, 4H,
JH,OH = 5.5 Hz), 3.96 (m, 4H), 4.14 (m, 2H), 6.93 (m, 4H), 7.8 (m,
4H).

1450
R. Cassano et al. / Tetrahedron Letters 48 (2007) 1447–1450
Figure 2. Photos in polarizing light of the SmC and N phases of 11.
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Acknowledgments
The present work was supported by Sampa Project and
University of Calabria funds.
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