New liquid crystalline stilbene derivatives containing 1,2-dienylalkoxy chains

Article abstract of DOI:10.1080/15421400701205784

New liquid crystalline stilbene derivatives containing 1,2-dienylalkoxy chains 1 (n=7, 9, 11) have been synthesized and their mesomorphic properties have been measured by polarizing optical microscopy, differential scanning calorimetry, and absorption spectroscopy. The effect of terminal alkoxy chain length and polimerisable function on the mesomorphic behaviour is discussed.

Full text of DOI:10.1080/15421400701205784

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New Liquid Crystalline Stilbene
Derivatives Containing 1,2-
Dienylalkoxy Chains
Giuseppe Chidichimo a , Giovanni De Filpo a ,
Giuseppe Salerno a , Lucia Veltri a , Bartolo Gabriele
b & Fiore Pasquale Nicoletta b
a Dipartimento di Chimica , Università della
Calabria , Arcavacata di Rende, Cosenza, Italy
b Dipartimento di Scienze Farmaceutiche ,
Università della Calabria , Arcavacata di Rende,
Cosenza, Italy
Published online: 22 Sep 2010.
To cite this article: Giuseppe Chidichimo , Giovanni De Filpo , Giuseppe Salerno ,
Lucia Veltri , Bartolo Gabriele & Fiore Pasquale Nicoletta (2007) New Liquid
Crystalline Stilbene Derivatives Containing 1,2-Dienylalkoxy Chains, Molecular Crystals
and Liquid Crystals, 465:1, 165-174, DOI: 10.1080/15421400701205784
To link to this article: http://dx.doi.org/10.1080/15421400701205784
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Mol. Cryst. Liq. Cryst., Vol. 465, pp. 165–174, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701205784
New Liquid Crystalline Stilbene Derivatives Containing
1,2-Dienylalkoxy Chains
Giuseppe Chidichimo
Giovanni De Filpo
Giuseppe Salerno
Lucia Veltri
`
Dipartimento di Chimica, Universita della Calabria, Arcavacata di
Rende, Cosenza, Italy
Bartolo Gabriele
Fiore Pasquale Nicoletta
`
Dipartimento di Scienze Farmaceutiche, Universita della Calabria,
Arcavacata di Rende, Cosenza, Italy
New liquid crystalline stilbene derivatives containing 1,2-dienylalkoxy chains 1
(n ¼ 7, 9, 11) have been synthesized and their mesomorphic properties have been
measured by polarizing optical microscopy, differential scanning calorimetry,
and absorption spectroscopy. The effect of terminal alkoxy chain length and poli-
merisable function on the mesomorphic behaviour is discussed.
Keywords: electro-optical devices; liquid crystals; stilbenes
INTRODUCTION
Liquid crystals represent an interesting class of materials for their appli-
cations in optics and electro-optics [1,2]. The anisotropic properties of
liquid crystals allow them to be easily aligned by an electric or magnetic
field, a mechanical action (e.g. shear flow), an alignment agent. The most
common alignments are homeotropic and homogeneous, i.e. the liquid
crystal directors form, respectively, a 0ꢀ and 90ꢀ angle with the normal
to the cell substrates. Generally, a preferential direction is obtained by
MIUR, the Italian Ministry for Research and University is acknowledged for finan-
cial supports (grants PRIN 2005 and EX-60%).
Address correspondence to Giuseppe Chidichimo, Dipartimento di Chimica, Univer-
`
sita della Calabria, Arcavacata di Rende, Cosenza, 87036, Italy. E-mail: chidichi@unical.it
165

166
G. Chidichimo et al.
mean of an alignment agent (inorganic layers, Langmuir-Blodgett films,
surface gratings, surfactants, lecithins, polyimides, which are coated
onto cell substrates) and then it is changed by an electric field with a
consequent change of the optical properties of the cell. Recently, it has
been investigated the effect of UV or visible light on alignment layers
doped with dyes [3] and on the agents bearing chemical groups, which
can undergo a trans!cis isomerization [4], a photo-dimerization [4], or
UV photo-polymerization [5]. The importance of such an investigation
is that the illuminated regions can change their preferential alignment
by a light beam and the original alignment can be gained by a subsequent
illumination. This light induced alignment change has been attributed to
a different dye orientation or to a change of molecular shape. More
recently, a great interest has aroused on polymers bearing both liquid
crystalline and photo-responsive units [6]. These molecules can be homo-
geneously oriented by alignment agents owing to the presence of liquid
crystalline units. This preferential alignment can be changed via
irradiation of photo-responsive groups with a suitable frequency light
beam [7]. Such technique has allowed the development of devices with
a response time of some microseconds [7]. The synthesis of such interest-
ing polymers has been carried out starting from a polymerizable mixture,
which showed mesomorphic behaviour. The mixture in its nematic state
was aligned by agents coated on glass supports and then thermo-
polymerized. The application of UV light allowed a phase transition for
the azobenzene pendants from a homogeneous state (characteristic of
trans isomers) to an isotropic state (or disordered state, characteristic
of cis isomers). However, it is known that cis isomers of azobenzenes
are unstable, so after irradiation they tend to relax to the trans form.
Another disadvantage in the use of azobenzenes is related to their
thermal and photo-chemical degradation with a consequent formation
of free radicals, which can destroy the device [8]. On the contrary, it is
known that stilbenes are thermally and photochemically more stable
than molecules bearing azobenzene groups. In fact, their relaxation back
to the original trans form can be achieved only by irradiating the cells
with a light beam at a different frequency [9]. In the framework of our
continous interest for the development of high quality liquid crystalline
FIGURE 1 Molecular structures of synthesized molecules.

New Liquid Crystalline Stilbene Derivatives
167
monomers bearing photo-responsive units, we have recently reported the
synthesis of new liquid crystalline monomers for cationic polymerization,
consisting of stilbene derivatives containing vinyloxyalkoxy chains [10].
We now wish to report the synthesis and the study of mesomorphic
properties of a novel class of liquid crystalline stilbene derivatives 1
(see Fig. 1), bearing a dienyl function suitable for radical polymerization.
This kind of polymerization may result more indicated than others for
the growth of aligned polymer as thin films.
EXPERIMENTAL
Methods
The transition temperatures and phase assignments were determined
with a Leitz Laborlux 12 POL polarizing optical microscope in conjunc-
tion with a Linkam TH600 hot stage. The observed textures were com-
pared with those reported in the literature [11]. The enthalpies of
transitions were measured by differential scanning calorimetry using
a Netzsch 2000-T4 calorimeter with a heating–cooling rate of
10ꢀC minꢁ1
.
An evaluation of the homogeneous alignment of compounds 1 and 2
(Fig. 2) was performed by filling glass cells (5 mm thick, anti-parallel
polyimide coating, Linkam) with compounds 1 and 2 in their nematic
state and measuring the transmittance of the polarized probing He-Ne
beam as a function of the rotational angle / for the cross configuration
of the polarizer and analyzer (see Fig. 3).
Photo-isomerisation of compounds 1 was monitored by UV-VIS absorp-
tion spectroscopy with a Yasco 550 V spectrometer as a function of energy
supplied at 340 nm and 260 nm by a 200 W high-pressure mercury lamp.
FIGURE 2 Synthesis of compounds 1.

168
G. Chidichimo et al.
FIGURE 3 Experimental set up. P, polarizer; A, analyzer, n, easy axis for
homogeneous alignment; /, tilt angle.
Synthesis
All compounds 1 were synthesized according to the general route shown
in Figure 2, involving the following steps: (a) Wittig reaction between
(4-cyanobenzyl)triphenylarsonium bromide and O-protected 4-hydroxy-
benzaldehyde, followed by deprotection to give 4-[2-(4-hydroxyphenyl)-
vinyl]benzonitrile; (b) Williamson reaction between the latter and the
appropriate bromoalcohol to give alcohols 2; (c) Swern oxidation [12]
of 2 to aldehydes 3; (d) Wittig reaction between the latter and allyltri-
phenylarsonium bromide. We have already reported the synthesis of
compounds 2 with n ¼ 7, 9, and 11, which showed liquid crystalline
behaviour [10]. In the present work, we have also synthesized com-
pound 2 with n ¼ 6, which also shows a liquid crystalline phase.
All reactions were analyzed by TLC on silica gel 60 F254 or by GLC
using a Shimadzu GC-14A gas chromatograph and capillary columns
with polymethylsilicone þ 5% phenylsilicone as the stationary phase
(HP-5). Column chromatography was performed on silica gel 60
(Merck, 70–230 mesh). Starting triphenylarsine, 4-cyanobenzyl bro-
mide, 4-hydroxybenzaldehyde, 3,4-dihydro-2H-pyran, hexane-1,6-diol,
heptane-1,7-diol, nonane-1,9-diol, 11-bromoundecan-1-ol, oxalyl chlor-
ide, allyl bromide were commercially available and were used without
further purification. 4-(Tetrahydropyran-2-yloxy)benzaldehyde [13],
6-bromohexan-1-ol [14], 7-bromoheptan-1-ol [14], 9-bromononan-1-ol
[14], (4-cyanobenzyl)triphenylarsonium bromide [10], 4-[2-(4-hydro-
xyphenyl)vinyl]benzonitrile [10],(E)-4-{2-[4-(x-hydroxyalkyloxy)
phenyl]vinyl}benzonitrile derivatives 2 (n ¼ 7, 9, 11) [10], and
allyltriphenylarsonium bromide [15] were prepared according to

New Liquid Crystalline Stilbene Derivatives
169
literature procedures. (E)-4-{2-[4-(6-hydroxyhexyloxy)phenyl]vinyl}-
benzonitrile (n ¼ 6, 70%) was prepared according to the general pro-
cedure reported in ref. 10, and purified by column chromatography
on silica gel using CH2Cl2 as eluent.
General Procedure for Swern Oxidation of Derivatives 2 to 4-
{2-[4-(x-Oxoalkyloxy)Phenyl]Vinyl}Benzonitrile Derivatives 3
A solution of DMSO (5.8 mL) in anhydrous CH2Cl2 (97 mL) was added
drop wise under nitrogen to a cooled (ꢁ78ꢀC), mechanically stirred sol-
ution of oxalyl chloride (3.5 mL) in anhydrous CH2Cl2 (97 mL). After a
few minutes, a solution of 2 (20.5 mmol) in anhydrous CH2Cl2 (70 mL)
was added drop wise and the mixture was allowed to stir for additional
40 min. at ꢁ78ꢀC. After the addition of Et3N (14.3 mL), the mixture
was allowed to warm to rt, water was added and phases were sepa-
rated. The aqueous layer was extracted with CH2Cl2, and the collected
organic phases were washed several times with water and brine and
eventually dried over Na2SO4. After elimination of the solvent by
rotary evaporation, crude products were purified by column chromato-
graphy on silica gel using as eluent 9:1 CHCl3=hexane (n ¼ 6, 82%),
9:1 CH2Cl2=hexane (n ¼ 7, 75%); 9:1 CHCl3=hexane (n ¼ 9, 72%);
9:1 CHCl3=hexane (n ¼ 11, 87%).
General Procedure for the Preparation of
4-{2-[4-(Dienylalkoxy)Phenyl]Vinyl}Benzonitrile Derivatives 1
The method of Tewari [16] was employed. To a stirred suspension of
NaOH (32 mg, 1.3 mmol) in anhydrous THF (30 mL) was added under
nitrogen allyltriphenylarsonium bromide (555 mg, 1.3 mmol). After
additional stirring at rt for 0.5 h, 3 (1.3 mmol) was added and the
resulting mixture was allowed to stir at rt for 48 h. After acidification
with 0.1 N HCl (7 mL), CHCl3 was added and phases were separated.
The aqueous layer was extracted with CHCl3, and the collected
organic phases were dried over CaCl2. After elimination of the solvent
by rotary evaporation, crude products were purified by column chro-
matography on silica gel using as eluent 9:1 CHCl3=hexane (n ¼ 6,
35%); 8:2 CHCl3=hexane (n ¼ 7, 42%); 7:3 CHCl3=hexane (n ¼ 9,
53%); 6:4 CHCl3=hexane (n ¼ 11, 49%).
Chemical Characterization of Representative Products
All products were fully characterized by spectroscopic techniques and
elemental analysis. Here we report data for compounds 1, and 3 with
n ¼ 9.

170
G. Chidichimo et al.
Melting points were determined on a Reichert Thermovar melting
point apparatus and are uncorrected. Elemental analyses were carried
1
out with a Carlo Erba Elemental Analyzer Mod. 1106. H NMR and
13C NMR spectra were taken on a Bruker AC300 spectrometer and
run on CDCl3 solutions with Me4Si as internal standard and recorded
at 300 MHz and 75 MHz, respectively. Chemical shifts and coupling
constants (J) are given in ppm (d) and in Hz, respectively. IR spectra
were taken on a Perkin-Elmer Paragon 1000 PC FT-IR spectrometer.
4-[2E-(Dodeca-9E,11-Dienyloxyphenyl)Vinyl]Benzonitrile 1
(n ¼ 9)
Colourless solid. IR (KBr) 2925 (m), 2853 (m), 2224 (m), 1598 (s), 1513
(m), 1251 (s), 1173 (m), 833 (m), 757 (m), 556 (m) cmꢁ1; 1H NMR
(CDCl3) d7.64–7.59 (m, 2 H aromatic), 7.57–7.52 (m, 2 H aromatic),
=
7.49–7.43 (m, 2 H aromatic), 7.17 (distorted d, J 16.1, 1 H, CHAr),
=
6.99–6.87 (m, 3 H, aromatic þ CHAr), 6.31 (dt, J 17.1, 10.3, 1 H
dienic), 6.11–5.99 (m, 1 H dienic), 5.77–5.65 (m, 1 H dienic), 5.08 (dd,
=
=
J 17.1, 1.5, 1 H, CHH), 4.98–4.92 (m, 1 H, CHH), 3.98 (t, J 6.6, 2
H, OCH2), 2.13–2.03 (m, 2 H, CH2), 1.85–1.74 (m, 2 H, CH2), 1.52–
1.24 (m, 10 H, 5 CH2); 13C NMR (CDCl3) d159.7, 142.3, 137.3, 135.5,
132.5, 132.1, 130.9, 129.2, 128.9, 128.2, 126.5, 124.4, 119.1, 114.9,
114.6, 68.1, 32.5, 29.4, 29.3, 29.22, 29.16, 29.12, 26.0. Anal. Calcd for
C27H31NO: C, 84.11; H, 8.10; N, 3.63. Found C, 84.02; H, 8.12; N, 3.61.
4-{2E-[4-(9-Oxononyloxy)Phenyl]Vinyl}Benzonitrile 3 (n ¼ 9)
Colourless solid, mp 128–129ꢀC. IR (KBr) 2927 (m), 2859 (m), 2225 (m),
1709 (s), 1598 (s), 1513 (s), 1467 (w), 1255 (s), 1215 (s), 1174 (s), 1023 (w),
839 (s), 555 (m) cmꢁ1; 1H NMR (CDCl3) d9.76 (t, J 2.0, 1 H, CHO), 7.61–
7.56 (m, 2 H aromatic), 7.55–7.49 (m, 2 H aromatic), 7.48–7.41 (m, 2 H
=
aromatic), 7.15 (distorted d, J 16.1, 1 H, CH), 6.93–6.86 (m, 2 H aro-
=
matic), 6.92 (distorted d, J 16.1, 1 H, CH), 3.96 (t, J 6.6, 2 H, OCH2),
2.42 (td, J 7.3, 2.0, 2 H, CH2CHO), 1.85–1.72 (m, 2 H, CH2), 1.69–1.57
(m, 2 H, CH2), 1.52–1.29 (m, 8 H, 4 CH2); 13C NMR (CDCl3) d202.8,
159.7, 142.3, 132.4, 132.0, 128.9, 128.3, 126.5, 124.4, 119.1, 114.8,
110.0, 68.0, 43.9, 29.26, 29.18, 29.14, 29.07, 26.0, 22.0. Anal. Calcd for
C24H27NO2: C, 79.74; H, 7.53; N, 3.87. Found C, 79.85; H, 7.51; N, 3.88.
RESULTS AND DISCUSSION
We have synthesized compounds 1 with n ¼ 6, 7, 9, 11. The transition
temperatures and enthalpy changes of compounds 1 are reported in

New Liquid Crystalline Stilbene Derivatives
171
TABLE 1 Transition Temperatures=ꢀC and Enthalpy Changes ()=Jgꢁ1 for
Compounds 1 with different Alkoxy chain length, n. Cr, Crystal; Sm A,
Smectic A phase; N, nematic phase; I, isotropic phase
6
Cr 107.2 (100.6) I
7
9
11
Cr 47.7 (17.9) N 50.9 (14.6) I
Cr 48.8 (24.2) N 74.8 (10.1) I
Cr 81.0 (15.8) SmA 127.8 (42.5) N 134.7 (21.1) I
Table 1. All compounds 1 with an alkoxy chain length larger than 6
exhibit a nematic phase. The absence of mesomorphism for n ¼ 6
can be attributed to the fact that a short chain length does not give
enough lateral cohesion. Compounds with a long alkoxy chain length
(n ¼ 9, n ¼ 11) show a large mesomorphic range. This is because the
lateral intermolecular cohesions increase with increasing alkoxy chain
lengths stabilising in such a way the mesophases. A long chain length
(n ¼ 11) stabilizes a smectic A phase, too. It is important to note that
even precursors 2 show liquid crystalline behaviour. The transition
temperatures and enthalpy changes of compounds 2 with n ¼ 5, 7, 9,
and 11 have already been reported [10]. The transition temperatures
and enthalpy changes of compounds 2 with n ¼ 2, 3 and 6 are shown
in Table 2 (results for n ¼ 5, 7, 9, and 11 are included for complete-
ness). All compounds 2 show a nematic phase. Compound 2 with
n ¼ 5 also shows a smectic A phase.
The evaluation of the homogeneous alignment of 1 gave a repeated
pattern of valleys and peaks with a typical separation of 90ꢀ (see
Fig. 4). This confirms that cells are well homogeneously aligned. It
is known that stilbenes show appreciable absorption in the solar
spectral region. UV spectra and changes induced by the indicated per-
iods of irradiation at 340 nm are shown in Figure 5 for n ¼ 11 (0.1 M in
CHCl3 at 25ꢀC, quantum yield 0.21).
Before illumination all samples consisted of only the trans form of
the stilbene derivative, because this isomer is thermodynamically
TABLE 2 Transition Temperatures=ꢀC and Enthalpy Changes ()=Jgꢁ1 for
compounds 2 with different Alkoxy Chain Length, n. Cr, crystal; Sm A, smectic
A phase; N, nematic phase; I, isotropic phase
2
3
5
6
Cr 166.1 (68.7) N 170.5 (50.1) I
Cr 86.9 (49.6) N 100.3 (17.3) I
Cr 88.8 (40.4) SmA 94.9 (53) N 104.8 (17.7) I
Cr 127.9 (20.0) N 131.0 (84.7) I
Cr 83.4 (46.4) N 91.3 (28.7) I
7
9
Cr 91.0 (79.2) N 98.4 (33.0) I
11
Cr 98.2 (36.3) N 101.2 (67.5) I

172
G. Chidichimo et al.
FIGURE 4 Repeated pattern of valleys and peaks in homogeneously aligned
compound 1 (n ¼ 11).
FIGURE 5 Changes in the UV spectra for compound 1 (n ¼ 11) induced by
irradiation at 340 nm for different time intervals. Dotted line represents the
UV spectra of compound after 5 minutes of irradiation at 260 nm.

New Liquid Crystalline Stilbene Derivatives
173
more stable than its cis form. Upon illumination with UV light
(340 nm), all compounds 1 clearly undergo a photo-transformation to
the cis isomer in an almost complete manner. No by-products were
formed in this process, as confirmed by TLC and NMR (CDCl3) analy-
sis. The photo-transformation is manifest by the decreasing intensities
of p!pꢂ absorptions at 340 nm. No thermal back reaction to trans
isomers was found in agreement with literature [14]. Upon irradiation
with UV light at 260 nm for 5 minutes, we observed the relaxation
back to the initial state as shown by the dotted curve in Figure 5.
The photo-isomerization was also confirmed by photo-irradiation of
aligned thin cells. In fact, if a UV beam (340 nm) impinged on a homo-
geneously aligned sample in its nematic phase we observed, by using
the experimental set up described in Figure 3, a phase transition to
the isotropic state, due to the fact that the cis isomers are no longer
liquid crystalline. As shown in Figure 6, the depolarized light intensity
decreases a lot as a function of irradiation time. If such a cell is after-
wards irradiated with a 260 nm light beam, one can induce a relax-
ation back to the trans form with a consequent increase of scattered
light.
These experiments confirm the potential use of stilbene molecules
as information processing and data storage materials.
FIGURE 6 Depolarized light scattering behaviour for compound 1 (n ¼ 11) as
a function of UV irradiation at 340 nm and 260 nm.

174
G. Chidichimo et al.
CONCLUSIONS
We have successfully synthesised a novel type of liquid crystalline stil-
bene molecules having a dienyl function suitable for radical polymer-
ization. All compounds with a rather long alkoxy chain (n > 6) show a
nematic phase. The temperature range of mesomorphism is rather
large for long alkoxy chain lengths. Further investigation is now in
progress to polymerize the obtained homogeneously aligned films
and to study their optical response to a light beam.
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