Z-isomers of 3,3'-disubstituted azobenzenes highly compatible with liquid crystals

Article abstract of DOI:10.1039/a808083f

It is known that mesophase changes can be induced by E-Z photoisomerization of azobenzenes doped in liquid crystals. Novel azobenzenes have been designed on the basis of Molecular Mechanics and Molecular Orbital calculation, aimed at exploiting novel photoresponsive guest-host liquid crystalline systems exhibiting no mesophase change despite the drastic structural alteration of the guest molecules. It was found that the introduction of alkanoyloxy groups at both the 3- and 3'-positions of azobenzene leads to phase stability of nematic systems upon the E-Z photoisomerization even at a dopant concentration as high as 20 wt%. However phase separation was brought about when 3,3'-dialkoxyazobenzenes and 4,4'- dialkoxyazobenzenes were employed as guest molecules. The relation between the conformational structures of the guests in their E- and Z-isomers and their compatibility with nematic hosts was examined thermodynamically. Experimental results were compared in some details with the simulations. It was shown that 3,3'-dialkanoyloxyazobenzene prefers a rod-like structure in both E- and Z-isomers.

Full text of DOI:10.1039/a808083f

J O U R N A L O F
Z-Isomers of 3,3∞-disubstituted azobenzenes highly compatible with
liquid crystals
Christian Ruslim and Kunihiro Ichimura*
Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta,
Midori-ku, Yokohama 226, Japan. E-mail: kichimur@res.titech.ac.jp
C H E M I S T R Y
Received 19th October 1998, Accepted 4th December 1998
It is known that mesophase changes can be induced by E–Z photoisomerization of azobenzenes doped in liquid
crystals. Novel azobenzenes have been designed on the basis of Molecular Mechanics and Molecular Orbital
calculation, aimed at exploiting novel photoresponsive guest–host liquid crystalline systems exhibiting no
mesophase change despite the drastic structural alteration of the guest molecules. It was found that the introduction
of alkanoyloxy groups at both the 3- and 3∞-positions of azobenzene leads to phase stability of nematic systems
upon the E–Z photoisomerization even at a dopant concentration as high as 20 wt%. However phase separation
was brought about when 3,3∞-dialkoxyazobenzenes and 4,4∞-dialkoxyazobenzenes were employed as guest
molecules. The relation between the conformational structures of the guests in their E- and Z-isomers and their
compatibility with nematic hosts was examined thermodynamically. Experimental results were compared in some
details with the simulations. It was shown that 3,3∞-dialkanoyloxyazobenzene prefers a rod-like structure in both E-
and Z-isomers.
The modification of mesophases with light has attracted
increasing attention because of providing access to a wide
variety of areas such as optical switching, holography and
optical information storage systems.1–3 Photoresponsiveness
in liquid crystal (LC) systems can be achieved by the manipu-
lation of photoreactive guest molecules, or, in some cases, the
liquid crystalline matrices themselves are photoreactive.
Generally, azobenzenes or stilbenes undergo E–Z isomeriz-
ation upon UV light irradiation. This process is reversible
through photoirradiation or heat exposure. The photoisomer-
ization is accompanied by prominent changes in the properties
of the guest molecules and consequently of the surrounding
mesophasic matrices so that the photochemistry leads to the
emergence of photo-optical effects in the systems.4 The geo-
metrical photoisomerization of stilbenes5 and azobenzenes6
dissolved in nematic LCs leads to the disordering of the LC
phase because of the predominant formation of Z-isomers
with bent structures. This is the principle for the isothermal
photochemical phase transition which has been proposed with
a view to application in optical information storage.2 Another
exploitation of these photoisomerization characteristics was
demonstrated by Sackmann7 who reported the reversible
change of the helical pitch of an azobenzene–cholesteric liquid
crystal system upon photoisomerization. Heppke et al. intro-
duced chiral para- and ortho-substituted azobenzenes in a
nematic LC and investigated the molecular twisting power of
the system which, in some cases, showed a reversible inversion
of the helical sense on isomerization.8 Hermann et al. reported
the photoinduced change of flexoelectric anisotropy as a result
of E–Z configurational change of an azobenzene dopant in a
nematic liquid crystal and hence of the distribution of the
dipole moment in the system.9 In this novel photoresponsive
LC system, a mesophase change needs to be avoided, in sharp
contrast to cases based on the structural modification of
photoresponsive guests directed towards isothermal photoch-
emical phase transition. In other words, the mesophase has to
be preserved in this case both before and after the photoisomer-
ization of guests such as azobenzenes.
stability upon photoisomerization. Our motivation in selecting
3,3∞-disubstitution was the anticipation that an appropriate
choice of substituents would give rise to the preferential
formation of conformers in both E- and Z-isomers in which
the terminal chains are stretched out to make the molecule
rod-shaped. Accordingly, without disorganizing or destroying
the liquid crystalline phase, changes in the guest molecular
structure, i.e. changes in the properties of the guest or the
entire system, can be manipulated so that they can be magnified
by high concentration doping. Although some studies of the
effect of molecular structures of azobenzenes on the nematic
phase have been made,10 all the compounds used have been
4-substituted or 4,4∞-disubstituted such as
5
and 6.
Furthermore, there has been no report on the effect of 3,3∞-
disubstitution on the E–Z photoisomerization. To our knowl-
edge, there are few detailed studies emphasizing confor-
mational changes of photoresponsive molecules upon
irradiation with light with respect to the stability of the liquid
crystalline phase.
The effect of the E–Z photoisomerization of azobenzene
dopants at high concentrations on a nematic phase is reported
first. Then, the guest–host interaction is discussed through the
In this work, we report the effect of photoisomerization of
guest 3,3∞-disubstituted azobenzenes 1–4 on the stability of
liquid crystalline hosts. Our interest is to design and develop
photoresponsive liquid crystalline systems that exhibit phase
J. Mater. Chem., 1999, 9, 673–681
673

the solvent and excess dihydropyran, the product was obtained
as a brown oil. 1H NMR (CDCl ): d(ppm) 1.68–2.08 (m, 6
3
H), 3.60–3.95 (m, 2 H), 5.50 (m, 1 H), 7.29–7.90 (m, 4 H).
3,3∞-Bis(tetrahydropyran-2-yloxy)azobenzene
This was prepared according to the literature.11 To a 1 L
three-necked flask equipped with a thermometer, a mechanical
stirrer and a dropping funnel were added dry ether (350 mL)
and 4.4 g (116 mmol) of LiAlH . The solution was kept at
4
−70 °C by using an ethanol–dry ice bath and stirred, while 3-
(tetrahydropyran-2-yloxy)nitrobenzene, diluted with dry ether,
was slowly added through a dropping funnel. After turning to
a dark-brown suspension, the reaction was allowed to warm
to room temperature gradually. After the disappearance of
the reactant, monitored by TLC, the product was extracted
with ether, and the organic phase was washed with distilled
water, dried over MgSO , concentrated and dried in vacuo to
4
give 11.4 g of product (58% from m-nitrophenol) as a dark
orange powder (mp 96–98 °C). 1H NMR (CDCl ): d(ppm)
3
1.65–1.91 (m, 12 H), 3.61–3.94 (m, 4 H), 5.54 (m, 2 H),
7.14–7.60 (m, 8 H). 13C NMR (CDCl ): d(ppm) 18.7, 25.2,
3
30.4, 62.1, 96.4, 109.6, 117.5, 119.4, 129.7, 153.8, 157.8.
3,3∞-Dihydroxyazobenzene
A solution of 3,3∞-bis(tetrahydropyran-2-yloxy)azobenzene
(11.4 g, 30 mmol) in methanol (150 mL) was refluxed with an
excess of 2% aqueous oxalic acid for 1 h, and then extracted
with ether. The product was purified through flash chromatog-
raphy (silica gel, ethyl acetate–hexane (151)) and recrystallized
from methanol to give 2.2 g (19% from m-nitrophenol) of
product as a yellow powder (mp 211–213 °C). 1H NMR
Fig. 1 Structures of nematic liquid crystal hosts and their phase
transition temperatures.
nematic–isotropic phase equilibria. The correlation of the
results with the photoinduced structural change of guest
azobenzenes is evaluated, together with the calculated stable
conformations provided by Molecular Mechanics and
Molecular Orbital calculation. The chemical formula and the
(DMSO-d ): d(ppm) 6.94–7.40 (m, 8 H), 9.84 (s, 2 H).
6
13C NMR (DMSO-d ): d(ppm) 107.1, 115.3, 118.6, 130.2,
6
153.2, 158.2. IR (KBr, cm−1): 3290. Found: C, 67.71; H, 4.83;
nematic–isotropic phase transition temperature T of the host
N, 12.75. Calc. for C H N O : C, 67.30; H, 4.67; N, 13.08%.
NI
12 10 2 2
liquid crystals used in this experiment are shown in Fig. 1.
3,3∞-Bis(hexyloxy)azobenzene 1
Experimental
This was prepared from 3,3∞-dihydroxyazobenzene (0.32 g,
1.5 mmol), 1-bromohexane (0.60 g, 3.6 mmol) and K CO
Synthesis and characterization of materials
2
3
(0.62 g, 4.5 mmol) by means of the Williamson reaction in
DMF (10 mL) at 75 °C. The product was purified through
column chromatography (silica gel, ethyl acetate–hexane
(153)) to give 0.43 g of 1 as a brownish orange solid (yield:
m-Nitrophenol, p-nitrophenol, 3,4-dihydro-2H-pyran, pyridin-
ium toluene-p-sulfonate, 1-bromobutane and 1-bromohexane
were purchased from Tokyo Chemical Industry. Butanoyl
chloride, hexanoyl chloride and dodecanoyl chloride were
purchased from Kanto Chemical and 2-methyl-3-nitrophenol
was from Sigma Chemical. All reagents were used without
further purification. Diethyl ether and THF were distilled over
76%). 1H NMR (CDCl ): d(ppm) 0.92 (t, J=6.8, 6H),
3
1.35–1.50 (m, 12H), 1.82 (quintet, J=7.8, 4 H), 4.05 (t, J=
6.6, 4 H), 7.01–7.55 (m, 8 H). 13C NMR (CDCl ): d(ppm)
3
14.1, 22.6, 25.8, 29.2, 31.6, 68.3, 106.5, 117.0, 118.3, 129.7,
lithium aluminum hydride (LiAlH ), DMF over molecular
153.9, 159.9. Found: C, 75.62; H, 8.88; N, 7.24. Calc. for
4
sieves, and dichloromethane over phosphorus pentaoxide. All
C H N O : C, 75.39; H, 8.90; N, 7.33%.
˚
24 34 2 2
solvents were stored over 4 A molecular sieves.
The chemical structures of intermediates and final products
were characterized by 1H and 13C NMR spectra, recorded on
a Bruker AC-200 NMR Spectrometer with TMS as an internal
standard (J values are given in Hz), and elemental analysis.
Melting points were determined on a Yanaco MP-S3 micro
melting apparatus, and phase transition temperatures of the
final products were determined on a DSC 22C (Seiko Densi
Kogyo) and a Polarizing Optical Microscope Olympus BH-2
equipped with a Mettler FP800 hot stage.
3,3∞-Bis(butanoyloxy)azobenzene 3a
The esterification of 3,3∞-dihydroxyazobenzene (0.43 g,
2.0 mmol) with butanoyl chloride (0.53 g, 5.0 mmol) in the
presence of triethylamine (0.61 g, 6.0 mmol) was carried out
in diethyl ether. A conventional work-up gave an orange solid
which was then purified by recrystallization from ethyl acetate
to produce 0.58 g of 3a (yield: 82%). 1H NMR (CDCl ):
d(ppm) 1.07 (t, J=7.2, 6 H), 1.76–1.88 (m, 4 H), 2.58 (t, J=
7.4, 4 H), 7.20–7.66 (m, 8 H). 13C NMR (CDCl ): d(ppm)
13.68, 18.47, 36.26, 114.96, 121.79, 124.33, 129.77, 151.45,
153.42, 171.90. Found: C, 67.66; H, 6.11; N, 7.86. Calc. for
3
3
3-(2-Tetrahydropyranyloxy)nitrobenzene
m-Nitrophenol (14.5 g, 104 mmol) was dissolved in 45 mL
CH Cl . To this solution were added 3,4-dihydro-2H-pyran
C H N O : C, 67.77; H, 6.27; N, 7.91%.
20 22 2 4
2
2
(10 g, 119 mmol) and pyridinium toluene-p-sulfonate (2.8 g,
11 mmol). The mixture was stirred at 0 °C for 30 min and
then for 2 h at room temperature. The mixture was washed
several times with 3 mol dm−3 NaOH solution and distilled
3,3∞-Bis(hexanoyloxy)azobenzene 3b
This was prepared in the same way in 85% yield. 1H NMR
(CDCl ): d(ppm) 0.94 (t, J=6.6, 6H), 1.38–1.86 (m, 12 H),
3
water, respectively, and dried over MgSO . After removal of
2.59 (t, J=7.2, 4 H), 7.19–7.56 (m, 8 H). 13C NMR (CDCl ):
4
3
674
J. Mater. Chem., 1999, 9, 673–681

d(ppm) 13.9, 22.3, 24.6, 31.3, 34.4, 115.0, 121.8, 124.3, 129.8,
151.5, 153.4, 172.1. IR (KBr, cm−1): 2952, 2930, 2870, 1758.
6.8, 6 H), 1.35–1.48 (m, 12 H), 1.82 (quintet, J=6.4, 4 H),
4.03 (t, J=6.5, 4 H), 6.98 (d, J=9.0, 4 H), 7.86 (d, J=9.2, 4
Found: C, 70.07; H, 7.66; N, 6.67. Calc. for C H N O : C,
H). 13C NMR (CDCl ): d(ppm) 14.1, 22.6, 25.7, 29.2, 31.6,
24 30 2 4
3
70.21; H, 7.38; N, 6.82%.
68.4, 114.7, 124.3, 147.0, 161.2. Found: C, 75.02; H, 8.89; N,
7.32. Calc. for C H N O : C, 75.34; H, 8.98; N, 7.32%.
24 34 2 4
3,3∞-Bis(dodecanoyloxy)azobenzene 3c
4,4∞-Bis(hexanoyloxy)azobenzene 6
This was prepared in a similar way as described above in 50%
The esterification of 4,4∞-dihydroxyazobenzene with hexanoyl
chloride gave an orange solid in 74% yield. 1H NMR (CDCl ):
d(ppm) 0.94 (t, J=7.2, 6 H), 1.38–1.86 (m, 12 H), 2.59 (t,
J=7.6, 4 H), 7.24 (d, J=8.8, 4 H), 7.94 (d, J=9.0, 4 H).
yield. 1H NMR (CDCl ): d(ppm) 0.88 (t, J=6.6, 6H),
3
1.27–1.85 (m, 36H), 2.59 (t, J=7.4, 4H), 7.19–7.84 (m, 8H).
3
13C NMR (CDCl ): d(ppm) 14.2, 22.7, 25.0, 29.2, 29.3, 29.4,
3
29.5, 29.6, 32.0, 34.5, 114.9, 121.8, 124.4, 129.8, 151.5, 153.4,
13C NMR (CDCl ): d(ppm) 13.9, 22.4, 24.6, 31.3, 34.4, 122.3,
172.1. Found: C, 74.52; H, 9.42; N, 4.85. Calc. for
3
124.1, 150.1, 152.9, 172.0. IR (KBr, cm−1): 2953, 2930, 2870,
C H N O : C, 74.69; H, 9.42; N, 4.84%.
36 54 2 4
1748. Found: C, 70.11; H, 7.42; N, 6.86. Calc. for C H N O :
24 30 2 4
C, 70.21; H, 7.38; N, 6.82%.
3,3∞-Dihydroxy-2,2∞-dimethylazobenzene
This was prepared by reductive coupling of 2-methyl-3-nitro-
phenol after protecting the hydroxy groups with 3,4-dihydro-
2H-pyran in a way similar to that for 3,3∞-dihydroxyazoben-
zene. In this case dry THF was used as solvent.
Recrystallization from methanol–ethyl acetate (151) gave a
dark orange solid (yield: 32% from 2-methyl-3-nitrophenol)
Sample preparation
Samples used in evaluating the effect of photoisomerization
on LC phase stability were prepared by doping the azobenzenes
at different concentrations in host LCs. Those for the nematic–
isotropic equilibrium analysis were prepared by varying the
concentration from 0 to 2.0 mol%. The mixtures were heated
of mp 237–238 °C. 1H NMR (DMSO-d ): d(ppm) 2.51 (s,
6
6H), 6.95–7.16 (m, 6H), 9.68 (s, 2H). 13C NMR (DMSO-d ):
6
above their T to obtain homogeneous solutions. The solu-
NI
tions were then injected to 5 mm thick LC cells, fabricated by
d(ppm) 9.8, 106.2, 116.7, 124.5, 126.2, 151.5, 156.4. IR
glass substrates covered with a uniaxially rubbed PVA film.
(KBr, cm−1): 3291. Found: C, 69.65; H, 5.90; N, 11.38. Calc.
for C H N O : C, 69.39; H, 5.84; N, 11.56%.
14 14 2 2
Photoisomerization
3,3∞-Bis(hexyloxy)-2,2∞-dimethylazobenzene 2
E–Z Photoisomerization was performed using 350 nm light
from a diffraction grating irradiator (Jasco CRM-FD) and
was confirmed by tracing the electronic absorption spectra
recorded on an HP8452A diode array spectrometer.
This was prepared from 3,3∞-dihydroxy-2,2∞-dimethylazoben-
zene and 1-bromohexane in 77% yield. 1H NMR (CDCl ):
d(ppm) 0.92 (t, J=6.6, 6 H), 1.34–1.92 (m, 12 H), 2.61 (s, 6
3
H), 4.02 (t, J=6.3, 4 H), 6.89–7.28 (m, 6 H). 13C NMR
Results and discussion
(CDCl ): d(ppm) 10.0, 14.1, 22.7, 25.9, 29.4, 31.6, 68.6, 108.2,
3
112.8, 126.1, 127.8, 151.9, 158.2. Found: C, 75.71; H, 9.32; N,
Molecular design
6.69. Calc. for C H N O : C, 76.03; H, 9.35; N, 6.82%.
26 38 2 2
Calamitic (rod-like) liquid crystals are usually composed of
rigid core units, which are linked through a linear group or
directly, with terminal chains attached to the cores so that
molecular shapes should be long and narrow.13 The E-isomer
of azobenzene is a representative mesogen with two phenyl
rings as core units that are linked through a linear -NLN-
bond, and provides a range of liquid crystalline molecules
through substitution with suitable terminal chains at the 4-
positions. On the other hand, the Z-isomers of azobenzene
and the related stilbenes possess a V-shaped structure and have
been recognized as non-mesogenic units. In fact, the introduc-
tion of terminal substituents at both of the 4-positions of these
molecules gives rise to liquid crystallinity, and this fact has
given rise to a general scheme for achieving photochemical
mesophase changes.4
Our basic idea for making the Z-isomers of azobenzenes
compatible with LC hosts was to control the conformation of
3,3∞-disubstituted azobenzenes to give the desired conformer,
in which two terminal substituents at the 3,3∞-positions stretch
out in such a way that the molecule has a rod-like shape. 3,3∞-
Disubstituted azobenzenes may have three conformations for
each isomer. The three conformations of E- and Z-isomers,
named here as l- (linear), nl- (non-linear) and sl- (semi-linear)
type, are shown in Fig. 2. The l-type is the most rod-like
structure so that it should be highly compatible with LCs,
whereas the nl-type is bent while the sl-type possesses an
intermediate structure. Taking into account the steric hindrance
due to optional substituents, it was anticipated that the intro-
duction of small substituents at the 2- and 2∞- positions would
enhance the possibility of generating rod-like structures.
On the basis of the rough sketch of the molecular design,
stable conformations of the isomers were optimised using
CAChe (Computer Aided Chemistry) version 3.0.14 The
3,3∞-Bis(hexanoyloxy)-2,2∞-dimethylazobenzene 4
This was prepared from 3,3∞-dihydroxy-2,2∞-dimethylazoben-
zene and hexanoyl chloride (0.47 g, 3.5 mmol) in 67% yield.
1H NMR (CDCl ): d(ppm) 0.95 (t, 6 H), 1.35–1.90 (m, 12
3
H), 2.55 (s, 6 H), 2.64 (t, J=7.4, 4 H), 7.11–7.53 (m, 6 H).
13C NMR (CDCl ): d(ppm) 10.6, 14.0, 22.4, 24.8, 31.4, 34.3,
3
113.6, 124.4, 126.3, 131.0, 150.4, 152.0, 172.0. IR (KBr, cm−1):
2955, 2928, 2870, 1758. Found: C, 70.87; H, 8.01; N, 6.22.
Calc. for C H N O : C, 71.19; H, 7.83; N, 6.39%.
26 34 2 4
4,4∞-Dihydroxyazobenzene
Bogoslovskii’s method was applied.12 In 50 mL of distilled
water was dissolved 14.2 g of CuSO Ω5H O. Aqueous
4
2
ammonia was added to the solution until a dark-blue water-
soluble complex was formed. Then, aqueous NH OHΩHCl
was introduced to form the colorless Cu() complex. To this
solution was added dropwise the diazonium salt of p-amino-
phenol (5.5 g, 50.5 mmol) solution prepared in advance. After
stirring for 1 h, the product was extracted with ether, washed
2
with water and dried over MgSO . The solvent was removed,
4
and the collected solid was recrystallized from MeOH to give
1.35 g (25%) of reddish-brown solid of mp 223–224 °C.
1H NMR (DMSO-d ): d(ppm) 6.90 (d, J=8.8, 4H), 7.70 (d,
6
J=8.6, 4H), 10.10 (s, 2H). 13C NMR (DMSO-d ): d(ppm)
6
115.8, 124.1, 145.3, 160.0. IR (KBr, cm−1): 3186.
4,4∞-Bis(hexyloxy)azobenzene 5
This was prepared from 4,4∞-dihydroxyazobenzene and
1-bromohexane and purified by recrystallization from ethyl
acetate in 67% yield. 1H NMR (CDCl ): d(ppm) 0.91 (t, J=
3
J. Mater. Chem., 1999, 9, 673–681
675

Table 1 Phase transition temperatures of the guests
Compounds
Phase-transition temperature/°C
1
K
K
K
K
K
K
K
K
31
58
96
89
97
54
86
120
I
S
I
I
I
S
S
S
2
105
I
3a
3b
3c
4
5
6
76
98
126
I
N
I
110
I
aK: crystalline, S: smectic, N: nematic, I: isotropic.
ited LC phases, which were identified as the smectic phases
through DSC measurement and polarizing optical microscopy.
Further discussion of the LC phases of these materials will be
presented elsewhere.
Photoisomerization characteristics
Absorption spectra of 3,3∞-disubstituted azobenzenes (Fig. 3(a)
and (b)) were compared to those of azobenzene and 4,4∞-
disubstituted azobenzenes (Fig. 3 (c)) in hexane. As can be
seen, l
values for the p–p* absorptions of 3,3∞-disubstituted
max
azobenzenes and their molar extinction coefficients e at l
are similar to those of azobenzene, whereas 4,4∞-disubstituted
max
Fig. 2 Three conformational structures of E- and Z-3,3∞-disubstituted
azobenzenes and the model compounds used in the Molecular
Mechanics and Molecular Orbital calculation. The arrows indicate
the rotation of the flexible spacers used as search parameters (dihedral
angles) for optimization.
Molecular Mechanics calculations were performed using the
MM2 parameter, whereas the Molecular Orbital calculations
were performed with the semi-empirical MOPAC PM3 param-
eter. Special efforts were made in the Molecular Mechanics
calculation to take into account the rotations about the two
C–O bonds between the benzene rings and the spacers as
shown by the arrows in Fig. 2. Four model compounds, 1∞–4∞,
were used in the calculation (see also Fig. 2). The conformations
with the lowest energy calculated by MM2 were then used as
initial structures for the PM3 calculation in order to obtain
the optimized structures as well as the magnitudes of the heats
of formation.
Synthesis and characterization of azobenzene derivatives
The synthesis of the intermediate 3,3∞-dihydroxyazobenzene
has already been reported by Ruggli and Hinovker,15 who
used zinc–NaOH solution as a reductive coupling agent to
reduce m-nitrophenol. The same procedure was carried out,
but it was found that the product also contained 3,3∞-
dihydroxyazoxybenzene. This led us to develop an alternative
method using LiAlH reduction of the aromatic nitro com-
4
pounds.16 This method was applied successfully in the synthesis
of 3,3∞-dihydroxyazobenzene, after first protecting the hydroxy
groups of m-nitrophenol with 3,4-dihydro-2H-pyran.17
Alkylation and esterification gave the desired azobenzenes
(1–4).
The precursors to 4,4∞-disubstituted azobenzenes, the 4,4∞-
dihydroxyazobenzenes, can be prepared by the diazo-coupling
of phenol with p-aminophenol.18 However, the yield was
unsatisfactory (less than 10%). Instead the Bogoslovskii reac-
tion,12 which is a specific reaction for the preparation of o,o∞-
dihydroxyazo compounds, was used to prepare 4,4∞-dihydroxy-
azobenzene in a relatively good yield and this was transformed
into 5 and 6.
The thermal analyses of the azobenzene derivatives were
performed using DSC at heating and cooling rates of 2 °C per
minute. The results are summarized in Table 1. It should be
stressed here that some 3,3∞-disubstituted azobenzenes exhib-
Fig. 3 Absorption spectra of ca. 10−5 M hexane solution of (a) 1
(solid line), 2 (dashed line) and 4 (dotted line); (b) 3a (solid line), 3b
(dashed line) and 3c (dotted line); (c) azobenzene (solid line), 5
(dashed line) and 6 (dotted line).
676
J. Mater. Chem., 1999, 9, 673–681

derivatives, UV light irradiation induced only a change in T
NI
for all cases, whereas the LC phases were maintained. A
distinctive effect of Z-isomer conformations between 3,3∞-
disubstituted and 4,4∞-disubstituted azobenzene on LC phase
stability was observed in systems loaded with 20 wt% of
azobenzenes. Fig. 6 shows the photoinduced phase change of
an LC system containing 20 wt% of 4,4∞-disubstituted azobenz-
enes 5 and 6, respectively, as a result of the formation of Z-
isomers. Because of the solubility limitation in the higher
loading, LC cells were heated at temperatures below T of
NI
the corresponding mixtures and subjected to UV irradiation.
The micrographs in this experiment were taken at the tempera-
ture where the LC phase showed the best homogeneity, which
was approximately several degrees below the T of the mixed
NI
systems. It was obvious that phase separation occurred in both
systems irrespective of the nature of the p-substituents. Cooling
the systems resulted in the appearance of crystalline domains.
Fig. 7 shows the results for the systems with 3,3∞-bis(hexyl-
oxy)azobenzene 1 and its derivative substituted with two
methyl groups at the 2- and 2∞-positions 2, respectively. Unlike
the systems containing 5 or 6, which lost almost all the LC
domains, these systems, although showing phase separation,
still maintained large proportions of the LC domains.
Although a 20 wt% loading seemed to exceed the solubility
limit of the 2–DON-103 system and resulted in crystalline
domains in the homogeneous cell before light exposure, the
solubility increased after UV irradiation due to Z-isomer
formation.
Essentially, no phase change was induced photochemically
when the Z-isomer was substituted with hexanoyloxy groups
at the 3- and 3∞-positions (3b), as shown in Fig. 8, although
the preirradiation phase was not totally homogeneous due to
the limited solubility. The introduction of two methyl groups
at the 2- and 2∞-positions displayed interesting results, as
expected. The 4–DON-103 system showed amazingly stable
phase maintenance during photoisomerization, as shown in
Fig. 8(c) and (d). The solubility in the LC was quite high so
that homogeneous alignment without any defect was observed
before UV irradiation. After UV irradiation, the homogeneous
alignment was mostly maintained, suggesting that the methyl
substitution at the ortho-position leads to the stabilization of
nematic phase to a larger extent, despite the E–Z photoisomer-
ization. Another interesting feature of this system is the fact
that the temperature range of the nematic phase after photoiso-
Fig. 4 Absorption spectra of 3b during irradiation with 350 nm UV
light at initial state (solid line), in the middle of reaction (dashed
line), at photostationary state (dotted line), in (a) hexane solution and
(b) nematic EXP-CIL within an LC cell.
azobenzenes show a larger e with l
at a longer wavelength.
max
Z-Fractions at photostationary states under illumination with
350 nm light in hexane were more than 90% for all the
azobenzenes. The absorption spectral changes during photoiso-
merization for 3b in hexane and in a nematic EXP-CIL are
shown in Fig. 4. The photoisomerization characteristics in the
nematic phase were similar to those in solutions.
Effect of photoisomerization on guest–host systems
Each guest azobenzene (concentration: 5, 10 and 20 wt%) was
doped in DON-103 (T =73.5 °C), and these binary systems
NI
merization was around room temperature (T =35.7 °C for
were injected into empty LC cells, the inner surfaces of which
NI
20 wt% doped system). These results suggested that Z-isomer
were covered with a rubbed poly(vinyl alcohol) thin film to
generate a homogeneous alignment. The E–Z photoisomeriz-
ation in these highly doped systems was monitored by the
n–p* absorption changes upon irradiation with 350 nm light,
as shown in Fig. 5. It was confirmed that sufficient conversion
into the Z-isomer takes place in all cases.
could fit well with the host.
Nematic–isotropic equilibria of a guest–host LC
Though many studies have been carried out on the mesophase
changes induced by E–Z photoisomerization of doped azob-
enzenes from an empirical standpoint to the best of our
knowledge, no quantitative discussion has been made of the
guest–host liquid crystal systems displaying photoinduced
mesophase changes. This has led us to perform quantitative
studies on photoresponsive guest–host liquid crystal systems
exhibiting a photo-optical effect by means of thermodynamic
measurements, in order to obtain further insight into the
structure–property relationship of the systems.
In binary systems containing less than 10 wt% of azobenzene
With minor modifications, the Flory-Huggins theory can be
adapted to describe nematic–isotropic phase equilibria.19,20
The change of the nematic–isotropic phase transition tempera-
ture at an infinitely low guest concentration is the function of
heat of transition of a nematic host and the activity coefficient
of a dopant in both nematic and isotropic phases. At low
concentrations of azobenzene, there exists a linear relationship
between the concentration of the azobenzene guest and the
change of T , defined as reduced temperature [eqn. (1)],
NI
Fig. 5 Absorption spectra of ca. 20 wt% 3b in DON-103 before (solid
line) and after (dotted line) irradiation with UV light.
T
=T /T
red
(1)
0
J. Mater. Chem., 1999, 9, 673–681
677

Fig. 6 Polarized micrographs of (a) 5–DON-103 before irradiation at 75 °C, (b) after irradiation at 20 °C; and (c) 6–DON-103 before irradiation
at 75 °C, (d) after irradiation at 20 °C, with UV light. The dopant concentration is ca. 20 wt%. h indicates the angle between analyzer and
rubbing direction.
Fig. 7 Polarized micrographs of (a) 1–DON-103 before irradiation at
20 °C, (b) after irradiation at <20 °C; and (c) 2–DON-103 before
Fig. 8 Polarized micrographs of (a) 3b–DON-103 before irradiation
irradiation at 57 °C, (d) after irradiation at 22 °C, with UV light. The
at 45 °C, (b) after irradiation at <20 °C; and (c) 4–DON-103 before
dopant concentration is ca. 20 wt%. h indicates the angle between
irradiation at 46 °C, (d) after irradiation at 30 °C, with UV light. The
analyzer and rubbing direction.
dopant concentration is ca. 20 wt%. h indicates the angle between
analyzer and rubbing direction.
where T and T are T of the guest–host LC system and of
0
NI
the pure nematic host, respectively. Fig. 9 shows this linear
relation for the 3b–DON-103 system. The existence of a two-
phase region where the nematic and isotropic phases coexisted
was confirmed in all of our systems, as first reported by de
Kock.11 The slope of the nematic–isotropic boundary, b, is a
parameter connected with the compatibility of guests with the
host. The higher the b value, the more compatible the guest is
with the host. This fact has also been described from the
viewpoint of the order parameter, S, by other workers.10,19
Here, TN and TI are temperatures at which the first drop
red
red
of isotropic liquid appears and the last nematic drop disappears
on heating, or the last drop of isotropic liquid disappears and
the first nematic drop appears on cooling, and x is the mole
g
fraction of a dopant (see Fig. 9). Regardless of the difference
in broadness of the two-phase areas, we derived the values of
the slopes for our systems. In this case the slope b is the
average value of bN on heating and bI on cooling. The systems
were then exposed to 350 nm UV light. After confirming that
the E–Z photostationary state is reached by monitoring UV
av
Fig. 9 The boundary of nematic–isotropic phase of 3b–EXP-CIL
system, (%) T N , (#) T I on heating and (1) T I , (6) T N
red.
red.
red.
red.
on cooling.
678
J. Mater. Chem., 1999, 9, 673–681

as host, thermal Z–E isomerization took place during measure-
ments because of the high T
.
NI
To discuss guest–host compatibility in more detail, the
broadness of the two-phase region and the heat of transition
of the host have to be considered. Therefore, the transform-
ation of thermodynamic properties to those of the infinitely
dilute solution, which was described by Kronberg et al.,19 is
necessary. The transformation formulas are given in eqn. (2)
and (3):
bN
bN2=
(2)
k−k2
1+
A
(bN)−1+(bI)−1
B
bI
bI2=
(3)
k−k2
1−
A
(bN)−1+(bI)−1B
where, k=(bN)−1−(bI)−1, and k2=DH /RT . DH is the
0
0
0
heat of nematic–isotropic transition of the pure host. DH
0
values for nematic EXP-CIL, 5CB and DON-103 are 956.34,
748.20 and 978.24 J mol−1, respectively. Another useful par-
ameter r Dx is related to activity coefficients of the guest c
2
g
and b through eqn. (4)
1n (bN2/bI2)=1n (c N2/c I2)=r Dx
(4)
g
g
g
r Dx corresponds to the interchange free energy parameter of
the guest and host. The thermodynamic data are listed in
Table 2. As can be seen from the Table, 4,4∞-disubstituted
azobenzene–EXP-CIL systems (runs 2 and 3) enhanced the
stability of the system strongly, being indicated by small values
g
Fig. 10 Average nematic–isotropic phase boundaries of the guests (#)
azobenzene, (%) 6 and (6) 4 in nematic EXP-CIL (a) before and
(b) after UV light irradiation.
absorption spectra, T was again measured. The values of
of r Dx. However, when photoisomerized, a large increase in
the value was observed. This can be understood by considering
the conformational change from the E- to Z-form which
NI
g
the slope b before and after irradiation are shown in Fig. 10.
av
The thermal Z–E reversion during the measurements can be
ignored in the case of EXP-CIL or 5CB as host, due to their
induces the disordering of the system and may lead to phase
T
being close to room temperature. In the case of DON-103
separation as already shown above. The difference in r Dx for
NI
g
Table 2 Thermodynamic data of guest–host nematic LC systems before and after UV light irradiation
Before UV light irradiation
Run
Guest
Host
bN
bI
bN2
bI2
r Dx
g
1
2
3
4
5
6
7
8
9
Azobenzene
EXP-CIL
EXP-CIL
EXP-CIL
EXP-CIL
EXP-CIL
EXP-CIL
EXP-CIL
EXP-CIL
EXP-CIL
EXP-CIL
5CB
−0.46
0.23
0.16
−0.42
0.29
0.20
−0.48
0.25
0.18
−0.40
0.27
0.19
0.17
5
−0.10
−0.07
0.31
0.19
0.31
0.33
0.35
—
6
1
−0.90
−0.58
−0.93
−0.94
−1.05
—
−0.60
−0.66
−0.43
−0.77
−0.45
−0.75
−0.80
−0.83
—
−0.51
−0.59
−0.37
−0.98
−0.56
−0.98
−1.03
−1.11
—
−0.62
−0.69
−0.43
−0.71
−0.47
−0.71
−0.74
−0.78
—
−0.50
−0.53
−0.37
2
3a
3b
3c
3b*a
4
10
11
12
0.21
0.27
0.15
4
4
DON-103
After UV light irradiation
Run
Guest
Host
bN
bI
bN2
bI2
r Dx
g
1
2
3
4
5
6
7
8
9
Azobenzene
EXP-CIL
EXP-CIL
EXP-CIL
EXP-CIL
EXP-CIL
EXP-CIL
EXP-CIL
EXP-CIL
EXP-CIL
EXP-CIL
5CB
−1.41
−1.44
−1.62
−1.18
−1.32
−1.13
−1.26
−1.26
−1.21
−0.85
−0.83
−0.65
−1.14
−1.09
−1.23
−0.97
−0.96
−0.89
−0.99
−0.83
−1.00
−0.69
−0.70
−0.56
−1.63
−1.60
−1.85
−1.31
−1.40
−1.21
−1.38
−1.24
−1.36
−0.88
−0.85
−0.67
−1.01
−1.00
−1.09
−0.88
−0.91
−0.83
−0.91
−0.84
−0.90
−0.66
−0.58
−0.53
0.48
0.47
0.53
0.40
0.42
0.38
0.42
0.38
0.41
0.29
0.39
0.23
5
6
1
2
3a
3b
3c
3b*a
4
10
11
12
4
4b
DON-103
aPure Z-isomer of 3b separated by column chromatography. bThe data after irradiation involved approximately 10% of thermal Z–E
isomerization.
J. Mater. Chem., 1999, 9, 673–681
679

these systems was very high (more than 0.5) compared to
other systems.
implying that the E–Z photoisomerization proceeds to a great
extent in the LC host.
Let us now compare 3a, 3b, and 3c–EXP-CIL systems (runs
6, 7 and 8) which are different only in the length of the flexible
chains. In their E-forms, extending the chain slightly increased
The effect of the host was examined by comparing systems
containing 4 (runs 10, 11 and 12). Obviously, hosts with higher
nematic–isotropic transition enthalpy DH (DON-103>EXP-
0
the r Dx. However, such a relationship was not observed after
CIL>5CB) are much more stable because of the high nematic
g
irradiation. It is proper to assume that in the range of these
phase ordering. However, in order to suppress the thermal
alkyl sizes, the difference of the chain length is not an essential
Z–E reverse isomerization, hosts with relatively low T were
preferable in the experiments. The features in the 4–DON-103
system (run 12) involved approximately 10% of this effect as
NI
factor influencing the r Dx. For a non-mesomorphic solute–
g
nematic system, the independence of n-alkane size and the
order parameter has been described.18 The same conclusion
seems to be appropriate also for our mesomorphic solute–
nematic systems. After UV irradiation, these samples possess
monitored by UV absorption spectra after T measurement.
NI
Correlation between calculated and experimental data
smaller values of r Dx compared to 5 or 6–EXP-CIL systems.
g
The difference from those before UV irradiation was less than
Rod-like structures of molecules are essential in creating a
calamitic LC phase, which can be induced by introducing
flexible substituents to the ends of a mesogenic unit. In the
case of photoresponsive azobenzenes, extending the 4,4∞-pos-
ition is a well-known method in creating liquid crystalline
substances, but the formation of its bent Z-isomers will destroy
the LC phase to a certain extent. We have performed molecular
orbital calculations and shown that LC systems with a new
type of dopants, the 3,3∞-disubstituted azobenzenes, can exhibit
phase stability upon E–Z isomerization. Good correlation was
obtained between calculated and experimental results, as men-
tioned above. The dihedral angles and the heat of formation
were calculated with the procedure described above, and the
results are summarized in Table 3. The signs of the dihedral
angles indicate the relative rotational position between the
two planes, positive being clockwise and negative being
anti-clockwise rotation.
0.1. The same result was also confirmed for the 4–EXP-CIL
system (run 10), in which methyl groups are attached to the
2- and 2∞-positions of the azobenzene core. This suggests that
the Z-conformers are nearly as compatible with the host as
the E-conformers. In fact, 4-containing systems show the
smallest r Dx among those with the 3,3∞-disubstituted azobenz-
g
enes, which support our prediction of the effectiveness of steric
hindrance caused by a methyl group to the stability of the
rod-like Z-structure.
In the case of the system containing 3,3∞-disubstituted
azobenzene with ether groups, 1 (run 4), the change of r Dx
g
was no larger than for those with ester groups, 3 or 4, while
that for 2 (run 5) was somewhat larger but still smaller than
those for 4,4∞-disubstituted azobenzenes, 5 and 6. These inter-
mediate values may be the reason for the slight photo-induced
phase separation as demonstrated above. This behavior will
be explained later by comparison with the theoretically stable
Z-conformers of the corresponding guests.
Another comparison was made for 3b and 3b*–EXP-CIL
systems (runs 7 and 9) where 3b* is a pure Z-isomer of 3b,
which was isolated and purified by column chromatography
on silica gel. The two systems were identical in all features,
In the model compound, 1∞, there were no significant
differences in the heat of formation between l-, nl- and sl-type
conformations in either the E- or Z-isomer. In the E-isomer
of 3∞, the differences seem to be more pronounced. The E-l-
and E-nl-type were likely to be more stable than the E-sl-type.
The same tendency was also observed among the Z-confor-
Table 3 Dihedral angles and heat of formation, calculated by PM3 for the four model compounds
Dihedral angles (°)
Heat of
Model
compound
formation/
kcal mol−1
C(1)-NLN-C(1∞)
C(4)-C(3)-O-C
C(4∞)-C(3∞)-O-C∞
NLN-C(1)-C(2)
NLN-C(1∞)-C(2∞)
1∞
E-l type
E-nl type
E-sl type
Z-l type
Z-nl type
Z-sl type
−180.0
−178.9
−180.0
−5.6
−3.7
0
−1.0
1.4
−0.9
−4.0
−4.6
176.6
−2.5
177.7
−0.6
−2.7
179.7
179.7
−169.8
0.8
171.0
179.0
−3.4
139
−44.2
45.8
15.99
15.80
15.79
19.29
18.92
18.51
3.9
141.3
135.9
51.3
2∞
E-l type
E-nl type
E-sl type
E-l type
Z-nl type
Z-sl type
−177.0
−178.9
179.6
−4.0
−1.7
6.8
−2.2
2.6
0
−8.8
−2.7
−92.3
−6.7
9.9
8.9
0.3
6.8
−157.0
−167.0
34.4
139.5
136.6
156.2
−14.0
−32.3
136.9
−61.4
−55.0
6.30
7.87
9.34
8.47
9.12
11.77
0.9
−56.6
3∞
E-l type
E-nl type
E-sl type
E-l type
Z-nl type
Z-sl type
175.4
−176.0
176.8
4.6
−109.3
−161.3
−51.0
−15.9
138.2
48.7
−50.6
26.6
28.3
10.7
−171.7
31.0
164.9
149.8
−18.0
−137.8
−135.5
−45.0
−69.62
−67.55
−68.04
−63.50
−64.94
−62.94
21.5
−141.3
0.4
2.3
48.4
−150.4
−146.2
−43.0
4∞
E-l type
E-nl type
E-sl type
E-l type
Z-nl type
Z-sl type
180.0
177.3
−177.6
0.9
0.9
−5.7
−49.5
38.8
60.3
59.7
45.3
42.9
48.3
43.4
48.4
64.8
40.9
49.4
−169.2
140.6
−24.9
−134.5
134.0
170.0
45.8
28.0
−78.30
−76.67
−75.29
−78.37
−74.81
−73.45
−138.4
61.2
57.9
57.8
680
J. Mater. Chem., 1999, 9, 673–681

reason that the compound with the ester groups is more
rod-like than that with the ether groups.
Conclusion
The molecular design and synthesis of novel type
photoresponsive dopants, 3,3∞-disubstituted azobenzenes, were
performed with the aim of developing a photoresponsive
nematic LC system that exhibits phase stability during photo-
isomerization. We discovered that appropriate substituents
realize our objective. The guests, 3 and 4, are the best
candidates since they, in both E- and Z-conformers, exhibit
the lowest interchange free energy in the host LC, and thus
maintain the phase stability of the system, which is not
observable in the case of 4,4∞-disubstituted azobenzenes.
Stable E- and Z-conformations of each guest were calculated
by Molecular Mechanics and Molecular Orbital methods. The
calculated molecular structures and their heats of formation
were compared to the thermodynamic properties of experimen-
tal data to reveal a reasonable correlation between them. The
results also suggested that substituent dipoles and steric hin-
drance play an important role in controlling the conformation
of the novel azobenzenes.
Fig. 11 Structures of E-4∞ and Z-4∞.
mers. Comparing these two model compounds, it is reasonable
to say that 3∞ was more likely to adopt a rod-like structure
(the l-type isomers), although some portion of non-rod-
like structures might coexist due to the small energy
differences. The non-rod-like structures can possibly be sup-
pressed to some extent when this compound is in an LC
environment, because of the interaction with the rod-like
LC molecules. This is in agreement with the experimental
results for 3 containing systems (Fig. 8 (a), (b) and run 7
in Table 2).
In accord with our expectations, the rod-like-generating
effect is intensely pronounced when relatively small substitu-
ents such as methyl are attached to the 2- and 2∞-positions of
the 3,3∞-disubstituted azobenzene. In model compounds, 2∞
without the dimethyl and 4∞ with dimethyl groups, it is obvious
that the E-l-types of both compounds are preferred. The Z-l-
and Z-nl-types of 2∞ may exist as relatively stable conformers
since the energy difference is somewhat small. However, the
Z-l-type of 4∞ was undoubtedly the most stable one among the
conformers of this compound, as indicated by the significant
difference in the heat of formation. This is also supported by
the fact that the E-l-type and Z-l-type of 4∞ exhibit equal
values of heat of formation. Notice also that these two isomers
have elongated rod-like structures as shown in Fig. 11. The
References
1
2
3
4
M. W. Gibbons, T. Kosa, P. P. Murohay and P. J. Shannon,
Nature, 1995, 377, 43.
T. Sasaki, T. Ikeda and K. Ichimura, Macromolecules, 1992, 25,
3807.
R. H. Berg, S. Hvilsted and P. S. Ramanujam, Nature, 1996,
383, 505.
K. Ichimura, in Photochromism; Molecules and Systems, ed.
H. Durr and H. Bouas-Laurent, Elsevier, Amsterdam, 1990,
¨
p. 903.
5
W. W. Haas, K. F. Nelson, J. E. Adams and G. A. Dir,
J. Electrochem. Soc., 1974, 121, 1667.
6
7
8
S. Tazuke, S. Kurihara and T. Ikeda, Chem. Lett., 1987, 911.
E. Sackmann, J. Am. Chem. Soc., 1971, 93, 7088.
G. Heppke, H. Marschall, P. Nurnberg, F. Oestreicher and
G. Scherowsky, Chem. Ber., 1981, 114, 2501.
D. S. Hermann, P. Rudquist, K. Ichimura, K. Kudo, L. Komitov
and S. T. Lagerwall, Phys. Rev. E, 1997, 55, 2857.
9
10 D. Bauman, Mol. Cryst. Liq. Cryst., 1988, 159, 197.
11 A. C. de Kock, Z. Phys. Chem., 1904, 48, 129.
12 B. M. Bogoslovskii, J. Gen. Chem. USSR, 1946, 16, 193.
13 P. J. Collings and M. Hird, in Introduction to Liquid Crystals;
Chemistry and Physics, Taylor and Francis, London, 1997.
14 Cache Reference, ver. 3.0, CaChe Scientific Inc., USA, 1992.
15 P. Ruggli and M. Hinovker, Helv. Chim. Acta, 1934, 17, 411.
16 R. F. Nystrom and W. G. Brown, J. Am. Chem. Soc., 1948, 70,
3738.
17 N. Grant, V. Perlog and R. P. A. Sneedan, Helv. Chim. Acta,
1963, 46, 415.
18 S. R. Sandler and W. Karo, in Organic Functional Group
Preparations, 2nd edn., Academic Press Inc., 1986, ch. 14.
19 B. Kronberg and D. Patterson, J. Chem. Soc., Faraday Trans. II,
1976, 72, 1687; B. Kronberg, D. F. R. Gilson and D. Patterson,
J. Chem. Soc., Faraday Trans. II, 1976, 72, 1673.
fact that r Dx of the 4–EXP-CIL system after irradiation is
g
much smaller than that of the 2–EXP-CIL system is due to
the relatively large difference in the energy between its Z-l-
type and Z-nl- or Z-sl-type conformers. The difference in the
phase stability for these two systems, as shown qualitatively
in Fig. 7 and 8, can also be explained by means of the
difference in the heat of formation. It is worth noticing also
that the optimized structures provide the information that the
dipole moments of substituents orient in such a position that
the total dipole moments are minimised. This may be the
20 G. R. Luckhurst and G. W. Gray, in The Molecular Physics of
Liquid Crystals, Academic Press, London, 1979, ch. 10 and 11.
Paper 8/08083F
J. Mater. Chem., 1999, 9, 673–681
681