Photosensitive chiral self-assembling materials: Significant effects of small lateral substituents

Article abstract of DOI:10.1039/c6tc01103a

Novel azobenzene-based photosensitive mesogens with lactate chiral units were synthesized. In order to modify the rate of the thermal Z-E isomerization of these compounds, small lateral substituents were introduced into their core in the proximity of the azo group. The influence of lateral substitution on the kinetics of the Z-E isomerization, mesomorphic behaviour, and UV-Vis absorption spectra was studied. It was found that the position of the substituents in the azobenzene core significantly affects the rate of their thermal isomerization. The stability of Z-isomers of several studied compounds is comparable to that of compounds with complex molecular structures designed for optical data storage. Although lateral substitution influences the breadth/length ratio of the core, liquid-crystalline properties of the studied materials have been preserved.

Full text of DOI:10.1039/c6tc01103a

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Photosensitive chiral selfꢀassembling materials: significant
effects of small lateral substituents
Martin Cigl,a,b Alexey Bubnov,a Miroslav Kašpar,a František Hampl, b Věra Hamplová,a Oliva
Pacherová a and Jiří Svobodab
a Institute of Physics, The Czech Academy of Sciences, Na Slovance 1999/2, CZ-182 21
Prague 8, Czech Republic
b Department of Organic Chemistry, University of Chemistry and Technology, CZ-166 28
Prague 6, Czech Republic
Novel azobenzeneꢀbased photosensitive mesogens with lactate chiral unit were
synthesized. In order to modify the rate of the thermal Z-E isomerization of these compounds,
small lateral substituents were introduced into their core in the proximity of the azo group.
The influence of lateral substitution on the kinetics of the Z-E isomerization, mesomorphic
behaviour, and UVꢀVis absorption spectra was studied. It was found that the position of the
substituents in the azobenzene core significantly affects the rate of their thermal
isomerization. The stability of Zꢀisomers of several studied compounds is comparable to that
of compounds with a complex molecular structures designed for optical data storage.
Although lateral substitution influences the breadth/length ratio of the core, liquidꢀcrystalline
properties of the studied materials have been preserved.
Introduction
Photosensitive materials which can change their structural and optical properties in
response to light irradiation as an external stimulus are intensively studied due to their high
application potential in photonics. Photonic devices combine utilization of photons and
electrons for signal processing which brings many advantages. The most significant ones are
e.g. higher signal to noise ratio, faster data processing or multiple channel processing. Such
materials can be used for construction of devices like spatial optical modulators
(holography)1,2, high density data storage3 media and many other devices based on molecular
switches.4 Because of the increasing requirements on volume and speed of data processing,
searching for materials enabling construction of the above mentioned devices has become an
important area in the field of materials chemistry. In this respect, azobenzene structural unit
represents the most intensively studied type of photosensitive moiety used in such materials.
In addition to photonics, its reversible E/Z photoisomerization has been utilized as a light
triggered switch in a variety of soft actuators5–7, sensors8, optical9 and biomedical systems.10,11
1

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A number of photonic applications can be set up on soft selfꢀassembling materials, i.e.
liquid crystals (LCs)12ꢀ15, utilizing their functional fluidity combined with the short range
orientation ordering that results in their selfꢀorganizing nature. Usually, alignment of liquid
crystals can be changed by external stimuli like electric field or temperature. In addition,
involving the photosensitive azobenzene moiety (either present in a substance doped in LC
matrix or incorporated in the molecular core of LC) external light irradiation can serve as a
cheap, clean and easyꢀguided stimulus.16
Despite the fact that azobenzene derivatives are one of the most intensively studied
photosensitive systems, some drawbacks still persist. They do not form a biꢀstable switching
system, since the photogenerated Z-form thermally re-isomerizes (thermal relaxation) to the
thermodynamically more stable E-form even in the dark. The rate of this first order process
strongly depends on the substitution pattern of the azobenzene moiety.17,18 Moreover,
practical applications require advanced systems exhibiting either extremely fast thermal
recovery (systems for data processing) or suppressed thermal relaxation (systems for data
storage). Systems exhibiting fast recovery are usually based on pushꢀpull type azobenzenes,
i.e. azobenzenes bearing electron donor group and electron withdrawing group in 4 and 4´ꢀ
positions resp. (also referred as pseudostilbenes19). One important disadvantage of such a
system is that the particular substitution patterns of derivatives (mostly commercial azo dyes)
with reasonably fast thermal relaxation usually disable their incorporation into the molecular
core of a liquid crystal. Therefore these compounds are mostly utilized as photochromic
guests in LC hosts. Numerous such dye-doped systems have been studied so far20,21 but they
still suffer from either low solubility of the azo dye in the LC host or aggregation of dye
molecules during switching cycles. Much effort has also been spent on development of azo
compounds with a long living Z-isomer suitable for data storage applications.22 A typical way
to increase the stability of Zꢀconfiguration involves an introduction of steric hindrance in the
proximity of azo group by means of bulky lateral substituents23 or by bridging the azobenzene
moiety thus forming cyclic or macrocyclic structures.24 In some special cases, the Z-isomer
becomes even thermodynamically more stable than the E-isomer and thermal relaxation
proceeds in opposite direction.25,26 However, the introduction of bulky substituents into the
molecular core of rodꢀlike LC may lead to the destabilization of mesophases or even to the
loss of mesomorphic behavior.27 Therefore these bulky structures can serve as photochromic
guests only, just like azo compounds with fast thermal relaxation. As stated above, such hostꢀ
guest systems suffer with many problems.
2

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Unlike the above cited studies dealing mainly with complex macrocyclic structures
requiring sophisticated multiꢀstep syntheses, we have recently studied28 photosensitive chiral
azo compound Ia (see Fig. 1 for the general structure) with a simple molecular structure
exhibiting rich mesomorphism in a broad temperature interval. Mesogen Ia represents a
typical pushꢀpull type azobenzene. Owing to their nonꢀsymmetric electron distribution, these
substances possess a longitudinal dipole moment which makes them very useful building
blocks of LCꢀcores.15 Pushꢀpull azobenzene derivatives have been reported as substances
exhibiting relatively fast thermal Z-E relaxation22,29 and therefore unsuitable for data storage
applications. A question appeared on the possibility of tuning the rate of thermal ZꢀE
relaxation. The finding that mesomorphic behaviour is maintained even in the presence of
lateral methyl substituent in the molecular core28 inspired us to design and study a series of
novel liquid crystalline materials IbꢀIi possessing two identical lateral substituents (CH3, F
Cl, Br) attached in ortho positions to azo linkage in one of the aromatic units. It was
reasonable to assume that a substitution in the close vicinity of the azo linkage in LCs of
general formula I may substantially influence the rate of their thermal ZꢀE isomerization due
to steric and electronic effects of the substituents. To the best of our knowledge, no similar
investigation of the influence of the pushꢀpull azobenzenes substitution on their relaxation has
been performed yet.
Fig. 1 Chemical structure of the studied compounds.
Experimental
Materials
The general synthetic route for the synthesis of compounds Ibꢀh is shown in Scheme 1.
Compound Ii was synthesized in somewhat different way, reported in the Scheme S4 in the
supporting information file. Ethyl 4ꢀaminobenzoate 1 and phenols 6, 7 and 10 were purchased
from SigmaꢀAldrich and Acros Organics. The nonꢀsubstituted material Ia was synthesized
according to the reported procedure.28 Aminobenzoic acids30 2ꢀ5 and phenols31,32 8 and 9
were obtained by modification of known synthetic procedures. All azobenzene derivatives
11bꢀi were prepared following the azo coupling procedure. In the next step, the hydroxy
3

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group of compounds 1bꢀh was protected in a form of methyl carbonate by a reaction with
methyl chloroformate to yield the protected acids 12aꢀg. Then, the chiral terminal chain was
introduced by acylation of hexyl (S)ꢀlactate with the chlorides of acids 11aꢀg. The carbonate
protecting group of 13aꢀg was removed by hydrolysis with aq. ammonia to release the
phenols 14aꢀg. Finally, the phenols 14ꢀg were coupled with 4ꢀ(dodecyloxy)benzoic acid in a
DCCꢀmediated reaction to yield the target compounds Iaꢀg. The detailed syntheses and
structural characterization of all intermediates and target compounds are summarized in the
supporting information file.
Scheme 1. Synthesis of compounds Ibꢀh.
Kinetic measurements
Kinetics of the thermal Z-E isomerizations were investigated using NMR spectroscopy,
starting from photostationary states. These were achieved by irradiation of the 10 mM
solutions of the studied compounds in CDCl3 in quartz cuvettes with a lowꢀpressure mercury
lamp (8 W sterilair BLBꢀ8, 366 nm) equipped with a filter. The changes of integral intensities
of several wellꢀseparated signals of E and Zꢀisomer were periodically monitored maintaining
the same phasing of the spectra (see supporting information file for details).
The acquired Zꢀisomer mole fraction time dependencies were fitted by the least square
procedure to the equation 1:
4

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

x0
xτ




(1)
ln
= kτ


where x0 and xτ are mole fractions of the Z-isomer at the time
τ = 0 and at the time
τ, respectively; k is the rate constant of the thermal Z-E relaxation. Equation corresponds to
the linearized form of a general first order rate equation. The reported rate constants are
averages of the values obtained from at least three measurements. Activation parameters were
obtained from Arrhenius plot of rate constants measured at least at three different
temperatures in the span from 20 °C to 50 °C.
UVꢀVis spectra
UVꢀVis spectra were recorded using Shimadzu UVꢀVIS spectrometer 1601. Spectra of 20 µM
solutions in chloroform were taken at room temperature in quartz cuvettes of optical length
1 cm.
Mesomorphic and structural properties
Observation of the characteristic textures and their changes by polarizing optical microscopy
(POM) was carried out in 12 ꢁm thick glass cells, which were filled with LC material in the
isotropic phase by means of capillary action in dark. The inner surfaces of the glass plates
were covered by IndiumꢀTinꢀOxide electrodes and polyimide layers unidirectionally rubbed,
which ensures planar alignment of the molecules. A LINKAM LTS E350 heating/cooling
stage with TMS 93 temperature programmer which enabled temperature stabilization within
±0.1 K was used for temperature control. Phase transition temperatures, melting points (m.p.),
clearing points (c.p.) and phase transition enthalpies (ꢂH) were determined by differential
scanning calorimetry (DSC ꢀ Pyris Diamond PerkinꢀElmer 7) on samples of 4ꢀ8 mg
hermetically sealed in aluminium pans in cooling/heating runs in a nitrogen atmosphere at a
heating/cooling rate of 5 K minꢀ1. The temperature was calibrated on extrapolated onsets of
melting points of water, indium and zinc. The enthalpy change was calibrated on enthalpies of
melting of water, indium and zinc.
Small angle Xꢀray scattering (SAXS) measurements have been performed to determine
structural properties of the identified mesophases. Bruker D8 DISCOVER SUPER SPEED
device with rotating Cu anode (wavelength λ = 1.5418Å) working with 12 kW power and
equipped with the elements as follows: the parabolic Göbel mirror conditioning the incident
beam and the soller slits, the LiF monochromator and the scintillation detector on the side of
the diffracted beam has been used for the studies. The Anton Paar chamber DCS 350 has been
used for the temperature control (accuracy in temperature stabilization is 0.1 K). The positions
5

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of the peaks and the intensity were determined by a computational simulation using Bruker
EVA 13 software. For compounds possessing smectic phases, the layer thickness, d, has been
determined using Bragg’s law n
λ = 2dsinθ, where d is calculated from the position of the
small angle (
θ
= 0.2o–4.5o) diffraction peaks.
Results and discussion
Thermal Z-E isomerization
There are two widely accepted mechanisms of the azobenzene thermal Z-E isomerization.29
The first one is the rotational pathway via a transition state with a single bond between
nitrogen atoms (Fig. 2, pathway a). The second one involves the inversion of the NNC angle
through a transition state with a spꢀhybridized nitrogen atom (Fig. 2, pathway b). Polarization
of the N=N bond typical for pushꢀpull azobenzenes increases the rate of the thermal Z-E
isomerization since it stabilizes the charge separation in transition states of both pathways.29,33
We expected that steric hindrance of the substituents in ortho positions to the azo linkage
combined with their electronic effects may considerably influence the rate of thermal
relaxation of Zꢀisomers. However, there were limitations for the choice of lateral substituents,
especially as far as their size was concerned – they should not supress the ability of the
substances to form mesophases. Therefore, the following set of substituents was involved into
our study: methyl group combining steric hindrance with a weak positive inductive effect and
halogens (F, Cl, Br) as electron withdrawing substituents with variable size. Iodine was
omitted because of possible photolability of iodinated materials.
Fig. 2 Isomerization of azobenzene: (a) rotation pathway, (b) inversion pathway.
The course of thermal ZꢀE isomerization fits very well the firstꢀorder reaction kinetics. As
examples, the linearized plots of isomerizations performed at 20 °C according to Eqn. 1 are
shown in Fig. 3. All the obtained kinetic data (the rate constants and activation parameters) of
the thermal Z-E isomerization in chloroform for all the studied compounds IaꢀIi are
6

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summarized in Table 1. For better clearness, halfꢀlives of isomerizations are given as well.
Since the kinetics of thermal isomerization comprises of two most likely simultaneously
operating pathways with different kinetic parameters, the data in Table 1 represent overall
kinetic parameters of the isomerization process. The data unambiguously corroborate our
assumption on the influence of lateral substitution on the rate of thermal ZꢀE isomerization.
Fig. 3 First order plots for thermal Z-E isomerization at 20 °C: (a) Ia (×), Ib(○), Ic(◊), Id(□)
Ie(+), Ii(ꢂ); (b) Ia (×), If(●), Ig (♦), Ih(■). Solid lines are fits to eqn. 1.
Depending on the substituent and its position, the rate constants of thermal ZꢀE relaxation
vary in the span of almost five orders of magnitude (Table 1). Lateral substitution in ortho
positions of the pꢀbenzoyloxy (“push”) ring of the azobenzene moiety decelerates
substantially the ZꢀE relaxation. The observed slowing down is a resultant of steric and
electronic effects of the substituents. In the case of dimethylꢀderivative Ib the deceleration
can be attributed to steric hindrance. Among the dihaloꢀderivatives IcꢀIe the maximum
slowing down was found in the case of dichloroꢀ and difluoroꢀderivatives Id and Ie,
respectively. We suppose it is the electronꢀwithdrawing effect of these substituents supressing
the charge separation in the transition state of thermal relaxation29,33 which is a predominant
factor responsible for the observed decrease in the reaction rate. On the other hand, electronꢀ
withdrawing substituents (Br and Cl) in the ring possessing “pull” ester function facilitate the
thermal ZꢀE isomerization of materials Ig and Ih due to their stabilizing effect on charge
separation in the transition state. However, relaxation of the difluoroꢀderivative Ii is much
slower in comparison with that of the unsubstituted derivative Ia. Most likely, the positive
mesomeric effect of fluorine compensates its negative inductive effect as ensues from its σm
and σ
p Hammett constants values34 (0.34 and 0.06, respectively).
Table 1 Kinetic parameters of thermal Z-E isomerization.
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comp.
k20 [sꢀ1]
Ea [kJ/mol] logA [sꢀ1] *x(Z)PSS t1/2 (20°C)
Ia
Ib
Ic
Id
Ie
If
3.6 ꢃ 10ꢀ6
2.2 ꢃ 10ꢀ7
2.2 ꢃ 10ꢀ7
9.7 ꢃ 10ꢀ8
9.5 ꢃ 10ꢀ8
7.9 ꢃ 10ꢀ6
1.1 ꢃ 10ꢀ4
7.0 ꢃ 10ꢀ5
3.6 ꢃ 10ꢀ7
88.8
107.8
103.7
97.2
10.4
12.6
11.7
10.3
11.9
13.3
12.8
13.5
13.7
0.84
0.74
0.52
0.52
0.74
0.70
0.57
0.43
0.63
53.5 h
36.5 days
36.5 days
82.7 days
84.4 days
24.4 h
106.2
101.2
94.2
Ig
Ih
Ii
1.8 h
99.1
2.8 h
113.1
22.3 days
* molar fraction of Zꢀisomer at photostationary state
Despite of the remarkable effect of the lateral substituents on isomerization rates, typical
increase in activation energy (Ea) as a consequence of lateral substitution was lower than 20
kJ molꢀ1 (except of the fluorinated mesogen Ii with 24.3 kJ molꢀ1). Unfortunately, because of
the correlation between Ea and log A given by the fact that they were obtained simultaneously
from the Arrhenius equation, it is difficult to decide, whether there is a trend between Ea and
any substituent parameter.
At this place, it must be stated that the rate constant values measured in solution can be
considered as a good estimation of the values measured in mesophases.18,35 It was already
mentioned above, that the thermal stability of the Zꢀisomers is a crucial factor for optical data
storage applications of azo compounds.22 Thermal stability of Zꢀisomers of derivatives IbꢀIe
was found to be comparable with that found for compounds of complex molecular structure
designed as materials for optical data storage23,36–42 (Table 2). From this point of view our
results are of high value considering that all the studied derivatives IaꢀIi belong to a pushꢀpull
type of azoꢀcompounds where a fast thermal Z-E relaxation is expected. It must also be
mentioned that all materials given in Table 2 exhibited only a small solvent effect on the
observed rate constants.33 This makes the comparison of the halfꢀlives in different solvents
reasonable.
Table 2. Zꢀisomer halfꢀlives compared with published materials.
τ1/2 (25°C)
[days]
comp.
solvent
Ref.
Ib
chloroform
chloroform
chloroform
chloroform
acetonitrile
toluene
17.0
20.0
Ic
Id
38.7
35.8
Ie
azophane 1
azophane 2
tBuBBAB
azopeptide
macrocycle
13.7
371.4
40.1
27.7
7.0
36
37
38
39
40
solid film
chloroform
THF
8

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xanthene hinge
1637.3
7.4
toluene
41
23
2,2´,6,6´ꢀtetraiꢀPrAB
isooctane
4,4´ꢀbis(AcNH)ꢀ2,2´,6,6´ꢀ
tetraMeOAB
14.1
DMSO
42
An indisputable advantage of the reported new materials is the preservation of liquid
crystalline behaviour. Therefore they can be used as both photosensitive LCs and
photosensitive guest with expected good miscibility with a LCꢀhost. For dichloroꢀderivative
Id we have confirmed an excellent compatibility with commercial nematic matrix E48
(Merck).43 Materials Ia and Id along with several other laterally substituted azocompounds
were also tested as chiral photosensitive dopants for induction of a cholesteric mesophase
with a phototunable helix pitch.43 Although their helical twisting power was low the
photoinduced changes of helical pitch were significant and extremely stable due to the
suppressed thermal ZꢀE isomerization and excellent compatibility with nematic host.
Mesomorphic and structural properties
For the studied materials, the sequence of mesophases and phase transition temperatures were
determined by characteristic textures and their changes observed in polarizing optical
microscope and from DSC measurements. The mesomorphic properties are summarized in
Table 3. On cooling from the isotropic phase (Iso), the nonꢀsubstituted Ia compound
possesses blue phase (BPII), the chiral nematic (cholesteric) phase (N*), frustrated twist grain
boundary smectic A phase (TGBA), orthogonal paraelectric smectic A* phase (SmA*), and
tilted ferroelectric smectic C* phase (SmC*). Microphotographs of the characteristic textures
obtained under crossed polarizers in POM for Ia compound are presented on Figure 4.
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Fig. 4 Microphotographs of the characteristic textures for Ia compound: (a) platelet texture of
the BPII phase; (b) cholesteric fanꢀshaped texture; (c) filament texture at the TGBA*ꢀSmA*
phase transition; (d) texture of the TGBA* phase; (e) fanꢀshaped texture of the SmA* phase;
(f) broken fan texture of the SmC* phase. All microphotographs (aꢀb) and (dꢀf) were obtained
on 4 µm thick sample cell with planar alignment; microphotograph (c) was obtained on a oneꢀ
surfaceꢀfree sample. Width of all the microphotographs is about 150 µm.
Lateral substitution by methyl group, bromine and chlorine atom at different positions of the
molecular core strongly affects the mesomorphic behaviour. Independently on the type and
place of the lateral substituent, the melting point and clearing point significantly decreases
with respect to that of the nonꢀsubstituted Ia compound. Similar behaviour in the case of
laterally substituted compounds with different molecular core has already been reported.44–47
All the compounds possessing methyl group (Ib, If) or chlorine atom (Id, Ih) as a lateral
substituent exhibit the orthogonal paraelectric SmA* phase down to room temperature. The
lateral substitution by bromine atom close to the chiral centre makes the ferroelectric SmC*
phase preferential down to low temperatures similarly as in ref 48. However, both laterally
dibromoꢀsubstituted compounds Ic and Ig possess fully monotropic behaviour.
Table 3 Melting (m.p.) and clearing points (c.p.) on the second heating, as well as the phase
transition temperatures (Tc) on the second cooling determined by DSC. All the temperatures
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are shown in °C. The corresponding enthalpies [ꢂH] indicated in square brackets are in
kJ/mol. “―” the phase does not exist
.
m.p
[ꢂH]
69
c.p.
[ꢂH]
131
Tc
Tc
Tc
Tc
Tc
Tc
comp.
[ꢂH]
[ꢂH]
[ꢂH]
[ꢂH]
[ꢂH]
[ꢂH]
54
[ꢀ40.25]
ꢀ34
[0.28]
ꢀ32
94
[ꢀ0.04]
120
[ꢀ0.01]
48
123
[ꢀ0.48]
56
130
[ꢀ0.76]
69
131
[a]
Ia
Ib
Ic
Id
Ie
Ie
If
Cr
Tg
Tg
Cr
Cr
Cr
Cr
Cr
Cr
SmC*
―
SmA*
SmA*
―
TGBA
TGBA
―
N*
N*
―
BPII
―
Iso
Iso
Iso
Iso
Iso
Iso
Iso
Iso
Iso
[43.69] [0.55]
44 71
[39.75] [1.07]
46 46
[50.68] [50.68]
45 56
[32.20] [1.28]
70.2 52
[39.83] [36.72]
68 73
[35.82] [0.79]
59 59
[51.95] [51.95]
51 103
[33.93] [0.23]
64.2 111.6
[40.12] [ꢀ1.23]
[ꢀ0.01]
[ꢀ0.64]
[ꢀ1.14]
42
[ꢀ1.01]
SmC*
―
―
[0.24]
21b
55
SmA*
SmA*
SmA*
―
―
―
―
[ꢀ47.69]
52
[ꢀ36.72]
14
[ꢀ1.51]
102.7
[ꢀ0.80]
35
67.6
[ꢀ0.05]
110.4
[ꢀ0.72]
72.00
[ꢀ1.36]
55.00
[ꢀ0.76]
101.00
[ꢀ0.23]
111.60
SmC*
―
―
N*
N*
N*
N*
N*
―
―
―
[ꢀ5.43]
[ꢀ0.64]
32b
―
―
―
[ꢀ39.87]
31b
[ꢀ30.53]
44.2
63
[ꢀ1.35]
81
[ꢀ0.34]
Ig
―
SmA*
TGBA
―
Ii
―
―
―
―
[ꢀ34.05]
[ꢀ1.23]
.
a can be determined from POM only; b crystallization occurs on further heating as opposite peak on DSC curve
To confirm the structure, and hence the type of the mesophases, SAXS measurements have
been performed. As an example, the results of SAXS for nonꢀsubstituted compound Ia,
namely the temperature dependence of the layer spacing, d, and SAXS signal intensity within
the whole temperature range of the smectic phases are shown in Figure 5. The compound Ia
exhibits a slight increase of d(T) in the SmA* phase on approaching the phase transition to the
SmC* phase which can be explained by the stretching of the aliphatic chains with the
°
temperature decrease. On the SAXS intensity curve there is a drop up at about 120 C
corresponding to the TGBAꢀSmA* phase transition. In the vicinity of the SmA*–SmC* phase
transition, there is a pronounced drop of the SAXS intensity at the SmA*–SmC* phase
transition followed by a continuous increase below the phase transition temperature. For about
20 K below the SmA*–SmC* phase transition, there is a decrease in d values due to increase
of the tilt angle and also due to increasing order of the long molecular axes in the plane of the
smectic layers. This decrease is followed by an increase in layer spacing values of the SmC*
phase on cooling due to stretching of the aliphatic molecule chains while approaching
crystallization.
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Fig. 5 Temperature dependence of the smectic layer spacing, d(T), determined from SAXS
measurements and related intensity of the peak maximum for compound Ia. The dashed line
stands for the temperature of the SmA*ꢀ SmC* phase transition on cooling.
UVꢀVis spectra
UVꢀvisible spectra of the studied azobenzene derivatives IaꢀIi typically show one strong
absorption band in the near UV region corresponding to the πꢀπ* transition, and one weak
absorption band in the visible region of the spectra, corresponding to the nꢀπ* transition for all
compounds of the series. As example the comparison of spectra of compounds Ia, Id and Ih
is given in Figure 6.
Fig. 6 UVꢀVis spectra of Eꢀisomers of Ia (solid curve), Id (dash and dot curve) and Ih
(dashed curve).
Lateral substitution resulted in lowering the πꢀπ* absorption and also to a slight blue shift of
the πꢀπ* band was observed for all materials except Ib (Table 4). The group of materials with
faster thermal Z-E isomerization (Ieꢀg) shows more noticeable hypsochromic effect than the
substances Ibꢀd with more stable Z-isomer. In the case of the nꢀπ* absorption band, a small
bathochromic effect of lateral substituents is observed for all compounds. All these
observations are in agreement with general principles describing substituent effects on UVꢀ
Vis absorption spectra of organic compounds.49
Table 4 Positions of UVꢀVis absorption bands of Eꢀisomers of studied compounds.
λmax π−π*
[nm]
λ
max n-π*
[nm]
comp.
Ia
Ib
Ic
Id
Ie
If
333
334
317
315
349
313
316
310
328
431
474
471
465
456
459
460
449
454
Ig
Ih
Ii
12

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DOI: 10.1039/C6TC01103A
Upon illumination with UVꢀlight (366 nm), typical changes in the UV spectra of all materials
occur due to the gradual E-Z photoisomerization, i.e. drop of the πꢀπ* absorption and a small
increase and blue shift of the nꢀπ* band as depicted for compound Ib in Figure 7.
Fig. 7 UVꢀVIS spectra of Ib before (solid curve) and after (dashed curve) illumination by
UVꢀlight (366 nm) for 15 min.
Conclusion
A series of novel azoꢀbased liquid crystalline materials has been synthesized and studied. It
has been found that the thermal relaxation can be either enhanced or significantly suppressed
by the same lateral substituents in different positions of the azobenzene unit. Resulting
suppression of the isomerization rate is comparable to reported materials with sophisticated
molecular structures designed for optical data storage. Lateral substitution also resulted in
decrease of the absorption in the near UV region, compared to nonꢀsubstituted parent
compound Ia. Preservation of liquid crystalline properties is a further important feature of the
studied compounds, however, the typical consequences of lateral substitution was lowering
the transition temperatures and reduction of the mesomorphic polymorphism. The
combination of high stability of Zꢀisomer, mesomorphic behaviour, and simple molecular
structure makes derivatives Ibꢀd very good candidates for optical data storage materials. To
the best of our knowledge, there have not been reported any rodꢀlike liquid crystalline azoꢀ
materials yet which have so stable Zꢀisomers like compounds Ibꢀd.
Acknowledgements
This work was partially funded by the Czech Science Foundation (project No. 16ꢀ12150S),
Ministry of Education, Youth and Sports of the Czech Republic (project MEYS LH15305)
and specific university research (project MSMT No 20/2015).
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