The intrinsic photoferroelectric effect in the smectic C* phase of a chiral azobenzene

Article abstract of DOI:10.1039/b602060g

The ferroelectric properties in the liquid-crystalline smectic C* phase of the chiral mesogenic azobenzene (S)-4-(4-decyloxy-phenylazo)benzoic acid 4-(1-methylhexyloxy)-3-nitro-phenyl ester ('W470') were studied and found to be strongly dependent on the i

Full text of DOI:10.1039/b602060g

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www.rsc.org/materials | Journal of Materials Chemistry
The intrinsic photoferroelectric effect in the smectic C* phase of a chiral
azobenzene{
Alexander Saipa,a Mikhail A. Osipov,b Kenneth W. Lanham,c Catherine H. Chang,c David M. Walbac and
Frank Giesselmann*a
Received 10th February 2006, Accepted 15th August 2006
First published as an Advance Article on the web 4th September 2006
DOI: 10.1039/b602060g
The ferroelectric properties in the liquid-crystalline smectic C* phase of the chiral mesogenic
azobenzene (S)-4-(4-decyloxy-phenylazo)benzoic acid 4-(1-methylhexyloxy)-3-nitro-phenyl ester
(‘W470’) were studied and found to be strongly dependent on the intensity of incident light. The
light-induced trans to cis isomerization of the azobenzene chromophore which is an intrinsic
constituent of this SmC* mesogen leads to a pronounced photoferroelectric effect since the
ferroelectric SmC* ordering is considerably disturbed by the presence of bent-shaped cis-isomer
molecules: At increasing light intensity a strong decrease of both SmC* order parameters,
spontaneous electric polarization Ps and director tilt angle h, is observed. It is further seen that
even the nature of the ferroelectric transition is affected by the photoisomerization which leads to
a crossover from the nearly tricritrical transition observed in the absence of light to a typical
second-order transition. This result is interpreted theoretically taking into account a coupling
between the director tilt and the biaxial order parameter which is strongly affected by the
light-induced generation of the bent-shaped cis-isomers. The dynamics of the intrinsic
photoferroelectric effect in W470 was studied by time-resolved measurements of spontaneous
polarisation and director tilt.
SmA* phase at higher temperatures. As temperature T
Introduction
increases h and Ps continuously decrease until the non-tilted
SmA* phase is reached at the SmA*–SmC* phase transition
Ferroelectric materials which, as a result of interaction with
light, change their ferroelectric properties such as spontaneous
temperature Tc*. The connection between director tilt h and
electric polarization P can be obtained from the standard
Landau expansion of the free energy density DF around Tc*.5,6
In the absence of an external electric field (E = 0) the
electric polarization Ps or coercive field are denoted as
’’photoferroelectrics’’.1 In liquid crystals photoferroelectric
behaviour can be induced by dissolving small amounts of
photochemically isomerizable dopants in ferroelectric liquid
spontaneous polarization Ps can be expressed as:
crystalline host phases like the chiral smectic C* (SmC*)
phase.2
Ps = e0xCh
(2)
In the SmC* phase of elongated molecules the average
direction of the long molecular axis n is inclined at a certain
director tilt angle h with respect to the smectic layer normal k.
The full rotational symmetry about k is broken by the director
tilt leading in the case of chiral molecules to the polar C2
point group symmetry.3–5 As a result the spontaneous electric
polarization Ps is observed along the two-fold symmetry axis,
perpendicular to h and k:
where Ps increases linearly with the director tilt h and basically
becomes temperature dependent as a result of the temperature
dependence of h.
It has been experimentally verified that homogeneous
mixtures of photoisomerizable dyes with these ferroelectric
SmC* phases are photoresponsive ferroelectric materials where
macroscopic properties like Ps, h, or Tc* can be modulated by
irradiation with light of the appropriate wavelength and
intensity.7–15 In certain cases the ferroelectric SmC* phase can
be completely suppressed by light-induced phase transitions
into the SmA* phase or even the isotropic phase. Re-
isomerization of the photosensitive dopant molecules restores
the initial properties of the original state before irradiation
(‘dark state’). This opens the possibility to control the
properties and even the phase behaviour of ferroelectric liquid
crystal materials by light irradiation.
Ps y k 6 n
(1)
In the most common ferroelectric liquid crystals the SmC*
phase undergoes a second-order transition to the non-tilted
aUniversity of Stuttgart, Institute of Physical Chemistry, D-70569
Stuttgart, Germany. E-mail: f.giesselmann@ipc.uni-stuttgart.de;
Fax: +49 711 685-2563; Tel: +49 711 685-4460
bUniversity of Strathclyde in Glasgow, Department of Mathematics,
Livingstone Tower, 26 Richmond Street, Glasgow, UK G1 1XH
cUniversity of Colorado at Boulder, Department of Chemistry and
Biochemistry, Campus Box 215, Boulder, Colorado, 80309-0215, USA
{ Electronic supplementary information (ESI) available: full synthesis
details. See DOI: 10.1039/b602060g
The mechanism of the induced photoferroelectric effect in
these guest/host systems has recently been investigated in our
group.14,15 A basic result of these investigations is the distinc-
tion between the primary and secondary photoferroelectric
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effects.15 The primary effect originates from light-induced
changes of the ferroelectric ordering. The secondary effect is a
pure thermodynamic consequence of the shift in Tc* which
results from the photoinduced variation in the isomer com-
position of the photoferroelectric phase. We further reported
that the light-induced changes in the spontaneous polarization
and the molecular isomerization process both follow simulta-
neous mono-exponential rate laws with the same rate con-
stants and activation energies. According to eqn (2), changes
in Ps could be a result of light-induced variations in h, in C, or
both. However, it was found that the primary photoferro-
electric effect actually observed in our guest/host systems
mainly results from a light-induced variation of the bilinear
coupling coefficient C which is related to the bias of molecular
rotations in the SmC* phase.14 At least for low concentrations
of the photoisomerizable dopant no indications of a significant
photoinduced change of the director tilt were found. One of
the primary goals of the present work has been the study of a
different class of photoisomerizable smectic materials where
the director tilt can be significantly modified by the irradiation.
In this paper we now report our studies on the photo-
ferroelectric effect in the SmC* phase of a chiral azobenzene
mesogen. As such mesogens contain a photoisomerizable
azobenzene linkage as an inherent part of their mesogenic
core it is not necessary to further induce photoferroelectric
behaviour through the addition of photoisomerizable dyes.
Instead, photoferroelectricity is expected and proven to be
an intrinsic property of SmC* phases constituted by photo-
isomerizable mesogens.
observed in the polarisation microscope on cooling and under
red light (633 nm) to avoid the photoisomerization of the
azobenzene moiety is given by:
X 57.1 uC SmC* 87.3 uC SmA* 117.1 uC isotr.
The low-temperature phase designated as ‘X’ is a peculiar
optically isotropic fluid, the nature of which has not been
conclusively clarified so far.17 The W470 material was filled by
capillary action into LC test cells (E.H.C. Ltd., Tokyo, Japan)
with a 2 mm cell gap and 16 mm2 indium tin oxide (ITO)
electrodes. The sample was thermostatted and irradiated with
various intensities of monochromatic light. All experiments in
this study were carried out with light of 450 nm wavelength
where the maximum photoresponse of the W470 sample was
obtained. Light- and temperature-dependent values of the
spontaneous polarization Ps and the director tilt angle h were
measured in an electro-optic setup which was described
previously.14
Results and discussion
Photostationary state
The formation of bent-shaped cis-isomers is stimulated by
constant illumination of the W470 sample with light of fixed
intensity I and wavelength 450 nm. Typically after a couple of
minutes the photostationary state is reached where the balance
between the photoinduced trans A cis isomerization and the
thermally as well as photochemically activated cis A trans
re-isomerization leads to a stationary amount of cis-isomer
molecules, the concentration x of which increases with the
intensity I of incident light.13,14 To study the properties of
W470 in its photostationary states the spontaneous polariza-
tion Ps and the tilt angle h were continuously measured as a
function of the temperature T during cooling the sample at a
rather slow rate of 0.25 K min21. The choice of a sufficiently
slow cooling rate ensures that the sample readjusts its
photostationary state at any temperature in the range of
In clear contrast to the induced photoferroelectric effect in
guest/host systems we show that the intrinsic photoferro-
electric effect in this chiral azobenzene mesogens is mainly
connected with substantial changes of the director tilt.
Moreover, this may lead to irreversible re-orientations of the
smectic layer configuration (i.e., from the initial ‘bookshelf’ to
the ‘chevron’ configuration).
We also show that the photoisomerization may even affect
the nature of the SmA* to SmC* transition in such materials
transforming the initial tricritical behaviour—observed
without irradiation—into a typical second-order transition.
This transition is interpreted theoretically taking into account
a coupling between the director tilt and the biaxial order
parameter of the SmC phase which is very sensitive to the
presence of cis-isomers.
Experimental
In the experiments on the intrinsic photoferroelectric effect we
used the chiral azobenzene (S)-4-(4-decyloxyphenylazo)ben-
zoic acid 4-(1-methylhexyloxy)-3-nitro-phenyl ester (abbre-
viated as ‘W470’):16
Fig. 1 UV-vis spectra (absorbance A vs. wavelength l) of W470
dissolved in chloroform at 25 uC. Solid line: trans-isomer before
irradiation; dashed line: cis-isomer as obtained after 40 min irradiation
with light of 366 nm wavelength.
UV absorption spectra of the trans and cis isomers dissolved in
chloroform are depicted in Fig. 1. The phase sequence of W470
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based on the following expansion of the free energy of the
SmC* phase in terms of tilt angle h and polarization P:5,6
ꢀ
{CPhz
ꢁ
1
1
1
DF~ a T{Tcꢀ h2z bh4z ch6
2
4
6
(3)
1
P2{PE
2e0x
The first three terms in eqn (3) represent the free energy of the
non-chiral SmC phase with Landau coefficients a, b, and c.
The polarization-tilt coupling is reflected by the 2CPh term,
introducing the bilinear coupling coefficient C, which measures
the orientational ordering of transverse molecular dipoles in
the chiral SmC* phase. The P2/(2e0x) term introducing the
generalised susceptibility x and the vacuum permittivity e0 is
related to the decrease in entropy due to the polar ordering.
The last term in eqn (3) describes the coupling to a non-zero
electric field E.
Minimisation of the free energy in eqn (3) with respect to h
leads to the following equation:
ꢂ
ꢃ
1 e0xCE
T~Tcꢀz
{bh2{ch4
(4)
a
h
which relates the temperature T and the equilibrium tilt angle h
at that temperature.18,19 According to eqn (2) the product e0xC
was calculated from the ratio of the experimental Ps and h
data. The plot of the ratios Ps/h vs. temperature T in Fig. 3
shows a negligible temperature dependence so that for each
data set an average value of e0xC is taken into account. The
decrease of e0xC at increasing light intensity mainly reflects the
light-induced reduction of C due to the primary photoferro-
electric effect.14
With given values of e0xC, Tc*, and E = 2 V mm21 (the field
amplitude applied to measure h and Ps) eqn (4) is fitted to the
experimental h(T) data with the Landau coefficients a, b, and c
remaining the parameters to fit. The fit parameters a, b, and c
obtained for different intensities of incident light are listed in
Table 1 together with the respective values of Tc* and e0xC.
Fig. 2 Temperature-dependent values of (a) spontaneous polariza-
tion Ps(T) and (b) tilt angle h(T) measured in the photostationary state
at different intensities of incident light (l = 450 nm).
measurement. These temperature scans of Ps(T) and h(T) were
repeated at different light intensities I.
The results of these measurements are depicted in Fig. 2.
The Ps(T) curves in Fig. 2(a) show that W470 has its highest
polarization in the absence of light (I = 0 W cm22) where Ps
reaches 240 nC cm22 at 65 uC. This remarkably high value of
spontaneous polarization is substantially reduced under the
illumination with light [‘negative photoferroelectric effect’,
(hPs/hI) , 0]. Even far below the ferroelectric transition at
65 uC a moderate light intensity of 0.84 mW cm22 stimulates
a 55% drop in polarization from 240 nC cm22 down to
110 nC cm22. Close to the SmA*–SmC* phase transition the
secondary photoferroelectric effect is noticed from the light-
induced lowering of the transition temperature Tc*(I). Far
below the ferroelectric transition the primary photoferro-
electric effect is reflected by the flattening of the Ps(T) curve at
increasing I. The corresponding h(T) curves depicted in
Fig. 2(b) are quite similar to the behaviour of Ps(T) and give
clear evidence that also the director tilt is substantially reduced
by the illumination with light and the subsequent formation
of cis-isomers. These light-induced variations of the director
tilt angle deserve a closer analysis since they are in clear
contrast to the induced photoferroelectric effect in LC–dopant
mixtures where only negligible changes of the tilt had been
observed.14
Light-induced variations of the director tilt angle
Fig. 3 The ratio Ps/h (= e0xC) vs. temperature T of the non-irradiated
(0.00 mW cm22) and two irradiated (0.42 and 0.84 mW cm22) samples.
In the range of interest the temperature dependence of Ps/h can be
neglected in a first approximation.
Deeper insight into the consequences of the light-induced
reduction of the director tilt is obtained by a detailed analysis
of the tilt and polarization data in terms of Landau theory,
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therefore their influence on the SmA*–SmC* phase transition
may be related to the biaxial ordering in the SmC* phase. As
discussed in ref. 21 typically low values of the Landau
coefficient b in various smectic C materials19 can be explained
taking into consideration a coupling between tilt and biaxial
order of short molecular axes. In the phenomenological theory
of ferroelectric smectic C* liquid crystals tilt is described by the
pseudovector:6
Table 1 Summary of Landau parameters. Expansion coefficients a, b,
and c and phase transition temperature Tc* result from the fitting of
eqn (4) to the experimental tilt data in Fig. 2. Values of e0xC are
directly taken from the experiments in Fig. 3
I/mW cm22
a/103 J m23 K21
b/103 J m23
c/103 J m23
Tc*/uC
0.00
56
y0
13
87.4
4.3
0.42
22
64
7
83.5
4.0
0.84
9
400
2
80.5
2.9
e0xC/1023 C m22
w = [n 6 k](n?k)
(5)
where n is the director and k is the smectic layer normal. The
absolute value of the order parameter w describes the
magnitude of the tilt, i.e. w = (1/2) sin 2h. At small tilt angles
h2 % 1, w # h. The direction of w corresponds to that of the C2
axis of the SmC* phase, i.e. it is perpendicular to the tilt plane.
According to eqn (2) the spontaneous polarisation in the
SmC* phase is proportional to w, i.e. Ps = e0xCw where
the coefficient C may be positive or negative depending on the
molecular structure.
It is well known that the SmC phase is biaxial, and therefore
the molecular orientational ordering cannot be completely
specified by the nematic tensor order parameter Qab
=
S(nanb 2 dab/3). In particular, the molecular short axes should
be partially ordered in the direction perpendicular (or parallel)
to the tilt plane even in a nonchiral SmC phase without any
polarisation (see Fig. 5). This biaxial ordering is nonpolar and
can be described by the tensor order parameter Bab which can
be written as:21
Fig. 4 T(h) curves of the non-irradiated sample and two irradiated
samples. Solid curves are the fit results according to eqn (4) with
parameters listed in Table 1.
The comparison in Fig. 4 proves the good agreement between
the fit results (solid lines) and the experimental data points in
the vicinity of the smectic A*–C* transition point.
Bab = Sbabb 2 cacbT =
(6)
ˆ ˆ
C(nanb 2 dab/3) + B(wawb 2 mamb)
As seen in Table 1, all Landau coefficients are significantly
changed in the irradiated states. Both coefficients a and c
decrease by a factor of 6 within the experimental range of the
light intensity I. It should be noted that this decrease only leads
to some quantitative changes of the order parameters in the
transition. However, most remarkable is the dramatic change
of the coefficient b, the sign of which controls the order of the
SmA–SmC transition.19,20 In the dark state b # 0 and thus
the system is close to the tricritical point. In contrast, in the
irradiated state the coefficient b is positive and large, and thus
a pronounced second-order transition takes place. This light-
induced crossover indicates that the nature of the ferroelectric
transition is indeed changed by the light-induced formation of
the cis isomers. We will discuss this most noteworthy observa-
tion from the theoretical point of view in the following section.
where the unit vectors b and c, (b?c), are in the direction of
ˆ
short molecular axes, the unit vector w is parallel to P and the
unit vector m is perpendicular to both w and n (see Fig. 5).
Here B is the scalar biaxial order parameter and the order
parameter C describes the tendency of short molecular axes
to order along the director. One notes that in the limit of
perfect nematic order S A 1 and C A 0. In this limit the
biaxial order parameter B = Scos QT where the angle Q
specifies the orientation of the short molecular axis about the
long axis a.
Now the part of the total free energy of the SmC* phase
which does not depend on polarisation can be expanded in
Light-induced variation of biaxial order
In this section we show that the light-induced change in the
order of the transition can be described theoretically in a
qualitative way taking into account that the irradiation with
light induces a transformation of trans into cis isomers. Thus
in a photostationary state there exists some (molar) fraction x
of cis isomers which act as a nonmesogenic dopant. cis Isomers
are bent-shaped molecules with a weakly pronounced ‘long
axis’, and thus one cannot expect a large degree of nematic
orientational ordering of such molecules in the SmC* phase.
On the other hand, cis isomers are strongly biaxial and
Fig. 5 Ordering of the molecular axes a, b, and c of a biaxial molecule
with respect to the reference frame n, w, and m where n denotes the
director, w is along the direction of spontaneous polarization P and m
is orthogonal to both n and w; k is the normal to the smectic layers.
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powers of the primary vector order parameter w and the
secondary tensor order parameter Bab
should be partially ordered in the biaxial SmC phase thus
contributing to the average biaxial order. If the fraction of cis
isomers x is small, the parameter A can be expanded in powers
of x keeping the linear term: A # A0 + xA1, where we assume
that A1 . 0. Now the effective Landau coefficient b can be
expressed as:
:
1
1
1
DFðw,BÞ~ a(T{Tcꢀ)w2z bw4z cw6
2
1
2
4
6
(7)
ꢀ
ꢁ
z
AB2zG waBabwb z. . . ,
2
2ð1{xÞ G2
where the coefficient G describes a coupling between tilt and
the biaxial ordering. Substituting eqn (5) and (6) into eqn (7)
one obtains for small tilt angles h:
b&b{
(11)
A0zxA1
In the dark state x = 0 and, according to our experimental data,
the coefficient b vanishes. This means that the coefficient b in
eqn (11) can be expressed in terms of G and A0, i.e. b # 2G2/A0
and:
ꢀ
ꢁ
1
1
1
DFðh,BÞ& a T{Tcꢀ h2z bh4z ch6
2
1
2
4
6
(8)
z
AB2zGBh2
!
2
ð1{xÞ
bðxÞ&b 1{
(12)
1zkx
Minimising the free energy (8) with respect to B and
substituting back into eqn (8) one obtains the full expansion
of the free energy in powers of the tilt angle h:
where k = A1/A0 is on the order of 1.
One can readily see from eqn (12) and from Fig. 6 that the
Landau coefficient b(x) vanishes at x = 0 and becomes positive
for all values of the molar fraction x of cis-isomers which are
created by the irradiation of light. The coefficient b(x) is
also an increasing function of x, at least for small x. These
qualitative results enable one to explain why the irradiation
with light results in a positive value of the coefficient b (in this
particular material) which grows with the increasing light
intensity. One also notes that the rapid growth of the
coefficient b, which follows from the experiment, is determined
by the fact that b is given by a difference between two large
terms in eqn (11). In the pure smectic C* without light
irradiation these two terms cancel each other. By contrast, in
the irradiated sample the balance between the two terms is
shifted producing a large effect even at small values of x.
However, it is seen from Table 1 that absolute values of
b remain relatively small compared, for example, to the
coefficient c. It is also seen in Fig. 6 that if we assume b to be of
the same order as c (# 104 kJ m23) the maximum experi-
mental value of b # 400 kJ m23 is achieved at x # 0.014
ꢀ
ꢁ
1
1
1
DFðhÞ& a T{Tcꢀ h2z bh4z ch6
(9)
2
4
6
where:
2G2
b~b{
(10)
A
Thus the Landau coefficient b is renormalised due to a
coupling between tilt and biaxiality order parameters. In the
context of this model the nearly tricritical natue of the SmA–
SmC transition, observed in many materials,19,20 is related to
large values of the coupling coefficient G which may lead to
abnormally small values of b according to eqn (10).
In the smectic C* material investigated in this paper the
coefficient b approximately vanishes in the dark state, i.e. in
the system of predominantly trans isomers b = 0. Irradiation
with light creates a certain fraction of strongly biaxial bent-
shaped cis isomers which grows with the increasing light
intensity. In the mixture of trans and cis isomers the coupling
between the average tilt and the biaxial order parameters B
and B1 of the two isomers can approximately written as:
(1 2 x)GBh2 + xG1B1h2
where x is the molar fraction of cis isomers and B1 is the
corresponding biaxial order parameter. The coefficient G describes
the coupling between tilt and the biaxial order parameter in the
system of pure trans isomers while the coefficient G1 describes the
corresponding contribution of cis isomers. One notes, however,
that cis isomers do not possess a well defined long axis, and
therefore the orientational order parameter of such molecules must
be low. Thus the cis isomers may only make a small contribution
to the stabilization of the average tilt in the SmC phase. The
contribution of these isomers to the coupling between tilt and
the biaxial order parameter is also expected to be small and thus
G1 % G. For simplicity we assume that G1 = 0 and thus the
coupling between average tilt and average biaxial ordering can
approximately be expressed as G(1 2 x)Bh2. The coefficient A in
eqn (8) should also depend on the fraction of cis isomers because
these molecules are strongly biaxial, and their molecular planes
Fig. 6 Increase of the fourth-order Landau coefficient b with the
molar fraction
x of bent-shaped cis-isomers formed during the
irradiation with light. The curve has been calculated according to
eqn (12) with b # 104 kJ m23 and k # 1.
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which seems to be a small enough value to maintain the
stability conditions of the SmC* phase. Finally, it should be
noted that all Landau coefficients may depend on the molar
fraction x due to a dilution effect determined by the presence
of nonmesogenic molecules. This additional dependence, which
has not been taken into account in the present simple model, is
expected, however, to be qualitatively the same for all coeffi-
cients, and thus it cannot explain the specific behaviour of the
coefficient b which determines the order of the transition.
configuration is recovered only after a high E-field treatment,
where the tilted layers are straightened up by the application of
a strong electric field with an amplitude of about 20 V mm21
.
The reason for this light-induced bookshelf to chevron
transition is not clear to us at all. Since the common
explanation for the tilting of smectic layers into the chevron
configuration is given by the shrinkage of the smectic layer
spacing at increasing tilt of the molecules the origin might be
related to light-induced variations of the layer spacing due to
the formation of the cis-isomers.
Light-induced bookshelf to chevron transitions
Time-resolved measurements
Another interesting consequence of the photoisomerization
process seems to be the induction of irreversible changes in the
smectic layer arrangement. An example of these light-induced
smectic layer re-orientations is depicted in Fig. 7. In the initial
dark state the W470 sample was aligned to be in the so-called
‘bookshelf’ configuration where the smectic layers are all more
or less perpendicular to the glass substrates of the LC cell.4
The sample is continuously switched by a low-amplitude
electric field of 2 V mm21 to measure its polarization. Hence,
no SmC* ferroelectric domains are seen in this picture. After
the irradiation with light (450 nm, 0.42 mWcm22) and the
subsequent formation of the cis-isomers the initial bookshelf
configuration transformed into the so-called ‘chevron’ con-
figuration which is recognised by the characteristic ‘zig-zag’
defect lines in the texture micrograph.5,22 The chevron
configuration is signified by smectic layers tilted in opposite
directions. Domains of opposite layer tilt directions are
separated by zig-zag defects. The chevron configuration is
preserved even after the irradiation has been stopped and
the polarization relaxed to its initial value. The bookshelf
After the irradiation is stopped, the cis-isomers that have been
formed in the photostationary state thermally re-isomerize
to the rod-shaped trans-state and, simultaneously, the initial
dark state properties (except the layer configuration, see
above) of the SmC* phase are restored. The thermal cis A
trans isomerizations of azobenzenes in diluted isotropic
solutions23,24 as well as in liquid-crystalline guest/host
systems13,14 are known to follow simple mono-exponential
rate laws, the rate constants k of which are described by the
Arrhenius law with activation energies typically on the order
of EA # 102 kJ mol21
.
We studied the thermal relaxation of the W470 sample after
irradiation by time-resolved measurements of Ps and h at
several temperatures ranging from 85 uC, close to the SmA*–
SmC* phase transition, down to 65 uC, 20 K below the
transition. The results for five selected temperatures are
shown in Fig. 8 where the solid lines represent the best fits
Fig. 7 Texture micrographs in the SmC* phase of the W470 sample
at 65 uC showing a light-induced bookshelf to chevron transition of the
smectic layers. All pictures are taken with light of 589 nm wavelength
to avoid photoisomerization during the microscopic observation. For
further explanations see text.
Fig. 8 Relaxation of (a) the spontaneous polarization Ps and (b)
director tilt h after ending the irradiation with light at 65, 70, 75, 80,
and 85 uC. Solid curves are the best fits to mono-exponential rate laws
with rate constants listed in Table 2.
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intrinsic photoferroelectric effect in the one-component
SmC* phase of W470 is mainly determined by the light-
induced change of the director tilt angle which then also gives
rise to a change of the spontaneous polarization. In addition, a
light-induced decrease of the bilinear coupling coefficient C
(between polarisation and tilt) has also been observed.
The temperature variation of the tilt angle h(T) in the dark
state and in the irradiated states has been fitted using the
standard Landau expansion of the free energy. All coefficients
of this expansion have been found to depend significantly on
the intensity of light. In particular, a dramatic change of the
coefficient b has been observed which signifies a change in the
order of the SmA*–SmC* transition from the tricritical one in
the dark state to the pronounced second-order one in the
irradiated states. This change in the nature of the transition
has been interpreted theoretically taking into account that the
bent-shaped cis-isomer molecules (generated through the
irradiation with light) possess a strong biaxiality. The effective
coefficient b, which determines the deviation from the
tricritical point, is strongly affected by the coupling between
the tilt and the biaxial order parameters. The corresponding
coupling coefficient depends on the molar fraction of cis-
isomers, and this enables us to explain qualitatively the
experimentally observed strong dependence of the coefficient
b on the intensity of light.
Table 2 Summary of rate constants k observed in the mono-
exponential rate laws of the Ps and h relaxations at different
temperatures T
T/uC
65
49
53
70
59
60
75
80
78
80
111
95
85
115
128
Ps relaxation k/1023 s21
h relaxation k/1023 s21
It has been found that the light-induced variation of the tilt
angle is connected with an irreversible change of the smectic
layer configuration from the initial bookshelf geometry to the
chevron one. In addition, close to the SmA*–SmC* transition
point the secondary photoferroelectric effect has been
observed which corresponds to a shift of the transition
temperature.
Fig. 9 Arrhenius plots of the temperature-dependent rate constants
listed in Table 2. The slopes of the regression lines (2Ea/R) lead to
activation energies in the order of 31 kJ mol21 for both the relaxation
of the spontaneous polarization Ps (a) and the relaxation of the tilt
h (b).
Acknowledgements
This work was supported by the Deutsche Forschungs-
gemeinschaft (grant no. GI 243/3-3) and the Fonds der
Chemischen Industrie, and by the U.S. National Science
Foundation MRSEC award no. DMR-0213918.
to mono-exponential rate laws. Indeed, both Ps(t) and h(t) are
well-described by these rate laws with similar time constants
of, e.g. 12–13 s at 75 uC. All rate constants obtained from
fitting the Ps (t) and the h(t) relaxations are listed in Table 2.
The temperature dependence of these rate constants is
analysed in the Arrhenius plots shown in Fig. 9. It is readily
seen from Fig. 9 that both processes follow the Arrhenius law
with almost identical activation energies around 31 kJ mol21
which is roughly one third of the typical activation energies
found in diluted azobenzenes with similar substituents at the
p and p9 positions.23,14
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