A non-standard temperature dependence of the order parameter of the 5CB liquid crystal doped with an azo-derivative

Article abstract of DOI:10.1080/15421400701206121

The effects of the cis and trans form of the 4-OMe-phenylazobenzene at 1% and 7% mole fraction on order, mesophase stability and dynamics of the 5CB liquid crystal were studied using the ESR spin probe technique. The presence of the trans azo-derivative induced a downshift of the nematic-isotropic transition temperature TNI which increased with the concentration but did not change the dependence of the order parameter on the reduced temperature. The cis isomer caused even larger TNI shifts, determining a marked decrease of the order parameter with a peculiar dependence on the reduced temperature, quite different from the standard Haller-type behavior. The spin probe dynamics appeared instead to be essentially unchanged by the presence of the azo-derivative.

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A Non-Standard Temperature
Dependence of the Order
Parameter of the 5CB Liquid
Crystal Doped with an Azo-
Derivative
Ilaria Vecchi a , Alberto Arcioni a , Corrado
Bacchiocchi a , Giustiniano Tiberio a , Claudio
Zannoni a & Paolo Zanirato b
a Dipartimento di Chimica Fisica e Inorganica and
INSTM, Viale Risorgimento, Bologna, Italy
b Dipartimento di Chimica Organica, “A. Mangini”
Università, Viale Risorgimento, Bologna, Italy
Available online: 22 Sep 2010
To cite this article: Ilaria Vecchi, Alberto Arcioni, Corrado Bacchiocchi, Giustiniano
Tiberio, Claudio Zannoni & Paolo Zanirato (2007): A Non-Standard Temperature
Dependence of the Order Parameter of the 5CB Liquid Crystal Doped with an Azo-
Derivative, Molecular Crystals and Liquid Crystals, 465:1, 271-281
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Mol. Cryst. Liq. Cryst., Vol. 465, pp. 271–281, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701206121
A Non-Standard Temperature Dependence of the Order
Parameter of the 5CB Liquid Crystal Doped with an
Azo-Derivative
Ilaria Vecchi
Alberto Arcioni
Corrado Bacchiocchi
Giustiniano Tiberio
Claudio Zannoni
Dipartimento di Chimica Fisica e Inorganica and INSTM, Viale
Risorgimento, Bologna, Italy
Paolo Zanirato
`
Dipartimento di Chimica Organica ‘‘A. Mangini’’ Universita, Viale
Risorgimento, Bologna, Italy
The effects of the cis and trans form of the 4-OMe-phenylazobenzene at 1% and 7%
mole fraction on order, mesophase stability and dynamics of the 5CB liquid crystal
were studied using the ESR spin probe technique. The presence of the trans azo-
derivative induced a downshift of the nematic-isotropic transition temperature
TNI which increased with the concentration but did not change the dependence
of the order parameter on the reduced temperature. The cis isomer caused even
larger TNI shifts, determining a marked decrease of the order parameter with a
peculiar dependence on the reduced temperature, quite different from the
standard Haller-type behavior. The spin probe dynamics appeared instead to be
essentially unchanged by the presence of the azo-derivative.
Keywords: biphasic systems; complex fluids; ESR spin probe; nematics; phase transi-
tions; photoisomerization
PACS Numbers: 42.70.Df, 42.79.Kt, 61.30.Gd, 64.70.Md, 76.30.-v, 82.70.-y
We thank MIUR (PRIN ‘‘Modelling and characterisation of liquid crystals for nano-
organised structures’’ and FIRB ‘‘Molecular and organic=inorganic hybrid nanostruc-
tures for photonics’’), INSTM and the University of Bologna for support.
Address correspondence to Claudio Zannoni, Dipartimento di Chimica Fisica e
Inorganica and INSTM, Viale Risorgimento 4, Bologna I-40136, Italy. E-mail: claudio.
zannoni@cineca.it
271

272
I. Vecchi et al.
I. INTRODUCTION
In recent years a great deal of technological interest has been gener-
ated by photoresponsive materials formed by a nematic liquid crystal
(LC) with a variable amount (usually a few percent) of an azobenzene
derivative dissolved. Besides the possible direct applications of these
materials, ranging from optical switching to information storage
devices [1], systems of this type have also attracted attention as sim-
pler models for the study of the fundamental working principles of
photoresponsive polymers [2,3]. Depending upon the nature of the
components, the photomodulation of the nematic phase allows either
to modify the LC director orientation (‘‘order–order’’ change) or to
induce a phase transition (‘‘order–disorder’’ change) [4]. Both these
processes can be performed via the well-known azo-derivative trans-
cis reversible photoisomerization.
In particular, the ‘‘order–disorder’’ change, which is the object of the
present study, has been interpreted in terms of a change of the ordered
phase stability. The rod-like shape of the para-substituted trans iso-
mer, being similar to that of the LC host, has a limited influence on
the nematic phase, while the bent, V-shape of the cis azo-derivative
introduces a large destabilization which causes a marked shift of the
nematic-isotropic (N–I) phase transition to a lower temperature. The
trans–cis photoisomerization leads to a lowering of the nematic–
isotropic transition temperature ðTNIÞ of the mixture and thus, when
the TNI is reduced below the current sample temperature, the system
will effectively undergo an isothermal N–I (‘‘order–disorder’’) phase
transition.
II. EXPERIMENTAL
The structure of the 4-OMe-phenylazobenzene (OMPAB) photoactive
unit used in this study is shown in Figure 1.
FIGURE 1 Chemical structure of 4-OMe-phenylazobenzene (OMPAB) together
with the trans-cis photoisomerization scheme.

Non-Standard Behavior of the 5CB Order Parameter
273
OMPAB was synthesized according to the Mills reaction [5], in
which nitrosobenzene combines with primary arylamines in glacial
acetic acid to give unsymmetrical azo-derivatives. The compound
obtained did not show any LC phase.
The host matrix employed was the LC 4-cyano-40-n-pentylbiphenyl
(5CB), having a nematic range between 295.7 and 308.5 K. It was
obtained from Merck KGaA (Darmstadt, Germany) and used without
any further purification. To monitor the order and the dynamics of the
LC system, we chose the nitroxide spin probe 3b-DOXYL-5a-choles-
tane (CSL), purchased from Aldrich. Its size, rigidity and rod-like
shape similar to that of the 5CB make this molecule a suitable probe
for our purposes [6,7]. The CSL structure together with the standard
choice for the molecular frames and the corresponding definition of the
principal components D (reorientation of the molecular long axis) and
?
D (rotation around the long axis) of the rotational diffusion tensor
k
have been presented in a previous work [7].
The samples were prepared by adding OMPAB to a solution of CSL
spin probe in 5CB with a concentration of 6 ꢀ 10ꢁ4 gCSL=g5CB. Two
values of the azo-derivative mole fraction were studied: x ¼ 1% and
x ¼ 7%. The good solubility of the spin probe and of the OMPAB in
5CB, at the concentrations studied, made it possible to prepare uni-
form samples just by mechanical mixing at a temperature slightly
above TNI, without adding any solvent.
The samples not requiring photoisomerization (trans isomers) were
stored in the dark at room temperature into glass capillaries of 1.8 mm
internal diameter, until ESR measurements. To obtain the cis iso-
mers, according to the trans–cis photoisomerization reaction (see
Fig. 1), the samples were exposed to UV-light (k ¼ 365 nm) in a quartz
cuvette with a UVGL-58 MINERALIGHT Lamp (UVP). The trans–cis
photoisomerization was monitored by recording, during the exposure,
a series of UV-visible absorption spectra with a Cary 1E (Varian) spec-
trophotometer. UV-visible spectra were taken before any irradiation
and after different times of exposure, dissolving 5CB azo-doped sam-
ples in ciclohexane to achieve a concentration of the azo-derivative
of 2.5 ꢀ 10ꢁ4 M.
The trans isomer spectra exhibited a strong absorption band at
about 340 nm, due to p ! pꢂ transition, and a much weaker band cen-
tered at 450 nm, due to n ! pꢂ transition. Both these features are
characteristic of trans-azobenzene type molecules. Irradiation at
365 nm caused a decrease of the p ! pꢂ band intensity and a build-
up of the n ! pꢂ band, which was slightly shifted toward shorter wave-
lengths. This was an indication that the isomerization was taking
place. We decided to consider the process successful when we could

274
I. Vecchi et al.
FIGURE 2 Typical absorption spectra recorded after different times of UV
exposure for 5CB doped with 4-OMe-phenylazobenzene (OMPAB) dissolved
in ciclohexane (see text for details).
not observe any further change by comparing two consecutive UV-
visible spectra (see Fig. 2). This typically required between 10 and
24 hours, depending on the OMPAB concentration. Immediately after
the isomerization, the samples doped with the cis azo-derivative were
inserted into the capillaries and ESR spectra were recorded. After the
ESR measurements, a further UV-visible absorption spectrum was
taken and compared to the last one taken immediately before the
ESR measurements. In all the cases, these spectra were essentially
identical, indicating that the cis isomers did not significantly reverse
to the trans configuration during the measurements.
The ESR spectra were acquired with a Bruker ESP300E spec-
trometer equipped with an ER 041XG microwave X-band (9.5 GHz)
Gunn Diode bridge and a rectangular ER 4102 cavity. The samples
were thermostated with a nitrogen flux through a variable tempera-
ture unit Bruker B-VT 2000. The temperature, monitored with a
calibrated type T thermocouple (Comark Ltd.) located in contact
with the capillary containing the sample, showed a stability better
than ꢃ0.05 K. We recorded ESR spectra on heating taking the
samples from the N to the I phase, in a range of about 20 K around
the TNI
.

Non-Standard Behavior of the 5CB Order Parameter
275
III. RESULTS AND DISCUSSION
The ESR spectra simulation program adopted was based on a set of
Fortran routines implementing the ‘‘slow tumbling’’ theory for a spin
probe reorienting in a LC, developed by Freed and collaborators
[8–10], combined with a software package [11] that optimizes the fit
parameters for a series of spectra at the same time (global target
analysis) using the Gauss-Newton-Marquardt non-linear least squares
method [12].
We started our analysis by applying the same globalization scheme
proposed in previous works [7,13], where, for each sample, the spectra
recorded at different temperatures, within the same phase (N or I), are
fitted simultaneously in a global analysis. The ESR spectra of each
sample are reproduced considering as variable, not global, fit para-
meters the orientational order parameter of the probe hP2i and the
perpendicular component of the probe rotational diffusion tensor D .
?
To minimize the correlation between the components of this tensor,
the ratio Dr ¼ D =D was also introduced.
?
k
Spectra in the I phase were fitted by keeping the values of hP2i fixed
to zero and considering Dr as a global variable. This approach gave
stable and uncorrelated values for the parameters with a Dr of 9.9.
In a series of preliminary fits, the temperature dependence of Dr in
the N phase, for each sample, was described by a Haller-type equation:
c
DrðTÞ ¼ D0r ð1 ꢁ T=TcÞ þDIr;
ð1Þ
where Tc is the temperature at which the hP2i drops to zero, DIr is the
diffusion coefficient ratio of the I phase which was kept fixed to the
value indicated above, whereas c and D0r , the limiting deviation of Dr
from the isotropic value at zero temperature, were adjustable global
parameters. Unfortunately, this choice for the temperature depen-
dence of Dr did not help significantly to reduce the correlation among
the parameters. A more appropriate approach seemed to be consider-
ing Dr simply independent of the temperature, as done for the I phase.
As expected, the presence of the azo-derivatives induced a decrease
of TNI, as reported in Table 1. We also observed that the temperature
transition downshift increased with the solute concentration and was
larger for the cis-doped samples.
In samples containing the trans isomer, at both concentrations, the
temperature dependence of the hP2i (on the reduced temperature,
Tꢂ ¼ T=TNI) and of the dynamics (on the absolute temperature) were
essentially identical to those observed in the pure 5CB.
Unfortunately, in samples doped with the cis isomer, despite
the apparent good fit of the experimental spectra (not shown), the

276
I. Vecchi et al.
TABLE 1 Nematic–isotropic Phase Transition Temperatures (TNI) and
Corresponding Shifts with Respect to the Pure 5CB (DTNI ¼ TNI;5CBþOMPAB
TNI;5CB) for the Liquid Crystal Samples Doped with trans or cis 4-OMe-
ꢁ
Phenylazobenzene (OMPAB) at the two Concentrations Studied (1% and 7%)
TNI=K DTNI=K
trans
7%
cis
trans
7%
ꢁ1.0
cis
1%
1%
306.0
7%
1%
ꢁ0.2
1%
7%
5CB þ OMPAB
308.3
307.5
308.5
289.5
ꢁ2.5
ꢁ19.0
pure 5CB
globalization scheme presented above did not seem to be able to
recover reasonable parameters. In particular, at 1% cis there was only
a slight decrease of the order parameter, with respect to the pure 5CB,
across almost the whole nematic phase but not in a range of tempera-
tures close to TNI, where the deviation was significantly larger and the
hP2i seemed to approach the N–I phase transition in a continuous
fashion, despite its first order nature. At 7% cis the orientational order
was significantly reduced, showing, toward the end of the N phase, an
almost linear dependence on the Tꢂ, quite peculiar for an order para-
meter, and no transition at all. Finally, the recovered parameters for
the rotational dynamics were also difficult to rationalize exhibiting
unusual correlations and unreasonably large errors.
Altogether, these results suggested that the adopted model was not
appropriate to describe the cis-doped samples whose behavior turned
out to be more complex than expected (see below). In fact, LC mix-
tures containing non-mesomorphic solutes can be affected not only
by a downshift of the TNI, as we have seen in our samples, but also
by a broadening of the transition temperature, due to the onset of a
biphasic (N=I) region as the system approaches the TNI [14,15]. In
the biphasic region, the nematic and the isotropic phases coexist in
equilibrium and the solute is distributed between them, according
to its activity coefficients.
A more detailed examination of the ESR spectra relative to the cis-
doped samples, acquired near the TNI, indeed suggested that those
line shapes could be related to the superposition of a nematic and an
isotropic spectrum. The presence of a biphasic region was then con-
firmed with a polarizing microscope, by observing the temperature
behavior of the samples throughout the N phase up to the N–I tran-
sition. The cholestane partitioning between the two phases could in

Non-Standard Behavior of the 5CB Order Parameter
277
principle affect the analysis of the ESR spectra, and thus the recovery
of meaningful parameters, particularly if the probe concentration in
the nematic (or isotropic) phase became so low to hinder the detection
of the corresponding contribution to the spectra. However, here we did
not encounter this problem: in fact, all the ESR spectra relative to the
biphasic regime clearly showed the features characteristic of both
the phases, including the line positions that determine the order
parameters (see Fig. 3). We thus expect the CSL spin probe to be parti-
tioned essentially uniformly between the phases, reporting the LC
behavior of the ordered nematic and the isotropic environment.
In the final fits, we adopted, therefore, a different globalization
scheme where the spectra corresponding to a biphasic system were fit-
ted to a weighted sum of two spectra, one for each phase, modeled with
the same set of parameters described above but keeping, for the
nematic contribution, Dr fixed to 7.1, the value obtained for the pure
5CB, to further minimize the correlation. For consistency, nematic
spectra that did not show phase coexistence, were also fitted by keep-
ing Dr fixed to 7.1. Isotropic spectra were fitted as described in the
FIGURE 3 Typical experimental ESR spectra (dashed line) and fits (solid
line) obtained with the second model proposed. For each spectrum, the tem-
perature, the azo-derivative concentration, the isomer type and the corre-
sponding phase are indicated. Spectra labeled 7% cis were recorded at
temperatures close to the N–I phase transition where the N and the I phase
coexist (see text for details).

278
I. Vecchi et al.
FIGURE 4 Order parameter hP2i of the spin probe against reduced tempera-
ture Tꢂ for the samples doped with (a) the trans and (b) the cis 4-OMe-
phenylazobenzene (OMPAB) at 1% and 7% mole fractions, compared to the
pure 5CB. The results shown were obtained with the second model considered
(see text for details).

Non-Standard Behavior of the 5CB Order Parameter
279
previous globalization scheme. The good agreement between the
experimental and the calculated spectra is shown in Figure 3.
In Figure 4 we show the results relative to the probe orientational
order, plotted against the reduced temperature Tꢂ to easily compare
the behavior of the pure LC and the ones relative to the azo-doped
samples. The hP2i temperature dependence of the pure 5CB and of
the trans-doped samples, at both concentrations (Fig. 4 (a)), was found
to be, as expected, identical to the one obtained with the first globaliza-
tion scheme. The reduced temperature dependence of the hP2i of cis-
doped samples (Fig. 4 (b)) showed a discontinuity at the TNI, typical
of a first order transition.
At the larger concentration of 7% (Fig. 4 (b)), for Tꢂ > 0.98, the hP2i
values showed a peculiar behavior, quite different from the standard
Haller-type decrease of the order parameter, normally observed. This
can be explained considering that approximately at Tꢂ ¼0.98 the sys-
tem enters the biphasic region, where the azo-derivative is distributed
between the two coexisting phases, resulting in a decrease of its con-
centration in the N phase. A lower concentration of the azo-derivative
will reduce the destabilization of the N fraction, causing the observed
small increase of the hP2i. As the system approaches the phase tran-
sition, the hP2i behavior is therefore determined essentially by two
effects: a further decrease of the concentration of the azo-derivative
in the N phase and the increase of the temperature. The first effect
would increase the order of the N phase even further but the second
will, of course, act in the opposite direction, resulting in the observed
peculiar behavior of the hP2i.
The dynamics appeared essentially independent of the nature, the
configuration and the concentration of the azo-derivative, while was
found to have a different trend in the N and in the I phase. In fact,
the temperature dependence of the tumbling diffusion coefficient D
?
of the spin probe is well represented by an Arrhenius-type behavior,
ðD ¼D0 ꢀ exp½ꢁE=RTꢄÞ, with D0 ¼ ð6 ꢃ 2Þ ꢀ 107 nsꢁ1 and E ¼ 53 ꢃ
?
2 kJ=mol for the N and D0 ¼ (3 ꢃ 1) ꢀ 105 nsꢁ1 and E ¼ 41 ꢃ 2 kJ=mol
for the I phase.
IV. CONCLUSIONS
The presence of the azo-derivative, even at the lower mole fraction of
1%, induces a shift of the nematic-isotropic transition to a lower tem-
perature. The shift increases, as expected, with the concentration of
the azo-derivative and is always larger for the cis isomer.
A comparison of the reduced temperature dependence of hP2i for
each sample reveals that at 1% and 7% trans (Fig. 4 (a)) and 1%

280
I. Vecchi et al.
cis (Fig. 4 (b)), the destabilization caused by the azo-derivative
results essentially in a shift of the whole phase diagram to a lower
temperature. In fact, in these cases, the hP2i dependence on Tꢂ is
identical to that of the pure 5CB and it begins to show only a small
decrease for the cis at 1%. At 7% cis (Fig. 4 (b)), instead, the order
of the phase is clearly lower than that of the pure 5CB, indicating
that the shift of the phase diagram to a lower temperature is no
longer able to compensate the destabilization introduced by the
azo-derivative.
Moreover, in this case the system shows a region of phase separ-
ation which begins approximately at Tꢂ ¼ 0.98. The occurrence of a
biphasic region within what was supposed to be a purely N phase, is
suggested by a peculiar ESR line shape which looks significantly dif-
ferent from a typical nematic one and is confirmed by the polarizing
microscope observations. The fit of the biphasic spectra is then suc-
cessfully carried out using the second model proposed. The hP2i depen-
dence on Tꢂ shows a peculiar behavior where the order parameter
slightly increases as the system enters the biphasic region. In the
cases studied, the spin probe dynamics in the presence of the azo-
derivative is essentially unchanged with respect to that observed in
the pure 5CB. A more systematic study of the effects of different
azo-derivatives on the order, the mesophase stability and the occur-
rence of a biphasic region in 5CB is under way.
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