Kinetico-mechanistic study of the thermal cis-to-Trans isomerization of 4,4a?2-dialkoxyazoderivatives in nematic liquid crystals

Article abstract of DOI:10.1021/jp909557h

4,4a?2-Dialkoxy-substituted azobenzenes are usually required for technical applications. Here, we study the mechanism through which the azo-dye thermally isomerizes from the unstable cis isomer to the more stable trans isomer when it is incorporated in the nematic liquid-crystalline state. We have determined the kinetic and thermal activation parameters for this process in different nematic environments. Their comparison with those values obtained in isotropic media demonstrates that the mechanism through which the thermal cisto-trans isomerization takes place is the inversion pathway in all the physical states studied. The nematic order increases the rate of the thermal cis-to-trans isomerization process. This fact is related to a cooperative interaction established between the mesogen molecules and the azo-dye. This effect is not present in the isotropic state. ? 2010 American Chemical Society.

Full text of DOI:10.1021/jp909557h

J. Phys. Chem. B 2010, 114, 1287–1293
1287
Kinetico-Mechanistic Study of the Thermal Cis-to-Trans Isomerization of
4,4′-Dialkoxyazoderivatives in Nematic Liquid Crystals
Jaume Garcia-Amoro´s,† Manuel Mart´ınez,‡ Heino Finkelmann,§ and Dolores Velasco*,†
Departament de Qu´ımica Orga`nica, Facultat de Qu´ımica, UniVersitat de Barcelona, Mart´ı i Franque`s, 1-11,
E-08028, Barcelona, Spain., Departament de Qu´ımica Inorga`nica, Facultat de Qu´ımica, UniVersitat de Barcelona,
Mart´ı i Franque`s, 1-11, E-08028, Barcelona, Spain, Institut fu¨r Makromolekulare Chemie, Albert-Ludwigs-
UniVersita¨t Freiburg, Hermann-Staudinger-Haus, Stefan-Meier-Strasse 31, D-79104 Freiburg, Germany
ReceiVed: October 6, 2009; ReVised Manuscript ReceiVed: December 11, 2009
4,4′-Dialkoxy-substituted azobenzenes are usually required for technical applications. Here, we study the
mechanism through which the azo-dye thermally isomerizes from the unstable cis isomer to the more stable
trans isomer when it is incorporated in the nematic liquid-crystalline state. We have determined the kinetic
and thermal activation parameters for this process in different nematic environments. Their comparison with
those values obtained in isotropic media demonstrates that the mechanism through which the thermal cis-
to-trans isomerization takes place is the inversion pathway in all the physical states studied. The nematic
order increases the rate of the thermal cis-to-trans isomerization process. This fact is related to a cooperative
interaction established between the mesogen molecules and the azo-dye. This effect is not present in the
isotropic state.
1. Introduction
Liquid crystals are unique materials that exhibit the molecular
order characteristic of the crystalline state and the molecular
mobility typical of the liquid phase.1 The potential application
of liquid-crystalline systems arises from the possibility to change
the alignment of the mesogens as a response to a diversity of
external stimuli, such as light,2,3 heat,4-6 mechanical or elec-
tromagnetic fields,7 etc. Light is probably the best alternative
to control and modify the properties of liquid-crystalline systems
(instead of electricity or heat) because it is a clean and cheap
energy source; furthermore, this type of triggering can be
controlled quickly and remotely.8 For this reason, nowadays,
there is a growing interest in the study of the properties of
photoactive, dye-doped, liquid-crystalline systems, given their
possible application as information processing materials.
Azobenzenes are photochromic organic molecules that un-
dergo a clean and reversible isomerization process. The
thermodynamically stable trans isomer can be transformed to
the metastable cis isomer by irradiation with UV light. The
inverse process (cis-to-trans isomerization) can be carried out
either by heating or by irradiation with visible light.9 The
conformational changes associated with these isomerization
processes make azobenzenes ideal candidates to produce opti-
cally controlled materials. The mechanism of its thermal cis-
to-trans isomerization has attracted much attention for many
years and has given rise to controversy. For the process, two
different mechanisms have been proposed (Figure 1): one
involving a simple rotation around the N-N bond10,11 and
another implying an inversion, in-plane lateral shift, through a
linear state.12-14
Figure 1. Rotation and inversion mechanisms proposed for the thermal
cis-to-trans isomerization processes of azobenzenes.
investigation of its cis-to-trans isomerization mechanism in
isotropic solvents. To the best of our knowledge, only a few
studies about this mechanism in liquid-crystalline solvents have
been carried out,16-20 this field being still rather unexplored.
Very recently, even the isomerization process has been studied
on microcrystals in the absence of any solvent.21 It is thus clear
that, given the great interest generated on azobenzene-doped
nematic liquid crystals, a thorough and comprehensive kinetico-
mechanistic study about the azobenzene cis-to-trans thermal
isomerization process in a mesomorphic environment is needed.
The knowledge of the rate and the actuating mechanism of the
cis-to-trans isomerization process in azobenzene-doped liquid
crystals is of great importance for designing promising new
photoactive materials to be used in optical switching, informa-
tion storage, and artificial muscles, among other applications.22-25
This contribution tries to establish the mechanism through
which the azobenzene molecules thermally isomerize from the
thermodynamically unstable cis isomer to the more stable trans
isomer when they are immersed in the nematic liquid-crystalline
state. The kinetic and thermal activation parameters for the
Since the first observation about azobenzene isomerization
from Hartley in 1937,15 many efforts have been put forth in the
* To whom correspondence should be addressed. Phone: 00 34 93 403
92 60. Fax: 00 34 93 339 78 78. E-mail: dvelasco@ub.edu.
† Departament de Qu´ımica Orga`nica, Universitat de Barcelona.
‡ Departament de Qu´ımica Inorga`nica, Universitat de Barcelona.
§ Albert-Ludwigs-Universita¨t Freiburg.
10.1021/jp909557h  2010 American Chemical Society
Published on Web 01/05/2010

1288 J. Phys. Chem. B, Vol. 114, No. 3, 2010
Garcia-Amoro´s et al.
Figure 3. Changes in the electronic spectrum of a 1 cis-to-trans
isomerizing toluene solution at 45 °C (∆t ) 360 s, [2] ) 3 × 10-5 M).
Figure 2. Chemical structure of the azoderivatives 1 and 2 and of the
nematic mesogen 5CB.
analyzer of the microscope with respect to the rubbing direction.
On reaching 45°, the expected change from darkness to
brightness associated with the monodomain was observed. The
absence of characteristic textures was also detected by POM.
Polydomain samples were prepared using the above solid
solutions in 1 mm optical path cells. The order parameter of
the monodomain and polydomain solutions was analyzed by
means of polarized UV-vis spectroscopy. POM microphoto-
graphs of a monodomain and polydomain of a solid solution of
2 in 5CB before irradiation at room temperature are shown in
Figures S1 and S2 in the Supporting Information section.
2.3. Kinetic Experiments. For the experiments in isotropic
solvents, ∼3 × 10-5 M solutions of the azo-dye in the
corresponding solvent were used. In this case, 1 cm optical path
quarz cells were used. For the solid solutions, the concentration
was ∼4 × 10-3 M (see above). Irradiation of the samples was
carried out with a Philips high-pressure mercury lamp (500 W
nominal power) and using a 0.5 M solution of Co(NO3)2 as an
optical filter; irradiation was pursued until no changes were
observed in the electronic spectrum of the sample on further
irradiation; the usual irradiation times were 10 min for isotropic
solutions and 45 min for solid solutions in 5CB. Afterward,
solutions were thermostatted in the dark at the desired temper-
ature, and the thermal cis-to-trans isomerization was monitored
by the change in the electronic spectrum of the sample.
Atmospheric pressure runs were monitored on a Varian Cary
500E spectrophotometer. For runs at variable pressure, a
previously described pressurizing system and pillbox cell was
used, which was connected to a J&M TIDAS spectrophotom-
eter.30
thermal cis-to-trans isomerization process have been determined
in nematic monodomain and polydomain phases for two
different 4,4′-dialkoxy-substituted azoderivatives and compared
with those obtained in isotropic solutions. In isotropic solvents,
the pressure activation parameters have been determined as a
definite proof of the actuating mechanism for all the processes.26
The mesogen choice is based on its rodlike structure, similar to
that of the dyes and the existence of its nematic phase between
20 and 35 °C, which is convenient for technical applications.
2. Experimental Methods
2.1. Instruments and Materials. Two different azo-dyes
have been chosen as photoactive molecules for the kinetic study:
4,4′-dimethoxyazobenzene (1) and 4,4′-di-(5-hexenyloxy)-
azobenzene (2) (Figure 2), which differ in the length of the
lateral alkoxy chains; their preparation has been carried out
according to the procedures described in the literature.27,28 All
compounds were characterized by 1H NMR (400 MHz) and 13
C
NMR (100 MHz) spectra collected on a Varian Mercury
spectrophotometer. All the solvents used in the kinetic
studiessethanol (Sharlau), toluene (Sharlau), cyclohexane (Al-
drich), acetonitrile (Aldrich), and the nematic mesogen 4-cyano-
4′-n-pentylbiphenyl (5CB, Figure 2)29 (Alpha Aesar)swere used
as received.
Polarized optical microscopy (POM) was carried out using a
Nikon Eclipse polarizing microscope at room temperature.
Polarized electronic spectra of the azobenzene-doped liquid-
crystalline mixtures were measured with a Varian Cary 500E
instrument using a Glan-Thompson polarizer between the light
source and the sample.
Observed rate constants were derived in all cases from
absorbance versus time traces at the wavelengths of maximum
absorption for the corresponding trans isomers using standard
software packages.31 No dependence of the values of the
observed rate constants on the selected wavelengths was
detected, as expected for reactions in which a good retention
of isosbestic points is observed, and in all cases, the absorbance
versus time traces fit perfectly to a first-order rate profile (Figure
3). No dependence of the rate constants on the concentration
of the azo-dye was observed, as expected for first-order
processes. All postrun fittings for the determination of the
thermal and pressure activation parameters32 were carried out
by the standard available commercial programs. Table S1
(Supporting Information) collects all the observed rate constants
measured as a function of the solvent, temperature, and pressure.
2.2. Preparation and Order Analysis of the Nematic
Solutions. For the kinetic experiments in the liquid-crystalline
state, solid solutions of the different azocompounds in the
nematic mesogen 5CB were used. Samples were prepared by
mixing the desired amounts of 5CB and the corresponding azo-
dye, followed by homogenization by magnetic stirring for 10
min in the isotropic state. Standard concentration of the solid
solutions was 4 × 10-3 M, to avoid possible effects derived
from self-aggregation of the dye.
Monodomain samples were prepared in 10 and 4 µm optical
path quarz cells. The cell surface was rubbed with a piece of
cloth in a single direction, causing the alignment of the mesogens
due to their electrostatic interaction with the cell surface.
Homogeneity of the samples was checked by local probe
microscopy. POM experiments were run by rotation of the

Isomerization of 4,4′-Dialkoxyazoderivatives
J. Phys. Chem. B, Vol. 114, No. 3, 2010 1289
TABLE 1: Electronic Spectral Data for the Trans Isomer of
Azocompounds 1 and 2 in Different Solvents in the
Thermodynamically Stable State and Extent of the
Photoisomerization of 1 and 2 at 298 Ka
solvent
azocompound
λ
max/nm
Y/%
cyclohexane
1
2
1
2
1
2
1
2
1
2
352
354
355
358
354
356
357
356
364
365
83
83
84
84
79
84
84
84
37
40b
45
acetonitrile
ethanol
toluene
5CB monodomain
Figure 4. Electronic spectrum of a 2 (3 × 10-5 M) solution in
cyclohexane before irradiation (solid line) and once the photostationary
state has been reached (dotted line).
a [azocompound] ) 3 × 10-5 M in isotropic solvents and
[azocompound] ) 4 × 10-3 M in nematic solvents. b Obtained in a
measurement using a cell with an optical path of 4 µm.
3. Results
(Y, eq 1) for azocompounds 1 and 2 in all the solvents studied.
For the four isotropic solvents studied, the extent of the trans-
to-cis photoisomerization for azocompounds 1 and 2 at 298 K
is in the 80-85% range, whereas for the monodomain nematic
phase (10 µm) of mesogen 5CB, lower values of Y around 40%
have been found. The measurement of Y in a thinner cell of 4
µm of optical path afforded the same value, indicating that the
incident light power is enough to perform the photoisomerization
throughout the nematic phase cells without gradient concentra-
tion. It has been reported for thin polystyrene films that there
is no difference in the experimental Y value for the azo-
photoisomerization in film thicknesses among the range from
40 nm to 1.5 µm.37 Important differences in the rate of the trans-
to-cis photoisomerization were also observed and qualitatively
analyzed. Although the photostationary state was reached after
5 min in isotropic solvents, 30 min was needed to reach the
corresponding equilibrium in the nematic phase of the liquid
crystal 5CB for our setup (see Experimental section).
The global order parameter of the nematic solutions, S, has
been determined to account for possible differences in the
kinetics of the cis-to-trans isomerization and in the actuating
mechanism for this process. Figure 5 (left) shows the polarized
electronic spectra of azocompound 2 in the nematic mesogen
5CB at 298 K; the dichroic ratio, DR ) A||/A⊥, was calculated
for all the samples using the corrected absorbance of the
host-guest mixture at λmax for the trans isomer of the corre-
sponding dye with respect to the polarized absorption of the
pure nematic mesogen 5CB in the corresponding direction.
The nematic global order parameter of the nematic solutions
can be then calculated as S ) (DR -1)/(DR + 2).40-43 For
the monodomain solutions, values of S of 0.46 and 0.55 were
determined for azocompounds 1 and 2, respectively, whereas
for the polydomain solutions, the value of S was effectively
zero. Although for the polydomain solutions, the value of the
global nematic order parameter is null, it is evident that the
local order parameter should be larger and approaching the S
value of the corresponding monodomain nematic solutions.
Figure 5 (right) shows the dependence of S for a 2-5CB mixture
with the reduced temperature, Tred ) T/TLC. The global order
parameter diminishes with an increase in the reduced temper-
ature, as expected, until the nematic-to-isotropic phase transition
(Tred ) 1) is reached. At this point, the global order parameter
has a value around S ) 0.4 (as predicted by Maier-Saupe
theory44), and it becomes S ) 0 when the isotropic phase is
obtained.
The thermodynamically stable trans form of azobenzenes 1
and 2 can be converted to the metastable cis isomers by
irradiation with UV light. To quantify the relative stability of
the metastable cis form, the fractional amount of cis isomer
present in the system after the photostationary state is reached
has been determined. Different methodologies, such as nuclear
magnetic resonance (NMR),33,34 high-pressure liquid chroma-
tography (HPLC),35 or UV-vis spectroscopy, can be used for
this purpose.33,36,37 In our case, the extent of the trans-to-cis
photoisomerization of azocompounds 1 and 2 in the isotropic
solvents used (cyclohexane, toluene, ethanol, and acetonitrile)
as well as in a monodomain solution of the nematic mesogen
5CB was determined via UV-vis spectroscopy at 298 K. The
cis isomer fraction in the photostationary state, Y, can be
determined by eq 1.38
1 - Aph/A0
1 - εcis/εtrans
Y )
(1)
A0 and Aph correspond to the initial and the photostationary
state absorbance values measured at λmax (trans form), and εcis
and εtrans are the molar absorption coefficients of the cis and
trans isomers at this wavelength (λmax). We have used the
εcis/εtrans ) 0.056 literature value38 determined for 4,4′-
dimethoxyazobenzene in DMSO, which has been considered
to be independent of the length of the alkoxy lateral chains and
the solvent used.
Figure 4 shows the changes observed in the absorption spectra
of azocompound 2 in cyclohexane at 298 K. The spectra of the
stable trans isomer exhibit the expected peak at ∼355 nm,
corresponding to the π-π* transition, plus a weak broad band
signal at ∼450 nm, associated to the n-π* transition. Similar
absorption spectra have been registered for both azocompounds
in all the solvents studied. A minor red-shift of the absorption
maximum and a broadening of the π-π* band was detected
for the nematic samples. It is associated with the influence of
the host on the dye-host aggregation equilibria.39 On UV light
irradiation, the trans-to-cis photoisomerization occurs accom-
panied by a decrease in the intensity of the 355 nm band and
an increase in the 450 nm signal until the photostationary state
is reached; that is, the inverse process of that shown in Figure
3. Table 1 collects the wavelength of the maximum absorption
for the trans isomer and the extent of the photoisomerization

1290 J. Phys. Chem. B, Vol. 114, No. 3, 2010
Garcia-Amoro´s et al.
Figure 5. Electronic spectra of a solid solution of azocompound 2 (4 × 10-3 M) in 5CB at 298 K with the incident light polarized parallel (solid
line) and perpendicular (dotted line) to the nematic director (left). Evolution of the global order parameter, S, with the reduced temperature Tred
T/TLC (right).
)
TABLE 2: Kinetic and Thermal and Pressure Activation Parameters for the Thermal Cis-to-Trans Isomerization of
Azocompounds 1 and 2 in Different Isotropic and Nematic Solvents
solvent
105 × 298kth/s-1
∆H+/kJ mol-1
∆S+/J K-1 mol-1
∆V+/cm3 mol-1
acetonitrile (ε ) 37.5)
ethanol (ε ) 24.6)
2
1
2
1
2
2
1
2
2
1
2
1.47
1.90
2.30
1.81
1.95
2.38
2.81a
2.72a
3.42
3.55
4.28
92 ( 1
92 ( 1
91 ( 1
91 ( 1
91 ( 1
89 ( 1
88 ( 1
88 ( 1
82 ( 2
77 ( 1
68 ( 1
-30 ( 3
-29 ( 3
-30 ( 4
-33 ( 3
-32 ( 1
-39 ( 3
-37 ( 2
-38 ( 1
-59 ( 7
-73 ( 3
-102 ( 2
∼0
∼0
∼0
∼0
toluene (ε ) 2.4)
cyclohexane (ε ) 2.0)
5CB (isotropic)
5CB (nematic) polydomain
5CB (nematic) monodomain
a Extrapolated from the thermal activation parameters.
Figure 6. Eyring plot for the thermal cis-to-trans isomerization of azocompound 1 in toluene solution (left) and ln kth vs P plots for azocompound
2 in the different solvents studied at 313 K (right).
Finally, the kinetico-mechanistic study of the thermal cis-to-
trans isomerization process for the 4,4′-dialkoxyazoderivatives
1 and 2 in different media at different temperatures and pressures
was conducted. No dependence of the rate of the thermal process
on the use of scanning or diode-array instruments has been
observed, which is indicative of the fact that, under the
conditions used for the thermal kinetic study, all the photo-
chemical processes can be neglected, and first-order rate
constants for the thermal cis-to-trans isomerization, kth, can be
derived from first-order absorbance versus time traces indicated
in Figure 3. The variation of these rate constants with temper-
ature and pressure for each solvent allows for the determination
of the enthalpies, entropies, and volumes of activation, ∆H+,
∆S+, and ∆V+, using Eyring and ln k versus P plots.45-47 Table
2 collects all the relevant kinetic and activation parameters
determined in all the solvents used; Figure 6 collects the Eyring
plot of azocompound 1 in toluene (left) and the ln k versus P
plots of azocompound 2 in ethanol and toluene (right).
4. Discussion
Our main objective was the study of the kinetic and the
actuating mechanism of the thermal cis-to-trans isomerization
process of 4,4-dialkoxy-substituted azobenzenes in nematic
mesophases and the analysis of the differences with respect to
the conventional isotropic solvents. Given the fact that a nematic
mesophase is an anisotropic environment in which the mesogens
are aligned along the director direction, any deviation of this

Isomerization of 4,4′-Dialkoxyazoderivatives
J. Phys. Chem. B, Vol. 114, No. 3, 2010 1291
Figure 7. Schematic representation of the disorganization produced
in the nematic mesophase as a consequence of the azocompound
isomerization.
Figure 8. Plot of ∆H+ versus ∆S+, for the cis-to-trans thermal
isomerization process of azocompounds 1 and 2 in all the solvents
studied.
mean orientation will reduce the state of order of this phase. In
this respect, trans-4,4′-dialkoxy-substituted azobenzenes, with
a rodlike geometry, can be easily incorporated in the liquid-
crystalline phase, thereby contributing to the nematic potential.
In contrast, the cis form of the same chromophores presents a
bent shape that contributes less to the nematic potential and
reduces the local order parameter. When the trans-to-cis
photoisomerization takes place (Figure 7), a decrease in the local
order parameter occurs, and it is extended to all the nematic
mesophase (domino effect).48 This effect is evident, for example,
in the decrease in the nematic-to-isotropic phase transition
temperature, TN-I, of a 2-5CB mixture with an azobenzene
molar fraction of x ) 0.1 after irradiation with UV light. When
the trans isomer of azocompound 2 is present in the mixture, it
shows a TN-I value of 38.5 °C, but when the sample is irradiated
with UV light, the TN-I of the mixture decreases to 34.3 °C
due to the presence of the bent cis isomer in the nematic
solution.28 Similar results were obtained with mixtures of
azocompound 1 with 5CB at the same molar fraction of the
dye. This mixture decreased its TN-I value from 43.0 to 37.1
°C under irradiation with UV light. Otherwise, no changes were
detected by POM in the nematic samples used in the kinetic
study due to the small content of the dye. In this way, these
nematic samples exhibited the same TN-I value as the pure 5CB
(35 °C), and they did not experience any change in their TN-I
under irradiation with UV light. The trans-to-cis photoisomer-
ization produces a decrease in the local order parameter of the
nematic mesophase. Therefore, in isotropic solvents, the trans-
to-cis isomerization of the chromophore can be attained more
easily than in nematic liquid crystals. Consequently, the trans-
to-cis photoisomerization will be more favorable and faster in
an isotropic environment. The data indicated in Table 1, as well
as the time needed for achieving the photostationary state (see
above), fully agree with this fact.
As for the thermal cis-to-trans kinetico-mechanistic study,
the solvents have been chosen to cover different structural
features: polar protic solvents, aprotic solvents with strong polar
functions, nonpolar solvents, nonpolar solvents able to establish
π-π interactions with the azo-dye, and the monodomain and
polydomain nematic phase and isotropic phase of 5CB. As a
whole, the processes clearly occur via the same intimate mechanism
for all the solvents and phases used in the study, as can be seen in
the compensation plot of ∆H+ versus ∆S+ shown in Figure 8. A
common isokinetic relationship has been found to be indicative of
the existence of a single reaction mechanism.49-51 According to
this, the mechanism actuating in the isotropic solvents is the
same as in the liquid crystalline matrix, independent of whether
it is in the isotropic or in the nematic state or if it is a
monodomain or a polydomain mesophase. The isokinetic
temperature of the plot, that is, the temperature at which the
rate of the process is coincident for all the environments studied,
corresponds to ∼40 °C. This value is close to the nematic-to-
isotropic phase transition temperature of the 5CB-azo mixtures
(35 °C).
The measurement of the volumes of activation produces for
these processes unequivocal evidence about the operation of
the inversion or rotation isomerization mechanism (Figure 1).49,52
The sensitiveness to medium polarity changes of the azocom-
pound during the activation process should produce an accelera-
tion of the thermal cis-to-trans isomerization on increasing the
pressure for the rotational mechanism on polar solvents; that
is, electrostriction. In contrast, this effect should not be observed
in either polar or nonpolar solvents if the reaction takes place
via the inversional mechanism depicted in Figure 1.53 The
thermal cis-to-trans isomerization process of the azobenzene-
bridged crown ether 3,3′-[1,10-diaza-4,7,13,16-tetraoxa-18-
crown-6]-biscarbonylazobenzene cannot occur via the rotational
mechanism due to structural restrictions, and it is normally taken
as a defined standard for the inversion mechanism.54 The
determination of the volumes of activation for the thermal cis-
to-trans isomerization process of this azo-crown ether in several
solvents has been carried out, and the values found in the
literature were all close to zero, as expected.53 In contrast, for
push-pull azobenzenes, which undergo isomerization via the
rotation mechanism, the values of the volumes of activation have
been reported in the -20 cm3 mol-1 range in several solvents.26,55
It is thus clear from the experimental data collected in Table 2
that the isomerization of azocompounds 1 and 2 occurs via the
inversional mechanism (∆V+ ∼ 0, Figure 6 right).
Further evidence that azocompounds 1 and 2 undergo their
thermal isomerization through the inversion mechanism comes
from the analysis of the dependence of the first-order rate
constant values with the dielectric constant of the solvent used.
The values of the kinetic and activation parameters in Table 2
show no dependence of the rate constant on the dielectric
constant of the solvent. For the push-pull 4-N,N-(dimethyl-
amino)-4′-nitroazobenzene, which has been proposed to isomer-
ize through the rotational mechanism, rate constants have been
reported that show a dramatic dependence on the dielectric
constant of the solvent (2.71 × 10-3 s-1 in pentane; 298 s-1 in
dimethylsulfoxide).55,56
Once the isomerization mechanism actuating for azocom-
pounds 1 and 2 has been unequivocally established as the
inversional one (Figure 1) and found to be invariable for all
the solvents studied, a detailed examination of the differences
in Table 2 on the solvent nature is needed. Whereas the cis
form has a t1/2 between 8 and 13 h at 298 K in all the isotropic

1292 J. Phys. Chem. B, Vol. 114, No. 3, 2010
Garcia-Amoro´s et al.
state of the isomerization process has a non-polar character,
indicating the operation of an inversion mechanism. The analysis
of the kinetic and activation parameters obtained for the thermal
cis-to-trans isomerization process of azo-dyes 1 and 2 in
isotropic and in a liquid-crystalline environment indicates that,
even though the actuating mechanism is the same, there is a
great influence of the system order in the rate of the process.
An acceleration of the thermal cis-to-trans isomerization process
in the liquid crystalline media is observed, and it is related to
a cooperative interaction established between the mesogenic
molecules and the azo-dye; the effect is lost when the isotropic
state is reached.
Acknowledgment. Financial support from the European
project: Functional Liquid-Crystalline Elastomers (FULCE-
HPRN-CT-2002-00169) and from the Ministerio de Educacio´n
y Ciencia (Projects CTQ-2009-13797-C02-02 and CTQ-2009-
14443-C02-02) is gratefully acknowledged. J. Garcia-Amoro´s
is grateful for the award of a doctoral grant from the Universitat
de Barcelona. The authors thank Carlos Rodr´ıguez del R´ıo for
his help in the measurement of volumes of activation.
Figure 9. Eyring plot for the thermal cis-to-trans isomerization process
of azocompound 2 in the nematic and isotropic phases of mesogen
5CB.
solvents, this value decreases to around 4-5 h in the oriented
nematic phase of the mesogen 5CB at the same temperature.
In a polydomain nematic solution, the value for t1/2 is slightly
larger than that registered in the monodomain sample. This
difference is accompanied by changes in both enthalpies and
entropies; that is, a more organized transition state, including
solvent environment, implies lesser enthalpic demands.
Figure 9 shows the Eyring plot for the thermal isomerization
process carried out in a 2-5CB mixture in a temperature range
that covers both nematic and isotropic phases. Two well-defined
regions that correspond to the nematic and isotropic phases can
be identified. The crossing between the two lines is about at 37
°C which is, within error, the measured TN-I value of the 2-5CB
mixture (35 °C). Below this temperature, the sample is in the
nematic phase with thermal activations parameters (Table 2)
distinct from those observed at higher temperatures in the
isotropic state and enthalpy and entropy of activation in line
with the other isotropic solvents used in this study. Similar
results were obtained for 1-5CB mixtures.
Supporting Information Available: Microphotographs of
a monodomain and a polydomain sample obtained with a solid
solution of azocompound 2 in the nematic mesogen 5CB at 298
K. Values of the observed rate constants for all the reactions
studied as a function of solvent, temperature and pressure for
azoderivatives 1 and 2. This material is available free of charge
via the Internet at http://pubs.acs.org.
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