Kinetics of thermal: Cis - Trans isomerization in a phototropic azobenzene-based single-component liquid crystal in its nematic and isotropic phases

Article abstract of DOI:10.1039/c7cp06820d

Single-component azobenzene-based phototropic liquid crystals (PtLC) are promising materials that have started to be explored for photonic applications. One of the essential factors determining the applicability of these materials is the rate of the thermally driven cis-trans isomerization. In this paper, the kinetics of the thermal back cis-to-trans reaction in a pure 4-hexyl-4′-methoxyazobenzene (6-AB-O1) compound in its isotropic liquid and nematic phases is studied (the undoped LC). The reaction rate constants, activation energies and thermal activation parameters were determined based on spectroscopic studies. The reaction kinetics is compared to that measured for the compound dissolved in chloroform. The results demonstrate that the thermal back reaction depends on the phase and molecular environment of the cis-isomer. Moreover, the effect of temperature on the absorption spectra of 6-AB-O1 in its isotropic, nematic and crystalline phases is examined. The changes in the compound's absorption spectra in the respective phases have been correlated to the positional order parameter S.

Full text of DOI:10.1039/c7cp06820d

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Kinetics of thermal cis-trans isomerization in phototropic azobenzene-
based single-component liquid crystal in its nematic and isotropic phases
Andrzej Miniewicz, Hanna Orlikowska, Anna Sobolewska* and Stanislaw Bartkiewicz
Advanced Materials Engineering and Modelling Department, Faculty of Chemistry, Wroclaw
University of Science and Technology, Wybrzeze Wyspianskiego 27, 50ꢀ370 Wroclaw,
Poland, Fax: (048) 71 320 33 64
Andrzej Miniewicz, andrzej.miniewicz@pwr.edu.pl Phone: +48 71 320 35 00
Hanna Orlikowska: 221300@student.pwr.edu.pl Phone: +48 71 320 24 12
Anna Sobolewska, Corresponding author: anna.sobolewska@pwr.edu.pl Phone: +48 71 320
39 24,
Stanislaw Bartkiewicz: stanislaw.bartkiewicz@pwr.edu.pl Phone: +48 71 320 24 12
1

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Abstract
Singleꢀcomponent azobenzeneꢀbased phototropic liquid crystals (PtLC) are promising
materials that have been started to be explored for photonic applications. One of the essential
factors determining the applicability of these materials is the rate of the thermally driven cis-
trans isomerization. In this paper, the kinetics of thermal back cis-to-trans reaction in a pure
4ꢀhexylꢀ4’ꢀmethoxyazobenzene (6ꢀABꢀO1) compound in its isotropic liquid and nematic
phases is studied (the undoped LC). The reaction rate constants, activation energies and
thermal activation parameters were determined based on spectroscopic studies. The reaction
kinetics is compared to that measured for the compound dissolved in chloroform. The results
demonstrate that the thermal back reaction depends on the phase and molecular environment
of the cisꢀisomer. Moreover, the effect of temperature on the absorption spectra of 6ꢀABꢀO1
in its isotropic, nematic and crystalline phases is examined. The changes in the compound’s
absorption spectra in the respective phases have been correlated to the positional order
parameter S.
2

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Introduction
Azobenzene and its derivatives belong to the large family of photochromic organic molecules.
Their role, however, is much more important than any other group of photochromic
compounds due to the proven role in a multitude of the molecular as well as macroscopic
processes that can be controlled by their photoisomerization property. Azobenzene and its
derivatives undergo a clean photophysical and reversible isomerization process1 that depends
on their substituents as well as the molecular environment. Photoisomerization of
azobenzenes could be initiated almost in any matrix including organic solvents,2 liquid
crystals (LC’s),3ꢀ5 polymeric liquid crystals,6 polymers,7 bioꢀpolymers,8ꢀ10 organic glasses,11
nanoparticles,12 large molecules13ꢀ15 and even solid microcrystals.16,17 Azobenzene derivatives
are used to remotely (by light) trigger or control:
the helical twisting power,18
fluorescence,19,20 the geometrical structure or molecular machines,21,22 the surface
wettability,23,24 polymer surface reliefs,25ꢀ28 the surface command for ordering LCs,29,30
artificial muscleꢀlike photoactuators,31 the polymer elastomer film bending and shrinking. 32,33
They are widely used in optical processing, memory and data storage,34ꢀ37 holography and
nonlinear optics,38ꢀ40 optical phase conjugation,41,42 etc. just to mention only the most
spectacular examples of a very broad list reported in the literature.
Upon absorption of a photon by an azobenzene molecule the thermodynamically
stable rodꢀlike trans-isomer is transformed to the metastable bend cis-isomer. The inverse
process cisꢀtoꢀtrans isomerization can be initiated by an absorption by the cisꢀisomer of
another photon of a suitable energy or can proceed spontaneously in darkness driven by heat.
The conformational changes that occur in azobenzene derivatives have been studied
experimentally and theoretically.43 Illumination of a solution or solid sample containing
azobenzene derivatives with UV light for some time interval leads to a photostationary state,
in which the fraction of cisꢀisomers depends on the quantum yields of the two isomerization
processes, light fluence and on the thermal relaxation rate. The molecular electronic structure
of the particular azobenzeneꢀdye strongly affects the rate of thermal reconversion. In
azobenzeneꢀtype molecules the photoinduced cisꢀform can persist in darkness for hours
whereas the cisꢀlifetimes of aminoazobenzenes and pseudostilbenes are in the order of
minutes and seconds, respectively.44 It is widely accepted that thermal conformational
transformation from cisꢀtoꢀtrans occurs either via outꢀofꢀplane rotation around the N–N
bond45,46 or via inꢀplane inversion around the N=N bond.47,48 The former isomerization
process is characterized by nonꢀzero volume of activation ꢀV≠ whereas the isomerization
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through inversion is characterized by almost zero volume of activation ꢀV≠ ≈ 0 indicating that
the reaction rate should depend on the matrix properties especially on the character of voids
enabling particular mechanism of the reaction. We did not observed the volume of activation
in our experiments, but we are convinced that the mechanism of cisꢀtoꢀtrans back reaction is
largely determined by the phase in which the reaction occurs. The mechanisms of thermal
back cisꢀtoꢀtrans reaction in various azobenzene derivatives have been extensively studied in
a series of excellent papers by GarciaꢀAmoros et al.49ꢀ52 who observed kinetics of this reaction
in various organic solvents, nematic liquid crystals and liquid crystal elastomers. Depending
on the molecular structure and the isomerization mechanism the characteristic relaxation time
(τ = 1/k, here k is the reaction rate constant) for thermal cisꢀtoꢀtrans isomerization process, of
the lowꢀmolecular mass azobenzene derivatives, could extend from hundreds of milliseconds
to several tens of hours.52 The investigations also pointed out on the important influence of
the ordered nematic matrix on the back reaction relaxation time.52
Most of the previous studies devoted to the mechanisms and kinetics of thermal back cisꢀtoꢀ
trans reaction of azobenzene derivatives, that were performed for liquid crystal matrices, have
been done in at least twoꢀcomponent systems, e.g. liquid crystal with an azoꢀdye as a dopant
molecule.12,49ꢀ53 However, there is a large family of azobenzeneꢀbased liquid crystals for
which the photochromic reactions were not systematically and in detail studied, e.g. ref. 54.
Azobenzeneꢀbased liquid crystals are interesting because of their promising nonlinear optical
and holographic properties as well as optomechanical ones. Absorbed light can induce the
same processes, i.e. disruption of the nematic phase or change of the refractive index due to
the appearance of lightꢀinduced nonꢀmesogenic cisꢀisomers in otherwise ordered nematic
phase, i.e. the same phenomenon that was observed in nonꢀazobenzene nematic crystals doped
with azobenzene nematogenes. Our recent publications55ꢀ57 devoted to study of the behaviour
of phototropic azobenzeneꢀbased liquid crystals under the laser light irradiation have largely
stimulated this work. Phototropic liquid crystals belong to a new class of liquid crystals that
are controlled using light.58 The understanding of the photochromism mechanisms involved in
these processes cannot be completed without knowledge of thermal back cisꢀtoꢀtrans
isomerization process in the phototropic liquid crystals.
Therefore, in this work, for the first time, to the best of our knowledge, we undertook a study
of the thermal back reaction occurring in an exemplary single component phototropic
azobenzene liquid crystalline compound 4ꢀhexylꢀ4’ꢀmethoxyazobenzene (6ꢀABꢀO1).59 Our
intention was to measure the kinetics of thermal back cisꢀtoꢀtrans reaction in a pure
compound using UVꢀVis spectroscopy technique in the samples thermostated during
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photoisomerization process within a wide temperature range encompassing the isotropic
liquid phase (Iso), through the nematic liquid crystal phase (N) and ending with the solid state
polycrystalline phase (Cr). For the reference purpose we also measured the kinetics of the
thermal back reaction of the 6ꢀABꢀO1 compound when diluted in weakly polar solvent
chloroform (CHCl3). We provide the discussion of the obtained results.
Experimental Section
Materials
4ꢀhexylꢀ4’ꢀmethoxyazobenzene (6ꢀABꢀO1) from the family of 4ꢀalkylꢀ4’ꢀalkoxyazobenzenes
(mꢀABꢀOn, m, n = 1,2,3 …) has been obtained from Authors of publications54,59 who
synthesized it and was used directly for our kinetics study.54,59 The chemical formula of 6ꢀ
ABꢀO1 is shown in the inset to Figure 1. According to Zienkiewicz and Galewski59, the final
product of synthesis (i.e. 6ꢀABꢀO1) was two times purified by flash column chromatography
(Fluka silica gel 60 mesh, CHCl3 as an eluent), recrystallized from acetone and few times
melted in vacuum, the compound purity was estimated as 99%. 59 The 6ꢀABꢀO1 compound
is a thermotropic and phototropic singleꢀcomponent liquid crystal.60 The sequence of 6ꢀABꢀ
O1 phase transitions on cooling from its isotropic liquid phase has been determined by
Zienkiewicz and Galewski59 using the polarizing microscope (POM) technique and
differential scanning calorimetry (DSC) method: Cr 286.1 N 324.6 Iso, where Cr is an
abbreviation for a crystal, N is the nematic phase and Iso is the isotropic liquid phase.
Sample preparation
The pure 6ꢀABꢀO1 compound was heated above 328 K and then was introduced into the
liquid crystal cell by capillary forces. The cell was composed of two bare glass plates without
ordering layers and separated by 5 ꢁm. Schlieren texture in the nematic phase has been
observed under the polarizing microscope (POM). From these observations we concluded that
the liquid crystal alignment is degenerate planar at least in the nematic phase (cf. Fig. 1 e).
Measurements of the kinetics of the cis-trans thermal relaxation reaction were performed after
sample photoexcitation with UV light separately for the material in its isotropic and liquid
crystalline phases. In addition, the kinetics of photochromic thermal back reaction was studied
for 6ꢀABꢀO1 dissolved in chloroform (C = 7.6×10ꢀ5 M). In this case, 1 cm optical path quartz
cuvette was used.
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Instrumentation
The measurements of UVꢀVis absorption spectra of the compound in its isotropic, nematic
and crystalline phases were carried out with a Cary 3 UVꢀVisible spectrophotometer (Varian).
Temperature controller unit comprised of the Peltier cell and the sample holder was placed in
the spectrophotometer chamber. The temperature of the sample was controlled using Digital
Jabel thermoregulator connected to a power supply. The samples were allowed to equilibrate
at the desired temperature for few minutes before the start of the data collection.
The effect of temperature on the shape and the intensity of the absorption bands of 6ꢀ
ABꢀO1 in different phases was studied by placing the filled LC cell in the sample holder,
heating above the LC clearing temperature (TNI = 324.6 K) and then measuring absorption
spectra while the temperature was decreasing. As a result of the gradual temperature decrease
the phase transitions from the isotropic phase, through the nematic liquid crystal phase to the
solid crystalline phase occurred in the sample.
For studies of the kinetics of photochromic reaction, series of measurements of
absorption spectra of the 6ꢀABꢀO1 compound in the isotropic and nematic phases were
performed in the temperature ranges of their stability. The procedure was following: firstly,
the sample was heated up to the desired temperature and the absorption spectrum of the nonꢀ
irradiated by UV sample was measured. Subsequently, the sample area was irradiated for 2
minutes using a semiconductor laser of wavelength 405 nm and the power of 70 mW (beam
diameter 3 mm). During this time interval the photoisomerization reaction has reached
photostationary state dominated with cis-isomers of 6ꢀABꢀO1. After switching off the laser
light, thermal cisꢀtoꢀtrans isomerization was monitored by changes in the UVꢀVis absorption
spectra of the sample, thermostated in the dark at the desired temperature. Due to extremely
high absorbance of the compound only the absorption of the n–π* band peaking at about 440
nm could be measured, while usually for diluted azobenzene derivatives the absorption
changes of the π*–π* centered at about 350 nm are analysed. For each of the phases (isotropic
liquid or nematic liquid crystal) the measurements were carried out at several different
temperatures. The cis-trans thermal relaxation kinetic of 6ꢀABꢀO1 in the isotropic phase was
measured at temperatures 333, 348 and 363 K while in the nematic phase at 307, 315 and 319
K. The registered spectra were normalized by dividing the absorbance A(λ) by the value of
the absorbance A(410 nm) at the isosbestic point.
Measurements of the UVꢀVis spectra of thermal cisꢀtoꢀtrans back isomerization for
the solution of 6ꢀABꢀO1 in chloroform (C = 7.6×10ꢀ5 M) were performed using PerkinꢀElmer
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Lambda 20 UVꢀVis spectrophotometer with buildꢀin temperature controller unit. The
absorption spectrum of the nonꢀirradiated quartz cuvette filled with the solution was measured
at first. Then the cuvette was irradiated for 10 minutes in the spectral range of 390 – 415 nm
using a xenon lamp and the bandꢀpass glass filter. The cis-trans thermal relaxation was
studied at temperatures 296, 303, 313, 323 and 333 K. The registered spectra were normalized
by dividing the absorbance by the value of the absorbance at the isosbestic point, which in this
case appeared at λ = 416 nm. The process of spectra measurement with Lambda 20 UVꢀVis
spectrophotometer in the range of wavelengths 380 to 600 nm did not influence the spectra
shapes or band intensities, possibly due to short illumination time and relatively weak light
intensity in the spectrophotometer.
Results and Discussion
The effect of temperature on the absorption spectra of the 6-AB-O1 compound in its
different phases
The studied 6ꢀABꢀO1 compound is the azobenzeneꢀbased thermotropic liquid crystal and
thereby its UVꢀVis absorption spectrum depends on the temperature. Fig. 1a shows the
influence of the temperature on the shape and the intensity of the absorption bands of 6ꢀABꢀ
O1. The temperature decrease causes that the material undergoes the phase transitions from
the isotropic liquid phase, through the nematic LC phase to the crystalline phase which can be
observed in changes of the compound’s absorption spectrum. The absorption spectra of 6ꢀABꢀ
O1 measured in the range 380 – 600 nm in the isotropic phase exhibit the band with
maximum at ca. λ = 436 nm, that corresponds to the n–π* transition band. The absorbance
weakly changes with temperature decrease until its sudden drop at the IsoꢀN transition
temperature TIꢀN = 324.65 K. Within the nematic phase the change (decrease) in the
absorbance with temperature of the n–π* band is much more pronounced than in the isotropic
phase. When 6ꢀABꢀO1 becomes a solid crystal the intensity of the n–π* band significantly
decreases and finally the single band is replaced by two weak bands centered at 420 nm and
460 nm. Apparently, the absorption from the ground trans form to the excited trans* form
through the n–π* transition is prohibited in the crystal. The further temperature decrease does
not affect the absorbance spectrum when the material is in the crystal form. It is clear from
Fig. 1 that the absorbance depends on the phase at which the material is, as well as, that the
absorption of the compound in its crystalline form is significantly different from those
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observed in its both liquid phases. The light scattering in the solid state becomes important,
i.e. reaching up to 10 % of the maximum absorbance in this phase, therefore we decided to
correct the spectra by subtracting the measured absorbance of a solidified compound in the
cell by the value of absorbance at wavelength of 600 nm, i.e. outside its absorption range.
Because the absorbance significantly drops at the crystalline phase of 6ꢀABꢀO1 the compound
changes a colour from being brownish in its liquid (isotropic) and the LC phases to yellowish
in the solid state (cf. Fig. 1b, c and d). Analysis of the absorbance data shown in Fig. 1a leads
to an interesting findings. The abrupt jumps in the absorption measured at 436 nm when 6ꢀ
ABꢀO1 undergoes the phase transitions from the isotropic to the nematic phase and then to the
crystalline phase evidently suggest that the positional order parameter S of mesogenes plays
an important role and is somehow correlated with the absorbance magnitude (cf. Fig. 2a). As
it is known for calamitic liquid crystals S = 0 in the isotropic phase, jumps to about 0.4461 at
the IsoꢀN phase transition temperature, next smoothly increases with temperature and finally
jumps to S = 1 when the nematic phase disappears and the crystal is formed. Moreover inside
the nematic phase one observes a systematic, temperature dependent change of the absorption
on cooling the sample from high temperature to room temperature.
a
)
3
2
1
0
Iso
LC
Cr
400
450
500
550
600
Wavelength [nm]
b) Cr
c) LC
d) Iso
T
286 K
325 K
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Fig. 1 a) Changes in the UVꢀVis absorption spectra of 4ꢀhexylꢀ4’ꢀmethoxyazobenzene (6ꢀABꢀ
O1) compound in the LC cell during temperature decrease from 340 to 280 K. The inset
shows the chemical structure of 6ꢀABꢀO1. (b, c and d) Photos of the LC cell containing 6ꢀABꢀ
O1 in different phases b) crystalline (Cr), c) liquid crystalline (LC) and d) isotropic (Iso). e)
Schlieren texture of 6ꢀABꢀO1 taken under the polarizing microscope (POM) in its nematic
phase. The compound was placed in 5 ꢁm thick bare glass liquid crystal cell.
The minute changes in the spectra of the crystalline form with temperature decrease as
observed in Fig. 1 a) we attribute to the defects at which remnant nematic phase can still exist
during cooling process. At low temperatures (i.e. 286 K) the spectrum is fully stabilized and
no changes with time were detected by absorbance measurements which allowed us to draw a
conclusion that the cisꢀform transformation is blocked at least in the crystal bulk.
By assigning the value of the absorbance measured within the isotropic phase at 325 K
to the S = 0 value and the absorbance of the crystalline phase at 286 K to S = 1 and replotting
correspondingly within that scale the remaining points we get a plot showing temperature
dependence of the nematic order parameter in function of reduced temperature ꢀꢁꢂꢃ ꢄ ꢀ/ꢀꢅꢆ
(cf. Fig. 2b). Analysis of this plot resembles plots of an order parameter temperature
dependence S(T) of typical nematic liquid crystals (cf. Fig. 5 of the reference 49) in
accordance with MaierꢀSaupe theory61 ꢇ ꢀ ꢄ 1 ꢊ ꢋ ∙ ꢀ ꢀꢅꢆ , where c ≈ 1 and
β
is a
ꢈ ꢉ
ꢈ
⁄
ꢉꢌ
parameter dependent on the mesogene structure and usually smaller than 0.25.
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2.0
a)
1.5
1.0
Iso
Cr
N
0.5
280
300
320
340
360
Temperature [K]
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
b)
6ABO1
TNI = 324.65 K
Tm = 286.15 K
Cr
Iso
N
0.9
1.0
1.1
T/TNI
Fig. 2 a) Absorbance of 6ꢀABꢀO1 measured at λ = 436 nm in function of temperature on
cooling the LC cell from 370 K down to 275 K. Shadowed area shows the temperature range
of existence of the nematic phase as established from DSC measurements. b) Absorbance
from Fig. 2a replotted in the scale 0 – 1 according to the order parameter S characteristic for
the isotropic liquid (S = 0) to the crystal (S = 1) in function of the reduced temperature
ꢀꢁꢂꢃ ꢄ ꢀ/ꢀꢅꢆ. The order parameter S(T) within the nematic phase behaves as expected in
terms of its temperature dependence. Tm – melting temperature, TNI –temperature of phase
transition from nematic to isotropic phase.
The drop of the absorbance upon the IsoꢀN phase transition observed in 6ꢀABꢀO1 (cf. Fig. 1
and Fig. 2a) could be related to the few phenomena. One can mention: (i) effect of molecular
ordering, (ii) the formation of molecular Hꢀaggregates, (iii) observation of temperature
equilibrium of cisꢀisomers or (iv) a process of yet unknown origin. Neither of them can be
unequivocally confirmed at this moment. The first (i) mechanism could be explained only on
supposition that the strong anchoring of homeotropic type imparts almost perpendicular to the
substrate alignment of transꢀisomers when in the nematic phase. In that case, on the phase
ꢏ ꢈ
transition Iso to N the absorbance is expected to drop from the value ꢍꢎ ꢄ ꢐ ꢍ∥ + 2ꢍꢑ to
ꢉ
the value of A with a condition that: ꢍ∥ > A > ꢑ. Here ꢍ∥ and ꢍꢑ represent the absorbances
ꢍ
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measured at perpendicular directions to the involved transition dipole moment of the molecule
in its transꢀisomer form. Then A(T) should decrease with a decrease of temperature toward
the value of ꢍꢑ. On the basis of POM observation of typical nematic texture (cf. Fig. 1 e) we
decided to rule out this mechanism. The second possible mechanism (ii) could be related with
the formation of Hꢀtype dimers. Such a situation is favoured when transꢀisomers are axially
aligned. In this case the splitting of electronic excited states occurs and the light absorption
transition to the lower state is forbidden, then the absorption band is hypsochromically shifted
toward shorter wavelengths. The observation of the respective absorption band in this case is
impossible due to the already large absorbance in a 5 ꢁm thick layer of 6ꢀABꢀO1, but we plan
to perform quantum chemical calculations to predict whether such dimerization is possible.
The third mechanism (iii) related to absorbance of cisꢀisomers would be possible under
assumption that the cisꢀisomers are activated by elevated temperature or by the weak light of
spectrophotometer during process of spectra measurements. Additional measurements,
especially of the refractive index in homogenic LC cells using linearly polarized light and
their correlation with absorption spectra, are planned to find out which mechanism is
responsible for the observed changes in the spectra upon temperature decrease.
The kinetics of photochromic back reaction of the 6-AB-O1 compound in the isotropic
and nematic phases
The irradiation of 6ꢀABꢀO1 compound in its isotropic liquid phase or in the nematic phase by
light of wavelength 405 nm induces the trans-cis isomerization reaction which leads to an
increase of the concentration of the cis-form in the sample volume. As a result the absorbance
of the π−π∗ band in the UV spectral range (~ 370 nm) decreases and the absorbance of the
n–π* band centered at around 436 nm increases. After switching off the light, the thermally
driven cis-trans isomerization of 6ꢀABꢀO1 occurs in the sample which can be investigated by
monitoring changes in the maximum absorbance of the n–π* band in function of time. The
intensity of the band decreases due to thermal relaxation process. In Fig. 3 are shown the
results of kinetic studies of the cis-trans thermal relaxation of the 6ꢀABꢀO1 compound in the
isotropic phase carried out at three different temperatures 363, 348 and 333 K. Based on the
measured absorption spectra (cf. upper row of Fig. 3), the rate constants (cf. lower row of Fig.
3) and the activation energies of thermal cis-trans isomerization reaction were determined.
Note that the isosbestic point is clearly shown in any of the studied phases.
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a) T = 363 K
b) T = 348 K
c) T = 333 K
1.5
1.0
0.5
0.0
1.5
1.5
1.0
0.5
0.0
Time
1.02
1.00
0.98
1.02
1.00
0.98
1.02
Time
Time
1.00
1.0
0.5
0.0
0.98
409
409
410
411
410
411
409
410
411
Wavelength [nm]
Wavelength [nm]
Wavelength [nm]
400
450
500
550
600
400
450
500
550
600
400
450
500
550
600
Wavelength [nm]
Wavelength [nm]
Wavelength [nm]
d) T = 363 K
e) T = 348 K
f) T = 333 K
0
-2
-4
-6
0
-1
-2
-3
0
-1
-2
-3
kꢀ = 1.08*10-2 s-1
-1
kꢀ = 2.90*10-3 s-1
kꢀ = 9.90*10-4
s
0
200 400 600 800 1000
Time [s]
0
200
400
Time [s]
600
0
1000
2000
3000
Time [s]
Fig. 3 (a, b and c) Changes in the absorbance due to the thermally driven cis-trans
isomerization of the 6ꢀABꢀO1 compound in the isotropic liquid (Iso) phase measured at
different temperatures: a) 363 K, b) 348 K and c) 333 K; insets show a closer look at the
isosbestic point. (d, e and f) The method of determination of the rate constant of thermal cis-
ꢖ ꢘꢖ
ꢗ
∞
ꢈ ꢉ
trans isomerization reaction (ꢒꢀ ) of 6ꢀABꢀO1 in the Iso phase, where ꢓ ꢄ ꢔꢕ ꢖ ꢘꢖ
.
ꢙ
∞
The rate constants of the cis-trans isomerization reaction in the dark (kꢀ), obtained for each
mentioned above temperature, were determined from the equation derived on the assumption
of the first order kinetics of the isomerization reaction and the applicability of the Lambertꢀ
Beer law:
ꢖ ꢘꢖ
ꢗ
∞
ꢈ ꢉ
ꢚ ꢓ ꢄ ꢔꢕ ꢖ ꢘꢖ ꢄ ꢊꢒꢀꢓ
(1)
ꢙ
∞
where A0 corresponds to the initial absorbance (at the beginning of the back reaction), At is the
momentary absorbance (changing during the reaction) and A∞ is the absorbance in the
equilibrium (after the reaction was finished).53 The values of A0, At and A∞ were measured at
the wavelength of λ = 440 nm (corresponding to the maximum of the n–π* transition band).
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The temperature dependences of the rate constants of the thermal cis-trans isomerization are
expect to follow the Arrhenius equation:
ꢞ
ꢠꢡ
ꢟ
ꢢ
ꢒꢀꢈꢀꢉ ꢄ ꢛ
⋅
ꢜꢝꢘ
(2)
where Ea is the activation energy of the reaction,
ꢛ
is the preꢀexponential factor, R is the gas
constant and T is temperature in the absolute scale. The analogical measurements and the
same analysis of the results were performed for 6ꢀABꢀO1 in the liquid crystalline phase. Fig.
4 shows the results of studies of the cis-trans thermal reaction of 6ꢀABꢀO1 in the nematic
phase performed at temperatures 319, 315 and 307 K.
a) T = 319 K
b) T = 315 K
c) T = 307 K
1.01
1.01
1.5
1.0
0.5
0.0
1.5
1.0
0.5
0.0
1.5
1.0
0.5
0.0
Time
Time
Time
1.02
1.00
1.00
1.00
0.99
410
0.98
410
0.99
410
411
412
411
412
411
412
Wavelength [nm]
Wavelength [nm]
Wavelength [nm]
400
450
500
550
600
400
450
500
550
600
400
450
500
550
600
Wavelength [nm]
Wavelength [nm]
Wavelength [nm]
e) T = 315 K
d) T = 319 K
f) T = 307 K
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
0.0
-0.1
-0.2
-0.3
-0.4
kꢀ = 1.75*10-4 s-1
kꢀ = 3.15*10-4 s-1
kꢀ = 4.87*10-5 s-1
0
1000
2000
3000
0
1000
2000
3000
0
2000 4000 6000 8000
Time [s]
Time [s]
Time [s]
Fig. 4 (a, b and c) Changes in the absorbance due to the thermally driven cis-trans
isomerization of the 6ꢀABꢀO1 compound in the liquid crystalline (nematic) phase measured at
different temperatures: a) 319 K, b) 315 K and c) 307 K; insets show a closer look at the
isosbestic point. (d, e and f) The plots allowing the determination of the rate constants kꢀ of
thermal cis-trans isomerization reaction of 6ꢀABꢀO1 at different temperatures in the nematic
phase.
The kinetics of photochromic back reaction of 6-AB-O1 in chloroform solution
The absorption spectrum of the solution of the 6ꢀABꢀO1 compound in chloroform exhibits
two distinct bands, this was possible due to wider than before measurement spectral range
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because of much smaller extinction (cf. Fig. 5). The stronger one located at ca. λ = 345 nm,
corresponds to the π–π* transition, and the weaker at λ = 445 nm is related to the n–π*
transition band. In this case no influence of the temperature on the absorption spectrum of the
6ꢀABꢀO1 chloroform solution was observed. The irradiation of the 6ꢀABꢀO1 solution by light
from the spectral range 390 – 415 nm induces the trans-cis isomerization reaction. It leads to
a decrease of the concentration of the trans form which is manifested as a significant decrease
of the intensity of the π–π* band and a small rise of the intensity of the n–π* band (cf. Fig. 5).
After switching off the UV light, thermal cis-trans isomerization of 6ꢀABꢀO1 occurs which
was examined this time by monitoring changes in the absorbance of the π–π* band. Due to
thermal relaxation and the recovery of thermodynamically more stable trans form, the
intensity of the π–π* band increases. The thermal cis-trans isomerization kinetics of the 6ꢀABꢀ
O1 compound dissolved in a solvent was studied at five temperatures: 296, 303, 313, 323 and
333 K. Fig. 5 depicts the results obtained for temperatures 333, 323 and 303 K.
a) T = 333 K
b) T = 323 K
c) T = 303 K
2.5
2.0
1.5
1.0
0.5
0.0
2.5
2.0
1.5
1.0
0.5
0.0
2.5
2.0
1.5
1.0
0.5
0.0
0.129
0.126
0.132
0.129
0.132
Time
0.129
0.126
0.123
Time
Time
0.123
0.126
0.123
0.120
414 415 416 417
418
414 415 416 417
418
414 415 416 417
418
Wavelength [nm]
Wavelength [nm]
Wavelength [nm]
300
400
500
600
300
400
500
600
300
400
500
600
Wavelength [nm]
Wavelength [nm]
Wavelength [nm]
d) T = 333 K
e) T = 323 K
f) T = 303 K
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
0.0
-0.2
-0.4
-0.6
-0.8
0.0
-0.2
-0.4
-0.6
kꢀ = 5.50*10-6 s-1
kꢀ = 1.02*10-4 s-1
kꢀ = 4.89*10-4 s-1
0
500
1000
1500
0
2000
4000
6000
0
10000
20000
Time [s]
Time [s]
Time [s]
Fig. 5 (a, b and c) Changes in the absorbance due to the thermally driven cis-trans
isomerization of 6ꢀABꢀO1 dissolved in chloroform (C = 7.6×10ꢀ5 M) measured at different
temperatures: a) 333 K, b) 323 K and c) 303 K; insets show a closer look at the isosbestic
point. (d, e and f) The plots allowing the determination of the rate constants kꢀ of thermal cis-
trans isomerization reaction of 6ꢀABꢀO1 in chloroform solution.
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A good retention of isosbestic points in all studied phases excludes the photochemical
degradation of the compound. Absorbance versus time traces fit well the first order reaction
mechanism. The results of the thermal cis-trans isomerization kinetics measurements of the
investigated compound in all studied cases: the isotropic liquid (Iso), nematic liquid crystal
(NLC) phase and in a solution are compared in Fig. 6 in a form of Arrhenius plot and gathered
in Table 1. The dotted lines in Fig. 6 represent the linear fit of the data, and the slopes of the
lines have been used to estimate the activation energies Ea.
-4
-6
77.4 kJ mol-1
-8
127.0 kJ mol-1
-10
NLC
Iso
Solution
-12
-14
117.9 kJ mol-1
0.0028 0.0030 0.0032 0.0034
T-1 K-1
[
]
Fig. 6 Comparison of the kinetics of the thermal cis-trans isomerization of the 6ꢀABꢀO1
compound in the isotropic phase (Iso) (circles), in the nematic liquid crystal phase (NLC)
(squares) and in a solution (triangles): a plot of the natural logarithm of the rate constant kꢀ
(T) of thermal relaxation as a function of inverse temperature (Arrhenius plot). The values of
activation energies Ea are indicated, respectively.
The variation of the rate constants with temperature for each phase and for chloroform
solution of 6ꢀABꢀO1 allow determination of the enthalpies and entropies of activation: ∆ꢣ‡
and ∆ꢇ‡, respectively, if one assumes that the transition state theory (activatedꢀcomplex
theory)62,63 is approximately valid in the described cases. This is not obvious as the activatedꢀ
complex theory has been elaborated for reactions in gases. Here, we use this theory in order to
compare the obtained by us results with those published in papers of GarciaꢀAmoros et al.49,50
From the EyringꢀPolanyi equation of the form:
‡
‡
∆ꢩ
ꢒ∆ꢈꢀꢉ ꢄ ꢤ ꢦ ꢜ∆ꢨ ꢜꢘ
(3)
ꢥ
ꢠ
ꢠꢡ
ꢧ
where ∆ꢣ‡ is enthalpy of activation, ∆ꢇ‡ is entropy of activation, h is Planck’s constant and
ꢤ
∆
ꢒ
ꢪ is Boltzmann constant, by plotting ꢔꢕ ꢝ ꢦ ꢢ versus 1/T both enthalpy and entropy of
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activation can be determined in the isotropic as well as nematic phases. Results of these
calculations are also gathered in Table 1. The energy barrier for the azobenzene thermal
isomerization is on the order of 100 – 150 kJ molꢀ1,64 so the data with estimation of activation
energies for 6ꢀABꢀO1 shown in Table 1 seem to be correct.
However, the measurements are loaded with uncertainties. The main source of them is linked
with inaccuracy of temperature, absorbance and time measurements, but also the possible
influence of the photostationary state on the results. The latter was minimized by procedure of
normalization of the spectra with respect to the value of isosbestic point absorbance. We
always use the same irradiation time and the same intensity of 405 nm light coming from laser
diode to ensure similar initial concentration of cisꢀisomers, however this can be inaccurate
due to fact that the absorbance is different in different phases. The temperature measurements
were loaded with systematic error of temperature readings as the temperature sensor was
glued to the glass of the LC cell. Therefore we expect the maximum shift between the
measured temperature and the temperature of the LC up to ± 3 degrees. Absorption
measurements were influenced in the nematic phase by light scattering that increasing
measured absorbance at some extent evaluated on the level of below 2 percent. However,
light scattering in the solid state become important, i.e. up to 10 % and this was partially
corrected by subtracting the measured absorbance of a solidified compound in the cell by the
value of absorbance at wavelength of 600 nm, i.e. outside of absorption range of the studied
compound. The results, especially those related to measurements of activation energies EA
were also influenced by the temperature range available for rate constants determination.
When this range is narrow the larger uncertainties are expected. Basing on the above
mentioned discussion we evaluated and supplied estimation of measurement uncertainties to
the Table 1 for each kinetic measurement separately.
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Table 1. The rate constants (kꢀ), characteristic times (τth), the activation energies (Ea) and
thermal activation parameters (enthalpy ∆ꢣ‡ and entropy ∆ꢇ‡) for the thermal cis-trans
isomerization process of 6ꢀABꢀO1 in the dark in different environments: liquid crystalline
(nematic) (NLC), isotropic liquid (Iso) and in chloroform solution.
Environment
T [K]
Ea [kJ molꢀ1]
127±5
∆ꢇ‡[JKꢀ1 molꢀ1]
kꢀ [sꢀ1]
τth [min]
∆ꢣ‡[kJ molꢀ1]
4.87×10-5
1.75×10-4
3.15×10-4
9.90×10-4
2.90×10-3
1.08×10-2
2.34×10-6
5.90×10-6
2.52×10-5
1.02×10-4
4.89×10-4
NLC
NLC
307
315
319
333
348
363
296
303
313
323
333
342
95
124±3
78±9
NLC
53
Iso
17
Iso
6
77±4
77±3
ꢀ72±9
36±4
Iso
1.5
7123
2825
661
163
34
Solution
Solution
Solution
Solution
Solution
118±3
115±2
From the analysis of the results shown in Table 1 one can conclude that the kinetics of
thermal cis-trans isomerization reaction of the 6ꢀABꢀO1 compound depends on the phase and
molecular environment. Comparing the reaction characteristic time τth in the nematic phase at
307 K which amounts to 342 min with that at 303 K in solution amounting 2825 min one can
notice that the reaction rate is almost one order slower in solution than in the ordered nematic
phase.
The thermal cis-trans isomerization process of azobenzeneꢀderivatives has been
extensively studied by GarciaꢀAmoros and coꢀworkers.49ꢀ52 At first, they investigated the cis-
trans isomerization kinetics of azoꢀdyes dissolved in different organic solvents and in liquid
crystalline solvent (5CB).49 The results revealed a great influence of the system order on the
rate of the process. The nematic order S increased the rate of the thermal cis-trans
isomerization. An acceleration of the process in the liquid crystalline media in comparison to
the reaction in isotropic solvents was explained on the basis of a cooperative interaction
established between the mesogenic molecules and the azoꢀdye (the effect was not present in
the isotropic phase). Further, they analyzed in detail the role of the local order on the thermal
cis-trans isomerization of the azoꢀdye by using as a host four different nematogens of
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different nature.50 They demonstrated that azoꢀderivatives show faster thermal cis-trans
isomerization kinetics when they are under the influence of the nematic mean field than
dissolved in isotropic solution. Authors proved that the local order parameter induced in the
azoꢀdye by the host mesogen determines the rate of its thermal cis-to-trans isomerization
process. Moreover, the studies of the thermal cis-trans isomerization of the azoꢀdye were
conducted in both the nematic and isotropic phases of two host mesogens revealing that the
reaction occurs faster when the azoꢀdye is dispersed in the isotropic phase than when it is in
the nematic phase. Summarizing, among all aforementioned systems, the thermal cis-trans
isomerization of the azoꢀdye occurs with the highest rate when it is placed in the isotropic
phase of the host and most slowly when the azoꢀdye is dissolved in isotropic solution. These
results are in general in accordance with the results reported in the present study, despite the
fact that GarciaꢀAmoros et al. investigated exclusively the guestꢀhost systems, while we
focused attention to the pure azoꢀLC compound, i.e., a singleꢀcomponent system though in
different phases.
Our results are in partial agreement with those reported for 4,4’ꢀdiꢀ(5ꢀhexenyloxy)azobenzene
dissolved in matrices of several common liquid crystals including a standard 5CB and various
organic solvents.50 The thermal cis-trans reaction in 6ꢀABꢀO1 has the highest activation
energy (127 kJ molꢀ1) when the cisꢀmolecule is within uniaxially ordered nematic liquid
crystalline phase with an order parameter of S ≈ 0.6. The same cisꢀmolecule in disordered
isotropic liquid characterized with an order parameter S = 0 shows for the thermal cis-trans
isomerization the much smaller activation energy of only 77 kJ molꢀ1. This result is different
from that reported in ref. 50. This may be an indication that specific interaction or
aggreagation induced interactions may play a decisive role in the behaviour of phototropic
LCs. In our case the reaction enthalpy is larger in the nematic phase (∆ꢣ‡ = 124 kJ molꢀ1)
than in the isotropic one (∆ꢣ‡ = 77 kJ molꢀ1) while for the azobenzeneꢀdye diluted in nematic
liquid crystals the reverse behaviour has been observed (∆ꢣ‡ = 68 kJ molꢀ1 in the nematic
phase of 5CB versus 88 kJ molꢀ1 in its isotropic phase). Surprisingly, the same cisꢀmolecule in
chloroform solution for which the order parameter is S = 0, as in the isotropic liquid, shows
again a very high activation energy of 118 kJ molꢀ1 and ∆ꢣ‡ = 115 kJ molꢀ1. From the Eyringꢀ
Polanyi equation we got qualitatively different values for activation entropies, namely
negative (∆ꢇ‡ = ꢀ72 JKꢀ1 molꢀ1) for the isotropic phase and positive (∆ꢇ‡ = +78 JKꢀ1 molꢀ1) for
the nematic phase. The negative value of the activation entropy change suggests that the
transition between the cisꢀisomer to the transꢀisomer is restricted and has a substantial degree
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of order. Respectively the same transition in the ordered nematic phase is preferred due to the
action of the mean filed of the nematic LC. It is well known that the cis-azobenzene in
opposition to the trans-azobenzene is a poor mesogen. The free space around the cisꢀisomer is
necessary for the geometrical changes which accompany the isomerization reaction. The cis
form of the molecule, due to its bent shape, occupies more space than the trans form. The
return of the cis isomer to the elongated trans form, that entails a significant conformational
change, requires some free space available around. In the liquid crystalline state, which is a
partially longꢀrange ordered system, the free space around the molecules is limited, thereby
thermal relaxation occurs slower. The difference in the rate of the thermal relaxation process
is reflected in the activation energy, which is around 40 % higher for the reaction in the liquid
crystalline state than in the isotropic one. This indicates that the molecular organization
produced within the system to reach the corresponding transition state is larger in the nematic
phase than in the isotropic liquid phase.
Comparing the results obtained for the liquid crystalline matrices to those obtained for
a solution, it follows that the thermal cis-trans isomerization occurs the most slowly for the
6ꢀABꢀO1 compound dissolved in a solvent. Evidently, the environment of chloroform
effectively slows down the process of the thermal relaxation. A faster reaction in the case of
the liquid crystalline state can indicate the occurrence of coꢀoperative interactions in it
resulting from the shortꢀrange orientational arrangement of the molecules which tend to
recover the initial alignment along the nematic director. Despite the fact that the reaction rate
constants for the solution and for the liquid crystalline state are evidently different, the
activation energies for these two different environments are comparable.
Conclusions
The thermal cisꢀtrans isomerization of a single component azobenzeneꢀbased liquid crystal in
its isotropic liquid and nematic phases has been investigated and compared to the one
obtained for the compound dissolved in chloroform. Taking into account all studied systems,
one can conclude that the cis-trans isomerization reaction is the fastest for 6ꢀABꢀO1 in the
isotropic liquid phase (few minutes), slower in the nematic one (from around one hour to few
hours) and the slowest when it is dissolved in a solvent (at comparable temperature the
reaction rate is almost one order slower than for the nematic phase). These results clearly
show that the thermal back reaction strongly depends on the phase of the PtLC and molecular
environment. Note that for the azobenzeneꢀdye doped PtLCs the acceleration of the thermal
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cis-trans isomerization process in the liquid crystalline media is observed while in the studied
singleꢀcomponent PtLC the nematic order slows down the reaction.
The difference in the rate of the thermal relaxation process is reflected in the activation
energy and thermal activation parameters. The thermal cis-trans reaction in 6ꢀABꢀO1 has the
highest activation energy (127 kJ molꢀ1) when the cisꢀmolecule is in the nematic phase, while
it is much smaller (77 kJ molꢀ1) when the cisꢀform is in the isotropic liquid. A very high
activation energy (118 kJ molꢀ1) is obtained for the reaction when the cisꢀmolecule is in
chloroform solution. The reaction enthalpy is larger in the nematic phase than the isotropic
one which is the opposite behavior than observed for the azobenzeneꢀdye diluted in nematic
liquid crystals. The negative activation entropy has been measured for the isotropic phase
while the positive one has been found for the nematic phase.
The temperature decrease causes that 6ꢀABꢀO1 undergoes the phase transitions from
the isotropic liquid phase through the nematic LC phase to the crystalline phase which is
reflected in changes of the compound’s absorption spectrum. The differences in the
absorption magnitude in the respective phases have been correlated to the positional order
parameter S of mesogens.
Acknowledgment
This work was financially supported by the National Science Centre, Poland
2014/15/B/ST8/00115 and the Wroclaw University of Science and Technology
statutory fund.
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Physical Chemistry Chemical Physics
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DOI: 10.1039/C7CP06820D
Table of Contents Entry
Kinetics of thermal cis-trans isomerization of the single-component azobenzene-based
phototropic liquid crystal in its isotropic liquid and nematic phases is studied.
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