Conformation of Novel Azo-Dyes Bearing End-Capped Oligo(ethylene glycol) Studied by UV-vis and NMR Spectroscopy

Article abstract of DOI:10.1021/acs.jpcb.5b05853

Two novel azo-dyes bearing an end-capped oligo(ethylene glycol) chain were synthesized and then studied by UV-visible and NMR spectroscopy. For both azobenzenes, the end-capped oligo(ethylene glycol) segment is on the para position of the first phenyl ring. On the second phenyl ring, a methoxy group is added on the para position for one azo-dye and no substitution group on the other, which made them electronically a push-push and a push system, respectively. The presence of the methoxy group changes significantly the absorption and the photoisomerization behaviors and results in a much less intense absorbance for the trans isomer and a shift from 350 to 360 nm. In the kinetic studies the azobenzene bearing a methoxy group shows a zero-order and a first-order kinetics as a function of the time scale of the study as well as an aggregation phenomenon. This azo-dye in different solvents has been studied by ¹H NMR and pulsed gradient NMR experiments to understand the effects of the photoisomerization and the aggregation on the self-diffusion of these molecules in solutions.

Full text of DOI:10.1021/acs.jpcb.5b05853

Article
pubs.acs.org/JPCB
Conformation of Novel Azo-Dyes Bearing End-Capped
Oligo(ethylene glycol) Studied by UV−vis and NMR Spectroscopy
Cassandre Kouvatas,† Wilms E. Baille,† Jesus Ortíz-Palacios,‡ Edgar Aguilar-Ortíz,‡ Ernesto Rivera,*,‡
́
and X. X. Zhu*,†
†Dep
‡Instituto de Investigaciones en Materiales, Universidad Nacional Auton
04510 Mexico D.F., Mexico
́
artement de chimie, Universite
́
de Montrea
́
l, C.P. 6128, Succursale Centre-ville, Montrea
́ ́
l, Quebec H3C 3J7, Canada
́
oma de Mexico, Circuito Exterior Ciudad Universitaria, C.P.
́
́
́
ABSTRACT: Two novel azo-dyes bearing an end-capped oligo(ethylene glycol) chain were
synthesized and then studied by UV−visible and NMR spectroscopy. For both azobenzenes,
the end-capped oligo(ethylene glycol) segment is on the para position of the first phenyl
ring. On the second phenyl ring, a methoxy group is added on the para position for one azo-
dye and no substitution group on the other, which made them electronically a push−push
and a push system, respectively. The presence of the methoxy group changes significantly
the absorption and the photoisomerization behaviors and results in a much less intense
absorbance for the trans isomer and a shift from 350 to 360 nm. In the kinetic studies the
azobenzene bearing a methoxy group shows a zero-order and a first-order kinetics as a
function of the time scale of the study as well as an aggregation phenomenon. This azo-dye
in different solvents has been studied by 1H NMR and pulsed gradient NMR experiments to
understand the effects of the photoisomerization and the aggregation on the self-diffusion of
these molecules in solutions.
INTRODUCTION
■
Azobenzene molecules, initially used as dyes, have been widely
studied due to their reversible isomerization properties. The
light-induced isomerization of the azo dye N-ethyl-N-(2-
hydroxyethyl)-4-(4-nitrophenylazo)aniline (Disperse Red 1)
grafted onto thin polymer films was first reported by two
research groups.1,2 Azobenzenes can switch from the trans (E)
to cis (Z) isomers when they are irradiated preferably with
linear polarized light at the appropriate wavelength. Their
photochromic behavior makes them potential molecular
switches due to the photoinduced motions.3,4 Polymers may
be made to be light-responsive after the incorporation or
grafting of azo dyes.2,5,6 The substitution groups on azo-dyes
have a significant influence on the absorption spectra and the
formation of H or J aggregates.7 The position and the shape of
the characteristic bands π−π* (more intense for the trans
isomer) and n−π* (more intense for the cis isomer)3,7,8
strongly depend on the nature of the substitution groups.
Figure 1 shows a typical absorption spectrum for azobenzene
molecules with two main bands, a π → π* transition at 350 nm
and another n → π* transition at 450 nm. If the azobenzene
unit is substituted by an electron-donor group (R1) and an
electron-withdrawing group (R2), known as “push−pull”
systems,3 a red shift of the π → π* band and a blue shift of
the n → π* band are observed.
Figure 1. UV−vis absorption spectrum of a para-substituted
azobenzene.
diffusion of small molecules in polymers systems such as
modified polysaccharides,13−15 and self-diffusion of linear, star,
and dendritic poly(ethylene glycol)s in water16 and in
hydrogels.17,18 The self-diffusion behavior of the azo-dyes in
different media may provide additional information on the
intermolecular interaction and aggregation depending on the
The incorporation of hydrophilic or amphiphilic polymer
chains into azo-dyes can induce their micellization in water.9−11
The photochemical behavior of such polymeric materials may
be studied by UV−vis spectroscopy and by pulsed field gradient
(PFG) NMR techniques.12 PFG NMR has been used to study
Received: June 18, 2015
Revised: August 24, 2015
© XXXX American Chemical Society
A
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Article
Scheme 1
molecular configuration (trans or cis) and the substitution
groups.
(2C, Cf), 114.58 (2C, Cb), 113.94 (2C, Cg), 72.32 (1C,
HOCH2CH2), 70.61−70.11 (4C, OCH2 of the tetra(ethylene
glycol) chain), 69.42 (1C, PhNNPhOCH2CH2), 67.46
(PhNNPhOCH2), 61.45 (1C, HOCH2CH2), 55.32 (1C,
PhOCH3).
Herein, we synthesized two azobenzene derivatives bearing a
TEG segment on the para position of the first phenyl ring and a
similar azo-dye bearing a methoxy group on the para position of
the other phenyl ring, which made them a push system and a
push−push system, respectively. First, UV−vis spectroscopy
was used to characterize these molecules in aqueous media and
their cis-to-trans thermal relaxation kinetics. Then, PFG NMR
experiments were done with the azo-dyes dissolved in different
solvents before irradiation after trans-to-cis and after cis-to-
trans photoisomerizatios to study the self-diffusion and the
dynamics of the azobenzene units.
Synthesis of the 4′-Tetra(ethylene glycol)-azobenzene
(AB-TEG). The same synthetic procedure (replacing p-anisidine
1 by aniline 2) was used for the synthesis of 1-
hydroxyazobenzene with a yield of 85%. Then, Ts-TEG (0.83
g, 2.52 mmol) was reacted with 1-hydroxyazobenzene (0.5 g,
2.52 mmol) by using the same procedure as previously
described for the synthesis of MeOAB-TEG to give the desired
1
azo-compound AB-TEG. Yield: 92%. H NMR (400 MHz,
CDCl3) (Scheme 1): δ = 7.90 (d, J = 8.9 Hz, 2 H, H2), 7.86 (d,
J = 7.75 Hz, 2 H, H3), 7.51−7.41 (m, 3 H, H1), 7.03 (d, J = 8.96
Hz, 2 H, H4), 4.22 (t, J = 4.6 Hz, 2H, PhNNPhOCH2), 3.89
(t, J = 4.92 Hz, 2 H, PhNNPhOCH2CH2), 3.75−3.60 (m, 12
H, OCH2 of the tetra(ethylene glycol) chain). 13C NMR (100
MHz, CDCl3) (Scheme 1): δ = 160.94 (1C, Ch), 152.33 (1C,
Cd), 146.65 (1C, Ce), 130.07 (1C, Ca), 128.69 (2C, Cb), 124.39
(2C, Cf), 122.23 (2C, Cc), 113.48 (2C, Cg), 72.21 (1C,
HOCH2CH2), 70.41−69.91 (4C, OCH2 of the tetra(ethylene
glycol) chain), 69.20 (1C, PhNNPhOCH2CH2), 67.33 (1C,
PhNNPhOCH2CH2), 61.21 (1C, HOCH2CH2).
EXPERIMENTAL SECTION
■
Synthesis. All reagents used in the synthesis of the
azobenzenes were purchased from Aldrich and used as received.
The syntheses of the azobenzenes bearing a tetra(ethylene
glycol) (TEG) segment are shown in Scheme 1. The diazonium
salts of the two azobenzenes were prepared in situ because
these salts are not commercially available. Precursor Ts-TEG
was synthesized according to a method previously reported.19
Synthesis of the 4-Methoxy-4′-tetraethylene Glycol-
azobenzene (MeOAB-TEG). p-Anisidine 1 (1 g, 8.13 mmol)
and NaNO2 (0.56 g, 8.13 mmol) were dissolved in a HCl
aqueous solution (30%, 50 mL) at 0 °C with vigorous stirring.
Then, phenol (0.764 g, 8.13 mmol) was added to the solution
and was stirred for 1 h at room temperature. Afterward, the
reaction mixture was neutralized with a NaOH solution (10%)
and stirred for 3 h. The product was filtered and dried to a
constant weight to yield azo-compound MeOAB. Yield: 75%.
The intermediate Ts-TEG (0.72 g, 2.19 mmol) was reacted
with MeOAB (0.5 g, 2.19 mmol), K2CO3 (1.21 g, 8.77 mmol),
and a catalytic amount of 18-crown-6 in dry acetone (25 mL).
The reaction mixture was heated to reflux for 48 h; then, it was
cooled to room temperature, filtered, and concentrated at
reduced pressure. The crude product was purified by column
chromatography on silica gel using a mixture of ethyl acetate/
hexane 9:1 and pure ethyl acetate as eluent, to give the
azobenzene derivative MeOAB-TEG. Yield: 90%.1H NMR
(400 MHz, CDCl3) (Scheme 1): δ = 7.83 (d, J = 8.7 Hz, 2 H,
H2), 7.82 (d, J = 8.7 Hz, 2 H, H3), 6.97 (d, J = 8.5 Hz, 2 H, H4),
6.95 (d, J = 8.0 Hz, 2 H, H1), 4.18 (t, J = 4.25 Hz, 2 H, PhN
NPhOCH2), 3.84 (s, 3H, PhOCH3), 3.11−3.55 (m, 14 H,
OCH2 of the tetra(ethylene glycol) chain). 13C NMR (100
MHz, CDCl3) (Scheme 1): δ = 161.36 (1C, Ch), 160.55 (1C,
Ca), 146.90 (1C, Cd), 146.80 (1C, Ce), 124.12 (2C, Cc), 124.07
Samples Irradiation. Absorption spectra were acquired on
an OmniCure S2000 Spot UV Curing System. The UV light for
irradiation was generated by a high-pressure 200 W mercury
vapor short arc UV lamp. The azobenzene photoisomerization
experiments were performed in 20 mL glass bottles. To obtain
light sources in the range of desired wavelengths, we introduced
320−390 and 400−500 nm filters successively into the light
guide for the trans-to-cis and the cis-to-trans isomerization,
respectively.8,19,20
Absorption Spectroscopy. The kinetics of relaxation were
studied after the trans-to-cis and cis-to-trans photoisomerisa-
tion for both azobenzenes in water solutions.21 Samples for
absorption spectroscopy measurements were prepared in
distillated water, with an azo-compound concentration of 0.04
and 0.20 mg/mL for AB-TEG and MeOAB-TEG, respectively.
The absorption spectra were recorded in 1 cm quartz cells on a
Cary 5000 UV−vis-NIR spectrophotometer at room temper-
ature.
NMR Experiments. 1H and 13C NMR spectra of the
compounds in CDCl3 were recorded at room temperature on a
Bruker Avance 400 spectrometer operating at 400 and 100
1
1
MHz for H and 13C nuclei, respectively. The H PFG NMR
experiments were performed at 25 °C using 5 mm anti-UV
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Article
NMR tubes on the same NMR spectrometer equipped with an
ultrastable temperature controller and also with a Diff 60
diffusion probe, a 5 mm autotuning broad-band probe with a z-
gradient coil permitting a maximum gradient of 3000 G/cm.
For self-diffusion experiments, 6 mg of the azobenzene sample
was dissolved in 1 mL of a deuterated solvent such as D2O
show three main absorption bands for both MeOAB-TEG and
AB-TEG in water: The first two strong bands at 240 and 350
nm are assigned to the π → π* transition, which are associated
with the transition dipole moments along the short and long
axis of the azobenzene molecules, respectively.24 Only the band
at 350 nm was used to characterize both MeOAB-TEG and AB-
TEG molecules because the trans-to-cis isomerization is
promoted by irradiation with wavelengths between 320 and
390 nm (using a filter). The third band corresponding to n →
π* transition at 420 nm (forbidden transition) represents the
cis isomer.
2
2
(99.9 atom % H), CD3OD (99.8 atom % H), and DMSO-d6
2
(99.9 atom % H), all purchased from Sigma-Aldrich.
A stimulated echo pulse sequence (STE: 90-t1-90-t2-90-t1-
echo)16 was used for the NMR experiments. The self-diffusion
coefficients of the azobenzenes in different solvents were
determined according to the following equation
The deconvoluted absorption spectra of both azobenzenes
exhibit four characteristic bands (Figures 2A,B). The trans
isomer is more stable at the equilibrium than the cis and is the
dominant species. In fact, the π → π* band intensity is higher
than the n → π* band in each spectrum because the latter
transition is not allowed according to the symmetry rules.4 The
AB-TEG spectrum shows a much intense π → π* band than
MeOAB-TEG, even though AB-TEG is five times less
concentrated than MeOAB-TEG. Therefore, the molar
absorption coefficient of AB-TEG should be substantially
higher than that of MeOAB-TEG. Furthermore, the absorption
band π → π* shifts from 350 to 360 nm (Figure 2, vertical
dashes) when a methoxy group is added to the phenyl ring.
This is due to the electron-donor nature of this substituent,
which red-shifts the π → π* band. The effect would have been
even more pronounced if an electron-withdrawing group had
been attached in the place of TEG because a push−pull system
instead of a push−push system would have induced a stronger
red shift of the π → π* absorption band, thereby causing a total
overlap with the absorption n → π* band.3,4
2
⎛
⎞
⎟
δ
3
I = I0e−D(γδG) Δ −
⎜
⎝
⎠
(1)
where I0 and I are the NMR signals in the absence and in the
presence of the gradient pulses, respectively, D is the self-
diffusion coefficient in the Z direction, γ is the gyromagnetic
ratio of the observed nucleus (γH = 2.67522128 × 107 red T−1
s−1), δ is the gradient length, G is the gradient strength, and Δ
is the time interval between two gradient pulses.22,23 The self-
diffusion coefficients D were calculated with a coefficient of
determination (R2) in the range of 0.98 to 0.99.
RESULTS AND DISCUSSION
■
Absorption Spectroscopy. Prior to the irradiation study,
the UV−vis spectrum of each azobenzene is obtained to
determine the wavelength of excitation, which mainly depends
on the nature of the substation groups, The spectra in Figure 2
Photoisomerization. The aqueous solutions of AB-TEG
and MeOAB-TEG were irradiated for 15 min at 320−390 nm
for the trans-to-cis and at 400−500 nm for the cis-to-trans
isomerizations (Figure 3). In the spectra of AB-TEG, a
significant decrease in intensity of the π → π* band (from
75.8 to 11.3) is observed after irradiation for the trans-to-cis
isomerization (Figure 3A and Table 1), while the irradiation of
the cis isomer induces an increased intensity of the same band
(Figure 3A), but the π → π* band area (55.3) does not recover
its original value (75.8) before irradiation (Table 1). The area
may be fully recovered through cis-to-trans thermal relaxation,
with the complete formation of the trans isomer.3 The n → π*
band has a small change in intensity during the trans-to-cis
isomerization, increasing from 16.6 to 22.0 (Table 1), but it is
not considered to be significant because this absorption band
does not correspond to an allowed transition.
The spectra of MeOAB-TEG show a different behavior after
the trans-to-cis isomerization. The intensities of the trans and
cis bands both increase (Figure 3B). The area under the trans
isomer band rises from 32.3 to 44.0 (Table 1) in contrast with
the decrease in the π → π* absorption band due to the trans-to-
cis transformation of AB-TEG. The area under the n → π*
band also rises by almost three-fold from 13.5 to 52.0, much
larger than the increase for the trans-to-cis isomerization of AB-
TEG (Table 1). The π → π* band intensity further increases
after the cis-to-trans isomerization (Figure 3B), and the area
increases from 44.0 to 114.4, about 3.5 times higher than the
initial value (32.3) (Table 1). Once again, this is very different
from the 73% recovery obtained for AB-TEG. These changes in
the adsorption spectra are typically related to the packing of
trans isomers. The methoxy group added on the phenyl ring
induces the formation of aggregates when MeOAB-TEG is in
Figure 2. UV−vis spectra of (A) MeOAB-TEG (0.04 mg/mL) and
(B) AB-TEG (0.20 mg/mL) and their deconvolution spectra (dashed
spectra) before irradiation. A red shift of the trans band from 350 to
360 nm is observed when the azobenzene is para-substituted with an
electron-donor group, as shown by the vertical dashes in panels A and
B.
C
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promoting the disaggregation and the dissolution of azo-
dyes.9,24,27,28 The irradiation liberates individual trans isomers
from the aggregates and thus induces an increase in the n → π*
band intensity (Figure 3B) because more single trans isomers
are available for the trans-to-cis isomerization.
The intensity of the π → π* band is higher after the
irradiation at 400−500 nm (Figure 3B) than at the initial
equilibrium because the proportion of single trans isomers is
higher after cis-to-trans photoisomerization due to the delay in
forming the aggregates. These results are further confirmed by
the NMR experiments.
Kinetics. AB-TEG and MeOAB-TEG in water were
irradiated for 15 min at 320−390 nm for the trans-to-cis
isomerization. The absorption spectra were recorded every
hour to study the kinetics of cis-to-trans thermal relaxation,
which can be qualitatively evaluated by the apparent extent of
29
photoisomerization Yapp
I
trans(0) − Itrans(t)
Y (t) =
app
I
trans(0)
(2)
where Itrans(0) and Itrans(t) are the intensities of the trans
absorption band at 349 nm before (assuming all azo-dyes are in
the trans form) and after irradiation at 320−390 nm at time t,
respectively. In addition, this thermal relaxation can be
described by the ratio of isomerization Ri, that is, the mole
fraction of trans form of the azo-dyes25
Figure 3. Deconvoluted UV−vis spectra for (A) AB-TEG and (B)
MeOAB-TEG. Solid lines, before the trans-to-cis irradiation; blue dots,
after the trans−cis irradiation at 320−390 nm; red dashes, after the cis-
to-trans irradiation at 400−500 nm. The peaks close to 350 nm are the
π → π* band, while the ones close to 430 nm correspond to the n →
π* band. For MeOAB-TEG, the π → π* band after the cis-to-trans
irradiation (red dashes) will decrease with time due to the aggregation
of the azo-dye, while AB-TEG does not show this aggregation process.
I
trans(0) − Itrans(t)
Ri(t) =
I
trans(0) − Itrans(∞)
(3)
where Itrans(0), Itrans(t), and Itrans(∞) are the intensities of the
absorption band at 349 nm (more intense for the trans) before
(assuming all azo-dyes are in the trans form) and after
irradiation at 320−390 nm at time t and at infinity, respectively.
Figure 4A,B shows the evolution of Yapp and 1 − Ri with time
after irradiation of AB-TEG at 320−390 nm. A first order for
the cis-to-trans thermal relaxation is observed, as described by
eqs 4 and 525,29
its trans form. Usually this behavior results from intermolecular
hydrophilic, hydrophobic, van der Waals interactions, and
hydrogen bonding.7,9,24−26 The planar (E) conformation of the
azo groups allows the molecules to interact with each other by
van der Waals forces through the phenyl ring and by hydrogen
bonds between the methoxy group and the TEG chain.7,26
At the equilibrium state, the MeOAB-TEG molecules tend to
stack, forming H-aggregates before the trans-to-cis isomer-
ization. (A hypsochromic shift is observed when azobenzenes
aggregate under the influence of a solvent, an additive, or a
change in concentration.)7,10,26 When the molecules are
irradiated at 320−390 nm, the aggregates may break up
under the heat generated by the irradiation as a function of time
(total of 15 min for all measurements), thus releasing free
MeOAB-TEG molecules, which are instantly isomerized to the
cis form. The cis isomer has a bent structure,3 hindering the
packing and the interaction between these molecules and
Y (t) = Y (0) − kt
app
app
(4)
kt
1 − Ri(t) = 1 −
I
trans(0) − I trans(∞)
(5)
where Yapp(0) and Yapp(t) are the apparent extents of
photoisomerization at the photostationary state and at time t,
respectively, and k represent the rate constant for the thermal
cis-to-trans isomerization. The half lives t1/2 for the two
approaches can be expressed by30,31
Y (0)
app
t1/2
=
(6)
2k
Table 1. Position and Area of trans and cis Absorption Bands of AB-TEG and MeOAB-TEG before and after the trans-to-cis and
the cis-to-trans Isomerization
MeOAB-TEG isomer
AB-TEG isomer
π→π*
λmax (nm)
n→π*
π→π*
λmax (nm)
n→π*
λmax (nm)
transitions
area
λmax (nm)
area
area
area
before irradiation
361
361
361
32.3
44.0
428
436
428
13.5
52.0
57.5
350
345
350
75.8
11.3
55.3
425
432
433
16.6
22.0
16.6
trans→cis 320−390 nm
cis→trans 400−500 nm
114.4
D
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aggregation, which means that the latter is the rate-limiting
reaction. The isomerization from cis- to trans-MeOAB-TEG is
faster than the packing of the freshly isomerized trans isomers.
This could explain the higher proportion of trans isomers at the
end of the relaxation compared with the initial value.
NMR Diffusion Experiments. For a better understanding
of the aggregation of MeOAB-TEG in the trans form, we
studied the self-diffusion of MeOAB-TEG in three different
1
solvents by H PGSE NMR experiments. The experiments
were done on MeOAB-TEG in D2O and in DMSO-d6 before
and after 15 min of irradiation at 320−390 and at 400−500 nm.
In CD3OD the irradiation was limited to 5 min because
methanol is more volatile. The spectra in Figure 5 show
different peaks for the trans and cis isomers in the three
solvents used. Thus, the characteristic peaks for each isomer
were chosen to calculate the self-diffusion coefficients presented
in Table 3. (Aromatic peaks are used due to overlapping peaks
from the aliphatic part of the cis and trans isomers.) The
solubilities of trans and cis isomers in D2O are different. Figure
5A shows that MeOAB-TEG is not detectable before
irradiation; after irradiation at 320−390 nm, however, the cis
isomer is detectable and remains visible even after irradiation at
400−500 nm (cis to trans isomerization). In Figure 5B,C, the
trans and cis isomers are visible both before and after
irradiation at 320−390 and at 400−500 nm, respectively.
The aromatic peaks of both azobenzenes were used to
determine the self-diffusion coefficients because the peaks of
the aliphatic parts of the cis and trans isomers have overlapping
signals. The self-diffusion coefficients of MeOAB-TEG
determined in the different deuterated solvents are shown in
Table 3. Because a very low percentage of cis isomers exist at
the equilibrium or before irradiation (Figure 5) the measure-
ment of the self-diffusion coefficient for this isomer is not
precise in DMSO-d6 and CD3OD. The azobenzene molecules
are mainly in their trans conformation before the irradiation.
The van der Waals interactions between the azo-dyes are
stronger because the trans isomers are more stretched, which
induces the packing of molecules so that the solubility was too
low to be detected by NMR (Figure 5A, before irradiation).
MeOAB-TEG becomes soluble in D2O after the trans-to-cis
irradiation because the cis isomers have a bent structure and
cannot pack as easily and therefore have a slightly higher
solubility, which allowed the calculation of the self-diffusion
coefficient of the cis isomer (3.79 × 10−10 m2/s, Table 3).
In DMSO-d6, MeOAB-TEG is more soluble. Before
irradiation, the Dcis and Dtrans values of MeOAB-TEG are
similar ((2.19 and 2.15) × 10−10 m2/s, respectively). After
irradiation at 320−390 nm a more significant difference in D
values of the two isomers is observed ((2.19 and 1.97) × 10−10
m2/s for cis and trans isomers, respectively). In this case,
MeOAB-TEG exists mainly in the cis conformation, and
aggregated trans isomers are dissociated. After irradiation at
Figure 4. Evolution of the values of Yapp (A and C, fits to eq 4) and 1
− Ri (B and D, eq 5) with time for AB-TEG (A,B) and MeOAB-TEG
(C,D) in water.
I
trans(0) − Itrans(∞)
2k
t1/2
=
(7)
Table 2 lists the characteristic constants of the cis-to-trans
thermal relaxation of AB-TEG for these two methods, and both
lead to similar rate constants k and half lives t1/2
.
For MeOAB-TEG, the thermal cis-to-trans relaxation has
two stages. The first 30 h are dominated by the cis-to-trans
isomerization, which was determined as a zero-order reaction
(Figures 4C,D). Another trend stands out after 50 h and
globally appears to have first-order kinetics. It is actually
composed of two reactions, a cis-to-trans isomerization and the
aggregation of the individual trans isomers. If both reactions are
described by zero-order kinetics the rate coefficient and the
half-life time can be extracted from k = kcis→trans − kaggr in eq 8
(Table 2).32 The velocity of the competition (Vcomp) between
the aggregation and the cis-to-trans relaxation can be expressed
by
V
comp = kY = (kcis→trans − kaggr)Y
app
app
(8)
where kcis→trans, kaggr, and k are the rate constants of pure cis-to-
trans relaxation, aggregation and the combination of the
relaxation and aggregation for MeOAB-TEG, respectively.
If we consider both reactions as zero-order reactions,
kcis‑to‑trans for MeOAB-TEG is 3 times higher than that of AB-
TEG (Tables 1 and 2). Thus, the relaxation of MeOAB-TEG is
faster. The half life for the thermal relaxation of MeOAB-TEG
is 5.43 h until aggregation appears, which induces a competition
between these two processes. Furthermore, the rate constant of
the pure cis-to-trans relaxation is higher than that of the
Table 2. Determination of Zero-Order Rate Constant k and Half Life t1/2 with the Yapp and Ri Methods for MeOAB-TEG and
MeOAB-TEG in Water
MeOAB-TEG
aggregation and cis-to-trans
AB-TEG
pure cis-to-trans relaxation
relaxation
aggregation
k (h−1
)
t1/2 (h)
kcis→trans (h−1
)
t1/2 (h)
k (h−1
)
t1/2 (h)
kagg (h−1
)
t1/2 agg (h)
withYapp eq 4
with Ri eq 5
5.3 × 10−3
6.2 × 10−3
75.3
74.4
1.71 × 10−2
2.49 × 10−2
5.4
5.4
2.3 × 10−3
3.4 × 10−2
119.6
117.8
1.48 × 10−2
2.15 × 10−2
18.6
18.9
E
DOI: 10.1021/acs.jpcb.5b05853
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The Journal of Physical Chemistry B
Article
400−500 nm, however, the D values of the two isomers
become similar again.
Both trans and cis isomers are soluble in CD3OD. The higher
self-diffusion coefficient values obtained in CD3OD (generally
over 3 times higher in value, Table 3) are mainly due to the
higher solubility and the lower viscosity of the solution in
comparison with DMSO-d6 and D2O solutions. Right after
irradiation at 400−500 nm, the trans isomers are not yet
aggregated and behave similarly as the cis isomers, and a small
percentage of the cis isomers still remains (Table 1). Therefore,
the Dcis of MeOAB-TEG is higher in D2O than in DMSO-d6.
The packing in D2O occurs in a few hours, as shown in a kinetic
study.
CONCLUSIONS
■
The azobenzene derivatives bearing an end-capped oligo-
(ethylene glycol) show a very different absorption behavior
when a methoxy group was added to the phenyl ring at the para
position of the TEG group. In fact, the intensity obtained for
the 1-methoxy-azobenzene is much lower than its non-
methoxylated counterpart, even though its concentration is 5
times higher. Therefore, the molar absorption coefficient of AB-
TEG is substantially higher than that of MeOAB-TEG. Both
molecules exhibit trans-to-cis and cis-to-trans transitions when
they are irradiated at 320−390 nm and then at 400−500 nm,
respectively. After the cis-to-trans photoisomerization, the
azobenzene bearing a methoxy group tends to stack, forming
H aggregates. The formation of aggregates after cis-to-trans
isomerization leads to a decreased absorbance as a function of
time.
The study of cis-to-trans thermal relaxation shows that the
presence of the methoxy group changes significantly the
relaxation process. For example, AB-TEG shows a first-order
reaction with the two methods, one based on the apparent
extent of photoisomerization and the other on isomerization
ratio. In contrast, two trends were observed in the case of
MeOAB-TEG: a first-order process during the first 30 h and
another first-order trend starting around 50 h; however, the
spectral change observed indicates that the preferential cis-to-
trans isomerization of both azobenzene proceeds spontaneously
at room temperature, even without light irradiation. Fur-
thermore, the rate constant determined for MeOAB-TEG is
faster than AB-TEG, which leads to a shorter half life for
MeOAB-TEG.
The PFG NMR experiments did not show any significant
difference in the self-diffusion coefficients for AB-TEG and
MeOAB-TEG. The D values vary in the different solvents, and
important differences were observed before and after irradiation
due to the disruption of the aggregation through irradiation and
the slow kinetics of the aggregation of the trans isomers.
Figure 5. 1H NMR spectra of MeOAB-TEG in D2O (A), CD3OD (B),
and DMSO-d6 (C) before and after irradiations at 320−390 and 400−
500 nm, respectively. The intensities of these peaks were used to
determine the self-diffusion coefficients.
Table 3. Self-Diffusion Coefficients for MeOAB-TEG in Various Solvents before and after Irradiation
Dcis (10−10 m2/s)
after irradiation
Dtrans (10−10 m2/s)
after irradiation
solvent
D2O
before irradiation
320−390 nm
400−500 nm
before irradiation
320−390 nm
400−500 nm
N/A
7.22
2.19
3.79
7.14
2.15
3.68
7.26
2.12
N/A
7.17
2.15
N/A
7.32
1.97
N/A
6.85
2.16
CD3OD
DMSO-d6
F
DOI: 10.1021/acs.jpcb.5b05853
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The Journal of Physical Chemistry B
Article
(16) Wang, Y. J.; Therien-Aubin, H.; Baille, W. E.; Luo, J. T.; Zhu, X.
X. Effect of molecular architecture on the self-diffusion of polymers in
aqueous systems: A comparison of linear, star, and dendritic
poly(ethylene glycol)s. Polymer 2010, 51, 2345−2350.
AUTHOR INFORMATION
Corresponding Authors
*E.R.: E-mail: riverage@unam.mx.
*X.X.Z.: E-mail: julian.zhu@umontreal.ca.
Notes
■
́
(17) Therien-Aubin, H.; Zhu, X. X.; Moorefield, C. N.; Kotta, K.;
Newkome, G. R. Effect of Ionic Binding on the Self-Diffusion of
Anionic Dendrimers and Hydrophilic Polymers in Aqueous Systems as
Studied by Pulsed Gradient NMR Techniques. Macromolecules 2007,
40, 3644−3649.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
́
(18) Therien-Aubin, H.; Baille, W. E.; Zhu, X. X. Diffusion of
DGAPA-UNAM (PAPIIT IN100513) and NSERC of Canada
are acknowledged for financial support. We also thank Mr.
Sylvain Essiembre and Dr. Cedric Malveau for their help in
selected experiments.
molecular probes and the effects of their interactions with polymer
matrices as studied by pulsed-field gradient NMR spectroscopy. Can. J.
Chem. 2008, 86, 579−585.
̂
(19) Rivera, E.; Belletete, M.; Natansohn, A.; Durocher, G. Synthesis,
characterization, and optical properties of a novel azo-dye bearing an
oligo(ethylene glycol) methyl ether side chain in solution and in the
solid state. Can. J. Chem. 2003, 81, 1076−1082.
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DOI: 10.1021/acs.jpcb.5b05853
J. Phys. Chem. B XXXX, XXX, XXX−XXX