Role of Solvent and Effect of Substituent on Azobenzene Isomerization by Using Room-Temperature Ionic Liquids as Reaction Media

Article abstract of DOI:10.1021/acs.joc.5b00898

The effects of a para substituent, as the electron-donating -OCH₃ and -OtBu groups and the electron-withdrawing -Br and -F atoms, on azobenzene isomerization have been investigated in a series of imidazolium ionic liquids (BMIM PF₆,

Full text of DOI:10.1021/acs.joc.5b00898

Article
pubs.acs.org/joc
Role of Solvent and Effect of Substituent on Azobenzene
Isomerization by Using Room-Temperature Ionic Liquids as Reaction
Media
Guido Angelini,† Nadia Canilho,‡ Melanie Emo,§ Molly Kingsley,∥ and Carla Gasbarri*,†
́
†Dipartimento di Farmacia, Universita
‡Universite
§Institut J. Barriol, FR 2843, CNRS, Universite
̀
“G. d’Annunzio” di Chieti-Pescara, via dei Vestini, 66100 Chieti, Italy
de Lorraine, SRSMC, UMR7565, Vandœuvre-les-Nancy F-54506 Cedex, France
de Lorraine, BP 70239, Vandœuvre-les-Nancy F-54506 Cedex, France
́
̀
́
̀
∥University of Colorado Denver, Anschutz Medical Campus, 13001 East 17th Place, Aurora, Colorado 80045, United States
S
* Supporting Information
ABSTRACT: The effects of a para substituent, as the electron-donating
−OCH3 and −OtBu groups and the electron-withdrawing −Br and −F
atoms, on azobenzene isomerization have been investigated in a series of
imidazolium ionic liquids (BMIM PF6, BMIM BF4, BMIM Tf2N, EMIM
Tf2N, BM2IM Tf2N, and HMIM Tf2N). The thermal cis−trans conversion
tends to be improved in the presence of the substituent, as pointed out by
the first-order rate constants measured at 25 °C. Both the rotation and the
inversion mechanisms occur in BMIM Tf2N, EMIM Tf2N, and HMIM
Tf2N, as highlighted by typical V-shape Hammett plots, but only rotation
takes place in BMIM PF6, BMIM BF4, and BM2IM Tf2N. The possible
interactions between the cation and the anion of the solvent and both the
isomers of the azobenzene derivatives have been studied by small-wide-
angle X-ray scattering (SWAXS). The calculated cis population in the
photostationary state and the hardness parameter η of the trans isomer show that azobenzene and F-azobenzene are the less
reactive molecules for the trans−cis conversion in all the investigated ionic liquids.
take place by using RTILs as reaction media since both the
cation and the anion can be involved.30 The effect of the
solvent on the kinetics and the dynamics of azobenzene
isomerization has been widely investigated,31−33 but to the best
of our knowledge, the most recent studies in ionic liquids have
concerned push−pull derivatives or a functionalized azoben-
zene moiety.34−36 The aim of this work has been the evaluation
of the effect induced by one electron-donating or -withdrawing
substituent (−OCH3, −OtBu, −Br, and −F) in the para
position on the azobenzene isomerization by using a BMIM+/
Tf2N−-based RTIL as solvent to highlight the role of the cation
and the anion in the reaction. The structure of the investigated
azobenzenes and RTILs are reported in Figure 1.
1. INTRODUCTION
Since Hartley’s identification1 of the cis isomer in 1937, the
azobenzene molecule has been widely studied and successfully
employed as a component of a large variety of systems, such as
molecular devices,2,3 surface-modified materials,4,5 and light-
sensitive biomolecules.6,7 The trans−cis isomerization can be
induced by UV irradiation8 and electrostatic9 and mechanical
stress10 leading to typical changes in the spectroscopic
properties of azobenzene.11 The thermal cis−trans conversion
takes place spontaneously in the dark due to the higher stability
of the trans isomer.12 The mechanism by which the reaction
occurs is not yet unequivocally established since competition
between rotation and inversion has been proposed.13−15
Generally, the solvent properties, including polarity and
viscosity, and the presence of substituents could promote one
mechanism over the other16−19 so that azobenzene and
derivatives find many applications as kinetic or spectroscopic
probes in organic solvents20−22 and aqueous solutions.23−25
Room-temperature ionic liquids (RTILs) have attracted much
attention in the last years for their unique properties26 and their
potential high recyclability.27,28 RTILs can be considered as
designer solvents due to the possibility to change the cation or
the anion in a wide range of combinations to induce strong
changes in density, viscosity, melting point, and water
miscibility.29 Furthermore, different solute−solvent interactions
2. RESULTS AND DISCUSSION
It was assumed that the thermodynamically stable trans was the
dominant isomer in the azobenzene solution before irradi-
ation.37,38 The spectral changes induced by the exposition to
UV light in BMIM Tf2N are shown in Figure 2 as an example.
The kinetic profile of the thermal cis−trans isomerization
follows a first-order decay, and the first-order rate constants kobs
determined at 25 0.1 °C for the investigated azobenzenes in
Received: April 22, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.joc.5b00898
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The Journal of Organic Chemistry
Article
mechanisms, but linear plots have been determined in BMIM
BF4, BMIM PF6, and BM2IM Tf2N, showing that only rotation
takes place. The Hammett plots in EMIM Tf2N and BMIM BF4
are reported in Figure 3 as examples.
Cimiraglia and co-workers13 have highlighted that in the case
of 4-monosubstitued azobenzenes with an electron-withdraw-
ing substituent the nitrogen atom connected with the
substituted phenyl ring is involved in the rehybridation and
the inversion occurs by a perpendicular displacement of the
substituted ring to the azo bond. In the cis isomer the phenyl
rings are twisted of about 53° because of steric reasons, but the
electron-withdrawing substituents increase the torsion angle by
the conjugation between the nitrogen lone-pair and the
substituted phenyl ring. Since translational movements are
limited in ionic liquids50 rotation becomes the dominant
pathway for the cis−trans isomerization.34,36 The high
viscosity51 of BMIM BF4, BMIM PF6, and BM2IM Tf2N
(103.8, 273.0, and 115.2 mPa·s, respectively) in comparison to
BMIM Tf2N (50.9 mPa·s), EMIM Tf2N (32.0 mPa·s), and
HMIM Tf2N (68.0 mPa·s) may explain the change of
mechanism from inversion to rotation in the case of Br- and
F-azobenzene. Moreover, higher kobs values for F-azobenzene
than Br-azobenzene have been measured in the RTILs in which
only the rotation mechanism occurs (Table 1), suggesting a
higher efficacy for the more electronegative atom to stabilize
the dipolar transition state of the reaction. Since rotation is the
prevalent mechanism in polar solvents, while inversion
generally occurs in nonpolar media,52−54 BMIM BF4, BMIM
PF6, and BM2IM Tf2N seem to act as polar solvents in
comparison to BMIM Tf2N, EMIM Tf2N, and HMIM Tf2N.
Even if the reaction mechanism can be influenced by
viscosity and polarity, the solvent properties seem to have no
effect on the thermal isomerization rate.55 The conversion of
the cis into the trans isomer of the investigated azobenzenes is
faster in the ionic liquids in comparison to methanol and
ethanol.25 High values of kobs have been obtained in the
presence of −OCH3 and −OtBu (Table 1), in agreement with
the higher stabilization of the polar transition state generated in
the rotation mechanism.20,56 Furthermore, this charged
transition state is promoted in BMIM BF4, as suggested by
the increasing of the kobs values of the alkoxy derivatives from 3
up to 14 times in comparison to BMIM Tf2N and BMIM PF6.
This effect is probably due to the properties of the BF4− anion,
as the lower dispersion charge, the higher Kamlet−Taft β
value,57 and the smaller van der Waals radius58 than Tf2N− and
Figure 1. Structures of the investigated azobenzenes and RTILs.
Figure 2. UV−vis spectra of azobenzene in BMIM Tf2N before (solid
line) and after (dash line) UV irradiation.
the ionic liquids are reported in Table 1. Each value
corresponds to the mean between at least three different
determinations.
The presence of a substituent and its position in the
molecule can strongly influence kinetically and thermodynami-
cally a large variety of reactions.39−45 The electronic properties
of an atom or a group are quantified by the Hammett constant
σ values.46 It is well-known47−49 that the electron-donating
groups promote the rotation mechanism for the cis−trans
azobenzene isomerization, while the electron-withdrawing
groups favor the inversion pathway. Typical V-shaped
Hammett plots have been obtained in EMIM Tf2N, HMIM
Tf2N, and BMIM Tf2N, suggesting the coexistence of both the
−
PF6 .
Previous studies59 about the structural organization of
imidazolium ionic liquids have demonstrated an extended
connection between anions and cations through the formation
of hydrogen bonds. SWAXS spectra of EMIM Tf2N and BMIM
BF4 in the presence of Br- and OCH3-azobenzene before and
after UV irradiation show that the q position of the peaks seems
Table 1. First-Order Rate Constants for the Cis−Trans Isomerization of the Investigated Azobenzenes at 25 0.1 °C
kobs (10‑6 s−1
EMIM Tf2N
)
para-substituent
HMIM Tf2N
BM2IM Tf2N
BMIM Tf2N
BMIM PF6
BMIM BF4
H
2.75 0.1
4.16 0.3
3.54 0.1
2.69 0.1
0.72 0.3
2.76 0.3
3.12 1.2
3.89 0.1
1.61 0.1
1.70 0.1
0.89 0.2
3.08 0.3
3.98 0.1
1.23 0.1
0.93 0.9
3.02 1.2
4.19 0.7
3.70 0.1
3.36 0.1
1.99 0.1
0.98 0.1
6.83 0.1
4.78 0.1
1.37 0.1
2.00 0.1
14.2 0.2
22.7 0.1
35.7 0.3
5.55 0.1
11.0 0.1
OCH3
OtBu
Br
F
B
DOI: 10.1021/acs.joc.5b00898
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Article
Figure 3. Hammett plots in EMIM Tf2N (a) and BMIM BF4 (b).
Figure 4. SWAXS spectra for Br- and OCH3-azobenzene isomers at 3 × 10−5 M in EMIM Tf2N (a) and BMIM BF4 (b).
Table 2. Hardness Parameter (eV) of the Trans Isomer and Cis Population after UV Irradiation in the Different Ionic Liquids
cis isomer population (%)
ηtrans
para-substituent
HMIM Tf2N
BM2IM Tf2N
EMIM Tf2N
BMIM Tf2N
BMIM PF6
BMIM BF4
1.98
1.86
1.93
1.93
1.97
H
49.4
93.8
62.8
80.8
58.4
2
1
4
1
2
41.2
67.8
81.8
77.4
65.3
4
6
1
2
1
55.1
96.1
84.6
81.4
63.4
2
3
1
1
1
53.5
84.9
91.2
76.9
60.9
1
9
1
1
2
60.7
97.4
91.3
86.8
61.7
4
2
5
1
2
78.8
94.8
85.9
98.1
69.9
2
2
2
2
1
OCH3
OtBu
Br
F
to be not affected by the azobenzenes at the investigated
concentrations, suggesting that the structural network of
cations and anions of the RTILs is retained (Figure 4).
The rate of the thermal isomerization is independent of the
initial concentration of the cis isomer in solution,60 but the
solvent and the nature of the substituents could influence the
amount of the cis azobenzene formed by irradiation.19,61 The
reactivity of the trans isomer to form the less stable cis by UV
irradiation can be related to the η value. Generally, a higher η
corresponds to a less reactive system.62,63 The cis population in
the photostationary state obtained by using eq 1 and the
hardness parameter η of the trans isomer for the investigated
azobenzenes are reported in Table 2.
The formation of the cis isomer is strongly favored in the
presence of the substituent in all the RTILs. The amount of the
cis isomer of azobenzene increases from 49.4% in HMIM Tf2N
and 60.7% in BMIM PF6 up to 93.8% and 97.4%, respectively,
in the presence of the methoxy group.
The azobenzene molecule (X = H) is the less reactive
system, as confirmed by the highest η, while the OCH3-
azobenzene derivate is the most reactive between the
derivatives, as confirmed by the lowest η. Interestingly, F-
azobenzene is the system closer to azobenzene for cis
population and η value, suggesting a similar low reactivity in
the trans−cis conversion.
3. CONCLUSIONS
Both rotation and inversion take place for the cis−trans
azobenzene isomerization in BMIM Tf2N, EMIM Tf2N, and
HMIM Tf2N, according to the electronic nature of the
substituent. A change of mechanism from inversion to rotation
has been observed for Br- and F-azobenzene in BMIM BF4,
BMIM PF6, and BM2IM Tf2N, according to the viscosity of the
solvent. The structural organization of cations and anions
seems to be not affected by the presence of the azobenzene
isomers as suggested by SWAXS measurements. The cis
population of the azobenzene derivatives increases in the
photostationary state and the calculated η values of the trans
isomer suggest that OCH3-azobenzene is the most reactive
system of the investigated molecules for the trans−cis
conversion. The azobenzene isomerization still represents a
model reaction and the results presented herein could
C
DOI: 10.1021/acs.joc.5b00898
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
contribute to the understanding and the improving of RTILs as
reaction media.
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/acs.joc.5b00898.
AUTHOR INFORMATION
Corresponding Author
*E-mail: c.gasbarri@unich.it. Tel: +39-0871-3554786.
Notes
4. EXPERIMENTAL SECTION
■
General Information. Azobenzene and ethanol (99% spectrosco-
py grade) were purchased from Fluka and used without further
purification; p-OCH3-azobenzene, p-OtBu-azobenzene, p-Br-azoben-
zene and p-F-azobenzene were obtained as previously described.64
BMIM Tf2N (1-butyl-3-methylimidazolium bis(trifluoromethyl-
sulfonyl)imide), BMIM PF6 (1-butyl-3-methylimidazolium hexafluor-
ophosphate), BMIM BF4 (1-butyl-3-methylimidazolium tetrafluoro-
borate), EMIM Tf2N (1-ethyl-3-methylimidazolium bis-
(trifluoromethylsulfonyl)imide), BM2IM Tf2N (1-butyl-2,3-
dimethylimidazolium bis(trifluoromethylsulfonyl)imide) and HMIM
Tf2N (1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-
imide) were purchased from IoLiTec and Solvionics and stirred
under vacuum at 60 °C overnight before the preparation of the
samples.
Sample Preparation for the Kinetics Measurements. Stock
ethanol solutions from the investigated azobenzenes were prepared at
a concentration of 2.6 × 10−3 M and kept in the dark at room
temperature at least for 4 days before use. Then, an appropriate
amount was transferred into a 1 cm light pass quartz cuvette
containing 200 μL of the proper ionic liquid to obtain a final
concentration of 2.6 × 10−5 M. The sample was irradiated for 45 min
by using a Hg−Xe arc lamp (150 W) equipped with a band-pass
interference filter centered at 365.0 +2/-0 nm wavelength and 10.0
+2/-2 nm bandwidth: the spectral changes, as the decreasing of the
high-intensity absorption band centered in the wavelength range 315−
349 nm (depending both on the substituent and the solvent) and the
increasing of the low-intensity band at about 440 nm (due to an n →
π* transition) were used as evidence for the trans−cis isomerization.
The first-order rate constants kobs were spectrophotometrically
calculated by monitoring in the dark the absorption change at or
near the absorption maximum of the trans isomer over a period of
about 24 h.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work has been supported by MIUR (PRIN 2010-2011,
prot. 2010N3T9M4) and University “G. d’Annunzio” of Chieti-
Pescara.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Hammett plots in the RTILs, HOMO and LUMO energies of
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SWAXS spectra, and computational data. The Supporting
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