Photoresponsive ionic liquids with an azobenzene moiety

Article abstract of DOI:10.1246/cl.160048

We examined photoresponsive ionic liquids containing an azobenzene moiety. Ionic liquids with a p,p-azobenzene or o,p- tetrafluoroazobenzene moiety exhibit a reversible solid-to-liquid phase transition induced by photo- and thermal-stimulation. o,p-Azobenzene, which exists as an ionic liquid with a bent structure, is in a liquid state even as a trans-isomer. Such photoresponsive ionic liquids may be useful as novel functional materials, such as in optical sensors.

Full text of DOI:10.1246/cl.160048

Received: January 18, 2016 | Accepted: January 26, 2016 | Web Released: February 4, 2016
CL-160048
Photoresponsive Ionic Liquids with an Azobenzene Moiety
Kaoru Nobuoka,*1,2 Satoshi Kitaoka,3 Toyokazu Yamauchi,1 Thomas Harran,1 and Yuichi Ishikawa1
1Department of Applied Chemistry, Faculty of Engineering, Oita University, 700 Dannoharu, Oita 870-1192
2Research Promotion Institute, Oita University, 700 Dannoharu, Oita 870-1192
3Department of Biotechnology and Chemistry, Faculty of Engineering, Kindai University,
1 Umenobe, Takaya, Higashihiroshima, Hiroshima 739-2116
(E-mail: nobuokak@oita-u.ac.jp)
We examined photoresponsive ionic liquids containing an
azobenzene moiety. Ionic liquids with a p,p¤-azobenzene or o,p¤-
tetrafluoroazobenzene moiety exhibit a reversible solid-to-liquid
phase transition induced by photo- and thermal-stimulation.
o,p¤-Azobenzene, which exists as an ionic liquid with a bent
structure, is in a liquid state even as a trans-isomer. Such
photoresponsive ionic liquids may be useful as novel functional
materials, such as in optical sensors.
and stilbene.14 The azobenzene moiety was adopted as an
ionic material in this study. In the ionic liquids, the effect of
photoisomerism induced a change in the aggregation behavior
on solvent-dependent photoresponsive conductivity in ionic
liquids with an azobenzene moiety15 and the photochromic
properties of ionic liquids with an azobenzene moiety as an
anion in ethanol solution or in a thin film16 have been reported.
In addition, HPLC stationary phases functionalized by ionic
liquids with azobenzene moieties17 and photoresponsive visco-
elastic worm-like micelles formed by the mixture of an ionic
liquid and an azobenzene derivative18 have been reported.
Ionic liquid crystals with azobenzene moieties have also been
prepared.5 We selected azobenzene as a photoresponsive ligand
due to its simple structure and its conventional synthesis. In
addition to the fact that the introduction of bulky and symmetric
moieties to the ionic structure results in high melting point and
high viscosity, the trans-cis isomerization of azobenzene
changes the polarity and the molecular volume. Furthermore,
the presence of an isomerization mechanism that does not
include bond cleavage or bond creation makes the ionic liquids
stable. An azobenzene moiety was introduced to the cation
structure because we selected bis(trifluoromethanesulfonimide),
which readily produces room-temperature ionic liquids, as an
anion. Furthermore, we selected an imidazolium moiety as the
basic skeleton for the cation.
Keywords: Ionic liquid (IL)
| Azobenzene | Photoresponsive IL
Room-temperature ionic liquids composed of cations and
anions have been widely used in many fields, including
synthesis,1 catalysis,1,2 and electrochemistry.3 The most attrac-
tive property of ionic liquids is their high designability. Diverse
task-specific ionic liquids can be produced by the introduction
of various functional groups and proper anion-cation combi-
nations.4
Stimuli-responsive compounds are a type of functional
compounds. Their physical and chemical properties change in
response to external stimuli such as light, temperature, pH,
and electric or magnetic fields. Various studies have examined
photoresponsive ionic materials like ionic liquid crystals,5
ionogels,6 and ionic liquids.7 The application of photoisomeri-
zation of an ionic liquid for the isolation of Z-cinnamic acids has
also been reported.8 In addition, ionic liquids have been used as
a medium for photoisomerization.9 Herein, we synthesized task-
specific ionic liquids with photoresponsive azobenzene moieties
and investigated their physical properties under thermal- and
photoresponsive isomerization conditions.
Ionic liquids with azobenzene moieties were synthesized
according to Scheme 1. p-(3-Bromopropoxy)nitrobenzene (2a)
was synthesized in 40% yield via a Williamson ether synthesis
with p-nitrophenol and dibromopropane, and the aromatic nitro
group was reduced by treatment with 5% Pd/C to give the
corresponding p-(3-bromopropoxy)aniline (3a). 4-(3-Bromopro-
poxy)-4¤-hydroxyazobenzene (4a) was synthesized in 34% yield
Various photoresponsive ligands have been identified, in-
cluding azobenzene,10 dithienylethene,11 fulgide,12 spiropyran,13
O
(CH2)3Br
OH
O
(CH2)3Br
O (CH2)3Br
1. HCl aq, NaNO2, 5oC
Pd/C, H2
methanol
Br(CH2)3Br, KOH
ethanol, reflux
N
O
N
O2N
O2N
H2N
2. phenol, NaOHaq
Na2CO3, 5oC
1a
1b
2a:
2b:
3a:
3b:
4a (p, p') 34%
4b
: p-nitrophenol
: o-nitrophenol
para 40%
ortho 67%
para
ortho
(o, p') 37%
HO
N
N
O
(CH2)3Br
(CH2)3
O
(CH2)3
N
N
Li(CF3SO2)2N
water
Br(CH2)3Br, KOH
THF, reflux
N-methylimidazole
N
N
N
N
N
N
reflux
2Br-
2(CF3SO2)2N-
5a
5b
6a
6b
7a
7b
(p, p') 34%
(o, p') 60%
(p, p') 64%
(o, p') 95%
(p, p'-azo IL) 88%
(o, p'-azo IL) 90%
N
N
Br(CH2)3
O
(CH2)3
O
(CH2)3 O
N
N
Scheme 1. Preparation of ionic liquids 7a and 7b with an azobenzene moiety.
Chem. Lett. 2016, 45, 433–435 | doi:10.1246/cl.160048
© 2016 The Chemical Society of Japan | 433

N
N
im+
hν
O
N
N
O
im+
Δ
trans:cis = 34:66
Tg = -29.0oC
O
O
2(CF3SO2)2N-
im+
im+
2(CF3SO2)2N-
trans:cis = 100:0
Tm = 94.7oC
Photoirradiation
(330~400 nm)
-160 -120 -80
-40
0
40
80
120
trans 34% : cis 66%
trans 100%
Temeprature / oC
Figure 1. Photoisomerization of ionic liquid 7a.
¹1
Figure 2. DSC thermograms of ionic liquid 7a at a rate 10 °C min
before and after photoirradiation.
Table 1. Melting points and glass-transition temperature of ionic
liquids with an azobenzene moiety before and after photoirradiation
The disruption of molecular symmetry due to the bent
structure lowered the melting point. Thus, we synthesized o,p¤-
azo 7b, which is a stereoisomer of p,p¤-azo 7a with less
molecular symmetry (Scheme 1). The synthesized o,p¤-azo ionic
liquid 7b was a pure trans-isomer. The trans-o,p¤-azo-isomer is
a viscous liquid (4290 cP at 25 °C) at room temperature despite
photoirradiation unlike the p,p¤-azo because the structure of the
trans-o,p¤-azo-isomer has low molecular symmetry which makes
it difficult to crystallize. The glass-transition temperature of pure
trans-o,p¤-azo is ¹28.0 °C. The photoirradiation of pure trans-
o,p¤-azo 7b for two hours resulted in a photostationary state
consisting of 7% trans-o,p¤-azo 7b and 93% cis-o,p¤-azo 7b,
which remained in a liquid state at room temperature. We
determined the activation energy of the thermal cis-trans
isomerization from the Arrhenius plot of the isomerization
rate constant against the different temperatures (see Supporting
Information Figures S7 and S8). The activation energy of the
thermal cis-trans isomerization of o,p¤-azo 7b in DMF
(96 kJ mol¹1) was higher than that of p,p¤-azo 7a (82 kJ mol¹1).
This is due to the steric hindrance of the substituents. After
photoirradiation, the glass-transition temperature changed slight-
ly to ¹29.5 °C (Table 1). It is reasonable to deduce that since
trans-o,p¤-azo 7b originally has a bent structure, the molecular
structure does not show remarkable change in the structural
formula unlike p,p¤-azo 7a, even if it isomerizes to cis-o,p¤-
isomer 7b.
Ionic liquids
p,p¤-azo 7a
trans:cisa
Tm/°Cb
Tg/°Cb
100:0
34:66
100:0
7:93
80:20
16:84
94.7
94.7
®
®
84.4
84.4
®
¹29.0
¹28.0
¹29.5
®
o,p¤-azo 7b
o,p¤-tetrafluoro-azo
dipyridinium 8
¹22.0
aCalculated from 1H NMR spectra. bDetermined from DSC
¹1
thermogram at a heating rate of 10 °C min
.
via a diazo-coupling reaction, and the hydroxy group was
modified via Williamson synthesis with dibromopropane to give
the corresponding ether 5a in 34% yield. 5a reacted with N-
methylimidazole and a subsequent anion-exchange reaction of
the resulting imidazolium bromide salt 6a afforded the corre-
sponding imidazolium salt, 4,4¤-bis[3-(3-methylimidazolium-
1-yl)propoxy]azobenzene di[bis(trifluoromethanesulfonimide)]
(7a, p,p¤-azo), in relatively high yield (88%). We confirmed
that the synthesized p,p¤-azo ionic liquid 7a was a pure trans-
isomer by examining its 1H NMR spectrum. The trans-isomer is
a solid at room temperature.
Photoirradiation was performed in a methanol-dichloro-
methane (1:10, v/v) solution with light at a wavelength of 330-
400 nm at 15 °C for 2 h. Figure 1 shows the cis-trans photo-
isomerization of ionic liquids with azobenzene moieties and the
photograph is the neat p,p¤-azo 7a before and after photo-
irradiation. After photoirradiation, neat p,p¤-azo 7a became a
viscous liquid at room temperature. Table 1 summarizes the
ratio of each isomer and the thermal properties before and
after photoirradiation. Photoirradiation of pure trans-p,p¤-azo 7a
resulted in a photostationary state consisting of 34% trans-p,p¤-
azo 7a and 66% cis-p,p¤-azo 7a. As shown in Figure 2, the
melting temperature of pure trans-p,p¤-azo, which is a linear
molecule, is 94.7 °C. In addition to the melting temperature, the
glass transition temperature (Tg = ¹29.0 °C) was also observed
as the temperature at which the cis-isomer bends after photo-
irradiation. Repeated DSC scans showed a decrease in the area
under the endothermic peak at Tg = ¹29 °C from the cis-p,p¤-
isomer because of cis-trans thermal isomerization. The endo-
thermic Tg peak from the cis-p,p¤-isomer completely disappeared
and only the endothermic Tm peak from the trans-p,p¤-isomer
was observed in the fourth repeated DSC scan (see Supporting
Information Figure S6).
The introduction of fluorine atoms to decrease intermolec-
ular forces can effectively reduce the viscosity of ionic liquids.
We synthesized an ionic liquid with fluorinated azobenzene
8 (Figure 3), as shown in Scheme 1. We adopted a stable
pyridinium ring as a cation moiety of 8 because the imidazolium
ring decomposed due to the strong electron-withdrawing
property of fluorine atoms. The synthesized o,p¤-tetrafluoro-azo
dipyridinium ionic liquid 8 is a mixture of trans- and cis-isomer
(trans:cis = 80:20). The ionic liquid 8 is a solid at room
N (CH2)3
O
F
N N
F
F
(CH2)3
2(CF3SO2)2N-
O
F
N
Figure 3. trans-o,p¤-Tetrafluoro-azo dipyridinium ionic liquid 8.
434 | Chem. Lett. 2016, 45, 433–435 | doi:10.1246/cl.160048
© 2016 The Chemical Society of Japan

temperature and its melting point is 84.4 °C. The highly
symmetric structure of the pyridinium cation seems to increase
its crystallinity. Although the ionic liquid 8 is sensitive to light
and heat, photoisomerization of the ionic liquid 8 could be
observed with the use of its ethanol solution under nitrogen
atmosphere. Photoirradiation of the ionic liquid 8 resulted in
a photostationary state consisting of 16% trans-p,p¤-tetrafluoro-
azo dipyridinium 8 and 84% cis-tetrafluoro-azo dipyridinium 8,
and the glass-transition temperature was ¹22.0 °C. It was
predicted that delocalization of the electron density through the
introduction of strongly electron-withdrawing fluorine atoms
would reduce the thermal cis-trans isomerization energy of 8.
MacFarlane, H. Ohno, B. Scrosati, Nat. Mater. 2009, 8, 621.
a) J. H. Davis, Jr., Chem. Lett. 2004, 33, 1072. b) R. Giernoth,
Angew. Chem., Int. Ed. 2010, 49, 2834. c) S. Tang, G. A.
Baker, H. Zhao, Chem. Soc. Rev. 2012, 41, 4030. d) K.
Nobuoka, S. Kitaoka, T. Kojima, Y. Kawano, K. Hirano, M.
Tange, S. Obata, Y. Yamamoto, T. Harran, Y. Ishikawa, Org.
Chem. Int. 2014, 836126.
a) B. Soberats, E. Uchida, M. Yoshio, J. Kagimoto, H. Ohno,
T. Kato, J. Am. Chem. Soc. 2014, 136, 9552. b) K. Ishiba, M.
Morikawa, C. Chikara, T. Yamada, K. Iwase, M. Kawakita, N.
Kimizuka, Angew. Chem., Int. Ed. 2015, 54, 1532. c) K.
Stappert, J. Muthmann, E. T. Spielberg, A.-V. Mudring, Cryst.
Growth Des. 2015, 15, 4701. d) E. Wuckert, M. D. Harjung,
N. Kapernaum, C. Mueller, W. Frey, A. Baro, F. Giesselmann,
S. Laschat, Phys. Chem. Chem. Phys. 2015, 17, 8382.
a) M. Tamada, T. Watanabe, K. Horie, H. Ohno, Chem.
Commun. 2007, 39, 4050. b) A. Kavanagh, R. Byrne, D.
Diamond, K. J. Fraser, Membranes 2012, 2, 16. c) M.
Czugala, C. O’Connell, C. Blin, P. Fischer, K. J. Fraser, F.
Benito-Lopez, D. Diamond, Sens. Actuators, B 2014, 194,
105.
4
5
¹1
However, the energy of 8 was actually higher (125 kJ mol in
ethanol) than the values for azo ionic liquids 7a and 7b, which
do not contain fluorine atoms (see Supporting Information
Figure S9). Although it is difficult to compare the activation
energies because these values were measured in different
solvents, we can surmise that the symmetric structure of
pyridinium cations affects the isomerization energy.
In conclusion, we have demonstrated the photoisomeriza-
tion of ionic liquids with azobenzene moieties. The linear trans-
p,p¤-azo 7a isomer, which is solid at room temperature, was
photoisomerized to the liquid cis-p,p¤-azo 7a isomer. The bent
trans-o,p¤-azo 7b isomer is a liquid at room temperature, and
the glass-transition temperature after photoisomerization was
slightly decreased. The introduction of fluorine atoms into the
azobenzene moiety was not effective for reducing the viscosity
or the melting point, but did destabilize the cation ring.
Additional experiments are underway to prepare azo ionic
liquids that exhibit both physical and chemical stability, a low
melting point, and low viscosity.
6
7
a) K. Vijayakrishna, S. K. Jewrajka, A. Ruiz, R. Marcilla,
J. A. Pomposo, D. Mecerreyes, D. Taton, Y. Gnanou,
Macromolecules 2008, 41, 6299. b) Y. Kohno, H. Ohno,
Phys. Chem. Chem. Phys. 2012, 14, 5063. c) Z.-L. Xie, X.
Huang, A. Taubert, Adv. Funct. Mater. 2014, 24, 2837.
M. L. Salum, C. J. Robles, R. Erra-Balsells, Org. Lett. 2010,
12, 4808.
8
9
T. Nakashima, K. Miyamura, T. Sakai, T. Kawai, Chem.®Eur.
J. 2009, 15, 1977.
10 a) G. S. Kumar, D. C. Neckers, Chem. Rev. 1989, 89, 1915.
b) T. Schultz, J. Quenneville, B. Levine, A. Toniolo, T. J.
Martínez, S. Lochbrunner, M. Schmitt, J. P. Shaffer, M. Z.
Zgierski, A. Stolow, J. Am. Chem. Soc. 2003, 125, 8098.
c) H. M. D. Bandara, S. C. Burdette, Chem. Soc. Rev. 2012,
41, 1809.
This work was supported by JSPS KAKENHI Grant
Numbers 25810109, 26600028, 15K05598 and JST A-STEP
Grant Number AS242Z02498M.
Supporting Information is available on http://dx.doi.org/
10.1246/cl.160048.
11 H. Tian, S. Yang, Chem. Soc. Rev. 2004, 33, 85.
12 Y. Yokoyama, Chem. Rev. 2000, 100, 1717.
13 G. Berkovic, V. Krongauz, V. Weiss, Chem. Rev. 2000, 100,
1741.
References
1
2
3
a) M. A. P. Martins, C. P. Frizzo, D. N. Moreira, N. Zanatta,
H. G. Bonacorso, Chem. Rev. 2008, 108, 2015. b) H. Olivier-
Bourbigou, L. Magna, D. Morvan, Appl. Catal., A 2010, 373,
1. c) J. P. Hallett, T. Welton, Chem. Rev. 2011, 111, 3508.
a) P. J. Dyson, Appl. Organomet. Chem. 2002, 16, 495. b) J.
Dupont, R. F. de Souza, P. A. Z. Suarez, Chem. Rev. 2002,
102, 3667. c) T. Welton, Coord. Chem. Rev. 2004, 248, 2459.
d) V. I. Pârvulescu, C. Hardacre, Chem. Rev. 2007, 107, 2615.
a) M. C. Buzzeo, R. G. Evans, R. G. Compton, ChemPhys-
Chem 2004, 5, 1106. b) P. Hapiot, C. Lagrost, Chem. Rev.
2008, 108, 2238. c) N. V. Plechkova, K. R. Seddon, Chem.
Soc. Rev. 2008, 37, 123. d) M. Armand, F. Endres, D. R.
14 D. H. Waldeck, Chem. Rev. 1991, 91, 415.
15 S. Zhang, S. Liu, Q. Zhang, Y. Deng, Chem. Commun. 2011,
47, 6641.
16 L. C. Branco, F. Pina, Chem. Commun. 2009, 6204.
17 a) H. Qiu, M. Takafuji, T. Sawada, X. Liu, S. Jiang, H. Ihara,
Chem. Commun. 2010, 46, 8740. b) H. Qiu, A. K. Mallik, M.
Takafuji, H. Ihara, Chem.®Eur. J. 2011, 17, 7288. c) H. Qiu,
A. K. Mallik, M. Takafuji, X. Liu, S. Jiang, H. Ihara,
J. Chromatogr. A 2012, 1232, 116. d) H. Qiu, S. Jiang, M.
Takafuji, H. Ihara, Chem. Commun. 2013, 49, 2454.
18 Y. Bi, H. Wei, Q. Hu, W. Xu, Y. Gong, L. Yu, Langmuir 2015,
31, 3789.
Chem. Lett. 2016, 45, 433–435 | doi:10.1246/cl.160048
© 2016 The Chemical Society of Japan | 435