Photochromism of 3-butyl-1-methyl-2-phenylazoimidazolium in room temperature ionic liquids

Article abstract of DOI:10.1016/j.jphotochem.2009.10.002

We studied the photochromism of a newly synthesized ionic liquid, [2PA-Bmim]Tf₂N ([2PA-Bmim]⁺: 3-butyl-1-methyl-2-phenylazoimidazolium, Tf₂N^-: bis(trifluoromethanesulfonyl)-amide) which is characterized by a phenylazo group substituted on the imidazolium ring. The melting point of [2PA-Bmim]Tf₂N is 329 K. The absorption spectrum of [2PA-Bmim]⁺ dissolved in conventional organic solvents or in ionic liquids changes drastically upon UV-light irradiation, which is attributed to the photoisomerization of the phenylazo group from E- to Z-forms during irradiation and the backward thermal isomerization from Z- to E-forms in the dark. The E-Z photoisomerization quantum yield, Φ_iso, was determined by 355 nm laser photolysis. The Φ_iso value slightly depends on solvent viscosity, from 0.12 in 3-butyl-1-methylimidazolium PF₆^- (η = 241 cP) to 0.19 in toluene (η < 1 cP). On the other hand, no solvent dependence was observed for Arrhenius parameters of the backward Z-E thermal isomerization. We discuss the isomerization mechanism and the reason why the E-Z photoisomerization yield depends on solvent viscosity.

Full text of DOI:10.1016/j.jphotochem.2009.10.002

Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 12–18
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photochromism of 3-butyl-1-methyl-2-phenylazoimidazolium in room
temperature ionic liquids
Tooru Asaka, Nobuyuki Akai, Akio Kawai∗, Kazuhiko Shibuya∗∗
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 H89, Ohokayama, Meguro-ku, Tokyo 152-8551, Japan
a r t i c l e i n f o
a b s t r a c t
We studied the photochromism of a newly synthesized ionic liquid, [2PA-Bmim]Tf2N ([2PA-Bmim]+:
3-butyl-1-methyl-2-phenylazoimidazolium, Tf2N−: bis(trifluoromethanesulfonyl)-amide) which is char-
Article history:
Received 8 July 2009
Received in revised form
17 September 2009
Accepted 2 October 2009
Available online 12 October 2009
acterized by
a phenylazo group substituted on the imidazolium ring. The melting point of
[2PA-Bmim]Tf2N is 329 K. The absorption spectrum of [2PA-Bmim]+ dissolved in conventional organic
solvents or in ionic liquids changes drastically upon UV-light irradiation, which is attributed to the pho-
toisomerization of the phenylazo group from E- to Z-forms during irradiation and the backward thermal
isomerization from Z- to E-forms in the dark. The E–Z photoisomerization quantum yield, ˚iso, was
determined by 355 nm laser photolysis. The ˚iso value slightly depends on solvent viscosity, from 0.12
Keywords:
Ionic liquid
Photoisomerization
Photochromism
−
in 3-butyl-1-methylimidazolium PF6 (ꢀ = 241 cP) to 0.19 in toluene (ꢀ < 1 cP). On the other hand, no sol-
vent dependence was observed for Arrhenius parameters of the backward Z–E thermal isomerization.
We discuss the isomerization mechanism and the reason why the E–Z photoisomerization yield depends
on solvent viscosity.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
tion around NN bond of E and Z conformers accompanying a large
conformational change and there should be solvent effects on
Ionic liquids which are salts having low melting points have
been studied in various fields to understand their interesting
properties such as high viscosity, non-volatility, high ion con-
ductivity and so on [1]. These properties are controlled by a
choice of anion and/or cation components, which enables us to
create novel properties of solvent by introducing a functional
group into an ionic liquid. In this view point, we are inter-
ested in creating photofunctional ionic liquids, and planning
to synthesize imidazolium based ionic liquids with a pheny-
lazo group which is known to show photochromism [2]. As
the first step to create photochromic ionic liquids, we synthe-
sized 3-butyl-1-methyl-2-phenylazoimidazolium (=[2PA-Bmim]+)
bis(trifluoromethanesulfonyl)amide (=Tf2N−) (see Scheme 1) and
studied photochromic reaction of [2PA-Bmim]+ both in organic
solvents and in ionic liquids. Unfortunately, the melting point of
[2PA-Bmim]Tf2N was found to be 329 K which is above room tem-
perature, but some modification such as replacements of alkyl
group and/or anion may lower the melting point and give us
phenylazoimidazolium-based room temperature ionic liquids. The
photochromism of phenylazo compounds is owing to isomeriza-
the reaction. Since viscosity of ionic liquids is remarkably high,
rotational [3–9] and translational [10–16] diffusion motions of
solutes are different from those in conventional solvents. Solva-
tion [3,4,6,17–23] in ionic liquids is another interesting subject
and it has been reported that polarity of ionic liquids is close to
those of alcohols [17–19]. To develop phenylazo-substituted ionic
liquids with excellent photochromic character, we should eval-
uate how viscous and polar ionic liquids inhibit isomerization.
Although there are a few studies [24–26] about photoisomerization
of E- and Z-forms in ionic liquids, we intend to understand photo-
and thermal-isomerization of a phenylazo group attached to the
imidazolium ring, which should be important when we develop
photochromic ionic liquids. As the first step, we examined the pho-
tochromism of [2PA-Bmim]+ as a solute. This knowledge will be
useful when we study phenylazoimidazolium-based room temper-
ature ionic liquids whose developments are under progress.
Photo- and thermal-isomerization of azocompounds are rather
complicated and the isomerization mechanism has been argued
for long time. Here we briefly review isomerization of azobenzene
and its derivatives [2,27–43], which is important to understand
photochromism of [2PA-Bmim]+. Both E- and Z-forms undergo
photoisomerization and Z-form thermally isomerizes to E-form
[2]. Photoisomerization mechanism [27,28] has not been fully
understood yet and there remains unknown whether photoiso-
merization proceeds through in-plane bending of CNN moiety
(inversion mechanism) or an out-of-plane torsion of CNNC moi-
∗
Corresponding author. Tel.: +81 3 5734 2231; fax: +81 3 5734 2231.
Corresponding author. Tel.: +81 3 5734 2224; fax: +81 3 5734 2224.
E-mail addresses: akawai@chem.titech.ac.jp (A. Kawai),
∗∗
kshibuya@chem.titech.ac.jp (K. Shibuya).
1010-6030/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2009.10.002

T. Asaka et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 12–18
13
2.1.2. 3-Butyl-1-methyl-2-phenylazoimidazolium iodide
(=[2PA-Bmim]I)
[2PA-Bmim]I was synthesized by the similar synthetic pro-
cedure of 3-butyl-1-methylimidazolium chloride (BmimCl) [46].
Pai-Me and butyl iodide were dissolved in toluene. The solution
was heated to 383 K and refluxed for 5 h. The orange precipitation
was washed with ethyl acetate and toluene.
2.1.3. 3-Butyl-1-methyl-2-phenylazoimidazolium Tf2N
(=[2PA-Bmim][Tf2N])
Scheme 1. Two structural forms of [2PA-Bmim]Tf2N.
The synthesis of [2PA-Bmim]Tf2N followed the synthetic pro-
cedure of 3-butyl-1-methylimidazolium Tf2N (Bmim Tf2N) [46].
[2PA-Bmim]I and lithium bis(trifluoromethanesulfonyl)amide
were dissolved in water. After the solution was stirred for 24 h at
room temperature, orange precipitation was corrected and washed
with water.
Melting points of synthetic products were measured using
a differential scanning calorimeter (Shimadzu, DSC-60) and the
identification was done by 1H NMR (JEOL, JNM-AL300) in CDCl3.
Viscosity of ionic liquid solvents were measured by a viscometer
(BROOKFIELD, DV-II+ Pro).
ety (rotation mechanism). In these arguments, solvent viscosity is
certainly important and ethylene glycol which has lower viscosity
than ionic liquids gives remarkable viscosity effects on the photoi-
somerization. As for the thermal process, isomerization proceeds
through the inversion but, some azobenzene derivatives such as 4-
dimethylamino-4ꢀ-nitroazobenzene (=DNAB), the rotation is major
channel. This feature is schematically shown in Fig. 1. The activated
state of DNAB in rotation mechanism has charge transfer (CT) char-
acter and rotation is dominant in polar solvents [39–42] and in
ionic liquids [43]. Since our sample is an azobenzene derivative,
we should consider solvent polarity effects on its isomerization in
ionic liquids.
2.1.4. 1H NMR (300 MHz, CDCl3)
Pai-Me (m.p. = 393 K): 4.17 (s, 3H, NCH3), 7.26 (s, 1H, CH), 7.39
(s, 1H, CH), 7.60 (m, 3H, phenyl), 8.11 (d, 2H, phenyl).
[2PA-Bmim]I (m.p. = 441 K): 1.00 (t, 3H, CH3), 1.46 (m, 2H, CH2),
1.95 (m, 2H, CH2), 4.34 (s, 3H, NCH3), 4.66 (t, 2H, NCH2), 7.60–7.75
(m, 3H, phenyl), 8.00 (d, 2H, phenyl), 8.12 (d, 1H, CH), 8.33 (d, 1H,
CH).
[2PA-Bmim]Tf2N (m.p. = 329 K): 0.98 (t, 3H, CH3), 1.43 (m, 2H,
CH2), 1.90 (m, 2H, CH2), 4.19 (s, 3H, NCH3), 4.54 (t, 2H, NCH2),
7.59–7.74 (m, 5H, phenyl, CH), 7.95 (d, 2H, phenyl).
Recently,
a notable study was reported on 1-methyl-2-
phenylazoimidazole (=Pai-Me) [44], which has similar structure to
[2PA-Bmim]+. Photoisomerization quantum yield (˚iso) of Pai-Me
is 0.25 in toluene, which seems promising that our synthetic sam-
ple will have photochromic activity. In this study, we measured ˚iso
value to evaluate if [2PA-Bmim]+ shows photochromism. Arrhenius
parameters of the thermal isomerization were also measured. We
discuss how the viscosity and polarity of ionic liquids influences
the isomerization of [2PA-Bmim]+.
2.2. Optical measurements
The absorption spectra of [2PA-Bmim]Tf2N were measured by
a UV–vis spectrometer (Shimadzu, UV-2450). A light at 365 nm
from an ultrahigh pressure mercury lamp (USHIO, USH-500SC)
was used for the photoisomerization from the E-form to the Z-
form. The thermal isomerization of the samples was monitored
by a UV–vis absorption method at various temperatures using a
temperature controller (Shimadzu TCC-240A). The ˚iso values of
E–Z photoisomerization were determined by a transient absorp-
tion method [47]. A Xe-flash lamp was used for a monitor light
and the third harmonics of a Nd3+-YAG laser (Continuum, Sure-
light) was used for an excitation light source. A laser energy meter
(Gentec, QE-25-SP-S) was used to measure relative laser power.
2. Experimental
2.1. Synthesis and sample characterization
2.1.1. 1-Methyl-2-phenylazoimidazole (=Pai-Me)
2-Phenylazoimidazole (=Pai-H) was synthesized by a pre-
viously reported method [45]. Pai-H, methyl iodide, and
1,8-diazabicyclo[5.4.0]undec-7-ene (=DBU) were dissolved in
methanol. The solution was heated to 338 K, refluxed for 6 h,
and then evaporated. The separated crystal was washed with
water.
Fig. 1. Mechanisms proposed for the thermal isomerization of an azobenzene derivative, 4-dimethylamino-4ꢀ-nitroazobenzene (=DNAB) [40–43].

14
T. Asaka et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 12–18
Degassed acetonitrile solution of benzophenone was used as a stan-
dard sample to determine ˚iso values. The transient absorption
was measured at 516.5 nm for the standard sample triplet–triplet
absorption and 376.5 nm for the photoisomerization of the present
interest. For the determination of quantum yield, the transient
absorbance change by only a single laser shot was accumulated
for the data acquisition and the sample was replaced after every
laser shot to avoid the photodegradation of solvents, i.e. ionic
liquids.
The spectroscopic grade toluene and methanol, the fluorometry
grade acetonitrile, and ionic liquids were purchased from Kanto
Chemical Co., Ltd. (Tokyo, Japan) and used as received for mea-
surements. All spectroscopic experiments were performed at room
temperature, except for the measurements of the thermal isomer-
ization (293–308 K).
2.3. Quantum chemical calculations
The energies of the E- and Z-forms of [2PA-Bmim]+ in the ground
state were obtained by a DFT calculation. The molecular geometry
optimizations and frequency calculations were performed at the
B3LYP/6-31G* level using the Gaussian 03 program [48].
3. Results and discussion
3.1. Observation of spectral changes upon UV-light irradiation
Fig. 2 shows changes in the absorption spectra of [2PA-
Bmim]Tf2N in BmimTf2N upon the UV-light irradiation. The
spectral carrier is surely [2PA-Bmim]+ without doubt since neither
Bmim+ nor Tf2N− have absorption in this spectral region. Before the
Fig. 2. Absorption spectral change of [2PA-Bmim]Tf2N in BmimTf2N (2.6 × 10−5 M).
(a) Spectral changes upon the UV-light (365 nm) irradiation at room temperature.
The spectral carrier is [2PA-Bmim]+. Three isosbestic points are recognized at 265,
312 and 423 nm. (b) Reproduced absorption spectra of E-form and Z-form [2PA-
Bmim]+. See the text for details.
UV-light irradiation, the strong ␲–␲* band (ε ∼ 23,000 M−1 cm−1
,
M = mol dm−3) at 361 nm and the weak n–␲* band around 455 nm
(ε ∼ 1800 M−1 cm−1) were observed. The spectral feature is quite
similar to that of E-form Pai-Me [44]. After the UV-light irradiation,
the 361 nm band decreased in intensity and the maximum wave-
length shifted from 361 to 311 nm (ε ∼ 7000 M−1 cm−1). Besides,
the n–␲* band at 455 nm increased in intensity. The spectral carrier
generated after the UV irradiation is attributed to the Z-form based
on the spectrum of Z-form Pai-Me [44]. It is notable the S1(n–␲*)
band intensity of Z-form is higher than that of E-form. This may
be due to more twisted structure of CNNC moiety in Z-form which
results in partially allowed optical transition for n–␲* excitation.
Optical properties of E- and Z-form [2PA-Bmim]+ are summarized
in Table 1. Three isosbestic points were observed at 265, 314, and
423 nm. The similar results were obtained in other solvents. Z-form
produced by the UV irradiation of E-form undergoes thermal Z–E
isomerization. The absorption spectrum recorded at 60 min after
the UV-light irradiation finished is exactly the same as the initial
one of E-form. From these results, [2PA-Bmim]+ is concluded as a
T-type photochromic compound [49].
It is worthwhile to evaluate purity of Z-form spectrum in Fig. 2a
to compare pure spectra of E- and Z-forms. As explained later, E-
form energy of [2PA-Bmim]+ is much lower than Z-form and we
safely attributed the absorption spectrum before UV-light irradia-
tion to pure E-form. On the other hand, pure Z-form spectrum was
hardly obtained under steady state condition of the mixture solu-
tion of E- and Z-forms during 365 nm irradiation. Here we assume
that photoisomerization quantum yield from E-form to Z-form at
365 nm excitation equals to that from Z-form to E-form, which
means that a half of absorbance at 365 nm of absorption spectrum
at 600 s after UV-light irradiation is due to pure E-form. Under this
assumption, we reproduced pure Z-form spectrum by subtracting
E-form spectrum from the absorption spectrum at 600 s after UV-
Table 1
Optical properties of E- and Z-form [2PA-Bmim]+.
Solvent
E-form
␲–␲*
Z-form
␲–␲*
ꢁisoa/nm
n–␲*
ꢁmaxb/nm
ε/104 M−1 cm−1
ꢁmaxb/nm
ε/103 M−1 cm−1
ꢁmaxb/nm
ε/103 M−1 cm−1
Toluene
367
360
360
365
360
361
358
359
1.9
2.4
2.5
2.2
2.1
2.3
2.3
2.2
–
310
311
315
311
311
311
310
–
459
450
455
455
450
455
453
452
1.8
2.1
2.0
2.0
1.7
1.8
1.9
1.8
–, 321, 442
Methanol
Acetonitrile
Ethylene glycol
EmimBF4
BmimTf2N
BmimBF4
8.2
7.5
7.1
6.5
7.0
6.9
6.8
266, 314, 426
263, 313, 421
266, 319, 429
265, 313, 421
265, 314, 423
265, 315, 423
265, 314, 420
BmimPF6
a
Wavelength of isosbestic point. ꢁiso around 265 nm in toluene were not observed because of electronic absorption of solvent toluene.
Wavelength of band peak.
b

T. Asaka et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 12–18
15
Fig. 4. Difference spectra of [2PA-Bmim]+ upon the UV irradiation. Black circles
represent the absorbance change spectrum of [2PA-Bmim]+ in toluene measured
at 2 ␮s after 355 nm laser excitation obtained by a transient absorption method.
The solid line shows the difference spectrum of [2PA-Bmim]+ in toluene before and
after UV irradiation for 1 h obtained by a UV–vis spectrometer and the spectrum
was scaled to fit the black circles. The sample solution was prepared by dissolving
[2PA-Bmim]Tf2N in toluene (2.8 × 10−5 M).
Fig. 3. Time profile of absorbance of [2PA-Bmim]+ in BmimTf2N in the dark mea-
sured at 360 nm. Temperature was kept at 307 K. The fitting line reproduces the data
points measured at 360 nm. The AE value (0.39) corresponds to the absorbance of
[2PA-Bmim]+ for this sample solution when all the Z-form returns to the E-form.
See the text for details.
ward isomerization of [2PA-Bmim]+ certainly occurs within several
hours even in highly viscous solvent.
light irradiation. These E- and Z-form spectra are shown in Fig. 2b. It
is clearly shown that pure Z-form spectrum is essentially the same
with the absorption spectrum at 600 s after UV-light irradiation as
given in Fig. 2a. Therefore, we consider that extinction coefficients
given in Table 1 are reasonable values.
As described before, the thermal isomerization process of DNAB
depends on polarity of solvents: The Ea for DNAB in polar solvents
(ca. 40 kJ mol−1) are smaller than those in non-polar solvents (ca.
80 kJ mol−1) [43]. The difference of the Ea value is very large of
about 40 kJ mol−1. This is because the activated state of DNAB has
strong CT character and is stabilized in polar solvents. On the other
hand, the Ea for [2PA-Bmim]+ are almost the same in non-polar
toluene, polar acetonitrile, ethylene glycol and four ionic liquids.
This may mean that the activated state of [2PA-Bmim]+ does not
have CT character. According to the previous studies on the ther-
mal isomerization of azobenzene, the reaction takes place through
inversion mechanism and there are few differences in the acti-
vation energy (Ea = 80–100 kJ mol−1) among non-polar and polar
solvents. Although the Ea of about 80 kJ mol−1 for [2PA-Bmim]+
is slightly lower than that of azobenzene, a characteristic point
of [2PA-Bmim]+ isomerization, namely no dependence on solvent
polarity, is quite similar to that of azobenzene. From these reasons,
we concluded that Z-form [2PA-Bmim]+ thermally isomerizes by
inversion mechanism.
3.2. Thermal backward isomerization
After the long-time UV-light irradiation to prepare Z-form,
absorbance around 360 nm was dominantly due to Z-form of [2PA-
Bmim]+. Fig. 3 shows a time profile of 360 nm absorbance in the
dark. The thermal isomerization in BmimTf2N takes place slowly,
and the reaction rate is characterized by a single exponential func-
tion. The unimolecular rate constant of the thermal backward
isomerization (k) was obtained by the analysis of the measured
absorbance, Abs(t), using Eq. (1).
Abs(t) = (AbsZ − AbsE)e−kt + AbsE
(1)
where AbsE and AbsZ are absorbances corresponding to pure sam-
ples of E- and Z-forms, respectively. The experimental data were
fitted by a nonlinear algorithm with Eq. (1). The activation energy,
Ea, of thermal backward isomerization, and the frequency factor,
A, were determined by fitting the temperature dependent k val-
ues and the results are listed in Table 2. We measured viscosities
of the solvents at 298 K and the values were also tabulated. Both
the Ea and the A values are essentially identical within experimen-
tal error for seven solvents of largely different viscosity. This result
is consistent with the observation in the thermal isomerization of
azobenzene whose Arrhenius parameters show no clear correlation
with solvent viscosity [43]. We addressed here that thermal back-
3.3. Photoisomerization from E-form to Z-form
The photoisomerization of azobenzene takes place by both S1
and S2 excitation [2]. Here, we determined the quantum yields of
the photoisomerization of [2PA-Bmim]+ by a transient absorption
method after S2(␲–␲*) excitation. Fig. 4 shows the difference spec-
trum of [2PA-Bmim]+ in toluene obtained by the spectra before
and 2 ␮s after laser excitation. The spectral shape is quite similar to
the spectrum obtained by subtracting the initial E-form spectrum
from the product of Z-form spectrum measured by the UV–vis spec-
trometer. The isosbestic point around 440 nm is consistent with the
result (442 nm, see Table 2) in absorption spectral change in toluene
under the continuous-wave lamp irradiation. The depletion around
375 nm and the increase around 450 nm are thus attributed to the
photoisomerization from E-form to Z-form within [2PA-Bmim]+.
The spectral change occurs promptly within a laser pulse (ca. 10 ns).
A previous femtosecond spectroscopic study [29–33] reported the
lifetime of excited E-azobenzene is around 10 ps or much shorter.
Thus, we consider that the initial spectral change of [2PA-Bmim]+
certainly reflects the photoisomerization.
Table 2
Arrhenius parameters for backward Z–E thermal isomerization in various solvents
and measured viscosities, ꢀ of solvents at 298 K.
Solvent
Ea/kJ mol−1
A/s−1
ꢀ/cP
Toluene
77
80
79
90
85
90
85
2
4
5
5
3
4
4
(1.4 1.0) × 1010
(3.4 3.2) × 1010
(2.5 2.4) × 1010
(1.3 1.2) × 1011
(1.3 1.0) × 1011
(1.3 1.2) × 1012
(1.8 1.7) × 1011
<1
<1
17
34
50
Acetonitrile
Ethylene glycol
EmimBF4
BmimTf2N
BmimBF4
93
241
Fig. 5 shows absolute value of absorbance change, |ꢂAbs| of
BmimPF6
[2PA-Bmim]+ monitored at 376.5 nm as a function of the laser

16
T. Asaka et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 12–18
ined ˚iso value for wide range of solvent viscosity, ca. 1–241 cP as
listed in Table 3 and found two prominent features. One is that ˚iso
value does not become zero even though we have used remarkably
viscous solvents. This feature is very important for development of
photofunctional ionic liquids. In general, the viscosity of ionic liq-
uids is high, and one might be afraid that isomerization rate with
large amplitude motion like phenyl torsion is slow. However, the
present results indicate that isomerization certainly occurs in vis-
cous ionic liquids. The other feature is that ˚iso value gradually
decreases as viscosity increases. This may be related to the iso-
merization mechanism and we discuss this feature on the basis of
photoisomerization mechanism of azobenzene in the following.
Although a large number of studies have been reported for pho-
toisomerization of azobenzene, its mechanism seems still under
debates. Here, we consider the photoisomerization mechanism
after S2(␲–␲*) excitation proposed by recent ultra-fast time-
resolved measurements and theoretical calculations. Fujino et al.
determined the quantum yield of S2 → S1 relaxation as a unity
by a femtosecond fluorescence spectroscopy, and suggested that
both S1 and S2 excitations result in isomerization with the same
mechanism along S1 surface [31]. The ˚iso value is smaller for
S2 excitation because vibrationally hot S1 produced after internal
conversion undergoes fast relaxation to S0 without isomeriza-
tion. According to theoretical calculations, this relaxation occurs
through conical intersection between S1 and S0 states along con-
certed CNN-bending coordinate (concerted inversion mechanism)
[35]. To find direct evidence which well explains isomerization
mechanism clearly, Chang et al. measured femtosecond fluores-
cence anisotropy relaxation times of the excited state in hexane and
in ethylene glycol under S1 excitation [33]. They evaluated contri-
bution of rotation and concerted inversion processes for S1 relaxation
from the time dependence of the direction of transition moment.
They suggested that isomerization due to rotation occurs in non-
viscous hexane, while the rotation process is suppressed in ethylene
glycol of viscous solvent and ˚iso drops due to relaxation through
concerted inversion mechanism.
Fig. 5. Laser power dependence of absolute values of absorbance changes. Black
triangles are data for [2PA-Bmim]Tf2N in toluene, while white circles are for the
standard sample, benzophenone in acetonitrile. Each fitting line was obtained by
the least square fitting method.
power. E–Z photoisomerization is a one-photon process and the
linear dependence of absorbance change was observed. We also
examined the laser power dependence of triplet–triplet absorbance
change of benzophenone at 516.5 nm in acetonitrile, |ꢂAbsbp|, to
evaluate an apparatus factor. The relation between absorbance
changes and laser power are described by the following equations,
ꢀ
ꢀ
ꢂAbs = ˚iso˛l ꢂε(1 − 10−Abs355 )x
(2)
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢂAbsbp = ˚ISC ˛lεT−T (1 − 10−Abs355 )xbp
(3)
ꢀ
for [2PA-Bmim]+ and benzophenone, respectively. In these equa-
tions, ˛ means an apparatus factor, l a monitoring light pass length,
x and xbp relative values of the laser power for [2PA-Bmim]+ and
benzophenone samples, respectively, and ˚ISC (=1) is an S1–T1
ISC yield. The ꢂε of 15,000 M−1 cm−1 is the difference of extinc-
tion coefficients between E- and Z-forms at 376.5 nm. This value
was determined from the absorption spectra of E- and Z-forms in
toluene. The εT–T of 6200 M−1 cm−1 is an extinction coefficient of
triplet benzophenone at 516.5 nm [50]. The Abs355 is an absorbance
at 355 nm before the laser irradiation for both samples, and they
were equalized (Abs355 < 0.5) for [2PA-Bmim]+ and benzophenone
when we prepared the samples. Hence we combined Eqs. (2) and
(3) as
We discuss the photoisomerization mechanism of [2PA-Bmim]+
according to these previous studies for azobenzene to rationalize
the dependence of the quantum yields on solvent viscosity. Fig. 6
shows schematic potential surfaces along E–Z isomerization coor-
dinate for rotation, inversion and concerted inversion mechanisms.
Since both phenyl and imidazolium groups are included in bend-
ing motion of [2PA-Bmim]+, concerted inversion as discussed in
azobenzene dynamics does not exist. Here, we consider pseudo
concerted inversion for [2PA-Bmim]+ where phenyl and imida-
zolium groups undergoes bending with having the same CNN angle.
The result obtained by a DFT calculation is used for the energy dif-
ference of ca. 80 kJ mol−1 between E- and Z-forms at S0 state. The
εT−T (|ꢂAbs|/x)
˚
=
(4)
iso
ꢂε(|ꢂAbsbp|/xbp
)
where (|ꢂAbs|/x) and (|ꢂAbsbp|/xbp) are the slopes for [2PA-Bmim]+
and benzophenone in Fig. 5, respectively. From Eq. (4), ˚iso value in
toluene was determined as 0.19 0.02. ˚iso values in other solvents
were also determined and the results were summarized in Table 3.
It is noteworthy that ˚iso value in ionic liquids are 0.12–0.14
and are slightly smaller than those in organic solvents. We exam-
S1 energy is roughly estimated from absorption spectrum. The acti-
vated state energy along the S0 surface is 160 kJ mol−1 which is
estimated from the Ea value of ca. 80 kJ mol−1. The thermal back-
ward isomerization may proceed through inversion mechanism as
shown in Fig. 6a. This is because no solvent polarity dependence
was found on Ea values as discussed before and theoretical cal-
culation predicted much lower barrier height for inversion in case
of azobenzene [37]. On the other hand, photoisomerization mech-
anism is rather complicated. After S2 excitation, the E-form of
[2PA-Bmim]+ relaxes to S1 state very rapidly. Radiation process
is reasonably ignored because the natural radiation lifetimes esti-
mated by absorption spectra are 8–9 ns and 200–300 ns for S2–S0
and S1–S0 transitions, respectively, which are too long for relax-
ation. According to the studies on azobenzene, S1 surface along
rotation coordinate has no barrier and S1/S0 conical intersection
exists. Therefore, [2PA-Bmim]+ at S1 also undergoes conforma-
tional change along rotation coordinate and isomerization occurs
at the conical intersection. However, this process is suppressed
Table 3
Quantum yields, ˚iso of E–Z photoisomerization of [2PA-Bmim]+ under 355 nm
excitation in several solvents and measured viscosities, ꢀ of solvents at 298 K.
Solvent
˚
iso
ꢀ/cP
Toluene
0.19 0.02
0.16 0.02
0.16 0.02
0.15 0.02
0.14 0.02
0.13 0.02
0.13 0.02
0.12 0.02
<1
<1
<1
17
34
50
93
241
Acetonitrile
Methanol
Ethylene glycol
EmimBF4
BmimTf2N
BmimBF4
BmimPF6

T. Asaka et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 12–18
17
in conventional organic solvents, and is smaller in ionic liquids,
BmimX (X = Tf2N, BF4, PF6). These non-zero ˚iso values in viscous
solutions promise that [2PA-Bmim]+ can be used as photochromic
material. The ˚iso value decreases as solvent viscosity increases,
which suggests that isomerization at S1–S0 internal conversion
along rotation coordinate becomes slow in viscous solvent like
ionic liquids. We discussed concerted inversion mechanism as S1
relaxation channel in these viscous solvents.
Acknowledgements
The authors express their thanks to Mr. Y. Karibe for his kind aid
in NMR measurements and Mr. T. Kawamori for his kind assistance
in optical measurements. This work is partly supported by Grant-
in-Aids for scientific research (17073006 and 1935008) from the
Ministry of Education, Science, Culture and Sports of Japan.
References
[1] J.F. Wishart, E.W. Castner Jr., J. Phys. Chem. B 111 (2007) 4639–4640.
[2] N. Tamai, H. Miyasaka, Chem. Rev. 100 (2000) 1875–1890.
[3] H. Jin, G.A. Baker, S. Arzhantsev, J. Dong, M. Maroncelli, J. Phys. Chem. B 111
(2007) 7291–7302.
[4] A. Chakraborty, D. Seth, D. Chakrabarty, P. Setua, N. Sarkar, J. Phys. Chem. A 109
(2005) 11110–11116.
[5] N. Ito, R. Richert, J. Phys. Chem. B 111 (2007) 5016–5022.
[6] A. Paul, A. Samanta, J. Phys. Chem. B 111 (2007) 4724–4731.
[7] A.M. Funston, T.A. Fadeeva, J.F. Wishart, E.W. Castner Jr., J. Phys. Chem. B 111
(2007) 4963–4977.
[8] A. Kawai, T. Hidemori, K. Shibuya, Mol. Phys. 104 (2006) 1573–1579.
[9] Y. Miyake, T. Hidemori, N. Akai, A. Kawai, K. Shibuya, S. Koguchi, T. Kitazume,
Chem. Lett. 38 (2009) 124–125.
[10] H. Tokuda, K. Hayamizu, K. Ishii, M.A.B.H. Susan, M. Watanabe, J. Phys. Chem. B
108 (2004) 16593–16600.
[11] A. Skrzypczak, P. Neta, J. Phys. Chem. A 107 (2003) 7800–7803.
[12] O. Borodin, G.D. Smith, W. Henderson, J. Phys. Chem.
16879–16886.
B 110 (2006)
[13] N. Byrne, P.C. Howlett, D.R. MacFarlane, M. Forsyth, Adv. Mater. 17 (2005)
2497–2501.
[14] S.H. Chung, R. Lopato, S.G. Greebaum, H. Shirota, E.W. Castner Jr., J.F. Wishart,
J. Phys. Chem. B 111 (2007) 4885–4893.
[15] Y. Nishiyama, M. Fukuda, M. Terazima, Y. Kimura, J. Chem. Phys. 128 (2008)
164514–164522.
Fig. 6. Schematic drawings of the potential surfaces of [2PA-Bmim]+ along inver-
sion, rotation and concerted inversion coordinates. Potential surfaces for (a) thermal
isomerization and (b) photoisomerization are described and solid arrows represent
relaxation pathways. CI means conical intersection.
[16] D. Morgan, L. Ferguson, P. Scovazzo, Ind. Eng. Chem. Res. 44 (2005) 4815–4823.
[17] L. Crowhurst, P.R. Mawdsley, J.M. Perez-Arlandis, P.A. Salter, T. Welton, Phys.
Chem. Chem. Phys. 5 (2003) 2790.
in viscous ionic liquids (ꢀ > 30 cP) and S1–S0 relaxation through
concerted inversion may occur. Perhaps, this is the reason why
[18] C. Reichardt, Green Chem. 7 (2005) 339.
[19] A. Kawai, T. Hidemori, K. Shibuya, Chem. Lett. 33 (2004) 1464–1465.
[20] A. Kawai, T. Hidemori, K. Shibuya, Chem. Phys. Lett. 414 (2005) 378–383.
[21] J.F. Wishart, S.I. Lall-Ramnarine, R. Raju, A. Scumpia, S. Bellevue, R. Ragbir, R.
Engel, Radiat. Phys. Chem. 72 (2005) 99–104.
[22] K. Iwata, M. Kakita, H. Hamaguchi, J. Phys. Chem. B 111 (2007) 4914–4919.
[23] M. Halder, L.S. Headley, P. Mukherjee, X. Song, J.W. Petrich, J. Phys. Chem. A 110
(2006) 8623–8626.
[24] R. Ozawa, H. Hamaguchi, Chem. Lett. 30 (2001) 736–737.
[25] K.S. Mali, G.B. Dutt, T. Mukherjee, J. Chem. Phys. 128 (2008) 124515.
[26] T. Akitsu, Y. Einaga, Inorg. Chem. Commun. 9 (2006) 1108–1110.
[27] H. Rau, E. Luddecke, J. Am. Chem. Soc. 104 (1982) 1616–1620.
[28] S. Monti, G. Orlandi, P. Palmieri, Chem. Phys. 71 (1982) 87–99.
[29] I.K. Lednev, T.-Q. Ye, R.E. Hester, J.N. Moore, J. Phys. Chem. 100 (1996)
13338–13341.
[30] T. Fujino, T. Tahara, J. Phys. Chem. A 104 (2000) 4203–4210.
[31] T. Fujino, S.Y. Arzhantsev, T. Tahara, J. Phys. Chem. A 105 (2001) 8123–8129.
[32] T. Schultz, J. Quenneville, B. Levine, A. Toniolo, T.J. Martinez, S. Lochbrunner,
M. Schmitt, J.P. Shaffer, M.Z. Zgierski, A. Stolow, J. Am. Chem. Soc. 125 (2003)
8098–8099.
˚
value is smaller in viscous ionic liquids. It is unlikely that
iso
photoisomerization through inversion mechanism dominants the
relaxation, because (1) no conical intersection has been found
along inversion coordinate for azobenzene according to the recent
theoretical calculations and (2) isomerization through inversion
mechanism may not depend on solvent viscosity as compared to
rotation mechanism. For the complete understanding of photoi-
somerization mechanism of [2PA-Bmim]+, photochemical process
after S1(n–␲*)–S0 excitation should be examined by a ultra-fast
time-resolved spectroscopy. The continuing work along this line is
now in progress.
4. Conclusion
[33] C.-W. Chang, Y.-C. Lu, T.-T. Wang, E.W.-G. Diau, J. Am. Chem. Soc. 126 (2004)
10109–10118.
A
novel ionic liquid, [2PA-Bmim]Tf2N having azo-
photochromophoric group was synthesized for the first time
and was confirmed as a T-type photochromic compound by the
present spectroscopic studies. Although the conformational change
of the thermal isomerization is very large, Arrhenius parameters
do not depend on the solvent properties such as viscosity and
polarity. We concluded that the thermal isomerization from Z to E-
form of [2PA-Bmim]+ proceeds through inversion mechanism. The
[34] Y.-C. Lu, E.W.-G. Diau, H. Rau, J. Phys. Chem. A 109 (2005) 2090–2099.
[35] E.W.-G. Diau, J. Phys. Chem. A 108 (2004) 950–956.
[36] T. Ishikawa, T. Noro, J. Chem. Phys. 115 (2001) 7503–7512.
[37] C.R. Crecca, A.E. Roitberg, J. Phys. Chem. A 110 (2006) 8188–8203.
[38] A. Cembran, F. Bernardi, L. Garavelli, L. Gagliardi, G. Orlandi, J. Am. Chem. Soc.
126 (2004) 3234–3243.
[39] T. Asano, T. Okada, S. Shinkai, K. Shigematsu, Y. Kusano, O. Manabe, J. Am. Chem.
Soc. 103 (1981) 5161–5165.
[40] T. Asano, J. Am. Chem. Soc. 102 (1980) 1205–1207.
[41] D.G. Whitten, P.D. Wildes, J.G. Pacifici, G. Irick Jr., J. Am. Chem. Soc. 93 (1971)
2004–2008.
quantum yield of E–Z photoisomerization of [2PA-Bmim]+, ˚iso
was determined in various solvents. The ˚iso value is 0.15–0.19
,

18
T. Asaka et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 12–18
[42] K.S. Schanze, T.F. Mattox, D.G. Whitten, J. Am. Chem. Soc. 104 (1982)
1733–1735.
[43] K. Baba, H. Ono, E. Itoh, S. Itoh, K. Noda, T. Usui, K. Ishihara, M. Inamo, H.D.
Takagi, T. Asano, Chem. Eur. J. (2006) 5328–5333.
[44] J. Otsuki, K. Suwa, K. Narutaki, C. Sinha, I. Yoshikawa, I.K. Araki, J. Phys. Chem.
A 109 (2005) 8064–8069.
[45] U.S. Ray, G. Mostafa, T.H. Lu, C. Sinha, Crystal Eng. 5 (2002) 95–104.
[46] J.G. Huddleston, A.E. Visser, W.M. Reichert, H.D. Willauer, G.A. Broker, R.D.
Rogers, Green Chem. 3 (2001) 156–164.
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P.
Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Strat-
mann, A.J. Yazyev, R. Austin, C. Cammi, J.W. Pomelli, P.Y. Ochterski, K. Ayala, G.A.
Morokuma, P. Voth, J.J. Salvador, V.G. Dannenberg, S. Zakrzewski, A.D. Dapprich,
M.C. Daniels, O. Strain, D.K. Farkas, A.D. Malick, K. Rabuck, J.B. Raghavachari, J.V.
Foresman, Q. Ortiz, A.G. Cui, S. Baboul, J. Clifford, B.B. Cioslowski, G. Stefanov,
A. Liu, P. Liashenko, Piskorz, R.L. Komaromi, D.J. Martin, T. Fox, M.A. Keith, C.Y.
Al-Laham, A. Peng, M. Nanayakkara, P.M.W. Challacombe, B. Gill, W. Johnson,
M.W. Chen, C. Wong, J.A. Gonzalez, Pople, Gaussian 03, Revision C.02, Gaussian,
Inc., Wallingford, CT, 2004.
[47] N.K. Shrestha, E.J. Yagi, Y. Takatori, A. Kawai, Y. Kajii, K. Shibuya, K. Obi, J.
Photochem. Photobiol. A: Chem. 116 (1998) 179–185.
[49] H. Bouas-Laurent, H. Dürr, Pure Appl. Chem. 73 (2001) 639–665.
[48] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheese-
man, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S.
Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
[50] R.V. Bensasson, J.-C. Gramain, J. Chem. Soc., Faraday Trans.
1801–1810.
1 76 (1980)