Photochromic reaction of a novel room temperature ionic liquid: 2-Phenylazo-1-hexyl-3-methylimidazolium bis(pentafluoroethylsulfonyl)amide

Article abstract of DOI:10.1246/cl.2010.230

We synthesized a new room temperature ionic liquid, [2PA-Hmim][Pf ₂N], and characterized the photochromic behavior by absorption spectroscopy. [2PA-Hmim][Pf₂N] was found to be a photochromic liquid where solvent itself undergoes photochromic reaction.

Full text of DOI:10.1246/cl.2010.230

doi:10.1246/cl.2010.230
230
Published on the web February 6, 2010
Photochromic Reaction of a Novel Room Temperature Ionic Liquid:
2-Phenylazo-1-hexyl-3-methylimidazolium Bis(pentafluoroethylsulfonyl)amide
Akio Kawai,* Daiki Kawamori, Tomoo Monji, Tooru Asaka, Nobuyuki Akai, and Kazuhiko Shibuya*
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
2-12-1-H89 Ookayama, Meguro-ku, Tokyo 152-8550
(Received November 27, 2009; CL-091053; E-mail: akawai@chem.titech.ac.jp, kshibuya@chem.titech.ac.jp)
¹1
We synthesized a new room temperature ionic liquid, [2PA-
80 kJ mol more stable than the Z form. Phenylazo moiety
absorbs light in the UV and vis region and the E to Z
photoisomerization occurs. The Z form undergoes backward
thermal isomerization and [2PA-Bmim]+ has thus been classified
as a T-type photochromic ion. We expect similar photochromic
reaction for the newly-synthesized liquid, [2PA-Hmim][Pf2N].
Generally speaking, absorption spectroscopy has been
applied as a powerful method for the study of isomerization.
However, neat [2PA-Hmim][Pf2N] has extremely strong absorp-
tion and therefore the conventional absorption method using
a cubic cell with 1 cm optical path length was no longer
applicable. We prepared a thin layer sample of about 1 ¯m
thickness by utilizing two quartz plates, which was a similar set
up employed for thin layer absorption spectroscopy of RTILs.10
Figure 1a shows the absorption spectra of [2PA-Hmim][Pf2N]
thin layer at 298 K. We observed absorption spectral changes
during 436 nm light irradiation of a mercury high-pressure lamp
(USHIO USH-500SC) with appropriate filters. Although the
spectral change is rather small, we could recognize the isosbestic
points at 265 and 428 nm. The transition bands to S2(³³*) and
S1(n³*) appear around 350 and 460 nm, respectively. The
absorbance due to the S2(³³*) band was much larger than unity
even though we used a thin layer sample. Meanwhile, the
S1(n³*) forbidden band shows absorbance less than unity,
which allows us to make quantitative absorption measurements.
Hmim][Pf2N], and characterized the photochromic behavior by
absorption spectroscopy. [2PA-Hmim][Pf2N] was found to be a
photochromic liquid where solvent itself undergoes photo-
chromic reaction.
Since the discovery of air- and water- stable room temper-
ature ionic liquids (RTILs), various solvent properties have been
created by combinations of anion and cation.1 This variety of
RTIL attracts much interest from researchers who aim to
discover a novel solvent with some unique properties that have
never been created.2-8 In this study, we show recent results
through our challenging intention to introduce photochromic
character into RTIL and to obtain new photochromic solvent
which shows photochromism as solvent itself. For this purpose,
a series of phenylazo-substituted imidazolium-based ionic
compounds, [2PA-Rmim][anion] (2PA-Rmim: 2-phenylazo-1-
R-3-methylimidazolium, R= methyl, n-butyl, or n-hexyl) were
newly synthesized according to a similar preparation procedure
¹
described for [2PA-Bmim][Tf2N] {B: n-butyl, [Tf2N] : bis(tri-
fluoromethylsulfonyl)amide}.8,9 Table 1 lists the melting point
(mp) of these compounds measured by differential scanning
calorimetry (Shimadzu DSC-60). It is noteworthy that the mp
¹
of [2PA-Hmim][Pf2N] {H: n-hexyl, [Pf2N] : bis(pentafluoro-
ethylsulfonyl)amide} is 306 K being close to room temperature
and more interestingly, [2PA-Hmim][Pf2N] was found to be
quite stable supercooled liquid. Since our interest lies in
photochromic reaction of the liquid, we measured photoisome-
rization and thermal isomerization of [2PA-Hmim][Pf2N] in the
liquid phase at around 298 K.
(a)
Irradiation time
0.0 min
1.0
0.5
0.0
2.0 min
10.0 min
Photochromic reaction of cation components, [2PA-Rmim]+
may be described by the following scheme,
Isosbestic points
+
N
N
R
200
300
400
500
600
CH3
+
N
CH3
h
ν1
N
Wavelength / nm
R
N
N
ð1Þ
N
(b)
(c)
N
∆
, hν2
0.02
0.06
E form
Z form
At delay-time t2
1 min
0.00
-0.02
-0.04
-0.06
0.04
0.02
0.00
-0.02
according to our previous study for photochromism of [2PA-
Bmim]+ dissolved in various solvents.8 DFT calculation
(B3LYP/6-31G*) suggested the E form of this cation is ca.
5 min
30 min
600 min
Isosbestic point
After irradiation time t1
0.5 min
Isosbestic point
2.0 min
10.0 min
Table 1. Melting points of a series of [2PA-Rmim][anion]
mp/K
[Tf2N]
Cations
R =
400
450
500
550
600
400
450
500
550
600
¹
¹
¹
Wavelength / nm
Wavelength / nm
Anions:
I
[Pf2N]
Methyl
n-Butyl
n-Hexyl
>523
441
397
362
329
321
393
313
306
Figure 1. Absorption spectra of [2PA-Hmim][Pf2N] liquid (a),
and difference spectra for photoincuced E to Z reaction time t1
(b) and for dark Z to E reaction time t2 (c).
Chem. Lett. 2010, 39, 230-231
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

231
1.1 © 10 and 2.2 © 10¹3 s¹1. It is noteworthy that even the
slower component rate is about 10 times faster than the
corresponding dark reaction rate of [2PA-Bmim]+ in solution.8
As neat [2PA-Hmim][Pf2N] is a viscous RTIL and internal
motions are restricted,6,12-14 it is not straightforward to explain
such a fast rate. Since [2PA-Hmim]+ concentration in neat liquid
is very much higher than those in solution, we speculate
that certain intermolecular interaction among [2PA-Hmim]+
may contribute to fast and nonexponential isomerization. To
evaluate the mechanism, we need detailed experiments such as
Arrhenius parameter measurements which are now under
progress. One important revelation from the results is that this
fast recovery from Z to E forms is perhaps one of the reasons
why fZ is low. Another realization is that the fast thermal
isomerization gives one advantage to [2PA-Hmim][Pf2N] liquid;
this photochromic liquid recovers to the initial form quickly
after the light irradiation ceases. This characteristic will be
useful when building photochromic liquid that shows fast
recovery in the dark.
¹2
at 463nm
298K
0.24
0.23
0.22
0
500
1000
Time / s
1500
2000
Figure 2. Time profile of dark Z to E isomerization reaction of
[2PA-Hmim][Pf2N] liquid.
Figure 1b shows the difference absorption spectra of the S1
observed before and after 436 nm light irradiation. The clear
isosbestic point at 428 nm is recognized, which indicates that
photoisomerization from E to Z forms occurs in the neat liquid
of [2PA-Hmim][Pf2N]. After the light irradiation, the backward
isomerization from Z to E forms took place under dark
conditions. Figure 1c shows difference spectra observed with
different delay times in the dark. The isosbestic points again
appear, which suggests that the Z form produced by the 436 nm
light irradiation undergoes thermal isomerization to the E form
without yielding any by-product. Therefore, we conclude that
[2PA-Hmim][Pf2N] neat liquid shows T-type photochromism
around room temperature.
Second, we evaluated a steady-state molar fraction of Z
form, fZ, under the 436 nm light irradiation. For determination
of fZ, we observed absorbances, Abs1 = ¾ECd before irradiation
and Abs2 = {¾E(1 ¹ fZ) + ¾Z fZ}Cd under photostationary con-
ditions with photon flux of 2.4 © 10¹2 W cm¹2. The C and d
are the concentration of [2PA-Hmim]+ in the neat liquid and
In this letter, we reported on photochromic liquid, [2PA-
Hmim][Pf2N] with a mp of nearly room temperature. This liquid
undergoes E to Z photoisomerization with vis light irradiation
and Z to E thermal isomerization in the dark, which indicates
that the liquid is a T-type photochromic compound.
We thank Prof. Tomoya Kitazume (Tokyo Institute of
Technology) for his unfailing advice. This work was supported
in part by a Grant-in-Aid for Scientific Researches (Nos.
17073006 and 19350008) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan.
References and Notes
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E
¾Z, of E and Z forms [2PA-Hmim]+, we adopt the correspond-
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The low fZ is due to a relation of ¾E < ¾Z at 436 nm. The reverse
4
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relation of ¾E º ¾ holds at 360 nm and therefore the fZ value
Z
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© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/