Photo-, solvent-, and ion-controlled multichromism of imidazolium- substituted diarylethenes

Article abstract of DOI:10.1002/chem.200801192

Cationic diarylethenes with an imidazolium ring are synthesized for the first time. The imidazolium cationic moiety is connected directly to the ethene unit as one of the aryl units that take part in the photoinduced pericyclization reaction. The imidazol

Full text of DOI:10.1002/chem.200801192

FULL PAPER
DOI: 10.1002/chem.200801192
Photo-, Solvent-, and Ion-Controlled Multichromism of Imidazolium-
Substituted Diarylethenes
Takuya Nakashima, Kentaro Miyamura, Toshiyuki Sakai, and Tsuyoshi Kawai*[a]
Abstract: Cationic diarylethenes with
an imidazolium ring are synthesized for
the first time. The imidazolium cationic
moiety is connected directly to the
ethene unit as one of the aryl units that
take part in the photoinduced pericycli-
zation reaction. The imidazolium-sub-
stituted diarylethenes undergo reversi-
ble photochromic reactions in a variety
of organic media, including ionic liq-
uids, even though they have a delocal-
ized cationic charge in one of the five-
membered aromatic rings. The closed-
ring isomer shows solvatochromism de-
pending on the solvent donor numbers.
Addition of some tetraalkylammonium
salts, such as tetrabutyl ammonium ni-
trate, into the colored organic solution
of diarylethene also causes a color
change, indicating its ionochromic
property. These solvato- and ionochro-
mic properties are considered in con-
nection with the shift of chemical equi-
librium between the closed-ring iso-
mers, one with an extended p-conjuga-
tion system and one with limited p-
conjugation due to the strong interac-
tion with solvent molecules and anions
with high donor number.
Keywords:
pounds
photochromism · solvatochromism
imidazolium
com-
·
molecular switches
·
Introduction
of absorption profiles depending on the geometry, the bind-
ing condition with protein, the protonation-deprotonation of
the Schiff base, and the interaction with counteranions,
giving additional multichromic nature, including halochrom-
ism (pH-dependent), solvatochromism, and ionochromism.
The presence of Schiff base directly on the conjugated p-
backbone of retinal would be predominantly responsible for
such multichromic nature. To achieve such a multichromic
function in an artificial system, we have designed a series of
photochromic molecules based on the structure of diaryl-
In vision, the photoisomerization of visual pigments, such as
retinal, triggers the molecular events of the phototransduc-
tion cascade.[1] 11-cis-Retinal is one of the chromophores of
the visual pigments that combines with the apoprotein opsin
through a Schiff base linkage to form rhodopsin. The ab-
sorbance maximum of 11-cis-retinal is about 370 nm in solu-
tion, which shifts to longer wavelength when it binds to
opsin. The protonation of the Schiff base linkage between
11-cis-retinal and opsin is known to be responsible for the
bathochromic shift.[2] The environmental effects of the pro-
tein on the protonated Schiff base have been considered to
give rise to a further absorbance shift (opsin shift). For ex-
ample, the negatively charged groups of some amino acid
residues near the Schiff base could induce a greater degree
of delocalization of the p-electron system of retinal through
electrostatic interaction, and consequently a red shift in the
absorbance maximum occurs.[3] Thus, retinal shows a variety
AHCTUNGTRENNUNG
interest for their photoswitching capability as well as their
photomechanical behavior.[9,10] Photoswitching effects in di-
AHCTUNGTREGaNNNU rylethenes have been extensively studied for controlling
various chemical and physical properties, such as fluores-
cence intensity and wavelength,[11–14] p-conjugation connec-
tion pathways,[15,16] electronic conduction,[17–19] electrochemi-
cal response,[20–22] magnetic interactions,[23] chemical reactivi-
ty,[24] and self-assembling behavior.[25,26] Most of these photo-
switching effects are based, at least partly, on changes in the
extent of p-conjugation in diarylethene upon photochromic
reactions. On the other hand, the molecular designs have
been limited to the addition of functional units to the aryl
rings and the substitution of the central ethene
group.[4–8,15,22,27–29] Since the heteroaromatic rings of diaryl-
[a] Dr. T. Nakashima, K. Miyamura, T. Sakai, Prof. T. Kawai
Graduate School of Materials Science
Nara Institute of Science and Technology, NAIST
8916–5 Takayama, Ikoma, Nara 630–0192 (Japan)
Fax : (+81)743-72-6179
E-mail: tkawai@ms.naist.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801192.
AHCTUNGTREGeNNNU thene undergo reversible deformation into a nonaromatic,
Chem. Eur. J. 2009, 15, 1977 – 1984
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1977

fused heterocycle, as shown in Scheme 1, the direct substitu-
tion of the heteroaromatic ring with a functional aryl group
would give rise to dramatic changes in the physicochemical
of its positive charge would be modulated significantly and
reversibly upon the photochromic reactions, since the five-
membered ring is no longer the aromatic unit in the closed-
ring form isomer (b-form, Scheme 2). Various chemical and
electrical properties of molecules 1–4 are hence expected to
be modulated with the photochromic reactions. Further-
more, the presence of cationic quaternized nitrogen directly
on the p-conjugation backbone of the closed-ring isomers of
diarylethenes is expected to affect significantly the absorp-
tion spectra in a similar manner to the Schiff base on retinal
through interaction with counteranions and bases.
Scheme 1. Photochromism of diarylethene.
Results and Discussion
properties upon photochromic reactions. In this study, we
have replaced an aryl group of diarylethene with a cationic
imidazolium ring. The imidazolium ring is one of the N-het-
eroaromatics, and is also known as the most popular and in-
vestigated class of cationic structures of room-temperature
ionic liquids.[30] The delocalization or burying of cationic
charge and the low reduction potential of the imidazolium
cation make ionic liquids chemically stable towards many
organic and inorganic substances, which enables a number
of organic chemical reactions[31,32] to be carried out in the
ionic liquids.
Photochromism in organic solvents: Imidazolium-substituted
diarylethene 1 showed reversible photochromic reactions in
a wide range of organic solvents, including chloroform, tolu-
ene, 2-methyltetrahydrofuran (MeTHF), dimethyl sulfoxide,
pyridine, and even in ionic liquids. As shown in Figure 1, the
There are some reports on cationic diarylethenes contain-
ing pyridinium cation units as the side group connected to
the photoreactive thienyl moieties.[20,21,33] For example, Lehn
and co-workers have demonstrated p-conjugation switching
in some pyridinium-substituted diarylethenes.[20a] Some of
them exhibit characteristic photon-mode switching in their
electrochemical properties upon photochromic ring-cycliza-
tion and cycloreversion reactions. Matsuda et al. have re-
ported that the ionic-repulsive interaction in the dithienyl-
ACHTUNGTRENNUNGethene with two pyridinium side groups results in its im-
proved photochromic properties in methanol.[33] As shown
in Scheme 2, the present molecules, imidazolium-substituted
diarylethenes 1–4, are very different from these previous
molecules based on dithienylethenes.[20,21,33] A cationic imi-
dazolium group is connected directly to the central ethene
unit as an aryl group and participates in the photochromic
hexatriene–cyclohexadiene reaction. The delocalized nature
Figure 1. Absorption spectral change of 1 in MeTHF solution ([1]=3.3ꢁ
10À5 m): open-form 1a (dotted line) and photostationary state under irra-
diation with 313 nm light (solid line).
absorption band at 270 nm with a molar absorbance coeffi-
cient of 2.4ꢁ104 mÀ1 cmÀ1 observed in the original state was
suppressed, and a new absorption band appeared at around
510 nm after UV-light irradiation in MeTHF. The absorption
spectral change upon photoirradiation was accompanied by
an isosbestic point at 290 nm, which clearly indicates a two-
component photochromic reaction, the composition ratio of
which changes with light irradiation. It should also be noted
that the colored state can be stored under dark conditions
without remarkable spontaneous bleaching at room temper-
ature, whereas it can be completely bleached upon irradia-
tion with visible light to give an absorption spectrum identi-
cal to that of the initial solution of 1a. The coloration and
decoloration cycles can be repeated at least twenty times
with no marked photodegradation. Thus, compound 1 basi-
cally shows a similar photochromic nature to that of previ-
Scheme 2. Photochromic reaction of imidazolium-substituted diaryl-
ACHTUNGTRENNUNGethenes 1–4.
1978
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Chem. Eur. J. 2009, 15, 1977 – 1984

Multichromism of Diarylethenes
FULL PAPER
ously reported diarylethenes,[4] reversible photochromic re-
activity in solution, and thermal stability in both bleached
and colored states, despite having a cationic moiety in the
reactive unit.
rizes lmax of open- and closed-ring isomers of 1 in various
solvents together with their donor numbers (DN) and die-
lectric constants (e).[36] Whereas the peak positions are
almost identical independently of solvents for open-ring
isomer 1a, the absorption profiles for closed-ring isomer 1b
were significantly dependent on the medium. The lmax of 1b
shows a good correlation with the solvent DNs rather than
the solvent polarity. With the increase of DN, the absorption
peak shifts to shorter wavelength. The spectral shift of 1b
upon the addition of a solvent with high DN, for example,
pyridine, to the solution of 1b in toluene was studied in
order to investigate the origin of the observed solvato-
chromism. As shown in Figure 2, the absorption peak of 1 at
The closed-ring isomer 1b was difficult to isolate from the
colored solution of 1 by normal- and reverse-phase HPLC
due to its ionic nature and the low conversion ratio, even
though the colored state is thermally stable. To confirm the
1
photoisomerization reaction product, the H NMR spectral
change was studied carefully.[34] Briefly, characteristic signals
of methyl protons of the open-ring form isomer 1a were ob-
served at 2.12, 2.37, 2.42, 3.44, and 3.71 ppm in CDCl3. After
UV irradiation, five additional peaks were observed at 1.87,
2.07, 2.14, 3.14, and 3.47 ppm, and were attributed to the
five methyl groups in the closed-ring isomer 1b shown in
Scheme 2. The conversion ratio between 1a and 1b at the
photostationary state achieved by irradiation with UV light
(l=313 nm) was estimated to be 25% from the integrated
1
peak intensity of the H NMR spectrum. The molar absorb-
ance coefficient of the ring-closed isomer 1b was estimated
to be 1.2ꢁ104 mÀ1 cmÀ1 at the peak maximum, 510 nm from
the conversion of the photochromic reaction. Upon visible
irradiation the colored solution turned colorless and the five
methyl peaks disappeared in the 1H NMR spectrum. The
photochromic reaction quantum yields were also evaluated
by the standard procedure with bis(1-benzothiophene-3-yl)-
hexafluorocyclopentene in hexane as the reference.[35] Rela-
tively high quantum yields were found for both photocycli-
zation and photocycloreversion reactions: 0.3 (313 nm) and
0.4 (546 nm), respectively. The relatively large cyclorever-
sion quantum yield is responsible for the low conversion
ratio at the photostationary state. Since imidaozlium-substi-
Figure 2. Absorption spectral change of 1 at the photostationary state
(achieved by irradiation with 313 nm light) by the addition of pyridine to
the solution of 1 in toluene ([1]=4.0ꢁ10À5 m).
tuted diarylACHTUNGTRENNUNGethenes 2–4 have structures analogous with 1,
they also showed similar photochromic behavior to 1. The
substitutents on the imidazolium ring hardly affected their
photochromic performance except for the absorption
maxima.[34]
the photostationary state in toluene shifted to shorter wave-
length, accompanied by a decrease of the absorbance at
520 nm, and with the successive peak progression at 460 nm
by the addition of a small amount of pyridine. The presence
of an isosbestic point at 495 nm clearly demonstrates that
the mechanism of the solvatochromism should be attributed
to the shift of the two-component chemical equilibrium at
the ground state of the closed-ring isomer 1b, like that ob-
served for some merocyanine dyes, the photoisomers of spi-
roheterocyclic compounds.[37] For clarity, the components
with absorption peak positions at around 520 nm and
460 nm are hereinafter referred to as “red” and “yellow
components”, respectively. The ratio of red and yellow com-
ponents should be dominated by the DN of the solvent,
which determines the apparent absorption peak position of
the closed-ring isomer. Since the yellow component is likely
to appear in solvents with high DN, the closed-ring isomer
1b is regarded to interact strongly with donor solvents.
Solvent-controlled chromism: The imidazolium-substituted
diarylethenes showed characteristic solvent-dependent ab-
sorption spectra in their closed-ring states. Table 1 summa-
Table 1. Absorption maxima of the open- and closed-ring isomers of 1,
together with the donor numbers (DN) and dielectric constants (e) of sol-
vents.
solvents
lmax (1a)
[nm]
lmax[a] (1b)
[nm]
DN[b]
e[b]
[kcalmolÀ1
]
chloroform
toluene
acetonitrile
acetone
tetrahydrofuran
dimethyl sulfoxide
pyridine
emimTf2N
emimEtSO4
270
–
521
520
515
509
470
463
461
521
462
0
4.8
[c]
0.1
14.1
17.0
20.0
29.8
33.1
–
2.4
38.0
20.7
7.6
45.0
12.3
–
270
[c]
–
270
271
Ion-controlled chromism: The solvatochromic behavior was
also observed in ionic liquids. As shown in Table 1, the
closed-ring isomer 1b exhibits different absorption spectra
depending on the anion species of the ionic liquid. The red
component dominates in 1-ethyl-3-methylimidazolium bis-
[c]
–
273
[c]
–
–
–
[a] Absorption maximum in the visible region. [b] Ref. [36]. [c] n.d.=not
detected due to the absoption of solvent.
Chem. Eur. J. 2009, 15, 1977 – 1984
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1979

T. Kawai et al.
A
These solvent- and ion-controlled chromisms were also
observed for other imidazolium-substituted diarylethenes,
including 3 with an ethyl group as the R2 group, which clear-
ly indicates that the source of these chromisms should not
be attributed to the conformational twisting between the R2
phenyl ring and the fused heterocycles, but rather to the dif-
ference in electronic structure of the p-conjugation system.
yellow one is the major component in 1-ethyl-3-methylimi-
dazolium ethylsulfate (emimEtSO4). Although the electro-
static interaction in ionic liquids should be substantially
lower than in conventional organic solvents, the observed
difference in absorption profile may be attributable to the
difference in ionic interactions between closed-ring isomer
1b, possessing localized cationic charge, and the anionic
components of the ionic liquids, Tf2NÀ and EtSO4 . Such
1H NMR study for the yellow and red components: To ex-
ionochromic behavior of 1b was demonstrated clearly by
the titration of nucleophilic anions in the MeTHF and also
reproduces the spectral shift observed for solvatochromims
described above. As shown in Figure 3, the visible absorp-
plore the electronic structures of the red and yellow compo-
nents, we conducted a H NMR study for 2–4 in CDCl3 and
[D5]pyridine, giving the red and yellow components as
closed-ring photoisomers, respectively. In contrast to diaryl-
1
AHCTUNGTREGeNNUN thene 1, the N1 and N3 nitrogen atoms of which were both
methylated, diarylethenes 2–4 possess an ethyl group as the
R1–R3 substituents, respectively, on the imidazolium ring,
which enables us to take advantage of homonuclear proton-
proton correlation spectroscopy (COSY). Because the ethyl
group is only detectable for these molecules by 1H–1H
COSY (methyl–methylene correlation), the change of elec-
tronic structure around the imidazolium ring upon the pho-
tochromic reaction could be evaluated by following the
À
À
chemical shift of the methylene ( CH2 ) of the ethyl group
for 2–4 in each medium. Table 2 and Figure 4 summarize the
chemical shifts of the methylene of the ethyl group for 2–4
before and after UV irradiation in CDCl3 and
[D5]pyridine.[34] These chemical shifts directly reflect the
electron density of adjacent atoms (N1 and N3 nitrogen
Figure 3. Absorption spectral change of 1 at the photostationary state
(achieved by irradiation with 313 nm light) by the addition of TBANO3
to the solution of 1 in MeTHF ([1]=3.7ꢁ10À5 m).
Table 2. Chemical shifts of the methylene protons of the ethyl group for
2–4 and their change upon photocyclization reaction in CDCl3 and
[D5]pyridine.
in CDCl3
in [D5]pyridine
d [ppm]
before irrad. after irrad.
d [ppm]
Dd
d [ppm]
d [ppm]
Dd
ACHTUNGTRENNUNG[ppm]
tion band of 1b dramatically changed with the addition of
tetrabutylammonium nitrate (TBANO3) in a similar fashion
to that observed in Figure 2. The isosbestic point also ap-
peared at 495 nm, identical to that observed for the tolu-
ene–pyridine system. The components in equilibrium giving
the absorption peaks at longer and shorter wavelength
should have substantially similar p-conjugation structures to
the red and yellow components, respectively. The present
ionochromism could also be explained by the DNs of the
A
2
3
4
4.16
3.12
3.73
3.40
3.11
3.85
À0.76 4.15
À0.01 3.15
3.20
2.12
3.55
À0.95
À1.03
À0.55
0.12
4.10
À
anions. The DN for the hard base NO3 was estimated to be
around 20,[38] which is substantially larger than that of Tf2NÀ
(DNꢀ5.4),[39] the original bulky counteranion of 1, even
though these values are dependent on the medium. The ad-
dition of 0.8 equivalents (against the concentration of 1) of
TBANO3 to the solution of 1b in MeTHF was enough to
reach saturation. The binding constant between closed-ring
À
isomer 1b and NO3 could be estimated to be roughly of
the order of 107 mÀ1. The addition of an equimolar amount
À
of other anions with strong basicity, such as BrÀ and HSO4 ,
also gave rise to the spectral shift at shorter wavelength,
whereas the equimolar addition of bulky anions with small
Figure 4. Change of the chemical shifts of the methylene protons for 2–4
upon photocyclization reaction in CDCl3 (a) and in [D5]pyridine (b). For
clarity, only parts of the structures are depicted.
À
À
DNs, such as ClO4 and PF6 , had almost no effect on the
absorption profile of 1b.[34]
1980
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Chem. Eur. J. 2009, 15, 1977 – 1984

Multichromism of Diarylethenes
FULL PAPER
atoms and C2 carbon of imidazolium). In CDCl3, which pre-
dominantly gives the red component as a photoproduct, the
chemical shift of the methylene of compounds 2–4 showed
upfield shift, almost no shift, and downfield shift, respective-
ly. The photochromic reaction induces the loss in aromatic
character of aryl units, leading to slight upfield shifts (less
than 0.1 ppm) for protons that were lying within the de-
shielding region of the aromatic ring in general. Despite
this, the methyl signal for R2-ethyl-substituted 3 showed
almost no peak shift and that for 4 even exhibited a down-
field shift of 0.12 ppm. The photocyclization reaction
changes the localization state of cationic charge of the imi-
dazolium ring. In the open-ring isomer, the cationic charge
delocalized around the five-membered imidazolium ring,
which is responsible for the weak interaction with nucleo-
philic species. Upon photocyclization, the cationic charge
preferentially localized on the N3 nitrogen rather than the
N1 nitrogen atom, leading to the enhancement of the elec-
tron-withdrawing nature of the N3 nitrogen and C2 carbon
atoms. On the other hand, in [D5]pyridine, all methylene sig-
nals around the imidazolium ring showed significantly large
upfield shifts, which is beyond the estimation taking the loss
of aromaticity of the imidazolium ring into account. One
plausible explanation for this would include the strong inter-
action between the cationic charge of the closed-ring isomer
and the nucleophilic pyridines, which may neutralize the cat-
ionic charge on the imidazolium ring to reduce the electron-
withdrawing character of the heterocycle. The resulting en-
hancement of electron density of each methylene carbon by
the photocyclization reaction in pyridine should cause con-
siderable upfield shifts of the methylene proton signals.
Since the degree of upfield shift was most significant for the
R2-ethyl group, the nucleophilic species should predomi-
nantly interact with the carbon atom between two nitrogen
atoms.
of 1b–4b seem to have a limited p-conjugation structure
due to the strong interaction with donors at the C2 posi-
tion,[40] which leads to the absorption peak wavelength
shorter than those of the red components. Interestingly, the
yellow component of 3b also showed a similar peak wave-
length, of about 460 nm, to those of 1b, 2b, and 4b.[34] Thus,
the presence of a phenyl substitutent at the C2 position of
the imidazolium ring has almost no effect on the absorbance
peak position, which also supports the limited p-conjugation
system in the yellow component disconnected at the C2 po-
sition of the imidazolium ring. Because these two compo-
nents are in equilibrium, the addition of a strong acceptor,
such as trifluoroacetic acid (acceptor number=105), reversi-
bly converts the yellow component to the red one by inter-
acting with donor species. Scheme 4 shows the synthetic
pathways for imidazolium substituted diarylethenes 1–4.
Conclusion
We have designed novel diarylethenes that possess an imida-
zolium ring as an aryl unit. These imidazolium-substituted
diarylethenes undergo reversible photochromic reactions in
a wide range of solvents, including ionic liquids, even though
they contain a cationic aryl unit. The presence of positively
charged nitrogen directly on the conjugated p-system pro-
vides characteristic solvent- and anion-dependent absorption
profiles in a similar manner to 11-cis-retinal. The spectral
shift of the closed-ring isomer controlled by solvents and co-
existing anions was attributed to the shift of the two-compo-
nent chemical equilibrium at the ground state. The red and
yellow components were studied by solvent-dependent
1
1H NMR and H–1H COSY measurements to be character-
ized by the extent of the effective p-conjugation length. The
red component has an extended p-conjugation system from
one side to the other, whereas the yellow one possesses lim-
ited p-conjugation due to the strong binding of donors such
as nucleophilic solvent molecules and anions. The precise
control of the position of interaction of the nucleophilic spe-
cies would enable us to design a multicolor dye system by
using a single artificial chromophore.
Taking the explanations described above into account, the
canonical structures for the red and yellow components are
depicted in Scheme 3. The red component has a cationic ni-
trogen directly on the p-conjugation system, stabilizing the
cationic charge. The extended p-conjugated structure is in
good agreement with the absorbance peak appearing at
longer wavelength. In fact, the red component of 3b, which
does not have a phenyl group at the C2 position of the imi-
dazolium ring, has shorter effective p-conjugation length, re-
sulting in the absorbance peak, at 490 nm, considerably
shorter than those of 1, 2, and 4.[34] The yellow components
Experimental Section
1
General: H NMR spectra were recorded on a JEOL AL-300 spectrome-
ter (300 MHz). Separative HPLC was performed on a JASCO LC-2000
Plus Series. Mass spectra were measured with a mass spectrometer JEOL
JMS-T100 LC AccuTOF. Absorption spectra in solution were studied
with a JASCO V-550 spectrophotometer. Photoirradiation was carried
out using a USHIO 500 W Ultra-High-Pressure Mercury Lamp or a
500 W Xenon Short-Arc Lamp as the exciting light sources. Monochro-
mic light was obtained by passing the light through a monochromator
(Shimazu SPG-120S, 120 mm, f=3.5).
2,4-Dimethyl-3-(perfluorocyclopent-1-enyl)-5-phenylthiophene (5): n-Bu-
tyllithium (nBuLi, 1.6m in hexane, 12 mL, 16 mmol) was added slowly at
À788C under Ar to a solution of 3-bromo-2,4-dimethyl-5-phenylthio-
phene (4.0 g, 15 mmol) in dry THF (100 mL), and the mixture was stirred
for 2 h at that temperature. Perfluorocyclopentene (3.8 mL, 28 mmol)
Scheme 3. Equilibrium between the red and yellow components of
closed-ring isomer 1b.
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1981

T. Kawai et al.
5-Iodo-1,4-dimethyl-2-phenyl-1H-imi-
dazole (8): Iodine (3.8 g, 15 mmol) and
H5IO6 (1.7 g, 7.5 mmol) were added
sequentially to a solution of 6 (5.1 g,
30 mmol) in AcOH (30 mL), water
(30 mL), and conc. H2SO4 (5.2 mL).
After stirring for 7 h at 708C, the reac-
tion mixture was neutralized with
aqueous NaOH, followed by extrac-
tion with ethyl acetate. The organic
layer was dried over anhydrous mag-
nesium sulfate, filtered, and concen-
trated. Silica gel column chromatogra-
phy (ethyl acetate) afforded 6.9 g of 8
in 63% yield as a yellowish powder.
1H NMR (300 MHz, CDCl3, TMS):
d=2.32 (s, 3H), 3.65 (s, 3H), 7.40–7.48
(m, 3H), 7.55 ppm (d, J=9.0 Hz, 2H).
5-Iodo-1,3,4-trimethyl-2-phenyl-1H-
imidazolium bis(trifluoromethanesul-
fonyl)imide (10): Compound 8 (8.7 g,
29 mmol) was dissolved in acetonitrile
(45 mL).
Iodomethane
(8.8 mL,
150 mmol) was added to the solution
at room temperature under N2. After
stirring for two days, the reaction mix-
ture was filtered off from insoluble
substances, and then evaporated under
reduced pressure. The residue was re-
precipitated from ethyl acetate to give
the product 5-iodo-1,3,4-trimethylimi-
dazolium iodide (9) as
a colorless
powder. This powder was re-dissolved
in chloroform and an aqueous solution
of lithium bis(trifluoromethanesulfo-
nyl)imide (LiTf2N, 9.0 g, 32 mmol) was
added, followed by vigorous stirring
for 12 h. The organic layer was dried
over anhydrous magnesium sulfate,
and then evaporated in vacuo to leave
a yellowish powder (10, 11 g) in 94%
yield. 1H NMR (300 MHz, CDCl3,
TMS): d=2.48 (s, 3H), 3.65 (s, 3H),
3.66 (s, 3H) 7.61–7.75 ppm (m, 5H).
Scheme 4. Synthesis of imidazolium-substituted diarylethenes 1–4. Reagents: a) 1. nBuLi, 2. perfluorocyclopen-
tene; b) CH3I, tBuOK, [18]crown-6; c) I2, H5IO6; d) CH3I; e) LiTf2N, f) nBuLi; g) CH3CH2Br.
4-(2-(2,4-Dimethyl-5-phenylthiophene-
3-yl)-3,3,4,4,5,5-hexafluorocyclopent-1-
enyl)-1,3,5-trimethyl-2-phenyl-1H-imi-
dazolium Tf2N (1a): nBuLi (1.6m in
was then quickly added and the mixture stirred for another 2 h at À788C.
The reaction mixture was allowed to warm up to room temperature.
Methanol was added to the reaction mixture and extracted with ethyl
acetate. The combined organic fraction was washed with water, dried
with anhydrous magnesium sulfate, and concentrated. The residue was
subjected to silica gel column chromatography (hexane) to give product
hexane, 4.0 mL, 6.8 mmol) was added in a dropwise manner to a solution
of 10 (2.1 g, 3.5 mmol) in dry THF (20 mL) at À788C under Ar and
stirred for 0.5 h at that temperature. Then compound 5 (1.1 g, 2.9 mmol)
in dry THF (5 mL) was added slowly and the mixture was stirred for an-
other 1 h. The reaction mixture was allowed to warm up to room temper-
ature, and then methanol was added, followed by extraction with ethyl
acetate. The combined extract was washed with brine, dried over anhy-
drous magnesium sulfate, and evaporated under reduced pressure to give
an oily residue, which was purified with alumina gel column chromatog-
raphy (hexane/acetone). The resulting oily product was further purified
with reversed phase HPLC (methanol) to give 1.0 g of 1a in 49% yield.
1H NMR (300 MHz, CDCl3, TMS): d=2.13 (s, 3H), 2.37 (s, 3H), 2.42 (s,
3H), 3.45 (s, 3H), 3.72 (s, 3H), 7.26–7.72 ppm (m, 10H); ESI HRMS: m/
z calcd for C29H25F6N2S+ [MÀTf2N]+: 547.1637; found: 547.1643; elemen-
tal analysis calcd (%) for C31H28F12N3O4S3: C 44.98, H 3.04, N 5.08;
found: C 45.03, H 2.89, N 4.98.
1
5 (2.1 g) as a colorless powder in 37% yield. H NMR (300 MHz, CDCl3,
TMS,): d=2.11 (s, 3H), 2.38 (s, 3H), 7.28–7.40 ppm (m, 5H).
1,4-Dimethyl-2-phenyl-1H-imidazole (6) and 1,5-dimethyl-2-phenyl-1H-
imidazole (7): Iodomethane (4.0 mL, 65 mmol) was added to a solution
of 4-methyl-2-phenyl-1H-imidazole (10 g, 64 mmol), [18]crown-6 (1.5 g,
6.4 mmol) and potassium-tert-butoxide (7.6 g, 64 mmol) in dry THF
(110 mL) under N2. After stirring for 12 h at ambient temperature, the
reaction mixture was extracted with ethyl acetate and washed with water.
The organic layer was dried over anhydrous magnesium sulfate, filtered,
and concentrated. The residue was purified by alumina gel column chro-
matography (hexane/ethylacetate=2:1) to give 5.1 g of 6 (Rf =0.3) in
47% yield as a yellowish oil and 2.8 g of 7 (Rf =0.1) in 25% yield as a
1-Ethyl-4-iodo-3,5-dimethyl-2-phenyl-1H-imidazolium Tf2N (12): Bromo-
ethane (1.9 mL, 25 mmol) was added to a solution of 8 (3.0 g, 10 mmol)
in acetonitrile (25 mL) and the mixture was stirred for 24 h at 808C. The
reaction mixture was evaporated and the residue was re-precipitated
1
colorless solid. H NMR (300 MHz, CDCl3, TMS) 6: d=2.26 (s, 3H), 3.63
(s, 3H), 6.62 (s, 1H), 7.31–7.59 ppm (m, 5H); 7: d=2.27 (s, 3H), 3.59 (s,
3H), 6.88 (s, 1H), 7.29–7.60 ppm (m, 5H).
1982
www.chemeurj.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 1977 – 1984

Multichromism of Diarylethenes
FULL PAPER
from ethyl acetate to give the product, 1-ethyl-4-iodo-3,5-dimethyl-2-
phenyl-1H-imidazolium bromide (11), as a colorless powder. This powder
was re-dissolved in chloroform and an aqueous solution of LiTf2N (4.6 g,
16 mmol) was added, followed by vigorous stirring for 12 h. The organic
layer was dried over anhydrous magnesium sulfate, and then evaporated
in vacuo to leave a yellowish powder (12, 3.8 g) in 78% yield. 1H NMR
(300 MHz, CDCl3, TMS): d=1.28 (t, J=7 Hz, 3H), 2.48 (s, 3H), 3.59 (s,
3H), 4.04 (q, J=7 Hz, 2H), 7.58–7.73 ppm (m, 5H).
Acknowledgements
We thank Prof. Y. Kobuke (Kyoto University) for fruitful discussions.
The authors also thank Y. Nishikawa and F. Asanoma for mass spectro-
scopic and 1H–1H COSY measurements. This work was partly supported
by Ministry of Education, Culture, Sports, Science and Technology
(MEXT) of Japan with Grants-in-Aid for Scientific Research on Priority
Areas, “Super-Hierarchy Structures” (No. 17067011) and “New Frontiers
in Photochromism” (471) (No. 20044019).
4-(2-(2,4-Dimethyl-5-phenylthiophene-3-yl)-3,3,4,4,5,5-hexafluorocyclo-
pent-1-enyl)-1-ethyl-3,5-dimethyl-2-phenyl-1H-imidazolium Tf2N (2a):
This compound was prepared by the same procedure as used as for 1a,
except that 10 was replaced by 12. From 12 (2.0 g, 3.2 mmol), 5 (1.2 g,
3.2 mmol), and nBuLi (2.1 mL, 3.4 mmol), 2a (1.2 g, 44%) was obtained
as a colorless solid. 1H NMR (300 MHz, CDCl3, TMS): d=1.26 (t, J=
7 Hz, 3H), 2.13 (s, 3H), 2.36 (s, 3H), 2.44 (s, 3H), 3.39 (s, 3H), 4.15 (q,
J=8 Hz, 2H), 7.32–7.76 ppm (m, 10H); ESI HRMS: m/z calcd for
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+
C62H54F18N5O4S4 [2MÀTf2N]+: 1402.2766; found: 1402.2778.
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5 (1.0 g,
2.6 mmol), and nBuLi (2.1 mL, 3.3 mmol), 3a (0.50 g, 27%) was obtained
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8 Hz, 3H), 2.08 (s, 3H), 2.27 (s, 3H), 2.37 (s, 3H), 3.13 (q, J=8 Hz, 2H),
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This compound was prepared by the same procedure as used for 1a,
except that 10 was replaced with 21. From 21 (1.0 g, 1.6 mmol), 5 (0.68 g,
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+
(m, 10H); ESI HRMS: m/z: calcd for C62H54F18N5O4S4 [2MÀTf2N]+:
1402.2766; found: 1402.2775.
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Received: June 18, 2008
Published online: September 9, 2008
1984
www.chemeurj.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 1977 – 1984