Photomodulation of ionic interaction and reactivity: Reversible photoconversion between imidazolium and imidazolinium

Article abstract of DOI:10.1021/ja802986y

An interconversion system between imidazolium and imidazolinium has been proposed for the first time. Imidazolium and imidazolinium cations exhibit different reactivity due to the difference in the aromaticity and charge localization structure, which is successfully controlled by means of photoirradiation in the present system. A 4,5-dithiazolylimidazolium salt was prepared and studied as a new class of photochromic materials modulating electrostatic interactions and chemical reactivities. The photochromic 4,5-dithiazolylimidazolium showed reversible photoconversions between imidazolium open-form and imidazolinium closed-form upon successive irradiation with UV and visible light. The imidazolinium closed-form exhibited characteristic solvato- and ionochromisms in which the absorption maximum shifted by more than 80 nm depending on the solvent polarity and counteranions, whereas the imidazolium open-form showed no such solvent-dependent property. Because the corresponding nonionic 4,5-dithiazolylimidazole also did not exhibit the solvent-dependent absorption profile both in open-and closed-forms, the appearance of these chromisms in imidazolinium closed-form was attributed to the change in the extent of ionic interaction, which was brought about by the photoconversion of imidazolium to imidazolinium. The photoderived strong ionic interaction of imidazolinium with counteranion was further applied to the photocontrolled nucleophilic reaction system. Whereas the imidazolium open-form was inert to nucleophiles such as sodium methoxide, the imidazolinium closed-form was reactive to the nucleophilic reaction, demonstrating a photogated reaction system.

Full text of DOI:10.1021/ja802986y

Published on Web 10/09/2008
Photomodulation of Ionic Interaction and Reactivity:
Reversible Photoconversion between Imidazolium and
Imidazolinium
Takuya Nakashima, Masako Goto, Shigekazu Kawai, and Tsuyoshi Kawai*
Graduate School of Materials Science, Nara Institute of Science and Technology,
8916-5 Takayama, Ikoma, Nara 630-0192, Japan
Received April 23, 2008; E-mail: tkawai@ms.naist.jp
Abstract: An interconversion system between imidazolium and imidazolinium has been proposed for the
first time. Imidazolium and imidazolinium cations exhibit different reactivity due to the difference in the
aromaticity and charge localization structure, which is successfully controlled by means of photoirradiation
in the present system. A 4,5-dithiazolylimidazolium salt was prepared and studied as a new class of
photochromic materials modulating electrostatic interactions and chemical reactivities. The photochromic
4,5-dithiazolylimidazolium showed reversible photoconversions between imidazolium open-form and
imidazolinium closed-form upon successive irradiation with UV and visible light. The imidazolinium closed-
form exhibited characteristic solvato- and ionochromisms in which the absorption maximum shifted by more
than 80 nm depending on the solvent polarity and counteranions, whereas the imidazolium open-form
showed no such solvent-dependent property. Because the corresponding nonionic 4,5-dithiazolylimidazole
also did not exhibit the solvent-dependent absorption profile both in open- and closed-forms, the appearance
of these chromisms in imidazolinium closed-form was attributed to the change in the extent of ionic
interaction, which was brought about by the photoconversion of imidazolium to imidazolinium. The
photoderived strong ionic interaction of imidazolinium with counteranion was further applied to the
photocontrolled nucleophilic reaction system. Whereas the imidazolium open-form was inert to nucleophiles
such as sodium methoxide, the imidazolinium closed-form was reactive to the nucleophilic reaction,
demonstrating a photogated reaction system.
Introduction
Imidazoles are an important class of aromatic heterocycles
because of their attractive chemical properties, such as acid-base
reactions and metal complex forming reactivity.1 Apart from
their importance in biological systems as potential bridging
ligands between metal ion centers in metalloenzymes, recent
studies have focused on their use as building blocks for extended
π-conjugation molecules.2-5 Imidazole is readily converted into
cationic imidazolium by the protonation or disubstitution of two
N-positions. The 1,3-disubstituted imidazolium ring (Figure 1a),
a cationic N-heteroaromatic ring, is well-known as the most
popular and investigated class of the cationic structure of room-
temperature ionic liquids.6,7 The reaction of 1,3-disubstituted
Figure 1. Chemical structures of 1,3-disubstituted imidazolium (a) and
imidazolinium (b) cations.
imidazolium cations is restricted due to the chemical stability
of the imidazolium ring derived from the delocalization or
burying of cationic charge, the aromaticity, and the low
reduction potential. The reactivity of imidazolium cations mainly
stems from the relatively high acidity of the H(2) hydrogen of
the imidazolium ring. 1,3-Disubstituted imidazolium groups
have been used as subunits for several anion receptors that bind
anions by forming a C(2)-H· · ·X- hydrogen bond.8-10 It is
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14570 J. AM. CHEM. SOC. 2008, 130, 14570–14575
10.1021/ja802986y CCC: $40.75
2008 American Chemical Society

Photoconversion between Imidazolium and Imidazolinium
A R T I C L E S
Scheme 1
also well-known that deprotonation at the C(2) position of
imidazolium generates N-heterocyclic carbene ligands11 to form
the carbene-metal complexes being used in homogeneous
catalysis.12-16 Except for the case of such reaction conditions
as those described above, 1,3-disubstituted imidazolium ionic
liquids are principally stable toward many organic and inorganic
substances, which enables a number of organic chemical
reactions,17,18 including nucleophilic reactions19-22 to be carried
out in ionic liquids. Alternatively, the imidazolinium cations,
having the structure of C(4)-C(5) saturated imidazolium (4,5-
dihydroimidazolium, Figure 1b), show similar but different
chemical reactivity from imidazolium cations due to their
nonaromaticity and their partly localized cationic structure.
Similar to imidazoliums, imidazoliniums are also famous
precursors for free Arduengo-type carbenes.23 The acidity of
H(2) hydrogen of imidazolinium is higher than that of imida-
zolium, and the pKa value is roughly estimated to be less than
20,24 which is apparently smaller than that of a simple
imidazolium cation (pKa ) 21-23).25 Because of the high
electrophilicity at the C(2) position, imidazolinium cations are
potential substrates for some nucleophilic reactions,24,26 which
makes the imidazolinium-based ionic liquids uncommon.27,28
Thus, imidazolium and imidazolinium exhibit different reac-
tivities due to the difference in aromaticity and the localization
state of cationic charge, while they are both N-heterocyclic
cations. In this study, we propose the reversible photointercon-
version system between imidazolium and imidazolinium by
means of photochromic reactions.
toswitching effects, especially in photochromic diarylethenes,31
have been extensively studied for controlling various chemical
and physical properties.31-34 Some recent studies highlight the
gated photochromic systems, in which the photochromic
reactivity is controlled with other stimuli, such as chemicals,
with an eye toward applications in display and memory
technologies.35-39 However, there have been few reports on the
photogated reactivity of diarylethenes.37b,40,41 In photogated
reactivity, light triggers a structural change in the molecule and
imparts chemical reactivity unique to the photogenerated isomer.
We describe herein a photomodulated electrostatic interaction
and reactivity by taking advantage of the photochromic reaction
of 4,5-diarylarylene, which rearranges the π-conjugation system
with light irradiation.42-44 As shown in Scheme 1, the open-
ring isomer (a-form) consists of three heteroaromatic rings, so-
called “triangle terarylene”, and includes a hexatriene component
in their molecular structure, which undergoes reversible pho-
toinduced cyclization reaction to form a closed-ring isomer (b-
form) in a similar manner to diarylethenes31 and fulgides.45 The
three heteroaromatic rings collapse simultaneously with the
rearrangement of the π-connection, and our strategy is to
incorporate a cationic imidazolium ring into the terarylene
structure as one of the heteroaromatics. The open-ring form
contains the chemically stable imidazolium ring with delocalized
cationic charge, while closed-ring photoisomer possesses the
reactive imidazolinium structure. Thus, by controlling the
Considerable interest has been focused on organic photo-
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J. AM. CHEM. SOC. VOL. 130, NO. 44, 2008 14571

A R T I C L E S
Nakashima et al.
Scheme 2
Figure 2. Absorption spectral change of 1(a) and 2 (b) in acetonitrile and
hexane, respectively, upon UV irradiation (λ ) 313 nm). Each trace was
measured after the irradiation for 0, 5, 10, 20, 30, 50, 100, and 200 s. The
concentrations were 2.4 × 10-5 M for 1 and 2.1 × 10-5 M for 2.
aromaticity and the localized state of the cationic charge of
N-heterocyclic ring with the photochromic reaction, the elec-
trostatic ineteraction with anions and the reactivity with nu-
cleophiles are expected to be modulated. A guiding principal
for designing future photoresponsive ionic assemblies and ionic
liquids relating to photochromism is proposed for the first time.
Table 1. Absorption Maxima and Coefficients of the Open- and
Closed-Ring Isomers of 1 and 2, together with the Quantum Yields
d
φa-b
e
φb-a
λ
max/nm (ꢀ/104 M-1cm-1
)
1aa
1ba
2ab
2bb
293 (3.7)
639c (2.2)
293 (4.6)
654c (2.3)
0.56
0.65
Results and Discussion
5.7 ×10-2
6.3 ×10-2
Reversible Photochromism in Solution. 4,5-Dithiazolyl-
imidazolium iodide 1a was prepared and investigated in this
study. Thiazole units bridged by the central imidazolium ring
were introduced to improve the photochromic reactivity of ter-
arylene.42b Corresponding 4,5-dithiazolyl imidazole 2a was a
synthetic intermediate of 1a and used as a reference compound.
Compound 2a was synthesized by conventional cross-coupling
reactions of corresponding thiazolyl pinacol borate and 4,5-
diiodoimidazole unit and then the N(3) position of 2a was
methylated with iodomethane to afford the cationic compound
1a. Their chemical structures were confirmed by high-resolution
a In acetonitrile. b In hexane. c Absorption maximum in the visible
region. λirrad ) 313 nm. λirrad ) 517 nm.
d
e
Table 2. Solvent Effects on the Absorption Maxima (λmax) of 1a
acetonitrile
acetone
dichloromethane
2-methylTHF
toluene
1a
293
639
294
654
296
566
645
1bb
642
565
1b-ClO4
a Solvent dielectric constants are as follows: acetonitrile
) 37;
1
mass spectroscopy, H NMR and 13C NMR. Both terarylenes
acetone ) 21; dichloromethane ) 9.1; 2-methylTHF ) 7.0; toluene )
1a and 2a showed thermally irreversible,42b,46 but photochemi-
cally reversible, photochromism in various organic media, as
shown in Scheme 2. The closed-ring isomer 2b formed by the
UV light irradiation was isolated from the colored solution by
reversed-phase HPLC using methanol as an eluent, whereas the
closed-ring 1b could not be isolated by the chromatographic
methods because of its ionic nature. Formation of both closed-
2.4. b Absorption maximum in the visible region.
pericyclic reaction, indicating a certain degree of localization
of the double bond on the C(4)-C(5) position. Similar reversible
photochromic behavior was also observed for neutral compound
2a (Figure 2 b). The conversion ratios from 1a to 1b and 2a to
2b at the photostationary state, which was achieved by irradia-
tion with UV light (λ ) 313 nm) were estimated to be 93%
and 86%, respectively. The values of λmax and ꢀ of 1 and 2 are
summarized in Table 1 with photochromic reaction quantum
yields (φa-b and φb-a) evaluated with the standard procedure using
1,2-bis(5-(2,4-diphenylphenyl)-2,4-dimethyl-3-thienyl)perfluoro-
cyclopentene in hexane47 as a standard. Both 1a and 2a
demonstrated quite high quantum yields in cyclization reaction
due to the reduced steric hindrance with the introduction of
thiazole units.42b
1
ring isomers, 1b and 2b, was confirmed by H NMR spectro-
scopic study in CDCl3 solution.
As shown in Figure 2, 1a had no absorption band in the
visible range in acetonitrile. Upon irradiation with UV light (λ
) 313 nm), the colorless solution of 1a turned blue, and a new
absorption band appeared at around 640 nm. The colored
solution can be completely bleached upon irradiation with visible
light, giving an absorption spectrum identical to that of the initial
solution of 1a.46 An isosbestic point appeared at 306 nm and
supported the two-component photochromic reaction. The
coloration and bleaching cycles can be repeated at least 20×
without any photodegradation. That is, 4,5-dithiazolylimid-
azolium 1a underwent reversible photochromic reactions, even
though positively charged imidazolium rings participate in the
Photomodulation of Ionic Interaction. Interestingly, 4,5-
dithiazolylimidazolium 1 showed characteristic solvent-depend-
ent absorption spectra in the closed-ring imidazolinium state.
Table 2 summarizes the values of λmax of open- and closed-
ring isomers of 1 in various solvents. In polar solvents such as
(46) See Supporting Information.
(47) Kim, M.-S.; Kawai, T.; Irie, M. Chem. Mater. 2003, 15, 4539–4543.
9
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Photoconversion between Imidazolium and Imidazolinium
A R T I C L E S
acetonitrile, acetone, and dichloromethane, the λmax of closed-
ring isomer 1b appeared at around 640 nm, which shifted to a
shorter wavelength with increasing dielectric constants of
solvents, indicating the stronger stabilization of the ground-state
of 1b than that of the excited-state in polar media. However,
that is not the case with the λmax in low polar media such as
2-methyltetrahydrofuran (2-MeTHF) and toluene. The absorp-
tion peak shifted to a shorter wavelength by more than 80 nm,
and the color of solution changed from blue to red. Such a large
blue-shift cannot be explained with ordinary solvatochromism,
in which the absorption maximum responds to the solvent
dielectric constants in a manner consistent with changes in the
polarity of the chromophore upon the transition from the ground-
state to the excited state.48
The nonionic 4,5-dithiazolylimidazole 2 never showed such
a solvent-dependent absorption behavior in both open- and
closed-ring states. The optical response observed for the closed-
ring isomer 1b, therefore, should be attributed to the solvent-
modulated ionic interaction specifically between the imidaz-
olinium unit in 1b and the iodide anion. In polar media, each
imidazolinium cation 1b and iodide anion would be well
solvated with solvent molecules to result in weak electrostatic
interaction, while the insufficient solvation of ionic species in
low polar media may give rise to the formation of a certain
kind of contact ion pair in which the iodide anion strongly binds
to the imidazolinium cation at ground state. This explanation
was also supported by the anion-exchange measurement. The
counteranion I- was readily exchanged with perchlorate anion
(ClO4-) by the addition of silver perchlorate to capture the
iodide anion as silver iodide. By changing the anion from iodide
to perchlorate, the absorption peak of 1b-ClO4 in 2-MeTHF
appeared at around 650 nm corresponding to those of 1b in
polar solvents. Once the anion was replaced with bulky and
soft perchlorate anion, the electrostatic interaction between
imidazolinium cation and anion should be weakened even in
less polar media to give the absorption profile identical with
those of 1b(I-) in well solvating polar media.
Alternatively, the λmax of 4,5-dithiazolylimidazolium 1a, open-
ring isomer remained almost constant against the solvent polarity
(Table 2), which clearly demonstrates the difference in the
electrostatic interaction of imidazolium and imidazolinium rings
with iodide anion. As described above, the cationic charge is
more localized in the nonaromatic imidazolinium ring than in
the aromatic imidazolium ring, leading to the stronger electro-
philicity for imidazolinium 1b than 1a. Thus, the enhancement
of electrophilic nature seems to be indispensable for the
observation of solvatochromism, which was achieved by the
photoconversion of imidazolium to imidazolinium. The similar
effect of electrophilicity of C(2)-carbon on the electronic
transition has been recently reported by Bielawski49 as the
solvatofluorochromism of benzobis(imidazolium) salts with
electron-withdrawing substituents. However, in that case, the
nucleophiles bond to benzobis(imidazolium) cation only at the
electronic excited state.49
Figure 3. A: Absorption spectral change of 1 with photochemical and
chemical reactions in methanol. (a) dotted line: 1a, (b) broken line: after
UV irradiation to spectrum a, (c) solid line: after the addition of MeONa to
spectrum b. ([1] ) 1.6 × 10-5 M). B: Absorption spectra of 1b (7.7 ×
10-6 M) in the presence of MeONa (0 M to 4.0 × 10-4 M).
Scheme 3
as alkoxide anions, and the reaction was studied with absorption
spectra and 1H NMR measurements. Figure 3A shows the
absorption spectral change of 1 in anhydrous methanol with
UV irradiation, followed by addition of an excess amount of
sodium methoxide (MeONa) as a nucleophile. The photo-
generated product in methanol was also colored in blue,
reflecting the high polarity of methanol and showed absorption
band at around 640 nm as shown in Figure 3A(b). The blue-
colored solution of 1b turned red with the addition of MeONa/
methanol, giving an absorption peak at around 560 nm (Figure
3A(c)). These two absorption bands coincided with those
observed in the solvatochromic behavior as summarized in Table
2. The titration measurement gave the continuous progress and
suppression of the absorption bands at 560 and 640 nm,
respectively, accompanied by an isosbestic point at 571 nm,
supporting two-component chromic reaction of the colored
isomer (Figure 3B). The red-colored substance was easily
transferred to nonpolar hexane by the simple biphasic extraction
process with methanol. However, the blue color remained in
the methanol phase, indicating the different polarity between
the red- and blue-colored species. Because the ionic 1a was
never soluble in hexane, the nonionic 1b-OMe depicted in
Scheme 3 seems to be the most plausible structure as the red-
colored photoproduct in the presence of MeONa.
Photomodulation of Reactivity. To demonstrate the different
reactivity of imidazolium and imidazolinium, especially the
strong electrophilicity of imidazolinium, we then investigated
the reaction of imidazolinium 1b with a strong nucleophile such
We then confirmed the structure at each state with 1H NMR
measurements. Figure 4 shows 1H NMR spectra of the aliphatic
region, where a, b, and c correspond to each absorption spectrum
a, b, and c in Figure 3A, respectively. Because the methyl
protons of 1 are of note here, chloroform-d for a, b and hexane-
d14 for c were used as the solvents of 1H NMR measurements.
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Stochkett, A. S.; Ritschdorff, E. T.; Shear, J. B.; Bielawski, C. W.
J. Am. Chem. Soc. 2008, 130, 3143–3156.
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A R T I C L E S
Nakashima et al.
Figure 5. Reversible color change and phase transfer of 1b by the
successive addition of MeONa and TFA in the hexane/ionic liquid biphasic
system. (a) 1b in emimTFSI after UV irradiation, [1] ) 0.30 mM; (b) 1b-
OMe in hexane by the addition of excess amount of MeONa to (a); and (c)
1b in emimTFSI by the addition of excess amount of TFA to (b).
dependent density functional theory (TDDFT)50 calculation at
the B3LYP/6-31G*51 level for 1b+ (without I-) and 1b-OMe
were in good agreement with the experimentally observed
wavelength of the absorption maxima for 1b+ and 1b-OMe.46
Because the nucleophile adducts have unstable, distorted struc-
tures with a quarternary imidazolidine C(2) atom, the neutral
products reversibly converted to a blue-colored ionic compound
1b by the addition of acids such as hydrochloric acid and
trifluoroacetic acid (TFA) to eliminate the methoxy group as
methanol (Scheme 3). The red-colored product 1b-OMe still
showed reversible photochromism in hexane between the red and
colorless states, whereas the reversibility of photoconversion
gradually diminished with repetitive UV-vis irradiations.
Similar to the case of solvatochromism, the nucleophilic
addition was not observed for open-ring isomer 1a and 4,5-
dithiazolylimidazole 2a and 2b, demonstrating the specific
reactivity of imidazolinium cation against nucleophiles. Fur-
thermore, the nucleophilic reaction of imidazolinium 1b with
MeO- was also observed in an imidazolium-based ionic liquid,
1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide
(emimTFSI), which directly represents the higher reactivity of
imidazolinium than imidazolium. The 4,5-dithiazolylimidaz-
olium 1 showed reversible photochromic reaction even in
emimTFSI and was a blue color, forming closed-ring isomer
1b by UV irradiation (Figure 5a). Upon addition of MeONa
into the blue-colored ionic liquid phase, the color turned to red
and transferred into an upper hexane layer by vigorous stirring
(Figure 5b). The color reversibly changed to blue and transferred
into the lower ionic liquid phase through the addition of acid
(TFA) to again form ionic 1b (Figure 5c).
Figure 4. 1H NMR spectra of methyl group protons. (a) 1a in chloroform-
d, (b) after UV irradiation to a, (c) the extract in hexane-d14 from the mixture
of photoproduct of 1 in the presence of MeONa in methanol. For the clarity
of assignment, the parts of the structure were depicted at the corresponding
peaks.
For the measurement of spectrum b, methanol was evaporated
and replaced with chloroform-d after the photoconversion to
1b, while spectrum c was measured in hexane-d14 after the
extraction of the product from the methanol solution of 1b with
MeONa. Two characteristic singlet methyl signals at 3.83 and
2.32 ppm were observed for the open-ring isomer 1a (Figure
4a), and each signal was assigned as described in Figure 4a.
These methyl signals slightly shifted to the high magnetic field
after the photoconversion to 1b (Figure 4b). The shift was
significant for the methyl groups attached to the thiazole groups
because the hybrid orbital of the R-carbon (C(5) in thiazole ring)
was converted from sp2 to sp3 with the rearrangement of the
π-connection. A further shift of these signals to the high
magnetic field and peak splitting was observed, together with
the appearance of a new singlet signal at 3.24 ppm for the extract
in hexane-d14 (Figure 4c) after the addition of MeONa to 1b.
All of these signals were assigned as depicted in Figure 4c for
the following reasons. The new signal at 3.24 ppm was assigned
as the methoxy group bound to the C(2) position of the
imidazolinium ring because the MeO- ion was insoluble in
hexane and the measurement of the differential nuclear Over-
hauser effect (1D-NOE) exhibited the strong correlation of this
peak with the splitting of two peaks of N-Me groups at 3.02
and 3.08 ppm.46 The significant upfield shift of methyl peaks
should be explained by the absence of positive charge and
accompanying increase in electron density on the fused hetero-
cycle. The closed-ring isomer 1b has a planar structure of C2
symmetry, and the nucleophilic addition of MeO- to the C(2)
position of the imidazolinium ring breaks it, giving the split of
signals of the methyl groups on the fused heterocycle. All of
these findings described above clearly indicate that the red-
colored product that gave the absorbance peak at around 560
nm is 1b-OMe, as depicted in Scheme 3. The results of time-
Thus, the reactivity of the imidazolium cation has been readily
controlled by combination with the photochromic reaction,
which reversibly converts the stable imidazolium to the reactive
imidazolinium. Such a compound could be demonstrated as one
of the few examples of a photogated reaction system,41 in which
light triggers a structural change in the molecule and imparts
chemical reactivity unique to the photogenerated isomer.
Conclusions
We have designed a reversible transformation system between
chemically stable imidazolium and reactive imidazolinium on
the basis of the design of a photochromic terarylene that
undergoes reversible aromatic/nonaromatic conversions with the
(50) (a) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys.
1998, 109, 8218–8224. (b) Bauernschmitt, R.; Ahlrichs, R. Chem.
Phys. Lett. 1996, 256, 454–464. (c) Casida, M. E.; Jamorski, C.;
Casida, K. C.; Salahub, D. R. J. Chem. Phys. 1998, 108, 4439–4429.
(51) Gaussian 03, revision D.01, Gaussian, Inc., Wallingford CT, 2004.
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Photoconversion between Imidazolium and Imidazolinium
A R T I C L E S
Scheme 4. Synthetic Route to Compounds 1a and 2ba
stirred for 30 min. After the addition of iodomethane (1.6 mL, 26
mmol), the solution was further stirred for 10 h. The reaction
mixture was extracted with ethylacetate and washed with brine.
The organic layer was dried over anhydrous magnesium sulfate,
filtered, and concentrated. The residue was dissolved in chloroform,
and the insoluble part was removed by filtration. The filtrate was
concentrated to give 3 (7.6 g, 74%) as a colorless powder. 1H NMR
(CDCl3, TMS, 300 MHz) δ 3.75 (s, 3H), 7.44-7.50 (m, 3H),
7.54-7.57 (m, 2H).
4,4′-(1-Methyl-2-phenyl-1H-imidazole-4,5-diyl)bis(5-methyl-2-
phenylthiazole) (2a). A 100-mL four-necked flask was charged with
5-methyl-2-phenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)thiazole42b (0.82 g, 2.7 mmol), 3 (0.43 g, 1.0 mmol), 2′-
(dicyclohexylphosphino)-N,N-dimethylbiphenyl-2-amine (10 mg,
0.030 mmol), Pd(OAc)2 (0.020 g, 0.11 mmol), and CsF (0.43 g,
2.8 mmol) in DMF/1,4-dioxane (1.5 mL/10 mL) solution. The
mixture was then stirred at 80 °C under N2. After having been stirred
for a day, the reaction mixture was extracted with diethylether, and
the organic layer was dried with anhydrous magnesium sulfate,
filtered, and concentrated. Alumina gel column chromatography
(hexane/ethylacetate 8:1) and reversed phase HPLC (methanol)
afforded 2a (0.044 g, 9.0%) as a colorless solid. 1H NMR (CDCl3,
TMS, 300 MHz) δ 2.13 (s, 3H), 2.45 (s, 6H), 3.76 (s, 3H),
7.30-7.35 (m, 3H), 7.40-7.57 (m, 6H), 7.78-7.84 (m, 4H),
7.96-8.00 (m, 2H); 13C NMR (CDCl3, 75 MHz) δ 12.2, 29.1, 33.6,
125.7, 126.2, 128.4, 128.6, 128.7, 128.9, 129.1, 129.3, 129.9, 130.2,
130.8, 133.7, 133.9, 134.4, 135.7, 142.8, 146.4, 148.6, 163.5, 164.3;
ESI-HRMS (m/z) [M+H]+ Calcd. for C30H25N4S2, 505.1521;
Found, 505.1527. Anal. Calcd. for C30H24N4S2: C, 71.39; H, 4.79;
N, 11.10. Found: C, 71.44; H, 4.53; N, 11.17.
a Reagents and conditions: (a) NaH, iodomethane, dry THF; (b) 5-methyl-
2-phenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiazole, Pd(OAc)2,
2′-(dicyclohexylphosphino)-N,N-dimethylbiphenyl-2-amine, CsF, DFM/1,4-
dioxane; (c) iodomethane, acetonitrile.
rearrangement of the π-conjugation structure. The 4,5-dithiaz-
olylimidazolium terarylene showed reversible photochromism
with good performance in organic media and exhibited the
characteristic solvato- and ionochromisms responding to the
extent of ionic interaction between the photogenerated imida-
zolinium cation and counteranions. In less polar solvents, the
iodide anion strongly bonds to the electrophilic imidazolinium
closed-form at the ground state. The mechanism of solvato- and
ionochromisms was clearly demonstrated by the nucleophilic
addition of methoxide to the electrophilic C(2) position of
imidazolinium cation. Because the imidazolium open-ring
isomer was stable to strong nucleophiles, such as sodium
methoxide, the reaction only occurred for the imidazolinium
closed-ring isomer, demonstrating the photogated reactivity. The
present concept of the photoswitching of charge delocalization-
localization states would give us a new strategy for the design
of photoresponsive ionically self-assembling dye systems,52 as
well as for the photomodulation of chemical reactivity.
1,3-Dimethyl-4,5-bis(5-methyl-2-phenylthiazol-4-yl)-2-phenyl-
1H-imidazolium iodide (1a). To a solution of 2a (13 mg, 0.030
mmol) in 100 mL of acetonitrile, an excess amount of iodomethane
was added and stirred for a day. The solution was concentrated,
and the residue was purified with reversed-phase HPLC (methanol)
to give 1a (14 mg, 73%) as colorless solid.1H NMR (300 MHz,
TMS, CDCl3) δ 2.32 (s, 6H), 3.83 (s, 6H), 7.27-7.46 (m, 6H),
7.70-7.72 (m, 3H), 7.87-7.92 (m, 4H), 8.14-8.17 (m, 2H); 13C
NMR (CDCl3, 75 MHz) δ 13.0, 35.4, 126.3, 127.6, 129.1, 129.8,
130.5, 131.6, 132.7, 137.3, 138.3, 146.5, 166.4; ESI-HRMS (m/
z) [M]+ Calcd. for C31H27N4S2, 519.1677; Found, 519.1673.
Experimental Section
1
General. H NMR and 13C NMR spectra were recorded on a
JEOL AL-300 spectrometer. Separative HPLC was performed on
a HITACHI LaChrom ELITE HPLC system and a JASCO LC-
2000 Plus Series. Mass spectra were measured with a mass
spectrometer JEOL JMS-T100LC AccuTOF. Absorption spectra
in solution were studied with a JASCO V-670 spectrophotometer.
Photoirradiation was carried out using an USHIO 500 W ultrahigh-
pressure mercury lamp as an exciting light source. Monochromatic
light was obtained by passing the light through a monochromator
(Shimazu SPG-120S, 120 mm, f ) 3.5). The intensity of incident
light after passing the monochromator was found to be 10 mW
Acknowledgment. The authors thank Mrs. Y. Nishikawa and
F. Asanoma for mass spectroscopic and 1D-NOE measurements,
respectively. This work was partly supported by Grant-in-Aids for
Scientific Research (B) (No. 17350069) and Scientific Research
on Priority Areas, “Super-Hierarchy Structures” (No. 17067011)
and “New Frontiers in Photochromism”(471) (No. 20044019) from
the Ministry of Education, Culture, Sports, Scientific and Technol-
ogy (MEXT) of Japan.
cm-2
.
Supporting Information Available: Complete ref 51; 1H NMR
spectra of 1a, 1b, 2a, and 2b, 13C NMR spectra of 1a and 2a,
photocycloreversion and thermalcycloreversion study of 1b and
2b, 1H NMR and 1D NOE difference spectra of 1b-OMe, and
DFT/TDDFT calculations for 1b+ and 1b-OMe at B3LYP/6-
31G*. This material is free of charge via the Internet at http://
pubs.acs.org.
Synthesis. Compounds 1a and 2b were synthesized according
to the reaction scheme depicted in Scheme 4.
4,5-Diiodo-1-methyl-2-phenyl-1H-imidazole (3). To a solution
of 4,5-diiodo-2-phenyl-1H-imidazole (10 g, 25 mmol) in dry THF
was added NaH (0.60 g, 25 mmol) at 0 °C under N2, which was
(52) (a) Faul, C. F. J.; Antonietti, M. AdV. Mater. 2003, 15, 673–683. (b)
Morikawa, M.; Yoshihara, M.; Endo, T.; Kimizuka, N. J. Am. Chem.
Soc. 2005, 127, 1358–1359. (c) Ma, T.; Li, C.; Shi, G. Langmuir 2008,
24, 43–48.
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