Photochromic reaction of diarylethenes having phenol moiety as an aryl ring

Article abstract of DOI:10.1246/bcsj.20130252

Three types of diarylethenes having one phenol moiety as an aromatic unit were synthesized in order to control pH change upon irradiation with UV and visible light. Upon irradiation with 313nm light, these open-ring isomers were transformed to thermally unstable colored isomers in methanol solution. During the storage of the photoirradiated diarylethene solutions in the dark for 10min, the thermally unstable closed-ring isomers were converted to thermally stable keto isomers. The structures of these isomers were characterized by 1HNMR and X-ray crystallography. The keto isomers were transformed to the initial phenol isomer upon irradiation with visible light. Additionally, we also found the formation of a novel photochromic diarylethene derivative having one keto group on the cyclopentane ring during UV irradiation. A pH change between the two isomers was observed in a mixture of ethanol and water.

Full text of DOI:10.1246/bcsj.20130252

528
Bull. Chem. Soc. Jpn. Vol. 87, No. 4, 528-538 (2014)
© 2014 The Chemical Society of Japan
Photochromic Reaction of Diarylethenes Having
Phenol Moiety as an Aryl Ring
Tadatsugu Yamaguchi,*1 Yusuke Kamihashi,1 Toru Ozeki,1 Ayaka Uyama,2 Jun-ichiro Kitai,2
Megumi Kasuno,2 Kimio Sumaru,3 Yoshiro Kimura,4 Satoshi Yokojima,5
Shinichiro Nakamura,6 Masakazu Morimoto,7,8 and Kingo Uchida*2
1Hyogo University of Teacher Education, 942-1 Shimokume, Kato, Hyogo 673-1494
2Department of Materials Chemistry, Faculty of Science and Technology, Ryukoku University,
Seta, Otsu, Shiga 520-2194
3Research Center for Stem Cell Engineering, National Institute of Advanced Industrial Science and Technology,
Tsukuba, Ibaraki 305-8565
4Department of Biomolecular Engineering, Tokyo Institute of Technology,
4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8501
5School of Pharmacy, Tokyo University of Pharmacy and Life Sciences, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392
6RIKEN Research Cluster for Innovation, Nakamura Lab, 2-1 Hirosawa, Wako, Saitama 351-0198
7Department of Chemistry, College of Science, Rikkyo University,
3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501
8Research Center for Smart Molecules, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501
Received September 11, 2013; E-mail: tyamagu@hyogo-u.ac.jp, uchida@rins.ryukoku.ac.jp
Three types of diarylethenes having one phenol moiety as an aromatic unit were synthesized in order to control pH
change upon irradiation with UV and visible light. Upon irradiation with 313 nm light, these open-ring isomers were
transformed to thermally unstable colored isomers in methanol solution. During the storage of the photoirradiated
diarylethene solutions in the dark for 10 min, the thermally unstable closed-ring isomers were converted to thermally
stable keto isomers. The structures of these isomers were characterized by 1H NMR and X-ray crystallography. The keto
isomers were transformed to the initial phenol isomer upon irradiation with visible light. Additionally, we also found the
formation of a novel photochromic diarylethene derivative having one keto group on the cyclopentane ring during UV
irradiation. A pH change between the two isomers was observed in a mixture of ethanol and water.
Diarylethenes are organic photochromic compounds that
The resonance stabilization control of phenol may lead to
interesting properties. Diarylethenes having phenol rings as
an aryl group were considered to be good candidates because
photoswitchable pH changes are expected. Accompanied by the
change of π-conjugation during photochromism, photoinduced
pH changes are expected. pH control using photochromism is
a very interesting property in water12-19 because the photo-
switching property is applicable in the biological field.20 It is
anticipated that increasing acidity caused by the generation of
a photochromic isomer leads to cell death or suppresses the
multiplication of bacteria. We synthesize diarylethenes 1o-3o
in order to control the pH change upon irradiation with UV and
visible light (Scheme 1).
are most promising for optical-memory and optical-switch use
because they have thermal irreversibility and fatigue resist-
ance.1-7 Photochromic diarylethene is based on a reversible
structural change between 1,3,5-hexatriene and cyclohexa-
diene. Although diarylethenes such as 1,2-diphenylethene
generate a thermally unstable product upon UV-light irradi-
ation, those having heterocyclic five-membered rings, such as
1,2-di(3-thienyl)ethene, have a thermally stable closed-ring
isomers.1 Recently, hybrid diarylethenes having phenyl and
thiophene units have shown photochromism in solution and
thermal stability at 80 °C for 100 min.8
Phenol is weakly acidic, it yields the phenolate anion at
high pH. Phenol acidity arises from the resonance stabilization
of the phenoxide anion by the aromatic ring.9,10 Phenol also
exhibits keto-enol tautomerism with its unstable keto tautomer
cyclohexadienone. The equilibrium constant for enolization
Results and Discussion
We synthesized the diarylethenes 1o-3o by the general
procedure for diarylethene derivatives (Scheme 2). In order to
enhance the conversion to the closed-ring isomer by increment
of the cyclization quantum yield upon UV irradiation, com-
pound 1o having a 2-methyl-5-phenylthiophene-3-yl group as
is approximately 1013«1 11
and only a tiny fraction of phenol
,
exists as the keto isomer. Phenol therefore exists entirely in the
enol isomer.
Published on the web April 15, 2014; doi:10.1246/bcsj.20130252

T. Yamaguchi et al.
Bull. Chem. Soc. Jpn. Vol. 87, No. 4 (2014)
529
another aromatic ring was designed and synthesized.21 The
coupling reaction of 3-bromo-2-methyl-5-phenylthiophene (6)
with 1-(4-methoxymethoxy-2,6-dimethylphenyl)-2,3,3,4,4,5,5-
heptafluorocyclopentene (5) and the following acidification
gave 1o. In order to enhance the solubility in ethanol/water
mixture for pH measurement, the diarylethenes 2o and 3o
whose substituent is smaller or more polar than the 5-phen-
ylthienyl group of 1o were synthesized. The diarylethene 2o
has a smaller hydrophobic hydrocarbon group than 1o, and 3o
has a polar carbonyl group. Similarly, the coupling reaction
of 3-bromo-2,5-dimethylthiophene (7) with 5, and the follow-
ing acidification gave 2o. The coupling reaction of 3-bromo-5-
(5,5-dimethyl-1,3-dioxacyclohex-2-yl)-2-methylthiophene (10)
with 5 followed by acidification gave 3o. The compounds 1o-
3o were confirmed by 1H NMR, high-resolution mass spec-
troscopy, X-ray crystallographic analysis, and elemental analy-
sis. These results confirmed that the initial open-ring isomers
1o-3o have phenol structures. We predicted that these com-
pounds show isomerization upon UV irradiation, but unusual
behavior was observed during irradiation with 313 nm light.
Photochromism of Diarylethene Having a Phenol Group.
Figure 1 shows the absorption spectral changes of 1 in meth-
anol solution. After irradiation with UV light for 15 s, the open-
ring isomer 1o was transformed into a colored isomer, how-
ever the absorption maximum of this colored isomer was
F F
F F
F
F
F
F
F
F
F
F
hν
R
S
R
S
OH
OH
1o: R = Ph
2o: R = Me
1c: R = Ph
2c: R = Me
3c: R = CHO
3o: R = CHO
Scheme 1. Photoisomerization upon UV irradiation.
F2
F2
F2
Br
1) n-BuLi / THF
2) C5F8
F
OMOM
OMOM
5
F2
F2
S
F2
Br
1) n-BuLi / THF
R
S
2)
5
R
6: R = Ph
7: R = Me
8: R = Ph
9: R = Me
OMOM
F2
2,2-dimethyl-1,3-
propanediol,
p-toluenesulfonic
F2
F2
Br
Br
1) n-BuLi / THF
2)
acid
O
S
O
OHC
S
5
O
S
toluene
O
OMOM
10
11
F2
F2
F2
S
F2
F2
S
F2
H2SO4/ MeOH
R
R
OMOM
OH
1o: R = Ph
2o: R = Me
3o: R = CHO
8: R = Ph
9: R = Me
O
11: R=
O
Scheme 2. Syntheses of 1o, 2o, and 3o.

530
Bull. Chem. Soc. Jpn. Vol. 87, No. 4 (2014)
Diarylethenes with Phenol Moiety
shifted to a shorter wavelength in the dark. At the same time,
the absorption intensity at the absorption maximum was
slightly increased. After 10 min in the dark, the colored band
reached a stable stationary state whose absorption maximum
was observed at 534 nm. The closed-ring enol isomer 1c is
unstable at room temperature. This means that an initial colored
isomer 1c did not yield a single compound. We considered that
the photogenerated closed-ring enol isomer 1c changed to the
stable keto isomer 1k (Scheme 3).
The stable structures of 1o and 1k were assigned by 1H NMR
in CDCl3. The phenol proton for 1o was observed at 4.91 ppm,
and two protons attached to the phenol ring were observed at
6.49 ppm. Upon irradiation with 313 nm light, 1k was separated
by silica gel column chromatography (eluent hexane/ethyl
acetate = 4:1). Two singlet protons of the methylene moiety of
1k appeared at 2.37 and 3.03 ppm, and 1k has a keto structure.
After irradiation with visible (>400 nm) light, 1k was imme-
diately transformed into the open-ring isomer 1o, which has a
phenol structure. 1k in methanol was thermally stable in a tube
cell at 70 °C for 24 h. No thermal isomerization from 1k to 1o
was observed (see Supporting Information). After heat treat-
ment at 70 °C, the characteristics of 1k in photoisomerization
were maintained.
keto isomer of 1o by density functional theory (DFT) and time-
dependent DFT (TDDFT) with B3LYP exchange correlation
functional (Figure 2). The solvent effect due to methanol was
considered by the polarizable continuum model. In Figure 2,
the free energy relative to that of 1o is shown as ¦G. The
calculated wavelengths of the excitation energies of the phenol
isomer 1o were 360 and 331 nm, and these values are in agree-
ment with the experimental one (-max = 379 nm in methanol).
Upon irradiation with UV light, the closed-ring enol isomer 1c
will be initially formed from the phenol isomer 1o. However,
¦G of the enol isomer 1c is 35.5 kcal mol¹1, which is much
larger than ¦G of the keto isomer 1k (20.3 kcal mol¹1). Thus, it
is expected that the enol isomer 1c will isomerize to the stable
keto isomer 1k. The observed blue shift in absorption spectra
is further explained computationally based on Scheme 3, i.e.,
the calculated wavelengths of the lowest excitation energies
of 1c and 1k were 629 and 546 nm, respectively. We therefore
conclude that the observed blue shift in absorption spectra is
due to the conformational change from the enol isomer 1c to
the stable keto isomer 1k.
When the keto isomer 1k was irradiated with visible light,
the stable isomer 1o was regenerated. This is because the
initially formed keto isomer (1o-keto) of 1o upon irradiation
with visible light to the keto isomer 1k is unstable in com-
parison with the phenol isomer 1o, i.e., the free energy of the
In order to further confirm the validity of Scheme 3, we
performed ab initio quantum chemical calculations for the free
energies and the excited state energies of 1o, 1c, 1k, and the
¹1
keto isomer 1o was 12.2 kcal mol larger than that of the
phenol isomer of 1o. Consequently, the structure of keto isomer
1o-keto is immediately converted to the phenol isomer 1o as
shown in Figure 2.
1.0
0.8
0.6
As in the case of 1, the diarylethene 2, which has the 2-
methyl-5-phenylthiophene ring, also shows photochromism in
methanol (Scheme 4). Figure 3a shows photochromism of 2 in
methanol solution. The physical properties of 2 were similar to
those of 1 (Table 1). The closed-ring isomer 2k also has a keto
group and does not show thermal stability at 70 °C for 24 h
in ethanol. The cyclization quantum yield (ΦCO = 0.40) of 2
in methanol is slightly larger than that of 1 (ΦCO = 0.28) in
methanol, while the cycloreversion quantum yield of 2 is large
(ΦOC = 0.022). These results may be due to the differences
between the extensions of π-conjugated systems.
0.4
0.2
0.0
after 10 min
after 0 min
300
400
500
600
700
The diarylethene 3o has the same photoreactivity as the
compounds 1 and 2 in methanol. The separation of 3k after
UV irradiation is accomplished by reversed-phase column
chromatography (methanol:water = 4:1). Figure 3b shows the
photochromism of 3 in a mixture of methanol and water.
In Table 1, the absorption spectral properties were collected
Wavelength / nm
Figure 1. Absorption spectra of 1o (dotted line), 1k (solid
line), and after irradiation with 313 nm light for 15 s and
holding in the dark for 0 min followed by 10 min in
methanol (dashed line).
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
ν
h
in dark
F
S
S
S
O
OH
OH
1o
1c (enol)
1k (keto)
ν
h '
Scheme 3. Isomerization upon photoirradiation of 1.

T. Yamaguchi et al.
Bull. Chem. Soc. Jpn. Vol. 87, No. 4 (2014)
531
F2
F2
F2
F2
F2
S
F2
UV
S
OH
1o
1c
OH
λ
max 542 nm
λ
max 379 nm
ΔG = 35.5 kcal mol−1
Δ
G = 0 kcal mol−1
(stable)(calculated: excitation
wavelength 360 nm, 331 nm)
(calculated: excitation
wavelength 629 nm)
F2
F2
F2
S
F2
F2
S
F2
Vis.
O
O
1o-keto
ΔG = 12.2 kcal mol−1
1k
λ
max 532 nm
ΔG = 20.3 kcal mol−1
(calculated: excitation
wavelength 546 nm)
(stable)
Figure 2. The calculated free energies relative to that of 1o and the calculated excitation wavelengths for 1o, 1c, 1k, and 1o-keto
isomers.
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
h
ν
in dark
R
R
S
S
R
S
O
2k: R = Me
3k: R = CHO
OH
OH
2o: R = Me
3o: R = CHO
2c: R = Me
3c: R = CHO
(keto)
(enol)
hν'
Scheme 4. Isomerization upon photoirradiation of 2 and 3.
in the mixed solvent because the water is difficult to re-
move in vacuo. The absorption spectra were also obtained in
methanol. The absorption coefficient for 3o was 2.00 © 104
(Table 1) was larger than that of 12 (-max = 552 nm, 4.8 © 103
mol dm¹3) in hexane.
X-ray Crystallographic Analysis. To support the results
of the absorption spectra, we confirm the above structures by
X-ray structure analysis. Suitable crystals were obtained for
the diarylethene 3o and the closed-keto isomer 2k. The ORTEP
drawings for these structures are shown in Figures 4a and 4b,
respectively.
Figure 4a shows the ORTEP drawing for 3o, which has a
phenol structure. The distance between O1 and C5 is 0.1355
nm, which is comparable to the length of a C-O single bond.
The bond lengths of C1-C6 and C6-C5 are 0.1387 and 0.1389
nm, respectively. The distances are typical of aromatic carbon
atoms. Two aryl moieties, which are phenol and thiophene
rings, are in an antiparallel conformation, and the distance
between the reactive carbon atoms (C1 and C12) is 0.3887 nm.
¹1
dm3 mol¹1 cm at 255 nm in methanol. The keto isomer 3k is
¹3
thermally stable at 70 °C for 24 h in methanol. 10¹7 mol dm
3o can be dissolved in water, and photochromism was observed
in water.
The cyclization quantum yield of 1o (Φoc = 0.28) was
almost the same as that of 1-(2-methyl-5-phenyl-3-thienyl)-
2-(2-methylphenyl)-3,3,4,4,5,5-hexafluorocyclopentene (12)
(Scheme 5) (Φoc = 0.25).8 However, the cycloreversion quan-
tum yield of 1 (ΦCO = 0.0019) differed from that of the
analogue 12 (ΦCO = 0.31).8 This means that the cycloreversion
reaction of 1c was suppressed by the tautomerization to the
keto isomer 1k. The molar coefficient of the closed-ring keto
isomer 1k (-max = 534 nm, 2.16 © 104 mol dm¹3) in methanol

532
Bull. Chem. Soc. Jpn. Vol. 87, No. 4 (2014)
Diarylethenes with Phenol Moiety
However, this compound did not show photochromism in the
crystalline phase in spite of the favorable conditions, that is, the
distance within 0.42 nm and two aryl moieties oriented to the
antiparallel conformation.22 This is attributable to the larger
dihedral angle of phenol ring and ethene moiety (68.8°) of 3o
compared to those of diarylethenes with five-membered hetero-
aromatic rings (e.g., 62.5° for 1,2-bis(2-methyl-5-phenylthio-
phen-3-yl)-3,3,4,4,5,5-hexafluorocyclopentene (13o)) (see Sup-
porting Information).
Figure 4b shows the ORTEP drawing for 2k. The distance
between O1 and C5 is 0.1223 nm, which is comparable to that of
a C-O double bond, and the drawing suggests the keto struc-
ture. The bond lengths for C1-C6 and C6-C5 are 0.1539 and
0.1499 nm, respectively. These results suggest that the bonds
are single bonds and that C6 is an sp3 carbon atom. The lengths
of the C5-C4, C4-C3, C3-C2, C2-C7, and C7-C11 bonds
changed by turn. This suggests the existence of π-conjugation
beyond the bonds. The lengths of the C5-C4, C3-C2, and
C7-C11 bonds are 0.1461, 0.1468, and 0.1455 nm, and those
of the C4-C3 and C2-C7 bonds are 0.1352 and 0.1368 nm,
respectively.
1.0
(a)
0.8
0.6
0.4
0.2
0.0
Generation of By-Product upon Irradiation with UV
Light. A solution of diarylethene 2o was irradiated with 313
(a)
200
300
400
500
600
700
Wavelength / nm
1.0
(b)
0.8
0.6
0.4
0.2
0.0
(b)
200
300
400
500
600
700
Wavelength / nm
Figure 3. (a) Absorption spectra of 2o (dotted line), 2k
(solid line), and after irradiation with 313 nm light for
15 s and holding in the dark for 0 min followed by 10 min
in methanol (dashed line). (b) Absorption spectra of 3o
(dotted line), 3k (solid line), and after irradiation with 313
nm light for 15 s and holding in the dark for 0 min followed
by 10 min in mixed solvent of ethanol and water (4:1 in
volume ratio) (dashed line).
Figure 4. ORTEP drawings of (a) 3o and (b) 2k.
Table 1. Absorption Spectral Properties of Diarylethenes 1-4 in Solution
¹1
¾/104 dm3 mol¹1 cm
Quantum yield
Compound
(solvent)
o
k
Cyclization
Cycloreversion
1
3.19
(261 nm)
0.875
2.16
(534 nm)
0.846
0.28
(313 nm)
0.40
0.0019
(517 nm)
0.022
(CH3OH)
2
(CH3OH)
3
(259 nm)
2.0
(511 nm)
1.07
(313 nm)
0.22
(517 nm)
0.0018
(C2H5OH:H2O = 4:1)
4
(255 nm)
1.0
(511 nm)
0.639
(313 nm)
0.34
(517 nm)
0.067
(hexane)
(293 nm)
(552 nm)
(313 nm)
(517 nm)

T. Yamaguchi et al.
Bull. Chem. Soc. Jpn. Vol. 87, No. 4 (2014)
533
F
F
1.0
0.8
0.6
0.4
F
F
F
F
F
F
F
F
F
F
(a)
h
ν
h '
ν
S
S
12o
12c
F
F
F
F
F
F
F
F
F
F
F
F
0.2
0.0
h
ν
h '
ν
S
S
S
S
200
300
400
500
600
700
800
13o
13c
Wavelength / nm
Scheme 5. Photochromism of diarylethenes 12 and 13.
1.0
(b)
nm light for 3 min in methanol. The predominant product upon
photoirradiation was the closed-ring isomer 2c. After prolonged
irradiation with UV light, the by-product 4o was obtained when
alcoholic solvents such as methanol and ethanol were applied.
When the ethanol solution (70 mL) containing the starting
material 2o (129 mg) was irradiated with UV light for 300 min,
29 mg of 4o was obtained.
0.8
0.6
0.4
Figure 5a shows the absorption spectra of 4 in methanol.
No absorption spectral change was observed upon irradiation
with 365 nm light. Figure 5b shows the absorption spectra of
4 in hexane. Upon irradiation with 365 nm light, the open-
ring isomer 4o was converted to the closed-ring isomer 4k
(Scheme 6). Upon irradiation with >440 nm light, the closed-
ring isomer was converted back to the initial 4o. The photo-
chromic behavior is similar to that of the diarylethene, which
has maleimide or maleic anhydride. The formation of twisted
intramolecular charge transfer (TICT)4,23 in open-ring isomer
is suspected because of the decrease of cyclization efficiency
with increasing the polarity of the solvents (see Supporting
Information Figure S14). The thermal stability test for 4k was
carried out in hexane at 60 °C for 24 h, and no thermal
isomerization was observed under these conditions. Separated
4k was dissolved in methanol, but it was thermally unstable in
methanol at 60 °C for 3 h.
A single crystal of 4o was obtained by recrystallization from
hexane, and the crystal was analyzed by X-ray crystallography.
The ORTEP drawing of 4o is shown in Figure 6. The diaryl-
ethene 4o has an α,β-unsaturated ketone structure by introduc-
tion of a ketone group at the fluorinated cyclopentene ring of
2o. The keto moiety introduced at the cyclopentene ring has a
bond distance of 0.1213 nm (C8-O2) for 4o. The bond length
is similar to that of O1-C5 (0.1223 nm) of 2k. Compound 4o
also has a phenol structure. The distance between O1 and C5
is 0.1373 nm, which is comparable to that of a C-O single
bond. The unsymmetrical structure on the cyclopentene ring is
valuable in photochromic diarylethene studies. The perfluoro-
cyclopentene derivative 2o does not depend on the polarity
in photochromism. The unsymmetrical cyclopentene derivative
4o strongly depends on the polarity of the solvent in photo-
chromism.
0.2
0.0
after 10 min.
after 0 min.
800
200
300
400
500
600
700
Wavelength / nm
Figure 5. (a) Absorption spectra of 4o in methanol (solid
line) and after irradiation of 365 nm light for 3 min (dotted
line). (b) Absorption spectra of 4o (solid line), 4k (bold
solid line), and after irradiation with 313 nm light for 15 s
and holding in the dark for 0 min followed by 10 min in
hexane (dashed line).
ethene 4o was obtained in an alcohol solvent such as meth-
anol and ethanol. The compound was not obtained in hexane
or in ethyl acetate. The diarylethene 4o was generated in the
photostationary state upon irradiation with 365 nm light. In
addition, 365 nm light irradiation of 2k leads to the generation
of 4o. The UV light energy and the electron withdrawal from the
keto structure induced the removal of the fluorine atoms from
the diarylethene, and the surrounding alcohol solvents attacked
this moiety. Finally, the dialkoxy groups at the cyclopentene
moiety were removed as ether to produce the isomer 4o. Similar
alcohol-sensitive products were obtained from the system of
pyridine derivatives, and solvation in methanol was observed
for this system.24 However, we could not find a dialkoxy
product such as the intermediate product in 4o generation. After
the removal of the ether structure, the stability of 4k in polar
solvents caused it to transform to the open-ring 4o.
pH Measurement in Mixed Solvent. Since the acidic
phenol moiety bearing an electron-withdrawing perfluoro-
cyclopentene ring at the para-position converts to enol or α,β-
unsaturated ketone in the cyclization reaction, reversible pH
changes are expected to occur during the photoisomerization.
Figure 7 shows the proposed mechanism behind the con-
version of the starting material 2o to the product 4o. The diaryl-

534
Bull. Chem. Soc. Jpn. Vol. 87, No. 4 (2014)
Diarylethenes with Phenol Moiety
An experiment on pH changes upon photoirradiation was
accomplished in a mixture of ethanol and water (ethanol:
water = 2:3 v/v) at 23-25 °C. We use the ethanol/water
mixture because the solubility of the diarylethenes in water is
low. We used this solvent ratio due to the tradeoff between low
solubility and the stability of pH measurement. The compound
1o did not dissolve in the mixed solvent. The pH was not
measured under the experimental conditions. The compounds
2o, 3o, and 4o were dissolved in the mixed solvent. The con-
centration of 2o was 1.0 © 10¹3 mol dm¹3. In the initial state of
2o, the pH of the solution was 6.64 « 0.03. After irradiation
with UV light (313 nm), the pH of the solution changed to
5.13 « 0.03. The ratio of conversion from 2o to 2k was
estimated to be 13% by absorption band analysis. After irradi-
ation with visible light, the pH did not return to its original
value. Prolonged UV light irradiation leads to the production
Figure 6. ORTEP drawing of 4o.
F
F
F
F
F
F
F
F
O
F
F
O
F
F
O
h
ν
in dark
S
S
S
O
OH
OH
4c (enol)
4k (keto)
4o
hν'
Scheme 6. Isomerization upon photoirradiation of 4.
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
+H+
F
UV
S
S
S
H
S
F
O
OH
H
H
OH
OH
2k
H
2o
2c
F
F
F
F
F
F
F
HOEt
O Et
OH
F
H OEt
F
F
F
F
F
F
F
F
F
-H-
-H+
-EtOEt
-F2
S
S
S
S
H
F
H
H
H
OH
OH
OH
OH
H
H
H
H
F
F
F
O
O
F
F
F
F
light
S
S
H
4k
O
OH
H
4o
Figure 7. The proposed mechanism of the formation of 4o.

T. Yamaguchi et al.
Bull. Chem. Soc. Jpn. Vol. 87, No. 4 (2014)
535
of a by-product 4o, and other products. By-product generation
makes it difficult to analyze these results. The pH of pure 2k
could not be identified due to the instability of the value (6.00-
6.65) in the same measuring condition (mixed solution with
1.0 © 10¹3 mol dm¹3).
Aldrich, TCI, Wako, or Nacalai Tesque and used without
purification. The silica gel used was silica gel 60N (spherical,
neutral) 63-210 ¯m from Kanto Chemical Co., Ltd. The
TLC used was MERCK 25TLC aluminum sheets 20 © 20 cm
silica gel 60 F254 1.05567.0001. HPLC was carried out on
a Shimadzu LC-10AD liquid chromatography coupled with
a Shimadzu SPD-10A spectrophotometric detector. Columns
(Wako Wakosil 5SIL and GL Science Inertsil PERP ODS) were
Similar data was observed on 3o systems (concentration;
c = 1.0 © 10¹3 mol dm¹3) in the mixture of ethanol and water
(ethanol:water = 2:3 v/v). In the initial state of the solution of
3o, the pH was 5.55 « 0.15. After irradiation with UV light
(313 nm), the pH change was not observed. The ratio of con-
version from 3o to 3k was estimated to be 13% by absorption
1
used to analyze diarylethene isomers. H NMR spectra were
recorded on a Bruker DPX 400 (400 MHz) spectrometer and
Gemini 2000 (200 MHz) at ambient temperature. Chemical
shifts are denoted in ¤ units (ppm) relative to the CHCl3 solvent
signals.25 Mass spectra were obtained from Shimadzu Parvum
2 (DI), and JEOL MS-700 (EI). Melting points were measured
on a BUCHI B-545, and absorption spectra were measured on a
Shimadzu U-2500PC spectrophotometer. The pH measurement
was performed on a Horiba D-51 pH meter with a 6377-10D
electrode. UV light irradiation was carried out with a Moritex
MUV-202U xenon-mercury lamp (200 W), or a Topcon Fi5L
hand lamp (12 W, 365 nm). Visible light was irradiated with
a 500 W Xe-arc lamp (USHIO SX-Ul501XQ) fitted with a
Toshiba color filter (Y-50). The cyclization and cycloreversion
quantum yields were determined in comparison with the
photoisomerization rate of 1,2-bis(2-methyl-1-benzothiophen-
3-yl)-3,3,4,4,5,5-hexafluorocyclopentene in hexane as a refer-
ence.26 The geometry was optimized by density functional
theory (DFT) with the B3LYP27-29 exchange correlation func-
tional. The effect of methanol solvent was taken into account
by the polarizable continuum model (PCM).30,31 The free
energies at room temperature, 298.15 K, were obtained by
using a rigid-body rotator and a harmonic oscillator model. The
excitation wavelengths were calculated by the time-dependent
DFT (TDDFT)32 with the B3LYP. All the calculations were
performed by Gaussian 0933 with the 6-31G(d) basis set.34-36
X-ray crystallography was carried out using a CCD-based
X-ray diffractometer (Bruker, SMART APEX) with Mo Kα
radiation (- = 0.71073 ¡). The temperature was controlled by
a nitrogen-gas applying system (Japan Thermal Engineering,
TC-190CP-CS-K). The data were collected as a series of ½-
scan frames, each with a width of 0.3°/frame. Data reduction
was performed using SAINT software, which corrects for
Lorentz and polarization effects, and decay. The cell constants
were determined by the global refinement. The structure was
solved by direct methods using SHELXS-9737 and refined by
full-matrix least-squares on F2 using SHELXL-97.38
¹3
band analysis. The pH of 1.0 © 10¹3 mol dm solution of 3k
was measured to be 5.67 « 0.11. As a result, the pH was not
controlled by irradiation with UV light.
The pH of the mixture of ethanol and water (ethanol:water =
2:3 v/v) solution containing pure 4k was 4.35 « 0.07 (1.0 ©
10¹3 mol dm¹3). Upon irradiation with >500 nm light for 6 h,
the keto isomer 4k was transformed into 4o. The photo-
¹3
generated 4o shows the pH value of 4.15 « 0.06 (1.0 © 10
mol dm¹3) in the mixed solution. A small reduction of pH
value change was observed by the recovery of phenol moiety
of 4o under these conditions. Although the structure of 2k
has one carbonyl (>C=O), that of 4k has two carbonyls. The
expansion of π-conjugation between the two carbonyl units on
4k leads to the dissociation of the hydrogen ion, and the pH of
4k becomes higher than that of 2k. The decrease in the pKa
for 4k is similar to that for acetyl acetone, which is caused by
α-hydrogen. In the case of 4o, a similar π-conjugation effect
between a carbonyl on cyclopentene and an alcohol on phenol
is reflected in the pKa, and the release of the proton on the
phenol moiety may be induced by the cyclopentene and phenol
rings being in the same plane.
The cyclization and cycloreversion quantum yields were
revised after the peer review of the manuscript due to the recent
revision of the standard quantum yields of diarylethenes
(Ref. 26).
Conclusion
We have synthesized diarylethenes 1o, 2o, and 3o having
a phenol unit as an aryl unit. They showed photochromism
in methanol solution upon alternating irradiation with UV and
visible light. The closed-ring isomers 1o and 3o are thermally
stable at 70 °C for 24 h in methanol solution. 2o and 3o were
dissolved in a mixture of ethanol and water to observe the
pH changes upon photoirradiation. Only small photoinduced
pH changes were observed during the photochromism of 2o
and 4o. Prolonged UV photoirradiation of 2o generated the by-
product 4o, which was a product of alcohol decomposition. The
diarylethene 4o shows photochromism in hexane, but not in
methanol solution. The characteristics of 4o are similar to those
of diarylethene having maleimide or maleic anhydride deriv-
atives in polar solvent. In order to achieve the stable pH control
by photoirradiation, it is necessary to synthesize the derivative
with water solubility.
1-(4-Methoxymethoxy-2,6-dimethylphenyl)-2,3,3,4,4,5,5-
heptafluorocyclopentene (5). To a 200 mL flask, 70 mL of
THF anhydrous solution containing 5.99 g (2.45 © 10¹2 mol)
of 2-bromo-5-methoxymethyl-1,3-dimethylbenzene was added.
The flask was placed on a dry ice-methanol bath and the solu-
¹2
tion was cooled to ¹78 °C. Then, 18.4 mL (1.6 M, 2.94 © 10
mol) of n-BuLi hexane solution was added gradually and
stirred for 1 h at ¹78 °C. The solution was transferred to
another flask containing 4 mL (1.89 © 10¹2 mol) of octafluoro-
cyclopentene in 100 mL of anhydrous THF kept at ¹78 °C
under argon atmosphere via an opposing needle tube. After
stirring at that temperature for 1.5 h, 10 mL of water was added
to the mixture and the mixture was warmed to ambient tem-
perature. To the mixture, 100 mL of ether was added and the
Experimental
General. All the solvents used were of reagent grade and
were distilled before use. Reagents were purchased from

536
Bull. Chem. Soc. Jpn. Vol. 87, No. 4 (2014)
Diarylethenes with Phenol Moiety
1
organic layer was washed with water (80 mL © 4). The water
layer was extracted with ether (100 mL © 3), and the combined
organic layer was dried over sodium sulfate, and the solvent
was evaporated in vacuo. The mixture was purified by column
chromatography (hexane) to obtain 7.15 g, 2.00 © 10¹2 mol of
the oily compound 5 in 82% yield. 5: 1H NMR (400 MHz,
CDCl3): ¤ 2.20 (s, 6H), 3.49 (s, 3H), 5.18 (s, 2H), 6.82 (s, 2H).
MS (DI): 358 (M+). Anal. Calcd for C15H13F7O2: C, 50.29; H,
3.66%. Found: C, 50.69; H, 3.71%.
116.5 °C. H NMR (400 MHz, CDCl3): ¤ 2.08 (s, 6H), 2.21
(s, 3H), 4.91 (brs, 1H), 6.49 (s, 2H), 6.87 (s, 1H), 7.18-7.31 (m,
5H). Anal. Calcd for C24H18F6OS: C, 61.53; H, 3.87%. Found:
1
C, 61.35; H, 3.99%. 1k: H NMR (400 MHz, CDCl3): ¤ 1.32
(s, 3H), 1.70 (s, 3H), 2.36 (s, 3H), 2.37 (d, J = 15 Hz, 1H),
3.03 (d, J = 15 Hz, 1H), 6.21 (s, 1H), 6.91 (s, 1H), 7.45-7.65
(m, 5H).
1-(4-Hydroxy-3,5-dimethylphenyl)-2-(2,5-dimethyl-3-
thienyl)-3,3,4,4,5,5-hexafluorocyclopentene (2o).
Com-
1-(4-Methoxymethoxy-2,6-dimethylphenyl)-2-(2-methyl-
5-phenyl-3-thienyl)-3,3,4,4,5,5-hexafluorocyclopentene (8).
A 500 mL reaction flask containing 150 mL of anhydrous
THF solution and 4.95 g (1.95 © 10¹2 mol) of 3-bromo-2-meth-
yl-5-phenylthiophene (6) was placed in a dry ice-methanol
bath and the solution was kept at ¹78 °C under argon atmos-
phere. To the solution, 14.7 mL (1.6 M, 2.35 © 10¹2 mol) of n-
BuLi hexane solution was gradually added and stirred for 1 h at
that temperature, followed by the addition of THF anhydrous
solution (50 mL) containing 7.00 g (1.95 © 10¹2 mol) of 5.
After stirring for 3.5 h at the temperature, 10 mL of water was
added and left to warm to ambient temperature. To the reaction
mixture, 100 mL of ether was added, and the organic layer was
washed with water (80 mL © 4). The water layer was extracted
with ether (80 mL © 3), the combined organic layer was dried
over sodium sulfate, and the solvent was evaporated in vacuo.
The mixture was purified by column chromatography (hexane:
dichloromethane = 4:1 v/v) to obtain 3.64 g, 7.10 © 10¹3 mol
of the compound 8 in 36% yield. 8: mp 59.4-61.2 °C. 1H NMR
(400 MHz, CDCl3): ¤ 2.18 (s, 6H), 2.30 (s, 3H), 3.46 (s, 3H),
5.15 (s, 2H), 6.76 (s, 2H), 6.92 (s, 1H), 7.31-7.38 (m, 5H).
Anal. Calcd for C26H22F6O2S: C, 60.93; H, 4.33%. Found: C,
60.61; H, 4.48%.
pound 2o was synthesized from 1.74 g (3.86 © 10¹3 mol) of
9 by the same procedure as used for 1o. After purification,
0.286 g of 2o was obtained in 90% yield.
2o: 1H NMR (400 MHz, CDCl3): ¤ 2.12 (s, 6H), 2.15 (s, 3H),
2.30 (s, 3H), 4.71 (brs, 1H), 6.40 (s, 1H), 6.54 (s, 2H). MS
(FAB): Calcd for C19H16F6OS: 406.0826. Found: 406.0826. MS
(DI): 406 (M+). Anal. Calcd for C19H16F6OS: C, 56.15; H,
1
3.97%. Found: C, 56.20; H, 4.29%. 2k: H NMR (400 MHz,
CDCl3): ¤ 1.25 (s, 3H), 1.62 (s, 3H), 2.26 (d, J = 15 Hz, 1H),
2.33 (m, 6H), 2.96 (d, J = 15 Hz, 1H), 6.18 (s, 1H), 6.30 (s, 1H).
3-Bromo-5-(5,5-dimethyl-1,3-dioxacyclohexan-2-yl)-2-
methylthiophene (10). To a 200 mL reaction flask with a
Dean Stark tube, 1.90 g (9.24 © 10¹3 mol) of 3-bromo-5-meth-
yl-2-thiophenecarbaldehyde, 0.963 g (9.24 © 10¹3 mol) of 2,2-
dimethyl-1,3-propanediol, 0.350 g (9.24 © 10¹4 mol) of p-
toluenesulfonic acid, and 50 mL of toluene were added and
refluxed for 1 h. The solvent was evaporated, and the residue
was purified by column chromatography (chloroform) to obtain
2.190 g, 7.52 © 10¹3 mol of the compound 10 in 81% yield.
10: 1H NMR (200 MHz, CDCl3): ¤ 0.78 (s, 3H), 1.25 (s, 3H),
2.37 (s, 3H), 3.66 (q, J = 16 Hz, 4H), 5.61 (s, 1H), 6.93 (s, 1H).
MS (DI): 290 (M+). Anal. Calcd for C11H15BrO2S: C, 45.37;
H, 5.19%. Found: C, 45.36; H, 5.21%.
1-(4-Methoxymethoxy-2,6-dimethylphenyl)-2-(2,5-di-
1-[5-(5,5-Dimethyl-1,3-dioxacyclohexane-2-yl)-2-methyl-
3-thienyl]-2-(4-methoxymethoxy-2,6-dimethylphenyl)-
3,3,4,4,5,5-hexafluorocyclopentene (11). To a stirred THF
solution (30 mL) containing 11 (1.020 g, 3.56 © 10¹3 mol),
methyl-3-thienyl)-3,3,4,4,5,5-hexafluorocyclopentene
(9).
¹3
The compound 9 was synthesized from 0.450 g (3.86 © 10
mol) of 3-bromo-2,5-dimethylthiophene (7) by the same proce-
dure as 8. After purification, 0.481 g of 9 was obtained in 54%
yield.
¹3
2.45 mL of 1.6 M butyllithium hexane solution (3.91 © 10
mol) was slowly added at ¹78 °C, and the solution was stirred
for 15 min at ¹78 °C. Then, the compound 5 (1.274 g, 3.56 ©
10¹3 mol) was added slowly to the reaction mixture at ¹78 °C,
and which was left stirring at ¹78 °C and then gradually
warmed to 30 °C over 12 h. The reaction mixture was poured
into concentrated sodium chloride solution and extracted with
diethyl ether. The organic phase was dried over anhydrous
magnesium sulfate and evaporated in vacuo. The crude product
was purified by column chromatography on silica gel (hexane:
chloroform = 1:1 v/v) to give 1.458 g of 11 in 74% yield. 11:
1H NMR (200 MHz, CDCl3): ¤ 0.77 (s, 3H), 1.23 (s, 3H), 2.13
(s, 6H), 2.20 (s, 3H), 3.47 (s, 3H), 3.63 (q, J = 15 Hz, 4H), 5.13
(s, 2H), 5.45 (s, 1H), 6.71 (s, 2H), 6.81 (s, 1H). MS (DI): 550
(M+). Anal. Calcd for C26H28F6O4S: C, 56.72; H, 5.13%.
Found: C, 56.40; H, 5.19%.
9: 1H NMR (200 MHz, CDCl3): ¤ 2.14 (s, 9H), 2.29 (s, 3H),
3.47 (s, 3H), 5.15 (s, 2H), 6.14 (s, 1H), 6.73 (s, 2H). MS (DI):
450 (M+). Anal. Calcd for C21H20F6O2S: C, 56.00; H, 4.48%.
Found: C, 55.93; H, 4.51%.
1-(4-Hydroxy-2,6-dimethylphenyl)-2-(2-methyl-5-phenyl-
3-thienyl)-3,3,4,4,5,5-hexafluorocyclopentene (1o).
To a
200 mL reaction flask, 1.0 g (1.95 © 10¹3 mol) of 8 and 75 mL
of methanol was added and heated to 62 °C (refluxing). To this
flask, a drop of concd sulfuric acid was added every 10 min
while checking the progress of the reaction by TLC. After the
addition of 9 drops of sulfuric acid, the spot of the starting
compound disappeared on TLC. After the reaction mixture was
cooled to ambient temperature, 80 mL of ether was added and
the organic layer was washed with water (100 mL). The water
layer was extracted with ether (80 mL © 5), the combined
organic layer was dried over sodium sulfate, and the solvent
was evaporated in vacuo. The mixture was purified by column
chromatography (hexane:dichloromethane = 4:1 v/v followed
by dichloromethane 100%) to obtain 0.82 g, 1.75 © 10¹3 mol
of the compound 1o in 90% yield. After recrystallization from
hexane, 0.60 g of 1o was obtained in 66% yield. 1o: mp 115.4-
1-(2-Formyl-5-methyl-3-thienyl)-2-(4-hydroxy-2,6-di-
methylphenyl)-3,3,4,4,5,5-hexafluorocyclopentene
(3o).
The compound 3o was synthesized from 0.930 g (1.69 ©
10¹3 mol) of 11 by the same procedure as used for 1o. After
purification, 0.286 g of 3o was obtained in 40% yield. 3o:
1H NMR (400 MHz, CDCl3): ¤ 2.12 (s, 6H), 2.35 (s, 3H), 5.36
(brs, 1H), 6.56 (s, 2H), 7.44 (s, 1H), 9.69 (s, 1H). MS (DI): 420

T. Yamaguchi et al.
Bull. Chem. Soc. Jpn. Vol. 87, No. 4 (2014)
537
Table 2. X-ray Crystallographic Data of 3o, 2k, and 4o
References
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2
M. Irie, Chem. Rev. 2000, 100, 1685.
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3o
2k
4o
Formula
Formula weight
Temp/K
Crystal system
Space group
Unit cell dimensions
a/¡
C19H14F6O2S C19H16F6OS C19H16F4O2S
420.36
93(2)
Monoclinic Monoclinic triclinic
P2(1)/c
Wiley VCH, Weinheim, 2011. doi:10.1002/9783527634408.
406.38
93(2)
384.39
93(2)
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91.1500(10) 133.0140(10) 109.560(2)
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¹3
Density(calcd)/g cm 1.503
Goodness-of-fit on F2 1.059
Final R indices [I/2·(I)]
1.611
1.032
R1
wR2
0.0359
0.0858
0.0303
0.0766
0.0421
0.0991
R indices (all data)
R1
wR2
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0.0880
0.0324
0.0783
0.0513
0.1033
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Found. C, 54.15; H, 3.43%. 3k: H NMR (400 MHz, CDCl3):
¤ 1.31 (s, 3H), 1.68 (s, 3H), 2.34-2.38 (m, 4H), 3.01 (d, J =
14 Hz, 1H), 6.33 (s, 1H), 7.20 (s, 1H), 9.93 (s, 1H).
3-(2,5-Dimethyl-3-thienyl)-4,4,5,5-tetrafluoro-2-(4-hy-
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1
droxy-2,6-dimethylphenyl)cyclopent-2-enone (4o).
To a
stirred methanol solution (80 mL) containing 2o (0.129 g,
0.317 © 10¹3 mol) was irradiated with 313 nm light for 15 h.
After the solution was held in the dark for 2 h, the solution was
evaporated. The residue was purified by column chromatog-
raphy on silica gel (hexane:ethyl acetate = 4:1 v/v), and the
compound was purified by HPLC column chromatography
(column: Wakosil 5SIL, hexane:ethyl acetate = 4:1 v/v) to
give 0.029 g of the compound 4o in 24% yield.
4o: 1H NMR (400 MHz, CDCl3): ¤ 1.92 (s, 3H), 1.93 (s, 6H),
2.37 (s, 3H), 4.80 (brs, 1H), 6.55 (s, 2H), 6.69 (s, 1H). MS
(FAB): Calcd for C19H16F4O2S: 384.0807. Found: 384.0807.
MS (DI): 384 (M+). Anal. Calcd for C19H16F4O2S: C, 59.37;
H, 4.20%. Found: C, 59.41; H, 4.24%.
X-ray Crystal Structure. Crystallographic data for the
structural analysis of compounds 3o, 2k, and 4o (Table 2) have
been deposited at Cambridge Crystallographic Data Centre
as Nos. CCDC 928989, 928990, and 928991, respectively.
These data can be obtained free of charge by contacting The
Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge, CB2 1EZ, U.K.; fax: +44 1223 336033; E-mail:
deposit@ccdc.cam.ac.uk; or http://www.ccdc.cam.ac.uk/conts/
retrieving.html.
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Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P.
Supporting Information
The thermal stability data of newly synthesized compounds
are included in Supporting Information. This material is availa-
ble free of charge on the Web at: http://www.csj.jp/journals/
bscj/.

538
Bull. Chem. Soc. Jpn. Vol. 87, No. 4 (2014)
Diarylethenes with Phenol Moiety
Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg,
M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida,
T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A.
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J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R.
Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
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