Photochromism of a diarylethene having an azulene ring

Article abstract of DOI:10.1021/jo202673z

Diarylethene derivatives incorporating an azulene ring at the ethene moiety were synthesized. One derivative having thiazole rings showed the expected coloration reaction by excitation at 313 nm (to a higher singlet state) but not when excited at 635 nm (

Full text of DOI:10.1021/jo202673z

Article
pubs.acs.org/joc
Photochromism of a Diarylethene Having an Azulene Ring
Jun-ichiro Kitai,† Takao Kobayashi,*,‡ Waka Uchida,§ Makoto Hatakeyama,§ Satoshi Yokojima,∥,⊥
Shinichiro Nakamura,*,⊥ and Kingo Uchida*,†
†Department of Materials Chemistry, Faculty of Science and Technology, Ryukoku University, Seta, Otsu 520-2194, Japan
‡Mitsubishi Chemical Group, Science and Technology Research Center, Inc., 1000 Kamoshida, Yokohama 227-8502, Japan
§Department of Biomolecular Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
∥Tokyo University of Pharmacy and Life Sciences, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan
⊥Nakamura Laboratory, RIKEN Research Cluster for Innovation, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
S
* Supporting Information
ABSTRACT: Diarylethene derivatives incorporating an azulene
ring at the ethene moiety were synthesized. One derivative having
thiazole rings showed the expected coloration reaction by
excitation at 313 nm (to a higher singlet state) but not when
excited at 635 nm (S0 to S1 excitation). The system demonstrates
that the cyclization reaction can be controlled by selective
excitation at different wavelengths of the absorption spectrum.
On the other hand, another derivative having thiophene rings did
not show any photochromism. The results clearly show the
importance of the coplanarity of the system for the photoisomerization.
thus, their photoreactivity is independent of the wavelength of
the incident light. Over the past decade, Miyasaka et al.
observed the extraordinarily high efficient cycloreversion (ring-
opening) reaction of a diarylethene by the multiphoton
excitation using a pulsed laser at high photon densities. They
concluded that the orbital parentage of the electronic state
plays an important role in the enhancement of the cyclo-
reversion reaction.7
Here, we have designed new diarylethene derivatives with an
azulene ring fused to the ethene moiety in which the reactivity
of the cyclization (ring-closing) reaction of the diarylethene can
be controlled by tuning the wavelength of irradiation without
the need for high peak power pulsed excitation. Because the
absorption spectra of the open-ring isomers should be
composed of the absorption of the azulene ring and the aryl
rings, comparison of UV and visible light excitation for
cyclization of the open-ring isomer is interesting to examine.
Such a molecular design in which a photophysically active unit
was introduced to a diarylethene to deliver multiple-
functionality has been described before for example in a
stilbene diarylethene system;8 however, in that case a complete
loss of the photoreactivity of one of the units was observed.
INTRODUCTION
■
Recently, the photoresponsive system in which a photochromic
molecule is combined with the other chromophores has been
paid much attention from the viewpoint of novel photo-
switchable systems.1 More than 100 years have passed since
azulene was first isolated by Piesse from the distillation of
chamomile oil. Azulenes constitute the most well-known class
of polycyclic nonbenzenoid aromatic compounds with their
unusual electronic structure and remarkable colors.2 The blue
color is due to absorption from the S0 to the S1 state. Azulene
shows an extremely weak “normal” S1 → S0 fluorescence and a
significant “anomalous” S2 → S0 fluorescence, which violates
Kasha’s rule3 for emission. There have been several studies4 on
the photophysics of azulene and the mechanism of the
anomalous fluorescence is well understood. The lack of normal
fluorescence from S1 reflects the very fast nonradiative decay
from S1 to S0 (the short lifetime of the S1 state of azulene has
been experimentally and theoretically confirmed), and the
observation of significant fluorescence from S2 is due to the
slow internal conversion from S2 to S1, caused by the relatively
large energy gap between S2 and S1.
In the field of photochromism, Tokumaru et al. introduced
the azulene ring as one of the aromatic rings of a photochromic
azobenzene, which underwent cis to trans one-way isomer-
ization on a triplet manifold.5 However, the specific excited
states were not well applied in the photochromism. In the
family of photochromic compounds, diarylethenes have
excellent photochromic properties with high thermal stability
of both isomers and a high fatigue resistant under multiple
photochromic switching cycles.6 Here, they obey Kasha’s rule;
RESULTS AND DISCUSSION
■
Diarylethenes 1o and 2o (Scheme 1) were newly synthesized
by the Suzuki coupling between five-membered aryl halides and
5,6-dibromoazulene which is prepared by Lemal’s method.9
Received: January 7, 2012
Published: March 12, 2012
© 2012 American Chemical Society
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Scheme 1. Photochromic Reaction of Diarylethenes 1 and 2
Scheme 2. Synthetic Procedure of Derivatives 1o and 2o
Figure 1. (a) Photoinduced spectral changes of diarylethene 1 in
hexane (1.3 × 10−5 M). Inset shows absorption spectrum of 1o in the
500−750 nm range. Solid line: before UV irradiation. Broken line:
after 313 nm UV irradiation for 16 min (1o/1c = 13:87). (b)
Absorption spectrum of 2o in hexane.
The route is shown in Scheme 2. The derivative 1o showed the
photochromism in hexane solution as well as the other
derivatives having thiazole rings as aryl groups,10 and the
spectral changes are shown in Figure 1a. Diarylethene 1o has
two absorption bands; a smaller band corresponding to S0 to S1
excitation of the azulene ring was observed at 594 nm (ε 1.0 ×
103 M−1 cm−1), and an intense band corresponding to a S0 to a
higher level singlet state excitation was observed at 288 nm (ε
9.5 × 104 M−1cm−1). Because of the S0 to S1 excitation band of
the azulene ring, the color of the hexane solution of 1o was
blue. Upon UV (313 nm) irradiation, new absorption bands at
495 nm (ε 1.6 × 104 M−1cm−1) and around 700 nm appeared
(broken line in Figure 1a), and the color changed to brownish
pink. The bands disappeared upon irradiation with visible light
(λ> 500 nm) and regenerated the original spectra, as shown in
the solid line in Figure 1a. To ascertain that the new bands were
attributed to the closed-ring isomer 1c, the photoreaction was
monitored by H NMR spectroscopy. Before UV irradiation,
the two methyl signals of 1o were observed at 2.25 and 2.29
ppm. After UV irradiation, two new methyl signals appeared at
2.11 and 2.15 ppm and disappeared upon irradiation with
visible light (Figure S1, Supporting Information). Compound
1c was thermally stable in hexane solution at room temperature
for 24 h but reverted to 1o with a half-life of 13 h at 70 °C in
the dark. The activation energy for the thermal cycloreversion
was 82.8 kJ/mol (Figure S2, Supporting Information).
The absorption spectrum of 2o in hexane is shown in Figure
1b. Diarylethene 2o has also two absorption bands; the smaller
structured band is observed around 594 nm (ε 1.0 × 103 M−1
cm−1), which is assigned to the S0 to S1 excitation of azulene
ring, and another intense band is observed at 290 nm (ε 9.5 ×
104 M−1 cm−1), which is attributed to the excitation from S0 to
the higher excited singlet state of the molecule. No absorption
spectral changes were observed upon photoirradiation at any
wavelength in hexane or methanol solutions. Considering the
corresponding perfluorocyclopentene derivative,12 the reason
was attributed to the nonplanarity of the structure of 2o due to
the steric repulsion between the two hydrogen atoms on the
azulene and thiophene rings (Figure S3, Supporting Informa-
tion). Kawai et al. have proposed that the coplanarity of
aromatic rings is important in enhancing photoreactivity and
achieved a cyclization quantum yield of 1.0 by introducing
thiazole units which are effective for fixing the conformation of
the three aromatic rings by intramolecular hydrogen bonding.13
Therefore, the difference of the photoreactivity of 1o and 2o is
attributed the planarity of the open-ring isomer structures due
to intramolecular hydrogen bonding.
11
1
The photochemical quantum yield for ring closing was
measured at wavelengths corresponding to the two absorption
bands to ascertain the wavelength dependence of the
cyclization of 1o. The cyclization quantum yield (ϕc) at 313
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nm, which corresponds to the excitation from S0 to the higher
singlet state, was measured to be 1.7 × 10−3, which is much
smaller than that of the corresponding perfluorocyclopentene
analogue (0.32 (at 313 nm) of 1,2-bis(5-methyl-2-phenyl-
thiazol-4-yl)perfluorocyclopentene).10a In contrast, no changes
in the absorption spectrum was observed upon excitation into
the S0 → S1 band (635 nm, 2.45 mW) for 60 h. This shows that
the cyclization quantum yield is estimated to be ≪10−6 and
apparently zero. From these results, the photocyclization
reaction of 1o was controlled by the selection of the wavelength
of excitation. For the cycloreversion reaction, the quantum
yield (ϕo) was determined to be 1.0 × 10−4 at 492 nm. The
value was also much smaller than that of the perfluorocyclo-
pentene analogue (0.02 at 492 nm).10a
To understand why the photochromic quantum yields are so
low, we analyzed the geometries (Figure 2) and electronic
Figure 3. Kohn−Sham orbitals of 1o obtained at the S0 structure. The
S1−S10 TDDFT excitations are expressed mainly by singly excitations
from HOMO-2, HOMO-1, or HOMO to LUMO, LUMO+1, or
LUMO+2. Colored spheres represent each atom, similar to Figure 2.
Geometries are optimized by B3LYP/6-31G(d) in hexane solvent with
PCM.
the accuracy of excitation energies by the B3LYP method in
other diarylethene systems and found the excitation energies by
TD-B3LYP was quite accurate.20 We thus employed B3LYP in
the following calculations. For 1o, the excitation energies by
CAM-B3LYP and M06-2X are blue-shifted in comparison with
those by B3LYP. See the Supporting Information for the
excitation energies of 1o by the CAM-B3LYP and M06-2X
functionals.) The effect of hexane solvent was taken into
account by polarizable continuum model (PCM).21
The relative free energies of each conformer are shown in
parentheses of Figure 2. The notations “parallel” and “anti-
parallel” represent the conformers distinguished by the
geometry (rotational angle) of the two thiophene rings.
Previous studies concerning diarylethene photochromes
showed that interconversion between the two conformers is
much slower than photocyclization,7,11 and photocyclization
only occurs22 by the antiparallel conformer A in a conrotatory
fashion because the distance between the two reactive carbon
atoms of the antiparallel conformer B is larger than 0.4 nm, and
therefore, no possibility exists for the cyclization reaction.23 In
the case of 2o, the free energy of “anti-parallel conformer A” is
higher than those of the other conformers; on the other hand,
in the case of 1o it is the lowest. Therefore, considering the
Boltzmann populations of the conformers which can undergo
ring closure, the calculations rationalize that cyclization can be
expected in 1o but not in 2o.
We measured the fluorescence quantum yield (QF). The
fluorescence spectra of 1o and 2o indicate they fluoresce from
the S2 state, as is the case of the azulene molecule. The
fluorescence quantum yields of 1o and 2o were determined to
be 0.002 and 0.005, respectively, by comparison with the
spectrum of rhodamine B (see the Supporting Information).
Here, the fluorescence quantum yield and the cyclization
quantum yield are given by QF = kRτ and Qclo = kcloτ,
respectively, where an excited state lifetime is given by τ = (kR +
Figure 2. Optimized structures of conformers for 1o (a) and 2o (b).
Colored spheres represent atoms: yellow = sulfur, blue = nitrogen,
gray = carbon, white = hydrogen. For notations of “parallel” and “anti-
parallel” see text. “G” in parentheses indicates the Gibbs free energy
(kcal/mol) relative to that of “anti-parallel conformer A”. Geometries
are optimized by B3LYP/6-31G(d) in hexane solvent with PCM. Free
energies are obtained at 298.15 K.
structures (Figure 3) using density functional theory with the
B3LYP14 exchange-correlation functional and with the 6-
31G(d) basis set15 in Gaussian 09.16 (It has been reported
that the valence excitation wavelengths given by the TD-B3LYP
method agree with the experimental UV/vis spectrum better
than those by the long-range corrected (TD-CAM-B3LYP)17
and meta-GGA(TD-M06-2X)18 functionals.19 We also assessed
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kNR + kclo)−1, the radiative decay rate is kR, the nonradiative
decay rate is kNR, and the cyclization reaction rate is kclo.
Considering that the fluorescence emits from the S2 state and
that the cyclization reaction might start not from the S2 (charge
transfer (Az→DAE/DAE→Az)) state but from a higher Sn
(intra-DAE excitation) state, which will be explained later, no
cyclization reaction from S2 with the small QF means that
kNR(S2→S1) is very large and no fluorescence from Sn(n > 2)
with the small Qclo indicates that kNR(Sn(n > 2)→S1) is quite
large.
The second question concerns why the quantum yield of the
cyclization of 1o depends on the excitation wavelength. We
calculated the vertical excitation energies of 1 as a function of
the C−C distance of two reactive carbon atoms for the
cyclization reaction.24 Since the ground and excited states
around the pericyclic minimum where HOMO and LUMO
energies of DAE cross each other cannot be described by
conventional (TD-)DFT methods, in other words the TDDFT
analysis of the cyclization reaction is valid only from the FC
region before a DAE 1B/2A conical intersection point which
remains unknown in this study, in the following we quite
roughly discuss only the initial stage of cyclization reaction path
of 1 using TDDFT potential energy surfaces (PESs) along the
C−C distance of two reactive carbon atoms.
Figure 5. Kohn−Sham orbitals of 1 obtained at 2.4 Å of C−C
distance. The S1−S10 TDDFT excitations are expressed mainly by
singly excitations from HOMO-2, HOMO-1, or HOMO to LUMO,
LUMO+1, or LUMO+2. Colored spheres represent each atom, similar
to Figure 2. Geometries are optimized by B3LYP/6-31G(d) in hexane
solvent with PCM.
Figure 4 shows the PESs of the ground state S0 and the
vertical excited states of S1−S10. The excitation energies, the
Figure 4. Potential energy surfaces of 1 along C−C distance (distance
of two carbon atoms that form a single bond by photocyclization).
Solid line shows energy of S0 state, and each dot represents energy of
S1−S10 vertically excited. Geometries are optimized by B3LYP/6-
31G(d) in hexane solvent with PCM. Excitation energies are calculated
by time-dependent DFT with B3LYP/6-31G(d) and with PCM.
oscillator strengths, and the Kohn−Sham (KS) orbitals at the
equilibrium geometry of 1o in the ground state (3.67 Å in
Figure 4) as well as at the C−C distances with 3.0, 2.7, and 2.4
Å are summarized in Tables S1−S4 of the Supporting
Information. The KS orbitals in Tables S1−S4 are shown in
Figures 3 and 5 and also Figures S4 and S5 in the Supporting
Information. We found that the S1 Franck−Condon (FC) state
of 1o is mainly due to the HOMO−LUMO transition and
localized around the azulene ring (Figure 3). Since the
character of the S1 state of 1o primarily corresponds to that
of the S1 state (mainly due to the HOMO−LUMO transition)
of azulene (Figure 6) whose S1 lifetime is very short (around 1
ps),4 the S1 lifetime of 1o is considered too short to overcome
the energy barrier of the S1 state for the cyclization reaction
(around 14.0 kcal/mol shown in Figure 4). Regarding the Sn
Figure 6. Kohn−Sham orbitals of azulene molecule. These orbitals
become occupied/unoccupied by the electronic excitation to
fluorescence-emitting S2 state. Geometries are optimized by B3LYP/
6-31G(d) in hexane solvent with PCM.
states above S1, roughly characterized, the S2−S6 states have
charge transfer (CT) (Az→DAE/DAE→Az) excitation char-
acter and the S7−S8 states have intra-DAE excitation character
in the TD-B3LYP results. Both S7 and S8 states, whose energies
are very close to each other, contain a HOMO-1→LUMO+2
excitation configuration which corresponds to a HOMO→
LUMO excitation (1B state) of the DAE part and leads to a
cyclization reaction. Following the TD-B3LYP results, it is
considered that the cyclization reaction of 1o starts not from
the CT (Az→DAE/DAE→Az) excitation but from the intra-
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DAE excitation of the S7 and/or S8 state. Considering that the
TD-B3LYP amplitude of the HOMO-1→LUMO+2 excitation
in the S8 state is larger than that in the S7 state, we assumed that
the cyclization reaction starts from the S8 FC state in the
following. (The excited state characters of 1o at the TD-CAM-
B3LYP and TD-M06-2X levels are shown in Tables S5 and S6
of the Supporting Information.)
showed that the force of the closed-ring isomer of diarylethene
at the Franck−Condon state is correlated with the cyclo-
reversion reaction.25)
In conclusion, we synthesized diarylethene derivatives with
an azulene ring as the ethene moiety, and a derivative having
thiazole rings showed photochromism. By introducing the
azulene ring, the diarylethene gains several excited states, and
the cyclization reaction proceeds from a higher excited state but
not from the lowest state. Theoretical calculations showed that
the cyclization reaction proceeded through a diabatic potential
energy surface from higher electronic states, which are similar
to the excited states of the diarylethenes, and the S1 state and
the lower Sn states correspond to the S1 state of azulene and the
CT(Az→DAE/DAE→Az) states, respectively. A photochromic
system with such selective excitation will be useful for optical
logic gate and nondestructive readout materials.
Now we will see the reactive C−C bond length dependency
of energies and characters of the excited states to make a
qualitative discussion about the initial stage of the cyclization
reaction path of 1o. Excited states which has a character similar
to the S8 FC state of 1o appear in the excited state S6 (HOMO-
1−LUMO+1 transition) at C−C distances of 3.3 and 3.0 Å
(Figures 4 and S4, Supporting Information). Then the excited
states start to mix with the azulene localized S1 state, as seen in
the S4 state (HOMO−LUMO+2 transition) at a C−C distance
of 2.7 Å (Figure S5, Supporting Information) and finally
reaches the S1 state (HOMO−LUMO transition) at a C−C
distance of 2.4 Å with a significant mixture of the azulene S1
state but with the C−C bond character of the LUMO orbital
(Figure 5). Thus, we expect that the cyclization reaction will
proceed through the diabatic potential energy surface indicated
by the arrows after the initial excitation to the S8 state in Figure
4. To further confirm this view, we calculated the forces of 1o
at the S8 FC state and found that they are directed toward the
cyclization reaction (Table 1 and Figure 7). (We previously
EXPERIMENTAL SECTION
■
General Methods. All solvents used were reagent grade and were
distilled before use. 1H NMR spectra were recorded at ambient
temperature. Chemical shifts are denoted in δ units (ppm) relative to
the solvent signals CHCl3 (1H NMR: δ = 7.24 ppm, 13C: δ = 77.0
ppm). Mass spectra (FAB-MS) were obtained by using 3-nitrobenzyl
alcohol and glycerol as the matrix. UV light was irradiated by a hand
lamp SPECTROLINE Model EB-280C/J (λ: 312 nm). Visible light
was irradiated by a 500 W Xe-arc lamp (USHIO SX-Ul501XQ). For
the absorption spectral measurements, optical cells with 1 cm light
pass lengths were used for the absorption spectral measurement of the
solutions. A diode laser (Neoark DMSH-3S, λ 635 nm, 2.45 mW) was
applied for the irradiation of the S0 to S1 excitation of a diarylethene
derivative in hexane solution.
Table 1. Forces in Internal Coordinate (Hartree/Bohr or
Hartree/Radian) for Atoms (N1) of 1o at the S8 State with
a
S0-Optimized Structure
Synthesis. 5,6-Bis(5-methyl-2-phenylthiazol-4-yl)azulene (1o).
To a THF anhydrous solution (20 mL) containing 1.39 g (5.46
mmol) of 4-bromo-5-methyl-2-phenylthiazole (3)10a was
gradually added 4.10 mL (6.55 mmol) of 1.6 N n-BuLi hexane
solution at −78 °C in an argon gas atmosphere followed by
stirring for 1 h at that temperature. Then, 1.77 mL (6.55 mmol)
of B(OBu)3 was added and the mixture stirred at that
temperature for 30 min, followed by stirring at room
temperature. To the reaction mixture were added 5,6-
dibromoazulene (4) (0.60 g, 2.10 mmol), Pd(PPh3)4 (0.14 g,
0.126 mmol), 10.5 mL of 2 M Na2CO3 aqueous solution, and
20 mL of THF, and then the solution was heated by
microwave. The heating was carried out by using a microwave
reactor (Shikoku Instrumentation Co., Ltd.; ZMW-023) for 2 h
with divided irradiation periods of 1 min. In the middle period,
additional Pd(PPh3)4 (0.073 g, 0.063 mmol) was added. After
the reaction was over, 50 mL of water and 20 mL of ether were
added, and the organic layer was separated. The water layer was
extracted with ether (30 mL × 3), and the combined organic
layer was dehydrated over sodium sulfate anhydrous. After the
sodium sulfate was filtered off, the solvent was evaporated then
the residue was purified by column chromatography (hexane/
ethyl acetate = 95:5) to obtain 0.30 g (0.63 mmol) of as a deep-
blue colored oil. Recrystallization from the hexane solution at 5
°C gives 0.076 g of deep-colored crystals of 1o in 7.6% yield.
N1
C5
N2 length/force N3
angle/force
N4
C4
dihedral/force
C1
C5
C2
C7
+1.497
C3
C1
C1
C2
+112.908
+0.018234
+125.361
−0.069662
+120.147
−0.021733
+125.468
−0.055300
−174.912
+0.002191
+116.838
+0.015019
+170.477
+0.014336
−62.337
−0.008598
+1.377
C6
C7
C8
C3
C3
C3
+0.041416
+1.496
−0.002836
+1.377
+0.029992
+0.010420
a
N2, N3, and N4 are the atoms for bond length (N1−N2), bond angle
(N1−N2−N3), and dihedral angle (N1−N2−N3−N4). Positive
(negative) force operates to increase (decrease) the corresponding
internal coordinate. Labels for each atom (C1−C8) correspond to that
of Figure 7. Those values are obtained by B3LYP/6-31G(d) in hexane
solvent with PCM.
1
1o. Mp: 138.1−139.9 °C. H NMR (400 MHz, CDCl3) δ: 2.25 (s,
3H), 2.29 (s, 3H), 7.32−7.35 (m, 6H), 7.43 (d, J = 9.6 Hz, 1H), 7.47
(d, J = 3.7 Hz, 2H), 7.79−7.84 (m, 4H), 7.97 (t, J = 3.8 Hz, 1H), 8.42
(d, J = 9.6 Hz, 1H), 8.62 (s, 1H). 13C NMR (100 MHz, CDCl3) δ:
12.4*, 12.5*, 118.9, 120.0, 125.7, 126.3*, 126.4*(4C), 128.9 (4C),
129.56*, 129.62*(2C), 129.8*, 130.1*, 133.9*, 134.0*, 135.4, 138.0,
139.2*, 139.3*, 139.6(2C), 144.2(2C), 155.15*, 155.23*, 163.0*,
163.3* (signals with marked with an asterisk are split due to
atropisomerization.10 Underlines with (nC) indicate the total number
of carbon atoms for split lines is n). IR (KBr): 753, 830, 1140, 1402,
1440, 1577, 2856, 2916, 2944 cm−1, Anal. Calcd for C30H22N2S2: C,
75.91; H, 4.67; N, 5.90. Found: C, 76.09; H, 4.70; N, 5.84.
Figure 7. Molecular Structure of 1o. Labels for carbon atoms (C1−
C8) correspond to that of Table 1.
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5,6-Bis(2,5-dimethylthiophene-3-yl)azulene (2o). To a THF
anhydrous solution (10 mL) containing 0.83 g (3.50 mmol) of 3-
iodo-2,5-dimethylthiophene (5)26 was gradually added 3.07 mL (4.90
mmol) of 1.6 N n-BuLi hexane solution at −78 °C in an argon gas
atmosphere followed by stirring for 1 h at that temperature. Then, 1.23
mL (4.55 mmol) of B(OBu)3 was added and the mixture stirred at that
temperature for 30 min, followed by additional stirring for 2 h at room
temperature. To the reaction mixture were added 5,6-dibromoazulene
(4) (0.40 g, 1.40 mmol), Pd(PPh3)4 (0.194 g, 0.168 mmol), 7.0 mL of
2 M Na2CO3 aqueous solution, and 10 mL of THF, and then the
solution was refluxed for 14 h. After the reaction was over, 20 mL of
water and 10 mL of ether was added, and the organic layer was
separated. The water layer was extracted with ether (10 mL × 3), and
the combined organic layer was dehydrated over sodium sulfate
anhydrous. After the sodium sulfate was filtered off, the solvent was
evaporated and then the residue was purified by column
chromatography (hexane/ethyl acetate = 95:5) to obtain 0.10 g
(0.29 mmol) of 2o as a deep-blue colored oil in 20.5% yield.
*(K.U.) Tel: +81-77-543-7462. Fax: +81-77-543-7483. E-mail:
uchida@rins.ryukoku.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This study was partially supported by Grants-in-Aids for
Scientific Research on Priority Area ‘‘New Frontiers in
Photochromism (No. 471)’’ from the Ministry of Education,
Culture, Sports, Science, and Technology (MEXT), Japan. All
calculations were performed on TSUBAME, a supercomputer
at the Tokyo Institute of Technology.
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2o. 1H NMR (400 MHz, CDCl3) δ: 2.14 (s, 3H), 2.17 (s, 3H), 2.31
(s, 3H), 2.33 (s, 3H), 6.25 (s, 1H), 6.29 (s, 1H), 7.18 (d, J = 10.4 Hz,
1H), 7.34 (d, J = 3.6 Hz, 1H), 7.38 (d, J = 2.8 Hz, 1H), 7.89 (t, J = 3.6
Hz, 1H), 8.28 (d, J = 10.4 Hz,1H), 8.37 (s, 1H). 13C NMR (100 MHz,
CDCl3) δ: 13.9, 14.0, 15.08, 15.15, 117.8, 118.8, 126.0, 127.7*, 128.1*,
131.7, 134.5(2C), 134.6, 137.3, 138.4, 138.8*, 140.1(2C), 141.4*,
142.0*, 146.6(2C). EI-MS (m/z): 348 (M+). HRMS: calcd for C22 H20
S2 348.1006, found 348.1008. IR (KBr): 601, 689, 760, 848, 971, 1241,
1398, 1435, 1462, 1501, 1514, 1577, 2852, 2916, 3020, 3061 cm−1.
Computational Method. Geometries were optimized by using
B3LYP/6-31G(d) with a polarizable continuum model (PCM) to
consider hexane solvent. Free energies were obtained at 298.15 K.
Excitation energies were calculated by using time-dependent DFT with
B3LYP/6-31G(d) and with PCM under the Franck−Condon
conditions. All calculations were performed by using Gaussian 09.16
Photochromic Quantum Yield Measurement. The quantum
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were determined in hexane by measuring the rate of isomerization in
the initial stage of the reactions, comparing the reaction yields of 1
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irradiating wavelength was controlled at 0.4.
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ASSOCIATED CONTENT
* Supporting Information
■
S
1H NMR spectral changes of diarylethene 1 by alternative
irradiation with UV and visible light, the temperature
dependence of the thermal fading of 1c in hexane, Kohn−
Sham orbitals of 1, calculated excitation energy, oscillator
strength, and the change of orbital occupation for the electronic
vertical excitation of 1o, 1c, 2o, and azulene, Cartesian
coordinates of 1o, 1c, 2o, and azulene, calculated free energies
of 1o and 2o, calculated potential energies of 1o, 1c, and
azulene, fluorescence emission spectra, NMR spectra, elemental
analysis, and HRMS of new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Authors
*(T.K.) E-mail: tkoba@rsi.co.jp.
*(S.N.) E-mail: snakamura@riken.jp.
■
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Article
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NOTE ADDED AFTER ASAP PUBLICATION
■
This paper was published ASAP on March 20, 2012. The
Supporting Information file was updated. The revised paper
was reposted on March 23, 2012.
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