Self-assembly of photochromic diarylethenes with amphiphilic side chains: Reversible thermal and photochemical control

Article abstract of DOI:10.1021/jo060505t

(Graph Presented) Diarylethene derivatives with hexaethylene glycol side chains were synthesized and their self-assembling and photochromic reactivity were investigated. The diarylethenes showed photochromism in organic solvents and even in water. The aqueous solution of the compounds turned turbid quickly upon heating. The clouding behavior was investigated using ¹H NMR spectroscopy, dynamic light scattering, and absorption spectroscopy. It was revealed that, in the aqueous solution, the compounds self-assembled into aggregates, and the aggregates were loosened by raising the temperature. The cloud-point temperature of the closed-ring isomer was 5-7°C lower than that of the open-ring isomer. When asymmetric methyl groups were introduced in the amphiphilic side chains, induced circular dichroism (ICD) was observed upon irradiation with UV light in water. This ICD was explained by the difference in the self-assembling behavior between the open- and the closed-ring isomers. It was suggested that the closed-ring isomers assembled into a chiral nanostructure.

Full text of DOI:10.1021/jo060505t

VOLUME 71, NUMBER 20
SEPTEMBER 29, 2006
© Copyright 2006 by the American Chemical Society
Self-Assembly of Photochromic Diarylethenes with Amphiphilic Side
Chains: Reversible Thermal and Photochemical Control
Takashi Hirose,† Kenji Matsuda,*,†,‡ and Masahiro Irie*,†
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu UniVersity and
PRESTO, JST, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
irie@cstf.kyushu-u.ac.jp; kmatsuda@cstf.kyushu-u.ac.jp
ReceiVed March 8, 2006
Diarylethene derivatives with hexaethylene glycol side chains were synthesized and their self-assembling
and photochromic reactivity were investigated. The diarylethenes showed photochromism in organic
solvents and even in water. The aqueous solution of the compounds turned turbid quickly upon heating.
1
The clouding behavior was investigated using H NMR spectroscopy, dynamic light scattering, and
absorption spectroscopy. It was revealed that, in the aqueous solution, the compounds self-assembled
into aggregates, and the aggregates were loosened by raising the temperature. The cloud-point temperature
of the closed-ring isomer was 5-7 °C lower than that of the open-ring isomer. When asymmetric methyl
groups were introduced in the amphiphilic side chains, induced circular dichroism (ICD) was observed
upon irradiation with UV light in water. This ICD was explained by the difference in the self-assembling
behavior between the open- and the closed-ring isomers. It was suggested that the closed-ring isomers
assembled into a chiral nanostructure.
Introduction
core induce the formation of the stacked structure in polar
solvents. When chiral PEG chains are used, chiral nanostructures
can be formed.3 Furthermore, the introduction of such polar side
chains makes it possible for hydrophobic compounds to dissolve
into water.4 Solubility in water is a very important property to
be applied to a biological field such as biosensors or drug
deliveries.
Hydrophobic interaction is the key driving force for the
formation of three-dimensional structures of biopolymers and
also is widely used for the construction of highly organized
nanostructures in materials chemistry.1 A poly(ethylene glycol)
(PEG) side chain and an aromatic core are popular components
for this strategy.2 A flexible polar side chain and a rigid apolar
(2) (a) Hill, J. P.; Jin, W.; Kosaka, A.; Fukushima, T.; Ichihara, H.;
Shimomura, T.; Ito, K.; Hashizume, T.; Ishii, N.; Aida, T. Science 2004,
304, 1481-1483. (b) van Herrikhuyzen, J.; Syamakumari, A.; Schenning,
A. P. H. J.; Meijer, E. W. J. Am. Chem. Soc. 2004, 126, 10021-10027. (c)
Hou, J.-L.; Jia, M.-X.; Jiang, X.-K.; Li, Z.-T.; Chen, G.-J. J. Org. Chem.
2004, 69, 6228-6237. (d) Li, A. D. Q.; Wang, W.; Wang, L.-Q. Chem.s
Eur. J. 2003, 9, 4594-4601. (e) Lee, M.; Cho, B.-K.; Zin, W.-C. Chem.
ReV. 2001, 101, 3869-3892.
† Kyushu University.
‡ PRESTO, JST.
(1) (a) Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore, J. S.
Chem. ReV. 2001, 101, 3893-4012. (b) Nelson, J. C.; Saven, J. G.; Moore,
J. S.; Wolynes, P. G. Science 1997, 277, 1793-1796. (c) Hirschberg, J. H.
K. K.; Brunsveld, L.; Ramzi, A.; Vekemans, J. A. J. M.; Sijbesma, R. P.;
Meijer, E. W. Nature 2000, 407, 167-170.
10.1021/jo060505t CCC: $33.50 © 2006 American Chemical Society
Published on Web 05/16/2006
J. Org. Chem. 2006, 71, 7499-7508
7499

Hirose et al.
SCHEME 1. Photochromism of Diarylethene with PEG Side Chains
SCHEME 2. Synthesis of Compound 1aa
a Reagents and conditions: (a) TsCl, aq NaOH, THF, 95%; (b) HO(CH2CH2O)3H, aq KOH, 90%; (c) TsCl, aq NaOH, THF, 72%; (d) 6, K2CO3, DMF,
quantitative; (e) n-BuLi, dry THF, and then (BuO)3B; (f) 2,5-dibromo-3-methylthiophene, aq Na2CO3, and then Pd(PPh3)4, 70% in two steps; (g) n-BuLi,
dry THF, and then perfluorocyclopentene, 21%.
Diarylethenes are photochromic compounds that are poten-
tially applicable to optical memory media because of the
thermally stable and the fatigue-resistant properties.5 Chiroptical
property is of particular interest because the property can be
used for a nondestructive readout.6 The photochemical con-
rotatory cyclization of the diarylethene produces two enantio-
meric closed-ring isomers ((R,R)- and (S,S)-) originating from
two asymmetric carbon atoms. The stereoselective cyclization
in a chiral environment has been accomplished in solution and
in the single-crystalline phase.7
The stereoselective cyclization is not an exclusive strategy
for the photoswitching of the chiroptical properties. Reversible
formation of chiral structures along with photochromism can
bring about the switching of chiroptical properties. The control
of the self-assembled structures by using various types of
photochromic compounds has been reported. Azobenzene is a
very convenient photoactive molecule to control the self-
assembled structures because it has a large conformational
change along with its cis-trans isomerization.8 Feringa et al.
reported the design of the photoresponsive self-assembling
system by using photochromic diarylethene derivatives.7a The
difference in the preference of the helical structures between
the open- and the closed-isomers controls the structure of the
gel states. In a diarylethene system, the open-ring isomer has a
relatively loose twisted conformation, whereas the closed-ring
isomer has a stiff planar structure. Although the geometrical
structure change of diarylethene is relatively small, the difference
in rigidity and π conjugation associated with photochromism
is anticipated to induce the change in the formation/dissociation
of the self-assembled structures.
In this study, we synthesized diarylethenes having PEG side
chains and investigated their photochromic property and self-
(6) (a) Feringa, B. L.; Delden, R. A. V.; Koumura, N.; Geertsema, E.
M. Chem. ReV. 2000, 100, 1789-1816. (b) Yokoyama, Y. Chem.sEur. J.
2004, 10, 4338-4394. (c) Yokoyama, Y.; Uchida, S.; Yokoyama, Y.;
Sagisaka, T.; Uchida, Y.; Inada, T. Enantiomer 1998, 3, 123-132.
(7) (a) Jong, J. J. D. D.; Lucas, L. N.; Kellogg, R. M.; Esch, J. H. V.;
Feringa, B. L. Science 2004, 304, 278-281. (b) Kose, M.; Shinoura, M.;
Yokoyama, Y.; Yokoyama, Y. J. Org. Chem. 2004, 69, 8403-8406. (c)
Yamamoto, S.; Matsuda, K.; Irie, M. Angew. Chem., Int. Ed. 2003, 42,
1636-1639. (d) Yokoyama, Y.; Shiraishi, H.; Tani, Y.; Yokoyama, Y.;
Yamaghchi, Y. J. Am. Chem. Soc. 2003, 125, 7194-7195. (e) Takeshita,
M.; Yamato, T. Angew. Chem., Int. Ed. 2002, 41, 2156-2157. (f)
Yokoyama, Y.; Hosoda, N.; Osano, Y. T.; Sasaki, C. Chem. Lett. 2001,
30, 1112-1113. (g) Kodani, T.; Matsuda, K.; Yamada, T.; Kobatake, S.;
Irie, M. J. Am. Chem. Soc. 2000, 122, 9631-9637. (h) Yamaguchi, T.;
Inagawa, T.; Nakazumi, H.; Irie, S.; Irie, M. Chem. Mater. 2000, 12, 869-
871. (i) Yamaguchi, T.; Uchida, K.; Irie, M. J. Am. Chem. Soc. 1997, 119,
6066-6071. (j) Yokoyama, Y.; Saito, M. Chirality in Photochromism. In
Chiral Photochemistry; Inoue, Y., Ramamurthy, V., Eds.; Marcel Dekker:
New York, 2004; pp 235-259.
(3) (a) Prince, R. B.; Brunsveld, L.; Meijer, E. W.; Moore, J. S. Angew.
Chem., Int. Ed. 2000, 39, 228-230. (b) Prince, R. B.; Moore, J. S.;
Brunsveld, L.; Meijer, E. W. Chem.sEur. J. 2001, 7, 4150-4154. (c)
Brunsveld, L.; Meijer, E. W.; Prince, R. B.; Moore, J. S. J. Am. Chem.
Soc. 2001, 123, 7978-7984. (d) Barbera´, J.; Iglesias, R.; Serrano, J. L.;
Sierra, T.; de la Fuente, M. R.; Palacios, B.; Pe´rez-Jubindo, M. A.; Va´zquez,
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K. Bull. Chem. Soc. Jpn. 1998, 71, 985-996. (c) Lucas, L. N.; van Esch,
J.; Kellogg, R. M.; Feringa, B. L. Chem. Commun. 1998, 2313-2314. (d)
de Jong, J. J. D.; Lucas, L. N.; Hania, R.; Pugzlys, A.; Kellogg, R. M.;
Feringa, B. L.; Duppen, K.; van Esch, J. H. Eur. J. Org. Chem. 2003, 1887-
1893.
7500 J. Org. Chem., Vol. 71, No. 20, 2006

Self-Assembly of Photochromic Diarylethenes
SCHEME 3. Synthesis of Compound (S,S)-2aa
a Reagents and conditions: (a) 3,4-dihydro-2H-pyran, PPTS, dry CH2Cl2, quantitative; (b) LiAlH4, Et2O, 91%; (c) 6, dry THF, and then PPTS, EtOH,
78%; (d) TsCl, aq NaOH, THF, 75%; (e) 14, K2CO3, DMF, 57%; (f) n-BuLi, dry THF, and then (BuO)3B; (g) 2,4-dibromo-3-methylthiophene, aq Na2CO3,
and then Pd(PPh3)4, 40% in two steps; (h) n-BuLi, dry THF, and then perfluorocyclopentene, 20%.
assembling behavior. The application to the reversible CD and
ORD signal switching has also been examined.
by LiAlH4, and then reacted with tosyl-protected Hxg chain 6
to give chiral side chain 14. Bromothiophene derivatives 9 and
16 were synthesized by Suzuki coupling of 2,4-dibromo-5-
methylthiophene with phenyl boronic acid derivatives having a
PEG chain. To obtain compounds 1a and (S,S)-2a, the bro-
mothiophene derivatives were lithiated and reacted with a
perfluorocyclopentene. The structures of 1a and (S,S)-2a were
confirmed by NMR, MASS spectroscopy, and elemental
analysis.
Results and Discussion
Synthesis. As described before, to employ the hydrophobic
interaction for the self-assembly, the molecule is required to
have amphiphilic side chains and an aromatic core. For the
flexible amphiphilic side chains, a hexaethylene glycol (Hxg)
group was adopted because of the facile preparation and the
solubility in many solvents, including water.4 For the aromatic
photochromic core, 1,2-bis(2-methyl-5-phenyl-3-thienyl)per-
fluorocyclopentene was selected because of the established
photochromic performance. In this manner, amphiphilic di-
arylethene 1a was designed. Chiral sources were introduced in
(S,S)-2a to probe the chiroptical properties (Scheme 1).
Compounds 1a and (S,S)-2a were synthesized according to
Schemes 2 and 3. Tosyl-protected Hxg side chain 6 was
synthesized according to the literature.4 To introduce asymmetric
methyl groups as chiral sources nearby the aromatic core, (S)-
ethyl lactate was protected by tetrahydropyranyl group, reduced
Photochromic Reactions. The photochromic reactivity of
diarylethenes 1 and (S,S)-2 was studied in water, ethyl acetate,
and hexane solution (Figure 1). Compounds 1 and (S,S)-2 show
an excellent photochromic performance in organic solvent and
even in water. Under irradiation with 313-nm light, the colorless
open-ring isomers 1a and (S,S)-2a turned blue, and the blue
color disappeared upon irradiation with 578-nm light. The blue
color is due to the photogenerated closed-ring isomers 1b and
(S,S)-2b. The closed-ring isomers were isolated using reversed-
phase HPLC. The conversion ratios under irradiation with 313-
nm light, which were determined by comparing the spectra of
the isolated closed-ring isomer 1b with that in the photosta-
tionary state, were 90.9, 99.0, and 97.6% in water, ethyl acetate,
and hexane, respectively. The spectroscopic properties were
summarized in Table 1. In aqueous solution, the absorption
spectra showed a structured absorption band, and the conversion
ratio was lower than that in ethyl acetate. Furthermore, absorp-
tion maxima are blue-shifted in water. The spectral shape and
conversion ratio in hexane are similar to that in ethyl acetate.
The unique behavior in water implied the formation of ag-
gregates of compounds 1 and (S,S)-2. The cyclization and the
cycloreversion quantum yields of compound 1 in water were
0.48 (313 nm) and 3.3 × 10-3 (517 nm), respectively.
(8) (a) Murakami, H.; Kawabuchi, A.; Matsumoto, R.; Ido, T.; Na-
kashima, N. J. Am. Chem. Soc. 2005, 127, 15891-15899. (b) Yang, W.-
Y.; Ahn, J.-H.; Yoo, Y.-S.; Oh, N.-K.; Lee, M. Nat. Mater. 2005, 4, 399-
402. (c) Sakai, H.; Orihara, Y.; Kodashima, H.; Matsumura, A.; Ohkubo,
T.; Tsuchiya, K.; Abe, M. J. Am. Chem. Soc. 2005, 127, 13454-13455.
(d) Kadota, S.; Aoki, K.; Nagano, S.; Seki, T. J. Am. Chem. Soc. 2005,
127, 8266-8267. (e) Yagai, S.; Karatsu, T.; Kitamura, A. Chem.sEur. J.
2005, 11, 4054-4063. (f) Sidhaye, D. S.; Kashyap, S.; Sastry, M.; Hotha,
S.; Prasad, B. L. V. Langmuir 2005, 21, 7979-7984. (g) Yagai, S.;
Nakajima, T.; Karatsu, T.; Saitow, K.; Kitamura, A. J. Am. Chem. Soc.
2004, 126, 11500-11508. (h) Sumaru, K.; Kameda, M.; Kanamori, T.;
Shinbo, T. Macromolecules 2004, 37, 4949-4955. (i) Rakotondradany, F.;
Whitehead, M. A.; Lebuis, A.-M.; Sleiman, H. F. Chem.sEur. J. 2003, 9,
4771-4780. (j) Jeong, K.-S.; Chang, K.-J.; An, Y.-J. Chem. Commun. 2003,
1450-1451. (k) Ichimura, K.; Oh, S.-K.; Nakagawa, M. Science 2000, 288,
1624-1626. (l) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata,
H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994,
116, 6664-6676.
Clouding Behavior. Upon heating, the aqueous solutions of
compound 1a and (S,S)-2a turned turbid9 at 40-50 °C (Figure
2). The turbid solutions returned to clear solutions upon cooling
J. Org. Chem, Vol. 71, No. 20, 2006 7501

Hirose et al.
FIGURE 1. Absorption spectra of diarylethenes along with photochromism: (a) 1 in water (2.2 × 10-5 M); (b) 1 in hexane (1.6 × 10-5 M); (c)
1 in ethyl acetate (2.0 × 10-5 M); (d) (S,S)-2 in water (2.0 × 10-5 M); (e) (S,S)-2 in ethyl acetate (1.6 × 10-5 M). Solid line denotes the open-ring
isomer, dashed line denotes the closed-ring isomer, and dotted line denotes the sample in the photostationary state under irradiation with 313-nm
light.
TABLE 1. Absorption Maxima of the Open- and the Closed-Ring Isomers and the Conversion from the Open- to Closed-Ring Isomers under
Irradiation with 313-nm Light
compound
solvent
1
(S,S)-2
water
ethyl acetate
hexane
water
ethyl acetate
open-ring isomer λmax (ꢀ; nm)
closed-ring isomer λmax (ꢀ; nm)
conversion (%)
289 (2.8 × 104)
573 (7.9 × 103)
90.9
296 (4.8 × 104)
592 (2.4 × 104)
99.0
294 (4.5 × 104)
583 (2.2 × 104)
97.6
294 (3.5 × 104)
583 (1.6 × 104)
86.1
297 (5.0× 104)
594 (2.5 × 104)
97.2
to room temperature. This clouding behavior is thermally
reversible and sensitive to temperature change.
The cloud-point temperature of the solutions was determined
for the open-ring isomer and the sample in the photostationary
state under irradiation with 313-nm light by monitoring the
change of the absorbance at 800 nm, where the diarylethene
chromophore has no absorption. Figure 3 shows the absorption
spectral changes upon heating and the plot of the absorbance
at 800 nm against temperature for compound 1. At higher
temperatures, both absorption bands in the ultraviolet and the
visible region were red-shifted and sharpened. This behavior
suggests the collapse of aggregate upon heating. The spectral
change occurred in a narrow temperature range. The aqueous
solution of the open-ring isomer 1a turned turbid at 47-48.5
°C, while the photostationary-state solution changed at 40-42
°C. Thus, the cloud-point temperature is dependent on the
configuration of the diarylethene units, open- or closed-forms.
The temperature was lowered by about 5-7 °C upon UV
irradiation.
FIGURE 2. Photographs of the H2O solution of compound 1a (a) at
room temperature and (b) at 60 °C.
(9) (a) Aathimanikandan, S. V.; Savariar, E. N.; Thayumanavan, S. J.
Am. Chem. Soc. 2005, 127, 14922-14929. (b) Chen, H.; Li, J.; Ding, Y.;
Zhang, G.; Zhang, Q.; Wu, C. Macromolecules 2005, 38, 4403-4408. (c)
Arotc¸are´na, M.; Heise, B.; Ishaya, S.; Laschewsky, A. J. Am. Chem. Soc.
2002, 124, 3787-3793. (d) Lele, A. K.; Badiger, M. V.; Hirve, M. M.;
Mashelkar, R. A. Chem. Eng. Sci. 1995, 50, 3535-3545.
To investigate the molecular motion in the aggregate, a
1
variable-temperature H NMR measurement was carried out.
At room temperature, broadened peaks were observed in D2O,
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Self-Assembly of Photochromic Diarylethenes
FIGURE 3. Absorption spectral changes of an aqueous solution of compound 1 upon heating. (a) The open-ring isomer (3.0 × 10-5 M) measured
at 20, 46, 47.4, 47.8, 48.5, 50, 60, 70, and 80 °C from bottom to top; (b) the sample in the photostationary state under irradiation with 313-nm light
(2.8 × 10-5 M) measured at 20, 40, 41, 42, 44.2, 46.2, 50, 60, 70, and 80 °C from bottom to top; (c) the plot of absorbance at 800 nm against
temperature. The open-ring isomer (circle plot), the sample at the conversion ratio of 50% (delta plot), and the sample in the photostationary state
under irradiation with 313-nm light (crisscross plot).
FIGURE 5. The size distribution of the open-ring isomer 1a in water
(2 × 10-5 M) at 30 °C (solid line) and heated to 70 °C (dashed line).
To obtain supporting information about the aggregated
structure, AFM measurements were performed (Figure 6). The
1
observed heights of the aggregates were 30-50 nm. Considering
that these aggregations were measured in the absence of solvent,
the observed size is substantially compatible with DLS measure-
ments.
FIGURE 4. Variable-temperature H NMR spectra of compound 1a
in D2O (a) at 20 °C and (b) at 60 °C.
while corresponding peaks in CDCl3 were sharp. When the D2O
solution was heated, the broadened peaks turned sharp. The
peaks appeared at about 3.5-4.5 ppm and at about 7.0-8.0
ppm and were assignable to the protons of the Hxg side chain
and the diarylethene core, respectively (Figure 4). It should be
noted that the compounds in the D2O solution at high temper-
ature showed sharp peaks even though the solution was turbid.
This result indicates that the aggregated structures formed at
low temperature were collapsed upon heating, and the collapsed
loosened structures should cause the light scattering.
Dynamic Light Scattering (DLS) and Atomic Force
Microscopy (AFM) Measurements. To examine the size
distribution of the aggregates of compounds 1 and (S,S)-2, DLS
measurements were carried out (Figure 5). The peak corre-
sponding to the size of around 100 nm was observed in water.
However, in ethyl acetate, the light scattering signal was too
weak to obtain the size distribution. Both the diarylethene core
and the PEG side chains are soluble in ethyl acetate. Therefore,
compounds 1 and (S,S)-2 should be molecularly dispersed in
ethyl acetate. The peak in water at room-temperature turned
sharp when the solution was heated. The transition temperature
was 45-50 °C for the open-ring isomer 1a and 42-46 °C for
the photostationary-state sample. The observed transition tem-
perature agreed well with the temperature obtained by 1H NMR
and UV-vis absorption spectra.10
Switching of the CD and ORD Spectra. A CD spectral
measurement is the most convenient way to detect chiral
structural change, because the spectrum reflects supramolecular
structures very sensitively. CD spectral changes along with
photochromism were measured for compound (S,S)-2 in water
and in ethyl acetate. No CD signal was observed before and
after the photocyclization reaction in ethyl acetate, where (S,S)-2
is molecularly dispersed. In water, the colorless open-ring isomer
(S,S)-2a did not show any CD signal, while intense CD signal
was observed upon UV irradiation (Figure 7). To know the
origin of the CD signal, diastereomeric excess (de) of the
photoproduct was measured using analytical HPLC, but de was
not discerned in the product.11 This indicates that the CD signal
is not due to the stereoselective cyclization. The CD signal of
(S,S)-2b in water corresponding to the absorption band of the
closed-ring isomer in the visible region shows an exciton-
splitting type with a positive first Cotton effect around 670 nm
and a negative second Cotton effect around 530 nm. The
observed CD signal is due to the intermolecular torsion of the
transition moments in the visible absorption band. It is known
(10) Though the change of the size distribution associated with photo-
irradiation was measured for the compound (S,S)-2, no significant change
was perceived. See Supporting Information.
(11) See Supporting Information.
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Hirose et al.
FIGURE 6. Tapping mode AFM image of the open-ring isomer 1a on mica in the absence of solvent.
FIGURE 7. CD spectra of (S,S)-2 (a) in water (2.6 × 10-5) and (b) in ethyl acetate (1.6 × 10-5). Red line, the open-ring isomer; blue line, the
closed-ring isomer; green line, the sample in the photostationary state under irradiation with 313-nm light.
FIGURE 8. Model of the self-assembled nanostructure of (S,S)-2 in water. (a) A nanostructure of the open-ring isomer and (b) that of the closed-
ring isomer. Only the nanostructure composed of the closed-ring isomers has a chiral property. The orange arrows show the torsion of the transition
moments along the long axis of the diarylethene chromophore.
that the transition moment of the closed-ring isomer of a
diarylethene exists along the long axis of the molecule.12 The
self-assembled structure model that explains the results is shown
in Figure 8. In this figure, the hydrophobic diarylethene moiety
is stacked with torsion of their transition moment and surrounded
by an amphiphilic PEG side chain. Thus, these CD spectral
changes indicate that the nanostructure of the photogenerated
(S,S)-2b in water has a chiral environment and that the
nanostructural chirality can be switched upon photoirradiation.
The induced CD signal intensity depends on the twist sense
bias of the transition moment and the distance between the two
chromophores. The twist sense bias and the distance are
expected to depend on the interaction between the chro-
mophores, which regulate the relative conformation of the
chromophores. The closed-ring isomer has a planar structure
so that the stacked structure is favorably formed by the
solvophobic effect. On the other hand, the open-ring isomers
have a bulky twisted conformation, which prevents the formation
of the stacked structure. The difference is considered to cause
the different CD spectra between the open- and closed-ring
isomers.
(12) Kobatake, S.; Shibata, K.; Uchida, K.; Irie, M. J. Am. Chem. Soc.
2000, 122, 12135-12141.
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Self-Assembly of Photochromic Diarylethenes
FIGURE 9. CD spectra of (S,S)-2 after separating the diastereomers (a) in water (2.6 × 10-5) and (b) in ethyl acetate (1.4 × 10-5). Red line, the
open-ring isomer; blue line, the closed-ring isomer; green line, the sample in the photostationary state under irradiation with 313-nm light; black
line, the closed-ring isomer of the other diastereomer.
FIGURE 10. CD spectra of (S,S)-2 in water and the calculated spectra. (a) Green dashed line [A], original CD spectrum of a diastereomer (the
blue line in Figure 9a); black dashed line [B], original CD spectrum of the other diastereomer (the black line in Figure 9a); red solid line [C], the
calculated spectrum by averaging A and B; light blue solid line [D], original spectrum of the racemic closed-ring isomer (the blue line in Figure
7a). (b) Calculated CD spectra in water corresponding to Figure 9b. Green solid line, the calculated spectrum by subtracting D from A; black solid
line, subtracting D from B.
To investigate the effect of the chiral source at the reactive
carbon, two diastereomers, (S,S,S,S)-2b and (S,R,R,S)-2b, were
separated by chiral HPLC, and the CD spectra were measured
as shown in Figure 9. Figure 9b shows the results in ethyl
acetate. The separated diastereomers showed strong CD signals
without exciton coupling. The CD spectra were similar to those
reported for the enantiomerically separated closed-ring isomers
of the diarylethenes.7 The diastereomers showed signals with
the same intensity and opposite signs. After irradiation with
visible light, the CD signals disappeared, and successive
irradiation with UV light did not regenerate the CD signals.
This result is consistent with the fact that stereoselective
cyclization does not take place, and compound (S,S)-2 is
molecularly dispersed in ethyl acetate. Thus, the CD signal
originating from asymmetric carbon atoms at the reactive center
observed in ethyl acetate does not have exciton splitting.
Figure 9a shows the spectra in water. In this case, the spectra
of the two diastereomers were not mirror images. Upon
irradiation with visible light, the CD signals of each diastereomer
disappeared in a similar manner as in ethyl acetate. Successive
irradiation with UV light gave the exciton-coupled CD signal
identical with that observed before separation of the diastere-
omers. Regardless of the starting diastereomers, (S,S,S,S)-2b
and (S,R,R,S)-2b, the regenerated spectra were identical,
suggesting the racemization took place when the sample was
converted to the open-ring isomer.
components should have canceled out each other when the
spectra was added together. Interestingly, calculated averaged
spectrum (spectrum C in Figure 10a: [A + B]/2, where A is
the green dashed line, and B is the black dashed line) reproduced
the exciton-splitting shape of the observed spectrum before
separation of diastereomers in water (spectrum D in Figure 10a).
Furthermore, a pair of CD signals with the same intensity and
opposite signs were obtained when subtracting spectrum D from
each original spectra A and B (Figure 10b). The subtraction
enables the exclusion of the effect of the exciton-coupled signal
that was always observed in aqueous solution as a result of the
assembled structure. The resulting signals with mirror symmetry
are ascribed to the genuine component of the reactive-center
chirality in the spectra of (S,S)-2b measured in water.
These facts support that the CD spectra of the diastereomer
in water consist of two different components: one is an exciton-
coupled CD signal originating from a chiral environment of the
self-assembled nanostructure; the other is a signal without
exciton splitting originating from asymmetric carbon atoms at
the reactive center.
In general, photochromic reactions have no light-intensity
threshold. Even very weak light can induce the reaction.
Therefore, when used in optical memory, the memory is
destroyed during readout with weak reading light. The destruc-
tive readout can be avoided by using optical rotation as a
detection signal, because the refractive index change can be read
with light longer than the absorption band. To evaluate the
possibility of a nondestructive readout, ORD spectral change
was measured for compound (S,S)-2 in water (Figure 11). The
The CD signals originating from the chirality of the reactive
center, which has no exciton splitting, should have the same
intensity and opposite signs even in water, so that these
J. Org. Chem, Vol. 71, No. 20, 2006 7505

Hirose et al.
As described before, in the photocyclization reaction, no
diastereoselection was observed. This also suggests that the
intermolecular interaction is weak. When the intermolecular
interaction is strong, the conformation of the open-ring isomer
would be restricted to (P)-helix or (M)-helix, which would lead
to stereoselective cyclization. In turn, the stable aggregates,
which are formed by strong intermolecular interactions, would
have less sensitivity to external stimuli, such as light or
temperature.
Conclusions. In this study, we have synthesized diarylethene
derivatives with Hxg side chains as an amphiphilic group and
investigated their photochromic properties and self-assembling
behavior. The compounds 1 and (S,S)-2 were molecularly
dispersed in common organic solvents such as ethyl acetate,
however, they self-assembled into nanostructures in water. The
aqueous solution turned turbid in a narrow temperature range
upon heating, and the cloud-point temperature is dependent on
the configuration of the diarylethene units, open- or closed-
forms. The temperature decreased as much as 5-7 °C upon
UV irradiation. When an asymmetric methyl group was
introduced nearby the aromatic core, induced circular dichroism
(ICD) was observed for the closed-ring isomer. Though both
isomers form the self-assembled nanostructures around 100 nm,
only the closed-ring isomer has a chiral environment. The chiral
environmental change between the open- and the closed-ring
isomers enables reversible photocontrol of the ICD by irradiation
with the appropriate light. This concept gives us a new strategy
for the photoswitching of the chiroptical properties. Although
the solid-state system is ideal, the results reported here can be
a new candidate for the nondestructive readout method without
stereoselective cyclization.
FIGURE 11. ORD spectra of (S,S)-2 in water (6.6 × 10-5 M). The
black line denotes the open-ring isomer, and the red line denotes the
sample in the photostationary state under irradiation with 313-nm light.
FIGURE 12. The plot of molar ellipticity at 670 nm against the mole
percentage of (S,S)-2.
change of ORD intensity was observed even at 800 nm, where
the closed-ring isomer of the diarylethene chromophore has no
absorption, and the change was completely reversible by
photoirradiation. This result indicates that a nondestructive
readout is possible by the detection of optical rotation using
the light of long wavelength, which does not induce the
photochromic reaction.
Experimental Section
A. Materials. Compounds 4, 5, and 6 were prepared according
to the method reported previously.4 Triethylene glycol monomethyl
ether (3), p-iodophenol (7), and ethyl lactate (10) were purchased
from commercial sources. Compound 12 was prepared from
compound 10 via compound 11, according to the literature.14
p-Iodo-{2-[2-(2-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}ethoxy)-
ethoxy]ethoxy}benzene (8). Hxg monomethyl ether 6 (6.00 g, 13.3
mmol) and p-iodophenol 7 (0.504 g, 4.0 mmol) were added to a
solution of K2CO3 (5.5 g, 40 mmol) in DMF (40 mL). The solution
was stirred for 7 h at 70 °C. After cooling, the mixture was poured
into aqueous HCl (pH ) 2, 200 mL) and extracted with CH2Cl2
(100 mL × 2). The organic layer was washed with water (100 mL
× 3) and brine (100 mL × 1) and dried over magnesium sulfate,
and the solvent was evaporated in vacuo. The crude product was
purified by silica gel column chromatography (ethyl acetate) to yield
8 (2.04 g, 4.09 mmol, quantitatively) as a viscous colorless oil. 1H
NMR (CDCl3, TMS, 200 MHz) δ 3.38 (s, 3H, Me), 3.50-4.20
(m, 24H), 6.70 (d, J ) 9 Hz, 2H, Ar), 7.54 (d, J ) 9 Hz, 2H, Ar);
FAB HRMS (m/z) [M]+ calcd for C19H31IO7, 498.1115; found,
498.1138.
3-Bromo-5-(4-{2-[2-(2-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}-
ethoxy)ethoxy]ethoxy}phenyl)2-methylthiophene (9). To a solu-
tion of compound 8 (300 mg, 0.602 mmol) in dry THF (10 mL)
was slowly added n-butyllithium (BuLi) hexane solution (1.6 M,
0.395 mL, 0.632 mmol) at -78 °C under an argon atomosphere.
The solution was stirred for 1 h at -78 °C. After the addition of
boric acid tri-n-butyl ester (0.242 mL, 0.903 mmol), the reaction
mixture was further stirred for 1 h. The reaction was stopped by
the addition of water. 2,4-Dibromo-5-methylthiophene (231 mg,
“Sergeants and Soldiers” Character. The above results
indicate that (S,S)-2b self-assembled into chiral nanostructures
in aqueous solution. To know the mechanism of the chiral
induction, CD spectra were measured for the mixture of achiral
1 and chiral (S,S)-2. This experiment is the so-called “Sergeants
and Soldiers”3b,13 experiment. If compounds 1 and (S,S)-2
aggregate independently, an equimolar mixture solution should
show a half intensity of the CD signal. On the other hand, if
compound (S,S)-2 assembles into a chiral structure with achiral
compound 1, it is possible that a small amount of optically active
substance can give the chiral environment to the allover system.
Open-ring isomers 1a and (S,S)-2a were dissolved in water,
and a CD spectrum was measured in the photostationary state
under irradiation with 313-nm light. Figure 12 shows the plot
of molar ellipticity at maximum wavelength in the visible region
against the mole percentage of (S,S)-2. Although the molar
ellipticity tends to deviate from the linear relationship, the effect
is very weak. The weak effect implied that the intermolecular
interaction between 1 and (S,S)-2 is insignificant.
(13) (a) van Gestel, J.; Palmans, A. R. A.; Titulaer, B.; Vekemans, J. A.
J. M.; Meijer, E. W. J. Am Chem. Soc. 2005, 127, 5490-5494. (b) Wilson,
A. J.; Masuda, M.; Sijbesma, R. P.; Meijer, E. W. Angew. Chem., Int. Ed.
2005, 44, 2275-2279. (c) Brunsveld, L.; Schenning, A. P. H. J.; Broeren,
M. A. C.; Janssen, H. M.; Vekemans, J. A. J. M.; Meijer, E. W. Chem.
Lett. 2000, 292-293.
(14) Chiellini, E.; Galli, G.; Carrozzino, S. Macromolecules 1990, 23,
2106-2112.
7506 J. Org. Chem., Vol. 71, No. 20, 2006

Self-Assembly of Photochromic Diarylethenes
0.903 mmol), Pd(PPh3)4 (34.8 mg, 0.030 mmol), and aqueous
Na2CO3 (20 w %, 10 mL) were added to the solution. The solution
was refluxed for 16 h. The reaction product was extracted with
ethyl acetate (× 2), and the organic layer was washed with brine
(× 3), dried over MgSO4, filtered, and evaporated. The crude
product was purified by silica gel column chromatography (ethyl
acetate) to yield 9 (230 mg, 0.42 mmol, 69.8%) as a yellow oil. 1H
NMR (CDCl3, TMS, 200 MHz) δ 2.40 (s, 3H, Me), 3.38 (s, 3H,
Me), 3.52-4.20 (m, 24H), 6.91 (d, J ) 9 Hz, 2H, Ar), 6.98 (s,
1H), 7.42 (d, J ) 9 Hz, 2H, Ar); FAB HRMS (m/z) [M]+ calcd for
C24H35BrO7S, 546.1287; found, 546.1284.
(S)-p-Iodo-(2-{2-[2-(2-{2-[2-(2-methoxy-ethoxy)-ethoxy]-
ethoxy}-ethoxy)-ethoxy]-ethoxy}-propoxy)-benzene (15). The
compound 14 (0.80 g, 1.58 mmol) and p-iodophenol (0.41 g, 1.74
mmol) was added to a solution of K2CO3 (2.14 g, 15.5 mmol) in
DMF (15 mL). The solution was stirred for 7 h at 70 °C. After
cooling, the mixture was poured into aqueous HCl (pH ) 2, 200
mL) and extracted with CH2Cl2 (× 2), and the organic layer was
washed with brine (× 3) and dried over MgSO4. The solvent was
evaporated in vacuo. The crude product was purified by silica gel
column chromatography (AcOEt) to yield 15 (0.96 g, 1.78 mmol,
quant.) as a pale red oil. 1H NMR (CDCl3, TMS, 200 MHz) δ 1.29
(d, J ) 6 Hz, 3H, Me), 3.38 (s, 3H, Me), 3.45-4.60 (m, 27H),
6.71 (d, J ) 9 Hz, 2H, Ar), 7.53 (d, J ) 9 Hz, 2H, Ar); FAB
HRMS (m/z) [M]+ calcd for C22H37IO8, 556.1533; found, 556.1532.
(S)-3-Bromo-5-[4-(2-{2-[2-(2-{2-[2-(2-methoxy-ethoxy)-ethoxy]-
ethoxy}-ethoxy)-ethoxy]-ethoxy}-1-methyl-ethoxy)-phenyl]-2-
methyl-thiophene (16). To a solution of compound 15 (900 mg,
1.67 mmol) in dry THF (30 mL) was slowly added n-butyllithium
hexane solution (1.6 M, 1.15 mL, 1.84 mmol) at -78 °C under an
argon atomosphere. The solution was stirred for 1 h at -78 °C.
After the addition of boric acid tri-n-butyl ester (0.672 mL, 2.51
mmol), the reaction mixture was further stirred for 1 h. The reaction
was quenched by the addition of water. 2,4-Dibromo-5-methyl-
thiophene (642 mg, 2.51 mmol), Pd(PPh3)4 (96.5 mg, 0.084 mmol),
and aqueous Na2CO3 (20 w %, 30 mL) were added to the solution.
The solution was refluxed for 12 h. The reaction product was
extracted with AcOEt (× 2), and the organic layer was washed
with brine (× 3), dried over MgSO4, filtered, and evaporated. The
crude product was purified by silica gel column chromatography
(AcOEt) to yield 16 (400 mg, 0.66 mmol, 39.5%) as a pale yellow
1,2-Bis[5-(4-{2-[2-(2-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}-
ethoxy)ethoxy]ethoxy}phenyl)2-methylthiophene-3-yl]perfluo-
rocyclopentene (1). To a solution of compound 9 (383 mg, 0.678
mmol) in dry THF (5 mL) was slowly added n-butyllithium (BuLi)
hexane solution (1.6 M, 0.461 mL, 0.737 mmol) at -78 °C under
an argon atomosphere. The solution was stirred for 1 h at -78 °C.
After the addition of perfluorocyclopentene (0.042 mL, 0.302
mmol), the reaction mixture was further stirred for 1 h at that
temperature. The reaction was stopped by the addition of water.
The reaction product was extracted with ethyl acetate (× 2), and
the organic layer was washed with brine (× 3), dried over MgSO4,
filtered, and evaporated. The crude product was purified by silica
gel column chromatography (ethyl acetate/acetone ) 1:1) to yield
1
1 (70 mg, 0.063 mmol, 20.9%) as a blue oil. H NMR (CDCl3,
TMS, 400 MHz) δ 1.94 (s, 6H, Me), 3.37 (s, 6H, Me), 3.52-3.76
(m, 40H), 3.84-3.90 (m, 4H), 4.12-4.18 (m, 4H), 6.93 (d, J ) 9
Hz, 4H, Ar), 7.16 (s, 2H), 7.45 (d, J ) 9 Hz, 4H, Ar). FAB MS
(m/z) [M]+ 1108.83. Anal. Calcd for C53H70F6O14S2: C, 57.39; H,
6.36. Found: C, 57.19; H, 6.34. UV-vis (H2O) λmax (ꢀ) 289
(28 000); (AcOEt) λmax (ꢀ) 296 (48 000); (hexane) λmax (ꢀ) 294
(45 000).
1
oil. H NMR (CDCl3, TMS, 200 MHz) δ 1.32 (d, J ) 6 Hz, 3H,
Me), 2.40 (s, 3H, Me), 3.38 (s, 3H, Me), 3.52-3.80 (m, 26H), 4.57
(q, J ) 6 Hz, 1H), 6.92 (d, J ) 9 Hz, 2H, Ar), 6.98 (s, 1H), 7.41
(d, J ) 9 Hz, 2H, Ar); FAB HRMS (m/z) [M]+ calcd for C27H41-
BrO8S, 604.1706; found, 604.1728.
(S)-1-{2-[2-(2-{2-[2-(2-Methoxy-ethoxy)-ethoxy]-ethoxy}-
ethoxy)-ethoxy]-ethoxy}-propan-2-ol (13). To a solution of (S)-
2-(tetrahydropyran-2-yloxy)-propan-1-ol 12 (3.02 g, 18.7 mmol)
and p-toluenesulfonic acid Hxg monomethyl ether (12.8 g, 21.1
mmol) in dry THF (100 mL) was added NaH (1.27 g, 52.9 mmol).
The solution was refluxed for 12 h. After cooling, the reaction was
quenched by the addition of water, and the solution was evaporated
to remove THF. The reaction product was extracted with CH2Cl2
(× 2), and the organic layer was washed with brine (× 3), dried
over MgSO4, and evaporated. The crude product was dissolved in
MeOH (50 mL), and then TsOH‚H2O (0.6 g, 3.18 mmol) was added
at 0 °C. The solution was stirred for 4 h at room temperature and
quenched by aqueous NaHCO3. The reaction product was extracted
with AcOEt (× 2), and the organic layer was washed with brine
(× 3), dried over MgSO4, and evaporated. The crude product was
purified by silica gel column chromatography (AcOEt/acetone )
(S,S)-1,2-Bis-[5-(4-{2-[2-(2-{2-[2-(2-{2-methoxy-ethoxy}-ethoxy)-
ethoxy]-ethoxy}-ethoxy)-ethoxy]-1-methyl-ethoxy}-phenyl)-2-
methyl-thiophene-3-yl]-perfluorocyclopentene ((S,S)-2a). To a
solution of compound 16 (200 mg, 0.330 mmol) in dry THF (2
mL) was slowly added n-butyllithium hexane solution (1.6 M, 0.227
mL, 0.363 mmol) at -78 °C under an argon atomosphere. The
solution was stirred for 1 h at -78 °C. After the addition of a
solution of perfluorocyclopentene (0.021 mL, 0.149 mmol) in dry
THF (1.2 mL), the reaction mixture was further stirred for 1 h at
that temperature. The reaction was quenched by the addition of
water. The reaction product was extracted with AcOEt (× 2), and
the organic layer was washed with brine (× 3), dried over MgSO4,
filtered, and evaporated. The crude product was purified by silica
gel column chromatography (AcOEt/acetone ) 1:1) and separative
RP-HPLC (CH3CN/MeOH/H2O ) 9:1:2) to yield (S,S)-2a (50 mg,
1
1:1) to yield 13 (2.0 g, 5.64 mmol, 30.2%) as a colorless oil. H
NMR (CDCl3, TMS, 200 MHz) δ 1.13 (d, J ) 6 Hz, 3H, Me),
3.38 (s, 3H, Me), 3.45-4.02 (m, 27H); FAB HRMS (m/z) [M +
H]+ calcd for C16H35O8+, 355.2332; found, 355.2333.
1
0.041 mmol, 19.5%) as a blue oil. H NMR (CDCl3, TMS, 200
MHz) δ 1.32 (d, J ) 6 Hz, 6H, Me), 1.93 (s, 6H, Me), 3.38 (s, 6H,
Me), 3.45-3.80 (m, 52H), 4.50-4.68 (m, 2H), 6.93 (d, J ) 9 Hz,
4H, Ar), 7.16 (s, 2H), 7.45 (d, J ) 8 Hz, 4H, Ar); FAB MS (m/z)
[M]+ 1224.74. Anal. Calcd for C59H82F6O16S2: C, 57.83; H, 6.74.
Found: C, 57.53; H, 6.70. UV-vis (H2O) λmax (ꢀ) 295 (35 000);
(AcOEt) λmax (ꢀ) 294 (50 000).
Corresponding Closed-Ring Isomer (S,S)-2b (Mixture of
(S,R,R,S)-2b and (S,S,S,S)-2b). 1H NMR (CDCl3, TMS, 200 MHz)
δ 1.33 (d, J ) 6 Hz, 6H, Me), 2.14 (s, 6H, Me), 3.38 (s, 6H, Me),
3.45-3.80 (m, 52H), 4.50-4.68 (m, 2H), 6.57 (s, 2H), 6.94 (d, J
) 9 Hz, 4H, Ar), 7.49 (d, J ) 9 Hz, 4H, Ar); UV-vis (H2O) (ꢀ)
583 (16 000); (AcOEt) (ꢀ) 594 (25 000).
B. Photochemical Measurements. Photoirradiation was carried
out using a 500-W super-high-pressure mercury lamp or a 500-W
xenon lamp. Mercury lines of 313 and 578 nm were isolated by
passing the light through a combination of band-pass filters or sharp-
cut filters and a monochromator. Closed-ring isomers were separated
by reversed-phase HPLC (CH3CN/MeOH/H2O ) 9:1:4, flow )
(S)-Toluene-4-sulfonic Acid 2-{2-[2-(2-{2-[2-(2-Methoxy-
ethoxy)-ethoxy]-ethoxy}-ethoxy)-ethoxy]-ethoxy}-1-methyl-eth-
yl Ester (14). Sodium hydroxide (218 mg, 5.44 mol) was dissolved
in water (0.4 mL). A solution of 13 (1.5 g, 4.23 mmol) in THF
(0.4 mL) was added at 0 °C. Then a solution of p-TsCl (887 mg,
4.65 mmol) in THF (1.0 mL) was slowly dropped into the solution.
After warming to room temperature, the mixture was stirred for 7
h. Water (40 mL) was added to the reaction mixture and acidified
by 6 M H2SO4. The reaction product was extracted with CH2Cl2
(× 2), and the organic layer was washed with brine (× 3), dried
over MgSO4, and evaporated. The crude product was purified by
silica gel column chromatography (AcOEt/acetone ) 1:1) to yield
14 (1.6 g, 3.15 mmol, 74.5%) as a colorless oil. 1H NMR (CDCl3,
TMS, 200 MHz) δ 1.13 (d, J ) 6 Hz, 3H, Me), 2.45 (s, 3H, Ts),
3.38 (s, 3H, Me), 3.45-4.02 (m, 27H), 7.35 (d, J ) 8 Hz, 2H, Ar),
7.80 (d, J ) 8 Hz, 2H, Ar); FAB HRMS (m/z) [M + H]+ calcd for
C23H41O10S+, 209.2420; found, 509.2417.
J. Org. Chem, Vol. 71, No. 20, 2006 7507

Hirose et al.
1.5 mL/min), and diastereomers were separated by chiral HPLC
(hexane/2-propanol/ethanol ) 47:47:6, flow ) 1.5 mL/min).
Quantum yields were determined by using bis(1-benzothiophen-
3-yl)hexafluorocyclopentene in hexane solution as the reference.15
The absorbance of the sample and the reference at the irradiation
wavelength (313 nm) were adjusted to be the same for the
cyclization quantum yield measurement. The reaction rate of the
sample and the reference were measured in the same conditions
and compared. For the cyclization quantum yield measurement,
absorbance (A) at the absorption maximum of the closed-ring isomer
was plotted against time. For the cycloreversion quantum yield
measurement, log(10A - 1) at the absorption of the irradiated
wavelength was plotted against time. The measurement was carried
out three times, and the value was determined by averaging.
C. DLS Measurements. Particle size distribution was measured
on a Nicomp 380ZLS particle sizer equipped with a 785-nm red
laser as the light source, using a fixed angle (90°). The samples
were filtrated by a Millipore Millex membrane filter (0.20 µm)
before measurement. The samples were kept at 30 °C during the
measurement.
D. AFM Measurements. AFM images were acquired with a
Digital Instruments Nanoscope III. Damage to the tip or sample
was minimized by using tapping mode AFM. The aqueous solution
of 1 (ca. 10-5 M, 10 µL) in the photostationary state was dropped
on mica substrate. The sample was left about 6 h in a dark place
and dried in vacuo for about 2 h at room temperature.
Acknowledgment. This work was supported by PRESTO,
JST and by a Grant-in-Aid for Scientific Research (S) (No.
15105006) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
Supporting Information Available: Detailed variable-temper-
1
ature H NMR spectra, the plot of the change of absorbance on
cooling, and the HPLC chart for the separation of diastereomers.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(15) Uchida, K.; Tsuchida, E.; Aoi, Y.; Nakamura, S.; Irie, M. Chem.
Lett. 1990, 63-64.
JO060505T
7508 J. Org. Chem., Vol. 71, No. 20, 2006