Photochromism of 1,2-Bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene in a single-crystalline phase. Conrotatory thermal cycloreversion of the closed-ring isomer

Article abstract of DOI:10.1021/ja001996g

1,2-Bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a) was found to undergo a thermally reversible photochromic reaction in solution as well as in the single-crystalline phase. Upon irradiation with ultraviolet light the hexane or toluene solution

Full text of DOI:10.1021/ja001996g

J. Am. Chem. Soc. 2000, 122, 12135-12141
12135
Photochromism of
1,2-Bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene in a
Single-Crystalline Phase. Conrotatory Thermal Cycloreversion of the
Closed-Ring Isomer
Seiya Kobatake,† Katsunori Shibata,† Kingo Uchida,‡ and Masahiro Irie*,†
Contribution from the Department of Chemistry and Biochemistry, Graduate School of Engineering,
Kyushu UniVersity, and CREST, Japan Science and Technology Corporation, Higashi-ku,
Fukuoka 812-8581, Japan, and Department of Materials Chemistry, Ryukoku UniVersity, Seta,
Otsu 520-2194, Japan
ReceiVed June 5, 2000. ReVised Manuscript ReceiVed September 27, 2000
Abstract: 1,2-Bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a) was found to undergo a thermally
reversible photochromic reaction in solution as well as in the single-crystalline phase. Upon irradiation with
ultraviolet light the hexane or toluene solution containing 1a and single-crystal 1a turned blue along with the
formation of closed-ring isomer (1b). The blue solution and single crystal returned colorless by irradiation
with visible light (λ > 500 nm) or heating above 100 °C. The bleaching is due to the cycloreversion of 1b to
1a. The thermal cycloreversion activation energies were measured in the toluene solution and in the single-
crystalline phase above 100 °C. The activation energy from photogenerated 1b to 1a in the solution was 128
kJ mol-1, while it decreased to 120 kJ mol-1 in the crystal of the open-ring isomer 1a. In the crystal of the
closed-ring isomer 1b the energy increased to 137 kJ mol-1. The cycloreversion reaction was found to be
controlled by the environmental conditions. The thermal cycloreversion reaction in the crystal was directly
followed by X-ray crystallography and revealed to proceed in a conrotatory mode, which is opposed to the
general Woodward-Hoffmann rules.
Introduction
The thermal stability of the diarylethenes depends on the aryl
groups.8 When the aryl groups are thiophene or benzothiophene
groups, such as 2,3-bis(2-methyl-1-benzothiophen-3-yl)maleic
anhydride, the closed-ring isomers are thermally stable, while
the closed-ring isomers of diarylethenes having phenyl, pyrrolyl,
or indolyl groups, such as 2,3-bis(1,2-dimethyl-3-indolyl)maleic
anhydride, are thermally unstable (half-life time at 80 °C, 3
h).8-11 The thermal stability difference is not due to the steric
hindrance in the thermal disrotatory cycloreversion reaction as
proposed by Heller et al. for furylfulgide,5 because the steric
effect is similar between the above bisbenzothienylethene and
bisindolylethene. The difference is ascribed to the electronic
structural difference. When the aryl groups have low aromatic
stabilization energies, the energy difference between the open-
and closed-ring isomers becomes small and the energy barrier
in the cycloreversion reaction becomes large, such as bisben-
zothienylethene. On the contrary, when the aryl groups have
aromatic stabilization energies, the closed-ring isomers become
thermally unstable, such as bisindolylethene.
Various types of photochromic compounds have been so far
developed in an attempt to apply the compounds to optoelec-
tronic devices.1-4 For the applications, such as optical memories
and photooptical switches, the compounds are required to
undergo thermally irreversible and fatigue-resistant photochro-
mic reactions. In most photochromic compounds, however,
photogenerated isomers are thermally unstable and return to the
initial isomers in the dark. Recently, three types of thermally
irreversible photochromic compounds, furylfulgide,5 diaryl-
ethenes,6 and phenoxynaphthacenequinones,7 have been devel-
oped. Among the compounds, the diarylethenes are the most
promising photochromic compounds for the applications because
of their fatigue-resistant character.4
† Graduate School of Engineering, Kyushu University and CREST.
‡ Ryukoku University.
(1) Photochromism; Brown, G. H., Ed; Wiley-Interscience: New York,
1971.
(2) Photochromism Molecules and Systrems; Du¨rr, H., Bouas-Laurent,
H., Eds.; Elsevier: Amsterdam, 1990.
(3) Applied Photochromic Polymer Systems; McArdle, C. B., Ed.;
Blackie, Glasgow, 1992.
(4) Photo-ReactiVe Materials for Ultrahigh-Density Optical Memory; Irie,
M., Ed.; Elsevier: Amsterdam, 1994.
The above discussion assumes that the cycloreversion reac-
tions proceed in a conrotatory mode. The conrotatory thermal
cycloreversion process is opposite to the general Woodward-
Hoffmann rules12 of electrocyclic reactions of 6-π electronic
(5) (a) Heller, H. G.; Oliver, S. J. Chem. Soc., Perkin Trans. 1 1981,
197. (b) Darcy, P. J.; Heller, H. G.; Strydom, P. J.; Whittall, J. J. Chem.
Soc., Perkin Trans. 1 1981, 202. (c) Yokoyama, Y. Chem. ReV. 2000, 100,
1717.
(6) (a) Irie, M.; Mohri, M. J. Org. Chem. 1988, 53, 803. (b) Irie, M.
Chem. ReV. 2000, 100, 1685.
(7) Buchholtz, F.; Zelichenok, A.; Krongauz, V. Macromolecules 1993,
26, 906.
(8) Nakamura, S.; Irie, M. J. Org. Chem. 1988, 53, 6136.
(9) (a) Nakayama, Y.; Hayashi, K.; Irie, M. J. Org. Chem. 1990, 55,
2592. (b) Nakayama, Y.; Hayashi, K.; Irie, M. Bull. Chem. Soc. Jpn. 1991,
64, 789.
(10) Uchida, K.; Nakamura, S.; Irie, M. Res. Chem. Intermed. 1995, 21,
861.
(11) Uchida, K.; Matsuoka, T.; Sayo, K.; Iwamoto, M.; Hayashi, S.; Irie,
M. Chem. Lett. 1999, 835.
10.1021/ja001996g CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/22/2000

12136 J. Am. Chem. Soc., Vol. 122, No. 49, 2000
Kobatake et al.
Figure 1. Absorption spectral change of 1 in hexane (1.7 × 10-5 M)
by photoirradiation: 1a (- - -), 1 in the photostationary state under
irradiation with 290 nm light (- - -), and 1b (s).
systems. It is indispensable to confirm the conrotatory thermal
cycloreversion process to determine which mechanism, steric
or electronic, plays a role in the thermal stability of the closed-
ring isomers of diarylethenes. In this paper we report on the
photochromism of 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluo-
rocyclopentene (1a) in solution and in a single-crystalline phase.
The thermal stability of the closed-ring isomer 1b was measured
above 100 °C, and the effect of the environment on the rate
was examined. In addition, the thermal cycloreversion process
was directly followed by X-ray crystallography to elucidate
which mode, conrotatory or disrotatory, takes place.
Figure 2. Polarized absorption spectra of the photogenerated blue
crystal: (a) direction of polarizer, (b) polarized absorption spectra, and
(c) the polar plots of the absorbance at 630 nm.
Results and Discussion
Photochromism in Solution. Figure 1 shows the absorption
spectral change of 1a by irradiation with 290 nm light. 1a has
the absorption maximum at 286 nm (ꢀ ) 4.0 × 104 M-1 cm-1
)
respectively. The cyclization quantum yield of 1a was slightly
lower than that of the methyl substituent dithienylethene (2a)
(0.5913).
in hexane. Upon irradiation with 290 nm light, the colorless
hexane solution of 1a turned blue, in which a visible absorption
band was observed at 600 nm. The blue photoproduct was
isolated by HPLC (silica gel; hexane as the eluent), and the
1
structure was analyzed by mass spectrum, H NMR spectrum,
elemental analysis, and X-ray structural analysis. The mass
number and elemental analysis were the same as for 1a. This
indicates that the blue material is an isomer of 1a. The ethyl
protons in the 1H NMR spectrum shifted to lower magnetic field
and the thienyl proton shifted to higher magnetic field, in
comparison with those of 1a. This result indicates that this
product is the closed-ring isomer 1b. The X-ray crystallographic
analysis also confirmed that the product is 1b having ethyl
groups in a trans configuration (see Table 3 and Figure 8).
The absorption spectrum of 1b is also shown in Figure 1.
The spectrum was almost the same as that of the solution in
the photostationary state upon 290 nm light irradiation, indicat-
ing high conversion from 1a to 1b. The ꢀ value of 1b at 600
nm was determined to be 1.7 × 104 M-1 cm-1. The conversion
from 1a to 1b in the photostationary state under irradiation with
290 nm light was 96%. The blue color disappeared upon
irradiation with visible light (λ > 500 nm), and the absorption
spectrum returned to that of 1a.
Photochromism in a Single-Crystalline Phase. A plate
single crystal having a hexagonal surface was obtained by
recrystallization of 1a from hexane. The hexagonal surface had
angles of two 92° and four 134°, as shown in Figure 2a. Single-
crystal 1a turned blue upon irradiation with 366 nm light. The
light could penetrate the crystal as deep as 400 µm and change
the color, as shown in Figure 3. The photogenerated blue crystal
was dissolved into hexane and the absorption spectrum was
measured. The absorption maximum in the visible region was
the same as that of 1b shown in Figure 1. The crystal of the
isolated colored photoproduct was confirmed to be 1b having
ethyl groups in a trans configuration by X-ray crystallographic
analysis. This indicates that the blue color is due to the formation
of 1b. The blue color of the crystal disappeared upon irradiation
with visible light (λ > 500 nm).
The cyclization and cycloreversion quantum yields were
determined to be 0.52 (286 nm) and 0.0081 (600 nm),
(12) The ConserVation of Orbital Symmetry; Woodward, R. B.; Hoff-
mann, R.; Verlag Chemie: Weinheim, 1970.
(13) Irie, M.; Lifka, T.; Kobatake, S.; Kato, N.; Irie, M. J. Am. Chem.
Soc. 2000, 122, 4871.

1,2-Bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene
J. Am. Chem. Soc., Vol. 122, No. 49, 2000 12137
Figure 4. ORTEP drawings of top (a) and side (b) views of 1a showing
50% probability displacement ellipsoids. Only a half of the molecule
is independent. The CF2 groups in the cyclopentene ring were disordered
equally. The ethyl groups were also disordered in the ratio of 76:24.
Only the major structure is illustrated for clarity.
Figure 3. Photographs of top (a) and side (b) views of crystal 1a
irradiated with 366 nm light.
Table 1. Crystal Data and Structure Refinement for 1a
empirical formula
formula weight
temperature
crystal system
space group
C29H22F6S2
548.59
80(2) K
orthorhombic
Pbcn
unit cell dimensions
a ) 22.277(3) Å
b ) 10.951(2) Å
c ) 10.569(2) Å
2578.3(7) Å3
4
volume
Z
density (calculated)
goodness-of-fit on F2
final R indices [I > 2σ(I)]
1.413 g/cm3
1.092
R1 ) 0.050
wR2 ) 0.120
R1 ) 0.066
wR2 ) 0.127
R indices (all data)
Figure 5. Packing diagram of 1a (a) and the typical molecular structure
of packed 1a (b). The arrow showed the maximum direction of the
polar plots shown in Figure 2.
To confirm that the photochromic reaction proceeds in the
crystal lattice, the coloration of the crystal was observed under
linearly polarized light, as shown in Figure 2b. Before photo-
irradiation the crystal was colorless. Upon irradiation with 366
nm light, the crystal turned blue at a certain angle (θ ) 90°).
When the crystal was rotated as much as 90°, the color almost
disappeared. The blue color intensity at 630 nm changed by
rotating the crystal sample, as shown in Figure 2c. This indicates
that the closed-ring isomers are regularly oriented in the crystal.
In other words, the photochromic reaction proceeds in the crystal
lattice. If the cyclization reaction proceeded in an amorphous
phase, the color intensity change should not be observed.
X-ray Crystallographic Analysis of 1a. Table 1 shows the
X-ray crystallographic analysis data of 1a. Figure 4 shows the
ORTEP drawings of 1a. In solution diarylethenes have two
conformations, parallel and antiparallel, and they interconvert.
In crystals, on the other hand, there is no exchange between
the two conformers. The ORTEP drawing of 1a indicates that
1a is packed in an antiparallel conformation in the crystal. The
distance between the reacting carbon atoms was estimated to
be 0.371 nm, which is close enough for the reaction.14
hexagonal surface. The arrow in Figure 5b shows the maximum
direction of the 630 nm absorption band of the closed-ring
isomer estimated from the absorption anisotropy. The direction
of the arrow coincides with the direction of the long axis of the
open-ring isomer. The photocyclization reaction proceeded in
the crystal lattice of the open-ring isomer 1a without changing
the center of gravity of the molecule.15
Thermal Stability of 1b. Diarylethenes having 2-methyl-
thiophene or benzothiophene aryl groups undergo thermally
irreversible photochromic reactions even at 100 °C. In a previous
paper13 the half-life time of the closed-ring isomer of 2a was
estimated to be 1900 years at 30 °C. Introduction of bulky
substituents at the 2- and 2′-positions of the thiophene or
benzothiophene aryl groups is expected to change the thermal
stability of the closed-ring isomer. Introduction of isopropyl
substituents at the 2- and 2′-positions of benzothiophene aryl
groups decreased the thermal stability.16 1a has ethyl substituents
The hexagonal surface corresponds to the (100) plane. Figure
5a shows the molecular packing of 1a viewed from the
(15) Kobatake, S.; Yamada, T.; Uchida, K.; Kato, N.; Irie, M. J. Am.
Chem. Soc. 1999, 121, 2380.
(16) Kobatake, S.; Uchida, K.; Tsuchida, E.; Irie, M. Chem. Lett. 2000,
1340.
(14) Ramamurthy, V.; Venkatesan, K. Chem. ReV. 1987, 87, 433.

12138 J. Am. Chem. Soc., Vol. 122, No. 49, 2000
Kobatake et al.
Table 3. Crystal Data and Structure Refinement for 1b and 1b′
1b
1b′
empirical formula
formula weight
temperature
crystal system
space group
C29H22F6S2
548.59
123(2) K
monoclinic
C2/c
unit cell dimensions
a ) 29.762(6) Å
b ) 12.427(2) Å
c ) 13.424(3) Å
â ) 100.719(3)°
4878.2(16) Å3
8
a ) 29.804(4) Å
b ) 12.391(2) Å
c ) 13.489(2) Å
â ) 101.289(3)°
4885.0(12) Å3
8
volume
Z
density (calculated)
goodness-of-fit on F2
final R indices [I > 2σ(I)]
1.494 g/cm3
0.969
1.492 g/cm3
1.017
Figure 6. Temperature dependence of thermal cycloreversion reaction
rates of 1b in toluene (0), 1b photogenerated in crystal 1a (O), and 1b
in crystal 1b (b).
R1 ) 0.041
wR2 ) 0.095
R1 ) 0.068
wR2 ) 0.106
R1 ) 0.046
wR2 ) 0.111
R1 ) 0.077
wR2 ) 0.128
R indices (all data)
Table 2. Arrhenius Parameters of Thermal Cycloreversions from
1b to 1a
Ea/kJ mol-1
A/s-1
closed-ring isomer photogenerated in crystal 1a is considered
to be less stable than the closed-ring isomer in toluene and in
crystal 1b. Also, the open-ring isomer produced by the thermal
cycloreversion reaction is less stable than the open-ring isomer
in toluene and in crystal 1a. In a previous paper it was confirmed
that the open-ring isomer photogenerated in the closed-ring
crystal has a constrained conformation.17 The thermally gener-
ated open-ring isomer is also considered to be in the constrained
conformation (see next section and Figure 10).
Mechanism of the Thermal Cycloreversion Reaction. The
crystal was not shattered even after the photocyclization/thermal
cycloreversion reactions were repeated many times. This means
that the photogenerated colored crystal returned to the initial
crystal by the thermal cycloreversion reaction. The photocy-
clization reactions of diarylethenes in crystals proceed in a
conrotatory mode.15,18,19 This means that photogenerated closed-
ring isomer 1b returns to the open-ring isomer 1a in a con-
rotatory mode by the thermal cycloreversion reaction.
To confirm the conrotatory thermal cycloreversion reaction
of 1b to 1a, X-ray crystallographic analysis was carried out
before and after the thermal cycloreversion reaction. The
crystalline data of 1b are shown in Table 3. The R1 value for
the reflection with I > 2σ(I) and the wR2 value for all data
were 0.041 and 0.106, respectively. The residual highest electron
peaks remain in the middle of covalent bonds. The heights of
the peaks are below 0.463 e Å-3, which indicates the absence
of disordered structure in the closed-ring isomer. Figure 8 shows
ORTEP drawings of 1b. Bond lengths for double and single
bonds are within a range of 1.355-1.360 and 1.415-1.449 Å,
respectively. The three five-membered rings are almost coplanar
in the closed-ring isomer.
The crystallographic data for crystal 1b′ after heating at 100
°C for 256 h are also shown in Table 3. The space group was
the same as that of crystal 1b. The crystal structure 1b was
used as the initial model. After the difference Fourier synthesis,
two new peaks, Q1 and Q2, appeared around the sulfur atoms
as shown in Figure 9. The heights of the peaks Q1 and Q2 were
1.13 and 1.00 e Å-3, respectively, which are much larger than
other residual peaks (less than 0.47 e Å-3). At the same time
negative counter regions were observed at S1 and S2 of the
closed-ring isomer. This indicates that the S1 and S2 atoms of
in crystal 1a
in toluene
in crystal 1b
120
128
137
2.7 × 1012
4.0 × 1012
5.3 × 1012
at the 2- and 2′-positions of thiophene groups. It is of interest
to investigate the thermal stability of 1b.
First, 1b was heated in toluene for 50 h at 120 °C. The blue
color of the solution was bleached. The absorption spectrum of
the bleached solution was the same as that of the open-ring
isomer 1a. This clearly indicates that at 120 °C 1b thermally
returned to the open-ring isomer 1a. The thermal cycloreversion
reaction rates of 1b were followed at various temperatures by
the absorption spectral measurement. The half-life time at 100
°C was estimated to be 52 h. Figure 6 shows the temperature
dependence of the bleaching. From the plots, the activation
energy (Ea) and frequency factor (A) in toluene were determined
to be 128 kJ mol-1 and 4.0 × 1012 s-1, respectively.
Next, the thermal stability of the closed-ring isomers photo-
generated in crystal 1a was measured. The thermal cyclorever-
sion rate was faster than that observed in toluene. Figure 6 also
shows the temperature dependence of the thermal cycloreversion
reaction rate in the crystal. The Ea and A values for the
photogenerated 1b in crystal 1a were determined to be 120 kJ
mol-1 and 2.7 × 1012 s-1, respectively. The Ea value was smaller
than that in toluene. This indicates that 1b in crystal 1a is less
stable than 1b in toluene.
Finally, the thermal stability of the closed-ring isomers in
crystal 1b was measured. Crystal 1b was heated for 110 h at
130 °C, and the bleached product was measured by H NMR
1
spectroscopy. The 1H NMR spectrum indicated that the bleached
product was 1a and any byproduct was not formed. The thermal
cycloreversion rate was much slower than that of 1b photo-
generated in crystal 1a. Figure 6 shows the temperature depen-
dence of the thermal cycloreversion rate of crystal 1b. The Ea
and A values for the 1b crystal were determined to be 137 kJ
mol-1 and 5.3 × 1012 s-1, respectively. The Ea value was much
larger than that in toluene. Table 2 summarizes the Arrhenius
parameters in toluene, in crystal 1a, and in crystal 1b.
From the above kinetic studies the energy diagrams for the
reactions of 1 in the ground state can be deduced as shown in
Figure 7. In the diagrams it was assumed that the energy levels
or the conformations of the molecules in the crystalline phase
reflect the levels or conformations in solution. The energy level
of the open-ring isomer in crystal 1a is similar to that in toluene
and that of the closed-ring isomer in crystal 1b is also similar
to that in toluene, as shown in Figure 7. On the other hand, the
(17) Yamada, T.; Kobatake, S.; Muto, K.; Irie, M. J. Am. Chem. Soc.
2000, 122, 1589.
(18) Kodani, T.; Matsuda, K.; Yamada, T.; Kobatake, S.; Irie, M. J. Am.
Chem. Soc. 2000, 122, 9631.
(19) Yamada, T.; Kobatake, S.; Irie, M. Bull. Chem. Soc. Jpn. 2000, 73,
2179.

1,2-Bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene
J. Am. Chem. Soc., Vol. 122, No. 49, 2000 12139
Figure 7. Energy diagram of open- and closed-ring isomers.
presence of ethyl groups at the reactive carbons. The distance
between C1B and C10B of the open-ring isomer generated by
thermal cycloreversion reaction in the crystal was 2.81 Å, which
is shorter than the value for the most stable open-ring isomer
in the open-ring isomer crystal 1a, 3.71 Å, which is shown in
Figure 10c. The distance between S1B and S2B was 5.46 Å,
which is smaller than the distance between two sulfur atoms of
the stable open-ring isomer, 6.07 Å. The open-ring isomer
(Figure 10b) generated by thermal cycloreversion reaction in
the closed-ring isomer crystal has a conformation more planar
than the most stable open-ring isomer (Figure 10c). This
confirms that the thermally generated open-ring isomer had a
constrained conformation. This is the reason the open-ring
isomer generated thermally in crystal 1b is less stable than the
open-ring isomer in solution.
Conclusions
It has been demonstrated that 1,2-bis(2-ethyl-5-phenyl-3-
thienyl)perfluorocyclopentene (1a) undergoes a thermally re-
versible photochromic reaction in solution as well as in the
single-crystalline phase. Upon irradiation with ultraviolet light
the colorless toluene solution containing 1a and the single crystal
1a turned blue, and the blue color disappeared upon heating
above 100 °C. The activation energies of the thermal cyclo-
reversion reaction from 1b to 1a were determined to be 128 kJ
mol-1 in toluene, 120 kJ mol-1 in the crystal of 1a, and 137 kJ
mol-1 in the crystal of 1b. The activation energies and thermal
cycloreversion rates depended on the environmental conditions.
The X-ray crystallography of crystal 1b during the thermal
reaction indicated that the thermal cycloreversion reaction
proceeded in the conrotatory mode.
Figure 8. ORTEP drawings of top (a) and side (b) views of 1b showing
50% probability displacement ellipsoids.
the closed-ring isomer moved to Q1 and Q2 by heating. These
peaks were assigned to two sulfur atoms S1B and S2B. The
S1B and S2B were refined as disordered atoms of S1 and S2,
respectively. Furthermore, two new peaks appeared around the
carbon atoms of C1 and C10, respectivelty. The new peaks were
also assigned to two carbons C1B and C10B at the reacting
points of the open-ring isomer. The C1B and C10B were refined
isotropically. The heights of the residual peaks were below 0.43
e Å-3. The R1 for the reflection with I > 2σ(I) and the wR2 for
all data were 0.046 and 0.111, respectively. The occupancy
factor for the open-ring isomer converged to 0.078, which means
that around 8% of the closed-ring isomers thermally converted
to the open-ring isomers.
Figure 10 shows ORTEP drawings of the open-ring isomer,
which was thermally generated in the closed-ring isomer crystal.
The four peaks, S1B, S2B, C1B, and C10B, clearly indicated
that the thermal cycloreversion reaction proceeded in a conro-
tatory mode. The reason the disrotatory cycloreversion was not
observed is that the reaction was sterically prohibited by the
Experimental Section
General. Solvents used were spectroscopic grade and purified by
1
distillation before use. H NMR spectra were recorded on a Varian
Gemini 200 spectrometer (200 MHz). Tetramethylsilane was used as
an internal standard. Mass spectra were taken with a Shimadzu GCMS-
QP5050A gas chromatograph-mass spectrometer. The melting points
were measured by using a Perkin-Elmer Pyris 1 differential scanning
calorimeter. Absorption spectra in solution were measured with a
Hitachi U-3410 absorption spectrophotometer. Absorption spectra in
the single-crystalline phase were measured by using a Leica DMLP
polarizing microscope connected with a Hamamatsu PMA-11 photo-
detector. Polarizer and analyzer were set in parallel to each other. A
Mettler-Toledo FP82HT hot stage was used to maintain constant

12140 J. Am. Chem. Soc., Vol. 122, No. 49, 2000
Kobatake et al.
Figure 9. Fo - Fc difference Fourier electron density maps through peaks Q1 and Q2.
temperature of the crystal. Photoirradiation was carried out using a
Ushio 500 W super-high-pressure mercury lamp or a Ushio 500 W
xenon lamp as the light sources. Monochromic light was obtained by
passing the light through a monochromator (Ritsu MV-10N). X-ray
crystallographic analysis was carried out with a Bruker SMART1000
CCD-based diffractometer (50 kV, 40 mA) with Mo KR radiation. The
data collection was performed using SAINT, SHELXS-86,20 and
SHELXL-97.21
2,4-Dibromo-5-ethylthiophene (3). Bromine (25 mL; 0.49 mol) was
added slowly into a flask containing 2-ethylthiophene (25 g; 0.22 mol)
and acetic acid (500 mL). The reaction mixture was stirred overnight
at room temperature. The mixture was neutralized and extracted with
ether. The ether extract was dried over MgSO4, filtrated, and evaporated.
The residue was purified by silica gel column chromatography using
hexane as the eluent. 3 was obtained as a colorless oil of 39 g (65%
yield): 1H NMR (200 MHz, CDCl3) δ 1.25 (t, J ) 7.5 Hz, 3H), 2.75
(q, J ) 7.5 Hz, 2H), 6.86 (s, 1H); MS m/z (M+) 268, 270, 272. Anal.
Calcd for C6H6Br2S: C, 26.69; H, 2.24. Found: C, 26.61; H, 2.22.
3-Bromo-2-ethyl-5-phenylthiophene (4). To 340 mL of dry ether
containing 3 (20 g; 0.074 mol) was added 49 mL of 15% n-BuLi hexane
solution (0.080 mol) at -78 °C under nitrogen atmosphere, and the
solution was stirred for 1 h at the low temperature. Tri-n-butylborate
(30 mL; 0.11 mol) was slowly added to the reaction mixture at -78
°C, and the mixture was stirred for 2 h at that temperature and then
overnight at room temperature. The reaction mixture was neutralized
with concentrated HCl and then extracted with ether. The ether solution
was extracted with aqueous sodium HCl. The aqueous layer was
acidified with concentrated hydroxide. The resulting precipitate was
filtrated. The solid material (3-bromo-2-ethyl-5-thiopheneboronic acid)
(12 g; 0.051 mol) was added into a flask containing THF (300 mL),
20 wt % Na2CO3(aq) (100 mL), and iodobenzene (9.6 g; 0.047 mol).
The mixture was refluxed for 5 h at 70 °C. The product was extracted
with ether. The organic layer was dried over MgSO4, filtrated, and
concentrated. The residue was purified by silica gel column chroma-
tography using hexane as the eluent to give 9.1 g of 4 in 46% yield as
a colorless oil: 1H NMR (200 MHz, CDCl3) δ 1.31 (t, J ) 7.5 Hz,
3H), 2.82 (q, J ) 7.5 Hz, 2H), 7.12 (s, 1H), 7.2-7.7 (m, 5H); MS m/z
(M+) 266, 268. Anal. Calcd for C12H11BrS: C, 53.95; H, 4.15. Found:
C, 53.97; H, 4.19.
Figure 10. ORTEP drawings of top (a) and side (b) views of the
thermally generated open-ring isomer showing 50% probability dis-
placement ellipsoids. The reaction was carried out at 100 °C for 256
h. C1B, C10B, S1B, and S2B were determined based on difference
Fourier synthesis and other atoms based on the structure before
photoirradiation. (c) ORTEP drawing of the open-ring isomer in the
open-ring isomer crystal 1a. The perfluorocyclopentene rings in parts
b and c were horizontally placed. The thiophene rings are red for clarity.
1,2-Bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a). To
80 mL of dry THF solution containing 4 (9.0 g; 0.034 mol) was added
24 mL of 15% n-BuLi hexane solution (0.039 mol) at -78 °C under
nitrogen atmosphere, and the solution was stirred for 2 h at that low
(20) Sheldrick, C. M. Acta Crystallogr., Sect. A 1990, 46, 467.
(21) Sheldrick, C. M. SHELXL-97, Program for Crystal Structure
Refinement; Universita¨t Go¨ttingen: Go¨ttingen, 1997.

1,2-Bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene
J. Am. Chem. Soc., Vol. 122, No. 49, 2000 12141
temperature. Perfluorocyclopentene (2.3 mL; 0.017 mol, Nippon Zeon)
was slowly added to the reaction mixture at -78 °C, and the mixture
was stirred for 3 h at that temperature. The reaction was stopped by
the addition of methanol. The product was extracted with ether. The
organic layer was dried over MgSO4, filtrated, and concentrated. The
residue was purified by column chromatography on silica gel using
hexane as the eluent and by recrystallization from hexane to give 6.3
(dq, J ) 7.3, 13.8 Hz, 2H), 6.64 (s, 2H), 7.3-7.7 (m, 10H); MS m/z
(M+) 548. Anal. Calcd for C29H22F6S2: C, 63.49; H, 4.04. Found: C,
63.46; H, 4.05.
Acknowledgment. This work was supported by CREST
(Core Research for Evolutional Science and Technology) of
Japan Science and Technology Corporation (JST). We also thank
NIPPON ZEON CO., Ltd. for their supply of perfluorocyclo-
pentene.
1
g of 1a in 68% yield as a colorless crystal: mp 164 °C (DSC); H
NMR (200 MHz, CDCl3) δ 0.98 (t, J ) 7.4 Hz, 6H), 2.33 (q, J ) 7.4
Hz, 4H), 7.20 (s, 2H), 7.2-7.6 (m, 10H); MS m/z (M+) 548. Anal.
Calcd for C29H22F6S2: C, 63.49; H, 4.04. Found: C, 63.51; H, 4.04.
Closed-Ring Isomer of 1a (1b). 1b was isolated by passing a
photostationary solution containing 1a and 1b through a HPLC
(Shimadzu LC-6AD) pump system equipped with Shimadzu SPD-10AV
detector and silica gel column (Wako, Wakosil 5SIL) with hexane as
Supporting Information Available: X-ray structural infor-
mation on 1a, 1b, and 1b′ (PDF) and an X-ray crystallographic
file (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
1
the eluent: mp 170 °C (DSC); H NMR (200 MHz, CDCl3) δ 1.16
(t, J ) 7.3 and 7.5 Hz, 6H), 2.54 (dq, J ) 7.5 and 13.8 Hz, 2H), 3.04
JA001996G