Conformational control of photochromic reactivity in a diarylethene single crystal

Article abstract of DOI:10.1002/anie.200462426

(Chemical Equation Presented) The deck is stacked in favor of future photonic devices. Novel photochromic crystals comprise alternating layers of two conformations of diarylethene 1 a. Irradiation with UV light induces isomerization of only one conformer

Full text of DOI:10.1002/anie.200462426

Communications
Photochemistry
Conformational Control of Photochromic
Reactivity in a Diarylethene Single Crystal**
Seiya Kobatake,* Yoshimichi Matsumoto, and
Masahiro Irie*
Photochromism is the result of reversible photoisomerization
between two isomers that have distinct absorption spectra.[1,2]
The two isomers differ from one another not only in their
absorption spectra but also in various physical and chemical
[*] Dr. S. Kobatake
Department of Applied and Bioapplied Chemistry
Graduate School of Engineering, Osaka City University
Sugimoto 3-3-138, Sumiyoshi-ku, Osaka 558-8585 (Japan)
Fax: (+81)6-6605-2797
E-mail: kobatake@a-chem.eng.osaka-cu.ac.jp
Y. Matsumoto, Prof. M. Irie
Department of Chemistry and Biochemistry
Graduate School of Engineering, Kyushu University
Hakozaki 6-10-1, Higashi-ku, Fukuoka, 812-8581 (Japan)
Fax: (+81)92-642-3568
E-mail: irie@cstf.kyushu-u.ac.jp
[**] This work was supported in part by Grants-in-Aid for Scientific
Research on Priority Area (417) (15033252), Young Scientists (A)
(16685014), Scientific Research (S) (15105006), and the 21st
Century COE Program from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan. We thank NIPPON ZEON
CO., Ltd. for the supply of octafluorocyclopentene.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200462426
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Chemie
properties, such as refractive index, dielectric constant,
oxidation–reduction potential, and geometric structure.
Among a number of photochromic compounds, diarylethenes
with heterocyclic aryl groups such as thiophene or benzo-
thiophene are the most promising candidates for applications
in optical memories and switches, owing to their thermally
irreversible and fatigue-resistant photochromic perform-
ance.[3,4]
Photochemical reactions in the crystalline state have been
intensively investigated since the pioneering work of Schmidt
and co-workers in the early 1960s.[5–11] Although many
photochromic compounds have been reported already, com-
pounds that exhibit photochromism in the crystalline state are
rare.[12,13] Typical photochromic compounds such as spiro-
pyran and azobenzene are not photochromic in the crystalline
phase, as large geometric structural changes are prohibited in
crystals. However, some diarylethene derivatives can exhibit
photochromism even in the single-crystalline phase.[14–16]
Depending on the particular structure, diarylethene crystals
turn yellow, red, blue, or green upon irradiation with UV
light. The color of the crystal is thermally stable, and never
returns to the initial colorless state in the dark. Diarylethene
derivatives are the only photochromic chromophores that
exhibit thermally irreversible photochromism in the single-
crystalline phase.[17]
in hexane. The solution in hexane showed a visible absorption
maximum at 617 nm, which is the same as that of the isolated
closed-ring isomer 1b. The blue color of the crystal disap-
peared upon irradiation with visible light (l > 500 nm). The
crystal of 1a underwent photochromism.
The color of the crystal was observed with a polarizing
microscope in which the polarizer and analyzer were set in
parallel with each other. The UV-irradiated, colored crystal
was placed on the sample stage and rotated. Viewed from the
(001) surface at a given starting angle, the crystal showed a
strong blue color. Upon rotation by 908, the color of the
crystal disappeared; the blue color reappeared at a rotation of
1808. The clear dichroism indicates that the photogenerated
closed-ring isomers are regularly oriented in the crystal and
that the photochromic reaction takes place in the crystal
lattice.[14]
The crystal and molecular structures of 1a were deter-
mined by X-ray crystallographic analysis. The crystal belongs
to a monoclinic system with space group P21/c, and Z = 8
(Experimental Section). The two diarylethene conformers A
and B are independent in the crystal. Figure 1 shows ORTEP
Recently, our group prepared alternating nanolayered or
mosaic structures composed of two kinds of diarylethene
derivatives and examined their photochromic performance.[18]
When the cocrystals were irradiated with UV light, only one
of the derivatives underwent photochromism. As a result, the
irradiated co-crystals consisted of an alternating layered
structure of open- and closed-ring isomers, or of a three-
dimensional alternating alignment structure of open- and
closed-ring isomers. During the course of our studies into
single-crystalline photochromism, we found that crystals of
1,2-bis(2-methoxy-4-methyl-5-phenyl-3-thienyl)perfluorocy-
clopentene (1a) have a novel molecular-packing structure in
which two different conformations are alternately packed.
Herein we report single-crystalline photochromism and the
important role that conformation plays in photochromic
reactivity.
Figure 1. ORTEP renderings of a) conformer A, and b) conformer B in
crystals of 1a with 50% probability displacement ellipsoids. Distances
between reactive carbon atoms are indicated, and highlighted with
dashed lines.
Single crystals of 1a were obtained by recrystallization
¯
from acetone. The crystals have well-developed (001), (101),
¯
¯
(001), and (101) surfaces. Other surfaces are not well-
developed and were not determined. The crystals of 1a
turned blue upon irradiation at a wavelength of 366 nm
(Scheme 1). To confirm that the blue color is a result of the
closed-ring isomer 1b, the blue-colored crystal was dissolved
renderings of the two conformers. The two geometric
structures are different from each other, but in either case
the molecules are arranged in an antiparallel orientation.
Figure 2 shows the molecular packing diagrams viewed from
the b and c axes. Blue- and red-rendered structures represent
conformers A and B, respectively. In the crystal, the two
conformations are alternately packed in layered structures
with a layer thickness of ꢀ 1 nm.
Our group previously reported that the photocyclization
reactivity of diarylethenes in the single-crystalline phase is
controlled by the distance between the reactive carbon atoms
in the antiparallel orientation.[19] If the distance exceeds
4.2 ꢀ, the photocyclization reaction does not take place.
However, if distance is shorter than 4.0 ꢀ, the diarylethenes
can undergo an efficient photocyclization reaction. The
Scheme 1. Photochromic isomerization of diarylethene 1a to the
closed-ring structure 1b.
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Communications
ring isomer 1b was detected by X-ray diffractometry. The
space group was not changed, and the unit-cell dimensions
were similar to those initially determined (Experimental
Section). The molecular structure is shown in Figure 4. The
Figure 2. Molecular-packing diagrams of 1a viewed from a) the b axis and
b) the c axis. Blue- and red-colored molecules show photoactive and photo-
inactive diarylethene conformers A and B, respectively.
distance dependence of photochromic reactivity is similar to
À
the observation of photochemical C C bond formation in the
crystalline phase reported by Schmidt and co-workers.[5] As
shown in Figure 1, conformer A has a distance of 3.65 ꢀ
between the reactive carbon atoms, whereas the analogous
distance in conformer B is 4.93 ꢀ.
Thin microcrystals of diarylethene 1a were prepared by
sublimation on a thin glass plate by heating at 1048C. The
typical crystal size was ꢀ 10 ꢁ 10 ꢁ 5 mm3. Crystal shapes and
reversible color changes were similar to those of crystals
obtained from a solution in acetone. Microcrystals were
irradiated at 366 nm ( ꢀ 26 mWcmÀ2). The molar ratio of
open- to closed-ring isomers (1a/1b) after irradiation with
UV light was determined by HPLC (silica gel column,
hexane/ethyl acetate (98:2) as eluent). Figure 3 shows the
Figure 4. ORTEP renderings of a) conformer A and b) conformer B in
photoirradiated crystals of 1a with 30% probability displacement
ellipsoids. The photogenerated closed-ring isomer 1b is indicated in
blue. The conversion ratio obtained by X-ray crystallographic analysis
was 8%.
positions of the sulfur atoms, the reactive carbon atoms, and
the methoxy groups in the closed-ring isomer were obtained
only from conformer A. No peak that corresponds to the
closed-ring isomer of conformer B was observed. This result
indicates that only conformer A underwent the photochromic
reaction upon irradiation with UV light, and that confor-
mer B remained inactive. The difference in the distance
between the reactive carbon atoms explains the different
reactivity of the two conformers. The conversion saturation at
50% shown in Figure 3 is also explained by the selective
photoreaction. The conformational difference controls the
reactivity, as observed for chiral organic salts.[21]
In conclusion, diarylethene 1a is packed in two different
conformations in the crystalline state, and the two conformers
exhibit different photochemical reactivities. The periodic
refractive index change in the crystal structure could be
potentially useful in future photonic devices, such as one-
dimensionally periodic photonic crystals and three-dimen-
sional optical memory.[22]
Figure 3. The progress of the photocyclization reaction over time in
the diarylethene microcrystal of 1a by irradiation at l=366 nm.
time dependence of the conversion ratio for the photo-
cyclization reaction in the crystal. Upon irradiation at 366 nm,
1a was efficiently converted into closed-ring isomer 1b by as
much as 50% after irradiation for 4 h. This indicates that half
the molecules packed in the crystal undergo the photo-
chromic reaction.
Experimental Section
Single crystals of 1a were prepared by recrystallization
from acetone, and were used for X-ray crystallography in situ.
An appropriate size for crystallographic analysis was obtained
by cutting the crystals to final dimensions of ꢀ 0.3 ꢁ 0.2 ꢁ
0.05 mm3. A single crystal was then irradiated with UV
light. A high conversion ratio for such a thick crystal cannot
be expected, owing to the inner-filter effect of absorption.[20]
After irradiation at 366 nm ( ꢀ 5 mWcmÀ2) for 5 h, the closed-
Single crystals were observed with a Nikon E600POL polarizing
microscope. Polarizer and analyzer were set in parallel with each
other. Photoirradiation was carried out with a mercury short arc lamp
(100 W). The wavelength (l = 366 nm) was selected by passing the
light through a band-pass filter. X-ray crystallographic analysis of
single crystals was performed with a Bruker SMART CCD X-ray
diffractometer or a Rigaku RAXIS X-ray diffractometer with MoKa
radiation (l = 0.71073 ꢀ) and a graphite monochromator. Crystal cell
constants were calculated by global refinement. The structure was
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solved by direct methods with SHELXS86[23] and refined by full least-
squares on F2 with SHELXL97.[24]
Eur. J. 2003, 9, 621; e) M. Morimoto, S. Kobatake, M. Irie, J. Am.
Chem. Soc. 2003, 125, 11080.
1a (Synthesis details in the Supporting Information): m.p. 117.5–
118.58C; UV/Vis (n-hexane): lmax (e) = 288 nm (28600mÀ1 cmÀ1);
1H NMR (200 MHz, CDCl3/TMS): d = 2.10 (s, 6H; Me), 3.78 (s, 6H;
MeO), 7.2–7.5 ppm (m, 10H; Ph); 13C NMR (100 MHz, CDCl3): d =
14.1 (Me), 61.3 (MeO), 110.2 (3-thienyl), 111.5 (CF2), 116.0 (CF2),
124.4 (5-thienyl), 127.1 (Ph), 128.6 (Ph), 129.1 (Ph), 130.5 (4-thienyl),
[15] a) T. Kodani, K. Matsuda, T. Yamada, S. Kobatake, M. Irie, J.
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[18] a) M. Morimoto, S. Kobatake, M. Irie, Chem. Rec. 2004, 4, 23;
b) M. Morimoto, S. Kobatake, M. Irie, Photochem. Photobiol.
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[19] a) S. Kobatake, K. Uchida, E. Tsuchida, M. Irie, Chem.
Commun. 2002, 2804; b) K. Shibata, K. Muto, S. Kobatake, M.
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=
134.1 (Ph), 139.5 (C C), 163.4 ppm (2-thienyl); FAB-HRMS: m/z:
calcd for C29H22F6O2S2 [M]+: 580.0965; found: 580.0966.
Crystal data for 1a: C29H22F6S2O2; monoclinic; space group P21/c;
a = 24.036(5), b = 10.017(2), c = 24.141(6) ꢀ, b = 114.104(4)8, V=
5306(2) ꢀ3; Z = 8; T= 123(2) K; M = 580.61; m(MoKa) = 0.269 mmÀ1
,
30557 reflections measured, 11264 independent reflections. Final R1
[I > 2s(I)] = 0.0456, wR2 (all data) = 0.1202.
Crystal data for 1a/1b (photoirradiated crystal): C29H22F6S2O2;
monoclinic; space group P21/c; a = 24.040(7), b = 9.929(3), c =
24.191(7) ꢀ, b = 113.897(7)8, V= 5279(3) ꢀ3; Z = 8; T= 123(2) K;
[20] T. Yamada, S. Kobatake, M. Irie, Bull. Chem. Soc. Jpn. 2000, 73,
2179.
M = 580.61; m(MoKa) = 0.271 mmÀ1
, 38733 reflections measured,
12116 independent reflections. Final R1 [I > 2s(I)] = 0.0794, wR2 (all
data) = 0.1690.
[21] E. Cheung, T. Kang, M. R. Netherton, J. R. Scheffer, J. Trotter, J.
Am. Chem. Soc. 2000, 122, 11753.
CCDC-253671 (1a) and -253672 (1a/1b) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[22] a) J. D. Joannopoulos, R. D. Meade, J. N. Winn, Photonic
Crystals, Princeton University Press, Princeton, 1995; b) J. D.
Joannopoulos, P. R. Villeneuve, S. Fan, Nature 1997, 386, 143.
[23] G. M. Sheldrick, Acta Crystallogr. Sect. A 1990, 46, 467.
[24] G. M. Sheldrick, SHELXL-97, Program for Crystal Structure
Refinement, Universitꢃt Gꢄttingen, Gꢄttingen, 1997.
Received: October 26, 2004
Published online: February 23, 2005
Keywords: chromophores · crystal engineering · isomerization ·
.
photochromism · X-ray diffraction
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