Large geometrical structure changes of photochromic diarylethenes upon photoirradiation

Article abstract of DOI:10.1016/j.tetlet.2003.12.005

Photochromic diarylethene derivatives having phenylethynyl and pent-1-ynyl groups at the reactive carbons have been synthesized. These ethynyl groups enhance the cycloreversion quantum yields of diarylethenes without affecting the absorption maxima and th

Full text of DOI:10.1016/j.tetlet.2003.12.005

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 1155–1158
Large geometrical structure changes of photochromic diarylethenes
upon photoirradiation
Kentaro Morimitsu, Seiya Kobatake and Masahiro Irie*
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Hakozaki 6-10-1,
Higashi-ku, Fukuoka 812-8581, Japan
Received 10 September 2003; revised 8 November 2003; accepted 3 December 2003
Abstract—Photochromic diarylethene derivatives having phenylethynyl and pent-1-ynyl groups at the reactive carbons have been
synthesized. These ethynyl groups enhance the cycloreversion quantum yields of diarylethenes without affecting the absorption
maxima and the absorption coefficients of the closed-ring isomers. The derivatives exhibited large geometrical structural changes in
the photoisomerization process.
ꢀ 2003 Elsevier Ltd. All rights reserved.
Various types of photochromic compounds have been
developed so far to apply them to optoelectronic
devices.1 Among these compounds, diarylethenes have
attracted much attention because they undergo ther-
mally irreversible and fatigue-resistant photochromic
reactions.2–5 Diarylethenes alter their structures between
open- and closed-ring isomers along with the photoiso-
merization. As confirmed by X-ray crystallographic
analysis, however, the motion of reactive moieties is very
small and the molecular size remains similar in the
photoisomerization process, which enables the photo-
chromic reaction even in the single-crystalline state.4 In
this letter, we report on a new feature of diarylethenes
that the molecular size remarkably changes upon photo-
irradiation.
Four types of novel diarylethene derivatives (1a, 2a, 4a,
and 5a) shown in Scheme 1 were synthesized. 1,2-Bis(2-
phenylethynyl-5-phenyl-3-thienyl)perfluorocyclopentene
(1a)6 was prepared from 2,3-dibromo-5-phenylthioph-
ene and ethynylbenzene in Sonogashira reaction condi-
tion7 followed by coupling with octafluorocyclopentene.
1,2-Bis(2-phenylethynyl-1-benzothien-3-yl)perfluorocy-
clopentene (4a)8 was prepared from 2,3-dibromo-
benzothiophene and ethynylbenzene according to the
procedure similar to that used for 1a. 1,2-Bis[2-(pent-1-
ynyl)-5-phenyl-3-thienyl]perfluorocyclopentene
(2a)9
and 1,2-bis[2-(pent-1-ynyl)-1-benzothien-3-yl]perfluoro-
cyclopentene (5a)10 were synthesized using 1-pentyne
instead of ethynylbenzene in the same procedure as 1a
and 4a, respectively.
Scheme 1.
Keywords: Photochromism; Diarylethene; Substituent effect.
* Corresponding author. Tel.: +81-92-642-3556; fax: +81-92-642-3568; e-mail: irie@cstf.kyushu-u.ac.jp
0040-4039/$ - see front matter ꢀ 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.12.005

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K. Morimitsu et al. / Tetrahedron Letters 45 (2004) 1155–1158
that of 1a. The conversion from 1a to 1b in the photo-
stationary state upon irradiation with 313-nm light was
56%. Thermal decoloration of 1b was not observed even
after a day at room temperature. The visible absorption
maximum of 1b was observed at 575 nm, which is the
same as the methyl derivative 3b.12 In addition, the
absorption coefficient of 1b was also almost similar to
that of 3b.12 These results mean that the phenylethynyl
groups at the reactive carbons do not affect the main
electronic structure of the closed-ring isomer compared
with diarylethenes having alkoxy13–15 and cyano16
groups at the reactive carbons. Diarylethenes 2,17 4,18
and 519 showed the photochromic behavior similar to 1,
and ethynyl substituents did not cause any spectral
change of the closed-ring isomers even in benzothienyl-
ethenes 4 and 5. The spectral data of each diarylethene
are summarized in Table 1. The absorption spectra of
the closed-ring isomers were not affected by the ethynyl
groups at the reactive carbons.
To examine the substituent effect on photochromic
reactivity we measured cyclization and cycloreversion
quantum yields of the diarylethenes in hexane at room
temperature. Table 1 summarizes the quantum yields of
1–6. The cyclization quantum yields decreased by the
introduction of the ethynyl groups. On the other hand,
the substituents tend to increase the cycloreversion
quantum yields. Introduction of ethynyl groups at the
reactive carbons is an efficient approach to enhance the
cycloreversion reactivity.
Figure 1. Absorption spectral change of 1 (2.2 · 10ꢀ5 M) (a) and 4
(2.0 · 10ꢀ5 M) (b) in hexane by photoirradiation: open-ring isomers
(- - -), closed-ring isomers (––), and the mixture in the photostationary
state under irradiation with 313-nm light (–– - ––).
The ethynyl derivatives have long arms at the reactive
carbons, which is different from most of the diaryleth-
enes synthesized so far. X-ray crystallographic analysis
of 1a and 1b was carried out to know the geometrical
structure of the open- and closed-ring isomers. Figure 2
shows the ORTEP drawings of 1a21 and 1b.22 As can be
seen from the side views, the geometrical shapes of 1a
and 1b are quite different. The distance between the
reactive carbon atoms of 1a was determined to be
0.344 nm, which is close enough for a photocyclization
reaction to take place inside the crystal.23 However, 1
did not show any photochromism in the crystalline
state. The unit cell volume per molecule expands from
Figure 1a shows the absorption spectra of 1a and 1b in
hexane. 1a has an absorption maximum at 309 nm in
hexane. Upon irradiation with 313-nm light, the color-
less solution of 1a turned to blue, which is due to the
formation of the closed-ring isomer 1b.11 The photo-
generated 1b was isolated by HPLC and the spectrum is
shown in the figure. The blue color of 1b in hexane
disappeared upon irradiation with visible light
(k > 500 nm) and the absorption spectrum returned to
3
21
3
22
ꢀ
ꢀ
803.4 A (1a) to 852.8 A (1b) in the photoisomer-
Table 1. Absorption characteristics and photoreactivitya of diarylethenes in hexane
c
d
f
g
kmax (nm)b
e (Mꢀ1 cmꢀ1
)
Ua!b
kmax (nm)e
e (Mꢀ1 cmꢀ1
)
Ub!a
Conversion (%)h
1a
2a
309
298
280
309
278
258
49,000
48,000
35,600
49,000
33,000
14,000
0.17
0.41
0.59
0.13
0.21
0.35
1b
2b
575
575
575
520
520
517
16,000
16,000
15,600
11,000
9600
0.32
0.27
56
76
97
55
41
45
3a12
3b12
0.013
0.55
0.58
0.35
4a
5a
6a20
4b
5b
6b20
9100
a Standard deviation of the experimental error: 10%.
b Absorption maxima of the open-ring isomers in the UV region.
c Absorption coefficients of the open-ring isomers at the absorption maxima in the UV region.
d Photocyclization quantum yields.
e Absorption maxima of the closed-ring isomers in the visible region.
f Absorption coefficients of the closed-ring isomers at the absorption maxima in the visible region.
g Photocycloreversion quantum yields.
h Conversion from the open- to the closed-ring isomers in the photostationary state under irradiation with 313-nm light.

K. Morimitsu et al. / Tetrahedron Letters 45 (2004) 1155–1158
1157
Figure 2. ORTEP drawings of top (a) and side (b) views of 1a, and top (c) and side (d) views of 1b, showing 50% probability displacement ellipsoids.
6. 1a: Yellow crystals: mp 176.0–177.0 ꢁC; 1H NMR
ization process. The atomic orbitals of the reactive
(200 MHz, CDCl3) d 7.1–7.3 (m, 22H); MS m=z (Mþ)
692. Anal. Calcd for C41H22F6S2: C, 71.09; H, 3.20.
Found: C, 71.07; H, 3.16.
carbon atoms convert from sp2 to sp3, and the bond
angle changes in the cyclization process. The phenyle-
thynyl groups which stack with phenyl groups on
opposite side of thiophene moieties by intramolecular
aromatic interactions in 1a (Fig. 2b) leave far from the
phenyl groups and locate nearly perpendicular to the
main molecular plane of 1b (Fig. 2d).
7. Pereira, R.; Iglesias, B.; de Lera, A. R. Tetrahedron 2001,
57, 7871–7881.
8. 4a: Two polymorphic crystals, colorless and yellow
crystals, were obtained from 4a. Both were identified by
4a by X-ray crystallography: mp 213.5–214.5 ꢁC (colorless
crystals); 1H NMR (200 MHz, CDCl3) d 6.9–7.8 (m, 18H);
MS m=z (Mþ) 640. Anal. Calcd for C37H18F6S2: C, 69.37;
H, 2.83. Found: C, 69.03; H, 2.74.
9. 2a: Yellow crystals: mp 98.0–99.0 ꢁC; 1H NMR (200 MHz,
CDCl3) d 0.8 (t, J ¼ 7 Hz, 6H), 1.4 (sex, J ¼ 7 Hz, 4H), 2.1
(t, J ¼ 7 Hz, 4H), 7.3–7.6 (m, 12H); MS m=z (Mþ) 624.
Anal. Calcd for C35H26F6S2: C, 67.29; H, 4.20. Found: C,
67.36; H, 4.24.
10. 5a: Colorless crystals: mp 149.0–150.0 ꢁC; 1H NMR
(200 MHz, CDCl3) d 1.1 (t, J ¼ 7 Hz, 6H), 1.6 (sex,
J ¼ 7 Hz, 4H), 2.4 (t, J ¼ 7 Hz, 4H), 7.2–7.8 (m, 8H); MS
m=z (Mþ) 572. Anal. Calcd for C31H22F6S2: C, 65.02; H,
3.87. Found: C, 65.03; H, 3.85.
Acknowledgements
The present work was partly supported by Grant-in-
Aids for Scientific Research (S) (no 15105006), Scientific
Research on Priority Areas (no 15033252 and 12131211)
and the 21st Century COE Program from the Ministry
of Education, Culture, Sports, Science and Technology
of Japan. K.M. thanks to Research Fellowships of the
Japan Society for the Promotion of Science for Young
Scientists. We thank NIPPON ZEON CO., Ltd. for
their supply of octafluorocyclopentene.
11. 1b was isolated by HPLC (silica gel; hexane/ethyl ace-
tate ¼ 90/10 as the eluent). 1a and 1b were eluted at 20 and
32 min, respectively.
12. Irie, M.; Lifka, T.; Kobatake, S.; Kato, N. J. Am. Chem.
Soc. 2000, 122, 4871–4876.
13. Shibata, K.; Kobatake, S.; Irie, M. Chem. Lett. 2001, 618–
619.
References and notes
14. Morimitsu, K.; Shibata, K.; Kobatake, S.; Irie, M. J. Org.
Chem. 2002, 67, 4574–4578.
€
1. Durr, H.; Bouas-Laurent, H. Photochromism: Molecules
15. Morimitsu, K.; Shibata, K.; Kobatake, S.; Irie, M. Chem.
Lett. 2002, 572–573.
16. Morimitsu, K.; Kobatake, S.; Nakamura, S.; Irie, M.
Chem. Lett. 2003, 32, 858–859.
17. 2b was isolated by HPLC (silica gel; hexane/ethyl ace-
tate ¼ 99/1 as the eluent). 2a and 2b were eluted at 29 and
41 min, respectively.
and Systems; Elsevier: Amsterdam, 1990.
2. Irie, M.; Uchida, K. Bull. Chem. Soc. Jpn. 1998, 71, 985–996.
3. Irie, M. Chem. Rev. 2000, 100, 1685–1716.
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Nature 2002, 420, 759–760.

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K. Morimitsu et al. / Tetrahedron Letters 45 (2004) 1155–1158
18. 4b was isolated by HPLC (silica gel; hexane/ethyl ace-
tate ¼ 90/10 as the eluent). 4a and 4b were eluted at 26 and
35 min, respectively.
lographic data have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publica-
tion numbers CCDC 219008.
19. 5b was isolated by HPLC (silica gel; hexane/ethyl ace-
tate ¼ 99/1 as the eluent). 5a and 5b were eluted at 27 and
30 min, respectively.
20. Uchida, K.; Tsuchida, E.; Aoi, Y.; Nakamura, S.; Irie, M.
Chem. Lett. 1999, 63–64.
22. Crystal data for 1b: C41H22F6S2, MW ¼ 692.73, mono-
clinic, space group P21/n, Z ¼ 4, T ¼ 123ð2Þ K,
ꢀ
ꢀ
ꢀ
a ¼ 11:015ð4Þ A, b ¼ 26:499ð10Þ A, c ¼ 11:902ð4Þ A, b ¼
3
ꢀ
100:955ð6Þꢁ, V ¼ 3411ð2Þ A , goodness of fit ¼ 1.020,
R1½I > 2rðIÞꢁ ¼ 0:0623, wR2ðall dataÞ ¼ 0:1690. Crystal-
lographic data have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publica-
tion numbers CCDC 219009.
21. Crystal data for 1a: C41H22F6S2, MW ¼ 692.73, mono-
clinic, space group P21/c, Z ¼ 4, T ¼ 123ð2Þ K,
ꢀ
ꢀ
ꢀ
a ¼ 16:522ð4Þ A, b ¼ 10:594ð2Þ A, c ¼ 18:586ð4Þ A, b ¼
3
ꢀ
98:949ð4Þꢁ, V ¼ 3213:6ð12Þ A , goodness of fit ¼ 0.964,
R1½I > 2rðIÞꢁ ¼ 0:0527, wR2ðall dataÞ ¼ 0:1339. Crystal-
23. Kobatake, S.; Uchida, K.; Tsuchida, E.; Irie, M. Chem.
Commun. 2002, 2804–2805.