Photochromism of diarylethene derivatives having cyclohexyl and cyclohexenyl groups in single-component crystals and a two-component mixed crystal

Article abstract of DOI:10.1016/j.tetlet.2009.02.139

Diarylethene derivatives 1a having two cyclohexyl groups and 2a having a cyclohexyl and a cyclohexenyl group formed a mixed crystal composed of almost equal amounts of the two components, and underwent photochromism in the mixed crystal as well as in the

Full text of DOI:10.1016/j.tetlet.2009.02.139

Tetrahedron Letters 50 (2009) 3404–3407
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Tetrahedron Letters
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Photochromism of diarylethene derivatives having cyclohexyl and cyclohexenyl
groups in single-component crystals and a two-component mixed crystal
a,b,
Masakazu Morimoto a,b,c, , Masahiro Irie
*
*
a Department of Chemistry, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan
b Research Center for Smart Molecules, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan
c Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Honcho 4-1-8, Kawaguchi-shi, Saitama 332-0012, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Diarylethene derivatives 1a having two cyclohexyl groups and 2a having a cyclohexyl and a cyclohexenyl
group formed a mixed crystal composed of almost equal amounts of the two components, and underwent
photochromism in the mixed crystal as well as in the single-component crystals.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 3 January 2009
Revised 6 February 2009
Accepted 19 February 2009
Available online 25 February 2009
Keywords:
Photochromism
Diarylethene
Cyclohexyl group
Mixed crystal
Photochromism is defined as a photoinduced reversible trans-
formation of a chemical species between two isomers having dif-
ferent absorption spectra.1 Although various types of
photochromic molecules have been reported so far, molecules
which show photochromic reactivity in the crystalline state are
rare.2–7 In addition, most photochromic crystals undergo thermally
reversible photochromic reactions and photogenerated isomers re-
turn to the initial isomers in the dark. In contrast to them, diary-
lethene derivatives having heterocyclic aryl rings undergo
thermally irreversible and fatigue-resistant photochromic reac-
tions not only in solution but also in the single-crystalline phases.8
They undergo photocyclization and photocycloreversion reactions
in the crystal lattices by alternate irradiation with ultraviolet
(UV) and visible light, and show reversible color changes between
colorless and colored states. The photogenerated color and the
quantum yield of the photoreaction vary depending on the molec-
ular conformation in the crystal as well as on the chemical struc-
ture of the diarylethene molecule.9
photochromism. A successful example of multicolor photochromic
crystals is a three-component crystal composed of dioxazolyl-,
dithiazolyl-, and dithienylethene derivatives, closed-ring isomers
of which exhibit yellow, red, and blue, respectively.10e The mole-
cules are quite similar in the geometrical structure except the dif-
ference in the atoms in the heterocyclic aryl rings. The similarity in
the geometrical structures helps the formation of well-mixed crys-
tals, and the crystal shows multicolor photochromism.
Based on the above strategy utilizing the similarity in the
molecular geometrical structure, we tried to prepare another mul-
ti-component photochromic crystal by using diarylethene deriva-
tives
1 and 2 having cyclohexyl and cyclohexenyl groups
(Scheme 1). We expected that 1 and 2 form mixed crystals with
3 having two phenyl groups11 because the geometry and size of
the molecules are similar to each other. In addition, 3 having con-
jugative phenyl groups is known to turn blue upon UV irradiation,
while 1 and 2 are expected to exhibit red color. Unfortunately, both
1 and 2 did not form mixed crystals with 3, but we succeeded in
the preparation of a two-component mixed crystal composed of al-
most equal amounts of 1 and 2. We report on photochromic reac-
tions of 1 and 2 in the single-component crystals and the two-
component mixed crystal.
The syntheses of open-ring isomers 1a and 2a were performed
according to Scheme 2. The details are described in the Supple-
mentary data. First, 1-(4-bromo-5-methylthiophen-2-yl)cyclohex-
anol (5)12 was prepared by a reaction of lithiated 3,5-dibromo-2-
methylthiophene (4) with cyclohexanone. Then, 5 was reduced
by adding LiAlH4 under the presence of AlCl3 to be transformed
Several examples of two- or three-component diarylethene
mixed crystals which contain different kinds of diarylethene deriv-
atives have been reported.10 If the diarylethene components exhi-
bit different absorption spectra, the photoreaction of each
component can be selectively controlled by irradiation with light
of appropriate wavelength, and the crystals show multicolor
* Corresponding author. Tel./fax: +81 3 3985 2397.
E-mail addresses: m-morimoto@rikkyo.ac.jp (M. Morimoto), iriem@rikkyo.ac.jp
(M. Irie).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.139

M. Morimoto, M. Irie / Tetrahedron Letters 50 (2009) 3404–3407
3405
Scheme 1. Photochromism of diarylethenes 1 having two cyclohexyl groups, 2
having a cylcohexyl and a cyclohexenyl group, and 3 having two phenyl groups.
Figure 1. Absorption spectra of 1 (a, 1.90 Â 10À5 M) and 2 (b, 1.32 Â 10À5 M) in
hexane. Dotted lines: open-ring isomers 1a and 2a, solid lines: closed-ring isomers
1b and 2b, dashed lines: photostationary states under irradiation with 300 nm light.
to 3-bromo-5-cyclohexyl-2-methylthiophene (6).13 On this reac-
tion, 3-bromo-5-cyclohexenyl-2-methylthiophene (7) was also
generated as a dehydrated by-product. 6 and 7 could not be sepa-
rated by flash column chromatography, and were obtained as a
mixture in the molar ratio of 6:7 = 4:1. The characterization of 6
and 7 in the mixture was confirmed by NMR spectroscopy. Finally,
the mixture of 6 and 7 was lithiated and coupled with octafluoro-
cyclopentene. On a TLC analysis of the reaction mixture, two major
products were found to be photochromic. They were separated by
flash column chromatography and identified to be 1a having two
cyclohexyl groups and 2a having a cyclohexyl and a cyclohexenyl
group by 1H NMR, MS, and elemental analysis.14,15
1 and 2 underwent photochromism in solution. Figure 1 shows
absorption spectra of 1 and 2 in hexane. Upon irradiation with UV
light, colorless solutions of the open-ring isomers 1a and 2a turned
red and reddish purple, respectively. These color changes are due
to the generation of the closed-ring isomers 1b and 2b. 1b and
2b exhibit absorption maxima at 512 and 538 nm, respectively.
The maximum of 2b showed a bathochromic shift in comparison
with that of 1b by as much as 26 nm. The shift means that the ole-
fin moiety in the cyclohexenyl group in 2b participates in the
p
conjugation of the central photochromic backbone. The conversion
ratios from the open- to the closed-ring isomers under irradiation
with 300 nm light were 64% and 65% for 1 and 2, respectively. The
colored states were stable in the dark at room temperature and
bleached by irradiation with visible light (k > 440 nm).
Slow evaporation of a methanol solution of 1a afforded color-
less plate-like crystals. The crystal structure was determined by
X-ray crystallographic analysis. The crystal belongs to a triclinic
crystal system with a space group P1 and Z = 8.16 An asymmetric
unit in the unit cell contains four 1a molecules shown in ORTEP
drawings in Figure 2a. The four molecules in Figure 2a adopt differ-
ent conformations concerning the cyclohexane rings. The thio-
phene rings of the four molecules are fixed in anti-parallel
orientations, and the distances between the reacting carbons are
Scheme 2. Syntheses of 1a and 2a.

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M. Morimoto, M. Irie / Tetrahedron Letters 50 (2009) 3404–3407
Figure 2. ORTEP drawings for single-component crystals of 1a (a) and 2a (b).
3.465–3.501 Å. This fulfills the requirement for diarylethene deriv-
atives to undergo photochromic reactions in the crystalline state.9
Single crystals of 2a were grown by recrystallization from hexane.
The crystal of 2a is triclinic P1 with Z = 2.16 Figure 2b shows an OR-
TEP drawing of 2a. C22–C27 is the olefin double bond in the cyclo-
hexenyl group. 2a molecule also adopts an anti-parallel
conformation, and the distance between the reacting carbons is
3.493 Å. The crystal of 2a is also expected to show photochromic
reactivity.
Photochromism of the single-component crystals of 1a and 2a
was examined. Upon irradiation with UV light (k = 365 nm), the
colorless single crystals of 1a and 2a turned red and reddish purple,
respectively. Figure 3 shows polarized absorption spectra of the
colored crystals. Absorption maxima for 1b and 2b in the colored
crystals are located at 525 and 550 nm, respectively. Anisotropy
in polar plots of the absorbance reflects the regular orientation of
the photogenerated closed-ring isomers in the crystals and means
that photocyclization reactions in both the crystals of 1a and 2a
proceed in the single-crystalline phase.17 The colored crystals were
bleached by visible irradiation (k > 440 nm).
did not yield mixed crystals, probably due to the difference in the
planarity between the cyclohexyl and phenyl groups or due to the
difference in the solubility to the solvent. However, we found that
1a and 2a form a mixed crystal composed of almost equal amounts
of the two components. The mixed crystal was prepared by recrys-
tallization of a 1:1 (molar ratio) mixture of 1a and 2a from meth-
anol. The composition ratio of the crystal was found to be
1a:2a = 53:47 by HPLC analysis. Molecular structures of 1a and
2a are quite similar to each other except that 2a has the olefin moi-
ety in the cyclohexenyl group. This enables the formation of mixed
crystals composed of 1a and 2a in almost equal amounts. Accord-
ing to X-ray crystallographic analysis, the mixed crystal has the
same unit cell as that of the single-component crystal of 2a, tri-
clinic P1 with Z = 2.16 On the refinement of the molecular structure,
however, high peaks of residual electron density were observed
around the cyclohexenyl group in 2a. These peaks were assigned
to the carbon atoms in the cyclohexyl group in 1a, which is incor-
We tried to prepare mixed crystals composed of 1a and 3a or
composed of 2a and 3a. Unfortunately, upon recrystallization from
mixed hexane solutions, the molecules crystallized separately and
Figure 3. Polarized absorption spectra and polar plots of the absorbance for UV-
irradiated single-component crystals of 1a (a) and 2a (b).
Figure 4. ORTEP drawings for 1a and 2a in two-component mixed crystal of 1aÁ2a.
The occupancy ratio is 1a:2a = 59:41.

M. Morimoto, M. Irie / Tetrahedron Letters 50 (2009) 3404–3407
3407
References and notes
1. (a) Brown, G. H. Photochromism; Wiley-Interscience: New York, 1971; (b) Dürr,
H.; Bouas-Laurent, H. Photochromism: Molecules and Systems; Elsevier:
Amsterdam, 2003.
2. Golden, J. H. J. Chem. Soc. 1961, 3741–3748.
3. Hadjoudis, E.; Vittorakis, M.; Moustakali-Mavridis, I. Tetrahedron 1987, 43,
1345–1360.
4. Trozzolo, A. M.; Leslie, T. M.; Sarpotdar, A. S.; Small, R. D.; Ferraudi, G. J.;
DoMinh, T.; Hartless, R. L. Pure Appl. Chem. 1979, 51, 261–270.
5. Maeda, K.; Hayashi, T. Bull. Chem. Soc. Jpn. 1970, 43, 429–438.
6. Ichimura, K.; Watanabe, S. Bull. Chem. Soc. Jpn. 1976, 49, 2220–
2223.
7. Sixl, H.; Warta, R. Chem. Phys. 1985, 94, 147–155.
8. (a) Irie, M. Chem. Rev. 2000, 100, 1685–1716; (b) Kobatake, S.; Irie, M. Bull.
Chem. Soc. Jpn. 2004, 77, 195–210; (c) Morimoto, M.; Irie, M. Chem. Commun.
2005, 3895–3905; (d) Irie, M. Bull. Chem. Soc. Jpn. 2008, 81, 917–926.
9. (a) Shibata, K.; Muto, K.; Kobatake, S.; Irie, M. J. Phys. Chem. A 2002, 106, 209–
214; (b) Kobatake, S.; Uchida, K.; Tsuchida, E.; Irie, M. Chem. Commun. 2002,
2804–2805; (c) Morimoto, M.; Kobatake, S.; Irie, M. Chem. Eur. J. 2003, 9, 621–
627; (d) Morimoto, M.; Kobatake, S.; Irie, M. Cryst. Growth Des. 2003, 3, 847–
854.
10. (a) Yamada, T.; Kobatake, S.; Irie, M. Bull. Chem. Soc. Jpn. 2002, 75, 167–173; (b)
Morimoto, M.; Kobatake, S.; Irie, M. Adv. Mater. 2002, 14, 1027–1029; (c)
Morimoto, M.; Kobatake, S.; Irie, M. J. Am. Chem. Soc. 2003, 125, 11080–11087;
(d) Kuroki, L.; Takami, S.; Shibata, K.; Irie, M. Chem. Commun. 2005, 6005–6007;
(e) Takami, S.; Kuroki, L.; Irie, M. J. Am. Chem. Soc. 2007, 129, 7319–7326.
11. Irie, M.; Lifka, T.; Kobatake, S.; Kato, N. J. Am. Chem. Soc. 2000, 122, 4871–4876.
12. Compound 5: pale yellow crystals; 1H NMR (CDCl3, 400 MHz) d 1.26–1.86 (m,
10H, aliphatic H), 1.77 (s, 1H, OH), 2.36 (s, 3H, CH3), 6.77 (s, 1H, thienyl H).
13. Compound 6: pale yellow oil; 1H NMR (CDCl3, 400 MHz) d 1.26–2.66 (m, 11H,
aliphatic H), 2.33 (s, 3H, CH3), 6.57 (s, 1H, thienyl H). Compound 7: pale yellow
oil; 1H NMR (CDCl3, 400 MHz) d 1.26–2.66 (m, 8H, aliphatic H), 2.34 (s, 3H,
CH3), 6.07 (m, 1H, olefinic H), 6.70 (s, 1H, thienyl H).
Figure 5. Absorption spectrum of UV-irradiated two-component mixed crystal of
1aÁ2a.
porated into the crystal lattice of 2a. The final molecular structures
after the refinement are shown as ORTEP drawings in Figure 4. The
cyclohexyl group in 1a is observed as a disordered structure on the
cyclohexenyl group in 2a. The occupancy ratio of the cyclohexyl
and cyclohexenyl groups is 59:41, which is consistent with the
composition ratio determined from HPLC analysis.
Photochromism of the two-component mixed crystal composed
of 1a and 2a was also examined. Upon irradiation with UV light
(k = 340 nm), the colorless single crystal turned red. Figure 5 shows
an absorption spectrum of the UV-irradiated colored crystal. The
spectrum has a maximum at 535 nm, and is located between the
spectra of the single-component crystals of 1a and 2a. HPLC anal-
ysis of the colored crystal revealed that both of the closed-ring iso-
mers 1b and 2b were generated in the molar ratio of 1b:2b = 57:43
by the UV irradiation. There is no remarkable selectivity in the
photocyclization of 1a and 2a in the mixed crystal.
14. Compound 1a: colorless crystals; mp 115–116 °C; 1H NMR (CDCl3, 400 MHz) d
1.24–2.70 (m, 22H, aliphatic H), 1.81 (s, 6H, CH3), 6.68 (s, 2H, thienyl H); MS
(EI) m/z 532 (M+); elemental Anal. Calcd for C27H30F6S2: C 60.88, H 5.68. Found:
C 60.90, H 5.67. Compound 2a: colorless crystals; mp 120–121 °C; 1H NMR
(CDCl3, 400 MHz) d 1.25–2.71 (m, 19H, aliphatic H), 1.81 (s, 3H, CH3), 1.85 (s,
3H, CH3), 6.07 (m, 1H, olefinic H), 6.71 (s, 1H, thienyl H), 6.81 (s, 1H, thienyl H);
MS (EI) m/z 530 (M+); elemental Anal. Calcd for C27H28F6S2: C 61.11, H 5.32.
Found: C 60.87, H 5.31.
15. Another photochromic compound, which was supposed to be a diarylethene
derivative having two cyclohexenyl groups, 1,2-bis(5-cyclohexenyl-2-methyl-
3-thienyl)perfluorocyclopentene, was detected on the TLC analysis, but could
not be isolated because of the low yield and the instability of the compound.
The compound decomposed during the isolation and purification
process.
In conclusion, 1a and 2a formed a mixed crystal composed of al-
most equal amounts of the two components owing to the similar-
ity in the molecular structures, and both underwent
photochromism in the mixed crystal as well as in the single-com-
ponent crystals.
16. Crystal data for 1a: C27H30F6S2, M = 532.65, T = 93(2) K, triclinic P1,
a = 15.6169(12),
b = 92.9890(10)°,
b = 16.5614(13),
c = 20.4041(16) Å,
a
= 91.6290(10)°,
c
= 104.878(2)°, V = 5088.6(7) Å3, Z = 8, R1 (I > 2
r
) = 0.0511,
Acknowledgments
wR2 (all data) = 0.1027. Crystal data for 2a: C27H28F6S2, M = 530.63, T = 93(2) K,
triclinic P1, a = 8.3804(14), b = 11.4709(19), c = 13.424(2) Å, = 86.426(3)°,
b = 73.184(3)°, ) = 0.0545, wR2
= 87.983(3)°, V = 1232.7(4) Å3, Z = 2, R1 (I > 2
(all data) = 0.0965. Crystal data for 1aÁ2a: (C27H30F6S2)0.589(C27H28F6S2)0.411
M = 531.82, T = 93(2) K, triclinic P1, a = 8.6118(13), b = 11.4971(17),
a
This work was supported by PRESTO, JST, and Grant-in-Aids for
Scientific Research for Priority Areas ‘New Frontiers in Photochro-
mism (471)’ (No. 19050008) and Young Scientists (B) (No.
19750023) from the Ministry of Education, Culture, Sports, Science
and Technology (MEXT), Japan.
c
r
,
c = 13.344(2) Å,
Z = 2, R1 (I > 2
a
r
= 87.125(2)°, b = 73.441(2)°,
) = 0.0754, wR2 (all data) = 0.1922. Crystallographic data
c
= 83.191(2)°, V = 1257.3(3) Å3,
(excluding structure factors) for the structures in this Letter have been
deposited with the Cambridge Crystallographic Data Centre as supplementary
publication nos. CCDC 714780–714782. Copies of the data can be obtained, free
of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax:
+44-(0)1223-336033 or e-mail: deposit@ccdc.cam.ac.Uk).
Supplementary data
17. (a) Kobatake, S.; Yamada, T.; Uchida, K.; Kato, N.; Irie, M. J. Am. Chem. Soc. 1999,
121, 2380–2386; (b) Kobatake, S.; Yamada, M.; Yamada, T.; Irie, M. J. Am. Chem.
Soc. 1999, 121, 8450–8456.
Supplementary data (experimental and syntheses) associated
with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2009.02.139.