The allomorphism of a photochromic diarylethene

Article abstract of DOI:10.1016/S0040-4039(02)02739-9

The photochromic diarylethene, 1,2-bis(2-methyl-5-(2,2′-dicyanovinyl)-thien-3-yl)perfluorocyclopentene (1a), was synthesized by a novel method. Two kinds of single crystals of the compound were obtained depending on the different recrystallization conditions and their structures were determined by X-ray crystallographic analysis. Allomorphism of the photochromic diarylethene was discovered. The compound underwent a photochromic reaction both in solution and in the single crystalline phase of the anti-parallel conformer.

Full text of DOI:10.1016/S0040-4039(02)02739-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 1011–1014
The allomorphism of a photochromic diarylethene
Shou-Zhi Pu,a Fu-Shi Zhang,a,* Fan Sun,a Ru-Ji Wang,a Xin-Hong Zhoua and Shek-Kiu Chanb
aDepartment of Chemistry, Tsinghua University, Beijing 100084, China
bFaculty of Science and Technology, University of Macau, Macau 3001, China
Received 14 October 2002; revised 21 November 2002; accepted 29 November 2002
Abstract—The photochromic diarylethene, 1,2-bis(2-methyl-5-(2,2%-dicyanovinyl)-thien-3-yl)perfluorocyclopentene (1a), was syn-
thesized by a novel method. Two kinds of single crystals of the compound were obtained depending on the different
recrystallization conditions and their structures were determined by X-ray crystallographic analysis. Allomorphism of the
photochromic diarylethene was discovered. The compound underwent a photochromic reaction both in solution and in the single
crystalline phase of the anti-parallel conformer. © 2003 Elsevier Science Ltd. All rights reserved.
Much attention has been devoted to photochromic
compounds because of their potential applicability to
optical memories and photonic switching devices.1
Diarylethenes with heterocyclic aryl groups are most
promising compounds for such applications due to the
good thermal stability of both isomers and their high
fatigue resistance and rapid response time.2–7
any crystalline phase is determined by the crystalliza-
tion conditions, such as the recrystallization solvent,
concentration, temperature, the velocity of crystalliza-
tion, and so on. In this letter, it is our goal to report
allomorphism of the photochromic compound 1,2-bis-
(2-methyl-5-(2,2%-dicyanovinyl)-thien-3-yl)perfluoro-
cyclopentene (1a), synthesized in very high yield by a
novel method. Two types of crystals of the compound
were obtained by controlling different recrystallization
conditions. Using the X-ray crystallographic analysis
method, we found that all molecules of the compound
are packed either in an active anti-parallel conforma-
tion (1a-a) or in an inactive parallel conformation
(1a-b) in the corresponding crystalline phase, therefore,
Although many photochromic compounds have been
reported so far, compounds which show strong pho-
tochromic reactivity in the crystalline phase are very
rare.2,8 Generally, diarylethenes have interconverting
parallel and anti-parallel conformations in solution, in
almost equal amounts. Only anti-parallel conforma-
tions can undergo effective photocyclization reactions
by a conrotatory mechanism, while the parallel confor-
mations are photochemically inactive.9 However, in
crystals, there is no exchange between the two conform-
ers, and the molecules are regularly oriented and
packed in fixed conformations depending on the recrys-
tallization solvent.10 A diarylethene which has two
kinds of crystals under normal conditions is very rare.
To the best of our knowledge, only a unique example
has been reported.10 The two conformations of a
diarylethene can interconvert with each other and
achieve an equilibrium state in solution, but, in the
crystalline phase, one conformation must transform
completely to the other conformation during the course
of the recrystallization, and the final conformation in
Keywords: diarylethene; photochromic; allomorphism; X-ray crystal-
lographic analysis.
* Corresponding author. Tel.: +86-010-62782596/+86-010-62770304;
e-mail: zhangfs@mail.tsinghua.edu.cn
Scheme 1. Photochromism of the diarylethene.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02739-9

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S.-Z. Pu et al. / Tetrahedron Letters 44 (2003) 1011–1014
visible light (u>600 nm), and absorption maxima of 1a
were observed at 358 nm.
allomorphism of this diarylethene derivative exists. Its
photochromism is shown in Scheme 1.
The crystal structures were determined by X-ray dif-
fraction. The two types of crystal structures found are
shown in Figure 214,15 and their packing diagrams are
shown in Figure 3. The ORTEP drawing of 1a-a indi-
cates that 1a-a is packed in a photoactive anti-parallel
conformation in the crystal. The ORTEP drawing of
1a-b in the crystal indicates that 1a-b is packed in the
parallel conformation, which cannot undergo a photo-
cyclization reaction.16
Although the compound was reported by Galit in
1993,7 we synthesized it by a different method (Scheme
2). Firstly, the compound 3 was prepared by hydrolyz-
ing
1,2-bis[2-methyl-5-(2-(1,3–dioxoane))-thien-3-yl]-
perfluorocyclo-pentene 2 in 95% yield.11 Then, a
Knoevenagel12 condensation was used to convert 3 to
the symmetric compound 1a in 89% yield by employing
malonodinitrile, and the structure was confirmed by
melting point, NMR and mass spectrometry.13 Finally,
two kinds of yellow crystals were obtained from chloro-
form by controlling the concentration of the com-
pound, the crystallization temperature and the velocity
of crystallization.
For the crystal of 1a-a, the refinement converged with
R1=0.0584 and wR2=0.1050 for all data. The confor-
mation of the cyclopentene is an envelope and one of
the five carbon atoms, C(3), is dipositional in disorder.
It has two positions located on each side of the plane
defined by the other four carbon atoms. The disordered
carbon atom and the two fluorine atoms, F(3) and F(4),
connecting with it were assigned occupancies to be 0.5
in the refinement which gives the molecule a pseudo-
symmetry of a 2-fold axis to meet the requirement of
the space group. All non-hydrogen atoms were sub-
jected to anisotropic refinement. The dihedral angles
between the cyclopentene ring and the two thiophene
rings were both 44.9°. Bond lengths for double and
single bonds were within a range of 1.343–1.376 and
Figure 1 shows the absorption spectral changes of
alkene 1a in chloroform solution. Upon irradiation
with 365 nm UV light, the colorless solution of 1a
turned green, and absorption maxima of 1b were
observed at 360, 438 and 724 nm. The green colored
solution of 1b returned to colorless on irradiation with
,
1.331–1.718 A. The distance of C(4)ꢀC(4A) which are
the reactive carbon atoms, was estimated to be 0.3589
nm. It is close enough for the reaction theoretically to
take place in the crystalline phase.17 For the crystal of
1a-b, the hydrogen atoms were generated theoretically
and rode on their parent atoms in the final refinement.
All non-hydrogen atoms were subjected to anisotropic
refinement. The final full-matrix least-square refinement
on F2 converged with R1=0.0584 and wR2=0.1050 for
2380 observed reflections [I]2|(I)]. The final differ-
ence electron density map showed no features. The
dihedral angles between the cyclopentene ring and the
two thiophene rings are 55.0 and 60.2°, respectively.
Bond lengths for double and single bonds are within
Scheme 2. Reagents and conditions.
,
the range of 1.336–1.368 and 1.408–1.518 A. The dis-
tance of C(6)ꢀC(15), which should be the reactive car-
bon atoms, was estimated to be 0.4216 nm, which is far
too large for the reaction.17
The two types of crystals showed different pho-
tochromic reactivity coincident with the theoretical
analysis. Table 1 shows the color and umax changes of
the two kinds of crystals on UV light irradiation. On
irradiation with 365 nm UV light, the yellow single
crystal of 1a-a turned green. When observed under
polarized light, the green color intensity changed on
rotation of the crystalline sample. This phenomenon
suggested that the closed-ring isomers were regularly
packed in the crystal. When the green crystal was
dissolved in chloroform, the solution turned to green,
and the absorption maximum was observed at 724 nm,
which is the same as that of the closed-ring form 1b
shown in Figure 1. The green color disappeared upon
irradiation with 700 nm light. However, the yellow
single crystals of 1a-b did not change color on irradia-
tion by UV light. On dissolving the crystal in chloro-
Figure 1. Absorption spectra of alkene 1a in chloroform
solution (4.5×10−5M) open-ring form (solid-line), closed-ring
form (dotted-line), and in the photostationary state under
irradiation with 365 nm UV light (dashed-line).

S.-Z. Pu et al. / Tetrahedron Letters 44 (2003) 1011–1014
1013
Figure 2. ORTEP drawings for C23H10F6N4S2 with 35%
probability ellipsoids, showing the atomic numbering scheme.
(a) The crystal of 1a-a—anti-parallel conformation. Selected
Figure 3. Packing views along the b direction. (a) The crystal
of 1a-a—anti-parallel conformation; (b) the crystal of 1a-b—
parallel conformation.
,
bond distances (A) and bond angles (°): S(1)ꢀC(4) 1.7177(11),
C(1)ꢀC(1A) 1.3428(16), C(1)ꢀC(5) 1.4647(14), C(1)ꢀC(2)
1.4961(16), C(4)ꢀC(5) 1.3612(12), C(4)ꢀC(8) 1.4889(13),
C(5)ꢀC(6) 1.4159(13); C(1A)ꢀC(1)ꢀC(5) 130.28(5), C(1A)ꢀ
C(1)ꢀC(2) 110.55(5), C(5)ꢀC(1)ꢀC(2) 119.16(8), F(1A)ꢀ
C(2)ꢀC(1) 113.54(8), C(5)ꢀC(4)ꢀC(8) 129.26(10), C(5)ꢀC(4)ꢀ
S(1) 111.12(7), C(4)ꢀC(5)ꢀC(1) 125.14(8). Symmetry code A:
1+x−y, 2−y, −z. (b) The crystal of 1a-b—parallel conforma-
Table 1. Color and umax changes of the two kinds of crys-
tals
Crystal
Before UV irradiation
After UV irradiation
,
tion. Selected bond distances (A) and bond angles (°):
a
a
Color
umax (nm) Color
umax (nm)
S(1)ꢀC(6) 1.701(3), S(1)ꢀC(9) 1.729(3), S(2)ꢀC(15) 1.709(3),
S(2)ꢀC(18) 1.738(3), C(1)ꢀC(2) 1.336(4), C(1)ꢀC(7) 1.473(4),
C(1)ꢀC(5) 1.500(4), C(2)ꢀC(16) 1.274(4), C(2)ꢀC(3) 1.502(5),
C(6)ꢀC(7) 1.379(4), C(15)ꢀC(16) 1.373(4), C(15)ꢀC(19)
1.504(4), C(16)ꢀC(17) 1.410(4); C(6)ꢀS(1)ꢀC(9) 92.63(16),
C(2)ꢀC(1)ꢀC(7) 127.4(3), C(2)ꢀC(1)ꢀC(5) 111.5(3), C(7)ꢀC(1)ꢀ
C(5) 121.0(3), C(1)ꢀC(2)ꢀC(16) 126.9(3), C(1)ꢀC(2)ꢀC(3)
110.7(3), F(6)ꢀC(3)ꢀC(2) 115.6(3), F(1)ꢀC(5)ꢀC(1) 111.4(3),
C(7)ꢀC(6)ꢀC(10) 129.4(3), C(7)ꢀC(6)ꢀS(1) 111.1(2), C(10)ꢀ
C(6)ꢀS(1) 119.5(3), C(16)ꢀC(15)ꢀS(2) 111.8(2), C(19)ꢀC(15)ꢀ
S(2) 118.9(2), C(15)ꢀC(16)ꢀC(2) 126.3(3).
1a-a
1a-b
Yellow
Yellow
358
358
Green
Yellow
724
358
a The maximal wavelength was measured in chloroform.
open-ring form 1a. The difference in reactivity between
1a-a and 1a-b is ascribed to the different conformations
of the compound in the solid state.
In conclusion, the compound described here represents
an example of allomorphism within the family of
diarylethenes. It demonstrates that diarylethenes can
possess two conformations in the crystalline phase, and
that only one conformation exists in any crystal. Fur-
ther utilization of this phenomenon is currently in
progress.
form after irradiation by UV light, the color in solution
did not change, and the maximum absorption was still
observed at 358 nm, which is same as that of the

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S.-Z. Pu et al. / Tetrahedron Letters 44 (2003) 1011–1014
12. (a) Nakayama, J. Synthesis 1975, 38; (b) Degani, I.;
Acknowledgements
Fochi, R. Synthesis 1976, 471; (c) Akiba, K.-Y.;
Ishikawa, K.; Inamoto, N. Bull. Chem. Soc. Jpn. 1978,
51, 2674.
This work was supported by the Projects of Develop-
ment of the State Key Fundamental Research
(G199903305) and the Fund of Fundamental Research
of Tsinghua University.
13. Selected data for 1a: yellow solid; mp=206–208°C; 1H
NMR (500 MHz, CDCl3) l 2.14 (s, 6H), l 7.64 (s, 2H),
l 7.78 (s, 2H); MS m/z (M+) 520.
14. Crystal data of 1a-a: C23H10F6N4S2, M=520.47, hexago-
,
nal, a=8.405(3), b=8.405(3), c=57.853(16) A, h=90.00,
3
References
,
i=90.00, k=120.00°, V=3539(2) A , T=295(2) K, space
group P6122 (no. 178), Z=6, Dcalcd=1.465 g cm−3
,
F(000)=1572, v(Mo Ka)=2.92 cm−1, R1=0.0479, wR2=
0.0815. Further details on the crystal structure investiga-
tion have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publica-
tion number CCDC 191842. Copies of the data can be
obtained, free of charge, on application to: CCDC, 12
Union Road, Cambridge CB2 1EZ, UK [fax: +44 1223
336033; e-mail: deposit@ccdc.cam.ac.uk].
1. (a) Brown, G. H. Photochromism; Wiley-Interscience:
New York, 1971; (b) Du¨rr, H.; Bouas-Laurent, H. Pho-
tochromism: Molecules and Systems; Elsevier: Amster-
dam, 1990.
2. Irie, M. Chem. Rev. 2000, 100, 1685.
3. Takeshita, M.; Soong, C. F.; Irie, M. Tetrahedron Lett.
1998, 39, 7717.
4. Takeshita, M.; Irie, M. Tetrahedron Lett. 1998, 39, 613.
5. Tyler, B.; Neil, R. Adv. Mater. 2001, 13, 347.
6. Acebes, A. F.; Lehn, J.-M. Adv. Mater. 1998, 10, 1519.
7. Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. J. Chem. Soc.,
Chem. Commun. 1993, 1439.
8. Scheffer, J. R.; Pokkuluri, P. R. In Photochemistry in
Organized & Constrained Media; Ramamurthy, V., Ed.;
VCH: New York, 1990.
15. Crystal data of 1a-b: C23H10F6N4S2, M=520.47, mono-
,
clinic, a=37.069(10), b=7.7397(12), c=16.349(3) A, h=
3
,
90.00, i=102.490(14), k=90.00°, V=4579.5(16) A ,
T=295(2) K, space group C2/c (no. 15), Z=8, Dcalcd
=
1.510 g cm−3, F(000)=2096, v(Mo Ka)=3.01 cm−1, R1=
0.0584, wR2=0.1050. Further details on the crystal
structure investigation have been deposited with the
Cambridge Crystallographic Data Centre as supplemen-
tary publication number CCDC 191843. Copies of the
data can be obtained, free of charge, on application to:
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax:
+44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk].
16. Nakamura, S.; Irie, M. J. Org. Chem. 1988, 53, 6136.
17. Ramamurthy, V.; Venkatesan, K. Chem. Rev. 1987, 87,
433.
9. Yamada, T.; Muto, S. K.; Irie, M. J. Am. Chem. Soc.
2000, 122, 1589.
10. Kobatake, S.; Yamada, M.; Yamada, T.; Irie, M. J. Am.
Chem. Soc. 1999, 121, 8450.
11. (a) Shouzhi, P.; Fushi, Z.; Xinhong, Z.; Fan, S.; Haobo,
G.; Sikao, C.; Ruji, W. Z. Kristallogr. NCS 2002, 217,
413; (b) Shouzhi, P.; Sikao, C.; Ruji, W.; Fushi, Z.;
Xinhong, Z.; Fan, S.; Peng, Y. Z. Kristallogr. NCS 2002,
217, 415.