1,2-Bis[5-(1,3-dioxolan-2-yl)-2-ethyl-3-thienyl]-3,3,4,4,5, 5-hexafluoro-cyclopent-1-ene, a photochromic dithienylethene

Article abstract of DOI:10.1107/S0108270105025758

The title compound, C₂₃H₂₂F₆O ₄S₂, a photochromic dithienylethene, is a promising material for optical storage and other optoelectrical devices. The molecule adopts a photoactive antiparallel conformation in the crystalline state. The distance between the two reactive C atoms which are involved in potential ring closure is 3.829 (4) A. The dihedral angles between the central cyclopentene ring and the adjacent thiophene rings are 55.38 (7) and 54.81 (9)°. The colourless crystals turn magenta when exposed to UV radiation and the process is reversible.

Full text of DOI:10.1107/S0108270105025758

organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
photochromic diarylethene crystals with an antiparallel
conformation can reversibly turn various colours (yellow, red,
blue or green) from colourless, depending on their molecular
structure, upon irradiation with UV and appropriate wave-
length visible light (Kobatake & Irie, 2004). These crystals also
exhibit good thermal stability of the two isomers and
remarkable fatigue resistance. Crystals of diarylethenes in the
antiparallel conformation are thus very promising for practical
applications.
ISSN 0108-2701
1,2-Bis[5-(1,3-dioxolan-2-yl)-2-ethyl-
3-thienyl]-3,3,4,4,5,5-hexafluoro-
cyclopent-1-ene, a photochromic
dithienylethene
Shou-Zhi Pu,a* Tian-She Yang,a Ru-Ji Wangb and Jing-Kun
Xua
aJiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology
Normal University, Nanchang 330013, People's Republic of China, and
bDepartment of Chemistry, Tsinghua University, Beijing 100084, People's Republic
of China
Correspondence e-mail: pushouzhi@tsinghua.org.cn
Received 28 July 2005
Accepted 11 August 2005
Online 31 August 2005
The title compound, C23H22F6O4S2, a photochromic dithienyl-
ethene, is a promising material for optical storage and other
optoelectrical devices. The molecule adopts a photoactive
antiparallel conformation in the crystalline state. The distance
between the two reactive C atoms which are involved in
In the present work, the title photochromic diarylethene,
(Ia), was synthesized. We have previously reported the
structure of the 2-methyl analogue, (II), of this compound (Pu
et al., 2002). In order to investigate systematically the substi-
tuent effect at the 2-position of the thiophene of diarylethenes
on their photochemical properties, we have now determined
the structure of (Ia). The 2-methyl and 2-ethyl compounds
differ from each other, not only in their crystal structures but
also in their various photochemical properties, such as
absorption maxima, ease of cyclization, cycloreversion
quantum yield and oxidation±reduction potentials.
The molecular structure of (Ia) is shown in Fig. 1, a packing
diagram is shown in Fig. 2 and selected geometric parameters
are given in Table 1. As shown in Fig. 1(a), the molecule has
approximate twofold symmetry, with the two thiophene rings
in a photoactive antiparallel conformation. The hexa¯uoro-
cyclopentene ring adopts a C4-envelope conformation, with
the ¯ap atom (C4) equally disordered over two sites (C4 and
C40) and concomitant disorder of the attached F atoms [see
Fig. 1(b), with details in the Experimental section]. In the
cyclopent-1-ene ring, the C1ÐC2 bond is clearly a double
bond, and the other bonds from C1 and C2 (Table 1) are
clearly single bonds.
Ê
potential ring closure is 3.829 (4) A. The dihedral angles
between the central cyclopentene ring and the adjacent
thiophene rings are 55.38 (7) and 54.81 (9)ꢀ. The colourless
crystals turn magenta when exposed to UV radiation and the
process is reversible.
Comment
Photochromic compounds have been extensively investigated
for their possible application in optoelectronics, viz. as optical
memories, photoswitches and waveguides (Fan et al, 1999).
Among all photochromic systems, diarylethene derivatives are
the most promising candidates for optical memories and other
optoelectronic devices because of the good thermal stability of
the two isomers and their high sensitivity, fast response and
high fatigue resistance (Irie, 2000; Tian & Yang, 2004).
Generally, diarylethenes with thiophene heterocyclic rings
have two interconverting conformations in almost equal
amounts in solution, i.e. antiparallel and parallel conforma-
tions (Irie & Mohri, 1988; Uchida et al., 1990); only antiparallel
conformations can undergo effective photocyclization reac-
tions by a conrotatory mechanism, while the parallel confor-
mations are photochemically inactive (Yamada et al., 2000).
However, there is no interconversion between the two
conformers in the crystalline phase of diarylethenes and the
molecules are regularly oriented in a ®xed conformation (Pu et
al., 2003, 2004, 2005), except for one example (Kobatake et al.,
2005) where there are two independent molecules with
different conformations in the asymmetric unit. In addition,
The two independent planar thiophene ring systems have
essentially identical geometries and the dihedral angles
between the central cyclopent-1-ene ring and the adjacent
thiophene rings are 55.38 (7) and 54.81 (9)ꢀ. The corre-
sponding values in the methyl analogue, (II), are both 49ꢀ.
This conformation leads to a C12Á Á ÁC22 separation of
o568 # 2005 International Union of Crystallography
DOI: 10.1107/S0108270105025758
Acta Cryst. (2005). C61, o568±o570

organic compounds
Ê
Ê
3.829 (4) A [compared with 3.67 A in (II)], which is short
enough, theoretically, for a ring-closure reaction to take place
in the crystalline phase (Ramamurthy & Venkatesan, 1987) to
generate compound (Ib) (see scheme). The orientations of the
ethyl groups at C12 and C22 are de®ned by the torsion angles
C13ÐC12ÐC16ÐC17 [ 139.8 (4)ꢀ] and C23ÐC22ÐC26Ð
C27 [120.8 (4)ꢀ].
the magenta colour changed on rotation of the crystalline
sample. This phenomenon suggests that the closed-ring mol-
ecules of (Ib) are packed regularly in the crystal, but we have
not been able to establish the crystal structure of (Ib). When
the magenta crystal was dissolved in hexane, the solution was
magenta-coloured and the absorption maximum was observed
at 542 nm, consistent with the presence of the closed-ring
isomer (Ib). The magenta colour disappeared upon irradia-
tion with light of wavelength 510 nm or daylight, and the
absorption spectrum of the solution containing the colourless
material was the same as that of solutions of the open-ring
isomer (Ia).
The two terminal 1,3-dioxolane rings both have envelope
conformations; the O31/C32/O33/C34/C35 ring has a C32-
envelope conformation, while the O41/C42/O43/C44/C45 ring
adopts an O41-envelope form. Their orientations relative to
the thiophene rings are presumably largely determined by
crystal-packing considerations and lead to the torsion angles
H32ÐC32ÐC15ÐS11 = 166ꢀ and H42ÐC42ÐC25ÐS21 =
176ꢀ.
In the crystal structure, molecules related by a 21 screw axis
are linked by weak CÐHÁ Á ÁF intermolecular interactions
from one of the dioxolane CÐH groups to generate chains
extending along the b direction, as shown in Fig. 2 (details are
given in Table 2).
Crystals of (Ia) showed photochromic reaction, in accor-
dance with the expected ring closure, to form (Ib). Upon
irradiation with light of wavelength 254 nm, the colourless
crystals of (Ia) quickly became magenta-coloured, as shown in
Fig. 3. When observed under polarized light, the intensity of
Figure 2
A view along the b direction, showing the CÐHÁ Á ÁF interactions in (Ia)
1
2
(dashed lines). [Symmetry codes: (i) x + 32, y + 21, z + 21; (ii) x + 23, y
1
z + .]
2
,
Figure 1
(a) The structure of the title compound, shown with 20% probability
displacement ellipsoids. For clarity, only one orientation of the disordered
cyclopent-1-ene ring is shown. (b) A ball-and-stick diagram, showing the
disorder at C4 in the cyclopent-1-ene ring.
Figure 3
A view of crystals of the colourless compound, (Ia), and the same crystals
after exposure to UV radiation, viz. (Ib).
ꢁ
Acta Cryst. (2005). C61, o568±o570
Pu et al. C23H22F6O4S2 o569

organic compounds
Table 2
Hydrogen-bond geometry (A, ).
Experimental
ꢀ
Ê
The title diarylethene, (Ia), was originally derived from 5-ethyl-
thiophene-2-carbaldehyde, (1). First, 4-bromo-5-ethylthiophene-
2-carbaldehyde, (2), was afforded in 73.3% yield by brominating
compound (1) in acetic acid at room temperature. The dioxolane
acetal, (3), was then prepared in 84.0% yield by re¯uxing under
Dean±Stark conditions in the presence of compound (2), glycol and
p-toluenesulfonic acid (TsOH) in benzene. Finally, to a stirred solu-
tion of compound (3) (2.296 g, 8.73 mmol) in tetrahydrofuran (50 ml)
was added dropwise a 2.5 M n-BuLi solution in hexane (3.5 ml,
8.75 mmol) at 195 K under a nitrogen atmosphere. Stirring was
continued for 30 min and then octa¯uorocyclopentene (0.59 ml,
4.36 mmol) was added slowly to the reaction mixture. The mixture
was then stirred for 2.5 h at 195 K. The reaction was quenched by the
addition of water. After a series of routine operations, the title
compound, (Ia) (1.2 g, 2.2 mmol), was obtained in 50.9% yield by
column chromatography on SiO2 using CHCl3 as eluent. The
compound was crystallized from chloroform±hexane (1:2 v/v) at room
temperature and produced crystals suitable for X-ray analysis. The
crystals of (Ia) had the following elemental analysis and NMR data:
m.p. 400.4±400.6 K; analysis calculated for C23H22F6O4S2: C 51.11,
DÐHÁ Á ÁA
DÐH
HÁ Á ÁA
DÁ Á ÁA
DÐHÁ Á ÁA
C35ÐH35AÁ Á ÁF52i
0.97
2.45
3.361 (5)
157
Symmetry code: (i) x ‡ 32; y ‡ 21; z ‡ 21.
It was obvious from electron-density maps that the CF2 group at
C4 was disordered, corresponding with two orientations of the
cyclopent-1-ene ring in a C4-envelope conformation. This disorder
was readily modelled with atom C4 disordered over two sites (C4 and
C40), with F atoms F41/F410 and F42/F420. DFIX restraints were used
to keep the C4ÐF, C40ÐF and FÁ Á ÁF separations in agreement with
the observed CÐF geometry at the C3 and C5 sites. Initially, the two
disordered orientations were re®ned with tied occupancy parameters,
but as these re®ned occupancy values were not signi®cantly different
from 0.5, the occupancies were then ®xed at 0.5 for the ®nal re®ne-
ment cycles. All H atoms were clearly de®ned in difference maps and
were allowed for as riding atoms, with CÐH distances in the range
Ê
0.96±0.98 A and with Uiso(H) = 1.2 and 1.5Ueq(C).
1
Data collection: XSCANS (Bruker, 1997); cell re®nement:
XSCANS; data reduction: XSCANS; program(s) used to solve
structure: SHELXTL (Bruker, 1997); program(s) used to re®ne
structure: SHELXTL; molecular graphics: PLATON (Spek, 2003);
software used to prepare material for publication: SHELXTL.
H 4.10%; found: C 51.23, H 4.20%; H NMR (400 MHz, CDCl3):
ꢀ 0.924±0.961 (t, 6H, J = 7.8 Hz, ±CH3), 2.192±2.248 (q, 4H, J = 7.5 Hz,
±CH2), 4.002±4.036 (t, 4H, J = 6.8 Hz, ±CH2), 4.101±4.136 (t, 4H, J =
7.0 Hz, ±CH2), 6.021 (s, 2H, ±CH), 7.094 (s, 2H, thiophene-H).
Crystal data
3
C23H22F6O4S2
Mr = 540.53
Monoclinic, P21=n
a = 13.353 (2) A
b = 13.370 (2) A
Dx = 1.489 Mg m
Mo Kꢂ radiation
Cell parameters from 24
re¯ections
This work was ®nancially supported by the Science Funds of
the Education Of®ce of Jiangxi, China (grant No. [2005] 140),
and by the Natural Science Foundation of Jiangxi, China
(grant No. 050017).
Ê
Ê
Ê
ꢃ = 4.9±12.9ꢀ
ꢄ = 0.30 mm
T = 295 (2) K
1
c = 13.600 (3) A
ꢁ = 96.796 (11)ꢀ
3
Ê
V = 2411.0 (7) A
Z = 4
Grain, colourless
0.5 Â 0.5 Â 0.4 mm
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: FG1872). Services for accessing these data are
described at the back of the journal.
Data collection
Bruker P4 diffractometer
! scans
h = 1 ! 15
k = 1 ! 15
5323 measured re¯ections
4231 independent re¯ections
3377 re¯ections with I > 2ꢅ(I)
Rint = 0.026
l = 16 ! 16
References
3 standard re¯ections
every 97 re¯ections
intensity decay: none
Bruker (1997). XSCANS (Version 2.2) and SHELXTL (Version 5.10). Bruker
AXS Inc., Madison, Wisconsin, USA.
ꢃmax = 25.0ꢀ
Fan, M. G., Yu, L. & Zhao, W. (1999). Organic Photochromic and Thermo-
chromic Compounds, edited by J. C. Crano & R. Guglielmetti, Vol. 1, pp.
195±197. New York: Plenum Press.
Re®nement
Re®nement on F2
R[F2 > 2ꢅ(F2)] = 0.049
wR(F2) = 0.144
S = 1.05
4231 re¯ections
w = 1/[ꢅ2(Fo2) + (0.0639P)2
+ 1.2878P]
Irie, M. (2000). Chem. Rev. 100, 1685±1716.
Irie, M. & Mohri, M. (1988). J. Org. Chem. 53, 803±808.
Kobatake, S. & Irie, M. (2004). Bull. Chem. Soc. Jpn, 77, 195±210.
Kobatake, S., Matsumoto, Y. & Irie, M. (2005). Angew. Chem. Int. Ed. 44,
2148±2151.
Pu, S.-Z., Xu, J.-K., Shen, L., Xiao, Q., Yang, T.-S. & Liu, G. (2005). Tetra-
hedron Lett. 46, 871±875.
where P = (Fo2 + 2Fc2)/3
(Á/ꢅ)max < 0.001
3
Ê
Áꢆmax = 0.39 e A
3
Ê
0.29 e A
340 parameters
H-atom parameters constrained
Áꢆmin
=
Extinction correction: SHELXTL
(Bruker, 1997)
Extinction coef®cient: 0.0021 (7)
Pu, S.-Z., Zhang, F.-S., Fan, S., Wang, R.-J., Zhou, X.-H. & Chan, S.-K. (2003).
Tetrahedron Lett. 44, 1011±1015.
Pu, S.-Z., Zhang, F.-S., Wang, R.-J. & Liang, Q. (2004). Acta Cryst. C60, o305±
o307.
Pu, S.-Z., Zhang, F.-S., Zhou, X.-H., Sun, F., Guo, H.-B., Chan, S.-K. &
Wang, R.-J. (2002). Z. Kristallogr. New Cryst. Struct. 217, 413±414.
Ramamurthy, V. & Venkatesan, K. (1987). Chem. Rev. 87, 433±481.
Spek, A. L. (2003). J. Appl. Cryst. 36, 7±13.
Tian, H. & Yang, S.-J. (2004). Chem. Soc. Rev. 33, 85±97.
Uchida, K., Nakayama, Y. & Irie, M. (1990). Bull. Chem. Soc. Jpn, 63, 1311±
1315.
Yamada, T., Muto, S. K. & Irie, M. (2000). J. Am. Chem. Soc. 122, 1589±
1592.
Table 1
Selected bond lengths (A).
Ê
S11ÐC15
S11ÐC12
S21ÐC25
S21ÐC22
C1ÐC2
1.713 (3)
C2ÐC23
C2ÐC3
C3ÐC40
C3ÐC4
C4ÐC5
C40ÐC5
1.472 (3)
1.496 (3)
1.523 (11)
1.528 (12)
1.498 (11)
1.529 (12)
1.719 (3)
1.717 (3)
1.724 (3)
1.344 (3)
1.467 (3)
1.505 (3)
C1ÐC13
C1ÐC5
ꢁ
o570 Pu et al. C23H22F6O4S2
Acta Cryst. (2005). C61, o568±o570