Syntheses and photochromic properties of metacyclothiophenophan-1-enes

Article abstract of DOI:10.1246/cl.2009.982

A small thia[3]metacyclo[2]thiophenophan-1-ene, cyclophane composed of a thiophene ring, a benzene ring, and their bridge, has been synthesized and displays photochromism, while a metacyclothiophenophan-1-ene bearing a long bridge was nonphotochromic due

Full text of DOI:10.1246/cl.2009.982

982
Chemistry Letters Vol.38, No.10 (2009)
Syntheses and Photochromic Properties of Metacyclothiophenophan-1-enes
Michinori Takeshita,Ã Hirotsugu Jin-nouchi, and Jun Ishikawa
Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University,
1 Honjo, Saga 840-8502
(Received July 29, 2009; CL-090709; E-mail: michi@cc.saga-u.ac.jp)
A small thia[3]metacyclo[2]thiophenophan-1-ene, cyclo-
The only difference between these cyclophan-1-enes is the
length of the bridge. An unsymmetrical diarylethene 3 was a pre-
cursor for preparing 1 and 2.6
phane composed of a thiophene ring, a benzene ring, and their
bridge, has been synthesized and displays photochromism, while
a metacyclothiophenophan-1-ene bearing a long bridge was non-
photochromic due to little conjugation between three double
bonds, that was estimated by DFT calculations.
The inner methyl groups of 1a appeared at 0.96 and
1.37 ppm which are assignable to methyl protons of the benzene
and the thiophene rings and they were shifted to higher magnetic
field compared to those of acyclic 3a with 0.77 and 0.60 ppm, re-
spectively. These shifts result from shielding effects of the oppo-
site aromatic rings, therefore, the conformation of 1a is the pho-
toactive anti conformation. On the other hand, the chemical
shifts of the inner methyl groups of 2a were 1.43 and
1.78 ppm and they were 0.30 and 0.19 ppm upfield compared
with those of 3a. These chemical shifts show that these methyl
protons were a little shielded by the opposite aromatic rings
and the conformation of 2a was also assignable to the photoac-
tive anti parallel.
Upon irradiation with UV light (330 Æ 20 nm), the pale yel-
low solution of 1a and 3a in CH2Cl2 turned red and orange, re-
spectively. The colors and spectra returned to the initial by the
visible (>460 nm) irradiation. Spectral changes are shown in
Figure 2. The conversions of 1 and 3 upon irradiation with
330 Æ 20 nm were 53 and 52%, respectively.7 In contrast, nei-
ther color nor spectral changes were observed in the solution
of 2a, indicating 2 is non-photochromic. It is reported that when
the distances between two reactive carbon atoms are longer than
Dithienylethenes have been studied intensely since they dis-
play P-type photochromism and are relatively fatigue resistant.1
As a consequence of these properties dithienylethenes are fre-
quently considered as candidates for organic photomemory sys-
tems.2 Stilbenes, which are composed of two benzene rings
linked by an ethene bridge, have been well known as thermally
reversible photochromic compounds for several decades.3 In
contrast, diarylethenes having two different membered aromatic
rings, for example composed of 5-membered and 6-membered
rings, have been scarcely reported.4 Recently, we have reported
photochromic cyclophan-1-enes which have one additional
bridge to the diarylethenes.5 The cyclophan-1-enes have specific
photochromic features5 however cyclophan-1-ene with different
aromatic rings has not yet been reported. These facts encouraged
us to prepare cyclophan-1-enes composed of a benzene ring and
a thiophene ring and to investigate the photochromic properties
of the synthesized cyclophan-1-enes.
˚
Figure 1 shows the prepared metacyclothiophenophan-1-
enes with a benzene ring and a thiophene ring. We have designed
two types of metacyclothiophenophan-1-enes, one with a short
–CH2SCH2– bridge 1, the other with a long ether bridge 2.
4.2 A, photocyclization reaction does not occur in crystalline
state.8 Our cyclophane system is also rigid enough to apply this
empirical law. Then the most stable structures of metacyclo-
thiophenophan-1-enes were estimated by DFT:B3LYP/6-31GÃ
in Spartan ’08 and the distances and the dihedral angles of each
reactive bond, which are indicated as bold lines in Figure 1, are
summarized with those of photochromic thiophenophan-1-enes
F2
F2
F2
F2
F2
F2
hν
Me
Me
hν ' or ∆
But
S
Me
Me
S
But
Me
Me
1a
F2
1b
S
S
F2
F2
F2
F2
F2
But
Me
Me
S
S
Me
But
Me
Me
Me
3a
O
S
O
2a
O
F2
F2
F2
F2
F2
F2
S
S
Me
S
Me
Me
Me
Me
Me
Me
Me
O
O
Figure 2. Absorption spectra of 3a (broken, plain), photosta-
tionary state of 3 (solid, plain), 1a (broken, bold), photostation-
ary state of 1 (solid, bold), and 2a (dotted, plain) in CH2Cl2
(5:0 Â 10À5 mol dmÀ3).
S
O
4
5
Figure 1. Prepared metacyclothiophenophan-1-enes and thio-
phenophan-1-enes. Bold lines indicate reactive double bonds.
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.10 (2009)
983
˚
Table 1. Distances of reactive carbon atoms (A) and dihedral
angles of the double bonds of each aromatic ring (degree) esti-
mated with DFT calculations
little conjugation. This is a specific feature of the metacyclothio-
phenophan-1-enes containing both 5-membered and 6-mem-
bered aromatic rings.
C–C
distance
CyP–Ben Th–Ben
(Th)b
Photo-
chromism
Th–CyPa
(Th)c
This work is supported by a Grant-in-Aid for Science Re-
search in a Priority Area New Frontiers in Photochromism
(471) (No. 21021021) from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan and a Grant-
in-Aid for Scientific Research (C) (No. 21550119) from Japan
Society for the Promotion of Science (JSPS). The authors are
also grateful to Zeon Co., Ltd. for providing perfluorocyclopen-
tene.
1a
2a
4
3.00
3.84
3.15
3.67
36.7
47.5
38.7
48.2
50.3
57.3
52.0
47.2
66.2
88.3
75.3
79.3
yes
no
yes
yes
5
aDihedral angles of the reactive double bonds of thiophene
rings (Th) and cyclopentene rings (CyP). bThose of cyclopen-
tene rings (CyP) and benzene (Ben) or thiophene rings (Th).
cThose of thiophene rings (Th) and benzene (Ben) or thio-
phene rings (Th).
References and Notes
1
2
M. Irie, Chem. Rev. 2000, 100, 1685.
a) M. Irie, in Photo-reactive Materials for Ultrahigh Density
Optical Memory, Elsevier, Amsterdam, 1994. b) T. Tsujioka,
M. Kume, M. Irie, Jpn. J. Appl. Phys., Part 1 1995, 34, 6439.
c) A. J. Myles, N. R. Branda, Adv. Funct. Mater. 2002, 12,
167. d) G. M. Tsivgoulis, J.-M. Lehn, Angew. Chem., Int.
Ed. Engl. 1995, 34, 1119. e) T. Tsujioka, Y. Hamada, K.
Shibata, A. Taniguchi, T. Fuyuki, Appl. Phys. Lett. 2001,
78, 2282. f) M. Irie, T. Fukaminato, T. Sasaki, N. Tamai,
T. Kawai, Nature 2002, 420, 759.
4 and 59,10 in Table 1.11 The distance of 2a was 3.84 A and this
˚
value is longer than the others, however still short enough to car-
ry out the cyclization. On the other hand, the dihedral angle of
the reactive double bonds of the benzene and the thiophene rings
of 2a was estimated to be nearly perpendicular at 88.3 degrees.
Therefore, there is little conjugation between them. In contrast,
the dihedral angles of the other cyclophan-1-enes, which show
photochromism, were smaller than that of 2a. In the 2a system,
the three double bonds are less conjugated than those of other
thiophenophan-1-enes sterically. Absorption spectrum of 2a in
Figure 2 also supports this consideration. The absorption maxi-
mum of 2a was 315 nm, which is 10 nm blue shifted compared
with that of 1a. The nonphotochromism of 2a is due to not only
the large separation of the two reactive carbon atoms, but also
the scant conjugation of the reactive double bonds.
3
For example, a) K. A. Muszkat, Top. Curr. Chem. 1980, 88,
89. b) M. Takeshita, M. Ogawa, K. Miyata, T. Yamato,
J. Phys. Org. Chem. 2003, 16, 148.
4
5
For example: S. Pu, C. Fan, W. Miao, G. Liu, Tetrahedron
2008, 64, 9464.
a) M. Takeshita, T. Yamato, Tetrahedron Lett. 2001, 42,
4345. b) M. K. Hossain, M. Takeshita, T. Yamato, Eur. J.
Org. Chem. 2005, 2771. c) M. Takeshita, A. Tanaka, T.
Hatanaka, Opt. Mater. 2007, 29, 499. d) M. Takeshita, T.
Yamato, Angew. Chem., Int. Ed. 2002, 41, 2156. e) M.
Takeshita, T. Yamato, Chem. Lett. 2004, 33, 844.
Experimental details and spectral data are shown in Support-
ing Information. Supporting Information is available elec-
tronically on the CSJ-Journal Web site, http://www.csj.jp/
journals/chem-lett/index.html.
The quantum yield for the photocyclization reaction of the
cyclophan-1-ene 1a (0.56)12 upon irradiation with 313 nm light
is larger than that for the acyclic one 3a (0.49). This is due to the
conformational rigidity of the small cyclophan-1-ene. The con-
formation of the cyclophan-1-ene is fixed to the photoactive anti
conformation and this is one of the advantages of cyclophan-1-
ene systems compared to diarylethenes as we have already men-
tioned.5,9,10 On the other hand, the quantum yield of 1a (0.56) is
smaller than that of thiophenophan-1-ene 4a (0.70).10 This phe-
nomenon is probably due to their potential energy surfaces.13
The thermal stabilities of the closed forms have been also
studied. The activation energies and pre-exponential factors
were estimated by the Arrhenius plots of the rate constants for
the thermal ring-opening reaction to give the open forms at sev-
eral temperatures of each closed form. The activation energy,
pre-exponential factor, and the lifetime at 25 ꢀC of the thermal
6
7
8
The conversions were determined by HPLC analyses of the
solutions at photostationary states.
a) V. Ramamurthy, K. Venkatesan, Chem. Rev. 1987, 87,
433. b) M. Morimoto, S. Kobatake, M. Irie, Chem.—Eur.
J. 2003, 9, 621.
9
M. Takeshita, C. Tanaka, T. Miyazaki, Y. Fukushima, M.
Nagai, New J. Chem. 2009, 33, 1433.
10 M. Takeshita, M. Nagai, T. Yamato, Chem. Commun. 2003,
1496.
11 The most stable structures estimated with DFT calculations
are indicated in Supporting Information.
ring-opening reaction of 1b were 67.7 kJ molÀ1, 1:6 Â 106 sÀ1
,
and 89 h, respectively, and those of the closed form 3b were
86.8 kJ molÀ1, 3:1 Â 109 sÀ1, and 432 days. The thermal stability
of metacyclothiophenophan-1-en is lower than that of diaryl-
ethene 3b and these phenomena also occur in thiophenophan-
1-enes.9
12 The quantum yields of the photocyclization reactions of
thiophenophan-1-enes were obtained by comparing the ini-
tial rates for the photocyclizations of thiophenophan-1-enes
with that of bis(2-methyl-1-benzothien-3-yl)hexafluoro-
cyclopentene: K. Uchida, E. Tsuchida, Y. Aoi, S. Nakamura,
M. Irie, Chem. Lett. 1999, 63.
13 S. Nakamura, T. Kobayashi, A. Takata, K. Uchida, Y.
Asano, A. Murakami, A. Goldberg, D. Guillaumont, S.
Yokojima, S. Kobatake, M. Irie, J. Phys. Org. Chem. 2007,
20, 821.
In summary, two types of metacyclothiophenophan-1-enes
have been synthesized and the small one underwent photochro-
mic reaction, while the large one was non-photochromic. This
phenomenon is due to the dihedral angles of the reactive double
bonds of aromatic rings estimated by DFT calculations. The di-
hedral angles between the aromatic units of the non-photochro-
mic compounds are nearly perpendicular and hence show very
Published on the web (Advance View) September 12, 2009; doi:10.1246/cl.2009.982