Invisible photochromism of diarylethene derivatives

Article abstract of DOI:10.1039/b804137g

Diarylethene derivatives with oxidized thiophene rings shift their absorption band to a shorter wavelength in the UV region upon photocyclization; no color change was observed during the photochromic reaction, and the invisible photochromism is advantageous for devices used under room light. The Royal Society of Chemistry.

Full text of DOI:10.1039/b804137g

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Invisible photochromism of diarylethene derivativesw
Tuyoshi Fukaminato,*a Masaaki Tanaka,a Lumi Kurokia and Masahiro Irie*b
Received (in Cambridge, UK) 11th March 2008, Accepted 16th May 2008
First published as an Advance Article on the web 2nd July 2008
DOI: 10.1039/b804137g
Diarylethene derivatives with oxidized thiophene rings shift their
absorption band to a shorter wavelength in the UV region upon
photocyclization; no color change was observed during the
photochromic reaction, and the invisible photochromism is ad-
vantageous for devices used under room light.
Photochromism is defined as ‘‘a reversible transformation of a
chemical species induced in one or both directions by absorp-
tion of light between two forms, A (before) and B (after),
having different absorption spectra’’.1
hn
ꢀ!
Scheme 1 Photochromism of bis(3-thienyl)ethene and bis(2-thienyl)-
ethene derivatives.
A
B
ꢀ
hn0;D
In general, photochromic molecules are initially in a colorless
or pale yellow form (A) and convert to a colored form (B)
upon photoirradiation. Although the color changes are the
most eminent performance of photochromic molecules, the
molecules can modulate or switch not only the color but also
other properties, such as p-conjugation length,2 refractive
indices,3 geometrical structures,4 chiral properties,5 and solu-
bility.6 These property changes are applied to photo-switches
of conductance and magnetic interactions,7 light-driven ac-
tuators,8 photoresponsive liquid crystals9,10 and others. For
such applications, the color changes are not necessary condi-
tions and perturb the performance in some cases. Therefore,
the development of photochromic systems, in which both
isomers are colorless, is of significance.11 The insensitivity to
visible light is also advantageous for devices used under room
light. To realize such invisible photochromic systems, both the
A and B isomers should have their longest absorption bands in
the UV region, and A and B should be exchanged repeatedly
by UV light irradiation with different wavelengths.
the p-conjugation length becomes longer, the absorption
maximum shifts to a longer wavelength. Therefore, it is
anticipated that the absorption band of 3-thienylethene deri-
vatives shifts to a longer wavelength upon photocyclization,
while the band of 2-thienylethene derivatives shifts to a shorter
wavelength. Predictions based on the chemical structure
changes are not always correct. All photochromic 2-thienyl-
ethene derivatives so far reported showed bathchromic shifts
upon UV irradiation (see Table S1, ESIw).13
Here we report on unusual photochromic diarylethene
derivatives, which shift the absorption band to a shorter
wavelength in the UV region upon photocyclization. We have
synthesized diarylethene derivatives, in which oxidized thio-
phene rings are attached to the ethene moiety through the 2-
positions (1 and 2) (Scheme 2), and have examined the
photochromic behavior in detail.
Compounds 1 and 2 were prepared by oxidation of 1,2-bis-
(3-methyl-5-phenyl-2-thienyl)perfluorocyclopentene and 1,2-
bis-(3,5-dimethyl-2-thienyl)perfluorocyclopentene
using
Diarylethenes undergo thermally irreversible and fatigue
resistant photochromic reactions.12 There exist two types of
diarylethene derivatives, 3- and 2-thienylethene derivatives, as
shown in Scheme 1. The color change in diarylethene deriva-
tives is based on the change in the p-conjugation length along
with the photocyclization/photocycloreversion reactions. As
3-chloroperoxybenzoic acid (m-CPBA) in 85% and 79%
yields, respectively.14 The structures were identified by 1H
NMR spectroscopy, MS spectra, elemental analysis, and X-
ray crystallographic analysis (see ESIw).
Fig. 1 shows the absorption spectral change of 1 in
1,4-dioxane solution upon irradiation with visible (l 4 430
nm) and UV light. Before photoirradiation, the absorption
a Department of Chemistry and Biochemistry, Graduate School of
Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka
819-0395, Japan. E-mail: tuyoshi@cstf.kyushu-u.ac.jp; Fax: +81-
92-802-2873; Tel: +81-92-802-2873
b Department of Chemistry, Rikkyo University, Nishi-Ikebukuro 3-34-
1, Toshima-ku, Tokyo 171-8501, Japan. E-mail: iriem@rikkyo.ac.jp;
Fax: +81-3-3985-2397; Tel: +81-3-3985-2397
w Electronic supplementary information (ESI) available: Experimental
details, synthetic procedures, NMR spectra, crystallographic analysis
data, and calculations. CCDC reference numbers 662776–662779. For
ESI and crystallographic data in CIF or other electronic format, see
DOI: 10.1039/b804137g
Scheme 2 Molecular structures and photochromism of the oxidized
bis(2-thienyl)ethene derivatives (1 and 2).
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are the open-ring isomer (1a). Fig. 2b shows the ORTEP
drawings of the photogenerated isomer. The photogenerated
isomer is the closed-ring isomer (1b). The result clearly in-
dicates that 1a undergoes a photocyclization reaction to give
the closed-ring isomer (1b), which has an absorption band at a
shorter wavelength compared to that of the open-ring isomer
(1a).
Similar and more marked absorption change was observed
for compound 2 in 1,4-dioxane, as shown in Fig. 3. Before
photoirradiation, the absorption maximum was observed at
317 nm (e = 5 ꢂ 103 Mꢀ1 cmꢀ1) and the solution was
colorless. Upon irradiation with 390 nm light, the absorption
at 317 nm gradually decreased and a new absorption appeared
at 284 nm. Upon UV light (284 nm) irradiation, the initial
absorption band was restored. The spectral changes could be
repeated more than 100 times. In compound 2, no color
change was observed along with the photocyclization and
photocycloreversion reactions. The photochromic behaviour
can be termed as ‘‘invisible photochromism’’.
Fig. 1 Absorption spectra of the open-ring isomer (solid line), the
photostationary state under irradiation with 430 nm light (dashed
line), and the closed-ring isomer (broken line) of 1 in 1,4-dioxane
solutions (2.4 ꢂ 10ꢀ5 M) at room temperature. The conversion from
the open- to the closed-ring isomer under irradiation with 430 nm light
was estimated to be 92%. Inset: photographs of the solution of 1a and
the photostationary state upon irradiation with 430 nm light.
maximum was observed at 356 nm (e = 1.5 ꢂ 104 Mꢀ1 cmꢀ1
)
and the solution was pale yellow as shown in the inset of
Fig. 1. Upon irradiation with visible light (l 4 430 nm), the
pale yellow color disappeared and a new absorption band
grew up in the UV region at 260 nm. Upon irradiation with
UV (313 nm) light, the colorless solution returned to yellow
and the absorption band at 356 nm was restored. The photo-
generated isomer was stable even at 80 1C and the spectral
change could be repeated for more than 100 times.
The molecular structures of both isomers of 2 were also
determined from X-ray crystallographic analysis. The single
crystals of both isomers were obtained by recrystallization
from a methanol solution. Fig. 4 shows ORTEP drawings of
both isomers of 2. The analysis confirmed that the initial
isomer with the absorption band at 317 nm is the open-ring
isomer and the photogenerated isomer is the closed-ring
isomer.
The molecular structures of both isomers were determined
using X-ray crystallographic analysis. The yellow colored
isomer (i.e. before visible light irradiation) and the photogen-
erated colorless isomer were isolated by HPLC and their
respective single crystals were obtained by recrystallization
from methanol. Fig. 2a shows ORTEP drawings for the yellow
colored isomer. There exist two conformers and both isomers
The photocyclization and photocycloreversion quantum
yields were determined by applying the standard procedure.15
The results are summarized, along with the absorption char-
acteristics of both open- and closed-ring isomers, in Table 1.
Although the quantum yield of the cyclization reaction of 1a is
quite low, the value of 2a is moderate. The low yield of 1a is
attributed to the extended p-conjugation in the open-ring
isomer.2
In conclusion, we have prepared diarylethene derivatives (1
and 2), which exhibit thermally irreversible inverse photochro-
mic reactions. The absorption maxima shift to shorter wave-
lengths with the photocyclization reactions. In compound 2, no
color change was observed during the photochromic reaction.
This photochromic performance is termed ‘‘invisible
Fig. 3 Absorption spectra of the open-ring isomer (solid line), the
photostationary state under irradiation with 390 nm light (dashed
line), and the closed-ring isomer (broken line) of 2 in 1,4-dioxane
solutions (1.5 ꢂ 10ꢀ4 M) at room temperature. The conversion from
the open- to the closed-ring isomer under irradiation with 390 nm light
was estimated to be 83%. Inset: photographs of the solution of 2a and
the photostationary state upon irradiation with 390 nm light.
Fig. 2 ORTEP drawings of (a) 1a, conformer A (anti-parallel) and B
(parallel) and (b) 1b showing 50% probability displacement ellipsoids.
Hydrogen atoms and minor structures have been omitted for clarity.
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Quantum yields
Cyclization
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photochromism’’. The invisible photochromism is advantageous
for opto- and electromolecular devices used under room light.
This work was partly supported by the Grant-In-Aid for
Fundamental Research Program (S) (No. 15105006), Grant-
in-Aids for Young Scientist (B) (No. 18750119), Grant-in-Aids
for Scientific Research on Priority Areas (471) (No. 19050009)
from the Ministry of Education, Culture, Sports, Science, and
Technology (MEXT) of Japanese Government and also by the
Grant-in-Aid for The Global COE Program, ‘‘Future Mole-
cular Systems’’ from the Ministry of Education, Culture,
Science, Sports and Technology of Japan. L. Kuroki also
acknowledges the Grant-in-Aid for JSPS Fellows.
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3926 | Chem. Commun., 2008, 3924–3926