Novel thermally reversible photochromic compounds: Dihydrobenzothiophene and dihydronaphthalene

Article abstract of DOI:10.1080/15421400590946163

Novel thermally reversible photochromic dihydrobenzothiophenes 1 and 2, and dihydronaphthalenes 3-6 were synthesized. The colored form 1Q returned to the colorless 1R within 150 min, while the decoloration of 2Q was much slower. On the other hand, dihydronaphthalenes returned very quickly. The decoloration rate was controlled by introducing a methoxy group on the phenyl ring.

Full text of DOI:10.1080/15421400590946163

Mol. Cryst. Liq. Cryst., Vol. 430, pp. 53–58, 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590946163
Novel Thermally Reversible Photochromic Compounds:
Dihydrobenzothiophene and Dihydronaphthalene
Yasushi Yokoyama
Hitoshi Nagashima
Kensaku Takada
Tetsuya Moriguchi
Sujen Man Shrestha
Department of Advanced Materials Chemistry, Graduate School of
Engineering, Yokohama National University, Tokiwadai, Hodogaya,
Yokohama, Japan
Yayoi Yokoyama
Faculty of Home Economics, Tokyo Kasei Gakuin University,
Aiharacho, Machida, Tokyo, Japan
Novel thermally reversible photochromic dihydrobenzothiophenes 1 and 2, and
dihydronaphthalenes 3–6 were synthesized. The colored form 1Q returned to the
colorless 1R within 150 min, while the decoloration of 2Q was much slower. On
the other hand, dihydronaphthalenes returned very quickly. The decoloration rate
was controlled by introducing a methoxy group on the phenyl ring.
Keywords: adamantylidene group; dihydrobenzothiophene; dihydronaphthalene;
hydrogen rearrangement; photochromism; thermal reversibility
INTRODUCTION
Although a great number of thermally reversible photochromic com-
pounds based on the cyclization-cycloreversion reaction, such as spir-
opyrans, spirooxazines, dihydroindolizines, and nahpthopyrans, have
been reported [1–3], few are known to have the cyclohexadiene–hexa-
triene system. Only several fulgide derivatives [4] and diarylethenes
[5] are well-known, and a plain dihydronaphthalene system [6], if it
Address correspondence to Y. Yokoyama, Department of Advanced Materials
Chemistry, Graduate School of Engineering, Yokohama National University, Tokiwadai,
Hodogaya, Yokohama, 240-8501, Japan. E-mail: yyokoyam@ynu.ac.jp
53

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Y. Yokoyama et al.
SCHEME 1 Thermally reversible photochromism of 1.
can be regarded as photochromic, is known. In the course of our
research on closely related thermally irreversible photochromic com-
pounds [7], we found that a dihydrobenzothiophene 1R bearing an
adamantylidene group showed the thermally reversible and steady
photochromism (Scheme 1) [8]. We here report the thermally revers-
ible photochromism of several related compounds.
RESULTS AND DISCUSSION
1. Synthesis of 2O
Synthesis of 2O was done in 32% yield from 1-(1-adamantylidene-1-
phenylmethyl)-2,3,3,4,4,5,5-heptafluorocyclopentene and 2-lithiothio-
phene, according to the similar procedures described elsewhere [8].
2. Photoreaction of 2
Photoreaction of 2O (Scheme 2, Fig. 1) was carried out as previously
reported for 1O [8]. When a toluene solution of 2O was irradiated with
313-nm light, an absorption band at 487 nm appeared (Scheme 3).
When the irradiation was continued, it became larger while the
absorption maximum moved gradually to the shorter wavelength.
After 150 min (photostationary state), the maximum was 468 nm. It
is attributed to the initial generation of 2C (487 nm) and the final
entire conversion to 2Q (468 nm).
When the irradiation was stopped, the absorption band started to
shrink. However, in the first 20 min, the rate of decoloration was fast

Novel Thermally Reversible Photochromic
55
SCHEME 2 Thermally reversible photochromism of 2.
and blue shift of the absorption maximum was observed. After that,
the docoloration occurred as a first order reaction. The behavior of
initial 20 min may be the result of cyclization of less stable geometric
isomer of the terminal double bond of 2Q.
One of the reasons to synthesize 2 was to accelerate the decoloration
rate of 1. However, the change of the connecting position of the thio-
phene ring from C-3 to C-2 brought about the wrong effect. We thus
undertook the synthesis of new compounds, which should loose the
large aromatic stabilization energy when generating Q-forms.
3. Synthesis and Photoreaction of 3R
Synthesis of 3O was done in 64% yield from 1-(1-adamantylidene-1-
phenylmethyl)-2,3,3,4,4,5,5-heptafluorocyclopentene and phenyllithium.
FIGURE 1 Absorption spectral change of 2 in toluene (c: 1.01 Â 10À4 mol
dmÀ3). a) 2O to 2Q (313 nm, 0 to 150 min); b) 2Q to 2R (33.2ꢀC, 0 to 320 min).

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FIGURE 2 a) Photocoloration of 3R in amorphous polyolefin film by 254 nm
at 173 K. b) Thermal decoloration of 3Q in amorphous polyolefin film at 173 K.
Irradiation of 313-nm light to the toluene solution of 3O afforded
1
3R, which was confirmed by H NMR of the product. However, differ-
ent from 1 and 2, no color development was observed during the photo-
irradiation. This is because the hydrogen rearrangement reaction of
3C and decoloration by cyclization of the colored 3Q is so fast that
the concentration of the colored species 3C and 3Q are quite low.
Therefore we carried out the photoreaction at low temperature.
Figure 2 shows the absorption spectral change of an amorphous poly-
olefin film [9] containing 3R at 173 K.
Now the decoloration is too fast at room temperature. That is
because the loss of aromatic stabilization energy when 3R is changed
to 3Q is too large. Therefore, in order to slow down the decoloration
cyclization rate, the deformation of the o-quinodimethane structure
by electronic perturbation was investigated.
4. Synthesis and Photoreaction of 4R, 5R, and 6R
Introduction of 4- and 3-methoxyphenyl groups to 1-(1-adamantylidene-
1-phenylmethyl)-2,3,3,4,4,5,5-heptafluorocyclopenteneafforded4Oand
5O in 55 and 55% yields, respectively. Irradiation of toluene solution
of 4O by 313-nm light afforded 4R. On the other hand, similar treat-
ment on 5O afforded 5R and 6R because the electrocyclization can
occur on either of the two ortho carbon atoms.
In order to evaluate the thermal reactivity of the colored forms of
methoxy-substituted dihydronaphthalenes, 3Q, 4Q, 5Q, and 6Q were
dissolved in 2-methylTHF and their photochemical ant thermal reac-
tivity were examined. At low temperature, the coloration was not
observed by 366-nm light irradiation. When the temperature was
raised, coloration was observed because the photochemical ring open-
ing could occur by overcoming the constraint of the cold environment.
When the temperature was raised more, the coloration was no longer

Novel Thermally Reversible Photochromic
57
SCHEME 3 Photoreactions and thermal reactions of 4, 5, and 6.
observed because the decoloration by cyclization became faster than
the ring opening. Therefore, the compound has its specific tempera-
ture range to show the coloration.
It is interesting that the coloration range depends on the substi-
tution pattern. The unsbustituted 3Q has the lowest, 4Q next, and
5Q and 6Q have the highest. The participation of the lone pair on
the methoxy oxygen in the conjugation may perturb the unstable
o-quinodimethane structure to stabilize it. Therefore the decoloration
is retarded. The difference of the temperature range in 100 K between
3Q and 5Q=6Q is noteworthy. The schematic presentation of the
coloration temperature range is shown in Figure 3.
FIGURE 3 Photocoloration temperature range of dihydronaphthalenes in
2-methylTHF with 366-nm light irradiation.

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CONCLUSION
New thermally reversible photochromic dihydrobenzothiophene 2R
and dihydronaphthalenes 3R, 4R, 5R, and 6R were synthesized.
The rate of decoloration cyclization reaction of the latter was con-
trolled by the substitution pattern of the methoxy group on the phenyl
group.
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