Metal-ion stabilization of photoinduced open colored isomer in crowned spirobenzothiapyran

Article abstract of DOI:10.1039/a903877i

A spirobenzothiapyran derivative bearing a monoaza-12-crown-4 moiety affords significant thermal stability enhancement in the UV light induced colored merocyanine form of its photochromic moiety by metal-ion complexation of its crown ether moiety, although the complexation hardly induces the isomerization without photoirradiation.

Full text of DOI:10.1039/a903877i

Metal-ion stabilization of photoinduced open colored isomer in crowned
spirobenzothiapyran
Mutsuo Tanaka,a Kenji Kamada,a Hisanori Ando,a Takashi Kitagaki,b Yasuhiko Shibutani,b Setsuko Yajima,c
Hidefumi Sakamotoc and Keiichi Kimura*c
a Osaka National Research Institute, AIST, 1-8-13, Midorigaoka, Ikeda, Osaka 563-8577, Japan
b Department of Chemistry, Faculty of Engineering, Osaka Institute of Technology, 5-16-1, Ohmiya, Asahi-ku, Osaka
535-8585, Japan
c Department of Applied Chemistry, Faculty of Systems Engineering, Wakayama University, 930, Sakae-dani,
Wakayama 640-8510, Japan. E-mail: kimura@sys.wakayama-u.ac.jp
Received (in Cambridge, UK) 14th May 1999, Accepted 23rd June 1999
A spirobenzothiapyran derivative bearing a monoaza-
12-crown-4 moiety affords significant thermal stability
enhancement in the UV light induced colored merocyanine
form of its photochromic moiety by metal-ion complexation
of its crown ether moiety, although the complexation hardly
induces the isomerization without photoirradiation.
phenolic hydroxy group of crowned nitrosalicylaldehyde4a to
the corresponding thiol group was attained by the reaction with
(CH3)2NCSCl at 0–25 °C in the presence of NEt3, by refluxing
in toluene, and by hydrolysis with KOH.2d The crowned
thiosalicylaldehyde was then reacted with 2-methylene-
1,3,3-trimethylindoline in refluxing EtOH to yield the spiro-
benzothiapyran bearing a monoaza-12-crown-4 moiety, 1,
which was purified by recrystallization from EtOH after
preparative gel-permeation chromatography.† For comparison,
we also synthesized a spirobenzothiapyran derivative bearing
an oligooxyethylene moiety, 2, in a similar way to 1.
Absorption spectra of 1 and 2 were measured in MeCN in the
presence and absence of an alkali metal perchlorate. The spectra
of 1 and 2 were hardly changed by adding an equimolar amount
of Li+, Na+, and K+, unless irradiated by UV light. This means
that isomerization to their merocyanine form by the thiapyran
ring opening cannot proceed by metal ion complexation of their
crown ether ring under dark conditions [Scheme 1(b)]. This
isomerization behavior of 1 is very different from that of the
spirobenzopyran bearing a monoaza-12-crown-4 moiety, the
pyran ring of which can be opened readily by metal ion
complexation even without UV light irradiation4 [Scheme 1(a)],
probably due to the polarity increase induced by the metal ion
binding.
Spirobenzopyrans1 and spirobenzothiapyrans2 are well-known
photochromic compounds that isomerize to their corresponding
merocyanine forms under UV light, and vice versa under visible
light or heat. Such kinds of compounds have been studied
extensively in view of photochromic devices.3 However, the
application of spirobenzothiapyrans to photochromic devices
does not seem to be very easy due to the poor thermal stability
of their open colored form. In our previous studies4 on
spirobenzopyrans bearing a monoazacrown ether moiety, we
found that the spiropyran ring opening was facilitated by metal
ion complexation of the crown ether moiety in the presence of
metal ions. This resulted in the spiropyran ring opening even
without UV light irradiation [Scheme 1(a)]. This finding
prompted us to design spirobenzothiapyrans bearing a crown
ether moiety, which we call crowned spirobenzothiapyrans,
expecting facilitation of the thiapyran ring opening by metal ion
complexation of the crown ether moiety. This in turn may allow
the resulting spirobenzothiapyrans to be applicable for photo-
chromic devices. Here we report preliminary results on
significant stabilization of the photoinduced open colored
isomer of a crowned spirobenzothiapyran in the presence of
alkali metal ions, especially Li+.
It should be noted that irradiation of 365 nm UV light‡ on the
1 solution containing Li+ causes a drastic change in the
absorption spectrum (Fig. 1). On the other hand, hardly any
significant spectral change was observed in the 1 solution
without any metal ion and with an equimolar amount of K+. The
Na+ addition afforded only a slight spectral change (Fig. 1).
Definitely, the metal ion complexing ability of the crown ether
The synthesis of crowned spirobenzothiapyran 1 was carried
out in accordance with Scheme 2. The conversion of the
Scheme 1
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ion, even Li+, scarcely induced such UV light-induced iso-
merization to the open colored isomer as seen in the 1
system.
In order to elaborate the thermal stability of the open colored
form of crowned spirobenzothiapyran 1 in MeCN, thermal
decoloration of the 1 solution in the presence or absence of Li+
was followed at room temperature by turning off UV the light.§
In the absence of Li+, the thermal isomerization back to the
spiropyran form was complete within 5 s. On the contrary, the
thermal back-isomerization in the presence of Li+ was very
sluggish even by visible light irradiation, the half-life of the
UV-induced merocyanine isomer being about 2 min. This again
proves the Li+ complexation-induced high stability of the
colored merocyanine form in crowned spirobenzothiapyran 1.
In conclusion, the crowned spirobenzothiapyran 1 exhibits
characteristic photochromism as follows: i) the isomerization of
1 to its corresponding colored merocyanine form does not
proceed even in the presence of any crown ether complexing
metal ion, unless otherwise irradiated by UV light; ii) the Li+-
selective complexation of the crown ether moiety facilitates UV
light induced isomerization (thiapyran ring opening) dramat-
ically; and iii) the metal ion complexation stabilizes the colored
merocyanine isomer thus formed to a great extent. Thus the
photochromic crown ether seems to be promising for applica-
tions such as photochromic devices.
Notes and references
† Selected data for 1 : mp 146–148 °C; dH(CDCl3, 500 Hz) 1.23 (3H, s,
CH3), 1.37 (3H, s, CH3), 2.68 (3H, s, NCH3), 2.71 (4H, t, J 4, NCH2),
3.53–3.77 (14H, m, OCH2 and PhCH2N), 5.98 (1H, d, J 6, CHN), 6.50 (1H,
d, J 7.5, ArH), 6.85 (1H, t, J 7, ArH), 6.93 (1H, d, J 6, CHN), 7.06 (1H, d,
J 6.5, ArH), 7.17 (1H, t, J 7.5, ArH), 7.93 (1H, s, ArH), 8.73 (1H, s, ArH).
Calc. for C28H35N30O5S: H, 6.67, C, 64.00; N, 8.00; S, 6.10. Found: H,
6.63; C, 63.86; N, 7.86; S, 5.84%.
‡ The UV light, obtained by passing light of a 250 W Hg lamp through a
light filter (wavelength 363.25 nm, Dm/2 9.5 nm, transmittance 0.53), was
introduced to the cell compartment of a spectrophotometer by using a glass
fiber guide and was irradiated on the quartz cell containing a solution. The
absorption spectra were therefore taken, while irradiating the measurement
cell in the perpendicular direction to the measuring incident light.
§ The thermal coloration was followed by measuring the absorbance at 550
nm after UV light irradiation for 1.5 min.
Scheme 2
moiety of 1 stabilizes the colored merocyanine isomer that is
formed only on UV light irradiation, owing to the polarity
enhancement. It is worth noting that the stabilization is highly
selective for Li+. In addition to the selective Li+ binding with a
monoaza-12-crown-4 moiety of 1, some interaction between the
thiolate anion and Li+ complexed by the crown ether moiety
probably contributes to the stabilization of the colored mero-
cyanine isomer of 1, as is the case with the Li+ complex for
merocyanine form of crowned spirobenzopyran (Scheme 1).4c
This Li+-specific stabilization of the photoinduced colored form
of 1 is also supported by photochromism for a model compound
carrying a linear oligooxyethylene moiety instead of a crown
ether moiety, 2. Since the linear oligooxyethylene moiety of 2
cannot bind a metal ion very powerfully, the presence of a metal
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Fig. 1 Absorption spectra of crowned spirobenzothiapyran 1 under UV
irradiation in the absence and presence of an alkali metal ion. 1 and alkali
metal perchlorate: 2 3 1024 mol dm23 in acetonitrile.
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Chem. Commun., 1999, 1453–1454