Received: September 28, 2013 | Accepted: November 5, 2013 | Web Released: November 9, 2013
CL-130904
Photochromic Property of Anionic Spiropyran with Sulfonate-substituted Indoline Moiety
Akira Sugahara,1 Naru Tanaka, Atsushi Okazawa, Nobuyuki Matsushita, and Norimichi Kojima*1
,³
1
1
2,3
1
Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo,
3
-8-1 Komaba, Meguro-ku, Tokyo 153-8902
Department of Chemistry, College of Science, Rikkyo University,
-34-1 Nishi-ikebukuro, Toshima-ku, Tokyo 171-8501
Research Center for Smart Molecules, Rikkyo University,
-34-1 Nishi-ikebukuro, Toshima-ku, Tokyo 171-8501
2
3
3
3
(
E-mail: cnori@mail.ecc.u-tokyo.ac.jp)
For fabricating photoresponsive multifunctional materials,
we have developed the potassium salt of a new anionic
However, many functional metal complexes such as spin-
crossover complexes are cationic or neutral because of the
presence of transition-metal ions with positive charge. In fact,
there are few anionic spin-crossover complexes, which are
iron(III) complexes of tridentate salicylaldehyde thiosemicarba-
spiropyran,
1¤,3¤,3¤-trimethyl-6-nitrospiro[2H-1-benzopyran-
¹
2,2¤-indoline]-5¤-sulfonate (1 ), with the sulfonate group present
on the indoline moiety and whose crystal structure was revealed
by single-crystal X-ray diffraction analysis. The photochromism
of 1¢K was demonstrated at room temperature in the KBr-
diluted state, as well as in solution. The protonated spiropyran
9
10
zones and related ligands and a few others. The development
of anionic spiropyrans is desired for expanding the molecule
design for the photoresponsive materials based on the organic
inorganic hybrid systems. Sulfonate- or carboxylate-substituted
1
H shows negative photochromism.
1
1
spiropyrans have been reported for use as anionic spiropyrans.
Some examples of photochromism of anionic spiropyrans have
1
1c
Photochromic molecules continue to attract much attention
been reported in some matrices such as reversed micelles,
1
12
13
because of their practical applicability; e.g., in storage media,
LangmuirBlodgett films, silica, and layered double hydrox-
drug-delivery systems, ophthalmic lenses,3 and chemo- and
biosensors. Moreover, a large number of photochromic mole-
2
ides.
14
4
On the basis of the above-mentioned observation, we have
designed novel anionic spiropyrans for further developing the
photoresponsive materials. We report herein a novel anionic
cules have been developed on organicinorganic hybrid sys-
3
,5
tems
having photoresponsivity coupled with transport,
optical, and magnetic properties. Spiropyrans are one of the
best investigated organic photochromic classes. As shown in
Scheme 1 (left), photochromism of spiropyrans is based on the
photocleavage of the CspiroOpyran bond on the spirocyclic (SP)
form, inducing the structural conversion of a colorless (or
weakly colored) SP form to a deep colored merocyanine (MC)
form. The MC form, which is ordinarily less stable than the
closed form, reverts to the SP form via both photochemical and
thermal processes. Yu and co-workers have developed cationic
spiropyran,
1¤,3¤,3¤-trimethyl-6-nitrospiro[2H-1-benzopyran-
¹
2,2¤-indoline]-5¤-sulfonate (1 ), and its protonated molecule
(1H) (Scheme 1).
Spiropyran salt 1¢K was synthesized by the reaction of
sulfonate-substituted indolenium with salicylaldehyde, and was
successfully characterized by single-crystal X-ray structural
1
5
1
analysis, in addition to elemental analysis and H NMR, mass,
1
6
and IR spectroscopies. The protonated spiropyran 1H was
prepared by a reaction of the potassium salt with hydrochloric
acid.
6
7
spiropyrans and spirooxazines, including a pyridinium moiety,
and demonstrated the remarkable photochromism in the crystal-
line state at room temperature. By using the photochromic
compounds as a counter cation, several photoresponsive func-
Figure 1 shows the molecular structure of 1¢K¢H2O.17 The
single-crystal structural analysis reveals the closed form of 1,
and there is a unique independent spiropyran molecule in the
asymmetric unit. The C1(spiro)O1 bond length is 1.485 ¡,
8
tional organicinorganic hybrid systems have been developed.
1
8
which is a typical value (1.451.51 ¡) for spiropyrans. The
five-membered ring of the indoline half is bent at N1C1C2
(
the C8N1C1C2 torsion angle is ¹28.9(3)°). All the sul-
fonate oxygen atoms bridge two potassium ions in the μ-oxo-
bridged manner, building a rigid two-dimensional Á Á ÁOKOÁ Á Á
arrayed sheet structure. On the opposite side of the sheet, the
nitro group makes a one-dimensional network among the
spiropyran molecules by weak noncovalent bonds of O2£O3
1
9
(
2.966(5) ¡) and O2£N2 (3.067(6) ¡). The coordinated water
molecules of O7 and O8 are disordered with an occupancy ratio
of 0.774(12):0.226(12) on the potassium ion.
To confirm the photochromism of the spiropyran, we
measured the UVvis absorption spectra of 1¢K at 300 K in
methanol solution; the spectra are shown in Figure 2 at several
periods of UV light irradiation time. The absorption peaks
corresponding to the ππ* transition of the SP form were
Scheme 1. Photochromic reaction of protonated and deproto-
nated spiropyrans: 1 (left) and 1H (right).
¹
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