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Chemistry Letters Vol.36, No.12 (2007)
Photoisomerization of 2-(2-Hydroxyphenyl)benzothiazole Derivatives
as an Adequate Model to Reveal Photochromism of Schiff Bases in the Solid State
Masatsugu Taneda, Yoshinori Kodama, Yusaku Eda, Hiroyuki Koyama, and Toshio Kawatoꢀ
Department of Chemistry, Faculty of Sciences, Kyushu University, 4-2-1 Ropponmatsu, Chuo-ku, Fukuoka 810-8560
(Received September 5, 2007; CL-070962; E-mail: Kawato@chem.rc.kyushu-u.ac.jp)
Photochromic behavior of 2-(2-hydroxyphenyl)benzothia-
to prove that the pedal motion of the molecule is not always
necessary for the occurrence of photocoloration of Schiff bases,
whose photochrome is ascribed to the formation of a cis-keto
isomer.
zole derivatives, whose photoinduced deformation was restricted
by the cyclic structure, was first observed to suggest the exis-
tence of a cis-keto form as a certain structure for the photo-
chrome of N-salicylideneanilines in the crystal state.
2-(2-Hydroxyphenyl)benzothiazole derivatives used in
this study are shown in Figure 2. Compounds 3, 4, and 5 were
prepared by condensation of 2-aminobenzenethiol with salicylal-
dehyde, 3,5-di-tert-butylsalicylaldehyde, or 3,5-bis(1-methyl-1-
phenylethyl)salicylaldehyde7 in dimethyl sulfoxide, respective-
ly.8 The resultant products were purified by recrystallization
from methanol to yield fine crystals, which were not suited
for X-ray analysis but were identified by satisfactory elemental
analytical data and spectral data.9 Crystals of 5 included a small
amount of water molecules, which could not be removed com-
pletely by heating at 50 ꢁC in vacuo. Although 3 can be regarded
as an analogue of 1, whose aniline ring carbon and azomethine
carbon are connected by a sulfur atom, crystals of 3 did not show
any color change on irradiation with UV light. The ESIPT of
2-(2-hydroxyphenyl)benzothiazoles has been extensively inves-
tigated.10 After the photo-excitation of an enol conformer,
ESIPT from the hydroxy group to the ring nitrogen atom occurs
to give a keto form, which returns to the original enol form in the
electronic ground state. The lifetime of the keto form is known to
be very short in gas phase or in solution.11 Thus far the lifetime
of such a keto form in the crystal state has not been reported.
Crystal structure of 3 was analyzed by Stenson,12 who reported
that 3 was planar and packed face to face with short intermolec-
ular contacts in the crystal. The most important condition for ex-
hibiting crystalline photochromism of Schiff bases is to secure
reaction room for the photoinduced isomerization in the crystal
lattice.13 Thus, 3 undergoes ESIPT in crystals; however, a little
more room in the crystal is needed for 3 to form distorted cis-
keto form that is different from the form derived from a fast equi-
librium between the OH form and the NH form in the electronic
ground state. On the other hand, color of crystals of 4 and 5 with
bulky substituents in the molecules changed from light yellow to
deeper color on UV light irradiation and returned to the starting
yellow after placing them in the dark. The bulky substituents
were effective to act as a space opener, which maintained
room for the molecular deformation of a keto form after ESIPT
to stabilize the photochrome in the excited state as observed
Photochromism of organic molecules has been the subject of
a number of previous investigations owing to its scientific inter-
est as well as technical applications.1 The crystalline photo-
chromism of N-salicylideneaniline (1) and its analogues is of in-
terest owing to the usefulness for a model system to elucidate
molecular tautomerism in the solid state (Figure 1).2,3 There is
general agreement that the coloration processes involve excited-
state intramolecular proton transfer (ESIPT) from the o-hydroxy
group to the imine nitrogen atom followed by framework
changes in the molecule.3 However, the mechanistic diversity
has not completely been solved yet. An unambiguous mechanis-
tic study, made by Ohashi et al. using X-ray diffraction analysis,
shed light on a trans-keto form as the photocolored species of N-
(3,5-di-tert-butylsalicylidene)-3-nitroaniline (2).4 Considering
from the result, the photochromism is caused by pedal motion
of two phenyl rings of the molecule. This suggests that the inter-
atomic distances at the peripheral part of the molecule may not
be so important for the photochromism. In contrast with these as-
pects, we have reported that the stability of the photochrome de-
pends on the circumstances of molecules in the crystal lattice.
Furthermore, our previous communication described the coexis-
tence of another type of isomer, which was tentatively assigned
to a cis-keto form, in the photocolored crystals.5 Even if a small
amount of a cis-keto species is present in the crystal, its structur-
al resemblance to the original enol–imine form can lead to
serious difficulties in solving the X-ray diffraction data with
satisfactory accuracy. The clarification of the mechanism of
color change processes of Schiff bases and the development of
new compounds with photochromic properties continue to be
of interest in the search for practical utilization of ESIPT.6 In
order to be able to tackle this problem we needed additional
reliable or preparative information on the structure of the
photochrome. In this study, we employed cyclic Schiff bases
R1
R2
O
hν
R2
R2
R1
R1
O
H
N
R
R
O
H
R
R
H
O
∆
N
N
S
N
S
hν
R1
R1
H
trans-keto form
∆
hν
H
O
R1
R2
3: R = H
∆
4: R = tBu
N
1
2
H
H
5: R = C(CH3)2(C6H5)
C(CH3)3
NO2
cis-keto form
R1
Figure 2. Photochromism of 2-(2-hydroxyphenyl)benzothia-
zoles.
Figure 1. Photochromism of N-salicylideneanilines.
Copyright Ó 2007 The Chemical Society of Japan
Taneda, Masatsugu
