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Chemistry Letters Vol.38, No.1 (2009)
Novel Photoisomerization of N-(2-Nitrobenzylidene)-3-amino-5-methylpyrazole Triggered by
Intramolecular Proton Transfer from Azomethine Group to Nitro Group in the Crystal State
ꢀ1
ꢀ2
Kiichi Amimoto and Toshio Kawato
Department of Science Education, Graduate School of Education, Hiroshima University,
-1-1 Kagamiyama, Higashi-Hiroshima 739-8524
Department of Chemistry, Faculty of Sciences, Kyushu University, 4-2-1 Ropponmatsu, Chuo-ku, Fukuoka 810-8560
1
1
2
(Received September 24, 2008; CL-080918; E-mail: kawato@chem.rc.kyushu-u.ac.jp)
Excited-state intramolecular proton transfer from the azo-
methine group to the nitro group of N-(2-nitrobenzylidene)-3-
amino-5-methylpyrazole induced the formation of 2-nitroso-
benzamide attended by vivid color change from yellow to
reddish brown in the crystal state.
Figure 1. Photographs of crystals of 1 and 1P.
Excited-state intramolecular proton transfer (ESIPT) phe-
nomena continue to receive significant attention because of var-
ious applications to materials design and processes in the fields
1
of chemistry and biology. Certain organic molecules exhibit
2
color changes through ESIPT in the crystal state. A great deal
of chemistry on such photosensitive crystals is now progressing
3
because of their potential usefulness for photoelectron systems.
In the most accessible compounds of this type are included
N-salicylideneanilines,
2
,4
whose coloration process involves
ESIPT from the phenolic hydroxy group to the imino nitrogen
atom, and 2-(2,4-dinitrobenzyl)pyridine, whose photocoloration
originates from a nitro group-assisted ESIPT from the methylene
5
group to the nitrogen atom of the pyridine ring. As for the prac-
Figure 2. UV–vis spectra of 1 before (red line) and after 365-
nm light irradiation for 1 h (blue line); (a) solid-state reflectance
spectra and (b) absorption spectra in CH3CN (½1ꢂ ¼ 1:0 ꢃ
tical usefulness, the resultant photochromes are thermally unsta-
ble and the colored species can return easily to the original form
in the dark. Accordingly, it is required to develop a new ESIPT
system that produces stable photochrome. Now we report that
Schiff base 1 (Scheme 1) bearing a nitro group and a pyrazole
ring is very sensible to light to yield a highly stable photochrome
in the crystal state.
ꢁ4 ꢁ3
mol dm ). Inset in (b) shows absorption spectra in the case
1
0
of a 50-fold concentrated solution.
and in CH3CN solution are shown in Figure 2, which exhibits
that 1P absorbs a wide range of visible light around 400–
650 nm and has a broad absorption maximized at around
760 nm, which is suggestive of formation of an aromatic nitroso
compound by the photoreaction.
Schiff base 1 was prepared by condensation of 2-nitrobenz-
aldehyde with 3-amino-5-methylpyrazole in ethanol. The prod-
uct was purified carefully by recrystallization from ethanol in
the dark to yield pale yellow fibers, whose structure was identi-
Direct information from the photochemical transformation
of the nitro group in 1 to the nitroso group in 1P was obtained
from ATR-FTIR spectroscopy (Figure 3). The intense absorption
1
fied as 1 by satisfactory elemental analysis and H NMR spectral
6
data. Upon exposure of crystals of 1 to room light, the color of
ꢁ1
the crystals turned to bright brown, whose tone became much
deeper with time. Photographs of the powdered crystals of 1 be-
fore and after irradiation with 365-nm light at room temperature
are shown in Figure 1.
Photocolored species 1P was so stable in the crystal state
that neither heating it on a hot plate nor irradiation with an incan-
descent light above 500 nm returned it to 1. The photoreaction of
bands at 1530 and 1340 cm for 1 were indexed to the asym-
metric and symmetric stretching vibration of the nitro group,
respectively. As the photoreaction of 1 proceeded, these charac-
teristic bands of 1 became weaker and new bands appeared at
ꢁ1
1668 and 3280 cm . The former is reasonably assigned to the
characteristic N=O stretching band of a nitroso group, while
the latter is ascribed to the N–H stretching vibration of amide.
Promising clues to explain the photoreactivity of 1 and to imply
the structure of 1P were obtained through NMR analysis
1 to 1P proceeded in CH3CN smoothly. UV–vis spectra of 1 be-
fore and after irradiation with 365-nm light in the crystal state
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1
(
Figure 4). Judging from H NMR spectral change due to the
conversion of 1 to 1P by UV irradiation in CD3CN, the signal
at 9.10 ppm for the azomethine proton of 1 disappeared and
the new broad resonance appeared at 9.73 ppm. This observation
implicates proton abstraction from the azomethine group and
formation of an amide-like group. Remarkable changes of chem-
ical shifts for aromatic protons were also observed concurrently.
Scheme 1. Schiff base 1.
Copyright Ó 2009 The Chemical Society of Japan
Amimoto, Kiichi
