New photo-responsive unit of biindenylidene with TEMPO radical substituents

Article abstract of DOI:10.1016/j.tetlet.2007.06.067

Incorporation of organic radical TEMPO into photochromic biindenylidene system leads to the synthesis of a novel dual functional compound 4. Its photochromism, structure, magnetic properties as well as ESR spectroscopy were deliberately investigated. Rema

Full text of DOI:10.1016/j.tetlet.2007.06.067

Tetrahedron Letters 48 (2007) 6044–6047
New photo-responsive unit of biindenylidene with TEMPO
radical substituents
Xu Li,a,b Jie Han,a,* Mei-Li Pang,a Yong Chen,a Jian-Xin Zhang,a Hong Ma,c
Zheng-Jie Hea and Ji-Ben Menga,*
aDepartment of Chemistry, Nankai University, Tianjin 300071, China
bThe Chinese People’s Armed Police Forces Academy, Langfang 065000, China
cDepartment of Materials Science and Engineering, University of Washington, Seattle, WA 98195, USA
Received 29 December 2006; revised 11 June 2007; accepted 15 June 2007
Available online 20 June 2007
Abstract—Incorporation of organic radical TEMPO into photochromic biindenylidene system leads to the synthesis of a novel dual
functional compound 4. Its photochromism, structure, magnetic properties as well as ESR spectroscopy were deliberately investi-
gated. Remarkable changes of the ESR spectra were observed for 4 upon photoirradiation with different light sources. The inter-
molecular interactions were interpreted with regard to its crystal structure.
ꢀ 2007 Elsevier Ltd. All rights reserved.
The development of novel magnetic material, which
responds to light, heat, pressure or other stimuli, is of
significance from the viewpoints of both basic science
and new technology emergence.1 Currently, there is
much interest in the preparation of organic photochro-
mic and photomagnetic materials, in which color and
magnetic properties can be controlled by light. Such
an optically induced tuning of color and magnetic
behaviors may have potential applications in molecular
photoswitching devices for memory and information
processing.2 Up to date, several types of organic photo-
chromic and photomagnetic compounds such as azo-
benzene, diarylethene, spiropyran, terphenoquinone,
anthracene, naphthopyran, and hexaarylbiimidazole
have been discovered, and their properties have been
extensively investigated.3 Aminoxyl radical moieties
have been widely adopted in these compounds, because
of their versatility and good stability in the field of mate-
rial chemistry as building blocks for molecule-based
magnetic materials.4 As disclosed in recent reports from
our group and others,5 the biindenylidene derivatives
are a unique class of photochromic organic compounds,
which undergo photochromism and simultaneously
generate radicals in the crystalline state. If the stable
aminoxyl radical moiety is incorporated into such a
photochromic compound like biindenylidene, a new
multifunctional molecule could be designed with two
kinds of magnetic moieties. This new photochromic
compound could serve as a good candidate to investi-
gate inter- or intra-molecular magnetic interaction,
which could be controlled by irradiation with light.
With this idea in mind, we have successfully synthesized
a new dual-functional biindenylidene derivative 4 by
introducing a nitroxide radical (TEMPO) to the biinden-
ylidene system (Scheme 1). The structure of 4 was
confirmed by single crystal X-ray diffraction, and its
photochromic and photomagnetic properties were inves-
tigated, and characterized with UV–vis absorption and
ESR techniques. For the purpose of comparison, the
precursor 3 was investigated accordingly as well. Herein,
we report the results from these investigations.
The photocolor reaction of 3 and 4 was monitored by
UV–vis absorption. As expected, the precursor 3 shows
photochromic property. Yellow powder 3 turns to
brown, when exposed to sunlight for a few minutes,
and the brown powder returns slowly to yellow solid
in the dark or upon heating. The new radical compound
4 shows photochromism in a crystalline state, Scheme 2
shows the supposed photochromic interconversion
between 4a and 4b. The yellow crystals of 4a turn to
Keywords: Photochromism; Magnetic; Biindenylidene; Radicals.
*
Corresponding authors. Tel.: +86 22 2350 9933; fax: +86 22 2350 2230
(J.-B.M.); e-mail addresses: hanjie@nankai.edu.cn; mengjiben@
nankai.edu.cn
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.06.067

X. Li et al. / Tetrahedron Letters 48 (2007) 6044–6047
6045
OCH2CH3
CH
0.8
OH
O
H3CH2CO
a)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
O
OH
O
HO
N
1
N
HC
O
OH
O
2
b)
O
H3CH2CO CH
OH
CHO
OCH2CH3
O
HO
OH
O
200
300
400
500
600
700
800
Wavelength / nm
c)
HC
OH
N
O
Figure 1. UV spectral changes of 4 (Á Á Á) before and (—) after
N
O
irradiation.
4
3
OHC
Scheme 1. Synthetic route of the radical biindenylidene 4. (a) p-
benzaldehyde diethyl acetal magnesium bromide in dry THF, then air
oxidation H+, 46%; (b) p-toluenesulfonic acid in chloroform, 90%; (c)
4-amino-TEMPO and AcOH in dichloromethane, 60%.
changes were recorded at room temperature. Figure
2a–c show the corresponding ESR spectra of 4 in differ-
ent states upon unirradiation and upon irradiation with
different light source in solid state.6 The ESR spectrum
of the unirradiated 4 shows just one ESR signal—that
of the TEMPO (g = 2.0055, Fig. 2a). When 4 is exposed
to sunlight for several minutes, the ESR spectrum
N
N
N
N
HC
HC
O
O
changes apparently, and
a
new radical signal
(g = 2.0056, Fig. 2b) appears besides that of TEMPO.
Apparently, there exist two different kinds of radicals
in the irradiated compound 4, one kind of radical gener-
ated from two indanione moieties,5b as expected in
Scheme 1, the other originated from the stable TEM-
POs, and the former kind of radicals is photo-respon-
sive, so the photochromism and radical behaviors of
compound 4 can be controlled by light. It is worthy to
note that the two expected ESR signals merge, forming
a broad pattern, when 4 is irradiated with a high pres-
sure Hg lamp for 10 min (g = 2.0075, Fig. 2c), which
may be explained by the interactions existing between
the two kinds of radicals.
O
O
OH
OH
hv
heat
OH
OH
O
O
HC
HC
N
N
N
N
O
O
4a
4b
Scheme 2. Photochromic reaction of 4.
reddish brown 4b, when exposed to sunlight for a few
minutes. Compound 4b is thermally labile, and the
reddish brown crystals fade and return to the original
yellow gradually in the dark or quickly upon heating
under nitrogen atmosphere. Also, UV–vis absorption
of 4 considerably increases in the range of 500–800 nm
wavelength after irradiation (Fig. 1). This result is
consistent with the proposed photochromism mecha-
nism (Scheme 2).
When the irradiated reddish brown 4 was dissolved in
dichloromethane, there is only a typical TEMPO signal
(Fig. 2d) in the ESR spectrum of the resultant solution,7
and the ESR spectra of compound 4 did not change in
solution under irradiation, which also means that the
radicals originated from the indanione moieties are also
quenched in the solution. Further research discovered
that the family compounds in this work display photo-
chromism and simultaneously generate radicals in solid
or crystalline state; however, the radicals generated from
the two indanione moieties can be easily quenched in
solution and the photochromism also does not appear
in solution.
The significant changes of the ESR spectra of 4 were
observed along with the photochromism in solid state
after irradiation with different light source. For the com-
parison with compound 4, the precursor 3 was tested by
ESR technique before and after irradiation, respectively.
The unirradiated yellow 3 does not show ESR signals at
room temperature, and the irradiated brown 3 shows
ESR signals. While the irradiated brown 3 is dissolved
in dichloromethane, the resultant solution does not
show any ESR signals. The results clearly indicate that
the radicals generated from two indanione moieties are
quenched in the solution. In the ESR measurement, 4
was irradiated with UV or visible light, and its ESR
The magnetic susceptibility of the photochromic com-
pound 4 was measured by SQUID susceptometer within
the temperature range of 2–300 K (Fig. 3). A Curies–
Weiss plot of 1/v versus T yields Curies constant of
0.62 emu K molÀ1 and a Weiss constant of À9.38 K.
The data have been deduced after calibration. Accord-
ing to Curie–Weiss law, both constants are in favor of
antiferromagnetic interaction.8

6046
X. Li et al. / Tetrahedron Letters 48 (2007) 6044–6047
0.06
500
400
300
200
100
0
0.05
0.04
0.03
0.02
0.01
3360 3390 3420 3450 3480 3510 3540 3570 3600 3630
0.00
0
50
100
150
200
250
300
a Magnetic Field / G
T (k)
Figure 3. Temperature dependence of the magnetic susceptibility (v)
and 1/v for 4 is shown in the solid line.
3360 3390 3420 3450 3480 3510 3540 3570 3600 3630
b Magnetic Field / G
Figure 4. Molecular structure of isomer 4.
analysis was carried out on 4 (Fig. 4).9 Compound 4
has a central symmetry with two indanione moieties
˚
linked by a double bond C1A–C1AA (1.342(6) A).
Two loops of indanione are almost perfectly parallel in
˚
very short perpendicular distance (0.2321 A), hence the
double bond has little distortion. This kind of confor-
mation could be crucial to its photochromic and photo-
magnetic properties. There is also intermolecular
hydrogen bonding among the molecules, which could
stabilize the radicals (Fig. 5). The shortest distance
between the neighboring TEMPO radical molecules is
3360 3390 3420 3450 3480 3510 3540 3570 3600 3630
c Magnetic Field / G
3460
3480
3500
3520
3540
d Magnetic Field / G
Figure 2. ESR spectra of 4 at different states at room temperature.
In order to investigate the relationship between the pho-
toreactivity and the structure, X-ray crystal structure
Figure 5. Stacking of compound 4, the dotted line means the H-
bonding interaction.

X. Li et al. / Tetrahedron Letters 48 (2007) 6044–6047
6047
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found to be 5.391 A for oxygen–oxygen, which is a fair
distance as to allow only weak antiferromagnetic inter-
action between the spins.10 According to the McConnell
mechanism,11 the relative signs of the spin densities on
those oxygen atoms are opposite to each other, which
leads to the intermolecular antiferromagnetic result.
On the other hand, the shortest distance between the
˚
adjacent molecular carbonyl oxygen atoms is 6.923 A,
which is too long for any possible interaction.
In conclusion, we have first synthesized a novel biinde-
nylidene derivative having two TEMPO radicals in the
molecule, and studied its photochromism and photo-
magnetism in the crystalline state. ESR results clearly
show that two kinds of radicals exist in the crystalline
molecule 4, and the radical generated from two indani-
one moieties is photo-responsive. Incorporation of
organic photochromism into magnetic system may not
only lead to interesting photochromic behaviors, but
may also provide a new strategy to develop photo-con-
trol magnetic materials. Herein we have made a small
step in the direction. Further detailed studies of the
mechanism of photomagnetism of 4 are in progress in
our laboratory.
Acknowledgement
This work was financially supported by grants from the
National Natural Science Foundation of China (Nos.
20490210, 20372039).
6. The ESR test was carried out on a Bruker EMX-6/1 EPR
Spectrometer. The g value was calculated by the known
parameters of g3 and g4 of Mn-mark. X-band EPR spectra
were obtained at room temperature. Measurement condi-
tions: center field 3510.000 G, sweep width 600.000 G,
modulation frequency 100.00 kHz, modulation amplitude
0.50 G, and microwave frequency 9.854 GHz.
7. Measurement conditions: center field 3510.000 G, sweep
width 200.000 G, modulation frequency 100.00 kHz, mod-
ulation amplitude, 0.50 G, and microwave frequency
9.854 GHz.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2007.06.067.
8. Kaneko, T.; Akutusu, H.; Yamada, J.-I.; Nakatsuji, S.
Org. Lett. 2003, 5, 2127–2129.
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c = 10.2948(16) A, b = 105.032(3)ꢁ, T = 294(2) K, Dc =
3
1.298 g cmÀ3
,
V = 2499.9(7) A , crystal size = 0.22 ·
˚
0.18 · 0.14 mm, Z = 4, F(000) = 1028, l = 0.290 mmÀ1
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,
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