Non-destructive erasable molecular switches and memory using light-driven twisting motions

Article abstract of DOI:10.1039/c0cc02685a

Novel types of chiroptical switches and memory with non-destructive readout that are entirely optically controlled for molecular devices in solution and neat films.

Full text of DOI:10.1039/c0cc02685a

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Non-destructive erasable molecular switches and memory using
light-driven twisting motionsw
a
ab
a
b
Masuki Kawamoto,* Natsuki Shiga, Kazuto Takaishi and Takashi Yamashita
Received 20th July 2010, Accepted 26th August 2010
DOI: 10.1039/c0cc02685a
7
b
Novel types of chiroptical switches and memory with non-
destructive readout that are entirely optically controlled for
molecular devices in solution and neat films.
polarized light using trans–cis isomerization of the azo-
benzene moiety. If azobenzene derivatives are completely fixed
in the cis conformation in the dark, they can be characterized
as thermally irreversible and fatigue-resistant azobenzenes.
Herein we report chiroptical switches and memory with non-
destructive readout that are entirely optically controlled for
molecular devices in solution and neat films. The switching
behaviour exhibits high fatigue resistance and stored informa-
tion remained stable over 1.7 months.
Artificial molecular machines are currently of interest because
of their response to external stimuli such as electric field,
temperature, pH, ionic strength, and light, which leads to
1
changes in their molecular motions. Photoresponsive molecular
machines have received a great deal of attention in the past
decade; their high sensitivity and fast response are of great
interest for molecular devices. These compounds are expected
to find application in the fields of molecular switches, memory,
Fig. 1 shows the chemical structures of R3A, S3A, and R2A
used in this study. The thermal behaviour of the compounds
was investigated by differential scanning calorimetry (DSC) at
2
ꢀ1
and logic gates.
a heating rate of 10 1C min . On the first heating cycle, an
endothermic peak (change in enthalpy (DH) = 30 kJ mol )
ꢀ
1
Chiroptical switches for light-driven molecular devices are
one of the most promising strategies for optical data storage
3
and processing. An ideal chiroptical compound should simul-
caused by melting was observed at 205 1C. Upon cooling to
room temperature, the compound formed a transparent glass
via a supercooled liquid state. We detected an endothermic
event corresponding to the glass transition temperature at
112 1C on the second heating cycle. The formation of the
glassy state was also evident in the X-ray diffraction patterns
of the samples. Before thermal treatment, the sample exhibited
sharp diffraction peaks because of its crystalline nature. After
thermal treatment above the melting point, the diffraction
pattern of the sample showed only a broad halo. Neat films
with a thickness of 120 nm were prepared by spin coating a
solution of R3A and S3A in chloroform onto a fused silica
substrate. The samples exhibited isotropic properties because
they lacked grain boundaries under a polarizing microscope.
The maximum absorption wavelengths (lmax/nm) of R3A
taneously possess the properties of photobistability and fatigue
resistance as well as high sensitivity and spatial resolution.
Non-destructive readout is desirable for high-performance
molecular switches and memory, because this method does
4
not erase stored information.
Azobenzene is a typical photochromic compound that changes
its shape upon photoirradiation. It exhibits a large change in
length upon photoisomerization, with the distance between the
0
˚ ˚
- and 4 -carbons decreasing from 9.0 A (trans form) to 5.5 A
4
5
cis form). Several groups have demonstrated optical switching
(
6
of azobenzene derivatives upon trans–cis photoisomerization.
Unfortunately, cis–trans thermal back isomerization of azo-
benzenes takes place even at room temperature, resulting in
deterioration of the stored information. Azobenzene derivatives
have lifetimes for cis–trans thermal back isomerization at room
ꢀ
1
ꢀ1
and S3A in 1,4-dioxane were 231 (e/M cm 171 800), 295
(23 800), 331 (26 400), 365 sh (14 400) and 436 nm (850). Some
5
1
of these can be assigned as the B
1
(231 nm), L
temperature ranging from seconds to days. There has been an
b
a
(293 nm), and
1
1
(331 nm) transitions of the binaphthyl group. The B
ever-growing effort to find effective compounds for inducing the
cis-azobenzene with high thermal stability. However, the strategy
for the thermally stable azobenzenes is still unclear at the
present stage.
L
L
b
b
and
1
b
bands correspond to the long axis of the naphthyl group,
8
while the L band corresponds to the short axis. The trans
a
1
form of the azobenzene moiety in the compounds exhibited
We previously reported novel light-driven chiral compounds
containing a photoresponsive azobenzene moiety and a chiral
7
binaphthyl moiety in a single species. It was found that
the compounds exhibit photomodulation of liquid-crystalline
7a
helical structures and photoinduced chirality by elliptically
a
RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
E-mail: mkawamot@riken.jp; Fax: +81 48 467 9389;
Tel: +81 48 467 2745
Department of Pure and Applied Chemistry,
b
Faculty of Science and Technology, Tokyo University of Science,
2641 Yamazaki, Noda, Chiba 278-8510, Japan
w Electronic supplementary information (ESI) available: Synthetic
procedures, experimental and analytical details, spectral and kinetic
data. CCDC 776373 and 776374. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0cc02685a
Fig. 1 Chemical structures of the photoresponsive chiral compounds
R3A, S3A, and R2A, respectively.
8
344 Chem. Commun., 2010, 46, 8344–8346
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absorption maxima at 365 nm due to a p–p* transition and
36 nm from a n–p* transition. Circular dichroism (CD)
spectra of R3A and S3A in 1,4-dioxane exhibited features that
molecular shape significantly alters the stabilization of these
compounds upon cis–trans isomerization.
4
The change in the molar circular dichroism (De) of a
binaphthyl-based transition is dependent on the dihedral angle
between the two naphthyl moieties. The change in the value of
mirrored one another. Exciton couplets at ca. 240 and 440 nm
1
were derived from B transitions of the two naphthyl moieties
b
8
ꢀ1
ꢀ1
and n–p* transitions of the azobenzene moieties, respectively.
The superimposed CD spectra of the films of R3A and S3A
|Deafter ꢀ Debefore| at 238 nm was 210 M cm . Rosini et al.
calculated the relationship between the value of De and the
1
1
also formed a symmetrical image, providing evidence for the
1
dihedral angle.
From the results of the calculation,
opposite chirality of the binaphthyl moieties, with the
B
b
the change in the dihedral angle of the compound is estimated
to be about 301 after photoirradiation. DFT calcula-
tions revealed that the dihedral angles of trans-R3A and
cis-R3A are 1031 and 751, respectively, which is a change of
transitions appearing at 244 nm and a weak exciton couplet
occurring between 400 and 600 nm.
Irradiation of R3A and S3A in 1,4-dioxane at 365 nm
induced trans–cis isomerization of the azobenzene moiety.
The ratio of the trans : cis isomers in the photostationary state
1
2
approximately 301.
R3A and S3A exhibited specific suppression of cis–trans
thermal back isomerization of the azobenzene moiety. Because
of these desirable characteristics, the non-destructive molecular
switches and memory of the compounds were investigated.
When the chiral cyclic compounds are irradiated at 365 and
436 nm, the molecular twisting motion can be induced by
trans–cis and cis–trans photoisomerization of the azobenzene
1
9
was determined to be 15 : 85 using H NMR spectroscopy.
On the other hand, the ratio after irradiation at 436 nm for the
cis–trans photoisomerization process was 99 : 1, indicating
that recovery of the initial state could be achieved. First-order
rate constants (k) and thermodynamic parameters such as
a
enthalpy of activation (DH ) and entropy of activation
a
(
DS ) for thermal cis–trans isomerization were determined
in 1,4-dioxane according to the Eyring equations (k at 298 K:
D
moiety, respectively. In contrast, optical rotation ([a] ) can be
1
0
monitored by irradiation at 589 nm without any substantial
changes, because the chiral compounds are inactive toward
photoisomerization at this wavelength. It is confirmed that the
7
ꢀ1
a
ꢀ1
a
ꢀ1 ꢀ1
2
.2 ꢁ 10ꢀ
s
; DH : 25 kcal mol ; DS : ꢀ5.1 cal mol
K ).
The lifetime (t: 1/k) of R3A at 298 K is 1263 h (52.6 days)
2
0
(
Fig. 2a), which is extremely long compared with those of
change in the value of the molar optical rotation ([F] ,
D
c = 0.1 in chloroform) of R3A before and after irradiation
azobenzene derivatives previously reported. The value of t of
0
20
D
20
the corresponding 2,2 -disubstituted model compound, R2A,
at 365 nm in a photostationary state, [F]
ꢀ [F] after,
D
before
20
1
3
is 122 h at 298 K, even though these compounds have the same
molecular weight. The specific difference is interpreted in terms
was 15401. A specific large change in the [F]D of R2A can be
obtained by means of photoinduced molecular twisting
a
20
D
after photo-
of the molecular shape of the structures. DS reflects the
difference in the degree of freedom between the ground and
motions. The magnitude of modulations of [F]
irradiation at 365 nm was approximately 97001. These results
indicate that the chiral cyclic compounds possess both a large
a
transition states. The value of DS is derived from the cyclic
structure by assuming that R3A shows limited formation
2
0
change in the value of [F]
D
upon irradiation and efficient
a
ꢀ1
ꢀ1
compared with R2A (DS : ꢀ1.3 cal mol
K ). In fact, a
reversibility. Because of these favorable properties, the com-
pounds are regarded as promising for chiral switching
applications.
crystal structure of trans-R3A exhibited a strained structure in
comparison to that of trans-R2A (Fig. 2b and c). Because the
0
distance between the 3- and 3 -carbon atoms in the azobenzene
0
Optically controllable neat films of the chiral compounds
(thickness: 500 nm), which are driven by photon-mode
processes with a non-destructive readout, were achieved by
exploiting the dynamic twisting motion of the molecules.
Photoinduced changes in the optical rotations, |abefore ꢀ aafter|,
moiety is longer than that of the 2- and 2 -carbon atoms, the
cyclic linkage of R3A provides strong communication between
the terminal groups of the azobenzene moiety attached to the
binaphthyl moiety. In contrast, cis-R3A has greater steric
hindrance and a distorted conformation compared to cis-R2A
according to the results of density functional theory (DFT)
calculations at the B3LYP/6-31(d) level. This difference in
ꢀ1
were about 1001 cm and these values can be switched by
Fig. 3 (a) Chiroptical switching with non-destructive readout of the
values of a of neat films of R3A (red) and S3A (blue) (monitoring
wavelength: 589 nm) at 298 K. (b) Photoswitching of the absorbance of a
neat film of R3A (monitoring wavelengths: 365 nm (circles) and 436 nm
Fig. 2 (a) Lifetimes of cis–trans thermal back isomerization of R3A
ꢀ2
(red) and R2A (blue) in 1,4-dioxane at various temperatures. Crystal
(diamonds)) at 298 K. Irradiation wavelengths: 365 nm (10 mW cm ,
ꢀ2
structures of (b) trans-R3A and (c) trans-R2A.
600 s) and 436 nm (10 mW cm , 600 s), film thickness: 500 nm.
This journal is c The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 8344–8346 8345

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alternating irradiation between 365 and 436 nm (Fig. 3a).
Although the photoinduced changes in the values are not
equal, the recovery of the initial state could be achieved by
irradiation with visible light to cause cis–trans photoisomeri-
zation of the azobenzenes (Fig. 3b). The value of t of the chiral
cyclic compound in a neat film with respect to cis–trans
thermal back isomerization of the azobenzene moiety at
they will allow the fabrication of high-performance solid
molecular devices that are driven by incident light.
We thank Dr D. Hashizume and Dr T. Fukunaga of
RIKEN for X-ray measurements and their valuable comments
and suggestions. This research was partially supported by the
Sumitomo Foundation and the Kurata Memorial Hitachi
Science and Technology Foundation (to M. K.).
2
98 K is 620 h (25.8 days). This means that neat films of
R3A and S3A exhibit good fatigue resistance at the irradiated
site. These results indicate that the chiral cyclic compounds
can be used as optical switches as well as for optical data
storage in the solid state.
Notes and references
1 For reviews, see: (a) V. Balzani, A. Credi and M. Venturi,
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Mater., 2006, 18, 1239–1250; (e) B. L. Feringa, J. Org. Chem.,
In this work, molecular structures and device parameters
such as the film thickness were not optimized. According to
the absorption spectra, incident light at 365 and 436 nm is
absorbed by the bulk of the neat film and photoisomerization
of the compounds takes place over almost the whole area
2
007, 72, 6635–6652.
(a) T. Muraoka, K. Kinbara and T. Aida, Nature, 2006, 440,
12–515; (b) A. Credi, Angew. Chem., Int. Ed., 2007, 46,
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2
(
Fig. 3b). As mentioned above, R3A and S3A formed smooth,
5
uniform thin films that do not scatter light at 589 nm. The net
population of the excited molecules formed by the absorption
of photons could be controlled by choosing a suitable
film thickness. Irie reported a new class of photochromic
diarylethenes that show open- and closed-form isomers with
¨
(
J. Am. Chem. Soc., 2010, 132, 4191–4196.
3
4
B. L. Feringa, R. A. van Delden and M. K. J. Ter Wiel, in
Molecular Switches, ed. B. L. Feringa, WILEY-VCH, Weinheim,
2001, pp. 123–163.
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Branda, Angew. Chem., Int. Ed., 2001, 40, 1752–1755;
1
4
high thermal stability and high fatigue resistance. However,
diarylethenes have two conformations, parallel or antiparallel,
and cyclization can proceed only from the antiparallel con-
former. The population ratio of the two conformations is
essentially 1 : 1, so the cyclization quantum yield cannot
(b) B. Kippelen, P.-A. Blanche, A. Schulzgen, C. Fuentes-Hernandez,
¨
G. Ramos-Ortiz, J.-F. Wang, N. Peyghambarian, S. R. Marder,
´
A. Leclercq, D. Beljonne and J.-L. Bredas, Adv. Funct. Mater., 2002,
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1
Proc. Natl. Acad. Sci. U. S. A., 2003, 100, 8109–8112; (d) V. Garcia,
1
4
exceed 0.5. When the ratio of the antiparallel conformation
is increased, the quantum yield is also expected to increase. An
interesting approach to achieving selective cyclization is to use
chiral diarylethenes that show highly diastereoselective ring
S. Fusil, K. Bouzehouane, S. Enouz-Vedrenne, N. D. Mathur,
A. Barthelemy and M. Bibes, Nature, 2009, 460, 81–84.
´ ´
For reviews, see: (a) K. Ichimura, Chem. Rev., 2000, 100,
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6
1
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5
closure. On the other hand, the advantage of using azo-
benzenes is their simple isomerization reversibility, which
ensures dynamic change in the molecular structure as mentioned
above. It is believed that cyclic linkage of the molecular
structure would be a simple and effective approach to realize
photochromic compounds with higher thermal stability. New
cyclic molecules with good performance are expected to be
developed using this line of molecular design.
1
2037–2057, and references therein.
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˜
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In conclusion, novel chiral cyclic compounds containing
binaphthyl and azobenzene functionalities were demonstrated
to behave uniquely as non-destructive erasable chiroptical
memory through photoinduced switching of the dihedral angle
of the binaphthyl moiety. Chiroptical switches that exhibited
high fatigue resistance and stable information storage for over
1
ꢀ2
9
R3A or S3A in 1,4-dioxane-d was irradiated at 365 nm (10 mW cm )
8
for 100 s. The change in the doublet at 7.5 ppm from the 5,50-position
of the azobenzene moiety before and after irradiation was monitored.
1
1
0 H. Eyring, Chem. Rev., 1935, 17, 65–77.
1 C. Rosini, S. Superchi, H. W. I. Peerlings and E. W. Meijer, Eur. J.
Org. Chem., 2000, 61–71.
1
.7 months in 1,4-dioxane at 298 K were achieved by alter-
nating the photoirradiation wavelength. Furthermore, the
values of the optical rotations of the neat film can be switched
without deterioration of the stored information by means of
the dynamic molecular twisting motions of the photoresponsive
chiral compounds. Because both isomers of the compounds
show good film-forming properties and high thermal stability,
12 According to the crystal structure of trans-R3A, the dihedral angle
of the binaphthyl moiety is about 1101.
2
0
20
1
1
1
3 The value of [F]
D
is defined as [a]
D
/100 ꢁ molecular weight.
4 M. Irie, Chem. Rev., 2000, 100, 1685–1716, and references therein.
5 Y. Yokoyama, T. Shiozawa, Y. Tani and T. Ubukata, Angew.
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8
346 Chem. Commun., 2010, 46, 8344–8346
This journal is c The Royal Society of Chemistry 2010