A fully photoreversible supramolecular polymer having a diarylethene photoswitch

Article abstract of DOI:10.1246/cl.2010.82

The particle size of a supramolecular polymer composed of a diarylethene photoswitch and two quadruple hydrogenbonding sites having C₁₇ chain changes photoreversibly in tetrahydrofuran at room temperature, while that in a CHCl₃ solution is not photoreversible but is reversible with the assistance of heat.

Full text of DOI:10.1246/cl.2010.82

doi:10.1246/cl.2010.82
82
Published on the web December 26, 2009
A Fully Photoreversible Supramolecular Polymer Having a Diarylethene Photoswitch
Michinori Takeshita,* Miyuki Hayashi, and Takashi Miyazaki
Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, 1 Honjo, Saga 840-8502
(Received October 15, 2009; CL-090927; E-mail: michi@cc.saga-u.ac.jp)
The particle size of a supramolecular polymer composed
of a diarylethene photoswitch and two quadruple hydrogen-
bonding sites having C17 chain changes photoreversibly in
tetrahydrofuran at room temperature, while that in a CHCl3
solution is not photoreversible but is reversible with the
assistance of heat.
Moreover, the compound that we have reported was difficult
to dissolve in most organic solvents except for chloroform or a
mixture of chloroform and ethanol. Therefore, fully photo-
reversible supramolecular formation has not yet been accom-
plished, and this fact has encouraged us to develop a fully
photoreversible supramolecular polymer in solution.
Figure 1 shows a schematic for photochromic diarylethene
that has two quadruple hydrogen-bonding sites. To enhance the
solubility, we have introduced two C17 chains at the ureidopyr-
imidinone moieties shown as compound 2.5 In this system, since
the open form is flexible and can form weak intramolecular
hydrogen bonding, the supramolecule would be difficult to form.
On the other hand, in the closed form, which is generated from
the open form upon irradiation with UV light, the diarylethene
moiety takes place at the site of the cyclization reaction, and two
hydrogen-bonding moieties would be fixed at both sides.
Therefore, intermolecular hydrogen bonding becomes possible
and a supramolecular polymer would be formed. Alternate
irradiation of UV (330 « 20 nm) and visible light (>460 nm)
reversibly changed the absorption spectra of a CHCl3 solution of
2, showing that compound 2 is photochromic.5 Since the
absorption spectrum of the solution of the closed form 2b did
not change when it was stored in the dark at room temperature,
the closed form 2b was thermally stable at room temperature.
The ratio of the open form 2a to the closed form 2b in the
photostationary state at 330 « 20 nm was >99:1, which was
determined with 1H NMR spectroscopy. In contrast to the
diarylethene with hydrogen bonding sites, on which we have
previously reported,4 the solubility of 2a in organic solvents
improved. For example, 2a can be dissolved in THF and a non-
polar solvent such as toluene.
Supramolecular polymers have been attracting much atten-
tion recently since they are a novel material that may rival
conventional macromolecules.1 One of the most important
differences between supramolecular polymers and conventional
macromolecules is reversibility. A supramolecular polymer can
be associated and/or dissociated with external stimuli such as
temperature change, solvent polarity change, and the addition of
chemical substances.1 Some reports have been presented on the
photoswitching of hydrogen-bonded supramolecular polymers;
however, the switching was not fully photoreversible, and
formation or decomposition of the supramolecule took place
with heating since the photoswitches were composed of
thermally reversible azobenzene.2 One report indicates a photo-
switchable self-assembly with diarylethene photoswitch, but
it failed to show a significant viscosity change.3 We have
previously reported a supramolecular polymer that has a
thermally irreversible diarylethene as a photoswitch 1.4 The
supramolecular formation of 1 was photoreversible; however,
the dissociation was stepwise. With the assistance of heat, it has
become possible to switch between monomer and supramolec-
ular polymers with photoirradiation. This phenomenon is due to
stable intermolecular quadruple hydrogen bonding and requires
high temperatures to fully dissociate such strong binding.
H
N
H
N
F
2
O
N
R
F
R
F
2
2
R
Me
N
Me
O
H
N
F
Me
2
H
H
H
N
N
O
H
N
Me
Me
O
S
O
F
2
O
N
Me
N
N
S
N
S
O
N
N
H
N
H
1,2a
F
H
O
N
2
S
H
H
Me
hν
hν'
1,2a'
R
F
2
F
1: R=C11H23
2: R=C17H35
F
2
2
Me
Me
Me
R
R
H
H
S
S
O
N
N
O
Me
1,2b
N
H
N
H
N
H
N
O
O
N
N
H
supramolecular polymer
Figure 1. Photoreversible supramolecular formation.
© 2010 The Chemical Society of Japan
Chem. Lett. 2010, 39, 82-83
www.csj.jp/journals/chem-lett/

83
Since the hydrogen bonding is weakened in a polar solvent such
as THF, the particle size became smaller than that in toluene. The
particle size did not change in the dark at the same temperature.
Upon visible irradiation, the solution became colorless, and
compound 2 returned to the open form 2a. Simultaneously, the
particle size decreased to its initial value. These phenomena were
caused by the high polarity of the solvent. In the higher polar
solvent (THF), the quadruple hydrogen bonding become weak-
ened compared with that in the lower polar solvent (CHCl3).
Therefore, the dissociation of the hydrogen bonding became
faster in a THF solution. Substitution of C11 chain of 1 with C17
chain enhanced the solubility of 2 in the polar solvents like THF.
The particle size was also dependent on the concentration of the
Figure 2. Particle size change of the supramolecule upon irradiation
with UV and visible light in a toluene solution (20 °C, [2] = 3.0 ©
10¹4 mol dm¹3).
solution due to the intermolecular hydrogen bonding, as the
¹3
particle size in the concentrated solution (3.0 © 10¹4 mol dm
)
was almost 1.5 times larger than that in the dilute solution
(2.0 © 10¹4 mol dm¹3). Nevertheless, fully photoreversible su-
pramolecular formation at room temperature was accomplished.
In summary, a THF solution of diarylethene having two
quadruple hydrogen-bonding sites changes the particle size fully
photoreversibly at room temperature.
This work is supported by a Grant-in-Aid for Science
Research in a Priority Area New Frontiers in Photochromism
(471) (No. 21021021) from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan and a Grant-in-
Aid for Scientific Research (C) (No. 21550119) from Japan
Society for the Promotion of Science (JSPS). The authors are also
grateful to Zeon Co., Ltd. for providing perfluorocyclopentene.
Figure 3. Particle size change of the supramolecule upon irradiation
with UVand visible light in THF solutions (20 °C, circles: [2] = 3.0 ©
10¹4 mol dm¹3, squares: [2] = 2.0 © 10¹4 mol dm¹3).
References and Notes
1
For reviews, see: C. Schmuck, W. Wienand, Angew. Chem. 2001,
113, 4493; C. Schmuck, W. Wienand, Angew. Chem., Int. Ed.
2001, 40, 4363; T. Kato, N. Mizoshita, K. Kanie, Macromol.
Rapid Commun. 2001, 22, 797; L. Brunsveld, B. J. B. Folmer,
E. W. Meijer, R. P. Sijbesma, Chem. Rev. 2001, 101, 4071; A. J.
Wilson, Soft Matter 2007, 3, 409; S. Yagai, A. Kitamura, Chem.
Soc. Rev. 2008, 37, 1520; J. D. Fox, S. J. Rowan, Macromolecules
2009, 42, 6823.
Measurements of the particle size changes of toluene
and THF solutions of 2a were performed with dynamic light
scattering, the results of which are shown in Figures 2 and 3.6
As shown in Figure 2, in a toluene solution, the particle size of
the supramolecular polymer increased drastically according to
the photocyclization reaction upon irradiation with UV light;
hence, the color of the solution turned to blue-violet. Finally, the
particle size reached about 180 nm. The length of compound 2b
was about 3 nm, at which point more than 60 molecules formed
the supramolecular polymer. The particle size and the absorption
spectrum were maintained when the solution was stored in the
dark. Upon visible irradiation, the solution became colorless,
and the ring-opening reaction took place. However, the particle
size decreased slightly and did not return to its initial value. This
can be explained by the stable multiple hydrogen bonding
in a nonpolar solvent such as toluene, as we have previously
reported.4 The supramolecular polymer composed of the open
form was, therefore, present in this conditions. After the solution
was heated at 70 °C for 1 h, the supramolecular polymer
dissociated, and the particle size returned to its initial value
(<10 nm). These particle size changes were repeated at least 5
times. The system reached thermodynamic equilibrium after
obtaining the activation energy by heating.
2
S. Shinkai, T. Yoshida, K. Miyazaki, O. Manabe, Bull. Chem. Soc.
Jpn. 1987, 60, 1819; S. Yagai, T. Iwashima, K. Kishikawa, S.
Nakahara, T. Karatsu, A. Kitamura, Chem.®Eur. J. 2006, 12,
3984.
3
4
5
L. N. Lucas, J. van Esch, B. L. Feringa, R. M. Kellogg, Chem.
Commun. 2001, 759.
M. Takeshita, M. Hayashi, S. Kadota, K. H. Mohammed, T.
Yamato, Chem. Commun. 2005, 761.
Synthesis of the compounds will be shown elsewhere. Compound
1a: colorless powder, mp >300 °C; 1H NMR (300 MHz, CDCl3,
25 °C, TMS): ¤ 13.01 (s, 2H, pyrimidinone N-H), 12.34 (s, 2H,
urea N-H), 12.28 (s, 2H, urea N-H), 7.72 (d, J = 8 Hz, 4H;
benzene-H), 7.34 (d, J = 8 Hz, 4H, benzene-H), 5.93 (s, 2H,
pyrimidinone-H), 2.49 (s, 4H, alkyl-H), 2.36 (s, 6H, thiophene-H),
2.06 (s, 6H, thiophene-H), 1.60 (m, 4H, alkyl-H), 1.29-1.16 (m,
56H, alkyl-H), 0.86-0.83 (m, 6H, alkyl-H); Anal. Calcd for
C73H98F6N8O4S2¢2H2O: C, 64.20; H, 7.53; N, 8.20%. Found: C,
64.18; H, 7.16; N, 8.10%. 1H NMR spectrum and absorption
spectral change upon irradiation with UV light are shown in
Supporting Information. Supporting Information is available
electronically on the CSJ-Journal Website, http://www.csj.jp/
journals/chem-lett/index.html.
Figure 3 shows the particle size change of a THF solution of
compound 2. The particle size of the supramolecular polymer of
the closed form 2b in THF was about 90 nm, which was almost
half of that in toluene despite having similar concentrations.
6
Dynamic light scattering measurements were performed with an
Otsuka Electronics ELS-800 and the minimum limit for the
measurement is 10 nm.
Chem. Lett. 2010, 39, 82-83
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/