Light-driven molecular-crystal actuators: Rapid and reversible bending of rodlike mixed crystals of diarylethene derivatives

Article abstract of DOI:10.1002/anie.201105585

Limber but strong: Two-component mixed crystals of diarylethene derivatives exhibit reversible, rapid, and fatigue-resistant bending upon alternate irradiation with UV and visible light (see picture). The crystals showed reversible curling into a hairpin shape while remaining crystalline. Copyright

Full text of DOI:10.1002/anie.201105585

Angewandte
Chemie
DOI: 10.1002/anie.201105585
Photochromic Actuators
Light-Driven Molecular-Crystal Actuators: Rapid and Reversible
Bending of Rodlike Mixed Crystals of Diarylethene Derivatives**
Fumitaka Terao, Masakazu Morimoto, and Masahiro Irie*
Piezoelectric crystals are useful in industrial and consumer
applications because of their special characteristics: they can
be bent by electricity. But the crystals require a wire
connection so that the electricity can be supplied. This wire
connection prevents their use in water and reduction of their
size to the micrometer scale. Herein, we report on wireless
molecular-crystal actuators, which work upon photoirradia-
tion. Rodlike mixed crystals of 1-(5-methyl-2-phenyl-4-thia-
zolyl)-2-(5-methyl-2-p-tolyl-4-thiazolyl)perfluorocyclopen-
tene (1a) and 1,2-(5-methyl-2-p-tolyl-4-thiazolyl)perfluoro-
cyclopentene (2a; Scheme 1) with sizes ranging from micro-
level, the movement fails to be effectively linked to macro-
scopic motion of materials. Polymer artificial muscles primar-
ily depend upon the response of bulk materials rather than
upon individual molecular behavior. It is a challenge to
construct molecular materials that perform macroscopic
mechanical motion that stems from stimuli-responsive geo-
metrical structure changes of individual molecules.
In previous reports,[22] we demonstrated that photochem-
ical geometrical structure changes of individual diarylethene
molecules in crystals induced changes in molecular packing
and alignment, resulting in the anisotropic deformation of the
bulk crystals. For example, a rodlike crystal of 1,2-bis(5-
methyl-2-phenyl-4-thiazolyl)perfluorocyclopentene exhibits
reversible bending upon alternate irradiation with UV and
visible light.[22a] For practical applications, the crystal actua-
tors should have sufficient durability and substantial mechan-
ical properties. The above single-component diarylethene
crystal lacks this durability and breaks in less than 100
deformation cycles. To improve fatigue resistance, we pre-
pared a platelike two-component cocrystal composed of 1,2-
bis(2-methyl-5-(1-naphthyl)-3-thienyl)perfluorocyclopentene
and perfluoronaphthalene.[22d] The cocrystal exhibits reversi-
ble bending for up to 250 cycles. During the course of the
study on such multicomponent crystals it was found that the
durability is further improved by mixing two diarylethene
derivatives, 1a and 2a, in a crystal. The bending can be
repeated more than 1000 times without any crystal damage,
and the crystals exhibit light-driven bending in all directions
toward the UV light source in wide temperature range from
4.6 K to 370 K. Moreover, the photogenerated maximum
stress of the crystals was found to be comparable to that of
piezoelectric crystals.
Scheme 1. Photochromism of diarylethenes 1 and 2.
meters to millimeters were found to exhibit rapid, reversible,
and fatigue-resistant bending upon alternate irradiation with
UV (365 nm) and visible (> 500 nm) light.
Various types of artificial molecular muscles have been
reported, such as rotaxanes,[1–7] catenanes,[8–12] polymer films
and gels,[13–16] conductive polymers,[17] liquid-crystal elasto-
mers,[18–21] molecular crystals,[22–26] and nanotubes.[27] Although
the supramolecular systems (e.g., bistable rotaxanes and
catenanes) exhibit musclelike sliding motion at the molecular
Microcrystals were prepared by recrystallization from
ethanol. Platelike crystals are mainly formed from the ethanol
solution containing only 1a. When 2a is added to the solution
in amounts exceeding 30 mol%, formation of rodlike crystals
becomes dominant. In the ethanol solution containing
equimolar amounts of 1a and 2a, rodlike crystals composed
of both 1a (63 mol%) and 2a (37 mol%) grew. The rodlike
two-component mixed crystals bend toward UV (365 nm)
light sources and become straight upon irradiation with
visible (> 500 nm) light, while the shape of the single-
component crystals of 1a remains unchanged. The surface
skin layers were peeled off the crystals (see Figure S1 in the
Supporting Information). When the content of 2a in the
mixed crystals was increased over 60 mol%, the photostimu-
lated bending was suppressed, and the bending was not
observed for the rodlike crystal of 2a. Figure 1 shows the
bending cycles of the rodlike mixed crystal (1.3 mm ꢀ 25 mm ꢀ
13 mm) containing both 1a and 2a (1a:2a = 63/37) upon
[*] F. Terao, Dr. M. Morimoto, Prof. Dr. M. Irie
Department of Chemistry and Research Center for Smart Molecules
Rikkyo University
Nishi-Ikebukuro 3-34-1, Toshima-ku, Tokyo 171-8501 (Japan)
Fax: (+81)3-3985-2397
E-mail: iriem@rikkyo.ac.jp
[**] The present work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas “New Frontiers in Photochromism
(471)” (No. 19050008) from the Ministry of Education, Culture,
Sports, Science, and Technology (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105585.
Angew. Chem. Int. Ed. 2012, 51, 901 –904
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
901

.
Angewandte
Communications
Figure 2. Bending of the rodlike two-component mixed crystal contain-
ing 1a and 2a (1a:2a=63/37) in all directions. a) Schematic illustra-
tion of the bending rodlike crystal. b) The crystal
Figure 1. Reversible bending of a rodlike two-component mixed crystal
of 1a and 2a (1a:2a=63/37) upon alternate irradiation with UV
(365 nm) and visible (>500 nm) light. a) Pairs of photographs of the
crystal (1.3 mmꢀ25 mmꢀ13 mm) showing the 1st, 500th, and 1000th
cycles. b) Tip displacement of the crystal during reversible bending.
c) An approximately 10ꢀ (left) and 80ꢀ (right) expanded photograph of
the crystal after the 1000th cycle. The surface of the crystal remained
clear even after 1000 bending cycles, and no damage was observed.
(1.3 mmꢀ40 mmꢀ15 mm) was irradiated from the right, lower, and left
sides with UV (365 nm) and visible (>500 nm) light, and movement
of the rodlike crystal edge was monitored. The edge moved toward the
UV light source and then returned to the original position upon
irradiation with visible light. c) First, the crystal was irradiated from the
lower side with UV light, and then from the right side with controlled-
intensity UV and visible light. The edge of the crystal exhibited
rotation. d) Reversible curling to a hairpin shape upon irradiation with
UV light (crystal length: 3.0 mm). The crystal kept the crystalline state
even after the curling and returned to the original straight shape upon
irradiation with visible light. See also Movies S1, S2, and S3 in the
Supporting Information.
alternate irradiation with UV and visible light. The edge of
the rod reversibly moved as much as 0.56 mm upon photo-
irradiation, and the bending could be repeated more than
1000 times. The surface of the crystal remained clear even
after 1000 cycles, and no damage was discerned, as shown in
Figure 1c, right.
The melting point of the mixed crystals depends on the
ratio of the two components. It showed the minimum of
1318C at the molar ratio of 1:1 and increased when the
content of one of the components increased (see Figure S2 in
the Supporting Information). X-ray crystallographic analysis
of the mixed crystals indicates that the crystal system and
space group are the same as for the single-component crystal
of 1a, even when the content of 2a is as large as 60 mol% (see
Table S1 in the Supporting Information). The low melting
point of the crystal containing equal molar amounts of the two
component molecules suggests that intermolecular interac-
tion among the component molecules is weakened in the
mixed crystals. The weakened intermolecular interaction is
considered to favor the macroscopic mechanical motion and
improve the durability of the crystals.
The rodlike crystal has a rectangular (or distorted
hexagonal) cross-section, as shown in Figure 2a. The rod
was irradiated from the right, lower, and left sides with UV
and visible light, and bending behavior was monitored. The
rodlike crystal bends toward the UV (365 nm) light source
irrespective of the irradiation direction, as shown in Figure 2b
and Movie S1 in the Supporting Information. The bent
rodlike crystal returns to a straight position upon irradiation
with visible (> 500 nm) light. When the light intensity of UV
and visible light is controlled, the edge of the rodlike crystal
exhibits rotation movement, as shown in Figure 2c and
Movie S2 in the Supporting Information. The bending is
ascribed to a gradient in the extent of photoreaction caused
by high absorbance of the crystal.
Contraction of the irradiated part of the crystal results in
bending toward the light source, as in bimetals. The bending
irrespective of the irradiation direction indicates that con-
traction of the surface region takes place along the long axis
of the rod. To confirm the bending mechanism, in situ X-ray
crystallographic analysis was carried out. X-ray crystallo-
graphic data before and after UV (365 nm) light irradiation,
and after subsequent visible (> 500 nm) light irradiation, are
shown in Table S2 in the Supporting Information. Upon
irradiation with UV light for 10 s, 6.1% of component
diarylethene molecules convert from the open- to the
closed-ring isomers, and the cell length of b axis shows a
small but significant decrease (0.12%). The lengths of the a
and c axes increase by 0.23 and 0.09%, respectively. Upon
visible-light irradiation, the cell parameters return to the
initial values. The b axis corresponds to the long axis of the
rodlike crystal (see Figure S3 in the Supporting Information).
The decrease of the length of the b axis explains the
contraction of the irradiated surface region and accounts for
the bending toward the UV light source. Shrinkage by 0.12%
is the average change of the bulk crystal lattice when the
average conversion reaches 6.1%. The photoreaction upon
UV (365 nm) light irradiation is limited to the surface layer of
the crystal because of high absorbance of the crystal in the
UV region. Most of the bulk crystal remains unchanged. On
the other hand, X-ray crystallographic analysis measures the
bulk crystal. Therefore, the conversion in the surface region is
considered to be over a few tens of percent, and the b axis
shrinks over 0.5%. Shrinkage of 0.5% is enough to induce the
902
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 901 –904

Angewandte
Chemie
large bending.[28] The decrease of the length of the b axis is
ascribed to the changes of molecular packing and alignment,
as reported.[22]
transition temperatures.[18] The present molecular-crystal
actuators are superior to the existing polymer muscles in
their response time (< 5 ms) and their wide range of working
temperature (4.6 K < T< 370 K). Another characteristic fea-
ture of the molecular-crystal actuator is light-driven bending
motion in water. Figure 3b shows the photostimulated
reversible bending of the rodlike crystal in water. The
bending behavior is similar to that observed in air.
The rodlike crystal can bring about gearwheel rotation
(Figure 4 and Movie S4 in the Supporting Information). Upon
UV light irradiation, the crystal bends and hits the gear,
The rodlike crystal that was 3.0 mm long showed curling
into a hairpin shape upon irradiation with UV light, and the
shape remained stable after switching off the light (Figure 2d
and Movie S3 in the Supporting Information). The crystal
remained crystalline even after the curling and returned again
to its straight shape upon irradiation with visible light. The
curling and recovery to the straight shape could be repeated
many times while crystallinity was maintained. The two-
component crystal is flexible.
A noticeable feature of the molecular-crystal actuators is
the performance at low temperature. The crystal was cooled
to 4.6 K in a cryostat and irradiated with UV (365 nm) and
visible (> 500 nm) light. The rodlike crystal exhibits light-
driven bending even at 4.6 K (Figure 3a). The bending
behavior was monitored using a high-speed camera (Vision
Figure 4. Gearwheel rotation operated by a light-driven molecular-
crystal actuator. The two-component mixed crystal containing 1a and
2a (1a:2a=63/37) (1.3 mmꢀ60 mmꢀ12 mm) was fixed on the tip of a
metal needle. The gear (diameter: 3.2 mm) was rotated by the crystal,
which showed reversible bending upon alternate irradiation with UV
(365 nm) and visible (>500 nm) light. See also Movie S4 in the
Supporting Information.
resulting in rotation of the gear. The crystal returns to the
straight shape upon controlled irradiation with UVand visible
light. Then, irradiation with UV light bends the crystal and
again induces the rotation of the gearwheel. The cycles can be
repeated many times. This is actual photomechanical work of
the rodlike crystal.
Figure 3. Bending of the rodlike two-component mixed crystal contain-
ing 1a and 2a (1a:2a=63/37) at 4.6 K and in water. a) Reversible
bending of the crystal (1.5 mmꢀ20 mmꢀ10 mm) upon irradiation with
UV (365 nm) and visible (>440 nm) light at 4.6 K. The crystal was
fixed to a copper sample holder in a cryostat. b) Reversible bending of
the rodlike crystal (2.3 mmꢀ30 mmꢀ15 mm) in water upon irradiation
with UV and visible light.
We also tried to use the rodlike crystal to lift a metal load
(see Movie S5 and Figure S5 in the Supporting Information).
The rodlike crystal (2.5 mg) was fixed at the edge of a glass
plate as a cantilever arm, and a metal weight (2.2710 mg) was
loaded onto the rod. The weight is 908 times heavier than the
crystal. Upon irradiation with UV (365 nm) light, the weight
was lifted as high as 0.10 mm. The tiny rodlike crystal, which
weighs only 2.5 mg, performs lifting work as large as 2.2 nJ.
This mechanical work is ascribed to a large Youngꢁs
modulus of the crystal materials. The modulus of the crystal
was measured by means of a manual beam-bending test to be
as large as 8.5 GPa (see Figure S6 in the Supporting Informa-
tion). This value is much larger than those of typical polymer
materials (ca. 1 GPa)[29] and similar to other organic crys-
tals.[30] The large Youngꢁs modulus enables the crystal to
generate strong force and carry out large mechanical work. It
Research, PhantomV710; Figure S4 in the Supporting Infor-
mation). The crystal already showed bending in the first frame
after irradiation with pulsed laser light (355 nm, 8 ns pulse
width). The exposure time of each frame is five microseconds.
Therefore, the light-driven bending takes place in less than
five microseconds at 4.6 K. Polymer artificial muscles, such as
polymer gels, conductive polymers, and liquid-crystal elasto-
mers, are rather slow-acting (typically in seconds to minutes),
and the working temperature must be above the glass-
Angew. Chem. Int. Ed. 2012, 51, 901 –904
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
903

.
Angewandte
Communications
[14] L. Matejka, M. Ilavsky, K. Dusek, O. Wichterle, Polymer 1981,
22, 1511 – 1515.
[15] M. Irie, Adv. Polym. Sci. 1990, 94, 28 – 67.
[16] S. Juodkazis, N. Mukai, R. Wakaki, A. Yamaguchi, S. Matsuo, H.
Misawa, Nature 2000, 408, 178 – 181.
[17] K. Kaneto, M. Kaneko, Y. Min, A. G. MacDiarmid, Synth. Met.
1995, 71, 2211 – 2212.
[18] H. Finkelmann, E. Nishikawa, G. G. Pereira, M. Warner, Phys.
Rev. Lett. 2001, 87, 015501.
[19] Y. Yu, M. Nakano, T. Ikeda, Nature 2003, 425, 145.
[20] H. Jiang, S. Kelch, A. Lendlein, Adv. Mater. 2006, 18, 1471 –
1475.
[21] M. Yamada, M. Kondo, J. Mamiya, Y. Yu, M. Kinoshita, C. J.
Barrett, T. Ikeda, Angew. Chem. 2008, 120, 5064 – 5066; Angew.
Chem. Int. Ed. 2008, 47, 4986 – 4988.
[22] a) S. Kobatake, S. Takami, H. Muto, T. Ishikawa, M. Irie, Nature
2007, 446, 778 – 781; b) M. Irie, Bull. Chem. Soc. Jpn. 2008, 81,
917 – 926; c) L. Kuroki, S. Takami, K. Yoza, M. Morimoto, M.
Irie, Photochem. Photobiol. Sci. 2010, 9, 221 – 225; d) M.
Morimoto, M. Irie, J. Am. Chem. Soc. 2010, 132, 14172 – 14178.
[23] M. A. Garcia-Garibay, Angew. Chem. 2007, 119, 9103 – 9105;
Angew. Chem. Int. Ed. 2007, 46, 8945 – 8947.
[24] a) R. O. Al-Kaysi, A. M. Mꢆller, C. J. Bardeen, J. Am. Chem.
Soc. 2006, 128, 15938 – 15939; b) R. O. Al-Kaysi, C. J. Bardeen,
Adv. Mater. 2007, 19, 1276 – 1280; c) J. T. Good, J. J. Burdett, C. J.
Bardeen, Small 2009, 5, 2902 – 2909; d) L. Zhu, A. Agarwal, J.
Lai, R. O. Al-Kaysi, F. S. Tham, T. Ghaddar, L. Mueller, C. J.
Bardeen, J. Mater. Chem. 2011, 21, 6258 – 6268; e) L. Zhu, R. O.
Al-Kaysi, C. J. Bardeen, J. Am. Chem. Soc. 2011, 133, 12569 –
12575.
[25] a) K. Uchida, S. Sukata, Y. Matsuzawa, M. Akazawa, J. J. D.
de Jong, N. Katsonis, Y. Kojima, S. Nakamura, J. Areephong, A.
Meetsma, B. L. Feringa, Chem. Commun. 2008, 326 – 328; b) S.
Kobatake, H. Hasegawa, K, Miyamura, Cryst. Growth Des.
2011, 11, 1223 – 1229.
[26] H. Koshima, N. Ojima, H. Uchimoto, J. Am. Chem. Soc. 2009,
131, 6890 – 6891.
[27] A. E. Aliev, J. Oh, M. E. Kozlov, A. A. Kuznetsov, S. Fang, A. F.
Fonseca, R. Ovalle, M. D. Lima, M. H. Haque, Y. N. Gartstein,
M. Zhang, A. A. Zakhidov, R. H. Baughman, Science 2009, 323,
1575 – 1578.
is possible to estimate the maximum stress generated by UV
irradiation from the photostimulated bending of the crystal
cantilever beam (see Figure S7 in the Supporting Informa-
tion). The maximum stress was estimated to be 56 MPa, which
is 180 times larger than that of muscles (ca. 0.3 MPa) and
comparable to that of piezoelectric crystals, such as lead
zirconate titanate (PZT, ca. 50 MPa). The robust light-driven
molecular-crystal actuators having substantial mechanical
properties comparable to piezoelectric crystals and can be
potentially used in various micro- and nanomechanical
applications.[31]
Received: August 7, 2011
Published online: October 26, 2011
Keywords: light-driven actuators · mechanical properties ·
.
molecular crystal · photochromism · X-ray diffraction
[1] P. L. Anelli, N. Spencer, J. F. Stoddart, J. Am. Chem. Soc. 1991,
113, 5131 – 5133.
[2] R. A. Bissell, E. Cꢂrdova, A. E. Kaifer, J. F. Stoddart, Nature
1994, 369, 133 – 137.
[3] M. C. Jimꢃnez, C. Dietrich-Buchecker, J.-P. Sauvage, Angew.
Chem. 2000, 112, 3422 – 3425; Angew. Chem. Int. Ed. 2000, 39,
3284 – 3287.
[4] Y. Liu, A. H. Flood, P. A. Bonvallet, S. A. Vignon, B. H.
Northrop, H.-R. Tseng, J. O. Jeppesen, T. J. Huang, B. Brough,
M. Baller, S. Magonov, S. D. Solares, W. A. Goddard, C.-M. Ho,
J. F. Stoddart, J. Am. Chem. Soc. 2005, 127, 9745 – 9759.
´
[5] J. D. Badjic, V. Balzani, A. Credi, S. Silvi, J. F. Stoddart, Science
2004, 303, 1845 – 1849.
[6] L. Fang, M. Hmadeh, J. Wu, M. A. Olson, J. M. Spruell, A.
Trabolsi, Y.-W. Yang, M. Elhabiri, A.-M. Albrecht-Gary, J. F.
Stoddart, J. Am. Chem. Soc. 2009, 131, 7126 – 7134.
[7] P. G. Clark, M. W. Day, R. H. Grubbs, J. Am. Chem. Soc. 2009,
131, 13631 – 13633.
[8] D. B. Amabilino, C. O. Dietrich-Buchecker, A. Livoreil, L.
Pꢃrez-Garcꢄa, J.-P. Sauvage, J. F. Stoddart, J. Am. Chem. Soc.
1996, 118, 3905 – 3913.
[28] A 0.5% contraction of the surface region is enough to induce
bending by as much as 208 in the crystal that is 1.5 mm long and
10 mm thick.
[29] C. L. van Oosten, K. D. Harris, C. W. M. Bastiaansen, D. J.
Broer, Eur. Phys. J. E 2007, 23, 329 – 336.
[9] D. J. Cꢅrdenas, A. Livoreil, J.-P. Sauvage, J. Am. Chem. Soc.
1996, 118, 11980 – 11981.
[10] M. Asakawa, P. R. Ashton, V. Balzani, A. Credi, C. Hamers, G.
Mattersteig, M. Montalti, A. N. Shipway, N. Spencer, J. F.
Stoddart, M. S. Tolley, M. Venturi, A. J. P. White, D. J. Williams,
Angew. Chem. 1998, 110, 357 – 361; Angew. Chem. Int. Ed. 1998,
37, 333 – 337.
[11] D. A. Leigh, J. K. Y. Wong, F. Dehez, F. Zerbetto, Nature 2003,
424, 174 – 179.
[12] J. V. Hernꢅndez, E. R. Kay, D. A. Leigh, Science 2004, 306,
1532 – 1537.
[30] T. Danno, T. Kajiwara, H. Inokuchi, Bull. Chem. Soc. Jpn. 1967,
40, 2793 – 2795.
[31] CCDC 770597, 770598, 770599, 770600, 770601, 770602, 770603
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[13] C. D. Eisenbach, Polymer 1980, 21, 1175 – 1179.
904
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 901 –904