Photomechanical motion of furylfulgide crystals

Article abstract of DOI:10.1246/cl.2012.107

Plate-like microcrystals of a photochromic furylfulgide bend toward the light upon UV irradiation and then straighten upon visible light irradiation. The reversible bending was observed over 200 cycles of alternating irradiation with UV and visible light. The mechanism of bending could be explained by the X-ray crystallographic data.

Full text of DOI:10.1246/cl.2012.107

doi:10.1246/cl.2012.107
Published on the web December 30, 2011
107
Photomechanical Motion of Furylfulgide Crystals
Hideko Koshima,* Hidemitsu Nakaya, Hidetaka Uchimoto, and Naoko Ojima
Department of Materials Science and Biotechnology, Graduate School of Science and Engineering,
Ehime University, Matsuyama, Ehime 790-8577
(Received September 28, 2011; CL-110798; E-mail: koshima.hideko.mk@ehime-u.ac.jp)
Plate-like microcrystals of a photochromic furylfulgide bend
toward the light upon UV irradiation and then straighten upon
visible light irradiation. The reversible bending was observed
over 200 cycles of alternating irradiation with UV and visible
light. The mechanism of bending could be explained by the
X-ray crystallographic data.
Scheme 1. Photochromic reaction of furylfulgide 1.
Creating mechanical motion in bulk materials based on
geometric structural changes in individual molecules in response
to external stimuli presents a scientific challenge. Artificial
molecular mechanical systems, however, have not been linked to
macroscale mechanical motion.1 Large-scale mechanical motion
of molecular materials has been reported only in liquid-crystal
elastomers, in which a photoinduced order-disorder phase
transition was used as the driving force.2 Recently, mechanical
bending of photochromic diarylethene crystals was reported, and
the macroscale bending of the crystals was found to be caused
by molecular-scale motion.3 Additionally, several photomechan-
ical crystals composed of azobenzenes and salicylideneanilines
have been reported to provide promising opportunities for
artificial molecular machinery.4-9
Fulgides constitute a class of photochromic compounds that
undergo reversibly electrocyclic ring-closure and ring-opening
reactions.10 The furylfulgide (E)-2-[1-(2,5-dimethyl-3-furyl)-
ethylidene]-3-isopropylidenesuccinic anhydride (E1) in E-form
exhibits photochromism in the crystalline state (Scheme 1).11-13
This letter describes plate-like microcrystals of E1 that reveal
reversible bending upon alternate irradiation with UVand visible
light. The bending mechanism is based on changes in crystal
structure before and after photoirradiation.
Microcrystals of E1 were prepared by sublimation of
crystalline powders in a small platinum pan covered with a
glass plate. Plate-like microcrystals grew on the surface of
the glass plate after heating to approximately 20 °C below
the melting point (126 °C) and holding for several hours
(Figure S1). X-ray diffractograms of the microcrystals contained
two peaks, which were assigned to the 101 and 202 reflections
based on consistencies with existing crystallographic data
(Figure S115).11,13
Figure 1a shows the hexagonal face of a plate-like E1
microcrystal (115 © 60 © 2 ¯m3) with the left end fixed to an
adjacent crystal while the other portion is free. The top surface
of the plate-like microcrystal was identified as the (101) face
with its longitudinal direction along the b axis, based on
comparisons with the bulk crystal having a hexagonal surface,
which exhibits a (101) face along the b axis (Figure 1d). When
the (101) face of the microcrystal was irradiated from the
diagonal underside at 365 nm (10 mW cm¹2) with a UV-light-
emitting diode (LED) for 1 s, the crystal curled from the right
upper corner toward the light, reaching a maximum twisted curl
Figure 1. The (101) face of the plate-like E1 microcrystal
before (a) and after UV irradiation for (b) 1 and (c) 2 s. Scale
bar: 50 ¯m. The (101) face of the bulk single crystal of E1
before (d) and after UV irradiation (e). Scale bar: 1 mm.
after 2 s with a color change from pale yellow to red due to
the formation of the closed C1 isomer (Figures 1a-1c and
Video S115). In contrast, a millimeter-scale bulk single crystal
changed color upon UV irradiation, but the crystal shape did not
change (Figures 1d and 1e).
When the (101) face of a narrow plate-like microcrystal
(109 © 6 © 2 ¯m3) was irradiated at 365 nm from the lower side,
the crystal bent toward the light, reaching a maximum tip
displacement angle of 9° after 2 s (Figures 2a and 2b, and
Video S215). Subsequent illumination with a halogen lamp
equipped with a filter (>390 nm, 10 mW cm¹2) returned the
crystal to its initial straight shape after 30 s (Figure 2a). This
reversible bending was observed over 200 cycles of alternating
irradiation with UV (2 s) and visible light (30 s) (Figure 2c). The
bending motion was accompanied by a color change from pale
yellow to red (-max = 512 nm) due to the formation of the closed
C1 isomer in the crystals (Figure S215). The red crystal also
returned to the initial pale yellow color due to the photochemical
ring-opening reaction, generating the E1 isomer.
The bending effect was ascribed to a gradient with respect to
the extent of electrocyclic ring-closure as a function of light
penetration, such that shrinkage of the irradiated crystal surface
along the b axis resulted in a bent macrostructure. In the E1
crystal, the E1 molecules have torsional conformation with a
Chem. Lett. 2012, 41, 107-109
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108
Figure 4. (A) Hexagonal (101) top face of an E1 microcrystal,
(B) atomic force microscopy images of the (101) surface and
(C) the b-b¤ section before (a, d) and after irradiation with UV
for 2 s (d, e) and then visible light for 20 min (c, f).
Figure 2. Bending of a narrow, plate-like crystal of E1 (a)
before and (b) after UV irradiation from the lower side. Scale
bar: 20 ¯m. (c) Reversible bending was repeatable over as many
as 200 cycles.
both the E1 and the C1 isomers at a ratio of 0.945:0.055
(Figure 3c). The length of the b axis of the unit cell decreased
0.13% from 9.6203(5) to 7.6101(8) ¡ after two-photon excita-
tion (Figure 3d), bending the plate-like microcrystal toward the
light source.
The (101) top surface of the E1 microcrystal was smooth
before irradiation, as observed by atomic force microscopy
(Figures 4a and 4d). After UV irradiation for 2 s, uneven
features appeared with a height of 200 nm and a relative
roughness of 18% of the crystal thickness (1100 nm) (Figures 4b
and 4e). The relative intensity of X-ray diffraction peaks 101 and
202 of the microcrystals decreased to 63% and 50%, respec-
tively, due to ring-closure photoisomerization to the C1 isomer
(Figure S1). Subsequent visible light irradiation for 20 min did
not decrease the uneven features (Figures 4c and 4f). On the
other hand, the diffraction peaks completely recovered their
initial intensity due to ring-opening photoisomerization to the
E1 isomer (Figure S1), suggesting that the (101) smooth surface
of the E1 single crystal before irradiation changed to the E1
polycrystalline state near the surface after irradiation with visible
light. However, the correlation between the formation of the
polycrystalline state and the bending behavior is not clear at the
present time.
Figure 3. Molecular structures of (a) the E1 isomer and (b) the
C1 isomer in the crystals of E1 and C1, respectively. (c) ORTEP
drawings show the disordered structures of the E1 (black) and
the C1 (red) isomers after irradiation at 742 nm at the 25%
probability level. (d) Molecular arrangement on the (101) face.
Hydrogen atoms are omitted for clarity.
dihedral angle of 38.3° between the succinic anhydride portion
and the furan ring (Figure 3a) and are arranged at the (101) face
in a twofold screw along the b axis.11 Upon UV irradiation, the
torsional E1 molecules underwent electrocyclic ring-closure to
C1 molecules on the (101) crystal surface. The crystalline C1
isomer is nearly planar with a dihedral angle of 13.8° between
the succinic anhydride portion and the furan rings (Figure 3b).13
Therefore, the ring-closure photoisomerization from the tor-
sional E1 isomer to the nearly planar C1 isomer shrank the
length of the b axis of the unit cell. By contrast, because
photoisomerization does not occur in the absence of light, the
unit cell dimensions remained constant for the nonilluminated
surface, causing the microcrystals to bend toward the light
source.
In conclusion, the photomechanical bending of plate-like
microcrystals of the furylfulgide (E)-2-[1-(2,5-dimethyl-3-fur-
yl)ethylidene]-3-isopropylidenesuccinic anhydride was repeated
upon alternate irradiation with UV and visible light. The
mechanism of bending could be explained by the X-ray
crystallographic data.
This work was supported by a Grant-in-Aid for Scientific
Research in a Priority Area “New Frontiers in Photochromism”
from MEXT, Japan.
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