Bending, Curling, Rolling, and Salient Behavior of Molecular Crystals Driven by [2+2] Cycloaddition of a Styrylbenzoxazole Derivative

Article abstract of DOI:10.1002/anie.201705325

We report interesting photomechanical behaviors of the dynamic molecular crystals of (E)-2-(2,4-dichlorostyryl)benzo[d]oxazole (BOACl24). The photosalient effect of the rod-like crystal based on a metal-free olefin driven by photodimerization is observed.

Full text of DOI:10.1002/anie.201705325

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Accepted Article
Title: Bending, curling, rolling and salient behaviors of molecular
crystals driven by [2+2] cycloaddition of styrylbenzoxazole
derivative
Authors: Haoran Wang, Peng Chen, Zhu Wu, Jinyu Zhao, Jingbo Sun,
and Ran Lu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201705325
Angew. Chem. 10.1002/ange.201705325
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Angewandte Chemie International Edition
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COMMUNICATION
Bending, curling, rolling and salient behaviors of molecular
crystals driven by [2+2] cycloaddition of styrylbenzoxazole
derivative
Haoran Wang, Peng Chen, Zhu Wu, Jinyu Zhao, Jingbo Sun, and Ran Lu*
[
10]
Abstract: We report interesting photomechan ic al behaviors of the
dynamic molecular crystals of (E)-2-(2,4-
leaping
in response to light.
Among the photo-triggered structural transformations, [2+2]
cycloaddition reaction has gained increasing interests since
Schmidt and co-w orkers revealed the criterion for the
dichlorostyryl)benzo[d]oxazole (BOACl24). The photosalient effect
of the rod-like crystal based on metal-free olefin driven by
photodimerization is observed. Moreover, the needle-like crysta ls of
BOACl24 exhibit a reversible bending away from UV light source.
The nanofibers curl easily under UV irradiation in organogel, in
wh ic h the photo-induced rolling of a small slice undergoes. It
suggests that the rapid release of the accumulated strain during
photodimerization may lead to photosalient effect, and the bending
or curling happens when the strain is released slow ly . Notably, [2+2]
cycloaddition takes place between two d if ferent conformational
isomers of BOACl24 on account of the rotation of benzoxazole ring
around CC single bond in excited state before photodimerization.
Such topophotochemical reaction has not been reported elsew here.
[
11]
topophotochemical reaction of olefin pairs. Recently, Naumov
et al. found the single crystals based on metal complexes
bearing styrylpyridine ligands exploded and shattered violently
upon UV irradiation.
photosalient effect driven by [2+2] cycloaddition in the molecular
crystals formed from metal-free organic compounds is rare
[
9d]
To the best of our know ledge, the
[
5a,6a]
reported.
In general, the extended -conjugated systems
prefer to pack into w ell-ordered arrangements via -
interactions. If the packing mode of the conjugated olefin
molecules in crystalline state satisfies the geometric criteria for
[2+2] cycloaddition, the topophotochemical reaction may take
place, and the photomechanical motion becomes possible. As a
member of diarylethene family, styrylbenzoxazoles are attractive
building blocks in supramolecular assemblies, luminescent dyes,
N
ature creates many kinds of actuators to transform light,
CO
2
storage and optical data storage materials, but their
[
12]
thermal, or chemical energy into mechanical motion, w hich is
one of the basic processes in life activities. Recently, the smart
materials w ith mechanical effects under light, thermal or
chemical stimuli have received much attention since they are
essential for the generation of mechanically tunable components
for actuation, flexible electronics, artificial muscles, and soft
topophotochemistry is seldom studied. With these in mind, w e
synthesized styrylbenzoxazole derivatives BOACl24, BOACl4
and BOACl2 (Scheme 1). The reason for the introduction of
chlorine in different position of benzene w as that some crystals
formed via halogen···halogen as w ell as - interactions have
been found to exhibit photomechanical effects. It is interesting
that the molecular crystals of BOACl24 show ed photo-induced
bending, curling, rolling and salient behaviors depending on the
shapes of the dynamic molecular crystals.
[
13]
[
1]
microfluidic
devices.
Particularly,
the
photo-induced
mechanical motion can be remotely controlled w ithout any w ire
connections, w hich may simplify the structure and reduce the
Firstly, controllable photo-induced bending w as observed for
the slender needle-like crystal of BOACl24. As show n in Figure
, before UV irradiation, tw o needle-like crystals w ere stacked
[
2]
weight of the device.
actuators driven by light are fabricated from photoactive
Most of the current macroscopic
1
[
3-4]
elastomeric or liquid-crystalline materials.
How ever, the
together almost in parallel. Upon irradiation perpendicularly from
dow nside by 365 nm light, the thinner crystal bent upw ard
obviously, w hile the thicker one bent upw ard slightly (photos in
upper row in Figure 1). After that, the irradiation from upside
could pull the tw o bent needle-like crystals into straight ones
(photos in middle row and the front four ones in bottom row in
Figure 1). Further changing the irradiation direction into
dow nside, the crystals bent upw ard again (the last tw o photos in
bottom row in Figure 1). The above bending cycle could be
repeated for several times. Since the photo-induced bending
aw ay from the light source for the thinner needle-like crystals
was more significant than the thicker ones, a bunch of needle-
like crystals based on BOACl24 could be bloomed and merged,
like a flow er, by changing the irradiation direction (Video S1).
mechanical responsive materials w ith fast energy transfer, good
resistance to fatigue and controllable motility are required in
future applications. Coincidentally, dynamic molecular crystals
emerged as a new platform to achieve rapid energy transfer
through the dense and ordered packing crystalline lattices on
account of low energy dissipation and enhanced coupling
[
5]
betw een light and mechanical energy. It has been found that
the crystals based on photochemical active organic compounds
of azobenzenes, anthracenes, diarylethenes and furylfulgides
[
6]
[7]
[8]
[9]
are capable of bending, curling, tw isting, craw ling and
[
*]
H. Wang, Z. Wu, J. Zhao, Dr. J. Sun, and Prof. R. Lu
State Key Laboratory of Supramolecular Structure and Materials,
College of Chemistry, Jilin University, Changchun, China
E-mail: luran@mail.jlu.edu.cn
Dr. P. Chen
Key Laboratory of Functional Inorganic Material Chemistry (MOE),
School of Chemistry and Materials Science, Heilongjiang University,
Harbin, China
Scheme 1. Molecular structures of styrylbenzoxazole derivatives..
Supporting information for this article is given via a link at the end of
the document.
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Angewandte Chemie International Edition
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appeared at 5.32 ppm and 5.12 ppm w hen irradiating the
crystals of BOACl24 for 1 min. They could be ascribed to the
signals for H in cyclobutane formed via [2+2] cycloaddition, and
their integral values increased gradually w ith prolonging the
irradiation time on account of the further dimerization.
Meanw hile, the integral values for the peaks at 8.16 ppm and
8.00 ppm corresponding to H in vinyl unit decreased gradually. It
was clear that the new ly emerging peaks in the range of 7.1-7.9
ppm for the sample gained by irradiating the crystals of
BOACl24 at 365 nm for 5 min (Figure S2f) w ere quite similar to
those for DBOACl24 (the dimer of BOACl24, Figure S2g).
Therefore, the photo-induced [2+2] cycloaddition reaction w as
the driving force for the transformation of the light energy into
motilities in crystals of BOACl24.
Figure 1. Photos of the needle-like crystals of BOACl24 before and after
irradiation by 365 nm light for d if ferent time observed by naked eyes (the
arrows indicate the irradiation direction).
This is an interesting example of the reversible bending of the
crystals aw ay from the light source driven by [2+2] cycloaddition,
which is important in the generation of controllable
photomechanical devices.
Besides the bending of needle-like crystals of BOACl24, w e
found that the crystals could jump aw ay from its original
positions under UV irradiation for a w hile as show n in Videos S2
and S3. Further prolonging the irradiation time, the bent crystals
exploded into many pieces (Video S3). Such photosalient effect
could also be observed for the rod-like crystal. As show n in
Figure 2, the fluorescence for the longest rod-like crystal of
BOACl24 w as intensified gradually w ith increasing the UV
irradiation time. When the crystal w as irradiated by UV light for
35 s, the domains w ith strong bluish violet fluorescence w ere
observed. Finally, the crystal exploded into fragments, some of
which moved out of the visual field w hen the irradiation time w as
prolonged to 40 s. It should be noted that the photo-induced
curling of a straight slim fiber in low er right side in Figure 2 w as
detected, and the small crystal hang onto the fiber could move
together w ith the fiber during the curling. It meant that the photo-
induced curling of the fiber could flick light object, w hich w as
similar to the small bud of plants to push the soil.
Figure 2. Optical microscopy images of the rod-like crystal of BOACl24 before
(
a, under daylight) and after irradiation by 365 nm light for 5 s (b), 10 s (c), 15
s (d), 20 s (e), 25 s (f), 30 s (g), 35 s (h) and 40 s (i).
To reveal the driving forces for the above photomechanical
In view of the fact that the different photomechanical effects
are alternative channels for the relaxation of the energy
1
effects, the fluorescence emission, UV-vis absorption and
H
[
5]
NMR spectral changes before and after UV irradiation of the
accumulated during photo-induced structural transformation.
crystals of BOACl24 w ere studied. As show n in Figure S1,
we make an assumption that the w ay for the release of strain
affected the mode of photomechanical motion. The photosalient
behavior occurs as a result of instantaneous release of the
colossal strain that has accumulated in the rod-like crystal
interior, and the photo-induced bending or curling undergoes via
slow release of the internal strain in soft needle-like
before irradiation the crystals of BOACl24 gave
a broad
emission centered at ca. 470 nm, w hich blue-shifted to ca. 420
nm after irradiation for 20 s w ith 365 nm UV light, so that the
emitting colors turned from cyanine blue to bluish violet. The
emission intensity at ca. 420 nm increased w ith prolonging the
irradiation time. Meanw hile, the crystals split into smaller pieces
during UV irradiation, suggesting photomechanical motion
[
6c,10b]
crystals.
In order to confirm that, w e prepared more flexible
1D nanofibers via the organogelation of BOACl24, w hich is a
[
14]
(
insets in Figure S1a and Video S4). Additionally, BOACl24
facile method to fabricate soft materials. Figure S7 illustrated
that 3D netw ork consisting lots of nanofibers w as generated in
methanol organogel of BOACl24. It w as found that the
nanofibers curled drastically, even into a circle, w hen irradiated
by UV light (Figure 3b and Video S5). Meanw hile, the rolling of a
small slice w as observed in organogel of BOACl24 under UV
light (Figure 3a). Therefore, the ordered self-assemblies formed
via the organogelation of photochemical active gelators are
candidates as photomechanical materials, w hich is rare reported.
gave the maximal absorption at ca. 330 nm in original crystals,
which decreased gradually w hen irradiated by UV light,
accompanied w ith the appearance of new absorption at ca. 275
nm (Figure S1b). Notably, an isoabsorptive point at 290 nm
appeared, meaning the formation of new species. According to
[
6a]
the reported literatures, w e deemed that the photodimerization
of olefin molecules happened, leading to the above
photochromism. As show n in Figure S2, tw o new double peaks
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Additionally, the single crystal structure of the dimer
DBOACl24 w as obtained so as to reveal the photomechanical
mechanism for the molecular crystals of BOACl24 (Figure 4d-e
and Table S2). The bond length of CC in four-membered ring
were 1.580 Å and 1.574 Å, respectively, in single crystal. It
meant that each carbon atom in original C═C had to move ca.
1.1 Å during photodimerization. Meanw hile, the new ly formed
cyclobutane impel benzoxazole and benzene to move aw ay
from four-membered ring, like a bird tilting the w ings. As a result,
the distance betw een the outmost Cl atoms reached 7.783 Å
and the outmost carbon atoms in benzoxazole became 6.435 Å.
The movement of atoms during the photodimerization creates
and accumulates strain in crystals, w hich w ould be released via
Figure 3. Optical microscope images of the assemblies in methanol organogel
of BOACl24 before and after irradiation by 365 nm light. (a) rolling of a small
slice and (b) curling of the nanofibers.
Different from BOACl24, no photomechanical motion w as
detected for the crystals of BOACl4 and BOACl2. In order to
reveal the different mechanical responsive behaviors to light, w e
determined the single crystal structures of BOACl24, BOACl4
and BOACl2 (Table S1, Figures 4, S5 and S6). Figure 4
illustrated that the dihedral angle betw een benzene and
benzoxazole in BOACl24 is 32.10°, and the molecules pack into
triclinic space group P 1̅ in single crystal. The adjacent molecules
[
15]
mechanical motion.
The crystals w ith high length-diameter ratio are normally soft
and flexible. When the light source w as applied vertically, the
dimerization of the molecules took place, leading to the
outspread of the new ly formed DBOACl24. Therefore, the
needle-like crystals w ould bend or the nanofibers w ould curl
aw ay from the light source w hen the strain accumulated during
[
6c]
photo-induced structural transformation w as released slow ly.
are aligned in parallel w ith a distance of 3.865 Å (Figure 4a-c),
which is less than 4.20 Å, so that [2+2] cycloaddition reaction
It should be noted that the bending and unbending could be
controlled by irradiating the needle-like crystal from different side
for a short time, such as several seconds (Figure 1 and Video
S1-S3). During one cycle of bending/unbending, the molecules
on the surface of crystal underw ent [2+2] cycloaddition reaction.
At this stage the molecules inside the crystal may not be fully
dimerized since the crystal could be further bent. This point can
be supported by the fact that lots of BOACl24 molecules w ere
not dimerized after UV irradiation for 5 min (Figure S2f) and 20
min of irradiation time w as needed for the generation of
DBOACl24 (Figure S2g) from BOACl24. In the case of rod-like
crystal, more olefin pairs w ere transformed into cyclobutanes
upon UV irradiation on account of large lateral area, yielding
strong strain. When the rigid crystals could not w ithstand the
colossal strain, they exploded rapidly and show ed photosalient
behavior.
[
3d-e,11]
might occur in crystal.
How ever, the distances betw een the
olefin pairs in adjacent molecules are 4.630 Å and 6.123 Å,
respectively, in single crystals of BOACl2 and BOACl4 (Figure
S5 and S6), w hich are larger than 4.20 Å. Thus, no
[
3d-e,11]
photodimerization is possible.
Besides the distance
betw een olefin pair, the angles of θ2 and θ3 w ere also
considered to be geometric criteria for photodimerization (Chart
S1 and Table S2). The more closer to 90° of θ2 and θ3, the
more easier for photochemical reaction. The θ2 and θ3 in single
crystal of BOACl24 are 71.75° and 82.39°, but they w ere 61.09°
and 60.70° in single crystal of BOACl4, as w ell as 24.65° and
76.13° in single crystal of BOACl2. Therefore, the
photomechanical effect cannot be observed from the crystals of
BOACl2 and BOACl4 on account of the absence of [2+2]
cycloaddition.
Additionally, Reddy and Desiraju have designed flexible
molecular crystals formed from halogenbenzene-containing
compounds, w hich could bend upon stimulation by mechanical
[
16a-b]
stress.
Herein, w e also observed similar mechanical
deformation of the needle-like crystal of BOACl24. As show n in
Video S6, w hen one straight needle-like crystal, w hich w as held
w ith a pair of forceps, w as pushed w ith a metallic needle from
the opposite side in silicon grease, it bent obviously w ithout
breaking. Upon w ithdraw ing the external force, the crystal w as
unbent quickly into a straight one w ithout cracking. The above
bending/unbending process could be repeated several times.
Therefore, the bending of needle-like crystal of BOACl24 w as
[
16c]
elastic instead of plastic
anti-fatigue.
deformation w ith a certain degree of
It is clear that [2+2] cycloaddition of BOACl24 yields at least
four kinds of cyclobutane derivatives if the product
stereochemistry is not defined by the original crystal structure.
Herein, only single product of DBOACl24 w as detected.
Carefully observing the molecular packing in the single crystal of
BOACl24, the oxygen atoms in tw o adjacent parallel
benzoxazole units in the precursors are located at the same side
view ed along molecular long axis (Figure 4a-c), but the tw o
Figure 4. Single crystal structure of BOACl24 viewed along b-axis (a) and c-
axis (b), the distance between two adjacent parallel molecules (c), molecular
configuration of DBOACl24 in single crystal (d and e).
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Angewandte Chemie International Edition
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oxygens are situated at the opposite sides of cyclobutane plane
in single crystal of DBOACl24 (Figure 4d-e). It is know n that the
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BOACl24 to afford DBOACl24. To best of our know ledge, such
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In summary, shape-dependent photo-induced bending, curling,
rolling and salient behaviors of the molecular crystals of
BOACl24 w ere observed. Interestingly, the reversible
controllable bending of the needle-like crystals aw ay from the
light source driven by [2+2] cycloaddition w as observed, w hich
was resulted from the slow release of the strain accumulated by
the movement of the atoms during photodimerization of
BOACl24. We also found the photosalient effect of the crystals
based on metal-free olefin BOACl24 on account of the rapid
release of the strong internal strain originated from the photo-
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the
3
28.
photomechanical motion has seldom been reported in organogel,
the nanofibers in methanol organogel of BOACl24 w ere found to
curl under UV irradiation. In particular, w e found a rotation of
benzoxazole ring around CC single bond upon UV irradiation
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This w ork w as financially supported by the National Natural
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Keywords:photomechanical effects•dynamic molecular
crystals•topophotochemical•styrylbenzoxazole•[2+2]
cycloaddition
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The dynamic molecular crystals of
Haoran Wang, Peng
Chen, Zhu Wu, Jinyu
Zhao, Jingbo Sun, and
Ran Lu*
(
dichlorostyryl)benzooxazole
BOACl24exhibited photomechanic
motion induced by topophotochemical
dimerization.
Page No. – Page No.
Bending, curling, rolling
and salient behaviors of
molecular crystals
driven by [2+2]
cycloaddition of
styrylbenzoxazole
derivative
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