Photoinduced bending of a large single crystal of a 1,2-bis(4-pyridyl) ethylene-based pyridinium salt powered by a [2+2] cycloaddition

Article abstract of DOI:10.1002/anie.201301207

Rolled-up crystals: Photoinduced bending of large single crystals has been realized based on a simple organic small molecule (see picture). The bending process is accompanied with high visual fluorescence contrast, which is essential for remote detection of photomechanical work. Copyright

Full text of DOI:10.1002/anie.201301207

Angewandte
Chemie
Communications
Photomechanical Motion
Rolled-up crystals: Photoinduced bend-
ing of large single crystals has been
realized based on a simple organic small
molecule (see picture). The bending
process is accompanied with high visual
fluorescence contrast, which is essential
for remote detection of photomechanical
work.
J. K. Sun, W. Li, C. Chen, C. X. Ren,
D. M. Pan, J. Zhang*
&&&& — &&&&
Photoinduced Bending of a Large Single
Crystal of a 1,2-Bis(4-pyridyl)ethylene-
Based Pyridinium Salt Powered by a [2+2]
Cycloaddition
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DOI: 10.1002/anie.201301207
Photomechanical Motion
Photoinduced Bending of a Large Single Crystal of a 1,2-Bis(4-
pyridyl)ethylene-Based Pyridinium Salt Powered by a
[2+2] Cycloaddition**
Jian-Ke Sun, Wei Li, Cheng Chen, Cai-Xia Ren, Dan-Mei Pan, and Jie Zhang*
Transducing an input energy stimulus into output energy that
can power or accomplish mechanical work is a pervasive topic
of many scientific disciplines. One of the current issues is to
harness the natural and renewable energy sources which have
spawned an ever-burgeoning pursuit of novel materials. In the
last few years, the research on molecular machines based on
the conversion of the molecular geometry change into
mechanical motion of macroscale materials has attracted
much attention.[1] Although elaborate design and synthesis of
various types of supramolecular machines have been
reported, to amplify this nanoscale molecular motion in
large-scale bulk materials is still a great challenge. Owing to
the advantages in remote, temporal and spatial detection,
light-induced deformable materials have emerged as excel-
lent candidates for photomechanical applications.[2] Recently,
some solid-state photoreactions in highly ordered molecular
crystals, such as open-closed ring isomerization,[3] cis–trans
isomerization[4] and photodimerization[5] have been actively
investigated since these reactions are often accompanied by
molecular motion, and thus cause morphological changes on
the surfaces of single crystals. These achievements show great
promise in linking the molecular-scale deformation to macro-
scale movement of the densely packed crystal. Nevertheless,
the reported crystalline materials usually are quite small (on
the nanometer or micrometer scale), which may limit their
practical applications in real world. Until now, the rational
design and assembly of small organic molecules that can
generate large macroscopic photomechanical movement still
remain a challenge.
try. Although numerous examples of the solid-state
[2+2] cycloaddition have been demonstrated,[6] the explora-
tion of the photomechanical effect on this molecule is quite
scarce. The reason is that the restricted motion of this rigidly
conjugated small molecule in solid-state reactions limits the
microscopic motion to be amplified to macroscopic move-
ment of the bulk materials (especially in a large-scale single
crystal). Furthermore, to promote favorable stacking for
a cycloaddition reaction, template-induced and coordination-
driven self-assembly techniques have been commonly used,
which also bring about much more restrictions on the freedom
of molecular motion. Herein we report an interesting example
of photomechanical motion based on the 4,4’-bpe derivative,
1-(4-carboxybenzyl)-4-[2-(4-pyridyl)-vinyl]-pyridinium chlo-
ride (HBCbpeCl; Scheme 1). Compared with the conven-
The trans-1,2-bis(4-pyridyl)ethylene (4,4’-bpe) molecule is
a kind of photoactive molecule that can undergo photo-
induced [2+2] cycloaddition under a proper packing geome-
Scheme 1. The cationization of one end of the 4,4’-bpe molecule,
a head-to-tail arrangement and photo-cycloaddition.
[*] J. K. Sun, C. Chen, C. X. Ren, D. M. Pan, Prof. J. Zhang
State Key Laboratory of Structural Chemistry
Fujian Institute of Research on the Structure of Matter
CAS Fuzhou, Fujian 350002 (P.R. China)
tional 4,4’-bpe molecule, the incorporation of the carboxy-
benzyl group onto one end of this molecule affords some
advantages that may facilitate the cycloaddition and motion
amplification: 1) The addition of the methylene group
effectively increases molecular flexibility. 2) A cation–p
interaction between the pyridinium ring and the terminal
pyridine ring is likely to form and induce a tight molecular
stacking in a head-to-tail (HT) fashion,[7] which is beneficial
not only in facilitating cycloaddition reactions, but also in
ensuring freedom of motion of the substituted side group.
3) The terminal groups of BCbpe can be connected by
hydrogen bonds to each other upon the degree of protona-
tion, or form hydrogen bonds with some solvent molecules,
which may allow the cohesive force between the molecules to
E-mail: zhangjie@fjirsm.ac.cn
Dr. W. Li
Department of Materials Science and Metallurgy
University of Cambridge, Cambridge CB2 3QZ (UK)
[**] We thank the National Natural Science Foundation of China for
financial support (grant numbers 21271173 and 20973171). W. Li is
grateful to the University of Cambridge and the European Research
Council for providing financial support. Prof. A. K. Cheetham at the
University of Cambridge is acknowledged for the great help in
measuring the Young’s modulus.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301207.
2
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be finely tuned by environmental manipulation, and thus
render the crystal resistance to mechanical stress to avoid
fragmentation. Attractively, the needle-like single crystal 1
(BCbpe·2H2O) obtained by self-assembly of the HBCbpeCl
compound can undergo remarkable deformation in the
centimeter scale upon photoirradiation, and become lumi-
nescent on distortion, which gives a big advantage for remote
detection of mechanical movements.
Single crystals of 1 were obtained by slow evaporation of
the NaOH-neutralized HBCbpeCl in methanol–water mixed
solvents within a few days. The typically long needlelike
crystals (with the size of 0.5–3.6 cm in length) grew along the
a axis (for the face indexing see the Supporting Information).
Mounted at one end with a tweezers and exposed to a Xe
lamp for about 2 s (200 mWcmꢀ2), the needlelike crystal
(1.5 ꢀ 0.08 ꢀ 0.12 cm3) undergoes a dramatic deformation that
is perpendicular to its longest dimension, bending towards the
light source without breaking (Figure S1). To further test the
deformation ability of the single crystal, a free-standing single
crystal (0.82 ꢀ 0.07 ꢀ 0.05 cm3) on a filter paper is irradiated
with a Xe lamp in a controlled manner (Figure 1). The
The observed photomechanical effect should be related to
the [2+2] cycloaddition reaction of the BCbpe molecules in 1.
1
To verify the [2+2] photo-cycloaddition reaction, H NMR
measurements on the crystals were carried out before and
after photoirradiation (Figure S2). It has shown that about
9% of the initial reaction was performed after irradiating
a single crystal (3 mg) for 10 s (200 mWcmꢀ2). The small yield
of the photoproduct is similar to those observed in the
azobenzene or diarylethene derivatives with photomechan-
ical effects,[2–4] which is reasonable since the photoreaction
generally occurs on the surface of the crystal because of the
high absorbance of the crystals in the UV/Vis region.
However, such a proportion is large enough to induce
1
bending of the crystals. H NMR spectroscopy revealed that
the olefinic groups have reacted to form cyclobutane rings in
about 85.4% yield after photoirradiation for 48 h. To further
demonstrate the photodimerization reaction, the powdered
crystalline 1 was irradiated for a prolonged time, and then
dissolved in mixed water–methanol solvents. X-ray single-
crystal diffraction analysis on the recrystallized yellow com-
pound definitely confirmed the formation of a HT-type s-
dimer after photoirradiation (Figures S3 and S4).
For a [2+2] cycloaddition reaction to occur in the solid
state, the adjacent reactive double bonds should be packed in
parallel with a distance less than 4.2 ꢁ. X-ray crystallographic
analysis reveals that 1 crystallizes in the monoclinic space
group P21/c. The bpe moieties stack in columns along the
crystallographic a axis and are oriented in a HT manner with
alternate distances of 3.60 and 3.65 ꢁ between the reactive
olefin carbon atoms, and the distances of 3.50 and 3.74 ꢁ
between adjacent pyridine and pyridinium rings, suggesting
a significant contribution and role of the cation–p interaction
in controlling the molecular packing (Figure 2 and Figure S5
in the Supporting Information). The corresponding geomet-
[8]
rical parameters q1, q2, q3, D1, and D2 (see Table S1 in the
Supporting Information) are in accordance with the high
photoactivity of the crystal 1. The carboxybenzyl group
substituted on a nitrogen atom of the bpe molecule is twisted
by 88.948 from the bpe plane, hanging alternately on the two
sides of the bpe column with larger interval. Interestingly, two
carboxylate oxygen atoms from different columns, together
with four symmetry-related water molecules, form a hydro-
gen-bonded six-membered ringlike buckle (O···O distances of
2.77–2.79 ꢁ), which associates with each other through weak
hydrogen-bonding interactions (the O···O distance: 2.85 ꢁ)
and holds the 1D column units into 2D undulating layer.
These layers are stacked in a -ABAB- pattern with the six-
membered ring unit and bpe moiety staggered relative to each
other.
Although numerous solid-state [2+2] cycloaddition reac-
tions based on 4,4’-bpe derivatives have been demonstrated,
no photomechanical effect has been observed for their
molecular assemblies. Besides a disadvantage caused by the
intrinsic rigidity of the 4,4’-bpe molecule, the prevalent
strategies to align the olefinic bonds in optimal orientation
for cycloaddition, including template or coordination-driven
self-assembly, also restrict the mobility of molecules to some
extent. From the structurally well-characterized examples
that undergo photoinduced cycloaddition in the solid state,[9]
Figure 1. The roll up of a single crystal (0.82ꢀ0.07ꢀ0.05 cm3) with
controlled irradiation. The irradiation light toward the top side of the
crystal is perpendicular to its (001) face. The light is focused on
different areas in the (001) face at each time.
irradiation light toward the top side of the crystal is
perpendicular to the (001) face. Firstly, we focus the
irradiation on the top of right side of the crystal. The edge
is bent toward the light source. Then we do the same
procedure on the top of the left side of the single crystal to roll
up another end. Finally, we direct the concentrated light
irradiation on the top of the middle of the crystal. The crystal
tends to bend around its middle part and shows a tendency to
enclose two ends of the crystal. Further effort to fabricate the
crystalline ring is failed because of the cracking of the whole
crystal, which may be caused by the release of large
accumulated structural stress in the crystal lattice. However,
the magnitude of bending is unprecedented in deformable
crystalline materials especially in view of its large scale (in the
centimeter range). It is noted that such deformation is plastic,
so that the original shape of the crystals is not restored, even
after they have been aged in the dark for a few days.
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macroscopic crystal growth directions (Figur-
es S6–S8), we deduce that the photo-cycloaddi-
tion generates a contraction force, which is
accumulated and amplified through the infinite
p-stacked parallel column, resulting in the
anisotropic contraction of the (001) plane.
Since the high absorbance of the dense crystal
hinders a deeper penetration of light, the photo-
reaction only occurs in the surface region of the
crystal. The difference in the degree of contrac-
tion leads to the bending of the crystal toward
the irradiated light (Figure 2). This bending
process is reminiscent of the bilayer cantilever
effect. The cantilever becomes curved by a bend-
ing moment caused by the mismatch of the stress
in each layer.[11] The crystal presented here may
be also considered to be composed of reacted
and unreacted layers with different local
stresses, which induce the bending of the crystal.
The molecular deformation induced by [2+2]
cycloaddition can also be reflected in the micro-
scopic surface of the single crystal. We use the
real-time AFM images to record the surface
morphological changes after photoirradiation.
The original surface of the crystal is relatively
Figure 2. Illustrations of the bending direction of crystal 1 and related crystal
structure. 1) The 2D supramolecular layers are stacked in an -ABAB- manner. 2) The
molecular arrangement in the (001) face. 3) The [2+2] cycloaddition induces the
contraction of the planes parallel to the (001) face, leading to the bending
perpendicular to the a axis of the crystal. All hydrogen atoms in the structures were
omitted for clarity.
it can be found that the 4,4’-bpe molecules are anchored to the
molecular skeleton by coordination or hydrogen-bonding
interactions, their molecular movements are difficult to be
propagated out because of the restriction of a rigid geometry.
By engineering 4,4’-bpe molecule to meet the condition for
forming cation–p interactions, the derivative BCbpe mole-
cules are directed into a parallel head-to-tail close arrange-
ment suitable for cycloaddition without the assistance of
a template, and stacked in infinite columns that are favorable
for transferring nanoscale cycloaddition motion along a spe-
cific direction. It should be noted that the photoreactivity of
the bpe-based derivatives arranged in an infinite parallel
array congenial for a cycloaddition reaction is still less
explored and no deformation phenomenon of crystals has
been reported.[6a,10] Compared with these molecular assem-
blies, the present compound possesses additional structure
advantages responsible for its macroscopically photomechan-
ical deformation: 1) The introduction of the carboxybenzyl
group increases the molecular flexibility because of the freely
rotating CH2 junction, allowing large conformational change
to occur in response to environmental stress. 2) The adjacent
BCbpe molecules are joined together by hydrogen bonds
between the carboxylate oxygen atoms and water molecules.
Each junction is present as six-membered ringlike structure
and associates with each other through weak hydrogen-
bonding interactions in a parallelogramlike linkage pattern.
By virtue of these deformable hexagon and parallelogram
structures, the hydrogen-bonded molecular layer possesses
a high binding strength and flexibility to withstand greater
tensile strain. 3) The BCbpe molecules and flexible joints in
adjacent layers are arranged in a staggered pattern. This
arrangement is expected to increase the deformation toler-
ance of the molecular aggregates to mechanical stress. By
analyzing the relationship between microscopic packing and
smooth with an average roughness of 1.1 nm. Upon irradi-
ation, the surface of initial crystal becomes rugged and the
roughness has increased gradually to 2.6 nm then to 6.7 nm
with increasing irradiation time, accompanied by the appear-
ance of ridge-like structures (Figures S9 and S10).
To reveal the mechanical strength of the single-crystal
materials, the Youngꢂs modulus is measured via nanoinden-
tation which is established as an effective method to assess the
mechanical response of solids with high precision. Nano-
indentation measurements are performed using a sharp
Berkovich tip (tip radius ꢁ 100 nm) in a continuous stiffness
measurement (CSM) mode. The indenter axis is aligned
À
Á
ꢀ
normal to the artificially polished (011) and 110 orientated
facets of single crystal 1. Representative P–h curves obtained
on both facets are shown in Figure 3. The loading part of both
facets is smooth, indicating that the plastic deformation,
which occurs underneath the Berkovich tip during indenta-
tion, is relatively homogeneous in nature. The average values
of elastic modulus (E) and hardness (H) normal to (011) and
À
Á
ꢀ
110 of 1, extracted from the P–h curves, are calculated over
depths of 200–1000 nm in order to minimize the imperfection
of the Berkovich tip.[12] Although the loading during inden-
tation is not perfectly uniaxial and the stress field generated
underneath the indenter is nonuniform, indentation can be
used to probe the mechanical properties of single crystals
since the measured modulus is strongly dependent on the
elastic response along the indenter axis and is weakly affected
by the transverse directions.[13] The average elastic moduli
normal to the orthogonal facets were found to be E(011)
=
10.284 ꢂ 0.204 GPa and E
¼9.670 ꢂ 0.371 GPa, which are
ꢀ
110
ð
Þ
much larger than those of typical polymeric materials (ca.
1 GPa) and are comparable to a handful of organic crystals
(8.5–15 GPa).[3b,13a,b,14] The high Youngꢂs modulus and large
deformation of 1 indicate that the photocycloaddition reac-
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contrast with usual on–off luminescence photoswitching
systems, the examples involving the photoswitching from
nonfluorescent to fluorescent state are scarce,[15] although
great interest has been aroused due to their high contrast ratio
(102) and powerful applications in super-resolution fluores-
cence imaging that requires initial dark background.[16] In
particular, such distortion-promoted luminescence process is
essential for remote detection of photomechanical work
because of its high visual contrast. To explore the emission
mechanism of this process, the photoluminescence behavior
of the BCbpe molecule in solution is studied. Remarkably, the
emission profile in methanol is quite similar to that of the
photoirradiated sample, and exhibits the wavelength depen-
dent emission behaviors (Figure S12). Moreover, the pure s-
dimer is almost nonluminescent under the same condition.
These results reveal that the photoluminescence switching
occurring simultaneously during the deformation process
should originate from the liberated BCbpe monomers. The
excitation wavelength-dependent emission profiles from dual
to single emission behavior are attributed to the enhancement
of intramolecular charge transfer (ICT) emission as it is
consistent with an observation on the cationic stilbazolium
derivatives incorporating both donor and acceptor in solution
state,[17] and it is further demonstrated by solvent polarity-
dependent emission (Figure S13). We speculate that the [2+2]
cycloaddition may induce the changes in molecular confor-
mation or packing pattern in the crystals and thus weaken the
intermolecular interaction of partial BCbpe molecules, lead-
ing to a fluorescence change from quenching to emitting state.
In summary, we have successfully realized photodriven
mechanical movement of crystalline materials on the macro-
scopic scale based on a derivate of the 4,4’-bpe molecule. The
deformation scale of the single crystal is by far the largest
among the reported photomechanical crystalline materials.
Attractively, the deformation process is accompanied with
high fluorescence visual contrast, which makes it an excellent
candidate for remote detection of photomechanical work.
The present work may provide some clues for developing
macroscopically photomechanical crystalline materials based
on small organic molecules.
Figure 3. Representative P (Load)ꢀh (h=indentation depth) curves for
ꢀ
1 with the ð110Þ and (011) orientated facets measured by a Berkovich
tip. Inset: elastic moduli E of 1 as a function of the indentation depth,
wherein each error bar represents the standard deviation from
20 indents.
tion could generate a strong force to exceed the elastic
threshold of the single crystal and carry out large mechanical
work.
The photoirradiation not only generates a photomechan-
ical effect, but also induces the luminescence switching. The
crystalline sample is nonluminous in the initial state but shows
highly visual red luminescence emission under UV light (l =
365 nm) after several minutes of irradiation (Figure 4 and
Figure S11). Spectral measurements reveal that the photo-
irradiated sample exhibits a wavelength-dependent dual
emission behavior. Upon excitation at 385 nm, two emission
peaks centered at 458 and 647 nm can be detected. With
increasing the excitation wavelength, the spectra are domi-
nated by the emission in the long-wavelength region. In
Received: February 11, 2013
Revised: April 19, 2013
Published online: && &&, &&&&
Keywords: cycloaddition · photomechanical properties ·
.
pyridinium salts · single crystals
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